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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.

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.

Currently there is a high level of interest in the development of ultraviolet (UV) light sources for solid state lighting, optical sensors, surface decontamination and water purification. III-V semiconductor UV LEDs are now successfully manufactured using the AlGaN material system; however, their efficiency is still low. The majority of UV LEDs require AlxGa1-xN layers with compositions in the mid-range between AlN and GaN. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1-xN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of AlxGa1-xN bulk crystals by any standard bulk growth techniques has not been developed so far. There are very strong electric polarization fields inside the wurtzite (hexagonal) group III-nitride structures. The charge separation within quantum wells leads to a significant reduction in the efficiency of optoelectronic device structures. Therefore, the growth of non-polar and semi-polar group III-nitride structures has been the subject of considerable interest recently. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende GaN bulk crystals by anystandard bulk growth techniques were not successful. Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used plasma-assisted molecular beam epitaxy (PA MBE) and have produced for the first time free-standing layers of zinc-blende GaN up to 100 μm in thickness and up to 3-inch in diameter. We have shown that our newly developed PA-MBE process for the growth of zinc-blende GaN layers can also be used to achieve free-standing wurtzite AlxGa1-xN wafers. Zinc-blende and wurtzite AlxGa1-xN polytypes can be grown on different orientations of GaAs substrates - (001) and (111)B respectively. We have subsequently removed the GaAs using a chemical etch in order to produce free-standing GaN and AlxGa1-xN wafers. At a thickness of ∼30 μm, free-standing GaN and AlxGa1-xN wafers can easily be handled without cracking. Therefore, free-standing GaN and AlxGa1-xN wafers with thicknesses in the 30–100 μm range may be used as substrates for further growth of GaN and AlxGa1 xN-based structures and devices. We have compared different RF nitrogen plasma sources for the growth of thick nitride AlxGa1-xN films including a standard HD25 source from Oxford Applied Research and a novel high efficiency source from Riber. We have investigated a wide range of the growth rates from 0.2 to 3 μm/h. The use of highly efficient nitrogen RF plasma sources makes PA-MBE a potentially viable commercial process, since free-standing films can be achieved in a single day. Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.

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.

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.

Two In x Ga1-xN nanorod samples with graded In compositions of x = 0.5-0 (Ga-rich) and x = 0.5-1 (In-rich) grown by molecular beam epitaxy were studied using transmission electron microscopy. The nanorods had a wurtzite crystal structure with growth along and core-shell structures with an In-rich core and Ga-rich shell. Energy-dispersive x-ray analysis confirmed grading over the entire compositional range and showed that the axial growth rate was primarily determined by the In flux, and the radial growth rate by the Ga flux. There was no evidence of misfit dislocations due to grading, but the strain due to the lattice mismatch between the In-rich core and Ga-rich shell was relaxed by edge dislocations at the core-shell interface with Burgers vectors and.

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.

Plasma-assisted molecular beam epitaxy (PA-MBE) is now widely used for the growth of group III-nitrides. Many years ago it became clear that during PA-MBE there is unintentional doping of GaN with boron (B) due to decomposition of the pyrolytic boron nitride (PBN) cavity of the RF plasma source. In this paper we discuss the unintentional B incorporation for PA-MBE growth of GaN and AlxGa1−xN using a highly efficient RF plasma source. We have studied a wide range of MBE growth conditions for GaN and AlxGa1−xN with growth rates from 0.2 to 3 µm/h, RF powers from 200 to 500 W, different nitrogen flow rates from 1 to 25 sccm and growth times up to several days. The chemical concentrations of B and matrix elements of Al, Ga, N were studied as a functions of depth using secondary ion mass spectrometry (SIMS). We demonstrate that boron incorporation with this highly efficient RF plasma source is approximately 1×1018 to 3×1018 cm−3 for the AlxGa1−xN growth rates of 2 – 3 µm/h.

