Deposition of the low resistive Ag–N dual acceptor doped p-type ZnO thin films
Introduction
In recent years ZnO has become a promising material for solar cells, surface acoustic wave devices [1], optoelectronic devices such as ultraviolet light-emitting diodes, laser diodes and photodetectors [2]. Among the available wide band gap semiconductors, ZnO is considered as the brightest emitter even at room temperature as it has wide band gap (3.37 eV) and large excitation binding energy (60 meV) [3]. Recent reports on the p-type conductivity in ZnO are broadening the scope for homo- and heterojunction ZnO devices. For the development of ZnO based devices, it is necessary to have p-type and n-type ZnO [4]. First principles total energy calculations showed that the oxygen vacancy (VO) and zinc interstitial (Zni) defects in ZnO have low formation energies and these intrinsic defects act as the sources for the unintentional n-type conductivity [5]. Hence, achieving p-type conduction in ZnO is difficult due to the self-compensation effect and low solubility of the acceptor dopants in ZnO [6]. There are many reports claiming the realization of p-type conduction in ZnO films by mono-doping of group V elements such as N [7], [8], P [9], [10], As [10], [11] and Sb [12], or group I elements like Ag [13] and Li [14]. Recently, a dual-doping method using two acceptor agents namely Li–N [15], [16] or As–N [17] was proposed to prepare p-type ZnO. In this line, Zhang et al. [18] suggested that the dual-doping method is the best channel to overcome the difficulties in achieving p-type ZnO.
Furthermore, reproducible and long-time stable p-type ZnO has been realized by the dual-acceptor doping of silver and nitrogen. Ag and N are the two best candidates for producing p-type ZnO considering the strain effects and energy levels of substitutional AgZn and NO acceptors. However, there are only a few studies on ZnO:(Ag, N) films reported in literature. Recently, Yan et al. [4] reported the fabrication of p-type ZnO:(Ag, N) films with good electrical properties by ion beam assisted deposition. Similarly, Bin et al. [19] fabricated the p-type ZnO films by dual-acceptor doping of Ag and N using the ultrasonic spray pyrolysis technique. In the present study, p-type ZnO:(Ag, N) films with low resistivity were achieved by spray pyrolysis. The spray pyrolysis technique is a simple, economical and more versatile than other chemical deposition techniques. In addition, it allows the possibility of obtaining films with the required properties for different applications. Further, it can be easily adapted for the production of large-area films [20]. In this paper, the authors concentrate on the effect of doping concentrations of Ag and N on the structural, morphological, compositional, electrical transport and optical properties of ZnO films deposited by the spray pyrolysis technique.
Section snippets
Experimental
In the present work the ZnO:(Ag, N) films were deposited using a spray solution mainly comprises of 0.1 M Zn(CH3COO)2·2H2O (Sigma-Aldrich, 99.5%, Germany) dissolved in a mixture of deionized water and ethanol (Merck, 99.9%, Germany) taken in the proportion of 9:1 respectively. The dopant concentration was varied from 0 to 4 at% for both Ag and N. The Ag and N dual doping was achieved by dissolving an appropriate quantity of AgNO3 (Sigma-Aldrich, 99.9%, USA) and CH3·COONH4 (Sigma-Aldrich, 99.9%,
Structural analysis
Fig. 1 shows the XRD patterns of ZnO:(Ag, N) thin films with Ag and N dopants concentration varied from 0 at% to 4 at%. It can be observed from Fig. 1 that all the films exhibit the characteristic diffraction peaks of ZnO (JCPDS card: 75-0576) indicating that the films are in polycrystalline form with wurtzite structure. Further, no characteristic peaks corresponding to the presence of secondary phases are observed implying that the dopants have not destroyed the ZnO structure and act as typical
Conclusions
In the present study, the structural, morphological, electrical and optical properties of ZnO:(Ag,N) films deposited by spray pyrolysis (SP) were investigated in detail as a function of dopant concentration. The conductive type of the ZnO:(Ag, N) films showed p-type with increasing Ag and N dopant concentration. The p-type ZnO films with high hole concentration (2.17×1018 cm−3) and low resistivity (8.70×10−2 Ω cm) were obtained at a dopant concentration of 4 at%. The XRD results revealed a change
Acknowledgments
Author M.C.S. Kumar is thankful to the Department of Science and Technology (DST), Govt. of India for the financial support through SERC-Fast Track project for young Scientists.
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