JP2005327907A - Semiconductor laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly reliable semiconductor laser element having good temperature characteristics and oscillating in 650 nm wavelength band. <P>SOLUTION: In the semiconductor laser element where an active layer and a clad layer are composed of an AlGaInP based material, a clad layer arranged on the p side has such a structure as a nondoped layer 51, a p-type first doped layer 52, a tensile strain layer 531 for suppressing diffusion of impurities into the active layer, and a p-type second doped layer 54 are formed in layers sequentially from the active layer side. Impurity concentration of the p-type first doped layer 52 is set lower than that of the p-type second doped layer 54. Preferably, impurity concentration of the p-type first doped layer 52 is set in the range of 5×10<SP>16</SP>cm<SP>-3</SP>to 3×10<SP>17</SP>cm<SP>-3</SP>. Element resistance is decreased by making the nondoped layer thin and threshold current is prevented from increasing by suppressing heat generation. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a semiconductor laser element suitable for use in optical communication.

  The AlGaInP-based semiconductor laser is a semiconductor laser usually used as a light source for optical recording on a DVD (Digital Versatile Disc) or the like. As a problem related to the reliability of this AlGaInP-based semiconductor laser, zinc introduced as a p-type impurity diffuses during crystal growth or energization to reach the vicinity of the active layer, thereby reducing the light emission efficiency of the device or crystal defects. The problem of causing is known. Since zinc is particularly easily diffused with AlGaInP-based materials, the problem of zinc diffusion is particularly serious with AlGaInP-based semiconductor lasers.

  As a solution to the above problem, a method of providing a lattice mismatch layer (strain layer) between the active layer and the impurity introduction layer has been proposed. This method utilizes the phenomenon that zinc does not easily move from the unstrained layer to the compressive strained layer and from the tensile strained layer to the unstrained layer.

For example, Patent Document 1 discloses a structure in which an undoped guide layer, an undoped cladding layer, a lattice mismatch layer, an undoped cladding layer, and a p-type cladding layer are sequentially stacked on the active layer. In this structure, since the cladding layer, guide layer, and active layer below the lattice mismatch layer are kept in a non-doped state, it is possible to prevent zinc from reaching the active layer. From the viewpoint of preventing zinc diffusion, it is considered that the thickness of the non-doped layer below the p-type cladding layer should be as thick as possible.
Japanese Patent Laid-Open No. 10-209573

  In recent years, low-cost and easy-to-mount plastic fibers have attracted attention in the field of optical communications. However, the plastic fiber has a problem that the internal loss is larger than that of the silica-based fiber.

  In order to secure a transmission distance similar to that of a silica-based fiber in optical communication using a plastic fiber, it is necessary to suppress the internal loss in consideration of the wavelength dependence of the transmitted light intensity of the fiber. For this reason, it has been studied to use the AlGaInP semiconductor laser having the 650 nm wavelength band in place of the semiconductor laser having the wavelength band of 1.3 μm or more which has been conventionally used for optical communication.

  However, since the AlGaInP semiconductor laser was originally developed for indoor use, the existing products are reliable enough to operate stably as a communication light source in a poor environment (especially in a high temperature environment). Not equipped. For this reason, an improvement for obtaining higher reliability is required.

  However, in the structure disclosed in Patent Document 1, when the lattice mismatching layer and the non-doped layer thereunder are made thicker in order to enhance the effect of preventing the diffusion of zinc, the element resistance increases. An increase in element resistance causes an increase in the temperature of the element and deteriorates temperature characteristics. In addition, the threshold current also increases, and the lifetime of the device is shortened.

  In view of the above circumstances, an object of the present invention is to provide a semiconductor laser device having a wavelength range of 650 nm that can be used as a light source for optical communication and has a long temperature and a long lifetime.

  The first semiconductor laser device of the present invention is a semiconductor laser device in which the active layer and the cladding layer are made of an AlGaInP-based material, and the active layer is formed as a layer constituting the cladding layer disposed on one side of the active layer. In order from the closer side, a non-doped layer, a p-type first doped layer, a strained layer, and a p-type second doped layer are provided, and the impurity concentration of the p-type first doped layer is lower than the impurity concentration of the p-type second doped layer. It is characterized by a concentration.