Currently there is high level of interest in developing of vertical device structures based on the group III nitrides. We have studied n- and p-doping of free-standing zinc-blende GaN grown by plasma-assisted molecular beam epitaxy (PA-MBE). Si was used as the n-dopant and Mg as the p-dopant for zinc-blende GaN. Controllable levels of doping with Si and Mg in free-standing zinc-blende GaN have been achieved by PA-MBE. The Si and Mg doping depth uniformity through the zinc-blende GaN layers have been confirmed by secondary ion mass spectrometry (SIMS). Controllable Si and Mg doping makes PA-MBE a promising method for the growth of conducting group III-nitrides bulk crystals.

Optical properties of free standing cubic gallium nitride grown by molecular beam epitaxy system are investigated by a polarized infrared (IR) reflectance technique. A strong reststrahlen band, which reveals the bulk-like optical phonon frequencies, is observed. Meanwhile, continuous oscillation fringes, which indicate the sample consists of two homogeneous layers with different dielectric constants, are observed in the non-reststrahlen region. By obtaining the first derivative of polarized IR reflectance spectra measured at higher angles of incidence, extra phonon resonances are identified at the edges of the reststrahlen band. The observations are verified with the theoretical results simulated based on a multi-oscillator model.

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)

Molecular beam epitaxy has been used to grow continuous overlayers of In 0.5 Ga 0.5 N and InN on (1 1 1)Si, via growth of an intermediate nanorod arrays. Transmission electron microscopy showed that few threading dislocations are generated during nanorod growth, and are subsequently confined to twist grain boundaries 1–2 μm apart in the overlayer. In the In 0.5 Ga 0.5 N growth, the nanorods formed core–shell hexagonal wurtzite structures with In-poor surfaces, with lateral growth leading to grains with cubic zinc blende structure. High spatial resolution cathodoluminescence imaging showed that the In 0.5 Ga 0.5 N overlayers luminesced strongly, with evidence of compositional variations on the 1–2 μm scale. The reasons for compositional variations and the growth of cubic material are discussed.

In-situ Auger electron spectroscopy was used to determine the 1:1 flux ratios of Ga and N during growth of GaN by molecular beam epitaxy at low substrate temperatures. By linearly ramping the Ga-flux, while keeping the N-flux constant, and simultaneously measuring the chemical composition by monitoring N and Ga Auger peaks, the time of deviation from stoichiometry could be determined. The method was applied at very low substrate temperatures where reflection high-energy electron diffraction does not reveal clear growth mode changes. The importance of the N- vs Ga-rich conditions were confirmed with transmission electron microscopy which showed a distinct change in crystallinity between material at the top and bottom of the film, which are in agreement with previous findings.

The electronic structure of the conduction and valence bands of highly mismatched alloys (HMAs) such as GaN 1− x Sb x are well described by the band anticrossing model. The properties of this alloy, which has a large band gap range and controllable valence band positions, make it a candidate material for efficient solar energy conversion devices. We have examined the growth and structural properties of amorphous and crystalline GaN 1− x Sb x . These HMAs were grown by low temperature molecular beam epitaxy (MBE) under Ga-rich conditions. While there is a monotonic linear increase of Sb incorporation with Sb overpressure, there was no obvious dependence of Sb incorporation with growth temperature for the range of 10–470 °C. At growth temperatures lower than 100 °C, GaN 1− x Sb x HMAs lose crystallinity and become amorphous for Sb compositions at or exceeding ~5%. Ga-rich growth resulted in strong absorption at energies as low as 1 eV for GaN 1− x Sb x alloys of all compositions. The strong low energy absorption may arise from a Ga-related defect band.

We have studied a novel approach for the growth of GaN layers, namely plasma-assisted electroepitaxy (PAEE). We have designed and built a new growth chamber which allowed us to combine a plasma-assisted molecular beam epitaxy process with a liquid phase electroepitaxy. We have demonstrated that it is possible to grow continuous GaN layers by PAEE from a liquid Ga melt at growth temperatures as low as ∼650 °C, with low nitrogen overpressures of ∼3×10−5 Torr.

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)

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.

In this paper we describe detailed transmission electron microscopy studies of GaN1−xBix with 0.05 < x < 0.18 grown by low-temperature molecular beam epitaxy under Ga-rich conditions. Microstructural transformation from columnar growth separated by thin amorphous areas in the films with lowest Bi content (5%) to pseudo-amorphous structure with crystalline grains embedded in the amorphous matrix in the samples with higher Bi content (13% to 18%) was observed. In addition, metallic Bi segregation occurred in the samples with the highest Bi concentration. An abrupt decrease in absorption edge energy is found in samples with higher Bi content.

