The quantum efficiency of GaN-based light-emitting diodes (LEDs) is investigated at temperatures 77–300 K. It is found that the efficiency droop is due to a decrease in the internal quantum efficiency (IQE) in the low-energy part of the emission spectrum. The efficiency starts to decrease at a temperature independent forward voltage of Umax≈2.9 V. At this voltage tunneling current through the LED-structure begins to dominate. It is suggested that the external quantum efficiency droop is related to reduction of the IQE due to tunneling leakage of carriers from the quantum well (QW) to defect states in barriers, and to reduction of the injection efficiency by excess tunneling current under QW through deep defect states in barriers.

A femtosecond laser focused inside bulk GaN was used to slice a thin GaN film with an epitaxial device structure from a bulk GaN substrate. The demonstrated laser slicing lift-off process did not require any special release layers in the epitaxial structure. GaN film with a thickness of 5 µm and an InGaN LED epitaxial device structure was lifted off a GaN substrate and transferred onto a copper substrate. The electroluminescence of the LED chip after the laser slicing lift-off was demonstrated.

GaN films with thicknesses up to 3 mm were grown in two custom-made halide vapor phase epitaxy (HVPE) reactors. V-shaped defects (pits) with densities from 1 cm −2 to 100 cm −2 were found on the surfaces of the films. Origins of pit formation and the process of pit overgrowth were studied by analyzing the kinematics of pit evolution. Two mechanisms of pit overgrowth were observed. Pits can be overgrown intentionally by varying growth parameters to increase the growth rate of pit facets. Pits can overgrow spontaneously if a fast-growing facet nucleates at their bottom under constant growth conditions. *

GaN films with thickness up to 3 mm were grown by halide vapour phase epitaxy method. Two growth modes were observed: the high temperature (HT) mode and the low temperature (LT) mode. Films grown in HT mode had smooth surface, however the growth stress was high and caused cracking. Films grown in LT mode had rough surface with high density of V-defects (pits), however such films were crack-free. The influence of growth parameters on the pit shape and evolution was investigated. Origins of pits formation and process of pit overgrowth are discussed. Crack-free films with smooth surface and reduced density of pits were grown using combination of the LT and HT growth modes.

Представлены результаты исследования зависимостей тока и емкости от прямого смещения диодов Шоттки Au/n-GaN, а также спектров подзонного оптического поглощения и дефектной фотолюминесценции объемных кристаллов и тонких слоев n-GaN. Показано, что туннелирование с участием дефектов является доминирующим механизмом транспорта при прямых смещениях контактов Шоттки на n-GaN. Зависимости тока и емкости от прямого смещения отражают энергетический спектр дефектов в запрещенной зоне n-GaN: рост с энергией плотности глубоких состояний, ответственных за желтую фотолюминесценцию в GaN, и резкий экспоненциальный хвост состояний с урбаховской энергией EU=50 мэВ вблизи края зоны проводимости. Уменьшение частоты перескоков электронов вблизи интерфейса Au/n-GaN приводит к широкому распределению локальных времен диэлектрической релаксации и изменению распределения электрического поля в области пространственного заряда при прямых смещениях. DOI: 10.21883/FTP.2017.09.44888.8528