  The second semiconductor laser device of the present invention is a semiconductor laser device in which the active layer and the cladding layer are made of an AlGaInP-based material, and is active as a layer constituting the cladding layer disposed on one side of the active layer. A p-type first doped layer, a strained layer, and a p-type second doped layer are provided in this order from the side closer to the layer, and the impurity concentration of the p-type first doped layer is lower than the impurity concentration of the p-type second doped layer. It is characterized by being.

In the first and second semiconductor laser elements, the AlGaInP-based material is a material represented by (Al _x Ga _1-x ) _y In _1-y P (0 ≦ x <1, 0 <y <1). It is. That is, each semiconductor laser device of the present invention has a structure in which AlGaInP layers having different compositions (values of x and y) are laminated on a GaAs substrate. The AlGaInP-based material includes GaInP (when x = 0).

In addition, the impurity concentration of the p-type doped layer is preferably 5 × 10 ¹⁶ cm ⁻³ or more and 3 × 10 ¹⁷ cm ⁻³ or less for the p-type first doped layer. The p-type second doped layer may be set freely so that the concentration profile of the layer becomes a profile necessary for obtaining desired characteristics of the semiconductor laser element.

  The strain layer is a compression strain layer or a tensile strain layer. Or it is good also as a multilayer structure which consists of at least 2 types of layers among a tensile strain layer, a compression strain layer, or a non-strain layer. Examples of a multilayer structure composed of two or more layers include a multilayer structure in which tensile strain layers and unstrained layers are alternately stacked, a multilayer structure in which compression strain layers and unstrained layers are alternately stacked, and tensile strain layers and compressive strains. A multilayer structure in which layers are alternately stacked can be considered as a multilayer structure in which three types of layers are alternately stacked.

The strained layer preferably has a tensile strained layer or a multilayer structure including a tensile strained layer. The strained layer may be a non-doped layer, but preferably a p-type impurity is introduced so that the impurity concentration is 5 × 10 ¹⁶ cm ⁻³ or more and 3 × 10 ¹⁷ cm ⁻³ or less. Good.

  The first and second semiconductor laser elements of the present invention have a structure in which a strained layer is disposed between the p-type second doped layer and the active layer, so that impurities introduced into the p-type second doped layer are in the vicinity of the active layer. Is prevented from spreading to In addition, a layer into which a low-concentration impurity is introduced is provided between the strained layer and the active layer to reduce the element resistance. As a result, it is possible to prevent deterioration of the active layer due to the diffusion of zinc, suppress heat generation due to element resistance, and obtain good temperature characteristics.

  If impurities are also introduced into the strained layer, the total thickness of the non-doped layer is further reduced, so that better temperature characteristics can be obtained.

  Furthermore, if the strained layer has a tensile strained layer or a multilayered structure including a tensile strained layer, the band gap between the strained layer and the active layer increases, so that the overflow of carriers that occurs during energization is suppressed, and even better characteristics are achieved. can get.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic diagram showing the structure of a semiconductor laser device according to the first embodiment of the present invention. As shown in the figure, this semiconductor laser device is composed of an n-type GaAs substrate 1 whose main surface is a plane inclined by 5 to 15 degrees in the (111) A plane direction with respect to a normal crystal orientation plane ((100) plane). An element having the following structure formed thereon.

On the n-type GaAs substrate 1, an n-type GaAs buffer layer 2, an n-type (Al _0.7 Ga _0.3 ) _0.51 In _0.49 P lower cladding layer 3, a light emitting layer 4, an AlGaInP upper first A clad layer 5 and a p-type Ga _0.51 In _0.49 P etching stop layer 6 are laminated.

