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This note compares the resonator uniformity for devices with several different Tcs. Devices which are closer to stoichiometric appear to have better uniformity than the lower Tc TiN devices, although the ~0.65K NbTiN device also showed good uniformity. We also show data from the 2.4K TiN device where long negative tails are seen in the phase pulses.
We've tested several higher Tc TiN and NbTiN devices to see if we can improve the uniformity of the devices by using films which are closer to stoichiometric. Fig. 1 compares the frequency shift versus temperature for the 20 resonators on each device. Variations in the fraction frequency shift vs. tempearture between resonators on the same device indicate variation in the gap or alpha across the device.
The stoichiometric (Tc = 4.2K) TiN film has the most uniform temperature response. The largest variation is seen for the 0.5K TiN, while the films with higher nitrogen content have less resonator-to-resonator variation. However, one resonator on the 2.4K TiN device has anomalously high responsivity.
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Fig. 2 shows the Qi variation for each resonator between the different films. The stoichiometric film has the highest Qis as well. The 2.4K film had unusually low internal Qs (~20k).
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In addition to affecting the relative responsivity between resonators, changes in alpha across the device causes the resonant frequencies to spread out from their design bandwidth of 200 MHz. Fig. 3 shows the resonant frequencies of the 20 resonators for each device. All 20 resonators fit within the 340 MHz bandwidth of the readout for each device except the 0.5K TiN, which has a significant dispersion in the resonant frequencies. The median frequency with no kinetic inductance is ~5.3 GHz, so the devices have &alpha=1-(fr/fd)2 varying from ~0.3-0.8.
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From these comparisons, it's clear that the higher Tc films have better uniformity than the 0.5K TiN device. The 0.65K NbTiN device also has good uniformity in the frequency spacing and high internal Qs (unfortunately temperature sweep data wasn't taken on this device, so the uniformity of the responsivity is unclear). We should check if this uniformity is repeatable since the lower Tc NbTiN device has significantly higher responsivity than the higher Tc films. I plan to test another chip from the same wafer in an upcoming run.
During the cooldown of the 2.4K TiN device, a strange power dependence in the phase response was observed for phonon-mediated pulses from the 109Cd source. Figs. 4a/b show averaged pulses from the source for a single resonator as a function of drive power (varying by nearly 20 dB, with the highest power just past the point where discontinuities are visible in the resonance curve). At high power, phase and amplitude pulses following the qp trajectory are observed with a falltime ~30 us. As the power is decreased, a slow (~700 us) negative tail begins to show up in the phase direction only, without a corresponding component in the amplitude direction.
It is not clear if this behavior is unique to the 2.4K device since I usually don't take pulse data at such low powers. I'll check to see if this is reproducible with the lower Tc devices on the next cooldown.
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