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The spectral dependences of the nonequilibrium carrier lifetimes obtained by using time-resolved photoluminescence (PL) spectroscopy reveal new features in the carrier dynamics in AlGaN epilayers with high aluminum content. Three decay components can be traced in the PL decay. The fast, intermediate, and slow components are attributed to the decay of the free carriers, localized carriers, and the carriers trapped in deep states, respectively. The spectral dependence of the fast component is flat with a very weak dependence on temperature. At low temperatures, the intermediate decay component has the longest effective decay times in the vicinity of the PL band peak and exhibits a decrease on both high and low energy slopes. The spectral dependence of the lifetimes flattens at elevated temperatures. To interpret the decrease in the carrier lifetime at lower energies, we suggest the modification of the double-scaled potential profile in AlGaN alloys by introducing a gradual potential decrease to the low-potential regions instead of the abrupt walls of the regions previously assumed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The photoluminescence (PL) spectroscopy of AlGaN epilayers and heterostructures under quasi-steady-state conditions at different excitations in a wide temperature range from 8 to 300 K established the connection between the internal quantum efficiency, efficiency droop, and carrier localization. The carrier localization conditions were estimated from the temperature dependences of the PL band peak positions. The temperature dependences of the PL intensity were fitted using the Arrhenius-type equation with two activation energies, which were attributed to the carrier redistribution within the localized states and carrier delocalization, respectively. The delocalization of the localized carriers affects the luminescence efficiency via the enhancement of the two mechanisms of opposite signs: the bimolecular radiative recombination and nonradiative recombination due to an increased probability to reach nonradiative recombination centers. [ABSTRACT FROM AUTHOR]

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 ~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. [ABSTRACT FROM AUTHOR]

Stimulated emission (SE) is studied in AlGaN/AlGaN multiple quantum wells (MQWs) with different Al content grown on sapphire substrate. The spectra of spontaneous and stimulated emission and their transformations with increasing temperature as well as stimulated emission thresholds were measured in the temperature range from 8 to 300 K. Phonon-assisted band broadening in low-Al-content MQWs and double-scaled potential profile in high-Al-content MQWs were observed in the samples and linked with carrier localization conditions. The temperature dependence of the stimulated emission threshold was similar in the samples where the stimulated transitions occur between extended states and in the samples where the transitions occur in localized states. The stimulated emission threshold depends predominantly on the density of nonradiative recombination centers. [ABSTRACT FROM AUTHOR]

A set of Al0.35Ga0.65N/Al0.49Ga0.51N multiple quantum wells (MQWs) with fixed barrier width and well widths varying from 1.65 to 5.0 nm has been grown by metal-organic chemical vapor deposition. Carrier dynamics in the MQWs were studied using time-resolved photoluminescence (PL) spectroscopy and light-induced transient grating (four wave mixing) technique. The authors observed that the lifetime of nonequilibrium carriers (excitons) increases with decreasing well width and interpreted the effect by stronger localization preventing their migration to nonradiative recombination centers. Meanwhile the radiative decay time is also influenced by screening of the built-in electric field, which spatially separates the electrons and holes. It is shown that this effect affects the initial part of PL intensity decay after pulsed excitation. It becomes more pronounced with increase in the initial carrier density but saturates when the carrier density is high enough to completely screen the built-in electric field. The screening effect on PL decay is stronger in wider quantum wells. [ABSTRACT FROM AUTHOR]

AlGaN epilayers with different aluminum content have been studied by photoluminescence (PL) and light-induced transient grating (LITG) techniques. The epilayers were grown by conventional metal organic chemical vapor deposition (MOCVD) and by migration enhanced MOCVD (MEMOCVDTM). The carrier lifetime and PL intensity in epilayers containing the same amount of aluminum are shown to be inversely proportional to the dislocation density. which was determined using etch pit technique. The carrier diffusion length is shown to be close to the average distance between two first-neighbor dislocations. Enhancement of the lifetime and PL intensity in epilayers grown on sapphire by MEMOCVDTM is demonstrated. [ABSTRACT FROM AUTHOR]

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 excita-tion into potential minima formed by band gap fluctuations, localization of electrons was observed. Excitation energy dependence of the pump-probe transient shape allows estimating locali-zation potential, which is about 80 meV independently of the well width, and is prob-ably caused by fluctuations of AlN molar fraction. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Time-resolved photoluminescence and light-induced transient grating measurements of GaN epilayers show that the photoluminescence decay can be described by two coupled exponential terms and that carrier mobility and lifetime in GaN epilayers are correlated within the model which accounts for nonradiative carrier recombination predominantly at dislocations. The obtained results demonstrate that migration-enhanced metalorganic chemical vapor deposition (MEMOCVD™) allows for growth of high-quality GaN epilayers on sapphire substrates with the dislocation density close to 108 cm-2, carrier lifetime as long as 2 ns, and ambipolar diffusion coefficient of 2.1 cm2 s-1 corresponding to the hole mobility of approximately 40 cm2 V-1 s-1. [ABSTRACT FROM AUTHOR]

