We report the effect of substrate thickness on the optical and thermal characteristics of InGaN/GaN light emitting diodes (LEDs), operating at l $ 450 nm, with different mesa sizes. For various mesa sizes with different substrate thicknesses, the junction temperature (T j ) is measured as a function of injection current by the forward voltage method and the characteristic temperature is also investigated by measuring the temperaturedependent electroluminescence spectrum. Based on the experimentally determined heat source density, the junction temperature, heat flux, and thermal resistance (R th ) are calculated theoretically by using a three-dimensional anisotropic heat dissipation model. The use of a thin substrate thickness of 150 mm effectively improves the heat extraction capability due to the shorter heat transfer path length. For 150 mm (443 mm) thick substrate, the T j value of LEDs is experimentally estimated as 307.7 K (322.6 K), 311.6 K (329.5 K), and 323 K (355.9 K) under an injection current of 80 mA for 450 Â 450, 400 Â 400, and 300 Â 300 mm 2 , respectively. At a high injection current of 400 mA, the T j of 450 Â 450 mm 2 LED with 150 mm thick substrate is reduced by $25.7% compared to that of LED with 443 mm substrate thickness, indicating a value of T j ¼ 384.5 K. The R th value is decreased for larger mesa size and thinner substrate thickness, resulting in R th < 30 K/W for 450 Â 450 mm 2 LED with 150 mm thick substrate. The theoretically calculated R th values indicate a good agreement with the experimentally measured data.
Diffraction efficiencies of three different types of subwavelength grating (SWG) structures were simulated for optoelectronic device applications using rigorous coupled wave analysis method. The effect of height and period of SWG structures on the reflectance were investigated. Also, the internal and external reflection from the SWG structures were simulated and analyzed.
We design the InGaP/GaAs dual-junction (DJ) solar cells by optimizing shortcircuit current matching between top and bottom cells using the Silvaco ATLAS. The relatively thicker base layer of top cell exhibits a larger short-circuit current density (J sc ) while the thicker base layer of bottom cell allows for a smaller J sc . The matched J sc of 10.61 ± 0.05 and 13.25±0.06 mA/cm 2 under AM1.5G and AM0 illuminations, respectively, are obtained, leading to the increased conversion efficiency. The base thicknesses of top InGaP cells are optimized at 0.8 and 0.65 µm for AM1.5G and AM0 illuminations, respectively, and the base thicknesses of bottom GaAs cells are optimized at 2 µm. For the optimized solar cell structure, the maximum J sc = 10.66 mA/cm 2 (13.31mA/cm 2 ), V oc = 2.34 V (2.35 V), and fill factor = 87.84%(88.1%) are obtained under AM1.5G (AM0) illumination, exhibiting a maximum conversion efficiency of 25.78% (23.96%). The effect of tunnel diode structure, i.e, GaAs/GaAs, AlGaAs/AlGaAs, and InGaP/InGaP, on the characteristics of solar cells is investigated. The photogeneration rate in the DJ solar cell structure is also obtained by incident light of different wavelengths.
We report the enhancement of the minority carrier lifetime of GaInP with a lateral composition modulated (LCM) structure grown using molecular beam epitaxy (MBE). The structural and optical properties of the grown samples are studied by transmission electron microscopy and photoluminescence, which reveal the formation of vertically aligned bright and dark slabs corresponding to Ga-rich and In-rich GaInP regions, respectively, with good crystal quality. With the decrease of V/III ratio during LCM GaInP growth, it is seen that the band gap of LCM GaInP is reduced, while the PL intensity remains high and is comparable to that of bulk GaInP. We also investigate the minority carrier lifetime of LCM structures made with different flux ratios. It is found that the minority carrier lifetime of LCM GaInP is ∼37 times larger than that of bulk GaInP material, due to the spatial separation of electrons and holes by In-rich and Ga-rich regions of the LCM GaInP, respectively. We further demonstrate that the minority carrier lifetime of the grown LCM GaInP structures can easily be tuned by simply adjusting the V/III flux ratio during MBE growth, providing a simple yet powerful technique to tailor the electrical and optical properties at will. The exceptionally high carrier lifetime and the reduced band gap of LCM GaInP make them a highly attractive candidate for forming the top cell of multi-junction solar cells and can enhance their efficiency, and also make them suitable for other optoelectronics devices, such as photodetectors, where longer carrier lifetime is beneficial.
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