bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fabrication and Measurement of 4H-Silicon Carbide Avalanche Photodiodes
Authors: Burr Kent , Sandvik Peter , Arthur Stephen , Brown Dale , Matocha Kevin 
Published: April 2011

ABSTRACTAvalanche photodiodes (APDs) that are sensitive in the ultraviolet (UV) from approximately 250 – 350 nm have been fabricated from 4H-SiC. The SiC APDs, which use hole-initiated avalanche multiplication, were produced using n-type SiC epitaxial layers grown on a p-type substrate. In order to achieve avalanche breakdown in the bulk of the material, a dry ion-based etching technique was used to form sloped sidewalls on the devices. The devices had an area of approximately 1 mm2, and they had maximum breakdown voltages of approximately 500 V. The APDs had a positive temperature coefficient for avalanche breakdown voltage and showed excellent stability for multiplication factors in excess of several hundred. Dark current, photo responsivity, and multiplication measurements from room temperature to 150°C will be presented. The dark noise performance of the APDs has also been characterized using a standard nuclear spectroscopy system consisting of a charge sensitive preamplifier, a shaping amplifier, and a multichannel analyzer. The input equivalent dark noise charge and excess noise factor for the dark current was measured over a range of shaping times, temperatures, and bias voltages. The noise performance of SiC APDs in applications such as gamma ray or x-ray spectroscopy will be highly dependent on the achievement of low bulk leakage current at the operating point. Here, an ionization coefficient ratio (k=α/β) of 0.078 was found.
Observations of electron velocity overshoot during high-field transport in AlN
Authors: Collazo Ramón , Schlesser Raoul , Roskowski Amy , Davis Robert F. , Sitar Z. 
Published: April 2011

ABSTRACTThe energy distribution of electrons transported through an intrinsic AlN film was directly measured as a function of the applied electric field. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. The electron energy distribution featured kinetic energies higher than that of completely thermalized electrons. Transport through 80 nm thick layers indicated the onset of quasi-ballistic transport. This was evidenced by symmetric energy distributions centered at energies above the conduction band minimum for fields greater than 530 kV/cm. Drifted Fermi-Dirac energy distributions were fitted to the measured energy distributions, with the energy scale referenced to the bottom of the AlN conduction band. The drift energy and the carrier temperature were obtained as fitting parameters. Overshoots as high as five times the saturation velocity were observed and a transient length of less than 80 nm was deduced. In addition, the velocity-field characteristic was derived from these observations. This is the first experimental demonstration of this kind of transport in AlN.
Time Resolved Optical Studies of InGaN Layers Grown on LGO
Authors: Cheung Maurice , Namkoong Gon , Furis Madalina , Chen Fei , Cartwright Alexander. N. , Doolittle W. Alan , Brown April 
Published: April 2011

ABSTRACTRadiative recombination processes in bulk InGaN grown by molecular beam epitaxy (MBE) on lithium gallate (LGO or LiGaO2) substrates were investigated using microscopic PL and time-resolved photoluminescence (TRPL). The improved structural quality resulting from a better lattice match of the LGO substrate to III-V nitride materials simplifies these investigations because well-defined composition phases can be analyzed for both homogeneous and phased separated InGaN samples. Epilayers of InGaN intentionally grown with and without indium segregation were studied. X-ray diffraction measurements showed that the homogeneous epilayer was high quality In0.208Ga0.702N and the segregated epilayer exhibited peaks corresponding to both In0.289Ga0.711N and In0.443Ga0.557N indicating the presence of higher In concentration regions in this sample. Spatially resolved photoluminescence spectra confirm the existence of these regions. The photoluminescence intensity decay is non-exponential for both samples and a stretched exponential fit to the decay data confirms the existence of local potential fluctuations in which carriers are localized before recombination.
Femtosecond pump and probe spectroscopy of optical nonlinearities in an InGaN/GaN heterostructure
Authors: Chen Fei , Cartwright A. N. 
Published: April 2011

