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Challenges for Group III Nitride Semiconductors for Solid State Lighting and Beyond

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Due to its extremely versatile and interesting properties, Gallium Nitride has emerged into the most important semiconductor material in the world - besides silicon. Gallium Nitride has developed into a core technology in areas like solid state lighting, high power and high frequency electronics and sensing, with tremendous impact on modern society. The symposium will bring together the international Gallium Nitride community in order to discuss problems, prospects and future developments of this fascinating material.

 

Scope:

Solid State Lighting (SSL) technology is expected to penetrate everyday life with affordable devices in the next 10 to 15 years. SSL sources, completely based on the semiconductor material Gallium Nitride, will replace inefficient and environmentally harmful conventional light sources, and therefore contribute significantly to a sustainable energy saving and carbon footprint reduction. In order to further support this transition, white LEDs with higher efficiencies and lower cost per lumen have to be developed.. In addition, Gallium Nitride devices will enter the high power electronics as well as sensor market, making Gallium Nitride the most important semiconductor material in the world – besides silicon. In contrast to silicon, however, Gallium Nitride material quality is still suffering from major problems, like high defect densities, limited capability of producing highly resistive material, challenges concerning MOS technology, as well as problems in obtaining high quality InGaN independent of indium concentration.
In the recent past, the growth of 3-dimensional Gallium Nitride as well as Gallium Nitride nanorods has further opened a whole new area of nitride materials research. Gallium Nitride is challenging the materials science community worldwide.

The symposium will focus on the most challenging problems of nowadays Gallium Nitride technology: the green gap, green and blue laser diodes, cost efficient fabrication of high power LEDs, fabrication of 3-dimensional Gallium Nitride, growth of low defect density material for GaN based electronics, alternative growth techniques, hybrid Gallium Nitride – silicon technology, nanometrology of 3D nitrides. Beyond that, the symposium will address challenges concerning suitable phosphor materials for white lighting, as well as novel developments for a completely new approach for producing white light: laser lighting. The symposium will take advantage of a substantial participation of research groups from industry.

 

Hot topics to be covered by the symposium:

  • 3D Gallium Nitride technology
    3D Gallium Nitride has emerged into a very interesting sub-field of nitride technology. 3D GaN (nanrods, nanowires) offers various advantages in contrast to thin films: zero defect density, large surface area, photonic crystal control on the electromagnetic modes etc.- Low defect density materialFor many applications (especially in electronics) the growth of low defect density Gallium Nitride is still a big challenge.
  • Overcoming the green gap
    The reduction of quantum efficiency in the green spectral range is still a big challenge for the development of green light emitters like laser diodes and high efficiency LEDs.
  • High efficiency phosphors
    In a white LED, suitable phosphor materials are converting blue light from an GaN LED into “white” light. These phosphors are extremely important for the efficiency of the LED. There is a large effort worldwide to improve the characteristics of the phosphors in terms of efficiency at higher temperatures and a suitable adaptation of the absorption spectrum to the emission of a blue LED.
  • Green laser diodes
    The lack of green emitting semiconductor laser diodes is still a big challenge for material scientists. The availability of all three colors would boost the development of compact, mobile beamer technology as well as other applications in the field of biosensing, medicine etc.
  • Laser lighting
    Laser lighting is one of the new directions which is explored in order to supply white light with high directionality. Laser lighting suffers from limited phosphor performance in terms of temperature stability, as well as limited powered of respective laser diodes. In order to make laser lighting happen, substantial progress concerning material development for phosphors and laser diodes has to be made.

 

List of confirmed invited speakers:

  • ALEDIA (name to be confirmed)
  • Gerd Bacher (University of Duisburg)
  • Jürgen Christen (University of Magdeburg)
  • Hiroshi Fujioka (University of Tokyo)
  • Nicolas Grandjean (EPFL Lausanne)
  • Andreas Hangleiter (Braunschweig University of Technology)
  • Michael Heuken (Aixtron)
  • Axel Hoffmann (Berlin University of Technology)
  • Gregor Koblmueller (TUM Munich)
  • Alois Krost (University of Magdeburg)
  • Martin Kuball (University of Bristol)
  • Robert Martin (University of Strathclyde)
  • Eva Monroy (CEA, France)
  • OSRAM AG (name to be confirmed)
  • Daniel Prades (University of Barcelona)
  • Henning Riechert (Paul Drude Institute Berlin)
  • Angela Rizzi (University of Göttingen)
  • Carsten Ronning (University of Jena)
  • Lars Samuelson (Lund University)
  • Jim Speck (UCSB)
  • Holger von Wenckstern (University of Leipzig)
  • Tobias Voss (University of Bremen)
  • Bernd Witzigmann (University of Kassel)
  • C.C. Yang (National Taiwan University)

 

Scientific Committee:

  • Bruno Daudin (CNRS/UJF Grenoble)
  • Jürgen Gutowski (University of Bremen)
  • Colin Humphreys (Cambridge University)
  • Sergey Ivanov (IOFFE St. Petersburg)
  • Eoin O´Reilly (Tyndall National Institute, Ireland)
  • Ferdinand Scholz (University of Ulm)

 

Proceedings:

The proceedings of this symposium will be published in Physica Status Solidi (c). High quality manuscripts will be published as part of a special issue on GaN technology in Physica Status Solidi (a).

 

Symposium organizers:

 

Andreas Waag
Braunschweig University of Technology
Hans-Sommer-Strasse 66
38106 Braunschweig
Germany
Phone: +49 531 391 3773
Fax: +49 531 391 5144
a.waag@tu-bs.de

 

Enrique Calleja
ISOM, Polytechnical University
Ciudad Universitaria
Madrid
Spain
Phone: +34 913367315
Fax: +34 913367323
calleja@die.upm.es

 

Martin Strassburg
OSRAM Opto Semiconductors GmbH
Leibnizstr. 4
93055 Regensburg
Germany
Phone: +49 15114513421
Fax: +49 9418504445615
martin.strassburg@osram-os.com

 

Dave Cherns
University of Bristol, UK
School of Physics
Tyndall Av.
BS8 1TL
UK
Phone: +44 11792 88702
Fax: +44 11792 55624
d.cherns@bristol.ac.uk

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09:40
Authors : Che-Hao Liao, Charng-Gan Tu, Chia-Ying Su, Wen-Ming Chang, Horng-Shyang Chen, Yu-Feng Yao, Chieh Hsieh, Hao-Tsung Chen, Chih-Kang Yu, Yean-Woei Kiang, Chih-Chung (C. C.) Yang
Affiliations : National Taiwan University, Taipei, Taiwan

Resume : With the nano-imprint lithography and the pulsed growth mode of metalorganic chemical vapor deposition, a regularly-patterned, c-axis nitride nanorod (NR) light-emitting diode (LED) array of uniform geometry with m-plane core-shell InGaN/GaN quantum wells (QWs) is formed. In the pulsed growth for an NR of a constant cross section size, the sources of groups III and V are switched on and off alternatively with fixed supply durations. By varying the supply duration of group III source (TMGa) in the pulsed growth process, the NR cross section can be tapered for growing another section of NR of a different cross-sectional size. Based on this growth technique, a multiple-section GaN NR of changing cross-sectional size can be obtained. When InGaN/GaN QWs are deposited on the sidewalls of the NR, the indium contents and QW thicknesses are different in different sections of different cross-sectional sizes due to different strain relaxation conditions. In this situation, the emission wavelengths of the QWs from different sections are different, leading to multiple-color emission of such an NR array. The pulsed growth conditions for controlling the cross section tapering are to be discussed. Also, the NR growth speeds of different cross-sectional sizes will be compared. The mixture of multiple-color emissions from such an NR into quasi-white light will be demonstrated.

K.K.I.2
11:45
Authors : C.J. Lewins 1, S.M. Lis 1, E.D. Le Boulbar 1, I. Girgel 1, P.R. Edwards 2, R.W. Martin 2, P.A. Shields 1, and D.W.E. Allsopp 1*
Affiliations : 1. Dept. Electrical & Electronic Engineering, University of Bath, Bath, BA2 7AY, UK; 2. Dept. of Physics, SUPA, University of Strathclyde, Glasgow, G4 0NG, UK; *Email: D.Allsopp@bath.ac.uk

Resume : III-nitride core-shell LEDs are considerable topical interest. Their advantages include providing a low cost means of fabricating non-polar LEDs in which the quantum efficiency is not degraded by the Quantum-Confined Stark Effect and increasing the area of the active region over its planar equivalent, potentially reducing the current density in the device and hence droop. A further potential advantage is the ability to engineer the direction of the light output through photonic crystal (PhC) effects, but this requires creating more regular shaped core-shell structures than hitherto achieved. This paper reports the demonstration of previously unobserved strong PhC effects in core shell nanorod LEDs by measurements of the angle-resolved photoluminescence. Highly ordered arrays of regularly shaped InGaN/GaN core-shell LEDs were fabricated by inductively coupled plasma etching GaN cores of different height from an n-GaN template and subsequent MOVPE growth. The resulting uniformly shaped core-shell structures had m-plane sidewalls capped by a {10-11} faceted nanopyramid with a widened region at their intersection with the sidewalls. It was found that shorter core-shell structures operate in a diffractive PhC regime as the in-plane wave vectors of the optical modes before diffraction were independent of azimuthal direction. In arrays of taller core-shell devices light emission occurs at wave vectors that vary with PhC lattice direction, indicating a strong photonic operating regime. FDTD simulations confirm the existence of this strong PhC regime which is associated with the formation in the core-shell array of Bloch modes that couple only weakly with the underlying GaN buffer layer.

K.K.I.7
13:30
Authors : Jonas Ohlsson 1 3, Zhaoxia Bi1 , Rafal Ciechonski 2, Kristian Storm 1, Bo Monemar1, and Lars Samuelson 1´2 3
Affiliations : 1. Lund University, Solid State Physics/Nanometer Structure Consortium, Lund, Sweden 2. Glo AB, Ideon Science Park, Lund, Sweden and Sunnyvale, CA, USA 3. QuNano AB, Ideon Science Park, Lund, Sweden

Resume : Nanowires offer a generic method for realization of dislocation-free GaN grown on either silicon or sapphire substrates, using conventional, high dislocation density, planar GaN as seeding layer. We are using a thin silicon-nitride mask with holes in the range 80-250 nm in diameter and initiate growth under selective-area-growth conditions yielding preferential axial growth of dislocation-free GaN nanowires in the c-direction, [0001]. By modifying growth conditions such that radial growth dominates, we can have the thin nanowires act as ideal substrates on top of which we grow radial LED-device structures on the m-planes (10-10) of the original needle, leading to ideal radial pn-junctions containing single or multiple quantum wells for carrier recombination. We can also choose to grow the second step in a mode that yields 3D nanostructures, which offers very interesting opportunities for growth of ternary µ-substrates and device layers of high crystal quality, for instance InGaN for realization of longer-wavelength emission, or AlGaN for UV-emission. In this talk we show the status of this technology as well as results of a multitude of characterization methods as performed within the EU-funded project “Nanowires for Solid State Lighting – NWs4LIGHT”.

