In this paper, we report the results of investigation of 9.5 µm AlGaAs/GaAs and strain compensated 4.7 µm AlInAs/InGaAs/InP QCLs. We also show the results for 9.5 µm lasers based on lattice matched AlInAs/InGaAs/InP structures. The developed GaAs/AlGaAs lasers show the record pulse powers of 6 W at 77 K and up to 50 mW at 300 K. This has been achieved by careful optimization of the MBE growth process and by applying a high reflectivity metallic coating to the back facet of the laser. The 9.5 µm AlInAs/InGaAs/InP lasers utilize AlInAs waveguide and were grown exclusively by MBE without MOCVD regrowth. The short wavelength, strain compensated QCLs were grown by MOCVD. They represent state‐of‐the‐art parameters for the devices of their design. For epitaxial process control, the atomic‐force microscopy (AFM), high resolution X‐ray diffraction (HR‐XRD) and transmission electron microscopy (TEM) were used to characterize the morphological and structural properties of the layers. The basic electro‐optical characterization of the lasers is provided. We also present results of Green's function modeling of mid‐IR QCLs and demonstrate the capability of non‐equilibrium Green's function (NEGF) approach for sophisticated but still computationally effective simulation of laser's characteristics.

The development of charge coupled device thermoreflectance (CCD TR) instrumentation for accurate and rapid evaluation of the thermal characteristics of quantum cascade lasers is demonstrated. The thermal characterization of such devices provides a mode for comparing different operating conditions, geometries and device designs. The method allows for registration of the high-resolution maps of the temperature distribution in a time not exceeding several seconds. The capabilities of the CCD TR are compared with standard TR spectroscopy.

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Switchable, double wavelength generation is demonstrated from a single vertical external cavity surface-emitting laser chip. Power of ~0.5 W for two wavelengths λ≈967 nm and 1,018 nm i.e. within the spectral distance of 51 nm were registered. In the semiconductor heterostructure a single set of nominally identical quantum wells was enclosed in a single, two-mode resonant microcavity. The wavelength switching was induced by the change of the pump power. The increase or decrease of the pump power changes the active region temperature and thus tunes spectrally the gain spectrum to the one of two modes.

We investigate molecular beam epitaxy (MBE) growth conditions of micrometers-thick In0.52Al0.48As designed for waveguide of InGaAs/InAlAs/InP quantum cascade lasers. The effects of growth temperature and V/III ratio on the surface morphology and defect structure were studied. The growth conditions which were developed for the growth of cascaded In0.53Ga0.47As/In0.52Al0.48As active region, e.g., growth temperature of Tg = 520 °C and V/III ratio of 12, turned out to be not optimum for the growth of thick In0.52Al0.48As waveguide layers. It has been observed that, after exceeding ~1 µm thickness, the quality of In0.52Al0.48As layers deteriorates. The in-situ optical reflectometry showed increasing surface roughness caused by defect forming, which was further confirmed by high resolution X-ray reciprocal space mapping, optical microscopy and atomic force microscopy. The presented optimization of growth conditions of In0.52Al0.48As waveguide layer led to the growth of defect free material, with good optical quality. This has been achieved by decreasing the growth temperature to Tg = 480 °C with appropriate increasing V/III ratio. At the same time, the growth conditions of the cascade active region of the laser were left unchanged. The lasers grown using new recipes have shown lower threshold currents and improved slope efficiency. We relate this performance improvement to reduction of the electron scattering on the interface roughness and decreased waveguide absorption losses.

Abstract:In this paper, we report on the experimental investigation of the thermal performance of lattice matched AlInAs/InGaAs/InP quantum cascade lasers. Investigated designs include double trench, single mesa, and buried heterostructures, which were grown by combined Molecular Beam Epitaxy (MBE) and Metal Organic Vapor Phase Epitaxy (MOVPE) techniques. The thermal characteristics of lasers are investigated by Charge-Coupled Device CCD thermoreflectance. This method allows for the fast and accurate registration of high-resolution temperature maps of the whole device. We observe different heat dissipation mechanisms for investigated geometries of Quantum Cascade Lasers (QCLs). From the thermal point of view, the preferred design is the buried heterostructure. The buried heterostructures structure and epi-layer down mounting help dissipate the heat generated from active core of the QCL. The experimental results are in very good agreement with theoretical predictions of heat dissipation in various device constructions.

In order to adjust the highly controllable and optimum growth conditions, the multi-step interrupted-growth MBE processes were performed to deposit a series of GaAs/Al0.45Ga0.55As QCL structures. The additional calibrations of MBE system were carried out during the designed growth interruptions. This solution was combined with a relatively low growth rate of active region layers, in order to suppress the negative effects of elemental flux instabilities. As a result, the fabricated QCL structures have yielded devices operating with peak optical power of ∼12 mW at room temperature. That is a better result than was obtained for comparable structures deposited with a growth rate kept constant, and with the only initial calibrations performed just before the epitaxy of the overall structure.