J W Orton

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.

Acceptor binding energy in GaN and related alloys

The recent achievement of p-type doping in GaN epitaxial films emphasizes the importance of the acceptor ionization energy with relevance to the application of group III nitrides in visible-light-emitting devices. Measured values of approximately 200 meV suggest that doping efficiencies at room temperature may be no more than 1%, resulting in undesirable series resistance in LEDs and lasers. Consideration of the effective mass hydrogen model of the acceptor ground state in GaAs, GaP and GaN suggests that this large value of GaN results from the strong electronegativity of the nitrogen atom and the corresponding reduction in dielectric constant, compared with the other compounds. This is further emphasized by the need to use the high-frequency dielectric constant rather than the static value when the ionization energy is greater than the TO phonon energy. Using in ( infinity )=5.35 and an effective mass of 0.4 m provides good agreement with the measured ionization energy of GaN, this value of mn being consistent with estimates based on recent hole mobility measurements. It is also consistent with smoothly increasing hole mass in the sequence GaAs (0.33 m), GaP (0.37 m), GaN (0.40 m). Finally, the opportunity is taken to speculate about likely values of acceptor ionization energies in AlGaN and AlGaAsN alloys.

Photoluminescence studies of InGaN/GaN multi-quantum wells

We report measurements of photoluminescence, photoluminescence excitation spectroscopy and photoluminescence time decay on three MOVPE-grown InGaN/GaN multiple quantum well structures with 13% In in the wells and well widths Lz = 1.25, 2.5 and 5.0 nm. The PL spectra are dominated by single emission peaks, together with phonon sidebands spaced by a GaN LO phonon energy (92 meV). The peak energies are red-shifted with respect to energies calculated for exciton recombination in square quantum wells and the wide well sample also shows a significant Stokes shift between emission and absorption. Recombination lifetimes measured at 6 K are energy dependent, increasing as the photon energy is scanned downwards through the emission line. They also depend strongly on well width. On the low energy side of the 5 nm well emission line we measure lifetimes as long as 100 ns. Raising the temperature from 6 to 300 K results in a strong reduction of emission intensity for all samples and reduction of the lifetimes, though by a much smaller factor. The peak positions shift slightly to lower energy but by far less than the shift in the band edge. We consider three different theoretical models in an attempt to interpret this data, an exponential tail state model, a model of localization due to In/Ga segregation within the wells and the quantum confined Stark effect model. The QCSE model appears able to explain most of the data reasonably well, though there is evidence to suggest that, in addition, some degree of localization occurs.

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.

Optical and photoelectric study of mirror facets in degraded high power AlGaAs 808 nm laser diodes

Mirror facets of broad area AlGaAs SQW 808 nm lasers with fast degradation of the output power have been investigated by photoluminescence microscopy (PLM), electroluminescence microscopy, and micro-photocurrent spectroscopy techniques. Over 2000 images of the photoluminescent light emitted from the facets of approximately 20 degraded laser bars have been made. Repeating patterns associated with the degradation have been noted and classified. Degraded lasers have dark lines on the mirror facet surface which are inclined from the epitaxial plane by 55°. Pairs of such lines often form V-shaped patterns. The orientation of the observed features suggest they are planar or line defects lying the the {111} crystallographic planes. The defects demonstrate a reduction in the lifetime of nonequilibrium electron–hole pairs and increased density of states in the forbidden gap of the active region of the laser structure, and are directly related to laser failure. Observations of the evolution in size and contrast of ...

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.

Band gap of GaN films grown by molecular‐beam epitaxy on GaAs and GaP substrates

There has been controversy concerning the energy gap of the cubic (zinc‐blende) form of the wide‐gap semiconductor GaN. Measurements are reported of the band gaps of both hexagonal (wurtzite) and cubic thin films deposited by a modified molecular‐beam‐epitaxy process on (001) GaAs and GaP substrates. The important difference from conventional MBE lies in the method of supplying nitrogen to the growing film. Here a rf nitrogen plasma source operating at 13 MHz is used. The structure of the films was monitored by x‐ray diffraction and controlled by the addition of an As beam which results in growth of the cubic form—otherwise films grow with the hexagonal structure. The band gaps were measured at room temperature by optical reflectivity, as evidenced by the sharp reduction in interference oscillations as the photon energy approached the band edge, and confirmed by the observation of band‐edge photoluminescence. The results can be summarized as Eg=3.42±0.02 eV for the hexagonal and 3.22±0.02 eV for the cubic...