JP2008047692A - Self-induced oscillating semiconductor laser and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a self-induced oscillating semiconductor laser which stably APC drives. <P>SOLUTION: There is provided a semiconductor layer 20, which is formed of an n-type clad layer 21, an active layer 22, a p-type clad layer 23 that includes a stripe shaped embedded ridge portion 28, an n-type buffer layer 24, a p-type contact layer 25, an etching stop layer 26, and a gap layer 27, laminated in this order. In the semiconductor layer 20, an emission side reflection film 33 is formed on a front end face perpendicular to a direction in which the embedded ridge 28 extends, and reflectivity Rf of the front end face is adjusted so as to satisfy P2<Pop<P1. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a self-pulsation type semiconductor laser that self-oscillates by interaction between a gain region that amplifies emitted light emitted by injected carriers and a saturable absorption region that absorbs light emitted from the gain region. About.

  In recent years, self-pulsation type semiconductor lasers (pulsation lasers) have attracted attention as low-noise semiconductor lasers (LDs). The self-excited oscillation type semiconductor laser is a laser that oscillates while undergoing self-excited oscillation, has low coherence and low return light noise, and is particularly useful for optical disc applications. This self-excited oscillation type semiconductor laser includes, for example, two p-side electrodes separated in the direction of the resonator as described in Patent Document 1, and one p-side electrode is grounded (grounded). Or by applying a reverse bias to the p-side electrode and applying a forward bias to the other p-side electrode, a saturable absorption region is formed in a region corresponding to the first electrode, and a region corresponding to the second electrode. Gain regions are formed, and self-oscillation is caused by interaction between these regions.

  In a self-excited oscillation type semiconductor laser, the level of light output decreases due to self-excited oscillation. Therefore, if the reflectance on the front end face side is increased, it becomes difficult to extract laser light from the front end face. For this reason, the reflectance on the front end face side is generally set to be equal to or lower than the reflectance of the cleavage plane.

JP 2004-186678 A

  By the way, in an optical disc application, APC (Auto Power Control) driving that keeps the light output constant by controlling the current value is usually performed. In order to perform the APC driving stably, the driving power in the IL characteristic is used. It is essential that the fluctuation (ΔL / ΔI) of the light output with respect to the fluctuation of the nearby current is gentle.

  However, since a self-pulsation type semiconductor laser generally has a very steep kink in the vicinity of the threshold current in the IL characteristic, the drive power has a power at the inflection point of the kink. If it falls below, the change in the optical output with respect to the change in the current becomes large, and there is a problem that the APC drive becomes unstable.

  The present invention has been made in view of such problems, and an object thereof is to provide a self-pulsation type semiconductor laser capable of performing stable APC driving and a method for manufacturing the same.

  A self-pulsation type semiconductor laser according to the present invention includes a semiconductor layer formed by laminating a first conductivity type layer, an active layer, and a second conductivity type layer including a stripe-shaped current confinement structure thereon in this order. is there. An outgoing side reflective film is formed on the front end surface of the semiconductor layer perpendicular to the extending direction of the current confinement structure, and the reflectance on the front end surface side is adjusted to satisfy P2 <Pop <P1. .

  Here, Pop is the driving power, and P1 is the kink of the kink generated in the vicinity of the threshold current of the IL characteristic when the exit-side reflecting film is not provided on the front end face, that is, when the front end face is a cleavage plane. The power at the inflection point, P2, represents the power at the inflection point of the kink that occurs in the vicinity of the threshold current of the IL characteristic when the output side reflection film is provided on the front end face.

  In the self-excited oscillation type semiconductor laser of the present invention, the reflectance on the front end face side is adjusted so as to satisfy P2 <Pop <P1, and therefore, an extremely high rise that occurs near the threshold current in the IL characteristic. The power at the inflection point of the steep kink becomes lower than the driving power.

