bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of AlGaAs Oxidation in MBE and MOCVD Grown Samples
Authors: Chen Y. , Roshko A. , Bertness K. A. , Readey D. W. , Allerman A. A. , Tan M. , Tandon A.
Published: April 2011

Simultaneous wet-thermal oxidation of MBE and MOCVD grown AlxGa1−xAs layers (× = 0.1 to 1.0) showed that the epitaxial growth method does not influence the oxidation rate. Nearly identical oxidation depths were measured for samples grown by both techniques. It was found, however, that the oxidation rate is very sensitive to non-uniformities in the Al concentration in the AlxGa1−xAs layers, and that maintaining consistent and uniform Al concentrations is critical to achieving reproducible oxidation rates. The study also showed that the oxidation rate was not affected by the V/III ratio during growth nor by impurities at concentrations less than or equal to 10 ppm.
Exciton Diamagnetic Shifts and Magnetic Field Dependent Linewidths in Ordered and Disordered InGaP Alloys
Authors: Jones E. D. , Bajaj K. K. , Coli G. , Crooker S. A. , Zhang Yong , Mascarenhas A. , Olsen J. M.
Published: April 2011

We have measured the diamagnetic shifts and photoluminescence linewidths of excitonic transitions in ordered and disordered In0.48 Ga0.52 P alloys, lattice matched to GaAs, in pulsed magnetic fields at 4 and 76K. The pulsed magnetic field ranged between 0 and 50T. The variations diamagnetic shifts with magnetic field in disordered and weakly ordered samples are considerably smaller than those calculated using a free exciton model. For a given magnetic field, the value of the diamagnetic shifts are found to increase with increasing order parameter. Furthermore, for all samples, the diamagnetic shifts at 76K are larger than at 4K suggesting that the excitons are strongly localized.
GaInNAs Material Properties for Long Wavelength Opto-Electronic Devices
Authors: Gambin Vincent , Ha Wonill , Wistey Mark , Kim Seongsin , Harris James S.
Published: April 2011

Dilute nitrogen GaInNAs is a new promising material as an active region for use in 1.3 and 1.55 μm opto-electronic devices. It has been commonly observed that increasing the nitrogen content generally reduces the optical emission intensity and increases laser threshold. However, some non-radiative recombination defects are removed from the material during a post-growth anneal. One drawback to the anneal is that nitrogen out-diffuses from the quantum wells and blue-shifts optical emission. Using a modified active region structure, we have decreased nitrogen out-diffusion and reduced the luminescence blue-shift while still improving crystal quality. The growth consists of high nitrogen GaNAs barriers grown between lower nitrogen GaInNAs quantum wells. As an added benefit, the nitride barriers strain compensate for the compression in the high In content GaInNAs wells. Furthermore, in order to improve luminescence at long wavelengths, we have added Sb to GaInNAs and have observed high intensity photoluminescence (PL) out to 1.6 μm. We have grown and fabricated in-plane GaInNAs lasers that emit at 1.3 μm with a current threshold density of 1.2 kA/cm2 and GaInNAsSb lasers with emissions at 1.46 μm with a current threshold of 2.8 kA/cm2.
Excitonic Diamagnetic Shifts and Magnetic Field Dependent Linewidths in AlxGa1-xAs Alloys
Authors: Coli G. , Bajaj K. K. , Reno J. L. , Jones E. D.
Published: April 2011

We report measurements of both the diamagnetic shifts and the linewidths of excitonic transitions in AlxGa1-xAs alloys as a function of Al concentration and magnetic field at 1.4 K using photoluminescence spectroscopy. The magnetic field was varied from 0 to 13 tesla and Al composition in our samples ranged from 0 to 30%. The samples were grown on GaAs substrates oriented along [001] direction using molecular beam epitaxy at 590°C. We find that for a given value of alloy composition, both the diamagnetic shift and excitonic linewidth increase as a function of magnetic field. To explain our experimental data we propose that the excitons are localized in a very specific manner. To simulate this localization, we assume that the exciton reduced mass is effectively increased and is obtained by using the alloy dependent heavy-hole mass along (001) direction treated isotropically. The calculated values of the variations of the diamagnetic shift and excitonic linewidth as a function of magnetic field obtained using this model agree very well with those reported here.
Novel AlGaAs/CaF2 SESAM Device for Ultrashort Pulse Generation
Authors: Schön Silke , Gallmann Lukas , Haiml Markus , Keller Ursula
Published: April 2011

A novel ultrabroadband AlGaAs/CaF2 semiconductor saturable absorber mirror (SESAM) covering nearly the entire Ti:sapphire gain spectrum is demonstrated. This device supports sub-10-fs pulse operation of a laser. In contrast to previous SESAMs of comparable bandwidth, our device can be monolithically grown by molecular beam epitaxy and requires no post-growth processing. GaAs is used as semiconductor saturable absorber material. The high defect concentration of the material is due to the lattice-mismatched growth on a fluoride surface with (111) orientation. With a time response of 1.2 ps for carrier trapping, a saturation fluence of 36 μJ/cm2 and a modulation depth of up to 2.2% , the GaAs saturable absorber is well-suited for all-optical switching in SESAM devices used for ultrashort pulse generation.
Effect of Marangoni Convection on InSb Single Crystal Growth by Horizontal Bridgman Method
Authors: Kodera K. , Kinoshita A. , Arafune K. , Nakae Y. , Hirata A.
Published: April 2011

