## 3 Results
### Devices Faturing Pt-ALD Dielectric-Pt Junctions
#### 3.1.2 O\({}_{3}\)-Based vs. H\({}_{2}\)O-Based ALD of Al\({}_{2}\)O\({}_{3}\) in SETs
Interestingly, the shape of the Coulomb blockade \"dip\", which is a function of the charging energy of the island, did not change noticeably with annealing, indicating that the junction capacitance was not noticeably changed [14]. Analysis by TEM and energy dispersive x-ray spectroscopy (EDX) showed a disproportionate amount of oxygen (compared to aluminum) at the bottom Pt/Al\({}_{2}\)O\({}_{3}\) interface, suggesting that the bottom platinum layer was slightly oxidized during the ALD process [14]. Since platinum oxide is thermodynamically unstable, its presence would explain the change in conductance after annealing for both ozone- and water-based devices and the long-term instability of water-based devices [24; 25]. According to calculations of the change in Gibbs free energy, PtO\({}_{2}\) should be readily reduced by hydrogen even at room temperature. However, exposing ozone-based devices to forming gas (5% H\({}_{2}\) in Ar) at near room temperature (32 \({}^{\circ}\)C) for 1 h caused an increase in the device conductance by about a factor of 30 (0.5 to 15 nS) which is a smaller change compared to that observed after high-temperature annealing in Ar. This suggests not all of the platinum oxide is reduced at low temperatures [14], probably due to slower diffusion of molecular hydrogen and slower reaction rates. Upon raising the temperature, further increases in conductance were observed as can be seen in Figure 4.  
Another observation that supports the idea of platinum oxide formation was the time-dependent decrease in conductance following the forming gas treatments. Figure 5 shows the change of the room temperature conductance of a device over a 23-h period immediately after treatment in forming gas at 90 \({}^{\circ}\)C for 20 min; as can be seen, the conductance drops by almost an order of magnitude. This can be explained by re-oxidation of the Pt electrodes from the edge of the tunnel barriers in the device. As mentioned above, treatment in forming gas at lower temperatures does not appear to completely reduce the parasitic oxide as further increases in conductance were observed when subsequently higher temperatures were used (Figure 4). The time-dependent drop in conductance suggests that  
Figure 4: Measurement of three Pt-Al\({}_{2}\)O\({}_{3}\)-Pt devices from the same die displaying how successive treatments using longer exposures to forming gas and higher temperatures caused further increases in conductance. when the devices are taken out of the anneal chamber into the oxygen-containing ambient, a thin ring of platinum around the edge of the tunnel barrier begins to re-oxidize and \"pinch-off\" the current tunneling through the barrier as shown in Figure 6. At higher temperatures, when most of the platinum oxide is decomposed, this effect is negligible because the majority of the tunneling area is free of the parasitic oxide. Figure 5: Single electron transistor (Pt-Al\({}_{2}\)O\({}_{3}\)-Pt) conductance at room temperature for an O\({}_{3}\)-based device as a function of time immediately after an anneal in forming gas. The decrease in conductance is most likely caused by reoxidation near the edge of the tunnel barrier (Figure 6). Figure 6: Illustration showing a top view of the reduction and reoxidation of Pt-Al\({}_{2}\)O\({}_{3}\)-Pt tunnel barriers. In as fabricated devices (**a**), electrons must travel through both the Al\({}_{2}\)O\({}_{3}\) and the PtO\({}_{x}\) (blue region). This thicker barrier causes low conductance. Treatment with forming gas at 90 \({}^{\circ}\)C only partially reduces the PtO\({}_{x}\) (**b**) but this provides a region where the electrons can tunnel through just the Al\({}_{2}\)O\({}_{3}\) layer (brown area). When brought back out into the oxygen-containing ambient, this more conductive region shrinks as the edge of the tunneling region is re-oxidized (**c**). The same process happens when the devices are treated at 300 \({}^{\circ}\)C but, since all the parasitic oxide is reduced in this case (**d**), the effect of re-oxidation is negligible (**e**).