Magnetoresistance ratio in magnetic tunnel junction with silicon diffused MgO barrier

We demonstrated a magnetic tunnel junction (MTJ) consisting of Fe/MgO-Si-MgO/Fe. Si layer was deposited at room temperature and at 700 °C; when deposited at 700 °C, Si diffused into the MgO layer. The MTJ with silicon deposited at 700 °C attained high MR ratios of up to 38.7 and 2.9% at t Si = 0.19 and 1.3 nm, respectively. Low-temperature measurements established that the temperature dependence of the MR ratio and resistance between MTJs with and without diffused silicon are significantly different. This behavior confirms that the Si-MgO channel acts as an impurity semiconductor in the MTJ.


S
][3][4][5] The theoretical concept of spin MOSFETs was first proposed by Sugahara et al. in 2004. 6)In the early days, spin MOSFETs using GaAs or GaOx channels were demonstrated by Kanaki et al. 7,8) However, practical applications were not realized, with the low magnetoresistance (MR) ratio being one of the primary reasons for the non-realization.Recently, studies have demonstrated the implementation of several spin valves using GaAs or La 0.67 Sr 0.33 MnO 3 channels that had large MR ratios of up to 140% at low temperatures. 9,10)However, for practical applications of the spin MOSFET, it is necessary to fabricate a device with a high MR ratio at room temperature rather than at low temperature.
Lateral spin valves using silicon semiconductors as the channel have been reported.In these studies, MR ratios of up to approximately 1% have been obtained at room temperature. 11,12)Design guidelines of silicon-based lateral spin valves are important for further increase of the MR ratio. 13)One of the factors responsible for the increase of the MR ratio is the reduction of the channel length.Thus, a vertical structure in which electrons in the channel conduct along a direction perpendicular to the film surface is considered to be promising for this purpose.This is because the vertical structure enables a shorter channel length, and spin relaxation in the channel is suppressed.Consequently, a high MR ratio is also expected.Vertical spin valves using semiconductors, such as GaAs, GaO x , or Ge as the channel have been reported, [14][15][16] and MR ratios of up to approximately 1% have been obtained at room temperature. Mreover, silicon-based vertical spin valves have also been implemented; however, their silicon thicknesses are of the order of micrometers owing to the fabrication process.17) Furthermore, improvement of the spin injection efficiency is crucial to achieve an improved MR ratio.It has been reported both theoretically and experimentally that the insertion of an insulator as a tunnel barrier between a magnetic material and a semiconductor is effective in improving spin injection efficiency.11,[18][19][20][21] This is because insulator insertion can solve the conductivity mismatch of the junction.22) On the other hand, the magnetic tunnel junction (MTJ) is another device with a similar vertical structure and a high MR ratio.Here, the MTJ is a device with a vertical structure in which a MgO barrier is sandwiched between two ferromagnetic materials.High MR ratios have been reported because the MgO barrier allows coherent tunneling.It was theoretically predicted that a high MR ratio of over 1000% could be obtained in Fe/MgO/Fe MTJ devices, 23,24) and an MR ratio of 200% was indeed realized at room temperature in 2004.[25][26][27] Furthermore, MR ratios of over 600% have now been achieved at room temperature.28) When an MTJ with the aforementioned features is applied to spin MOSFET, a further improvement of MR ratio can be expected because both the short channel length due to the vertical structure and efficient spin injection due to the MgO barrier can be achieved simultaneously.
However, such an MTJ-applied silicon-based vertical device, which we call a Si-MTJ in this paper, has not yet been reported.Because silicon has high compatibility with conventional MOS technology and a long spin lifetime, 6,29) early realization of MTJ-applied silicon-based vertical devices is required.
In this study, Si-MTJs with silicon deposited between the MgO barriers were fabricated at room temperature as well as at 700 °C, and their crystal structures are discussed with the aid of scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) images.Subsequently, MR measurements were performed, and it was shown that Si-MTJ with silicon deposited at 700 °C has a very high MR ratio at room temperature.In addition, the semiconductor channel properties are discussed with reference to low-temperature measurements.Our results show that Si-MTJs with silicon deposited at 700 °C are effective for achieving a high MR ratio and suggest that gate control would be possible in the future.
A multilayer film consisting of MgO(5)/Fe(30)/MgO (1.2)/Si(t Si )/MgO(1.2)/Fe(10)/Co(5)/Au(10) (described in nm) was deposited on a single crystal MgO(001) substrate by molecular beam epitaxy method.Figure 1(a) shows the temperature settings of the heater for the deposition and annealing of each layer.For instance, the MgO(001) substrate was annealed at 300 °C for 30 min and then at 800 °C for 10 min.Similarly, the MgO(5) buffer layer and the bottom Fe (30) layer were deposited at 200 °C and room temperature, respectively.The substrate was then annealed at 350 °C for 30 min.Further, the bottom MgO barrier, upper MgO barrier, Co(5) layer, and top electrode were deposited at room temperature while the upper Fe (10) layer was deposited at 200 °C.The Si(t Si ) layer deposited between MgO barriers was deposited at room temperature as well as at 700 °C.The silicon layer was wedge-structured and varied in film thickness.From now on, the Si-MTJs with silicon deposited at room temperature and 700 °C are referred to as RT-Growth-Si-MTJs and 700 °C-Growth-Si-MTJs, respectively.
Si-MTJs were fabricated from multilayers using a combination of photolithography, Argon-ion milling, and lift-off methods.Elliptical Si-MTJs were fabricated on a 2 cm square substrate.The sizes of the Si-MTJs are approximately 5 μm on the short axis and approximately 6 μm on the long axis.The magnetic field H was applied in the Fe[100] direction, i.e. the long-axis direction, along the film surface.
