Magnetism And Electrical Transport Of Group ? Amorphous Semiconductor Films

With the rapid development of the microelectronics technology,the integration density of the devices increases at the speed of Moore's law,which is challenging due to problems of the power consumption and the quantum confinement effect,and so on.In general,the conventional microelectronics devices are concentrated on the use of charge freedom of electrons.Compared with conventional charge-based semiconductor devices,utilizing charge and spin degrees of freedom together increases the data processing speed and decreases the power consumption.Under the background,a new field named spintronics has attracted people' great attention.Spin-orbit coupling?SOC?,which allows the manipulation of electron spins in semiconductors heterostructures,has drawn great interest due to its potential applications in spintronic devices.Two mechanisms of SOC were proposed,one is Dresselhaus SOC originating from the broken inversion symmetry of a crystal,and the other one is Rashba SOC arising from the structure inversion asymmetry.The intrinsic Rashba effect is related to interfaces and surfaces and is proportional to the electric potential gradient across the interface or surface.The extrinsic Rashba effect allows tuning the spin-orbit coupling by the applied voltage.Therefore,the Rashba SOC may serve as an important control parameter for the spintronic devices.The spin-orbit interaction is responsible for many physical properties and phenomena,including spin Hall effect?SHE?,topological insulators,skyrmions,anomalous Hall effect?AHE?,and so on.In particular,AHE usually observed in ferromagnetic materials is essentially determined by SOC and spin polarization of the carriers.The mechanisms of AHE mainly include the intrinsic mechanism,side-jump and skew scattering.The intrinsic contribution to AHE is only dependent on the band structure of the perfect crystal and is largely independent of any impurity and phonon scattering within a framework of momentum-space Berry phase.The side-jump and the skew scattering belong to the extrinsic contributions to AHE.The side-jump is referred that the new trajectory is displaced by a finite distance from the old trajectory.The skew scattering is a left-right asymmetric scatter:ing due to the effective SOC of electrons or impurities.The above three conventional mechanisms of AHE are based on SOC of the conducting carriers.However,a majority of studies on AHE are focused on single ferromagnetic films,and therefore the interfacial effect on AHE owing to the Rashba SOC is seldom considered.And AHE in the ferromagnet induced by SOC has been still confused with nonliner Hall effect described by a two-carrier conduction model.According to above research,we prepared Si-based semiconductor materials by doping the transitional metal Mn into the Si semiconductors to make nonmagnetic Si magnetism.A certain concentration of Mn ions is needed,since it is known that a single Mn atom prefers to occupy a tetrahedral interstitial position of the silicon crystal lattice and forms localized magnetic moments?Mn2+ with the effective magnetic moment ?3 ?B?-Meanwhile,Mn dopants introduce free carriers?holes?into the system,which could mediate the long range exchange coupling between localized magnetic moments.We firstly studied the magnetic and electrical transport properties of MnxSi1-x semiconductor films and MnxSi1-x/SiO₂/Si p-i-n junctions.The mechanism of the enhanced AHE on the Mn_0.48Si_0.52/SiO₂/Si p-i-n junctions is the result of the interfacial Rashba SOC,rather than that of the two-band conduction mechanism.Moreover,we demonstrated the electrical transport properties of the?FeCo?_0.67Ge_0.33/Ge p-p junction.The Hall effect of the p-p junction can be induced by the interfacial Rashba SOC and two-band conduction model.Our work will promote the application of the Si and Ge-based magnetic semiconductors in spintronic devices.The main results are as follows:?.The magnetism and anomalous Hall effect of amorphous Mn_0.48Si_0.52 films prepared on glass substrates with a high Mn concentration were studied.Mn_0.48Si_0.52 films show the weak ferromagnetic phase below the blocking temperature of 17 K and superparamagnetism above 17 K.The relation between the anomalous Hall coefficient and the longitudinal resistivity indicates that AHE originates from the skew scattering mechanism?Refer to J.Alloys Compd.,2015,623:438?.In order to overcome the low solubility of Mn ions in Si semiconductors and the second phase precipitation and/or clusters formed under the thermal nonequilibrium condition at 20 °C,we prepared amorphous MnxSi1-x semiconductor films with different film thicknesses on glass substrates and various Si?100?substrates.With increasing the Mn concentration,the magnetization increases and reaches to a maximum in the Mn_0.48Si_0.52 films.As the Mn concentration further increases from x = 0.48,the magnetization decreases due to the short-range anti-ferromagnetic