IOP PUBLISHING JOURNAL OFPHYSICSD: APPLIEDPHYSICS J. Phys. D: Appl. Phys. 42 (2009) 035001 (4pp) doi:10.1088/0022-3727/42/3/035001
Influence of the substrate temperature on
the electrical and magnetic properties of
ZnO : N thin films grown by pulse laser
deposition
Chang-Feng Yu
1, Sy-Hann Chen
1, Shih-Jye Sun
2and Hsiung Chou
31Department of Applied Physics, National Chiayi University, Chiayi 60004, Taiwan 2Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan 3Department of Physics and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
E-mail:cfyu@mail.ncyu.edu.tw
Received 15 August 2008, in final form 19 November 2008 Published 18 December 2008
Online atstacks.iop.org/JPhysD/42/035001
Abstract
We investigated the effect of the substrate temperature on the magnetic, electrical and optical properties of nitrogen-doped ZnO thin films. Experiments showed that a high substrate temperature depresses the deposition rate and produces much thinner films displaying more robust ferromagnetism. In addition, the resistivity decreased as the substrate temperature increased and all of the nitrogen-doped films, at different substrate temperatures, had larger gaps than the pure ZnO film.
1. Introduction
Magnetic ion doped ZnO thin films [1,2] have attracted the interest of many researchers because of their room temperature ferromagnetism [3,4]. This large band gap transparent material [5,6] also has extremely high potential in industrial applications [7]. Interestingly, non-magnetic ion doped ZnO thin films also reveal robust ferromagnetism, e.g. nitrogen-doped ZnO thin films (ZnO : N) [8,9]. The mechanism behind the enhancement of ferromagnetism by the doped nitrogen is still unclear. Because of the completely filled d-orbits of the non-magnetic ion doped ZnO thin films, the origin of the ferromagnetism is out of the ordinary. Some researchers believe that the magnetic mechanism of non-magnetic ion doped thin films of transition metal oxide comes from the localized states of oxygen defects [10–12], which is different from the bound magnetic polaron (BMP) mechanism proposed by Coey [3] for magnetic ion doped transition metal oxides. Undoped zinc oxide films generally exhibit natural n-type conduction due to the presence of intrinsic donor-type defects induced by deviation from stoichiometry. Therefore, one of the major obstacles in the development of ZnO material is the difficulty encountered in finding an efficient p-type
dopant. Theoretical calculation predicts that nitrogen is an outstanding candidate in current research for p-type doping of ZnO. A number of groups have been trying to realize p-type ZnO employing nitrogen as a dopant source by various methods [13,14]. Despite the many reports on the successful growth of p-type ZnO films by nitrogen doping, there were also a number of reports in which the groups were not able to reproduce and maintain the same results. Unfortunately, this p-type doping is unstable [15], but reveals the particular doping level of nitrogen in ZnO. In fact, some contradictory results represented in ZnO films by nitrogen doping have revealed n-type electricity [16,17]. The nature of conduction (n- or p-type) for ZnO : N seems to depend on the fabrication process and conditions. We think that the substrate temperature is an important parameter in ZnO : N thin film deposition.
We utilized pulsed laser deposition (PLD) under a mixed atmosphere of O2 or N2O to deposit ZnO : N thin films on
glass substrates at different substrate temperatures. Our results showed that the magnetic, electrical and optical properties of the ZnO : N thin films were dependent on the substrate temperature during the film deposition. Although the parameters of the material’s properties are entangled and complicated, through an investigation of our results, we propose some mechanisms for determining them.
J. Phys. D: Appl. Phys. 42 (2009) 035001 C-F Yu et al
Figure 1. X-ray diffraction spectra of ZnO : N thin films of various thicknesses on glass substrates.
2. Experiments
The reliable method of PLD was used to deposit thin films of undoped and nitrogen-doped ZnO as transparent electrodes on glass substrates. Ablation PLD targets were fabricated with high purity (5N) powder by the solid state reaction technique. The dimension and the thickness of the target are about 1 and 0.125 inch. ZnO : N films were deposited on the glass substrates at 150–300◦C in a N2O (99.99%) atmosphere
of 200 mTorr. A double frequency Q switched Nd : YAG pulsed laser operated at 532 nm, with a pulse duration of about 7 ns and an 18.5 mJ cm2 energy density, was focused
on the target to generate a plasma plume. The target-to-substrate distance was 5.0 cm. To explore the effects of various film thicknesses, films with thicknesses of 45–185 nm were prepared under a fixed pressure and various substrate temperatures. The crystallinity and surface morphology of the ZnO films were characterized by x-ray diffraction (Rigaku Multiflex CD3684N diffractometer). The thickness of thin films was measured by an α-step profiler. Hysteresis loops were measured with an alternating gradient magnetometer. The conductivity, carrier concentration and mobility were measured at room temperature by the Hall effect measurement. The composition of thin films was analysed by secondary ion mass spectroscopy (SIMS).
