Abstract:
A semiconductor light-emitting device includes a light generation unit generating light with an oscillation wavelength λ, a light outgoing facet from which light generated at the light generation unit emerges, a light reflecting facet at which light generated at the light generation unit is reflected, and a high reflection film at the light reflecting facet and made of a dielectric multilayered film of at least three layers. The high reflection film includes a first layer which is in contact with the light reflection facet, is constituted of Al 2 O 3 , and has a thickness smaller than λ/4n, wherein n is the refractive index of Al 2 O 3 , a second layer which is in contact with the first layer, and a third layer which is in contact with the second layer and has a refractive index different from the refractive index of the second layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a semiconductor light-emitting device and more particularly, to a semiconductor laser device suited for use in short wavelength and high output applications. 
     2. Background Art 
     With optical recording devices such as an optical disc, a further increase in information recording amount has been demanded. In order to increase the information recording amount in optical recording devices, it is needed that the semiconductor light-emitting devices be shorter in wavelength and higher in output power. 
     Semiconductor laser devices have a problem in that if light absorption in coating films takes place, an optical absorption loss becomes so great that degradation of characteristics, such as a rise in operating current, is induced and heat is generated by the optical absorption of the coating film, thereby causing degradation at the facet called COD (catastrophic optical damage) and making it impossible to obtain desired optical output power. COD is a phenomenon wherein a facet of a semiconductor laser device locally absorbs light and becomes high in temperature, thereby melting and recrystallization of the semiconductor material at the facet of the semiconductor laser device. 
     Semiconductor laser devices have at least two facets including a light output facet called “front facet” and a light reflection facet called “rear facet”. Generally, in high power semiconductor lasers, a low reflectance coating film is formed on the front facet, and a high reflectance coating film is formed on the rear facet. Specifically, the reflectance of the coating film on the rear end face is typically 70% or higher, preferably 90% or higher. The reflectance of the coating film on the front end face, on the other hand, is determined based on the required characteristics of the semiconductor laser (Lower reflectance does not necessarily guarantee higher laser performance.) General high power semiconductor lasers, on the other hand, employ a coating film having a reflectance of approximately 3-7%, or a coating film having a reflectance of approximately 7-20% when it is necessary to prevent return of light. 
     The characteristics of this high reflection film are important so as to suppress the characteristic degradation of a semiconductor laser, COD and the like. The respective reflection films serve to protect the facet as well. 
     The high reflection film (high reflection coating) of a semiconductor laser disclosed in JP-A-Hei-6-224514 has the following configuration so as to improve a heat spreading characteristic and prevent COD at the rear facet on which the high reflection film is formed. More particularly, a Si film whose thermal conductivity is high is made thick and an Al 2 O 3  film whose thermal conductivity is low is made thin. The high reflection film of the semiconductor laser disclosed in JP-A-Hei-6-224514 is formed in this way and is improved in heat spreading, thereby preventing COD. 
     The reflection film of the semiconductor laser set out hereinabove in JP-A-Hei-6-224514 relating to a conventional technique is excellent in heat spreading in the semiconductor laser. However, lowering of optical absorption of the high reflection film is not taken into account. Blue semiconductor lasers also have another problem with regard to the coating films. Since the wavelength of the laser beam of these semiconductor lasers is short, the coating films have a high optical absorption coefficient and hence tend to degrade, as compared to conventional semiconductor lasers. Further, since conventional coating films do not adequately function as passivation films, the crystalline structure of the end face portions of the laser may be degraded, resulting in a reduced COD threshold. This has inhibited high output power of the semiconductor laser. 
     [Patent Literature 1] JP-A-Hei-6-224514 
     [Patent Literature 2] JP-A-Hei-9-326527 
     [Patent Literature 3] JP-A-2003-264333 
     [Patent Literature 4] JP-A-2004-327678 
     [Patent Literature 5] JP-A-2002-305348 
     [Patent Literature 6] JP-A-2006-165478 
     [Patent Literature 7] JP-A-2005-175111 
     [Patent Literature 8] JP-A-2004-327581 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished so as to solve the above problem and its object is to provide a semiconductor laser device wherein optical absorption of a high reflection film formed at rear facet is suppressed, thereby suppressing COD of the semiconductor laser device. 
