www.elsevier.com/locate/optmat

Optical Materials 27 (2005) 1050–1054

On the mechanism of electroluminescence excitation in Er-dopedSiO2 containing silicon nanoclusters

J.M. Sun a,*, W. Skorupa a, T. Dekorsy a, M. Helm a, A.N. Nazarov b

a Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf e.V., P.O. Box 510119, D-01314 Dresden, Germanyb Institute of Semiconductor Physics, NASU, Kyiv, Ukraine

Available online 8 October 2004

Abstract

The effect of the density of silicon nanoclusters on both electroluminescence (EL) and photoluminescence (PL) of Er3+ ions in

indium-tin oxide/SiO2:Er/n-type silicon metal-oxide-semiconductor structures was studied by co-implantation of excess silicon into

a 200nm SiO2 layer with a concentration in the range of 1–15%. Contrary to the PL, the EL from both the green and infrared peaks

of Er3+ shows a dramatic quenching when the average distance between the silicon clusters decreases below 3nm. In addition, elec-

tric-field-induced quenching of the photoluminescence from silicon clusters and Er3+ is observed. These results indicate that the EL

excitation process of Er3+ ions is governed by the direct impact excitation by hot electrons. An increase of the silicon nanocluster

density causes direct tunneling of electrons between silicon clusters, thus reducing the population of energetic hot electrons for

impact excitation of Er3+ ions.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Erbium; Electroluminescence; Impact excitation; Silicon dioxide; Nanoclusters

1. Introduction

Since the first report in 1983 [1] both scientific and

technological research has been focused on the lumines-

cence of Er3+ from silicon-based materials due to its

light emission at the optical telecommunication wave-

length and material compatibility to modern silicontechnology. A light-emitting diode (LED) based on

Si:Er:O has been demonstrated, which operates at room

temperature at l.54lm [2]. However, due to the strong

energy back transfer from excited Er3+ to free carriers

[3,4], the electroluminescence (EL) efficiency is low

(10�5–10�4) at room temperature. Recently, strong cou-

pling between Er3+ and silicon nanoclusters has been

found in Er-doped Si-rich SiO2, where the energy trans-fer from excited silicon nanoclusters to the Er3+ results

0925-3467/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2004.08.061

* Corresponding author.

E-mail address: j.sun@fz-rossendorf.de (J.M. Sun).

in a strong infrared photoluminescence (PL) from Er3+

[5]. Efficient EL from the Er-doped Si-rich SiO2 metal-

oxide-semiconductor (MOS) structures has been re-

ported by Iacona et al. [6]. Contrary to the PL behavior,

the incorporation of excess silicon in SiO2 causes a

strong quenching of the EL in the MOS structure, but

results in a better operation stability [7]. This indicatesa different excitation process and new concepts should

be applied for optimization of both material and struc-

ture in EL devices. However, up to now, the excitation

mechanism of EL from Er3+ in SiO2 and its quenching

caused by introducing silicon clusters has not been well

understood.

In this paper, the different effects of silicon nanoclus-

ter density on EL and PL of Er3+ ions have been studiedin Si-rich SiO2:Er MOS structures with excess silicon

content varying from 0 to 15%. An increase of the

green/infrared ratio of the EL from Er3+ lines and simul-

taneous quenching of the PL from both Er3+ and Si

nanoclusters are observed by increasing electric field.

J.M. Sun et al. / Optical Materials 27 (2005) 1050–1054 1051

These results give strong evidence that the EL of Er3+

ions is generated by direct impact excitation by hot elec-

trons. In addition, a strong decrease of the EL efficiency

from Er3+ was observed when the average distance

between the silicon clusters decrease below 3nm. The

current–electric field characteristics show that the popu-lation of direct tunneling electrons between silicon clus-

ters increases with increasing the density of silicon

clusters. Therefore the fraction of energetic hot electrons

for impact excitation of Er3+ ions is reduced, thus lead-

ing to strong quenching of the EL from Er3+.

2. Experimental

Samples were prepared by implanting Si+ and Er+

into thermally oxidized SiO2 with thickness of 200nm.

