Growth Of Low-dimensional Metal(cd, Sn, Ge) Selenide Semiconductors And Investigation Of The Electrical Transport Property Under Irradiation

Recently, the in-situ characterization of physical properties of the low-dimensiional semiconductor nanomaterials by using in-situ electron microscope has attracted much attention all over the world. It is important to reveal the effect of electron-beam irradiation on the properties(e.g. electrical transport and optoelectrical properties) of the low-dimensional semiconductor nanomaterials for the electron-beam irradiation is inevitable in the in-situ process. In this paper, low-dimensional metal selenide semiconductors were synthesized and then their electrical and optoelectrical properties were investigated before, during and after electron-beam irradiation in a SEM.(1) Low-dimensional nanostructured II-VI binary(Cd Se) and ternary(Cd SxSe1-x) compounds have potential applications in the field of electronics and optoelectronics due to their unique characteristics. However, it is known that intrinsic Cd Se and Cd SxSe1-x usually have low conductivity which limits their performance in certain device applications. Doping is an effective method to improve the electrical transport property of semiconductor nanostructures. The Sb doped Cd Se and In, Cl co-doped Cd SxSe1-x nanostructures were synthesized by using chemical vapor deposition(CVD) process, and then high-performance field effect transistors(FET), photodetectors and Schottky diodes were fabricated by using these nanostructures. The results indicate that the doped Cd Se nanobelts have n-type field effect behaviour and high conductivity of approximately five orders of magnitude larger than that of the intrinsic nanobelts. The photodetectors based on single doped nanobelt exhibit high responsivity(6.1×104 A/W), high external quantum efficiency(1.2×105) and large Ilight/Idark ratio(~253) for a 650-nm-light at 1 V. A Schottky diode based on a doped-nanobelt/gold contact has a rectification ratio of ~2×106 and excellent optoelectrical property. A n-type FET based on individual In and Cl co-doped Cd SxSe1-x nanowire exhibits high Ion/Ioff ratio of ~200 and carrier mobility of 62.2 cm2/(V×s). The carrier density of the nanowire is obtained to be 1.9×1017/cm3. The conductivity and photoconductivity distribution(yellow light with power density of 1.53 m W/cm2) the doped Cd SxSe1-x nanowires are 1-10 and 0.1-20 S/cm respectively. The electrical measurement and XPS results indicate that the In and Cl are successfully doped into the Cd SxSe1-x nanostructures. In addition, part of Cd and S(or Se) are probably substituted by In and Cl, respectively. Therefore, the In and Cl as donor impurities improve the conductivity of the nanostructures. The temperature dependent I-V curves of the doped Cd SxSe1-x nanowires were investigated, indicating that the conductivity of the nanostructures decreases with decreasing the temperature. The above results indicate that the as-synthesized Cd Se and Cd SxSe1-x nanostructures are excellent building blocks for high-performance electronics and optoelectronics.(2) The Sn Se, Sn Se2 and Ge Se2 are important IV-VI semiconductors. The plate-like Sn Se and Sn Se2 structures were synthesized by a one-step CVD process. The growth mechanism and electrical properties of Sn-Se structures was investigated. The Sn Se and Sn Se2 can be obtained at the high and low temperature zone respectively due to their difference of melting point. The growth of Sn Se2 hexagonal-plate depends on the concentration gradient of gaseous reactants. The electrical measurement results indicate that the resistivity obtained along [001] is ~15 times higher than that obtained along the direction perpendicular to [001] for Sn Se2 nanoplates and the resistivity obtained along [100] is ~105 times higher than that obtained along the direction perpendicular [100] for Sn Se micro-plates. In addition, the Ge Se2 nanobelts with zigzag structure were synthesized and their growth mechanism was discussed, indicating that the growth including vapor-liquid-solid and vapor-solid process.(3) The electrical and optoelectrical properties of the Cd Se、Cd S、Cd SxSe1-x、Ge Se2 and Zn Se low-dimensional nanostructures were investigated before, during and after electron-beam irradiation. The results indicate that the conductivity of the Cd Se、Cd S and Cd SxSe1-x can be perpetually improved due to the electron-beam irradiation. The performance of a FET based on a single Cd Se nanobelt is significantly improved by the electron-beam irradiation. Comparing with the FET before irradiation, the FET after irradiation has a higher Ion/Ioff ratio, carrier mobility and carrier concentration(e.g. from ~12 to 5×107, from 0.15 to 155.3 cm2V-1s-1 and from ~1.1×1014 to ~1.3×1016 cm-3, respectively). The conductivity of Cd S nanobelt after irradiation is ~106 times larger than that before irradiation. Besides the conductivity of Cd S nanobelts after irradiation increases firstly and then decreases with increasing the electron-beam energy. The carrier concentration of the Cd SxSe1-x nanowires increases due to electron beam irradiation(e.g. from 7.62×1017 to 2.13×1018 cm-3). The electron-beam induced effect depends on the diameter of the Cd SxSe1-x nanowires, i.e. the increased conductivity benefits from the small diameter. In addition, the increased conductivity benefits from the conductivity before irradiation. Comparing with before irradiation, both photocurrent and external quantum efficiency of a Cd S nanobelt device increase to ~10 times but its response time also increase greatly after irradiation. The electrical and optoelectrical performance can be back to the state before irradiation when these nanostructures are placed in the air for long time.A possible mechanism of the above electron-beam induced effect is proposed below. Large number of secondary-electrons and electron-hole pairs produce in the nanostructures under the electron-beam irradiation( ≤ 30 ke V). Therefore the conductivity of the nanostructures is enhanced greatly during irradiation. The recombination of electron-hole pairs occurs after irradiation, but most of the secondary-electrons reside in the nanostructures or on their surface. These residual electrons must reside in the conduction band as all lower energetic state are filled. These electrons are free to move under the influence of an external field, and hence, improve the conductivity and change the optoelectrical properties.However, the electrical properties of Zn Se and Ge Se2 are not changed significantly after electron-beam irradiation. Therefore, the electron-beam induced effect on electrical transport properties depends on the nature of materials.