TIDS 15, Sant Feliu de Guixols, 1-5 September 2013
Electron tunneling between surface states and implanted Ge
atoms in Si-MOS structures with Ge nanocrystals
Issai Shlimak
Jack and Pearl Resnick Institute of Advanced Technology,
Department of Physics, Bar-Ilan University, Ramat-Gan, Israel
In cooperation with:
Itamar Baron, Shai Levy, Avraham Chelly, Zeev Zalevsky
Bar-Ilan University, Israel
Tiecheng Lu
Sichuan University, China
Alexander Ionov
A.F. Ioffe Physical-Technical Institute, Russia
Outline:
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4/8/2015
Motivation
Sample preparation
Influence of neutron irradiation
Results and explanatory model
Suggestion of a new logic device
I. Shlimak "C-V characteristics..."
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Motivation
MOS structures with NC-Ge embedded into an insulating SiO2 layer on a Si
substrate are considered a perspective material for nano-size novel electronic
devices, such as nonvolatile memory units. This is due to hysteresis of C-V
characteristics caused by charge trapping in nanocrystals.
In most of the reviewed works, the thickness of SiO2 layer was relatively small
(30-50nm) and NC-Ge were distributed throughout the SiO2 layer, (sometimes
even penetrating into the Si substrate).
This leads to conclusion that the main mechanism of NC-Ge charging is
electron transfer from the Si substrate. Vice versa, properties of Si substrate
could be influenced by implanted Ge atoms.
Typical high frequency C-V of p-Si MOS
without and with NC-Ge, (adopted from
Appl. Phys. Lett. 82,1212 (2003).
The main goal of this work was to eliminate the mutual influence NC-Ge and Si substrate, for which a special
series of samples have been fabricated with NC-Ge layer spatially separated from Si/SiO2 interface.
4/8/2015
I. Shlimak "C-V characteristics..."
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Motivation
MOS structures with NC-Ge embedded into an insulating SiO2 layer on a Si
substrate are considered a perspective material for nano-size novel electronic
devices, such as nonvolatile memory units. This is due to hysteresis of C-V
characteristics caused by charge trapping in nanocrystals.
In most of the reviewed works, the thickness of SiO2 layer was relatively small
(30-50nm) and NC-Ge were distributed throughout the SiO2 layer, (sometimes
even penetrating into the Si substrate).
This leads to conclusion that the main mechanism of NC-Ge charging is
electron transfer from the Si substrate. Vice versa, properties of Si substrate
could be influenced by implanted Ge atoms.
Typical high frequency C-V of p-Si MOS
without and with NC-Ge, (adopted from
Appl. Phys. Lett. 82,1212 (2003).
The main goal of this work was to eliminate the mutual influence NC-Ge and Si substrate, for which a special
series of samples have been fabricated with NC-Ge layer spatially separated from Si/SiO2 interface.
Sample preparation
1)Thick ( 640 nm) amorphous SiO2 films were initially grown on the <100> oriented p-Si substrates.
2) Ge+ ions with high dose 1017 cm-2 were implanted into SiO2 layer. The ion energy was 150 keV which corresponds
to the projection range 100 nm and the half-width of spatial distribution 30 nm.
As a result, implanted Ge layer in the form of single atoms and amorphous clusters was located under the
SiO2 surface and far from Si/SiO2 interface.
3) Gold spots with diameter 0.3 mm were deposited on top of the SiO2 layer to form the MOS structures.
4) Ge nanocrystals (NC-Ge) with average
diameter of 4-10 nm appear only after
sample annealing at 8000C
HR-TEM image of NC-Ge layer.
inset shows the image of a single Ge NC
4/8/2015
I. Shlimak "C-V characteristics..."
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MOS structures under study
Four types of MOS structures were studied
1) “Initial” samples (a p-Si MOS structure with thick
SiO2 layer, before ion implantation)
2) “Implanted” samples, (after Ge+ ion implantation,
before annealing), without NC-Ge
3) “NC-Ge” samples, with NC-Ge (after annealing at
800oC)
4) “Irradiated” samples, NC-Ge samples that were
irradiated with neutrons dose1020 cm-2 followed by
annealing of radiation damage at 8000C.
4/8/2015
I. Shlimak "C-V characteristics..."
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MOS structures under study
Four types of MOS structures were studied
1) “Initial” samples (a p-Si MOS structure with thick
SiO2 layer, before ion implantation)
2) “Implanted” samples, (after Ge+ ion implantation,
before annealing), without NC-Ge
3) “NC-Ge” samples, with NC-Ge (after annealing at
800oC)
4) “Irradiated” samples, NC-Ge samples that were
irradiated with neutrons dose1020 cm-2 followed by
annealing of radiation damage at 8000C.
C-V characteristics were measured at high frequencies (100kHz and 1MHz)
S-shape
4/8/2015
U-shape
U-Hysteresis
I. Shlimak "C-V characteristics..."
return of S-shape
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Explanatory model
accumulation V
C = C0
S-shape
<0
depletion V  0
C  Cmin
(1/C)=(1/ C0)+(1/Cdep)
U-shape
To the best of our knowledge, U-shape of C-V
has never been observed in MOS structures at
high frequencies.
inversion V > 0
C  C0 at low f (<104Hz)
C = Cmin at high f (>105Hz)
minority carriers
4/8/2015
I. Shlimak "C-V characteristics..."
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Explanatory model
accumulation V
C = C0
S-shape
<0
depletion V  0
C  Cmin
(1/C)=(1/ C0)+(1/Cdep)
U-shape
To the best of our knowledge, U-shape of C-V
has never been observed in MOS structures at
high frequencies.
This effect can be understood taking into account :
location of Ge atoms close to the surface and far from
the Si/SiO2 interface.
