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Effect of oxygen on photoluminescence properties of a-Si:C:O:H film
Journal of Non-Crystalline Solids 59 & 60 (1983) 585-588 North.HoUand PublishingCompany
585
EFFECT OF OXYGENON PHOTOLUMINESCENCEPROPERTIESOF a-Si:C:O:H FILM Hiroshi ITO, Yoshito KAWAKYU, Toyoki HIGUCHI and KyozohIDE Toshiba Research and DevelopmentCenter, ToshibaCorporation Komukai Toshiba-cho, Saiwai-ku, Kawasaki-city, 210 Japan Photoluminescence properties of a-Si:C:O:H films were investigated. a-Si:C:O:H films were prepared by glow discharge decomposition of TMS and 02 gas mixture. The PL peak position shifts to higher photon energy and relative PL intensity increases. Also, the optical band gap shifts towards higher photon energy. These tendencies are caused by oxygen atom incorporation into the films. I. INTRODUCTION A broad luminescence band in the visible region for a-Si:C:H films at room temperature has been reported by H. Munekata et al. l .
They prepared these films
by the plasma decomposition of tetramethylsilane (TMS). The luminescenceof a-Si:C:H has been interpreted as a transition betweenband t a i l states, similar to the case for a-Si:H films. In the case of a-Si:H from SiH4, the addition of oxygen (02) caused an optical absorption edge and a luminescencepeak to shift to higher photon energy 2 .
This paper reports the optical properties and luminescence characteristics for a-Si:C:O:H films prepared by plasma decomposition of TMS and 02 gas mixture. 2. EXPERIMENTALRESULTS The samples used in this study were prepared by RF glow discharge decomposition of TMS and 02 gas mixture. similar to that used in Ref. I .
The deposition system was constructed
Figure l shows a block diagram of the
deposition system. TMS in the bubbler was vaporized by carrier gas (He) and introduced into the reaction chamber. 02 was added from another gas line into the chamber. Thus, the atomic ratio between oxygen and silicon (O/Si) was determined by controlling the 02 and TMS gas ratio. In this study, all samples were made at constant TMS flow rate (4cm3/min). Pressure in the chamberwas about l.O Torr during the deposition. Si wafers (FZ) and fused quartz were used for the substrates. RF power (13.56 MHz) was supplied with capacitive coupled electrodes.
Typically, power density was 0.25 W/cm2.
The films were deposited
at substrate temperature of 350°C. 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
586
tl. Ito et al. / Effect o f oxygen on photoluminescence properties
l
i
q
60
6O
Si Pressure(~ CMmber
5o~
~ 50 _,2
40_~ i'
~
~ 2o He Gas Cylinder
"-,
/ ' o / 1 $ i , c.o)
30
H
o-
10 0
O= GosCylind~-
"'..~ C/[$i÷c.o)
o"
~'
50 20 10 ~
i i i i I LO 02 04 0.6 0.8 12 O/Si (Atomic Ratio in Gas)
Fig. 2 Deposition rate and composition as a function of O/Si ratio in gas
Fig. 1 Block diagram of deposition system
The samples of 0.5-2.0 ]Jm thick were used for the measurements. Figure 2 shows the Si, C and 0 contents in the films analyzed by EPMAand the deposition rate. in O/Si ratio.
The deposition rate increased monotonically with increase
Si content was almost same for all samples studied here.
content was increased as O/Si ratio increased.
0
On the other hand, C content
was decreased. In order to confirm the film structure and the composition, IR absorption spectra of the films on Si wafer were measured by Fourier transform IR spectroscopy. Typical IR spectra are shown in Fig. 3.
Assignments for the vibration
modes were tentatively determined3. IR spectra showed the absorption peak correlated to Si-O bonds as shown in Fig. 3.
Also, 0 atoms were detected by EPMA, as shown in Fig. 2.
results, 0 atoms seem to be easily incorporated into the films.
From these Absorbanceof
Si-O band, in proportion to the number of Si-O bonds, was increased as O/Si ratio increased. On the other hand, absorbances of Si-Hn and C-Hn (n=l-3) bands were decreased rapidly. Photoluminescence (PL) spectra for a-Si:C:O:H films deposited onto Si wafers are shown in Fig. 4.
