Growth of InGaN, having high Indium composition without compromising crystal quality has always been a great challenge to obtain efficient optical devices. In this work, we extensively study the impact of non-radiative defects on optical response of the plasma assisted molecular beam epitaxy (PA-MBE) grown InGaN nanowires, emitting in the higher wavelength regime (
λ
>
520
nm). Our analysis focuses into the effect of defect saturation on the optical output, manifested by photoluminescence (PL) spectroscopy. Defect saturation has not so far been thoroughly investigated in InGaN based systems at such a high wavelength, where defects play a key role in restraining efficient optical performance. We argue that with saturation of defect states by photo-generated carriers, the advantages of carrier localization can be employed to enhance the optical output. Carrier localization arises because of Indium phase segregation, which is confirmed from wide PL spectrum and analysis from transmission electron microscopy (TEM). A theoretical model has been proposed and solved using coupled differential rate equations in steady state to undertake different phenomena, occurred during PL measurements. Analysis of the model helps us understand the impact of non-radiative defects on PL response and identifying the origin of enhanced radiative recombination.
Non-radiative defects play a deterministic role in regulating the performance of LEDs. Yet, defect saturation in LEDs is relatively unexplored in the literature. Here, we establish the theoretical background of carrier-induced defect saturation from the band structure of quantum well (QW)-based InGaN LEDs after solving Poisson and Schrodinger's equations self-consistently. Time dynamics of defect saturation are demonstrated through solving a set of coupled differential rate equations iteratively, considering carrier transitions between different energy levels in the QW region. They indicate an increasing degree of defect saturation with higher carrier injection at steady state. Capacitance versus voltage (CV) measurements on fabricated InGaN MQW LEDs, conducted at low frequencies clearly demonstrate the considerable effect of defect saturation at higher bias. We propose a correction term in the typical RC circuit model for LEDs, considering defect saturation, and solved it analytically to explain the frequency-dependent CV characteristics. Analytical calculation of CV response, based on the modified RC model, shows a fairly satisfactory matching with the experimental data at different frequencies. Also, the frequency dependence of negative capacitance at a higher bias regime is explained through the conductance versus voltage (GV) characteristics.
N-side up thin-AlGaInP epilayers based on vertical light-emitting diodes (VLEDs) (light emitting area: 44 mil × 44 mil) with Si and composite metal (copper/Invar/copper; CIC) substrates were obtained by wafer bonding and epilayer transferring technologies. The coefficients of thermal expansion of the Si and CIC substrates were about 2.6 × 10−6 /K and 6.1 × 10−6 /K respectively. The coefficient of thermal expansion of CIC was matched to the GaAs substrate and AlGaInP epilayer. After the epilayer transferred to the CIC substrate, a small amount of bending occurred. The performance of the packaged LEDs/CIC was almost the same with that of LEDs/Si. The redshift phenomenon was from 623 to 643 nm for the CIC/LEDs while that of the LEDs/Si chip was from 625 to 643 nm with an injected current from 100 mA to 1 A and high output power (251 mW at 1 A) as compared to packaged LEDs/Si (299.5 mW at 1 A). The variation in light-emitting wavelengths of the LEDs/CIC was the same with that of the LEDs/Si. The distribution of the temperature of the LEDs on the CIC substrate was less than the LEDs on Si throughout the surface. The obtained data suggested that CIC could be extended instead of a Si substrate for high-power thin film LEDs applications.
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