Abstract:
For a light emitting device using gallium nitride (GaN), on a substrate are sequentially formed a GaN-based layer, an AlGaN-based layer, and a light emitting layer. To prevent cracks in the AGaN-based layer, the AlGaN-based layer is formed before planarization of the surface of the GaN layer on a surface of the GaN layer which is not planar. For a laser, the AlGaN-based layers serve as clad layers which sandwich the light emitting layer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a gallium nitride-based light emitting device and a manufacturing method for the same, and in particular to a device with fewer cracks and a method of suppressing formation of cracks.  
           [0003]    2. Description of the Related Art  
           [0004]    Gallium nitride (GaN)-based compound semiconductors are applied to short wavelength light emitting devices, such as LEDs.  
           [0005]    For efficient enclosure of light in a light emitting devices of an LED, a laser, and so forth, an AlGaN layer having a thickness 0.4 μm or greater or a strained layer super lattice layer constituting of alternately stacked AlGaN and GaN is grown on a GaN layer. Because an AlGaN layer has a smaller refractive index than that of a GaN layer, sandwiching the light emitting layer by AlGaN layers encloses light from the light emitting layer within the light emitting layer.  
           [0006]    [0006]FIG. 5 shows a structure of a general short wavelength light emitting device (370 nm to 450 nm). Specifically, an n-GaN layer  12  having a thickness 1 μm or greater is formed on a sapphire substrate  10 , an n-AlGaN layer having a thickness of approximately 0.5 μm is formed as an n-clad  14  on the n-GaN layer  12 , an InGaN layer is formed as an active layer  16  on the n-AlGaN layer, and an AlGaN layer is formed as a p-clad layer  18  on the InGaN layer. That is, the light emitting layer  16  made of InGaN is sandwiched by the clad layers  14  and  18 . In such a structure, light from the active layer  16  is reflected by the clad layers  14  and  18 , thus enclosed in the active layer  16 .  
           [0007]    However, because a lattice constant of AlGaN, the material of the clad layers, is smaller than that of GaN, lateral tensile stress is caused in the AlGaN layer as indicated by the arrows in the drawing. Larger tensile stress is caused in a thicker AlGaN layer, and tensile stress in excess of a critical value causes a crack  14   a  to form.  
           [0008]    Japanese Patent Laid-open Publication No. Hei 11-68256 proposes formation of an InGaN layer and formation thereon of an AlGaN layer serving as a crack preventing layer.  
           [0009]    [0009]FIG. 6 shows a structure of a light emitting device including an InGaN layer. A difference from the structure of FIG. 4 lies in that an InGaN crack preventing layer  13  is formed between the n-GaN layer  12  and the n-clad layer  14 . The crack preventing layer  13  has a thickness in the range of between 10 nm and 0.5 μm. In the publication, it is explained that this range is selected because a thickness thinner than 10 nm will have little effect on crack prevention and a thickness larger than 0.5 μm may affect the crystals themselves.  
           [0010]    Such a crack preventing layer  13 , however, can prevent cracks only when the AlGaN clad layer has a thickness approximately 0.5 μm or less. Cracks may be caused in an AlGaN clad layer which is thicker than this value. Moreover, when InGaN is used for a crack preventing layer  13 , (UV) light from the active layer  16  is absorbed by the InGaN, and light emitting efficiency is thereby deteriorated.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention aims to provide a light emitting device which suppresses the formation of cracks, and a method for manufacturing such a light emitting device.  
           [0012]    According to the present invention, there is provided a gallium nitride-based light emitting device, comprising a substrate; a GaN-based layer formed on the substrate; an AlGaN-based layer formed on the GaN-based layer; and a light emitting layer formed on the AlGaN-based layer. The surface of the GaN-based layer at a boundary relative to the AlGaN-based layer is uneven, that is not smooth or planar.  
           [0013]    More specifically, an AlGaN-based layer is formed on a GaN-based layer which has not yet grown enough to have a planar surface, rather than on a GaN-based layer which has grown sufficiently to have a planar surface. Because the surface of the GaN-based layer is non-planar, the surface of the AlGaN-based layer formed on the GaN-based layer is also not planar at an early stage of the formation, and therefore a plurality of inclined faces are formed. Accordingly, stress is not concentrated in a direction parallel to the layer, but is also created and transmitted in directions along the inclined faces. Therefore, the compound stress vector over the whole AlGaN-based layer is smaller than when an AlGaN-based layer is formed on a planar surface of a GaN-based layer. Cracks can therefore be prevented, in thicker AlGaN-based layers. It should also be noted that the AlGaN-based layer may be a strained layer super lattice layer, or a SLS layer, instead of a single AlGaN layer.  
           [0014]    In one embodiment, a GaN-based layer is a GaN layer, and clad layers which sandwich the light emitting layer are formed on the GaN layer. The clad layers contain AlGaN.  
