Transcript Exotic Coulomb blockade oscillations

Origin of Coulomb Blockade Oscillations in
Single-Electron Transistors
Fabricated with Granulated Cr/Cr2O3
Resistive Microstrips
Xiangning Luo, Alexei O. Orlov, and Gregory L. Snider
University of Notre Dame, Dept. of Electrical Engineering, Notre Dame,
IN 46556
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Outline
 Purpose: to understand single-electron devices
with resistive microstrips instead of tunnel junctions
 Can single-electron transistor be built using only
resistors with no tunnel junctions?
 SETs with metal islands and resistive microstrips
are fabricated and tested. Coulomb blockade
oscillations are observed, but what is the origin of
these oscillations?
 Possible mechanisms for Coulomb blockade
oscillations are investigated and discussed
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Fabrication of CrOx SETs by Two Steps of
E-beam Lithography and Deposition
 Why two steps? To eliminate junctions!
 First layer e-beam lithography and metal deposition
define the Au electrodes and island (2 nm Ti and 10
nm Au)
 The CrOx resistive microstrips connecting the island
to the electrodes are formed in the second e-beam
lithography and deposition step.
 Cr (8 nm-10 nm or 40 nm) was evaporated in the oxygen
ambient. By controlling the oxygen pressure and
deposition rate, different values of sheet resistance of
CrOx film were achieved.
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Different Contact Designs (Type #1)
Gate
Gate
Cr
Au
SiO2
Island
Cr
Source
Drain
Source
Island
Cr
Drain
Cr
Cr
Type #1: large tabs (wider than 300nm in two dimensions) on both
ends cover all of the steps where the two layers of metal overlap.
Au
Cr
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Measurements on type #1 (tabs everywhere) SETs
 Over 95% devices showed conductance at room
temperature.
 The CrOx films were very uniform and lasted for a
long time when exposed to air.
 In the low temperature measurement (300mK)
 R<2
kΩ/□, weak temperature dependence
 2 kΩ /□<R<7 kΩ /□, significant nonlinearities and a
temperature dependence characteristic of variable range
hopping were observed; however, none of the devices
exhibited Coulomb blockade oscillations.
 R>7 kΩ /□, all of the devices were frozen out, showing
no conductivity below 5 KΩ.
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Different Contact Designs (Type #2)
Gate
Au
Cr
Gate
Cr
Au
SiO2
Island
Cr
Source
Drain
Source
Island
Cr
Cr
Drain
Cr
Type #2: large tabs only cover the steps of source and drain and no tabs
appear on the island.
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Measurements on type #2 SETs
Coulomb blockade oscillations were only observed in devices with
NARROW LINES touching the island
The yield vs. resistance of type #2 devices
Resistance range
R<100k 
100k<R<200k
200k<R<1M 
R>1M
Total number of devices
12
9
5
4
Number of devices showed CBO
0
5
3
3
Yield
0%
56%
60%
75%

 Coulomb blockade oscillations were only observed when the resistance of
devices was greater than 100 k Ω.
 Devices with higher resistance were more likely to show Coulomb
blockade oscillations
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Low Temperature Measurements (Type#2)
(a)
(b)
(a) I-V curves of an SET in open state and blocked state. (b) I-Vg modulation
curve of the same SET of (a) measured at 300 mK showed deep modulation
by the gate.
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Low Temperature Measurements (Type#2)
Charging diagram of an SET measured at 300 mK showed a
charging energy of ~ 0.4 meV.
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AFM Images
(a)
(b)
Au island
CrOx wire
Large tab
Gate
CrOx wire
with tabs
Au island
(a) AFM image of a CrOx wire deposited on the edge of Au island.
(b) The AFM image of an abnormal SET revealed that only two edges were
covered by large tabs in the sample with a pattern shift.
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Step Edge Junctions or Resistive microstrips
with “right” resistance and capacitance?
(b)
Au island
Cr
(a)
Au island
Cr
SiO2
Au
Cr
(c)
Cr
SiO2
Au island
R>RQ
C « e2/2kBT
Top view (a), cross section (b) of step edge junction, the areas where step
edge junctions formed are marked by circles, and cross section (c) showing
resistive microstrips with “right” resistance and capacitance.
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AFM Image
Island
Gate
Au layer
CrOx
Gate
The abnormal devices which had a very rough surface of CrOx films.
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Multiple Frequencies in I-Vg Modulation Curves
Multiple frequencies in I-Vg modulation curve of abnormal devices with a
very rough CrOx surface.
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SETs with Thicker CrOx Wires (Type #2)
 SETs with thicker (~ 40 nm) CrOx wires were also fabricated using
pattern design type #2 with different widths of island (80 nm 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 (8-10nm)
CrOx wires.
 Among those devices having significant nonlinearity, about 95% (21
out of 22) exhibited Coulomb blockade oscillations, which is much
higher than that of SETs with thinner CrOx wires.
 Tunnel barriers other than step edge junction formed at the interface of
Au island and CrOx providing small enough capacitance and resistance
lager than RQ to fulfilled the two requirements of Coulomb blockade
oscillations
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Low Temperature Measurements SETs with
Thicker CrOx Wires (Type #2)
(a)
(b)
(a) I-Vg modulation curves of an SET with 40 nm thick CrOx strips showed deep
modulation by the gate. (b) Charging diagram of the same SET of (a) measured
at 12 mK
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Low Temperature Measurements SETs with
Thicker CrOx Wires (Type #2)
Temperature dependence of an SET with thicker CrOx wires
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Low Temperature Measurements of a CrOx
Wire Crossed Over Two Au wires
Au
Cr
Source
Gate
Drain
(a) Schematic of the layer of a CrOx wire crossed over two Au wires. (b)
Coulomb blockade oscillations observed on this structure at 300 mK.
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SETs with Pt as the First Layer
 SETs with Pt instead of Au as the first layer and thicker (~
40 nm) CrOx as the second layer were also fabricated
using pattern design type #3.
 Most of the devices showed significant nonlinearity in I-V
curves at 300 mK.
 None of these devices showed any gate dependence.
 More experiments are needed.
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Conclusions
 Two basic requirements to observe single electron tunneling
effects:
 the total capacitance of the island, C, must be small enough
that the charging energy EC = e2/2C >> kBT.

 the resistance of the tunnel barriers, RT > RQ = 25.8 k to
suppress quantum charge fluctuations.
 Resistive microstrip itself does not provide localization of
electrons in the island - first requirement may not be fulfilled.
 Two possible explanations:
 Step edge “break junctions” with low C are formed at the
connecting interface between CrOx wires and Au wires
 Microstrips with small overlapping area and high resistance
may satisfy both requirements
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