     According to one aspect of the present invention, semiconductor laser device comprises a cavity formed to extend in a propagation direction of a laser beam. A first facet is formed on one end of the cavity to emit the laser beam. A second facet is formed on the other end of the cavity to reflect the laser beam. And a high reflection film formed at a side of said light reflecting facet, which made of an insulating and coating multi-layered film of at least three layers or more. The high reflection film includes, a first layer which is in contact with said light reflection facet, is constituted of Al 2 O 3  and has a thickness smaller than λ/4n wherein λ is the wavelength of the laser beam and n is a refractive index of Al 2 O 3 , a second layer which is in contact with said first layer and is constituted of any of SiO 2 , MgF 2 , NaF, Ca 2 F, CeF 3 , LaF 3 , LiF and NdF 3 , and a third layer which is in contact with said second layer and has a refractive index different from said second layer. 
     Other and further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a semiconductor laser according to a first embodiment; 
         FIG. 2  is a diagram illustrating the arrangement of the high reflection film; 
         FIG. 3  is a graph showing the dependence of the reflectance of the high reflection film on the thickness of the first layer; and 
         FIG. 4  shows an influence of thinning of the final layer on the reflectance of the high reflection film. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a diagram illustrating a semiconductor laser according to a first embodiment of the present invention. The semiconductor laser according to the first embodiment has a substrate  16 . An electrode  18  is formed on a back side of the substrate  16 . The substrate  16  has a cladding layer  15  of a first conduction type formed on the surface thereof. An active layer  12  is formed on the cladding layer  15 . The active layer  12  is a layer wherein light is emitted by recombination of carriers. A cladding layer  14  of a second conduction type is formed on the active layer  12 . The cladding layers  15 ,  14  are each a layer for increasing a carrier density in the active layer  12 . An electrode  19  is formed on the cladding layer  14 . After completion of the process of forming the semiconductor laser device having such a structure as set forth above, the wafer is cleft in position. 
     According to the cleavage of the wafer as mentioned above, there appear a light output facet called “front facet  21 ” formed on one end thereof to emit the laser beam and a light reflection facet called “rear facet  22 ” formed on the other end. A low reflection film  20  is formed on the front facet  21 . The low reflection film  20  is a insulating and coating multi-layered film. The low reflection film  20  protects the front facet  21  of the semiconductor laser device. In general, the low reflection film  20  is formed so that its reflectance is lower than that of the semiconductor laser device. A high reflection film  10  is formed on the rear facet  22 . The high reflection film  10  is a insulating and coating multi-layered film. One of the purposes of the formation of the high reflection film  10  is to protect the semiconductor laser device. The high reflection film is required to have a high reflectance and suppress an optical loss. The high reflection film generally has a reflectance of not lower than 70%, more preferably not lower than 90%. The reflectance of the high reflection film as described above is required to be high in order to extend the life of the semiconductor laser device. 
     The present invention is characterized in the high reflection film.  FIG. 2  is a diagram illustrating the arrangement of the high reflection film  10 . As stated hereinabove, the high reflection film  10  is a insulating and coating multi-layered film. A first layer  23  is formed at a position in contact with the rear facet  22  of this embodiment. The main purpose of forming the first layer  23  is to attain good adhesion to the rear facet  22 . This first layer  23  is in the form of a film made of Al 2 O 3 . The thickness of the first layer  23  of the embodiment is any value smaller than a value obtained by dividing the oscillation wavelength λ of a laser by four times the refractive index of the Al 2 O 3  film. Hence, when the thickness of the first layer  23  is taken as d, d satisfies the following equation (1)
 
 d&lt;λ/ 4 n   equation (1)
 
     The refractive index of the Al 2 O 3  film is about 1.6 with the case of a laser beam having an oscillation wavelength of 405 nm. If λ is 405 nm and n is 1.6, λ/4n is equal to about 63 nm. In this embodiment, the first layer  23  has a thickness of 10 nm, which meets the equation (1). 
     A second layer  24  is formed in contact with the first layer  23 . The second layer  24  is constituted of SiO 2 . 
