JP7406244B2 - Vertical cavity surface emitting laser and its manufacturing method - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act Announced in the "Web program of the 80th Autumn Academic Conference of the Japan Society of Applied Physics 2019" published on the website on July 16, 2019 [Publications, etc.] Reiwa 1 Published in the ``2019 80th Japan Society of Applied Physics Autumn Academic Conference Proceedings,'' published on September 4, 2019 [Publications, etc.] Presented at the Autumn Academic Conference

The present invention relates to vertical cavity surface emitting lasers.

One type of semiconductor laser is a vertical cavity surface emitting laser (VCSEL). A VCSEL has a resonator formed perpendicularly to a semiconductor substrate, and emits a beam perpendicularly to the substrate. Since VCSEL is cheaper than other semiconductor lasers, its applications and market size are expanding year by year.

FIG. 1 is a cross-sectional view of a conventional VCSEL. The VCSEL 100r includes a substrate 102, a lower DBR (Distributed Bragg Reflector) layer 104, an active layer 106, an upper DBR layer 108, a lower electrode 110, and an upper electrode 112. An oxidized confinement layer 114 is formed above the active layer 106. A current flows through the current injection region A defined by the oxidized confinement layer 114.

Japanese Patent Application Publication No. 2011-003725

R. Jager, M. Grabherr, C. Jung, R. Michalzik, G. Reiner, B. Weigl, K.J. Ebeling, "57% wallplug efficiency oxide-confined 850 nm wavelength GaAs VCSELs," Electronics Letters, vol. 33, Issue 4, pp. 330-331, 13 February 1997. J.-F. Seurin, G. Xu, V. Khalfin, A. Miglo, J. D. Wynn, P. Pradhan, C. L. Ghosh, L. A. D'Asaro, "Progress in high-power high-efficiency VCSEL arrays," Proc. SPIE , vol. 7229, 722903, 6 February 2009.

To increase the efficiency of a VCSEL, an approach can be taken to increase the doping concentration of the semiconductor layer and reduce the resistance between the electrodes. However, increasing the doping concentration increases light absorption, which works against improving efficiency. Therefore, conventional VCSELs have limited efficiency improvement.

It is in this context that the present invention has been made, and one exemplary object of certain aspects thereof is to provide a surface emitting laser with improved efficiency.

Certain aspects of the present invention relate to vertical cavity surface emitting lasers. A vertical cavity surface emitting laser includes a lower DBR (Distributed Bragg Reflector) layer, an active layer formed on the lower DBR layer, and an upper DBR layer formed on the active layer. The upper DBR layer includes a proton injection region in a portion that overlaps the current injection region of the active layer.

Note that the terms "upper" and "lower" (upper and lower) in this specification are merely for convenience, and the upper DBR layer in which the proton injection region is formed can be understood as the DBR layer through which the drive current flows.

The current injection region may be defined by a current confinement structure including a high resistance region and a low resistance region surrounded by the high resistance region.

Another aspect of the invention relates to a method of manufacturing a vertical cavity surface emitting laser. The manufacturing method includes the step of injecting protons into a portion of the upper DBR layer that overlaps with a current injection region of the active layer.

Note that arbitrary combinations of the above components, or expressions of the present invention converted between methods, devices, etc., are also effective as aspects of the present invention.

According to certain aspects of the present invention, the efficiency of vertical cavity surface emitting lasers can be improved.

1 is a cross-sectional view of a conventional VCSEL. 1 is a cross-sectional view of a VCSEL according to an embodiment. FIG. 3 is a diagram showing the relationship (simulation results) between the depth of a proton injection region, resistance value, and light absorption. FIG. 3 is a diagram showing the structure of a sample. FIGS. 5(a) and 5(b) are diagrams showing the IV characteristics and current dependence of output power (measurement results) of sample A and sample B. 3 is a distribution diagram of output power of sample A and sample B. FIG. FIGS. 7A to 7D are diagrams showing configuration examples 1 to 4 of the VCSEL. FIGS. 8(a) and 8(b) are diagrams showing analysis results of electrical resistance and light absorption in configuration examples 1 to 4 shown in FIGS. 7(a) to 7(d). FIGS. 9(a) to 9(f) are diagrams showing a method for manufacturing a VCSEL. FIGS. 10(a) to 10(d) are diagrams showing the proton injection process of FIG. 9(b).

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on preferred embodiments with reference to the drawings. Identical or equivalent components, members, and processes shown in each drawing are designated by the same reference numerals, and redundant explanations will be omitted as appropriate. Further, the embodiments are illustrative rather than limiting the invention, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

FIG. 2 is a cross-sectional view of the VCSEL 100 according to the embodiment. VCSEL 100 includes a substrate 102, a lower DBR layer 104, an active layer 106, an upper DBR layer 108, a lower electrode 110, and an upper electrode 112.