We believe that the U-shape of C-V is due to electron
tunneling between surface states and Ge atoms.
inversion V > 0
C  C0 at low f (<104Hz)
C = Cmin at high f (>105Hz)
minority carriers
The applied voltage V tilts the top of potential barrier between
surface and the nearest Ge atoms which leads to enhancement of
the electron tunneling from the surface states to Ge atoms. This
increases the positive surface charge Q and therefore also the
measured capacitance C = ∂Q/∂V.
Only Ge atoms nearest to the surface can participate in the
charging because the probability of tunneling exponentially drops
with distance. This limits the increase of Q and leads to the
capacitance saturation.
As a result, the layer of Ge atoms becomes negatively charged
and partially screens the electric field at the Si/SiO2 interface.
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I. Shlimak "C-V characteristics..."
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NC-Ge samples
Formation of NC-Ge makes tunneling
easier and leads to C-V hysteresis due
to charge trapping in NC-Ge.
The fact that hysteresis is not observed
in “implanted” samples shows that
single Ge atoms and amorphous
clusters cannot hold the charge
I-V characteristics
Electron tunneling between surface and NC-Ge leads to
appearance of an additional transport channel along the surface
“initial” sample, R0 = 2 GΩ
4/8/2015
“NC-Ge” sample, R0 = 400 MΩ
and falls down to 80 MΩ at |V| > 5V
I. Shlimak "C-V characteristics..."
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Neutron irradiation and second annealing
Ge space distribution
in “NC-Ge”” samples
Return of S-shape
Before neutron irradiation
After neutron irradiation and
annealing at 8000C
Restoring the S-shape indicates
that tunneling between surface
states and nearest NC-Ge is
suppressed.
This means that Ge atoms and
nanocrystals are removed from
the vicinity of the surface!.
Removal of NC-Ge from the surface was directly observed in HR-TEM images and using EDS (energy dispersive
elemental spectroscopy).
What happened to the Ge atoms?
HR-TEM of “irradiated” sample
4/8/2015
EDS of “irradiated” sample
I. Shlimak "C-V characteristics..."
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Neutron irradiation and second annealing
Ge space distribution
in “NC-Ge”” samples
Return of S-shape
Before neutron irradiation
After neutron irradiation and
annealing at 8000C
Restoring the S-shape indicates
that tunneling between surface
states and nearest NC-Ge is
suppressed.
This means that Ge atoms and
nanocrystals are removed from
the vicinity of the surface!.
Removal of NC-Ge from the surface was directly observed in HR-TEM images and using EDS (energy dispersive
elemental spectroscopy).
What happened to the Ge atoms?
Ge atoms were detected on the SiO2 surface of “irradiated” samples in the form of Ge oxide and elemental Ge by XPS .
This means that redistribution of NC is caused by an enhanced diffusion of Ge atoms toward SiO 2 surface during
annealing of radiation damage after fast neutron irradiation.
HR-TEM of “irradiated” sample
4/8/2015
EDS of “irradiated” sample
XPS (X-ray photoelectron spectroscopy) of SiO2
surface. (a) – “NC-Ge” sample, (b) – “irradiated” sample
I. Shlimak "C-V characteristics..."
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Possible application
Light illumination
White light generates the electron-hole pairs
across NC-Ge and Si forbidden bands which
significantly decreases variation of the surface
charge due to the applied voltage.
Combination of U-shape and Hysteresis suggests a possibility of a novel 2-bit (4-digit)
logic memory cell controlled by either electrical switching or light illumination
τ ~ t 0.76
Voltage and light-induced switching
of “NC-Ge” MOS structure
4/8/2015
Kinetics of the capacitance decay
after the gate voltage is switched off
I. Shlimak "C-V characteristics..."
Current time of relaxation τ as a
function of time of observation t.
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Summary
A series of p-Si based MOS structures were fabricated with NC-Ge embedded into a thick SiO2 layer.
Three distinctive features of the samples should be noted:
• thick SiO2 (640 nm) layer;
• high density of implanted Ge ions (1017 cm-2);
• location of implanted Ge atoms near the surface and far from Si/SiO2 interface
C-V measurements of the obtained MOS structures showed the following effects:
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S-shape of C-V characteristics in “initial” samples was replaced by U-shape after Ge ion implantation;
Formation of NC-Ge in “implanted” samples led to C-V hysteresis due to charge trapping in NC-Ge;
An explanatory model of U-shape based on assumption of electron tunneling between SiO 2 surface states
and nearest Ge atoms is discussed.
S-shape of C-V was recovered after neutron irradiation and annealing of radiation damage which was
explained by suppression of electron tunneling caused by removal of Ge atoms away from the surface.
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Combination of U-shape and hysteresis opens the possibility for a new
optoelectronic device based on optically controlled 2-bit (4-digit)
memory retention unit
4/8/2015
I. Shlimak "C-V characteristics..."
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THANK YOU!
THANK YOU!
C-V characteristics
of Si-MOS
structures
withimplanted
Ge nanocrystals
Electron
tunneling between
surface
states and
Ge atoms in
Si-MOS structures with Ge nanocrystals
IssaiIssai
Shlimak
Shlimak
Jack and Pearl Resnick Institute of Advanced Technology,
Jack and Pearl Resnick Institute of Advanced Technology,
Department of Physics, Bar-Ilan University, Ramat-Gan, Israel
Department of Physics, Bar-Ilan University, Ramat-Gan, Israel
4/8/2015
I. Shlimak "C-V characteristics..."
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Temperature dependence of I-V and C-V in “NC-Ge” samples
I-V characteristics,
Temperature increases (from bottom to top)
from 300C to 1200C
4/8/2015
C-V characteristics,
Temperature increases (from bottom to top)
from 300C to 2500C
I. Shlimak "C-V characteristics..."
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