JP2008211013A - Surface-emitting semiconductor laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the stability of the oscillation of the single lateral mode of an ion implantation type photonic crystalline surface-emitting laser. <P>SOLUTION: A photonic crystalline surface-emitting laser device has a current constriction layer 14 which comprises one or a plurality of layers in an upper DBR mirror 15 and which is formed by ion implantation. A point defect in which a circular hole is not formed is formed on the center of a two-dimensional circular hole arrangement 20. When the width Z of the point defect is set to D, the width of the current opening of a current constriction structure is set to 0.3×D<Z<0.7×D. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a surface emitting semiconductor laser device, and more particularly to a vertical cavity surface emitting semiconductor laser device having a photonic crystal structure and a current confinement structure by an ion implantation method.

  Vertical cavity surface emitting semiconductor laser (VCSEL: Vertical Cavity Surface Emitting Semiconductor Laser, hereinafter simply referred to as surface emitting laser), as its name suggests, the direction in which light resonates is perpendicular to the substrate surface It has been attracting attention as a light source for communication such as optical interconnection, or as a device for various applications such as sensor applications.

  The reason why surface emitting lasers are attracting attention is that surface emitting lasers can form a two-dimensional array structure of elements more easily than conventional edge emitting semiconductor lasers, and they do not need to be cleaved before forming a mirror. It is possible to test the wafer level, and because the volume of the active layer is remarkably small, it can oscillate with a very low threshold current and has low power consumption.

  The surface emitting laser has an advantage that the fundamental mode oscillation can be easily obtained with respect to the longitudinal mode of the oscillation spectrum because the cavity length is extremely short. However, on the other hand, since there is no control mechanism for the transverse mode, a plurality of higher order modes oscillate. The laser light oscillated by the plurality of higher-order transverse modes causes significant deterioration in proportion to the transmission distance during optical transmission, particularly during high-speed modulation. Therefore, various structures have been proposed for surface emitting lasers that realize laser oscillation in the fundamental transverse mode.

  In the surface emitting laser, as a means for widening the light emitting area and obtaining the fundamental transverse mode oscillation, for example, a structure as disclosed in Patent Document 1 can be cited. FIG. 8 is a partial sectional perspective view of the conventional surface emitting laser. This surface emitting laser has an optical confinement structure called a photonic crystal structure, and forms a periodic two-dimensional circular hole array in an upper reflecting mirror structure (upper DBR mirror). The two-dimensional circular hole array slightly reduces the refractive index perceived by light, and acts as a clad for a point defect region where no central circular hole exists. In this surface emitting laser, since the transverse mode control by such a weak refractive index confinement is performed, the area of the light emitting region that oscillates only the fundamental transverse mode can be increased. Therefore, attention has been focused on the possibility that the surface-emitting laser can have a high output and a low resistance. A surface emitting laser having this structure is also called a photonic crystal surface emitting laser.

As the current confinement structure, either an AlAs selective oxidation method (for example, Non-Patent Document 1) or an ion implantation method (for example, Non-Patent Document 2) is mainly used. The AlAs selective oxidation method has an advantage that an increase in device resistance due to the constriction structure can be suppressed because the thickness of the current confinement structure is as small as about 20 nm, for example. However, on the other hand, there is a disadvantage that the device reliability is impaired, for example, stress strain is generated by selective oxidation, and the device is broken during oscillation by the stress strain. On the other hand, the ion implantation method has an advantage that such stress strain does not occur.
Special table 2007-502029 IEEE Journal of Selected Topics in Quantum Electronics, Vol.9, No.5, pp.1439-1445, September / October 2003 IEE Electronics Letters, Vol.41, No.6, pp.326-328, March 2005

  However, even when the ion implantation method is used, as described in Non-Patent Document 2, there is a problem that oscillation of a higher mode occurs during high current implantation or high speed modulation, and the oscillation mode tends to become unstable. It was.

