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Appl. Sci. 2017, 7(3), 246; https://doi.org/10.3390/app7030246

Review
Metal-Insulator-Metal Single Electron Transistors with Tunnel Barriers Prepared by Atomic Layer Deposition
1
Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA
2
Lashkaryov Institute of Semiconductor Physics, 03028 Kyiv, Ukraine
3
Present address: Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, CA 94720-1770, USA
4
Present address: Intel Corp, 2501 NW 229th Ave., Hillsboro, OR 97124, USA
5
Present address: Cypress Semiconductor Corp, 2401 East 86th St., Bloomington, MN 55425, USA
*
Author to whom correspondence should be addressed.
Academic Editor: Antonio Ficarella
Received: 16 January 2017 / Accepted: 27 February 2017 / Published: 3 March 2017

Abstract

:
Single electron transistors are nanoscale electron devices that require thin, high-quality tunnel barriers to operate and have potential applications in sensing, metrology and beyond-CMOS computing schemes. Given that atomic layer deposition is used to form CMOS gate stacks with low trap densities and excellent thickness control, it is well-suited as a technique to form a variety of tunnel barriers. This work is a review of our recent research on atomic layer deposition and post-fabrication treatments to fabricate metallic single electron transistors with a variety of metals and dielectrics.
Keywords:
single electron transistor; atomic layer deposition; tunnel barrier

1. Introduction

Single electron tunneling transistors (SETs) utilize the Coulomb blockade effect experienced by electrons travelling through a nanoscale “island” between the source and drain. The transport mechanism responsible for transfer of charge is quantum mechanical tunneling through thin layers of dielectrics connecting the island to the external electrodes, thus forming metal-insulator-metal (MIM) junctions. (In actuality, the devices discussed in this paper have M-I-M-I-M structure. We will refer to them as “MIM” SETs for the devices made using the same metal for the electrodes and island; if two different metals are used for the island and source/drain electrodes, the full sequence of materials will be presented; we will use the generic term “metal-based SET” for any SET device which uses metals (as opposed to semiconductors) for the island and electrodes.) Since the first experimental demonstration almost 30 years ago [1], the dominant technique for making the ultrathin tunnel barriers required for SET operation has remained the controlled oxidation of the metals forming the devices. The performance of SETs is critically dependent on the quality of the dielectrics forming the tunnel barriers. While oxidation of metals (Al, Nb, Cr, Ni, and Ti) to form tunnel junctions is possible, the quality of the resulting dielectric suffers from several detrimental effects. A very limited number of metals can form stable defect-free oxide dielectrics suitable for quantum tunneling. Devices fabricated using chromium [2,3] and titanium [4] oxides exhibit instabilities of junction resistance and excess noise caused by charge traps in the barriers. In addition, metal oxides such as TiOx and NiO have the disadvantage of low tunnel barrier heights (~100 meV for TiOx [4] and ~200 meV for NiO [5]), which makes it difficult to operate devices at high temperatures. The most widely used metal for nanoscale MIM junctions is aluminum. It has the advantage of easily forming a stable oxide with a large band gap and band offset suitable for forming MIM junctions. Consequently, Al-based SETs have by far the most mature fabrication process, with the lowest noise [6] and the largest charging energy [7] among all MIM SET devices. Nonetheless, Al-based SETs still suffer from several detrimental effects. Zimmerman et al. [8] demonstrated that alumina (Al2O3) formed by controlled oxidation exhibits unquenchable long term drift in device characteristics. Rippard et al. [9] showed that there is a broad energy distribution of electron states inside Al2O3 and these states can provide low-energy single-electron channels through the oxide. Therefore, the search for suitable high quality dielectrics needs to be continued.
One promising fabrication method for the controllable formation of tunnel barriers is Atomic Layer Deposition (ALD), a self-limiting process that ideally allows the deposition of films, including non-native oxides, with single monolayer accuracy. In this paper we summarize experimental results of MIM SETs fabricated with electrodes of either noble metals (Pt and Pd) or a metal that allows chemical reduction of its native oxide (Ni). The formation of the tunnel barriers, which are not a native oxide of the metals used, is by ex-situ atomic layer deposition of ALD using Al2O3, SiO2, their combination, as well as SiNx. Two ALD tools were used in fabrication: a Savannah ALD system from Ultratech/Cambridge NanoTech with an integrated ozone generator and a FlexAl ALD system from Oxford Instruments with a remote plasma reactor which enables plasma-enhanced ALD (PEALD).

