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VUV excimer light source for deposition of amorphous semiconductors
Applied Surface Science 54 (1992) 430-434 North-Holland
applied surface science
VUV excimer light source for deposition of amorphous semiconductors F. Kessler a n d G . H . B a u e r Institut für Physikalische Elektronik, Universität Stuttgart, Pfaffenwaldring 47, D-7000 Stuttgart 80, Germany Received 28 May 1991; accepted for publication 31 May 1991
Two different types of a large-area high-intensity VUV dielectric-barrier-discharge lamp for VUV emission have been constructed and operated over a wide range of voltage (0.5-20 kV), frequency (0.1-160 kHz), gas pressure (0.2-3 bar), and electrode spacing (0.7-2 mm). VUV spectra were monitored for pure Ar, Kr, and Xe and their respective mixtures. The spatial excimer photon flux has been calculated versus distance x from the lamp window. Due to diffusive spatial radiation the intensity rapidly drops with distance x. With Xe excimer photo-decomposition of Si2 Hó we have deposited intrinsic and p-type doped high quality photosensitive amorphous hydrogenated silicon (a-Si:H) (Ophoto/Odark = 106).
1. Introduetion Usually amorphous semiconductors such as aSi:H, a-Ge:H, a-SiC:H, and a-Ge:H are deposited in glow discharge (GD), where process gases are decomposed very effectively and high film growth rates can be achieved, combined with surface bombardments by high energetic electrons, " h o t radicals", and ions which are supposed to deteriorate the growing film surface. Photochemical vapour deposition (photo-CVD) on the other hand, is a very promising soft method which offers the possibility of kinetic control of the film deposition process according to very selective photolytic dissociation. The type of primary radical is determined by supplied photon energies and thus film properties can be influenced. Due to the absence of high energetic radicals and ions an improvement of the optoelectronic quality of films and interfaces e.g. in solar cells by means of photo-CVD is expected [1-4]. Absorption cross sections of SiH 4 and Si2H 6 are negligible for hv < 8 eV and hv< 6.4 eV, respectively. In this VUV region usually low-pressure D 2 and Xe lamps [5,6] which provide rather low fluxes (i.e. low deposition rates) have been applied. In this paper we present design and fea-
tures of operation of a high-intensity incoherent VUV source, tunable in photon energy as proposed by Eliasson and Kogelschatz [7].
2. Experimental
2.1. Barrier discharge lamps In a dielectric barrier discharge lamp one or both electrodes have to be electrically insulated, e.g. covered by a dielectric. We have designed both options (figs. l a and lb): (i) A planar configuration where the high-voltage electrode is covered by a synthetic quartz hat (thickness 0.25 or 1 mm) and light is transmitted through the ground electrode (fine wire mesh) at 0.7-2 mm located from the dielectric. In order to use backward radiation too, AI has been evaporated onto the inner surface of the cap (fig. la). (ii) A parallel plate lamp with 15 quartz incapsulated electrodes at 0.85 mm distance and lateral light emission (fig. lb). Both types are incapsulated in a PTFE body enclosed by a 6 mm thick MgF 2 (or CaF 2) window. The lamps have been operated at (1-.3) bar and at AC power of 0.1-15 kHz or 160-175 kHz (depending upon the power supply). The output flux
0169-4332/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
F. Kessler, G.H. Bauer / VUV excimer light source for deposition of amorphous serniconductors
~)!
h,.
I
~b)
.
"f
431
.
Fig. 1. Dielectricbarrier discharge lamps: (a) planar; (b) parallel plate configuration.
in 0.1-15 kHz is approximately proportional to the applied voltage (0.6-20 kV) and frequency (0.1-15 kHz). 2.2. Spectral distributions
The formation of rare-gas molecules is based upon the excitation of a ground-state atom (~S0) by inelastic collision with an electron; due to the high-pressure neutral density and collision frequency at 1-3 bar higher excitations ra~idly thermalize to the lowest excitation levels P1 or P2, respectively. From these states either atomic resonance transitions to the ~round stare occur or an excited dimer (lE+ or ~Y~+) in a three-particle process is formed:
(~Y.g) and its slightly bonding van der Waals state is converted into VUV photons of the second and first noble gas continuum (see fig. 2) [7,8]. High pressure favours the formation of excimers and consequently supports the yield of VUV photons according to lack of ground-state molecules. With decreasing neutral (gas or) particle density (e.g. from 2-0.5 bar, see fig. 3a) the peak position of the first continuum shifts towards the atomic line 3P1 --* 1So which cuts off the spectrum by self-absorption. Simultaneously, the photon flux of the second continuum is reduced (three-body reaction), but the peak maximum remains at constant energy.
