@article{bungaro_rapcewicz_bernholc_2000, title={Ab initio phonon dispersions of wurtzite AlN, GaN, and InN}, volume={61}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.61.6720}, abstractNote={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 substantial potential of the group-III nitrides and their alloys for applications in optoelectronic and high speed devices has attracted a great deal of interest. AlN, GaN, and InN have direct energy gaps which span a substantial range, from the visible to the ultraviolet region of the spectrum. Consequently, their alloys have direct gaps that can be tuned to any value within this range simply by varying the alloy composition. This tunability offers many possibilities for device engineering. In particular, InGaN alloys were used in the realization of light emitting diodes and laser diodes operating in the blue and UV spectral region. Under ambient conditions, the III nitrides crystallize in a hexagonal, wurtzite ~2H! structure, although thin films having a cubic, zincblende ~3C! structure have also been grown. 1 Properties of interest for device engineering and design, such as electronic transport, nonradiative electron relaxation processes, lattice specific heat, etc., are strongly influenced by phonon excitations. Furthermore, a number of nondestructive experimental techniques of sample characterization, for instance, Raman spectroscopy or IR reflectivity, involve phonon measurements. A characterization of the phonon dispersions and densities of states for the group-III nitrides is therefore desirable. However, since it is very challenging to grow single crystals of suitable size for neutron-scattering experiments, there are no experimental data for the phonon dispersions of these compounds. Only very recently have the phonon density of states for AlN and GaN been obtained from time-of-flight neutron spectroscopy using bulk powders. 2,3 In addition, numerous studies of the zone-center phonons in GaN and AlN films have been conducted using Raman and IR spectroscopy. Due to the lack of latticematched substrates, these samples are affected by the high density of defects and strain present in the films and it is therefore unclear how well these data represent the true bulk values. The least studied of the three nitrides is InN for which there are only few Raman studies. 4,5}, number={10}, journal={PHYSICAL REVIEW B}, author={Bungaro, C and Rapcewicz, K and Bernholc, J}, year={2000}, month={Mar}, pages={6720–6725} }
 @article{bernholc_briggs_bungaro_nardelli_fattebert_rapcewicz_roland_schmidt_zhao_2000, title={Large-scale applications of real-space multigrid methods to surfaces, nanotubes, and quantum transport}, volume={217}, ISSN={["1521-3951"]}, DOI={10.1002/(sici)1521-3951(200001)217:1<685::aid-pssb685>3.0.co;2-3}, abstractNote={The development and applications of real-space multigrid methods are discussed. Multigrid techniques provide preconditioning and convergence acceleration at all length scales, and therefore lead to particularly efficient algorithms. When using localization regions and optimized, non-orthogonal orbitals, calculations involving over 1000 atoms become practical on massively parallel computers. The applications discussed in this chapter include: (i) dopant incorporation and ordering effects during surface incorporation of boron, which lead to the formation of ordered domains at half-monolayer coverage; (ii) incorporation of Mg into GaN during growth, and in particular the conditions that would lead to maximum p-type doping; (iii) optical fingerprints of surface structures for use in real-time feedback control of growth: and (iv) mechanisms of stress release and quantum transport properties of carbon nanotubes.}, number={1}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Bernholc, J and Briggs, EL and Bungaro, C and Nardelli, MB and Fattebert, JL and Rapcewicz, K and Roland, C and Schmidt, WG and Zhao, Q}, year={2000}, month={Jan}, pages={685–701} }
 @article{boguslawski_rapcewicz_bernholc_2000, title={Surface segregation and interface stability of AlN/GaN, GaN/InN, and AlN/InN {0001} epitaxial systems}, volume={61}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.61.10820}, abstractNote={segregation energies are similar and an order of magnitude larger than in the arsenides or Si/Ge systems. The largest segregation energy of about 3.6 eV is found for the AlN/InN surface. In contrast, segregation effects for the N-adatom 232 reconstruction on the ~0001! surfaces are strongly suppressed. At AlN/GaN and GaN/InN interfaces the segregation effects are very weak and should not affect their morphology. Our results suggest the choice of optimal conditions of growth that lead to sharp interfaces and longer lifetimes of carriers in III-nitride heterosystems.}, number={16}, journal={PHYSICAL REVIEW B}, author={Boguslawski, P and Rapcewicz, K and Bernholc, JJ}, year={2000}, month={Apr}, pages={10820–10826} }
