Optical waveguide device

This invention aims at providing an optical waveguide device capable of stably operating for an extended period of time. The optical waveguide device comprises an optical waveguide path formed inside a surface of an electro-optical substrate, a buffer layer formed on the optical waveguide path, and a driving electrode for impressing an electric field so as to change a refractive index of the optical waveguide path, wherein the buffer layer is made of a transparent dielectric or insulator of a mixture between silicon dioxide and an oxide of at least one element selected from the group consisting of the metal elements of the Groups III to VIII, Ib and IIb of the Periodic Table and semiconductor elements other than silicon, or a transparent dielectric or insulator of an oxide between silicon and at least one of the metal elements and semiconductor elements described above.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention generally relates to an optical waveguide device. More 
particularly, it relates to a structure of an optical waveguide device in 
an optical communication system for which particularly high reliability is 
required, and having a structure that will be used for a high speed 
optical switch for exchanging multi-channel large capacity data in optical 
form or will be used for an optical external modulator for ultra-high 
speed ultra-distance communication, and which improves stability of the 
modulator and switch for an extended period of time. 
2. Description of the Related Art 
In ordinary optical waveguide devices used for optical switches and optical 
modulators, an electric field is applied to an optical waveguide path 
formed inside a surface of an electro-optical crystal substrate such as 
lithium niobate (LiNbO.sub.3) so as to change the refractive index of this 
optical waveguide path. In this way, the switching of optical signals 
travelling inside the waveguide path, and their phase modulation and 
intensity modulation are carried out. 
In such waveguide devices, a buffer layer of a transparent dielectric film 
having a smaller refractive index than that of the waveguide path is 
sandwiched between the waveguide path and the electrode so as to prevent 
light propagating through the waveguide path from being absorbed by the 
electrode metal. When an electrode is formed on this buffer layer and a 
voltage is applied to the electrode, an electric field is applied to the 
waveguide path formed in the substrate crystal and the refractive index of 
the waveguide path changes in proportion to the intensity of the electric 
field. Thus, functions such as switching and modulation are provided. 
In this case, the intensity of the electric field applied to the waveguide 
path and its change with time are greatly affected by characteristics of 
the buffer layer. Since the optical output changes in proportion to the 
refractive index of the waveguide path, that is, in proportion to the 
intensity of the electric field applied to the waveguide path, a technique 
for accurately controlling the electric field applied to the waveguide 
path is very important in devices of this kind. 
The waveguide devices using such an electro-optical crystal substrate 
include optical switches, modulators, branching filters, polarized wave 
controllers, and so forth, but for the safe of convenience the following 
explanation will be for a Mach-Zehnder type optical modulator using a 
LiNbO.sub.3 waveguide path for use in a ultra-high speed optical 
communication modulator. 
FIG. 23 shows the appearance of a conventional Mach-Zehnder type modulator. 
In the drawing, reference numeral 1 denotes a lithium niobate 
(LiNbO.sub.3) crystal substrate that is cut in such a manner that an X 
axis of the crystal axis extends in a longitudinal direction of a chip, a 
Z axis extends in the direction of thickness so as to use an 
electro-optical coefficient r.sub.33 and a Y axis extends in the direction 
perpendicular to the X and Z axes. A semi-circular optical waveguide path 
2 having a greater refractive index than that of the substrate 1 and 
having a diameter of about 7 .mu.m is formed on a surface of the substrate 
1 by first forming a metal titanium (Ti) film by a known film formation 
means, such as electron beam deposition, then patterning this titanium 
(Ti) deposition film into a belt-like form in an X direction shown in the 
drawing, and thermally diffusing titanium into the substrate 1. 
Next, in order to prevent absorption of light propagating through the 
optical waveguide path 2 by the electrode, silicon dioxide (SiO.sub.2) 
having a specific dielectric constant of 4.0 and a refractive index of 
about 1.45 is deposited to a thickness of 0.5 .mu.m over the entire 
surface of the waveguide substrate 1 by a film formation technique, such 
as electron beam deposition, thereby forming a buffer layer. (To 
facilitate an understanding, the optical waveguide path 2 is shown as if 
it existed on the surface of the buffer layer 3 in FIG. 23.) Furthermore, 
a signal electrode 4 and a ground electrode 5 consisting of a thin gold 
(Au) film having a width of 7 .mu.m and a thickness of 10 .mu.m, for 
example, are formed by vacuum deposition and plating at positions on the 
surface of the buffer layer 3 corresponding to the optical waveguide path 
2. A travelling wave electrode and a signal source 6 are connected by a 
coaxial cable 7. Similarly, a terminal resistor 8 is connected by the 
coaxial cable 7. A lithium niobate crystal block 9 is bonded to the end 
surface of the optical waveguide path 2, and the waveguide path is 
connected to a fiber 11 by a fiber fixing jig 10. 
