In the course of developing microcalorimeters as detectors for astronomical X-ray spectroscopy, we have undertaken an empirical characterization of ''non-ideal" effects in the doped semiconductor thermometers used with these detectors, which operate at temperatures near 50 mK. We have found three apparently independent categories of such behavior that are apparently intrinsic properties of the variable-range hopping conduction mechanism in these devices: 1/f fluctuations in the resistance, which seems to be a 2D effect; a departure from the ideal coulomb-gap temperature dependence of the resistance at temperatures below T 0 /24; and an electrical nonlinearity that has the time dependence and extra noise that are quantitatively predicted by a simple hot electron model. This work has been done largely with ion-implanted Si : P : B, but similar behaviors have been observed in transmutation doped germanium.
The electrical conductivity in doped semiconductors in the strongly localized variable range hopping regime is currently explained as phonon-assisted electron hopping. While investigating the non-Ohmic behavior of doped silicon at temperatures of 0.05-1 K, we found strong evidence for the existence of separate temperatures for the electron and phonon systems analogous to the hot-electron effect in metals. This behavior cannot easily be explained by phonon-assisted hopping and seems to favor instead a direct electron-electron interaction at low temperature. A hot-electron model makes definite predictions for the dependence of the electrical conductivity on the bias power, the frequency dependence of the resistance nonlinearities, and for an additional noise term. We have made a systematic investigation of these quantities, and find all of them in good agreement with the model predictions over a wide range of parameters.
Achieving optimum spectral energy resolution in conventional scanning electron microscopes (SEM) can be accomplished by using microcalorimeters. Improvements in device design are being studied, although many research groups have developed fabrication techniques that produce consistent results [1]. Here, we discuss the fabrication method that we are using to produce microcalorimeters using superconducting transition edge sensors (TES) as thermometers, and some of the roadblocks we are observing.
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