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Title: Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces

Abstract

The surface orientation can have profound effects on the atomic-scale processes of crystal growth and is essential to such technologies as GaN-based light-emitting diodes and high-power electronics. We investigate the dependence of homoepitaxial growth mechanisms on the surface orientation of a hexagonal crystal using kinetic Monte Carlo simulations. To model GaN metal-organic vapor phase epitaxy, in which N species are supplied in excess, only Ga atoms on a hexagonal close-packed (HCP) lattice are considered. The results are thus potentially applicable to any HCP material. Growth behaviors on c-plane (0001) and m-plane (011¯0) surfaces are compared. We present a reciprocal space analysis of the surface morphology, which allows extraction of growth mode boundaries and direct comparison with surface X-ray diffraction experiments. For each orientation, we map the boundaries between 3-dimensional, layer-by-layer, and step flow growth modes as a function of temperature and growth rate. Two models for surface diffusion are used, which produce different effective Ehrlich-Schwoebel step-edge barriers and different adatom diffusion anisotropies on m-plane surfaces. Simulation results in agreement with observed GaN island morphologies and growth mode boundaries are obtained. These indicate that anisotropy of step edge energy, rather than adatom diffusion, is responsible for the elongated islands observed onmore » m-plane surfaces. As a result, island nucleation spacing obeys a power-law dependence on growth rate, with exponents of –0.24 and –0.29 for the m- and c-plane, respectively.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22), Scientific User Facilities Division
OSTI Identifier:
1373393
Grant/Contract Number:
AC02-06CH11357
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Chemical Physics
Additional Journal Information:
Journal Volume: 146; Journal Issue: 14; Journal ID: ISSN 0021-9606
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; kinetic Monte Carlo; semiconductor; thin film growth

Citation Formats

Xu, Dongwei, Zapol, Peter, Stephenson, G. Brian, and Thompson, Carol. Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces. United States: N. p., 2017. Web. doi:10.1063/1.4979843.
Xu, Dongwei, Zapol, Peter, Stephenson, G. Brian, & Thompson, Carol. Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces. United States. doi:10.1063/1.4979843.
Xu, Dongwei, Zapol, Peter, Stephenson, G. Brian, and Thompson, Carol. Wed . "Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces". United States. doi:10.1063/1.4979843. https://www.osti.gov/servlets/purl/1373393.
@article{osti_1373393,
title = {Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces},
author = {Xu, Dongwei and Zapol, Peter and Stephenson, G. Brian and Thompson, Carol},
abstractNote = {The surface orientation can have profound effects on the atomic-scale processes of crystal growth and is essential to such technologies as GaN-based light-emitting diodes and high-power electronics. We investigate the dependence of homoepitaxial growth mechanisms on the surface orientation of a hexagonal crystal using kinetic Monte Carlo simulations. To model GaN metal-organic vapor phase epitaxy, in which N species are supplied in excess, only Ga atoms on a hexagonal close-packed (HCP) lattice are considered. The results are thus potentially applicable to any HCP material. Growth behaviors on c-plane (0001) and m-plane (011¯0) surfaces are compared. We present a reciprocal space analysis of the surface morphology, which allows extraction of growth mode boundaries and direct comparison with surface X-ray diffraction experiments. For each orientation, we map the boundaries between 3-dimensional, layer-by-layer, and step flow growth modes as a function of temperature and growth rate. Two models for surface diffusion are used, which produce different effective Ehrlich-Schwoebel step-edge barriers and different adatom diffusion anisotropies on m-plane surfaces. Simulation results in agreement with observed GaN island morphologies and growth mode boundaries are obtained. These indicate that anisotropy of step edge energy, rather than adatom diffusion, is responsible for the elongated islands observed on m-plane surfaces. As a result, island nucleation spacing obeys a power-law dependence on growth rate, with exponents of –0.24 and –0.29 for the m- and c-plane, respectively.},
doi = {10.1063/1.4979843},
journal = {Journal of Chemical Physics},
number = 14,
volume = 146,
place = {United States},
year = {Wed Apr 12 00:00:00 EDT 2017},
month = {Wed Apr 12 00:00:00 EDT 2017}
}