     A third layer  26  is formed in contact with the second layer  24 . The third layer  26  is formed of Ta 2 O 5 . 
     A fourth layer  28  is formed in contact with the third layer  26 . The fourth layer  28  is formed of SiO 2 . 
     A fifth layer  30  is formed in contact with the fourth layer  28 . The fifth layer  30  is formed of Ta 2 O 5 . 
     A sixth layer  32  is formed in contact with the fifth layer  30 . The sixth layer  32  is formed of SiO 2 . 
     A seventh layer  34  is formed in contact with the sixth layer  32 . The seventh layer  34  is formed of Ta 2 O 5 . 
     An eighth layer  36  is formed in contact with the seventh layer  34 . The eighth layer  36  is formed of SiO 2 . 
     A ninth layer  38  is formed in contact with the eighth layer  36 . The ninth layer  38  is formed of Ta 2 O 5 . 
     A tenth layer  40  is formed in contact with the ninth layer  38 . The tenth layer  40  is formed of SiO 2 . 
     In this way, the first layer  23  to the tenth layer  40  are successively laminated on the side of the rear facet  22 , thereby forming the high reflection film  10 . 
     The thicknesses of the second layer  24  to ninth layer  38  are, respectively, determined in the following manner. That is, the thicknesses of the second layer  24  to ninth layer  38  are those values obtained by dividing the oscillation wavelength λ of the semiconductor laser device by four times the refractive indices of the constituent materials of the respective layers. With a laser beam having an oscillation wavelength of 405 nm, the refractive index of SiO 2  is at about 1.5. On the other hand, the refractive index of Ta 2 O 5  is at about 2.2. In this embodiment, SiO 2  is used as a low refractive index material and Ta 2 O 5  used as a high refractive index material. The SiO 2  films and the Ta 2 O 5  films are alternately superposed to increase the reflectance of the high reflection film  10 . 
     The tenth film  40 , which is a finally formed film of the high reflection film  10 , is formed so as to increase the moisture resistance of the semiconductor laser device. The moisture resistance is one of parameters indicative of reliability of a laser device and indicates reliability of a laser device under environments such as of high humidity, high temperature and the like. The tenth layer  40  of this embodiment is so formed as to provide a thickness that is any value smaller than a value obtained by dividing the oscillation wavelength λ of the semiconductor laser device by four times the refractive index of the SiO 2  film. 
     The respective layers covering from the first layer  23  to the tenth layer  40  making up of the high reflection film  10  are formed by a CVD (chemical vapor deposition) method, a vacuum deposition method or a sputtering method and the like. 
     In general, problem concerning the high power semiconductor laser, COD is a most important problem to be solved. COD is a phenomenon which occurs due to the vicinity of a facet of a semiconductor laser becoming an optical absorption region. If the optical output density becomes high, local generation of heat in the vicinity of the facet becomes so great that the band gap is reduced. Eventually, positive feedback is established in which the absorption becomes greater, thereby inviting a further rise in temperature. Finally, the temperature of the absorption region arrives at a melting point thereby causing the crystals to be melted and result in a reduction in the COD threshold. This phenomenon may, in some case, be caused by the coating film becoming high in temperature, leading to deteriorated reliability. This problem of COD becomes more pronounced as the high output power continues. In order to ensure reliability of a semiconductor laser, care should be taken so as to avoid COD. 
     As one of causes for COD, mention is made of the high reflection film per se formed at the rear facet, which undergoes optical absorption and generation of heat. If optical absorption takes place in the high reflection film, an optical absorption loss and reflection loss are so great that not only characteristic degradation such as of an increase in operating current is invited, but also heat is generated in the high reflection film in itself by the optical absorption to cause local generation of heat to occur in the vicinity of the facet, finally making it impossible to obtain a desired optical output power as a result of reduction in a COD threshold. To avid this, it is necessary to suppress optical absorption of the high reflection film per se. In a conventional semiconductor laser as disclosed, for example, in the foregoing Patent Literature 1, however, an effective measure for preventing a temperature rise of the high reflection film is not taken and particularly, a difficulty is involved in realizing a high output performance of a 405 nm semiconductor laser. 
