Self-assembled strain-free growth of In droplets on GaN via droplet epitaxy (DE) technique was investigated. Controlling In droplet size and density as a function of substrate temperature is described. The highest density of 1.36 × 108 cm-2 was observed at a very low substrate temperature of 30 °C. The resulting droplets are crystalline at room temperature and are characterized ex-situ by atomic force microscopy (AFM) and x-ray diffraction (XRD). The formation of quantum dots (QDs) through crystallization of In droplets grown using the DE method has many advantages over the strain driven Stranski-Krastinow technique, including the ability to control a wide range of QD shapes, sizes, and densities, as well as overcoming the limitations of lattice mismatched with substrates. This study explains the first stage of forming a controlled InN QD on GaN.
The optical properties of Λ-graded indium gallium nitride (InGaN) solar cells are studied. Graded InGaN well structures with the indium composition increasing to xmax and then decreasing in a Λ-shaped pattern have been designed. Through polarization doping, this naturally creates alternating p- and n-type regions. Separate structures are designed by varying the indium alloy profile from GaN to maximum indium concentrations ranging from 20% to 90%, while maintaining a constant overall structure thickness of 100 nm. The solar cell parameters under fully strained and relaxed conditions are considered. The results show that a maximum efficiency of ≅5.5 % under fully strained condition occurs for xmax = 60 % . Solar cell efficiency under relaxed conditions increases to a maximum of 8.3% for xmax = 90 % . Vegard’s law predicts the bandgap under relaxed conditions, whereas a Vegard-like law is empirically determined from the output of nextnano™ for varying indium compositions to calculate the solar cell parameters under strain.
The optical properties of periodic graded GaN/InGaN are studied. We have designed graded InGaN quantum well (QW) structures with the indium composition increasing then decreasing in a zigzag pattern. Through polarization doping, this naturally creates alternating p-type and n-type regions. Separate structures are designed by varying the number of repeating periods (1 to 3), while maintaining constant overall structure thicknesses. Calculation of the transition probabilities and the electron and hole wave-functions between the conduction band and the valence band reveals a complex energy structure which predicts the photoluminescence peaks for band to band transitions.
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