(invited talk) Sulfur-hyperdoped silicon by ultrashort laser processing

Hyperdoping of semiconductors can be used to shift the optical band gap towards lower energies and thereby extend the optical response of a material like Si further into the infrared. This makes the approach attractive for opto-electronic applications, e.g. photodiodes or intermediate band solar cells. Here, we study the hyperdoping of Si with sulfur that we obtain by placing the Si substrate in a sulfur containing atmosphere and subsequent irradiation by a multitude of ultrashort laser pulses. We present an overview of our key findings regarding the so obtained material system and elaborate on remaining challenges and promising perspectives.

We evaluate the correlation of laser process parameters like effective pulse density, laser fluence and pulse duration as well as different and customized post-hyperdoping treatments on structural, optical, and electrical material properties. The structural properties are studied by transmission electron microscopy and Raman spectroscopy, optical properties by absorptance measurements and electrical properties by the effective minority carrier lifetime and current-voltage (I-V) measurements. In case of the carrier lifetime, we apply an Al2O3 surface passivation by atomic layer deposition (ALD) to reduce the surface recombination rate and, thus, to become more sensitive to changes within the hyperdoped bulk. For I-V measurements, we deposit ohmic metal contacts. Challenges for both surface passivation and metallization that result from the surface roughness of the material are discussed. We furthermore implement the material into a photodetector device and characterize its performance in the sub-bandgap spectral region up to 5µm.