     The semiconductor laser device of this embodiment is able to suppress generation of heat by optical absorption at the high reflection film formed on the light reflection facet. This is because of the effect of this embodiment wherein the first layer  23  is formed as thin as about 10 nm. It is usual that the thicknesses of the respective layers forming the high reflection film are so selected as to be at values, each obtained by dividing the laser oscillation wavelength λ by four times the refractive index n of the constituent material of each layer, i.e. at λ/4n. If, in this embodiment, the thickness of the first layer  23  is calculated in such a way as set out above, it becomes about 63 nm as mentioned hereinbefore. The first layer  23  of the embodiment has a thickness d that satisfies the equation (1) as described hereinbefore. Accordingly, the thickness d of the first layer  23  of the embodiment is, at least, smaller than 63 nm. In this embodiment, the thickness of the first layer  23  is set at 10 nm. The reliability of a laser device in case where the thickness d of the first layer  23  is smaller than λ/4n is better than the reliability in the case where the thickness d is set at λ/4n. This is because when the thickness d of the first layer  23  is smaller than λ/4n, an increasing effect of preventing COD is achieved over the case where the thickness is at λ/4n. In this sense, it is favorable from the standpoint of reliability to set the thickness of the first layer  23  at smaller than λ/4n. In this embodiment, the first layer  23  satisfies this requirement and can suppress COD. It will be noted that the number symbol  63  that is a criterion used to determine the thickness of the first layer  23  may be changed to another value if the oscillation wavelength λ or the refractive index n changes and thus, is not an absolute one. 
     In order to ensure adhesion that is one of roles of the first layer  23 , it is sufficient that the first layer  23  has a thickness of about 1 nm. In view of the film formation control and ease in film formation of the Al 2 O 3  film, the thickness of the first layer  23  is set at 10 nm in this embodiment. In this way, it is balanced to secure the adhesion between the high reflection film  10  and the light reflection facet  22  and to improve reliability of a laser device by improving the effect of preventing COD. 
     Al 2 O 3 , which has been accepted as not absorbing light when used in a conventional 780 nm semiconductor laser for CD and 660 nm semiconductor laser for DVD, may become a light absorber in a 405 nm semiconductor laser. It is suggested in Journal of Applied Physics, Vol. 88, No. 11, Dec. 1, 2000, pp. 6413-6417 that Al 2 O 3  may become a light absorber. In  FIG. 3  of the above non-patent literature, the intensity of cathode luminescence of Al 2 O 3  depend on wavelength is shown. From this, it will be seen that Al 2 O 3  has a light absorption characteristic over 300-500 nm. Hence, it is considered that the Al 2 O 3  film formed at the light reflection facet generates heat by absorption of light and causes a reduction in the COD threshold. In this sense, the COD suppressing effect resulting from the thinning of the first layer  23  constituted of Al 2 O 3  is not restricted within 405 nm semiconductor laser. More particularly, in all types of the semiconductor laser devices whose oscillation wavelength ranges about 300-500 nm, the thinning of the Al 2 O 3  layer has a good effect on a reduction of absorption in the film and suppressing the COD threshold. 
       FIG. 3  is a graph showing the dependence of the reflectance of the high reflection film  10  on the thickness of the first layer  23 . The results shown in  FIG. 3  are those values obtained from theoretical calculation. It will be noted that “pair” of “pair numbers” indicated in graph legends of  FIG. 3  means a double-layered structure made of a pair of SiO 2  and Ta 2 O 5 . This double-layered structure is one wherein the Ta 2 O 5  thin film is placed on the SiO 2  thin film. For instances “pair number=3” means that three pairs of SiO 2  and Ta 2 O 5  are placed between the first-layered Al 2 O 3  film and the final-layered SiO 2  film. The final layer is intended to mean a finally formed layer of the high reflection film. The final layer indicates the tenth layer  40  in this embodiment. The meaning of the provision of a plurality of pairs of SiO 2  and Ta 2 O 5  is to obtain a desired reflectance. As will be apparent from  FIG. 3 , a greater number of pairs generally result in a higher reflectance. 
