More light... from silicon

Silicon is the key material for microelectronics. A major technological breakthrough would be the efficient emission of light from silicon based materials. This would allow to manufacture silicon based light emitters with the same technology used in the highly advanced silicon microelectronics industry. As a result light emitters for short distance data transmission could be realized which can be produced more cost efficient than compound semiconductor materials like GaAs and InP. The light emitters could form the basis for silicon optoelectronic components. A long term goal is the realization of an optical interconnect technology, as it is pursued e.g. by leading chip manufacturers like Intel Corp. (http://www.intel.com/technology/itj/index.htm). The last words of the famous German poet Goethe (1749-1832) were “more light...” which is now echoed as “more light from silicon” from researchers worldwide.

Dresden, July 12, 2004.

The Forschungszentrum Dresden-Rossendorf (FZD) has been active in research on silicon based light emitters for several years. Most recently the first silicon based light emitter for the UV could be realized which finds strong interest for bio-sensor applications (press release from FZR, 29.06.2004). For data communication, however, longer wavelengths are of interest, especially in the near infrared wavelength range. From a fundamental physics point of view silicon is a most inappropriate material for light emission (due to its so called “indirect bandgap”). This fact has stimulated worldwide research activities in the last decade to circumvent this fundamental problem.

Now researchers from FZD succeeded to increase the light emission from silicon at a wavelength of 1100 nm. The electrically driven devices are based on silicon diodes which are modified by ion implantation of Boron atoms at high doses. The microscopic modifications of the silicon lead to an enhancement of the electroluminescence by a factor 1000 (1). However, one fundamental problem still exists: only a small portion of the generated light is able to escape the semiconductor with more than 95% being fenced in. This rather fundamental detriment could be circumvented by employing a new microresonator concept, which further enhances the emission at a desired wavelength and yield a higher directionality of the light output. For this purpose the silicon diode is prepared on a metallic reflector layer made from cobaltsilizide which is buried some 100 nanometer beneath the silicon surface. This metallic layer forms the bottom mirror of the microresonator and at the same time the electric contact to the diode. The preparation of the buried silizide mirror was done by the Forschungszentrum Jülich. The top mirror of the microresonator is formed by multilayers of silicon and silicon dioxide, which are deposited with nanometer accuracy on top of the diode in the cleanroom of the FZD.

“Microresonator concepts are well known since the early 90s”, says Thomas Dekorsy, head of the spectroscopy division at the FZD. “The primary difficulty is that mirrors for microresonators usually consist of isolating materials, thus hampering the preparation of electrical contacts to the diode. We solved this problem with our concept while keeping the compatibility with silicon process technology.”

With this microresonator concept the efficiency of the silicon diodes could be further enhanced, whereas the theoretical limits of this concept are still much higher. These limits can be easily reached by changes to the presently used process technology. The microresonator concept has been filed as a patent (2) and has first been published in Electronics Letters, July 8th 2004 (3).

(1) Origin of anomalous temperature dependence and high efficiency of silicon light-emitting diodes, J. Sun et al., Applied Physics Letters vol. 83, pp. 3385-3887 (2003)
(2) Silicon based optoelectronic device, T. Dekorsy et al., filed at Deutsches Patentamt (10348269.5)
(3) Silicon based electrically driven microcavity LED, J. Potfajova et al., Electronics Letters vol. 40, pp. 904-906 (2004)

Part of a wafer with silicon diodes

Contact at FZD:
Prof. Manfred Helm
Institute of Ion Beam Physics and Materials Research
Tel.: ++ 49 351 260-2260

Contact for the press:
Dr. Christine Bohnet
Public Relations
Tel.: ++ 49 351 260 2450
Fax: ++ 49 351 260 2700

P.O. Box 51 01 19 / 01314 Dresden/Germany