Optical studies of strain and defect distribution in semipolar (1101) GaN on patterned Si substrates

摘要

Formation of defects in semipolar (1101)-oriented GaN layers grown by metal-organic chemical vapor deposition on patterned Si (001) substrates and their effects on optical properties were investigated by steady-state and time-resolved photoluminescence (PL) and spectrally and spatially resolved cathodoluminescence (CL). Near-band edge emission is found to be dominant in the c~+-wings of semipolar (1101)GaN, which are mainly free from defect-related emission lines, while the c~- wings contain a large number of basal stacking faults. When the advancing c~+ and c~- fronts meet to coalesce into a continuous film, the existing stacking faults contained in c~- wings continue to propagate in the direction perpendicular to the c-axis and, as a result, the region dominated by stacking fault emission is extended to the film surface. Additional stacking faults are observed within the c~+ wings, where the growing c~+ wings of GaN are in contact with the SiO_2 masking layer. Out-diffusion of oxygen/silicon species and concentration of strain near the contact region are considered as possible causes of the stacking fault formation. CL linescans performed along the surface and across the thickness of the non-coalesced and coalesced layers revealed that, while most of the material in the near-surface region of the non-coalesced layers is relaxed, coalescence results in nonuniform strain distribution over the layer surface. Red-shifted near-band-edge emission from the near-surface region indicates tensile stress near the surface of a coalesced layer, reaching a value of 0.3 GPa. The regions near the GaN/AlN/Si(111) interface show slightly blue shifted, broadened near-band-edge emission, which is indicative of a high concentration of free carriers possibly due to incorporation of shallow-donor impurities (Si and/or O) from the substrate or SiO_2 mask. Steady-state and time-resolved PL results indicate that semipolar (1101)GaN on patterned Si exhibits optical properties (PL intensity and carrier lifetimes) approaching to those of the state-of-the-art c-plane GaN grown using in situ SiN_x nanonetwork mask on c-plane sapphire. Long PL lifetimes (～2ns) for the (1101)GaN layers show that the semipolar material holds promise for light emitting and detecting devices.