JPWO2005074080A1 - Surface emitting laser and manufacturing method thereof - Google Patents

Abstract

On the substrate (1), the first DBR layer (2), the first cladding layer (3), the active layer (4), the second cladding layer (5), the oxidation current confinement portion forming layer (6), the second It has a stacked structure (20) in which DBR layers (7) are sequentially stacked, and an upper electrode (8) and a lower electrode (9). In addition, a structure modulation region (12) extending from the vicinity of the step (11) formed on the substrate (1) exists in a partial region of the laminated structure (20) that is separated from the optical axis (10) by a predetermined distance or more. . This structural modulation region (12) differs from other regions in at least one of parameters such as layer thickness, interface flatness, and inclination of the interface with respect to the substrate surface, and is reflected in comparison with the central portion of light emission including the optical axis (10). The rate is low. The effective width (A) of the inner high reflection region surrounded by the structure modulation region (12) is set substantially equal to the width of the fundamental transverse mode light. On the other hand, the diameter (B) of the non-oxidized region of the current confinement portion is set wider than this. With such a configuration, a surface emitting laser having a simple structure and excellent single mode oscillation characteristics can be obtained without significantly increasing the number of steps.

Description

  The present invention relates to a vertical cavity surface emitting laser that outputs fundamental transverse mode light and a method of manufacturing the same.

  A vertical cavity surface emitting laser (VCSEL, hereinafter abbreviated as VCSEL) is lower in manufacturing cost, higher in manufacturing yield, and easier to form a two-dimensional array than an end surface laser. In recent years, it has been actively developed.

  As the surface emitting laser, a high-power single fundamental transverse mode laser is required. However, for example, in an oxidation current confinement type surface emitting laser, in order to obtain a single fundamental transverse mode, the current confinement region having a low electric resistance must be reduced to about 5 μmφ or less. When the current confinement region is reduced, both the element resistance and the thermal resistance increase, and there is a problem that a sufficient output cannot be obtained due to the influence of heat generation.

  As a conventional surface emitting laser having a high output in the basic transverse mode, there is a surface emitting laser as shown in FIG. This device includes a lower electrode 1111, a substrate 1011, a lower reflecting mirror structure 1021, a lower cladding layer 1031, a light emitting layer 1041, an upper cladding layer 1051, an upper reflecting mirror structure 1061, a low reflectivity zone 1071 formed by ion implantation, a loss, and the like. The laser beam is emitted along the optical axis 1091 and has a structure including a determining element 1081 and an upper electrode 1101. The loss determining element 1081 is processed into a concave shape in order to gradually increase the optical loss of the resonator in the direction orthogonal to the optical axis 1091 as the distance from the optical axis 1091 increases. The loss determining element 1081 increases the resonator loss as the distance from the optical axis 1091 in the direction orthogonal to the optical axis 1091 increases, so that the light reflection direction moves away from the center of the light emitting layer 1041. In this surface-emitting laser, fundamental transverse mode oscillation occurs close to the optical axis 1091. On the other hand, higher-order transverse mode oscillation occurs at a position away from the optical axis 1091. Therefore, the resonator loss of the higher-order transverse mode increases. As a result, the fundamental transverse mode light output is greatly increased (see, for example, Japanese Patent Laid-Open No. 10-56233).

  As a conventional surface emitting laser, there is a surface emitting laser as shown in FIG. This device has a structure in which an electrode 1112, a substrate 1012, a multilayer mirror 1022, an active layer 1042, an oxide layer 1062, a multilayer mirror 1072, a spacer layer 1082, a multilayer mirror 1092, and an electrode 1102 are sequentially stacked. The current confinement structure is formed by partially oxidizing the oxide layer 1062. In addition, the spacer layer 1082 is oxidized simultaneously with the oxide layer 1062. Since this spacer layer 1082 gives a resonator loss to the light in the high-order transverse mode, the high-order transverse mode in the peripheral portion of the emission center region can be suppressed. That is, the current confinement structure is formed to be large, and the lateral mode control is realized by the spacer layer 1082 (see, for example, JP-A-2002-353562).

  However, the conventional surface emitting laser shown in FIG. 8 has a drawback that the process for forming the recesses is complicated. In addition, the laser characteristics are greatly affected by the deviation between the center of the light emitting layer and the center of the recess and the variation in the radius of curvature of the recess, so that there is a limit to improving the yield. Further, since the reflectance is lowered even for light slightly deviated from the center of the recess, the threshold value is also increased for the fundamental transverse mode light.

  On the other hand, in the structure shown in FIG. 9, not only the oxide layer 1062 that is a current confinement layer but also a spacer layer 1082 that is oxidized to suppress higher-order transverse modes is present in the current path between the electrodes. In some cases, the element resistance increases, causing an increase in operating voltage and a problem of heat generation. In order to select and oscillate only the fundamental transverse mode light more efficiently, if the width of the non-oxidized region in the spacer layer 1082 in FIG. 9 is set narrower than the width of the non-oxidized region in the oxide layer 1062, the current path (FIG. The solid arrow in FIG. 10 becomes narrower, and the influence of an increase in element resistance is further increased.

