bluuebunny/crossref_metadata_2025_split huggingface dataset:
Engineering CuInGaSSe2 Surface Properties to Enhance Device Performance
Authors: Menezes Shalini , Li Yan , Menezes Sharmila J. 
Published: April 2011

ABSTRACTThe CuInGaSSe2/CdS heterostructure interface has a special effect on the performance of an important thin film photovoltaic device. The CdS buffer layer is essential to stabilize the performance of CuInGaSSe2 based devices. It adjusts the lattice mismatch at the absorber/window interface, repairs CuInGaSSe2 surface defects and protects it from air oxidation. Unfortunately, the CdS material has many environmental issues. This paper reports an alternate chemical approach to engineer the interface defects in CuInGaSSe2 and maximize its PV output. It describes a simple processing step to manipulate the defect density. This step could potentially reduce sensitivity to the ambience, widen the surface bandgap and replace the current hazardous processes used in state-of-the-art CuInGaSSe2 modules. Photocurrent and spectral response measurement in an electrolytic medium monitor the effects of surface modification, specific metal ions and time. The CuInGaSSe2 films respond easily to a number of external stimuli with either positive or negative changes in the electro-optic properties. Strong time dependence of the photocurrent suggests a dynamic equilibrium of point defects in the CuInGaSSe2 film. The results provide new insights into the effects of stoichiometry, deposition methods and oxide formation, on the defect chemistry. They also provide directions for reconfiguring the deep defects for enhanced device performance without the need for toxic etchants or buffer layers, and the environmental hazards associated with these steps.
Design and Fabrication of GaN-based Permeable-Base Transistors
Authors: Cabalu Jasper S. , Gunter Liberty L. , Friel Ian , Bhattacharyya Anirban , Fedyunin Yuri , Chu Kanin , Bellotti Enrico , Eddy Charles , Moustakas Theodore D. 
Published: April 2011

ABSTRACTIn this work, we present the first successful fabrication of an etched grooved GaN-based permeable-base transistor structure. Growth of the device structures was done by Molecular Beam Epitaxy (MBE) on thick HVPE GaN quasi-substrates. The fabrication process took advantage of isolations pads via He implantation and SiN deposition, as well as submicron ICP etching of collector fingers patterned via e-beam lithography. SEM of the finished devices shows smooth etched finger structures and base layer surface with finger sidewall angles of ∼85° for 1:1 and 1:3 finger spacing. Specific contact resistivities of ∼ 3 × 10-6 Ω•cm2 for the ohmic contacts were achieved with Ti/Al/Ni/Au metallization scheme. Preliminary DC testing of the devices show a collector current IC = 140 mA/mm at VCE of 5V and VBE of +0.5V. The maximum transconductance gm is ∼111 mS/mm in the measured collector-emitter bias range. These values are comparable, within the measurement tolerance, to physics-based modeling results.
MOCVD AlGaN/GaN HFET's Material Optimization and Devices Characterization
Authors: Demchuk Alexander , Olson Don , Olson Dan , Shin Minseub , Munns Gordon 
Published: April 2011

ABSTRACTAn optimization of growth parameters of AlxGa1*xN/AlN/GaN heterostructure field effect transistors (HFET) grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique on SiC and sapphire substrates with relatively high Al mole fraction in the barrier layer (0.3 < x < 0.5) has been presented. The properties of the two-dimensional electron gas (2DEG) forming at the AlxGa1-xN/GaN heterojunction can be tuned by careful adjustments of AlxGa1-xN barrier layer thickness and Al mole fraction, x. The 2DEG sheet conductivities (μ ns) as high as 2.6 × 1016 V-1s-1 at μ ∼ 2200 cm2/Vs and ns ∼ 1.2 × 1013 cm-2 has been achieved on AlxGa1-xN/AlN/GaN HFET structures on SiC substrate at x = 0.47. HFET devices processed on these structures exhibited improved low field conductivities and DC and high frequency performance. Saturation currents above 1.2 A/mm at 0 V gate bias, transconductance as high as 340 mS/mm at Lg = 0.25 μm and FT × Lg > 20 GHz × μm were demonstrated on HFET structure grown on SiC substrates.
Recombination Related to Two-Dimensional Electron Gas of AlxGa1-xN/GaN Single Heterostructures Studied with Picosecond Time-Resolved Photoluminescence
Authors: Yang Qing , Armitage Rob , Weber Eicke R. , Birkhahn Ronald , Gotthold David , Guo Shiping , Albert Brian 
Published: April 2011

