Abstract:
In a nitride semiconductor light-emitting device having an active layer between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer, the active layer has a multiple quantum well structure including a plurality of In x Ga 1-x N (0&lt;x≦1) quantum well layers and a plurality of In y Ga 1-y N (0≦y&lt;1) barrier layers stacked alternately, and at least one of the plurality of barrier layers has a super-lattice structure in which a plurality of barrier sub-layers having mutually different In composition ratios are stacked periodically.

Description:
This nonprovisional application is based on Japanese Patent Application No. 2006-315582 filed with the Japan Patent Office on Nov. 22, 2006, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a nitride semiconductor light-emitting device and particularly to improvement of its luminous efficacy. 
     2. Description of the Background Art 
     In a nitride semiconductor light-emitting device such as a nitride semiconductor LED (Light Emitting Diode), according to Japanese Patent Laying-Open No. 06-268257, an active layer interposed between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer has a multiple quantum well (MQW) structure in which a quantum well layer of In 0.2 Ga 0.8 N and a barrier layer of In 0.04 Ga 0.96 N are repeated alternately for 5 periods. 
     Further, according to Japanese Patent Laying-Open No. 11-330554, in an MQW active layer of a nitride semiconductor light-emitting device, a well layer of In 0.4 Ga 0.6 N and a barrier layer of GaN are repeated alternately for 5 periods. 
     As described above, in the prior art according to Japanese Patent Laying-Open Nos. 06-268257 and 11-330554, each barrier layer is formed of a single layer of GaN or InGaN. 
     Incidentally, since InN has a smaller energy band gap compared to GaN, a layer having relatively high In concentration can act as a quantum well layer, while a layer having relatively low In concentration can act as a barrier layer. From a viewpoint of improving luminous efficacy by trapping and recombining carriers in the well layer of the MQW active layer, it is preferable that there is larger difference in band gap between the quantum well layer and the barrier layer, and it is also preferable that the thickness of the barrier layer can be increased to some extent. 
     As a factor of improving the luminous efficacy by enhancing the quality of MQW active layer in the nitride semiconductor light-emitting device, it may be effective to reduce influence of distortion caused at the interface between the well layer and the barrier layer by lattice mismatch between these layers. 
     When GaN is used for the barrier layer as described in Japanese Patent Laying-Open No. 11-330554, it is difficult to avoid the influence of distortion caused by lattice mismatch between the InGaN well layer and the GaN barrier layer. Particularly, when a well layer having a high In concentration or a barrier layer having a large thickness is desired in an MQW active layer in a nitride semiconductor light-emitting device for emitting blue light or green light, the influence of distortion caused by lattice mismatch becomes more noticeable. 
     When InGaN is used both for the well layer and the barrier layer as in Japanese Patent Laying-Open No. 06-268257, it is not possible to increase In concentration in the barrier layer that is required to have sufficiently higher band gap compared to the well layer. Specifically, the In concentration must be made different between the well layer and the barrier layer, and thus it is difficult to avoid the influence of distortion caused by lattice mismatch between the well layer and the GaN barrier layer. 
     SUMMARY OF THE INVENTION 
     In spite of the difficulty of the problem related to lattice mismatch in the MQW active layer of nitride semiconductor light-emitting device as described above, a main object of the present invention is to provide a nitride semiconductor light-emitting device having improved luminous efficacy, by reducing the influence of distortion caused by lattice mismatch at the interface between the well layer and the barrier layer. 
     According to an aspect of the present invention, in a nitride semiconductor light-emitting device having an active layer between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer, the active layer has a multiple quantum well structure including a plurality of In x Ga 1-x N (0&lt;x≦1) quantum well layers and a plurality of In y Ga 1-y N (0≦y&lt;1) barrier layers stacked alternately and then at least one of the barrier layers has a super-lattice barrier structure in which a plurality of barrier sub-layers having mutually different In composition ratios are stacked periodically. 
     According to another aspect of the present invention, among the plurality of barrier layers, a barrier layer other than the one in contact with the p-type nitride semiconductor layer has the super-lattice barrier structure. Preferably, the well layer has an In composition ratio larger than that of any of the barrier sub-layers included in the barrier layer. Further, in the barrier layer, it is preferable that the barrier sub-layer having relatively smaller In composition ratio has larger thickness compared to the one having relatively larger In composition ratio. 
     According to a still further aspect of the present invention, the n-type nitride semiconductor layer includes an n-side super-lattice layer in contact with the active layer; and the n-side super-lattice layer includes n-side super-lattice sub-layers of In z1 Ga 1-z1 N (0&lt;z1&lt;1) and of In z2 Ga 1-z2 N (0≦z2&lt;1) stacked alternately. 
