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Amorphous-to-crystalline transition of indium oxide films deposited by reactive evaporation
Thin Solid Films, 151 (1987) 2 1S-222 PREPARATION
215
AND CHARACTERIZATION
AMORPHOUS-TO-CRYSTALLINE TRANSITION OF INDIUM FILMS DEPOSITED BY REACTIVE EVAPORATION K. ITO, T. NAKAZAWA Department (Received
of Elecfronics,
OXIDE
AND K. OSAKI Faculty of Engineering, Shinshu University. Naguno 380 (Japan
July 1, 1986; revised December
15, 1986; accepted
March
J
17, 1987)
Amorphous indium oxide films deposited by reactive evaporation of indium in oxygen onto unheated glass substrates were transformed into polycrystalline films by annealing. It was found that the oxygen pressure during deposition affected the optical and electrical characteristics of the polycrystalline films as follows. The film deposited at a low oxygen pressure (0.02 Pa) and subsequently crystallized by annealing had high transmittance and resistivity. In contrast, the film deposited at a high oxygen pressure (0.2 Pa) had low transmittance and resistivity after the amorphous-to-crystalline transition which was accompanied by precipitation of metallic indium in the film. The optimum transparent conductive film which had a resistivity of 2.1 x lo- 3 R cm and a transmittance of 84% or higher was obtained from an amorphous indium oxide film which was deposited at an oxygen pressure of 0.07 Pa and then annealed in nitrogen at 250 “C.
1. INTRODUCTION
Indium oxide is one of the most popular materials for transparent conductive films. Indium oxide films, which are usually doped with tin, have been fabricated by sputteringlp4, electron beam deposition’ or ion beam sputtering6. Pan and Ma’, however, deposited undoped In,O, film by reactive evaporation of a mixture of indium and In,O,. We have used In,O, films deposited by reactive evaporation of indium as windows for heterojunction solar cellss-‘O. The reactive evaporation method has an advantage over the first three methods because it does not cause radiation damage to the substrates. Polycrystalline transparent conductive films can be obtained by heating the substrates to about 200 “C during deposition, but this process causes some solar cells to be less efficient, as reported by Ito and Nakazawa9~io. These researchers showed, however, that solar cell performance was not degraded if an amorphous indium oxide film with low conductivity and transmittance is initially deposited onto the unheated substrate and subsequently transformed into a crystalline transparent conductive film by annealing. In this work we studied the amorphous-to-crystalline transition of the indium oxide films deposited by reactive evaporation in order to determine the optimum 0040-6090/87/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
216
K. ITO, T. NAKAZAWA,
evaporation and annealing conditions under which the amorphous transformed into a good window film for solar cells. 2.
EXPERIMENTAL
K. OSAKI
film can be
PROCEDURE
Indium oxide films were deposited by reactive evaporation of indium. The vacuum system used was provided with a variable leak valve through which pure oxygen was bled. The evaporation boat consisted of a quartz crucible heated by a tantalum heater. Metallic indium (purity 99.999%) was charged in the crucible as the source material. Glass microscope slides were used as substrates. The substrate was loaded in the vacuum chamber after being degreased ultrasonically in organic solvents. The vacuum chamber was initially evacuated to 4 x 10e3 Pa using a diffusion pump. Oxygen was then admitted to the chamber. By adjusting the valve the chamber pressure could be varied between 0.02 and 0.2 Pa during deposition. The source temperature was measured using an alumel-chrome1 thermocouple sheathed in a thin quartz tube and was maintained at 800 “C. The deposition rate of the film was about 5 nm mini. The thickness of the films was in the range 70-l 50 nm. The deposited films were annealed isochronally (for 30 min) in a nitrogen flow at temperatures from 100 to 300 “C. After rapidly cooling the films to room temperature we measured various properties of the films as follows. The X-ray diffraction in the film was measured using an X-ray diffractometer (Rigaku Denki RAD-1A). The spectral transmittance of the film was measured in the visible region using a spectrophotometer (Shimadzu MPS-5000). The resistivity and the Hall effect were measured using the four-point probe and Van der Pauw methods respectively. 3.
RESULTS
3.1. X-ray diffraction Figure 1 shows the effects of annealing temperature on the X-ray diffraction curves of the indium oxide film deposited at an oxygen pressure POxYof 0.07 Pa. The as-deposited films were amorphous because no sharp peak could be found in the
12221 annealed
at 250-C
IL001 2
J\
LO
30 2 0
(degree)
Fig. 1. X-ray diffraction
patterns for the film deposited
at P,,., = 0.07 Pa.