     It has been generally accepted that a practically satisfactory reflectance is at about 90%. If the reflectance is 90% or over, an optical loss in the high reflection film can be suppressed. In this sense, the pair number n has been, in most cases, set at “4” or over and this embodiment provides a structure wherein the pair number is at 4. When the results of calculation of  FIG. 3  are checked in case of the pair number being at 4, it will be seen that when the film thickness of Al 2 O 3  exceeds 40 nm, the reflectance is below 90%. According, from the standpoint of the reflectance, the thickness of the Al 2 O 3  film is preferably at 40 nm or below. 
     Summarizing the foregoing, while preventing COD with the high reflection film having adequate adhesion and satisfying the conditions where the reflectance of the high reflection film is 90% or over, the possible thickness of the Al 2 O 3  serving as the first layer  23  is any value ranging 1 nm-40 nm. According to the arrangement of this embodiment, the thickness of the first layer  23  is formed at 10 nm, for which the above-stated effect can be obtained, thus enabling COD to be prevented while keeping a high reflectance of not smaller than 90%. 
     The thickness of the tenth layer  40 , which is a final layer of this embodiment, is set at a value that is smaller than a value obtained by dividing the oscillation wavelength λ by four times the refractive index of the SiO 2  film as described hereinabove. If the thickness of the tenth layer  40  is set at a value obtained by dividing the oscillation wavelength λ by four times a refractive index like the second layer  24  to ninth layer  38 , about 67 nm is obtained. Comparing a semiconductor laser which has a tenth layer  40  constituted of SiO 2  and having a thickness of any value smaller than 67 nm with a laser device wherein a tenth layer  40  is constituted of SiO 2  and has a thickness of 67 nm with respect to reliability (COD preventing effect), the former is better. Accordingly, the tenth layer  40  of this embodiment is set at a thickness of either smaller than 67 nm or a value obtained by dividing the oscillation wavelength λ by four times the refractive index of SiO 2 . 
       FIG. 4  shows an influence of thinning of the final layer  40  on the reflectance of the high reflection film  10 .  FIG. 4  shows the results of theoretical calculation. According to  FIG. 4 , in order to obtain a reflectance of 90% or over in case where the pair number is 4 as adopted in this embodiment, it is necessary that the thickness of the tenth layer  40  be any value of not larger than 40 nm. Accordingly, the thickness of the tenth layer  40  is preferably smaller than 67 nm from the standpoint of the COD preventing effect, and is preferably not larger than 40 nm from the standpoint of the reflectance. 
     Although the pair number of this embodiment is set at 4, the invention is not limited thereto. More particularly, the pair number n may take any value so far as a necessary reflectance is obtained. 
     Although SiO 2  is used as a low reflectance film in the even-number layers including the second layer  24 , fourth layer  28 , sixth layer  32  and eighth layer  36  in this embodiment, the invention should not be construed as limited thereto. More particularly, the effects of the invention can be obtained if the even-number layers forming the low reflectance film are made of any of MgF 2 , NaF, Ca 2 F, CeF 3 , LaF 3 , LiF, NdF 3  and other types of fluorides. Moreover, using Al 2 O 3  in place of SiO 2  as low reflectance films except for the second layer  24 , the effects of the invention can also be obtained. 
     Although, in this embodiment, odd-number layers except for the first layer  23  are used as the high reflectance film including the third layer  26 , fifth layer  30 , seventh layer  34  and ninth layer  38 , the invention should not be construed as limited thereto. More particularly, the effects of the invention can be obtained if using high reflectance materials such as Nb 2 O 5 , HfO 2 , ZrO 2 , Y 2 O 3 , AlN, SiN and the like. 
     In this embodiment, the semiconductor laser has been illustrated, to which the invention is not limited. More particularly, the feature of the invention resides in facet coating, and the effects of the invention can be obtained using, as a light-emitting device, not only a semiconductor laser, but also those light-emitting devices such as LED (light-emitting diode) and the like. 
     The features and advantages of the present invention may be summarized as follows. According to the invention, the reliability of the semiconductor light-emitting device can be improved. 
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
     The entire disclosure of a Japanese Patent Application No. 2007-121055, filed on May 1, 2007 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.