2. Materials and Methods

To investigate the applicability of ALD to tunnel barrier formation, we fabricate “cross-tie” SETs, as illustrated in Figure 1. The process involves fabrication of two metal layers defined by electron-beam lithography (EBL, Vistec, Jena, Germany) with ALD dielectric in between. The general approach for fabrication of SETs in the cross-tie geometry is outlined below, while more fabrication details are given within the results subsections.
In the first step, the bottom metal layer defining the source and drain electrodes (20–40 nm thick, 50 nm wide) is deposited by electron beam evaporation through the EBL-defined mask on 500 nm of thermally grown SiO2, as shown in Figure 1a. After liftoff and cleaning, the dielectric barrier is deposited using ALD (Figure 1b). Finally, the top metal layer forming a cross-tie island (20–50 nm thick) is defined by evaporating the metal through the EBL-defined mask and liftoff (Figure 1c). In cross-tie SETs, inert metals such as platinum (Pt) and palladium (Pd) were studied as electrode materials to avoid the influence of native metal oxides in the MIM junctions.
The resulting structures feature two nominally identical junctions, the capacitance of which can be estimated from a parallel-plate approximation Cd = Cs = εε0A/d where ε0 and ε are the vacuum permittivity and the dielectric constant of a chosen ALD dielectric, A is the junction area defined by the overlap between the island and each lead, and d is the thickness of the barrier. For a 1 nm thick SiO2 (Al2O3) tunnel barrier with a junction area of ~4000 nm2 (Figure 1d), the parallel plate model gives a capacitance value of ~180(350) aF (using bulk values of ε = 3.9 for SiO2 and ε = 9 for Al2O3). The charge population of the island is controlled by an electrostatic gate situated 0.5 to 5 um away from the island (shown in Figure 1d) resulting in an experimentally determined capacitance of 0.1 to 30 aF. Assuming a gate capacitance much smaller than the junction capacitance, the total island capacitance for device is, therefore, ~350–400 aF for an SiO2 barrier and ~700–900 aF for an Al2O3 barrier.
This sum of the capacitances CΣ defines the figure of merit for an SET, the charging energy EC = e2/2CΣ [10], where e is electron charge, the calculated value of which is ~0.2 meV for an SiO2 barrier or 0.1 meV for Al2O3. Thus, the chosen design enables observation of well-developed Coulomb blockade at an experimentally attainable temperature T = 0.3 K << EC/kB where kB is the Boltzmann constant.
A modification of this process, known as “half damascene” fabrication, is used to decrease the overlap of the source and drain with the island by fabricating an island inlaid in the substrate dielectric (Figure 2) [11]. This lowers the capacitance of the junctions by narrowing the width of the island and avoiding the side overlap always present in cross-tie design and thus raises the maximum operating temperature of the device. To fabricate the inlaid island devices, the pattern of the island is etched into the oxide in an inductively coupled plasma (ICP, Oxford, Abingdon, UK) etcher [12] (Figure 2b). Metal is deposited by e-beam evaporation to fill the trench in the SiO2 (Figure 2c) and chemical mechanical polishing (CMP, Logitech, Glasgow, UK) is then used to remove any residual metal on the oxide field, while leaving the island trench filled with metal (Figure 2d). CMP enables a very significant decrease in the junction area and can be used to fabricate a full-damascene metallic SET [13]. Next, ALD is used to form the tunnel barrier over the island (Figure 2e). Finally, the source and drain are defined using the same process as for the cross-tie devices (Figure 2f). The overlap area of the junctions prepared by these techniques was in the range 500–1000 nm2 (Figure 2g, gate not shown). Source and drain junctions of 1000 nm2, together with a gate capacitance of 0.1 aF, result in an island capacitance of 70 aF. With this capacitance, the charging energy is expected to exceed 1 meV, allowing well-developed Coulomb blockade to be observed at temperatures up to ~3 K.
The metal used in half-damascene and full-damascene SET structures was Ni due to its good adhesion to SiO2, relative oxidation resistance, and well-established CMP process.
The thickness of the barrier is estimated using the known deposition rate for the ALD process and has been validated by transmission electron microscopy (TEM, FEI, Hillsboro, OR, USA) [11,14]. In order for the electron wave function to be localized to the island the junction resistance RJ must satisfy the following condition: RJ > RQ [10] (where RQ = h/e2 is the quantum resistance, h is Planck’s constant, and e is the elementary charge). Our experiments show that to fulfill this condition at least 9 Al2O3 ALD cycles or 12 SiO2 ALD cycles (~1 nm in both cases) are required to form the tunnel barrier dielectric; otherwise, the number of devices with short circuited junctions exceeds 50%.
Following fabrication, devices are wire bonded to a ceramic chip carrier and placed in a closed-cycle refrigerator (CCR) for electrical characterization. Two CCRs were used in this study: a 4He CCR with a base temperature ~3.5 K (Advanced Research Systems, Macungie, PA, USA), and a 3He CCR with a base temperature of ~0.3 K (Janis Research Company, Woburn, MA, USA); the temperature of the sample is monitored by a thermometer attached to the chip carrier. Low temperature characterization was performed on a total of more than 100 devices (Pt-ALD dielectric-Pt and Ni-ALD dielectric-Ni structures) resulting from more than 10 fabrication runs.
Electrical measurements of differential conductance of the devices, G = dI/dVds, are performed using standard lock-in techniques with a 10–1000 μV excitation voltage at low frequencies (8–38 Hz). For spectral analysis of noise, a spectrum analyzer (based on a UHF lock-in amplifier from Zurich Instruments, Zurich, Switzerland) connected to the output of the current amplifier was used.

3. Results

3.1. Devices Featuring Pt-ALD Dielectric-Pt Junctions

3.1.1. Initial Experiments: SETs with H2O-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 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 etcher for 2–3 min to remove any e-beam resist residue from the previous step. Next, a layer of Al2O3 with a target thickness of ~1 nm was deposited in a Savannah S100 ALD system by Cambridge Nanotech at 200 °C using nine cycles of trimethylaluminum Al(CH3)3 (TMA) and H2O as precursors. Finally, the SET island was defined by a second EBL and deposition of 10/30 nm of Ti/Pt at 1 × 10−6 Torr base pressure. The resulting devices (with Pt-Al2O3-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 TEC/kB the differential conductance at Vds = 0 mV (where Vds is the drain to source voltage) decreases towards G0/2, where G0 = G(|Vds| >> 2EC/e) due to a reduction in the island occupation probability imposed by Coulomb blockade [17]. For φB/e >> |Vds| > 2EC/e (where φB is the tunnel barrier height), the Coulomb blockade is lifted and the SET should exhibit an almost constant differential conductance that approaches G0. However, the zero-bias conductance of Pt-Al2O3-Ti-Pt SETs in [15] at low temperatures was significantly lower than G0/2, 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 G0 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 TixAlyO compound. This observation is consistent with reports on ternary TixAlyO compounds [18,19], where a 1000-fold increase of resistance as compared to TiO2 and a dielectric constant of 62 was reported. Such a large dielectric constant results in a charging energy smaller than kBT 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. A Coulomb diamond plot or “charge stability diagram” [10] of a typical Pt-Al2O3-Pt device fabricated with nine cycles of ALD Al2O3 (~1 nm) is shown in Figure 3a. One immediately noticeable difference between this device (with Pt-Al2O3-Pt junctions) and the one reported in [15] (with Pt-Al2O3-Ti-Pt junctions) is that the value of the zero-bias differential conductance measured in a peak of the CBOs is much closer to the expected value G(Vds = 0 mV) ≈ G0/2 (Figure 3b). Further analysis of the plot by fitting it to the orthodox model [16,20] enables estimation of the junction parameters (CS = 240 aF; GS = 35 μS; Cd = 160 aF; Gd = 45 μS; Cg = 32 aF (the value of gate capacitance can be extracted from the period of CBOs, Cg = e/dVg, where dVg is a voltage difference between adjacent CBO peaks) and charging energy EC = 0.185 meV. The total capacitance (C = 432 aF) is lower than expected from the parallel plate model estimate (~700–900 aF) potentially due to the low dielectric constant of very thin Al2O3 films compared to the bulk value [21]. Unexpectedly, the differential conductance of the devices steadily increased over the course of several months, approaching values expected for shorted metal structures with no dielectric barriers (~1 mS). This clearly indicates the instability of the Al2O3 tunnel barrier on a Pt substrate using TMA and H2O precursors. We will discuss a probable cause for this below.
Another metal studied in combination with H2O-based ALD Al2O3 was Pd. However, devices fabricated with Pd as a metal onto which ALD Al2O3 was deposited exhibited a much broader spread of junction resistances compared to Pt-based devices, possibly due to greater surface roughness of the Pd film compared to the Pt film, as shown by atomic force microscopy measurements [22].

