Abstract:
A nitride based semiconductor laser diode device comprising a selective growth mask with a grating structure is proposed. The island-like stacked epitaxial layers including the P-type cladding layer is formed from the selective growth mask upon the active layer of the semiconductor laser structure. This proposed structure can reduce the strain by the deformation due to the isolate structure. Thus, increase of thickness of the cladding layer and/or increase of composition difference can be achieved without crack existing in the island-like stacked epitaxial layers. The optical confinement can be effectively improved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims the priority benefit of Taiwan application serial no. 91121531, filed on Sep. 20, 2002.  
         BACKGROUND OF INVENTION  
         [0002]    1. Field of Invention  
           [0003]    The present invention relates to a semiconductor device by gallium nitride compound. More particularly, the present invention relates to a structure capable of applying on a gallium nitride laser diode device for increasing a thickness of a surface epitaxial layer. In the specification, the surface epitaxial layer is referring to a patterned epitaxial layer in the gallium nitride laser diode device.  
           [0004]    2. Description of Related Art  
           [0005]    The typical optoelectronics device is a hetero structure. Since the crystal structure between the hetero epitaxial layers is not matched and the thermal expansion coefficients are also different, a strain energy exists at the interface. During the fabrication process or in operation of the device, this strain energy will be partially released by a form of dislocation or other defect forms. When the thickness of the epitaxial layer is greater than a certain critical value, the material then release the energy by a crack form, resulting in a crack of the epitaxial layer. For this consideration, the hetero epitaxial crystal layer cannot be grown in overlarge thickness. If the thickness of the epitaxial layer is large, a poor function of the device or even a severe damage would occur due to the crack. However, if the thickness of the epitaxial layer is not sufficient large, a poor performance may also occur.  
           [0006]    In the conventional technology, it has several methods can reduce the problems caused by the strain energy in the hetero structure. For example, (1) a method of epitaxial lateral overgrowth (ELOG) is proposed. In addition, (2) a buffer layer is also introduced, wherein an aluminum nitride layer or a gallium nitride layer in a rather small thickness is formed on a substrate at low temperature, to serve as a buffer layer, so as to reduce the problem of not being match for the crystal lattice between the epitaxial layer and the substrate. As a result, the condition for subsequently growing the gallium nitride at high temperature is improved and the quality of epitaxial crystal structure is also improved. In addition, (3) a strain layer superlattice is also proposed.  
           [0007]    The conventional gallium nitride device in the current status is mostly using the C-plane of the sapphire (Al 2 O 3 ) as the substrate. It has about an amount of 16% in lattice mismatch existing between the substrate and the gallium nitride epitaxial crystal, resulting in a rather large strain energy inside the gallium nitride thin film, which is grown on the sapphire, in which the density of the dislocation is high up to 10 9  10 11 /cm 2 . The foregoing technology can only be used for solving the strain energy or effect, which are caused by epitaxial layer between substrate and the gallium nitride, or inside the device.  
           [0008]    Taking the gallium nitride laser diode as an example, the application for the foregoing conventional technologies is basically limited to the epitaxial layer under the active layer. The epitaxial layer above the active layer still has the problem of hetero material structure. In other words, the conventional method still cannot effectively solve the problem of crack in the epitaxial layer above the active layer.  
           [0009]    [0009]FIG. 1 is a cross-sectional view, schematically illustrating the structure of a conventional gallium nitride laser diode, which is sequentially formed with a substrate  101 , a buffer layer  102  with gallium nitride compound semiconductor formed at relative low temperature, an N-type gallium nitride compound semiconductor layer  103 , a set of bar mask  104 , an N-type gallium nitride compound semiconductor layer  105 , a heavily doped layer  106 , an N-type gallium nitride compound semiconductor superlattice cladding layer  107 , an N-type gallium nitride compound semiconductor light guiding layer  108 , a gallium nitride compound semiconductor active layer  109 , a P-type gallium nitride compound semiconductor cap layer  110 , a P-type gallium nitride compound semiconductor light guiding layer  111 , a P-type gallium nitride compound semiconductor superlattice cladding layer  112 , and a P-type metal electrode contact layer  113 . In order to improve the quality of the epitaxial crystal and prevent the crack from occurring, the conventional structure for the gallium nitride laser diode structure has included the low-temperature buffer layer  102 , the method of ELOG ( 104 ), the strain layer super lattice structure ( 107 ,  112 ). However, these kinds of technologies for reducing the strain and the defects is not effective with respect to strain energy existing in the surface hetero epitaxial layer. FIG. 2, is a picture, schematically illustrating the crack occurring on the surface of the epitaxial layer for the conventional gallium nitride laser diode structure.  
