Kunimichi Omae

Room-Temperature CW Lasing of a GaN-Based Vertical-Cavity Surface-Emitting Laser by Current Injection

We report the demonstration of CW lasing at room temperature in a GaN-based vertical-cavity surface-emitting laser (VCSEL) by current injection. The active region of the VCSEL consisted of a two-pair InGaN/GaN quantum well active layer. The optical cavity consisted of a 7-λ-thick GaN semiconductor layer and an indium tin oxide layer for p-contact embedded between two SiO2/Nb2O5 dielectric distributed Bragg reflectors. The VCSEL was mounted on a Si substrate by wafer bonding and the sapphire substrate was removed by laser lift-off. Under CW operation for an 8-µm aperture device, the threshold current was 7.0 mA and the emission wavelength was approximately 414 nm.

Improvement in Lasing Characteristics of GaN-based Vertical-Cavity Surface-Emitting Lasers Fabricated Using a GaN Substrate

We compared the lasing characteristics of GaN-based vertical-cavity surface-emitting lasers (VCSELs) fabricated using a GaN substrate with those fabricated using a sapphire substrate. The original substrates are removed from the devices after the devices have been bonded to Si substrates. Consequently, with the exception of the cavity length, the two kinds of fabricated VCSELs have almost the same structures. The VCSELs fabricated using a GaN substrate have a higher maximum output power (0.62 mW) and longer lifetimes than those fabricated using a sapphire substrate. Even for the VCSELs fabricated with a GaN substrate, 10-min operation causes their threshold current to increase.

Radiative and nonradiative recombination processes in GaN-based semiconductors

Time-resolved optical characterization is an indispensable tool to study the recombination mechanisms of excitons and/or carriers based on radiative, non-radiative, localization and many-body processes. In this paper, we review the instrumentation of various spectroscopic techniques for the assessment of In x Ga 1 -x N-based semiconductors such as time-resolved photoluminescence (TRPL), time-resolved electroluminescence (TREL), transient grating (TG) method to probe photothermal processes, microscopic TRPL using optical microscope, submicroscopic TRPL using scanning near field optical microscopy (SNOM) and pump-and-probe spectroscopy for the measurement of transient absorption/gain spectra. The obtained results are cited in the references.

Dimensionality of excitons in InGaN-based light emitting devices

Temperature dependence of radiative and non-radiative recombination times has been investigated in InxGa1–xN based light emitting devices by employing time-resolved luminescence spectroscopy. The mean In-composition (x value) assessed in this study is 10%, 20% and 30% whose emissions at 300 K correspond to near ultraviolet (390 nm), violet (422 nm) and blue (471 nm), respectively. It was found that the degree of exciton localization was enhanced with increasing In-composition in InxGa1–xN active layers, and that the zero-dimensional feature was revealed best of all in the sample with x = 30%, where radiative recombination time was almost constant (4 to 6 ns) in the temperature range from 20 to 300 K. The internal quantum efficiency of this sample was estimated to exceed 70% at 300 K. It is likely that such high efficiency is a result of zero-dimensionality because capture cross-sections to non-radiative recombination centers are greatly reduced once excitons are trapped at deep localization centers.

Dynamics of optical gain in InxGa1-xN multi-quantum-well-based laser diodes

Dynamical behavior of optical gain formation has been assessed at room temperature in the InxGa1−xN multi-quantum-well (MQW) based laser diodes (LDs) by employing pump and probe spectroscopy with a pulse width of 150 fs. The LDs are composed of (a) In0.1Ga0.9N–In0.02Ga0.98N MQW and (b) In0.3Ga0.7N–In0.05Ga0.95N MQW, whose stimulated emissions correspond to near ultraviolet (390 nm) and blue (440 nm), respectively. The optical gain was contributed from the nearly delocalized states [the lowest-quantized MQW levels (LQL)] in the sample (a), while it was from highly localized levels with respect to the LQL by 500 meV for the sample (b). It was found that the photogenerated carriers rapidly (less than 1 ps) transferred to the LQL, and then relaxed to the localized tail within the time scale of about 5 ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of the LQL under higher photoexcitation.

