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In this letter, TiO2 nanocrystalline film was prepared on SrTiO3 (001) substrate to form an n-n heterojunction active layer. Interdigitated Au electrodes were deposited on the top of TiO2 film to fabricate modified HMSM (heterojunction metal-semiconductor-metal) ultraviolet photodetector. At 10 V bias, the dark current of the detector was only 0.2 nA and the responsivity was 46.1 A/W at 260 nm. The rise and fall times of the device were 3.5 ms and 1.4 s, respectively. The TiO2/SrTiO3 heterojunction contributed a lot to the high responsivity and reduced the fall time, which improved the device performance effectively. These results demonstrate the excellent application of TiO2/SrTiO3 heterojunction in fabricating high performance UV photodetectors.
©2012 Optical Society of America
1. Introduction
Wide band gap semiconductor ultraviolet (UV) photodetectors have drawn much attention owing to civil and military applications [1–3]. At present, the substrate materials of UV detectors were mainly concentrated on SiC, diamond and ZnO films and extended to TiO2 and perovskite oxides [4–7]. As a member of perovskite oxide, impressive results regarding the photoelectric effect properties of SrTiO3 (3.2 eV) have been achieved. While photodetectors based on SrTiO3 substrate still exhibit a low responsivity [8]. On the other hand, due to its outstanding physical and optical properties, wide-bandgap semiconductor TiO2 (3.2 eV for anatase) has been studied extensively in UV detecting. But TiO2 based detectors show a slow time response [6]. The combination of the two materials provides a promise advantage in improving the UV photodetector performance. However, up to now, little attention has been paid to the related research on TiO2/SrTiO3 heterojunction (TSH).
As is known, structures play an important role in the device performance. Various structures were applied in detector fabrication, such as photoconduction, p-i-n junction and heterojunction metal-semiconductor-metal (HMSM) [9–11]. Among these, HMSM structure indicates obvious advantages for its ease fabrication and low dark current [12]. In this letter, we demonstrated the TiO2/SrTiO3 UV photodetector based on a modified HMSM structure, which exhibits high internal gain. The gain mechanism of the device was analyzed in detail.
2. Experiments
In the experiment, TiO2 thin film was prepared by sol-gel method. The procedure included dissolution of 10 ml tetrabutyl titanate [Ti(OC4H9)4] in 100 ml ethanol (C2H5OH), followed by adding 10 ml CH3COOH, then 10 ml acetylacetone, and 10 ml de-ionized water last. The mixture was stirred at room temperature for 30 min after each reagent was added. To prepare TSH active layer, the TiO2 solution was directly smeared on the commercial available SrTiO3 (001) substrate by spin coating at the rate of 3000 rpm for 20 s to ensure uniformity. After that, the sample was annealed for 2 h at 650 °C to form TiO2/SrTiO3 heterojunction. The thicknesses of the TiO2 films measured by a XP-2 profilometer were 20 nm. Interdigitated Au/TSH/Au circular structure was designed and fabricated. The Au films were deposited by radio frequency magnetron sputtering on top of the TSH active layer. Then the planar interdigitated electrodes were prepared by standard photolithography and lift-off technique. Both of the finger width and the spacing were equal to 20 µm, and the total active area was 0.38 mm2. With SrTiO3 (001) substrate and TiO2 films prepared on quartz substrate as the base materials separately, Au/SrTiO3/Au and Au/TiO2/Au detectors were fabricated in the same way for comparison. In order to verify the formation of TSH, Al and Au electrodes were used to provide ohmic contact on TiO2 film and SrTiO3 substrate, respectively.
The dark current and the I-V characteristics of the devices under irradiation of 260 nm UV light were measured by a Keithley 2601 Source Meter. A 30 W deuterium lamp and a monochromator were used to provide monochromatic light. The responsivity of the device was measured by a UV power meter together with a Keithley 2601 Source Meter. An oscilloscope and a 5 MΩ load resistance were used to measure the response time of the devices.
3. Results and discussions
In order to study and demonstrate the formation of TSH diode, the energy level diagrams of TiO2/SrTiO3 heterojunction are shown in Fig. 1(a) . Since the conduction band edge of SrTiO3 is 200 meV more negative than that of TiO2 [13], the electrons would diffuse from SrTiO3 into TiO2 while holes should diffuse from TiO2 into SrTiO3, thus a built-in electric field could be formed at the TiO2/SrTiO3 interface [14]. Under illumination, photo-generated carriers are separated by the built-in field, hence the recombination is reduced and collection efficiency of the carriers is improved accordingly. The I-V characteristics of the TSH diode in dark and under illumination of 260 nm are shown in Fig. 1(b). Obvious rectification behavior was observed. Since ohmic contacts are formed between the metal electrodes (Al or Au) and the substrate materials (TiO2 or SrTiO3) [8,15], the barrier should be caused by the TiO2/SrTiO3 heterojunction.
Fig. 1 (a) The schematic band diagram of TiO2/SrTiO3 heterojunction (b) The I-V characteristics of the TSH diode in dark and under illumination of 260 nm UV light.
Figure 2 shows the I-V characteristics of the Au/TSH/Au, Au/TiO2/Au and Au/ SrTiO3/Au detectors in dark and under the irradiation of 260 nm UV light. At 10 V bias, the TSH detector shows a small dark current of 0.2 nA. Considering the active area of the device (0.38 mm2), low dark current density of 5.3×10−8 A/cm2 was obtained, which is much lower than those of other reported detectors [1,2]. Under the irradiation of 9.7 μW/cm2, the photocurrents of TSH, TiO2 and SrTiO3 detectors are 1.7 μA, 1.1μA and 6.1 nA, respectively. It is noteworthy that the TSH detector exhibited the largest photocurrent and the ratio of photocurrent to dark current is more than three orders of magnitude, which is larger than that of TiO2 and SrTiO3 detectors. These results indicated the good performance of the TSH detector.
