Vertical cavity surface emitting laser, vertical cavity surface emitting laser device, optical transmission device, and information processing apparatus

A vertical cavity surface emitting laser that includes: a substrate; a first semiconductor multilayer reflector; an active region; a second semiconductor multilayer reflector; a columnar structure formed from the second semiconductor multilayer reflector to the first semiconductor multilayer reflector; a current narrowing layer formed inside of the columnar structure and having a conductive region surrounded by an oxidization region; a first electrode formed at a top of the columnar structure, electrically connected to the second semiconductor multilayer reflector and defining a beam window; a first insulating film comprised of a material with a first refractive index and formed on the first electrode to cover the beam window; and a second insulating film comprised of a material with a second refractive index and formed on the first insulating film, of which a radius is smaller than a radius of the conductive region.

BACKGROUND

(i) Technical Field

The present invention relates to a vertical cavity surface emitting laser, a vertical cavity surface emitting laser device, an optical transmission device, and an information processing apparatus.

(ii) Related Art

A vertical cavity surface emitting laser (VCSEL) is used as a light source in a communication device and an image forming apparatus. Single lateral mode, high power and long service life are required for such vertical cavity surface emitting laser used as a light source. In an exemplary selective oxidation type vertical cavity surface emitting laser, a single lateral mode is achieved by reducing a radius of the oxidized aperture of a current narrowing layer to about 2 through 3 μm, but it becomes difficult to obtain an optical output greater than or equal to 3 mW stably.

SUMMARY

According to an aspect of the present invention, there is provided a vertical cavity surface emitting laser device including: a substrate; a first semiconductor multilayer reflector of a first conductive type formed on the substrate; an active region formed on the first semiconductor multilayer reflector; a second semiconductor multilayer reflector of a second conductive type formed on the active region; a columnar structure that is formed from the second semiconductor multilayer reflector to the first semiconductor multilayer reflector on the substrate; a current narrowing layer that is formed inside of the columnar structure, and has a conductive region surrounded by an oxidization region selectively oxidized; a first electrode that is annular, is formed at a top of the columnar structure, is electrically connected to the second semiconductor multilayer reflector, and defines a beam window; a first insulating film that is comprised of a material which has a first refractive index capable of transmitting an oscillation wavelength, and formed on the first electrode to cover the beam window; and a second insulating film that is circular, is comprised of a material which has a second refractive index that is able to transmit an oscillation wavelength and greater than the first refractive index, and is formed on the first insulating film, and of which a radius is smaller than a radius of the conductive region.

DETAILED DESCRIPTION

A description will now be given, with reference to the accompanying drawings, of exemplary embodiments of the present invention. In the following description, a selective oxidation type vertical cavity surface emitting laser will be exemplified, and a vertical cavity surface emitting laser is abbreviated as a VCSEL. The scale in drawings is exaggerated to understand the feature of the present invention, and is not same as the scale of actual devices.

First Exemplary Embodiment

FIG. 1is a schematic cross-section view of a VCSEL in accordance with the first exemplary embodiment of the present invention. As illustrated inFIG. 1, a VCSEL10of the exemplary embodiment is formed by stacking an n-type lower Distributed Bragg Reflector (hereinafter, abbreviated as DBR)102, an active region104, and a p-type upper DBR106on an n-type GaAs substrate100. The n-type lower DBR102is formed by stacking AlGaAs layers with different Al composition alternately. The active region104includes a quantum well layer sandwiched between upper and lower spacer layers. The p-type upper DBR106is formed by stacking AlGaAs layers with different Al composition on the active region104alternately.

The n-type lower DBR102is a multi-layer stack formed by a pair of an Al0.9Ga0.1As layer and an Al0.3Ga0.7As layer for example. The thickness of each layer is λ/4nr(λ is an oscillation wavelength, and nris a refractive index of the medium), and the Al0.9Ga0.1As layer and the Al0.3Ga0.7As layer are stacked alternately 40 periods. A carrier concentration after doping an n-type impurity (silicon) is 3×1018cm−3for example.

A lower spacer layer of the active region104is an undoped Al0.6Ga0.4As layer, quantum well active layers are an undoped Al0.11Ga0.89As quantum well layer and an undoped Al0.3Ga0.7As barrier layer, and an upper spacer layer is an undoped Al0.6Ga0.4As layer.

