2007_FTIR characterization of light emitting Si-rich nitride films prepared by low pressure chemical vapor deposition

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FTIR characterization of light emitting Si-rich nitride films prepared

by low pressure chemical vapor deposition

V. Em. Vamvakas ⁎, S. Gardelis

Institute of Microelectronics, NCSR “ Demokritos” , P. O. Box 60228, 15310 Aghia Paraskevi, Athens, Greece

Available online 4 May 2007

Abstract

We report on the infrared transmission and light emission of Si-rich nitride (SRN) films prepared by low pressure chemical vapor deposition

(LPCVD) from dichlorosilane (SiH2Cl2, DCS) and ammonia (NH3) mixtures. The main absorption band at about 830 cm−1, attributed to Si– N

vibration mode and observed in stoichiometric silicon nitride, shifted to slightly higher wavenumbers with increasing Si content in the SRN films.

Annealing at temperatures higher than the deposition temperature induced a further shift of the main band to higher wavenumbers. Additionally, a

new band appeared as a “shoulder ” at about 1080 cm−1, attributed to partial oxidation of the silicon nanocrystals. Photoluminescence (PL)

obtained from the SRN films increased considerably and shifted to shorter wavelengths as the Si content decreased whereas annealing caused

further enhancement and a slight shift to shorter wavelengths in comparison with the as-grown films.

© 2007 Elsevier B.V. All rights reserved.

PACS: 78.30.-j; 78.55.-m; 78.67.Bf

Keywords: Silicon nitride; Silicon nanoparticles; FTIR spectroscopy; Photoluminescence

1. Introduction

Stoichiometric silicon nitride films are widely used in silicon

based micro- and nanotechnology as barriers to sodium diffusion

and as masking layers for the local oxidation of Si in ULSI

technology [1,2]. In addition their excellent optical properties

make them suitable for optical waveguides and for antireflective

and protective coatings for solar cells [3–6]. The introduction of

extra silicon in silicon nitride films was firstly used to lower or

reverse the residual stresses of these films [7–9] making possible

the development of suspending membranes in micro-mechanical

systems. Other applications of Si-rich nitride (SRN) films in-clude the typical Oxide– Nitride–Oxide (ONO) memory device

[10–14] and photonic devices [15–22].

In this work SRN films were grown by low pressure chemical

vapor deposition (LPCVD) using SiH2Cl2 (DCS) and NH3 mix-

tures in order to study their light emission properties in com-

bination with FTIR analysis. All films were deposited at 800 °C

and then annealed at 950 °C and 1100 °C in dry nitrogen for times

ranging between 30 min and 4 h. FTIR analysis showed that

annealing caused the formation of Si–O bonds even though

precaution was taken to avoid the presence of oxygen during the

annealing process. This indicated that SRN films could be very

easily oxidized. The films emitted light in the visible at room

temperature. The light emissioncharacteristics dependedon the Si

content of the films and the post-annealing treatment.

2. Experimental details

All depositions were carried out in a Tempress Systems Inc.

(model omega junior), horizontal hot wall reactor at 800 °C,

230 mTorr. NH3 flow ratio was kept constant whereas DCS flowratio varied in order to deposit films with different stoichio-

metries. The deposition parameters are summarized in Table 1.

Before deposition Si substrates were cleaned in a 1:1 H2SO4:

H2O2 solution followed by a dip in hydrofluoric acid (HF)

solution, rinsed in de-ionized water and blown dry with dry

nitrogen. All studied films had thickness of about 100 nm. Post-

annealings were performed at 950 °C and at 1100 °C for 30 min,

1, 2 and 4 h in a furnace in dry nitrogen (N2 99.996%, O2 b

1 ppm, H2O b1 ppm) flowing at a rate of 3.5 slm (standard liters

per minute). Before annealing all samples were cleaned in 1:1

H2SO4:H2O2 solution, rinsed in de-ionized water, blown dry

Surface & Coatings Technology 201 (2007) 9359–9364

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⁎ Corresponding author. Tel.: +30 210 6503117; fax: +30 210 6511723.

E-mail address: [email protected] (V.Em. Vamvakas).

