• W. L. Sarney, S. P. Svensson, M. Ting, N. Segercrantz, W. Walukiewicz, K. M. Yu, R. W. Martin, S. V. Novikov, and T. C. T. Foxon, “Intermixing studies in GaN₁₋ₓSbₓ highly mismatched alloys,” Applied Optics, vol. 56, iss. 3, p. B64–B69, 2016.
    [BibTeX] [Abstract] [Download PDF]

    GaN1?xSbx with x{\texttt{\char126}} 5-7\% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photo-electrochemical cell for solar water splitting. In this study, we grew GaN1?xSbx under conditions intended to induce phase segregation. Prior experiments with the similar alloy GaN1?xAsx, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb, all suggested that GaN1?xSbx alloys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case, and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions, and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.

    @Article{strathprints58170,
    author = {Wendy L. Sarney and Stefan P. Svensson and Min Ting and Natalie Segercrantz and Wladek Walukiewicz and Kin Man Yu and Robert W. Martin and Sergei V. Novikov and C.T. Thomas Foxon},
    title = {Intermixing studies in {GaN₁₋ₓSbₓ} highly mismatched alloys},
    journal = {Applied Optics},
    year = {2016},
    volume = {56},
    number = {3},
    pages = {B64--B69},
    month = {September},
    note = {{\copyright} 2016 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.},
    abstract = {GaN1?xSbx with x{\texttt{\char126}} 5-7\% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photo-electrochemical cell for solar water splitting. In this study, we grew GaN1?xSbx under conditions intended to induce phase segregation. Prior experiments with the similar alloy GaN1?xAsx, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb, all suggested that GaN1?xSbx alloys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case, and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions, and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.},
    keywords = {semiconductor materials, optoelectronics, thin films, Optics. Light, Atomic and Molecular Physics, and Optics},
    url = {http://strathprints.strath.ac.uk/58170/}
    }

  • K. M. Yu, W. L. Sarney, S. V. Novikov, N. Segercrantz, M. Ting, M. Shaw, S. P. Svensson, R. W. Martin, W. Walukiewicz, and C. T. Foxon, “Highly mismatched GaN₁₋ₓSbₓ alloys : synthesis, structure and electronic properties,” Semiconductor Science and Technology, vol. 31, iss. 8, p. 83001, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Highly mismatched alloys (HMAs) is a class of semiconductor alloys whose constituents are distinctly different in terms of size, ionicity and/or electronegativity. Electronic properties of the alloys deviate significantly from an interpolation scheme based on small deviations from the virtual crystal approximation. Most of the HMAs were only studied in a dilute composition limit. Recent advances in understanding of the semiconductor synthesis processes allowed growth of thin films of HMAs under non-equilibrium conditions. Thus reducing the growth temperature allowed synthesis of group III-N-V HMAs over almost the entire composition range. This paper focuses on the GaNxSb1-x HMA which has been suggested as a potential material for solar water dissociation devices. Here we review our recent work on the synthesis, structural and optical characterization of GaN1-xSbx HMA. Theoretical modeling studies on its electronic structure based on the band anticrossing (BAC) model are also reviewed. In particular we discuss the effects of growth temperature, Ga flux and Sb flux on the incorporation of Sb, film microstructure and optical properties of the alloys. Results obtained from two separate MBE growths are directly compared. Our work demonstrates that a large range of direct bandgap energies from 3.4 eV to below 1.0 eV can be achieved for this alloy grown at low temperature. We show that the electronic band structure of GaN1-xSbx HMA over the entire composition range is well described by a modified the BAC model which includes the dependence of the host matrix band edges as well as the BAC model coupling parameters on composition. We emphasize that the modified BAC model of the electronic band structure developed for the full composition of GaNxSb1-x is general and is applicable to any HMA.

