Thin film growth and study of physical properties for 2D layered materials

概要

1 Introduction
Since the experimental exemplification of graphene [1], two-dimensional (2D) layered materials have attracted keen interests as potential applications to atomically thin-film devices using their wide variety of physical properties. It has been found that 2D layered materials exhibit a strong thickness- or number of layer-dependent physical properties, such as electronic band structures, optical properties, superconductivity, and magnetism, in which the ‘dimensionality’ plays a vital role. In addition, thin film growth has been proved to be one of the effective approaches to realize electronic and optoelectronic devices based on thin film superlattices, owing to the precise controllability of the thickness and applicability to the large-scale fabrication.

In this thesis, we explored the physical properties in 2D layered materials, including semiconductor indium selenide (InSe), semi-metal ultra-thin Sn films (or stanene), and superconductor iron selenide (FeSe), by thin film growth techniques. By using thin film growth, the thickness of thin film is well controlled by the deposition duration, which enables the investigation of thickness dependent properties in 2D layered materials with a precise thickness. Here, physical properties, including optics, electrical transport, and superconductivity have been investigated.

2. Results and discussion
Thickness-dependent optical and electrical properties in InSe
InSe is a semiconductor with a layered crystal structure [Fig. 1(a)], and holds a bandgap of about 1.26 eV in bulk form. The density functional theory calculation suggests that its optical bandgap increases to the value ranging between 2.4 and 3.0 eV by reducing the thickness down to monolayer due to the quantum confinement effect [2]. Such thickness dependent optical bandgap makes InSe a candidate for an efficient photodetector sensitive to the wavelength selected by thickness.

To examine its thickness dependence of the optical and electrical properties, we applied a PLD technique for the growth of InSe thin films on Al2O3 (0001) and InP (111) substrates as shown in Fig. 1(b). The transmittance spectra of d-nm-thick InSe films on Al2O3 were measured to detect the thickness-dependent bandgap modification. With decreasing d, the systematic shift of the absorption edge in the absorbance spectra was clearly observed.

This blue shift with the thickness reduction is consistent with the previous photoluminescence studies in exfoliated flakes and calculations [2]. In addition, we obtained a high sheet conductance and room-temperature mobility reaching 730 cm2V−1s−1 for InSe on InP, while InSe on Al2O3 is highly resistive. From the weak thickness dependence of carrier density in d > 10 nm region, we concluded that the high conductance originates from the interface between InSe and InP. The possible origin for the interface conduction should be charge transfer originated from band offset and valence mismatch [3]. The PLD growth technique of the layered InSe thin films will accelerate its applications towards the multifunctional optical and electrical devices based on 2D materials.

Two-dimensional growth of conductive ultra-thin Sn films
Thin film growth of α-Sn or its 2D form (stanene) is prerequisite to extract their intriguing transport properties stemming from the unique band topology. However, it has been difficult to grow the Sn thin film directly on insulating substrate because of the 3D island growth on oxide substrates. A phase transition between a low- temperature phase α-Sn (diamond structure) to a high-temperature phase β-Sn (tetragonal) occurring at around 13 °C probably accounts for this 3D growth.

We have investigated the effect of an Fe buffer layer on the growth mode of the Sn film on an insulating Al2O3 (0001) substrate at room temperature using MBE. The inset of Fig. 2 illustrates the growth mode evolution of the Sn film. Atomic force microscopy revealed that the 3D growth of the Sn films on Al2O3 varied to a 2D layer-based mode by insertion of the 2-nm-thick Fe layer, signaling the effectiveness of the Fe insertion to stabilize 2D growth of Sn. The 3D growth mode reappeared when the thickness of Sn (dSn) exceeds the critical dSn of about 1.0 nm. This growth mode change can be seen as a systematic increase and saturating behavior of the sheet conductance (1/Rxx) of the dSn-nm Sn/2-nm-thick Fe bilayer (denoted as dSn-Sn/2-Fe) with increasing dSn (Fig. 2). From the slope of dSn dependent 1/Rxx, we extracted the resistivity of the 2D grown Sn (ρSn) of about 10−5 Ω·cm, which is rather consistent with that of α-Sn. We can expect the emergence of topological features in α-Sn thin films since the enhanced anomalous Hall effect (AHE) Hall angle in Sn/Fe bilayers might indicate an existence of large proximity effect between ferromagnetic Fe and topological α-Sn [4]. Further optimization of the 2D growth of conductive ultra-thin Sn, such as low-temperature growth or different buffer layers, may offer a platform to stabilize α-Sn or 2D stanene films for investigation of their topological properties.

Tunneling spectroscopy in FeSe with insulating capping layer
FeSe is a layered superconductor with superconducting transition temperature Tc about 8 K in bulk form. Electrical transport measurements for a monolayer FeSe on SrTiO3 substrate or a multilayer FeSe doped with electron demonstrate that the Tc is largely enhanced to ~40 K. In contrast, spectroscopic measurements have reported the gap closing temperature of ~65 K, which is much higher than the Tc in resistivity measurement [5]. To resolve the inconsistency between transport and spectroscopic measurements, we attempt to perform simultaneous measurements for evaluation of the gap size by the tunneling spectroscopy and detection of Tc by electrical transport for high-Tc ultra-thin FeSe films with the electric-double layer (EDL) transistor configuration as shown in Fig. 3(a). We utilized the insulating amorphous 2-nm-thick InSe as a tunneling barrier as shown in Fig. 3(b). In the InSe-capped 2-nm-thick FeSe (2-InSe/2-FeSe) on SrTiO3, a high Tc up to 41 K was achieved by applying gate voltage. In high-Tc condition, the tunneling spectra (dI/dV–V) curves showed a signature of superconducting gap at low temperatures, which gradually closes with increasing temperature up to a value comparable to the onset Tc in the electrical transport measurement.

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