JP2006253212A - Semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor laser which can reduce an absorption loss without reducing an internal quantum efficiency. <P>SOLUTION: The semiconductor laser includes a p-type cladding layer, an n-type cladding layer, an activity layer provided between the p-type cladding layer and the n-type cladding layer, a first optical confinement layer provided between the activity layer and the p-type cladding layer and consisting of an i-type semiconductor, a second optical confinement layer provided between the activity layer and the n-type cladding layer and consisting of an i-type semiconductor, and an inhibiting spread inhibition layer provided between the first optical confinement layer and the p-type cladding layer to inhibit the dopant from being spread from the p-type cladding layer to the activity layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a semiconductor laser.

  Conventionally, a semiconductor laser in which a p-type cladding layer is doped with Zn is known. Since p-type impurities such as Zn are easily diffused by heat, this Zn diffuses to the active layer during crystal growth. When Zn diffuses to the active layer, absorption loss due to valence band absorption, free carrier absorption and the like increases. Therefore, in the semiconductor laser described above, degradation of laser characteristics such as an increase in threshold current and a decrease in efficiency has occurred.

In order to prevent such diffusion of Zn into the active layer, the semiconductor laser disclosed in Patent Document 1 uses a separate confinement heterostructure (SCH), and any of the optical confinement layers provided on both sides of the active layer. Is also doped and becomes n-type. In the semiconductor laser disclosed in Patent Document 1, since the optical confinement layer provided between the p-type cladding layer doped with Zn and the active layer is n-type, diffusion of Zn into the active layer is suppressed. Therefore, in the semiconductor laser of Patent Document 1, deterioration in laser characteristics such as an increase in threshold current due to Zn diffusion and a decrease in efficiency is reduced.
JP-A-6-244492

  However, if the entire optical confinement layer provided between the active layer and the p-type cladding layer is doped n-type like the semiconductor laser of Patent Document 1, the pn junction is not the position of the active layer. Thus, since it is formed between the optical confinement layer and the p-type cladding layer, efficient carrier injection is suppressed into the active layer located spatially away from the pn junction. Specifically, since the holes injected from the p-type electrode side pass through the pn junction and reach the active layer while recombining with the electrons injected from the n-type electrode side, the hole concentration is In the active layer that is maximum near the pn junction and located away from the pn junction, the hole concentration decreases.

  As a result, the overlap between the electron concentration distribution and the hole concentration distribution in the active layer becomes unbalanced. In general, emission recombination of carriers for obtaining laser light is likely to occur where the electron concentration distribution and the hole concentration distribution overlap. Therefore, in the semiconductor laser of Patent Document 1, the light emission recombination of carriers is reduced. Therefore, in the semiconductor laser of Patent Document 1, the internal quantum efficiency is reduced due to the suppression of the injection efficiency of carriers, particularly holes, into the active layer.

  As described above, in the semiconductor laser disclosed in Patent Document 1, laser characteristics such as a decrease in light emission efficiency are deteriorated.

  Accordingly, an object of the present invention is to provide a semiconductor laser capable of reducing absorption loss without reducing internal quantum efficiency.

  The semiconductor laser of the present invention includes a p-type cladding layer, an n-type cladding layer, an active layer provided between the p-type cladding layer and the n-type cladding layer, and between the active layer and the p-type cladding layer. A first optical confinement layer made of i-type semiconductor; a second optical confinement layer made of i-type semiconductor; and a first optical confinement layer and a p-type clad. And a diffusion suppression layer that is provided between the layers and suppresses diffusion of impurities of the p-type cladding layer into the active layer.

  According to the present invention, since the diffusion suppression layer provided between the p-type cladding layer and the first optical confinement layer includes the n-type impurity, the diffusion of impurities from the p-type cladding layer reaches the active layer. Absent. The first optical confinement layer between the diffusion suppression layer and the active layer is made of an i-type semiconductor. Here, an i-type semiconductor (or an i-type semiconductor layer) generally refers to a semiconductor (or a semiconductor layer) having a lower electrically active carrier density than an n-type semiconductor or a p-type semiconductor.

