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The influence of substrate temperature on the deposition rate and optical properties of a-Si:H thin films prepared by RF-glow discharge
Journal of Non-Crystalline North-Holland, Amsterdam
Solids 89 (1987)
13
13-23
THE INFLUENCE OF SUBSTRATE TEMPERATURE ON THE DEPOSITION RATE AND OPTICAL PROPERTIES OF a-Si : H THIN FILMS PREPARED BY RF-GLOW DISCHARGE G. MYBURG Deportnrent
Received
and R. SWANEPOEL
of Physics,
Rand Afrikaans
Unioersity.
Johannesburg
South Africa
27 May 1986
The deposition rate and the optical properties such as the homogeneity parameter Ad, the refractive index n(X). the optical absorption a(X) and the optical gap .!?a of rf-glow dischargeproduced a-Si : H thin films have been studied as a function of the substrate temperature T,. The substrate temperature 7% in the range -100 to 4OO’C was found to be a very important factor determining the above-mentioned parameters. All these material properties/parameters of each film were calculated from their transmission spectra obtained over the wavelength region of 400 to 900 nm by means of a spectrophotometer. As the substrate temperature r, increases from - 100 to 4OO”C, while keeping all other parameters constant, the deposition rate and the optical gap decrease non-linearly from 12.2 to 1.55 A/s and 2.46 to 1.69 eV. respectively. AI the same time the density (from 1.64 g/cm’ at T, = 23°C IO 2.14 g/cm’ at T, = 200°C). the refractive index n (thus also the dielectric constant tr) and the optical absorption (I as a function of photon energy, increase. At substrate temperatures T, of 300 and 400°C the refractive index n(X), 400 nm < h 4 900 run, was even higher than that of single crystal silicon reported in the literature. The same applies to the dielectric constant cr.
1. Introduction Although hydrogenated amorphous silicon is already used commercially for solar cell fabrication amongst other things [l-3], the physics of this material is not yet well understood. The high conversion efficiency of these cells results from the unique electrical and optical properties which are present only in the discharge-produced a-Si: H films. The properties of this material are influenced by a multitude of preparation parameters, such as the pressure in the plasma chamber, the rf power, the gas flow rate, the geometry of the chamber [4,5] and many more. While some of the electrical properties of such films have been reported, their optical properties (except maybe for the optical absorption a) and the effect of the various deposition parameters on these properties of a-Si: H films, have not been systematically and extensively studied. We report on the substrate temperature T, dependence, -100°C < q Q 400°C of the deposition rate, the homogeneity parameter Ad, the refractive index n(A), 400 MI < h Q 900 run, the optical absorption (u(h) and the optical gap ,??aof thin a-Si : H films deposited in a rf-glow discharge system. 0022-3093/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
14
G. Myburg.
R. Swanepoel
/ RF-glow
discharge-produced
a-Si:
H thin film
2. Experimental 2.1. Apparatus
The films referred to were deposited in a capacitively coupled rf-glow discharge system (13.56 MHz) of which the plasma chamber was based on that of Knights et al. [6,7]. The chamber pressure was monitored simultaneously with McLeod and Penning meters. The advantage of a McLeod meter is that the reading is independent of the type of gas of which the pressure is being measured. The cathode (diam. 42 mm) and anode (diam. 60.5 mm) were 20 mm apart. All the films referred to in this article were deposited on the anode. In order to reduce the effect of unknown parameters as far as possible, pure silane (SiH,) was used undiluted [8], instead of diluting it with a noble gas. Corning 7059 glass was used for the substrates. A chromel-alumel thermocouple was used to determine the temperature of the anode and thus also the substrate temperature T,. 2.2. Method
The values of d, Ad, n(X) and a(h) for the various films were determined from their transmission spectra according to the nondestructive technique proposed by Swanepoel [9-111. Finer details of the method being used are
or.........~.........~.........~.......~.~~..~....~~ 400
500
600
Wavelength
Fig. 1. The transmission spectrum of film temperature of -50°C, with Ad = 22.8 nm nm (dashed line). The effect of Ad is easily in
700 (nm
600
900
)
0.225-2014 which was deposited at a substrate (solid line) and the simulated spectrum with Ad = 0 visible from this figure. (n(X) and a(h) are identical both cases.)
G. Myhurg,
1.0
.
*.
R. Swonepoel
.
,.
/ RF-glow
*.
.
8,.
discharge-produced
. ,
I,
I*,
o-Si:
. .
