Excess carrier generation in femtosecond-laser processed sulfur doped silicon by means of sub-bandgap illumination

Abstract

With Fourier-transform photocurrent spectroscopy and spectral response measurements, we show that silicon doped with sulfur by femtosecond laser irradiation generates excess carriers, when illuminated with infrared light above 1100 nm. Three distinct sub-bandgap photocurrent features are observed. Their onset energies are in good agreement with the known sulfur levels S +, S0, and S2 0. The excess carriers are separated by a pn-junction to form a significant photocurrent. Therefore, this material likely demonstrates the impurity band photovoltaic effect

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

“…Obviously, the band is much narrower than the band, indicating the corre sponding ratio of neutral and charged donor centers in the hyperdoped layer. Furthermore, the observation of the pronounced characteristic S bands in the infrared spectra, as compared to structureless and anomalously broadened absorption bands for microstructured, hyperdoped, and amorphized Si in previous works [5][6][7], demonstrates the nondegeneracy of the hyper doped layer and its high crystallinity. The latter con clusion is supported by Raman spectroscopy in Fig.…”

“…This contradiction can be resolved, considering the broad energy spectrum of shallow and deep donor impurity states of sulfur in Si [7,11]. Shallow impurity states are characterized by energies less than 15 meV [21], while in this work in the hyperdoped Si layer mid infrared interband transitions occur from narrow donor and bands arranged at 0.188 and 0.37 eV below the conduction band minimum [7,11].…”

“…Moreover, since the optical probing depth at 488 nm (≈0.7 μm [17]) is much larger than δ S ≈ 0.12 μm, such Raman spectra may indicate significant Si surface coverage by sulfur. [7,11], respectively. Obviously, the band is much narrower than the band, indicating the corre sponding ratio of neutral and charged donor centers in the hyperdoped layer.…”

“…Such hyperdoped materials dem onstrate a number of attractive properties, for exam ple, ICT at abnormally high impurity concentrations of ~10 20 -10 21 cm -3 [9] and anomalous absorbance over the visible and near and mid infrared ranges (i.e., in the transparency range of intrinsic Si) [5][6][7] owing to broadband impurity and free carrier absorp tion. Meanwhile, even after annealing, such hyper doped materials exhibit structureless and anomalously broad absorption bands in the near and mid infrared ranges (up to 10 μm [7]) owing to their spontaneous amorphization within hyperdoped layers [10], dis abling infrared spectral characterization of structural impurity states in Si matrix. It is known for chalco gens, e.g., for sulfur, that doping atoms may reside in substitutional, interstitial, and cluster (dimer) posi tions, starting from the solubility limit of 3 × 10 16 cm -3 , constructing multiple donor bands in the range from 0.1 to 0.6 eV below the conduction band [5,9,11].…”

“…During the last decade, unprecedented doping of silicon at a level of ~0.1-1 at % (impurity concentra tion of 10 20 -10 21 cm -3 ) [5][6][7], well exceeding its equi librium solubility limit (≈3 × 10 16 cm -3 for sulfur [8]), became possible in chemically active gases or from pre deposited interfacial thin solid films in course of Si surface remelting by ultrashort (sub and picosec ond) laser pulses, as well as by laser annealing of ion implanted samples. Such hyperdoped materials dem onstrate a number of attractive properties, for exam ple, ICT at abnormally high impurity concentrations of ~10 20 -10 21 cm -3 [9] and anomalous absorbance over the visible and near and mid infrared ranges (i.e., in the transparency range of intrinsic Si) [5][6][7] owing to broadband impurity and free carrier absorp tion. Meanwhile, even after annealing, such hyper doped materials exhibit structureless and anomalously broad absorption bands in the near and mid infrared ranges (up to 10 μm [7]) owing to their spontaneous amorphization within hyperdoped layers [10], dis abling infrared spectral characterization of structural impurity states in Si matrix.…”