0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.04.069

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with dry nitrogen and then inserted in the furnace where they

remained for 30 min at 300 °C. This procedure was followed in

order to exclude the possibility of partial oxidation of the

samples due to remaining humidity after the last rinse.

Fourier transform infrared (FTIR) spectra were recorded in

transmission mode using a Bruker (model Tensor 27) single

beam spectrometer. Before recording the spectrum, the

background was taken placing a freshly cleaned piece of silicon

cut from the same silicon wafer used as the substrate. This was

performed in order to eliminate absorption of the substratecaused by the vibration of the Si–Si bond which gives a peak at

611 cm−1 and the interstitial oxygen which gives a peak at

1108 cm−1 [23,24]. Photoluminescence was performed at room

temperature using for excitation the 458 nm line of an Ar +-ion

laser. The signal was analyzed by a Jobin-Yvon spex HR-320

spectrometer and detected by a photomultiplier tube.

3. Results and discussion

The increase of the Si content of silicon nitride films results

in an increase of the refractive index of the films compared to

the stoichiometric ones [1]. The last two columns of Table 1

give the refractive index for our films at 632.8 nm and theexpected Si/N ratio according to the literature [1].

Fig. 1 shows the FTIR transmission spectra of the as-grown

SRN films investigated in this study. The strong absorption

band located at about 830 cm−1 corresponds to the asymmetric

stretching mode of vibration of the Si– N bond. There is also a

weak band located at about 480 cm−1 which corresponds to the

rocking mode of vibration of the Si– N bond. However, the

study of this band is difficult with our equipment since it is

located close to the lower band limit of our spectrometer. In

addition the study of the low energy band does not offer any

extra information thus we focus our study on the main

absorption band. No Si–H o r N–H or any other impurityrelated vibration mode was detected between 4000 cm−1 and

400 cm−1.

The main absorption band of the stoichiometric silicon

nitride film was located at 832 cm−1. For the SRN films this

band shifted slightly to higher wavenumbers. Specifically, in the

film with Si/N ratio 1.1 this band appeared at 840 cm−1 whereas

in the film with Si/N ratio 1.35 it appeared at 842 cm−1. For

films with Si/N ratio 0.86 it appeared at almost the same

position as that of the stoichiometric film, indicating that FTIR

transmission measurements could not distinguish films with

these stoichiometries. The observed shift of the Si– N vibration

mode in the SRN films is expected to be due to the presence of

the extra Si in the films.

Annealing of SRN films with Si/N ratios 1.35 and 1.10 at

temperatures higher than the deposition caused a slight shift of

the main absorption band to higher wavenumbers followed by a

broadening. In addition, a second band appeared, as a

“shoulder ” to the main band, at 1080 cm−1. We note here that

spectra obtained from stoichiometric silicon nitride films and

SRN films with Si/N equal to 0.86 annealed together with the

Table 1

Deposition parameters, refractive index n at 632.8 nm and Si/N ratio of the

deposited films

T (°C) P (mTorr) φDCS (sccm) φ NH3 (sccm) n Si/N

Stoichiometric 800 230 20 60 2.02 0.75

Silicon rich 800 230 13.8 5.5 2.15 0.86

800 230 27.5 5.5 2.28 1.10800 230 55.0 5.5 2.51 1.35

Fig. 1. FTIR transmission spectra of the as-grown SRN films with different Si

content.

Fig. 2. FTIR transmission spectra of SRN films with Si/N=1.35 after depositionand after annealing for 4 h at 950 °C and 1100 °C.

9360 V.Em. Vamvakas, S. Gardelis / Surface & Coatings Technology 201 (2007) 9359 – 9364





the bulk of the films, far away from their boundaries. Therefore,

it is logical to assume that the formation of the “shoulder ” at

1080 cm−1 after annealing is due to the formation of Si–O–Si

bridges caused by the partial oxidation of the extra silicon.

