Z. D. Kovalyuk

High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe

A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103 cm2 V-1 s-1 and 104 cm2 V-1 s-1 at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayers mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides and black phosphorus.

Tuning the Bandgap of Exfoliated InSe Nanosheets by Quantum Confinement

Strong quantization effects and tuneable near-infrared photoluminescence emission are reported in mechanically exfoliated crystals of γ-rhombohedral semiconducting InSe. The optical properties of InSe nanosheets differ qualitatively from those reported recently for exfoliated transition metal dichalcogenides and indicate a crossover from a direct to an indirect band gap semiconductor when the InSe flake thickness is reduced to a few nanometers.

High Broad‐Band Photoresponsivity of Mechanically Formed InSe–Graphene van der Waals Heterostructures

High broad‐band photoresponsivity of mechanically formed InSe–graphene van der Waals heterostructures is achieved by exploiting the broad‐band transparency of graphene, the direct bandgap of InSe, and the favorable band line up of InSe with graphene. The photoresponsivity exceeds that for other van der Waals heterostructures and the spectral response extends from the near‐infrared to the visible spectrum.

Electrical and photoelectrical properties of photosensitive heterojunctions n-TiO2/p-CdTe

Photosensitive heterojunctions n-TiO2/p-CdTe were fabricated by dc reactive magnetron deposition of TiO2 thin films with n-type conductivity onto freshly cleaved p-CdTe single-crystal substrates (1 1 0). Their electrical properties were investigated and the dominating current mechanisms were analyzed at forward and reverse biases in the scope of the tunnel-recombination and emission-recombination models. The obtained surface-barrier structures n-TiO2/p-CdTe possessed the following photoelectrical parameters under 100 mW cm?2 illumination: the open-circuit voltage Voc = 0.69 V, the short-circuit current Isc = 6 mA cm?2 and the fill factor FF = 0.42.

The direct-to-indirect band gap crossover in two-dimensional van der Waals indium selenide crystals

The electronic band structure of van der Waals (vdW) layered crystals has properties that depend on the composition, thickness and stacking of the component layers. Here we use density functional theory and high field magneto-optics to investigate the metal chalcogenide InSe, a recent addition to the family of vdW layered crystals, which transforms from a direct to an indirect band gap semiconductor as the number of layers is reduced. We investigate this direct-to-indirect bandgap crossover, demonstrate a highly tuneable optical response from the near infrared to the visible spectrum with decreasing layer thickness down to 2 layers, and report quantum dot-like optical emissions distributed over a wide range of energy. Our analysis also indicates that electron and exciton effective masses are weakly dependent on the layer thickness and are significantly smaller than in other vdW crystals. These properties are unprecedented within the large family of vdW crystals and demonstrate the potential of InSe for electronic and photonic technologies.

Light-dependent I–V characteristics of TiO2/CdTe heterojunction solar cells

A new approach was proposed for the analysis of light current?voltage (I?V) characteristics of nonideal heterjunctions in the case of the dominating current transport mechanisms induced by multistep tunnel-recombination processes via interface states. I?V characteristics of the anisotype abrupt heterojunction n-TiO2/p-CdTe were investigated under different light conditions. The values of the series Rs?and shunt Rsh?resistance, as well as those of the coefficients ? and B, which quantitatively describe the dominating tunnel-recombination current transport mechanism, were established to be strongly dependent on light conditions.

Quantum confined acceptors and donors in InSe nanosheets

We report on the radiative recombination of photo-excited carriers bound at native donors and acceptors in exfoliated nanoflakes of nominally undoped rhombohedral γ-polytype InSe. The binding energies of these states are found to increase with the decrease in flake thickness, L. We model their dependence on L using a two-dimensional hydrogenic model for impurities and show that they are strongly sensitive to the position of the impurities within the nanolayer.

Mechanisms of charge transport in anisotype n -TiO 2 / p -CdTe heterojunctions

Surface-barrier anisotype n-TiO2/p-CdTe heterojunctions are fabricated by depositing thin titanium-dioxide films on a freshly cleaved surface of single-crystalline cadmium-telluride wafers by reactive magnetron sputtering. It is established that the electric current through the heterojunctions under investigation is formed by generation-recombination processes in the space-charge region via a deep energy level and tunneling through the potential barrier. The depth and nature of the impurity centers involved in the charge transport are determined.

Electrical properties of anisotype heterojunctions n-CdZnO/p-CdTe

Anisotype surface-barrier n-Cd0.5Zn0.5O/p-CdTe heterojunctions are fabricated by the high-frequency sputtering of a Cd0.5Zn0.5O alloy film onto a freshly cleaved single-crystal CdTe surface. The main electrical properties of the heterojunctions are studied and the dominant mechanisms of charge transport are established, namely, the multistage tunnel-recombination mechanism under forward bias, Frenkel-Pool emission, and tunneling under forward bias. The influence of the surface electrically active states at the heterojunction interface is analyzed and their surface concentration is evaluated: Nss ∼ 1014 cm−2.

Formation of nanostructure on the surface of layered InSe semiconductor caused by oxidation under heating

Variations in the morphology of an InSe surface caused by air oxidation and the influence of the oxide-layered-semiconductor heteroboundary on the photoelectric properties of the In2O3-InSe structure were studied. It is found that an ordered nanosized heteroboundary can form as a result of oxidation of the layered InSe semiconductor and that this heteroboundary determines the high photosensitivity of the In2O3-InSe heterostructure in the spectral region of exciton absorption at room temperature.