bluuebunny/crossref_metadata_2025 huggingface dataset:
A Damage-Reduced Process Revealed by Photoluminescence in Photoelectrochemical Etching GaN
Authors: Hwang J. M. , Hsieh J. T. , Hwang H. L. , Hung W. H.
Published: September 2012

Photoelectrochemical (PEC) etching technique has been proven to be an effective method to etch GaN. Despite its success, investigations on etching-induced damage are still scare. In this work, the damage induced by PEC etching of GaN in KOH electrolyte was studied. Photoluminescence (PL) spectroscopy was used to explore the origin of etching-induced damaged layer. From the variable temperature PL measurements, the origin of etching-induced damage was attributed to be the defect complex of VGa-ON (gallium vacancy bonds to oxygeon on nitrogen antisite). With determination of the defect origin, the electronic transition in the etch damage-related yellow luminescence (YL) band was suggested to be deep donor-like state to shallow-acceptor transition. In addition, a post-treatment method with boiled KOH chemical etching was developed to remove the thin damaged layer. In this method, crystallographic etching characteristics of boiled KOH was observed to assist in the formation of smooth sidewall facets. As revealed by the reduction of yellow luminescence, we propose this novel technique as a near damage-free etching method.
Fabrication and Characterization of InGaN Nano-scale Dots for Blue and Green LED Applications
Authors: Kim K.S. , Hong C.-H. , Lee W.-H. , Kim C.S. , Cha O.H. , Yang G.M. , Suh E.-K. , Lim K.Y. , Lee H.J. , Cho H.K. , Lee J.Y. , Seo J.M.
Published: September 2012

Thin layers of InGaN were grown by metalorganic chemical vapor deposition and characterized with atomic force microscopy and high-resolution transmission electron microscopy. InGaN deposited on GaN exhibits a Stranski-Krastanov growth mode, including 2D wetting layer and 3D self-assembled quantum dots. Besides, we observed that the formed InGaN nano-scale dots have a trapezoidal shape with a {1-102} facet with respect to (0002) surface. Visible spectral range from UV to green was easily obtained by changing InGaN quantum well thickness up to 2.3 nm.
The Microstructure and Electrical Properties of Directly Deposited TiN Ohmic Contacts to Gallium Nitride.
Authors: Ruterana P. , Nouet G. , Kehagias Th. , Komninou Ph. , Karakostas Th. , Poisson M.A. di Forte , Huet F.
Published: September 2012

When the stoichiometric TiN was deposited directly on GaN, we obtained columnar TiN grains of 5-20 nm section which cross the whole film thickness and are rotated mostly around the [111] axis. The conventional epitaxial relationship is obtained and no amorphous patches are observed at the interface. The deposition of TiN on Si doped GaN layers lead to the formation of an ohmic contact, whereas we obtain a rectifying contact on p type layers.
Highly Chemical Reactive Ion Etching of Gallium Nitride
Authors: Karouta F. , Jacobs B. , Moerman I. , Jacobs K. , Weyher J.L. , Porowski S. , Crane R. , Hageman P.R.
Published: September 2012

: A highly chemical reactive ion etching process has been developed for MOVPE-grown GaN on sapphire. The key element for the enhancement of the chemical property during etching is the use of a fluorine containing gas in a chlorine based chemistry. In the perspective of using GaN substrates for homo-epitaxy of high quality GaN/AlGaN structures we have used the above described RIE process to smoothen Ga-polar GaN substrates. The RMS value, measured by AFM, went from 20 Å (after mechanical polishing) down to 4 Å after 6 minutes of RIE. Etching N-polar GaN resulted in a higher etch rate than Ga-polar materials (165 vs. 110 nm/min) but the resulting surface was quite rough and suffers from instability problems. Heat treatment and HCl dip showed a partial recovery of Schottky characteristics after RIE.
Improved Low Resistance Contacts of Ni/Au and Pd/Au to P-Type GaN Using a Cryogenic Treatment
Authors: Park Mi-Ran , Anderson Wayne A. , Park Seong-Ju
Published: September 2012

A low resistance Ohmic contact to p-type GaN is essential for reliable operation of electronic and optoelectronic devices. Such contacts have been made using Ni/Au and Pd / Au contacts to p-type Mg-doped GaN (1.41×1017 cm−3) grown by metalorganic chemical vapor deposition ( MOCVD ) on ( 0001 ) sapphire substrates. Thermal evaporation was used for the deposition of those metals followed by annealing at temperatures of 400 ∼ 700 °C in an oxygen and nitrogen mixed gas ambient, then subsequently cooled in liquid nitrogen which reduced the specific contact resistance from the range of 9.46∼2.80×10−2 ωcm2 to 9.84∼2.65×10−4 ωcm2 for Ni/Au and from the range of 8.35∼5.01×10−4 ωcm2 to 3.34∼1.80×10−4 ωcm2 for Pd/Au. The electrical characteristics for the contacts were examined by the current versus voltage curves and the specific contact resistance was determined by use of the circular transmission line method (c-TLM). The effects of the cryogenic process on improving Ohmic behavior (I-V linearity) and reducing the specific contact resistance will be discussed from a microstructural analysis which reveals the metallurgy of Ohmic contact formation.
Deep Centers and Persistent Photoconductivity Studies in Variously Grown GaN Films
Authors: Polyakov Alexander Y. , Smirnov Nikolai B. , Govorkov Anatoliy V. , Usikov Alexander S. , Shmidt Natalie M. , Pushnyi Boris V. , Tsvetkov Denis V. , Stepanov Sergey I. , Dmitriev Vladimir A. , Mil'vidskii Mikhail G. , Pavlov Vladimir F.
Published: September 2012