H thin films
.,*,
-------------------------------,~ .9 :
15
. . I\
? .6 :
f
:
” P
.7 :
Wavelength
(nm)
Fig. 2. The transmission spectrum of film 0.225-2014 which was deposited at a substrate temperature of 400°C. with Ad = 9 nm (solid line) and the simulated spectrum with Ad = 0 om (dashed line).
described in ref. [4]. Figures 1 and 2 show typical interference spectra. The films referred to in this article were between 800 and 1400 nm thick. The error limits applicable to the values of the thicknesses (that is to say also the corresponding deposition rates) and refractive indices, are less than rt 2% and that for OLk 5%. The values for the optical gap E,, were determined with the aid of the following two equations: ahv = B( hv - ‘q2 and nahv=B(hv-E0)3, by using the graphs of (cyh~)‘/~ versus hv and (ncuh~)‘/~ versus hv [12,13]. 2.3. Paramerer
Ad
The parameter Ad, covered comprehensively by Swanepoel [lo], appears to be a very interesting and also important quantity in that it contains the total effect of at least the following four factors, all due to the non-homogeneities in the film: - The product of the average refractive index and the variation in the thickness of the area (typically 0.1 X 1 mm2) over which a single transmission spectrum was obtained. - The product of the average thickness and the variation in refractive index (specifically parallel to the plane of the substrate) of the area over which a single transmission spectrum was obtained. - The effect of slit width.
16
G. Myburg.
R. Swonepoel
/ RF-glow
dischorge-produced
a-S:
H rhinfilnrr
- The effect of refractive index variations with depth (specifically perpendicular to the substrate) in the film. The effect of Ad on a transmission spectrum is illustrated by the spectra in figs. 1 and 2. Typical experimental values obtained for Ad lie between 7.5 and 22.8 nm (without taking into account the variation in film thickness between 806 and 1396 nm). In the case of fig. 1 two simulated transmission spectra of film 0.225 - 20 - 14 (q = -50°C) are shown with Ad = 0 nm (the dashed curve) and Ad = 22.8 nm (the solid curve) with the values of d, n(X) and a(h) identical in both cases. Theoretically the first three of the above-mentioned factors can be calculated. By taking transmission spectra at various points on a film, the variation in n can be determined. In practice it could not definitely be shown that any variations in n parallel to the plane of the substrate occurred. This was impossible since the error limits of k2% were greater than the differences which occurred. Thus the product of the variation in n (which mainly occurred due to experimental errors) and the average thickness which has a value typically of the order of 1000 nm, is consequently very inaccurate. By choosing the slit width sufficiently narrow, for example 1 nm or less, the effect of slit width on the value of Ad can be considered to be negligibly small. A mathematical expression describing the fourth factor does not exist at this stage. 3. Experimental
results and discussion
All relevant information in respect of the seven fiis deposited at different substrate temperatures (varying from -100°C up to and including 400°C) is given in table 1. For convenience the identification numbers of the various films were chosen in such a way that they corresponded precisely with the preparation conditions (pressure, power and gas flow rate) at which the
Table 1 This table contains all the important different substrate temperatures. Film
0.225-20-14 0.225-20-14 0.225-20-14 0.225-20-14 0.225-20-14 0.225-20-14 0.225-20-14
Temperature
Thickness
T, (“(3
(m)
400 300 200 100 25 -50 - 100
1396 1247 1052 1044 910 1015 806
“Ad” (nm)
experimental
Deposition rate
data
of the films
,I (633 m)
;I000 nm)
4.21 4.09 3.72 3.38 2.64 2.17 1.85
3.70 3.62 3.36 3.12 2.51 2.12 1.84
(r
which
were
Optical (eV)
gap
(nahv)“’
(ahv)“2
1.59 1.63 1.71 1.74 1.96 2.27 -
1.69 1.73 1.82 1.87 2.14 2.46
(A/s) 9.0 7.5 8.0 13.5 16.0 22.8 8.0
1.55 1.89 2.92 3.63 8.79 9.40 12.20
13.7 13.1 11.3 9.7 6.3 4.5 3.4
deposited
at
G. Myburg.
R. Swanepoel
/ RF-glow
discharge-produced
a-.%: H thin films
17
Fig. 3. The reflection interference patterns of six films which were deposited at the same conditions of pressure (0.225 to@, power (20 W) and gas flow rate (14 cm3/min), but at different substrate temperatures T,: (a) - lCOeC. (b) -50°C. (c) 23OC. (d) loO°C, (e) 2oO°C and (f) 300°C.
various films were deposited. By way of illustration, the film under consideration in table 1 was deposited at a pressure of 0.225 Tot-r, a power of 20 W and a gas flow rate of 14 cm3/min (at normal temperature and pressure) of silane. The reflection interference patterns from six of these films are shown in fig. 3. Reflection interference patterns were obtained by illuminating the substrates on which the a-Si : H films were deposited, with a He-Ne laser (632.8 nm). A
18
G. Myburg,
R. Swmepoel
Fig. 4. The deposition
/ RF-glow
discharge-produced
rate as a function
of the substrate
o-Si:
H thin films
temperature
T,.