Depth profiles of implanted silicon and Er atoms were

calculated using TRIM 98 as a first approximation, as

shown in Fig. 1. Two implantation energies were used

for Si+ at 35 and 80keV, which generated a flat profile

of excess silicon inside a depth from 65 to 150nm. Theimplantation doses were varied to get excess silicon con-

centration from 0 to 15%. Er+ ions were implanted with

an energy of 280keV and a dose 1 · 1015cm�2, with the

profile located inside the depth region with constant ex-

cess silicon concentration. The concentration of Er3+ for

all the samples was constant at 0.24%. After implanta-

tion, samples were annealed at 1100 �C in a flowing N2

atmosphere for 1 h to form silicon nanocrystals. Latera 100nm transparent conductive indium-tin oxide layer

was deposited and processed into electrodes with diam-

eter of 0.5mm.

The PL of Er3+ and of Si nanoclusters was measured

using a 5mW He–Ne laser and a 532nm laser, respec-

tively, for excitation. EL spectra were measured under

a constant current by applying a positive voltage to

the ITO layer of the MOS structure. The PL and EL

0 100 200 3000

5

10

15

20

Er 0.24 %

15 %

11 %

5.6 %

3.0 %1.2%

Si SiO2

Er c

once

ntra

tion

(ato

mic

% )

Exce

ss S

i con

cent

ratio

n (a

tom

ic %

)

Depth (nm)

0.0

0.1

0.2

0.3

0.4

Si+ 35keV +80 keV

Er+280 keV

Fig. 1. Depth profiles of as-implanted Si and Er atoms in our samples

calculated by TRIM 98.

from the samples were collected by a Triax 320 mono-

chromator and detected by a Ge detector in the infrared

spectral range and by a photomultiplier in the visible

range. The current–voltage and EL-current characteris-

tics were measured at the same time with a computer

controlled multi-channel data acquisition system.

3. Results and discussion

Fig. 2 shows the PL spectra for samples with different

implanted silicon concentration of 3, 5.6 and 10.5%. A

strong increase of the PL from silicon clusters is ob-

served for implanted excess silicon above 3%, and,simultaneously, the PL line from Er3+ is strongly in-

creased as well. This reveals that the excitation of Er3+

is due to the energy transfer from Si nanoclusters to

Er3+.

The PL peak in the red region comes from recombi-

nation of excitons in silicon nanoclusters, considering

that the peak energy of silicon nanocluster increases

with decreasing the diameter due to the quantum size ef-fect. According to the theoretical calculation by Delerue

et al., the band gap, Eg, of a silicon nanocluster is related

to diameter d as: Eg = Eg0 + d�1.39 [8] where Eg0 is the

band gap of bulk Si. From the PL peak of the nanoclus-

ters, we can estimate that the average diameter of the sil-

icon clusters is around 2nm. This is consistent to the

direct with the value deduced from transmission electron

microscopy image of silicon-rich SiO2 annealed at thesame temperature [9].

EL from Er3+ can be detected by injection of hot elec-

trons from silicon by applying an electric field above

8MV/cm across the oxide layer. Fig. 3 shows the EL

spectra in the visible and infrared range from the

MOS structure under a constant current of 1lA for

600 700 800 1400 16000

1

2

3

4

5

6Excitation532 nm

Si 3 %

Si 5.6 %

Si 10.5%

Excitation633 nm

Si 3 %

Si 5.6 %

Si 10.5%

PL in

tens

ity (a

.u.)

Wavelength (nm)

Fig. 2. PL spectra from the ITO/Si-rich SiO2:Er/Si MOS structures

containing different excess Si concentrations of 3, 5.6 and 10.5%.

400 500 600 700 800 1500 16000

10

20

30

40

excess Si content

Current=1µA

0 % 1.2 % 3.0 % 5.6 % 10.5 % 15 %

EL in

tens

ity (a

. u.)

