Method for manufacture of photo-semiconductor

A photo-semiconductor is produced by melting Bi.sub.2 O.sub.3 or its mixture with a small amount of an oxide or fluoride and then quenching the resultant melt in the form of a film.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
This invention relates to bismuth oxide and its solid solution which 
exhibit the property of a remarkable decrease in the electric resistance 
when exposed to light (hereinafter referred to as "photoconductivity or 
photo-semiconductivity"). A substance showing such property is a 
photo-semiconductor. This invention, therefore, relates to a method for 
preparing bismuth oxide and its solid solution which are 
photo-semiconductors. The photo-semiconductors can be expected to be 
applied chiefly for photo-detecting materials and photo-switching 
materials in the field of optoelectronics. 
Now, the photo-semiconductors will be described in more detail. 
Photo-semiconductivity refers to the phenomenon that a certain 
semiconductor or insulator gains in electronconductivity when exposed to 
light. Such photo-semiconductivity is widely utilized in television 
cameras, infrared detectors, photometers, etc. The photo-semiconductive 
substances which have been actually used at present are chalcogen 
compounds such as cadmium sulfide (CdS), lead sulfide (PbS), lead selnide 
(PbSe) and cadmium mercury tellulride (CdHgTe), and silicon (Si) and 
germanium (Ge) single crystals incorporating certain kinds and amounts of 
impurities. They are virtually wholly non-oxide type substances. They are 
generally used in the form of a film. Although they possess fairly high 
levels of photo-semiconductivity, they are thermally and chemically 
unstable. Further, they are required to possess an exactly defined 
composition and, therefore, necessitates a highly advanced technique for 
their production. The conditions under which they are effectively used 
have their own limits because they exhibit their photo-semiconductivity at 
low temperatures. Generally, oxides are stable as compared with the 
aforementioned substances. Thus, the desirability of developing an oxide 
which is excellent in photo-semiconductivity and is easy to manufacture 
has been finding growing recognition. At present, only a few 
.gamma.-Bi.sub.2 O.sub.3 solid solutions (.gamma.-BSO and .gamma.-BGO) of 
the compositions of 6Bi.sub.2 O.sub.3.SiO.sub.2 (abbreviated as BSO) and 
6Bi.sub.2 O.sub.3.GeO.sub.2 (abbreviated as BGO) have been known to the 
art as oxides exhibiting photo-semiconductivity. In the cases of 
.gamma.-BSO and .gamma.-BGO single crystals, it has been demonstrated that 
they show an increase in their magnitudes of electroconductivity by 
roughly hundreds of times when irradiated with a visible ray (R. E. 
Aldrich et al: Journal of Applied Physics, Vol. 42, p 493 (1971) and S. L. 
Hou et al: Applied Physics Letters, Vol. 18, p 325 (1971)). 
Bi.sub.2 O.sub.3 exists in several polymorphous types such as .alpha., 
.beta., and .delta. types in addition to the .gamma. type. For example, 
.delta.-Bi.sub.2 O.sub.3 assumes a face-centered cubic fluorite type 
structure abounding with oxygen vacancy and is held to possess higher 
oxygen ion conductivity than stabilized zirconia (ZrO.sub.2). Thus, the 
possibility of .delta.-Bi.sub.2 O.sub.3 being used as an oxygen sensor or 
a solid electrolyte at low temperatures is drawing growing attention. 
The inventors have found after various related studies that materials 
excelling in photo-semiconductivity can be produced by a certain treatment 
of .alpha. type Bi.sub.2 O.sub.3 or its mixture with small amounts of 
appropriate oxide or fluoride, which is described in detail afterward. The 
present invention has been perfected on the basis of this discovery. 
OBJECT AND SUMMARY OF THE INVENTION 
An object of this invention is to provide an easy method for the 
manufacture of a .delta.-Bi.sub.2 O.sub.3 photo-semiconductor. 
Another object of this invention is to provide a method for the manufacture 
of a photo-semiconductor consisting of a solid solution of Bi.sub.2 
O.sub.3 with other oxide, which is easy to carry out. 
To accomplish the objects described above, the method of this invention 
comprises melting .alpha.-Bi.sub.2 O.sub.3 alone or the mixture thereof 
with a small amount of a specific oxide or fluoride, dropping the 
resultant melt onto two rollers kept in rotation and thereby allowing the 
melt to be suddenly cooled in the form of a film. The film thus prepared 
exhibits excellent photo-semiconductivity.

EXAMPLE 
Bi.sub.2 O.sub.3 photo-semiconductors were prepared by using such secondary 
additives as silicon dioxide (SiO.sub.2), vanadium pentoxide (V.sub.2 
O.sub.5), niobium pentoxide (Nb.sub.2 O.sub.5), tantalum pentoxide 
(Ta.sub.2 O.sub.5), molybdenum oxide (MoO.sub.3), tungsten oxide 
(WO.sub.3), zirconium oxide (ZrO.sub.2), cerium oxide (CeO.sub.2), bismuth 
fluoride (BiF.sub.3) and yttrium fluoride (YF.sub.3). A raw material 
powder thoroughly mixed with the given components in a stated ratio was 
pressed into tablets. The tablets were fired in an electric furnace at 
750.degree. C. for five hours, finely ground, again pressed into rods 5 
mm.times.5 mm.times.30 mm. The rods were fired at 750.degree. C. for five 
hours and used as a sample for rapid quenching treatment. A rapid 
quenching device was composed of an infrared image furnace using a halogen 
lamp of 1.5 KW and two metallic rollers of 50 mm in diameter operated at a 
rate of 2,800 rotations per minute. The sample rods were melted in the 
furnace to become liquid beads. The molten beads were dropped between the 
two rollers in rotation to be spread and rapidly quenched. Consequently, 
there were obtained pieces of film about 20 .mu.m in thickness, about 15 
mm in width, and about 15 mm in length. The crystal phases precipitated in 
the rapidly quenched films with various compositions and the 
crystallographic orientations were examined by X-ray diffraction. The 
results are shown in Table 1. 
