Patent Publication Number: US-2009218593-A1

Title: Nitride semiconductor light emitting device and method of frabicating nitride semiconductor laser device

Description:
This nonprovisional application is based on Japanese Patent Application No. 2005-363590 filed with the Japan Patent Office on Dec. 16, 2005, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to nitride semiconductor light emitting devices and methods of fabricating nitride semiconductor laser devices. 
     2. Description of the Background Art 
     It is generally known that a semiconductor laser device having a cavity with a facet degraded is impaired in reliability. It is considered that a cavity has a facet degraded when a nonradiative recombination level exists and accordingly the facet excessively generates heat. A major cause of such level is that the cavity has a facet oxidized. 
     Accordingly Japanese Patent Laying-Open No. 09-162496 (Patent Document 1) discloses a method coating a facet of a cavity with a coating film formed of a nitride not excluding oxygen to prevent the cavity from having the facet oxidized. Furthermore, Japanese Patent Laying-Open No. 2002-237648 (Patent Document 2) discloses a method coating a facet of a cavity of a nitride semiconductor laser device with a coating film of dielectric material of the same nitride as the facet to reduce a nonradiative recombination level. 
     Thus coating a facet of a cavity with a coating film of nitride has conventionally been known. Among nitrides, aluminum nitride (AlN) is particularly chemically and thermally stable and is an insulator of good quality, and is also high thermal conductivity and significantly effectively dissipates heat. As such, it exhibits an excellent feature as a coating film coating a facet of a cavity of a semiconductor laser (see Japanese Patent Laying-Open No. 03-209895 (Patent Document 3) for example). A coating film which does not contain oxygen, however, is generally high in stress and considered to lead to degradation such as dark line for example. 
     SUMMARY OF THE INVENTION 
     The present inventors have studied to develop a technique to form a coating film of AlN aforementioned on a facet of a cavity to implement a nitride semiconductor laser device that can be driven for high output without impairing reliability as the cavity has the facet degraded. 
     Initially, ECR sputtering is performed with aluminum (Al) and gaseous nitrogen to deposit a coating film of AlN of 50 nm thick at 100° C. on a light emitting facet of a cavity of a nitride semiconductor laser device. Furthermore, the nitride semiconductor laser device is provided at a light reflecting facet of the cavity with a highly reflective film formed of a pair of silicon oxide film and titanium oxide film to obtain a high reflectance of at least 95%. 
     The nitride semiconductor laser device thus fabricated was examined for catastrophic optical damage (COD) level before and after it was aged (for 300 hours at 70° C. at 100 mW, CW driven). A COD level indicates a maximum optical output value of a device that is obtained when it receives a gradually increasing current and finally has a facet destroyed thereby and thus stops lasing. Note that in the present specification a COD level is evaluated by a maximum optical output value of a device that is obtained when the device&#39;s optical output-current characteristic is measured with the device CW driven. Herein a COD level was evaluated by an average value of the COD levels of five such nitride semiconductor laser devices. 
     As a result, before they were aged they thermally saturated at an optical output of 400 mW, and after they were aged their cavities thus have their facets destroyed with COD, and their COD level was approximately 230 mW, as shown in  FIG. 11 . 
     The aged devices provided a decreased COD level probably because a nitride semiconductor forming the light emitting facet of the cavity and AlN forming the coating film had their mutual contact impaired and thus at their interface there was generated an intense heat, which degraded the facet of the cavity. The impaired contact was caused probably by the internal stress that the coating film has, a difference between the facet of the cavity and the coating film in thermal expansion coefficient, and the heat generated when the device lases. 
     Furthermore, if a coating film formed of AlN is provided at a light extraction plane serving as a light emitting portion of a nitride semiconductor light emitting diode device, and the light extraction plane, formed of nitride semiconductor, and the coating film formed of AlN have their mutual contact impaired, a nonradiative center or the like formed at their interface can absorb light, resulting in inefficiency in extracting light. 
     Accordingly the present invention contemplates a nitride semiconductor light emitting device that can provide an improved COD level while it is aged, and a method of fabricating a nitride semiconductor laser device. 
     The present invention is a nitride semiconductor light emitting device having a light emitting portion coated with a coating film, wherein the light emitting portion is formed of a nitride semiconductor, and the coating film in contact with the light emitting portion is formed of an oxynitride. 
     Herein the present nitride semiconductor light emitting device may be a nitride semiconductor laser device and the light emitting portion may be a facet of a cavity. 
     Furthermore in the present nitride semiconductor light emitting device the oxynitride can be aluminum oxynitride or silicon oxynitride. 
     Furthermore in the present nitride semiconductor light emitting device the oxynitride preferably contains oxygen in an amount of at least 2 atomic % and at most 35 atomic %. 
     Furthermore in the present nitride semiconductor light emitting device the oxynitride more preferably contains oxygen in an amount of at least 2 atomic % and at most 15 atomic %. 
     Furthermore in the present nitride semiconductor light emitting device a film formed of an oxide or a nitride can be deposited on the coating film. 
     Furthermore in the present nitride semiconductor light emitting device the film formed of the nitride can be a film formed of a nitride of at least one of silicon and aluminum. Herein if the film formed of the nitride is a film formed of the nitride of silicon the film preferably has a thickness of at least 5 nm. 
     Furthermore in the present nitride semiconductor light emitting device the film formed of the oxide can be a film formed of at least one selected from the group consisting of aluminum oxide, silicon oxide, titanium oxide, hafnium oxide, zirconium oxide, niobium oxide, tantalum oxide, and yttrium oxide. 
     Furthermore in the present nitride semiconductor light emitting device on the coating film a film formed of an oxynitride can be deposited. Herein the film formed of the oxynitride can be a film formed of an oxynitride of silicon or aluminum. 
     Furthermore in the present nitride semiconductor light emitting device on the coating film a film formed of magnesium fluoride can be deposited. 
     Furthermore the present invention is a method of fabricating a nitride semiconductor laser device with a cavity having a facet coated with a coating film, comprising the steps of: providing cleavage to form the facet of the cavity; and coating the facet of the cavity with a coating film formed of an oxynitride. 
