| It has been a long time to search for magnetoresistive(MR)devices that combine the advantages of high performance and low cost.At present,a large MR effect is found not only in magnetic materials,but also in non-magnetic materials.The physical mechanisms of the MR effect for the non-magnetic semiconductor materials are diverse because the large MR effect has been observed in a variety of materials.Among these,the MR effect based on avalanche ionization has the advantages of relatively larger MR and a simpler device structure,which make it particularly attractive.In this paper,the silicon-based semiconductor heterojunction devices were studied as examples to investigate the separation of the interface MR and the bulk MR of the heterojunction device,the influence of interface contact barrier on the total MR of the device,and the negative differential resistance(NDR)and the non-saturated MR effect based on avalanche ionization.The room temperature MR effect of germanium-based semiconductor devices was also studied and the mechanisms of the MR were analyzed after excluding the Hall effect.The main results are as follows:The MR effect of Ag/Si O2/p-Si Schottky heterojunction device at 150?300 K was investigated.Combining the two-probe and the four-probe methods,the interface MR was successfully separated from the bulk MR.We found that all of the I-V curves could be divided into two regions at threshold voltage:the Schockley region and the avalanche ionization region.The MR values in the avalanche ionization region are much larger than that in the Schockley region.In the avalanche ionization region,the total MR effects are much larger than the bulk MR,indicating that the total MR effect based on avalanche ionization mainly comes from the interface MR effect.The mechanism analysis indicates that the bulk MR effect is a normal one due to the existence of the Lorentz force on the carriers.For the interface MR effect based on avalanche ionization,the MR effect is caused by the suppression of the local plasmas by applying magnetic fields.By reducing the electrode distance,we find that a large total MR effect can be obtained at room temperature.In the avalanche ionization region,the total value can be as high as 1847%@1 T.The total MR value can be large about1185%under 0.1 T,and the corresponding MR sensitivity S(S=MR/B)is about 118.5T-1.Electrical transport properties and MR effects of the symmetrical structure devices of Ag/Si O2/p-Si/Si O2/Ag and Pt/Si O2/p-Si/Si O2/Pt were compared.And the effect of Schottky barrier height on the MR of silicon-based Schottky heterojunction devices was investigated.The TEM characterization analysis confirmed that the structures of electrodes of the two devices were indeed Ag/Si O2/Si and Pt/Si O2/Si,respectively.At different temperatures,the V-I curves of the Ag/Si O2/p-Si/Si O2/Ag device exhibits nonlinear transport characteristics and have significant initial voltage,indicating that the device is a heterojunction structure one.For the Pt/Si O2/p-Si/Si O2/Pt device,the V-I curves are straight lines passing through the origin point under different magnetic field conditions,indicating that the device is in ohmic transport characteristics.By studying the MR effect,we find that the avalanche ionization-based MR values of Ag/Si O2/p-Si/Si O2/Ag devices increase linearly with increasing magnetic field strength without any saturation.The largest MR at 10 K could reach about 1080%@1 T.The lower the temperature is,the larger the MR value is.The MR values of Ag/Si O2/p-Si/Si O2/Ag devices are much larger than those of the Pt/Si O2/p-Si/Si O2/Pt devices,suggesting that the MR effect of the silicon-based heterojunction device is large with a Schottky barrier height.The NDR(i.e.,the measurement voltage decreased as the applied current increases)and unsaturated MR effect were observed simultaneously in In/Si O2/p-Si Schottky heterojunction devices,and their physical mechanisms were investigated.It is found that under zero magnetic field condition,the NDR effect can be observed at 25 K which is much higher than the temperature of NDR occurring in intrinsic silicon.The NDR effect is significantly enhanced by applying a small magnetic field of 0.1 T.Meanwhile,the unsaturated positive MR effect is obtained.The MR value is relatively larger in the vicinity of the avalanche ionization region than in the NDR region,and the largest MR value can reach about 530%@1 T at 10 K near the avalanche ionization region.And even in a small magnetic field of 0.05 T,a higher S(about 20 T-1)is obtained.Analysis shows that the NDR effect and the unsaturated MR effect can be explained on the basis of the same physical mechanism of the avalanche ionization in the In/Si O2/p-Si Schottky heterojunction.The NDR effect is mainly resulted from complex effects containing the carrier injection after avalanche ionization in the Schottky heterojunction,the reduction of the Schottky barrier height,and the Joule heating effect.The large unsaturated MR effect derives from the suppression of the plasma due to avalanche ionization by the applied magnetic field.In addition,the total NDR and bulk NDR effects and the corresponding MR effects of Ag/Si O2/p-Si Schottky heterojunction devices were studied by two-probe and four-probe methods.We find that the total NDR effect of the device is difficult to observe under the two-probe method and zero magnetic field conditions.The semiconductor bulk NDR effect based on the avalanche ionization can be easily detected by the four-probe method.This is related to the existence of a unique voltage detection loop structure which is relatively independent of the current source in the four-probe structure.The room temperature electrical transport and MR effect of In/p-Ge:Ga/In germanium-based semiconductor devices were investigated,and the influence of carrier inhomogeneity on MR effect was analyzed by combining the Hall effect.We find that all V-I characteristic curves present straight lines passing through the zero point in a small current range,indicating that the interface effect of the device can be completely ignored.When the magnetic field is parallel to the surface of the sample,the MR value increases first and then decreases with the increasing current,and the MR value based on avalanche ionization reaches the largest of about 40%@1 T.The mechanism analysis shows that the larger the Hall voltage,the stronger the carrier inhomogeneity and the larger the MR effect.At a smaller electrode distance,a larger MR based on avalanche ionization of about 91%@1 T is obtained.This is related to the change of the electric field distribution with the electrode spacing changing,leading to the change of the degree of the carrier inhomogeneity.In addition we find that the change in the direction of the magnetic field has a very significant effect on the MR effect of the device.This is not only derives from geometric effects but also comes from carrier inhomogeneity. |
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