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The incorporation of phosphorus in amorphous silicon
Journal of Non-CrystallineSolids 59 & 60 (1983) 613-616 North-Holland Publishing Company
613
THE INCORPORATION OF PHOSPHORUS IN AMORPHOUSSILICON* D. LEIDICH, E. LINHART, E. NIEMANN, H.W. GRUENINGER, R. FISCHER, AND R.R. ZEYFANG AEG-TELEFUNKEN Forschungsinstitut, 6000 Frankfurt 71, W.Germany Amorphous f i l m s of Si-P a l l o y s were prepared by plasma d e p o s i t i o n in an r f gas discharge of mixtures of SiH 4 and PH3. The r e l a t i v e gas phase content of PH3 was varied between 5-10 -5 and u n i t y . The phosphorus i n c o r p o r a t i o n r a t i o was detemfined by neutron a c t i v a t i o n analysis and atomic absorption spectroscopy; i t decreases from about 6 at low concentrations to u n i t y at high conc e n t r a t i o n s . The f i l m s were characterized by measurements of the dark conduct i v i t y and i t s a c t i v a t i o n energy. - To e x p l a i n our r e s u l t s we propose t h a t , in the range of P contents i n v e s t i g a t e d , most of the P atoms are 3 - f o l d coo r d i n a t e d ; a small f r a c t i o n is 4 - f o l d coordinated and accounts f o r the doping. I.
INTRODUCTION Both c r y s t a l l i n e and amorphous s i l i c o n can be doped by the i n c o r p o r a t i o n of
phosphorus, the phenomena are, thus, the same in both cases. There are, however, some q u a l i t a t i v e d i f f e r e n c e s :
in the amorphous f i l m s the i n c o r p o r a t i o n of
dopants takes g e n e r a l l y place during growth of the f i l m s , the doping e f f i e n c y is very low 1'2, and a high concentration of hydrogen is present. We w i l l r e p o r t here on measurements of the r e l a t i v e phosphorus concentration x incorporated in a-Si:H f i l m s during plasma d e p o s i t i o n from PH3-SiH
mixtures.
The total range from gas phase doping concentrations as low as 5-10-
up to
pure a-P:H was covered. The films were characterized by measurements of the dark conductivity and i t s activation energy. 2. EXPERIMENTAL
The doped a-Si:H and a l l o y f i l m s were prepared in a conventional c a p a c i t i v e l y coupled high-frequency plasma deposition reactor under the f o l l o w i n g condit i o n s : frequency 13.65 MHz, power d e n s i t y 0.02 W/cm2, t o t a l flow r a t e 20 sccm, t o t a l pressure 0.6 mbar, substrate temperature 280°C f o r a l l compositions app l i e d . The PH3 content of the process gas was determined using capacitance absolute pressure gauges. Above a gas phase r a t i o y = p(PH3)/P(PH 3 + SiH4) = 0.01 the phosphorus con*This work was supported under the technological program of the Federal Department of Research and Technologyof the FRG. The authors alone are responsible for the contents. 0022-3093/83/0000-0000/$03.00 © 1983 North-HoUand/Physical Society of Japan
D. Leidich et al. / The incorporation o f phosphorus in amorphous silicon
614
10 0
/J'
=-
~ i0-I
t e n t of the f i l m s was de-
S
Lermined by atomic absorpLion spectroscopy (AAS), below t h a t by neutron act i v a t i o n a n a l y s i s . The mass of the f i l m s was meas-
-~ m-2
x/
/-
i0-3
ured by weighing and, in a d d i t i o n at low P-content, by determining the dimensions of the f i l m s and using a d e n s i t y of 2.2 g/ cm3. The mass of the f i l m s
10 -4
',
I0-5
'
,,,,,,i
,
,
,
,,,;,,i
10-4
,,
10-3
,
,,,,,,l
,,
I0-2
,
,,,,,q
,
,
b,,,,,,
10 q
[PH~,] / [ PH3,,SiHJ
10 0
determined the toLal accuracy of the analysis to roughly ~ 10%. For the e l e c L r i c a l
FIGURE I Relative P-content of the f i l m s as function of the r e l a t i v e gas phase P-content
measurements, samples prepared in the same run as those f o r the Pdetermina-
t i o n were covered with two 20 mm long coplanar Cr electrodes I mm apart. 3. RESULTS AND DISCUSSION The r e s u l t s on the phosphorus i n c o r p o r a t i o n are shown in Fig. I .