3. Results and discussion
Figure 1 shows the XRD data for films with various thicknesses, grown at different substrate temperatures. All the films exhibited a polycrystalline structure with (1 0 0), (0 0 2) and (1 0 1) peaks. However, no preferred orientation was observed in most of the films except for the one grown at 200◦C in which the (0 0 2) reflection peak became intense and sharper as compared with the other samples, indicating a tendency of preferential growth in the films. When films were deposited within 250–300◦C, there was a decrease in the intensity of all the peaks, which suggests a degradation of the quality of the films grown at higher temperatures. By using the strongest peak, (0 0 2), as a reference for comparing the lattice constants in the c direction, it is found that the pure ZnO films exhibit
a larger lattice constant of 5.20 Å than N-doped ZnO : N films which have lattice constants of around 5.19 Å.
Secondary ion microscopic spectrum depth profile measurements were performed to verify the presence of Zn, O and N in the ZnO and ZnO : N films. Figure2shows the depth profile of the main elements in the deposited ZnO and ZnO : N films. As expected, measurements showed that the concentrations of Zn and O were constant across the ZnO and ZnO : N films. The oxygen concentration was low, indicating that a high level of oxygen vacancies was created during the film’s growth with the introduced O2 or N2O gas. Moreover,
the nitrogen concentration was well detected and showed a uniform distribution throughout the ZnO : N film, as shown in figure2(b).
Bulk ZnO and very thick ZnO films do not reveal magnetism, but a very weak ferromagnetism is displayed in very thin films [8]. Since the valence configuration of the d-orbitals of Zn is completely filled, most researchers believe that magnetism results from serious oxygen vacancies, which lead to the formation of an impurity band that crosses the Fermi level and splits spin-dependently [10]. Experiments have shown that nitrogen doping in ZnO thin films enhances their ferromagnetism. Although some mechanisms have been proposed to explain the appearance of ferromagnetism in ZnO : N thin films [9], until now the enhancement mechanism has been unclear. Therefore, investigating ZnO : N films in all respects is helpful and necessary to realize the role that nitrogen plays in magnetism.
Figure 3 shows the temperature dependence of the magnetization of films deposited at different substrate temperatures. The most robust ferromagnetism appeared for the film grown at the highest temperature, 300◦C, which manifests the thinnest films. Obviously, the thickness of the films also depended on the substrate temperature. This revealed that a high substrate temperature, e.g. 300◦C, depressed the deposition rate and produced much thinner films. Because the chemical interaction of the gases was temperature dependent, we propose that when the substrate temperature was high the thermal activation was high enough to effectively decompose N2O into N and O atoms. Although
the decomposed O from the N2O also participated in film
deposition, the increased N concentration strongly depleted the O2 and eventually decreased the film deposition rate.
Similarly, we believe that this thinnest film had high oxygen vacancies. The very high oxygen vacancies and the high level of Zn content [18], as shown in the SIMS measurement, contributed to the most robust ferromagnetism in all films. At medium temperatures (150 and 200◦C) the competition between the O2 depletion and the thermal activation for film
deposition was complicated, giving rise to different substrate temperatures, e.g. 150 and 250◦C, producing films with the same thickness. Comparing the two films (150 and 200◦C) based on the aforementioned proposal, the film deposited at the much lower substrate temperature (150◦C) should have more oxygen vacancies, giving rise to more robust ferromagnetism than the film deposited at 200◦C.
The electrical properties of the ZnO : N films were examined by Hall measurements at room temperature. Figure4
J. Phys. D: Appl. Phys. 42 (2009) 035001 C-F Yu et al
Figure 2. Secondary ion microscopic spectra of (a) undoped ZnO film and (b) N-doped ZnO film.
Figure 3. Temperature dependence of the magnetization of films deposited at different substrate temperatures. The thickness of the films depended on the substrate temperature.
shows the electrical resistivity of the ZnO : N thin films as a function of the substrate temperature at a working pressure of 200 mTorr, using an N2O flow rate of∼40 sccm.
The results of various substrate temperatures for the thin films showed a resistivity of about 0.001–0.1 cm, mobility of about 2–20 cm2V−1s−1 and concentrations of about
1019–1020cm−3. We found that the resistivity in the ZnO : N
thin films decreased as the substrate temperature increased from 150 to 200◦C and then increased exponentially to higher levels. The minimum resistivity for the ZnO : N thin films was found to occur when the substrate temperature was 200◦C, for which the minimum magnetic moment was obtained.
