bluuebunny/crossref_metadata_2025_split huggingface dataset:
Optical Properties of Ingan/GaN Multi Quantum Well Structures
Authors: Bergman J. P. , Saksulv N. , Dalfors J. , Holtz P. O. , Monemar B. , Akasaki I , Amano H. 
Published: April 2011

AbstractA set of GaN/InGaN multiple quantum wells (QWs) with well thickness 30 Å and barrier thickness 60 Å were grown by MOCVD on sapphire substrates. The n-type Si doping of the InGaN QWs was varied, in order to produce a different electron concentration in the QWs for the different samples. Optical spectra were obtained by time resolved photoluminescence spectroscopy. The data show weak excitonic spectra from the QWs as well as a broad deeper emission with a much stronger intensity. The spectral shape becomes narrower and the energy position shifts to higher energies with increasing doping. The two different emissions are not easily separated in CW or time integrated spectra, but are clearly observed in a time resolved spectral measurement due to their different recombination rates. The deeper emission has a long and non-exponential decay, with an average decay time in the order of several hundred nanoseconds. The higher energy exciton emission has a much faster decay of about 1 ns. The lower energy band is tentatively explained as due to separately localized electron-hole (e-h) pairs in the QW.
High Temperature Photoluminescence and Photoluminescence Excitation Spectroscopy of Er Doped Gallium Nitride
Authors: Hömmerich U. , Thaik Myo , Robinson-Brown T. , MacKenzie J. D. , Abemathy C. R. , Pearton S. J. , Wilson R. G. , Schwartz R. N. , Zavada J. M. 
Published: April 2011

AbstractWe report results of high temperature photoluminescence and photoluminescence excitation studies of Er doped GaN. Er was doped into GaN either by ion-implantation or during growth by metalorganic molecular beam epitaxy (MOMBE). Using above gap excitation (λex=325nm), GaN:Er showed strong 1.54 μm Er3+ luminescence up to 550K which indicates the potential of this material for high temperature opto-electronic applications. In addition, we performed timeresolved photoluminescence excitation (PLE) measurements over the wavelength range 420 to 680 nm using a Nd:YAG pumped Optical Parametric Oscillator (OPO). Similar to our previous PLE results of Er doped AIN, we observed that Er3+ ions in GaN can be excited either through resonant pumping of Er3+ energy levels or through energy-transfer processes involving defect states. The relative contribution of direct and indirect Er3+ excitation, however, seems to depend on the material preparation method.
Cubic Inn Inclusions as the Cause for the Unusually Weak Pressure Shift of the Luminescence in InGaN
Authors: Perlin Piotr , Weinstein Bernard A. , Christensen Niels E. , Gorczyca Iza , Iota Valentin , Suski Tadeusz , Wisniewski Przemek , Osinski M. , Eliseev P. G. 
Published: April 2011

AbstractWe propose a new approach to explain the unusually low pressure coefficients of the luminescence peaks observed in single-quantum-well InGaN-based light emitting diodes manufactured by Nichia Chemical Industries. In view of the most recent first principles band structure calculations for InN under hydrostatic pressure, we find that it is possible to reproduce the measured low (12–16meV/GPa) pressure coefficients of the luminescence by assuming the formation of zincblende InN inclusions in the InGaN quantum well layers. These cubic inclusions, surrounded by the usual wurtzite material, should act like quantum dots giving rise to enhanced electron localization. Obtained this way, the pressure shift of the luminescence peaks in blue and green InGaN-based emitters should be close to 14meV/GPa, in good agreement with our experimental results. This explanation of the observed low pressure coefficients in these devices is consistent with recent independent evidence for InN inclusions in InGaN epilayers.
Observation Of Native Ga Vacancies In Gan By Positron Annihilation
Authors: Saarinen K. , Laine T. , Kuisma S. , Nissilä J. , Hautojärvi P. , Dobrzynski L. , Baranowski J. M. , Pakula K. , Stepniewski R. , Wojdak M. , Ysmolek A. , Suski T. , Leszczynski M. , Grzegory I. , Porowski S. 
Published: April 2011

AbstractPositron annihilation experiments have been performed to identify native point defects in GaN bulk crystals as well as in epitaxial layers. The results show that Ga vacancies are present at concentrations of 1017 – 1018 cm−3 in undoped GaN bulk crystals and layers, whereas the Mgdoped samples are free of Ga vacancies. The Ga vacancies are negatively charged and their concentration correlates with the intensity of the yellow luminescence. We conclude that the Ga vacancies contribute to the electrical compensation of n-type GaN and their acceptor levels are involved in the yellow luminescence transition.
Stress Gradients In Heteroepitaxial Gallium Nitride Films
Authors: Ager J. W. , Conti G. , Romano L. T. , Kisielowski C. 
Published: April 2011

