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Nanoscience and
Nanotechnology Letters
Vol. 2, 7–10, 2010
Comparison of Charge Storage Behavior of
Electrons and Holes in a Continuous
Ge Nanocrystal Layer
Ming Yang1 , Tu Pei Chen1 ∗ , Wei Zhu1 , Jen It Wong1 , and Sam Zhang2
1
2
School of Electrical and Electronic Engineering, Nanyang Technological University, 639798, Singapore
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 639798, Singapore
The charge storages behaviors of electrons and holes injected into a continuous nc-Ge layer embedded in SiO2 synthesized by the Ge ion implantation technique are investigated. The hole injection
is found to exhibit a much higher charging
speed
well as ato:
better retention in the nc-Ge layer as
Delivered
byasIngenta
compared to the electron injection.
The Technological
better hole retention
is explained in terms of slower charge
Nanyang
University
loss to the Si substrate as well as slowerIP
lateral
charge diffusion as a result of the larger oxide barrier
: 155.69.2.12
height. The faster hole injection isFri,
also30
attributed
to the
less significant lateral diffusion of injected
Jul 2010
05:36:07
holes during the charging period. The results indicate that the oxide barrier height plays an important
role in the charge storage behaviors of electrons and holes injected into the continuous nc-Ge layer.
Keywords: Ge Nanocrystals, Non-Volatile Memory, Charge Injection, Charge Retention, Lateral
Charge Diffusion.
Memory structures based on Ge nanocrystals (nc-Ge)
have received much attention for the next-generation nonvolatile memory (NVM) devices due to their extended
scalability and improved memory performance, such as
low operating voltage, fast program/erase speeds, good
endurance and long data retention time.1–10 Such memory
structure usually employs a nanocrystal layer located in
between of the control oxide and the tunnel oxide to replace
the floating gate in a conventional NVM. The memory
states can be maintained by the three-dimensional confinement of charges, i.e., electrons or holes, in nanocrystals.11
There have been many efforts devoted to the structural
properties,2–4 memory effects5 6 and degradation of memory performance,7 8 but few research has been done to compare the charge storage behaviors of electrons and holes
injected into the nc-Ge layer. Akca et al. studied the charge
and discharge dynamics of electrons and holes injected
into the nc-Ge based NVM structure.9 However, their work
did not provide the information of the performance difference between the injected electrons and holes in terms of
charging speed and charge retention, which are two important attributes for the NVM devices. One of the promising techniques to synthesize nc-Ge embedded in SiO2 is
the low-energy ion implantation technique which is fully
∗
Author to whom correspondence should be addressed.
Nanosci. Nanotechnol. Lett. 2010, Vol. 2, No. 1
compatible with the mainstream Si-based technology and
is able to achieve well-controlled size and depth distributions of nc-Ge.4 In our previous work, we synthesized the
nc-Ge layer embedded in SiO2 using the Ge ion implantation technique at an ion energy of 2 keV.10 The effect of
high temperature annealing on the charge storage behavior of electrons in the metal-oxide-semiconductor (MOS)
structures containing a continuous nc-Ge layer has been
reported. In this work, we further investigate the charge
storage behaviors of electrons and holes injected into the
continuous nc-Ge layer using capacitance–voltage (C–V )
technique. The different behaviors of injected electrons and
holes in terms of charging speed and charge retention are
compared and discussed in details.
Ge ion implantation into a 11 nm SiO2 thin film thermally grown on a p-type Si substrate was carried out at
the energy of 2 keV with the ion dose of 3 × 1016 cm−2 .
Subsequently, a 30 nm SiO2 layer was deposited onto the
implanted oxide by the plasma-enhanced chemical vapor
deposition (PECVD) technique. High-temperature annealing at 800 C was then performed in N2 for 30 minutes
to transform the narrow excess-Ge region into a continuous nc-Ge layer. Details about the sample fabrication
and the structural properties of the annealed sample have
been published somewhere else.10 The nc-Ge layer contains evenly distributed nc-Ge near the Si substrate with
an average size of ∼6 nm. The effective thicknesses of
1941-4900/2010/2/007/004
doi:10.1166/nnl.2010.1055
7
Comparison of Charge Storage Behavior of Electrons and Holes in a Continuous Ge Nanocrystal Layer
Yang et al.
