JP2009246035A - Long-wavelength-band surface-emitting laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-reliabile long-wavelength-band surface-emitting laser element reduced in the occurrence of transition even when an undoped DBR mirror is stacked on n-type GaAs. <P>SOLUTION: Concentration occupied by Al having a large lattice constant in a group III site of a semiconductor material forming a low-refractive-index layer of a lower DBR mirror 2 and containing Al, Ga and As is set to 80-100%; the average value of donor or acceptor impurity concentration in the whole of the lower DBR mirror 2 and an upper DBR mirror 12 is set not larger than 5×10<SP>17</SP>cm<SP>-3;</SP>and silicon concentration having a small lattice constant in an n-GaAs substrate 1 is set to 1×10<SP>16</SP>cm<SP>-3</SP>to 1.5×10<SP>18</SP>cm<SP>-3</SP>, whereby the occurrence of transition by lattice mismatch between the n-GaAs substrate 1 and the lower DBR mirror 2 is reduced, and reliability of the long-wavelength-band surface-emitting laser element can be enhanced. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a lower multilayer reflector formed of a periodic structure of a high refractive index layer and a low refractive index layer, and an upper multilayer film formed of a periodic structure of a high refractive index layer and a low refractive index layer. The present invention relates to a long-wavelength surface emitting laser device formed on a GaAs substrate.

  It is considered that a large amount of surface emitting laser (VCSEL) devices that output laser light in a long wavelength band of 1000 to 1700 nm and have an operation speed of 10 Gbps or more will be required in the future in the field of optical interconnection and optical communication. ing.

  As a conventional surface emitting laser element, a DBR (Distributed Bragg Reflector) mirror is configured by laminating an active layer between upper and lower multilayer mirrors made of a semiconductor, and a current path is provided to further increase current injection efficiency. A vertical cavity surface emitting laser element that forms a current confining layer to be limited and emits laser light in a direction perpendicular to a semiconductor substrate is disclosed (see Patent Document 1).

Here, in order to reduce the light absorption in the DBR mirror, a research institution adopting an undoped DBR mirror in which the concentration of impurities caused by light absorption is 5 × 10 ¹⁷ cm ⁻³ or less in the recent long wavelength band VCSEL There are many. In this case, the substrate is a semi-insulating substrate (SI substrate), and the electrodes are taken from the epi side for both n-type and p-type. Further, there has been proposed a surface emitting laser element employing an n-type GaAs substrate to which an n-type impurity such as silicon is added in order to improve reliability. However, until now, no long wavelength band VCSEL employing an undoped DBR mirror with an impurity concentration of 5 × 10 ¹⁷ cm ⁻³ or less on an n-type GaAs substrate has not been reported.

JP 2005-252032 A

  However, when an undoped DBR mirror is stacked on n-type GaAs, dislocation occurs due to lattice mismatch between the n-type GaAs substrate and the undoped DBR mirror, and the reliability of the surface emitting laser element is reduced. The first experiment revealed this.

  An object of the present invention is to provide a highly reliable long-wavelength-band surface emitting laser element in which generation of dislocations is reduced even when an undoped DBR mirror is stacked on n-type GaAs.

In order to solve the above-described problems and achieve the object, a long-wavelength surface emitting laser device according to the present invention includes an n-type GaAs substrate, stacked on the substrate, and a high refractive index layer and a low refractive index layer. An optical resonator having a lower multilayer reflector formed of a periodic structure; an upper multilayer reflector formed of a periodic structure of a high refractive index layer and a low refractive index layer; the lower multilayer reflector; In the long wavelength band surface emitting laser device provided between the upper multilayer reflector and comprising an active layer for generating light, and a p-type electrode and an n-type electrode drawn out from the resonator, The low refractive index layer of the lower multilayer mirror is formed of a semiconductor material containing Al, Ga, As, and the concentration of the Al in the group III site of the semiconductor material is 80% or more and 100% or less, Lower multilayer reflector and upper multilayer reflector The average value of the donor or acceptor impurity concentration in the entire mirror is 5 × 10 ¹⁷ cm ⁻³ or less, and the n-type GaAs substrate is 1 × 10 ¹⁶ cm ⁻³ or more and 1.5 × 10 ¹⁸ cm ⁻³ or less. It is characterized by containing a concentration of silicon.

Further, in the long wavelength band surface emitting laser device according to the present invention, the average Al composition X of Al occupying the group III of the lower multilayer mirror and the silicon concentration Ycm ⁻³ of the n-type GaAs substrate are 1 × 10 ¹⁶ ≦ Y ≦ −8.8 × 10 ¹⁹ X ² + 6.4 × 10 ¹⁹ X−1 × 10 ¹⁹ is satisfied.

In the long wavelength band surface emitting laser device according to the present invention, the silicon concentration of the n-type GaAs substrate is 1 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less.

Further, in the long wavelength band surface emitting laser device according to the present invention, the lower multilayer reflector has an average value of donor or acceptor impurity concentration in the substrate side region of 1 × 10 ¹⁸ to 1 × 10 ²¹ cm ⁻³ . The average value of the donor or acceptor impurity concentration in the active layer side region is 1 × 10 ¹⁵ to 1 × 10 ¹⁸ cm ⁻³ , and the average value of the donor or acceptor impurity concentration in the entire lower multilayer reflector is 5 × 10 ^{It is 17} cm- ³ or less.