ABSTRACTThe magnitudes and evolutions of two photoinduced absorption nonlinearities, absorption bleaching and field screening, were compared and investigated by employing electroabsorption and femtosecond pump-probe spectroscopy in a biased InGaN/GaN p-i-n double heterostructure. Steady state electroabsorption measurements indicate the field induced absorption coefficient changes in this structure are caused by the Franz-Keldysh effect. The temporal resolution of the absorption bleaching spectra suggests that the photoinduced carriers rapidly relaxed to the InGaN band edge within several picoseconds. As the applied reverse bias field was increased, the transition of the differential absorption spectral signature from the signature for absorption bleaching to the signature for field screening was observed. The magnitude of the change in absorption due to photoinduced carrier screening of the applied field is quantified and compared to absorption bleaching.
Structural Defects in Mg-doped GaN and AlGaN grown by MOCVD
Authors: Tomiya S. , Goto S. , Takeya M. , Ikeda M. 
Published: April 2011

ABSTRACTWe have investigated the structural defects formed in Mg-doped GaN and AlGaN epitaxial layers grown by metal organic chemical vapor deposition. These defects have an inverse pyramidal shape and appear when the Mg concentration [Mg] is higher than ∼4×1019/cm3. The density of the defects increases as [Mg] increases, but the size of the defects becomes smaller as [Mg] increases. The density of the defects also has a strong correlation with the hydrogen concentration in the epitaxial layers. Transmission electron microscope analysis reveals that the defects have an inversion operation to the matrix and that their boundaries are Mg-rich. We also propose a model for defect formation.
Distinguishing negatively-charged and highly conductive dislocations in gallium nitride using scanning Kelvin probe and conductive atomic force microscopy
Authors: Simpkins Blake S. , Yu Edward T. , Waltereit Patrick , Speck James S. 
Published: April 2011

ABSTRACTScanning Kelvin probe microscopy (SKPM) and conductive atomic force microscopy (C-AFM) are used to image surfaces of GaN grown by molecular beam epitaxy (MBE). Numerical simulations are used to assist in the interpretation of SKPM images. Detailed analysis of the same area using both techniques allows imaging of surface potential variations arising from the presence of negatively charged dislocations and dislocation-related current leakage paths. Correlations between the charge state of dislocations, conductivity of leakage current paths, and possibly dislocation type can thereby be established. Approximately 25% of the leakage paths appear to be spatially correlated with negatively charged dislocation features. This is approximately the level of correlation expected due to spatial overlap of randomly distributed, distinct features of the size observed, suggesting that the negatively charged dislocations are distinct from those responsible for localized leakage paths found in GaN. The effects of charged dislocation networks on the local potential profile is modeled and discussed.
Characterization of Dark-Block Defects in Cantilever Epitaxial GaN on Sapphire
Authors: Provencio P. P. , Follstaedt D. M. , Missert N. A. , Koleske D. D. , Mitchell C. C. , Allerman A. A. , Ashby C. I. H. 
Published: April 2011

ABSTRACTCantilever epitaxy of GaN on sapphire has been augmented by the use of initial facetted GaN growth on narrow sapphire mesas (< 1μm) in order to turn remaining threading dislocations from vertical to horizontal, and thus reduce the overall dislocation density at the surface where devices would be placed. With this modification, isolated non-radiative block-like defect areas have been introduced that hinder optical and electronic material performance. Here we characterize these defects with microscopy, and show that they are arrays of lateral dislocations, with cracks and voids along their centerlines. We deduce that they result when tilted GaN is joined to neighboring oriented material. Their presence is independent of the type of nucleation layer used.
Study of the Origin of Misorientation in GaN Grown by Pendeo-Epitaxy
Authors: Zakharov D. N. , Liliental-Weber Z. , Roskowski A. M. , Einfeldt S. , Davis R. F. 
Published: April 2011

ABSTRACTGrowth of pendeo-epitaxial (PE) layers introduces misorientation between the seed layers and the overgrown wing layers. The origin of this misorientation has been studied by Transmission Electron Microscopy (TEM) using a set of samples in which subsequent procedures utilized in PE were applied, i.e. growth of GaN template, stripe etching, annealing at the growth temperature of the PE layers and final PE growth. It was shown that etching of seed-stripes did not change the type of defects or their distribution. However, heating to the PE growth temperature drastically modified the surface and V-shaped pits were formed. The surface became smooth again after the PE growth took place. Overgrowth of the V-shaped pits resulted in formation of edge threading dislocations over a seed-stripe region with a dislocation density of 8.0×108 cm−2. Formation of new edge dislocations over the seed can have an influence on the misorientation between the PE grown regions.
Characterization of Cubic GaN Films Using An AlN/GaN Ordered Alloy on GaAs (100) by RF-MBE
Authors: Shike Junichi , Shigemori Atsushi , Ishida Koichi , Takahashi Kiyoshi , Kimura Ryuhei 
Published: April 2011

