Al doped ZnO based metal–semiconductor–metal and metal–insulator–semiconductor–insulator–metal UV sensors
Introduction
In recent times, ZnO thin film based metal–semiconductor–metal (MSM) UV-sensors (UVSs) have made much progress because of their simple fabrication step and high gain. These UVSs have wide applications, such as flame detection, chemical agent detection, space communications, and solar astronomy [1], [2], [3]. Variety of UVSs has been developed according to the requirements of applications. For fabricating UVSs working in the wavelength range (200–380 nm), Al doped ZnO (Al:ZnO) or MgZnO thin films are being used by researchers [4], [5], [6]. Because the bandgap of Al:ZnO or MgZnO materials can be tuned (from 3.3 to 7.8 eV) by changing the doping concentration [7], [8], [9]. Al:ZnO thin films have attracted much attention because of its some merits like high refractive index, non-toxicity, high stability under hydrogen plasma and low-cost synthesis [4], [5], [7].
To grow high-quality ZnO or Al:ZnO thin films, a lot of fabricating techniques were applied, such as pulsed laser deposition (PLD) [10], sol–gel synthesis [11], [12], spray-pyrolysis [13], sputtering [14] and thermal evaporation of metallic zinc [15]. Among them RF sputtering method is most common and reliable method for deposition of ZnO or Al:ZnO thin films.
Researches are being carried out by many groups to improve the performance of the MSM UVSs to increase their photocurrent, contrast ratio and to decrease the dark current. Young et al. reported RF-sputtered ZnO-based MSM and metal–insulator–semiconductor (MIS) UVSs with responsivities 0.089 and 0.0083 A/W, respectively [16]. Zhou et al. reported ZnO nanowire (NW) UV nanosensors (NSs), they found the sensitivity of ZnO NW NSs has been improved for four orders of magnitude by fabricating Schottky type devices instead of Ohmic type devices [17]. Ali et al. reported sol–gel deposited ZnO based MSM UVSs, with responsitivity value 0.056 A/W. They fabricated MISIM UVSs by inserting 5 nm oxide (SiO2) layer above ZnO layer, which showed better responsivity 0.206 A/W than its MSM counterpart [18].
Although many research-groups are working on ZnO MSM UVSs, however, study on the Al:ZnO based UVSs are very less. In this paper, we have fabricated Al:ZnO based MSM and MISIM UVSs. For fabricating MISIM structure, one thin insulating layer of SiO2 is inserted in between the metal and semiconducting layer of MSM structure. The effect of this SiO2 layer on the UV sensing properties of MISIM sensors (Ss) are investigated and compared with the properties of MSM Ss.
Section snippets
Preparation of Al:ZnO thin films
Al:ZnO thin films were deposited on p-type Si substrates by RF magnetron sputtering method. Before deposition of Al:ZnO thin films, Si substrates were cleaned by two cleaning solutions named RCA-1 and RCA-2, for removing various contaminants from the surface of Si substrates. DI water with resistivity ∼18 MΩ cm was used for rinsing the substrates after RCA solutions cleaning. At a base pressure of 10−5 mbar and sputtering pressure 2.6 × 10−3 mbar, AZO thin film was grown on silicon substrates by
Structural properties
The XRD plot of Al:ZnO thin film is shown in Fig. 2. Diffraction angle (2θ) was plotted with respect to the intensity of X-ray. The unit of intensity was kept arbitrary and the diffraction angle (2θ) was scanned in the range 30° to 55°. The variation of diffraction angle was 0.2 degrees per step with a time step of 10 s. It can be seen from diffraction pattern in Fig. 2 that Al:ZnO films has a strong peak at 34.4° at the (0 0 2) plane with preferred c-axis orientation. Other peaks (1 0 1) and (1 0 0)
Conclusion
In summary, RF-sputtered Al:ZnO thin films were grown on p-type Si substrates at room temperature. MSM and MISIM UV-sensors with Pd interdigitated metal electrodes have been fabricated. It was found that by adding a thin insulating-layer of SiO2 below the Schottky contact, the parameters of the photodetector can be improved. The dark current of MISIM was reduced in comparison to dark current of MSM UVSs. Similarly, the contrast ratio of MISIM improved and became higher than that of MSM UVSs.
Acknowledgements
This work is done at IIT- Bombay Nano Fabrication (IITBNF) lab, Mumbai, India. The author gratefully acknowledge Prof. Anil Kottantharayil for his valuable suggestions and discussions for this work.
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