  In the long wavelength band surface emitting laser device according to the present invention, the lower multilayer reflector includes 0.01% or more of nitrogen or phosphorus.

In the long wavelength band surface emitting laser device according to the present invention, a part of the lower multilayer reflector includes carbon of 1 × 10 ¹⁸ cm ⁻³ or more.

Further, in the long wavelength band surface emitting laser device according to the present invention, a part of the lower multilayer reflector includes 1 × 10 ¹⁸ cm ⁻³ or more of silicon.

In the long wavelength band surface emitting laser device according to the present invention, the donor or acceptor impurity concentration in the vicinity of the node of the light distribution of the lower multilayer reflector is 1 × 10 ¹⁸ to 1 × 10 ²¹ cm ^−3. It is characterized by.

  In the long wavelength band surface emitting laser device according to the present invention, at least a part of the upper multilayer mirror is made of a dielectric.

According to the present invention, the concentration of Al having a large lattice constant in the group III site of the semiconductor material containing Al, Ga, As that forms the low refractive index layer of the lower multilayer reflector is 80% or more and 100% or less, Further, even when the average value of the donor or acceptor impurity concentration in the entire lower multilayer reflector and upper multilayer reflector is 5 × 10 ¹⁷ cm ⁻³ or less, silicon having a small lattice constant in the n-type GaAs substrate is used. By setting the concentration to 1 × 10 ¹⁶ cm ⁻³ or more and 1.5 × 10 ¹⁸ cm ⁻³ or less, the occurrence of dislocation due to lattice mismatch between the n-type GaAs substrate and the lower multilayer mirror is reduced. It becomes possible to improve the reliability of the wavelength band surface emitting laser element.

  Embodiments of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the embodiments. In the description of the drawings, the same parts are denoted by the same reference numerals. Furthermore, it should be noted that the drawings are schematic, and the relationship between the thickness and width of each layer, the ratio of each layer, and the like are different from the actual ones. Also in the drawings, there are included portions having different dimensional relationships and ratios.

  FIG. 1 and FIG. 2 are diagrams showing the main configuration of the long-wavelength surface emitting laser element 100 according to the present embodiment. 1 is a plan view, and FIG. 2 is a cross-sectional view taken along the line II-II shown in FIG. As shown in these drawings, the surface emitting laser element 100 includes an undoped lower DBR mirror 2 and an n-type contact layer that function as a lower multilayer reflector that is stacked on an n-GaAs substrate 1 having a plane orientation (001). 3, n-type electrode 4, n-type cladding layer 5, active layer 6, p-type cladding layer 7, current confinement layer 8, p-type cladding layer 9, p-type contact layer 10, p-type electrode 11 and upper multilayer reflector The upper DBR mirror 12 functioning as Among these, the n-type clad layer 5 and the active layer 6, the current confinement layer 8, the p-type clad layer 7 and the p-type clad layer 9 laminated thereon are mesa posts formed into a columnar shape by an etching process or the like. Is formed. The mesa post diameter is, for example, 30 μm.

The lower DBR mirror 2 is formed on an undoped GaAs buffer layer stacked on the n-GaAs substrate 1 with a thickness of 0.1 μm, for example. The undoped lower DBR mirror 2 is a composite semiconductor layer in which a semiconductor layer containing Al, Ga, As that functions as a low refractive index layer and a semiconductor layer containing Ga, As that functions as a high refractive index layer are paired. Are formed as a semiconductor multilayer mirror in which a plurality of pairs are stacked. The thickness of each layer constituting the composite semiconductor layer of the undoped lower n-type DBR mirror 2 is λ / 4n (λ: oscillation wavelength, n: refractive index). The lower DBR mirror 2 is an undoped DBR mirror in which the average value of the impurity concentration in the entire lower DBR mirror 2 is 5 × 10 ¹⁷ cm ⁻³ or less.

  The n-type contact layer 3 is formed on the undoped lower DBR mirror 2 using n-GaAs. The thickness of the n-type contact layer 3 is (3λ) / 4. The n-type cladding layer 5 is formed on the n-type contact layer 3 using n-GaAs as a material. The p-type cladding layer 7 is formed using p-GaAs as a material on an active layer 6 described later. The p-type cladding layer 9 is formed using p-GaAs as a material on a current confinement layer 8 described later. The p-type contact layer 10 is formed on the p-type cladding layer 9 using p-GaAs as a material. The thickness of the p-type contact layer 10 is λ / 4.

The current confinement layer 8 is formed on the p-type cladding layer 7 and is composed of an opening 8a as a current confinement opening and a selective oxidation layer 8b. The current confinement layer 8 is formed of an Al-containing layer made of, for example, Al _0.98 Ga _0.02 As having a thickness of 30 nm. The selective oxidation layer 8b is formed on the annular zone by oxidizing the Al-containing layer by a predetermined range from the outer peripheral portion along the laminated surface. The selective oxidation layer 8b has an insulating property and narrows the current injected from the p-type electrode 11 and concentrates it in the opening 8a, thereby increasing the current density in the active layer 6 immediately below the opening 8a. Yes. The current opening diameter of the opening 8a is, for example, 6 μm.

