Electrical Properties And Reversible Phase Transition Of CdS, PbSe And PbTe Under High Pressure

Researches of the properties of semiconductors, the main objectiveare make known of their electric characteristics. We use Van der Pauwmethod arrangement in our experiment. This arrangement can eliminate most of contact resistance between the sample and the probes. With the designed diamond anvil, we perform in-situ high-pressure electrical resistance measurements on CdS, PbSe and PbTe.Electron transport is very sensitive to lattice. Under external pressure the structure of matter will be changed, which results in the change of electron transport accordingly. In the measurement of CdS, it is found that a significant drop (about 4 orders of magnitude) of resistivity can be observed most of contact resistance between the sample and the probes. With the designed diamond anvil, we perform in-situ high-pressure electrical resistance measurements on CdS, PbSe and PbTe. Electron transport is very sensitive to lattice. Under external pressure the structure of matter will be changed, which results in the change of electron transport accordingly. In the measurement of CdS, it is found that a significant drop (about 4 orders of magnitude) of resistivity can be observed between 2 GPa and 3 GPa in compression process. The pressure at which the resistance drops suddenly is related to the phase transition from wurtzitestructure to rock-salt structure in CdS. When the transform occur, theresistivity rise between 2-5GPa,this is because wurtzite structure androck-salt structure coexist during this range .Seen from the plot of temperature verse resistivity , we make sure that the mixed phase in CdS is metallic .Resistivity variation during decompression is also studied. Theresistivity can not recover to original value. From the Raman scattering measurement we known that the structure transition is reversible, the resistivity cannot recover is because the electron transition or defects during compression cannot recover.In order to discover the mechanism of conduction in different structures, we investigated the narrow gap semiconductors- PbSe and PbTe. As the pressure increases from ambient to 27GPa, two and three abnormal conductivity changes can be found at room temperature in PbSe and PbTe, respectively. The abnormal conductivity changes in PbSe is close to the phase transition pressure which reported previously, while in PbTe ,only two phase transition was reveled before. We conclude that may be a new phase transition in it. Aftesition, the conductivity in both PbSe and PbTe rise equal to mer, so we think that they are metallization. In our experiment, we did not see positive temperature coefficencies; this is because in diamond anvil cell (DAC), the sample chambes and the results have large r the second tranetal order is so small that the volume of sample in there can not be expanded in these temperatures. (300K-450k)After the first transition, the conductivity rise with increasing temperature ,this is typical semiconductor characteristic .However after the second transition, the conductivity vary disorder with temperature ,the reason is that at these point the resistance of sample is so small that can analogy with electroderror . Resistivity variation during decompression is also studied in PbTe, only one abnormal point in the decompression process, therefore we think that PbTe recover to NaCl-structure directly without undergo intermediated phase, the transition is reversible.Seen from the results, the mixed phase of CdS exhibits metal characteristic, this is helpful to us to explore the mechanism of phase transition. Also, the measurement of resisitivity of PbSe and PbTe in small sample chamber (DAC) do not stretch energy gap when the temperature is rising, these results may bring out new subject in the field of thermoelectric materials.