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MATERIALS SCIENCE: ON INTEGRATED SEMICONDUCTORS

The following points are made by M.G. Lagally and R.H. Blick (Nature 2004 432:450):

1) Computers rely on silicon. Although other semiconductors have desirable features, in this context the materials properties of silicon are so outstanding that it is really the only choice for the large-scale integration of fast electronic devices. But there is a dark shadow over silicon, in that it produces no light. Many devices use light -- simple ones, such as the solid-state diode lasers typically found in CD players, for example; and more complex ones, such as the light amplifiers used in long-range optical-fiber communication. Gallium arsenide or other more exotic semiconductors must be incorporated into these devices to generate light.[1]

2) Why is the lack of optical activity in silicon a problem? The ever-decreasing size of transistors made of silicon means that the transit time for charge carriers through them (and hence their switching time) is decreasing rapidly. The overall speed of a microprocessor will be more and more limited by the time delay inherent in the connections between individual transistors[2]. According to the International Technology Roadmap for Semiconductors[3], the traditional copper/dielectric-material system for interconnects will have to be replaced by some novel on-chip interconnect scheme beyond the year 2010.

3) Optical communication is very fast, so if communication between the far reaches of a chip were possible by optical means, the full advantage of size scaling could be realized. Hence there is a demand for faster on-chip data communication using opto-electronic components, such as light-emitting diodes made from aluminium, gallium, arsenic and phosphorus (elements from groups III and V of the periodic table that are commonly paired in semiconducting "III-V" materials). These materials are optically active and can be grown as heterostructures (of different materials that are crystallographically linked) to form quantum wells and quantum boxes that emit coherent light through electrical stimulation. Their optical activity can be designed, as can the crystal growth sequence required to obtain it.

4) Because the fabrication of silicon devices and the process of heterostructure growth for lasers are well developed, devices that require both are typically manufactured using a hybrid technology -- one that contains different chip sets of silicon and III-V circuits connected by wires. But it is easy to see that this approach fails to speed on-chip communication. For that, direct crystallographic integration of III-V heterostructures onto silicon is needed, preferably at the nanoscale.

5) Exactly this approach has now been demonstrated by Maertensson et al[1], who have grown optically active III-V nanowire heterostructures on silicon. The nanowires -- which sometimes look like a bed of nails -- are typically 2 microns long, with a base diameter of 50-100 nm. They have light-emitting sections grown into them and thus function effectively as light towers on the silicon substrate. These nanopillars, as well as being a beacon of hope for on-chip optical communication, open a variety of other communication applications by linking optical components with silicon-based circuitry.[4,5]

References (abridged):

1. Maertensson, T. et al. Nano Lett. 4, 1987-1990 (2004)

2. Peercy, P. S. Nature 406, 1023-1026 (2000)

3. http://public.itrs.net

4. Wagner, R. S. & Ellis, W. C. Appl. Phys. Lett. 4, 89-90 (1964)

5. Woll, A. R., Rugheimer, P. & Lagally, M. G. Int. J. High-Speed Electron. Syst. 12, 45-78 (2002)

Nature http://www.nature.com/nature

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APPLIED PHYSICS: ON OPTICAL SILICON CHIPS

The following points are made by Graham T. Reed (Nature 2004 427:595):

1) Research into optical circuits for communications began in the 1970s. Early visions of optical circuits were as "optical superchips", containing light emitters, modulators, amplifiers, optical isolators, detectors and, latterly, electronic intelligence(1). However, there are still differing views about the optimum material for such components. This is sometimes articulated in the phrase "optical circuits have yet to find their silicon" -- a reference to silicon's dominance in the microelectronics industry. Despite its success as an electronic material, silicon has received rather modest attention as an optical material. But that may be about to change: Liu et al(2) of the Intel Corporation have fabricated the first silicon-based optical modulator with a bandwidth that exceeds 1 gigahertz (GHz).

2) In technology terms, research into silicon as an optical material has been under way for a long time -- since the mid-1980s. However, relatively little progress has been made in comparison with more exotic materials such as III-V compounds (indium phosphide, gallium arsenide and related compounds), the insulator lithium niobate, or even silica (which typically uses silicon as a substrate material). That is not to say that little has been achieved, but the global technical effort has largely been concentrated on materials other than silicon.

3) There are two main reasons for this. First, silicon does not have an inherent mechanism for the emission of light: it is an indirect bandgap material, which means that its crystal structure makes it impossible to fabricate an efficient light-emitting device, such as a laser, from this material in the conventional way. The III-V compounds, in contrast, are direct bandgap materials and for this reason are often used in semiconductor lasers.

