113119-3
Lau, Tan, and Trigg
Appl. Phys. Lett. 89, 113119 ͑2006͒
observed that the values of Nt and the nanocrystal density
͑Nnc͒ were similar. For solid state reaction Si nc, other than
the potential well existing in the Si nc,8,16 defect states also
plays a major role in the contribution to the trap density. It
has been reported17 that traps present at the interfaces, grain
boundaries, and defects in Si nc ͑Ref. 5͒ are capable of in-
ducing charge trapping as well. Lu and Lai18 have reported
that high energy ball milling is often capable of creating
structural defects such as surface defects, dislocations, grain
boundaries, etc. Figure 4 shows high resolution TEM images
revealing ͑a͒ dislocations and ͑b͒ grain boundaries in the
solid state reaction Si nc. Dislocations and grain boundaries
are evident in the Si nc. These dislocations and grain bound-
aries may be deep traps that contribute to the approximately
100 traps per Si nc. It has been reported that in CdSe nano-
crystals, an Nt/Nnc ratio of 100 was observed for deep
traps.19 This is similar to solid state reaction where disloca-
tions are prominent. This explains why a higher Et at
0.17 eV, as a result of higher Tt at 2057 K, is required to
“free” the trapped holes as compared to the Et of 0.14 eV for
Si nc by plasma decomposition8 and 0.08 eV for amorphous
Si nc.7 Though defects in Si nc are presented, they are re-
ported to be efficient charge trapping centers.17
FIG. 4. High resolution TEM images revealing ͑a͒ dislocations and ͑b͒ grain
boundaries in the solid state reaction Si nc.
In conclusion, the SCLC theory can be used to model the
macroscopic electronic conduction mechanism of solid state
reaction Si nc thin film. Nt and Et values are found to be
1.46ϫ1018 cm−3 and 0.17 eV. The Nt/Nnc ratio is ϳ100 due
to the existence of structural defects. This is attributed to the
dislocations which exist as deep traps. The higher Et value,
as compared to Si nc synthesized by plasma deposition and
amorphous Si,8 shows that higher trap energy is required for
the release of holes in these deep traps. The high Nt/Nnc ratio
and Et values provide evidences of larger charge trapping
ability and longer retention times which is of high impor-
tance to Si nc memory devices.
simplified into a power law dependence where JϳVm, where
the exponent factor m=l+1. In this way, the gradient in the
log-log plot of the J-V relationship will directly give the
characteristic temperature and thus the characteristic energy.
Figure 3 shows ͑a͒ the SCLC power law fits to the data
in Fig. 2͑a͒ from 305 to 400 K and ͑b͒ the variation of ex-
ponent factor m with respect to inverse temperature. Upon
the power law fitting of Fig. 2͑a͒ results, Fig. 3͑a͒ reveals
that m decreases as temperature increases ͑or inverse tem-
perature decreases͒. This is as predicted by the SCLC theory.
Using Fig. 3͑b͒, the gradient of the straight line reveals that
the values of Tt and Et are 2056 K and 0.17 eV, respectively.
Kumar et al.15 has further approximated Eq. ͑1͒ to an
Arrhenius form,
1S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E. F. Crabbe, and K. Chan,
Appl. Phys. Lett. 68, 1377 ͑1996͒.
2U. Avci and S. Tiwari, and I. Khan, Appl. Phys. Lett. 84, 2406 ͑2004͒.
3Z. J. Horváth, Current Appl. Phys. 6, 145 ͑2006͒.
4H. W. Lau, O. K. Tan, B. C. Ooi, Y. Liu, T. P. Chen, and D. Lu, J. Cryst.
Growth 288, 92 ͑2006͒.
1 qVpNv
Et
qNtd2
J =
exp −
ln
,
͑3͒
ͩ ͪ
ͩ ͪ
ͫ
ͬ
2
d
kT
2 oV
s
5H. W. Lau, O. K. Tan, Y. Liu, C. Y. Ng, T. P. Chen, K. Pita, and D. Lu, J.
Appl. Phys. 97, 104307 ͑2005͒.
where the activation energy is
6H. W. Lau, O. K. Tan, Y. Liu, D. A. Trigg, and T. P. Chen,
Nanotechnology 17, 4078 ͑2006͒.
Et
Ea = ln
k
qNtd2
.
͑4͒
ͩ ͪ
7Z. Shen, U. Kortshagen, and S. A. Campbell, J. Appl. Phys. 96, 2204
͑2004͒.
2 oV
s
Using Eq. ͑3͒ a plot of ln J vs 1/T at a constant voltage will
give a gradient of Ea, as shown in Eq. ͑4͒ From Eq. ͑4͒, the
total trap density can be determined. Using this method, a
graph using ln I vs 1/T is plotted and Nt is found to be
1.46ϫ1018 cm−3 at a bias of 1 V. By a further examination
of Eq. ͑3͒, it is revealed that the current is almost indepen-
dent of temperature, where Ea=0, at a crossover voltage,7,8
8M. A. Ratiq, Y. Tsuchiya, H. Mizuta, S. Oda, S. Uno, Z. A. K. Durrani,
and W. I. Milne, Appl. Phys. Lett. 87, 182101 ͑2005͒.
9H. Grabert, and M. H. Devoret, Single Charge Tunneling: Coulomb Block-
age Phenomena in Nanostructures ͑Plenum, New York, 1992͒, Vol. 294.
10K. Nishiguchi, X. Zhao, and S. Oda, J. Appl. Phys. 92, 2748 ͑2002͒.
11H. E. Romero and M. Drndic, Phys. Rev. Lett. 95, 156801 ͑2005͒.
12T. A. Burr, A. A. Seraphin, E. Werwa, and K. D. Kolenbrander, Phys. Rev.
B 56, 4818 ͑1997͒.
13A. Rose, Phys. Rev. 97, 1538 ͑1955͒.
qNtd2
14P. Mark and W. Helfrich, J. Appl. Phys. 33, 205 ͑1962͒.
15V. Kumar, S. C. Jain, A. K. Kapoor, W. Greens, T. Aernauts, J. Poortmans,
and R. Mertens, J. Appl. Phys. 94, 1283 ͑2003͒.
Vc =
.
͑5͒
2
s
o
16T. Kamiya, K. Nakahata, Y. T. Tan, Z. A. K. Durrani, and I. Shimuzu, J.
Appl. Phys. 89, 6265 ͑2001͒.
Extrapolation of the curves from Fig. 2͑a͒ results in the con-
vergence of the curves. The corresponding voltage is the
crossover voltage Vcϳ100 V, as shown in Fig. 3͑a͒. From
Eq. ͑5͒, it is derived that Nt is 1.44ϫ1018 cm−3, which is
close to the Nt determined from Eq. ͑4͒.
17Y. Shi, K. Saito, H. Ishikuro, and T. Hiramoto, J. Appl. Phys. 84, 2358
͑1998͒.
18L. Lu and M. O. Lai, Mechanical Alloying ͑Kluwer Acedemic, Boston,
1998͒.
19R. A. M. Hikmet, D. V. Talapin, and H. Weller, J. Appl. Phys. 93, 3509
The value of Nt is 100 times larger than the Si nc density.
8
This is quite unlike the results reported by Ratiq et al. who
͑2002͒.
128.143.199.160 On: Sun, 14 Dec 2014 10:23:26