K.K.I.8
15:15
Authors : Marcus Müller1, Benjamin Max1, Gordon Schmidt1, Silke Petzold1, Peter Veit1, Frank Bertram1, Jürgen Christen1, Martin Mandl2, Tilman Schimpke2, and Martin Strassburg2
Affiliations : 1 Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg, Germany 2 OSRAM Opto Semiconductors GmbH, Regensburg, Germany

Resume : During the last years research on III-nitride core-shell microrods has become more intense due to the possibilities for high efficient optoelectronic devices. By taking advantage of the core-shell geometry with a high aspect ratio, the effective light emitting area can be dramatically increased in comparison to conventional planar heterostructures. The core-shell microrod sample was grown by MOVPE on a GaN/sapphire template. Selective area growth using an SiO2 structured masking layer has been applied for the formation of the GaN microrods. The change of growth parameters to conditions adequate for 2-dimensional growth forms large pyramidal cappings. Finally, a single InGaN quantum well (SQW) was deposited. In this study we correlate the optical properties with the crystalline real structure using low temperature cathodoluminescence spectroscopy (CL) directly performed in a STEM. We observe two distinct luminescence contributions of the InGaN SQW which are directly correlated to its morphology. Highly spatially resolved CL mappings of single microrods exhibit an InGaN SQW emission at 400 nm from the non-polar facets. In contrast, the InGaN SQW on the semi-polar facets shows a longer wavelength emission.

K.K.I.12
16:30
Authors : C. Tessarek 1 2, M. Heilmann 1, G. Sarau 1, and S. Christiansen 1 3
Affiliations : 1. Max Planck Institute for the Science of Light, Günther-Scharowski-Str. 1, 91058 Erlangen, Germany; 2. University Erlangen-Nuremberg, Institute of Optics, Information and Photonics, Staudtstr. 7/B2, 91058 Erlangen, Germany; 3. Helmholtz Centre Berlin for Materials and Energy, Hahn-Meitner Platz 1, 14109 Berlin, Germany

Resume : Self-catalyzed GaN rods on sapphire substrates have been grown by metal-organic vapor phase epitaxy [1]. Cathodoluminescence (CL) was used to investigate the optical properties. High quality (Q) factor whispering gallery modes (WGMs) were observed in regular, hexagonal shaped GaN micro- and nanorods [2]. WGMs with Q-factors of up to 700 and 100 are visible in the spectrum of a microrod and nanorod, respectively. Utilizing µ-photoluminescence, even higher Q-factors of up to 4000 have been observed in microrods demonstrating the high morphological quality of the rod structures [3]. The spectral position of the WGMs in a slightly tapered GaN wire can be tuned by changing the position of the fixed electron beam at the sidewall of a wire [4]. Good agreement between the calculated and measured WGMs is achieved. InGaN quantum wells (QWs) are deposited on GaN microrods. Dependent on the growth conditions, the QW emission can be adjusted from ~400-450 nm. CL is detected if the electron beam is scanning the nonpolar InGaN quantum wells. The QWs on the top facets are not optically active. It will be shown that WGMs are still present in GaN microrods covered with InGaN quantum wells. [1] C. Tessarek et al., J. Appl. Phys. 114, 144304 (2013). [2] C. Tessarek et al., Phys. Status Solidi C, accepted. [3] C. Tessarek et al., Opt. Express 21 (2013) 2733. [4] C. Tessarek et al., Jpn. J. Appl. Phys. 52 (2013) 08JE09.

K.K.I.14
17:00
Authors : Yong Tae Kim 1, Ji -Ho Park 2, and Akihiro Wakahara 2
Affiliations : 1. Semiconductor Materials and Devices Lab., Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seoul, Korea; 2. Department of Electronics and Information Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Toyohashi, Japan

Resume : Rare earth ion (REI)-doped III-nitride has been very attractive for next generation high luminescence photonics. In this work, Eu-doped GaN (GaN:Eu) films are grown by plasma assisted molecular beam. RHEED pattern indicates that when the III/V ratio exceeds 1 the growth mode changes from 3D to step-flow/2D and the Eu content in GaN film is abruptly decreased from 0.75 to 0.02 at.%. Luminescence properties have been investigated with relation to the Eu concentration in the GaN:Eu film. Transition peaks from the 4f n shell of Eu3+ ions are observed at 585-595, 596-610, 611-629, 630-645 and 660-670 nm, which are assigned as 5D07F0, 5D0→7F1, 5D0→7F2, 5D1→7F4 and 5D0→7F3. Among these transitions, the dominant one is the 5D0→7F2 transition due to the selection rule, and the two peaks observed at 621 and 622.6 nm in the 5D0→7F2 transition are indexed as α and β. The intensity of β is relatively stronger than peak α when the Eu concentration is below 0.4 at.%. However, when the Eu concentration exceeds 0.4 at.% the α peak becomes stronger than that the β peak. These results suggest that the competition of relative intensities between α and β depends on the Eu concentration and the luminescence sites. From the luminescence efficiency of 5D0→7F2 transition, it is found that the PL efficiency is obviously increased by the 2 order of magnitude when the Eu concentration does not exceed 1 at.%. Therefore, we will discuss relationship between the luminescence efficiency and the Eu concentration in detail.

K.K.Pl.2
17:00
Authors : C. Tessarek 1 2, M. Heilmann 1, A. Haab 3, H. Hardtdegen 3, C. Dieker 4, E. Spiecker 4, S. Christiansen 1 5
Affiliations : 1. Max Planck Institute for the Science of Light, Günther-Scharowski-Str. 1, 91058 Erlangen, Germany; 2. University Erlangen-Nuremberg, Institute of Optics, Information and Photonics, Staudtstr. 7/B2, 91058 Erlangen, Germany; 3. Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich, 52425 Jülich, Germany; 4. University Erlangen-Nuremberg, Center for Nanoanalysis and Electron Microscopy, Erlangen; 5. Helmholtz Centre Berlin for Materials and Energy, Hahn-Meitner Platz 1, 14109 Berlin, Germany.

Resume : Self-assembled GaN micro- and nanorods on sapphire substrates have been grown by metal-organic vapor phase epitaxy. It will be shown that the rods grow via a self-catalyzed vapor-liquid-solid (VLS) growth mode [1]. A large variety of rod structures can be achieved with heights up to 50 µm, diameters from 10 nm to a few µm and densities up to 10^8 cm-2. The presence of Si during growth of the GaN rods has a strong influence on the aspect ratio. Furthermore, Si improves the rod morphology yielding to rods with a regular hexagonal shape, smooth sidewall facets and sharp edges. Several experiments will be presented to clarify the role of Si during the growth of GaN rods. Structural investigations were carried out utilizing energy dispersive X-ray spectroscopy in combination with transmission electron microscopy. A thin SiN layer exists on the sidewall facets of the GaN rods. Si-N has a larger binding energy compared to GaN and the SiN layer acts as an antisurfactant for GaN. These effects stabilize the sidewall facets promoting vertical growth. The influence of the SiN layer on the InGaN quantum well growth, on the thermal stability and on physical etching experiments will be discussed. Finally, a model will be presented summarizing the role of Si during the VLS GaN rod growth [2]. [1] C. Tessarek et al., J. Appl. Phys. 114, 144304 (2013). [2] C. Tessarek et al., Cryst. Growth Des., submitted.

K.K.Pl.5
17:00
Authors : J. Rodrigues 1, M. Felizardo 2, E. Alves 2 3, A. J. Neves 1, G. Tourbot 4, T. Auzelle 4, B. Daudin 4, M. Boćkowski 5, K. Lorenz 2 3, T. Monteiro 1
Affiliations : 1. Departamento de Física & I3N, Universidade de Aveiro, 3810-193 Portugal; 2. IST, Instituto Superior Técnico, Campus Tecnológico e Nuclear, Universidade de Lisboa, EN10, 2695-066 Bobadela LRS, Portugal; 3. IPFN, IST, Lisboa, Portugal; 4. CEA/CNRS Group,“Nanophysique et Semiconducteurs”, INAC, CEA/Grenoble, 17 rue des Martyrs, Grenoble Cedex 9, 38054, France; 5. Institute of High Pressure Physics, Polish Academy of Sciences, 01-142 Warsaw, Poland

Resume : Gallium nitride (GaN) is a wide band gap semiconductor exhibiting exceptional properties for solid state lighting. Moreover, GaN appears as an excellent host for the incorporation of lanthanides allowing covering a wide emission range of the electromagnetic spectrum by an appropriate choice of the dopant ion. With an intra-4f 2 electronic configuration, Pr3+ is known to be an efficient red emitter in wide band gap hosts. In GaN films the intensity of the dominant transition between the 3P0->3F2 multiplets is known to suffer a strong reduction with increasing temperature (~80% from 14 K to room temperature (RT)) in implanted samples annealed at ~1000 ºC. In order to increase the luminescence efficiency by minimizing the nonradiative processes alternative, approaches were adopted such as the annealing at high temperatures and high N2-pressures (HTHP) and the use of low dimensional structures, namely GaN nanowires (NWs) and quantum dots (QDs). In particular, NWs offer some advantages over traditional layered structures, mostly due to their ability to be grown on various and cheap substrates such as silicon. Higher extraction efficiency is also expected when compared with the planar structures. In this work, a comparative study of the red Pr3+ emission is performed for GaN films, NWs and QDs. All the samples exhibit the sharp 3P0->3F2 transition at RT and in the HTHP-annealed films the emission can be observed with the naked eye. The role of the implantation and thermal annealing parameters on the red luminescence will be discussed.

