Photoemission Study of the Electronic Structure of Wurtzite GaN(0001) Surfaces
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DOWNES,* D. DOPPALAPUDI,** and THEODORE D. MOUSTAKAS.**
• Department of Physics, Boston University, Boston, MA 02215 •* Department ofElectrical and Computer Engineering, Boston University, Boston, MA 02215.
ABSTRACT The surface electronic structure of wurtzite GaN (0001) (1 x 1) has been investigated using angle-resolved photoemission spectroscopy. Surfaces were cleaned by repeated cycles of N 2 ion bombardment and annealing in ultra-high vacuum. A well-defined surface state below the top of the valence band is clearly observed. This state is sensitive to the adsorption of both activated H 2 and 02, and exists in a projected bulk band gap, below the valence band maximum. The state shows no dispersion perpendicular or parallel to the surface. The symmetry of this surface state is even with respect to the mirror planes of the surface and polarization measurements indicate that it is of sp, character, consistent with a dangling bond state.
INTRODUCTION Despite the technological importance of refractory III-V nitrides as materials for wide band gap semiconductor devices, there is a significant lack of experimental data concerning the basic electronic structure of such films [ 1]. Only recently have the intrinsic band structure, the
density of states, and the electronic properties of surfaces come under scrutiny. A fundamental understanding of electronic structure issues is required ifthese materials are to achieve their full technological potential. Of particular importance is to understand the electronic properties of both clean and adsorbate covered nitride surfaces. We report here a high resolution angle resolved photoemission study of the electronic structure of clean wurtzite GaN(0001) surfaces. Surfaces were cleaned by a sputter/anneal technique. We have observed an occupied surface state which exists below the valence band maximum (VBM) and not in the direct gap. This state has sp, character, and exhibits little or no dispersion along the high symmetry directions of the surface Brillouin zone. The ARP spectra indicate that this state is essentially destroyed by adsorption of 02 or activated H2, or by ion bombardment of the surface.
ANGLE RESOLVED PHOTOEMISSION SPECTROSCOPY In angle resolved photoemission spectroscopy (ARP) the sample is illuminated with monochromatic ultraviolet radiation, leading to the photoemission of electrons. The kinetic energy and momentum of these electrons is then measured typically using a movable hemispherical electrostatic analyzer. The kinetic energy of the emitted electron can be related to the binding energy of the electron state (band) inside the solid, while the electron momentum
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in vacuum can similarly be related to the band momentum (k vector). Given the short mean free path of electrons in solids at the kinetic energies typically encountered in ARP, this spectroscopy is inherently surface sensitive, with a sampling depth of on the order of 5 - 10 A. Since bulk wavefunctions extend continuously
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