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Hydrogen content and optical properties of LPCVD amorphous silicon films
Journal of Non.CrystallineSolids77 & 78 0985) 575-578 North-HoUand,Amsterdam
575
HYDROGEN CONTENT AND OPTICAL PROPERTIES OF LPCVD AMORPHOUSSILICON FILMS
C. MANFREDOTTI, G.SCARANO Istituto
di Fisica Superiore, Corso M. D'Azeglio 46, 10125 Torino, I t a l y
G. DELLA MEA and C. ROSSI ALVAREZ Dipartimento di F i s i c a , Via Marzolo 8, 53100 Padova, I t a l y Optical properties of LPCVD amorphous s i l i c o n f i l m s deposited both from SiH4 and from SigH_ are i n v e s t i g a t e d at very low H contents. The o p t i c a l energy gap v a r i e s wi~h deposition temperature even i f H concentration is constant. The non-homogeneous H p r o f i l e seems not influence o p t i c a l p r o p e r t i e s , which d i s p l a y features q u i t e s i m i l a r to those of GD samples. I . INTRODUCTION Optical properties of LPCVD amorphous s i l i c o n f i l m s are c e r t a i n l y less extens i v e l y i n v e s t i g a t e d with respect to GD deposited f i l m s ,
in p a r t i c u l a r f o r growth
temperatures below 600°C I . Recently 2 i t has been proved t h a t H i n c o r p o r a t i o n is possible with Si2H 6 LPCVD, but the o p t i c a l gap Eg is q u i t e independent of H cont e n t CH and i t is never l a r g e r than 1.6 eV even f o r CH of 20%. In the present work, i t is indicated t h a t i t is possible to get E = 1.7 eV g by Si2H 6 LPCVD at low temperature ( 460°C ) and pressure ( 0.2 t o r r ) even f o r CH below I% and t h a t Eg depends only on deposition temperature Td. The exponential
absorption edge displays a l o g a r i t h m i c slope E0 not strongly d i f f e r e n t from
what reported in GD l i t e r a t u r e 3. However, the absorption c o e f f i c i e n t is l a r g e r by more than one order of magnitude below 2 eV. F i n a l l y , by nuclear techniques it
is proved t h a t CH is not homogeneous in depth, even i f a bulk content may be
always determined. 2. EXPERIMENTAL The a-Si f i l m s have been grown in an h o r i z o n t a l 5" quartz tube by a hot wall reactor with a 50 cm long hot zone. Electronic grade SiH 4 or Si2H 6 was admitted at a flow r a t e of 50 SCCM and at temperatures between 430°C and 550°C. The gas pressure was held at 0.1 t o r r ( SiH 4) and at 0.2 t o r r ( Si2H 6 ). The growth rates displayed approximately the same a c t i v a t i o n energy and varied from 0.5 up to 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C Manfredottietal./Hydrogen contentand opticalproperties
576 o
22 A/min. Films were deposited on sapphire or 7059 Corning glass 3" substr~tes and thickness was accurately determined by etching using an alpha-step apparatus. Films turned out to be rather homogeneousin thickness both r a d i a l l y and longitud i n a l l y ( along the oven axis ). For H p r o f i l e measurements, 60~cm Si substrates were used, being the thicko
hess of the f i l m in the range 400 - 6000 A. The CH was determined by using 7 MeV van der Graaf of Legnaro Laboratories ( I t a l y ) by the reaction N15 + Hl and by monitoring the 4.43 MeV resonant y-rays. Beamenergy resolution and intensity o at the were 4 KeV and l pA/cm2 respectively.Depth resolution varied from 30 A o
surface to lO0 A down to l um. The estimated precision in CH was of the order of O.1%. The absorption coefficientawas determined either by Valeev's method4 or by a more accurate method5 depending on f i l m thickness. 3. RESULTS AND DISCUSSION Tauc's plots for a-Si grown from SiH4 are shown in Fig. l for deposition tempeo
o
ratures of 450°C, 500°C and 550°C. Thicknesses are 420 A, 6000 A and I pm respectively. The optical gaps derived by usual f i t t i n g procedures are 1.6, 1.55
(cm-I/2eV I / 550°C
Fig. 1
400
Tauc's plots for SiH. grown samples at ~hree different deposition temperatures.