 @article{bungaro_rapcewicz_bernholc_1999, title={Surface sensitivity of impurity incorporation: Mg at GaN (0001) surfaces}, volume={59}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.59.9771}, abstractNote={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 ~0001! 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. @S0163-1829~99!02615-6# Controlled incorporation of impurities is critical to semiconductor science and technology. Specific dopant concentrations are necessary to achieve desired conductivities, recombination rates in light emitting devices, or other electrical characteristics. In general, doping of bulk materials can be performed in three different ways: ~i! by in-diffusion following growth, ~ii! by ion implantation, and ~iii! during growth. The in-diffusion of impurities is limited by the maximum solubility, which is a thermodynamic quantity that can be determined from either experiments or calculations. Ion implantation has no such limit, but in many materials the implantation damage is severe and cannot be annealed out, limiting the usefulness of this technique. During growth, the incorporation of an impurity occurs at the surface and depends sensitively upon the surface and its environment. For example, Tersoff 1 has emphasized that impurity energetics and consequently its solubility in the near-surface region can be significantly different from that in the bulk. This is because under typical growth conditions, diffusion at the surface is much faster than in the bulk and the impurity density of the near-surface region can be frozen in as the film grows,}, number={15}, journal={PHYSICAL REVIEW B}, author={Bungaro, C and Rapcewicz, K and Bernholc, J}, year={1999}, month={Apr}, pages={9771–9774} }
 @article{rapcewicz_chen_yakobson_bernholc_1998, title={Consistent methodology for calculating surface and interface energies}, volume={57}, ISSN={["1550-235X"]}, DOI={10.1103/physrevb.57.7281}, abstractNote={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 ~1992!; 45, 6089 ~1992!# to provide a methodology for the determination of surface energies. The surface energies of the unrelaxed, unreconstructed GaAs ~001! and ~111! surfaces are calculated as a test. As an example of the application of the formalism to a low symmetry system, the energies of selected ~0001! surfaces of the wide-gap semiconductors GaN and SiC are determined. @S0163-1829~98!02012-8#}, number={12}, journal={PHYSICAL REVIEW B}, author={Rapcewicz, K and Chen, B and Yakobson, B and Bernholc, J}, year={1998}, month={Mar}, pages={7281–7291} }
 @article{nardelli_rapcewicz_bernholc_1997, title={Polarization field effects on the electron-hole recombination dynamics in In0.2Ga0.8N/In1-xGaxN multiple quantum wells}, volume={71}, DOI={10.1063/1.120269}, abstractNote={The effect of the polarization field in wurtzite In0.2Ga0.8N/In1−xGaxN (x>0.8) multiple quantum wells is studied from first principles. The pyroelectric and piezoelectric fields naturally present in the system due to its wurtzite structure are strong enough to reduce the interband recombination rate in an ideal quantum well. We suggest that composition fluctuations, observed in the active region of actual devices, provide the necessary confinement for an improved recombination rate and lasing.}, number={21}, journal={Applied Physics Letters}, author={Nardelli, M. B. and Rapcewicz, K. and Bernholc, Jerzy}, year={1997}, pages={3135–3137} }
