Optical and electrical properties of Ge-implanted SiO2
layers on n-Si and p-Si
W.S. Leea, J.Y. Jeonga, H.B. Kimb, K.H. Chaeb, C.N. Whangb,S. Imb,*, J.H. Songc
aDepartment of Metallurgical Engineering, Yonsei University, Seoul 120-749, South KoreabDepartment of Physics, Atomic-scale Surface Science Research Center, Yonsei University, Seoul 120-749, South Korea
cAdvanced Analysis Center, Korea Institute of Science and Technology, Seoul, 130-650, South Korea
Received 2 August 1999; accepted 29 October 1999
Abstract
Ge ions of 100 keV were implanted into a 120 nm-thick SiO2 layer on n-Si at room temperature while those of 80 keV were
into the same SiO2 layer on p-Si. Samples were, subsequently, annealed at 5008C for 2 h to effectively induce radiative defects
in the SiO2. Maximum intensities of sharp violet photoluminescence (PL) from the SiO2/n-Si and the SiO2/p-Si samples were
observed when the samples have been implanted with doses of 1� 1016 and 5� 1015 cmÿ2, respectively. According to
current±voltage (I±V) characteristics, the defect-related samples exhibit large leakage currents with electroluminescence (EL)
at only reverse bias region regardless of the type of substrate. Nanocrystal-related samples obtained by an annealing at 11008Cfor 4 h show the leakage at both the reverse and the forward region. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Ge; SiO2; Implantation; Photoluminescence (PL); Carrier-transport
1. Introduction
The visible photoluminescence from porous Si at
room temperature has been studied to open an attrac-
tive ®eld of Si-based optoelectronics [1]. Recently, Si
or Ge nanocrystals embedded in SiO2 matrix have also
been of interest for the same purpose [2±5]. For a
fabrication technique of nanocrystals, ion implanta-
tion is a good candidate in that a given number of ions
can be placed at a controlled depth by changing ion
¯uences and acceleration energies. As byproducts of
the technique, radiative defects are usually induced to
SiO2 during implantation. Zhang et al. reported var-
ious PL bands peaked at 400 and 700 nm from Ge-
implanted SiO2 ®lms and attributed the PL to the
radiative defect centers and the quantum con®nement
effect of Ge nanocrystals in the SiO2 layers, respec-
tively [6]. The PL band near 400 nm was assigned the
T1 ! S0 (triplet to singlet) transition [7,8]. Electro-
luminescence (EL) study has also been started to
realize light-emitting devices using Si and Ge-nano-
crystals or radiative defects embedded in SiO2 layers
[4±7]. However, intense debates about the origins of
the EL still exist. Experimental and theoretical studies
using current±voltage (I±V) characteristics on the
metal-insulator-semiconductor (MIS) structures are
necessary to further understand the mechanisms of
Applied Surface Science 169±170 (2001) 463±467
* Corresponding author. Tel.: �82-2-2123-2842;
fax: �82-2-312-5375.
E-mail address: [email protected] (S. Im).
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 0 4 - 2
the EL and the carrier-transport [5,9±11]. In this paper,
we present the PL results from Ge-implanted SiO2
layers and the I±V results from various MIS structures
(Au/SiO2/n-Si and Au/SiO2/p-Si) obtained by Ge-
implantation and post-annealing. This explains the
mechanisms of the EL and the carrier-transport.
2. Experiment
Thermally grown 120 nm-thick SiO2 layers on
(1 0 0) n-type Si wafers (1 W cm) and p-type Si
wafers (7ÿ25 W cm) were implanted by Ge ions of
100 and 80 keV, respectively, at room temperature
(RT) with doses of 5� 1015, 1� 1016, and 5�1016 cmÿ2. According to TRIM95, most Ge atoms
exist within the oxide layer because the projected
range of Ge is about 50 nm for 80 keV and 60 nm
for 100 keVof ion energy. The peak concentrations are
expected to range from 1 to 10 at.%. After implanta-
tion, the samples were annealed in nitrogen ambient
for 2 h at 5008C to maximize the density of radiative
defects [7,9]. For Ge-nanocrystal formation in the
oxide, some of the samples (one oxide with 5�1016 Ge cmÿ2 on n-Si, the other with 5� 1015
Ge cmÿ2 on p-Si) were chosen to be annealed at
11008C for 4 h. Cross-sectional transmission electron
microscopy (XTEM) was performed to con®rm the
presence of Ge-nanocrystals in the SiO2 layer. Photo-
luminescence (PL) was obtained by using an UV laser
as a light source with an excitation wavelength of
351 nm and a power of 120 mW. All the spectra were
taken at room temperature by using a grating spectro-
meter and a photomultiplier tube (PMT) detector. A
cutoff ®lter to pass only long wavelength above
375 nm was used to block the light scattered from
the source. Metal-insulator-semiconductor (MIS) dot
structures for current±voltage (I±V) measurements
were prepared using semitransparent Au layers as
front electrodes with a thickness of 20 nm and a dot
diameter of 300 mm. Backside electrodes were
obtained using an indium paste. Electroluminescence
was observed using a current source. The current±
voltage characteristics were measured under both
forward and reverse bias conditions. In the present
study, the forward bias was de®ned by applying a
positive voltage to the semitransparent Au electrode
with respect to Si substrates.
