Towards realization of an all solid-state quantum-well cooler for sub-dilfridge temperatures

The design for a novel cryogenic solid-state heat pump was recently proposed by this group, based on cyclic expansion and compression of electron subband degeneracy in a 2D quantum well (QW) for sub-Kelvin cooling. The proposed device cools by a factor of T_c/T_h = g₁/g₂ per cycle by electrically tuning the QW subband degeneracy from g₁ to g₂ with a central `paddle’ gate. To realize this device, careful consideration is required when designing the GaAs heterostructure. Doping concentration and QW width will determine how energetically close the QW is to the g₁:g₂ = 1:2 subband crossover when no gate voltage is applied. Spacer thickness should be large enough to prevent significant scattering but small enough to provide sufficient electron doping. The gate voltage required to tune the degeneracy must also be below the dielectric breakdown limit. Simulations of the subband degeneracy and electron density according to front and back spacer thickness and doping are shown to align well with experimental results. Preliminary experimental data suggests the ability to populate/depopulate the 2nd subband as a function of ‘paddle’ gate voltage. Experimental results from device designs with higher electron density will be reported.