Chapter 3 Metal oxide superlattice
3.2 Distribution Patterns of dopants
3.2.1 Conventional versus mixed ALD doping processes
unfavorable to enhance the electrical conductivity, so we developed a new ALD doping process, i.e. mixed ALD doping process. In the mixed ALD doping process, the dopant precursor, TDMAHf, was introduced into reaction chamber along with DEZn
simultaneously while depositing dopant layers. By this way, two precursors were going to compete with each other to react with surface hydroxyl groups, so the decrement of Hf number density in dopant layers was obtained. The schematic illustration of
depositing dopant layer by conventional process and mixed ALD doping process is shown in Figure 3.2. We also deposited types of mixed ALD doping process Hf:ZnO (HZO), the thermoelectric properties of them are summarized in Table 3.4.
First, it could be discovered that the carrier concentration of HZO deposited by mixed ALD doping process was much higher than that deposited by conventional process at the same repetition rate of dopant layers. Since the actual dopant
concentration was quite low in mixed ALD doping process, the number density of Hf in mixed ALD doping process was only around 0.6~0.7 nm-2, which was much lower than that in conventional process, i.e. ~ 4.80 nm-2. The dopant concentration measured by XPS and calculated number density is summarized in Table 3.5. In this situation,
dopants were much freer to donate free carriers and then became ionized ions without severe repulsion from other ionized dopants in the same dopant layer. For this reason, we could increase the carrier concentration of HZO by increasing repetition rate of dopant layer until dopants were no longer such free to donate free carrier owing to the repulsion from ionized dopants in the adjacent dopant layers, i.e. from mix 4:1 to mix 3:1. With regard to the electrical mobility, the decrement of dopants in mixed ALD doping process cut down the dopant scattering effect and reduced the suppression of crystallinity, so the electrical mobility in mixed ALD doping was higher than that in conventional process. Figure 3.3(a) shows the XRD patterns for comparing crystallinity of the same dopant layer period length deposited by two different process, i.e. con 24:1 versus mix 24:1. Nevertheless, Figure 3.3(b) reveals that densely inserted dopant layers still decreased the crystallinity in (002), especially. Besides, an abundance of free carriers raised the electron scattering, which may retard the transport of each other, so the electrical mobility decreased with carrier concentration, i.e. mix 3:1. Luckily, the overall effect of mixed ALD doping process on electrical conductivity was positive, the best value was almost doubled in comparison to conventional process, i.e. mix 14:1 versus con 24:1.
Referring to Seebeck coefficient, the benefit of mixed ALD doping process on promoting doping efficiency was unfavorable to maintain the high absolute value of
Seebeck coefficient, so the absolute value of Seebeck coefficient decreasd severely and the lowest value was only remain 40% compared to undoped ZnO, -44.62 and -107.49, respectively. Therefore, the PF value was quite low when dopant layers were densely inserted. Luckily, the PF value was greater than undoped ZnO when the period length of dopant layers was long, such as mix 34:1, mix 29:1 and mix 24:1. The highest PF
occured at mix 24:1 with the value of 1.90×10-4 W m-1 K-2. Due to the low degradation to the electrical mobility in these structures, high carrier concentration was inessential for acquiring high electrical conductivity, so decrement on the absolute value of Seebeck coefficient could be suppressed. As the period length of dopant layers kept increasing, e.g. from mix 24:1 to mix 34:1, the PF value decreased and was close to the value of undoped ZnO.
Unlike its benefit to electrical performance, mixed ALD doping process showed a poor effect on reducing the thermal conductivity of ZnO. It could be seen that the degree of reducing thermal conductivity of mixed ALD doping process was much lower than that of conventional process at the same dopant layer repetition rate, i.e. ~2.13 for con 9:1 and ~8.48 for mix 9:1. As for the similar dopant concentration, e.g. mix 9:1 and con 49:1, conventional process was able to suppress thermal conductivity more
effectively even densely inserted mixed ALD doping dopant layers reduced the crystallinity more severely, which can be seen in Figure 3.3(c). This result was
attributed to the outstanding effect of more distinct interface on phonon scattering, so the conventional process could reduce the thermal conductivity much more effectively.
With regard to the overall effect on ZT, the best performance occured at mix 3:1, whose thermal conductivity was the lowest, and the degree of enhancement was about 4-fold.
Summing up two distribution patterns of dopant, it could be realized that close-packed dopants deposited by conventional process were able to form complete
heterogeneous interfaces and then had an advantage in reducing thermal conductivity.
On the other hand, sparsely distributed dopants deposited by mixed ALD doping process were able to increase carrier concentration efficiently and less harmed the electrical mobility. These two characteristics ensured a high PF value, such as mix 24:1.
In order to obtain the advantage of each process simultaneously for further enhancing ZT value, we were going to combine two processes and found out the best distribution pattern of the dopants.
Table 3.4: The thermoelectric characteristic of undoped ZnO, conventional process and mixed ALD doping process HZO. n: carrier concentration and the unit is ×1019 cm-3; μ:
electrical mobility and the unit is cm2 V-1 s -1 ; σ: electrical conductivity, S cm-1 ;κ:
Table 3.5: The concentration of dopant measured by XPS and atomic number density of Hf. The number density is calculated by using composition ratio, film thickness and density.
O:Zn:Hf Number density of Hf
(nm-2)
Con 9:1 41.75 : 54.46 : 3.79 4.82
Con 24:1 45.71 : 52.87 : 1.42 4.83
Con 49:1 45.50 : 53.81 : 0.64 4.76
Mix 24:1 44.63 : 55.17 : 0.20 0.69
Mix 19:1 44.29 : 55.45 : 0.26 0.68
Mix 9:1 45.59 : 53.83 : 0.58 0.72
Mix 4:1 44.75 : 53.99 : 1.26 0.71
Mix 3:1 43.82 : 54.58 : 1.60 0.73
Figure 3.2: The schematic illustration of depositing dopant layer by conventional process and mixed ALD doping process.
Figure 3.3: XRD comparison for (a) same period length; (b) densely inserted mixed ALD doping; (c) same Hf content.