FIG. 24A shows a sectional structure on a cut line in the modulator shown 
in FIG. 23. In terms of an electrical equivalent circuit, this section can 
be expressed, as shown in FIG. 24B, by a buffer layer resistance R.sub.b, 
the resistance R.sub.LN of the lithium niobate crystal and their 
capacitances C.sub.B and C.sub.LN. In this equivalent circuit, the voltage 
V.sub.LN applied to lithium niobate is substantially determined by the 
capacitance C alone in the equivalent circuit at the instant of the 
application of voltage to the electrodes 4 and 5, and has a voltage value 
given by the following equation (1): 
##EQU1## 
After the passage of a sufficient period of time, V.sub.LN is substantially 
determined by the resistance R in the equivalent circuit, and is given by 
the following equation (2): 
##EQU2## 
Accordingly, the voltage applied to the waveguide path changes between the 
instant of the application of voltage to the electrodes of the modulator 
and after the passage of sufficient time. In consequence, the outgoing 
light from the modulator also changes, which change is referred to as a 
"DC drift" in lithium niobate waveguide devices. 
FIG. 25 is a diagram showing the relation between the impressed voltage and 
the intensity of outgoing light. In the diagram, when a voltage V.sub.1 is 
applied, there is an optical output P.sub.1 at the instant of the 
application, but this optical output decreases with time. Assuming that 
the optical output reaches P.sub.2, this state is equivalent to the state 
where only a voltage V.sub.2 is effectively applied to the electrodes, and 
this drift quantity S can be evaluated by the following equation (3): 
EQU S=(V.sub.1 -V.sub.2)/V.sub.1 ( 3) 
This DC drift is the phenomenon that is generated by the DC component of 
the voltage applied to the electrodes, and is proportional to the degree 
of the impressed voltage. In other words, assuming that a 0.3V DC drift 
occurs when a 1 V voltage is applied, the DC drift of 3 V occurs when a 10 
V voltage is applied. It is therefore convenient to express the DC drift 
quantity by percentage to the impressed voltage when the DC drift is 
discussed. Accordingly, the DC drift quantity will be expressed by 
percentage in the following description. 
FIG. 26 shows the relation between the resistance R and the capacitance C 
on the basis of the equations (1) and (2) and the occurrence of the DC 
drift. When the voltage determined by the resistance of the equation (2) 
is smaller than the voltage determined by the capacitance of the equation 
(1), a positive drift occurs and is represented by (a). At this time, the 
voltage (or the electric field) applied to the waveguide path gradually 
decreases, and when the voltage determined by the resistance is greater 
than the voltage determined by the capacitance, a negative drift occurs 
and is represented by (c). At this time, the voltage (or the electric 
field) applied to the waveguide path gradually increases. Needless to say, 
the practical DC drift is not as simple as described above. 
Here, the resistance, capacitance, etc., such as the resistance of the 
interface layer between the LiNbO.sub.3 substrate 1 and the buffer layer 
3, the resistance of the buffer layer 3 in the horizontal direction, the 
resistance arising from the difference of the peripheral portion of the 
optical waveguide path 2 from the substrate 1, etc., are equivalently 
expressed by C.sub.B, C.sub.LN, R.sub.B and R.sub.LN. As is known well, 
the electric resistance of a dielectric (an insulator) changes with the 
voltage impression time. When mobile ions exist in the buffer layer 3 and 
in the crystal, a spatial charge distribution owing to their migration 
must also be taken into consideration. When the DC drift is examined, 
therefore, these factors must be collectively taken into consideration. 
However, it is extremely complicated to clarify in detail the mechanisms 
for all these phenomena and to classify and describe same. Therefore, an 
explanation will be directed primarily to a method of improving the DC 
drift characteristics and the characterizing results obtained by such a 
method. 