  Therefore, the n-type cladding layer, the second optical confinement layer (i layer), the active layer (i layer), the first optical confinement layer (i layer), the diffusion suppression layer (i layer or p type), and the p type cladding When a p-i-n structure is formed with the layer and current is passed through the semiconductor laser, both electron and hole carriers are efficiently injected from the n-side and p-side electrodes into the active layer that is the i layer. Is done. As a result, the electron concentration distribution and the hole concentration distribution in the active layer overlap, and carrier recombination tends to occur in the active layer.

  As described above, the semiconductor laser of the present invention has a pin structure in which the increase of the light absorption coefficient is suppressed by suppressing the diffusion of impurities from the p-type cladding layer and at the same time the active layer is maintained as the i layer. Therefore, efficient emission recombination in the active layer can be obtained, and a semiconductor laser having high emission efficiency can be obtained.

  In addition, since the first optical confinement layer that is the light propagation path is made of an i-type semiconductor, free carrier absorption in the first optical confinement layer is reduced. Therefore, in the semiconductor laser of the present invention, effects such as a reduction in threshold current and an increase in light emission efficiency can be expected.

  The diffusion suppressing layer of the semiconductor laser of the present invention is made of the same material as that of the p-type cladding layer and is preferably made of an i-type semiconductor.

  Further, the diffusion suppression layer of the semiconductor laser of the present invention is the same material as that of the first optical confinement layer, and may be made of an i-type semiconductor.

  The diffusion suppression layer of the semiconductor laser of the present invention has a thickness of 5 nm to 30 nm, the diffusion suppression layer includes an n-type impurity, and the n-type impurity is added to the p-type cladding layer. It is preferable to add at a concentration lower than the impurity concentration.

  Further, the impurity concentration distribution of the p-type cladding layer of the semiconductor laser of the present invention may be inclined in the direction from the active layer to the p-type cladding layer.

  According to the present invention, a semiconductor laser having a high internal quantum efficiency and a low threshold current is provided.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals.

(First embodiment)
FIG. 1 is a sectional view showing the structure of a semiconductor laser according to the first embodiment of the present invention. As shown in FIG. 1, the semiconductor laser 10 has a plurality of semiconductor layers stacked on an n-type InP substrate 12 as described below. An n-type InP cladding layer 14 is provided on the n-type InP substrate 12, an InGaAsP second optical confinement layer 16 is provided on the n-type cladding layer 14, and an i-type InGaAsP active layer is provided on the second optical confinement layer 16. The i-type InGaAsP first optical confinement layer 20 is provided on the active layer 18, the InP diffusion suppression layer 22 is provided on the i-type first optical confinement layer 20, and the diffusion suppression layer 22 is provided. Is provided with a p-type InP first clad layer 23, a p-type InP second clad layer 24 is provided on the p-type InP first clad layer 23, and a contact layer 27 on the p-type second clad layer 24. Is provided. The second optical confinement layer 16 can be made of, for example, an i-type semiconductor.

  The second optical confinement layer 16, the active layer 18, and the i-type first optical confinement layer 20 constitute a separate confinement heterostructure (SCH). Since the active layer 18 is located between the second optical confinement 16 and the i-type first optical confinement layer 20, the second optical confinement layer 16 and the i-type first optical confinement layer 20 have carriers ( Acts to confine electrons and holes).

  The second optical confinement layer 16, the active layer 18, and the first optical confinement layer 20 are located between the n-type cladding layer 14 and the p-type cladding layers 23 and 24. Since the n-type cladding layer 14 and the p-type cladding layers 23 and 24 are made of a material having a refractive index smaller than that of the material of the second optical confinement layer 16, the active layer 18, and the first optical confinement layer 20, these layers 16 , 18 and 20 to confine light.