AbstractHeteroepitaxial GaN films grown on sapphire are usually in a compressive residual biaxial stress state that is typically 100 – 500 MPa but can be as high as 1 GPa. Spatially resolved measurements of GaN film stresses obtained via micro-Raman spectroscopy using shifts in the frequency of the E2 phonon are presented. The spatial resolution of this stress measurement technique is ca. 2 μm laterally and 10 μm in the axial (growth) direction. Two interesting stress gradients are found near cleaved edges. (1) On the surface of the cleaved edge the compressive film stress is a maximum at the GaN/sapphire interface and relaxes to a nearly stress-free state towards the free edge. (2) In the lateral direction, the stress increases away from the edge, reaching a steady-state value at approximately 10x the film thickness. It is shown that the effects are consistent with existing finite element models used to treat strained growth of surface structures in other semiconductor systems. An axial stress gradient that is not associated with edge effects is found in some thick (>20 μm) HVPEgrown films. The interface region is found to be up to 500 MPa in compression, but this stress declines toward the film surface, which is found to be nearly stress free.
Compositionally Dependent Band Offsets In Aln/AlxGa1-xN Heterojunctions Measured By Using X-Ray Photoelectron Spectroscopy
Authors: Beach R.A. , Piquette E.C. , Grant R.W. , McGill T.C. 
Published: April 2011

AbstractAlthough GaN has been extensively studied for applications in both light emitting and high power devices, the AlN/GaN valence band offset remains an area of contention. Values quoted in the literature range from 0.8eV (Martin)[1] to 1.36eV (Waldrop)[2]. This paper details an investigation of the AIN/AlxGa1-xN band offset as a function of alloy composition. We find an AlN/AlxGa1-xN valence band offset that is nearly linear with Al content and an end point offset for AlN/GaN of 1.36 ± 0.1 eV. Samples were grown using radio frequency plasma assisted molecular beam epitaxy and characterized with x-ray photoelectron spectroscopy(XPS). Core-level and valence-band XPS data for AIN (0001) and AlxGa1-xN (0001) samples were analyzed to determine core-level to valence band maximum (VBM) energy differences. In addition, oxygen contamination effects were tracked in an effort to improve accuracy. Energy separations of core levels were obtained from AlN/AlxGa1-xN(0001) heterojunctions. From this and the core-level to valence band maximum separations of the bulk materials, valence band offsets were calculated.
Photoemission Study of The Electronic Structure of Wurtzite Gan(0001) Surfaces
Authors: Smith Kevin E. , Dhesi Sarnjeet S. , Stagarescu Cristian B. , Downes James , Doppalapudi D. , Moustakas Theodore D. 
Published: April 2011

AbstractThe surface electronic structure of wurtzite GaN (0001) (1 × 1) has been investigated using angle-resolved photoemission spectroscopy. Surfaces were cleaned by repeated cycles of N2 ion bombardment and annealing in ultra-high vacuum. A well-defined surface state below the top of the valence band is clearly observed. This state is sensitive to the adsorption of both activated H2 and O2, and exists in a projected bulk band gap, below the valence band maximum. The state shows no dispersion perpendicular or parallel to the surface. The symmetry of this surface state is even with respect to the mirror planes of the surface and polarization measurements indicate that it is of spz character, consistent with a dangling bond state.
An Investigation of the Electron Escape Time within a Biased AlGaN/GaN Quantum Well
Authors: Lefebvre Kevin R. , Anwar A. F. M. 
Published: April 2011

AbstractIn this paper, the electron escape time from an Al.25Ga.75N/GaN/Al.25Ga.75N quantum well system is calculated as a function of electric field. The calculation includes the piezoelectric field induced by the material system, the continuous density of states within a quantum well as a result of the applied electric field, the change in the group velocity and the proper partitioning of between the tunneling and thermionic emission currents. The calculation of the escape time is achieved by solving Schroedinger equation, through the logarithmic derivative of the wavefunctions, and Poisson's equation self-consistently.
Acceptor Binding Energies in GaN and AIN
Authors: Mireles Francisco , Ulloa Sergio E. 
Published: April 2011

