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We have developed a novel technique that creates highly doped and structured silicon. Focusing a train of femtosecond laser pulses on silicon wafers in the presence of heavy chalcogens (e.g. S, Se, or Te) dopes a thin layer of silicon at the surface to non-equilibrium levels. This optical hyperdoping process creates black silicon, a highly absorbing surface with extended spectral sensitivity. This material offers new opportunities for silicon-based optoelectronic devices.

Black silicon is strongly light-absorbing. During hyperdoping, the polished surface of a silicon wafer is transformed from shiny gray to deep black. In addition to near-unity absorption in the visible, black silicon absorbs over 80 percent of below band-gap, infrared light for wavelengths as long as 2500 nm. This can be used to make photodiodes with remarkable responsivity in both the visible and infrared. It is also possible that this extended absorption range can improve the efficiency of silicon solar cells.

Black silicon surface viewed under a scanning electron microscope.

Black silicon surface viewed at high magnification

The surface morphology of black silicon depends strongly on the laser pulses and surrounding environment. Irradiating a silicon surface with a train of intense femtosecond laser pulses in the presence sulfur hexafluoride gas transforms the flat, mirror-like surface into a forest of microscopic spikes. Under some conditions, the spikes are tens of micrometers tall and have tip sizes on the order of hundreds of nanometers. However, the morphology of black silicon does not determine the doping levels. In fact, with careful control of the laser parameters, we have developed a method to hyperdope silicon while maintaining a mirror-like surface. This flat hyperdoped silicon also exhibits strong infrared light absorption and is advantageous for material characterization studies and device fabrication.

Current work focuses on generalizing optical hyperdoping techniques, understanding the fundamental physics of optically hyperdoped materials, and finding useful applications for these novel materials.

Plainly Speaking
Silicon is the material of computer chips and solar cells. It is ubiquitous throughout the high-tech world. Ordinarily, silicon absorbs a moderate amount of visible light, but a substantial amount of visible light is reflected as well, and infrared and ultraviolet light are transmitted through silicon or reflected from it with very little absorption.

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Silicon wafer with patch of black silicon (actual size).

Black silicon is made by shining a series of very short, very intense laser pulses at a silicon surface in a chamber filled with sulfur-rich gas. Black silicon, in contrast, absorbs nearly all light at wavelengths ranging from the ultraviolet to the infrared. This suggests it may be very useful in improving the performance of existing silicon devices, such as detectors and photovoltaics.

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