  The diffusion suppression layer 22 is located between the i-type first optical confinement layer 20 and the p-type first cladding layer 23. Since the diffusion suppression layer 22 is made of the same material as that of the p-type cladding layer 23, it acts to confine light in the second light confinement layer 16, the active layer 18, and the first light confinement layer 20. That is, in this embodiment, light is confined between the n-type cladding layer 14 and the diffusion suppression layer 22 and the p-type cladding layer 23.

  The active layer 18 has, for example, a multiple quantum well (MQW) structure. Electrons and holes are injected into the active layer 18 from the n-type cladding layer 14 and the p-type cladding layers 23 and 24 via the second optical confinement layer 16 and the i-type first optical confinement layer 20. In the active layer 18, these electrons and holes are recombined to generate light.

  In addition, the i-type second optical confinement layer 16, the active layer 18, the i-type first optical confinement layer 20, the diffusion suppression layer 22, and the p-type first cladding layer 23 are Z orthogonal to the XY plane shown in FIG. It forms a mesa extending in the direction. This mesa is embedded in the current blocking layer 26. The current blocking layer 26 blocks current flowing from the p-type cladding layer 23 to the n-type cladding layer 14. Accordingly, electrons and holes injected from the n-type cladding layer 14 and the p-type cladding layer 23 are efficiently guided to the active layer 18.

If an example of the semiconductor layer of such a semiconductor laser 10 is shown, the diffusion suppression layer 22 will consist of an i-type semiconductor. The thickness of the diffusion suppression layer 22 is 5 nm or more and 30 nm or less. This layer 22 is doped with n-type impurity Si. The n-type impurity concentration of the n-type substrate 12 is about 2 × 10 ¹⁸ cm ⁻³ . The thickness of the n-type cladding layer 14 is about 500 nm, the n-type impurity of this layer 14 is Si, and its concentration is about 1 × 10 ¹⁸ cm ⁻³ . The thicknesses of the second optical confinement layer 16 and the i-type first optical confinement layer 20 are each about 50 nm, and these layers 16 and 20 have an impurity concentration of 1 × 10 ¹⁷ cm ⁻³ or less, or are undoped semiconductors. is there. The active layer 18 has an impurity concentration of 1 × 10 ¹⁷ cm ⁻³ or less, or is an undoped semiconductor. The p-type impurity of the p-type first cladding layer 23 and the p-type second cladding layer 24 is Zn, and the concentration thereof is 1 × 10 ¹⁷ cm ⁻³ or more and 2 × 10 ¹⁸ cm ⁻³ or less. The total thickness of these layers 23 and 24 is about 500 nm.

Note that the concentration of the impurity Si doped in the diffusion suppression layer 22 is doped below the p-type impurity concentration of the p-type cladding layer, and is compensated for impurities diffused from the p-type cladding layer. The conductivity type is p-type, and the electrically active carrier density is adjusted to be 5 × 10 ¹⁷ cm ⁻³ or less.

  According to the semiconductor laser 10 of the present embodiment, since the diffusion suppression layer 22 provided between the p-type cladding layer 23 and the first optical confinement layer 20 contains the n-type impurity Si, the p-type cladding layer The diffusion of the impurity Zn from 23 and 24 does not reach the active layer 18. The first optical confinement layer 20 between the diffusion suppression layer 22 and the active layer 18 is made of an i-type semiconductor. Therefore, the electron concentration distribution and the hole concentration distribution in the active layer 18 overlap, and carrier recombination easily occurs in the active layer 18. Therefore, in the semiconductor laser 10, the absorption loss due to the diffusion of the impurity Zn from the p-type cladding layer is reduced without causing a decrease in internal quantum efficiency due to the difference in the spatial distribution of electrons and holes.

  Further, if the second optical confinement layer 16 is made of an i-type semiconductor in addition to the first optical confinement layer 20 that is a light propagation path, free carrier absorption in the first optical confinement layer 20 and the second optical confinement layer 16 can be prevented. Reduced. Therefore, the semiconductor laser 10 has effects such as an increase in threshold current being reduced and an increase in light emission efficiency.

  Further, since the diffusion suppression layer 22 is made of an InP semiconductor, the p-type impurity Zn does not diffuse into the first optical confinement layer 20. For this reason, there is an advantage that light absorption in the light confinement layer where a large amount of light is distributed can be reduced.