I t is seen
t h a t in the whole range of gas mixtures i n v e s t i g a t e d the concentration of phosphorus in the f i l m s rises s t e a d i l y with the gas phase r a t i o y according to a power law yV, with v = 0.82. The c o e f f i c i e n t of i n c o r p o r a t i o n is, of course, u n i t y f o r x = y = I and is l a r g e r than I below t h a t , i . e . P is more r e a d i l y built
into the growing f i l m than Si. In contrast to p r e v i o u s l y published re-
ports 3, we f i n d no i n d i c a t i o n of a s a t u r a t i o n of the phosphorus content x with the gas phase r a t i o y. Figs. 2 and 3 show the r e s u l t s of the e l e c t r i c a l measurements. Typical Arrhenius plots of the dark c o n d u c t i v i t y are seen in Fig. 2 f o r several compos i t i o n s out of the Si-P a l l o y system. The dark c o n d u c t i v i t y at room temperature (an e x t r a p o l a t e d value was used at y = I ) as well as the a c t i v a t i o n energies EA are p l o t t e d vs. y in Fig. 3. The series begins at a gas phase r a t i o of 5.10-5; there the dark c o n d u c t i v i t y od has already reached the high value of 5.10-3 Q-I cm-1. With increasing y, ad very slowly increases and reaches a broad maximum at y = I0 -2. Above t h a t a d r a s t i c decrease of od sets in which stops only at 1.5-10 -16 ~ - I cm-l, the e x t r a p o l a t e d value f o r pure a-P:H. The
D. Leidich et al. / The incorporation o f phosphorus in amorphous silicon
1
615
m-1
phosl~o~usce~ltent:
10"1
-I,1
10"3 _ "A
10.2 -
=>. >
" ~ 1 0
-2
10-3_
\
10-t.
No.,
0.9
>~
"0 10-9
§
i0.5 -
o 1 0 TM
0
"o 10-6_
10-? _
io
10.8 10-9 1 0 "10
1 0 -I1 _
0.5
~
1613t 0.3
16'51
io-171
i 1
10-5
0.1
[PH3) /[SiH4. +PH3]
FIGURE 3 Dark c o n d u c t i v i t y and a c t i v a t i o n energy vs. r e l . gas phase P-content
10-12. 1 0 "13
'i lO's"T
i
2
I
3
I
t. 103KIT
FIGURE 2 ( l e f t ) Dark c o n d u c t i v i t y vs. r e c i p r o c a l temperature in the Si-P system
corresponding EA values f o l l o w an almost r e c i p r o c a l curve, except t h a t there is no minimum at 10-2; EA varies between 0.15 eV at y = 5.10 -5 and 1 . 1 7 e V a t y = I . From these data, one can e a s i l y estimate t h a t the concentration of P donors r e l a t i v e to the t o t a l P concentration, i . e . the doping e f f i c i e n c y ,
is very low.