  When the element resistance increases, the operating voltage increases and a sufficient output cannot be obtained due to the influence of heat generation. Therefore, although the maximum optical output (about 2 mW to about 5 mW) is obtained in the structure of FIG. 9, the same level of output cannot be obtained in the structure of FIG. Therefore, in the case of aiming at high output, the non-oxidized region in the central region of the spacer layer 1082 cannot be narrowed as shown in FIG. That is, the problem of suppressing the high-order transverse mode light and outputting the fundamental transverse mode and the problem of achieving high output were in a trade-off relationship.

  In order to solve the above problems, an object of the present invention is to emit a fundamental transverse mode light at a high output while suppressing a higher order transverse mode.

  In order to achieve the above object, the VCSEL of the present invention includes a laminated structure in which at least a first conductivity type Bragg reflector layer, an active layer, and a second conductivity type Bragg reflector layer are sequentially laminated on a substrate, A current confinement structure that concentrates the current flowing through the structure in the light emitting region of the active layer, and a structure that is formed in a region separated from the optical axis in the stacked structure and has a lower reflectance than the central portion of light emission including the optical axis And a modulation region.

  The VCSEL manufacturing method of the present invention includes a step of sequentially laminating at least a first conductivity type Bragg reflector layer, an active layer, and a second conductivity type Bragg reflector layer on a substrate to form a laminated structure. And a step of forming a structure modulation region having a lower reflectance than a central portion including the central axis in a region separated from the central axis in the laminated structure.

  According to the present invention, a surface emitting laser having a simple structure and excellent single mode oscillation characteristics can be obtained without significantly increasing the number of steps.

FIG. 1A is a sectional view of a VCSEL which is a first embodiment of the present invention. FIG. 1B is an enlarged cross-sectional view of a step portion of a VCSEL that is the first embodiment of the present invention. FIG. 2A is a schematic diagram showing a manufacturing process of the VCSEL in the first embodiment of the present invention. FIG. 2B is a schematic diagram illustrating a manufacturing process subsequent to FIG. 2A. FIG. 2C is a schematic diagram illustrating a manufacturing process subsequent to FIG. 2B. FIG. 2D is a schematic diagram illustrating a manufacturing process subsequent to FIG. 2C. FIG. 3A is a diagram of a VCSEL that is a second embodiment of the present invention. FIG. 3B is an enlarged cross-sectional view of an uneven portion of a VCSEL that is a second embodiment of the present invention. FIG. 3C is a plan view showing another example of the uneven portion of the VCSEL according to the second embodiment of the present invention. FIG. 4 is a sectional view of a VCSEL which is the third embodiment of the present invention. FIG. 5 is a cross-sectional view of a VCSEL that is a fourth embodiment of the present invention. FIG. 6 is a cross-sectional view of a VCSEL that is a fifth embodiment of the present invention. FIG. 7 is a cross-sectional view of a VCSEL that is a sixth embodiment of the present invention. FIG. 8 is a cross-sectional view of a conventional VCSEL. FIG. 9 is a cross-sectional view of a conventional VCSEL. FIG. 10 is a schematic diagram showing a problem of the conventional VCSEL in FIG.

  In the present invention, a structure modulation region having a reduced reflectivity is formed in a part of the laminated structure, thereby obtaining a high output while enhancing the effect of suppressing the high-order transverse mode.

  In the case of a laminated structure, the reflectance decreases due to the difference in layer thickness, interface flatness, or the distribution of the inclination of the interface with respect to the substrate surface. One best mode of the present invention is to provide a high-order lateral width by providing a structural modulation region having a reduced reflectivity in the non-current confinement region of the current confinement portion, away from the optical axis in a direction orthogonal to the optical axis. High output is obtained while enhancing the mode suppression effect.

  According to another preferred embodiment of the present invention, the reflectance is higher than that of the central portion of the light emission due to mutual diffusion or high-concentration impurity diffusion at the periphery of the second conductivity type Bragg reflector (DBR) layer. It is characterized in that a structure modulation region in which the resistance is lowered is provided.

  As a result of the above, the width of the high reflectivity region inside the structural modulation region where the reflectivity is reduced, the higher order mode is suppressed, and can be freely narrowed to the optimum width for single fundamental transverse mode oscillation, The suppression effect of higher order modes can be maximized.

  In addition, since the width of the current confinement portion can be set wider, the resistance of the current path can be reduced and saturation due to heat can be suppressed.

  Furthermore, since a complicated etching process or the like on the emission surface is not required to obtain the fundamental transverse mode laser beam, it can be manufactured by a relatively simple process.

(First embodiment)
A first embodiment of a VCSEL according to the present invention will be described with reference to FIGS. 1A and 1B. This laser includes a first DBR (Distributed Bragg Reflector) layer 2, a first cladding layer 3, an active layer 4, a second cladding layer 5, an oxidation current confinement layer forming layer 6, and a second DBR layer 7 on a substrate 1. , And an upper electrode 8 and a lower electrode 9. Further, a structure modulation region 12 extending from the vicinity of the step 11 formed on the substrate 1 exists in a partial region of the laminated structure 20 that is separated from the optical axis 10 by a predetermined distance or more in a direction orthogonal to the optical axis 10. This is one of the characteristics. In this embodiment, the structure modulation region 12 has at least one distribution of parameters that affect the reflectance, such as layer thickness, interface flatness, and inclination of the interface with respect to the substrate surface, in other regions (light emission including the optical axis 10). Unlike the central portion, the reflectance is lower than in other areas.

