- Nano Express
- Open Access
Effect of Oxidation Condition on Growth of N: ZnO Prepared by Oxidizing Sputtering Zn-N Film
© The Author(s). 2016
- Received: 1 February 2016
- Accepted: 16 May 2016
- Published: 1 June 2016
Nitrogen-doped zinc oxide (N: ZnO) films have been prepared by oxidizing reactive RF magnetron-sputtering zinc nitride (Zn-N) films. The effect of oxidation temperature and oxidation time on the growth, transmittance, and electrical properties of the film has been explored. The results show that both long oxidation time and high oxidation temperature can obtain the film with a good transmittance (over 80 % for visible and infrared light) and a high carrier concentration. The N: ZnO film exhibits a special growth model with the oxidation time and is first to form a N: ZnO particle on the surface, then to become a N: ZnO layer, and followed by the inside Zn-N segregating to the surface to oxidize N: ZnO. The surface particle oxidized more adequately than the inside. However, the X-ray photoemission spectroscopy results show that the lower N concentration results in the lower N substitution in the O lattice (No). This leads to the formation of n-type N: ZnO and the decrease of carrier concentration. Thus, this method can be used to tune the microstructure, optical transmittance, and electrical properties of the N: ZnO film.
- N-doped ZnO
Zinc oxide (ZnO) has a wide direct bandgap of 3.4 eV at room temperature and a high exciton binding energy of 60 meV . Additionally, it also has features of being low cost, non-toxic, stable, and transparent [2, 3]. The ZnO film has become one of the most important semiconductor materials and exhibits a wide potential application in the fields of light-emitting diodes , spintronic device , solar cells , thermoelectric materials [7, 8], and so on. As a semiconductor material, it is important to form a p-n junction and to increase the concentration and mobility of a carrier. Now, it has been a challenge to build a high-efficiency and stable homo ZnO p-n junction  since the carriers are easy to trap at the p-n heterojunction [10, 11]. Although there are reports about the ZnO homo-junction [12–14], a stable and high-efficiency homo-junction is still in the laboratory research stage. The key point is to prepare p-type ZnO and to improve the concentration and mobility of the carrier.
It is generally recognized that the conductivity type of intrinsic ZnO is n-type because there are many donor impurities that exist, such as oxygen vacancy, zinc interstitial, and hydrogen [1, 15]. Furthermore, the concentration and mobility of the carrier in the n-type ZnO is easy to tune by doping the donor impurity (Al, Ga, In, etc.) [16–18]. However, the dopant of acceptor impurity in ZnO is very difficult . Many elements have been selected as the dopant to obtain a more stable p-type ZnO semiconductor, such as N [20, 21], P , As , Sb , Cu , Li , Na , Ag , and Au . In addition, a multi-element co-dopant has also been used to improve the acceptor concentration [29, 30] and to increase the whole concentration so as to compensate the intrinsic defect. But this makes the doping theory become complex. At the same time, the doping result is more difficult to control. Because the elements N and O have similar characters of extranuclear electron structure, ionic radius, and shallow acceptor level, N is one of the more effective dopants for obtaining the p-type ZnO [1–3, 21]. However, the problems of low doping concentration and poor stability still remain for the nitrogen-doped zinc oxide (N: ZnO). The effect of the N dopant and microstructure of the ZnO film on the semiconductor properties should be studied further.
In this study, a method has been developed to prepare N: ZnO by thermal oxidation of the reactive radio frequency (RF) magnetron-sputtering zinc nitride (Zn-N) film. Similar oxidation method has been used in our previous studies [31, 32]. That study focuses on the diluted magnetic semiconductor properties of Co: ZnO film by oxidizing a thermal-evaporated Co/Zn bilayer. In this study, the effect oxidation temperature and oxidation time on the crystal structure, surface morphology, chemical state, transmittance, and electrical properties have been studied by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoemission spectroscopy (XPS), UV–vis spectrophotometry, and the Hall effect measurement. A new growth model of ZnO was found.
