Surface light emitting semiconductor laser element

Abstract

A surface light emitting semiconductor laser element, comprises a substrate, a lower reflector including a semiconductor multi-layer disposed on the substrate, an active layer disposed on the lower reflector, an upper reflector including a semiconductor multi-layer disposed on the active layer, a compound semiconductor layer having a first opening for exposing the upper reflector and extending over the upper reflector, and a metal film having a second opening for exposing the upper reflector disposed inside of the first opening and extending over the compound semiconductor layer, wherein the metal film and the compound semiconductor layer constitute a complex refractive index distribution structure where a complex refractive index is changed from the center of the second opening towards the outside. A method of emitting laser light in a single-peak transverse mode is also provided.

Claims

1. A surface light-emitting semiconductor laser element, comprising: a substrate, a lower reflector having a first-type semiconductor multi-layer structure disposed on the substrate; an active layer disposed on the lower reflector; a current confinement layer including a current injection region and an oxidation region disposed on the active layer; an upper reflector having a second-type semiconductor multi-layer structure disposed on the current confinement layer; a multi-layer structure disposed on the upper reflector, the multi-layer structure comprises a first layer defining a first opening and a second layer disposed on the first layer and defining a second opening, wherein a step-wise shape is formed by the first layer and the second layer in a cross-section view; and a first electrode disposed on the multi-layer structure.

2. The surface light-emitting semiconductor laser element of claim 1, wherein the first opening defined by the first layer has a first diameter in the cross-section view, and wherein the second opening defined by the second layer has a second diameter greater than the first diameter in the cross-section view.

3. The surface light-emitting semiconductor laser element of claim 2, wherein the first diameter is measured at an uppermost portion of the first layer, and wherein the second diameter is measured at an uppermost portion of the second layer.

4. The surface light-emitting semiconductor laser element of claim 1, wherein an aperture is formed by the first opening defined by the first layer and the second opening defined by the second layer, and wherein light generated in the active layer propagates through the aperture.

5. The surface light-emitting semiconductor laser element of claim 1, wherein the multi-layer structure comprises a plurality of semiconductor layers.

6. The surface light-emitting semiconductor laser element of claim 1, wherein the first layer and the second layer of the multi-layer structure have different optical properties.

7. The surface light-emitting semiconductor laser element of claim 1, wherein the multi-layer structure is configured to direct light as a concave lens.

8. The surface light-emitting semiconductor laser element of claim 1, wherein the multi-layer structure is configured to focus light generated in the active layer.

9. The surface light-emitting semiconductor laser element of claim 1, wherein the multi-layer structure is a complex refractive index distribution structure.

10. The surface light-emitting semiconductor laser element of claim 9, wherein the complex refractive index distribution structure has a complex refractive index that changes in an outward direction from a center of the first opening defined by the first layer.

11. The surface light-emitting semiconductor laser element of claim 9, wherein the first layer has a first refractive index, wherein the second layer has a second refractive index, and wherein the second refractive index is larger than the first refractive index.

12. The surface light-emitting semiconductor laser element of claim 1, wherein the first layer and the second layer have different compositions.

13. The surface light-emitting semiconductor laser element of claim 1, wherein the first layer is directly disposed on the upper reflector, and wherein the first electrode is directly disposed on the second layer of the multi-layer structure.

14. The surface light-emitting semiconductor laser element of claim 1, wherein the first electrode is a metallic layer surrounding the first opening defined by the first layer, and the second opening defined by the second layer.

15. The surface light-emitting semiconductor laser element of claim 14, wherein the first opening defined by the first layer has a first sidewall surface, wherein the second opening defined by the second layer has a second sidewall surface, and wherein the first sidewall surface, and the second sidewall surface, are in direct contact with the metallic layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a sectional view showing the structure of a surface light emitting semiconductor laser element according to a first embodiment of the present invention;

(2) FIG. 2 is a top view of the surface light emitting semiconductor laser element in FIG. 1;

(3) FIG. 3A is a schematic sectional view showing a main part of the surface light emitting semiconductor laser element in according to a first embodiment of the present invention;

(4) FIG. 3B is a schematic sectional view for illustrating functions of the main part corresponding to FIG. 3A;

(5) FIG. 4 is a graph showing a far-field pattern (FFP) of a surface light emitting semiconductor laser element in according to a first embodiment of the present invention;

(6) FIG. 5A is a sectional view showing a step of manufacturing a surface light emitting semiconductor laser element in according to a second embodiment of the present invention;

(7) FIG. 5B is a sectional view showing a step of manufacturing the surface light emitting semiconductor laser element in according to a second embodiment of the present invention;

