JP2007329487A - Laser element, and optical recording reproducing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor laser element with less spontaneous emission light. <P>SOLUTION: This laser element includes: a substrate; a semiconductor which is formed on the top surface of the substrate while containing active layer and a ridge structure of stripe shape; and a light absorbing part provided above the semiconductor. In the laser element, an oscillating direction of a laser beam is the stripe direction of the ridge structure. The light absorbing part is provided apart from a lower end of the ridge structure in a transverse direction so as not to absorb a basic mode or a higher-order mode of the laser beam, has a characteristics absorbing the light in a wavelength range including the wavelength of the laser beam, and is made of a metal or a semiconductor having a refraction index smaller than that of the above semiconductor. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a laser device made of semi-conductors, in particular, to a structure for suppressing the spontaneous emission light.

  As a light source for the next-generation optical recording / reproducing apparatus, a light source that emits light having a short wavelength capable of further reducing a condensing diameter is used in order to realize a high recording density. However, in order to reduce the cost, it is desirable to use an inexpensive plastic material for a lens in an optical recording / reproducing apparatus or an optical disk as a recording medium. Generally, such a material has an absorption edge of about 390 nm. The wavelength of the light emitted from is required to be around 400 nm. A semiconductor laser element made of a nitride represented by gallium nitride is suitable for such a short wavelength light source.

A typical structure of a nitride semiconductor laser device is disclosed in Patent Document 1 and is as shown in FIG. The laser element includes an N electrode 111, an SiC substrate 101, an AlN buffer layer 102, an n-GaN layer 103, an n-AlGaN cladding layer 104, an InGaN active layer 105, a p-AlGaN cladding layer 106, and a p- A GaN contact layer 107, an Al ₂ O ₃ protective film 109, a P electrode 110, and a SiO ₂ insulating film 112 are provided. The wavelength of the laser beam emitted from this laser element having the active layer 105 made of InGaN is about 405 nm. In the upper part of the laser element, the ridge of the current flowing in the element is narrowed in a direction perpendicular to the resonance direction of light in the active layer 105, as in the case of a general long wavelength laser element. A structure (a convex portion of the p-cladding layer) is formed, and an insulating SiO ₂ film 112 is provided on the side.
JP-A-9-289358

  Nitride semiconductor layers 102 to 107 and SiC substrate 101 in the laser element are almost transparent to the oscillation wavelength, and absorb little spontaneous emission light generated during laser oscillation. It is common to use sapphire as the substrate, and the active layer may be sandwiched from above and below by a light guide layer of nitride semiconductor, and these are also the same. Therefore, the ratio of spontaneous emission light that is emitted together with the laser light from the emission end face of the laser element is high. When the inventor fabricated the laser device having the above structure and measured the output intensity from one of the emission end faces, the intensity of the spontaneous emission light component was 0.5 to 1 mW.

  Since spontaneous emission light has a wide wavelength range (spectral width), noise is large. For this reason, when the laser element is operated at a low output of, for example, 10 mW or less, the spontaneous emission light component is relatively increased and the overall noise is also increased. In addition, when controlling the output of the laser light by the detection current of a monitor PD (photodiode) packaged with a stem or the like, if the ratio of the spontaneous emission light component is large, the fluctuation of the output becomes large and the control becomes difficult. Become. The internal PD installed directly behind the laser element receives a large amount of spontaneous emission light, and the external PD installed via the condensing optical system in front of the laser element has relatively high noise. Because. Furthermore, in a hologram laser, spontaneous emission light emitted from the side surface of the laser element or the like becomes stray light, which can make control difficult.

In a nitride semiconductor laser device having a ridge structure, the refractive index of the insulating film provided on the side of the ridge structure is generally smaller than the refractive index of the nitride semiconductor, and therefore, the active layer is directed to the side of the ridge structure. The spontaneous emission light is easily reflected at the interface between the insulating film and the nitride semiconductor layer. For example, when light is incident from GaN having a high refractive index to SiO ₂ having a low refractive index, total reflection occurs when the incident angle is about 40 ° or more. The spontaneously emitted light reflected in this way returns to the active layer side, and is easily emitted from the emission end face together with the original laser light.

  On the other hand, a semiconductor such as GaAs in a relatively long wavelength laser element currently used as a light source of an optical recording / reproducing apparatus absorbs an oscillation wavelength, and spontaneous emission light at the time of laser oscillation of the current laser element is This is significantly less than that of a nitride semiconductor laser device. If the nitride semiconductor laser element having a large amount of spontaneous emission as described above is used as the light source of the optical recording / reproducing apparatus as it is, fine adjustment of the optical output becomes difficult, and noise characteristics at the time of low output oscillation are reduced.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a nitride semiconductor laser element with less spontaneous emission light. It is another object of the present invention to provide a high performance optical recording / reproducing apparatus including such a laser element.