the tunnel oxide and control oxide are ∼5 and 31 nm,
where SiO2 and Ge are the permittivity of the SiO2 and
respectively. To study the charge storage behavior, the
Ge, respectively, tC is the control oxide thickness between
MOS structure was fabricated by the deposition of Al gate
the gate electrode and the nc-Ge layer, and dnc-Ge is the
electrodes and Al backside contact. Figure 1(a) shows the
diameter of nc-Ge. The negative sign in Eq. (1) accounts
schematic diagram of the MOS structure with a continuous
for the fact that the hole injection (Qnc-Ge > 0) leads to
nc-Ge layer embedded in the SiO2 . All the electrical chara negative VFB , while the electron injection (Qnc-Ge < 0)
acterizations were performed with a Keithley 4200 semileads to a positive VFB . With tC = 30.9 nm and dnc-Ge =
conductor characterization system at room-temperature in
6 nm as determined from the TEM image,10 the Qnc-Ge
a light-shielded environment.
corresponding to different VCHARGE can be obtained.
The charge injection behaviors were investigated by conFigure 2 shows the dependence of Qnc-Ge on the chargducting the high-frequency (1 MHz) C–V measurements
ing time for the electron and hole injection with different
before and after a constant charging voltage (VCHARGE ) was
VCHARGE . For both electron and hole injection, the magniapplied to the gate electrode. The memory effects were contude of Qnc-Ge increases with the charging time. However,
firmed by the flatband voltage shifts (VFB in the C − V
it is interesting to note that under the same magnitude of
characteristics. The selective injection of electrons or holes
VCHARGE , the significant hole injection can be achieved at
can be achieved by choosing the right polarity of VCHARGE .
a much shorter charging time as compared to the electron
As shown in Figure 1(b), by applying a positive VCHARGE
injection. In other words, the hole injection shows a higher
of 18 V to the gate electrode for 1 s, a positive VFB of
charging speed than the electron injection. For example,
∼2.1 V can be observed from the C–V characteristics. The
the hole injection
at VCHARGE = −18 V is noticeable for a
Delivered by Ingenta
to:
positive VFB indicates the trapping of electrons in the conshort
charging
time
Nanyang Technological University of 100 s, but the electron injection at
duction band of nc-Ge.11 12 On the other hand, as shown
VCHARGE = 18 V is significant only after a relatively long
IP : 155.69.2.12
in Figure 1(c), by applying a negative VCHARGE of −18 V
charging
time of 100 ms. As explained later, this phenomFri, 30 Jul 2010 05:36:07
for 1 s, the VFB is −2.4 V. The negative VFB indicates
ena is associated with the lateral charge diffusion along the
the trapping of holes in the valence band of nc-Ge.11 12 The
continuous nc-Ge layer during the charging period. With a
relationship between the VFB and the density (Qnc-Ge of
sufficient long charging time, all positive VCHARGE lead to
trapped electrons/holes in nc-Ge can be described by11
the electron injection into nc-Ge (i.e., Qnc-Ge < 0), while
all
negative VCHARGE lead to the hole injection into nc-Ge
Q
1 SiO2
(1)
VFB = − nc-Ge tC +
dnc-Ge
(i.e.,
Qnc-Ge > 0). This suggests that all electrons/holes are
SiO2
2 Ge
injected from the Si substrate to the nc-Ge layer which is
very close to the Si substrate. Due to the relatively thick
(a)
(∼31 nm) control oxide between the nc-Ge layer and the
Al gate
Al gate, the electron injection from the Al gate under a
Control oxide
negative VCHARGE is effectively prevented.
