Investigation On The Preparation And Photovoltaic Properties Of ZnMgO Thin Films And ZnMgO/p-Si Heterojunction

In this thesis, we investigated the structural and photovoltaic properties of Zn_1-xMg_xO（x=0.1、0.3、0.5、0.7、0.9）thin films and Zn_0.5Mg_0.5O/p-Si heterostructure prepared by pulsed laser deposition（PLD） method. Based on the fabrication of ZnMgO/p-Si heterojunction, the laser energy dependence of rectifying and photovoltage properties of the heterojunction has been systematically investigated. In the following, the primary achievements in this work are described.（1） Mg-doped ZnO sintered body needs to be incubated at both 500℃ and 1100℃ for 2 hours and sintered at 1300℃ for 5 hours in favor of the volatilization of water vapor and organic components to obtain a homogeneous, dense target.（2） The structural of Zn_1-xMg_xO films were measured by XRD and Raman spectrum, the result reveal that there are two kinds of crystal structure for different Mg content. When the Mg doping amount is between x=0 and x=0.3, the crystal structure of Zn_1-xMg_xO films still maintained a hexagonal wurtzite structure, when x is higher than 0.5, the crystal structure of Zn_1-xMg_xO films changing from hexagonal structure to cubic structure. Along with the increase of the content of Mg, the main diffraction peaks of Zn_1-xMg_xO films shifted to higher angles.（3） The photoluminescence spectra of Zn_1-xMg_xO thin films was mainly compsite of two emission peaks near at 375 nm and 525 nm respectively. As the Mg content increased from 10% to 90%, the ratio of ultraviolet emission peak and visible emission peak R=PU/PV increased firstly and decreased when Mg content is above 50%. When the Mg doping amount was 90%, the ratio had a maximum which is about 8.5.（4） Both transmission and absorption spectra were used to calculate the band gap of Zn_1-xMg_xO thin films, whichever is averaged to reduce the error. The results show that when x= 0.1, 0.3and 0.5, the band gap of Zn_1-xMg_xO thin films is 3.37 eV, 4.29 eV and 5.19 eV respectively, namely when the doping amount of Mg is up to 50%, the band gap of Zn_1-xMg_xO thin films has reached to sun-blind band.（5） The I-V curves exhibit a good rectifying behavior with a rectification ratio（IF/IR） of above 2.5×102 at a 1V bias, indicating the formation of a diode at room temperature.（6） The ideality factor n obtaind from the I-V curve are much higher than the values of an ideal p-n heterojunction. To clarify the much higher ideality factor of the special heterojunction diode, an equivalent diode model was proposed. Based on the comprehension of the metal-semiconductor contact,the In/n-Zn_0.5Mg_0.5O/p-Si/In heterojunction device can be modeled by the actual p-Si/n-ZnO heterojunction diode, the In/p-Si Schottky diode and the negligible In/ZnO contact resistance in the interim bias voltage range. The ideality factor measured externally is the sum of the ideality factors of the individual rectifying junctions, each with different values of the reverse current and ideality factors. The In/Si Schottky diode is reverse-biased and its ideality factor n is much larger than 2（n>>2）, thus, the high ideality factors of the device can be observed.（7）The n-Zn_0.5Mg_0.5O/p-Si heterojunctiion device shows good photoresponse properities in ultraviolet wavelength range at room temperature. As the laser energy decrease from192 mJ to 78 mJ, the heterojunction shows a negative laser energy coefficient of peak photovoltage（ECP） and has maximum peak voltage of 100 mJ at 78 mJ. It is reasonable for the negative ECP due to the thickening of the depletion layer and the reduction of the carrier concentration.