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Hexagonal boron nitride is a large band-gap insulating material which complements the electronic and optical properties of graphene and the transition metal dichalcogenides. However, the intrinsic optical properties of monolayer boron nitride remain largely unexplored. In particular, the theoretically expected crossover to a direct-gap in the limit of the single monolayer is presently not confirmed experimentally. Here, in contrast to the technique of exfoliating few-layer 2D hexagonal boron nitride, we exploit the scalable approach of high-temperature molecular beam epitaxy to grow high-quality monolayer boron nitride on graphite substrates. We combine deep-ultraviolet photoluminescence and reflectance spectroscopy with atomic force microscopy to reveal the presence of a direct gap of energy 6.1 eV in the single atomic layers, thus confirming a crossover to direct gap in the monolayer limit., Insulating hexagonal boron nitride (hBN) is theoretically expected to undergo a crossover to a direct bandgap in the monolayer limit. Here, the authors perform optical spectroscopy measurements on atomically thin epitaxial hBN providing indications of the presence of a direct gap of energy 6.1 eV in the single atomic layer.

Integration of graphene and hexagonal boron nitride (hBN) into lateral heterostructures has drawn focus due to the ability to broadly engineer the material properties. Hybrid monolayers with tuneable bandgaps have been reported, while the interface itself possesses unique electronic and magnetic qualities. Herein, we demonstrate lateral heteroepitaxial growth of graphene and hBN by sequential growth using high-temperature molecular beam epitaxy (MBE) on highly oriented pyrolytic graphite (HOPG). We find, using scanning probe microscopy, that graphene growth nucleates at hBN step edges and grows across the surface to form nanoribbons with a controlled width that is highly uniform across the surface. The graphene nanoribbons grow conformally from the armchair edges of hexagonal hBN islands forming multiply connected regions with the growth front alternating from armchair to zigzag in regions nucleated close to the vertices of hexagonal hBN islands. Images with lattice resolution confirm a lateral epitaxial alignment between the hBN and graphene nanoribbons, while the presence of a moiré pattern within the ribbons indicates that some strain relief occurs at the lateral heterojunction. These results demonstrate that high temperature MBE is a viable route towards integrating graphene and hBN in lateral heterostructures.

Single-photon emitters (SPEs) in hexagonal boron nitride (hBN) have garnered increasing attention over the last few years due to their superior optical properties. However, despite the vast range of experimental results and theoretical calculations, the defect structure responsible for the observed emission has remained elusive. Here, by controlling the incorporation of impurities into hBN via various bottom-up synthesis methods and directly through ion implantation, we provide direct evidence that the visible SPEs are carbon related. Room-temperature optically detected magnetic resonance is demonstrated on ensembles of these defects. We perform ion-implantation experiments and confirm that only carbon implantation creates SPEs in the visible spectral range. Computational analysis of the simplest 12 carbon-containing defect species suggest the negatively charged $${\rm{V}}_{\rm{B}}{\rm{C}}_{\rm{N}}^ -$$ defect as a viable candidate and predict that out-of-plane deformations make the defect environmentally sensitive. Our results resolve a long-standing debate about the origin of single emitters at the visible range in hBN and will be key to the deterministic engineering of these defects for quantum photonic devices. Comparison of hexagonal boron nitride samples grown with different techniques and with varying carbon-doping content provides evidence that the defects emitting single photons in the visible range are carbon related.

Lattice-matched graphene on hexagonal boron nitride is expected to lead to the formation of a band-gap but requires the formation of highly strained material and has not hitherto been realised. We demonstrate that aligned, lattice-matched graphene can be grown by molecular beam epitaxy using substrate temperatures in the range 1600-1710 °C and co-exists with a topologically-modified moiré pattern, and with regions of strained graphene which have giant moiré periods up to ~80 nm. Raman spectra reveal narrow red-shifted peaks due to isotropic strain, while the giant moiré patterns result in complex splitting of Raman peaks due to strain variations across the moiré unit cell. The lattice-matched graphene has a lower conductance than both the Frenkel-Kontorova-type domain walls, and also the topological defects where they terminate. We relate these results to theoretical models of band-gap formation in graphene/boron nitride heterostructures.

