n- GaN surface treatments for metal contacts studied via x-ray photoemission spectroscopy

Abstract

The surface chemistry and electronic properties of n-GaN surfaces were studied via x-ray photoemission spectroscopy before and after wet chemical treatments. Shifts of the surface Fermi level were measured with the change in position of the Ga 3d core level peak. HCl treatment of n-GaN led to a 0.9 eV shift of the surface Fermi level toward the conduction band minimum, while KOH treatment led to a 0.3 eV shift of the surface Fermi level toward the valance band maximum. These shifts lead to a reduction in the surface barrier for HCl-treated n-GaN and for KOH-treated p-GaN, potentially improving contact resistance. The changes in surface chemistry indicate that a N (or Ga) deficiency with HCl(KOH) treatment alters the surface state density through the formation of donor (acceptor)-like states.

X-ray photoemission determination of the Schottky barrier height of metal contacts to n–GaN and p–GaN

“…These effects could be due to a surface photovoltage effect or to a resistive voltage drop at the contacts of the sample and at the depletion region below the surface 25 and are mentioned in Ref. 4. These effects lead to a change in the band bending and can complicate the measurement of the surface-barrier height as previously observed during the investigation of Au deposition on GaN via photoemission.…”

“…For n-GaN, HCl has been shown previously to decrease the surface-barrier height presumably through creation of N vacancies that pin E F close to the conduction-band minimum ͑CBM͒. 4 For aqua regia treatment of p-GaN, it has been shown that E F pins near the valence-band maximum ͑VBM͒, coincident with the Ga vacancy states, decreasing the surface-barrier height. 7 Al and Ti may be expected to behave similarly, since both are strong nitride formers ͑AlN and TiN͒.…”

“…3,4 The effect of these treatments on the surface chemistry was correlated with the electronic properties through monitoring changes in the movement of the Ga 3d core level. The KOH treatments led to a decreased Ga/N ratio on the surface for both n-and p-GaN.…”

“…Contacts to p-type GaN have been particularly challenging, since it is difficult to grow samples with a high enough carrier concentration to promote field emission through the Schottky barrier. Due to the complexity of metallurgical contacts, various approaches have been tried, including chemical surface treatments, [3][4][5][6][7] plasma cleaning, [8][9][10] metal deposition techniques ͑sputtering, electron beam, or thermal deposition͒, [11][12][13] contact annealing in a variety of ambients, 14,15 and use of bilayers 16 -18 or multilayers [19][20][21] of various metals.…”

Scanning probe investigation of surface charge and surface potential of GaN-based heterostructures

“…Therefore, it appears that in addition to a surface dipole, the HCl process must modify the band bending at the N-face surface. 14 Considering all of the results here, the band bending at the as-received N-face surface is initially flat or slightly upward and increases as a result of the HCl clean. The deduced net charge is not large enough to account for the observed change in surface potential.…”