On the etching stop layer 6, a mesa stripe-shaped ridge structure 14 parallel to the resonator direction and a current blocking layer 10 formed so as to sandwich the ridge structure 14 are disposed. As shown in the drawing, the ridge structure portion 14 includes a p-type (Al _0.7 Ga _0.3 ) _0.51 In _0.49 P upper second cladding layer 7 and a p-type Ga _0.51 In _0.49. The P hetero buffer layer 8 and the p-type GaAs cap layer 9 are stacked.

  A p-type GaAs contact layer 11 formed so as to cover the ridge structure portion 14 and the current blocking layer 10 is disposed. An n-side electrode 13 is provided on the back surface of the n-type GaAs substrate 1, and a p-side electrode 12 is provided on the p-type GaAs contact layer 11.

Among the layers shown in FIG. 1, the light emitting layer 4 includes an (Al _0.5 Ga _0.5 ) _0.51 In _0.49 P first optical waveguide layer, an active layer, and (Al _0.5 Ga _0.5 ). _0.51 In _0.49 P is a layer in which a second optical waveguide layer is laminated in order. Among these, the active layer has a multiple quantum well (MQW) structure in which a plurality of quantum well layers and barrier layers made of ultrathin films of 100 mm or less are alternately stacked.

  The AlGaInP upper first cladding layer 5 has a layer structure described below. FIG. 2 is a diagram showing the layer structure (left of the figure) and the energy level (right of the figure) of the AlGaInP upper first cladding layer 5.

As shown in the left of the figure, the AlGaInP upper first cladding layer 5 is specifically composed of a non-doped (Al _0.7 Ga _0.3 ) _0.51 In _0.49 P layer 51, p doped with zinc as an impurity. It comprises a first type doped layer 52, an (Al _0.7 Ga _0.3 ) _0.6 In _0.4 P tensile strained layer 531, and a p-type second doped layer 54 into which zinc is introduced as an impurity. The zinc concentration of the p-type first doped layer 52 is 5 × 10 ¹⁶ cm ^{−3 to} 3 × 10 ¹⁷ cm ⁻³ , and the zinc concentration of the p-type second doped layer is 3 × 10 ¹⁷ cm ⁻³ to 2 ×. It is in the range of 10 ¹⁸ cm ⁻³ . That is, the p-type first doped layer 52 is doped so as to have a lower concentration than the p-type second doped layer. Further, the AlGaInP tensile strained layer 531 is also a layer into which zinc is introduced, and its concentration is in the range of 5 × 10 ¹⁶ cm ^{−3 to} 3 × 10 ¹⁷ cm ⁻³ , similar to the p-type first doped layer 52. .

  Also, as shown on the right side of the figure, the AlGaInP tensile strained layer 531 has a higher energy level than the layer that is lattice-matched with the substrate, so in this structure, the band gap between the active layer and the cladding layer is the tensile strained layer. Will be larger than the structure without.

  As described with reference to FIGS. 1 and 2, the feature of this semiconductor laser device is that a strained layer is provided in the cladding layer on the p side of the light emitting layer, and zinc is low between the strained layer and the light emitting layer. The point is that a layer doped in concentration is arranged. Another feature is that the strained layer itself is doped at a lower concentration than the layer above it.

The graph of FIG. 3 shows the influence of the layer thickness on the element resistance of each of the non-doped layer and the low-concentration (low-doped) layer. As the graph shows, the device resistance increases by about 0.1Ω every time the thickness of the non-doped layer included in the device increases by 100 nm. On the other hand, if the non-doped layer is replaced with a low-doped layer having a zinc concentration of 5 × 10 ¹⁶ cm ^{−3 to} 3 × 10 ¹⁷ cm ⁻³ , as shown in the graph, the device resistance increases when the layer thickness increases by 100 nm. Can be suppressed to about 0.05Ω. From this relationship, it is clear that the device resistance can be reduced in the above structure as compared with the structure disclosed in Patent Document 1.

  The graph of FIG. 4 represents a change in temperature characteristics when the total thickness of the non-doped AlGaInP layer 51 and the p-type first doped layer 52 is 200 nm and the ratio is changed. The horizontal axis of the graph represents the thickness of each of the non-doped AlGaInP layer 51 and the p-type first doped layer 52. In order to indicate that the total of the two layers is 200 nm, two types of scales are attached to the horizontal axis. The vertical axis of the graph represents the characteristic temperature T0.

