Single-electron devices with granulated film cotunneling suppressors

Single-electron devices where a well-determined number of electrons can be controllably transferred continue to attract significant attention as one of the promising devices for applications in future digital circuits and metrology. However, the family of devices based upon precise transfer of single electrons (single-electron pumps, turnstiles, traps, latches and Quantumdot Cellular Automata (QCA) ) suffers from errors caused by cotunneling [1]. Cotunneling, a macroscopic quantum process, impairs the operation of devices where charge transfer is actuated by controllable Coulomb barriers, because it opens a classically prohibited channel for charge transfer under that barrier. A traditional way to reduce the cotunneling [2] is to increase the number of tunnel junctions in the device, N, because cotunneling current scales as I V 2N-1. For small biases the cotunneling current is thus significantly reduced. In practice, this approach has a significant drawback due to the inevitable presence of random background charges on the islands of the array. The need to individually adjust the random offset charges drastically complicates the operation and tuning of the devices. Here we present a different approach to suppress the cotunneling using a granulated metal film (Cr evaporated in 02 ambient) (Fig. 1) with weakly insulating properties. First, we fabricated and tested a single-electron transistor (SET) where suppression of the cotunneling is achieved by replacing the traditional metal island with a granulated metal film (Fig. 2). The SET has a characteristic charging energy defined by the welldefined Al/AlOx junctions while strongly suppressing the cotunneling by electron scattering in the granulated metal island (Fig 3). The value of the charging energy of the SET, EcSET z 0.27 meV exceeds the activation energy within the film, AE, by more than one order of magnitude. Therefore, with respect to the external gate, the granulated metal island can be viewed as a "good" metal and CBOs are completely defined by the larger scale parameter, EcSET >> AE. Second, using this design, we fabricated and tested a single-electron latch which utilizes this technology to achieve cotunneling suppression. A single-electron latch [3] consists of three dots connected in series by tunnel junctions. To achieve memory function in a latch the cotunneling must be suppressed. In previous demonstrations of single electron latches lithographically defined multiple tunnel junctions (MTJ) were used to connect the dots in a latch. The use of MTJs, however, requires complicated cancellation of the background charges affecting parasitic islands of the MTJs. In this work we use a granulated metal oxide film as the material for the middle dot (Fig. 4) that also acts as a cotunneling suppressor, replacing lithographically defined MTJs. The granulated metal film acts as a network of tunnel junctions and thus hampers the cotunneling while the presence of random offset charges localized in the oxidized grain boundaries makes this network of junctions insensitive to the external gate [4]. The experiments demonstrate the operation of the latch at low temperature (Fig. 5), with latching times greater than 1 s, and unlike devices fabricated using lithographically defined MTJs, no background charge compensation is required. This work was supported by the MRSEC Center for Nanoscopic Materials Design of the National Science Foundation under Award No. DMR-0080016.