     Preferably, the well layer has an In composition ratio larger than that of any of the n-side super-lattice sub-layers included in the n-side super-lattice layer. Further, in the n-side super-lattice layer, it is preferable that the n-side super-lattice sub-layer having relatively smaller In composition ratio has larger thickness compared to the one having relatively larger In composition ratio. 
     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 showing a stacked-layer structure of an LED device according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, a nitride semiconductor light-emitting device according to an embodiment of the present invention will be described in detail, together with some comparative examples. 
     Embodiment 
       FIG. 1  is a schematic cross-section illustrating a stacked-layer structure of an LED device according to an Embodiment of the present invention. Needless to say, the present invention is not limited to this Embodiment. 
     The LED device of  FIG. 1  includes a GaN buffer layer  2 , an n-type GaN contact layer  3 , an n-side super-lattice layer  4 , an active layer  5 , a p-type AlGaN clad layer  6 , and a p-type contact layer  7  stacked successively on a sapphire substrate  1 . 
     P-type contact layer  7  is covered with a p-side transparent electrode  8 , on a partial area of which a p-side pad electrode  9  is formed. Further, an n-side pad electrode  10  is formed on a partially exposed area of n-type contact layer  3 . 
     The LED device of  FIG. 1  according to the Embodiment of the present invention as described above can be fabricated through the following process. 
     (Substrate Preprocessing) 
     First, a sapphire (C-plane) substrate  1  is set in a reaction chamber of an MOCVD (Metal Organic Chemical Vapor Deposition) apparatus. The substrate temperature is increased to 1050° C. letting hydrogen flow in the reaction chamber, whereby the substrate is dry-cleaned. 
     (Buffer Layer  2 ) 
     The cleaned substrate is cooled to 510° C. and then GaN buffer layer  2  is grown to about 20 nm thickness on substrate  1 , using ammonia and TMG (trimethyl gallium) for a source gas as well as hydrogen for a carrier gas. 
     (N-Type Contact Layer  3 ) 
     After forming GaN buffer layer  2 , the substrate temperature is increased to 1050° C. and then n-side GaN contact layer  3  doped with Si at a concentration of 1×10 18 /cm 3  is grown to 6 μm thickness, using ammonia and TMG (trimethyl gallium) for a source gas, hydrogen for a carrier gas, and silane for an impurity gas. 
     (N-Side Super-Lattice Layer  4 ) 
     After forming n-side GaN contact layer  3 , the substrate temperature is decreased to 800° C. and then n-side super-lattice layer  4  is formed using ammonia, TMG and TMI (trimethyl indium) for a source gas as well as nitrogen for a carrier gas. At this time, a first kind of n-side super-lattice sub-layer of In 0.05 Ga 0.95 N is grown to 2 nm thickness and then a second kind of n-side super-lattice sub-layer of GaN is grown thereon to 15 nm thickness. These first and second kinds of n-side super-lattice sub-layers are grown alternately for 9 periods, thereby forming n-side super-lattice layer  4 . 
     It is natural that the In composition ratio, thickness and the like of n-side super-lattice sub-layers are not limited to those exemplarily shown in this Embodiment. It is preferred, however, that the n-side super-lattice sub-layer having relatively smaller In composition ratio is made thicker compared to the one having relatively larger In composition ratio. The reason for this is that n-side super-lattice layer  4  should desirably have larger band gap compared to active layer  5 . More specifically, the n-side super-lattice sub-layer having relatively smaller In composition ratio should preferably have its thickness in the range of 5 to 20 nm, and the one having relatively larger In composition ratio should preferably have its thickness in the range of 1 to 5 nm. 
     (Active Layer  5 ) 
     After formation of n-side super-lattice layer  4 , the substrate temperature is decreased to 750° C. and then a quantum well layer of In 0.25 Ga 0.75 N is grown to 2.5 nm thickness, using ammonia, TMG and TMI for a source gas as well as nitrogen for a carrier gas. 
     Thereafter, a first kind of barrier sub-layer of GaN is grown to 5 nm thickness and a second kind of barrier sub-layer of In 0.1 Ga 0.9 N is grown thereon to 1.5 nm thickness. The first and second kinds of barrier sub-layers are grown alternately for 3 periods, thereby forming the super-lattice barrier structure having a thickness of 19.5 nm. 
     Then, the quantum well layers and the barrier layers having the super-lattice barrier structure are grown alternately for 5 periods. Thereafter, one more well layer is grown, and a barrier layer of a single GaN layer is grown to 20 nm thickness, thereby finishing MQW active layer  5 . 
     It is natural that the In composition ratio, thickness and the like of the barrier sub-layers are not limited to those exemplarily shown in this Embodiment. Preferably, however, the barrier sub-layer having relatively smaller In composition ratio should have larger thickness compared to the one having relatively larger In composition ratio. The reason for this is that the barrier layer should have larger band gap than the well layer. Specifically, the barrier sub-layer having relatively smaller In composition ratio should preferably have the thickness in the range of 2 to 10 nm, and the one having relatively larger In composition ratio should preferably have the thickness in the range of 1 to 3 nm. 
