Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to single-electronics.
More specifically, it relates to single-electron devices
useful as diodes and transistors or as other electronic
devices, and to chemical processes for their preparation.
A single-electron device, as the term is
generally understood, is a device which controls the
movement of individual electrons in solids. They commonly
take the form of tunnel junctions, consisting of two
conductors separated by an ultrathin (less than 100
nanometers) layer of insulating material. When a voltage
is applied across the tunnel junction, inducing current
(electron movement) to flow in the conductors, a surface
charge accumulates on one surface of the conductor against
the insulating layer, with an equal and opposite charge on
the other insulator/conductor surface. When this surface
charge exceeds a predetermined value, an electron tunnels
through the insulating layer, thereby reducing the surface
charge. When the surface charge is less than the
predetermined value, tunnelling is suppressed (the Coulomb
blockade). As a surface charge increases due to continued
current flow in the conductor, i.e. continued application
of the voltage across the insulator, the charge increases
again, until the Coulomb blockade is overcome, whereupon
another single electron tunnels through. The phenomenon
can be observed experimentally, as a fl~n~mpntal relation
between applied voltage and the frequency of oscillation in
the current flowing through the junction. The height of
the Coulomb blockade depends upon the conductance and
capacitance of the insulating layer.
A general description of single electronics and
single-electron devices and their method of operation
appears in "Scientific American", June 1992, page 80, in an
article by Likharev and Claeson.
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These single-electron phenomena show promise for
application to digital integrated circuits. Currently
computer chips can have a density of about ten million
devices per square centimetre, to handle the commonly used
digital electrical pulses. Using single electronics, even
further reductions in size are possible, since bits of
information can be represented through the passage of
individual electrons. For this to happen, however,
techniques need to be developed to fabricate complex
structures whose smallest dimension is, controllably and
reproducibly, less than 100 nanometers. Such techniques
also need to be capable of operation economically and
efficiently, for the production of single-electronic
devices incorporating one or more such tunnel junctions.
It is an object of the present invention to
provide novel, single-electron devices.
It is a further object of the invention to
provide novel processes for making single-electron devices.
In the present invention, single-electron devices
are made by a technique of electro deposition. A layer of
porous oxide is deposited on the surface of a conductive
substrate of a metal selected from aluminum, titanium,
niobium and tantalum, by electrolytic anodizing. The pores
formed in the oxide layer are of extremely small but very
uniform shape, diameter and depth, controllable within
close limits by the conditions of anodizing and/or by
subsequent chemical etching. These highly uniform,
nanometer-sized, densely packed pores are used in the
present invention as templates, for the formation therein
of conducting or semiconducting materials, to form an
active region ("nanowire") of a tunnel junction device.
The insulating layer or layers of the device can be formed
by oxidation or other chemical conversion of the or each
end portion of the, nanowire. Alternatively, the residual
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oxide layer at the bottom of each pore, separating the pore
from the electrically conducting metal substrate, can form
an insulating layer, with the metal substrate itself acting
as the second conductor separated by this insulating oxide
layer from the nanowire which is formed in the pore.
The invention thus takes advantage of the very
small but highly uniform and controllable sizes of the
pores formed in the oxide layer, by anodizing and if
desired by subsequent chemical etching, to make single-
electron devices. The pore sizes can be controlled to adiameter of about 90 Angstroms up to several thousand
Angstroms, and to a height of several hundred Angstroms to
several thousand Angstroms, to give devices with an array
of conductor-barrier arrangements of dimensions suitable to
exhibit single-electron tunnelling effects. Moreover, the
pores and hence the nanowires formed in them can be
extremely densely packed, allowing the preparation of
arrays of tunnel junction devices, of a density up to the
order of 101 per square centimetre.
FIGURE 1 is an idealized illustration of a pores
template formed as a part of the process of the invention;
FIGURE 2 is a schematic view of a cross section
of a nickel nanowire prepared and studied in the present
inventlon;
FIGURE 3 is a diagrammatic cross section of an
alternative and preferred embodiment of the present
invention.
FIGURE 4 is a voltage-current curve derived from
devices according to the present invention as described in
Example 2 below.
Devices according the present invention can take
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a variety of different forms, and incorporate one or more
insulating barrier layers and correspondingly two or more
conductors, at least one of which is a nanowire. The
conductive metal substrate may constitute a conductor of
the finally produced single-electron device, in which case
the metal substrate is chosen from among aluminum,
titanium, niobium, tantalum and alloys thereof on the basis
of desired electrical properties. In other embodimentsl
the conductive metal substrate may be fully removed and
replaced during the manufacturing process, and not
constitute any part of the final device. In such cases,
the initial choice of substrate is dictated by cost and by
ease of manufacture, and is preferably all~m;nllm.
