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Deposition of a-Si:H with the hot-wire technique
Journa.l of Non-Crystalline Solids 164-166 (1993) 83-86 North-Holland
~ ~
r ~0hIl~
Deposition of a-Si:H with the hot-wire technique R. Zedlitz, F. Kessler and M. Heintze Institut fiir Physikalische Elektronik, Universit~t Stuttgart Pfhffenwaldring 47 D-70569 Stuttgart, Germany Amorphous hydrogenated silicon (a-Si:H) was deposited by Sill 4 decomposition on a hot tungsten filament. A substrate temperature of Ts=400°C was chosen since an improved degradation behaviour is expected as a result of low hydrogen incorporation. The effects of gas pressure, substrate-to-filament distance ds_f and filament temperature Tf on film properties are discussed. Material of good optoelectronic quality is obtained at high deposition rates. 1. INTRODUCTION Wiesmann et a1.(1979)[1] were the first to report a-Si:H deposition by thermal dissociation of Sill4. Unfortunately at their deposition conditions of extremely low silane pressure (~-5.10"2pa) and relative low dissociation temperature (1600°C) the film properties obtained were discouraging when compared to glow discharge (GD)samples. On the other hand the pronounced light degradation in optimized a-Si:H films from GD has been linked to hydrogen content. Due to the high H-incorporation ( ~ 1 0 % ) of device quality GD films, alternative methods with better gas phase control and allowing higher substrate temperatures, resulting in films of less pronounced StaeblerWronski effect, are desired, Matsumura[2][3] was the first to obtain acceptable hot-wire (HW) film qualities at (50-3000)Pa. The H-content of these films was less than that of comparable GD samples. Nevertheless for T s > 300°C trph and the ratio Oph/Od rapidly decreased. Also Doyle et al.[4] who decreased the gas pressure to p~.0.SPa demonstrated the possibility of preparing films with optoelectronic properties comparable to the GD technique, but unfortunately these exhibited the same light soaking effects, The most encouraging HW samples have been fabricated by Mahan et al. [5][6] at low gas pressure: By varying the substrate temperature from 40°C to 630°C, a-Si:H with 0.1at%
indicating a relaxed network. In this regime the HW technique is superior to GD. The low H content material exhibited significantly less metastability than GD samples[6] demonstrating the potential of this deposition method. Following mass spectrometry data[4], the silane dissociation path is assigned to the adsorption of Sill 4 at the hot filament and subsequent evaporation of atomic Si and H. Also an additional catalytic process[5] cannot be ruled out. Due to the remote gas dissociation two pathways for film growth can be distinguished: i) At high pressures[2][3] or large substrate distances the primarily generated radicals undergo gas phase reactions resulting in secondary long-lived species responsible for film growth. ii) On the other hand at low pressures or small wire-to-substrate distances [4-6] the primary radicals reach the substrate without any gas reactions. In the present contribution we have studied HW deposition and film properties at Ts=400°C, that is outside the temperature range for device quality GD samples. 2. EXPERIMENTAL The HW deposition system used in this study comprises a high vacuum chamber with a base pressure below 10-4pa. A tungsten filament, 4cm in length and 0.25mm in diameter, is centered over the substrate holder and a nozzle directs the gas to the filament in a flow direction parallel to the substrate plane. A shutter between filament and sample holder allows baking out the wire prior to depositions
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
84
R. Zedlitz et al. / Deposition of a-Si:H with hot-wire-technique
