  The top-down synthesis of inherently ferroelectric semiconductors and their
integration with traditional material platforms have the potential to enable
new low power logic devices, and to harness the bulk photoelectric effect for
more efficient photovoltaic cells. InSe is a layered van der Waals compound
exhibiting multiple polytypes, with semiconducting gamma-InSe revealing a
non-centrosymmetric space group and showing a high carrier mobility at room
temperature. Here we report the growth of InSe films on close to lattice
matched semi-insulating GaAs(111)B substrates by molecular beam epitaxy (MBE).
Excellent nucleation behavior resulted in the growth of smooth, single phase
InSe films. The dominant polytype determined from X-ray diffraction was the
targeted gamma-InSe, however Raman spectroscopy revealed spatial variations in
the overall low-intensity non-centrosymmetric vibration modes. Transmission
electron microscopy uncovered the presence of the three bulk polytypes beta,
gamma, and epsilon-InSe coexisting in the films arranging in nanosized domains.
The different polytypes can be interpreted as sequences of stacking faults and
rotational twin boundaries of gamma-InSe made from individual
non-centrosymmetric Se-In-In-Se layers with P-6m2 symmetry. A second,
centrosymmetric Se-In-In-Se layer polymorph was identified with P-3m symmetry,
which is typically not present in InSe bulk phases. First principles
calculations revealed small formation energy differences between the InSe
polymorphs and polytypes, yet sizeable differences in their electronic
properties. Nanoscale domain sizes of varying polytypes thus resulted in
sizeable electronic disorder in the grown films that dominated the electronic
transport properties. Our results indicate that bottom-up thin film synthesis
is a viable synthesis route towards stabilization of InSe polytypes not present
in the bulk.