FIG. 27 shows the evaluation result of the DC drift of the modulator having 
the prior art structure shown in FIG. 23. FIG. 27 shows the DC drift 
characteristics changing with time, which are evaluated at atmospheric 
temperature of 20.degree. C., 60.degree. C., 100.degree. C. and 
140.degree. C. It can be appreciated from the diagram that the DC drift is 
an extremely slow phenomenon occurring at rates of 5% a day at room 
temperature (20.degree. C.), 30% per 10 days and 100% per 200 days. (In 
the conventional modulators, too, there is, of course, the case where this 
phenomenon reaches 100% within several minutes if a process condition is 
incomplete, such as when there is damage to the crystal.) 
Even if the DC drift is such an extremely slow phenomenon, the 
characteristics of the components for optical communication must be 
compensated for, for at least a period of 15 years, and the 
characteristics described above are not sufficient. Furthermore, it is 
practically difficult to evaluate such a phenomenon in the course of 15 
years and then to produce a product, but fortunately, it is known that 
this phenomenon is accelerated by temperature, as shown in the drawing. In 
other words, it is known that when evaluation is made at 100.degree. C., 
the phenomenon can be evaluated in acceleration of 1,000 times at room 
temperature. It is therefore possible to evaluate and estimate long term 
characteristics for periods of more than 15 years by carrying out the 
evaluation at 100.degree. C., and for this reason, the following 
description will be based on the evaluation result at 100.degree. C. or 
140.degree. C. as the reference. 
Several methods for improving this DC drift have been proposed in the past. 
Since the existence of the buffer layer 3 is the main cause for the DC 
drift as described above, a structure using a transparent electrode for 
preventing the absorption of propagating light by the electrodes without 
forming the buffer layer 3 has been proposed (KOKAI (Japanese Unexamined 
Patent Publication) No. 55-69122). However, there is no material that is 
transparent at a wavelength of 1.3 .mu.m oar 1.55 .mu.m, which is 
important for optical communication, and that has a sufficiently smaller 
refractive index than that of the waveguide path. Accordingly, the 
structure proposed by the reference described above is not disposed 
immediately above the waveguide path but is disposed in the proximity of 
the waveguide path, so as to avoid the problem of optical absorption. In 
practice, there is a device in which the electrodes must be formed 
immediately on the waveguide path, such as a Z-cut substrate device. As a 
counter-measure for such a case, (KOKAI (Japanese Unexamined Patent 
Publication) No. 61-198133), which reduces the optical absorption by 
mixing an electrically conductive material with a transparent insulator 
film has been proposed. According to this method, an optical wavelength 
range, which is effective as a transparent electrode, can assuredly shift 
to a longer wavelength side compared to the case where the electrically 
conductive material is used alone. 
To retain the function as the electrode, however, the proportion of this 
electrically conductive material must be increased, but because generally 
known conductive materials have strong optical absorptivity at a 
wavelength of 1 .mu.m or above, it is difficult to form a transparent 
electrode in this region. Particularly when beams of light pass 
perpendicularly through the film, they are almost fully absorbed in most 
cases if the film is used as the buffer layer 3 on the optical waveguide 
path 2, even though the absorption loss is small. Furthermore in this 
case, the buffer layer 3 must be divided into the shapes of the electrodes 
4, 5 because it fundamentally plays a role equivalent to that of the 
electrodes. 
Another proposal (KOKAI (Japanese Unexamined Patent Publication) No. 
1-155631: Article A: Electronics Lett. Vol. 26, No. 17, pp. 1409-1410) 
contemplates trapping mobile ions by assuming that ions in the buffer 
layer 3 move and cause localization of ions because of the impressed 
voltage, and thus generate the DC drift. The introduction of the trap to 
prevent localization of the ions is common in semiconductor technology. 
According to the results of the Article A executing this method, an 
improvement of the DC drift has been attained by doping P. Eventually, 
however, the stable state can be kept for only two hours, and at least 80% 
of the DC drift occurs in the course of three hours. FIG. 27 shows the DC 
drift characteristics of the SiO.sub.2 buffer layer 3 to which nothing is 
added, according to the prior art structure. Compared to the results of 
the Article A, the characteristics are much better, and the doping effect 
of P as the trap cannot be observed. 
