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Formation of microcrystalline structure in a-Si:H films prepared by RF sputtering
Journal of Non.CrystaUineSolids 59 & 60 (1983) 823-826 North-Holland Publishing Company
823
FORMATION OF MICROCRYSTALLINE STRUCTURE IN a-Si:H FILMS PREPARED BY RF SPUTTERING Mikio NODA, Hideki SHIMIZU, Hisashi KOHNOand Hiroyuki ISHIDA Aichi University of Education, 448 Kariya, Aichi, JAPAN. Formation of m i c r o c r y s t a l l i n e structure in a-Si:H films prepared by RF sputtering has been investigated by TEM and FT-IR. The Films deposited in pure H2 have g r a i n - l i k e structure and tend to c r y s t a l l i z e with increasing the gas pressure, while the films deposited in Ar containing 10 % H2 usually become amorphous. With increasing input power and the gas pressure, however, the l a t t e r films tend to have amerphous-microcrystalline mixed s t r u c t u r e , and corresponding to t h i s s t r u c t u r a l change, s h i f t of o p t i c a l absorption edge to lower energy and decreases of r e s i s t i v i t y and a c t i v a t i o n energy of i t s temperature dependence are observed. Considerable a t t e n t i o n has been paid to hydrogenated m i c r o c r y s t a l l i n e s i l i c o n (~C-Si:H) f i l m s , because of t h e i r high c o n d u c t i v i t y , high doping e f f i c i e n c y and suitable properties for window side layer of solar c e l l .
The ~C-Si:H films are
usually fabricated by high power glow discharge (GD) in hydrogen based si]ane. Reports on the formation of ~C-Si:H films by RF sputtering (SP) are, however, few, though t h e i r preparation conditions can change more widely than those of GD.
SP films prepared in He/H2 mixture investigated by Imura et a l . have shown
that the films c r y s t a l l i z e when p a r t i a l pressure of H2 is high.
Our previous
results have shown that SP films prepared in pure H2 are composed of fine grains and tend to have ~C structure with increasing the gas pressure 2.
These
results show p o s s i b i l i t i e s on the formation of uC structure for SP f i l m s , and then systematic i n v e s t i g a t i o n of the preparation conditions to make ~C structure is i n t e r e s t i n g .
In t h i s paper, trends of the formation of uC structure and
correlated some o p t i c a l and e l e c t r i c a l properties have been investigated by changing gas atomosphere, input power (Wi ) and gas pressure (Pg). Preparations of the samples were performed at a substrate temperature of 200°C in a diode SP system (ANELVA SPF-210A) as reported before 2'3.
The films
were deposited in pure H2 gas (SPH f i l m ) , or in Ar gas containing 10 % H2 (SPAH film).
Samples f o r measurements of IR absorption using a FT-IR (JEOL JIR-4OX)
were prepared on a s i l i c o n wafer substrate.
The films for observations by TEM
(JEOL-IOOB) were deposited on a KBr substrate, or on a carbon f i l m supported by copper g r i d .
The measuring and operating conditions of TEM and FT-IR were the
same as those reported before 3.
The electron micrographs were taken at under
focus p o s i t i o n and then low density portions in the films are w h i t i s h . 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
824
M. Noda et al. / Microcrystalline structure in a-Si.H Films
SPAH
SPH
500 A FIGURE 1 TEM micrographs of SPH and SPAH films prepared at Wi = 8 W/cm2 and Pg = 7 Pa. Figure 1 shows t y p i c a l TEM micrographs of the SPH and SPAH f i l m s . has g r a i n - l i k e structure.
The SPH f i l m With increasing
~T 3 E u
%
Pg, the grain size increased and the electron d i f f r a c t i o n pattern became sharp.
I-.-
Then the SPH films have g r a i n - l i k e pC structure.
While the SPAH f i l m has clear
c r a c k - l i k e voids.
The d i f f r a c t i o n patterns
of the SPAH films were amorphous even
~0
2400
2200 2000 1800 WAVENUMBER (cm-1)
though Pg was increased to 14 Pa. These results show that Ar used f o r sputtering introduces the voids and d i s t o r t i o n in the f i l m s . Figure 2 shows differences of IR absorption spectrum between the SPH and
FIGURE 2 IR absorption i n t e n s i t y ( I / t ) I n ( I / T ) in the range of stretching mode for SPH and SPAH films deposited under the same conditions as in Fig. I , where t is the thickness and T the transmittance of the f i l m s .
SPAH films in the range o f stretching mode. All the peaks containing the other modes could be assigned to SiH, SiH 2 and SiH 3 v i b r a t i o n modes reported by Lucovsky et al 4, as reported before 2.
