Patent Publication Number: US-2011058584-A1

Title: Semiconductor laser device and fabrication method for the same

Description:
TECHNICAL FIELD 
     The present invention relates to a semiconductor laser device and a fabrication method for the same, and more particularly to a blue to ultraviolet semiconductor laser device using a nitride and the fabrication method for the same. 
     BACKGROUND ART 
     Conventionally, III-V compound semiconductor laser devices, such as aluminum gallium arsenic (AlGaAs) infrared lasers and indium gallium phosphorus (InGaP) red lasers, have been widely used as lasers for communication and as read and write elements for compact discs (CDs) and digital versatile discs (DVDs). 
     In recent years, blue and ultraviolet semiconductor laser devices having a shorter wavelength have been implemented using nitride semiconductors represented by a general formula Al x Ga y In (1-x-y) N (0≦x≦1, 0≦y≦1, 0≦1−x−y≦1). Such semiconductor laser devices have been increasing put in practical use as light sources for read and write of high-density optical discs such as next-generation DVDs (Blu-Ray Discs). At present, as blue semiconductor laser devices, low-power ones of several tens of mW for playback and high-power ones of 100 mW class for recording are commercially available. To improve the recording speed, further increase in the power of blue semiconductor laser devices has been attempted; even 200 mW-class semiconductor laser devices are now coming on the market. 
     In general, in a nitride semiconductor laser device, narrowing of an injected current in a direction horizontal to the substrate is performed with a ridge structure formed by etching part of a nitride semiconductor material into a stripe shape. Also, light trapping in a direction horizontal to the substrate is performed with a real index guided structure. Hence, laser light propagating inside a stripe-shaped light waveguide region is likely to leak in a substrate horizontal direction. Leak light (stray light) is subjected to multiple reflection in a laser cavity direction and finally released from the output end face. As a result, stray light interferes with a principal laser beam, resulting in appearance of ripples in a far field pattern (FFP) and deviation from a Gaussian shape. If such a laser beam is used for read and write of an optical disc, the light use efficiency decreases, resulting in occurrence of noise and occurrence of a readout error. 
     Methods for suppressing ripples caused by leak light in a substrate horizontal direction to obtain a good FFP shape have been so far examined. 
     For example, disclosed is a method in which a p-side electrode is also formed consecutively on a portion outside a ridge stripe where a p-type cladding layer is exposed (see Patent Document 1, for example). Stray light leaking outside the ridge waveguide portion is therefore absorbed with the p-side electrode, and hence the FFP shape can be improved. 
     Also disclosed is a technique in which a light absorption region made of a dielectric, a metal or a semiconductor having a refractive index larger than that of a waveguide is formed in a portion apart from the ridge waveguide portion, to thereby improve the FFP shape (see Patent Documents 2 and 3, for example). 
     Yet another technique disclosed is forming a plurality of concave portions near the light output end face to scatter leak light from the ridge waveguide portion, to thereby improve the FFP shape (see Patent Document 4, for example). 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 11-186650 
     Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-237661 
     Patent Document 3: Japanese Laid-Open Patent Publication No. 2006-216731 
     Patent Document 4: Japanese Laid-Open Patent Publication No. 2005-311308 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, the conventional techniques for improving the FFP shape have the following problems. In the case of forming the p-side electrode, not only on the ridge waveguide portion, but also on the exposed face of the p-type cladding layer, a current may also flow in a portion other than the ridge waveguide portion. The current flowing in a portion other than the ridge waveguide portion is merely about a millionth of that flowing in the ridge waveguide portion. However, if the injected current is increased to increase the power of the semiconductor laser, the leak current flowing in a portion other than the ridge waveguide will be no more negligible. The leak current will worsen the FFP shape and cause new ripples. 
     To form the p-side electrode also in a portion other than the ridge waveguide portion, the width of the p-side electrode must be far greater than the width of the ridge waveguide portion. Such a wide p-side electrode will cause a cleavage failure with which a crack, a minute step and chip and the like may occur at the cavity end face portions. Also, electrode coming-off is likely to occur at the end face portions. As a result, the yield will decrease. Moreover, light is likely to scatter at the end face portions, and this will become a new cause for a faulty FFP shape. 
     No leak current will occur if an absorption layer made of a dielectric is formed. However, with no dielectric material having a sufficiently large absorption coefficient existing, a sufficient light absorption effect is unavailable. 
     If the absorption layer is made of a semiconductor, at least two times of growth of a semiconductor layer is necessary, and this is very disadvantageous in terms of cost. Also, a semiconductor layer active in light absorption tends to be a layer having many defects, a layer rich in impurity, a layer high in In content or the like. Hence, such an absorption layer will facilitate deterioration of the semiconductor laser device, degrading the reliability. 
     In the case of forming concave portions to scatter leak light without forming an absorption layer, the above problems relating to the absorption layer do not occur. However, since the leak light is merely scattered, part of the scattered light returns to the waveguide portion as light of a different phase, causing noise. In a nitride semiconductor laser device, there arise band bending due to spontaneous polarization and a piezoelectric effect caused by strain, band shrinkage due to high injection carrier density, a red shift of the oscillation wavelength due to heat generation and the like. For this reason, the oscillation wavelength in the ridge waveguide portion tends to be longer than the wavelength at an absorption edge in the surrounding region. Hence, by merely scattering leak light from the waveguide, light cannot be absorbed but is repeatedly scattered inside the cavity as stray light. Finally, such light leaks out from an end face, possibly interfering with the FFP or returning to inside the waveguide region causing noise. 