simple layout [4] (similar to that used observing Newton’s rings) made the photographing (with a Polaroid camera) of the interference patterns formed in the thin a-Si : H film possible. These images provide information on a macroscopic scale regarding the product of the film thickness d and the refractive index n(X) of the thin film according to the well-known relation: 2nd = mh. The dark bands represent whole number orders of interference and are consequently exactly 180” out of phase with respect to the transmission spectrum (at 632.8 nm) taken at the same point on the film. It was also further found that the refractive index remained constant at least up to a 5 mm radius from the midpoint of the substrate. This consequently implies that the interference patterns were only due to a thickness variation that occurred in the a-Si : H film .on the surface of the substrate [4]. An important implication following from this is that this technique provides the opportunity to measure thickness variations - thus uniformity with respect to film thickness - in a simple non-destructive way and especially over relatively large surfaces and even on opaque substrates. 3.1. The values of the deposition rates appearing in table 1 were determined according to the film thickness at the central point of the substrate. The relationship between deposition rate and the substrate temperature (fig. 4) shows an approximately exponential decrease with an increase in the substrate temperature T,. This behaviour can be ascribed to the increase in the kinetic energy on the surface of the substrate. Even over the temperature range 100°C < q < 400°C, there was a decrease in the rate of deposition of more than 55% from 3.63 A/s to 1.55 A/s. Conversely, the results published by
G. Myburg,
R. Swanepoel
/ RF-glow
discharge-produced
o-Si: H rhinjilmc
19
Matsuda [14], indicate an approximately constant value for the deposition rates over the temperature range 100°C < T, < 500°C. From the published data of Matsuda [14] concerning deposition rate versus substrate temperature, it is unclear according to which method the film thicknesses were determined. Also the error limits applicable to the empirical values are not indicated. 3.2. The decrease in Ad from 22.8 to 7.5 nm that occurred with an increase of T, from - 50 to 400°C indicates an increase in homogeneity of the a-Si : H films with an increase in q. This increase in homogeneity can be explained by means of the decrease in deposition rate with increase in substrate temperature. A decrease in deposition rate will, without doubt, lead to a denser and more homogeneous microstructure. 3.3..The influence of the substrate temperature, Ts, on the refractive index, n(A), is depicted in figs. 5 and 6. One would already be able to predict an increase in refractive index with an increase in substrate temperature from paragraph 3.1. The experimentally obtained values of n of the various films are indicated with different symbols in fig. 5. The curves through the various sets of data points represent quadratic fits of the data, according to the equation: n = A/X2 + B/X + C, where A, B and C are constants. The extrapolated sections of these curves on the short wavelength side cannot be guaranteed. On the long wavelength side, the values of n tend towards a constant value and on this side, these curves can in most cases be used up to x = 1000 run [4].
1.71 400
500
600
700
Wavelength
Fig. 5. The relationship deposited at different
between the refractive substrate temperatures crystal
800
900
I 1000
(nm)
index and the wavelength for the films which were in comparison with the refractive index of singlesilicon [15].
20
G. Mvburg,
R. Swanepoel
/ RF-glow
discharge-produced
.
3.6 c
_
.
0k! 3.2 .E .$ i
2.8-
= $
2.4-
a-Si:
H rltin films
.
.
.
.
..
_ 2.0 -
:
cn:n l :n=n
(1000nm) (633nm)
_
, 1.6 -200
1
0 -100
’
’ 0
’
1 loo
’
Temperature
Fig.
6. The
refractive
indices,
n(633
nm)
and
’ 200
’
’ 300
I
’ 400
’
. 500
(‘C)
n(1000
nm),
as a function
of the substrate
1emperature r,. A surprising result is the fact that the values obtained for the refractive indices at T, = 300” and 400°C, are higher than those quoted for single crystal silicon by Koltun [15]. The refractive index of single crystal silicon at 300 K is indicated with a dashed line in fig. 5. The corresponding values for single crystal silicon at 633 nm and 1000 nm are 3.94 and 3.49 respectively, while those for a-Si: H were 4.09 and 3.62 at T, = 300°C and 4.21 and 3.70 at T, = 400°C. A similar tendency has also been observed by Swart et al. [16] and this was with a-Si obtained by means of high-dosage ion implantation. Sakata et al. [17] published data for n versus T, but omitted to describe the method by which n and d were determined. Apart from the fact that the wavelength at which n was determined was also not specified, no mention is made of the error limits applicable to n and d. The rising tendency which the refractive index shows with an increase in temperature (fig. 6) implies that the relative dielectric constant, er= n2, n(lOOO nm) will show a sharply rising tendency with an increase in substrate temperature. The behaviour of r,(lOOO nm) against substrate temperature is shown in fig. 7. er reached a value of 13.7 at the substrate temperature of 400°C. This value is much larger than the 11.8 of single crystal silicon [18]. 3.4. The optical absorption, (Y, as a function of photon energy showed a dramatic increase with increase in substrate temperature (fig. 8). The curve for T, = 2OO’C is comparable with that of Knights et al. [8] at 230°C. Figure 8 includes only the curves of the films deposited at substrate temperatures of -50°C and higher. The determination of (Y for the remaining film (T, =
G. Myburg,
R. Swanepoel
/ RF-glow
discharge-produced
u-S:
21
H thin film
67-
.
65-
.
4. 3200
-100
0
100
200
Temperature
Fig. 7. The relative
dielectric
consIant
r,(lONl
300
400
500
(‘C )
nm) as a function
of the substrate
temperature
T,
-1OOOC) was not possible due- to the absorption in the substrate itself at wavelengths shorter than 350 run. The optical absorption as a function of photon energy for single crystal silicon at 300 K is indicated with a dashed line
Photon
energy
(eV)
Fig. 8. The relationship between the optical absorption a and the photon energy for the films which were deposited at different substrate temperatures in comparison with the optical absorption of single crystal silicon at 300 K [18].
