Controlled Growth of GaN Nanowires
H. Temkin and M. Holtz
||Semiconductor nanowires are quasi-one-dimensional structures exhibiting unique electrical
and optical properties. The purpose is to obtain highly perfect wires, free of dislocations,
in order to explore the intrinsic optical and electrical properties of technologically
important materials and the possibility of novel devices based on 1D structures. One
of the approaches to formation of nanowires is the vapor-liquid-solid (VLS) method.
The process, as illustrated here, starts with self-assembly of nanodots of a metal
catalyst, e.g. Ni on a substrate such as Si(111) or sapphire (0001). Once the heat
treatment required for the metal self-assembly is completed, the metal (or molten
eutectic) is exposed to source materials, e.g. Ga and active nitrogen, delivered in
vapor form, and supersaturated with them. The source material is diffused from the
vapor-liquid to the liquid-solid interface. Precipitation of the material at the liquid-solid
interface drives the growth of the wire directly under the metal nanodot, with the
metal droplet riding over the growing wire. The size of growing nanowires, and their
density, is controlled by the size and density of metal nanodots.
Our nanowire growth begins with a ~ 2 nm thick layer of Ni deposited on substrate
sapphire. Annealing at high temperatures causes the Ni to self assemble into regular
nanodots. By controlling the diameter of these nanodots, we control the diameter of
the nanowires. Furthermore, the growth time allows us to control the nanowire length.
|A representative composite scanning electron microscope image of GaN nanowires is
shown on the right. The nanowires grow perpendicular to the substrate, with fairly
constant diameters, smooth sidewalls, and hexagonal cross-sections. Nanowires grow
selectively from islands of the metal catalyst and no deposits of GaN could be seen
between the wires. A slight increase in the diameter of the growing wire, visible
in Fig. 7(a), may be indicative of sidewall growth. Characteristic metal caps at nanowires
tips, shown in the inset (b) of Fig. 7, confirm the VLS growth mechanism. The shape
of the caps is typical of Ni nanodots formed in high temperature, 830oC, anneals.
Research support: NSF