  In order to achieve the above object, a laser device of the present invention includes a substrate, a semiconductor formed on the upper surface of the substrate and having a stripe-shaped ridge structure including an active layer, and light provided above the semiconductor. A laser element having an absorption portion, wherein the oscillation direction of the laser beam is a stripe direction of the ridge structure, wherein the light absorption portion is a fundamental mode or a higher-order mode of the laser beam in a lateral direction from a lower end of the ridge structure. It is made to be made of a metal or semiconductor having a property of absorbing light in a wavelength range including the wavelength of the laser light and having a refractive index smaller than the refractive index of the semiconductor. .

  In the laser element of the present invention, preferably, the separation distance is 0.5 μm or more and 10 μm or less.

  In this laser element, a part of spontaneous emission light traveling in all directions from the active layer is incident on the light absorbing portion and absorbed. Moreover, since the light absorption part is close to the constricted part of the current path, and therefore close to the part of the active layer that resonates the light to become laser light, there is much spontaneous emission light incident on the light absorption part, and the laser There is little spontaneous emission outside the device. In addition, since the light absorption part absorbs light in a wavelength range including the wavelength of the laser light, the spontaneous emission light emitted outside the laser element has particularly few light having the same wavelength as the laser light. Therefore, the spontaneous emission light that tends to adversely affect the use of laser light is greatly reduced.

  The above laser element includes a ridge structure extending in the resonance direction of light in the active layer and an insulating film located on the side of the ridge structure and in contact with the ridge structure in order to form a narrowed portion of the current path. The absorber may be configured to be located between the insulating film and the active layer. The formation of a narrowed portion in the current path by the ridge structure and the insulating film is an established and excellent method, but as described above, depending on the relationship between the refractive index of the insulating film and the nitride semiconductor layer, the interface between the two The spontaneous emission light reflected by the laser beam increases, and the reflected spontaneous emission light is easily emitted from the emission end face together with the laser light. However, in this configuration in which the light absorbing portion is located between the insulating film and the active layer, the spontaneous emission light is absorbed before reaching the insulating film, and is reflected by the insulating film and emitted along with the laser light. There is little emitted light.

  In order to form a constricted portion of the current path, the active layer includes a ridge structure extending in the resonance direction of light in the active layer, and an insulating film located on the side of the ridge structure and in contact with the ridge structure. An intermediate layer may have a groove, and the light absorption portion may fill the groove of the layer between the insulating film and the active layer. In this configuration, by deepening the groove of the layer between the insulating film and the active layer, the light absorption part can be brought close to the active layer, and spontaneous emission light having a small angle with respect to the active layer can be absorbed.

  In order to form a constricted portion of the current path, the active layer includes a ridge structure extending in the light resonance direction and an insulating film located on the side of the ridge structure and in contact with the ridge structure. It is also possible to adopt a configuration that is located on the side of the insulating film and in contact with the insulating film. In this configuration, it is not necessary to process a portion deeper than the lower end of the ridge structure, and manufacturing is easy. Since the light absorbing portion is an insulator, it can also be used to form a narrowed portion in the current path, and the width of the insulating film located on the side of the ridge structure can be reduced. Accordingly, much of the spontaneous emission light can be absorbed by the light absorbing portion, similarly to the configuration in which the light absorbing portion is located between the insulating film and the active layer.

  It is preferable that the absorption coefficient κ of the light absorption unit with respect to light having a wavelength of laser light generated in the active layer is 0.1 or more. If it does in this way, it is not necessary to make a light absorption part very thick, and manufacture becomes easy.

The distance between the narrowed portion of the current path and the light absorbing portion is 0. It should be 5 μm or more. If the light absorption part is too close to the current path, the threshold current for laser oscillation increases, but if this distance is reached, the increase in threshold can be kept low.

  In the present invention, an optical recording / reproducing apparatus for recording and reproducing information by guiding light from a light source to an optical recording medium is provided with any of the above laser elements as a light source. Since the laser beam has a short wavelength and can form minute spots, the recording density is high and the noise due to spontaneous emission is low, so that information can be recorded and reproduced accurately, and the laser Fine adjustment of the light intensity is also facilitated.

  A laser element made of a nitride semiconductor, wherein one portion of a current path in the element reaching an active layer that generates laser light is constricted in a direction perpendicular to the resonance direction of light in the active layer As in the present invention, it has a characteristic of absorbing light in a wavelength range including the wavelength of laser light generated in the active layer, and is slightly separated from the constricted portion of the current path. If a light absorption part extending in the resonance direction is provided, a part of spontaneous emission light can be absorbed by the light absorption part, and inconveniences such as noise and difficulty in control due to spontaneous emission light are reduced. be able to.

  In particular, in a configuration including a ridge structure extending in the resonance direction of light in the active layer and an insulating film located on the side of the ridge structure and in contact with the ridge structure in order to form a narrowed portion of the current path, Spontaneous emission light reflected from the interface of the nitride semiconductor layer and emitted from the emission end face together with the laser light can be reduced, and while taking advantage of the configuration, inconvenience caused by the spontaneous emission light emitted together with the laser light. It can be certainly reduced.