     An object of the present invention is implementing a nitride semiconductor laser device in which ripples are reduced and the far field pattern shape is close to a Gaussian shape. 
     Means for Solving the Problems 
     To attain the above object, according to the present invention, the semiconductor laser device is configured to include light absorption layers formed on both sides of a ridge waveguide portion. 
     The semiconductor laser device of the present invention is directed to a semiconductor laser device provided with a cavity structure having a pair of cavity end faces opposed to each other, including: a semiconductor multilayer structure including an n-type semiconductor layer, an active layer and a p-type semiconductor layer sequentially formed on a substrate in this order and having a stripe-shaped ridge waveguide portion extending in a direction intersecting the cavity end faces; a dielectric layer formed on the semiconductor multilayer structure to cover at least part of both side faces of the ridge waveguide portion; light absorption layers formed on both sides of the ridge waveguide portion on the semiconductor multilayer structure so as to be spaced from the ridge waveguide portion and the cavity end faces; and a p-side electrode formed on the ridge waveguide portion. 
     The semiconductor laser device of the present invention includes light absorption layers formed on both sides of the ridge waveguide portion on the semiconductor multilayer structure so as to be spaced from the ridge waveguide portion and the cavity end faces. With this configuration, the intensity of leak light leaking sideways from the ridge waveguide portion can be reduced. Hence, the far field pattern shape of light emerging from the semiconductor laser device can be improved. Also, being spaced from the cavity end faces, the light absorption layers will not obstruct cleavage. 
     In the semiconductor laser device of the invention, the light absorption layers are preferably conductive and electrically insulated from the p-side electrode. With this configuration, even when the light absorption layers are made of a conductive material, the leak current from the light absorption layer can be made null. Hence, a material such as a metal high in effective absorption coefficient can be used without causing stray light due to a leak current or degrading the reliability. 
     The semiconductor laser device of the invention may further include an insulating layer formed between the light absorption layers and the p-side electrode. With this configuration, the length of the optical absorption layers in the cavity direction can be increased. 
     In the semiconductor laser device of the invention, the light absorption layers may be axially symmetric with respect to the ridge waveguide portion. 
     In the semiconductor laser device of the invention, the light absorption layers may be formed near a light output end face, out of the cavity end faces, from which light emerges. With this configuration, the light absorption layers and the p-side electrode can be insulated from each other without forming an insulating film. 
     In the semiconductor laser device of the invention, each of the light absorption layers may have a first portion formed near a light output end face from which light emerges and a second portion formed near a cavity end face opposite to the light output end face. With this configuration, the length of the light absorption layers in the cavity direction can be increased. 
     In the semiconductor laser device of the invention, the spacing between the first portion and the light output end face may be equal to the spacing between the second portion and the cavity end face opposite to the light output end face. In this case, the first portion and the second portion may be axially symmetric with respect to a center line of the cavity structure in an end face direction. 
     In the semiconductor laser device of the invention, the semiconductor multilayer structure may have a step portion formed at least at a region of the cavity end faces excluding the ridge waveguide portion. With this configuration, correct cleavage can be made at the time of cleavage of the semiconductor laser device. 
     In the semiconductor laser device of the invention, the distance between the light absorption layers and the center of the ridge waveguide portion may be 10 μm or less. 
     In the semiconductor laser device of the invention, the length of the light absorption layers in a direction parallel to the ridge waveguide portion may be 5 μm or more. 
     In the semiconductor laser device of the invention, the light absorption layers may be made of a material whose refractive index n and extinction coefficient k at an oscillating wavelength satisfy n≧1 and n+2k≧2. In this case, it may be made of a material whose refractive index n and extinction coefficient k at an oscillating wavelength satisfy n&gt;2 and 0.001&lt;k&lt;2.5. 
     In the semiconductor laser device of the invention, the light absorption layers may be made of a material whose extinction coefficient at an oscillating frequency is 1 or more. 
     In the semiconductor laser device of the invention, the light absorption layers may include at least one of Cu, Pd, Zr, Nb, Cr, Ni, Au, Pt, Ti, Ta, W, Mo and amorphous Si. 
     In the semiconductor laser device of the invention, the light absorption layers may include at least one of CrN, TiN, ZrN, NbN, TaN and MoN. 
     In the semiconductor laser device of the invention, the light absorption layers may include the same metal material as that included in the p-side electrode. 
     The fabrication method for a semiconductor laser device of the present invention includes the steps of: forming a semiconductor multilayer structure including an n-type semiconductor layer, an active layer and a p-type semiconductor layer on a substrate sequentially by crystal growth; forming a ridge waveguide portion extending in a cavity direction in the p-type semiconductor layer; forming a dielectric layer on the p-type semiconductor layer; forming a first opening exposing the top face of the ridge waveguide portion in the dielectric layer and also forming a second opening exposing the p-type semiconductor layer in at least part of a region of the dielectric layer excluding the ridge waveguide portion and a portion becoming a cavity end face by cleavage; and forming a p-side electrode and a light absorption layer by filling the first opening and the second opening with a metal material. 
     The fabrication method for a semiconductor laser device of the present invention includes a step of forming a p-side electrode and a light absorption layer by filling the first opening and the second opening with a metal material. Hence, since the p-side electrode and the light absorption layer can be formed simultaneously, the number of process steps can be reduced. 