K.K.Pl.13
17:00
Authors : Mufasila M. Muhammed, Idris A. Ajia, Yoshikatsu Morishima, Yoshihiro Yamashita, Shinkuro Sato, Akito Kuramata, and Iman Roqan
Affiliations : Mufasila M. Muhammed; Idris A. Ajia; Iman Roqan - King Abdullah University of Science and Technology, Thuwal, Saudi Arabia. Yoshikatsu Morishima; Yoshihiro Yamashita; Shinkuro Sato; Akito Kuramata -Tamura Corporation, Sayama, Saitama 350-1328, Japan

Resume : The efficiency of III-Nitride-based devices mainly depends on the line defect density. It is crucial to reduce the density of the line defects. β-Ga2O3 has recently been proposed as an alternative to the more ubiquitous substrates, including Al2O3 and SiC, because it combines the high transparency of sapphire with the conductivity of SiC and circumvents their undesirable properties. In addition, the lattice mismatch in β-Ga2O3 is estimated to be just 10%. Here, we show that high internal quantum efficiency (IQE) is reached in a high-quality crystal GaN film, grown on a monoclinic (-201)-oriented β-Ga2O3 substrate by metalorganic vapour phase epitaxy. Photoluminescence measurement shows very weak yellow band and intense bandedge emission with a high IQE of ~40%, which is quite remarkable compared with the IQE of GaN epilayers grown on sapphire. Time-resolved spectroscopy and Raman measurements were used to investigate the effect of nonradiative recombination due to defects. Atomic force microscopy (AFM) analysis shows that the mean square value of the roughness is 0.68 nm, which is indicative of a very smooth surface. AFM shows the dislocation density of the film to be in the range of 2 to 8 × 108 cm2. X-ray diffraction (XRD) measurements evaluate the epitaxial structure and strain, indicating that GaN films grown on β-Ga2O3 are single crystalline wurtzite along the c-axis. The width of the rocking curve (0.16 arc) indicates that the quality of the crystal is high.

K.K.Pl.14
17:00
Authors : Johannes Ledig, Merten Popp, Hergo–H. Wehmann, Andreas Waag
Affiliations : Institut für Halbleitertechnik, Technische Universität Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany.

Resume : Three dimensional light emitting diodes (LEDs) with a core-shell geometry are supposed to be substantially advantageous over conventional planar LEDs. With pitches of several µm and sidewall dimensions larger than the wavelength, photonic crystal effects are not expected to dominate and wave simulations are not needed to calculate the light propagation. However, the photons will undergo a multiple reflection on their way out which needs to be known in detail if such µm-structures are to be optimized for light extraction. Photon path simulations of a single 3D structure are performed by means of a ray tracing software with a specially implemented camera. This delivers orthogonal images of the modeled structure and by rotating it around the model in a series of observation angles it is also used to create Lambert’s plots of the angle resolved emission data comparable to the far-field. Hexagonal columns with an aspect ratio of 5 have been modeled including a truncated pyramid at the top - representing the semi-polar facets and the c-plane top facet of the GaN columns grown by MOVPE. The ray tracing simulations were performed for an air ambient and a small spherical light source placed in the center of the column or close to its sidewall facet at 90% distance from the center. 2D images and Lambert's plots obtained by the orthogonal camera shows the emission characteristic with respect to reflection and refraction from a point source at different positions inside the structure.

K.K.Pl.19
17:00
Authors : Kuldeep Takhar, P. Bhattacharya, K. Ghosh, S. Ganguly, D. Saha and Apurba Laha
Affiliations : Department of Electrical Engineering and Center of Excellence in Nanoelectronics, Indian Institute of Technology Bombay, Mumbai 400076, India

Resume : AlGaN/GaN based HEMT are widely used for high power switching, microwave power and radio frequency (RF) low noise amplifier applications due to its unique material properties. However, there are several challenges that require significant research to meet the modern technological demands. One of the most critical parameters that significantly degrades the performance of AlGaN/GaN based HEMT is large leakage current caused by inferior Schottky gate contact and buffer induced leakage. In present study, we carried out systematic investigation of large number of metals (e.g. Ni, Ni/Au, Pt, Pt/Al, Pd, Ag, Cr and Ti) and metal/semiconductor multilayers (e.g Ge/Ni/Ge/Ni/Au) based Schottky contacts on AlGaN/GaN heterostructure grown by PA-MBE on Sapphire substrates. Interestingly, estimated Schottky barrier heights for various metals exhibit no systematic correlation with respect to metal work functions, inferring pinning of Fermi level at interface due to metal-induced-gap-states (MIGS). Introduction of several buffer layers such as MgO, Gd2O3, TiO2 and Ge between AlGaN and metal contacts shows significant impact on Schottky barrier height which, we believe, could be due to de-pinning of Fermi levels. Optimized Schottky contact shows reverse bias (1V) leakage current as low as <1x10^-7A at 30°C. Furthermore, use of Al0.3Ga0.7N (~10nm) as back barrier beneath Al0.2Ga0.8N/GaN HEMT improves breakdown voltage up to 200V which is almost 4 times higher than that of without back barrier.

K.K.Pl.21
17:00
Authors : Takeshi Ohgaki, Ken Watanabe, Isao Sakaguchi, Shunichi Hishita, Ohashi Naoki, Hajime Haneda
Affiliations : Environment and Energy Materials Research Division, National Institute for Materials Science

Resume : Scandium nitride (ScN) films were grown on m-face sapphire substrates by a molecular beam epitaxy method, and their crystalline orientation, crystallinity and their electric properties were examined. Epitaxial ScN films with an orientation relationship (110)ScN || (10-10)a-Al2O3 and [001]ScN || [1-210]a-Al2O3 were obtained regardless of difference of crystal structure, and their crystal orientation anisotropy were negligible small. The crystallinity and Hall mobility of the films were drastically improved by applying high-temperature growth. The carrier concentration and Hall mobility of the ScN films grown at 900°C ranged from 1019–1021 cm-3 and 80–140 cm2•V-1•s-1, respectively. The increase in carrier concentration with decrease in Hall mobility caused by carrier scattering of the ionized shallow donors was observed in the film grown under Sc-rich conditions. These results indicate that high-temperature growth and an m-face sapphire substrate were suitable for growth of high-quality ScN films.

K.K.Pl.23
17:00
Authors : K.P.O'Donnell
Affiliations : Strathclyde U.

Resume : The temperature dependences of the emission spectra of luminescent semiconductors is of interest from both a fundamental and a practical viewpoint. It has recently been shown that a slight alteration of the accepted R/NR recombination model for the dependence of intensity on temperature and excitability on power density can produce experimental results that are far from intuitively obvious [1]. On the occasion of the 25th anniversary of the publication of Favennec?s rule [2], that widegap RE-doped semiconductors should be better light emitters at higher temperatures, we should be careful to avoid the pitfalls of extrapolation from a small set of known results. I shall explore several examples of light emission by GaN-related semiconductors in order to illustrate these points and provide an alternative description of luminescence quenching in semiconductors with particular relevance to GaN doped with rare-earth ions. [1] Michael A. Reshchikov, PHYSICAL REVIEW B 85, 245203 (2012) [2] P Favennec et al. ELECTRONC LETTERS 25, 718-9 (1989)

K.K.Pl.31
17:00
Authors : Bong Kyun Kang, Sung Ryul Mang, Keun Man Song and Dae Ho Yoon
Affiliations : Sungkyunkwan University

Resume : III-nitrides nanostructures have attracted extensive attention due to their unique electronic and optical properties. Gallium nitride (GaN) has a direct wide bandgap of 3.4 eV at room temperature, and is a promising candidate material for short wavelength optoelectronic devices, such as light emitting diodes and laser diodes, as well as high power and high temperature operation devices. Compared with film and nanowires, GaN nanoparticles could be alternative hybrid integration materials with a variety of optical and electrical properties because of the flexible powder form and controlled shape or size, as well as low fabrication cost. In recent years, the fabrication of monodispersed semiconductor nanostructures has attracted extensive attention due to their distinct low effective densities, high specific surface area and potential scale dependent application in catalyst, optical devices, chemical sensor and drug delivery. The semiconductor-based photocatalysts such as TiO2, SrTiO3 and GaN have attracted extensive attention and have been used extensively to decompose water under UV light irradiation while efficiency is still low. To improve the photocatalytic activity, the semiconductor-based photocatalysts have to minimize recombination between electrons and holes as well as enhance photogenerated charge carrier via doping, large surface area and changed surface modification. In this work, monodispersed GaN nanostructures were synthesized successfully nitridation of Ga(OH)3 templates by using hydrothermal method. The GaN@ZnO nanocomposites were synthesized via hydrothermal methods. In addition, the crystal structure, morphology, optical properties and particle size of the monodispersed GaN@ZnO nanocomposites were analyzed using powder X-ray diffraction (XRD), high resolution transition electron microscope (HT-TEM), photoluminescence spectrometry (PL) and energy dispersive X-ray analysis (EDX).

K.K.Pl.32
17:00
Authors : A. Nikolenko 1, B. Sadovyi 2 3, V. Strelchuk 1, A. Romanyuk 1, A. Belyaev 1, S. Porowski 2, J. Weyher 2, I. Grzegory 2, I. Petrusza 4, V. Turkievich 4 and V. Kapustianyk 3
Affiliations : 1. V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 45, prospect Nauky, 03028 Kyiv, Ukraine; 2. Institute of High Pressure Physics PAS, Sokolowska str., 29/37, 01-142 Warsaw, Poland 3. Department of Physics, Ivan Franko National University of Lviv, 50, Dragomanova str., Lviv, 79005, Ukraine 4. V. N. Bakul Institute for Superhard Materials NAS Ukraine, 2, Avtozavodska str., Kyiv, 04074, Ukraine

Resume : In this work, the bulk diffusion of oxygen in wurtzite-type GaN crystals at temperatures up to 3400 K and pressures up to 9 GPa is studied. For this purpose the GaN crystals grown by hydride vapor phase epitaxy (HVPE) and having strongly nonuniform distribution of oxygen (as revealed by photo-etching and SIMS measurements), the main donor in GaN, are used. Confocal micro-Raman spectroscopy is applied to estimate free electron concentration from the analysis of plasmon-LO-phonon coupled modes (LOPC). Thereby spatial distribution of free electron concentration is studied by lateral scanning along the cleaved surfaces of the investigated GaN crystals. Thus the HVPE GaN crystals studied are shown to contain heavily doped ( n ~ 2.0÷4.0•10^19 cm-3) and undoped (n ≤ 10^17 cm-3) areas having sharp step-like carrier concentration profiles in micrometer scale. Annealing at high temperatures and high pressures results in only slight diffusion blurring of the carrier profiles at a distance less than 10 um from the interface. Extremely small values of diffusion coefficient D ≤ 6.3•10-13 m2/s (T = 3400 K and P = 9 GPa) estimated from the measured diffusion length are in good agreement with K. Harafuji's molecular dynamic calculations [1], confirming the anomalously small diffusion coefficient in the N-sublattice of GaN. 1. K. Harafuji, T. Tsuchiya, K. Kawamura Molecular dynamics simulation for evaluating melting point of wurtzite-type GaN crystal // J. Appl. Phys. 96, 2501 (2004).