ABSTRACTHigh quality cubic GaN films were successfully grown on an AlN/GaN ordered alloy by RF-MBE. AlN/GaN ordered alloy is here employed instead of a AlGaN nucleation layer formed by nitridation of an AlGaAs buffer layer. Dominant cubic GaN epilayer (1.0 μm) growth was confirmed by insitu RHEED observations, AFM, TEM, PL and X-ray diffraction measurements.
Background Impurity Reduction and Iron Doping of Gallium Nitride Wafers
Authors: Vaudo Robert P. , Xu Xueping , Salant Allan D. , Malcarne Joseph A. , Hutchins Edward L. , Brandes George R. 
Published: April 2011

ABSTRACTBackground impurities and the resulting electrical characteristics were studied for GaN wafers grown using hydride vapor phase epitaxy at various growth conditions. The electron concentration was found to decrease with increasing GaN thickness, by orders of magnitude in the first few microns of growth, but continuing gradually for thousands of microns. Physical removal of the backside degenerate layer enabled improved analysis of the electrical properties. Secondary ion mass spectroscopy was used to determine that the presence of oxygen and silicon accounted for the electron concentration for unintentionally n-type doped material. The concentration of oxygen was found to vary more than that of silicon and increased with decreasing growth temperature. The resistivity was measured to be as high as 1 ohm-cm, corresponding to a carrier concentration of 1016 cm−3. Iron was demonstrated to effectively compensate the residual donors and increased the resistivity to greater than 109 ohm-cm at room temperature and greater than 3×105 ohm-cm at 250 °C. An activation energy for the iron-doped GaN was determined by variable temperature resistivity measurements to be 0.51 eV.
Lattice constant variation in GaN:Si layers grown by HVPE
Authors: Usikov A. , Kovalenkov O. V. , Mastro M. M. , Tsvetkov D. V. , Pechnikov A. I. , Soukhoveev V. A. , Shapovalova Y. V. , Gainer G. H. 
Published: April 2011

ABSTRACTThe structural, optical, and electrical properties of HVPE-grown GaN-on-sapphire templates were studied. The c and a lattice constants of the GaN layers were measured by x-ray diffraction. It was observed that the c and a lattice constants vary non-monotonically with Si-doping. The proper selection of Si-doping level and growth conditions resulted in controllable strain relaxation, and thus, influenced defect formation in GaN-on-sapphire templates. It was also observed that HVPE homoepitaxial GaN layers grown on the templates have better crystal quality and surface morphology than the initial templates.
High-Spatial-Resolution Cathodoluminescence Measurement of InGaN
Authors: Kanie Hisashi , Okado Hiroaki , Yoshimura Takaya 
Published: April 2011

ABSTRACTThis paper described observation of cathodoluminescence (CL) of microcrystalline InGaN bulk crystals under a scanning electron microscope (SEM) with a high-spatial-resolution (HR) CL measuring apparatus. HR-CL spectra from facets of InGaN crystals vary from facet to facet and are single peaked. Histogram analysis of the CL peak positions of HR spectra from the facets of the crystals in the area scanned during a low-resolution CL measurement shows a two-peaked form with comparable peak wavelengths. The diffusion length of a generated electron- ho le pair or an exciton from the recombination centers with a higher-energy-level state to that with a lower state is estimated to be 500 nm at the longest by the comparison of two monochromatic HR-CL images of adjoining facets.
Phase Transitions on Gan Surfaces
Authors: Adelmann C. , Lymperakis L. , Brault J. , Mula G. , Neugebauer J. , Daudin B. 
Published: April 2011