Fig. 2 (a) The I-V characteristics of the TSH, TiO2 and SrTiO3 detectors in dark (b) The I-V characteristics of the three detectors under the illumination of 260 nm UV light. Inset of Fig. 2(a) shows the picture of the final devices.
The introduction of TiO2/SrTiO3 heterojunction may play an important role in the evident photocurrent gain. The only difference between TiO2 and TSH detector is that a TSH active layer is introduced in the latter. In the case of TiO2 detector, under illumination and a certain bias, photo-generated electrons are collected by the positive electrodes, while photo-generated holes drift towards the TiO2/Au interface and are captured by the surface trapping states in TiO2 film due to band bending. These trapped holes will lead to the accumulation of the net positive charge at the TiO2/Au interface, and negative mirror charges will generate at the metal side, resulting in a reduction in the Schottky barrier height [16,17]. When a TSH active layer is introduced, photocarriers generated near the TiO2/SrTiO3 interface would be separated by the heterojunction electric field. Then photo-generated holes in the heterojunction would drift towards the negatively biased depletion region and would be trapped by defects in TiO2 film. This leads to more negative mirror charges, resulting in the further decrease of Schottky barrier height. Thus more carriers could get across the barrier and large photocurrent is obtained. Therefore, the photocurrent of TSH detector is obviously improved compared with that of TiO2 detector. The inset of Fig. 2 is the microscope picture of the final TSH detector.
Figure 3 is the spectral response of the Au/TSH/Au detector at 10 V bias under different wavelengths. The peak responsivity Rp could be determined by [18]
Where I is the photocurrent, Rλ is the normalized response function, Pλ is the incident power spectrum from the lamp, and λ is the wavelength. The photocurrent can be calculated by integrating the product of the responsivity spectrum (the normalized response function multiplied by the peak responsivity) and the incident power spectrum over all wavelengths. Therefore, the peak responsivity could be obtained from Eq. (1). Under irradiation of 260 nm UV light, the peak responsivity of TSH detector is 46.1 A/W, indicating a large internal gain in the detector. The inset shows the bias dependence of responsivity for the three detectors. It can be seen that the TSH detector has the highest responsivity. The high responsivity could be attributed to the internal gain, which is introduced by the wide neutral region between the electrodes working in a photoconductive mode [1]. On the whole, the photoconductive gain is achieved by trapping one type of carriers from recombination while the other type of carriers could transit freely in the photoconductor repeatedly. Therefore, the gain g can be expressed as [6]where τ is the hole (minority) lifetime, t is the transit time of electrons, which could be written aswhere mμE is the electron drift velocity, L is the interelectrode spacing, mμ is the electron mobility and E is the electric field. Since E could be described as V/L, where V is the applied bias, the whole equation can be expressed as [19]As V, L, and mμ are constant in the experiment, the lifetime τ of the holes plays a major role in obtaining a large gain. Owing to being trapped at the TiO2/Au interface by the defects, the holes are difficult to recombine with the electrons. Moreover, the barrier at the TiO2/SrTiO3 interface would separate the photo-carriers and are helpful in reducing the recombination of carriers. Therefore the lifetime of the holes would be longer. Hence a larger gain could be obtained. The calculated internal gain of the TSH and TiO2 detectors are 219.9 and 142.3 respectively. Compared with other similar devices [8,10], a significant improvement in the internal gain is obtained when a TSH active layer is introduced.Fig. 3 The spectral response of the TSH detectors at the bias of 10 V. Inset shows the bias dependence of responsivity for TSH, TiO2 and SrTiO3 detectors.
The time response characteristic of the TSH detector is shown in Fig. 4 . The inset shows the time response of TiO2 detector. The response time of the device was obtained by measuring the voltage variation in a 5 MΩ load resistance in the test circuit. The rise time is 3.5 ms and the fall time is 1.4 s, which is faster than that of other similar devices [2,6]. And compared with TiO2 detector (17.5 ms and 21.1 s), there is an obvious improvement in the time response performance in TSH detector. It is known that the response time of photoconductive gain detectors is mainly decided by the energy level of the trap states—the shallower the trap states exist, the quicker the thermal emission assisted de-trapping process is, which manifests as a shorter response time [20]. Since the energy levels of trap states in SrTiO3 is shallower than that in TiO2 [21,22], a faster response time in the TSH detector is expectable.
4. Conclusion
In conclusion, TiO2 nanocrystalline has been prepared on SrTiO3 (001) substrate to form an n-n heterojunction. Interdigitated Au electrodes were deposited on the heterojunction active layer to fabricate UV photodetector. At 10 V bias, the dark current of the detector was 0.2 nA and the responsivity was 46.1 A/W at 260 nm. The ratio of photocurrent to dark current is about three orders of magnitude, which is much higher than that of TiO2 or SrTiO3 MSM detector. The rise time and the fall time are 3.5 ms and 1.4 s, respectively. The modified HMSM structure enhanced the responsivity and reduced the fall time. The results expose that the TiO2/SrTiO3 heterojunction active layer is beneficial for the improvement of device performance.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 60977031, 5097703, 61007022); Chinese National Programs for High Technology Research and Development (Grant No. 2009AA03Z402); Doctoral Found of Ministry of Education of China (Grant No. 20090061110040).
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