The Influence of Oxygen and Carbon Contaminants on the Valence Band of p-GaN(0001)

Abstract

Mg-doped GaN(0001) epitaxial layers, grown by molecular beam epitaxy in a setup interconnected with an analytic chamber, were analysed by means of X-ray photoelectron spectroscopy to study their physicochemical properties under different preparation methods. Investigations were carried out for the following samples: asgrown, air-exposed and treated with isopropanol (IPA) or HCl, and in situ cleaned. The X-ray photoelectron spectroscopy results confirmed that the air-exposed samples are contaminated with oxygen and carbon atoms, which can be removed only by in situ cleaning. The valence band maximum varies in the range from 1.2 eV to 2.7 eV below the Fermi level for the as-grown and ex situ prepared samples, respectively. The valence band maximum for in situ cleaned sample is located at 1.7 eV below the Fermi level.

Properties of Bare and Thin-Film-Covered GaN(0001) Surfaces

“…Cleaning the surface of GaN crystals under UHV is problematic because it can lead to changes in the surface stoichiometry, regardless of whether it is an ion bombardment or annealing. Ion bombardments may introduce defects and preferentially remove nitrogen atoms [62,70,71]. To eliminate this effect, the postbombardment annealing method or low-energy N-ion bombardment can be utilized [72][73][74].…”

“…Despite the wide availability of professional literature on these topics and some very good and extensive reviews [59][60][61], there are still papers containing misinterpretations of the basic results obtained in photoemission experiments. This is, among other things, due to (i) the relatively difficult cleaning procedure of the GaN surface, which may lead to changes in its stoichiometry-the bare and metal-covered surface can easily be enriched with gallium [29,34,62], which can lead to a flawed interpretation of the deconvoluted Ga-3d components; (ii) the presence of a large number of Ga Auger lines, which may overlap other core level spectra; (iii) surface photovoltage (SPV) effects, which can cause changes in the Fermi level position versus the valence band maximum [63,64]. Furthermore, SVP can sometimes even lead to the appearance of a quasi-Fermi level in the metal/GaN system [30], which can be mistakenly interpreted as a chemical shift.…”

Interfacial Polarization of Thin Alq3, Gaq3, and Erq3 Films on GaN(0001)

“…[ 30 , 31 ], which is most likely due to the fact that the initial surface of the substrates used in this report is depleted of holes. On GaN(0001) close to the conduction band minimum, there is a surface state which derives from Ga dangling bonds [ 32 , 33 ], thus, in the case of p-type GaN, the Fermi level pinning to this state leads to a strong band banding, which is the common observation [ 34 , 35 , 36 , 37 ]. Giving that the substrate is Mg-doped, the formation of depletion region is shown.…”

Properties of Thin Film-Covered GaN(0001) Surfaces

“…Cleaning the surface of GaN crystals under UHV is problematic because it can lead to changes in the surface stoichiometry, regardless of whether it is an ion bombardment or annealing. Ion bombardments may introduce defects and preferentially remove nitrogen atoms [62,70,71]. To eliminate this effect, the post-bombardment annealing method or low-energy N-ion bombardment can be utilized [72][73][74].…”

“…This is, among other things, due to: (i) the relatively difficult cleaning procedure of the GaN surface, which may lead to changes in its stoichiometry. The bare and metal-covered surface can easily be enriched with gallium [29,34,62], which can lead to a bad interpretation of the deconvoluted Ga-3d components; (ii) the presence of a large number of Ga Auger lines which may overlap others' core level spectra; and (iii) surface photovoltage (SPV) effects which can cause changes in the Fermi level position versus the valence band maximum [63,64]. Furthermore, SVP can sometimes even lead to the appearance of a quasar Fermi level in the metal/GaN system [30], which can be mistakenly interpreted as a chemical shift.…”