Making Monolithic Integrated Systems With GaN
GaN produces great LEDs, lasers and transistors. Just imagine the possibilities when two or more of these classes of devices are united on the same chip by Hoi Wai Choi, Wai Yuen Fu, Kwai Hei Li and Yuk Fai Cheung from the University of Hong Kong
GaN has breathed new life into LEDs. This wide bandgap material has propelled the status of this device from indicator lights to illumination lamps that are pushing incandescent lamps to the brink of existence, while slashing carbon footprints. And the penetration of this device still has further to go, due to rising sales of LEDs that combine GaN with AlGaN, rather than InGaN, the key material for visible emission. This alternative alloy widens the bandgap, enabling the production of devices stretching into the ultraviolet, which can be used to disinfect air, water and surfaces. There is also the cousin of the GaN LED, the GaN laser. It is already being widely used in highdensity optical storage, commonly known as BluRay.
Following in the footsteps of these optoelectronic devices is an electronic industry based on the same family of materials. Their great intrinsic material properties has led the GaN HEMT to be widely touted as the next generation of power electronic device. Further commercial success may follow from other types of GaN device, including microelectromechanical systems and resonators, which are both under development.
In hindsight, it would have been better to develop the broad portfolio of GaN devices along similar lines. But that's not been the case. Instead, devices have often been developed individually, due to the lack of a common structure or platform. Nevertheless, optoelectronics and electronics often go hand-inhand; an LED requires an electronic driver for stable operation, and this driver is built from electronic components. So it is clear that the GaN platform can become even more useful, functional and efficient when multiple components are tightly integrated to form an integrated system, just like that found with silicon ICs (see Figure 1, which illustrates this concept).
Let's begin by considering photonic integrated systems. A promising building block for these is the LED, which combines high quantum efficiencies with fast response times, making it suitable for transmitting signals at high speeds. However, the LED needs to be paired "“ on the same chip "“ with a photodetector capable of detecting the light that it emits. Although the GaN epitaxial layer underneath the active region is capable of making a good photodetector, this would not detect visible light. To address this, a thick InGaN layer may be grown over the LED structure to provide light absorption. However, there is a penalty to pay "“ an increase in the complexity of the process flow.
To avoid this stumbling block, our team at Hong Kong University adopts a different approach, using multiple quantum wells to detect the light that is emitted by this active region. There is much merit in this idea, given luminescence and absorption are complementary optical processes.
With this approach, the detection sensitivity depends on the relative shift between emission and absorption spectra, which is known as the Stokes shift. Despite this limitation, typical InGaN/GaN multiple quantum wells can still have a spectral overlap between 20 nm and 30 nm.