K.K.Pl.37
17:00
Authors : M. Bouzidi*, Z. Benzarti, I. Halidou, Z. Chine, B. El Jani
Affiliations : Université de Monastir, Faculté des Sciences Unité de recherche sur les Hétéro-Epitaxies et Applications (URHEA), 5000 Monastir, Tunisia. E-mail: * elbouzidimed16@yahoo.com

Resume : We investigated the silicon doping effects on GaN layers grown on sapphire substrate by metalorganic chemical vapor phase deposition (MOCVD). We have used silane (SiH4) to intentionally incorporate silicon during the crystal growth of GaN. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were used to study the structural and surface morphology of the films. Room temperature photoluminescence spectra of Si doped GaN layers (GaN:Si) exhibited a decrease in the intensity of yellow luminescence (YL) with increasing SiH4 flow rate, which could be due to a decrease in the concentration of gallium vacancy (VGa) or VGa-related complexes. Fundamental optical band gap measured by photoreflectance showed a redshift up to a concentration of electrons of about n = 6×10^18 cm-3. Above this value, a sudden blueshift of the band gap energy was observed. This result was interpreted by the competing effects of Burstein-Moss band filling and band gap renormalization. Keywords: GaN:Si, Photoreflectance, Burstein-Moss effect, band gap renormalization, yellow luminescence, XRD, SEM.

K.K.Pl.39
17:00
Authors : Jana Hartmann 1, Helena Franke 2, Matin Sadat Mohajerani 1, Johannes Ledig 1, Xue Wang 1, Frederik Steib 1, Martin Straßburg 3, Hergo-Heinrich Wehmann 1, Rüdiger Schmidt-Grund 2, Marius Grundmann 2, Andreas Waag 1
Affiliations : 1. Technical University Braunschweig, Institute of Semiconductor Technology, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany; 2. University Leipzig, Institute of Experimental Physics III, Linnéstr. 5, 04103 Leipzig, Germany; T 3. OSRAM Opto Semiconductors GmbH, Leibnizstr. 4, 93055 Regensburg, Germany

Resume : Defect free GaN material is a prerequisite for high efficiency light emitting diodes (LEDs) and laser diodes. Therefore, high aspect ratio vertical growth of three-dimensional (3D) GaN has been developed recently. These 3D structures are not only interesting for high efficiency LEDs, but might also be applied for vertical-cavity surface emitting lasers (VCSELs). Based on our existing 3D core-shell InGaN/GaN LED technology we are evolving structures with diameters smaller than 500 nm and combine them with a distributed Bragg reflector (DBR) in order to investigate the potential for vertically emitting 3D devices. The DBR consisting of aluminium oxide (Al2O3) and yttria-stabilized zirconia (YSZ) are fabricated by pulsed laser deposition (PLD) at 650°C. The vertical GaN based LED columns are grown by selective area growth (SAG) by metal organic vapour phase epitaxy (MOVPE) at much higher temperatures. Therefore the thermal strain and stability of the DBR has to be taken into account for combining these two technologies. As the epitaxial growth and the electrical connection of the later device is not possible directly on the dielectric DBR, we developed several approaches to position the DBR at the bottom of the 3D structures. In this work we will discuss first results of the two most promising approaches: Firstly, LED structures were re-grown on columns that have been truncated after DBR deposition and, secondly, a foil with a DBR was pressed onto the surface of the LED sample.

K.K.Pl.40
17:00
Authors : K. K. Abgaryan, D.L. Reviznikov, I. V. Mutigullin
Affiliations : Dorodnicyn Computing Centre of RAS

Resume : Three-scale model is proposed for the calculation of 2DEG mobility in AlGaN/GaN heterostructures. First of all Fang-Howard approximation is used for the calculation of electron wavefunctions in a triangle potential well in the vicinity of the heterostructure interface. The value of 2DEG concentration is required for this calculation. This value can be estimated from the first-principles calculations. This computational model allows to calculate following 2DEG properties: energy levels, corresponding wavefunctions, potential distribution, electron concentration distribution. Knowledge of the electron concentration in 2DEG, of the wavefunctions and of the heterointerface characteristics allows one to calculate electron mobility in 2DEG. To do so, it is necessary to take into account various scattering mechanisms. Therefore there is relatively simple connection between electronic structure calculations of AlGaN/GaN heterostructure and 2DEG electron mobility calculation. Data obtained in electronic structure calculations are used in the calculation of electron mobility. Thus our three-scale model is complete.

K.K.Pl.44
17:00
Authors : M. Cuniot-Ponsard 1, I. Saraswati 2 4 S-M. Ko 3, M. Halbwax 2 Y-H. Cho 3, N-R. Poespawati 4, E. Dogheche 2
Affiliations : 1. Laboratoire Charles Fabry, IOGS, CNRS, Univ Paris-Sud, 2 Avenue Augustin Fresnel, 91127 Palaiseau cedex, France; 2. Institut d’Electronique, Microélectronique et Nanotechnologie, Groupe Optoélectronique, IEMN UMR 8520 CNRS, Avenue Poincaré, 59652 Villeneuve d’Ascq, France; 3. Department of Physics and KI for the Nano-Century, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea; 4. Electrical Engineering Department, Faculty Engineering, Universitas Indonesia, 42435, Depok, Indonesia

Resume : In order to take advantage in all-optic optoelectronic devices, we have investigated the optical and piezoelectric properties of GaN films deposited on (111) silicon. Films are epitaxially grown by MOCVD, thanks to a buffer made of (Al, Ga) N intermediate layers [1]. Structural properties of GaN are analyzed using TEM and the influence of threading dislocations density is discussed. Optical properties are investigated using a prism coupling [2]. Electrooptic measurements are performed using an original technique [3]. A semi-transparent gold electrode is deposited on top of GaN layer and an alternating voltage is applied between top and bottom electrodes. The electro-optic, converse piezoelectric, and electro-absorptive coefficients are simultaneously determined from the measurement of the electric field induced variation ΔR(θ) in the reflectivity of the Au/GaN/buffer/Si stack versus incident angle. The method also enables to determine the GaN layer polarity. The results obtained for a Ga-face [0001] GaN layer when using a modulation frequency of 230 Hz are for the electro-optic coefficients r13 = +1 pm/V, r33 = +1.60 pm/V at 633 nm, and for the transverse piezoelectric coefficient d33 = +4.59 pm/V. The value measured for the electro-absorptive variation is Δko/ΔE = +0.77 pm/V. The electro-optic coefficients for GaN /Si and the electro-absorptive coefficient are measured for the first time. The converse piezoelectric value agrees with values previously reported.

K.K.Pl.47
17:00
Authors : M.P. Chauvat 1, Y. Wang 1, M. Morales, S. Valdueza-Felip 2, E. Monroy 2, and P. Ruterana 1
Affiliations : 1. CIMAP, CNRS-ENSICAEN-CEA-UCBN, 6 Blvd. Maréchal Juin, 14050 Caen, France; 2. CEA-Grenoble, INAC/SP2M/NPSC, 17 rue des Martyrs, 38054 Grenoble, France.

Resume : The InGaN alloys theoretically constitute the best family for covering the largest part of the solar spectrum for the application in photovoltaics. However, many challenges lie still across the development of nitride based solar cells: 1) InN and GaN grow at different temperatures, especially using metalorganic vapor phase epitaxy (MOVPE), ~ 500 and above 1000°C, respectively; 2) The lattice mismatch between GaN and InN is about 11%. 3) InN and GaN exhibit phase separation due mainly to the large differences in atomic radii. Therefore, the growth need to be carried out using well controlled conditions in order to deposit good crystalline quality layers. This may better be realized by molecular beam epitaxy (MBE) which uses lower temperatures than MOVPE. In this work, we have investigated the structure of InGaN layers. InGaN films with RT PL emission ranging from 450–670 nm (0.9–0.6 of Ga content) have been synthesized on (0001)GaN/sapphire templates through a continuous search of the best quality of the layer and efficiency of the devices. The TEM investigations show typical differences between the MOVPE and MBE layers. In MBE, a good control of the growth conditions may lead to smooth surfaces and homogeneous composition layers. By MOVPE a systematic phase separation is seen to take place and characteristic defects form when the In composition is above 25-30%. From this investigation, various strategies for improving the layers quality will be discussed.

K.K.Pl.48
17:00
Authors : M. Auf der Maur and A. Di Carlo
Affiliations : Dipartimento di Ingegneria Elettronica Universita` di Roma "Tor Vergata", Via del Politecnico 1, 00133 Roma

Resume : In this work we present fully self-consistent electro-thermo-mechanical simulation results for typical AlGaN/GaN HEMT structures, including the effects due to the passivation layers and the contact metallizations. We analyze the mechanical stress state under different dc operating conditions, looking at both mechanical energy density in the strained layers and at the resolved shear stresses on different wurtzite slip systems. We show that the mechanical stress state for different bias conditions is qualitatively and quantitatively different. In the off state, converse piezoelectric effect leads to a strong increase in energy density under the gate. The self-heating under medium and high dc power dissipation results in a relaxation of the elastic energy density. However, the stress field becomes strongly anisotropic on the (0001) plane, which induces appreciable resolved shear stress on most slip planes. In both cases, the amount of stress is strongly bias-dependent. The energy densities and shear stresses are compared with theoretical predictions of critical values, showing that both converse piezoelectric effect and dislocation glide might be relevant for device degradation.

K.K.Pl.49
17:00
Authors : Jincheng Zhang, Xiangdong Li, Chunfu Zhang, Wei Ha, Xing Chen, Shuai Zhang, Shenglei Zhao, Xiaohua Ma, Yue Hao
Affiliations : Key Laboratory of Wide Band Gap Semiconductor Materials and Devices, School of Microelectronics, Xidian University, Xi’an 710071, People’s Republic of China

Resume : Recently, great attention has been paid onto using AlGaN as channel material in order to obtain higher breakdown voltage and output power. In this poster, we grew nine epitaxial samples from sample A to I with MOCVD. To improve the crystalline quality of the samples, different Al-component channel layers and different buffer layers were investigated. Test results demonstrate that both the crystalline quality and sheet carrier density decrease dramatically as the Al component of AlGaN channel increases. Besides, graded high Al-component AlGaN buffer layers suffer serious parasitic effects. To suppress the parasitic effects and obtain a high sheet carrier density, sample E (Al0.4Ga0.6N/Al0.18Ga0.18N/GaN) was grown and a high sheet carrier density of 1.04×10^13 cm-2 and a high mobility of 852 cm2/V•s were thus obtained. The (002) and (102) FWHMs of the Al0.18Ga0.18N channel layer of sample E are 255 arc sec and 1002 arc sec, respectively. Finally, AlGaN channel HEMTs with LG = 1.5 μm, LGS = 2 μm, and LGD = 2.5 μm were fabricated on sample I (Al0.4Ga0.6N/Al0.18Ga0.18N/ AlxGa1-xN/GaN, x = 0~0.18), taking advantage of the high crystalline quality of the composite buffer layer of graded AlGaN and GaN. Electron beam evaporated Ti/Al/Ni/Au (22/140/55/45 nm) and Ni/Ti/Ni (70/80/30 nm) were then made for source/drain and gate contacts, respectively. A very low specific contact resistance of 1.327×10^-6 Ω•cm2 was obtained by TLM. To the best of our knowledge, this is the best result ever reported. Besides, a peak ID of 441.3 mA/mm with VG = 2 V, a high breakdown voltage of 225 V, and a high transconductance of 91.7 mS/mm were also obtained. These results mentioned above indicate that the AlGaN channel HEMTs are very promising in the power electronics field.

