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Figure 6.1(a) shows an AFM image of a CVD SLG after being transferred to the SiO₂/Si substrate and chemically etched. The surface is very rough and has many defects, such as particles, ripples, folds, and kinks. The CVD graphene can be large-scale patterned at certain locations, afterwards the electrodes are fabricated above the graphene flakes. Figure 6.1(b) shows an optical image of a 5×5 array of CVD SLG spin valve devices on the SiO₂/Si substrate. This fabrication process is highly compatible with the industrial semiconductor manufacturing nowadays. Figure 6.1(c) shows an SEM image of a spin valve device (device A3-4) with a patterned CVD SLG. Besides the advantage of wafer-scale fabrication, due to the CVD graphene can be patterned into different shapes, therefore the layouts of the devices are no longer limited by the size and shape of the graphene flakes, and the structures can be fabricated as densely as possible.

Figure 6.2(a) shows the measurement of the CVD SLG (device A3-2-1 H) resistance as a function of gate voltage. In this device, the Dirac point lo-cates at VD=−17 V. The charge carrier mobilities can be extracted from the conductivity as a function of gate voltage, as shown in Fig. 6.2(b), and are µe ∼1290 cm²/Vs and µh ∼1070 cm²/Vs. Figure 6.2(c) shows a spin valve ex-periment of device A3-2-1 H at VG=0 V, the spin valve signal is about 4.6 Ω.

In addition, the Hanle spin precession measurement at VG=0 V is shown in

Figure 6.1: (a) AFM image of a CVD SLG. (b) Optical microscopic image of a 5×5 array of CVD SLG spin valve devices (sample A4). (c) SEM image of a CVD SLG spin valve device (sample A3-4) with 17 parallel Co electrodes.

Figure 6.2: (a) Measurement of CVD SLG (device A3-2-1 H) graphene resistance as a function of gate voltage at room temperature. (b) Gate voltage dependence of conductivity of device A3-2-1 H. The carrier mobilities (estimated by the red fitting lines) are µ_e ∼1290 cm²/Vs and µ_h ∼1070 cm²/Vs. (c) Spin valve and (d) Hanle spin precession measurements of device A3-2-1 H at zero gate voltage. Note that the backgrounds in the Hanle precession data are subtracted. The blue (black) spheres represent the non-local resistance when the magnetizations of the injector and detector are parallel (antiparallel), and the red curves are the fits to Eq. 2.18.

Figure 6.3: (a) CVD SLG (device A3-2-1 H) resistivity as a function of carrier density at room temperature. (b) Carrier density dependence of spin relaxation time (blue squares) and momentum scattering time (red hollow spheres).

Fig. 6.2(d). The blue (black) spheres represent the non-local resistance when the magnetizations of the injector and detector align in parallel (antiparal-lel) configuration, and the red curves are the fits to Eq. 2.18. By fitting the Hanle precession data, the spin relaxation time τ_s ≈182.2 ps, and diffu-sion constant D ≈0.00703 m²/s are extracted, and thus the spin relaxation length λ_s ≈1.1 µm is obtained. The spin transport quantities, including τ_s, D, and λ_s, of the CVD SLG are very similar to those of the exfoliated natural SLG [20, 28, 30, 32] (also see Chapter 5).

The carrier density dependence of τs (blue squares) of device A3-2-1 H is shown in Fig. 6.3(b). The τs has a minimum value at the Dirac point, corresponding to the maximum of the graphene resistivity in Fig. 6.3(a). As the carrier density increases, the τs also increases. In order to identify the spin relaxation mechanism in the CVD SLG, the analysis methods used in Chapter 4 are again applied: investigating (1) the correlation between τs and τp; and (2) the diffusion constant dependence of τs, and (3) the carrier density dependence of the product τsτp and the ratio τs/τp. The τp in the Boltzmann transport regime (n ≥ 10¹¹ cm⁻²) can be estimated from the conductivity and carrier density [113]:

Figure 6.4: (a) Spin relaxation time as a function of momentum scattering time of device A3-2-1 H at room temperature. The red line is a linear fit to τs = a0τp, where a₀ is a constant. (b) The product τ_sτ_p (blue color) and the ratio τ_s/τ_p (green color) as a function of carrier density.

τ_p = σ e²

h

v_Fpπg_vg_sn(E), (6.1) where h is the Planck constant, g_v is the two-fold valley degeneracy (g_v=2 ), g_s is the spin degeneracy (g_s=2), and v_F is the Fermi velocity (v_F=10⁶ m/s).

The red hollow spheres in Fig. 6.3(b) represent the τ_p as a function of carrier density of device A3-2-1 H. One can see the spin relaxation time is propor-tional to momentum scattering time, as shown in Fig. 6.4(a). The red line is a linear fit to τ_s = a₀τ_p, where a₀ is a constant. Additionally, the carrier density dependences of the product τ_sτ_p and the ratio τ_s/τ_p show that τ_s/τ_p is much more constant, see Fig. 6.4(b). The results so far suggest that the Elliott-Yafet mechanism dominates the spin relaxation in CVD SLG [28, 47].

Next the temperature dependence of graphene resistivity and spin relax-ation time are discussed, as shown in Fig. 6.5. Similar to the natural SLG, the resistivity of the CVD SLG (device A3-1-1 L) shows no significant tem-perature dependence near the Dirac point. And the spin relaxation times are linearly scaled with the carrier density at temperatures 300 K to 5 K. The results suggest that the dominant spin relaxation mechanism in CVD SLG does not change with the temperature and remain the Elliott-Yafet type at

Figure 6.5: (a) Graphene resistivity, (b) spin relaxation time, and (d) spin relax-ation length of the CVD SLG device A3-1-1 L as a function of carrier density at various temperatures, T=300 K, 200 K, 100 K, and 5 K.

Figure 6.6: Contact mode AFM image and line scan profile of a CVD SLG.

temperatures 300 K to 5 K. Furthermore, both the spin relaxation time and spin relaxation length have weak temperature dependences, which indicate that the CVD SLG device is nicely thermal stable. From the above, the CVD SLG has shown a nice agreement with the natural SLG. The results strongly suggest that the CVD SLG is promising for large-scale spintronic devices fab-rication and is possible to be integrated into the conventional semiconductor manufacturing.