It is necessary to clarify the effect of Marangoni convection on single crystal growth from a melt in order to improve the quality of the grown crystal. Particularly, the deviation of crystalmelt (C-M) interface from a planar shape is a major problem because it may deteriorate the quality of the grown crystal. In this paper, we investigated the effect of thermal and solutal Marangoni convection on C-M interface shape in an In-Sb binary system by the horizontal Bridgman (HB) method. The C-M interface concavity strongly depends on the cooling rate and the temperature gradient under uniform concentration distribution conditions in the melt. A large concavity was observed at low cooling rates and high temperature gradient conditions. The concavity was found to be caused by thermal Marangoni convection, by taking Péclet number into account. Then, we varied the composition of the In-Sb binary system to induce solutal Marangoni convection intentionally. The C-M interface was kept planar in case solutal Marangoni convection occurred in the direction opposite to the thermal one. Therefore, we believe that the utilization of solutal Marangoni convection will be a new control technique to make the C-M interface planar for the HB system. From these results, it was clarified that Marangoni convection plays a significant role in the HB crystal growth system.
Luminescence from erbium oxide grown on silicon
Authors: Nogales E. , Méndez B. , Piqueras J. , Plugaru R. , García J. A. , Tate T. J.
Published: April 2011

The luminescence properties of erbium oxide grown on crystalline and amorphous silicon substrates were studied by means of photo- and cathodoluminescence techniques. Differences in the luminescence spectra for samples grown on the two types of substrates used are explained in terms of the different types of erbium centers formed by taking into account the substrate properties and the thermal treatments during growth. For comparison, erbium implanted and oxygen coimplanted crystalline and amorphous silicon have been also investigated by luminescence techniques. In the implanted samples, the sharp transitions from erbium ions in the visible range were quenched and the main emission corresponds to the intraionic transitions in Er3+ ions in the infrared range peaked at 1,54 μm.
Shallow-donor states in spherical quantum dots with parabolic confinement
Authors: Duque C. A. , Porras-Montenegro N. , Dios-Leyva M. de , Oliveira L. E.
Published: April 2011

The evidence of a parabolic potential well in quantum wires and dots was reported in the literature, and a parabolic potential is often considered to be a good representation of the “barrier” potential in semiconductor quantum dots. In the present work, the variational and fractionaldimensional space approaches are used in a thorough study of the binding energy of on-center shallow donors in spherical GaAs-Ga1-xAlxAs quantum dots with potential barriers taken either as rectangular [Vb (eV) ??1.247 x for r >] or parabolic [Vb (r) ??β2?r2] isotropic barriers. We define the parabolic potential with a β?parameter chosen so that it results in the same E0 groundstate energy as for the spherical quantum dot of radius R and rectangular potential in the absence of the impurity. Calculations using either the variational or fractional-dimensional approaches both for rectangular and parabolic potential result in essentially the same on-center binding energies provided the dot radius is not too small. This indicates that both potentials are alike representations of the quantum-dot barrier potential for a radius R quantum dot provided the parabolic potential is defined with?β?chosen as mentioned above.
Effects of Electric Fields on Cathodoluminescence from II-VI Quantum Well Light Emitting Diodes
Authors: Nikiforov A. Y. , Cargill G. S. , Tamargo M. C. , Guo S. P. , Chen Y.-C.
Published: April 2011

Effects of electrical bias on the cathodoluminescence (CL) have been investigated for a blue II-VI quantum well (QW) light emitting diode structure of ZnCdMgSe, lattice-matched to InP. In CL wavelength scans, the observed effects include largely reversible changes in QW CL intensity and wavelength and changes in cladding CL intensity. In CL time-based scans, the QW CL intensity showed both immediate and long term changes with bias. Irreversible, degradationrelated decreases in QW CL intensity were also observed. Effects of bias on CL were modeled by calculating the rates of carrier production by electron bombardment and the resulting electron and hole currents with different applied bias fields. These model calculations do not explain many of the experimental observations, because the model does not include effects of bias on carrier escape and redistribution in the QW and effects of bias on generation and transport of atomic scale defects.
Strategies For Direct Monolithic Integration of AlxGa(1−x)As/InxGa(1−x)As LEDS and Lasers On Ge/GeSi/Si Substrates Via Relaxed Graded GexSi(1−x) Buffer Layers
Authors: Groenert Michael E. , Leitz Christopher W. , Pitera Arthur J. , Yang Vicky K. , Lee Harry , Ram Rajeev J. , Fitzgerald Eugene A.
Published: April 2011

AlxGa(1−x)As/GaAs quantum well lasers have been demonstrated via organometallic chemical vapor deposition (OMCVD) on relaxed graded GexSi(1−x) virtual substrates on Si. Despite unoptimized laser structures with high series resistance and large threshold current densities, surface threading dislocation densities as low as 2×106 cm−2 enabled cw room-temperature lasing at a wavelength of 858nm. The laser structures are oxide-stripe gain-guided devices with differential quantum efficiencies of 0.16 and threshold current densities of 1550A/cm2. Identical devices grown on commercial GaAs substrates showed differential quantum efficiencies of 0.14 and threshold current densities of 1700A/cm2. This comparative data agrees with our previous measurements of near-bulk minority carrier lifetimes in GaAs grown on Ge/GeSi/Si substrates. A number of GaAs/Ge/Si integration issues including thermal expansion mismatch and Ge autodoping behavior in GaAs were overcome.
Optical Constants of Annealed a-Si:H from Transmittance at Normal Incidence
Authors: Doi Atsutoshi , Matsumoto Yoshiyuki
Published: April 2011

