A comprehensive first-principles investigation was made of single and multiple

gallium and nitrogen vacancies in gallium nitride nanowires. Nanowires in the

[00•1] growth direction were considered, with diameters of 9.5 and 15.9Å, and the

stability of multiple vacancies was investigated for a wide range of configurations

in order to determine the preferred spatial distribution. The effect of saturating the

dangling bonds at the edge of the nanowires was also investigated. For one, two

and three nitrogen vacancies, it was found that the most favorable configuration

was with the vacancies at the edge of the nanowires. It was also found that, for

multiple (two and three) nitrogen vacancies, the vacancies preferred to cluster

together rather than remain well-separated. For one and two gallium vacancies, the

preferred vacancy location was also at the edge of the nanowires, with clustering

favoured for two vacancies. Examining the band structure of unsaturated nanowires

(with and without vacancies), states in the band-gap were observed that could be attributed to edge states. These edge states were removed when the dangling bonds

at the edge of the nanowires were saturated with hydrogen. For unsaturated wires,

the nature of the single Ga and N vacancy states was similar to that in the bulk;

acting as a triple acceptor and single donor, respectively. The position of the

vacancy states relative to the two regions of edge-induced states was similar to

their location relative to the conduction band minimum and valence band

maximum in the bulk, suggesting that any conductivity arising from the vacancies

would be confined to the outer region of the nanowires. The two- and threenitrogen

vacancy complexes induced additional states in the band-gap, acting as

double and triple donors respectively, while the two-gallium vacancy complex

reconstructed to an N3-like structure and induced several fully occupied and

unoccupied singlet states into the band-gap. Examining the defect-induced states

for the gallium vacancy in the saturated wire versus in bulk GaN, a similar result

was found in terms of the number, location and occupation of the defect states,

except that the states in the wire were slightly deeper in the band-gap. For the

nitrogen vacancy in the saturated wire versus in bulk GaN, a similar behaviour was

again found in terms of the number and occupation of the defect states. However,

the higher-lying singly occupied state was closer to the conduction band in the

wire, and the location of the fully occupied singlet state was below the valence

band maximum in bulk GaN, and above it in the saturated nanowire. Considering

the formation energy of gallium and nitrogen vacancies, it was found that nitrogen

vacancies were significantly more stable than gallium vacancies, and these were

thus expected to be the major defect in GaN nanowires.

Atomic and Electronic Structure of Single and Multiple Vacancies in GaN

Nanowires from First-Principles. D.J.Carter, C.Stampfl: Physical Review B, 2009,

79[19], 195302