High Pressure Studies Of Different Conduction Mechanisms In Semiconductors

In this paper,electrical transport properties of the electron conducting material VO₂ were measured utilizing high pressure in situ direct current measurements,We have shown the pressure-dependent activation energy,resistivity,carrier concentration,mobility and Hall coefficient,and systematically discuss the physical mechanism causing these changes.Afterward,the electrical transport properties of BaH₂?H^-conducting?,KH₂PO4?proton conducting?and MWCNT/PVDF composites?mixed ionic-electronic conductiviting?were measured utilizing high pressure in situ alternating current?AC?impedance spectroscopy measurements.We have also investigated the pressure regulation mechanism to both the carrier transportation and dielectric properties at the bulk and grain boundaries.The specific experiments and analysis are as follows:1.Effect of pressure on electrical transport properties of VO₂ has been investigated by accurate in situ resistivity,Hall-effect,and the temperature dependence of resistivity measurements.The value of RH is negative in the whole pressure range,which indicates the electrons are the dominant carrier under compression.The electrical transport parameters including Resistivity,Hall coefficient,carrier concentration,and mobility changes obviously around 10.4 GPa,which is attributed to the structural phase transition of VO₂,accompanied by the V-V dimmers rearrangement.The temperature dependence of resistivity reveals that the phase transition is a semiconductor-to-semiconductor transformation,not the pressure-induced metallization as previous reported by Raman and IR experiments observation.And the abrupt increasing of activation energy at 10.4 GPa indicates VO₂ has an increasing n-type insulating behavior during the structural phase transition.The negative slope of Et vs P indicates that the donors energy levels move toward ? bands?conduction band?of V atoms and the energy gap decreases with increasing pressure,which reduces the energy barriers height and makes carriers transport easier.In addition,in situ measuring carries transportation is beneficial to achieve unique properties of high-pressure phases and optimize VO₂ properties based applications under ambient or extreme conditions.2.BaH₂,as a kind of pure H^-ionic conducting material,is widely concerned because hydride ions?H^-?have an appropriate size for fast transport.Pressure is an effective mean to improve the electrical transport properties of materials,however whether it also improve H^-ions transport pathways has not been determined.Here,Pressure-induced H^-ions transport properties of BaH₂ have been investigated under pressure up to 11.2 GPa by in situ impedance spectroscopy measurement.The H^-ions transport properties including resistance,relaxation frequency,and relative permittivity varys signigicantly around 2.3 GPa,which can be attributed to structural phase transition of BaH₂ from pnma phase to P63/mmc phase.In pnma phase,pressure has little effect on H^-ions transport in the diffusion layer.The diffusion coefficient is only slightly decreased with the increase of pressure.In P63/mmc phase,the diffusion coefficient increases rapidly with pressure increasing,which indicate pressure induces H^-ions transport easier in the diffusion layer.The effect of pressure on the ion diffusion properties is different,which is because the effect of pressure on the diffusion path length of H^-ions is different in BaH₂ diffusion layers.In pnma phase,from the pressure dependence of the conductivity,we can know that pressure can suppress the H^-ions transport channel,which impede the H^-ions transpot in the transport channel and slow down the speed of the H^-ions movement,and finally leads to the decrease of the grain conductivity with the increase of pressure.While in P63/mmc phase,pressure broadens the H^-ion channel,which leads to grain conductivity increases with the increase of pressure.Under alternating electric field,from the frequency dependence of M''and Z'',we can see that the electrical transport process of H^-ions in the grain boundaries mainly shows a large resistance response and the capacitance response is very weak.3.The electrical transport properties of KH₂PO4 sample is measured by the in situ AC measurement technique with the pressure up to 30.0 GPa.The resistance and relaxation frequency varys signigicantly around 2.5 GPa,8.5 GPa and 15.6 GPa,which can be attributed to the changes of proton?H+?electrical transport process caused by structural phase transition of KH₂PO4 sample.In ? phase,the d Ea/d P is positive,which indicate pressure reduces the proton?H+?mobility,hindering the transport of protons?H+?in grains.In ?,? and ?' phase,the negative d Ea/d P value means that the activation energy decreases with increasing puressure.Pressure broadens the transport channels of proton?H+?,increases mobility,and makes proton?H+?migration in transport channel easier.Under alternating electric field,from the frequency dependence of M''and Z'',we can give a conclusion as follows: in ? phase,The electrical properties of KDP grains are dominated by proton?H+?local conduction,and the effect of pressure on proton?H+?local conduction is very small;in ? phase,the proton?H+?local conduction of KDP grains is gradually destroyed with increasing pressure,and proton?H+?long-distance mobile is gradually enhanced;in ? phase,Proton?H+?local conduction is very weak,long-distance transportation plays a dominant role;in ?' phase,all proton transforms into long-distance transportation in KDP grains.4.The electrical transport properties of MWCNT/PVDF composites have been investigateded by the high pressure in situ AC impedance spectroscopy with the pressure up to 25.0 GPa.The diffusion coefficient and relaxation frequencies have discontinuous changes around 14.0 GPa and 20.0 GPa,which are attributed to the electronic phase transition of multi-walled carbon nanotubes?MWCNT?and structural phase transformation from ? phase to ? phase in Polyvinylidene fluoride?PVDF?.At the pressure below 14.0 GPa,the proportion of ionic conduction is much larger than the electronic conduction,which indicates the ions are the dominant carrier in MWCNT/PVDF composites.And the pressure has little effect on the ratio of the ionic to electronic conductivity.The pressure-dependent diffusion coefficient changes of the ions are highly dependent on the diffusion path length in the ions diffusion layer.The diffusion velocity of ions decreases with increasing pressure.The defects in the interface is the main foctor which effect the electron transport process in MWCNT/PVDF composites,and through analysis of our pressure-dependent grain boundary resistances,we find that compression attenuates the defects scattering of electrons in interface.In 14.0-20.0 GPa,Compression dose not has significant effect of on the ions diffusion in diffusion layer,but weakens the defects scattering of electrons in interface.Pressure reduces the proportion of ionic conduction,and simultaneously increases the electronic conduction share.At about 20.0 GPa,in the composite material,the proportion of ionic and electronic conductivity are reversed,and the dominant charge carriers have a change from ions to electrons.And the lattice scattering in the matrix and the defects scattering at the interface co-dominate the electron transport ion in the MWCNT/PVDF composites.At this puressure,the diffusion path of ions is drastically shortened,and the diffusion velocity of ions is rapidly accelerated.