Rapid Communications are intended for the accelerated publication of important new results and are therefore gtven prt ort t'y' treatment both in the editorial ofhce and in production AR. apid Communication in physical Review n shouid be no ionger than 4 printed pages and must be accompanied by an abstract Pa.ge proofs are sent to authors Fluctuation attraction in condensed matter: A nonlocal functional approach By exploiting the analogy between the correlation charge surrounding an electron in an interacting electron gas and the electronic charge localized about nuclei in condensed atomic systems, we are led to a functional approach which partially preserves the nonlocality of fluctuations in an inhomogeneous system. In this way, the van der Waals and Axilrod-Teller interactions are determined solely from the fluctuation properties of the electron gas. The approach gives further insight into the local-density approximation.

Phonon excitations play an important role in electronic transport, nonradiative electron-relaxation processes, and other properties of interest for materials characterization, device engineering, and design. We have calculated the phonon dispersions and density of states for wurtzite AlN, GaN, and InN using state-of-the-art density-functional perturbation theory. The calculations are in good agreement with the existing experimental data for zone-center modes and predict the full phonon dispersions throughout the Brillouin zone. In particular, it is found that the three-phonon decay of the LO phonon in two acoustic phonons is not allowed in GaN and InN, since the LO frequency is much larger than the acoustic frequencies over the entire spectrum.

The influence of growth conditions and surface polarity upon the morphology of ͑0001͒ GaN surfaces is studied from first principles. The charge transfer between the Ga and N atoms in GaN and the very large electronegativity of nitrogen are found to play decisive roles in determining the stable reconstructions. Further, hydrogen stabilizes the ideally cleaved surface irrespective of polarity. For both polarities, adsorption of 3/4 of a monolayer of hydrogen results in a very stable surface with a 2ϫ2 symmetry. ͓S0163-1829͑97͒51544-X͔The commercial production of high-brightness blue LED's and the recent demonstration of laser diodes 1 have realized the expectations that have been held out for the III-V nitrides as optoelectronic materials. Advances in the application of this material to devices have been very closely tied to progress in the growth of epitaxial layers and heterostructures. Thus, a refined understanding of its growth is necessary for a further improvement of the quality of the material. In spite of the great effort that has been expended on the study of growth, even the polarity of the surface, i.e., whether the surface is Ga or N terminated, remains to be fully determined.2 The surface morphology depends critically upon polarity, 3 and the uncertainty about the surface polarity will hinder progress in growth technology.The two chief methods employed for epitaxial growth of GaN are metalorganic chemical vapor deposition ͑MOCVD͒ and molecular-beam epitaxy ͑MBE͒. Further, a variety of substrates and a spectrum of buffer layers have been used to grow GaN, making interpretation of the experimental data difficult. This is reflected in the fact that MBE-grown samples of unknown polarity exhibit a 2ϫ2 reconstruction, 4 while samples grown by MOCVD on sapphire have been shown to have different terminations depending upon the preparation of the buffer layer or growth parameters. 3,5,6 It is generally agreed that the Ga-terminated MOCVD-grown samples are smooth, which is consistent with the observed two-dimensional growth in MBE of a presumed Gaterminated surface. However, conflicting conclusions have been made about the N-terminated surface; it has been show to have inversion domains, 3 to be faceted, 5 and to have a 1ϫ1 reconstruction with a 3/4 monolayer of hydrogen coverage. 6In this situation, a theoretical characterization of the surface morphology can provide a framework for the interpretation of the experimental results. Thus, we have studied the dependence upon growth conditions of the 2ϫ2 reconstructions of clean GaN ͑0001͒ surfaces with both polarities. We find that the ionic nature of GaN and the very large electronegativity of nitrogen determine the adsorption sites for the lowest-energy adatom reconstructions and their subsequent behavior. Because the presence of hydrogen in MOCVD constitutes a major difference between the growth parameters of MBE and MOCVD, we have also studied the influence of adsorbed hydrogen on the surface morphology of GaN.All calculations were carried out within the frame...

A consistent approach to the calculation of the surface energy valid for all crystal systems is presented. Voronoi polyhedra are introduced and used in conjunction with the energy-density formalism of Chetty and Martin ͓Phys. Rev. B 45, 6074 ͑1992͒; 45, 6089 ͑1992͔͒ to provide a methodology for the determination of surface energies. The surface energies of the unrelaxed, unreconstructed GaAs ͑001͒ and ͑111͒ surfaces are calculated as a test. As an example of the application of the formalism to a low symmetry system, the energies of selected ͑0001͒ surfaces of the wide-gap semiconductors GaN and SiC are determined.

The interplay of surface termination, reconstruction patterns, and availability of species involved determines the incorporation of impurities during growth. We study ab initio this interplay for Mg at the GaN ͑0001͒ surfaces and find that optimal incorporation conditions strongly depend upon surface orientation and cannot be predicted using bulk stoichiometric arguments. With reasonable assumptions on the kinetics, high densities of Mg can be achieved in the absence of hydrogen and the Ga surface displays superior incorporation characteristics to that of the N surface. ͓S0163-1829͑99͒02615-6͔