  When the absorption coefficient κ of the light absorption part with respect to the light having the wavelength of the laser light generated in the active layer is 0.1 or more, it is not necessary to make the light absorption part very thick and the manufacture is easy.

  If the distance between the narrowed portion of the current path and the light absorbing portion is 0.3 μm or more, the increase in the laser oscillation threshold can be suppressed low, the increase in driving power can be avoided, and the deterioration of the element can also be suppressed. .

  In the optical recording / reproducing apparatus provided with the laser element of the present invention as a light source, the laser light has a short wavelength and can form a minute spot. Therefore, the recording density is high and noise due to spontaneous emission light is small. Information can be recorded and reproduced accurately, and the fine adjustment of the intensity of the laser beam is facilitated.

  Embodiments of a nitride semiconductor laser element and an optical recording / reproducing apparatus according to the present invention will be described below with reference to the drawings. The configuration of the laser device 1 of the first embodiment is schematically shown in the longitudinal sectional view of FIG. The laser element 1 includes an N electrode 10, an n-GaN substrate 11, an n-GaN layer 12, an n-InGaN crack prevention layer 13, an n-AlGaN cladding layer 14, an n-GaN guide layer 15, an n-InGaN active layer 16, A p-AlGaN barrier layer 17, a p-GaN guide layer 18, a p-AlGaN cladding layer 19, a p-GaN contact layer 20, an insulating film 21, an absorption film 22, and a P electrode 23.

  The nitride semiconductor layers from the n-GaN layer 12 to the p-GaN contact layer 20 are stacked in this order on the upper surface of the n-GaN substrate 11, of which the upper portion of the p-cladding layer 19 and the p-contact layer are stacked. Reference numeral 20 denotes a striped ridge structure. FIG. 1 shows a cross section perpendicular to the ridge structure. The insulating film 21 is provided so as to cover the side surface of the ridge structure and the upper surface of the p-cladding layer 19 positioned on the side of the ridge structure, and the absorption film 22 is provided between the p-cladding layer 19 and the insulating film 21. Is provided. The N electrode 10 is provided so as to cover substantially the entire lower surface of the substrate 11, and the P electrode 23 is provided so as to cover substantially the entire upper surface and side surfaces of the ridge structure and the upper surface of the p-cladding layer 19.

  The current flows between the P electrode 23 and the N electrode 10, but the current path is restricted by the ridge structure and the insulating film 21, and the portion of the current path from the ridge structure to the active layer 16 is narrowed, and the lower end of the ridge structure The width is substantially equal to the width of. Hereinafter, this narrowed portion of the current path is referred to as a current confinement region. A portion of the active layer 16 located below the current confinement region is a portion involved in the generation of laser light. The resonance direction of light in the active layer 16 is perpendicular to the paper surface of FIG. 1, and the generated laser light is emitted from both end faces parallel to the paper surface of FIG. Since the current path is narrowed, the width of the generated laser light is narrowed, the density of the current flowing through the active layer 16 is increased, and the driving power necessary for laser oscillation is reduced.

  During laser oscillation, spontaneously emitted light that does not resonate and exits in all directions from the active layer 16 is also generated. In the laser element 1, part of the light is absorbed by the absorption film 22. The absorption film 22 is made of Mo and has a predetermined absorption rate with respect to a wavelength range including the wavelength of the laser beam. Further, the absorption film 22 made of Mo has a higher refractive index than the p-cladding layer 19 made of AlGaN. If the refractive index of the absorbing film 22 is smaller than the refractive index of the p-cladding layer 19, the reflectance at the interface between the two becomes high and total reflection occurs if the incident angle is large. Such a reflection can be prevented by setting the rate to be equal to or higher than the refractive index of the p-cladding layer 19.

FIG. 2 shows an enlarged view of the periphery of the ridge structure. The absorption film 22 is slightly separated from the lower end of the ridge structure. When the width of the lower end of the ridge structure is W1 and the width between the two absorption films 22 located on both sides of the ridge structure is W2, the separation distance D between the absorption film 22 and the lower end of the ridge structure is D = (W2−W1). / 2
It is. In this region of distance D, the insulating film 21 is in direct contact with the upper surface of the p-cladding layer 19. Since the width of the current confinement region is substantially equal to the width of the lower end of the ridge structure, D is also the distance between the absorption film 22 and the current confinement region. The width of the lower end of the ridge structure, that is, the width W1 of the current confinement region is W1 = 2.0 μm.

  In order to absorb more spontaneous emission light, the smaller the separation distance D, the better. However, the separation distance D is related to the laser oscillation characteristics. For example, if the distance D is too small, the absorption film 22 affects the oscillation mode, the internal absorption αi increases, and the threshold current for laser oscillation increases. To rise. Therefore, in the laser element 1, D = 0.5 μm.

