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Scanning near-field PL spectroscopy was applied to study spatial variations of the emission spectra of AlGaN epilayers with AlN molar fractions between 0.3 and 0.7. Experiments were performed at 300 K with 100 nm spatial resolution. In general, photoluminescence spectra were found to be highly uniform with the peak energy deviation of 2 to 6 meV for different alloy compositions. In the 30% and 42% Al layers, a slightly lower Al content and a higher point defect concentration at the boundaries of growth domains were detected. These features were attributed to the higher mobility of Ga adatoms during growth. The inhomogeneous broadening beyond the random alloy distribution was found negligible for the 30% and 42% Al samples, and about 40 - 50 meV for the layers with a larger Al content., QC 20170125

Scanning near-field photoluminescence (PL) spectroscopy was applied to study spatial variations of emission spectra of AlxGa1-xN epilayers with 0.6 <= x <= 0.7. PL spectra were found to be spatially uniform with peak wavelength standard deviations of only similar to 2 meV and ratios between peak intensity standard deviations and average peak intensity values of 0.06. The observed absence of correlation between the PL peak wavelength and intensity shows that spatial distribution of nonradiative recombination centers is not related to band potential fluctuations. Our results demonstrate that the homogeneous broadening and the random cation distribution primarily determine PL line-widths for layers grown under optimized conditions., QC 20150127

Photoexcited carrier dynamics and localization potentials in Al0.86In0.14N/GaN heterostructures have been examined by time-resolved and scanning near-field photoluminescence (PL) spectroscopy. The large GaN and AlInN PL intensity difference, and the short AlInN PL decay and GaN PL rise times indicate efficient photoexcited hole transfer from AlInN to GaN via sub-band-gap states. These states are attributed to extended defects and In clusters. Near-field PL scans show that diameter of the localization sites and the distance between them are below 100 nm. Spatial variations of the GaN PL wavelength have been assigned to the electric field inhomogeneities at the heterostructure interface., QC 20120711

Scanning near-field photoluminescence spectroscopy has been applied to evaluate bandgap fluctuations in epitaxial AlGaN films with the AlN molar fraction varying from 0.30 to 0.50. A dual localization pattern has been observed. The potential of the small-scale (<100 nm) localization, evaluated from the width of the photoluminescence spectra, is between 0 and 51 meV and increases with increased Al content. These potential variations have been assigned to small-scale compositional fluctuations occurring due to stress variations, dislocations, and formation of Al-rich grains during growth. Larger area potential variations of 25-40 meV, most clearly observed in the lower Al-content samples, have been attributed to Ga-rich regions close to grain boundaries or atomic layer steps. The density, size, and bandgap energy of these domains were found to be composition dependent. The lower bandgap domains were found to be strongly correlated with the regions with efficient nonradiative recombination., QC 20110721

Aging under high current stress of AlGaN quantum well based light emitting diodes with high and low Al content in the wells emitting at 270 nm and 335 nm, respectively, has been studied by scanning near field optical spectroscopy and far field electroluminescence, photoluminescence and time-resolved photoluminescence. In the high Al content devices emission band related to optical transitions in the cladding involving nitrogen vacancies has been found. Evolution of this band during aging suggests that the role of N vacancies is crucial in the aging process by aiding defect generation and formation of high conductivity channels., QC 20101213

We report on photoluminescence (PL) studies of AlGaN/AlGaN multiple quantum well (MQW) structures with well widths spanning from 1.65 to 5.0 nm under various excitations and temperatures. The samples were fabricated by migration-enhanced metal-organic chemical vapor deposition technique (MEMOCVD (R)). Screening of the built-in electric field by photogenerated carriers reduced quantum-confined Stark effect (QCSE). This is confirmed by solving the Poisson and Schrodinger equations for AlGaN-based quantum wells (QWs) under study. Analysis of the PL internal quantum efficiency under different excitations in MQWs with different widths shows strong influence of the carrier localization on the radiative properties of MQWs., QC 20110121

Emission from a 285 nm AlGaN quantum well light emitting diode has been studied by scanning near-field optical spectroscopy. The scans revealed micrometer-size domainlike areas emitting with a higher intensity and at a longer wavelength; presumably, because of a lower AlN molar fraction in these regions. Experiments performed on different days have shown that with time, intensity from these spots increases and emission wavelength shifts to the red, indicating a further change in the quantum well alloy composition. This has allowed distinguishing an aging mechanism that involves locally increased current, heating, and atom migration., QC 20100525

