In a forthcoming issue of Solid State Communications (vol. 115), a group from Max Planck Institute for Solid State Research, Stuttgart, Germany, reports a substantial enhancement of the thermal conductivity in a bulk silicon crystal made from only one isotope (28Si) [1]. At room temperature, single-isotope silicon (SISSI) is a 60% better heat conductor than natural silicon, which consists of three stable isotopes (92.2% 28Si, 4.7% 29Si, 3.1% 30Si). At 77K, the temperature of liquid nitrogen, the enhancement amounts to a factor of 2.4, while at 20K, where the thermal conductivity has its maximum, it is close to a factor of six. The maximum thermal conductivity of SISSI (30000 Wm-1K-1) exceeds that of the best natural heat conductor, namely diamond, by a factor of 2.5.
The thermal conductivity enhancement in SISSI is based on the effect that phonons, the quantized lattice vibrations, are scattered by fluctuations of the atomic masses, as present in most conventional crystals, which contain a mixture of different stable isotopes. The suppression of this scattering in isotopically enriched crystals results in an enhancement of the thermal conductivity which can be very strong. For example, the removal of about 8% "isotopic impurities", present in natural silicon, in the nominally isotopically pure SISSI crystal (enriched to 99.8588% 28Si) investigated by the Max Planck researchers is sufficient to increase the thermal conductivity by almost an order of magnitude. The thermal conductivity enhancement varies with temperature due to the presence of other phonon scattering mechanisms, such as boundary scattering (at low temperatures) and "umklapp" scattering (at high temperatures).
Most elements contain considerable fractions of many different stable isotopes. With the end of the cold war, highly isotopically enriched (and chemically pure) elements have become available at prices which allow large single crystals with masses of several grams to be grown. While originally used predominantly for basic research, isotopically modified materials are now entering applications.
Already at present, but even more so in near-future microelectronics applications with still larger integration densities and switching speeds, thermal problems severely limit the chip performance. Appropriate measures are thus necessary to prevent destruction. Due to their enhanced thermal conductivity, thin epitaxial SISSI layers on natural silicon substrate or bulk SISSI wafers should very effectively remove heat from critical regions, the so-called "hot spots", of a device. SISSI provides an elegant way to address thermal problems since its benefits can be obtained without changing existing technology. Other applications can be envisioned in the area of X-ray optics, since conventional Bragg-reflecting mirrors will not withstand the enormous heat load of highly brilliant next-generation synchrotrons. In this area and for special applications in microelectronics it might even be worthwhile to exploit the still larger thermal conductivity enhancement of SISSI at lower temperatures. Of particular interest is the fact that the maximum effect occurs at a temperature at which the thermal expansion of silicon nearly vanishes. Consequently, SISSI is mechanically very stable with respect to deformations induced by variations in temperature.
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[1] Thermal conductivity of isotopically enriched silicon T. Ruf, R. W. Henn, M. Asen-Palmer, E. Gmelin, M. Cardona, H.-J. Pohl, G. G. Devyatych, and P. G. Sennikov Solid State Communications, in press (2000)
Published: May 10, 2000
Further information:
Dr. Tobias Ruf
Max-Planck-Institut für Festkörperforschung
Heisenbergstr. 1
D-70569 Stuttgart, Germany
Tel.: +49-(0)711-689-1735
Fax: +49-(0)711-689-1712
e-mail: ruf@cardix.mpi-stuttgart.mpg.de
Journal
Solid State Communications