           [0015]    The present invention can be more clearly understood with reference to the following description of embodiments, to which the present invention is not limited. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The above and other objects, features, and advantages of the present invention will become further apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings wherein:  
         [0017]    [0017]FIG. 1 is a diagram showing a structure of a light emitting device in an embodiment of the present invention;  
         [0018]    [0018]FIG. 2 is a diagram explaining growth of a GaN layer on a substrate;  
         [0019]    [0019]FIG. 3 is a diagram showing growth of an AlGaN layer on a GaN layer, FIG. 3(A) showing an initial stage of growth, and FIG. 3(B) showing completion of growth;  
         [0020]    [0020]FIG. 4 is a graph showing relationships between stress and thickness of GaN layers, FIG. 4(A) relating to a GaN layer having a non-planar surface, FIG. 4(B) relating to a GaN layer having a planar surface;  
         [0021]    [0021]FIG. 5 is a diagram showing a structure of a conventional UV light emitting device; and  
         [0022]    [0022]FIG. 6 is a diagram showing a structure of another conventional UV light emitting device. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    In the following, an embodiment of the present invention will be described based on the drawings.  
         [0024]    [0024]FIG. 1 is a diagram showing a structure of an embodiment of a GaN-based light emitting device. Specifically, on the substrate  10 , there are sequentially formed an n-GaN layer  12 , an n-clad layer  14 , an active layer (a light emitting layer)  16 , a p-clad layer  18 , and a p-GaN layer  20 . A p-electrode  22  is formed on the p-GaN layer  20 , and an n-electrode  24  is formed on a part of the n-GaN layer  12  which is etched to be exposed. The clad layers  14  and  16  each are either a single AlGaN layer or a strained layer super lattice (SLS) layer constituting of alternately stacked AlGaN and GaN layers. The active layer (light emitting layer)  16  is an InGaN layer or the like. In this embodiment, the surface of the n-GaN layer  12  is not planar but uneven, and the n-clad layer  14  is thus formed on the uneven surface of the n-GaN layer  12 . The unevenness of the n-GaN layer  12  can be realized by adjusting a growth time or thickness of the n-GaN layer  12 .  
         [0025]    [0025]FIG. 2 shows growth of GaN in formation of an n-GaN layer  12  on the substrate  10 . Specifically, in the process of growing GaN on a substrate made of sapphire or the like at 1070° C. using an MOCVD, initially, GaN grows into an island shape, rather than uniformly, (see the dotted (single dot) line in FIG. 2) due to the presence of a region with a coarse crystal lattice along the boundary relative to the substrate  10 . As growth proceeds, growth parallel to the direction in which the layer lies (a lateral direction) becomes dominant until a GaN layer  12  is ultimately completed as a continuous film (see the solid line in FIG. 2).  
         [0026]    That is, while the n-GaN layer  12  is still thin, the surface of the layer  12  is not yet planar, and island growth can be observed on the surface. In this embodiment, at the stage where the surface of the GaN is yet to be planar, in other words, during the stage of island growth, growth of the n-GaN layer  12  is arrested and an n-clad layer  14  is formed thereon.  
         [0027]    [0027]FIGS. 3A and 3B show growth of the n-clad layer  14 . In these drawings, an n-GaN layer  12  does not constitute a continuous film or a planar surface in the lateral direction, as shown by the dotted line, with its surface being not-planar but like an island or uneven. The surface of the n-GaN layer  12  is substantially planar when the grown n-GaN layer  12  has grown to have a thickness 1 μm or greater (for example, 2 μm) but is rather uneven, or like an island or archipelago, when the thickness is about 0.4 μm. In the example of this embodiment, an n-clad layer  14  is grown on the n-GaN layer  12  at the stage where its surface is yet to be planar, as shown in FIG. 3A.  
         [0028]    Tensile stress is created in respective regions of the n-clad layer  14  (indicated by the arrows a, b in the drawing). The direction of tensile stress caused along the inclined faces of the island portion is not parallel to the direction along which the layer lies. Therefore, although the stress increases as the n-clad layer  14  grows thicker, the compound stress does not increase in proportion to the thickness of the AlGaN layer as the stress is a vector. That is, as in the ultimate state shown in FIG. 3B, the compound stress does not increase, despite the growth in the thickness of the n-clad layer  14 , and occurrence of cracks can be prevented.  
         [0029]    In this embodiment, because the n-clad layer  14  can be made thicker than was conventionally possible, light or carrier enclosure effect can be further improved. It should be noted that an n-clad layer  14  maybe either a single AlGaN layer or an SLS layer constituting of AlGaN layers and GaN layers.  
