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Recent developments with group III nitrides suggest AlxGa1-xN based LEDs can be new alternative commer-cially viable deep ultra-violet light sources. Due to a sig-nificant difference in the lattice parameters of GaN and AlN, AlxGa1-xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE) using a novel RF plasma source. Thick wurtz-ite AlxGa1-xN films were grown by PA-MBE on 2-inch GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1-xN samples. Growth rates of AlxGa1-xN up to 3 μm/h have been demonstrated. Our novel high efficiency RF plasma source allowed us to achieve free-standing bulk AlxGa1-xN layers in a single day’s growth, which makes our MBE bulk growth technique commercially vi-able.

We have studied time-integrated and time-resolved photoluminescence (PL) spectra and the transport properties of cubic GaN layers doped with different amounts of either magnesium (p-type) or silicon (n-type). In Mg doped material a broad blue peak was observed whose origin in cubic material has not been well established. Our temperature dependent and time resolved PL results as well as the observation of strong self-compensation in Mg-doped cubic GaN have allowed us to present a model for the origin of this emission in cubic material. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper demonstrates the growth of InGaN nanorods and lateral growth over nanorod arrays using molecular beam epitaxy. It is shown that nitrogen rich growth conditions result in a nanorod array and that, by changing to metal rich conditions, lateral growth may be enhanced to coalesce the nanorods into a continuous overgrown film. Energy dispersive X-ray spectroscopy has been used to demonstrate that the nanorods display a core-shell structure with In-rich cores and In-poor edges. Transmission Electron Microscopy has shown that the nanorods are free of dislocations. However, when lateral growth occurs basal plane stacking faults are generated. It is shown that this stacking fault generation leads to a change in structure from hexagonal to cubic. When coalescence has occurred large angle grain boundaries are present. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied the growth of GaN layers by plasma-assisted electroepitaxy (PAEE). We have used an RF-plasma source to produce high concentrations of N species in the Ga melt and a high DC electric current to transfer the N species from the Ga surface to the growth interface. We have achieved continuous GaN layers by PAEE at growth temperatures as low as ∼650oC. We have also investigated PAEE growth of GaN layers on GaN nanocolumn templates. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transmission Electron Microscopy of Mg doped GaN1-xAsx samples, grown by MBE at low temperatures, show substantial structural changes for samples that are semi-insulating and those with high or low conductivity. The conductive samples show p-type conductivity as evidence from the positive thermopower values. All the Mg doped samples show phase segregation: cubic GaAs and GaN grains (a mixture of cubic and some hexagonal) phases within an amorphous matrix. The best conductive samples show cubic GaAs grains with high density of stacking faults embedded into an amorphous matrix. The samples that are less conductive have lower ratio of the amorphous to the crystalline phase of the samples and much lower density of stacking faults. Higher Mg concentration is expected in the amorphous parts of the samples The semi-insulating samples that have either low Mg concentration or low As show grains of GaAs and GaN attached to each other with no evidence of the amorphous phase between them. There are no SFs in these grains. It is possible that the presence of the GaN between the GaAs grains lead to semi-insulating material properties since p-type doping of GaN is more difficult. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have discovered materials with a wide tunability of the band gap that opens the possibility to achieve a desirable material for multi-junction solar cells. Two different alloy systems have been studied. GaN1-xAsx alloys over the entire composition range were grown by plasma-assisted MBE at low temperature on crystalline and amorphous (glass) substrates. For growth on sapphire the alloys are amorphous in the composition range of 0.17

