Preparation and characterization of atomically clean, stoichiometric surfaces of n- and p-type GaN(0001)

It is demonstrated that in situ exposure of the (0001) surface of n- and p-type GaN thin films to flowing ammonia at 860 deg. C and 10-4 Torr removes hydrocarbon and oxygen/hydroxide species below the detectable limits of x-ray and ultraviolet photoelectron spectroscopies (UPS) and decreases the Ga/N ratio from 1.3 to 1.0. Additional indications of the efficacy of this ammonia-based chemical vapor cleaning (CVC) process were the shifts in the Ga 3d and the N 1s core level positions from the as-loaded to the CVC surfaces of the n-type samples from 21.0±0.1 to 20.6±0.1 eV and from 398.3±0.1 to 398.0±0.1 eV, respectively, and the change in the UPS measured low energy spectrum turn on from 3.9±0.1 (as-loaded samples) to 3.0±0.1 eV (cleaned samples) below the Fermi level. Analogous changes in the p-type samples were from 19.6±0.1 to 18.9±0.1 eV and from 397.1±0.1 to 396.3±0.1 eV for the Ga 3d and the N 1s core levels, and from 3.0±0.1 to 1.1±0.1 eV for the UPS valence band maximum (VBM). The VBM values of the CVC samples indicate band bending of ∼0.3 eV upward on n type and ∼0.8 eV downward on p type. Electron affinities of 2.8±0.1 and 2.6±0.1 eV were determined for the clean n-type and p-type surfaces, respectively. Irrespective of doping, the CVC process left unchanged the (1x1) low energy diffraction pattern, the terraced microstructure, and the root mean square roughness observed for the surfaces of the as-loaded samples, i.e., the surface microstructure was not damaged during the high temperature exposure to ammonia at low pressure