Mocular Beam Epitaxial Growth,optical Spectra And Photoelectric Properties Of CdTe Thin Films On SrTiO₃?111? Substrates

Cadmium telluride?CdTe?is an important II-VI semiconductor material,which has been widely used in X-ray and?ray detectors and thin film solar cells.In addition,due to the same crystal structure and lattice matching,Zn-doped CdTe crystal(Cd_0.96Zn_0.04Te)is the best substrate for epitaxial growth of the infrared material Hg_xCd_1-XTe?MCT?.SrTi O₃?STO?is an important material for epitaxial growth of the functional oxides,such as superconducting,ferroelectric,multiferroic,and giant magnetoresistive.In recent years,STO has been deposited on some substrates,such as Si and GaAs,as well as IV,III-V,and II-VI semiconductor materials has also been deposited on STO substrates,which facilitates the development of inter-integrated and multi-functional perovskite oxide and semiconductor materials.It was found that high quality CdTe thin can be deposited on STO?001?substrates,and?111?growth orientaition is preferred.In contrast,?111?STO and?111?CdTe show the similar structure and symmetry,which is more conducive to epitaxial growth.Therefore,in this dissertation,the CdTe thin film was epitaxially grown on STO?111?,the epitaxial processes were optimized and the possible epitaxial mechanism was studied.The photoluminescence spectrum and optical-electro properties of CdTe epitaxial thin film were further studied.Some results were obtained as follows.First,the epitaxial growth of high-quality CdTe single-crystal thin films was successfully achieved on a largely mismatch STO?111?substrate by molecular beam epitaxy.The STO substrates were treated with a chemical solution and then a high temperature annealing,where a?7×7?reconstructed surface with atomical flatness was obtained.The CdTe epitaxial growth was achieved by adjusting the substrate temperatures and the flux ratios,and the best epitaxial process was obtained.The in-situ detection by reflection high energy electron diffraction?RHEED?combined with Atomic Force Microscopy?AFM?revealed that the growth mode of CdTe epitaxial films changed from 3D to 2D.Second,the epitaxial growth mechanism of CdTe on the STO?111?substrate is different from the one on the STO?001?.The epitaxial growth mechanism of CdTe on the STO?111?substrate is that the stress induced by the lattice mismatch was released through a formation of a strained layer.The critical thickness of the strained layer is⁴5 nm by RHEED,XRD and AFM.Thirdly,the photoluminescence spectra of CdTe epitaxial films show that the strong exciton emission peaks and band-band transition peaks appear in the spectral lines at 1.598 eV and 1.603 eV,respectively,indicating that the CdTe epitaxial films show high crystal quality.A new transition peak at 1.486 eV is found,which is probably caused by stress layer.In addition,the transmission spectra show that there are two transition peaks,which are 1.597 e V and 1.621 eV at 5 K,respectively.One is related to the band gap of the relaxed CdTe layer,and another may be attributed to the band gap of the stressed CdTe layer.Fourth,the conductivity of CdTe epitaxial film increased linely with the increase of temperature,showing a typical semiconductor property.Additionally,the photoelectric response of the CdTe epitaxial film indicated that the dark current of the CdTe/STO material system is much larger than that of the CdTe/Mica material system,while the photocurrent of the CdTe/STO material system is far smaller than that of the CdTe/Mica material system.This may be attributed to the existence of a lot of defects because of the formation of the stressed buffering layer.Moreover,as the optical power of the 637 nm laser increased from 19.2 mW/cm² to 30 mW/cm²,the switching ratios of the CdTe/STO varies from 1.7 to 2.1,while ones of the CdTe/Mica increases from 15 to 30.