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WP 2 - Design and Growth

Fig. 1. Typical state-of-the-art ICL active region design [1].
Fig. 2. Optical mode simulations of a typical substrate-emitting ring ICL device [2].

Workpackage 2 is about the design and growth of the ICL structures. By using a computational method based on a Schrödinger-Poisson solver specific designs can be obtained for the target wavelengths of ~4.32/4.36 µm (CO2), ~4.53 µm (N2O) and 4.74 µm (O3). The such designed devices are grown by solid-source molecular beam epitaxy (MBE) and test structures are characterized for relevant figures of merrit concerning their emission wavelength: electrical luminescence spectra, laser emission wavelength and spectral bandwidth.

In order to define design rules for ICL gain material which is suitable for the processing of ring laser cavities, theoretical calculations regarding the band structure and the corresponding probability densities of the quantum mechanical states are performed. For this an 8 bands k·p model is used. This enables the exact tailoring of the emission wavelength for the mentioned target wavelengths as well as the transport properties within the active region of the structure. The main goal within this part of the work is the variation of the layer thicknesses and compositions in the active region to optimize laser parameters such as threshold current density, temperature stability and electrical properties. Additionally, optical mode simulations are performed using a custom mode solver that utilizes the transfer matrix method to solve the Helmholtz equation in one or two dimensions. This is of particular importance for the implementation of design rules that evolve from the ring laser geometry for surface emission.

All samples are grown by MBE on GaSb substrates. To achieve optimal device performance, high epitaxial quality has to be ensured in all funtional parts of the laser structure. Hence, the strain that builds up in some of the layers with a lattice constant different from that of the substrate have to be carefully balanced ("strain compensation") in order to avoid relaxation effects. To achieve the best device results, various growth optimizations need to be performed to define the best growth parameters for the designed structures. This especially includes the growth temperature, the appropriate fluxes as well as the growth speed. For the characterization of the grown layer structures different tools and methods such as Photoluminescence (PL), high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and secondary ion mass spectroscopy (SIMS) are utilized.

Once the first complete ICL structures are grown, test laser must be processed. To achieve a fast feedback loop, edge emitting broad area and ridge waveguide based devices will first be processed to extract the basic parameters like emission wavelengt, spectral bandwidth and threshold current density. Also the temperature performance will be evaluated in detail.

[1] I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell,
     J. R. Lindle, and J. R. Meyer, "Rebalancing of internally generated carriers for mid-infrared
     interband cascade lasers with very low power consumption", Nat. Comm. 2, 585, 2011.
[2] M. Holzbauer, R. Szedlak, H. Detz, R. Weih, S. Höfling, W. Schrenk, J. Koeth, and
     G. Strasser, "Substrate-emitting ring interband cascade lasers", Appl. Phys. Lett. 111,
     171101, 2017.