The active layer 6 has a multiple quantum well (MQW) structure in which, for example, three composite semiconductor layers made of Ga _0.65 In _0.35 N _0.01 As _0.99 / GaAs are stacked, Spontaneously emitted light of 1 μm to 1.7 μm is emitted based on the current injected from the p-type electrode 11 and constricted by the current confinement layer 8. The Ga _0.65 In _0.35 N _0.01 As _0.99 layer functions as a quantum well and has a thickness of, for example, 7.3 nm. The GaAs layer functions as a barrier layer and has a thickness of 9 nm, for example. The spontaneous emission light is amplified by being resonated and amplified in a direction perpendicular to each layer including the active layer 6 between the lower DBR mirror 2 and the upper DBR mirror 12 serving as a resonator, and then lasered from above the upper DBR mirror 12. Injected as light. The current confinement layer 8 is formed at a position separated from the active layer 6 by 3λ / 4. The n-type contact layer 3, the n-type cladding layer 5, the active layer 6, the p-type cladding layer 7, the current confinement layer 8, the p-type cladding layer 9 and the p-type contact layer 10 constitute a 3λ optical resonator.

The upper DBR mirror 12 is formed on the p-type contact layer 10. The upper DBR mirror 12 is formed as a dielectric multilayer mirror in which a plurality of pairs (for example, 5 pairs) of composite dielectric layers made of, for example, SiN / SiO ₂ are stacked, and the thickness of each layer is the same as that of the lower DBR mirror 2. Is λ / 4n. For example, the upper DBR mirror 12 forms a dielectric multilayer film having a predetermined number of layers in a range including a mesa post, and etches a peripheral region other than the portion directly above the opening 8a in the dielectric multilayer film (etching step). Is formed.

  The p-type electrode 11 is stacked on the p-type contact layer 10 and is formed in a ring shape so as to surround the upper DBR mirror 12. On the other hand, the n-type electrode 4 is laminated on the n-type contact layer 3, and is formed in a C shape so as to surround the bottom surface portion of the mesa post along the laminated surface. The p-type electrode 11 and the n-type electrode 4 are electrically connected to an external circuit (such as a current supply circuit) (not shown) by a p-type extraction electrode 14 and an n-type extraction electrode 13, respectively. The growth of each layer may be performed using any one of the growth methods of MBE, gas source MBE, CBE, and MOCVD.

  Here, in the conventional surface emitting laser element, when a doped DBR mirror is stacked on n-type GaAs, the threshold value increases and the slope efficiency increases due to free carrier absorption in the n-type GaAs substrate and DBR. There was a problem of reduction and light output reduction.

First, a conventional surface emitting laser element will be described. FIG. 3 is a cross-sectional perspective view schematically showing a surface emitting laser element according to the prior art. As shown in FIG. 3, the surface emitting laser element according to the prior art includes an n-GaAs substrate 101 having a plane orientation (100), an n-type lower DBR mirror 102, an n-GaAs cladding layer 105, an active layer 106, p- A current confinement layer 108 composed of a GaAs cladding layer 107, an opening 108a and a selective oxidation layer 108b, a p-GaAs cladding layer 109, and a p-type upper DBR mirror 112 are sequentially stacked. The n-GaAs cladding layer 105 to the upper DBR mirror 112 have a mesa post structure, and this mesa post is filled with polyimide 115, for example. A p-side electrode 111 having an opening is formed on the upper surface of the upper DBR mirror 112 and the polyimide 115, and an n-side electrode 104 is formed on the lower surface of the n-GaAs substrate 101. Here, the lower DBR mirror 102 is formed by laminating 35.5 pairs of multilayer films in which a pair of an Al _0.9 Ga _0.1 As layer and a GaAs layer each having a layer thickness of λ / 4n.

A conventional surface emitting laser element employs a doped DBR mirror having a dopant concentration of about 1 × 10 ¹⁸ cm ⁻³ , and further ensures that crystal defects (EPD: Etch Pit Density) are less than 500 cm ^−2. An n-GaAs substrate 101 to which silicon (Si) that is an n-type impurity is added in an amount of about ³ to 4 × 10 ¹⁸ cm ⁻³ is employed. Therefore, development of a long wavelength band VCSEL using an undoped DBR as shown in FIGS. 1 and 2 was started.

Here, AlGaAs forming the low refractive index layer of the lower DBR mirror 2 contains a high composition of Al, and therefore has a lattice constant much larger than that of GaAs. Further, since Si has a function of reducing the lattice constant of GaAs, the n-GaAs substrate 1 doped with a large amount of Si has a smaller lattice constant than GaAs without addition of Si. Therefore, when the lower DBR mirror 2 including the AlGaAs layer as the low refractive index layer is stacked on the n-GaAs substrate 1, a large difference is generated between the lattice constant of the n-GaAs substrate 1 and the lattice constant of the lower DBR mirror 2. Therefore, there is a problem that dislocation occurs due to the accumulation of strain due to the lattice mismatch, and the reliability of the surface emitting laser element is lowered. Actually, when the undoped lower DBR mirror 2 is formed on the n-GaAs substrate 101 having a Si concentration of 4 × 10 ¹⁸ cm ⁻³ , in the case of AlAs / GaAs (31 pairs), 1 × 10 ⁷ cm ^{−. Two} transitions occurred, and in the case of AlGaAs / GaAs (37 pairs), a transition of 1 × 10 ⁵ cm ⁻² occurred, and a desired yield could not be achieved.