     Furthermore in the present method of fabricating the nitride semiconductor laser device the oxynitride can be provided by employing aluminum oxide as a target. 
     Furthermore in the present method of fabricating the nitride semiconductor laser device the oxynitride can be provided by employing aluminum oxynitride as a target. 
     The present invention can thus provide a nitride semiconductor light emitting device that can provide an improved COD level while it is aged, and a method of fabricating a nitride semiconductor laser device. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross section of one preferred example of the present nitride semiconductor laser device in a first embodiment. 
         FIG. 2  is a schematic side view of the present nitride semiconductor laser device of the first embodiment shown in  FIG. 1 , as seen along the length of its cavity. 
         FIG. 3  schematically shows a configuration of an ECR sputtering deposition apparatus. 
         FIG. 4  represents a result of an analysis in composition, as seen depthwise, by AES of an aluminum oxynitride film separately prepared under a condition identical to that adopted in the first embodiment of the present invention. 
         FIG. 5  shows a result of examining a COD level of the present nitride semiconductor laser device in the first embodiment before and after it is aged. 
         FIG. 6  is a graph representing an optical output-current characteristics of the present nitride semiconductor laser device in the first embodiment after it is aged. 
         FIG. 7  is a graph representing an optical output-current characteristics of a conventional nitride semiconductor laser device after it is aged. 
         FIG. 8  shows a result of examining a COD level dependency of a content of oxygen of a coating film formed of aluminum oxynitride in the present nitride semiconductor laser device of the first embodiment. 
         FIG. 9  represents a result of an analysis in composition, as seen depthwise, by AES of a coating film deposited on a light emitting facet of a cavity of the present nitride semiconductor laser device in a second embodiment. 
         FIG. 10  represents a result of an analysis in composition, as seen depthwise, by AES of a coating film deposited on a light emitting facet of a cavity of the present nitride semiconductor laser device in a third embodiment. 
         FIG. 11  compares a COD level of a conventional nitride semiconductor laser device that is obtained before it is aged and a COD level thereof that is obtained after it is aged. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter embodiments of the present invention will be described. Note that in the figures, identical reference characters denote identical or corresponding components. 
     The present invention is a nitride semiconductor light emitting device having a light emitting portion coated with a coating film. The light emitting portion is formed of nitride semiconductor and the coating film contacting the light emitting portion is formed of an oxynitride. Thus in the present invention the light emitting portion can be coated with a coating film formed of an oxynitride to allow the light emitting portion formed of the nitride semiconductor and the coating film formed of the oxynitride to contact each other more closely to allow the nitride semiconductor light emitting device to achieve an improved COD level while it is aged. 
     Herein the present nitride semiconductor light emitting device for example includes a nitride semiconductor laser device, a nitride semiconductor light emitting diode device, and the like. Furthermore, if the present nitride semiconductor light emitting device is a nitride semiconductor laser device, its light emitting portion corresponds to a facet of a cavity. If the present nitride semiconductor light emitting device is the nitride semiconductor light emitting diode device, then its light emitting portion corresponds to a light extraction plane. Note that in the present invention the nitride semiconductor laser device indicates a semiconductor laser device at least having an active layer and a clad layer formed of material containing Al x In y Ga z N (a compound of at least one group-III element selected from the group consisting of aluminum, indium and gallium, and nitrogen, a group-V element) as a main component, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z≠0. Furthermore in the present invention a facet of a cavity means a mirror plane formed at least by cleaving a stack including an active layer and a clad layer stacked on the substrate. 
     Furthermore, the oxynitride employed in the prevent invention can be implemented for example by aluminum oxynitride or silicon oxynitride. Herein the oxynitride preferably contains oxygen as a constituent in an amount of at least 2 atomic % and at most 35 atomic %. If the oxynitride contains oxygen in an amount of less than 2 atomic %, it provides a tendency that the light emitting portion formed of the nitride semiconductor and the coating film formed of the oxynitride contact each other less closely and heat is generated and thus impairs the light emitting portion. If the oxynitride contains oxygen in an amount of more than 35 atomic %, the oxygen contained in the coating film oxidizes the light emitting portion formed of the nitride semiconductor and a nonradiative recombination level is caused, and a tendency to provide a reduced COD level is provided. More preferably the oxynitride contains oxygen as a constituent in an amount of at least 2 atomic % and at most 15 atomic %. 
     Furthermore the coating film formed of the oxynitride employed in the present invention preferably has a thickness of at least 1 nm. If the coating film has a thickness of less than 1 nm it is hardly controllable in thickness and may fails to completely coat the light emitting portion. In contrast, if the coating film is excessively large in thickness, there is a possibility that stress becomes an issue to be considered. However, the present invention&#39;s effect would not be impaired because the coating film is excessively large in thickness. 
     Furthermore, the coating film formed of the oxynitride employed in the prevent invention may underlie a film formed of an oxide (e.g., at least one type of oxide selected from the group consisting of aluminum oxide, silicon oxide, titanium oxide, hafnium oxide, zirconium oxide, niobium oxide, tantalum oxide, and yttrium oxide), a nitride (e.g., at least one nitride of aluminum nitride and silicon nitride) and the like to control reflectance. Furthermore in the present invention on the coating film formed of the oxynitride there may be deposited a film formed of magnesium fluoride (MgF) or a similar fluoride or a film formed of silicon oxynitride, aluminum oxynitride or the like as a film formed of an oxynitride different in composition in oxygen from the oxynitride forming the underlying coating film. 
     For example, as shown Table 1, a variety of types layers may be combined together to control reflectance. Note that while Table 1 shows that a first layer adjacent to a surface of the nitride semiconductor to serve as a coating film is formed of aluminum oxynitride, it may alternatively be formed of silicon oxynitride. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 1st layer adjacent to nitride semiconductor 
                 2nd layer overlying 1st layer 
                 3rd layer overlying 2nd layer 
               
