Abstract:
A semiconductor light emitting device includes a semiconductor multilayer structure comprising a plurality of Group III–V nitride semiconductor layers including two semiconductor layers of different conductivity types, and a transparent electrode formed on the semiconductor multilayer structure. The transparent electrode contains an impurity element developing the same conductivity type as that of an impurity element introduced into a semiconductor in the semiconductor multilayer structure, which semiconductor has an interface with the transparent electrode. Therefore, contact resistance between the transparent electrode and the semiconductor having the interface with the transparent electrode is decreased.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to semiconductor light emitting devices made of Group III–V nitride semiconductors, which are capable of emitting light in the blue to ultraviolet regions. 
   Recently, light emitting diodes (GaN-based LEDs), using a Group III–V nitride (hereinafter, referred to simply as a “nitride”) expressed by a general formula B z  ,Al x Ga l−x−y−z In y N, where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1, have found wide application in various kinds of display panels, large display apparatus and traffic lights, for example. White LEDs, in which a GaN-based LED is combined with a fluorescent substance, have also been put into practical use, and are expected to replace the currently used lighting equipment, if their luminous efficiency is improved in the future. 
     FIG. 18  illustrates a cross-sectional structure of a known blue light emitting diode in which nitride semiconductors are used (See Japanese Laid-Open Publication Nos. 07-094782, 10-173224, and 2000-5891.) As shown in  FIG. 18 , in the known blue light emitting diode, a first semiconductor layer  102  made of an n-type nitride semiconductor, and a second semiconductor layer  103  made of a p-type nitride semiconductor are sequentially stacked on a substrate  101  made of sapphire. 
   A first electrode  104  made of nickel and gold with a thickness of from about 2 nm to about 5 nm is formed on the second semiconductor layer  103 . The first electrode  104  can make a good ohmic contact with the p-type nitride semiconductor. 
   A second electrode  105  made of gold is formed on the first electrode  104 . The second electrode  105 , which is for wire bonding, passes through the first electrode  104  to reach the second semiconductor layer  103 . An n-type ohmic electrode  106  is formed on an exposed portion of the first semiconductor layer  102 . 
   With this structure, in the known blue light emitting diode, recombination radiation (generated light), emitted by the pn junction formed by the interface between the first and second semiconductor layers  102  and  103 , is transmitted through the second semiconductor layer  103  and the first electrode  104 , and then extracted. 
   However, a problem with the known blue light emitting diode is that the recombination radiation produced by the pn junction is partially absorbed by the first electrode  104  made of the metals. To deal with this problem, if the thickness of the first electrode  104  is reduced significantly, the amount of radiation transmitting through the first electrode  104  can be increased. In that case, however, a trade-off occurs in which series resistance (sheet resistance) in the first electrode  104  is increased, which makes it difficult to significantly increase the optical electric characteristics of the device, that is, the device characteristics. 
   Alternatively, instead of the metals, transparent material may be used to form the first electrode  104  in order to increase the light-extraction efficiency. Nevertheless, a problem also arises in this case, in which contact resistance between the p-type nitride semiconductor layer and the transparent electrode formed thereon is large. 
   In addition, there is another problem in that nitride semiconductors, in which the activation ratio of an impurity, particularly of a p-type impurity, that determines the conductivity type of the semiconductor is small, have large sheet resistance. 
   SUMMARY OF THE INVENTION 
   In view of the above-mentioned problems, it is therefore an object of the present invention to decrease contact resistance with respect to a transparent electrode in a nitride semiconductor device. 
   In order to achieve the above object, an inventive semiconductor light emitting device employs a structure in which a transparent electrode that has an interface with a nitride semiconductor layer is doped with an impurity developing the same conductivity type as that of an impurity introduced into the nitride semiconductor layer, or is doped with a metal that can adsorb hydrogen. 
   Further, in order to achieve the above object, an inventive semiconductor light emitting device employs a structure in which a passivation film that has an interface with a nitride semiconductor layer is doped with an impurity developing the same conductivity type as that of an impurity introduced into the nitride semiconductor layer, or is doped with a metal that can adsorb hydrogen. 
   Specifically, a first inventive semiconductor light emitting device includes a semiconductor multilayer structure comprising a plurality of Group III–V nitride semiconductor layers including two semiconductor layers of different conductivity types, and a transparent electrode formed on the semiconductor multilayer structure. The transparent electrode contains an impurity element developing the same conductivity type as that of an impurity element introduced into a semiconductor in the semiconductor multilayer structure. The semiconductor has an interface with the transparent electrode. 
   In the first inventive semiconductor light emitting device, the impurity element contained in the transparent electrode is diffused into the semiconductor layer having the interface with the transparent electrode by a heat treatment performed during the fabrication process, so that the impurity element diffused from the transparent electrode into the semiconductor layer causes a decrease in the value of resistance in the semiconductor layer where the semiconductor layer is near the interface with the transparent electrode. Thus, the contact resistance of the semiconductor layer with respect to the transparent electrode is decreased. 
   A second inventive semiconductor light emitting device includes a semiconductor multilayer structure comprising a plurality of Group III–V nitride semiconductor layers including two semiconductor layers of different conductivity types, and a transparent electrode formed on the semiconductor multilayer structure. The transparent electrode contains a metal element that adsorbs hydrogen. 