ABSTRACTRecent experimental and theoretical studies highlighted the importance of the growing surface structure on the final morphology of GaN. Actually, optimum morphology is achieved by growth in presence of a Ga bilayer adsorbed on the GaN surface. The threshold fluxes limiting the region of the Ga bilayer adsorption have been measured as a function of the GaN substrate temperature, giving rise to a Ga adsorption phase diagram. The Ga flux limiting the regions in the adsorption phase diagram exhibit a linear behavior in an Arrhenius plot. However, both energy activation (about 5 eV) and prefactor (in the 1025 range) are surprinsingly high. These questions were adressed by studying the adsorption/desorption of Ga adatoms and small islands (consisting of 2 and 3 Ga adatoms) on the Ga bilayer surface employing first principle density functional theory calculations. We find a desorption barrier of 2.1 eV and a binding energy between two Ga atoms of approximately 0.3 eV. Using these numbers we derived a simple growth model (based on rate equations). An analysis of the experimental data with the model revealed the origin of the large difference in the activation energies and the unusually large prefactor. We find that the nucleation of the droplets cannot be described by a simple Arrhenius behavior (as commonly assumed to fit experimental data) but that the nucleation energy is temperature dependent.
GaN epilayers and AlGaN/GaN multiple quantum wells grown on freestanding [1100] oriented GaN substrates
Authors: Chen C. Q. , Gaevski M. E. , Sun W. H. , Kuokstis E. , Yang J. W. , Simin G. , Khan M. A , Maruska H. P. , Hill D. W. , Chou M. M. C. , Gallagher J. J. , Chai B. H , Song J. H. , Ryu M. Y. , Yu P. W. 
Published: April 2011

AbstractWe report on the homoepitaxial growth of GaN on freestanding [1100] oriented GaN substrates using metalorganic chemical vapor deposition. A proper pretreatment of the substrates was found to be essential for the GaN homoepitaxy. The influence of growth conditions such as V/III molar-ratio and temperature on the surface morphology and optical properties of epilayers was investigated. Optimized pretreatment and growth conditions led to high quality [1100] oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Based on these GaN epilayer, AlGaN/GaN multiple quantum wells have been grown on the freestanding M-plane GaN. Photoluminescence data confirm that built-in electric field for M-plane structures is very weak, and this situation results in a stronger PL intensity in comparison with C-plane multiple quantum wells in tests at low excitation level.
Role of polarization in the photoluminescence of C- and M-plane oriented GaN/AlGaN multiple quantum wells
Authors: Kuokstis E. , Chen C. Q. , Gaevski M. E. , Sun W. H. , Yang J. W. , Simin G. , Asif Khan M. , Maruska H. P. , Hill D. W. , Chou M. M. C. , Gallagher J. J. , Chai B. H. 
Published: April 2011

ABSTRACTThe comparative study of photoluminescence (PL) dynamics of wurtzite-type GaN/AlGaN multiple quantum wells (MQWs) fabricated using low-pressure metalorganic chemical vapor deposition technique over GaN coated [0001]-sapphire (C-plane) and single crystalline [1100]-oriented freestanding GaN (M-plane) substrates is presented. The MQWs on C-plane sapphire at low excitation exhibited much lower (∼30 times) PL intensity in comparison with M-plane samples. The C-plane MQWs showed a strong excitation intensity-induced PL spectrum line blueshift (up to 140 meV). Meanwhile identical MQW structures on M-plane substrates demonstrated no PL peak shifts indicating an absence of polarization fields. At higher excitation (>50 kW/cm2) the PL intensity and spectra peak positions for both the C- and the M-plane MQWs become nearly the same and do not differ with subsequent increase of pumping. Theoretical analysis and comparison with PL experimental data revealed strong (up to ∼1.2–1.3 MV/cm) built-in electrostatic fields in the C-plane structures whereas M-plane structures are almost non-polar.
Cathodoluminescence of MBE-grown cubic AlGaN/GaN multi-quantum wells on GaAs (001) substrates
Authors: As D. J. , Potthast S. , Köhler U. , Khartchenko A. , Lischka K. 
Published: April 2011