Wavelength (nm)

Fig. 3. EL spectra from the ITO/Si-rich SiO2:Er/Si MOS structure

containing different excess Si concentrations of 0, 1.2, 3, 5.6, 10.5 and

15%.

0 1 2 3 4 5 60

10

20

30

40Heat up distance of hot electrons ~3 nm

2.64 1.68 1.22 0.92 0.55

EL Er 1540 nm EL Er 550 nm

PL

inte

nsity

(a.u

.)

Average Si cluster distance (nm)

EL in

tens

ity (a

. u.)

0

1

2

3

4

5

PL Er 1540 nm PL Si clusters

Density of Si clusters (1019cm-3)

0.71

Fig. 4. EL and PL intensity from the different peaks of Er3+ and Si

nanoclusters as a function of the density of the silicon clusters. The

average diameter of the Si nanoclusters is taken as 2nm from an

estimate of the average distance between Si clusters.

0.10 0.15 0.20 0.25 0.3010-9

10-7

10-5

10-3 Excess Si % d (nm)

15 0,57 10.6 0,94 5.6 1,68 3.0 2,63 1.2 4,34 0 Er only SiO2 No Er3+

and excess Si

1/E (cm/MV)

J/E2 (A

/(MV)

2 )

Fig. 5. Dependence of the injected current density on the inverse

electric field in ITO/Si-rich SiO2:Er/Si MOS structure containing

different excess silicon concentration.

1052 J.M. Sun et al. / Optical Materials 27 (2005) 1050–1054

samples with different silicon concentrations. In addi-

tion to the infrared peak at 1.54lm, two green bands

from the transitions of 2H11/2 and4S3/2 ! 4I15/2 of Er

3+

are also detected in the visible range. The blue band at

450–470nm can be attributed to the point defects ofneutral oxygen vacancies created by ion implantation

in SiO2 [10]. The EL intensity of both the green lines

and the infrared lines of Er3+ decreases dramatically

with increasing the content of the excess silicon above

3%, where a high density of silicon nanoclusters is

formed in SiO2. Contrary to the well resolved peaks

from Si clusters in PL spectra, no EL peak from the sil-

icon nanoclusters can be detected in the EL spectra be-fore breakdown of the devices, which shows that the

silicon clusters are not effectively excited under the

high-field low-current injection through a thicker oxide

layer. EL from undoped silicon nanoclusters were ob-

served in MOS structures with thin Si-rich gate oxide

less than 30nm and a high excess Si content above

10%. In these MOS structures, the carrier injection into

Si nanoclusters is mainly due to the efficient doubleinjection of carriers through tunneling in thin oxide lay-

ers. High current injection at a lower field enables the

excitation of PL from silicon nanoclusters. This is differ-

ent from our MOS structure with a thicker gate oxide.

In Fig. 4 the PL and EL intensity of the 1.54lm line

of Er3+ are plotted as a function of the density and the

average distance between silicon nanoclusters. One can

see that the EL decreases strongly when the average dis-tance between the silicon clusters drops below the mean

free path of the hot electrons (�3nm) in SiO2 as deter-

mined by different methods [11]. The electrons, which

travel between two silicon clusters with a distance below

such a distance, will not get sufficient energy from the

electric field to overcome the energy loss due to scatter-

ing processes. The average energy of the electrons will

decrease with an increase of the direct tunneling between

the silicon clusters.

Fig. 5 shows the current density J versus the inverse

of electric field E plotted in order to verify the Fow-

ler–Nordheim tunneling expressed through J =

AE2exp(�B/E), where A and B are constants. The tun-neling at lower electric field region can be observed from

the increase of the current density with nanocluster den-

sity, where the average distance of the silicon clusters

drops below the electron mean free path in SiO2. Due

to the small accelerating distance in the electric field,

electrons cannot get sufficient energy to overcome the

barrier for re-injection into the conduction band of

SiO2 during the tunneling between silicon nanoclusters.When the distance of silicon clusters is above 3nm or

at very high electric field, the electrons can get sufficient

6.5 7.0 7.5 8.0 8.5 9.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Excess Si 10.5 %No Si implantation

Rat

io o

f Gre

en a

nd in

frare

d pe

ak (a

. u.)