AbstractWe present binding energy calculations for Mg, Zn, and C substitutional shallow acceptors in GaN and AIN for both, wurtzite (WZ) and zincblende (ZB) crystal phases. The calculations are performed within the effective mass theory through the 6 × 6 Rashba-Sheka-Pikus and the Luttinger- Kohn matrix Hamiltonians for WZ and ZB bulk crystals, respectively. An analytic representation for the pseudopotential is used to introduce the nature of the impurity atom. The energy shift due to polaron effects is also considered in this approach. The estimated ionization energies are in good agreement with those reported experimentally and those reported theoretically employing other methods. We find that the binding energies for ZB GaN acceptors are shallower than the corresponding impurities in the WZ crystalline phase. The binding energy dependence upon the crystal field splitting in the WZ compounds is analyzed.
Uniaxial Stress Effects On Valence Band Structures Of GaN
Authors: Yamaguchi A. A. , Mochizuki Y. , Sasaoka C. , Kimura A. , Nido M. , Usui A. 
Published: April 2011

AbstractValence band modification by uniaxial stress in GaN is investigated by reflectance spectroscopy. It is observed that the energy separation between the A and B valence bands increases with the applied uniaxial stress in the c-plane. Changes of the wavefunctions by the stress are also investigated by the polarization characteristics of the reflectance spectra. The experimental results are analyzed on the basis of k•p theory, and deformation potential D5 is experimentally determined as -3.3 eV. It is indicated that the uniaxial strain effect could be utilized for improving GaN-based laser performance.
Theory of Ga, N and H terminated GaN (0001)/(0001) surfaces
Authors: Eisner J. , Haugk M. , Gutierrez R. , Frauenheim Th. 
Published: April 2011

AbstractWe present a theoretical study of atomic structures, electrical properties and formation energies for a variety of possible reconstructions with 1×1 and 2×2 periodicity of the GaN(0001) and (0001) surfaces. We find that during MBE growth in the (0001) direction 2×2 structures become stable under N rich growth conditions while Ga rich environment should yield structures with 1×1 periodicity. Considering MBE growth on (0001) surfaces, among the investigated structures only those with 1×1 periodicity are predicted to be stable. During MOCVD growth, where H terminated surfaces may occur, only structures with lx1 periodicity are found to be stable for both growth directions.
Selective Area Etching of GaN and AlGaN by Thermally Chemical Reaction in Hydrogen Ambient
Authors: Hiramatsu K. , Matsushima H. , Hanai H. , Sawaki N. 
Published: April 2011

AbstractThe selective area etching of wurtzite GaN (0001) and AlGaN (0001) with SiO2 masks is investigated for different temperatures of 800 to 1 100°C and different ambient gases of H2, N2 and Ar including NH3. The etching rate of GaN increases with increasing annealing temperature under H2 ambient. This etching is attributed to the chemical reaction between Ga-N and H2. On the other hand, the etching of GaN does not occur in the N2 or Ar ambient gas. The NH3 gas in H2 ambient suppresses the chemical reaction, while the NH3 gas in N2 enhances it. The surface etching of Al0.1Ga0.9N is not observed even in H2 ambient.
Silicon Carbide And Gallium Nitride Rf Power Devices
Authors: Weitzel C. E. , Moore K. E. 
Published: April 2011

AbstractImpressive RF power performance has been demonstrated by three radically different wide bandgap semiconductor power devices, SiC MESFET's, SiC SIT's, and AlGaN HFET's. AlGaN HFET's have achieved the highest fmax 97 GHz. 4H-SiC MESFET's have achieved the highest power densities, 3.3 W/mm at 850 MHz (CW) and at 10 GHz (pulsed). 4H-SiC SIT's have achieved the highest output power, 450 W (pulsed) at 600 MHz and 38 W (pulsed) at 3 GHz. Moreover a one kilowatt, 600 MHz SiC power module containing four multi-cell SIT's with a total source periphery of 94.5 cm has been demonstrated.
Growth Control And Characterization Of Wide Band Gap Silicon-Carbon Films
Authors: Kerdiles S. , Rizk R. , Pérez-Rodríguez A. , Garrido B. , González-Varona O. , Calvo-Barrio L. , Morante J.R. 
Published: April 2011