  On the other hand, the surface emitting laser element 100 according to the present embodiment includes an Al concentration indicating a group III site of a semiconductor material containing Al, Ga, and As that forms a low refractive index layer of the lower DBR mirror 2, and n -By controlling the Si concentration contained in the GaAs substrate 1, the difference between the lattice constant of the n-GaAs substrate 1 and the lattice constant of the lower DBR mirror 2 is reduced, and the strain accumulated in the substrate and DBR mirror is reduced. Thus, dislocations due to strain are reduced.

  First, the Al concentration of the semiconductor material of the low refractive index layer constituting the lower DBR mirror 2 will be described. When the Al concentration of the low refractive index layer of the lower DBR mirror is set to a low concentration of less than 80%, the thermal resistance per layer of the low refractive index layer increases and the reflectance difference from the high refractive index layer is increased. Since the number of pairs must be increased in order to ensure, the thermal resistance of the lower DBR mirror 2 becomes high, and it becomes difficult to ensure a sufficient output especially during high temperature operation. For this reason, the Al concentration of the low refractive index layer of the lower DBR mirror 2 is to ensure a difference in reflectance from the high refractive index layer formed of GaAs, to suppress the number of pairs of low refractive index layers, to suppress an increase in thermal resistance, 80% or more is necessary to ensure output during high temperature operation.

  When the Al concentration of the low refractive index layer of the lower DBR mirror 2 is set to 80% or more in this way, the lattice constant is significantly larger than that of GaAs because it contains high composition Al. Further, when the n-GaAs substrate 1 to which a large amount of Si is added is used, a large difference is generated between the lattice constant of the n-GaAs substrate 1 and the lattice constant of the lower DBR mirror 2, and this lattice mismatch. Dislocation occurs due to the accumulation of strain due to, and the reliability of the surface emitting laser element is lowered. Therefore, in the present embodiment, each concentration of Si is actually added to the n-GaAs substrate 1 to verify the dislocation density when the lower DBR mirror 2 having different Al concentrations is formed. The concentration of silicon contained in the n-GaAs substrate 1 is adjusted so as to be less than the dislocation density.

FIG. 4 is a diagram showing the relationship between the Si concentration in the n-type substrate and the dislocation density when AlGaAs layers having various Al concentrations are stacked as the low refractive index layer of the lower DBR mirror 2. A curve l1 in FIG. 4 shows the relationship between the Si concentration in the n-type substrate and the dislocation density when an AlAs layer having an Al concentration of 100% is stacked as the low refractive index layer of the lower DBR mirror 2, and the curve l2 in FIG. These show the relationship between the Si concentration in the n-type substrate and the dislocation density when an Al _0.8 Ga _0.2 As layer having an Al concentration of 80% is stacked as the low refractive index layer of the lower DBR mirror 2. Curve 11 corresponds to the case where 31 pairs of AlAs layers and GaAs layers are stacked, and curve 12 corresponds to the case where 37 pairs of Al _0.8 Ga _0.2 As layers and GaAs layers are stacked.

As shown in FIG. 4, when the Al concentration is 100%, if Si concentration of n-GaAs substrate is 1 × ¹⁰ 17 ^{cm -2} dislocation density of 1 × ¹⁰ 5 ^{cm -2} next, Si concentration Is 1.5 × 10 ¹⁸ cm ⁻² , the dislocation density is 1 × 10 ⁶ cm ⁻² , and when the Si concentration is 4 × 10 ¹⁸ cm ⁻³ , the dislocation density is 4 × 10 ¹⁸ cm ^−3. The density increases to 1 × 10 ⁷ cm ⁻² . When the Al concentration is 80%, when the Si concentration of the n-GaAs substrate is 1 × 10 ¹⁷ cm ⁻² , the dislocation density is reduced to 1 × 10 ³ cm ⁻² , and the Si concentration is 1. When the density is 5 × 10 ¹⁸ cm ⁻² , the dislocation density is 1 × 10 ⁴ cm ⁻² , but when the Si concentration is 4 × 10 ¹⁸ cm ⁻³ , the dislocation density is 1 It becomes x10 ^{< 5} > cm ^<-2> . Thus, when the Al concentration of the low refractive index layer is 80% and when the Al concentration is 100%, the difference in dislocation density is about two digits.

