Us researchers report 8.1 μm wavelength quantum cascade laser (QCL) grown on silicon (Si) by MOCVD [S. Xu et al., Applications. Physics Letters, v123, p031110, 2023]. "There are no previous reports of QCL growth on silicon substrates by metal-organic chemical vapor deposition (MOCVD)," commented the team from the University of Wisconsin-Madison, the University of Illinois at Urbana-Champaign and MicroLink Devices Inc.

This integration on silicon could lead to the development of chip scale, reliable and mass-producible photonic integrated circuits (PIC). The researchers contrast this with other integration methods such as wafer bonding: "Hybrid integration methods rely on precise alignment to achieve efficient waveguide to laser optical coupling, which in turn requires tight fabrication machining tolerances. Direct integration onto silicon through heteroepitaxy enables mid-infrared (IR) optoelectronic devices to be integrated with mature CMOS-compatible silicon platforms at low cost and high throughput."

Mid-infrared QCL is usually grown on indium phosphide (InP). The team paid particular attention to creating a virtual InP substrate on silicon by combining molecular beam epitaxy (MBE) and MOCVD. MOCVD is superior to MBE in production. "The remaining technical challenge is to overcome the defects and epitaxial growing-related problems caused by large lattice constant and thermal expansion mismatches (e.g., about 8% lattice mismatches) and about 50% thermal expansion coefficient mismatches between InP and primary substrates such as silicon," the researchers comment.

The arsenide portion of the template structure (Figure 1) is a limited company grown on a commercial (001) GaP/Si template (supplied by NAsP III/V) using a solid source MBE. The substrate is nominally coaxial and compatible with high-throughput industrial-scale CMOS electronics production. The initial layer consists of an Indium Gallium Arsenide (InGaAs) dislocation filter layer (DFL) sandwiched in GaAs. By keeping the thickness of the initial arsenide layer at 0.5 μm, the researchers sacrificed some of the potential for reducing the penetration dislocation density (TDD). The GaAs layer grows in two steps, first at low temperatures of 500°C and then at higher temperatures (580/610°C for the lower/upper layers, respectively). As far as the upper layers are concerned, one motivation for doing so is to avoid the escape of indium in InGaAs DFL.

The upper InP metaseptic buffer (MBL) portion of the template grows through MOCVD and includes four additional DFLS, consisting of three 2nm/37nm InAs/InP pairs.

The QCL is completed using MOCVD and has a total epitaxial thickness (including the metamorphic buffer layer and the laser layer) of approximately 13 μm. QCL/Si did not show cracks, which the team believes could be due to two factors: the small sample size of 1.7cmx1.7cm, and the curvature accumulation mitigated by the 800 μm thick silicon substrate. The TDD for the arsenide portion of the template was estimated to be 1.0 x109 / cm-2. InP MBL reduces this to 7.9x108 /cm 2.

Under pulsed operation, the threshold current density on silicon is 22% lower than that of devices grown on bulk InP substrates during the same process run: in the figure, 1.50kA/cm 2 and 1.92kA /cm 2, respectively. The researchers comment: "This may reflect reduced incorporation of silicon dopants within the active nuclear superlattices due to pre-existing defects or differences in the growth temperatures of the silicon and InP substrate surfaces. In addition, uneven growth around the defect site may reduce carrier mobility and tunneling efficiency, which would explain the higher series resistance observed in devices grown on silicon."

The higher the voltage required to provide a given current injection in a silicon-based QCL, the higher the series resistance. Despite the higher series resistance, silicon-based QCL also provides higher peak optical output power: 1.64W for silicon-based devices and 1.47W for INP-based devices. The corresponding slope efficiency is 0.72W/A and 0.65W/A, and the electro-optical conversion efficiency is 2.85% and 2.50%, respectively.

The emission spectral analysis showed a variety of modes in the wavelength range 7.6-8.3 μm. The maximum peak values of InP and Si based devices are about 8.1 μm and 8.0 μm, respectively. These wavelengths are slightly shorter than the design target of 8.2 μm. The researchers believe that this difference may be due to local growth changes affecting layer thickness, as shown in X-ray diffraction analysis.

Source: Laser Network