22
G. hfvburg,
R. Swoneporl
/ RF-glow
discharge-produced
o-Si:
H thin films
. 2.4. 2.2 .
z
z il
2.0 -
4
1.8 -
0"
-
.: ( ahv) '/2 n:(na hv) 4
. .
. .
.
1.6 -
1.4 -200
'1
- 100
'
" 0
' 100
Temperature Fig. 9. The optical
gap ITo as a function
'
" 200
. .
.
" 300
" 400
.
500
( ‘C ) of the substrate
temperaiure
T,,
in fig. 8. The amorphous silicon film, deposited at T, = 4OO”C, shows a higher optical absorption than single crystal silicon at photon energies higher than 1.75 eV (wavelengths of around 700 nm and shorter). 3.5. The experimental data in respect of the optical gap, Ea, is given in table 1 and fig. 9. An increase in the substrate temperature, T,, from - 50 to 400°C caused the optical gap to shrink by 0.68 eV, from 2.27 eV at T, = -50°C to 1.59 eV at 400°C. The optical gap of the remaining film could not be determined due to absorption in the substrate itself at wavelengths shorter than 350 nm. This decrease in E,, with an increase in T, is ascribed to the decrease in the hydrogen concentration in the a-Si : H films with an increase in r, [19,20].
3.6. The densities of the films deposited at 23°C and 200°C were determined by means of the well-known flotation technique [4,5]. The values were 1.64 and 2.14 g/cm3 respectively. These values represent densities that are respectively 30% and 8% lower than the density of single-crystal silicon. Brodsky et al. [21], with the aid of nuclear techniques, also observed an increase in the density with an increase in q. The work of Zanzucchi et al. [22] shows great similarities as regards the general behaviour of n, a and E, with an increase in T,. This latter publication, however, only contains data for the temperature interval, 195°C < T, Q 420°C and also gives no indication of what error limits are applicable to the values of d and n. Judging from the techniques used for the determination of d and n, the error limits applicable to these parameters are likely to be
G. Myburg.
R. Swonepoel
/ RF-glow
discharge-produced
o-Si:
H thin film
23
rather larger than *2%. A further gap in the literature is that no mention is made of the effect of thickness variation, or more comprehensively, the effect of Ad, as described in this publication, that of Swanepoel [lo] and that of Myburg [4]. By ignoring the effect of Ad, large errors can be made. The financial support of the Council of Scientific and Industrial Research, South Africa, is acknowledged and thanks are due to J.F. Myburg for her assistance in the preparation of the manuscript.
References [I] Y.J. Hamakawa. J. Non-Cryst. Solids 59&60 (1983) 1265. [2] W.E. Spear, in: Poly-Micro-Crystalline and Amorphous Semiconductors. eds. P. Pinard and S. Kalbitzer (Editions de Physique. France, 1984) p. 527. [3] I. Shimizu, J. Non-Cryst. Solids 77&78 (1985) 1363. [4] G. Myburg, Thesis, RAU, Johannesburg (1985). [S] G. Myburg, to be published. [6] J.C. Knights, Phil. Msg. 34 (1976) 663. (71 J.C. Knigbl~. G. Lucovsky and R.J. Nemanich, J. Non-Cryst. Solids 32 (1979) 393. [8] J.C. Knights, R.A. Lujan. M.P. Rosenblum, R.A. Street, D.K. Bieglesen and J.A. Reimer, Appl. Phys. Lett. 38 (1981) 331. [9] R. Swanepoel. J. Phys. E 16 (1983) 1214. [lo] R. Swancpoel, J. Phys. E 17 (1984) 896. [ll] R. Swanepoel. J. Opt. Sot. Am. 2 (1985) 1339. [12] CC. Tsai and H. Fritzsche, Solar Energy Mat. 1 (1979) 29. 1131 R.H. Klazes. M.H.L.M. van den Broek. J. Bezemer and S. Radelaar, Phil. Mag. B 45 (1982) 377. [14] A. Matsuda. J. Non-Cryst. Solids 59&60 (1983) 767. [lS] M.M. Koltun, Selective Optical Surfaces for Solar Energy Converters (Allerton, New York, 1981) p. 14. [16] P.L. &art. H. Aharoni and B.M. Lacquet. Nucl. Instr. and Meth. B 6 (1985) 365. [17] I. Sakata. Y. Hayasbi, M. Yamanaka and H. Karasawa, J. Appl. Phys. 52 (1981) 4334. [18] S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981) p. 850. [19] J.C. Bruyere. A. Deneuville, A. Mini, J. Fontenille and R. Danielou, J. Appl. Phys. 51 (1980) 2199. [20] M.H. Brodsky, Thin Solid Films 40 (1977) L23. [21] M.H. Brodsky, M.A. Frisch and J.F. Ziegler, Appl. Phys. Lett. 30 (1977) 561. [22] P.J. Zanzucchi, C.R. Wronski and D.E. Carlson, J. Appl. Phys. 48 (1977) 5227.
