Patent Application
20060056473
(a) Fields of the Invention
The present invention relates to semiconductor light-emitting elements applicable to surface emitting semiconductor laser elements or the like, and to methods for fabricating such an element.
(b) Description of Related Art
In recent years, rapid proliferation of the Internet has pushed the amount of data transmission sharply, so that high-capacity data communications networks have been required.
Surface emitting laser elements (or vertical cavity surface emitting laser elements (VCSELs)) are characterized in that laser light can be emitted in a perpendicular direction to the principal surface of a substrate formed with the element and in that the element has a low threshold current and a high power conversion efficiency. In addition to this, the surface emitting laser elements have various advantages that: they can output circular outgoing light whose cross section perpendicular to the optical axis is circular; two-dimensional arrangement of them is facilitated; and inspection of them is carried out easily; other advantages. Moreover, the surface emitting laser elements are expected to be fabricated at low cost. Thus, in the future, the surface emitting laser elements are expected to obtain more increasing demand as light sources for high-capacity data communications.
In an optical communications system made by combining a surface emitting laser element with a plastic optical fiber (POF), the surface emitting laser element and the plastic optical fiber can both be fabricated at low cost, so that this system offers prospects for use as a system for short-range communications.
As is apparent from the above, the surface emitting laser element holding great promise as a light source for optical communications has a structural characteristic in that it includes a cavity and an active layer sandwiched by two reflecting mirror films with high reflectivities and in that a portion of the element forms a current confinement region (see, for example, Japanese Unexamined Patent Publications Nos. 2004-055688, 2003-188475, and 2003-086896).
In general, a reflecting mirror film used for a surface emitting laser element is a reflecting mirror of a multilayer film formed by epitaxial growth. The multilayer-film reflecting mirror is formed by alternately stacking two types of thin films made of different materials with different refractive indices, and the formed mirror can provide a reflectivity as high as 99.9% or more. As the material for the reflecting mirror film, a multilayer film of semiconductor is typically employed because this film has a high uniformity and can be formed simultaneously with formation of an active layer and the like during crystal growth. Other than this film, a dielectric multilayer film or the like is employed as the reflecting mirror film.
Current confinement regions for confining current injected into an active layer are classified into two broad categories: a proton implantation type in which protons are implanted into a cavity to form an insulating region and the formed region defines a current injection region; and a selective oxidation type in which an insulating oxide film of high resistance is formed around an active layer and the formed film defines a current injection region. The previous mainstream was the former, that is, the proton implantation type, but the recent mainstream has been shifting to the latter, that is, the selective oxidation type because of ease of its fabrication process, its high reproducibility, and other advantages.
Hereinafter, a selective oxidation type surface emitting laser element according to a first conventional example will be described with reference to
In order to use a surface emitting laser element for a light source for data communications, it is important for the element to have a structure capable of operating at high speed in a high frequency range. In order for the surface emitting laser element to accomplish a high-speed operation above 10 Gbit/s in the future, it is especially important to reduce the parasitic capacitance of the element.
A structural approach of the surface emitting laser element currently used in general is to employ the structure as shown in
However, the inorganic insulating film has a comparatively high relative permittivity. For example, SiO2 has a relative permittivity of about four. Further, there is a limit (an upper limit) on the maximum formable thickness thereof, which causes the problem of an increase in the parasitic capacitance of the surface emitting laser element.
As one of solutions to this problem, the structure shown in
However, when the technique in which the mesa structure is buried in the polyimide resin layer 215 is employed like the second conventional example shown in
Furthermore, if even part of the polyimide resin layer 215 is brought into contact with the semiconductor surface or exposed from the surface of the element, alteration of the polyimide resin layer 215 such as oxidation arises in a fabrication process of the element. As a consequence, the altered polyimide resin layer 215 chemically reacts with the semiconductor material. In addition, in the mesa structure having a certain side shape, an ununiform stress is applied to the polyimide resin layer 215, so that the gap mentioned above is more likely to be generated therebetween. These troubles cause a problem of degradation of reliability of the surface emitting laser element.