The p-type upper DBR106is a multi-layer stack formed by a pair of an Al0.9Ga0.1As layer and an Al0.3Ga0.7As layer for example. The thickness of each layer is λ/4nr, and the Al0.9Ga0.1As layer and the Al0.3Ga0.7As layer are stacked alternately 24 periods. A carrier concentration after doping a p-type impurity (carbon) is 3×1018cm−3for example. A contact layer106A comprised of p-type GaAs is formed at a top layer of the upper DBR106, and a current narrowing layer108comprised of p-type AlAs is formed at a bottom layer of the upper DBR106or inside of the upper DBR106.

A cylindrical mesa (a columnar structure) M is formed on the substrate100by etching a semiconductor layer from the upper DBR106to the lower DBR102. The current narrowing layer108is exposed on the side surface of the mesa M, and has an oxidization region108A which is selectively oxidized from the side surface, and a conductive region (oxidized aperture)108B surrounded by the oxidization region108A. In the oxidization process of the current narrowing layer108, the oxidation rate of an AlAs layer is faster than that of an AlGaAs layer, and the oxidization proceeds from the side surface of the mesa M to the inside at an almost constant rate. Therefore, the planar shape of the surface, which is parallel to the principal surface of the substrate100of the conductive region108B, becomes a round shape which reflects the outer shape of the mesa M, and the center of the conductive region108B corresponds to the axial center of the mesa M which means an optical axis. The radius of the conductive region110B may have the size at which the high-order lateral mode oscillation occurs. For example, the radius of the conductive region110B may be equal to or larger than 5 μm in a wavelength range of 780 nm.

An annular metallic p-side electrode110is formed at the top layer of the mesa M. The p-side electrode110is comprised of a metal formed by stacking Au or Ti/Au for example, and is ohmic connected to the contact layer106A of the upper DBR106. The outside diameter of the p-side electrode110is larger than the radius of the conductive region108B. The circular opening is formed at the center of the p-side electrode110, and this opening defines a beam window110A which emits a beam. The center of the beam window110A corresponds to the optical axis of the mesa M, and the radius of the beam window110A is larger than the radius of the conductive region108B.

A circular first insulating film112is formed on the p-side electrode110to cover the beam window110A. The first insulating film112is comprised of a material that is able to transmit the oscillation wavelength. The outside diameter of the first insulating film112is smaller than the outside diameter of the p-side electrode, and larger than the radius of the beam window110A. Therefore, the beam window110A is covered by the first insulating film112completely, but a part of the p-side electrode110is exposed by the first insulating film112.

An interlayer insulating film114covering the edge of the bottom, side, and top of the mesa M is formed. The edge of the interlayer insulating film114covers the part of the p-side electrode110, and a annular contact hole116which exposes the p-side electrode110is formed between the interlayer insulating film114and the first insulating film112.

A circular second insulating film118comprised of a material that is able to transmit the oscillation wavelength is formed on the first insulating film112. The center of the second insulating film118corresponds to the optical axis, and the outside diameter of the second insulating film118is smaller than the radius of the conductive region108B. For example, when the radius of the conductive region108B is about 5 μm, the radius of the second insulating film118is about 3 μm. In the preferable exemplary embodiment, the second insulating film118can be formed with the same process as that of the interlayer insulating film114by using the same material as that of the interlayer insulating film114. An n-side electrode120that is electrically connected to the lower DBR102is formed on the back side of the substrate100.

FIG. 2is an enlarged cross-section views of the top of the mesa of the VCSEL inFIG. 1. In the present exemplary embodiment, the refractive index of the first insulating film112is n1, the refractive index of the second insulating film118is n2, the refractive index of the semiconductor layer (the contact layer106A) of the upper DBR106is n3. Then, the relation among n1, n2, and n3is n1<n2<n3. Each film thickness of the first insulating film112and the second insulating film118is odd multiples of the wavelength of the medium λ/4, which means (2n−1) λ/4 (n is positive integer).

