S. Bidnyk’s research while affiliated with Oklahoma State University and other places

We present the results of an experimental study on stimulated emission (SE) in optically pumped InGaN/GaN multi-quantum well (MQW) structures in the temperature range of 175 K to 575 K. Samples used in this work consisted of 12 QWs and the GaN barriers were intentionally doped with different Si concentrations. The effect of doping on the SE thresholds of the MQWs were investigated. We observed that the SE spectra were comprised of many narrow peaks of less than I Å full width at half maximum (FWHM). No broadening of the FWHMs of the peaks occurred as the temperature was raised from 175 to 575 K. The SE threshold was measured as a function of temperature and compared with that of a thin GaN film. Low SE thresholds were attributed to high quantum efficiency of the MQWs, possibly associated with large carrier localization. A characteristic temperature of 162 K was derived from the temperature dependence of the SE threshold. The integrated emission intensity versus pumping density was examined for different temperatures. This study shows that InGaN/GaN MQWs are suitable for the development of laser diodes that can operate well above room temperature.

We report a comprehensive study on the optical properties of GaN- and AlGaN-based lasing structures at high-levels of optical excitation (carrier densities of 1017-1020 cm-3) and identify critical issues necessary for the development of near- and deep-UV light emitting devices. We successfully achieved room temperature stimulated emission (SE) with emission wavelengths ranging from 351 nm to 373 nm in a variety of samples. Through an analysis of the temperature-dependent lasing characteristics, combined with absorption and time-resolved photoluminescence measurements, we estimated the carrier density required to achieve the SE threshold in GaN epilayers. We found that in AlGaN epilayers, the onset of SE (∼1019 cm-3) occurs at carrier densities one order of magnitude higher than in thick GaN epilayers, indicating that an electron-hole plasma is the dominant gain mechanism over the entire temperature range studied (10 K to 300 K). A remarkably low lasing threshold was observed in GaN/AlGaN heterostructures over the temperature range of 10 K to 300 K. Our experimental results indicate that GaN/AlGaN heterostructures could be used to efficiently generate laser emission with wavelengths shorter than 373 nm. The implications of this study on the development of UV laser diodes is discussed.

The optical properties of (In, Al) GaN thin films and heterostructures have been compared under the conditions of strong nanosecond excitation. The stimulated emission (SE) threshold from AlGaN epilayers was found to increase with increasing Al content compared to GaN, in contrast to InGaN epilayers, where an order of magnitude decrease is observed. Optically pumped SE has been observed from AlGaN films with aluminum concentrations as high as 26%. Room temperature SE at wavelengths as low as 327 nm has been achieved. In contrast to the increase of SE threshold seen for AlGaN films, we found that AlGaN/GaN heterostructures which utilize carrier confinement and optical waveguiding drastically enhance the lasing characteristics. We demonstrate that AlGaN/GaN heterostructures are suitable for the development of deep ultraviolet laser diodes.

We have systematically studied the influence of Si doping on the optical characteristics of InGaN/GaN multiple quantum wells (MQWs) using photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy combined with studies of optically pumped stimulated emission and structural properties from these materials. The MQWs were grown on 1.8-µm-thick GaN layers on c-plane sapphire films by metalorganic chemical vapor deposition. The structures consisted of 12 MQWs with 3-nm-thick InGaN wells, 4.5-nm-thick GaN barriers, and a 0.1-µm-thick Al 0.07 Ga 0.93 N capping layer. The Si doping level in the GaN barriers was varied from 1 x 10 17 to 3 x 10 19 cm -3 . PL and PLE measurements show a decrease in the Stokes shift with increasing Si doping concentration. The 10 K radiative recombination lifetime was observed to decrease with increasing Si doping concentration (n), from ~ 30 ns (for n < 1 x 10 17 cm -3) to ~ 4 ns (for n = 3 x 10 19 cm -3). To elucidate whether non-radiative recombination processes affect the measured lifetime, the temperature-dependence of the measured lifetime was investigated. The reduced Stokes shift, the decrease in radiative recombination lifetime, and the increase in structural and interface quality with increasing Si doping indicate that the incorporation of Si in the GaN barriers results in a decrease in carrier localization at potential fluctuations in the InGaN active regions and the interfaces.

