Metalorganic vapour phase epitaxy of InGaAs/InAlAs and GaAs/AlGaAs quantum cascade laser structures

Metalorganic vapour phase epitaxy (MOVPE) has been successfully introduced by our group as an alternative growth technology of mid-IR InGaAs/InAlAs/InP quantum cascade lasers (QCLs) [1, 2]. Later on, we have transferred this technology to a production type multi wafer MOVPE reactor [3]. Many research groups and industrial companies have since followed our technological approach. The crystalline quality of the MOVPE grown material meets the stringent requirements imposed by the QCLs designs for operation in a wide spectral range of ~5-16 µm. However, developing an epitaxial process of highly strain-compensated QCL structures for operation at shorter wavelengths of ~3-5 µm appeared to be extremely challenging. Careful tuning the growth temperature regime was used to produce 30-period In0.7Ga0.3As/In0.34Al0.66As structures with ~1.2% mismatch from InP in the individual constituent layers. Fig. 1 shows an STEM image of a part of the QCL core. The measured length of one cascaded period of 51.5 nm is identical to the intended value. The same period length was derived from the X-ray diffraction data. 10 µm wide and 3 mm long devices with as-cleaved facets operate at λ ≈ 4 µm and deliver more than 2.4 W of peak optical power from both facets at 300 K with threshold current density of 2.5 kA/cm2 (Fig. 2). The lasers operate up to at least 400 K with characteristic temperature of 153 K. The developed epitaxial process represents a solid platform for engineering straincompensated QCLs structures for shorter emission wavelengths around 3.5 µm. Another direction of our recent research efforts was revisiting GaAs-based QCLs to develop a robust and costeffective growth technology of devices operating around 9 µm. InGaP and InAlP waveguides were used to improve optical confinement and reduce waveguide losses. STEM confirmed the intended thickness of individual GaAs and Al0.45Ga0.55 layers in the laser core. The amplitude of the interface roughness is less than 0.5 nm (the nominal thickness of the thinnest layer in the active region is 0.9 nm). QCLs with In0.47Al0.53P waveguides demonstrate record low threshold current densities for the GaAs/AlxGa1-xAs materials system. Under pulsed operation, threshold current densities of 2.2 and 4.4 kA/cm2 were observed at 240 and 300 K respectively, and laser emission was maintained up to temperatures of at least 330 K. The laser emitted peak optical powers of 0.57 W at 240 K and 0.16 W at 300 K. The presented laser performance should greatly increase the prospects of mid-IR GaAs-based QCLs for technological applications.