Effect of strain modification on crystallinity and luminescence of InGaN/GaN multiple quantum wells grown by MOCVD

Abstract

The effects of compressive strain modification on green-emitting InGaN/GaN multiple quantum wells (MQWs) are comprehensively investigated by inserting a bulk InGaN prestrain underlying layer with different indium doping contents. Systematic measurements of the structural, morphological and optical properties are conducted. Better crystallinities are obtained for both modified samples, in which the density of V-pits is reduced from 4.0 × 108 /cm2 in the original sample to 1.9 × 108 /cm2 and 3.3 × 108 /cm2. Compared with the conventional InGaN/GaN MQWs, the photoluminescence (PL) emission wavelength of the strain-modified MQW structure with an InGaN (In = 0.03) prestrain layer is blueshifted by ~ 6 nm as a result of the reduced built-in piezoelectric field, while the other MQW structure with an InGaN (In = 0.1) layer has an ~ 15 nm redshift, which is attributed to not only the ~ 1% increase in the In content in the QWs and the ~ 0.6 nm increase in the well width due to the composition pulling effect, but also the ~ 0.19 degree of strain relaxation in the InGaN QWs. The low-temperature PL (LTPL) intensities of both samples are approximately twice as large as that of the conventional one, whereas the internal quantum efficiency is three times larger at the maximum. These tremendous improvements are mainly due to the weakened quantum confined stark effect and stronger carrier localization states, as confirmed by LTPL and temperature-dependent PL (TDPL) measurements.