TABLE 1 
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Results of rapid quenching treatment 
Starting composition 
Formed 
Run No. 
(mol ratio) phase Grain-orientation 
______________________________________ 
1 Bi.sub.2 O.sub.3 
.delta. Slightly incomplete 
2 95Bi.sub.2 O.sub.3.5SiO.sub.2 
.delta. Complete 
3 90Bi.sub.2 O.sub.3.10SiO.sub.2 
.delta. Complete 
4 97.5Bi.sub.2 O.sub.3.2.5V.sub.2 O.sub.5 
.delta. Slightly incomplete 
5 95Bi.sub.2 O.sub.3.5V.sub.2 O.sub.5 
.delta. Slightly incomplete 
6 97.5Bi.sub.2 O.sub.3.2.5Nb.sub.2 O.sub.5 
.delta. Slightly incomplete 
7 95Bi.sub.2 O.sub.3.5Nb.sub.2 O.sub.5 
.delta. Complete 
8 90Bi.sub.2 O.sub.3.10Nb.sub.2 O.sub.5 
.delta. Complete 
9 97.5Bi.sub.2 O.sub.3.2.5Ta.sub.2 O.sub.5 
.delta. Slightly incomplete 
10 95Bi.sub.2 O.sub.3.5Ta.sub.2 O.sub.5 
.delta. Complete 
11 90Bi.sub.2 O.sub.3.10Ta.sub.2 O.sub.5 
.delta. Complete 
12 95Bi.sub.2 O.sub.3.5MoO.sub.3 
.delta. Complete 
13 95Bi.sub.2 O.sub.3.5WO.sub.3 
.delta. Complete 
14 95Bi.sub.2 O.sub.3.5ZrO.sub.2 
.beta. Incomplete 
15 6Bi.sub.2 O.sub.3.CeO.sub.2 
.beta. Incomplete 
16 Bi.sub.2 O.sub.2.9.F.sub.0.2 
.beta. Incomplete 
17 95Bi.sub.2 O.sub.3.5YF.sub.3 
.beta. Incomplete 
______________________________________ 
The crystal orientation was rated on the basis that a sample showing only 
peaks based on the (111) and its higher order (hhh) is "complete" and that 
a sample showing any other peak even to a slight extent is "slightly 
incomplete". Though samples containing ZrO.sub.2, CeO.sub.2 or YF.sub.3 
formed a tetragonal .beta. phase and showed a high crystallographic 
orientation on the (111) plane, they are to be "incomplete" because their 
orienting property is inferior more or less. 
The rapid-quenched film thus produced was coated with an electroconductive 
silver paste to form an electrode. This electrode was tested for electric 
resistance (surface inherent resistivity) under exposure to light. The 
irradiating light used for this purpose had a wavelength varying from 300 
to 700 nm and the irradiation energy of the light was 5.times.10.sup.5 to 
19.times.10.sup.5 erg/cm.sup.2.sec. The results are shown in Table 2. The 
relation between the wavelength (nm) of the irradiating light and the 
surface inherent resistivity (.OMEGA.) is shown in the drawing. In the 
diagram, the results of Runs No. 2, 4, 7, 10, 12, 13, and 15 are shown. 
TABLE 2 
______________________________________ 
Results of irradiation 
Photo Wavelength of 
Minimum number 
sensiti- light at lowest 
of digits of 
Run No. 
vity electric resistance 
electric resistance 
______________________________________ 
1 yes 520 nm 0.5 
2 yes 490 1 
3 no -- -- 
4 yes 490 1.5 
5 no -- -- 
6 yes 485 1 
7 yes 510 1.6 
8 no -- -- 
9 yes 500 1 
10 yes 490 2.2 
11 no -- -- 
12 yes 500 1.6 
13 yes 510 2.0 
14 yes 510 0.7 
15 yes 510 1.3 
16 yes 515 2.5 
17 yes 490 2.3 
______________________________________ 
From the foregoing results obtained in the example, it is noted that 
.delta.-Bi.sub.2 O.sub.3 type solid solutions exhibit 
photo-semiconductivity. The value of photo-semiconductivity reached their 
peaks generally in the neighborhood of 500 nm in wavelength. This tendency 
has a bearing on the color of the rapid-quenched film. Virtually all the 
rapid-quenched films assumed orange to dark orange colors. This fact 
indicates that they have absorption of greenish light (about 500 nm in 
wavelength). It is, therefore, logical to conclude that their 
photo-semiconductivity is caused by the lattice vibration excited by the 
absorption of this light. Regarding the relation between the 
photo-semiconductivity and the composition of film, the 
photo-semiconductivity is weak when the amount of the second additives is 
insufficient and is totally absent when the amount of the added second 
additive is excessive. The optimum composition of the solid solution 
varied from one kind to another of the second additive used. In the 
present example, the film composition was fixed very roughly. There are 
indications that manufacture of a photo-semiconductor with a still better 
quality will be materialized by appropriate selection of composition. In 
fact, 95Bi.sub.2 O.sub.3.5Ta.sub.2 O.sub.5 (Run No. 10) and 95Bi.sub.2 
O.sub.3.5WO.sub.3 (Run No. 13) were obtained in the example which showed 
decreases of electric resistance by at least a few hundred times. These 
photo-semiconductors possessed properties sufficient to meet applications 
as light switching elements.