Single Electron transistors and their applications

1
Single Electron transistors and their applications

• One of the early Single-Electron Transistors
  fabricated at ND
• by Dr. Islamshah Amlani (now with Motorola
  Research Division)

2
Basics of Single-Electron Tunneling

• Capacitor stores energy
•  EC C V2/2
• To charge a capacitor by 1e requires potential
  difference
• V e/C,
• The charging energy  EC
• To observe SET effects the following condition
  must be fulfilled
• ECgtgtkT 
• To pass the current capacitor must be leaky,
  but not too leaky (tunnel junction)
• Typical charge/discharge time DtRC
• From Heisenberg uncertainty principle

3
Why dont we see Coulomb blockade effects
everyday?

• For any "practical" capacitance used in
  commercial electronics, this energy is negligibly
  small compared to thermal energy at room
  temperature
• Charging of 1 pF capacitor by 1 electron requires
  
• EC e (1.60210-19 Coulomb/10-12 Farad) 0.1602
  meV,
• whereas thermal energy kT/e 26 meV at 300K
  (room temperature).

For 300K operation
C 1 aF
4
Basic facts about Coulomb blockade

• For isolated island the charge is quantized Qne
• How to make a single-electron device?
• The IV characteristic of an isolated nanoscale
  tunnel junction.
• What if we make an isolated island with two
  junctions?
• Non linear, 2 terminal element could be useful

No Vds bias applied
Vds bias applied
Warning! Very hard to obtain such an I-V from
single junction!!!
Can we control the current through the junctions
by external electric field?
5
Single-Electron Tunneling transistor ( SET)

• Not only source-drain bias can supply energy for
  charge transfer, but also the gate

• Transition from n to n1
• Number of electrons on the island changes by one
• Current only flows when n changes to n1
• Charge transfer one electron at a time

6
Single electron transistor in Vg and Vds
coordinates

• Stability plot Coulomb diamond

Current flows when Vg ne/2Cg
7
How to use SET as a supersensitive electrometer?

• An ultimate electrometer achievable charge
  sensitivity dQ10-6 e/Hz1/2
• Most of the noise comes from the environment
  (traps nearby, etc)
• Solution RF SET

8
How temperature affects the performance of SET
9
Measurement Setup for testing SETs

• Differential conductance measurement
• Two parameters are scanned

10
Single-Electron Transistors fabricated at Notre
Dame
11
Recent developments

• Device by Hubert George MSc (2008)
• Charging energy 3 meV
• Measurements are taken at 300 mK

12
What SETs are good for?

• The best electrometer possible
• Can detect extremely small change of charge
  (ltlt10-3 e/Hz1/2 ) with extremely small current
  flows (the only current from sensor is to charge
  the coupling capacitor CClt10-15 F)
• Coulomb blockade and single-electron effects are
  specifically important for molecular electronics,
  where the size is intrinsically small.
• Drawback of SETs is their sensitivity to
  fluctuations of the background charges

13
Problems shrinking the current-switch an emerging
of QCA idea
14
Quantum-dot Cellular Automata
Old Paradigm
New Paradigm
Current switch
Tunneling between dots
Polarization P 1 Bit value 1
Neighboring cells tend to align. Coulomb coupling
15
Metal-dot electronic QCA cells how do we make
them, and how do we test them?
Simple 4-dot cell is shown

• Dots small metal (Al) islands separated by
  tunnel junctions (Al203)
• Junctions area of about 0.1 x 0.1 mm2
  thickness is 0.1-0.5 nm
• Charging energy is small, so that operation
  temperature is low (lt1K)
• But easy to make and good for the proof of concept

16
All-metal SET fabrication
2nd evaporation
1st evaporation

• Aluminum Tunnel junction technology combining E
  beam lithography with a suspended mask technique
  and double angle evaporation
• Oxide layer between two layers of Aluminum forms
  tunnel junctions.