            
           
           
               
               
               
               
               
               
            
               
                 material 
                 thickness 
                 material 
                 thickness 
                 material 
                 thickness 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 aluminum oxynitride 
                 20 nm 
                 aluminum oxide (Al 2 O 3 ) 
                 140 
                 nm 
                 — 
                 — 
               
               
                 (oxygen content: 2%) 
               
               
                 aluminum oxynitride 
                 10 nm 
                 aluminum oxynitride 
                 5 
                 nm 
                 aluminum oxide (Al 2 O 3 ) 
                 140 nm 
               
               
                 (oxygen content: 5%) 
                   
                 (oxygen content: 10%) 
               
               
                 aluminum oxynitride 
                 50 nm 
                 silicon oxide (SiO 2 ) 
                 120 
                 nm 
                 — 
                 — 
               
               
                 (oxygen content: 11%) 
               
               
                 aluminum oxynitride 
                  6 nm 
                 titanium oxide (TiO 2 ) 
                 50 
                 nm 
                 — 
                 — 
               
               
                 (oxygen content: 15%) 
               
               
                 aluminum oxynitride 
                 10 nm 
                 tantalum oxide (Ta 2 O 5 ) 
                 100 
                 nm 
                 — 
                 — 
               
               
                 (oxygen content: 5%) 
               
               
                 aluminum oxynitride 
                 10 nm 
                 yttrium oxide (Y 2 O 5 ) 
                 230 
                 nm 
                 — 
                 — 
               
               
                 (oxygen content: 5%) 
               
               
                 aluminum oxynitride 
                 80 nm 
                 zirconium oxide (ZrO 2 ) 
                 100 
                 nm 
                 — 
                 — 
               
               
                 (oxygen content: 5%) 
               
               
                 aluminum oxynitride 
                 20 nm 
                 silicon nitride (Si 3 N 4 ) 
                 20 
                 nm 
                 silicon oxide (SiO 2 ) 
                 140 nm 
               
               
                 (oxygen content: 2%) 
               
               
                 aluminum oxynitride 
                 20 nm 
                 silicon nitride (Si 3 N 4 ) 
                 20 
                 nm 
                 aluminum oxide (Al 2 O 3 ) 
                 120 nm 
               
               
                 (oxygen content: 4%) 
               
               
                 aluminum oxynitride 
                 25 nm 
                 silicon nitride (Si 3 N 4 ) 
                 150 
                 nm 
                 — 
                 — 
               
               
                 (oxygen content: 3%) 
               
               
                 aluminum oxynitride 
                 20 nm 
                 silicon nitride (Si 3 N 4 ) 
                 150 
                 nm 
                 aluminum oxide (Al 2 O 3 ) 
                 120 nm 
               
               
                 (oxygen content: 4%) 
               
               
                 aluminum oxynitride 
                 20 nm 
                 silicon oxynitride 
                 150 
                 nm 
                 — 
                 — 
               
               
                 (oxygen content: 7%) 
                   
                 (oxygen content: 5%) 
               
               
                 aluminum oxynitride 
                 20 nm 
                 silicon nitride (Si 3 N 4 ) 
                 20 
                 nm 
                 silicon oxynitride 
                 150 nm 
               
               
                 (oxygen content: 7%) 
                   
                   
                   
                   
                 (oxygen content: 5%) 
               
               
                   
               
            
           
         
       
     
