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Being a flexible wide band gap semiconductor, hexagonal boron nitride (h-BN) has great potential for technological applications like efficient deep ultraviolet light sources, building block for two-dimensional heterostructures and room temperature single photon emitters in the ultraviolet and visible spectral range. To enable such applications, it is mandatory to reach a better understanding of the electronic and optical properties of h-BN and the impact of various structural defects. Despite the large efforts in the last years, aspects such as the electronic band gap value, the exciton binding energy and the effect of point defects remained elusive, particularly when considering a single monolayer. Here, we directly measured the density of states of a single monolayer of h-BN epitaxially grown on highly oriented pyrolytic graphite, by performing low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The observed h-BN electronic band gap on defect-free regions is $(6.8\pm0.2)$ eV. Using optical spectroscopy to obtain the h-BN optical band gap, the exciton binding energy is determined as being of $(0.7\pm0.2)$ eV. In addition, the locally excited cathodoluminescence and photoluminescence show complex spectra that are typically associated to intragap states related to carbon defects. Moreover, in some regions of the monolayer h-BN we identify, using STM, point defects which have intragap electronic levels around 2.0 eV below the Fermi level., 50 Pages, 8 Figures, 100+ references

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.

In this work we present our recent results on the high-temperature plasma-assisted molecular beam epitaxy (PA-MBE) of hBN monolayers with atomically controlled thicknesses for 2D applications and on the growth of significantly thicker hBN layers for potential DUV applications. We also report our recent results on the high-temperature PA-MBE growth of hBN layers using a high-efficiency RF plasma source with high active nitrogen fluxes. Our results demonstrate that PA-MBE growth on highly oriented pyrolytic graphite (HOPG) substrates at temperatures ~1390°C can achieve mono- and few-layer thick hBN with a control of the hBN coverage and atomically flat hBN surfaces which is essential for 2D applications of hBN layers. The hBN monolayer coverage can be reproducible controlled by the PA-MBE growth temperature, time and B:N flux ratios. Significantly thicker hBN layers have been achieved at higher B:N flux ratios. We observed a gradual increase of the hBN thickness by decreasing the growth temperature from 1390°C to 1080°C. However, by decreasing the MBE growth temperature below 1250°C, we observe a rapid degradation of the optical properties of hBN layers. Therefore, high-temperature PA-MBE, above 1250°C, is a viable approach for the growth of high-quality hBN layers for 2D and DUV applications.

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.

Monolayer hexagonal boron nitride (hBN) tunnel barriers investigated using conductive atomic force microscopy reveal moiré patterns in the spatial maps of their tunnel conductance consistent with the formation of a moiré superlattice between the hBN and an underlying highly ordered pyrolytic graphite (HOPG) substrate. This variation is attributed to a periodc modulation of the local density of states and occurs for both exfoliated hBN barriers and epitaxially grown layers. The epitaxial barriers also exhibit enhanced conductance at localized subnanometer regions which are attributed to exposure of the substrate to a nitrogen plasma source during the high temperature growth process. Our results show clearly a spatial periodicity of tunnel current due to the formation of a moiré superlattice and we argue that this can provide a mechanism for elastic scattering of charge carriers for similar interfaces embedded in graphene/hBN resonant tunnel diodes.

We demonstrate direct epitaxial growth of high-quality hexagonal boron nitride (hBN) layers on graphite using high-temperature plasma-assisted molecular beam epitaxy. Atomic force microscopy reveals mono- and few-layer island growth, while conducting atomic force microscopy shows that the grown hBN has a resistance which increases exponentially with the number of layers, and has electrical properties comparable to exfoliated hBN. X-ray photoelectron spectroscopy, Raman microscopy and spectroscopic ellipsometry measurements on hBN confirm the formation of sp2-bonded hBN and a band gap of 5.9 ± 0.1 eV with no chemical intermixing with graphite. We also observe hexagonal moiré patterns with a period of 15 nm, consistent with the alignment of the hBN lattice and the graphite substrate.