  In view of the above, the present invention has improved a photonic crystal surface emitting laser having a current confinement structure using an ion implantation method, and has a stable single mode in a basic mode even when high current injection or high-speed modulation is adopted. An object of the present invention is to provide a photonic crystal surface emitting laser that can easily oscillate in a transverse mode.

The present invention has been made to solve the above problems. That is, the present invention relates to a surface emitting semiconductor laser device including an upper reflecting mirror structure and a lower reflecting mirror structure formed on a substrate, and a light emitting layer disposed between the upper reflecting mirror structure and the lower reflecting mirror structure. ,
The upper reflecting mirror structure is formed with a two-dimensional hole array having a point defect in the center of which no hole exists, and at least one of the upper reflecting mirror structure and the lower reflecting mirror structure is provided with the light emitting layer. In the vicinity, a current confinement layer is formed in which ions are implanted around the current leaving a current opening in the center,
The surface emitting semiconductor laser device is characterized in that the width (Z) of the current opening is smaller than the diameter (D) of a circle connecting the hole centers closest to the point defect.

  In the surface emitting laser element of the present invention, Z <0.7 × D is preferable, and 0.3 × D <Z <0.5 × D is more preferable.

  Further, it is possible to adopt a configuration in which the holes of the two-dimensional hole array are circular holes. The two-dimensional hole array may be a triangular lattice or a square lattice.

  The ions implanted into the current confinement layer may be hydrogen ions, oxygen ions, or other ions as long as an insulating region can be formed.

  The substrate may be an n-type GaAs substrate.

  The current confinement layer may be configured as a single semiconductor layer having a thickness of ((m / 2) +1/4) λ, where m is a natural number of 1 or more and the oscillation wavelength in the medium is λ. Alternatively, it may be composed of a plurality of semiconductor layers.

  The present invention is particularly applicable to a surface emitting laser element having an oscillation wavelength of 1 μm or more and 1.6 μm or less.

  In the surface emitting laser having the photonic crystal structure of the present invention, the opening width (Z) of the insulating layer by ion implantation is smaller than the width (D) of point defects in the two-dimensional vacancy array (Z < By adopting D), it becomes difficult to obtain a gain in a higher-order mode having a wider electric field intensity distribution than in the fundamental mode. Therefore, even in the case of high current injection or high-speed modulation, only the fundamental mode is stable. Oscillation is possible.

  In a surface emitting laser element having a conventional structure that does not have a photonic crystal structure, when an oxide confinement structure is adopted, a fundamental mode is considered in view of a large difference in refractive index between a current opening and an insulating region (current confinement). Efforts have been made to reduce the area of the opening due to the single transverse mode oscillation by. On the other hand, when a current confinement structure using an ion implantation method is employed in a surface emitting laser element having a conventional structure, efforts have been made to increase the area of the opening in order to reduce element resistance. In a surface emitting laser element having a photonic crystal structure, when forming a current confinement structure using an ion implantation method, there is a structure in which the opening width is increased for the purpose of reducing element resistance, by analogy with the conventional structure. It was adopted. In the present invention, for the photonic crystal surface emitting laser, it was experimentally confirmed that a single transverse mode oscillation was stably obtained by adopting the above-described structure of Z <D and suppressing the area of the opening. The configuration of the present invention is obtained.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a partial sectional perspective view of a light emitting laser according to a first embodiment of the present invention. The surface emitting laser 100 is designed so that the oscillation wavelength is 850 nm. The surface-emitting laser 100 is formed on an n-type GaAs substrate 11 and has an active layer (light emission) formed between a lower DBR mirror 12, an upper DBR mirror 15, and a lower DBR mirror 12 and an upper DBR mirror 15. Layer) 13, an upper electrode 16 formed on the upper DBR mirror 15, and a lower electrode 17 formed on the back surface of the n-type GaAs substrate 11.

