'Holey' Silicon Brightens Future For Computers, Optical Devices

February 05, 1998

WEST LAFAYETTE, Ind. -- A bright but frail member of the silicon family has found new vigor through a process developed at Purdue University, lighting the path to faster, smaller computers and new types of sensing devices.

Jillian Buriak, assistant professor in Purdue's Department of Chemistry, has developed a way to stabilize the surface of porous silicon, a light-emitting material that offers great promise for combining light and electronics to build new types of computers and other optical devices.

The method also may lead to fine-tuned sensors that can be used to perform real-time measurements in manufacturing and medicine.

"This is the most stable porous silicon surface to date," Buriak says. "Using our treatment, we can produce an incredibly stable surface that should stand up to the rigors of use."

Purdue is pursuing a patent on the method. Details of the discovery will be published in the Feb. 17 issue of the Journal of the American Chemical Society.

Porous silicon is identical in makeup to the silicon used in many technological applications today, but its surface contains tiny openings -- or pores. The pores contain microscopic structures made of silicon that emit light when ultraviolet light is applied. This type of silicon has been known to scientists since the 1950s, when they discovered that silicon could not always be polished smooth during manufacturing.

It wasn't until 1990 that this "rough" or porous silicon was found to have photoluminescent properties. In 1992, scientists discovered that it also emits light when electric current is applied, a finding that opened the door to coupling light and electronics to build computers and other devices.

"Because most of our current technology is based on silicon, it may be relatively easy to develop the optical applications and combine them with current technologies, as the manufacturing processes are already in place," Buriak says.

For example, porous silicon could serve as a flat, millimeter-thick display area for computer screens, replacing large, bulky computer screens that depend on cathode-ray tubes.

The properties of porous silicon also make it an ideal material to develop computers based on light signals instead of electrical signals. Such computers would be faster, as beams of light can transmit information much more quickly than electrons making their way through a solid material.

Using light to transmit data also would eliminate heat buildup in computers, allowing scientists to design smaller computers by stacking multiple layers of chips made of porous silicon.

Though the properties of porous silicon offer promises of powerful new technologies, Buriak says that until now, the untreated material was too fragile to hold up to these applications. Oxygen and water molecules in the air interact with the surface of porous silicon to create a glass-like coating that disrupts its photoluminescent properties.

"Within a few weeks, the material will oxidize, or 'rust,'" Buriak says. "But in this case, instead of leaving a brownish rough coating, the oxidation process produces a smoother, glass-like surface that limits the function of the material."

Buriak, working with undergraduate researcher Matthew Allen of Swartz Creek, Mich. , found a way to prevent this oxidation using a chemical process that works in liquids.

"A lot of reactions involve chemical bonds similar to the ones that develop on the surface of porous silicon. So I listed these reactions and came up with one that I thought had the best chance of working without damaging the surface," Buriak says.

"What we came up with is a very clean, very easy, room-temperature, one-hour reaction that allows us to stabilize the surface."

Buriak coats the porous surface of the silicon with Lewis acid, a solution that brings about a reaction that produces a greasy coating that protects the surface while allowing the porous silicon to maintain its photoluminescent properties.

To test how well the treatment stands up to environmental stresses, Buriak boiled samples of treated and untreated porous silicon in a highly basic solution of potassium hydroxide for an hour.

"Silicon and silica compounds generally dissolve in a solution with a pH greater than 7," she says. "By boiling it, we are accelerating the aging process to test how well this stabilizing method will stand up to rigorous conditions over a period of time."

The treated surfaces showed no oxidation and only minor changes in photoluminescent properties, while the surfaces of the untreated samples dissolved.

"This indicates that, once it is treated, the surface will remain stable for long periods of time," she says.

The new treatment also will allow scientists to add other compounds to the surface, so that the light-emitting properties of porous silicon can be manipulated to respond to certain chemicals or conditions.

This feature can be exploited to develop new types of medical or industrial sensing devices, Buriak says.

"When UV light strikes the surface of porous silicon, it reradiates back in the red wavelength, producing a bright orange color," Buriak says. "But if we add, for example, a chemical that binds to sodium ions, when sodium is present it will cause the reradiated wavelength to shift, producing a different color such as yellow or red. So you could look at the color difference and see whether sodium is present, and at what concentration it's present."

Using this knowledge, scientists could design sensing devices that could be used in a doctor's office, eliminating the need to send blood and other tissue samples to a laboratory for testing.

The same techniques could be applied to develop sensors that respond immediately to chemical changes in the environment. Such sensors could be used in factories to perform real-time, on-line quality control measurements.

"Currently, if you want to check a chemical mixture during the manufacturing process, you have to go through a time-consuming process of taking a sample and sending it to a quality control lab where it is tested," Buriak says. "The ideal situation would be to have a sensor placed in the vat where the chemical mixtures are prepared, so that the mixture is continuously monitored during the process."

Buriak says that with the development of a stable form of porous silicon, such applications may be in place within three to five years. Her research was funded by Purdue.

Purdue University

Related Silicon Articles from Brightsurf:

Single photons from a silicon chip
Quantum technology holds great promise: Quantum computers are expected to revolutionize database searches, AI systems, and computational simulations.

For solar boom, scrap silicon for this promising mineral
Cornell University engineers have found that photovoltaic wafers in solar panels with all-perovskite structures outperform photovoltaic cells made from state-of-the-art crystalline silicon, as well as perovskite-silicon tandem cells, which are stacked pancake-style cells that absorb light better.

Surprisingly strong and deformable silicon
Researchers at ETH have shown that tiny objects can be made from silicon that are much more deformable and stronger than previously thought.

A leap in using silicon for battery anodes
Scientists have come up with a novel way to use silicon as an energy storage ingredient.

Flexible thinking on silicon solar cells
Combining silicon with a highly elastic polymer backing produces solar cells that have record-breaking stretchability and high efficiency.

No storm in a teacup -- it's a cyclone on a silicon chip
University of Queensland researchers have combined quantum liquids and silicon-chip technology to study turbulence for the first time, opening the door to new navigation technologies and improved understanding of the turbulent dynamics of cyclones and other extreme weather.

Black silicon can help detect explosives
Scientists from Far Eastern Federal University (FEFU), Far Eastern Branch of the Russian Academy of Sciences, Swinburne University of Technology, and Melbourne Center for Nanofabrication developed an ultrasensitive detector based on black silicon.

2D antimony holds promise for post-silicon electronics
Researchers in the Cockrell School of Engineering are searching for alternative materials to silicon with semiconducting properties that could form the basis for an alternative chip.

Silicon technology boost with graphene and 2D materials
In a review published in Nature, ICFO researchers and collaborators report on the current state, challenges, opportunities of graphene and 2D material integration in Silicon technology.

Light and sound in silicon chips: The slower the better
Acoustics is a missing dimension in silicon chips because acoustics can complete specific tasks that are difficult to do with electronics and optics alone.

Read More: Silicon News and Silicon Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.