nc-Ge
After the charge injection, the Qnc-Ge as a function of the
waiting time (up to 5 × 104 s) was measured. During the
Tunnel oxide
waiting period, both the gate electrode and the backside
Si substrate
contact were grounded. The C–V measurements and the
Al back contact
4
After VCHARGE = 18 V
for 1 s
(c)
15
10
5
15
10
5
Initial
Charged
Hole injection
3
Initial
Charged
20
Capacitance (pF)
Capacitance (pF)
20
After VCHARGE = –18 V
for 1 s
Qnc-Ge (×10–7 C/cm2)
(b)
VCHARGE = –18 V
VCHARGE = –14 V
VCHARGE = –10 V
2
1
0
–1
Electron injection
–2
VCHARGE = 10 V
VCHARGE = 14 V
VCHARGE = 18 V
–3
–4
–2
0
2
Bias voltage (V)
4
–4
–2
0
2
4
Bias voltage (V)
Fig. 1. (a) Schematic cross-sectional diagram of the MOS structure with
a continuous nc-Ge layer embedded in the SiO2 . The high-frequency
C–V characteristics show a positive VFB (b) and a negative VFB
(c) after the charging at VCHARGE = 18 V or −18 V for 1 s, respectively.
8
–4
10–6
10–5
10–4
10–3
10–2
10–1
100
101
Charging time (s)
Fig. 2. Evolution of Qnc-Ge as a function of the charging time for different VCHARGE .
Nanosci. Nanotechnol. Lett. 2, 7–10, 2010
Yang et al.
Comparison of Charge Storage Behavior of Electrons and Holes in a Continuous Ge Nanocrystal Layer
|Qnc-Ge| (×10–7 C/cm2)
extraction of Qnc-Ge were automated by a computer procharge diffusion leads to an extension of charge distribution from one charged MOS structure to its surrounding
gram. Figure 3 shows the charge retention characteristics
uncharged nc-Ge outside the MOS structure. In our previfor the electron and hole injections at VCHARGE = 18 V and
ous study,10 the comparison between MOS structures with
−17 V, respectively. The charging time was fixed at 1 s.
and without a continuous nc-Ge layer has confirmed that
Both of the two charging voltages resulted in a similar
the fast charge decay during the initial waiting period of
magnitude of Qnc-Ge immediately after the charging opera∼100 s is due to the lateral charge diffusion along the contion. As the waiting time increases, the magnitude of Qnc-Ge
tinuous nc-Ge layer, while the slower logarithmic decay
tends to return to its uncharged state (i.e., Qnc-Ge = 0),
after the initial 100 s is mainly contributed by the charge
indicating the loss of injected electrons/holes during the
leakage from the nc-Ge to the Si substrate.
waiting period. Moreover, the loss of injected electrons is
Knowing that the decay of Qnc-Ge for the MOS structure
faster than that of injected holes during the waiting period.
with a continuous nc-Ge layer is caused by two charge
For a total waiting time of 5×104 s, the percentage of
leakage mechanisms, the charge retention characteristics
electron loss is ∼43%, while the percentage of hole loss is
in Figure 3 can be further examined. The dependence of
∼33%. The result clearly demonstrates that the hole injecQnc-Ge on the waiting time (t) after the initial 100 s can be
tion leads to a better charge retention as compared to the
fitted
by a logarithmic relationship
electron injection.
Two distinct regions in the charge decay curves after both
Qnc-Ge t = a + b ln t
(2)
electron and hole injection can be observed. Over the initial
100 s, the charge decay is relatively fast. However,
afterby Ingenta
Delivered
where a to:
and b are fitting parameters. Such a logarithmic
the initial 100 s, it can be observed thatNanyang
the chargeTechnological
decay
University
relationship
between Qnc-Ge and the waiting time is consisis relatively slower and exhibits a logarithmic relationship
IP : 155.69.2.12
tent with the work of Busseret et al.14 The fitting indicates
between Qnc-Ge and the waiting time. For the MOS
Fri, structure
30 Jul 2010that
05:36:07
for both electron and hole injection, the dominant
with a continuous nc-Ge layer, two charge loss mechanisms
charge loss mechanism after the 100 s is the charge leakcan be identified during the charge retention measurements.
age to the Si substrate. Moreover, the logarithmic decay for
the injected holes is slower than that for the injected elecThe first one is the charge leakage from the nc-Ge to the Si
trons. This is because the holes injected into the valence
substrate across the tunnel oxide.13 14 This mechanism usuband of nc-Ge embedded in SiO2 theoretically encounter
ally leads to a logarithmic decay of trapped charges with
a larger oxide barrier height as compared to the electrons
waiting time due to the variation of the tunneling probainjected into the conduction band of nc-Ge.9 As a result,
bility when more charges tunnel out to the Si substrate.14
the loss of the injected holes to the Si substrate tends to
The second charge loss mechanism is the lateral charge difbe slower than that of the injected electrons.