     EFFECT OF THE INVENTION 
     According to the semiconductor laser and the fabrication method for the same of the present invention, a nitride semiconductor laser device in which ripples are reduced and the far field pattern shape is close to a Gaussian shape can be implemented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1(   a ) to  1 ( d ) show a semiconductor laser device of Embodiment 1 of the present invention, wherein (a) is a plan view, (b) is a cross-sectional view taken along line Ib-Ib in (a), (c) is a cross-sectional view taken along line Ic-Ic in (a), and (d) is a cross-sectional view taken along line Id-Id in (a). 
         FIG. 2  is an enlarged plan view of a light absorption layer of the semiconductor laser device of Embodiment 1 of the present invention. 
         FIGS. 3(   a ) and  3 ( b ) show characteristics of a semiconductor laser device provided with no light absorption layer, wherein (a) shows the measurement results of a horizontal far field pattern, and (b) shows the simulation results of a near field pattern obtained from the horizontal far field pattern. 
         FIG. 4  is a graph showing the relationship between the length of the light absorption layer in the cavity direction and the leak light absorptance thereof. 
         FIG. 5  is a graph showing the relationship between the effective absorptance of the light absorption layer and the length thereof in the cavity direction. 
         FIG. 6  is a graph showing the results of calculation of the effective absorptance of the light absorption layer based on the refractive index and the extinction coefficient. 
         FIG. 7  is a table of the refractive index and the extinction coefficient of various materials. 
         FIGS. 8(   a ) and  8 ( b ) show characteristics of the semiconductor laser device of Embodiment 1 of the present invention, wherein (a) shows the measurement results of a horizontal far field pattern, and (b) shows the simulation results of a near field pattern obtained from the horizontal far field pattern. 
         FIGS. 9(   a ) to  9 ( c ) are plan views showing alterations of the light absorption layer of the semiconductor laser device of Embodiment 1 of the present invention. 
         FIGS. 10(   a ) to  10 ( d ) show a semiconductor laser device of Embodiment 2 of the present invention, wherein (a) is a plan view, (b) is a cross-sectional view taken along line Xb-Xb in (a), (c) is a cross-sectional view taken along line Xc-Xc in (a), and (d) is a cross-sectional view taken along line Xd-Xd in (a). 
         FIG. 11  is a plan view showing a state of the semiconductor laser device of Embodiment 2 of the present invention before primary cleavage. 
         FIG. 12  is a plan view showing a state of an alteration of the semiconductor laser device of Embodiment 2 of the present invention before primary cleavage. 
         FIGS. 13(   a ) and  13 ( b ) show the alteration of the semiconductor laser device of Embodiment 2 of the present invention, wherein (a) is a plan view and (b) is a cross-sectional view at a cavity end face. 
         FIGS. 14(   a ) to  14 ( d ) show a semiconductor laser device of Embodiment 3 of the present invention, wherein (a) is a plan view, (b) is a cross-sectional view taken along line XIVb-XIVb in (a), (c) is a cross-sectional view taken along line XIVc-XIVc in (a), and (d) is a cross-sectional view taken along line XIVd-XIVd in (a). 
         FIGS. 15(   a ) to  15 ( d ) show a semiconductor laser device of Embodiment 4 of the present invention, wherein (a) is a plan view, (b) is a cross-sectional view taken along line XVb-XVb in (a), (c) is a cross-sectional view taken along line XVc-XVc in (a), and (d) is a cross-sectional view taken along line XVd-XVd in (a). 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               10  Substrate 
               12  Semiconductor multilayer structure 
               12 A Light output end face 
               12 B Rear end face 
               12   a  Ridge waveguide portion 
               13  N-type semiconductor layer 
               14  Active layer 
               15  P-type semiconductor layer 
               16  Dielectric layer 
               17  Light absorption layer 
               17 A First portion 
               17 B Second portion 
               18  Insulating layer 
               21  P-side electrode 
               22  N-side electrode 
               23  P-side electrode pad 
               31  N-type GaN layer 
               32  N-type cladding layer 
               33  N-type guide layer 
               51  Optical guide layer 
               52  P-type cladding layer 
               53  P-type contact layer 
               61  Primary cleavage line 
               62  Cleavage guide groove 
               62   a  Step portion 
           
         
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     It is known that in a ridge waveguide-type semiconductor laser device, a near field pattern (NFP) is worsened with leak light from the ridge waveguide portion. The inventors of the present invention have first made in-depth examination on how leak light from a ridge waveguide portion occurs. As a result, it has been found that leak light occurs because light propagating inside the ridge waveguide portion is scattered due to minute fluctuation of the ridge waveguide portion and the morphology of the surface of a semiconductor layer. Also found has been that as the ridge is narrower, the effect of the scattering is larger and hence leak light increases. 
     Leak light exists on both sides of the ridge waveguide portion in the NFP. It has been found that when the relative intensity of leak light standardized with respect to the peak intensity of the NFP becomes about 1×10 −3 , ripples significantly appear in the far field pattern (FFP). Conversely, it has also been found that when the relative intensity of leak light is 5×10 −4  or less, ripples in the FFP considerably decreases, and that when the relative intensity is 1×10 −4  or less, no ripple is observed at all in the FFP, permitting attainment of a good FFP shape. 