When the silicon content was low or the annealing time was

short then the oxidation of the films resulted in the formation of

isolated Si–O–Si bridges surrounded by silicon nitride or silicon clusters. The asymmetrical stretching mode of vibration

of these isolated bridges is similar to those boundary bridges

located at the interfaces of SiO2 films thus resulting in the

appearance of the peak at 1090 cm−1 (Table 3). When annealing

time is longer or/and the silicon content of the films is higher,

the possibility of the formation of Si–O–Si bridges surrounded

by other Si–O–Si bridges becomes higher. These isolated

bridges which do not “feel” the silicon nitride matrix are

vibrating similarly with those located in the bulk of SiO2 films

thus giving a peak at 1070 cm−1. However, the origins of the

oxygen causing the partial oxidation of these films are not clear.

It is possible that part of this oxygen enters during thedeposition of the films, since the same furnace is also used for

the deposition of SiO2 films, although great care was taken to

avoid any oxygen contamination. We note here that FTIR

transmission spectra obtained from stoichiometric silicon

nitride films annealed together with the SRN films did not

reveal the “shoulder ”. In addition, second derivative of

transmission spectra obtained from the as-grown SRN films

did not clearly reveal the existence of Si–O bonds. It is possible

that the inserted oxygen during deposition is making bonds with

the silicon nitride matrix which are infrared inactive. During

annealing oxygen is redistributed in the silicon nitride films and

for the case of SRN films it is easy to oxidize the extra silicon

and form Si–O–Si bridges. This most probably is not hap- pening for stoichiometric silicon nitride films since before the

formation of Si–O–Si bridges a break of Si– N bonds must

occur. This is not thermodynamically favorable at temperatures

as low as 950 °C while temperatures as high as 1100 °C must be

considered close to the lowest limit of the oxidation in dry

ambient of silicon nitrides. However, the scope of this discussion

is not to find the origins of the oxygen which is responsible for

the oxidation of the films, but it is the fact that SRN films can be

oxidized relatively easy when traces of oxygen are present at

temperatures starting from at least 950 °C.

The light emission properties of all films were investigated.

The light emission characteristics were mainly sensitive to thesilicon content of the films. Significant improvement in the

intensity of the emitted light was realized by post-annealing of

the films at temperatures higher than the growth temperature.

Specifically, the as-grown film with Si/N ratio 1.35 did not give

any detectable luminescence in the region of measurements

between 470 nm and 900 nm. The film with Si/N ratio 1.10

demonstrated a broad PL spectrum peaking at 600 nm (Fig. 4).

The film with Si/N ratio 0.86 showed the most efficient

luminescence peaking at 570 nm. (Fig. 4) We note that the PL

spectra obtained from the as-grown films shifted to shorter

wavelengths compared with the indirect band gap of bulk

silicon (about 1100 nm). A considerable enhancement of the PL

peak accompanied by a shift to higher energies with decreasing T a b

l e 3

P r e s

e n c e

/ l a c

k o

f p e a

k s o

f t h e

2 n

d d e r i v a t i v e o

f t r a n s m

i s s i o n

i n t h e r a n g e

1 2 0 0 –

9 5 0 c m −

1

o f s i l i c o n n

i t r i d e

f i l m s w

i t h v a r i o u s s t o i c h i o m e t r i e s

2 n d

d e r i v a t i v e o

f t r a n s m

i s s i o n

( 1 2 0 0 –

9 5 0 c m −

1 )

A f t e r

d e p o s

i t i o n

A n n e a

l e d

9 5 0 °

C

1 1 0 0 ° C

3 0 m

i n

1 h

2 h

4 h

3 0 m

i n

1 h

2 h

4 h

1 0 9 0 c m −

1

1 0 7 0 c m −

1

1 0 9 0

c m −

1

1 0 7 0 c m −

1

1 0 9 0 c m −

1

1 0 7 0 c m −

1

1 0 9 0 c m −

1

1 0 7 0 c m −

1

1 0 9 0 c m −

1

1 0 7 0 c m −

1

1 0 9 0 c m −

1

1 0

7 0 c m −

1

1 0 9 0 c m −

1

1 0 7 0 c m −

1

1 0 9 0 c m −

1

1 0 7 0 c m −

1

1 0 9 0 c m −

1

1 0 7 0 c m −

1

S t o i c

h i o m e t r i c ( S i / N )

0 . 7

5

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

S R N

( S i / N )