    @Article{strathprints56601,
    author = {K M Yu and W L Sarney and S V Novikov and N Segercrantz and M Ting and M Shaw and S P Svensson and R W Martin and W Walukiewicz and C T Foxon},
    title = {Highly mismatched {GaN₁₋ₓSbₓ} alloys : synthesis, structure and electronic properties},
    journal = {Semiconductor Science and Technology},
    year = {2016},
    volume = {31},
    number = {8},
    pages = {083001},
    month = {June},
    abstract = {Highly mismatched alloys (HMAs) is a class of semiconductor alloys whose constituents are distinctly different in terms of size, ionicity and/or electronegativity. Electronic properties of the alloys deviate significantly from an interpolation scheme based on small deviations from the virtual crystal approximation. Most of the HMAs were only studied in a dilute composition limit. Recent advances in understanding of the semiconductor synthesis processes allowed growth of thin films of HMAs under non-equilibrium conditions. Thus reducing the growth temperature allowed synthesis of group III-N-V HMAs over almost the entire composition range. This paper focuses on the GaNxSb1-x HMA which has been suggested as a potential material for solar water dissociation devices. Here we review our recent work on the synthesis, structural and optical characterization of GaN1-xSbx HMA. Theoretical modeling studies on its electronic structure based on the band anticrossing (BAC) model are also reviewed. In particular we discuss the effects of growth temperature, Ga flux and Sb flux on the incorporation of Sb, film microstructure and optical properties of the alloys. Results obtained from two separate MBE growths are directly compared. Our work demonstrates that a large range of direct bandgap energies from 3.4 eV to below 1.0 eV can be achieved for this alloy grown at low temperature. We show that the electronic band structure of GaN1-xSbx HMA over the entire composition range is well described by a modified the BAC model which includes the dependence of the host matrix band edges as well as the BAC model coupling parameters on composition. We emphasize that the modified BAC model of the electronic band structure developed for the full composition of GaNxSb1-x is general and is applicable to any HMA.},
    keywords = {highly mismatched alloys (HMAs), semiconductor alloys, semiconductor synthesis, band anticrossing , Solid state physics. Nanoscience, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/56601/}
    }

  • S. V. Novikov, C. R. Staddon, R. Martin, A. J. Kent, and T. C. Foxon, “Molecular beam epitaxy of free-standing wurtzite AlₓGa₁₋ₓN layers,” Journal of Crystal Growth, vol. 425, p. 125–128, 2015.
    [BibTeX] [Abstract] [Download PDF]

    Recent developments with group III nitrides present AlxGa1xN based LEDs as realistic devices for new alternative deep ultra-violet light sources. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick wurtzite AlxGa1xN films were grown by PA-MBE on 2-in. GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1xN samples. X-ray microanalysis measurements confirm that the AlN fraction is uniform across the wafer and mass spectroscopy measurements show that the composition is also uniform in depth. We have demonstrated that free-standing wurtzite AlxGa1xN wafers can be achieved by PA-MBE for a wide range of AlN fractions. In order to develop a commercially viable process for the growth of wurtzite AlxGa1xN substrates, we have used a novel Riber plasma source and have demonstrated growth rates of GaN up to 1.8 mm/h on 2-in. diameter GaAs and sapphire wafers

    @Article{strathprints56186,
    author = {Sergei V. Novikov and C.R. Staddon and Robert Martin and A.J. Kent and C. Thomas Foxon},
    title = {Molecular beam epitaxy of free-standing wurtzite {AlₓGa₁₋ₓN} layers},
    journal = {Journal of Crystal Growth},
    year = {2015},
    volume = {425},
    pages = {125--128},
    month = {February},
    abstract = {Recent developments with group III nitrides present AlxGa1xN based LEDs as realistic devices for new alternative deep ultra-violet light sources. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick wurtzite AlxGa1xN films were grown by PA-MBE on 2-in. GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1xN samples. X-ray microanalysis measurements confirm that the AlN fraction is uniform across the wafer and mass spectroscopy measurements show that the composition is also uniform in depth. We have demonstrated that free-standing wurtzite AlxGa1xN wafers can be achieved by PA-MBE for a wide range of AlN fractions. In order to develop a commercially viable process for the growth of wurtzite AlxGa1xN substrates, we have used a novel Riber plasma source and have demonstrated growth rates of GaN up to 1.8 mm/h on 2-in. diameter GaAs and sapphire wafers},
    keywords = {substrates, molecular beam epitaxy, nitrides, semiconducting III-V materials, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/56186/}
    }

  • S. V. Novikov, M. Ting, K. M. Yu, W. L. Sarney, R. W. Martin, S. P. Svensson, W. Walukiewicz, and C. T. Foxon, “Tellurium n-type doping of highly mismatched amorphous GaN₁₋ₓAsₓ alloys in plasma-assisted molecular beam epitaxy,” Journal of Crystal Growth, vol. 404, p. 9–13, 2014.
    [BibTeX] [Abstract] [Download PDF]

    In this paper we report our study on n-type Te doping of amorphous GaNi1-xAsx layers grown by plasma assisted molecular beam epitaxy. We have used a low temperature PbTe source as a source of tellurium. Reproducible and uniform tellurium incorporation in amorphous GaNi1-xAsx layers has been successfully achieved with a maximum Te concentration of 9 x 10(20) cm(-3). Tellurium incorporation resulted in n-doping of GaN1-xAsx layers with Hall carrier concentrations up to 3 x 10(19) cm(-3) and mobilities of similar to 1 cm(2)/V s. The optimal growth temperature window for efficient Te doping of the amorphous GaNi1-xAsx layers has been determined.

