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Temperature dependence of glow discharge polycrystalline silicon films and thin film transistor
Journal of Non.Crystalline Solids 59 & 60 (1983) 1191-1194 North-Holland PublishingCompany
1191
TEMPERATURE DEPENDENCE OF GLOW DISCHARGE POLYCRYSTALLINE SILICON FILMS AND THIN FILM TRANSISTOR
Y. HIRAI, K. SAKAI, Y. OSADA, K. AIHARA AND T. NAKAGIRI CANON RESEARCH CENTER, CANON INC. Nakane, Meguro-ku, Tokyo 152, Japan
We observed that the conductivity of glow discharge polycrystalline silicon films changed from 10 -5 to 10 -7 (~'~-cm)-i at substrate temperature of about 350-400°C, which was associated with the change of activation energy. At substrate temperatures lower than about 350°C, the films showed broad ring patterns with randomly oriented microcrystallites, and at higher temperatures above 400°C the preferred (II0) orientation appeared. The field effect mobility of polycrystalline silicon increased as substrate temperature increased.
i. iNTRODUCTION A hydrogenated amorphous silicon (a-Si) is one of the attractive materials for the thin film transistor (TFT) switching devices on glass substrates [. However, a-Si is to{) small in mobility and unstable for TFT's to construct amplifiers or shiftregisters.
On the contrary, the mobility of polycrystalline
silicon is larger than that of a-Si and polycrystalline silicon is expected to form stable active devices.
Polycrystalline Si (P-Sf) or microcrystalline Si
was prepared by glow discharge method at lower substrate temperature 2 than by other methods such as CVD, LPCVD and MBD 3.
P-Si films grown on thermal oxidiz-
ed Si substrate at 500°C have thickness dependence of the conductivity, the activation energy and the field effect mobility 4.
In this paper we report
about the substrate temperature dependence of the electrical, crystalline and optical properties in P~Si films prepared by glow discharge method, together with the thickness dependence, and report about its application for thin film transistors.
2. PROPERTIES OF THE P-Si FILM
2.1. Sample preparation P-Si films were prepared by RF glow discharge decomposition of silane in a capacitive coupled 8ro~uh apparatus. shown in Table I.
The preparation condition of films is
The reaction chamber was evacuated with diffusion pump down
to about 1 x 10 -6 Torr.
Substrate temperature was measured at the surface of
the substrate by a small thermocouple.
We used Corning #7059 and quartz sub-
strates except for single crystal Si substrates used for IR absorption measurements.
0022-3093/83/0000--0000/$03.00 © 1983 North-Holland/Physical Society of Japan
1192
Y. Hirai et al. / Glow discharge polycrystalline silicon films TABLE 1 Preparation condition Substrate temperature Gas Flow rate Pressure Discharge power
250°C - 500°C SiH 4 1% diluted in H 2 40.0 sccm 0.05 Torr 20 W
2.2. Crystalline properties There was strong dependence of the crystalline structure of P-Si on substrate temperature and film thickness. tron diffraction temperatures
Fig.l(a)-(d)
show the reflection elec-
(RHEED) patterns from the films deposited at several different
(250-500°C).
As the substrate temperature is increased,
the elec-
tron diffraction pattern changes from a weak diffused ring to a ring with strong orientation. as in Fig.l(d),
The (220) diffused spots clearly appear at about 400°C,
and they show a preferred
(Ii0) oriented crystallite.
The
(Ii0) orientation appears at 250°C, and it becomes stronger with increasing substrate temperatures.
However,
only at about 400°C it becomes weaker owing
to inclined oriented crystallites.
It was observed to change in crystalline
properties at about 400°C.
(a)
(b)
(a)
(b)
(c)
(d)
(c)
(d)
FIGURE 1 RHEED patterns from P-Si films at various temperatures; (a) 250°C, (b) 350°C, (c) 400°C, (d) 500°C when the thickness is 0.25 ~m.
FIGURE 2 RHEED patterns from P-Si films with different thickness; (a) 0.05 ~m, Ts= 250°C, (b) 0.43 ~m, Ts=250°C, (c) 0.05 ~m, Ts=500°C and (d) 0.43 ~m, Ts=500°C.
The thickness dependence of crystalline
structure was found in films on
glass substrates as well as on thermal oxdized Si substrates. at 250 and 500°C show a considerable Fig.2(a)-(d).
Films deposited
difference in the diffraction patterns in
In films deposited at 250°C, the pattern shows almost no differ-
ence in the electron diffraction as the film thickness increases.