AMORPHOUS-TO-CRYSTALLINE
TRANSITION
OF I@,
217
FILMS
diffraction curve. No change was observed in the diffraction patterns on annealing the films at temperatures lower than 200 “C. However, after annealing at 250 “C the (222) and (400) peaks of In,O, emerged from the diffuse diffraction background. The crystallization temperature T, is defined as the annealing temperature at which the diffraction peaks of In,O, emerge. The value of T, increased from 200 to 300 “C on increasing Pox,, from 0.02 to 0.2 Pa. Successive annealing at 300 “C had no effect on the diffraction peak heights. We chose 30 min for the annealing time because observations made in situ showed that an irreversible change in the resistivity of the film was almost completed after annealing at a temperature of lOO-300°C for several minutes. Table I shows the relationship between Poxy and the X-ray diffraction peak heights of In,0,(222) and In,0,(400). The (222) peak height for the film deposited at Poxy of 0.02 Pa was considerably less than that for the film deposited at higher Pox,, (0.04-0.2 Pa). In particular, the film deposited at 0.07 Pa showed the largest ratio of the (222) peak height to the (400) peak height: the (222) plane of crystallites of In,O, would be preferentially oriented parallel to the substrate surface. This film had the most suitable properties for a transparent and conductive layer, as described in Section 3.2. TABLE
I
RELATION
BETWEEN
THE
X-RAY
DIFFRACTION
PEAK
HEIGHT
AND
THE
OXYGEN
PRESSURE
Poxy DURING
DEPOSITION
Peak height” (counts s - ‘)
0.02 0.04 0.07 0.09 0.2
200 250 250 250 300
a The heights of the (222) and (400) peaks of In,03
(2221
(400)
620 3300 4600 1540 1260
270 290 300 490 510
were measured
after annealing
at T,
Another effect of PO._was as follows. A (101) diffraction peak corresponding to metallic indium was observed for the as-deposited film produced at Poxy = 0.02 Pa. This peak disappeared after the film was annealed at T,. The height of the In(lO1) peak was comparable with that of the In,0,(222) peak which emerged after crystallization. The indium was oxidized by a trace amount of oxygen contained in the annealing atmosphere of nitrogen. When the film was annealed and crystallized in a vacuum of about 4 x 10m3 Pa, the metallic indium still remained in the stoichiometric polycrystalline film. In the case of the film deposited at Poxy = 0.2 Pa, however, the peak of metallic indium was observed only after crystallization: the amorphous-to-crystalline transition of the film was accompanied by precipitation of indium. 3.2. Electrical characteristics and transmittance The resistivity p of the as-deposited films depends on Pox,,, as shown in Fig. 2. It
218
K. ITO, T. NAKAZAWA,
K. OSAKI
10 -
1 -
-E c Q
Ol-
!
001' 001
I
,!
05
005 01
P oxy
Fig. 2. Resistivity
(PaI
of as-deposited
films us. oxygen pressure
during deposition.
increased as Poxr increased from 0.01 to 0.09 Pa. The high resistivity of up to 1.5 R cm is most probably caused by low carrier mobility in these amorphous films. Figure 3 shows the spectral transmittance of the indium oxide films. The asdeposited films showed low transmittance (about 10% at a wavelength of 500 nm) which tended to increase with increasing wavelength. Annealing at temperatures
(a)
WAVELENGTH
WAVELENGTII
(nmi
WAVELENGTH
1 nml
(b)
inmi
(cl
Fig. 3. Per cent transmission (c) 0.2 Pa.