     Furthermore, if the present nitride semiconductor light emitting device is a nitride semiconductor laser device, then the present nitride semiconductor laser device can be fabricated by a method including the steps of: forming a facet of a cavity by cleavage; and coating the facet of the cavity with a coating film formed of an oxynitride. 
     Herein if the coating film is formed of aluminum oxynitride then the coating film can be formed for example by providing a target of aluminum oxide in a film deposition chamber, introducing only gaseous nitrogen into the film deposition chamber, and performing reactive sputtering. Thus employing the target of aluminum oxide can eliminate the necessity of intentionally introducing gaseous oxygen into the film deposition chamber in depositing the oxynitride. 
     Furthermore, if a reactive sputtering apparatus is employed, the target formed of aluminum oxide can be dispensed with: instead, a target formed of aluminum can be placed in the film deposition chamber and gaseous oxygen can subsequently be introduced thereinto, and a microwave can be applied to generate a plasma of oxygen to oxidize a surface of the target of aluminum to prepare on the surface a target formed of aluminum oxide. 
     For example, the aluminum oxynitride can be produced from a target of aluminum through such steps as follows: 
     Step 1: A target of aluminum is placed in the film deposition chamber of the reactive sputtering apparatus and gaseous oxygen is introduced into the chamber. A microwave is applied to expose the target of aluminum to a plasma of oxygen to oxidize the target of aluminum from a surface by approximately several nm to prepare a target formed of aluminum oxide; and 
     Step 2: Subsequently, gaseous nitrogen and gaseous argon are introduced in the film deposition chamber and a microwave is applied to cause a plasma thereof and in that condition the target of aluminum oxide is sputtered to form the aluminum oxynitride. 
     Between steps 1 and 2 there may be introduced the step of exposing a surface of the nitride semiconductor to a plasma of argon, a plasma of nitrogen, or a plasma of a gaseous mixture of argon and nitrogen to clean the surface of the nitride semiconductor. 
     Furthermore, if the coating film is formed of aluminum oxynitride, aluminum is prone to oxidation, and if gaseous oxygen is introduced into the film deposition chamber, there is a tendency that it is difficult to exert control to provide an oxynitride controlled in composition to have a small content of oxygen, and to reproduce such oxynitride. However, this can be addressed by using as a target of an aluminum oxide represented by Al x O y  and less oxidized, and avoiding introducing gaseous oxygen into the film deposition chamber and instead introducing gaseous nitrogen alone thereinto so that an aluminum oxynitride having a small content of oxygen can be relatively readily deposited, wherein 0&lt;x&lt;1, 0&lt;y&lt;0.6, and x+y=1. Furthermore, replacing the target of the aluminum oxide represented by Al x O y  and less oxidized with a target of an aluminum oxynitride having a small content of oxygen can be similarly effective, wherein 0&lt;x&lt;1, 0&lt;y&lt;0.6, and x+y=1. 
     Furthermore, modifying a degree of vacuum in the film deposition chamber and/or modifying temperature and/or the like condition(s) therein for film deposition can also provide an oxynitride variable in content of oxygen and hence composition. Note that when the film deposition chamber has lower degrees of vacuum it tends to help the oxynitride to introduce oxygen, and that higher temperatures for film deposition tend to prevent the oxynitride from introducing oxygen thereinto. 
     Furthermore, if the film deposition chamber has an internal wall oxidized or is provided at the internal wall with aluminum oxide, and thereafter gaseous argon and gaseous nitrogen are introduced into the film deposition chamber, and a target of Al is employed and sputtering is thus performed to deposit a film, then the internal wall has oxygen departed by a plasma, and a coating film formed of aluminum oxynitride can thus be formed. 
     Furthermore while in the above a nitride semiconductor laser device having a ridged stripe has been exemplified, the gist of the present invention relates to a film coating a light extraction plane allowing light generated in nitride semiconductor to be extracted from the nitride semiconductor externally. As such, the present invention is not limited to the nitride semiconductor laser device having the ridged stripe. It is also effectively applicable for example to a film coating a light extraction plane of a surface emitting laser, a film coating a light extraction plane of a nitride semiconductor light emitting diode (i.e., a surface of nitride semiconductor, or a side surface cleaved or divided in dividing into chips), and the like. 
     First Embodiment 
       FIG. 1  is a schematic cross section of one preferred example of a nitride semiconductor laser device of the present embodiment. More specifically, the present embodiment provides a nitride semiconductor laser device  100  including an n type GaN substrate  101 , an n type AlGaInN buffer layer  102  deposited thereon, an n type AlGaInN clad layer  103  deposited thereon, an n type AlGaInN guide layer  104  deposited thereon, an AlGaInN multi quantum well active layer  105  deposited thereon, a p type AlGaInN guide layer  106  deposited thereon, a p type AlGaInN clad layer  107  deposited thereon, and a p type AlGaInN contact layer  108  deposited thereon. It should be noted that each layer has a composition ratio adjusted as appropriate and thus irrelevant to the essence of the present invention. Furthermore in the present embodiment the nitride semiconductor laser device lases at a wavelength which can be adjusted, as appropriate, within a range for example of 370 nm to 470 nm depending on the composition ratio of AlGaInN multi quantum well active layer  105 . The present embodiment is adjusted to allow lasing at a wavelength of 405 nm. 
     Note that while the present embodiment employs a substrate formed of GaN, the present invention may be implemented with a substrate formed for example of AlGaN or AlN. 
     Furthermore in the present embodiment nitride semiconductor laser device  100  has p type AlGaInN clad layer  107  and p type AlGaInN contact layer  108  partially removed to have ridged stripe portion  111  extending in the direction of the length of a cavity. Herein ridged stripe portion  111  has a width for example of approximately 1.2 μm to 2.4 μm, typically approximately 1.5 μm. Note that while the present specification describes that ridged stripe portion  111  has a width of approximately 1.2 μm to 2.4 μm by way of example the present invention is also suitably applicable to a broad-area nitride semiconductor laser device used for example in applications for illumination. (The “broad area” means that ridged stripe portion  111  has a width of approximately 2 μm to 100 μm.) 
     Furthermore, p type AlGaInN contact layer  108  has a surface provided with a p electrode  110  and under p electrode  110  an insulation film  109  is provided except for a portion provided with ridged stripe portion  111 . Furthermore on n type GaN substrate  101  at a surface opposite that having the aforementioned layers stacked thereon an n electrode  112  is deposited. 
       FIG. 2  is a schematic side view of the nitride semiconductor laser device of the present embodiment shown in  FIG. 1 , as seen in the direction of the length of the cavity. Herein in the present embodiment nitride semiconductor laser device  100  has the cavity with a light emitting facet  113 , which serves as a light emitting portion of laser device  100 , coated with a coating film  114  formed of aluminum oxynitride and having a thickness of 50 nm, and a light reflecting facet  115  provided with a 6 nm thick aluminum oxynitride film  116 , a 80 nm thick aluminum oxide film  117 , a 71 nm thick silicon oxide film and a 46 nm thick titanium oxide film paired and deposited by four pairs in layers, with the silicon oxide film first deposited, and thereafter a 142 nm thick, frontmost silicon oxide film deposited to provide a highly reflective film  118 . 
