Electrical and optical properties of bulk GaN substrates studied by Kelvin probe and photoluminescence

Abstract

We have investigated the N‐ and Ga‐polar faces of bulk GaN substrates with photoluminescence (PL) and the surface photovoltage (SPV) technique using a Kelvin probe attached to an optical cryostat. Experiments were conducted in vacuum. Some of the surfaces were mechanically polished (MP), while others were epi‐ready after a chemical‐mechanical polish (CMP). From the SPV measurements, the band bending in a sample having both surfaces treated with the CMP method was calculated to be about 0.83 and 0.70 eV for the Ga‐ and N‐polar surfaces, respectively. The restoration of the SPV after ceasing the UV illumination showed that the SPV from CMP‐treated surfaces behaved as predicted by a thermionic model, whereas the SPV from MP‐treated surfaces restored with a much faster‐than‐predicted rate. This result can be interpreted by the hopping of charge carriers in the highly‐defective near‐surface layer of the MP‐treated samples. Remarkably, removing the top 700 nm defective layer by dry etching restored the quality of the electrical and optical properties of GaN. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Green luminescence in Mg-doped GaN

“…The properties of the GL2 band with a maximum at 2.35 eV in the studied Mg-doped GaN samples are identical to those in undoped high-resistivity GaN grown under very Ga-rich conditions. Moreover, a very weak GL2 band with the same properties appeared in high-quality undoped freestanding GaN grown by HVPE after the surface was mechanically polished [25]. The quantum efficiency of the GL2 band in Mg-doped GaN samples is the highest among the three groups of samples, which allowed us to study it in more detail and for a wider range of temperatures and excitation intensities.…”

“…As for the bulk GaN templates, the vacancies could be formed along with other structural defects in the near-surface region during the mechanical polishing. This near-surface, defective layer in these samples is thin (about 1 to 2 μm) and can be removed by dry etching [25]. Finally, the transition energy levels of the nitrogen vacancy (calculated in Sec.…”

Fine structure of the red luminescence band in undoped GaN

“…We previously observed the same (but structureless) RL band in high-quality thick or freestanding GaN grown by HVPE at different facilities. 5,23,24 In all of these works, the fine structure could not be observed, because it was masked by adjacent PL bands (usually the GL band). Note that the red band appearing in GaN grown by molecular beam epitaxy in extremely Ga-rich conditions (labeled in Ref.…”

“…5 as the RL2 band) is clearly related to a different defect. In contrast to the RL band, the RL2 band is quenched at temperatures above 100 K and disappears at about 200 K. 5 The fact that the RL band is observed in very pure undoped GaN samples 23,24 indicates that some native defects or the most common contaminations in the HVPE growth are involved in the related defect. Among possible candidates for the defect responsible for RL band, we suggest carbon, silicon, oxygen, and gallium vacancy.…”

Identification of point defects in HVPE-grown GaN by steady-state and time-resolved photoluminescence

“…7 It can also be detected after mechanical polishing of the surface in high-purity freestanding GaN grown by HVPE. 17 The shape of the GL2 band at low temperature can be modeled with Eq. (1).…”