Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With reflector – opaque mask – or optical element integral...
Reexamination Certificate
2000-05-31
2004-01-06
Everhart, Caridad (Department: 2825)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With reflector, opaque mask, or optical element integral...
C438S038000, C438S031000, C438S046000, C257S091000
Reexamination Certificate
active
06674095
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the stabilization of surfaces of a compound semiconductor to be used for a semiconductor device and, in particular, to a fabrication technique appropriate to prevent the deterioration of the light-emitting end surface of a semiconductor laser device.
In recent years, semiconductor laser devices have been broadly used as key components for optical disks and optical communications.
In a typical conventional semiconductor laser device, a reflecting film (typically, a dielectric film) for regulating the reflectance is formed at a light-emitting portion, and this film has been used for protecting the light-emitting portion and controlling the reflectance. For this film, Al
2
O
3
(alumina), SiOx (silicon oxide), and SiNx (silicon nitride) have been typically used. Also, a process such as electron beam evaporation (EB evaporation), plasma CVD, ECR-CVD, or sputtering has been typically used to form the reflecting film.
A semiconductor interface state is generated at the light-emitting end surface formed by the aforementioned conventional process. In addition to this, a very intense light passes through this end surface. This has led to the problem that the light-emitting end surface tends to deteriorate, particularly during a high-power operation.
As a solution to this problem, there has been a method in which a bar obtained by cleaving a semiconductor laser device wafer is immersed in a sulfur-containing solution (ammonium sulfide) to thereby form a film containing sulfur of several atomic layers on a cavity end surface, and further a protective layer of Si
3
N
4
or the like is formed on the film, as taught in, for example, the Japanese Patent Laid-Open Publication No. HEI 3-149889.
However, the conventional method of forming the end surface protective layer has the following problems.
That is, if the sulfur atoms adhere to an AlGaAs semiconductor surface, then the interface states are restrained, whereby photoabsorption is inhibited. However, the sulfur layer formed by the solution treatment exhibits weak bond to AlGaAs, and this disadvantageously leads to the detachment of the greater part of the sulfur layer from AlGaAs during the protective layer depositing process.
When forming the protective layer by an electron beam evaporation method on the semiconductor laser device light-emitting end surface that has undergone sulfur treatment, the electron beam impinges on the source of evaporation to heat it up to an elevated temperature to thereby carry out the evaporation. In this stage, the ionized molecules for deposit, part of the electron beam, intense light and so on reach the semiconductor laser device end surface, and these matters act to remove the sulfur layer from the AlGaAs surface.
Furthermore, in case that a compact dielectric protective layer having good adhesion is formed, the electron beam is intensified, and this has caused the problem that the effect of the sulfur treatment is disadvantageously reduced by a large quantity.
When forming a protective layer by a method using plasma, more specifically, an ECR-CVD method, a plasma CVD method, or a sputtering method, instead of the electron beam evaporation method, the plasma impinges on the sulfur layer, removing the sulfur layer from the AlGaAs surface. Therefore, this technique also has the problem that the effect of the sulfur treatment is disadvantageously reduced by a large quantity.
Besides the above method of forming a sulfur layer, a method as a second prior art technique is proposed by, for example, the Japanese Patent Laid-Open Publication No. HEI 7-176819 in which a bar obtained by cleaving a semiconductor laser device wafer is immersed in an ammonium sulfide solution while being irradiated with light, to thereby form a polymolecular layer of sulfur on the semiconductor laser light-emitting end surface.
The polymolecular layer of sulfur formed in this method serves to prevent the sulfur layer from coming off even when ultraviolet rays are applied in the subsequent process.
However, the process of applying light to the bar in the solution has had a problem that, due to difficulties in uniformly dispersing light and the fact that the sulfur polymolecular layer tends to volatilize on an elevated temperature condition (the melting point of monoclinic sulfur is 119° C.) similarly to the aforementioned sulfur layer, the polymolecular layer does not provide sufficient protection for the end surface in depositing a reflecting film at an elevated substrate temperature.
As a third prior art technique, for example, the Japanese Patent Laid-Open Publication No. HEI 4-345079 proposes a method in which after the semiconductor laser device light-emitting end surface has been subjected to an ammonium sulfide solution treatment, a II-VI semiconductor single crystal (such as ZnS) is formed in a high vacuum by an MBE (molecular-beam epitaxy) method.
However, this method requires use of an expensive MBE apparatus. Furthermore, the method using the MBE apparatus has a problem that a technique for crystal growth, which is very hard to control, is needed.
The crystal growth by the MBE method is generally carried out after the formation of an electrode, and this has caused a problem that the satisfactory growth of the II-VI semiconductor single crystal is hard to achieve due to contamination by the electrode or substances adhering to the electrode.
There is a further problem as follows. It is difficult to grow a uniform single crystal II-VI semiconductor on the cleaved surface of AlGaAs, so that unevenness called the hillock frequently occurs. Furthermore, if a single crystal is formed on the semiconductor laser device end surface, there may occur a distortion in the inside of the semiconductor laser device due to differences in coefficient of thermal expansion and lattice constant, eventually causing deterioration of the laser device.
SUMMARY OF THE INVENTION
The present invention has been made to solve the aforementioned problems and has an object to stabilize a surface of a compound semiconductor, such as light-emitting end surfaces of a semiconductor laser device, so that sulfur provided on the surface of the compound semiconductor surface is not detached by the influence of evaporation and the like.
The present invention has another object to increase the lifetime of the semiconductor laser device particularly during a high-power operation.
The above objects are achieved by a compound semiconductor surface stabilizing method comprising steps of:
immersing a region that includes a surface of a compound semiconductor in a solution containing sulfur ions; and
immersing the region that includes the surface of the compound semiconductor in a solution containing cations which react with sulfur to form a sulfide.
Note that the “region that includes a surface” of a compound semiconductor may be part of or whole the compound semiconductor.
According to the above arrangement, the sulfur layer is formed by immersing the region that includes a surface of the compound semiconductor in the solution containing sulfur ions, and thereafter the sulfide layer for protecting the sulfur layer is formed by immersing the sulfur layer in the cationic solution that generates a sulfide through reaction with sulfur. The sulfide layer prevents the sulfur layer from being detached from the surface by application of heating, electrons, ions inside plasma, and light or the like. Therefore, the problem of the coming-off, or detachment, of sulfur that has been caused by the prior art technique (as disclosed in, for examples Japanese Patent Laid-Open Publication No. HEI 3-149889) can be solved. Furthermore, the sulfide layer formed in accordance with the invention is superior in stability at high temperatures to the sulfur polymolecular layer formed by the second prior art technique (as disclosed in, for example, Japanese Patent Laid-Open Publication No. HEI 7-176819), and this can prevent the detachment of sulfur even if a high-temperature treatment is performed. Taking advantage of the solution react
Everhart Caridad
Sharp Kabushiki Kaisha
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