JP2010219261A - Semiconductor laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-reliability semiconductor laser element that suppresses a deterioration phenomenon at both ends of a stripe. <P>SOLUTION: The semiconductor laser element 200 includes a pair of an electrode layer 40 and an electrode layer 50, and an epitaxial layer 20 formed between a pair of the electrode layer 40 and the electrode layer 50 and including an active layer 20c, light-guide layers 20b, 20d, and clad layers 20a, 20e. The electrode layer 40 includes a stripe 41 conductive with the epitaxial layer 20, and each current injection part 42 respectively located on both sides of the stripe 41 and configured such that an insulating layer 30 is interposed between the electrode layer and the epitaxial layer 20. Resistance per unit area of the epitaxial layer 20 in a direction orthogonal to the electrode layer 40 is set larger on the side of each end of the stripe 41 than on the side of the central part of the stripe. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

The present invention relates to a semiconductor laser device.

  Conventionally, in a semiconductor laser element, characteristic deterioration due to crystal defects or the like has been observed at the time of output, and there has been a problem that sufficient output characteristics and lifetime have not been obtained. Several studies have been conducted on this characteristic deterioration. As causes of the characteristic deterioration, optical damage destruction (COD: Catastrophic Optical Damage) and dark line defect (DLD: Dark Line Defect), which are deterioration phenomena caused by crystals, are known.

  COD and DLD are known as a phenomenon in which irreversible destruction that lowers light output occurs, and is considered to occur as follows. That is, non-radiative recombination occurs through defect levels inherent in the crystal during the operation of the semiconductor laser, resulting in a local decrease in carrier density. Therefore, with respect to the oscillation wavelength at which the gain inside the resonator can be obtained, the gain cannot be obtained due to the decrease in carrier density in that region, and light absorption occurs. Temperature rise occurs due to light absorption, and the band gap Eg decreases. When the band gap is reduced, absorption is further increased, and a positive feedback that the temperature rises is applied, crystal melting and dislocation are generated, and deterioration and destruction of the device occur.

  Until now, as in Japanese Patent Laid-Open No. 6-338657 (Patent Document 1), in order to improve the resistance of the semiconductor laser to this deterioration phenomenon, the active layer near the reflector, that is, the resonator end face, which is a high light density region, is centered. A window structure composed of a crystal material having a larger band gap than that of the active layer is proposed.

This window structure forms a non-absorbing mirror (NAM) on the cavity end face by regrowth. After a crystal stacked structure, which is the basic structure of a laser, is fabricated, the cavity end is etched until it reaches the active layer, and the cavity facet is buried and grown again by crystal growth. Here, by increasing the band gap of the regrown crystal relative to the active layer, the buried layer at the end of the resonator becomes transparent with respect to the oscillation wavelength, and a window structure is formed.
In addition, a non-absorbing mirror based on impurity diffusion has been devised as disclosed in JP-A-5-218593 (Patent Document 2). This utilizes the fact that the thermal diffusion of Zn, Si, etc. is utilized, the crystal is disordered in the vicinity of the active layer, the effective refractive index in the vicinity of the end face is lowered, and it becomes transparent to the oscillation wavelength.
In any case, by adopting an absorption structure at the end of the resonator, a window structure can be formed, a deterioration phenomenon near the reflecting surface can be suppressed, and the output characteristics and life of the laser can be improved to some extent.

JP-A-6-338657 Japanese Patent Laid-Open No. 5-218593

However, a semiconductor laser element (especially a high-power semiconductor laser element) has a problem that the light density at both ends of the stripe increases in addition to the above-described light density distribution at the cavity end. Therefore, there is a problem that the above-described deterioration phenomenon is likely to occur at both ends of the stripe.
This is due to the injection current distribution of the semiconductor laser element. In general, a semiconductor laser device can suppress the effects of gain saturation and heat by increasing the width of the stripe, and can obtain a high optical output with respect to the injected current. However, when current is injected from both outer sides of both ends of the stripe, since the electrode area is large, the resistance current increases the injected current at both ends of the stripe and decreases at the center. According to this current distribution, the light density also increases at both ends of the stripe. As a result, the above-described deterioration phenomenon tends to occur at both ends of the stripe.
On the other hand, it is conceivable to bond a wire that conducts current in the stripe. However, bonding a wire in the stripe damages the active layer under the stripe, and reduces the laser output characteristics and lifetime. It will cause a decline.

