Femtosecond-Laser-Induced Formation of Visible-Light-Emitting Structures Inside Silicon

We report a 680-nm photoluminescence (PL) from a femtosecond-laser-modified region inside silicon. Light-emitting structures were formed inside silicon by femtosecond laser irradiation in an air atmosphere. The formation of light-emitting structures arose from laser-induced oxygen doping, and the doping depths of a few hundred micrometers could be reached. PL intensities increased with increasing depth in the laser-modified region. The effects of the laser powers and scanning velocities were investigated. The peak positions of PL spectra showed almost no change after annealing. According to the Raman spectroscopy analysis, the defect states between the silicon nanocrystals and SiO₂ matrix contribute to the PL.

I. Introduction

Silicon has been one of most important materials in the field of optoelectronics. However, the luminescence efficiency of bulk silicon is very low because of its indirect band gap structure. Numerous Si-based light emitters have been developed by producing low-dimensional silicon systems, such as porous silicon [1], silicon nanoparticles [2], [3] and silicon nanocrystals embedded in a SiO₂ matrix [4]–[8]. Recently, femtosecond-laser processing has become an attractive method for enhancing the luminescence of silicon. One method of producing light-emitting silicon is to form silicon nanoparticles with a size of a few nanometers by fs-laser ablation of a solid target in a liquid environment [3]. The other method is to form femtosecond-modified structures on a bulk silicon surface [4]–[6] or silicon compound films [7]. As compared to silicon nanoparticles, the light-emitting silicon produced on bulk and film substrates has the advantage that it could be used to realize light-emitting devices through the standard semiconductor fabrication process. Wu et al. first reported visible photoluminescence (PL) with a peak wavelength varying between 540 nm and 630 nm from black silicon, which was fabricated by 1-kHz fs-laser irradiation of bulk Si in an air atmosphere [5]. Later, two PL bands at 680 nm and 600 nm were observed from the black silicon that was fabricated by using high-repetition-rate fs-laser irradiation in air [6]. Emelyanov et al. reported visible luminescence from hydrogenated amorphous silicon modified by femtosecond laser radiation [7]. Although the emission wavelengths were different, the emission mechanisms for the above observed PL were attributed to the formation of Si nanocrystals embedded in a silica matrix. A consensus has been reached that highly localized defects at the Si/SiO₂ interface [8] and the quantum confinement (QC) of excitons both play important roles in the radiative emission from silicon nanocrystals embedded in a silica matrix [9], [10]. However, it is difficult to distinguish these two mechanisms experimentally [11]. Beyond luminescence in the visible range, strong infrared luminescence was also observed from black silicon formed by femtosecond laser radiation in an SF₆ atmosphere and consequent thermal annealing [12]. Until now, most research has reported on luminescence from silicon surface layers modified by femtosecond laser radiation; no reports have been published on luminescence from laser-induced structures inside silicon. In our previous study, we found that an 800-nm femtosecond laser could induce structure changes inside silicon, including embedded microchannels [13] and deep oxygen-doped regions [14]. Recently, we developed a new technology for fabricating high-aspect-ratio grooves by using wet etching to remove materials in oxygen-doped regions [15]. However, the physical properties of the femtosecond-modified region inside silicon (for example, the luminescence) are still unclear.