The
peaks at 1994 and 2094 cm- I due to SiH and SiH 2 configurations are observed for -I the SPAH f i l m . While the SPH f i l m has two peaks at about 2100 and 2180 cm due to SiH2 and SiH3, but the peak due to SiH is not observed.
The SiH 2 and
SiH 3 peaks of the SPH f i l m and the SiH 2 peak of the SPAH f i l m increased with increasing the voids observed by TEM. These results show that Ar produces defects such as dangling bonds and then the SiH configuration is formed in the uniform portions of the SPAH f i l m as a r e s u l t of termination of the dangling bond by hydrogen.
While the SPH f i l m
which is not d i s t o r t e d by Ar has the g r a i n - l i k e ~C structure.
M. Noda et al. / Microcrystalline structure in a-Si.'H Films
825
. . . . . . . ;L
2.5 W/cm2
H
10 W/cm2
o
17.5 W/cm2 500 A
FIGURE 3 TEM micrographs of SPAH films prepared at Pg = 3 Pa, when Wi is changed from 2.5 to 17.5 W/cm2.
3 Pa I
14 Pa
I°
500 A
FIGURE 4 TEM micrograph of SPAH film prepared at Wi = 17.5 W/cm2 and Pg = 14 Pa.
FIGURE 5 Electron diffraction patterns of SPAH films deposited at Wi = 17.5 W/cm2, when Pg is changed from 3 to 14 Pa.
The SPAH films are usually amorphous due to the distortion by Ar as described above, but tend to have pC structure with increasing Wi .
As shown in Fig. 3,
the microstructure of SPAH films deposited at 3 Pa, which have more uniform structure comparedto the films deposited at higher pg3, changes from r e l a t i v e l y rough to uniform structure with increasing Wi .
Furthermore, at very high Wi of
17.5 W/cm2, aggregates of 10 to 100 A in size are observed. As shown in Fig. 4, the aggregates becomemore clear when Pg increases to 14 Pa.
Corresponding to
these structural changes, the electron diffraction pattern became sharp with increasing Wi and Pg, and changes from amorphous to uC patterns as shown in Fig. 5 when the clear aggregates appears. Dark f i e l d micrograph taken for the sample of Fig. 4 showedthat the clear aggregates are crystalline.
The IR absorption
due to SiH3 configuration, which appears strongly in SPH films, also appeared as a feature of the formation of the aggregates. These results mentioned above clearly show that the formation of pC phase in the amorphous phase takes place continuously with increasing Wi and Pg.
M. Noda et al. / Microcrystalline structure in a-Si.'H Films
826
600
i
i
i
i
~
i
i
5OO
i~ -5- \ "~,,, 10 Wlcrn2 ~
~
a
=0.21eVJ
E u
~300
u_ 6
2"5W/c m2~,~,
"'-,
(Ea:O-46eV),,~,~,.
= 200
!
v
~
Wlcm2
100
"~ i0
L 2.4
I X'' l" 2.8 3.2
I 3.6
IO001T (K-~)
%
I
q
1.6
I
tll
l
2.0
I
2.4
I
2.6
h V (eV) FIGURE 6 Optical absorption of SPAH films, when Wi is changed from 2.5 to 10 W/cm2.
FIGURE 7 Temperature dependence of conductivity of SPAH films, when Wi is changed from 2.5 to 10 W/cm2.
Corresponding to this formation of microcrystalline-amorphous mixed structure, s h i f t of optical absorption edge to lower energy and decreases of r e s i s t i v i t y and activation energy of its temperature dependence also take place continuously, as shown in Fig. 6 and 7, respectively. Then continuous control of the properties of the films are possible by u t i l i z i n g the formation of ~C phase. In summary, the SPH films have grain-like structure and tend to have ~C structure with increasing Pg due to the increase of the grain size. While the SPAH films tend to have microcrystalline-amorphous mixed structure when Wi and Pg are high. The formation of the ~C phase is continuous for the increases of Wi and Pg, and corresponding to this structural changes, properties of optical absorption and temperature depenoence of conductivity are also changes continuously. We would like to thank to Mr. Yoshichika IWAMOTOand Yuji SAIGO for t h e i r assistance. This work was supported p a r t i a l l y by THE ISHIDA FOUNDATION. REFERENCES I) T. Imura, K. Mogi and A. Hiraki, Solid State Commun. 40 (1981) 161. 2) H. Ishida, M. Noda and H. Shimizu, Jpn. J. Appl. Phys. 22 (1983) L 73. 3) M. Noda and H. Ishida, Jpn. J. Appl. Phys. 21 (1982) L 195. 4) G. Lucovsky, R. J. Nemanich and J. C. Knights, Phys. Rev. B 19 (1979) 2064.