Here, in the case of a dislocation density of 1 × 10 ⁴ cm ⁻² , dislocations exist at a density of 0.01 per 10 μm square. That is, in the case of a dislocation density of 1 × 10 ⁴ cm ⁻² , there are about 0.01 dislocations in the mesa structure of the surface emitting laser element, and the yield per channel is 99%. . For example, in order to reduce the yield in a 10-channel array to 90% or less, the yield per channel needs to be 99% or less. For this reason, in order to secure a yield of 90% or more in a 10-channel array, the dislocation density needs to be 1 × 10 ⁴ cm ⁻² or less.

Therefore, as shown by the curve l2 in FIG. 4, in order to obtain a dislocation density of 1 × 10 ⁴ cm ⁻² , when the Al concentration of the low refractive index layer of the lower DBR mirror 2 is 80%, the n-GaAs substrate 1 Si concentration needs to be 1.5 × 10 ¹⁸ cm ⁻³ . That is, it can be said that the upper limit of the Si concentration of the n-GaAs substrate 1 is 1.5 × 10 ¹⁸ cm ⁻³ . Furthermore, when the Al concentration is 100%, in order to achieve the dislocation density of 1 × 10 ⁴ cm ⁻² , the Si concentration needs to be about 1 × 10 ¹⁶ cm ⁻³ or less.

And the crystal defect (EPD: Etch Pit Density) in a GaAs substrate needs to be suppressed to 5000 or less, desirably 500 cm ^-2 or less. This EPD amount decreases as the Si addition amount increases. Specifically, when the Si addition amount is 1 to 4 × 10 ¹⁸ cm ⁻³ , the EPD is surely 500 cm ⁻² or less, and when the Si addition amount is less than 1 × 10 ¹⁶ cm ^−3. EPD exceeds 5000 cm- ² . Therefore, in order to suppress EPD to 5000 cm ⁻² or less, the Si concentration of the n-GaAs substrate needs to be 1 × 10 ¹⁶ cm ⁻³ . That is, it can be said that the lower limit of the Si concentration of the n-GaAs substrate 1 is 1 × 10 ¹⁶ cm ⁻³ . Thus, in the present embodiment, the Si concentration of the n-GaAs substrate 1 is set to 1 × 10 ¹⁶ cm ⁻³ or more and 1.5 × 10 ¹⁸ cm ⁻³ or less to prevent both dislocation generation and EPD suppression. Realize.

  Furthermore, actually, not only when the Al concentration of the low refractive index layer of the lower DBR mirror 2 is 80% or 100%, the Si concentration of the n-GaAs substrate 1 is allowed for each concentration of 80 to 100%. The capacity was verified. FIG. 5 shows the result of obtaining the upper limit value of the Si concentration of the n-GaAs substrate 1 with respect to each Al concentration of the low refractive index layer of the lower DBR mirror 2. The horizontal axis represents the average Al composition of the lower DBR mirror 2 that combines both the low refractive index layer and the high refractive index layer, so the actual Al concentration of the low refractive index layer is twice the average Al composition. It becomes.

A curve l3 in FIG. 5 shows a relationship between each Al concentration of the low refractive index layer of the lower DBR mirror 2 and an allowable Si concentration value of the n-GaAs substrate 1. In addition, the curve l3 of FIG. 5 sets the upper limit of a dislocation density to 1 * 10 ^{< 4} > cm ^<-2> . In the figure, Lu is the upper limit value 1.5 × 10 ¹⁸ cm ⁻³ of the Si concentration of the n-GaAs substrate 1 described above, and Li is the lower limit value 1 × 10 ¹⁶ of the Si concentration of the n-GaAs substrate 1 in the figure. cm- ³ .

From the result of FIG. 5, by adjusting the Si concentration of the n-GaAs substrate 1 and each Al concentration of the low refractive index layer of the lower DBR mirror 2 so as to be equal to or lower than the curve l3, the dislocation density is 1 × 10. It can be suppressed to less than ⁴ cm ⁻² . Then, by setting the Si concentration of the n-GaAs substrate 1 to be the lower limit value Li1 × 10 ¹⁶ cm ⁻³ or more of the Si concentration, the EPD of the n-GaAs substrate 1 can be set to 5000 cm ⁻² or less. That is, when the average Al composition X of Al occupying the group III of the lower DBR mirror 2 and the Si concentration of the n-GaAs substrate 1 are Y, the average Al composition X and Si concentration Y are as follows: It only has to satisfy the relationship of the formula.
1 × 10 ¹⁶ ≦ Y ≦ −8.8 × 10 ¹⁹ X ² + 6.4 × 10 ¹⁹ X-1 × 10 ¹⁹ (1)

Furthermore, in order to ensure the reflectivity of the high refractive index layer and the low refractive index layer, when the average Al composition X of Al occupying group III of the lower DBR mirror 2 is 0.45, The Al concentration of the low refractive index layer of the DBR mirror 2 needs to be 90%. For this reason, the upper limit value of the optimum range of the Si concentration of the n-GaAs substrate 1 is 1 × 10 ¹⁸ corresponding to the average Al composition 0.45 of Al occupying the group III of the lower DBR mirror 2 from the curve l3 of FIG. cm ⁻³ . It is desirable to suppress the EPD of the n-GaAs substrate 1 to 500 or less. Since the EPD can be reduced to 500 cm ⁻² or less by increasing the Si concentration of the n-GaAs substrate to at least 1 × 10 ¹⁷ cm ⁻³ , the lower limit value of the optimum range of the Si concentration of the n-GaAs substrate 1 is 1 × 10 ¹⁷ cm ⁻³ . Therefore, the optimum range of the Si concentration of the n-type GaAs substrate is 1 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less.

