Piezo-phototronic effect in 2D III-VI compound based heterostructures for optoelectronic device applications

Abstract

The piezo-phototronic effect is known as a three-way coupling among photoexcitation, piezoelectric and semiconducting properties, which attracts significant attention owing to the potential modulation to the generation, transportation, separation and recombination of the photo-excited charge carriers near the metal-semiconductor Schottky contact and p-n junction by the strain-induced piezo-potential. Comparing to the rest of heterojunctions, p-n junction has been broadly investigated as it plays a vital role for numerous modern commercial optoelectronics through the beneficial characteristic combination of each material to enhance the device performance. In contrast to one-dimensional (1D) and three-dimensional (3D) materials, two-dimensional (2D) materials present various advantages involving distinctive and simple crystal morphology, adjustable energy band gaps and the absence of traditional consideration of lattice mismatch during contact, beneficial for building up van der Waals (vdW) heterostructures in multiple applications. Besides, photoelectric conversion becomes more efficient in 2D crystals as the recombination of carriers has been largely suppressed with a reduction of the interface defects. Although p-n junction based on 2D materials have remarkable ductility and mechanical behaviours, making them suitable for flexible applications in a wide range, including drug delivery, wearable communication, and sensors, most of the research studied in recent years focuses on fabricated devices on hard substrate. Due to the restriction of transferring procedures and extra complex fabrication process, little work has been done on the strain-modulation performance of low-dimensional based flexible p-n junction photodetectors. Indium Selenide (In2Se3) owns high electrical mobility, excellent optical sensitivity and piezoelectric properties, which is highly promising as a piezophototronic material for the construction of flexible p-n heterojunction photodetector with p-type 2D TMDs. In2Se3 is known as a group III-VI multi-phase chalcogenide compound showing numerous benefits and has attracted extensive research interest in practical applications such as optoelectronic and photovoltaic devices. Among all existing phases, n-type α-In2Se3 has a moderate optical band gap of 1.4eV in bulk state and increase with decreasing thickness in 2D limit, demonstrating notable advances in photodetection prospective for a wide wavelength range. Also, on account of fast response time, efficient light absorption, outstanding photoresponse and high on/off ratio, α-In2Se3 is considered as an ideal material for heterostructure photodetectors. This thesis presents the fabrication of flexible α-In2Se3/WSe2 vdW heterostructure photodetector with a favourable structure design aiming to better optoelectronic behaviour by the piezo-phototronic effect. When the external mechanical deformation is applied to the flexible substrate, piezoelectric charges are generated at the In2Se3 side. The band slope near the p-n junction interface can be modulated by the charge induced piezo-potential. Also, the efficiency of photo-generated electron-hole pairs separation and transport can be further enhanced, which leads to an increase in photocurrent. The output current under 0.433% strain and 782 μW/cm2 optical intensity can achieve 304 times larger than strain-free dark conditions. Also, the responsivity and detectivity can reach up to 4.61×105 A/W and 4.34×1014 Jones respectively. These results introduce a scheme in which flexible photodetectors' behaviour based on 2D materials can be improved by applying moderate external mechanical deformation. Furthermore, the novel 2D material based vdW heterostructure design can be inspired and extended to other synthetic optoelectronic applications.