Growth, characterisation and surface structures of MnSb and NiSb thin films
2011
Epitaxial growth of NiSb on GaAs(111)B substrates has been achieved for the
first time. X-ray diffraction confirms the films are of high quality and oriented (0001)
with respect to the (111) substrate. Surface reconstructions have been observed on
NiSb thin films through electron diffraction performed in situ during and after growth.
Three different surface reconstructions have so far been observed. Two of these, the
(4×4) and (4×6) reconstructions, are entirely new to the binary pnictides. The latter,
however, is only metastable. The third reconstruction is a td(1×3) and is believed to
be related to similar reconstructions seen on MnSb and MnAs.
The epitaxial growth of MnSb on GaAs(111)B substrates has been the subject
of a J dependent growth study. It is seen that the final surface morphology is highly
sensitive to the local beam flux ratio, J, with changes of a few percent leading to sharp
and abrupt changes in the morphology. An XRD investigation of these films reveal varied
and complex behaviour, with the appearance of a large number of reflections which do
not originate from either GaAs(111) or MnSb(0001). Cubic MnSb(111) crystallites have
been seen in some thin films in a variety of strain states with evidence of tetragonal
distortions.
Surface preparation methods of air-exposed MnSb has been investigated using a
combination of x-ray photoelectron spectroscopy (XPS), scanning electron microscopy
(SEM) and x-ray mangetic circular dichroism (XMCD). A thick Mn-oxide layer, which is
resistant to conventional in situ ion-bombarding and annealing (IBA) methods, is formed
after exposure to air. Such surfaces are found to be non-magnetic. A simple combination
of HCl acid etching followed by in situ IBA treatments is found to result in a well ordered
(2×2) surface, with the recovery of a magnetic surface. A new antimony capping
procedure has been investigated and found to be effective in preventing oxidation of the
surface even after prolonged exposure to air. Such samples are found to retain a magnetic
surface. It will also be shown that detailed analysis of the XMCD is not possible due
to jj coupling which precludes the use of the sum rules, whilst theoretical calculations
within the SPR-KKR DFT framework fail to adequately describe some aspects of the
bulk magnetic behaviour.
Quantitative surface structure determination using co-axial impact collision scattering
spectroscopy (CAICISS) and low energy electron diffraction (LEED I-V) has been
performed on the MnSb(0001)-(2×2) reconstruction. In total, 68 unique surface structures
have been trialled, with none of them fitting the experimental data to any degree
of satisfaction. A number of key observations have however still been made. Firstly,
the LEED I-V suggests the MnSb is bulk-terminated with antimony and consists of a
manganese-rich surface layer. This agrees with RHEED observations made during and
after growth. However, both the CAICISS and LEED I-V data show the surface region
to be six-fold symmetric, in direct contradiction with the bulk symmetry. In addition,
the CAICISS data indicates the bulk structure is not preserved all the way to the surface, with the Sb–Sb and Mn–Mn layer separations being approximately equal. This suggests
the structure of the outermost MnSb layers deviate significantly from the bulk structure
and has profound implications for the surface magnetic and electronic properties, as well
as for epitaxial growth with MnSb acting as the substrate.
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