 @article{bernholc_briggs_sullivan_brabec_nardelli_rapcewicz_roland_wensell_1997, title={Real space multigrid methods for large scale electronic structure problems}, volume={65}, DOI={10.1002/(SICI)1097-461X(1997)65:5<531::AID-QUA18>3.0.CO;2-5}, abstractNote={We describe the development and applications of a new electronic structure method that uses a real-space grid as a basis. Multigrid techniques provide preconditioning and convergence acceleration at all length scales and therefore lead to particularly efficient algorithms. The salient points of our implementation include: (i) new compact discretization schemes in real space for systems with cubic, orthorhombic, and hexagonal symmetry and (ii) new multilevel algorithms for the iterative solution of Kohn–Sham and Poisson equations. The accuracy of the discretizations was tested by direct comparison with plane-wave calculations, when possible, and the results were in excellent agreement in all cases. These techniques are very suitable for use on massively parallel computers and in O(N) methods. Tests on the Cray-T3D have shown nearly linear scaling of the execution time up to the maximum number of processors (512). The above methodology was tested on a large number of systems, such as the C60 molecule, diamond, Si and GaN supercells, and quantum molecular dynamics simulations for Si. Large-scale applications include a simulation of surface melting of Si and investigations of electronic and structural properties of surfaces, interfaces, and biomolecules. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 531–543, 1997}, number={5}, journal={International Journal of Quantum Chemistry}, author={Bernholc, Jerzy and Briggs, E. L. and Sullivan, D. J. and Brabec, C. J. and Nardelli, M. B. and Rapcewicz, K. and Roland, C. and Wensell, M.}, year={1997}, pages={531–543} }
 @article{nardelli_rapcewicz_bernholc_1997, title={Strain effects on the interface properties of nitride semiconductors}, volume={55}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.55.r7323}, abstractNote={the range from the deep ultraviolet to the visible. 2 Not surprisingly the potential technological importance of these materials has elicited the interest of a number of theoretical groups. 3‐7 In spite of this, the strained interfaces of these lattice mismatched materials have not been studied. We find that strain effects are significant, inducing changes of 20% to 40% in the value of the band offset and that these changes increase with decreasing in-plane lattice parameter. The AlN/GaN/InN interfaces are all of type I, while the Al 0.5Ga 0.5N/AlN zinc-blende ~001! interface is found to be of type II. Finally, we studied the GaN/AlN wurtzite interface, where qualitatively new features, namely pyroelectric and piezoelectric effects, appear due to the low symmetry of the wurtzite lattice. The standard ab initio plane-wave pseudopotential method 8‐10 was employed in the calculations. The energy cutoff for the plane-wave expansion was 50 Ry to ensure convergence of the nitrogen pseudopotential. We used the equivalent of ten k points for bulk and superlattice calculations in the zinc-blende structure 11 and six k points for calculations of the wurtzite structure. 12 Convergence both in the size of the plane-wave basis and in the number of special points has been carefully checked. The Perdew-Zunger parametrization 13 of the Ceperley-Alder form 14 of the exchange-correlation energy was used. For interface calculations, we employed 414 superlattices ~16 atoms! along the ~001! and ~0001! directions. Nonlocal, norm-conserving pseudopotentials 15‐17 were included using the KleinmanBylander approach. 18 For nitrogen, we used a neutral con}, number={12}, journal={PHYSICAL REVIEW B}, author={Nardelli, MB and Rapcewicz, K and Bernholc, J}, year={1997}, month={Mar}, pages={R7323–R7326} }
 @article{nardelli_rapcewicz_bernholc_1997, title={Theory of interfaces and surfaces in wide-gap nitrides}, volume={15}, DOI={10.1116/1.589429}, abstractNote={A selection of the results of a theoretical investigation of the properties of interfaces and surfaces of the wide-gap III–V nitride semiconductors is reviewed. The electronic properties of wurtzite heteroepitaxial interfaces of AlN and GaN, incorporating the effects of strain, are discussed. In particular, we find that this interface is of type I and have calculated the valence-band offset to be −0.57 eV. The surface energies and atomic geometries of the 2×2 reconstructions of the (0001) face of GaN are also presented. In conditions which are rich in a given species, an adatom reconstruction of that species is found to be the most energetically favorable: for gallium-rich conditions, the reconstruction with a gallium adatom on a T3 site is the most stable, while for nitrogen-rich conditions the reconstruction with a nitrogen adatom on the H3 site is energetically the most favorable.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Nardelli, MB and Rapcewicz, K and Bernholc, J}, year={1997}, pages={1144–1147} }