  A method for manufacturing the laser element 1 will be described with reference to FIG. The epitaxial growth method described below is a method for growing a crystal film on a substrate, and includes a VPE (vapor phase epitaxial) method, a CVD (chemical vapor deposition) method, and a MOVPE (organometallic vapor phase epitaxial) method. , MOCVD (metal organic chemical vapor deposition) method, Halide-VPE (halogen chemical vapor deposition) method, MBE (molecular beam epitaxial) method, MOMBE (organic metal molecular beam epitaxial) method, GSMBE (gas source molecular beam epitaxial) ) Method and CBE (Chemical Beam Epitaxial) method.

First, a GaN wafer as the GaN substrate 11 is manufactured. In the GaN wafer, steps of about 10 to 50 nm are provided at intervals of several μm on a GaN single crystal film (wafer original plate) having a thickness of about 500 μm, and a new GaN layer of about 4 μm is laminated by epitaxial growth. This is to remove the history of the penetrating dislocation or the like having a single-crystal film is obtained by utilizing the lateral selective growth stepped GaN single crystal film. The obtained GaN wafer has a structure in which a region with a high defect density and a region with a very low density are periodically repeated, and the laser structure is formed on a region with a very low defect density.

Next, each gallium nitride semiconductor layer is epitaxially grown on the GaN wafer. First, a wafer is set in the MOCVD apparatus, and a low-temperature GaN buffer layer (not shown) is grown by 25 nm at a substrate (wafer) temperature of 550 ° C. using NH _{3 as a} group V material and TMGa (trimethyl gallium) as a group III material. Let Next, SiH ₄ is added to the raw material, and an n-GaN layer 12 (Si impurity concentration 1 × 10 ¹⁸ / cm ³ ) is grown by 3 μm at a substrate temperature of 1075 ° C.

Subsequently, TMIn (trimethylindium) is added as a group III material, the substrate temperature is lowered to about 700 to 800 ° C., and the n-In _0.07 Ga _0.93 N crack prevention layer 13 is grown to 50 nm. Further, TMIn as a group III raw material is changed to TMAl (trimethylaluminum), the substrate temperature is raised again to 1075 ° C., and the n-Al _0.1 Ga _0.9 N cladding layer 14 (Si impurity concentration 1 × 10 ¹⁸ / cm ³ ) is reduced to 0. Then, the n-GaN guide layer 15 is grown by 0.1 μm.

Thereafter, the substrate temperature is lowered to 730 ° C., and an active layer 16 having a multiple quantum well structure composed of three periods of a 4-nm-thick In _0.15 Ga _0.85 N well layer and a 6-nm-thick In _0.05 Ga _0.95 N barrier layer is grown. The layer structure is in the order of barrier layer / well layer / barrier layer / well layer / barrier layer / well layer / barrier layer. Note that the growth interruption may be performed for 1 second or more and 180 seconds or less between the barrier layer and the well layer or between the well layer and the barrier layer. In this way, the flatness of each layer is improved and the light emission half width is reduced.

Next, the substrate temperature is raised again to 1050 ° C., and the p-Al _0.25 Ga _0.75 N barrier layer 17 is grown to 18 nm and the p-GaN guide layer 18 is grown to 0.1 μm. Further, at the same substrate temperature, the p-Al _0.1 Ga _0.9 N cladding layer 19 is grown to 0.5 μm and the p-GaN contact layer 20 is grown to 0.1 μm. In growing these layers, Mg is added as a p-type impurity at a concentration of 5 × 10 ¹⁹ to 2 × 10 ²⁰ / cm ³ using Cp ₂ Mg (biscyclopentadienyl magnesium).

After the nitride semiconductors are stacked in this way, the p-GaN contact layer 20 and the p-AlGaN cladding layer 19 are dry etched to form a ridge structure. Thereafter, the absorption film 22 is provided with a thickness of about 100 nm by vacuum deposition, and the insulating film 21 is further provided. Mo is used as the material of the absorption film 22, and SiO ₂ is used as the material of the insulating film 21. Note that the absorption film 22 may be provided by any method, for example, a sputtering method may be employed. Further, the insulating film 21 is required to have little absorption with respect to the oscillation wavelength, and besides SiO ₂ , TiO ₂ , Al ₂ O ₃ , Ta ₂ O ₅ , ZrO ₂ , In ₂ O ₃ , Nd ₂ O _3. Sb ₂ O ₃ , CeO ₂ , ZnS, Bi ₂ O _{3 and the} like can also be used.

  After the insulating film 21 is provided, the P electrode 23 is provided by vapor deposition. The P electrode 23 is, for example, Pd / Mo / Au from the insulating film 21 side. A film for improving adhesion may be provided between the insulating film 21 and the P electrode 23.

