Carrier Transport Mechanisms Of N-ZnO/ZnMgO/p-GaN And Terahertz Transmission Properties Of VO₂Thin Films

Many metal oxides with excellent physical and chemical properties are widely applied in many fields, such as photoelectric conversion, microelectronic devices. Zinc oxide (ZnO) and vanadium dioxide (VO2) have been considered as more promising functional materials due to excellent photoelectrical properties and unique metal-insulator phase transition (MIT) characteristic. Recently, ZnO has been regarded as a new research hotspot in the field of short-wavelength optoelectronics because it has many advantages, such as wider bandgap semiconductor (3.37eV) and larger exciton binding energy (60meV), e.g. But the realization of stable and reproducible p-type ZnO has long been the bottle-neck of ZnO-base optoelectronic devices. To achieve practical applications of ZnO light emitting devices, The researchers have prepared ZnO-base heteroj unction devices (such as, n-ZnO/p-GaN). However, in most cases, the glowing effect is not very good in n-ZnO layer, while the electroluminescence originate dominantly from the GaN layer because electron can be injected into p-GaN layer more easily than hole be injected into n-ZnO layer. In order to solve this dilemma, electron-blocking layer (EBL, such as, ZnMgO, A1N, MgO) are generally introduced to achieve better light inside ZnO layer. ZnMgO is more suitable for n-ZnO/p-GaN as EBL due to it has similar rystal structure and smaller lattice mismatch with ZnO. In addition, among various vanadium oxides, the VO2thin films, which has became the research hotspot in these fields of phase transition caused by hot and electricity because phase transition temperaturecan is the most close to room temperature (68℃) and can be integrated with microelectronics technology. This peculiar transition characteristic lead to crystal phase, electrical, optical and magnetic properties of VO2exhibit abrupt changes. Therefore, VO2films has great application prospects in these fields of transition storage, IR intelligent window and terahertz (THz) switch, e.g. The main results in this thesis as follows:(1) The heteroj unction diodes with n-ZnO/p-GaN and n-ZnO/ZnMgO/p-GaN structures have been deposited on the p-GaN by ultrasonic spray pyrolysis (USP) technique. The carrier transport mechanism of n-ZnO/ZnMgO/p-GaN heteroj unction was systemically investigated by temperature-dependent current-voltage measurements. The results revealed that the multistep tunneling is the dominant carrier transport mechanism in n-ZnO/ZnMgO/p-GaN heteroj unction under forward current. This means that this structure not only restraints electrons in the n-ZnO layer but also keeps the holes flowing from GaN into ZnO, and increases the carrier recombination rate so that achieves injection luminescence in the ZnO layer.(2) VO2films were grown on c-and m-plane sapphire substrates by pulsed laser deposition (PLD) technique. The VO2films with preferred growth orientation and uniform dense distribution have been achieved on both substrates, as confirmed by X-ray diffraction (XRD) and scanning electron (SEM). The THz transmission properties of VO2thin films were studied by terahertz time-domain spectroscopy (THz-TDS). The results indicate that the THz transmission properties of VO2films are strongly influenced by the sapphire substrate orientation.