JP2005310938A - Semiconductor laser and epitaxial wafer therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface-emitting epitaxial wafer for semiconductor laser, capable of reducing the threshold current with higher efficiency than the conventional, by making the place where the time average of light intensity distribution is maximum coincide with an active layer. <P>SOLUTION: On a substrate 1, at least a first first-conductivity-type clad layer 3, an active layer 4, and a second second-conductivity-type clad layer 5 are provided by epitaxial-growth. When the film thickness of the first clad layer 3 is set as d<SB>1</SB>, the film thickness of the second clad layer 5 is set as d<SB>2</SB>, the refractive index of the first clad layer 3 is set as n<SB>1</SB>, and the refractive index of the second clad layer 5 is set as n<SB>2</SB>; the Formula (d<SB>1</SB>/n<SB>1</SB>)=(d<SB>2</SB>/n<SB>2</SB>) is established. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an epitaxial wafer for a semiconductor laser and a semiconductor laser.

  In recent years, a surface emitting laser called VCSEL (Vertical Cavity Surface Emitting Laser) has attracted attention. Compared with edge-emitting lasers with a conventional structure, VCSELs have low threshold current, low power consumption, high efficiency, circular narrow-angle emission light, high-speed modulation, easy integration and mounting, and performance tests on a wafer basis It is expected to become an indispensable device in the optical communication field in the future.

  The basic structure of the VCSEL will be briefly described below. On the semiconductor substrate having the first conductivity type, the first reflecting mirror having the first conductivity type, the first cladding layer having the first conductivity type, the active layer, and the second having the second conductivity type. The second clad layer and the second reflecting mirror having the second conductivity type are sequentially formed. Electrodes are formed on the lower surface of the substrate and the upper surface of the reflecting mirror having the second conductivity type, and the current flows from the electrode to the active layer through the first and second reflecting mirrors and the first and second cladding layers. Make an injection. The injected current is converted into light by luminescence recombination. The light is confined and resonated by the first and second reflecting mirrors in a region composed of the first and second cladding layers and the active layer, and laser oscillation occurs. At this time, the light having a wavelength that causes laser oscillation becomes a standing wave in a region composed of the first and second cladding layers and the active layer, and the antinode of the standing wave is just the position of the active layer.

  The antinode of the standing wave means the place where the time average of the light intensity distribution is the maximum. That is, the position of the antinode of the standing wave in the active layer means strong light confinement in the active layer. This strong optical confinement makes it possible to realize a laser with high efficiency and low threshold current.

  Details of this principle are described in, for example, Kenichi Iga and Fumio Koyama, “Fundamentals and Applications of Surface-Emitting Lasers” (Kyoritsu Shuppan, 1999).

  In a more realistic structure, a contact layer is provided between the second reflecting mirror and the electrode in order to make an ohmic contact, or a high resistance region or an oxidation current confinement layer by proton injection is used to lower the threshold current density. The basic principle and structure of laser oscillation are as described above.

Examples of VCSEL structures according to literature include a red surface emitting laser (see Non-Patent Document 1, for example) in which an InGaAlP multiple quantum well active layer is sandwiched between GaAlAs multilayer DBR mirrors (Bragg reflectors), and a light emitting active layer of AlGaAs. A surface-emitting laser having a structure sandwiched between a first clad layer and a second clad layer made of, and further sandwiched between a first Bragg reflector and a second Bragg reflector made of AlAs and GaAs (for example, see Patent Document 1) )
Toshiba Review Vol. 56No. 10 (2001), p. 36-38 JP-A-8-213693

  By the way, in the existing VCSEL, the same material is used for the first and second cladding layers, and the thickness is also the same. In this case, when the first and second cladding layers have the same refractive index, the antinode of the above-mentioned standing wave is located in the active layer, and laser oscillation occurs.

  However, in general, in the VCSEL, the electrical characteristics of the first and second cladding layers are formed to be in conflict. For example, if the first cladding layer is an n-type compound semiconductor, the second cladding layer is formed of a p-type compound semiconductor, and if the first cladding layer is a p-type compound semiconductor, the second cladding layer is It is formed of an n-type compound semiconductor. At this time, even if the same material is used, the refractive index is different between p-type and n-type. Is known. According to this book, the refractive index change Δn due to the change in the number of free carriers ΔN is given by the following equation.

Δn = (− ΔNe ² ) / ( ² nmε ₀ ω ² )
e: elementary charge of electrons, n: refractive index, m: effective mass, ε ₀ : dielectric constant of vacuum, ω: frequency of light The first and second cladding layers have thermally excited free carriers. Since it exists, the refractive index changes. At this time, since the electrical characteristics of the first and second cladding layers are contradictory, electrons are dominant as carriers in one cladding layer and holes are dominant in the other layer, respectively.

