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Silicon-based Diluted Magnetic Semiconductor Thin Films And Its Structure And Transport Properties

Posted on:2011-02-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:W F SuFull Text:PDF
GTID:1118330335492460Subject:Condensed matter physics
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Semiconductor, as one of the four greatest inventions in the twentieth century, is the base material for information processing devices. Ferromagnetic materials have been widely used in the information storage area. Semiconductor devices generally take advantage of the electron charge, whereas the recording information exploits the electron spin of magnetic materials. It is then quite natural to ask if both the charge and spin of electrons in the same material can be put into use to further improve the performance of devices. Mass storage and processing of information can then be realized within the same device. Magnetic semiconductor material is such a material which is a combination of the conventional semiconductor and the magnetic material. One possible way to fabricate such material is to dope magnetic elements into a semiconductor lattice. Nowadays, preparation of the magnetic semiconductor material and its physical properties have become a hot research field.We investigated the preparation of magnetic semiconductors on Si-substrate by molecular beam epitaxy (MBE). Fe or Mn-doped Si magnetic semiconductor films were prepared on Si(100) substrates under different growth conditions. Fe concentrations, film growth rate, substrate temperatures were varied for comparison studies. In addition, both p-and n-type substrates were employed. In-situ film composition analysis was carried out in the analysis chamber with Auger electron spectroscopy (AES). The depth profiles of atomic concentrations in the film were measured with the secondary ion mass spectroscopy (SIMS). The surface morphologies of the films were investigated by using atomic force microscope (AFM). X-ray diffraction (XRD), Raman spectroscopy and cross-sectional transmission electron microscope (TEM) were also used to study the film characterization.We found that the higher the Fe concentration, the rougher the surface is. The films grown on p-type Si substrates have smoother surfaces than those on n-type substrates. Compared with the film grown at 200℃, the surface of the film grown at 250℃is a little rougher.40 nm-thick films possess high quality crystal structures. However, the films are amorphous at a thickness of 80 nm. The SIMS depth profiles reveal that the obtained films have a uniform distribution of Fe concentration along the growth direction and no significant surface segregation of Fe. From the high-resolution cross-sectional TEM image of the 40 nm-thick Fe0.04Si0.96 film, no Fe cluster is observed in the film. The XRD measurements and Raman spectroscopy demonstrate that there are no Fe or Fe-related compounds contained in the films for the Fe concentration below 7%. We studied the transport properties of Fe0.04Si0.96 thin films at low temperature. From the relationship between the film resistivity and its temperature, we obtained two activation energy Ea, i.e.40 and 2.5 meV, for temperatures below 50 K. The former corresponds to thermally activated conduction involving the electron transition from valence band to shallow acceptor levels; while the later corresponds to holes hopping within the impurity band.Magnetoresistance (MR) measurement in the temperature range of 15-300 K illustrates a temperature dependent feature with a highest ratio of 935% at the 30 K (where the perpendicular magnetic field is 3 T). The MR was carried out for the magnetic field applied perpendicular to the film plane(MR-P), and in the film plane either perpendicular(MR-T) or parallel (MR-L) to the current. MR-P is the largest and MR-L is the smallest. We make a possible explanation by the curving of the carriers trajectories in a magnetic field and the theory of shrinkage of the wave function.Only normal Hall effect was observed for the temperatures above 30 K. The positive Hall coefficient shows that the carriers are holes in the film. The anomalous Hall effect appears when the temperature is lower than 30 K and the Hall coefficient turns to negetive below 27 K. The complex relationship between the Hall coefficient and magnetic filed at various temperatures is well explained by the two-carriers model, which shows the two kinds of carriers are both holes. The temperature dependence of density and mobility of the carriers are obtained and the activation energy of these two kinds of carriers are 45 and 2.3 meV, respectively, which is similar to the result of 40 and 2.5 meV deduced by the relationship between the resistivity and reciprocal temperature.In addition, Mn-doped Si thin films on Si (100) substrates show different behaviors with Fe dopant. On the one hand, low concentration of Mn dopant results in the amorphous of Si, on the other hand, it is easier to form MnxSi1-x compound. Low-temperature annealing can improve the crystalline structure and magnetic properties of films. We also investigated their microstructures by using XRD and Raman spectroscopy. Compared with the conventional XRD, Raman spectroscopy is more sensitive to investigate the microstructures of Mn silicides, especially at the initial stage of formation of the Mn silicides. Two phases of Mn silicides, MnSi1.73 and MnSi were identified and their Raman spectra were reported.
Keywords/Search Tags:Magnetic semiconductor, Molecular beam epitaxy, FexSi1-x, MnxSi1-x
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