In the current study we have demonstrated the feasibility of a novel approach for the growth of GaN layers, namely plasma-assisted electroepitaxy (PAEE). In this method, we have combined the advantages of the plasma process for producing high concentrations of active N species in the Ga melt with the advantages of electroepitaxy in transferring the N species from the Ga surface to the growth interface, without spontaneous crystallisation on the surface or within the solution. We have designed and built a new growth chamber which allows us to combine the plasma-assisted molecular beam epitaxy process with a liquid phase electroepitaxy system. We have demonstrated that it is possible to grow GaN layers by PAEE at growth temperatures as low as ∼650 oC and with low nitrogen overpressures of ∼3×10-5 Torr. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have analysed bismuth incorporation into GaN layers using plasma-assisted molecular beam epitaxy (PA-MBE) at extremely low growth temperatures of less than ∼100 °C under both Ga-rich and N-rich growth conditions. The formation of amorphous GaN1−xBix alloys is promoted by growth under Ga-rich conditions. The amorphous matrix has a short-range order resembling random crystalline GaN1−xBix alloys. We have observed the formation of small crystalline clusters embedded into amorphous GaN1−xBix alloys. Despite the fact that the films are pseudo-amorphous we observe a well defined optical absorption edges that rapidly shift to very low energy of ∼1 eV.

We have studied the growth of zinc-blende (cubic) GaN and AlxGa1-xN layers and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced GaN layers up to 100 μm in thickness. Thick, cubic GaN films were grown on semi-insulating GaAs (001) substrates by a modified plasma-assisted molecular beam epitaxy (PA-MBE) and were removed from the substrate after the growth. The resulting free-standing GaN wafers with thicknesses in the 30–100 μm range may be used as substrates for further epitaxy of cubic GaN-based structures and devices. We have demonstrated the scalability of the process by growing GaN layers up to 3” in diameter. We have developed procedures to cleave the wafers and to polish them to produce epi-ready surfaces. We have demonstrated that the PA-MBE process we have developed also allows us to achieve free-standing zinc-blende AlxGa1-xN wafers over the entire composition range from GaN to nearly pure AlN. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated the growth of bulk, free-standing zinc-blende (cubic) GaN layers by plasma-assisted molecular beam epitaxy (PA-MBE). The PA-MBE technique was used for bulk crystal growth and GaN layers up to 100 µm in thickness were produced. We have established that the best structural properties of free-standing zinc-blende GaN can be achieved with initiation under Ga-rich conditions, but without Ga droplet formation. The procedure to produce free-standing bulk zinc-blende substrates from thick GaN layers grown on GaAs substrates has been developed. We have demonstrated the scalability of the process by growing free-standing GaN layers up to 3 in. in diameter. Growth of free-standing bulk GaN layers has allowed us to refine the values for the basic parameters of zinc-blende GaN. We have demonstrated also that the PA-MBE process we have developed has allowed us to achieve free-standing AlxGa1−xN wafers.

