1. Introduction

The issue of how to manipulate electrical resistance is quite old, fundamental and always attractive due to its huge potential for applications. The existing methods of manipulating resistance mainly include controlling resistance by changing temperature and application of external field. For example, superconductivity below critical temperature [1–3] and magnetoresistance (MR) controlled by external magnetic field [4–7], both are very successful in many research and application areas. We recently discovered a laser-induced resistance effect based on metal-oxide-semiconductor (MOS) structures with some significant features including excellent spatial sensitivity and large resistance change ratio [8] and a bias-induced offset effect overlapped on this effect [9], which refers to electrical controlling of the laser-induce resistance effect. We also find some other photoelectric effect in similar MOS-type structures, e.g. lateral photovoltaic effect (LPE), infrared laser induced and bias-assisted improved LPE [10–14]. However, to our knowledge, seldom research of magnetic modulation on laser-induced resistance was made.

In this paper, we report a laser-induced resistance effect in Cr/SiO₂/Si structures with a large resistance change ratio of 92% and high spatial sensitivity of 0.79 MΩ/mm. Moreover, we find the effect could be modulated by magnetic field applied perpendicular to the film (the modulation is negligibly small as the magnetic field applied parallel to the film). This result is quite similar to our previous studies on modulation of lateral photovoltage (LPV) by magnetic field in MOS-type structures [15], in which the relative variation of LPV can reach up to 93.2% with 2T of external magnetic field applied was found. We think both modulation of resistance and LPV can be ascribed to the Lorentz force acting on the photo-generated carriers gathering in the inversion layer of the structure. This work provides an approach for the development of new type photoelectric devices which are sensitive to magnetic field, such as magnetically sensitive photoelectric sensors, magnetic switches, magnetically modulated transistors, diodes, even storage devices.

2. Experimental results and discussion

The experiment is performed in Cr/SiO₂/Si structures. The Cr films of varying thickness are fabricated on n-type single crystal Si (111) substrates by DC magnetron sputtering at room temperature. The thickness of the wafers is ~0.3 mm and the resistivity of the wafers is in the range of 50-80 Ω·cm at room temperature. The Si substrates are covered with thin native SiO₂ layers of 1.2 nm measured by transmission electron microscopy (TEM), which are semitransparent to electrons at room temperature [16]. The base pressure of the vacuum system prior to deposition is lower than 1.0 × 10⁻⁴ Pa, and an argon gas pressure of 0.80 Pa is maintained during deposition. The deposition rate of chromium metal is determined by a stylus profile meter on thick calibration samples which is controlled in the range of 0.1 nm/s-0.2 nm/s in this experiment. All the contacts to the films are formed by alloying indium which is less than 1 mm in diameter showing no measurable rectifying behavior. The samples are scanned by laser (635 nm and 5 mW) focused into roughly 50-μm-diameter spot at the metal surface without any spurious illumination reaching the sample.

Initially we discuss the results for the structure with Cr films of 4.5 nm, which exhibits the strongest laser-induced resistance effect. In this structure, the original resistance on the Cr film between two point (A and B in Fig. 1(a)) 1 mm apart is 0.37 MΩ (R₀). When the laser (5mW, 635nm) illuminates on point A (with a positive bias of A⁺, B^-), the resistance remarkably increases to 0.66 MΩ (R_max), while the laser point turns to point B, the resistance is reduced to 0.03 MΩ (R_min). And with laser position changing in the area between A and B, the resistance changes almost linearly with the laser position as shown in Fig. 1(b).

 

Fig. 1 (a) Measurement diagram of laser-induced resistance. (b) Laser-induced resistance on metal side as a function of laser (635 nm and 5 mW) position in Cr(4.5 nm)/SiO₂(1.2 nm)/Si structure with AB = 1mm. The lines are linear fitting.

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Define the resistance change ratio ($\Delta$) as

Then the change ratio in this case varies from −78% ~92%. While the applied bias reverses from positive to negative (A^-, B⁺), the resistance changes conversely as shown in Fig. 1(b). Furthermore, the spatial sensitivity can reach up to 0.79 MΩ/mm, which shows that even a very small laser position displacement can change the resistance significantly.

To gain additional insight into the laser-induced resistance effect, we also investigated the AB distance-dependence. As shown in Fig. 2(a), a shorter distance leads to not only a larger change ratio but also a higher sensitivity. When the AB distance is reduced to 1 mm, the change ratio can reach up to 92% with a spatial sensitivity of 0.79 MΩ/mm. In contrast, when the distance is increased to 7 mm, the change ratio drops to 17.5% and the spatial sensitivity drops to 0.27 MΩ/mm.

 

Fig. 2 (a) The laser-induced resistance change ratio and spatial sensitivity with different AB distance and Cr thickness of 4.5 mm. The lines are linear fitting. (b) The laser-induced resistance change ratio and spatial sensitivity with different Cr thickness and AB = 1 mm. The lines are B-spline fitting.