         [0030]    [0030]FIGS. 4A and 4B show changes in stress along the direction of the thickness of the n-clad layer  14 , which is an AlGaN layer in this example. FIG. 4A shows stress changes in an example wherein an AlGaN layer is formed on a GaN layer  12  having an uneven or island-shaped surface. FIG. 4B shows stress changes in an example wherein an SLS is formed on a GaN layer  12  having a planar or continuous surface. These drawings show that compressive stress is generated in the GaN layer  12 , and the compressive stress is changed to tensile stress at the boundary relative to the n-clad layer  14 , so that tensile stress is generated in the n-clad layer  14 . In the example of FIG. 4A, because stress changes relatively slowly at the boundary between the n-GaN layer  12  and the n-clad layer  14  crack-causing maximum stress is inhibited. That is, σa&lt;σb, wherein σa and σb are the maximum stresses of the n-GaN layer  12  and the n-clad layer  14 , respectively, is maintained. In other words, in this embodiment, a critical film thickness of the n-clad layer  14 , referring to the maximum thickness in which no cracks are formed, is increased.  
         [0031]    In the following, actual examples of application of the present embodiment will be described.  
         [0032]    On a sapphire C surface substrate  10 , a GaN layer  12  having a thickness t was grown at 1070° C., and an SLS layer  14  of Al 0.2 Ga 0.8 N/GaN was formed thereon in N cycles. An MOCVD was used for the growth. Specifically, a sapphire substrate  10  was placed in a reaction tube, and heated to 1150° C. under H 2  atmosphere using a heater. Then, trimethylgallium (TMG), NH 3 , and H 2  were introduced into the tube via a gas introducing section for growth of a GaN layer  12 , while maintaining the substrate at 1075° C. Thereafter, trimethyl aluminum was further introduced into the tube for growth of an SLS layer  14  of AlGaN/GaN, in which the thickness of each Al 0.2 Ga 0.8  layer was 2 nm and that of each GaN layer was 2 nm. After growth was complete, whether or not any cracks were formed on the surface of the n-clad layer  14  was observed using both an optical microscope and an atomic force microscope (AFM). The combination of an optical and AFM microscopic evaluation should detect any cracks, even when none are observed by the naked eye. The observation results are shown in the Table below.  
                                             TABLE                               SLS Total Thickness           t (μm)   N   (= 4 nmx N) (μm)   Presence/absence of Crack                                0.2   500   2   Absent       0.4   500   2   Absent       0.6   450   1.8   Present       2   250   1   Present                  
 
         [0033]    As can be seen from the Table, no cracks were observed in thick GaN layers  12 , even with thicknesses of 0.2 μm or 0.4 μm, and even when the total thickness of the n-clad layer  14  is on the order of 2 μm. On the contrary, cracks were detected when the n-GaN layer  12  is thick, such as 0.6 μm or greater, but the total thickness of the n-clad layer  14  was on the order of 1.8 μm or less. This data provides evidence that formation of an n-clad layer on an n-GaN layer which is still thin, or has yet to be grown thick enough to have a planar surface, rather than on an n-GaN layer  12  which has grown to have a planar surface, as conventionally, enables formation of a thick SLS layer  14  (2 μm or greater thick), which in turn ensures reliability of the n-clad layer  14 . Obviously, a GaN layer  12  can be formed with a thickness 0.2 μm or 0.4 μm through adjustment of growth time.  
         [0034]    When a single Al 0.2 Ga 0.8 N layer, instead of an SLS layer  14 , is grown on the GaN layer  12 , the critical thickness, the borderline thickness at which no cracks are formed, is approximately ⅓ of the thickness of the SLS. This is believed to be because the average Al composition of the SLS layer  14  is as small as 0.1 and because distortion distributed within the SLS serves to further reduce the stress. The critical thickness of a single AlGaN layer is small compared to an SLS, but large compared to formation on a planar surface of the GaN layer  12 .  
         [0035]    It is observed that, when a thickness t of a GaN layer  12  is 0.4 μm or greater, density of dislocation caused within the SLS layer  14  is as low as that for a thickness t of 2 μm. However, for a thickness t of a GaN layer  12  of 0.2 μm or less, the dislocation density sharply increases.  
         [0036]    In view of the above, it can be known that, in order to ensure preferable crystalline state, the thickness of the GaN layer  12  must be approximately 0.2 μm or greater, preferably approximately 0.3 μm or greater, and that, in order to prevent cracks in the clad layer  14  formed on the GaN layer  12 , the thickness is preferably approximately 0.5 μm or less. That is, the relationship 0.2 μm≦thickness of GaN layer  12  t≦0.5 μm is maintained. Here it should be noted that the upper limit of the thickness t of the GaN layer  12  may vary depending on the required thickness of the clad layer  14 . In order to grow a clad layer  14  having a thickness 2 μm or greater, the upper limit value of the thickness t is 0.5 μm. When a thickness 2 μm or smaller is acceptable for the clad layer  14 , the upper limit value of the thickness t is 0.5 μm or greater. That is, the possibility of crack occurrence depends on the thickness of the clad layer  14 . An AlInGaN layer may be used for the clad layer  14 .  
         [0037]    It should also be noted that, although a laser is described in the example this embodiment, the present invention is similarly applicable to an LED and other similar technologies.