S. V. Novikov

Defect reduction in GaN/(0001)sapphire films grown by molecular beam epitaxy using nanocolumn intermediate layers

Transmission and scanning electron microscopies are used to examine the epitaxial lateral overgrowth of GaN on GaN nanocolumns grown on AlN/(0001)sapphire by molecular beam epitaxy. Initially, N-rich growth gave a bimodal morphology consisting of defect-free Ga-polar nanocolumns emanating from a compact, highly defective N-polar layer. Under subsequent Ga-rich conditions, the nanocolumns grew laterally to produce continuous Ga-polar overlayers. Threading dislocation (TD) densities in the overlayer were in the range of 108–109cm−2, up to two orders of magnitude less than in the N-polar underlayer. It is proposed that the change in polarity is a key factor controlling the reduction in TD density.

The growth and properties of group III nitrides

Abstract We have studied a novel material system (AlGa)(AsN), which can be lattice matched to GaP (or more importantly Si), grown using a low temperature modified molecular beam epitaxy (MBE) technique to reduce the density of native defects. Active nitrogen is provided by an Oxford Applied Research, RF activated plasma source. This source has enabled us to prepare binary films of GaN and InN and alloy layers of Ga(AsN) and In(AsN) at growth rates of approximately 0.3 monolayers/s. The films have been studied using in-situ reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES) and ex-situ using X-ray diffraction, C–V profiling and photoluminescence/excitation measurements. We have obtained clear evidence for the existence of films with significant concentrations of N (∼ 20%), using appropriate growth conditions.

Growth and characterization of free-standing zinc-blende (cubic) GaN layers and substrates

In this paper, we describe bulk, free-standing, zinc-blende (cubic) GaN wafers grown by plasma-assisted molecular beam epitaxy. We have grown GaN layers of up to 60 ?m in thickness. We present the data from characterization measurements that confirm the cubic nature of the GaN crystals and show that the fraction of the material that is hexagonal in nature is not more than about 10% in the best thick samples. Cubic (0?0?1) GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0?0?0?1) c-axis wurtzite GaN. Therefore, the free-standing GaN wafers we have grown would make ideal lattice-matched substrates for the growth of cubic GaN-based structures for blue and ultraviolet optoelectronic devices, and high-power and high-frequency electronic applications.

Highly mismatched crystalline and amorphous GaN1−xAsx alloys in the whole composition range

Alloying is a commonly accepted method to tailor properties of semiconductor materials for specific applications. Only a limited number of semiconductor alloys can be easily synthesized in the full composition range. Such alloys are, in general, formed of component elements that are well matched in terms of ionicity, atom size, and electronegativity. In contrast there is a broad class of potential semiconductor alloys formed of component materials with distinctly different properties. In most instances these mismatched alloys are immiscible under standard growth conditions. Here we report on the properties of GaN1−xAsx, a highly mismatched, immiscible alloy system that was successfully synthesized in the whole composition range using a nonequilibrium low temperature molecular beam epitaxy technique. The alloys are amorphous in the composition range of 0.17<x<0.75 and crystalline outside this region. The amorphous films have smooth morphology, homogeneous composition, and sharp, well defined optical absorp...

Molecular beam epitaxy of GaBiAs on (311)B GaAs substrates

We report the growth by molecular beam epitaxy of GaBixAs1−x epilayers on (311)B GaAs substrates. We use high-resolution x-ray diffraction (HRXRD), transmission electron microscopy, and Z-contrast imaging to characterize the structural properties of the as-grown material. We find that the incorporation of Bi into the GaBiAs alloy, as determined by HRXRD, is sizably larger in the (311)B epilayers than in (001) epilayers, giving rise to reduced band-gap energies as obtained by optical transmission spectroscopy.

Carrier localization in GaBiAs probed by photomodulated transmittance and photoluminescence

The carrier localization phenomenon has been investigated for GaBiAs by photomodulated transmittance (PT) and photoluminescence (PL). In the case of PT measurements, a decrease in the energy-gap related PT signal has been clearly observed below 180 K. In PL spectra a broad emission band very sensitive to the excitation power has been found. In comparison to the energy-gap related transition, this band is shifted to red. The recombination time for this band at low temperature decreases from 0.7 to 0.35 ns with the increase in the emission energy. All the findings are clear evidences for strong carrier localization in this alloy.

Strong blue emission from As doped GaN grown by molecular beam epitaxy

Arsenic doped GaN grown by molecular beam epitaxy has been studied by room temperature photoluminescence. In addition to the wurzite band edge transition, luminescence from the cubic phase and very strong blue emission at ∼2.6 eV are observed. The intensities of the blue and the cubic band edge emissions have a power law dependence on the As2 flux. The formation of the cubic phase has been explained by the initial formation of GaAs before substitution of the As by the more reactive N. The intensity of the blue emission at room temperature of the As doped samples is more than an order of magnitude stronger than the band edge emission in undoped samples.

Far-infrared transmission in GaN, AlN, and AlGaN thin films grown by molecular beam epitaxy

We present a far-infrared transmission study on group-III nitride thin films. Cubic GaN and AlN layers and c-oriented wurtzite GaN, AlN, and AlxGa1−xN (x<0.3) layers were grown by molecular beam epitaxy on GaAs and Si(111) substrates, respectively. The Berreman effect allows us to observe simultaneously the transverse optic and the longitudinal optic phonons of both the cubic and the hexagonal films as transmission minima in the infrared spectra acquired with obliquely incident radiation. We discuss our results in terms of the relevant electromagnetic theory of infrared transmission in cubic and wurtzite thin films. We compare the infrared results with visible Raman-scattering measurements. In the case of films with low scattering volumes and/or low Raman efficiencies and also when the Raman signal of the substrate material obscures the weaker peaks from the nitride films, we find that the Berreman technique is particularly useful to complement Raman spectroscopy.

The growth and properties of GaN:As layers prepared by plasma-assisted molecular beam epitaxy

We have studied the growth and properties of GaN:As layers prepared by molecular beam epitaxy, using a plasma source for active nitrogen. We have demonstrated that arsenic doping during growth produces films showing blue emission at room temperature. The blue emission is centred at 2.6 eV and is more than a decade stronger than the band edge emission. The films are predominantly wurtzite, but with a small cubic content which exists mainly close to the substrate-epilayer interface. We have investigated the influence of growth conditions on the intensity of this blue emission. In films grown under optimum conditions, the blue emission is strong enough to be clearly visible under normal room lighting. We have also discussed the transition from As-doped GaN showing blue emission to the formation of GaN1-xAsx alloys. We have determined that for a fixed arsenic flux, increasing the N/Ga ratio leads to the formation of alloy films. Our results suggest that this materials system may have potential applications in electronic and opto-electronic devices.

Nitrogen species from radio frequency plasma sources used for molecular beam epitaxy growth of GaN

We have made a detailed study of the optical spectroscopy of two different RF plasma sources used for the growth of GaN by molecular beam epitaxy. Our studies show that for both sources the predominant species present in the cavity are nitrogen atoms. The strongest optical emission occurs at 869 nm. We have also studied, in detail, the factors which influence the ion content of the flux. Two key parameters are the temperature of the wall of the cavity and the size of the holes in the aperture plate from which the species emerge into the vacuum. We have identified conditions under which the ion content can be made negligibly small and show that this results in films with improved optical properties.