  In general, the effective mass of electrons and holes is greatly different. For example, in GaAs, the effective mass of electrons is 0.067 me, and the mass of holes is 0.45 me (me: electron mass). Therefore, the refractive index change Δn differs between a layer in which electrons are dominant as carriers and a layer in which holes are dominant as carriers. From the above, it is clear that the refractive index is generally different between the first and second cladding layers.

  The effective masses of electrons and holes in compound semiconductors are described in, for example, the above-mentioned Haruo Nagai, Sadao Adachi and Takashi Fukui, “III-V Group Semiconductor Mixed Crystals” (Corona, 1988) p. Details are described in FIG.

  Thus, when the physical thicknesses of the first and second cladding layers are the same and the refractive indexes are different, the antinodes of the above-mentioned standing waves are shifted from the active layer, and the optical confinement is weakened. . This not only causes an increase in threshold current, but in some cases, laser oscillation itself is impossible.

  Therefore, an object of the present invention is to solve the above-mentioned problems, and by matching the location where the time average of the light intensity distribution is maximum with the active layer, it is a surface-emitting type that has higher efficiency and lower threshold current than conventional ones. An object is to provide an epitaxial wafer for a semiconductor laser.

  In order to achieve the above object, the present invention is configured as follows.

An epitaxial wafer for a semiconductor laser according to the invention of claim 1 is formed by epitaxially growing at least a first conductivity type first cladding layer, an active layer, and a second conductivity type second cladding layer on a substrate. And the thickness of the first cladding layer is d ₁ , the thickness of the second cladding layer is d ₂ , the refractive index of the first cladding layer is n ₁ , and the refractive index of the second cladding layer. Where n ₂ is the following formula (d ₁ / n ₁ ) = (d ₂ / n ₂ )
Is established.

  An epitaxial wafer for a semiconductor laser according to a second aspect of the invention is the epitaxial wafer for a semiconductor laser according to the first aspect, wherein the first reflecting mirror is provided between the first cladding layer and the substrate. A second reflecting mirror is provided on the substrate, and a lower electrode is formed on the substrate as an electrode for current injection, and an upper electrode is formed on the second reflecting mirror.

  A semiconductor laser epitaxial wafer according to a third aspect of the invention is the semiconductor laser epitaxial wafer according to the second aspect, wherein a contact layer for making an ohmic contact is provided between the second reflecting mirror and the upper electrode. It is characterized by being.

  A semiconductor laser epitaxial wafer according to a fourth aspect of the present invention is the semiconductor laser epitaxial wafer according to any one of the first to third aspects, wherein the active layer is at least one of gallium, indium, and aluminum as a group III element. It is characterized in that it includes at least one compound semiconductor layer using various kinds of elements and using at least one kind of element among phosphorus, arsenic, nitrogen, and antimony as a group V element.

  The semiconductor laser epitaxial wafer according to claim 5 is the semiconductor laser epitaxial wafer according to any one of claims 1 to 3, wherein the active layer includes gallium and indium as group III elements, And nitrogen and arsenic at the same time.

  A semiconductor laser according to a sixth aspect of the present invention is manufactured by cutting the semiconductor wafer epitaxial wafer according to any one of the first to fifth aspects.

<Key points of the invention>
As already mentioned, when the physical thicknesses of the first and second cladding layers are the same and the refractive indexes are different, the above-mentioned standing wave antinodes are displaced from the active layer, and the optical confinement is prevented. become weak. This not only causes an increase in threshold current, but in some cases, laser oscillation itself is impossible.

  In order to explain this phenomenon, light has the property that the optical length is different even if the physical length is the same in a medium having a different refractive index. Specifically, the optical length is given by a value obtained by dividing the physical length by the refractive index. Therefore, when the physical thicknesses of the first and second cladding layers are equal and the refractive indexes are different, the optical distance from the active layer to the first reflecting mirror and the second distance from the active layer The optical distance to the reflecting mirror is different, and the active layer is not located at the center of the first and second reflecting mirrors when viewed at the optical distance. For this reason, the antinode of the above-mentioned standing wave, that is, the place where the time average of the light intensity distribution is maximum exists at a position shifted from the active layer, and the light confinement in the active layer becomes weak. .