0 . 8

6

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

1 . 1

0

–

–

y

–

y

–

y

–

y

–

y

y

y

y

y

y

y

y

1 . 3

5

–

–

y

–

y

y

y

y

y

y

y

y

y

y

y

y

y

y

A l l f i l m s

h a v e a t h i c k n e s s o

f a

b o u t 1 0 0 n m

( y :

f o r p r e s e n c e o

f t h e

c o r r e s p o n

d i n g p e a

k , – :

f o r

l a c

k o

f p r e s e n c e

) .




silicon content in the films was observed. We note that de-

creasing silicon content in the films is expected to lead to

formation of silicon clusters of smaller sizes. Considering all of

the above, the most probable explanation for the light emission

from the as-grown films is the quantum confinement of the

carriers recombining in the silicon clusters which may be al-

ready formed during the growth of the films due to the high

temperature of the growth reaction (800 °C). Similar light

emission has been observed in SRN films grown by techniques

other than the one used in this study [15–22]. There is though a

debate as to what is the origin of the light emission. Some

attribute the effect to the quantum confinement of carriers in the

silicon nanocrystals [18, 22] whereas others suggest that theeffect originates from nitrogen-related localized surface states

introduced within the optical gap of silicon nanocrystals [19,

20]. In either case silicon nanocrystals are necessary to confine

the electron and the hole which then recombine radiatively.

All as-grown films were examined for their light emission

properties after annealing. The sample with Si/N ratio 1.35

emitted light only after annealing at 1100 °C in which, as TEM

images demonstrated, silicon nanocrystals of sizes between

1.5 nm and 5 nm were formed [25]. Annealing of the films with

lower Si/N ratio resulted in PL enhancement accompanied by a

shift of the PL peak to shorter wavelengths. Fig. 4 shows this

effect in the case of the sample with Si/N ratio 1.10. This effect coincided with the appearance of the Si–O–Si vibration modes

in the FTIR spectrum as discussed above in detail. The effect

could be well explained by the enhancement of the localization

of the carriers in the silicon nanocrystals as their size was

reduced due to their oxidation.

4. Conclusions

SRN films with different Si content were prepared by LPCVD

from DCS and NH3 mixtures. Infrared transmission spectra of the

SRN films revealed the existence of an absorption band at about

830 cm−1 attributed to the Si– N asymmetrical stretching mode.

This band slightly shifted to higher wavenumbers as the silicon

content of the films increased. Annealing at temperatures higher

than the growth temperature caused further shift of the main

absorption band to higher wavenumbers whereas at the same time

a new absorption band at about 1080 cm−1 appeared as a

“shoulder ” to the main absorption band. Calculations of the

second derivative of the “shoulder ”, revealed the existence of Si–

O–Si bridges due to partial oxidation of the extra Si from traces of oxygen which may be inevitably present during annealing. Light

emission properties of the SRN films were sensitive to the Si

content, the annealing temperature and the duration of the

annealing. Enhancement of PL accompanied by a blue shift with

decreasing Si content was observed in the as-grown SRN films.

Similar effects were observed with increasing annealing temper-

ature and annealing time which coincided with the partial oxi-

dation of the Si nanocrystals resulting in a reduction in their sizes.

All these effects could well be attributed to the confinement of the

carriers in the Si nanocrystals which might recombine radiatively

in the Si nanocrystals perhaps via surface states.

References

[1] A.C. Adams, in: S.M. Sze (Ed.), VLSITechnology, 2nd edn,McGraw-Hill,

1988, p. 233, International edition.

[2] F.H.P.M. Habraken, A.E.T. Kuiper, Mater. Sci. Eng. R12 (3) (1994) 123.

[3] D. Davazoglou, Thin Solid Films 437 (2003) 266.

[4] K. Misiakos, E. Tsoi, E. Halmagean, S. Kakabakos, Technical Digest,

International Electron Devices Meeting, 1998, p. 25.

[5] P. Wu, P. Hogrebe, D.W. Grainger, Biosens. Bioelectron. 21 (2006) 1252.

[6] O. Schultz, M. Hofmann, S.W. Glunz, G.P. Willeke, 31st IEEE PVSC

Orlando, Florida, 2005, p. 872.