without contaminating the substrate and a defined start of the process after the heating current is adjusted. The filament temperature was measured by a pyrometer. All films were prepared with pure Sill 4 at a sample temperature of 400°C on glass (7059) or c-Si substrates. The effect of ds_f, p and Tf on film properties was investigated. The gas pressure was adjusted by the Sill 4 flow rate. A slight pressure rise of about 10-20% was observed with the filament on, resulting from the dissociation of Sill 4. The tungsten content c w in the films was determined by X-ray photo-emission spectroscopy (XPS) with a detection limit of 0.008at% (-4×1018cm-3). The films were characterized by measurements of optical transmission (band gap, thickness), IR spectroscopy (CH), PDS (Urbach energy, defects) and CPM (degradation). The temperature dependent dark conductivity and AM 1 photoconductivity were measured using coplanar AI contacts. For rapid degradation the samples were held at 40°C and illuminated by a focused tungsten halogen lamp (800W) using a piece of the same sample as a filter to achieve homogeneous absorption. 3. RESULTS AND DISCUSSION The possibility of a heat rise of the growing film surface due to radiation can be checked by comparing the filament radiation absorbed and the emission of the substrate (Ts=673K). Under the assumption of a grey body, the filament and substrate emissions are Ff=¢w-os.T 4 and Fs=¢s.os.T~s respectively (~w, es " emissivities of tungsten and substrate surface; a s StefanBoltzmarm constan 0. Assuming an infinitely long wire the radiation intensity decreases linearly with ds!f, that is the intensity arrivin~ at the substrate decreases by a factor rfC(d~_f+~) 0"5. With Tf=2173K, T~=673K, filament radius rf=0.0125cm, substrate radius rs= lcm, ds_f=l.Scm , os=5.67-10-12W-cm-2.K-4 and the rough estimation ~w=~s the ratio of absorbed to emitted radiation at the substrate surface is: (If/Is).rfC(d2_f+~)°'5 =0.75, that is, both energy fluxes are near thermal equilibrium. The crucial parameters are ¢w and es, which additionally depend on hp. On the other hand a-Si:H absorbs less than
10% of the light intensity. Therefore we conclude that even at the minimum distance of ds_f= 1.5cm no significant film heating by radiation occurs. Another possible energy flux towards the substrate is caused by thermal conduction and by atomic hydrogen recombination, if Si and H are the primary radicals. At a gas pressure p=2.7Pa, with the reaction constant kH=2.68.10-12cm3-s'l[7] and mean gas temperature Tg=Ts=673K, the H-lifetime is ~'H=k.Tg/(kH-P) = 1.3.10-3s (k Boltzmann constant). The mean velocity v x perpendicular to the substrate surface (x-direction)is a function of gas flux and gas density. With our low gas density vx=l.2.104cm.s-l,givingapathlengthofH-radical of lrl=Vx.r~15cm. Taking into account the small rate constant of Si[8] (ksi < kH) the film precursors should be atomic Si and H at all distances ds_f investigated in this work. 50
~
~
1
L
40 ~ 30 ~ ,~ 20 ~ 10 0 , , . . . . . t500 1700 1900 2100
.-, 0~, -1 _ .
-8
2300
filament temp. [°C] Fig. 1: Deposition rate rd and tungsten content cw as a function of filament temperature. DL denotes the detection limit. (p=2.7Pa, ds_f=2cm) Figure 1 shows the deposition rate r d and the tungsten content c w versus the filament temperature Tf. r a increases almost linearly with Tf up to 1900°C and attains a saturation a little below 2000°C. A detectable tungsten contamination above 1950°C is observed. These results match with earlier findings [9] and ensure that the samples prepared at Tf=1660°C are free of tungsten. In order to reduce W incorporation into the films we have threaded a 0.2 thick W filament into a 0.5mm ceramic capillary (A1203). In this configuration even at elevated temperatures of Tf> 1900°C no film growth was observed, hence a catalytic process on the filament surface seems necessary as has been proposed by Matsumura[2,3]. In figures 2, 3 and 4b the influence of substrate-
85
R. Zedlitz et al. / Deposition of a-Si:H with hot-wire-technique
to-filament distance ds_f on film properties is shown. 50 40
-4
1.78
7-~-B
1.76 ,%
7
o
3O •+<.
•o 20
1.74 ~
I0
1.72
o
,
1.0
,
,
,
2.0
,
,.
3 0
(.)