    @Article{strathprints51167,
    author = {S. V. Novikov and M. Ting and K.M. Yu and W.L. Sarney and R.W. Martin and S.P. Svensson and W. Walukiewicz and C.T. Foxon},
    title = {Tellurium n-type doping of highly mismatched amorphous {GaN₁₋ₓAsₓ} alloys in plasma-assisted molecular beam epitaxy},
    journal = {Journal of Crystal Growth},
    year = {2014},
    volume = {404},
    pages = {9--13},
    month = {October},
    abstract = {In this paper we report our study on n-type Te doping of amorphous GaNi1-xAsx layers grown by plasma assisted molecular beam epitaxy. We have used a low temperature PbTe source as a source of tellurium. Reproducible and uniform tellurium incorporation in amorphous GaNi1-xAsx layers has been successfully achieved with a maximum Te concentration of 9 x 10(20) cm(-3). Tellurium incorporation resulted in n-doping of GaN1-xAsx layers with Hall carrier concentrations up to 3 x 10(19) cm(-3) and mobilities of similar to 1 cm(2)/V s. The optimal growth temperature window for efficient Te doping of the amorphous GaNi1-xAsx layers has been determined.},
    keywords = {GaNAs, GaN, tellurium, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/51167/}
    }

  • M. Shaw, K. M. Yu, M. Ting, R. E. L. Powell, W. L. Sarney, S. P. Svensson, A. J. Kent, W. Walukiewicz, C. T. Foxon, S. V. Novikov, and R. W. Martin, “Composition and optical properties of dilute-Sb GaN₁₋ₓSbₓ highly mismatched alloys grown by MBE,” Journal of Physics D: Applied Physics, vol. 47, iss. 46, p. 465102, 2014.
    [BibTeX] [Abstract] [Download PDF]

    In this work the compositional and optical characterization of three series of dilute-Sb GaN1 ? xSbx alloys grown with various Sb flux, under N and Ga-rich conditions, are presented. Using wavelength dispersive x-ray microanalysis and Rutherford backscattering spectroscopy it is found that the N-rich samples (Ga flux {\ensuremath{<}} 2.3 {$\times$} 10?7 Torr) incorporate a higher magnitude of GaSb than the Ga-rich samples (Ga flux {\ensuremath{>}} 2.3 {$\times$} 10?7 Torr) under the same growth conditions. The optical properties of the Ga-rich samples are measured using room temperature cathodoluminescence (CL), photoluminescence (PL) and absorption measurements. A broad luminescence peak is observed around 2.2 eV. The nature and properties of this peak are considered, as is the suitability of these dilute-Sb alloys for use in solar energy conversion devices.

    @Article{strathprints50168,
    author = {Martin Shaw and K.M. Yu and M. Ting and R. E. L. Powell and W. L. Sarney and S. P. Svensson and A. J. Kent and W. Walukiewicz and C. T. Foxon and S. V. Novikov and Robert W. Martin},
    title = {Composition and optical properties of dilute-{Sb} {GaN₁₋ₓSbₓ} highly mismatched alloys grown by {MBE}},
    journal = {Journal of Physics D: Applied Physics},
    year = {2014},
    volume = {47},
    number = {46},
    pages = {465102},
    month = {October},
    abstract = {In this work the compositional and optical characterization of three series of dilute-Sb GaN1 ? xSbx alloys grown with various Sb flux, under N and Ga-rich conditions, are presented. Using wavelength dispersive x-ray microanalysis and Rutherford backscattering spectroscopy it is found that the N-rich samples (Ga flux {\ensuremath{<}} 2.3 {$\times$} 10?7 Torr) incorporate a higher magnitude of GaSb than the Ga-rich samples (Ga flux {\ensuremath{>}} 2.3 {$\times$} 10?7 Torr) under the same growth conditions. The optical properties of the Ga-rich samples are measured using room temperature cathodoluminescence (CL), photoluminescence (PL) and absorption measurements. A broad luminescence peak is observed around 2.2 eV. The nature and properties of this peak are considered, as is the suitability of these dilute-Sb alloys for use in solar energy conversion devices.},
    keywords = {dilute-Sb GaN1 ? xSbx alloys, solar energy conversion devices, Highly Mismatched Alloys (HMAs), semiconductor alloys, Optics. Light, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials},
    url = {http://strathprints.strath.ac.uk/50168/}
    }

  • K. M. Yu, S. V. Novikov, M. Ting, W. L. Sarney, S. P. Svensson, M. Shaw, R. W. Martin, W. Walukiewicz, and C. T. Foxon, “Growth and characterization of highly mismatched GaN₁₋ₓSbₓ alloys,” Journal of Applied Physics, vol. 116, iss. 12, p. 123704, 2014.
    [BibTeX] [Abstract] [Download PDF]