   In the second inventive semiconductor light emitting device, when the metal element that adsorbs hydrogen, contained in the transparent electrode, is diffused into a semiconductor layer having an interface with the transparent electrode by a heat treatment performed during the fabrication process, the metal element that adsorbs hydrogen, diffused from the transparent electrode into the semiconductor layer, adsorbs (binds to) the hydrogen atoms that have been bound to an impurity element introduced into the semiconductor layer. This increases the activation ratio of the impurity element introduced into the semiconductor layer where the semiconductor layer is near the interface with the transparent electrode, resulting in a decrease in the resistance of the semiconductor layer. Accordingly, the contact resistance of the semiconductor layer with respect to the transparent electrode is reduced. 
   A third inventive semiconductor light emitting device includes a semiconductor multilayer structure comprising a plurality of Group III–V nitride semiconductor layers including two semiconductor layers of different conductivity types, and a passivation film formed on the semiconductor multilayer structure. The passivation film contains an impurity element developing the same conductivity type as that of an impurity element introduced into a semiconductor in the semiconductor multilayer structure. The semiconductor has an interface with the passivation film. 
   In the third inventive semiconductor light emitting device, the impurity element contained in the passivation film is diffused into the semiconductor layer having the interface with the passivation film by a heat treatment performed during the fabrication process, so that the impurity element diffused into the semiconductor layer from the passivation film causes a reduction in the value of resistance in the semiconductor layer where the semiconductor layer is near the interface with the passivation film. As a result, the value of resistance (sheet resistance) in the upper portion of the semiconductor layer is allowed to be small. 
   A fourth inventive semiconductor light emitting device includes a semiconductor multilayer structure comprising a plurality of Group III–V nitride semiconductor layers including two semiconductor layers of different conductivity types, and a passivation film formed on the semiconductor multilayer structure. The passivation film contains a metal element that adsorbs hydrogen. 
   In the fourth inventive semiconductor light emitting device, when the metal element contained in the passivation film is diffused into a semiconductor layer having an interface with the passivation film by a heat treatment performed during the fabrication process, the metal element that adsorbs hydrogen, diffused from the passivation film into the semiconductor layer, adsorbs (binds to) the hydrogen atoms that have been bound to an impurity element introduced into the semiconductor layer. This increases the activation ratio of the impurity element introduced into the semiconductor layer where the semiconductor layer is near the interface with the passivation film, resulting in a decrease in the value of resistance in the semiconductor layer. Accordingly, the value of resistance (sheet resistance) in the upper portion of the semiconductor layer is reduced. 
   In the first or third inventive semiconductor light emitting device, the impurity elements are preferably magnesium, zinc, beryllium, or silicon. 
   In the second or fourth inventive semiconductor light emitting device, the metal element is preferably nickel, palladium, or platinum. 
   The third or fourth inventive semiconductor light emitting device preferably further includes a transparent electrode formed on the semiconductor multilayer structure where the passivation film is not formed. 
   In the first through fourth inventive semiconductor light emitting devices, the transparent electrode is preferably made of indium tin oxide or gallium oxide. 
   Further, the first through fourth inventive semiconductor light emitting devices preferably further include, on the transparent electrode, a multilayer film that reflects light emitted from the semiconductor multilayer structure, and includes a plurality of dielectric layers. 
   The first through fourth inventive semiconductor light emitting devices preferably further include a multilayer film, which is formed to the side of the semiconductor multilayer structure opposite to the transparent electrode, and which reflects light emitted from the semiconductor multilayer structure, and includes a plurality of dielectric layers or a plurality of semiconductor layers. 
   In this case, the multilayer film is preferably made of at least two substances among silicon oxide, silicon nitride, niobium oxide, hafnium oxide, titanium oxide and tantalum oxide. 
   A first inventive method for fabricating a semiconductor light emitting device includes the steps of forming, on a substrate, a semiconductor multilayer structure comprising a plurality of Group III–V nitride semiconductor layers including two semiconductor layers of different conductivity types; and forming a transparent electrode on an electrode-formation face of the semiconductor multilayer structure by using material that contains an impurity element developing the same conductivity type as that of an impurity element introduced into a semiconductor having the electrode-formation face, and then heat-treating the transparent electrode. 
   In accordance with the first inventive semiconductor-light-emitting-device fabrication method, the impurity element contained in the transparent electrode formed on the semiconductor multilayer structure is diffused into the semiconductor layer having the interface with the transparent electrode by the heat treatment. The impurity element diffused from the transparent electrode into the semiconductor layer causes a reduction in the value of resistance in the semiconductor layer where the semiconductor layer is near the interface with the transparent electrode. Accordingly, the contact resistance of the semiconductor layer with respect to the transparent electrode is permitted to be small. 
   A second method for fabricating a semiconductor light emitting device includes the steps of forming, on a substrate, a semiconductor multilayer structure comprising a plurality of Group III–V nitride semiconductor layers including two semiconductor layers of different conductivity types; and forming a transparent electrode on the semiconductor multilayer structure by using material that contains a metal element that adsorbs hydrogen, and then heat-treating the transparent electrode. 
   According to the second inventive semiconductor-light-emitting-device fabrication method, the metal element that adsorbs hydrogen, contained in the transparent electrode formed on the semiconductor multilayer structure, is diffused into a semiconductor layer having an interface with the transparent electrode by the heat treatment. The metal element that adsorbs hydrogen, diffused from the transparent electrode into the semiconductor layer, adsorbs (binds to) the hydrogen atoms that have been bound to an impurity element introduced into the semiconductor layer. This increases the activation ratio of the impurity element introduced into the semiconductor layer where the semiconductor layer is near the interface with the transparent electrode, leading to a decrease in the resistance of the semiconductor layer. As a result, the contact resistance of the semiconductor layer with respect to the transparent electrode is reduced. 