     The ridge width of a nitride semiconductor laser device is comparatively small, which is generally about 1 μm to 2 μm. Hence, a nitride semiconductor laser device tends to cause leak light. By increasing the ridge width, improvement in FFP shape is expected. However, an increased ridge width may possibly worsen other characteristics such as the threshold current, kinking and the angle of divergence. Changing of the ridge width is therefore not easy. Also, in a nitride semiconductor laser device, there arise phenomena such as band bending due to spontaneous polarization and the piezoelectric effect caused by strain, band shrinkage due to high injection carrier density and a red shift of the oscillation wavelength due to heat generation. For this reason, the oscillation wavelength in the ridge waveguide portion is often longer than the wavelength at an absorption edge in the surrounding region. Hence, light that has leaked from the ridge waveguide portion becomes stray light propagating outside the ridge waveguide portion without being absorbed. Such stray light finally emerges from the light output end face, appearing as ripples in the FFP shape. 
     As described above, in a nitride semiconductor laser device, it is structurally difficult to prevent occurrence of leak light. Hence, for improving the NFP and the FFP, it is necessary to prevent leak light from being released from the light output end face in one way or another. The present inventors have found as a result of examination that leak light can be effectively absorbed by forming a light absorption layer made of a material large in light absorption property on a semiconductor layer in a region other than the ridge waveguide portion. 
     Leak light from the ridge waveguide portion may occur roughly symmetrically with respect to the ridge waveguide portion. Hence, it is considered effective to form light absorption layers on both sides of the ridge waveguide portion symmetrically. 
     Also, it has been found most effective to form light absorption layers near the light output end face. The reasons for this seem to be that leak light mainly propagates in the cavity direction and is finally released from the light output end face and that the light density distribution in the cavity direction is highest near the light output end face. 
     Hereinafter, a semiconductor laser device improved in FFP shape implemented based on the examination results described above will be described in concrete ways. 
     Embodiment 1 
     Embodiment 1 of the present invention will be described with reference to the relevant drawings.  FIGS. 1(   a ) to  1 ( d ) show a semiconductor laser device of Embodiment 1, wherein (a) shows a plan structure, and (b), (c) and (d) respectively show cross-sectional structures taken along line Ib-Ib, line Ic-Ic and line Id-Id in (a). 
     The semiconductor laser device of this embodiment, formed by cleavage, is provided with a cavity structure having a pair of cavity end faces opposed to each other. The cavity structure, formed on a substrate, is composed of a semiconductor multilayer structure  12  having a stripe-shaped ridge waveguide portion  12   a  extending in the cavity direction and multilayer dielectric reflection films formed on the cavity end faces. The semiconductor laser device of this embodiment is also provided with light absorption layers  17  formed on both sides of the ridge waveguide portion  12   a  on the semiconductor multilayer structure  12 . With this placement, leak light from the ridge waveguide portion  12   a  can be absorbed, permitting improvement of the FFP shape. 
     Specifically, the semiconductor multilayer structure  12  has an n-type semiconductor layer  13 , an active layer  14  and a p-type semiconductor layer  15  formed sequentially on a substrate  10  made of n-type GaN. The semiconductor multilayer structure  12  can be in any form as long as a laser cavity can be formed. For example, it may be configured as follows. 
     The n-type semiconductor layer  13  includes an n-type GaN layer  31  having a thickness of 1 μm, an n-type cladding layer  32  made of n-Al 0.04 Ga 0.96 N having a thickness of 2.5 μm and an n-type guide layer  33  made of n-type GaN (n-GaN) having a thickness of 150 nm. The active layer  14  is of a triple quantum well structure including well layers made of In 0.10 Ga 0.90 N having a thickness of 3 nm and barrier layers made of In 0.02 Ga 0.98 N having a thickness of 7.5 nm. The p-type semiconductor layer  15  includes a p-type optical guide layer  51  made of p-type GaN (p-GaN) having a thickness of 120 nm, a p-type cladding layer  52  made of p-Al 0.05 Ga 0.95 N having a thickness of 0.5 μm and a p-type contact layer  53  made of p-GaN having a thickness of 100 nm. 
     A buffer layer may be formed between the semiconductor multilayer structure  12  and the substrate  10 . The buffer layer may be made of n-GaN having a thickness of 200 nm, for example. The semiconductor multilayer structure  12  may be formed by metal-organic chemical vapor deposition (MOCVD) or the like. In formation of an n-type layer, silicon (Si) may be doped as a donor impurity at a density of about 5×10 17  cm −3 . In formation of a p-type layer, magnesium (Mg) may be doped as an acceptor impurity at a density of about 1×10 20  cm −3 . 
     The ridge waveguide portion  12   a  extending in the cavity direction is formed in the semiconductor multilayer structure  12 . The ridge waveguide portion  12   a  may be formed by selectively removing part of the p-type cladding layer  52  and the p-type contact layer  53 . A dielectric layer  16  made of silicon dioxide (SiO 2 ) having a thickness of 200 nm is formed on the side faces of the ridge waveguide portion  12   a  and on the p-type cladding layer  52  excluding the portion constituting the ridge waveguide portion  12   a . Note that the dielectric layer  16  does not necessarily cover the entire side faces of the ridge waveguide portion  12   a  but may just cover at least part thereof. 
     The dielectric layer  16  has openings for exposing the p-type cladding layer  52  at predetermined positions on both sides of the ridge waveguide portion  12   a , and the openings are filled with amorphous silicon forming the light absorption layers  17 . The position, size, material and the like of the light absorption layers  17  will be described later in detail. 
     A p-side electrode  21  is formed on the ridge waveguide portion  12   a  via the p-type contact layer  53 . An n-side electrode  22  is formed on the face (back face) of the substrate  10  opposite to the face on which the semiconductor multilayer structure  12  is formed. A p-side electrode pad  23  is formed on the dielectric layer  16  so as to cover the ridge waveguide portion  12   a  and is electrically connected with the p-side electrode  21 . 
