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 Al0.07Ga0.93N capping layer. The Si doping level in the GaN barriers was varied from 1 × 1017 to 3 × 1019 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 × 1017 cm−3) to ∼ 4 ns (for n = 3 × 1019 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.

We report the results of nondegenerate optical pump-probe absorption experiments performed on GaN and InGaN thin films and quantum wells under the conditions of strong optical band to band excitation. The evolution of the band edge in these materials was monitored as the number of photoexcited free carriers was increased beyond that required to achieve population inversion and observe stimulated emission. The band edge of InGaN is shown to exhibit markedly different high excitation behavior than that of GaN, explaining in part the reduction in stimulated emission threshold that typically accompanies the incorporation of indium into GaN to form InGaN. A comparison of the band edge absorption changes observed in pump-probe experiments to the gain spectra measured in variable-stripe gain experiments is also given.

We have systematically studied both the spontaneous and stimulated emission properties in blue-light-emitting InxGa1-xN/GaN multiple quantum well structures using various linear and nonlinear optical techniques. Our experimental observations are consistently understandable in the context of localization of carriers associated with, large potential fluctuations in the InxGa1-xN active regions and at heterointerfaces. The studies have been done as a function of excitation power density, excitation photon energy, excitation length, and temperature. The results show carrier localization features for spontaneous emission and demonstrate the presence of potential fluctuations in the InxGa1-xN active region of the InxGa1-xN/GaN structures and its predominant role in spontaneous emission. In addition, the experimental observations strongly indicate that the stimulated emission has the same microscopic origin as spontaneous emission, i.e., radiative recombination of localized states. Therefore, we conclude that carriers localized at potential fluctuations in InxGa1-xN active layers and interfaces can play a key role in not only spontaneous but also stimulated emission of state-of-the-art blue-light-emitting InxGa1-xN/GaN quantum structures.

We have systematically studied both the spontaneous and stimulated emission properties in blue-light-emitting ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ multiple quantum well structures using various linear and nonlinear optical techniques. Our experimental observations are consistently understandable in the context of localization of carriers associated with large potential fluctuations in the ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{}$ active regions and at heterointerfaces. The studies have been done as a function of excitation power density, excitation photon energy, excitation length, and temperature. The results show carrier localization features for spontaneous emission and demonstrate the presence of potential fluctuations in the ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ active region of the ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ structures and its predominant role in spontaneous emission. In addition, the experimental observations strongly indicate that the stimulated emission has the same microscopic origin as spontaneous emission, i.e., radiative recombination of localized states. Therefore, we conclude that carriers localized at potential fluctuations in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ active layers and interfaces can play a key role in not only spontaneous but also stimulated emission of state-of-the-art blue-light-emitting ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{GaN}$ quantum structures.

We report the results of an experimental study on stimulated and spontaneous emission from high-quality single-crystal GaN films grown on 6H-SiC and (0001) sapphire substrates in the temperature range of 300–700 K. We observed edge-emitted stimulated emission (SE) at temperatures as high as 700 K for samples grown on both SiC and sapphire substrates. The energy position of the SE and spontaneous emission peaks were shown to shift linearly to longer wavelengths with temperature and empirical expressions for the energy positions are given. We demonstrate that the energy separation between the spontaneous and SE peaks gradually increases from 90 meV at 300 K to 200 meV at 700 K indicating that an electron-hole plasma is responsible for the SE mechanism in this temperature range. The temperature sensitivity of the SE threshold for different samples was studied and the characteristic temperature was found to be 173 K in the temperature range of 300–700 K for one of the samples studied. We suggest that the unique properties of SE in GaN thin films at high temperatures could potentially be utilized in optoelectronic devices.

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.

Optically pumped stimulated emission (SE) from InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition has been systematically studied as a function of excitation length (Lexc). Two distinct SE peaks were observed from these structures: one that originates at 425 nm at 10 K (430 nm at 300 K) and another that originates at 434 nm at 10 K (438 nm at 300 K). The SE threshold for the high energy peak was observed to always be lower than that of the low energy peak, but the difference was found to decrease greatly with increasing Lexc. A detailed study of the emission intensity of these two SE peaks as a function of excitation density shows that the two peaks compete for gain in the MQW active region.

We present the results of a detailed study of the band edge absorption changes in GaN and InGaN thin films induced by the presence of excess photo-generated free carriers. The 1s A and B free excitonic resonances in GaN are shown to decrease with increasing above-gap nanosecond optical excitation due to screening by free carriers and exciton–exciton scattering. The decrease in excitonic absorption with increasing above-gap excitation is accompanied by a significant increase in the below-gap absorption coefficient. To further explore this behavior, femtosecond nondegenerate optical pump–probe experiments were also performed using an amplified Ti : sapphire laser. Exciton saturation due to screening by free carriers and excitonic phase space filling was again observed, along with a modest amount of below-gap induced absorption attributed to band gap renormalization. Similar experiments were performed on InGaN thin films. The band edge absorption changes observed in InGaN films were found to be significantly different than those observed in GaN.

