Fabrication and Performance of Advanced Photovoltaic and Plasmonic Devices

In this work, two avenues for potentially improving the performance of future electronic and optoelectronic devices have been explored. Backside contact technology for high-performance III-V solar cells and the exploration of plasma-wave effects in GaN have been developed and evaluated. For the backside contact solar cell technology, we analyzed the performance of an advanced novel design for III-V multi-junction solar cells —the backside contact III-V triple-junction solar cell. For the study of plasma-wave effects, we have focused on observing and studying plasma-wave effects in III-N plasmonic devices at room temperature. The fabrication of both backside contact solar cells and III-N plasma-wave devices is discussed.

Backside contact technology has been demonstrated successfully in silicon solar cells, but has not been explored carefully or demonstrated in III-V multi-junction solar cells so far. In this work, a numerical model was developed to evaluate and optimize both the electrical and thermal properties of backside-contact triple-junction solar cells. The optimization of the epitaxial structure resulted in a 14% (relative) efficiency improvement and the backside-contact technology results in another 5% (relative) efficiency enhancement. For the fabrication and ultimately the demonstration of backside-contact triple-junction solar cells, the critical process step — via-hole fabrication — is demonstrated. The developed full-wafer via-hole fabrication process, which is compatible with the process flow for backside-contact solar cells, resulted in uniform and smooth etch morphologies and near vertical sidewall profiles.

For the study of plasma-wave effects, in this work we designed and fabricated GaN-based devices for observation of plasma-wave effects at room temperature. While plasma-wave effects have been reported previously at cryogenic temperatures, this study seeks to characterize the potential of these effects for room temperature operation. The devices explored here use grating-gate structures to enhance the plasma-wave signatures in the device response. The device geometries were optimized and nanoscale fabrication techniques were developed to fabricate the devices. On-wafer electrical testing showed clear signatures of plasma-wave effects at room temperature. These signatures were found to agree very well with analytical models of plasma-wave propagation. The observation of electrically-significant plasma-wave effects in GaN opens the possibility of future devices that exploit this physics for enhanced functionality.