Review of Structure of Bare and Adsorbate-Covered GaN(0001) Surfaces

Abstract

A review of surface structures of bare and adsorbate-covered GaN (0001) and (000) surfaces is presented, including results for In, Mg, Si, and H adsorbates. Emphasis is given to direct determination of surface structure employing experimental techniques such as scanning tunneling microscopy, electron diffraction, and Auger electron spectroscopy, and utilizing first principles computations of the total energy of various structural models. Different surface stoichiometries are studied experimentally by varying the surface preparation conditions (e.g. Ga-rich compared to N-rich), and the stoichiometry is included in the theory by performing calculations for various chemical potentials of the constituent atoms. Based on the work reviewed here, surface reconstructions for plasma-assisted molecular beam epitaxy growth of GaN (0001) and (000) surfaces are fairly well understood, but reconstructions for reactive molecular beam epitaxy or for metal-organic vapor phase epitaxy (both involving H, at moderate and high temperatures, respectively) are less well understood at present.

In‐situ ellipsometry: Identification of surface terminations during GaN growth

“…However, the surface roughening after continuous growth under N-rich conditions disable the stabilization of smooth N-rich surfaces and LEED measurements were difficult in many cases. The surface morphologies after growth under Ga-and N-rich conditions, however, agree well with prior observations [1,2,11,12]. The Ga-rich case shows smooth step-flow growth together with Ga droplets on a larger length scale while the N-rich case displays rough, 3-dimensional growth.…”

“…Thereby, the spectra with the higher overall amplitude are correlated to higher Ga beam fluxes using a Ga effusion cell temperature of 950°C. Low-energy electron diffraction (LEED) images of surfaces stabilized under these conditions reveal in all cases the presence of a "1×1" 1 + 1/6 LEED pattern with the six characteristic spots around the hexagonal bulk spots as known for Ga-bilayer on the surface [1]. In contrast, for N-rich conditions the áε 2 ñ spectra show a lower overall amplitude which refer to Ga effusion cell temperature of 910°C.…”

“…For the latter case much is already known concerning the surface structures during growth by varying the surface stoichiometry from Ga-rich to N-rich conditions [1,2]. Under Ga-rich conditions it has been established that slightly more than 2 ML (ML = monolayer =1.14×10 15 atoms/cm 2 ) of Ga resides on the surface in a metallic fluid-like state [2].…”

“…Under Ga-rich conditions it has been established that slightly more than 2 ML (ML = monolayer =1.14×10 15 atoms/cm 2 ) of Ga resides on the surface in a metallic fluid-like state [2]. This Ga bilayer has been shown to have a dramatic influence on the kinetics of the growing surface [1]. Finally, it is known, that the presence of this Ga bilayer is necessary for a smooth layer by layer growth of GaN in PAMBE.…”

“…Moreover, it turns out that ellipsometry can be used to characterize the surface reconstruction in wurtzite GaN similar as reflectance anisotropy does for cubic III -V compounds. The optical spectra for the PAMBE reveal clear differences between growth under Ga-rich and N-rich conditions, which are attributed to the presence of a Ga-bilayer and various N-rich reconstructions on the surface [1]. …”