Christopher W. Burrows

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

Epitaxial films including bulklike 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 half-metallicity of the cubic polymorph and compare its spin polarization as a function of reduced magnetization with that of the well known half-metal NiMnSb. In both cases, half-metallicity is lost at a threshold magnetization reduction, corresponding to a temperature T* 350 K, making epitaxial cubic MnSb a promising candidate for efficient room temperature spin injection into semiconductors.

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

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.

Realisation of magnetically and atomically abrupt half-metal/semiconductor interface : Co2FeSi0.5Al0.5/Ge(111)

Halfmetal-semiconductor interfaces are crucial for hybrid spintronic devices. Atomically sharp interfaces with high spin polarisation are required for efficient spin injection. In this work we show that thin film of half-metallic full Heusler alloy Co2FeSi0.5Al0.5 with uniform thickness and B2 ordering can form structurally abrupt interface with Ge(111). Atomic resolution energy dispersive X-ray spectroscopy reveals that there is a small outdiffusion of Ge into specific atomic planes of the Co2FeSi0.5Al0.5 film, limited to a very narrow 1 nm interface region. First-principles calculations show that this selective outdiffusion along the Fe-Si/Al atomic planes does not change the magnetic moment of the film up to the very interface. Polarized neutron reflectivity, x-ray reflectivity and aberration-corrected electron microscopy confirm that this interface is both magnetically and structurally abrupt. Finally, using first-principles calculations we show that this experimentally realised interface structure, terminated by Co-Ge bonds, preserves the high spin polarization at the Co2FeSi0.5Al0.5/Ge interface, hence can be used as a model to study spin injection from half-metals into semiconductors.

Epitaxial growth of cubic MnSb on GaAs AND InGaAs(111)

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.

Spatial regularity of InAs-GaAs quantum dots : quantifying the dependence of lateral ordering on growth rate

The lateral ordering of arrays of self-assembled InAs-GaAs quantum dots (QDs) has been quantified as a function of growth rate, using the Hopkins-Skellam index (HSI). Coherent QD arrays have a spatial distribution which is neither random nor ordered, but intermediate. The lateral ordering improves as the growth rate is increased and can be explained by more spatially regular nucleation as the QD density increases. By contrast, large and irregular 3D islands are distributed randomly on the surface. This is consistent with a random selection of the mature QDs relaxing by dislocation nucleation at a later stage in the growth, independently of each QD’s surroundings. In addition we explore the statistical variability of the HSI as a function of the number N of spatial points analysed, and we recommend N > 103 to reliably distinguish random from ordered arrays.

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

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 A, 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.

Enhanced spin polarization at n-MnSb(0001)/InP(111) interface

We studied the electronic and structural properties of the interfaces between the niccolite (n-) polymorph of ferromagnetic MnSb and the binary semiconductor InP along the MnSb(0001)/InP(111) direction. Plane-wave pseudopotential ab-initio electronic structure calculations were used. The Mn-to-P interface becomes 63.0% spin-polarized, much higher than the bulk polarization. The other three possible interfaces for contacts retain their ferromagnetism at the interface. Adhesion energy of the Mn-to-P interface is 63meV/Å, larger than the other terminations, and it has the shortest interlayer bond distance of 2.43 Å. The high interface spin polarization of the favorable Mn-to-P interface is advantageous for spin transport.

Synchrotron X-ray diffraction in air and vacuum: Strain and structure at the nano-scale

The simultaneous use of symmetric and grazing incidence X-ray diffraction in the characterisation of a pnictide material is demonstrated using MnSb epi-layers grown on compound semiconductor substrates. This combination of diffraction geometries enables a comprehensive determination of the lattice parameters and complex structural behaviour of MnSb. For example, in the ultra-thin limit (<;5 nm) of layers grown on GaAs an evolution from coherently strained to relaxed islands is observed with increasing thickness. However, a mixture of both strained and relaxed islands is seen beyond the calculated critical thickness. As film thickness increases, a cubic polymorph is observed and the relative content of this polymorph was probed using reciprocal space maps. Finally, recent surface X-ray diffraction work performed under ambient conditions is compared with similar data obtained at ultra-high vacuum conditions. The information found using these complementary diffraction geometries can readily be applied to a range of compound semiconductors.

Nanostructures and surface reconstructions in Mn / III–V systems and MnSb

The deposition of Mn on to different III-V surfaces can generates both self-assembled nanostructures and alloyed surface reconstructions. These are analyzed by electron diffraction, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) for different InSb and GaAs surfaces. The overall behavior can be explained by mass transport and thermodynamics, provided the group V content of the initial surface reconstruction is accounted for. The Mn-GaAs reconstructions are described in the framework of the generalized electron counting rule (GECR). Conversely, the reconstructions of MnSb, a ferromagnetic weak metal, cannot be described by the GECR. A joint quantitative electron diffraction, STM and density functional theory study reveals the favored structure of MnSb(0001)-(2×2) to be Sb-terminated with additional Sb adatoms on HCP hollow sites.

Surface X-ray diffraction during GaAs/MnSb/Ga(In)As epitaxial growth

Multilayer structures comprised of a central layer of the binary transition metal pnictide MnSb and outer layers of standard semiconductors have been grown using molecular beam epitaxy (MBE). The growth of the GaAs overlayers was characterized using a combination of reflection high energy electron diffraction (RHEED) and in situ surface x-ray diffraction (SXRD). It was found that a GaAs overlayer can be grown via MBE onto a MnSb(1 1̅01)/GaAs(001) virtual substrate. For both GaAs(111)A and InGaAs(111)A starting substrates, it was found that the GaAs overlayer altered the final strain state of the central MnSb layer causing a change from tensile strain to compressive strain in the out of plane direction.