Thermoelectric properties of epitaxial Ge thin films on Si(001) with strong crystallinity dependence

Abstract

The thermoelectric properties of epitaxial Ge films on Si(001) substrates were investigated, where the film crystallinity was controlled using various growth techniques established in large-scale integration research. The films exhibited substantially lower thermal conductivities than that of bulk Ge. Unlike carrier mobility with strong crystallinity dependence, the Seebeck coefficient was similar to the high bulk value and exhibited less crystallinity dependence. This demonstrates that inherently high-carrier-mobility Ge films with high crystallinity can exhibit reduced thermal conductivity and a high thermoelectric power factor simultaneously, leading to the realization of thermoelectric films on Si for reusing low-temperature waste heat from Si-based electronic devices.

Operation of thin-film thermoelectric generator of Ge-rich poly-Ge1-xSnx on SiO2 fabricated by a low thermal budget process

“…equential 3D integration of a thin-film energy-harvesting device in a silicon (Si) chip [1][2][3][4][5][6] is strongly desired for realizing a standalone sensing network system, called the internet-of-things society and/or trillion sensor universe, coupled with low-power consumption technologies. The thermoelectric generator (TEG), which could convert waste heat into electricity, is one of the options.…”

High Thermoelectric Power Factor Realization in Si-Rich SiGe/Si Superlattices by Super-Controlled Interfaces

“…31 We also calculated S curves of unstrained Si with ionized impurity scattering (scattering parameter, r, is 1.5 (black dashed curve)) and of compressively strained Si 1-x Ge x (s-Si 1-x Ge x ) with ionized impurity scattering and alloy scattering 53 (colored curves) using Boltzmann's transport equation under the relaxation time and the parabolic band approximation. 20,23 In the case of fully strained Si 1-x Ge x , lifting of the degeneracy of E C occurs due to the strain. 54 Lowering the degeneracy degree from 6 to 4 caused by compressive strain affected the effective density of states in the conduction band, N C in the S calculation of strained layers.…”

“…One of the biggest problems for TE use of eco-friendly light-element materials has been their high κ. Therefore, nanostructured materials have been intensively studied to reduce κ drastically via interface phonon scattering. − …”

Low thermal conductivity in single crystalline epitaxial germanane films

“…3(b). [32][33][34][35] It should be noted that the κ i is quite lower than other layered materials (MoS 2 and Bi 2 Te 3 ) although the MoS 2 and Bi 2 Te 3 composed of heavy atoms which bring low κ in general. Compared with layered material MoS 2 , κ i is twice lower in GeH although GeH has 2% shorter interlayer distance and lighter constituent atoms than those of MoS 2 shown in Fig.…”

“…The value of κ i was obtained to be 1.1 ± 0.3 W m −1 K −1 at room temperature, which is extremely-low compared with the previously-reported κ value of epitaxial Ge films with diamond cubic structure on Si (15 W m −1 K −1 ). 32) This result came from layered structures in GeH that are binding with a weak vdW interaction, resulting in suppression of phonon transport between the layers. Next, we compare with κ values of other layered materials with weak vdW interlayer interaction, as shown in Fig.…”