  A photonic crystal structure is formed in the upper DBR mirror 15, and a point defect having no circular hole is formed in the central portion of the two-dimensional circular hole array 20 constituting the photonic crystal structure. A current confinement layer 14 is formed in the upper DBR mirror 15 and has a central current opening that forms a conductive portion formed by p-type or n-type impurity implantation, and impurity implantation that cancels the conductivity due to impurity implantation. And a surrounding insulating portion (current constriction portion) formed so as to have insulating properties. The current confinement layer 14 includes one or more specific layers among the layers formed in the upper DBR mirror 15.

In this embodiment, the relationship between the diameter D of the circle obtained by connecting the centers of the plurality of circular holes closest to the central point defect and the diameter Z of the current opening of the current confinement layer is
0.3 × D <Z <0.5 × D
The arrangement of the circular holes and the size of the current openings are determined so that Z may be 0.7 × D or less. By making Z smaller than 0.7 × D, single transverse mode oscillation by fundamental mode oscillation is facilitated, and by making it less than 0.5 × D, single transverse mode oscillation is further facilitated. . Further, by increasing Z to be larger than 0.3 × D, an increase in element resistance is prevented.

The structure of the said embodiment is obtained by the following manufacturing processes, for example. First, n-type Al _0.8 Ga _0.2 As layers and n-type AlAs layers each having a thickness of λ / 4n (where λ is an oscillation wavelength and n is a refractive index) are alternately formed on the n-type GaAs substrate 11 by MOCVD. The lower DBR mirror 12 made of 35 pairs of multilayer films is formed. The carrier concentration of each layer is, for example, 1 × 10 ¹⁸ cm ⁻³ . Next, a lower cladding layer (not shown), an active layer 13 having a four-layer GaAs / AlGaAs quantum well structure, and an upper cladding layer (not shown) are sequentially stacked on the stacked layer.

Further thereon, p-type Al _0.8 Ga _0.2 As layers and p-type Al _0.2 Ga _0.8 As layers each having a thickness of λ / 4n are alternately laminated to form an upper DBR mirror 15 consisting of 30 pairs. The carrier concentration of each layer is, for example, 1 × 10 ¹⁸ cm ⁻³ . A p-type GaAs layer is formed on the surface of the uppermost layer of the upper DBR mirror 15 to constitute the entire layer structure. This state is shown in FIG.

  Next, after the SiNx film 21 is formed on the surface of the laminated structure by using the plasma CVD method, the photoresist film 9 is formed into a cylindrical shape having a diameter of 5 μm to 7 μm by photolithography using a normal photoresist thereon. To process. This width of 5 μm to 7 μm corresponds to the diameter (Z) of the current opening formed by peripheral ion implantation using the photoresist film 9 as a mask, that is, the current opening width. The current opening width is smaller than 10 μm in the diameter (referred to as D) of a circle connecting the centers of a plurality of circular holes closest to the point defect formed in the central portion of the two-dimensional hole array. Note that Au (gold) may be used for the ion implantation mask instead of the photoresist film 9.

Thereafter, using the photoresist film 9 as an implantation mask by an ion implantation apparatus, hydrogen ions (protons) are implanted at an acceleration energy of 400 keV and a dose amount of 4 × 10 ¹⁴ cm ⁻² . A narrowed region and an opening (light emitting region) 18 are formed (FIG. 3). Note that ions to be implanted are not limited to hydrogen, but may be any ion that can form a high-resistance insulating layer, and may be oxygen or the like. After removing the photoresist, annealing is performed at 450 ° C., and then the SiNx film 21 is removed.

  Next, after a new SiNx film is formed on the surface of the laminated structure using the plasma CVD method, SiNx is etched by photolithography using an ordinary photoresist and RIE (reactive ion etching). A hard mask for dimensional circular hole arrangement is formed. The two-dimensional circular hole array is a triangular lattice-shaped two-dimensional circular hole array structure having a point defect in which a circular hole does not exist in the center, a period of 5 μm, and a diameter of each circular hole of 2 μm. Here, the width of the point defect is defined as the diameter of a circle formed by connecting the centers of the circular holes closest to the point defect, and the diameter of the point defect is 10 μm.