(8) FIG. 6C is a sectional view showing a step of manufacturing the surface light emitting semiconductor laser element in according to a second embodiment of the present invention;

(9) FIG. 6D is a sectional view showing a step of manufacturing the surface light emitting semiconductor laser element in according to a second embodiment of the present invention;

(10) FIG. 7E is a sectional view showing a step of manufacturing the surface light emitting semiconductor laser element in according to a second embodiment of the present invention;

(11) FIG. 7F is a sectional view showing a step of manufacturing the surface light emitting semiconductor laser element in according to a second embodiment of the present invention;

(12) FIG. 8 is a sectional view showing the structure of a surface light emitting semiconductor laser element in according to a third embodiment of the present invention;

(13) FIG. 9 is a sectional view showing the structure of the surface light emitting semiconductor laser element in according to a third embodiment of the present invention;

(14) FIG. 10A is a sectional view showing the structure of a surface light emitting semiconductor laser element in according to a fourth embodiment of the present invention;

(15) FIG. 10B is a plan view showing the structure of the surface light emitting semiconductor laser element in according to a fourth embodiment of the present invention;

(16) FIG. 10C is a waveform of a transverse mode in according to a fourth embodiment of the present invention;

(17) FIG. 11 is a sectional view showing the structure of a surface light emitting semiconductor laser element in a Comparative Example;

(18) FIG. 12 is a sectional view showing the structure of a conventional surface light emitting semiconductor laser element;

(19) FIG. 13A is a sectional view showing a step of manufacturing the conventional surface light emitting semiconductor laser element;

(20) FIG. 13B is a sectional view showing a step of manufacturing the conventional surface light emitting semiconductor laser element;

(21) FIG. 14A is a schematic sectional view showing a main part of a surface light emitting semiconductor laser element according to one embodiment of the present invention; and

(22) FIG. 14B is a schematic sectional view illustrating a function of the main part shown in FIG. 14A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(23) The invention will be described in more detail which referring to the attached drawing. The conductivity type, the film type, the film thickness, the film forming method, the size and the like cited in the following embodiments are offered to aid in understanding of the present invention and are not to be construed as limiting the scope thereof.

(24) Embodiment 1

(25) FIG. 1 shows a sectional view of a surface light emitting semiconductor laser element according to the present invention. FIG. 2 is a top view of the surface light emitting semiconductor laser element. FIG. 3A is a schematic sectional view showing a main part of the surface light emitting semiconductor laser element. FIG. 3B is a schematic sectional view for illustrating functions of the main part corresponding to FIG. 3A.

(26) As shown in FIG. 1, a surface light emitting semiconductor laser element 10 comprises a laminated structure sequentially comprising an n-type GaAs substrate 12, a lower diffractive bragg reflector (hereinafter lower DBR) 14 comprising an n-type semiconductor multi-layer, an Al.sub.0.3Ga.sub.0.7As lower clad layer 16, a GaAs light emitting layer (active layer) 18, an Al.sub.0.3Ga.sub.0.7As upper clad layer 20, an upper diffractive bragg reflector 22 (hereinafter upper DBR) comprising a p-type GaAs cap layer, and a p-type GaAs contact layer 24 with a film thickness of 150 nm having an impurity concentration of 510.sup.18 cm.sup.3.

(27) The lower DBR 14 has a semiconductor multi-layer structure with a total film thickness of about 4 m including 35 pairs of n-type AlAs layers and n-type GaAs layers. The upper DBR 22 has a semiconductor multi-layer structure with a total film thickness of about 3 m including 25 pairs of p-type Al.sub.0.9Ga.sub.0.1As layers and p-type Al.sub.0.1Ga.sub.0.9As layers.

(28) A cylindrical mesa post 26 having a mesa diameter of 40 m is formed by etching the contact layer 24, the upper DBR 22, the upper clad layer 20, the active layer 18, the lower clad layer 16, and the lower DBR 14, as shown in FIGS. 1 and 2.

(29) On the active layer 18 in the upper DBR 22, an oxidized current confinement layer 28 is disposed instead of the p-type Al.sub.0.9Ga.sub.0.1As layer. The AlAs layer 28 has a film thickness of 30 nm, and comprises a circular AlAs layer 28A having a diameter of 12 m disposed at the center and an oxidized-Al layer 28B disposed around the circular AlAs layer 28A.

(30) The AlAs layer 28A is a p-type AlAs layer formed instead of the p-type Al.sub.0.9Ga.sub.0.1As layer. The oxidized-Al layer 28B is formed by selectively oxidizing Al in the p-type AlAs layer. The oxidized-Al layer 28B has high electrical resistance and functions as a current confinement area, while the circular AlAs layer 28A functions as a current injection area having electrical resistance lower than that of the oxidized-Al layer 28B.