3.1.2. O3-Based vs. H2O-Based ALD of Al2O3 in SETs

Initial experiments described above indicated that H2O-based ALD of tunnel barriers resulted in functional devices, but these suffered from intrinsic barrier instabilities resulting in irreversible detrimental changes in the device performance. An alternative method for forming Al2O3 in the Cambridge Savannah ALD reactor is to use a different oxidizer by replacing H2O with O3 from a dedicated ozone generator. To investigate the difference between H2O-based ALD and O3-based ALD, two batches of Pt-Al2O3-Pt SET devices were fabricated with an equal number of ALD cycles using ozone and water [14]. It was discovered that SETs that used H2O-based ALD showed much lower resistance than SETs fabricated with O3-based ALD for a given number of ALD cycles. For both batches, nine cycles of ALD (~1 nm in thickness) were used as this was found to be an optimal balance between complete coverage of the barrier over the electrodes and measureable tunneling currents. Structures fabricated using H2O showed an average resistance of around 1 MΩ with a very significant spread of resistances ranging from tens of kΩ to several MΩ. Again, after a few weeks at ambient conditions, the resistance of these devices dropped to about 10 kΩ. In contrast, the as-prepared, O3-based devices showed an average resistance of almost 1 GΩ. Low temperature (~5 K) testing of both O3- and H2O-based devices revealed, in addition to a “dip” near Vds = 0 mV related to Coulomb blockade, a temperature dependent monotonic increase of conductance with increasing bias outside of the Coulomb gap (|Vds| > 2Ec/e) contrary to orthodox Coulomb blockade theory [16]. This indicates the presence of a parasitic non-metallic component in series with the tunnel barrier in the junctions.
One technique frequently used to improve the quality of thin films is annealing, which can passivate fabrication defects [23]. However, the results of annealing the SETs after fabrication (375 °C for 3 min in an Ar environment) differed drastically depending on the oxidant used in the ALD process for devices fabricated with the same number of deposition cycles. Namely, most of the H2O-based devices developed electrical shorts after annealing. In contrast, the resistance of devices fabricated with O3-based ALD dropped by approximately two orders of magnitude after annealing under the same conditions, yet they did not develop any shorts. Instead, the resistance became quite uniformly reduced to a few MΩs and remained stable over time (for at least six months, the longest observation time available). When tested again at ~5 K with |Vds| > 2EC/e applied, the annealed O3-based devices displayed almost constant differential conductance, as expected from orthodox Coulomb blockade theory. (The value of the charging energy in these experiments−as estimated above from the parallel plate model−is on the order of 5 K, precluding observation of developed Coulomb blockade at the temperature of experiment (~5 K).) 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/Al2O3 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, PtO2 should be readily reduced by hydrogen even at room temperature. However, exposing ozone-based devices to forming gas (5% H2 in Ar) at near room temperature (32 °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 °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 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 7 shows the Coulomb diamond plot measured at 0.35 K for O3-based devices treated with forming gas at 90 °C in a single post-fabrication step. The performance of these devices, however, was hampered by unusually high levels of excess noise far exceeding Johnson and shot noise levels typical for MIM SETs [26]. This noise is identified as random telegraph signal (RTS) noise [27], a topic that will be discussed below. Figure 7b shows a series of G(Vds) characteristics measured in the peak of CBO at several temperatures for an O3-based device treated in forming gas. It is clear that the conductance in the peaks of CBOs, instead of saturating at a level of G0/2 (~55 nS in Figure 7b) as expected for MIM SETs, continues to drop with the lowering of temperature. An estimation of activation energy in the peak of blockade gives a value of less than 0.05 meV, typical for granular metals [28,29]. This observation suggests that a thin layer of platinum oxide survived the forming gas treatment and its presence in series with the Al2O3 tunnel barrier results in the weak activation behavior in the peaks of CBO at Vds = 0 mV [29].
A comparison of experimental results obtained for two batches of devices featuring Pt-Al2O3-Pt junctions (Figure 3 and Figure 7) with the exact same number of ALD cycles (nine) and similar overlap areas for the junctions shows a significant difference in charging energy. Namely, the charging energy for the H2O-based device of Figure 3 (EC ≈ 0.185 meV) as compared to the O3-based device of Figure 7 (EC ≈ 0.37 meV) was about two times smaller, while its conductance was about two orders of magnitude larger. This difference in charging energy is evidently caused by a difference in the junction capacitances and for the same area of the junctions a doubling of the dielectric thickness would result in half the capacitance and exponentially larger resistance, in reasonable agreement with the experiments. (This is admittedly just an “order of magnitude” estimate, because in both cases (Figure 3 and Figure 7) the shape of the diamonds also indicates a significant difference in junction capacitances in each device, most likely caused by non-uniformities in the dielectric thickness and uncertainty in the junction area.) This observation suggests that the actual thickness of a dielectric is different for the two different oxidant precursors with the same number of deposition cycles. Namely, the actual thickness of the dielectric in the water-based devices was about half of what was expected from the number of deposition cycles. The likely cause for this is the different effects of H2O and O3 oxidizing pulses on the nucleation delay and metal surface oxidation. This again provides evidence for the more aggressive action of O3 in forming PtOx (and potentially more uniform coverage) compared to H2O. Note that in both cases values of junction capacitances are consistent with a reduced dielectric constant, ε < 5, compared to the bulk value (ε ≈ 9) [21,30]. The above experiments also provide evidence that the formation of native oxide and interfacial layers, even on noble metals, can drastically change the properties of the MIM junctions.
PEALD of SiO2 using O2 plasma and Bis(diethylamino)silane was also attempted on platinum. Using a stencil mask to pattern 140 μm diameter capacitors, it was found that a continuous layer of SiO2 was not formed until roughly 100 cycles of SiO2 (~10 nm) had been deposited. This suggests the formation of a non-uniform SiO2 film (in the form of islands) on the Pt substrate. It was unknown, however, whether the density of pinholes was high enough to prevent the fabrication of pinhole-free nanoscale SET junctions. To test this, a single experiment was run using 15 cycles of SiO2 (~1.5 nm) to form Pt-SiO2-Pt SETs using the cross-tie fabrication process. All of these devices (~40) were shorted, supporting the theory that the film has a high density of pinholes where SiO2 never nucleated. This, together with the previous experiments, supports the hypothesis that ALD nucleation on inert metal substrates, such as platinum and palladium, is a challenge and limits the choice of materials and methods for deposition.

3.2. Devices Featuring Ni-ALD Dielectric-Ni Junctions

Our experimental results obtained with Pt electrodes indicate that a fundamental issue with the use of ALD tunnel barriers in MIM junctions is that the presence of the oxidizing pulse (H2O or O3) will likely lead to partial oxidation of the metals in the SET. While the formation of native oxide promotes the nucleation and growth of the ALD dielectric, it drastically reduces the transparency of tunnel barriers making them unusable for target applications.
In order to solve the problem of ALD nucleation and the presence of parasitic native oxide, two different approaches were considered: (1) to use a metal that is more susceptible to oxidation than noble metals but whose oxide can be chemically reduced after the ALD dielectric step; and (2) to use an ALD process in which the tunnel barrier formation does not require an oxidizing pulse. Nickel was selected for the first approach because of its relative inertness, compared to Al and Ti, and the reducibility of its native oxide at temperatures that will not damage the barrier dielectric or cause deformation of the metal layer. SiNx ALD was selected for the oxygen-free tunnel barrier formation where the N2 plasma replaces the O2, O3, or H2O pulse and therefore parasitic formation of the native oxide can be minimized. Below we discuss the results obtained using Ni-dielectric-Ni junctions with tunnel dielectric deposited in the Oxford FlexAL PEALD reactor.

