Morphology and surface reconstructions of GaN(1 1 00) surfaces

GaN is grown by plasma assisted molecular beam epitaxy on ZnO(1 1 00) substrates. Well-oriented (1 1 00) GaN surfaces are obtained, and (1 1 01) oriented facets are also observed. On the GaN(1 1 00) surfaces under Ga-rich conditions a surface reconstruction with approximate symmetry of "4×5" is found. A model is proposed in which this reconstruction consists of 2 ≥ monolayers of Ga terminating the GaN surface. Reconstructions of GaN have been extensively studied in the past five years, primarily for (0001) and (000 1 ) surfaces [1]. Films with these surface orientations have well known pyroelectric and piezoelectric properties [2,3]. Although such properties can have useful device applications (such as confining carriers in a two-dimensional electron gas [2]), for other applications these properties are undesirable. For this reason a few growth studies of (10 1 0) oriented (m-plane) GaN have been performed since for this orientation the crystal symmetry precludes pyroelectric and piezoelectric effects (at least in the absence of shear stresses in the growth plane) [4]. In one recent study Waltereit et al. grew GaN on γ-LiAlO2(100) [4]. Well oriented (1 1 00) films with a slate like surface morphology were observed. No surface reconstructions were found on the films. In this work we have used plasma assisted molecular beam epitaxy (PAMBE) to grow GaN films on ZnO(1 1 00) substrates. With lattice mismatches of 2.0% and 0.5% in the a and c directions, respectively, ZnO offers an attractive substrate for GaN heteroepitaxy. We obtain well oriented (1 1 00) GaN films, although we also observe the formation of (1 1 01) facets (from which we deduce that the energies of (1 1 00) surfaces and (1 1 01) facets are comparable). Several reconstructions on the GaN(1 1 00) surfaces are found, with the most common having approximate symmetry of "4×5". We propose a model in which this structure consists of 2 ≥ monolayers of Ga terminating the GaN surface. ZnO(1 1 00) substrates were obtained from Cermet, Inc. The MBE growth and subsequent scanning tunneling microscopy (STM) was performed using a system previously described [1]. The ZnO substrates were cleaned prior to growth simply by heating them to about 550°C for 20 min. Growth was initiated by exposing the ZnO surface simultaneously to incident Ga and N fluxes. A ≈20 nm thick GaN layer is grown at a relatively low temperature of about 500°C, and subsequent growth was performed at about 700–750°C. (Following the growth, and with our MBE