Known techniques may be used for the VCSEL structure and materials, and although there are no particular limitations, an example will be described. For example, the substrate 102 is a III-V group semiconductor and may be a GaAs substrate. A lower electrode 110, which is an n-type electrode, is formed on the back surface of the substrate 102. The lower DBR layer 104 has a laminated structure of an Al _0.92 Ga _0.08 As layer and an Al _0.16 Ga _0.84 As layer (AlGaAs=aluminum gallium arsenide) doped with n-type impurities. % reflectance. As the n-type impurity, silicon (Si), selenium (Se), etc. can be used, for example.

The active layer 106 has a multiple quantum well structure of In _0.2 Ga _0.8 As/GaAs (indium gallium arsenide/gallium arsenide). For example, the active layer 106 may have a three-layer quantum well structure. A lower spacer layer and an upper spacer layer, which are undoped Al _0.3 Ga _0.7 As layers, are formed on both sides of the multiple quantum well structure as necessary. The upper DBR layer 108 may have a laminated structure of an Al _0.92 Ga _0.08 As layer and an Al _0.16 Ga _0.84 As layer (AlGaAs=aluminum gallium arsenide) doped with p-type impurities. As the p-type impurity, carbon (C), zinc (Zn), magnesium (Mg), beryllium (Be), or the like can be used.

In the upper DBR layer 108, protons are injected into a portion B that overlaps the current injection region A of the active layer 106, thereby forming a proton injection region 120. The current injection region A of the active layer 106 is defined by a current confinement structure including a high resistance region X and a low resistance region Y surrounded by the high resistance region X. Specifically, the low resistance region Y is defined by the current injection region A. corresponds to

VCSEL 100 may include an oxide confinement layer 114 that includes AlAs. By selectively oxidizing this oxidized confinement layer 114 from the side surface of the mesa, a high resistance region X is formed and a current confinement structure is generated.

The upper electrode 112, which is a P-type electrode, is formed above the upper DBR layer 108.

The above is the structure of the VCSEL 100.

Next, its operation will be explained. In the upper DBR layer 108, the drive current _ID injected from the upper electrode 112 flows to a region with a high carrier concentration outside the proton injection region 120, as shown by the dashed line, and from the vicinity of the oxidized confinement layer 114 flows into the active region. It begins to flow towards the current injection region A of layer 106.

When the drive current _ID flows through the current injection region A of the active layer 106, population inversion is formed in the active layer 106, causing laser oscillation. The laser light is amplified by reciprocating between the lower DBR layer 104 and the upper DBR layer 108 in a portion overlapping with the current injection region A, and a portion of the laser light is extracted from the upper DBR layer 108.

By injecting protons into the portion B of the upper DBR layer 108 that overlaps the current injection region A, the carrier concentration of this portion B is reduced, and therefore light absorption, ie, loss, is reduced. This allows the efficiency of the VCSEL 100 to be improved.

Note that as the carrier concentration in the proton injection region 120 of the upper _DBR layer 108 decreases, the resistance of that portion increases; The increase in resistance due to the formation of the proton injection region 120 can be ignored because the proton can flow through a high concentration path. Alternatively, even if the increase in resistance cannot be ignored, if the improvement in efficiency due to the decrease in light absorption exceeds the decrease in efficiency due to the increase in resistance, the efficiency of the VCSEL 100 as a whole can be improved.

Furthermore, by proton injection, the carrier concentration in the proton injection region 120, which is the optical path, can be controlled independently of the carrier concentration in the non-injection region, which is the drive current path. It is also possible to increase the concentration above that conventionally and thus further reduce the resistance of the current path. Therefore, in addition to the effect of reducing light absorption, the efficiency can be improved by the combination of the effect of current path resistance.

In addition, proton injection can be easily introduced into the manufacturing process of semiconductor lasers, as can be seen from the fact that it has been used in the production of current confinement structures. Therefore, the technical hurdles for forming the proton injection region 120 are low, and the increase in cost is very small.

In this embodiment, proton implantation is performed after manufacturing the VCSEL stacked structure, but as a comparative technique, impurities are implanted in the center and outer periphery of the upper DBR layer 108 during the manufacturing stage of the VCSEL stacked structure. It is also possible to adopt a method of changing the doping concentration. However, in VCSEL devices, which have a huge structure compared to transistors, in order to selectively change the doping concentration in each region, it is necessary to combine crystal growth, lithography, and etching multiple times, which inevitably increases costs. . On the other hand, the proton injection used in the embodiment does not require a complicated process, so it can be said to be suitable for large devices such as semiconductor lasers.