Using a oo of 104 Q-I cm-1 we obtain t h a t the Fermi level at y = 5-10 -5 l i e s 0.3 eV below the conduction band edge or 0.5 eV above i t s p o s i t i o n in the undoped case. The t o t a l concentration of donors is then estimated using a value of 1017 eV-I cm-3 f o r the d e n s i t y of states 4 which gives 5-1016 cm-3, at the lowest t o t a l P concentration of 3-10 -4 ( r e l a t i v e ) or 1.5-1019 cm-3 (absolute). Comparison of these f i g u r e s gives a doping e f f i c i e n c y of roughly o.3%, which is in f a i r agreement with other r e s u l t s I . According t o F i g . 3 , the slow increase of ~d with increasing P-content, and the decrease above y = 10-2 r e f l e c t a decreasing doping e f f i e n c y of the phosphorus atoms. From the smooth curve in Fig. I of the function x = f (y) and from the re-
616
D. Leidich et al. / The incorporation o f phosphorus in amorphous silicon
s u l t (Fig. 3) t h a t there are no abrupt changes in the e l e c t r i c a l p r o p e r t i e s , the conclusion may be drawn t h a t no change occurs, in the whole range of conc e n t r a t i o n s , of the manner the m a j o r i t y of P atoms are accommodated in the Si-P network. I t is well known, t h a t in a-Si the s i l i c o n atoms are 4 - f o l d coordinated, whereas in a-P the phosphorus atoms are 3 - f o l d coordinated. Therefore, in the amorphous Si-P a l l o y system, there must be a continuous change in the coo r d i n a t i o n of the amorphous network, ranging from 4 - f o l d coordinated Si on the S i - r i c h side of the system through a mixed 3-4 coordinated network in the intermediate range to a pure 3 - f o l d phosphorus network. We suggest t h a t in the whole a l l o y system most of the P atoms are 3 - f o l d coordinated, and do not generate defect states w i t h i n the gap 5. Only a small f r a c t i o n of P atoms are i n corporated as 4 - f o l d coordinated donor atoms or as P-related defects 6'7 into the network. The decrease of the doping e f f i c i e n c y with increasing P-content can be explained in two ways, by an increasing number of P-induced defects, or by an increasing f l e x i b i l i t y
of the Si-P network which makes the i n c o r p o r a t i o n of the
4 - f o l d coordinated P atoms less l i k e l y . ACKNOWLEDGEMENTS We thank Dr. Reese f o r the neutron a c t i v a t i o n analysis and Mr. Pilz f o r the AAS measurements. REFERENCES I) D.V. Lang, J.D. Cohen, J.P. Harbison, Phys. Rev. L e t t . 48 (1982) 421 2) R.A. Street, Phys. Rev. L e t t . 49 (1982) 1187 3) W.E. Spear, P.G. LeComber, P h i l . Mag. 33 (1976) 935 4) A. Madan, P.G. LeComber, W.E. Spear, J. Non-Cryst. Solids 20 (1976) 239 5) D. Adler, Solar Cells 9 (1983) 133 6) D.V. Lang, J.D. Cohen, J.P. Harbison, Phys. Rev. B25 (1982) 5285 7) H. Dersch, J. Stuke, J. Beichler, phys. s t a t . s o l .
(b) 105 (1981) 265
613
THE INCORPORATION OF PHOSPHORUS IN AMORPHOUSSILICON* D. LEIDICH, E. LINHART, E. NIEMANN, H.W. GRUENINGER, R. FISCHER, AND R.R. ZEYFANG AEG-TELEFUNKEN Forschungsinstitut, 6000 Frankfurt 71, W.Germany Amorphous f i l m s of Si-P a l l o y s were prepared by plasma d e p o s i t i o n in an r f gas discharge of mixtures of SiH 4 and PH3. The r e l a t i v e gas phase content of PH3 was varied between 5-10 -5 and u n i t y . The phosphorus i n c o r p o r a t i o n r a t i o was detemfined by neutron a c t i v a t i o n analysis and atomic absorption spectroscopy; i t decreases from about 6 at low concentrations to u n i t y at high conc e n t r a t i o n s . The f i l m s were characterized by measurements of the dark conduct i v i t y and i t s a c t i v a t i o n energy. - To e x p l a i n our r e s u l t s we propose t h a t , in the range of P contents i n v e s t i g a t e d , most of the P atoms are 3 - f o l d coo r d i n a t e d ; a small f r a c t i o n is 4 - f o l d coordinated and accounts f o r the doping. I.
INTRODUCTION Both c r y s t a l l i n e and amorphous s i l i c o n can be doped by the i n c o r p o r a t i o n of
phosphorus, the phenomena are, thus, the same in both cases. There are, however, some q u a l i t a t i v e d i f f e r e n c e s :
in the amorphous f i l m s the i n c o r p o r a t i o n of
dopants takes g e n e r a l l y place during growth of the f i l m s , the doping e f f i e n c y is very low 1'2, and a high concentration of hydrogen is present. We w i l l r e p o r t here on measurements of the r e l a t i v e phosphorus concentration x incorporated in a-Si:H f i l m s during plasma d e p o s i t i o n from PH3-SiH
mixtures.