We report the results of an experimental study on near-threshold gain mechanisms in optically pumped GaN epilayers in the temperature range of 20–700 K. High-quality single-crystal GaN films grown on 6H–SiC and (0001) sapphire were used in this study. We show that the dominant near-threshold gain mechanism is inelastic exciton–exciton scattering for temperatures below ∼150 K, characterized by band-filling phenomena and a low stimulated emission (SE) threshold. An analysis of both the temperature dependence of the SE threshold and the relative shift between stimulated and band-edge-related emission indicates electron–hole plasma is the dominant gain mechanism for temperatures exceeding 150 K. Based on our results, we discuss possibilities of reducing the room-temperature lasing threshold in laser diode structures with a GaN active medium.

Summary form only given. At present, the main focus of III-V nitride research is the optimization of current-injected laser diodes in order to achieve a low lasing threshold and extend the lifetime of working devices. Whereas a significant amount of work has been dedicated to such issues as the choice of substrate, facet formation, measurement of far-field emission patterns, as well as the study of temperature effects on lasing characteristics, the subject of optical confinement has not been adequately addressed. In this work we introduce a novel technique for investigation of optical confinement in GaN-based lasing structures, which utilizes both high spatial and spectral resolution of sample emission.

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 AIGaN 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 AIGaN 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 AIGaN/GaN heterostructures are suitable for the development of deep ultraviolet laser diodes.

Femtosecond pump-probe measurements were performed in GaN epilayers to study carrier dynamics in the band edge region. Excitonic absorption was found to begin saturating at a pump fluence of 20 mu J/cm(2) which corresponds to an estimated carrier density of 1 x 10(18) cm(-3). At zero delay between pump and probe, induced absorption is observed below the unpumped band gap due to ultrafast bandgap renormalization. After 375 fs, a large induced transparency is observed just below the excitonic resonance which is due to a transient electron-hole plasma. After 1 ps, the absorption has partially recovered to a level associated with excitonic phase-space filling. The absorption then recovers with a characteristic time of similar to 20 ps, a value which increases with increasing excitation density.

We present a technique for evaluating optical confinement in GaN-based lasing structures by studying their spectrally resolved near-field pattern under high optical excitation. Emission spectra were found to be strongly dependent on the position of the collection optics relative to the active region when the sample was excited above the lasing threshold. The spatially resolved spectra contain a modulation signature that can be used to deduce the optical confinement characteristics. We show that the observed index-guided modes result from multiple internal reflections at angles very close to the critical angle for total internal reflection between the semiconductor layers with different refractive indices. This technique was used to evaluate the degree of optical confinement in GaN epilayers and GaN/AlGaN separate confinement heterostructures. The implications of this study on the design of GaN-based laser diodes are discussed.

Summary form only given. The nonlinear optical properties of highly excited InGaN/GaN multiple quantum well (MQW) blue laser structures have been studied using nondegenerate optical pump-probe spectroscopy, energy selective optically pumped stimulated emission (SE) spectroscopy, and variable stripe gain spectroscopy. The evolution of the near band edge transitions of the InGaN active layers were monitored using nondegenerate optical pump-probe transmission spectroscopy as the number of photoexcited free carriers was increased beyond that necessary to achieve population inversion. Clear bleaching of band tail states was observed with increasing optical excitation.

We report the results of an experimental study on near-threshold gain mechanisms in optically pumped GaN epilayers in the temperature range of 20–700 K. High-quality single-crystal GaN films grown on 6H–SiC and (0001) sapphire were used in this study. We show that the dominant near-threshold gain mechanism is inelastic exciton–exciton scattering for temperatures below ∼150 K, characterized by band-filling phenomena and a low stimulated emission (SE) threshold. An analysis of both the temperature dependence of the SE threshold and the relative shift between stimulated and band-edge-related emission indicates electron–hole plasma is the dominant gain mechanism for temperatures exceeding 150 K. Based on our results, we discuss possibilities of reducing the room-temperature lasing threshold in laser diode structures with a GaN active medium.