As illustrated inFIG. 2, there are a circular first region Z1where the first insulating film112is formed and the second insulating film118is stacked on the first insulating film112, and an annular second region X2where only the first insulating film112is formed around the first region Z1, on the contact layer106A exposed by the beam window110A. The center of the first region Z1corresponds to the center of the conductive region110B (the optical axis), but the size of the first region Z1is smaller than the radius of the conductive region110B. As the second insulating film118of which the refractive index n2is greater than the refractive index n1of the first insulating film112is formed in the first region Z1, the reflection ratio r1of the upper DBR106including the first region Z1is higher than the reflection ratio r2of the upper DBR106including the second region Z2. Therefore, a high-order lateral mode oscillation is suppressed in the upper DBR106including the second region Z2, and a fundamental transverse mode oscillation is accelerated in the upper DBR106including the first region Z1. Therefore, it is possible to increase the optical output by making the radius of the conductive region108B (the radius of the oxidized aperture) be the size at which a high-order lateral mode oscillates.

Preferably, it is desirable to select a material that makes the difference between the refractive indexes n1and n2large. This makes it possible to make a difference of reflection ratio between the first region Z1and the first region Z2(r1−r2) large. For example, the first insulating film112may be comprised of SiON, and the second insulating film118may be comprised of SiN. In addition to this, the first insulating film112and the second insulating film118may be comprised of combinations indicated inFIG. 3. For example, when the first insulating film112is comprised of SiON, the second insulating film118may be comprised of TiO2. When the first insulating film112is comprised of SiO2, the second insulating film118may be comprised of SiN.

A description will now be given of a simulation result of a reflection ratio of the upper DBR of the VCSEL of the present exemplary embodiment. Suppose that the upper DBR106is composed by stacking Al0.9Ga0.1As layer and Al0.3Ga0.7As layer24periods.FIG. 4Aillustrates a reflection ratio r2of the upper DBR including the second region Z2when SiON with a film thickness of λ/4 is formed as the first insulating film112.FIG. 4Billustrates a reflection ratio r1of the upper DBR including the first region Z1when SiON with a film thickness of λ/4 is formed as the first insulating film112, and SiN with a film thickness of λ/4 is formed as the second insulating film118. In the second region Z2illustrated inFIG. 4A, the reflection ratio r2is about 99.2% in a wavelength range of 780 nm. In the first region Z1illustrated inFIG. 4B, the reflection ratio r1is about 99.7% in a wavelength range of 780 nm. A reflection ratio needed for a laser oscillation typically is about 99.5%. Therefore, in the first region Z1, the fundamental lateral mode generated on the optical axis is easily oscillated, and in the second region Z2the high-order lateral mode oscillation away from the optical axis is suppressed. The fundamental transverse mode oscillation is selectively accelerated and the high-order lateral mode oscillation is suppressed, by making the difference between reflection ratios r1of the first region Z1and r2of the second region Z2. Ordinary skilled persons in the art can understand that it is possible to adjust reflection ratios r1and r2by selecting the periodic number of the upper DBR106and materials of first and second insulating films112and118.

Second Exemplary Embodiment

FIG. 5is a cross-section view of a main part of a VCSEL in accordance with a second exemplary embodiment of the present invention. In a VCSEL10A in accordance with the present exemplary embodiment, a taper is formed in the mesa M, and the radius of the mesa gradually narrows along to the top. Such taper of the mesa can be formed by selecting proper etching condition. In addition to the taper of the mesa M, the end surface of the first insulating film112, the end surface of the interlayer insulating film114, and the end surface of the second insulating film118are inclined in a tapered shape. By making the mesa M have a taper structure, the step coverage of the adherent interlayer insulating film114is improved, and it is possible to prevent the disconnection of the interlayer insulating film114. In addition, it is possible to make the film thickness of the interlayer insulating film114that is formed on the side and top of the mesa M almost uniform. When the second insulating film118is formed with the same process as that of the interlayer insulating film114, it is possible to uniformly-control both film thicknesses to be odd multiples of λ/4. Furthermore, it is possible to prevent the disconnection of metallic wiring that is coupled to the p-side electrode110through the contact hole116.