The total range from gas phase doping concentrations as low as 5-10-
up to
pure a-P:H was covered. The films were characterized by measurements of the dark conductivity and i t s activation energy. 2. EXPERIMENTAL
The doped a-Si:H and a l l o y f i l m s were prepared in a conventional c a p a c i t i v e l y coupled high-frequency plasma deposition reactor under the f o l l o w i n g condit i o n s : frequency 13.65 MHz, power d e n s i t y 0.02 W/cm2, t o t a l flow r a t e 20 sccm, t o t a l pressure 0.6 mbar, substrate temperature 280°C f o r a l l compositions app l i e d . The PH3 content of the process gas was determined using capacitance absolute pressure gauges. Above a gas phase r a t i o y = p(PH3)/P(PH 3 + SiH4) = 0.01 the phosphorus con*This work was supported under the technological program of the Federal Department of Research and Technologyof the FRG. The authors alone are responsible for the contents. 0022-3093/83/0000-0000/$03.00 © 1983 North-HoUand/Physical Society of Japan
D. Leidich et al. / The incorporation o f phosphorus in amorphous silicon
614
10 0
/J'
=-
~ i0-I
t e n t of the f i l m s was de-
S
Lermined by atomic absorpLion spectroscopy (AAS), below t h a t by neutron act i v a t i o n a n a l y s i s . The mass of the f i l m s was meas-
-~ m-2
x/
/-
i0-3
ured by weighing and, in a d d i t i o n at low P-content, by determining the dimensions of the f i l m s and using a d e n s i t y of 2.2 g/ cm3. The mass of the f i l m s
10 -4
',
I0-5
'
,,,,,,i
,
,
,
,,,;,,i
10-4
,,
10-3
,
,,,,,,l
,,
I0-2
,
,,,,,q
,
,
b,,,,,,
10 q
[PH~,] / [ PH3,,SiHJ
10 0
determined the toLal accuracy of the analysis to roughly ~ 10%. For the e l e c L r i c a l
FIGURE I Relative P-content of the f i l m s as function of the r e l a t i v e gas phase P-content
measurements, samples prepared in the same run as those f o r the Pdetermina-
t i o n were covered with two 20 mm long coplanar Cr electrodes I mm apart. 3. RESULTS AND DISCUSSION The r e s u l t s on the phosphorus i n c o r p o r a t i o n are shown in Fig. I .
I t is seen
t h a t in the whole range of gas mixtures i n v e s t i g a t e d the concentration of phosphorus in the f i l m s rises s t e a d i l y with the gas phase r a t i o y according to a power law yV, with v = 0.82. The c o e f f i c i e n t of i n c o r p o r a t i o n is, of course, u n i t y f o r x = y = I and is l a r g e r than I below t h a t , i . e . P is more r e a d i l y built
into the growing f i l m than Si. In contrast to p r e v i o u s l y published re-
ports 3, we f i n d no i n d i c a t i o n of a s a t u r a t i o n of the phosphorus content x with the gas phase r a t i o y. Figs. 2 and 3 show the r e s u l t s of the e l e c t r i c a l measurements. Typical Arrhenius plots of the dark c o n d u c t i v i t y are seen in Fig. 2 f o r several compos i t i o n s out of the Si-P a l l o y system. The dark c o n d u c t i v i t y at room temperature (an e x t r a p o l a t e d value was used at y = I ) as well as the a c t i v a t i o n energies EA are p l o t t e d vs. y in Fig. 3. The series begins at a gas phase r a t i o of 5.10-5; there the dark c o n d u c t i v i t y od has already reached the high value of 5.10-3 Q-I cm-1. With increasing y, ad very slowly increases and reaches a broad maximum at y = I0 -2. Above t h a t a d r a s t i c decrease of od sets in which stops only at 1.5-10 -16 ~ - I cm-l, the e x t r a p o l a t e d value f o r pure a-P:H. The
D. Leidich et al. / The incorporation o f phosphorus in amorphous silicon
1
615
m-1
phosl~o~usce~ltent:
10"1
-I,1
10"3 _ "A
10.2 -
=>. >
" ~ 1 0
-2
10-3_
\
10-t.