Monolayers of hexagonal boron nitride (hBN) are grown on graphite substrates using high-temperature molecular beam epitaxy (HT-MBE). The hBN monolayers are observed to grow predominantly from step edges on the graphite surface and exhibit a strong dependence of the morphology, including the dominant crystallographic edge, of the hBN monolayers, on the growth temperature, as well as systematic variations in growth rate and coverage, and significant differences in the growth at monolayer and multilayer graphite steps. At graphite monolayer steps hBN grows laterally across the surface on the lower terrace, but hBN growth on the upper side of the graphite step is more limited and is nucleated by three-dimensional clusters. Multilayer graphite steps exhibit a much higher density of non-planar hBN aggregates and growth on both the upper and lower terraces occurs. The results show that the hBN monolayer growth edge type, hBN island shape and the presence of hBN aggregates can be controlled in HT-MBE, with the highest quality layers grown at a substrate temperature of about 1390 °C. Sequential HT-MBE growth of hBN, graphene (G) and a second cycle of hBN growth results in the formation of monolayer thick lateral hBN–G–hBN heterostructures, in which a strip of G is embedded between monolayers of hBN.

We have used phonon imaging to study the drag current induced by non-equilibrium acoustic phonons in a low-mobility n-type Gallium Nitride epilayer. Current densities up to j=4.7×10 3 A/m 2 were induced by transverse acoustic phonon drag of electrons in the structure. We also carried out numerical calculations of the phonon-drag current including the effects of phonon focussing in the sapphire substrate, acoustic anisotropy in the GaN and acoustic mismatch between the two materials. We show that the theory is able to account for the qualitative features of the drag image as well as the magnitude of the phonon-drag-induced current.

Aluminium layers deposited onto GaN/(0 0 0 1) sapphire and AlN/(0 0 0 1) sapphire substrates by molecular beam epitaxy have been characterised and compared with aluminium grown directly onto (0 0 0 1) sapphire. Aluminium layers with very flat surfaces having RMS roughness in the range 0.2–0.7 nm as measured by atomic force microscopy were grown. Complementary scanning electron microscopy, reflection high energy electron diffraction and conventional transmission electron microscopy observations confirmed that the Al layers adopted a twinned island microstructure, exhibiting a {1 1 1}-oriented growth surface and epitaxial relationships of the form 〈1 1 2〉 Al ∥〈1 1 0 0〉 nitride ∥〈1 1 2 0〉 sapphire and 〈1 1 2〉 Al ∥〈1 1 2 0〉 sapphire .

The epitaxial growth of GaN layers on sapphire substrates by molecular beam epitaxy at low temperatures (500°C) has been investigated. Samples exhibited a transition from hexagonal to mixed hexagonal/cubic phase under conditions of increasing Ga flux as determined using a TEM-RHEED technique with complementary SEM and PL observations. Embedded cubic grains adopted two domain variants with additional evidence for twinning.

We have observed strong blue emission at room temperature from arsenic doped GaN samples grown by molecular beam epitaxy. Similar results were obtained for samples doped with both arsenic dimers and tetramers. The origin of this blue emission is discussed and a growth model proposed to account for our observations. We propose that arsenic doped GaN may be a suitable replacement for (InGa)N as the active region for blue light emitting devices.

We have investigated arsenic doping of GaN films grown at high temperature (∼800°C) by molecular beam epitaxy. Growth in an arsenic environment changes the growth mode and promotes the growth of the cubic polytype. Growth under arsenic also produces films which show strong blue emission at room temperature, whose intensity is dependent on both the As flux and the Ga : N ratio. We propose a growth model based on As incorporation onto the Ga sublattice, which can explain all of our observed data.

Quantum well solar cells (QWSC) have been proposed as a way to achieve a higher efficiency than conventional homogeneous cells. A key quantity for understanding the potential of the QWSC is the spatial variation of the quasi-Fermi level separation (Δ φ F ) which is directly linked to the radiative losses. For the first time we have successfully measured Δ φ F in asymmetric double QW (DQW) p–i–n diodes in the AlGaAs/GaAs system at temperatures between 200 and 295 K. We have made several significant observations: (1) radiative losses through the QW decrease with increasing temperature and are clearly below the detailed balance-based predictions at room temperature. (2) Δ φ F is not the same in both wells which contradicts the assumption made by some researchers. (3) Δ φ F shows a dependence on the separation between the two QWs at low temperatures.