1.80
,
2
5.0
r d decreases as (ds_f)-2, while the band gap Eg remains almost constant (fig.2), indicating a comparable hydrogen content c H for all samples, This is in agreement with the model mentioned above, that at all distances ds_f the same primary radicals contribute to the film growth and that T s does not change significantly due to radiation. ,-,,
~
""
6
o 54
4
r~
~
1.0
2
2'.0
3[0 ds-t [ c m ]
era
~
,
,
410
1
2
3
4
2
p r e s s u r e [Pa]
3
d,-¢ [cm]
4
5
Fig. 4: photo- and dark conductivity versus gas pressure (a) (ds_f=2cm) and ds_f (b) (p=2.7Pa).
reactor for optimum optoelectronic quality material are a relatively short substrate-to-filament spacing of ds_f=2cm and a gas pressure of p = 2 . 7 P a adjusted by a flow of 10sccm Sill 4. 6.0 ~ ~
4.0
=
o
,
-to
0
(log(a)=3.5) versus substrate-to-filament distance, (p=2.7Pa, T f = 1660°C)
,
qph
-12 + . . . . . . . . . . . . . . . . . .
Fig. 2: Deposition rate rd and optical band gap Eg
50
.~/
.<=. - e
d,_f [cm]
52
~ph
,.70
4 0
58
(b)
~
2.0
¢,/ ._~"~
0
5.0
Fig. 3: Urbach energy E o and ot0av= 1.2eV) as a function of ds_f. (p=2.7Pa, T f = 1660°C)
0.0
,
,
,
,
,
,
,
,
1.80
40.0
1.78
"~. 30.0
1.76
.-. '~'
m
20.0
1.74
1o.o
1.72
0.0
From the undeconvoluted PDS spectra of l # m thick samples a minimum in Urbach energy E o and defect absorption ct(hv=l.2eV) was observed for ds_f=2cm (fig.3). Also for the ratio of photo- aph to dark conductivity a d (fig.4b) at this distance a maximum of aph/a d ,.~ 105 was found. The correlation between conductivities and gas pressure p is shown in figure 4a. aph increases with p, whereas for a d a minimum at p = 2 . 7 P a is observed, The favorable deposition parameters in our
,
,
0
,
1
,
,
,
2 3 p r e s s u r e [Pa]
,
,
,
4
,
5
'~'
1.70
Fig. 5: Hydrogen content c H, deposition rate r d and band gap Eg (1og(~)=3.5) as function of gas pressure. (ds_f=2cm, Tf= 1660°C) Figure 5 shows that both the hydrogen content c H and the deposition rate rd increase linearly with gas pressure p. Due to the rising c H, Eg also increases as is already known from glow discharge films, c H for the highest pressure investigated (4.4Pa) is close to 6%, the value for GD films prepared at 400°C.
86
R. Zedlitz et al. / Deposition of a-Si:H with hot-wire-technique
The influence of gas pressure p and substrate-tofilament distance ds.f on deposition rate rd is easy to understand: r d is proportional to p (Sill 4 density) at constant ds.f; and r d is proportional to ds_2f for constant p. The hydrogen content c H is not determined alone by rd, since c H remains constant for different ds_f and increases to p. We propose the following mechanism to explain this effect: A balance between r d and the thermalization of the H atoms evaporated from the filament controls c H in the films. Upon increasing p, r a and simultaneously the thermalization of atomic H evaporated from the filament are enhanced resulting in less hydrogen abstraction from the growing surface; consequently c H increases with p. On the other hand, when (Is.f is decreased at constant p, r d increases and the thermalization decreases; hence c H remains constant since the more effective H abstraction by 'hotter' H atoms is balanced by the faster growth of the film. -6.0 DC
~-. < ,~
CPM on both samples prior to and after the degradation show that the defect absorption for HW samples increases by about 3 times for the HW and 10 times for the GD film. The lower hydrogen content and a more relaxed silicon network of the HW material has been regarded as being responsible for this effect[6]. 4. CONCLUSIONS In the present study we have demonstrated that a-Si:H films of good optoelectronic properties at high deposition rates (25A/s), high substrate temperature (400°C) and considerably low hydrogen content cr1=(2-6)% can be achieved by the HW method. The degradation of this material is less pronounced than in comparable glow discharge films. Hydrogen content is controlled by a balance of growth rate and H abstraction from the film surface. Tungsten contamination as well as substrate heating by radiation are negligible at filament temperatures below 1900°C. The dissociation of Sill 4 occurs by a catalytic process at the filament surface. ACKNOWLEDGEMENTS Technical assistance by H. Wagner and funding by the BMFT under contract no. 0328327-D is acknowledged.
