We all know that light travels faster than sound, that’s why we see lightning first before hearing the thunder. Since light travels so fast, why not use light for information transfer?

Scientists worldwide have been researching on “photonics” – the technology that uses light to send information from one place to another. It simply translates the information through pulses of a laser beam, fire it down a thin optic glass fibre, translates it back at the other end and hey, you’re downloading the latest movies in a fraction of a second.

The main application of photonics is in telecommunications. Optic fibres are a fraction of the cost of copper cables and can carry much more. These days the internet gets faster every year by taking advantage of the ongoing advances in photonics. By replacing electronics with photonics, we will have circuits that use light instead of electricity. Computers that run on light can achieve incredible speeds of over 100 Gbps.

However, the main challenge for widespread adoption of photonics is cost, which mainly arises from the fibre connector parts. The solution to this cost issue is “silicon photonics”, which will enable 10 to 100 times reduction in cost than currently available. Imagine if optical functions could be manufactured and integrated in silicon wafers, just as electronic devices are today, the photonics industry could take advantage of the infrastructure and R&D know-how of silicon manufacturing over the past 40 years.

By “siliconising” photonics, the benefits of optical communication could be brought to future networks, servers and computers. Recently, silicon photonics has made great progress. For example, researchers in IME have demonstrated low loss light transmission in silicon waveguides, fast electro-optical effects and light emission in silicon – which are the important building blocks towards realising optical communication.

Such research works are part of IME’s Nano-Electronics & Photonics (NanoEP) Programme launched in August 2007. By leveraging the advanced silicon micro and nano-fabrication technologies in IME, the activities of NanoEP Programme focus on the CMOS platform-based silicon nanowire devices with applications in electronics and bio-electronics, novel storage memory, and silicon photonic devices with applications in high-speed optical interconnects and light-emitters.

Some significant research achievements of IME in NanoEP are:

• CMOS-compatible silicon nanowire logic gates

IME researchers have demonstrated, for the first time, the functional logic circuits for CMOS inverters, which show excellent transfer characteristics down to very low voltage (0.2V) featuring appreciable noise margin, good gain and low switching currents. Further integration has been demonstrated by realising multi-stage ring oscillators. A 3-stage NW CMOS ring oscillator is shown below. This technology brings a step closer to the integration of nanostructure-based electronic building blocks with other wire-based applications such as bio-electronics.



Tilted view SEM image of fabricated Si-nanowire 3-stage ring-oscillator with 2-stage buffer for signal probing.

• Monolithically integrated Ge-Photodetector on Si-CMOS compatible photonics platform

  Germanium-on-Silicon (Ge-on-Si) photodiodes are critical for low-cost Si-based optical-electronic-integrated-circuits. However, it is challenging to integrate Ge on Si due to Ge/Si lattice mismatch. Using a novel low-temperature SEG-Ge deposition on Si waveguide, IME researchers have demonstrated waveguide photodetectors in Si and Ge with excellent responsivity and speed (up to 1.12A/W and 3dB bandwidth of 3.4GHz from lateral Ge PIN detectors) as well as low dark currents (< 1uA at reverse bias at -1V).

  These devices can be readily integrated with existing Si-CMOS platform for applications such as optical transceivers. In the future, this will enable the industry to design multi-channel data chip without incurring extra expenses for assembly of external photodetectors.



TEM image of the selective epi grown (SEG) Ge on Si/SiGe buffer on Si

• Electrically pumped Si light emitting devices

  IME has demonstrated the first electrically excited light emitting device from a silicon-based material using a unique nano-superlattice structure of Si/SiN stacking. Light emission is facilitated through quantum confinement of the injected carriers within the nanoscale silicon structures.

  The picture shows a square device lighted when an electrical bias is applied to it (inset shows the same chip taken under lighted conditions before electrical bias). Tuning of the emission wavelength (colour) has been demonstrated although the tuning range is small and is still limited within the orange colour spectrum. While the intensity of the emitted light is still too low for practical applications, this represents a major step in the pursuit of a silicon-based light source.



Picture of a square device lighted when biased. The inset shows the same die taken under lighted condition, showing the details of the die.

IME’s NanoEP Programme has built an excellent infrastructure for silicon technology with 8” wafer processing capability. Its competencies include nanoscale CMOS platform technologies for new materials and device research, post-CMOS process technologies, backend processes on prefabricated CMOS process technologies, Si-photonic crystal process baselines for Si-photonics devices and Si-SOI platform. Moving forward, the Programme aims to be a turn-key technology provider to help customers materialise concepts into reality in the emerging nanotechnology industry.