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Surface structure of glow discharge a-Si:H: implications for multilayer film growth
Journal of Non-Crystalline Solids 97&98 (1987) 1439-1442 North.Holland, Amsterdam
1439
SURFACE STRUCTURE OF GLOW DISCHARGE a-Si:H: IMPLICATIONS MULTILAYER FILM GROWTH
FOR
R.W. COLLINS and J.M. CAVESE Standard Oil Research. 4440 Warrensville Ctr. Rd.. Cleveland. Ohio 44128 USA Ellipsometry studies have been performed using inert and reactive gas plasmas as probes of the near-surface structure of plasma-deposited hydrogenated amorphous silicon (a-Si:H). By exposing freshly-prepared a-Si:H to a weak Ar plasma, in situ ellipsometry permits detection of a ~ 4-8 ~ thickness decrease within the first second. This is interpreted as a dynamic transition layer between the plasma and the static film. Further plasma exposure generates ~ 20 ~ additional surface roughness after 0.5 hr. Various results are used to argue that this bombardment-generated roughness is in the form of atomic scale microvoids. In contrast, roughness detected on high quality surfaces consists of larger scale modulation (> 50 ~. in the plane of the surface). Implications of these results for multilayer film growth are discussed. 1. INTRODUCTION Many recent studies of thin film amorphous semiconductors have concentrated on the electro-optical properties and microstructure of interfaces. 1 In this work. in situ and spectroscopic ellipsometry experiments have been used to study contributions to the near-surface structure of a-Si:H that may influence interface abruptness. Ellipsometry is sensitive to subrnonolayer changes in the thickness of oxides or roughness, both of which suppress the dielectric discontinuity at a surface. Surface or interface roughness can be modeled as a separate layer with a dielectric function determined by effective medium theory using a mixture of under- and over-lying materials, providing that the scale of the roughness, in the plane of the surface, is within bounds set by the theory. 2 2, EXPERIMENT To obtain the in situ data, a rf glow discharge reactor was mounted at the axis of a spectroscopic rotating analyzer ellipsometer set at 3.4 or 3.5 eV. In the early stages of plasma surface modification, ellipsometry angles. (II,A). were collected at 1 s intervals, and were converted to the pseudo-dielectric function. ( < e l > , < e 2 > ) .
The pseudo-
dielectric function is the dielectric function of a hypothetical, isotropic, opaque film with a "mathematically" abrupt surface that gives the same (II.A} data as the real sample.2 ( < E l > . < ~ 2 > ) spectra, from 2.5 to S.0 eV, were analyzed with rnultilayer optical and linear regression analysis, using reference dielectric functions 3 for a-Si and SiO 2. The Bruggeman effective medium approximation was used in modeling the roughness layers.2 Nearly identical ( w i t h i n ~ 2 .~) roughness thicknesses have been obtained independently on the same surface by modeling in situ data during growth and spectroscopic data after growth. This consistency has lent confidence to both the in situ analysis and the ability of the reference dielectric function to describe the a-Si:H. 0022-3093/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
1440
R. IV. Collins, J.M. Cavese / Surface structure of glow discharge a-Si:H
3. RESULTS AND DISCUSSION In Fig. l[a], data are presented for the first 45 s of Ar plasma exposure of a freshly-deposited high quality a-Si:H thin film (points). Analysis of data obtained during growth of the a-Si:H leads to an estimate of the starting film thickness of 60 ~ with a roughness layer of ~ 8 ~. At this thickness, the initial growth microstructure has just converged. 4 Throughout Ar exposure, there is a decrease in ( < ~ 1 > , < ~ 2 > ) that can be explained with a gradual increase in surface roughness, from 8 to 16 ~. To obtain the close fit (broken line) to the data in Fig l[a]. however, it was also necessary to assume that the total film thickness decreased from 60 to 53 ~ in the first second of plasma exposure, with no detectable loss thereafter. This accounts for the initial increase in <~2 > for the first data point. The solid line in the Fig, l[a] shows the trajectory expected if the film thickness decreased with no change in the roughness thickness. The initial film loss. 4-8 ~ from analysis of several identical runs using thin a-Si:H, is attributed to a more weakly-bonded surface layer, separating the static film from the plasma. The thickness estimate is reasonably close to that from mass spectrometry studies. 