K.K.Pl.50
17:00
Authors : S. R. Xu, J. C. Zhang, T. Jiang, X. W. Zhou, L. A. Yang, Y. Hao
Affiliations : Key Laboratory of Wide Band Gap Semiconductor Materials and Devices, School of Microelectronics, Xidian University, Xi’an 710071, People’s Republic of China

Resume : Gallium nitride and its alloys with indium and aluminum nitride are very attractive materials especially due to their wide application in electronic and optoelectronic devices. The majority of conventional GaN devices are grown with respect to the c-plane. The polarization fields in multiple quantum well structures along the polar c-axis cause a significant band bending and thus give rise to the spatial separation of electrons and holes. This situation leads to a reduced optical emission efficiency of light-emitting diodes (LEDs), as well as an undesirable redshift in the emission spectra from the quantum wells. So there have been considerable interests in the growth of nonpolar and semipolar gallium nitride based on epitaxial films, heterostructures, and devices. The polar properties of GaN make the behaviors of the different polar directions distinctly different, especially when there is impurity incorporatied. However, the influence of the polar direction on the impurity incorporation and luminescence properties is lacking. We have investigated the unintentional impurities oxygen and carbon in GaN films grown on c-plane, r-plane as well as m-plane sapphire by metal-organic chemical vapor deposition. The GaN layer was analyzed by secondary ion mass spectroscopy. The different trend of the incorporation of oxygen and carbon have been explained in the polar(0001) nonpolar (11-20) and semipolar (11-22) GaN by combination of the atom bonding structure and the origin direction of the impurities. Furthermore, it is found there is stronger yellow luminescence (YL) in GaN with higher concentration of carbon, suggesting that C-involved defects are the origin responsible for the YL.

K.K.Pl.51
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09:30
Authors : G. Sarau, M. Heilmann, M. Latzel, S. Christiansen
Affiliations : Max Planck Institute for the Science of Light, Günther-Scharowsky-Str. 1, 91058 Erlangen, Germany

Resume : The scattering in the emission wavelength of InGaN/GaN MQW heterostructures is generally attributed to local variations in strain, thickness and In content. Here, we employed a combination of nanosphere lithography and reactive ion etching to produce tightly size-controlled nano-LEDs with a mean top diameter of 188.11±16.92 nm demonstrated to result in a high efficient and collimated light emission towards the surface normal. A statistically relevant study of the strain state in individual nano-LEDs was performed using non-resonant micro-Raman spectroscopy based on a clear splitting (nanorod and substrate) of the E2(high) phonon mode of GaN. We found a consistent strain relaxation with regard to the strain in the initial film and the nanostructuring procedure that was also confirmed by the blue shift of the CL emission. Complementary micro-Raman–CL spectroscopic measurements on the same, isolated nanorods covering the entire range of stress states showed that the same strain level does not necessarily result in the same QW emission wavelength for all nanorods. Band profile calculations agreed well with the optical transitions observed in the CL experiments, namely a mean emission at 441.09±5.82 nm, when fluctuations in the well thickness of 2–2.4 nm and in the In composition of 0.18 – 0.22 were considered with respect to the nominal values of 2.2 nm and 0.2, respectively. Our work establishes a powerful methodology for basic understanding of light emission from nano-emitters.

K.K.II.3
09:45
Authors : Pawel Strak, Pawel Kempisty, Agnieszka Jamroz, Konrad Sakowski, Stanislaw Krukowski
Affiliations : Institute of High Pressure Physics of the Polish Academy of Sciences

Resume : We have studied the bound electron and hole states in nitride superlattice heterostructures using a density functional theory. The systems are assumed to be differently strained in lateral directions, with a lattice vector changing evenly from one to the second a lattice constants of two compounds forming structure. The c lattice vector is allowed to relax fully. When considering heterostructures composed of a ternary compound, the a and c lattice vectors are taken from Vegard's law. The effects of these external influences as well as of changes in the geometry of the heterostructures on the localization of bound states are discussed in detail. Direct inspection of the obtained energy band profiles suggest that the localization of bound states differs significantly, due to differences in band offsets of valence and conduction bands, which are related not only to differences in strain conditions. When passing from binary to ternary compounds we have found unexpected almost zero valence band offset between well and a barrier, which could be easily changed from positive to negative values by small changes in strain conditions. These had strong influence on localization of bound valence states and accordingly strong influence on reduction of optical transition rates.

K.K.II.4
10:30
Authors : E. Monroy 1, M. Beeler 1, Bellet-Amalric 1, C. Bougerol 1, P. Hillev 2, J. Schörmann 2, M. Eickhoff 2, M. Tchernycheva 3, F. H. Julien 3, A. Vardi 4, G. Vahir 4.
Affiliations : 1. CEA-CNRS Group Nanophysics and Semiconductors, CEA/INAC/SP2M and CNRS-Institute Néel, 17 rue des Martyrs, 38054 Grenoble cedex 9, France; 2. I. Physikalisches Institut, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany; 3. Photis Dept., Institut d’Electronique Fondamentale, Université Paris-Sud, 91405 Orsay cedex, France; 4. Solid State Institute and Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel.

Resume : III-nitrides have recently emerged as promising materials for new intersubband (ISB) technologies with potential applications for fiber-optic communications and throughout the THz spectral range. ISB transitions in GaN/AlGaN quantum wells (QWs) can be tuned from 1.0 µm to 10 µm, and ISB absorption in the THz range (> 20 µm) has also been reported. For certain devices, it is interesting to replace QWs by laterally confined systems like quantum dots (QDs) to increase the intraband recovery times. In the case of GaN/AlN QDs, near-infrared TM-polarized intraband absorption has been observed, and is attributed to transitions from the ground state of the conduction band to the first excited electronic state confined along the growth axis. Furthermore, the lateral confinement in the QDs gives rise to additional mid-infrared transitions reacting to TE-polarized excitation. However, in the QD system the material choice and the nanostructure dimensions, which determine the operational wavelength, are limited by the elastic requirements for Stranski-Krastanov growth. In the case of nanowires (NWs), their large surface-to-volume ratio allows misfit strain to be elastically released, extending the viable active region size and composition beyond the limits of planar systems or QDs. In this work we present and discuss the intraband performance of these three kinds of nanostructures (QWs, QDs, NWs), studying the effect of their geometries and strain distribution on their optical properties.

K.K.II.5
11:15
Authors : Ž. Gačević 1, S. Lazić 2, N. García-Lepetit 1, E. Chernysheva 2, S.Albert 1, A. Bengochea-Encabo 1, S. Metzner 3, M. Müller 3, F. Bertram 3, J. Christen 1, J.M. Calleja 2, and E. Calleja 1
Affiliations : 1. ISOM-DIE, Universidad Politécnica de Madrid Spain; 2. Departamento de Física de Materiales, Universidad Autónoma de Madrid, E-28049 Madrid, Spain; 3. Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg, Germany

Resume : An InGaN quantum dot (QD) embedded into a GaN nanowire (NW) is a promising system for realization of efficient quantum light emitters. Among those, single photon sources (SPSs) based on self-assembled (SA) GaN QDs and InGaN disks embedded into SA GaN NWs have been recently reported. We demonstrate here the emission of single photons from the QD-like states observed in the 20 nm thick InGaN disks embedded into GaN NWs ordered in a two-dimensional array. The structure was fabricated on (0001)GaN-on-sapphire templates using nanohole masks prepared by colloidal lithography. Scanning electron microscopy confirms the formation of NWs with pyramidal tops and highly uniform diameters and heights (200 and 500 nm, respectively), ordered in hexagonal matrices (with 270 nm pitch). Photoluminescence (PL) and transmission electron microscopy combined with cathodoluminescence reveal an intense emission band around 500 nm, originating from the apices of the InGaN sections. They further confirm that the luminescence originates from excitons confined in a strong electric field, with their initial lifetimes (~1 ns) nearly constant up to the room temperature. Micro-PL measurements reveal intense and narrow (< 500 μeV) QD-like emission lines. Photon correlation measurements performed on these emission centers using a Hanbury Brown and Twiss interferometer show pronounced antibunching. We find the g(2)(0) < 0.3, which is a clear signature of single photon emission.

K.K.II.7
11:45
Authors : John Buckeridge, C. Richard A. Catlow, A. Walsh, D. O. Scanlon, A. A. Sokol
Affiliations : University College London, Kathleen Lonsdale Materials Chemistry, Department of Chemistry, 20 Gordon Street, London WC1H 0AJ, United Kingdom

Resume : The effects of doping on the minority charge carrier concentration in GaN remains a controversial issue, with many theoretical studies producing contradicting results. The standard method to model defects in crystals is the plane-wave supercell approach, which has disadvantages related to unwanted interaction between periodic images of defects. We present results of calculations based on a hybrid quantum mechanical/molecular mechanical embedded cluster approach to modelling defect formation associated with Group 2 dopants in GaN. As our approach does not employ periodic boundary conditions, provides access to the vacuum level, and allows the use of high level quantum chemistry approximations, accurate and unambiguous defect levels can be determined. From our calculations we find that a substantial amount of experimentally determined optical data can be attributed to the N vacancy in GaN, which is a charge-compensating defect for Group 2 dopant incorporation. Our calculated defect levels associated with the dopants are in excellent agreement with experiment where available. Furthermore, we definitively show that standard hybrid density functionals are inadequate for treating these defects and that double exchange is necessary for an accurate description.

