Black silicon
Black silicon (english black silicon ) is a surface modification of the crystalline silicon. In this case, caused by high-energy bombardment with ions or ultra-short laser pulses of the needle -like structures on the surface, which greatly reduce the reflection of the substrate. Originally this change was observed mid-1980s, at that time she was a negative side effect in reactive Ionentiefenätzen ( DRIE ).
Properties
Black silicon is a needle-shaped surface structure ( with a length> 10 microns with a diameter of < 1 micron on single crystal silicon ), and therefore the structure of the form is also referred to as "silicon grass" or " RIE grass".
Essential feature is an increased absorption of incident light. Similar to the moth-eye - effect ( mechanism of certain anti- reflection layer ) of the high reflectance of the silicon is reduced ( as a function of the wavelength of 20 to 30 % by a quasi- normal incidence ) significantly (to about 5%) by the structures. This is due to the formation of a so-called effective medium through the microstructure. They cause a continuous transition of the effective refractive index of the medium, so that no optical interface exist sharp, on which the light may be reflected (as the Fresnel formulas ). Instead, the light is " gently " led into the material and hardly reflects what can appear to be black the object.
Application
The unusual optical properties of the semiconductor make the material for sensor applications interesting. As of February 2009 the applications are in development. Applications are:
- Image sensors with increased sensitivity
- Residual light amplifier
- Thermal Imagers
- Photovoltaics with increased efficiency through increased absorption. Due to the increased surface area increased long-term stability is to be expected .. In January 2012, researchers have been able to achieve an improvement in efficiency and a simplified production of solar cells using extremely short laser pulse treatment of black silicon solar cells.
- Mechanical contacts and interfaces.
At other applications in the terahertz -, nano- pores and mounting area being researched.
Production during reactive Ionentiefenätzen
The reactive Ionentiefenätzen in semiconductor technology, a standard procedure for the preparation of trenches and holes ( with a depth of up to several 100 micrometers, with some very high aspect ratios ). This is achieved by repeatedly switching between an etching and a Passivierungschritt.
During etching, however small deposits of passivation can remain on the ground and this "mask". With a displacement of the process towards the passivation arise auszuformende structures that are not removed even when the etching steps. Thus there arise vertical surfaces at which a polymer layer can be deposited. Thus, covered by the deposition of the top and remain available to the polymer from the sides, long silicon pillars. The process can be adjusted so that the needles can be formed on a square millimeter million.
Prepared according to Mazur method
In 1999, a group developed at Harvard University (by Eric Mazur and James Carey ), a process in which black silicon produced by bombardment with extremely high energy pulsed femtosecond lasers. By laser bombardment, the three-dimensional structure is altered, and there is a needle-shaped surface (300 nm long), which is relatively uniform and highly reproducible.
In the presence of sulfur hexafluoride, in the laser irradiation, a significantly larger amount of sulfur can be incorporated into the silicon ( doping), so that the band gap is reduced, and thus change the electrical and optical properties of the material. Due to the lower band gap also reaches low-energy light (up in the infrared range ) in order to excite electrons in the conduction band (see photoelectric effect ). By additionally applying a small DC bias voltage, the sensitivity and thus the current produced can be increased by a factor greater than 100. The reason is that a photon out solves many electrons here.