The energy relaxation rate for hot electrons in n-type GaN epilayers has been measured over the temperature range 1.5–300 K. Several samples grown by molecular-beam epitaxy and having different electron concentrations have been studied. At low electron temperatures (Te

We demonstrate, for the first time, very strong blue emission at room temperature in GaNAs grown by molecular beam epitaxy (MBE). We have studied the MBE growth of GaN at approximately 800°C in the presence of a wide range of arsenic fluxes from 10−9 to 10−5 mbar using both As2 and As4. We have shown that the surface reconstructions in the absence and presence of arsenic are significantly different. Using atomic force microscopy, we have shown that arsenic acts as a surfactant, at low fluxes we obtain large flat islands showing atomic steps. In addition the surface roughness improves with increasing arsenic overpressure. Using Auger electron spectroscopy, we have shown that the surface concentration of As is independent of the As flux for a wide range of arsenic fluxes from 10−9 to 10−5 mbar and is practically the same for both As2 and As4. Our photoluminescence studies have allowed us to propose a growth mechanism for As-doped GaN in which cubic GaN is formed from GaAs by N atoms replacing As. This study suggests that GaNAs may replace InGaN in MBE grown opto-electronic devices.

GaN films grown by RF plasma source molecular beam epitaxy (MBE) were investigated by low-temperature photoluminescence (PL) and secondary ion mass spectroscopy (SIMS). A peak that is generally accepted as being the donor bound exciton was observed in all spectra. The energy of the transition varied from sample to sample, but was always in the range of 3.466-3.473 eV. In some of the samples, an additional peak was observed. The energy of the transition also varied and was in the range of 3.417-3.428 eV. An attempt was made to correlate the presence of this peak to the oxygen and silicon content of the samples using SIMS. We conclude that, contrary to some previous reports, the peak seen in the 4K PL spectra of GaN in the region of 3.424 eV cannot always be attributed to oxygen in the film. We also find no correlation between the line and the Si or C concentrations. The position of the DoX peak was also found to be independent of the Si and C concentrations.

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.

Room-temperature photoenhanced wet chemical etching of MBE grown GaN has been studied during the processing of devices for a physics research programme. The process uses a 0.5 M KOH solution with illumination provided by a mercury arc lamp emitting at 365 nm. The maximum etch rate achieved was ∼45 nm min −1 , much lower than that for MOCVD material etched under similar conditions. Surface morphologies vary significantly for wafers etched under nominally identical conditions, from surfaces with whiskers to highly crystallographic faces.

An investigation of the conditions giving rise to surface reconstruction for GaN grown by molecular beam epitaxy on (0 0 0 1) bulk GaN substrates has been carried out. The results of different surface reconstructions were investigated by supplying Ga and active nitrogen separately to GaN surfaces. A (2×2) reconstruction on (0 0 0 1)GaN substrates and for GaN layers grown on such substrates was observed. 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. The (2×2) reconstruction is considered to be due to an additional amount of relatively tightly bound Ga on this (1×1) surface. Adding further Ga to the (2×2) reconstructed surface resulted in a return to a (1×1) reconstruction.

The paper is devoted to the investigation of low-temperature ( T =1.8 and 4.2 K) luminescence spectra of double GaAs/Al 0.33 Ga 0.67 As asymmetric quantum wells (DQW) under the changing electric field V dc , applied normally to the plane of layers of DQWs. It was found that the full width of half maximum (FWHM) of the indirect exciton luminescence line (IX) in some interval of V dc values narrows sharply (up to 3.5 times). In the transition range of V dc values, in which there arises a sharp decrease (or increase) of FWHM, abnormally large fluctuations at the time of IX intensity are observed. The dependence of FWHM on the optical pumping level I P reveals a sharp FWHM decrease in some interval of I P values. The results are discussed using the assumption that the observed phenomena are due to the appearance of a condensed state in collective interacting spatially indirect excitons in the DQW.