FIG. 3 is a diagram showing the relationship (simulation results) between the depth of the proton injection region 120, resistance value, and light absorption. The carrier concentrations of the proton injection region 120 are 5×10 ¹⁶ (cm ⁻³ ), 1×10 ¹⁷ (cm ⁻³ ), 2×10 ¹⁷ (cm ⁻³ ), 5×10 ¹⁷ (cm ⁻³ ), 1 ×10 ¹⁸ (cm ⁻³ ). The proton injection region 120 was formed to have a depth in the range of 0 to 2.5 μm. The carrier concentration in the upper DBR layer 108 other than the proton injection region 120 (non-injection region) is 2×10 ¹⁸ (cm ⁻³ ). If the carrier concentration in the proton injection region is lowered too much, the resistance value will increase excessively, so the carrier concentration in the proton injection region is preferably about 2×10 ¹⁷ to 1×10 ¹⁸ (cm ⁻³ ). This corresponds to about 1/10 to 1/2 of the carrier concentration in the non-injected region.

A sample of the VCSEL 100 that was actually produced will be explained. FIG. 4 is a diagram showing the structure of the sample. The design parameters are as follows.
Mesa height h=3.45μm
Mesa diameter φ ₁ = 30μm
Output window diameter _φ2 = 15μm
Selective oxidation aperture diameter φ ₃ = 10μm
Proton injection diameter φ ₄ = 10 μm

Sample A with the same structure but without the proton injection region 120 and sample B with the proton injection region 120 were prepared, and their characteristics were compared. Regarding proton implantation, the implantation energy is 310 keV, the dose is 1×10 ¹² (cm ⁻² ), and a proton implantation region 120 is formed at a depth of 2.0 to 2.5 μm. In addition, in ion implantation (proton implantation), the implantation distribution has a peak concentration at a depth according to the implantation energy, and a concentration distribution that spreads to some extent in the depth direction is formed, and 2.0 to 2.5 μm is Refers to a region into which a significant concentration of protons has been injected. Therefore, within that depth range, the proton injection concentration (more essentially carrier concentration) is not necessarily uniform. Note that the simulation assumes that the carrier concentration is uniform over the specified depth range.

FIGS. 5(a) and 5(b) are diagrams showing the IV characteristics and current dependence of output power (measurement results) of sample A and sample B. It can be seen that in sample B in which the proton injection region 120 was formed, the output power when the same drive current was applied was dramatically increased compared to sample A in which the proton injection region 120 was not formed.

FIG. 6 is an output power distribution diagram of sample A and sample B. The horizontal axis represents slope efficiency SE.

In addition, sample A created in the experiment has the carrier concentration of the upper DBR layer determined so that good characteristics can be obtained in a conventional configuration, and sample B is a device with the same conditions as sample A, but with protons. It is injected with.

Here, in the conventional structure (that is, sample A), if the carrier concentration is made too high, light absorption increases, so it was necessary to determine the carrier concentration in consideration of these trade-offs. In contrast, in the VCSEL 100 according to the embodiment, even if the carrier concentration in the non-injected region is increased, light absorption does not increase. In other words, it becomes possible to further increase the carrier concentration in the upper DBR layer than in the past. Therefore, there is room for further improvement in characteristics compared to sample B when an optimized design is performed.

Next, the results of the study regarding the formation position of the proton injection region 120 in the depth direction will be explained. FIGS. 7A to 7D are diagrams showing configuration examples 1 to 4 of the VCSEL 100. In configuration example 1 shown in FIG. 7(a), the proton injection region 120 is formed at a depth of 0-2 μm. In configuration example 2 shown in FIG. 7(b), the proton injection region 120 is formed at a depth of 0-1 μm. In configuration example 3 shown in FIG. 7(c), the proton injection region 120 is formed at a depth of 0.5-1.5 μm. In configuration example 4 shown in FIG. 7(d), the proton injection region 120 is formed at a depth of 1-2 μm.

FIGS. 8(a) and 8(b) are diagrams showing analysis results of electrical resistance and light absorption in configuration examples 1 to 4 shown in FIGS. 7(a) to 7(d). i is no proton injection, ii is the configuration example of FIG. 7(b), iii is the configuration example of FIG. 7(c), iv is the configuration example of FIG. 7(d), v is the analysis of the configuration of FIG. 7(a) Show the results.

From this analysis result, it can be seen that the characteristics can be further improved by adopting a structure in which the implantation is performed at a deeper position rather than in a region close to the surface of the device.

