A study on MSM ultraviolet photodetectors using micro-spherical Zinc-Tin-Oxide based transferable monolayer

Abstract

Over the past several decades, ultraviolet (UV) photodetectors have attracted much attention in a variety of applications, including missile launch and corona detection, space communications, military applications, biological research, and various sensors. During the last decade, research has been actively conducted on ultraviolet photodetectors using nano- and microstructure-based semiconductors which have high surface area to volume ratios. Typically, metal oxides having not only excellent photo-detection performance but also chemical stability and reliability based photodetectors have been reported largely. Among them, zinc-tin-oxide (ZTO) has attracted a great deal of attention in recent years due to its high electron mobility, electrical conductivity and outstanding optical properties as well as the characteristics mentioned above. ZTO is a ternary compound material having two bonding structures, ZnSnO3 and Zn2SnO4. ZTO based photodetectors reported in abundance have mostly used wires, cubes, and particles without a regular structure. In the case of wires, the most of investigated photodetectors using ZTO used single nanowire which cause the small amount of current such as pico-scale and several nano-amperes due to tiny effective area from a single nanowire, make it difficult to apply to the actual device. Therefore, the large area devices using nanomaterials which can be used in practical device level were continuously required. Moreover, among the various morphologies of nanomaterials, a spherical structure has many advantages to be used in optoelectronic devices. Although it can increase the optical trap state of the surface and prolong the life of the photo-carrier due to surface-to-volume ratio and high effective-area coverage ratio, there are still little reports. In this study, I synthesized amorphous ZnSnO3 (amor-ZTO) and crystalline Zn2SnO4 (crys-ZTO) with inverse spinel structure into microspheres of various sizes and used them to form a transferable monolayer. Then, metal-semiconductor-metal photodetectors were fabricated and its electrical and optical properties were evaluated. The amor-ZTO and crys-ZTO were formed by heat treatment of ZnSn (OH)6 (ZTHO) prepared by ethanol precipitation method which can be synthesized with high speed. The amor-ZTO and crys-ZTO based monolayers were formed on the polydimethylsiloxane (PDMS) template by unidirectional rubbing method. Subsequently, the ZTO monolayer formed on PDMS easily transferred to double polished Al2O3 (0001) substrate via poly(vinyl alcohol) assisted transfer process. X-ray diffraction and Raman spectroscopy were measured and analyzed to confirm the changes in crystal characteristics and phase of synthesized ZTHO, amor-ZTO and crys-ZTO microspheres. In addition, the current-voltage and photo-response characteristics of photodetectors fabricated Metal-Semiconductor-Metal (MSM) photodetectors using ZTO microsphere based monolayer as building blocks were measured and identify the cause of the measurement results through the presence of an apparent heterojunction inside of crys-ZTO microsphere. The ZTO-based photodetector fabricated by the crys-ZTO device showed a large difference according to the coupling structure. The photodetector based on the crys-ZTO produced about 77 times the photocurrent as compared with the amor-ZTO under 254nm UV irradiation with 5 V forward bias. Simultaneously, the external quantum efficiency of the ZTO device was about 16.7% (320 nm) for amor-ZTO and about 580.9% (310 nm) for crys-ZTO based photodetector. In addition, I investigated the change of photo-detection performance according to the diameter of crys-ZTO spheres with relatively high performance compare to amor-ZTO microspheres by synthesizing crys-ZTO microspheres in various diameters. As a result, it showed the highest photo-detection performance in the 1.24 μm diameter spheres, which could be explained from XPS analysis by defects such as oxygen vacancy and surface hydroxide at the crys-ZTO microspheres. The newly proposed metal oxide nano-spheres synthesis method and transferable monolayer formation through this paper can be applied to the new structure of optoelectronic devices.