3.2.1. SET Devices with Ni-SiO2-Ni Tunnel Junctions

As discussed above, two issues must be addressed in order to successfully fabricate MIM SETs using an ALD process: the unavoidable oxidation of metals and determination of the appropriate number of cycles to form a pinhole-free tunnel barrier with desired transparency (RQ < RJ < 104 RQ). For that purpose two types of test structures were fabricated on an oxidized Si wafer: (1) single layer nanowires covered with PEALD SiO2; and (2) liftoff-based cross-tie devices made of Ni-SiO2-Ni tunnel junctions (as described earlier). The PEALD of SiO2 was performed using Bis(diethylamino)silane and oxygen plasma with a nominal growth rate of 0.09 nm/cycle.
For cross-tie devices, reference structures (“metal short circuits”) with no PEALD dielectric and without any associated oxygen plasma exposure were first fabricated and exhibited conductance of G ≈ 400 μS, which is comparable with the conductance of a Ni nanowire with similar dimensions. However, devices with only two cycles of SiO2 PEALD (≈0.2 nm) displayed a conductance below G ≈ 5 nS. This value is about four orders of magnitude smaller than the expected value G ≈ 50 μS based on the Simmons approximation for 0.2 nm of uniform SiO2 [31]. Furthermore, at low temperatures, strong suppression of conductance at Vds = 0 mV along with a significant decrease of conductance over a broad range of Vds was observed (Figure 8), which contradicts orthodox Coulomb blockade theory for MIM SETs [16]. This behavior is, as it was in the case of Pt-Al2O3-Pt devices, consistent with a parasitic NiO layer (or a granular metal NiO-Ni mix; we will refer to it as NiO for short) in series with the tunnel junctions (i.e., [Ni-NiO]-SiO2-[NiO-Ni] instead of Ni-SiO2-Ni), as it modifies the overall barrier for electron transport and leads to a thermal activation of conductance at low bias [5]. The oxygen plasma step in PEALD has been previously reported to oxidize the surface of the substrates [32,33,34]. Measurements of the decrease in conductance in single layer nanowires after being covered with 10 cycles of PEALD SiO2 (~0.9 nm) indicate that the NiO layer is about 2 nm thick. Fortunately, NiO can be reduced to Ni by annealing in hydrogen at moderately high temperatures ≥300 °C [35,36,37]. We have found that a forming gas anneal (FGA) at 400 °C for 2 min in 5% H2–95% Ar causes the conductance of the cross-tie structures with two cycles of PEALD SiO2 to increase from G ≈ 5 nS to G > 600 μS [11], while a 30 min FGA is required to increase the conductance of the Ni nanowire covered by ~1 nm of SiO2 back to that of the as-deposited nanowires [11,38].
This is consistent with hydrogen-promoted reduction of the parasitic NiO to Ni: H2 + NiO = H2O + Ni; the H2O is removed during annealing [11,38]. Further measurements of cross-tie devices by TEM and EDX revealed that in addition to the oxide formation on the bottom Ni layer, the top Ni electrodes, deposited on the PEALD SiO2, are likewise oxidized and that a second annealing step is required to reduce the parasitic NiO at the lower interface of the top Ni layer [11].
Based upon our observations, all cross-tie devices with less than 12 cycles of SiO2 (<1.1 nm) subjected to two FGA steps ((1) for bottom electrode NiO reduction: 30 min, 400 °C and (2) for top electrode NiO reduction: 10 min, 300 °C both in 5% H2–95% Ar) exhibited fairly high conductance, G > 50 μS > 1/RQ. This is most likely due to non-uniformities in the deposited SiO2, which lead to “pinholes” short-circuiting the top and bottom Ni electrodes [39]. However, for devices with 12 or 13 cycles treated with two anneal steps, the conductance matched the Simmons equation within an order of magnitude, suggesting the ALD growth had filled in the pinholes and both NiO layers had been mostly reduced.
The cause of oxidation in the top Ni electrodes remains to be investigated, but the oxidation of metals when deposited on SiO2 has been reported for Cu, Mo, and W and has been attributed to oxygen containing contaminants, such as water, on the oxide surface [40,41]. Furthermore, substituting aluminum for nickel as the top electrode (Ni-SiO2-Al) resulted in devices that were all electrical opens (i.e., no measurable tunneling current). This can be explained as resulting from the aluminum layer oxidizing at the surface where it makes a contact with the SiO2 film, just as the nickel layer had, but causing an even greater increase in resistance due to the large band gap of aluminum oxide. Likewise, substituting palladium for nickel on the top layer (Ni-SiO2-Pd) shows behavior indicative of the top palladium layer oxidizing at the metal-SiO2 interface but unlike Al2O3, this oxide can be reduced. These experiments suggest that the oxidation of the top metal layer is unavoidable (at least for oxide dielectrics such as Al2O3 and SiO2) and that reducing the NiO at the top interface is therefore just as necessary as doing so at the bottom interface [11].
From the experiments described above it was concluded that 12–13 PEALD cycles of SiO2 (~1.1 nm) are suitable for cross-tie SETs because the resulting conductance is sufficiently below the quantum conductance, GQ = 1/RQ ≈ 40 μS, required to suppress quantum fluctuations [16], but high enough (>10 nS) to avoid signal-to-noise ratio problems.

