Black Silicon: 23 Years Later

In 1996 Eric Mazur, SPIE Member and a physics professor at Harvard University in Cambridge, Massachusetts, and several of his graduate students discovered a material they later called "black silicon." The group placed a silicon wafer in a vacuum chamber, filled it with chalcogen-containing gas, and irradiated the silicon with ultrashort pulses from a femtosecond laser. Upon examination, the team discovered the blackened surface was covered with a vast array of nanoscale spikes.

This altered material eventually proved to be much more sensitive to light than a standard silicon chip. It was announced to the world at the American Physical Society Centennial meeting in 1999 in Atlanta, Georgia. In 2003, Mazur and his team built the first black silicon photodetector. Additional testing of black silicon revealed its high light absorption at wavelengths between 400 nm and 2.5 µm-far beyond the infrared response of regular silicon.

Mazur and then-graduate student James Carey of Harvard University founded the startup SiOnyx in 2006, with the intent of using black silicon to improve the infrared sensitivity of silicon-based photodetectors and image sensors in low-light and night-vision applications, suitable for a wide range of industries.

"What Mazur and SiOnyx discovered is that the increase in light absorption could lead to a useful gain in detectable electrical current, resulting in a highly sensitive photodetector that includes infrared wavelengths (lower photon energies) that silicon usually does not absorb," says SPIE Fellow David Dickensheets, a professor of electrical and computer engineering at Montana State University in Bozeman. "Their end goal goes beyond simply absorbing all the light to actually converting those absorbed photons into useful electrons."

SiOnyx's black-silicon-based technology is a significant breakthrough in the development of smaller, lower-cost, and higher-performing photodetection or optoelectronic devices for a variety of applications. In 2012, SiOnyx received venture funding from Coherent and In-Q-Tel, the venture wing of the US Central Intelligence Agency (CIA). A year later SiOnyx developed a night-vision camera for the US Army that achieved its expected performance goals when operated in lab conditions that simulated a moonless night. Building on these successes, in 2018 SiOnyx brought to market the Aurora camera, which captures high-definition video in near total darkness.

"It has been quite a long road," says Mazur. "The idea has always been for black silicon technology to benefit consumer technology. Early support from the Department of Defense helped make the transition from prototype to proven technology possible. The research we did for the Army Research Office validated our work-there was a lot of skepticism initially."

That's not the case now: a search for "black silicon" on Google Scholar will show nearly 9,000 records.

With the increased sensitivity of black silicon detectors in the near infrared, the most obvious commercial applications are for low-light situations.

"Any market that benefits from near-infrared-enhanced performance and/or better low-light sensitivity is a target for our technology," adds Carey, now principal engineer for SiOnyx. The number of applications continues to grow and includes commercial applications (night vision, machine vision, automotive, gesture, time of flight, security, defense, and spectrometry) and military applications (night vision, targeting, laser "see-spot"). "The increased near-infrared sensitivity of our products provides value in all of these markets," he states.

The Aurora camera is the first SiOnyx product specifically aimed at the consumer electronics space. While most commercial night vision cameras capture images in grayscale or monochromatic green, black silicon technology can uniquely produce full-color photos and video in low-light conditions, including near-total darkness-ideal for a variety of military, security, and consumer applications. High-end night vision technology is either based on analog intensifier tubes or thermal sensors that cost thousands of dollars, and sometimes tens of thousands of dollars-the Aurora is a digital alternative that is based on complementary metal-oxide-semiconductors (CMOS).

In April 2018 the company launched a Kickstarter^TM campaign to support the future launch of the camera. "We felt that using Kickstarter was a great way to engage our new consumer audience," says Carey. "It is a perfect platform to get early customer feedback, prioritize features, market products that are still in a formative stage, and create advocacy through the enthusiastic backers that use Kickstarter."

After the first four days of the launch SiOnyx had tripled the $50,000 goal it had hoped to meet for that time period. The crowdsourcing campaign raised a total of about $300,000 and generated more than 600 backers.

"We were both humbled and extremely excited by the incredible response Aurora has received through its Kickstarter," indicates Stephen Saylor, president and CEO of SiOnyx. "Aurora is tapping into the massively underserved market for action cameras with real night vision at consumer prices and we are grateful to our current and future backers for helping us continue this journey forward."

The term "black silicon" actually dates back at least to the 1980s as a generic term for describing high-aspect-ratio features formed on the surface of silicon.

Black silicon can be formed in a variety of ways, including wet etching, dry etching, and laser ablation. The resulting geometry of the nanotexures can vary dramatically; small changes at the nanoscale can have large impacts on performance at the macroscale. The spikes (also called silicon nanowires) can be customized for parameters such as diameter, density, and length, which can significantly alter the optical reflection and absorption of the module.