  The present invention has been made in view of such a problem, and an object of the present invention is to provide a highly reliable semiconductor laser device that can suppress the deterioration phenomenon at both ends of the stripe.

  The semiconductor laser device of the present invention includes a pair of electrode layers A and B, and an epitaxial layer formed between the pair of electrode layers A and B, including an active layer, a light guide layer, and a cladding layer. The electrode layer A includes a stripe conducting to the epitaxial layer, and a current injection portion located on both sides of the stripe and having an insulating layer interposed between the epitaxial layer and the electrode layer A. The resistance per unit area of the epitaxial layer in the direction orthogonal to the electrode layer A is larger on the end side than on the central part of the stripe.

In the semiconductor laser device of the present invention, the resistance per unit area of the epitaxial layer has the distribution as described above, whereby the injection current can be prevented from becoming high at both ends of the stripe. As a result, the deterioration phenomenon at both ends of the stripe can be prevented. The principle will be described later.
Further, since it is not necessary to bond a wire for conducting current in the stripe, the active layer under the stripe is not damaged, and the output characteristics and lifetime of the laser are not reduced.

  In the present invention, it is preferable that the resistance per unit area of the epitaxial layer in the direction perpendicular to the electrode layer A is increased from the central part of the stripe toward the end side. By doing so, the effect of making the injection current uniform and preventing the deterioration phenomenon at both ends of the stripe becomes even more remarkable. Note that the resistance per unit area of the epitaxial layer may change continuously or stepwise as it goes from the central part of the stripe to the end side.

  In the present invention, it is preferable that the resistance per unit area of the epitaxial layer in the direction orthogonal to the electrode layer A follows a quadratic function distribution having a minimum value in the central portion of the stripe. By doing so, the effect of making the injection current uniform and preventing the deterioration phenomenon at both ends of the stripe becomes even more remarkable.

  As the form in which the resistance per unit area of the epitaxial layer has the above distribution, for example, the film thickness of the entire epitaxial layer or one or more layers constituting the epitaxial layer is larger than the central portion of the stripe. , And a large one at the end side. In particular, it is preferable that the film thickness of the entire epitaxial layer or one or more layers constituting the epitaxial layer increase as it goes from the central part of the stripe to the end side.

  In addition, as another form, for example, one in which the resistivity of the entire epitaxial layer or one or more layers constituting the epitaxial layer is larger on the end side than the central portion of the stripe can be cited. In particular, it is preferable that the resistivity of the whole epitaxial layer or one or more layers constituting the epitaxial layer increases as it goes from the central part of the stripe to the end side.

2 is a perspective view illustrating a configuration of a semiconductor laser element 200. FIG. It is explanatory drawing showing the film thickness distribution of the contact layer 20f. 6 is an explanatory diagram illustrating a method for manufacturing the semiconductor laser element 200. FIG. 3 is an equivalent circuit diagram showing an operating state of the semiconductor laser element 200. FIG. 3 is a graph showing a resistance distribution of a semiconductor layer 100. 4 is a graph showing a distribution of injected current flowing through a semiconductor layer 100. 2 is a perspective view illustrating a configuration of a semiconductor laser element 200. FIG. 6 is a process diagram illustrating a method of manufacturing the semiconductor laser element 200. FIG.

  An embodiment of the present invention will be described.