We have shown that for GaN films grown directly on sapphire substrates using plasma-assisted molecular beam epitaxy, the application of a localised mechanical stress on the films can produce metallic droplets on the surface. By using X-ray photoelectron spectroscopy measurements we have demonstrated that the droplets consist of Ga, which can be removed by etching in HCl. This problem can be avoided using nitridation of the surface combined with a low-temperature buffer layer, or by using alternative substrates such as GaAs or Si.

RHEED studies of the reconstructions obtained during growth of GaN on a range of different substrates are reported. The effects of pre-nitridation of the substrate, of growth temperature and also of surface contamination by oxygen and arsenic are discussed. During stoichiometric growth, an unreconstructed surface is usually observed. For all substrates, reconstruction is generally observed upon subsequent cooling. These reconstructions, which vary according to the polarity of the surface, are found to be associated with the presence on the surface of excess gallium which undergoes an order/disorder transition in the temperature range 400–500°C.

We have been studying the structural properties of Group III-nitrides by EXAFS. Previously we have investigated bulk Group III-nitrides and its alloys and have demonstrated that we can grow the full range of alloys from AlN to InN at low temperatures. Here we extend these investigations to study the local environment and lattice site occupied by dopant atoms in such samples.

We report on an investigation by Auger Electron Spectroscopy (AES) of Mg-doped GaN films grown by Molecular Beam Epitaxy (MBE). This study shows that there is a finite concentration gradient in a region close to the surface (10–80 nm) in which the Mg concentration decreases rapidly from a few percent of a monolayer to a bulk value of about (2–3)×10 19 cm −3 for heavily doped samples. Such behaviour can be understood in terms of a kinetically limited migration of the dopant to the surface during the growth.

We report on an investigation of the mechanisms giving rise to surface reconstruction for GaN grown by molecular beam epitaxy (MBE) on a range of different substrates. We have studied the effects of surface contamination by oxygen or arsenic and demonstrate that both can influence the surface reconstruction. We show that surface reconstruction measured by reflection high-energy electron diffraction (RHEED) is associated with excess Ga on the surface, which undergoes an order–disorder transition at about 400–500°C. For MBE, growth on hydride vapour phase epitaxy (HVPE) GaN/SiC composite substrates having the Ga polarity the 2×2 reconstruction is intrinsic. This intrinsic reconstruction can be destroyed by heating to a high temperature or by oxidation. The intrinsic 2×2 reconstruction can also be destroyed by adding an additional monolayer of more weakly bound Ga, which can be removed by desorption at high temperature. For growth by MBE on sapphire with nitridation, a 3×3 reconstruction is observed on cooling the sample to

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.

We report on an investigation of the initial stages of growth by molecular beam epitaxy (MBE) of GaN layers on composite substrates consisting of hydride vapour phase epitaxial (HVPE) GaN films on SiC substrates. In situ reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) were used to monitor the surface, reconstruction of the GaN and to measure the structure of the grown surface, respectively. A 1×3 streaky RHEED pattern was observed on the HVPE GaN layers from room temperatures up to 750°C. This corresponds to a (√3×√3)R30° reconstruction of the GaN surface. At a substrate temperature of 600–650°C, growth by MBE of GaN layers on the HVPE GaN substrate leads to a change of the RHEED pattern from 1×3 to hexagonal 2×2 during the first few minutes of epitaxy. RHEED oscillations were observed at the beginning of growth at 400°C. MBE growth at higher temperatures ∼730–750°C significantly enhances the flatness of the surface of the GaN layer. In this case, RHEED patterns were more intense and streaky than those seen for GaN layers grown at lower temperatures. The surface of the MBE GaN layers grown at high temperatures were atomically flat within the sensitivity of our AFM system.

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.

Current–voltage and capacitance-voltage characteristics of an n-GaN/n+-GaAs substrate heterojunction have been measured over the temperature range from 120 to 360 K. It was found that, depending on the GaAs substrate temperature, the heterojunction behaves either as a back-to-back diodes structure or as a metal–insulator–semiconductor (MIS) structure. From the reverse bias saturation currents, an activation energy was estimated and was found to be 0.25 and 0.16 eV for the samples which behaved as MIS structures.