The self-pulsation type semiconductor laser manufacturing method of the present invention includes the following steps.
(A) A semiconductor layer is formed by laminating a first conductivity type layer, an active layer, and a second conductivity type layer including a stripe-shaped current confinement structure on the substrate in this order, and then extending the current confinement structure. Step (B) of forming a pair of cleavage planes perpendicular to the current direction Step (C) of measuring the IL characteristics of the front end face among the cleavage planes (C) P2 based on the IL characteristics obtained by the measurement <Pop <P1> The process of forming the output side reflective film of the reflectance which satisfy | fills on the front end surface

  In the method for manufacturing a self-excited oscillation type semiconductor laser according to the present invention, an output-side reflective film satisfying P2 <Pop <P1 based on the IL characteristic obtained by measurement is formed on the front end face. For this reason, the power at the inflection point of the kink that occurs in the vicinity of the threshold current in the IL characteristic and has a very steep rise becomes lower than the drive power.

  According to the self-excited oscillation type semiconductor laser of the present invention and the manufacturing method thereof, the reflectance at the front end face side is adjusted so as to satisfy P2 <Pop <P1, and therefore the power at the inflection point of the kink is It becomes less than the driving power. As a result, the slope of the IL characteristic (ΔL / ΔI) in the vicinity of the drive power, that is, the change in the optical output with respect to the change in the current becomes moderate, and the APC drive can be stabilized.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a perspective view showing the structure of a semiconductor laser 1 according to an embodiment of the present invention. 2 shows a cross-sectional configuration in the direction of arrows AA in FIG. 1, and FIG. 3 shows a cross-sectional configuration in the direction of arrows BB. Moreover, FIG. 1 thru | or FIG. 3 is represented typically and differs from an actual dimension and shape. This semiconductor laser 1 is a self-excited oscillation type semiconductor laser having two electrodes arranged in a cavity direction on a ridge portion.

  The semiconductor laser 1 is obtained by growing a semiconductor layer 20 on a substrate 10. This semiconductor layer 20 is a laser in which an n-type cladding layer 21, an active layer 22, a p-type cladding layer 23, an n-type buffer layer 24, a p-side contact layer 25, an etching stop layer 26, and a cap layer 27 are stacked in this order. It has a structure. Here, the n-type cladding layer 21 is the “first conductivity type layer” of the present invention, and the p-type cladding layer 23, the p-side contact layer 25, the etching stop layer 26 and the cap layer 27 are the “second conductivity type” of the present invention. It corresponds to each "layer". Hereinafter, a direction in which the semiconductor layers are stacked is referred to as a vertical direction, a laser beam emission direction is referred to as an axial direction, and a direction perpendicular to the axial direction and the vertical direction is referred to as a horizontal direction.

  Here, the semiconductor layer 20 is not particularly limited, but is made of, for example, an AlGaAs-based (780 nm band), AlGaInP-based (650 nm band), or AlGaInN-based (405 nm band) compound semiconductor. In addition, each said layer is n type impurity which consists of IV group and VI group elements, such as Si (silicon), Ge (germanium), O (oxygen), Se (selenium), or Mg (magnesium), as needed. It contains p-type impurities composed of Group II and Group IV elements such as Zn (zinc) and C (carbon).

  A layer including the p-type cladding layer 23, the n-type buffer layer 24, the p-side contact layer 25, the etching stop layer 26, and the cap layer 27 has a strip-shaped buried ridge portion 28 ( Current confinement structure). A strip-shaped ridge is formed in the p-type cladding layer 23, and an n-type buffer layer 24 and a p-side contact layer 25 are laminated in this order so as to cover the ridge. An etching stop layer 26 and a cap layer 27 are stacked in this order from the side surface of the ridge of the p-side contact layer 25 to the surface where the ridge is not formed.

  On the semiconductor layer 20, a strip-shaped groove 29 extending perpendicularly to the axial direction across the buried ridge 28 is provided. The groove 29 is for preventing the p-side electrode 30 and the p-side electrode 31 from being electrically short-circuited with each other.

  Further, of the surfaces of the p-side contact layer 25 and the cap layer 27, a p-side electrode 30 is formed on the later-described outgoing-side reflective film 33 side with reference to the groove portion 29, and a later-described rear-side reflective film 34 side has the p-side electrode 31 is formed. The p-side electrode 30 and the p-side electrode 31 have a structure in which, for example, Ti (titanium), Pt (platinum), and Au (gold) are stacked in this order.

