Optoelectronic interconnects for integrated circuits - Achievements 1998 – 2001 - Silicon based interconnects

Silicon is the leading semiconductor in the microelectronic industry.Not only silicon is dominating the past. Its technology is predicted to improve with the increasing demand for higher complexity integrated circuits and to maintain its role for a long time to go. At the same time the enormous progress of communication technologies in the last years has increased the demand for efficient and low-cost optoelectronic functions. For several present and future applications, photonic materials - in which light can be generated, guided, modulated, amplified, and detected - need to be integrated with standard electronic circuits in order to combine the information-processing capabilities of electronics data transfer and the speed of light. In particular, chip-to-chip or even intrachip optical communications all require the development of efficient optical functions and their integration with state-of-the-art electronic functions.The use of optical interconnects among devices is indeed a solution to what is known as the "interconnects bottleneck". The total length of interconnecting conductors within a chip is continuously increasing towards values of several km in 1 cm2. This produces problems related to the Joule effect, heat dissipations and slow down of the system related to resistance and capacity of the system. The replacement of electrical interconnects with optical ones would solve the problem. However Si is characterized by an indirect bandgap and by a weak electro-optic effect. It is therefore not suitable, in its bulk form, for the implementation of fundamental optical functions such as light sources and modulators. Several approaches need hence to be investigated. The European Commission focussed a research program on the interconnects problem launching several projects within the Microelectronic Advanced Research Initiative (MEL-ARI).In the case of Si optoelectronics we are faced with a material problem to circumvent the physical inability of bulk silicon to emit light. Several strategies have been considered and explored. They can be divided in three different groups: (i) low dimensional systems, (ii) emitting impurities and (iii) semiconductor silicides. For low dimensional systems the cases of porous silicon, silicon nanocrystals, Si/CaF2 and Si/SiO2 quantum wells have been explored. In all of these cases nanometer sized silicon is embedded within an insulating host. The quantum confinement has several effects: it increases the radiative probability, it decreases the non-radiative recombination routes, it increases the energy of emission. Indeed intense room temperature photoluminescence can be achieved. The main problem here is the carrier injection to achieve electroluminescence. Several routes have been followed and several devices operating at room temperature were fabricated. The challenges here are to efficiently inject carriers in a semi-insulating material and to have sufficiently low operating voltages. The most promising case of light emitting impurities is that of Er in Si. Room temperature operating devices were fabricated. A great effort was spent in comprehending the basic physical mechanisms. The main advantage here is that standard technology based on single crystal silicon can be used introducing erbium as a dopant. Particularly interesting is the light emitting transistor in which erbium, introduced in the collector-base region, is excited through electrons injected from the emitter. The third approach of semiconducting silicides is based on the observation that these silicides can be grown on Si and should have a direct bandgap. Indeed, luminescent devices have been fabricated with beta-FeSi2 precipitates formed by ion implantation in a Si diode. The quantum efficiency obtained from these devices is however not yet sufficiently high. Though none of the approaches is at a stage ready for application, this European effort can be considered extremely successful. In fact, this booklet reports a picture of the state-of-the-art in Si-based optoelectronics with major improvements with respect to the past both in terms of materials properties, understanding and device performances. Still some work is needed to obtain real applications; nevertheless the obtained results represent a solid basis for future developments.