  After providing the P electrode 23, the lower surface side of the GaN wafer is partially removed by mechanical processing or etching, so that the thickness of the wafer is about 80 to 200 μm. This is for facilitating the process of dividing the laser elements 1 to be individual later. In particular, when the laser light emitting end face of the active layer 16 is formed at the same time as the division, it is preferable to make it thin to about 80 to 150 μm. The lower surface of the wafer after partial removal is made smooth to improve the adhesion of the N electrode 10. When mechanical processing is employed, polishing may be performed from the beginning, but it is efficient to perform polishing after a certain thickness is obtained by grinding.

  Thereafter, the N electrode 10 is thinly provided on the lower surface of the GaN wafer. The N electrode 10 is, for example, Ti / Al / W / Pt / Au from the substrate 11 side. A vacuum deposition method is suitable for providing such a thin metal film with good controllability, but other methods such as an ion plating method and a sputtering method may be used. After providing the N electrode 10, annealing is performed at a temperature of about 500 ° C. to make the P electrode 23 and the N electrode 10 good ohmic electrodes.

After annealing, the GaN wafer is divided into individual laser elements 1. Dividing the wafer is performed, for example, by putting a scribe line at a diamond point on the lower surface and breaking the wafer by applying an appropriate force along the scribe line. The scribe line is formed by a method using a wire saw or a thin blade, a laser scribing method in which a laser beam such as an excimer laser is heated and rapidly cooled to cause cracks in the laser beam irradiation site, and a laser beam with high energy density is used. It is also possible to employ a laser aflation method that irradiates and evaporates the irradiated part. Further, it is also possible to cut directly by a wire saw or thin blade.

  The individual laser elements 1 obtained by the division are mounted on a heat sink by a die bonding method, and the P electrode 23 is connected to a power source by a wire bonding method to be put into practical use. The mounting to the heat sink is performed firmly by junk-up with the N electrode 10 side as the bonding surface. Note that the heat sink is a stem or the like.

  The characteristics of the laser device 1 manufactured as described above were examined. As a result, continuous oscillation was performed at 40 mA at an ambient temperature of 25 ° C., and the oscillation wavelength was 405 ± 5 nm. The oscillation mode was a basic mode. The resonator length is 500 μm, and the reflectance of the emission end face is about 20%.

  The relationship between the injection current I and the output L of the laser beam is shown in FIG. In FIG. 3, an IL curve represented by a solid line is that of the laser element 1. Spontaneous emission light at the time of laser oscillation was 0.5 mW or less. Further, when the noise characteristic when the output L was 2 mW was measured, it was RINmax <−125 dB / Hz.

  For comparison, a laser device having a configuration in which only the absorption film 22 is omitted was manufactured, and the characteristics were examined. The laser element of this comparative example, like the laser element 1, continuously oscillated at 40 mA at an ambient temperature of 25 ° C., and its IL curve is represented by a broken line in FIG. That is, the spontaneous emission light at the time of laser oscillation is about 1 mW, which is about twice that of the laser element 1. Further, the noise characteristics when the output L was 2 mW deteriorated as RINmax <−115 dB / Hz.

  In order to investigate the influence of the separation distance D between the absorption film 22 and the lower end of the ridge structure on the characteristics, several laser elements 1 having different separation distances D were manufactured. When the distance D was set to 0.15 μm, continuous oscillation occurred at 45 mA at an ambient temperature of 25 ° C., resulting in an increase in threshold current. Spontaneously emitted light at the time of laser oscillation was 0.5 mW or less, and noise characteristics when the output L was 2 mW were also RINmax <−125 dB / Hz, and no change was observed. The increase in the threshold current is considered to be due to the increase in internal absorption due to the decrease in the separation distance D. When the distance D was set to 2.0 μm, continuous oscillation was performed at 40 mA at an ambient temperature of 25 ° C., and spontaneous emission light at the time of laser oscillation was 0.6 mW or less.

  FIG. 4 shows the relationship between the separation distance D and the intensity of spontaneous emission light. In FIG. 4, each point is that of the laser element 1, and a straight line (1 mW) represented by a solid line is that of the laser element of the above-described comparative example. The amount of spontaneously emitted light tends to increase as the separation distance D increases. However, even if the separation distance D is 10 μm, the amount of spontaneous emission light remains only about half that of the configuration without the absorption film 22.

  FIG. 5 shows the relationship between the separation distance D and the threshold current necessary for laser oscillation. The measurement was performed at an ambient temperature of 25 ° C. When the separation distance D becomes extremely small, the threshold current increases, but it can be seen that the threshold current can be suppressed when D ≧ 0.3 μm.

To summarize the above results, the separation distance D between the absorption film 22 and the lower end of the ridge structure is D ≧ 0.3 μm from the viewpoint of threshold current.
It is preferable to satisfy the above, and from the viewpoint of reducing spontaneous emission light, it is preferable that the lower limit is within this range.

