Cover Image: Issue 5, Materials Today.
Cover Image: Issue 5, Materials Today.

GaN is an important semiconductor material for high-temperature and high-frequency opto-electronic devices. The possession of a highly polar c-axis, which exhibits strong internal fields, and non-polar m- anda-axes, which are free from internal fields, has motivated a considerable amount of research on the anisotropic optical and electrical properties of GaN. The internal fields limit the radiative recombination efficiencies of opto-electronic systems based on GaN, and so such devices are fabricated based on non-polar substrates. GaN nanostructures also show a preferred growth direction and can be used as building blocks for integrated nano-photonic and nano-electronic devices [1].

Straight and smooth nanowires, grown using the celebrated vapor–liquid–solid (VLS) process in the presence of a catalytic particle, is the chief nanostructure morphology reported by several research groups. Different morphologies would however be useful for several applications [2] and [3]. Yet, the GaN nanostructure morphologies reported in literature are limited in comparison with other semiconductor nanostructures, and there are hardly any reports on the optical properties of polyhedrons grown using chemical vapor deposition (CVD).

The motivation behind the work featured on this month's cover was the control of GaN nanostructure morphology, and the investigation of the optical and electrical properties of the structures. The growth of GaN nanostructures was carried out in a homemade thermal CVD system. High purity gallium metal was placed on a Tungsten boat and inserted into the center of a quartz tube, which was then placed inside a horizontal tube furnace. A substrate was also placed into the quartz tube, 1 – 25 mm from the source (with or without 2 nm of thermally evaporated gold for catalysis). At the center (single zone) of the furnace the temperature is uniform and constant over a distance of up to 100 mm. The quartz tube was purged with ammonia and the flow was maintained at 500 standard cubic centimeters per minute (SCCM) for one hour prior to growth. The flow rate was then slowly reduced to the desired value and the temperature was subsequently increased at a rate of 30 K/min, from room temperature to the growth temperature (1173 – 1323 K), and maintained for 3 hours under a constant flow of ammonia (10 – 100 SCCM).

After the reaction, the furnace was cooled to room temperature and the resulting GaN nanostructures were analyzed. The nanostructures included smooth-come-straight nanowires, zig-zag nanowires, nano-whiskers, laterally overgrown polyhedra, stacked and branched pyramids, flower-like, stalagmite-like, and obelisk-like structures, etc. We have controlled the morphology of GaN nanostructures grown with and without catalysis; studying the whole spectrum of morphologies helps us to understand the growth mechanism in detail. The morphologies were highly controllable and repeatable, with the determining factors being the growth temperature, flow rate of ammonia, and the distance between the source and the substrate [4].

The image on the cover shows a GaN nano-flower grown without catalysis on a silicon substrate at 1323 K, with a 25 SCCM ammonia flow rate and the substrate placed at a distance of 2 mm from the gallium source. The energy dispersive x-ray analysis of the composition indicates that the Ga/N ratio of the nano-flower is almost 1. The structure clearly shows laterally overgrown hexagonal facets with sharp tips, indicating N-polarity. The high temperature and low ammonia flow rate suppresses one dimensional growth and leads to lateral overgrowth, due to limited surface diffusion of gallium on different planes. The top facet of the nano-flower shown on the cover is about 1.2 μm, but this can be controlled and we achieved as small as 180 nm. The nanostructures can be selectively grown on various substrates and used for the fabrication of solar cells. We are currently investigating the p-n junction characteristics of these GaN polyhedra on commercial GaN templates for such applications. The image was recorded using a field emission scanning electron microscope (Carl Zeiss, NTS GmbH SUPRA 40VP, Germany) operated at 10 keV.

Further Reading
[1] T. Kuykendall et al. Nat Mater, 3 (2004), p. 524
[2] C.Y. Nam et al. Appl Phys Lett, 85 (2004), p. 5676
[3] X.M. Cai et al. Appl Phys Lett, 87 (2005), p. 183103
[4] Chander et al., Unpublished.

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DOI: 10.1016/S1369-7021(11)70123-7