__ e. _ BB I-- EISEVIER Nuclear Instruments and Methods in Physics Research A 370 (1996) 21 I-212 NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SectIonA Progress in low temperature thin film thermometers* L. Dumoulin*, L. Berg& S. Marnieros, J. Lesueur Centrr zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA de Spectrornh-ir NuclGaire et de Spectromitrie de Masse. lNZP3-CNRS, Bdt. 108, 9140-T Orxzy Campus, Frat~c zyxwvutsrqponml Abstract Amorphous NbSi thin films are suitable as thermometric material for bolometric applications. We give detailed results about their transport properties and their electron-phonon coupling. In a composite bolometer, the thermometer and its coupling with the absorber play a crucial role. Thin films directly deposited on the absorber are very attractive because of their strong, direct thermal coupling and their easy patterning. With superconducting thin films, one can study non-equilibrium effects, leading to an improved sensitivity of massive bolometers and possibly to some event discrimination (I]. On the other hand, doped semi- conductors such as Ge NTD are very good thermometers for standard JFET electronics. We have shown [2] that amorphous heavily doped thin film semiconductors such as Nb,Si,_\ behave like Ge NTD (i.e., as an Anderson insulator) with larger available bias power densities (50 times). We have now strong indications [3] that such thin film thermometers are also sensitive to non-equilibrium phonons. In this contribution we report on our new results on transport properties of NbtSi,_ , thin films (from 70 to 300 nm thick). The films are prepared by coevaporation of pure Nb and Si in an ultra high vacuum chamber, the rate of each source being accurately tuned to get the expected com- position x. Fig. la illustrates the strong sensitivity of the low temperature properties with .r. A noticeable improve- ment in the film homogeneity was obtained by rotating the sample during the deposition (3 revolutions per second for an evaporation rate of 3 A s ’ )_ The comparison of the two curves (x = 8.4% and x = 8.5% in Fig. la) indicates that the composition reproducibility is around & = 0. I %. The resistive behavior for a composition range leading to useful resistances (I MO near 30 mK) is described by the Coulomb gap variable range hopping theory. However for samples very near the Metal-Insulator Transition (MIT) we observe, as expected, a crossover from exp(T, / T)“q to exp(T,,lT)“’ laws when decreasing the tempera- ’ Work related to the EDELWEISS Collaboration. * Corresponding author. Tel. +33 1 69 41 52 08. fax +33 I 69 41 52 68. e-mail dumoulin@csncls,in2p3,fr. 10’ dependence for different composition ratio! g lo’ G c 10’ s h r 102 > z $ 101 zi 6 i . Ofemperatur;(K) O8 04 4 Nb, Si,., resistance temperature dependence for different annealing temperature size: 5 mm x 5 mm x 300nm R= R, exp(T,/T )’ ‘* xx ~XXXXXXXX, ++++ + + + +++*+++++++*+* +I 0.1 0.2 0.3 0.4 0.5 0.6 0.) Temperature (K) Fig. I (a) Resistivity versus temperature dependence for different composition ratios of Nb,Si,_, thin films. (b) Successive anneal- ings of a given film at increasing temperatures. The best fits were obtained keeping R,, -5 n as a constant. The increase of T,, indicates a reduction of the localization length, and therefore an evolution to a more insulating state. 016%9002/96/$15.00 0 1996 Elsevier Science B.V All rights reserved SSDI 0168-9002(95)01120-X V. BOLOMETERS