| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 98 === Diluted magnetic semiconductors (DMSs) is one of the most promising candidates to realize spintronics devices with the potentials to control both charge and spin degrees of freedom in one material. Among various systems, group IV-based (Si, Ge) DMSs specially attract much attention due to the compatibility with existing industry of modern electronics. In our works, we aimed at exploring to develop new Si-based DMSs materials, and the corresponding distinct physical properties. These included microstructure, ferromagnetism, and transport properties. Three main topics have been involved.
In the first topic, we have demonstrated room-temperature ferromagnetism in Cr-doped hydrogenated amorphous Si (a-Si1-xCrx:H) thin films grown by sputtering method. The microstructure of these samples was identified by XRD and HRTEM, in which all as-deposited films are amorphous without any clusters or second phases. Pronounced anomalous Hall effect (AHE) and temperature dependent magnetization (M-T) curve both suggest the origin of the ferromagnetism may arise from percolation of magnetic polarons. Furthermore, the local environments around Cr atom were analyzed by EXAFS, confirming that successful incorporation of an unusually high concentration of Cr magnetic ions in a-Si:H thin films nearly free of Cr clusters and oxides. In addition, the effect of hydrogenation shows precipitous influence on magnetic and electrical properties of the samples. Only when dangling bonds in the sample are largely hydrogenated, will robust ferromagnetic order be observed.
In the second topic, we have developed Mn-doped a-Si:H thin films, which also show room-temperature ferromagnetic order. The structural, magnetic and electrical properties were also studied by the analyses discussed in Cr-doped samples. Moreover, the enhancement of these properties by hydrogenation was observed again. However, it is noted that the behaviors in M-T and AHE measurements are quite different from those of Cr-doped ones. The M-T curve fits very well by combination of Curie-Weiss law and 3D spin-wave model and AHE was reproducibly obtained at 150 K. These results suggest that the origin of ferromagnetism may arise from the itinerant carrier mediated mechanism.
Finally, in the last topic, we have studied the effect of post-annealing treatment on the properties of a-Si1-xMnx:H thin films. Annealing manifestly enhances saturation magnetization, electrical conductivity, and carrier concentration. Moreover, the hydrogen concentration was determined by ERD methods. The results revealed that there is no hydrogen effusion during low temperature annealing. The reason of property enhancement by annealing arises from the thermal energy which promotes hydrogen diffusion, hence a more homogeneous distribution and lessens defect density. In particular, AHE was clearly and reproducibly observed at room temperature in annealed samples, which is the highest temperature ever observed in Si-based DMSs. These results indicate that the origin of ferromagnetism arises from hole-mediated mechanism, which is essential for the realization of practical Si-based spintronics devices.
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