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(Color online) (a) Control sample layer structure. (b) Strained sample layer structure. (c) Optical image of the cooler device (false color). The central mesa is 222 μm long by 6 μm wide and approximately 100 nm high, with the electron gas restricted to the degenerately doped region in the top 30 nm. Interdigitated aluminum (200 nm thick) contacts form two arrays of 6 μm × 6 μm cooler junctions with the central junction pair used as a thermometer to measure the electron temperature in the middle of the mesa. The cooler voltage VC is applied so that there is no parasitic current through the thermometer. The thermometer is biased using a high impedance battery powered current source (Itherm = 80 pA) and its voltage (Vtherm ) is monitored using a floating differential voltage amplifier.
(Color online) Electron temperature Te vs. cooler bias VC . The green broken curve is a fit to the experimental data (black squares) from the unstrained control. Blue circles are data for the strained sample and the red curve is a fit to the model. The inset shows Te versus VC at different Tb for the strained sample. Note that Tb = Te when VC = 0.
(Color online) Cooling performance of strained (solid circles) and unstrained (open circles) samples for a range of bath temperatures Tb . Minimum electron temperature Tmin at each Tb is obtained from the Te-Vc cooling curves (see Fig. 2). The values shown by the points are Tmin in mK. Solid blue line: calculated Tmin /Tb for the strained sample. Broken blue line: calculated Tmin /Tb for the control sample. Red points from Ref. 3, broken red line is a guide to the eye. Junction cooling powers PC were calculated for VC = 0.323 mV.
(Color online) Calculated cooling power versus electron temperature Te for control (broken curves) and strained (solid curves) devices at a bath temperature Tb of 300 mK. (1) junction cooling power PC at VC = 0.323 mV using Γ from Table I. (2) junction cooling power PC at VC = 0.323 mV for Γ = 0. (A) Pe-ph . (B) Pe-ph + PJ . (C) Pe-ph + PJ + PQP . Filled circle: Tmin for the strained device. Open circle: Tmin for the control device.
(Color online) Calculated Tmin versus RT using different cooling calculations for the strained sample. (1) PC + Pe-ph = 0, Γ = 0, curve (2) PC + Pe-ph = 0, Γ = 0.01 Δ. (3) PC + Pe-ph + PJ = 0, Γ = 0.01 Δ. (4) PC + Pe-ph + PJ + PQP = 0, Γ = 0.01 Δ, β = 0.02. The magenta cross shows the cooling measurement result for the strained sample. Junction cooling powers PC were calculated for VC = 0.323 mV.
Model parameters. Also, Δ = 0.19 meV, ν = 4 × 10−17 m3, and β = 0.02 for both devices. For the strained (control) sample, the sheet resistance was 569 (383) Ω/square.9 Rsm results from 14 squares in parallel between the cooler contacts.
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