coupling between Mn ions.The magnetic properties of the Mn_0.48Si_0.52 film do not depend on substrates and film thicknesses above 20 nm.The conducting character of Mn_0.48Si_0.52 films is on the metallic side of the metal-insulator transitionand and the electron-electron and electron-phonon interactions play a dominated role.The relation between the anomalous Hall coefficient and the longitudinal resistivity indicates that AHE originates from the skew scattering mechanism.?.We found that an enhanced negative AHE in Mn_0.48Si_0.52/SiO₂/Si p-i-n and Mn_0.48Si_0.52/Si p-n junctions due to the interfacial Rashba SOC around 200 K?Refer to RSC Adv.,2016,6:55930?.As the temperature increases from 5 to 150 K,the magnitude of the positive Hall resistivity of Mn_0.48Si_0.52/SiO₂/Si p-i-n junctions gradually decreases,but at 150 K,the sign of the Hall resistivity becomes negative and the magnitude of the Hall resistivity increases rapidly to a maximum of 10 ?-cm up to 200 K.Further increasing temperature from 200 to 300 K,the magnitude of the Hall resistivity gradually decreases again.The positive AHE of Mn_0.48Si_0.52/SiO₂/Si p-i-n junctions below 150 K is due to a bulk effect of Mn_0.48Si_0.52 films.The p-i-n junction shows the greatly enhanced negative AHE above 150 K,which is attributed to the interfacial Rashba SOC of p-i-n junctions with low barriers,rather than the two-carrier conduction mechanism.By removing the SiO₂ native oxide layer of Si substrates,the interfacial Rashba SOC of Mn_0.48Si_0.52/Si p-n junctions is further enhanced owing to the lower barrier of p-n junctions.?.We further studied the effect of the interface barrier on the Hall effect of Mn_0.48Si_0.52/SiO₂/Si p-i-n and p-i-p juctions,which were prepared by sputtering the Mn_0.48Si_0.52 films on the n-Si+SiO₂ and p-Si+SiO₂ substrates with the high conducting carriers density.Mn_0.48Si_0.52/SiO₂/Si p-i-n and p-i-p juctions show the positive anomalous Hall resistivity at the low temperature range from 5 to 150 K and from 5 to 180 K,which is contributed to the bulk effect of Mn_0.48Si_0.52 films,because the high interfacial barrier serve as an excellent insulating layer.At the high temperature ranges from 150 to 300 K and from 180 to 300 K,the weak AHE of p-i-n and p-i-p juctions induced by the interfacial Rashba SOC is coved up by the OHE of the substrates as a consequence of the strong shunting effect of the substrates.The AHE of Mn_0.48Si_0.52/Si p-n and p-p juctions removing the SiO₂ native oxide layer of Si substrates mainly originates from the bulk effect of Mn_0.48Si_0.52 films at the low temperature from 5 to 150 K and from 5 to 175 K and is due to the OHE of the n-Si and p-Si substrates from 150 to 300 K and from 175 to 300 K,respectively.?.The longitudinal conductance,magnetoresistance and Hall resistance of amorphous?FeCo?_0.67Ge_0.33 magnetic films and?FeCo?_0.67Ge_0.33/Ge p-p juctions were reported systematicly.We found that the nonlinear Hall effect of?FeCo?_0.67Ge_0.33/Ge p-p juctions is attributed to both the two-carrier conduction mechanism and Rashba SOC.On the one hand,the conducting character of?FeCo?_0.67Ge_0.33 films prepared on glass substrates is on the metallic side of the metal-insulator transition from 5 to 300 K,which indicates that the electron-electron and electron-phonon interactions play a dominated role.And the Hall resistance of?FeCo?_0.67Ge_0.33/glass samples is mainly attributed to the anomalous Hall resistance of single?FeCo?_0.67Ge_0.33 films,rather than glass substrates.On the other hand,the longitudinal conductance,magnetoresistance and Hall resistance of?FeCo?_0.67Ge_0.33/Ge p-p juctions fabricated on Ge substrates increase with increasing temperature from 5 K and reach to the maximum up to around 60 K.As temperature further increases,the longitudinal conductance,magnetoresistance and Hall resistance decrease to about 270 K.Below 60 K,the conducting character of?FeCo?_0.67Ge_0.33/Ge p-p juctions mainly originates from the single?FeCo?_0.67Ge_0.33 film,because the electrical shunt effect of Ge substrates is negligible and the carriers are blocked by thicher interfacial potential barriers.At the temperature of 60 K,the longitudinal conductance,magnetoresistance and Hall resistance of p-p juctions reach to a maximum,because the carriers cross the barrier to run up to the Ge substrates.And at 60 K,Hall effect of p-p junctions is mainly attributed to the interfacial Rashba SOC of the junction with a low barrier.At high temperature ranges,the conducting character of?FeCo?_0.67Ge_0.33/Ge p-p junctions is due to the electrical shunting of Ge sbustrates.Moreover,the nonlinear Hall effect of?FeCo?_0.67Ge_0.33/Ge p-p junctions can be also explained by the two-carrier conduction mechanism between the?FeCo?_0.67Ge_0.33 film and Ge substrate.But we can not identify the contribution to the nonlinear Hall effect of?FeCo?_0.67Ge_0.33/Ge p-p junctions from the interfacial Rashba SOC and from the two-carrier conduction mechanism quantitatively.