Mid-infrared detection with semiconductor based pixel arrays attracted constant research interest over the past years. Remaining challenges for intersubband detectors are high device performance at elevated temperatures in combination with cost effective scalability to large pixel counts needed for applications in remote sensing and high resolution infrared imaging.
In this field, quantum cascade detectors may offer promising advantages such as photovoltaic room temperature operation at a designable operation wavelength with compatibility to stable material systems and growth technology.
We present a high performance InGaAs/InAlAs quantum cascade detector design suitable for pixel devices. The design is based on a vertical optical transition and resonant tunneling extraction. The 20 period active region is optimized for a high device resistance and thereby high detectivity up to room temperature. The pixels are fully compatible with standard processing technology and material growth to provide scalability to large pixel counts. An enhanced quantum cascade detector simulator is used for design optimization of the resistance and extraction efficiency while maintaining state of the art responsivity. The device is thermo-compression bonded to a custom read out integrated circuit with substrate bottom side illuminated pixels utilizing a metal grating coupling scheme. The operation wavelength is designed to align with the strong CO2 absorption around 4.3µm. A room temperature responsivity of 16mA/W and a detectivity of 5∙10^7 cm√Hz/W was achieved in good agreement with our simulation results. Device packaging and thermo-electric cooling in an N2 purged 16 pin TO-8 housing has been investigated.
The integration of electrical and optical components on a single chip, favourable silicon, is a major goal in research. Thereby, a bottleneck is the integration of active and passive optical elements. Graphene, with its electrically tuneable absorption and ultrafast photoresponse, is a promising candidate to move a step closer towards high-speed on-chip integration.
We fabricated a dual-gate tuneable pn-junction graphene phototdetector to investigate the relevant conversion mechanisms. The photodector is integrated on a silicon slot waveguide, which has twofold function. First, the two silicon strips of the slot waveguide are utilized as dual gate electrodes to create an electrically controllable pn-junction in the graphene. Second, the slot waveguide design allows confinement of light in subwavelength dimension. The confined light is directly absorbed in the slot between the n- and p-doped regions. At zero bias the conversion is dominated by the photo-thermoelectric effect, where we achieved a responsivity of 35 mA/W. While by applying a low bias of 300 mV, the responsivity increased to 76 mA/W due to an additional photoconductive contribution.
The photoresponse of photodetectors based on the photo-thermoelectric effect arises from hot electrons, rather than lattice heating. Therefore, we could demonstrate that our graphene integrated photodetector based on a tuneable pn-junction reaches a setup-limited 3dB-bandwidth of 65 GHz, which is the highest value reported for a graphene-based phototedetector.
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