Study of MgB2 and YBa2Cu3O7-x Microbolometers for THz Sensing Applications
Abstract: In this thesis, two novel THz detectors based on magnesium diboride (MgB2) and yttrium barium copper oxide (YBa2Cu3O7-x) are presented. In particular, MgB2 superconductor is used for the investigation of hot-electron bolometer (HEB) mixers, while YBa2Cu3O7-x is explored in microbolometers for room-temperature coherent and incoherent detection. Superconducting NbN hot electron bolometer (HEB) mixers are widely used in terahertz radio astronomy. HEB mixers are the most sensitive mixers at frequency above 1THz. However, their drawback is a limited IF bandwidth. Due to the short electron-phonon interaction time, MgB2 is a promising superconductor for an improved gain bandwidth in HEB mixers. MgB2 HEBs integrated with spiral antenna were fabricated, characterised and studied. The gain bandwidth was investigated with respect to the thickness and the critical temperature of the film. A gain bandwidth of 1.3GHz, 2.3GHz and 3.4GHz was measured in 30 nm, 15 nm and 10 nm MgB2 films, respectively. Using the two temperature model the experimental gain bandwidths data were analysed and, the electron-phonon interaction time, τe−ph of 7 ps to 15 ps, the phonon escape time, τesc of 4.8 ps to 42 ps were extracted resulting on the first model for HEB mixers made of MgB2 films. At 600GHz and 1.6THz, the lowest noise temperature was measured to be 800K and 1150K at a bath temperature of 4.2K, respectively. A noise bandwidth as large as 6-7GHz was measured for HEB mixers fabricated from 10 nm films with a critical temperature of 15K. YBa2Cu3O7-x microbolometers on bulk substrate integrated with planar antennas were investigated at room-temperature. The highest electrical and optical responsivity was measured to be 230V/W and 45V/W at 1mA bias current while the minimum noise equivalent power was 50 pW/Hz0.5. A large variety of bolometers with different areas, geometries, resistances and volumes were studied showing that the noise voltage normalised to the voltage drop over the bolometers was constant and equal to (VN/V)2=6×10−11 ×1/f×Hz−1. It is expected that this value can be used as a rule of thumb for the noise estimation of any YBa2Cu3O7-x bolometers in the limit of 1/f noise. Heterodyne mixer measurements of YBa2Cu3O7-x bolometers showed a response time of 2.5 ns which is the lowest reported among room-temperature bolometers.
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