  As shown in this graph, in the structure in which only the non-doped layer 51 is below the AlGaInP tensile strained layer 531, the characteristic temperature of the semiconductor laser element is lower than 60K. On the other hand, when the thickness of the non-doped layer is 100 nm and the thickness of the p-type first doped layer 52 is 100 nm, that is, when the ratio of the two types of layers is 1: 1, the characteristic temperature is about 100K.

As described above, in the above structure, the zinc introduced into the p-type second doped layer may move to the tensile strained layer 531, but the p-type first doped layer is unstrained from the tensile strained layer 531. Since the movement to 52 is limited, the possibility that the diffused zinc reaches the vicinity of the light emitting layer 4 is reduced. Although the p-type first doped layer 52 is on the active layer side of the tensile strain layer 531, the zinc concentration is as low as 5 × 10 ¹⁶ cm ^{−3 to} 3 × 10 ¹⁷ cm ^−3. There is no adverse effect.

  Also, as shown in the graph of FIG. 4, the temperature characteristics are better than those of the conventional structure in which all layers from the strained layer to the active layer are non-doped, and can operate stably even in a high temperature environment.

  In general, AlGaInP-based semiconductor lasers have a smaller band gap between the active layer and the cladding layer than other material-based lasers. Therefore, it is said that carrier overflow is likely to occur when operated in a high-temperature environment. However, in the semiconductor laser device of the present embodiment, since the band gap between the active layer and the cladding layer is widened by providing the tensile strain layer 531, the overflow of carriers is suppressed, and the device operates with a low driving current even in a high temperature environment. be able to.

Hereinafter, the manufacturing process of the semiconductor laser device of FIG. 1 will be described in order. First, an n-type GaAs wafer substrate whose main surface is a surface inclined by 5 to 15 degrees in the (111) A plane direction with respect to a normal crystal orientation plane ((100) plane) is set in a crystal growth apparatus, and its main A GaAs film to be an n-type GaAs buffer layer 2 and an n-type (Al _0.7 Ga _0.3 ) _0.51 In _{0. 49} P film, AlGaInP first optical waveguide layer (Al _0.5 Ga _0.5 ) _0.51 In _0.49 P film, thin film stack structure constituting multiple quantum well active layer, AlGaInP second optical waveguide layer (Al _0.5 Ga _0.5 ) _0.51 In _0.49 P film is formed.

Subsequently, each layer constituting the AlGaInP upper first cladding layer 5 is formed. That is, the (Al _0.7 Ga _0.3 ) _0.51 In _0.49 P film that becomes the non-doped layer 51 and the p-type (Al _0.7 Ga _0.3 ) that becomes the p-type first doped layer 52 _{. 51} In _0.49 P film, AlGaInP tensile strained layer 531 (Al _0.7 Ga _0.3 ) _0.6 In _0.4 P film, p-type second p-type layer 54 (Al _{0. 7} Ga _0.3) to form a _0.51 an In _0.49 P layer.

Next, a p-type Ga _0.51 In _0.49 P film functioning as an etching stop layer 6 in a process to be described later is formed, and an AlGaInP upper second cladding layer 7 is formed thereon (Al _0.7 Ga _{0 .3} ) A _0.51 In _0.49 P film, a p-type Ga _0.51 In _0.49 P film to be the hetero buffer layer 8, and a p-type GaAs film to be the cap layer 9 are formed.

Next, an SiO ₂ film is formed on the p-type GaAs film so as to cover only the portion where the ridge structure portion 14 is formed, and the p-type GaAs film and the p-type Ga ₀ are formed using the SiO ₂ film as a mask. Etching the _.51 In _0.49 P film and the (Al _0.7 Ga _0.3 ) _0.51 In _0.49 P film. At this time, the layer below the etching stop layer 6 is not etched by the function of the etching stop layer 6. As a result, a mesa stripe-shaped ridge structure, that is, the aforementioned ridge structure portion 14 is formed on the etching stop layer 6.

