Communication
Structural investigation of InSe layered semiconductors

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Highlights

  • Structure and morphology of InSe prepared by Bridgman method have been investigated.

  • XRD, TEM and Raman techniques have been employed to characterize InSe samples.

  • By crossing the information of different techniques, it is possible to unambiguously discern between InSe polytypes.

Abstract

During the last decade, III–VI layered semiconductors (GaSe, InSe, GaS, etc.) have emerged as potential candidates for various applications, such as FET and optoelectronic devices. The properties of this class of layered materials are strongly dependent on their structure, and the existence of different polytypes makes it necessary the identification of the structural phase. In this work, we have performed a detailed investigation of the crystal structure and morphology of bulk InSe, by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. The combination of the employed techniques allowed to identify the structural phase of InSe samples (ϵ polytype). Most importantly, we show that only by crossing the information of each technique it is possible to unambiguously discern between similar polytypes.

Introduction

Two-dimensional (2D) layered materials have attracted extensive interest in the past years due to their unique optical, electronic and mechanical properties [1], [2]. Besides graphene, some typical 2D materials, which have been explored both experimentally and theoretically, include transition metal dichalcogenides (TMDCs), black phosphorus (BP), hexagonal boron nitride (h-BN) and III–VI semiconductors [3], [4].

Among the mono-chalcogenides, InSe and GaSe, which have been synthesized via physical and chemical methods [5], [6], [7], [8], exhibit promising properties in the field of microelectronic, optical and optoelectronic applications [9], [10]. For both compounds, the high tunability of the band gap in the few-layers regime, due to strong quantum confinement effects, has been theoretically and experimentally proved [11], [12], [13], [14]. Moreover, InSe exhibits a carrier mobility reaching the value of 103 cm2 V−1 s−1 at room temperature, highest among the other TMDCs and superior to BP [15]. It has been demonstrated also that III–VI materials can be used in solar energy conversion devices, field-effect transistors and photodetectors. Recently, these materials have been included into novel heterostructure for the development of new transistors, sensors etc. [10], [16], [17], [18].

Since III–VI semiconductors possess various polytypes with different stoichiometry [19], depending on the arrangements of atoms and on the stacking layers, the phase identification of the bulk material is of primary importance. Indeed, as well known, besides the confirmation of the stoichiometric ratio, the accurate knowledge of the structural phase is required in order to understand and simulate all the macroscopic properties of the crystal.

We report here an experimental investigation of semiconductors InSe, by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and Raman spectroscopy. We show that, due to the existence of similar polytypes, only combining the findings of each technique the structural phase can be unambiguously determined. Overall, our study provides a useful guide to discern between polytypes of mono-chalcogenides layered semiconductors.

Section snippets

Methods

The InSe crystals were grown in double wall ampoules by means of the Bridgman method starting from a non-stoichiometric melt, containing an In excess of about 5% [20]. The high quality of the as-grown ingot was characterized by X-ray diffraction (XRD) by using a Bruker AXS D8 Discover diffractometer with Cu Kα radiation (λ = 1.5418 Å). Measurements were taken at room temperature for 2 θ values over 2 ° ÷ 70 ° in steps of 0.01°/0.004°, with a step time of 1 s.

In order to study the layered

Results and discussion

The crystal structure of mono-chalcogenides III–VI compounds consists of basic quadruple-layers (QLs) (about 8 Å of thickness), held together by weak van der Waals forces; each QL is formed by covalently bonded atoms (Se-In-In-Se); this peculiar structure makes easy the mechanical exfoliation of the sample. The three-dimensional lattices are built by stacking QLs in different way, yielding selected polytypes.

InSe crystals, with 1:1 indium/selenium ratio, exhibit three common polytypes, ϵ, β and

Conclusion

In this work, we provide a detailed investigation of the crystal structure and morphology of bulk InSe, by means of XRD, TEM and Raman techniques. XRD results allowed to identify the hexagonal phase (typical of ϵ and β polytypes) and discard the rhombohedral one (γ polytype); in addition to this, we proved the single-phase and good crystalline quality of the sample, and provide the value of the lattice parameters: c = (16.672 ± 0.045) Å and a = (4.005 ± 0.022) Å. TEM measurements lead us to

Acknowledgments

M. M. Pipita and M. Castriota wish to thank POR CALABRIA FESR-FSE 2014-2020-Asse I-Promozione Della Ricerca e Dell’innovazione, Italy , Obiettivo specifico 1.2 Azione 1.2.2 Project: Meraviglie.

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