Miniature Laser Grown on Silicon Chip

1. Recently, Scientists from the US and Europe have successfully grown miniature lasers directly on silicon wafers.
2. This marks a major progress in silicon photonics, a technology that uses light (photons) instead of electricity (electrons) to transmit information on chips.

What is Silicon Photonics?

1. Silicon photonics uses photons to carry data, replacing electrons used in traditional chips.
   1. This is about using light instead of electricity inside computer chips.
   2. Traditional chips work by moving electrons through circuits.
   3. The new method uses photons, or light particles, to carry information — this is called silicon photonics.
2. Advantages of photons over electrons:
   1. Travel faster
   2. Provide higher data bandwidth
   3. Cause lower energy losses
3. Applications:
   1. Already used in data centres and optical sensors
   2. Has potential in quantum computing and high-speed data transmission

The Main Challenge in Photonic Chips

1. A laser is required as a light source on the chip.
2. Traditionally, lasers are manufactured separately and then attached to the chip.
3. This method causes:
   1. Slower performance
   2. Manufacturing mismatches
   3. Higher cost
4. Integrating the laser directly on the chip has been a long-standing technological problem.

The Progress

1. Researchers grew lasers directly on a silicon chip using a scalable, cost-effective method.
2. The process was conducted in a standard CMOS (Complementary Metal-Oxide-Semiconductor) facility.
   • CMOS is widely used in current semiconductor manufacturing.
3. This makes the technique compatible with existing manufacturing processes.

Components of a Photonic Chip

A typical photonic silicon chip consists of four main components:

1. Laser (light source) – generates photons.
2. Waveguides – guide photons, like wires guide electrons.
3. Modulators – encode or decode data by altering light properties like intensity, phase, or wavelength.
4. Photodetectors – convert light signals into electrical signals.

How Lasers Work (Stimulated Emission)

1. Laser stands for Light Amplification by Stimulated Emission of Radiation.
2. In this process:
   1. A photon causes an excited electron to release energy and drop to a lower level.
   2. This releases another identical photon.
   3. The chain reaction creates a coherent light beam – the laser.

Why Silicon Alone Doesn’t Work

1. Silicon has an indirect bandgap, so it is not efficient at emitting light.
2. Materials like Gallium Arsenide (GaAs) have a direct bandgap and can emit light efficiently.
3. Problem: GaAs and silicon have different crystal structures, causing defects when grown together.
   • These defects make the laser less efficient by turning light energy into heat.

The Solution – 'Trench Design'

1. Inspired by a 2007 AmberWave Systems study.
2. Researchers:
   1. Created deep trenches in the silicon wafer.
   2. Filled trenches with silicon dioxide (an insulator).
   3. Deposited GaAs at the trench bottom so defects remained trapped and didn’t affect the laser.
   4. Grew defect-free GaAs above the trench.
3. Added three thin layers of Indium Gallium Arsenide (InGaAs) to act as the laser.
   1. InGaAs is GaAs with 20% gallium replaced with indium for better light emission.
4. Covered with Indium Gallium Phosphide for protection.
5. Electrical contacts were added to power the laser.

Key Data and Results

1. 300 functional lasers were grown on a single 300-mm silicon wafer.
   1. 300 mm is the industry-standard wafer size.
2. Laser emitted light at a wavelength of 1,020 nm.
   1. Suitable for short-distance communication between chips.
3. Threshold current to power the laser: 5 mA (same as an LED in a computer mouse).
4. Laser output: approximately 1 milliwatt (mW).
5. Continuous operation:
   1. Worked for 500 hours at room temperature (25°C).
   2. Efficiency dropped at 55°C.
   3. Other optical chips have worked up to 120°C, so further stability improvements are needed.

Significance of the Breakthrough

1. First monolithic laser diode successfully grown on a 300-mm silicon wafer.
2. This approach:
   1. Solves the long-standing issue of integrating lasers with chips.
   2. Can lead to faster computing, lower energy consumption, and reduced costs.
3. Especially beneficial for data centres, AI systems, and green computing.
4. The process is scalable, efficient, and can be used with existing chip-making facilities.