AbstractSilicon-carbon films have been grown by reactive hydrogen magnetron sputtering at a substrate temperature of 730°C, with different values of carbon-to-silicon sputtered area ratio, rc. The layers were investigated by infrared spectroscopy, Raman scattering, x-ray photoelectron spectroscopy and optical absorption. For rc below 30%, almost only Si nanocrystallites were formed with a few fraction of amorphous SiC, whereas for rc exceeding 30%, a drastic change was noticed, leading to the achievement of SiC crystals in the layers. These latter were found of near-stoichiometric composition with an atomic ratio C/Si ˜1.04. The results suggest that the excess C is of graphitic-like configuration being likely located in the intergrain regions, in addition to some silicon-oxygen bonds. These features are accompanied by an abrupt widening of the band gap in the transition region that is consistent with the formation of SiC nanocrystals. The large value measured for the band gap (≥3 eV) is thought to be due to more than one origin, such as size effect of SiC, Si-O bonds and possible presence of different SiC polytypes.
A Novel Technique For Rtp Annealing Of Compound Semiconductors
Authors: Fu M. , Sarvepalli V. , Singh R. K. , Abernathy C. R. , Cao X. , Pearton S. J. , Sekhar J. A. 
Published: April 2011

AbstractWe introduce for the first time a novel rapid thermal processing (RTP) unit called ZapperTM, which has recently been developed by MHI Inc. and the University of Florida for high temperature thermal processing of semiconductors. This ZapperTM unit is capable of reaching much higher temperatures (>1500 °C) than conventional tungsten-halogen lamp RTP equipment and achieving high ramp-up and ramp-down rates. We have conducted implant activation annealing studies of Si+-implanted GaN thin films (with and without an AIN encapsulation layer) using the ZapperTM unit at temperatures up to 1500 °C. The electrical property measurements of such annealed samples have led to the conclusion that high annealing temperatures and AIN encapsulation are needed for the optimum activation efficiency of Si+ implants in GaN. It has clearly been demonstrated that the ZapperTM unit has tremendous potential for RTP annealing of semiconductor materials, especially for wide bandgap compound semiconductors that require very high processing temperatures.
Electronic Properties of Wide Bandgap Materials
Authors: Yater J.E. , Shih A. , Abrams R. 
Published: April 2011

AbstractSecondary electron emission spectroscopy is used to investigate the generation and transport of impact-ionized electrons in wide bandgap material. Secondary electron yield and energy distribution measurements from C(100) and CVD diamond samples are analyzed to obtain information about the internal gain and electron energy distribution following impact ionization, as well as the effects of the transport process on the internal electron distribution. By studying the emission from surfaces having a negative electron affinity (NEA), the total transmitted intensity and the full energy spectrum of the internal electrons are revealed in the measurements. Energy spectra measured from the diamond samples contain a low-energy peak whose energy position and width are independent of incident beam energy. This suggests that the peak represents the electron distribution produced by impact-ionization events. A large percentage of the total emitted electrons lie within this peak, indicating that the impact-ionization process is very efficient at generating low-energy electrons. Very high yields are measured from both samples, establishing the presence of high internal gain and efficient electron transport in the material. From the linear slope of the yield curves, the escape depth of the low-energy electrons is deduced to be much larger than ˜ 0.1 μm in both diamond samples.
Metalorganic Chemical Vapour Deposition of CuInSe2 From Copper and Indium Diselenocarbamates for Solar Cell Devices
Authors: McAleese John , O'Brien Paul , Otway David J. 
Published: April 2011

AbstractThin film(s) of chalcopyrite CulnSe2 have been grown by low-pressure metal-organic chemical vapour deposition (LP-MOCVD) using the precursors In(Se2CNMenHexyl)3 and precursors Cu(Se2CNMenHexyl)2. The precursors were prepared from carbon diselenide. Films were grown on glass between 400 – 450 °C, and characterized by X-ray diffraction, optical spectroscopy (UV/Vis), EDAX and scanning electron microscopy.
Surface Morphology of LPE SiGe Layers Grown on (100) Si Substrates
Authors: Sembian A. M. , Silier I. , Davies K. , Gutjahr A. , Lyutovich K. , Konuma M. , Banhart F. 
Published: April 2011

AbstractWe have investigated the surface morphology of thick SiGe layers grown on Si(100) substrates. SiGe layers containing different Ge concentrations (from 0 to 16 at.%) and having thickness of about 15μm are prepared by liquid phase epitaxy (LPE) method using various growth conditions. The wavelength of undulation of SiGe layers is found to be increasing when we adopt low cooling rates during LPE process. The roughness of the layer does not show any significant change with cooling rate.
Mocvd as a Technique for the Growth of CdTe/CdS Solar Cells
Authors: Berrigan R. A. , Irvine S. J. C. , Stafford A. , Ellis D. , Colehamilton D. J. 
Published: April 2011

