Short time transient thermal behavior of solid-state microrefrigerators

We present detailed experimental and theoretical studies of the short time transient thermal behavior of SiGe superlattice microrefrigerators on a chip. Transient temperature profiles of microrefrigerator devices of different sizes are obtained using thermoreflectance technique. Thermal imaging with submicron spatial resolution, 0.1 K temperature resolution, and 100 ns temporal resolution is achieved. The dynamic behavior of the microrefrigerators shows an interplay between Peltier and Joule effects. Peltier cooling appears first with a time constant of about 10-30 mu s, then Joule heating in the device starts taking over with a time constant of about 50-150 mu s. The experimental results agree very well with the theoretical predictions based on thermal quadruple method. The difference in the two time constants can be explained considering the three-dimensional thermal resistances and capacitances of the microrefrigerator. In addition this shows that the Joule heating at the top metal/semiconductor interface does not dominate the microrefrigerator performance. Experimental results show that under high current pulsed operation, the microrefrigerator device can provide cooling for about 30 mu s, even though steady state measurements show heating.