Growth and characterization of non-polar (11-20) GaN and AlGaN/GaN MQWs on R-plane (10-12) sapphire

The majority of GaN films and related devices have been grown along the polar [0001] direction, and epitaxial growth along non-polar directions has received much less attention. In this paper we report the study of material properties of GaN and AlGaN/GaN multiple quantum wells (MQWs) deposited on R-plane (10-12) sapphire substrates using RF plasma-assisted molecular beam epitaxy (MBE). In this growth direction, III-Nitrides grow along the non-polar [11-20] direction, with the c-axis in the plane of growth. Various nucleation steps such as surface nitridation, as well as GaN and AlN buffer layers were investigated. Our results indicate that surface nitridation of R-plane sapphire is an undesirable nucleation step, contrary to what has been observed in the case of (0001) sapphire. The AlN buffer layer leads to well-oriented films along the [11-20] direction with many threading defects and faceted surface morphology whereas the GaN buffer leads to the formation of mis-oriented domains close to the buffer region. However, these domains are overgrown and the films have smoother surface morphology with fewer threading defects. These structural findings are supported by photoluminescence and Hall effect measurements done on the same films. Photoluminescence (PL) measurements of (11-20) AlGaN/GaN MQWs show much higher intensity than for similar structures grown on the C-plane sapphire, consistent with the absence of internal fields in the non-polar direction.