(31) On the mesa post 26, the contact layer 24 has a first opening 30 having an inner diameter of 20 m at the center. The contact layer 24 is annular to expose the upper DBR 22 through the first opening 30.

(32) An insulation layer, i.e., a SiO.sub.2 film 32 having a film thickness of 300 nm, is extended over the periphery of the contact layer 24, the side of the mesa post 26, and the lower DBR 14. The SiO.sub.2 film 32 on the contact layer 24 has a circular third opening 34 having an inner diameter of 35 m that is greater than the first opening 30 to expose the contact layer 24.

(33) A p-side electrode 36 comprising a Ti/Pt/Au metal lamination film having a film thickness of 500 nm is extended over the upper DBR 22, the contact layer 24, and the SiO.sub.2 film 32, and has a circular second opening 38 having an inner diameter of 14 m on the upper DBR 22 to expose the upper DBR 22.

(34) As shown in FIG. 2, the AlAs layer (current injection area) 28A has slightly smaller diameter than the third opening 38 of the p-side electrode 36. The AlAs layer 28A has a diameter of 12 m, and the p-side electrode has an inner diameter of 14 m.

(35) At an opposite surface of the n-type GaAs substrate 12, an n-side electrode 40 comprising AuGe/Ni/Au is formed.

(36) FIG. 3 schematically shows optical elements of the upper DBR 22. In the surface light emitting semiconductor laser element 10, the contact layer 24, the SiO.sub.2 film 32, and the p-side electrode 36 on the upper DBR 22 provide both electrical and optical functions.

(37) As shown in FIG. 3A, the contact layer 24 having the first opening 30 extended in a ring shape and the SiO.sub.2 film 32 having the third opening 34 extended in a ring shape on the contact layer 24 are formed step-wise. Accordingly, the complex refractive index is increased isotropically from the center of the first opening 30, i.e., the center of a light emitting surface, towards the outside. As shown in FIG. 3B, there is provided a complex refractive index distribution structure that acts as a convex lens.

(38) The p-side electrode 36 having the second opening 38 has an aperture through which the light passes. As shown in FIG. 3B, the p-side electrode 36 provides optical functions similar to the complex refractive index distribution structure having an absorption opening 44 and a convex lens 46, since the metal in the p-side electrode 36 provides the complex refractive index.

(39) For example, gold (Au) has a real-part refractive index of 0.2 and an imaginary-part (absorption coefficient) refractive index of 5.6 for a laser light with a wavelength of 0.85 m.

(40) In the surface light emitting semiconductor laser element 10, the contact layer 24 having the first opening 30 has a refractive index greater than that of the opening. The p-side electrode 36 having the second opening 38 has an absorption coefficient greater than that of the opening.

(41) A combined optical system of the convex lens 46, the absorption opening 44, and the concave lens 42 is provided on the light-emitting surface. In addition, the combined optical system is disposed on a resonator of the surface light emitting semiconductor laser element 10 and thus acts as one part of the resonator.

(42) In the surface light emitting semiconductor laser element 10, laser resonance modes are selected to some degree by the current confinement action of the current confinement layer 28. Light in the high-order mode having a wide light-emitting angle is scattered at the concave lens 42, absorbed in the absorption opening 44, and converged in the convex lens 46, as shown in FIG. 3B.

(43) By combining these conditions with the effects of the aperture of the current confinement layer 28, almost one mode is forcedly selected, thereby oscillating in a single-peak transverse mode.

(44) When the optical output is increased, almost one mode is forcedly selected by the convex lens 46, the absorption opening 44, and the concave lens 42, as well as by the aperture of the current confinement layer 28, whereby multiple transverse modes become a single-peak transverse mode, even if light is oscillated in the multiple transverse modes.

(45) The full width at half maximum (FWHM) of the surface light emitting semiconductor laser element 10 produced using the method described below was measured. As shown in FIG. 4, the FWHM is 5.5, which is half or less that of the conventional surface light emitting semiconductor laser element having a constriction diameter of about 4 m. Thus, the surface light emitting semiconductor laser element 10 is in a single-peak transverse mode. FIG. 4 is a graph showing a far-field pattern (FFP) of the surface light emitting semiconductor laser element 10. In the graph, H and V waveforms are intensity distributions of irradiated beams in planes orthogonal to each other.

(46) In EMBODIMENT 1, the contact layer 24, the SiO.sub.2 film 32 and the p-side electrode 36 are formed step-wise, whereby a complex refractive index changing from the center of the second opening 38, i.e., the center of a light emitting surface, towards the outside is formed to provide a single-peak transverse mode.

