| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Wednesday Sessions |
| Session TF-WeE |
| Session: | Microstructure & Surface Morphological Evolution in Organic & Inorganic Films |
| Presenter: | Zakaria Al Balushi, The Pennsylvania State University, USA |
| Authors: | Z. Al Balushi, The Pennsylvania State University, USA J. Redwing, The Pennsylvania State University, USA |
| Correspondent: | Click to Email |
InGaN quantum wells (QWs) have been well established as active layers in LEDs and laser diodes. Recently, there has been interest in the growth of thick InGaN to serve as strain-reducing layers for deep-green and red emitters. The growth of thick layers is, however, challenging. The miscibility gap between InN and GaN leads to InGaN phase separation and indium clustering. In addition to low indium incorporation at high growth temperatures, a high density of V-pits are observed in InGaN films grown in the III-polar direction. Alternatively, the growth of films in the N-polar direction offers advantages that are attractive for the growth of thick InGaN. N-polar growth enables higher indium incorporation and V-pits are less favorable to form. Despite these advantages, limited studies on thick N-polar InGaN films have been reported. This is because N-polar InGaN grown by MOCVD typically suffer from high densities of hexagonal hillocks that originate from low quality N-polar GaN templates. Therefore, in order to assess the quality of thick N-polar InGaN films, it is necessary to probe the local structure and optical properties of high quality grown films.
In this study, we compare the structural and optical properties of thick III-polar and N-polar InGaN. Both III-polar and N-polar InGaN growth was carried out by MOCVD under identical conditions (130 nm, 780°C, 300 Torr, V/III=2450) on high quality GaN base layers. The N-polar InGaN films were free of hexagonal hillocks and exhibited a reduced RMS roughness of 0.66 nm in comparison to the RMS roughness of 2.30 nm for III-polar InGaN. High resolution XRD, SIMS and EDX measurements revealed higher indium incorporation in the N-polar films (13%) when compared to III-polar InGaN (7.5%). From PL measurements, two distinct peak emissions located at different depths of the film were observed for both film polarities, one peak originating from a pseudomorphically strained region and the other from a partially relaxed region. To further investigate the optical properties of InGaN films, we performed both steady state and time-resolved cathodoluminescence mapping of the III-polar and N-polar films at room and liquid helium temperatures. From CL measurements, distinct emission from indium clusters and threading dislocation around V-pits were observed in III-polar InGaN films. In the case of N-polar InGaN, CL emission was homogenous across the film surface. With the combination of spectroscopic techniques, this study elucidates the differences in the luminescence in thick InGaN as a function of film polarity and gives new insights into possible mechanisms of luminescence quenching commonly observed N-polar InGaN.