Defect-induced ferromagnetism in silicon


Defect-induced ferromagnetism in silicon

Liu, Y.; Zhang, X.; Yuan, Q.; Han, J.; Helm, M.; Zhou, S.; Song, B.

I. INTRODUCTION
Defect-induced ferromagnetism provides an alternative for organic and semiconductor spintronics. Though it is weak, it can be stable above room temperature. Till now it has been confirmed at least in oxides [1, 2] and carbon based materials [3, 4]. Interestingly, the relation between magnetism and defects in Silicon was demonstrated decades ago [5]. Since then, some progresses were made [6-9] and push forward the research of magnetic Mn doped Si a lot but it is drawn little attention itself. Here, with the latest growth purifying technique and sensitive measurements, we investigated the magnetism in Silicon after neutron irradiation and try to correlate the observed magnetism to particular defects in Si.

II. RESULTS
Commercially available p-type Si wafer (Hefei Ke Jing) is cut into pieces for performing neutron irradiations. The magnetic impurities are ruled out as they can not be detected by secondary ion mass spectroscopy. The concentration of the main impurity Boron is around 4 × 1014 cm-3. Pieces are irradiated for varying durations, corresponding doses in the ranges of 1.91 × 1017 - 2.29 × 1018 n/cm2. Each piece is irradiated only once and pristine pieces are kept for the purpose of comparison.
Raman spectra show the patterns of Si crystals irradiated similar to that of the pristine one. The relative intensity variation confirms the slight damage induced by irradiation. Positron annihilation lifetime spectroscopy is performed to investigate the defect types. All the measured spectra are fit into an exponential function of three components. The lifetime τ2, usually corresponding to defects, takes value of 375 ps, independent of irradiation dose. This positron trapping center is assigned to a kind of stable vacancy clusters of hexagonal rings (V6) [10]. The fraction of the longer lifetime (τ2) component I2 is closely correlated with irradiation dose, which means the concentration of V6 is enhanced by increasing neutron doses.
After irradiation, the samples still show strong diamagnetism. Only weak paramagnetism can be found in zero-field-cooled / field-cooled magnetization. The ferromagnetic signal in Silicon after irradiation enhances and then weakens with increasing irradiation doses as shown in Fig. 1. The saturation magnetization can reach 6 × 10-5 emu/g at 5 K. At room temperature it still remains as much as 5 × 10-5 emu/g. In such a transition metal free system, the ferromagnetism in neutron irradiated Si is closely associated with V6 defects. A Silicon 2 × 2 × 2 supercell with a defect of V6 is built to understand the magnetism in neutron irradiated Silicon. Unfortunately, the V6 shows no spin-polarized state. The change of charge state or Boron doping can not make it spin-polarized, either. At this end, more efforts are needed to comprehend this phenomenon.

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Keywords: defect-induced ferromagnetism; silicon; neutron irradiation; semiconductors

  • Lecture (Conference)
    INTERMAG 2015, 11.-15.05.2015, Beijing, China

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