   The first or second inventive semiconductor-light-emitting-device fabrication method preferably further includes, before the transparent-electrode formation step, the steps of: forming a passivation film on the semiconductor multilayer structure, and removing from the passivation film a portion in which the transparent electrode is to be formed. The passivation film is preferably formed using material that contains an impurity element developing the same conductivity type as that of an impurity element introduced into a semiconductor in the semiconductor multilayer structure. The semiconductor has an interface with the passivation film. 
   The first or second inventive semiconductor-light-emitting-device fabrication method preferably further includes, before the transparent-electrode formation step, the steps of: forming a passivation film on the semiconductor multilayer structure, and removing from the passivation film a portion in which the transparent electrode is to be formed. The passivation film is preferably formed using material that contains a metal element that adsorbs hydrogen. 
   A third inventive method for fabricating a semiconductor light emitting device includes the steps of: forming, on a substrate, a semiconductor multilayer structure comprising a plurality of Group III–V nitride semiconductor layers including two semiconductor layers of different conductivity types; forming a first electrode made of metal on the semiconductor multilayer structure; removing the substrate from the semiconductor multilayer structure; and forming a transparent electrode on a second-electrode-formation face of the semiconductor multilayer structure by using material that contains an impurity element developing the same conductivity type as that of an impurity element introduced into a semiconductor having the second-electrode-formation face, wherein the second-electrode-formation face opposes the first electrode, and then heat-treating the transparent electrode. 
   In accordance with the third inventive semiconductor-light-emitting-device fabrication method, even in a case where unconductive insulative material such as sapphire is used as the material for the substrate, it is possible to respectively form a first electrode and a second electrode on the upper and lower faces of the semiconductor multilayer structure so that the first and second electrodes oppose each other. In addition, the impurity element diffused from the transparent electrode into the semiconductor layer having the interface with the transparent electrode causes a reduction in the value of resistance in the semiconductor layer where the semiconductor layer is in the vicinity of the interface with the transparent electrode. Accordingly, the contact resistance of the semiconductor layer with respect to the transparent electrode is allowed to be small. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  illustrate a blue light emitting diode in accordance with a first embodiment of the present invention.  FIG. 1A  illustrates the plan configuration, while  FIG. 1B  illustrates the cross-sectional view taken along the line Ib—Ib of  FIG. 1A . 
       FIGS. 2A through 2C  are cross-sectional views illustrating sequential process steps for fabricating the blue light emitting diode in accordance with the first embodiment of the present invention. 
       FIGS. 3A and 3B  are cross-sectional views illustrating sequential process steps for fabricating the blue light emitting diode in accordance with the first embodiment of the present invention. 
       FIG. 4  is a cross-sectional view of a blue light emitting diode in accordance with a second modified example of the first embodiment of the present invention. 
       FIG. 5  is a cross-sectional view of an ultraviolet light emitting diode in accordance with a second embodiment of the present invention. 
       FIGS. 6A through 6C  are cross-sectional views illustrating sequential process steps for fabricating the ultraviolet light emitting diode in accordance with the second embodiment of the present invention. 
       FIGS. 7A through 7C  are cross-sectional views illustrating sequential process steps for fabricating the ultraviolet light emitting diode in accordance with the second embodiment of the present invention. 
       FIG. 8  is a cross-sectional view of a blue light emitting diode in accordance with a third embodiment of the present invention. 
       FIGS. 9A through 9C  are cross-sectional views illustrating sequential process steps for fabricating the blue light emitting diode in accordance with the third embodiment of the present invention. 
       FIGS. 10A through 10C  are cross-sectional views illustrating sequential process steps for fabricating the blue light emitting diode in accordance with the third embodiment of the present invention. 
       FIG. 11  is a cross-sectional view of a blue light emitting diode in accordance with a second modified example of the third embodiment of the present invention. 
       FIG. 12  is a cross-sectional view of a blue light emitting diode in accordance with a fourth embodiment of the present invention. 
       FIGS. 13A through 13D  are cross-sectional views illustrating sequential process steps for fabricating the blue light emitting diode in accordance with the fourth embodiment of the present invention. 
       FIGS. 14A through 14C  are cross-sectional views illustrating sequential process steps for fabricating the blue light emitting diode in accordance with the fourth embodiment of the present invention. 
       FIG. 15  is a cross-sectional view of a blue-light surface-emitting laser device in accordance with a fifth embodiment of the present invention. 
       FIGS. 16A through 16C  are cross-sectional views illustrating sequential process steps for fabricating the blue-light surface-emitting laser device in accordance with the fifth embodiment of the present invention. 
       FIGS. 17A through 17C  are cross-sectional views illustrating sequential process steps for fabricating the blue-light surface-emitting laser device in accordance with the fifth embodiment of the present invention. 
       FIG. 18  is a cross-sectional view of a known blue light emitting diode. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   (First Embodiment) 
   Hereinafter, a first embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIGS. 1A and 1B  illustrate a blue light emitting diode in accordance with the first embodiment of the present invention.  FIG. 1A  illustrates the plan configuration, while  FIG. 1B  illustrates the cross-sectional structure taken along the line Ib—Ib of  FIG. 1A . 
