Electrical properties and device applications of atomic layer deposited ZnO and GaN thin films
Okyay, Ali Kemal
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Zinc oxide (ZnO), a semiconducting material with a wide band gap of 3.37 eV, has become a promising material for wide range of electronic and optoelectronic applications. One of the most important properties of this material is its large exciton binding energy of 60 meV, which makes ZnO a strong candidate for ultraviolet light emitting diodes and lasers. In addition, potentially high electron mobility and the transparency in the visible region strengthen the future of the ZnO based transparent electronics. Although several applications of ZnO have taken their places in the literature, use of ZnO in the thermal imaging applications is yet to be explored. In the parts of this thesis related to ZnO, the temperature coefficient of resistance and electrical noise together with resistivity and contact resistance properties of atomic layer deposition based ZnO are investigated. Due to its remarkably high temperature coefficient of resistance value and suitable 1/f noise corner frequency, this material is proposed as an alternative material to be used in the active layers of uncooled microbolometers. GaN is another wide gap semiconductor which has been intensely investigated throughout the last decades for its potential usage in both optical and electrical applications. Especially, high saturation velocity of the electric carriers of this material has made it a strong candidate to be used in high power applications. Furthermore the high electron mobility transistors based on the 2-dimensional electron gas region formed between the AlGaN and GaN, have found wide range of applications in radio frequency (RF) electronics area. Currently, most commonly used techniques for growing GaN, are metal organic chemical vapor deposition and molecular beam epitaxy. Both of these techniques offer single crystalline layers; however, the process temperatures used in the growth of the GaN disable the use of this material in low temperature flexible electronic/optoelectronic applications. In order to solve this problem, hollow cathode plasma assisted atomic layer deposition technique is utilized and GaN thin films with polycrystalline structures are successfully grown at 200°C. In the parts of this thesis related to GaN, the electrical properties, the effect of contact annealing on the resistivity of the GaN thin films and the contact resistance between this material and Ti/Au metallization scheme are investigated. Afterwards, we present the world’s first thin film transistor with atomic layer deposition based GaN channel and discuss its electrical characteristics in detail. Finally, the GaN thin film transistors are fabricated by performing all fabrication steps at temperatures below 250°C. This is the lowest process thermal budget for the GaN based thin film transistors reported so far. Electrical characteristics as well as the stability of the proposed device are investigated and the results obtained are discussed. Proposed devices are believed to pave the way for the GaN-based stable flexible/transparent electronics after further materials and process optimization.