S.E. Hooper

Selective growth of zinc‐blende, wurtzite, or a mixed phase of gallium nitride by molecular beam epitaxy

We report on the growth of GaN with a zinc‐blende, wurtzite, or a mixed phase structure on (001)GaP and (001)GaAs substrates by a low‐temperature modified molecular beam epitaxy technique. By systematically varying the incident arsenic overpressure, films grown at a moderate substrate temperature of ≊620 °C show predominately wurtzite α‐GaN, zinc‐blende β‐GaN, or a mixed phase of the two. Films containing only the metastable phase β‐GaN were achieved by using a relatively high growth temperature of ≊700 °C and with an arsenic overpressure of ≊2.4×10−5 Torr. X‐ray diffraction measurements indicate an improved crystalline quality for the layers grown at ≊700 °C compared to those grown at ≊620 °C as evident by a narrower full width at half‐maximum of 35 min for β‐GaN, which is among the narrowest reported to date.

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.

Photoluminescence of MBE grown wurtzite Be-doped GaN

We report new lines in the photoluminescence (PL) spectrum of lightly Be-doped GaN. The low-temperature PL spectrum of the lightly doped sample is dominated by a transition at 3.385 eV with first and second LO phonon replicas. Power-resolved PL measurements showed that the peak at 3.385 eV narrowed in width and shifted to higher energies with increasing excitation intensity. Thus the transition is attributed to donor-to-acceptor recombination, involving a Be acceptor of optical ionization energy of between 90 and 100 meV. This is much shallower than the acceptor level of 250 meV induced by Mg doping. Increasing the doping, however, resulted in a quenching of the band-edge luminescence and the appearance of a broad transition centred around 2.4 eV which we assign to a complex involving Be. Undulations on the peak were consistent with interference effects. On increasing the doping level even further all luminescence was quenched.

Low-temperature luminescence study of GaN films grown by MBE

We report the results of low-temperature photoluminescence measurements on GaN films grown by molecular beam epitaxy on (0001) sapphire substrates. Samples were either nominally undoped or doped with Si. The spectra are generally dominated by a sharp peak at 3.47 eV which is attributed to excitons bound to neutral donors. A much weaker peak (or shoulder) near 3.45 eV probably arises from excitons bound to neutral acceptors. On raising the temperature to 50 K, in some samples free exciton peaks can be partially resolved on the high-energy side of the main line. In others we believe that these free excitons are recaptured onto neutral acceptors, thus enhancing the low-energy side of the line. A broader emission line appears in many samples at an energy near 3.42 eV which shows significant variation in position between samples. Our data show that it represents a free-to-bound, probably a free hole-to-donor, transition. This donor has previously been associated with oxygen. Of particular interest is the fact that some samples show a second sharp peak at 3.27 eV, together with a second broader peak at about 3.17 eV (also variable in energy). The sharp peak is energetically consistent with its being either a donor - acceptor or a free electron-to-bound hole transition, but subsidiary measurements rule out both these possibilities. We suggest that it may represent an exciton bound to a deep donor or a shallow donor-bound exciton in zinc blende GaN inclusions contained within the mainly wurtzite material. We tentatively interpret the 3.17 eV line as a phonon replica of this zinc blende line, the phonon energy being perturbed by the small size of the inclusions and by strain effects within these inclusions.

Morphology of luminescent GaN films grown by molecular beam epitaxy

GaN thin films were grown by molecular beam epitaxy on sapphire substrates. Scanning electron (SE) and atomic force microscopies reveal that on a typical film an assembly of oriented hexagonal microcrystallites rises above a background of polycrystalline or amorphous material. Cathodoluminescence (CL) spectra of the films feature bright UV exciton peaks and a broad green emission band. We identify the exciton peaks as those of the wurtzite form of GaN. A comparison of SE and CL micrographs of the same sample area shows that the luminescence emanates almost entirely from the hexagonal crystallites.