  The p-side electrode 31 is electrically connected to a portion of the p-side contact layer 25 and the cap layer 27 on the rear reflective film 34 side, and a direct current source (not shown) through a wire (not shown). ) To supply a current by forward bias to the semiconductor layer 20. As a result, the p-side electrode 31 has a role of causing laser oscillation with respect to the semiconductor layer 20. Therefore, when a portion of the p-side electrode 31 that is in electrical contact with the p-side contact layer 25 is a contact portion 31 </ b> A, A region corresponding to the contact portion 31A in the active layer 22 functions as a so-called gain region 22a.

  On the other hand, the p-side electrode 30 is electrically connected to a portion of the p-side contact layer 25 and the cap layer 27 on the output-side reflective film 33 side and is connected to a DC voltage source (not shown) via a wire (not shown). And a voltage by zero bias or reverse bias is applied to the semiconductor layer 20. Accordingly, the p-side electrode 30 has a role of causing self-excited oscillation with respect to the semiconductor layer 20, and therefore, a portion of the p-side electrode 30 that is in electrical contact with the p-side contact layer 25 is referred to as a contact portion 30A. In the active layer 22, a region corresponding to the contact portion 30A functions as a so-called saturable absorption region 22b.

  Here, the area of the contact portion 30A is set within a range in which the self-excited oscillation in the semiconductor layer 20 can be continued, and the length in the axial direction is, for example, 1 μm to 100 μm. On the other hand, the area of the contact portion 31A is set within a range in which laser oscillation in the semiconductor layer 20 can be continued, and its axial length is, for example, 150 μm to 1 mm.

  Further, the contact portion 30A only needs to be in a region sandwiched between the resonators composed of the emission-side reflection film 33 and the rear-side reflection film 34, and therefore, the contact portion 30A is formed corresponding to any portion above the embedded ridge portion 28. However, as in the present embodiment, it is preferable that the upper portion of the embedded ridge portion 28 is formed so as to correspond to the emission-side reflection film 33 side.

Of the side surface (cleavage surface) perpendicular to the extending direction (axial direction) of the embedded ridge portion 28, the output side reflection film 33 is formed on the front end surface, and the rear side reflection film 34 is formed on the rear end surface. The output side reflection film 33 and the rear side reflection film 34 are, for example, SiO ₂ (silicon oxide), Al ₂ O ₃ (alumina), α-Si (amorphous silicon), TiO ₂ (titanium oxide), TiNbO ₂ (niobium titanate). ), HfO ₂ (hafnium oxide) and ZrO ₂ (zirconium oxide). Here, the output-side reflecting film 33 is adjusted so that the reflectance Rf on the front end face side is higher than the reflectance R1 on the cleavage plane, and the rear reflecting film 34 has a reflectance Rr on the rear end face side of the front end. It is adjusted to be higher than the reflectance Rf on the surface side. That is, the reflectance Rf on the front end face side and the reflectance Rr on the rear end face side are adjusted so as to satisfy the following expressions. As a result, the light generated in the gain region of the active layer 22 is amplified by reciprocating between the pair of emission-side reflection films 33 and the rear-side reflection film 34, and as a beam from the emission-side reflection film 33 and the rear-side reflection film 34. It comes to be injected.

R1 <Rf <Rr (1)

  On the other hand, an n-side electrode 32 is provided on the entire back surface of the substrate 10 and is electrically connected to the substrate 10 and the n-type cladding layer 21. The n-side electrode 32 has, for example, a structure in which an alloy of Au and Ge (germanium), Ni (nickel), and Au are laminated in this order. Since the n-side electrode 32 is electrically connected to the heat sink when the semiconductor laser 1 is mounted on the heat sink (not shown), it has the same potential as the ground (not shown) electrically connected to the heat sink. (Zero volts).

  The self-excited oscillation type semiconductor laser 1 having such a configuration can be manufactured as follows.

  4A and 4B show the manufacturing method in the order of steps. In order to manufacture the self-excited oscillation type semiconductor laser 1, the semiconductor layer 20A on the substrate 10A is formed by, for example, the MOCVD (Metal Organic Chemical Vapor Deposition) method.

  Specifically, first, an n-type cladding layer 21A, an active layer 22A, and a p-type cladding layer 23A are stacked in this order on the substrate 10A (see FIG. 4A).