-6.5
_q
-7.0
t t , 3 4 5 6 log(t. Is]) Fig. 6: Degradation of Iphoto during illumination for a HW and a device quality (Ts=225°C) GD sample. 0
~ 1
i 2
In figure 6 the degradation behaviour of the optimum HW material (ds_f=2cm, p=2.7Pa) is compared to a device quality film from glow discharge. Although the initial photocurrent Iphot o o f the HW sample is less, the degradation is slower and results in a higher endpoint. Measurements of
REFERENCES 1. H. Wiesmarm, A.K. Ghosh, T. McMahon, and M. Strongin, J.Appl.Phys. 50 (1979) 3752. 2. H. Matsumura, J.Appl.Phys. 65 (1989) 4396. 3. H. Matsumura, Appl.Phys.Lett. 51 (1987) 804. 4. J. Doyle, R. Robertson, G.H. Lin, M.Z. He, and A. Gallagher, J.Appl.Phys. 64 (1988) 3215. 5. A.H. Mahan, J. Carapella, B.P. Nelson, R.S. Crandall and I. Balberg, J.Appl.Phys. 69 (1991) 6728. 6. A.H. Mahan, M. Vanecek, AlP Conf.Proc. 234 (1991) 195. 7. M.J. Kushner, J.Appl.Phys. 62 (1987)2803. 8. M.E.Coltrin, R.J.Kee, and J.A.Miller, J.Electrochem.Soc. 131 (1984) 425. 9. C. Horbach, W. Beyer and H. Wagner, J.Non-Cryst.Sol. 137/138 (1991)661.
~ ~
r ~0hIl~
Deposition of a-Si:H with the hot-wire technique R. Zedlitz, F. Kessler and M. Heintze Institut fiir Physikalische Elektronik, Universit~t Stuttgart Pfhffenwaldring 47 D-70569 Stuttgart, Germany Amorphous hydrogenated silicon (a-Si:H) was deposited by Sill 4 decomposition on a hot tungsten filament. A substrate temperature of Ts=400°C was chosen since an improved degradation behaviour is expected as a result of low hydrogen incorporation. The effects of gas pressure, substrate-to-filament distance ds_f and filament temperature Tf on film properties are discussed. Material of good optoelectronic quality is obtained at high deposition rates. 1. INTRODUCTION Wiesmann et a1.(1979)[1] were the first to report a-Si:H deposition by thermal dissociation of Sill4. Unfortunately at their deposition conditions of extremely low silane pressure (~-5.10"2pa) and relative low dissociation temperature (1600°C) the film properties obtained were discouraging when compared to glow discharge (GD)samples. On the other hand the pronounced light degradation in optimized a-Si:H films from GD has been linked to hydrogen content. Due to the high H-incorporation ( ~ 1 0 % ) of device quality GD films, alternative methods with better gas phase control and allowing higher substrate temperatures, resulting in films of less pronounced StaeblerWronski effect, are desired, Matsumura[2][3] was the first to obtain acceptable hot-wire (HW) film qualities at (50-3000)Pa. The H-content of these films was less than that of comparable GD samples. Nevertheless for T s > 300°C trph and the ratio Oph/Od rapidly decreased. Also Doyle et al.[4] who decreased the gas pressure to p~.0.SPa demonstrated the possibility of preparing films with optoelectronic properties comparable to the GD technique, but unfortunately these exhibited the same light soaking effects, The most encouraging HW samples have been fabricated by Mahan et al. [5][6] at low gas pressure: By varying the substrate temperature from 40°C to 630°C, a-Si:H with 0.1at%
indicating a relaxed network. In this regime the HW technique is superior to GD. The low H content material exhibited significantly less metastability than GD samples[6] demonstrating the potential of this deposition method. Following mass spectrometry data[4], the silane dissociation path is assigned to the adsorption of Sill 4 at the hot filament and subsequent evaporation of atomic Si and H. Also an additional catalytic process[5] cannot be ruled out. Due to the remote gas dissociation two pathways for film growth can be distinguished: i) At high pressures[2][3] or large substrate distances the primarily generated radicals undergo gas phase reactions resulting in secondary long-lived species responsible for film growth. ii) On the other hand at low pressures or small wire-to-substrate distances [4-6] the primary radicals reach the substrate without any gas reactions. In the present contribution we have studied HW deposition and film properties at Ts=400°C, that is outside the temperature range for device quality GD samples. 