ABSTRACTNearbandgap radiative recombination in undoped AlxGa1-xN/GaN (x = 0.28 to 0.35) single heterostructures grown by metalorganic chemical vapor deposition is investigated using picosecond time-resolved photoluminescence at 11K.Room temperature Hall effect measurements show both high sheet carrier concentrations (∼1×1013 cm-2) and high mobilities (∼1000 cm2/Vs), suggesting the formation of a two-dimensional electron gas (2DEG) at the heterointerface. The luminescence transient of the 3.481 eV emissions, which is usually assigned to donor-bound exciton emission of GaN, consists of an initial fast component and a slow second component. Samples with higher aluminum concentrations show broad luminescence peaks in the spectral range of 3.41—3.45 eV with long decay lifetimes over 1 ns. In addition, the below bandgap emissions saturate at high excitation power and shift toward lower energies with increasing time delay. The observed emissions are explained by the recombination processes involving the 2DEG at the heterointerface.
Effect of Buffer Design on AlGaN/AlN/GaN Heterostrucutres by MBE
Authors: Namkoong Gon , Alan W. , Brown A. S. , Losurdo M. , Giangregorio M. M. , Bruno G. 
Published: April 2011

ABSTRACTThe effect of the buffer layers on the subsequent GaN epitaxial layers and electrical properties of AlGaN/AlN/GaN heterojunction structures nitrided at various temperatures was investigated. For AlN buffer layers, two different growth conditions of AlN buffer layers were introduced to avoid Al droplets. We found that etch pit density and structural quality of GaN epitaxial layer strongly depends on the growth conditions of AlN buffer layers. When using a double buffer layer (low temperature GaN on high temperature AlN) for 200 °C nitridation, the etch pit density was measured to high 107 cm-2 in GaN epitaxial layers. Furthermore, we observed that electrical properties of AlGaN/AlN/GaN heterostructures depend on growth conditions of buffer layers and nitridation temperatures. The mobility in Al0.33Ga0.67N/AlN/GaN structures grown on single AlN buffer layers for 200 °C nitridation were 1300 cm2/Vs at a sheet charge of 1.6×1013 cm-2. Using the double buffer layer for 200 °C nitridation, the mobility increased to 1587 cm2/Vs with a sheet charge of 1.25×1013 cm-2.
Optical and Microstructural Characterisation of InN Grown by PAMBE on (0001) Sapphire and (001) YSZ
Authors: Anderson P. A. , Kendrick C. E. , Lee T. E. , Diehl W. , Reeves R. J. , Kennedy V. J. , Markwitz A. , Kinsey R. J. , Durbin S. M. 
Published: April 2011

ABSTRACTInN continues to be a topic of great interest, particularly with respect to the issues surrounding its bandgap energy. To further explore this material and its properties, we have grown 200–300 nm InN films by a plasma-assisted molecular beam epitaxy (PAMBE) technique on a variety of substrates, including (0001) sapphire, (100) InAs, and both (100) and (111) YSZ. Single-crystal films regardless of quality all show the commonly reported broad luminescence feature in the range of 0.7 to 0.8 eV, although we have also observed this feature in polycrystalline films. Growth on (100) InAs and (100) YSZ was motivated by a desire to explore cubic InN; in both cases growth appears to be initially cubic, but a mixture of hexagonal and cubic phases is detected in the final layer.
Growth and Characterization of Bulk GaN Crystals at High Pressure and High Temperature
Authors: D'Evelyn M. P. , Narang K. J. , Park D.-S. , Hong H. C. , Barber M. , Tysoe S. A. , Leman J. , Balch J. , Lou V. L. , LeBoeuf S. F. , Gao Y. , Teetsov J. A. , Codella P. J. , Tavernier P. R. , Clarke D. R. , Molnar R. J. 
Published: April 2011

ABSTRACTWe report the growth and characterization of bulk GaN single crystals by temperature-gradient recrystallization at high pressure and high temperature (HPHT), using apparatus adapted from that used to synthesize gem-grade diamond crystals. The bulk crystals are grown on seeds that were synthesized by hydride vapor phase epitaxy (HVPE) and subsequently removed from their sapphire substrate. Our largest crystals to date are 15×18 mm in diameter; however, the process is scalable to 50 mm and above. The crystals are transparent and well faceted, and dislocation densities below 100 cm−2 have been achieved. Additional characterization of the GaN crystals is also presented.
Electron Field Emission from GaN Nanotip Pyramids
Authors: Ng Hock M. , Shaw Jonathan , Chowdhury Aref , Weimann Nils G. 
Published: April 2011

