High-efficiency oxide VCSEL with improved light extraction, and manufacturing method thereof

Abstract

The present invention relates to a vertical cavity surface emitting laser (VCSEL) and a manufacturing method thereof, and more specifically, to a high-efficiency oxide VCSEL which emits laser beams having a peak wavelength of 860 nm, and a manufacturing method thereof.

Claims

1. An oxide vertical cavity surface emitting laser (VCSEL) comprising a conductive current spreading layer formed between a top electrode and a top distributed Bragg reflector to pass laser beams having a peak wavelength of 86010 nm, having a refractive index lower than that of the top distributed Bragg reflector, and having a rough top surface.

2. The VCSEL according to claim 1, wherein the current spreading layer is a GaP layer.

3. The VCSEL according to claim 2, wherein the GaP layer includes a metallic and/or non-metallic dopant.

4. The VCSEL according to claim 3, wherein one or more among a group including Mg, Zn and carbon are selected as the dopant.

5. The VCSEL according to claim 1, wherein the current spreading layer has a thickness of 1 m or larger.

6. The VCSEL according to claim 1, wherein the rough layer is a wet or dry-etched rough layer.

7. The VCSEL according to claim 1, wherein roughness of the rough layer is >1.

8. The VCSEL according to claim 1, wherein the oxide vertical cavity surface emitting laser includes a bottom electrode, a substrate, a bottom distributed Bragg reflector, an active layer, a top distributed Bragg reflector, a top electrode, and an oxidized layer.

9. The VCSEL according to claim 8, wherein the active layer is configured of a GaAs quantum well and an AlGaAs quantum barrier layer; the top and bottom DBRs are distributed Bragg reflectors having a structure repeatedly stacking two layers of an Al.sub.xGa.sub.1-xAs layer of 0.8<x<1 and an Al.sub.yGa.sub.1-yAs layer of 0<y<0; the oxidized layer is configured of an outer oxidized layer of a ring shape and an inner current window of a center circle shape; and the top electrode is an electrode of a ring shape.

10. The VCSEL according to claim 8, wherein the oxidized layer is positioned between layers of top p-DBRs.

11. The VCSEL according to claim 9, wherein the oxide VCSEL operates at a current of 10 to 40 mA.

12. The VCSEL according to claim 9, wherein the top electrode is a multilayer electrode of a ring shape including a Pt layer.

13. The VCSEL according to claim 9, further comprising an anti-reflection layer on a top of the current spreading layer.

14. A method of manufacturing an oxide VCSEL having a peak wavelength of 86010 nm, wherein a GaP layer having a thickness of 1 m or larger and a rough top surface is positioned between a top electrode and a top DBR.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a view showing an exploded cross-section of a conventional oxide VCSEL.

(2) FIG. 2(a) is a view showing an SEM image of a DBR damaged when oxidation is progressed, and FIG. 2(b) is a view showing a shape of a current window, in which the black stripe is a trench (pitted) area, and the area in the middle is a pillar area for emitting light, and the brighter area is an oxidized area.

(3) FIG. 3 is a cross sectional view showing an oxide VCSEL according to an embodiment of the present invention.

(4) FIG. 4 is a cross sectional view showing an oxide VCSEL without a GaP layer according to comparative embodiment 1 compared to the present invention.

(5) FIG. 5 is a cross sectional view showing an oxide VCSEL having a non-rough GaP layer according to comparative embodiment 2 compared to the present invention.

(6) FIG. 6 is a view comparing the structures and top surfaces of a rough GaP layer of an embodiment of the present invention, a state without a GaP layer of comparative embodiment 1, and a non-rough GaP layer of comparative embodiment 2.

(7) FIG. 7 is a view comparing light intensity of a rough GaP layer of an embodiment of the present invention, a state without a GaP layer of comparative embodiment 1, and a non-rough GaP layer of comparative embodiment 2 at a current of 20 mA.

(8) FIG. 8 is a view comparing light intensity of a rough GaP layer of an embodiment of the present invention, a state without a GaP layer of comparative embodiment 1, and a non-rough GaP layer of comparative embodiment 2 at a current in a range of 0 to 50 mA.

(9) FIG. 9 is a view showing the factors related to intensity of light generated by a VCSEL.

(10) FIG. 10 is a view describing light reflectivity of a rough GaP layer of an embodiment of the present invention, a state without a GaP layer of comparative embodiment 1, and a non-rough GaP layer of comparative embodiment 2.

(11) TABLE-US-00001 DESCRIPTION OF SYMBOLS 100: Oxide VCSEL 110: Bottom electrode 120: Substrate 130: Bottom DBR 140: Active layer 150: Top DBR 160: GaP layer 170: Top electrode 180: Oxidized layer

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(12) Hereafter, the present invention will be described in detail through embodiments. The embodiments described below are intended not to limit, but to illustrate the present invention.

