Note: Descriptions are shown in the official language in which they were submitted.
~ arlle~Kyle~Van U~te~ 1-2-~0
GLASS COATING FOR SEMICONDUCTOR OPTICAL ~EVICES
Back~round of the Invention
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1. F_eld of__he Invent on
The invention is concerned with the protection of
se!miconductor optical devices and the ad~ustment of facet
5 reflectivity.
2. Description_of the Prior Art
The term "semiconductor optical device" is used
in the following to designate any device which comprises
a body of semiconductor material which either emits light
10 in response to an applied voltage or detects light by
producing a voltage in response to incident light.
Examples of semiconductor optical devices are light
emitting diodes, superradiant diodes, laser diodes,
detectors, opto~isolators, and phototransistors, as
15 described, e.g., in A. A. Bergh and P. J. Dean, "Light~
Emitting Diodes", Clarenden Press, 1976.
The development of semiconductor optical devices
has reached a level at which their use in optical
communications systems appears likely. Particularly well
20 developed among such devices are laser diodes consisting
of a gallium arsenide substrate on which a light emitting
p~n junction is formed in epitaxial layers of germanium -
or tellurium doped gallium arsenide and gallium aluminum
arsenide are deposited. Methods such as liquid phase
25 epitaxy and molecular beam epitaxy have been successfully J
used for deposition.
Laser diodes are typically produced in the form
of tiny chips comparable in size and shape to grains of
salt and having electrodes attached to those two facets
30 which are parallel to the epitaxial layers. Of the four
facets, which are perpendicular to the epitaxial layers,
two are typically roughened and two are cleaved to act as
partially reflecting mirrors, thus forming a Fabry~Perot
cavity. A survey of the state of the art of laser diode
35 technology may be found in the paper by M. B. Panish,
"Heterostructure Injection Lasers", Proceedings of_the
IEEE, Vol. 64, No. 10, October 1976, pages 1512~1540.
Practical application of semiconductor optical
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devices in communications systems depends on a number of
device characteristics, some o~ which are critically
dependent on facet qualities such as resistance to
atrnospheric influence and the degree of facet
5 re~lectivity. For example, in the case of light emitting
diodes, low facet reflectivity is desirable to maximize
available light output. Similarly, low facet
reflectivity is desirable in photodiode detectors to
maximize diode sensitivity. In the case of laser diodes
10 it has been realized that, even at relatively modest
power levels, mirror facet erosion may shorten useful
diode life. To alleviate this problem, the application
of protective dielectric coatings to light emitting
facets of laser diodes has been advocated.
In particular, coatings of SiO2 and A1203
have been proposed, respectively, by M. Ettenberg, H. S.
Sommers, H. Kressel, and H. F. Lockwood in "Control of
Facet Damage in GaAs Laser Diodes", Applied Phys cs
_tters, Vol. 18, No. 12, 15 June 1971, pages 571~573,
20 and by I. Ladany, M. Ettenberg, H. F. Lockwood and H.
Kressel in "A1203 Halfwave Films for Long~Life CW
Lasers", Applied Physics Letters, Vol. 30, No. 2, 15
January 1977, pages 87~88.
Other aspects of long~term reliability of laser
25 diodes have been reported on by I. Ladany and H. Kressel
in "Influence of Device Fabrication Parameters on Gradual
Degradation of (AlGa)As CW Laser Diodes", Applied Physics
Letters, Vol. 25, No. 12, 15 December 1974, pages 708~710
and by H. Kressel and I. Ladany in "Reliability Aspects
30 and Facet Damage in High Power Emission From (AlGa)As CW
Laser Diodes at Room Temperature", RCA Rev., Vol. 36,
June 1975, pages 231~239.
Summary of the Invention
It has been discovered that lead silicate glass
35 is particularly suited as a coating material to protect
semiconductor optical devices and to adjust surface
reflectivity. The coating is easily applied to
;~ semiconductor material by sputtering from a preformed
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glass body whose composition is chosen so as to achieve
suitable thermal and optical properties.
In accordance with an aspect of the invention there is
provided a semiconductor optical device comprising a body
of at least one semiconductor material at least a portion
of which is coated with at least a first layer of a
dielectric material characterized in that said dielectric
material consists essentially of a glass, at least 90
percent by weight of said glass being composed of PbO and
SiO2, the molar ratio between PbO and SiO2 being in
the range of from 20:80 to 70:30.
Brief Description of the Drawinq
Fig. 1 shows schematically and greatly magnified a
lasar diode equipped with a lead silicate protective
coating;
Fig. 2 shows graphically the index of refraction and
the linear expansion coefficient of lead silicate glass as
a function of glass composition; and
Fig. 3 shows graphically the light output of a GaAs
laser diode before and after coating of a light emitting
facet with lead silicate glass.
Detailed Description
Fig. 1 shows GaAs substrate 1, electrically active
layer 2, waveguiding strip 3, lead silicate glass coatings
4, electrical contact pad 5, and electrical wire 6.
Coatings 4 are conveniently deposited by sputtering from a
preformed glass body having the desired composition. The
resulting coatings have uniform composition, they adhere
well, and they are free of pinholes.
Fig. 2 which is based on G. W. Morey, Properties of
Glass, Reinhold Press, 1954 ~2nd ed.), page 375 and G. J.