Figures 1(b)-1(g) show the reflection high-energy electron diffraction (RHEED) patterns for each layer.Figures 1(d   show the MR curves for devices at t Si = 0, 0.37, and 0.74 nm in the 700 °C-Growth-Si-MTJ, respectively.The resistance change near the zero field corresponds to the magnetization reversal of the free Fe(30) layer, and the resistance change near the 4-10 mT corresponds to the magnetization reversal of the Fe(10)/Co(5) layer with high coercive force.It is worth noting that the Fe(10)Co( 5) layer had a magnetic vortex structure because of its elliptical shape.In such a structure, the center of the vortex moves gradually owing to the external magnetic field thereby resulting in a step-like resistance change.The difference in the behavior of each MR curve is due to the stochastic magnetization reversal process.Similar shaped MR curves were obtained for both RT-Growth-Si-MTJ and 700 °C-Growth-Si-MTJ over the entire silicon thickness range.Figure 3(g) shows the silicon thickness dependence of the MR ratio.The MR ratios exhibit similar behaviors regardless of whether the magnetic field is applied in the long-or shortaxis direction of the ellipse.The MR ratios when the magnetic field is applied in the long-axis direction are slightly higher; hence, only those values are shown here.MR ratios of MTJ samples with thicker silicon layers were also examined; however, significant MR ratios were not obtained.Therefore, these MR ratios are not shown here.MR ratio of MTJ without silicon layer was approximately 60%.Moreover, the silicon thickness dependence of the MR ratio differs significantly between RT-Growth-Si-MTJ and 700 °C-Growth-Si-MTJ.In the RT-Growth-Si-MTJ, it was found that the MR ratio significantly decreases by a factor of approximately 1/10 when approximately one atomic layer of silicon (approximately 0.135 nm) is inserted.In contrast, in 700 °C-Growth-Si-MTJ, the MR ratio remains above 20% even when approximately one atomic layer of silicon is inserted.Furthermore, very high MR ratios of up to 38.7 and 2.9% were obtained at t Si = 0.19 and 1.3 nm, respectively.One concern is the possibility that a parallel circuit consisting of the MgO barrier and the Si-MgO channel due to the discontinuous silicon layer is formed in the case of 700 °C-Growth-Si-MTJ.In this case, even if electrons do not conduct through the Si-MgO channel, the obtained MR ratio was attributed to the MgO barrier; thus, the spin-dependent transport of the Si-MgO channel would not be evaluated.However, as shown in Fig. 3(h), the resistance of 700 °C-Growth-Si-MTJ at t Si = 0 nm (that is, 700 °C-Growth-Si-MTJ without the silicon layer), is approximately one order of magnitude larger than that of 700 °C-Growth-Si-MTJ at approximately t Si = 1.5 nm.Therefore, most of the electrons flow in the region where the silicon layer is diffused, and the MR ratio measured in this study evaluates the spin-dependent transport in the Si-MgO channel.Additionally, to further investigate the properties of the 700 °C-Growth-Si-MTJ that exhibit such high MR ratios, low-temperature measurements were performed.for 700 °C-Growth-Si-MTJ at t Si = 0, 0.37, and 0.74 nm.In the MTJ without the silicon layer, the MR ratio did not change significantly with a decrease in temperature.This behavior is consistent with the temperature dependence of the MR ratio in conventional Fe/MgO/Fe MTJ. 25)However, when silicon is diffused, the MR ratio increases up to five times at low temperatures in comparison with that at room temperature.This phenomenon can be explained by considering that part of the Si-MgO channel possesses semiconductor properties.In other words, higher temperature increases the number of carriers thereby inducing spin flips consequently decreasing the MR ratio.
Next, Figs.4(d)-4(f) show the temperature dependence of the resistance for 700 °C-Growth-Si-MTJ.It can be seen that the resistance of the MTJ without silicon decreases slightly with increasing temperature, whereas the resistance of the Si-MTJ decreases significantly.This significant decrease in the resistance of the Si-MTJ can be caused by semiconductor carriers.With increasing temperature, carriers are generated, and the resistance decreases.These behaviors indicate that the Si-MgO channel acts as an impurity semiconductor.
In this study, RT-Growth-Si-MTJs and 700 °C-Growth-Si-MTJs were fabricated, and the structural, electrical, and magnetic properties were evaluated.Based on the RHEED images, it can be construed that the silicon layer formation is amorphous in RT-Growth-Si-MTJ and polycrystalline in 700 °C-Growth-Si-MTJ.Additionally, in 700 °C-Growth-Si-MTJ, the epitaxial Si-MgO channel was successfully formed while the barrier maintained the crystalline of the MgO substrate.MR measurements revealed that the silicon thickness dependence of the MR ratio behaved very differently in RT-Growth-Si-MTJs and 700 °C-Growth-Si-MTJs.In the RT-Growth-Si-MTJ, the MR ratio decreased significantly by a factor of approximately 1/10 when approximately one atomic layer of silicon was diffused.In contrast, for 700 °C-Growth-Si-MTJ, very high MR ratios of up to 38.7 and 2.9% were obtained at t Si = 0.19 and 1.3 nm, respectively.Furthermore, low-temperature measurements of 700 °C-Growth-Si-MTJ demonstrated that the temperature dependence behavior of the MR ratio and the resistance between MTJs with and without diffused silicon are significantly different, thereby confirming that the Si-MgO channel acts as an impurity semiconductor in 700 °C-Growth-Si-MTJ.
In conclusion, this study shows that 700 °C-Growth-Si-MTJ is effective for improving the MR ratio of spin valves.Moreover, because the Si-MgO channel acts as an impurity semiconductor, gate control can be expected to be a definite possibility in the future.
)-1(g) show that the RHEED images of the bottom MgO layer show streak patterns in both RT-Growth-Si-MTJ and 700 °C-Growth-Si-MTJ, which indicates that a single crystal is formed.In contrast, the RHEED images of the silicon layer, shown in Figs.1(c)-1(f), display different patterns between RT-Growth-Si-MTJ and 700 °C-Growth-Si-MTJ.The RHEED image of the RT-Growth-Si-MTJ shown in Fig.1(c)shows both streak and diffuse patterns.The streak pattern reflects the crystalline nature of the bottom MgO layer, while the diffuse pattern suggests that the silicon layer is amorphous.Conversely, the RHEED image of the 700 °C-Growth-Si-MTJ, shown in Fig.1(f), presents a ring pattern which suggests polycrystalline formation.However, in the upper MgO layer, streak patterns appear again in both RT-Growth-Si-MTJ and 700 °C-Growth-Si-MTJ, which indicates that crystalline has been recovered.As aforementioned, it is suggested that the silicon layer formation in RT-Growth-Si-MTJ and 700 °C-Growth-Si-MTJ is amorphous and polycrystalline, respectively.Figures2(a) and 2(b) show STEM and EDX images of RT-Growth-Si-MTJ, and Figs.2(c) and 2(d) show STEM and EDX images of 700 °C-Growth-Si-MTJ at t Si = 0.4 nm.The lattice images of the STEM images show that the MgO layer is in a single crystal state and the silicon and MgO layers are indistinguishable in both RT-Growth-Si-MTJ and 700 °C-Growth-Si-MTJ.The yellow region in the EDX image shows silicon atoms, thus indicating that the silicon layer is a

