The thermal conductivity predictions from the isotropic and direct summation meth- ods in various materials with different supercell sizes will be presented in this section. We first verify the simulation code by comparing the results in this work to literature papers, we then calculate the thermal conductivity results in bulk Si, Si/Ge QDSLs, and Ge/Si QDSLs.
10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 10 100 1000 R educed k (W/cm K ps) Temperature (K) Si Ge 1×1 2×2 Si [39] Ge [39] 1×1 [39] 2×2 [39] (a) 0 200 400 600 800 1000 1200 100 150 200 250 300 350 400 Thermal Conductivity (W/mK) Temperature (K) Si Si [31] (b)
Figure 2.3: (a) Reduced thermal conductivity for Si, Ge and Si/Ge superlattices with different periodicity; (b) Thermal conductivity for bulk Si. Lines are results from this work, markers in (a) are results from Ref. (Tamura et al. 1999), and markers in (b) are results from Ref. (Hopkins et al. 2009).
2.3.1
Verification of Modeling Methods
Before examining the validity of the isotropic assumption, we first verified the direct summation and isotropic methods against benchmark data in the literature. The direct summation results were compared to the reduced thermal conductivity data computed by Tamura et al. (Tamura et al. 1999) for bulk Si, bulk Ge, and Si/Ge superlattices with different atom layer thicknesses as shown in Fig.2.3(a). In Fig.2.3 (a), the superlattice unit cell geometry is denoted nm, where n and m refer to the thicknesses of the Si and Ge layers, respectively, in terms of the number of atomic layers in the superlattice unit cell. Using the same potentials, geometries, and lattice constants, we obtained very good agreement with Tamura’s results. Additionally, we compared our isotropic method thermal conductivity calculations to those of Hopkins
et al. (Hopkins et al. 2009) for bulk Si. Excellent agreement between the two is found
(Fig. 2.3 (b)).
2.3.2
Bulk Silicon
After verifying our method, the thermal conductivity of bulk Si with different supercell sizes was calculated using both direct summation and isotropic methods. Convergence
tests with different q-space resolution show that the thermal conductivities achieve stability for q-space intervals dq below 0.025 rad/nm, so the value 0.025 rad/nm was
used for all calculations. Figure 2.4 shows that the direct summation method gives a constant thermal conductivity for all supercell sizes, as expected. In contrast, the isotropic thermal conductivity results decrease with supercell size, showing a similar trend to that observed in Ref. (Davis & Hussein 2011). The decreasing behavior predicted from the isotropic method is not physically meaningful, as the choice of supercell size should not affect the thermal conductivity of a bulk material. Moreover, the difference between the direct summation and the isotropic methods for bulk Si increases as the supercell size increases. This implies that the isotropic method is increasingly unreliable for predicting thermal conductivities when the supercell size increases and indicates that great care must be taken when using it to predict the thermal conductivity of supercell-based systems such as superlattices, nano phononic crystals, and other periodic structures.
0
1
2
3
4
5
6
Supercell size in CCs
0
50
100
150
200
Thermal Conductivity (W/mK)
Direct Summation [100] Isotropic [110] Isotropic [100]Figure 2.4: Thermal conductivity of bulk Si with different supercell sizes using the direct summation and isotropic methods.
2.3.3
Silicon/Germanium Quantum Dot Superlattices
To examine the validity of the isotropic assumption further, we compared the results of direct summation and isotropic calculations on various quantum dot superlattices (QDSL) configurations. Si/Ge QDSLs with supercell side length varying from two to five CCs of Si and a single CC of Ge in the center, as seen in Fig. 2.1, were in- vestigated first. Ge/Si QDSLs with the same geometry as the Si/Ge systems but with reversed atom positions were also investigated in order to explore the effects of material configuration on thermal conductivity. The thermal conductivity results for Si/Ge, Ge/Si QDSLs in the [100] direction, and Si/Ge QDSLs in the [110] direction are shown in Fig. 2.5(a), (b), (c), respectively. For both methods and in both QDSL types, the thermal conductivity increases toward the bulk value of the host material as the supercell size increases. This increase occurs because the nano particle relaxation time decreases relative to the Umklapp and impurity relaxation time as supercell size increases, leading to longer phonon lifetimes and higher thermal conductivities. It is also observed that the Si/Ge QDSL thermal conductivity increases at a higher rate than that of the Ge/Si QDSL. This is thought to occur because the increase in vol- ume fraction of host material with supercell size is another factor, in addition to the decrease in phonon-nanoparticle relaxation time, that impacts thermal conductivity. In the Si/Ge QDSL case, the host material (Si) thermal conductivity is higher than that of the QD (Ge). This leads to thermal conductivity enhancement beyond that determined by nanoparticle scattering alone. In contrast, the lower thermal conduc- tivity of the Ge host material in Ge/Si QDSL leads to a suppression of overall thermal conductivity below that determined by nanoparticle scattering alone. This suppres- sion is not strong enough to offset the increase with supercell size due to reduction in nanoparticle relaxation time, so the Ge/Si QDSL thermal conductivity follows the same trend as that of Si/Ge QDSL. More interestingly, the thermal conductivities from direct summation grow more rapidly with supercell size than those from the isotropic method. This is similar to the case of bulk Si, where the isotropic thermal conductivity deviates more strongly from the direct summation results as supercell
2 3 4 5 Supercell size in CCs 20 40 60 80
Thermal condcutivity (W/mK) Material: Si/Ge QDSL Direct (FBZ) [100] Isotropic (Iso-BZ) [100] (a) 2 3 4 5 Supercell size in CCs 20 40 60 80
Thermal condcutivity (W/mK) Material: Ge/Si QDSL Direct (FBZ) [100] Isotropic (Iso-BZ) [100] (b) 2 3 4 5 Supercell size in CCs 20 40 60 80
Thermal condcutivity (W/mK) Material: Si/Ge QDSL Direct (FBZ) [110]
Isotropic (Iso-BZ) [110]
(c)
Figure 2.5: Thermal conductivity of (a) Si/Ge QDSLs in [100] direction, (b) Ge/Si QDSLs in [100] direction and (c) Si/Ge QDSLs in [110] direction.
size increases.
The difference between the isotropic assumption and the direct summation meth- ods can be more clearly quantified by calculating the ratio of kISO (Eq. 1.15) and
kF BZ (Eq. 1.12), as shown in Fig. 2.6 for Si/Ge QDSL, Ge/Si QDSL, and bulk Si
in the [100] direction. For all three cases, the isotropic thermal conductivity results drift further away from the direct summation results as the supercell size increases. The change of the ratio from the smallest supercell to the largest supercell is 15% to 25% in all three cases.