    A systematic investigation on the effects of growth temperature, Ga flux, and Sb flux on the incorporation of Sb, film structure, and optical properties of the GaN1-xSbx highly mismatched alloys (HMAs) was carried out. We found that the direct bandgap ranging from 3.4 eV to below 1.0 eV for the alloys grown at low temperature. At the growth temperature of 80 degrees C, GaN1-xSbx with x{\ensuremath{>}}6\% losses crystallinity and becomes primarily amorphous with small crystallites of 2-5 nm. Despite the range of microstructures found for GaN1-xSbx alloys with different composition, a well-developed absorption edge shifts from 3.4 eV (GaN) to close to 2 eV for samples with a small amount, less than 10\% of Sb. Luminescence from dilute GaN1-xSbx alloys grown at high temperature and the bandgap energy for alloys with higher Sb content are consistent with a localized substitutional Sb level E-Sb at similar to 1.1 eV above the valence band of GaN. The decrease in the bandgap of GaN1-xSbx HMAs is consistent with the formation of a Sb-derived band due to the anticrossing interaction of the Sb states with the valence band of GaN.

    @Article{strathprints51371,
    author = {K.M. Yu and S. V. Novikov and Min Ting and W.L. Sarney and S.P. Svensson and M. Shaw and R.W. Martin and W. Walukiewicz and C.T. Foxon},
    title = {Growth and characterization of highly mismatched {GaN₁₋ₓSbₓ} alloys},
    journal = {Journal of Applied Physics},
    year = {2014},
    volume = {116},
    number = {12},
    pages = {123704},
    month = {September},
    note = {{\copyright} 2015 AIP Publishing},
    abstract = {A systematic investigation on the effects of growth temperature, Ga flux, and Sb flux on the incorporation of Sb, film structure, and optical properties of the GaN1-xSbx highly mismatched alloys (HMAs) was carried out. We found that the direct bandgap ranging from 3.4 eV to below 1.0 eV for the alloys grown at low temperature. At the growth temperature of 80 degrees C, GaN1-xSbx with x{\ensuremath{>}}6\% losses crystallinity and becomes primarily amorphous with small crystallites of 2-5 nm. Despite the range of microstructures found for GaN1-xSbx alloys with different composition, a well-developed absorption edge shifts from 3.4 eV (GaN) to close to 2 eV for samples with a small amount, less than 10\% of Sb. Luminescence from dilute GaN1-xSbx alloys grown at high temperature and the bandgap energy for alloys with higher Sb content are consistent with a localized substitutional Sb level E-Sb at similar to 1.1 eV above the valence band of GaN. The decrease in the bandgap of GaN1-xSbx HMAs is consistent with the formation of a Sb-derived band due to the anticrossing interaction of the Sb states with the valence band of GaN.},
    keywords = {GaN, GaSb, HMAs, film structures, growth temperature, Ga flux, Sb flux, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/51371/}
    }

  • S. V. Novikov, K. M. Yu, A. Levander, D. Detert, W. L. Sarney, Z. Liliental-Weber, M. Shaw, R. W. Martin, S. P. Svensson, W. Walukiewicz, and C. T. Foxon, “Molecular beam epitaxy of highly mismatched N-rich GaNSb and InNAs alloys,” Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures, vol. 31, iss. 3, p. 03C102, 2013.
    [BibTeX] [Abstract] [Download PDF]

    GaN materials alloyed with group V anions form the so-called highly mismatched alloys (HMAs). Recently, the authors succeeded in growing N-rich GaNAs and GaNBi alloys over a large composition range by plasma-assisted molecular beam epitaxy (PA-MBE). Here, they present first results on PA-MBE growth and properties of N-rich GaNSb and InNAs alloys and compare these with GaNAs and GaNBi alloys. The enhanced incorporation of As and Sb was achieved by growing the layers at extremely low growth temperatures. Although layers become amorphous for high As, Sb, and Bi content, optical absorption measurements show a progressive shift of the optical absorption edge to lower energy. The large band gap range and controllable conduction and valence band positions of these HMAs make them promising materials for efficient solar energy conversion devices.

    @Article{strathprints47230,
    author = {S.V. Novikov and K.M. Yu and A. Levander and D. Detert and W.L. Sarney and Z. Liliental-Weber and M. Shaw and R.W. Martin and S.P. Svensson and W. Walukiewicz and C.T. Foxon},
    title = {Molecular beam epitaxy of highly mismatched N-rich GaNSb and InNAs alloys},
    journal = {Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures},
    year = {2013},
    volume = {31},
    number = {3},
    pages = {03C102},
    month = {May},
    abstract = {GaN materials alloyed with group V anions form the so-called highly mismatched alloys (HMAs). Recently, the authors succeeded in growing N-rich GaNAs and GaNBi alloys over a large composition range by plasma-assisted molecular beam epitaxy (PA-MBE). Here, they present first results on PA-MBE growth and properties of N-rich GaNSb and InNAs alloys and compare these with GaNAs and GaNBi alloys. The enhanced incorporation of As and Sb was achieved by growing the layers at extremely low growth temperatures. Although layers become amorphous for high As, Sb, and Bi content, optical absorption measurements show a progressive shift of the optical absorption edge to lower energy. The large band gap range and controllable conduction and valence band positions of these HMAs make them promising materials for efficient solar energy conversion devices.},
    keywords = {molecular beam epitaxy, mismatched, N-rich GaNSb, InN1xAsx , alloys, Electrical engineering. Electronics Nuclear engineering, Physics, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/47230/}
    }