     The p-side electrode  21  may be a multilayer film of palladium (Pd) and platinum (Pt), for example. In formation of the p-side electrode  21 , it is preferred to subject the substrate to electron beam evaporation while being overheated to 70° C. to 100° C. and thereafter to heat treatment at about 400° C. With this formation, the contact resistance of the p-side electrode  21  can be reduced and also the cohesion thereof can be improved. The p-side electrode pad  23  and the n-side electrode  22  may be a multilayer film of titanium (Ti), platinum (Pt) and gold (Au). 
     The p-side electrode pad  23  is formed to be spaced from the light absorption layers  17 , so that the p-side electrode  21  and the light absorption layers  17  are electrically insulated from each other. Hence, no leak current will flow to the semiconductor multilayer structure  12  via the light absorption layers  17  that are in contact with the p-type cladding layer  52 . 
     A multilayer dielectric reflection film (not shown) having a reflectance of 50% is formed on a light output end face  12 A from which laser light in the semiconductor multilayer structure  12  emerges, and a multilayer dielectric reflection film (not shown) having a reflectance of 95% is formed on a rear end face  12 B opposite to the light output end face  12 A. The semiconductor multilayer structure  12  thus functions as a laser cavity. 
     Hereinafter, the formation position and size of the light absorption layers  17  will be described in more detail.  FIG. 2  shows the planar positional relationship between the light absorption layer  17  and the ridge waveguide portion  12   a . From the viewpoint of absorbing leak light from the ridge waveguide portion  12   a , the light absorption layer  17  should be formed on the entire surface of the p-type cladding layer excluding the portion constituting the ridge waveguide portion  12   a . However, various restrictions are imposed on the region in which the light absorption layer  17  is formed. 
     It is theoretically considered most effective to form the light absorption layer  17  in contact with the light output end face  12 A. However, if the light absorption layer  17  is formed over a cleavage line before the cleavage process into elements, it may obstruct the cleavage causing a crack and a step at the cleavage end face and thus degrading the effect of improving the FFP shape. The light absorption layer  17  must be made of a material excellent in light absorption property. Hence, a metal material is preferably used as will be described later. A metal layer formed by general evaporation, which is rich in amorphous ingredients, has spreading nature and lacks in cleavage nature. Therefore, if a metal layer is formed over a cleavage portion of an element, it will obstruct the cleavage; a crack and a step may occur at the cleavage end face, or only the metal layer may be divided at a position deviated from the cleavage line. As a result, the FFP shape may be worsened instead of being improved. In view of the above, the light absorption layer  17  must be formed to be spaced from the light output end face. 
     Also, if the light absorption layer  17  and the ridge waveguide portion  12   a  are in contact with each other, the principal beam propagating inside the ridge waveguide portion  12   a  will be absorbed by the light absorption layer  17 . For this reason, the light absorption layer  17  must be spaced from the ridge waveguide portion  12   a . However, if the light absorption layer  17  is excessively apart from the ridge waveguide portion  12   a , it may possibly fail to absorb leak light that affects the FFP shape. 
       FIGS. 3(   a ) and  3 ( b ) respectively show the intensity distribution of a horizontal FFP obtained from a conventional semiconductor laser device having a width of a ridge waveguide portion of 1 μm and the intensity distribution of a NFP estimated from the intensity distribution of the horizontal FFP. The NFP estimation was made by executing inverse Fourier transformation for the FFP distribution. A correct NFP intensity distribution is unavailable from the FFP intensity distribution because light phase information has disappeared. In other words, the axial asymmetry of the NFP distribution cannot be estimated because no phase information is available. However, it has been confirmed that from comparison with the NFP observation results, the position and relative intensity of the estimated NFP roughly agree with the observed ones. 
     As shown in  FIG. 3(   b ), the principal beam propagates within the range of about 2 μm from the center of the ridge waveguide portion. It is therefore not preferred to set a distance d 1  from the center of the ridge waveguide portion to the light absorption layer  17  shown in  FIG. 2  at less than 2 μm. Also, as described earlier, the FFP shape is hardly affected by leak light when the relative intensity of the leak light is 1×10 −4  or less. It is therefore preferred to set the distance d 1  from the center of the ridge waveguide portion  12   a  to the light absorption layer  17  at 10 μm or less. Since leak light is considered highest in the range of about 2 μm to 10 μm from the center of the ridge waveguide portion, d 1  is more preferably set at about 2 μm to 3 μm. 
     Next, a length d 2  of the light absorption layer  17  in the cavity direction shown in  FIG. 2  will be examined. The length d 2  must be at least a length with which the leak light absorption effect is sufficiently exerted.  FIG. 4  shows the relationship between the absorptance and d 2  observed when the absorption coefficient of the light absorption layer  17  varies. 
     As shown in  FIG. 4 , when the effective absorption coefficient αe is 100 cm −1 , d 2  must be about 70 μm for reducing leak light to ½, and about 250 μm for reducing leak light to 1/10. When the effective absorption coefficient αe is 1000 cm −1 , d 2  must be about 7 μm for reducing leak light to ½, and about 25 μm for reducing leak light to 1/10. The effective absorption coefficient αe as used herein refers to the effective light absorption coefficient of the lower part of the light absorption layer  17  formed on the p-type cladding layer  52 . 