Similarly, the insulating metal oxide formed
initially on the conductive metal substrate may or may not
form one or more of the insulating barrier layers in the
final device. This will influence the choice of metal
substrate, for similar reasons.
The simplest device according to the present
invention, and the device most easily prepared (although
not the most preferred embodiment) is one which has a
single insulating layer (barrier) comprised of the oxide of
the metal substrate, disposed between the metal substrate
constituting one conductor, and a nanowire conductor formed
in a pore in the oxide layer. Such a device is prepared by
cleaning and anodizing the metal substrate, under carefully
controlled conditions to provide thereon oxide film with
the dense array of uniformly sized pores, the depth of the
pores being controlled to leave a suitably thin oxide layer
between the pore and the substrate to act as an insulating
layer of a single-electron device. Metal, semi-conductor
or other conducting material is subsequently deposited in
the pores of the oxide layer, to contact the insulating
oxide layer. A thin metal film (also referred to macro-
metal), for electrical connection purposes, is deposited
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over the nanowire in the pores, and in electrical contacttherewith.
Other single-barrier devices according to the
invention can be prepared by the process outlined above,
followed by a removal and replacement of one or both of the
metal substrate and the residual oxide separating the metal
substrate from the pore. This can be effected chemically,
by known metal and metal oxide removing techniques, e.g.
chemical etching. After removal of only the metal
substrate, another metal can be deposited over the metal
oxide insulating barrier layer. After removal of both the
metal substrate and the metal oxide insulating barrier
layer, another insulating barrier layer can be deposited on
one end of the nanowire, e.g. by controlled oxidation or
other chemical treatment thereof. Then another conductor,
e.g. metal, can be deposited over the newly formed barrier
layer, to complete the single barrier device according to
the invention.
Double barrier devices according to the invention
can be prepared by depositing or oxidizing an insulating
barrier layer on the end of the nanowire remote from the
metal substrate, followed by deposition of another
conductor over this insulating barrier layer, in the case
where the original substrate and intervening oxide layer
below the pores are left intact to constitute one barrier
layer and conductor. In the case where the original
substrate and the intervening oxide layer below the pores
are both removed, double barrier devices according to the
invention can be prepared after their r~ ovdl, by
deposition onto, or oxidation of, both ends of the residual
nanowire of the insulating barrier layer, e.g. an oxide
layer, followed by deposition thereover of conductive
layers, e.g. of metal.
In order to constitute single electron devices,
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the em~bodiments of this invention only need to include one
nanowire, and in all embodiments the pores formed in the
oxide layers formed on the metal substrates provide the
templates for these nanowires. Then one or two barrier
layers can be deposited, of a size suitable for exhibiting
single electron tunnelling effects, on account of the
extremely small sizes of these nanowires, particularly
their end surface areas. The uniformity and density of the
pores allows the preparation of useful devices with
practical applications.
A further, preferred embodiment of the invention
is a multiple barrier device having a thin conductive
intermediate layer, to which electrical connections can
readily be made. This is prepared by initially providing
an electrically conductive metal substrate of one of the
aforementioned metals, e.g. all~m;n~m~ and providing a
microporous oxide coating as previously described, on both
sides. If desired, the substrate in this em.bodiment can
comprise a conductive core, chosen on the basis of its
electrical characteristics for a suitable application,
carrying a deposit on both sides of one of the
aforementioned metals, e.g. aluminum, this metal layer
having a micro porous oxide coating as previously
described. Metal can then be deposited in the pores to
form nanowires, separated from the metal substrate by
residual oxide forming the insulating barrier, on each
side, thereby providing a double barrier device. A thin
metal film can then be applied over the nanowires at their
ends remote from the substrate, for electrical contact
purposes, thereby providing a double barrier, three contact
device comprising a very large array of nanowire-barrier
"pathways", operable in the manner of a transistor.
This embodiment of the invention, utilizing a two
sided porous substrate, can be further expanded from a two-
barrier device to a three-barrier device or a four-barrier
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device, if desired, by creation of another barrier layer,
e.g. by controlled oxidation, on the distal end of one or
both of the sets of nanowires protruding from each side of
the substrate. In such cases, the metal film is applied
over the or each of the barrier layer, to provide
electrical contact means.
