GB2268625A - Nanofabricated logic device - Google Patents

Abstract

A nuclear structure, comprising two atoms 1a, 1b, between source 3 and drain 4 regions on a substrate 2, is assembled with a STM. The structure exhibits quantised nuclear states, which are utilised to define logic states, for use as a logic device. The atoms are preferably silver, but can be tungsten or platinum. <IMAGE>

Description

Logic Device DESCRIPTION This invention relates to a logic device.

Prior nano-scale logic devices have utilised quantum wells for retaining single or small numbers of electrons. The quantum mechanical state of the well can exhibit a number of discrete quantised energy levels that can be used to define logic levels in a logic device.

The present invention provides a new approach for defining logical conditions in a nano-scale device, wherein the quantum mechanical nuclear state is used for defining logical conditions.

In accordance with the invention there is provided a logic device comprising a nuclear structure with a plurality of quantised nuclear states, and means for defining output logical conditions in response to the quantised nuclear state of the structure.

When considering permissible quantum mechanical states for an atomic or molecular system, it may be necessary to consider not only the electron states but also the spin states of the nucleus. It is well known that different combinations of nuclear spin states produce fine and hyperfine structure in the states of atomic and in particular molecular systems. For example, considering the hydrogen molecule, it can exist in a para or ortho state in which the nuclear spin states of the protons of the hydrogen molecule align in different configurations. The ortho and para states have different configurations of quantised nuclear energy levels due to the different interactions of the nuclear spin states. Thus, the permitted transitions between states are different for the ortho and para forms of the molecule.In accordance with the invention, this difference may be used to define different logical levels.

In nature, most naturally occurring molecules which exhibit ortho and para states, in their liquid and gaseous form, produce an averaging of the two permitted sets of transitions due to molecular collisions within the liquid or gas. However, in accordance with the invention, it has been appreciated that in a solid or solid state environment, nuclear structures can be fabricated with preset nuclear spins in a predetermined configuration, e.g. in an ortho or para state. The resulting permitted transitions associated with the preset nuclear state can be used to define predetermined logical conditions.

In order that the invention may be more fully understood an embodiment thereof will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic sectional view of a logic device in accordance with the invention; and Figure 2 is a schematic diagram of the energy levels associated with the device of Figure 1.

Referring to Figure 1, an artificial "molecule" comprising atoms la, lb is assembled on a substrate 2.

The atoms which constitute the molecule 1 may be moved into position by the use of the tip of a probe of a scanning tunneling microscope (STM). A general review of techniques for manipulating single atoms on the surface of a substrate is given in "Atomic and Molecular Manipulation with the Scanning Tunneling Microscope" J. A. Stroscio and D. M. Eigler Science Volume 254 29 November 1991 p. 1319 - 1326.

The molecular structure 1 is disposed between source and drain regions 3, 4 which may themselves be formed from conductive atoms or molecules arranged using an STM tip.

During manufacture, the substrate 2 may be rendered in a conductive condition to allow a tunneling current to be established between the STM tip and the substrate, and thereafter may be switched to a relatively non-conducting condition as described in our co-pending application No. 9213423.8 filed on 24th June 1992. The source and drain regions 3, 4 are separated from the molecule 1 by gaps 5a, 5b that act as tunneling junctions. The molecular structure 1 is physisorbed (Van der Waals bonded) to the surface of the substrate 2 rather then chemisorbed, which would alter the electronic states of the structure to a much greater extent.

As will be explained in more detail hereinafter, the molecule 1 is written into either its ortho or para molecular state, with the spin states of its nuclei arranged accordingly. Figure 2 shows the energy configuration of the device with the moelcule 1 in one of its two possible states e.g. its ortho state.

Figure 2 shows quantised energy levels 6, 7 associated with the ortho state between which an electron transition can occur. A corresponding transition cannot occur when the molecule is in its para state. The tunneling junctions 5a, 5b shown in Figure 1 present potential barriers 8, 9 shown in Figure 2. The source region 3 of Figure 1 is configured to have its Fermi level so arranged that its conduction band overlaps the lower quantised energy level 6 of the molecule 1. The drain region 4 of Figure 1 has a corresponding conduction band overlapping the uppermost quantised energy level 7 of the molecule 1.