2. EXPERIMENTAL The HW deposition system used in this study comprises a high vacuum chamber with a base pressure below 10-4pa. A tungsten filament, 4cm in length and 0.25mm in diameter, is centered over the substrate holder and a nozzle directs the gas to the filament in a flow direction parallel to the substrate plane. A shutter between filament and sample holder allows baking out the wire prior to depositions
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.
84
R. Zedlitz et al. / Deposition of a-Si:H with hot-wire-technique
without contaminating the substrate and a defined start of the process after the heating current is adjusted. The filament temperature was measured by a pyrometer. All films were prepared with pure Sill 4 at a sample temperature of 400°C on glass (7059) or c-Si substrates. The effect of ds_f, p and Tf on film properties was investigated. The gas pressure was adjusted by the Sill 4 flow rate. A slight pressure rise of about 10-20% was observed with the filament on, resulting from the dissociation of Sill 4. The tungsten content c w in the films was determined by X-ray photo-emission spectroscopy (XPS) with a detection limit of 0.008at% (-4×1018cm-3). The films were characterized by measurements of optical transmission (band gap, thickness), IR spectroscopy (CH), PDS (Urbach energy, defects) and CPM (degradation). The temperature dependent dark conductivity and AM 1 photoconductivity were measured using coplanar AI contacts. For rapid degradation the samples were held at 40°C and illuminated by a focused tungsten halogen lamp (800W) using a piece of the same sample as a filter to achieve homogeneous absorption. 3. RESULTS AND DISCUSSION The possibility of a heat rise of the growing film surface due to radiation can be checked by comparing the filament radiation absorbed and the emission of the substrate (Ts=673K). Under the assumption of a grey body, the filament and substrate emissions are Ff=¢w-os.T 4 and Fs=¢s.os.T~s respectively (~w, es " emissivities of tungsten and substrate surface; a s StefanBoltzmarm constan 0. Assuming an infinitely long wire the radiation intensity decreases linearly with ds!f, that is the intensity arrivin~ at the substrate decreases by a factor rfC(d~_f+~) 0"5. With Tf=2173K, T~=673K, filament radius rf=0.0125cm, substrate radius rs= lcm, ds_f=l.Scm , os=5.67-10-12W-cm-2.K-4 and the rough estimation ~w=~s the ratio of absorbed to emitted radiation at the substrate surface is: (If/Is).rfC(d2_f+~)°'5 =0.75, that is, both energy fluxes are near thermal equilibrium. The crucial parameters are ¢w and es, which additionally depend on hp. On the other hand a-Si:H absorbs less than
10% of the light intensity. Therefore we conclude that even at the minimum distance of ds_f= 1.5cm no significant film heating by radiation occurs. Another possible energy flux towards the substrate is caused by thermal conduction and by atomic hydrogen recombination, if Si and H are the primary radicals. At a gas pressure p=2.7Pa, with the reaction constant kH=2.68.10-12cm3-s'l[7] and mean gas temperature Tg=Ts=673K, the H-lifetime is ~'H=k.Tg/(kH-P) = 1.3.10-3s (k Boltzmann constant). The mean velocity v x perpendicular to the substrate surface (x-direction)is a function of gas flux and gas density. With our low gas density vx=l.2.104cm.s-l,givingapathlengthofH-radical of lrl=Vx.r~15cm. Taking into account the small rate constant of Si[8] (ksi < kH) the film precursors should be atomic Si and H at all distances ds_f investigated in this work. 50
~
~
1
L
40 ~ 30 ~ ,~ 20 ~ 10 0 , , . . . . . t500 1700 1900 2100
.-, 0~, -1 _ .
