Relationship of basal plane and prismatic stacking faults in GaN to low temperature photoluminescence peaks at approx 3.4 eV and approx 3.2 eV

摘要

The relationship between the optical properties and microstructure of GaN is of great interest due to the important optical and electronic applications of this material.Several different studies have been reported attempting to link the low temperature photoluminescence (PL) peak at approx 3.4 eV to the presence of various microstructural defects.However,no clear systematic studies have been reported establishing such a link for the PL peak observed at approx 3.2 eV.In this paper,we present evidence linking the approx 3.4 eV PL peak to the presence of a thin layer of cubic phase associated with basal plane stacking faults (BSF).This relationship is mainly established by studying a series of ammonothermally-grown GaN bulk crystals.The existence and strength of the approx 3.4 eV peak are found to be related to the I_2 type BSF (R_(I2)=l/3<1010>) observed in these samples.To investigate the relationship between the approx 3.2 eV peak and structural defects,a series of GaN epilayers grown on either SiC or sapphire (of various off-cut angles) was investigated by TEM and PL spectroscopy.Samples grown on 3.5~° off-cut SiC and 5~° and 9~° off-cut sapphire substrates exhibit PL peaks near approx 3.2 and approx 3.4 eV,which are absent in the on-axis SiC and sapphire cases.TEM shows that the former group of samples has defect configurations consisting of prismatic stacking faults (PSFs) folding back and forth between two different {1120} planes connected by stair rod dislocations,which in turn fold onto to I_1 type BSFs again with stair rod dislocations at the fault intersections.The approx 3.2 eV PL peaks are proposed to possibly arise from transitions involving the PSFs and the stair rods associated with their mutual intersections and their intersections with the BSFs.The -3.4 eV peak is again attributed to the thin layer of cubic phase associated with the I_1 type BSF (three bilayers as opposed to four bilayers for the I_2 type BSF).