  Next, a photoresist is formed on the p-type cladding layer 23A, and a photoresist layer (not shown) is formed in a region where a ridge is to be formed based on the photolithography technique. Subsequently, part of the p-type cladding layer 23A is selectively removed by dry etching using the photoresist layer as a mask. Thereafter, the photoresist layer is removed. Thereby, a ridge is formed in the p-type cladding layer 23A.

  Next, the n-type buffer layer 24A, the p-side contact layer 25A, the etching stop layer 26A, and the cap layer 27A are stacked in this order on the entire surface. Next, the entire surface is polished until the p-side contact layer 25A is exposed. As a result, a strip-shaped buried ridge portion 28 extending in the axial direction is formed (see FIG. 4B).

  Next, a photoresist is formed on the entire surface, and a photoresist layer (not shown) having an opening in a region where the groove 29 is to be formed is formed based on the photolithography technique. Subsequently, the region where the groove 29 is to be formed is increased in resistance by, for example, an etching method. Thereafter, the photoresist layer is removed.

  Next, a photoresist is formed over the entire surface, and a photoresist layer (not shown) is formed in a region where the groove 29 is to be formed based on the photolithography technique. Subsequently, Ti, Pt, and Au are laminated in this order by using, for example, a vapor deposition method. Thereafter, the photoresist layer is removed. Thereby, the p-side electrode 30 is formed on the front end face side, and the p-side electrode 31 is formed on the rear end face side.

  Next, if necessary, the back surface of the substrate 10 is polished, and Au, Ge, Ni, and Au are laminated in this order on the surface by using, for example, a vapor deposition method. Thereby, the n-side electrode 32 is formed. Next, after forming a pair of cleaved surfaces (front end surface and rear end surface) perpendicular to the extending direction of the embedded ridge portion 28, the rear reflective film 34 is formed on the rear end surface by, for example, sputtering.

  Next, after measuring the IL characteristic on the front end face side in a state where the front end face is a cleavage plane, and determining the reflectance Rf on the front end face side according to the procedure described later, the front end face side has such reflectance Rf. In this manner, the exit side reflection film 33 is formed on the front end face. Then, the self-pulsation type semiconductor laser 1 is manufactured by finally forming a chip.

  In this semiconductor laser 1, when a forward biased current is supplied between the p-side electrode 31 and the n-side electrode 32, the current confined by the buried ridge portion 28 becomes a gain region 22 a (light emission) of the active layer 22. This causes light emission due to recombination of electrons and holes. This light is reflected by the pair of reflecting mirror films, causes laser oscillation at a wavelength at which the phase change when reciprocating once is an integral multiple of 2π, and is emitted to the outside as a beam.

  At this time, when a voltage of zero bias or reverse bias is applied between the p-side electrode 30 and the n-side electrode 32, the emitted light emitted from the gain region 22 a corresponds to the p-side electrode 30 in the active layer 22. It is absorbed in the saturable absorption region 22b and converted into a current (photocurrent). This current is swept out to the outside through a wire (not shown) connected to the p-side electrode 30. The gain region 22a and the saturable absorption region 22b interact to cause self-excited oscillation.

  FIG. 5A shows the IL characteristics of the self-pulsation type semiconductor laser according to the comparative example and the self-pulsation type semiconductor laser 1 of the present embodiment, and FIG. An area surrounded by a broken line in FIG. Here, the self-pulsation type semiconductor laser according to the comparative example is provided with an emission-side reflection film adjusted so that the reflectance Rf is equal to or less than the reflectance R1 of the cleavage plane, and the reflectance Rf is a cleavage plane. This is different from the self-pulsation type semiconductor laser 1 of the present embodiment, which includes the exit-side reflection film 33 adjusted to be greater than the reflectance R1 of the first embodiment.

  In the self-oscillation type semiconductor laser according to the comparative example, as shown in FIGS. 5A and 5B, a very steep kink occurs in the vicinity of the threshold current of the IL characteristic. . At this time, the kink inflection point a may exceed the optical output Pop in the use of the optical disk.

