Optical properties of polar, nonpolar, and semipolar InGaN/GaN multiple quantum wells on sapphire

The polarization fields in the c-axis-oriented hexagonal GaN system cause spatial separation of electrons and holes in quantum wells, reducing the quantum efficiency, and resulting in a red shift of the emission as well as a blue shift with increasing injected carrier density. In this paper, we report on the growth and optical characterization of InGaN/GaN multiple quantum wells (MQWs) on nonpolar (11 (2) over bar0) a- and polar (0001) c-planes, as well as two sernipolar planes, (11 (2) over bar2) and (1 (1) over bar 01) of GaN. There are two kinds of a-plane used in this study. One of the (11 (2) over bar0) a-planes was obtained on (I TOO) m-plane sapphire substrates during the epitaxial lateral overgrowth (ELO) of (11 (2) over bar2) oriented sernipolar GaN films, while the other one was planar a-plane GaN gown on (1 (1) over bar 01) r-plane sapphire substrates. The sernipolar (11 (2) over bar2) and (1 (1) over bar 01) planes were obtained as sidewall facets during the ELO of c-plane GaN with the mask stripes aligned along the GaN m-axis and a-axis, respectively. InGaN/GaN multiple quantum wells (MQWs) with a nominal well thickness of 4 nm and a barrier thickness of 8 nm were grown on these five GaN samples by metalorganic chemical vapor deposition. Excitation power dependent photoluminescence (PL) measurements were carried out on these quantum well structures to study the effect of polarization-induced electric field on the band-edge emission. The quantum-well emission from the two a-plane MQWs showed no shift, compared to a 74 meV blue shift for the c-plane MQWs when the excitation power density was increased from 4.1x10(3) mW/cm(2) to 1.2x10(5) mW/cm(2). The sernipolar (11 (2) over bar2) showed a blue shift of 35 meV with increased excitation power density, suggesting reduced polarization compared to that in c-plane. No quantum-well emission could be observed for the MQWs on (1101) semipolar planes. The shift in the quantum-well emission energy was attributed to the polarization field screening by photo-generated carriers in the quantum wells.