Y. H. Kwon

Time-resolved study of yellow and blue luminescence in Si- and Mg-doped GaN

Time-resolved photoluminescence has been employed to study the donor-acceptor pair recombination kinetics of the yellow (∼2.3 eV) and blue (∼2.8 eV) luminescence bands in Si- and Mg-doped GaN layers, respectively. As the Si doping concentration in Si-doped GaN increases, the lifetime τ1/e of the yellow luminescence decreases, indicating that a shallow Si donor is the origin of the yellow luminescence. The blue luminescence is most likely due to a shallow Mg acceptor and a deep donor composed of a Mg acceptor-nitrogen vacancy complex, as seen by the independence of τ1/e on the Mg concentration measured by secondary ion mass spectroscopy in the range (2.5–6.0)×1019 cm−3. As the temperature is increased from 10 to 300 K, the lifetimes for the yellow and blue luminescence remain nearly constant, indicating that the distribution of electrons and holes bound to donors and acceptors does not change much with increasing temperature.

Structural and optical characteristics of InxGa1−xN/GaN multiple quantum wells with different In compositions

We have studied the structural and optical properties of InxGa1−xN/GaN multiple quantum wells with different In compositions of 8.8%, 12.0%, and 13.3% by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), PL excitation (PLE), stimulated emission (SE), and time-resolved PL spectroscopy. As the In composition increases, the superlattice peaks in HRXRD measurements and the PLE band edge broaden, indicating the deterioration of interface quality due to the difficulty of uniform In incorporation into the GaN layer. However, the lower room-temperature SE threshold densities of the higher In concentration samples indicate that the effect of In suppressing nonradiative recombination overcomes the drawbacks associated with increasing interface imperfection.

Mechanism of efficient ultraviolet lasing in GaN/AlGaN separate-confinement heterostructures

We report the results of an experimental study on efficient laser action in an optically pumped GaN/AlGaN separate-confinement heterostructure (SCH) in the temperature range of 10–300 K. The lasing threshold was measured to be as low as 15 kW/cm2 at 10 K and 105 kW/cm2 at room temperature. Strongly polarized (TE:TM⩾300:1) lasing peaks were observed over the wavelength range of 358–367 nm. We found high-finesse lasing modes that originate from self-formed microcavities in the AlGaN and GaN layers. Through analysis of the relative shift between spontaneous emission and lasing peaks, combined with the temperature dependence of the lasing threshold, we conclude 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. Based on our results, we discuss possibilities for the development of ultraviolet laser diodes with a GaN active medium.

Well-thickness dependence of emission from GaN/AlGaN separate confinement heterostructures

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.

Optical Properties and Lasing in (In, Al)GaN Structures

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 he readily applied to the fabrication of InGaN/(In)GaN laser diodes.

Effect of Well Thickness on GaN/AlGaN Separate Confinement Heterostructure Emission

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.

Comparative Study of Emission from Highly Excited (In, Al) GaN Thin Films and Heterostructures

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, the authors found that AlGaN/GaN heterostructures which utilize carrier confinement and optical waveguiding drastically enhance the lasing characteristics. They demonstrate that AlGaN/GaN heterostructures are suitable for the development of deep ultraviolet laser diodes.

Comparison study of structural and optical properties of InxGa1-xN/GaN quantum wells with different In compositions

The effect of In on the structural and optical properties of In x Ga 1−x N/GaN multiple quantum wells (MQWs) was investigated. These were five-period MQWs 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.

Microstructure-Based Lasing in GaN/AlGaN Separate Confinement Heterostructures

The authors report on an experimental study of microstructure-based lasing in an optically pumped GaN/AlGaN separate confinement heterostructure (SCH). They achieved low-threshold ultra-violet lasing in optically pumped GaN/AlGaN separate confinement heterostructures over a wide temperature range. The spacing, directionality, and far-field patterns of the lasing modes are shown to be the result of microcavities that were naturally formed in the structures due to strain relaxation. The temperature sensitivity of the lasing wavelength was found to be twice as low as that of bulk-like GaN films. Based on these results, the authors discuss possibilities for the development of ultra-violet laser diodes with increased temperature stability of the emission wavelength.