The metallic substrate used in the process of the
present invention, and in one embodiment serving as the
conductor contacting the insulating layer, is normally
all]m;nl~m or an aluminum alloy in which aluminum is the
predominant metal, in foil or sheet form, or as a deposited
alllm;nllm film on an inert substrate. A surface layer of
alnm;n-lm oxide is deposited thereon, to constitute the
insulating layer, by anodizing the metal sheet or foil.
Firstly, the sheet or foil should be cleaned, ultra-
sonically and/or with organic solvent to remove grease
residues from its surface. Then it is preferably washed
with an alkaline chemical liquid so as to achieve a degree
of surface etching, followed by neutralization to remove
alkali excess. A wide variety of different chemical
reagents may be used for these purposes. Sodium carbonate
solution is an example of a suitable base with which to
effect the etching. Nitric acid is a suitable neutralizing
agent for subsequent use. Thus, the preferred cleaning
process is ultrasonic cleaning in dichloromethane, followed
by treatment with dilute sodium carbonate and then with
dilute nitric acid. However, the cleaning process can be
conducted with a very wide variety of different reagents.
Electrolytic anodizing of the cleaned sheet or
foil, for the purposes of depositing thereon a porous
surface of oxide, is suitably accomplished by making the
metal the anode of an electrolytic cell, using suitable
(e.g. lead) counter-electrodes in an acid bath. Suitably,
the bath is a relatively dilute solution of strong acids
such as sulfuric acid, phosphoric acid, oxalic acid, or
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chromic acid. Direct current is passed through the cell
between the electrodes, at a current or voltage suitably
adjusted to provide the correct film deposition. This can
be accomplished at room temperatures.
After suitable anodizing as described, the
substrate is removed from the bath and rinsed. It is
desirable to rid the surface of the treated substrate of
residual acid, and neutralize it at this point. It is,
however, undesirable to neutralize the acid chemically,
since this might have the effect of damaging the deposited
film. Accordingly, it is preferred to rinse the anodized
film with suitable quantities of water to remove acid and
effect appropriate neutralization.
A substrate so prepared has micro-pores in the
oxide surface film, of substantially uniform size in the
nanometer size range, and extending substantially
perpendicularly from the substrate surface and therefore
substantially parallel to one another. At the bottom of
each such micropore is a very thin layer of alllm;nllm oxide,
an electrical insulating material, separating the body of
the pore from the conducting metal alllm;nllm or aluminum
alloy substrate. By appropriate means according to the
present invention, this pore structure is utilized to
provide another conductor, separated from the conducting
all~m;mlm or aluminum alloy substrate, by a nanometer-size
range thickness of insulating layer to form the principle
of the single-electron devices according to the present
invention.
In a next step, therefore, an appropriate
conductor is applied to the interior of the pores. A
typical example of such a conductor is nickel. Before or
after deposition of the metal such as nickel into the
pores, the height and size of the pores can be chemically
adjusted, by chemical etching. The length of this "column"
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g
of deposited metal defined by the size of pore, is not
critical, but is typically in the size range from 1-10
microns .
The diameter, the density and the height of the
pores is determined to some extent by the choice of acid in
the anodizing bath. Suitable acids from which to choose
are phosphoric acid, oxalic acid, sulphuric acid and
chromic acid. The use of phosphoric acid leads the
formation of relatively large pores. The use of oxalic
acid or chromic acid leads towards the formation of smaller
pores, while the use of sulphuric acid leads the formation
of pores of the smallest diameters. The relative
concentration of the acid used also has a minor effect on
the pore sizes produced, but this effect is of little
significance in comparison with that derived from the
choice of acid.
Pore density, i.e. the distance between
individual pores in the substrate, can be further varied by
choosing the DC anodizing voltage appropriately. High
anodizing voltages lead to low pore densities, whilst low
anodizing voltages tend towards the film with high poor
densities. The anodizing voltage has a lesser effect on
the individual pore sizes.
After the anodizing process has been completed,
the diameters and the depths of the pores can be further
adjusted by etching the oxide film with an appropriate acid
medium. As noted above, this can be done before or after
deposition of metal into the pores. This acid etching thus
makes the final adjustment in the thickness of the oxide
layer which eventually separates the metal substrate from
the deposited conductor metal in the pore.
The acid chosen for etching purposes, to remove
controlled amounts of the oxide film, can be selected from
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the same group of acids as chosen in respect of the
anodizing bath.