   As shown in  FIGS. 1A and 1B , a first semiconductor layer  12 , a multi-quantum well (MQW) active layer  13 , and a second semiconductor layer  14  are sequentially formed on a substrate  11  made of sapphire, for example. The first semiconductor layer  12  is made of n-type gallium nitride (GaN) having a thickness of about 4 μm and a carrier density of about 1×10 17  cm 2 . The MQW active layer  13  is formed by stacking three pairs of an about 7 nm thick barrier layer of GaN and an about 3 nm thick well layer of In 0.3 Ga 0.7 N. The second semiconductor layer  14  is made of p-type gallium nitride (GaN) having a thickness of about 0.8 μm and a carrier density of about 1×10 18  cm 2 . 
   A transparent electrode  15  having a thickness of about 100 nm and made of indium tin oxide (ITO) is formed on the second semiconductor layer  14 . 
   A bonding pad  16  of gold (Au) is formed selectively on the transparent electrode  15 , and an n-type ohmic electrode  17 , made of a multilayer structure of titanium (Ti) and gold (Au), is formed on a selectively exposed portion of the first semiconductor layer  12 . 
   By this structure, blue light, generated and emitted from the MQW active layer  13 , and passing through the second semiconductor layer  14  and the transparent electrode  15 , is taken out to the exterior. 
   The first embodiment is characterized in that the impurity element introduced into the ITO that forms the transparent electrode  15  is magnesium (Mg), which is the impurity element introduced into the p-type second semiconductor layer  14 . As will be described later, the magnesium introduced into the ITO is diffused into the second semiconductor layer  14  by an annealing performed during fabrication process, causing contact resistance between the second semiconductor layer  14  and the transparent electrode  15  to decrease. 
   It should be noted that the impurity element to introduce into the transparent electrode  15  is not limited to magnesium, but zinc (Zn), beryllium (Be), or any other dopant that makes the conductivity type of gallium nitride be p-type, may be used. 
   In addition, instead of the MQW active layer  13 , a single-quantum well (SQW) active layer of indium gallium nitride with a thickness of about 20 nm may be provided. 
   Moreover, as shown in  FIG. 1A , since the transparent electrode  15  is conductive, the location and shape of the electrode may be determined arbitrarily. 
   As described above, in the first embodiment, magnesium, which is the impurity element introduced into the p-type second semiconductor layer  14 , is introduced into the transparent electrode  15 . Therefore, contact resistance between the second semiconductor layer  14  and the transparent electrode  15  is decreased, thereby allowing the operating voltage to be reduced. 
   Hereinafter, referring to the accompanying drawings, it will be described how to fabricate a blue light emitting diode having the above-mentioned structure. 
     FIGS. 2A through 2C  and  FIGS. 3A and 3B  are cross-sectional views illustrating sequential process steps for fabricating a blue light emitting diode in accordance with the first embodiment of the present invention. 
   First, as shown in  FIG. 2A , a low-temperature buffer layer (not shown) made of gallium nitride is grown on a substrate  11  made of sapphire by a metal organic vapor phase epitaxy (MOVPE) process. The substrate  11  is about 5.1 cm (=2 inches) in diameter, and the plane orientation of the principal surface thereof is a (0001) plane. In the MOVPE process, trimethylgallium (TMG) is used as a gallium source, ammonia (NH 3 ) is used as a nitrogen source, and hydrogen (H 2 ) is used as a carrier gas, while the growth temperature is set at about 500° C. The buffer layer buffers a lattice mismatch between the sapphire and a first semiconductor layer  12 , for example, grown on the sapphire. Subsequently, while mono-silane (SiH 4 ), which is a source material containing silicon serving as a donor impurity, is introduced, and with the growth temperature being set to about 1030° C., the first semiconductor layer  12  made of n-type gallium nitride with a thickness of about 4 μm is grown on the low-temperature buffer layer. Then, the supply of the mono-silane is stopped, and a barrier layer made of gallium nitride with a thickness of about 7 nm is grown on the first semiconductor layer  12 . The carrier gas is then changed to nitrogen (N 2 ), and at the same time the growth temperature is lowered to about 800° C., and while trimethylindium (TMI) as an indium source is also supplied, a well layer is grown on the barrier layer. The well layer has a thickness of about 3 nm, and is made of indium gallium nitride, in which indium proportion is 30%. The barrier layer and the well layer are grown alternately in three pairs, thereby forming a MQW active layer  13 . By this quantum well structure, the MQW active layer  13  generates blue light with a wavelength of about 470 nm. As mentioned above, when the barrier layers of gallium nitride are grown, hydrogen is used as the carrier gas, and the growth temperature is set at about 1030° C. On the other hand, when the well layers of indium gallium nitride are grown, nitrogen is used as the carrier gas, and the growth temperature is set at about 800° C. 
   Next, cyclopentadienyl magnesium (Cp 2 Mg), which is a source material containing magnesium as an acceptor impurity, is introduced into the respective source gases of trimethylgallium and ammonia, and a second semiconductor layer  14  made of p-type gallium nitride with a thickness of about 0.8 μm is grown on the MQW active layer  13 . After the second semiconductor layer  14  has been grown, the second semiconductor layer  14  is subjected to an annealing process performed using an annealing furnace for 20 minutes in a nitrogen ambient at a temperature of about 750° C. Through the annealing process, the p-type dopant introduced into the second semiconductor layer  14  is activated, which further reduces the resistance of the second semiconductor layer  14 . 
   Subsequently, as shown in  FIG. 2B , the second semiconductor layer  14 , the MQW active layer  13 , and upper portions of the first semiconductor layer  12  are removed selectively by dry etching, such as reactive ion etching (RIE) using, e.g., chlorine (Cl 2 ) as an etching gas, or inductively coupled plasma (ICP) etching, thereby forming n-type electrode formation regions  12   a  in the first semiconductor layer  12 . 