  The laser element 1 is suitable as a light source for an apparatus for converging a laser beam in a minute range on an irradiation target because the spontaneous emission light is small. However, since the emission angle of the spontaneous emission light is not the same as the emission angle of the laser light, when the optical path length from the laser element 1 to the irradiation target is very short, the spontaneous emission light is collected around the spot formed by the laser light. It will be lighted. Of the spontaneous emission light emitted together with the laser light, the peripheral light is mainly the light reflected by the insulating film 21 located between the absorption film 22 and the ridge structure, and therefore the size of the pattern formed by the spontaneous emission light. Depends on the distance D.

Considering this point, the separation distance D between the absorption film 22 and the lower end of the ridge structure is
D ≦ 10μm
It is preferable to satisfy. When the distance D is 10 μm, the upper limit of this range, even when the optical system is set so that the spot diameter of the laser light is 2 μm and the substantial optical path length from the laser element 1 to the irradiation target is 3 cm. The size of the condensed pattern of emitted light is suppressed to about 3 to 4 μm. Therefore, if the separation distance D satisfies the above range, in the optical disc apparatus provided with this optical system, the amount of light coupled outside the pits of the optical disc is small, and the detected signal intensity is prevented from greatly fluctuating. Can do.

  In order to examine the influence of the material and thickness of the absorption film 22 and the width W1 of the lower end portion of the ridge structure on the characteristics, these different laser elements 1 were manufactured.

As a result, it has been found that the material of the absorption film 22 may be anything as long as it absorbs the wavelength of the oscillating laser beam. However, in order to suppress the thickness of the absorption film 22 to about 100 nm, the absorption coefficient κ of the material of the absorption film 22 is κ ≧ 0.1.
It is desirable to satisfy. The absorption coefficient κ is an imaginary component of the refractive index represented by a complex number.

As such a material, metals including Mo, Si, Ge, a semiconductor such as GaAs, SiO, insulator such as TiO _2, polyamide and the like. These materials are effective in suppressing higher-order mode oscillation, have a refractive index equal to or higher than that of the p-cladding layer 19, and favorably suppress reflection at the interface with the p-cladding layer 19. You can also. When κ <0.1, unless the absorption film 22 has a thickness exceeding 100 nm, spontaneous emission light at the time of laser oscillation does not decrease so much and it takes time to form the absorption film 22.

As for the width W1 of the ridge structure, as long as the separation distance D satisfies D ≧ 0.3 μm,
0.5μm ≦ W1 ≦ 5μm
It has been found that the above condition does not affect the amount of spontaneous emission light or the threshold current of laser oscillation.

The feature of the structure of the laser element 1 that reduces spontaneous emission light is in the portion including the absorption film 22 above the active layer 16, and the portion below the active layer 16 is not involved in reduction of spontaneous emission light. Accordingly, sapphire substrate 1 1, SiC, be made of material other than GaN, such as Si, not instead any that may reduce the spontaneous emission light. In the laser element 1, the absorption film 22 is positioned between the p-cladding layer 19 and the insulating film 21, but the absorption film 22 is provided between the p-guide layer 18 and the p-cladding layer 19. Also good. Furthermore, even if an oxide film or the like generated during the process is interposed between the absorption film 22 and the p-cladding layer 19 or between the absorption film 22 and the p-guide layer 18, it affects the reduction of spontaneous emission light. There is no.

  Further, the vicinity of the ridge structure does not need to be flat, and it is only necessary that the absorption film 22 exists and the absorption film 22 is separated from the current confinement region. One modification of the laser element 1 is shown in FIG. This is because the entire portion of the p-cladding layer 19 and the p-contact layer 20 located on the side of the ridge structure is not removed by etching, but two cross-sections V-shaped reaching the p-cladding layer 19. A groove is provided and a portion between the two grooves has a ridge structure. The absorption film 22 is provided on the entire upper surface of the p-contact layer 20 except for the ridge structure.

  The configuration of the laser element 2 of the second embodiment is schematically shown in the longitudinal sectional view of FIG. The laser element 2 includes an N electrode 30, an n-GaN substrate 31, an n-GaN layer 32, an n-InGaN crack prevention layer 33, an n-AlGaN cladding layer 34, an n-GaN guide layer 35, an n-InGaN active layer 36, A p-AlGaN barrier layer 37, a p-GaN guide layer 38, a p-AlGaN cladding layer 39, a p-GaN contact layer 40, an insulating film 41, an absorption film 42, and a P electrode 43. Except for the shape of the insulating film 41 and the material of the absorption film 42, the material and arrangement of each component of the laser element 2 are the same as those of the laser element 1 of the first embodiment.

  The insulating film 41 covers the side surface of the ridge structure, but only a small portion of the p-cladding layer 39 is located on the side portion of the ridge structure, and the absorption film 42 is formed of the insulating film 41. It is located on the side, in contact with the insulating film 41, and in contact with the P electrode 43. The absorption film 42 is made of n-Si and is an insulator. In the laser device 2 having such a configuration, the absorption film 42 also serves as a part of the configuration in which the P electrode 43 and the p-cladding layer 39 are insulated to form a current confinement region.

