skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Surface oxidation of GaN(0001): Nitrogen plasma-assisted cleaning for ultrahigh vacuum applications

Abstract

The cleaning of metal-organic vapor-phase epitaxial GaN(0001) template layers grown on sapphire has been investigated. Different procedures, performed under ultrahigh vacuum conditions, including degassing and exposure to active nitrogen from a radio frequency nitrogen plasma source have been compared. For this purpose, x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and scanning tunneling microscopy have been employed in order to assess chemical as well as structural and morphological surface properties. Initial degassing at 600 °C under ultrahigh vacuum conditions only partially eliminates the surface contaminants. In contrast to plasma assisted nitrogen cleaning at temperatures as low as 300 °C, active-nitrogen exposure at temperatures as high as 700 °C removes the majority of oxide species from the surface. However, extended high-temperature active-nitrogen cleaning leads to severe surface roughening. Optimum results regarding both the removal of surface oxides as well as the surface structural and morphological quality have been achieved for a combination of initial low-temperature plasma-assisted cleaning, followed by a rapid nitrogen plasma-assisted cleaning at high temperature.

Authors:
 [1]; ; ; ; ;  [2]
  1. Institute of Solid State Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany and Department of Physics, Birla Institute of Technology and Science, Pilani, 333031 Rajasthan (India)
  2. Institute of Solid State Physics, University of Bremen, P.O. Box 330440, 28334 Bremen (Germany)
Publication Date:
OSTI Identifier:
22318058
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Vacuum Science and Technology. A, Vacuum, Surfaces and Films; Journal Volume: 32; Journal Issue: 5; Other Information: (c) 2014 American Vacuum Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; DEGASSING; ELECTRON DIFFRACTION; GALLIUM NITRIDES; NITROGEN; ORGANOMETALLIC COMPOUNDS; OXIDATION; OXIDES; PLASMA; PRESSURE RANGE BELOW 1 NANO PA; PRESSURE RANGE MICRO PA; PRESSURE RANGE NANO PA; SAPPHIRE; SURFACE CLEANING; X-RAY PHOTOELECTRON SPECTROSCOPY