fusion along the continuous nc-Ge distributed layer.8 Due
The amount of charge loss that is solely due to the
to the blanket Ge ion implantation, the continuous nc-Ge
charge leakage to the Si substrate during the initial 100 s
layer is also present in the oxide areas not under the Al gate
can be extrapolated based on the fitting. After subtracting
electrodes. Thus, a channel of lateral charge diffusion exists
the charge loss due to the charge leakage to the Si substrate
due to the charge transfer through the dissolved Ge atoms
from the measured charge loss, the charge decay characin the oxide between two adjacent nanocrystals. The lateral
teristic due to the lateral charge diffusion was obtained
and is found to follow an exponential decay. The exponential decay is consistent with previous studies about the
2.5
lateral charge decay using the technique of electrostatic
Charge leakage to the Si substrate
force microscopy (EFM).14 15 The percentage of charge
2.0
loss (Q/Q0 ) as a result of the exponential charge decay
due to the lateral charge diffusion is given by
1.5
Q
t
(3)
= 1 − exp −
Lateral charge diffusion
Q0
1.0
After hole injection (VCHARGE = –17 V for 1 s)
After electron injection (VCHARGE = 18 V for 1 s)
Fitting with logarithmic decay
0.5
0.0
101
102
103
104
105
Waiting time (s)
Fig. 3. Absolute Qnc-Ge as a function of waiting time for the electron
injection at VCHARGE = 18 V and the hole injection at VCHARGE = −17 V.
The solid lines represent the fittings based on Eq. (2).
Nanosci. Nanotechnol. Lett. 2, 7–10, 2010
where Q is the amount of charge loss due to the lateral
charge diffusion, Q0 is the total initial charges that contribute to the lateral charge diffusion, t is the waiting time
after the charge injection, and is the characteristic decay
time. As shown in Figure 4, the percentage of charge loss
caused by the lateral charge diffusion during the initial
waiting period for both electron and hole injections can be
well fitted with Eq. (3). The result confirms the existence
of the lateral charge diffusion during the initial waiting
9
Comparison of Charge Storage Behavior of Electrons and Holes in a Continuous Ge Nanocrystal Layer
Yang et al.
employed to synthesize a continuous nc-Ge layer embedded in the gate oxide of a NVM structure. The charge
storages behaviors in terms of charging speed and charge
80
retention for electrons and holes injected into such continuous nc-Ge layer have been studied. It has been found that
τ = 28.4 s
60
the hole injection has a much higher charging speed than
the electron injection, and the injected holes show a bet40
ter charge retention as compared to the injected electrons
after charging. Two charge loss mechanisms have been
After electron injection (VCHARGE = –17 V for 1 s)
20
identified. The better hole retention has been explained in
After hole injection (VCHARGE = 18 V for 1 s)
terms of slower charge loss to the Si substrate as well
Fitting with Eq. (3)
as slower lateral charge diffusion due to the larger oxide
0
barrier height encountered by holes. The faster hole injec0
50
100
150
200
tion is also attributed to the less significant lateral diffuWaiting time (s)
sion of the injected holes during the charging operation.
Fig. 4. Percentage of charge loss (Q/Q0 due to the lateral charge
The results indicate that the oxide barrier height signifidiffusion in the continuous nc-Ge layer after the charging at VCHARGE = 18
cantly affects the charge storage behaviors of the electrons
and −17 V, respectively. The characteristic decay time for the injected
and holes injected in the continuous nc-Ge layer. For the
electrons and holes obtained from the fittings based on Eq. (3) is 19.9
NVM application of the ion-implantation-synthesized ncand 28.4 s, respectively.
