The Influence of Crystal Structure on the Lattice Sites and Formation Energies of Hydrogen in Wurtzite and Zinc-Blende GaN Page: 4 of 19
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Hydrogen is a common impurity in GaN films grown via metalorganic vapor-phase
deposition (MOCVD) where it is introduced either from the source compounds or from H2
when used as a carrier gas.1 Hydrogen may also be introduced after growth for purposes
of device isolation,2 as a byproduct of various processing steps,' or simply to study its
annealing behavior.24 Understanding the annealing behavior of hydrogen in GaN is
particularly important because either low-energy electron-beam irradiation5 or a post-growth
anneal step6 is needed to dissociate Mg-H complexes, and activate p-type doping in
MOCVD-grown material. Moreover, hydrogen is retained in GaN up to a temperature of
approximately 900 C, and is therefore expected to play a significant role in nearly all
processing stages of GaN-based devices.7
Because of its importance to emerging nitride-based technologies, hydrogen in GaN
has been the focus of a number of recent experimental and theoretical studies. (See
chapters 6 and 11 in Ref. 1 for reviews of theoretical and experimental work in this area.)
Many questions remain unanswered, however, regarding topics such as the annealling
behavior of hydrogen and its interaction with point and extended defects. A starting point
for answering these questions can be provided by theoretical studies of hydrogen in defect-
free GaN. Neugebauer and Van de Walle (NVdW) performed such a study for zinc-blende
GaN.8, 9 They found several interesting features including a large negative-U effect and a
preference for the nitrogen anti-bonding site by H instead of the bond-centered site favored
in other semiconductors. Based on their previous studies of native point defects in GaN,10
they also suggested that hydrogen should display similar behavior in the more
technologically relevant wurtzite structure. However, there are notable differences in the two
crystal structures that could affect interstitial hydrogen (see also Chap. 6 in Ref.1). The
lower symmetry of wurtzite, for instance, results in there being two different Ga-N bond
lengths and bond angles. This could affect the relative stabilities of anti-bonding and bond-
centered sites. An even more noticeable difference is revealed by examining the two crystal
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Wright, A.F. The Influence of Crystal Structure on the Lattice Sites and Formation Energies of Hydrogen in Wurtzite and Zinc-Blende GaN, article, February 1, 1999; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc687412/m1/4/: accessed November 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.