In this current paper we have studied 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 allowed 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 continuous GaN layers by PAEE from liquid Ga melt at growth temperatures as low as ∼650 °C, with low nitrogen overpressures of ∼3×10 −5 Torr.

There is a significant difference in the lattice parameters of GaN and AlN and for many device applications AlxGa1-xN substrates would be preferable to either GaN or AlN. We have studied the growth of free-standing zinc-blende and wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick (similar to 10 mu m) zinc-blende and wurtzite AlxGa1-xN films were grown by PA-MBE on 2-in. GaAs (0 0 1) and GaAs (1 1 1)B substrates respectively and were removed from the GaAs substrate after the growth. We demonstrate that free-standing zinc-blende and wurtzite AlxGa1-xN wafers can be achieved by PA-MBE for a wide range of Al compositions. (C) 2011 Elsevier B.V. All rights reserved.

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 GaN and AlxGa1-xN layers, structures and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for 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 mu m in thickness. Thick, undoped, cubic GaN films were grown on semi-insulating GaAs (0 0 1) substrates by a modified plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The resulting free-standing GaN wafers may be used as substrates for further epitaxy of cubic GaN-based structures and devices. We have demonstrated that the PA-MBE process, we had developed, also allows us to achieve free-standing zinc-blende AlxGa1-xN wafers.

We have studied the growth of wurtzite 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 2 in diameter wurtzite AlxGa1−xN layers up to 10 μm in thickness. Undoped wurtzite AlxGa1−xN films were grown on GaAs (1 1 1)B substrates by a plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The fact that free-standing ternary AlxGa1−xN wafers can be grown is very significant for the potential future production of wurtzite AlxGa1−xN substrates optimized for AlGaN-based device structures.

Using low temperature MBE, we have shown that it is possible to grow amorphous GaN1-xAsx layers with a variable As content (0 < x < 0.8) on both crystalline (sapphire and silicon) and amorphous (glass and Pyrex glass) substrates. Despite the fact that the samples with high As content are amorphous, we observe a gradual continuous decrease of bandgap from similar to 3.4 to similar to 0.8 eV with increase in As content. To the best of our knowledge this is the first demonstration of homogeneous amorphous GaN-based alloys over a wide composition range. The large band gap range of the amorphous phase of GaNAs covers much of the solar spectrum. The amorphous nature of the GaNAs alloys is particularly advantageous since low cost substrates such as glass and Pyrex glass can be used for solar cell fabrication.

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)

In the current study we have demonstrated the feasibility of a novel approach for the growth of the GaN layers namely plasma-assisted electroepitaxy (PAEE). In this method, we have tried to combine 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 crystallization on the surface or within the solution. We have designed and built a new growth chamber which allows us to combine the 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 ∼500 °C and with low nitrogen overpressures of ∼3×10−5 Torr. We have shown that the combination of the two processes, an N-plasma flux and a DC current, are vital for the successful growth of GaN layers by PAEE. The exact mechanisms, which lead to the growth of GaN by PAEE, still need to be investigated.

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.

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.

In this article, we propose a new complementary geometrical growth mechanism, which may partially explain some of the apparent anomalies in our understanding of the growth of GaN nanocolumns by plasma-assisted molecular beam epitaxy (PA-MBE). This geometrical addition to any complete model for nanocolumn growth is based on the fact that most samples are grown using substrate rotation and it predicts an enhanced growth rate in the plane normal to the surface, i.e. vertically compared with the lateral growth rate of the columns. It also suggests a mechanism for the enhanced diffusion of gallium on the sidewalls of the columns even under strongly nitrogen-rich conditions. Finally, geometrical considerations also predict the growth of non-(0 0 0 1) oriented samples from the same mechanism. Some experimental evidence supporting this complementary geometrical model is presented.

