Constructing Interconnects With Nitride Nanowire Arrays
Throughout the twentieth century, engineers improved the performance of the silicon IC by simply reducing the size of the transistors. But in recent times, progress has been far harder to come by. It has hinged on the introduction of exotic materials and architectures to ensure that the miniaturisation of the transistor does not produce dire consequences "“ and it will not be long before there is a need to add light-based communication, to enable faster transfer of information within and between CMOS chips.
The latter technology, often referred to as silicon photonics, has much promise, because light travels far faster than electrons (see Figure 1 for an example of a simple photonic circuit). Due to this hike in speed, circuits that incorporate photonics can operate at far higher communication rates than their electronic cousins, shortening on-chip and off-chip delays.
Fulfilling the promise of the optically equipped IC is not easy, however. Silicon will undoubtedly remain the material for the electronics, but its indirect bandgap hampers its use in the components needed for optical communication. While impressive progress has been made in research and commercial development of silicon-based detectors, modulators and passive waveguides, III-Vs are the essential ingredient for making a light source.
The current solution is to bond either GaAs-based or InP-based lasers to the silicon wafer. That's not ideal, though, because it creates thermal issues: the silicon gets hot, pegging back laser output and efficiency. A far better approach is the epitaxial growth and fabrication of a diode laser on silicon. Ideally, this source must emit at around 1.3 Î¼m, the zerodispersion wavelength for the SiO2 waveguide. Note that another advantage of this spectral range is that it is in the regime of eye-safe operation.
Unfortunately, success with this approach is hampered on three fronts: there is a large lattice mismatch, giving rise to a high density of dislocations; there is a thermal expansion coefficient mismatch that can cause strain in the device; and there is the potential for unwanted antiphase domains, which can be created during the growth due to the polar/non-polar nature of the epitaxial layers and the substrate.
Despite all these drawbacks, encouraging progress has been made by using devices that sport suitable buffer layers. At the University of Michigan, we have led the way, demonstrating the first InAs/GaAs quantum dot laser that is grown directly on silicon. And other groups are now following in our footsteps, improving the performance of this device on (001) silicon substrates.
However, progress is slow, and problems remain. The silicon that's used as a foundation for the lasers is offcut at 4Â° towards the (011) direction in order to avoid anti-phase domains and resulting defects.
Meanwhile, CMOS circuits are formed on (001), a situation that's not going to change. In addition, fabrication of the lasers involves the growth of complicated buffers and intermediate layers, making the whole process cumbersome.
Switching to nitrides
Our view is that instead of trying to address all of these problems, it is far better to switch to another material system. We chose the nitrides, because they combine tremendous tunability of the bandgap with growth on a variety of substrates, such as sapphire, SiC and silicon "“ including the form that's used for CMOS circuits.
Figure 1. The simplest example of a photonic integrated circuit on a silicon platform. At the University of Michigan, diode lasers and photodiodes are fabricated with III-nitride heterostructure disk-in-nanowire arrays grown directly on silicon by MBE. The waveguide is formed by dielectric deposition.
One distinguishing feature of GaN and its related alloys is its large, intrinsic polarisation. The fields that result aid the design of HEMTs, but are a drawback to LEDs, hampering radiative recombination efficiency and limiting the thickness of the quantum well. A further complication is that increasing the indium content of an InGaN well to propel its emission from the blue to the green creates compositional inhomogeneities. This makes it very challenging to produce light sources with acceptable efficiency beyond the blue. And magnifying this issue is the high density of defects, arising from the substantial lattice mismatch with the substrate.
To sidestep these issues, we are avoiding planar III-nitride semiconductors, and instead working with nanowires. Arrays of these nanowires can be grown directly, without masks or catalysts, on various substrates including CMOS-compatible (001) silicon. The density of the nanowires in these selforganized random arrays may range from 108 cm-2 to 1011 cm-2, and their diameter can be between 20 nm and 80 nm (see Figure 2).