Deposition of metal or other conductor into the
uniform pores to form the nanowires is suitably
accomplished by immersing the previously anodized alllm;nllm
surface into an appropriate electrolyte containing the
desired metal in anionic form, then applying alternating
current between the all]m;nllm and a suitable counter
electrode such as graphite. The metallic particles
eventually formed are faithful replicas of the interiors of
the pores in the metallic oxide layer. There do not appear
to be any surface effects, inhibiting a true filling of the
pores to the lower and side extremities thereof with the
deposited metal. Individual metallic deposits are
initially formed within the pores, and grow from the bottom
of the pores upwardly and outwardly. The deposits
eventually coalesce into compact metal rods, posts or
cylinders, i.e. nanowires. The length of the nanowires
depends on the duration of the AC electrolysis process,
among other parameters.
In one em-bodiment of the present invention, after
the template has been etched back to expose the tips of the
nanowires, the tips of the nanowires are oxidized at their
ends remote from the all~m;nllm substrate, to form insulating
electrical barrier layers thereon. The oxidation is
conducted carefully, in a controlled manner, so that an
insulating, oxide layer of extremely thin but constant
thickness is formed. On top of this oxide layer there is
deposited a further conductor such as a metal (e.g. gold)
so that a double-barrier device is created, having 2
insulating layers through which single-electron tunnelling
can occur upon application of suitable voltage, in large
number arrays. The uniformity of the pores initially
formed in the all]m;nllm oxide on the substrate allows for
the high degree of uniformity of metal deposited in such
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pores, which in turns allows for the high degree of
uniformity of the oxide deposited on top of the metal, as
the second very thin insulating layer.
For preparing an insulating barrier layer on the
end of a nanowire, formed in the pores of the oxide as
described above, a process of oxidation can be conducted in
order to form an oxide insulating barrier layer. This can
be done in the presence or absence of the initial metal
substrate. Oxidation of metals is a simple, well
controlled process, which can be conducted by heating the
metal in the presence of oxygen, e.g. in an oven or under
a heat lamp. The temperature to which the metal is heated,
the time of exposure to oxygen and the nature of the
atmosphere determine the depth of formation of the
insulating oxide layer on the end of the nanowire, and is
readily and easily controllable, since the oxidation is
generally a slow process. Normally, barrier layers of a
thickness of from about 10 to 1,000 Angstroms, most
preferably from 10 to 100 Angstroms are formed. The best
thickness depends to some extent upon the nature of the
oxide material, e.g. on its dielectric constant and
bandgap. The higher the dielectric constant, the thinner
the oxide layer needs to be.
Instead of oxide formation to provide these
barrier layers, other methods can be adopted, provided that
they are controllable to give appropriate thicknesses of
layers. Thus, insulating or semiconducting materials can
be applied by solution deposition, for example when a
polymeric insulating material is chosen. Deposition can
also be conducted by vapour deposition by chemical means,
sputtering or direct vapourization, all of which processes
are capable of exact thickness control. When the initial
all]m;mlm substrate and residual oxide has been removed, so
that it is required to provide a new barrier layer on both
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ends of the nanowire, these can be provided simultaneously,
especially in cases where the same insulating material to
substantially the same thickness is being applied to both
ends, or sequentially in the case where different
insulating materials are being applied to each end.
Any such insulating barrier layer provided on a
device according to the invention is overlayed by a
conducting layer, preferably a metal layer, to which
external electrical contact is made. In the case where the
alllm;nllm substrate remains in place as one of the external
connecting conductors, only the distal end of the nanowire
carrying the insulating barrier layer needs to be provided
with a metal deposit. Suitable methods for applying metal
over the insulating barrier layers are known in the art,
and include electrical chemical methods, sputtering and
other metal vapour deposition techniques. In the case
where the initial aluminum substrate and its associated
oxide layer have been removed, it is usual but not
necessary to apply the same metal to both ends of the
nanowire, by one or other of these techniques, depending
upon the electrical characteristics desired in the final
product.
Reference has been made above to embodiments in
which the initial substrate material, normally alllm;nllm is
removed from the device, and replaced by another metal for
electrical connection purposes. This can be performed by
chemical methods, e.g. by etching with appropriate acids
under carefully controlled conditions. By suitable choice
of chemicals, within the skill of the art, the metal
substrate can be etched away, and subsequently if desired,
the residual metal oxide on the end of the nanowires can be
etched away also, for replacement with another chosen
barrier material. In this way, the metal material for use
as the conductor, and the barrier material, can be chosen
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from a wide range of different materials selected on the
basis of their electrical characteristics for use in the
chosen application.
