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Publications

Bulk crystal growth and surface preparation of NiSb, MnSb and NiMnSb

I Maskery, C W Burrows, M Walker, R P Singh, G Balakrishnan, J A Duffy and G R Bell, J. Vac. Sci. Technol. B 34, 041219 (2016)

Bulk single crystal and polycrystalline samples of NiSb, MnSb, and NiMnSb have been grown and characterized. The lattice parameter of NiMnSb was found to be 5.945 ± 0.001 Å, around 0.25% larger than previous reports. The surface preparation of these materials was investigated using x-ray photoelectron spectroscopy. Wet etching with HCl and argon ion sputtering were used in tandem with vacuum annealing. For both binary materials, a clean and stoichiometric surface could be regained by HCl etching and annealing alone. However, clean and stoichiometric ternary NiMnSb was not successfully prepared by these methods. The transition metal 2p and 3p levels are analyzed for all three materials.

Read More: http://scitation.aip.org/content/avs/journal/jvstb/34/4/10.1116/1.4953549

Epitaxial Growth of Cubic MnSb on GaAs and InGaAs(111)

G R Bell, C W Burrows, T P A Hase, M J Ashwin, S R C McMitchell, A M Sanchez and J D Aldous, SPIN 04, 1440025 (2014)

The cubic polymorph of the binary transition metal pnictide (TMP) MnSb, c-MnSb, has been predicted to be a robust half-metallic ferromagnetic (HMF) material with minority spin gap ≳1 eV. Here, MnSb epilayers are grown by molecular beam epitaxy (MBE) on GaAs and In0.5Ga0.5As(111) substrates and analyzed using synchrotron radiation X-ray diffraction. We find polymorphic growth of MnSb on both substrates, where c-MnSb co-exists with the ordinary niccolite n-MnSb polymorph. The grain size of the c-MnSb is of the order of tens of nanometer on both substrates and its appearance during MBE growth is independent of the very different epitaxial strain from the GaAs (3.1%) and In0.5Ga0.5As (0.31%) substrates.

Read More: http://www.worldscientific.com/doi/abs/10.1142/S2010324714400256


Interaction of Mn with GaAs and InSb: incorporation, surface reconstruction and nano-cluster formation

C W Burrows, S A Hatfield, F Bastiman, G R Bell, Journal of Physics: Condensed Matter 26, 395006 (2014)

The deposition of Mn on to reconstructed InSb and GaAs surfaces, without coincident As or Sb flux, has been studied by reflection high energy electron diffraction, atomic force microscopy and scanning tunnelling microscopy. On both Ga- and As-terminated GaAs(0 0 1), (2 × n) Mn-induced reconstruction domains arise with n = 2 for the most well ordered reconstructions. On the Ga-terminated (4 × 6), the Mn-induced (2 × 2) persists up to around 0.5 ML Mn followed by Mn nano-cluster formation. For deposition on initially β2(2 × 4)-reconstructed GaAs(0 0 1), the characteristic trench structure of the reconstruction is partially preserved even beyond 1 monolayer Mn coverage. On both the β2(2 × 4) and c(4 × 4) surfaces, MnAs-like nano-clusters form alongside the reconstruction changes. In contrast, there are no new Mn-induced surface reconstructions on InSb. Instead, the Sb-terminated surfaces of InSb (0 0 1), (1 1 1)A and (1 1 1)B revert to reconstructions characteristic of clean In-rich surfaces after well defined coverages of Mn proportional to the Sb content of the starting reconstruction. These surfaces are decorated with self-assembled MnSb nanoclusters. These results are discussed in terms of basic thermodynamic quantities and the generalized electron counting rule.

Read more: http://dx.doi.org/10.1088/0953-8984/26/39/395006


Heteroepitaxial Growth of Ferromagnetic MnSb(0001) Films on Ge/Si(111) Virtual Substrates

C W Burrows, A Dobbie, M Myronov, T P A Hase, S B Wilkins, M Walker, J J Mudd, I Maskery, M R Lees, C F McConville, D R Leadley and G R Bell, Crystal Growth & Design 13 (11), 4923-4929 (2013)

Heteroepitaxial Growth of Ferromagnetic MnSb(0001) Films on Ge/Si(111) Virtual Substrates
Abstract - Molecular beam epitaxial growth of ferromagnetic MnSb(0001) has been achieved on high quality, fully relaxed Ge(111)/Si(111) virtual substrates grown by reduced pressure chemical vapor deposition. The epilayers were characterized using reflection high energy electron diffraction, synchrotron hard X-ray diffraction, X-ray photoemission spectroscopy, and magnetometry. The surface reconstructions, magnetic properties, crystalline quality, and strain relaxation behavior of the MnSb films are similar to those of MnSb grown on GaAs(111). In contrast to GaAs substrates, segregation of substrate atoms through the MnSb film does not occur, and alternative polymorphs of MnSb are absent.

