Investigation On P- And Sb-doped P-type ZnO Thin Films

ZnO is a semiconductor with a direct wide band gap of 3.37 eV at room temperature. The large exciton binding energy of 60 meV paves the way for an intense near-band-edge excitonic emission at room temperature or higher, because this value is 2.3 times of that the room-temperature thermal energy (k_BT=26 meV). The applications of ZnO in optoelectronic devices require both high quality p- and n-type materials. However, ZnO has largely failed to live up to its potential due to its "asymmetric" doping limitation. ZnO intrinsically shows n-type conductivity, and it is difficult to dope the materials p-type. A major obstacle in achieving p-type material is generally thought to be the self-compensation from native donor defects such as V_O and Zn_i or hydrogen as an unintentional extrinsic donor, and the low solubility of the p-type dopants. Therefore, one important issue should be overcome before ZnO could potentially make inroads into the world of optoelectronics devices: the growth of p-type ZnO crystals with good properties. Group-V dopants have been considered as the most possible dopants for p-type ZnO. Recently, several research groups have reported p-type conductivity in ZnO with large-size-mismatched dopants such as P, As and Sb, providing a new way for p-type dopant research of ZnO thin films.In this thesis, based on a comprehensive review of the theories and fabrication of p-type ZnO, we conducted a detailed study of p-type ZnO doping with P and Sb. In addition, based on our early research on P doping, we have fabricated the P doped p-type ZnO thin films by MOCVD, and the effects of P doping source evaporating temperatures on properties of p-type ZnO thin films are discussed in this thesis. An optimal result is achieved at the evaporating temperature of 450℃. The structural, electrical and optical properties of the films are readily presented in this thesis. Guided by XPS analysis and a model for large-sized-mismatched group-V dopant in ZnO, a P_Zn-2V_Zn complex is believed to be the most possible acceptor in the P doped p-type ZnO thin films.On the other hand, Sb-doped p-type ZnO thin films have been realized by PLD. The effect of substrate temperature, oxygen flow on structural, electrical and opticalproperties of the films have been investigated. The Sb-doped films obtained show good crystallinity with (002) orientation, high transmittance (90%) in the visible region, and good optical quality in PL spectra. Hall measurements indicate that p-type conduction can be achieved at the growth temperatures from 500 °C to 600 ℃. Silicon wafer, quartz and glass are employed for substrates to growth ZnO thin films, and it is concluded that silicon wafer is the optional substrate from XRD analysis.Finally, optical quality of P- and Sb-doped p-type ZnO thin films is compared. We find that P-doped thin film shows higher transmittance than Sb-doped film. Compared with free excitonic luminescence peak of 375 ran for intrinsic ZnO, the peak of PL spectra exhibits a small shift in ultra-violet region for P and Sb doped ZnO, which may be attributed to lattice distortion or related to defect. Compared with P doping, Sb-doped ZnO thin films show severe distortion of lattice.