Electrical Transport Property Of Ⅱ-Ⅵ Group CdX (X=S, Se, Te) Under High Pressure

With the development of science and technology, more and more measurement can be performed in diamond anvil cells (DACs), such as X-ray diffraction, Raman scattering, Brillouin scattering et can all be in-situ measured at high pressure and high temperature. These increasingly mature techniques have led to a great improvement in the research of high pressure physics. However, the development of high pressure electrical measurement is slow and it focuses on the DC (direct current) measurement. The high pressure AC (alternate current) impedance spectroscopy (IS) measurement technology is still a blank. Although there are some reports about the high pressure IS measurement performed in high pressure apparatus with big sample chamber, the pressure and temperature can be realized is restricted, usually not beyond 10GPa and 1500K. In this thesis, for solving above problems, using the mature film deposition and photolithograph techniques, we introduce IS measurement technique into DACs, set up a high pressure IS measurement system, and realize high pressure ACIS measurement. By this technology innovation, the electrical transport behavior is investigated systematically at high pressure on the samples of CdS, CdSe and CdTe.Using thin film sputtering and photolithograph techniques we adopt integration method and directly integrate the measurement circuit on the anvil of DAC. We substitute film electrode for bulk electrode like metal foil or lead and check the contact condition using the V-I (voltage-ampere) test all-the-time. The results indicate that the electrode test remains linear. This proves that the electrode was stable at high pressure and has no effect on the IS measurement. In addition, by open and short circuit tests, we supply the parameters of parasitic impedance of our system. The results indicate that the inductance from leads appears obvious only at the frequency high than 104Hz, and is only 60ohm at the highest frequency. So when the sample to be measured has a low conductance, the leads inductance can be neglected. But when the conductance is higher than several hundreds ohm-1, it must be concerned. The parasitic impedance from sample chamber is 100kohm at frequency low than 1MHz. For the sample with high conductance, it can be neglected. For the sample with low conductance or in high frequency region, the parasitic impedance can be eliminated by a parallel capacitance. So for different matters measured, we adopt corresponding method, and the effect of measurement system on the impedance measurement is excluded.The investigation results of the electrical transport property of II-VI group semiconductors CdX(X=S,Se,Te)under high pressure is followed: From the DC measurement results we find that our experimental results are much more abundant than before. We observe the sharp drop of resistivity at 2GPa (CdS), 2.6GPa (CdSe), 3.5GPa and 10GPa (CdTe) which is agreeable with previous results. They are due to the structural phase transitions of samples. For CdS three new inflexions of resistivity are observed at 8GPa, 14GPa and 21GPa, respectively. They are caused by the change of energy band structure of CdS at high pressure. For CdSe and CdTe, abnormal changes of resistivity appear at 9.8GPa, 17GPa and 7GPa, 15GPa, 22GPa, respectively, which are never reported. We think they attribute to the electronic phase transition of CdSe and CdTe.The temperature dependence of the DC resistance at high pressure shows that for CdS, its high pressure phases all exhibit semiconducting characteristic. For CdSe, it has a positive temperature coefficient in the temperature range from ambient temperature to 150K. For CdTe, its rock-salt phase shows typical semiconducting property. At 11GPa CdTe still has positive temperature coefficient due to the existence of mixed phases. By fitting to the curve of the temperature dependence of resistivity, the activation energy and band gap can be figured out. For CdSe, we find that the decrease trend of band gap with pressure is more and more slow, and deduce that the metallization pressure is above 70GPa. For CdTe, the activation energy of rock-salt phase increases with pressure, and the band gap at 6.4GPa is figured out to be about 445meV.From the ACIS measurement results we find that for the three samples, there are all two impedance semicircle arcs in the Nyquist representation. It indicates that there exist two conduction processes, grain interior conduction and grain boundary conduction. Just because of the different resistivity of samples at high pressure, the ratio of the two impedance arcs is different. By choosing appropriate representation and equal circuit, the pressure dependence of grain interior conductance is obtained. The results indicate that the phase transitions observed by DC measurement can be observed by ACIS measurement. This indicates that these two measurement methods obtain the same results. They all reflect the electrical property of sample itself. The ACIS can distinguish the grain boundary effect and obtain the accurate sample resistance.The impedance spectroscopy study of CdS under high pressure shows that the grain boundary resistance and grain boundary relaxation frequency which all descript the grain boundary property appear obvious change before and after phase transition occurred. Before the phase transition the grain boundary resistance decreased with pressure promptly, while after the phase transition it decreased much smoothly. The relaxation frequency of grain boundary firstly increased with pressure, and was followed by a transition region and then increased with pressure again. By fitting to the pressure dependence of grain boundary relaxation frequency we obtained the pressure dependence of grain boundary activation energy. Before phase transition the activation energy of the grain boundary increases with pressure and then remains constant from 8.0GPa to 11.60GPa. After 11.60GPa, on the contrary, the activation energy decreases with pressure. This indicates that in the pressure range of 3.7GPa ~ 8.0GPa, the pressure has a positive contribution to the activation energy and makes the transport of charge carriers difficult. After 11.60GPa the activation energy decreases with increasing pressure and the transport of charge carriers through the boundary becomes easier. So it indicates that pressure and the change of crystal itself all affect the property of grain boundary obviously, and induce the change of gain boundary resistance and relaxation frequency.From the dielectric property study of CdS under high pressure we find that under high pressure two extrinsic relaxation processes were observed: contact relaxation and grain boundary relaxation processes. The grain boundary relaxation always existed at high pressure, while the contact relaxation disappeared after 11.59GPa. By excluding these two extrinsic relaxation processes, the pressure dependence of static average dielectric constant was obtained, which reflected the intrinsic dielectric property of CdS under high pressure. Between 2.7GPa and 3.69GPa, the dielectric constant rises with pressure, this attributes to the structural phase transition from wurtzite to rock-salt phase [21], after that it has a slight decrease with pressure up to about 10GPa. At 10.53GPa the dielectric constant rises again and then followed by a slow decrease until 13.89GPa. With the pressure further increasing, the dielectric constant decreases gradually from 1.9×105 at 13.89GPa to 7.5×104 at 19.70GPa and levels above 19.70GPa. The complex change of dielectric constant is due to the change of band gap fromΣν→Χc to Lν→Χc at high pressure.In summary, we integrate a microcircuit on DACs for high pressure impedance spectroscopy measurement, and have a test on CdS, CdSe and CdTe. The result of this thesis not only expands the pressure range of impedance spectroscopy measurement from several GPa to decade GPa, but also set up the technology basis of high pressure and high temperature impedance spectroscopy measurement.