As described above, in the surface emitting laser element 1 according to this embodiment, Al having a large lattice constant occupies the group III site of the semiconductor material containing Al, Ga, and As that forms the low refractive index layer of the lower DBR mirror 2. By setting the concentration to 80% or more and 100% or less, even when the necessary number of pairs are stacked, the thermal resistance of the lower DBR mirror 2 is lowered to enable output during high temperature operation, and in the n-GaAs substrate 1 Dislocation generation due to lattice mismatch between the n-type GaAs substrate and the lower multilayer mirror is achieved by setting the silicon concentration having a small lattice constant to a concentration of 1 × 10 ¹⁶ cm ⁻³ or more and 1.5 × 10 ¹⁸ cm ⁻³ or less. Achievement of high yield of long-wavelength surface emitting laser device even when undoped DBR mirrors of the required number of pairs are formed on an n-GaAs substrate because reduction is possible. It is possible to realization of high-temperature operation at achieved and the low thermal resistance of high reliability.

  The surface emitting laser element 1 has an average Al composition X of Al occupying the group III of the lower DBR mirror 2 and an average Al composition X and Si concentration Y, where Y is the Si concentration of the n-GaAs substrate 1. By adjusting the Al concentration of the lower DBR mirror 2 and the Si concentration of the n-GaAs substrate 1 so as to satisfy the relationship of the expression (1), any one of the Al concentrations in the range of 80% to 100% Even with the concentration, it is possible to achieve a high yield of the long-wavelength surface emitting laser element, achieve high reliability, and realize high-temperature operation with low thermal resistance.

Furthermore, by setting the Si concentration of the n-GaAs substrate 1 to 1 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less, both the high performance of the surface emitting laser element 1 and further suppression of EPD are achieved. Can be realized.

Actually, the lower DBR mirror 2 is a case where 34.5 pairs of n-Al _0.9 Ga _0.1 As layers and GaAs layers each having a layer thickness of λ / 4n are stacked as a pair, and the n-GaAs substrate 1 In the case of a surface emitting laser element that oscillates 1.3 μm band light having a Si concentration of 9 × 10 ¹⁷ to 1 × 10 ¹⁸ cm ⁻³ , the threshold current is 1 mA, the slope efficiency is 0.25 W / A, and 100 ° C. or more. In addition to CW oscillation of 10 Gbps, it is possible to operate at 10 Gbps at 85 ° C., and further increase the 10-channel array yield to 90% or more and suppress the number of FITs indicating the failure rate of accidental failure to 100 or less per channel. Became.

  Further, when the Al concentration of the low refractive index layer of the lower DBR mirror 2 is high, a semiconductor material further added with an impurity having an action of reducing the lattice constant is adopted as a constituent semiconductor material, and the n-GaAs substrate 1 The difference between the lattice constant and the lattice constant of the lower DBR mirror 2 may be reduced.

  For example, nitrogen (N) is further added to a group V site of a semiconductor material containing Al, Ga, and As constituting the low refractive index layer. Since N has an effect of reducing the lattice constant, by adopting a semiconductor material having a low refractive index layer containing N, the lattice constant of the low refractive index layer can be reduced. It is possible to further reduce dislocation generation due to lattice mismatch between the n-type GaAs substrate and the lower multilayer reflector by further reducing the lattice constant difference with the GaAs substrate 1. Note that when N is added to the semiconductor material constituting the low refractive index layer in an amount of 0.01% or more, the effect of reducing the lattice constant of the low refractive index layer actually occurs. It is desirable that it is 0.01% or more. On the other hand, if N is added at a concentration exceeding 1%, the difference in lattice constant from the substrate increases. Therefore, N contained in the low refractive index layer is preferably 1% or less.

  It is also possible to add phosphorus (P) to a group V site of a semiconductor material containing Al, Ga, and As constituting the low refractive index layer. Since P also has the effect of reducing the lattice constant in the same manner as N, by adopting the semiconductor material of the low refractive index layer containing this P, the lattice constant of the low refractive index layer can be reduced. It is possible to further reduce the occurrence of dislocation due to lattice mismatch between the n-type GaAs substrate and the lower multilayer mirror by further reducing the lattice constant difference with the n-GaAs substrate 1 included. In addition, since the effect of shrinking the lattice constant of the low refractive index layer actually occurs when P is added to the semiconductor material constituting the low refractive index layer by 0.03% or more, P contained in the low refractive index layer is: It is desirable that it is 0.03% or more. On the other hand, if P is added at a concentration exceeding 3%, the difference in lattice constant from the substrate increases. Therefore, P contained in the low refractive index layer is preferably 3% or less. Since N and P do not absorb light, the semiconductor material to which N and P are added can be used for the low refractive index layer at any position of the lower DBR mirror 2. Further, N or P may be added to the high refractive index layer to reduce the strain amount of the entire DBR.

