Physica B 194-196 (1994) 27-28 Noah-Holland
A N e w T e m p e r a t u r e S e n s o r in L o w - T e m p e r a t u r e C o m p o s i t e B o l o m e t e r s f o r H i g h R e s o l u t i o n Spectroscopy of Nuclear Radiation. P.Delsing 1, C.D.Chen 1, T.Claeson 1, P.Davidsson 1 , B.Jonson 1, M.Lindroos 1, S.Norrman 2, G.Nyman 1 , and S.Qutaishat 1. 1Department of Physics, Chalmers Univ. of Technology and Univ. of Gbteborg, S-412 96 Grteborg, Sweden 2Department of Solid State Electronics, Chalmers Univ. of Technology, S-412 96 G6teborg, Sweden We suggest a new type of temperature sensor in low temperature composite bolometers for detection of nuclear radiation. The new sensor is a two-dimensional (2D) array of ultrasmall (-0.01ktm 2) aluminum Josephson junctions, situated on a micro machined silicon absorber. The incident radiation creates phonons in the absorber, which in turn can excite either charge solitons or vortex solitons in the 2D array. The benefit of this sensor is the low operation temperature, which is determined by the activation energy for the solitons, and can easily be as low as 20 mK. A low operation temperature is essential since the specific heat of the absorber has a T 3 dependence. The activation energy can be set to the desired value by changing the individual junction sizes as well as the junction resistances. Furthermore, the optimal operation point may be trimmed by a moderate magnetic field. Since the atomic mass of aluminum is very close to that of silicon the Kapitza resistance between the absorber and the sensor is small. By keeping the aluminum electrodes in the superconducting state, the amount of additional electronic heat capacity from the sensor may also be kept low. 1. I N T R O D U C T I O N Low temperature bolometers are predicted to have a very high energy resolution for detecting nuclear particles. Theoretically, the energy resolution can be better than leV at very low temperatures [ 1]. There are three general features of this kind of detector, i) An absorber in which the kinetic energy of the particle generates phonons (raises the temperature), ii) A temperature sensor which measures the temperature rise. iii) Heat leaks through which the heat (phonons) can escape from the absorber. Several different types of detectors have been proposed and operated [1-3]. Energy resolution down to - 1 0 eV has been achieved  by McCammon et al. The sensor can either be attached to the absorber (composite bolometer), or the sensor can be an integrated part of the absorber (integrated bolometer). The composite bolometer has the advantage that different types of sensors can used. For instance, one can use a semiconducting thermistor or the quasi particle resistance of a Josephson junction. However, at very low t e m p e r a t u r e the sensor may be thermally disconnected from the absorber by the Kapitza resistance. For the integrated bolometer the choices of absorber and type of sensor are restricted, but the problem of Kapitza resistance between the absorber and the sensor is avoided. The sensor in an integrated bolometer can be made by doping part of a silicon absorber . In this paper we suggest a new
type of temperature sensor, namely a 2D ultrasmall Josephson junctions, which evaporated onto the absorber. Using a micro-machined Si as the absorber, the with Kapitza resistance can be minimized. 2. P H Y S I C S
OF T H E 2D A R R A Y
If the capacitance C of a tunnel junction is made small, the charging energy Ec=e2/2C, can be larger than the thermal energy kBT. Furthermore, if the junction resistance RN is larger than the quantum resistance RQ=6.45 kfL the quantum fluctuations of the charge become small. This leads to a Coulomb blockade of tunneling, such that no tunneling occurs for low voltages at T=0 . In a 2D array of small Josephson junctions a single electron or a single Cooper pair can polarize the junction capacitances of the array . These charge solitons can be created in pairs by thermal fluctuations. The motion of these charge solitons gives rise to a current, which is temperature dependent. If the Josephson coupling energy Ej is small, the zero bias resistance can be approximately described by the semi empirical formula :
R 0 = N R N ( 1 + e (0"25EC+A)/kBT)
N is the number of junctions in series, M is the
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array of can be piece of problem
300 400 500 600 T (mK) Fig.1 R(T) for an 80x80junction array. R=35 k~, EC/kB=0.88 K, A/kB=2.32 K. number of junctions in parallel, R N is the normal state resistance of the individual tunnel junction. In other words, the array goes insulating at low temperature. The temperature at which this happens is the activation energy Ea---0.25Ec+A. By applying a magnetic field to the array, A can be continuously decreased to zero. For the arrays of aluminum junctions, which we have fabricated, A/k B is of the order of 2K and EC/kB can be varied from 4K and downwards by changing the junction sizes. This means that we can chose an arbitrarily low Ea. If on the other hand the Josephson coupling energy is large and the junction resistance is small the array goes superconducting at low temperature. The same behavior, a transition to either an insulating or a superconducting state, has been observed also for ultra thin films. The temperature dependences of these transitions can be used as temperature sensors, but here we focus on the insulating transition for 2D arrays. The R(T) dependence is similar to the theoretical prediction for the quasiparticle current for Josephson junctions. However, at low temperature the prediction for Josephson junctions does not hold.
3. DESIGN CONSIDERATIONS The benefit of this sensor is that the operation temperature can be chosen easily by changing the size and resistance of the tunnel junctions. Going to lower temperature the energy resolution of the detector is increased. Furthermore, the optimal working point can be tuned with an external magnetic field. In Fig.l we show the zero bias resistance for one array as a function of temperature and for different magnetic fields. At fields above
800 G the electrodes are no longer superconducting. This array contained 6400 junctions (M=N=80) and covered an area of -50x50pm. Note that it is possible to change the resistance of the array at a given temperature by several orders of magnitude with a magnetic field. The quite high resistance shown in Fig. 1 is a drawback since it, together with the lead capacitance CL, gives a large RCL time constant, which limits the speed of the detector. This can be compensated for by using a cooled preamplifier which reduces not only the lead capacitance but also amplifier noise. Furthermore, it is possible to reduce the array resistance by making a rectangular array with M>>N. On the other hand, a relatively high resistance of the sensor is an advantage, since it is better matched to low noise FETpreamplifiers than a low resistive sensor. For composite bolometers the sensor itself may have a non negligible heat capacity so that it lowers the sensitivity of the detector. The 2D array which we suggest is an evaporated thin film structure and the volume fraction of the sensor to that of the total bolometer is roughly the film thickness -0.1 ~tm divided by the thickness of the absorber -250pm. If the sensor is kept in the superconducting state the electronic specific heat can be kept low. Another problem is that of Kapitza resistance between the absorber and the sensor. Using aluminum which has an atomic mass m=27 for the sensor and silicon with m=28 as the absorber, the reflection of phonons at the interface is very small, and therefore the Kapitza resistance is reduced. We have used the Swedish Nanometer Facility and we gratefully acknowledge financial support from the Swedish Research Council for Engineering Sciences and the Wallenberg Foundation.
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