The 4.88 eV (254 nm) line of an Hg lamp was used as the
excitation source. The peak energy of emission spectra, almost single broad band, was shifted to higher photon energy as O/Si ratio in gas increased, but the half width was not significantly changed within a region where O/Si ratio was from 0 to 0.2. Further introducing O, the emission spectra becamenarrower and peak energy was shifted to higher photon energy. temperature.
So blue-white PL spectra were observed at room
In order to investigate these PL characteristics from another point of view,
11. lto et al. / Effect o f oxygen on photolurninescence properties
~3.0~
' '
I,
587
.
,
!
,
,
,
,
, siLo.si $i-C
! =
i
Si-Hn Si-CH~
3600
2800 2000 1200 Wavenumbers (crn"1)
400
Fig. 3 IR spectra for a-Si:C:O:H films
,£
2'o
3.0 Photon Energy (eV)
35
Fig. 4 PL spectra for various a-Si:C:O:H films. Inset; Relations between EpL and E04 the excitation energy dependenceof PL spectra was measured for sample whose O/Si ratio was O.l.
Excitation energies were varied from 4.88 eV (254 nm) to
2.84 eV (436 nm). PL spectra were similar when excitation energies were above about 3.3 eV, that is, the peak energy stood at about 2.6 eV and the half width remained in about 0.8 eV.
As the excitation energies were lowered below about
3.3 eV (sub-band gap excitations), PL spectra became narrower and asymmetrical, that i s , intensity on the higher energy side decreased4. -3. DISCUSSIONS Amorphous films deposited from TMS contained O, although which were prepared without adding 02.
Data on the bonds from IR spectra and the composition from
EPMA showed that incorporated O atoms mainly formed bonds with Si.
I t should be
noted that the increase in 0 content caused a decrease in C content and Si-Hn bonds. Therefore, the incorporation mechanism for 0 is considered as follows.
TMS
(containing He as carrier) was decomposedby RF glow discharge and new bonds,
H. Ito et al. / Effect o f oxygen on photoluminescence properties
588
such as Si-H, Si-Si, Si-C-Si, Si-C-C-Si etc., would be formed. Adding 02 to TMS, other bonds, such as Si-O-Si etc., would be formed rather easily. On the other hand, Si-Hn bond formation would be avoided. In the inset in Fig. 4, the relations between PL peak energy EpL and the energy correlated to band gap E04 (photon energy where absorption coefficient was lO4 cm- l ) is plotted. This figure shows that increase in 0 content caused E04 to broaden, but EpL to shift to higher energy slightly. Thus, larger Stokes s h i f t occurred i f 0 content increased. Theseresults show that the electronphonon coupling effect increases2'5, and that the band t a i l states become deeper in the energy gap. Samples containing more 0 absorbed less photons, that is, the absorption coefficient became smaller.
However,larger PL signal in the visible region
was observed for l ~m thick films with larger 0 content.
The relative
efficiency, estimated by, Relative Efficiency =
Area of PL Spectrum A---bsorbedNumberof Photons
became larger as 0 content increased. 0 content would have the optimum value for the maximumefficiency. I f excitations were madeat sub-band gap, PL spectra changed the shape to decrease the intensity on the higher energy side.
Tail states seem to be acted
as efficient radiative centers from this effect. 4. CONCLUSION For a-Si:C:O:H films added 02 to TMS compared to without 02, PL peak shifts to higher energy were realized and blue-white PL emissions at room temperature were observed. Furthermore, the efficiency becamelarger. ACKNOWLEDGEMENT The authors would like to thank Prof. H. Kukimoto and Mr. H. Munekatafor their helpful discussions. measurement of EPI~A.
Theywould like to thank Mr. O. Hirao for the
REFERENCES I) H. Munekataet al. Appl. Phys. Letters 37 (1980) 536 2) J. C. Knights et al. J. of Non-crystalline Solids 35/36 (1980) 279 3) R. Szeto and D. W. Hess J. Appl. Phys. 52 (1981) 903 4) B. A. Wilson and T. P. Kerwin Phys. Rev. B25 (1982) 5276 5) H. Munekataet al.
