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

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5.1 Crystal Structure
:\ crucial obstacle impeding the growth of high quality, defect free GaN films is the
lack of a suitable substrate. Traditionally, GaN has grown on the [0001] face of
sapphire, despite the large lattice mismatch and thermal incompatibility [64].
In
general, GaN crystallizes in the wurzite phase when grown on a hexagonal substrate
and the zincblende phase when grown on cubic substrates (except for the [Ill] face
of cubic substrates). Examples of substrates for wurzite GaN include :\1 2 0 3 [0001]
[65], Ah03 [1120] [15], Al 2 0 3 [0110] [66], SiC 6H [67, 68, 69] and Ga:\s [111] [70].
Zincblende GaN has been grown on GaAs [001], 3C SiC [71], MgO [66], and Si [001]
[72. 73] For the remainder of this thesis, we will confine ourselves to wurzite Ga.\"
grown on :\1 2 0 3 [0001]. All of our GaN samples have this structure. which is the
most common structure for GaN.
Before discussing the physical structure of GaN, a few definitions are required.
Cnlike cubic systems, the structure of GaN and Al 2 0 3 are referred to using a four
index label. The convention for hexagonal systems is a four axis system, e.g. [hkil]
where the h, k, and i refer to the reciprocal of the intercept of the ~Iiller
plane with
the lattice vector in the hexagonal plane [74]. The three indices are related to one
another according to
i = -(h + k). (,5.1 )
The fourth index, 1, corresponds to the reciprocal of the Miller plane with the crystal
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