Synthesis and characterization of Germanium quantum dots for thermoelectric applications

Abstract:

Energy resources are a main factor for the development of industry and human life, however, the use of fusil fuels as energy is harmful to the environment. Taking these two matters into consideration, the use of waste energy is a good response. The thermoelectric phenomena, which was, discovered in the 18th century plays a main role in converting waste heat energy to electricity and vice-versa.

Germanium quantum dots (Ge QDs) have received special attention due to their unusual electrical and optical properties, which are correlated to the quantum confinement effect. In thermoelectric devices amazing electrical property of Ge QDs are utilized. Ge QDs can be applied in thermoelectric devices to increase the electrical conductivity while decreasing the thermal conductivity, resulting in an increasing of the figure of merit (ZT); a characteristic for thermoelectric devices that should be as high as possible.

In this study, Reduced Pressure Chemical Vapor Deposition (RPCVD) was used to synthesize Ge QDs utilizing GeH4 gas on silicon at a temperature of 450℃ with deposition times of 23s, 25s, 30s, 60s, 120s and 240s, and at a total and partial pressure of 20 Torr and 20 mTorr respectively. RPCVD was used to fabricate multi-layer Ge dots on silicon wafers, which were sandwiched between thin silicon films. Process parameters used in this study to deposit thin interlayers silicon film were as follows: Total pressure: 20 Torr, temperature: 500℃ and partial pressure of 10 mTorr. Deposition times of 150s, 300s and 600s were used to deposit interlayers of silicon utilizing Si₂H₆ gas to connect and disconnect carrier transfer between Ge QDs perpendicularly and to investigate the surface roughness. Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Energy Dispersive Spectroscopy (EDS), and High Resolution X-Ray Diffraction (HRXRD) were employed to investigate the Ge dots and interlayers silicon films. These characterizations showed that the smallest dots are obtained from 23s deposition time which means higher tunneling of electrons and an increase of electrical conductivity. The data also showed that a shorter deposition time results in a higher relative strain which means higher carrier mobility and higher electrical conductivity. Finally, multilayers of Si/strained Ge-dots analyzed to find the smoothest surface, and the smoothest surface was obtained with 23s deposition time of Ge dots, which means less electrical noise in thermoelectric devices. Such structures are ready to be grown on silicon on insulator (SOI) wafer to make advanced coupled or uncoupled dots for future thermoelectric applications.