Fig. 1 .
Fig. 1.(a) Film structure of the device and set temperature of the heater during deposition and annealing.Values in parenthesis are thickness with a unit of nanometer.(b)-(g) RHEED pattern of each layer surface.The direction of electron beam is aligned to the MgO[100] direction.

Fig. 2 . 2 ©
Fig. 2. (a) STEM and (b) EDX images for RT-Growth-Si-MTJ, and (c) STEM and (d) EDX images for 700 °C-Growth-Si-MTJ.Both junctions consisted of Fe/MgO/Si (t Si = 0.4 nm)/MgO/Fe.The yellow dots in the EDX images show silicon atoms.EDX images are normalized so that the total signal intensity of the silicon atom is 100%.

Figures 4 (
a)-4(f) show the results of low-temperature measurements for 700 °C-Growth-Si-MTJ.Figures 4(a)-4(c) present the temperature dependence of the MR ratio

Fig. 3 . 3 ©
Fig. 3. (a)-(f) Magnetic field dependence of the junction resistance for RT-Growth-Si-MTJ and 700 °C-Growth-Si-MTJ at each silicon thickness.Each measurement is plotted using a different color.(g) Silicon thickness dependence of MR ratio for RT-Growth-Si-MTJ and 700 °C-Growth-Si-MTJ.(h) Silicon thickness dependence of resistance for 700 °C-Growth-Si-MTJ.Error bars denote the maximum and minimum values at each silicon thickness.