3. Result and discussion
Fig. 1 shows the PL spectra obtained from the SiO2
layers annealed after implantation with different doses
of Ge. Sharp peaks of violet luminescence are
observed around 420 nm from all the samples. As
the dose increases up to 1� 1016 cmÿ2, the PL inten-
sity from the SiO2 on n-Si increases. But as the dose
increases above 1� 1016 cmÿ2, the intensity begins to
decrease. At the dose of 1� 1016 cmÿ2, the violet
luminescence is strong enough to be clearly observed
by the naked eye. Many investigators have reported
the origin of this luminescence [7±9]. Germanium
oxygen de®cient centers (GODCs) existing in Ge-
implanted SiO2 may be responsible for the violet
luminescence. According to Rebohle et al. a radiative
transition from a ground singlet state to a triplet state
shows the PL. The transition may occur at the oxygen
de®cient centers with O3BSi-GeBO3 structure [7].
These centers can be achieved by optimal thermal
Fig. 1. PL spectra obtained from samples annealed at 5008C for
2 h after RT-implantation at an energy of 100 and 80 keV with
doses of 5� 1015, 1� 1016, and 5� 1016 cmÿ2. The sample with
the highest dose, 5� 1016 cmÿ2 has again been characterized after
annealed at 11008 for 4 h.
464 W.S. Lee et al. / Applied Surface Science 169±170 (2001) 463±467
annealing processes for defect relaxation because
right after implantation, most of the implantation-
induced defects form non-radiative asymmetrical
structures like E0 centers (O3 � Si� �Si � O3). If
the dose is so high (over 1� 1016 cmÿ2) that the
non-radiative defects may be oversaturated in the
layer, the defect relaxation process may be limited.
In the case of the sample with p-Si, the maximum
intensity of the PL is obtained from the sample
implanted with a dose of 5� 1015 cmÿ2. It is thought
that for the sample implanted with the ion energy of
80 keV, the GODC radiative defects are most effec-
tively formed with 5� 1015 Ge cmÿ2.
Nanocrystal-related PL was expected to appear at
600ÿ630 nm but that luminescence is not detected at
all and only a small defected-related peak is observed
as shown in Fig. 1 (the sample legend is 5�1016 cmÿ2, 11008C, 4 h). It is thought that the prob-
ability for radiative transition is very small in the
system of Ge-nanocrystal/SiO2 matrix due to a high
density of dangling bonds at the interface between
nano-Ge and SiO2.
The current±voltage characteristics of Au/SiO2/n-
Si MIS structures prepared with various conditions are
shown in Fig. 2. No visible current leakage is observed
in unimplanted raw SiO2. Strong rectifying behaviors
are observed in the samples which have been
implanted and then annealed at 5008C for 2 h. Under
a forward bias, no current leakage occurs in the range
of applied voltage. Large current leakage is found at a
reverse bias and the EL was also observed (we did not
measure the EL spectra). The onset of light emission
was at a reverse current ofÿ15 mA. When the reverse
current was ÿ20 mA, a clear EL was observed with
the naked eye in a dark room. Compared to the defect-
related samples, nanocrystal-related sample annealed
at 11008C for 4 h exhibits different I±V characteristics
which show a slow current rising at about �10 V.
Fig. 3 shows I±V characteristics obtained from Au/
SiO2/p-Si MIS structures where the oxide layers have
been implanted with 5� 1015 Ge cmÿ2. The I±V char-
acteristics are similar to the case of the MIS structure
with n-Si substrate. However, the reverse leakage
current increases more rapidly in this sample than
in the sample with n-Si. The EL was also observed
under a reverse bias. The sample with Ge nanocrystals
on p-Si also shows a similar I±V behavior compared to
the nanocrystal sample with n-Si except for one
difference that the forward current rise is slower in
the sample with p-Si. It is probably due to the differ-
ence between the implanted Ge doses or nanocrystal
densities in the samples. The SiO2 on n-Si may have
ten times more Ge nanocrystals to help the injected
electrons tunnel than the oxide on p-Si. The presence
of nanocrystals was con®rmed by XTEM on both
samples with n-Si and p-Si.