  The characteristics of the laser element 2 in which the distance D between the absorption film 42 and the lower end of the ridge structure was 0.4 μm were examined. As a result, continuous oscillation was performed at 41 mA at an ambient temperature of 25 ° C., and the oscillation wavelength was 405 ± 5 nm. The oscillation mode was a basic mode. Spontaneously emitted light at the time of laser oscillation was 0.5 mW or less, and noise characteristics when the output was 2 mW were RINmax <−125 dB / Hz.

Further, similarly to the first embodiment, various laser elements 2 having different separation distances D and materials of the absorption film 42 were manufactured, and the influence of these on the characteristics was examined. As a result, like the laser element 1, the separation distance D is D ≧ 0.3 μm.
And the absorption coefficient κ of the absorption film 42 with respect to the oscillation wavelength is κ ≧ 0.1
If the above condition is satisfied, the spontaneous emission light at the time of laser oscillation is 0.5 mW or less, and the noise characteristic when the output is 2 mW is also RINmax <−125 dB / Hz.

  The laser element 2 can be manufactured in the same manner as the laser element 1 only by using a mask when forming the insulating film 41. It is also possible to reverse the order of forming the insulating film 41 and the absorption film 42 and form the former first. A modification of the laser element 2 manufactured in this way is shown in FIG.

  The configuration of the laser element 3 of the third embodiment is schematically shown in the longitudinal sectional view of FIG. The laser element 3 includes an N electrode 50, an n-GaN substrate 51, an n-GaN layer 52, an n-InGaN crack prevention layer 53, an n-AlGaN cladding layer 54, an n-GaN guide layer 55, an n-InGaN active layer 56, A p-AlGaN barrier layer 57, a p-GaN guide layer 58, a p-AlGaN cladding layer 59, a p-GaN contact layer 60, an insulating film 61, an absorption wall 62, and a P electrode 63. The material and arrangement of each component of the laser element 3 are the same as those of the laser element 1 of the first embodiment except that an absorption wall 62 is provided instead of the absorption film 22.

Two grooves are formed in the p-cladding layer 59 and the p-guide layer 58 along the current confinement region, and an absorption wall 62 is provided so as to fill these grooves. The groove has a width of 20 μm and extends from the upper surface of the p-cladding layer 59 to the middle of the p-guide layer 58. The absorption wall 62 is made of TiO ₂ and is an insulator.

  The laser element 3 can be manufactured in substantially the same manner as the laser element 1. That is, when the p-cladding layer 59 is grown and the groove is formed by etching and the absorption wall 62 is provided, a mask having a window facing the groove may be used.

  The characteristics of the laser element 3 in which the distance D between the absorption wall 62 and the lower end of the ridge structure was 0.8 μm were examined. As a result, continuous oscillation was performed at 40 mA at an ambient temperature of 25 ° C., and the oscillation wavelength was 405 ± 5 nm. The oscillation mode was a basic mode. Spontaneously emitted light at the time of laser oscillation was 0.5 mW or less, and noise characteristics when the output was 2 mW were RINmax <−125 dB / Hz.

Further, in the same manner as in the first embodiment, various laser elements 3 having different separation distances D and materials of the absorption wall 62 were manufactured, and the influence of these on the characteristics was examined. As a result, like the laser element 1, the separation distance D is D ≧ 0.3 μm.
And the absorption coefficient κ of the absorption wall 62 with respect to the oscillation wavelength is κ ≧ 0.1
If the above condition is satisfied, the spontaneous emission light at the time of laser oscillation is 0.5 mW or less, and the noise characteristic when the output is 2 mW is also RINmax <−125 dB / Hz.

Since the absorption wall 62 is close to the active layer 56, the laser element 3 can absorb spontaneous emission light having a small angle with respect to the active layer 56. The position of the lower end of the absorption wall 62 (the depth of the groove) is not limited, and may reach the active layer 56. However, the absorption wall 62 reaching the active layer 56 must be an insulator. Such a modification is shown in FIG. In this configuration, the absorption wall 62 passes through the active layer 56 and reaches the n-GaN guide layer 55. When the active layer 56 is not reached, the absorption wall 62 does not need to be an insulator and may be an n-type semiconductor.

  The width of the absorption wall 62 is not limited, and the width may be increased. Such a modification is shown in FIG. In this configuration, the absorption wall 62 reaches the side surface of the laser element 3, and the p-cladding layer 59 only exists around the ridge structure and the current confinement region.

  In order to prevent reflection, as described above, it is desirable that the refractive index of the absorption wall 62 be equal to or higher than the refractive index of each layer in contact with the absorption wall 62. However, the refractive index of the absorption wall 62 is ridge structure. And the vertical mode refractive index between the absorption wall 62 and the absorption wall 62 is satisfied.

