## 3 Results  
### Devices Faturing Pt-ALD Dielectric-Pt Junctions  
#### 3.1.1 Initial Experiments: SETs with H\({}_{2}\)O-Based ALD of the Tunnel Barrier  
The first experimental demonstration of a metal-based SET fabricated using ALD dielectric was reported in [15]. In those experiments, the source/drain leads of the SET were first defined by  
Figure 2: Process steps in fabricating the “half-damascene” cross-tie SET: (**a**) Si substrate (dark grey) is oxidized (SiO\({}_{2}\) in blue); (**b**) a trench for the island is etched through an EBL defined polymethylglutarimide (PMGI) mask (yellow); (c) metal (light grey, e.g. Ni) is deposited; (**d**) the chemical mechanical polishing (CMP) step is performed and metal island is formed; (**e**) ALD dielectric (green) is deposited; (**f**) metal source and drain electrodes (light grey) are deposited; and (**g**) micrograph of half-damascene SET (gate not shown). The nearly vertical line is the inlaid island and the slightly wider, nearly horizontal lines are the source and drain. evaporating 10/20 nm of Ti/Pt through an EBL-defined pattern on a double layer methyl methacrylate (MMA)/polymethyl methacrylate (PMMA) resist, followed by a lift-off in acetone. Before the dielectric deposition, the sample was cleaned in an oxygen plasma barrel etched for 2-3 min to remove any e-beam resist residue from the previous step. Next, a layer of Al\({}_{2}\)O\({}_{3}\) with a target thickness of \(\sim\)1 nm was deposited in a Savannah S100 ALD system by Cambridge Nanotech at 200 \({}^{\circ}\)C using nine cycles of trimethylaluminum Al(CH\({}_{3}\))\({}_{3}\) (TMA) and H\({}_{2}\)O as precursors. Finally, the SET island was defined by a second EBL and deposition of 10/30 nm of Ti/Pt at \(1\times 10^{-6}\) Torr base pressure. The resulting devices (with Pt-Al\({}_{2}\)O\({}_{3}\)-Ti-Pt tunnel junctions) showed characteristic Coulomb blockade oscillations (CBOs) of conductance with applied gate voltage [15], but it was also observed that the electrical characteristics of these devices strongly deviated from that expected for metal-based SETs. According to the orthodox model of Coulomb blockade [16], when a metal-based SET is cooled to a temperature \(T\approx E_{C}/k_{B}\) the differential conductance at \(V_{ds}=0\) mV (where \(V_{ds}\) is the drain to source voltage) decreases towards \(G_{0}/2\), where \(G_{0}=G(\,|\,V_{ds}\,|>>2E_{C}/e)\) due to a reduction in the island occupation probability imposed by Coulomb blockade [17]. For \(\phi_{\text{B}}/e>>|\,V_{ds}\,|>2E_{C}/e\) (where \(\phi_{\text{B}}\) is the tunnel barrier height), the Coulomb blockade is lifted and the SET should exhibit an almost constant differential conductance that approaches \(G_{0}\). However, the zero-bias conductance of Pt-Al\({}_{2}\)O\({}_{3}\)-Ti-Pt SETs in [15] at low temperatures was significantly lower than \(G_{0}/2\), and the temperature dependence of this conductance indicated the presence of an in-series thermally activated component. Moreover, the device was retested 5 weeks after the initial fabrication, and it was found that its conductance at room temperature decreased significantly (by a factor of \(>\)100) while at low temperature G\({}_{0}\) was too low to be measured, indicating a freeze out of carriers, and no evidence of CBOs was observed. It appears reasonable that over time the Ti layer at the bottom of Ti/Pt island reacted with the ALD layer and altered the tunnel barriers, most likely leading to a formation of an insulating ternary Ti\({}_{x}\)Al\({}_{y}\)O compound. This observation is consistent with reports on ternary Ti\({}_{x}\)Al\({}_{y}\)O compounds [18; 19], where a 1000-fold increase of resistance as compared to TiO\({}_{2}\) and a dielectric constant of 62 was reported. Such a large dielectric constant results in a charging energy smaller than k\({}_{\text{B}}\)T and attenuates gate modulation so that CBOs disappear. To avoid this problem, in subsequent experiments the Ti adhesion layer was excluded from the top metal layer, and pure Pt was used as the SET island material.