Monolithically integrated QCLs on silicon

Low-strain quantum cascade lasers with optimised metamorphic buffer layers are closing the gap to realising reliable, high-power, infrared sources on silicon

BY Enrique Sanchez Cristobal, Ahmad Azim and Luke Millbocker from IRGlare, Matthew Fetters, Amy Liu and Joel Fastenau from IQE, and Alejandro Villalobos meza and Arkadiy Lyakh from The University of Central Florida

For designers of semiconductor lasers, quantum wells are a fundamental building block for these nanoscale devices, as well as an incredible asset. Not only do these devices serve as effective generators of laser light through radiative recombination processes, but the use of quantum well engineering also allows for control of emission wavelengths through changes in material composition.

Common semiconductor lasers, such as laser diode and the VCSEL, emit at wavelengths determined by the difference between the lowest energy levels of the electrons in the conduction band and the highest energy of the holes in the valence band of the quantum well material. This is a limitation that can be overcome by another unique semiconductor structure, the quantum cascade laser (QCL), which achieves lasing through inter-sub-band transitions associated with different excited states of the conduction bands of two materials. (see Figure 1).

Figure 1. Optical transitions in laser diodes (left) and QCLs (right). While transitions in laser diodes are limited by the tuneability of the material band gap, QCL optical transitions occur between excited conduction band states of two materials which make up a periodic quantum well structure. Engineering of these layers and their compositions allow QCLs to emit light in the range of 3-14 µm.

At the heart of this semiconductor device is a superlattice composed of two alternating materials whose conduction bands work together to create a periodic quantum well structure, which emits infrared photons under an applied electrical bias. The material composition of the superlattice constituents can be varied to achieve emission in a broad infrared range, and the periodic nature of this structure enables a single electron to generate multiple photons as it cascades through the superlattice (see Figure 2).

Figure 2. Diagram of a stage, or period, of the periodic quantum well structure that is generated from the conduction bands of the two alternating superlattice constituents. Injected electrons experience optical transitions before undergoing several non-radiative transitions and reaching the end of the QCL stage. Electrons are then injected into the next stage, where they repeat the process and continue to generate infrared photons.


While QCLs are not as well-known as diode lasers and VCSELs, they are produced commercially, and are used in a range of critical sectors, including defence, spectroscopy, and free-space communication. In these applications they serve as compact, high-powered infrared sources that span the mid- to long-wave infrared range, thanks to a highly tuneable quantum well superlattice structure. More recent designs can deliver a continuous-wave output power of several watts at room temperature, enabling them to play pivotal roles in applications such as defence, where their low atmospheric losses at critical spectral windows allow them to be used in direct infrared countermeasure systems. Another important application is in the field of imaging, where QCL-based infrared microscopy has been successfully deployed for the rapid, precise classification of different cancer tissues.

Up until now, QCLs have usually been produced with lattice-matched material systems, such as InP. However, while this is conducive to maximising material quality and device performance, it also significantly limits the opportunities for this device, due to the high cost and low scale of native substrates.

The solution to overcoming this limitation is to modify the fabrication of QCLs to be able to account for the lattice mismatch between non-native substrates and the active layers. In this regard, our team at IRGlare, working in partnership with engineers at The University of Central Florida and IQE, has broken much new ground. Our triumphs include the first demonstration of monolithic integration of QCLs onto lattice mismatched substrates, and pioneering efforts related to the growth of these lasers on silicon. We are one of two groups to provide the first demonstration of integration of QCLs onto silicon – our team and another reported this success around the same time – and we trailblazed the integration of these lasers onto large-diameter silicon substrates. Another noteworthy claim of ours is that, until recently, our QCLs on silicon produced the highest performance on this foundation.

Figure 3. The metamorphic buffer design for a medium-wave infrared QCL on silicon alleviates strain from lattice mismatch by gradually transitioning the lattice constant between adjacent materials along the growth direction.

Silicon integration
In the remainder of this article, we detail our efforts at producing high-performance QCLs on silicon. These lasers employ an InP cladding, surrounded by an active region that’s composed of alternating InGaAs and AlInAs layers that serve as quantum wells and barriers, respectively. By adjusting the composition of these materials, we either match the lattice-constant to the surrounding InP, or we target zero accumulated strain to maximise the quality of the epitaxial material – a crucial factor in ensuring laser performance.

One of the big attractions of growing QCLs on silicon is the opportunity to take advantage of the scalability and cost-efficiency of silicon technology. Thanks to the ubiquity of silicon in modern electronics, substrates made from this material are cheaper and far more accessible than the InP substrates that III-V-based QCLs traditionally rely on for good performance. Realising the direct epitaxial growth of QCLs on large-diameter silicon substrates offers the tantalising prospect of manufacturing compact infrared platforms that support both the electronic and photonic components on a single chip.

As well as the technical benefits of integrating complex components on a single compact chip, silicon substrates offer the benefit of massively upscaling the production and yield of standard QCLs, as silicon wafers with diameters that are larger than 200 mm are readily available. These large-diameter wafers, coupled with the exceptionally compact nature of standard QCLs, promise to enable the processing of significantly more QCLs on a single wafer, all while reducing the cost of production.