An electron transition between the quantised energy level 6, 7 can be stimulated by an input photon e.g.

from a laser source (not shown). Thus, in use, electrons from the source 9 can tunnel through the barrier 8 into the energy level 6 of the molecule 1 and thereafter can be stimulated to energy level 7 by the laser radiation so that the electrons can tunnel through the barrier 9 into the conductive band 10 of the drain 4, so as to produce current flow.

If however, the molecule 1 is switched to its other molecular state i.e. the para state, selection rules dictate that the transition between energy levels 6, 7 would not be permitted and so no current would flow.

Since the transitions between levels 6, 7 are in the optical range, the device may be operable at room temperature. The laser illumination is only necessary during reading of the device and so it would be suitable for long term storage and archiving.

Current flow through the device can be controlled by adjusting the tunneling barriers 8 and/or 9.

As previously mentioned, in order for the device to operate, the nuclear spins of the molecule 1 need to be given a predetermined orientation. This can be achieved with a fast pulse from the tip of the STM, using a ultra fast laser pulse combined with an STM tip, to generate a local high field pulse and thereby write the nuclear spins. The resulting local field can be enhanced using a single crystal ferromagnetic tip or a suitable diamagnetic material. It would also be possible to arrange the artificial molecule 1 vertically and apply a strongly anisotropic magnetic field.

A preferred way of making the molecular structure is by direct writing using a ferromagnetic tip such as Cur02.

Such tips have been used hitherto for observation but as far as we are aware there have been no reports of their use for fabrication. Thus, in use, the CrO2 tip is installed in an STM and the tip field ensures that individual atoms deposited with the STM are configured with a preset magnetic state and so an ortho or para molecule can be fabricated as desired. The nuclear spin state can be changed by flipping the state of one of the atoms#.

Suitable atoms for constructing the molecule 1 comprise Ag, W, Pt. The element Ag is particularly convenient as it is the easiest to predict in terms of its properties as it is net hydrogenic with a single electron in its outer orbital. An Ag, two atom system or dimer will show allowed J odd-odd transitions for the ortho state and allowed even-even transitions for the para state. Its ground state, whether odd or even, has the same electronic configuration for the both the ortho and para states. However, the first excited state for each of the ortho and para states is at a different energy level, providing suitable energy selection for the system shown in Figure 2, and the conductive band 10 of the drain can thus be tuned to select one or the other of excited states.

Claims (12)

1. A logic device comprising; a nuclear structure with a plurality of quantised nuclear states, and means defining output logical conditions in response to the quantised nuclear state of the structure.

2. A logic device according to claim 1 wherein the structure is writeable in a first or second of said nuclear states and in the first thereof exhibits an allowable transition between first and second energy levels, the device including a source for supplying carriers at said first energy level to the structure, and a drain to receive carriers from the structure at said second energy level, whereby to permit carrier flow from the source to the drain selectively when the structure is in said first nuclear state.

3. A device according to claim 2 including means for stimulating transition of said carriers from first to said second energy levels so as to produce carrier flow from the source to the drain.

4. A device according to claim 3 including means for supplying photons to the structure for stimulating said transition.

5. A device according to any preceding claim wherein said nuclear structure comprises an assembly of atoms having their nuclear spins arranged in a predetermined configuration.

6. A device according to claim 5 wherein said atoms are Ag atoms.

7. A device according any one of claims 2 to 6 including tunneling barriers between said structure and said source and drain respectively.

8. A logic device substantially as hereinbefore described with reference to the accompanying drawings.

9. A method of fabricating a logic device according to any preceding claim including using a probe to manipulate individual atoms into a predetermined configuration to define said nuclear structure.

10. A method of configuring the nuclear spins of said nuclear structure in a logic device according to any one of claims 1 to 8, including applying a magnetic field thereto so as to produce a given nuclear spin alignment.

11. A method of aligning the spins of a nuclear structure in a logic device according to any of claims 1 to 8, including applying a laser pulse to the structure.

12. A method of fabricating a logic device substantially as hereinbefore described with reference to the accompanying drawings.