  FIG. 12 shows a schematic configuration of the optical recording / reproducing apparatus according to the fourth embodiment. The optical recording / reproducing apparatus 4 includes a semiconductor laser element 71, a collimator lens 72, a beam splitter 73, a condenser lens 74, a photodiode 75, and a photodetector 76. The laser element 71 is any one of the nitride semiconductor laser elements 1 to 3 according to the above-described embodiments. The photodiode 75 is provided outside the laser element 71 and receives a part of the laser light L divided by the beam splitter 73. The output intensity of the laser light L is adjusted based on the intensity detected by the photodiode 75. Instead of providing the external photodiode 75 as described above, an intensity detection photodiode may be provided inside the laser element 71.

  When reproducing information, the laser light L emitted from the laser element 71 passes through the collimating lens 72, the beam splitter 73, and the condensing lens 74 in this order, and is condensed in the pits on the recording surface of the optical disc 77, and the signal recorded in the pits. Reflected light reflecting. This reflected light again passes through the condenser lens 74 and is guided to the photodetector 76 by the beam splitter 73, and a signal is reproduced by the photodetector 76. At present, the reflectivity of the optical disk and the detection efficiency of the optical detector are low, and the output intensity of the laser beam L is, for example, about 30 mW during recording and about 5 mW during reproduction. The strength is thought to decrease.

  In the optical recording / reproducing apparatus 4 provided with the nitride semiconductor laser element 1, 2, or 3 having the absorption films 22, 42 or the absorption wall 62 as the laser element 71 as the light source, the laser light L is formed on the optical disc 77. There is little spontaneous emission incident around the spot. For this reason, errors are unlikely to occur during recording and reproduction. In addition, since there is little noise due to spontaneously emitted light, the accuracy of reproduction performed at a low output is further increased, and the intensity detected by the photodiode 75 becomes accurate, and the output intensity of the laser light is accurately adjusted. It is possible.

1 is a longitudinal sectional view schematically showing a configuration of a laser element according to a first embodiment. FIG. 3 is an enlarged longitudinal sectional view showing the periphery of the ridge structure of the laser device of the first embodiment. The figure which shows the relationship between the injection current and laser beam output in the laser element of 1st Embodiment, and a comparative example. The figure which shows the relationship between the separation distance of the absorption film and ridge structure in the laser element of 1st Embodiment, and the intensity | strength of spontaneous emission light. The figure which shows the relationship between the separation distance of the absorption film and ridge structure in the laser element of 1st Embodiment, and the threshold current of laser oscillation. The longitudinal cross-sectional view which shows typically the structure of the modification of the laser element of 1st Embodiment. The longitudinal cross-sectional view which shows typically the structure of the laser element of 2nd Embodiment. The longitudinal cross-sectional view which shows typically the structure of the modification of the laser element of 2nd Embodiment. The longitudinal cross-sectional view which shows typically the structure of the laser element of 3rd Embodiment. The longitudinal cross-sectional view which shows typically the structure of the modification of the laser element of 3rd Embodiment. The longitudinal cross-sectional view which shows typically the structure of another modification of the laser element of 3rd Embodiment. The block diagram which shows typically schematic structure of the optical recording / reproducing apparatus of 4th Embodiment. The longitudinal cross-sectional view which shows the structure of the conventional laser element typically.

Explanation of symbols

1, 2, 3 Laser element 10, 30, 50 N electrode 11, 31, 51 n-GaN substrate 12, 32, 52 n-GaN layer 13, 33, 53 n-InGaN crack prevention layer 14, 34, 54 n- AlGaN cladding layers 15, 35, 55 n-GaN guide layers 16, 36, 56 n-InGaN active layers 17, 37, 57 p-AlGaN barrier layers 18, 38, 58 p-GaN guide layers 19, 39, 59 p- AlGaN cladding layers 20, 40, 60 p-GaN contact layers 21, 41, 61 Insulating films 22, 42 Absorbing film 62 Absorbing walls 23, 43, 63 P electrode 4 Optical recording / reproducing apparatus 71 Laser element 72 Collimating lens 73 Beam splitter 74 Condenser lens 75 Photo diode 76 Photo detector 77 Optical disc

Claims (3)

  A substrate,
  A semiconductor formed on an upper surface of the substrate and having a stripe-shaped ridge structure including an active layer;
  A light absorption part provided above the semiconductor;
  A laser element in which the oscillation direction of laser light is the stripe direction of the ridge structure,
  The light absorbing portion is spaced apart from the lower end of the ridge structure so as not to absorb the fundamental mode or higher order mode of the laser light, and absorbs light in a wavelength range including the wavelength of the laser light. A laser element comprising a metal or a semiconductor having characteristics and a refractive index smaller than that of the semiconductor. The separation distance is 0.5μm or more 10μm or less, according to claim 1 Symbol placement laser device. In an optical recording / reproducing apparatus for recording and reproducing information by guiding light from a light source to an optical recording medium,
Optical recording and reproducing apparatus comprising: a laser device as a light source according to claim 1 or 2.

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