However,
500°C, a film 0.05 ~ m thick has a weak ring and a film 0.25 ~ m thick shows a strongly oriented pattern. The surface of these films was very smooth.
The average grain size was
at
Y. Hirai et al. / Glow discharge polycrystalline silicon films
determined
1193
from the half width of the (ii0) X-ray diffraction pattern by means
o of the Scherrer method. respectively.
The grain sizes at 250 and 500°C were 60 and 500 A,
X~ray diffraction measurements
showed that the diffraction peak
intensity became stronger and the half width became narrower as the substrate temperature
increased.
larger with increasing
It indicates that the grain size of P-Si becomes temperature.
2.3. Electrical and optical properties The dependence of electrical properties on substrate temperature and film thickness was observed.
As shown in Fig.3,
the conductivity changes at about
350-400°C from I0 ~5 to 10 -7 (~'2"cm)-1 and the activation energy from 0.32 to 0.6 (eV) at room temperature.
A plot of conductivity versus reciprocal
ature was straight line unlike the result previously reported 2.
temper-
The higher
conductivity and the lower acivation energy in the lower temperature region below 350°C agreed well with the results of microcrystalline earlier 5,6.
silicon reported
It was found that the conductivity and the activation energy in
the higher temperature
region above 400°C were 1-2 x 10 -7 ( ~ ' c m ) -I and 0.52-
0.63 (eV), respectively. The optical absorption ature increased,
coefficient was measured.
As the substrate temper-
the absorption edge shifted a little to lower energies.
As
compared with a-Si and crystalline Si, in higher energy region the absorption coefficient of P-Si films was larger than that of crystalline Si and smaller than that of a-Si.
Cl
~
In lower energy region,
T-..,. Eo
the absorption coefficient of P-Si
"~ 2.0
~ys,
S~U~CE
0.6 o LLI
~n
> ,6°
----"°
I
tJJ
INSU.ATOR
~ 1.0
0,2 ~ Z
0 u
1# 200
300
400
SUBSTRATE TEMR
/ 500
ILl
o
I s ( "C )
FIGURE 3 Conductivity and activation energy vs. subtrate temperature
9
~
l
o
4OO
I
45O SUBSTRATE TEMP.
I 5OO Ts ( ~ )
FIGURE 4 Field effect mobility u ~ vs. substrate temperature. FThe inset is the schematic structure of TFT.
1194
Y. Hirai et al. / Glow discharge polycrystalline silicon films
films was larger than that of a-Si and crystalline Si. The hydrogen contents had substrate temperature dependence and changed from 7% to about 0.3% as the substrate temperature
raised from 250 to 500°C.
3. P-Si TFT P-Si TFT's were prepared photolithographically conductor technology as previously reported 4. section through the TFT.
by standard crystalline semi-
The inset in Fig.4 shows a
The TFT has a coplanar structure with an upper gate,
a 20 ~ m channel length and a 650 ~ m channel width.
In the P-Si TFT composed of
a film prepared at 400°C, drain current in excess of 0.1 mA was obtained with a 30 V gate voltage and a 20 V source-drain voltage. current ~ 10 -7 (A).
The TFT has an off-
Fig.4 shows the field effect mobility UFE versus the sub-
strate temperatures between 400 and 500°C. with increasing substrate temperature.
It can be seen that UFE increases
It was also observed
that unlike an a-Si
TFT, the source-drain current I d and the threshold voltage VTH in the P-Si TFT did not change with time under steady application of a gate bias at a constant temperature.
It should be emphasized
that the stability of P-Si TFT was
extremely better than that of a-Si TFT, and the stability was of primary importance in application.
REFERENCES i) A. J. Snell, K. D. Mackenzie, W. E. Spear and P. G. Le Comber, Appl. Phys. 24 (1981) 357. 2) F. Morin and M. Morel, Appl. Phys. Lett.
35 (1979) 686.