Bair, "The Constitution of Lead Oxide Silica Glasses:II,
the Correlation of Physical Properties with Atomic
Arrangement", Journal of the American Ceramic Society, Vol
19, pages 341-358 (1936), may be helpful in selecting
glass composition with respect to index of refraction or
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linear expansion coefflcient. It can be seen, for
example, that a molar ratio of approximately 34:66 for
g}ass constituents PbO and SiO2, leads to a glass whose
linear expansion coefficient closely matches that of
GaAs. Alternatively, a molar ratio o~ approximately
49.5:50.5 results in a glass having a refractive index of
n2=1.9 which is particularly desirable for an
antireflection coating on GaAs which has
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a refractive index of nl=3.61.
Lead silicate glass may also be applied
beneficially to other semiconductor materials such as
GaAlAs, GaP, GaAsP, GaInAsP, and GaAsSb. More generally,
5 semiconductor materials whose linear expansion
coefficient lies in a preferred range of from
qlx10~6/C to 14x10~6/C may be coated with a
t:hermally compatible lead silicate glass. The composition
of lead silicate glass should preferably lie in the range
10 Of from essentially 20 mole percent PbO and 80 mole
percent SiO2 to essentially 70 mole percent PbO and 30
mole percent SiO2. Amounts of at least 20, and
preferably 30, mole percent PbO are desirable to ensure .-;~
proper fusing of the lead silicate glass at relatively
15 low temperatures. Amounts of at least 30, and preferably
40, mole percent SiO2 are desirable to ensure glass
formation. The presence of Cu, Na, or K ions is known to
shorten the useful life of optical diodes and should be
minimized in the glass coating. Transparent oxides such
20 as B203, A1203, and ZrO2 can be tolerated in a
combined amount of up to ten percent by weight and may be
helpful to maintain the coating in a glassy state, e.g.,
where high temperatures are encountered. Such glasses
are discussed in Robert H. Dalton, "Solder Glass
25 Sealing", Journal of the American Ceramic Society, Vol.
-
39, pages 109~112.
FIG. 3 shows a beneficial effect realized by theapplication of a lead silicate glass coating on a GaAs
laser diode. Solid curves labelled A and B correspond to
30 light output from the two light emitting facets of an
uncoated GaAs laser diode. Dashed curves labelled A' and
B' correspond to light output from a laser diode having
one light emitting facet coated with a layer of lead
silicate glass. Specifically, curve A' corresponds to
35 light output from the uncoated facet and curve B' to light
output from the coated facet of the laser diode. It can
be seen that, in the presence of the coating, laser light
is emitted at greater intensity and more nearly as a
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linear function of diode current. This advantage is due
to reduced facet reflectivity in the presence of the
coating and can be realized not only with laser diodes
but also with other light emitting semiconductor optical
5 devices. In the case of photodiodes a corresponding
advantage is increased sensitivity of the coated diode
and, in all cases, coated semiconductor optical devices
are effectively protected from harmful atmospheric
influence.
In general, coatings having a thickness equal to
one~half of the wavelength of light traversing the
surface do not materially affect the reflectivity of the
coated surface. Such coatings may be used for surface
protection and may have a composition selected primarily
15 with regard to thermal compatibility with the
semiconductor material. Coatings having an optical
thickness equal to one~fourth of the wavelength are
particularly suited as antireflection coatings. Such
coatings, when made of a glass whose refractive index
20 approximately equals the square root of the refractive
index of the semiconductor material, may serve to reduce
surface reflectivity virtually to zero. More generally,
surface reflectivity may be adjusted to any value between
zero and the reflectivity of the uncoated surface by a
25 coating having a thickness in the range of from
one~quarter to one~half of the wavelength and having a
composition which results in a glass whose refractive
index is approximately the square root of the refractive
index of the semiconductor material.
While a molar ratio of 49.5:50.5 of glass
constituents PbO:SiO2 is optimal in an antireflection
coating on GaAs, glasses in the compositional range of
from 30:70 to 60:40 mole percent, when deposited in an
optical thickness of one~fourth of the wavelength, can
35serve to reduce surface reflectivity of a coated GaAs
surface to a value of less than approximately one
percent. To avoid undue strain between coating and GaAs
substrate, PbO contents should preferably be in the range
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- 6 - Barnes-Kyle-Van Uitert 1-2-90
of from 30 to 40 mole percent. If an increase in surface
reflectivity is desired, an additional layer of a highly
reflective material such as gold may be deposited on the
g]Lass layer, e.g., by conventional evaporation
5 techniques.
Escample:
Lead silicate glass containing 54 mole percent
PbO and 46 mole percent SiO2 was cast as a disc having
a diameter of 15 cm. The disc was mounted in a radio
10 frequency sputtering apparatus equipped with an oil
diffusion pump. An atmosphere of 80 percent argon and 20
percent oxygen at a total pressure Of 10~2 Torr was
maintained in the apparatus during sputtering. Radio
frequency power to the cathode was 100 Watts
15 corresponding to an average power density of 0.56
Watts/cm2. Lead silicate glass layers having a
thickness in the range of from 40 nm to 250 nm were
deposited on GaAs~AlGaAs laser diodes of the double
heterostructure type. The distance between the target
20 and the substrate was 38 mm and a deposition rate of two
nm per minute was obtained. Uniform, tightly adhering
films free of pinholes were obtained. Measurements
carried out on a laser diode having a glass coating
thickness of 113 nm are depicted in FIG. 3. This
25 thickness corresponds to an optical thickness of 0.27
wavelengths.
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