Monday, September 1, 2014
Thermal boundary resistance in YBCO THz microbolometers at room temperature
Göteborg : Chalmers tekniska högskola, 2014. 105 s.
Terahertz waves are utilized for a wide range of applications, from security, medical imaging to gas spectroscopy, etc. Previous investigations in the Teraherzt and Millimeter wave group at Chalmers, has shown that antenna-integrated Y Ba2Cu3O7 (YBCO) bolometer could serve as a potential detector for this range of the electromagnetic spectrum. The detector is composed of a 70 nm thick YBCO _lm with micron sized dimension and is deposited on a crystalline Al2O3 (sapphire) substrate. Phonons, the quasi-particles associated with the lattice vibrations, transport the heat from the _lm to the substrate, but are scattered in this process. This scattering is macroscopically represented by the thermal resistance. The thermal resistance is responsible for the response and speed of these bolometers. Two parameters are varied that we believe could affect this thermal resistance. The first parameter is the thickness of the CeO2 buffer layer. This layer is situated between the YBCO layer and the sapphire substrate. Its purpose is to provide a good lattice match and chemical isolation of the YBCO layer with respect to the substrate. The range studied is 10 - 50 nm. The second parameter is the deposition temperature during the deposition of the film using pulsed laser deposition, this parameter is known to affect the YBCO film quality, however no study has investigated its influence on the thermal properties of the detector. The range studied is 780◦C − 855◦C. The thermal resistance is experimentally studied by fabricating the bolometers with above mentioned parameters and measuring them using DC (IV, Resistance-Power) and RF techniques (voltage response versus modulation frequency). Results are analyzed and compared to reported measurements as well as with the Two Temperature model. Considerable variability is present for all devices, even when fabrication parameters are kept constant. The performance was not improved by either the buffer thickness or the temperature deposition in the studied range. The effective thermal resistance ex- tracted from the DC measurements is found to be situated between 0.1.10−3cm2K/W − 1.0.10−3cm2K/W (excluding outliers). These values are not in accordance with the RF-measurements. The study suggests that additional knowledge on the phenomena involved in the heat transport is required.