Electronic states of oxidized GaN(0001) surfaces

The structure and electronic properties of oxidized (0001) surfaces of GaN grown by plasma-assisted molecular beam epitaxy are investigated by scanning tunneling microscopy/spectroscopy, Auger electron spectroscopy, and first-principles theory. For oxygen exposure at room temperature an amorphous gallium oxide layer is found to form, resulting in a distribution of midgap electronic states extending out from the GaN valence band edge. The influence of these states on the electron concentration in buried AlGaN/GaN heterojunctions is discussed. The electronic properties of oxidized GaN and AlGaN surfaces are relevant for understanding the operation of heterojunction field-effect transistors (HFETs) with channel layers buried ≈50 nm below the surface [1-5]. For structures grown either by plasma-assisted molecular beam epitaxy (PAMBE) [2] or metal organic chemical vapor deposition (MOCVD) [1,3] it has been argued that the air-exposed surface possesses electronic states which act as donors, donating charge to the channel of the HFET. It is observed that the efficacy of this mechanism depends on the depth of the channel below the surface, which for studies of MOCVD-grown material leads to deduced energy level positions of 1.65 eV below the AlxGa1-xN (x=0.34) conduction band minimum (CBM) for an assumed single donor state [1], or 1.0–1.8 eV below the CBM for a band of such states [3]. For PAMBE-grown material the sensitivity of the transistor characteristics to the GaN surface properties has been employed for fabricating chemical sensors [4,5]. The (0001) surface of GaN grown by PAMBE under Ga-rich conditions is known to be covered by slightly more than 2 monolayers (ML=1.14×10 atoms/cm) of excess Ga [6-8]. Exposure of the PAMBE-grown surfaces to oxygen is thus expected to yield an oxide of Ga. A number of prior works have dealt with the structure of oxidized GaN surfaces for which the oxide is found to be generally of the Ga2O3 form [9], although, with one notable exception [10], these prior works did not start with a well defined Garich PAMBE-grown surface of the type studied here. Our studies reported here deal with surfaces that have been exposed to oxygen while held at room temperature (RT). We find a saturated oxygen coverage of about 2 ML, with a similar value for the excess Ga coverage. STM images reveal the formation of a disordered oxide on the surface. This metal oxide is found to have a band gap of ≈2 eV in size, although with tails of states extending into this gap. We find in particular that the spectrum of states extending in from the GaN valence band, with these states being normally filled i.e. donor-like, is