Electric field MV/cm

Fig. 7. Ratio of the green to infrared peak of EL from Er3+ as a

function of the average electric field in the oxide layer for two Er-

doped MOS structure without Si+ implantation (dots) and with 10.5%

excess Si atoms (squares).

J.M. Sun et al. / Optical Materials 27 (2005) 1050–1054 1053

energy from the electric field and are injected into the

conduction band of SiO2 from silicon nanoclusters.

Therefore, the current–electric field characteristic of

the SiO2 containing smaller excess silicon with average

distance of silicon clusters larger than 3nm is similar

to the pure SiO2 without Si+ implantation. The transi-tion of the conductance from Fowler–Nordheim tunnel-

ing to direct tunneling between silicon clusters can be

clear distinguished with decreasing the electric field or

reducing the average distance of silicon clusters below

3nm, as shown in Fig. 5.

There are also other possibilities for the quenching of

EL from Er3+ with increasing the density of silicon clus-

ters. One possibility is the lack of the excitation of thesilicon nanoclusters. This is evidenced by the missing

peaks from the nanoclusters in the EL spectra. The rea-

son is probably due to the difficulty of carrier injection

into the Si nanoclusters embedded in a SiO2 matrix,

i.e. the strong electron trapping that causes negatively

charged silicon clusters [12], and therefore, inhibit fur-

ther electron injection or impact excitation of the clus-

ters by Coulomb repulsion.The other possibility is the field-induced quenching:

excitons generated inside the silicon nanoclusters are

dissolved under the high electric field before diffusing

to Er3+. In order to elucidate this issue, we studied the

change of the PL intensity from both silicon nanocluster

and Er3+ 1.54lm peaks under different electric field, as

shown in Fig. 6. We can see that the PL from silicon

nanoclusters decreases to 90% with increasing the elec-tric field up to 5MV/cm. Since the PL from Er3+ in Si-

rich SiO2 is mainly generated by sensitization from sili-

con nanoclusters, the luminescence from Er3+ also de-

creases to 90% following the quenching of the PL

from the silicon clusters. These results indicate that the

quenching of the PL from Er3+ is also partially due to

0 1 2 3 4 5 63.0

3.2

3.4

3.6 PL Er 1540 nm PL Si 680 nm

PL in

tens

ity (a

.u.)

Electric field (MV/cm)

Fig. 6. Electric field-induced quenching of the PL from silicon

nanoclusters and Er3+ ions in ITO/Si-rich SiO2:Er/Si MOS structure

containing 10.5% excess silicon atoms.

the field-induced dissociation of excitons created in sili-

con nanoclusters.

Fig. 7 shows the ratio of the relative EL intensity of

the green peak (4S3/2–4I15/2) to the infrared peak (4I13/2

! 4I15/2) (G/I ratio), which increases strongly with

increasing the electric field for the sample with excess sil-

icon content of 10.5% and the sample without Si+

implantation. This is a strong evidence that the excita-

tion of the Er3+ stems from the impact excitation by

hot electrons, since the high energy levels of Er3+ are ex-

cited more efficiently when the energy distribution of hot

electrons shifts to high energy with increasing the elec-

tric field.

4. Conclusions

In summary, the difference in the PL and EL from Er-

doped SiO2 has been studied for different density of sil-

icon clusters. The sensitization from silicon clusters

dominates PL excitation of Er3+, while the impact exci-

tation play an important role in the EL excitation. When

the average distance of the silicon clusters becomes lessthan the mean free path of the electrons in SiO2, direct

tunneling of electrons between silicon nanoclusters be-

come significant and the average energy of hot electrons

decreases, thus leading to a quenching of the EL from

Er3+.

Acknowledgements

The authors would like to thank I. Winkler and J.

Schneider for the ion implantation, H. Felsmann, C.

Neisser, and G. Schnabel for the processing of the

MOS structures.

1054 J.M. Sun et al. / Optical Materials 27 (2005) 1050–1054

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