823
FORMATION OF MICROCRYSTALLINE STRUCTURE IN a-Si:H FILMS PREPARED BY RF SPUTTERING Mikio NODA, Hideki SHIMIZU, Hisashi KOHNOand Hiroyuki ISHIDA Aichi University of Education, 448 Kariya, Aichi, JAPAN. Formation of m i c r o c r y s t a l l i n e structure in a-Si:H films prepared by RF sputtering has been investigated by TEM and FT-IR. The Films deposited in pure H2 have g r a i n - l i k e structure and tend to c r y s t a l l i z e with increasing the gas pressure, while the films deposited in Ar containing 10 % H2 usually become amorphous. With increasing input power and the gas pressure, however, the l a t t e r films tend to have amerphous-microcrystalline mixed s t r u c t u r e , and corresponding to t h i s s t r u c t u r a l change, s h i f t of o p t i c a l absorption edge to lower energy and decreases of r e s i s t i v i t y and a c t i v a t i o n energy of i t s temperature dependence are observed. Considerable a t t e n t i o n has been paid to hydrogenated m i c r o c r y s t a l l i n e s i l i c o n (~C-Si:H) f i l m s , because of t h e i r high c o n d u c t i v i t y , high doping e f f i c i e n c y and suitable properties for window side layer of solar c e l l .
The ~C-Si:H films are
usually fabricated by high power glow discharge (GD) in hydrogen based si]ane. Reports on the formation of ~C-Si:H films by RF sputtering (SP) are, however, few, though t h e i r preparation conditions can change more widely than those of GD.
SP films prepared in He/H2 mixture investigated by Imura et a l . have shown
that the films c r y s t a l l i z e when p a r t i a l pressure of H2 is high.
Our previous
results have shown that SP films prepared in pure H2 are composed of fine grains and tend to have ~C structure with increasing the gas pressure 2.
These
results show p o s s i b i l i t i e s on the formation of uC structure for SP f i l m s , and then systematic i n v e s t i g a t i o n of the preparation conditions to make ~C structure is i n t e r e s t i n g .
In t h i s paper, trends of the formation of uC structure and
correlated some o p t i c a l and e l e c t r i c a l properties have been investigated by changing gas atomosphere, input power (Wi ) and gas pressure (Pg). Preparations of the samples were performed at a substrate temperature of 200°C in a diode SP system (ANELVA SPF-210A) as reported before 2'3.
The films
were deposited in pure H2 gas (SPH f i l m ) , or in Ar gas containing 10 % H2 (SPAH film).
Samples f o r measurements of IR absorption using a FT-IR (JEOL JIR-4OX)
were prepared on a s i l i c o n wafer substrate.
The films for observations by TEM
(JEOL-IOOB) were deposited on a KBr substrate, or on a carbon f i l m supported by copper g r i d .
The measuring and operating conditions of TEM and FT-IR were the
same as those reported before 3.
The electron micrographs were taken at under
focus p o s i t i o n and then low density portions in the films are w h i t i s h . 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
824
M. Noda et al. / Microcrystalline structure in a-Si.H Films
SPAH
SPH
500 A FIGURE 1 TEM micrographs of SPH and SPAH films prepared at Wi = 8 W/cm2 and Pg = 7 Pa. Figure 1 shows t y p i c a l TEM micrographs of the SPH and SPAH f i l m s . has g r a i n - l i k e structure.
The SPH f i l m With increasing
~T 3 E u
%
Pg, the grain size increased and the electron d i f f r a c t i o n pattern became sharp.
I-.-
Then the SPH films have g r a i n - l i k e pC structure.
While the SPAH f i l m has clear
c r a c k - l i k e voids.
The d i f f r a c t i o n patterns
of the SPAH films were amorphous even
~0
2400
2200 2000 1800 WAVENUMBER (cm-1)
though Pg was increased to 14 Pa. These results show that Ar used f o r sputtering introduces the voids and d i s t o r t i o n in the f i l m s . Figure 2 shows differences of IR absorption spectrum between the SPH and
FIGURE 2 IR absorption i n t e n s i t y ( I / t ) I n ( I / T ) in the range of stretching mode for SPH and SPAH films deposited under the same conditions as in Fig. I , where t is the thickness and T the transmittance of the f i l m s .
SPAH films in the range o f stretching mode. All the peaks containing the other modes could be assigned to SiH, SiH 2 and SiH 3 v i b r a t i o n modes reported by Lucovsky et al 4, as reported before 2.