Citation Formats

Gangopadhyay, Subhashis, Schmidt, Thomas, E-mail: tschmidt@ifp.uni-bremen.de, Kruse, Carsten, Figge, Stephan, Hommel, Detlef, and Falta, Jens. Surface oxidation of GaN(0001): Nitrogen plasma-assisted cleaning for ultrahigh vacuum applications. United States: N. p., 2014. Web. doi:10.1116/1.4886956.
Gangopadhyay, Subhashis, Schmidt, Thomas, E-mail: tschmidt@ifp.uni-bremen.de, Kruse, Carsten, Figge, Stephan, Hommel, Detlef, & Falta, Jens. Surface oxidation of GaN(0001): Nitrogen plasma-assisted cleaning for ultrahigh vacuum applications. United States. doi:10.1116/1.4886956.
Gangopadhyay, Subhashis, Schmidt, Thomas, E-mail: tschmidt@ifp.uni-bremen.de, Kruse, Carsten, Figge, Stephan, Hommel, Detlef, and Falta, Jens. Mon . "Surface oxidation of GaN(0001): Nitrogen plasma-assisted cleaning for ultrahigh vacuum applications". United States. doi:10.1116/1.4886956.
@article{osti_22318058,
title = {Surface oxidation of GaN(0001): Nitrogen plasma-assisted cleaning for ultrahigh vacuum applications},
author = {Gangopadhyay, Subhashis and Schmidt, Thomas, E-mail: tschmidt@ifp.uni-bremen.de and Kruse, Carsten and Figge, Stephan and Hommel, Detlef and Falta, Jens},
abstractNote = {The cleaning of metal-organic vapor-phase epitaxial GaN(0001) template layers grown on sapphire has been investigated. Different procedures, performed under ultrahigh vacuum conditions, including degassing and exposure to active nitrogen from a radio frequency nitrogen plasma source have been compared. For this purpose, x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and scanning tunneling microscopy have been employed in order to assess chemical as well as structural and morphological surface properties. Initial degassing at 600 °C under ultrahigh vacuum conditions only partially eliminates the surface contaminants. In contrast to plasma assisted nitrogen cleaning at temperatures as low as 300 °C, active-nitrogen exposure at temperatures as high as 700 °C removes the majority of oxide species from the surface. However, extended high-temperature active-nitrogen cleaning leads to severe surface roughening. Optimum results regarding both the removal of surface oxides as well as the surface structural and morphological quality have been achieved for a combination of initial low-temperature plasma-assisted cleaning, followed by a rapid nitrogen plasma-assisted cleaning at high temperature.},
doi = {10.1116/1.4886956},
journal = {Journal of Vacuum Science and Technology. A, Vacuum, Surfaces and Films},
number = 5,
volume = 32,
place = {United States},
year = {Mon Sep 01 00:00:00 EDT 2014},
month = {Mon Sep 01 00:00:00 EDT 2014}
}
  • Low-temperature remote plasma-assisted oxidation and nitridation processes for interface formation and passivation have been extended from Si and SiC to GaN. The initial oxidation kinetics and chemical composition of thin interfacial oxide were determined from analysis of on-line Auger electron spectroscopy features associated with Ga, N, and O. The plasma-assisted oxidation process is self-limiting with power-law kinetics similar to those for the plasma-assisted oxidation of Si and SiC. Oxidation using O{sub 2}/He plasma forms nearly pure GaO{sub x}, and oxidation using 1% N{sub 2}O in N{sub 2} forms GaO{sub x}N{sub y} with small nitrogen content, {approx}4-7 at. %. The interfacemore » and dielectric layer quality was investigated using fabricated GaN metal-oxide-semiconductor capacitors. The lowest density of interface states was achieved with a two-step plasma-assisted oxidation and nitridation process before SiO{sub 2} deposition.« less
  • The surface morphology and the spatial distribution of defect-related luminescence of GaN(0001) layers grown by plasma-assisted molecular-beam epitaxy under gallium-rich conditions has been investigated. Droplets of liquid gallium form on the surface during growth and lead to distinct spiral hillocks under the droplet. The droplets are surrounded by extended voids which point to an incomplete gallium adlayer on the GaN surface during growth at the droplet boundary. Cathodoluminescence spectra indicate an enhanced intensity in the yellow spectral range for the GaN under the droplets which is attributed to a change in the local density of point defects in the layer.more » {copyright} 2001 American Institute of Physics.« less
  • The growth mode of N-face GaN deposited on AlN(0001) by plasma-assisted molecular beam epitaxy has been investigated. Based on reflection high-energy electron diffraction experiments, we demonstrate that for appropriate Ga fluxes and substrate temperature, a self-regulated 1-ML-thick Ga excess film can be formed on the growing surface. Depending on the presence of this Ga monolayer, the growth can proceed following either the Stranski-Krastanow or the Frank Van der Merwe growth modes, hence enabling the synthesis of either quantum dots or quantum wells.
  • We present a study of the evolution of the Ga adlayer during plasma-assisted molecular-beam epitaxy of (0001) GaN as a function of both Ga flux and growth temperature. In situ quadrupole mass spectrometry was used to quantitatively determine the adsorbed Ga coverage by monitoring its subsequent desorption after GaN growth. Independent of the growth time, the Ga adlayer was found to form steady-state coverages that increase continuously from 0 to 2.5 monolayers when raising the Ga flux from N-rich to moderate Ga-rich growth conditions. At higher Ga fluxes or lower growth temperatures, macroscopic Ga droplets form on top of themore » Ga adlayer (Ga droplet regime). Based on the temperature dependency for the transition between the Ga adlayer and Ga droplet regime, we determined an apparent activation energy of 3.4 eV, which is discussed with respect to previously reported values.« less
  • The thermodynamic aspects of indium-face InN growth by radio frequency plasma-assisted molecular-beam epitaxy (rf-MBE) and the nucleation of InN on gallium-face GaN (0001) surface were investigated. The rates of InN decomposition and indium desorption from the surface were measured in situ using reflected high-energy electron diffraction and the rf-MBE 'growth window' of In-face InN (0001) was identified. It is shown that sustainable growth can be achieved only when the arrival rate of active nitrogen species on the surface is higher than the arrival rate of indium atoms. The maximum substrate temperature permitting InN growth as a function of the activemore » nitrogen flux was determined. The growth mode of InN on Ga-face GaN (0001) surface was investigated by reflected high-energy electron diffraction and atomic force microscopy. It was found to be of the Volmer-Weber-type for substrate temperatures less than 350 deg. C and of the Stranski-Krastanov for substrate temperatures between 350 and 520 deg. C. The number of monolayers of initial two-dimensional growth, in the case of Stranski-Krastanov mode, varies monotonically with substrate temperature, from 2 ML at 400 deg. C to about 12 ML at 500 deg. C. The evolution and coalescence of nucleated islands were also investigated as a function of substrate temperature. It was found that at higher temperature their coalescence is inhibited leading to porous-columnar InN thin films, which exhibit growth rates higher than the nominal value. Therefore, in order to achieve continuous InN layers on GaN (0001) a two-step growth approach is introduced. In that approach, InN is nucleated at low temperatures on GaN and the growth continues until full coalescence of the nucleated islands. Subsequently, this nucleation layer is overgrown at higher substrate temperature in order to achieve high-quality continuous films. The InN films grown by the two-step method were investigated by x-ray diffraction, Hall-effect measurements, and transmission electron microscopy. It was found that the lattice mismatch between InN and GaN is almost completely accommodated by the development of a misfit dislocation network at the interface. Optimum group-III to active nitrogen flux ratios and substrate temperature conditions were identified for the two-step growth process. Films, grown under those conditions, exhibited full width at half maximum of x-ray rocking curves at (0004) and (1015) diffractions equal to 360 and 435 arc sec, respectively. Room-temperature Hall mobility was found to depend sensitively on the group-III to active nitrogen flux ratio during growth of the main step and to be independent of the structural properties of the films. Mobilities up to 860 cm{sup 2}/V s at carrier concentration of 1.6x10{sup 19} cm{sup -3} were measured.« less