  The manufacturing method of the VCSEL shown in FIG. 1A will be described with reference to FIGS. 2A to 2D. The following description is an example of a short wavelength laser device, and a material having an oscillation wavelength of about 0.85 μm is selected.

  First, as shown in FIG. 2A, a mesa having a circular planar shape and a diameter of about 3 μm is formed on the n-type GaAs substrate 1 by using photolithography and etching techniques and surrounded by a step 11 of about 0.1 μm. (Step 1). The step 11 is arranged at a position separated from the central axis 10a by a predetermined distance or more.

  Although the planar shape of the mesa is a simple circular pattern, it is not limited to a circular shape, and any shape can be used according to the purpose. For example, anisotropy such as an ellipse or a rectangle can be given for polarization control. Moreover, although the level | step difference 11 was about 0.1 micrometer, it is not restricted to this. If the flattening action during epitaxial growth is strong, it may be larger, and conversely even a slight level difference can provide an effect to some extent. In addition, the step is not necessarily a steep step, and may be a step having a slope that is gently inclined with respect to the normal direction of the substrate surface, or a step in which the inclination angle of the slope changes.

Next, as shown in FIG. 2B, on the n-type GaAs substrate 1 on which the step 11 is formed, a pair of an n-type Al _0.2 Ga _0.8 As layer 2-2 and an n-type Al _0.9 Ga _0.1 As layer 2-1 is formed as a basic unit. A first DBR layer 2 in which a plurality of DBRs (n-type semiconductor mirror layers) are stacked, a first cladding layer 3 of n-type Al _0.3 Ga _0.7 As, an undoped GaAs quantum well, and an Al _0.2 Ga _0.8 As barrier layer Layer 4, p-type Al _0.3 Ga _0.7 As second cladding layer 5, p-type Al _x Ga _1-x As (where 0.9 <x <1) oxidation current confinement forming layer 6 and p-type Al _0.2 Ga _A second DBR layer 7 in which a plurality of DBRs (p-type semiconductor mirror layers) having a pair of a _0.8 As layer and a p-type Al _0.9 Ga _0.1 As layer as a basic unit is stacked is formed by metal organic chemical vapor deposition (MOCVD). The laminated structure 20 is formed by sequentially laminating (Step 2). Of course, other methods such as molecular beam epitaxy (MBE) may be used.

In each DBR layer 2, 7, the film thicknesses of the high refractive index Al _0.2 Ga _0.8 As layer 2-2 and the low refractive index Al _0.9 Ga _0.1 As layer 2-1 are set to be different from each other in the medium. The optical path length is set to be approximately 1/4 of the oscillation wavelength. Alternatively, the total thickness (thickness in DBR units) of the thickness of the Al _0.2 Ga _0.8 As layer 2-2 and the thickness of the Al _0.9 Ga _0.1 As layer 2-1 is such that the optical path length is ½ of the oscillation wavelength. You may set as follows. The reason why the Al composition x of the oxidation current confinement portion forming layer 6 is set to a high composition (0.9 <x) is that oxidation hardly occurs at 0.9 or less. On the other hand, the low refractive index Al _0.9 Ga _0.1 As layer 2-1 of the DBR layers 2 and 7 is set to a low Al composition in which little oxidation occurs, whereby selective oxidation of the oxidation current confinement portion forming layer 6 is performed. It becomes possible.

In step 2, near the step 11, as shown in the enlarged view of FIG. 1B, Al _0.9 Ga _0.1 As is mainly on the side where the step height is low (here, the outer side when viewed from the central axis 10a of the laminated structure 20). The layer 2-1 increases in layer thickness, crystal plane inclination, etc., and this is successively taken over to the upper layer, so that the structural modulation region 12 having a lowered reflectivity is on the side where the step height is mainly low in the stacking direction ( However, it depends on the growth conditions.

  Next, as shown in FIG. 2C, a photoresist 13 is applied onto the second DBR layer 7 to form a circular resist mask (step 3). Next, etching is performed by dry etching until the surface of the second cladding layer 5 is exposed to form a columnar structure having a diameter of about 30 μm (step 4), and the side surface of the oxidation current confinement portion forming layer 6 is exposed. Thereafter, the photoresist 13 is removed (step 5).

  Next, as shown in FIG. 2D, heating is performed at a temperature of about 400 ° C. for about 10 minutes in a furnace in a steam atmosphere (step 6). As a result, only the outer peripheral edge of the oxidation current confinement portion forming layer 6 is selectively oxidized in an annular shape to form an oxidized region. At the same time, a non-oxidized region having a diameter of about 8 μm is formed in the central portion including the optical axis 10 of the oxidation current confinement portion forming layer 6. Note that in this step, heating is stopped before the width B of the non-oxidized region becomes smaller than the width A of the central portion surrounded by the structure modulation region 12.

  Next, in order to form the upper electrode 8, a photoresist mask is formed at a predetermined position (Step 7), and titanium (Ti) and gold (Au) are deposited on the entire surface as an electrode (Step 8), and then the photoresist is removed. The upper electrode 8 is formed by lifting off (step 9). Finally, an AuGe (germanium) alloy is vapor-deposited on the entire back surface of the substrate, heated and alloyed to form the lower electrode 9 (step 10).