Deep levels studies on a set of n-GaN films grown by MOCVD and HVPE reveal the presence of electron traps with levels near Ec-0.25 eV, Ec-0.55 eV, Ec-0.8 eV, Ec-1 eV, hole traps with levels near Ev+0.9 eV and a band of relatively shallow states in the lower half of the bandgap. The total density of these latter states was estimated to be some 1016 cm−3 and they were tentatively associated with dislocations in GaN based on their high concentration and band-like character. None of the electron or hole traps could be unambiguously related with strong changes of diffusion lengths of minority carriers in various samples. It is proposed that such changes occur due to different surface recombination velocities. An important role of Ec-0.55 eV traps in persistent photoconductivity phenomena in n-GaN has been demonstrated.
Photocurrent Spectroscopy Investigations of Mg-Related Defects Levels in p-Type GaN
Authors: Chung S. J. , Cha O. H. , Cho H. K. , Jeong M. S. , Hong C-H. , Suh E-K. , Lee H. J.
Published: September 2012

The defect levels associated with Mg impurity in p-type GaN films were systematically investigated in terms of doping concentration by photocurrent spectroscopy. Mg-doped GaN samples were grown on sapphire substrate by metal organic chemical vapor deposition and annealed in nitrogen atmosphere at 850 for 10 minutes. At room temperature, PC spectra showed two peaks at 3.31 and 3.15 eV associated with acceptor levels formed at 300 and 142 meV above valence band in as grown samples. But, after the thermal annealing, PC spectra exhibited various additional peaks depending on the Mg concentration. In the GaN samples with Mg concentration around 6 7 1017 cm−3, we have observed PC peaks related to Mg at 3.31 as well as 3.02 eV and carbon acceptor at 3.17 eV. For moderately Mg doped GaN samples, i.e., the hole concentration p=3 4 1017 cm−3, additional peak was observed at around 0.9 eV which can be attributed to defects related to Ga vacancy. For relatively low Mg doped samples whose hole concentrations are 1 2 1017 cm−3, additional broad peak was observed at around 1.3 eV. This peak may be related to the yellow band luminescence. As the Mg concentration is increased, the concentration of Ga vacancies can be reduced because Mg occupies the substitutional site of Ga in GaN lattice. When the hole concentration is above 6 7 1017 cm−3, the yellow luminescence and Ga vacancy related peaks disappeared completely.
Characteristics of Deep Centers Observed in n-GaN Grown by Reactive Molecular Beam Epitaxy
Authors: Fang Z-Q. , Look D. C. , Kim Wook , Morkoç H.
Published: September 2012

Deep centers in Si-doped n-GaN samples grown on sapphire by reactive molecular beam epitaxy, using different ammonia flow rates (AFRs), have been studied by deep level transient spectroscopy. In addition to five electron traps, which were also found in n-GaN layers grown by both metalorganic chemical-vapor deposition and hydride vapor-phase epitaxy, two new centers C1 (0.43-0.48 eV) and E1 (0.25 eV) have been observed. C1, whose parameters show strong electric-field effects and anomalous electron capture kinetics, might be associated with dislocations. E1, which is very dependent on the AFR, exhibits an activation energy close to that of a center created by electron irradiation and is believed to be a defect complex involving VN.
Comparison Study of Structural and Optical Properties of InxGa1-xN/GaN Quantum Wells with Different in Compositions
Authors: Kwon Yong-Hwan , Gainer G. H. , Bidnyk S. , Cho Y. H. , Song J. J. , Hansen M. , Denbaars S. P.
Published: September 2012

The effect of In on the structural and optical properties of InxGa1−xN/GaN multiple quantum wells (MQWs) was investigated. These were five-period MQWs grown on sapphire by metalorganic chemical vapor deposition. Increasing the In composition caused broadening of the high-resolution x-ray diffraction superlattice satellite peak and the photoluminescence-excitation bandedge. This indicates that the higher In content degrades the interface quality because of nonuniform In incorporation into the GaN layer. However, the samples with higher In compositions have lower room temperature (RT) stimulated (SE) threshold densities and lower nonradiative recombination rates. The lower RT SE threshold densities of the higher In samples show that the suppression of nonradiative recombination by In overcomes the drawback of greater interface imperfection.
Emission at 247 nm from GaN Quantum Wells Grown by MOCVD
Authors: Someya Takao , Hoshino Katsuyuki , Harris Janet C. , Tachibana Koichi , Kako Satoshi , Arakawa Yasuhiko
Published: September 2012

Photoluminescence (PL) spectra were measured at room temperature for GaN quantum wells (QWs) with Al0.8Ga0.2N barriers, which were grown by atmospheric-pressure metal organic chemical vapor deposition (MOCVD). The thickness of the GaN QW layers was systematically varied from one monolayer to four monolayers. We clearly observed a PL peak at a wavelength as short as 247 nm (5.03 eV) from one monolayer-thick QWs. The effective confinement energy is as large as 1.63 eV.
Pendeo-Epitaxial Growth and Characterization of GaN and Related Materials on 6H-SiC(0001) and Si(111) Substrates
Authors: Davis Robert F. , Gehrke T. , Linthicum K.J. , Zheleva T. S. , Rajagopal P. , Zorman C. A. , Mehregany M.
Published: September 2012

Discrete and coalesced monocrystalline GaN and AlxGa1−xN layers grown via Pendeoepitaxy (PE) [1] originated from side walls of GaN seed structures containing SiNx top masks have been grown via organometallic vapor phase deposition on GaN/AlN/6HSiC(0001) and GaN(0001)/AlN(0001)/3C-SiC(111)/Si(111) substrates. Scanning and transmission electron microscopies were used to evaluate the external microstructures and the distribution of dislocations, respectively. The dislocation densities in the PE grown films was reduced by at least five orders of magnitude relative to the initial GaN seed layers. Tilting in the coalesced GaN epilayers was observed via X-ray diffraction. A tilt of 0.2° was confined to areas of mask overgrowth; however, no tilting was observed in the material suspended above the SiC substrate. The strong, low-temperature PL band-edge peak at 3.45 eV with a FWHM of 17 meV was comparable to that observed in PE GaN films grown on 6H-SiC(0001). The band-edge in the GaN grown on AlN(0001)/SiC(111)Si(111) substrates was shifted to a lower energy by 10 meV, indicative of a greater tensile stress.
Fabrication of GaN with Buried Tungsten (W) Structures Using Epitaxial Lateral Overgrowth (ELO) via LP-MOVPE
Authors: Miyake Hideto , Yamaguchi Motoo , Haino Masahiro , Motogaito Atsushi , Hiramatsu Kazumasa , Nambu Shingo , Kawaguchi Yasutoshi , Sawaki Nobuhiko , Iyechika Yasushi , Maeda Takayoshi , Akasaki Isamu
Published: September 2012

