Jack B. Lam’s research while affiliated with Oklahoma State University and other places

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.

We report experimental and theoretical studies of the excitonic optical Stark effect in GaN photoexcited below the excitonic resonances with various polarization configurations and pump detunings, using nondegenerate pump-probe spectroscopy at 10 K. We observed that the Stark effect in GaN is strongly dependent on pump and probe relative linear polarizations. We found that this dependence results from the small spin-orbit splitting in GaN and a mixing of A and B valence bands induced by a linearly polarized pump. Using two different circular polarization configurations, we also observed splitting of degenerate excitons because of different optical Stark shifts. Our experimental results are explained by a simple theoretical model.

Optical and X-ray studies of MOCVD-grown InGaN epilayers with low indium concentration G. H. Park, S. J. Hwang, S. K. Shee, T. Sugahara, J. B. Lam, G. H. Gainer and J. J. Song, Center for Laser and Photonics Research and Department of Physics, Oklahoma State University, Stillwater, OK 74078, USA; S. Sakai, Electrical and Electronic Department, University of Tokushima, Tokushima, Japan. In_xGa_1-xN epilayers with low indium concentration (x < 5%) were grown by low pressure metalorganic chemical vapor deposition on (0001) sapphire. These samples were characterized by optical techniques and high-resolution X-ray diffraction. Photoluminescence (PL) and stimulated emission (SE) were measured. The PL intensity of the InGaN epilayers is much higher than that of GaN, even for very small indium concentrations. The PL peaks show the S-shaped temperature dependence, and the stimulated emission threshold is also temperature dependent. The PL and SE also vary greatly with indium concentration. These observations indicate that the way indium incorporates into GaN varies with In concentration. The structural characteristics will be discussed in light of their possible relation to the optical characteristics. This work is supported by ONR, BMDO, and AFOSR.

We report the results of an experimental study on near- threshold gain mechanism in optically pumped GaN epilayers and GaN/AlGaN separate confinement heterostructures (SCHs) over the temperature range of 10 to 300 K. We show that in GaN epilayers the near-threshold gain mechanism is inelastic exciton-exciton scattering for temperatures below approximately 150 K, whereas at elevated temperatures an electron-hole plasma is the dominant gain mechanism. An analysis of the relative shift between the spontaneous emission and lasing peaks in SCH samples, combined with the temperature dependence of the lasing threshold, reveals that exciton-exciton scattering is the dominant gain mechanism leading to low-threshold ultraviolet lasing in the GaN/AlGaN SCH over the entire temperature range studied. Strongly polarized (TE:TM > 300:1) lasing peaks were observed in a wavelength range of 358 - 367 nm. We found that high finesse lasing modes originated from self-formed microcavities in the AlGaN and GaN layers. The lasing threshold was measured to be as low as 15 kW/cm2 at 10 K and 105 kW/cm2 at room temperature. Based on our results we suggest ways for the realization of GaN-active-medium UV laser diodes.

We report the results of an experimental study on near-threshold gain mechanisms in optically pumped GaN epilayers and GaN/AlGaN separate confinement heterostructures (SCHs) in the temperature range of 10 to 300 K and identify critical issues necessary for the development of near- and deep-UV laser diodes. We show that the near-threshold gain mechanism in GaN epilayers is inelastic exciton-exciton scattering for temperatures below ~150 K, whereas at elevated temperatures an electron-hole plasma is the dominant gain mechanism. The Mott density in GaN (density at which excitons dissociate) was estimated to be 1.1× 10^18 cm-3. By improving carrier and optical confinement we were able to extend the exciton-exciton scattering gain mechanism up to room temperature in GaN/AlGaN SCHs. A remarkably low lasing threshold (carrier density substantially below the Mott density) was achieved in SCH heterostructures over the temperature range of 10 to 300 K. The implications of this study on the development of UV laser diodes will be discussed. This work was supported by AFOSR and BMDO.

We report the results of an experimental study of Al_xGa_1-xN epilayers grown by metal-organic chemical vapor deposition under high optical excitation conditions in the temperature range of 30 to 300 K. Deep-ultraviolet stimulated emission (SE) was observed in samples with Al compositions as high as 26%. We found that at room temperature the SE thresholds in our samples were comparable to those of high-quality GaN epilayers. The bandgap and the energy position of spontaneous and stimulated emission peaks were measured over the entire temperature range studied. Through an analysis of the temperature dependence of the relative energy positions and the SE threshold, combined with time-resolved photoluminescence measurements, we estimated the carrier density at the threshold to be ~10^19 cm-3. Such a high carrier density indicates that an electron-hole plasma is responsible for the generation of gain in this material system from 30 to 300 K. Issues related to the development of short-wavelength AlGaN-based light emitting devices will be discussed. This work was supported by ONR and DARPA.