(47) The surface light emitting semiconductor laser element 10 can provide almost the same level of optical output as that provided by a conventional multi-mode surface light emitting semiconductor laser element. Since the surface light emitting semiconductor laser element 10 has the same electrical structure as that of the conventional multi-mode surface light emitting semiconductor laser element, the surface light emitting semiconductor laser element 10 has almost the same level of resistance and impedance.

(48) The surface light emitting semiconductor laser element 10 emits laser light in a single-peak transverse mode so that the surface light emitting semiconductor laser element 10 can be optically coupled to actual optical fibers with high optical-connection efficiency.

(49) Embodiment 2

(50) FIGS. 5A, 5B, 6C, 6D, 7E and 7F are sectional views showing steps of manufacturing the surface light emitting semiconductor laser element according to the present invention.

(51) As shown in FIG. 5A, a lower DBR 14, a lower clad layer 16, a light emitting layer (active layer) 18, an upper clad layer 20, an upper DBR 22, and a p-type GaAs contact layer 24 are sequentially laminated on an n-type GaAs substrate 12 using a MOCVD method or the like.

(52) Before the upper DBR 22 is formed, an AlAs layer 28 having a film thickness of 30 nm is formed instead of the p-type Al.sub.0.9Ga.sub.0.1As layer on the layer of the upper DBR 22 at the nearest side of the active layer 18.

(53) As shown in FIG. 5B, the contact layer 24, the upper DBR 22, the upper clad layer 20, the active layer 18, the lower clad layer 16, and the lower DBR 14 are etched by a dry etching method using a chlorine-based gas to form a cylindrical mesa post 26 having a mesa diameter of 40 m.

(54) The laminated structure having the mesa post 26 is heated at 400 C. under steam atmosphere to selectively oxidize only Al in the AlAs layer 28 from the peripheral to the internal side of the mesa post 26, leaving a circular AlAs layer 28A having a diameter of 12 m at the center, and disposing an oxidized-Al layer 26B around the AlAs layer 28A. Thus, a current confinement layer is formed.

(55) As shown in FIG. 6C, a SiO.sub.2 film 32 is formed over the contact layer 24 of the mesa post 26, the side of the mesa post 26, and the lower DBR 14.

(56) As shown in FIG. 6D, the SiO.sub.2 film 32 is etched to provide an opening 34 having an inner diameter of 35 m.

(57) As shown in FIG. 7E, the contact layer 24 exposed on the opening 34 is etched to provide an opening 34 having an inner diameter of 20 m.

(58) As shown in FIG. 7F, a Ti/Pt/Au metal lamination film 39 is formed on the mesa post 26.

(59) Furthermore, the metal lamination film 39 is etched to provide an opening 38, whereby a p-side electrode 36 is formed. After the n-type GaAs substrate 12 is polished to a predetermined thickness, an n-side electrode 40 is formed on the opposite surface of the n-type GaAs substrate 12. Thus, the surface light emitting semiconductor laser element 10 shown in FIG. 1 can be produced.

(60) As described above, the surface light emitting semiconductor laser element 10 can be produced with similar processes to those used for the conventional surface light emitting semiconductor laser element except for the sizes of the contact layer 24 and the p-side electrode 36.

(61) Embodiment 3

(62) FIG. 8 shows a sectional view of an alternative surface light emitting semiconductor laser element according to the present invention.

(63) The alternative surface light emitting semiconductor laser element has a similar structure in a main part 50 to the surface light emitting semiconductor laser element 10 except that a contact layer 52 and a p-side electrode 54 have different structures.

(64) As shown in FIG. 8, the contact layer 52 includes three layer: an upper contact layer 52A, a middle contact layer 52B, and a lower contact layer 52C. The impurity concentrations of respective contact layers gradually decrease step-wise from the upper contact layer to the lower contact layer.

(65) The lower contact layer 52C has, for example, an impurity concentration of 510.sup.18, which is the lowest among the three contact layers, and has an opening 56C which is the largest opening. The middle contact layer 52B has, for example, an impurity concentration of 110.sup.19, which is higher than the lower contact layer, but lower than the upper contact layer, and has an opening 56B which is smaller than the opening 56C of the lower contact layer, but greater than an opening 56A of the upper contact layer. The upper contact layer 52A has, for example, an impurity concentration of 310.sup.19, which is the highest among the three contact layers, and has the opening 56A which is the smallest among the three contact layers.

(66) The p-side electrode 54 is also formed step-wise so as to conform to the contact layers 52A, 52B and 52C, as well as the openings 56A, 56B and 56C.

