Fast and Uncooled Semiconducting Ca-Doped Y-Ba-Cu-O Thin Film-Based Thermal Sensors for Infrared.

YBa2Cu3O6+x (YBCO) cuprates are semiconductive when oxygen depleted (x < 0.5). They can be used for uncooled thermal detection in the near-infrared: (i) low temperature deposition on silicon substrates, leading to an amorphous phase (a-YBCO); (ii) pyroelectric properties exploited in thermal detectors offering both low noise and fast response above 1 MHz. However, a-YBCO films exhibit a small direct current (DC) electrical conductivity, with strong non-linearity of current-voltage plots. Calcium doping is well known for improving the transport properties of oxygen-rich YBCO films (x > 0.7). In this paper, we consider the performances of pyroelectric detectors made from calcium-doped (10 at. %) and undoped a-YBCO films. First, the surface microstructure, composition, and DC electrical properties of a-Y0.9Ca0.1Ba2Cu3O6+x films were investigated; then devices were tested at 850 nm wavelength and results were analyzed with an analytical model. A lower DC conductivity was measured for the calcium-doped material, which exhibited a slightly rougher surface, with copper-rich precipitates. The calcium-doped device exhibited a higher specific detectivity (D*=7.5×107 cm·Hz/W at 100 kHz) than the undoped device. Moreover, a shorter thermal time constant (<8 ns) was inferred as compared to the undoped device and commercially available pyroelectric sensors, thus paving the way to significant improvements for fast infrared imaging applications.

Measurable lower limit of thin film conductivity with parallel plate waveguide terahertz time domain spectroscopy.

Parallel plate waveguide (PPWG) terahertz (THz) time domain spectroscopy (TDS) is a powerful tool to investigate the properties of thin and low conductive materials. In this Letter, we determine the lower limit of detection of the PPWG-THz-TDS approach. We provide a closed-form expression of the minimal measurable conductivity by the system. The experimental results of amorphous YBa2Cu3O7-δ films indicate that the factor limiting the spectroscopic modality is the waveguide device misalignment. On the other hand, the expression of the minimal detectable conductivity provides a clear scheme of optimization by increasing the waveguide length and therefore enhancing the sensitivity of the system.