| Manganese telluride(MnTe)has two typical crystalline structures:nickel arsenide(NiAs)type structure(H-MnTe)and zinc-blende structure(ZB-MnTe).H-MnTe is a stable phase while ZB-MnTe is a metastable phase.H-MnTe and ZB-MnTe exhibit antiferromagnetic properties,i.e.magnetic semiconductors.It is reported that Neel temperature(TN)of H-MnTe is?310 K and the TN of ZB-MnTe ranges from 50 to 70 K.H-MnTe stacks along the c-axis in the form of Mn/Te/Mn/Te hexagonal dense stack,and the Mn spin vectors in the ab plane are arranged in parallel,thus showing the ferromagnetic exchange effect,while the spin vectors between the planes are arranged in inverse parallel,thus showing the antiferromagnetic exchange effect.This unique spin arrangement induces abundant magnetic and transport properties,such as anisotropic magnetism and anisotropic magnetoresistance effect.Recently,it has been reported that,unlike bulk materials,Hex-MnTe epitaxial films exhibit ferromagnetic.Transport properties depend on the thickness of the films and as the films become thinner,the behavior of H-MnTe changes from semiconductor to metalZB-MnTe is Heisenberg antiferromagnet exhibiting some fascinating physical phenomena,such as magnetization steps,spin glass phase,inelastic neutron scattering,one magnon Raman scattering and giant magneto-optical effect,etc,due to its unique magnetic structure.ZB-MnTe is thermodynamically unstable and exists in film form synthesized by nonequilibrium growth techniques.Furthermore,different from other Mn-related compounds(MnS,MnSe et al.),ZB-MnT is a wide-band-gap semiconductor with Eg of?3.4 eV.Therefore,it is an ideal material for studying d-d multiplet transitions in Mn2+ ions because of the fact that its transition energy is higher than 2 eVStable and Metastable phases of MnTe Single-crystalline Films were epitaxially grown by MBE.The surface,interface,crystal structure,stress state and epitaxial relationships were studied by RHEED,XRD,AFM and HRTEM.The phonon vibration characteristics of H-MnTe and the d-d multiplet transitions in Mn2+ ions of ZB-MnTe thin films were studied by Raman and PL techniques,respectively.Main results have been obtained as follows:1.Single crystalline H-MnTe thin films were epitaxially grown on InP(111)substrates by molecular beam epitaxy by optimizating the epitaxial processes and the optimized growth conditions were determined.The interface of H-MnTe/InP heteroepitaxial system is sharp with atomic-scale connection,and the surface shows atomic flatness,for example,the surface RMS roughness of the film with thickness of 60 nm is?1.845 nm.The epitaxial relationships between H-MnTe and InP is as follows:(001)H-MnTe ?(111)InP,[210]H-MnTe ?[110]InP,[010]H-Mn Te ?[112]InP.Temperature dependent Raman scattering spectra of NiAs-type MnTe epitaxial films were measured to explore the temperature evolution of Raman shifts,line-width at half maximum and intensity across Tn.A significant deviation from an anharmonic mode below Tn strongly supports the existence of spin-phonon interaction and its coupling strength is calculated to be-0.16 cm-1 from the E2g phonon.In addition,the emergence of 268.8 cm-1 and 345.8 cm-1 Raman scattering peaks indicates the existence of two magnons scattering caused by spin flip in the system,which may be due to the spin exchange between the nearest neighbor and the third nearest neighbor Mn2+,respectively.2.Single crystalline ZB-MnTe thin films were grown on InP(111)substrates by molecular beam epitaxy.The optimized growth technology were determined and the growth rate of the films were controlled at 0.75 ± 0.3 nm/min.The analysis of surface and interface microstructures shows that the interface of ZB-MnTe/InP is the "serrated" connection structure and the surface of the films also exhibits a wavy island structure.This is mainly due to the mixed polar surface of InP substrate which affects the formation energy of MnTe thin films,and thus inducing the formation of metastable ZB-MnTe single crystalline thin films.The crystallographic relationship between ZB-MnTe films and InP substrates is as follows:(111)ZB-MnTe 11(111)InP,[110]ZB-MnTe 11[110]InP,[112]ZB-MnTe 11[112]InP.3.Mn2+ d-d multiplet transitions in zinc-blende MnTe single-crystalline thin films were studied by temperature-dependent photoluminescence spectroscopy.The emission peaks of 1.715eV and 2.113 eV are assigned to the 4T1g(4G)to 6A1(6S)and 4T1(4G)to 6A1(6S)transitions related to octahedrally and tetrahedrally coordinated Mn2+ were observed.The peak position shifted toward high energy with the increase of temperature indicates the existence of electronic-phonon interaction in ZB-MnTe thin films and the corresponding phonon energies are about 21 meV and 42 meV,respectively.The emission peak of 2.393 eV originates from the 4T2(4G)to 6A1(6S)transition.The peak energy and intensity of this emission peak almost independent of temperature indicates that the transition from high excitation state to ground state is highly localized.In addition,the emission peak at 1.542 eV originates of Mn11 in an environment disturbed by defects or impurities,the emission peak at 1.602 eV originates from perturbed Mn2+ states in interaction with magnons.Both of them shifted toward low energy with increasing temperature and the intensity decreased exponentially.4.Metastable ZB-MnTe films with high-crystalline quality were deposited on highly dissimilar perovskite SrTiO3(001)substrates.It is found that ZB-MnTe crystallizeed in[111]direction and the epitaxial relationships between ZB-MnTe thin films and STO substrates are summaried as;(111)ZB-MnTe ?(001)STO,[110]ZB-MnTe ?[100]STO,[112]ZB-MnTe ?[010]sto or other equivalent directions.ZB-MnTe thin films were twelve-fold symmetric along[111]orientation.At the beginning of epitaxial growth,a 1-2 nm ultra-thin "pseudomorphic layer" was formed on the surface of STO substrates,which made the subsequent growth of thin films no longer bound by the stress of large mismatched substrates and possessed obvious characteristics of "Compliant Epitaxy" growth mechanism. |
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