4) Second, silicon does not exhibit an electro-optic effect known as the "Pockels effect", the traditional characteristic for fast modulation of light (that is, encoding data onto light by selectively changing its intensity). In other materials, such as lithium niobate, modulation is typically achieved via the Pockels effect, which causes a linear change in a material's refractive index with an applied electric field. Other optical modulators use electric-field effects known as electro-absorption and electro-refraction, but these are weak in silicon. Instead, modulation in silicon is achieved through thermal mechanisms, which are relatively slow (typically kilohertz), or through the introduction of free carriers (electrons or their positively charged counterparts, holes), which in turn results in both absorption and a change in refractive index. This latter mechanism, known as the "free-carrier dispersion effect", is still also relatively slow, as it is associated with the physical movement of charge within the device. Nevertheless, it has been predicted that silicon-based modulators using this effect could achieve bandwidths exceeding 1 GHz (3), although in practice working devices(4,5) have been limited to about 20 MHz.

References (abridged):

1. Soref, R. A. Proc. IEEE 81, 1687-1706 (1993)

2. Liu, A. et al. Nature 427, 615-618 (2004)

3. Png, C. E., Reed, G. T., Atta, R. M. H., Ensell, G. J. & Evans, A. G. R. Proc. SPIE 4997, 190-197 (2003)

4. Tang, C. K. & Reed, G. T. Electron. Lett. 31, 451-452 (1995)

5. Dainesi, P. et al. IEEE Photon. Technol. Lett. 12, 660-662 (2000)

Nature http://www.nature.com/nature

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ON THE LIMITS OF QUANTUM OPTICAL COMMUNICATION CHANNELS

The following points are made by J. Tworzydlo and C.W. Beenakker (Phys. Rev. Lett. 2002 89:043902):

1) To faithfully transmit information through a communication channel, the rate of transmission should be less than the capacity of the channel [1,2]. Although current technology is still far from the quantum limit, there is an active scientific interest in the fundamental limitations to the capacity imposed by quantum mechanics [3,4]. Ultimately, these limitations originate from the uncertainty principle, which is the source of noise that remains when all external sources have been eliminated.

2) An important line of investigation deals with strategies to increase the capacity. One remarkable finding of recent years has been the beneficial role of multiple scattering by disorder, which under some circumstances can increase the capacity by increasing the number of modes that effectively carry the information [5]. Quite generally, the capacity increases with increasing signal-to-noise ratio, so that amplification of the signal is a practical way to increase the capacity. When considering the quantum limits, however, one should include not only the amplification of the signal (e.g., by stimulated emission), but also the excess noise (e.g., due to spontaneous emission). The two are linked at a fundamental level by the fluctuation-dissipation theorem, which constrains the beneficial effect of amplification on the capacity.

3) While the effects of disorder and amplification on communication rates have been considered separately in the past, their combined effects are still an open problem. Even the basic question, "Does the capacity go up or down with increasing gain?", has not been answered. The authors were motivated to look into this problem by the recent interest in so-called "random lasers". These are optical media with gain, in which the feedback is provided by disorder instead of by mirrors. Below the laser threshold, these materials behave similar to linear amplifiers with strong intermode scattering, and this results in some unusual noise properties. As the authors demonstrate, the techniques developed in connection with random lasers can be used to predict under what circumstances the capacity is increased by amplification.

4) In summary: The authors report a study of the competing effects of stimulated and spontaneous emission on the information capacity of an amplifying disordered waveguide. At the laser threshold the capacity reaches a "universal" limit, independent of the degree of disorder. Whether or not this limit is larger or smaller than the capacity without amplification depends on the disorder, as well as on the input power. Explicit expressions are obtained for heterodyne detection of coherent states, and generalized for an arbitrary detection scheme.

References (abridged):

1. C.E. Shannon, Bell Syst. Tech. J. 27, 379 (1948); 27, 623 (1948)

2. T. M. Cover and J. A. Thomas, Elements of Information Theory (Wiley, New York, 1991)

3. C. M. Caves and P. D. Drummond, Rev. Mod. Phys. 66, 481 (1994)

4. M.A. Nielsen and I.L. Chuang, Quantum Computation and Quantum Information (Cambridge University, Cambridge, England, 2000)

5. G.J. Foschini, Bell Labs Tech. J. 1, 41 (1996)

Phys. Rev. Lett. http://prl.aps.org

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