  A configuration formed of the oxidized region and the non-oxidized region formed in the oxidized current confinement portion forming layer 6 is referred to as a current confinement portion (current confinement structure). The oxidized region at the outer peripheral edge has a high electric resistance, and the non-oxidized region at the center has a low electric resistance. Therefore, by making the width of the non-oxidized region substantially the same as that of the light emitting region that emits light in the active layer 4, current can be concentrated in the light emitting region. The oxidized region is also referred to as “non-current confinement region”, and the non-oxidized region is also referred to as “current confinement region” or “opening”.

  As described above, the VCSEL of FIG. 1A obtained by the manufacturing method of FIGS. 2A to 2D suppresses resistance and thermal resistance because the diameter of the non-oxidized region (arrow B in FIG. 1A) of the current confinement portion is as wide as about 8 μm. it can.

  On the other hand, the diameter of the circular mesa surrounded by the step 11 provided on the n-type GaAs substrate 1 is set to a sufficiently small value of about 3 μm, so that the structure modulation region 12 having a lowered reflectivity is non-oxidized in the current confinement portion. Higher-order transverse modes can be suppressed by being formed inside the region.

  In this embodiment, the effective width (arrow A in FIG. 1A) of the inner high reflection region surrounded by the structure modulation region 12 is substantially equal to the width of the fundamental transverse mode light (about 5 μm). A surface emitting laser device capable of increasing the output while maintaining one fundamental transverse mode could be provided, and a single fundamental transverse mode oscillation could be realized with a high output of about 5 mW or more.

  The fundamental transverse mode light has a maximum intensity at the center of the optical axis and is weak at the outer periphery. Here, the width of the fundamental transverse mode light is a width at which the light intensity is about 75% of the whole. It is appropriately determined between 60% and 90% depending on the required characteristics of the element.

  In step 1 of FIG. 2A, the step 11 is formed directly on the n-type GaAs substrate 1, but for example, the step 11 is formed after growing the buffer layer, or the stacking of the first DBR layer 2 is interrupted halfway. Then, the step 11 can be formed, and the rest can be continuously laminated.

  In the latter case, although the number of processes is increased, the structure modulation region 12 having a reduced reflectivity can be effectively generated in a region closer to the active layer 4, so that a larger higher-order transverse mode suppression effect can be expected.

  In step 1 of FIG. 2A, a single mesa is formed on the n-type GaAs substrate 1, but the number of processes is increased, but the mesa is formed in double and triple to form steps in a multi-step shape. As a result, the structural modulation region 12 having a reduced reflectance over a wider range can be formed by controlling the distribution thereof.

  Further, the step 11 is formed so as to become lower from the optical axis 10 toward the outside, because this is because the influence of the structure modulation region 12 does not easily reach the light emitting region of the active layer 4. However, when the step 11 and the active layer 4 are largely separated from each other, if the step is set low, the effect is hardly exerted on the active layer 4 because the step is sufficiently flattened during the lamination. In this case, conversely, it may be formed so as to be lowered on the inside or mixed.

  2A to 2D show how one VCSEL is formed, but a large number of VCSELs can be simultaneously formed in a matrix using a large-area substrate. Accordingly, not only can each apparatus be cut out and used, but also can be cut out from the substrate into a desired array (for example, 1 × 10, 100 × 100, etc.).

(Second embodiment)
A second embodiment of the VCSEL according to the present invention will be described with reference to FIGS. 3A to 3C.

  Differences from FIG. 1A and FIG. 1B shown in the first embodiment are as follows. That is, the present embodiment employs substantially the same manufacturing method as that of the first embodiment, but in step 1 shown in FIG. 2A of the first embodiment, a step 11 is formed on the n-type GaAs substrate 1. In contrast to the mesa having a circular planar shape surrounded by, a concavity and convexity 14 having a concentric planar shape is formed on the n-type GaAs substrate 1 as shown in FIG. 3A. Is different. In addition, if it is uneven | corrugated, it will not be restricted to a concentric circle shape, The multiple ellipse shape and rectangle which were formed in cyclic | annular form may be sufficient. Moreover, the plurality of island-shaped irregularities 14a shown in FIG. 3C may be used. Furthermore, it may be regular or irregular.

  With this configuration, it is possible to control and form the distribution of the structural modulation region 12 in a wider range with a simpler process than in the first embodiment (for example, a plurality of steps in the first embodiment). It is necessary to repeat photolithography many times to provide it).

  Therefore, in this embodiment, not only has the effect of the first embodiment, but also more appropriate reflectance distribution control can be easily performed, so that high-output single fundamental transverse mode light can be output more efficiently. Can do.

  Also in this embodiment, as in the first embodiment, the irregularities 14 and 14a are not directly formed on the n-type GaAs substrate 1, but for example, after the buffer layer is grown or the first DBR layer 2 is grown. It is also possible to form the irregularities 14 and 14a after interrupting the stacking of the intermediate layer and continue to stack the rest. In the latter case, as in the first embodiment, the number of processes is increased, but the structure modulation region 12 having a reduced reflectivity in a region closer to the active layer 4 can be effectively generated. An inhibitory effect can be expected.

(Third embodiment)
A third embodiment of the VCSEL according to the present invention will be described with reference to FIG.