3) M. Matsui, Y. Shiraki, Y. Katayama, K. L. I. Kobayashi, A. Shintani and E. ~ r u y a m a , Appl. Phys. Lett. 37 (1980) 936. 4) Y. Hirai, Y. Osada, T. Komatsu, S. Omata, K. Aihara and T. Nakagiri, Appl. Phys. Lett. 42 (1983) 701. 5) W. E. Spear, G. Willeke, P. G. Le Comber and A. G. Fitzgerald, JOURNAL DE PHYSIQUE, Colloque C4, supplement au n°10, Tome 42, Octobre 1981, P.C4-257. 6) Y. Mishima,
S. Miyazaki,
M. Hirose and Y. Osaka, Phil. Mag. 46 (1982) i.
1191
TEMPERATURE DEPENDENCE OF GLOW DISCHARGE POLYCRYSTALLINE SILICON FILMS AND THIN FILM TRANSISTOR
Y. HIRAI, K. SAKAI, Y. OSADA, K. AIHARA AND T. NAKAGIRI CANON RESEARCH CENTER, CANON INC. Nakane, Meguro-ku, Tokyo 152, Japan
We observed that the conductivity of glow discharge polycrystalline silicon films changed from 10 -5 to 10 -7 (~'~-cm)-i at substrate temperature of about 350-400°C, which was associated with the change of activation energy. At substrate temperatures lower than about 350°C, the films showed broad ring patterns with randomly oriented microcrystallites, and at higher temperatures above 400°C the preferred (II0) orientation appeared. The field effect mobility of polycrystalline silicon increased as substrate temperature increased.
i. iNTRODUCTION A hydrogenated amorphous silicon (a-Si) is one of the attractive materials for the thin film transistor (TFT) switching devices on glass substrates [. However, a-Si is to{) small in mobility and unstable for TFT's to construct amplifiers or shiftregisters.
On the contrary, the mobility of polycrystalline
silicon is larger than that of a-Si and polycrystalline silicon is expected to form stable active devices.
Polycrystalline Si (P-Sf) or microcrystalline Si
was prepared by glow discharge method at lower substrate temperature 2 than by other methods such as CVD, LPCVD and MBD 3.
P-Si films grown on thermal oxidiz-
ed Si substrate at 500°C have thickness dependence of the conductivity, the activation energy and the field effect mobility 4.
In this paper we report
about the substrate temperature dependence of the electrical, crystalline and optical properties in P~Si films prepared by glow discharge method, together with the thickness dependence, and report about its application for thin film transistors.
2. PROPERTIES OF THE P-Si FILM
2.1. Sample preparation P-Si films were prepared by RF glow discharge decomposition of silane in a capacitive coupled 8ro~uh apparatus. shown in Table I.
The preparation condition of films is
The reaction chamber was evacuated with diffusion pump down
to about 1 x 10 -6 Torr.
Substrate temperature was measured at the surface of
the substrate by a small thermocouple.
We used Corning #7059 and quartz sub-
strates except for single crystal Si substrates used for IR absorption measurements.
0022-3093/83/0000--0000/$03.00 © 1983 North-Holland/Physical Society of Japan
1192
Y. Hirai et al. / Glow discharge polycrystalline silicon films TABLE 1 Preparation condition Substrate temperature Gas Flow rate Pressure Discharge power
250°C - 500°C SiH 4 1% diluted in H 2 40.0 sccm 0.05 Torr 20 W
2.2. Crystalline properties There was strong dependence of the crystalline structure of P-Si on substrate temperature and film thickness. tron diffraction temperatures
Fig.l(a)-(d)
show the reflection elec-
(RHEED) patterns from the films deposited at several different
(250-500°C).
As the substrate temperature is increased,
the elec-
tron diffraction pattern changes from a weak diffused ring to a ring with strong orientation. as in Fig.l(d),
The (220) diffused spots clearly appear at about 400°C,
and they show a preferred
(Ii0) oriented crystallite.
The
(Ii0) orientation appears at 250°C, and it becomes stronger with increasing substrate temperatures.
However,
only at about 400°C it becomes weaker owing
to inclined oriented crystallites.
It was observed to change in crystalline
properties at about 400°C.
(a)
(b)
(a)
(b)
(c)
(d)
(c)
(d)
FIGURE 1 RHEED patterns from P-Si films at various temperatures; (a) 250°C, (b) 350°C, (c) 400°C, (d) 500°C when the thickness is 0.25 ~m.
FIGURE 2 RHEED patterns from P-Si films with different thickness; (a) 0.05 ~m, Ts= 250°C, (b) 0.43 ~m, Ts=250°C, (c) 0.05 ~m, Ts=500°C and (d) 0.43 ~m, Ts=500°C.
The thickness dependence of crystalline
structure was found in films on
glass substrates as well as on thermal oxdized Si substrates. at 250 and 500°C show a considerable Fig.2(a)-(d).
Films deposited
difference in the diffraction patterns in
In films deposited at 250°C, the pattern shows almost no differ-
ence in the electron diffraction as the film thickness increases.
However,
500°C, a film 0.05 ~ m thick has a weak ring and a film 0.25 ~ m thick shows a strongly oriented pattern. The surface of these films was very smooth.