We report on the electrical characterization of AlN/GaN/AlN double-barrier resonant tunnelling diodes (RTDs) using steady-state current-voltage and capacitance-voltage (C-V) characteristics in a wide frequency range with 2 kHz steps. The C-V characteristics of a double-barrier RTD show different behaviour under forward and reverse polarities and a strong dependence on frequency. The monotonous growth of capacitance at forward bias was registered, while a more complicated dependence was observed at reverse voltages. In order to analyse this dependence, a self-consistent calculation of the potential profile of the structure was performed taking into account polarization effects at the AlN/GaN interfaces. The peculiarities are analysed in the model of possible charge trapping at the interface states. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We use cathodoluminescence and Raman spectroscopy to investigate GaN layers grown by plasma-assisted molecular beam epitaxy with different Bi fluxes. We find that the growth of GaN with an additional Bi flux favours the formation of submicron cubic domains, which results in layers with reduced crystalline quality and lower luminescence emission. We detect a Bi-related Raman feature at low frequencies that we tentatively assign to local Bi modes in GaN, which indicates that a fraction of Bi atoms incorporates into the GaN lattice. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We have used bolometric detection techniques to study the phonons generated by ultrafast optical excitation of a gallium nitride/aluminium nitride superlattice (SL) structure. For excitation wavelengths shorter than 435 nm, we observe a strong longitudinal acoustic phonon signal. Angular dependence measurements show that this signal enhancement becomes channelled closer to the SL growth direction for excitation wavelengths shorter than the peak photoluminescence wavelength. The results are consistent with similar measurements using GaAs/AlAs SL's, where ultrafast optical excitation resulted in the generation of monochromatic phonons when the structure was resonantly excited. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated the effect of the internal electric fields on radiative and non-radiative processes in a series of AlN/GaN superlattices. The results of measurements involving direct detection of phonons emitted as a result of non-radiative recombination and carrier energy relaxation are compared with time-resolved photoluminescence studies. Using these complimentary techniques we have observed directly the effect of free carrier screening on the radiative and non-radiative recombination processes in the superlattice samples. We find that, at high excitation power, photoinjected carriers screen the strong internal fields, resulting in an enhanced radiative recombination. As the carriers recombine, the de-screening effect results in an increase in recombination via non-radiative processes, which we observe directly as a delayed phonon signal in the time of flight measurements. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN/AlGaN double barrier resonant tunnelling structures grown by molecular beam epitaxy on GaN templates have been studied. Peaks in the I(V) characteristics are observed, which are similar to resonant peaks seen in conventional III–V based devices. However, the behaviour of the peaks in I(V) depend upon the previous charge-state of the device produced by electrical bias. Current instabilities are also observed at low voltages. The possible origin of the peaks in the I(V) at room temperature and 4 K is discussed. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The application of chemical microanalysis techniques to profile the reaction pathways of metallic diffusion couple ohmic contacts to AlGaN/GaN field effect transistors and to appraise the influence of As on the development of GaN heterostructures grown by plasma assisted molecular beam epitaxy is illustrated. 30 nm of Ti within AuPdAlTi contacts is necessary for TiN formation and contact activation following rapid thermal annealing at 950 °C, whilst the supply of excess Ti leads to over-development of the contact and consumption of the AlGaN layer. The presence of As during GaN growth leads to the development of a high density of cubic GaN stacking disorders. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Isoelectronic doping of Al x Ga 1-x N alloys with arsenic in films grown by molecular beam epitaxy has been investigated. In photoluminescence spectra of these Al x Ga 1-x N layers, there is a gradual increase of the band gap and a smaller progressive shift of the position of blue emission band to the higher energies as the Al content increases. To explain the change in the energy of the blue band in As-doped Al x Ga 1-x N alloys a model has been proposed, which predicts the shift in the blue band energy to be equal to the valence band offset from AlGaN to GaN. This prediction is consistent with the experimental results.

We report on observation of photoluminescence in 1.2-1.4 eV spectral range from As-doped GaN layers grown by plasma-assisted molecular beam epitaxy. We attribute this emission to recombination in GaAs inclusions formed during the growth process in the GaN(As) layers. This near-infrared emission is efficient under UV photoexcitation, but its intensity is low for excitation energies below the band gap of GaN. Low-temperature photoluminescence spectra reveal a set of sharp emission lines with maxima from 1.516 eV to 1.436 eV. We attribute some of these sharp lines to the emission of excitons bound to nitrogen incorporated into the GaAs crystallites and also to their GaAs-like optical phonon replicas. Photoluminescence excitation spectra show a series of maxima, which can be attributed to the formation of bound and free excitons and also to the formation of excitons with the simultaneous emission of GaAs-like optical phonons.

We have investigated the influence of an additional bismuth flux during growth on the properties of GaN films prepared by plasma-assisted molecular beam epitaxy. A wide range of bismuth fluxes have been used, at the highest Bi flux this was larger than the flux of Ga. X-ray and photoluminescence studies demonstrate that the structural quality and optical properties of the GaN films grown at ∼800 °C are practically unchanged. X-ray and Auger studies indicated that the concentration of Bi in the bulk of the films grown at ∼800 °C is rather small. We have demonstrated that it is possible to grow conductive metallic Bi films epitaxially onto GaN films in-situ after epitaxy by reducing the deposition temperature. Bismuth may thus potentially be a new material for in-situ contacts to GaN layers and device structures.

We have investigated the influence of the growth conditions on the intensity of the blue emission from arsenic doped GaN films grown by plasma-assisted molecular beam epitaxy. We have demonstrated, that for fixed Ga, N and As fluxes there is an optimum growth temperature, at which the highest blue emission intensity is observed. Possible models explaining this effect were discussed.

