gura 2.5 Componentes sometidos al proceso de borurización, a) cojinetes, b) anillos par
4.2 Evaluación del exponente de Hurst por los métodos de trazado auto-afín
The process and the transfer of the large-scale CVD synthesized bilayer graphene (CVD BLG) are similar to the CVD SLG’s. By controlling the post-annealing procedure after the deposition, one is able to obtain either single layer or bilayer graphene films. After the synthesis, the bilayer graphene films are partially formed on the substrate. The single layer graphene surrounding the bilayer graphene is dry etched by using the e-beam lithography and RIE. Fig-ure 6.7(a) shows an optical microscopic image of a spin valve device with a CVD BLG (the darker area). To simplify the fabrication process, the locally deposited Co/MgO stacked films are used on this sample (sample JB BI-2).
Figure 6.8(a) shows the CVD BLG (device JB BI-2 BC) conductivity as a function of gate voltage at room temperature, and the Dirac point (the local minimum) locates at VD=−12 V. Due to the surface inhomogeneity of this bilayer graphene, the minimum conductivity does not locate at the Dirac point exactly but keep decreasing while the gate voltage sweeps further than
Figure 6.7: (a) Optical microscopic image of a spin valve device (sample JB BI-2) with CVD BLG (the darker area). The surrounding CVD SLG is chemically etched.
(b) Raman spectrum of the CVD BLG.
Figure 6.8: (a) Measurement of CVD BLG (device JB BI-2 BC) conductivity as a function of gate voltage at room temperature. The Dirac point locates at VD=−12 V. (b) Non-local Hanle spin precession measurement of device JB BI-2 BC at VG=+28 V. The spin relaxation time τs ≈454 ps and λs ≈1.85 µm are obtained by fitting the Hanle precession data.
Figure 6.9: (a) Momentum scattering time dependence of spin relaxation time in the CVD BLG device JB BI-2 BC at room temperature. (b) Carrier density dependences of the product τsτp (blue color) and the ratio τs/τp (green color).
−30 V. Although the bilayer graphene is nonuniform, however, the 2D-band (∼2700 cm−1) in the Raman spectrum [see Fig. 6.7(b)] confirms that most of the graphene is bilayer [101, 104] while only a small part is either SLG and/or FLG. It is expected that another Dirac point can be revealed if the gate voltage increases further to the negative direction, similar to the previous observation in the exfoliated BLG [see Fig. 4.11(d)]. From the gate voltage dependence of conductivity measurement, the carrier mobility in the electron conduction regime is estimated to be µe ∼1700 cm2/Vs. Figure 6.8(b) shows a Hanle spin precession experiment of device JB BI-2 BC at VG=+28 V. The spin relaxation time is found to be ≈454 ps and the spin relaxation length λs ≈1.85 µm. The quantities of the µe, τs, and λsin CVD BLG are comparable to those in natural BLG observed previously. Furthermore, the momentum scattering time dependence of τs, and the carrier density dependences of the product τsτp and the ratio τs/τp are shown in Fig. 6.9. The results indicate:
(1) the τs has a negative correlation with the τp, which is similar to the observation in exfoliated BLG [see Fig. 4.10(a)]; and (2) the product τsτp has weaker carrier density dependence than the ratio τs/τp does. Therefore the D’yakonov-Perel’ mechanism is suggested to be dominant in CVD BLG at room temperature [28, 47].
Next the temperature dependence of CVD BLG resistivity and spin
relax-Figure 6.10: (a) Graphene resistivity and (b) spin relaxation time a function of carrier density at various temperatures, from 290 K to 5 K. (c) Temperature dependence of spin relaxation time at various gate voltages, VG= VD, VD+10 V, VD+20 V, VD+30 V.
ation time are examined, as shown in Figs. 6.10(a) and (b). The temperature dependence of CVD BLG (device JB BI-2 BC) resistivity indicates that the resistivities near the Dirac point increase with the decrease of temperature.
The insulating behavior near the Dirac point might be due to the reduction of the thermally excited carriers [22, 81] and has been observed in natural BLG as well [see Figs. 2.14(a) and 4.23(a)]. On the left side of the Dirac point (n<0), possibly due to the inhomogeneity of the graphene surface, the resistivity does not decrease with the increase of carrier density but keep increasing instead, especially at higher temperatures. Therefore only the phenomena in the elec-tron conduction regimes are emphasized. Figure 6.10(b) shows the τs as a function of carrier density at various temperatures and Fig. 6.9(c) shows the temperature dependence of τs at different carrier densities. At room tempera-ture, the τsis linearly scaled with the carrier density as well as the observation in natural BLG (see Chapter 4). As the temperature decreases, the trend of τs on the carrier density begins being suppressed. At 260 K, the τs still in-creases with the carrier density at lower carrier densities (n<1.0×1012 cm−2), but turns to a constant at higher carrier densities (n>1.0×1012 cm−2). At 230 K, the τs increases with carrier density at n<1.0×1012 cm−2, but
de-comparable performances to natural BLG, including the µ, τs, and λs at room temperature and low temperatures. Recall the correlation between the τs and µe in Fig. 4.12. The data point in blue hollow square represents the CVD BLG device JB BI-2 BC, and indeed it locates around the fit line. Addition-ally, the carrier density dependence of τs is also very similar to that of natural BLG. The above suggests the D’yakonov-Perel’ type of spin relaxation is also valid and dominant in CVD BLG. However, for a better systematic study of the spin transport in CVD BLG, it is needed to examine more samples.
