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.
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This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2009-279328 filed on Dec. 9, 2009.
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.
SUMMARYAccording 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.
Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein:
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 EmbodimentThe n-type lower DBR 102 is 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 nr is 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 3×1018 cm−3 for example.
A lower spacer layer of the active region 104 is 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 DBR 106 is 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 3×1018 cm−3 for example. A contact layer 106A comprised of p-type GaAs is formed at a top layer of the upper DBR 106, and a current narrowing layer 108 comprised of p-type AlAs is formed at a bottom layer of the upper DBR 106 or inside of the upper DBR 106.
A cylindrical mesa (a columnar structure) M is formed on the substrate 100 by etching a semiconductor layer from the upper DBR 106 to the lower DBR 102. The current narrowing layer 108 is exposed on the side surface of the mesa M, and has an oxidization region 108A which is selectively oxidized from the side surface, and a conductive region (oxidized aperture) 108B surrounded by the oxidization region 108A. In the oxidization process of the current narrowing layer 108, 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 substrate 100 of the conductive region 108B, becomes a round shape which reflects the outer shape of the mesa M, and the center of the conductive region 108B corresponds to the axial center of the mesa M which means an optical axis. The radius of the conductive region 110B may have the size at which the high-order lateral mode oscillation occurs. For example, the radius of the conductive region 110B may be equal to or larger than 5 μm in a wavelength range of 780 nm.
An annular metallic p-side electrode 110 is formed at the top layer of the mesa M. The p-side electrode 110 is comprised of a metal formed by stacking Au or Ti/Au for example, and is ohmic connected to the contact layer 106A of the upper DBR 106. The outside diameter of the p-side electrode 110 is larger than the radius of the conductive region 108B. The circular opening is formed at the center of the p-side electrode 110, and this opening defines a beam window 110A which emits a beam. The center of the beam window 110A corresponds to the optical axis of the mesa M, and the radius of the beam window 110A is larger than the radius of the conductive region 108B.
A circular first insulating film 112 is formed on the p-side electrode 110 to cover the beam window 110A. The first insulating film 112 is comprised of a material that is able to transmit the oscillation wavelength. The outside diameter of the first insulating film 112 is smaller than the outside diameter of the p-side electrode, and larger than the radius of the beam window 110A. Therefore, the beam window 110A is covered by the first insulating film 112 completely, but a part of the p-side electrode 110 is exposed by the first insulating film 112.
An interlayer insulating film 114 covering the edge of the bottom, side, and top of the mesa M is formed. The edge of the interlayer insulating film 114 covers the part of the p-side electrode 110, and a annular contact hole 116 which exposes the p-side electrode 110 is formed between the interlayer insulating film 114 and the first insulating film 112.
A circular second insulating film 118 comprised of a material that is able to transmit the oscillation wavelength is formed on the first insulating film 112. The center of the second insulating film 118 corresponds to the optical axis, and the outside diameter of the second insulating film 118 is smaller than the radius of the conductive region 108B. For example, when the radius of the conductive region 108B is about 5 μm, the radius of the second insulating film 118 is about 3 μm. In the preferable exemplary embodiment, the second insulating film 118 can be formed with the same process as that of the interlayer insulating film 114 by using the same material as that of the interlayer insulating film 114. An n-side electrode 120 that is electrically connected to the lower DBR 102 is formed on the back side of the substrate 100.
As illustrated in
Preferably, it is desirable to select a material that makes the difference between the refractive indexes n1 and n2 large. This makes it possible to make a difference of reflection ratio between the first region Z1 and the first region Z2 (r1−r2) large. For example, the first insulating film 112 may be comprised of SiON, and the second insulating film 118 may be comprised of SiN. In addition to this, the first insulating film 112 and the second insulating film 118 may be comprised of combinations indicated in
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 DBR 106 is composed by stacking Al0.9Ga0.1As layer and Al0.3Ga0.7As layer 24 periods.
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 to
A resist pattern is formed on the contact layer 106A by the photolithography process conventionally known, and the annular p-side electrode 110 comprised of Au/Ti is formed on the contact layer 106A by the liftoff process. Then, SiON is deposited on whole surface of the substrate 100 by CVD, and the circular first insulating film 112 covering the beam window 110A which is the opening of the p-side electrode 110 is formed by patterning SiON. At this time, the inside of the p-side electrode 110 is covered by the first insulating film 112, and the outside is exposed. The beam window 110A is protected from an exposure and particles generated in subsequent processes by being covered by the first insulating film 112.