A buried tungsten (W) mask structure with GaN is successfully obtained by epitaxial lateral overgrowth (ELO) technique via low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). The selectivity of GaN growth on the window region vs. the mask region is good. An underlying GaN with a striped W metal mask is easily decomposed above 500 °C by the W catalytic effect, by which radical hydrogen is reacted with GaN. It is difficult to bury the W mask because severe damage occurs in the GaN epilayer under the mask. It is found that an underlying AlGaN/GaN layer with a narrow W stripe mask width (mask/window = 2/2 νm) leads the ELO GaN layer to be free from damage, resulting in an excellent W-buried structure.
Advanced Pendeoepitaxy™ of GaN and AlxGa1−xN Thin Films on SiC(0001) and Si(111) Substrates via Metalorganic Chemical Vapor Deposition
Authors: Gehrke T. , Linthicum K. J. , Rajagopal P. , Preble E. A. , Davis R. F.
Published: September 2012

Growth of GaN and AlxGa1−xN thin films on 6H-SiC(0001) and Si(111) substrates with low densities of defects using the PENDEO™ process and the characterization of the resulting materials are reported. The application of a mask on the GaN seed structures hinders the vertical propagation of threading dislocations of the seed material during regrowth, but introduces a misregistry in the overgrowing material resulting in low quality crystal growth. This misregistry has been eliminated due to advanced processing and the exclusion of the masking layer. The new generation of samples do not show any misregistry, as shown by transmission electron microscopy.
GaN and AlN Layers Grown by Nano Epitaxial Lateral Overgrowth Technique on Porous Substrates
Authors: Mynbaeva M. , Titkov A. , Kryzhanovski A. , Zubrilov A. , Ratnikov V. , Davydov V. , Kuznetsov N. , Mynbaev K. , Stepanov S. , Cherenkov A. , Kotousova I. , Tsvetkov D. , Dmitriev V.
Published: September 2012

Defect density and stress reduction in heteroepitaxial GaN and AlN materials is one of the main issues in group III nitride technology. Recently, significant progress in defect density reduction in GaN layers has been achieved using lateral overgrowth technique. In this paper, we describe a novel technique based on nano-scale epitaxial lateral overgrowth.

GaN layers were overgrown by hydride vapour phase epitaxy (HVPE) on porous GaN. Porous GaN was formed by anodization of GaN layers grown previously on SiC ŝubstrates. Pore's size was in nano-scale range.

Thickness of overgrown layers ranged from 2 to 120 microns. It was shown that GaN layers overgrown on porous GaN have good surface morphology and high crystalline quality. The surface of overgrown GaN material was uniform and flat without any traces of porous structure. Raman spectroscopy measurements indicated that the stress in the layers grown on porous GaN was reduced down to 0.1 - 0.2 GPa, while the stress in the layers grown directly on 6H-SiC substrates remains at its usual level of about 1.3 GPa.

Preliminary experiments were done on HVPE growth of AlN layer on porous substrates. Improvement of surface morphology and crack density reduction has been observed.
Mass Transport, Faceting and Behavior of Dislocations in GaN
Authors: Nitta S. , Kashima T. , Kariya M. , Yukawa Y. , Yamaguchi S. , Amano H. , Akasaki I.
Published: September 2012

The behavior of threading dislocations during mass transport of GaN was investigated in detail by transmission electron microscopy. Mass transport occurred at the surface. Therefore, growing species are supplied from the in-plane direction. The behavior of threading dislocations was found to be strongly affected by the mass transport process as well as the high crystallographic anisotropy of the surface energy of the facets particular to GaN.
Integration of PLZT and BST Family Oxides with GaN.
Authors: Osinsky Andrei V. , Fuflyigin Vladimir N. , Wang Feiling , Vakhutinsky Peter I. , Norris Peter E.
Published: September 2012

Recent advances in the processing of complex-oxide materials has allowed us to monolithically grow ferroelectrics of lead lanthanum zirconate titanate (PLZT) and barium strontium titanate (BST) systems on a GaN/sapphire structure. High quality films of PLZT and BST were grown on GaN/c-Al2O3 in a thickness range of 0.3-5 [.proportional]m by a solgel technique. Field-induced birefringence, as large as 0.02, was measured from a PLZT layer grown on a buffered GaN/sapphire structure. UV illumination was found to result in more symmetrical electrooptic hysteresis loop. BST films on GaN demonstrated a low frequency dielectric constant of up to 800 with leakage current density as low as 5.5.10−8 A/cm2.
HVPE and MOVPE GaN Growth on Slightly Misoriented Sapphire Substrates
Authors: Parillaud Olivier , Wagner Volker , Bühlmann Hans-Jörg , Lelarge François , Ilegems Marc
Published: September 2012

