Black silicon is a new type of silicon material with excellent optoelectronic properties. This article summarizes the research work on black silicon by Eric Mazur and other researchers in recent years, detailing the preparation and formation mechanism of black silicon, as well as its properties such as absorption, luminescence, field emission, and spectral response. It also points out the important potential applications of black silicon in infrared detectors, solar cells, and flat-panel displays.Crystalline silicon is widely used in the semiconductor industry due to its advantages such as ease of purification, ease of doping, and high-temperature resistance. However, it also has many drawbacks, such as high reflectivity of visible and infrared light on its surface. Furthermore, due to its large band gap, crystalline silicon cannot absorb light with wavelengths greater than 1100 nm. When the wavelength of incident light is greater than 1100 nm, the absorption and response rate of silicon detectors are greatly reduced. Other materials such as germanium and indium gallium arsenide must be used to detect these wavelengths. However, the high cost, poor thermodynamic properties and crystal quality, and incompatibility with existing mature silicon processes limit their application in silicon-based devices. Therefore, reducing the reflection of crystalline silicon surfaces and extending the detection wavelength range of silicon-based and silicon-compatible photodetectors remains a hot research topic.
To reduce the reflection of crystalline silicon surfaces, many experimental methods and techniques have been employed, such as photolithography, reactive ion etching, and electrochemical etching. These techniques can, to some extent, change the surface and near-surface morphology of crystalline silicon, thus reducing silicon surface reflection. In the visible light range, reducing reflection can increase absorption and improve device efficiency. However, at wavelengths exceeding 1100 nm, if no absorption energy levels are introduced into the silicon band gap, reduced reflection only leads to increased transmission, because the band gap of silicon ultimately limits its absorption of long-wavelength light. Therefore, to extend the sensitive wavelength range of silicon-based and silicon-compatible devices, it is necessary to increase photon absorption within the band gap while simultaneously reducing silicon surface reflection.
In the late 1990s, Professor Eric Mazur and others at Harvard University obtained a new material—black silicon—during their research on the interaction of femtosecond lasers with matter, as shown in Figure 1. While studying the photoelectric properties of black silicon, Eric Mazur and his colleagues were surprised to discover that this microstructured silicon material possesses unique photoelectric properties. It absorbs almost all light in the near-ultraviolet and near-infrared range (0.25–2.5 μm), exhibiting excellent visible and near-infrared luminescence characteristics and good field emission properties. This discovery caused a sensation in the semiconductor industry, with major magazines vying to report on it. In 1999, Scientific American and Discover magazines, in 2000 the Los Angeles Times science section, and in 2001 New Scientist magazine all published feature articles discussing the discovery of black silicon and its potential applications, believing it to have significant potential value in fields such as remote sensing, optical communications, and microelectronics.
Currently, T. Samet from France, Anoife M. Moloney from Ireland, Zhao Li from Fudan University in China, and Men Haining from the Chinese Academy of Sciences have all conducted extensive research on black silicon and achieved preliminary results. SiOnyx, a company in Massachusetts, USA, has even raised $11 million in venture capital to serve as a technology development platform for other companies, and has begun commercial production of sensor-based black silicon wafers, preparing to use the finished products in next-generation infrared imaging systems. Stephen Saylor, CEO of SiOnyx, stated that the low cost and high sensitivity advantages of black silicon technology will inevitably attract the attention of companies focused on research and medical imaging markets. In the future, it may even enter the multi-billion dollar digital camera and camcorder market. SiOnyx is also currently experimenting with the photovoltaic properties of black silicon, and it is highly likely that black silicon will be used in solar cells in the future. 1. Formation Process of Black Silicon
1.1 Preparation Process
Single-crystal silicon wafers are cleaned sequentially with trichloroethylene, acetone, and methanol, and then placed on a three-dimensionally movable target stage in a vacuum chamber. The base pressure of the vacuum chamber is less than 1.3 × 10⁻² Pa. The working gas can be SF₆, Cl₂, N₂, air, H₂S, H₂, SiH₄, etc., with a working pressure of 6.7 × 10⁴ Pa. Alternatively, a vacuum environment can be used, or elemental powders of S, Se, or Te can be coated onto the silicon surface in a vacuum. The target stage can also be immersed in water. Femtosecond pulses (800 nm, 100 fs, 500 μJ, 1 kHz) generated by a Ti:sapphire laser regenerative amplifier are focused by a lens and irradiated perpendicularly onto the silicon surface (the laser output energy is controlled by an attenuator, which consists of a half-wave plate and a polarizer). By moving the target stage to scan the silicon surface with the laser spot, large-area black silicon material can be obtained. Changing the distance between the lens and the silicon wafer can adjust the size of the light spot irradiated on the silicon surface, thereby changing the laser fluence; when the spot size is constant, changing the moving speed of the target stage can adjust the number of pulses irradiated on a unit area of the silicon surface. The working gas significantly affects the shape of the silicon surface microstructure. When the working gas is constant, changing the laser fluence and the number of pulses received per unit area can control the height, aspect ratio, and spacing of the microstructures.