Solids 89 (1987)
13
13-23
THE INFLUENCE OF SUBSTRATE TEMPERATURE ON THE DEPOSITION RATE AND OPTICAL PROPERTIES OF a-Si : H THIN FILMS PREPARED BY RF-GLOW DISCHARGE G. MYBURG Deportnrent
Received
and R. SWANEPOEL
of Physics,
Rand Afrikaans
Unioersity.
Johannesburg
South Africa
27 May 1986
The deposition rate and the optical properties such as the homogeneity parameter Ad, the refractive index n(X). the optical absorption a(X) and the optical gap .!?a of rf-glow dischargeproduced a-Si : H thin films have been studied as a function of the substrate temperature T,. The substrate temperature 7% in the range -100 to 4OO’C was found to be a very important factor determining the above-mentioned parameters. All these material properties/parameters of each film were calculated from their transmission spectra obtained over the wavelength region of 400 to 900 nm by means of a spectrophotometer. As the substrate temperature r, increases from - 100 to 4OO”C, while keeping all other parameters constant, the deposition rate and the optical gap decrease non-linearly from 12.2 to 1.55 A/s and 2.46 to 1.69 eV. respectively. AI the same time the density (from 1.64 g/cm’ at T, = 23°C IO 2.14 g/cm’ at T, = 200°C). the refractive index n (thus also the dielectric constant tr) and the optical absorption (I as a function of photon energy, increase. At substrate temperatures T, of 300 and 400°C the refractive index n(X), 400 nm < h 4 900 run, was even higher than that of single crystal silicon reported in the literature. The same applies to the dielectric constant cr.
1. Introduction Although hydrogenated amorphous silicon is already used commercially for solar cell fabrication amongst other things [l-3], the physics of this material is not yet well understood. The high conversion efficiency of these cells results from the unique electrical and optical properties which are present only in the discharge-produced a-Si: H films. The properties of this material are influenced by a multitude of preparation parameters, such as the pressure in the plasma chamber, the rf power, the gas flow rate, the geometry of the chamber [4,5] and many more. While some of the electrical properties of such films have been reported, their optical properties (except maybe for the optical absorption a) and the effect of the various deposition parameters on these properties of a-Si: H films, have not been systematically and extensively studied. We report on the substrate temperature T, dependence, -100°C < q Q 400°C of the deposition rate, the homogeneity parameter Ad, the refractive index n(A), 400 MI < h Q 900 run, the optical absorption (u(h) and the optical gap ,??aof thin a-Si : H films deposited in a rf-glow discharge system. 0022-3093/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
14
G. Myburg.
R. Swanepoel
/ RF-glow
discharge-produced
a-Si:
H thin film
2. Experimental 2.1. Apparatus
The films referred to were deposited in a capacitively coupled rf-glow discharge system (13.56 MHz) of which the plasma chamber was based on that of Knights et al. [6,7]. The chamber pressure was monitored simultaneously with McLeod and Penning meters. The advantage of a McLeod meter is that the reading is independent of the type of gas of which the pressure is being measured. The cathode (diam. 42 mm) and anode (diam. 60.5 mm) were 20 mm apart. All the films referred to in this article were deposited on the anode. In order to reduce the effect of unknown parameters as far as possible, pure silane (SiH,) was used undiluted [8], instead of diluting it with a noble gas. Corning 7059 glass was used for the substrates. A chromel-alumel thermocouple was used to determine the temperature of the anode and thus also the substrate temperature T,. 2.2. Method
The values of d, Ad, n(X) and a(h) for the various films were determined from their transmission spectra according to the nondestructive technique proposed by Swanepoel [9-111. Finer details of the method being used are
or.........~.........~.........~.......~.~~..~....~~ 400
500
600
Wavelength
Fig. 1. The transmission spectrum of film temperature of -50°C, with Ad = 22.8 nm nm (dashed line). The effect of Ad is easily in
700 (nm
600
900
)
0.225-2014 which was deposited at a substrate (solid line) and the simulated spectrum with Ad = 0 visible from this figure. (n(X) and a(h) are identical both cases.)
G. Myhurg,
1.0
.
*.
R. Swonepoel
.
,.
/ RF-glow
*.
.
8,.
discharge-produced
. ,
I,
I*,
o-Si:
. .
H thin films
.,*,
-------------------------------,~ .9 :
15
. . I\
? .6 :
f
:
” P
.7 :
Wavelength
(nm)
Fig. 2. The transmission spectrum of film 0.225-2014 which was deposited at a substrate temperature of 400°C. with Ad = 9 nm (solid line) and the simulated spectrum with Ad = 0 om (dashed line).
described in ref. [4]. Figures 1 and 2 show typical interference spectra. The films referred to in this article were between 800 and 1400 nm thick. The error limits applicable to the values of the thicknesses (that is to say also the corresponding deposition rates) and refractive indices, are less than rt 2% and that for OLk 5%. The values for the optical gap E,, were determined with the aid of the following two equations: ahv = B( hv - ‘q2 and nahv=B(hv-E0)3, by using the graphs of (cyh~)‘/~ versus hv and (ncuh~)‘/~ versus hv [12,13]. 2.3. Paramerer
Ad
The parameter Ad, covered comprehensively by Swanepoel [lo], appears to be a very interesting and also important quantity in that it contains the total effect of at least the following four factors, all due to the non-homogeneities in the film: - The product of the average refractive index and the variation in the thickness of the area (typically 0.1 X 1 mm2) over which a single transmission spectrum was obtained. - The product of the average thickness and the variation in refractive index (specifically parallel to the plane of the substrate) of the area over which a single transmission spectrum was obtained. - The effect of slit width.