We present preliminary results on gallium nitride growth by HVPE on C-plane sapphire with 2, 4 and 6 degrees misorientation towards M and A directions. A nucleation GaN buffer layer is deposited prior the growth by MOVPE. Surface morphology and growth rates are compared with those obtained on exact C-plane oriented sapphire, for various growth conditions. As expected, the steps already present on the substrate surface help to initiate a directed step-flow growth mode. The large hillocks, which are typical for HVPE GaN layers on (0001) sapphire planes, are replaced by more or less parallel macro-steps. The width and height of these steps, due to step bunching effect, depend directly on the angle of misorientation and on the growth conditions, and are clearly visible by optical or scanning electron microscopy. Atomic force microscopy and X-ray diffraction measurements have been carried out to quantify the surface roughness and crystal quality.
Hydride Vapour Phase Homoepitaxial Growth of GaN on MOCVD-Grown ‘Templates’
Authors: Paskova T. , Tungasmita S. , Valcheva E. , Svedberg E.B. , Arnaudov B. , Evtimova S. , Persson P.Å , Henry A. , Beccard R. , Heuken M. , Monemar B.
Published: September 2012

We report on an improved quality of thick HVPE-GaN grown on MOCVD-GaN ‘template’ layers compared to the material grown directly on sapphire. The film-substrate interface revealed by cathodoluminescence measurements shows an absence of highly doped columnar structures which are typically present in thick HVPE-GaN films grown directly on sapphire. This improved structure results in a reduction of two orders of magnitude of the free carrier concentration from Hall measurements. It was found that the structure, morphology, electrical and optical properties of homoepitaxial thick GaN layers grown by HVPE were strongly influenced by the properties of the MOCVD-GaN ‘template’. Additionally the effect of Si doping of the GaN buffer layers on the HVPEGaN properties was analysed.
Visible and Infrared Emission from GaN:Er Thin Films Grown by Sputtering
Authors: Chen Hong , Gurumurugan K. , Kordesch M.E. , Jadwisienczak W.M. , Lozykowski H.J.
Published: September 2012

Erbium-doped films were grown on sapphire and silicon substrates by reactive sputtering, with different Er concentrations in the film. GaN films deposited at 800 K were determined to be polycrystalline by x-ray diffraction analysis, and retained their polycrystalline structure after annealing in nitrogen at 1250 K. The Er-doped films showed optical transmission beginning at about 360 nm, and the Er dose and film purity were determined with Rutherford backscattering spectroscopy. Photoluminescence and cathodoluminescence spectroscopy showed sharp emission lines corresponding to Er 3+ intra 4fn shell transitions over the range from 9 – 300 K. At above-bandgap optical and electron excitation, the 4S3/2 and 4F9/2 transition dominate, and are superposed on the “yellow band” emission. The infrared emission line at 1543 nm, corresponding to the Er 4I⅓ to 4I3/2 transition is also observed.
Growth and Characterization of GaN Thin Films on Si(111) Substrates Using SiC Intermediate Layer
Authors: Lim K. Y. , Lee K. J. , Park C. I. , Kim K.C. , Choi S. C. , Lee W.-H. , Suh E.-K. , Yang G. M. , Nahm K. S.
Published: September 2012

GaN films have been grown atop Si-terminated 3C-SiC intermediate layer on Si(111) substrates using low pressure metalorganic chemical vapor deposition (LP-MOCVD). The SiC intermediate layer was grown by chemical vapor deposition (CVD) using tetramethylsilane (TMS) as the single source precursor. The Si terminated SiC surface was obtained by immediately flow of SiH4 gas after growth of SiC film. LP-MOCVD growth of GaN on 3C-SiC/Si(111) was carried out with trimethylgallium (TMG) and NH3. Single crystalline hexagonal GaN layers can be grown on Si terminated SiC intermediate layer using an AlN or GaN buffer layer. Compared with GaN layers grown using a GaN buffer layer, the crystal qualities of GaN films with AlN buffer layers are extremely improved. The GaN films were characterized by x-ray diffraction (XRD), photoluminescence (PL) and scanning electron microscopy (SEM). Full width at half maximum (FWHM) of double crystal x-ray diffraction (DCXD) rocking curve for GaN (0002) on 3C-SiC/Si(111) was 890 arcsec. PL near band edge emission peak position and FWHM at room temperature are 3.38 eV and 79.35 meV, respectively.
Optical Properties of Manganese Doped Amorphous and Crystalline Aluminum Nitride Films
Authors: Caldwell M.L. , Richardson H.H. , Kordesch M.E.
Published: September 2012

An aluminum nitride (AlN) film deposited on silicon (100) was used as the substrate for growing manganese (Mn) doped AlN film by metal organic chemical vapor deposition (MOVCD). The (15.78 [.proportional]m) under layer of AlN was grown at 615°C at a pressure of 10−4 Torr. The (2.1 [.proportional]m) top layer of Mn-AlN was grown at the same temperature and pressure but doped with pulse valve introduction of the manganese decacarbonyl (100 ms on, 100 ms off). The film was then characterized ex situ with IR reflectance microscopy, X-ray diffraction, scanning electron microscopy imaging, cathodoluminescence, and X-ray fluorescence. The IR reflectance measurements showed a strong (A1) LO mode for AlN at 920 cm−1 and 900 cm−1 with a shoulder at 849 cm−1. X-ray Diffraction yielded three diffraction peaks at a 2ø position of 33, 36 and 38 degrees corresponding to 100, 002, and 101 lattice planes respectively. Cathodoluminescence results show strong visible emitted light from incorporated manganese. The relative percentage of manganese to aluminum was below the detection limit (0.01 %) of the Xray fluorescence spectrometer. Amorphous Mn doped AlN films have also been grown using a low temperature atomically abrupt sputter epitaxial system. The amorphous Mn doped AlN showed no cathodoluminescence.
Surface Morphology of GaN: Flat Versus Vicinal Surfaces
Authors: Xie M.H. , Seutter S.M. , Zheng L.X. , Cheung S.H. , Ng Y.F. , Wu Huasheng , Tong S.Y.
Published: September 2012