We have studied the low-temperature growth of gallium nitride arsenide (GaN)As layers on sapphire substrates by plasma-assisted molecular beam epitaxy. We have succeeded in achieving GaN1−xAsx alloys over a large composition range by growing the films much below the normal GaN growth temperatures with increasing the As2 flux as well as Ga:N flux ratio. We found that alloys with high As content x>0.1 are amorphous and those with x

Since c-GaN and ScN have near identical lattice parameters, we have studied the growth of ScN by molecular beam epitaxy, on different surfaces, as a possible buffer layer for improving the quality of c-GaN. Epitaxial growth of ScN on c-GaN(1 0 0), GaAs(1 0 0) and ScAs(1 0 0) surfaces has been demonstrated for the first time. ScN was found to grow in a (1 0 0) orientation on ScAs, whereas on c-GaN and GaAs it grew in a (1 1 1) orientation. Epitaxy of GaN on ScN(1 1 1) results in the hexagonal polytype, while GaN on ScN(1 0 0) produces the cubic polytype. AFM studies show smoother surfaces are attained for c-GaN grown on ScN(1 0 0), in comparison to conventional c-GaN growth on GaAs(1 0 0). This shows that ScN(1 0 0) layers offer the potential to improve the quality of c-GaN(1 0 0).

It has been demonstrated that it is possible to grow by molecular beam epitaxy bulk free-standing zinc-blende (cubic) GaN layers. Such layers have potential applications as lattice matched substrates for growth of non-polar cubic GaN device structures of improved performance. We present the data from characterisation measurements that confirm the cubic nature of the GaN crystals and show that the fraction of the hexagonal material is not more than about 10% in the best 50 μm thick free-standing GaN samples. Our research is aimed at increasing the size of the free-standing cubic GaN layers in order to make this technology suitable for the commercial production of substrates and we have now demonstrated the growth of 100 μm thick, 2-in diameter GaN substrates.

A series of 100-oriented ScN films was grown under N-rich conditions on 100-oriented Si using different Sc fluxes. The ScN films grew in an epitaxial cube-on-cube orientation, with [0 0 1] ScN //[0 0 1] Si and [1 0 0] ScN //[1 0 0] Si , despite the high (11%) lattice mismatch between ScN and Si. The film grain size increases and the film ω -FWHM decreases with increasing Sc flux, but the film roughness increases. Films grown under similar conditions on 111-oriented Si resulted in mixed 111 and 100 orientations, indicating that the 100 orientation is favoured both due to texture inheritance from the substrate and due to the growth conditions used.

We have used Raman scattering to investigate a series of undoped and As-doped GaN epilayers grown on sapphire substrates by plasma-assisted molecular beam epitaxy with different Ga-to-N ratios. We find that the frequency of the E2h phonon mode of GaN is very sensitive to the Ga-to-N ratio used to grow both types of samples, which may be attributed to strain effects arising from the different growth conditions. We observe Raman signatures corresponding to the formation of submicron cubic domains in some of the spectra of the As-doped samples. We discuss the origin of a broad band located at 232 cm−1 that appears in both the undoped and the As-doped epilayers. We speculate that this band, reported in previous Raman studies on As-doped and Bi-doped GaN, may be attributed to the formation of Ga clusters.

GaMnN layers have been grown on GaAs(0 0 1) substrates by plasma-assisted molecular beam epitaxy (PAMBE), under both N-rich and Ga-rich conditions, at temperatures from 680 °C down to 186 °C. Under N-rich conditions, a reduction in the growth temperature results in a transition from faulted single-crystalline zinc-blende layer growth to mixed-phase zinc-blende and wurtzite polycrystalline layer growth, whilst being also associated with increasing levels of Mn incorporation. Improved layer crystallinities were associated with PAMBE growth under Ga-rich conditions. In particular, a tilted, mixed-phase growth mode allowing for the deposition of comparably uniform GaMnN alloys, at a level of ~3.5 at% Mn, was identified for slightly Ga-rich conditions at a reduced temperature of ~265 °C. No correlation could be established between the GaMnN/GaAs (0 0 1) fine-scale defect structures and the layer functional properties, with all the free-standing layers showing p-type conductivities and comparable mobilities.