We demonstrate direct epitaxial growth of high-quality hexagonal boron nitride (hBN) layers on graphite using high-temperature plasma-assisted molecular beam epitaxy. Atomic force microscopy reveals mono- and few-layer island growth, while conducting atomic force microscopy shows that the grown hBN has a resistance which increases exponentially with the number of layers, and has electrical properties comparable to exfoliated hBN. X-ray photoelectron spectroscopy, Raman microscopy and spectroscopic ellipsometry measurements on hBN confirm the formation of sp2-bonded hBN and a band gap of 5.9 ± 0.1 eV with no chemical intermixing with graphite. We also observe hexagonal moiré patterns with a period of 15 nm, consistent with the alignment of the hBN lattice and the graphite substrate.

The growth and properties of hexagonal boron nitride (hBN) have recently attracted much attention due to applications in graphene-based monolayer thick 2D-structures and at the same time as a wide band gap material for deep-ultraviolet device (DUV) applications. We present our results on the high-temperature plasma-assisted molecular beam epitaxy (PA-MBE) of hBN monolayers on highly oriented pyrolytic graphite (HOPG) substrates. Our results demonstrate that PA-MBE growth at temperatures ~1390 oC can achieve mono- and few-layer thick hBN with a control of the hBN coverage and atomically flat hBN surfaces which is essential for 2D applications of hBN layers. The hBN monolayer coverage can be reproducible controlled by the PA-MBE growth temperature, time and B:N flux ratios. Significantly thicker hBN layers have been achieved at higher B:N flux ratios. We observed a gradual increase of the hBN thickness from 40 to 70 nm by decreasing the growth temperature from 1390 oC to 1080 oC.\ud However, by decreasing the MBE growth temperature below 1250 oC, we observe a rapid degradation of the optical properties of hBN layers. Therefore, high-temperature PA-MBE, above 1250 oC, is a viable approach for the growth of high-quality hBN layers for 2D and DUV applications.

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.

Techniques of low-temperature photoluminescence (PL), photoluminescence excitation, and photoreflectance were used to study the effect of hydrogen plasma treatment at 260°C on antimodulation Si-doped GaAs/AlGaAs heterostructures with near-surface single quantum wells (QWs) grown by molecular-beam epitaxy. It was found that, in the case of excitation below the AlGaAs band gap, exciton PL from the QW is quenched due to an increase in the electric field in the structure. The reason for this is that the passivation of surface states by hydrogen results in the Fermi level depinning from the midgap of the nominally undoped p-type GaAs cap layer, while shallow-level impurities present in the layers of the structure are not neutralized (their complexes with hydrogen dissociate under the influence of illumination and strong electric fields).

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.

We present here a study of the growth of AlN by molecular beam epitaxy on sapphire (0 0 0 1) substrates using reflection high energy electron diffraction and atomic force microscopy. The surface morphology and reconstruction have been monitored as a function of temperature using initially nitrided sapphire substrates. We obtained a (2×2) reconstruction on AlN after growth using two different procedures; either by cooling or by remaining at high temperature for an extended period of time. During the subsequent growth of AlN we can maintain this (2×2) reconstruction for some time, before it reverts to a (1×1) pattern. We have also studied the influence upon the reconstruction of post-growth deposition of Al at high temperature onto the AlN surface after growth. By desorbing the excess Al, the (2×2) surface reconstruction recovery time was found to be directly proportional to the Al deposition time. This strongly suggests, therefore, that the (2×2) reconstruction may be intrinsic to AlN.

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.

Deep levels in undoped GaN materials grown by modified molecular beam epitaxy (MBE) are investigated by photoluminescence (PL) and optical quenching of photoconductivity measurements. A broad band which extends from 2.1 to 3.0 eV with a maximum at about 2.7 eV is observed, and four prominent quenching bands were found located at 2.18, 2.40, 2.71, and 2.78 eV above the valence band, respectively. These levels are attributed to four holes trap levels existence in the material. The defects cannot be firmly identified at present. (C) 2000 Elsevier Science B.V, All rights reserved.