   Then, as shown in  FIG. 2C , ITO, into which magnesium, i.e., the same impurity element as the p-type dopant in the second semiconductor layer  14 , has been introduced, is selectively grown to a thickness of about 100 nm on the second semiconductor layers  14 , thereby forming transparent electrodes  15 . The ITO may be grown by a sputtering process, a pulsed laser deposition (PLD) method, an electron beam (EB) process, or a sol-gel method, for example. In terms of reducing the resistance of the ITO, a sputtering process or a PLD method is preferable. Further, if a sputtering process or a PLD method is employed, a target material is normally formed by sintering, thus making it easy to introduce an impurity element such as magnesium. Subsequently, after the transparent electrodes  15  have been grown, the transparent electrodes  15  are subjected to an annealing process performed at a temperature of about 500° C. Through the annealing process, part of the magnesium introduced into the ITO is diffused into the second semiconductor layers  14  through the interfaces between the ITO and the second semiconductor layers  14 . This leads to a decrease in the value of resistance in the second semiconductor layers  14  where the second semiconductor layers  14  are near the interfaces with the transparent electrodes  15 , such that the transparent electrodes  15  having small contact resistance with respect to the second semiconductor layers  14  are formed. In this embodiment, the impurity element to introduce into the ITO is not limited to magnesium, but zinc or beryllium may be used. Nevertheless, magnesium is preferable in terms of activation of the dopant. Further, the transparent electrodes  15  are not limited to ITO, but may be made of any substance that makes the transparent electrodes  15  transparent with respect to emitted light having a wavelength of 470 nm, and tin oxide (SnO 2 ) or zinc oxide (ZnO), for example, may be used. 
   Then, as shown in  FIG. 3A , bonding pads  16  for wire bonding are selectively formed on the respective transparent electrodes  15 . Subsequently, titanium and gold are sequentially grown on the n-type electrode formation regions  12   a  in the first semiconductor layer  12 , thereby forming n-type ohmic electrodes  17 . 
   Next, as shown in  FIG. 3B , the substrate  11  is divided into chips each about 300 μm square, thereby obtaining blue light emitting diodes. 
   In this manner, magnesium, that is, the impurity element with which the p-type second semiconductor layers  14  have been doped, is introduced beforehand into the transparent electrodes  15  (p-type electrodes), and diffused through the interfaces into the second semiconductor layers  14  by an annealing process. Therefore, in the resultant blue light emitting diode, in which emitted light is taken out through the p-type second semiconductor layer  14 , the contact resistance of the transparent electrode  15  with respect to the second semiconductor layer  14  is allowed to be small, so that the second semiconductor layer  14  has small resistance near its interface with the transparent electrode  15 . As a result, it is possible to lower the operating voltage. 
   First Modified Example of the First Embodiment 
   In the first embodiment, a dopant that makes the conductivity type of gallium nitride be p-type is introduced into the transparent material (ITO) that forms the transparent electrodes  15 . However, in addition to the p-type dopant, a metal element that tends to adsorb (bind to) hydrogen atoms, e.g., nickel (Ni), palladium (Pd), or platinum (Pt), may be introduced. In that case, in order to form the transparent electrodes  15 , a metal element, such as nickel, that easily adsorbs hydrogen atoms may be introduced beforehand into the target material for growing the transparent electrodes  15 . 
   Normally, a p-type dopant introduced into a p-type gallium nitride semiconductor tends to bind to hydrogen atoms, causing the p-type dopant to be deactivated. In view of this, if metal atoms that easily adsorb hydrogen atoms are diffused into the p-type semiconductor layer through the transparent electrode  15 , the metal atoms diffused into the p-type semiconductor layer attract the hydrogen atoms that have been taken into the p-type semiconductor layer. 
   In this manner, the metal atoms such as nickel atoms separate the hydrogen atoms that cause the p-type dopant to be deactivated, from the p-type dopant, so that activation of the p-type dopant such as magnesium is facilitated. Therefore, the p-type second semiconductor layer  14  has small resistance where the p-type second semiconductor layer  14  is in the vicinity of the interface with the transparent electrode  15 . As a result, it is possible to form the transparent electrode  15  with small contact resistance with respect to the second semiconductor layer  14 . 
   Second Modified Example of the First Embodiment 
   In the first embodiment, generated light is extracted through the p-type second semiconductor layer  14 . However, a flip-chip device, in which generated light is taken out through a substrate  11 , may be formed. 
   As shown in  FIG. 4 , in a blue light emitting diode, a transparent electrode  15  is secured onto a mounting substrate  20  with a high-reflectance film  21  and a first solder material  22  being interposed therebetween. The high-reflectance film  21  is made of a multilayer film composed of a plurality of dielectrics. Further, an n-type ohmic electrode  17  is secured onto the mounting substrate  20  with a second solder material  23  being interposed therebetween. 
   In this way, in the blue light emitting diode in accordance with a second modified example, the high-reflectance film  21 , instead of the bonding pad  16 , is provided on the transparent electrode  15 , so that light emitted toward the transparent electrode  15  is reflected by the high-reflectance film  21 , and extracted through the substrate  11 . In this modified example, the higher the reflectance of the high-reflectance film  21  the better, and it is preferable that the reflectance is at least 70% or more. 