1. Configuration of Semiconductor Laser Element 200 The configuration of the semiconductor laser element 200 will be described with reference to FIG. The semiconductor laser device 200 includes an epitaxial layer 20 including an N-type cladding layer 20a, an optical guide layer 20b, an active layer 20c, an optical guide layer 20d, a P-type cladding layer 20e, and a contact layer 20f on the upper surface of the N-type substrate 10. The insulating layer 30 and the P-type electrode 40 are stacked, while the N-type electrode 50 and the solder layer 60 are stacked on the back surface of the N-type substrate 10.

  Here, the N-type substrate 10 is an N-type GaAs substrate. The N-type cladding layer 20a is an N-type AlGaAs cladding layer. The light guide layer 20b is an AlGaAs light guide layer (N-type or undoped). The active layer 20c includes an AlGaAs / GaAs multiple quantum well active layer (an AlGaAs / GaAs multiple quantum well (MQW: Multi Quantum Well formed by alternately laminating an undoped AlGaAs barrier layer and an undoped GaAs well layer). Well) active layer). The light guide layer 20d is an AlGaAs light guide layer (P-type or undoped). The P-type cladding layer 20e is a P-type AlGaAs cladding layer. The contact layer 20f is a P-type GaAs contact layer.

  In the insulating layer 30, the central portion in the left-right direction (the left-right direction in FIG. 1 and hereinafter referred to as the stripe width direction) is cut away, and the P-type electrode 40 is formed in the epitaxial layer 20 in this central portion. (In particular, the contact layer 20f). A portion of the P-type electrode 40 that is in contact with the epitaxial layer 20 is a stripe 41. Let L be the width of the stripe 41 in the stripe width direction. The resonator length of the active layer 20c is W. In the P-type electrode 40, both sides of the stripe 41 (hereinafter referred to as a current injection portion 42) are separated from the epitaxial layer 20 by the insulating layer 30. Gold wire 70a, 70b for current injection is bonded to the current injection portion 42. The N-type substrate 10 and the epitaxial layer 20 are combined to form a semiconductor layer 100.

  Here, among the layers constituting the epitaxial layer 20, the refractive index of the active layer 20c is the highest, the refractive indexes of the light guide layers 20b and 20d are lower than the refractive index of the active layer 20c, and the N-type cladding layer 20a. The refractive index of the P-type cladding layer 20e is lower than that of the light guide layers 20b and 20d. Thereby, light can be confined in the active layer 20c.

2. Next, the thickness t of the contact layer 20f (hereinafter referred to as the contact layer thickness t) will be described with reference to FIG. As shown in FIG. 2, the contact layer thickness t is larger on the end E side than the central portion C of the stripe 41. More specifically, the contact layer thickness t increases as the distance from the central portion C of the stripe 41 toward the end portion E increases. More specifically, the contact layer thickness t follows a quadratic function distribution having a minimum value in the central portion C of the stripe 41.

In FIG. 2, only half of the contact layer 20f is shown, but the film thickness t of the contact layer 20f is set similarly in the other half not shown.
3. Manufacturing Method of Semiconductor Laser Element 200 Next, a manufacturing method of the semiconductor laser element 200 will be described. First, an N-type substrate 10 is prepared. Then, an epitaxial layer 20 composed of the N-type cladding layer 20a, the light guide layer 20b, the active layer 20c, the light guide layer 20d, the P-type cladding layer 20e, and the contact layer 20f is epitaxially grown on the upper surface of the N-type substrate 10.

  Epitaxial growth methods include methods such as liquid-phase epitaxy (LPE), molecular-beam epitaxy (MBE), and metal-organic chemical vapor epitaxy (MOCVD). is there. Further, as the structure of the active layer 20c, for example, a double hetero structure, a quantum well structure, or the like may be employed.

  The thickness distribution of the contact layer thickness t can be formed as follows. As shown in FIG. 3A, after the epitaxial layer 20 is formed, a photoresist layer 300 is formed on the upper surface of the contact layer 20f. The line width of the opening in the mask used for exposure is wide at the portion corresponding to the central portion of the stripe 41 and is narrowed as it approaches both ends of the stripe 41. As a result, the photoresist layer 300 is widely removed at the portion corresponding to the central portion of the stripe 41, and the width of the removed portion becomes narrower as the both ends of the stripe 41 are approached.