Thereafter, an n-type GaAs film to be the current blocking layer 10 is formed by crystal growth on the portion where the ridge structure portion 14 is not formed on the etching stop layer 6 by a selective growth method using a SiO ₂ mask. Further, after removing the SiO ₂ mask, a p-type GaAs film to be the contact layer 11 is formed so as to cover both the ridge structure portion 14 and the current blocking layer 10.

  Next, the material (Ti / Pt / Au) of the p-side electrode 12 is vapor-deposited on the contact layer 11 and heat-treated. Further, after the GaAs substrate 1 is polished until the total thickness becomes about 100 μm, the material (Au / Ge / Ni / Au) of the n-side electrode 13 is deposited on the polished surface and heat-treated.

  Finally, a laser bar having a resonator length of about 0.75 to 1.5 mm is cleaved from the wafer processed as described above, and a low reflectance optical film is provided on one of the cleavage surfaces, and a high reflectance is provided on the other. The optical film is coated. Further, each laser bar is separated into chips by cleavage. Thereby, the semiconductor laser device as shown in FIG. 1 is completed.

  Although the first embodiment of the present invention has been described in detail above, other embodiments will be exemplified below. FIG. 5 is a diagram showing the structure of the cladding layer of the semiconductor laser device according to the second embodiment of the present invention. As is clear from comparison with FIG. 2, in the semiconductor laser device of the second embodiment, there is no non-doped layer between the tensile strain layer 531 and the light emitting layer 4, and only the p-type first doped layer 52 is disposed. Has been.

  As described above, the device resistance increases as the non-doped layer is thicker. Therefore, in this structure, the device resistance can be further reduced as compared with the first embodiment.

FIG. 6 is a diagram showing the structure of the cladding layer of the semiconductor laser device according to the third embodiment of the present invention. The semiconductor laser device of the third embodiment is replaced with (Al _0.7 Ga _0.3 ) _0.6 In _0.4 P tensile strain layer 531 of the semiconductor laser device of the second embodiment (Al _0.7 Ga _0.3 ) _0.4 In _0.6 P compression strained layer 532 is provided.

  Since the compressive strain layer can also suppress the diffusion of zinc, if the proportion of the non-doped layer thereunder is reduced, better characteristics than the conventional one can be obtained. However, in the case of compressive strain, the band gap between the AlGaInP first cladding layer and the active layer does not widen as shown in FIG. For this reason, in order to suppress the overflow of the carrier at the time of energization, it is more preferable that the strain layer is a tensile strain layer.

As the structure of the strained layer, in addition to this, a structure in which a compression strained layer and an unstrained layer ((Al _0.7 Ga _0.3 ) _0.5 In _0.5 layer) are alternately laminated, a tensile strained layer and an unstrained layer A structure in which layers are alternately stacked, a structure in which compression strain layers and tensile strain layers are alternately stacked, and a structure in which three types of layers of compression strain layers, tensile strain layers, and non-strain layers are combined are also conceivable. Also in the case of a multilayer structure, a structure including a tensile strain layer is preferable because an effect of suppressing carrier overflow is obtained.

  It is confirmed that the characteristic temperature of the element is higher by about 10K in the structure in which the strain layer includes the tensile strain layer than in the structure in which the tensile strain layer is not included.

  Further, the strain amount of the strain layer may be a strain amount that can suppress the diffusion of zinc, and is not limited to the above embodiments. The above embodiment exemplifies the case where the strain amount of the strain layer is about 0.7%, but the effect of preventing the diffusion of zinc can be obtained even when the strain amount is about 0.2%, for example. Note that the effect of preventing the diffusion of zinc increases as the strain amount of the strained layer increases, but there is also a problem that the occurrence rate of crystal defects increases when the strain amount is large. Since the rate of occurrence of crystal defects depends on crystal growth conditions and the like, the strain amount of the strained layer is preferably determined in consideration of such factors.