J. of Luminescence 24/25 (1981) 43
585
EFFECT OF OXYGENON PHOTOLUMINESCENCEPROPERTIESOF a-Si:C:O:H FILM Hiroshi ITO, Yoshito KAWAKYU, Toyoki HIGUCHI and KyozohIDE Toshiba Research and DevelopmentCenter, ToshibaCorporation Komukai Toshiba-cho, Saiwai-ku, Kawasaki-city, 210 Japan Photoluminescence properties of a-Si:C:O:H films were investigated. a-Si:C:O:H films were prepared by glow discharge decomposition of TMS and 02 gas mixture. The PL peak position shifts to higher photon energy and relative PL intensity increases. Also, the optical band gap shifts towards higher photon energy. These tendencies are caused by oxygen atom incorporation into the films. I. INTRODUCTION A broad luminescence band in the visible region for a-Si:C:H films at room temperature has been reported by H. Munekata et al. l .
They prepared these films
by the plasma decomposition of tetramethylsilane (TMS). The luminescenceof a-Si:C:H has been interpreted as a transition betweenband t a i l states, similar to the case for a-Si:H films. In the case of a-Si:H from SiH4, the addition of oxygen (02) caused an optical absorption edge and a luminescencepeak to shift to higher photon energy 2 .
This paper reports the optical properties and luminescence characteristics for a-Si:C:O:H films prepared by plasma decomposition of TMS and 02 gas mixture. 2. EXPERIMENTALRESULTS The samples used in this study were prepared by RF glow discharge decomposition of TMS and 02 gas mixture. similar to that used in Ref. I .
The deposition system was constructed
Figure l shows a block diagram of the
deposition system. TMS in the bubbler was vaporized by carrier gas (He) and introduced into the reaction chamber. 02 was added from another gas line into the chamber. Thus, the atomic ratio between oxygen and silicon (O/Si) was determined by controlling the 02 and TMS gas ratio. In this study, all samples were made at constant TMS flow rate (4cm3/min). Pressure in the chamberwas about l.O Torr during the deposition. Si wafers (FZ) and fused quartz were used for the substrates. RF power (13.56 MHz) was supplied with capacitive coupled electrodes.
Typically, power density was 0.25 W/cm2.
The films were deposited
at substrate temperature of 350°C. 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
586
tl. Ito et al. / Effect o f oxygen on photoluminescence properties
l
i
q
60
6O
Si Pressure(~ CMmber
5o~
~ 50 _,2
40_~ i'
~
~ 2o He Gas Cylinder
"-,
/ ' o / 1 $ i , c.o)
30
H
o-
10 0
O= GosCylind~-
"'..~ C/[$i÷c.o)
o"
~'
50 20 10 ~
i i i i I LO 02 04 0.6 0.8 12 O/Si (Atomic Ratio in Gas)
Fig. 2 Deposition rate and composition as a function of O/Si ratio in gas
Fig. 1 Block diagram of deposition system
The samples of 0.5-2.0 ]Jm thick were used for the measurements. Figure 2 shows the Si, C and 0 contents in the films analyzed by EPMAand the deposition rate. in O/Si ratio.
The deposition rate increased monotonically with increase
Si content was almost same for all samples studied here.
content was increased as O/Si ratio increased.
0
On the other hand, C content
was decreased. In order to confirm the film structure and the composition, IR absorption spectra of the films on Si wafer were measured by Fourier transform IR spectroscopy. Typical IR spectra are shown in Fig. 3.
Assignments for the vibration
modes were tentatively determined3. IR spectra showed the absorption peak correlated to Si-O bonds as shown in Fig. 3.
Also, 0 atoms were detected by EPMA, as shown in Fig. 2.
results, 0 atoms seem to be easily incorporated into the films.
From these Absorbanceof
Si-O band, in proportion to the number of Si-O bonds, was increased as O/Si ratio increased. On the other hand, absorbances of Si-Hn and C-Hn (n=l-3) bands were decreased rapidly. Photoluminescence (PL) spectra for a-Si:C:O:H films deposited onto Si wafers are shown in Fig. 4.