16
G. Myburg.
R. Swonepoel
/ RF-glow
dischorge-produced
a-S:
H rhinfilnrr
- The effect of refractive index variations with depth (specifically perpendicular to the substrate) in the film. The effect of Ad on a transmission spectrum is illustrated by the spectra in figs. 1 and 2. Typical experimental values obtained for Ad lie between 7.5 and 22.8 nm (without taking into account the variation in film thickness between 806 and 1396 nm). In the case of fig. 1 two simulated transmission spectra of film 0.225 - 20 - 14 (q = -50°C) are shown with Ad = 0 nm (the dashed curve) and Ad = 22.8 nm (the solid curve) with the values of d, n(X) and a(h) identical in both cases. Theoretically the first three of the above-mentioned factors can be calculated. By taking transmission spectra at various points on a film, the variation in n can be determined. In practice it could not definitely be shown that any variations in n parallel to the plane of the substrate occurred. This was impossible since the error limits of k2% were greater than the differences which occurred. Thus the product of the variation in n (which mainly occurred due to experimental errors) and the average thickness which has a value typically of the order of 1000 nm, is consequently very inaccurate. By choosing the slit width sufficiently narrow, for example 1 nm or less, the effect of slit width on the value of Ad can be considered to be negligibly small. A mathematical expression describing the fourth factor does not exist at this stage. 3. Experimental
results and discussion
All relevant information in respect of the seven fiis deposited at different substrate temperatures (varying from -100°C up to and including 400°C) is given in table 1. For convenience the identification numbers of the various films were chosen in such a way that they corresponded precisely with the preparation conditions (pressure, power and gas flow rate) at which the
Table 1 This table contains all the important different substrate temperatures. Film
0.225-20-14 0.225-20-14 0.225-20-14 0.225-20-14 0.225-20-14 0.225-20-14 0.225-20-14
Temperature
Thickness
T, (“(3
(m)
400 300 200 100 25 -50 - 100
1396 1247 1052 1044 910 1015 806
“Ad” (nm)
experimental
Deposition rate
data
of the films
,I (633 m)
;I000 nm)
4.21 4.09 3.72 3.38 2.64 2.17 1.85
3.70 3.62 3.36 3.12 2.51 2.12 1.84
(r
which
were
Optical (eV)
gap
(nahv)“’
(ahv)“2
1.59 1.63 1.71 1.74 1.96 2.27 -
1.69 1.73 1.82 1.87 2.14 2.46
(A/s) 9.0 7.5 8.0 13.5 16.0 22.8 8.0
1.55 1.89 2.92 3.63 8.79 9.40 12.20
13.7 13.1 11.3 9.7 6.3 4.5 3.4
deposited
at
G. Myburg.
R. Swanepoel
/ RF-glow
discharge-produced
a-.%: H thin films
17
Fig. 3. The reflection interference patterns of six films which were deposited at the same conditions of pressure (0.225 to@, power (20 W) and gas flow rate (14 cm3/min), but at different substrate temperatures T,: (a) - lCOeC. (b) -50°C. (c) 23OC. (d) loO°C, (e) 2oO°C and (f) 300°C.
various films were deposited. By way of illustration, the film under consideration in table 1 was deposited at a pressure of 0.225 Tot-r, a power of 20 W and a gas flow rate of 14 cm3/min (at normal temperature and pressure) of silane. The reflection interference patterns from six of these films are shown in fig. 3. Reflection interference patterns were obtained by illuminating the substrates on which the a-Si : H films were deposited, with a He-Ne laser (632.8 nm). A
18
G. Myburg,
R. Swmepoel
Fig. 4. The deposition
/ RF-glow
discharge-produced
rate as a function
of the substrate
o-Si:
H thin films
temperature
T,.