Monte Carlo simulation of phonon-assisted localized exciton hopping has been employed to describe the photoluminescence linewidth variation with temperature and to reveal band potential profile of ternary AlGaN epilayers with different carrier lifetimes. The lifetimes of 30 and 190 ps were experimentally determined in the layers with AlN buffers grown by conventional metal-organic chemical vapor deposition (MOCVD) and by migration-enhanced MOCVD (MEMOCVD™), respectively. The potential profile in AlGaN is shown to consist of double-scaled fluctuations. Exciton hopping in Al0.26Ga0.74N occurs within the random potential fluctuations (on the scale σ≈19 meV) in isolated low-potential regions with the average localization energy dispersed on the scale Γ≈19 meV. Such a pattern of band potential profile was found to be independent on the growth technique used for the deposition of their AlN buffer layers. This implies that the large difference in carrier lifetimes estimated in the AlGaN epilayers with the same Al content is caused by different densities of nonradiative recombination centers rather than by carrier localization in the potential fluctuations. [ABSTRACT FROM AUTHOR]

GaN epilayers grown on sapphire and SiC substrates and exhibiting yellow and blue luminescence were studied using photoluminescence spectroscopy, light-induced grating, and Kelvin probe techniques. For epilayers on sapphire substrates, we observed practically no yellow luminescence (YL) and lifetimes exceeding 100 ps in samples with the Fermi level close (∼0.5 eV) to conduction band, while samples with Fermi level close to the midgap exhibited YL. A general trend is a decrease in the intensity of yellow and blue emission bands compared to the intensity of the band-to-band recombination in the samples with longer carrier lifetimes. [ABSTRACT FROM AUTHOR]

The photoluminescence (PL) of AlGaN quantum wells with AlN barrier layers deposited on substrates fabricated of AlN single crystals is studied in the temperature range from 11 to 300 K under pulsed band-to-band excitation of the well material. The abnormal temperature dependence of the PL peak position and differences in the character of the peak shift with increasing excitation power density observed at low and elevated temperatures are interpreted in terms of carrier/exciton localization and screening of the built-in electric field. The formation of these localized states with narrow energy distribution and high density is favorable for efficient light emission. [ABSTRACT FROM AUTHOR]

Conductive undoped and semi-insulating vanadium-doped 6H-SiC substrates were studied using the light-induced transient grating technique and photoluminescence (PL) spectroscopy. Carrier lifetime of 400±10 ps and diffusion coefficient of 2.7±0.2 cm[sup 2] s[sup -1] were obtained for the nominally undoped wafer, while the corresponding parameters for the V-doped wafer were estimated to be 130±5 ps and 0.9±0.5 cm[sup 2] s[sup -1], respectively. The peak PL intensity in the vanadium-doped wafers is more than three orders of magnitude lower than that in nominally undoped wafers. Low-temperature cw PL spectra revealed a band peaked at 507 nm, which is caused by V doping. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) properties of two GaN epilayers grown in identical conditions on substrates of Al face and N face of bulk single-crystal AlN are studied in the temperature range from 8 to 300 K under weak cw excitation and strong pulsed excitation up to the intensities when electron–hole heating and stimulated emission are observed. At low temperatures and pump intensities, PL of GaN on Al-face AlN is consistent with that of homoepitaxial Ga-face GaN, while GaN on N-face AlN exhibits features indicating the existence of tail localized states. At carrier densities high enough for band-to-band transitions to dominate, the room-temperature PL of GaN on N-face AlN is higher than that in GaN on Al-face AlN due to longer effective lifetime of photoexcited carriers. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Temperature and excitation power dependences of the photoluminescence Stokes shift and bandwidth were studied in quaternary AlInGaN epilayers as a function of indium content. At low excitation power, gradual incorporation of indium into AlGaN is shown to result in S- and W-shaped temperature dependences of the band peak position and bandwidth, respectively. At high excitation power, the S- and W-behavior disappears; however, increased indium molar fraction boosts the redshift of the luminescence band at high temperatures. Our results indicate that the incorporation of indium into AlGaN has a noticeable impact on the alloy transport properties. At low temperatures and low excitation power, the indium incorporation facilitates hopping of localized excitons, whereas at high temperatures and high excitation power, it sustains free motion of delocalized carriers that results in the band-gap renormalization via screening. [ABSTRACT FROM AUTHOR]