     In reality, however, leak light propagating inside the cavity passes the region of the light absorption layer  17  a number of times while being reflected from the light output end face and the rear end face. When the reflectance of the light output end face is 50% and the reflectance of the rear end face is 95%, leak light is estimated to pass the region of the light absorption layer about three times on average. It is therefore considered possible to reduce leak light to ½ with d 2  being only about 25 μm even when the effective absorption coefficient αe is 100 cm −1 . When the effective absorption coefficient αe is 1000 cm −1 , d 2  can further be reduced. 
       FIG. 5  shows the relationship between d 2  and the effective absorption coefficient ae in the cases of reducing leak light to ½ and 1/10. As is found from this graph, d 2  and the inverse of the effective absorption coefficient αe are in a proportional relationship. Hence, the minimum value of d 2  can be determined by 
         d 2 =c ×(1 /αe )
 
     where c is a coefficient determined with the reduction rate of leak light. For example, the value of c will be about 7000 when leak light is reduced to ½, and about 23000 when leak light is reduced to 1/10. These are values obtained under the condition that the Al content in the p-type cladding layer in this embodiment is 3%. The value of c varies with the Al content in the p-type cladding layer  52  and the like; the value of c tends to be smaller as the Al content in the p-type cladding layer  52  is higher. 
     From the above, the length d 2  of the light absorption layer  17  in the cavity direction, which must be changed with the material of the light absorption layer  17  and the required degree of reduction of leak light, is preferably about 2 μm to 50 μm. 
     The normal cavity length of a nitride semiconductor laser is about 400 μm to 800 μm. As long as the value of d 2  is in the range of about 2 μm to 50 μm, the light absorption layer  17  can be spatially separated from the p-side electrode pad  23 . Hence, even when the light absorption layer  17  is conductive, the light absorption layer  17  and the p-side electrode pad  23  can be isolated from each other without the necessity of providing an insulation film. As a result, the FFP shape can be improved in a simple process. Note that as long as the cavity length is long enough to permit the light absorption layer  17  to be spatially separated from the p-side electrode pad  23 , d 2  can be made further long without causing any trouble. 
     A width d 3  of the light absorption layer  17  in the end face direction shown in  FIG. 2  is preferably 10 μm or more, because with a width of about 10 μm, a major portion of the region in which leak light exists can be covered as shown in  FIG. 3 . Also, during secondary cleavage in the cavity direction, as during the primary cleavage, the cleavage precision will improve if the light absorption layer  17  has not been formed over a cleavage portion. Hence, the width d 3  of the light absorption layer  17  in the end face direction is preferably set so that the light absorption layer  17  does not reach the side face of the cavity. 
     The material of the light absorption layer  17  will be described. As described earlier, the effective absorption coefficient αe of the lower part of the light absorption layer  17  is preferably 100 cm −1  or more. The light absorption layer  17  is formed above the layer through which leak light propagates, and hence there is a spatial deviation from the light distribution of the leak light. The absorption effect thereof is therefore liable to be restrictive. However, the effective absorption coefficient αe of the light absorption layer can be roughly estimated from the refractive index n and extinction coefficient k of the light absorption layer and the material of the p-type cladding layer. 
       FIG. 6  shows the results of determination of the relationship among the refractive index n, the extinction coefficient k and the effective absorption coefficient αe at a wavelength of 405 nm. The effective absorption coefficient αe is an index representing the effective light absorption of lower part of the light absorption layer formed on the p-type cladding layer. This is therefore determined from the light absorption property of the light absorption layer, the difference in refractive index between the light absorption layer and the p-type cladding layer and the like. As shown in  FIG. 6 , to state broadly, the following materials can be used: a material having a refractive index n of about 2 or more and an extinction coefficient k of 0.001 or more and a material having a refractive index n of about 1 to 2 and an extinction coefficient k of about 1 to 4. More specifically, a material satisfying n≧1 and n+2k≧2 is preferable, and a material satisfying n&gt;2 and 0.001&lt;k&lt;2.5 is more preferable. It is assumed in this case that the Al content in the p-type cladding layer is 3%. If the Al content is greater, the effective absorption coefficient ae tends to be greater. 
       FIG. 7  shows the values of the refractive index n and extinction coefficient k of main materials. The following materials satisfying the conditions described above are preferably used: metal materials such as copper (Cu), palladium (Pd), zirconium (Zr), is niobium (Nb), chromium (Cr), nickel (Ni), gold (Au), platinum (Pt), titanium (Ti), tungsten (W), molybdenum (Mo) and tantalum (Ta); metal nitrides such as titanium nitride (TiN), chromium nitride (CrN), zirconium nitride (ZrN), niobium nitride (NbN), tantalum nitride (TaN) and molybdenum nitride (MoN); and amorphous silicon (a-Si). Multilayer films, alloys and the like of these materials may also be used. Materials such as silver (Ag) and aluminum (Al) falling outside the above range can also be used as long as the length d 2  of the light absorption layer in the cavity direction can be made large. 
       FIGS. 8(   a ) and  8 ( b ) respectively show the intensity distribution of a horizontal FFP obtained from the semiconductor laser device of this embodiment and the intensity distribution of a NFP estimated from the FFP.  FIG. 8  shows the measurement results obtained when the light absorption layer  17  is made of amorphous Si and has the distance d 1  from the center of the ridge waveguide portion  12   a  of 2 μm, the length d 2  in the cavity direction of 10 μm and the width d 3  in the end face direction of 50 μm. Compared with the case shown in  FIG. 3  provided with no light absorption layer, ripples are greatly reduced and a FFP shape close to a Gaussian shape is obtained. 