ABSTRACTElectron field emission was measured from GaN nanotip pyramids formed by polarity-selective chemical etching in KOH solution. The GaN samples were grown by plasma-assisted molecular beam epitaxy and consisted of regions of Ga- and N-polar GaN grown at the same time. The pyramids were formed only in the N-polar regions and have extremely sharp tips with diameters estimated to be less than 20 nm. Field emission measurements showed a characteristic Fowler-Nordheim behavior. The average turn-on field was 1.6 V/μm with a corresponding normalized field enhancement factor of about 1500.
Optimization of GaN/AlGaN Quantum Wells for Ultraviolet Emitters
Authors: Hangleiter A. , Fuhrmann D. , Greve M. , Rossow U. 
Published: April 2011

ABSTRACTWe have studied GaN/AlGaN and GaInN/GaN quantum well structures grown on sapphire and SiC substrates using low-pressure MOVPE. The emission wavelength of the GaN/AlGaN QW's was varied in the 320–400 nm range and that of the GaInN/GaN QW's in the 390–480 nm range, by adjusting the QW width. Using temperature dependent resonant-excitation photoluminescence measurements we have determined the internal quantum efficiency (IQE) and analyzed its temperature dependence. For the GaN/AlGaN QW's we achieve a reasonable IQE of in excess of 5 % only at fairly high excitation power, while GaInN/GaN QW's provide IQE's of up to 46 % at rather low power density, indicating a much stronger role of defects for the former. While thermal activation energies are dominated by intrinsic mechanisms for GaInN/GaN QW's, lower activation barriers evident from the GaN/AlGaN data also hint at a more import role of defects.
Structural Units and Energy of Grain Boundaries in GaN
Authors: Chen Jun , Ruterana Pierre , Nouet Gerard 
Published: April 2011

ABSTRACTThe energy of coincidence grain boundaries in wurtzite structure was investigated with the Stillinger-Weber potential after modification for GaN. A boundary can have two interfaces with a different period corresponding to the edge and diagonal of the unit cell of coincidence site lattices for rotations around [0001] in the range 0–60°. Their energy depends mainly on the atomic structure of the dislocation cores used for the boundary plane reconstruction. As a function of the structure units in the period, the energy of the edge and diagonal shows two minima which are observed for the edge boundaries.
Band bending near the surface in GaN as detected by a charge sensitive probe
Authors: Sabuktagin S. , Reshchikov M. A. , Johnstone D. K. , Morkoç H. 
Published: April 2011

ABSTRACTWe measured the absolute value of the surface band bending in GaN layers grown by molecular beam epitaxy with a charge sensitive surface microprobe. Surface potential measurements showed an upward band bending from 0.7 to 1.4 eV in undoped and Si-doped GaN. The samples stored in dark for one week showed an increase in band bending by up to 0.1 eV. The effect of ultraviolet (UV) exposure (with a lamp or a pulsed nitrogen laser) on band bending was also studied. Typically, the surface barrier decreased by about 0.2 – 0.5 eV under UV light. The barrier was restored very slowly (by a logarithmic law) in the dark at room temperature. These and other similar phenomena are tentatively attributed to thermionic transfer of free electrons from the bulk to the surface states. Photo-induced desorption of oxygen may also play a role in the observed effects.
Processing of rare earth doped GaN with ion beams
Authors: Lorenz K. , Wahl U. , Alves E. , Wojtowicz T. , Ruterana P. , Dalmasso S. , Martin R. W. , ÒDonnell K. P. , Ruffenach S. , Briot O. , Vantomme A. 
Published: April 2011

ABSTRACTGaN epilayers grown by MOCVD were implanted with different fluences of thulium at room temperature and at 500 °C in order to find the optimum implantation conditions. Rutherford backscattering spectrometry in the channeling mode was used to monitor the damage evolution in the Ga-sublattice and to establish the lattice site location of the thulium ions. The nature of structural defects was studied with transmission electron microscopy and the optical properties of the samples with room temperature cathodoluminescence. The introduced damage could be significantly reduced by implantation at high temperature for fluences up to 5×1015 Tm/cm2. Annealing was necessary for optical activation of the implanted samples, in all cases. After annealing, sharp rare earth related emissions were observed in the blue and in the near infra-red spectral region.
Photoluminescence from freestanding GaN with (1010) orientation
Authors: Reshchikov M. A. , Teke A. , Maruska H. P. , Hill D. W. , Morkoç H. 
Published: April 2011