  In each of the above embodiments, the tensile strain layer 531 is doped with zinc at a low concentration, but the strain layer may be a non-doped layer. Even if the strained layer is non-doped, if a layer doped at a low concentration is provided in the region between the strained layer and the active layer, the temperature characteristics can be improved over the conventional structure where the strained layer to the active layer are non-doped. it can.

In the above embodiment, although the zinc concentration of the p-type first doped layer 52 and tensile strain layer 531 and 5 × ¹⁰ 16 ^{cm -3} or more 3 × ¹⁰ 17 ^{cm -3} or less, the p-type first doped layer Even if the zinc concentration of 52 deviates from the above range, the semiconductor laser element is included in the scope of the present invention as long as the above effect can be obtained.

  Since the present invention has been made in the process of studying a solution to the zinc diffusion problem, the above embodiment describes the p-type impurity as zinc. However, if the total thickness of the non-doped layer is large, the device The problem of increased resistance can be solved not only by zinc but also by introducing other impurities such as magnesium. That is, the p-type impurity in the present invention is not limited to zinc.

1 is a schematic diagram showing the overall structure of a semiconductor laser device according to an embodiment of the present invention. The figure which shows the detailed structure (the figure left) and energy level (the figure right) of the clad layer of the semiconductor laser element in one embodiment of this invention A graph showing the relationship between the thickness of the non-doped layer and the low-doped layer and the element resistance Graph showing the relationship between the thickness of the p-type first doped layer and the characteristic temperature The figure which shows the detailed structure of the clad layer of the semiconductor laser element in other embodiment of this invention The figure which shows the detailed structure (the figure left) and energy level (the figure right) of the clad layer of the semiconductor laser element in other embodiment of this invention

Explanation of symbols

1 n-type GaAs substrate 2 n-type GaAs buffer layer 3 n-type AlGaInP lower clad layer 4 light emitting layer
5 AlGaInP upper first cladding layer 51 Non-doped layer 52 p-type first doped layer 531 Tensile strain layer 532 Compressive strain layer 54 p-type second doped layer 6 Etching stop layer 7 p-type AlGaInP upper second cladding layer 8 p-type GaInP hetero Buffer layer 9 p-type GaAs cap layer 10 current blocking layer 11 p-type GaAs contact layer 12 p-side electrode 13 n-side electrode 14 Ridge structure

Claims (6)

A semiconductor laser device in which an active layer and a cladding layer are made of an AlGaInP-based material,
As a layer constituting the cladding layer disposed on one side of the active layer, a non-doped layer, a p-type first doped layer, a strained layer, and a p-type second doped layer are provided in order from the side closer to the active layer. ,
2. The semiconductor laser device according to claim 1, wherein an impurity concentration of the p-type first doped layer is lower than an impurity concentration of the p-type second doped layer. A semiconductor laser device in which an active layer and a cladding layer are made of an AlGaInP-based material,
As a layer constituting a clad layer disposed on one side of the active layer, a p-type first doped layer, a strained layer, and a p-type second doped layer are provided in order from the side closer to the active layer,
2. The semiconductor laser device according to claim 1, wherein an impurity concentration of the p-type first doped layer is lower than an impurity concentration of the p-type second doped layer. 3. The semiconductor laser device according to claim 1, wherein an impurity concentration of the p-type first doped layer is 5 × 10 ¹⁶ cm ⁻³ or more and 3 × 10 ¹⁷ cm ⁻³ or less.   4. The semiconductor laser device according to claim 1, wherein the strain layer is a tensile strain layer.   4. The semiconductor laser device according to claim 1, wherein the strain layer has a multilayer structure formed by combining a tensile strain layer and a compression strain layer and / or a non-strain layer. The strained layer is a layer into which a p-type impurity is introduced, and the concentration of the impurity is 5 × 10 ¹⁶ cm ⁻³ or more and 3 × 10 ¹⁷ cm ⁻³ or less. 6. The semiconductor laser device according to any one of items 1 to 5.

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