  • S. V. Novikov, C. R. Staddon, F. Luckert, P. R. Edwards, R. W. Martin, A. J. Kent, and C. T. Foxon, “Zinc-blende and wurtzite AlₓGa₁₋ₓN bulk crystals grown by molecular beam epitaxy,” Journal of Crystal Growth, vol. 350, iss. 1, p. 80–84, 2012.
    [BibTeX] [Abstract] [Download PDF]

    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.

    @Article{strathprints40875,
    author = {S. V. Novikov and C. R. Staddon and F. Luckert and P. R. Edwards and R. W. Martin and A. J. Kent and C. T. Foxon},
    title = {Zinc-blende and wurtzite {AlₓGa₁₋ₓN} bulk crystals grown by molecular beam epitaxy},
    journal = {Journal of Crystal Growth},
    year = {2012},
    volume = {350},
    number = {1},
    pages = {80--84},
    month = {July},
    abstract = {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.},
    keywords = {zinc-blende , AlxGa1-xN, wurtzite , bulk crystals , molecular beam epitaxy , substrates , semiconducting III-V materials, nitrides, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/40875/}
    }

  • S. V. Novikov, K. M. Yu, A. X. Levander, Z. Liliental-Weber, R. dos Reis, A. J. Kent, A. Tseng, O. D. Dubon, J. Wu, J. Denlinger, W. Walukiewicz, F. Luckert, P. Edwards, R. Martin, and C. T. Foxon, “Molecular beam epitaxy of GaN₁₋ₓBiₓ alloys with high bismuth content,” Physica Status Solidi A: Applications and Materials Science, vol. 209, iss. 3, p. 419–423, 2012.
    [BibTeX] [Abstract] [Download PDF]

    We have analysed bismuth incorporation into GaN layers using plasma-assisted molecular beam epitaxy (PA-MBE) at extremely low growth temperatures of less than {$\sim$}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 {$\sim$}1 eV.

    @Article{strathprints39519,
    author = {S.V. Novikov and K.M. Yu and A.X. Levander and Z. Liliental-Weber and R. dos Reis and A.J. Kent and A. Tseng and O.D. Dubon and J. Wu and J. Denlinger and W. Walukiewicz and Franziska Luckert and Paul Edwards and Robert Martin and C.T. Foxon},
    title = {Molecular beam epitaxy of GaN₁₋ₓBiₓ alloys with high bismuth content},
    journal = {Physica Status Solidi A: Applications and Materials Science},
    year = {2012},
    volume = {209},
    number = {3},
    pages = {419--423},
    month = {January},
    abstract = {We have analysed bismuth incorporation into GaN layers using plasma-assisted molecular beam epitaxy (PA-MBE) at extremely low growth temperatures of less than {$\sim$}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 {$\sim$}1 eV.},
    keywords = {semiconducting III?V materials, molecular beam epitaxy, nitrides, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/39519/}
    }

  • R. E. L. Powell, S. V. Novikov, F. Luckert, P. R. Edwards, A. V. Akimov, C. T. Foxon, R. W. Martin, and A. J. Kent, “Carrier localization and related photoluminescence in cubic AlGaN epilayers,” Journal of Applied Physics, vol. 110, iss. 6, p. 63517, 2011.
    [BibTeX] [Abstract] [Download PDF]

    The steady state and time-resolved photoluminescence (PL) spectra of cubic AlxGa1-xN have been measured for 0 {\ensuremath{<}} x {\ensuremath{<}} 1. The intensity of the room temperature PL increases by an order of magnitude when the AlN content increases from x = 0 to x = 0.95. Additionally, the PL decay slows down with the decrease of temperature and increase of x. These results show that strong localization of carriers on alloy composition fluctuations plays a large role in determining the intensity and temporal evolution of the PL. The activation energy for the localized carriers increases with the increase of x and reaches the value of 55 meV at x = 0.95.