5 It is not clear if the later time roughness increase in Fig l[a] is a result of the disruption of the weak bonding that links separate nucleation structures. To determine the differences, if any, between the "intrinsic" roughness on the a-Si:H remaining after nucleation and that generated by the inert gas plasma, oxide growth on such surfaces has been examined. These experiments were performed on thicker (~ 2000 ~), opaque a-Si:H samples. Fig. lib] shows data obtained during Ar (30 min), and then 0 2 (150 min) plasma exposure of high quality a-Si:H. The decrease in ( < ~ 1 > . < ~ 2 > ) during Ar plasma exposure can be modeled as in Fig. l[a] with a surface roughness 3C 2-q
251 Ar plosrno exposure T = - - 2 5 0 ° C ; 8 W power
_/
[ol
Ar
:2
201
2~ 2;
start O2 plosrno
i~.~J#~ ~ ' ' ~ ~ end
!tort
151
p|ommo
[b]
2E 25 9
, 10
, , 11 <¢1> 12
, 13
,
14
10 -5
'
0
' <~>
'
10
15
FIGURE 1 In situ ellipsometry data obtained during: [a] Ar plasma, exposure of thin (60 ~) high quality a-Si:H. The solid and broken lines (crosses at 2 A increments) are models for a thickness decrease and an increase in surface roughness, respectively. [b] Ar. and then O.j plasma exposure of opaque a-Si:H (2000 ~). The crosses denote a model for an increase in roughness, then oxidation (in 4 J~ increments).
R. W. Collins, J.M. Cavese / Surface structure of glow discharge a-Si.'H
1441
TABLE I Structural data deduced from the fits to spectra before and after Ar and/or O~ plasma exposure for high quality a-Si:H (top) and poorer quality a-Si:H (bottom~. Oxide refractive indices (at 3.5 eV) of 1.40.0.03 and 1.51-0.10 were determined, respectively.
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Before plasma exposure
After plasma exposure
Surface Roughness Thickness
Oxide Thickness
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Bulk Composition .
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Interface Roughness Thickness .
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Bulk Composition
.
]pure SIR4; Ar (30 min) and 0 2 (150 min) plasmas] 15.0.5 ~
0.979*0.005 a-Si 0.021.0.005 void
128.6 ~
13.2 ~
0.976*0.007 a-Si 0.024*0.007 void
20*2 ~
0.955"0.01 0.045.0.01
[1:50 SiH4:Ar: 0 2 (150 min) plasma only] 29"1 ~
0.942*0.006 a-Si 0.058*0.006 void
59*5 ~
a-Si void
increase, from 15 to 34 ~ (crosses). Table I (top) provides results of a linear regression analysis of spectroscopic data before and after the treatments. The following model is proposed to explain the results. The in-plane scale of the 15.0.5 ~ roughness on the unexposed a-Si:H is greater than the thickness of the generated oxide (128 ~). Thus, the oxidation front locally conforms to the surface modulations. This accounts for the 13.2 ~ roughness at the a-Si:H/a-SiOx interface. In contrast, the in-plane scale of the 19 ~ roughness generated by the Ar plasma is less than the oxide thickness, possibly of atomic scale, and so is not reproduced at the interface. (This would leave a rough oxide surface, difficult to detect from the data.) The second segment fit in Fig. l[b] is based on this model. Other high quality samples follow this pattern reproducibly. The data of Figs. 2 also suggest differences between the "intrinsic" roughness and that generated by inert gas plasmas. First, from Fig. 2[a], the oxidation rate of plasmaexposed a-Si:H is greater than codeposited, unexposed a-Si:H. This result is consistent with an inert gas plasma-induced roughness of atomic-scale microvoids, which could influence diffusion of species to the oxidation front. Second, in Fig. 2[b], data for the growth of high quality a-Si:H on an Ar plasma-exposed surface are presented. The broken line is calculated assuming that the new a-Si:H grows uniformly atop the intact surface layer (ie. with a thickness-independent dielectric function). From the best fit model (solid line), it is found that the modified surface induces void formation in the first 30 ~ of the new a-Si:H. This would not occur if the plasma-induced roughness was large enough in scale for the new film to cover it conformally. Thus. after 30 ~ in this case, the new film has lost memory of the plasma-induced substrate microstructure. Finally. it is of interest to know whether the small scale roughness exists on