  Here, in a semiconductor laser for an optical disk, APC (Auto Power Control) driving that keeps a light output constant by controlling a current value is usually performed. In order to perform APC driving stably, IL is used. It is essential that the slope (ΔL / ΔI) of the light output with respect to the current value in the characteristics is gentle. However, as shown in FIGS. 5A and 5B, the self-pulsation type semiconductor laser generally has a very steep kink rising in the vicinity of the threshold current in the IL characteristic. Therefore, when the power P1 at the inflection point a of the kink exceeds the drive power Pop, the slope of the IL characteristic (ΔL / ΔI), that is, the change in the optical output with respect to the change in current becomes large, and the APC drive is performed. It becomes unstable.

  On the other hand, in the self-pulsation type semiconductor laser 1 of the present embodiment, the reflectance Rf on the front end face side is made higher than the reflectance R1 of the cleaved surface, so that the power at the inflection point b of the kink is driven. It is also possible to make it lower than the power Pop, and as a result, the slope of the IL characteristic (ΔL / ΔI) in the current range corresponding to the driving power, that is, the change in the light output with respect to the change in the current is made gentle. It becomes possible. As a result, stable APC driving can be performed, and therefore it can be suitably applied to the use of optical discs.

  Hereinafter, a method of formulating the reflectance Rf that can moderate the slope of the IL characteristic (ΔL / ΔI) in the light output Pop will be specifically described.

  When the power P1 at the inflection point a of the kink exceeds the drive power Pop, as shown by the solid lines in FIGS. 5A and 5B, the slope of the IL characteristic in the vicinity of the drive power Pop (ΔL / ΔI) becomes steep.

  However, if the reflectance Rf on the front end face side is gradually increased to reduce the emission power of light emitted from the front end face side, the power at the kink inflection point a also decreases. When the reflectance Rf on the front end face side exceeds a certain value, the power at the inflection point b of the kink becomes lower than the driving power Pop. As a result, by changing the inflection point of the kink in FIG. 5B from a to b, the current corresponding to the drive power Pop as shown by the one-dot chain line in FIG. 5A and FIG. The slope (ΔL / ΔI) of the IL characteristic in this range becomes gentle.

  From this, when the power at the inflection point b of the kink that occurs in the vicinity of the threshold current of the IL characteristic when the output side reflection film is provided on the front end face is P2, the following equation is satisfied. Thus, it can be seen that the reflectance Rf on the front end face side may be established.

P2 <Pop <P1 (2)

  In general, the relationship between the emission power (Pout) and the reflectance Rf on the front end face side is expressed by the following equation.

Pout = (1/2) × (C 0 / n eq) × In (1 / (RfRf)) × {(1-Rf) / ((1-Rf) + (1-Rr))} × S ... ( 3)

Here, C ₀ is the speed of light, n _eq is the transmission refractive index of the active layer 22, hν is the band gap energy of the active layer 22, and W is the axial width of the buried ridge portion 28, d is the thickness of the active layer 22 in the vertical direction, and S is the internal photon density.

  At this time, for example, if the reflectivity Rr on the rear end face side is 80%, the internal photon density S is constant, and the material of the self-excited oscillation semiconductor laser 1 is AlGaAs or AlGaInP, the Pout-Rf curve is shown in FIG. Thus, when the material of the self-excited oscillation type semiconductor laser 1 is AlGaInN, the Pout-Rf curve is as shown in FIG. 6 and FIG. 7, the vertical axis of FIG. 6 and FIG. 7 assumes that the output power from the front end face side is 1 when the front end face is not provided with the exit side reflection film, that is, the front end face is a cleavage plane. The normalized value is shown.

  6 and 7, for example, it is assumed that the light output at the inflection point of the kink when the exit-side reflecting film is not provided on the front end face is 5 mW, and the driving power for APC driving is 4 mW. At this time, from 4 mW / 5 mW = 0.8, it can be seen that the reflectance should be set so that the light output becomes smaller than 0.8 in FIGS. Therefore, when the material of the self-excited oscillation type semiconductor laser 1 is AlGaAs-based or AlGaInP-based, the reflectance Rf on the front end face side may be made larger than 40% as shown in FIG. When the material of the laser 1 is AlGaInN, it can be seen that the reflectance Rf on the front end face side should be larger than 27% as shown in FIG.

