how a single defect can affect silicon nano-devices …...how a single defect can affect silicon...
TRANSCRIPT
The Big Idea
• As MOS-FETs continue to shrink, single atomic scale defects are beginning to affect device performance
Source Drain
Gate
Outline
• The impact of a single atom on a MOSFET
• Locating a single atom in a transistor
• The potential for a single atom
Review of MOS-FETs I
Heavily n-doped Source and Drain S emi-
conductor
O xide
M etal
Lightly p-doped channel
Source Drain
Gate
e-
e- e-
e- e-
e- e-
e- e-
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h+ h+
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Review of MOS-FETs II Typical MOS-FET Curve - 300 K
0
1
2
3
-1.5 -1 -0.5 0 0.5 1
VGate (V)
I (n
A)
D
Gate
S
Negative
Holes Accumulate h+ h+ h+ h+
h+ h+ h+ h+ h+ h+
h+ h+ h+ h+ h+ e- e-
e- e-
e- e-
e- e- e-
e-
e- e- e-
e- e- e
-
e- e- e-
e-
e-
e-
e- e-
e-
S D
Gate
Positive
Electrons Invert e- e-
e- e-
e- e-
e- e-
e- e- e-
e-
e-
e-
e- e-
e- e- e-
e-
e- e- e-
e- e- e- e- e-
e- e- e- e- e-
e- e- e- e-
e-
Threshold Voltage (VT)
Not just shrinking….
High-κ dielectrics
Planar to 3D Strain
e- e- e- e-
e- e- e-
Source
Gate
Source Drain
Gate
3 nm Silicon Dioxide
Hafnium Oxide
Source Drain
Gate
Atomic Scale Defects
Source Drain
Dopant
Gate
e-
e- e-
e- e-
e- e-
e- e-
e- e-
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e- e-
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e- e-
e- e- e-
e-
e- e- e- e- e-
e- e- e- e-
e-
Leakage to gate
Random Dopants change VT
3 nm
Trap
e-
Threshold Voltage
M Pierre, et al. “Single-donor ionization energies in a nanoscale CMOS channel,” Nature Nano, 2010
VG
I
0 1 -1
ΔVT
10 nm
~ 10s of dopants
25 devices studied, ΔVT ≈ 1 V
Ordered Dopant Arrays
Heavily n-doped Source and Drain
Source Drain
Dopant
Shinada, et al. Nature (2005)
VT
N
Random
Std. Dev. = 0.3 V
VT
N
Ordered
Std. Dev. = 0.1 V
Outline
• The impact of a single atom on a MOSFET
• Locating a single atom in a transistor
• The potential for a single atom
Looking for a single atom
Annular dark-field scanning-TEM
Need to chop up device to look at it
K. Van Benthem, et al. “Three-dimensional imaging of individual hafnium atoms inside a semiconductor” Applied Physics Letters, 87 03104 (2005)
The Basic Idea: Cryogenic Temperatures
E
z EC
EF e- e-
Source Drain
e-
Dip: Quantum Dot
Peaks: Tunnel Junction
e- e-
12 Dopant
The Basic Idea: Coulomb Blockade
E
EC
EF eVSD ≈ 1 meV e-
VG
I G
GC
eV
13
aFV
C
V
eC
G
G 1001.0
10 19
Drain Source
Gate
Lower Gates • Heavily doped Poly • 10 – 40 nm long • 3 independent gates
Nanowire • ~20 nm x 20 nm x 500 nm • Surrounded by 20 nm SiO2
Upper Gate • Heavily doped Poly
A. Fujiwara, et al. APL 88, 053121 (2006)
14
Positive voltage on upper gate inverts wire
Poly Upper Gate
Poly Lower Gate
Silicon Dioxide
Crystalline Silicon
e-
e- e-
e- e-
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e-
LGS LGC LGD
Upper Gate
Source Drain
T = 4.4 K VSD = 2 mV VLGS,C,D = 0
Negative voltages on lower gates form tunnel barriers
Poly Upper Gate
Poly Lower Gate