An object of the present invention is to solve the conventional problems mentioned above and to provide a surface emitting laser element capable of highly reliable high-speed modulation.
To attain the above object, the present invention has the structure in which an insulating film covers a resin layer which fills a space surrounding a mesa structure in a semiconductor light-emitting element.
To be more specific, a semiconductor light-emitting element according to the present invention is characterized by including: a mesa structure of semiconductor formed to have a convex cross section; a first insulating film formed on a side surface of the mesa structure; a resin layer formed on the first insulating film to fill a space surrounding the mesa structure; a second insulating film covering the resin layer; and an electrode formed on the second insulating film to come into contact with part of the top surface of the mesa structure.
With the semiconductor light-emitting element of the present invention, the resin layer is prevented from coming into direct contact with the mesa structure and from being directly subjected to various atmospheres during fabrication processes, so that alteration of the resin layer can be avoided. This eliminates creation of a gap between the resin layer and the mesa structure to planarize the surrounding portion of the mesa structure by the resin layer. Therefore, a break in the electrode in contact with the top surface of the mesa structure is difficult to cause, and thereby a semiconductor light-emitting element with high reliability can be provided. That is to say, a surface emitting semiconductor laser element can be fabricated which has high reliability and excellent controllability of transverse mode and which can be modulated at high speed.
Preferably, in the semiconductor light-emitting element of the present invention, the resin layer is covered continuously with the first and second insulating films. With this element, degradation and alteration of the resin layer in fabrication steps can be prevented more reliably.
Preferably, in the semiconductor light-emitting element of the present invention, the resin layer has a relative permittivity of three or smaller. With this element, the parasitic capacitance associated with the contact portion of the resin layer with the electrode is decreased, which provides good high frequency characteristics.
Preferably, in the above case, the resin layer is made of benzocyclobutene (BCB) resin. Since BCB resin has a smaller relative permittivity and shrinks in volume less than polyimide resin, the mesa structure can be buried flat in the BCB resin with no gap created.
Preferably, in the semiconductor light-emitting element of the present invention, the first insulating film is formed to come into contact with the side surface of the mesa structure of the semiconductor and its surrounding portion, and the resin layer, the first insulating film, and the second insulating film are in contact with each other. With this element, alteration or the like of the resin layer is more difficult to cause, which provides a more stable resin layer.
Preferably, in the above case, at least part of the top surface of the resin layer is higher than the top surface of the mesa structure. With this element, the top surface of the mesa structure is lower than the top surface of the resin layer, and in addition, than the top surface of the second insulating film located on the resin layer. Therefore, jig or the like is unlikely to touch the top surface of the mesa structure in fabrication steps, so that the mesa structure can be protected from mechanical impact.
Preferably, in the semiconductor light-emitting element of the present invention, the mesa structure has a current confinement region formed selectively in a portion thereof. With this element, a current can be effectively injected into a desired portion of the mesa structure, so that a loss of the injected current can be reduced.
Preferably, in the above case, the current confinement region is formed so that the region is fully surrounded with an oxidized insulating region made by selective oxidation.
Preferably, in the above case, the current confinement region is formed so that the region is fully surrounded with an insulating region made by selective implantation of protons.
Preferably, in the case where the semiconductor light-emitting element of the present invention includes the current confinement region, the mesa structure has: lower and upper reflecting mirror films formed in lower and upper portions of the mesa structure, respectively; and an active layer formed between the lower and upper reflecting mirror films and in parallel with the top surface of the mesa structure. With this element, the lower and upper reflecting mirror films constitute a laser cavity. Therefore, a surface emitting semiconductor laser element can be provided which emits light of a predetermined wavelength perpendicularly to the principal surface of the active layer.