Time-resolved transmission and photoluminescence measurements were performed on Al0.35Ga0.65N/Al0.49Ga0.51N quantum well structures with different well widths. Comparison of transmission and luminescence data shows that dynamics of electrons and holes excited into extended quantum well states are governed by nonradiative recombination. For excitation into potential minima formed by band gap fluctuations, localization of electrons was observed. Excitation energy dependence of the pump-probe transient shape allows estimating localization potential, which is about 80 meV independently of the well width, and is probably caused by fluctuations of AlN molar fraction., QC 20100525

Photoexcited carrier dynamics in a 280 nm AlGaN quantum well (QW) light emitting diode has been studied by time-resolved photoluminescence at forward and reverse bias. Long ( for AlGaN QWs with high Al content) room temperature carrier lifetimes of about 600 ps were measured with only a slight dependence on bias. These lifetimes are much longer than calculated free carrier tunnelling and thermionic emission times, pointing out the importance of excitonic effects for carrier dynamics in AlGaN QWs., QC 20100525

Shift of the transition energy after pulsed optical excitation in Al0.35Ga0.65N/Al0.49Ga0.51N quantum well (QW) structures with varying well width has been studied by time-resolved photoluminescence. The shift dynamics, which is due to descreening of the intrinsic electric field, has characteristic times similar to carrier lifetimes revealing negligible influence of trapped carriers on screening. Comparison of the experimental spectral shifts with the calculations has shown that the intrinsic field in our AlGaN QWs is about 0.4-0.5 MV/cm, which is about a factor of two smaller than the value calculated using the theoretical polarization constants., QC 20110712

Intrinsic electric fields in AlxGa1-xN/AlyGa1-yN quantum wells embedded into p-i-n structures are studied using photoluminescence experiments. Spectral shifts induced by external bias and screening by photoexcited carriers allow evaluating the intrinsic fields caused by piezoelectric and spontaneous polarizations. In quantum wells with low Al content, the field is about 1 MV/cm, which is in agreement with theoretical estimations. For high Al molar fractions (35% well, 50% barrier), the extracted intrinsic field is lower and, most importantly, has the opposite sign to that predicted by the theory., QC 20100525

The project aimed at significant improvement of the III-nitride based epitaxial materials and device design and fabrication for high-power heterostructure field-effect transistors (HFETs). The key innovative approaches implemented in this program include novel pulsed atomic layer epitaxy (PALE) technique to grow the buffer layer with low defect density, improved epitaxial uniformity in multi-wafer MOCVD reactor, growing HFET wafers with the sheet resistance below 300 Ohm/square. Design improvements include double- heterostructure devices (DHFET) with InGaN electron confinement layer, insulated gate design using SiO2 gate insulator (MOSDHFETs) and innovative field-plate design. These new devices demonstrated high RF powers 15-20 W/mm at a drain bias of 50-65 V, and good parameter stability at 19 W/mm CW powers as confirmed by 100+ hours testing., The original document contains color images.

The electron delay time associated with the electron propagation across the FET gate barrier layer under high positive gate bias is expected to induce a dynamic negative differential conductance and enhance the growth of plasma waves in the channel 1. This dynamic negative conductance is related to the phase shift between the current and voltage waveforms caused by the electron time delay during the electron tunneling through the gate barrier. We present experimental investigations of the plasma wave detector responsivity at 200 GHz and 600 GHz radiation for long channel AlGaAs/GaAs and AlGaInN/GaN based HEMTs at 8 K and 300 K. The appearance of detector response correlated with an increase of the injected gate current under the forward gate bias is reported for both types of the investigated devices. Our results confirm that a large gate current can enhance the excitation of plasma waves., Pub. in IEEE Lester Eastman Conference on High Performance Devices, vPS-6, p135-142,2000. This article is from ADA423729 Proceedings IEEE Lester Eastman Conference on High Performance Devices at University of Delaware, Newark, Delaware, August 6, 7, and 8. 2002