6.3 Summary of the chapter
The spin valve devices with CVD SLG and BLG are demonstrated. The spin and charge transport properties of the CVD SLG and BLG are exam-ined at room temperature and at low temperatures. The results show that these CVD synthesized SLG and BLG have quite similar performances, in-cluding the carrier mobility, spin relaxation time, and spin relaxation length, compared to the exfoliated graphene. At room temperature, the possible dom-inant spin relaxation mechanism in CVD SLG is the Elliott-Yafet type, and is the D’yakonov-Perel’ type in CVD BLG. Therefore, despite of the structural defects (e.g., ripples and kinks), the CVD synthesized graphene can be very promising for large-scale spintronic device integration instead of exfoliated graphene.
spin relaxation length, and high carrier mobility. The spin relaxation mecha-nism in single layer graphene (SLG) has been studied in the past few years.
However, the importance of the spin transport in bilayer graphene (BLG) has not yet been investigated. The unique electronic properties of BLG, such as the finite carrier effective mass and electric field induced band gap, make it interesting to also study the spin transport phenomena. Therefore, in this thesis the spin transport in BLG is mainly emphasized and systematically studied. For this purpose, the spin valve devices with BLG are fabricated.
The BLG is deposited on the SiO2/Si substrate, then the spin valve de-vice in four-terminal non-local geometry is fabricated by a sequence of e-beam lithography, e-beam evaporation of MgO and Co, and standard lift-off tech-niques. The role of the Co electrodes in the device is to serve as the spin injector and detector, and the MgO film is for combating the conductivity mismatch between Co and graphene. For comparison, the SLG devices are fabricated and studied as well. In order to identify the dominant spin re-laxation mechanism in BLG, the correlation between the spin transport and electronic properties are mainly investigated, particularly the spin relaxation time (τs) and the charge carrier mobility (µ). 17 BLG devices from 6 different samples are collected and performed the carrier density dependence of
con-ductivity, spin valves, and Hanle spin precessions. Long spin relaxation times of ∼2 ns at room temperature is observed in one of the BLG devices, which is the first observation of nanosecond spin relaxation times in graphene. Fur-thermore, it is discovered that in BLG, the spin relaxation time is inversely proportional to the carrier mobility (τs ∝ µ−1) at room temperature. This result suggests that the dominant spin relaxation mechanism in BLG is the D’yakonov-Perel’ (DP) type, which is different from the Elliott-Yafet (EY) type, where τs ∝ µ, in SLG. Therefore, the solution for enhancing the τs in SLG, e.g., increasing the carrier mobility, could be a poison for BLG.
Next the temperature dependences of the charge and spin transport in BLG and SLG are examined. It is found that in BLG, the resistivity in the vicinity of the Dirac point increases while decreasing the temperature. This insulating behavior might be due to the decrease of the thermally excited carriers. From the spintronics point of view, the correlation between the τs
and µ is again investigated at 5 K. Interestingly, the general behavior of DP type of spin relaxation (i.e., τs ∝ µ−1) still dominates at high carrier den-sities. However, near the Dirac point, the DP and EY mechanism could be equally important. Furthermore, the carrier density dependence of τs is posi-tive correlaposi-tive at room temperature and becomes negaposi-tive correlaposi-tive while the temperature decreases, and the transition occurs at about 220 K. These observations imply that the dominant spin relaxation mechanism has a transi-tion from the DP type to the DP-EY-co-governed type while the temperature decreases. However, the origins of this transition need to be further investi-gated and clarified. On the other hand, both the spin and charge transport in SLG shows no significant difference between at room temperature and at 5 K.
To integrate graphene into the conventional semiconductor manufacturing, the processes of large-scale graphene devices must be developed. The spin valve devices with CVD SLG and BLG are demonstrated in this thesis. The charge and spin transport properties of the CVD SLG and BLG are examined at room temperature and low temperatures. The experimental results show that these CVD synthesized SLG and BLG have quite similar performances, including the carrier mobility, spin relaxation time, and spin relaxation length, compared to the natural graphene. Therefore, the CVD graphene can be a
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