  Next, as shown in FIG. 3B, when dry etching is performed, an etching rate difference due to the microloading effect occurs, and an etching shape having a deep central portion and shallow both end portions can be realized.

  Next, as shown in FIG. 3C, after removing the photoresist layer 300, the remaining portion is removed by isotropic etching, thereby forming the thickness distribution of the contact layer thickness t as described above. To do.

  Thereafter, the insulating layer 30 is formed on the upper surface of the contact layer 20f, and the P-type electrode 40 is formed by an electron beam evaporation method, a sputtering method, or the like. When patterning is required, it is processed into a predetermined pattern using photoresist processing, chemical etching, ion beam etching, or the like.

  Next, in order to facilitate cleavage during chip formation, the back surface of the N-type substrate 10 is polished and thinned. Note that this is not the case when a reflective surface is produced by dry etching without using a cleavage plane.

  Next, the N-type electrode 50 is formed on the back surface of the N-type substrate 10 to a predetermined thickness by an electron beam evaporation method, a sputtering method, or the like. After film formation, annealing is performed to form the N-type electrode 50. Next, the solder layer 60 is formed on the back surface of the N-type electrode 50. When a thin film by vapor deposition is used as the solder layer 60, the thin film is formed by a method such as electron beam vapor deposition, resistance heating vapor deposition, or sputtering.

  The semiconductor laser device 200 thus fabricated is bonded and mounted (die-bonded) on a heat sink (not shown), another semiconductor substrate, a circuit substrate, or the like. Next to the die bonding, in order to make electrical contact with the semiconductor laser element 200, the P-type electrode 40 and the drive circuit wiring are bonded with gold wire wires 70a and 70b such as Au and Pt. At this time, wire bonding is applied to the current injection portions 42 located on both sides of the stripe 41. It is not preferable to bond a wire in the stripe 41 because the epitaxial layer 20 is damaged and the semiconductor laser device 200 is deteriorated. Thereafter, the semiconductor laser element 200 is encapsulated as necessary.

The manufacturing method of the semiconductor laser element 200 is not limited to the above-described manufacturing method, and various methods can be adopted without departing from the gist of the present invention.
4). Effect produced by the semiconductor laser element 200 A current flowing through the semiconductor layer 100 in the thickness direction is defined as an injection current. In the semiconductor laser device 200, the variation of the injected current in the stripe width direction is small. That is, in the conventional semiconductor laser element, the injection current is small at the center of the stripe 41 and increases as it approaches the end of the stripe 41, but in the semiconductor laser element 200, the difference is small. This will be described with reference to FIGS.

FIG. 4 is an equivalent circuit diagram from the center portion to the end portion of the stripe 41 of the semiconductor laser element 200. 4, 0, 1,..., N are coordinates in the stripe width direction, 0 is a coordinate at the center of the stripe 41, and n is a coordinate at the end of the stripe 41. In FIG. m is an arbitrary value from 0 to n. R _L is a resistance corresponding to the length ΔL of the P-type electrode 40 in the stripe width direction. I _m (m = 0, 1,... N) is a current flowing through the P-type electrode 40 in the stripe width direction at the coordinate m. R _m (m = 0, 1,... N) is a resistance in the thickness direction corresponding to ΔL in the portion of the coordinate m in the semiconductor layer 100. V _m (m = 0, 1,... N) is a voltage applied to the resistor R _m . Here, the current in the thickness direction of the semiconductor layer 100 at the coordinate m is expressed as V _m / R _m .
From the equivalent circuit of FIG. 4, the following equations (1) and (2) are obtained.

In the above equations (1) and (2), assuming that the current V _m / R _m (injection current) in the thickness direction of the semiconductor layer 100 is equal at coordinates 0 to _n , R _m is calculated to obtain the following equations (3) and (4 ) Is required.