We study changes in the optical constants of a-Si:H films caused by the thermal annealing involved in solid phase crystallization. The aim is to examine the growth mechanism, since changes in refractive index are most probably caused by a change in the network structure. The refractive index change was studied from interference fringes in transmitted light at normal incidence, and shows differing dependence on temperature in different thermal ranges. DSC measurement was also performed to examine changes in the network structure with temperature. Changes in optical and thermal properties induced by an increase of temperature reveal frequent network changes of a-Si:H below 470°C and of a-Si in the range 470 to 570°C. We also found crystallization at about 570°C, and grain growth above the crystallization temperature. Knowledge of network changes in a-Si film allows orientation control by an external seed.
Simulations of Realizable Photonic Bandgap Structures with High Refractive Contrast
Authors: Gersten Bonnie , Synowczynski Jennifer
Published: April 2011

The transfer matrix method (TMM) software (Translight, A. Reynolds [1]) was used to evaluate the photonic band gap (PBG) properties of the periodic arrangement of high permittivity ferroelectric composite (40 wt% Ba0.45Sr0.55TiO3/60 wt% MgO composite, εR = 80, tanδ?= 0.0041 at 10 GHz) in air (or Styrofoam, εR ∼ 1) matrix compared to a lower permittivity material (Al2O3, εR = 11.54, tanδ?= 0.00003 at 10 GHz) in air. The periodic structures investigated included a one-dimensional (1D) stack and a three-dimensional (3D) face centered cubic (FCC) opal structure. The transmission spectrum was calculated for the normalized frequency for all incident angles for each structure. The results show that the bandgaps frequency increased and the bandgap width increased with increased permittivity. The effects of orientation of defects in the opal crystal were investigated. It was found by introducing defects propagation bands were introduced. It was concluded that a full PBG is possible with the high permittivity material.
Optical study of 2D photonic crystals in an InP/GaInAsP slab waveguide structure
Authors: Ferrini Rolando , Leuenberger David , Mulot Mikaël , Qiu Min , Moosburger Jürgen , Kamp Martin , Forchel Alfred , Anand Srinivasan , Houdré Romuald
Published: April 2011

We report on the optical properties of two dimensional (2D) photonic crystals (PCs) deeply etched in an InP/GaInAsP step-index waveguide. Transmission (T) measurements through simple PC slabs and through one-dimensional (1D) Fabry-Pérot (FP) cavities between PC mirrors are reported and compared to theory. A 2D finite difference time-domain (FDTD) method combined to a phenomenological out-of-plane loss model is used to assess different loss contributions. The PC optical properties are deduced from the FP peak analysis. The origin of the high T level observed inside the stopgap is investigated.
Impact of In content on the structural and optical properties of (In,Ga)N/GaN multiple quantum wells grown by plasma-assisted molecular beam epitaxy
Authors: Waltereit Patrick , Speck James S.
Published: April 2011

We have studied the structural and optical properties of a series of (In,Ga)/GaN multiple quantum wells with identical thicknesses but varied In content grown by plasma-assisted molecular beam epitaxy. Careful choice of the growth parameters returns samples with smooth and abrupt interfaces. The shift of the photoluminescence transition energy with externally applied biaxial tension was investigated. We observed a red-shift for small In contents while a blue-shift was detected for higher In contents. This result is in qualitative agreement with band profile calculations taking into account both the band gap deformation potentials and the piezoelectric polarization in these structures. However, the magnitude of the shift is well in excess of the calculated one. We attribute this finding to a substantial deviation of the piezoelectric constants from those calculated for unstrained material. Finally, we estimate the piezoelectric polarization of InGaN/GaN for linear and non-linear terms in strain.
Theory of Electron Energy Loss Spectroscopy and its Application to Threading Edge Dislocations in GaN
Authors: Fall C. J. , Jones R. , Briddon P. R. , Blumenau A. T. , Frauenheim T. , Heggie M. I.
Published: April 2011

The electronic structure of dislocations in GaN is controversial. Several experimental techniques such as carrier mobility studies and cathodoluminescence experiments have indicated that dislocations are charged while theoretical studies point to intrinsic states and/or point defect accumulation along the core as a source of electrical activity. Electron Energy Loss Spectroscopy (EELS) studies have the ability to probe the electronic structure of extended defects. Here we report rst principles calculations of the EELS spectrum applied to edge dislocations in GaN. It is found that the electrostatic potential at N atoms in the vicinity of the dislocation varies by the order of a volt and casts doubt on any simple interpretation of core loss spectroscopy. On the other hand, low loss spectroscopy leads directly to detailed information about any gap states. The low loss spectrum obtained by the theory is in good agreement with recent experimental work and indicates that threading dislocations in p-type GaN possess acceptor levels in the upper half of the gap.
Conductivity, photoconductivity and optical properties of amorphous GaN films
Authors: Koo A. , Lanke U. D. , Ruck B. J. , Brown S. A. , Reeves R. , Liem I. , Bittar A. , Trodahl H. J.
Published: April 2011

It has been predicted that amorphous GaN has a low density of states in the gap, and therefore has potential as a useful opto-electronic material in the blue-green spectral region. We have synthesised amorphous GaN films on various substrates by ion assisted deposition and investigated the effects of sample preparation conditions on the conducting and optical properties. The room temperature resistivity ρ0 of stoichiometric (Ga:N of 1:1) films is above 105 Ω cm, and these films exhibit a complex form for the temperature dependence of the resistivity. Films having an excess of Ga show a much lower ρ0. The optical absorption shows ar 0 band-gap of 3 eV, with the gap falling below that value when the amorphous network incorporates homopolar (Ga-Ga) bonds. The best films are thus transparent across the visible region with a low density of gap states, undetectable in optical absorption. The photoluminescence spectra obtained from these a-GaN films consist of a broad green light emission peaking at 528 nm. Preliminary photoconductivity measurements show sensitivity in the UV.
High-Temperature Hardness of Bulk Single-Crystal AlN
Authors: Yonenaga Ichiro , Nikolaev Andrey , Melnik Yuriy , Dmitriev Vladimir
Published: April 2011