  In addition, if the thickness of the diffusion suppression layer 22 is 5 nm or more, the diffusion of the p-type impurity Zn can be suppressed as the thickness is 5 nm or more.

  If the thickness of the diffusion suppression layer 22 is 30 nm or less, the series resistance of the laser is not increased so much and the influence of carrier overflow is small.

  A method for manufacturing a semiconductor laser according to the first embodiment of the present invention will be described below. 2 and 3 are cross-sectional views showing the manufacturing process of the semiconductor laser 10.

(First semiconductor layer deposition step)
First, a stacked structure of semiconductor layers is formed on the n-type InP substrate 52. As shown in FIG. 2A, on an n-type InP substrate 52, an n-type InP cladding layer 54, an i-type InGaAsP second optical confinement layer 56, an i-type InGaAsP active layer 58, and an i-type InGaAsP first light. A confinement layer 60, an n-type InP diffusion suppression layer 62, and a p-type InP first cladding layer 63 are sequentially formed. These semiconductor layers are grown using, for example, an organometallic vapor phase epitaxy (OMVPE).

(Mask formation process)
Next, an etching mask layer is formed on the i-type first optical confinement layer. As shown in FIG. 2A, the etching mask layer 65 is formed by depositing a mask layer on the p-type first cladding layer 63 and etching the mask layer into a predetermined shape by a photolithography technique. The mask layer 65 is formed in a strip shape extending in the Z direction orthogonal to the XY plane. For example, silicon dioxide (SiO ₂ ) or silicon nitride (SiN) is used as the material of the mask layer.

(Mesa formation process)
Next, as shown in FIG. 2B, the i-type second optical confinement layer 56, the active layer 58, the i-type first optical confinement layer 60, the n-type diffusion suppression layer 62, and the p-type first cladding layer 63. Is etched into a mesa composed of the i-type second optical confinement layer 56a, the active layer 58a, the i-type first optical confinement layer 60a, the n-type diffusion suppression layer 62a, and the p-type first cladding layer 63a. Form.

(Current block layer formation process)
Next, an InP current blocking layer 66 is formed on the n-type cladding layer 54 without removing the mask material, and the i-type second optical confinement layer 56a, the active layer 58a, and the i-type first optical confinement layer 60a, n A mesa composed of the type diffusion suppression layer 62a and the p-type first cladding layer 63a is embedded. As shown in FIG. 3A, the current blocking layer 66 is not deposited on the mask layer 65 but selectively grows only on the n-type cladding layer 54. Accordingly, the mesa left by etching is buried and flattened.

(Second semiconductor layer deposition step)
Next, after removing the mask material, a stacked structure of semiconductor layers is formed on the p-type first cladding layer 63a. As shown in FIG. 3B, a p-type InP second cladding layer 64 and a contact layer 67 are sequentially formed on the p-type first cladding layer 63a and the current blocking layer 66. These semiconductor layers are grown using, for example, OMVPE.

For example, the n-type impurity of the n-type diffusion suppressing layer 62a is Si, and the concentration thereof is preferably equal to or lower than the p-type impurity concentration of the p-type first cladding layer 63 and the p-type second cladding layer 64. . The p-type impurity of the p-type first cladding layer 63a and the p-type second cladding layer 64 is Zn, and the concentration of each is preferably 1 × 10 ¹⁷ cm ⁻³ or more. Moreover, it is preferable that each density | concentration is 2 * 10 < ¹⁸ > cm < ^-3 > or less.

  Since the environmental temperature when growing these semiconductor layers 63a and 64 using OMVPE is high, Zn doped into the p-type cladding layers 63a and 64 during the crystal growth of the p-type cladding layers 63a and 64 is Diffusion into the n-type diffusion suppression layer 62a. Therefore, the n-type diffusion suppression layer 62a includes this Zn and Si doped in itself. Preferably, the n-type diffusion suppression layer 62a is an i-type semiconductor.