In particular, reducing the lattice constant difference at the interface between the n-GaAs substrate 1 and the lower DBR mirror 2 is effective in preventing the occurrence of lattice mismatch, so that a semiconductor material containing a large amount of N and P is used on the n-GaAs substrate 1 side. It is desirable to employ as a low refractive index layer in the region. Further, in order to undoped the lower DBR mirror 2, the average value of the donor or acceptor impurity concentration in the entire lower DBR mirror 2 needs to be 5 × 10 ¹⁷ cm ⁻³ or less. Corresponding to the high concentration of impurities, the concentration of N and P in the active layer 6 side region may be lowered. Specifically, the average value of the donor or acceptor impurity concentration in the region of the lower DBR mirror 2 on the n-GaAs substrate 1 side is 1 × 10 ¹⁸ to 1 × 10 ²¹ cm ^−3, and the active layer 6 side of the lower DBR mirror 2 is used. The average value of the donor or acceptor impurity concentration in the region is 1 × 10 ¹⁵ to 1 × 10 ¹⁸ cm ^−3, and the average value of the donor or acceptor impurity concentration in the entire lower DBR mirror 2 mirror is 5 × 10 ¹⁷ cm ⁻³ or less. To do. The donor or acceptor impurity concentration in the n-GaAs substrate 1 side region of the lower DBR mirror 2 and the donor or acceptor impurity concentration in the active layer 6 side region of the lower DBR mirror 2 are changed stepwise or continuously. Also good.

It is also possible to add carbon (C) to the semiconductor material constituting the lower DBR mirror 2. Since C also has an effect of reducing the lattice constant, the lattice constant of the entire lower DBR mirror 2 can be apparently reduced by adopting a semiconductor material containing C as a constituent layer of the lower DBR mirror 2. It is possible to further reduce dislocation generation due to lattice mismatch between the n-type GaAs substrate and the lower DBR mirror by further reducing the lattice constant difference with the n-GaAs substrate 1 containing Si. Similarly, Si can be added to the semiconductor material constituting the lower DBR mirror 2. As described above, since Si also has an effect of reducing the lattice constant, by adopting this Si-containing semiconductor material for the constituent layer of the lower DBR mirror 2, the lattice constant of the entire lower DBR mirror 2 can be apparently reduced. It is possible to further reduce dislocation generation due to lattice mismatch between the n-type GaAs substrate and the lower DBR mirror by further reducing the lattice constant difference from the n-GaAs substrate 1 containing Si having a small lattice constant. Note that, when C or Si is added to the semiconductor material by 1 × 10 ¹⁸ cm ⁻³ or more, an effect of reducing the lattice constant of the semiconductor material actually occurs. Therefore, C included in a part of the lower DBR mirror 2, The concentration of Si is desirably 1 × 10 ¹⁸ cm ⁻³ or more.

Further, as described above, reducing the lattice constant difference at the interface between the n-GaAs substrate 1 and the lower DBR mirror 2 is effective in preventing the occurrence of lattice mismatch. It is desirable to employ as a constituent layer of the lower DBR mirror 2 in the region on the GaAs substrate 1 side. Further, in order to undoped the lower DBR mirror 2, the average value of the donor or acceptor impurity concentration in the entire lower DBR mirror 2 needs to be 5 × 10 ¹⁷ cm ⁻³ or less. The concentration of C and Si in the region on the active layer 6 side of the lower DBR mirror 2 may be lowered corresponding to the high concentration of impurities in the constituent layers of the lower DBR mirror 2. In particular, since C and Si have a function in which generated holes or electrons absorb light by free carrier absorption, lowering the C and Si concentration in the active layer 6 side region is an impurity in the active layer 6 having high light intensity. Therefore, it is desirable to set the C and Si concentrations on the active layer 6 side low. Specifically, the average value of the donor or acceptor impurity concentration in the region of the lower DBR mirror 2 on the n-GaAs substrate 1 side is 1 × 10 ¹⁸ to 1 × 10 ²¹ cm ^−3, and the active layer 6 side of the lower DBR mirror 2 is used. The average value of the donor or acceptor impurity concentration in the region is 1 × 10 ¹⁵ to 1 × 10 ¹⁸ cm ^−3, and the average value of the donor or acceptor impurity concentration in the entire lower DBR mirror 2 mirror is 5 × 10 ¹⁷ cm ⁻³ or less. To do. The donor or acceptor impurity concentration in the n-GaAs substrate 1 side region of the lower DBR mirror 2 and the donor or acceptor impurity concentration in the active layer 6 side region of the lower DBR mirror 2 are changed stepwise or continuously. Also good.

Here, since it is known that the amount of light loss is low if the concentration of C and Si is about 1 × 10 ¹⁸ cm ⁻³ , the case where C and Si are added to most of the region of the lower DBR mirror 2. It is desirable that the concentration of C and Si be about 1 × 10 ¹⁸ cm ⁻³ . In this case, the average value of the donor or acceptor impurity concentration in the entire lower DBR mirror 2 needs to be 5 × 10 ¹⁷ cm ⁻³ or less.