    @Article{strathprints44735,
    author = {R. E. L. Powell and S. V. Novikov and F. Luckert and P. R. Edwards and A. V. Akimov and C. T. Foxon and R. W. Martin and A. J. Kent},
    title = {Carrier localization and related photoluminescence in cubic AlGaN epilayers},
    journal = {Journal of Applied Physics},
    year = {2011},
    volume = {110},
    number = {6},
    pages = {063517},
    month = {September},
    abstract = {The steady state and time-resolved photoluminescence (PL) spectra of cubic AlxGa1-xN have been measured for 0 {\ensuremath{<}} x {\ensuremath{<}} 1. The intensity of the room temperature PL increases by an order of magnitude when the AlN content increases from x = 0 to x = 0.95. Additionally, the PL decay slows down with the decrease of temperature and increase of x. These results show that strong localization of carriers on alloy composition fluctuations plays a large role in determining the intensity and temporal evolution of the PL. The activation energy for the localized carriers increases with the increase of x and reaches the value of 55 meV at x = 0.95.},
    keywords = {solid-solutions, alloys, epilayers, spectra, Physics, Atomic and Molecular Physics, and Optics},
    url = {http://strathprints.strath.ac.uk/44735/}
    }

  • S. V. Novikov, C. R. Staddon, C. T. Foxon, K. M. Yu, R. Broesler, M. Hawkridge, Z. Liliental-Weber, J. Denlinger, I. Demchenko, F. Luckert, P. Edwards, R. Martin, and W. Walukiewicz, "Growth by molecular beam epitaxy of amorphous and crystalline GaNAs alloys with band gaps from 3.4 to 0.8 eV for solar energy conversion devices," Journal of Crystal Growth, vol. 323, iss. 1, p. 60–63, 2011.
    [BibTeX] [Abstract] [Download PDF]

    Using low temperature MBE, we have shown that it is possible to grow amorphous GaN1-xAsx layers with a variable As content (0 {\ensuremath{<}} x {\ensuremath{<}} 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.

    @article{strathprints32272,
    volume = {323},
    number = {1},
    month = {May},
    author = {S.V. Novikov and C.R. Staddon and C.T. Foxon and K.M. Yu and R. Broesler and M. Hawkridge and Z. Liliental-Weber and J. Denlinger and I. Demchenko and Franziska Luckert and Paul Edwards and Robert Martin and W. Walukiewicz},
    title = {Growth by molecular beam epitaxy of amorphous and crystalline GaNAs alloys with band gaps from 3.4 to 0.8 eV for solar energy conversion devices},
    journal = {Journal of Crystal Growth},
    pages = {60--63},
    year = {2011},
    keywords = {crystals, molecular beam epitaxy, amorphous, crystalline, GaNAs alloys , solar energy conversion devices, band gaps , Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/32272/},
    abstract = {Using low temperature MBE, we have shown that it is possible to grow amorphous GaN1-xAsx layers with a variable As content (0 {\ensuremath{<}} x {\ensuremath{<}} 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.}
    }

  • S. V. Novikov, C. R. Staddon, C. T. Foxon, F. Luckert, P. Edwards, R. Martin, and A. J. Kent, "Molecular beam epitaxy as a method for the growth of free-standing bulk zinc-blende GaN and AlGaN crystals," Journal of Crystal Growth, vol. 323, iss. 1, p. 80–83, 2011.
    [BibTeX] [Abstract] [Download PDF]

    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.

    @article{strathprints32270,
    volume = {323},
    number = {1},
    month = {May},
    author = {S.V. Novikov and C.R. Staddon and C.T. Foxon and Franziska Luckert and Paul Edwards and Robert Martin and A.J. Kent},
    title = {Molecular beam epitaxy as a method for the growth of free-standing bulk zinc-blende GaN and AlGaN crystals},
    journal = {Journal of Crystal Growth},
    pages = {80--83},
    year = {2011},
    keywords = {substrates, molecular beam epitaxy, nitrides, semiconducting III?V materials, free-standing bulk , zinc-blende GaN , AlGaN crystals, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/32270/},
    abstract = {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.}
    }

  • S. V. Novikov, C. R. Staddon, R. E. L. Powell, A. V. Akimov, F. Luckert, P. R. Edwards, R. W. Martin, A. J. Kent, and C. T. Foxon, "Wurtzite AlₓGa₁₋ₓN bulk crystals grown by molecular beam epitaxy," Journal of Crystal Growth, vol. 322, iss. 1, p. 23–26, 2011.
    [BibTeX] [Abstract] [Download PDF]

    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 [mu]m 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.