A wide range of metals is available to form the
nanowires in the pores of the oxide insulating layer,
according to the present invention. The basic criteria for
its selection are that it must be a good electrical
conductor, and that it must be depositable in the pores of
the oxide layer by practical, acceptable means. It is also
preferred that any chosen metal should be one which can
readily and controllably be converted to an electrically
insulating, or at least less electrically conducting,
compound thereof, by relatively simple chemical means such
as controlled oxidation. Examples of suitable such
preferred metals are nickel, cadmium, bismuth, iron,
titanium, niobium, silver, gold, and platinum.
Figure 1 of the accompanying drawings illustrates
diagrammatically the nature of the pores template formed in
anodic aluminum oxide, electrically chemically oxidized in
an acid electrolyte. Residual al-lm;nl~m substrate 10 has a
layer of oxide 12 thereon, with uniform, parallel pores 14
extending therethrough, substantially perpendicularly to
the plane of the residual aluminum layer. A portion 16 of
the al-lm;n-]m oxide layer separates each pore from the
alllm;n-]m substrate layer 10. Each of the pores 14 is open
at its distal end 18, remote from the alllm;nl]m substrate
layer 10. By tuning the key parameters of the
electrochemical oxidation process - voltage, temperature,
acid electrolyte and time, one can obtain pores of any
length up to several microns and any diameter between about
10 and 200 nanometers (100 and 2000 Angstroms).
In the next stage of the process according to the
invention, metal is electrochemically deposited into the
pores by transferring the pre-anodized alllm;nl~m to an
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appropriate electrolyte followed by AC electrolysis. As
electro deposition continues, metal fills the pore from the
bottom upward. This forms nanowires in the pores.
Figure 2 is a schematic view of the cross section
of such a nanowire made by depositing nickel into the pores
14 of the alllm;nllm oxide layer 12 generally illustrated in
Figure 1. The wires 22 are separated from the bottom most
aluminum substrate 10, which provides an electrical
contact, by an aluminum oxide thickness of insulating
barrier 16 of about 250 Angstroms. By a process of
controlled oxidation, a nickel oxide insulating layer 24
has been provided on the topmost portion of the nickel
nanowire 22, and by electro deposition, a layer of silver
metal 26 has been applied over the nickel oxide layer 24,
so that the silver layer and the alllm;nllm substrate provide
electrical contacts, separated by a double barrier device
formed of the nickel nanowire 22, the alllm;nllm oxide
barrier portion 16 and the nickel oxide barrier portion 24.
The thickness of the nickel oxide layer 24 is about 80
Angstroms. The nickel wire 22 is about 120 Angstroms in
diameter, and about 9,000 Angstroms in length. The
electrical performance of this device, and further detail
of its preparation, are described below in Example 1.
Figure 3 of the accompanying drawings illustrates
diagrammatically in cross section a different, preferred
embodiment of the present invention, comprising an alllm;mlm
substrate 30 having porous oxide layers 32,34 on each side
thereof. The pores 36,38 therein extend close to the
alllm;mlm substrate 30 but there is provided a barrier 40 of
insulating aluminum oxide residue, between the pores and
the alllm;nllm substrate. Metal e.g. nickel has been
deposited into the pores, to form nanowires 42,44 extending
away from the all~m;nllm substrate. A metal, e.g. silver has
been electro deposited over the distal ends of the
respective sets of nanowires to provide macromolecular
`~ - 15 - 2137013
metal contacts 46,48.
With the device illustrated in Figure 3,
electrical contact can be made to the aluminum substrate
sheet 30, and to each of the peripheral metal layers 46,48.
Thus, different signals and settings can be provided to the
central substrate 30. Conductivity through this substrate
between metal layers 46 and 48 depends upon single electron
tunnelling through the insulating barriers of oxide at each
side of the substrate sheet. Accordingly, the device shown
in Figure 3 can be operated in the manner of a transistor
and as such incorporated into electronic circuits, e.g. for
digital purposes.
The invention is further described, for
illustrative purposes, in the following specific examples.
Example 1
To prepare single-electron devices by the process
of the present invention, a super purity (99.99~) Alllm;nllm
sheet, after being degreased in trichloroethylene, was used
as the substrate, and anodized at 20V using a 0.5M sulfuric
acid electrolyte and a high purity lead foil cathodes.
