Günther Palfinger Low Dimensional Si / SiGe Structures Deposited by UHV - CVD for Thermophotovoltaics

Photocells low band gap, i. e. smaller than that of Si, can be applied in multi-junction solar cells and in thermophotovoltaics (TPV). Such band gaps can be achieved by embedding layers of SiGe nanostructures between Si spacers. Such a stack placed into the space charge region of a Si pin diode enhances the infrared sensitivity of the photocell. Compatibility to the mainstream Si technology is assured by using crystalline Si wafers as a substrate and UHV-CVD (Ultra High Vacuum-Chemical Vapour Deposition) epitaxy for material growth, promising more cost efficient mass fabrication in comparison with other low band gap materials as e. g. GaSb. Due to the lattice mismatch between Si and Ge of about 4%, the SiGe structures will be strained, limiting the amount of SiGe that can be epitaxially deposited. As absorption of light is a necessary condition for photovoltaic conversion and the total thickness of SiGe is small in comparison to the thickness of a Si photocell, the absorption coefficient of the SiGe nanostructures should be much higher than that of Si to achieve comparable quantum efficiency. The spacial confinement of charge carriers in the SiGe nanostructures causes a spreading of the wave function in momentum space, leading to a higher probability of a direct transitions of charge carriers from the valence to the conduction band. Thus, it was expected that these transition without the need for a phonon increase optical absorption significantly. This work investigates for the first time the optical absorption of SiGe nanostructures on an absolute scale and compares it with a photocurrent measurement. Additionally, it estimates the cost of electricity produced by TPV, a potential application of low band gap photocells. Optical absorption of SiGe structures was measured by internal reflection spectroscopy, where the light passes more than 400 times through the stack of SiGe layers. The total reflection of the light results in a standing electromagnetic wave. The absorption coefficient was obtained from the measurement data, taking the geometry and the electric field distribution into account. The experimental results are compared with a theoretical model, considering the band structure of strained SiGe and confinement effects. The comparison of the photocurrentwith the absorption measurement indicates that some of the absorbed photons with energies below 1eV are not are not converted into photocurrent. Without light trapping, more than 1000 layers of SiGe structures are needed to absorb 1% of the light with a photon energy of 1eV. Light trapping and further improvements in growth technology are necessary to obtain SiGe structures that are applicable for photocell production. A detailed cost estimate is performed for a Si photocell based TPV system, a projected GaSb photocell based system and a future, highly efficient system with inexpensive photocells, expected to be achievable with low band gap photocells based on Si compatible technology. For the calculation of the price of electricity, a lifetime of 20 years, an interest rate of 4.25% per year and maintenance costs of 1% of the investment is presumed. To