Fabrication and properties of coherent-structure In-polarity InN∕In0.7Ga0.3N multiquantum wells emitting at around 1.55μm

Abstract

In-polarity InN∕In0.7Ga0.3N multiquantum wells (MQWs) were fabricated on a thick In0.7Ga0.3N interlayer/Ga-polarity GaN template by radio-frequency plasma-assisted molecular beam epitaxy. We then investigated how the lattice relaxation and piezoelectric field in InN wells affect their structural and photoluminescence (PL) properties, respectively. It was found that the critical thickness of InN well on In0.7Ga0.3N barrier was about 1nm. A clear PL peak shift from 1.40to1.95μm was observed depending on the InN well thickness from 0.7to2.0nm. Correspondingly, PL-intensity reduction was also observed with increasing well thickness. No PL was observed for the sample with 4.1nm thick InN wells. On the basis of theoretical estimation of transition energies in InN∕In0.7Ga0.3N MQWs, it was confirmed that the quantum-confined Stark effect (QCSE) played an important role for both the observed PL peak shift and the decrease in intensity. The piezoelectric field in coherently grown InN wells was about 3MV∕cm but it was reduced to about 1–2MV∕cm for the samples with relaxed InN wells. It was confirmed that the InN wells must be thinner than the critical thickness (1nm) in following two points: to reduce defects arising from lattice relaxation and to reduce QCSE leading to emission-peak redshift and a decrease in intensity.