Therefore, in the present invention, since the active layer is the place where the time average of the light intensity distribution is maximum, the optical thicknesses of the first and second cladding layers are made equal. That is, the thickness of the first cladding layer is d ₁ , the thickness of the second cladding layer is d ₂ , the refractive index of the first cladding layer is n ₁ , and the refractive index of the second cladding layer is n ₂ . Then, the following formula (d ₁ / n ₁ ) = (d ₂ / n ₂ )
The configuration is established.

In the epitaxial wafer for a semiconductor laser of the present invention, the first cladding layer has a thickness of d ₁ (unit nanometer), the second cladding layer has a thickness of d ₂ (unit nanometer), and the first cladding layer Where n _{1 is} the refractive index of the second cladding layer and n ₂ is the refractive index of the second cladding layer, the following formula (d ₁ / n ₁ ) = (d ₂ / n ₂ )
The configuration is established. Therefore, according to the epitaxial wafer for a semiconductor laser of the present invention, the antinode of the standing wave, that is, the place where the time average of the light intensity distribution is maximum is located in the active layer. It becomes possible to provide a surface emitting semiconductor laser epitaxial wafer with reduced current.

  Hereinafter, the present invention will be described based on the illustrated embodiments.

As shown in FIG. 1, a semiconductor device according to the present invention includes at least a first conductivity type first clad layer 3 and an active layer 4 and a second conductivity type second clad on a predetermined substrate 1. Layer 5 is epitaxially grown. At this time, the film thickness of the first cladding layer 3 is d ₁ (unit nanometer), the film thickness of the second cladding layer 5 is d ₂ (unit nanometer), and the refractive index of the first cladding layer 3 is n. ₁ , where n ₂ is the refractive index of the second cladding layer 5, the following formula (d ₁ / n ₁ ) = (d ₂ / n ₂ )
Is formed to hold.

  A first reflecting mirror 2 is provided between the first cladding layer 3 and the substrate 1, a second reflecting mirror 6 is provided on the second cladding layer 5, and the substrate is used as an electrode for current injection. The lower electrode 15 is formed on the second reflecting mirror 6 and the upper electrode 16 is formed on the second reflecting mirror 6. If necessary, a contact layer 7 for making an ohmic contact can be provided between the second reflecting mirror 6 and the upper electrode 16. In order to lower the threshold current density, it is possible to provide a high resistance region by proton implantation or an oxidation current confinement layer.

  The current injected from the upper and lower electrodes 15 and 16 to the active layer 4 through the first and second reflecting mirrors 2 and 6 and the first and second cladding layers 3 and 5 into the active layer 4 4 is converted into light by luminescence recombination. The light is confined and resonated by the first and second reflecting mirrors 2 and 6 in a region composed of the first and second cladding layers 3 and 5 and the active layer 4 and oscillates. At this time, if the above equation is satisfied, strong light confinement is performed in the active layer 4 in the element, and a surface emitting laser with higher efficiency and lower threshold current can be realized.

  At this time, the material constituting the active layer 4 includes at least one element of gallium, indium and aluminum as a group III element and at least one element of phosphorus, arsenic, nitrogen and antimony as a group V element. Can be used.

  Furthermore, the active layer can contain gallium and indium as group III elements and can simultaneously contain nitrogen and arsenic as group V elements, and so-called GaInNAs-based materials can be used.

  As an example of the present invention, a surface-emitting type semiconductor laser epitaxial wafer (wafer A) having the structure shown in FIG. 1 was prototyped.

  The wafer A is formed on a Si-doped n-type GaAs substrate 1 as a first conductivity type semiconductor substrate, a lower multilayer reflection film 2 as a first reflection mirror of the first conductivity type, and a first conductivity type first substrate. Si-doped n-type GaAs cladding layer 3 as the cladding layer, undoped GaInNAs active layer 4, Zn-doped p-type GaAs cladding layer 5 as the second conductivity-type second cladding layer, second-conductivity-type second reflector The upper multilayer reflective film 6 is sequentially formed. An AuGeNi lower electrode 15 is formed on the lower surface of the substrate 1, and an AuZn upper electrode 16 is formed on the upper surface of the second reflecting mirror having the second conductivity type to constitute a surface emitting semiconductor laser.

More specifically, in this embodiment, a Si-doped n-type Al _0.1 Ga _0.9 As layer having a thickness of 98 nm and a Si-doped n-type Al _0.9 Ga _0.1 As having a thickness of 107 nm are formed on an n-type GaAs substrate 1 doped with Si. Lower multilayer reflective film 2 in which 30 layers are alternately stacked, Si-doped n-type GaAs cladding layer 3 with a thickness of 184 nm, Undoped Ga _0.72 In _0.28 N _0.01 As _0.99 active layer 4 with a thickness of 10 nm, Zn-doped with a thickness of 190 nm p-type GaAs cladding layer 5, upper multilayer reflection film 6 in which a Zn-doped p-type Al _0.1 Ga _0.9 As layer having a thickness of 1101 nm and a Zn-doped p-type Al _0.9 Ga _0.1 As layer having a thickness of 110 nm are alternately laminated for 25 periods, A Zn-doped p-type GaAs contact layer 7 having a thickness of 20 nm was grown in this order by metal organic vapor phase epitaxy to obtain a wafer A.