300 200 lO0
1.5
2.
2,5
hv(eV)
and 1.42 eV respectively. The same plots for a-Si obtained from Si2H6 are shown in Fig. 2. At deposition temperatures of 460°C, 480°C and 500°C, the thicknesses o
o
o
are 2000 A, 4200 A and 8000 A and E values are 1.7, 1.62 and 1.55 eV respective g ly. Only few representative results are reported, which are indicative of average data. The behaviour of Eg as a function of Td is reported in Fig. 3 separately for SiH4 and Si2H6 grown films. The indication is that, while the former ones
C. Manfredotti et al. / Hydrogen content and optical properties
(cm-I/2evl/
4/O°C i0°C
Si2H6
577
7 °C
Eg(eV)
800 700
•
1.5
600
Si2H6 Sill4
500 400
1.0
300 200 Qe
100
1.5
2.
I
I
450
500
I
550 Td(°C)
2.5 hv(eV)
Fig. 2 Tauc's plots for Si2H6 grown samples.
Fig. 3 Optical gap E versus deposition temperature T. b~th for SiH4 and Si2H6
grown samples.
tend to saturate t h e i r optical gap with decreasing Td, the l a t t e r ones show E g values which increase at lower Td. This increase cannot be due to a variation of CH, since in a l l investigated samples CH was constant at 0.6% approximately. As a matter of fact, the low pressures used and the low deposition velocities obtained allow for H effusion during deposition6, giving practically a background value for CH. I t must be noted, however, that H p r o f i l e is by no means constant: o an example is given in Fig. 4 for a sample 2000 A thick.
CH(%) 1.4
i
SiH4
Td= 550°C d = 1800
Fig. 4 Typical C.. p r o f i l e as a function of d~pth d for a Sill 4 grown sample.
1.2 I.O 0.8 0.6
. . . . . . .
500
I000
1500
2000 d(A)
In p a r t i c u l a r , CH increases steeply at the surface, because of the presence of radicals, and also at the interface with substrate, where i t may easily be l a r ger than I%. In all cases, however, CH values in excess of I% are limited to
C. Manfredotti et al. / Hydrogen content and optical properties
578 o
less than 200 A in depth, both at the surface and at the interface. I t is therefore unlikely that non-homogeneityin CH could influence absorption measurements o
particularly in Si2H6 samples, which are always thicker than 2000 A. Therefore, the only possible conclusion is that E dependsonly on deposition g temperature, at least at low growth pressures and that the f i l m precursor SiH2 in Si2H6 depositions is capable of giving a larger optical gap, even i f H effuses during the growth. Finally, i t has been noted that absorption coefficients displayed Urbach's t a i l s with logarithmic slope E0 between 60 and 140 meV. The dependenceof Eg on E0 is not strongly different from what reported for GD samples3. Therefore, our LPCVD films are in some way intermediate between a-Si films quoted in l i t e r a ture 3 and GD films: they are similar to the former ones as far as the behaviour of absorption coefficient above E is concerned, while display a valence band g t a i l not much different from the l a t t e r ones and E values which l i e between g the two. ACKNOWLEDGMENTS Thanks are due to Prof. R. r.lurri; F. Demichelis and Dr. A. Tagliaferro for optical absorption measurements and to Dr. M. Meliga for thickness measurements. REFERENCES l) J. Magarino, Poly-Micro-Crystalline and Amorphous Semiconductors, MRS Europe 1984, eds. P. Pinard and S. Kalbitzer, p. 651. 2) Y. Ashida, Y. Mishima, M. Hirose, Y. Osaka and K. Kojima, Jap. J. Appl. Phys. 23 (1984) L129. 3) L. Ley, ref. l , p. 451. 4) A. S. Valeev, Opt. Spectr. 15 (1963) 269. 5) F. Demichelis, E. Minetti-Mezzetti, A. Tagliaferro and E. Tresso, Fifth EC Photovoltaic Solar Energy Conf., eds. W. Palz and F. F i t t i p a l d i , D. Reidel Publishing Co., Boston 1984, p. 759. 6) M. Reinelt, S. Kalbitzer and G. MUller, Proc. lOth Int. Conf. on Amorphous and Liquid Semiconductors, Tokyo 1983, eds. K. tanaka and T. Shimizu, NorthHolland, Amsterdam]984, p. 169.