Note that the point defect is not limited to one in which one circular hole is removed from the two-dimensional circular hole arrangement as in the present embodiment, and may be one in which a plurality of (for example, seven) circular holes are removed. Next, by using the circular hole arrangement structure formed in the SiNx film as a mask, a circular hole arrangement structure having a depth of about 4 μm is formed in the laminated structure under the SiNx film by ICP (inductively coupled plasma) dry etching using Cl _2. Form. Further, the entire SiNx film is removed by RIE (FIG. 4).

  Next, after a new SiNx film is formed on the surface of the laminated structure using the plasma CVD method, SiNx is formed thereon by photolithography using a normal photoresist and RIE (reactive ion etching). Remove to form a ring-shaped opening. For example, AuZn is vapor-deposited on the portion from which the SiNx film has been removed to form the ring-shaped upper electrode 16. Thereafter, the n-type GaAs substrate 11 is polished from the back surface so that the substrate thickness is about 200 μm, and for example, Cr / Au is vapor-deposited on the polished back surface to form the lower electrode 17. Thus, the surface emitting laser according to the present embodiment is obtained. This structure is shown in FIG.

  A surface-emitting laser element according to the second embodiment of the present invention will be described. The surface emitting laser element of the present embodiment is designed so that the oscillation wavelength is 1300 nm, and the layer structure differs from that of the first embodiment due to the difference in the oscillation wavelength. A manufacturing process of the surface-emitting laser element according to the second embodiment will be described with reference to FIGS. 2 to 5 as in the first embodiment.

First, an n-type Al _0.9 Ga _0.1 As layer and an n-type GaAs layer each having a thickness of λ / 4n (where λ is an oscillation wavelength and n is a refractive index) are formed on the n-type GaAs substrate 11 by MOCVD. Are alternately stacked to form the lower DBR mirror 12 composed of 35 pairs of multilayer films. The carrier concentration of each layer is 1 × 10 ¹⁸ cm ⁻³ . Next, a lower clad layer, three layers of GaInNAsSb / GaNAs quantum well structure active layer 13 and an upper clad layer are sequentially laminated thereon using a gas source MBE method. Furthermore, by using the MOCVD method again, p-type Al _0.9 Ga _0.1 As layers and p-type GaAs layers each having a thickness of λ / 4n are alternately laminated, and an upper DBR mirror 15 consisting of 21 pairs is formed. Form (FIG. 2). The carrier concentration of each layer is 1 × 10 ¹⁸ cm ⁻³ .

Next, after a SiNx film 21 is formed on the surface of the laminated structure using a plasma CVD method, a photoresist film 9 having a diameter of 5 μm to 7 μm is formed thereon by photolithography using a normal photoresist. Process into a cylindrical shape. This width of 5 μm to 7 μm corresponds to the diameter (Z) of the opening of the insulating layer formed by ion implantation, that is, the current opening width. The opening width Z is smaller than the diameter (D) of 10 μm of the circle connecting the center of the hole closest to the point defect at the center of the two-dimensional hole array described later. Instead of the photoresist film used as the ion implantation mask, Au (gold) may be used. Thereafter, using the photoresist film 9 as an ion implantation mask by an ion implantation apparatus, hydrogen ions (protons) are implanted at an acceleration energy of 400 keV and a dose of 4 × 10 ¹⁴ cm ⁻² , and the current confinement layer 14 and the light emitting region 18 are implanted. (FIG. 3). The ions to be implanted are not limited to hydrogen, but may be any ion that can form a high-resistance insulating layer, and may be oxygen or the like. After removing the photoresist film 9, annealing is performed at 550 ° C., and then the SiNx film is removed.

