Role of Co 3d Impurity Band in Room-Temperature Ferromagnetism of Diluted magnetism semiconductors Co-doped ZnO Probed by Hard X-ray Photoelectron Spectroscopy
Pei-Yu Chuang1*, Yen-Fa Liao2, Chih-Hua Liu1, Jiunn Chen3, Hua-Su Hsu4, Yu-Han Wu2, Ku-Ding Tsuei2, Jung-Chun- Andrew Huang1
1Department of Physics, National Cheng Kung University, Tainan, Taiwan
2National Synchrotron Radiation Research Center, Hsinchu, Taiwan
3Thin Film Science, National Ping Tung University, Pingtung, Taiwan
4Department of Applied Physics, National Ping Tung University, Pingtung, Taiwan
* presenting author:Pei-Yu Chuang,
Magnetic and semiconductor materials have been extensively investigated and demonstrate significant results and applications in scientific and industrial categories. The spin property of electrons has been successfully utilized to achieve useful magnetic devices. Ferromagnetic and semiconductor properties coexist in magnetic semiconductors, such as Eu and Mn chalcogenides and Cr spinels have been researched in early 1960s. But the crystal structure of such materials is very different from traditional semiconductors like Si and GaAs used in semiconductor industry. In addition, the Curie temperature is quite low and the crystal growths of these materials are difficult. Instead, magnetic semiconductors based on non-magnetic semiconductors, so called diluted magnetic semiconductor (DMS). The DMS materials intensively investigated in the past years that can be classified into several classes. One is Ⅱ-Ⅵ and the other one is Ⅲ-Ⅴ semiconductor doped with transition metal. TM-doped oxides, such as ZnO and TiO2, are now the potential candidates for DMS host materials.
Most of the initial experimental work on transitional metal-doped ZnO has focused on thin films with Co doping. The first report of ferromagnetism in Co-doped ZnO by Ueda et al of the thin films of Zn1-xCoxO (X=0.05-0.25) were fabricate by plus laser deposition (PLD) on sapphire substrates and the XRD patterns revealed the change in lattice parameter as a function of Co concentration and indicated the Co atom can substitute the Zn site. The Ms and Tc increased with increase in the carrier concentration. However, the Co-doped ZnO with ferromagnetic behavior was Co concentration less than 10 at%
In our report, the room-temperature ferromagnetism (RTFM) of DMSs of Co doped-ZnO thin films were fabricated with as function as Co concentrate by PLD. The thin films were grown on the Al2O3(11-20) and Al2O3(0001) substrates at the same growth temperature, deposition rate, and working pressure. According to in-situ electron diffraction and ex-situ x-ray diffraction, high quality single crystal (SC) and bi-crystal (BC) Co:ZnO films were established on Al2O3(11-20) and Al2O3(0001), respectively. By superconducting quantum interference magnetometer and anomalous Hall effect studies, room-temperature (RT) ferromagnetism (FM) was observed in both samples. However, the SC sample show much weaker ferromagnetism than the BC sample. It is noticed that the RTFM of Co-doped ZnO DMSs is strongly correlated with structure defects, which play a crucial role in RT-ferrimagnetism of DMSs. The RTFM of DMSs has been explained by the BMP model, however, direct observation of the impurity band structure is not available. The hard x-ray photoelectron spectroscopy (HAXPES) provides deep depth penetration that is suitable to study this critical issue.
The HAXPES probes energy at 6468.612 eV for Co (1%, 5% and 7%) doped ZnO thin films on Al2O3(11-20) substrate. The corresponding valence band result clearly the Co 3d impurity band near the Fermi edge, which the trend of peak intensity response above the reference results of Co concentration doped in ZnO based. (Fig. 1). According to our local density approximation with First-principle calculation predicted the intrinsic ferromagnetic insulating (FMI) ground state of Co doped ZnO. The spin-polarized impurity level situ in the cobalt spin gap, with the sensation of optical and thermal excitation, essentially contribute to the semiconducting with ferromagnetic polarized behavior at room temperature. The Fig. 2 shows that schematic near the Fermi level have ed impurity band of Co 3d. Herein, the HAXPES results and LDA calculation predicted are correspond, in markedly contrast to that of the ZnO sample. The finding gives a direct support of the BMP percolated model for the RTFM in DMSs and provide a noble way to explore the valence electronic structures with bulk information.

Keywords: Diluted magnetism semiconductors, Hard X-ray Photoelectron Spectroscopy, First-principle calculation