Characteristics of germanium thin film thermometers for use at low temperatures

Abstract

Germanium, thin films to be used as resistance thermometers have been tested at liquid helium temperature. Germanium is deposited in vacuum on insulated substrates and then silver is deposited onto the germanium films as ohmic contacts. Thermometers with desired resistances and suitable sensitivities can be easily fabricated by choosing proper deposition conditions. These thermometers have a quick response time and can be used to measure rapidly changing surface temperatures. The film resistance can be expressed as a function of temperature by a simple correlation, log R = C 0 + C 1 log T, between 4.2 K and 20 K. The sensitivity is approximately 20 Ω/K − 1 at 4.2 K. After 50 thermal cycles, the resistance at liquid helium temperature increased slightly, but is reproducible to within 0.8%. An estimated heat capacity of the germanium film is 2 × 10 −8 J K − 1 , and a thermal relaxation time is of the order of 10 − 12 s at 4.2 K. A stainless steel ribbon with the thermometers deposited on its surfaces is heated by a direct current in a liquid helium pool, and the surface temperature is measured. Large temperature oscillations due to occasional liquid solid contacts are observed.