EPITAXIAL SI SENSORS AT LOW-TEMPERATURES - NONLINEAR EFFECTS

Cryogenic bolometric sensors made from epitaxially grown Si:As have be en tested down to 40 mK. The sensors were grown by chemical vapour dep osition with a doped layer 8.4 mu m thick. The dopant concentration wa s measured using SIMS and was constant, +/-1%, with an excellent box p rofile. Arsenic concentrations up to 7.5 x 10(18) cm(-3) were achieved . Above 100 mK the low power resistance R(T) followed the variable ran ge hopping law, or Efros-Shklovskii law for a Coulomb gap, R(T) = R-0 exp(T-0/T)(1/2) with T-0 approximate to 25 K, typically. A double sens or arrangement was used to measure the electron-phonon coupling in the sensors and the phonon coupling to the hear sink. As the de current b ias through a senses was increased, spontaneous voltage oscillations w ere observed across the sensor below 100 mK, which limited the sensiti vity of the sensors in this region. These are circuit-limited oscillat ions between high and low resistance states. A phase diagram was estab lished for the spatio-temporal coexistence of the two states, with a c ritical temperature T-c = 115 mK. We show that this is an intrinsic ph ase transition within a thermal model of the electron-phonon coupling. For a resistance-temperature characteristic given by the Efros-Shklov skii law we find T-c = 0.00512T(0), independent of R-0 and the couplin g strength. This predicts T-c = 115 +/- 4 mK in this case. The model g ives excellent agreement for the critical voltage and current, by assu ming that the breakdown occurred via the formation of a filamentary re gion of high current density and high electron temperature. At higher currents, the response was temperature independent and given by I(E) = I(0) exp{-(E-0/E)(1/2)} where E is the average applied electric field and E-0 approximate to 380 V/cm, in agreement with a thermal model wh ich includes the phonon-phonon coupling to the heat sink.