As processing power continues to keep up with the Moore’s law schedules, the need for higher bandwidth data bus connections between memory and processing is becoming a major engineering challenge. Conventional electrical connections are becoming unwieldy at such high rates; they are also costly and require extremely high driving power.
An alternative that has received a lot of research attention is optical data transmission using lasers. The technique has high power efficiency, but the major technical challenge has been the lack of the ability to integrate the optics and electronics. Previous implementations have tended to use compound semiconductors, such as gallium arsenide, that are expensive and hard to mass-produce. In addition, excessive heat production from lasers on chips, the need to construct the laser separately and graft onto the chip, and the lack of the ability to create precise alignments for optics at the chip level in a mass production environment, have also been significant challenges.
Now, Frost & Sullivan Technical Insights has reported that researchers at the electronic materials research group at the Massachusetts Institute of Technology (MIT) have demonstrated the first laser fabricated using germanium that can generate wavelengths suitable for optical communication and operates at room temperature. From a theoretical perspective, this is the first practical validation of the notion that indirect bandgap semiconductors can yield practical lasers. The results have been described in a paper in the journal Optics Letters.
The group doped germanium with phosphorous. The extra electron of phosphorus atoms fill up the lower energy state in the conduction band, causing excited electrons to spill over into the higher-energy, photon-emitting state. According to the group’s theoretical work, phosphorous doping works best at 1020 atoms per cubic centimetre of germanium. The group claims they have begun to observe lasing at the current doping levels achieved of 1019 phosphorous atoms per cubic centimetre of germanium. They also lowered the energy difference between the two conduction-band states to increase probability of electron spillover by straining the germanium. Straining is the process of increasing the distance between atoms slightly, achieved in this case by growing the germanium directly on top of a layer of silicon at high temperatures. The alteration in the angle and length of the bonds between germanium atoms also lowered the energy difference between conduction band states.
This development carries much promise, since the semiconductor industry has been successful in integrating germanium into its established manufacturing processes. The cost and complexity barriers to using compound semiconductors could potentially be overcome by using indirect bandgap semiconductor lasers, but this technology is in an incipient phase. Significant advances in efficiency, and a longer history of successful demonstrations to prove reliability would be crucial for progress.
Using light for communication
In a related but separate development, IBM scientists unveiled a significant step towards replacing electrical signals that communicate via copper wires between computer chips with tiny silicon circuits that communicate using pulses of light. As reported in the recent issue of the scientific journal Nature, this is an important advancement in changing the way computer chips talk to each other.
The device, called a nanophotonic avalanche photodetector, is the fastest of its kind and could enable breakthroughs in energy-efficient computing that can have significant implications for the future of electronics. It explores the ‘avalanche effect’ in germanium, a material currently used in production of microprocessor chips. Analogous to a snow avalanche on a steep mountain slope, an incoming light pulse initially frees just a few charge carriers which in turn free others until the original signal is amplified many times. Conventional avalanche photodetectors are not able to detect fast optical signals because the avalanche builds slowly.
“This invention brings the vision of on-chip optical interconnections much closer to reality,” said Dr T.C. Chen, vice president, Science and Technology, IBM Research. “With optical communications embedded into the processor chips, the prospect of building power-efficient computer systems with performance at the Exaflop level might not be a very distant future.”
The avalanche photodetector demonstrated by IBM is the world’s fastest device of its kind. It can receive optical information signals at 40 Gbps and simultaneously multiply them tenfold. Moreover, the device operates with just a 1,5 V voltage supply, 20 times smaller than previous demonstrations. Thus, many of these tiny communication devices could potentially be powered by just a small AA-size battery, while traditional avalanche photodetectors require 20–30 V power supplies.
“This dramatic improvement in performance is the result of manipulating the optical and electrical properties at the scale of just a few tens of atoms to achieve performance well beyond accepted boundaries,” said Dr Assefa, the lead author on the paper. “These tiny devices are capable of detecting very weak pulses of light and amplifying them with unprecedented bandwidth and minimal addition of unwanted noise.”
In IBM’s device, the avalanche multiplication takes place within just a few tens of nanometres and that happens very fast. The tiny size also means that multiplication noise is suppressed by 50%–70% with respect to conventional avalanche photodetectors. The IBM device is made of silicon and germanium, the materials already widely used in production of microprocessor chips. Moreover it is made with standard processes used in chip manufacturing. Therefore, thousands of these devices can be built side-by-side with silicon transistors for high-bandwidth on-chip optical communications.
The avalanche photodetector achievement, which is the last in a series of prior reports from IBM Research, is thought to be the last piece of the puzzle that completes the development of the ‘nanophotonics toolbox’ of devices necessary to build the on-chip interconnects.
In December 2006, IBM scientists demonstrated a silicon nanophotonic delay line that was used to buffer over a Byte of information encoded in optical pulses – a requirement for building optical buffers for on-chip optical communications.
In December 2007, IBM scientists announced the development of an ultra-compact silicon electro-optic modulator, which converts electrical signals into light pulses, a prerequisite for enabling on-chip optical communications.
In March 2008, IBM scientists announced the world’s tiniest nanophotonic switch for ‘directing traffic’ in on-chip optical communications, ensuring that optical messages can be efficiently routed.
The report of this work, entitled ‘Reinventing Germanium Avalanche Photodetector for Nanophotonic On-chip Optical Interconnects,’ by Solomon Assefa, Fengnian Xia and Yurii Vlasov of IBM’s T.J. Watson Research Centre in Yorktown Heights, New York, was published in the March 2010 issue of Nature.
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