Abstract:
A light-emitting element having: a gallium oxide substrate on a front surface of which a crystal of a semiconductor material having a light-emitting element part is grown; and a substrate protection layer formed on a back surface of the gallium oxide substrate. A method of making a light-emitting element having the steps of: forming a substrate protection layer on a back surface of a gallium oxide substrate; growing a crystal of a semiconductor material having a light-emitting element part on a front surface of the gallium oxide substrate; and assembling the light-emitting element so as to form a electrical connection for the light-emitting element part.

Description:
[0001]     The present application is based on Japanese patent application No. 2005-360442, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to a light-emitting element and, in particular, to a light-emitting element that is formed by growing a crystal on a gallium oxide substrate having a substrate protection layer. Also, this invention relates to a method of making the light-emitting element.  
         [0004]     2. Description of the Related Art  
         [0005]     Conventionally, a light-emitting element comprising a substrate composed of SiC, and laminated layers of n-type layer and p-type layer formed on the substrate, the layers composed of GaN is known (for example, JP-A-2002-255692 referred as a patent document 1).  
         [0006]     On the other hand, in order to obtain a light-emitting element which comprises a substrate transmitted by a light of an ultraviolet part, so that a colorless and transparent conductive material transmitting a light from a visible part to an ultraviolet part can be provided, a vertical electrode structure can be formed by using the conductive material as the substrate, and a surface of a substrate side can also function as a surface of taking out the light, a light-emitting element comprising a gallium oxide substrate and a light-emitting element formed on the substrate is developed (for example, JP-A-2004-56098 referred as a patent document 2).  
         [0007]     However, the light-emitting element shown in the patent document 1 uses NH3 as a nitrogen source and hydrogen gas as a carrier gas in a process of growing an epitaxial layer such as GaN layer, so that a gallium oxide substrate, in particular, a Ga 2 O 3  substrate is etched in a back surface thereof by the hydrogen gas, and a permeability is decreased and a planarity of the substrate is deteriorated.  
         [0008]     In the etching attack of the hydrogen gas, a damage by a high temperature heat treatment (for example, a heat treatment at 1100° C.) and a damage by a low temperature heat treatment (for example, a heat treatment at 650° C.) are can be observed. observing etching marks due to the damage by SEM, it is recognized that (010) face and (100) face in particular are to be etched easily. However, even if an epitaxial growth face is allocated as (001) face, the etching would become large from fine scratches in a back surface side of the substrate, so that it is difficult to grow a crystal without generating the etching marks in the back surface side of the substrate.  
         [0009]     Therefore, in order to take out an output light from the back surface of the substrate in a light-emitting element formed by growing a GaN layer etc. on a gallium oxide substrate, it is required that the back surface of the substrate is planarized by a polishing process, and further in a making process of the light-emitting element nonflatness of the back surface of the substrate adversely affects a growing process of a compound semiconductor variously.  
       SUMMARY OF THE INVENTION  
       [0010]     It is an object of the invention to provide a light-emitting element formed on a gallium oxide substrate which is not affected by an etching attack of a hydrogen gas used in a growing process of a compound semiconductor, and comprises a good flatness and transparency thereof.  
         [0011]     It is a further object of the invention to provide a method of making the light-emitting element. 
    (1) According to one embodiment of the invention, a light-emitting element comprises:    
 
         [0013]     a gallium oxide substrate on a front surface of which a crystal of a semiconductor material comprising a light-emitting element part is grown; and  
         [0014]     a substrate protection layer formed on a back surface of the gallium oxide substrate.  
         [0015]     In the above embodiment (1), the following modifications and changes can be made.  
         [0016]     (i) The substrate protection layer comprises an electrical conductivity so as to function as an-electrode.  
         [0017]     (ii) The gallium oxide substrate comprises a Ga 2 O 3  substrate.  
         [0018]     (iii) The substrate protection layer comprises a material selected from TiN, W, WSi, BP, Al 2 O 3 , Mo, Ta, GaN, and AlN. 
    (2) According to another embodiment of the invention, a method of making a light-emitting element comprises the steps of:    
 
         [0020]     forming a substrate protection layer on a back surface of a gallium oxide substrate;  
         [0021]     growing a crystal of a semiconductor material comprising a light-emitting element part on a front surface of the gallium oxide substrate; and  
         [0022]     assembling the light-emitting element so as to form a electrical connection for the light-emitting element part.  
         [0023]     In the above embodiment (2), the following modifications and changes can be made.  
         [0024]     (iv) The gallium oxide substrate comprises a Ga 2 O 3  substrate.  