The growth and properties of mixed group V nitrides

Bearing in mind the problems of finding a lattice-matched substrate for the growth of binary group III nitride films and the detrimental effect of the large activation energy associated with acceptors in GaN, we propose the study of the alloy system AlGaAsN. We predict that it may be possible to obtain a direct gap alloy, with a band gap as wide as 2.8eV, which is lattice-matched to silicon substrates. The paper reports our attempts to grow GaAsN alloy films by molecular beam epitaxy on either GaAs or GaP substrates, using a radio frequency plasma source to supply active nitrogen. Auger electron spectra demonstrate that it is possible to incorporate several tens of percent of nitrogen into GaAs films, though x-ray diffraction measurements show that such films contain mixed binary phases rather than true alloys. An interesting observation concerns the fact that it is possible to control the crystal structure of GaN films by the application of an As flux during growth. In films grown at 620°C a high As flux tends to increase the proportion of cubic GaN while also resulting in the incorporation of GaAs. Films grown at 700°C show no evidence for GaAs incorporation; at this temperature, it is possible to grow either purely cubic or purely hexagonal GaN depending on the presence or absence of the As beam.

Some aspects of GaN growth on GaAs(100) substrates using molecular beam epitaxy with an RF activated nitrogen-plasma source

We have investigated how supplying active nitrogen from an RF activated plasma source under various plasma conditions influences certain aspects of the growth of GaN films on GaAs(100) substrates, using molecular beam epitaxy. In the first instance, the quantity of active nitrogen generated by the source was found to have a strong dependence on both the RF power and amount of nitrogen gas supplied to the plasma. In addition, the degree of optical discharge from the plasma was observed to give a semi-quantitative measure of active nitrogen. No observable loss of nitrogen from the sample surface in the temperature range 450 to 680°C was found during GaN growth. Scanning electron microscopy on the cleaved edges of the GaNGaAs(100) samples showed the GaN layer to be polycrystalline with a columnar nature typical of a highly lattice mismatched material system. X-ray diffraction measurements indicated that the GaN layers were entirely wurtzite in structure, with the full width at half maximum of the GaN (0002) reflection in the range 9 to 11.5 arcmin. A broad peak centred at around 3.4 eV was recorded using room temperature photoluminescence measurements on the layers.

Incorporation of Mg in GaN grown by molecular beam epitaxy

We report Mg doping experiments in GaN grown by plasma-enhanced molecular beam epitaxy on sapphire and GaAs substrates. Secondary ion mass spectrometry was used to measure the Mg concentration as a function of Mg flux. Our data show a linear dependence at low fluxes and then tend to saturate, in agreement with the measurements at higher fluxes by Guha et al. We model this in terms of two interacting layers of Mg atoms on the GaN surface, one chemisorbed, the other physisorbed. We suggest that surface stoichiometry is an essential factor in determining doping efficiency and is responsible for the observed temperature-dependence of Mg incorporation.

Improvement of the photoluminescence from gallium nitride layers grown by MBE with an additional incident indium flux

The effect of using an indium flux during the MBE growth of GaN layers was investigated. The properties of these layers were studied using electron probe microanalysis, secondary ion mass spectroscopy, photoluminescence and cathodoluminescence. The optical properties of the GaN layers are shown to improve as compared with undoped GaN layers grown under nominally the same conditions but without an additional indium flux.

MBE growth and characterization of magnesium-doped gallium nitride

We describe measurements of the electrical and luminescence properties of Mg-doped GaN films grown by plasma-enhanced molecular beam epitaxy on sapphire and GaAs substrates. Secondary ion mass spectroscopy measurements were used to determine the total Mg concentration in each of the films and showed the Mg profiles to be flat throughout the films. At low Mg beam fluxes, there is a linear relationship between the Mg concentration in the film [Mg] and the Mg flux but, for fluxes above about , [Mg] saturates at (at a growth temperature of C). We outline a simple model of Mg incorporation which explains the experimental data. Hall effect measurements reveal that p-type conductivity is obtained only in films grown under slightly nitrogen-rich conditions and demonstrate that, in thin films , conductivity is dominated by high densities of donor-type defects. This is also borne out by photoluminescence (PL) results. Most of the samples showed bound exciton emission at low temperatures, in some cases involving neutral donors, in others neutral acceptors, the emission suggesting that the incorporation of Mg results in a lowering of the GaN band gap. All samples show donor-acceptor recombination and detailed analysis suggests that Mg doping suffers from self-compensation. Room-temperature PL is dominated by a free electron-bound hole emission line at 3.2 eV in p-type samples but by deep emission in samples grown under Ga-rich conditions.