The surface morphology of GaN films grown by molecular beam epitaxy (MBE) is investigated by scanning tunneling microscopy (STM). A comparison is made between flat and vicinal surfaces. The wurtzite structure of GaN leads to special morphological features such as step pairing and triangularly shaped islands. Spiral mounds due to growth at screw threading dislocations are dominant on flat surfaces, whereas for vicinal GaN, the surfaces show no spiral mound but evenly spaced steps. This observation suggests an effective suppression of screw threading dislocations in the vicinal films. This finding is confirmed by transmission electron microscopy (TEM) studies. Continued growth of the vicinal surface leads to step bunching that is attributed to the effect of electromigration.
Growth of InN by MBE
Authors: Chen W.-L. , Gunshor R. L. , Han Jung , Higashimine K. , Otsuka N.
Published: September 2012

A series of experiments were performed to explore the growth of InN by Molecular Beam Epitaxy (MBE). The growth conditions were optimized based on the study of RHEED during growth and InN dissociation experiments. Characterization of the InN thin films were performed by various techniques such as TEM and XRD.
The Effect of the Buffer Layer on the Structure, Mobility and Photoluminescence of MBE grown GaN
Authors: Sharma Nikhil , Tricker David , Keast Vicki , Hooper Stewart , Heffernan Jon , Barnes Jenny , Kean Alistair , Humphreys Colin
Published: September 2012

Although GaN has been grown mainly by metal organic chemical vapour deposition (MOCVD), molecular beam epitaxy (MBE) offers the advantages of lower growth temperatures and a more flexible control over doping elements and their concentrations [1]. We are growing GaN by MBE on sapphire substrates, using a GaN buffer layer to reduce the misfit strain, thus improving the structural quality of the epilayer. The quality of the GaN epilayers (in terms of their photoluminescence, mobility and structure) has been investigated as a function of the buffer layer thickness and annealing time.

The investigation showed that increasing the buffer layer thickness improved the mobility of the material because the defect density in the GaN epilayer decreased. Optical characterisation showed that the ratio of the donor band exciton (DBE) peak (3.47eV) to the structural peak (3.27eV) in the photoluminescence spectrum, measured at 10K, increased with decreasing defect density. The unwanted structural peak can be considered to originate from a shallow donor to a shallow acceptor transition, which is clearly related to the structural defects in GaN. Thus by increasing the buffer layer thickness and annealing time the structural quality, mobility and photoluminescence improves in the GaN epilayers.

Structural characterisation by transmission electron microscopy (TEM) showed that the observed increase in the DBE to structural peak ratio in the photoluminescence spectra could be correlated with a decrease in the density of stacking faults in the GaN epilayers. The detailed structure of these stacking faults was investigated by dark field and high resolution TEM. Their effect on the electrical and optical behaviour of GaN may be assessed by determining the local change in the dielectric function in the vicinity of individual stacking faults.
MBE Growth of GaN Films in Presence of Surfactants: The Effect of Mg and Si
Authors: Mula Guido , Daudin Bruno , Adelmann Christoph , Peyla Philippe
Published: September 2012

We present here a description and an analysis of the modifications in the growth behaviour of GaN induced by the presence of foreign species. The particular cases of Mg and Si are analysed. Profound changes, both in microscopic and macroscopic scales, occur in presence of Mg, even for fluxes of about 1/1000th of the Ga flux. The growth rate can be increased by almost 50%, depending of the III/V ratio and on the amount of Mg. A theoretical model is proposed to describe the observed effect. It is found that Mg induces changes in the Ga and N diffusion barriers and acts as a surfactant. The effect is stronger on the a-GaN than on the α-GaN, where N is more tightly bonded. The effect of Si is by far less pronounced, probably because it is more easily incorporated than Mg, and its effect on the surface kinetics is then strongly reduced.
Role of Arsenic Hexagonal Growth-Suppression on a Cubic GaNAs Growth Using Metalorganic Chemical Vapor Deposition
Authors: Yoshida S. , Kimura T. , Wu J. , Kikawa J. , Onabe K. , Shiraki Y.
Published: September 2012

The hexagonal domain suppression-effects in cubic-GaNAs grown by metalorganic chemical-vapor deposition (MOCVD) is reported. A thin buffer layer (20 nm) was first grown on a substrate at 853 K using trimethylgallium and dimethylhydrazine (DMHy), and GaNAs samples were grown at different AsH3 flow rates (0 ∼ 450 µmol/min) at 1193 K. As a result, three types of surface morphologies were obtained: the first was a smooth surface (AsH3 = 0 µmol/min); the second was a mirrorlike surface having small and isotropic grains (AsH3 : 45 ∼ 225 µmol/min ); and the third involved threedimensional surface morphologies (above 450 µmol/min of AsH3 flow rate). Furthermore, it was confirmed using X-ray diffraction that the mixing ratio of hexagonal GaNAs in cubic GaNAs decreased with an increase of the AsH3 flow rate. We could obtain GaNAs having a cubic component of above 85% at AsH3 flow rates above 20 µmol/min. Therefore, the MOCVD growth method using AsH3 and DMHy was mostly effective for suppressing hexagonal GaNAs. It was observed that the photoluminescence intensity of GaNAs was decreased with increase of arsine flow rate.
Microstructure and Physical Properties of GaN Films on Sapphire Substrates
Authors: Chen Zhizhong , Zhang Rong , Zhu Jianming , Shen Bo , Zhou Yugang , Chen Peng , Li Weiping , Shi Yi , Gu Shulin , Zheng Youdou
Published: September 2012

Transmission electron microscopy (TEM), x-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were applied to study the correlation between the microstructure and physical properties of the GaN films grown by light radiation heating metalorganic chemical vapor deposition (LRH-MOCVD), using GaN buffer layer on sapphire substrates. When the density of the threading dislocation (TD) increases about one order of magnitude, the yellow luminescence (YL) intensity is strengthened from negligible to two orders of magnitude higher than the band edge emission intensity. The full width of half maximum (FWHM) of the GaN (0002) peak of the XRD rocking curve was widened from 11 min to 15 min, and in Raman spectra, the width of E2 mode is broadened from 5 cm-1 to 7 cm-1. A “zippers” structure at the interface of GaN/sapphire was observed by high-resolution electron microscope (HREM). Furthermore the origins of TD and relationship between physical properties and microstructures combining the growth conditions are discussed.
Structural Properties of (GaIn)(AsN)/GaAs MQW Structures Grown by MOVPE
Authors: Giannini C. , Carlino E. , Tapfer L. , Höhnsdorf F. , Koch J. , Stolz W.
Published: September 2012

