Deposition of Na–N dual acceptor doped p-type ZnO thin films and fabrication of p-ZnO:(Na, N)/n-ZnO:Eu homojunction
Introduction
Today ZnO has become one of the hottest research fields in advanced materials and devices. It is emerging as one of the most important electronic and photonic materials. It has great potential applications in information technology, biotechnology, nanoscale science and engineering [1], [2], [3]. ZnO has been extensively studied for its promising applications in optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs), because of its wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature [4], [5]. For many advanced applications, the development of ZnO based devices such as p–n homojunctions can be realized by utilizing both n-type and p-type ZnO films. However, the difficulty in achieving p-type ZnO impedes the development of such advanced devices. ZnO occurs naturally as an n-type semiconductor due to a large number of intrinsic defects such as oxygen vacancies (VO), Zn interstitials (Zni) and Zn antisite defects (ZnO). Therefore, it is very difficult to form the shallow acceptor levels because these acceptors can be compensated by numerous ZnO native defects, resulting in the formation of deep donor level traps [6].
The p-type doping in ZnO can be realized by substituting either group V elements (N, P, As, and Sb) for O sites or group I elements (H, Li, Na, Ag and K) for Zn sites [7], [8], [9], [10], [11], [12]. Among these acceptors, nitrogen (0.146 nm) is the most suitable dopant because of its similar ionic radius to oxygen (0.132 nm). Although considerable efforts have been focused to realize N-doped p-type ZnO and ZnO based p-n diodes, it is still difficult to achieve reproducible and good quality p-type conduction in N-doped ZnO [13], [14]. One way to achieve a good solubility of N into ZnO is to use a dual acceptor method that uses two acceptors. Recently, Na doping gains more attention and the theoretical studies indicate that it produces shallow acceptor state for NaZn. Moreover, p–type ZnO thin films and ZnO based p–n homojunction fabricated by various methods prove Na as an effective p-type dopant in ZnO [15], [16], [17]. There have been several reports on the growth of dual acceptor p-type ZnO films such as dual acceptor doping of Li–N [18], Ag–N [19] and K–N [20] by various techniques. Though, there are a few reports on Na-N dual acceptor doping of ZnO films [21], [22], still there is a scope for an extensive investigation of the properties of the ZnO:(Na, N) films fabricated by various deposition techniques.
In this paper authors report a study of structural, electrical, optical properties and surface morphology of Na–N dual acceptor doped p-type ZnO films deposited on glass substrate by spray pyrolysis and the fabrication of p–n homojunction. Spray pyrolysis [23] has been developed as a powerful tool to prepare various kinds of thin films such as metal oxides, superconducting materials and nanophase materials. It has the advantages of low cost, easy-to-use, safe and can be implemented in a standard laboratory. ZnO based p–n homojunctions are characterized by current–voltage (I–V) measurements. The current–voltage characteristics of the p–n junction are studied at room temperature. The experimental data from the I–V measurements of homojunctions are analyzed to calculate the ideality factor, series resistance and the barrier height. However, electrical characteristics of this diode are often influenced by various non-idealities such as interface state, interfacial oxide layer and series resistance. Therefore, the interface states and series resistance play an important role in determining the barrier height.
Section snippets
Experimental
In the present work the ZnO films are prepared by, Holmarc spray pyrolysis equipment model HO-TH-04. A schematic diagram of the experimental setup is as shown in Fig. 1. The ZnO:(Na, N) thin films were deposited on glass substrates, with dimensions of 76.2 mm × 25.4 mm × 1.2 mm, at a substrate temperature of 623 K by spray pyrolysis technique. The precursor solution was prepared by dissolving 0.1 M of zinc acetate dihydrate [Zn(CH3COO)22H2O, Sigma–Aldrich, 99.5%] in a 100 ml mixture of 90 ml deionized
Structural studies
Fig. 2 shows the XRD pattern of ZnO:(Na, N) films with different doping concentration. The crystallinity of the films has been studied by using XRD. It is seen that all the ZnO:(Na, N) films are preferentially oriented along (0 0 2) plane (c-axis) with hexagonal wurtzite structure and free from the formation of secondary phases. The low intensity peaks corresponding to (1 0 0), (1 0 1) and (1 1 0) plane diffraction have also been observed. A shift in the preferential growth from the (1 0 0) plane to (0 0
Conclusions
Na–N dual acceptor doping approach has been proposed for the fabrication of low resistive with high hole concentration p-type ZnO thin films by spray pyrolysis technique. Hall measurements show that all the films exhibit p-type conductivity. Among the p-type ZnO films, 6 at.% Na–N doped ZnO shows low resistivity with high hole concentration. The presence of Na and N, which is responsible for p-type conduction, is confirmed by EDX analysis. The observed red shift in NBE emission well supports the
Acknowledgement
Author M.C. Santhosh Kumar is thankful to the Department of Science and Technology (DST), Govt. of India for the financial support through SERB-Fast Track project for young Scientists.
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