| Abstract [eng] |
The aim of this work is to fabricate monolithically integrated mid-infrared quantum cascade lasers (QCL) and quantum cascade detectors photonic (QCD) system. For this reason, it is crucial to improve the fabrication process. Mounting configuration of the QCL devices was changed from epi-side up to epi-side down. This decreases the etched area of the structure and reduces the probability of defects or micro-mask features forming on the etched surface. Moreover, a 1.2 µm thick SiO₂ hard mask, required for deep (8 µm) etching structures in the InAs substrate was formed. For this reason, various materials were investigated that could act as a protective mask during the dry etching of the SiO₂ layer. It was found that photoresist, used as a soft mask on the SiO₂ layer, is not suitable for forming reliable hard mask using the RIE method. The photoresist may either form uneven mask edges or melt and flow due to intense ion bombardment, further degrading the structure’s geometry. A chromium hard mask was tested as an alternative. During RIE etching at standard power (P = 110 W), it was observed that re-deposition of the hard mask material occurs. This results in the formation of features approximately 500 nm high at the bottom of the structure and a rough surface. By increasing RIE power (P = 150 W), this effect was eliminated, and vertical structure sidewalls were formed. Additionally, the etching of the SiN dielectric was changed to a two-cycle process, because different areas of the detector and top electrode lines caused uneven photoresist thickness, which happened during the photolithography process. The metallization process was also modified: magnetron sputtering was used instead of electron beam evaporation. This method ensured more uniform metal deposition on the sidewalls of the edge structure. Finally, in the manufactured QCL devices, which had a laser wavelength of 9.4 µm, the minimum threshold current density reached J = 7.76 kA/cm², at a threshold current of Ith = 2.92 A. However, such a high current density indicates possible current leakage or carrier overflow into the cladding layers. |