         [0025]     (v) The substrate protection layer comprises a material selected from TiN, W, WSi, BP, Al 2 O 3 , Mo, Ta, GaN, and AlN.  
         [0026]     (vi) The assembling step comprises a step of removing the substrate protection layer.  
         [0027]     (vii) The substrate protection layer comprises an electrically nonconductive material.  
         [0028]     (viii) The substrate protection layer comprises Al 2 O 3  or AlN.  
         [0029]     In the above embodiment (1) or (2), the following modifications and changes can be made.  
         [0030]     (ix) The light-emitting element further comprising: a submount on which the light emitting element is mounted, wherein the submount comprises a Zener diode.  
         [0031]     (x) The assembling step comprises a step of mounting the light-emitting element on a submount comprising a Zener diode.  
       Advantages of the Invention  
       [0032]     According to the invention, a light-emitting element formed on a gallium oxide substrate which is not affected by an etching attack of a hydrogen gas used in a growing process of a compound semiconductor, and comprises a good flatness and transparency thereof, and a method of making the light-emitting element can be provided.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     The preferred embodiments according to the invention will be explained below referring to the drawings, wherein:  
         [0034]      FIG. 1  is a cross sectional view showing a gallium oxide substrate as a growth substrate for forming a light-emitting element;  
         [0035]      FIG. 2  is an explanatory block diagram showing a MOCVD method and a cross sectional view showing a main part of a MOCVD device;  
         [0036]     FIG. 3  is a cross sectional view showing a LED as a light-emitting element in a first preferred embodiment according to the invention,;  
         [0037]      FIG. 4  is a cross sectional view showing a LED as a light-emitting element in a second preferred embodiment according to the invention;  
         [0038]      FIG. 5  is a cross sectional view showing a LED as a light-emitting element in a third preferred embodiment according to the invention; and  
         [0039]      FIG. 6  is a cross sectional view showing a LED as a light-emitting element in a fourth preferred embodiment according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0040]     Composition of gallium oxide Substrate  
         [0041]     FIG. 1  is a cross sectional view showing a gallium oxide substrate as a growth substrate for forming a light-emitting element. As examples of the gallium oxide substrate, a Ga 2 O 3  substrate, in particular, a β-Ga 2 O 3  substrate can be cited. Hereinafter, a case using a Ga 2 O 3  substrate as the gallium oxide substrate will be explained.  
         [0042]     On one surface of the Ga 2 O 3  substrate  1  a substrate protection layer  2  is formed by a CVD method, a sputtering method etc. It is preferable that the substrate protection layer  2  is composed of a material comprising a heat resistance of 1200° C., and as its example TiN, W, WSi, BP, Al 2 O 3 , Mo, Ta, GaN, or AlN can be cited. TiN, W, WSi, and BP comprising an electrical conductivity are more preferable among the materials. In this embodiment, TiN comprising an electrical conductivity is used as the substrate protection layer  2 . Further, it is preferable that the substrate protection layer  2  comprises at least such a degree of thickness as pin holes do not occur, for example, the thickness of 500 to 5000 angstrom is preferable.  
         [0043]     FIG. 2  is an explanatory block diagram showing a MOCVD method and a cross sectional view showing a main part of a MOCVD device. The MOCVD device  100  comprises a reaction chamber  101  to which an exhaust part  106  comprising a vacuum pump and an exhaust device (not shown) is connected, a susceptor  102  mounting a Ga 2 O 3  substrate  1 , a heater  103  heating the susceptor  102 , a control axis  104  rotating the susceptor  102  and moving it in a vertical direction, a quartz nozzle  105  supplying a material gas at a slant or at an evenness to the Ga 2 O 3  substrate  1 , and gas generation devices generating various material gases, such as a TMG (trimethylgallium) gas generation device  111 , a TMA (trimethylaluminum) gas generation device  112 , and a TMI (trimethylindium) gas generation device  113 . Further, according to need, a number of the gas generation device can be increased or decreased. As a nitrogen source NH 3  can be used and as a carrier gas a hydrogen gas can be used. When a GaN film is formed the TMG and NH 3 , are used, when AlGaN film is formed the TMA TMG and NH 3  are used, and when InGaN film is formed the TMI, TMG and NH 3  are used. The film is formed by the MOCVD device  100  as follows. First, the Ga 2 O 3  substrate  1  is held on the susceptor  102 , facing a surface where the substrate protection layer  2  is disposed to a downward side and facing a surface where the film is to be formed to an upward side, and is housed in the reaction chamber  101 .  