In this study, a Zn-N precursor film was first prepared by reactive RF magnetron sputtering. A high-purity Zn target (99.99 % purity) was used. High-purity N2 (99.99 %) and Ar (99.99 %) were selected as the sputtering gas. The gas partial pressures are 0.3 and 0.5 Pa for N2 and Ar, respectively. The base pressure is <2 × 10−3 Pa, and the working pressure is 0.8 Pa. The sputtering time is 15 min. The distance between the target and the substrate is 100 mm. RF sputtering power is 100 W. The substrate temperature remains at room temperature. Monocrystal Si(100) and quartz are selected as the substrate. The substrates are cleaned in an ultrasonic device for 15 min and in turn in acetone and alcohol bath and then dried by high-pressure Ar blowing.
The Zn-N films were then oxidized to become N: ZnO films at air atmosphere in a heat treatment furnace. The Zn-N specimens were put into the furnace when the temperature reached the oxidation temperature, and then, the N: ZnO films were obtained by oxidizing for the desired time. In this study, two series of oxidation conditions were considered. (1) The samples were oxidized for 60 min at different oxidation temperatures of 300, 400, 450, and 500 °C. (2) The samples were oxidized at 400 °C for different times at 30, 60, and 120 min.
The crystal structure of the films was examined by XRD (DMAX 2400, Rigaku) with a grazing incidence of 1° in 2θ mode with monochromatic Cu Kα1 radiation (λ = 0.154056 nm). Surface morphology was examined by SEM (SUPRA 35, Zeiss). The composition and chemical state of the elements were determined by energy-dispersive X-ray spectroscopy (EDX; Inca, Oxford), XPS (ESCALAB 250Xi, Thermo Scientific), and TEM (2100F, JEOL). Optical transmittance was recorded with a UV–vis spectrophotometer (Lambda 750S, PerkinElmer). The electrical property was investigated by the Hall effect measurements (Ecopia HMS-3000, Korea).
Electrical properties of N: ZnO film oxidized at 400 °C for different times
Resistivity (Ω cm)
Carrier mobility (cm2/V S)
Hall coefficient (cm3/C)
9.44 × 1017
6.82 × 1015
1.64 × 1017
However, this kind of growth proposes a new method to control the N: ZnO structure. Furthermore, there is a big difference for the carrier concentration and mobility at different oxidation stages. These provide possibility to prepare p-type ZnO and to improve the concentration and mobility of the carrier. In our study, there are several reasons that lead to the n-type formation and the low carrier concentration and mobility. (1) The substrate temperature maintains at room temperature the deposition of the as-deposited Zn-N film in this study. There are a lot of donor impurities (oxygen vacancy, interstitial zinc, etc.) , which lead to a strong self-compensation effect after oxidation. (2) N content is too low. The lower N concentration results in the lower N substitution in the O lattice (No). This leads to the formation of n-type N: ZnO and the decrease of carrier concentration. In addition, this leads to the electrical properties of the films which are similar with those of the pure ZnO thin film (n-type) and also the lower carrier concentration . Furthermore, the insulating quartz substrate is nonconductive to generate p-type semiconductors .
The N: ZnO film was fabricated by the thermal oxidation of the reactive RF magnetron-sputtering Zn-N film. Oxidation temperature and oxidation time have a significant effect on the crystal structure, surface morphology, and chemical state. The correlation of the optical transmittance and electrical properties with the structure was explored. The results show that the control of oxidation condition of N: ZnO film has made the film exhibit a special growth model. The 400 °C temperature and 120 min time is the best oxidation condition for obtaining a high-quality N: ZnO film. The film can be oxidized completely at this condition. Meanwhile, its transmittance is over 85 % for the visible and infrared light and has a higher carrier concentration. The lower N concentration results in the lower No substitution in the O lattice. This leads to the formation of n-type N: ZnO and the decrease of carrier concentration. This oxidation growth gives an effective method to obtain the needed structure and properties of the N: ZnO film.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 51425401 and 51101034) and the Fundamental Research Funds for the Central Universities (Grant Nos. N140902001).
XQ and GL designed the experiment and analyzed the results. QW and KW instructed the experimental design. GC performed the experiments and the measurements. LX conducted the SEM and TEM analysis. All authors contributed to compiling the manuscript and have approved it for publication.
The authors declare that they have no competing interests.
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