Since the range of m is 0 to n, it can be seen that f (m) increases in a quadratic function as n increases (in the stripe width direction, from the center to the end). Accordingly, since the equivalent circuit diagram of FIG. 4 represents the end portion (coordinate 0) to the central portion (coordinate n) of the stripe 41, the resistance distribution of R _m is the lowest in the center portion of the stripe 41, and It becomes higher as a quadratic function as it approaches both ends.

FIG. 5 shows a schematic diagram of R _m when it is assumed that the injection current is constant regardless of the position in the stripe width direction. The distribution of R _{m in} the stripe width direction shown in FIG.

From the above results, it can be seen that if the distribution of R _m is the distribution D, the injection current is constant regardless of the position in the stripe width direction.
In the semiconductor laser element 200, as described above, the contact layer thickness t increases as the distance from the central portion C of the stripe 41 toward the end portion E increases. Therefore, R _m in the semiconductor laser element 200 increases as the distance from the central portion C of the stripe 41 to the end portion E thereof is the same as the distribution D. Therefore, the variation of the injection current in the stripe width direction is small. In particular, if the distribution of the contact layer thickness t in the stripe width direction is set so that R _m matches the distribution D, the injection current can be set regardless of the position in the stripe width direction as shown by the solid line in FIG. , Can be constant. On the other hand, if R _m is constant regardless of the position in the stripe width direction as in the conventional semiconductor laser element, the injection current is small at the center in the stripe width direction as shown by the wavy line in FIG. It gets bigger in the department.

The distribution of the injected current in the stripe width direction can be variously changed by setting the distribution of the contact layer thickness t in the stripe width direction.
For example, in contrast to the distribution D, high R _m at both ends of the stripe 41, by setting the distribution in the stripe width direction of the contact layer thickness t as R _m is lower at the central portion, the injection current, It becomes higher at the center of the stripe 41. Moreover, in contrast to the distribution D, low R _m at both ends of the stripe 41, by setting the distribution in the stripe width direction of the contact layer thickness t as R _m is higher at the central portion, the injection current, It becomes lower at the center of the stripe 41.

5). Modification In this embodiment, the contact layer thickness t has a thickness distribution. For example, any one of the N-type cladding layer 20a, the P-type cladding layer 20e, and the N-type substrate 10, or Two or more may have the same film thickness distribution as that of the contact layer 20f. In that case, since the N-type cladding layer 20a and the P-type cladding layer 20e have a role of confining light, it is necessary to have a thickness that does not affect the confinement of light. Further, the entire semiconductor layer 100 may have the same film thickness distribution as that of the contact layer 20f.

  The configuration and operation of the semiconductor laser element 200 in this embodiment are basically the same as those in the first embodiment, but are partially different. Hereinafter, the difference will be mainly described, and the description of the same part will be omitted or simplified.

In the semiconductor laser device 200 of this embodiment, as shown in FIG. 7, the thickness of each layer constituting the semiconductor layer 100 is uniform, and the thickness of the semiconductor layer 100 is also uniform.
However, in the semiconductor laser device 200 of the present embodiment, the resistivity (resistance per unit thickness) of the semiconductor layer 100 is larger on the end E side than the center C of the stripe 41. More specifically, the resistivity of the semiconductor layer 100 increases as it goes from the central portion C of the stripe 41 to the end portion side E thereof. More specifically, the resistivity of the semiconductor layer 100 follows a quadratic function distribution having a minimum value in the central portion C of the stripe 41.

  The resistivity distribution of the semiconductor layer 100 as described above can be realized by decreasing the doping carrier concentration of the semiconductor layer 100 from the central portion C of the stripe 41 to the end portion side E thereof.