Furthermore, there has been still another proposal wherein an upper layer 
of the buffer layer is shaped into a structure where a metal or a 
semiconductor exists in granular form so as to permit easy injection of 
electrons into the buffer layer and mitigation of the DC drift (KOKAI 
(Japanese Unexamined Patent Publication) No. 3-127023). This structure is 
characterized by its two-layered structure wherein a metal or 
semiconductor element is locally contained in a granular and metallic 
state without being oxidized, in an inner electrode of the buffer layer 
and at an interface portion of the buffer layer. However, the DC drift 
characteristics of the optical waveguide device fabricated by this method 
have not yet reached the level of stability required by optical 
communication systems, as described in Article B (Papers of Electronic 
Data Communication Society, c-1, Vol. 1. J75-C-1, No. 1, pp. 17-26, 
January, 1992). 
In the field of the optical waveguide devices that operate when an electric 
field is applied from the electrodes formed on the buffer layer formed on 
the optical waveguide path formed inside the surface of the opto-electric 
crystal substrate, to the optical waveguide path, the change of outgoing 
light with time resulting from the impressed D.C. (direct current) voltage 
component is referred to as the "DC drift". Although a large number of 
studies have so far been made to solve this DC drift, no report or data 
thereby solving this problem has been forthcoming. 
SUMMARY OF THE INVENTION 
Recently, practical utilization of ultra-high speed external optical 
modulators and optical switches using optical waveguide devices, 
particularly, lithium niobate (LiNbO.sub.3) waveguide paths, have been in 
demand. The present invention solves the DC drift problem which has been 
the greatest problem preventing the practical utilization of waveguide 
devices using an electro-optical effect. 
To accomplish the object described above, the present invention provides an 
optical waveguide device comprising an optical waveguide path formed 
inside a surface of an electro-optical crystal substrate, a buffer layer 
formed on the optical waveguide path, and a driving electrode for 
impressing an electric field so as to change a refractive index of the 
optical waveguide path formed on the buffer layer, wherein the buffer 
layer is made of a transparent dielectric or insulator of a mixture 
between silicon dioxide and an oxide of at least one element selected from 
the group consisting of the metal elements of the Groups III to VIII, Ib 
and IIb of the Periodic Table and semiconductor elements other than 
silicon, or a transparent dielectric or insulator of an oxide between 
silicon and at least one of the metal elements and semiconductor elements 
described above. 
In the optical waveguide device according to the present invention, since 
the buffer layer has a structure as described above, negative DC drift 
characteristics are exhibited at the initial stage with the passage of 
time, and additives affect mobile electrons or ions so that an increase of 
the DC drift of the optical waveguide device can be flattened much more 
than in the prior art devices. Therefore, the DC drift characteristics can 
be reduced for an extended period of time.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a perspective view showing a structure of a waveguide type 
modulator according to an embodiment of the present invention, and like 
reference numerals are used to identify like constituents as in the prior 
art. In the drawing, reference numeral 1 denotes a lithium niobate 
substrate, which is cut in such a manner that a Z axis lies in a direction 
of thickness. A 950 .ANG.-thick titanium (Ti) layer is formed by an 
electron beam deposition method on the surface of the waveguide substrate 
1, and patterning is made so as to define an optical waveguide in an X 
direction of a crystal axis. The substrate 1 is then heated at 
1,050.degree. C. for 10 hours in an oxidizing atmosphere so as to 
thermally diffuse titanium into the waveguide substrate 1. In this way, a 
belt-like optical waveguide path 2 having a width of about 7 .mu.m is 
formed so that the waveguide path 2 branches at one of the ends thereof, 
couples at the other end and is parallel at the center with a gap of 15 
.mu.m. 
Silicon dioxide (SiO.sub.2) containing 5 mol % of In.sub.2 O.sub.3 and 5 
mol % of TiO.sub.2 is deposited on the entire surface of this optical 
waveguide 2 by the electron beam deposition method or by a sputtering 
method to form a buffer layer 3'. The buffer layer 3' thus formed is 
annealed at 600.degree. C. for 10 hours in a wet oxidizing atmosphere. 