simple layout [4] (similar to that used observing Newton’s rings) made the photographing (with a Polaroid camera) of the interference patterns formed in the thin a-Si : H film possible. These images provide information on a macroscopic scale regarding the product of the film thickness d and the refractive index n(X) of the thin film according to the well-known relation: 2nd = mh. The dark bands represent whole number orders of interference and are consequently exactly 180” out of phase with respect to the transmission spectrum (at 632.8 nm) taken at the same point on the film. It was also further found that the refractive index remained constant at least up to a 5 mm radius from the midpoint of the substrate. This consequently implies that the interference patterns were only due to a thickness variation that occurred in the a-Si : H film .on the surface of the substrate [4]. An important implication following from this is that this technique provides the opportunity to measure thickness variations - thus uniformity with respect to film thickness - in a simple non-destructive way and especially over relatively large surfaces and even on opaque substrates. 3.1. The values of the deposition rates appearing in table 1 were determined according to the film thickness at the central point of the substrate. The relationship between deposition rate and the substrate temperature (fig. 4) shows an approximately exponential decrease with an increase in the substrate temperature T,. This behaviour can be ascribed to the increase in the kinetic energy on the surface of the substrate. Even over the temperature range 100°C < q < 400°C, there was a decrease in the rate of deposition of more than 55% from 3.63 A/s to 1.55 A/s. Conversely, the results published by
G. Myburg,
R. Swanepoel
/ RF-glow
discharge-produced
o-Si: H rhinjilmc
19
Matsuda [14], indicate an approximately constant value for the deposition rates over the temperature range 100°C < T, < 500°C. From the published data of Matsuda [14] concerning deposition rate versus substrate temperature, it is unclear according to which method the film thicknesses were determined. Also the error limits applicable to the empirical values are not indicated. 3.2. The decrease in Ad from 22.8 to 7.5 nm that occurred with an increase of T, from - 50 to 400°C indicates an increase in homogeneity of the a-Si : H films with an increase in q. This increase in homogeneity can be explained by means of the decrease in deposition rate with increase in substrate temperature. A decrease in deposition rate will, without doubt, lead to a denser and more homogeneous microstructure. 3.3..The influence of the substrate temperature, Ts, on the refractive index, n(A), is depicted in figs. 5 and 6. One would already be able to predict an increase in refractive index with an increase in substrate temperature from paragraph 3.1. The experimentally obtained values of n of the various films are indicated with different symbols in fig. 5. The curves through the various sets of data points represent quadratic fits of the data, according to the equation: n = A/X2 + B/X + C, where A, B and C are constants. The extrapolated sections of these curves on the short wavelength side cannot be guaranteed. On the long wavelength side, the values of n tend towards a constant value and on this side, these curves can in most cases be used up to x = 1000 run [4].
1.71 400
500
600
700
Wavelength
Fig. 5. The relationship deposited at different
between the refractive substrate temperatures crystal
800
900
I 1000
(nm)
index and the wavelength for the films which were in comparison with the refractive index of singlesilicon [15].
20
G. Mvburg,
R. Swanepoel
/ RF-glow
discharge-produced
.
3.6 c
_
.
0k! 3.2 .E .$ i
2.8-
= $
2.4-
a-Si:
H rltin films
.
.
.
.
..
_ 2.0 -
:
cn:n l :n=n
(1000nm) (633nm)
_
, 1.6 -200
1
0 -100
’
’ 0
’
1 loo
’
Temperature
Fig.
6. The
refractive
indices,
n(633
nm)
and
’ 200
’
’ 300
I
’ 400
’
. 500
(‘C)
n(1000
nm),
as a function
of the substrate
1emperature r,. A surprising result is the fact that the values obtained for the refractive indices at T, = 300” and 400°C, are higher than those quoted for single crystal silicon by Koltun [15]. The refractive index of single crystal silicon at 300 K is indicated with a dashed line in fig. 5. The corresponding values for single crystal silicon at 633 nm and 1000 nm are 3.94 and 3.49 respectively, while those for a-Si: H were 4.09 and 3.62 at T, = 300°C and 4.21 and 3.70 at T, = 400°C. A similar tendency has also been observed by Swart et al. [16] and this was with a-Si obtained by means of high-dosage ion implantation. Sakata et al. [17] published data for n versus T, but omitted to describe the method by which n and d were determined. Apart from the fact that the wavelength at which n was determined was also not specified, no mention is made of the error limits applicable to n and d. The rising tendency which the refractive index shows with an increase in temperature (fig. 6) implies that the relative dielectric constant, er= n2, n(lOOO nm) will show a sharply rising tendency with an increase in substrate temperature. The behaviour of r,(lOOO nm) against substrate temperature is shown in fig. 7. er reached a value of 13.7 at the substrate temperature of 400°C. This value is much larger than the 11.8 of single crystal silicon [18]. 3.4. The optical absorption, (Y, as a function of photon energy showed a dramatic increase with increase in substrate temperature (fig. 8). The curve for T, = 2OO’C is comparable with that of Knights et al. [8] at 230°C. Figure 8 includes only the curves of the films deposited at substrate temperatures of -50°C and higher. The determination of (Y for the remaining film (T, =
G. Myburg,
R. Swanepoel
/ RF-glow
discharge-produced
u-S:
21
H thin film
67-
.
65-
.
4. 3200
-100
0
100
200
Temperature
Fig. 7. The relative
dielectric
consIant
r,(lONl
300
400
500
(‘C )
nm) as a function
of the substrate
temperature
T,
-1OOOC) was not possible due- to the absorption in the substrate itself at wavelengths shorter than 350 run. The optical absorption as a function of photon energy for single crystal silicon at 300 K is indicated with a dashed line
Photon
energy
(eV)
Fig. 8. The relationship between the optical absorption a and the photon energy for the films which were deposited at different substrate temperatures in comparison with the optical absorption of single crystal silicon at 300 K [18].