Note that this analysis example assumes that proton implantation is performed for the doping concentration of a normal VCSEL, and as a result of proton implantation, there will always be more high resistance regions, and the resistance will always be higher than that of the reference sample (i). Is high.
In this embodiment, by proton injection, the carrier concentration in the proton injection region 120, which is the optical path, can be controlled independently of the carrier concentration in the non-injection region, which is the drive current path. It is also possible to increase the carrier concentration in the implanted region compared to conventional ones, thus further reducing the resistance of the current path. Such a structure is not subject to the above analysis example, and there is room for further improvement in characteristics than this analysis.

Next, a method for manufacturing the VCSEL 100 will be explained. FIGS. 9(a) to 9(f) are diagrams showing a method for manufacturing the VCSEL 100. First, as shown in FIG. 9A, a stacked structure 101 including a substrate 102, a lower DBR layer 104, an active layer 106, an upper DBR layer 108, and a protective film 109 is formed. The laminated structure 101 may be manufactured using a known technique. The protective film 109 is formed to protect the semiconductor surface. Although not limited thereto, the protective film 109 may include SiO ₂ (glass), which is easy to remove and causes less stress and erosion on the semiconductor.

Subsequently, as shown in FIG. 9(b), a proton injection region 120 is formed. Subsequently, as shown in FIG. 9(c), a mesa structure 130 is formed. Subsequently, as shown in FIG. 9(d), an oxidized confinement layer 114 is formed by a selective oxidation process. Then, as shown in FIG. 9(e), a polymer 132 is applied to the mesa structure 130. Then, as shown in FIG. 9(f), an upper electrode 112 and a lower electrode 110 are formed.

FIGS. 10(a) to 10(d) are diagrams showing the proton injection process of FIG. 9(b). As shown in FIG. 10(a), a resist 140 is applied on the VCSEL stacked structure 101. Subsequently, as shown in FIG. 10(b), the resist 140 is exposed, developed, and patterned to form an opening 142. Then, in FIG. 10C, protons are irradiated from above the resist 140, and the protons are injected into the upper DBR layer 108 through the opening 142 of the resist 140, thereby forming a proton injection region 120. In FIG. 10(d), the resist 140 is removed, and the protective film 109 is also removed, resulting in the state shown in FIG. 9(c).

Note that in the manufacturing method of FIGS. 9A to 9F, protons are injected before forming the mesa, but this is not the case, and the order of proton injection is not limited. For example, proton implantation may be performed after mesa formation or even after formation of oxidized confinement layer 114.

Further, in FIGS. 10A to 10D, proton implantation was performed using a resist, but proton implantation may be performed using a metal mask.

In the embodiment, the lower electrode (n-type electrode) 110 is formed on the back surface of the substrate 102, but the invention is not limited thereto. For example, an n-type contact layer may be inserted between the substrate 102 and the lower DBR layer 104 or above the lower DBR layer, and an n-type electrode may be formed above the n-type contact layer exposed in the mesa portion. A dielectric DBR layer may be employed as the lower DBR layer.

The upper DBR layer 108 may have a stacked structure of a semiconductor DBR layer and a dielectric DBR layer provided thereon. In this case, a proton injection region may be formed in the semiconductor DBR layer.

Although the present invention has been described using specific words and phrases based on the embodiments, the embodiments merely illustrate the principles and applications of the present invention, and the embodiments do not include the scope of the claims. Many modifications and changes in arrangement are possible without departing from the spirit of the present invention.

100 VCSEL
102 Substrate 104 Lower DBR layer 106 Active layer 108 Upper DBR layer 110 Lower electrode 112 Upper electrode 114 Oxidation confinement layer 120 Proton injection region 130 Mesa structure 132 Polymer

Claims (2)

a lower DBR (Distributed Bragg Reflector) layer;
an active layer formed on the lower DBR layer;
an upper DBR layer formed on the active layer;
Equipped with
The upper DBR layer is
a current confinement structure that includes a high resistance region and a low resistance region surrounded by the high resistance region and defines a current injection region of the active layer;
a proton injection region formed at a shallower position than the current confinement structure;
including;
The proton injection region overlaps the current injection region when viewed from above in a direction perpendicular to the substrate, and is a continuous region including the center of the exit window. laser. A method for manufacturing a vertical cavity surface emitting laser, the method comprising:
forming a current confinement structure in an upper DBR (Distributed Bragg Reflector) layer that includes a high resistance region and a low resistance region surrounded by the high resistance region and defines a current injection region of the active layer;
Injecting protons into a region shallower than the current confinement structure in the upper DBR layer;
Equipped with
A manufacturing method characterized in that the region into which protons are injected is a continuous region that overlaps the current injection region and includes the center of the exit window when viewed from above in a direction perpendicular to the substrate.

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