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By varying the Cr film thickness, we find the effect has a great bearing on the film thickness, which is similar to our previously study [8, 17], there is an optimal value of thickness. As shown in Fig. 2(b), the largest resistance change ratio (92%) and highest spatial sensitivity (0.79 MΩ/mm) occurs at the optimal Cr film thickness of 4.5 nm.

To explore the modulation of this effect, we applied external magnetic field to the samples as shown in Fig. 3(a). When the magnetic field is parallel to the film, the modulation of laser-induced resistance effect is negligibly small. However, as magnetic field is applied perpendicular to the film, we find a significant modulation as shown in Fig. 3(b). The resistance changes obviously, the change ratio can reach up to 110% at 15000Oe.

 

Fig. 3 (a) Measurement diagram of laser-induced resistance effect with external magnetic field applied. (b) The resistance change ratio with different Cr thickness. The lines are B-spline fitting. The inset shows peak value of resistance change ratio modulated by magnetic field.

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The laser-induced resistance can be explained by a photo-generated carriers diffusion and scattering model [8]. As shown in Fig. 4(a), when a laser illuminates the MOS structure of Cr/SiO₂/Si, the electron–hole pairs are generated inside Si. These photo-generated electrons will thus have the possibility tunneling through the oxide layer into the Cr film by the Schottky field, and then diffuse along the metal surface. In the meanwhile, some of them will recombine with the holes during the diffusion. When a negative bias is added on AB, the photo-generated diffusion electrons move in an opposite direction against the field-driven drift electrons in the region between laser point and point A (high resistivity region) as shown in Fig. 4(a), resulting in a large possibility of scattering between them, thus the resistance is increased. However, in the region between laser point and point B (low resistivity region), the diffusion electrons move in the same direction with the drift electrons, thus the resistance is decreased. In fact, the total resistance between A and B consists of these two parts. As the laser moves along the line between A and B, the both length of high resistivity region and low resistivity region changes accordingly. This results in the linearly change in whole resistance, as shown in Fig. 1(b).

 

Fig. 4 (a) Schematic diffusion model in the Cr/SiO₂/Si structure. (b) Schematic simple equilibrium energy-band diagram of the Cr/SiO₂/Si structure. E_gi and E_gs denote the bandgaps of SiO₂ and Si, respectively. Φ_mi is the metal-to-insulator barrier height and is related to the work function of Cr. Φ_si is the semiconductor-to-insulator barrier height and is related to the work function of Si.

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As the measurement bias is reversed, the drift electrons immediately reverse their movement direction. This will cause a symmetrical exchange of the high and low resistivity regions, resulting in a symmetry feature of the effect.

It should be noted that this effect is strongly influenced by both AB distance and electron diffusion length l which is related to metal film thickness [8]. Our experimental results show that resistance change ratio and sensitivity decrease as AB distance increases (Fig. 2(a)), and there is an optimal value of Cr film thickness (Fig. 2(b)), both properties are consistent with our previous study [8, 9, 17].

As we have known, in MOS structures, the effective resistivity measured on the metal film is strongly determined by two parallel channels: the metal film channel and the Si inversion layer channel (shown in Fig. 4) [18–23]. When the Cr film is very thin, its resistance will be very high, thus the inversion layer plays a major role in whole effective resistivity. The high mobility carriers in the inversion layer can be strongly affected by an external magnetic field applied perpendicular to the film due to the Lorentz force [24] (a magnetic field perpendicular to the direction of movement of carriers as shown in Fig. 3(a) and Fig. 4(a) will generate a Lorentz force). As the magnetic field increases, the Lorentz force increases, the high mobility carriers will move spirally and increase the possibility of scattering with each other, thus the resistance is increased as shown in Fig. 3(b). However, as Cr film grows thicker, its resistance will become low. In this case, the metal film channel will play a major role in the whole effective resistivity, which always leads to a smaller effective resistance and a more pronounced shorting effect. The possibility of scattering between diffusing electrons and drifting electrons is so large that the effect of Lorentz force acting on electrons in Cr film is negligibly small. Therefore, as Cr film is thick, the magnetic modulation will become very small (see Fig. 3(b)).

3. Conclusions

In conclusion, we have discovered a laser-induced resistance effect in Cr/SiO2/Si structure with a high sensitivity of 0.79 MΩ/mm as the laser moves along the Cr surface, and a large resistance change ratio of 92%. We also found this effect can be modulated by a magnetic field. We attribute this phenomenon to the Lorentz force acting on the photo-generated carriers gathering in the inversion layer. This work provide a novel approach for the control of resistance by external field in a metal-oxide-semiconductor (MOS) structures, and may be useful for the development of new type photoelectric devices, such as magnetically sensitive photoelectric sensors, magnetic switches, and magnetically modulated transistors, diodes, etc.