As illustrated
Then, the mask MK1 is removed, and the interlayer insulating film 114 comprised of SiN is formed on whole surface of the substrate as illustrated in
According to the fabrication method of the present exemplary embodiment, it is possible to form the second insulating film 118 with an easy process only changing a mask pattern by forming the second insulating film 118 and the interlayer insulating film 114 simultaneously, and mass production at low cost becomes possible. In addition, as the process is processed under the condition that the beam window 110A is protected by the first insulating film 112, 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.
A rectangular hollow cap 350 is fixed on the stem 330 including the chip 310, and a ball lens 360 is fixed in an opening 352 located in the center of the cap 350. The ball lens 360 is laid out so that the optical axis of the ball lens 360 corresponds to the substantial center of the chip 310. When a forward current is applied between leads 340 and 342, a laser beam is emitted from the chip 310 to the vertical direction. The distance between the chip 310 and the ball lens 360 is adjusted so that the ball lens 360 is included within the spread angle θ of the laser beam from the chip 310. A light receiving element and a temperature sensor to monitor the emitting condition of the VCSEL can be included in the cap.
The laser beam emitted from the surface of the chip 310 is focused by the ball lens 360. The focused beam enters to the core of the optical fiber 440, and is transmitted. In above exemplary embodiments, the ball lens 360 is 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 device 400 can include a drive circuit to apply an electrical signal to leads 340 and 342. The optical transmission device 400 can also include a receiving function to receive an optical signal through the optical fiber 440.
The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various exemplary embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.
Claims
1. A vertical cavity surface emitting laser comprising:
- 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.
2. The vertical cavity surface emitting laser according to claim 1, wherein a third refractive index to an oscillation wavelength of a semiconductor layer which composes the second semiconductor multilayer reflector is greater than the second refractive index.
3. The vertical cavity surface emitting laser according to claim 1, further comprising a third insulating layer that covers a sidewall and a part of a top of the columnar structure, wherein an opening to connect to the first electrode is formed between the third insulating film and the first insulating film.
4. The vertical cavity surface emitting laser according to claim 3, wherein the third insulating film is comprised of a same material as that of the second insulating film.
5. The vertical cavity surface emitting laser according to claim 1, wherein the radius of the conductive region is at least 5 μm.
6. A fabrication method of a vertical cavity surface emitting laser having a columnar structure on a substrate, the fabrication method comprising:
- stacking a semiconductor layer including a first semiconductor multilayer reflector of a first conductive type, an active region, a current narrowing layer which is conductive, a second semiconductor multilayer reflector of a second conductive type on the substrate;
- forming a first electrode which is annular and defines a beam window on the second semiconductor multilayer reflector;
- forming a first insulating film that is comprised of a material having a first refractive index to an oscillation wavelength on the first electrode to cover a beam window of the first electrode;
- forming the columnar structure including the first electrode and the first insulating film at a top on the substrate by etching the semiconductor layer from the second semiconductor multilayer reflector to the first semiconductor multilayer reflector;
- forming an oxidization region and a conductive region surrounded by the oxidization region inside of a current narrowing layer by oxidizing the current narrowing layer inside of the columnar structure selectively;
- forming a second insulating film that is comprised of a material having a second refractive index that is greater than the first refractive index on whole area of the substrate including the columnar structure; and
- forming a pattern of the second insulating film that corresponds to a center of the conductive region and is smaller than a radius of the conductive region, and an opening that exposes the first electrode on the first insulating film by removing the second insulating film at the top of the columnar structure selectively.
7. The fabrication method according to claim 7 wherein a third refractive index to an oscillation wavelength of a semiconductor layer composing the second semiconductor multilayer reflector is greater than the second refractive index.
8. A vertical cavity surface emitting laser device comprising:
- a vertical cavity surface emitting laser 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; and
- an optical member that receives a beam from the vertical cavity surface emitting laser.
9. An optical transmission device comprising:
- a vertical cavity surface emitting laser device that comprises: a vertical cavity surface emitting laser that includes: 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; and an optical member that receives a beam from the vertical cavity surface emitting laser; and
- a transmission unit that transmits a laser beam emitted from the vertical cavity surface emitting laser device through an optical medium.
10. An information processing apparatus comprising:
- a vertical cavity surface emitting laser that includes: 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;
- a focusing unit that focuses a laser beam emitted from the vertical cavity surface emitting laser onto a record medium; and
- a structure which scans the laser beam focused by the focusing unit on the record medium.
Type: Application
Filed: May 17, 2010
Publication Date: Jun 9, 2011
Applicant: FUJI XEROX CO., LTD. (Tokyo)
Inventors: Kazutaka Takeda (Kanagawa), Masahiro Yoshikawa (Kanagawa), Kazuyuki Matsushita (Kanagawa)
Application Number: 12/781,175
International Classification: H04B 10/04 (20060101); H01S 5/183 (20060101); H01L 33/00 (20100101); G11B 7/00 (20060101);