Preferably, in the above case, the lower and upper reflecting mirror films include semiconductor multilayer films, respectively. With this element, the reflecting mirror film with high reflectivity can be provided. Therefore, a high-power semiconductor laser element with high efficiency can be provided.
Preferably, in the case where the semiconductor light-emitting element of the present invention includes the active layer, the side surface of a portion of the mesa structure at which the active layer is located has an inclined angle of 40° or smaller with respect to a principal surface of the active layer. With this element, heat generated in the active layer can be efficiently dissipated downwardly from the mesa structure, and a uniform stress can be applied to the resin layer in contact with the first insulating film formed on the side surface of the mesa structure. This makes it difficult to cause a gap between the resin layer and the mesa structure.
Preferably, in the semiconductor light-emitting element of the present invention, the first and second insulating films are made of silicon oxide, silicon nitride, silicon oxynitride, niobium oxide, zirconium oxide, or tantalum oxide.
A method for fabricating a semiconductor light-emitting element according to the present invention is characterized by including the steps of: forming a semiconductor multilayer film on a substrate; selectively etching the semiconductor multilayer film to form the semiconductor multilayer film into a mesa structure with a convex cross section; forming a first insulating film on a side surface of the mesa structure and its surrounding portion; forming a resin layer on the first insulating film to fill a space surrounding the mesa structure; forming a second insulating film to cover the top of the resin layer and the perimeter of the top surface of the mesa structure; and forming an electrode on the second insulating film to come into contact with part of the top surface of the mesa structure.
With the method for fabricating a semiconductor light-emitting element according to the present invention, the semiconductor light-emitting element of the present invention can be fabricated reliably.
Preferably, in the method for fabricating a semiconductor light-emitting element according to the present invention, in the step of forming a resin layer, a coating method is carried out at least twice to apply the resin layer, and after every coating, the applied resin layer is cured. With this method, in filling a space surrounding the mesa structure with the resin layer, a flatter filling can be made.
Preferably, in the method for fabricating a semiconductor light-emitting element according to the present invention, a photosensitive resin material is used for at least part of the resin layer. With this method, a lithography method can change the thickness of the resin layer selectively. Therefore, various filling shapes can be employed.
Preferably, in the method for fabricating a semiconductor light-emitting element according to the present invention, the step of forming a resin layer includes: the substep of selectively light-exposing the photosensitive resin material; and the substep of developing the light-exposed resin material.
Preferably, in the method for fabricating a semiconductor light-emitting element according to the present invention, the resin layer is covered continuously with the first and second insulating films. With this method, in the step of forming a resin layer, degradation and alteration of the resin layer can be prevented more reliably.
A first embodiment of the present invention will be described with reference to the accompanying drawings.
Referring to
The contact layer 107, the upper reflecting mirror film 106, the current confinement layer 105, the upper spacer layer 104, the quantum well layer 103, and the lower spacer layer 102 are etched in mesa shape to expose the lower reflecting mirror film 101, and thus the etched layers and films constitute a mesa structure 120.
On a side surface of the mesa structure 120 and a region of the lower reflecting mirror film 101 exposed beside the mesa structure 120, a first insulating film 108 is formed which is made of silicon dioxide (SiO2) and has a thickness of 200 nm. In this element, the first insulating film 108 is also formed on the perimeter of the contact layer 107.
On the first insulating film 108, a resin layer 109 of benzocyclobutene (BCB) resin is formed to fill a space surrounding the mesa structure 120 with no gap created and to have a higher level than the top surface of the mesa structure 120.
On the resin layer 109, a second insulating film 110 of silicon dioxide having a thickness of 200 nm is formed to come into contact with the first insulating film 108 at the perimeter of the contact layer 107.
On the second insulating film 110, an upper contact electrode 111 is formed which comes into ohmic contact with the perimeter of the contact layer 107 and which has an electrode opening 111a exposing the contact layer 107. The upper contact electrode 111 is formed to cover the top end and its vicinity of the mesa structure 120 and one corner of the second insulating film 110.