Third Exemplary Embodiment

A description will now be given of a third exemplary embodiment. The third exemplary embodiment relates to a preferable fabrication method of the VCSEL. A fabrication method is described with reference toFIGS. 6A through 7B. As illustrated inFIG. 6A, the n-type lower DBR102, the active region104, and the p-type upper DBR106are stacked on the n-type GaAs substrate100by the metal organic chemical vapor deposition (MOCVD) method. The n-type lower DBR102is composed by stacking Al0.9Ga0.1As and Al0.3Ga0.7As with a carrier concentration of 2×1018cm−3alternately 40 periods so that each film thickness becomes quarter of the wavelength of the medium. The active region104is comprised of an undoped Al0.6Ga0.4As lower spacer layer, an undoped Al0.11Ga0.89As quantum well layer, an undoped Al0.3Ga0.7As barrier layer, and an undoped Al0.6Ga0.4As upper spacer layer. The p-type upper DBR106is composed by stacking a p-type Al0.9Ga0.1As layer and a p-type Al0.3Ga0.7As layer with a carrier concentration of 3×1018cm−3alternately 24 periods so that each film thickness becomes quarter of the wavelength of the medium. The p-type GaAs contact layer106A with a carrier concentration of 1×1019cm−3is formed at the top layer of the upper DBR106, and a p-type AlAs layer is formed at the bottom of the upper DBR106or inside of the upper DBR106. It is not illustrated, but a buffer layer may be provided between the substrate100and the lower DBR102.

A resist pattern is formed on the contact layer106A by the photolithography process conventionally known, and the annular p-side electrode110comprised of Au/Ti is formed on the contact layer106A by the liftoff process. Then, SiON is deposited on whole surface of the substrate100by CVD, and the circular first insulating film112covering the beam window110A which is the opening of the p-side electrode110is formed by patterning SiON. At this time, the inside of the p-side electrode110is covered by the first insulating film112, and the outside is exposed. The beam window110A is protected from an exposure and particles generated in subsequent processes by being covered by the first insulating film112.

As illustratedFIG. 6B, a circular mask MK1is formed on a region including the p-side electrode110and the first insulating film112by the photolithography process. Then, a cylindrical mesa M is formed by etching a semiconductor layer from the upper DBR106to the lower DBR102by the reactive ion etching process using boron trichloride for example. Accordingly, an AlAs layer108inside of the upper DBR106is exposed on the side surface of the mesa M. Then the oxidization process that exposes the substrate to the water-vapor atmosphere with a temperature of 340° C. for a given time is carried out, and the oxidization region108A which is oxidized a certain distance from the side surface of the mesa M is formed inside of the AlAs layer108. The oxidation control is performed so that a radius of plane field of the conductive region108B surrounded by the oxidization region108A becomes larger than the radius needed for a conventional single lateral mode (e.g. 3 μm), and becomes the size at which the high-order lateral mode occurs (e.g. 5 μm).

Then, the mask MK1is removed, and the interlayer insulating film114comprised of SiN is formed on whole surface of the substrate as illustrated inFIG. 7A. The interlayer insulating film114is adjusted so that the film thickness of the top of the mesa M becomes quarter of the wavelength of the medium. Then, as illustrated inFIG. 7B, a mask MK2is formed by the photolithography process, and the interlayer insulating film114exposed by the mask MK2is removed by etching. Preferably, the interlayer insulating film114is etched under the etching condition that the selectivity between the interlayer insulating film114and the first insulating film112can be selected. For example, the reactive ion etching process using an etchant of SF6+O2is carried out. According to this, patterns of the contact hole116and the second insulating film118to the p-side electrode110is formed at the top of the mesa M. Then, a metallic wiring that is coupled to the p-side electrode110through the contact hole116is formed, and the n-side electrode is formed on the back side of the substrate.

According to the fabrication method of the present exemplary embodiment, it is possible to form the second insulating film118with an easy process only changing a mask pattern by forming the second insulating film118and the interlayer insulating film114simultaneously, and mass production at low cost becomes possible. In addition, as the process is processed under the condition that the beam window110A is protected by the first insulating film112, this makes it work for the reliability of the VCSEL. When the insulating layer is formed inside of the contact layer by etching the contact layer, it is difficult to stop the etching with high accuracy. If the film thickness of the etched layer is not uniform, there is a possibility that a reflection ratio changes, and this makes it difficult to obtain a reproducible composition.