[7] M. Sekimoto, H. Yoshihara, T. Ohkubo, J. Vac. Sci. Technol. 21 (4) (1982)

1017.

[8] J.G.E. Gardeniers, H.A.C. Tilmans, C.C.G. Visser, J. Vac. Sci. Technol., A

14 (5) (1996) 2879.

[9] E. Cianci, F. Pirola, V. Foglietti, J. Vac. Sci. Technol. B 23 (1) (2005) 168.

[10] M.C. Poon, Y. Gao, T.C.W. Kok, A.M. Myasnikov, H. Wong, Microelectron.

Reliab. 41 (2001) 2071.

[11] J. Chan, H. Wong, M.C. Poon, C.W. Kok, Microelectron. Reliab. 43

(2003) 611.

[12] T.C. Chang, S.T. Yan, P.T. Liu, M.C. Wang, S.M. Sze, Electrochem. Solid-

State Lett. 7 (7) (2004) G138.

[13] K.-H. Wu, H.-C. Chien, C.-C. Chan, T.-S. Chen, C.-H. Kao, IEEE Trans.

Electron. Devices 52 (5) (2005) 987.

[14] S. Choi, H. Yang, M. Chang, S. Baek, H. Hwanga, S. Jeon, J. Kim, C. Kim,

Appl. Phys. Lett. 86 (2005) 251901.

[15] T.-Y. Kim, N.-M. Park, K.-H. Kim, G.Y. Sung, Y.-W. Ok, T.-Y. Seong, C.-J.

Choi, Appl. Phys. Lett. 85 (22) (2004) 5355.

[16] K.S. Cho, N.-M. Park, T.-Y. Kim, K.-H. Kim, G.Y. Sung, J.H. Shin, Appl.

Phys. Lett. 86 (2005) 071909.

[17] L.-Y. Chen, W.-H. Chen, F.C.-N. Hong, Appl. Phys. Lett. 86 (2005)

193506.

[18] T.-W. Kim, C.-H. Cho, B.-H. Kim, S.-J. Park, Appl. Phys. Lett. 88 (2006)

123102.

[19] L. Dal Negro, J.H. Yi, L.C. Kimerling, S. Hamel, A. Williamson, G. Galli,

Appl. Phys. Lett. 88 (2006) 183103.

[20] L. Dal Negro, J.H. Yi, J. Michel, L.C. Kimerling, T.-W.F. Chang, V.

Sukhvatkin, E.H. Sargent, Appl. Phys. Lett. 88 (2006) 223109.

[21] L.B. Ma, R. Song, Y.M. Miao, C.R. Li, Y.Q. Wang, Z.X. Cao, Appl. Phys.

Lett. 88 (2006) 093102.

[22] K. Ma, J.Y. Feng, Z.J. Zhang, Nanotechnology 17 (2006) 4650.

[23] I.P. Herman, Optical Diagnostics for Thin Film Processing, Academic

Press Inc., 1996.

[24] H.R. Philipp, Properties of Silicon, INSPEC The Institution of ElectricalEngineers, 1987, p. 1019, EMIS Datareview RN=16133.

Fig. 4. Photoluminescence (PL) spectra of as-grown and annealed SRN films.

9363V.Em. Vamvakas, S. Gardelis / Surface & Coatings Technology 201 (2007) 9359 – 9364



[25] V.Em. Vamvakas, N. Vourdas, S. Gardelis, Microelectron. Reliab. 47

(2007) 794.

[26] M. Cardona, in: F. Seitz, D. Turnbull, H. Ehrenreich (Eds.), Modulation

Spectroscopy, Academic Press, New York, 1969, p. 105.

[27] D. Davazoglou, V.Em. Vamvakas, J. Electrochem. Soc. 150 (5) (2003)

F90.

[28] V.Em. Vamvakas, D. Davazoglou, J. Electrochem. Soc. 151 (5) (2004)

F93.

[29] V.Em. Vamvakas, D. Davazoglou, J. Vac. Sci. Technol. B 23 (5) (2005)

1956.


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2007_FTIR characterization of light emitting Si-rich nitride films prepared by low pressure chemical vapor deposition