     Note that coating film  114 , and aluminum oxynitride film  116 , aluminum oxide film  117  and highly reflective film  118  are deposited on facets  113  and  115 , respectively, of a cavity of a sample prepared that are exposed as a cleavage plane provided by cleaving a wafer having deposited on the n type GaN substrate the n type AlGaInN buffer layer and other aforementioned semiconductor layers in order in layers, and provided with a ridged stripe portion, followed by an insulation film and p and n electrodes. 
     It should be noted that preferably before coating film  114  is deposited, in the film deposition apparatus the cavity has facet  113  heated for example at least 100° C. and at most 500° C. to remove oxide film, impurity and the like from facet  113  to clean it, although the present invention may dispense with doing so. Furthermore the cavity may have facet  113  exposed to a plasma for example of argon or nitrogen and thus cleaned, although the present invention may dispense with doing so. Furthermore the cavity may also have facet  113  heated and simultaneously exposed to a plasma. Furthermore when the facet is exposed to the plasma, for example it may be exposed to a plasma of argon, followed by that of nitrogen, or vice versa. Other than argon and nitrogen, for example a rare gas for example of helium, neon, xenon, krypton or the like can also be used. Furthermore, while preferably the cavity is provided at facet  113  with coating film  114  deposited with the facet heated at least 100° C. and at most 500° C., the present invention may dispense with thus heating the facet in depositing coating film  114 . 
     While coating film  114  can for example be sputtered through electron cyclotron resonance (ECR) as described hereinafter, the film can also be formed by a variety of other sputtering techniques, chemical vapor deposition (CVD), electron beam deposition (EB) or the like. 
       FIG. 3  schematically shows an ECR sputtering deposition apparatus in structure. More specifically, the ECR sputtering deposition apparatus includes a film deposition chamber  200 , a magnetic coil  203 , and a microwave introduction window  202 . Film deposition chamber  200  has a gas inlet  201  and a gas outlet  209  and is internally provided with a target  204  of Al connected to an RF power supply  208 , and a heater  205 . Furthermore film deposition chamber  200  is also internally provided with a sample stage  207  and thereon a sample  206  as described above is set. Magnetic coil  203  is provided to generate a magnetic field required to generate a plasma and RF power supply  208  is used to sputter target  204  of Al. Through microwave introduction window  202  a microwave  210  is introduced into film deposition chamber  200 . 
     Then gaseous nitrogen is introduced through gas inlet  201  into film deposition chamber  200  at a flow rate of 5.5 sccm and gaseous oxygen is introduced thereinto at a flow rate of 1.0 sccm, and furthermore, gaseous argon is introduced thereinto at a flow rate of 20.0 sccm to efficiently generate a plasma to deposit a film faster. Note that the gaseous nitrogen and oxygen introduced into film deposition chamber  200  can be varied in ratio to allow coating film  114  to contain oxygen in a varied amount. Furthermore, in order to sputter target  204  made of Al, an RF power of 500 W is applied to target  204  of Al and a microwave power of 500 W required to generate a plasma is applied. As a result, at a film deposition rate of 1.7 Å/sec, coating film  114  formed of an aluminum oxynitride providing an index of refraction of 2.0 for light having a wavelength of 633 nm can be deposited. Coating film  114  is formed of aluminum, nitrogen and oxygen, and their respective contents (atomic %) can be measured for example by Auger electron spectroscopy (AES). Furthermore, the content of the oxygen forming coating film  114  can also be measured by transmission electron microscopy-energy dispersive x-ray spectroscopy (TEM-EDX). 
       FIG. 4  shows a result of a depthwise AES composition analysis of an aluminum oxynitride separately provided under a condition identical to that described above. The contents of aluminum, oxygen and nitrogen, respectively, were obtained as based on an AES signal in intensity, with the sensitivity of a peak of each element considered. Herein, the aluminum, the oxide and the nitrogen together assume 100 atomic % and an element other than the aluminum, oxygen and nitrogen and contained in a small amount, such as argon, is excluded therefrom. 
     As shown in  FIG. 4 , the aluminum oxynitride contains aluminum in an amount of 34.8 atomic %, oxygen in an amount of 3.8 atomic %, and nitrogen in an amount of 61.4 atomic % and has a substantially uniform composition as seen depthwise. It should be noted that, although not shown in  FIG. 4 , a negligible amount of argon was detected. 
     Furthermore, the light reflecting facet  115  of the cavity may also have aluminum oxynitride film  116 , aluminum oxide film  117 , and highly reflective film  118  deposited for example by ECR sputtering, similarly as done when coating film  114  is deposited. Preferably before these films are also deposited the facet is heated and thus cleaned and/or exposed to a plasma and thus cleaned. It should be noted, however, that having a light emitting facet degraded would be considered as a more serious concern, since the light emitting facet has large optical density, and it is often the case that having a light reflecting facet degraded would not be a concern, since the light reflecting facet has smaller optical density than the light emitting facet. Accordingly the present invention only requires that the cavity have the light emitting facet coated with a coating film formed of oxynitride, and the cavity may have the light reflecting facet without aluminum oxynitride film or the like covering it. Furthermore while in the present embodiment the cavity has the light reflecting facet  115  provided with aluminum oxynitride  116  having a thickness of 6 nm, it can without a problem in particular be replaced with aluminum oxynitride  116  having as large a thickness for example as 50 nm. 
     Furthermore the cavity having a facet coated with the aforementioned film may subsequently be heated. This can be expected to remove moisture contained in the aforementioned film and improve the film in quality. 
     Thus the aforementioned sample is provided at the cavity on the light emitting facet  113  with coating film  114 , and on the light reflecting facet  115  with aluminum oxynitride film  116 , then aluminum oxide film  117  and then highly reflected film  118 , and thereafter divided into chips to obtain the nitride semiconductor laser device of the present embodiment. 
     The nitride semiconductor laser device of the present embodiment was examined for COD level before and after it was aged (for 300 hours at 70° C. at 100 mW, CW driven). A result thereof is shown in  FIG. 5 . As shown in  FIG. 5 , before it was aged it had a COD level of approximately 400 mW and after it was aged it had a COD level of approximately 350 mW. It has been found to provide a hardly degraded COD level. 
     Furthermore  FIG. 6  shows a graph of an optical output-current characteristic of the nitride semiconductor laser device of the present embodiment after it is aged. As shown in  FIG. 6 , after the nitride semiconductor laser device of the present embodiment was aged for 300 hours it still thermally saturated and the cavity was not observed to have light emitting facet  113  destroyed. The nitride semiconductor laser device of the present embodiment before it was aged of course also thermally saturated at an optical output of 400 mW, and the cavity was not observed to have light emitting facet  113  destroyed. 
