E. Kuokštis

III–Nitride UV Devices

The need for efficient, compact and robust solid-state UV optical sources and sensors had stimulated the development of optical devices based on III–nitride material system. Rapid progress in material growth, device fabrication and packaging enabled demonstration of high efficiency visible-blind and solar-blind photodetectors, deep-UV light-emitting diodes with emission from 400 to 250 nm, and UV laser diodes with operation wavelengths ranging from 340 to 350 nm. Applications of these UV optical devices include flame sensing; fluorescence-based biochemical sensing; covert communications; air, water and food purification and disinfection; and biomedical instrumentation. This paper provides a review of recent advances in the development of UV optical devices. Performance of state-of-the-art devices as well as future prospects and challenges are discussed.

Excitation power dynamics of photoluminescence in InGaN/GaN quantum wells with enhanced carrier localization

Excitation-power dynamics of near-band-edge photoluminescence (PL) peak position in InxGa1−xN∕GaN multiple quantum wells (x∼0.15) was analyzed as a function of well width. The analysis was based on energy reference provided by photoreflectance (PR) spectra. The difference in spectral position of the PR feature and low-excitation PL band (the Stokes Shift) revealed carrier localization energy, which exhibited a remarkable sensitivity to the well width, increasing from 75meV in 2nm wells to about 250meV in 4nm wells. Meanwhile collating of the PR data with the flat-band model for the optical transition energy in quantum wells rendered a relatively weak (0.5MV∕cm) built-in piezoelectric field. The blueshift of the PL peak position with increasing photoexcitation power density was shown to be in qualitative agreement with the model of filling of the band-tail states with some contribution from screening of built-in field in the thickest (4nm) wells. Increased incident photon energy resulted in an additional b...

Well-width-dependent carrier lifetime in AlGaN∕AlGaN quantum wells

A set of Al0.35Ga0.65N∕Al0.49Ga0.51N multiple quantum wells (MQWs) with fixed barrier width and well widths varying from 1.65to5.0nm 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 s...

Confocal spectroscopy of InGaN LED structures

Photoluminescence of InGaN structures for green light-emitting diodes (LEDs) with multiple quantum wells as an active medium was studied with spatial and spectral resolution using confocal microscopy. Bright spots of ~200u2009nm diameter were observed. Emission from these bright areas was up to 8 times more intense than from the rest of the sample surface and the band peak position in these areas was blueshifted with respect to the band position in the background surface of lower photoluminescence intensity. The data on emission properties in bright and dark areas and the dependence of these properties on the excitation power density were interpreted by assuming inhomogeneous distribution of defects acting as nonradiative recombination centres.

Study of Charge Carrier Transport in GaN Sensors

Capacitor and Schottky diode sensors were fabricated on GaN material grown by hydride vapor phase epitaxy and metal-organic chemical vapor deposition techniques using plasma etching and metal deposition. The operational characteristics of these devices have been investigated by profiling current transients and by comparing the experimental regimes of the perpendicular and parallel injection of excess carrier domains. Profiling of the carrier injection location allows for the separation of the bipolar and the monopolar charge drift components. Carrier mobility values attributed to the hydride vapor phase epitaxy (HVPE) GaN material have been estimated as μe = 1000 ± 200 cm2/Vs for electrons, and μh = 400 ± 80 cm2/Vs for holes, respectively. Current transients under injection of the localized and bulk packets of excess carriers have been examined in order to determine the surface charge formation and polarization effects.

Optical Characterization of MBE-Grown ZnO Epilayers

Optical characterization of molecular-beam-epitaxy-grown ZnO and MgZnO epitaxial layers on sapphire (0001) substrates is presented. The parameters such as carrier recombination time and optical gain coefficient are analyzed. Radiative recombination mechanisms of ZnO in dense quasiparticle system are discussed. The ZnO epilayers even with lower structural quality are tolerable for applications in optoelectronics as light emitters.

Temperature dependent double blueshift of photoluminescence peak position in MgZnO epitaxial layers

Temperature dependent photoluminescence (PL) of MgZnO epitaxial layers with high Mg content were studied to understand the effect of carrier localization on the PL dynamics, including the PL dependence on excitation power density and temperature. A double blueshift of the PL peak position with increase of measurement temperature was discovered. The blueshift took place at low as well as high temperature and could be attributed to the effect of carrier localization. It has been deduced that the randomly distributed carrier localization centers in the MgZnO films create two energy separated Gaussian-shape density-of-states tails in the vicinity of the fundamental band gap edge. Filling of these tail states by the thermally activated carriers with increase of temperature causes the temperature-induced double blueshift of the PL peak position. By analyzing the temperature dependent PL spectra, two parameters, σ and γ were extracted, which characterize the average energy depth distribution of the localizing po...

Photoluminescence dynamics in highly nonhomogeneously excited GaN

The analysis of the room-temperature photoluminescence (PL) dynamics in GaN 4.5-μm-thick layers under different excitation intensities revealed nonexponentional PL decay with the fast and slow transients. The fast decay is clearly resolved under lower excitation ( 100MW∕cm2). Theoretical modeling and comparison of the calculated results with the experimental data showed that the observed PL behavior is determined by the space nonhomogeneity of carriers generated by strongly absorbed 25ps duration laser pulse, the diffusion processes, as well as by the surface recombination. The latter mechanism plays an important role in the PL decay at lower excitation intensities, whereas the surface recombination saturates at high excitation levels. The radiative bimolecular recombination along with the stimulated emission predominates under these circumstances. The extracted carrier parameters are in good agreement with the values measured using other techniques.

Stimulated Emission from Wide-Gap Semiconductors

Stimulated emission and optical gain problems are reviewed on the basis of analysis of highly photoexcited III-Nitride and ZnO wide-gap materials. Spontaneous and stimulated emission along with optical gain of these materials in UV region is analyzed. The possible mechanisms of inverted population in In-containing materials and ZnO semiconductors are discussed involving mechanisms of dense electron-hole plasma, carrier (exciton) localization, as well as various interaction processes in dense exciton gas.

Carrier Localization and Decay in Wide‐band‐gap AlGaN/AlGaN Quantum Wells

Carrier dynamics in wide‐band‐gap AlGaN/AlGaN multiple quantum wells (MQWs) has been investigated by studying a set of MQWs with well widths varying from 1.65 to 5.0 nm. The structures were grown by metal‐organic chemical vapor deposition and investigated using photoluminescence spectroscopy under transient and quasi‐steady‐state conditions and light‐induced transient grating technique. It is demonstrated that the fast initial PL intensity decay is caused by recovery of built‐in electric field, while the carrier lifetime is strongly influenced by carrier localization. The scale of the localization was estimated by fitting the results of Monte Carlo simulation of exciton hopping with experimental PL linewidth dependence on temperature.