of the films deposited
at various
oxygen pressures:
(a) 0.02 Pa;(b)
0.07 Pa;
AMORPHOUS-TO-CRYSTALLINE
TRANSITION
OF II@,
219
FILMS
lower than z caused only a relatively small increase in the transmittance over the whole wavelength range. After annealing at T,, however, a significant increase in the transmittance was observed, particularly in short wavelength regions (300-500 nm). The films deposited at oxygen pressures of 0.02 and 0.07 Pa showed a transmittance higher than 70% for a wavelength of 500 nm after annealing at T, (see Figs. 3(a) and of less 3(b)). In contrast, the film deposited at Pox,,= 0.2 Pa showed a transmittance than 30% even after crystallization (see Fig. 3(c)). The indium oxide films deposited by reactive evaporation at lower Poxymay be useful for unerasable optical recording because the transmittance of the film changes greatly the moment the amorphousto-crystalline transition occurs. Hebard et al.” successfully applied In/InO, composites to erasable optical recording using a reversible phase change in the thin film composites prepared by reactive ion beam sputter deposition. Figure 4 shows the effect of annealing temperature on the resistivity. The resistivity of the films deposited at oxygen pressures of 0.2 and 0.07 Pa decreased with increasing annealing temperature. After annealing at T, a more pronounced decrease in the resistivity was observed owing to crystallization. In contrast, the resistivity ofthe film deposited at Pox,,= 0.02 Pa decreased slightly after annealing at q. In any case, the films showed the lowest resistivity after annealing at T,. Successive annealing at temperatures above T, caused a slight increase in the resistivity. The Hall effect of the films annealed at 300 “C was investigated. The results are shown in Fig. 5. The films deposited at oxygen pressures of 0.07 Pa or higher had a concentration of Hall mobility of about 10cm2 V-’ SK’ and an electron (5-8) x 102’ cm-3. The Hall effect of the film deposited at P,,,, = 0.02 Pa was too small to be measured. The film deposited at low Poxy(0.04 Pa) had a lower electron r
1
0
1c
10
z
c
-2
a lo-
;
10
Q 10. 1c lo-
-*
*
ANNEALING
1 200
100
TEMPERATURE x as-deposi ted
Fig. 4. Effect of annealing Fig. 5. Hall mobility oxygen pressure.
300 i’C)
on the resistivity
P oxy
of films deposited
k (A), carrier concentration
at various
n (0) and resistivity
(Pa)
oxygen pressures.
p (0) of film annealed
at 300 “C vs.
74
(223) x10-j
Thermal
Evaporation
In
In,% In
Thermal evaporation
and annealing
from electron gun
evaporation
40
+ In
2 x 1om4
In@, In,O,
3x10-3 4 x 1om4
60-95 10
2.1 x 10e3
25-60
-
(8816) x 1o-4
1.34x10-3
P
(cm* V-i s- ‘)
In,O,
P
T~F
(Q cm)
D.c. sputtering Thermal evaporation Thermal evaporation Thermal evaporation
ANDTRANSMITTANCE
In,% In
technique
MOBILITY~
Deposition
~,ELE~TK~N
Source
RESI~T~~IT~P,ELE~TRON~~N~ENTRAT~~N
TABLE II
3 x 1020
(0.5-1.1) x 10”
4.69 x 10”
3x1020
;Cm-3)
FILMSREPORTEDBY
84
85
15
90
60
96
71
(L)
and Sakatai5 Ovadyahu et al.‘” Present work
Noguchi
Pan and Ma’
Mizuhashi“’
Fraser and Cooki’ Nath and Bunshaht3
Reseurcher(s)
VARIOUSRESEARCHERS
AMORPHOUS-TO-CRYSTALLINE
TRANSITION
OF II@,
FILMS
221
concentration. It is thought that oxygen vacancies in this film are annealed out because the film deposited at low I’,,,, can be oxidized easily, as described previously. The electron mobility shown in Fig. 5 is smaller than that of In,03 films deposited onto heated substrates by other workers (Table II). 4.
DISCUSSION
Evidence has been given in Section 3 that the oxygen pressure Poxy is an important parameter which affects the electrical and optical properties of the indium oxide films. The effect of Poxyis discussed below. As shown in Fig. 2, the resistivity of the as-deposited films depends strongly on Poxy.Hebard and Nakahara reported similar results for reactive ion beam sputterdeposited indiumjindium oxide films. i’ They argue that the pressure dependence of the resistivity can be attributed to a change in the microscopic crystalline phase of the films. The as-deposited film produced at low Poxycontains metallic indium which is dispersed as fine particles in the amorphous indium oxide matrix. The electrical conduction of the film is probably caused by electrons tunnelling between the metallic particles. The excess metallic indium is left in the crystalline In,O, matrix of the film deposited at high Poxr (0.2 Pa), causing the resistivity and transmittance of the polycrystalline film to be low. When the film was annealed in an oxygen flow the transmittance reached 90% or higher, but the resistivity was of the order of lo-’ R cm after crystallization. Table II shows the electrical and optical properties of In,O, films obtained so far by various workers. The resistivity of our recrystallized film, which can be applied to solar cells, is comparable with that of the film deposited onto a heated substrate by thermal evaporation ofindium’3”5 but is higher than that of In,O, l4 or a mixture of In,03 and indium’. The recrystallized film has the medium transmittance in comparison with the other films. 5.