     Although the shape of the light absorption layer  17  was a rectangle in plan, other shapes may also be adopted as shown in  FIGS. 9(   a ) to  9 ( c ). Considering the distribution of leak light and the light intensity distribution in the cavity direction, however, the shape is preferably such that the width in the end face direction is greater as the position is closer to the light output end face and that the length in the cavity direction is greater as the position is closer to the ridge waveguide portion. 
     The fabrication method for the semiconductor laser device of this embodiment is the same as that for normal semiconductor laser devices except for forming the light absorption layers  17 . First, the semiconductor multilayer structure  12  is formed on the substrate  10  according to a normally-employed method. The p-type cladding layer  52  and the p-type contact layer  53  are then selectively etched to form the ridge waveguide portion  12   a . The dielectric layer  16  is then formed on the semiconductor multilayer structure  12  including the ridge waveguide portion  12   a , and a first opening exposing the top face of the ridge waveguide portion  12   a  is formed. The p-side electrode made of Pd and Pt is then formed in the first opening. Thereafter, second openings exposing the p-type cladding layer  52  are formed in predetermined regions of the dielectric layer  16 . The formation of the second openings may be made by photolithography and with buffered hydrogen fluoride (BHF), for example. A material such as silicon or a metal is then evaporated to fill the second openings with the material to thereby form the light absorption layers  17 . Thereafter, the p-side electrode pad is formed at a predetermined position. After thinning the substrate  10 , the n-side electrode is formed on the back face of the substrate. Primary cleavage is then performed through breaking from the back face of the substrate to form cavity end faces, and multilayer dielectric reflection films are formed on the cavity end faces. Secondary cleavage is then performed in the cavity direction to thereby obtain a semiconductor laser device having a cavity structure. The resultant semiconductor laser device is packaged and routed. 
     The number of process steps may be reduced by forming the light absorption layers  17  in the following process. After the formation of the dielectric layer  16  on the semiconductor multilayer structure  12 , openings are selectively formed on the top face of the ridge waveguide portion  12   a  and in the regions in which the light absorption layers  17  are to be formed. Thereafter, Pd and Pt are sequentially formed in the openings by electron beam evaporation and the like, to thereby form the p-side electrode  21  and the light absorption layers  17  simultaneously. The thicknesses of Pd and Pt may be 40 nm and 35 nm, respectively, for example. In place of the multilayer film of Pd and Pt, other materials may be used. 
     By employing the above process, in which the p-side electrode and the light absorption layers are formed simultaneously, the number of process steps can be reduced. When the distance d 1  of the light absorption layers from the ridge center was set at 2 μm, the length d 2  thereof in the cavity direction at 25 μm and the width d 3  thereof in the end face direction at 50 μm in the case of using the multilayer film of Pd and Pt for the p-side electrode and the light absorption layers, the resultant leak light absorption effect was roughly identical to that obtained when the length d 2  in the cavity direction was 10 μm in the case of using amorphous silicon. 
     Embodiment 2 
     Embodiment 2 of the present invention will be described with reference to the relevant drawings.  FIGS. 10(   a ) to  10 ( d ) show a semiconductor laser device of Embodiment 2, wherein (a) shows a plan structure, and (b), (c) and (d) respectively show cross-sectional structures taken along line Xb-Xb, line Xc-Xc and line Xd-Xd in (a). In  FIG. 10 , the same components as those in  FIG. 1  are denoted by the same reference numerals, and description thereof is omitted in this embodiment. 
     In the semiconductor laser device of this embodiment, each of the light absorption layers  17  has a first portion  17 A formed near the light output end face  12 A and a second portion  17 B formed near the rear end face  12 B. 
     The effect of absorbing leak light by the light absorption layer depends on the length of the light absorption layer in the cavity direction. Hence, by forming a light absorption layer also near the rear end face, the light absorption effect can be roughly doubled. 
     From the standpoint of the process for fabricating a semiconductor laser, it is preferred that the first portion  17 A and the second portion  17 B have the same shape and that the spacing between the first portion  17 A and the light output end face is equal to the spacing between the second portion  17 B and the rear end face. In other words, in the process for fabricating a semiconductor laser device, the first portion  17 A and the second portion  17 B are preferably symmetric with respect to the primary cleavage line. 
     Specifically, as shown in  FIG. 11 , in two laser cavities formed consecutively in the cavity direction on the substrate, the first portion  17 A and the second portion  17 B are formed to be axially symmetric with respect to a primary cleavage line  61 . If there is a structure asymmetric with respect to a cleavage line, a cleavage failure such as cleavage deviation, a crack and a step are likely to occur. Such a cleavage failure causes deterioration of the FFP shape. By forming the first portion  17 A and the second portion  17 B symmetrically with respect to the primary cleavage line, therefore, occurrence of a cleavage failure can be reduced. This can not only improve the FFP shape but also improve the yield. In the resultant semiconductor laser device after the cleavage, the first portion  17 A and the second portion  17 B are axially symmetric with respect to the center line of the cavity structure in the end face direction. 
     To further facilitate the cleavage, cleavage guide grooves  62  may be formed as shown in  FIG. 12 . The cleavage guide grooves  62  may be of any shape, but correct guidance can be ensured if the top end thereof is V-shaped. Also, if the depth is as large as about 3 μm, reaching the n-type cladding layer  32  or the n-type GaN layer  31 , cleavage can be correctly guided. 
