Ultra-low-cost near-infrared photodetectors on silicon
We demonstrate Silicon-only near-infrared (NIR) photodetectors (sensitive up to 2000 nm) that meet large-scale ultralow-cost fabrication requirements. For the detection of infrared photons, we use metal nanoislands that form Schottky contact with Silicon. NIR photons excite plasmon resonances at metal nanoislands and plasmons decay into highly energetic charge carriers (hot electrons). These hot electrons get injected into Silicon (internal photoemission), resulting in photocurrent. Several groups have studied plasmonic nanoantennas using high resolution lithography techniques. In this work, we make use of randomly formed nanoislands for broad-band photoresponse at NIR wavelengths. We observe photoresponse up to 2000 nm wavelength with low dark current density about 50 pA/μm2. The devices exhibit photoresponsivity values as high as 2 mA/W and 600 μA/W at 1.3 μm and 1.55 μm wavelengths, respectively. Thin metal layer was deposited on low-doped n-type Silicon wafer. Rapid thermal annealing results in surface reconstruction of the metal layer into nanoislands. Annealing conditions control the average size of the nanoislands and photoresponse of the devices. An Al-doped Zinc Oxide (AZO) layer was deposited on the nanoislands using thermal atomic layer deposition (ALD) technique to acts as a transparent conductive oxide (TCO) and patterned using photolithography. AZO film creates electrical connection between the nanoislands and also makes a heterojunction to Silicon. Simple and scalable fabrication on Si substrates without the need for any sub-micron lithography or high temperature epitaxy process make these devices good candidates for ultra-low-cost broad-band NIR imaging and spectroscopy applications. © 2015 SPIE.