Thin films incorporating GaN, InGaN and AlGaN are presently arousing considerable excitement because of their suitability for UV and visible light-emitting diodes and laser diodes. However, because of the lattice mismatch between presently used substrates and epitaxial nitride thin films, the films are of variable quality In this paper we describe our preliminary studies of nitride thin films using electron backscattered diffraction (EBSD). We show that the EBSD technique may be used to reveal the relative orientation of an epitaxial thin film with respect to its substrate (a 90 degrees rotation between a GaN epitaxial thin film and its sapphire substrate is observed) and to determine its tilt (a GaN thin film was found to be tiltedby 13 +/- 1 degrees towards [1010]GaN), where the tilt is due to the inclination of the sapphire substrate (cut off-axis by 10 degrees from (0001)sapphire towards (1010)sapphire). We compare EBSD patterns obtained from As-doped GaN films grown by plasma-assisted molecular beam epitaxy (PA-MBE) with low and high As4 flux, respectively. Higher As4 flux results in sharper, better defined patterns, this observation is consistent with the improved surface morphology observed in AFM studies. Finally, we show that more detail can be discerned in EBSD patterns from GaN thin films when samples are cooled.

The introduction of arsenic (As) into GaN to produce a group-V ternary alloy has been of much recent interest, mostly because of the prospect of reducing the GaN bandgap. We have performed a systematic study of the role of As in GaN grown by molecular beam epitaxy (MBE). The As content of this series of samples varies from 3.4 x 10 17 to 4.2 x 10 18 cm -3 . The data are presented to show how As effects the optical properties of GaN. Our focus is on the nature of the strong luminescence band found at ∼475 nm. The intensity of the GaN near bandedge emission is shown to decrease and the 475 nm emission to increase with As content. This is attributed to the large As atoms disrupting the GaN lattice and creating defects or stacking faults that act as non-radiative centers. We have used scanning electron microscopy (SEM) and cathodoluminescence (CL) to investigate the spatial uniformity of the ∼475 nm emission in these materials and show that the luminescence is inhomogeneous indicating arsenic segregation.

The acquisition of electron backscattered diffraction (EBSD) (or Kikuchi diffraction) patterns in the scanning electron microscope is proving to be a useful technique with which to probe the structural properties of nitride thin films. In this paper we show that if a sample is cooled the patterns improve dramatically, an increase in intensity of the Kikuchi lines and a decrease in the intensity of the diffuse background is observed. Kikuchi lines from higher order planes become visible and the HOLZ rings become better defined. Such cooled patterns yield more information on the sample, particularly on non-centrosymmetric planes, from which the polarity of the nitride thin film under investigation may be deduced.

The transition from As-doped GaN showing strong blue emission (∼2.6 eV) at room temperature to the formation of GaN 1-x As x alloys for films grown by molecular beam epitaxy was investigated. This study demonstrates that with increasing N to Ga ratio there is first an increase in the intensity of blue emission at about 2.6 eV and then a transition to the growth of GaN 1-x As x alloy films. Several possible models, which can explain how this might occur are presented.

We have studied the influence of arsenic on the growth and optical properties of GaN films grown by plasma-assisted molecular beam epitaxy (PA-MBE) under different stoichiometric conditions. For GaN films grown without arsenic, there is an optimum N to Ga flux ratio, which results in films showing a good morphology and the highest room temperature band edge luminescence efficiency. However, with arsenic present, the highest blue and band edge luminescence efficiency is observed in GaN at a higher N to Ga ratio. The onset of GaNAs alloy formation is associated with a change in surface morphology. This suggests that arsenic is acting as an isoelectronic surfactant during the growth of GaN films by PA-MBE, changing the morphology, the optical properties and nature of the electrically active defects.

As-doped GaN films have been grown by plasma-assisted molecular beam epitaxy and their properties investigated using atomic force microscopy, X-ray diffraction and photoluminescence (PL) spectroscopy. The structural properties of the As-doped GaN films improve with increasing sample thickness. The room temperature PL is dominated by a strong blue emission band, exhibiting multiple peaks centered at 2.6 eV. The number of peaks increases monotonically with sample thickness. From this we conclude that the multiple peaks in the blue emission band of As-doped GaN samples arise mainly from optical interference effects. However, the possibility of several transitions involving As being responsible for the blue emission process cannot be excluded.

