Femtosecond laser-induced formation of spikes on silicon

Abstract

We find that silicon surfaces develop arrays of sharp conical spikes when irradiated with 500-fs laser pulses in SF 6 . The height of the spikes decreases with increasing pulse duration or decreasing laser fluence, and scales nonlinearly with the average separation between spikes. The spikes have the same crystallographic orientation as bulk silicon and always point along the incident direction of laser pulses. The base of the spikes has an asymmetric shape and its orientation is determined by the laser polarization. Our data suggest that both laser ablation and laser-induced chemical etching of silicon are involved in the formation of the spikes. PACS: 61.80.B; 79.20.D; 82.65.JVarious micron-sized surface features have been observed on silicon surfaces after ion beam or pulsed laser irradiation. For example, mounds and columns are formed when silicon is sputtered by energetic Ar ions [1,2]. When irradiated by nanosecond laser pulses with fluence close to the melting threshold, silicon surfaces develop ripples with submicron periodicity [3]. Laser pulses with even higher fluence can induce laser ablation of silicon, leaving a crater on the surface surrounded by irregular cones protruding above the surface [4]. Previously we reported that silicon surfaces spontaneously develop arrays of conical spikes when repeatedly irradiated with high-fluence fs laser pulses in SF 6 or Cl 2 [5]. Similar structures have recently been reported with ns laser irradiation [6]. The spikes we observe have a high aspect ratio, a quasi-periodic spatial distribution, and are only formed in an atmosphere containing a halogen. Besides their scientific interest, the silicon spikes are of interest because of their potential applications as light absorbers [7] for solar cells and photodetectors, and as microneedles for transdermal drug * delivery [8]. In this paper, we characterize the surface morphology and study the effect of laser fluence and pulse duration on the spikes. ExperimentalWe carried out our experiments on n-type (arsenic-doped) Si(100) wafers with resistivity less than 5 × 10 −5 Ω m. Each wafer is cleaned with trichloroethylene, rinsed in acetone, and then rinsed in methanol. The wafer is mounted on a three-axis translation stage in a vacuum chamber with a base pressure of less than 10 −4 Torr. During the experiments, the chamber is backfilled with 500-Torr SF 6 .The laser system, consisting of a Ti:sapphire oscillator and a chirped-pulse-regenerative amplifier, produces a 1-kHz train of 100-fs, 0.5-mJ pulses at 800 nm. Longer pulses are obtained by adjusting the pulse compressor. The laser pulses are focused with a 0.1-m focal-length lens and, except where noted, incident normal to the sample. The spatial profile of the laser pulse is nearly Gaussian, with a fixed beam waist of 200 µm at the sample. The fluence (energy per unit area) varies over the laser spatial profile; values quoted below refer to the fluence at the center of the spatial profile. The fluence is controlled by changing the total incident energy with a halfwav...

Femtosecond laser irradiation-induced infrared absorption on silicon surfaces

“…The absorptance at wavelength of 1.5 µm of the structured silicon is about 50% that is much higher than that of unstructured silicon substrate. In visible wavelengths part, the absorptance increment is mainly originated from the effect of multiple reflections, i.e., the incident light is reflected multiple times on the micro/ nanostructured surface and is absorbed by the silicon [7][8][9][10][11]. In near-infrared (NIR) region, there is an obvious drop in wavelength around 1.1 μm for the unstructured silicon absorptance curve, which is corresponding to the silicon band gap.…”

“…Here we report that the IR absorption of silicon can be directly obtained through fabricating a silicon substrate surface with a femtosecond laser irradiation in air without applying any special background gas [11][12][13]. The scanning electron microscope (SEM) images of the fabricated silicon surface have shown arrays of silicon hills in micrometers with smaller nano-scaled structures on the hills' surfaces, which were generated during the laser irradiation.…”

Microtextured Silicon Surfaces for Detectors, Sensors & Photovoltaics

“…The resulting surface is covered with microstructures that are 2-3 µm tall and spaced by 2-3 µm (Figure 1a). The microstructure size, aspect ratio, and spacing increase with increasing laser fluence [11]. The microstructures are crystalline silicon covered with a few-hundred nanometer thick laser-altered surface layer ( Figure 1b) [9].…”

“…We determined the composition of the samples by fitting the data to simulated spectra. 11 In all simulations, the thickness of the doped layer was taken as 200 nm.This thickness was chosen based on the best fit as well as the transmission electron microscopy results in Reference 10. Figure 1 shows scanning electron microscope images of surfaces prepared in H 2 S, SF 6 , H 2 , and SiH 4 .…”

“…We determined the composition of the samples by fitting the data to simulated spectra. 11 In all simulations, the thickness of the doped layer was taken as 200 nm.…”