Study of Low-Energy Ion Assisted Epitaxy of Gan Films: Influence of the Initial Growth Rate
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*Universitdt Augsburg, Institut ftir Physik, Universitatsstr. 1, D-86135 Augsburg, [email protected] "**Lawrence Berkeley National Laboratory, Berkeley, CA 94720
ABSTRACT The deposition of thin epitaxial hexagonal gallium nitride films on c-plane sapphire by lowenergy nitrogen ion assisted deposition is shown to result in films of high crystalline quality. The quality can be further heightened by using the concept of an isothermal growth rate ramp. Characterization of film structure, defect density distribution and surface topography by XRD, RBS/C, and AFM, respectively, reveals the importance of the nitrogen ion energy and the ion to atom ratio on the film properties.
INTRODUCTION Hexagonal gallium nitride films on sapphire have attracted increasing interest in various scientific and industrial fields because of their excellent intrinsic physical properties. However, for industrial applications there are still many unresolved problems caused by the extrinsic physical and chemical properties of gallium nitride films. For example, the crystalline properties of the prepared films are characterized by a large lattice mismatch, a high dislocation density and a poor surface topography. One possibility to overcome these difficulties could be to prepare GaN films by ion assisted deposition. Recently, the low-energy nitrogen or ammonia ion assisted
deposition has been successfully used for the preparation of GaN films [ 1-5]. In this paper the influence of the initial growth rate on the structural properties of hexagonal GaN films grown on c-plane sapphire by low-energy ion assisted deposition is examined. The experiments were carried out at ion energies smaller than those to produce atom displacements below the first monolayer but higher than the energy to initiate displacements in the first monolayer and to stimulate the adatom mobility at the surface. Therefore, the nitrogen ion energy was chosen to be smaller than 25 eV, because this energy is the bulk displacement threshold energy of GaN [6]. EXPERIMENT GaN films were grown in a high vacuum chamber with a base pressure of 5x10-6 Pa. A conventional effusion cell was used to evaporate gallium with fluxes ranging from 1.0 to 3.Ox 1014 at.cm-2s 1 . Nitrogen ions with energies below 25 eV were delivered by a constricted glow discharge plasma source with an ion current density of 100 jiA/cm 2 at the sample [7]. The ion beam consisted of atomic and molecular nitrogen ions, as well as excited atomic and molecular nitrogen neutrals. For formation of GaN atomic nitrogen is required, as gallium does not chemically react with molecular nitrogen. In case that the nitrogen molecule has a kinetic
239 Mat. Res. Soc. Symp. Proc. Vol. 585 © 2000 Materials Research Society
energy exceeding the dissociation energy of molecular nitrogen (; 10 eV), the molecule will dissociate when colliding with the sample surface, delivering two reactive nitrogen atoms to the growing film. C-plane sapphire (A120 3) was used as substrate. The substrate temperature during growth
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