Increasing efficiency in hyperdoped Si photodetectors for infrared detection and imaging*

Hyperdoped silicon is a promising material for infrared detection due to its extended infrared absorption compared to bulk Si. Supersaturated solutions of impurities in Si are produced to create intermediate bands in between the valence and conduction bands. This new IB serves to create optical transitions with energies lower than the band gap, resulting sub-band gap absorption and photoconductivity. Ion implantation followed by pulsed laser melting has been demonstrated as a method to produce concentrations of impurities in Si that are well above the solid solubility limit.

To achieve devices that could be commercialized for photodetectors or other demanding applications, significant optical absorption and high quality Ohmic contacts for carrier extraction will be required. We fabricated Si layers hyperdoped with Au or Ti at varying thickness, attempted to form Ohmic contacts to the layers, and fabricated prototype p-n junction photodiodes. The results show significant enhancement of optical absorption by increasing the implant energy. For making Ohmic contacts to hyperdoped materials, we tried several treatments, including boron or phosphorus shallow doping, rapid thermal annealing (RTA) of contact, etching off the top metallic layer, and modifying the PLM process to suppress dopant segregation. Recipes for Ohmic contacts to each material were demonstrated, and a low ohmic contact resistivity around 0.1 Ω-cm² was achieved. For photodiodes, the IV characterization shows a weak rectifying effect for the Si based junction. The conversion range is extended to 2um, and the power conversion efficiency can reach 1% below Si bandgap and 0.6% at 1550nm, which is a significant improvement over recent similar devices. Such high efficiency hyperdoped Si devices have potential for commercialized photodetectors. The origin of the subbandgap response arises from substitutional Au in Si:Au materials and interstitial Ti in Si:Ti materials. Surface microstructure of Si:Ti may also contribute to the sub bandgap response.