Embodiment 1

(13) FIG. 3 is a view showing a VCSEL layer structure for emitting a laser beam having a peak wavelength of 860 nm, in which a high conductive GaP barrier layer manufactured by a MOCVD system is applied. As shown in FIG. 3, an oxide VCSEL 100 having a peak wavelength or 860 nm, in which a high conductive GaP barrier layer according to the present invention is applied, is an oxide VCSEL 100 which emits laser beams toward the top of a substrate 120. The substrate 120 is an n-type GaAs substrate. A bottom electrode 110 is provided on the bottom of the substrate 120.

(14) A bottom n-DBR 130, in which pairs of a high refractive index AlGaAs layer and a low refractive index AlGaAs layer are repeatedly stacked, is provided or the top of the substrate 120. An Al.sub.0.85Ga.sub.0.15As layer and an Al.sub.0.15Ga.sub.0.85As layer are repeatedly stacked as many as forty times.

(15) An active layer 140 is provided on the bottom DBR 130. The active layer 140 includes a quantum well structure for generating light. The active layer 140 is a GaAs/AlSaAs active layer QW for emitting light having a center wavelength of 850 nm.

(16) A top p-DBR 150 including an oxidized layer 180 is provided on the active layer 140. To avoid damage of the active layer in an oxidization process, the oxidized layer 180 may be inserted between layers of the pairs configuring the p-DBR 150 and may avoid direct contact with the active layer 140. The active layer 140 is stacked on a pair or two pairs of the top DBRs among the twenty pairs, and the other pairs of the top DBR are stacked on the oxidized layer 180.

(17) Accordingly, the top DBR 150 is configured of a first top DBR 151 positioned on the bottom of the oxidized layer 180 and a second top DBR 152 positioned on the top of the oxidized layer 180.

(18) In the same manner as the bottom n-DBR, pairs of a high refractive index AlGaAs layer and a low refractive index AlGaAs layer are repeatedly stacked in the top p-DBR 150, and the top p-DBR 150 is configured of twenty pairs of an Al.sub.0.85Ga.sub.0.15As layer and an Al.sub.0.15Ga.sub.0.85As layer.

(19) The oxidized layer 180 is configured of a circular current window (oxidation aperture) 181 formed of Al.sub.0.98Ga.sub.0.02As having a thickness of about 30 nm at the center and an oxidized ring 182 at the periphery that is formed as the current window is oxidized by steam. The DBR reflectivity shows an excellent characteristic of the stop-band shape at almost 98%.

(20) A GaP layer 160 is grown on the top p-DBR 150 as much as 3 m in the MOCVD method. A top electrode 170 is formed on the top GaP 160 in a ring shape.

(21) A rough surface is formed at the center of the top electrode 170 through dry etching. As shown in FIG. 6(c), whether a rough surface is formed is confirmed by photographing the surface. The peak wavelength of the active layer 140 is about 850 nm, and the cavity peak is about 860 nm by the DBR reflection.

COMPARATIVE EXAMPLE 1

(22) As shown in FIG. 4, in the embodiment 1, a top electrode 170 of a ring shape is formed on the top p-DBR 150 without a GaP layer 160. As shown in FIG. 6(a), the surface is photographed and compared.

COMPARATIVE EXAMPLE 2

(23) As shown in FIG. 5, in the embodiment 1, the top electrode 170 of a ring shape is formed on the top p-DBR 150 without a GaP layer 160. As shown in FIG. 6(b), the surface is photographed and compared.

(24) As shown in comparative example 1, it is confirmed that the surface of a conventional VCSEL without a GaP layer is very smooth, and also in the case of a VCSEL to which a GaP layer is applied as shown in comparative example 2, the surface is almost similar to that of the conventional VCSEL. When a rough GaP layer is applied as shown in embodiment 1, it is confirmed that very high roughness has been applied. Such a surface roughness is applied only to a local area through which light is actually emitted.

(25) Performance Test 1

(26) A 20 mA current is applied to a VCSEL without a GaP layer (comparative example 1), a VCSEL to which a non-rough GaP barrier having a thickness of 3 um is applied (comparative example 2), and a VCSEL having a rough GaP layer, and emission intensity is measured. Its result is shown in FIG. 6. In the case of comparative example 1 in which a GaP layer is not applied, an intensity of about 0.45 is shown, and in the case of comparative example 2 in which the GaP layer is applied, the intensity is about 0.86, which is an increase of intensity of about 80%. On the contrary, as shown in embodiment 1, it may be confirmed that when a rough GaP is applied, the intensity is about 99, which is an increase of intensity of about 110%.

