We have conducted simultaneous photoacoustic (PA) and photoluminescence (PL) measurements to accurately estimate the internal quantum efficiency (IQE). The method detects light from radiative recombination through PL measurement and heat from non-radiative recombination through PA measurement. In this study, we have applied the method to an InGaN-QW sample on a “stripe-core” GaN substrate in which the dislocation density periodically changes. Considering that photo-excited carriers recombine either radiatively or non-radiatively, the heat generation will increase in the defective region where emission efficiency is weak. In the line-scan measurement, the position-dependent complementary relationship between the PA and PL intensity is clearly observed.
We report the latest progress of gallium nitride-based Vertical-Cavity Surface-Emitting Lasers (VCSELs) containing a curved mirror. Highly uniform and efficient devices were developed. The average threshold current was 0.64 mA with a standard deviation of 0.043 mA. The peak wall-plug efficiency and output power were 13.4 % and 7.6 mW at operating currents of 5.2 mA and 12.8 mA, respectively. We obtained green VCSELs with milliwatt-class outputs and a wall-plug efficiency of 3.7%. We also report the progress of VCSELs with a single-cavity filtering mirror and a cavity length of approximately 25 μm showing highly varying reflectivity spectra, to demonstrate their single-longitudinal mode operation.
Many applications benefit from single mode operation of vertical-cavity surface-emitting lasers (VCSELs) due to the need for a consistent beam profile and wavelength. While long cavity VCSELs have demonstrated good results in recent years, with examples of sub 1mA thresholds for devices with blue and green emission, the mode has not been fully controlled. Traditionally, VCSELs, with their short cavities, inherently maintain single longitudinal mode operation while struggling with multi-lateral modes. However, the use of a long cavity device with a curved mirror provides the opposite challenges for mode control, as the lateral mode can be controlled with the lens curvature, but the long cavity length allows for multiple longitudinal modes. In this paper, we will present the use of a filtering mirror with a highly varying reflectivity spectrum for obtaining single longitudinal mode operation up to 2mW and 50 kA/cm2. The filtering mirror reflectivity leads to significant mirror loss for adjacent longitudinal modes. Thus, by engineering the beam profile with the lens and filtering the longitudinal modes with the mirror structure, we are able to control the output shape, divergence angle, and wavelength stability of the device. This mode control, along with the low thresholds and lifetimes greater than 2000 hours, shows the potential of the long cavity structure for a variety of applications.
Internal quantum efficiency (IQE) of radiative recombination for photo-excited carriers in compound semiconductor materials is usually estimated from temperature dependence of photoluminescence (PL) intensity by assuming that the IQE at cryogenic temperature is unity. III-nitride semiconductors, however, usually have large defect-density, and the assumption is not necessarily valid. In this study, we developed a new method to estimate accurate IQE values by simultaneous PL and photo-acoustic (PA) measurements, and demonstratively evaluated the IQE values for an InGaN quantum-well sample. The results show that the conventional method cannot give accurate IQE values, and that our method is a promising way for accurate estimation of absolute IQE values, which could lead to the accurate estimation of radiative and nonradiative recombination lifetimes in carrier dynamics studies.
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