Investigation On Mechanism Of Al-N Co-doping P-ZnO And Its' Related Devices Application

In recent years, wide band gap semiconductor materials zinc oxide (ZnO) becomes to attract much more attention than ever before. ZnO is a kind of II-VI 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 has a high melting point (1975℃), high thermal and chemical stability. ZnO single crystal thin film can be obtained at a temperature under 500℃, which is much lower than GaN and other wide band gap semiconductors, so it can greatly reduce the defects formed in high temperature. Furthermore, ZnO is abundant, cheap, innoxious, easy to be prepared and with potential commercial value. As an important candidate of short-wave optoelectronic devices and low-threshold UV laser, ZnO has become a hotspot in the area of semiconductor optoelectronic devices.To realize the application of ZnO in optoelectronic devices, excellent epitaxy n-and p-type thin films are necessary. Based upon that, fabricating the ZnO homoj unction, band-gap engineering through Cd and Mg doping, and finally realized the quatum well and super lattice structure. The formation energy of the intrinsic defects (Zn;and Vo) is very low, so there is a heavy compensation in the p-type doping. At the same time, despite that the shallow acceptor energy in the group V and I, the solution of the group V element is very low. And other group V element, such as P, As, Sb, all has the large lattice mismatch with the substitution for the O atom in ZnO, which can lead to crystallinity distortion and in-plane stress. The group I element can easily form the interstitial state, and act as the shallow compensation donor. Donor (Al)-Acceptor (N) co-doping method is a good way to realize the p-type conduction in ZnO. Our research group, i.e., Professor Ye Zhi-Zhen group of state key laboratory of silicon materials is one of the earliest groups engaging in ZnO film studies. We first try and realize the co-doping p-type ZnO internationally. The co-doping method can increase the solution of the acceptor in fixed condition, so it can help to obtain the high hole concentration p-ZnO thin films.In this work, we've fabricated the p-type ZnO thin films with good electrical properties by the Al-N co-doping method, and we mainly focused on the p-type doping mechanism. Based on the deep investigation on the growth mechanism of C-orientation polycrystalline ZnO films, we've proposed the comparison of different acceptor dopants in p-ZnO doping, and analysis several important factor of affecting the p-type conduction. Based upon that, Si/ZnO heterojunction, ZnO-based pn junction, ZnO/ZnCdO heterojunction and ZnO/Au Schottky Barrier Diode have been successfully fabricated, and taken the fundamental research.The formation of the C-axis orientation ZnO thin films is a self-assembled process, and the preferential orientation can be obtained in many different kinds of substrates. It should be mentioned that various substrates can determine diverse formation mechanism. When grown on Si, it will preferential nucleate firstly;while in the case of glass, an amorphous ZnO layer initially forms. So, the crystal quality and electrical properties of the obtained ZnO thin films will depend on the substrate.We have fabricated the stable and reproducible p-ZnO based on the Al-N co-doping method using the DC Reactive Magnetron Sputtering. The as-deposited p-type ZnO thin films show an electrical property, such as resistivity of 20-30Q-cm, hole concentration of 1017-1018cm"3, and hall mobility of 0.5-1 cm2/V-s<= A moderate Al content is necessary for the p-type ZnO realization, instead of the higher or lower content both leading to n-type conduction. Due to the big size mismatch, in the suitable condition other group V elements are inclined to substitute the Zn place and at the same time induce two Zn vacancies, instead of the O place for the good p-type behavior. The active group I elements, such as Li and Na, can form the interstitial states, which always act as the shallow donors, compensate the hole concentration, and finally destroy the p-type properties. The III-V co-doping method can greatly promote the N solution in the ZnO thin films, and Al-N co-doping way is the best candidate.Substrate temperature (Ts) can provide the dynamic force of the nucleation, reaction and motivation during the growth process, responsible for the crystal quality and electrical properties. Higher or lower Ts are both unfavorable for the good as-received ZnO thin films. P-type ZnO can be realized in the range of 380°C-480°Cand 560°C-600°C, while n-type in others. We proposed the H passivationAl promotion combined activation model to explain the conduction type variation of n-p-n-p of Ts. The presentation of donor Al can sufficiently enhance the incorporation of N into the ZnO films. Using NH3 as the N source, the ZnO films show p-type when the oxygen partial pressure is 40% and 85%. In the instance of NO and NO2, all the ZnO thin films show n-type. The post annealing treatment under the O2 ambient can greatly improve the p-type properties, while the growth of high temperature buffer layer can obviously enhance the stability and reproducibility of the p-ZnO.Compared with the ZnO homoj unction, the double heteroj unction with n-ZnO/n-ZnCdO/p-ZnO exhibits a better rectifying characteristics, and reverse breakdown voltage can reach as high as 15V, and leakage current 10"6A. The n-ZnO/Au show a good Schottky contact property, while the barrier height is low and ideal factor is relatively high, due to the intrinsic defects and the surface states in the interface.