     (P-Type Clad Layer  6 ) 
     After formation of active layer  5 , the substrate temperature is increased to 950° C. and then a p-type Al 0.15 Ga 0.85 N clad layer  6  doped with Mg at a concentration of 5×10 19 /cm 3  is grown to about 30 nm thickness, using ammonia, TMA (trimethyl aluminum) and TMG for a source gas, hydrogen for a carrier gas, and CP 2 Mg (cyclopentadienyl magnesium) for an impurity gas. 
     (P-Type Contact Layer  7 ) 
     After formation of p-type clad layer  6 , while the substrate temperature is kept at 950° C., a p-type GaN contact layer  7  doped with Mg at a concentration of 1×10 20 /cm 3  is grown to 0.1 μm thickness, using ammonia and TMG for a source gas, hydrogen for a carrier gas, and Cp 2 Mg for an impurity gas. 
     (Annealing) 
     After formation of p-type contact layer  7 , the substrate temperature is lowered to 700° C. and then annealing is performed in order to activate Mg as the p-type impurity, introducing only nitrogen into the reaction chamber. 
     (Electrode Formation) 
     Thereafter, the wafer obtained by growing semiconductor layers  2  to  7  on substrate  1  is taken out from the reaction chamber, and a mask (not shown) patterned to a prescribed shape is formed on a surface of p-type contact layer  7 . Using the mask, the wafer is etched from the side of p-type contact layer  7  in an RIE (Reactive Ion Etching) apparatus, whereby n-type contact layer  3  is partially exposed as shown in FIG.  1 . 
     After the etching, a transparent electrode  8  including Pd is formed to 7 nm thickness on almost the entire surface of p-type contact layer  7 , and a p-side Au pad electrode  9  is formed to 0.5 μm thickness on a prescribed partial area of transparent electrode  8 . On the other hand, an n-side pad electrode  10  including Ti and Al is formed on the partially exposed area of n-type contact layer  3  exposed by the etching. An LED device is finished by the formation of these electrodes. 
     The LED device of this Embodiment obtained in the above-described manner had emission wavelength of 470 nm and emission output of 4.0 mW with forward current of 20 mA, and the amount of shift in emission wavelength was about 2 nm in the forward current range of 0.1 to 20 mA. 
     Comparative Example 1 
     An LED device according to Comparative Example 1 differs from the above-described Embodiment of the invention only in that the structure of active layer  5  is altered. The altered active layer  5  of Comparative Example 1 was formed in the following manner. 
     Specifically, at the substrate temperature of 750° C., a well layer of In 0.25 Ga 0.75 N was grown to 2.5 nm thickness, and a barrier layer of a single GaN layer was grown to 15 nm thickness, using ammonia, TMG and/or TMI for a source gas as well as hydrogen for a carrier gas. These well layer and barrier layer were formed alternately for 6 periods, thereby forming the MQW active layer  5  in Comparative Example 1. 
     The LED device of Comparative Example 1 obtained in this manner had emission wavelength of 465 nm and emission output of 3.0 mW with forward current of 20 mA, and the amount of shift in emission wavelength was about 10 nm in the forward current range of 0.1 to 20 mA. 
     From comparison between the LED devices of the Embodiment of the invention and Comparative Example 1, it can be seen that the emission output of the LED device of the Embodiment is significantly improved by including the super-lattice structure in the barrier layer of active layer  5 . This indicates that in the Embodiment of the invention, the influence of distortion is reduced in active layer  5  and this leads to improved emission efficacy. Further, in the LED device of the Embodiment, as the influence of distortion is reduced in active layer  5 , the amount of shift in emission wavelength becomes significantly smaller. 
     Comparative Example 2 
     An LED device according to Comparative Example 2 differs from the above-described Embodiment of the invention only in that n-side super-lattice layer  4  was omitted. 
     The LED device of Comparative Example 2 had emission wavelength of 470 nm and emission output of 4.0 mW with forward current of 20 mA, and the amount of shift in emission wavelength was about 4 nm in the forward current range of 0.1 to 20 mA. 
     From comparison between the LED devices of the Embodiment of the invention and Comparative Example 2, it can be seen that in the LED device of the Embodiment, the amount of shift in emission wavelength is made smaller by the provision of n-side super-lattice layer  4 . This suggests that in the LED device of the Embodiment, n-side super-lattice layer  4  also contributes to reducing the influence of distortion in active layer  5 . 
     As described above, the present invention can provide a nitride semiconductor light-emitting device in which its luminous efficacy is improved and the amount of shift in emission wavelength is made smaller in the range from low to high injection current, by reducing the influence of distortion caused by lattice mismatch at the interface between the well layer and the barrier layer. 
     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 scope of the present invention being interpreted by the terms of the appended claims.