The InNAs films have been grown on GaAs substrates by molecular beam epitaxy using an RF-activated plasma N source. The properties of these films have been investigated by X-ray diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy mapping. Two phases, InNAs and InAs, were found in the InNAs films. The InNAs phase consists of pyramid-like islands connected to each other at the base and that most of the InAs grains grew between these InNAs islands in the shape of an inverse pyramid. Both InNAs and InAs are cubic and have the same crystallographic direction as the substrates.

An InNAs ternary alloy which was predicted to have a very narrow or even a negative band-gap energy was grown on GaAs(1 0 0) substrate by molecular-beam epitaxy (MBE). A ternary alloy with a nitrogen content of about 38%, which was almost lattice-matched to GaAs(1 0 0) substrate, was successfully grown.

We have studied the growth by hydride vapour phase epitaxy (HVPE) of GaN on GaN layers grown by molecular beam epitaxy (MBE) on sapphire and GaAs(1 1 1)B substrates. For growth on the GaN/sapphire composite substrates, the properties of the HVPE GaN are significantly improved as evidenced from X-ray studies. Growth on GaAs has enabled us to produce thick free-standing layers of GaN which may be suitable as substrates for further epitaxy.

The incorporation of magnesium and carbon in GaN grown by molecular beam epitaxy (MBE) has been investigated by secondary ion mass spectroscopy (SIMS) and other techniques. We have grown Mg:GaN in a wide range of chemical concentrations -. Low temperature photoluminescence of Mg:GaN is dominated by the donor-acceptor transitions associated with Mg at eV. Carrier concentration for Mg:GaN in the range - with mobilities were measured by the Hall effect technique. In the C:GaN layers, it was found that carbon can be uniformly incorporated into the layer at a concentration . However, at this high concentration there is a tendency for carbon to diffuse into the undoped GaN buffer layer.

Films of GaN have been grown simultaneously on three different substrates, sapphire, silicon carbide and GaN, by molecular beam epitaxy. The structural and optical properties of the films have been studied using x-ray diffraction techniques and low-temperature photoluminescence spectroscopy respectively. There is a clear correlation between the structural and optical properties of the film and the nature of substrate. The lattice parameter of the film is shown to depend on the type of substrate. The incorporation of shallow donors is also strongly influenced. The effect of lattice parameter, mismatch and differential thermal expansion on film properties is discussed.

Gold Schottky diodes have been fabricated, for the first time, on n-type GaN grown by molecular beam epitaxy. These diodes have been studied by I - V, C - V and deep-level transient spectroscopy. The conduction process inside the diodes is shown to be by thermionic field emission. The barrier height of 1.1 eV compares well with Au Schottky diodes manufactured from n-GaN grown by metalorganic chemical vapour deposition. However, there is evidence to suggest that the grown-in deep levels are different for the two growth techniques.

The cyclotron resonance of very high-mobility holes in GaAs - (Ga, Al)As heterojunctions grown on (111), (311) and (100) substrates has been studied in high magnetic fields of up to 40 T. As the temperature is increased from to , the cyclotron resonance is found to shift to lower magnetic fields, the size of shift depending on the cyclotron frequency and the substrate orientation. These observations may be explained using the model of interacting hole subsystems developed by Cole et al.