  Thereafter, a step of forming an electrode is performed. In this way, the semiconductor laser 10 shown in FIG. 1 is manufactured.

(Second Embodiment)
Next, a semiconductor laser according to a second embodiment of the present invention will be described. FIG. 4 is a cross-sectional view showing the structure of a semiconductor laser according to the second embodiment of the present invention. A semiconductor laser 10a shown in FIG. 4 is different from the first embodiment in that the InP diffusion suppression layer 22 shown in FIG. 1 is an InGaAsP diffusion suppression layer 22a.

  The diffusion suppression layer 22 a is located between the i-type first optical confinement layer 20 and the p-type first cladding layer 23. The diffusion suppression layer 22 a is made of the same material as that of the second light confinement layer 16, the active layer 18, and the first light confinement layer 20. Therefore, in the semiconductor laser 10a, the n-type cladding layer 14 and the p-type cladding layer 23 form the i-type second optical confinement layer 16, the active layer 18, the i-type first optical confinement layer 20, and the diffusion suppression layer 22a. Light is trapped.

The diffusion suppression layer 22a is made of an n-type InGaAsP semiconductor at the initial stage of the second semiconductor layer deposition step. For example, the thickness of the n-type semiconductor layer is 5 nm or more and 30 nm or less, the n-type impurity of the semiconductor layer is Si, and the concentration thereof is 1 × 10 ¹⁷ cm ⁻³ or more and 2 × 10 ¹⁸ cm ^−3. It is as follows. This n-type semiconductor layer contains Zn diffused from the p-type cladding layers 23 and 24 during the crystal growth of the p-type cladding layers 23 and 24, and becomes a diffusion suppression layer 22a. The diffusion suppression layer 22a includes this diffused Zn and Si doped therein. Diffusion suppression layer 22a preferably comprises an i semiconductor. For example, the thickness of the diffusion suppression layer 22a is 5 nm or more and 30 nm or less, and the carrier density of this layer is 5 × 10 ¹⁷ cm ⁻³ or less.

  According to the semiconductor laser 10a of the present embodiment, since the diffusion suppression layer 22a provided between the p-type cladding layer 23 and the first optical confinement layer 20 contains the n-type impurity Si, the p-type cladding layer The diffusion of the impurity Zn from 23 and 24 does not reach the active layer 18. The first optical confinement layer 20 between the diffusion suppression layer 22a and the active layer 18 is made of an i-type semiconductor. Therefore, the electron concentration distribution and the hole concentration distribution in the active layer 18 overlap, and carrier recombination easily occurs in the active layer 18. Therefore, in the semiconductor laser 10a, the absorption loss due to the diffusion of the impurity Zn from the p-type cladding layer is reduced without causing a decrease in internal quantum efficiency due to the difference in the spatial distribution of electrons and holes.

  If the second optical confinement layer 16 and the diffusion suppression layer 22a are also made of an i-type semiconductor in addition to the first optical confinement layer 20 that is a light propagation path, the first optical confinement layer 20 and the second optical confinement layer 16 are provided. And free carrier absorption in the diffusion suppression layer 22a is reduced. Therefore, in the semiconductor laser 10a, an increase in threshold current is reduced.

  The present invention is not limited to the above-described embodiment, and various modifications can be made.

  In the semiconductor laser according to the present embodiment, the impurity concentration distribution of the p-type cladding layer may be inclined in the direction from the active layer to the p-type cladding layer.

  When the impurity concentration distribution of the p-type cladding layer increases as it goes from the active layer to the p-type cladding layer, the doping concentration on the active layer side where more light is distributed in the p-type cladding layer is thin, so p Absorption on the active layer side of the mold cladding layer can be reduced. Therefore, the entire loss can be equivalently reduced without greatly increasing the resistance of the semiconductor laser.

  On the other hand, when the impurity concentration distribution in the p-type cladding layer decreases from the active layer toward the p-type cladding layer, the p-type impurity concentration at the interface with the optical confinement layer in the p-type cladding layer is high. Compared with the case where this concentration is low, carrier overflow of electrons to the p-type cladding layer can be suppressed.