The hardness of single-crystal aluminum nitride (AlN) 0.5-mm-thick wafers was measured at elevated temperatures and compared with that of other semiconductors. A Vickers indentation method was used to determine the hardness under an applied load of 0.5 – 5 N in the temperature range 20 - 1400°C. The average hardness was measured as 17.7 GPa at room temperature, harder than GaN and InN. The fracture toughness is 0.5 MPa•m1/2. AlN exhibits the hardness higher than that of GaN in the entire temperature range investigated. Up to about 1100°C, AlN maintains its hardness and thus, a high mechanical stability for AlN at elevated temperatures is deduced.
Mg related Defect Formation during MOVPE Growth of GaN based Films studied by Transmission Electron Microscopy
Authors: Kröger Roland , Figge Stephan , Böttcher Tim , Ryder Peter L. , Hommel Detlef
Published: April 2011

P-type active incorporation of magnesium into the gallium nitride lattice is still a challenge for the realization of wide band gap light emitters. This work presents a detailed microstructural study of phenomena connected with the problem of Mg doping of films grown by metal organic vapor phase epitaxy. Transmission Electron Microscopy both in conventional and in high resolution mode are used to obtain a better understanding of the Mg related defect formation processes. It is shown that the defect formation never occurs instantaneously at the beginning of the doping but a defect free zone always precedes the onset of the defect formation. This effect cannot solely be addressed to the memory effect of the reaction chamber since it is found that the defect density oscillates during growth. A more detailed investigation reveals that segregation of Mg plays a crucial role in the formation of defects. The observations emphasize the fact that a critical surface concentration is necessary for the defects to form. Beside the known processes of inversion boundary formation it is found that small nitrogen polar GaN grains in GaN films are gradually increasing on the expense of Ga polar grains if a sufficiently high Mg coverage and film thickness is realized. This process is found to occur in alternating steps along the {1123} and {0001}planes.
Evaluation of (In,Ga)N Films as Optical Absorption Filters for Application in Integrated Fluorescence Detection Micro-Bioanalytical Systems
Authors: Chediak J. Alex , Kneissl Michael , Sands Timothy D.
Published: April 2011

Fluorescence detection in integrated micro-bioanalytical systems requires the selective exclusion of light from the excitation source (typically blue or UV) and transmission of the longer wavelength fluorescence signal. In the present research, the application of (In,Ga)N alloy thin films as optical filters for absorption of blue (470 nm) and transmission of green (530 nm) has been evaluated. Absorption spectra, photoluminescence (PL) and Rutherford backscattering spectroscopy (RBS) results are presented for 200-400 nm (In,Ga)N thin films grown by MOCVD on sapphire substrates with 2-4 μm GaN buffer layers. A sigmoidal function was used to model the absorption coefficient of (In,Ga)N as a function of energy, Eg, and the broadening (ΔE) associated with the Urbach tail. Experimental data showed that, as expected, the absorption band edge for (In,Ga)N films broadened with increasing InN mole%. An increase in ΔE of 35 meV was observed when the InN mole% was increased from 10 to 16%. The sigmoidal function model provided a good fit to the experimental data, which allowed the experimental data to be extrapolated to higher InN concentrations. Based on this analysis, it is predicted that a 5 μm thick In0.22Ga0.78N film should transmit 50% of green light (530 nm) and only 3.29 x 1 0-4 of blue light (470 nm). The (In,Ga)N films that have been evaluated range in InN mole% from ~2-16%. The higher InN mole% samples (10-16%) can successfully filter lower wavelengths (e.g., 400 nm). For a filter application, it is also important that photoluminescence be effectively suppressed. In MQW structures, high PL intensity is often obtained by means of InN segregation (InN-rich nanoclusters). But for our evaluation with 200-400 nm (In,Ga)N films, relaxation of coherency strain results in large densities of dislocations, and correspondingly low PL intensity.
Electrical Properties of GaN/InGaN MQW Heterojunction Diodes as Affected by Various Plasma Treatments
Authors: Kim J. , Luo B. , Mehandru R. , Ren F. , Lee K. P. , Pearton S.J. , Polyakov A.Y. , Smirnov N.B. , Govorkov A.V. , Osinsky A.V. , Norris P.E.
Published: April 2011

Effects of UV/O3 or deuterium plasma treatment, of annealing in air at 550°C, of annealing in N2 at 500°C and various combinations of these treatments on leakage current and resistance in the forward direction of GaN/InGaN multiquantum-well light emitting diodes MQW LEDs were studied. It was shown that the best results are achieved with 5 minutes long UV/O3 treatment. LED structures thus prepared showed effects of strong tunneling in their I-V characteristics. The space charge region was shown to be located in the GaN/InGaN superlattice SL. Passing of moderately high forward current through the structure for several hours enhanced the overall tunneling through the structure and facilitated faster tunneling between the layers in the GaN/InGaN SL.
Electrical Properties of n-GaN/p-SiC and n-AlGaN/p-SiC Heterojunction Diodes
Authors: Luo B. , Kim J. , Mehandru R. , Ren F. , Lee K. P. , Pearton S.J. , Polyakov A.Y. , Smirnov N.B. , Govorkov A.V. , Kozhukhova E.A. , Osinsky A.V. , Norris P.E.
Published: April 2011