   (Second Embodiment) 
   Hereinafter, a second embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIG. 5  illustrates a cross-sectional structure of an ultraviolet light emitting diode in accordance with the second embodiment of the present invention. In  FIG. 5 , the same members as those shown in  FIGS. 1A and 1B  are identified by the same reference numerals and the description thereof will be omitted herein. 
   As shown in  FIG. 5 , a first semiconductor layer  32 , a MQW active layer  33 , and a second semiconductor layer  34  are sequentially formed on a substrate  11  made of sapphire, for example. The first semiconductor layer  32  is made of n-type aluminum gallium nitride (Al 0.4 Ga 0.6 N) having a thickness of about 4 μm and a carrier density of about 1×10 17  cm 2 . The MQW active layer  33  is formed by stacking three pairs of an about 7 nm thick barrier layer of aluminum gallium nitride (Al 0.12 Ga 0.88 N) and an about 3 nm thick well layer of GaN. The second semiconductor layer  34  is made of p-type aluminum gallium nitride (Al 0.4 Ga 0.6 N) having a thickness of about 0.8 μm and a carrier density of about 1×10 18  cm 2 . 
   An n-type ohmic electrode  37  made of titanium and aluminum is formed on an exposed portion of the first semiconductor layer  32 . 
   On the second semiconductor layer  34 , formed is a transparent electrode  35  having a thickness of about 100 nm and made of gallium oxide (Ga 2 O 3 ), into which about 1 mol % of tin (Sn) has been introduced. It is preferable that β-(cubic) gallium oxide be used, in which case the conductivity becomes excellent. Further, the impurity element introduced into the gallium oxide that forms the transparent electrode  35  is magnesium (Mg), which is the impurity element introduced into the p-type second semiconductor layer  34 . As in the first embodiment, the magnesium introduced into the gallium oxide is diffused into the second semiconductor layer  34  through an annealing performed during fabrication process, causing contact resistance between the second semiconductor layer  34  and the transparent electrode  35  to be decreased. 
   ITO, normally used as a transparent electrode, has low transmissivity with respect to ultraviolet light with a wavelength of about 300 nm, and thus is not suitable for a transparent electrode. On the other hand, gallium oxide, particularly β-gallium oxide, into which tin oxide has been introduced, has high transmissivity with respect to ultraviolet light in the 300 nm wavelength range, and is thus suitable for the transparent electrode  35  formed in the ultraviolet light emitting diode in accordance with the second embodiment. 
   As described above, in the ultraviolet light emitting diode of the second embodiment, since the transparent electrode  35  with high transmissivity is used, light-extraction efficiency is increased. In addition, the impurity developing the same conductivity type as that of the impurity that makes the second semiconductor layer  34  be of p-type, is introduced into the transparent electrode  35 , such that contact resistance between the transparent electrode  35  and the second semiconductor layer  34  is small, allowing the operating voltage to be decreased. 
   It should be noted that the impurity element to introduce into the transparent electrode  35  is not limited to magnesium, but zinc, beryllium, or any other dopant that renders the conductivity of gallium nitride p-type may be used. 
   Moreover, instead of the MQW active layer  33 , a single-quantum well (SQW) active layer of gallium nitride with a thickness of about 20 nm may be provided. 
   Hereinafter, referring to the accompanying drawings, it will be described how to fabricate an ultraviolet light emitting diode having the above-mentioned structure. 
     FIGS. 6A through 6C  and  FIGS. 7A through 7C  are cross-sectional views illustrating sequential process steps for fabricating an ultraviolet light emitting diode in accordance with the second embodiment of the present invention. 
   First, as shown in  FIG. 6A , a low-temperature buffer layer (not shown) of aluminum gallium nitride is grown on a substrate  11  made of sapphire by a MOVPE process. The substrate  11  is about 5.1 cm in diameter, and the plane orientation of the principal surface thereof is a (0001) plane. In the MOVPE process, trimethylgallium is used as a gallium source, trimethylaluminum is used as an aluminum source, ammonia is used as a nitrogen source, and hydrogen is used as a carrier gas, while the growth temperature is set at about 500° C. The low-temperature buffer layer buffers a lattice mismatch between the sapphire and a first semiconductor layer  32 , for example, grown on the sapphire. In this process step, the low-temperature buffer layer may be made of gallium nitride. Subsequently, while mono-silane, which is a source material containing silicon as a donor impurity, is introduced, and with the growth temperature being set at about 1030° C., the first semiconductor layer  32  made of n-type aluminum gallium nitride having a thickness of about 4 μm is grown on the low-temperature buffer layer. Then, the supply of the mono-silane is stopped, and a barrier layer made of aluminum gallium nitride with a thickness of about 7 nm is grown on the first semiconductor layer  32 . Thereafter, the supply of the trimethylaluminum as the aluminum source is stopped, and a well layer made of gallium nitride having a thickness of about 3 nm is grown on the barrier layer. The barrier layer and the well layer are grown alternately in three pairs, thereby forming a MQW active layer  33 . By this quantum well structure, the MQW active layer  33  generates ultraviolet light with a wavelength of about 360 nm. Then, cyclopentadienyl magnesium, which is a source material containing magnesium as an acceptor impurity, is introduced into the respective source gases of trimethylgallium, trimethylaluminum, and ammonia, and a second semiconductor layer  34  made of p-type aluminum gallium nitride with a thickness of about 0.8 μm is grown on the MQW active layer  33 . 