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 developed a method which allows us to measure calibrated electroluminescence (EL) spectra from quantum well (QW) structures at moderate forward biases. This method has been successfully applied for the first time to asymmetric double QW (DQW) p-i-n diodes in the AlGaAs/GaAs system at temperatures between 200 and 295 K. We have measured the EL signal from both QW and a theoretical model has been used to extract the quasi-Fermi level separation (ΔφF) in the QWs and thus the n·p product of the excess electron and hole pairs. We have made several significant observations: (1) both ΔφF decrease relative to the applied bias (Vapp) with increasing temperature. (2) ΔφF is not the same in both wells in contradiction with the assumption of the detailed balance theory. (3) At lower temperatures the quantum well ΔφF show a dependence on the separation between the two QWs.

We report time-resolved studies of ballistic phonon absorption in the fractional quantum Hall regime at Landau level filling factors of ν= 1 3 , 2 5 and 1 2 . The technique used can resolve the interaction of the two-dimensional electron system with LA and TA phonons and has been used to measure the temperature variation of the heat capacity of a single layer of electrons at ν= 1 3 . The energy gaps at ν= 1 3 have also been measured and found to be in good agreement with theory. The roles of compressible and incompressible regions in the phonon absorption process are discussed. Angle resolved measurements at ν= 2 5 are also in good agreement with theory.

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.

Reflectance measurements under normal incidence of light and variable angle spectroscopic ellipsometry (SE) studies are applied for characterizing hexagonal GaN films grown by molecular beam epitaxy on both GaAs (111)B and 6H-SiC substrates. A comparison of the reflectance above the band gap with model calculations for a smooth film yields definitely the root mean square surface roughness. By analyzing the envelopes of the reflectance spectra, the influence of a buffer layer and/or a non-abrupt substrate/film interface is verified and qualitative conclusions about the interface properties are deduced which are emphasized by transmission electron microscopy studies. On this basis a suitable model for data fitting is established which contains only a small number of parameters to be adjusted. From the fit, the complex refractive index versus photon energy as well as the thicknesses of the active layer and the interlayers are obtained. A meticulous analysis of the SE data below the band gap shows that from those studies the extraordinary refractive index of hexagonal GaN can be determined for the first time.

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.

International audience; Electron transport in Si‐doped and unintentionally‐doped GaN films grown on sapphire by MOCVD and MBE has been analysed assuming that a parallel conducting channel, via an impurity band, is present. No dependence on growth method or dopant type was observed, but other trends were apparent: a) the activation energy for the impurity band fell with increased doping; b) the temperature of the minimum in the Hall carrier density versus temperature curves increased with doping, but did not depend strongly on the absolute value of mobility; c) the ratio of the mobility in the GaN conduction band to that in the impurity band also showed systematic behaviour, possibly arising from structure‐related scattering processes. An STM study of the surface characteristics of some of these samples suggests that potential variations associated with particular structural features may be important in influencing the electrical properties.

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.

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.

A highly sensitive method for characterizing heteroepitaxial group III nitride films on various substrates is developed on the basis of reflectivity studies below and just above the fundamental absorption edge. By analyzing the envelopes of the reflectivity spectrum, the influence of a buffer layer, and/or a nonabrupt substrate/film interface is verified, the refractive index of the active layer can be calculated independently of both the active layer thickness and the interface properties, and finally, qualitative conclusions about the interface properties are deduced. On this basis a suitable model for data fitting is established which contains only a small number of adjusted parameters. Moreover, the correlation between the refractive index and the thickness of the active layer is removed. From the fit, the energy dispersion of the refractive index and the thickness for both the active layer and the interlayers are obtained. The ability and high accuracy of the method is demonstrated by applying it to ...