Similarly, our proposal for the substrate temperature dependence of magnetism can well illustrate the electrical properties. During the deposition process, the N atoms are doped in the films. We assume that the doped N concentrations are relatively high for high substrate temperature films, easily leading to high oxygen vacancy levels. There are two competing distributions in these doped N, one are the scattering centres, which are similar to oxygen vacancies,
Figure 4. The resistivity, concentration and mobility of the films deposited at different substrate temperatures.
and the other is the extra carrier donation from the valence compensation for atomic O. Meanwhile, the oxygen vacancies in the films play the roles of carrier capture centres and scattering centres. Therefore, the thin film deposited at a 300◦C substrate temperature had a high oxygen vacancy concentration, accompanying a low carrier concentration, giving rise to low carrier mobility and high resistivity. The thin film deposited at a 200◦C substrate temperature showed the lowest resistivity because the film was thicker, accompanied by lower oxygen vacancies and giving rise to a higher carrier concentration and mobility. The films deposited at 150 and 250◦C, which had the same thickness, showed almost the same resistivity but had different carrier mobility values. Based on our assumption, the level of oxygen vacancies in the 250◦C film was lower than in the 150◦C film, therefore the carrier mobility of the former was larger than the latter.
Semiconductors can be classified into two types, namely, direct gap and indirect gap materials. ZnO is a direct gap semiconductor. The effects of the substrate temperature on the optical transmission and absorption properties were studied. Figure5(a) shows the optical transmission spectra, for a wavelength range of 300– 800 nm, of the optimized deposited zinc oxide thin films deposited at various substrate temperatures. All the ZnO : N and ZnO films show high transparency with transmittances over 85% in the visible region (400–700 nm). For the ZnO : N films, the transmission spectra exhibited a steep drop as the incident wavelength is shorter than 350 nm indicating the optical band gap. We also found that 3
J. Phys. D: Appl. Phys. 42 (2009) 035001 C-F Yu et al
Figure 5. (a) The transmission of the films deposited at different substrate temperatures. (b) The band gap of the films extracted from the differential transmittance spectra.
the transmission edges around 355–380 nm shifted to shorter wavelengths as the growth temperature increased. Similar results were also observed in ZnO films, whose absorption edge was around 375 nm, which is very close to the intrinsic band gap of ZnO (3.2 eV). The absorption edges for the N-doped ZnO films shift towards lower wavelengths compared with that of the undoped ZnO sample. The Eg (energy band
gap) values of the ZnO films are determined from the peak wavelength values of the differential transmittance versus wavelength curves [19–21] which are shown in figure5(b). The Eg values are about 3.25 eV, 3.29 eV, 3.37 eV, 3.34 eV
and 3.37 eV for substrate temperatures of 150◦C, 200◦C, 250◦C, 300◦C for ZnO : N and undoped ZnO thin films, respectively. All N-doped films have larger band gaps than the pure ZnO film. We believe that the gap increase comes from a decrease in the lattice constant by oxygen vacancies or smaller N-doped atoms [22] which has been confirmed in x-ray diffraction measurements. The decrease in the lattice constant increases the Coulomb interaction among atoms, giving rise to an increase in the band gap. On comparing the 300 and 250◦C films, it was found that they showed totally different levels of ferromagnetism, which means the ferromagnetism does not depend on the band gap.
4. Conclusion
The PLD deposited ZnO thin films formed at the constant partial pressure of a N2O atmosphere at various substrate
temperatures revealed different ferromagnetic magnetization strengths. Experiments confirmed that nitrogen elements were doped in these thin films, which induced ferromagnetism. In addition, the resistivity in the nitrogen-doped ZnO thin films decreased as the substrate temperature increased from the low growth temperature regime and then the resistivity increased exponentially by growing films at higher temperatures. All the band gaps in the nitrogen-doped ZnO thin films enlarged and could be modified by the substrate temperature for film deposition. We propose that the differences in the magnetic and electrical properties come from the competition between the depletion effect involving the nitrogen and oxygen molecules and the thermal activation in the N2O decomposition
during film deposition. At various substrate temperatures, this competition produces thin films with different nitrogen concentrations, oxygen vacancies and film thicknesses, leading to different magnetic and electrical properties.
Acknowledgments
This work was supported by the National Science Council in Taiwan through Grant No NSC-95-2112-M-390-002-MY3 (S J Sun), No NSC-95-2112-M-110-011-MY3 (H Chou) and No 96-2120-M-002-017. We acknowledge the useful discus-sions with Professors C D Hu and D J Huang and Lance Horng.
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