1442
R.W. Collins, J.M. Cavese / Surface structure of glow discharge a-Si.'H
inadvertently damaged surfaces. As an example, in Table I (bottom), data are listed for 0 2 plasma exposure of a-Si:H deposited from a 1:50 mixture of SiH4:Ar. The data show that, of the ~ 30 ~ thick roughness on this sample, ~ 10 ~ may be attributed to the microvoid-type, caused by Ar bombardment concurrent with deposition. To conclude, these results have special relevance for the abruptness and smoothness of high quality a-Si:H interfaces. First, it is expected that the plasma may provide atomic mixing throughout the weakly-bonded transition layer, leading to ~ 6 ~ alloyed interracial layers between films of different composition. The same thickness value has been deduced from high resolution electron microscopy. 6 Furthermore, the oxidation studies suggest that, although the surface appears to be atomically smooth locally, larger scale surface modulations exist which are conformally covered when a new film is deposited on the surface. This leads to a modulated interface (~ 5-15 ~ thick). Finally. under conditions leading to poorer quality materials or surfaces, microstructure in the substrate film is imparted to the new film to a thickness which depends on its scale. REFERENCES 1) See. for example: J. Non-Cryst. Solids 77 & 78, 969-1104 (1985). 2) D.E. Aspnes, Thin Solid Films 89, 249 (1982). 3) D.E. Aspnes. A.A. Studna, and E. Kinsbron, Phys. Rev. B 29, 768 (1984). 4) R.W. Collins and J.M. Cavese, these Proceedings. 5) A. Gallagher, Bull. Am. Phys. Soc. 32, 750 (1987). 6) C.C. Tsai, R.A. Street, F.A. Ponce, and G.B. Anderson, Mat. Res. Soc. Symp. Proc. 70. 351 (1986).
100
26r [O]
<
80
[b]
Inert gas
plosmo
exposure
before ,.~
(/1 O1
stort Ar ploemo 24lend Ar plosmo lstart high q u o l l t y ~
end o--SI:H "~
v
_.9. W a
R 2C o O'
18
C..,.
0
.... 50TIME (.min)lO0
•
16 =
150
2
'
'
4
6
~
~
<8.>¢t 10
'
12
14
FIGURE 2 Oxidation behavior of two a-Si:H samples (solid). The broken lines are best fit diffusion relationships. [b] In situ ellipsometry data for new a-Si:H growth on an a-Si:H surface exposed to an Ar plasma. The broken and solid lines (for a-Si:H growth) are models for uniform and non-uniform growth. [a]
1439
SURFACE STRUCTURE OF GLOW DISCHARGE a-Si:H: IMPLICATIONS MULTILAYER FILM GROWTH
FOR
R.W. COLLINS and J.M. CAVESE Standard Oil Research. 4440 Warrensville Ctr. Rd.. Cleveland. Ohio 44128 USA Ellipsometry studies have been performed using inert and reactive gas plasmas as probes of the near-surface structure of plasma-deposited hydrogenated amorphous silicon (a-Si:H). By exposing freshly-prepared a-Si:H to a weak Ar plasma, in situ ellipsometry permits detection of a ~ 4-8 ~ thickness decrease within the first second. This is interpreted as a dynamic transition layer between the plasma and the static film. Further plasma exposure generates ~ 20 ~ additional surface roughness after 0.5 hr. Various results are used to argue that this bombardment-generated roughness is in the form of atomic scale microvoids. In contrast, roughness detected on high quality surfaces consists of larger scale modulation (> 50 ~. in the plane of the surface). Implications of these results for multilayer film growth are discussed. 1. INTRODUCTION Many recent studies of thin film amorphous semiconductors have concentrated on the electro-optical properties and microstructure of interfaces. 1 In this work. in situ and spectroscopic ellipsometry experiments have been used to study contributions to the near-surface structure of a-Si:H that may influence interface abruptness. Ellipsometry is sensitive to subrnonolayer changes in the thickness of oxides or roughness, both of which suppress the dielectric discontinuity at a surface. Surface or interface roughness can be modeled as a separate layer with a dielectric function determined by effective medium theory using a mixture of under- and over-lying materials, providing that the scale of the roughness, in the plane of the surface, is within bounds set by the theory. 2 2, EXPERIMENT To obtain the in situ data, a rf glow discharge reactor was mounted at the axis of a spectroscopic rotating analyzer ellipsometer set at 3.4 or 3.5 eV. In the early stages of plasma surface modification, ellipsometry angles. (II,A). were collected at 1 s intervals, and were converted to the pseudo-dielectric function. ( < e l > , < e 2 > ) .