In this work, we investigate the structural properties of (GaIn)(AsN)/GaAs multiplequantum wells (MQW) grown at low temperature by metalorganic vapour phase epitaxy. The structural properties, in particular the In- and N-incorporation, the lattice strain (strain modulation), the structural perfection of the metastable (GaIn)(AsN) material system and the structural quality of the (GaIn)(AsN)/GaAs interfaces are investigated by means of high-resolution x-ray diffraction, transmission electron microscopy (TEM), and secondary ion mass spectrometry. We demonstrate that (GaIn)(AsN) layers of high structural quality can be fabricated up to lattice mismatches of 4%. Our experiments reveal that N and In atoms are localized in the quaternary material and no evidences of In-segregation or N-interdiffusion could be found. TEM analyses reveal a low defect density in the highly strained layers, but no clustering or interface undulation could be detected. High-resolution TEM images show that (GaIn)(AsN)/GaAs interfaces are slightly rougher than GaAs/(GaIn)(AsN) ones.
Microstructral Investigations on GaN Films Grown by Laser Induced Molecular Beam Epitaxy
Authors: Zhou H. , Phillipp F. , Gross M. , Schröder H.
Published: September 2012

Microstructural investigations on GaN films grown on SiC and sapphire substrates by laser induced molecular beam epitaxy have been performed. Threading dislocations with Burgers vectors of 1/3<1120>, 1/3<1123> and [0001] are typical line defects, predominantly the first type of dislocations. Their densities are typically 1.5×1010 cm−2 and 4×109 cm−2 on SiC and sapphire, respectively. Additionally, planar defects characterized as inversion domain boundaries lying on {1100} planes have been observed in GaN/sapphire samples with an inversion domain density of 4×109 cm−2. The inversion domains are of Ga-polarity with respect to the N-polarity of the adjacent matrix. However, GaN layers grown on SiC show Ga-polarity. Possible reasons for the different morphologies and structures of the films grown on different substrates are discussed. Based on an analysis of displacement fringes of inversion domains, an atomic model of the IDB-II with Ga-N bonds across the boundary was deduced. High resolution transmission electron microscopy (HRTEM) observations and the corresponding simulations confirmed the IDB-II structure determined by the analysis of displacement fringes.
GaN Decomposition in Ammonia
Authors: Koleske D.D. , Wickenden A.E. , Henry R.L.
Published: September 2012

GaN decomposition is studied as a function of pressure and temperature in mixed NH3 and H2 flows more characteristic of the MOVPE growth environment. As NH3 is substituted for the 6 SLM H2 flow, the GaN decomposition rate at 1000 °C is reduced from 1x1016 cm-2s-1 (i.e. 9 monolayers/s) in pure H2 to a minimum of 1x1014 cm-2s-1 at an NH3 density of 1x1019 cm-3. Further increases of the NH3 density above 1x1019 cm-3 result in an increase in the GaN decomposition rate. The measured activation energy, EA, for GaN decomposition in mixed H2 and NH3 flows is less than the EA measured in vacuum and in N2 environments. As the growth pressure is increased under the same H2 and NH3 flow conditions, the decomposition rate increases and the growth rate decreases with the addition of trimethylgallium to the flow. The decomposition in mixed NH3 and H2 and in pure H2 flows behave similarly, suggesting that surface H plays a similar role in the decomposition and growth of GaN in NH3.
Co-Doping Characteristics of Si and Zn with Mg in P-Type GaN
Authors: Kim K.S. , Oh C.S. , Han M.S. , Kim C.S. , Yang G.M. , Yang J.W. , Hong C.-H. , Youn C.J. , Lim K.Y. , Lee H.J.
Published: September 2012

We investigated the doping characteristics of Mg doped, Mg-Si co-doped, and Mg-Zn codoped GaN films grown by metalorganic chemical vapor deposition. We have grown p-GaN film with a resistivity of 1.26 Ωm and a hole density of 4.3 × 1017 cm−3 by means of Mg-Si co-doping technique. The Mg-Si co-doping characteristic was also explained effectively by taking advantage of the concept of competitive adsorption between Mg and Si during the growth. For Mg-Zn co-doping, p-GaN showing a low electrical resistivity (0.7 Ωm) and a high hole concentration (8.5 × 1017 cm−3) was successfully grown without the degradation of structural quality of the film. Besides, the measured specific contact resistance for Mg-Zn co-doped GaN film is 5.0 × 10−4 cm2, which is lower value by one order of magnitude than that for only Mg doped GaN film (1.9 × 10−3 Ωm2).
Doping Dependence of the Thermal Conductivity of Hydride Vapor Phase Epitaxy Grown n-GaN/Sapphire (0001) Using a Scanning Thermal Microscope
Authors: Florescu D.I. , Asnin V.A. , Mourokh L.G. , Pollak Fred H. , Molnar R. J.
Published: September 2012

We have measured the doping concentration dependence of the room temperature thermal conductivity (κ) of two series ofn-GaN/sapphire (0001) fabricated by hydride vapor phase epitaxy (HVPE). In both setsndecreased linearly with log n, the variation being about a factor two decrease in κ for every decade increase inn. κ ≈ 1.95 W/cm-K was obtained for one of the most lightly doped samples (n= 6.9×1016cm−3), higher than the previously reported κ ≈ 1.7-1.8 W/cm-K on lateral epitaxial overgrown material [V.A. Asninet al, Appl. Phys. Lett. 75, 1240 (1999)] and κ κ 1.3 W/cm-K on a thick HVPE sample [E.K. Sichel and J.I. Pankove, J. Phys. Chem. Solids 38, 330 (1977)]. The decrease in the lattice component of κ due to increased phonon scattering from both the impurities and free electrons outweighs the increase in the electronic contribution to κ.
High Temperature Hardness of Bulk Single Crystal GaN
Authors: Yonenaga I. , Hoshi T. , Usui A.
Published: September 2012

