Controlled growth of In₂Se₃ low-dimensional structure with ferroelectric properties by chemical vapor deposition

Abstract

Two dimensional (2D) indium selenide hasattracted much attention in ferroelectric switching and piezoelectric nano-generator device because of its promising ferroelectric properties and multiphase structure. In the thesis, we have presented the chemical vapor deposition (CVD) growth of different phases of In2Se3 followed by the systematic study of their morphological, structural and ferroelectric properties.The effect of growth time and Ar flow rate on the shape and size of the depositedflakes is studied. Optical microscopy study revealed that the flake changed from a circular shape to a sharp face triangle as the Ar flow rate and growth time increased. The Raman spectroscopy and high-resolution scanning transmission electron microscopy (HR-STEM) studies revealed that the flakes were of the α and β phases, each of which has a hexagonal crystal structure. Strong second harmonic generation (SHG) is observed from α-In2Se3, demonstrating its non-centrosymmetric structure. Piezo-force microscopic (PFM) study showed the presence of out-of-plane (OOP) ferroelectricity with no In Plane (IP) ferroelectricity in CVD grown α-In2Se3 indicating its vertically confined piezoresponse, which was tuned by the applied electric bias and the flake thickness. The simultaneous growth of β and γ-In2Se3 by changing the substrate distance (temperature) from the precursor in the same CVD system is also achieved. Many existing studies, however, have focused on the growth of α and β phase, while CVD growth of the γ-phase has only been achieved for micron-thick flakes with the assistance of iodine trichloride (ICl3) catalysts. Here, we present the “catalyst-free” CVD growth of β and γ phase In2Se3 flakes with thicknesses down to only a few nanometers. Optical microscope and Raman spectroscopy studies revealed that the β and γ phases grown at lower and higher substrate distance (temperatures) have triangular and hexagonal shapes, respectively. The SHG measurement confirmed that β has a centrosymmetric structure and γ phase has a non-centrosymmetric structure. The ferroelectric properties of the γ-In2Se3 were measured along in-plane (IP) and out-of-plane (OOP) directions, for the first time, by employing PFM. The non-monotonic thickness dependence of the ferroelectricity was found and explained with the combination of Raman and SHG measurements. The shape-controlled growth of α-In2Se3 with OOP ferroelectricity and catalyst-free growth of γ-In2Se3 with IP and OOP ferroelectricity will be a valuable addition in the field of ferroelectric and piezoelectric switching devices.