The pseudo-
dielectric function is the dielectric function of a hypothetical, isotropic, opaque film with a "mathematically" abrupt surface that gives the same (II.A} data as the real sample.2 ( < E l > . < ~ 2 > ) spectra, from 2.5 to S.0 eV, were analyzed with rnultilayer optical and linear regression analysis, using reference dielectric functions 3 for a-Si and SiO 2. The Bruggeman effective medium approximation was used in modeling the roughness layers.2 Nearly identical ( w i t h i n ~ 2 .~) roughness thicknesses have been obtained independently on the same surface by modeling in situ data during growth and spectroscopic data after growth. This consistency has lent confidence to both the in situ analysis and the ability of the reference dielectric function to describe the a-Si:H. 0022-3093/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
1440
R. IV. Collins, J.M. Cavese / Surface structure of glow discharge a-Si:H
3. RESULTS AND DISCUSSION In Fig. l[a], data are presented for the first 45 s of Ar plasma exposure of a freshly-deposited high quality a-Si:H thin film (points). Analysis of data obtained during growth of the a-Si:H leads to an estimate of the starting film thickness of 60 ~ with a roughness layer of ~ 8 ~. At this thickness, the initial growth microstructure has just converged. 4 Throughout Ar exposure, there is a decrease in ( < ~ 1 > , < ~ 2 > ) that can be explained with a gradual increase in surface roughness, from 8 to 16 ~. To obtain the close fit (broken line) to the data in Fig l[a]. however, it was also necessary to assume that the total film thickness decreased from 60 to 53 ~ in the first second of plasma exposure, with no detectable loss thereafter. This accounts for the initial increase in <~2 > for the first data point. The solid line in the Fig, l[a] shows the trajectory expected if the film thickness decreased with no change in the roughness thickness. The initial film loss. 4-8 ~ from analysis of several identical runs using thin a-Si:H, is attributed to a more weakly-bonded surface layer, separating the static film from the plasma. The thickness estimate is reasonably close to that from mass spectrometry studies. 5 It is not clear if the later time roughness increase in Fig l[a] is a result of the disruption of the weak bonding that links separate nucleation structures. To determine the differences, if any, between the "intrinsic" roughness on the a-Si:H remaining after nucleation and that generated by the inert gas plasma, oxide growth on such surfaces has been examined. These experiments were performed on thicker (~ 2000 ~), opaque a-Si:H samples. Fig. lib] shows data obtained during Ar (30 min), and then 0 2 (150 min) plasma exposure of high quality a-Si:H. The decrease in ( < ~ 1 > . < ~ 2 > ) during Ar plasma exposure can be modeled as in Fig. l[a] with a surface roughness 3C 2-q
251 Ar plosrno exposure T = - - 2 5 0 ° C ; 8 W power
_/
[ol
Ar
:2
201
2~ 2;
start O2 plosrno
i~.~J#~ ~ ' ' ~ ~ end
!tort
151
p|ommo
[b]
2E 25 9
, 10
, , 11 <¢1> 12
, 13
,
14
10 -5
'
0
' <~>
'
10
15
FIGURE 1 In situ ellipsometry data obtained during: [a] Ar plasma, exposure of thin (60 ~) high quality a-Si:H. The solid and broken lines (crosses at 2 A increments) are models for a thickness decrease and an increase in surface roughness, respectively. [b] Ar. and then O.j plasma exposure of opaque a-Si:H (2000 ~). The crosses denote a model for an increase in roughness, then oxidation (in 4 J~ increments).
R. W. Collins, J.M. Cavese / Surface structure of glow discharge a-Si.'H
1441
TABLE I Structural data deduced from the fits to spectra before and after Ar and/or O~ plasma exposure for high quality a-Si:H (top) and poorer quality a-Si:H (bottom~. Oxide refractive indices (at 3.5 eV) of 1.40.0.03 and 1.51-0.10 were determined, respectively.
.
Before plasma exposure
After plasma exposure
Surface Roughness Thickness
Oxide Thickness
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Bulk Composition .
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Interface Roughness Thickness .
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Bulk Composition
.
