高压下锗化镁的金属化相变研究

Pressure-induced metallization transition in Mg_2Ge

锗化镁是一种窄带半导体,压力作用可以使锗化镁导带底与价带顶的能隙变小.本文基于第一性原理计算了锗化镁在高压下的能带结构以及反萤石相(常压稳定相)和反氯铅矿相(高压相)的焓值,发现在7.5 GPa时反萤石结构锗化镁导带底与价带顶的能隙闭合,预示着半导体相转变为金属相,计算结果还预测在11.0 GPa时锗化镁发生从反萤石结构到反氯铅矿结构的相变.实验研究方面,本文采用长条形压砧在连续加压条件下测量了锗化镁高压下的电阻变化,采用金刚石对顶压砧测量了锗化镁的高压原位拉曼光谱,发现在8.7 GPa锗化镁的电阻出现不连续变化,9.8 GPa以上锗化镁的拉曼振动峰消失.由于金属相的自由电子浓度高会阻碍激发光进入样品,进而引起拉曼振动峰消失,因此我们推测锗化镁在9.8 GPa转变为金属相.

Mg2Ge with anti-fluorite structure at ambient pressure is characterized as a narrow band semiconductor and in-creasing pressure results in a decrease of the gap. In this work, the band structure of anti-fluorite Mg2Ge under high pressure is studied by first principles calculations, which suggests that Mg2Ge becomes metallic at 7.5 GPa as a result of band gap closure. The enthalpy difference between anti-fluorite phase and anti-cotunnite phase under high pressure is calculated by the first-principles plane-wave method within the pseudopotential and generalized gradient approximation. The results show that Mg2Ge undergoes a phase transition from the anti-fluorite structure to anti-cotunnite structure at 11.0 GPa. Then we investigate experimentally the pressure-induced metallization of Mg2Ge by electric resistance measurement in strip anvil cell and Raman spectroscopy by diamond anvil cell. The pressure distribution is homoge-neous along the central line of the strip anvil and the pressure is changed ccontinuously by using a hydraulically driven two-anvil press. Raman scattering experiment is performed at pressure up to 21.1 GPa on a back scattered Raman spectrometer. The wavelength of excitation laser is 532 nm. No pressure-transmitting is used and pressure is determined by the shift of the ruby luminescence line. It is found that neither a discontinuous change of electrical resistance at 8.7 GPa nor Raman vibration modes of Mg2Ge appear above 9.8 GPa. The disappearance of the Raman vibration mode is ascribed to the metallization since the the free carrier concentration rises after metallization has prevented the laser light from penetrating into the sample. We compare these results with those of resistivity measurements in diamond anvil cell. Li et al. [2015 Appl. Phys. Lett. 107142103] reported that Mg2Ge becomes metallic phase at 7.4 GPa and is transformed into metallic anti-cotunnite phase at around 9.5 GPa. We speculate that the discontinuous change in electric resistance at 8.7 GPa is ascribed to the gap closure of anti-fluorite phase and Mg2Ge may transform into the anti-cotunnite phase above 9.8 GPa.