Epitaxial Growth And Physical Properties Of The Layered Antimonides Thin Films

Antimony selenide(Sb₂Se₃)has been proven to be an emerging photovoltaic material,which possesses natural one-dimensional structure and high light absorption coefficient(>10⁵ cm^-1)in the visible light range.In addition,the advantages of Sb₂Se₃such as decent band gap for single-junction solar cells(1.2 e V),non-toxic and low-cost make it has a great application prospects in fields like thin-film solar cells,photodetectors,lithium/sodium ion batteries and thermoelectric devices.However,existing studies have shown that the physical properties of Sb₂Se₃ are closely related to its crystalline quality.Sb₂Se₃ with a preferred orientation of(120)or(221)generally show better photoelectric performance than its amorphous counterpart.Meanwhile,the strong anisotropy of Sb₂Se₃restricts the transport of carriers,so it exhibits near-insulating electrical properties.Our topic was put forward on this basis,Sb₂Se₃ epitaxial films with high crystallinity and single preferred orientation was prepared by fully making use of the features of molecular beam epitaxy such as atomic-level film growth,precise controllable beam and non-equilibrium growth.Further studies on the preparation of(Sb_1-xBi_x)₂Se₃ solid solution thin films and the influence of doping on the physical properties of these samples,especially the optoelectronic properties.The main work of this paper is as follows:(1)Several commonly used substrates/buffer layers are selected for the growth of Sb₂Se₃ thin films,and the real-time growth condition can be observed through reflective high-energy electron diffraction(RHEED).In addition,using several characterization instruments,the morphologies and qualities of these films are studied.Through the analysis of the test results,it is found that except for the amorphous films grown on Si substrate,the other substrates/buffer layers are all possess good crystallinity with nanorod morphology.And Sb₂Se₃ films prepared directly on the fluorophlogopite substrate have better preferred orientation of(120)-plane.The dimension of Sb₂Se₃ nanorods on the Bi₂Se₃ buffer layer is smaller and showing a trend of dispersed growth;the size of the nanorods on the Pt-coated mica substrate is further reduced,and the preferred orientation is not obvious.The nanorods on the HOPG substrate are stacked in aggregate parallel,while the surface morphology of the substrate is different,which indicate a modulation effect on orientation of the nanostructures.(2)To improve the conductivity of intrinsic Sb₂Se₃,a series of(Sb_1-xBi_x)₂Se₃(nominal doping rate 0<x<1)solid solution films were prepared on the fluorophlogopite mica substrate.With the increase of Bi content,the nano-rod structure and density of the film surface have been significantly improved,and the phase structure of the film has gradually changed from the orthogonal phase of Sb₂Se₃ to the two-phase coexistence with the rhombohedral phase of Bi₂Se₃,and the phase separation point appears at around x=0.46.(3)The focus is on the optoelectronic properties of intrinsic Sb₂Se₃ and(Sb_1-xBi_x)₂Se₃ solid solution films.Under simulated sunlight,the intrinsic film exhibits good photoelectric response characteristics and stability.After being doped with Bi,the conductivity of the film increases by 1 to 2 orders of magnitude,and the response time is not significantly reduced.Under a bias voltage of 10 V,the photocurrent(I_ph)of the intrinsic film is about 19.42 n A.For the(Sb_1-xBi_x)₂Se₃ film,when there is no phase separation,the I_ph increases with the increase of the Bi doping rate;When the two phases coexist,I_ph is positively correlated with the relative proportion of the Bi₂Se₃rhombohedral phase in the film.In addition,the Seebeck coefficient of(Sb_0.54Bi_0.46)₂Se₃film measured at room temperature is negative(-107.14?V K^-1),indicating that Bi doping causes the conductivity type of Sb₂Se₃ to change from p-type to n-type,and the power factor is slightly improved,about 14.462?W cm^-1 K^-2.This thesis starts from two aspects of materials and devices,and successfully prepares high-quality Sb₂Se₃ and(Sb_1-xBix)₂Se₃ films on fluorophlogopite substrates.The insulation properties of the intrinsic film have been successfully enhanced.Besides,the photoelectronic and thermoelectric performance of Sb₂Se₃ has also been improved,which has practical significance for the further development and technical application of Sb₂Se₃ materials.