NanoDevices Group
17
How to wire Single-Electron devices?
0.7 mm

• Interfacing cells using optical lithography and
  wire bonding

300 mm
1.5 mm
18
Ultra-sensitive electrometers for QCA

• Sub-electron charge detection is needed
• Single-electron transistors are the best choice

SET electrometers can detect lt1 of elementary
charge.
19
QCA Two-Stage Shift Register

• Two latches with two electrometers
• Coupling by means of Capacitors
• Two-phase Clock is used
• One latch serves as input for the other

SEM micrograph
VIN
VCLK1
VCLK2
-VIN
20
Operation of QCA Shift Register
VIN
VCLK1
VCLK2
-VIN

• Small external input applied SR remains in
  neutral state
• CLK1 applied
• 1st latch switches. Input now can be removed
• CLK2 applied
• 2nd latch switches
• Process is repeated for the inverted input

21
Clocked molecular QCA
22
Scaling of Flash Memory Devices

• How many electrons are there per bit TODAY?
• Between 1,000 and 10,000
• Microelectronics
• progresses
• 
  down
• 
  to
  nanoscale
• Few or ONE electron per bit!

23
A prototype of Single-Electron Memory cell

• What to use as a readout of such a tiny charge?
• A single-electron transistor is the best choice
• We make memory cells here!
• Operates at low temperature (lt4K), because cells
  are still relatively large
• To achieve room temperature operation the cell
  size must be below 5 nm

Single-Electron transistor
Floating Gate
24
Single Electron Memory Operation

• ONE electron stored on the floating gate
  represents a bit of information

25
Demonstration of a Memory Cell with 20
electrons per bit

• It is better to store more than 1 electron
• Few electrons memory

26
SETs in different material systems

• Metal-metal oxide SETs
• Semiconductor dot SETs (GaAs, Si, InAs)
• Carbon nanotube SETs
• Nanotube and nanowire SETs (CNTs, InAs nanowires)
• If quantization energy Eq is comparable to EC,
  quantized energy spectrum will modify the
  periodicity (price is now EqEC)

27
RF Carbon nanotube (CNT) SET

• CNT SET as alternative of Aluminum SET.
• Measurement was done at 5 K.

28
Si SET fabricated at ND
29
Other significant applications

• SET as a photon detector
• SET as a mixer
• RF SET
• SETs in micromechanical applications
• SETs in microscopy (Charge sensitive tips)

30
Single Electron transistor as a radio-frequency
mixerR. Knobel, C. S. Yung, and A. N.
ClelandAPL 81 (532) 2002

• Fig. 1. (a) Electron micrograph of a typical
  sample, showing the 5050 nm2 overlap of the
  junctions and the two gate capacitors. The scale
  bar is 1 µm. (b) dc normal-state currentvoltage
  characteristic of the SET at 30 mK, where the
  modulation due to gate 1 is shown the same
  behavior was seen when modulating gate 2. The
  device parameters extracted from this measurement
  are CG1 4.21016 F, CG2 2.81016 F, R R1
  R2 850 k , and C 21015 F

31
Experimental setup for mixer

• Fig. 2. (a) Schematic diagram of mixing. The gate
  charge is set for maximum current (dashed
  vertical line), and a signal and (for heterodyne
  mixing) local oscillator voltage are coupled to
  the gate. (b) Mixing circuit schematic of both
  homodyne and heterodyne mixing. The SET is in the
  dashed box in the center. (c) Homodyne detector
  signal at dc, measured as a function of the
  signal voltage. The signal at 20 MHz was applied
  to gate 2, while the SET was in the
  superconducting state. Points are experimental
  data, and the dashed line is the expected Bessel
  function response. (d) Heterodyne mixing spectral
  density about if 152.15 Hz, with CGVS 0.1e,
  CGVLO 0.3e, LO/2 20 MHz, and S LO if,
  with the SET in the normal state. The vertical
  axis is in units of input signal charge spectral
  density qS CGVS.

32
Heterodyne detection (mixing)

• Fig. 3. (a) Amplitude of the mixing signal as a
  function of the dc gate charge for a range of VSD
  (offset for clarity, with VSD 4, 50, 100, and
  150 µV bottom to top), with     LO/2      20
  MHz and     if/2      152.15 Hz. (b) Modeled
  mixing current as a function of the dc gate
  charge, VLOCG 0.2e, VSCG 0.25e at 30 mK
  varying VSD as in (a). (c) Measured if current as
  a function of the rf charge on the gate for
      LO/2      20 MHz and     if/2     
  152.15 Hz, with fixed sourcedrain bias VSD 90
  µV. The three traces are for varying VLOCG/e. (d)
  Modeled if current for VSD 100 µV at 30 mK
  varying VLOCG as in (c)

33
Bandwidth limitations

• Fig. 4. (a) Mixing signal amplitude as a
  function of LO at constant if 152.15 Hz the
  signal and LO powers were 68 and 61 dBm,
  respectively. The low-frequency rolloff is due to
  high-pass filtering, and the high-frequency
  rolloff is due to the ISD/e limitation. The
  dashed line is a guide to the eye. (b) Mixing
  signal as a function of if for LO 50 MHz. The
  dashed line is a fit that gives a 3 dB
  single-side bandwidth of 250 Hz.