1~u+
e + X(1S0) --, e + X ** thermalization to X * (3P1,2 ), X* ~ h v + X atomic (resonance) transition or, .
1,3
+
X* +X+X~,X2 ( Yu)+X excimer formation (three-partiele reaction),
aP~+ts o 3Pa+ ts o
o) ~3
7
atomic transition
0
X~'~X+X+hv excimer radiation. The energy difference between the 1,3 Z u+ molecular binding state to the repulsive ground-state
iSo+ISo Internuelear separation Fig. 2. Schematicpotential energydiagram of rare gases.
432
IK Kessler. G.H. B a u e r / VUV excimer light source for deposition of amorphous semi«onductors 1.0
2nd
0.8
a)
Kr
p=2.0bar_,_/~\Ist
0.6
E
0.4
.6
0.2
cont
2O
L
p=l.0bar
15
conL.
~ (3pI«IS0)
"~ I0
p=O o.o
x o
Ar
PR= 0.100 0050
PR= 0.012
Ne
o.4,
C~ 2"
(tpl«lS0)
o.2. o.o
Xe/Ar
Ne 3pierS0)
p=2bar
4 0.8'
p =2bar
I, «L
6
7
9
8 Photon
energy
i0
I
(eV)
Fig. 3. Excimer spectra of (a) Kr at different pressures; (b) pure Xe, Kr and Ar.
In fig. 3b excimer spectra of pure Xe, Kr, and Ar at p = 2.5 bar are outlined. The gaussianshaped second continua peaking at hv = 7.3 eV (Xe), 8.6 eV (Kr), or 9.8 eV (Ar) show half-widths of 0.56, 0.77, or 0.78 eV. The first continuum of the Ar emission has been cut oft by MgF2 window absorption. The emission of K r / A r at different compositions ( Ptot = 2 bar) is shown in figs. 4a and 4b. We
30-
p =2bar
20 "
~'..
M o
/~0.250
ù~~ù / r , , ~
Q
PR=
°.'~°
i ' " "-~~~'~,, ~~ ~ PR=
~ß~
p=Zbar
a
1' 0
7
8 9 10 1 Photon energy (er) Fig. 4. Spectra of K r / A r at different pressure ratios PR.
,
<~
,'-
9 10 Photon e n e r g y (eV) Fig. 5. Spectra of X e / A r at different pressure ratlos PR = PXe/PX«+Ar" 5
assume rapid energy transfer from Ar* and Ar* to Kr according to: A r * + Kr --* Ar + K r * ,
(1)
Ar2* + Kr ~ Ar + Ar + Kr *
(2)
Kr * + Kr + Ar --, Kr2* + Ar.
(31
The rate of reaction (2) is 1.5 × 1 0 - 9 cm 3 s ] [9] and is higher by 102 [101 than of reaction (1), so pathway (2) with (3) will be favoured. Consequently, we predominantly observe even at PR < 0.025 the first and weaker second Kr continuum whereas Ar radiation is totally quenched. Fig. 5 shows spectra of X e / A r mixtures at PR=Pxe/Pto,>O.O03. D u e to resonant energy transfer from the smaller to the larger noble gas atoms such as Ar* + Xe ~ Ar + Xe*, Ar2* + Xe --, Ar + Ar + Xe* a typical Xe spectrum appears at PR > 0.1, but in contradiction to K r / A r with decreasing PR the first continuum shifts towards and changes into the Xe resonance line (3P 1 ~ 1S0) at 8.44 eV (146.9 nm). The highest Xe photon flux (at the same power input and total pressure) was obtained in a dilution of N e at PR =Pxe/Px~+Ne = 0.375 (fig. 6). Lamp operation was exceptionally stable and fine microdischarges were distributed very homogeneously, presumably due to much
F. Kessler, G.H. Bauer / VUV excimer light source for deposition of amorphous semiconductors
p=2bar
Substituting 2p/G = 7,
PR=
60
ù2v »O
0.375
4, = c°nst'j0
o~
2O 0
>l
PR=
0 O~
p=2bar
_.--.-0.125 ~..0.050
0
4, = const.[ (VcJ(5 + R 2 - X ) / ( ¢ ~ +
7
Xe/Ne I
i
8 Photon
R2)
- 0 . 5 7 l n ( X 2 + R 2) + y I n ( X ) ] .
