R. J. Molnar

Growth of GaN by ECR-assisted MBE

Abstract 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.

Blue‐violet light emitting gallium nitride p‐n junctions grown by electron cyclotron resonance‐assisted molecular beam epitaxy

Blue‐violet gallium nitride (GaN) light emitting p‐n junctions were grown by the method of electron cyclotron resonance‐assisted molecular beam epitaxy. This method has been modified to minimize plasma induced defects. Contrary to similar devices grown by metalorganic chemical vapor deposition, these devices do not require any postgrowth annealing to activate the Mg acceptors in the p layer. These devices turn‐on at approximately 3 V and have a spectral emission peaking at 430 nm.

Electron transport mechanism in gallium nitride

The electron transport mechanism in autodoped gallium nitride films grown by electron cyclotron resonance microwave plasma‐assisted molecular beam epitaxy was investigated by studying the temperature dependence of the Hall coefficient and resistivity on samples with various concentrations of autodoping centers. The Hall coefficients go through a maximum as the temperature is lowered from 300 K and then saturate at lower temperatures. The resistivities in the same temperature range initially increase exponentially and then saturate at lower temperatures. These findings are accounted for if a significant fraction of electron transport, even at room temperature, takes place in the autodoping centers and that conduction through these centers becomes dominant at lower temperatures. The activation energy of these centers was found to be on the order of 20–30 meV. When the concentration of the autodoping centers becomes smaller than that of deep compensating defects, the material becomes semi‐insulating and tran...

GROWTH AND DOPING OF GaN FILMS BY ECR-ASSISTED MBE

We report on growth, doping, and characterization studies of GaN films produced by the Electron Cyclotron Resonance microwave plasma assisted Molecular Beam Epitaxy. The films were grown heteroepitaxially on sapphire (0001), whose surface was converted into atomically smooth AIN by plasma nitridation. The GaN films were grown in two temperature steps, a process found to promote the layer-by-layer growth mode. ECR plasma conditions to grow either n-type autodoped or semi-insulating GaN film were identified. The structure and microstructure as well as the electrical properties of these two classes of films are discussed. A systematic dependence between electron mobility and net carrier concentration was found, which predicts that the mobility of GaN with a net carrier concentration of 10 14 cm- 3 is about 10 4 cm 2 /V-S. The insulating films were intentionally doped either p-type or n-type by incorporation of Mg or Si during film growth. Hole or electron concentrations at 300K between 1018 - 10 19 cm- 3 have been obtained without requiring any post-growth treatment.

Operation of a compact electron cyclotron resonance source for the growth of gallium nitride by molecular beam epitaxy (ECR-MBE)

The operation of an ASTeX compact electron cyclotron resonance plasma source and its effect on the growth of GaN thin films by electron cyclotron resonancemolecular beam epitaxy has been investigated. The role a flow limiting orifice plays in increasing plasma stability as well as reducing ion damage and impurities in resultant films has also been studied. Both optical emission spectroscopy as well as electrostatic (Langmuir) probe studies have been employed to elucidate the generation and transport of charged and neutral species. With the introduction of the flow orifice, a substantial decrease in ion induced damage as well as surface roughening in the films is observed. This can be accounted for in terms of a collisionally induced relaxation of the grad-B acceleration of charged species toward the substrate in plasma sources employing axial solenoidal fields.

Reactive Ion Etching of GaN Thin Films

Reactive ion etching of GaN grown by electron-cyclotron-resonance, microwave plasma-assisted molecular beam epitaxy on (0001) sapphire substrates was investigated. A variety of reactive and inert gases such as CC1 2 F 2 , SF 6 , CF 4 , H 2 /CH 4 mixtures, CF 3 Br, CF 3 Br/Argon mixtures and Ar were investigated. From these studies we conclude that of the halogen radicals investigated, Cl and Br etch GaN more effectively than F. The etching rate was found to increase with decreasing pressure at a constant cathode voltage, a result attributed to larger mean free path of the reactive species.

Excitonic transitions in GaAsAlGaAs superlattices studied with lateral photoconductivity

Abstract We studied the temperature dependence of the excitonic transitions in GaAsAlGaAs superlattices using lateral photocurrent spectroscopy. Due to the high sensitivity of the method, we determined the binding energies of the ground heavy hole and light hole excitons to be 15 ± 1 meV and 18 ± 1 meV respectively for the superlattice consisting of 300 periods of undoped GaAs (25 A thick) and Al0.3Ga0.7As (40 A thick).