Properties of n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions prepared by HVPE on 4H SiC substrates were studied by means of C-V, C/G-T, C-f, I-V and DLTS. It is shown, in agreement with earlier publications, that the GaN/p-SiC HJ is staggered type II with ΔEc=-0.4 eV andΔEv=0.6 eV. Whenchanging GaN for AlGaN with Al mole fraction of x=0.25-0.3 the band alignment becomes normal type I with ΔEc=0.2 eV andΔEv=0.6 eV. I-V characteristics of both heterojunctions bear evidence of strong tunneling via defect states, particularly centers with activation energy of 1.25 eV for GaN/4H SiC HJ. The tunneling was found to be more pronounced in the AlGaN/SiC HJs even though these HJs showed no evidence of formation of dark line defects at the interface, in contrast to GaN/SiC. DLTS measurements on both types of HJs revealed the presence of broad bands whose behavior is indicative of these bands being related to continuous states in the gap, most likely near the nitride/carbide interface.
Effect of Ge in Cl2 Plasma for Reactive Ion Etching of GaN
Authors: Urushido Tatsuhiro , Yoshida Harumasa , Miyake Hideto , Hiramatsu Kazumasa
Published: April 2011

We investigated the effect of Ge and Si in Cl2 plasma on reactive ion etching (RIE) of GaN. The etched surfaces of GaN were smooth, and high etch rates of 0.63 m/min and 0.41 m m/min were obtained using a Ge plate and a Si plate, respectively. A rough surface was formed for the quartz plate without the Ge plate or Si plate. Optical emission spectroscopy revealed optical emissions related to GeClx+ions in Cl2 plasma with the Ge plate, to SiClx+ ions with the Si plate and to Cl+ ions without the Ge or the Si plate. It is considered that the GeClx+ions and SiClx+ ions for RIE of GaN plays an important role in obtaining a smooth etched surface of GaN, and that the high-energy Cl+ ion severely damages the GaN surface.
Fabrication and Investigation of the Metal-Ferroelectric-Semiconductor Structure with Pb(Zr0.53Ti0.47)O3 on AlxGa1-xN/GaN Heterostructures
Authors: Shen B. , Li W. P. , Wang X. S. , Yan F. , Zhang R. , Bi Z. X. , Shi Y. , Liu Z. G. , Zheng Y. D. , Someya T. , Arakawa Y.
Published: April 2011

An AlxGa1-xN/GaN-based metal-ferroelectric-semiconductor (MFS) structure is developed by depositing a Pb(Zr0.53Ti0.47)O3 film on a modulation-doped Al0.22Ga0.78N/GaN heterostructure. In high-frequency capacitance-voltage (C-V) measurements, the sheet concentration of the two-dimensional electron gas at the Al0.22Ga0.78N/GaN interface in the MFS structure decreases from 1.56 × 1013cm-2to 5.6 × 1012cm-2under the –10 V applied bias. A ferroelectric C-V window of 0.2 V in width near –10V bias is observed, indicating that the AlxGa1-xN/GaN MFS structure can achieve memory performance without the reversal of the ferroelectric polarization. The results indicate that AlxGa1-xN/GaN heterostructures are promising semiconductor channel candidates for MFS field effect transistors.
Extraction of Polarization-Induced Charge Density in Modulation-Doped AlxGa1-xN/GaN Heterostructures Based on Schottky C-V Simulation
Authors: Zhou Y.G. , Shen B. , Yu H.Q. , Liu J. , Zhou H.M. , Zhang R. , Shi Y. , Zheng Y.D. , Someya T. , Arakawa Y.
Published: April 2011

A method based on Schottky capacitance-voltage (C-V) simulation was developed to extract the polarization-induced charge density in modulation-doped AlxGa1-xN/GaN heterostructures. There are two characteristic slopes in the experimental and simulated C-V curves.The influences of the polarization-induced charge density, n-AlGaN doping level and the Schottky barrier height on the positions of the two slopes in the C-V curves are much different from each other. The polarization-induced charge density can be extracted accurately by fitting the experimental C-V curves. It is extracted to be 6.78 x 1012cm-2in modulation-doped Al0.22Ga0.78N/GaN heterostructures with the Al0.22Ga0.78N thickness of 30 nm or 45 nm. The charge density reducesto 1.30 x 1012cm-2in the heterostructure with the Al0.22Ga0.78N thickness of 75 nm. It is thought that the reduction of the polarization-induced charges at the heterointerface is due to the partial relaxation of the Al0.22Ga0.78N layer on GaN.
TEM assessment of GaN/AlGaN/TiAlTiAu and GaN/AlGaN/TiAlPdAu ohmic contacts
Authors: Fay M W , Moldovan G , Harrison I , Birbeck J C , Hughes B T , Uren M J , Martin T , Brown P D
Published: April 2011

TiAlTiAu and TiAlPdAu contacts to GaN/AlGaN, rapid thermal annealed at temperatures ranging from 650°C to 950°C, have been investigated using conventional and chemical TEM analysis. Ohmic behaviour was seen for TiAlTiAu contacts annealed at 750°C or higher, but was not observed in TiAlPdAu contacts annealed at up to 950°C. The effect of annealing temperature on the structural evolution of the contact is explained in terms of different extents of interfacial reaction. In particular, the formation of TiN after anneals at high temperatures is required to activate the contact. At anneals of 950°C, TiAlTiAu samples show a structure of TiN grains within an interfacial band, with TiN inclusions into the AlGaN preceded by an Al-Au diffusion front. Inclusion formation and the effect on the contact electrical performance is described.
Novel Sensor Applications of group-III nitrides
Authors: Eickhoff M. , Ambacher O. , Steinhoff G. , Schalwig J. , Neuberger R. , Palacios T. , Monroy E. , Calle F. , Müller G. , Stutzmann M.
Published: April 2011

