Temperature Dependence of Photoelectrical Properties of Single Selenium Nanowires
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Influence of temperature on photoconductivity of single Se nanowires has been studied. Time response of photocurrent at both room temperature and low temperature suggests that the trap states play an important role in the photoelectrical process. Further investigations about light intensity dependence on photocurrent at different temperatures reveal that the trap states significantly affect the carrier generation and recombination. This work may be valuable for improving the device optoelectronic performances by understanding the photoelectrical properties.
KeywordsSe nanowires Trap states Photoconductivity Temperature effects
Semiconductor nanowires have great potential applications as the building blocks for nanoscale electronic and optoelectronic devices. Selenium (Se) is an important semiconductor, and Se nanowires have attracted enormous attentions due to their unique physical properties, such as high photoconductivity, large piezoelectric, and thermoelectric effects [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. In particular, photodetectors and photoelectrical switchers based on individual Se nanowires have been fabricated [10, 11, 12, 13, 14]. In order to further improve the performance of the Se nanowires optoelectronic devices, well understanding of the photoelectrical properties is desirable. Measurement of the temperature dependence of photoconductivity is an efficient method to study the photoelectrical properties because it can yield more information about the carrier generation and recombination. However, the detailed temperature effects on photoconductivity of single Se nanowires are not yet reported. In this work, we study the photoconductivity of single Se nanowires by measuring the time response on photocurrent and the light intensity dependence on photocurrent at different temperatures. It is found that the trap states are sensitive to the temperature.
In order to perform the measurement of photoelectrical properties of individual Se nanowires, the Se nanowires were mechanically transferred onto the SiO2(500 nm)/Si substrate. Ti/Au (10/70 nm) electrodes contacting onto individual Se nanowires were fabricated by electron beam lithography, metal sputtering deposition, and lift-off. SEM image of the fabricated two-terminal nanowire device is shown in Fig. 1d. Conductance measurements were carried out using a Keithley 4200 Semiconductor Characterization System and the samples were placed on a Janis micro-cryostat under vacuum (~10−6 Torr). For photoconductivity measurements, illumination was provided by an Ar ion laser with 514 nm wavelength guided by a Renishaw Raman microscope.
Figure 2b shows the time course of photocurrent measured at 100 K and 0.2 V bias. The photocurrent has a quick response to the laser illumination switching. No transient decay of photocurrent was observed after the device was exposed under illumination. At low temperatures and at dark, the carriers captured in the trap states are almost frozen. Under illumination, the carriers generated from the trap states and the bandgap contribute to the photoconductivity. The dynamic balance between the carrier generation and recombination results in the relatively steady photocurrent but with some noise. After turning off the illumination, the current declines quickly, and then the dark current slowly decreases with a long relaxation time, as shown in Fig. 2b. The relaxation of the dark current is due to the recapture of the carrier by the trap states.
In summary, the temperature effects on photoelectrical properties of single Se nanowires have been investigated. The light intensity dependence of photocurrent suggests that the Se nanowire photoconductivity is dominated by trap states at low temperatures, while a weak dependence of photocurrent on incident light intensity was observed at room temperature.
We thank Prof. Yadong Li of Tsinghua University for supplying the Se nanowires. This work was supported by NSFC (No. 10804002), MOST (Nos. 2007CB936202, 2009CB623703), and the Research Fund for the Doctoral Program of Higher Education (RFDP).
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