Origin of Coulomb Blockade Oscillations in Single-Electron Transistors

1
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
2
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

3
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.

4
Different Contact Designs (Type 1)
Gate
Gate
Cr
Au
SiO2
Island
Island
Drain
Cr
Drain
Source
Cr
Cr
Source
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.
5
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)
• Rlt2 kO/?, weak temperature dependence
• 2 kO /?ltRlt7 kO /?, significant nonlinearities
  and a temperature dependence characteristic of
  variable range hopping were observed however,
  none of the devices exhibited Coulomb blockade
  oscillations.
• Rgt7 kO /?, all of the devices were frozen out,
  showing no conductivity below 5 KO.

6
Different Contact Designs (Type 2)
Gate
Gate
Au
Cr
Au
Cr
SiO2
Island
Island
Drain
Drain
Source
Cr
Cr
Cr
Source
Cr
Type 2 large tabs only cover the steps of
source and drain and no tabs appear on the island.
7
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 Rlt100k 100k ltRlt200k 200k ltRlt1M Rgt1M
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 O.
• Devices with higher resistance were more likely
  to show Coulomb blockade oscillations

8
Low Temperature Measurements (Type2)
(b)
(a)
(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.
9
Low Temperature Measurements (Type2)
Charging diagram of an SET measured at 300 mK
showed a charging energy of 0.4 meV.
10
AFM Images
(b)
(a)
Gate
Au island
CrOx wire
with tabs
CrOx wire
Au island
Large tab

1. AFM image of a CrOx wire deposited on the edge of
   Au island.
2. The AFM image of an abnormal SET revealed that
   only two edges were covered by large tabs in the
   sample with a pattern shift.

11
Step Edge Junctions or Resistive microstrips with
right resistance and capacitance?
(b)
Au island
Cr
SiO2
Au
(a)
Au island
Cr
Cr
(c)
Au island
Cr
SiO2
RgtRQ 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.
12
AFM Image
Island
Gate
CrOx
Gate
Au layer
The abnormal devices which had a very rough
surface of CrOx films.
13
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 kO/?, 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

15
Low Temperature Measurements SETs with Thicker
CrOx Wires (Type 2)
(b)
(a)
(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
16
Low Temperature Measurements SETs with Thicker
CrOx Wires (Type 2)
Temperature dependence of an SET with thicker
CrOx wires
17
Low Temperature Measurements of a CrOx Wire
Crossed Over Two Au wires
Au
Gate
Cr
Drain
Source
(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.
18
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.

19
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
  gtgt kBT.
• the resistance of the tunnel barriers, RT gt 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