1.2 Microscopic Characteristics
After femtosecond laser irradiation, the originally smooth crystalline silicon surface exhibits an array of quasi-regularly arranged tiny conical structures. The cone tops are on the same plane as the surrounding unirradiated silicon surface. The shape of the conical structure is related to the working gas, as shown in Figure 2, where the conical structures shown in (a), (b), and (c) are formed in SF₆, S, and N₂ atmospheres, respectively. However, the direction of the cone tops is independent of the gas and always points in the direction of laser incidence, unaffected by gravity, and also independent of the doping type, resistivity, and crystal orientation of the crystalline silicon; the cone bases are asymmetrical, with their short axis parallel to the laser polarization direction. The conical structures formed in air are the roughest, and their surfaces are covered with even finer dendritic nanostructures of 10–100 nm.
The higher the laser fluence and the greater the number of pulses, the taller and wider the conical structures become. In SF6 gas, the height h and spacing d of the conical structures have a nonlinear relationship, which can be approximately expressed as h∝dp, where p=2.4±0.1; both height h and spacing d increase significantly with increasing laser fluence. When the fluence increases from 5 kJ/m² to 10 kJ/m², the spacing d increases by 3 times, and combined with the relationship between h and d, the height h increases by 12 times.
After high-temperature annealing (1200 K, 3 h) in a vacuum, the conical structures of black silicon did not change significantly, but the 10–100 nm dendritic nanostructures on the surface were greatly reduced. Ion channeling spectroscopy showed that the disorder on the conical surface decreased after annealing, but most of the disordered structures did not change under these annealing conditions.
1.3 Formation Mechanism
Currently, the formation mechanism of black silicon is not clear. However, Eric Mazur et al. speculated, based on the change in the shape of the silicon surface microstructure with the working atmosphere, that under the stimulation of high-intensity femtosecond lasers, there is a chemical reaction between the gas and the crystalline silicon surface, allowing the silicon surface to be etched by certain gases, forming sharp cones. Eric Mazur et al. attributed the physical and chemical mechanisms of silicon surface microstructure formation to: melting and ablation of the silicon substrate caused by high-fluence laser pulses; etching of the silicon substrate by reactive ions and particles generated by the strong laser field; and recrystallization of the ablated part of the substrate silicon.
The conical structures on the silicon surface are spontaneously formed, and a quasi-regular array can be formed without a mask. M. Y. Shen et al. attached a 2 μm thick transmission electron microscope copper mesh to the silicon surface as a mask, and then irradiated the silicon wafer in SF6 gas with a femtosecond laser. They obtained a very regularly arranged array of conical structures on the silicon surface, consistent with the mask pattern (see Figure 4). The aperture size of the mask significantly affects the arrangement of the conical structures. The diffraction of the incident laser by the mask apertures causes a non-uniform distribution of laser energy on the silicon surface, resulting in a periodic temperature distribution on the silicon surface. This ultimately forces the silicon surface structure array to become regular.
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