The phenomena of little forward current leakage in
the defect-related samples, shown in Figs. 2 and 3,
Fig. 2. Current±voltage (I±V) characteristics of Au/SiO2/n-Si (MIS) structures obtained from the SiO2 layers annealed after implantation with
doses of 5� 1015, 1� 1016, and 5� 1016 cmÿ2 at RT. The I±V result from a MIS structure of raw SiO2 layer is also plotted as a reference.
W.S. Lee et al. / Applied Surface Science 169±170 (2001) 463±467 465
may be related to the location of the level of GODCs.
The location of the ground (singlet) state of the neutral
GODC (discussed as O3BSi-GeBO3 structure) is not
well de®ned within the bandgap of SiO2 but must be
way below the Fermi level of Au. According to
Fujimaki et al. [8], the ground state is inferred to
be located at �6 eV below the conduction edge of
SiO2. The excited state (triplet) is at about 3 eV above
the ground level. Hot electrons may be injected into
the triplet states from the conduction band of n-Si but
may not continuously tunnel through the thick oxide
under the applied electrical ®elds. The only way to
obtain current ¯owing may be Fowler±Nordheim
tunneling of injected hot electrons. A very high ®eld
is necessary for the electrons to tunnel through the
thick oxide.
Carrier transport in Au/SiO2/n-Si and Au/SiO2/p-Si
under reverse bias conditions can be explained using
the energy band diagrams in Fig. 4(a) and (b). An
inversion layer may form at the SiO2/n-Si interface.
This is very possible because the threshold voltage for
the inversion is estimated to be about ÿ1 V at the
unimplanted SiO2 thickness of 120 nm. The hot elec-
trons injected from Au metal to the triplet states of
GODCs fall down to the ground state recombining
with the holes injected from the inversion layer
as shown in the diagram of Fig. 4(a). Under high
electric ®elds, the neutral GODCs are excited or
impact-ionized by hot electrons and holes. The recom-
bination or the relaxation of the excited electrons can
be the major mechanisms of EL. In the case of Au/
SiO2/p-Si, holes are accumulated at the SiO2/p-Si
interface as shown Fig. 4(b) so that they may be
readily injected into the excited triplet states, and then
recombine with the hot electrons injected from Au.
During this recombination process, the EL is also
expected. According to the results from Figs. 2 and
3, the case of hole accumulation shows higher reverse
current than that of hole inversion at the same reverse
voltage-implying that the sample with p-Si needs less
voltage to achieve the EL than the sample with n-Si.
Fig. 3. Current±voltage (I±V) characteristics of Au/SiO2/p-Si (MIS) structures obtained from the SiO2 layers annealed after implantation with
doses of 5� 1015 cmÿ2 at RT.
Fig. 4. Schematic energy band diagrams under a reverse bias for
(a) Au/SiO2/n-Si and (b) Au/SiO2/p-Si structures with SiO2 layers
annealed at 5008C for 2 h after implantation at RT.
466 W.S. Lee et al. / Applied Surface Science 169±170 (2001) 463±467
4. Conclusion
In summary, we have implanted Ge ions of 100 and
80 keV with various doses into 120 nm-thick SiO2
layers in Si(1 0 0) and then have annealed the samples
to achieve the radiative defects and Ge nanocrystals.
Optimum dose windows exist for the most intense
violet PL probably because the samples implanted
with too high doses have the non-radiative defects
oversaturated in the oxide layer. The defect-related
samples show light emission and large leakage cur-
rents under reverse bias conditions where the carrier-
transport may occur through the recombination of
injected electrons and inversion holes regardless of
the type of the substrate. Possible EL mechanisms are
the impact-ionization of GODC states and the elec-
tron±hole recombination. The MIS structure with hole
accumulation (p-Si substrate) appears to need less
voltage for the EL than the one with hole inversion
(n-Si). The nanocrystal-related sample shows the
leakage at both the forward and the reverse conditions
because both the tunneling via the Ge nanocrystals
and the electron±hole recombination at the nanocrys-
tals are not so dif®cult.
Acknowledgements
Authors would like to thank Prof. W. Choi in
the electronic department of Yonsei University for
valuable discussion on the carrier-transport mechan-
ism. This work was supported in part by the Korea
Science and Engineering Foundation (KOSEF)
through the ASSRC at Yonsei University, and the
grants from KOSEF (1999-2-114-004-5).
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