The present paper gives an overview over different sensor applications of GaN thin films and AlGaN/GaN heterostructures. The response of Pt-GaN Schottky diodes towards hydrogen and hydrogen containing gases is analysed and their gas sensitivity is characterized from room temperature up to 600°C. In addition, the sheet carrier density of a two dimensional electron gas confined at the heterointerface in AlGaN/GaN high electron mobility transistors (HEMTs) is shown to be significantly influenced by changes in the electronic properties of the device surface. This effect is successfully exploited for the realization of ion detectors and sensors for fluid monitoring based on AlGaN/GaN HEMTs with non-metalized gate areas. Promising possibilities of fabricating monolithically integrated sensor devices for wireless signal transmission are demonstrated by the realization of SAW devices on epitaxial AlN-films.
Three-Dimensional Modeling of the High Pressure Organometallic Chemical Vapor Deposition of InN using Trimethylindium and Ammonia
Authors: McCall Sonya D. , Bachmann Klaus J.
Published: April 2011

A physico-chemical model of the High Pressure Organometallic Chemical Vapor Deposition (HPOMCVD) process that describes three dimensional transport phenomena as well as gas-phase and surface reactions underlying the growth of compound semiconductors is presented. A reduced-order model of the Organometallic Chemical Vapor Deposition of indium nitride (InN) from trimethylindium In(CH3)3 or TMI and ammonia (NH3) at elevated pressures has been developed and tested using the computational fluid dynamics code, CFD-ACE+. The model describes the flow dynamics coupled to chemical reactions and transport in the flow channel of the Compact Hard Shell Reactor, as a function of substrate temperature, total pressure and centerline flow velocity.
Quantitative Defect Analysis of GaN Thick Films by TEM and AFM
Authors: Bhaskara Praveena , Sung Changmo , Bliss David , Suscavage Michael
Published: April 2011

Ever since the discovery of the astonishing properties of GaN, many research groups have been involved in the processing of the perfect GaN crystal. Iodine Vapor Phase Growth (IVPG) technique was employed to grow GaN epilayer on a MOCVD pre-deposited buffer layer. This new epitaxial system was characterized by TEM, AFM and EFM. A complete AFM study involved the polarity measurements and the etch pit density measurements. For the first time a systematic study was performed of the dislocation density changing as a function of distance from the substrate. TEM performed on the cross-section, as well as the plan view, of the samples showed a remarkable decrease in the dislocations in the current system, compared to the samples that were solely deposited by MOCVD. Advanced analytical methods of polarity and dislocation density measurements have been established to understand the relation between microstructure and electrical properties of the thick film GaN. Electrostatic Force Microscopy has been suggested as a potential tool for obtaining polarity information.
Microstructural characterization of GaN-GaAs alloys grown on (001) GaAs by molecular beam epitaxy
Authors: Kim Hyonju , Andersson T. G. , Södervall U. , Jäger C. , Jäger W. , Albrecht M. , Bösker G. , Stolwijk N. A.
Published: April 2011

We have investigated microstructural properties of GaAs:N and GaN:As layers using transmission electron microscopy. The samples were grown onto (001)-oriented GaAs substrates by RF-plasma assisted molecular beam epitaxy. It has been found that during the GaAs/GaAs:N epitaxial growth the supplied active nitrogen atoms gave rise to nanometer-size GaN crystallites formed in the GaAs matrix. In addition, silicon incorporation showed abnormal behavior at the two interfaces of the thin GaAs:N layer embedded in GaAs. A model is proposed for the formation of GaN crystallites in GaAs during the growth. In the GaN:As growth, the layer exhibited columnar growth, resulting in domains with different crystallographic orientation. With an increase of the film thickness, the zincblende structure changed to the wurtzite phase of GaN. The distribution of arsenic through the layer thickness was found to be inhomogeneous and be much higher near the GaN/GaAs interface compared to the region near the surface.
Characterization Of G-R Noise In GaN Films Grown By RF-MBE On Intermediate-Temperature Buffer Layers
Authors: Fong W.K. , Leung B.H. , Zhu C.F. , Surya Charles
Published: April 2011

We report detailed investigations of generation-recombination (G-R) noise in GaN films grown by rf-plasma assisted molecular beam epitaxy on intermediate-temperature buffer layer (ITBL) in addition to conventional low-temperature buffer layer. To characterize the film quality affected by the use of ITBL, low-frequency noise measurements were performed. The voltage noise power spectra show a strong dependence on the thickness of the ITBL. A model has been presented to explain the observed G-R noise, which stipulates that the phenomenon arises from the thermally activated trapping and detrapping of carriers by traps. The process leads to the correlated fluctuations in both the carrier number and the coulombic scattering rate. Detailed numerical evaluation shows that number fluctuation dominates in our samples. The calculated trap densities show that the use of ITBL can effectively reduce defect density by over an order of magnitude.
Two Step Growth of InN Films on Sapphire (0001) Substrates Without Nitridation Process by RF-MBE
Authors: Yamaguchi Tomohiro , Saito Yoshiki , Kano Kenji , Muramatsu Tomo , Araki Tsutomu , Nanishi Yasushi , Teraguchi Nobuaki , Suzuki Akira
Published: April 2011

InN films were grown on sapphire (0001) substrates by radio-frequency plasma-assisted molecular beam epitaxy. The InN buffer layers deposited at low temperature were either grown on a substrate with nitridation or on a substrate without nitridation. The InN buffer layers on the nitridated substrates were always single crystalline, whereas the buffer layers on non-nitridated substrates were always polycrystalline. However, even without nitridation process, single crystalline InN films could be grown on the polycrystalline InN buffer layers; in this case, the orientation was always [1120] InN//[1120] sapphire epitaxy, which differed from the [1010] InN//[1120] sapphire epitaxy in films grown with nitridation.
Investigation of the Optimum Growth Conditions of Wide-Bandgap Quaternary InAlGaN for UV-LEDs
Authors: Yamabi T. , Kinoshita A. , Hirayama H. , Ainoya M. , Hirata A. , Araki T. , Nanishi Y. , Aoyagi Y.
Published: April 2011

