University of Rochester engineers working with colleagues at
the University of Minnesota have developed the fastest silicon
photodetector yet reported -- a device which can detect up to 75
billion light signals per second (75 gigahertz), compared to 1 or
2 gigahertz for today's best commercial detectors. The work is
reported in today's issue of Applied Physics Letters.
"We are unaccustomed to see silicon react that fast. But if you look carefully, there's no scientific reason why it shouldn't work so quickly," says Rochester graduate student Sotiris Alexandrou, who presented this work recently at the Conference on Lasers and Electro-Optics (CLEO) in Baltimore.
The group headed by Thomas Hsiang, professor of electrical engineering and senior scientist at Rochester's Laboratory for Laser Energetics, also determined exactly what limits such detectors. Using today's technology, it would take over two weeks to transmit the contents of the Library of Congress via phone line; using Hsiang's device, it would take one working day. The device would also be useful in transmitting phone conversations, computer data, even satellite images.
Silicon is the material of choice for the electronics industry: almost all the electronic circuitry that runs our computers, VCRs, CD players and other gadgets is made of silicon.
Hsiang says the new detector is easily manufactured (all of the electronics are on one side of the substrate) and fits well into existing electronic systems.
Last year Hsiang's group made the fastest photodetector ever reported (Aug. 17, 1992 Applied Physics Letters), a gallium arsenide device that operates at 510 gigahertz. But Hsiang believes the latest results are more noteworthy.
"Silicon technology is widespread in manufacturing," says Hsiang. "Gallium arsenide is faster, but it is highly specialized and hard to use. Silicon is much more practical and widespread. A silicon photodetector is worth getting excited about."
To understand what a photodetector is, think of old-time forest rangers using mirrors to flash signals from hilltop to hilltop to communicate. The human eye -- one type of light detector -- can detect about 60 such flashes per second before they blur together and appear as one continuous image, like the image we see at a movie theater. Hsiang's device can detect 75 billion such signals per second before the signals blur together.
Such detectors are commonly used in fiber-optic telecommunications systems and in local computer networks, where information is transmitted via a light pulse down an optical fiber. The detectors play a key role in translating signals from optical to electrical form, and vice versa. Such detectors are used in an array of devices: CD players use them to decode laser beam signals into music, and supermarket check-outs use them to translate UPC labels into a price that rings up at the cash register. The device would also be useful in optoelectronic computer chips which carry both optical and electrical signals.
Another possible use: an ultrafast electrical pulse generator for digital circuits. Pulse generators are used in all digital circuits to send a "clock" signal which synchronizes the components of a digital circuit. Today's fastest clock signals are about 200 megahertz. "The clock signal must be faster than all the devices on the chip," says Alexandrou. "A faster clock is needed with ultrafast circuits," such as the superconducting circuits being developed at Rochester.
The key to the new MSM (metal-semiconductor-metal) detector is how fast it recovers after detecting a signal. Less than 20 trillionths of a second (picoseconds) after it detects a light signal the detector is ready to detect another one.
The device itself is much smaller than a square millimeter and is made up of alternating strips of metal and silicon. The strips and spacings are less than one-third of a micron (a micron is millionth of a meter) wide, less than one-hundredth the width of a human hair. Light hitting the surface creates electrons which move from the semiconductor to the electrodes. The tiny widths of the strips means electrons must travel only an extremely short distance to create a signal.
Hsiang and colleagues studied the device's response to both red and violet light. Scientists have known that longer wavelength light such as red light penetrates silicon more deeply than violet. The Rochester team showed that this deeper penetration is what limits most detectors' performance. When photons penetrate deeply into the material, the resultant electrons must travel further to an electrode to create a signal -- thus the electrical signal lasts longer and the device takes longer to recover before the next signal.
In Hsiang's tests a silicon detector could detect pulses of violet light up to 75 gigahertz and pulses of red light up to 38 gigahertz. To improve the performance at longer wavelengths (which are commonly used in communications), Hsiang suggests burying an insulating material such as silicon oxide just below the surface of the photodetector. The extra layer would prevent unwanted electrical activity and should improve the detector's response to red light.
Hsiang's group is one of very few in the world able to measure such ultrafast electrical signals using optical pulses. The team used a tunable titanium-doped sapphire laser to produce powerful laser bursts lasting less than a trillionth of a second to study the detector, much like a very fast strobe camera.
Also working on the project were graduate student Chia-Chi Wang at the University of Rochester, and M.Y. Liu and S.Y. Chou at Minnesota. At Rochester this work was supported by the National Science Foundation, the Department of Energy and the New York State Energy Research and Development Authority; at Minnesota, support came through NSF and the Packard Foundation.
tr