Single and multi-mode room temperature laser action was observed in GaN pyramids under strong optical pumping. The 5- and 15-micron-wide hexagonal-based pyramids were laterally overgrown on a patterned GaN/AlN seeding layer grown on a (111) silicon substrate by metal-organic chemical vapor deposition. The pyramids were individually pumped, imaged, and spectrally analyzed through a high magnification optical system using a high density pulsed excitation source. We suggest that the cavity formed in a pyramid is of a ring type, formed by total internal reflections of light off the pyramids' surfaces. The mode spacing of the laser emission was found to be correlated to the size of pyramids. The effects of pyramid geometry and pulse excitation on the nature of laser oscillations inside of the pyramids is discussed. Practical applications of the results for the development of light-emitting pixels and laser arrays are suggested.

This chapter explains many issues related to gain mechanisms, pump-probe experiments, microstructure lasing, imaging, and high-temperature optical properties of the material system, as well as provides some background information on the general properties of III-nitride thin films. The capacity of optical storage is limited by the spot size of the laser beam. The minimum spot size to which a laser can be focused is proportional to the wavelength of the laser light. Because of the shorter emission wavelength of lasers based on GaN thin films, storage and high-resolution printing are natural choices for GaN-laser-diode (LD) applications. To lower production costs, shorter wavelength and higher power LDs are necessary. There are some limitations, however, as to how short the wavelength needs to be. The organic photoreceptors used in printers should be sensitive to the laser wavelengths, and expensive optics should not be used. At low temperatures, near-band-edge luminescence spectra observed from most GaN samples are dominated by strong, sharp emission lines that result from the radiative recombination of free and bound excitons.

We investigated the effects of well thickness on spontaneous and stimulated emission (SE) in GaN/AlGaN separate confinement heterostructures (SCHs), grown by low pressure metal organic chemical vapor deposition. The SCH wells are unstrained and lattice matched to a GaN buffer layer. Our series of SCHs had GaN well thicknesses of 3, 5, 9, and 15 nm. We explain the spontaneous emission peak energy positions of the SCHs in terms of spontaneous and strain-induced piezoelectric polarizations. At 10 K, the carrier lifetime was found to be lowest for a 3 nm well, and the SE threshold was lowest for a 5 nm well. We show that the screening of the piezoelectric field and the electron–hole separation are strongly dependent on the well thickness and have a profound effect on the optical properties of the GaN/AlGaN SCHs. The implications of this study on the development of near- and deep-ultraviolet light emitters are discussed.

The effect of In on the structural and optical properties of InGa{sub 1-x}N/GaN multiple quantum wells (MQWs) was investigated. These were five-period MQW's grown on sapphire by metalorganic chemical vapor deposition. Increasing the In composition caused broadening of the high-resolution x-ray diffraction superlattice satellite peak and the photoluminescence-excitation bandedge. This indicates that the higher In content degrades the interface quality because of nonuniform In incorporation into the GaN layer. However, the samples with higher In compositions have lower room temperature (RT) stimulated (SE) threshold densities and lower nonradiative recombination rates. The lower RT SE threshold densities of the higher In samples show that the suppression of nonradiative recombination by In overcomes the drawback of greater interface imperfection.

We investigated the effects of well thickness on spontaneous and stimulated emission (SE) in GaN/AlGaN separate confinement heterostructures (SCHs), grown by low-pressure metalorganic chemical vapor deposition. The SCH wells are unstrained and lattice-matched to a GaN buffer layer. Our series of SCHs had GaN well thicknesses of 3, 5, 9, and 15 nm. We explain the spontaneous emission peak energy positions of the SCHs in terms of spontaneous and strain-induced piezoelectric polarizations. At 10 K, the carrier lifetime was found to be lowest for a 3 nm well, and the SE threshold was lowest for a 5 nm well. We show that the screening of the piezoelectric field and the electron-hole separation are strongly dependent on the well thickness and have a profound effect on the optical properties of the GaN/AlGaN SCHs. The implications of this study on the development of near- and deep-ultraviolet light emitters are discussed. © 2001 American Institute of Physics.

We achieved low-threshold ultra-violet lasing in optically pumped GaN/AlGaN separate confinement heterostructures over a wide temperature range. Lasing modes of a single microcavity were examined from 20 to 300 K and gain mechanisms were compared to those of a thick GaN epilayer. We have also systematically studied InGaN/(In)GaN multiple quantum wells as a function of well and barrier thickness. We demonstrate that the stimulated emission threshold and photoluminescence (PL) decay time are strongly dependent on the well and barrier thickness. The experimental results indicate that the enhanced optical quality of samples with larger barrier thicknesses can be readily applied to the fabrication of InGaN/(In)GaN laser diodes.