Mixed polytype/polymorph formation and its effects on the electronic properties in InSe films grown by molecular beam epitaxy on GaAs(111)B

Abstract

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.