The cyclotron resonance of very high-mobility two-dimensional holes in GaAs - (Ga, Al)As heterojunctions grown on (111), (311) and (100) substrates has been studied over the frequency range 30 to 200 GHz. Although the presence of two hole spin subbands in the samples suggests that two cyclotron resonances should be observed, in practice only a single resonance occurs for a wide range of conditions (temperature, field) and sample properties (hole density, mobility). Furthermore, the cyclotron resonance spectra often exhibit a strong temperature dependence. In the case of a single, sharp cyclotron resonance, the resonance field may shift by as much as 20% when warming the sample from 1.4 to 4.2 K. In the case of spectra containing multiple cyclotron resonances, similar changes in temperature shift the resonance positions together to form a single cyclotron absorption. This behaviour is explained in terms of two interacting hole subsystems with different effective masses formed by the two spin subbands. An analytical expression for the contribution to the high-frequency conductivity due to coupled cyclotron motion of the two hole systems is derived and shown to encompass previous theories developed for more restricted ranges of conditions. The expression predicts the complex behaviour of the experimental spectra very well, and enables hole masses, hole - lattice scattering rates and hole - hole scattering rates to be extracted. Comparisons between theory and data also show that a reactive interaction dominates the coupling between the spin subsystems at low temperatures. This is the first of two papers dealing with correlated hole cyclotron resonance; the second shows that the model derived in this work can also be used to treat cyclotron resonance data recorded at very high magnetic fields .

Meltback etching and selective meltback etching of GaN layers have been achieved using both liquid-phase epitaxy (LPE) and liquid-phase electroepitaxy (LPEE) techniques with GaAs solutions. Both the uniformity and the etching rate (∼ 2.4 μm/h) were improved using the LPEE technique.

We report the results of photoluminescence measurements of four MBE samples of GaN doped with Si at levels in the range 10 17 –10 19 cm −3 . At the lower doping levels the near band edge emission is consistent with exciton recombination, being dominated by excitons bound either to neutral donors or to neutral acceptors. On increasing the doping above 10 18 cm −3 the evidence suggests that recombination may be dominated by free hole to donor-bound electron transitions which are considerably broadened by donor banding effects.

P-type doping of molecular beam epitaxy grown GaN has been investigated using beryllium, magnesium and carbon, the sample being characterized by luminescence under optical excitation by He - Cd laser light of energy 3.815 eV. Doping resulted in a strong reduction of the band-edge luminescence in some samples and the appearance of deep level bands at around 2.3 and 2.5 eV. Photoluminescence spectra in the region of the band gap using high gain revealed the presence of narrow lines at positions 3.541, 3.449, 3.357 and 3.266 eV in a wide variety of these samples. These lines were constant in position with respect to temperature over the range of 5 K to 300 K and it was noted that they are separated by the GaN longitudinal optical phonon energy of 92 meV. It is believed that these lines are caused by resonant Raman scattered replicas appearing in the photoluminescence spectra.

An experimental study of carrier escape in a series of p - i - n asymmetric double quantum well (ADQW) structures using time-resolved photoluminescence is presented. By performing complementary continuous-wave photoluminescence and photocurrent measurements the carrier escape times were extracted as a function of temperature and perpendicular applied electric field and compared with a quantitative model which includes the mechanisms of tunnelling, thermally assisted tunnelling and thermionic emission. Holes were found to dominate the escape rate, and in the light of this it is shown that if narrow wells are grown next to wider wells and nearer the p region a significantly enhanced carrier escape rate can result. This is explained by resonant hole tunnelling and may have implications in the design of QW modulator and solar cell devices with improved performance.

GaN films were grown by the MBE method on substrates. Photoluminescence (PL) of the epitaxial films has been studied to characterize the films ex situ. The room-temperature PL spectrum contains an edge PL band and also a broad band of deep-level emission at 2.6 eV. This deep-level emission becomes very small in comparison with the edge emission at helium temperatures. Analysis of the low-temperature data suggests that the films contain both hexagonal and cubic crystallites. The main PL lines at 5.7 K are due to emission of excitons bound on neutral donors in hexagonal and cubic phases of the epitaxial film (3.472 eV and 3.263 eV), D - A pair recombination (3.154 eV) and also free electron - neutral acceptor transition (3.185 eV) in c-GaN. The involved acceptor has a binding energy of about 115 meV.

Both zinc-blende and wurtzite GaN layers were grown by a modified molecular beam epitaxy (MBE) method. These layers were grown on semi-insulating GaAs(100) substrates and doped with silicon and beryllium as n- and p-type dopants. The structure of the materials was confirmed using X-ray diffraction (XRD). Photoluminescence (PL) measurement at 77 K shows a single sharp emission at 3.473 eV for the wurtzite GaN doped with silicon, and this peak is shifted downwards in energy by about 200 meV in the zinc-blende GaN doped with silicon. The donor ionisation energy is estimated to be ∼ 30 meV. For beryllium in GaN, PL shows a broad emission at about 2.55 eV for the wurtzite material, while two lines at 2.90 eV and about 2.2 eV appear in the zinc-blende sample. An acceptor ionisation energy for zinc-blende GaN of 200 meV is estimated for the first time.