34
Single Electron transistor as mechanical
displacement meter

• The mechanical resonator (beam ) is made of
  etched GaAs wafer

35
SET as a photodetector
36
RF-SET or SET reflectometer

• Problem conventional SET-based electrometers
  have been limited by slow operation speeds,
  typically 1 kHz or less. Furthermore, the charge
  sensitivity at these low frequencies, while
  typically much higher than that of conventional
  electrometers, is limited by 1/f noise due to the
  motion of background charges.
• SET acts as a load to the LC resonant tank
  circuit.
• The read-out is done by monitoring the reflected
  signal through the tank circuit.
• RF-SET can operate even at frequencies in excess
  of 100 MHz, where the 1/f noise due to background
  charge motion is completely negligible.

37
The Radio-Frequency Single-Electron Transistor
(RF-SET) A Fast and Ultrasensitive Electrometer
R. J. Schoelkopf, P. Wahlgren, A. A.
Kozhevnikov, P. Delsing, D. E. Prober
Science, 1998
38
RF SET operation

• High-frequency response of the RF-SET. (A)
  Time-domain response of RF-SET for a large (5.5
  electrons peak-to-peak) signal, 10 kHz
  triangle-wave (dotted line) applied to the gate.
  (B) Small-signal (0.01e rms) response for 1.1-MHz
  sine wave on gate. S/N is approximately
  42, corresponding to a charge sensitivity of
  5.2  10 5 e/ . (C) Response to a small (0.006e
  rms) signal at 137 MHz

39
Noise performance

• Gain versus frequency for the RF-SET, showing the
  extremely large (100 MHz) bandwidth of the
  device. Frequencies investigated were only
  limited by cable losses in the cryostat, and
  cause the somewhat larger error bars at the
  highest frequency (137 MHz, solid triangle)
  shown. Lower two traces show the system noise,
  expressed in e/ , for operation in both normal
  (N) and superconducting (S) states.

40
RF SET limits
41
RF SET as an RF mixer
42
Current measurement by real-time counting of
single electrons

• Jonas Bylander, Tim Duty and Per Delsing Nature
  434, 361-364 (17 March 2005)
• The idea measure frequency of electron transfer
  of a known current

43
I-V characteristics of the array

• Upper inset above threshold, the current onset
  is sharp in the normal state, but more gradual in
  the superconducting state owing to the subgap
  resistance RSG
• Lower inset comparison of d.c. current
  measurement by counting individual electrons,
  that is, measuring a frequency (filled circles),
  and a conventional d.c. current measurement (open
  circles) by voltage biasing through a Stanford
  Research 570 transimpedance amplifier. Note that
  for very low currents, the spread in the
  frequency measurements is smaller than in the
  conventional current measurements.

44
Real-time electron counting

• Each peak in the reflected signal corresponds to
  one electron tunnelling into the SET island. The
  applied currents were, from top to bottom, 40, 80
  and 120 fA, corresponding to the frequency (f
  I/e) 250, 500 and 750 kHz, respectively. \
• Power spectral densities of the reflected power
  from the SET, measured using a spectrum analyser
  with 100 kHz resolution bandwidth. The peak
  frequencies are f 236, 460 and 700 kHz,
  corresponding to the currents 38, 74 and 112 fA
• The deviation from the nominal bias currents is
  due to a small offset in the current source.

45
Power Spectra of Single-Electron transfer
46
Noise in the SETs

• Johnson noise
• - usually negligible (thanks to small kT)
• Shot noise
• - very important! The source of back-action
• -but is lower than 2eIBW due to correlations
  in tunneling electrons (crudely by a factor of 2)
• Flicker noise
• - dominant at low f, may shift the operational
  point and totally break down the operation
• -solution avoid metal-oxide devices which has a
  lot of glassy amorphous material surrounding the
  junctions