Next, a method for manufacturing the semiconductor laser device 200 of this embodiment (particularly, a method for controlling the doping carrier concentration) will be described with reference to FIG.
In the ion implantation method, ions can be implanted almost uniformly into a layer from the substrate surface to a desired depth by changing the energy of the implanted ions. Therefore, the resistivity of the semiconductor layer 100 can be changed to a desired distribution by performing ion implantation by such a method a plurality of times.

  For example, as shown in FIG. 8, with respect to a predetermined layer constituting the semiconductor layer 100, the first to third ion implantations are performed on the central portion in the stripe width direction, and 3 × Only the second ion implantation is performed, and the second and third ion implantations are performed on the intermediate portion between the center portion and both end portions. Thereafter, when activation annealing that also serves as doping carrier diffusion is performed, the distribution of the doping carrier concentration becomes gentle and becomes smaller from the central portion C of the stripe 41 toward the end side E thereof. By this method, a desired doping carrier concentration distribution (that is, resistivity distribution) can be obtained.

  The predetermined layer whose resistivity is changed by the ion implantation method may be any one of the contact layer 20f, the N-type cladding layer 20a, the P-type cladding layer 20e, and the N-type substrate 10, or two or more. it can. Further, as described above, a doping carrier concentration distribution (that is, a resistivity distribution) may be formed for the entire semiconductor layer 100.

In addition, this invention is not limited to the said embodiment at all, and it cannot be overemphasized that it can implement with a various aspect in the range which does not deviate from this invention.
For example, the semiconductor laser element 200 may be a semiconductor laser element using a P-type substrate. The present invention can also be applied to a semiconductor laser using other semiconductor materials such as AlGaInAs and InGaAsP.

  Further, a method of forming a film thickness distribution as in the first embodiment and a method of forming a doping carrier concentration distribution as in the second embodiment may be used in combination. For example, in the predetermined layer (which may be a single layer or a plurality of layers) constituting the semiconductor layer 100, a film thickness distribution is formed as in the first embodiment, and the implementation is performed. As in Example 2, a doping carrier concentration distribution may be formed. In addition, a film thickness distribution is formed in one or more layers constituting the semiconductor layer 100 as in the first embodiment, and a doping carrier concentration distribution is formed in other layers as in the second embodiment. May be.

10 ... N-type substrate, 20 ... Epitaxial layer, 20a ... N-type cladding layer,
20b: light guide layer, 20c: active layer, 20d: light guide layer,
20e ... P-type cladding layer, 20f ... contact layer, 30 ... insulating layer,
40 ... P-type electrode, 41 ... stripe, 42 ... current injection part,
50 ... N-type electrode, 60 ... solder layer, 70a ... gold wire,
DESCRIPTION OF SYMBOLS 100 ... Semiconductor layer, 200 ... Semiconductor laser element, 300 ... Photoresist layer

Claims (5)

A pair of electrode layers A and B,
An epitaxial layer formed between the pair of electrode layers A and B and including an active layer, a light guide layer, and a cladding layer;
The electrode layer A includes a stripe that is electrically connected to the epitaxial layer, and a current injection portion that is located on both sides of the stripe and in which an insulating layer is interposed between the epitaxial layer,
The semiconductor laser device according to claim 1, wherein a resistance per unit area of the epitaxial layer in a direction orthogonal to the electrode layer A is larger at an end portion side than a central portion of the stripe.   2. The semiconductor laser device according to claim 1, wherein a resistance per unit area of the epitaxial layer in a direction orthogonal to the electrode layer A increases from a central portion of the stripe toward an end thereof.   3. The semiconductor laser according to claim 1, wherein a resistance per unit area of the epitaxial layer in a direction orthogonal to the electrode layer A follows a quadratic function distribution having a minimum value in a central portion of the stripe. element.   4. The film thickness of one or more layers constituting the entire epitaxial layer or the epitaxial layer is larger on the end side than the central portion of the stripe. 5. Semiconductor laser device.   5. The resistivity of one or more layers constituting the entire epitaxial layer or the epitaxial layer is larger on the end side than the central portion of the stripe. 6. Semiconductor laser device.