Next, after forming a SiNx film on the surface of the above-mentioned laminated structure using a plasma CVD method, SiNx is etched two-dimensionally by photolithography using a normal photoresist and RIE (reactive ion etching). A circular hole array is formed. The two-dimensional circular hole array has a point defect in which no circular holes are formed in the center, and has a triangular lattice-shaped circular hole array structure with a period of 5 μm and a circular hole diameter of 2 μm. In this case, the diameter of the point defect is defined as the diameter of a circle connecting the centers of the circular holes closest to the point defect, and this diameter is 10 μm. The point defect is not limited to a point defect in which one circular hole is present in the two-dimensional circular hole array, and may be a point defect in which a plurality of (for example, seven) circular holes are not present. Using this circular hole arrangement structure made of SiNx film as a mask, a circular hole arrangement structure having a depth of about 4 μm is formed in the laminated structure under the SiNx film by ICP (inductively coupled plasma) dry etching using Cl ₂ (FIG. 4). Further, all SiNx is removed by RIE.

  Next, after a SiNx film is again formed on the surface of these layer structures by plasma CVD, SiNx is removed by photolithography and RIE (reactive ion etching) using a normal photoresist on the SiNx film. An opening having a shape is formed, and, for example, AuZn is vapor-deposited thereon to form an upper electrode 16 having a ring shape. Thereafter, the substrate is polished to a thickness of about 200 μm and, for example, Cr / Au is vapor-deposited on the back surface to form the lower electrode 17 (FIG. 5). Thus, the surface emitting laser according to this embodiment is completed.

  The characteristics of the surface-emitting laser of the first embodiment and the conventional surface-emitting laser were compared. As the surface emitting laser of the first embodiment, laser elements having an opening width Z of 0.7 μm and 0.5 μm with respect to D = 10 μm, which is the width of a point defect, were formed. Further, as a conventional surface emitting laser, a surface emitting laser having an aperture width M of M = 10 μm (= D) and the other configuration similar to that of the first embodiment was manufactured.

  As a characteristic comparison, the current dependency of the side mode suppression ratio (SMSR) was evaluated. As shown in FIG. 6, the SMSR indicates the ratio between the oscillation intensity in the fundamental mode and the oscillation intensity of the higher-order mode adjacent to the fundamental mode in dB, and is an index indicating the stability of the single mode. Conventionally used. The fact that the SMSR is stable with respect to the injection current means that stable single mode oscillation in the fundamental mode is possible. FIG. 7 shows the current dependency of the SMSR actually measured for the first embodiment and the conventional surface emitting laser.

  As can be seen from FIG. 7, in the conventional surface emitting laser element in which the current injection aperture width Z is 10 μm, which is the same as the point defect width D, the SMSR is reduced by current injection of 30 mA or more, and as a result, higher order Mode oscillation was observed. However, in the surface emitting laser element according to the first embodiment, in the element in which the current injection aperture width Z is 7 μm (0.7 × D), even in the case of current injection of 30 mA or more, the high-order mode In the surface emitting laser element of 5 μm (0.5 × D), there is almost no oscillation in the higher order mode even when a current of 50 mA or more is injected, resulting in a stable single unit. It was observed that mode oscillation was maintained.

  As described above, stable single mode oscillation can be obtained by setting the opening width Z to be smaller than the point defect width D, preferably 0.7 × D or less, more preferably 0.5 × D or less. . Note that the opening width Z is not simply reduced.If the opening width Z is too small, the element resistance increases and the power loss in the fundamental mode increases. It is desirable to set it to 0.3 × D or more.

  Note that it can be easily estimated that the single-mode oscillation similar to that of the surface emitting laser element of the second embodiment can be obtained according to the principle of the present invention. That is, the surface emitting laser element of the present invention has an effect that a single mode oscillation can be stably obtained in an ion implantation type photonic crystal surface emitting laser.

