Development of Thin Film Germanium-Gold Thermistors for Calorimetric Detection of Nuclear Radiation.

Physics

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Scientific paper

The present work is to produce thin film semiconductor thermistors which can be directly fabricated on radiation absorbers to act as ultra low-mass, highly sensitive cryogenic phonon sensors for detecting single nuclear radiation interaction invents. The specific application envisioned for these devices is in the search for galactic Dark Matter, which is proposed to exist in the form of weakly interacting massive particles in the galaxy. Thin film Au doped Ge thermistors were directly fabricated on single crystal silicon absorbers using vacuum filament evaporation and microfabrication techniques. The fabrication procedure developed in the present work gives micron-scale thin film GeAu thermistors with highly reproducible characteristics. Electrical and thermal properties of thin film Ge_{rm 1-x}Au _{rm x} for 0.019 < x < 0.17 were studied between room temperature and 0.019K and in magnetic fields up to 4.0T. Measurements indicated that variable-range-hopping dominates the conductivity of GeAu thin film at temperatures below 10K. Metal-insulator transition of the film is found to occur for x > 0.17. The observed magnetoresistance is explained by using a field-dependent hopping exponent proposed in the present work combined with Mott's hopping conductivity theory. A new treatment of electrical field-induced nonlinearity in variable-range-hopping is also given which quantitatively reproduced the observed nonlinear resistivity. Electrical heat pulse and particle detection measurements showed that the total effective heat capacity of the device was dominated by the silicon absorber substrate at a bath temperature of 1.5 K and by electron system of the thermistor itself of the device when the bath temperatures were below 100 mK. Excellent responsivity of GeAu thin film calorimeters has been demonstrated in AC electrical pulse and nuclear radiation detection experiments. At optimal bias power, the responsivity (|DeltaV/ DeltaE|) of the present devices reaches 10^8 V/J at 1.5K and 1010 V/J at 0.02 K bath temperature.

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