II The problem: Fragmentation of Technology Platforms
Semiconductor - based integrated circuits (IC`s) are build up to nowadays on monolithic crystalline material. As the properties of the materials are important for the performance of the electronic circuitry, different semiconductor materials became the technology platform of choice for the various, targeted applications. Si technology is the dominant semiconductor market driven by the thirst for digital processing capability. In optoelectronic applications near the 870 nm wavelength, however, the AlGaAs alloy system on GaAs is widely employed. And for telecommunications, InP substrates became the bulk semiconductor platform, as InGaAsP can be grown lattice matched to achieve the desired 1.3 and 1.55 µm wavelength emission required for low-loss transmission in optical fibers. Today, Si and III-V (as well as the upcoming II-VI) semiconductor worlds rarely intermix due to this separation of substrate platforms.
The aim of engineered wafer systems in combination with hybrid device concepts is to increase the functionality and / or performance of integrated circuits by overcoming these limitations of electronic systems, brought about by the separation of the different semiconductor substrate platforms [5]. The main objective in the field is certainly the monolithic integration of lattice matched or mismatched alternative semiconductor thin film materials on the mainstream Si platform, as the latter is the most mature and in consequence the commercially most dominant technology. A high number of SoC solutions (e.g. III-V / Si photonics) would certainly be triggered by the successful integration of alternative semiconductor materials into sophisticated silicon or strained silicon chips. To taste the flavour of the exciting new opportunities ahead, the vision of the monolithic integration of GaAs on Si is used as an example to briefly sketch typical advantages [5]:
• the integrated materials system is an active circuit element by itself
Example: GaAs based optoelectronics coupled to Si based signal processing
• the integrated materials system serves as a buffer layer
Example: GaAs buffer to integrate InSb based IR detectors
• exploit mature Si wafer technology
Example: GaAs wafers are in comparison to Si wafers limited in size
• best combination of mechanical, electrical, thermal & processing properties
Example: GaAs has lower thermal conductivity & mechanical stability than Si
• film properties differ by interface, strain & quantum size effects from bulk
Example: quantum size effects in thin GaAs layers
A lot of R&D work was devoted to the tricky problem of combining the best attributes of compound semiconductors, like high mobility and direct band gap, with the low-cost, mature manufacturing processes of Si CMOS technologies. These efforts have brought impressive success at the research level, but in many cases commercialization of hybrid devices is still a distant dream. However, as more and more aggressively scaled Si IC solutions are limited by cost explosions and fundamental material properties, worldwide R&D activities on engineered wafer and hybrid device approaches are intensified, resulting in important innovations in the field.