   Next, as shown in  FIG. 6B , β-gallium oxide is grown to a film thickness of about 100 nm on the second semiconductor layer  34  by a PLD method, for example, thereby forming a transparent electrode  35 , wherein tin for making the electrode itself conductive, and magnesium to be diffused into the second semiconductor layer  34  have been introduced into the β-gallium oxide. The gallium oxide film may be grown by a sputtering process, but a PLD method, which gives excellent crystallinity, is preferable. Subsequently, after the transparent electrode  35  has been grown, the second semiconductor layer  34  and the transparent electrode  35  are subjected to an annealing process performed using an annealing furnace for 20 minutes in a nitrogen ambient at a temperature of about 750° C. Through the annealing process, the resistance of the transparent electrode  35  is decreased, while at the same time the p-type dopant in the second semiconductor layer  34 , including the p-type dopant diffused from the transparent electrode  35 , is activated, thereby further lowering the resistance of the second semiconductor layer  34 . 
   Next, as shown in  FIG. 6C , patterning is performed to selectively remove the transparent electrode  35  where the transparent electrode  35  is located above n-type ohmic-electrode formation regions. 
   Subsequently, as shown in  FIG. 7A , the second semiconductor layer  34 , the MQW active layer  33 , and upper portions of the first semiconductor layer  32  are selectively removed by dry etching, such as RIE using, e.g., chlorine as an etching gas, or ICP etching, thereby forming n-type electrode formation regions  32   a  in the first semiconductor layer  32 . 
   Next, as shown in  FIG. 7B , bonding pads  16  for wire bonding are selectively formed on the respective transparent electrodes  35 . Titanium and aluminum are then sequentially grown on the n-type electrode formation regions  32   a  in the first semiconductor layer  32 , thereby forming n-type ohmic electrodes  37 . 
   Next, as shown in  FIG. 7C , the substrate  11  is divided into chips each about 300 μm square, thereby obtaining ultraviolet light emitting diodes. 
   As described above, in the fabrication method of the second embodiment, since tin-oxide-added gallium oxide having high transmissivity with respect to ultraviolet light is used to form the transparent electrode  35 , the light-extraction efficiency is extremely favorable, resulting in an increase in power conversion efficiency. 
   Additionally, since magnesium, that is, the p-type impurity element with which the p-type second semiconductor layer  34  has been doped, is introduced into the transparent electrode  35 , part of the magnesium is diffused into the second semiconductor layer  34  where the second semiconductor layer  34  is in the vicinity of the interface with the transparent electrode  35  by an annealing process performed after the formation of the transparent electrode  35  as in the first embodiment. This allows the second semiconductor layer  34  to have small resistance near the interface with the transparent electrode  35 , thereby reducing contact resistance between the second semiconductor layer  34  and the transparent electrode  35 . 
   It should be noted that the impurity element to introduce into gallium oxide is not limited to magnesium, but zinc, beryllium, or any other dopant that makes aluminum gallium nitride develop p-type conductivity, may be used. 
   Modified Example of the Second Embodiment 
   In the second embodiment, a dopant that renders the conductivity of aluminum gallium nitride p-type is introduced into the transparent material (gallium oxide) that forms the transparent electrodes  35 . However, in addition to the p-type dopant, a metal element that tends to adsorb (bind to) hydrogen atoms, e.g., nickel (Ni), palladium (Pd), or platinum (Pt), may be introduced. In that case, in order to form the transparent electrodes  35 , a metal element, such as nickel, that tends to adsorb hydrogen atoms may be introduced beforehand into the target material for growing the transparent electrodes  35 . 
   Then, the ultraviolet light emitting diode made of nitride semiconductors in accordance with this modified example achieves an increase in the light-extraction efficiency as well as a decrease in the operating voltage. 
   (Third Embodiment) 
   Hereinafter, a third embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIG. 8  illustrates a cross-sectional structure of a blue light emitting diode in accordance with the third embodiment of the present invention. In  FIG. 8 , the same members as those shown in  FIGS. 1A and 1B  are identified by the same reference numerals and the description thereof will be omitted herein. 
   As shown in  FIG. 8 , the blue light emitting diode in accordance with the third embodiment has a so-called N-up structure, in which a transparent electrode  45  is formed on a first semiconductor layer  12  made of n-type gallium nitride. 
   A second semiconductor layer  14  made of p-type gallium nitride is formed to the side of a MQW active layer  13  opposite to the first semiconductor layer  12 , that is, the second semiconductor layer  14  is formed under the MQW active layer  13 . 
   A p-type electrode  41  made of platinum having a thickness of about 100 nm is formed underneath the second semiconductor layer  14 . Underneath the p-type electrode  41 , a plated underlying layer  42  made of gold with a thickness of about 200 nm is formed. Underneath the plated underlying layer  42 , a plated layer  43  made of gold having a thickness of about 50 μm is formed. 
   The third embodiment is characterized in that the impurity element introduced into the ITO that forms the transparent electrode  45  is silicon (Si), which is the impurity element introduced into the n-type first semiconductor layer  12 . As will be described later, the silicon introduced into the ITO is diffused into the first semiconductor layer  12  through an annealing performed during fabrication process, so that contact resistance between the first semiconductor layer  12  and the transparent electrode  45  decreases. 
   It should be noted that the impurity element to introduce into the transparent electrode  45  is not limited to silicon, but a dopant, such as germanium (Ge), that renders the conductivity of gallium nitride n-type, may be used. 
   Further, in this embodiment, the substrate  11  of sapphire is removed from the semiconductor multilayer structure, and the plated layer  43  of gold is provided instead. Therefore, instead of the sapphire having inferior heat-dispersion characteristics, the plated layer  43  with excellent heat-dispersion characteristics is mounted on a submount, thus ensuring that the temperature characteristics of the device increases. 
