Spin Polarization Transport Of Zn_1-xCo_XO Concentrated Magnetic Semiconductor Films

Electrons have two degrees of freedom:charge and spin. The charge property of electrons has been taken great advantage in semiconductor materials, which motives the developments of electronic technology, computer technology and information technology. However, with the improvement of integrated degree and the decrease of the devices’size, semiconductor devices are approving the physical limit little by little. During the recent twenty years, scientists find that electron’s spin diffusion length is much longer than its mean free path, and the spin of electrons can stay the same for much longer. As both of these two degrees of freedom transport with electrons, another spin channel is formed in semiconductors. As a result, the density and velocity of the transported information will be greatly improved. Besides, the nonvolatility of spin can help us to achieve the combination of information storage and processing. With the assistance of spin property, a new generation of multifunctional devices which is named as spintronic device is emerging with faster processing speed, smaller size, lower energy consumption and nonvolatility.Magnetic semiconductor combines the electric and magnetic properties of semiconductor, therefore, with both of the electron’s two degrees’of freedom (charge and spin), magnetic semiconductor possesses the superiorities of both semiconductor electronics and magneto-electronics. Thus, it is reputed the most promising material at present. It is widely applied in communication, information processing, magnetic recording, spin injection and quantum computing. Recently, scientists have discovered that there are spontaneous magnetic order and many other novel magnetic and electric transport properties in (Ga,Mn)As,(In,Mn)As and MnGe. However, the curie temperature (Tc) is very low, for example,(In,Mn)As, Tc≈35K;(Ga,Mn)As, Tc≈190K; MnGe, Tc≈116K. Accordingly, room-temperature magnetic semiconductors have attracted lots of attention for their potential application in spintronics devices.Based on the Zener model, Dietl predicted that GaN and ZnO which are doped with5%Mn and have high carrier concentration (3.5×1020cm-3) may have room-temperature ferromagnetism. From then on, scientists started to research on GaN and ZnO based wide bandgap magnetic semiconductors. There had been lots of evidence that the robust ferromagnetism in diluted magnetic semiconductors, such as Ge1-xMnx, Ga1-xFexN and Zn1-xCoxO, comes from nanoscale volumes containing a large density of magnetic cations, i.e., concentrated magnetic semiconductors (CMSs) which embedded in the matrix. However, in order to apply the magnetic semiconductors in spintronics materials, we should get spin polarized carriers additional to curie temperature above room temperature. Currently, there are plenty of reports on ZnO based diluted magnetic semiconductors with room-temperature ferromagnetism. But few of them are about the injection, transport and detection about the spin polarized carriers. In this paper, we mainly discussed about the spin polarization of ZnO based magnetic semiconductors.Generally, homogeneous magnetic semiconductors and even the concentrated magnetic semiconductors (CMSs) which are embedded in the dilute matrix may lead to the spin polarization of carriers in the whole magnetic semiconductor. There are various methods to probe the spin polarization, such as optical magnetic circular dichroism, anomalous Hall effect, and Andreev reflection spectroscopy. However, from the point of direct application in devices, tunneling magnetoresistance (TMR) is more practical. But up to now, there are few reports on TMR to clarify the spin polarization of charge carriers of ZnO-based magnetic semiconductors. In ZnCoO/ZnO/ZnCoO and ZnCoO/MgO/ZnCoO magnetic tunneling junctions, a large positive magnetoresistance was observed in low temperature while the butterfly-like peaks were not found in R(H) curves. This phenomenon implies that the observed magnetoresistance is not the spin polarization related TMR. In Zn0.91Co0.09O(180nm)/Al2O3(100nm)/Co, the low field butterfly-like two peaks were observed in MR-H curves, demonstrating a pronounced spin related TMR. However, the quantitative estimation of the spin polarization of the charge carriers was not given. K. A. Yates et al., derived a spin polarization of the carriers of55%in the codoped ZnO with1%Al and2%Mn from point-contact Andreev reflection spectroscopy. S. S. Yan et al., figured out the spin polarization in Zn0.28Co0.72O films is29.6%by spin dependent variable range hopping. But quantitative works on spin polarization of ZnO-based semiconductors by TMR in magnetic tunneling junctions were still not given.By controlling growth conditions, we prepared nearly pure Zn1-xCoxO CMS films by magnetron sputtering device. And the spin polarization of Zn0.32Co0.68O1-v (v means oxygen vacancies) CMS was derived from the measurements of TMR and spin-dependent variable range hopping. We obtained a TMR of19.1%in Co/ZnO/Zn0.32Co0.68O1-v magnetic tunneling junction at2K, and figured out a low limit of the spin polarization in Co/ZnO/Zn0.32Co0.68O1-v was25%. The TMR decreases with the increasing of temperature and bias voltage because the tunneling via localized impurity states in the barrier. On the other hand, the spin polarization was36.1%in Zn1-xCoxO CMS estimated by spin-dependent variable range hopping. Our results demonstrate that Zn1-xCoxO CMS films can serve as a spin injector or spin detector in semiconductor-based spintronics devices.