Apr. 9, 2008 – Sunovia Energy Technologies and EPIR Technologies say their R&D efforts have achieved “breakthroughs” in infrared sensors that will enable multijunction solar cells with high efficiencies and lower costs, through using less expensive IR system materials and manufacturing processes.
Specifically, the achievement is a MCT IR focal plane array grown directly on a thin cadmium telluride (CdTe) epilayer, which in turn was grown directly on a silicon (Si) readout integrated circuit (ROIC). The infrared focal plane array that generates the electrical signals to be converted into a digital picture was directly and monolithically connected to the ROIC that interprets those signals to create a picture, without a need for externally applied contacts and interconnects. This monolithic integration also formed a proof-of-concept for the fabrication of a novel high-efficiency, two-junction and two-terminal solar cell.
T.S. Lee, who joined EPIR from the U. of Illinois/Chicago, noted that several technical hurdles were overcome, including MBE growth of CdTe and HgCdTe layers on heterostructural silicon wafers containing readout ICs to successfully mate II-VI semiconductor materials with Si for device fabrication for x-ray and infrared detection.
Other challenges included cleaning the Si ROICs without damaging the contacts or resorting to temperatures high enough to damage the ROICs; direct deposition of high quality single crystal CdTe directly on the Si ROICs; deposition of high-quality single-crystal MCT on the CdTe and the dopant activation and device processing to create an FPA, all at temperatures low enough not to harm the ROIC.
This and similar work done by EPIR on IR FPAs “has clearly demonstrated the ability to fabricate such solar cells with the necessary current matching and efficient current collection, without a buffer layer between the Si and the CdTe, with or without a thin zinc telluride tunneling barrier as needed for current matching,” the companies claim in a statement.
The material grown and the monolithic devices fabricated showed “excellent” carrier recombination time, and similarly “excellent” dynamic FPA impedance (at the standard 80K operating temperature) of 10(6) Ohm-sq. cm (at zero bias) for growth on Si, and 10(5) Ohm-sq. cm for ROICs. Device operability also was deemed “excellent,” although the companies noted a small fraction of the pixels lost operability during device fabrication.