-8
2300
filament temp. [°C] Fig. 1: Deposition rate rd and tungsten content cw as a function of filament temperature. DL denotes the detection limit. (p=2.7Pa, ds_f=2cm) Figure 1 shows the deposition rate r d and the tungsten content c w versus the filament temperature Tf. r a increases almost linearly with Tf up to 1900°C and attains a saturation a little below 2000°C. A detectable tungsten contamination above 1950°C is observed. These results match with earlier findings [9] and ensure that the samples prepared at Tf=1660°C are free of tungsten. In order to reduce W incorporation into the films we have threaded a 0.2 thick W filament into a 0.5mm ceramic capillary (A1203). In this configuration even at elevated temperatures of Tf> 1900°C no film growth was observed, hence a catalytic process on the filament surface seems necessary as has been proposed by Matsumura[2,3]. In figures 2, 3 and 4b the influence of substrate-
85
R. Zedlitz et al. / Deposition of a-Si:H with hot-wire-technique
to-filament distance ds_f on film properties is shown. 50 40
-4
1.78
7-~-B
1.76 ,%
7
o
3O •+<.
•o 20
1.74 ~
I0
1.72
o
,
1.0
,
,
,
2.0
,
,.
3 0
(.)
1.80
,
2
5.0
r d decreases as (ds_f)-2, while the band gap Eg remains almost constant (fig.2), indicating a comparable hydrogen content c H for all samples, This is in agreement with the model mentioned above, that at all distances ds_f the same primary radicals contribute to the film growth and that T s does not change significantly due to radiation. ,-,,
~
""
6
o 54
4
r~
~
1.0
2
2'.0
3[0 ds-t [ c m ]
era
~
,
,
410
1
2
3
4
2
p r e s s u r e [Pa]
3
d,-¢ [cm]
4
5
Fig. 4: photo- and dark conductivity versus gas pressure (a) (ds_f=2cm) and ds_f (b) (p=2.7Pa).
reactor for optimum optoelectronic quality material are a relatively short substrate-to-filament spacing of ds_f=2cm and a gas pressure of p = 2 . 7 P a adjusted by a flow of 10sccm Sill 4. 6.0 ~ ~
4.0
=
o
,
-to
0
(log(a)=3.5) versus substrate-to-filament distance, (p=2.7Pa, T f = 1660°C)
,
qph
-12 + . . . . . . . . . . . . . . . . . .
Fig. 2: Deposition rate rd and optical band gap Eg
50
.~/
.<=. - e
d,_f [cm]
52
~ph
,.70
4 0
58
(b)
~
2.0
¢,/ ._~"~
0
5.0
Fig. 3: Urbach energy E o and ot0av= 1.2eV) as a function of ds_f. (p=2.7Pa, T f = 1660°C)
0.0
,
,
,
,
,
,
,
,
1.80
40.0
1.78
"~. 30.0
1.76
.-. '~'
m
20.0
1.74
1o.o
1.72
0.0
From the undeconvoluted PDS spectra of l # m thick samples a minimum in Urbach energy E o and defect absorption ct(hv=l.2eV) was observed for ds_f=2cm (fig.3). Also for the ratio of photo- aph to dark conductivity a d (fig.4b) at this distance a maximum of aph/a d ,.~ 105 was found. The correlation between conductivities and gas pressure p is shown in figure 4a. aph increases with p, whereas for a d a minimum at p = 2 . 7 P a is observed, The favorable deposition parameters in our
,
,
0
,
1
,
,
,
2 3 p r e s s u r e [Pa]
,
,
,
4
,
5
'~'
1.70
Fig. 5: Hydrogen content c H, deposition rate r d and band gap Eg (1og(~)=3.5) as function of gas pressure. (ds_f=2cm, Tf= 1660°C) Figure 5 shows that both the hydrogen content c H and the deposition rate rd increase linearly with gas pressure p. Due to the rising c H, Eg also increases as is already known from glow discharge films, c H for the highest pressure investigated (4.4Pa) is close to 6%, the value for GD films prepared at 400°C.
