Elsevier

Microelectronics Journal

Volume 36, Issues 3–6, March–June 2005, Pages 308-312
Microelectronics Journal

Origin of Coulomb blockade oscillations in single-electron transistors fabricated with granulated Cr/Cr2O3 resistive microstrips

https://doi.org/10.1016/j.mejo.2005.02.076Get rights and content

Abstract

The purpose of our work is to evaluate single-electron devices fabricated using resistive microstrips and to investigate its applicability for single-electron logic. In this article, we present our work on the fabrication and characterization of SETs with gold islands and CrOx resistive microstrips. The electron transport mechanism of CrOx resistors is also discussed and hypothesis of two types of possible junctions are given as the explanation for the experimental results.

Introduction

The single electron tunneling transistor (SET) [1], [2] is a device based on Coulomb blockade of tunneling. Typical metal-film SET consists of a small (micron size) metal island connected to the external circuits with two tunnel junctions and a gate capacitively coupled to the island. For an SET to operate, the total capacitance of the island, C, must be small enough that the charging energy EC=e2/2C is much greater than the thermal energy, kBT. The second condition imposed by the uncertainty principle is that the resistance of the tunnel barriers, RT must exceed the resistance quantum RQ=h/2e2≈13 kΩ. If these conditions are satisfied, the number of electrons on the island is fixed, and extra energy is needed to add an extra electron to the island (Coulomb blockade of tunneling). The blockade conditions could be lifted by changing the gate bias. If a small bias is applied between the junction which form source and drain of a SET, the tunneling current through the SET changes periodically as a function of applied gate voltage with a period of Vg=e/Cg, where Cg is a capacitance between the island and a gate. The quantization of energy levels in the island could be neglected for metallic islands with the size greater than some 100 nm even at miliKelvin temperatures and the system could be completely described by an orthodox theory of Coulomb blockade [3].

Since the first implementation of the SET where tunneling barriers made of Al2O3 were sandwiched between thin layers of Al island using Dolan-bridge technique [1], a number of different geometries, materials and methods have been used for the fabrication of thin metal film single electron transistors [3], [4], [5]. Despite some differences all these devices share the same basic design—a metal island isolated by two tunnel junctions. Recently, a general theory of Coulomb blockade was proposed by Nazarov [6]. According to this theory, tunnel junctions are not necessary to provide insulation required for discrete charging effects and they may be replaced by arbitrary scatterers, including tunnel junctions, quantum point contacts, and diffusive conductors. The scatterer of the same resistance as a tunnel junction suppresses Coulomb blockade exponentially by a factor of exp(−αG/GQ), where α is a dimensionless parameter depending on the type of the conductor, and GQ=1/RQ. It is important to note, that the requirement on the capacitance, C«e2/2kBT, for the scatterers discussed in Ref. [6] is fulfilled by assumption.

Combination of junctions and resistors was studied in Ref. [7] where SETs were made with Cr resistive microstrips augmenting tunnel junctions, Nb SETs with ‘weak links’ made by combined shadow evaporation and anodization [8], and more recently single-electron transistors with metallic microstrips instead of tunnel junctions were reported [9]. One important advantage of the devices combining resistors and junctions is a strong suppression of co-tunneling (a second order coherent quantum process consisting of multiple simultaneous tunneling events). The experiments by Lotkhov et al. [7] showed that microstrips with resistance nRQ act as n tunnel junctions connected in series, strongly suppressing the cotunneling. In quantum-dot cellular automata (QCA) [10], [11] devices, cotunneling severely impairs the ability of cells to store information. To suppress cotunneling, complicated multiple-tunnel junction (MTJ) structures were used in Ref. [12]. By replacing MTJ with resistive microstrips, the number of junctions can be reduced. This eliminates the problem of junction random background charge compensation in extra dots of MTJs and greatly simplifies the design of cells.

Section snippets

Fabrication

The single-electron transistors, comprising the Au electrodes, island and CrOx microstrips (see Fig. 1) were fabricated using two steps of e-beam lithography and metal deposition. In the purpose of eliminating the problem of parasitic junction formation caused by native Al oxide, Au is chosen instead of Al as the island metal. Two versions of SET island wiring geometry (WG), with one continuous CrOx wire atop a gold island, WG1 (Fig. 1a), and two CrOx wires connecting a gold island to source

Characterization of CrOx resistors

To obtain high quality CrOx resistive microstrips with controllable resistivity, we performed extensive characterization of the deposition process. In situ measurements were taken to monitor the resistance of the microstrips during deposition of Cr in an O2 ambient at different oxygen pressures. The results showed that the wires became conducting when the film thickness reached ∼2 nm. For 10 nm thick CrOx, sheet resistances in the range of 1–10 kΩ/□ were formed when the oxygen pressure was around

Characterization of single-electron transistors with thin CrOx wires

To form SETs, and study the origin of the Coulomb blockade in our devices, three types of patterns (Fig. 2) in each of the two versions of Fig. 1 were fabricated with 8–10 nm thick CrOx. In all of these, the first layer of metal (Au) is the same, defining the islands and electrodes. The difference is only in the second layer (CrOx) at the overlap areas.

In the first type of CPD design (type #1, Fig. 2a), the CrOx layer consists of narrow lines (∼70 nm) without tabs on the overlap areas. Very small

Characterization of single-electron transistors with thicker CrOx wires

SETs with thicker (∼40 nm) CrOx wires were also fabricated using CPD type #3 (Fig. 2c) with different widths of the island (80 and 500 nm). The room temperature sheet resistance of the devices showing significant nonlinearity in I–V curves at 300 mK is around 5 kΩ/□, which is about the same as our previous SETs with thinner (8–10 nm) CrOx wires. However, among those devices having significant nonlinearity, about 95% (21 out of 22) exhibited Coulomb blockade oscillations (Fig. 4a), which is much

Discussion

Based on these observations, we conclude that in the devices with 8–10 nm thick CrOx microstrips which show Coulomb blockade oscillations some sort of weak links with small intrinsic capacitance and R>RQ are formed at the edges of the CrOx wires overlapping the Au islands. Indeed, the Coulomb blockade is observed only on those samples with narrow lines (∼70 nm) overlapping the island (type #3) (Fig. 5). The CrOx line must be thinner at the overlapping edges where it climbs onto the island. Taking

Summary

We present experimental investigation of the origin of Coulomb blockade in the SETs with resistive microstrips with no intentional tunnel junctions, fabricated using technology similar to [3]. By using Au instead of Al for SET island material we eliminate a possibility of unintentional oxidation of the island in the fabrication process. By varying the area and the thickness of the microstrip regions overlapping the Au island we show that the Coulomb blockade which leads to SET operation is not

Acknowledgements

We are very thankful to A. Korotkov (UC Riverside) for performing FASTCAP calculations and attention to this work. The authors wish to thank V.A. Krupenin, B.I. Shklovskii for multiple useful discussions. R. Kummamuru and K. Yadavalli's assistance in the measurements is gratefully acknowledged. This work was supported by the MRSEC Center for Nanoscopic Materials Design of the National Science Foundation under Award Number DMR-0080016.

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