(67) According to the configuration of the contact layer 52 and the p-side electrode 54, an effective complex refractive index distribution structure is formed to improve focusing of the light, whereby a single-peak transverse mode can be more easily provided.

(68) As described above, the contact layer 52 is formed such that three layers have respective openings in a step-wise fashion. Specifically, an etching mask 58 is disposed on the upper contact layer 52A having lower impurity concentration, as shown in FIG. 9. The three contact layers 52A, 52B and 52C are dry etched under the same etching conditions. Since the etching rates are different due to the different impurity concentrations, the openings 56A, 56B and 56C having the desired sizes are formed on the three contact layers 52A, 52B and 52C.

(69) Alternatively, the three contact layers may be formed so that the Al compositions decrease step-wise from the upper contact layer to the lower contact layer. The three contact layers 52A, 52B and 52C are dry etched under the same etching conditions. Since the etching rates are different due to the different Al compositions, the openings 56A, 56B and 56C having diameters that become smaller step-wise from the upper contact layer to the lower contact layer are formed on the three contact layers 52A, 52B and 52C.

(70) Embodiment 4

(71) FIGS. 10A and 10B are a sectional view and a plan view respectively showing the structure of a surface light emitting semiconductor laser element oscillating in a higher-order mode according to the present invention. FIG. 10C is a waveform of a transverse mode.

(72) The surface light emitting semiconductor laser element emits light in a TE.sub.01 mode (donut-like light emission pattern). As shown in FIGS. 10A and 10B, the surface light emitting semiconductor laser element comprises, as a main part 60, a p-side electrode 62 including a circular central electrode 64 and an annular electrode 68 disposed via an annular light emitting window 66, as in EMBODIMENT 1.

(73) The surface light emitting semiconductor laser element has a similar structure to the surface light emitting semiconductor laser element 10 in EMBODIMENT 1 except that the p-side electrode 62 has a different structure.

(74) The contact layer 24 and the p-side electrode 62 provide the same effects as the complex refractive index distribution structure described in the surface light emitting semiconductor laser element 10 oscillating in the single mode. The single basic mode lower than the desired high-order mode is suppressed, and at the same time, modes higher than the desired high-order mode are suppressed.

(75) In this EMBODIMENT, the basic mode is absorbed and suppressed at the circular central electrode 64 made of gold disposed at the center of the light emitting surface. The modes higher than the TE.sub.01 mode are scattered using the aperture of the current confinement layer 28 (see FIG. 1) and the concave lens of the contact layer 24 in the complex refractive index distribution structure. Thus, light is selectively emitted in the TE.sub.01 mode.

(76) As long as the constriction diameter of the current confinement layer is set to cut-off the transverse modes other than the TE.sub.01 mode, the selectivity of the TE.sub.01 mode is further improved.

(77) As to conventional high-order mode control, Japanese Unexamined Patent Application Publication No. 2002-359432 discloses, for example, a method of selecting a mode by forming a groove (or a convex-concave shape) having a depth of a wavelength or wavelength on a mesa surface to exclude any undesirable excited modes or to include the desirable modes.

(78) However, although some functions can be added to the mesa using post processing such as ion beam etching, the devices are processed only one-by-one, thus reducing production efficiency, and the groove depth, that is the interference optical path difference, should be precisely defined, even if the device is subjected to patterning etching. Accordingly, such a conventional semiconductor laser may not be applicable to commercial devices.

(79) In contrast, the laser resonance mode can be selected by providing the complex refractive index distribution structure on the uppermost side of the resonator according to the present invention. In addition, the complex refractive index distribution structure can be provided by adjusting the shape or the refractive index of the compound semiconductor layer on the mesa, the insulation film, or the electrode in the typical production processes without adding any steps. Respective parts of the complex refractive index distribution structure can be produced with such a precision that is required for typical surface light emitting semiconductor laser elements. No high precise production processes are required. Currently available general process precision is enough for producing the complex refractive index distribution structure according to the present invention. Therefore, the complex refractive index distribution structure can be produced with good reproducibility.

(80) Comparative Embodiment

(81) FIG. 11 is a sectional view showing the structure of a comparative surface light emitting semiconductor laser element.

(82) The comparative surface light emitting semiconductor laser element comprises, as a main part 70, a scattering structure that randomly scatters light to an upper surface of a mesa, and a contact layer 72 having a fine convex-concave surface.

(83) Scattering at the convex-concave surface of the contact layer 72 affects the oscillation mode. A number of modes oscillate randomly. The light emitted therefrom includes a number of modes, resulting in a random light emission pattern.