  Here, when Si-doped n-type GaAs having a thickness of about 100 nm was epitaxially grown under the same growth conditions as the Si-doped n-type GaAs cladding layer 3, the refractive index was measured by spectroscopic ellipsometry, and the value was 3.559. .

  Similarly, when Zn-doped p-type GaAs having a thickness of about 100 nm was epitaxially grown under the same growth conditions as the Zn-doped p-type GaAs cladding layer 5 and the refractive index was measured by spectroscopic ellipsometry, the value was 3.447. It was.

From this result, it can be said that the Si-doped n-type GaAs cladding layer 3 has a thickness of 190 nm and a refractive index of 3.559, and the Zn-doped p-type GaAs cladding layer 5 has a thickness of 184 nm and a refractive index of 3.447. At this time, when the value obtained by dividing the thickness by the refractive index is calculated for each of the Si-doped n-type GaAs cladding layer 3 and the Zn-doped p-type GaAs cladding layer 5, Si-doped n-type GaAs cladding layer 3: 190 nm ÷ 3.559 = 53.4 nm,
Zn-doped p-type GaAs cladding layer 5: 184 nm ÷ 3.447 = 53.4 nm
It becomes.

  On the other hand, as a comparative example, an epitaxial wafer (wafer B) for a surface emitting semiconductor laser having a conventional structure shown in FIG. This is because a lower multilayer reflective film 9 as a first reflecting mirror of the first conductivity type, a first cladding layer of the first conductivity type, on a Si-doped n-type GaAs substrate 8 as a first conductivity type semiconductor substrate. As a Si-doped n-type GaAs cladding layer 10, an undoped GaInNAs active layer 11, a Zn-doped p-type GaAs cladding layer 12 as a second-conductivity-type second cladding layer, and a second-conductivity-type second reflector The upper multilayer reflective film 13 is sequentially formed (wafer B), the AuGeNi lower electrode 15 is formed on the lower surface of the substrate 8, and the AuZn upper electrode 16 is formed on the upper surface of the second reflecting mirror having the second conductivity type. Are formed to form a surface emitting semiconductor laser.

Specifically, similarly to wafer A, a Si-doped n-type Al _0.1 Ga _0.9 As layer having a thickness of 98 nm and a Si-doped n-type Al _0.9 Ga _0.1 layer having a thickness of 107 nm are formed on an n-type GaAs substrate 8 doped with Si. Lower multilayer reflective film 9 in which As layers are alternately stacked for 30 periods, Si-doped n-type GaAs cladding layer 10 having a thickness of 190 nm, Ga _0.72 In _0.28 N _0.01 As _0.99 active layer 11 having a thickness of 10 nm, Zn-doped having a thickness of 190 nm p-type GaAs cladding layer 12, upper multilayer reflective film 13 in which a 95-m thick Zn-doped p-type Al _0.1 Ga _0.9 As layer and a 108-nm-thick Zn-doped p-type Al _0.9 Ga _0.1 As layer are alternately stacked for 25 periods, A Zn-doped p-type GaAs contact layer 14 having a thickness of 20 nm was sequentially grown by metal organic vapor phase epitaxy to obtain a wafer B.

At this time, for each of the Si-doped n-type GaAs cladding layer 10 and the Zn-doped p-type GaAs cladding layer 11, the value obtained by dividing the thickness by the refractive index is calculated.
Si-doped n-type GaAs cladding layer 10: 190 nm ÷ 3.559 = 53.4 nm
Zn-doped p-type GaAs cladding layer 12: 190 nm ÷ 3.447 = 55.1 nm
It becomes.

  The wafer A and the wafer B thus obtained are cut into chips of 300 μm square, and the lower electrode 15 made of gold / germanium / nickel alloy (AuGeNi) and the gold / zinc alloy (AuZn) are obtained. The upper electrode 16 made of was formed. The upper electrode 16 is a ring electrode having a circular window having a diameter of 10 μm. Further, a part of the upper multilayer reflective films 6 and 13 located under the formation part of the upper electrode 16 was selectively oxidized in advance to form a current constriction. Further, gold (Au) wire 17 was bonded to the upper electrode 16 to obtain surface emitting lasers A and B having vertical resonators.