--, ~ -~'I r
J0 (1 - "y/cos v) sinv dv d , .
Integrating over the solid angle ~ of the emitting area and substituting O = arctan(R/X) (R is the radius of the emissive area and X the distance from the emissive area) we get
0.012
i0
433
i
9
l
i0
Regarding moreover window thickness d and with x = X - d the axial photon flux 4, = 4,(x) can be written:
e n e r g y (eV)
Fig. 6. Spectra of X e / N e at different PR = Pxe/Pxe+Ne.
lower electronic ionization energy and elastic cross section of Ne in comparison to Xe, Kr, and Ar. Benefits for mixing noble gases are: (i) generation of broad continuous spectra, e.g. K r / A r (fig. 4); (ii) generation of a single line, e.g. X e / A r at PR < 0.012 (fig. 5); (iii) enhancement of photon flux, e.g. X e / N e (fig. 6). An additional benefit of diluting Xe with gas of lighter atoms like Ar and Ne consists in sputtering of the metallic electrode to be reduced considerably, so that continuous stable operation of > 40 h has been achieved.
{¢(xq-d)
2 q - R 2 - - X - - e
4 , ( x ) = const.
~xT2?;
R2
-0.5~, l n [ ( x + d ) 2 + R 2] +V l n ( x + d ) } .
In fig. 7a the strong decrease of 4, versus x is outlined ( T = 0 . 1 , R = 1.9 cm). Additionally, numerically calculated radial distributions of 14,1 parallel to the window versus r and at various X are shown in fig. 7b. Flux 4, is rather constant at small distances X close to the emitting area and at r < R but strongly drops towards the edges (r = R).
2.3. Spatial photon fluxes The spatial photon flux 4,(x, r) of the planar lamp (tig, la) has been calculated versus distance x form the lamp window, assuming a flat and homogeneously emitting surface and negligible reflections at the window. Assuming constant 4, per solid angle $2 we get: d4,/d$2 = const., and with d~2=sinvdvd~ we obtain 4 , o c f f s i n v d v dq,. The angular transmission of the wire mesh (ground electrode) has been taken into account. The relative mesh transmission T depends on wire radius p and mesh size G and has been approximated to
[111: T= 1 - 2 p / ( G cos v) > 0.
1.5
ù)
1.0
0.5
2 o ù=
I
m 0.0
:.,
.
:
ù
:
,I
R i
:i ~
R I
I Il
Distance x from
-i 0 I 2 Distance r from
window (cm)
l a m p axis (cm)
Œ
4
6
-3
-2
Fig. 7. Calcu;ated photon flux: (a) versus window distance x, (b) versus distance r from the lamp,
F. Kessler, G.H. Bauer / V U V excirner light source for deposition of amorphous semiconductors
434 2.2 2.1
/ o
2.0
A
1.6
o
//
/
1.8
I~5
/ /
/o A
B
//
/
/
//~
~~
ùg_l°_':J,~~aù~«~o--.._ o A Kr-
«
o Photo-CVD
10
2'0 at% H
3'0
ity in the dark and under illumination (AM1) of undoped and approximately intrinsic samples even at high band-gaps (E~ < 1.9 eV) amounts to om/o d 106 at oijI > 10--~ f2 • cm indicating mid-gap defect densities sufficiently low in order to provide optoelectronic applications.
