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Title: Room-temperature GaAs and polysilicon thermal sensors for infrared radiation
Author: Chong, N.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1997
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GaAs was introduced as a thermoelectric material in a micromachined thermal detector. GaAs/Au thermocouples were fabricated on air-bridges with lengths ranging from 40 to 650 μm by anisotropic etching to undercut the GaAs substrate with photoresist patterns delineated by optical lithography. The devices have been characterized with a 10.6 μm CO2 laser. The responsivity and time constant for various lengths were determined in both air and vacuum conditions. The time constants ranging from 50 μs up to 2.2 ms were among the fastest for micromachined thermal detectors; the lower responsitivities were inevitable trade-offs. An analytical thermal transport model of the air-bridges was used to evaluate the device performance and deduce useful physical parameters from the experimental data such as the heat-transfer coefficient. The spectral response in the wavelength band from 1 to 12 μm, measured by using a monochromator, also highlighted the importance of antireflection coatings on the responsivity. To achieve a fast response time without sacrificing the responsivity of the thermal detectors, antenna-coupled designs were considered. A small free-standing element with a short time constant was coupled to an antenna which collected the incoming radiation and provided gain to the self-heating element. Polysilicon air-bridges, instead of metallic ones, were demonstrated for the first time in such a system and provided a practical level of performance which was measured in the linear-polarized CO2 laser system. Log-periodic and bow-tie antenna designs were chosen to facilitate a broadband response in the mid-infrared regime. The devices were able to operate in both thermocouple and bolometer modes both of which exhibited a higher responsivity than previously reported studies. The time constant was kept as short as 4 μs. Measurements made at 10.2μm wavelength indicated that the linearly polarized antennas had an on-axis cross-polarization ratio of about -4.5 dB and the antenna patterns in free-space had a directivity of approximately 5-9 dB. The deduced coupling efficiency was 34%. Measured frequency responses for modulated optical signals also agreed with the thermal transport models.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available