    @Article{strathprints30548,
    author = {S.V. Novikov and C.R. Staddon and R.E.L. Powell and A.V. Akimov and F. Luckert and P.R. Edwards and R.W. Martin and A.J. Kent and C.T. Foxon},
    title = {Wurtzite {AlₓGa₁₋ₓN} bulk crystals grown by molecular beam epitaxy},
    journal = {Journal of Crystal Growth},
    year = {2011},
    volume = {322},
    number = {1},
    pages = {23--26},
    month = {May},
    abstract = {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 [mu]m 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.},
    keywords = {semiconducting III-V materials, substrates, molecular beam epitaxy, nitrides, wurtzite, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/30548/}
    }

  • K. M. Yu, S. V. Novikov, R. Broesler, I. N. Demchenko, F. Luckert, R. W. Martin, J. D. Denlinger, Z. Liliental-Weber, W. Walukiewicz, and C. T. Foxon, "Highly mismatched crystalline and amorphous GaN₁₋ₓAsₓ alloys in the whole composition range," Journal of Applied Physics, vol. 106, iss. 10, p. 103709, 2009.
    [BibTeX] [Abstract] [Download PDF]

    Alloying is a commonly accepted method to tailor properties of semiconductor materials for specific applications. Only a limited number of semiconductor alloys can be easily synthesized in the full composition range. Such alloys are, in general, formed of component elements that are well matched in terms of ionicity, atom size, and electronegativity. In contrast there is a broad class of potential semiconductor alloys formed of component materials with distinctly different properties. In most instances these mismatched alloys are immiscible under standard growth conditions. Here we report on the properties of GaN1?xAsx, a highly mismatched, immiscible alloy system that was successfully synthesized in the whole composition range using a nonequilibrium low temperature molecular beam epitaxy technique. The alloys are amorphous in the composition range of 0.17{\ensuremath{<}}x{\ensuremath{<}}0.75 and crystalline outside this region. The amorphous films have smooth morphology, homogeneous composition, and sharp, well defined optical absorption edges. The band gap energy varies in a broad energy range from {$\sim$} 3.4 eV in GaN to {$\sim$} 0.8 eV at x {$\sim$} 0.85. The reduction in the band gap can be attributed primarily to the downward movement of the conduction band for alloys with x{\ensuremath{>}}0.2, and to the upward movement of the valence band for alloys with x{\ensuremath{<}}0.2. The unique features of the band structure offer an opportunity of using GaN1?xAsx alloys for various types of solar power conversion devices.

    @Article{strathprints26799,
    author = {K.M. Yu and S.V. Novikov and R. Broesler and I.N. Demchenko and F. Luckert and R.W. Martin and J.D. Denlinger and Z. Liliental-Weber and W. Walukiewicz and C.T. Foxon},
    title = {Highly mismatched crystalline and amorphous GaN₁₋ₓAsₓ alloys in the whole composition range},
    journal = {Journal of Applied Physics},
    year = {2009},
    volume = {106},
    number = {10},
    pages = {103709},
    month = {November},
    abstract = {Alloying is a commonly accepted method to tailor properties of semiconductor materials for specific applications. Only a limited number of semiconductor alloys can be easily synthesized in the full composition range. Such alloys are, in general, formed of component elements that are well matched in terms of ionicity, atom size, and electronegativity. In contrast there is a broad class of potential semiconductor alloys formed of component materials with distinctly different properties. In most instances these mismatched alloys are immiscible under standard growth conditions. Here we report on the properties of GaN1?xAsx, a highly mismatched, immiscible alloy system that was successfully synthesized in the whole composition range using a nonequilibrium low temperature molecular beam epitaxy technique. The alloys are amorphous in the composition range of 0.17{\ensuremath{<}}x{\ensuremath{<}}0.75 and crystalline outside this region. The amorphous films have smooth morphology, homogeneous composition, and sharp, well defined optical absorption edges. The band gap energy varies in a broad energy range from {$\sim$} 3.4 eV in GaN to {$\sim$} 0.8 eV at x {$\sim$} 0.85. The reduction in the band gap can be attributed primarily to the downward movement of the conduction band for alloys with x{\ensuremath{>}}0.2, and to the upward movement of the valence band for alloys with x{\ensuremath{<}}0.2. The unique features of the band structure offer an opportunity of using GaN1?xAsx alloys for various types of solar power conversion devices.},
    keywords = {alloying, amorphous semiconductors, conduction bands, electronegativity, energy gap, gallium arsenide, molecular beam epitaxial growth, nitrogen compounds, solar energy conversion, solubility, valence bands, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/26799/}
    }

  • L. Zhao, R. Campion, P. Fewster, R. Martin, B. Ber, A. Kovarsky, C. Staddon, K. Wang, K. Edmonds, C. Foxon, and B. Gallagher, "Determination of the Mn concentration in GaMnAs," Semiconductor Science and Technology, vol. 20, iss. 5, p. 369–373, 2005.
    [BibTeX] [Abstract] [Download PDF]

    Three series of 1 mu m thick Ga1-xMnxAs films with different Mn composition have been characterized using high-resolution x-ray diffraction (HRXRD). The results show that they are highly-crystalline and the growth is reproducible. The Mn compositions have also been measured by other popular methods: in situ ion gauge, x-ray fluorescence (XRF), electron probe microanalysis (EPMA) and secondary ion mass spectrometry (SIMS). The results show that the Mn concentrations measured by different methods are different and the difference between them becomes smaller with increasing Mn content.