22
G. hfvburg,
R. Swoneporl
/ RF-glow
discharge-produced
o-Si:
H thin films
. 2.4. 2.2 .
z
z il
2.0 -
4
1.8 -
0"
-
.: ( ahv) '/2 n:(na hv) 4
. .
. .
.
1.6 -
1.4 -200
'1
- 100
'
" 0
' 100
Temperature Fig. 9. The optical
gap ITo as a function
'
" 200
. .
.
" 300
" 400
.
500
( ‘C ) of the substrate
temperaiure
T,,
in fig. 8. The amorphous silicon film, deposited at T, = 4OO”C, shows a higher optical absorption than single crystal silicon at photon energies higher than 1.75 eV (wavelengths of around 700 nm and shorter). 3.5. The experimental data in respect of the optical gap, Ea, is given in table 1 and fig. 9. An increase in the substrate temperature, T,, from - 50 to 400°C caused the optical gap to shrink by 0.68 eV, from 2.27 eV at T, = -50°C to 1.59 eV at 400°C. The optical gap of the remaining film could not be determined due to absorption in the substrate itself at wavelengths shorter than 350 nm. This decrease in E,, with an increase in T, is ascribed to the decrease in the hydrogen concentration in the a-Si : H films with an increase in r, [19,20].
3.6. The densities of the films deposited at 23°C and 200°C were determined by means of the well-known flotation technique [4,5]. The values were 1.64 and 2.14 g/cm3 respectively. These values represent densities that are respectively 30% and 8% lower than the density of single-crystal silicon. Brodsky et al. [21], with the aid of nuclear techniques, also observed an increase in the density with an increase in q. The work of Zanzucchi et al. [22] shows great similarities as regards the general behaviour of n, a and E, with an increase in T,. This latter publication, however, only contains data for the temperature interval, 195°C < T, Q 420°C and also gives no indication of what error limits are applicable to the values of d and n. Judging from the techniques used for the determination of d and n, the error limits applicable to these parameters are likely to be
G. Myburg.
R. Swonepoel
/ RF-glow
discharge-produced
o-Si:
H thin film
23
rather larger than *2%. A further gap in the literature is that no mention is made of the effect of thickness variation, or more comprehensively, the effect of Ad, as described in this publication, that of Swanepoel [lo] and that of Myburg [4]. By ignoring the effect of Ad, large errors can be made. The financial support of the Council of Scientific and Industrial Research, South Africa, is acknowledged and thanks are due to J.F. Myburg for her assistance in the preparation of the manuscript.
References [I] Y.J. Hamakawa. J. Non-Cryst. Solids 59&60 (1983) 1265. [2] W.E. Spear, in: Poly-Micro-Crystalline and Amorphous Semiconductors. eds. P. Pinard and S. Kalbitzer (Editions de Physique. France, 1984) p. 527. [3] I. Shimizu, J. Non-Cryst. Solids 77&78 (1985) 1363. [4] G. Myburg, Thesis, RAU, Johannesburg (1985). [S] G. Myburg, to be published. [6] J.C. Knights, Phil. Msg. 34 (1976) 663. (71 J.C. Knigbl~. G. Lucovsky and R.J. Nemanich, J. Non-Cryst. Solids 32 (1979) 393. [8] J.C. Knights, R.A. Lujan. M.P. Rosenblum, R.A. Street, D.K. Bieglesen and J.A. Reimer, Appl. Phys. Lett. 38 (1981) 331. [9] R. Swanepoel. J. Phys. E 16 (1983) 1214. [lo] R. Swancpoel, J. Phys. E 17 (1984) 896. [ll] R. Swanepoel. J. Opt. Sot. Am. 2 (1985) 1339. [12] CC. Tsai and H. Fritzsche, Solar Energy Mat. 1 (1979) 29. 1131 R.H. Klazes. M.H.L.M. van den Broek. J. Bezemer and S. Radelaar, Phil. Mag. B 45 (1982) 377. [14] A. Matsuda. J. Non-Cryst. Solids 59&60 (1983) 767. [lS] M.M. Koltun, Selective Optical Surfaces for Solar Energy Converters (Allerton, New York, 1981) p. 14. [16] P.L. &art. H. Aharoni and B.M. Lacquet. Nucl. Instr. and Meth. B 6 (1985) 365. [17] I. Sakata. Y. Hayasbi, M. Yamanaka and H. Karasawa, J. Appl. Phys. 52 (1981) 4334. [18] S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981) p. 850. [19] J.C. Bruyere. A. Deneuville, A. Mini, J. Fontenille and R. Danielou, J. Appl. Phys. 51 (1980) 2199. [20] M.H. Brodsky, Thin Solid Films 40 (1977) L23. [21] M.H. Brodsky, M.A. Frisch and J.F. Ziegler, Appl. Phys. Lett. 30 (1977) 561. [22] P.J. Zanzucchi, C.R. Wronski and D.E. Carlson, J. Appl. Phys. 48 (1977) 5227.