Delivered by Ingenta
Ge layer,to:
the hole injection scheme could be more promisNanyang Technological
ing asUniversity
it offers better memory performance as compared
period. The characteristic decay time obtained fromIP
the: fit155.69.2.12
to the electron injection.
tings is 19.9 s for the electron injection and 28.4
s
for
the2010 05:36:07
Fri, 30 Jul
hole injection, indicating that the lateral charge diffusion
References and Notes
of the injected holes is slower than that of the injected
electrons. This is because the oxide barrier height for holes
1. Y. C. King, T. J. King, and C. Hu, IEEE Trans. Electron Devices
in the valence band of nc-Ge is theoretically larger that for
48, 696 (2001).
2. W. K. Choi, Y. W. Ho, S. P. Ng, and V. Ng, J. Appl. Phys. 89, 2168
electrons in the conduction band of nc-Ge. As a result, the
(2001).
lateral diffusion of the injected holes from one nc-Ge to
3. J. Von Borany, R. Grötzschel, K. H. Heinig, A. Markwitz, W. Matz,
its adjacent uncharged nc-Ge outside the MOS structure is
B. Schmidt, and W. Skorupa, Appl. Phys. Lett. 71, 3215 (1997).
slower as compared to the case of the injected electrons.
4. A. Markwitz, L. Rebohle, H. Hofmeister, and W. Skorupa, Nucl.
Since the holes injected into nc-Ge have slower charge loss
Instr. Meth. B 147, 361 (1999).
5. H. Fukuda, S. Sakuma, T. Yamada, and S. Nomura, M. Nishino,
to the Si substrate and slower lateral charge diffusion, the
T. Higuchi, and S. Ohshima, J. Appl. Phys. 90, 3524 (2001).
charge retention for the injected holes is better than that
6. S. Duguay, J. J. Grob, A. Slaoui, Y. Le Gall, and M. Amann-Liess,
for the injected electrons.
J. Appl. Phys. 97, 104330 (2005).
On the other hand, the phenomenon that significant hole
7. M. Kanoun, C. Busseret, A. Poncet, A. Souifi, T. Baron, and
injection can be achieved with a much shorter charging
E. Gautier, Solid-State Electronics 50, 1310 (2006).
8. J. K. Kim, H. J. Cheong, Y. Kim, J.-Y. Yi, H. J. Bark, S. H. Bang,
time as compared to the electron injection can be explained
and J. H. Cho, Appl. Phys. Lett. 82, 2527 (2003).
as follows. Since the lateral charge diffusion is a fast
9. I. B. Akca, A. Dâna, A. Aydini, and R. Turan, Appl. Phys. Lett.
process that plays a major role during the initial wait92, 052103 (2008).
ing period, it causes the charge loss along the continu10. M. Yang, T. P. Chen, Z. Liu, J. I. Wong, W. L. Zhang, S. Zhang,
and Y. Liu, J. Appl. Phys. 106, 103701 (2009).
ous nc-Ge layer during the charging operation and affects
11. S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E. F. Crabbé, and
the charge injection. Since the lateral diffusion of holes
K. Chan, Appl. Phys. Lett. 68, 1377 (1996).
is slower than that of electrons, for the same charging
12. M. Yang, T. P. Chen, J. I. Wong, C. Y. Ng, Y. Liu, L. Ding, S. Fung,
time, the loss of the injected holes is less than that of the
A. D. Trigg, C. H. Tung, and C. M. Li, J. Appl. Phys. 101, 124313
injected electrons during the charging operation. Thus, the
(2007).
13. S. Huang, S. Banerjee, R. T. Tung, and S. Oda, J. Appl. Phys. 93, 576
minimum charging time required to achieve a noticeable
(2003).
Qnc-Ge for the hole injection is much shorter than that for
14. C. Busseret, A. Souifi, T. Baron, and G. Guillot, Superlattices
the electron injection.
Microstruct. 28, 493 (2000).
In summary, low-energy Ge ion implantation at 2 keV
15. C. Y. Ng, T. P. Chen, H. W. Lau, Y. Liu, M. S. Tse, O. K. Tan, and
followed by a high-temperature annealing has been
V. S. W. Lim, Appl. Phys. Lett. 85, 2941 (2004).
τ = 19.9 s
∆Q/Q (%)
100
Received: 11 January 2010. Accepted: 16 April 2010.
10
Nanosci. Nanotechnol. Lett. 2, 7–10, 2010