Growth and application of WSe2 single crystal synthesized by DVT in thin film hetero-junction photodetector

Abstract

Tungsten di-selenide (WSe2) belonging to the family of layered transition metal di-chalcogenides (TMDCs) is at present widely used in optoelectronic devices due to their adequate energy band gap suitable for photosensing applications. In the present investigation, WSe2 single crystals are grown by direct vapor transport (DVT) technique in a dual zone horizontal furnace maintaining a temperature difference of 50 K between source zone (SZ) and growth zone (GZ). The crystals thus obtained were thin, shiny and with an average thickness of 30 mm. The surface topography of crystals studied by optical microscope revealed hexagonal spirals on the crystal surface which seemed to arise due to screw dislocation defect. The structural properties of the as grown crystals studied by powder X-ray diffraction (XRD) elucidated good crystallinity, hexagonal structure and confirmed the orientation of crystals along crystallographic c-axis. P-type semiconductor nature of the crystals was affirmed by thermoelectric power (TEP) measurement. Optical and vibrational properties of the grown crystals were studied by UV–Visible, photoluminescence (PL) and Raman spectroscopy. An optical direct band gap of 1.41 eV was determined for the crystals which were micromechanically exfoliated upto few layers. The excitonic mechanism of the grown WSe2 crystal was explained by PL spectroscopy. The results of Raman spectroscopy disclosed A1g and E2g vibrational modes present in the crystals. The current–voltage characteristics of nSnSe/pWSe2 hetero-structure studied using Keithley 2400 SMU showed rectification behavior at low bias voltage in dark and illuminated conditions. The diode parameters like ideality factor and barrier potential were determined to be 2.54 and 0.4 eV, respectively, by following conventional lnI–V method. The photodetection properties of the fabricated device were studied using a Laser source (670 nm) having an intensity of 3 mWcm−2at different bias voltage ranging from 0.3 V to 3 V. Parameters such as responsivity, detectivity and external quantum efficiency (EQE) were calculated to substantiate the excellent detection properties shown by the prepared photodetector using time resolved pulsed photoresponse. Value of responsivity and EQE increased from 42.22 mA W−1 to 533.77 mA W−1 and 7% to 95%, respectively, with increase in bias voltage from 0.3 V to 3 V. Also, detectivity values were found to be of the order of 107 Jones.