     In the case of forming the cleavage guide grooves, steps  62   a  will be formed at the light output end face  12 A and the rear end face  12 B as marks of the cleavage guide grooves as shown in  FIG. 13 . Note that the cleavage guide grooves may also be formed in the case of Embodiment 1 in which the light absorption layers  17  are only formed near the light output end face  12 A. In this case, also, cleavage failures can be reduced. 
     In this embodiment, also, the number of process steps can be reduced by forming the light absorption layers  17  and the p-side electrode  21  using the same materials. 
     Embodiment 3 
     Embodiment 3 of the present invention will be described with reference to the relevant drawings.  FIGS. 14(   a ) to  14 ( d ) show a semiconductor laser device of Embodiment 3, wherein (a) shows a plan structure, and (b), (c) and (d) respectively show cross-sectional structures taken along line XIVb-XIVb, line XIVc-XIVc and line XIVd-XIVd in (a). In  FIG. 14 , the same components as those in  FIG. 1  are denoted by the same reference numerals, and description thereof is omitted in this embodiment. 
     In the semiconductor laser device of this embodiment, the light absorption layers  17  are covered with the dielectric layer  16 . Hence, the electrical insulation between the p-side electrode pad and the light absorption layers  17  can be secured even when the length of the light absorption layers  17  in the cavity direction is large. This permits formation of the light absorption layers  17  using a material small in effective absorption coefficient ae. For example, even with use of a material whose effective absorption coefficient αe is 100 cm −1 , the leak light intensity can be reduced to 1/10 or less by setting the length of the light absorption layers  17  in the cavity direction at 300 μm or more. 
     For a semiconductor laser device having a short cavity length, also, the light absorption layers can secure a large length in the cavity direction. For example, when the cavity length is 400 μm, a light absorption layer having a length of 390 μm in the cavity direction, i.e., the entire length excluding 5 μm each from both end faces, can be formed. 
     The semiconductor laser device of this embodiment may be fabricated in the following manner. The ridge waveguide portion  12   a  is formed as in Embodiment 1. Thereafter, the light absorption layers  17  made of amorphous silicon having a thickness of 200 nm are formed on both sides of the ridge waveguide portion  12   a . Subsequently, a SiO 2  film having a thickness of 200 nm is formed on the entire surface of the substrate  10 , and then a portion of the SiO 2  film formed on the ridge waveguide portion  12   a  is selectively etched, to thereby form the dielectric layer  16 . The p-side electrode  21  is then formed on the ridge waveguide portion  12   a . Subsequently, after formation of the p-side electrode pad  23 , the n-side electrode  22  and the like, cleavage, formation of reflection films and the like are performed. 
     By following the above process, in which the light absorption layers are formed immediately after formation of the ridge waveguide portion  12   a , insulation of the light absorption layers  17  from the p-side electrode  21  and the p-side electrode pad  23  can be secured with only the SiO 2  dielectric layer  16  for current narrowing and light trapping. Hence, light absorption layers with a higher leak light absorption effect can be formed with the same number of process steps as in Embodiment 1. With this configuration, a nitride semiconductor laser providing a more satisfactory FFP shape can be implemented. Note that in this embodiment, also, the cleavage guide grooves can be provided. 
     Embodiment 4 
     Embodiment 4 of the present invention will be described with reference to the relevant drawings.  FIGS. 15(   a ) to  15 ( d ) show a semiconductor laser device of Embodiment 4, wherein (a) shows a plan structure, and (b), (c) and (d) respectively show cross-sectional structures taken along line XVb-XVb, line XVc-XVc and line XVd-XVd in (a). In  FIG. 15 , the same components as those in  FIG. 1  are denoted by the same reference numerals, and description thereof is omitted in this embodiment. 
     The semiconductor laser device of this embodiment includes an insulating layer  18  formed between the light absorption layers  17  and the p-side electrode pad  23 . With placement of the insulating layer  18 , the electrical insulation between the p-side electrode pad and the light absorption layers  17  can be secured even when the length of the light absorption layers  17  in the cavity direction is large. Also, with placement of the insulating layer  18 , the light absorption layers  17  can be formed after the p-side electrode  21  is formed and heat-treated. This provides an additional effect of widening the range of selection of materials for the p-side electrode  21  and the light absorption layers  17 . 
     The semiconductor laser device of this embodiment may be fabricated in the following manner. After formation of the p-side electrode  21  and the light absorption layers  17  as in Embodiment 1, the insulating film  18  made of SiO 2  having a thickness of 50 nm is formed on the entire surface of the substrate  10 . Thereafter, an opening exposing the p-side electrode  21  is formed in the insulating film  18  by photolithography and wet etching with BHF. Subsequently, after formation of the p-side electrode pad  23 , the n-side electrode  22  and the like, cleavage, formation of reflection films and the like are performed. The insulating layer  18  may be made of a material other than SiO 2 . In this embodiment, also, the number of process steps can be reduced by forming the light absorption layers  17  and the p-side electrode  21  using the same materials. Also, cleavage guide grooves may be provided. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, a nitride semiconductor laser device in which ripples are reduced and the far field pattern shape is close to a Gaussian shape can be implemented. Hence, the present invention is useful for a blue to ultraviolet semiconductor laser device using a nitride and a fabrication method for the same, and in particular for a nitride semiconductor laser device serving as a write and read light source for a high-density optical disc and a fabrication method for the same.