Microstructure of ZnO:Mn films with various Mn concentration was investigated with XANES and XPS. The experimental results revealed a substitution of Mn in ZnO and also excluded the existence of Mn oxides or metallic manganese clusters. The substitutional Mn presented a divalent state and all the ZnO:Mn films were n-type. Room temperature ferromagnetism monotonously decreases with the decrease of the electron carrier concentration. The observed ferrmagnetism should come from the carrier-mediated exchange.
Recently, diluted magnetic semiconductors (DMSs) [1-3] have attracted much interest for their potential applications in spintronics. Among them, Mn-doped ZnO is the most favorable candidate because the Mn ion possesses the highest magnetic moment among 3d transition metals and can create a fully polarized stable state due to halffilled 3d bands. There are many reports on room temperature ferromagnetism (RTFM) in the system since the theoretical prediction of RTFM by Dietl et al. in 2000 [
X-ray absorption near-edge structure (XANES) spectroscopy is a powerful probe for providing a “fingerprint” of chemical states and local electronic structure of incorporated atoms in the host compounds even in a dilute concentration. In particular, the absolute energy position of the edge spectra contains information about the valence state of the absorbing elements. In this paper we employ XANES combining other analysis techniques to investigate the Mn local atomic and electronic structures as well as magnetic interactions in Mn-doped ZnO film.
Mn-doped ZnO films were deposited on (0001) sapphire substrates (99.999%) by radio frequency (RF) magnetron sputtering, with a composite target of ZnO (99.99%) and Mn (99.99%). According to the area ratio of ZnO and Mn, the Mn-doped content could be adjusted. High purity Ar (99.999%) was introduced into the sputtering chamber at the base pressure of ≤6.0 × 10−4 Pa. The Arflow rate was 20 SCCM (SCCM denotes cubic centimeter per minute at STP (standard temperature and pressure)). The working pressure was 0.5 Pa, the sputtering power 90 W and the substrate temperature 500˚C. Prior to a deposition, a pre-sputtering cleaning was performed for about 20 minutes to eliminate possible contaminants from the target surface.
The Mn K-edge XANES (6.539 keV) spectra were measured at the U7C beamline of National Synchrotron Radiation Laboratory of China. High resolution XPS spectra were detected using a Kratos AXIS Ultra DLD spectrometer with a high resolution of 0.48 eV. The monochromatic Al Kα X-ray (λ = 0.8339 nm) was used as the incident light. The binding energy scale was calibrated using the C 1s line at 284.8 eV. Electrical properties were carried out at room temperature by Hall effect measurements using a Van der Pauw four-point method. An In electrode was made by soldering indium at four corns of the sample surface. The linear I-V behavior for all samples indicated a good Ohmic contact between In electrode and film layer. The magnetic property at room temperature was measured by the Quantum Design MPMSXL7 SQUID magnetometer.
To detect Mn cluster and any secondary phases in the ZnO:Mn samples, we have measured Mn K-edge XANES of all the DMS samples and Mn metal. For clarity,
(Peak A). It implies that there are no Mn oxides or metallic manganese clusters in the films. The same results have been obtained using an extended X-ray absorption fine structure (EXAFS) and synchrotron radiation X-ray diffraction (SR-XRD) [
In order to further investigate Mn occupation sites, we have attempted to perform XANES calculations with the FDMNES 2007 code [
timizing the geometry of the studied configurations, i.e., the bond lengths of MnZn-O and MnZn-Zn is 1.97 and 3.20 Å, respectively, while “MnZn expand” represent the model in which the bond lengths of MnZn-O and MnZn-Zn is expanded to 2.03 and 3.28 Å, respectively. In the MnZn model, A, C and D peaks can be well reproduced except the B peak. On the other hand, the height of the Peak A in the MnZn model is much larger than that in the DMS samples. However, in the MnZn expand model, four characteristic peaks can be well reproduced and the calculated spectrum resembles those of the DMS samples. It implies that Mn atoms are incorporated into the ZnO crystal lattice by the substitution on Zn sites, accompanying with an expansion of the MnZn-O and MnZn-Zn bond lengths due to the larger atomic radius of Mn compared with Zn. The same results have been observed by an extended X-ray absorption fine structure (EXAFS) [
The valent state of Mn in the DMS films is detected by high resolution XPS. In