ABSTRACTFreestanding GaN templates with (1010) orientation (M-plane) were obtained by halide vapor phase epitaxy (HVPE) on nearly lattice-matched LiAlO2and subsequent removal of the substrate by wet chemical etching. Photoluminescence (PL) spectrum from both sides of the GaN template investigated is dominated by peaks at 3.47, 3.42 and 3.36 eV, tentatively attributed to an exciton bound to the neutral shallow donor and two unidentified structural defects, respectively. The quantum efficiency of the exciton-related emission exceeds 10%, whereas that of the combined emission from the defect-related bands (red, yellow and blue) is below 0.1%. The evolution of the PL spectrum with temperature and excitation intensity is analyzed in detail. Effects of polishing and etching on the PL properties are also discussed.
Confocal Photoluminescence and Cathodoluminescence Studies of AlGaN
Authors: Dierolf V. , Svitelskiy O. , Cargill G. S. , Nikiforov A. Yu. , Redwing J. , Acord J. 
Published: April 2011

ABSTRACTAlxGa1-xN films grown by MOCVD on sapphire and SiC substrates have been investigated by spatially resolved confocal photoluminescence microscopy and cathodoluminescence spectroscopy and mapping. The sample on SiC has a rougher topography, but it is much more uniform in emission intensity and wavelength than the sample on sapphire.
Above and below Bandgap Excitation of Er-defect Complexes and Isolated Er in Er-implanted GaN
Authors: Braud A. , Abouzaid M. , Wojdak M. , Doualan J. L. , Moncorge R. , Pipeleers B. , Vantomme A. 
Published: April 2011

ABSTRACTPhotoluminescence (PL) spectra, luminescence dynamics and luminescence saturation of the Er3+4I13/2 → 4I15/2 transition in Er-implanted GaN samples at 7K are investigated. Under below-gap excitation, different Er centers are identified. Several Er centers are clearly excited via local defects or impurities. Er-defect complexes excited by above or below bandgap light are compared. Luminescence dynamics study shows that the 4I13/2 manifold has a shorter lifetime when Er ions are part of Er-defect complexes than when Er ions are isolated from any defect. The saturation of Er luminescence is investigated for the different types of Er center corresponding to specific excitation wavelengths.
Magnetic properties of Mn-doped GaN, InGaN, and AlGaN
Authors: Reed M. L. , Berkman E. A. , Reed M. J. , Arkun F. E. , Chikyow T. , Bedair S. M. , Zavada J. M. , El-Masry N. A. 
Published: April 2011

ABSTRACTWe report on the growth and magnetic properties of single crystal Mn-doped GaN, InGaN, and AlGaN films. The III-Nitride films were grown by MOCVD, while the Mn doping was performed by solid-state diffusion of a surface Mn layer deposited by pulsed laser ablation. Mn-doped InxGa1-xN films were grown with x < 0.15, where the easy axis of magnetization rotates from in-plane to out-of-plane by changing the InxGa1-xN thickness/strain-state of the film from compressively strained to relaxed. Mn-doped AlxGa1-xN films were grown with x < 0.40 showing ferromagnetic behavior above room temperature. SQUID measurements ruled out superparamagnetism within these films. By optimizing the growth and annealing conditions of Mn-doped III-Nitrides, we have achieved Curie temperatures in the range of 228 to 500K. These ferromagnetic Mn-doped III-Nitride films exhibit hysteresis with a coercivity of 100–500 Oe. TEM analysis showed no secondary phases within these films.
Combined MOCVD and MBE growth of GaN on porous SiC
Authors: Sagar Ashutosh , Feenstra R. M. , Inoki C. K. , Kuan T. S. , Koleske D. D. 
Published: April 2011

ABSTRACTGaN films have been grown homoepitaxially by MOCVD on MBE-grown GaN template layers, using both porous and nonporous SiC substrates. The effect of the porous SiC substrates on dislocations in the MBE and MOCVD GaN layers has been studied using TEM and x-ray characterization. A reduction in dislocation density from ≥1×1010 cm-2 in the MBE template to 2.5×109 cm-2 at the top of the MOCVD film is found, with similar final values in the MOCVD films for both porous and nonporous substrates. We discuss various mechanisms by which dislocation density is reduced in the MOCVD layers.
Atomic Structure of Defects in GaN:Mg grown with Ga polarity
Authors: Liliental-Weber Z. , Tomaszewicz T. , Zakharov D. , Jasinski J. , O'Keefe M. A. , Hautakangas S. , Laakso A. , Saarinen K. 
Published: April 2011