[]
o Xen Xe- (B-doped films) i
3. Summary
i
150 200 250 Substrate t e m p e r a t u r e ('C)
Fig. 8. Band-gap Eg versus at% H and deposition temperature
2.4. Deposition of amorphous semiconductors The lamp window towards the reactor was coated by low vapour pressure oil (e.g. Fomblin) to avoid deposition. Due to oil absorption and dissociation at photon energies hv > 8.4 eV most a-Si:H films have been prepared by use of pure SiaH 6 (absorption edge at hv > 6.5 eV) at p = 0.5 mbar and Xe excimer radiation (distance from lamp window: 1 cm). Substrate deposition temperature Ts has been varied between 160 and 250 ° C and film growth rates of up to 5 n m / m i n have been achieved. In comparison to G D samples the optical band-gaps Eg of all photo-CVD samples were higher and showed a more pronounced dependence on T~ (tig, 8). SIMS measurements revealed a clear correlation between band-gap (Xe-lamp samples) and hydrogen content c H (c H up to 30 at% H, fig. 8) in accordance with the findings of G D films. The enhanced Eg found in " K r samples" (fig. 8) obviously results from dissociation of Fomblin oil and subsequent carbon incorporation ( % = 5 at%). Moreover, band-gap reduction as a consequence of boron doping (0.25% B2H ó in the gas) is less pronounced than in G D films. Photosensitivity in terms of ratio of conductiv-
High photon fluxes in the range of 6.5-10.8 eV have been achieved with large-area VUV source, of which spectral distribution and photon energy easily can be adjusted by type, density, and composition of the neutral gas. High intensities from noble gas continua allow direct photo-CVD of undoped and B-doped amorphous semiconductors like a-Si:H from Si~H 6 at relatively high deposition rates (5 n m / m i n ) and with excellent electronic and optoelectronic properties.
References [1] H. Takei, T, Tanaka, W.Y. Kim, M. Konagai and K. Takahashi, J. Appl. Phys. 58 (1985) 3664. [2] T. Fuyuki, K.-Y. DU, S. Okamoto, S. Yasuda, T. Kimoto, M. Yoshimoto and H. Matsunami, in: Conf. Rec. 19 IEEE PVSC, New Orleans, 1987, p. 569. [3] R.E. Rocheleau, S.C. Jackson, S.S. Hegedus and B.N. Baron, Mater. Res. Soc. Proc. 70 (1986) 37. [4] P.A. Robertson and W.I. Milne, Mater. Res. Soc. Proc. 70 (1986) 31. [5] Y.K. Bhatnagar and W.|. Milne, Thin Solid Films 163 (1988) 237. [6] H. Matsunami, T. Shirafuji, T. Fuyuki and M. Yoshimoto, Mater. Res. Soc. Proc. 192 (1990) 505. [7] B. Eliasson and U. Kogelschatz, Appl. Phys. B 46 (1988) 299. [8] E.N. Pavlovskaya and A.V. Yakovleva, Opt. Spectrosc. (USSR) 54 (2) (1983) 132. [9] A. Gedanken, J. Jortner, B. Raz and A. Szöke, J. Chem. Phys. 57 (1972) 3456. [101 M. Bourene and J. Le Calve, J. Chem. Phys. 58 (1973) 1452. [11] F. Kessler, H.-D. Mohring and G.H. Bauer, Mater. Res. Soc. Proc. 192 (1990) 559.
applied surface science
VUV excimer light source for deposition of amorphous semiconductors F. Kessler a n d G . H . B a u e r Institut für Physikalische Elektronik, Universität Stuttgart, Pfaffenwaldring 47, D-7000 Stuttgart 80, Germany Received 28 May 1991; accepted for publication 31 May 1991
Two different types of a large-area high-intensity VUV dielectric-barrier-discharge lamp for VUV emission have been constructed and operated over a wide range of voltage (0.5-20 kV), frequency (0.1-160 kHz), gas pressure (0.2-3 bar), and electrode spacing (0.7-2 mm). VUV spectra were monitored for pure Ar, Kr, and Xe and their respective mixtures. The spatial excimer photon flux has been calculated versus distance x from the lamp window. Due to diffusive spatial radiation the intensity rapidly drops with distance x. With Xe excimer photo-decomposition of Si2 Hó we have deposited intrinsic and p-type doped high quality photosensitive amorphous hydrogenated silicon (a-Si:H) (Ophoto/Odark = 106).