The average grain size was
at
Y. Hirai et al. / Glow discharge polycrystalline silicon films
determined
1193
from the half width of the (ii0) X-ray diffraction pattern by means
o of the Scherrer method. respectively.
The grain sizes at 250 and 500°C were 60 and 500 A,
X~ray diffraction measurements
showed that the diffraction peak
intensity became stronger and the half width became narrower as the substrate temperature
increased.
larger with increasing
It indicates that the grain size of P-Si becomes temperature.
2.3. Electrical and optical properties The dependence of electrical properties on substrate temperature and film thickness was observed.
As shown in Fig.3,
the conductivity changes at about
350-400°C from I0 ~5 to 10 -7 (~'2"cm)-1 and the activation energy from 0.32 to 0.6 (eV) at room temperature.
A plot of conductivity versus reciprocal
ature was straight line unlike the result previously reported 2.
temper-
The higher
conductivity and the lower acivation energy in the lower temperature region below 350°C agreed well with the results of microcrystalline earlier 5,6.
silicon reported
It was found that the conductivity and the activation energy in
the higher temperature
region above 400°C were 1-2 x 10 -7 ( ~ ' c m ) -I and 0.52-
0.63 (eV), respectively. The optical absorption ature increased,
coefficient was measured.
As the substrate temper-
the absorption edge shifted a little to lower energies.
As
compared with a-Si and crystalline Si, in higher energy region the absorption coefficient of P-Si films was larger than that of crystalline Si and smaller than that of a-Si.
Cl
~
In lower energy region,
T-..,. Eo
the absorption coefficient of P-Si
"~ 2.0
~ys,
S~U~CE
0.6 o LLI
~n
> ,6°
----"°
I
tJJ
INSU.ATOR
~ 1.0
0,2 ~ Z
0 u
1# 200
300
400
SUBSTRATE TEMR
/ 500
ILl
o
I s ( "C )
FIGURE 3 Conductivity and activation energy vs. subtrate temperature
9
~
l
o
4OO
I
45O SUBSTRATE TEMP.
I 5OO Ts ( ~ )
FIGURE 4 Field effect mobility u ~ vs. substrate temperature. FThe inset is the schematic structure of TFT.
1194
Y. Hirai et al. / Glow discharge polycrystalline silicon films
films was larger than that of a-Si and crystalline Si. The hydrogen contents had substrate temperature dependence and changed from 7% to about 0.3% as the substrate temperature
raised from 250 to 500°C.
3. P-Si TFT P-Si TFT's were prepared photolithographically conductor technology as previously reported 4. section through the TFT.
by standard crystalline semi-
The inset in Fig.4 shows a
The TFT has a coplanar structure with an upper gate,
a 20 ~ m channel length and a 650 ~ m channel width.
In the P-Si TFT composed of
a film prepared at 400°C, drain current in excess of 0.1 mA was obtained with a 30 V gate voltage and a 20 V source-drain voltage. current ~ 10 -7 (A).
The TFT has an off-
Fig.4 shows the field effect mobility UFE versus the sub-
strate temperatures between 400 and 500°C. with increasing substrate temperature.
It can be seen that UFE increases
It was also observed
that unlike an a-Si
TFT, the source-drain current I d and the threshold voltage VTH in the P-Si TFT did not change with time under steady application of a gate bias at a constant temperature.
It should be emphasized
that the stability of P-Si TFT was
extremely better than that of a-Si TFT, and the stability was of primary importance in application.
REFERENCES i) A. J. Snell, K. D. Mackenzie, W. E. Spear and P. G. Le Comber, Appl. Phys. 24 (1981) 357. 2) F. Morin and M. Morel, Appl. Phys. Lett.
35 (1979) 686.
3) M. Matsui, Y. Shiraki, Y. Katayama, K. L. I. Kobayashi, A. Shintani and E. ~ r u y a m a , Appl. Phys. Lett. 37 (1980) 936. 4) Y. Hirai, Y. Osada, T. Komatsu, S. Omata, K. Aihara and T. Nakagiri, Appl. Phys. Lett. 42 (1983) 701. 5) W. E. Spear, G. Willeke, P. G. Le Comber and A. G. Fitzgerald, JOURNAL DE PHYSIQUE, Colloque C4, supplement au n°10, Tome 42, Octobre 1981, P.C4-257. 6) Y. Mishima,
S. Miyazaki,
M. Hirose and Y. Osaka, Phil. Mag. 46 (1982) i.