D. Doppalapudi

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.

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.

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...

Broadening of near-band-gap photoluminescence in n-GaN films

This letter addresses the broadening mechanism of the near-band-gap photoluminescence in GaN films doped n type with silicon. The films were produced by plasma assisted molecular beam epitaxy and their carrier concentration was varied systematically from 1015 to 1020 cm−3. The photoluminescence was excited with a 10 mW He–Cd laser at 77 K. At low carrier concentration ( 1018 cm−3) the full width at half maximum increases monotonically with carrier concentration up to about 120 meV. The broadening of the line at high carrier concentration is attributed to tailing of the density of states caused by potential fluctuations due to randomly distributed impurities. The data were quantitatively analyzed, as a function of carrier concentration and compensation ratio, using the impurity band broadening model of Morgan [Phys. Rev. 139, A343 (1965)], and the agreement between model and experim...

Distributed Bragg reflectors based on AlN/GaN multilayers

A 20.5 period distributed Bragg reflector stack based on AlN/GaN has been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. Peak reflectance up to 95% was observed at a wavelength of 392.5 nm. Cross-section transmission electron microscopy studies indicate that the interface of GaN-on-AlN is always sharper and more distinct than the interface of AlN-on-GaN. This is attributed to the different growth modes of AlN and GaN. When AlN grows on GaN, it tends to grow in a two-dimensional mode (Frank–van der Merwe mode) whereas GaN grows on AlN in a three-dimensional mode (Stranski–Krastanov mode). Based on these findings, the experimental reflectance data were simulated using the transmission matrix method.

Epitaxial growth of gallium nitride thin films on A-Plane sapphire by molecular beam epitaxy

In this article, we propose a crystallographic model to describe epitaxy of GaN on (1120) sapphire (A plane). The (1102) cleavage plane in sapphire is shown to extend to the GaN lattice as the (1120) plane, facilitating the formation of cleaved facets. It is shown that, although the lattice mismatch is much smaller than in the case of epitaxy on (0001), the difference in the planar symmetry in this case results in high-strained bonds near the interface. The use of nitridation and a low temperature buffer is therefore necessary. A systematic study of GaN growth on the A-plane sapphire by plasma-assisted molecular beam epitaxy was carried out to study the effects of plasma nitridation of the substrate and the growth of a low temperature GaN buffer on the structure and optoelectronic properties of the films. Transmission electron microscopy (TEM) studies indicate that films grown on substrates which were not nitridated prior to growth have a significant fraction of zinc-blende domains and poor orientation...

Domain structure in chemically ordered InxGa1−xN alloys grown by molecular beam epitaxy

Observation of long range atomic ordering in InGaN films grown by molecular beam epitaxy on A-plane sapphire is reported, based on x-ray diffraction and transmission electron microscopy studies. The InGaN films have a domain structure, with alternating domains of ordered and disordered phases, close to the film/substrate interface. Closer to the film surface, disordered domains are no longer observed. The degree of ordering was found to increase with growth rate (at the low growth rates used in these materials), which is consistent with ordering being a surface phenomenon.

MBE growth and doping of III–V nitrides

We report on the growth and doping of GaN by molecular-beam epitaxy on the c-plane of sapphire. We find that the steps of nitridation and low-temperature buffer have a significant effect on the structure, microstructure, defects and opto-electronic properties of the grown GaN films. The electron mobility in Si-doped GaN films was found to be controlled both by the density of ionized impurities and the density of dislocations. This result is consistent with the model which assumes that dislocations introduce acceptor centers. Films were doped p-type with Mg with carrier concentration up to 6 x 10 18 cm -3 , but relatively low hole mobilities (0.3 cm 2 /V s). These low mobilities can be improved by thermal annealing, a result attributed to the removal of static disorder.

Growth and properties of InxGa1−xN/AlyGa1−yN multiquantum wells developed by molecular beam epitaxy

In0.09Ga0.91N/GaN and In0.35Ga0.65N/Al0.1Ga0.9N multiquantum well structures (MQW) were grown by molecular beam epitaxy on (0001) sapphire substrates. The thickness of the wells and the barriers were in the range of 80–120 A. The microstructure of these MQW structures was investigated by transmission electron microscopy. The room temperature photoluminescence spectra in these MQW structures peak at 387 and 463 nm with full width at half maximum of 16 and 28 nm, respectively.