Ultraviolet photodetectors based on ZnO nanorods-seed layer effect and metal oxide modifying layer effect
© Zhou et al; licensee Springer. 2011
Received: 5 October 2010
Accepted: 15 February 2011
Published: 15 February 2011
Pt/ZnO nanorod (NR) and Pt/modified ZnO NR Schottky barrier ultraviolet (UV) photodetectors (PDs) were prepared with different seed layers and metal oxide modifying layer materials. In this paper, we discussed the effect of metal oxide modifying layer on the performance of UV PDs pre- and post-deposition annealing at 300°C, respectively. For Schottky barrier UV PDs with different seed layers, the MgZnO seed layer-PDs without metal oxide coating showed bigger responsivity and larger detectivity (D λ*) than those of PDs with ZnO seed layer, and the reason was illustrated through energy band theory and the electron transport mechanism. Also the ratio of D 254* to D 546* was calculated above 8 × 102 for all PDs, which demonstrated that our PDs showed high selectivity for detecting UV light with less influence of light with long wavelength.
Recently, a one-dimensional (1D) nanomaterial has attracted a lot of attention both for fundamental research and potential nano-device applications because of its peculiar characteristics and quantum size effect [1, 2]. Among the various nano-structured materials, due to their direct and wide energy bandgap (3.37 eV), ZnO nanorods (NRs) are a promising functional material as potential candidates for short-wavelength optoelectronics applications such as nanoscale lasers , light-emitting diodes , and ultraviolet (UV) photodetectors (PDs) [5–9]. Although ZnO has many advantages, the existence of many defects of ZnO NRs prepared by hydrothermal method  may benefit the formation of ohmic contacts at the electrode/ZnO NRs interface, which is an obstacle to applications in PDs due to its slow response and recovery behaviors.
The Schottky barrier plays an important role in improving the performance of the PDs, and many researchers have investigated the Schottky contact between ZnO NRs and metal [11–15], but investigations on effects of metal oxide coating and seed layer on ZnO NW Schottky PDs using post-deposition thermal annealing treatment are scarce. In this study, to investigate the effect of the seed layer and oxide material on the performance of PDs, a simple route to gain Schottky barrier by deposition of Pt electrodes on the top of different oxide material-coated n-ZnO NRs, which are prepared by hydrothermal process on different seed layers is introduced. Then, the samples are treated by thermal annealing process to form Schottky contacts. In this article, the authors have discussed the effects of metal oxide-modified layer on the performance of UV PDs pre- and after post-deposition annealing at 300°C. The investigation of PDs with different seed layers shows that the MgZnO seed layer-PDs without metal oxide coating demonstrates bigger responsivity and larger detectivity than those of PDs with ZnO seed layer, and the reason has been illustrated through energy band theory and the electron transport mechanism. Also the ratio of detectivity (D λ*, D 254* to D 546*) is calculated above 8 × 102 for all PDs, which demonstrates that our PDs show high selectivity for detecting UV light with lesser influence of light with long wavelength. The attractiveness of this study is the simplicity of the fabrication process, which could easily be scaled up, and our results may pave the way for the application of low-cost ZnO NRs UV PDs.
Results and discussion
It is very well known that the metal oxides deposited at 100°C have some structure defects with high carrier density, which will benefit the formation of ohmic contacts and electron transport. Hence, the metal oxide, as an electron transport layer in PDs, can improve the contact resistance between metal and semiconductor. Therefore, the PDs with metal oxide coating can enhance photoresponse characteristic before annealing. After annealing, the structure defects decrease, and the electrical resistivity of all metal oxides will increase, the photogenerated electrons will be blocked, and very few can be collected by Pt electrode at forward bias. However, for PDs without oxide coatings, the contacts at Pt/ZnO NRs interfaces are improved by annealing process, and the photogenerated electrons can easily reach to Pt electrode at forward bias and get high photocurrent, and PDs without oxide coatings show fast response and recovery behavior after annealing. Therefore, it is concluded that the PDs without oxide coatings display better performance than those with oxide coatings.
where n is the ideal factor, K is the Boltzmann's constant, T is the absolute temperature, ΦB is the barrier height, A is the Schottky contact area, and A* is the effective Richardson coefficient constant. By means of forward biased I-V measurements and Equation (1), it can be deduced that for the PDs with ZnO and MgZnO seed layer, Schottky barrier heights ΦB at the Pt/ZnO NRs interface are, respectively, about 0.768 and 0.796 eV at dark and the respective ΦB values are about 0.738 and 0.734 eV under 365-nm light. From above, it can be seen that ΦB decreases under 365-nm light, and it decreases by 0.03 and 0.062 eV for the PDs with ZnO and MgZnO seed layer, respectively. The decrease of ΦB for the PDs with MgZnO seed layer is two times that for the PDs with ZnO seed layer, which illustrates that the larger increase of photocurrent will result in the larger decrease of ΦB.
where R is the responsivity of the photodiode, J d is the dark current, J ph is the photocurrent density, and L light is the light intensity. Detectivity is calculated and also plotted in Figure 5. From the curves of the detectivity of PDs, it can be noted that the Schottky barrier PDs exhibited spectral response mainly in the range from 250 to 400 nm, with the detectivity above 1011 Jones (1 Jones = 1 cmHz1/2/W), and the detectivity of the PDs with MgZnO seed layer is higher than that of the PDs with ZnO seed layer. At the wavelength above 400 nm, the PDs show little detectivity, and the detectivity decreases with the increase of the wavelength. The ratio of D 254* to D 546* is above 8 × 102, which shows that the PDs have high selectivity for detecting UV light with less influence of light with long wavelength.
In conclusion, Schottky barrier PDs based on ZnO NRs were prepared by varying seed layers and metal oxide-coating materials. Before annealing, PDs coated with metal oxide materials showed enhanced photoresponse compared to that without coatings. However, after annealing treatment, the metal oxides will block photogenerated electrons to electrodes and reduce photocurrent. Also, after annealing at 300°C, contacts at the electrode/ZnO NRs or electrode/oxide interface were Schottky type, and the performance of the PDs has improved with the great decrease of response and recovery times. For different seed layer-PDs without oxide coating, the PDs with MgZnO seed layer showed bigger responsivity and lager detectivity than those of PDs with ZnO seed layer, and the ratio of D 254* to D 546* was above 8 × 102 for all PDs. The results may provide a simple route to obtain low-cost high performance UV PDs.
This study was partially supported by the National High Technology Research and Development Program of China (2009AA03Z219), the National Basic Research Program (2011CB933300) of China, the National Natural Science Foundation of China (11074194), and the Special Fund of Ministry of Education for Doctor's Conferment Post under grant No. 20070486015.
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