Since resistance is sufficiently high in this buffer layer 3', the buffer 
layer 3' need not be separated particularly between electrodes. Next, a 
1,000 .ANG.-thick Si film 12 is formed by sputtering to improve 
temperature characteristics. Thereafter, electrodes 4 and 5 are formed on 
the optical waveguide 2. In other words, the buffer layer in this device 
consists of a composition (SiO.sub.2).sub.0.95 (TiO.sub.2).sub.0.05 as a 
base and containing 5 mol % of In.sub.2 O.sub.3. FIG. 6 shows DC drift 
characteristics of a Ti-diffusion LiNbO.sub.3 waveguide type external 
modulator thus formed. The diagrams evaluate the DC drift characteristics 
at 20.degree. C., 60.degree. C., 100.degree. C. and 140.degree. C., and it 
can be understood that this phenomenon is accelerated depending on 
temperature. The activation energy in this case is about 1 eV. 
Strictly speaking, the composition of the buffer layer film hereby formed 
is not a compound or mixture consisting of a composition ratio of 
SiO.sub.2, TiO.sub.2 and In.sub.2 O.sub.3, but is believed to be a 
compound or mixture consisting of a composition ratio of SiO.sub.x, 
TiO.sub.y and InO.sub.z. From the necessity for quantitatively expressing 
the contents, however, the foregoing and following description will 
express the contents in terms of mol % using the molecular weight. 
Several effective methods are available to fabricate these buffer layers as 
will be illustrated below. In any case, it is of importance to form the 
film while the substrate temperature is kept below 250.degree. C. in order 
not to generate a defective layer on the substrate surface. 
(1) A method of forming the buffer layer by electron beam deposition or 
sputtering by mixing in advance silicon dioxide and an oxide of other 
metals or semiconductors, then sintering the mixture, and using the 
resulting target. 
(2) A film formation method by multi-source deposition or multi-electrode 
sputtering by using an apparatus equipped with a plurality of electron 
beam sources or sputter targets for simultaneously effecting vacuum 
deposition (FIG. 2) or sputtering of silicon dioxide and an oxide of other 
metals or semiconductors. 
(3) A method of forming a film in an ultra-multiple-layered on an optical 
substrate from a plurality of sources or targets (FIG. 5) by using an 
apparatus equipped with a plurality of electron beam sources or sputter 
targets and capable of rotating (FIG. 3) or reciprocating the optical 
substrate. In this case, it is effective to shape each layer to a 
thickness of from 0.2 to 200 .ANG.. 
(4) A method of forming the buffer layer by sputtering by placing an oxide 
of other metals or semiconductors on a silicon dioxide target, or forming 
a hole and burying the oxide (FIG. 6). 
(5) A method of forming a buffer layer film under the state of an oxide, 
wherein a part of the whole of the electron beam sources or the sputter 
targets of items (1) to (4) described above are formed under the state of 
a semiconductor, and vacuum deposition or sputtering is carried out in an 
oxygen-containing reactive atmosphere. 
The technical content described above is similarly effective when the 
object device is a switch or a wide variety of other devices, and is also 
effective for waveguide devices using other electro-optical crystal such 
as a LiTaO.sub.3 crystal. 
One of the fundamental constructions in the present invention resides in 
that a metal element of the Group IIIb is contained in the buffer layer 
3'. FIG. 7 shows the result when an SiO.sub.2 buffer layer 3' containing 5 
mol % of indium (In) of an element of the Group IIIb as In.sub.2 O.sub.3 
is formed on the waveguide, which is formed in turn by thermally diffusing 
Ti on the LiNbO.sub.3 substrate. In comparison with the buffer layer 3 
consisting of the composition of SiO.sub.2 alone, as shown in FIG. 27, it 
can be understood that an increase in the DC drift is retarded 
significantly under the same conditions at 100.degree. C. 
As described above, in the present invention, the buffer layer is formed, 
as a whole, as the mixture of the oxides of metals or semiconductors, and 
the present invention is entirely different from the structure described 
in the article B, which divides the layer into two layers and moreover, 
the metal is as such contained in granular form in the interface layer 
between the electrode and the buffer layer. Furthermore, when experiments 
were carried out shaping only the interface layer by the oxide mixture 
film, no improvement was observed at all in the DC drift. This means that 
the influence of the interface structure on the DC drift characteristics 
is small, and an improvement of the buffer layer as a whole is necessary. 
The reason a negative DC drift of about 20% occurs at the initial stage as 
in FIG. 7 is because the resistance drops owing to the addition of In. 