We systematically investigated the optimum growth condition of wide-bandgap quaternary InAlGaN for ultraviolet (UV) light-emitting diodes (LEDs) grown on SiC by metal organic vapor phase epitaxy (MOVPE). We obtained intense UV emission in the wavelength of 315-370 nm from quaternary InAlGaN, which was as high as blue emission from InGaN. We found that the optimized growth temperature of quaternary InAlGaN is shifted from 790 to 870 °C as the emission peak wavelength is changed from 370-315 nm. The strongest PL was obtained with the wavelength around 330 nm. The growth condition for intense PL is found to be decided by two factors, i.e., the carrier confinement in In segregation region and crystalline quality. We also found that the small In mole fraction of 2-5 % is enough to obtain maximally PL intensity for InAlGaN, which is much smaller value than that for InGaN, due to the large carrier confinement with small In fluctuation.
Composition Control during Growth of AlGaN Cladding Layers for InGaN-MQW Lasers with Ridge Formed by Selective Re-growth (RiS-type Lasers)
Authors: Kimura Akitaka , Kuramoto Masaru , Yamaguchi A. Atsushi
Published: April 2011

We investigated the mask-pattern and trimethylalminium (TMAl)-flow-rate dependence of the Al composition in AlGaN selective growth for novel InGaN multiple quantum well lasers having selectively grown ridge structures (RiS-type lasers). The Al composition decreased with increases of the local coverage ratio. This result was explained quantitatively by a simple model in which the Al is deposited as polycrystals on the mask without migration occurring and the Ga is concentrated into the window. Under a high TMAl flow-rate condition, this effect of lowering the Al composition was weakened by the high Ga consumption on the mask, and an desirable Al composition for GaN-based lasers was realized. The model can be used to control the composition of p-AlGaN layers for RiS-type lasers.
Characterization of AlxGa1-xN/ AlyGa1-yN Distributed Bragg Reflectors Grown by Plasma Assisted Molecular Beam Epitaxy
Authors: Klausing H. , Fedler F. , Rotter T. , Mistele D. , Semchinova O. , Stemmer J. , Aderhold J. , Graul J.
Published: April 2011

AlxGa1-xN/ AlyGa1-yN Distributed Bragg Reflectors (DBRs) with up to 45 periods have been grown on (0001) sapphire substrates by r.f. plasma-assisted molecular beam epitaxy (PAMBE) with the aid of two Al effusion cells. Several samples were grown with an Al mole fraction varying between 0.38 ≤ x ≤ 1 (0 ≤ y ≤ 0.4) at temperatures of up to 890°C. In all samples, an AlxGa1-xN buffer layer was used to prevent cracking of the quarter wave stack and improving surface morphology by choosing the Al content so that strain energy in the DBR structure would be compensated.

X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations were performed to determine the thickness of the quarter wave layer periods and the Al mole fraction of corresponding AlxGa1-xN single layers.

Room-temperature calibrated reflection and transmission (R&T) measurements were performed. Thus stray and self-absorption of the DBRs were extracted from reflectance and transmittance. The thickness of the quarter wave layers was designed such that the measured peak reflectances appeared between 346 nm to 421 nm. The dispersion data, including refractive indices and absorption coefficients, used in the calculation were extracted from R&T measurements done on the above mentioned AlxGa1-xN single layers.
Effect of the Inter-Subband Scattering in Modulation-Doped AlxGa1-xN/GaN Heterostructures
Authors: Zheng Z. W. , Shen B. , Jiang C. P. , Guo S. L. , Liu J. , Zhou H. M. , Zhang R. , Shi Y. , Zheng Y. D. , Zheng G. Z. , Chu J. H. , Someya T. , Arakawa Y.
Published: April 2011

Magnetotransport properties of Al0.22Ga0.78N/GaN modulation-doped heterostructures have been studied at low temperatures and high magnetic fields. The inter-subband scattering of the two-dimensional electron gas was observed. The inter-subband scattering is very weak and depends weakly on temperature when temperature is between 1.3 K and 10 K and becomes stronger with increasing temperature when temperature is higher than 10 K. The strain relaxation of the Al0.22Ga0.78N layer influences the inter-subband scattering. It is suggested that the inter-subband scattering is dominant by the elastic scattering when temperature is lower than 10 K, and changes to be dominant by the inelastic scattering of the acoustic phonons when temperature is higher than 10 K.
Electrical and Optical Studies of Si-Implanted GaN
Authors: Fellows James A. , Yeo Yung Kee , Hengehold Robert L. , Krasnobaev Leonid
Published: April 2011

The electrical and optical properties of Si-implanted GaN have been investigated as a function of ion dose, anneal temperature, and implantation temperature using Hall-effect measurements and photoluminescence. Implantation of 200 keV Si ions was made at room temperature and 800°C into MBE-grown GaN capped with 500 Å AlN at six different doses ranging from 1x1013 to 5x1015 cm-2. The samples were proximity cap annealed from 1050 to 1350°C for 5 min to 20 s using either a conventional furnace or rapid thermal annealing. For a given dose, electrical activation efficiencies and mobilities increase as the anneal temperature increases from 1050 to 1350°C. Generally, the higher the dose, the greater the activation efficiency for any given anneal temperature. For a sample implanted with a dose of 1x1015 cm-2 and annealed at 1350°C for 20 s, an electrical activation efficiency of 100% was obtained. Exceptional carrier concentrations and mobilities were obtained on all Si-implanted samples, and a comparison of the results was made between room temperature and 800°C implantation. Photoluminescence measurements were also performed in an effort to better understand the electrical activation behavior of the Si implants in GaN.
Electron transport in AlN under high electric fields
Authors: Collazo Ramón , Schlesser Raoul , Roskowski Amy , Davis Robert F. , Sitar Z.
Published: April 2011