1. Introduetion Usually amorphous semiconductors such as aSi:H, a-Ge:H, a-SiC:H, and a-Ge:H are deposited in glow discharge (GD), where process gases are decomposed very effectively and high film growth rates can be achieved, combined with surface bombardments by high energetic electrons, " h o t radicals", and ions which are supposed to deteriorate the growing film surface. Photochemical vapour deposition (photo-CVD) on the other hand, is a very promising soft method which offers the possibility of kinetic control of the film deposition process according to very selective photolytic dissociation. The type of primary radical is determined by supplied photon energies and thus film properties can be influenced. Due to the absence of high energetic radicals and ions an improvement of the optoelectronic quality of films and interfaces e.g. in solar cells by means of photo-CVD is expected [1-4]. Absorption cross sections of SiH 4 and Si2H 6 are negligible for hv < 8 eV and hv< 6.4 eV, respectively. In this VUV region usually low-pressure D 2 and Xe lamps [5,6] which provide rather low fluxes (i.e. low deposition rates) have been applied. In this paper we present design and fea-
tures of operation of a high-intensity incoherent VUV source, tunable in photon energy as proposed by Eliasson and Kogelschatz [7].
2. Experimental
2.1. Barrier discharge lamps In a dielectric barrier discharge lamp one or both electrodes have to be electrically insulated, e.g. covered by a dielectric. We have designed both options (figs. l a and lb): (i) A planar configuration where the high-voltage electrode is covered by a synthetic quartz hat (thickness 0.25 or 1 mm) and light is transmitted through the ground electrode (fine wire mesh) at 0.7-2 mm located from the dielectric. In order to use backward radiation too, AI has been evaporated onto the inner surface of the cap (fig. la). (ii) A parallel plate lamp with 15 quartz incapsulated electrodes at 0.85 mm distance and lateral light emission (fig. lb). Both types are incapsulated in a PTFE body enclosed by a 6 mm thick MgF 2 (or CaF 2) window. The lamps have been operated at (1-.3) bar and at AC power of 0.1-15 kHz or 160-175 kHz (depending upon the power supply). The output flux
0169-4332/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
F. Kessler, G.H. Bauer / VUV excimer light source for deposition of amorphous serniconductors
~)!
h,.
I
~b)
.
"f
431
.
Fig. 1. Dielectricbarrier discharge lamps: (a) planar; (b) parallel plate configuration.
in 0.1-15 kHz is approximately proportional to the applied voltage (0.6-20 kV) and frequency (0.1-15 kHz). 2.2. Spectral distributions
The formation of rare-gas molecules is based upon the excitation of a ground-state atom (~S0) by inelastic collision with an electron; due to the high-pressure neutral density and collision frequency at 1-3 bar higher excitations ra~idly thermalize to the lowest excitation levels P1 or P2, respectively. From these states either atomic resonance transitions to the ~round stare occur or an excited dimer (lE+ or ~Y~+) in a three-particle process is formed:
(~Y.g) and its slightly bonding van der Waals state is converted into VUV photons of the second and first noble gas continuum (see fig. 2) [7,8]. High pressure favours the formation of excimers and consequently supports the yield of VUV photons according to lack of ground-state molecules. With decreasing neutral (gas or) particle density (e.g. from 2-0.5 bar, see fig. 3a) the peak position of the first continuum shifts towards the atomic line 3P1 --* 1So which cuts off the spectrum by self-absorption. Simultaneously, the photon flux of the second continuum is reduced (three-body reaction), but the peak maximum remains at constant energy.
1~u+
e + X(1S0) --, e + X ** thermalization to X * (3P1,2 ), X* ~ h v + X atomic (resonance) transition or, .
1,3
+
X* +X+X~,X2 ( Yu)+X excimer formation (three-partiele reaction),
aP~+ts o 3Pa+ ts o
o) ~3
7
atomic transition
0
X~'~X+X+hv excimer radiation. The energy difference between the 1,3 Z u+ molecular binding state to the repulsive ground-state
iSo+ISo Internuelear separation Fig. 2. Schematicpotential energydiagram of rare gases.
432
IK Kessler. G.H. B a u e r / VUV excimer light source for deposition of amorphous semi«onductors 1.0
2nd
0.8
a)
Kr
p=2.0bar_,_/~\Ist
0.6
E
0.4
.6
0.2
cont
2O
L
p=l.0bar
15
conL.
