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: semicon@yonsei.ac.kr (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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