Theodore D. Moustakas

Scattering of electrons at threading dislocations in GaN

A model to explain the observed low transverse mobility in GaN by scattering of electrons at charged dislocation lines is proposed. Filled traps along threading dislocation lines act as Coulomb scattering centers. The statistics of trap occupancy at different doping levels are investigated. The theoretical transverse mobility from Coulomb scattering at charged traps is compared to experimental data. Due to the repulsive potential around the charged dislocation lines, electron transport parallel to the dislocations is unaffected by the scattering at charged dislocation lines.

Phase separation in InGaN thick films and formation of InGaN/GaN double heterostructures in the entire alloy composition

We report the growth of InGaN thick films and InGaN/GaN double heterostructures by molecular beam epitaxy at the substrate temperatures 700–800 °C, which is optimal for the growth of GaN. X-ray diffraction and optical absorption studies show phase separation of InN for InxGal−xN thick films with x>0.3. On the other hand, InxGal−xN/GaN double heterostructures show no evidence of phase separation within the detection capabilities of our methods. These observations were accounted for using Stringfellow’s model on phase separation, which gives a critical temperature for miscibility of the GaN–InN system equal to 2457 K.

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

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.

The role of dislocation scattering in n-type GaN films

The lateral transport in GaN films produced by electron cyclotron resonance plasma-assisted molecular beam epitaxy doped n type with Si to the levels of 1015–1020 cm−3 was investigated. The room temperature electron mobility versus carrier concentration was found to follow a family of bell-shaped curves consistent with a recently proposed model of scattering by charged dislocations. The mechanism of this scattering was investigated by studying the temperature dependence of the carrier concentration and electron mobility. It was found that in the low carrier concentration region (<1017 cm−3), the electron mobility is thermally activated with an activation energy half of that of carrier concentration. This is in agreement with the prediction of the dislocation model.

Metal contacts to gallium nitride

We report measurements on the nature of aluminum and gold contacts to GaN. The GaN films were deposited onto the R‐plane of sapphire substrates by molecular beam epitaxy and are autodoped n‐type. Metal contacts were deposited by evaporation and were patterned photolithographically. Current‐voltage characterization shows that the as‐deposited aluminum contacts are ohmic while the as‐deposited gold contacts are rectifying. The gold contacts become ohmic after annealing at 575 °C, a result attributed to gold diffusion. The specific contact resistivity of the ohmic aluminum and gold contacts were found by transfer length measurements to be of device quality (10−7–10−8 Ω m2). The results of these studies suggest a direct correlation between barrier height and work function of the metal, consistent with the strong ionic character of GaN.

Epitaxial growth of zinc blende and wurtzitic gallium nitride thin films on (001) silicon

Zinc blende and wurtzitic 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 this process a thin buffer layer is grown at relatively low temperatures followed by a higher temperature growth of the rest of the film. GaN films grown on a single crystalline GaN buffer have the zinc blende structure, while those grown on a polycrystalline or amorphous buffer have the wurtzitic structure.

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.

Heteroepitaxy, polymorphism, and faulting in GaN thin films on silicon and sapphire substrates

The structure of GaN films grown by electron‐cyclotron‐resonance‐assisted molecular beam epitaxy on Si(111), Si(001), basal‐plane sapphire, a‐plane sapphire, and r‐plane sapphire substrates was studied with four‐circle x‐ray diffractometry. Phase content, domain size, inhomogeneous strain, and in‐plane and out‐of‐plane domain misorientations were measured and compared for films grown on each type of substrate. Wurtzite and zinc blende polymorphs were found to coexist in films grown on Si(111). The two structures grow in the (0002) and (111) orientations, respectively, so that they may transform into each other via stacking faults on close‐packed planes. Smaller amounts of zinc blende material were also found in predominately (0002) wurtzitic films on a‐plane sapphire and (1120) wurtzitic films on r‐plane sapphire.

Effect of nitrogen on the growth of diamond films

The incorporation of nitrogen in diamond films and its effect on film growth were investigated. The nitrogen doping efficiency was found to be very low, consistent with a model of film growth involving simultaneous deposition and etching, which predicts a doping efficiency of 10−4. The growth habit was found to change from (111) to (100) with increase of nitrogen in the gas phase from N/C = 0.1% to 10%. The growth rate of the diamond films increases, and the diamond Raman peak sharpens with the amount of nitrogen, a result consistent with the model on defect‐induced stabilization of diamond.

PHASE SEPARATION AND ORDERING IN INGAN ALLOYS GROWN BY MOLECULAR BEAM EPITAXY

In this study, we investigated phase separation and long-range atomic ordering phenomena in InGaN alloys produced by molecular beam epitaxy. Films grown at substrate temperatures of 700–750 °C with indium concentration higher than 35% showed phase separation, in good agreement with thermodynamic predictions for spinodal decomposition. Films grown at lower substrate temperatures (650–675 °C) revealed compositional inhomogeneity when the indium content was larger than 25%. These films, upon annealing to 725 °C, underwent phase separation, similar to those grown at the same temperature. The InGaN films also exhibited long-range atomic ordering. The ordering parameter was found to increase with the growth rate of the films, consistent with the notion that ordering is induced at the growth surface. The ordered phase was found to be stable up to annealing temperatures of 725 °C. A competition between ordering and phase separation has been observed, suggesting that the driving force for both phenomena is lattice...