A new alloy system, the GaN(sub 1-x)As(sub x) alloys in the whole composition range was successfully synthesized using the non-equilibrium low temperature molecular beam epitaxy method. 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 absorption edges. The bandgap energy varies in a broad energy range from 3.4 eV in GaN to0.8 eV at x0.85. The reduction of the band gap can be attributed primarily to the downward movement of the conduction band for alloys with x 0.2, and to the upward movement of the valence band for alloys with x 0.2.

In this paper we have studied the unintentional incorporation of arsenic into thick (up to ∼75 μm) zinc-blende (cubic) GaN and AlGaN layers grown by a plasma-assisted molecular beam epitaxy (PA-MBE) on (001) GaAs substrates. We present detailed studies of As incorporation into free-standing zinc-blende (Al)GaN by secondary ion mass spectroscopy (SIMS) and electron probe microanalysis (EPMA). We demonstrate that there is no significant arsenic contamination of zinc-blende GaN layers and substrates due to diffusion of arsenic during the prolonged growths by PA-MBE. Arsenic incorporation is limited to the first few hundred nanometers of the layer and it is due to initiation phase of the growth of zinc-blende material, which involves use of arsenic as a surfactant to promote the growth of the cubic phase. These data have allowed us to determine the thickness of the unintentionally arsenic contaminated GaN region, which needs to be removed by polishing during production of free-standing zinc-blende GaN substrates. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the mechanisms giving rise to surface reconstruction for homo-epitaxial GaN grown by molecular beam epitaxy on (0001-) bulk GaN substrates. We have studied the surface reconstruction observed during growth and investigated the effects of supplying Ga and active nitrogen separately to the GaN surface. During growth we can observe a (2 × 2) reconstruction on (0001-) GaN under appropriate conditions. We have observed a (2 × 2) reconstruction on the GaN(0001-) substrates induced by supplying Ga to that surface. We have shown that the (2 × 2) surface reconstruction is stable in the presence of an active nitrogen flux at high temperature, but it disappears on cooling the GaN sample below ≈400 °C. Finally, we propose two possible growth models for GaN which can explain our RHEED data.

In this paper bulk, free-standing, zinc-blende (cubic) GaN wafers grown by plasma-assisted molecular beam epitaxy (PA-MBE) are described. GaN layers of up to 60 microns in thickness have been grown. Characterisation measurements confirm the cubic nature of the GaN crystals and show that the fraction of hexagonal material present is not more than ∼10%. Cubic (001)GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0001) c-axis wurtzite GaN. Free standing GaN wafers grown by PA-MBE make ideal lattice-matched substrates for the growth of cubic GaN-based structures for optoelectronic devices, and high power and high frequency electronic applications. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Epitaxial lateral overgrowth (ELO) has been used to reduce threading dislocation (TD) densities in GaN. This paper reports transmission and scanning electron microscopy studies of TDs in GaN films grown by ELO on GaN nanocolumns produced either during growth by MBE (method 1), or by self-organised patterning on a uniform layer grown by MOCVD (method 2). In method 1, isolated nanocolumns grew without TDs, but some TDs formed when nanocolumns coalesced. Although some of these TDs extended through the overlayer, extensive lateral migration of others, depending on the dislocation type, led to dislocation annihilation, lowering the overall TD density. In method 2, a similar process of lateral migration of TDs was observed during ELO, resulting in a continuous GaN overlayer with TD densities less than 108 cm–2. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated the temperature dependence of the transition from single phase films of GaN 1-x As x to phase separated layers, which show regions of hexagonal [0001] oriented GaN, cubic [111] oriented GaAs and hexagonal [0001] oriented GaN 1-x As x . We see a strong temperature dependence of the arsenic flux at which GaAs inclusions are first observed. Finally the intensity of blue emission observed in As-doped GaN samples decreases strongly with decreasing growth temperature.

An effective method for the synthesis of nitrofurazans (NF) from aminofurazans (AF) has been developed based on H 2 O 2 -containing oxidizing mixtures with reduced acidity; a correlation between the ionization potentials of AF and the oxidative capacity of the mixtures has been found, and a calculated analytical method to determine the choice of oxidizing mixtures has been derived.