The energy distribution of electrons transported through an intrinsic AlN film was directly measured as a function of the applied field, and AlN film thickness. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. Transport through films thicker than 95 nm and applied field between 200 kV/cm -350 kV/cm occurred as steady-state hot electron transport represented by a Maxwellian energy distribution, with a corresponding carrier temperature. At higher fields (470 kV/cm), intervalley scattering was evidenced by a multi-component energy distribution with a second peak at the energy position of the first satellite valley. Electron transport through films thinner than 95 nm demonstrated velocity overshoot under fields greater than 550 kV/cm. This was evidenced by a symmetric energy distribution centered at an energy above the conduction band minimum. This indicated that the drift component of the electron velocity was on the order of the “thermal” component. A transient length of less than 80 nm was deduced from these observations.
Improvements of Structural and Optical Properties of GaN/Al0.10Ga0.9N Multi-Quantum Wells by Isoelectronic In-doping
Authors: Wang Lianshan , Chua Soo Jin , Sun Wenhong
Published: April 2011

Absratct:

The effects of isoelectronic In-doping were studied on the structural and optical properties of 3-periods and 10-periods of GaN/Al0.10Ga0.90N multi quantum wells (MQWs). The GaN/Al40.10Ga0.90N MQWs were grown on u-GaN/sapphire via metalorganic chemical deposition (MOCVD) at 1050°C in H2 carrier gas. X-ray diffraction (XRD), and micro-Photoluminescence (PL) measurements revealed that In-doping into well layers improves the crystalline and optical properties of MQWs relative to those samples without In-doping. With increasing Trimethylindium (TMIn) flow rates from 4.2 mol/min to 42.6 mol/min, PL peaks from well layers obviously redshifted, due to the improvement of the strain along the interfaces between MQWS, irrespective of 3-periods or 10 periods MQWs. The improvement of the crystal quality was also confirmed by XRD.
Infrared and Ultraviolet Raman Spectra of AlN Thin Films Grown on Si(111)
Authors: Naik V. M. , Haddad D. , Danylyuk Y. V. , Naik R. , Auner G. W. , Rimai L. , Weber W. H. , Uy D.
Published: April 2011

Infrared (IR) spectroscopy and ultraviolet near-resonance enhanced Raman scattering have been used as alternative techniques to characterize the AlN films grown on Si(111). Epitaxial AlN films have been prepared by plasma source molecular beam epitaxy (PSMBE) at growth temperatures of 400 and 650 oC. The AlN/Si grown at 650°C shows distinct reflection high-energy electron diffraction (RHEED) patterns with the following epitaxial relations: AlN [0001] ‖ Si [111], and AlN < 011 0 > ‖ Si < 112 >. This is in sharp contrast with the observation of a spotty RHEED pattern for AlN/Si sample grown at 400°C, which appeared the same when viewed along any Si azimuth. Furthermore, the X-ray diffraction (XRD) rocking curve width showed a significant reduction from 5.7ofor sample grown at 400°C to 2° for the one grown at 650°C. The observation of Raman active A1(LO) and E2, and IR active E1(TO) zone center phonons, in both the films, is in accordance with the c-axis orientation of the films. However, the observed phonon mode frequencies and line widths indicate that the AlN samples grown at 400°C have less strain but more disorder compared to the ones grown at 650°C in agreement with RHEED and XRD data.
Light Emission from Quantum-Dot-Like Structures in Cubic GaN/InGaN/GaN Double Heterostructures and Quantum Wells
Authors: Lischka K.
Published: April 2011

Recent results of experimental investigations of cubic GaN/InGaN/GaN double heterostructures grown by molecular beam epitaxy are reviewed. Micro-Raman effect, photoluminescence and photoluminescence excitation spectroscopy as well as high-resolution x-ray diffraction measurements reveal clear evidence that the photoluminescence of these double heterostructures is related to In-rich quantum dot-like structures which are embedded within the InGaN layers. Annealing experiments corroborate the assumption that the formation of the In-rich clusters is terminated already during the growth process.
Piezoelectric Field and its Influence on the Pressure Behavior of the Light Emission from InGaN/GaN and GaN/AlGaN Quantum Wells
Authors: Suski T. , Perlin P. , Lepkowski S.P. , Teisseyre H. , Gorczyca I. , Prystawko P. , Leszczynski M. , Grandjean N. , Massies J. , Kitamura T. , Ishida Y. , Chichibu S.F. , Okumura H.
Published: April 2011

In this paper we review studies aiming at elucidation of the mechanisms responsible for anomalously low pressure coefficients of the light emission energy, dEE/dP, observed in quantum structures of InGaN/GaN and GaN/AlGaN. We have established that in hexagonal InGaN/GaN and GaN/AlGaN structures the main mechanism involved is related to the pressure induced increase of the piezoelectric field which determines also the strong red shift of the emission energy with thickness of the quantum well. To reproduce the experimental findings in InGaN/GaN case, it is necessary to take into account the dependence of the piezoelectric constants on the volume-conserving strain. Whereas the experimental results on a decrease of dEE/dP in GaN/AlGaNstructures can be fully accounted for within the linear elasticity theory. In contrast to these findings, dEE/dP magnitude measured in cubic InGaN/GaN quantum structures shows value close to changes of the InGaN bangap with pressure obtained from first principle calculations. The latter result is consistent with the absence of the built-in electric fields in the cubic nitride structures.