Progress and prospects of group-III nitride semiconductors

Reactive‐ion molecular‐beam epitaxy has been used to grow epitaxial hexagonal‐structure α‐GaN on Al2O3(0001) and Al2O3(0112) substrates and metastable zinc‐blende‐structure β‐GaN on MgO(001) under the following conditions: growth temperature Ts=450–800 °C; incident N+2/Ga flux ratio JN+2/JGa=1–5; and N+2 kinetic energy EN+2=35–90 eV The surface structure of the α‐GaN films was (1×1), with an ≊3% contraction in the in‐plane lattice constant for films grown on Al2O3(0001), while the β‐GaN films exhibited a 90°‐rotated two‐domain (4×1) reconstruction Using a combination of in situ reflection high‐energy electron diffraction, double‐crystal x‐ray diffraction, and cross‐sectional transmission electron microscopy, the film/substrate epitaxial relationships were determined to be: (0001)GaN∥ (0001)Al2O3 with [2110]GaN∥[1100]Al2O3 and [1100]GaN∥[1210]Al2O3, (2110)GaN∥(0112)Al2O3 with [0001]GaN∥[0111]Al2O3 and [0110]GaN∥[2110]Al2O3, and (001)GaN∥(001)MgO with [001]GaN∥[001]MgOFilms with the lowest e

Heteroepitaxial wurtzite and zinc‐blende structure GaN grown by reactive‐ion molecular‐beam epitaxy: Growth kinetics, microstructure, and properties

GaN films have been epitaxially grown onto (001) Si by electron cyclotron resonance microwave‐plasma‐assisted molecular‐beam epitaxy, using a two‐step growth process, in which a GaN buffer is grown at relatively low temperatures and the rest of the film is grown at higher temperatures. This method of film growth was shown to lead to good single‐crystalline β‐GaN and to promote lateral growth resulting in smooth surface morphology. The full width at half‐maximum of the x‐ray rocking curve in the best case was found to be 60 min. Optical‐absorption measurements indicate that the band gap of β‐GaN is 3.2 eV and the index of the refraction below the absorption edge is 2.5. Conductivity measurements indicate that the films may have a carrier concentration below 1017 cm−3.

Epitaxial growth and characterization of zinc‐blende gallium nitride on (001) silicon

High-quality GaN films have been grown on a variety of substrates by electron cyclotron resonance microwave plasma-assisted molecular beam epitaxy (ECR-MBE). The films were grown in two steps. First, a GaN-buffer was grown at low temperature and then the rest of the film was grown at higher temperatures. We found that this method of growth leads to a relatively small two-dimensional nucleation rate (∼20 nuclei/μm2 h) and high lateral growth rate (100 times faster than the vertical growth rate). This type of quasi-layer-by-layer growth results in a smooth surface morphology to within 100 A. Growth on Si(1 0 0) leads to single-crystalline GaN films having the zinc-blende structure. Growth on Si(1 1 1) leads to GaN films having the wurtzitic structure with a large concentration of stacking faults. The crystallographic orientation and the surface morphology of GaN films on sapphire depends on the orientation of sapphire. To this date, the best films were grown on the basal plane of sapphire.

Growth of GaN by ECR-assisted MBE

The extreme thermal and electronic properties of diamond and of silicon carbide, and the direct band gap of gallium nitride, provide multiplicative combinations of attributes which lead to the highest figures of merit for any semiconductor materials for possible use in high power, high speed, high temperature and high frequency applications. The deposition of monocrystalline diamond, at or below 1 atm total pressure and at a temperature T , has been achieved on diamond substrates; the deposited film has been polycrystalline on all other substrates but the achievement is no less significant. For electronic applications, heteroepitaxy of single-crystal films of diamond, an understanding of mechanisms of nucleation and growth, methods of impurity introduction and activation, and further device development must be achieved. Stoichiometric gallium nitride free of nitrogen vacancies has apparently not been obtained. Thus, knowledge of the defect chemistry of this material, the growth of semiconducting films on foreign substrates, and the development of insulating layers and of their low temperature deposition as well as device fabrication procedures must be achieved. By contrast, all of these problems have already been solved for silicon carbide, including the operation of a MOSFET at 923 K — the highest operating temperature ever reported for a field-effect device. However, considerable research remains to be done regarding the development of large silicon carbide substrates, of ohmic and rectifying contacts, of new types of devices, and of low temperature techniques for the deposition of insulating layers. Fugitive donor and acceptor species in unintentionally doped samples must also be identified and controlled.

Critical evaluation of the status of the areas for future research regarding the wide band gap semiconductors diamond, gallium nitride and silicon carbide

We report the preparation of thin films of gallium phosphide and gallium nitride by deposition from low‐pressure rf‐excited plasmas in mixtures of trimethylgallium with ammonia and phosphine, respectively. With the deposition conditions used, the gallium phosphide is found to be amorphous, while the gallium nitride is polycrystalline. We present preliminary measurements of optical and electrical properties.

J. C. Knights

Papers

Plasma deposition of GaP and GaN