           [0010]    On the gallium nitride laser diode structure, the cladding layer is usually formed from Al x  Ga 1-x  N, where if the quantity of X is higher, then the refraction index is smaller, the energy gap is larger, and the light confinement is better. As a result; the lattice mismatch is larger, thus, the thickness can not be overlarge. If the Al x  Ga 1-x  N layer is too thick, the crack is then easily occurring, and causes a failure of device. However, if the cladding layer is not sufficiently thick, the effect of light confinement then is getting worse, and the performance of device is then poor. Thus, it is a difficult issue for fabrication that how to control the thickness of epitaxial layer and the composition, so as to reduce the cracking of the epitaxial layer and improve the performance of the device.  
         SUMMARY OF INVENTION  
         [0011]    The invention provides an epitaxial growing structure, which uses a specific mechanism for the surface, so as to increase the thickness of the epitaxial layer in the gallium nitride laser diode structure but reduce the occurrence of crack.  
           [0012]    As embodied and broadly described herein, the invention provides a laser diode structure with an epitaxial crystal growing structure, which comprises a P-type gallium nitride compound semiconductor light guiding layer, formed on a surface of the active layer. A set of bar mask substance is formed on the P-type gallium nitride compound semiconductor light guiding layer. An island-like stacked structure is formed on the P-type gallium nitride compound semiconductor light guiding layer and the set of bar mask substance. Theisland-like stacked structure is formed including a P-type gallium nitride compound semiconductor cladding layer and a P-type metal electrode contact layer.  
           [0013]    It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0015]    [0015]FIG. 1 is a cross-sectional view, schematically illustrating the structure of a conventional gallium nitride laser diode.  
         [0016]    [0016]FIG. 2 is a picture, schematically illustrating the crack occurring on the surface of the epitaxial layer for the conventional gallium nitride laser diode structure.  
         [0017]    [0017]FIG. 3 is a cross-sectional view, schematically illustrating the structure of a gallium nitride laser diode, according to the first preferred embodiment of this invention.  
         [0018]    [0018]FIG. 4 is a top view, schematically illustrating the structure of the bar mask substance  313  over the epitaxial layer, according to the first preferred embodiment of this invention.  
         [0019]    [0019]FIG. 5 is a cross-sectional view, schematically illustrating the structure of a gallium nitride laser diode, according to the second preferred embodiment of this invention.  
         [0020]    [0020]FIG. 6 is a cross-sectional view, schematically illustrating the structure of a gallium nitride laser diode, according to the third preferred embodiment of this invention.  
         [0021]    [0021]FIG. 7 is a cross-sectional view, schematically illustrating the structure of a gallium nitride laser diode, according to the fourth preferred embodiment of this invention.  
         [0022]    [0022]FIG. 8 is a cross-sectional view, schematically illustrating the relative position for the lower bar mask substance, the upper bar mask substance, the active layer, and the island-like stacked structure, according to the preferred embodiment of this invention. 
     
    
     DETAILED DESCRIPTION  
       [0023]    Several terms used in the specification of the invention are generally described as follows. The term of gallium nitride compound semiconductor (GNCS) is referring to a form of Al x In y Ga (1-x-y) N 0â         Xâ         1, 0â         Yâ         1, 0â         X+Yâ         1), which can be formed by, for example, metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxial (MBE) crystal growth, or other similar epitaxial crystal growth. The dopants doped in the N-type gallium nitride compound semiconductor of the invention include Si, Ge, or other like elements with the same function. The dopants doped in the P-type gallium nitride compound semiconductor of the invention include Mg, Zn, Be, or other like elements with the same function.  
         [0024]    Embodiment 1: FIG. 3 is a cross-sectional view, schematically illustrating the structure of a gallium nitride laser diode, according to the first preferred embodiment of this invention. A substrate  301  is provided. The substrate  301  includes, for example, sapphire (Al 2 O 3 ), silicon carbide (SiC), gallium nitride (GaN), Spinel (MgAl 2 O 4 ), gallium arsenide (GaAs), zinc oxide (ZnO), silicon (Si), and so on. A gallium nitride compound semiconductor buffer layer  302  is formed on substrate  301  with an amorphous and/or polycrystallized structure and a thickness of 50 500 Angstrom. An N-type GNCS  303  is formed on the buffer layer  302  with a thickness of about 37 microns.  