  While the present invention has been described with reference to the embodiment, the present invention is not limited to the above embodiment, and various modifications can be made.

  For example, in the above embodiment, two p-side electrodes are provided, but only one may be provided without separation, or three or more may be provided.

  In the above embodiment, the index guide type current confinement structure (embedded ridge portion 28) is provided on the semiconductor layer 20. However, another current confinement structure such as a gain guide type may be provided.

  Moreover, in the said embodiment and modification, although the upper part of the semiconductor layer 20 was made into the p-type and the lower part was made into the n-type polarity, it can also be made into the reverse polarity.

  Further, the present invention is not limited to the manufacturing method specifically described in the above embodiment, and may be another manufacturing method.

1 is a perspective view illustrating a configuration of a self-pulsation type semiconductor laser according to an embodiment of the present invention. It is sectional drawing showing the cross-sectional structure of the AA arrow direction of FIG. It is sectional drawing showing the cross-sectional structure of the BB arrow direction of FIG. It is sectional drawing for demonstrating the manufacturing process of a self-oscillation type semiconductor laser. It is a relationship figure showing an example of an IL characteristic. It is a related figure showing an example of Pout-Rf characteristic. It is a related figure showing other examples of Pout-Rf characteristic.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Self-oscillation type semiconductor laser, 10 ... Substrate, 20 ... Semiconductor layer, 21 ... N-type cladding layer, 22 ... Active layer, 22A ... Gain region, 22B ... Saturable absorption region, 23 ... P-type cladding layer, 24 ... n-type buffer layer, 25 ... p-side contact layer, 26 ... etching stop layer, 27 ... cap layer, 28 ... buried ridge portion, 29 ... groove portion, 30, 31 ... p-side electrode, 30A, 31A ... contact portion, 32... N-side electrode, 33... Exit-side reflecting film, 34.

Claims (4)

A semiconductor layer formed by laminating a first conductivity type layer, an active layer, and a second conductivity type layer including a stripe-shaped current confinement structure thereon in this order;
An emission-side reflective film formed on a front end surface of the semiconductor layer perpendicular to the extending direction of the current confinement structure;
The reflectance on the front end face side is adjusted so as to satisfy P2 <Pop <P1.
Pop: Drive power P1: Power at the inflection point of a kink that occurs in the vicinity of the threshold current of the IL characteristic when the exit-side reflection film is not provided on the front end surface P2: The power on the front end surface The power at the inflection point of the kink that occurs in the vicinity of the threshold current of the IL characteristic when the output side reflection film is provided A rear reflective film formed on the rear end surface of the semiconductor layer perpendicular to the extending direction of the current confinement structure;
2. The self-pulsation type semiconductor laser according to claim 1, wherein reflectivities on the front end face side and the rear end face side are adjusted so as to satisfy R1 <Rf <Rr, respectively.
R1: Cleaved surface reflectance Rf: Front end surface side reflectance Rr: Rear end surface side reflectance The exit side reflective film is made of SiO ₂ (silicon oxide), Al ₂ O ₃ (alumina), α-Si (amorphous silicon), TiO ₂ (titanium oxide), TiNbO ₂ (niobium titanate), HfO ₂ (hafnium oxide). ) And ZrO ₂ (zirconium oxide). The self-pulsation type semiconductor laser according to claim 1, wherein at least one kind is included. A semiconductor layer is formed by laminating a first conductivity type layer, an active layer, and a second conductivity type layer including a stripe-shaped current confinement structure on the substrate in this order, and then the extending direction of the current confinement structure Forming a pair of cleavage planes perpendicular to
Measuring the IL characteristic of the front end face of the cleavage plane;
A step of forming, on the front end face, an exit-side reflecting film having a reflectance satisfying P2 <Pop <P1 based on the IL characteristic obtained by the measurement. Manufacturing method.
Pop: Drive power P1: Power at the inflection point of the kink generated near the threshold current of the IL characteristic when the exit-side reflection film is not provided on the front end face P2: the front end face The power at the inflection point of the kink that occurs in the vicinity of the threshold current of the IL characteristic when the exit-side reflecting film is provided

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