AbstractThe MOCVD growth of cadmium sulphide on indium tin oxide (ITO) coated glass is investigated in this work. Optimum transmission characteristics and grain size are observed for films grown at 290° C at VI/MI ratios of 1–1.5. It is postulated that the nucleation delay observed during in situ growth monitoring via laser interferometry s a critical step in acieving these results. The nucleation delay is noted to vary with temperature and also VI/II ratio, allowing the rationalisation of the resulting film quality in terms of this observation. Structural analysis indicates polycrystalline CdS of hexagonal phase of (100) preferred orientation. Results of preliminary doping studies indicate that CdS can be n-type doped during growth with nbutylchloride.
Preparation And Photovoltaic Properties Of Tin Sulfide And Tin Oxysulfide Thin Films By Spray Pyrolysis Technique
Authors: Kosugi Tsuyoshi , Murakami Kenji , Kaneko Shoji 
Published: April 2011

AbstractTin sulfide and a related novel compound, tin oxysulfide, thin films were deposited from the aqueous solution containing tin (11) chloride and thiourea by a spray pyrolysis technique. The constituent atoms of the thin films have merits of nontoxicity and availability. Physical and photovoltaic properties of both films were also investigated. It has, therefore, been considered that these two compounds become a promising absorbing layer for solar cell on account photovoltaic ability with high absorption coefficient and appropriate bandgap energy.
Development of High Efficiency Thin Film Polycrystalline Silicon Solar Cells using Vest Process
Authors: Ishihara T. , Arimoto S. , Morikawa H. , Nishimoto Y. , Kawama Y. , Takami A. , Hamamoto S. , Naomoto H. , Namba K. 
Published: April 2011

AbstractThin film Si solar cell has been developed using Via-hole Etching for the Separation of Thin films(VEST) process. The process is based on SOI technology of zone-melting recrystallization (ZMR) followed by chemical vapor deposition (CVD), separation of thin film, and screen printing. Key points for achieving high efficiency are (1)quality of Si films, (2)back surface emitter (BSE), (3)front surface emitter etch-back process, (4)back surface field (BSF) layer thickness and its resistivity, and (5)defect passivation by hydrogen implantation. As a result of experiments, we have achieved 16% efficiency(Voc:0.589V, Jsc:35.6mA/cm2, F.E:0.763) with a cell size of 95.8cm2 and the thickness of 77μm. It is the highest efficiency ever reported for large area thin film Si solar cells.
Deposition of Thick Silicon Layers on Glass Substrate for Photovoltaic Application
Authors: Sarret M. , Mohammed-Brahim T. , Baudry G. , Briand D. , Bonnaud O. 
Published: April 2011

AbstractA new silicon deposition reactor fabricated in our laboratory is presented in this paper. It works at Sub-Atmospherical total pressure, so it is a SAPCVD reactor. The deposition rates and electronic transport properties are compared with a conventional LPCVD reactor.
Polycrystalline Si Films Fabricated by Low Temperature Selective Nucleation and Solid Phase Epitaxy Process
Authors: Chen Claudine M. , Atwater Harry A. 
Published: April 2011

AbstractWith a selective nucleation and solid phase epitaxy (SNSPE) process, grain sizes of 10 μm have been achieved to date at 620°C in 100 nrm thick silicon films on amorphous SiO2, with potential for greater grain sizes. Selective nucleation occurs via a thin film reaction between a patterned array of 20 rnm thick indium islands which act as heterogeneous nucleation sites on the amorphous silicon starting material. Crystal growth proceeds by lateral solid phase epitaxy from the nucleation sites, during the incubation time for random nucleation. The largest achievable grain size by SNSPE is thus approximately the product of the incubation time and the solid phase epitaxy rate. Electronic dopants, such as B, P, and Al, are found to enhance the solid phase epitaxy rate and affect the nucleation rate.
Low-Temperature Sputter-Deposition of Poly-Crystal Silicon Thin-Films
Authors: Yeh Wen-Chang , Matsumura Masakiyo 
Published: April 2011

AbstractPoly-Si films were deposited at temperatures as low as 325°C by a newly proposed method. In the proposed method, both Ni-enhanced crystallization effects and vapor-phase deposition were utilized to reduce the deposition temperature. The X-ray diffraction measurement shows that the films were (110)-oriented at temperatures among 325°C to 419°C. When the partial pressure of hydrogen gas in the deposition chamber exceeds 0.1%, crystallization was disrupted. The characteristics of TFTs and pn junction diodes showed its potential applicability to AMLCDs and sollar cells.