The 4.88 eV (254 nm) line of an Hg lamp was used as the
excitation source. The peak energy of emission spectra, almost single broad band, was shifted to higher photon energy as O/Si ratio in gas increased, but the half width was not significantly changed within a region where O/Si ratio was from 0 to 0.2. Further introducing O, the emission spectra becamenarrower and peak energy was shifted to higher photon energy. temperature.
So blue-white PL spectra were observed at room
In order to investigate these PL characteristics from another point of view,
11. lto et al. / Effect o f oxygen on photolurninescence properties
~3.0~
' '
I,
587
.
,
!
,
,
,
,
, siLo.si $i-C
! =
i
Si-Hn Si-CH~
3600
2800 2000 1200 Wavenumbers (crn"1)
400
Fig. 3 IR spectra for a-Si:C:O:H films
,£
2'o
3.0 Photon Energy (eV)
35
Fig. 4 PL spectra for various a-Si:C:O:H films. Inset; Relations between EpL and E04 the excitation energy dependenceof PL spectra was measured for sample whose O/Si ratio was O.l.
Excitation energies were varied from 4.88 eV (254 nm) to
2.84 eV (436 nm). PL spectra were similar when excitation energies were above about 3.3 eV, that is, the peak energy stood at about 2.6 eV and the half width remained in about 0.8 eV.
As the excitation energies were lowered below about
3.3 eV (sub-band gap excitations), PL spectra became narrower and asymmetrical, that i s , intensity on the higher energy side decreased4. -3. DISCUSSIONS Amorphous films deposited from TMS contained O, although which were prepared without adding 02.
Data on the bonds from IR spectra and the composition from
EPMA showed that incorporated O atoms mainly formed bonds with Si.
I t should be
noted that the increase in 0 content caused a decrease in C content and Si-Hn bonds. Therefore, the incorporation mechanism for 0 is considered as follows.
TMS
(containing He as carrier) was decomposedby RF glow discharge and new bonds,
H. Ito et al. / Effect o f oxygen on photoluminescence properties
588
such as Si-H, Si-Si, Si-C-Si, Si-C-C-Si etc., would be formed. Adding 02 to TMS, other bonds, such as Si-O-Si etc., would be formed rather easily. On the other hand, Si-Hn bond formation would be avoided. In the inset in Fig. 4, the relations between PL peak energy EpL and the energy correlated to band gap E04 (photon energy where absorption coefficient was lO4 cm- l ) is plotted. This figure shows that increase in 0 content caused E04 to broaden, but EpL to shift to higher energy slightly. Thus, larger Stokes s h i f t occurred i f 0 content increased. Theseresults show that the electronphonon coupling effect increases2'5, and that the band t a i l states become deeper in the energy gap. Samples containing more 0 absorbed less photons, that is, the absorption coefficient became smaller.
However,larger PL signal in the visible region
was observed for l ~m thick films with larger 0 content.
The relative
efficiency, estimated by, Relative Efficiency =
Area of PL Spectrum A---bsorbedNumberof Photons
became larger as 0 content increased. 0 content would have the optimum value for the maximumefficiency. I f excitations were madeat sub-band gap, PL spectra changed the shape to decrease the intensity on the higher energy side.
Tail states seem to be acted
as efficient radiative centers from this effect. 4. CONCLUSION For a-Si:C:O:H films added 02 to TMS compared to without 02, PL peak shifts to higher energy were realized and blue-white PL emissions at room temperature were observed. Furthermore, the efficiency becamelarger. ACKNOWLEDGEMENT The authors would like to thank Prof. H. Kukimoto and Mr. H. Munekatafor their helpful discussions. measurement of EPI~A.
Theywould like to thank Mr. O. Hirao for the
REFERENCES I) H. Munekataet al. Appl. Phys. Letters 37 (1980) 536 2) J. C. Knights et al. J. of Non-crystalline Solids 35/36 (1980) 279 3) R. Szeto and D. W. Hess J. Appl. Phys. 52 (1981) 903 4) B. A. Wilson and T. P. Kerwin Phys. Rev. B25 (1982) 5276 5) H. Munekataet al.
J. of Luminescence 24/25 (1981) 43