Temperature-dependent photoluminescence (PL) measurements were performed for A-plane and C-plane bulk AlN single crystals and epitaxial layers on sapphire. A strong near-band-edge (NBE) emission and deep-level luminescence were observed. At low excitations, the emission spectra are dominated by free and bound excitonic transitions and their LO-phonon replicas. At high excitations, the broadening and redshift of the NBE band is attributed to dense electron-hole plasma formation. The PL spectra differences of bulk single crystals and epilayers is explained by the electron-hole plasma expansion peculiarities. [ABSTRACT FROM AUTHOR]

We present the results of a comparative photoluminescence (PL) study of GaN and InGaN-based epilayers, and InGaN/GaN multiple quantum wells (MQWs). Room-temperature PL spectra were measured for a very broad range of optical excitation from 10 mW/cm2 up to 1 MW/cm2. In contrast to GaN epilayers, all In-containing samples exhibited an excitation-induced blueshift of the peak emission. In addition, the blueshift of the emission in the InGaN epilayers with the same composition as the quantum well was significantly smaller. The comparison of the blueshift in the “bulk” InGaN and in the MQWs allowed us to separate two different mechanisms responsible for this effect: (i) filling of the localized states in In-rich areas and (ii) screening of the polarization electric field in strained MQW structures. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on observing a long-wavelength band in low-temperature photoluminescence (PL) spectrum of quaternary Al[sub 0.22]In[sub 0.02]Ga[sub 0.76]N/Al[sub 0.38]In[sub 0.01]Ga[sub 0.61]N multiple quantum wells (MQWs), which were grown over sapphire substrates by a pulsed atomic-layer epitaxy technique. By comparing the excitation-power density and temperature dependence of the PL spectra of MQWs and bulk quaternary AlInGaN layers, we show this emission band to arise from the carrier and/or exciton localization at the quantum well interface disorders. PL data for other radiative transitions in MQWs indicate that excitation-dependent spectra position is determined by screening of the built-in electric field. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The emission efficiency droop and internal quantum efficiency (IQE) in AlGaN epilayers and heterostructures were investigated by studying photoluminescence intensity dependence on excitation power density at different temperatures in the range from 8 to 300 K in three AlGaN samples with similar Al content (33%-35%) and different strength of carrier localization: an epilayer and multiple quantum wells with well widths of 5.0 and 2.5 nm. It is shown that the phenomena leading to the efficiency droop strongly influence the photoluminescence intensity dependence on temperature and, therefore, affect the estimation of IQE based on this dependence. A procedure to optimize the determination of IQE is proposed. [ABSTRACT FROM AUTHOR]

Presents a correct reprint to the article 'Near-band-edge photoluminescence of wurtzite-type AlN,' published in the 2002 issue of the 'Applied Physics Letters' journal.

We report on the comparative photoluminescence studies of AlGaN/GaN, GaN/InGaN, and AlInGaN/InGaN multiple quantum well (MQW) structures. The study clearly shows the improvement in materials quality with the introduction of indium. Our results point out the localized state emission mechanism for GaN/InGaN structures and the quantum well emission mechanism for AlInGaN/InGaN structures. The introduction of indium is the dominant factor responsible for the observed differences in the photoluminescence spectra of these MQW structures. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on the spectral dynamics of the reflectivity, site-selectively excited photoluminescence, photoluminescence excitation, and time-resolved luminescence in quaternary AlInGaN epitaxial layers grown on GaN templates. The incorporation of a few percents of In into AlGaN causes significant smoothening of the band-bottom potential profile in AlInGaN layers owing to improved crystal quality. An abrupt optical bandgap indicates that a nearly lattice-matched AlInGaN/GaN heterostructure with large energy band offsets can be grown for high-efficiency light-emitting devices. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Results on time-resolved study of GaN photoluminescence (PL) in a power density range from 0.5 mW/cm2 under CW excitation by ultraviolet light emitting diode (UV LED) to 1 GW/cm2 under pulsed excitation by YAG:Nd laser in the temperature range from 8 to 300 K are presented. Measurements of PL response in the frequency domain by using amplitude-modulated emission of a UV LED as well as time-resolved PL measurements using a streak camera and light-induced transient grating technique have been used in the study. Yellow luminescence (YL) intensity increases with increasing temperature up to ∼120 K and faster components in YL decay switch to slower components with increasing temperature under UV LED excitation. At low carrier densities, the trapping decreases the carrier lifetime below 250 ps, while the carrier lifetime in the same GaN sample under excitation ensuring saturation of the traps equals 2 ns. [Copyright &y& Elsevier]

Photoluminescence spectroscopy of AlGaN/AlGaN multiple quantum wells under quasi-steady-state conditions in the temperature range from 8 to 300 K revealed a strong dependence of droop onset threshold on temperature that was explained by the influence of carrier delocalization. The delocalization at room temperature results predominantly in enhancement of bimolecular radiative recombination, while being favorable for enhancement of nonradiative recombination at low temperatures. Studies of stimulated emission confirmed the strong influence of carrier localization on droop. [ABSTRACT FROM AUTHOR]