Investigation of Built-in Electric Fields at the GaSe/GaAs Interface by Photoreflectance Spectroscopy
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Investigation of Built-in Electric Fields at the GaSe/GaAs Interface by Photoreflectance Spectroscopy O. S. Komkova,*, S. A. Khakhulina, D. D. Firsova, P. S. Avdienkob, I. V. Sedovab, and S. V. Sorokinb a St.
Petersburg Electrotechnical University “LETI”, St. Petersburg, 197376 Russia b Ioffe Institute, St. Petersburg, 194021 Russia *e-mail: [email protected] Received April 27, 2020; revised May 20, 2020; accepted May 20, 2020
Abstract—Built-in electric fields appear during the molecular-beam epitaxy of GaSe on a GaAs(001) substrate at the GaSe/GaAs interface, the presence of the fields is evidenced by Franz–Keldysh oscillations observed in photoreflectance spectra. The varying values of the strength of these fields (from 9.8 to 17.6 kV/cm) can be associated both with the diffusion of Se atoms into the GaAs substrate (buffer layer) and with the formation of transient submonolayers during epitaxial growth. Built-in fields are not observed at the interface of structures grown on GaAs(111)B and GaAs(112) substrates, which can be explained by the lower propagation efficiency of Se into the substrate with mentioned orientations when compared with GaAs(001). Keywords: GaSe, layered semiconductors, modulation optical spectroscopy, molecular-beam epitaxy, photoreflectance DOI: 10.1134/S1063782620100176
1. INTRODUCTION The unique structural, electrical, and optical properties of semiconductor compounds with extreme two-dimensionality (to which chalcogenides of posttransition metals such as GaSe, InSe, etc. belong) are of great interest for researchers [1–3]. In particular, GaSe is a promising material for the fabrication of field-effect transistors and highly effective photodetectors [4–7]. It was shown that it is possible to fabricate a highly sensitive photodetector based on a single layer of twodimensional GaSe, which is able to detect weak signals of the order of nW in the ultraviolet region and visible optical spectral region (250–600 nm) with a sensitivity maximum at a wavelength of 254 nm [7]. However, either exfoliated GaSe “scales” or small-area samples fabricated by chemical vapor phase deposition were used in most of the above-listed works. Molecularbeam epitaxy (MBE), which potentially makes it possible to implement high-quality large-area 2D crystals on commercially available substrates, is still at the developmental stage [8]. Strong anisotropy of the transport, mechanical, and optical properties of GaSe is a consequence of its layered crystalline structure. Bulk GaSe crystals comprise a set of vertically ordered tetralayers (TLs) connected to each other by means of weak van der Waals forces. Each TL ~0.8 nm in thickness consists of four covalently coupled Se–Ga–Ga–Se atomic layers [9].
It should be noted that GaSe can be crystallized in various polytypes differing from each other in terms of the sequence of TL packing: with a hexagonal (β, ε, and δ polytypes) or rhombohedral (γ polytype) crystal lattice [9]. It was shown that MBE-grown GaSe layers usually correspond to the
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