ABSTRACTCubic phase GaN/AlxGa1-xN Multi Quantum Well structures were grown by rf-plasma assisted molecular beam epitaxy (MBE) on GaAs (001) substrates. X-ray measurements showed a high phase purity of the epilayers and revealed an Aluminum incorporation between 9 % and 49 %, respectively. The QW luminescence was tuned between 3.25 eV and 3.4 eV by means of the variation of QW barrier Aluminum content and QW width. Strong Cathodoluminescence (CL) from the GaN QWs and the underlying cubic AlxGa1-xN bulk material was observed at room temperature. The spatial localization of the QW emission was unambiguously determined by depth-resolved CL measurements. Combined with a model of energy-dependent penetration, diffusion, and recombination, these variations indicate a value of about 20 nm for the minority carrier diffusion length within the AlxGa1-xN confinement layer. The assignment of AlxGa1-xN bulk and GaN luminescence was further supported by employing a simple effective-mass quantum mechanical model.
Microscopic Description of Radiative Recombinations in InGaN/GaN Quantum Systems
Authors: Morel Aurelien , Lefebvre Pierre , Taliercio Thierry , Gil Bernard , Grandjean Nicolas , Damilano Benjamin , Massies Jean 
Published: April 2011

ABSTRACTRecombination dynamics in a variety of InGaN/GaN quantum systems has been studied by time resolved photoluminescence (PL). We have discovered that the time-decay of PL exhibits a scaling law: the nonexponential shape of this decay is preserved for quantum wells and quantum boxes of various sizes while their decay time varies over several orders of magnitude. To explain these results, we propose an original model for electron-hole pair recombination in these systems, combining the effects of internal electric fields and of carrier localization on a nanometer-scale. These two intricate effects imply a separate localization of electrons and holes. Such a microscopic description accounts very well for both the decays shape and the scaling law.
Dielectric Function of “Narrow” Band Gap InN
Authors: Goldhahn R. , Shokhovets S. , Cimalla V. , Spiess L. , Ecke G. , Ambacher O. , Furthmüller J. , Bechstedt F. , Lu H. , Schaff W. J. 
Published: April 2011

ABSTRACTSpectroscopic ellipsometry studies in the energy range from 0.7 up to 5.5 eV were carried out in order to determine the dielectric function (DF) of ‘narrow’ band gap (< 1 eV) single-crystalline InN films grown by molecular beam epitaxy on sapphire substrates. The imaginary part of the DF is characterized by a strong increase immediately above the band gap and then by a nearly constant value up to 4 eV. Pronounced structures above 4 eV are attributed to transitions along the L-M direction in the Brillouin-zone as a comparison with first-principles calculations indicates. In contrast, sputtered layers (band gap ∼1.9 eV) studied for comparison show a completely different spectral shape of the DF. Finally, DF's of high In-content InGaN alloys are presented, providing further evidence that InN is a “narrow” band gap semiconductor.
Electron Microprobe and Photoluminescence Analysis of Europium-Doped Gallium Nitride Light Emitters
Authors: Martin R. W. , Dalmasso S. , O'Donnell K. P. , Nakanishi Y. , Wakahara A. , Yoshida A. 
Published: April 2011

ABSTRACTRare-earth doped GaN structures offer potential for optical devices emitting in the visible region [1,2]. We describe a study of MOVPE grown GaN-on-sapphire epilayers implanted with Europium ions, producing characteristic red emission lines between 540 and 680 nm due to intra-4f(n) electron transitions. As-implanted and subsequently annealed samples are investigated using a combination of wavelength dispersive x-ray analysis (WDX), electron microscopy, cathodoluminescence (CL) and photoluminescence (PL). WDX is shown to be a powerful technique for quantifying rare-earth concentrations in GaN, with varying electron beam voltages allowing a degree of depth profiling, further enhanced by the simultaneous collection of room temperature luminescence (CL) from the analysed region [3]. The intensities of the sharp lines observed in the luminescence spectrum are compared to the doping density (between 1014 – 1015 cm−2) and the Eu content measured by WDX, using a Eu-doped glass standard. Differences observed in the luminescence spectra produced by laser and electron beam excitation will be discussed along with the importance of the annealing conditions, which “heal” defects visible in the electron micrographs.
Efficient GaN-based Micro-LED Arrays
Authors: Choi H. W. , Jeon C. W. , Dawson M. D. , Edwards P. R. , Martin R. W. 
Published: April 2011

ABSTRACTHighly efficient, two-dimensional arrays of parallel-addressed InGaN blue microLEDs with individual element diameters of 8, 12 and 20μm have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallisation, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have I-V characteristics similar to those of broad-area reference LEDs fabricated from the same wafer, and give superior (3mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction.