Electron Transport in n-Type InSe van der Waals Crystals with Co Impurities

Intercalation and doping are promising routes to tune properties of van der Waals (vdW) semiconductors and pave the way for their applications in digital electronics beyond Moore’s law, sensors and spintronics. The indium selenide (InSe) vdW crystal shows great promise for use in next-generation semiconductor technologies. For these applications to be realized, the effects of impurities on properties of InSe must be understood. Here, we present a comparative experimental study of electron transport in n-type InSe semiconductor doped and electrochemically intercalated with magnetic cobalt (Co) impurities. It is shown that the presence of Co decreases the free electron density, the Hall mobility along layers and the conductivity anisotropy σ⊥C/σ‖C. Furthermore, this leads to a change of the behavior of σ⊥C(T) dependence from a metallic one in pristine samples to a semiconducting one in samples with Co. We also demonstrate that the interaction of electrons with space-charge regions is an effective scattering mechanism, which should be taken into account in doped and intercalated crystals. The present work is important for the basic physics knowledge of the effect of Co impurities on physical properties of InSe, which is needed to tailor the parameters of this semiconductor for applications in electronics and spintronics

Charge Carrier Transport in Van Der Waals Semiconductor InSe Intercalated with RbNO3 Probed by Direct Current Methods

Layered van der Waals (vdW) semiconductors show great promise to overcome limitations imposed by traditional semiconductor materials. The synergistic combination of vdW semiconductors with other functional materials can offer novel working principles and device concepts for future nano- and optoelectronics. Herein, we investigate the influence of the intercalation of semiconducting n-type InSe vdW crystals with ferroelectric rubidium nitrate (RbNO3) on the transport of charge carriers along and across the layers. The apparent maxima in the temperature dependences of the Hall coefficient are explained in the framework of a model that predicts, along with three-dimensional carriers, the existence of two-dimensional ones contributing only to the conductivity along the layers. The revealed increase of the conductivity anisotropy and its activation variation with temperature, which is mainly due to a decrease of the conductivity across the layers, confirm a two-dimensionalization of electron gas in n-InSe after insertion of the ferroelectric. From the numerical analysis, we determined the densities of carriers of both types, concentrations of donors and acceptors, as well as the value of the interlayer barrier

The Interaction of Hydrogen with the van der Waals Crystal γ-InSe

The emergence of the hydrogen economy requires development in the storage, generation and sensing of hydrogen. The indium selenide (γ-InSe) van der Waals (vdW) crystal shows promise for technologies in all three of these areas. For these applications to be realised, the fundamental interactions of InSe with hydrogen must be understood. Here, we present a comprehensive experimental and theoretical study on the interaction of γ-InSe with hydrogen. It is shown that hydrogenation of γ-InSe by a Kaufman ion source results in a marked quenching of the room temperature photoluminescence signal and a modification of the vibrational modes of γ-InSe, which are modelled by density functional theory simulations. Our experimental and theoretical studies indicate that hydrogen is incorporated into the crystal preferentially in its atomic form. This behaviour is qualitatively different from that observed in other vdW crystals, such as transition metal dichalcogenides, where molecular hydrogen is intercalated in the vdW gaps of the crystal, leading to the formation of "bubbles" for hydrogen storage

Room temperature electroluminescence from mechanically formed van der Waals III–VI homojunctions and heterojunctions

Room temperature electroluminescence from semiconductor junctions is demonstrated. The junctions are fabricated by the exfoliation and direct mechanical adhesion of InSe and GaSe van der Waals layered crystals. Homojunction diodes formed from layers of p- and n-type InSe exhibit electroluminescence at energies close to the bandgap energy of InSe (Eg= 1.26 eV). In contrast, heterojunction diodes formed by combining layers of p-type GaSe and n-type InSe emit photons at lower energies, which is attributed to the generation of spatially indirect excitons and a staggered valence band lineup for the holes at the GaSe/InSe interface. These results demonstrate the technological potential of mechanically formed heterojunctions and homojunctions of direct-bandgap layered GaSe and InSe compounds with an optical response over an extended wavelength range, from the near-infrared to the visible spectrum

Resonant tunnelling into the two-dimensional subbands of InSe layers

Two-dimensional (2D) van der Waals (vdW) crystals have attracted considerable interest for digital electronics beyond Si-based complementary metal oxide semiconductor technologies. Despite the transformative success of Si-based devices, there are limits to their miniaturization and functionalities. Here we realize a resonant tunnelling transistor (RTT) based on a 2D InSe layer sandwiched between two multi-layered graphene (MLG) electrodes. In the RTT the energy of the quantum-confined 2D subbands of InSe can be tuned by the thickness of the InSe layer. By applying a voltage across the two MLG electrodes, which serve as the source and drain electrodes to the InSe, the chemical potential in the source can be tuned in and out of resonance with a given 2D subband, leading to multiple regions of negative differential conductance that can be additionally tuned by electrostatic gating. This work demonstrates the potential of InSe and InSe-based RTTs for applications in quantum electronics.

Epitaxial growth of γ-InSe and α, β, and γ-In2Se3 on ε-GaSe

We demonstrate that γ-InSe and the α, β and γ phases of In2Se3 can be grown epitaxially on ε-GaSe substrates using a physical vapour transport method. By exploiting the temperature gradient within the tube furnace, we can grow selectively different phases of InxSey depending on the position of the substrate within the furnace. The uniform cleaved surface of ε-GaSe enables the epitaxial growth of the InxSey layers, which are aligned over large areas. The InxSey epilayers are characterised using Raman, photoluminescence, X-ray photoelectron and electron dispersive X-ray spectroscopies. Each InxSey phase and stoichiometry exhibits distinct optical and vibrational properties, providing a tuneable photoluminescence emission range from 1.3 eV to ~ 2 eV suitable for exploitation in electronics and optoelectronics

Photoluminescence dynamics in few-layer InSe

We study the optical properties of thin flakes of InSe encapsulated in hBN. More specifically, we investigate the photoluminescence (PL) emission and its dependence on sample thickness and temperature. Through the analysis of the PL lineshape, we discuss the relative weights of the exciton and electron-hole contributions. Thereafter we investigate the PL dynamics. Two contributions are distinguishable at low temperature: direct bandgap electron-hole and defect-assisted recombination. The two recombination processes have lifetime of τ1 ∼ 8 ns and τ2 ∼ 100 ns, respectively. The relative weights of the direct bandgap and defect-assisted contributions show a strong layer dependence due to the direct-to-indirect bandgap crossover. Electron-hole PL lifetime is limited by population transfer to lower-energy states and no dependence on the number of layers was observed. The lifetime of the defect-assisted recombination gets longer for thinner samples. Finally, we show that the PL lifetime decreases at high temperatures as a consequence of more efficient non-radiative recombinations

Interlayer Band-to-Band Tunneling and Negative Differential Resistance in van der Waals BP/InSe Field-Effect Transistors

© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Atomically thin layers of van der Waals (vdW) crystals offer an ideal material platform to realize tunnel field-effect transistors (TFETs) that exploit the tunneling of charge carriers across the forbidden gap of a vdW heterojunction. This type of device requires a precise energy band alignment of the different layers of the junction to optimize the tunnel current. Among 2D vdW materials, black phosphorus (BP) and indium selenide (InSe) have a Brillouin zone-centered conduction and valence bands, and a type II band offset, both ideally suited for band-to-band tunneling. TFETs based on BP/InSe heterojunctions with diverse electrical transport characteristics are demonstrated: forward rectifying, Zener tunneling, and backward rectifying characteristics are realized in BP/InSe junctions with different thickness of the BP layer or by electrostatic gating of the junction. Electrostatic gating yields a large on/off current ratio of up to 108 and negative differential resistance at low applied voltages (V ≈ 0.2 V). These findings illustrate versatile functionalities of TFETs based on BP and InSe, offering opportunities for applications of these 2D materials beyond the device architectures reported in the current literature