Deep level related yellow luminescence in p-type GaN grown by MBE on (0001) sapphire

Yellow luminescence (YL) has been studied in GaN:Mg doped with Mg concentra tions ranging from 10(19) to 10(21) cm(-3) by spectral CL (T=5K) and TEM an d explained by suggesting that a different mechanism could be responsible f or the YL in p-type GaN with respect to that acting in n-type GaN. Transitions at 2.2, 2.8, 3.27, 3.21, and 3.44 eV were found. In addition to the wurtzite phase, TEM showed a different amount of the cubic phase in th e samples. Nano tubes with a density of 3x10(9) cm(-2) were also observed b y approaching the layer/substrate interface. Besides this, coherent inclusi ons were found with a diameter in the nm range and a volume fraction of abo ut 1%. The 2.8 eV transition was correlated to a deep level at 600 meV below the c onduction band (CB) due to Mg-Ga-V-N complexes. The 3.27 eV emission was as cribed to a shallow acceptor at about 170-190 meV above the valence band (V B) due to Mg-Ga. The 2.2 eV yellow band, not present in low doped samples, increased by incr easing the Mg concentration. It was ascribed to a transition between a deep donor level at 0.8-1.1 eV below the CB edge due to N-Ga and the shallow ac ceptor due to Mg-Ga. This assumption was checked by studying the role of C in Mg compensation. CL spectra from a sample with high C content showed tra nsitions between a C-related 200 meV shallow donor and a deep donor level a t about 0.9-1.1 eV below the CB due to a N-Ga-V-N complex. In our hypothesi s this should induce a decrease of the integrated intensity in both the 2.2 and 2.8 eV bands, as actually shown by CL investigations.