CONCLUSIONS
To prepare transparent and conductive films suitable for heterojunction solar cells, the amorphous-to-crystalline transition was investigated for indium oxide films deposited by reactive evaporation of indium in oxygen. Annealing of the amorphous films at 200-300 “C gave rise to the transition. We found that the oxygen pressure Poxyduring deposition had a pronounced effect on the electrical and optical characteristics of the polycrystalline films. When the film was deposited at Poxy= 0.2 Pa the resistivity reached 5 x 10e4 Q cm but the optical transmission was lower than 30% after annealing at the crystallization temperature. When the film was deposited at Poxy= 0.02 Pa the transmittance reached 70% or higher for visible light but the resistivity was of the order of 1O-2 S2cm. The optimum transparent conductive film can be obtained by annealing an amorphous indium oxide film deposited at an intermediate oxygen pressure of 0.07 Pa. After crystallization the film had a resistivity of 2.1 x 10e3 Q cm and a transmittance of 84% at a wavelength of 500 nm.
222
K. ITO, T. NAKAZAWA,
K. OSAKI
ACKNOWLEDGMENT
This work was partially
supported
by the Hoso Bunka Foundation.
REFERENCES
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17
W. M. Molzen, J. VW. Sri. Technol., 12(1975)99. J. C. C. Fan and F. J. Bachner, J. Electrochem. Sot., 122 (1975) 1719. W. G. Haines and R. H. Bube, J. Appl. Phys., 49 (1978) 304. M. Buchanan, J. B. Webb and D. F. Williams, Appl. Phys. Left., 37(1980) 213. I. Hamberg and C. G. Granqvist, Appl. Phys. Left., 44 (1984) 721. J. C. C. Fan, Appl. Phys. Left., 34 (1979) 515. C. A. Pan and T. P. Ma, J. Electrochem. Sot., 128 (1981) 1953. K. Ito and T. Nakazawa, Surf. Sci., 86 (1979) 492. K. Ito and T. Nakazawa, Tech. Dig. 1st Int. Photovoltaic Science and Engineering Conf., Times, Tokyo, 1984, p. 759. K. Ito and T. Nakazawa, J. Appl. Phys., 58 (1985) 2638. A. F. Hebard, G. E. Blonder and S. Y. Sub, Appl. Phys. Lett., 44 (1984) 1023. D. B. Fraser and H. D. Cook, J. Electrochem. Sot., 119 (1972) 1369. P. Nath and R. F. Bunshah, Thin Solid Films, 69 (1980) 63. M.Mizuhashi, ThinSolidFilms,70(1980)91. S. Noguchi and H. Sakata, J. Phys. D, 13 (1980) 1129. Z. Ovadyahu, B. Ovryn and H. W. Kraner, J. Electrochem. Sot., 130 (1983) 917. A. F. Hebard and S. Nakahara, Appl. Phys. Lett., 41(1982) 1130.
215
AND CHARACTERIZATION
AMORPHOUS-TO-CRYSTALLINE TRANSITION OF INDIUM FILMS DEPOSITED BY REACTIVE EVAPORATION K. ITO, T. NAKAZAWA Department (Received
of Elecfronics,
OXIDE
AND K. OSAKI Faculty of Engineering, Shinshu University. Naguno 380 (Japan
July 1, 1986; revised December
15, 1986; accepted
March
J
17, 1987)
Amorphous indium oxide films deposited by reactive evaporation of indium in oxygen onto unheated glass substrates were transformed into polycrystalline films by annealing. It was found that the oxygen pressure during deposition affected the optical and electrical characteristics of the polycrystalline films as follows. The film deposited at a low oxygen pressure (0.02 Pa) and subsequently crystallized by annealing had high transmittance and resistivity. In contrast, the film deposited at a high oxygen pressure (0.2 Pa) had low transmittance and resistivity after the amorphous-to-crystalline transition which was accompanied by precipitation of metallic indium in the film. The optimum transparent conductive film which had a resistivity of 2.1 x lo- 3 R cm and a transmittance of 84% or higher was obtained from an amorphous indium oxide film which was deposited at an oxygen pressure of 0.07 Pa and then annealed in nitrogen at 250 “C.
