Induced Magnetic Properties in Doped Semiconductors

Diluted magnetic semiconductors (DMSs) are formed by doping magnetic transition metals (TMs) into semiconductor matrices. They are a type of new semiconductors with fraction of magnetic ions on the sites of semiconductor constituent ions. DMSs have some excellent properties which enhance their potentials for using in a wide range of industrial and commercial applications. The presence of substituted magnetic ions results in unique magnetic properties which distinguish them from the ordinary semiconductor materials. Room temperature ferromagnetic (FM) ordering as one of the unique properties, has been observed in many DMSs. However, the origin of FM ordering in the DMSs is still controversial due to the presence of intrinsic magnetic properties possessed by the magnetic TM dopants. It is difficult to determinate whether the obtained products is indeed a solid solution of the semiconductor matrix and the magnetic TM dopant or it remains as semiconductor matrix engulfing a magnetic TM clusters, precipitates, or second phases that are responsible for the observed magnetic properties. Moreover, most of the magnetic TMs have extremely low solubility in the semiconductor matrix. Therefore, an increasing interest is provoked on developing magnetic systems by doping non-TM dopants. In this new type of system, the origin of the FM signal from the dopant can be eliminated, and higher doping ratio in the system can be expected due to no magnetic attraction. Recently, such magnetic systems have been obtained, for examples C-doped ZnO and C-doped TiO 2.;Inspired by these ideas, we had selected the non-magnetic element Al and W as the dopants, and introducing them to the semiconductor matrices, B4C and SiC. The Al-doped B4C, Al-doped SiC and W-doped B4C compounds were synthesized by high temperature solid-state sintering of carbon, boron/silicon, and aluminum/tungsten powders at 2200°C. In the Al-doped B4C system, doping of Al had not altered the crystal structure. All of the doped samples retained the same structure as the un-doped sample, which corresponded to the B4C (ICDD-PDF: 75-0424, space group R-3m). All results, including results of XRD, Raman and XPS, indicated that the parts of the carbon atoms on the C-B-C chains were replaced by some of the doped Al atoms. The result of this replacement was that the system showed FM feature with increasing of the Al content. More important, a typical spin-glass behavior was observed in this system. While in the Al-doped SiC system, all of the samples, including un-doped and doped samples, were mixtures containing 6H-SiC, 4H-SiC and 15R-SiC polytype. By comparison, the changes in the FM ordering in this system were also different from those in the Al-B 4C system. This indicated that the induced mechanisms of FM ordering due to the doping effects on the two semiconductors were not the same. To further investigation, attempts were made by using Mn as dopants. The result of Mn-doped SiC confirmed that the FM feature in SiC systems indeed did not depend on with the dopants. The two doped SiC systems demonstrated that the FM behavior might be related to one type of SiC polytype (15R-SiC) in the mixture rather than dopants.;The W-doped B4C system was an interesting one. The lightly doped sample showed FM feature, while the FM ordering became weak and disappeared with the increase of W content. A second phase B2W5 was produced in the highly doped samples. It was demonstrated that B2W5 had no FM feature. To compare, (W, Al) co-doped B4C were also synthesized under the same heating condition. It was found that the doping effects of Al and of W played its own individual role, and facilitated a common outcome. A weak spin-glass feature was observed in the co-doped system but not in the W-doped system.;In summary, four systems were fabricated. All of them showed changes in magnetic behaviors. The Al-doped B4C system showed spin-glass features for the first time. The results of this work demonstrated that the non-magnetic dopants indeed could affect the magnetic properties of semiconductors, and urged the doped-semiconductors to exhibit FM features. This made the doped-semiconductors behaved as DMS materials even there was no 3d electron in the whole of systems. This work provides an ideal system to illustrate the origin of spin order in non-TM doped wide-gap semiconductors.