Structural and electrical characteristics of a hyperdoped silicon surface layer with deep donor sulfur states

Large-Scale Laser Fabrication of Antifouling Silicon-Surface Nanosheet Arrays via Nanoplasmonic Ablative Self-Organization in Liquid CS2 Tracked by a Sulfur Dopant

“…This implies that elemental S is dissolved in the amorphous Si "fingers" rather than sedimented on their nanocrystalline boundaries, which is in the former case favorable for the formation of S-donor states with strong IR absorption. 23,24,[26][27][28][29]35 In this specific case, the gaseous S-containing products of CS 2 decomposition mainly in the hot ablative plume and minorly on the laser-heated Si surface (i.e., in the relief trenches), which are present in the amorphous "fingers", appear as chemical markers, indicating the formation of Si amorphous "fingers" through vapor-phase redeposition rather than via potential resolidification of the multishot laser-molten Si. In the same line, the high S abudance in the "fingers", being also highly homogeneous on the nanoscale, potentially indicates the high degree of intermixing and codeposition of Si ablation and CS 2 decomposition products, which can be perfectly realized in the first expansion/collapse cycle of the interfacial vapor bubble.…”

“…12,16−18 In contrast, ultrafine, down to sub-60 nm wide, surface nanoripples were revealed during ultrashort-pulse laser nanopatterning as weak relief nanomodulations in subablative "dry" 19,20 and strong relief nanomodulations in "wet" ablative 21,22 regimes, indicating a potential "wet processing" manner for 2D nanopatterning of Si. Moreover, ultrashort-pulse laser nanopatterning of Si in chemically active fluids, e.g., carbon disulfide (CS 2 ), was demonstrated to result in its surface hyperdoping by a few atomic percents of sulfur (>1 × 10 21 S atoms/cm 3 ), 23 compared to a few tenths of percent of impurity atoms (≥10 20 S atoms/cm 3 ) introduced in S-containing gases 24 (in any case, well above the equilibrium S solubility level in Si, ∼10 15 S atoms/cm 325 ). This makes Si surfaces unprecedentedly "black" via spectrally broad (UV−mid-IR) absorption within their microstructured and hyperdoped surface layers and very promising for advanced complementary-metal−oxide−semiconductor-compatible IR-photonics applications.…”

Multifunctional Sulfur‐Hyperdoped Silicon Nanoparticles with Engineered Mid‐Infrared Sulfur‐Impurity and Free‐Carrier Absorption

“…Such extinction coefficient demonstrates over the spectral range a typical smooth spectral dependence, related to Si–NP scattering, free‐carrier absorption (≈10 3 cm −1 , concentration ≈10 19 cm −3 ) and broadband absorption (≈10 4 cm −1 ) between different, inhomogeneously broadened S‐donor states in the amorphous Si phase and its “conduction” band [ 1,10–14 ] (Figure 5b). Meanwhile, additionally various narrow and strong (value ≈(0.5–3) × 10 4 cm −1 ) absorption bands appear in mid‐IR range (Figure 5b), representing the different characteristic “S‐donor band−conduction band” transitions in S‐doped crystalline Si, [ 15–17,36 ] occurring in this study not across a hyperdoped surface layer of a bulk Si target, [ 15,16 ] but, for the first time, over the nanocrystalline grains of the Si NPs at the donor impurity concentration ≈10 20 cm −3 . The derived free‐carrier concentration ≈10 19 cm −3 in the hyperdoped Si NPs exceeds by two orders of magnitude the maximal free‐carrier concentration ≈10 17 cm −3 (conductivity ≈0.1 Om −1 cm −1 ) in Si NPs laser‐fabricated in water ambient, [ 4 ] breaking through a way to much lower, much more safe IR‐laser and RF treatment powers during hyperthermia therapy of tumors.…”

“…In contrast, n‐hyperdoped Si was actively studied for the last decade as a material platform, supporting fabrication of mid‐IR sensitizers for solar cells, [ 9 ] broadband active media for thin‐film photovoltaics, [ 1 ] broad‐band mid‐IR night‐vision and imaging devices for novel promising and emerging applications. The currently achieved strong (≈10 3 –10 4 cm −1 ) smooth [ 10–14 ] or band‐engineered [ 15,16 ] mid‐IR absorption in sulfur‐(or selenium and tellurium [ 12 ] ) doped Si was suggested for raising solar‐cell efficiency up to 49% through efficient harvesting of near‐and mid‐IR solar radiation. [ 17 ] Besides the donor‐based band‐to‐band absorption, considerable free‐carrier absorption via thermal ionization of shallow donor states was also reported in such hyperdoped Si surface layers.…”

“…This is much lower, than the 35% precipitated atomic fraction of the overall sulfur content in the parent Si nanopatterns, [ 20 ] apparently, because of the highly‐dispersed structure of the Si NPs. This estimate of precipitated sulfur concentration in the Si NPs indicates that its atomic concentration, available for optoelectronic applications as broadband [ 1,10–14 ] or selective band‐engineered [ 15,16 ] IR‐absorption in S‐donor bands in silicon and the resulting IR‐photoconductivity, could be as high as ≈10 20 S‐atoms cm −3 . For the pure (non‐diluted) CS 2 ambient during the laser fabrication of the hyperdoped Si NPs this dopant concentration apparently indicates the ultimate value, which could be reduced by diluting liquid CS 2 by proper solvents.…”