3.2.2. Limitations of the “Double Anneal” Reduction Technique and Search for Alternative Reduction Methods

As described above, the proposed process for fabricating Ni-SiO2/Al2O3-Ni SETs requires two reduction steps to eliminate the parasitic NiO in series with the tunnel junctions. However, prolonged exposure of the device to elevated temperatures (>375 °C) results in thermally induced deformation of the Ni nanowires, referred to as agglomeration [42] and consequently open circuits. Figure 9 shows a micrograph of agglomeration occurring in an SET device as a result of FGA at 400 °C.
In addition to the thermally-induced deformation of thin metal films during anneal, the experiments also reveal very significant “switching” RTS noise, similar to that observed in Pt-Al2O3-Pt devices (Figure 7). Figure 10a shows a measurement of conductance G(Vds) with high Vds resolution of an SET device subjected to two FGA treatments. The experimental data highlight the important feature of this noise: for |Vds| < 2Ec/e where Coulomb blockade is present, the conductance jumps between two or more states, but appears remarkably stable once Coulomb blockade is suppressed for |Vds| >> EC/e. By contrast, the sweep of gate bias, Vg, at Vds = 0 mV (Figure 10b) reveals very strong intensity of the RTS noise almost uniformly occurring along the Vg axis. This behavior is consistent with random charge trapping/detrapping processes in the vicinity of the island (e.g., in the barrier dielectric) acting as a strong modulation randomly shifting the CBOs along the Vg axis. However, once Coulomb blockade is overcome (by applying |Vds| >> EC/e), these fluctuations are suppressed because the gate can no longer modulate the conductance.
Because of the detrimental effects on device performance associated with FGA, hydrogen plasma treatment has been investigated as an alternative way to reduce NiO [43,44]. This technique has the advantages of using a lower temperature and less time than FGA, and simultaneously passivating the defects residing in the tunnel barrier dielectric [45]. With plasma, various hydrogen species (H+, H, H2) capable of reacting with NiO are introduced into the process. The remote radio frequency hydrogen plasma (RHP) source in the chamber of the Oxford FlexAl PEALD reactor enables this in-situ treatment of the as-prepared dielectric film. To investigate this technique, following fabrication of the island and 12 cycles of PEALD SiO2 (~1.1 nm), the sample was subjected to RHP (pressure of 10 mTorr, plasma power of 100 W at 300 °C) for 5 min, followed by definition of the source and drain electrodes using EBL as described above. The resulting average room temperature differential conductance (0.1 μS) was higher than any of the devices prepared with two FGA reduction steps and it had a broader distribution of conductance values within a batch. Despite larger conductance values at 300 K, at low temperature (<10 K) devices in which the ALD dielectric was exposed to RHP exhibit a monotonic increase of conductance with applied bias at |Vds|>>EC/e and a conductance G(Vds = 0 mV) << G0/2 significantly lower than expected at the peaks of CBOs (Figure 11a). Once again, this behavior is indicative of residual NiO at the interface of the oxide and top metal where thermal activation may become a limiting factor in conductance, as mentioned previously. Similar to the previously discussed devices (Figure 7b and Figure 10b), a high level of RTS noise was also observed in CBO characteristics of these devices (Figure 11b).
To achieve a more complete reduction of residual NiO in the tunnel junctions, a second RHP treatment was performed on the completed devices for 5 min under identical conditions. This treatment increased the conductance further, from an average ~0.1 μS to ~3 µS at 300 K, the highest average conductance among all fabricated batches. We attribute this relatively high conductance, observed both before and after the final RHP treatment, to hydrogen-plasma induced thinning of the PEALD dielectric during the time when the uncovered tunnel barrier is subjected to the first RHP step [46]. At low temperature the majority of devices subjected to two consecutive RHP exhibit significantly improved SET characteristics (Figure 12a), most notably, with a very strong suppression of RTS noise at Vds ~ 0, as compared to the single RHP treated devices in Figure 11, or FGA treated devices (Figure 7 and Figure 10). However, in the peaks of the CBOs, the conductance still remains noticeably lower than expected (G << G0/2) and the G(Vds) characteristic still exhibits signs of non-linearity even at |Vds| >> EC/e evidently due to incomplete reduction of NiO (Figure 12a).
Furthermore, above a certain voltage threshold VTh ~ 5EC/e, a very large noise “floods” the measurement bandwidth to the point that it exceeds the ability of a lock-in amplifier to reject the incoherent signals (Figure 13a). The spectral density of this noise abruptly becomes about two orders of magnitude larger than that at low Vds and it appears to be of Lorenzian shape, typical for RTS trap recombination noise [47]. However, unlike the RTS noise visible in in Figure 10a, this noise is only present outside of the Coulomb blockade region and has a distinctly different frequency cut off (several hundred Hz), suggesting that some other mechanism, other than trap-modulation of Coulomb blockade, is responsible for its appearance. For example, it may be caused by population/depopulation of relatively deep traps located about ~7.5 meV above the Fermi level in the remaining NiO layer.
To summarize, we have demonstrated that two hydrogen-based treatments are required to achieve restoration of MIM characteristics of the junctions: by using two FGA or two RHP we achieved improvements in the device performance but both techniques have their limitations. The double FGA technique results in the appearance of large switching RTS noise and may lead to agglomeration, resulting in discontinuities, while the double RHP technique, though mitigating the RTS noise, leads to incomplete NiO reduction accompanied with thinning of the ALD dielectric layer.

3.2.3. Optimization of Ni-SiO2-Ni Junction Fabrication

The optimization of Ni-based MIM SETs with SiO2 PEALD dielectric was investigated using an approach that exploits the benefits and minimizes the weaknesses of both FGA and RHP treatments. We developed a process flow (referred to as FGA+RHP) where the first hydrogen-based treatment performed after barrier deposition is an FGA at 400 °C, because the main purpose of this step is to achieve complete reduction of the relatively thick parasitic NiO on the first Ni layer without damaging the SiO2 barrier. At this stage of the fabrication, the byproduct of the process, H2O, easily diffuses through a thin dielectric layer and is removed. For the second treatment, the primary goal is mainly to anneal defects in the PEALD SiO2 dielectric with a secondary objective of reducing the NiO layer on the bottom of the upper electrodes.
For this purpose, we performed a study comparing double FGA with FGA+RHP treatments [48]. Using two dice originating from the same oxidized Si wafer, 50 nm-thick inlaid Ni islands are defined in the SiO2 substrate using EBL, ICP, Ni evaporation, and CMP. Next, 12 layers of SiO2 as the dielectric tunnel barrier are deposited by PEALD on one sample, and a stack of 10 SiO2 + 2 Al2O3 is deposited on the second sample. Subsequently, the samples are subjected to a 10-s O2 plasma ashing step to clear any residual carbon on the deposited tunnel barrier. Next, the samples are treated with a 25 min-FGA at 400 °C in 5% H2–95% Ar. The source and drain leads are then defined in a second EBL, followed by an O2 plasma de-scum, metal evaporation, and liftoff. After completion of the fabrication process, the devices were tested electrically at 300 K and they all exhibited very low conductance, averaging on the order of 100 pS, indicative of the additional NiO layer on the bottom of the source/drain electrodes [11]. At this stage, the dice are separated into two separate pieces for the final NiO reduction treatment: A1 and B1 with 12 layers of SiO2, and A2 and B2 with a stack of 10 layers of SiO2 + two layers of Al2O3. Overall, more than 400 devices were fabricated in these four pieces. The A1 and A2 pieces were then used as reference samples and subjected to a second FGA for 10 min at 375 °C. The samples B1 and B2 were subjected to RHP (with the same conditions as described in Section 3.2.2 of this report). The average conductance at 300 K in all samples after the second treatment increased by a factor of ~103.
Figure 14 shows the comparison of charging diagrams for devices fabricated using two FGA steps (batches A1 and A2, Figure 14a) vs. devices fabricated using FGA+RHP (batches B1 and B2, Figure 14b). Most importantly, all of the measured devices from batches B1 and B2 clearly show the disappearance of gate-independent excess noise typical of double-FGA devices from batches A1 and A2 (Figure 14a): diamonds are now clearly visible in Figure 14b. A comparison of G(Vds) scans also shows the significant decrease in noise as can be seen in the two side-by-side G(Vds) cross sections of Figure 14. It is quite clear that in the recipe that combines FGA treatment for the SiO2-covered island with subsequent RHP treatment of the finished devices, a drastic reduction of switching RTS noise is obtained along with electric characteristics much closer to that expected for MIM junctions [48].
Let us now compare the obtained experimental results with the expected performance based on the dimensions of imaged SETs (Figure 2g). In the case of identical junctions and a gate capacitance much smaller than that of the junctions, Cs = Cd >> Cg, the absolute values of the slopes of the Coulomb diamonds (positive slope, α = Cg/(Cs + Cg), and negative slope β = −Cg/Cd) are expected to be nearly the same, and the highly asymmetric shape of the diamonds (e.g., Figure 7a and Figure 11) thus indicates a variation in the thickness of the two tunnel junctions most likely due to non-uniformities in the barrier dielectric. Figure 15a shows experimentally obtained data from one FGA+RHP treated device with nearly identical slopes |α| ≈ |β|. Simulations performed based on an orthodox theory model [20] enable fairly accurate extraction of the junction parameters. While variations of junction capacitances (~20%) and conductances (<50%) are fairly reasonable given potential variations in the thickness and junction areas, the absolute values of the capacitances appear to be significantly larger than expected from the parallel-plate model: CdCs = ε0εA/d = 28–40 aF (ε = 3.9 for SiO2, A = 900–1200 nm2, and d = 1.17 nm, the nominal thickness of 13 cycles of PEALD SiO2). A comparison of dimensions for devices fabricated using the cross-tie technique (Figure 1) and half-damascene process (Figure 2) shows a very significant narrowing of the island produced by the half-damascene technique. This is clearly beneficial because it lowers the junction capacitance and thus increases the charging energy. However, 3D simulations of the island total capacitance in half-damascene geometry show that the island capacitance can be more than double what would be expected from the parallel plate model due to fringing fields (Figure 16b). Thus the 3D capacitance simulation of Figure 16b (CIsland = 121 aF) provides a good match to the fitted simulation of Figure 15b (CIsland = Cd + Cs + Cg = 110 aF) and shows that fringing fields cannot be ignored for very small junctions.