575
HYDROGEN CONTENT AND OPTICAL PROPERTIES OF LPCVD AMORPHOUSSILICON FILMS
C. MANFREDOTTI, G.SCARANO Istituto
di Fisica Superiore, Corso M. D'Azeglio 46, 10125 Torino, I t a l y
G. DELLA MEA and C. ROSSI ALVAREZ Dipartimento di F i s i c a , Via Marzolo 8, 53100 Padova, I t a l y Optical properties of LPCVD amorphous s i l i c o n f i l m s deposited both from SiH4 and from SigH_ are i n v e s t i g a t e d at very low H contents. The o p t i c a l energy gap v a r i e s wi~h deposition temperature even i f H concentration is constant. The non-homogeneous H p r o f i l e seems not influence o p t i c a l p r o p e r t i e s , which d i s p l a y features q u i t e s i m i l a r to those of GD samples. I . INTRODUCTION Optical properties of LPCVD amorphous s i l i c o n f i l m s are c e r t a i n l y less extens i v e l y i n v e s t i g a t e d with respect to GD deposited f i l m s ,
in p a r t i c u l a r f o r growth
temperatures below 600°C I . Recently 2 i t has been proved t h a t H i n c o r p o r a t i o n is possible with Si2H 6 LPCVD, but the o p t i c a l gap Eg is q u i t e independent of H cont e n t CH and i t is never l a r g e r than 1.6 eV even f o r CH of 20%. In the present work, i t is indicated t h a t i t is possible to get E = 1.7 eV g by Si2H 6 LPCVD at low temperature ( 460°C ) and pressure ( 0.2 t o r r ) even f o r CH below I% and t h a t Eg depends only on deposition temperature Td. The exponential
absorption edge displays a l o g a r i t h m i c slope E0 not strongly d i f f e r e n t from
what reported in GD l i t e r a t u r e 3. However, the absorption c o e f f i c i e n t is l a r g e r by more than one order of magnitude below 2 eV. F i n a l l y , by nuclear techniques it
is proved t h a t CH is not homogeneous in depth, even i f a bulk content may be
always determined. 2. EXPERIMENTAL The a-Si f i l m s have been grown in an h o r i z o n t a l 5" quartz tube by a hot wall reactor with a 50 cm long hot zone. Electronic grade SiH 4 or Si2H 6 was admitted at a flow r a t e of 50 SCCM and at temperatures between 430°C and 550°C. The gas pressure was held at 0.1 t o r r ( SiH 4) and at 0.2 t o r r ( Si2H 6 ). The growth rates displayed approximately the same a c t i v a t i o n energy and varied from 0.5 up to 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
C Manfredottietal./Hydrogen contentand opticalproperties
576 o
22 A/min. Films were deposited on sapphire or 7059 Corning glass 3" substr~tes and thickness was accurately determined by etching using an alpha-step apparatus. Films turned out to be rather homogeneousin thickness both r a d i a l l y and longitud i n a l l y ( along the oven axis ). For H p r o f i l e measurements, 60~cm Si substrates were used, being the thicko
hess of the f i l m in the range 400 - 6000 A. The CH was determined by using 7 MeV van der Graaf of Legnaro Laboratories ( I t a l y ) by the reaction N15 + Hl and by monitoring the 4.43 MeV resonant y-rays. Beamenergy resolution and intensity o at the were 4 KeV and l pA/cm2 respectively.Depth resolution varied from 30 A o
surface to lO0 A down to l um. The estimated precision in CH was of the order of O.1%. The absorption coefficientawas determined either by Valeev's method4 or by a more accurate method5 depending on f i l m thickness. 3. RESULTS AND DISCUSSION Tauc's plots for a-Si grown from SiH4 are shown in Fig. l for deposition tempeo
o
ratures of 450°C, 500°C and 550°C. Thicknesses are 420 A, 6000 A and I pm respectively. The optical gaps derived by usual f i t t i n g procedures are 1.6, 1.55
(cm-I/2eV I / 550°C
Fig. 1
400
Tauc's plots for SiH. grown samples at ~hree different deposition temperatures.
300 200 lO0
1.5
2.