The sample was then rinsed in distilled water and
then immersed in an aqueous electrolyte consisting of
120g/1 nickel sulfate and 45g/1 boric acid. The nickel
metal was deposited into the pores by A.C electrolysis at
23C , at 200Hz and 16 V rms using graphite counter-
electrodes to form the nanowires.
The alumina film was then etched to expose the
tips of the nickel wires, in a solution consisting of 0.5M
phosphoric acid and 0.2M chromic acid at 80C for a period
of time varying from 30 - 120 seconds. For some samples, to
create a parallel array of equal length wires, the exposed
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tips were then polished off with gamma micropolish alumina
(0.05). The samples were then left at room temperature for
48 hours in order to allow the formation of a nickel oxide
tunnel barrier on the end of the nickel wire. After
allowing the oxidation to take place, the samples were
sonicated for two minutes in ethanol, air dried, then
transferred to a vacuum chamber where silver pellets were
evaporated and then deposited on the surface to form a
contact.
Samples as prepared above provided nanowires
separated from the bottom Al contact by an alllm;mlm oxide
layer of about 250 Angstroms thickness, and from the top
silver contact by a nickel oxide layer of about 80
Angstroms thickness. The wires were about 120 Angstroms in
diameter, about 9000 A in length, and about 1olO per sq. cm
in area density. By controlling key parameters such as the
oxidizing medium, post-anodizing etch time, the time for
A.C. > deposition, temperature, and concentrations of
electrolytes one can obtain pores filled with various
materials with lengths up to several microns and any
diameter between ~10 and ~200nm. The standard deviations in
the wire diameter can be controlled to below ~15~ of the
mean diameter.
EXAMPLE 2
The samples were measured at room temperature,
using both Keithley 237 and HP4145. Most samples were
~0.5cm2 with a nickel wire density of ~3 x 10 10 cm~2.
Figure 4 of the accompanying drawings shows the current-
voltage, typical of this family of samples.
The clearly observable stair-case like I-V
characteristics on Figure 4, the rather sharp rise edges of
the stairs, the plateaus, and the very small current before
the onset of the first step are reminiscent of single
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electron tunnelling in double-junction systems predicted by
Coulomb blockade theory and observed in experiments at low
temperatures.
The very small ~;m~n~ions of the junctions are
consistent with the possibility of observing room-
temperature Coulomb blockade effects. The unusually large
onset voltage of the first current step (~0.9V) and the
large voltage intervals between steps are consistent with
the predictions of the conventional theory if the
geometrical and material parameters characteristic of these
structures are used. The sharpness of the steps on Figure
4 is also consistent with theory provided that there is a
large difference in the resistances and capacitances of the
two junctions.
Coulomb blockade theory predicts that the onset
voltage of the first stair occurs at V1~ e/(2C1), i.e. V1 ~
lV when C1 is of the order of 10-19F. Assuming that in the
samples according to the invention the top (nickel oxide)
barrier is a hemispherical cap with a tip diameter of
~lOnm, a thickness of ~8nm and has a relative dielectric
constant of ~4 its capacitance (C1) would be approximately
5xlO-19F. A similar calculation for the bottom (aluminum
oxide) junction assuming a thickness of ~25nm and a
dielectric constant of ~4.5 leads to a capacitance (C2) of
approximately 2xlO-19F. The ~;men~ions used in the
calculations are consistent with electron microscopy
measurements. The estimated capacitance values are small
enough to satisfy the inequality kT ~ e2, c = C1C2
2C c1+C2
the prerequisite for observing single electron tunnelling
at room temperature. With these values of capacitance the
calculated onset voltage would be approximately 0.2V, which
is of the same order of magnitude as the experimentally
measured 0.85V.
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It should be noted that for Coulomb blockade to
be observed, the wires should be well isolated from the
environment, i.e. the tunnelling resistance should be well
above the quantum resistance RQ = e2/h = 23.6 kQ. This
condition is satisfied for our samples.
It is to be noted that these results were
obtained at room temperatures. At those temperatures,
single electron events are rarely observable unless the
energy change of the system associated with one electron
crossing is much greater than kT. Structure parameters of
the devices described herein give capacitances for the two
junctions sufficiently low for this to occur. The
capacitance values found are consistent with those required
for the observed voltage differences between subsequent
lS current jumps in a double-junction model. The system
demonstrates Coulomb blockade effects.
