William Angerer - Department of Physics and Astronomy - University ...

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in various bonding geometries within the lattice, and hence the hyperpolarizabilities
of the individual complexes would tend to cancel in an assemble average. Third.
there are several symmetries and energy positions within the band gap of the defect
state that may contribute to X~~x but not X~~x. Fourth, the position of this state in
the band gap region is unknown. Unfortunately, second-harmonic generation does not
unambiguously determine one- or two-photon excitations. Fifth, the defect state must
be a bulk defect state and not an interface defect state. Although inhomogeneities
at the interface may form defect states with large nonlinearities, significantly fewer
defect states are produced at the interface than in the bulk. The volume density of
interface defect states. N i , is related to the surface density of interface defects .. v1surface.
as Ni = NFurfaced, where d is the number of defects perpendicular to the surface per
cm. If we assume that the defect layer extends over 10 unit cells, one defect exists
per unit cell. and the surface density of interface defect states is 10 1 -1
em -2. then
Ni = 10 15 cm -3. Thus, Ni is several orders of magnitude smaller than the estimated
volume density of bulk defects, Nb = 10 20 ern -3.
6.6 Effect of Interface Strain
Finally we consider the influence of the sample miscut on possible strain induced
defect states in GaN. Observation of strain and disorder effects on second-harmonic
generation from vicinal Si(1l1)/Si0 2 and Si(100)/Si3N-I interfaces [102] suggests that
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