         [0025]    After the crystal growth, the whole structure is shifted out from the crystal growing machine, and the coating, photolithography and etching processes are performed to form a lower bar mask substance  304  on the N-type GNCS layer  303 . The direction of the bar can be any direction. The thickness can be, for example, 100 2000 Angstroms and the width can be, for example, 2 20 microns. The distance between the parallel bars in the bar mask substance  304  is about 2 500 microns. The bar mask substance  304  is used to cause multiple local crystal growing regions during the regrowth on the epitaxial layer, so as to change the distribution of strain force in the epitaxial layer and the growing direction of dislocation. As a result, the epitaxial layer formed over the N-type GNCS layer  303  has less dislocation density and strain energy. The bar mask substance  304  by itself has to have sufficient stability without producing the chemical reaction with the peripheral epitaxial materials, or contaminating the peripheral epitaxial materials. In addition, during the process of epitaxial growing, the epitaxy of GNCS does not directly grow on the mask substance. Each bar of the lower bar mask substance  304  has a cross-sectional shape in rectangular or any shape with similar functions. The mask substance  304  preferably includes silicon oxide, silicon nitride, or other material with similar function.  
         [0026]    Then, the substrate  301  is transferred back to the epitaxial growing machine. An N-type GNCS layer  305  is formed on the surface of the N-type GNCS layer  303  and the lower bar mask substance  304 . The N-type GNCS layer  305  is grown to have a planar surface and a thickness of 3 20 microns. A heavily doped N-type GNCS layer  306  is formed on the N-type GNCS layer  305  with a thickness of, for example, between 500 Angstroms and 2 microns. An N-type GNCS cladding layer  307  is formed on the heavily doped N-type GNCS layer  306 . The N-type GNCS cladding layer  307  is a multi-layer structure, including a super lattice structure composed of N-type gallium nitride semiconductor layer/N-type aluminum gallium nitride semiconductor layer with a thickness of 35 200/35 200 Angstroms. Moreover, the pair number is about 3 to 100 pairs. As a result, the total thickness is about 210 40000 Angstroms. An N-type GNCS light guiding layer  308  is formed on the N-type GNCS cladding layer  307  with a thickness of about 100 2000 Angstroms. Even though this layer can be doped with N-type dopants, can also be non-doped. A GNCS active layer  309  is formed on the GNCS light guiding layer  308  with a single-layer structure or a multi-layer structure by a thickness of about 30 1000 Angstroms. A P-type GNCS light guiding layer  310  is formed on the GNCS active layer  309  by a thickness of about 100 2000 Angstroms. Even though this layer can be doped with P-type dopants, it can also be non-doped. A P-type GNCS cap layer  311  is formed on the GNCS light guiding layer  310  by a thickness of about 300 2000 Angstroms. A P-type GNCS light guiding layer  312  is formed on the GNCS cap layer  311  by a thickness of about 200 3000 Angstroms.  
         [0027]    Further still, the whole structure is transferred out from the epitaxial growing machine. The coating, photolithography, and etching processes are performed to form an upper bar mask substance  313  on the P-type GNCS light guiding layer  312 . The direction of the bars should be the same as the direction of the lower bar mask substance  304 . The thickness of the bars is about 100 2000 Angstroms and the width is about 50 500 microns. The distance between the parallel bars is about 1 10 microns. Similar to the lower bar mask substance  304 , the upper bar mask substance  313  by itself should have sufficient stability without producing the chemical reaction with the peripheral epitaxial materials, or contaminating the peripheral epitaxial materials. In addition, during the process of epitaxial growing, the epitaxy of GNCS does not directly grow on the mask substance. Each bar of the upper bar mask substance  313  has a cross-sectional shape in rectangular or any shape with similar functions. The mask substance  313  preferably includes silicon oxide, silicon nitride, or other material with similar function.  
