Magnesium incorporation during the molecular beam epitaxy growth of wurtzite GaN is found to invert the Ga-polar (0001) face to the N-polar face. The polarity is identified based on the two different sets of reconstructions seen on the film prior to and after about 1 monolayer Mg exposure. The inversion boundary is seen to lie on the (0001) plane from transmission electron microscopy images, and a structural model is presented for the inversion. On the Ga-polar face, Mg is also seen to stabilize growth in the N-rich regime.

Scanning tunneling spectra have been measured on the 6H-SiC(0001) p 3 3 p 3 surface for both p-and n-type materials. With the use of exceptionally low tunnel currents, the tunneling spectra reveal distinct bands of empty and filled states, separated by 2.0 eV. The states are located at the same spatial position, thereby supporting a silicon adatom model which predicts a Mott-Hubbard-type density of states. [S0031-9007(98)08314-8] PACS numbers: 73.20.At, 07.79.Cz, 73.40.Cg Silicon carbide is a wide band gap semiconductor with properties that make it useful in high temperature electronics and other areas. SiC crystals exist with various stacking arrangements of the atomic planes; of particular interest are the cubic 3C polytype and hexagonal 4H and 6H polytypes. Much work has been done on characterizing the 6H-SiC(0001) surface as a surface suitable for epitaxy and device development. Several workers have studied the atomic structure of the surface using the scanning tunneling microscope (STM) [1,2]. A set of reconstructions of the Si terminated (0001) surface has been discovered, one of which, the p 3 3 p 3, has evoked much experimental [3][4][5] and theoretical [6][7][8][9] interest.Theoretically, the lowest energy model for this reconstruction consists of Si adatoms at T 4 positions on a Si terminated bulk crystal. Here, three of the Si adatom bonding orbitals are back-bonded to Si atoms. The fourth bonding orbital extends into vacuum with only one electron in it. Local density functional calculations for this structural model [6,8] predict a half-filled and hence metallic band arising from the dangling bond. More refined computations [7] performed after the photoemission [4] and inverse photoemission [5] spectra became available employed a two-dimensional Hubbard model. These calculations indicate that the energy levels of this surface consist of a filled band and an empty band, separated by a Hubbard gap of U 1.6 eV, thus producing a semiconducting density of states (DOS). Such a surface would have at each site a single localized electron, forming a 2D system of spins which can take two values on a triangular lattice. This problem is of great theoretical interest because such systems can be frustrated and form 2D spin glasses. This structure has been questioned in the literature, and several alternatives have been proposed [1,3,10,11], but experimental results to date are unable to distinguish between these models.Photoemission spectroscopy (PES) experiments on n-type 6H-SiC(0001) wafers which show a p 3 3 p 3 low energy electron diffraction (LEED) pattern reveal a fully filled band 1.2 eV below the Fermi level [4]. Inverse photoemission spectroscopy (IPES) [5] on the same surface shows an empty surface state 1.1 eV above the Fermi level. Both these results are in moderate agreement with recent theoretical predictions of a Mott-Hubbard ground state for this surface [7] as mentioned above.Scanning tunneling spectroscopy (STS) has a great advantage over PES and IPES in that a single tunneling spectrum can sho...

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Surface reconstructions during homoepitaxial growth of GaN (0001) are studied using reflection high-energy electron diffraction and scanning tunneling microscopy. In agreement with previous workers, a distinct transition from rough to smooth morphology is seen as a function of Ga to N ratio during growth. However, in contrast to some prior reports, no evidence for a 2×2 reconstruction during GaN growth is observed. Observations have been made using four different nitrogen plasma sources, with similar results in each case. A 2×2 structure of the surface can be obtained, but only during nitridation of the surface in the absence of a Ga flux.