|Title:||Fabrication and device characterization of wafer-scale two-dimensional semiconductors by pulsed laser deposition|
|Advisors:||Hao, Jianhua (AP)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Applied Physics|
|Pages:||xxi, 158 pages : color illustrations|
|Abstract:||The emergence of two-dimensional (2D) layered materials offers a new platform for investigating the physics of nano-systems and developing novel industrial devices with amazing performance. With one dimension approaches atomic scale, the properties of this ultrathin layer exhibit largely distinct from their bulks. Since discovery of graphene in 2004, the family of 2D materials has rapidly expanded to a number of graphene-like materials. Among them, black phosphorus (BP) and indium selenide (InSe) are semiconductors with promising electrical conductivity and tunable band gaps, which make up the deficiency of graphene for logic device applications. However, the large area fabrication, as the pre-requisite for practical applications, has not been achieved for both these two materials up to now. In this thesis, firstly, an unexplored 2D form of BP, namely amorphous BP (a-BP) were grown on different substrates by pulsed laser deposition (PLD), with the film thickness ranging from 2 to 10 nm and in-plane size beyond 10 mm2. In contrast to crystalline BP, a-BP is defined as highly disordered phase with atoms arranged in BP framework. Although the processing conditions of BP crystal is tough, the wafer-scale a-BP film is obtained at a relatively low temperature of 150 °C. The thickness dependent band gap of a-BP films is observed from the optical analysis. By fabricating into field-effect transistors (FETs), p-type semiconductive feature with large tunable mobility are achieved. In addition, because of the low synthesis temperature, a-BP films can be directly grown on polymer substrates for fabricating into flexible devices. The performance of the devices is barely influenced by the uniaxial strain. Secondly, photocarrier dynamics of the ultrathin a-BP films with different thickness are studied. By characterizing with resolved pump-probe system, the 2 nm thick a-BP film exhibits the excitons diffusion coefficient of 5 cm²sˉ¹, which is much larger than that of amorphous silicon. Moreover, the diffusion length of exciton attains 450 nm with consideration for a long lifetime of 400 ps. Thirdly, high quality layered InSe nanosheets with large area are deposited by PLD. The crystal structure and chemical composition are investigated by various techniques. A large tunable band gap across from 1.26 eV to 2.20 eV is shown by optical measurements. Besides, the FETs and phototransistors based on InSe films are fabricated and characterized, revealing high electron mobility and broadband photodetection ability from UV to NIR. In conclusion, wafer-scale synthesis of high quality 2D BP and InSe has been achieved by PLD. The electrical and optical properties and device demonstrations based on these two materials have been investigated. These studies lead us to realize fruitful electronic and optoelectronics applications with outstanding performance.|
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