  The current confinement layer constituting the current confinement structure can be composed of one or more specific layers adjacent to the active layer and any number of specific layers among the semiconductor layers constituting the upper DBR mirror and / or the lower DBR mirror. For example, the specific layer disposed in the vicinity of the active layer is a single semiconductor layer having a thickness of ((m / 2) +1/4) λ, where m is a natural number of 1 or more, the oscillation wavelength in the medium is λ. It may be configured. Here, m is 5 or more, more preferably 8 or more.

  In the above embodiment, an example of a triangular lattice arrangement is shown as an example of the two-dimensional circular hole arrangement, but the same result can be obtained not only with a triangular lattice but also with a square lattice arrangement. Further, the holes are not limited to circular holes, but may be polygons or ellipses. Further, a medium made of an appropriate material may be embedded in the holes.

  Although the present invention has been described based on the preferred embodiments, the surface emitting laser element of the present invention is not limited to the configuration of the above embodiments, and various modifications and changes can be made to the configurations of the above embodiments. Changes are also included in the scope of the present invention.

FIG. 3 is a partial cross-sectional perspective view of the surface emitting laser element according to the first and second embodiments of the present invention. FIG. 2 is a partial cross-sectional perspective view showing one step in the manufacturing process of the surface emitting laser element of FIG. 1. FIG. 3 is a partial cross-sectional perspective view showing one step in the manufacturing process subsequent to FIG. 2. FIG. 4 is a partial cross-sectional perspective view showing one step in the manufacturing process subsequent to FIG. 3. FIG. 5 is a partial cross-sectional perspective view showing one step in the manufacturing process subsequent to FIG. 4. The graph which shows the definition of SMSR. The graph which shows the current dependence of SMSR in embodiment and the conventional surface emitting laser. The partial cross section perspective view of the conventional surface emitting laser.

Explanation of symbols

100: surface emitting laser element 11: substrate 12: lower DBR mirror 13: active layer 14: current constricting layer 15: upper DBR mirror 16: upper electrode 17: lower electrode 18: light emitting region 20: two-dimensional circular hole array

Claims (11)

In a surface-emitting semiconductor laser device comprising an upper reflector structure and a lower reflector structure formed on a substrate, and a light emitting layer disposed between the upper reflector structure and the lower reflector structure,
The upper reflecting mirror structure is formed with a two-dimensional hole array having a point defect in the center of which no hole exists, and at least one of the upper reflecting mirror structure and the lower reflecting mirror structure is provided with the light emitting layer. In the vicinity, a current confinement layer having a current confinement portion in which ions are implanted in the periphery while leaving a current opening in the center is formed,
The surface emitting semiconductor laser device according to claim 1, wherein a width (Z) of the current opening is smaller than a diameter (D) of a circle connecting a hole center closest to the point defect.   2. The surface emitting semiconductor laser device according to claim 1, wherein Z <0.7 × D.   The surface emitting semiconductor laser device according to claim 2, wherein 0.3 × D <Z <0.5 × D.   The surface emitting semiconductor laser device according to claim 1, wherein the holes of the two-dimensional hole array are circular holes.   The surface emitting semiconductor laser device according to claim 1, wherein the two-dimensional hole array is a triangular lattice.   The surface emitting semiconductor laser device according to claim 1, wherein the two-dimensional hole array is a square lattice.   The surface-emitting semiconductor laser device according to claim 1, wherein the implanted ions are hydrogen ions.   The surface-emitting semiconductor laser device according to claim 1, wherein the implanted ions are oxygen ions.   The surface-emitting semiconductor laser device according to claim 1, wherein the substrate is an n-type GaAs substrate.   The current confinement layer is formed of a single semiconductor layer having a thickness of ((m / 2) +1/4) λ where m is a natural number of 1 or more and an oscillation wavelength in the medium is λ. The surface emitting semiconductor laser device according to any one of 9.   The surface emitting semiconductor laser device according to claim 1, wherein the oscillation wavelength is 1 μm or more and 1.6 μm or less.

Images