The hardness of single crystal GaN (gallium nitride) at elevated temperature is measured for the first time and compared with other materials. A Vickers indentation method was used to determine the hardness of crack-free GaN samples under an applied load of 0.5N in the temperature range 20 - 1200°C. The hardness is 10.8 GPa at room temperature, which is comparable to that of Si. At elevated temperatures GaN shows higher hardness than Si and GaAs. A high mechanical stability for GaN at high temperature is deduced.
Fabrication and Characterization of GaN Junctionfield Effect Transistors
Authors: Zhang L. , Lester L. F. , Baca A. G. , Shul R. J. , Chang P. C. , Willison C. G. , Mishra U. K. , Denbaars S. P. , Zolper J. C.
Published: September 2012

Junction field effect transistors (JFET) were fabricated on a GaN epitaxial structure grown by metal organic chemical vapor deposition. The DC and microwave characteristics, as well as the high temperature performance of the devices were studied. These devices exhibited excellent pinch-off and a breakdown voltage that agreed with theoretical predictions. An extrinsic transconductance (gm) of 48 mS/mm was obtained with a maximum drain current (ID) of 270 mA/mm. The microwave measurement showed an fr of 6 GHz and an fmax of 12 GHz. Both the ID and the gm were found to decrease with increasing temperature, possibly due to lower electron mobility at elevated temperatures. These JFETs exhibited a significant current reduction after a high drain bias was applied, which was attributed to a partially depleted channel caused by trapped electrons in the semi-insulating GaN buffer layer.
Probing Nitride Thin Films in 3-Dimensions using a Variable Energy Electron Beam
Authors: Trager-Cowan Carol , McColl D. , Sweeney F. , Grimson S. T. F. , Treguer J-F. , Mohammed A. , Middleton P. G. , Manson-Smith S. K. , O'Donnell K. P. , Stricht W. Van der , Moerman I. , Demeester P. , Wu M. F. , Vantomme A. , Zubia D. , Hersee S. D.
Published: September 2012

In this paper we illustrate the application of electron beam techniques to the measurement of strain, defect and alloy concentrations in nitride thin films. We present brief comparative studies of CL spectra of AlGaN and InGaN epilayers and EBSD patterns obtained from two silicon-doped 3 μm thick GaN epilayers grown on an on-axis (0001) sapphire substrate and a sapphire substrate misoriented by 10° toward the m-plane (1010).
The Atomic Structure of Extended Defects in GaN
Authors: Ruterana P. , Nouet G.
Published: September 2012

GaN layers contain large densities (1010 cm−2) of threading dislocations, nanopipes, (0001) and { 1120 } stacking faults, and { 1010 } inversion domains. Three configurations have been found for pure edge dislocations, mainly inside high angle grain boundaries where the 4 atom ring cores can be stabilized. Two atomic configurations, related by a 1/6 < 1010 > stair rod dislocation, have been observed for the { 1120 } stacking fault in (Ga-Al)N layers. For the {1010} inversion domain boundaries, a configuration corresponding to the Holt model was observed, as well as another with no N-N or Ga-Ga bonds.
Thermal Expansion of GaN at Low Temperatures - a Comparison of Bulk and Homo- and Heteroepitaxial Layers
Authors: Kirchner Verena , Heinke Heidrun , Einfeldt Sven , Hommel Detlef , Domagala Jaroslaw Z. , Leszczynski Michal
Published: September 2012

The thermal expansion of different GaN samples is studied by high-resolution Xray diffraction within the temperature range of 10 to 600 K. GaN bulk crystals, a homoepitaxial layer and different heteroepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were investigated. Below 100 K the thermal expansion coefficients (TEC) were found to be nearly zero which has to be taken into account when estimating the thermal strain of GaN layers in optical experiments commonly performed at low temperatures. The homoepitaxial layer and the underlying GaN substrate with a lattice mismatch of –6×10−4 showed identical thermal expansion. The comparison between the temperature behavior of lattice parameters of heteroepitaxial layers and bulk GaN points to a superposition of thermally induced biaxial strain and compressive hydrostatic strain.
Physical Properties of Silicon Doped Hetero-Epitaxial MOCVD Grown GaN: Influence of Doping Level and Stress
Authors: Hageman P.R. , Kirilyuk V. , Zauner A.R.A. , Bauhuis G.J. , Larsen P.K.
Published: September 2012

Silicon doped layers GaN were grown with MOCVD on sapphire substrates using silane as silicon precursor. The influence of the silicon doping concentration on the physical and optical properties is investigated. A linear relationship is found between the silane-input molfraction and the free carrier concentration in the GaN layers. The morphology of the samples is drastically changed at high silicon concentrations. Photoluminescence was used to probe bandgap variations as function of the silicon concentration. Increasing of the doping concentration led to a continuous shift of the exciton related PL to lower energies, while the intensity of the UV emission was found to increase up to a carrier concentration of n=2.5×1018 cm−3.
Structural and Electronic Properties of Line Defects in GaN
Authors: Elsner Joachim , Blumenau Alexander Th. , Frauenheim Thomas , Jones Robert , Heggie Malcolm I.
Published: September 2012