]pure SIR4; Ar (30 min) and 0 2 (150 min) plasmas] 15.0.5 ~
0.979*0.005 a-Si 0.021.0.005 void
128.6 ~
13.2 ~
0.976*0.007 a-Si 0.024*0.007 void
20*2 ~
0.955"0.01 0.045.0.01
[1:50 SiH4:Ar: 0 2 (150 min) plasma only] 29"1 ~
0.942*0.006 a-Si 0.058*0.006 void
59*5 ~
a-Si void
increase, from 15 to 34 ~ (crosses). Table I (top) provides results of a linear regression analysis of spectroscopic data before and after the treatments. The following model is proposed to explain the results. The in-plane scale of the 15.0.5 ~ roughness on the unexposed a-Si:H is greater than the thickness of the generated oxide (128 ~). Thus, the oxidation front locally conforms to the surface modulations. This accounts for the 13.2 ~ roughness at the a-Si:H/a-SiOx interface. In contrast, the in-plane scale of the 19 ~ roughness generated by the Ar plasma is less than the oxide thickness, possibly of atomic scale, and so is not reproduced at the interface. (This would leave a rough oxide surface, difficult to detect from the data.) The second segment fit in Fig. l[b] is based on this model. Other high quality samples follow this pattern reproducibly. The data of Figs. 2 also suggest differences between the "intrinsic" roughness and that generated by inert gas plasmas. First, from Fig. 2[a], the oxidation rate of plasmaexposed a-Si:H is greater than codeposited, unexposed a-Si:H. This result is consistent with an inert gas plasma-induced roughness of atomic-scale microvoids, which could influence diffusion of species to the oxidation front. Second, in Fig. 2[b], data for the growth of high quality a-Si:H on an Ar plasma-exposed surface are presented. The broken line is calculated assuming that the new a-Si:H grows uniformly atop the intact surface layer (ie. with a thickness-independent dielectric function). From the best fit model (solid line), it is found that the modified surface induces void formation in the first 30 ~ of the new a-Si:H. This would not occur if the plasma-induced roughness was large enough in scale for the new film to cover it conformally. Thus. after 30 ~ in this case, the new film has lost memory of the plasma-induced substrate microstructure. Finally. it is of interest to know whether the small scale roughness exists on
1442
R.W. Collins, J.M. Cavese / Surface structure of glow discharge a-Si.'H
inadvertently damaged surfaces. As an example, in Table I (bottom), data are listed for 0 2 plasma exposure of a-Si:H deposited from a 1:50 mixture of SiH4:Ar. The data show that, of the ~ 30 ~ thick roughness on this sample, ~ 10 ~ may be attributed to the microvoid-type, caused by Ar bombardment concurrent with deposition. To conclude, these results have special relevance for the abruptness and smoothness of high quality a-Si:H interfaces. First, it is expected that the plasma may provide atomic mixing throughout the weakly-bonded transition layer, leading to ~ 6 ~ alloyed interracial layers between films of different composition. The same thickness value has been deduced from high resolution electron microscopy. 6 Furthermore, the oxidation studies suggest that, although the surface appears to be atomically smooth locally, larger scale surface modulations exist which are conformally covered when a new film is deposited on the surface. This leads to a modulated interface (~ 5-15 ~ thick). Finally. under conditions leading to poorer quality materials or surfaces, microstructure in the substrate film is imparted to the new film to a thickness which depends on its scale. REFERENCES 1) See. for example: J. Non-Cryst. Solids 77 & 78, 969-1104 (1985). 2) D.E. Aspnes, Thin Solid Films 89, 249 (1982). 3) D.E. Aspnes. A.A. Studna, and E. Kinsbron, Phys. Rev. B 29, 768 (1984). 4) R.W. Collins and J.M. Cavese, these Proceedings. 5) A. Gallagher, Bull. Am. Phys. Soc. 32, 750 (1987). 6) C.C. Tsai, R.A. Street, F.A. Ponce, and G.B. Anderson, Mat. Res. Soc. Symp. Proc. 70. 351 (1986).
100
26r [O]
<
80
[b]
Inert gas
plosmo
exposure
before ,.~
(/1 O1
stort Ar ploemo 24lend Ar plosmo lstart high q u o l l t y ~
end o--SI:H "~
v
_.9. W a
R 2C o O'
18
C..,.
0
.... 50TIME (.min)lO0
•
16 =
150
2
'
'
4
6
~
~
<8.>¢t 10
'
12
14
FIGURE 2 Oxidation behavior of two a-Si:H samples (solid). The broken lines are best fit diffusion relationships. [b] In situ ellipsometry data for new a-Si:H growth on an a-Si:H surface exposed to an Ar plasma. The broken and solid lines (for a-Si:H growth) are models for uniform and non-uniform growth. [a]