  The difference between the configuration of FIG. 4 and the configuration of the first embodiment shown in FIGS. 1A and 1B is that the step 11 is formed on the n-type GaAs substrate 1 in FIGS. 1A and 1B. In this embodiment, the step 11 is formed in the spacer layer 15 provided on the upper part of the oxidation current confinement portion forming layer 6. Accordingly, in FIG. 1A, the structural modulation region 12 is mainly present inside the first DBR layer 2, whereas in the present embodiment, the structural modulation region 12 is present inside the second DBR layer 7. In this embodiment, the same effect as in the first embodiment is obtained, but the influence of the structure modulation region 12 does not reach the active layer 4.

  The manufacturing method of the present embodiment is obtained by changing the position where the step of forming the step 11 is inserted in FIGS. 2A to 2D showing the manufacturing method of the first embodiment. That is, the first DBR layer 2, the first cladding layer 3, the active layer 4, and the first layer in the step 2 of FIG. 2B are directly formed on the n-type GaAs substrate 1 without performing the step 1 of forming the step 11 in FIG. 2A. The two clad layers 5 and the oxidation current confinement portion forming layer 6 are stacked, and the stacking is temporarily interrupted at an appropriate stage in the initial stacking of the second DBR layer 7. After adding a new step of forming the step 11 on the interrupted surface, the second DBR layer 7 is restarted to form the remaining portion. In this case, the spacer layer 15 in FIG. 4 corresponds to the initial stacking portion of the second DBR layer 7.

  In this embodiment, the spacer layer 15 is provided on the upper part of the oxidation current confinement part forming layer 6, but it may be provided on the lower part of the oxidation current confinement part forming layer 6 and the step 11 may be formed there. Further, the step 11 may be formed by interrupting the second DBR layer 7 when it is further laminated.

(Fourth embodiment)
A fourth embodiment of the VCSEL according to the present invention will be described with reference to FIG.

  The difference between the configuration of this embodiment and the configurations of FIGS. 3A to 3C as the second embodiment is the same as the configuration of FIG. 4 as the third embodiment and FIGS. 1A and 1B as the first embodiment. It is almost the same as the difference from the configuration of 1B. 3A to 3C, the irregularities 14 and 14a are formed on the n-type GaAs substrate 1, whereas in this embodiment, the irregularities are formed on the spacer layer 15 provided below the oxidation current confinement portion forming layer 6. 14 and 14a are formed. Therefore, in FIG. 3A, the structural modulation region 12 exists mainly inside the first DBR layer 2, whereas the structural modulation region 12 exists inside the second DBR layer 7 in this embodiment. Also in this embodiment, the influence of the structure modulation region 12 does not reach the active layer 4 as in the third embodiment.

  The manufacturing method is also almost the same as that of the third embodiment. In this embodiment, the unevenness 14 and 14a are formed with respect to the step 11 in the third embodiment, and the position of the spacer layer 15 is changed. The third embodiment is different from the upper portion of the oxidation current confinement portion forming layer 6 in this embodiment except that the lower portion of the oxidation current confinement portion formation layer 6 is used. As in the third embodiment, the spacer layer 15 may be provided on the oxidation current confinement portion forming layer 6 and is interrupted when the second DBR layer 7 is further thickly laminated. May be formed.

(Fifth embodiment)
A fifth embodiment of the VCSEL according to the present invention will be described with reference to FIG. The difference between the configuration of FIG. 6 and the configuration of FIG. 4 which is the third embodiment is that in FIG. 6 there is no oxidation current confinement portion forming layer 6 and the diameter of the cylindrical structure of the second DBR layer 7 is about 10 μm. This is the point that current confinement is realized by making it as small as possible. In this way, a structure in which the diameter of the second DBR layer 7 is reduced in order to realize current confinement is referred to as a current confinement structure 30.

  In this embodiment, since the transverse mode control is performed by the structure modulation region 12, the width of the current confinement can be relatively increased. Therefore, as in this embodiment, a cylindrical structure can be formed by the second DBR layer 7 with a realistic diameter that does not cause a significant increase in electrical resistance, and direct current confinement can be realized, and the selective oxidation process is omitted. can do.

  In this embodiment, the spacer layer 15 is provided immediately above the second cladding layer 5, but the step 11 may be formed by interrupting the second DBR layer 7 when it is further laminated. Further, in place of the step 11, the irregularities 14 and 14a may be formed as in the fourth embodiment.

  Further, the position where the step 11 or the irregularities 14 and 14a are formed is the same as in the first or second embodiment, on the n-type GaAs substrate 1, on the surface where the buffer layer is grown, or on the first DBR layer 2. It is good also as the surface which interrupted lamination | stacking on the way.

In the first to fifth embodiments described above, at least the structure modulation region 12 is formed in one of the first DBR layer 2 and the second DBR layer 7, and the inner side surrounded by the structure modulation region 12. The high reflection region has a width A that is narrower than the width B of the current confinement region. For this reason, a VCSEL that maintains a single basic transverse mode at a high output is obtained.
In particular, the structure modulation region 12 is preferably formed in the second DBR layer 7 on the emission side. Furthermore, it is more preferable that the width A of the inner high reflection region surrounded by the structure modulation region 12 formed in the second DBR layer 7 is smaller than the width B of the current confinement region.
The effective width A of the inner high reflection region surrounded by the structural modulation region 12 formed in the first or second DBR layer 2 or 7 is preferably equal to the width of the light in the fundamental transverse mode. .