We present density-functional theory based studies for several types of line defects in both hexagonal and cubic GaN. {10-10} type surfaces play an important role in hexagonal GaN since similar configurations occur at open-core screw dislocations and nanopipes as well as at the core of threading edge dislocations. Except for full-core screw dislocations which possess heavily strained bonds all investigated stoichiometric extended defects in hexagonal GaN do not induce deep acceptor states in the band-gap and thus cannot be responsible for the yellow luminescence. However, electrically active point defects in particular gallium vacancies and oxygen related defect complexes are found to be trapped at the stress field of the dislocations. Preliminary calculations for cubic GaN find the ideal stoichiometric 60°-dislocations to be electrically active. As in hexagonal material, vacancies and impurities like oxygen are likely to be trapped at the dislocation core.
Simulation of H Behavior in p-GaN(Mg) at Elevated Temperatures
Authors: Myers S. M. , Wright A. F. , Petersen G. A. , Seager C. H. , Crawford M. H. , Wampler W. R. , Han J.
Published: September 2012

The behavior of H in p-GaN(Mg) at temperatures >400°C is modeled by using energies and vibration frequencies from density-functional theory to parameterize transport and reaction equations. Predictions agree semiquantitatively with experiment for the solubility, uptake, and release of the H when account is taken of a surface barrier.
Optical Activation Behavior of Ion Implanted Acceptor Species in GaN
Authors: Skromme B.J. , Martinez G.L.
Published: September 2012

Ion implantation is used to investigate the spectroscopic properties of Mg, Be, and C acceptors in GaN. Activation of these species is studied using low temperature photoluminescence (PL). Low dose implants into high quality undoped hydride vapor phase epitaxial (HVPE) material are used in conjunction with high temperature (1300 °C) rapid thermal anneals to obtain high quality spectra. Dramatic, dose-dependent evidence of Mg acceptor activation is observed without any co-implants, including a strong, sharp neutral Mg acceptor-bound exciton and strong donor-acceptor pair peaks. Variable temperature measurements reveal a band-to-acceptor transition, whose energy yields an optical binding energy of 224 meV. Be and C implants yield only slight evidence of shallow acceptor-related features and produce dose-correlated 2.2 eV PL, attributed to residual implantation damage. Their poor optical activation is tentatively attributed to insufficient vacancy production by these lighter ions. Clear evidence is obtained for donor-Zn acceptor pair and acceptor-bound exciton peaks in Zn-doped HVPE material.
Electrochemical Properties of Solution Synthesized Undoped and Aluminum Doped Lithium Manganate Thin Films
Authors: Das S.R. , Karan N.K. , Majumder S.B. , Katiyar R.S.
Published: September 2012

The spinel structured lithium manganate (LMO) is a promising cathode material for lithium ion rechargeable micro-batteries due to its higher energy density, environmentally benign nature, and low cost. To date, self-discharge and capacity fading (4 and 3V range), especially at elevated temperatures, still remains major research issues of LMO based cathodes. In the present work we have successfully synthesized lithium manganate thin films by a cost effective solution growth technique. These films exhibited excellent reversible lithium ion intercalation behavior with a discharge capacity of about 55.08:Ahcm−2:m−1 at a load of 20:Acm−2. The Li+ diffusivity was found to increase by substituting a part of manganese with aluminum (Al) in LMO lattice. Al substituted LMO films exhibited better cycleability as compared to the undoped LMO films. Further studies are in progress to investigate the effect of Al substitution on the cycleability of the films.
X-ray Absorption Spectroscopy Investigation of the Sub-Nanoscale Strain in Thin-Film Lithium Ion Battery Cathodes
Authors: Alamgir Faisal M. , Vansluytman Jason , Carter Daniel , Whitacre Jay , Kao Chi-Chang , Greenbaum Steve , Denboer Marten
Published: September 2012

LiCoO2 and LiNiO2, two important cathode materials for Li-ion batteries, were studied in their respective bulk and thin-film form. X-ray absorption spectroscopy (XAS) has been used to probe the local atomic structure and structural defects in the thin-film and bulk cathodes. Results comparing Li(Co,Ni)O2 in the bulk and thin-film forms suggests a correlation between intrinsic stress and local strain in the thin-film. This local strain is manifested by a collapse of the six-fold rotational symmetry within the metal-metal layer of the Li(Co,Ni)O2 system into a two fold one. The relationship between annealing conditions and the resulting local strain in these films is examined.
Electrochemical Properties and Microstructure of Tin Base Thin Film Electrode for Lithium Secondary Batteries
Authors: Choi Wanuk , Lee Jeong Yong
Published: September 2012

Cu/Sn alloy electrode, as an anode for Li ion secondary batteries, was fabricated by annealing of Sn thin film electroplated on copper thin foil. The structure of Cu/Sn alloy electrode was determined using XRD, SEM and TEM. The thin film was consisted of Cu6Sn5, Cu3Sn and so on, the major component was a few ten nanometer sized Cu6Sn5 grains and the minor (Cu3Sn and β-(Cu,Sn)) was dispersed among the Cu6Sn5 grains. The electrochemical properties of Cu6Sn5 thin film were evaluated in coin type half-cell, which was configured with Cu6Sn5, thin film electrode, Li metal electrode and typical 1 mol LiPF6 in EC:DMC (1:1) electrolyte. Annealed Cu/Sn electrode took and released Li ions well during Li insertion and de-insertion. Cycle performance of annealed Cu/Sn electrode was better than that of as-deposited Sn electrode, which was caused by topotactic reaction of Cu6Sn5and composite structure of Cu/Sn alloys.
Controlled synthesis of anganese (oxihydr)oxides in queous olution by hermolysis
Authors: Cassaignon S. , Koelsch M. , Jolivet J-P.
Published: September 2012

Nanometric particles of manganese (oxihydr)oxides (Hausmannite, Manganite, Nsutite, Pyrolusite and Vernadite) have been synthesized by thermohydrolysis of MnCl2 and/or KMnO4in aqueous medium. The modification of various chemical parameters (pH and precursors concentration, nature of the reducer or oxidizer, temperature and duration of the thermolysis) allow the control of the nature, the size and the morphology of the obtained particles.