Optical measurements of substrate temperature during the growth by plasma-assisted molecular beam epitaxy of GaN on sapphire and GaAs (1 1 1)B were obtained using two commercial BandiT systems for visible and infrared wavelengths. With the visible system, an absorption edge at ∼400 nm was seen after 20–50 nm of GaN growth, and although this became clearer and more stable for the film on sapphire, strong interference oscillations precluded further measurements on the GaAs substrate. Using the infrared version, we observed a real initial increase in temperature for GaN films grown on GaAs substrates followed by a subsequent decrease to a steady-state value.

Zinc-blende Ga1−xMnxN epilayers grown by plasma assisted molecular beam epitaxy as a function of Mn flux have been assessed using a variety of structural characterisation techniques. Increasing Mn flux is associated with the build up of a Mn surfactant layer during the early states of growth and a transition from zinc-blende single phase growth to zincblende/ wurtzite mixed phase growth.

Zinc-blende Ga 1− x Mn x N epilayers grown by plasma-assisted molecular beam epitaxy as a function of Ga flux are assessed using a variety of structural characterisation techniques. The Ga:N ratio was found to have a dominant impact on the zinc-blende Ga 1− x Mn x N epilayer growth rate and resultant composition, morphology and microstructure. A maximum growth rate and an improved microstructure are associated with growth under slightly Ga-rich conditions. A reduced growth rate and enhanced Mn incorporation are associated with growth under slightly N-rich conditions. α-MnAs inclusions and voids extending into the GaAs buffer layer were observed in all cases, however they are considered not to affect the layer electrical and magnetic properties.

Using Mn as a dopant, we developed a new method for reproducible p-type doping of cubic GaN in plasma-assisted molecular beam epitaxy (PA-MBE). Hole densities >10 18 cm −3 and hole mobilities >300 cm 2 /V s have been achieved in our cubic GaMnN films. The Ga:N flux ratio during growth has a strong influence on the electrical properties of the films. PA-MBE growth of cubic GaMnN shows great potential for electrical and spintronic devices.

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.

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)

Cubic (zinc-blende) Ga1-xMnxN layers were grown by plasma-assisted molecular beam epitaxy on GaAs (001) substrates. Some of the structures also contained cubic AlN buffer layers. Auger Electron Spectroscopy and Secondary Ion Mass Spectroscopy studies clearly confirmed the incorporation of Mn into cubic GaN, the Mn being uniformly distributed through the layer. X-ray diffraction studies demonstrated that the Mn-doped GaN films are cubic and do not show phase separation up to a Mn concentrations of x, 7 pages, 3 figures

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 the results of the investigation of the photoluminescence (PL) from GaAs nanocrystallites embedded in a GaN host crystal and formed during the growth of GaN(As) by molecular beam epitaxy. The low-temperature PL spectra reveal two emission bands with maxima at 1.20 and 1.43 eV. The emission intensities of both PL bands show a characteristic 2/3 power-law dependence upon the optical excitation intensity, suggesting a strong contribution of Auger recombination in the total recombination rate of non-equilibrium carriers inside the GaAs nanocrystallites. The integral PL intensity demonstrates a sharp maximum in the growth temperature dependence at ~780 °C. This can be explained by the competition of several temperature-dependent processes, which influence the formation of the GaAs nanocrystallites.

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 (MBE). A wide range of bismuth fluxes have been used, the highest Bi flux was larger than the flux of Ga. We have demonstrated that using a Bi flux during the growth of GaN by MBE at a temperature ∼800°C improves the surface morphology of the films and decreases the deep emission. We have demonstrated for the first time the growth of GaN 1− x Bi x alloys by MBE, the Bi concentration was small and increased with decreasing growth temperature. We have studied the influence of an additional bismuth flux on the growth of As-doped GaN layers and observed an increase of blue emission from the layers at some optimum range of Bi fluxes. All the results allow us to conclude that Bi may be a potential new dopant for the growth of GaN by MBE.

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.

We have studied 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 plasma-assisted molecular beam epitaxy. We have demonstrated 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. We present a model based on thermodynamic considerations, which can explain how this might occur.