If it is necessary to add C and Si at a high concentration such as when the Al composition of the low refractive index layer is high, a high concentration of C and Si should be added to the node portion where the light distribution intensity is weak. Good. This is because if the node portion is weak and does not feel light, even if a large amount of impurities that absorb light are added, light absorption is unlikely to occur and the amount of light loss can be reduced. Specifically, by adding C or Si at a concentration of 1 × 10 ¹⁸ cm ⁻³ or more and 1 × 10 ²¹ cm ⁻³ or less to a film thickness portion having a thickness of 30 nm or less in the node portion of the light distribution, While reducing the loss amount, the lattice constant of the lower DBR mirror 2 can be reduced, and the occurrence of dislocation can be reduced.

  Further, as the surface emitting laser element according to the present embodiment, as shown in FIGS. 1 and 2, the entire upper DBR mirror 12 is described as an example of a dielectric film. Only a part may be composed of a dielectric film, and the other part may be composed of a semiconductor film.

It is a top view which shows the structure of the surface emitting laser element concerning embodiment. It is sectional drawing which shows the II-II cross section shown in FIG. It is the cross-sectional perspective view which showed typically the surface emitting laser element concerning a prior art. It is a figure which shows the relationship between Si density | concentration in a n-type board | substrate, and a dislocation density in the case of laminating | stacking AlGaAs layer of each Al density | concentration as a low refractive index layer of the lower DBR mirror shown in FIG. It is a figure which shows the relationship between each Al density | concentration of the low refractive index layer of the lower DBR mirror shown in FIG. 2, and the upper limit of Si density | concentration of an n-GaAs substrate.

Explanation of symbols

100 surface emitting laser element 1,101 n-GaAs substrate 2,102 lower DBR mirror 3 n-type contact layer 4,104 n-type electrode 5 n-type cladding layer 6,106 active layer 7,9 p-type cladding layer 8,108 current Narrowing layer 8a, 108a Opening 8b, 108b Selective oxidation layer 10 p-type contact layer 11, 111 p-type electrode 12, 112 Upper DBR mirror 13 n-type extraction electrode 14 p-type extraction electrode 105 n-GaAs cladding layer 107 p-GaAs Cladding layer

Claims (9)

an n-type GaAs substrate;
A lower multilayer reflector formed of a periodic structure of a high refractive index layer and a low refractive index layer, and an upper multilayer film reflection formed of a periodic structure of a high refractive index layer and a low refractive index layer. An optical resonator having a mirror;
An active layer provided between the lower multilayer reflector and the upper multilayer reflector to generate light;
A p-type electrode and an n-type electrode drawn out from the resonator;
In a long wavelength band surface emitting laser device comprising:
The low refractive index layer of the lower multilayer mirror is formed of a semiconductor material containing Al, Ga, As, and the concentration of the Al in the group III site of the semiconductor material is 80% or more and 100% or less,
The average value of the donor or acceptor impurity concentration in the whole lower multilayer reflector and the upper multilayer reflector is 5 × 10 ¹⁷ cm ⁻³ or less,
The n-type GaAs substrate includes a silicon having a concentration of 1 × 10 ¹⁶ cm ⁻³ or more and 1.5 × 10 ¹⁸ cm ⁻³ or less. The average Al composition X of Al occupying the group III of the lower multilayer mirror and the silicon concentration Ycm ⁻³ of the n-type GaAs substrate are:
1 × 10 ¹⁶ ≦ Y ≦ −8.8 × 10 ¹⁹ X ² + 6.4 × 10 ¹⁹ X-1 × 10 ¹⁹
The long-wavelength surface emitting laser element according to claim 1, wherein: 3. The long-wavelength surface emitting laser device according to claim 1, wherein a silicon concentration of the n-type GaAs substrate is 1 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less. The lower multilayer reflector has an average value of donor or acceptor impurity concentration in the substrate side region of 1 × 10 ¹⁸ to 1 × 10 ²¹ cm ⁻³ , and an average value of donor or acceptor impurity concentration in the active layer side region is The average value of the donor or acceptor impurity concentration in the entire lower multilayer reflector is 5 × 10 ¹⁷ cm ⁻³ or less, which is 1 × 10 ¹⁵ to 1 × 10 ¹⁸ cm ^−3. The long wavelength band surface emitting laser device according to any one of ˜3.   5. The long-wavelength surface emitting laser element according to claim 1, wherein the lower multilayer mirror includes 0.01% or more of nitrogen or phosphorus. 6. The long-wavelength band surface emitting laser element according to claim 1, wherein a part of the lower multilayer-film reflective mirror contains 1 × 10 ¹⁸ cm ⁻³ or more of carbon. 7. The long-wavelength surface emitting laser element according to claim 1, wherein a part of the lower multilayer-film reflective mirror contains 1 × 10 ¹⁸ cm ⁻³ or more of silicon. 8. The donor or acceptor impurity concentration in the vicinity of a node of the light distribution of the lower multilayer mirror is 1 × 10 ¹⁸ to 1 × 10 ²¹ cm ^−3. The long-wavelength surface emitting laser element described.   9. The long-wavelength surface emitting laser element according to claim 1, wherein at least a part of the upper multilayer mirror is made of a dielectric.

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