~ arlle~Kyle~Van U~te~ 1-2-~0 GLASS COATING FOR SEMICONDUCTOR OPTICAL ~EVICES
Back~round of the Invention _ _ ~__ __ ._ ___ _ __ _ _ 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 - . ' --, , , S40i - 2 - Barnes-Kyle-Van Uitert1-2-90 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 .. .. . . ...
,, :- .-. . . : :, :: . .
.. .: , . . . . ..
-: .. :. . .
.~
., . ,., ", ; :: : .
, ~: ~ : , . ... .
r ~
l~S~
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 . .~ , .
.. , , ;, .
: .. ' . . . :
:, . .: - . : -, . ,, ~ : :
' :' "' ~ ` ' , :' ' 40~
- 3a -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 . ::- .::: : , :
: , . :
: : . . ~, ,. ~: :
. .
4~)1 - 4 - Barnes-Xyle-Van Uitert 1-2-90 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 ,,, : .. ....
a~
- 5 - Barnes-Kyle-Van Uitert 1-2-90 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 .:. . . .:.,: :
.
.: .: :. :. .~.
l~S40i - 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.
,: , ' : ' ~ ' , -.,