The Electrical Properties Of ZnX(S,Se,Te) And TiO₂ Under High Pressure

The combination of thin film technology, lithography technology and high pressure technology were used as a new generation high pressure intergration technology. It had been overcame the the defects of electrode which were consisting of fracture and deformation in DAC. The use of high pressure intergration technology can be accurated measurement the electrical signal in DAC. The AC impedance spectrum can obtain more information than DC measurement which can distinguish the grain inter and grainboundary resistance. However, the measurement of AC impedance spectrum in DAC is still a difficult technique due to the strict measurement enviorment (such as the sample chamber must be insulation, the additional capacitor and inductance in system should be minimized and so on). Furthermore, AC impedance spectroscopy method can get more enrich contents than DC resistivity measurements under pressure, for example the resistance from grain and grain boundary, the dielectric properties and so on. Thus, it is important and significant to develope a suitable method to solve the problems in DAC.To resolve this problem, the T301 gasket was used as an electrode (for its exellent conductivity) and the other electrode is one of the DAC which was sputting thin film. Then the two-point method was used to measure the impedance under pressure. The problems which were brought in high pressure measurements can be resolved by such electrode configuration. The following is the advantages of this configuration: first the deformation of electrode was small; second the insulation of sample chamber was resolved; and third the additional captiance and inductance in system were minimized. In summary, the AC impedance can be measured accuraly in DAC. Using this system the electrical properties of ZnX(S,Se,Te) and TiO2 under pressure were investigated.Application the new elcrode mode, the high pressure electrical properties of ZnX (S,Se,Te) and TiO2 were investigated. The results indicted that both the DC electrical resistivity method and AC impedance method can well measurement the electrical properties under pressure. In ZnS, the DC resistivity dropped several orders of magnitude at 14GPa which corresponding to structure phase transition from zinc-blende to rock salt structure. After transition, the resistivity became small due to the narrow gap in rock-salt phase ZnS. In ZnSe, there is a step-like drop in resistivity at 11-12GPa following a smooth trend under pressure which is ascribed to the structure phase transition from zinc-blende to cinnabar, then to rocksalt phase. Under high pressure, the rockalt phase is a metal phase which has a small resistivity value. In ZnTe, the discontinue change-points occur at 5GPa, 8GPa and 12GPa, respectively. The latter two points are corresponding to the structure phase transition from zinc-blende to cinnabar to Cmcm phase. Because the Cmcm phase in ZnTe is metal phase, so the resistivity value under pressure is small. The discontinue point at 5GPa may be due to the new structure phase or electronic phase transition. In contrast with the structure phase transition in ZnS, ZnSe and ZnTe, the unbalance size in anion and cation in ZnTe lead to the high pressure phase is Cmcm structure phase instead of rocksalt structure.In the ZnS AC impedance spectrum results, both semicircles which sepetately represent the grain and grain boundary resistance reduced as the pressure increasing while the shape maintained. This indicated that the conductive mechanism was stable under pressure and the reduced resistance is due to the shrink of band gap. At 14GPa, the impedance turn from two semicircles to a 1/4 arc. This pressure point is the same with the structure phase transition point in ZnS. In ZnSe, the size of impedance spectrum increased with increasing pressure which is ascribed to the enlargement of the band gap. The shape of impedance spectrums were changed two times between 11-12GPa which is due to the structure phase transition in ZnSe from zinc-blende to cinabber-rocksalt structure. The changed of impedance spectrum at the phase transition point can reflect the occurrence of structure phase transition, the interior conductive mechanism of material can be also reflected by the shape changed of impedance under pressure.Recently, people pay more attention to the interface charaterisic of nanomaterials. Due to the advantages of impedance which can divide the resistance into grain and grain boundary,it is used in DAC for high pressure measurement which is a direct method to reaserch the interface characteristics of nanoparticles. In this paper, the impedance of ZnS nanoparticles was measured between 0-30GPa. Through the shape change of impedance spectrum, we can know the phase transition points. The pressure dependence of muludus spectrum curves indicated that the grain boundary played an important role under pressure. At the same time, the pressure dependence of grain and grain boundary resistance further proved the structure phase transition points at 8GPa and 15GPa. The dielectric constant also can be getting from the high pressure impedance measurements. The enlargement of dielectric constant at 15GPa is due to structure phase transition. The study of different structures TiO2 under pressure discovered that the impedance has close relation with structure. The high pressure impedance spectrum in TiO2 is different with different structures. For the rutile structure, only one semicircle can be observed at low pressure which was due to small or no acculation of electron in space charge region. With the pressure increasing, the arcs of impedance distorted indicate the conductive mechanism changed. The electron occurred in space charge region. This distortion has direct relation with structure phase transition. The pressure dependence of grain and grain boundary resistance curve indicated that the structure phase transion occurred in 12GPa, and the baddeleyite phase is stable at 25GPa. The space charge potential is stable at high pressure and the value is about 30 mV.In anatase TiO2, the pressure dependent of resistance curve indicated that the discontinue change points at 5GPa and 12GPa which corresponding to the structure phase transition from anatase to columibite, then to baddeleyie. And baddeleyite structure is a stable phase above 23GPa. There is no change in impedance spectrum from 5GPa to 23GPa which indicated that the conductive mechanism is unchanged. This is ascribed to the same structure (columibite phase) in that range. The varation of relaxation frequency and space charge potential have discontinued changes at the phase change points also have relation with phase transition.In summary, both the DC method and AC impedance spectrum are used to research the electrical properties of semiconductor materials under pressure. However, the AC impedance measurements can give more physical informations than DC method, such as the varied shape in impedance spectrum corresponding to the conductive mechanism has been changed. The relaxation frequency, space electron potential, dielectric constant can also obtain from impedance results. However, the impedance spectrum measurement has large error when applied on the solid state material with low resistance.