On a surface of the substrate 100 opposite to the side of the lower reflecting mirror film 101 (the back surface of the substrate 100), a lower contact electrode 112 is formed which comes into ohmic contact with the substrate 100.
The lower reflecting mirror film 101 has a stacked structure formed by alternately stacking a first layer of n-type Al0.12Ga0.88As and a second layer of n-type Al0.90Ga0.10As. As an n-type impurity, silicon (Si) is doped. The thicknesses of the first and second layers are λ/4 n (where λ is the oscillation wavelength of the laser element, and n is the refractive index of a medium. Hereinafter, these letters indicate the respective parameters). Taking a combination of the first and second layers as one cycle, 34.5 cycles of the combinations are stacked to form the lower reflecting mirror film 101.
The quantum well layer 103 is formed by alternately stacking a well layer of a non-doped GaAs layer and a barrier layer of an Al0.30Ga0.70As layer. The number of well layers is three.
The lower spacer layer 102 and the upper spacer layer 104 are made of an Al0.30Ga0.70As layer. The quantum well layer 103, the lower spacer layer 102, and the upper spacer layer 104 constitute a quantum well active layer, and the total thickness of the quantum well active layer is λ/n.
The current confinement layer 105 is made of a p-type Al0.98Ga0.02As layer. This raises the Al content of a second layer of p-type Al0.90Ga0.10As forming the upper reflecting mirror film 106 that will be describer later, and the raised value is 0.98. In the first embodiment, the current confinement layer 105 is subjected to selective oxidation utilizing a characteristic in that the oxidation rate increases as the Al content thereof is high. Thereby, the center portion of the current confinement layer 105 is formed into a current confinement region 105a, and the end portion thereof is formed into an oxidized insulating region 105b.
The upper reflecting mirror film 106 has a stacked structure formed by alternately stacking a first layer of p-type Al0.12Ga0.88As and a second layer of p-type Al0.90Ga0.10As. As a p-type impurity, carbon (C) is doped. The thicknesses of the first and second layers are λ/4 n. Taking a combination of the first and second layers as one cycle, 22.5 cycles of the combinations are stacked to form the upper reflecting mirror film 106.
The contact layer 107 is made of a p-type GaAs layer. In order to decrease the contact resistance with the upper contact electrode 111, carbon as a p-type impurity is doped at a concentration of 1×1019 or higher.
Note that the materials for the first and second insulating films 108 and 110 are not limited to an inorganic insulating film made of silicon dioxide. Alternatively, silicon nitride (SiN), silicon oxynitride (SiON), niobium oxide (Nb2O5), zirconium oxide (ZrO2), or tantalum oxide (Ta2O5) can be employed therefor.
A surface emitting laser element 20 shown in
Moreover, as shown in the photograph in
Hereinafter, a fabrication method of the surface emitting semiconductor laser element with the structure shown above will be described with reference to FIGS. 4 to 7. The components of FIGS. 4 to 7 that are the same as those shown in
Referring to
Next, by a chemical vapor deposition (CVD) method or a sputtering method, a silicon oxide film is deposited on the entire surface of the contact layer 107. Thereafter, as shown in
Subsequently, as show in
As shown in
Next, as shown in
Subsequently, as shown in
As shown in
Therefore, as shown in
Subsequently, as shown in
As shown in
Next, as shown in
By a lithography method, on the second insulating film 110 and the contact layer 107, a photoresist pattern (not shown) is formed which has an opening pattern corresponding to an electrode pattern for forming the upper contact electrode 111. Subsequently, on the formed photoresist pattern, a multilayer metal film made of metal capable of ohmic contact with the p-type contact layer 107, such as a multilayer metal film made by stacking titanium (Ti)/platinum (Pt)/gold (Au) from bottom to top, is formed by a vapor deposition method or the like. Thereafter, the photoresist pattern is removed together with the multilayer metal film deposited on the photoresist pattern, that is, by a so-called liftoff method. Thus, as shown in
Subsequently, the back surface of the substrate 100 opposite to the side of the lower reflecting mirror film 101 is polished or etched to thin the substrate to have a thickness of about 150 μm. Then, on the back surface of the thinned substrate, by a vapor deposition method or the like, the lower contact electrode is formed which is made of metal capable of ohmic contact with the n-type substrate 100, such as an alloy of gold (Au), germanium (Ge), and nickel (Ni) alloyed in this order from the substrate. Thermal treatment is performed in a nitrogen atmosphere at about 400° C. for 10 minutes to alloy the upper contact electrode 111 with the contact layer 107, and the lower contact electrode 112 with the substrate 100.