3.2.4. SET Devices with Ni-SiNx-Ni Tunnel Junctions

As mentioned earlier, two approaches can be used to fix the problem of native metal oxides forming during the ALD process: chemically reducing the oxide (as described in the previous section) or using a dielectric that does not contain oxygen. The devices studied in this section used the latter approach and were prepared using the half-damascene process flow described above with PEALD of 21 cycles of SiNx, to form a dielectric barrier (with approximate thickness of 1.05 nm). Bis(diethylamino)silane (C8H22N2Si) and H2 + N2 plasma were the precursors used in the PEALD process in an Oxford FlexAL system. In contrast with previous sections, where oxygen-containing dielectrics were used (Al2O3 and SiO2), no post-PEALD treatment was used in this process. An experimental diamond plot for a typical device is shown in Figure 17.
Reduction of the conductance at the CBO peak to nearly G0/2 along with constant G(Vds) ~ G0 when |Vds| >> 2Ec/e (Figure 17a) is indicative of negligible presence of thermally activated NiO in the junctions and confirms that the presence of the oxidizing pulse (O2, O3, or H2O) in the ALD process is the main origin of the parasitic native oxide formation on the island. Moreover, since SiNx is hydrophobic and does not contain oxygen, the oxidation of the top metal while being deposited on the SiNx barrier is negligible. Experiments once again reveal a very large level of switching RTS noise similar to that observed for Pt-Al2O3-Pt and Ni-SiO2-Ni devices described in the previous sections. The noise analysis performed in [49] pinpoints the location of the traps within the junctions (either in the dielectric or at the metal-dielectric interfaces). Silicon nitride is known as a trap-rich material where the abundance of traps is generally associated with silicon atoms replacing nitrogen atoms, resulting in silicon dangling bonds, referred to as K-center defects [50]. Other defects can be associated with hydrogen, which is typically present in the SiNx matrix [51].

4. Conclusions

In this work SETs have been fabricated using thermal ALD and plasma enhanced-ALD to deposit tunnel barriers of Al2O3, SiO2, combinations of the two, and SiNx. Table 1 provides a summary of the different metals, dielectrics, and treatments and the most important observations relating to each. This demonstrates the ability of ALD to controllably deposit a variety of films on metal substrates including Ni and Pt. The two major interrelated issues with this technique have been: (1) oxidation of the metal leads during ALD; and (2) the formation of traps near the island, most likely in the barrier dielectric or at the metal-dielectric interfaces that lead to strong RTS-type electrical noise. In the case of Ni-SiO2-Ni SETs, it has been shown that both issues can be mitigated by a combination of forming gas annealing during fabrication and hydrogen plasma treatment after fabrication. Here post-fabrication RHP treatment appeared the most important factor in noise mitigation, potentially due to annealing of charged defects in SiO2 dielectric. Likewise, due to the instability of platinum oxide, ridding platinum-based devices of their native oxide has proven possible using argon anneal, however these devices also exhibited time instabilities caused potentially by reoxidation of Pt at Pt/Al2O3 interfaces. In addition, the choice of SiNx as the tunnel barrier seems to avoid parasitic oxide formation in the tunnel junctions so that no in-fabrication or post-fabrication is needed to restore the MIM behavior in the SETs, but lowering the electrical noise has proved to be more elusive. Further research is needed to determine the source of the traps and to investigate how post-fabrication hydrogen plasma is effective in passivating these traps, and what species cause the oxidation of the metal layer deposited on top of the ALD oxide. Future work will also explore amorphous metal thin films [52] (unlike the polycrystalline films used in this work) and the effects of surface morphology on barrier uniformity.

Acknowledgments

The authors would like to acknowledge the National Science Foundation for funding this research (NSF DMR-1207394, CHE-1124762, DGE-1313583, and ECCS-1509087). The authors are grateful to Professor Alexander Mukasyan for multiple useful discussions.