(Sixth embodiment)
A sixth embodiment of the VCSEL according to the present invention will be described with reference to FIG.

  In this embodiment, a first DBR layer 102, a first cladding layer 103, an active layer 104, a second ladder layer 105, an oxidation current confinement portion formation layer 106, and a second DBR layer 107 are sequentially stacked on a substrate 101. And the first electrode 109 and the second electrode 111. The first DBR layer 102 includes a multilayer film of a low refractive index layer 102-1 and a high refractive index layer 102-2. The same applies to the second DBR layer 107. The number of pairs of the second DBR layer 107 on the emission side is usually set to be smaller than the number of pairs of the first DBR layer 102 in order to make the reflectance smaller than that of the first DBR layer 102.

  The resonance part is composed of a first cladding layer 103, an active layer 104 and a second cladding layer 105. The active layer 104 is disposed in a portion corresponding to the antinode of the electric field strength of the resonance part. The oxidation current confinement portion forming layer 106 is disposed between the resonance portion and the first or second DBR layers 102 and 107. In particular, when the current confinement portion is formed of an oxide film, the oxidation current confinement portion formation layer 106 has a high electric field strength so that the refractive index difference between the semiconductor and the oxide film is large and the light confinement effect does not become too large. It is arranged at the position.

  In order to suppress the occurrence of the high-order transverse mode, the structure modulation region 108 having a lower reflectance than the central portion of the light emission due to mutual diffusion or high-concentration impurity diffusion is formed around the second DBR layer 107. Form. The opening width 113 of the structure modulation region 108 is narrower than the opening width 112 of the current confinement portion. The light emitting region that emits light in the active layer at the time of current injection becomes an elliptical region 114 from the opening width 112 of the current confinement portion.

  The light emitted from the elliptical light emitting region 114 is fed back by the optical resonator constituted by the upper and lower DBR layers 102 and 107, and laser oscillation occurs. However, since the structure modulation region 108 having a low reflectance is formed in this embodiment, sufficient feedback is not applied to the peripheral portion of the light emission. For this reason, it oscillates in the fundamental transverse mode having the maximum light intensity at the center of light emission, but it is difficult to oscillate in the higher order transverse mode having the maximum light intensity in the peripheral part.

  As a method for forming the structure modulation region 108 having a lowered reflectivity, in this embodiment, the interdiffusion of the multilayer film constituting the second DBR layer 107, or the destruction of the periodic structure by high-concentration impurity diffusion is used. .

  Interdiffusion of multilayer films refers to a phenomenon in which atoms constituting a multilayer film diffuse into each other's layers. For example, the reflectance of a DBR film made of GaAs / AlAs with 24 periods is 99% or more. The DBR film is irradiated with an electron beam only in the outer periphery so that the DBR in the light emission center portion is not diffused, and the multilayer film is diffused by using the abnormal diffusion in the region where the electron beam is irradiated. When the DBR is changed to AlGaAs (Al: 0.4) / AlGaAs (Al: 0.6) in this way, the reflectance decreases to 77%.

On the other hand, when the structure modulation region 108 is formed by impurity diffusion, carrier absorption also occurs. Assuming that the absorption coefficient in each layer is 100 cm ^{−1 in} the multilayer film structure of the above example, the absorptance of the entire DBR is about 4%, and the reflectance is also reduced by about 74%.

  Next, the VCSEL according to the present embodiment will be described in detail including the manufacturing process. The following description is an example of a short wavelength laser device, and a material having an oscillation wavelength of about 0.85 μm is selected.

First, as shown in FIG. 7, on a Si-doped n-type GaAs substrate 101, a high refractive index layer 102-2 composed of an n-type Al _0.2 Ga _0.8 As layer and a low refractive index composed of an n-type Al _0.9 Ga _0.1 As layer. A first DBR layer 102 in which a plurality of n-type DBRs (n-type semiconductor mirror layers) having a pair of layers 102-1 as a basic unit are stacked, a first clad layer 103 of n-type Al _0.3 Ga _0.7 As, a non-doped GaAs quantum Active layer 104 comprising a well and an Al _0.2 Ga _0.8 As barrier layer, second cladding layer 105 of p-type Al _0.3 Ga _0.7 As, and oxidation of p-type Al _x Ga _1-x As (where 0.9 <x <1) A second DBR layer 107 in which a plurality of DBRs (p-type semiconductor mirror layers) having a basic unit of a pair of a current confinement portion forming layer 106, a p-type Al _0.2 Ga _0.8 As layer, and a p-type Al _0.9 Ga _0.1 As layer are stacked; , Metal organic chemical vapor deposition (MO Sequentially laminated by VD) method to form a laminated structure 120. Other growth methods such as molecular beam epitaxy (MBE) may be used. This step corresponds to the step shown in FIG.

In each of the DBR layers 102 and 107, the film thicknesses of Al _0.2 Ga _0.8 As having a high refractive index and Al _0.9 Ga _0.1 As having a low refractive index are such that the respective optical path lengths in these media have an oscillation wavelength of about 0. It is set to be approximately 1/4 of 85 μm. Alternatively, the total film thickness (thickness in units of DBR) of the thickness of Al _0.2 Ga _0.8 As and the thickness of Al _0.9 Ga _0.1 As is set to be half of the oscillation wavelength of about 0.85 μm. It may be set.