All black silicon increases light absorption and decreases reflectivity; however, some black silicon is blacker than others. With the explosion of interest in nanowires for studying 1D and quantum materials, several other "black" materials have been created, including carbon nanotubes. However, when compared to these other materials, silicon still holds its own in overall light absorption and "blackness" and is very straightforward to process.

Scanning-electron microscope image of the surface of a black silicon wafer.

A possible competing technology to black silicon is graphene-based near-infrared detectors; however, graphene-based optoelectronic devices face significant challenges with scaling up, including the production of low-cost and high-quality graphene. Carey notes, "Other materials, such as SiGe and InGaAs, have high near-infrared and shortwave infrared sensitivity, but don't enjoy the scale and low cost that a silicon-based technology does."

At Montana State University, Dickensheets has incorporated nanotextured black silicon as an optical absorbing material into silicon-based micro-optoelectromechanical systems (MOEMS) to reduce stray light and increase optical contrast during imaging. "Inadvertent reflections are often a problem in MOEMS devices," he says. "A direct and practical use for black silicon is as a light absorber or baffle that can be patterned with micron-scale precision and built into a device, as a self-aligned aperture around the active optical surface."

Dickensheets and his graduate student Tianbo Liu are impressed by how black this material can be. The black silicon they make using cryogenic ICP-RIE etching has much less light scatter (less than 1 percent of incident optical power) and specular reflection (less than .001 percent reflectivity) compared to black paints or other commercial "black" flocking materials.

Black silicon can also be cast into a polymer film spun onto the wafer after the black layer is made. "If the wafer is then etched from the bottom, until all the solid silicon is gone, the nanowires will remain embedded in the polymer film, forming a black layer there," says Dickensheets, describing work done in the NSF-supported Montana Nanotechnology Facility by former graduate student Mohammad Moghimi. "This could be useful for creating highly absorbing features in flexible optical devices. Naturally occurring optical systems, like insect eyes, rarely occur on a rigid substrate, so flexible optical platforms may open up all kinds of new applications."

One of the biggest trends in photovoltaic manufacturing today is using black silicon to increase efficiency. SPIE Member Fatima Toor, assistant professor of electrical and computer engineering at the University of Iowa in Iowa City, has developed various techniques to improve the front- and back-surface performance of nanostructured black silicon solar cells, with up to a 23% increase in short wavelength (400 to 600 nm) internal quantum efficiency and an increase in the external quantum efficiency in the long wavelength (>900 nm) region of up to 11%. Her team is also working on utilizing black silicon for biosensing and tandem PV applications.

Although black silicon is more than two decades old, Toor believes it is just now reaching its commercial glory. She points out that Trina Solar, one of the largest photovoltaic cell and module manufacturers in the world, recently released its black-silicon-based PV modules that are higher efficiency over wide view-of-angle than traditional solar cells that are pyramid textured and coated with vacuum-based silicon nitride antireflection coatings. Toor says, "Given that black silicon is highly versatile and silicon-based, I believe more commercial products based on black silicon will continue to enter the market."

SiOnyx R&D activities continue to focus on improving the sensitivity of its sensors and designing image sensors for future applications.

In January 2019 SiOnyx announced a $19.9-million deal with the US Army to deliver digital night-vision cameras. These will be part of the Integrated Visual Augmentation System (IVAS) program, which provides individual soldiers with head, body, and weapon technologies. The army is eager to gain a combat advantage by improving low-light detection of people at a distance of 150 meters. The contract, which was awarded through the army's specialist Night Vision and Electronic Sensor Directorate (NVESD), calls for SiOnyx to deliver low-light camera modules within two years.

"This is an exciting new development for SiOnyx," says Carey. "The application is aimed at providing tremendous low-light imaging capability, in a smaller form factor, than is currently possible. There are plenty of challenges in making digital night vision small, such as optics design, thermal management, and sensor performance. State-of-the-art night vision today is analog and the form factor for the [current military-issue night vision goggle] PVS14 is considered compact. For digital night vision, the Aurora is significantly smaller than previous offerings-the cameras we will build for IVAS will be even smaller."

This new contract with the US Army brings the government's early work with SiOnyx full circle. While the Department of Defense's early interest and support of their technology ultimately allowed SiOnyx to enter the consumer market with the Aurora night vision camera, this latest contract with the US Army will allow the government to benefit from the technology advancements and lessons learned during SiOnyx's years developing for the commercial market. "We always intended our black silicon image sensors to be dual-use technology," says Carey. "There is tremendous value in improving the low-light and night-time imaging performance of silicon image sensors, for both military and commercial purposes."

Mark Crawford is a science and technology writer based in Madison, Wisconsin.