    @article{strathprints31060,
    volume = {20},
    number = {5},
    month = {May},
    author = {LX Zhao and RP Campion and PF Fewster and RW Martin and BY Ber and AP Kovarsky and CR Staddon and KY Wang and KW Edmonds and CT Foxon and BL Gallagher},
    title = {Determination of the Mn concentration in GaMnAs},
    journal = {Semiconductor Science and Technology},
    pages = {369--373},
    year = {2005},
    keywords = {GaMnAs, semiconductors, x-ray diffraction, Optics. Light, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/31060/},
    abstract = {Three series of 1 mu m thick Ga1-xMnxAs films with different Mn composition have been characterized using high-resolution x-ray diffraction (HRXRD). The results show that they are highly-crystalline and the growth is reproducible. The Mn compositions have also been measured by other popular methods: in situ ion gauge, x-ray fluorescence (XRF), electron probe microanalysis (EPMA) and secondary ion mass spectrometry (SIMS). The results show that the Mn concentrations measured by different methods are different and the difference between them becomes smaller with increasing Mn content.}
    }

  • C. Trager-Cowan, F. Sweeney, J. Hastie, S. K. Manson-Smith, D. A. Cowan, D. McColl, A. Mohammed, K. P. O'Donnell, D. Zubia, S. D. Hersee, C. T. Foxon, I. Harrison, and S. V. Novikov, "Characterisation of nitride thin films by EBSD," Journal of Microscopy, vol. 205, iss. 3, p. 226–230, 2002.
    [BibTeX] [Abstract] [Download PDF]

    Thin films incorporating GaN, InGaN and AlGaN are presently arousing considerable excitement because of their suitability for UV and visible light-emitting diodes and laser diodes. However, because of the lattice mismatch between presently used substrates and epitaxial nitride thin films, the films are of variable quality. In this paper we describe our preliminary studies of nitride thin films using electron backscattered diffraction (EBSD). We show that the EBSD technique may be used to reveal the relative orientation of an epitaxial thin film with respect to its substrate (a 90? rotation between a GaN epitaxial thin film and its sapphire substrate is observed) and to determine its tilt (a GaN thin film was found to be tilted by 13{$\pm$}1? towards [10 0]GaN), where the tilt is due to the inclination of the sapphire substrate(cut off-axis by 10? from (0001)sapphire towards (10 0)sapphire). We compare EBSD patterns obtained from As-doped GaN films grown by plasma-assisted molecular beam epitaxy (PA-MBE) with low and high As 4 flux, respectively. Higher As 4 flux results in sharper, better defined patterns, this observation is consistent with the improved surface morphology observed in AFM studies. Finally, we show that more detail can be discerned in EBSD patterns from GaN thin films when samples are cooled.

    @article{strathprints3086,
    volume = {205},
    number = {3},
    month = {March},
    author = {C. Trager-Cowan and F. Sweeney and J. Hastie and S.K. Manson-Smith and D.A. Cowan and D. McColl and A. Mohammed and K.P. O'Donnell and D. Zubia and S.D. Hersee and C.T. Foxon and I. Harrison and S.V. Novikov},
    title = {Characterisation of nitride thin films by EBSD},
    journal = {Journal of Microscopy},
    pages = {226--230},
    year = {2002},
    keywords = {crystalline quality, diffraction, EBSD, electron backscatterdiffraction, epitaxial thin films, GaN, Kikuchi, nitride thin films, sapphire, temperature dependence, nanoscience, Solid state physics. Nanoscience, Pathology and Forensic Medicine, Histology},
    url = {http://strathprints.strath.ac.uk/3086/},
    abstract = {Thin films incorporating GaN, InGaN and AlGaN are presently arousing considerable excitement because of their suitability for UV and visible light-emitting diodes and laser diodes. However, because of the lattice mismatch between presently used substrates and epitaxial nitride thin films, the films are of variable quality. In this paper we describe our preliminary studies of nitride thin films using electron backscattered diffraction (EBSD). We show that the EBSD technique may be used to reveal the relative orientation of an epitaxial thin film with respect to its substrate (a 90? rotation between a GaN epitaxial thin film and its sapphire substrate is observed) and to determine its tilt (a GaN thin film was found to be tilted by 13{$\pm$}1? towards [10 0]GaN), where the tilt is due to the inclination of the sapphire substrate(cut off-axis by 10? from (0001)sapphire towards (10 0)sapphire). We compare EBSD patterns obtained from As-doped GaN films grown by plasma-assisted molecular beam epitaxy (PA-MBE) with low and high As 4 flux, respectively. Higher As 4 flux results in sharper, better defined patterns, this observation is consistent with the improved surface morphology observed in AFM studies. Finally, we show that more detail can be discerned in EBSD patterns from GaN thin films when samples are cooled.}
    }