  As is apparent from the above calculation results, the surface emitting laser A is a semiconductor laser according to the present invention, and the surface emitting laser B is a conventional semiconductor laser.

  When the current-voltage characteristics of these lasers were examined, the characteristics shown in FIG. 3 were obtained. In FIG. 3, the horizontal axis represents current in units [mA], and the vertical axis represents light intensity in arbitrary units [a. u. ]. From FIG. 3, it is clear that the surface emitting laser A according to the present invention has a rising curve that is located on the left side of the figure and a lower threshold current than the conventional surface emitting laser B. Further, it can be seen that the surface-emitting laser A according to the present invention has a sharper rise in current-voltage characteristics than the conventional surface-emitting laser B. This means that the surface emitting laser A according to the present invention is more efficient than the conventional surface emitting laser B.

  As described above, according to the present invention, it is possible to provide a surface-emitting laser having a higher efficiency and a lower threshold than conventional surface-emitting lasers.

<Other embodiments and modifications>
The light emitting device according to the present invention is not necessarily used in a single chip shape, and may be a laser diode array as necessary.

The materials of the first and second cladding layers according to the present invention are not necessarily the same. The refractive index of the first cladding layer is n ₁ , and the refractive index of the second cladding layer is n ₂ . Then, the following formula (d ₁ / n ₁ ) = (d ₂ / n ₂ )
If is true, different materials can be used.

  The active layer of the light-emitting element according to the present invention is not necessarily made of a GaAs-based, AlGaAs-based, or GaInNAs-based material, and for example, a GaN-based, InGaAlP-based, or InGaAsP-based material can be used.

  The reflecting mirror in the light emitting device according to the present invention is not necessarily made of a GaAs-based or AlGaAs-based material, and may be a GaN-based or AlGaN-based material, for example.

  The light emitting device according to the present invention can be applied to a laser diode for optical communication, for example.

It is typical sectional drawing of the surface emitting semiconductor laser in connection with the Example of this invention. It is a typical sectional view of a conventional surface emitting semiconductor laser. It is a figure which shows the current-voltage characteristic of the surface emitting semiconductor laser in connection with the Example of this invention, and the conventional surface emitting semiconductor laser.

Explanation of symbols

1 Si-doped n-type GaAs substrate 2 First reflecting mirror (lower multilayer reflecting film)
3 First conductivity type first cladding layer (Si-doped n-type GaAs cladding layer)
4 Active layer (undoped GaInNAs active layer)
5 Second conductivity type second cladding layer (Zn-doped p-type GaAs cladding layer)
6 Second reflector (upper multilayer reflective film)
7 Contact layer (Zn-doped p-type GaAs contact layer)
15 Lower electrode (AuGeNi lower electrode)
16 Upper electrode (AuZn upper electrode)
17 gold wire

Claims (6)

On the substrate, at least a first conductivity type first clad layer, an active layer, and a second conductivity type second clad layer are epitaxially grown, and the thickness of the first clad layer is d _1. When the film thickness of the second cladding layer is d ₂ , the refractive index of the first cladding layer is n ₁ , and the refractive index of the second cladding layer is n ₂ , the following formula (d ₁ / n ₁ ) = (d ₂ / n ₂ )
An epitaxial wafer for a semiconductor laser, wherein: In the epitaxial wafer for semiconductor lasers of Claim 1,
A first reflecting mirror is provided between the first cladding layer and the substrate, a second reflecting mirror is provided on the second cladding layer, and a lower electrode is provided on the substrate as an electrode for current injection. An epitaxial wafer for a semiconductor laser, wherein an upper electrode is formed on the second reflecting mirror. In the epitaxial wafer for semiconductor lasers of Claim 2,
An epitaxial wafer for a semiconductor laser, wherein a contact layer for taking ohmic contact is provided between the second reflecting mirror and the upper electrode. In the epitaxial wafer for semiconductor lasers in any one of Claims 1-3,
At least one layer in which the active layer uses at least one element of gallium, indium and aluminum as a group III element and at least one element of phosphorus, arsenic, nitrogen and antimony as a group V element An epitaxial wafer for a semiconductor laser comprising: a compound semiconductor layer. In the epitaxial wafer for semiconductor lasers in any one of Claims 1-3,
An epitaxial wafer for a semiconductor laser, wherein the active layer contains gallium and indium as group III elements and simultaneously contains nitrogen and arsenic as group V elements.   A semiconductor laser produced by cutting the semiconductor wafer epitaxial wafer according to claim 1.

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