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Strong infrared photoluminescence from black silicon
made with femtosecond laser irradiation
Quan Lü, Jian Wang, Cong Liang, Li Zhao, and Zuimin Jiang*
State Key Laboratory of Surface Physics, Key Laboratory of Micro and Nano Photonic Structures
(Ministry of Education) and Department of Physics, Fudan University, Shanghai 200433, China
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Significant efforts have been devoted to the development of light emitters based on Si. However bulk Si is a poor light emitter due to its
indirect band gap.
Black silicon (b-Si) has been receiving a great deal of attention, but a lot of published results on photoluminescence (PL) of b-Si is at the
wavelength of the visible region.
Strong infrared PL at about 1.53 μm was observed for the first time from b-Si formed by femtosecond (fs) laser and treated by rapid thermal
annealing, which is corresponding to the optical telecommunication wavelength.
I. Sample fabrication
IV. Mechanism of luminescence
FIG. 3. (a) XPS spectra of S 2s and (b) PL spectra from the samples before and
after etching in KOH solution, respectively.
 The p-Si(001) wafer was chemically cleaned using the method of Ishizaka and
Shiraki and then cleaned in deionized water for several times.
 An fs laser beam as a light source, which provided 800 nm, 125 fs laser pulses
with a repetition rate of 1 kHz, for the irradiation on the Si wafer surface to form
a forest of spikes on it in a chamber with 70 kPa SF6 gas.
II. Microstructure of the b-Si
FIG. 1. SEM image of the b-Si surface formed by fs laser radiation in SF6 gas ,
showing that the average height of spikes is about 4~5 μm.
III. PL properties of the b-Si samples
annealed at different temperatures
The sulfur impurities are dispersed in the surface region with a depth of ~200 nm,
while the dislocations may be located at deeper depth. By etching the surface region
of the b-Si in KOH solution, sulfur impurities can be removed.
The corresponding PL signal increased rather than decreased for the etched b-Si
sample. The possibility of sulfur impurities luminescence can totally be ruled out,
and then the strong infrared PL is attributed to dislocations-related luminescence.
V. Power and temperature dependence of
the integrated PL intensity
FIG. 4. (a) PL spectra of the b-Si sample at different excitation powers from 6 to
590 mW. (b) Integrated PL intensity as a function of the excitation power.
The coefficient m is 0.58 in I ∝ Pm, which suggests that the PL of the b-Si sample
originates from the recombinations of bound-to bound states, consistent with the
nature of D1 electronic states.
anomalous behavior
FIG. 5. (a) Integrated PL intensity as a function of reciprocal of temperature. (b)
Integrated PL intensity and peak energy as a function of temperature in the range of
15 to 50 K.
The PL signal could be observed at temperature as high as 250 K, showing a high
quenching temperature.
When the temperature increases from 15 to 25 K, the PL intensity shows an
anomalous behavior, which may be explained by the increase of diffusion length in
photogenerated carriers.
FIG. 2. (a) PL spectra of the b-Si samples unannealed and annealed at different
temperatures, (b) PL spectra of b-Si samples unannealed and annealed at 500°C,
(c) integrated PL intensity, and (d) PL peak energy as a function of annealing
temperature.
VI. Conclusions
No PL signal was detected for the unannealed sample, whereas the integrated PL
intensity increases 16 times after annealing at 1000 °C compared with that after
annealing at 500 °C.
The PL is attributed to D1 luminescence rather than sulfur-related impurity center
luminescence.
The characteristics of the D1 PL were studied systematically, including excitation
power dependence and temperature dependence.
Q. Lu, J. Wang, C. Liang, L. Zhao, and Z. Jiang, Opt. Lett. 38, 1274 (2013)