The
peaks at 1994 and 2094 cm- I due to SiH and SiH 2 configurations are observed for -I the SPAH f i l m . While the SPH f i l m has two peaks at about 2100 and 2180 cm due to SiH2 and SiH3, but the peak due to SiH is not observed.
The SiH 2 and
SiH 3 peaks of the SPH f i l m and the SiH 2 peak of the SPAH f i l m increased with increasing the voids observed by TEM. These results show that Ar produces defects such as dangling bonds and then the SiH configuration is formed in the uniform portions of the SPAH f i l m as a r e s u l t of termination of the dangling bond by hydrogen.
While the SPH f i l m
which is not d i s t o r t e d by Ar has the g r a i n - l i k e ~C structure.
M. Noda et al. / Microcrystalline structure in a-Si.'H Films
825
. . . . . . . ;L
2.5 W/cm2
H
10 W/cm2
o
17.5 W/cm2 500 A
FIGURE 3 TEM micrographs of SPAH films prepared at Pg = 3 Pa, when Wi is changed from 2.5 to 17.5 W/cm2.
3 Pa I
14 Pa
I°
500 A
FIGURE 4 TEM micrograph of SPAH film prepared at Wi = 17.5 W/cm2 and Pg = 14 Pa.
FIGURE 5 Electron diffraction patterns of SPAH films deposited at Wi = 17.5 W/cm2, when Pg is changed from 3 to 14 Pa.
The SPAH films are usually amorphous due to the distortion by Ar as described above, but tend to have pC structure with increasing Wi .
As shown in Fig. 3,
the microstructure of SPAH films deposited at 3 Pa, which have more uniform structure comparedto the films deposited at higher pg3, changes from r e l a t i v e l y rough to uniform structure with increasing Wi .
Furthermore, at very high Wi of
17.5 W/cm2, aggregates of 10 to 100 A in size are observed. As shown in Fig. 4, the aggregates becomemore clear when Pg increases to 14 Pa.
Corresponding to
these structural changes, the electron diffraction pattern became sharp with increasing Wi and Pg, and changes from amorphous to uC patterns as shown in Fig. 5 when the clear aggregates appears. Dark f i e l d micrograph taken for the sample of Fig. 4 showedthat the clear aggregates are crystalline.
The IR absorption
due to SiH3 configuration, which appears strongly in SPH films, also appeared as a feature of the formation of the aggregates. These results mentioned above clearly show that the formation of pC phase in the amorphous phase takes place continuously with increasing Wi and Pg.
M. Noda et al. / Microcrystalline structure in a-Si.'H Films
826
600
i
i
i
i
~
i
i
5OO
i~ -5- \ "~,,, 10 Wlcrn2 ~
~
a
=0.21eVJ
E u
~300
u_ 6
2"5W/c m2~,~,
"'-,
(Ea:O-46eV),,~,~,.
= 200
!
v
~
Wlcm2
100
"~ i0
L 2.4
I X'' l" 2.8 3.2
I 3.6
IO001T (K-~)
%
I
q
1.6
I
tll
l
2.0
I
2.4
I
2.6
h V (eV) FIGURE 6 Optical absorption of SPAH films, when Wi is changed from 2.5 to 10 W/cm2.
FIGURE 7 Temperature dependence of conductivity of SPAH films, when Wi is changed from 2.5 to 10 W/cm2.
Corresponding to this formation of microcrystalline-amorphous mixed structure, s h i f t of optical absorption edge to lower energy and decreases of r e s i s t i v i t y and activation energy of its temperature dependence also take place continuously, as shown in Fig. 6 and 7, respectively. Then continuous control of the properties of the films are possible by u t i l i z i n g the formation of ~C phase. In summary, the SPH films have grain-like structure and tend to have ~C structure with increasing Pg due to the increase of the grain size. While the SPAH films tend to have microcrystalline-amorphous mixed structure when Wi and Pg are high. The formation of the ~C phase is continuous for the increases of Wi and Pg, and corresponding to this structural changes, properties of optical absorption and temperature depenoence of conductivity are also changes continuously. We would like to thank to Mr. Yoshichika IWAMOTOand Yuji SAIGO for t h e i r assistance. This work was supported p a r t i a l l y by THE ISHIDA FOUNDATION. REFERENCES I) T. Imura, K. Mogi and A. Hiraki, Solid State Commun. 40 (1981) 161. 2) H. Ishida, M. Noda and H. Shimizu, Jpn. J. Appl. Phys. 22 (1983) L 73. 3) M. Noda and H. Ishida, Jpn. J. Appl. Phys. 21 (1982) L 195. 4) G. Lucovsky, R. J. Nemanich and J. C. Knights, Phys. Rev. B 19 (1979) 2064.