Stimulated Emission and Pump-Probe studies were performed in GaN, InGaN, and AlGaN epilayers as well as GaN/AlGaN separate confinement heterostructures (SCHs). We show that in GaN epilayers the near-threshold gain mechanism is inelastic exciton-exciton scattering for temperatures below similar to 150 K, whereas at elevated temperatures 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. We further performed optical pumping of AlGaN epilayers at different temperatures. Stimulated emission has been observed in AlxGa1-xN thin films for Al concentrations as high as x = 0.26, with a resultant stimulated emission wavelength as low as 328 nm at room temperature. This result indicated that AlGaN-based structures are suitable not only for use in deep-ultraviolet detectors, but also as a potential source of deep-ultraviolet laser radiation. The interband optical transitions in GaN and InGaN have also been studied at 10 K and room temperature using nondegenerate nanosecond optical pump-probe techniques. At low temperatures, strong, well-resolved features were seen in the absorption and reflection spectra corresponding to the Is A and B exciton transitions. Broadening and decrease in intensity of these features were studied as the function of excitation pump density. We found that values of induced transparency and induced absorption are extremely large in GaN epilayers. The pump-probe results in GaN epilayers were directly compared to ones obtained from InGaN films. Significant differences in near-bandedge absorption changes were clearly observed between the two materials.

The nonlinear optical properties of band tail states in highly excited InGaN/GaN multiple quantum wells have been studied using energy selective optically pumped stimulated emission spectroscopy, nanosecond nondegenerate optical pump-probe spectroscopy, and variable-stripe gain spectroscopy. Energy selective optically pumped spontaneous and stimulated emission studies show mobility edge type behavior of the spontaneous and stimulated emission peak positions as the excitation photon energy is tuned across the states responsible for the broadened absorption edge of the InGaN active regions. The relative position of the mobility edge with respect to the absorption edge and the spontaneous and stimulated emission peak positions indicates the emission originates from carriers localized by extremely large potential fluctuations in the InGaN active layers of the MQWs. Nanosecond nondegenerate optical pump-probe spectroscopy of the band edge transitions show strong bleaching of band tail states with increasing above-gap optical excitation. The magnitude of the bleaching was found to be significantly affected by the onset of stimulated emission, indicating the carriers responsible for the observed bleaching and stimulated emission share the same recombination channels. Optical gain studies show substantial blueshifts in the gain maximum with increasing above-gap optical excitation. The behavior is attributed to filling of localized states with increasing optical excitation. The spectral range covered by the blueshift further evidences the large magnitude of the potential fluctuations. The experimental results are compared with those obtained from an InGaN layer of comparable indium composition. This work illustrates the dominance of localized state recombination, both spontaneous and stimulated, in the emission spectra of state-of-the-art InGaN MQW structures.

We have studied both the spontaneous and stimulated emission (SE) properties as a function of excitation photon energy for InGaN/GaN multiple quantum wells (MQWs). A significant redshift of the SE peak with decreasing excitation photon energy was observed as the excitation photon energy was tuned below a certain photon energy (“mobility edge”) for the InGaN/GaN MQWs, with similar behavior observed for the spontaneous emission. The relative position of the mobility edge with respect to the absorption edge and the spontaneous and stimulated emission peak positions indicates the emission originates from carriers localized by extremely large potential fluctuations in the InGaN active layers of the MQWs. Therefore, carrier localization in the InGaN active regions explains the observed spontaneous and stimulated emission behaviors of these materials.

Stimulated emission (SE) was systematically studied in GaN epilayers and InGaN/GaN multiquantum wells (MQWs) over a wide temperature range. We demonstrate that at temperatures below 150 K excitonic-related gain mechanisms dominate the near-threshold emission behavior of GaN, For temperatures above 150 K, electron-hole plasma recombination is responsible for SE mechanism in GaN epilayers for all excitation densities employed. The emission spectra for the InGaN-based structures were found to be drastically different than those of GaN over the entire temperature range studied. The incorporation of In into GaN was found to reduce the SE threshold by an order of magnitude. The SE spectra from InGaN/GaN MQWs were comprised of extremely narrow lines and no broadening of the lines was observed as the temperature was raised by several hundred degrees. The SE behavior for the InGaN/GaN MQWs is explained in terms of carrier localization at potential fluctuations in the InGaN layers. The temperature sensitivity of the SE threshold of InGaN/GaN samples was measured and compared with GaN epilayers. This study demonstrates that the SE mechanisms in InGaN/GaN MQWs an GaN are distinctly different.

We have investigated the optical properties of InGaN, GaN, and AlGaN epilayers using photoluminescence (PL), PL excitation, time-resolved PL, and optically pumped stimulated emission (SE) spectroscopy. The InGaN layers had a larger (i) Stokes shift, (ii) spectral broadening, (iii) decay time, and (iv) PL redshift with time than AlGaN layers of comparable alloy composition. The optically pumped SE behavior of InGaN layers is significantly different than that of GaN and AlGaN layers. This is attributed to the suppression of nonradiative recombination and the elimination of below-gap induced absorption in highly excited InGaN alloys, effects that accompany the incorporation of indium into GaN. In contrast, the SE behavior of AlGaN layers is similar to that of highly excited GaN.