  Next, a photoresist is applied on the epitaxial growth film to form a circular resist mask. Next, etching is performed by dry etching until the surface of the second cladding layer 105 is exposed to form a columnar structure having a diameter of about 30 μm. By this step, the side surface of the oxidation current confinement portion forming layer 106 is exposed. Thereafter, the photomask is removed. This step corresponds to the step shown in FIG.

Next, the surface of the upper surface of the mesa is covered with a photoresist except for the concentric annular portion having an inner diameter of about 8 μm to 10 μm and an outer diameter of about 12 to 14 μm. Thereafter, a ZnO film (impurity layer) is formed in an annular shape by sputtering on the uppermost layer of the mesa and annealed at 580 ° C. for 10 minutes. As a result, Zn (second conductivity type impurity) diffuses to a peripheral portion excluding the central portion of the optical axis to a depth of about 2 μm, and the second DBR layer 107 is destroyed. As a result, the interface between the high refractive index Al _0.2 Ga _0.8 As layer and the low refractive index Al _0.9 Ga _0.1 As layer becomes smooth, and the reflectance of the region decreases. As a result, a structure modulation region 108 having a lower reflectance than the central portion of light emission is formed in a part of the second DBR layer 107.

  The uppermost layer may be a GaAs / 2 layer. Furthermore, it goes without saying that this step may be performed before the above-described mesa forming step and further after a selective oxidation step for forming a current confinement portion described later.

  Thereafter, heating is performed at a temperature of about 400 ° C. for about 10 minutes in a furnace in a steam atmosphere. As a result, the oxidation current confinement portion forming layer 106 is selectively oxidized in an annular shape to form an oxidized region. At the same time, a non-oxidized region having a diameter of about 8 μm is formed in the central portion of the oxidation current confinement forming layer 106.

  A configuration formed of the oxidized region and the non-oxidized region formed in the oxidized current confinement portion forming layer 106 is referred to as a current confinement portion. The current confinement portion is provided to concentrate the current in the active layer region having the same width as that of the non-oxidized region.

Thereafter, a titanium (Ti) / gold (Au) ring-shaped upper electrode 109 is formed on the outer periphery of the mesa, and an AuGe alloy lower electrode 111 is formed on the entire back surface of the substrate. This step corresponds to FIG.
The structure modulation region 108 has the same central axis as the current confinement portion, and the inner diameter 113 surrounded by the structure modulation region 108 is smaller than the opening width 112 of the current confinement portion. For this reason, even if the opening width 112 of the current confinement portion is increased to about 8 μm, the single fundamental mode is maintained, and a high output operation of about 5 mW or more is possible.

  In the VCSEL of the present embodiment, the shape of the structure modulation region 108 is annular, so that the output light has a circular cross section. However, if necessary, output light having a desired cross sectional shape such as an elliptical shape can be obtained. You may make it radiate | emit.

In the embodiment described above, non-doped GaAs or non-doped Al _0.2 Ga _0.8 As is used as the material of the active layers 4 and 104. However, the material of the active layers 4 and 104 is not limited to these, and GaAs or InGaAs is used. An infrared VCSEL can be configured, and can also be applied to a visible VCSEL such as InGaP or AlGaInP.

  Furthermore, a long-wave single-mode VCSEL can be configured using InGaAsP on an InP substrate, GaInNAs, GaInNAsSb, GaAsSb, or the like on a GaAs substrate. These VCSELs are very effective for relatively long distance communications using single mode fiber. Furthermore, a VCSEL for blue or ultraviolet light can be configured using a GaN system, a ZnSe system, or the like.

  In addition, depending on the material of these active layers 4 and 104, the material and composition of other layers including the DBR layers 2, 7, 102, and 107 and the number of periods of the DBR layers 2, 7, 102, and 107 are included. Needless to say, the thickness of each layer can be appropriately selected and set.

In the VCSELs in the first to fourth and sixth embodiments, the oxidized region of the current confinement portion is configured to oxidize aluminum (Al), but is not limited to Al, and when oxidized, compared to the non-oxidized region. Any substance can be used as long as the electrical resistance is greatly increased (desired to be an insulator).
In the above-described embodiment, the first and second conductivity may be reversed, that is, the n-type may be changed to the p-type and the p-type may be changed to the n-type. In this case, it is desirable to form the current confinement portion between the active layers 4 and 104 and the first DBR layers 2 and 102. Alternatively, in any case, the current confinement portion is between the active layer 4 104 and the first DBR layer 2 102 and between the active layer 4 104 and the second DBR layer 7 107. You may form in both.

  Moreover, as long as it functions sufficiently in combination with the above-described embodiment, current confinement is not necessarily required due to selective oxidation. For example, a method using proton injection is applied in addition to the method shown in the fifth embodiment. You can also

  In the above-described embodiments, the VCSEL is formed on the conductive substrates 1 and 101. However, the case where both the p-type and n-type electrodes are formed on the surface side is not necessarily limited to the conductive substrates 1 and 101. For example, a non-doped substrate or a semi-insulating substrate may be used. Further, in the above-described embodiment, the case where all the layers stacked on the substrates 1 and 101 are doped has been described.