Author Contributions

G.K. fabricated and measured all the Ni-based devices. L.C.S., H.G. and M.S.M. fabricated and measured Pt-based devices. M.J.F. ran simulations of the fabricated devices. A.O.O. and G.L.S. designed experiments and oversaw cryogenic testing. A.N.N. contributed to theoretical explanation of plasma annealing. The manuscript was written by G.K., A.O.O. and M.S.M. and edited by all authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. “Cross-tie” fabrication of single electron transistors (SETs): (a) the metal source and drain (shown in grey) are deposited on thermal SiO2 (shown in blue) using electron beam lithography (EBL) and liftoff; (b) the sample is blanket coated by a dielectric (shown in green) using atomic layer deposition (ALD) to form the tunnel barrier (usually about 1 nm thick); (c) EBL and liftoff are used to form the metal island of the single electron transistor (SET) (shown in grey); and (d) micrograph of two cross-tie SETs. The wide line on the far left is the gate electrode and the narrow vertical lines are the metal islands.
Figure 1. “Cross-tie” fabrication of single electron transistors (SETs): (a) the metal source and drain (shown in grey) are deposited on thermal SiO2 (shown in blue) using electron beam lithography (EBL) and liftoff; (b) the sample is blanket coated by a dielectric (shown in green) using atomic layer deposition (ALD) to form the tunnel barrier (usually about 1 nm thick); (c) EBL and liftoff are used to form the metal island of the single electron transistor (SET) (shown in grey); and (d) micrograph of two cross-tie SETs. The wide line on the far left is the gate electrode and the narrow vertical lines are the metal islands.
Applsci 07 00246 g001
Figure 2. Process steps in fabricating the “half-damascene” cross-tie SET: (a) Si substrate (dark grey) is oxidized (SiO2 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.
Figure 2. Process steps in fabricating the “half-damascene” cross-tie SET: (a) Si substrate (dark grey) is oxidized (SiO2 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.
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Figure 3. Electrical characterization of Pt-Al2O3-Pt SET prepared using nine ALD cycles of Al2O3 (~1 nm) with no post fabrication treatments: (a) Coulomb diamond plot and G(Vds) dependence for Vg = 2.3 mV (a peak of Coulomb blockade oscillation). Note that zero bias conductance G(Vds = 0 mV) is just slightly below the G0/2 value (indicated by the dashed line) expected from orthodox theory; (b) Coulomb Blockade Oscillations (CBOs) for Vds = 0 mV. T = 0.3 K, measurements were performed immediately after fabrication.
Figure 3. Electrical characterization of Pt-Al2O3-Pt SET prepared using nine ALD cycles of Al2O3 (~1 nm) with no post fabrication treatments: (a) Coulomb diamond plot and G(Vds) dependence for Vg = 2.3 mV (a peak of Coulomb blockade oscillation). Note that zero bias conductance G(Vds = 0 mV) is just slightly below the G0/2 value (indicated by the dashed line) expected from orthodox theory; (b) Coulomb Blockade Oscillations (CBOs) for Vds = 0 mV. T = 0.3 K, measurements were performed immediately after fabrication.
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Figure 4. Measurement of three Pt-Al2O3-Pt devices from the same die displaying how successive treatments using longer exposures to forming gas and higher temperatures caused further increases in conductance.
Figure 4. Measurement of three Pt-Al2O3-Pt devices from the same die displaying how successive treatments using longer exposures to forming gas and higher temperatures caused further increases in conductance.
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Figure 5. Single electron transistor (Pt-Al2O3-Pt) conductance at room temperature for an O3-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 5. Single electron transistor (Pt-Al2O3-Pt) conductance at room temperature for an O3-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).
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Figure 6. Illustration showing a top view of the reduction and reoxidation of Pt-Al2O3-Pt tunnel barriers. In as fabricated devices (a), electrons must travel through both the Al2O3 and the PtOx (blue region). This thicker barrier causes low conductance. Treatment with forming gas at 90 °C only partially reduces the PtOx (b) but this provides a region where the electrons can tunnel through just the Al2O3 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 °C but, since all the parasitic oxide is reduced in this case (d), the effect of re-oxidation is negligible (e).
Figure 6. Illustration showing a top view of the reduction and reoxidation of Pt-Al2O3-Pt tunnel barriers. In as fabricated devices (a), electrons must travel through both the Al2O3 and the PtOx (blue region). This thicker barrier causes low conductance. Treatment with forming gas at 90 °C only partially reduces the PtOx (b) but this provides a region where the electrons can tunnel through just the Al2O3 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 °C but, since all the parasitic oxide is reduced in this case (d), the effect of re-oxidation is negligible (e).
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Figure 7. Electrical characteristics for O3-based Pt-Al2O3-Pt SET after anneal in forming gas at 90 °C for 1 h: (a) diamond plot at 0.3 K with Vg (Vds = 0 mV, blue) and Vds (Vg = 5.4 mV, red) cross sections; and (b) G(Vds) dependence of SET conductance measured in the peak of Coulomb blockade at several temperatures. The peaks of CBOs are below G0/2 (black dashed line), contrary to Coulomb blockade theory for metal-insulator-metal SETs.
Figure 7. Electrical characteristics for O3-based Pt-Al2O3-Pt SET after anneal in forming gas at 90 °C for 1 h: (a) diamond plot at 0.3 K with Vg (Vds = 0 mV, blue) and Vds (Vg = 5.4 mV, red) cross sections; and (b) G(Vds) dependence of SET conductance measured in the peak of Coulomb blockade at several temperatures. The peaks of CBOs are below G0/2 (black dashed line), contrary to Coulomb blockade theory for metal-insulator-metal SETs.
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Figure 8. Evidence of parasitic oxide: Experimental and simulated G(Vds) dependences for an untreated Ni-SiO2-Ni SET at different temperatures.
Figure 8. Evidence of parasitic oxide: Experimental and simulated G(Vds) dependences for an untreated Ni-SiO2-Ni SET at different temperatures.
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Figure 9. Scanning electron micrograph of a Ni-SiO2-Ni device where strong agglomeration occurs after a forming gas anneal (FGA) at 400 °C for 30 min leading to the breaks in the source and drain electrodes (nearly horizontal broken lines).
Figure 9. Scanning electron micrograph of a Ni-SiO2-Ni device where strong agglomeration occurs after a forming gas anneal (FGA) at 400 °C for 30 min leading to the breaks in the source and drain electrodes (nearly horizontal broken lines).
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Figure 10. (a) Typical G(Vds) dependence observed in samples fabricated with two FGA steps; T = 0.4 K. The metal is Ni and the barrier composition is six cycles of ALD SiO2 (~0.7 nm) and four cycles of ALD Al2O3 (~0.4 nm) so the total barrier thickness is ~1.1 nm; (b) G(Vg) dependence for the same SET, showing the excess switching noise.
Figure 10. (a) Typical G(Vds) dependence observed in samples fabricated with two FGA steps; T = 0.4 K. The metal is Ni and the barrier composition is six cycles of ALD SiO2 (~0.7 nm) and four cycles of ALD Al2O3 (~0.4 nm) so the total barrier thickness is ~1.1 nm; (b) G(Vg) dependence for the same SET, showing the excess switching noise.