2,5
hv(eV)
and 1.42 eV respectively. The same plots for a-Si obtained from Si2H6 are shown in Fig. 2. At deposition temperatures of 460°C, 480°C and 500°C, the thicknesses o
o
o
are 2000 A, 4200 A and 8000 A and E values are 1.7, 1.62 and 1.55 eV respective g ly. Only few representative results are reported, which are indicative of average data. The behaviour of Eg as a function of Td is reported in Fig. 3 separately for SiH4 and Si2H6 grown films. The indication is that, while the former ones
C. Manfredotti et al. / Hydrogen content and optical properties
(cm-I/2evl/
4/O°C i0°C
Si2H6
577
7 °C
Eg(eV)
800 700
•
1.5
600
Si2H6 Sill4
500 400
1.0
300 200 Qe
100
1.5
2.
I
I
450
500
I
550 Td(°C)
2.5 hv(eV)
Fig. 2 Tauc's plots for Si2H6 grown samples.
Fig. 3 Optical gap E versus deposition temperature T. b~th for SiH4 and Si2H6
grown samples.
tend to saturate t h e i r optical gap with decreasing Td, the l a t t e r ones show E g values which increase at lower Td. This increase cannot be due to a variation of CH, since in a l l investigated samples CH was constant at 0.6% approximately. As a matter of fact, the low pressures used and the low deposition velocities obtained allow for H effusion during deposition6, giving practically a background value for CH. I t must be noted, however, that H p r o f i l e is by no means constant: o an example is given in Fig. 4 for a sample 2000 A thick.
CH(%) 1.4
i
SiH4
Td= 550°C d = 1800
Fig. 4 Typical C.. p r o f i l e as a function of d~pth d for a Sill 4 grown sample.
1.2 I.O 0.8 0.6
. . . . . . .
500
I000
1500
2000 d(A)
In p a r t i c u l a r , CH increases steeply at the surface, because of the presence of radicals, and also at the interface with substrate, where i t may easily be l a r ger than I%. In all cases, however, CH values in excess of I% are limited to
C. Manfredotti et al. / Hydrogen content and optical properties
578 o
less than 200 A in depth, both at the surface and at the interface. I t is therefore unlikely that non-homogeneityin CH could influence absorption measurements o
particularly in Si2H6 samples, which are always thicker than 2000 A. Therefore, the only possible conclusion is that E dependsonly on deposition g temperature, at least at low growth pressures and that the f i l m precursor SiH2 in Si2H6 depositions is capable of giving a larger optical gap, even i f H effuses during the growth. Finally, i t has been noted that absorption coefficients displayed Urbach's t a i l s with logarithmic slope E0 between 60 and 140 meV. The dependenceof Eg on E0 is not strongly different from what reported for GD samples3. Therefore, our LPCVD films are in some way intermediate between a-Si films quoted in l i t e r a ture 3 and GD films: they are similar to the former ones as far as the behaviour of absorption coefficient above E is concerned, while display a valence band g t a i l not much different from the l a t t e r ones and E values which l i e between g the two. ACKNOWLEDGMENTS Thanks are due to Prof. R. r.lurri; F. Demichelis and Dr. A. Tagliaferro for optical absorption measurements and to Dr. M. Meliga for thickness measurements. REFERENCES l) J. Magarino, Poly-Micro-Crystalline and Amorphous Semiconductors, MRS Europe 1984, eds. P. Pinard and S. Kalbitzer, p. 651. 2) Y. Ashida, Y. Mishima, M. Hirose, Y. Osaka and K. Kojima, Jap. J. Appl. Phys. 23 (1984) L129. 3) L. Ley, ref. l , p. 451. 4) A. S. Valeev, Opt. Spectr. 15 (1963) 269. 5) F. Demichelis, E. Minetti-Mezzetti, A. Tagliaferro and E. Tresso, Fifth EC Photovoltaic Solar Energy Conf., eds. W. Palz and F. F i t t i p a l d i , D. Reidel Publishing Co., Boston 1984, p. 759. 6) M. Reinelt, S. Kalbitzer and G. MUller, Proc. lOth Int. Conf. on Amorphous and Liquid Semiconductors, Tokyo 1983, eds. K. tanaka and T. Shimizu, NorthHolland, Amsterdam]984, p. 169.