~ (3pI«IS0)
"~ I0
p=O o.o
x o
Ar
PR= 0.100 0050
PR= 0.012
Ne
o.4,
C~ 2"
(tpl«lS0)
o.2. o.o
Xe/Ar
Ne 3pierS0)
p=2bar
4 0.8'
p =2bar
I, «L
6
7
9
8 Photon
energy
i0
I
(eV)
Fig. 3. Excimer spectra of (a) Kr at different pressures; (b) pure Xe, Kr and Ar.
In fig. 3b excimer spectra of pure Xe, Kr, and Ar at p = 2.5 bar are outlined. The gaussianshaped second continua peaking at hv = 7.3 eV (Xe), 8.6 eV (Kr), or 9.8 eV (Ar) show half-widths of 0.56, 0.77, or 0.78 eV. The first continuum of the Ar emission has been cut oft by MgF2 window absorption. The emission of K r / A r at different compositions ( Ptot = 2 bar) is shown in figs. 4a and 4b. We
30-
p =2bar
20 "
~'..
M o
/~0.250
ù~~ù / r , , ~
Q
PR=
°.'~°
i ' " "-~~~'~,, ~~ ~ PR=
~ß~
p=Zbar
a
1' 0
7
8 9 10 1 Photon energy (er) Fig. 4. Spectra of K r / A r at different pressure ratios PR.
,
<~
,'-
9 10 Photon e n e r g y (eV) Fig. 5. Spectra of X e / A r at different pressure ratlos PR = PXe/PX«+Ar" 5
assume rapid energy transfer from Ar* and Ar* to Kr according to: A r * + Kr --* Ar + K r * ,
(1)
Ar2* + Kr ~ Ar + Ar + Kr *
(2)
Kr * + Kr + Ar --, Kr2* + Ar.
(31
The rate of reaction (2) is 1.5 × 1 0 - 9 cm 3 s ] [9] and is higher by 102 [101 than of reaction (1), so pathway (2) with (3) will be favoured. Consequently, we predominantly observe even at PR < 0.025 the first and weaker second Kr continuum whereas Ar radiation is totally quenched. Fig. 5 shows spectra of X e / A r mixtures at PR=Pxe/Pto,>O.O03. D u e to resonant energy transfer from the smaller to the larger noble gas atoms such as Ar* + Xe ~ Ar + Xe*, Ar2* + Xe --, Ar + Ar + Xe* a typical Xe spectrum appears at PR > 0.1, but in contradiction to K r / A r with decreasing PR the first continuum shifts towards and changes into the Xe resonance line (3P 1 ~ 1S0) at 8.44 eV (146.9 nm). The highest Xe photon flux (at the same power input and total pressure) was obtained in a dilution of N e at PR =Pxe/Px~+Ne = 0.375 (fig. 6). Lamp operation was exceptionally stable and fine microdischarges were distributed very homogeneously, presumably due to much
F. Kessler, G.H. Bauer / VUV excimer light source for deposition of amorphous semiconductors
p=2bar
Substituting 2p/G = 7,
PR=
60
ù2v »O
0.375
4, = c°nst'j0
o~
2O 0
>l
PR=
0 O~
p=2bar
_.--.-0.125 ~..0.050
0
4, = const.[ (VcJ(5 + R 2 - X ) / ( ¢ ~ +
7
Xe/Ne I
i
8 Photon
R2)
- 0 . 5 7 l n ( X 2 + R 2) + y I n ( X ) ] .
--, ~ -~'I r
J0 (1 - "y/cos v) sinv dv d , .
Integrating over the solid angle ~ of the emitting area and substituting O = arctan(R/X) (R is the radius of the emissive area and X the distance from the emissive area) we get
0.012
i0
433
i
9
l
i0
Regarding moreover window thickness d and with x = X - d the axial photon flux 4, = 4,(x) can be written:
e n e r g y (eV)
Fig. 6. Spectra of X e / N e at different PR = Pxe/Pxe+Ne.
lower electronic ionization energy and elastic cross section of Ne in comparison to Xe, Kr, and Ar. Benefits for mixing noble gases are: (i) generation of broad continuous spectra, e.g. K r / A r (fig. 4); (ii) generation of a single line, e.g. X e / A r at PR < 0.012 (fig. 5); (iii) enhancement of photon flux, e.g. X e / N e (fig. 6). An additional benefit of diluting Xe with gas of lighter atoms like Ar and Ne consists in sputtering of the metallic electrode to be reduced considerably, so that continuous stable operation of > 40 h has been achieved.
