Band engineering for high-performance InSe and BP photodetectors

Abstract

Recently, as communication, autonomous driving, biometric security technology, and autonomous driving technology increased, the demand for IR sensors has also increased. In particular, near IR (NIR) has been given attention because of its properties, such as reflecting from objects and less scattering in air, which are suitable for high-speed real-time imaging sensors. Most commercialized NIR sensors based on Si and InGaAs still show moderate performance. However, Si, and InGaAs sensors have encountered problems such as expensive processing, poor flexibility, defects by lattice mismatch, limited pixel size, and high thermal noise. Furthermore, 2D semiconductors are good replacement candidates for Si and InGaAs in optoelectric applications. 2D semiconductors improve defects by lattice mismatch because they contact other materials by van der Waals bonds. In addition, 2D semiconductors have high flexibility and low thermal noise. The 2D InSe has high electrical performance and a direct band gap of 1.26 eV for high NIR detection. However, a few reports show that InSe photodetectors sense IR light, but their performance is limited. In this thesis, we introduce a method for the fabrication of high-performance InSe IR and discuss its performance and driving mechanism. In Chapter 2, high-performance visible to NIR (470−980 nm wavelength (λ)) photodetectors using surface-doped InSe as a channel and few-layer graphenes (FLG) as electrodes are reported, in which the InSe top region is relatively p-doped using AuCl3. The surface-doped InSe photodetectors show outstanding performance, achieving a photoresponsivity (R) of ~19,300 A W-1 and a detectivity (D*) of ~3 × 1013 Jones at λ = 470 nm, and R of ~7,870 A W-1 and D* of ~1.5 × 1013 Jones at λ = 980 nm, which is superior to previously reported 2D material-based IR photodetectors operating without an applied gate bias. Surface doping using AuCl3 renders a band bending at the junction between the InSe surface and the top FLG contact, which facilitates electron-hole pair separation and immediate photodetection. Multiple doped or undoped InSe photodetectors with different device structures were investigated, providing insight into the photodetection mechanism and optimizing performance. Encapsulation with a hexagonal boron nitride dielectric also allows for three-month stability. However, when a low or zero drain bias applied at AuCl3-doped InSe photodetector, the low efficient photodetection is observed by bidirectional built-in potential in InSe. To fabricate a high-performance photodetector with low-power consumption, the built-in potential in InSe must align in one direction. In Chapter 3, we show state-of-the-art self-powered NIR sensors using graphene/In/InSe/Au as a photoactive region. The self-powered NIR sensors show outstanding performance, achieving a photoresponsivity of ~8.5 A W-1 and a detectivity of ~1012 Jones at 850 nm light. Multiple self-powered InSe photodetectors with different device structures and contacts were systematically investigated. In particular, the asymmetrically assembled graphene/In/InSe/Au vertical heterostructure offers a high built-in field, which gives rise to efficient electron-hole pair separation and transit time that is shorter than the photocarrier lifetime. The built-in potential across the InSe was estimated using the Schottky barrier height at each metal contact with InSe, obtained using density functional theory calculations. We also demonstrate the InSe vertical field effect transistors and provide out-of-plane carrier mobility for InSe. Using the out-of-plane mobility and structural parameters of each device, the built-in field, drift velocity, and corresponding transit time were estimated. Although the self-powered NIR sensors with graphene/In/InSe/Au heterostructure show outstanding performance, the NIR sensor fails to take advantage of vertical transport. Out-of-plane mobility of InSe is much lower than in-plane mobility due to the layered structure causing the van der Waals gap in the vertical direction. In Chapter 4, we introduce a vertical photodetector based on black phosphorus (BP), which has better electrical properties than InSe and high out-of-plane mobility comparable to in-plane mobility. The out-of-plane mobility of BP reaches ~300 cm2/Vs, which is comparable to in-plane mobility. The vertical BP photodetector with In/BP/Pt, In/BP/Au, and FLG/BP/Pt show a high dark current, which is not able to detect the photocurrent because of the small band gap of BP causing a low energy barrier. An effective way to suppress the dark current is required to realize a high-performance BP photodetector with vertical transport.