(27) These results show that although light efficiency of the VCSELs is enhanced owing to the current spreading effect when a GaP barrier is primarily applied, when surface roughness of the GaP barrier is applied, the light efficiency is enhanced much more as the light extraction efficiency of the light generated by the current spreading effect is enhanced.

(28) Performance Test 2

(29) FIG. 8 confirms (a) an I-V curve and (b) an I-L curve of VCSELs of comparative example 1, comparative example 2 and embodiment 1 by applying a current of 0 to 50 mA in the present invention.

(30) Confirming the (a) I-V characteristic, the I-V characteristics of all the samples are the same without regard to application of a GaP barrier or a rough GaP barrier. This is since that conductivity of the GaP is relatively high compared with those of VCSEL materials.

(31) Confirming the (b) I-B characteristic, a considerably enhanced light efficiency characteristic is confirmed according to application of the GaP barrier or the rough GaP barrier. Light efficiency of a general VCSEL is confirmed to be about 17 mW at about 33 mA, and when a GaP barrier is applied, light efficiency is about 24.5 mW, which is an increase of about 44%, and when a rough GaP barrier is applied, light efficiency is about 30 mW, which is an increase of about 76%.

(32) Analysis of Results

(33) FIG. 9 shows (a) a computational model for the current spreading effect of a VCSEL to which a high conductivity GaP layer is applied and (b) current flow curves when two probes contact on GaP and AlGaAs, respectively, which is a representative material of the GaP barrier and the VCSEL. In addition, it shows a final equation obtained from the computational model. It is confirmed through FIG. 8(a) that a real emission area of the active layer is limited by the current window, and in addition, it is known that the current spreading area Ls of the current injected from an electrode becomes maximum (Ls=R) when the current spreading area grows toward the further inner side as much as the r (non-current spreading area) value becomes 0, i.e., when the current spreading area becomes the same as the real emission area. These values are expressed as an equation shown on the bottom, and the most important variables in the equation are t (thickness of the GaP barrier) and (nonresistance of the GaP barrier 1/conductivity). Therefore, it is understood that Ls (current spreading area) abruptly increases as t increases or decreases as shown in the equation. Here, it is understood that, the current spreading area becomes the maximum value when Ls=R although t or continuously increases by the nature of the VCSEL having an inner local light emitting area. The GaP material, which is used when the two probe contact current flow curve (b) is confirmed, shows a conductivity higher than that of AlGaAs, which is a representative material of the VCSEL, as much as four times or more, and it is understood that the GaP barrier, which is a high conductive material, may be used to be appropriate to the equation of FIG. 8.

(34) FIG. 10 shows a result of light emitting paths of a general VCSEL (comparative example 1), a VCSEL to which a non-rough GaP barrier is applied (comparative example 2), and a VCSEL to which a rough GaP barrier is applied (embodiment 1). The light emitting path result shows that additional increase of light efficiency can be obtained only by performing surface treatment of the GaP barrier on the VCSEL that already has accomplished improvement of light efficiency through a current spreading layer by applying the GaP barrier. Further enhanced light extraction efficiency of a VCSEL can be obtained by roughly processing the surface of a GaP barrier existing in a local light emitting area of the VCSEL through a wet or dry etching process, and the light extraction efficiency may be regarded as being affected by decrease of total internal reflection (TIR) resulting from roughness of the surface. Additionally, when an anti-reflection layer of SiN, SiO or the like is applied on the top of a rough GaP, further higher light efficiency can be obtained.

(35) According to the present invention, there is provided a new barrier, which is also a current spreading layer, capable of protecting a top DBR in an oxidization process, enhancing flow of current, and passing light of an 860 nm VCSEL.

(36) The oxide VCSEL having a GaP barrier layer according to the present invention enhances light efficiency up to 40% by improving electrode protection and current flow of an 860 nm VCSEL having an oxidation aperture, and an optimum range of efficiency between the applied high conductive material and the oxidation aperture of the VCSEL can be confirmed.

(37) According to the present invention, there is provided a method of enhancing light emission and extraction capability of an oxide VCSEL having low emission efficiency due to a narrow emission area and a top DBR.

(38) Although the present invention has been illustrated and described in detail in the above drawings and descriptions, it is considered that the drawings and descriptions are illustrative or exemplary and not restrictive. Other changes may be apparent to those skilled in the art from the present invention. These changes may accompany other features that can be used instead of or in addition to the features publicized in this field or described in this specification. Modifications of the disclosed embodiments may be understood and affected by those skilled in the art from learning of the drawings, the present invention and the appended claims. The term comprisingused in the claims does not exclude other, elements or steps, and description of an indefinite article does not exclude a plurality of elements or steps. The fact that particular actions are cited in different dependent claims does not mean that these actions cannot be used to make combinations of the actions advantageous. Arbitrary reference symbols in the claims should not be interpreted as limiting the scope of the claims.