Silicon Dioxide
Crystalline Silicon
e-
e- e-
e- e-
e- e- e-
e- e- e- e-
e- e-
e- e- e-
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e- e-
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e- e-
e-
LGS LGC LGD
Upper Gate
Source Drain
-0.8 -0.7 -0.6 -0.5 -0.4
10-2
100
10-1
10-3
VLGD
(V)
g (
S)
LGD
Peaks correspond to transport through QDs
e- e- e- e- e- e- e- e- e- e- e- e-
T = 40 mK VUG = 1 V VLGC,D = 0
Device 1: T =39 mK VSD = 1 mV VLGS,C = 0
Measure current while scanning VUG and VLGD
17
0.8
20
.86
0.9
0.9
4
10
-3
10
-2
VU
G (V
)
I (nA)
Periodic Coulomb blockade oscillations
A Device 1: T =39 mK VSD = 1 mV VLGS,C = 0
• 2 flavors of QDs
– A: few periods, more strongly coupled to LGD
– B: many periods, more strongly coupled to UG
18
B
Measure Gate Capacitances
LGD (aF) UG (aF) Ratio LGD/UG LGC (aF)
Dev. 1: Dot A 2.3 + 0.3 - 1.3 1.3 + 0.2 - 0.6 1.71 ± 0.02 < 0.1
Dev. 1: Dot B 3.2 ± 0.2 7.9 ± 0.3 0.41 ± 0.01 < 0.1
19
Locate the Dot
Simulated ½ device in FASTCAP
LGC LGD
UG
Si Wire
20
Locate the Dot
LGC LGD
UG
Si Wire
21
LGD (aF) UG (aF) Ratio LGD/UG LGC (aF)
Dev. 1: Dot B 3.2 ± 0.2 7.9 ± 0.3 0.41 ± 0.01 < 0.1
• Measure gate capacitances
• Simulate capacitances to
1 nm slices of wire
• Integrate between z1 and z2
• For what z1 and z2 does 𝐶𝑠𝑖𝑚 = 𝐶𝑚𝑒𝑎𝑠
for all gates
𝐶𝑠𝑖𝑚 = 𝑑𝐶
𝑑𝑧
𝑧2
𝑧1
𝑑𝑧
Location of Dots
Between LGD and UG
LGC LGD
UG
Si Wire B
0
-20 -50
A
A B
z1 = -40 ± 3 z2 = -19 ± 3
22
0
20 95
z1 = 17 ± 1
z2 = 87 ± 2
50 Location in nm
LGD
Dopant Location?
Deduced conduction band modulation
LGC LGD
UG
Si Wire A B
EF
EC
23
Dopants?
We see same QDs in multiple devices -The cause appears systematic -Strain from temperature change and oxide growth -Could help make faster finFETs
Finding a Dopant
• Very similar technique has been used to located individual dopants and interface traps
– Nathaniel Bishop, et al; “Triangulating tunneling resonances in a point contact” Arxiv 1107.5104 (2011)
Outline
• The impact of a single atom on a MOSFET
• Locating a single atom in a transistor
• The potential for a single atom
Ultimate Transistor?
Source Drain
Gate
Similar to:
Cheng Cen, et al. “Oxide Nanoelectronics on Demand” Science 323, 1026 (2009)
Martin Fueschsle, et al. “Spectroscopy of few-electron single-crystal silicon quantum dots” Nature Nano, 5, 502 (2010)
Dopant
Beyond the transistor
• World looks different on the atomic-scale
– Quantum regime
• This is a problem for current transistors
– Tunneling to the gate
• Could this “quantumness” become useful
Quantum Search
Number of Boxes
10
1000
Old Computer
10 μs
1000 μs
New Computer
10 ns
1000 ns
Quantum Computer
10 ns
100 ns
“Classical” Computer: To search x100 boxes takes x100 as long
“Quantum” Computer: To search x100 boxes takes x10 as long
Conclusions
• MOSFETs are reaching the point where the placement of a single atom can affect device performance
• New tools allow the location of a single atom to be determined within a MOSFET
• The quantum nature of a single atom could one day allow for much more powerful devices
