The Preparation And Optoelectronic Properties Of Narrow Band Gap Semiconductor PbSe Films

Lead selenide?PbSe?,lead sulfide?PbS?and lead telluride?PbTe?are representatives of the IV-VI compounds,also known as the lead salts semiconductors.As a typical group of narrow band gap semiconductors,band gap of the IV-VI compounds ranges from 0.2to 0.4eV,which makes them very suitable for the application of infrared detection.Owing to the advantages of robust optoelectronic response and simple material synthesis process,PbSe with a direct gap of 0.27eV at room temperature has demonstrated its great potential in the application of infrared detection,especially in the mid-infrared band with the wavelength ranging from 3 to 6?m.And,after alloying with SnSe,formation of PbSnSe alloy could broaden the detectable band to long wave infrared spectrum with the wavelength ranging from 6 to 30?m.However,its intrinsic shortcomings,such as the huge dielectric constant and thermal expansion coefficient,have limited the application scopes of PbSe,in comparison with other material systems?HgCdTe alloy and?-?group semiconductor?.As a group of material with both obvious shortcomings and advantages,fabrication of some novel prototypes of PbSe photodetectors could help us take the advantages and bypass the disadvantages,which is a challenging and valuable research field.Thus,based on the intrinsic properties of PbSe,several prototypes of PbSe photodetectors have been proposed in this work,with the purpose of exploring potential of PbSe and PbSnSe in the infrared detection.Main findings of this research are described as follow.?1?Photoconduction of the polycrystalline PbSe has been researched firstly.The PbSe thin films were grown by molecular beam epitaxy?MBE?.After the growth process,annealing under oxygen atmosphere?sensitization process?was conducted.Through the investigation on the microstructure,chemical composition and performances of as-prepared PbSe thin films,the mechanism of sensitization was explained.By introducing oxygen atoms into n-type PbSe lattice,surface conductivity of the pristine PbSe grain was converted into p-type,leading to the formation of a micro pn junction between oxidized shell and pristine core.Thus,inhabitation on the recombination of photocarriers was realized via the spatial separation of those photo-induced electron-hole pairs,which could enhance the photoconduction of PbSe films in turn.?2?Owing to its difficulties on precise control of the doping depth and concentration during sensitization process,we employed the O₂-plasma treatment process,through which conductivity of the pristine n-type PbSe was converted to p-type.Thus,fabrication of PbSe photodiodes was conducted via the O₂-plasma treatment of PbSe thin films.In this section,by introducing oxygen into the n-PbSe thin films through O₂-plasma treatment,a pn junction was thus fabricated between the treated area and the pristine area.The as-fabricated devices showed significant photovoltaic response.Responsivity and detectivity of the as-fabricated device are 0.66mA·W^-11 and 2.2×10¹⁰cmHz^1/2·W^-1 under zero bias,respectively.After the light-off,decay time of the light current was 75ms.?3?Owing to the intrinsic material shortcomings of PbSe,such as poor mobility and prominent recombination,performances of the as-fabricated pn junction PbSe photodiodes were not robust enough.Therefore,graphene-PbSe heterojunction photodetectors were fabricated in this section,by transferring a monolayer graphene sheet onto the surface of PbSe films.And with the assistance of lithography-etching process,the graphene-PbSe photodiodes and the graphene-PbSe phototransistor were fabricated.The graphene-PbSe photodiode was a typical photovoltaic detector.Under 0 bias,responsivity and detectivity of the as-fabricated devices were 67mA·W^-1and6.8×10¹²cmHz^1/2·W^-1respectively.Through the combination of fast charge transfer in graphene and significant photoresponse in PbSe,huge gain was achieved in the graphene-PbSe hetero-phototransistor.In the as-fabricated structure,responsivity and detectivity were 6613A·W^-1and1.16×10¹²cmHz^1/2W^-1respectively.After the light-off,decay time of the light current were longer than 150ms in both device structure.?4?Poor dynamic performances of the as-fabricated graphene-PbSe heterojunction photodetectors were induced by the poor interface between graphene and PbSe,in which abundant interface defects could deteriorate the dynamic performances of the as-fabricated devices.In order to solve this problem,the Bi₂Se₃-PbSe heterojunction photodiodes were fabricated in this section by depositing a thin layer of topological insulator Bi₂Se₃ on the surface of PbSe films.At zero bias,responsivity and detectivity of the as-fabricated devices were confirmed as 3.9A·W^-1and 8.7×10¹¹cmHz^1/2W^-1respectively.Benefitting from the clean interface between Bi₂Se₃ and PbSe,after the light-off,decay time of the light current was less than 25ms.?5?At this time,we hope to further extend the prototypes above to the detection of long wavelength infrared band.Thus,synthesis of PbSnSe alloy by alloying of SnSe with PbSe in the MBE process was conducted.Decrease on band gap of the PbSnSe was realized,extending detectable wavelength to the long wave infrared band.Confirming by the infrared spectrum results,minimal value of the band gap in the as-synthesized PbSnSe film was 0.1eV,corresponding to the cutoff wavelength of 12?m.Therefore,a potential candidate for the fabrication of photodetectors functioning in long wave infrared band was found.At the moment,fabrication of Bi₂Se₃-PbSnSe heterojunction photodetector is in progress in our group,which could provide a potential routine for the application of IV-VI narrow-gap semiconductors.