Growth Of P-ZnO Films And Study Of Junction Characteristics

ZnO is a promising wide direct-gap II-VI semiconductor, has remarkable optical and electrical characteristics. Its exciton bound energy is about 60meV at room temperature, it has high efficiency on stimulating emission at room temperature;it is a kind of materials that can be used in blue light-emitting devices and UV light-emitting devices at room temperature or higher temperature. ZnO thin films have good piezoelectric constant and good electromechanical coupling coefficient. Moreover, it can also be used in transparent electrodes,UV detectors,thin film transistors,surface acoustic wave devices, etc. Since the ZnO films'UV lasing property is reported for the first time in 1996, the study of ZnO has achieved major breakthrough. Currently, the studies of ZnO thin films focus on the application of optoelectronic devices.There are lots of methods have been used to deposit ZnO films now,such as pulsed laser deposition(PLD),molecule beam epitaxy(MBE),metal organic chemical vapor deposition(MOCVD),atomic layer epitaxy(ALE),sputtering,plasma enhanced chemical vapor deposition(PECVD),e-beam evaporation,sol-gel,spray pyrolysis. In this paper, we grow high quality ZnO films by MOCVD. DEZn and O2 were used as Zinc and Oxygen sources. The characteristics of ZnO films were measured by XRD,XPS,SEM,PL and Hall.Undoped ZnO films were always n-type,many researchers suggested that it was because of the inherent defects. D.C. Look believed that the dominant donor was zinc interstitial;W.Gregory suggested that oxygen vacancy caused n-type conductivity;Van de Valle suggested that Hydrogen acted as donor in ZnO films. However, no matter which model is dominant in ZnO films, the self-compensation is the key point to prevent the formation of p-type ZnO films.At present, the mainly method to obtain p-type ZnO films is doping with different dopants, such as GroupⅠelements and GroupⅤelements. The GroupⅤelements(N,P,As) is the most commonly used dopants. In this paper, we choose arsenic (As) as dopant for growing p-type ZnO films. Some people believe that Aso defect is acceptor in p-type ZnO films. However, the recent reports indicate that As dose not replace O. It occupies the position of zinc and combines with two zinc vacancies to form AsZn-2VZn acceptor complex. This complex model has lower formation energy than any of parent defects. Its ionization energy is about 0.15eV. In our experiments we use GaAs as source of arsenic.Firstly, we selected semi-insulating GaAs wafer as substrates, and then deposited ZnO films on GaAs substrates by MOCVD, the thickness was about 500nm. We prepared two different ZnO samples, the first one was deposited on the GaAs substrate at 650 oC(sample one), the second one was annealed at 680 oC under the protection of nitrogen(sample two). We characterized the electrical properties of samples by Hall measurement, the results showed that: sample one was n-type with electron concentration of 3.5×1019cm-3 and electron mobility of 20cm2/Vs;sample two is p-type with hole concentration of 1.2×1017cm-3 and hole mobility of 1.8 cm2/Vs. We also characterized the samples by temperature dependent PL measurement, and the results showed that: the donor-related emission peak(D0X) was at 3.367eV in the PL spectrum of sample one;emission peaks related to A0X and FA were found at 3.356eV and 3.296eV in the PL spectrum of sample two. We also calculated the arsenic-related acceptor level,it located at about 140meV above the maximum of the ZnO valence band.Secondly, based on the ideas above, we found an economic method to deposit p-type ZnO films. In this new method, the GaAs films were deposited on sapphire substrates by magnetron sputtering, then ZnO films were deposited by MOCVD. Electrical properties of ZnO films were characterized by Hall measurement, the results showed that: ZnO films were p-type with hole concentration of 2.18×1017cm-3 and hole mobility of 1.28 cm2/Vs. We also characterized ZnO films by XRD,XPS and PL. The result of the X-ray diffraction showed a strong peak around 2θ=34.4o, which suggested C-axis preferred orientation, XPS result showed that As atoms had already diffused into ZnO films, PL spectra indicated that the optical properties of the ZnO films degrade with increasing the GaAs thickness.Au was deposited on ZnO film by thermal evaporation process. Ohmic contact can be formed between Au and p-ZnO film. N-type silicon was selected as substrate, 8-nm-thick GaAs film was deposited by magnetron sputtering. The structure of the heterojunction was Au/ZnO/GaAs/n-Si/Al. And it showed an obvious rectifying behavior. The threshold voltage was about 4V. For another, gold-zinc alloy layer was evaporated as electrodes and ohmic contact could be formed between Au-Zn alloy layer and n-ZnO film. ITO was used as substrate and 8-nm-thick GaAs layer was deposited. And we fabricated the homojunction with the structure of AuZn/n-ZnO/p-ZnO/ GaAs/ITO. It showed an obvious rectifying behavior with the turn-on voltage of 1.8V under the forward bias voltage.