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Figure 11. (a) Coulomb charging diagram of an Ni-SiO2-Ni SET in which the ALD dielectric (~1 nm of SiO2) was exposed to remote hydrogen plasma (RHP) (measured at T = 0.4 K) along with a G(Vds) cross section (red curve) at a peak of CBO (Vg = 0.45 mV). Strong asymmetry, non-MIM SET conductance out of blockade region, as well as “switching noise” are visible. (b) CBOs of the same device.
Figure 11. (a) Coulomb charging diagram of an Ni-SiO2-Ni SET in which the ALD dielectric (~1 nm of SiO2) was exposed to remote hydrogen plasma (RHP) (measured at T = 0.4 K) along with a G(Vds) cross section (red curve) at a peak of CBO (Vg = 0.45 mV). Strong asymmetry, non-MIM SET conductance out of blockade region, as well as “switching noise” are visible. (b) CBOs of the same device.
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Figure 12. (a) Coulomb charging diagram and G(Vds) cross section at Vg = 10 mV of a Ni-SiO2-Ni device fabricated with two RHP steps (measured at T = 0.4 K). Barrier composition is 12 cycles (~1 nm) of SiO2 by plasma-enhanced ALD (PEALD); (b) CBOs of the same device.
Figure 12. (a) Coulomb charging diagram and G(Vds) cross section at Vg = 10 mV of a Ni-SiO2-Ni device fabricated with two RHP steps (measured at T = 0.4 K). Barrier composition is 12 cycles (~1 nm) of SiO2 by plasma-enhanced ALD (PEALD); (b) CBOs of the same device.
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Figure 13. (a) G(Vds) dependence observed in (Ni-SiO2-Ni) SET fabricated with two RHP steps at a CBO peak. For Vds above a threshold VTh ≈ 15 mV, very strong noise is observed, but below this threshold the switching noise is much lower than in double FGA samples. The peaks of CBOs never reach G0/2 indicating that some NiO remains. (b) The spectral density of noise as a function of Vds shows two orders of magnitude increase of noise above 15 mV (T = 0.4 K).
Figure 13. (a) G(Vds) dependence observed in (Ni-SiO2-Ni) SET fabricated with two RHP steps at a CBO peak. For Vds above a threshold VTh ≈ 15 mV, very strong noise is observed, but below this threshold the switching noise is much lower than in double FGA samples. The peaks of CBOs never reach G0/2 indicating that some NiO remains. (b) The spectral density of noise as a function of Vds shows two orders of magnitude increase of noise above 15 mV (T = 0.4 K).
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Figure 14. Comparison of electrical characteristics for Ni-SiO2-Ni devices measured at 0.4 K: (a) Coulomb diamonds of a device from batch A1 treated with double FGA along with a G(Vds) cross section at Vg = 100 mV; and (b) Coulomb diamonds of a device from batch B1 treated with FGA+RHP along with a G(Vds) cross section at Vg = 170 mV. The difference of CBO periods results from different device proximities to electrostatic gates, and hence a different gate capacitance.
Figure 14. Comparison of electrical characteristics for Ni-SiO2-Ni devices measured at 0.4 K: (a) Coulomb diamonds of a device from batch A1 treated with double FGA along with a G(Vds) cross section at Vg = 100 mV; and (b) Coulomb diamonds of a device from batch B1 treated with FGA+RHP along with a G(Vds) cross section at Vg = 170 mV. The difference of CBO periods results from different device proximities to electrostatic gates, and hence a different gate capacitance.
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Figure 15. (a) Coulomb diamonds of a Ni-SiO2-Ni device treated with FGA+RHP along with a G(Vds) cross section at Vg = 3.3 V; and (b) simulations of an MIM SET along with a G(Vds) cross section at Vg = 3.3 V using the following parameters: Cg = 0.106 aF, Cd = 47 aF, Cs = 63 aF, Gd = 0.35 μS, Gs = 0.50 μS, T = 0.5 K.
Figure 15. (a) Coulomb diamonds of a Ni-SiO2-Ni device treated with FGA+RHP along with a G(Vds) cross section at Vg = 3.3 V; and (b) simulations of an MIM SET along with a G(Vds) cross section at Vg = 3.3 V using the following parameters: Cg = 0.106 aF, Cd = 47 aF, Cs = 63 aF, Gd = 0.35 μS, Gs = 0.50 μS, T = 0.5 K.
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Figure 16. 3D simulation of the total island capacitance in a half-damascene geometry: (a) Dimensions of the island (horizontal grey line) and source and drain (vertical grey lines). The island is inlaid in thermal SiO2 (ε = 3.9) and the source and drain are on top of the PEALD dielectric layer (ε = 3.9) surrounded by air (ε = 1); (b) Cross section of island inlaid in SiO2 dielectric. The simulated total island capacitance CIsland is for 1 nm of PEALD SiO2 (approximately the same thickness as 12 cycles of SiO2, ε = 1). C is the expected island capacitance using the parallel plate model of the two tunnel junctions for the given dimensions. Based on our simulations, the actual island capacitance can be more than twice the calculated value using the parallel plate approximation.
Figure 16. 3D simulation of the total island capacitance in a half-damascene geometry: (a) Dimensions of the island (horizontal grey line) and source and drain (vertical grey lines). The island is inlaid in thermal SiO2 (ε = 3.9) and the source and drain are on top of the PEALD dielectric layer (ε = 3.9) surrounded by air (ε = 1); (b) Cross section of island inlaid in SiO2 dielectric. The simulated total island capacitance CIsland is for 1 nm of PEALD SiO2 (approximately the same thickness as 12 cycles of SiO2, ε = 1). C is the expected island capacitance using the parallel plate model of the two tunnel junctions for the given dimensions. Based on our simulations, the actual island capacitance can be more than twice the calculated value using the parallel plate approximation.
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Figure 17. (a) Typical Coulomb diamond plot for a Ni-SiNx-Ni SET with 21 cycles of SiNx (~1 nm) along with a G(Vds) cross section at Vg = 47 mV (a peak of CBOs); and (b) CBOs from the same device exhibiting RTS noise.
Figure 17. (a) Typical Coulomb diamond plot for a Ni-SiNx-Ni SET with 21 cycles of SiNx (~1 nm) along with a G(Vds) cross section at Vg = 47 mV (a peak of CBOs); and (b) CBOs from the same device exhibiting RTS noise.
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Table 1. Summary of SET fabrication processes, treatments, and observations. Post-fab indicates a treatment after fabrication and post-ALD indicates a treatment after the respective ALD process.
Table 1. Summary of SET fabrication processes, treatments, and observations. Post-fab indicates a treatment after fabrication and post-ALD indicates a treatment after the respective ALD process.
Fabrication ProcessSource/Drain MetalIsland MetalDielectricCycles (nm)TreatmentObservations
Cross-tiePtPtALD Al2O3 (H2O-based)9 (~1 nm)noneNon-MIM behavior, long-term instability
Cross-tiePtPtALD Al2O3 (H2O-based)9 (~1 nm)Post-fab: 5 min, Ar 375 °CAll shorts
Cross-tiePtPtALD Al2O3 (O3-based)9 (~1 nm)noneHighly resistive
Cross-tiePtPtALD Al2O3 (O3-based)9 (~1 nm)Post-fab: 5 min, Ar 375 °CStable but noisy
Cross-tiePtPtPEALD SiO215 (~1.5 nm)noneAll shorts
Cross-tieNiNiPEALD SiO22 (~0.2 nm)noneLow conductance, non-MIM behavior
Cross-tieNiNiPEALD SiO22 (~0.2 nm)Post-ALD: 2 min, 5% H2 in Ar 400 °CEvidence of “pinholes”
Cross-tie/Half-DamasceneNiNiPEALD SiO212 (~1.1 nm)Post-ALD: (FGA) 30 min, 5% H2 in Ar 400 °CEffectiveness limited by agglomeration and noisy
Post-Fab: (FGA) 10 min, 5% H2 in Ar 300 °C
Cross-tie/Half-DamasceneNiNiPEALD SiO212 (~1.1 nm)Post-ALD: (RHP) 5 min, H2 plasma 100W 300 °CNon-MIM behavior, noisy
Cross-tie/Half-DamasceneNiNiPEALD SiO212 (~1.1 nm)Post-ALD: RHPHighest conductance, non-MIM behavior, noisy when Vds > 15 mV
Post-Fab: RHP
Cross-tie/Half-DamasceneNiNiPEALD SiO212 (~1.1 nm)Post-ALD: FGABest Performance
Post-Fab: RHP
Half-DamasceneNiNiPEALD SiNx21 (~1 nm)noneNoisy, but no evidence of NiO
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