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  • Purpose: The scope of this study was to determine a complete set of correction factors for several detectors in static small photon fields for two linear accelerators (linacs) and for several detectors. Methods: Measurements for Monte Carlo (MC) commissioning were performed for two linacs, Siemens Primus and Elekta Synergy. After having determined the source parameters that best fit the measurements of field specific output factors, profiles, and tissue-phantom ratio, the generalized version of the classical beam quality correction factor for static small fields, k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}},more » were determined for several types of detectors by using the egs{sub c}hamber Monte Carlo user code which can accurately reproduce the geometry and the material composition of the detector. The influence of many parameters (energy and radial FWHM of the electron beam source, field dimensions, type of accelerator) on the value of k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}} was evaluated. Moreover, a MC analysis of the parameters that influence the change of k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}} as a function of field dimension was performed. A detailed analysis of uncertainties related to the measurements of the field specific output factor and to the Monte Carlo calculation of k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}} was done. Results: The simulations demonstrated that the correction factor k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}} can be considered independent from the quality beam factor Q in the range 0.68 {+-} 0.01 for all the detectors analyzed. The k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}} of PTW 60012 and EDGE diodes can be assumed dependent only on the field size, for fields down to 0.5 x 0.5 cm{sup 2}. The microLion, and the microchambers, instead, must be used with some caution because they exhibit a slight dependence on the radial FWHM of the electron source, and therefore, a correction factor only dependent on field size can be used for fields {>=}0.75 x 0.75 and {>=}1.0 x 1.0 cm{sup 2}, respectively. The analysis of uncertainties gave an estimate of uncertainty for the 0.5 x 0.5 cm{sup 2} field of about 0.7% (1{sigma}) for k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}} factor and of about 1.0% (1{sigma}) for the field output factor, {Omega}{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}}, of diodes, microchambers, and microLion. Conclusions: Stereotactic diodes with the appropriate k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}} are recommended for determining {Omega}{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}} of small photon beams.« less
  • Our study compares the physical, chemical and electrical properties of Al 2O 3 thin films deposited on gallium polar c- and nonpolar m -plane GaN substrates by atomic layer deposition (ALD). Correlations were sought between the film's structure, composition, and electrical properties. The thickness of the Al 2O 3 films was 19.2 nm as determined from a Si witness sample by spectroscopic ellipsometry. We measured the gate dielectric was slightly aluminum-rich (Al:O=1:1.3) from X-ray photoelectron spectroscopy (XPS) depth profile, and the oxide-semiconductor interface carbon concentration was lower on c -plane GaN. The oxide's surface morphology was similar on both substrates,more » but was smoothest on c -plane GaN as determined by atomic force microscopy (AFM). Circular capacitors (50-300 μm diameter) with Ni/Au (20/100 nm) metal contacts on top of the oxide were created by standard photolithography and e-beam evaporation methods to form metal-oxide-semiconductor capacitors (MOSCAPs). Moreover, the alumina deposited on c -plane GaN showed less hysteresis (0.15 V) than on m -plane GaN (0.24 V) in capacitance-voltage (CV) characteristics, consistent with its better quality of this dielectric as evidenced by negligible carbon contamination and smooth oxide surface. These results demonstrate the promising potential of ALD Al 2O 3 on c -plane GaN, but further optimization of ALD is required to realize the best properties of Al 2O 3 on m -plane GaN.« less
  • Purpose: In a previous work, output ratio (OR{sub det}) measurements were performed for the 800 MU/min CyberKnife{sup ®} at the Oscar Lambret Center (COL, France) using several commercially available detectors as well as using two passive dosimeters (EBT2 radiochromic film and micro-LiF TLD-700). The primary aim of the present work was to determine by Monte Carlo calculations the output factor in water (OF{sub MC,w}) and the k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}} correction factors. The secondary aim was to study the detector response in small beams using Monte Carlomore » simulation. Methods: The LINAC head of the CyberKnife{sup ®} was modeled using the PENELOPE Monte Carlo code system. The primary electron beam was modeled using a monoenergetic source with a radial gaussian distribution. The model was adjusted by comparisons between calculated and measured lateral profiles and tissue-phantom ratios obtained with the largest field. In addition, the PTW 60016 and 60017 diodes, PTW 60003 diamond, and micro-LiF were modeled. Output ratios with modeled detectors (OR{sub MC,det}) and OF{sub MC,w} were calculated and compared to measurements, in order to validate the model for smallest fields and to calculate k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}} correction factors, respectively. For the study of the influence of detector characteristics on their response in small beams; first, the impact of the atomic composition and the mass density of silicon, LiF, and diamond materials were investigated; second, the material, the volume averaging, and the coating effects of detecting material on the detector responses were estimated. Finally, the influence of the size of silicon chip on diode response was investigated. Results: Looking at measurement ratios (uncorrected output factors) compared to the OF{sub MC,w}, the PTW 60016, 60017 and Sun Nuclear EDGE diodes systematically over-responded (about +6% for the 5 mm field), whereas the PTW 31014 Pinpoint chamber systematically under-responded (about −12% for the 5 mm field). OR{sub det} measured with the SFD diode and PTW 60003 diamond detectors were in good agreement with OF{sub MC,w} except for the 5 mm field size (about −7.5% for the diamond and +3% for the SFD). A good agreement with OF{sub MC,w} was obtained with the EBT2 film and micro-LiF dosimeters (deviation less than 1.4% for all fields investigated). k{sub Q{sub c{sub l{sub i{sub n,Q{sub m{sub s{sub r}{sup f{sub c}{sub l}{sub i}{sub n},f{sub m}{sub s}{sub r}}}}}}}}} correction factors for several detectors used in this work have been calculated. The impact of atomic composition on the dosimetric response of detectors was found to be insignificant, unlike the mass density and size of the detecting material. Conclusions: The results obtained with the passive dosimeters showed that they can be used for small beam OF measurements without correction factors. The study of detector response showed that OR{sub det} is depending on the mass density, the volume averaging, and the coating effects of the detecting material. Each effect was quantified for the PTW 60016 and 60017 diodes, the micro-LiF, and the PTW 60003 diamond detectors. None of the active detectors used in this work can be recommended as a reference for small field dosimetry, but an improved diode detector with a smaller silicon chip coated with tissue-equivalent material is anticipated (by simulation) to be a reliable small field dosimetric detector in a nonequilibrium field.« less
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