The effective masses of two-dimensional GaAs/(Ga,Al)As hole systems grown with (100), (311)B, (211)B, (111)B and (110) substrate orientations have been measured in magnetic fields of up to 80 T. The measured anisotropy is discussed in terms of bulk effects using a newly developed theory. Low energy (millimetre wave) cyclotron resonance absorptions show a 15% shift to higher fields in the temperature range 1.4 to 4.2 K over two orders of magnitude of energy. Many-body effects are used to discuss this observation.

We report time-resolved measurements of phonon absorption in the fractional quantum Hall regime. Experiments have been conducted on an n-type heterojunction grown on a 1.8 mm GaAs wafer. The start of the absorption signal is consistent with the ballistic phonon time-of-flight. The initial rise time is consistent with the time constant of our device and external circuits. This is followed by unusually slow rise and decay times.

We have studied the incorporation of Be and Si in GaN grown using molecular beam epitaxy (MBE). From secondary-ion mass spectroscopy (SIMS) measurements, Be was found to have an enhanced diffusion rate at high concentration similar to the behaviour observed previously in GaAs. On the other hand, Si behaves as a conventional n-type dopant with no evidence from SIMS for either segregation or diffusion out of the layer. For Be-incorporated material we report new lines in the PL spectrum for both wurtzite and zinc blende polytypes, and we suggest that Be may be a suitable dopant for the zinc blende material.

Time-resolved photoluminescence (TRPL), continuous-wave photoluminescence (PL) and photocurrent (PC) measurements have been performed on a single quantum well (SQW) embedded in a p - i - n diode and, by combining these measurements, unambiguous carrier escape times have been extracted as a function of perpendicular applied electric field and temperature. The experimental data are compared with a quantitative model based on the competition between escape and recombination lifetimes which includes direct tunnelling, thermally assisted tunnelling and thermionic emission processes and which has no free fitting parameters. Good agreement was found for the escape times with the radiative recombination rate being determined by the carriers with the faster escape rate and hence the lowest steady-state concentration. These are holes in this sample.

Molecular beam epitaxy grown, Be-doped multiquantum well heterostructures with the Be atoms at the centre of the wells are studied by conventional photoluminescence techniques. The variation of the ground state binding energy of the shallow Be acceptor with the width of the well has been measured for a slightly wider range (between 28 and 300 A) of well thicknesses than previously. The data are in good agreement with theory, confirming the high degree of localization of the doping at the centre of the GaAs wells. Two-hole transitions of the acceptor-bound exciton involving the and excited states of the Be impurity are clearly observed using above-bandgap excitation for the 71 A wide sample. The energies of the transition from the ground to the excited states of the Be atom are measured and found to be in good agreement with other experimental data and with recent theoretical predictions.

Films of Ga(AsN) and Al(AsN) have been grown using a modified molecular beam epitaxy method. In both cases the concentration of nitrogen incorporated is directly proportional to the amount of active nitrogen reaching the sample surface. For the Ga(AsN) samples we present evidence that phase separation occurs into nitrogen doped GaAs and arsenic doped GaN regions with a grain size of less than 3 μm.

Cyclotron resonance of two-dimensional holes in high-mobility GaAs-(Ga,Al)As heterojunctions with the growth directions (011), (111), (211), (311) and (100) has been measured at magnetic fields of around 35 T, corresponding to Landau level occupancies deep in the ultraquantum limit. A manipulation of the standard four-band Luttinger Hamiltonian has been used to show that the behaviour of the hole ground state is dominated by the leading field-dependent term in a power series expansion for the Landau level dispersion. The experimentally observed trend in measured effective mass with substrate orientation can therefore be qualitatively explained in terms of the variation of bulk hole mass with crystallographic direction.