Thereafter, a positive drift occurs, presumably due to mobile electrons 
and mobile ions produced by an electric field. This positive drift is also 
drastically reduced owing to the effect of the oxide of a metal or 
semiconductor element admixed with silicon oxide. It has been confirmed 
experimentally that the metal elements of the Group IIIb of the Periodic 
Table provide a remarkable effect of reducing the increase in the DC 
drift. On the other hand, the DC drift characteristics of the case where 
tin (Sn) having the atomic number next to that of In the Periodic Table is 
added, is improved significantly compared to the case where tin is not 
added, as shown in FIG. 8, but the improvement is not as great as in the 
case of In, which means that the degree of the effect obtained depends on 
the element employed. 
FIG. 9 shows the result of the case when another Group IIIb element, Al, is 
added. In this case, too, the element provides the effect of improving the 
DC drift characteristics over a long period of time. The Group IIIb 
elements are particularly effective for improving the DC drift 
characteristics over a long period of period, among others, In. 
On the other hand, it is also effective to add a compound containing at 
least one kind of metal element other than the Groups Ia and IIa of the 
Periodic Table, that is, the metal elements of the Groups III to VIII, Ib 
and IIb of the Periodic Table, to silicon dioxide (SiO.sub.2). For 
example, FIG. 10 shows DC drift characteristics when up to 1 mol % of 
GeO.sub.2 is contained in the base composition of (SiO.sub.2).sub.0.95 
(TiO.sub.2).sub.0.05 in the same way as in the buffer layer in the device 
described above, and FIG. 11 shows DC drift characteristics when 5 mol % 
of ZnO is contained. Furthermore, FIG. 12 shows DC drift characteristics 
when Cr.sub.2 O.sub.3 is contained in the amount of 5 mol %. The 
improvement in the stability of the operation of the device can be 
observed in all of these cases. 
As shown in FIG. 14, when Ti of the Group IVa is added so that the 
proportion of the Ti element is 5 mol % to the total of Si and Ti, an 
improvement in the DC drift characteristics can be observed compared to 
the case of SiO.sub.2 alone (FIG. 27). About -30% negative DC drift in 
this case results from a drop in electric resistance of the buffer layer 
film. The resistivity of the buffer layer film in this case is about 
10.sup.15 .OMEGA.cm. Prior art that utilizes the buffer layer 3' under 
such a state so as to cope with such a negative DC drift is not known. The 
prior art reports have exclusively been directed to the accomplishment of 
the state (b) where the DC drift does not occur in both cases where a 
positive DC drift occurs (a) and where a negative DC drift occurs (C), as 
shown in FIG. 26. 
As to the elements of the Groups of Ia and IIa of the Periodic Table, in 
the case of an Na-containing buffer layer, for example, the effect becomes 
contrary when compared to the case of only silicon dioxide, as shown in 
FIG. 13. In this way, the alkali metal elements exert adverse influences 
on the improvement of the DC drift characteristics. 
However, when the DC drift of the device is compensated for externally in 
accordance with DC drift conditions, such as when the circuit shown in 
FIG. 13 is effectively utilized, a disparity occurs between the positive 
and negative DC drifts and in this case, the negative DC drift has an 
effective meaning. In other words, when a positive 100% DC drift occurs, 
no large voltage is completely effective when the voltage is applied to 
compensate for the DC drift, but when a negative DC drift occurs, complete 
compensation can be effected by a compensation voltage that is smaller 
than the impressed DC voltage. 
Therefore, the condition in which the positive DC drift occurs with 
difficulty is found effective by deliberately mixing an additive to 
generate a negative DC drift or increasing the thickness of the buffer 
layer so as to reduce the capacitance of the buffer layer. In the 
modulators having the structures shown in FIGS. 23 and 24, the DC drift 
quantity occurring at the initial stage is determined for a modulator 
having an electrode width of 7 .mu.m, an electrode gap of 15 .mu.m and an 
electrode thickness of 10 .mu.m using the thickness of the buffer layer 3' 
and its resistivity as the parameters, the result can be obtained as shown 
in FIG. 16. In other words, the negative DC drift can be obtained with 
stability when the buffer layer 3' is fabricated so that resistivity is 
below 10.sup.16. 
