Optical and electrical properties of textured sulfur-hyperdoped silicon: a thermal annealing study

Abstract

When the sulfur element is hyperdoped into crystalline silicon to a supersaturated density of ~10 20 cm -3 , it can enhance the sub-bandgap light absorption of silicon from 0 to 70%, with the antireflection of surface dome structures. On the other hand, the local Si: S configuration that can contribute to the strong sub-bandgap absorption is still unknown. In order to find more characteristics of this local Si:S configuration, we thermally annealed the textured sulfur hyperdoped silicon, and analyzed the changes of its optical and electrical properties. We find that the imaginary part ( 2 ) of the complex dielectric constant of this Si:S configuration is almost constant in the wavelength range from 1250 nm to 2500 nm, and it decreases quickly when thermally annealed at the a) E-mail: qsc@semi.ac.cn b) E-mail: wangyx@henu.edu.cn 2 temperature range from 400℃ to 700℃. To consider the annealing process as a decomposition reaction of this local Si:S configuration, we obtain the thermal activation energy of attenuated infrared light absorption is 0.356eV for this Si:S configuration transforming from optically active state to be inactive state. Hall measurements demonstrate that thermal annealing can convert this local Si:S configuration from electrically active state to inactive state-namely, to lower the electron density and meanwhile enlarge the electron mobility. These experimental results will be very helpful to clarify the local Si:S configuration that can absorb strongly the light at the silicon sub-bandgap wavelength.

Mid-IR-Sensitive n/p-Junction Fabricated on p-Type Si Surface via Ultrashort Pulse Laser n-Type Hyperdoping and High-Temperature Annealing

“…Broadband NIR–MIR spectral characterization of the Si surface spots patterned at N = 25–200 pulses/spot, which appear visibly black at the white-light illumination (Figure , inset), was performed in the transmission mode in the range of 500–7500 cm –1 (1.3–20 μm, Figure ). Similar to previous studies on other sulfur-hyperdoped Si structures, − the nanopatterned Si spots exhibit slowly and monotonously decreasing transmittance from MIR- to NIR ranges and monotonous decrease versus N (Figure a), with their optical density of the modified surface layer changing vice versa (Figure b). Therefore, the Si surface nanopatterns, flat on the micrometer-scale, provide the unique almost flat and non-structured spectral responses with the related high NIR–MIR absorbance and the corresponding ultimately high extinction coefficients ∼10 4 –10 5 cm –1 in the sub-micron thick modified layers owing to their efficient light trapping and sulfur-donor absorption.…”

“…Because the non-Lorentzian MIR band peaked at 1090 cm −1 strongly increases for the fast liquid nitrogen quenching (sample #1) in comparison to the stationary air-based quenching (sample #2), this rules out its predominating oxide origin. In fact, this is the first identification of the deep sulfur centers, predominating after such high-temperature stationary annealing and quenching in hyperdoped Si samples, 12,14,15 with its very intense but rather narrowband sulfur-based MIR absorbance paving the way to intermediate band engineering in thermal imaging and night vision devices, photovoltaic cells. Moreover, the direct comparison of the 1090 cm −1 band for the as-fabricated and its annealed/l-N quenched product demonstrates the importance of the stationary high-temperature annealing and fast quenching for fabrication of MIR-sensitive sulfur-doped Si layers.…”

“…High-temperature annealing of the nanopatterned/hyperdoped Si samples was performed in a muffle oven in ambient air at a temperature of 1150 °C during 30 min. In previous experiments on annealing of sub-ps-laser microtextured, sulfur-hyperdoped Si samples, it was shown that this and higher temperatures enable efficient dissolution of sulfur in Si, while at lower temperatures, sulfur impurity proceeds to segregation in cluster forms with drastic decrease of the MIR absorbance. ,, The following quenching occurred via (1) fast immersion of the samples taken from the oven into liquid nitrogen and the resulting sub-second cooling (high quenching rate, #1) and (2) quasi-static cooling directly inside the turned off, closed oven and hour-scale cooling (low cooling rate, #2). After quenching, the samples were ultrasonically cleaned for 10 min to delaminate the surface oxide layer.…”

“…In the latter case, non-equilibrium hyperdoping of Si surfaces by chalcogen impurities well above their equilibrium solubility levels crucially increases their near-IR (NIR) and mid-IR (MIR) absorption. − This provides, along with additional femtosecond/sub-picosecond (fs/sub-ps) laser-induced surface microstructuring, efficient IR-range harvesting (actually, strong broadbandUV–MIR spectral response − ) via light trapping and donor-impurity absorption, promising for diverse optoelectronic (photovoltaics, IR, and night vision) applications. In this still challenging approach, ultrabroad smooth (non-structured) donor absorption bands merge to similar ultrabroad smooth absorption bands due to light trapping, − laser ablative removal of IR-absorbing donor impurity from the hyperdoped Si surface layer proceeds during microtexturing, and multi-micron deep laser microtexturing of the IR-trapping relief rules out fabrication of thin-film photovoltaic devices.…”

“…In this still challenging approach, ultrabroad smooth (non-structured) donor absorption bands merge to similar ultrabroad smooth absorption bands due to light trapping, − laser ablative removal of IR-absorbing donor impurity from the hyperdoped Si surface layer proceeds during microtexturing, and multi-micron deep laser microtexturing of the IR-trapping relief rules out fabrication of thin-film photovoltaic devices. Moreover, different precautionsfor example, stationary thermal post-annealing − or pulsed laser annealingare usually taken into account to restore low-defect crystalline state on such microstructured, hyperdoped surfaces of silicon. This is required for its good optoelectronic performance, upon such strong perturbation of its crystalline lattice by a few tenths of percent of impurity atoms or disruptive fs-laser microstructuring .…”