K.K.II.9
13:30
Authors : S. Fernández-Garrido 1, V. Kaganer 1, X. Kong 1, K. K. Sabelfeld 1 2, J. K. Zettler 1, T. Gotschke 1, R. Calarco 1, J. Grandal 1 3, E. Calleja 3, A. Trampert 1, L. Geelhaar 1 and O. Brandt1
Affiliations : 1 Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5–7, 10117 Berlin, Germany; 2 Institute of Computational Mathematics and Mathematical Geophysics, Russian Academy of Sciences, Lavrentiev Prosp. 6, 630090 Novosibirsk, Russia; 3 ISOM and Dpt. de Ingeniería Electrónica, ETSI Telecomunicación Universidad Politécnica de Madrid, 28040 Madrid, Spain

Resume : In contrast to epitaxial films, single crystalline GaN nanowires (NWs) can be grown on dissimilar substrates because their high aspect ratio inhibits the propagation of dislocations along the NW axis. Consequently, GaN nanowires have attracted great interest for the monolithic integration of optoelectronic devices on silicon. However, the physical mechanisms governing the spontaneous nucleation and growth of NWs are not completely understood yet. In this work, we show that, in the absence of structural or morphological defects of the substrate, GaN NWs form spontaneously along the [000 1] direction. The in-situ investigation by line-of-sight quadrupole mass spectrometry of the desorbing Ga flux during NW formation allows us to elucidate the role of the growth parameters on both the nucleation and subsequent growth of GaN NWs. We demonstrate that a self-regulated process, which depends on the effective III/V flux ratio, determines the final NW radius. We also introduce an empirical model that provides a quantitative comprehensive description of the time evolution of the entire NW ensemble, where collective effects (shadowing and exchange of Ga atoms between adjacent NWs) must be taken into account. Furthermore, the present model also makes it possible to extract, from the time evolution of the desorbing Ga flux, relevant growth parameters such as the nucleation rate or the average incubation time that precedes the formation of GaN NWs.

K.K.II.10
14:00
Authors : M. Malinverni 1, N. Grandjean 1, J.-M. Lamy 1, N. Kaufmann 1, L. Lahourcade 1, D. Martin 1, J.-F. Carlin 1, M. Rossetti 2, A. Castiglia2, M. Duelk 2, C. Velez 2
Affiliations : 1. Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland; 2. Exalos AG, CH-8952 Schlieren, Switerland

Resume : The technology of III-V nitride based light emitting diodes (LEDs) is nowadays well established in the blue wavelength range with impressive performance, i.e. a maximum wall plug efficiency exceeding 50%. However, LED efficiency goes down at high current density and for longer wavelength range. There is therefore still a room for research to get rid of the efficiency droop, or at least to limit it, and to extend the operation wavelength toward longer wavelength, beyond the green, while keeping a reasonable efficiency. Long-wavelength InGaN quantum wells require high indium content, which in turn leads to materials degradation and strain issues. In addition, high In content InGaN alloys are subject to thermal degradation, which may occur during the growth of p-type layers. In this presentation, we will present the growth of Mg doped GaN layers at low-temperature (750°C) by molecular beam epitaxy (MBE). The electrical properties compare well with those obtained on state of the art metal organic vapour phase epitaxy grown layers. Successful implementation of MBE grown GaN p-type cladding layers in lasers and long-wavelength LEDs will be reported. In addition, we will show that the use of low temperatures limits the diffusion of doping species across the p-n junction interface allowing us to achieve tunnel junction with excellent characteristics.

K.K.II.11
14:30
Authors : Alois Krost 1, Armin Dadgar 1, and André Strittmatter 1 2
Affiliations : 1.Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg, Germany; 2. Technische Universität Berlin Institut für Festkörperphysik Sekretariat EW 5-2 Hardenbergstraße 36 D-10623 Berlin Germany

Resume : Nowadays GaN light emitters are predominantly grown by MOVPE on sapphire substrates although LEDs on SiC, Si and even GaN show competitive performances. As the LED market is driven mostly by cost reduction GaN-on-silicon technology is perhaps the only current alternative to GaN-on-sapphire because of available substrate size and price. In the last decade the development of GaN on Si LEDs led to steadily increasing performances by solving numerous difficulties for the growth of GaN on Si. Of these difficulties cracking is the most severe which can be solved by several methods. AlGaN buffers, AlN/GaN superlattices, Al(Ga)N interlayers as well as selective growth are all viable pathways to control strain in LED structures and thereby prevent cracking during cooling after growth. However, these methods cannot be applied straightforward. For example, AlN/GaN superlattices and AlGaN interlayers for strain engineering introduce misfit dislocations but may also lead to additional threading dislocations. It has been observed that such interlayers have different impact on the following GaN layer in dependence of their specific position in the layer stack. We will demonstrate that the function of these layers depends on the strain state of the previous layer. Because of the high absorption coefficient of Si, LEDs have to be processed as thin film devices to achieve maximum light output. Here, high n-type doping is advised to enable efficient n-contact metallization and low operation voltage. We developed germanium doping for GaN layers with ND>10^20 cm-3 for this purpose. Ge doping has also the advantage over Si doping in avoiding edge type dislocation related tensile stress generation during growth. Our investigations show that Ge-doping behaves quite different to Si enabling much higher doping concentrations while maintaining a smooth GaN surface.

K.K.II.12
15:15
Authors : M. Auf der Maur, A. Pecchia, D. Barettin, G. Penazzi, F. Sacconi, A. Di Carlo
Affiliations : University of Rome Tor Vergata; CNR-ISMN; University of Rome Tor Vergata; Bremen Center for Computational Materials Science; Tiberlab S.r.l.; University of Rome Tor Vergata

Resume : In this work we present a theoretical study of the effect of random alloy fluctuations in the quantum well (QW) of an InGaN/GaN LED on the spontaneous emission properties. The calculations are based on an empirical tight-binding (ETB) model, using an sp3d5s* parametrization. Strain is calculated with a valence force field (VFF), preconditioned with a continuous linear elasticity calculation. The devices we consider consist of a single 3 nm QW with varying In content, undoped GaN barriers and AlGaN EBL. We first seek a selfconsistent solution of the Schroedinger/drift-diffusion model at a realistic operating current, using a kp model. Then we calculate the first few electron and hole states using ETB, and from this the optical matrix elements. These calculations are repeated for a statistical ensemble of structures with uniform random In distributions in the QW for each mean In concentration. For a reasonable representation of the random alloy and the induced fluctuations of the particles' orbitals we use a periodic supercell in the plane of the QW of 6x6 nm^2. As a result we obtain statistical distributions of the transition energies and oscillator strengths for different In concentrations. We find on the one hand that the oscillator strength is correlated with the transition energy, and on the other hand that the mean oscillator strength of the random alloy calculations shows a stronger decrease with In concentration than that obtained using virtual crystal approximation.

K.K.II.14
15:30
Authors : Felix Nippert 1, Steffen Westerkamp 1, Anna Nirschl 2, Ines Pietzonka 2, Tobias Schulz 3, Martin Albrecht 3, Alexander Franke 1, Thomas Kure 1, Christian Nenstiel 1, Gordon Callsen 1, Martin Strassburg 2, Axel Hoffmann 1
Affiliations : 1. Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany; 2. OSRAM Opto Semiconductors GmbH, Leibnizstraße 4, 93055 Regensburg, Germany; 3. Leibniz-Institut für Kristallzüchtung, Max-Born-Straße 2, 12489 Berlin, Germany.

Resume : We study MOCVD-grown green-emitting c-plane InGaN/GaN QW -structures of very high quality. They show no structural defects such as thickness fluctuations, phase separation or Indium clustering. TEM analyses confirm the excellent quality of our samples. Using temperature-dependent PL, we observe a drastically reduced internal quantum efficiency (IQE) with increasing thickness. Using photoluminescence and exciting only the QWs, we show that this the result of three effects: Firstly, the increasing internal electric fields due to the quantum-confined Stark effect (QCSE) separate electrons and holes along the growth direction reducing the radiative rates. Secondly, we see a loss of in-plane localization which combined with the longer carrier lifetime enhances carrier diffusion to non-radiative recombination centers in the thickest QWs (4.3~nm). Thidly, when using a high excitation power density, we observe excited states in the QWs, which emit in the blue to UV region and have very short lifetimes of less than 30ps. 8-band k.p calculations support our interpretation. These states comprise an additional non-radiative loss channel competing with Auger processes.

K.K.II.15
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Nitride devices and their application : E. Calleja, M. Kuball and H. Fujioka
08:30
Authors : Martin Kuball
Affiliations : H. H. Wills Physics Department University of Bristol Tyndall Avenue BS8 1TL U.K.

Resume : GaN is presently being explored as new material for power electronic devices, aiming at achieving superior performances to Si based technology. Although good to excellent performances have been demonstrated, current GaN devices still do not exploit the full performance potential of this material system, neither the reliability of the devices is fully adequate. I will review the latest developments in this field, including the role of Carbon doped GaN buffer layers for the working of the devices, in the addition the integration of GaN with diamond material systems to enable even higher power densities than presently possible.

K.K.III.1
09:45
Authors : S.M. Lis, S.E.J. O?Kane, S.A. Fox, C.J. Lewins, Y.D. Zhuang, J. Sarma, P.A. Shields, D.W.E Allsopp
Affiliations : Dept. of Electronic and Electrical Engineering, University of Bath, Bath, BA2 7AY, UK

Resume : Light-emitting diodes (LEDs) based on arrays of nanorod (NR) emitters have been widely re-searched in recent years. Advantages of such LEDs include increased internal quantum efficiency due to strain relaxation, low defect densities and increased light extraction efficiency. Another ad-vantage of NR-LEDs is the potential for highly directional light emission, a desirable attribute for etendue limited applications. Factors that affect this directionality include light waveguiding in the NRs and in any underlying GaN buffer layer from which light can be diffracted by highly ordered NR arrays. This paper presents the results of a study by simulation and measurement of the direc-tionality of light emission from NR-LEDs ICP etched from epitaxy containing either a single InGaN/GaN quantum well or a MQW oriented in the c-plane. It is shown how the diameter, height and sidewall angle of an NR, the thickness of any ITO contact layer and location of the dipoles in-fluence the far-field radiation pattern, notably the directionality of the emitted light. The emissive properties of single NRs were simulated by a modal expansion technique that accounts for reflection and diffraction at the nanorod ends, while those of nanorod arrays were studied using the finite-difference time-domain (FDTD) method. The predictions of both models were compared with detailed measured far-field angular emission patterns from green and blue light emitting NRs over a wide range of wavelengths, with excellent agreement achieved. The comparisons with results obtained by modal expansion reveal the dominant waveguide modes, which in turn depend on the wavelength of the emission and its radial and axial location within the NRs. The FDTD study revealed the diffractive behaviour.

