The Structural And Electrical Transport Properties Of Compressed AlAs, SnSe, And GeTe

The structural and electrical transport properties of group-III-V and IV-VI family semiconductor compounds, including AlAs, SnSe, and GeTe were systematically investigated combining with the in situ high-pressure electrical transport properties measurement, synchrotron X-ray diffraction methods and first-principles calculation methods. O ur studies have found that AlAs, SnSe, and GeTe went a phase transition under high pressure. With the phase transition, the electrical transport parameters vary abnor mally. According to our results, we have discussed the physical mechanism of the phase transition and electrical transport properties. In addition, we have studied the connection between the phase transitions and the transport properties. The details are listed below:1. By combining the X-ray diffraction and AC impedance spectral measurements, we have studied the electrical transport and structural properties of III-V binary compound semiconductor AlAs. The resistances of both the grain boundaries and bulk vary abnormally at ~10.9 GPa, accompanied by the cubic–hexagonal structural transition of AlAs. With further compression, the boundary effect becomes undistinguished, and subsequently the electrical transport mechanism converts from boundary- to bulk-dominated, which gives rise to significant reduction about three orders of magnitude of the total resistance. After quenching to ambient pressure, resistances recover to the initial values followed by the re-emergence of the boundary effect.With the pressure increasing, the boundary effect becomes undistinguished. This phenomenon can be understood as: “the grain boundaries are composed of disordered atoms and contain large numbers of defects due to incomplete bonding, which usually trap the charge carriers and act as recombination centers, subsequently leaving few free carriers for electrical transportation. The trapped carriers usually result in boundary scattering effects, which consequently hinder the conduction in the grain boundaries. Under compression, the recombination centers should eventually become saturated with the increase in the number of free carriers due to the decrease in the energy band-gap, and hence no longer trap the free carriers. Consequently, the grain boundary effect becomes much weaker compared with that of bulk conduction. ”We found that AlAs maintained semiconductor properties through t he temperature dependence of the impedance spectral measurements. We obtained the carrier transport activation energy of the bulk transport process(Eg) by fitting the R–T curves. Eg decreases with increasing pressure and changes its pressure dependence at ~14.0 GPa, which rationalizes the anomalous variation of the electrical transport properties of AlAs. In addition, we found that the grain boundary effect could be modulated by compression and plays positive roles in devices such as increasing the difference in resistance between the two states; Moreover, grain boundaries are usually undesired in many applications. By compression, the grain boundary effect could be reduced and even undistinguished, which opens up new possible pathways for designing AlAs-based applications and may also be applicable in optimizing the performance of phase-change memories.2. The electrical transport and structural properties of SnSe under compression have been studied b y using temperature dependence of electrical resistivity, X-ray diffraction, in situ Hall-effect measurement methods and first-principles calculation methods. The results indicate that SnSe transforms from a semiconducting to semimetallic state at 12.6 GPa, followed by an orthorhombic to monoclinic structural transition. Hall-effect measurements indicate that both the carrier concentration and mobility vary abnormally accompanied by the semimetallic electronic transition. First-principles band structure calculations confirm the semiconductiong-semimetallic transition, and reveal that the semimetallic character of SnSe can be attributed to the enhanced coupling of Sn-5s, Sn-5p and Se-4p orbitals under compressio n that results in the broadening of the energy bands and subsequently the closure of the band gap. TEM images of the sample after decompression indicate that the average grain size of SnSe decreases significantly with increasing pressure. The pressure modulated variations of electrical transport and structural properties may provide an approach to improving the thermoelectric properties of SnSe.3. We also made a deep exploration to the high pressure electrical transport and structural propeties of IV-VI compound semiconductor GeTe. The in situ Hall-effect measurements reveal that the resistivity, Hall coefficient, carrier concentration and mobility of GeTe vary abnormally at 8.5 GPa and 14.1 GPa. We consider the abnormally variation is caused by the phase transitions which have been reported previously. In order to further exploring the transport properties of the high-pressure phase, we take the temperature dependence of electrical resistivity measurements which indicate that the resistivity of GeTe increases with the increasing pressure. The temperature(T)-resistivity(R) curves exhibit a positive temperature dependence, dρ/d T>0, indicating the metallic character in GeTe. GeTe behaves as a metal at ambient pressure because of its low band-gap and the impurity energy level in the band-gap. At high-pressure phase, GeTe keeps its metallic character due to pressure- induced band-gap closure. First-principles band structure calculations confirm our result. In addition, the Hall-effect measurement result also shows that the carrier-type inversion of GeTe have been confirmed at 8.5 GPa.