Surface chemistry and electronic properties of nitride semiconductors studied via synchrotron-based photoemission spectroscopy.

摘要

The effects of both KOH and HCl treatments on the surface chemistry and electronic properties of n-GaN were studied via synchrotron radiation-based photoemission spectroscopy. The HCl treatment increases the Ga/N atomic ratio at the surface, while KOH treatment decreases this ratio of surface atoms. The HCl treatment can be used to lower the surface barrier on n-GaN prior to metal deposition to aid in ohmic contact formation.; The Schottky barrier heights for Au, Al, Ni, Pd, Pt, and Ti on both n-GaN and p-GaN were measured via photoemission spectroscopy. For this set of six metals, two Fermi level pinning behaviors were observed. For Au, Ti, and Pt, the Fermi level position occurs at an energy 0.5 eV higher within the band gap of n-GaN than the Fermi level position for the same metal deposited on p-GaN. In contrast, for Al, Ni and Pd, the Fermi level lies at a single energy within the band gap when deposited on either n-GaN or p-GaN. None of the six metals is observed to grow layer-by-layer on the GaN surface; instead growth is consistent with either the Volmer-Weber or Stranski-Krastanov modes. In the case of Au, islanding is clearly observed through atomic force microscopy images.; Cleaning treatments, and metal deposition and the electron accumulation phenomenon were investigated on both InAs and InN via photoemission spectroscopy. Due to insufficient removal of oxides on the InAs surface, the valence band maximum could not be observed in the photoelectron spectrum and therefore the correct Fermi level position in the band gap of InAs could not be determined. This prevents any conclusions from being drawn about the presence of an electron accumulation on this InAs. Both annealing and sputter-cleaning of InN allowed the valence band maximum to be determined. The deposition of Au on annealed-InN yielded a Fermi level energy, 0.7 ± 0.1 eV above the valence band maximum. Ti deposited on a sputtered-InN sample had a Fermi level energy 1.6 ± 0.1 eV above the valence band maximum. The current controversy surrounding the band gap of InN affects the conclusions which can be drawn concerning the presence of an electron accumulation layer. If the smaller band gap of 0.7 eV is assumed, then an electron accumulation would exist on the samples with both Au and Ti overlayers. Au was observed to grow as islands on the InAs surface as observed through both photoemission and atomic force microscopy. Au also appeared to grow through the formation of islands on annealed-InN when observed through photoemission spectroscopy.