1. INTRODUCTION
Indium oxide is one of the most popular materials for transparent conductive films. Indium oxide films, which are usually doped with tin, have been fabricated by sputteringlp4, electron beam deposition’ or ion beam sputtering6. Pan and Ma’, however, deposited undoped In,O, film by reactive evaporation of a mixture of indium and In,O,. We have used In,O, films deposited by reactive evaporation of indium as windows for heterojunction solar cellss-‘O. The reactive evaporation method has an advantage over the first three methods because it does not cause radiation damage to the substrates. Polycrystalline transparent conductive films can be obtained by heating the substrates to about 200 “C during deposition, but this process causes some solar cells to be less efficient, as reported by Ito and Nakazawa9~io. These researchers showed, however, that solar cell performance was not degraded if an amorphous indium oxide film with low conductivity and transmittance is initially deposited onto the unheated substrate and subsequently transformed into a crystalline transparent conductive film by annealing. In this work we studied the amorphous-to-crystalline transition of the indium oxide films deposited by reactive evaporation in order to determine the optimum 0040-6090/87/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
216
K. ITO, T. NAKAZAWA,
evaporation and annealing conditions under which the amorphous transformed into a good window film for solar cells. 2.
EXPERIMENTAL
K. OSAKI
film can be
PROCEDURE
Indium oxide films were deposited by reactive evaporation of indium. The vacuum system used was provided with a variable leak valve through which pure oxygen was bled. The evaporation boat consisted of a quartz crucible heated by a tantalum heater. Metallic indium (purity 99.999%) was charged in the crucible as the source material. Glass microscope slides were used as substrates. The substrate was loaded in the vacuum chamber after being degreased ultrasonically in organic solvents. The vacuum chamber was initially evacuated to 4 x 10e3 Pa using a diffusion pump. Oxygen was then admitted to the chamber. By adjusting the valve the chamber pressure could be varied between 0.02 and 0.2 Pa during deposition. The source temperature was measured using an alumel-chrome1 thermocouple sheathed in a thin quartz tube and was maintained at 800 “C. The deposition rate of the film was about 5 nm mini. The thickness of the films was in the range 70-l 50 nm. The deposited films were annealed isochronally (for 30 min) in a nitrogen flow at temperatures from 100 to 300 “C. After rapidly cooling the films to room temperature we measured various properties of the films as follows. The X-ray diffraction in the film was measured using an X-ray diffractometer (Rigaku Denki RAD-1A). The spectral transmittance of the film was measured in the visible region using a spectrophotometer (Shimadzu MPS-5000). The resistivity and the Hall effect were measured using the four-point probe and Van der Pauw methods respectively. 3.
RESULTS
3.1. X-ray diffraction Figure 1 shows the effects of annealing temperature on the X-ray diffraction curves of the indium oxide film deposited at an oxygen pressure POxYof 0.07 Pa. The as-deposited films were amorphous because no sharp peak could be found in the
12221 annealed
at 250-C
IL001 2
J\
LO
30 2 0
(degree)
Fig. 1. X-ray diffraction
patterns for the film deposited
at P,,., = 0.07 Pa.