         [0028]    [0028]FIG. 4 is a top view, schematically illustrating the structure of the bar mask substance  313  over the epitaxial layer. The upper bar mask substance  313  is used to form the island-like stacked structure on the surface of the gallium nitride laser diode. Thereby, the absorption of the elastic strain from the material can release the strain energy inside the epitaxial crystal. As a result, the epitaxial layer over the bar mask substance  313  can be grown by a larger thickness without crack. This is one of the advantages of the invention. Further descriptions are as follows.  
         [0029]    The whole structure is then transferred back to the epitaxial growing machine. A P-type GNCS cladding layer  314  is formed on the P-type GNCS light guiding layer  312  and the upper bar mask substance  313 . Since the upper bar mask substance  313  produces a crystal growing selective effect, the P-type GNCS cladding layer  314  is grown as island-like stacked structure. In order to allow the elastic strain to be effective, the islands to each other should be separated without connection. The P-type GNCS cladding layer  314  can be a single layer of aluminum gallium nitride semiconductor or a composed layer of P-type gallium nitride semiconductor/P-type aluminum gallium nitride semiconductor with the super lattice structure. The total thickness is about 0.5 10 microns. A P-type metal electrode contact layer  315  is formed on P-type GNCS cladding layer  314  by a thickness of about 150 2000 Angstroms.  
         [0030]    Embodiment  2 : FIG. 5 is a cross-sectional view, schematically illustrating the structure of a gallium nitride laser diode, which includes a substrate  501  having the materials including aluminum oxide, silicon carbide, gallium nitride, Spinel, gallium arsenide, zinc oxide, silicon, so on. A GNCS buffer layer  502  is formed on the substrate  501  as an amorphous and/or polycrystallized structure by a thickness of 50 500 Angstroms. An N-type GNCS layer  503  is formed on the GNCS buffer layer  502  by a thickness of about 3 7 microns.  
         [0031]    After the crystal growth, the whole structure is shifted out from the crystal growing machine, and the coating, photolithography and etching processes are performed to form a lower bar mask substance  504  on the N-type GNCS layer  503 . The features of the lower bar mask substance  504  is similar to the lower bar mask substance  304  in Embodiment 1. Each bar of the lower bar mask substance  504  has a cross-sectional shape in rectangular or any shape with similar functions. The direction of the bar can be any direction. The thickness can be, for example, 100 2000 Angstroms and the width can be, for example, 2 20 microns. The distance between the parallel bars in the bar mask substance  504  is about 2 500 microns. The material includes, for example, silicon oxide, silicon nitride, or other material with similar function.  
         [0032]    Then, whole structure is transferred back to the epitaxial growing machine. An N-type GNCS layer  505  is formed on the surface of the N-type GNCS layer  503  and the lower bar mask substance  504 . The N-type GNCS layer  505  is grown to have a planar surface and a thickness of 3 20 microns. A heavily doped N-type GNCS layer  506  is formed on the N-type GNCS layer  505  with a thickness of, for example, between 500 Angstroms and 2 microns. An N-type GNCS cladding layer  507  is formed on the heavily doped N-type GNCS layer  506 . The N-type GNCS cladding layer  507  is a multi-layer structure, including a super lattice structure composed of N-type gallium nitride semiconductor layer/N-type aluminum gallium nitride semiconductor layer with a thickness of 35 200/35 200 Angstroms. Moreover, the pair number is about 3 to 100 pairs. As a result, the total thickness is about 210 40000 Angstroms. An N-type GNCS light guiding layer  508  is formed on the N-type GNCS cladding layer  507  with a thickness of about 100 2000 Angstroms. Even though this layer can be doped with N-type dopants, can also be non-doped. A GNCS active layer  509  is formed on the GNCS light guiding layer  508  with a single-layer structure or a multi-layer structure by a thickness of about 30 1000 Angstroms. A P-type GNCS light guiding layer  510  is formed on the GNCS active layer  509  by a thickness of about 100 2000 Angstroms. Even though this layer can be doped with P-type dopants, it can also be non-doped. A P-type GNCS cap layer  511  is formed on the GNCS light guiding layer  510  by a thickness of about 300 2000 Angstroms. A P-type GNCS light guiding layer  512  is formed on the GNCS cap layer  511  by a thickness of about 200 3000 Angstroms. A P-type GNCS cladding layer  513  is formed on the P-type GNCS light guiding layer  512 . The P-type GNCS cladding layer  513  is a multi-layer structure composed of P-type gallium nitride semiconductor layer/P-type aluminum gallium nitride semiconductor layer by thickness of 35 200/35 200 Angstroms, and the pair number is about 3 to 10, resulting in a total thickness of about 210 4000 Angstroms.  
