Investigation On Li Doped P-type Zn_1-xMg_xO Films, ZnO And ZnMgO Nanostructures

ZnO is a kind ofâ…¡-â…¥compound semiconductor with a wide direct band gap of 3.37 eV at room temperature (RT) and a hexagonal wurtzite structure. Its high exciton binding energy (60 meV at RT), which is much higher than RT heat energy (26 meV), will theoretically favor efficient UV excitonic emission processes at RT. In addition, ZnO is abundant, cheap, innoxious, easy to be prepared and with potential commercial value. Thus, ZnO has been considered as a promising material for optoelectronic devices such as UV light- emitting diodes (LEDs), Laser diodes (LDs) and photodetectors. It is well known that the band gap of ZnO can be modulated from 3.3 eV to 4.3 eV by alloying different concentrations of MgO and the Zn_1-xMg_xO alloy still maintains the wurtzite structure. Moreover, the ionic radius of Zn²⁺(0.74 (?)) is close to that of Mg²⁺ (0.72(?)), therefore, the substitution of Zn²⁺ by Mg²⁺ does not induce a significant change in lattice constant. ZnMgO is a suitable material for ZnO/ZnMgO superlattices and quantum wells. These ZnMgO/ZnO heterosystem can improve the emission efficiency of devices and modulate the working waveband. However, in order to realize such optoelectronic applications based on ZnO/ ZnMgO heterostructures, one of the critical issues is to achieve stable p-type ZnMgO. Recently, several works have been reported on p-type ZnMgO films. They all focused on p-type doping ZnMgO with group V elements, whereas few reports have considered groupâ… elements on substitutional Zn or Mg sites. Compared with other groupâ… and groupâ…¤dopants, Li is considered as a relative shallow acceptor, and for Li_Zn, almost no lattice relaxations occurs around the impurity atom. Therefor, in this paper, we choose Li as a dopant to realize the p-type ZnMgO thin films using pulsed laser deposition (PLD).With reduction in size, nanostructures ZnO materials exhibits more excellent performance (such as higher conductance, transparency and electronic quantum transport) compared with the bulk counterparts, ZnO nanostructures have promising potentials in nanosized gas sensors, transducers, and field emitters etc. Thus, the syntheses of ZnO nanomaterials have become a new hotspot. Many kinds of ZnO nanostructures such as nanowires, nanotubes, nanobelts and nanorings have been obtained so far. Moreover, the band-gap of ZnO can be modulated by doping with Mg for further applications, which makes it possible for fabricating ZnO/Zn_1-xMg_xO nano-heterostructures. So ZnO and Zn_1-xMg_xO nanostructures have promising potential applications in nano-optoelectronics and nano- electronics devices.According to the hotspot and difficult questions existed in the study of ZnMgO thin films and ZnO nanostructures, p-type Li doped and Li-N codoped Zn_1-xMg_xO thin films have been realized by PLD. ZnO and ZnMgO one-dimensional nanostructures were also successfully fabricated using thermal evaporation method. The main content of this thesis is as follows:[1] We have grown Li doped p-type ZnMgO films on glass substrates with different Mg content (11-28 at. %) by pulsed laser deposition. Hall measurements suggest that the resistivity increases with Mg concentration. Acceptor levels related to Li_Zn located at about 150 meV and 174 meV above the valence band maximum was discriminated in photoluminescence spectra for Li doped Zn_0.89Mg_0.11O and Zn_0.72Mg_0.28O films, respectively. The conversion of donor-acceptor pair (DAP) to a free-to-neutral-acceptor (e, A⁰) transition was also observed in Zn_0.89Mg_0.11O: Li film. The optical band gap and the acceptor binding energy increase with an increase of Mg content in the films, which leads to a reduction in the hole concentration and an increase in the resistivity.[2] Li doped p-type Zn_0.95Mg_0.05O thin films have been achieved on glass substrates by pulsed laser deposition. The results of the Hall measurements indicate that p-type conduction in Li doped Zn_0.95Mg_0.05O films is strongly dependent on the oxygen pressure. Increasing oxygen pressure from 5 Pa to 25 Pa, the Li concentration in Zn_0.95Mg_0.05O firstly increases and then decreases. SIMS result suggests that a high Li acceptor concentration in the films grown at an oxygen pressure of 15 Pa and 20 Pa is responsible for the definitive p-type conductivity in these films.[3] p-type Zn_1-xMg_xO thin films were realized via monodoping Li by analyzing the influence of experimental parameters (such as the glass substrate temperatures, the laser pulse energy, the distance between target and substrate and the Li content in the targets) on the films. The optimal growth conditions are achieved. Stable and reproduceable p-type Zn_1-xMg_xO thin films have been obtained under optimized conditions. [4] Li-N dual-acceptor codoped p-type Zn_0.89Mg_0.11O films have been deposited on glass substrates by pulsed laser deposition under an ionized N₂O ambient. Hall measurements reveal that the films grown under a moderate N₂O pressure (23 Pa) have the lowest room-temperature resistivity of 133Ω·cm, with a hole concentration of 3.62×10¹⁶ cm^-3 and a Hall mobility of 1.3 cm²/Vs. The enhancement of Li and N incorporations was responsible for the good p-type conduction of this film, which was demonstrated by secondary ion mass spectroscopy. Zn_0.89Mg_0.11O/ZnO p-n heterojunction was fabricated. The rectifying current-voltage curve confirmed that Li-N dual-acceptor codoping is a promising method for p-type doping of ZnO.[5] Rose-like and sisal-shaped ZnO nanostructures have been synthesized by thermal evaporation method. Zinc acetate dihydrate powders were used as one of the source materials, which is deemed as the main reason for the formation of flower-like ZnO structures. The field-emission properties indicate that the sisal-shaped ZnO structures have better field emission properties than the rose-like structures. Our experiment results suggest that sisal-shaped ZnO structures are promising materials for applications in a flat panel display and brightness electron source.[6] ZnO nanorods and nanonails have been synthesized on silicon wafers by evaporating Zn powders. All the samples are hexagonal phase ZnO with highly c-axis preferential orientation. The analysis results demonstrated that the caps of nanonails possess a large number of oxygen vacancies, which may play a key role in determining the formation of nanonails and the high intensity of green emission.[7] ZnO/cubic ZnMgO coaxial heterostructure nanorods with wire-shaped tips have been synthesized via a three-step catalyst-free thermal evaporation process on the Si (111) substrates. The results of the measurements demonstrate that the nanorod with a cubic-phased tip consists of a wurtzite ZnO core surrounded by a cubic ZnMgO shell. We suggest that the growth temperature and the process control are responsible for the formation of the ZnO/ZnMgO heterostructure nanorods.