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A6: Characterization

Observation of Nonlinear Peltier Coefficient in Low doped n-type InGaAs at Cryogenic Temperatures

Amirkoushyar Ziabari1, Je-Hyeong Bahk1, Hong Lu3, Zhixi Bian2, Art Gossard3, and Ali Shakouri1,2
1. Birck Nanotechnology Center, Purdue University, 2. University of California Santa Cruz, 3. University of California Santa Barbara

We report experimental observation of nonlinear Peltier effect in thin film microrefrigerators at cryogenic temperatures. Current-dependent Peltier coefficient of low-doped n-type InGaAs and its associated Peltier cooling are extracted from electrical and thermal characterization techniques. At high electric fields the linear relation between Peltier cooling/heating with current is no longer valid. Peltier coefficient depends nonlinearly on current and rises significantly as current increases. Monte Carlo simulations show that the nonlinear Peltier coefficient is temperature independent, proportional to the effective mass, and inversely proportional to the square of carrier concentration in the semiconductor. Although the non-linear term is temperature independent, significant cooling at low temperature due to weaker electron-phonon coupling is predicted.  In an effort to measure the Peltier coefficient and Peltier cooling in nonlinear thermoelectrics, we designed and fabricated a set of low-doped n-type InGaAs microrefrigerators.

We used thermoreflectance thermal imaging to obtain the temperature profile of the microrefrigerators under bipolar biasing conditions at different current densities ranging from 100-1500A/cm2. The measurements conducted at cryogenic temperatures (30,50,70K). Temperature profiles as well as DC electrical measurements are input to a hybrid analytical-numerical model, based on the full heat balance equation, in order to extract the Peltier coefficient of the thin film. At cryogenic temperatures, due to the large current and temperature non-uniformity at the top surface of the microrefrigerators, one can extract a coarse estimate of the Peltier coefficient. Strong nonlinearity as a function of current density is observed.

We extracted temperature changes due to the Peltier cooling/heating and the Joule heating. About 8-9K Peltier cooling is achieved at a current density of about 1200A/cm2. Due to excessive Joule heating, the overall cooling performance of the device is not significant. We will discuss design requirements for a nonlinear thermoelectric device to achieve large coolings.