No.,
0.9
>~
"0 10-9
§
i0.5 -
o 1 0 TM
0
"o 10-6_
10-? _
io
10.8 10-9 1 0 "10
1 0 -I1 _
0.5
~
1613t 0.3
16'51
io-171
i 1
10-5
0.1
[PH3) /[SiH4. +PH3]
FIGURE 3 Dark c o n d u c t i v i t y and a c t i v a t i o n energy vs. r e l . gas phase P-content
10-12. 1 0 "13
'i lO's"T
i
2
I
3
I
t. 103KIT
FIGURE 2 ( l e f t ) Dark c o n d u c t i v i t y vs. r e c i p r o c a l temperature in the Si-P system
corresponding EA values f o l l o w an almost r e c i p r o c a l curve, except t h a t there is no minimum at 10-2; EA varies between 0.15 eV at y = 5.10 -5 and 1 . 1 7 e V a t y = I . From these data, one can e a s i l y estimate t h a t the concentration of P donors r e l a t i v e to the t o t a l P concentration, i . e . the doping e f f i c i e n c y ,
is very low.
Using a oo of 104 Q-I cm-1 we obtain t h a t the Fermi level at y = 5-10 -5 l i e s 0.3 eV below the conduction band edge or 0.5 eV above i t s p o s i t i o n in the undoped case. The t o t a l concentration of donors is then estimated using a value of 1017 eV-I cm-3 f o r the d e n s i t y of states 4 which gives 5-1016 cm-3, at the lowest t o t a l P concentration of 3-10 -4 ( r e l a t i v e ) or 1.5-1019 cm-3 (absolute). Comparison of these f i g u r e s gives a doping e f f i c i e n c y of roughly o.3%, which is in f a i r agreement with other r e s u l t s I . According t o F i g . 3 , the slow increase of ~d with increasing P-content, and the decrease above y = 10-2 r e f l e c t a decreasing doping e f f i e n c y of the phosphorus atoms. From the smooth curve in Fig. I of the function x = f (y) and from the re-
616
D. Leidich et al. / The incorporation o f phosphorus in amorphous silicon
s u l t (Fig. 3) t h a t there are no abrupt changes in the e l e c t r i c a l p r o p e r t i e s , the conclusion may be drawn t h a t no change occurs, in the whole range of conc e n t r a t i o n s , of the manner the m a j o r i t y of P atoms are accommodated in the Si-P network. I t is well known, t h a t in a-Si the s i l i c o n atoms are 4 - f o l d coordinated, whereas in a-P the phosphorus atoms are 3 - f o l d coordinated. Therefore, in the amorphous Si-P a l l o y system, there must be a continuous change in the coo r d i n a t i o n of the amorphous network, ranging from 4 - f o l d coordinated Si on the S i - r i c h side of the system through a mixed 3-4 coordinated network in the intermediate range to a pure 3 - f o l d phosphorus network. We suggest t h a t in the whole a l l o y system most of the P atoms are 3 - f o l d coordinated, and do not generate defect states w i t h i n the gap 5. Only a small f r a c t i o n of P atoms are i n corporated as 4 - f o l d coordinated donor atoms or as P-related defects 6'7 into the network. The decrease of the doping e f f i c i e n c y with increasing P-content can be explained in two ways, by an increasing number of P-induced defects, or by an increasing f l e x i b i l i t y
of the Si-P network which makes the i n c o r p o r a t i o n of the
4 - f o l d coordinated P atoms less l i k e l y . ACKNOWLEDGEMENTS We thank Dr. Reese f o r the neutron a c t i v a t i o n analysis and Mr. Pilz f o r the AAS measurements. REFERENCES I) D.V. Lang, J.D. Cohen, J.P. Harbison, Phys. Rev. L e t t . 48 (1982) 421 2) R.A. Street, Phys. Rev. L e t t . 49 (1982) 1187 3) W.E. Spear, P.G. LeComber, P h i l . Mag. 33 (1976) 935 4) A. Madan, P.G. LeComber, W.E. Spear, J. Non-Cryst. Solids 20 (1976) 239 5) D. Adler, Solar Cells 9 (1983) 133 6) D.V. Lang, J.D. Cohen, J.P. Harbison, Phys. Rev. B25 (1982) 5285 7) H. Dersch, J. Stuke, J. Beichler, phys. s t a t . s o l .
(b) 105 (1981) 265