When the resistance of the buffer layer 3' is gradually lowered, absorption 
of light owing to the free electrons of the buffer layer 3' occurs at an 
optical wavelength of 1 to 2 .mu.m. Therefore, the resistivity of the 
buffer layer 3' must be kept at a value not lower than 10.sup.9 .OMEGA.cm. 
It is advantageous in this case to fabricate the buffer layer 3' to a 
thickness of at least 0.25 .mu.m in order to avoid optical absorption by 
the electrodes. As a result, the DC drift quantity is below -20%. It can 
also be appreciated from FIG. 16 that when the buffer layer was as thick 
as 2.5 .mu.m, the DC drift percentage was nearly to -200%. FIG. 18 shows 
the experimental results when the film thickness varied, and it can be 
appreciated from this diagram that the negative DC drift quantity changes 
with film thickness. 
Accordingly, the drift quantity can be set to the range of -20% to -200% by 
selecting the material (resistivity, dielectric constant) of the buffer 
layer 3' and its thickness, and in this case, the magnitude of the DC 
drift becomes smaller in such a manner as to correspond to the shift on 
the negative side. Needless to say, this phenomenon not only provides the 
effect of reducing resistance but also inhibits the mobility of the mobile 
electrons and the mobile ions by the mixture of a plurality of metals and 
semiconductors when compared to the case where only silicon dioxide is 
used. By this effect, a significant improvement is attained not only in 
providing a short-term effect in that the DC drift is shifted to the 
negative side but also in providing a long-term effect in that the DC 
drift gradually shifts in the positive direction. 
The addition of further material to the buffer layer 3' containing the 
Group IIIb element effectively improves the DC drift characteristics. FIG. 
17 shows the effect when 5 mol % of TiO.sub.2 is further added to the 
SiO.sub.2 buffer layer containing 5 mol % of the In.sub.2 O.sub.3 element 
to the total of the metal or semiconductor in the film. It can be seen 
from the diagram that the quantity of the negative DC drift at the initial 
stage is further reduced. 
The DC drift over a long period of time (more than ten days) is also 
further improved. When other metal elements are added to the Group IIIb 
element, a synergistic effect is believed to occur. It is possible to 
reduce the resistance by increasing the amount of In, but this is not 
effective because optical absorption occurs in the 1 to 2 .mu.m wavelength 
range, which is important for optical communication. 
The content in this case is preferably from 0.001 to 35 mol % because such 
a content does not affect the absorption of light. Judging from the result 
of FIG. 19 showing the relationship between the In.sub.2 O.sub.3 content 
and the DC drift quantity, however, a significant improvement could be 
obtained by increasing the content more than ten times to 5 mol % from a 
trace amount of 0.3 mol % of In.sub.2 O.sub.3. FIG. 20 shows the 
evaluation result of the DC drift quantity at 140.degree. C. after one 
day. An optimum value existed for the content, and the characteristics 
reached the maximum near 4 mol %. FIG. 21 shows the relationship between 
the In.sub.2 O.sub.3 concentration and the electric resistance in the 
buffer layer film consisting of the SiO.sub.2 --In.sub.2 O.sub.3 mixture 
film. The higher the concentration, the lower the resistivity. Therefore, 
the existence of the optimum content obviously indicates that the DC drift 
does not improve by merely lowering the resistivity with the aim of 
obtaining a transparent electrode. Furthermore, the resistivity was 
10.sup.9 .OMEGA.cm at a content of 35 mol %, and this resistance value 
does not represent a conductive film. It can thus be understood that the 
present invention does not improve the DC drift by introducing of the 
transparent conductive film simply by the addition of substances. 
The addition of the Group IIIb elements of the Periodic Table to the buffer 
layer 3' according to the present invention has a close relation with 
annealing in the oxidizing atmosphere after film formation by electron 
beam deposition, resistance heating deposition, sputtering and other 
techniques. FIG. 22 shows a change in the DC drift quantity when the 
annealing temperature and the annealing time are changed. Annealing at a 
temperature of 300.degree. C. to 700.degree. C. for a time of from 2 to 10 
hours is effective. 
As described above, the present invention can significantly improve the 
conventional DC drift characteristics, can obtain more effective DC drift 
characteristics by effectively utilizing the negative DC drift, and can 
thus accomplish ultra-high speed external modulators and matrix switches 
satisfying the requirements for optical communication devices, for which a 
very high degree of reliability is a requisite.