Growth, characterisation and surface structures of MnSb and NiSb thin films

摘要

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.