In above exemplary embodiments, a description was given of a current narrowing layer comprised of AlAs, but a current narrowing layer may be an AlGaAs layer of which the Al composition is higher than the Al composition of other DBRs. In addition, the radius of the conductive region (the oxidized aperture) of the current narrowing layer can be changed appropriately according to required optical output. Furthermore, in above exemplary embodiments, the description was given of an GaAs-based VCSEL, but the present invention can be applied to other VCSELs using other III-V group compound semiconductors. In above exemplary embodiments, the description was given of a single spot VCSEL, but the VCSEL can be a multi-spot VCSEL where multiple mesas (emission portion) are formed on the substrate, or a VCSEL array.

A description will be given of a vertical cavity surface emitting laser device, an optical information processing apparatus, and an optical transmission device using the VCSEL of exemplary embodiments with reference to drawings.FIG. 8Ais a cross-section view illustrating a composition of a vertical cavity surface emitting laser device in which the VCSEL and an optical component are packaged. A vertical cavity surface emitting laser device300fixes a chip310, on which a long resonator VCSEL is formed, to a disk-shaped metal stem330via a conductive bond320. Conductive leads340and342are inserted in a through hole (not illustrated) provided to the stem330, the lead340is electrically connected to the n-side electrode of the VCSEL, and the lead342is electrically connected to the p-side electrode.

A rectangular hollow cap350is fixed on the stem330including the chip310, and a ball lens360is fixed in an opening352located in the center of the cap350. The ball lens360is laid out so that the optical axis of the ball lens360corresponds to the substantial center of the chip310. When a forward current is applied between leads340and342, a laser beam is emitted from the chip310to the vertical direction. The distance between the chip310and the ball lens360is adjusted so that the ball lens360is included within the spread angle θ of the laser beam from the chip310. A light receiving element and a temperature sensor to monitor the emitting condition of the VCSEL can be included in the cap.

FIG. 8Bis a diagram illustrating a composition of another vertical cavity surface emitting laser device. A vertical cavity surface emitting laser device302illustrated inFIG. 8Bfixes a plane glass362in the opening352located in the center of the cap350instead of using the ball lens360. The plane glass362is laid out so that the center of the plane glass362corresponds to the substantial center of the chip310. The distance between the chip310and the plane glass362is adjusted so that the opening radius of the plane glass362becomes equal to or larger than the spread angle θ of the laser beam from the chip310.

FIG. 9is a diagram illustrating a case where the VCSEL is applied to a light source of an optical information processing apparatus. An optical information processing apparatus370is provided with a collimator lens372which receives the laser beam from the vertical cavity surface emitting laser device300or302, in which the long resonator VCSEL is packaged, illustrated inFIG. 8Aor8B, a polygon mirror374which rotates at constant speed and reflects a beam of light from the collimator lens372at constant spread angle, a fθ lens376which receives the laser beam from the polygon mirror374and irradiates the laser beam to a reflection mirror378, the linear reflection mirror378, and a photoreceptor drum (a record medium)380which forms latent images based on the reflection beam from the reflection mirror378. As described above, the laser beam from the VCSEL can be used as a light source of the optical information processing apparatus such as a copier and a printer provided with an optical system which focuses the laser beam from the VCSEL onto the photoreceptor drum and a structure which scans the focused laser beam on the photoreceptor drum.

FIG. 10is a cross-section view illustrating a composition where the vertical cavity surface emitting laser device illustrated inFIG. 8Ais applied to an optical transmission device. An optical transmission device400includes a cylindrical chassis410fixed to the stem330, a sleeve420integrally-formed on the end surface of the chassis410, a ferrule430held in an opening422of the sleeve420, and an optical fiber440held by the ferrule430. The end portion of the chassis410is fixed to a flange332which is circumferentially-formed of the stem330. The ferrule430is laid out in the opening422of the sleeve420accurately, and the optical axis of the optical fiber440is matched to the optical axis of the ball lens360. The core of the optical fiber440is held in a through hole432of the ferrule430.

The laser beam emitted from the surface of the chip310is focused by the ball lens360. The focused beam enters to the core of the optical fiber440, and is transmitted. In above exemplary embodiments, the ball lens360is used, but other lenses such as a biconvex lens and a plane-convex lens can be used besides a ball lens. Furthermore, the optical transmission device400can include a drive circuit to apply an electrical signal to leads340and342. The optical transmission device400can also include a receiving function to receive an optical signal through the optical fiber440.