XPS Study of Oxygen Adsorption on (3X3) Reconstructed MBE Grown GaN Surfaces

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erformance of these devices, however, is still limited by a number of materials problems as well as difficulties in producing low resistance contacts. This is particularly true of p-type GaN where extremely high doping densities are required to produce p-type material. Reasonably good ohmic contacts to n-GaN have been found, and the interface chemistry of these contacts investigated. [ 1]

The effects of adsorbates on Fermi-level pinning and Schottky barrier heights, however, is not well understood. A study of the (I x I) GaN surface has shown an increase in electron affinity of -0.58 eV due to oxygen chemisorption. [2] These types of surface effects have strong implications for device fabrication. We report here a study of the (3x3) reconstructed surface of GaN(0001) including effects due to oxidation. EXPERIMENT GaN was grown on sapphire (0001) substrates by RF assisted molecular beam epitaxy using a low temperature GaN buffer layer. The substrate temperature was held at 800°C during growth under slightly metal-rich conditions. The surface morphology and reconstruction were characterized by reflection high energy electron diffraction (RHEED). The RHEED pattern was streaky during growth, and showed a clear (3x3) reconstruction after cool down. The (3x3) reconstruction has been determined to correspond to N-polar GaN. [31 Nitrogen polarity G 6.26

Mat. Res. Soc. Symp. Proc.Vol. 537 © 1999 Materials Research Society

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(DE 0 .2

c° 0 6Ga3d

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Ga3d Surf Bulk Error

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1000 L

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14 20 18 16 22 18 16 14 24 20 Binding Energy (eV) Binding Energy (eV) Figure 1: a) XPS spectrum of Ga3d area showing relaxed Ga side peak and corresponding fit. 24

22

b) Spectra showing metallic Ga3d peak removal by oxygen exposure.

corresponds to a crystal structure in which the bond parallel to the c-axis is from a nitrogen to a gallium atom, and results in a Ga terminated surface having three dangling bonds. The polarity of our sample was confirmed after data acquisition ex-situ by etching in molten KOH. [7] After growth, the samples were moved under vacuum to an XPS chamber via a UHV transfer tube. Data was then taken over 4 regions of interest: the Ga3d core level, N Is core level, O 1s core level, and the valence band region. Data acquisition was alternated with in-place exposure to 02 at room temperature via a leak valve. The sample is in line of sight of a hot filament and the oxygen can therefore be considered excited. Oxygen exposure was measured by monitoring system pressure, and is given in Langmuirs (L), were 1L=10-6Torr s. ANALYSIS Several trends were detected in the XPS spectra during oxidation of the (3x3) surface. It was found that the (3x3) reconstruction of the GaN surface results in a second peak on the lower binding energy side of the bulk Ga3d peak in GaN (Fig. la). The shift to lower binding energy is characteristic of metallic bonding between Ga atoms. This implies that the reconstruction corresponds to a fractional Ga adlayer atop a Ga terminated GaN surface. This is in agreement w