         [0044]     Composition of LED Element  
         [0045]      FIG. 3  is a cross sectional view showing an LED as a light-emitting element in a first preferred embodiment according to the invention.  
         [0046]     A LED element  10  comprises a Ga 2 O 3  substrate  1  comprising a conductivity type of n-type. And the LED element  10  comprises a n + -GaN layer  12  being Si-doped, a n-AlGaN layer  13  being Si-doped, a MQW (Multiple-Quantum-Well)  14  comprising a multiquantum well structure of a InGaN/GaN, a p-AlGaN layer  15  being Mg-doped, a p + -GaN layer  16  being Mg-doped and a p-electrode  17  composed of ITO (Indium Tin Oxide), being laminated on the Ga 2 O 3  substrate  1  in order. And further the LED element  10  comprises a substrate protection layer  2  disposed on an under surface of the Ga 2 O 3  substrate  1 .  
         [0047]     The n + -GaN layer  12  and the p + -GaN layer  16  are formed by supplying NH 3  and a trimethylgallium (TMG) gas into the reactor where the Ga 2 O 3  substrate  1  is disposed, using N 2  as a carrier gas in a growth temperature condition of 1100° C. As to the n + -GaN layer  12 , a monosilane (SiH 4 ) as a dopant for giving a conductivity type of n-type is used as a Si material, and as to the p + -GaN layer  16 , a cyclopentadienyl magnesium (Cp 2 Mg) as a dopant for giving a conductivity type of p-type is used as a Mg material. The n-AlGaN layer  13  and the p-AlGaN layer  15  are formed by supplying a trimethylaluminum (TMA) gas to the reactor in addition to the materials described above.  
         [0048]     The MQW  14  is formed by supplying a trimethylindium (TMI) gas and a trimethylgallium (TMG) gas into the reactor using H 2  as a carrier gas in a growth temperature condition of 1100° C. When the InGaN is formed the TMI gas and the TMG gas are supplied, and when the GaN is formed the TMG gas are supplied.  
         [0049]     Making Process of LED Element  
         [0050]     First, a Ga 2 O 3  substrate  1  is mounted on a susceptor  102  in a MOCVD device  100 , facing a surface where the substrate protection layer  2  is disposed to a downward side.  
         [0051]     Forming of GaN  
         [0052]     Next, after being raised to a predetermined temperature (400° C.), a supply of N 2  is started. Subsequently, a temperature rising in the reactor is started, and the temperature rising is stopped at 1100° C., and then the temperature is maintained and the TMG gas of 60 sccm is supplied, so that the n + -GaN layer  12  of 1 μm thickness is formed. Next, a supply of N 2  is stopped and H 2  is supplied.  
         [0053]     Subsequently, the n-AlGaN layer  13 , the MQW  14 , the p-AlGaN layer  15 , the p + -GaN layer  16 , and the p-electrode  17  are formed in order.  
         [0054]     A plurality of the light-emitting elements formed on the Ga 2 O 3  substrate  1  according to the process described above are cut to an individual light-emitting element by a dicing process etc., so that a bear chip is produced.  
         [0055]     Further, a light-emitting element comprising a MQW structure has been explained, but the invention can also be applied to a hetero structure, a double hetero structure, and a single quantum well structure similarly.  
         [0056]     Assembly of Light-emitting Element  
         [0057]     Each of the bear chips cut out from the Ga 2 O 3  substrate  1  is assembled into a light-emitting device according to the following process.  
         [0058]     A light-emitting element comprising the Ga 2 O 3  substrate  1 , the epitaxial layer  21 , and the p-electrode  17  is mounted on a submount  30  comprising lead pins  31  inserted and connected to a circuit substrate etc. through a conductive metal paste etc. The submount  30  is composed of a silicon substrate of n-type so as to operate as a zener diode for protecting the LED element  1  from a static electricity. The substrate protection layer  2  comprising an electrical conductivity is electrically connected to a p-type semiconductor layer  30   a  formed on the submount  30 . The p-electrode  17  is electrically connected to the submount  30  through a bonding part  20  by a bonding wire  22 . According to the process described above, a light-emitting element unit capable of being mounted to a circuit board etc. is completed.  
       Advantages of the First Embodiment  
       [0059]     According to the first preferred embodiment, the Ga 2 O 3  substrate  1  is not affected by an etching attack of a hydrogen gas used in an epitaxial growing process of the Ga 2 O 3  substrate  1  so as to maintain a good flatness and realize a high transparency, so that a light-emitting element comprising the Ga 2 O 3  substrate  1  and a method of making the element can be provided.  
