Fig. 1. Scanning electron microscopy image of a typical device. The island of about 80 nm which constitutes the quantum dot is contacted by aluminium electrodes (scale bar: 200nm)
Starting with a self-assembled SiGe island, we have fabricated a device in which we can precisely control the number of charge carriers. Thanks to the properties of the material in use, this device provides a means to manipulate the spin of the charge carriers in an unprecedented way.
One of the most fascinating challenges for future electronics is to produce devices whose operation relies on the degree of spin of the carriers, rather than their charge. Electron spins, which form a quantum mechanical two-level system, might be exploited for quantum information processing. While charge carriers are subject to long-range electrostatic interactions (Coulomb interaction), spins interact much less among themselves. Their coherence time is therefore relatively long, thus making them a promising candidate for qubits.
So far, most of the experimental work has focused on GaAs-based Quantum Dots (QDs). In GaAs QDs the quantum coherence of electron spins is lost on relatively short time scales (~30 ns), due to hyperfine interaction with the nuclear spins (both Ga and As have non-zero nuclear spin moments). We have moved towards SiGe for which hyperfine interaction is expected to be smaller. Before manipulating the spins of the charge carriers, it is necessary to prepare a quantum dot out of this material system. In this way, we can control the number of charge carriers within the quantum dot.
Fig. 2. Differential conductance measured at 16 mK as a function of the source-drain and backgate voltage. The dark background corresponds to zero conductance, while the coloured zones to large values of it. By fixing the backgate voltage at 5.3V (yellow arrow), the differential conductance stays very small until the source-drain voltage becomes larger than approximately 15mV. At this value, a charge can travel through the quantum dot.
Self assembled SiGe islands
Silicon and Germanium have a different lattice constant, which renders unstable a thin film of Ge epitaxially-deposited on Si. The system minimizes its elastic energy by the spontaneous formation of SiGe islands on the surface. This is the way our samples are fabricated at the Max-Planck Institute (now at IFW-Dresden).
In order to control the number of charge carriers within the QD formed by the island, one has to deposit electrodes – here made out of aluminium -- that form the drain and source of a transistor (Fig. 1). The fabrication is performed at PTA (Plateforme Technologique Amont). For the gate, we have employed a different method that replaces the deposition of an insulator and a metal on top of the islands. The islands are grown on silicon on insulator substrates (see tri-layer in the scheme of Fig. 1) with a heavily doped substrate that can be used as a backgate. This SOI has been specially fabricated at LETI starting with a heavily doped Si substrate, since standard SOI is not doped highly enough, and would not work for the low temperature experiments.
These measurements need to be done at low temperatures in order that the thermal energy of the charge carriers, which are holes in our case, is low enough. Fig. 2 shows the conductance between the source and drain as a function of the source-drain and backgate voltage. In fact, it is the differential conductance that is measured around a potential value applied between the source and the drain. The obtained figure, which shows coulomb diamonds, is the signature of the coulomb blockade phenomenon (see insert). Starting from the centre of the figure, the crossing line with large conductance corresponds to the addition or removal of a carrier from the island. These measurements also allow for determination of the energy spectrum of the charge carriers within the QD. The next step, which will open the way towards spin manipulation, consists in measuring precisely the Lande factor.
The coulomb blockade phenomenon is caused by the coulomb repulsion that, for example, “feels” an electron approaching a very small object already negatively charged. It can be observed when the energy needed to add one electron inside this object is important, more specifically, when larger than its thermal energy. Because of the dimensions of our quantum dots, we need to place them at low temperatures in order to observe this phenomenon.
Maj : 20/02/2014 (975)