Thickness-dependent carrier transport and optically enhanced transconductance gain in III-VI multilayer InSe

Abstract

Multilayer InSe with a thickness above ∼20 nm, is a direct semiconductor with a peak absorption wavelength approaching λ = 1000 nm, which is a promising candidate for solar-energy conversion and 2D optoelectronics devices. We present herein the experimental observations of thickness-dependent conductivity and photoconductive-responsivity spectrum in multilayer InSe as well as optically enhanced transconductance gain in the multilayer InSe metal-semiconductor-field-effect transistor (MESFET) illuminated by a halogen lamp. The voltage-current (V-I) measurement result shows multilayer InSe belongs to a p-type semiconductor, which can form a p-channel FET device. Thickness (t) dependent conductivity (σ) of multilayer InSe reveals about six-order variation from 5076 (Ω-cm)−1 (t = 5 nm) to 2.56 × 10−3 (Ω-cm)−1 (t = 184 μm, bulk) following a relationship of σ ∝ t−1.38. The highest conductivity in a thin InSe (e.g. t = 5 nm) is due to the increase of carrier density when the thickness is decreased. The photoresponsivity spectrum of a Ag-InSe-Ag multilayer photoconductor demonstrates a prominent peak absorption at 1.1 ∼ 1.3 eV, matches well with the direct-free-exciton energy of the InSe. A multilayer p-InSe MESFET was tested by V-I measurement. The transconductance was measured and determined to be The gm value will enhance about three times when the MESFET was placed under the illumination of a tungsten halogen lamp of a lower power density ∼0.5 mW · cm−2. All the experimental results demonstrate multilayer InSe a promising 2D material available for microelectronics and optoelectronics applications.