Fabrication and ultraviolet photoresponse characteristics of ordered SnO x (x ≈ 0.87, 1.45, 2) nanopore films
© Li et al; licensee Springer. 2011
Received: 3 July 2011
Accepted: 6 December 2011
Published: 6 December 2011
Based on the porous anodic aluminum oxide templates, ordered SnO x nanopore films (approximately 150 nm thickness) with different x (x ≈ 0.87, 1.45, 2) have been successfully fabricated by direct current magnetron sputtering and oxidizing annealing. Due to the high specific surface area, this ordered nanopore films exhibit a great improvement in recovery time compared to thin films for ultraviolet (UV) detection. Especially, the ordered SnO x nanopore films with lower x reveal higher UV light sensitivity and shorter current recovery time, which was explained by the higher concentration of the oxygen vacancies in this SnO x films. This work presents a potential candidate material for UV light detector.
PACS: 81.15.Cd, 81.40.Ef, 81.70.Jb, 85.60.Gz.
Keywordshighly ordered tin oxide nanopores films anodized aluminum oxide(aao) ultraviolet(uv) response oxygen vacancies
The AAO templates were prepared through stable high-field anodization in a H3PO4-H2O-C2H5OH electrolyte system . Anodization was carried out in a H3PO4-H2O-C2H5OH electrolyte system (concentration of H3PO4, 0.25 M) at 195 V. The temperatures of the electrolytes were kept at -10°C to 0°C with a powerful low-constant temperature bath. Sn films were deposited on AAO substrates by direct current (DC) magnetron sputtering using a circular tin target (diameter, 60 mm; purity, 99.99%) at room temperature. The base pressure, deposition pressure, substrate-target distance, sputtering power, and the Ar flux were 1 × 10-3 Pa, 0.85 Pa, 6 cm, 30 W, and 10 sccm, respectively. The sputtering time (t) was fixed at 3 min. To obtain the ordered porous SnO x films and to perform its electrical measurements under UV irradiation, three same samples was annealed at 350°C, 450°C, and 550°C in a quartz tube furnace system for 120 min at a heating rate of 10°C/min, respectively. The quartz tube was evacuated to about 50 Pa before heating and the flow rates of Ar and O2 are both fixed at 100 sccm during annealing. Then a 300-nm-thick gold electrodes was evaporated on the surface of SnO x nanopore films through a shadow mask and copper wires were connected to the electrodes at two contact pads by conducting silver glue. The spacing between the electrodes was 1 mm, and the length of the electrodes was 5 mm. The device structure is depicted in Figure 1d. What's more, SnO x thin films UV device on the quartz substrate were prepared under the same deposition and post-annealing condition as mentioned above for the purposes of comparison. Electrical measurements of all devices were carried out with a Keithley 2400 source-measure unit under ambient conditions. For UV detection, a xenon lamp was used as the light source and an excitation filter centered at 254 nm and the bias voltage was fixed at 1 V. The structural properties were determined using a D8 DISCOVER X-ray diffractometer (XRD) with Cu Κα radiation. The growth and surface morphologies were observed using a field-emission scanning electron microscope (FE-SEM, Philips Sirion 200, Philips, Holland, Netherlands). The Raman spectra of the SnO x nanostructures were measured using a Jobin Yvon LabRam HR 800 UV system with a 325 nm He-Cd laser.
Results and discussions
Composition evolution after annealing at different temperature
Photoresponse of the SnO x nanopore films under UV irradiation
The decreased recovery time of the ordered SnO x nanopore films compared to the thin films can be attributed to the increased surface areas. It is known that the oxygen molecules are absorbed onto SnO x surface by capturing free electrons from the n-type SnO x [O2(g) + e- → O2 -(ad)], which decrease the carrier density in the films and hence the porous films show a higher resitance. Upon UV illumination, electron-hole pairs are generated. The holes migrate to the surface along the potential slope produced by the band bending and recombine with the negatively charged adsorbed oxygen ions [h++O2 - → O2(g)], resulting in an enhancement of photocurrent. When the illumination is turned off, the films with higher surface area make O2 readsorbed on the surface easier, which lead to a shorter recovery time.
For the sample with a lower annealing, temperature shows a shorter recovery time, which could be attributed to below two main processes. First, it is known that oxygen vacancies in SnO x act as electron donors and the number of oxygen vacancies is expected to increase in lower annealing temperature under certain oxygen flows and annealing time (confirmed by the results of XRD pattern and Raman spectra above), higher concentration of the oxygen vacancies will give higher probability of the adsorption of oxygen molecules onto the surface of SnO x films, leading to the fast decreasing of the photocurrent. Second, the increase in the oxygen vacancies is expected to decrease the bending of the semiconductor near the surface . Electrons and holes recombine more easily with less bended band, inducing a shorter carrier lifetime. So the photocurrent decay after switching off UV is faster for the sample at lower annealing temperature.
In conclusion, we firstly report an effective method for the fabrication of ordered SnO x nanopore films. Annealing temperature is the key factor to control Sn/O ratio. Reversible photoconductive switching characteristics of the films were exhibited by switching UV light on/off, which is ascribed to the oxygen desorption/reabsorption on the surface of SnO x film. It is noted that the ordered SnO x nanopore films with lower x value possess more excellent ability to detect weak UV light, which could be attributed to the higher concentration of the oxygen vacancies in this SnO x films. Especially, this ordered nanopore films exhibit shorter recovery time compared to the thin films, which can be attributed to the increased surface areas. This study presents a new approach for fabricating UV light sensors based on Tin oxide films.
anodized aluminum oxide
field-emission scanning electron microscope
energy disperse spectroscopy
This work was supported by the National Major Basic Research Project of 2010CB933702, Natural Science Foundation of China (grant No. 11174197, 10874115, 10804071, and 10734020), National 863 Program 2011AA050518863, Shanghai Nanotechnology Research Project 0952nm01900, Research fund for the Doctoral Program of Higher Education of China. We thank Instrumental Analysis Center of SJTU for SEM analysis.
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