{¢(xq-d)
2 q - R 2 - - X - - e
4 , ( x ) = const.
~xT2?;
R2
-0.5~, l n [ ( x + d ) 2 + R 2] +V l n ( x + d ) } .
In fig. 7a the strong decrease of 4, versus x is outlined ( T = 0 . 1 , R = 1.9 cm). Additionally, numerically calculated radial distributions of 14,1 parallel to the window versus r and at various X are shown in fig. 7b. Flux 4, is rather constant at small distances X close to the emitting area and at r < R but strongly drops towards the edges (r = R).
2.3. Spatial photon fluxes The spatial photon flux 4,(x, r) of the planar lamp (tig, la) has been calculated versus distance x form the lamp window, assuming a flat and homogeneously emitting surface and negligible reflections at the window. Assuming constant 4, per solid angle $2 we get: d4,/d$2 = const., and with d~2=sinvdvd~ we obtain 4 , o c f f s i n v d v dq,. The angular transmission of the wire mesh (ground electrode) has been taken into account. The relative mesh transmission T depends on wire radius p and mesh size G and has been approximated to
[111: T= 1 - 2 p / ( G cos v) > 0.
1.5
ù)
1.0
0.5
2 o ù=
I
m 0.0
:.,
.
:
ù
:
,I
R i
:i ~
R I
I Il
Distance x from
-i 0 I 2 Distance r from
window (cm)
l a m p axis (cm)
Œ
4
6
-3
-2
Fig. 7. Calcu;ated photon flux: (a) versus window distance x, (b) versus distance r from the lamp,
F. Kessler, G.H. Bauer / V U V excirner light source for deposition of amorphous semiconductors
434 2.2 2.1
/ o
2.0
A
1.6
o
//
/
1.8
I~5
/ /
/o A
B
//
/
/
//~
~~
ùg_l°_':J,~~aù~«~o--.._ o A Kr-
«
o Photo-CVD
10
2'0 at% H
3'0
ity in the dark and under illumination (AM1) of undoped and approximately intrinsic samples even at high band-gaps (E~ < 1.9 eV) amounts to om/o d 106 at oijI > 10--~ f2 • cm indicating mid-gap defect densities sufficiently low in order to provide optoelectronic applications.
[]
o Xen Xe- (B-doped films) i
3. Summary
i
150 200 250 Substrate t e m p e r a t u r e ('C)
Fig. 8. Band-gap Eg versus at% H and deposition temperature
2.4. Deposition of amorphous semiconductors The lamp window towards the reactor was coated by low vapour pressure oil (e.g. Fomblin) to avoid deposition. Due to oil absorption and dissociation at photon energies hv > 8.4 eV most a-Si:H films have been prepared by use of pure SiaH 6 (absorption edge at hv > 6.5 eV) at p = 0.5 mbar and Xe excimer radiation (distance from lamp window: 1 cm). Substrate deposition temperature Ts has been varied between 160 and 250 ° C and film growth rates of up to 5 n m / m i n have been achieved. In comparison to G D samples the optical band-gaps Eg of all photo-CVD samples were higher and showed a more pronounced dependence on T~ (tig, 8). SIMS measurements revealed a clear correlation between band-gap (Xe-lamp samples) and hydrogen content c H (c H up to 30 at% H, fig. 8) in accordance with the findings of G D films. The enhanced Eg found in " K r samples" (fig. 8) obviously results from dissociation of Fomblin oil and subsequent carbon incorporation ( % = 5 at%). Moreover, band-gap reduction as a consequence of boron doping (0.25% B2H ó in the gas) is less pronounced than in G D films. Photosensitivity in terms of ratio of conductiv-
High photon fluxes in the range of 6.5-10.8 eV have been achieved with large-area VUV source, of which spectral distribution and photon energy easily can be adjusted by type, density, and composition of the neutral gas. High intensities from noble gas continua allow direct photo-CVD of undoped and B-doped amorphous semiconductors like a-Si:H from Si~H 6 at relatively high deposition rates (5 n m / m i n ) and with excellent electronic and optoelectronic properties.
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