The present invention is not limited to the configurations and methods specifically shown in these examples, and various variations are conceivable as long as they are within the spirit of the invention.

Claims (26)

A laminated structure in which at least a first conductivity type Bragg reflector layer, an active layer, and a second conductivity type Bragg reflector layer are sequentially laminated on a substrate;
A current confinement structure that concentrates the current flowing through the stacked structure in the light emitting region of the active layer;
A surface-emitting laser comprising: a structure modulation region formed in a region separated from the optical axis in the stacked structure and having a lower reflectance than a central portion of light emission including the optical axis.   The current confinement structure is formed between the first conductive type Bragg reflector layer and the active layer and between the second conductive type Bragg reflector layer and the active layer, and A current confinement region comprising a current confinement region formed in a central portion including an axis and a non-current confinement region formed around the current confinement region and having a higher electric resistance than the current confinement region, The surface emitting laser according to claim 1.   The surface emitting laser according to claim 2, wherein a width of a central portion surrounded by the structure modulation region is narrower than a width of the current confinement region.   2. The surface emitting laser according to claim 1, wherein the structure modulation region is formed in at least one of the first conductivity type Bragg reflector layer and the second conductivity type Bragg reflector layer. 3.   The surface emitting laser according to claim 1, wherein the structure modulation region has a parameter that affects the reflectance different from that of the other region of the multilayer structure.   6. The surface emitting laser according to claim 5, wherein the parameter is at least one of layer thickness, flatness, and inclination with respect to the surface of the substrate. It further comprises a step formed on the surface of any one of the layers included in the substrate and the laminated structure,
The surface emitting laser according to claim 1, wherein the structure modulation region is formed starting from the step.   The surface emitting laser according to claim 7, wherein the step is inclined with respect to a normal direction of a surface of the substrate.   The surface emitting laser according to claim 7, wherein the step is defined by a mesa structure.   The surface emitting laser according to claim 9, wherein a planar shape of the mesa structure is any one of a circle, an ellipse, and a rectangle.   The surface emitting laser according to claim 9, wherein the step is lowered outward from the optical axis.   8. The surface emitting laser according to claim 7, wherein the step is defined by multiple unevenness formed in an annular shape.   The surface emitting laser according to claim 12, wherein the planar shape of the multiple unevenness is any one of a circle, an ellipse, and a rectangle.   The surface emitting laser according to claim 7, wherein the step includes a plurality of island-shaped irregularities formed in a region separated from the optical axis.   2. The surface emitting laser according to claim 1, wherein the structural modulation region is formed by mutual diffusion of multilayer films constituting the second conductivity type Bragg reflector layer.   2. The surface emitting laser according to claim 1, wherein the structure modulation region is formed by high concentration impurity diffusion. Forming a laminated structure by sequentially laminating at least a first conductivity type Bragg reflector layer, an active layer, and a second conductivity type Bragg reflector layer on a substrate;
Forming a structural modulation region having a lower reflectance than a central portion including the central axis in a region spaced apart from the central axis in the laminated structure. The step of forming the laminated structure includes oxidizing between the first conductive type Bragg reflector layer and the active layer and between the second conductive type Bragg reflector layer and the active layer. A step of forming a current confinement portion forming layer made of a material that increases electrical resistance by
The method of manufacturing a surface emitting laser according to claim 17, further comprising a step of oxidizing a peripheral portion of the current confinement portion forming layer after the step of forming the stacked structure.   The oxidation step is characterized in that the oxidation treatment is stopped before the width of the non-oxidized central portion of the current confinement portion forming layer becomes narrower than the width of the central portion surrounded by the structure modulation region. A method for producing the surface emitting laser according to claim 18.   18. The step of forming the structure modulation region includes forming the structure modulation region in at least one of the first conductivity type Bragg reflector layer and the second conductivity type Bragg reflector layer. A method for producing the surface emitting laser according to claim 1.   The step of forming the structure modulation region includes a step of forming a structure modulation region that differs from other regions of the stacked structure in at least one of layer thickness, flatness, and inclination with respect to the surface of the substrate. Item 18. A method for producing a surface emitting laser according to Item 17. The step of forming the structure modulation region includes:
Forming a step in a region separated from the central axis of the surface of any one of the layers included in the substrate and the laminated structure;
The method of manufacturing the surface emitting laser according to claim 17, further comprising: forming a remaining layer of the laminated structure on the surface on which the step is formed.   23. The method of manufacturing a surface emitting laser according to claim 22, wherein the step of forming the step includes forming a step that decreases from the central axis toward the outside.   23. The method of manufacturing a surface emitting laser according to claim 22, wherein the step of forming the step includes forming a plurality of steps that decrease from the central axis toward the outside and a plurality of steps that increase.   The step of forming the structure modulation region includes a step of diffusing atoms constituting each layer of the stacked structure to other layers by irradiating an electron beam to a region separated from the central axis in the stacked structure. The method of manufacturing a surface emitting laser according to claim 17. The step of forming the structure modulation region includes:
Forming an impurity layer containing a second conductivity type impurity so as to surround the central axis on a region separated from the central axis in the second conductivity type Bragg reflector layer;
The method of manufacturing a surface emitting laser according to claim 17, further comprising a step of diffusing impurities from the impurity layer to the Bragg reflector layer of the second conductivity type.

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