         [0033]    Then, the whole structure is transferred out from the epitaxial growing machine. The coating, photolithography, and etching processes are performed to form an upper bar mask substance  514  on the P-type GNCS cladding layer  513 . The features of the bars in the upper bar mask substance  514  are similar to those in the bar mask substance  313  in Embodiment 1 with the cross-sectional shape in rectangular or any shape with similar functions. The direction of the bars should be same as the direction of the lower bar mask substance  504 . The thickness of the bars is about 100 2000 Angstroms and the width is about 50 500 microns. The distance between the parallel bars is about 1 10 microns.  
         [0034]    The whole structure is transferred back to the epitaxial growing machine. A P-type GNCS cladding layer  515  is formed on the P-type GNCS cladding layer  513  and the upper bar mask substance  514 . Since the upper bar mask substance  514  produces a crystal growing selective effect, the P-type GNCS cladding layer  515  is grown as island-like stacked structure. The islands to each other should be separated independently without connection, so as to have the elastic strain effect. The P-type GNCS cladding layer  515  can be a single layer of aluminum gallium nitride semiconductor or a composed layer of P-type gallium nitride semiconductor/P-type aluminum gallium nitride semiconductor with the super lattice structure. The total thickness is about 0.5 10 microns. A P-type metal electrode contact layer  516  is formed on P-type GNCS cladding layer  515  by a thickness of about 150 2000 Angstroms.  
         [0035]    Embodiment 3: FIG. 6 is a cross-sectional view, schematically illustrating the structure of a gallium nitride laser diode, according to the third preferred embodiment of this invention. The structure is formed by photolithography, etching, and coating processes on the structure of gallium nitride laser diode (see FIG. 3) in Embodiment  1 . In addition to the gallium nitride laser diode in epitaxial structure, the structure further includes an insulating layer  601  from, for example, silicon oxide. A P-type metal electrode  602  is formed on the P-type metal electrode contact layer  315 . An N-type electrode  603  is formed on the heavily doped N-type GNCS layer  306 .  
         [0036]    Embodiment 4: FIG. 7 is a cross-sectional view, schematically illustrating the structure of a gallium nitride laser diode, according to the fourth preferred embodiment of this invention. The structure is formed by photolithography, etching, and coating processes on the structure of gallium nitride laser diode (see FIG. 5) in Embodiment 2. In addition to the gallium nitride laser diode in epitaxial structure, the structure further includes an insulating layer  701  from, for example, silicon oxide. A P-type metal electrode  702  is formed on the P-type metal electrode contact layer  516 . An N-type electrode  703  is formed on the heavily doped N-type GNCS layer  506 .  
         [0037]    If the lower bar mask substance  304 ,  504  are properly associating with the upper bar mask substance  313 ,  514 , then the gallium nitride laser diode can have better efficiency. FIG. 8 is a cross-sectional view, schematically illustrating the relative position for the lower bar mask substance, the upper bar mask substance, the active layer, and the island-like stacked structure, according to the preferred embodiment of this invention. In FIG. 8, “A” represents the island-like stacked structure. B 1  represents the upper bar mask substance at a left side of the island-like stacked structure A. B 2  represents the upper bar mask substance at a right side of the island-like stacked structure A. C represents the active layer. D represents the lower bar mask substance. E represents the substrate. The dashed line  1  represents the center line of the short side of the lower bar mask substance. The dashed line  2  represents the center line of the window region between the bars of the lower bar mask substance. “b10” represents a side of the upper bar mask substance B 1  close to the island-like stacked structure A. “b20” represents a side of the upper bar mask substance B 1  close to the island-like stacked structure A. The relative locations for the gallium nitride laser diode structure are described as follows: 1. The upper bar mask substance B 1 , B 2  are located between the island-like stacked structure A and the active layer C. 2. The lower bar mask substance D is located between the substrate E and the active layer C. 3. The side b 10  of the upper bar mask substance B 1  has to cross over the center line  1  (dashed line  1 ) of the short side of the lower bar mask substance D. 4. The side b 20  of the upper bar mask substance B 2  has to cross over the center line  2  (dashed line  2 ) of the window region between the bars of the lower bar mask substance.  
         [0038]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.