Growth and characterization of Ag/n-ZnO/p-Si/Al heterojunction diode by sol–gel spin technique
Highlights
► Ag/n-ZnO/p-Si/Al heterojunction diode was grown via sol–gel technique. ► The characterization of ZnO material was investigated. ► The heterojunction structure showed a rectification behavior. ► Ideality factor and barrier height were found to be 2.03 and 0.71 eV, respectively.
Introduction
In transparent conducting oxide films, Zinc Oxide (ZnO) has gained substantial interest in the research community in part and widely studied in electronic and optical applications due to its excellent opto-electrical properties. ZnO is used in a wide range of optoelectronic applications such as light-emitting diodes (LEDs), flat display panels, solar cells, thin film photovoltaic cells, sensors and detectors [1], [2], [3], [4]. It is one of the most promising candidate material for optoelectronic devices such as photodetectors and heterojunction diodes, due to its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature [5] and suitable bandgap (3.37 eV) [6], [7].
Ohmic and Schottky contacts are essential for ZnO-based diodes to obtain some electrical parameters, almost all of these devices mentioned above require high-quality ohmic and rectifying contacts to get maximum efficiency out of them. Fabricating a good quality rectifying contacts on n-type ZnO is more difficult compared to obtaining Ohmic contacts, due to the high donor existence at the surface region, caused by defects such as zinc interstitials (Zni) and oxygen vacancies (Vo) [8]. The chemical reactions between metal and semiconductor, the defects in the surface region and the metal diffusion into the semiconductor are well-known problems of preventing the formation of Schottky contacts. Özgür et al. had reported that a high work function has to be applied to the surface of a ZnO crystal in order to create Schottky barrier with undoped ZnO [5]. Although Gold (Au) has some serious problems at temperatures higher than 330 K [9], [10], it has been widely used to form Schottky barriers to ZnO. Silver (Ag) has been used as a Schottky contact metal by some researchers [11], [12], [13] as well. Sheng et al. studied the Ag/ZnO Schottky diode and determined the flatband barrier height to be 0.89 and 0.92 eV by current–voltage and capacitance–voltage measurements, respectively. The ideality factor was also found to be 1.33 [13]. Since the use of heterostructure provides an advantage in the control of the electronic and optoelectronic properties of semiconductor devices [14], [15], a number of significant studies, especially on photodiode properties of n-ZnO/p-Si heterojunctions, have been carried out on the fabrication of ZnO-based heterojunctions using materials, such as p-Si and p-AlGaN. ZnO/Si heterojunctions are particularly interesting due to their more cost effectiveness and flexibility for optoelectronic device fabrications [16]. The fabrication and electroluminescence of an n-ZnO nanorod/p-Si heterojunction have been reported by Sun et al. [17] and Chen et al. [18].
Recently, there are reports about high quality ZnO films grown by the well-known techniques such as metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), radio frequency magnetron sputtering, pulsed laser deposition (PLD), sol–gel method and etc. [6], [19], [20], [21], [22], [23], [24], [25]. In this study, the sol–gel method was used to synthesize ZnO thin films on p-Si due to some advantages such as low cost, simplicity and versatility of its experimental procedure, large area production and homogeneity. To the best of our knowledge, there are only few reports [2], [16] on ZnO heterojunction diodes. In this paper, we report the device performance of the Ag/n-ZnO/p-Si/Al diode with structural and optical properties of ZnO thin film.
Section snippets
Experimental
ZnO thin films have been grown by sol–gel spin coating method onto p-type Si with resistivity of ∼0.01–0.03 Ω-cm and with [1 0 0] orientation and onto glass substrate. Zinc acetate dihydrate [Zn(CH3COO)2·H2O] was used as a starting material. 2-Methoxyethanol (C3H8O2) and Monoethanolamine (C2H7NO, MEA) were used as solvent and stabilizer, respectively. The solution was prepared as 0.5 M and the molar ratios of metals to MEA were taken in the ratio of 1:1. The sol was stirred at 60 °C for 2 h to obtain
Results and discussion
The XRD result of the ZnO film on p-type Si substrate is shown in Fig. 1. As can be seen in Fig. 1, there appears ZnO (0 0 2) peak at about 2θ = 33.8o indicating that the hexagonal wurtzite structure is predominant in ZnO film. The low intensity peak located at 2θ = 61.20° is attributed to (1 0 3) plane of hexagonal ZnO. Periasamy et al. [26] found almost similar result for ZnO thin film prepared on p-type Si substrate by vacuum coating technique. The crystallite sizes of the ZnO thin film were
Conclusion
We report fabrication of an Ag/n-ZnO/p-Si/Al heterojunction diode by sol–gel spin coating technique. The film exhibits hexagonal wurtzite structure with a strong (0 0 2) preferred direction perpendicular to the substrate. In addition, the low intensity (1 0 3) peak located at 2θ = 61.20° was observed in the XRD pattern. The crystallite size of ZnO is found 31.4 nm. The c and a lattice constants of the sample are 5.306 and 3.353 Å, respectively. The band gap of the ZnO film on glass substrate was found
Acknowledgements
This work was supported by the Atatürk University Research Fund, Project no 2011/98. One of the authors (E.F. Keskenler) would like to thank to Mustafa Furkan Keskenler for technical support.
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