     In contrast  FIG. 7  shows a graph of an optical output-current characteristic of a conventional nitride semiconductor laser device with a cavity having a light emitting facet provided with a coating film of AlN, as observed after it is aged for 300 hours at 70° C. at 100 mW, CW driven. As shown in  FIG. 7 , when the conventional nitride semiconductor laser device provided an optical output of approximately 250 mW, the cavity had the facet destroyed by COD. The above result thus reveals that the nitride semiconductor laser device of the present embodiment with a cavity having a light emitting facet provided with a coating film of aluminum oxynitride is compared with the conventional nitride semiconductor laser device with a cavity having a light emitting facet provided with a coating film of AlN, as observed after they are aged, the former provides an improved COD level. 
     A nitride semiconductor laser device with a cavity having a light emitting facet provided a coating film formed of oxynitride is not observed to provide a reduced COD level after it is aged probably because the facet of the cavity and the coating film can contact each other more closely. Nitride semiconductor contacts a coating film formed of oxynitride more closely than a coating film formed of AlN or a similar nitride probably because a strong bonding energy of aluminum, gallium or a similar group III element and oxygen acts. More specifically the oxynitride contains oxygen, which is considered to have a strong bond with aluminum, gallium or a similar group III element contained in the nitride semiconductor and thus contribute to enhanced contact. It can be said that the oxygen serves as a kind of adhesive. In other words this would be considered a technique that allows the nitride semiconductor and the coating film formed of the oxynitride to contact more closely. 
     The COD level dependency of a content of oxygen in the coating film of the aluminum oxynitride in the nitride semiconductor laser device of the present embodiment, was examined. A result thereof is shown in  FIG. 8 . More specifically, the nitride semiconductor laser device of the present embodiment having the coating film of the aluminum oxynitride with oxygen contained therein in an mount variable from 0 atomic % to 50 atomic % was aged for 300 hours (at 70° C. at 100 mW, CW driven) and its COD level was measured. Basically, oxygen and nitrogen both bond to aluminum. As such, while the coating film contains oxygen in a variable amount, the coating film contains aluminum in a substantially invariable amount (in atomic %), and nitrogen will accordingly be contained in an amount (in atomic %) decreased by that (in atomic %) of oxygen contained that is increased. 
     Furthermore in  FIG. 8  a circle indicates a position of a peak of an optical output in a optical output-current characteristic provided when the nitride semiconductor laser device does not have COD and thermally saturates, and its COD level will be higher than the indicated value, and a cross indicates that the nitride semiconductor laser device has COD, and the indicated value will be its COD level. 
     As shown in  FIG. 8 , it can be seen that if the laser device has a coating film formed of aluminum oxynitride containing oxygen in an amount of at least 2 atomic % and at most 35 atomic %, the laser device thermally saturates at an optical output of 300 mW or larger and it can be seen that the laser device exhibits an excellent characteristic. Accordingly the cavity preferably has the light emitting facet coated with a coating film containing oxygen in an amount of at least 2 atomic % and at most 35 atomic %. This provides a tendency that the laser device provides an improved COD level while it is aged. This is probably because the cavity&#39;s facet formed of the nitride semiconductor and the coating film formed of the aluminum oxynitride contact each other more closely and the cavity thus does not have the facet so oxidized as to cause a nonradiative recombination level and accordingly affect the device&#39;s COD level. Furthermore, when the coating film of the aluminum oxynitride contained oxygen in an amount of less than 2 atomic %, the cavity had the facet degraded, possibly because the facet of the nitride semiconductor and the coating film of the aluminum oxynitride contacted each other less closely, and this caused heat and thus degraded the facet. Furthermore, when the coating film contained oxygen in an amount of more than 35 atomic %, the laser device provided a decreased COD level, possibly because the oxygen contained in the coating film oxidized the cavity&#39;s facet of the nitride semiconductor and thus caused a nonradiative recombination level. 
     Second Embodiment 
     The present embodiment provides a nitride semiconductor laser device similar in configuration to that of the first embodiment except that the former has a cavity with a light emitting facet coated with a coating film modified in configuration and a light reflecting facet provided with a film modified in configuration. 
     More specifically in the present embodiment the nitride semiconductor laser device has a cavity with a light emitting facet provided with a coating film formed of aluminum oxynitride and having a thickness of 6 nm. Furthermore the cavity has a light reflecting facet provided with a 12 nm thick aluminum oxynitride film deposited under the same conditions as the coating film coating the light emitting facet of the cavity and on the aluminum oxynitride film a 81 nm thick silicon oxide film and a 54 nm thick titanium oxide film serving as one pair are deposited by four pairs in layers, with the silicon oxide film first deposited, and thereafter on a frontmost surface a 162 nm thick silicon oxide film is deposited to provide a highly reflective film. 
       FIG. 9  represents a result of an analysis in composition, as seen depthwise, by AES of the coating film deposited on the light emitting facet of the cavity of the nitride semiconductor laser device of the present embodiment. As shown in  FIG. 9 , the coating film contained aluminum, oxygen and nitrogen in amounts of 33.6 atomic %, 35.2 atomic %, and 31.2 atomic %, respectively, and had a substantially uniform composition as seen depthwise. 
     The nitride semiconductor laser device of the present embodiment was also aged for 300 hours (at 70° C. at 100 mW, CW driven) and its COD level was thereafter examined, similarly as has been described in the first embodiment. The nitride semiconductor laser device of the present embodiment after it was aged thermally saturated at an optical output of 330 mW, and has been confirmed to provide an improved COD level in comparison with that of the conventional nitride semiconductor laser device having a cavity with a light emitting facet provided with a coating film of AlN, as also observed after it was aged for 300 hours. 
     Third Embodiment 
     The present embodiment provides a nitride semiconductor laser device similar in configuration to that of the first embodiment except that the former has a cavity with a light emitting facet coated with a coating film modified in configuration and a light reflecting facet provided with a film modified in configuration. 
     More specifically in the present embodiment the nitride semiconductor laser device has a cavity with a light emitting facet provided with a coating film formed of aluminum oxynitride and having a thickness of 100 nm. Furthermore the cavity has a light reflecting facet provided with a 20 nm thick aluminum oxynitride film deposited under the same conditions as the coating film coating the light emitting facet of the cavity and on the aluminum oxynitride film a 81 nm thick silicon oxide film and a 54 nm thick titanium oxide film serving as one pair are deposited by four pairs in layers, with the silicon oxide film first deposited, and thereafter on a frontmost surface a 162 nm thick silicon oxide film is deposited to provide a highly reflective film. 
       FIG. 10  represents a result of an analysis in composition, as seen depthwise, by AES of the coating film deposited on the light emitting facet of the cavity of the nitride semiconductor laser device of the present embodiment. As shown in  FIG. 10 , the coating film contained aluminum, oxygen and nitrogen in amounts of 34 atomic %, 12 atomic %, and 54 atomic %, respectively, and had a substantially uniform composition as seen depthwise. 