With the fabrication method described above, the surface emitting laser element according to the first embodiment can be provided.
One modification of the first embodiment of the present invention will be described below with reference to the accompanying drawings.
A second embodiment of the present invention will be described below with reference to the accompanying drawings.
Referring to
On a side surface of the contact hole 122, the second insulating film 110 is formed to expose the substrate 100 at the bottom of the hole.
The contact hole 122 is formed with the lower contact electrode 112 with the second insulating film 110 interposed therebetween. At the bottom surface of the contact hole 122, a lower end of the lower contact electrode 112 comes into ohmic contact with the substrate 100, and an upper end thereof reaches the top surface of the resin layer 109.
Thus, in the second embodiment, the contact hole 122 reaching the substrate 100 is formed beside the mesa structure 120, and the lower contact electrode 112 is formed on the upper side of the element. This forms the structure of the element in which the lower contact electrode 112 and the upper contact electrode 111 do not interpose the substrate 100, so that the parasitic capacitance of the surface emitting semiconductor laser element 30 is further reduced.
Formation of the contact hole 122 creates irregularities on the surface of the laser element 30. However, since the resin layer 109 can be formed to have any thickness, the top surfaces of the upper contact electrode 111 and the lower contact electrode 112 which are located above the resin layer 109 can be formed flat.
In the first and second embodiments, description has been made of the case where the mesa structure 120 and the electrode opening 111a are circular. Alternatively, they can also be formed in any shape other than circular shape. Also in this case, the effects of the present invention can be effectively exerted.
In the first and second embodiments, the n-type lower reflecting mirror film 101, the p-type upper reflecting mirror film, and the p-type contact layer 107 are sequentially formed on the substrate 100 of n-type GaAs, but the combination of components of the element is not limited to this. For example, a p-type lower reflecting mirror film, an n-type upper reflecting mirror film, and an n-type contact layer can be sequentially formed on a substrate of p-type GaAs.
In the first and second embodiments, BCB resin is employed for the resin layer 109. However, the material for this layer is not limited to BCB resin, and a resin material with low permittivity can be employed therefor. As the resin material with low permittivity (low dielectric constant), use can be made of, for example, polyarylene-based resin material, “SiLK” (trademark) manufactured by Dow Chemical Company, or “FLARE” (trademark) manufactured by Honeywell International Incorporated.
In the first and second embodiments, description has been made of the surface emitting semiconductor laser element with an oscillation wavelength of about 850 nm, but irrespective of the oscillation wavelength, the effects of the present invention can be exerted also by a surface emitting semiconductor laser element with any oscillation wavelength.
As described above, in the surface emitting semiconductor laser element according to the present invention, the resin layer is prevented from being oxidized or altered in the element formation steps. This eliminates creation of a gap between the resin layer and the mesa structure. Thus, a surface emitting semiconductor laser element can be fabricated which has high reliability and excellent controllability of transverse mode and which can be modulated at high speed, so that it is useful for a light source for optical communications requiring high speed modulation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-240485 | Aug 2004 | JP | national |
This application claims priority under 35 U.S.C. § 119 on patent applications No. 2004-240485 filed in Japan on Aug. 20, 2004, and No. 2005-160254 filed in Japan on May 31, 2005, the entire contents of which are hereby incorporated by reference.