AMORPHOUS-TO-CRYSTALLINE
TRANSITION
OF I@,
217
FILMS
diffraction curve. No change was observed in the diffraction patterns on annealing the films at temperatures lower than 200 “C. However, after annealing at 250 “C the (222) and (400) peaks of In,O, emerged from the diffuse diffraction background. The crystallization temperature T, is defined as the annealing temperature at which the diffraction peaks of In,O, emerge. The value of T, increased from 200 to 300 “C on increasing Pox,, from 0.02 to 0.2 Pa. Successive annealing at 300 “C had no effect on the diffraction peak heights. We chose 30 min for the annealing time because observations made in situ showed that an irreversible change in the resistivity of the film was almost completed after annealing at a temperature of lOO-300°C for several minutes. Table I shows the relationship between Poxy and the X-ray diffraction peak heights of In,0,(222) and In,0,(400). The (222) peak height for the film deposited at Poxy of 0.02 Pa was considerably less than that for the film deposited at higher Pox,, (0.04-0.2 Pa). In particular, the film deposited at 0.07 Pa showed the largest ratio of the (222) peak height to the (400) peak height: the (222) plane of crystallites of In,O, would be preferentially oriented parallel to the substrate surface. This film had the most suitable properties for a transparent and conductive layer, as described in Section 3.2. TABLE
I
RELATION
BETWEEN
THE
X-RAY
DIFFRACTION
PEAK
HEIGHT
AND
THE
OXYGEN
PRESSURE
Poxy DURING
DEPOSITION
Peak height” (counts s - ‘)
0.02 0.04 0.07 0.09 0.2
200 250 250 250 300
a The heights of the (222) and (400) peaks of In,03
(2221
(400)
620 3300 4600 1540 1260
270 290 300 490 510
were measured
after annealing
at T,
Another effect of PO._was as follows. A (101) diffraction peak corresponding to metallic indium was observed for the as-deposited film produced at Poxy = 0.02 Pa. This peak disappeared after the film was annealed at T,. The height of the In(lO1) peak was comparable with that of the In,0,(222) peak which emerged after crystallization. The indium was oxidized by a trace amount of oxygen contained in the annealing atmosphere of nitrogen. When the film was annealed and crystallized in a vacuum of about 4 x 10m3 Pa, the metallic indium still remained in the stoichiometric polycrystalline film. In the case of the film deposited at Poxy = 0.2 Pa, however, the peak of metallic indium was observed only after crystallization: the amorphous-to-crystalline transition of the film was accompanied by precipitation of indium. 3.2. Electrical characteristics and transmittance The resistivity p of the as-deposited films depends on Pox,,, as shown in Fig. 2. It
218
K. ITO, T. NAKAZAWA,
K. OSAKI
10 -
1 -
-E c Q
Ol-
!
001' 001
I
,!
05
005 01
P oxy
Fig. 2. Resistivity
(PaI
of as-deposited
films us. oxygen pressure
during deposition.
increased as Poxr increased from 0.01 to 0.09 Pa. The high resistivity of up to 1.5 R cm is most probably caused by low carrier mobility in these amorphous films. Figure 3 shows the spectral transmittance of the indium oxide films. The asdeposited films showed low transmittance (about 10% at a wavelength of 500 nm) which tended to increase with increasing wavelength. Annealing at temperatures
(a)
WAVELENGTH
WAVELENGTII
(nmi
WAVELENGTH
1 nml
(b)
inmi
(cl
Fig. 3. Per cent transmission (c) 0.2 Pa.
of the films deposited
at various
oxygen pressures:
(a) 0.02 Pa;(b)
0.07 Pa;
AMORPHOUS-TO-CRYSTALLINE
TRANSITION
OF II@,
219
FILMS
lower than z caused only a relatively small increase in the transmittance over the whole wavelength range. After annealing at T,, however, a significant increase in the transmittance was observed, particularly in short wavelength regions (300-500 nm). The films deposited at oxygen pressures of 0.02 and 0.07 Pa showed a transmittance higher than 70% for a wavelength of 500 nm after annealing at T, (see Figs. 3(a) and of less 3(b)). In contrast, the film deposited at Pox,,= 0.2 Pa showed a transmittance than 30% even after crystallization (see Fig. 3(c)). The indium oxide films deposited by reactive evaporation at lower Poxymay be useful for unerasable optical recording because the transmittance of the film changes greatly the moment the amorphousto-crystalline transition occurs. Hebard et al.” successfully applied In/InO, composites to erasable optical recording using a reversible phase change in the thin film composites prepared by reactive ion beam sputter deposition. Figure 4 shows the effect of annealing temperature on the resistivity. The resistivity of the films deposited at oxygen pressures of 0.2 and 0.07 Pa decreased with increasing annealing temperature. After annealing at T, a more pronounced decrease in the resistivity was observed owing to crystallization. In contrast, the resistivity ofthe film deposited at Pox,,= 0.02 Pa decreased slightly after annealing at q. In any case, the films showed the lowest resistivity after annealing at T,. Successive annealing at temperatures above T, caused a slight increase in the resistivity. The Hall effect of the films annealed at 300 “C was investigated. The results are shown in Fig. 5. The films deposited at oxygen pressures of 0.07 Pa or higher had a concentration of Hall mobility of about 10cm2 V-’ SK’ and an electron (5-8) x 102’ cm-3. The Hall effect of the film deposited at P,,,, = 0.02 Pa was too small to be measured. The film deposited at low Poxy(0.04 Pa) had a lower electron r
1
0
1c
10
z
c
-2
a lo-
;
10
Q 10. 1c lo-
-*
*
ANNEALING
1 200
100
TEMPERATURE x as-deposi ted
Fig. 4. Effect of annealing Fig. 5. Hall mobility oxygen pressure.