In inhomogeneity and emission characteristics of InGaN

Recombination dynamics of spontaneous and stimulated emissions have been assessed in InGaN-based light emitting diodes (LEDs) and laser diodes (LDs), by employing time-resolved photoluminescence and pump and probe spectroscopy. As for an In0.02Ga0.98N ultraviolet LED, excitons are weakly localized by 15 meV at low temperature, but they become almost free at room temperature (RT). It was found that addition of a small amount of In results in the reduction of nonradiative recombination centres originating from point defects. The internal electric field does exist in InGaN active layers, and induces a large modification of excitonic transitions. However, it alone does not explain the feature of spontaneous emission observed in an In0.3Ga0.7N blue LED such as an anomalous temperature dependence of peak energy, almost temperature independence of radiative lifetimes and mobility-edge type behaviour, indicating an important role of exciton localization. The lasing mechanism was investigated for In0.1Ga0.9N near ultraviolet (390 nm), In0.2Ga0.8N violet-blue (420 nm) and In0.3Ga0.7N blue (440 nm) LDs. The optical gain was contributed from the nearly delocalized states (the lowest quantized levels (LQLs) within quantum wells) in the violet LD, while it was from highly localized levels with respect to the LQL by 250 meV for the violet-blue LD, and by 500 meV for the blue LD. It was found that the photo-generated carriers rapidly (less than 1 ps) transferred to the LQL, and then relaxed to the localized tail within the timescale of a few ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of the LQL under higher photo-excitation.

Nondegenerated Pump and Probe Spectroscopy in InGaN-Based Semiconductors

Carrier dynamics in two types of InGaN-based semiconductors were investigated using nondegenerated pump and probe spectroscopy. The samples consist of an In 0.1 Ga 0.9 N active layer, 30 nm thick (sample a) or an In 0.1 Ga 0.9 N/GaN multiple quantum well with ten periods, 3 nm/10 nm thick (sample b). Pump and probe spectroscopy revealed that photo-generated carriers pumped at 370 nm (3.350 eV) rapidly reach the bottom of levels in the active layers within a time scale of several ps for both samples. For the sample a, a positive peak at 3.21 eV was sandwiched between two negative peaks at 3.29 and 3.17 eV in the spectra of photo-induced change of absorption coefficient (Aa) because Franz-Keldysh and Stark effects on excitonic absorption were almost eliminated due to the screening of internal electric field. However, for the sample b, only negative signals were observed in Δα, indicating that the band filling of localized states plays a more important role. This is because the effect of electric field on the optical transition property is different depending on well widths.

Dynamics of spontaneous and stimulated emissions in GaN-based semiconductors

Recombination dynamics of spontaneous and stimulated emissions have been assessed in InGaN-based light emitting diodes (LEDs) and laser diodes (LDs), by employing time- resolved photoluminescence and pump and probe spectroscopy. As for an In0.02Ga0.98N-ultraviolet-LED, excitons are weakly localized by 15 meV at low temperature, but they become almost free at room temperature. It was found that addition of small amount of In results in the reduction of nonradiative recombination centers originating from point defects. The internal electric field does exist in InGaN active layers, and induces a large modification of excitonic transitions. However, it alone does not explain the feature of spontaneous emission observed in an In0.3Ga0.7N- blue-LED such as an anomalous temperature dependent of peak energy, almost temperature independence of radiative lifetimes and mobility-edge type behavior, indicating an important role of exciton localization.

Degenerate four-wave-mixing spectroscopy on epitaxially laterally overgrown GaN: Signals from below the fundamental absorption edge

We performed photoluminescence (PL) and degenerate four-wave-mixing (FWM) spectroscopy in epitaxially laterally overgrown GaN at 10 K. Optical transitions based on exciton complexes such as biexciton emission, exciton–exciton scattering, electron–hole plasma, and so on, were revealed by PL under a wide range of excitation densities. The FWM signals were observed from states below the fundamental excitonic absorption edge, showing that nonlinear photoswitching can be performed with a transmittance geometry. The origin of such nonlinearlity was discussed by correlating with feasible many-body effects between excitons, biexcitons, and free carriers.

Pump and probe spectroscopy of InGaN multi quantum well based laser diodes

Dynamical behavior of dense carriers has been assessed at room temperature (RT) in the InGaN multi quantum well (MQW) based laser diodes (LDs) by employing pump and probe (PP (b), x=0.2, y=0.05 and (c), x=0.3, y=0.05], whose stimulated emissions correspond to near ultraviolet (390 nm), violet (420 nm) and blue (440 nm), respectively. The optical gain was contributed from the nearly delocalized states [the lowest-quantized MQW levels (LQL)] in the sample (a), while it was from highly localized levels with respect to LQL by 250 meV for the sample (b), and by 500 meV for the sample (c). It was found that the photo-generated carriers rapidly (less than 1 ps) transferred to LQL, and then relaxed to the localized tail within the time-scale of several ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of LQL under higher photo-excitation.