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The Physical Characteristics Of ?-?/?-? Telluride Compound Semiconductors And Heterostructures

Posted on:2021-03-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:S S MaFull Text:PDF
GTID:1368330602493493Subject:Radio Physics
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Group ?-? tellurides refer to compounds composed of group ? elements(Ge,Sn,Pb)and group ? Te element,which are narrow-gap semiconductors;group ?-? tellurides refer to compound semiconductors consisting of group ? elements(Zn,Cd,Hg)and group ? Te element.In recent years,the topological nontrivial phase and spintronic properties of ?-?/?-? telluride semiconductors have attracted extensive attentions because of the fairly strong spin-orbital coupling in this material class,such as SnTe,PbTe,PbSnTe,HgTe and HgCdTe,etc.From a technical point of view,the growth methods for ?-?/?-? semiconductors are well-developed,which means high-quality films and heterostructures can be realized through a variety of epitaxy techniques.For instance,the first 2D TI(quantum spin Hall effect)was realized in HgTe/Hgi?xCdxTe quantum wells and the first topological crystalline insulator(TCI)was discovered in SnTe crystals.Moreover,HgCdTe and PbSnTe are also important narrow-gap semiconductors for infrared photodetectors.Therefore,?-?/?-? telluride semiconductors and heterostructures are of great significance in both device applications and fundamental researches.We firstly focused on the molecular bean epitaxy(MBE)of PbTe,CdTe and ZnTe,and obtained high-quality films as well as heterostructures.On this basis,we realized band inversion by introducing Te antisites in PbTe,turning PbTe into a TCI,and then explored the PbTe:Tepb TCI through ARPES and SdH oscillations.Furthermore,we studied the temperature dependence of carrier concentrations and mobilities in PbTe/ZnTe and PbTe/CdTe heterojunctions.The PbTe/ZnTe interfacial 2DEG exhibited apparent weak anti-localization(WAL)features in magnetoresistance(MR).By optimizing growth conditions,we suppressed the formation of interfacial defects in the van der Waals(vdW)epitaxial PbTe/CdTe heterojunctions,and the MR features evolved from the weak localization(WL)type into the WAL type with SdH oscillations emerging.The PbTe/CdTe interfacial 2DEG manifested Dirac fermion nature with a nontrivial ?Berry phase extracted from SdH oscillations.The main results of our work are as follows:1.We introduced Te antisite defects into PbTe by maintaining Te-rich condition in MBE growth process,and the diffraction peak shift on HRXRD spectrum indicated approximate 1.5%shrink of the lattice constant.According to the ab initio calculation of electronic band structures,the incorporation of Te antisites caused band inversion in PbTe,turning it into a TCI.The valence bands of PbTe:Tepb and unintentionally doped PbTe(111)films were compared via ARPES measurements,and non-paraboiic dispersions along with a sagging structure in the vicinity of valence band maximum(VBM)were observed on Te antisites incorporated sample,which were probably related with band inversion.2.The SdH oscillations of PbTe:Tepb(1 11)films demonstrated a nontrivial Berry phase attesting to the Dirac fermions from topological surface states.Combining the measured pressure dependence of SdH oscillation frequency with theoretically calculated band structures of strained PbTe:Tepb,we illuminated that the pressure induced Fermi surface structure evolution was a Lifshitz transition.3.We proposed and realized a novel lattice-mismatched PbTe/ZnTe(111)heterojunction and characterized the interfacial atom arrangement by cross sectional high-resolution TEM.The half-rocksalt and half-zincblende coordinating configuration of interfacial Te atoms resulted in excessive electrons which accumulated at the interface forming a layer of 2DEG.At room temperature the 2DEG showed carrier mobility as high as about 900 cm2V-1s-1.Cooling down to 2K,the mobility of PbTe/ZnTe 2DEG exceeded 40000 cm2V-1s-1,and the surface electron density was around 4X l013 cm-2.Obvious WAL features were revealed on the MR curve,which ought to be attributed to the contribution from up-shifted PbTe T Dirac points caused by interfacial biaxial strain.4.The PbTe/ZnTe(1 11)heterojunctions demonstrated the infrared sensitive lateral photovoltaic effect(LPE),which was the buildup of a photovoltage parallel to the junction interface induced by nonuniform illumination.Benefiting from the high mobility of interfacial electrons,the response speed of LPE output signal was very fast with a rise time less than 5 ns,manifesting the potential for high-speed infrared photodetectors.5.To overcome the shortcomings of the BaF2 substrates for growing ?-?/?-? telluride semiconductors,we realized the vdW epitaxy of PbTe(111)films and PbTe/CdTe(111)heterojunctions on flexible layered mica substrates,and revealed the negative MR of WL features arising from interfacial defects.By adjusting the Te flux ratio to higher level during PbTe growth process,we suppressed the formation of triangle pit defects on PbTe surface and consequently the mobility of PbTe/CdTe interfacial 2DEG was greatly enhanced.The optimized PbTe/CdTe heterojunctions demonstrated recognizable SdH oscillations along with MR cusp of WAL features around zero magnetic field at the temperature of 2K.From Landau fan diagram plot,a nontrivial? Berry phase was demonstrated,claiming the existence of Dirac fermions in the 2DEG.It was reported that magnetic films like Fe3O4 and CoFe2O4 had been successfully grown on mica,and we believe that combining these magnetic films with PbTe/CdTe heterojunctions or ?-? telluride compound TCIs would provide an excellent platform for practical spintronic device researches.
Keywords/Search Tags:MBE, PbTe, ZnTe, CdTe, semiconductor heterostructures, 2DEG, SdH oscillations, Berry phase, topological crystalline insulators, lateral photovoltaic effect, infrared photodetection
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