room temperature to the short-wavelength infrared (SWIR) range (1.4-3 µm, i.e., 0.89-0.41 eV) has the potential to revolutionize silicon-based optoelectronic devices. The introduction of a complementary metaloxide semiconductor (CMOS) compatible process would enable the integration of optical and electronic functionality on a single chip. [1] Nowadays, several approaches are under intensive research to enhance Si SWIR photoresponse, such as the integration of III-V compound semiconductors with silicon, [2,3] photodetectors based on SiGe alloys [4,5] or HgCdTe/Si heterostructures. [6] Despite their high performances, these devices suffer from several drawbacks: they are based on nonabundant or contaminant materials, they usually require cryogenic temperatures to operate, and they are hardly integrated into the very mature Si-CMOS fabrication routes, adding considerable cost and complexity to photonic circuit manufacturing.An alternative path to increase the Si SWIR photoresponse that would avoid the hybrid integration of Si technology with unconventional materials is the direct modification of its electronic band structure. From this perspective it is possible to find interesting approaches. One is the use of a laser-crystallization process to induce an anisotropic tensile stress in silicon optical fibers. This way the bandgap can be reduced from 1.11 to 0.59 eV. [7] Unfortunately, the fiber structure hinders its incorporation into planar imaging arrays. A different method, that has attracted recently great attention, [8] is the extrinsic sub-bandgap photoresponse obtained from the incorporation of deep-level impurities at concentrations far above their equilibrium values. Single-crystalline silicon layers with transition metals (Ti, V, Au, and Ag) or chalcogens (S, Se, and Te) at concentrations above 10 19 cm -3 have been obtained by means of ion implantation and subsecond annealing techniques (in a process denominated as supersaturation or hyperdoping). We have shown that silicon hyperdoped with Ti [9,10] or V [11] presents outstanding properties such as a sub-bandgap optical absorption coefficient in the 10 4 cm -1 range [12] or a photoconductive response extended down to 0.2 eV (6.2 µm) at cryogenic temperatures. [13] Gold hyperdoped [14,15] or silver hyperdoped [16] This work deepens the understanding of the optoelectronic mechanisms ruling hyperdoped-based photodevices and shows the potential of Ti hyperdoped-Si as a fully complementary metal-oxide semiconductor compatible material for room-temperature infrared photodetection technologies. By the combination of ion implantation and laser-based methods, ≈20 nm thin hyperdoped single-crystal Si layers with a Ti concentration as high as 10 20 cm −3 are obtained. The Ti hyperdoped Si/p-Si photodiode shows a room temperature rectification factor at ±1 V of 509. Analysis of the temperature-dependent current-voltage characteristics shows that the transport is dominated by two mechanisms: a tunnel mechanism at low bias and a recombination process in the space charge...

In the scope of supersaturated semiconductors for infrared detectors, we implanted Si samples with Ti at high doses and processed them by Rapid Thermal Annealing (RTA) to recover the crystal quality. Also, for comparative purposes, some samples were processed by Pulsed-Laser Melting (PLM). We measured the electronic transport properties at variable temperature and analysed the results. Results indicate that for RTA samples the surface layer with a high Ti concentration has a negligible conductivity due to defects. On the contrary, the implantation tail region, which has a measurable conductivity due to a very high electron mobility. This region shows the activation of a very shallow donor and of a deep donor level. While the deep level has been previously reported for Ti in Si, such a shallow level has never been measured, and we suggest that it originates from Ti-Si complexes. Finally, a decoupling effect between the implanted layer and the substrate seems to be present, and a bilayer model is applied to fit the measured properties. The fitted parameters follow the Meyer-Neldel rule. The role of the implantation tails in Si supersaturated with Ti is revealed in this work.

According to intermediate band (IB) theory, it is possible to increase the efficiency of a solar cell by boosting its ability to absorb low-energy photons. In this study, we used a hyperdoped semiconductor approach for this theory to create a proof of concept of different silicon-based intermediate band solar cells. Preliminary results show an increase in the external quantum efficiency (EQE) in the silicon sub-bandgap region. This result points to sub-bandgap absorption in silicon having not only a direct application in solar cells but also in other areas such as infrared photodetectors. To establish the transport mechanisms in the hyperdoped semiconductors within a solar cell, we measured the J-V characteristic at different temperatures. We carried out the measurements in both dark and illuminated conditions. To explain the behavior of the measurements, we proposed a new model with three elements for the IB solar cell. This model is similar to the classic two-diodes solar cell model but it is necessary to include a new limiting current element in series with one of the diodes. The proposed model is also compatible with an impurity band formation within silicon bandgap. At high temperatures, the distance between the intermediate band and the n-type amorphous silicon conduction band is close enough and both bands are contacted. As the temperature decreases, the distance between the bands increases and therefore this process becomes more limiting.

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In this work we have explored the growth by high pressure sputtering (HPS) of materials intended for novel selective contacts for photovoltaic cells. This technique shows promise for the low-damage low-temperature deposition of PV materials. We studied the deposition of ITO, MoOx and TiOx using pure Ar and mixed Ar/O2 atmospheres as well as ceramic or metallic targets. We show that HPS deposition of these materials is feasible. The growth rate is greatly reduced when oxygen is added to the argon sputtering atmosphere. The best sputtering RF power was 20-45 W for the pressure range studied. Finally, as-deposited films present high surface recombination, but a mild hot plate anneal at 200ºC recovers long effective lifetimes.