The growth of high In content InGaN material is notorious for being challenging because of high mechanical strain and thermodynamic instability of the system. It has been shown that one can improve the growth quality by using variable surface miscut. In this study, we demonstrate the use of micropatterning of bulk GaN substrates in order to improve the quality of high In content layers. During MOVPE growth the quality of the InGaN layers and the In content depends on the local shape of the substrate surface, reaching the highest emission intensity at the top of every pattern. We study patterns with characteristic sizes ranging from 1 to 6 µm created using two methods: binary photolithography combined with a thermal reflow process as well as multilevel photolithography. The latter allows for achieving lower inclination of the sidewall of the pattern which in turn supports a more stable growth process. The properties of the samples are studied using fluorescence microscopy, microphotoluminescence mapping and carrier diffusion measurements.
A micro-photoluminescence setup is used to investigate the ambipolar diffusion of charge
carriers in InGaN quantum wells (QW) grown by molecular beam epitaxy. The thickness of
the active region varies between 2.6 and 25 nm.
Our results show for all samples diffusion in the range of a few μm. Additionally, a larger
QW thickness is accompanied by a smaller luminescence spot radius in our experiment.
However, a larger dark carrier diffusion with increasing QW thickness cannot be excluded
due to an increasing carrier lifetime. Moving away from the excitation center leads to a
stronger tilt of the QW potential due to lower carrier density, consequently suppressing
radiative recombination.
Wide well blue InGaN LEDs and laser diodes show high internal quantum efficiency and high differential optical gain. We investigate the contribution of recombination from ground and excited confined states in MBE grown LED or laser structures with 2.6 nm, 7.8 nm and 15 nm wide InGaN wells by low and room temperature µEL and µPL. For the 7.8 nm well, the blue-shift caused by piezoelectric field screening (QCSE) and transition from ground to higher confined states are observed separably. The 25 nm emits at the band edge of the fully screened InGaN region and shows pronounced state filling.
Micro-photoluminescence (μPL) pinhole scans of an InGaN multi-quantum well (MQW) and single-quantum well (SQW) LED structure show large PL spots compared to the relatively small excitation area. We observe a lateral diffusion over a wide range in the order of 10 µm in both samples. The excitation density is varied to identify a dependence of the diffusion length on the amount of injected charge carriers. Overall the MQW sample shows a smaller diffusion length compared to the SQW sample, which is attributed to the different sample quality. In the MQW sample no significant change in diffusion length with varying excitation power is observed, whereas in the SQW sample the charge carrier diffusion increases with smaller excitation densities. Our spectral resolved μPL setup allows to measure the spectra as a function of the distance, which gives the possibility to investigate the energetic shift. As a result the PL spectra of both samples show a pronounced blue-shift at the center of the PL spots which is assigned to the screening of the quantum confined Stark effect (QCSE) and the band-filling.
Degradation of GaN-based laser diodes after some time of operation appears usually as raised threshold current, lower slope efficiency, or increased voltage. We investigate stressed and non-stressed laser diodes using micro-electroluminescence (μEL) and micro-photoluminescence (μPL). In μEL, the stressed device exhibits darker regions, which are correlated with a red-shifted emission. Both observations indicate a lower carrier density in these darker areas. Our μPL measurements do not suggest a corresponding increased defect density in these regions. This study shows inhomogeneous pumping of the active region that can be explained by current path formation from non-uniform hydrogen distribution. Additionally a background of increased nonradiative recombination is found in the stressed device, which is unrelated to the conductivity degradation.
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