Optically Induced Magnetotransport Properties of the FeSiO2/p-Si and FeSiO2/n-Si
Hybrid Structures
I. A. Bondarev1,2, A. S. Tarasov2, and N. V. Volkov2
1
Siberian Federal University, Institute of Engineering Physics and Radio Electronics, Krasnoyarsk, 660041 Russia
2Kirensky Institute of Physics, Russian Academy of Sciences, Siberian Branch, Krasnoyarsk, 660036 Russia
E-mail: fbi1993@mail.ru
We report the results of investigations of the metal/insulator/semiconductor hybrid structure. In our experiments, an
optically induced giant magnetoresistance of 104% was observed. Some assumptions concerning possible mechanisms of this
phenomenon are made. The observed effect makes the investigated structure promising for application in high-sensitivity
magnetic field detectors. The data obtained make it possible to build a theoretical model for describing the observed phenomenon.
The hybrid nanostructures are combinations of nonmagnetic semiconductors and ferromagnetic elements. The response of such structures to the spin-polarized current makes it possible to use the electron spin as an active element for data storage and transfer. This gave rise
to the new branch of condensed matter physics called spintronics. Along with the huge potential of semiconductor electronics, the hybrid structures will allow controlling the electron
transport in a system via the electron spin state. However, to apply these structures in real
spintronic devices, it is necessary to understand such fundamental phenomena as spin injection to the semiconductor, spin diffusion, spin accumulation, etc.
Study of the magnetoresistance effect in hybrid structures gives a new insight in the nature of the fundamental phenomena that take place in such systems and seem extremely important. In addition, the giant magnetoresistace effect attracts attention of researches as a
promising tool for creation of high-sensitivity magnetic field detectors.
In this work, we report the results of investigations of the metal/insulator/semiconductor
(MIS) structure using a dc current. The semiconductor was a boron-doped silicon substrate
and the insulator was a SiO2 layer with a thickness of 2 nm. The sample geometry used in the
experiments is illustrated in Fig. 1a. In addition, we fabricated a simple planar device based
on this structure. On the structure’s surface, two iron-film electrodes separated with a 20-μm
gap were formed (Fig. 1b).
Figure 1. (a) Experimental geometry and (b) schematic of the structure.
Figure 2 shows temperature dependences of the resistance Rph(T) obtained under optical
radiation in zero magnetic field and in the field H = 6 kOe and the corresponding temperature
dependences of the photoinduced magnetoresistance. The results were obtained for the bias
currents J = +20µA (Figs. 2а and 2b) and J = −20 µA (Figs. 10c and 10d), because, in this
case, the maximum influence of the magnetic field is observed. The first thing that attracts
attention is the huge value of photoinduced magnetoresistance ΔR/R, which can exceed many
times the value of 104% in H = 6 kOe. Without optical radiation, no noticeable variation in the
resistance of the device in magnetic field is observed. The other surprising result is the extremely high sensitivity of the photoinduced magnetoresistance to the magnetic field sign.
Here, the magnetic field was applied in the plane of the device perpendicular to the J direction.
Figure 2. Temperature dependences Rph(T) and ΔR/R(T) obtained at (a, b) J = 20 µA
and (c, d) J = −20µA.
Asymmetry of the photoconductivity response with respect to the bias current and magnetic field signs can be clearly seen in the I−V characteristics presented in Fig. 2a. Without
optical radiation, the I−V characteristics obtained at low temperatures are symmetrical and
their shape is similar to the reverse branch of a tunnel MIS diode; i.e., the tunnel conductivity
is implemented, probably, with the surface states involved [1]. Under irradiation of one of the
MIS junctions in zero field, the conductivity increases due to the tunneling through the interface centers. An increase in voltage U at fixed J values is related to the current saturation determined by the concentration of nonequilibrium carriers, i.e., by the rate of generation and
recombination of photoelectrons.
Figure 3. (a) I−V characteristics of the device under optical radiation measured in zero magnetic field and
in magnetic fields of +9kOe and −9 kOe; (b) bias dependence of magnetoresistance (ΔR/R)ph in the positive and
negative magnetic fields; and (ΔR/R)ph as a function of the magnetic field for bias currents of (c) +10 and −10
μA and (d) +50 and −50 μA.
Figures 3c and 3d show the magnetic-field dependences of (ΔR/R)ph for the high ( J =
±50 μA) and low (J = ±10 μA) bias current values. For J = ±10 μA, at the beginning of the
magnetic field sweeping, the (ΔR/R)ph value increases insignificantly in weak fields and
sharply grows in the fields above 3 kOe. For J = ±10 μA and H =10 kOe, (ΔR/R)ph attains 1.5
· 105% and the saturation is not observed. On the contrary, for J = ±50 μA, the rapid growth
of (ΔR/R)ph starts at H = 0 and slows down in the fields H = 5−7 kOe; with a further increase
in the magnetic field, the magnetoresistive effect tends to saturation. However, in this case,
the maximum (ΔR/R)ph value is smaller than 4 · 104 %.
Having analyzed the photoconductivity response to the magnetic field, we suppose that
the key role in the optically induced giant magnetoresistive effect is played by localized surface states and, perhaps, by the features in the behavior of nonequilibrium carriers in the device under study. In addition, we cannot exclude the contribution to the magnetoresistance
made by possible spin-dependent tunneling of photoexited electrons through the SiO2 potential barrier into the ferromagnetic electrode.
References
1) S.M. Sze. Semiconductor Devices // Wiley. – New York – 1985.