Infrared probe of metal-insulator transition in Si1−xGdx and Si1−xYx amorphous alloys in magnetic field

– Carrier dynamics in amorphous a-Si1−xREx (RE = Gd, Y) films has been studied in the doping regime close to the metal-insulator transition by means of infrared spectroscopy. Optical constants throughout the entire intra-gap region ( ω < 1 eV) have been found to be anomalously sensitive to changes of temperature and/or magnetic field. The observed behavior is consistent with the model of hopping transport where the interaction of carriers with both the lattice and large core spin of Gd ions is taken into account. Magnetic dopants may radically alter the properties of prototype semiconductors, such as Si or GaAs, and may lead to extraordinary effects even in these fairly simple systems [1–3]. For example, III-V materials doped with Mn develop ferromagnetism with the Curie temperature exceeding 110K [1] and amorphous Si alloyed with a high concentration of Gd shows gigantic negative magnetoresistance [3]. Present understanding of the role of magnetic impurities in carriers dynamics, especially in the regime where a material is close to the boundary of the metal-insulator transition (MIT), is rather poor, in part because the experimental results are very scarce. This motivated us to perform studies of the electromagnetic response of aSi1−xGdx in the vicinity of the MIT using the techniques of infrared (IR) optics and magnetooptics, which are ideally suited for the task. We have measured the IR spectra of heavily doped a-Si1−xGdx and its non-magnetic counterpart a-Si1−xYx close to the boundary of the metal-insulator transition. The amorphous Si matrix allows one to increase the concentration of magnetic Gd ions beyond the solubility limit in crystalline silicon. At x 0.11–0.13 these materials are insulating at T → 0 in zero magnetic field but become conducting in the field H 5T [3]. The effect is reminiscent of the behavior of diluted magnetic semiconductors [4]. However, in the latter case negative