86
R. Zedlitz et al. / Deposition of a-Si:H with hot-wire-technique
The influence of gas pressure p and substrate-tofilament distance ds.f on deposition rate rd is easy to understand: r d is proportional to p (Sill 4 density) at constant ds.f; and r d is proportional to ds_2f for constant p. The hydrogen content c H is not determined alone by rd, since c H remains constant for different ds_f and increases to p. We propose the following mechanism to explain this effect: A balance between r d and the thermalization of the H atoms evaporated from the filament controls c H in the films. Upon increasing p, r a and simultaneously the thermalization of atomic H evaporated from the filament are enhanced resulting in less hydrogen abstraction from the growing surface; consequently c H increases with p. On the other hand, when (Is.f is decreased at constant p, r d increases and the thermalization decreases; hence c H remains constant since the more effective H abstraction by 'hotter' H atoms is balanced by the faster growth of the film. -6.0 DC
~-. < ,~
CPM on both samples prior to and after the degradation show that the defect absorption for HW samples increases by about 3 times for the HW and 10 times for the GD film. The lower hydrogen content and a more relaxed silicon network of the HW material has been regarded as being responsible for this effect[6]. 4. CONCLUSIONS In the present study we have demonstrated that a-Si:H films of good optoelectronic properties at high deposition rates (25A/s), high substrate temperature (400°C) and considerably low hydrogen content cr1=(2-6)% can be achieved by the HW method. The degradation of this material is less pronounced than in comparable glow discharge films. Hydrogen content is controlled by a balance of growth rate and H abstraction from the film surface. Tungsten contamination as well as substrate heating by radiation are negligible at filament temperatures below 1900°C. The dissociation of Sill 4 occurs by a catalytic process at the filament surface. ACKNOWLEDGEMENTS Technical assistance by H. Wagner and funding by the BMFT under contract no. 0328327-D is acknowledged.
-6.5
_q
-7.0
t t , 3 4 5 6 log(t. Is]) Fig. 6: Degradation of Iphoto during illumination for a HW and a device quality (Ts=225°C) GD sample. 0
~ 1
i 2
In figure 6 the degradation behaviour of the optimum HW material (ds_f=2cm, p=2.7Pa) is compared to a device quality film from glow discharge. Although the initial photocurrent Iphot o o f the HW sample is less, the degradation is slower and results in a higher endpoint. Measurements of
REFERENCES 1. H. Wiesmarm, A.K. Ghosh, T. McMahon, and M. Strongin, J.Appl.Phys. 50 (1979) 3752. 2. H. Matsumura, J.Appl.Phys. 65 (1989) 4396. 3. H. Matsumura, Appl.Phys.Lett. 51 (1987) 804. 4. J. Doyle, R. Robertson, G.H. Lin, M.Z. He, and A. Gallagher, J.Appl.Phys. 64 (1988) 3215. 5. A.H. Mahan, J. Carapella, B.P. Nelson, R.S. Crandall and I. Balberg, J.Appl.Phys. 69 (1991) 6728. 6. A.H. Mahan, M. Vanecek, AlP Conf.Proc. 234 (1991) 195. 7. M.J. Kushner, J.Appl.Phys. 62 (1987)2803. 8. M.E.Coltrin, R.J.Kee, and J.A.Miller, J.Electrochem.Soc. 131 (1984) 425. 9. C. Horbach, W. Beyer and H. Wagner, J.Non-Cryst.Sol. 137/138 (1991)661.