We report on AlxInyGal-x-yN-GaN heterojunctions grown on sapphire and 6H/4H SiC substrates using a low pressure MOCVD. Based on a linear extrapolation of lattice constants as functions of molar fractions, we estimate that quaternary AlxInyGal-x-yN layers with Al/In mole fraction ratio of 5 should be nearly lattice-matched to GaN. The independent control of strain and of the band offset has been confirmed by X-ray and photoluminescence data. For AlxInyGal-x-yN-GaN junctions with barrier thickness less than 50 nm and varying alloy compositions (x from 0.1 to 0.2 and y from 0.00 to 0.02), room temperature sheet-carrier density and mobility values ranging from 0.5-1.5 x 1013cm ~2 and 1000-1500 cm2V-1sec-1 were measured. The mobility values increase by about a factor of 5 upon cooling to 77 K. This establishes the presence of the two-dimensional electron gas at the AlxInyGal-x-yN-GaN heterojunction interface and confirms a high quality of heterointerfaces. We also fabricated AlxInyGal-x-yN-GaN HFETs with the state-of-the art performance. The fabrication technology for these devices is substantially different from that for conventional AlGaN/GaN HEMTs. We will report on DC and microwave characteristics of these devices and compare these characteristics with those for conventional GaN-based HFETs.

We report on AlxInyGal-x-yN-GaN heterojunctions grown on sapphire and 6H/4H SiC substrates using a low pressure MOCVD. Based on a linear extrapolation of lattice constants as functions of molar fractions, we estimate that quaternary AlxInyGal-x-yN layers with Al/In mole fraction ratio of 5 should be nearly lattice-matched to GaN. The independent control of strain and of the band offset has been confirmed by X-ray and photoluminescence data. For AlxInyGal-x-yN-GaN junctions with barrier thickness less than 50 nm and varying alloy compositions (x from 0.1 to 0.2 and y from 0.00 to 0.02), room temperature sheet-carrier density and mobility values ranging from 0.5-1.5 x 1013cm ~2 and 1000-1500 cm2V-1sec-1 were measured. The mobility values increase by about a factor of 5 upon cooling to 77 K. This establishes the presence of the two-dimensional electron gas at the AlxInyGal-x-yN-GaN heterojunction interface and confirms a high quality of heterointerfaces. We also fabricated AlxInyGal-x-yN-GaN HFETs with the state-of-the art performance. The fabrication technology for these devices is substantially different from that for conventional AlGaN/GaN HEMTs. We will report on DC and microwave characteristics of these devices and compare these characteristics with those for conventional GaN-based HFETs.

We discuss the properties of the two dimensional (2D) electron gas at AlGaN/GaN heterointerface. The density of the 2D gas is affected by piezoelectric effects and by doping levels in both AlGaN and GaN layers. The record vatues of the sheet carrier density have been achieved wfth sheet electron densities of the two-dimensional electron gas on the order of 1.5 x 10(exp13)/cm2 and the sheet carrier concentration in the surface heterostructure channel as high as 4 x 10(exp13)/cm2. At high electron concentration, electrons are divided between the 2D and 3D states, and this division affects the electron mobility in the channeL Optical polar scattering, impurity scattering, and piezoelectric scattering limit the mobility. The largest electron mobility was observed in AIGaN/GaN heterostructures grown on SiC. At room temperature, the mobility is over 2,000 cm2/V-s; at cryogenic ten1peratures, the highest n%obdity is over 10,000 cm2/V-s. These values are high enough for the observation of the Quantum Hall Effect. Multichannel 2D electron structures are being developed that will allow us to achieve a much higher current density and much higher power., Pres: 6th Int Symp Nanostructures: Physics and Technology, St Petersburg, Russia. 22-26 Jun 1998. p524-530. This article is from ADA406591 Nanostructures: Physics and Technology International Symposium (6th) held in St. Petersburg, Russia on June 22-26, 1998 Proceedings

The selection is described of the powered support system for longwall 124, which is located under the goafs of the top layer seam no 510 at the Myslowice-Wesola coal mine in Poland. Issues are discussed which occurred due to significant variations in the thickness of the bottom layer extracted by longwall 124, and also resulting from the incorrect alignment of the powered support and the rock mass and the excessive wear of some elements in the longwall complex., The selection is described of the powered support system for longwall 124, which is located under the goafs of the top layer seam no 510 at the Myslowice-Wesola coal mine in Poland. Issues are discussed which occurred due to significant variations in the thickness of the bottom layer extracted by longwall 124, and also resulting from the incorrect alignment of the powered support and the rock mass and the excessive wear of some elements in the longwall complex.