K.K.III.4
11:00
Authors : S. Suresh 2, P. Renaud 1 2, X. Li 1 2, Y. El. Gmili 2, K. Panztas 1 2, G. Orsal 2 3, G. Patriache 4, J. P. Salvestrini 2 3, A. Ougazzaden 1 2.
Affiliations : 1. Georgia Institute of Technology/GTL, UMI 2958 GT/CNRS, 57070 Metz, France; 2. CNRS UMI 2958 Georgia Tech-CNRS, 57070 Metz, France; 3. Université de Lorraine, LMOPS, EA4423, 57070 Metz, France; 4. LPN CNRS, UPR, Route de Nozay, F-91460 Marcoussis, France.

Resume : There is a tremendous need for developing novel approaches to realize good quality thick InGaN epilayers to significantly improve the efficiency of concentrating photovoltaic systems. One possible solution for obtaining thicker and better InGaN is Nano Selective Area Growth (NSAG). Applying NSAG technology, we have grown successfully, 120 nm thick, perfectly selective InGaN nanorod and nanowire arrays over the dielectric mask using metal organic chemical vapor deposition. Uniform stripes and dots nanopatterns of SiO2 with 100 nm openings were realized using E-beam lithography. In the unmasked area InGaN is stymied with 3D growth, clusters of In and defects whereas the InGaN nanostructures in the patterened area are perfectly hexagonal, monocrystalline and homogenous in size, shape and composition exposing the smooth semipolar facets, which is sign of high quality nanostructures. The approximate indium composition and the strain state in the field were determined by the HRXRD analysis and the photoluminescence near band emission peak. Further structural properties were studied using cross sectional transmission electron microscope. The optical properties of these nanostructures were studied using the depth resolved cathodoluminescence spectroscopy, which showed considerable redshift in emission peaks confirming increase of In incorporation in the nanostructures. The interesting results of the structural and optical characterizations and possible growth mechanism of these InGaN

K.K.III.6
11:45
Authors : B. Alshehri 1, K. Dogheche 1, P.I. Seetoh 3, J.H. Teng 2, S.J. Chua 3, D. Decoster 1, E. Dogheche 1
Affiliations : 1 Institute of Electronics, Microelectronics and Nanotechnology, Optoelectronics Group (IEMN CNRS) Villeneuve d’Ascq, France 2 Institute of Materials Research and Engineering (IMRE), Singapore 117602, Singapore, 3 Department of Electrical and Computer Engineering, National University of Singapore (NUS), Singapore 117576

Resume : This work is focused both on InxGa1-xN single-layer and InxGa1-xN/GaN multilayered structures, with high Indium content (x>35%). In this study, InGaN/GaN films are epitaxially grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD). The microstructure of GaN and InxGa1-xN thin films are characterized by SEM/AFM/TEM [1] and optically by prism coupling/ ellipsometry for the refractive indices [2]. We focus here on the design and the fabrication of ultra-fast photodiodes operating in visible wavelength range. Different configurations for UTC photodiodes have been fabricated (sizes ranging from 2.5 to 50µm) and the technological processes optimized including dry etching processes for patterning GaN/InGaN layers. Ti/Al/Ni/Au and ITO materials for p and n type contacts are also investigated. [1] A. Gokarna, A. Gauthier-Brun, W. Liu, Y. Androussi, E. Dumont, E. Dogheche, J. H. Teng, S.J. Chua, D. Decoster, Applied Physics Letters ,Vol.96 , Issue: 19, May 2010. [2] A. Gauthier- Brun, J.H. Teng, E. Dogheche, Wei Liu, M Tonouchi A. Gokarna, S.J. Chua, D. Decoster, Properties of InxGa1-xN films in Terahertz range, Applied Physics Letter, 100, 071913 (2012)

K.K.III.9
13:30
Authors : Jincheng Zhang*, Fanna Meng, Junshuai Xue, Juncai Ma, Linyu Shi, Zhongfen Zhang, Huijuan Wen, Zhiyu Lin, Hao Zhou, Yue Hao
Affiliations : Key Lab of Wide Band Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi’an 710071, China

Resume : Due to strong polarization properties of III-nitrides, GaN-based heterostructures, such as AlGaN/GaN heterostructure, InAlN/GaN heterostructure, etc, have a special two dimensional electron gas (2DEG) with high density and high mobility. However, since the 2DEG density is very high, a large quantity of electron can overflow from the triangle potential well of 2DEG interface under higher temperature. This overflow of 2DEG will lead to reduction of the total electron mobility [1], and the HEMT device cannot be pinched off. In this report, to confine the overflow of high-density 2DEG, many novel heterostructures, such as high Al-content AlGaN/GaN with deeper potential well [2], AlGaN/GaN/AlGaN and InAlN/GaN/AlGaN double heterostructures with a back-barrier[3,4], AlGaN/GaN/AlGaN/GaN and InAlN/GaN/InAlN/GaN hetero -structure with double channels[5], were studied. GaN-based devices with these structures exhibit excellent transport characteristics under high temperature, so they are more suitable for the applications in high temperature electronic devices. References: [1] Zhang Jinfeng, Wang Chong, Zhang Jincheng, Hao Yue. Chinses Physics, 15(5):1060, 2006. [2] Zhang ZhongFen, Zhang JinCheng, et al, Science in China-Series G: Physics Mechanics and Astronomy, 52(12):1879-1884, 2009. [3] Meng, Fanna, Zhang, Jincheng, Zhou, Hao, et al, Journal of Applied Physics, 112(2), 2012. [4] Juncai Ma, Jincheng Zhang, Junshuai Xue, Chinese Journal of Semiconductors, 33(1), 2012. [5] Xue, JunShuai, Zhang, JinCheng, et al, Journal of Applied Physics, 111(11), 2012.

K.K.III.10
14:00
Authors : Martin W. G. Hoffmann 1 2 3, Jordi Sama 1, Olga Casals 1, Francisco Hernandez-Ramirez 1 3, Andreas Waag 2, Hao Shen 2, J. Daniel Prades 1
Affiliations : 1. Department of Electronics, University of Barcelona, Barcelona, Spain; 2. Institut fur Halbleitertechnik, Technische Universitaet Braunschweig, Braunschweig, Germany; 3. Department of Advanced Materials for Energy Applications, Catalonia Institute for Energy Research (IREC), Barcelona, Spain.

Resume : Low power consumption, long-term stability and high specificity are some of the most sought after requirements for future gas sensors technologies. In this contribution, the potential of GaN-based devices to contribute to this field will be discussed. First, new optically-driven sensor device concepts featuring zero power consumption have shown that this is an open field for optically active materials[1]. Light emitters and absorbers active in the visible range, such as InGaN, are required to operate these new technologies in ambient light. Second, the traditional strategy to maximize the sensor response by choosing chemically and thermally unstable materials leads to well know long-term stability issues. Additionally, this choice produces vigorous and highly unspecific responses to gases. Following recent finding[2], an alternative strategy could be relying on highly stable gas-immune materials to build up the fundamental elements of the sensor platform in combination with highly specific functionalizations. The well-established microelectronic GaN processes would provide the necessary fine control over the electrical and optical properties of the device platform. The flexibility of molecular chemistry would deliver sufficient flexibility to tailor the specific response of the sensor almost at will. Finally, the first attempts to implement this strategy in new devices will be presented. [1]NanoEnergy 2013, 2, 514–522. [2]Adv.Funct.Mater. 2013, DOI: 10.1002/adfm.201301478

K.K.III.11
14:30
Authors : Holger von Wenckstern
Affiliations : Universität Leipzig, Institut für Experimentelle Physik II, Halbleiterphysik

Resume : The detection of ultraviolet irradiation is important in environmental research and monitoring, flame detection and monitoring of industrial processes as UV curing of glues, adhesives or disinfection of drinking water by UV irradiation. Besides (Al,Ga)N semiconducting oxides present a material class that is very suited for realization of detectors operating in the UV-A, UV-B and even the UV-C spectral range. For most applications the determination of spectrally integrated UV radiation is not sufficient and a spectrally resolved detection of UV radiation is desired. Here, we demonstrate an approach towards monolithic wavelength-selective UV-A photo-detectors by using a continuous composition spread approach.

K.K.III.12
15:15
Authors : X. Li 1 2, S. Suresh 2, Y. El Gmili 2, F. Genty 3, P. Voss1, J-P. Salvestrini 2 3, A. Ougazzaden 1 2.
Affiliations : 1. Georgia Institute of Technology / GTL, UMI 2958, Georgia;Tech-CNRS, 57070 Metz, France; 2. CNRS UMI 2958, Georgia Tech-CNRS, 57070 Metz, France; 3. Université de Lorraine, LMOPS, EA4423, 57070 Metz, France

Resume : Boron containing III-nitrides are attractive system for deep-UV LEDs and LDs because of their wide bandgaps and flexible lattice. However the crystallinity and boron content have been limited due to large mismatch between BN and other nitrides. In this work, BAlN layers with boron composition from 1% to 5% were successfully grown on AlN template substrates by low-pressure organometallic vapor phase epitaxy. The samples were grown at 650˚C and then annealed at 1020˚C for recrystallization. Growth techniques such as temperature, growth time and TEB/III ratio in the gas phase were investigated. High quality BAlN layers were grown using flow-modulate epitaxy method that allows to enhance surface migration of boron atoms. 70 nm-thick layers show a good surface morphology. For the first time, clear XRD peak relating to 5% boron for this new material was observed, which suggests the formation of single-phase solid solution. Adding more boron to the AlN produced a shift in the peak positions to greater angles. Further results by TEM and optical characterizations will be presented. This new material is promising for deep-UV applications and gives more freedom for bandgap engineering of multi-structure devices.

K.K.III.14
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09:00
Authors : Frank Bertram, Juergen Christen
Affiliations : Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg, Germany

Resume : For a detailed understanding of complex semiconductor heterostructures a systematic determination of the structural, chemical, and optical properties on a nanometer scale is essential. The combination of luminescence spectroscopy – in particular at liquid He temperatures - with the high spatial resolution of a scanning transmission electron microscope (STEM) (dx<1 nm at RT,dx <5 nm at 10K), as realized by the technique of low temperature scanning transmission electron microscopy cathodoluminescence spectroscopy (STEM-CL), provides a unique, extremely powerful tool for the optical nano-characterization. In this study we will present our STEM-CL results from an ordered array of InGaN/GaN nano rods (NR) grown in top-down fabrication on GaN/sapphire template: the GaN template was structured using standard photolithography and reactive ion etching. Subsequently, the NRs were overgrown by MBE with a GaN buffer followed by a thick InGaN layer. We observe the highest CL intensity in the upper part of each NR. Spectrally resolved CL measurements at 15 K reveal distinct luminescence contributions originating from the different InGaN sections. Highly resolved linescans of individual NRs exhibit a characteristic luminescence from the bottom InGaN followed by a middle InGaN region. In addition, the upper part of the NR layer shows a broad, strongly red-shifted luminescence giving a detailed understanding of the In composition with distinct In gradients in vertical and radial directions.