A two-dimensional electron system (2DES) is excited with ballistic phonons. Using the 2DES as a thermometer we directly measure the specific heat of the electron system. At a Landau level filling factor $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/3$ we deduce a strongly reduced specific heat compared to $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$ which we can assign to a small but finite density of states in the fractional quantum Hall gap. Furthermore, at $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/3$ the phonons are shown to excite finite wave vector magnetorotons across a gap, in excellent agreement with theoretical predictions.

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.

A new optical method for the characterisation of substrate/film interface properties of GaN grown on GaAs is presented. It is based on reflectivity measurements over an extended spectral range. The influence of surface roughness is separated by a comparison of the experimental data with calculations for a smooth film above the band gap of GaN, while at lower energies, the deviations from the calculated one-layer behaviour (without interface) are strongly correlated to the effective refractive index of the interface nint and its thickness. It is shown that for GaAs substrates under certain MBE growth conditions, nint is mainly determined by the void fraction.

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.

Low-temperature (T=1.8, 4.2 K) luminescence of GaAs/Al0.3Ga0.7As double-coupled asymmetric quantum wells (DQW) is observed under variation of an electric field V dc applied along the normal to the DQW plane. The FWHM of the indirect exciton (IX) emission line was found to undergo, within a certain interval of V dc, a strong (up to a factor 3.5) narrowing, accompanied by anomalously large IX intensity fluctuations in time within the V dc region where a strong decrease (or increase) of the FWHM is observed to occur. The dependence of the FWHM on the pumping level I p also exhibits a substantial decrease of the FWHM within a certain I p interval. The results obtained are discussed in the frame of an assumption which relates the observed phenomena to the onset of a condensed state in the interacting ensemble of spatially indirect excitons in DQWs.

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.

Undoped hexagonal GaN films have been grown by molecular beam epitaxy on (111)B GaAs substrates. Both Ga flux and nitrogen plasma conditions were fixed, while the growth temperature was varied in the range from 650 up to 720C. Reflectivity measurements below and above the GaN band gap were employed in order to determine optical parameters characterizing the substrate/film interface properties. The refractive index of the interface layer was found to decrease with increasing growth temperature indicating the formation of a mixed medium in this region. This effect is related to the columnar growth of GaN and outdiffusion of substrate atoms via channels between the columns which promotes the formation of voids in the substrate/film interface. Low temperature initiated growth or nitridation of the substrate were observed to reduce the influence of elevated growth temperatures.

We report the results of a study of the absorption of non-equilibrium ballistic phonons by a two-dimensional electron system, in strong magnetic fields, by measuring its change in resistance. At the even denominator Landau level filling factors, m, of 1/2 and 3/2 the electron‐phonon interaction is observed to be cut-oA above a particular value of the component of the wavevector perpendicular to the low dimensional system. This cut-oA is consistent with that due to the finite thickness of the 2DES. At fractional quantum Hall eAect minima (1/3, 2/5, 2/3) we observe that the cut-oA to the interaction depends on the filling factor. At ma 1/3, the energy gap determined from these experiments is consistent with the theoretical prediction of the magnetoroton energy gap. ” 1998 Elsevier Science B.V. All rights reserved.

We have performed the first cyclotron resonance studies of high mobility two-dimensional hole systems (2DHS) under pressure. Experiments were carried out using millimetre wave radiation on GaAs-(Ga, Al)As heterojunctions. Pressures were applied using a clamp cell specially designed for millimetre wave measurements. Our data at ambient pressure show collectively coupled resonances arising from interactions between the two hole systems with different effective masses formed by the spin-split halves of the lowest heavy-hole subband. The application of pressure appears to completely suppress the collective effect and only a resonance corresponding to the lighter cyclotron mass from the heavy-hole spin-split subband is observed. (C) 1998 Elsevier Science B.V. All rights reserved.