Metallic Transition And Electical Properties Of Gap Under High Pressure

In this paper, by the alternating-current impedance spectrum measurements, in situ electrical resistivity measurements and the temperature dependence of the electrical resistivity measurements of compressed on the diamond anvil cell (DAC) incombination with first-principles calculations, we have studied systematically resistivity, the resistance of bulk and grain boundary, relaxation frequency of III-V compound semiconductor gallium phoshpide(GaP) to 35 GPa. At the same time we explained the micro-mechanism of GaP. Detailed experimental and theorecal observations are as following:The AC impedance spectroscopy of GaP sample is measured under high pressure to pressure. The experimental results show the resistance of GaP is consist of grain boundary resistance and bulk resistance. Both the grain boundary resistance and the bulk resistance significantly diminish with the pressure. The grain boundry resistance defects, the resistance of sample is equal to bulk resistance at 5.8 GPa. The resistance of sample sharply decreased by four orders of magnitude, and the relaxation strength sharply decreases. when the pressure increases from 24.5 GPa to 25.5 GPa. Because the structure of GaP has changed from zb to Cmcm under pressure. The resistance of GaP increases in decompression, when the pressure decrease from 10.3GPa to 9.2GPa, the resistance increased three orders of magnitude, the change results from the structural phase transition from Cmcm to zb.Secondly, we have carried accuately on the high-pressure in situ electrical resistivity of GaP sample, including compression process up to 35 GPa and decompression process. We observed, during the compression process, the electrical resistivity sharply decreased by four orders of magnitude from 23.5 GPa to 26.1 GPa. The sudden change in the electrical resistivity was derved from structural phase transition from zb phase to Cmcm phase. In the process of phase transition, the atomic position rearrangement of GaP sample caused the change of carrier concentration and mobility, making conductivity enhancing. During the decompression process, a rapid resistivity increase of about four orders of magnitude from 11.5 GPa to 8.5 GPa. This phenomenon was in accordance with the result of the AC impedance spectroscopy.Thirdly, the metallic of GaP was confirmed under high pressure. The experience results of temperature dependence of the electrical resistivity measurements shows the electrical resistivity have a positive temperature coefficient below the structural phase transition, GaP displays the properties of semiconductor. After the structural phase transition, the electrical resistivity have a negative temperature coefficient, GaP displays the properties of metal. The result declare Zb transform to Cmcm by the abnormal electrical resistivity is a typical semiconductor-to-metal transition. The activation energy can be fitted by Arrhenius formula. Between 85 and 185 K, the activation energy is relatively low; thus, the carriers are readily excited to a lower impurity level. Between 185 K and room temperature, the carriers are from a higher impurity level primarily. When the pressure close to 25.0 GPa, the difference value of two part activation energies close to zero, the band is broadening, the band gap is narrowing to close. The first-principles calculations of GaP shows the zb phase transforms into the Cmcm phase at 12.0 GPa based on the changing enthalpy vs. pressure relationships of the zb and Cmcm phases. The volume collapsed 18.6% at the same time. The calculated energy band structure indicated that the band gap across the Fermi level, Ⅱ phase of GaP sample is metallic.