         [0060]     Further, the substrate protection layer  2  is composed of a material comprising an electrical conductivity and is able to function as an n-electrode so as to perform two functions, so that a light-emitting element comprising a high productivity and a low cost can be provided. Furthermore, the transparency of the Ga 2 O 3  substrate  1  is maintained, so that a composition of taking out an output light of the light-emitting element from a back surface of the substrate can be adopted.  
       Second Embodiment  
       [0061]     FIG. 4  is a cross sectional view showing a LED as a light-emitting element in a second preferred embodiment according to the invention.  
         [0062]     A light-emitting element  10  according to the second preferred embodiment is different from the first preferred embodiment in a composition that a vertical location of a p-side and a n-side to the submount  30  are disposed in a direction opposite to the first preferred embodiment. That is, each of the bear chips cut out from the Ga 2 O 3  substrate  1  is assembled into a light-emitting device according to the following process. The p-electrode  17  is mounted on the submount  30  through a conductive metal paste etc. The substrate protection layer  2  composed of TiN comprising an electrical conductivity can function as an n-electrode, so as to be electrically connected to the p-type semiconductor layer  30   a  formed on the submount  30  through a bonding electrode  19  and a bonding part  20  by a bonding wire  22 . According to the process described above, a light-emitting element unit capable of being mounted to a circuit board etc. is completed.  
       Third Embodiment  
       [0063]     FIG. 5  is a cross sectional view showing a LED as a light-emitting element in a third preferred embodiment according to the invention.  
         [0064]     A light-emitting element  10  according to the third preferred embodiment is different from the first preferred embodiment in a composition that AlN of a nonconductive material is used as the substrate protection layer  2 . The epitaxial layer  21  is formed by a MOCVD method as same as the first preferred embodiment, after that the substrate protection layer  2  is removed by removing methods such as a polishing method, a CMP (Chemical Mechanical Polishing) method, an etching method.  
         [0065]     After the removal of the substrate protection layer  2 , a patterning is formed on both surfaces of the epitaxial layer  21  by using a photolithography technique, and the p-electrode  17  and n-electrode  18  are formed by a vapor deposition method.  
         [0066]     The light-emitting element formed on the Ga 2 O 3  substrate  1  according to the process described above are cut to an individual light-emitting element by a dicing process etc., so that a bear chip is produced.  
         [0067]     Assembly of Light-emitting Element  
         [0068]     Each of the bear chips cut out from the Ga 2 O 3  substrate  1  is assembled into a light-emitting device according to the following process.  
         [0069]     A light-emitting element comprising the Ga 2 O 3  substrate  1 , the epitaxial layer  21 , the p-electrode  17  and the n-electrode  18  is mounted on a submount  30  comprising lead pins  31  inserted and connected to a circuit substrate etc. through a conductive metal paste etc. The submount  30  is composed of a silicon substrate of n-type so as to operate as a zener diode for protecting the LED element  10  from a static electricity. The n-electrode  18  is electrically connected to a p-type semiconductor layer  30   a  formed on the submount  30 . The p-electrode  17  is electrically connected to the submount  30  through a bonding electrode  19  and a bonding part  20  by a bonding wire  22 . According to the process described above, a light-emitting element unit capable of being mounted to a circuit board etc. is completed.  
       Advantages of the Third Embodiment  
       [0070]     According to the third preferred embodiment, in addition to the advantages of the first preferred embodiment, a nonconductive material can be used as the substrate protection layer  2  so that a range of choices for the material can be expanded and a restriction on a making process can be decreased.  
       Fourth Embodiment  
       [0071]     FIG. 6  is a cross sectional view showing a LED as a light-emitting element in a fourth preferred embodiment according to the invention.  
         [0072]     A light-emitting element  10  according to the fourth preferred embodiment is different from the third preferred embodiment in a composition that a vertical location of a p-side and a n-side to the submount  30  are disposed in a direction opposite to the third preferred embodiment. That is, each of the bear chips cut out from the Ga 2 O 3  substrate  1  is assembled into a light-emitting device according to the following process. The p-electrode  17  is mounted on the submount  30  through a conductive metal paste etc. The n-electrode 18  is electrically connected to the p-type semiconductor layer  30   a  formed on the submount  30  through a bonding electrode  19  and a bonding part  20  by a bonding wire  22 . According to the process described above, a light-emitting element unit capable of being mounted to a circuit board etc. is completed.  
         [0073]     Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.