K.K.IV.2
09:45
Authors : X. Kong 1, S. Albert 2, A. Bengoechea-Encabo 2, M. Hanke 1, M.A. Sanchez-Garcia 2, E. Calleja 2, A. Trampert 1
Affiliations : 1. Paul-Drude-Institut fur Festkorperelektronik, Hausvogteiplatz 5–7, D-10117 Berlin, Germany; 2. ISOM and Dpto. Ingenieria Electronica, ETSI Telecomunicacion, Universidad Politecnica, Ciudad Universitaria, 28040 Madrid, Spain.

Resume : Axial (In,Ga)N/GaN nanowire (NW) heterostructures are considered as promising building blocks for the realization of novel high performance light-emitting diodes. Here we will report on the interplay between interface morphology, epitaxial strain and local chemical composition distribution in axial (In,Ga)/GaN NW heterostructures investigated by transmission electron microscopy (TEM). Two sets of NW samples are grown by plasma-assisted molecular beam epitaxy on Si(111) in a self-assembled way and on patterned GaN templates in an ordered way, respectively. Local electron energy-loss spectroscopy measurements show that both sets of NWs have a similar final indium concentration of 35%, but remarkably different interface profiles. The difference of interface profile is strongly linked to the interface morphology: i) In self-assembled NWs showing a large chemical interface width (~40 nm), the interface geometry exhibits a pencil-like shape, where 60°-type misfit dislocations are formed (i.e., with Burgers vector b=a/3[11-20]). ii) On the other hand, the ordered NWs containing flat boundaries and a small interface width of ~10 nm, are plastically relaxed by the formation of partial dislocations associated with stacking faults. Additionally, the residual strain distribution along the (In,Ga)N NWs is studied by high-resolution TEM and geometric phase analysis. The influence of interface morphology and chemical composition profile on local strain distribution and strain relaxation mechanism is further discussed by calculations based on finite-element method.

K.K.IV.4
11:00
Authors : Johannes Ledig, Xue Wang, Jana Hartmann, Markus Bähr, Andreas Fahl, Frederik Steib, Hergo–H. Wehmann, Andreas Waag
Affiliations : Institut für Halbleitertechnik, Technische Universität Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany.

Resume : Three dimensional light emitting diodes (LEDs) with a shell geometry around a columnar GaN core are supposed to have substantial advantages over conventional planar LEDs. The active area along the sidewalls of the GaN pillars can substantially be increased by high aspect ratios. Due to the 3-dimensional shape, the electrical and optical characterization of such device structures is a substantial problem. Here we demonstrate that a nano-manipulator setup inside a scanning electron microscope can be used in combination with a cathodoluminescence (CL) system to characterize the electro-optical properties by directly contacting single facets of the 3D structure. Electron beam induced current (EBIC) images clearly prove that the pn-junction is completely wrapped around the core. By comparing spatially resolved CL and EBIC, the rate of charge carrier generation, trapping and separation in different regions is discussed. We will present electroluminescence (EL) spectra from single core-shell LEDs obtained at different injection currents, in both the region of MQW as well as defect related emission. A wavelength shift of the MQW emission by 60 nm is observed along the structure height for both excitation methods (CL and EL), indicating a gradient of the indium incorporation. In addition, metal contacts have been fabricated in order to get a defined contact area By evaluating the contact area and the EL spectra we gain an insight to the internal efficiency versus current density.

K.K.IV.6
11:15
Authors : E. M. L. D. de Jong 1, W. D. A. M. de Boer 1, T. Gregorkiewicz 1, A. Koizumi 2, D. Timmerman 2, and Y. Fujiwara 2
Affiliations : 1. Van der Waals-Zeeman Institute, University of Amsterdam, The Netherlands; 2. Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Japan.

Resume : The rare-earth doped gallium nitride (GaN) materials have shown to be a promising candidate for LEDs, lasers and full-colour displays due to their ability to emit sharp and stable spectral lines. Based on these materials, efficient green and blue LEDs have been successfully developed. For realization of red emission Eu doping is considered. Here, we report on ultrafast optical spectroscopy investigations of state-of-the-art red-emitting GaN doped with trivalent europium (Eu3+) fabricated by organometallic vapor phase epitaxy. We employ experimental techniques of transient induced absorption (IA), using a conventional femtosecond pump-probe setup, and photoluminescence (PL) emission and excitation spectroscopy. In that way, carrier decay dynamics measured by IA is cross-correlated with photon generation responsible for PL. The spectrally and temporally resolved data allow for unique insights into the complex energy transfers in this system and their dynamics. In particular, we managed to separate carrier trapping from the build-up of the excited state population of Eu3+ ions, responsible for the red emission band. On this basis, an energy transfer model including dynamics of individual steps can be proposed for the first time. These findings provide fundamental information on the excitation and de-excitation mechanisms of Eu3+ ions in GaN and will serve for further optimization of this material towards lighting applications.

K.K.IV.7
11:30
Authors : Gordon Schmidt, Marcus Müller, Anja Dempewolf, Silke Petzold, Peter Veit, Frank Bertram, Christoph Berger, Armin Dadgar, Alois Krost, Jürgen Christen
Affiliations : Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg, Germany

Resume : III-Nitride based microcavities (MC) are one of the most promising candidates for the realization of polariton lasers operating at room temperature since they exhibit highly stable excitons and large oscillator strength. These lasers operate without inversion in the strong coupling regime (SCR) where the squared strength of the light-matter coupling g has to overcome the mean value of the squared line widths of exciton and cavity mode. Since g is proportional to the square root of the number of quantum wells (QWs), the SCR can be reached by embedding a large number of QWs.We present the optical and the structural properties at the nanometer scale of a MOVPE grown MC structure comprising a large number of embedded InGaN QWs by cathodoluminescence in a scanning transmission electron microscope (STEM-CL). The sample consists of a 28-fold InGaN/GaN QWs embedded in a GaN λ cavity on top of an AlInN/GaN distributed Bragg reflector (DBR) grown on sapphire substrate. Direct comparison of the STEM images with simultaneously recorded CL mappings resolve the complete layer sequence. In particular, the DBR layer stack is proven to be laterally and vertically homogeneous with sharp GaN/AlInN interfaces. A dominant emission with a broad spectral range of the InGaN MQW can be observed. Spectrally resolved linescans across the active region exhibit a redshift from the bottom (425 nm) to the top (465 nm) visualizing strain relaxation, higher In incorporation, and/or increasing QW thickness.

K.K.IV.8
11:45
Authors : B. Alshehri 1, S-M Lee 2, J-H Kang 2, K. Dogheche, S-H Gong 3, S-W Ryu2, E. Dumont 4, Y-H Cho 3, E. Dogheche 1
Affiliations : 1. Institute of Electronics, Microelectronics and Nanotechnology, Optoelectronics Group (IEMN CNRS UMR 8520) Villeneuve d?Ascq, France; 2, Department of Physics, Chonnam National University, Gwangju 500-757, Republic of Korea; 3. Advanced Institute of Science Technology (KAIST), Daejeon 305-701, Republic of Korea; 4. Energy Research Centre, University of Mons, 7000 Mons, Belgium.

Resume : Gallium Nitride (GaN) thin films have been prepared on sapphire by metal organic chemical vapor deposition (MOCVD) technique and a chemical etching method using KOH is used to develop nanoporous structures [1]. We present comparative investigations of porous and nonporous GaN layers. While the pores density is determined, we have investigated the microstructures in GaN films by using transmission electron microscopy (TEM). The refractive index dispersion has been evaluated through different techniques, ellipsometry and guided-wave prism coupling [2]. We have correlated the microstructure with the refractive index of the material. The aim of this research is to demonstrate that optical properties of GaN can be tuned by controlling the pores size and spacing. For a pores density of 20%, we report an index variation ∆n= -12*10-3. The control of the refractive index into GaN is therefore fundamental for the design of active and passive optical devices [1] S-H Gong, A. Stolz,G-H Myeong, E. Dogheche, A. Gokarna, S-W Ryu, D. Decoster, Y-H Cho, Optics letters, vol 36 no.21, pp4272-4274, nov 2011. [2] Y-H Lee, J-H Kang, S-W Ryu, Thin Solid Films, vol.540 150 (2013).

K.K.IV.9
15:30
Authors : A. Vilalta-Clemente1, M.P. Chauvat1, P. Gamarra2, M. Morales1, M. Tordjman2, J. F. Carlin3, M. A. di Forte-Poisson2, N. Grandjean3, and P. Ruterana1
Affiliations : 1 CIMAP UMR 6252 CNRS-ENSICAEN-CEA-UCBN, 6, Boulevard du Maréchal Juin, 14050 Caen Cedex, France, 2 Alcatel-Thales, III-V Lab. Route de Nozay, 91460 Marcoussis, France, 3 Institute of Condensed Matter Physics (ICMP), Ecole Polytechnique de Lausanne (EPFL) 1015 Lausanne, Switzerland, 4 LPN, Route de Nozay, 91460 Marcoussis, France

Resume : The nitride aluminium and indium alloy (InAlN), when lattice matched (LM) with GaN (i. e. In content at around 18%) is of great interest for opto- and micro-electronic applications. However, due to thermal mismatch between InN and AlN, the growth of good quality InAlN layers is still difficult and many workers have reported crystalline degradation versus thickness even at LM to GaN. The mechanisms governing this behaviour are not still clear, and many proposals are still under debate. In this work, we investigated InAlN layers, close to the LM composition, grown on GaN/Al2O3 by metal organic vapour phase epitaxy at different substrate temperatures and V/III ratios. Scanning transmission electron microscopy, X Ray diffraction, RBS and energy dispersive spectroscopies were used to determine the structure and composition. Our results show that the layers quality depends critically on the growth conditions. During our observations, structural disruption has been systematically observed versus thickness and depending on the growth conditions even completely polycrystalline layers were obtained. Therefore, it appears that obtaining good crystallinity and homogeneous composition layers probably needs a continuous layer evolution monitoring and corrections in the conditions during the growth. The influence and role of various parameters such as III/V ratios, growth temperature, sources control and overall pressure on the quality of the grown layers will be discussed.

K.K.IV.15

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