     The nitride semiconductor laser device of the present embodiment was also aged for 300 hours (at 70° C. at 100 mW, CW driven) and its COD level was thereafter examined, similarly as has been described in the first embodiment. The nitride semiconductor laser device of the present embodiment after it was aged thermally saturated at an optical output of 333 mW, and has been confirmed to provide an improved COD level in comparison with that of the conventional nitride semiconductor laser device having a cavity with a light emitting facet provided with a coating film of AlN, as also observed after it was aged for 300 hours. 
     Fourth Embodiment 
     The present embodiment provides a nitride semiconductor laser device similar in configuration to that of the first embodiment except that the former has a cavity with a light emitting facet coated with a coating film modified in configuration and a light reflecting facet provided with a film modified in configuration. 
     More specifically in the present embodiment the nitride semiconductor laser device has a cavity with a light emitting facet provided with a coating film formed of aluminum oxynitride and having a thickness of 12 nm (with oxygen contained in an amount of 35 atomic %) and thereon a 60 nm thick aluminum oxide film. Furthermore the cavity has a light reflecting facet provided with a 6 nm thick aluminum oxynitride film deposited under the same conditions as the coating film coating the light emitting facet of the cavity and on the aluminum oxynitride film, a 80 nm thick aluminum oxide film is deposited, and furthermore, a 81 nm thick silicon oxide film and a 54 nm thick titanium oxide film serving as one pair are deposited by four pairs in layers, with the silicon oxide film first deposited, and thereafter on a frontmost surface a 162 nm thick silicon oxide film is deposited to provide a highly reflective film. 
     The nitride semiconductor laser device of the present embodiment was also aged for 300 hours (at 70° C. at 100 mW, CW driven) and its COD level was thereafter examined, similarly as has been described in the first embodiment. The nitride semiconductor laser device of the present embodiment after it was aged thermally saturated at an optical output of 336 mW, and has been confirmed to provide an improved COD level in comparison with that of the conventional nitride semiconductor laser device having a cavity with a light emitting facet provided with a coating film of AlN, as also observed after it was aged for 300 hours. 
     Fifth Embodiment 
     The present embodiment provides a nitride semiconductor laser device similar in configuration to that of the first embodiment except that the former is adjusted to lase at a wavelength of 460 nm and that it has a cavity with a light emitting facet coated with a coating film modified in configuration and a light reflecting facet provided with a film modified in configuration. Note that the wavelength is adjusted by changing in composition ratio the AlGaInN of an AlGaInN multi quantum well active layer. 
     More specifically in the present embodiment the nitride semiconductor laser device has a cavity with a light emitting facet provided with a coating film formed of aluminum oxynitride and having a thickness of 50 nm thick to provide a reflectance adjusted to be approximately 10%. Furthermore the cavity has a light reflecting facet provided with a 6 nm thick aluminum oxynitride film deposited under the same conditions as the film coating the light emitting facet of the cavity and on the aluminum oxynitride film a 80 nm thick aluminum oxide film is deposited, and furthermore a 81 nm thick silicon oxide film and a 54 nm thick titanium oxide film serving as one pair are deposited by four pairs in layers, with the silicon oxide film first deposited, and thereafter on a frontmost surface a 162 nm thick silicon oxide film is deposited to provide a highly reflective film. 
     The nitride semiconductor laser device of the present embodiment was also aged for 300 hours (at 70° C. at 100 mW, CW driven) and its COD level was thereafter examined, similarly as has been described in the first embodiment. The nitride semiconductor laser device of the present embodiment after it was aged thermally saturated at an optical output of 343 mW, and has been confirmed to provide an improved COD level in comparison with that of the conventional nitride semiconductor laser device having a cavity with a light emitting facet provided with a coating film of AlN, as also observed after it was aged for 300 hours. 
     The nitride semiconductor laser device of the present invention that lases at a wavelength of 460 nm can be employed as an excitation source for an illumination device. 
     Sixth Embodiment 
     The present embodiment provides a nitride semiconductor laser device similar in configuration to that of the first embodiment except that the former has a cavity with a light emitting facet coated with a coating film modified in configuration and a light reflecting facet provided with a film modified in configuration. 
     More specifically in the present embodiment the nitride semiconductor laser device has a cavity with a light emitting facet provided with a coating film formed of silicon oxynitride and having a thickness of 12 nm. Furthermore the cavity has a light reflecting facet provided with a 20 nm thick silicon oxynitride film deposited under the same conditions as the coating film coating the light emitting facet of the cavity and on the silicon oxynitride film a 81 nm thick silicon oxide film and a 54 nm thick titanium oxide film serving as one pair are deposited by four pairs in layers, with the silicon oxide film first deposited, and thereafter on a frontmost surface a 162 nm thick silicon oxide film is deposited to provide a highly reflective film. 
     The coating film provided on the light emitting facet of the cavity of the nitride semiconductor laser device of the present embodiment was measured in composition by AES depthwise. It contained aluminum, oxygen and nitrogen in amounts of 34 atomic %, 3 atomic % and 63 atomic %, respectively, and had a substantially uniform composition as seen depthwise. 
     The nitride semiconductor laser device of the present embodiment was also aged for 300 hours (at 70° C. at 100 mW, CW driven) and its COD level was thereafter examined, similarly as has been described in the first embodiment. The nitride semiconductor laser device of the present embodiment after it was aged thermally saturated at an optical output of 302 mW, and has been confirmed to provide an improved COD level in comparison with that of the conventional nitride semiconductor laser device having a cavity with a light emitting facet provided with a coating film of AlN, as also observed after it was aged for 300 hours. 
     The present embodiment was also examined for COD level dependency of content of oxygen in the coating film, similarly as described in the first embodiment. More specifically, the nitride semiconductor laser device of the present embodiment having the coating film of the silicon oxynitride with oxygen contained therein in an mount variable from 0 atomic % to 50 atomic % was aged for 300 hours (at 70° C. at 100 mW, CW driven) and its COD level was thereafter measured. Basically, oxygen and nitrogen both bond to silicon. As such, while the coating film contains oxygen in a variable amount, the coating film contains silicon in a substantially invariable amount (in atomic %), and nitrogen will accordingly be contained in an amount (in atomic %) decreased by that (in atomic %) of oxygen contained that is increased. 
     The COD level dependency of an amount of oxygen contained in the coating film of the silicon oxynitride showed substantially the same tendency as that for the aluminum oxynitride as shown in  FIG. 8 . More specifically the laser device having a cavity with a light emitting facet provided with a coating film containing oxygen in an amount of at least 2 atomic % and at most 35 atomic % provided a tendency to provide an improved COD level while it was aged. 
     Seventh Embodiment 
     The present embodiment provides a nitride semiconductor laser device similar in configuration to that of the first embodiment except that the former has a cavity with a light emitting facet coated with a coating film modified in configuration and a light reflecting facet provided with a film modified in configuration. 