300 i’C)
on the resistivity
P oxy
of films deposited
k (A), carrier concentration
at various
n (0) and resistivity
(Pa)
oxygen pressures.
p (0) of film annealed
at 300 “C vs.
74
(223) x10-j
Thermal
Evaporation
In
In,% In
Thermal evaporation
and annealing
from electron gun
evaporation
40
+ In
2 x 1om4
In@, In,O,
3x10-3 4 x 1om4
60-95 10
2.1 x 10e3
25-60
-
(8816) x 1o-4
1.34x10-3
P
(cm* V-i s- ‘)
In,O,
P
T~F
(Q cm)
D.c. sputtering Thermal evaporation Thermal evaporation Thermal evaporation
ANDTRANSMITTANCE
In,% In
technique
MOBILITY~
Deposition
~,ELE~TK~N
Source
RESI~T~~IT~P,ELE~TRON~~N~ENTRAT~~N
TABLE II
3 x 1020
(0.5-1.1) x 10”
4.69 x 10”
3x1020
;Cm-3)
FILMSREPORTEDBY
84
85
15
90
60
96
71
(L)
and Sakatai5 Ovadyahu et al.‘” Present work
Noguchi
Pan and Ma’
Mizuhashi“’
Fraser and Cooki’ Nath and Bunshaht3
Reseurcher(s)
VARIOUSRESEARCHERS
AMORPHOUS-TO-CRYSTALLINE
TRANSITION
OF II@,
FILMS
221
concentration. It is thought that oxygen vacancies in this film are annealed out because the film deposited at low I’,,,, can be oxidized easily, as described previously. The electron mobility shown in Fig. 5 is smaller than that of In,03 films deposited onto heated substrates by other workers (Table II). 4.
DISCUSSION
Evidence has been given in Section 3 that the oxygen pressure Poxy is an important parameter which affects the electrical and optical properties of the indium oxide films. The effect of Poxyis discussed below. As shown in Fig. 2, the resistivity of the as-deposited films depends strongly on Poxy.Hebard and Nakahara reported similar results for reactive ion beam sputterdeposited indiumjindium oxide films. i’ They argue that the pressure dependence of the resistivity can be attributed to a change in the microscopic crystalline phase of the films. The as-deposited film produced at low Poxycontains metallic indium which is dispersed as fine particles in the amorphous indium oxide matrix. The electrical conduction of the film is probably caused by electrons tunnelling between the metallic particles. The excess metallic indium is left in the crystalline In,O, matrix of the film deposited at high Poxr (0.2 Pa), causing the resistivity and transmittance of the polycrystalline film to be low. When the film was annealed in an oxygen flow the transmittance reached 90% or higher, but the resistivity was of the order of lo-’ R cm after crystallization. Table II shows the electrical and optical properties of In,O, films obtained so far by various workers. The resistivity of our recrystallized film, which can be applied to solar cells, is comparable with that of the film deposited onto a heated substrate by thermal evaporation ofindium’3”5 but is higher than that of In,O, l4 or a mixture of In,03 and indium’. The recrystallized film has the medium transmittance in comparison with the other films. 5.
CONCLUSIONS
To prepare transparent and conductive films suitable for heterojunction solar cells, the amorphous-to-crystalline transition was investigated for indium oxide films deposited by reactive evaporation of indium in oxygen. Annealing of the amorphous films at 200-300 “C gave rise to the transition. We found that the oxygen pressure Poxyduring deposition had a pronounced effect on the electrical and optical characteristics of the polycrystalline films. When the film was deposited at Poxy= 0.2 Pa the resistivity reached 5 x 10e4 Q cm but the optical transmission was lower than 30% after annealing at the crystallization temperature. When the film was deposited at Poxy= 0.02 Pa the transmittance reached 70% or higher for visible light but the resistivity was of the order of 1O-2 S2cm. The optimum transparent conductive film can be obtained by annealing an amorphous indium oxide film deposited at an intermediate oxygen pressure of 0.07 Pa. After crystallization the film had a resistivity of 2.1 x 10e3 Q cm and a transmittance of 84% at a wavelength of 500 nm.
222
K. ITO, T. NAKAZAWA,
K. OSAKI
ACKNOWLEDGMENT
This work was partially
supported
by the Hoso Bunka Foundation.
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