# Stability of Single Electron Devices: Charge Offset Drift
## 2 Experimental Observations of Charge Offset Drift
### Metal-Based Devices
Reproduced with permission from reference [41], Copyright 2009, American Physical Society. associated with the presence of an amorphous insulator. Clearly, the blue curve's low frequency noise is much lower than that of the red curve, indicating that this is indeed the case. Other tests have been performed, including: (i) metal deposition method (thermal vs e-beam); (ii) annealing in forming gas or inert gas; (iii) the use of ozone when forming tunnel junctions; (iv) thermal cycling to room temperature; (v) a nano-Faraday cage; (vi) cooling the device with or without electrical protection; and (vii) measuring in the normal _vs._ superconducting state. Unfortunately, none of these other tests substantially reduced the charge offset drift characteristics in metal devices [42]. The above are the basic characteristics of charge offset drift in metal devices. The features shown in the data of Figure 3, particularly that data of Figure 3b, indicate a two-level system as the dominant noise source contributing to \(\Delta Q_{0}\). This conclusion is reinforced with the power spectral density data of Figures 4 and 5. The spectrum shown in Figure 4 follows \(S_{Q}\propto\frac{1}{1+\left(\frac{f}{0}\right)^{2}}\), indicating that these devices are dominated by a single or few TLSs. However, the short-term noise in Figure 5 is dominantly \(1/f\) in character. It has already been shown that the defects can be associated with the presence of amorphous insulators [41]. Since metal devices are usually fabricated on insulating materials but also use AlO\({}_{x}\) to form the tunnel barriers, it is natural to ask in what part of the device these defects are typically located. Presumably, TLSs located anywhere near the device could change their charge by \(e\) and affect the charge offset noise. An appealingly simple argument by [PERSON] et al. [43] concludes that the dominant TLSs must be located in the tunnel barriers of the device. Since the electric field is largest in the tunnel barriers, the defects residing there would induce the largest change in the electrostatic potential of the island. Defects located elsewhere would experience far smaller fields and, therefore, be incapable of producing such large (\(\Delta Q_{0}\geq 0.1e\)) excursions. Within this argument, monotonic or continuous drift would come from capacitively coupled defects located elsewhere but near the device and contribute to higher frequency \(1/f\) noise. Several experiments attempt to locate the defects. The work of [PERSON] et al. [44] used two separate SEDs located 100 nm apart from each other and measured the low-frequency (1-10 Hz) correlations in the charge fluctuations. The expectation is that the experiment would find uncorrelated noise for defects entirely in the barrier and correlated noise for defects residing in the substrate (sputter deposited Al\({}_{2}\)O\({}_{3}\)). They found an appreciable level of correlation between the devices, as much as 20%. Thus, this experiment shows that the level of noise coming from the substrate material is comparable to that coming from the barriers and renders the argument of [PERSON] et al. incomplete. The experiment depicted in Figure 4 (ref. [40]) also located the defects outside of the tunnel barriers by comparing the switching rates as a function of the plunger gate voltage to that expected for TLSs located in the barriers. As can be seen in the inset of Figure 4b, the switching rate is a monotonic and roughly exponential function of the gate voltage. This is in contrast to the voltage across a tunnel junction in the device, which should have a periodic dependence on gate voltage with period \(e/C_{g}=0.4\) mV for this device. Whatever the identity and the functional form of the defects' switching rate, the dependence on gate voltage should have some periodic character and not a large-scale dependence as shown. Thus, many of the defects contributing to \(\Delta Q_{0}\) are outside the tunnel barriers. Additionally, the data were found to be consistent with a cluster of TLSs which