     More specifically in the present embodiment the nitride semiconductor laser device has a cavity with a light emitting facet provided with a coating film formed of aluminum oxynitride and having a thickness of 25 nm and thereon a 150 nm thick silicon nitride film. Furthermore the cavity has a light reflecting facet provided with a 25 nm thick aluminum oxynitride film deposited under the same conditions as the coating film coating the light emitting facet of the cavity, and on the aluminum oxynitride film, a 50 nm thick silicon nitride film is deposited, and thereon, a 81 nm thick silicon oxide film and a 54 nm thick titanium oxide film serving as one pair are deposited by four pairs in layers, with the silicon oxide film first deposited, and thereafter on a frontmost surface a 162 nm thick silicon oxide film is deposited to provide a highly reflective film. 
     The coating film of aluminum oxynitride provided on the light emitting facet of the cavity of the nitride semiconductor laser device of the present embodiment was measured in composition by AES depthwise. It contained aluminum, oxygen and nitrogen in amounts of 34 atomic %, 3 atomic % and 63 atomic %, respectively, and had a substantially uniform composition as seen depthwise. 
     The nitride semiconductor laser device of the present embodiment was also aged for 300 hours (at 70° C. at 100 mW, CW driven) and its COD level was thereafter examined, similarly as has been described in the first embodiment. The nitride semiconductor laser device of the present embodiment after it was aged thermally saturated at an optical output of 350 mW, and has been confirmed to provide an improved COD level in comparison with that of the conventional nitride semiconductor laser device having a cavity with a light emitting facet provided with a coating film of AlN, as also observed after it was aged for 300 hours. 
     Note that if film of silicon nitride is deposited to have a thickness of 5 nm or smaller it is hardly formed uniformly. As such, preferably it is deposited to have a thickness of at least 5 nm. 
     Furthermore, if the nitride semiconductor laser device of the present embodiment is aged for a long period of time, it may have the coating film of aluminum oxynitride oxidized by moisture or the like present in the ambient in which the laser device is placed, and thus varied in index of refraction. Accordingly, when protecting the coating film against moisture is considered, the film of silicon nitride is preferably deposited to have a thickness of at least 40 nm. Furthermore when film deposition time and the like are considered the film of silicon nitride is preferably deposited to have a thickness of at most 300 nm n. 
     Furthermore while in the present embodiment a coating film formed of aluminum oxynitride underlies a film formed of silicon nitride, the present invention may be implemented with the silicon nitride film replaced with a silicon oxynitride film. If the coating film formed of aluminum oxynitride underlies the silicon oxynitride film, and protecting the coating film against moisture is considered, the silicon oxynitride film is preferably provided to contain oxygen in an amount of at most 40 atomic %. Furthermore the silicon oxynitride film is preferably deposited to have a thickness of at least 5 nm, more preferably at least 40 nm for a ground similar to that for the silicon nitride film described above. Furthermore the silicon oxynitride film is preferably deposited to have a thickness of at most 300 nm for a ground similar to that for the silicon nitride film described above. 
     Furthermore a nitride semiconductor laser device is fabricated (as a first nitride semiconductor laser device) to have a cavity with a light emitting facet provided with a coating film formed of aluminum oxynitride and having a thickness of 25 nm, a 150 nm thick silicon nitride film overlying the coating film, and a 30 nm thick aluminum oxide film overlying the silicon nitride film. 
     Furthermore a nitride semiconductor laser device is fabricated (as a second nitride semiconductor laser device) to have a cavity with a light emitting facet provided with a coating film formed of aluminum oxynitride and having a thickness of 25 nm, a 150 nm thick silicon nitride film overlying the coating film, and a 150 nm thick aluminum oxide film overlying the silicon nitride film. 
     Furthermore a nitride semiconductor laser device is fabricated (as a third nitride semiconductor laser device) to have a cavity with a light emitting facet provided with a coating film formed of aluminum oxynitride and having a thickness of 25 nm, a 150 nm thick silicon nitride film overlying the coating film, and a 30 nm thick silicon oxide film overlying the silicon nitride film. 
     Furthermore a nitride semiconductor laser device is fabricated (as a fourth nitride semiconductor laser device) to have a cavity with a light emitting facet provided with a coating film formed of aluminum oxynitride and having a thickness of 25 nm, a 150 nm thick silicon nitride film overlying the coating film, and a 150 nm thick silicon oxide film overlying the silicon nitride film. 
     The first to fourth nitride semiconductor laser devices thus fabricated also provided a result similar to that described above. 
     Note that the first to fourth nitride semiconductor laser devices each had the cavity with a light reflecting facet provided with films identical in configuration to those provided to the light reflecting facet of the cavity of the nitride semiconductor laser device of the seventh embodiment. 
     It should be noted that the aluminum oxynitride discussed in the present invention includes AlN with aluminum oxide mixed therewith and thus present therein, AlN with aluminum oxynitride present therein in the form of a crystal, and AlN with aluminum oxide and aluminum oxynitride present therein, and the present invention may adopt any form thereof. 
     Furthermore the above embodiments have been described with a coating film formed of aluminum oxynitride containing oxygen in an amount substantially uniform as seen depthwise, the coating film may be structured in multiple layers that contain oxygen in an amount graded and thus varying as seen depthwise or contain oxygen in different amounts, respectively. 
     Furthermore the above embodiments have mainly been described with a coating film formed of film of a silicon oxynitride or an aluminum oxynitride, the coating film may be a film of an aluminum-silicon oxynitride which is obtained by sputtering a target formed of a silicon-aluminum mixture and thus contains aluminum and silicon. 
     In doing so, controlling the silicon and the aluminum in the target or the silicon-aluminum mixture in composition can control the coating film in composition. Furthermore while in the above description a silicon-aluminum mixture is used as a target, sputtering to deposit film or the like can dispense with the silicon-aluminum mixture as the target; a target formed of silicon and that formed of aluminum may be arranged for example in a honeycomb, alternately, or the like, and simultaneously sputtered to provide a film formed of an aluminum and silicon-containing, aluminum-silicon oxynitride to serve as the coating film. 
     The present invention is applicable for example to a nitride semiconductor laser device lasing at a wavelength of the ultraviolet range to the green range, a nitride semiconductor laser device of a broad area type having a stripe of approximately several tens μm in width, a nitride semiconductor light emitting diode device lasing at a wavelength of the ultraviolet range to the red range, or the like. 
     Furthermore the coating film of the present invention would also be effectively applied on a facet of a nitride semiconductor laser device that is provided with a window structure (a structure averaging a composition in a vicinity of a facet of an active layer, such as used for example in a GaAs semiconductor laser device, to provide an increased bandgap therearound to provide an improved COD level). 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.