ABSTRACTElectron microscope phase images, produced by direct reconstruction of the scattered electron wave from a focal series of high-resolution images, were used to determine the nature of defects formed in GaN:Mg crystals. We studied bulk crystals grown from dilute solutions of atomic nitrogen in liquid gallium at high pressure and thin films grown by the MOCVD method. All the crystals were grown with Ga-polarity. In both types of samples the majority of defects were three dimensional Mg-rich hexagonal pyramids with bases on the (0001) plane and six walls on {1123} planes seen in cross-section as triangulars. Some other defects appear in cross-section as trapezoidal (rectangular) defects as a result of presence of truncated pyramids. Both type of defects have hollow centers. They are decorated by Mg on all six side walls and a base. The GaN which grows inside on the defect walls shows polarity inversion. It is shown that change of polarity starts from the defect tip and propagates to the base, and that the stacking sequence changes from ab in the matrix to bc inside the defect. Exchange of the Ga sublattice with the N sublattice within the defect leads to 0.6±0.2Å displacement between Ga sublattices outside and inside the defects. It is proposed that lateral overgrowth of the cavities formed within the defect takes place to restore matrix polarity on the defect base.
Influence of Growth Parameters on the Deep Level Spectrum in MBE-Grown n-GaN
Authors: Arehart A. R. , Poblenz C. , Heying B. , Speck J. S. , Mishra U. K. , DenBaars S. P. , Ringel S. A. 
Published: April 2011

ABSTRACTThe impact of growth temperature and Ga/N flux ratio on deep levels in GaN grown by molecular beam epitaxy (MBE) is systematically investigated using both deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) in a study designed to map out the presence and concentration of defects over a defined region of the MBE GaN growth phase diagram. A series of Si-doped GaN films were grown to cover a substrate temperature range and a Ga/N flux ratio range that spans from the N stable to the Ga droplet regimes along both variables. Identical growth templates were used to eliminate variations in dislocations between samples so that point defect variations could be tracked. For these samples, traps are detected at EC-Et=0.25, 0.60, 0.90, 1.35, 2.40, 3.04, and 3.28 eV. The near valence bands states at EC–3.04 and EC–3.28 eV are found to be strongly dependent on Ga/N flux with decreased concentrations as a function of increasing Ga flux toward the Ga droplet regime, but with little effect from growth temperature. The EC-1.35 eV level shows a strong dependence on growth temperature and only slight dependence on Ga/N flux ratio. In contrast, the concentration of the EC-Et=0.25, 0.90 eV levels increased with increasing Ga flux toward the Ga droplet regime, while the EC-Et=0.60 shows no dependence. The variation in concentration of the EC-2.40 eV level that has been related to VGa was difficult to quantify, but tends to increase towards nitrogen rich growth. The dependencies for the detected states with respect to growth temperature and Ga/N flux ratio suggest different physical point defect sources.
The Effects of Atmosphere, Temperature, and Bandgap on the Annealing of GaInNAs for Solar Cell Applications
Authors: Ptak A. J. , Kurtz Sarah , Young M. , Kramer C. 
Published: April 2011

ABSTRACTThe effects of thermal annealing on the minority-carrier diffusion lengths and depletion widths of GaInNAs are studied. We find that diffusion lengths in as-grown, Be- and Si-doped GaInNAs are limited to less than about 0.2 μm for samples with low concentrations of nitrogen. For higher concentrations of nitrogen, the diffusion lengths are not measurable. Annealing under a variety of temperatures and atmospheres typically makes the diffusion lengths even shorter. These short diffusion lengths are not yet long enough for GaInNAs to be useful in a next-generation, four-junction structure. Using undoped GaInNAs in a p–i–n structure is a promising approach to increase device performance. Currently, however, the depletion widths are too small at the bandgaps necessary for solar cells, and annealing does not appear to improve them.
A study of anion exchange reactions at GaAs surfaces for heterojunction interface control
Authors: Losurdo Maria , Giuva Danilo , Capezzuto Pio , Bruno Giovanni , Brown Terence , Triplett Greg , May Gary , Brown April S. 
Published: April 2011

ABSTRACTGaPyAs1-y/GaAs, GaAsySb1-y/GaSb and GaSbyAs1-y/GaAs superlattices (SLs) grown by MBE, by exposure of GaAs to phosphorus and antimonide fluxes, and by exposure of GaSb to an arsenic flux, respectively, have been investigated. The focus is on the abruptness of interfaces and understanding the mechanisms associated with anion incorporation and exchange. In the case of the Sb flux interaction with the GaAs surface, the Sb segregation at the GaAs surface inhibits anion exchange. For the case of As over GaSb reactions, anion exchange results in the formation not only of the ternary alloy GaAsySb1-y, but also of isoelectronic compounds AsSbx that segregate at the GaSb/GaAs interface. In the case of the P flux interfaction with the GaAs surface, fast in-diffusion of P results in graded GaPyAs1-y layer formation.