In addition, the semiconductor laser according to the present embodiment may have a configuration in which a p ⁺ -type InP carrier stop layer 28 is provided between the diffusion suppression layer 22 or 22a and the p-type cladding layer 23 (FIG. 5A). . For example, the thickness of the carrier stop layer 28 is about 20 nm, and the impurity Zn concentration of the carrier stop layer 28 is 2 × 10 ¹⁸ cm ⁻³ or more.

  According to this configuration, an electron carrier overflow can be suppressed as in a normal carrier stop layer.

  The semiconductor laser according to the present embodiment may be a distributed period type semiconductor laser (FIG. 5B). In this semiconductor laser, an InGaAsP diffraction grating formation layer 30 is provided between the diffusion suppression layer 22 or 22a and the first optical confinement layer 20, and a boundary surface between the diffraction grating formation layer 30 and the diffusion suppression layer 22 or 22a. A diffraction grating 32 is formed. For example, the thickness of the diffraction grating forming layer 30 is about 20 nm.

  In the semiconductor laser according to the present embodiment, the light confinement layers 16 and 20 and the active layer 18 may be made of an AlGaInAs semiconductor. The layers 16, 18, and 20 may be made of an AlGaAs semiconductor, and the substrate 12, the cladding layers 14, 23, and 24, and the diffusion suppression layer 22 may be made of a GaAs semiconductor. The semiconductor laser according to this embodiment has a structure in which a semiconductor layer is stacked on an n-type substrate, but may have a structure in which a semiconductor layer is stacked on a p-type substrate.

  In the semiconductor laser according to the present embodiment, Si is exemplified as the n-type impurity, but the present invention is not limited to this. Moreover, in the semiconductor laser according to the present embodiment, the thickness of the semiconductor layer is exemplified, but the value is not limited to the above-described value.

1 is a cross-sectional view showing a structure of a semiconductor laser according to a first embodiment of the present invention. FIG. 2A and FIG. 2B are cross-sectional views showing the manufacturing process of the semiconductor laser according to the embodiment of the present invention. FIG. 3A and FIG. 3B are cross-sectional views showing the manufacturing steps of the semiconductor laser according to the embodiment of the present invention. It is sectional drawing which shows the structure of the semiconductor laser which concerns on the 2nd Embodiment of this invention. FIGS. 5A and 5B are cross-sectional views showing the structure of a variation of the semiconductor laser according to the embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Semiconductor laser, 12 ... Substrate, 14 ... N-type cladding layer, 16 ... Second optical confinement layer, 18 ... Active layer, 20 ... First optical confinement layer, 22 ... Diffusion suppression layer, 23, 24 ... P-type cladding Layer 26, current blocking layer, contact layer 27.

Claims (5)

a p-type cladding layer;
an n-type cladding layer;
An active layer provided between the p-type cladding layer and the n-type cladding layer;
A first optical confinement layer formed between the active layer and the p-type cladding layer and made of an i-type semiconductor;
A second optical confinement layer provided between the active layer and the n-type cladding layer and made of an i-type semiconductor;
A diffusion suppression layer provided between the first optical confinement layer and the p-type cladding layer and suppressing diffusion of impurities from the p-type cladding layer into the active layer;
Comprising
Semiconductor laser. The diffusion suppression layer is made of the same material as that of the p-type cladding layer and is made of an i-type semiconductor.
The semiconductor laser according to claim 1. The diffusion suppression layer is made of the same material as that of the first optical confinement layer and is made of an i-type semiconductor.
The semiconductor laser according to claim 1. The thickness of the diffusion suppressing layer is 5 nm or more and 30 nm or less,
The diffusion suppression layer includes an n-type impurity, and the n-type impurity is added at a concentration lower than the concentration of the p-type impurity added to the p-type cladding layer.
The semiconductor laser according to claim 1. The impurity concentration distribution of the p-type cladding layer is inclined in a direction from the active layer toward the p-type cladding layer.
The semiconductor laser according to claim 1.

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