Read more: http://pubs.acs.org/doi/abs/10.1021/cg4011136


Growth and characterisation of NiSb(0001)/GaAs(111)B epitaxial films

J D Aldous, C W Burrows, I Maskery, M Brewer, D Pickup, M Walker, J Mudd, T P A Hase, J A Duffy, S Wilkins, C Sanchez-Hanke and G R Bell, Journal of Crystal Growth 357, 1-8 (2012).

Growth and characterisation of NiSb(0001)/GaAs(111)B epitaxial films
Abstract - Thin films of NiSb(0001) have been grown using molecular beam epitaxy on GaAs(111)B substrates and characterized with a variety of structural and surface-specific techniques supported by density functional theory calculations. Several differences were observed between NiSb and the more widely studied MnSb. A new (4 x 4) surface reconstruction was seen on NiSb(0001), along with other reconstructions common to MnSb or MnAs. Strain relaxation differs between NiSb and MnSb, with strained layers 10 nm thick persisting in NiSb and some crystallites of (1 (1) over bar 01) orientation appearing in thick (0001) films. Ga segregation through NiSb(0001) films does not occur, unlike in MnSb, and the native oxide of NiSb is more benign than the Mn-rich oxides of MnSb.

Read more: http://dx.doi.org/10.1016/j.jcrysgro.2012.07.010


Depth-dependent magnetism in epitaxial MnSb thin films: effects of surface passivation and cleaning

J D Aldous, C W Burrows, I Maskery, M Brewer, T P A Hase, J A Duffy, Martin R Lees, C Sanchez-Hanke, T Decoster, W Theis, A Quesada, A K Schmid and G Bell, Journal of Physics: Condensed Matter 24, 146002 (2012).

Depth-dependent magnetism in epitaxial MnSb thin films: effects of surface passivation and cleaning
Abstract - Depth-dependent magnetism in MnSb(0001) epitaxial films has been studied by combining experimental methods with different surface specificities: polarized neutron reflectivity, x-ray magnetic circular dichroism (XMCD), x-ray resonant magnetic scattering and spin-polarized low energy electron microscopy (SPLEEM). A native oxide 4.5 nm thick covers air-exposed samples which increases the film’s coercivity. HCl etching efficiently removes this oxide and in situ surface treatment of etched samples enables surface magnetic contrast to be observed in SPLEEM. A thin Sb capping layer prevents oxidation and preserves ferromagnetism throughout the MnSb film. The interpretation of Mn L3;2 edge XMCD data is discussed.

Read more: http://dx.doi.org/10.1088/0953-8984/24/14/146002


Cubic MnSb: Epitaxial growth of a predicted room temperature half-metal

James D. Aldous, Christopher W. Burrows, Ana M. Sánchez, Richard Beanland, Ian Maskery, Matthew K. Bradley, Manuel dos Santos Dias, Julie B. Staunton, and Gavin R. Bell, Physical Review B 85, 060403(R) (2012).

Cubic MnSb: Epitaxial growth of a predicted room temperature half-metal
Abstract - Epitaxial films including bulk-like cubic and wurtzite polymorphs of MnSb have been grown by molecular beam epitaxy on GaAs via careful control of the Sb4/Mn flux ratio. Nonzero-temperature density functional theory was used to predict ab initio the spin polarization as a function of reduced magnetization for the half-metals NiMnSb and cubic MnSb. In both cases, half-metallicity is lost at a threshold magnetization reduction, corresponding to a temperature T*< Tc. This threshold is far higher for cubic MnSb compared to NiMnSb and corresponds to T* > 350K, making epitaxial cubic MnSb a promising candidate for room temperature spin injection into semiconductors.

Read more: http://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.060403