   In this embodiment, although the N-up structure is employed as shown in  FIG. 8 , a so-called P-up structure, in which a transparent electrode is formed on the face of the p-type second semiconductor layer  14  opposite to the MQW active layer  13 , may be adopted. In that case, magnesium is introduced into the transparent electrode as in the first embodiment. Then, as in the case of the N-up structure, a device having excellent heat-dispersion characteristics, and capable of operating at low operating voltage, is obtained. 
   Moreover, as in the second embodiment, aluminum gallium nitride may be used to form the semiconductor multilayer structure, so that the MQW active layer emits ultraviolet light. In that case, it is preferable that the transparent electrode  45  be made of gallium oxide, in particular, β-gallium oxide, into which tin oxide and an impurity that develops the same conductivity type as that of an impurity introduced into a semiconductor layer having an interface with the transparent electrode  45 , have been introduced. 
   Furthermore, instead of the MQW active layer  13 , a single-quantum well (SQW) active layer of indium gallium nitride having a thickness of about 20 nm may be provided. 
   Hereinafter, referring to the accompanying drawings, it will be described how to fabricate a blue light emitting diode having the above-mentioned structure. 
     FIGS. 9A through 9C  and  FIGS. 10A through 10C  are cross-sectional views illustrating sequential process steps for fabricating a blue light emitting diode in accordance with the third embodiment of the present invention. 
   First, as shown in  FIG. 9A , a low-temperature buffer layer (not shown) is grown on a substrate  11  made of sapphire by a MOVPE process. The substrate  11  is about 5.1 cm in diameter, and the plane orientation of the principal surface thereof is a (0001) plane. In the MOVPE process, trimethylgallium is used as a gallium source, ammonia is used as a nitrogen source, and hydrogen is used as a carrier gas, while the growth temperature is set at about 500° C. The buffer layer buffers a lattice mismatch between the sapphire and a first semiconductor layer  12 , for example, grown on the sapphire. Subsequently, while mono-silane, which is a source material containing silicon as a donor impurity, is introduced, and with the growth temperature being set at about 1030° C., the first semiconductor layer  12  made of n-type gallium nitride having a thickness of about 4 μm is grown on the low-temperature buffer layer. Then, the supply of the mono-silane is stopped, and a barrier layer made of gallium nitride having a thickness of about 7 nm is grown on the first semiconductor layer  12 . The carrier gas is then changed to nitrogen (N 2 ), and at the same time the growth temperature is lowered to about 800° C., and while trimethylindium as an indium source is also supplied, a well layer is grown on the barrier layer. The well layer has a thickness of about 3 nm and is made of indium gallium nitride, in which indium proportion is 30%. The barrier layer and the well layer are grown alternately in three pairs, thereby forming a MQW active layer  13 . Then, cyclopentadienyl magnesium, which is a source material containing magnesium as an acceptor impurity, is introduced into the respective source gases of trimethylgallium and ammonia, and a second semiconductor layer  14  made of p-type gallium nitride having a thickness of about 0.8 μm is grown on the MQW active layer  13 . After the second semiconductor layer  14  has been grown, the second semiconductor layer  14  is subjected to an annealing process performed for 20 minutes in a nitrogen ambient at a temperature of about 750° C. Through the annealing process, the p-type dopant introduced into the second semiconductor layer  14  is activated, which further reduces the resistance of the second semiconductor layer  14 . 
   Subsequently, as shown in  FIG. 9B , a p-type electrode  41  made of platinum is formed on the entire surface of the second semiconductor layer  14  by an EB deposition method, for example. In this process step, the material for the p-type electrode  41  is not limited to platinum, but may be any material that has excellent ohmic with respect to the p-type second semiconductor layer  14 , and has high reflectance. For example, rhodium (Rh) or silver (Ag) may be used. 
   Next, as shown in  FIG. 9C , a plated underlying layer  42  made of gold is grown on the entire surface of the p-type electrode  41  by an EB deposition method, for example. Then, a plated layer  43  made of gold having a thickness of about 50 μm is grown on the entire surface of the plated underlying layer  42  by a plating process. 
   Thereafter, as shown in  FIG. 10A , the substrate  11  is removed from the semiconductor multilayer structure with the plated layer  43  formed thereon. The substrate  11  may be removed, for example, by a polishing method, in which the substrate  11  is polished mechanically, or by a laser lift-off method, in which the substrate  11  is peeled off by irradiating the substrate  11  through its reverse face (that is, its face opposing the first semiconductor layer  12 ) with a laser beam having a wavelength that passes through sapphire and is absorbed by gallium nitride. In a case of using a laser lift-off method, since metal gallium is produced by the thermal decomposition of the gallium nitride, and adheres to the face of the first semiconductor layer  12  from which the substrate  11  has been peeled off, the attached metal gallium has to be removed using hydrochloric acid. 
   Subsequently, as shown in  FIG. 10B , ITO, into which silicon, i.e., the same impurity element as the n-type dopant in the first semiconductor layer  12 , has been introduced, is grown to a thickness of about 100 nm by a PLD method, for example, on the exposed face of the first semiconductor layer  12 , that is, on the face of the first semiconductor layer  12  opposite to the MQW active layer  13 , thereby forming a transparent electrode  45 . In this process step, the impurity element to introduce into the material for the ITO may be a dopant that makes the first semiconductor layer  12  develop n-type conductivity. For example, germanium may be used in place of silicon. 
   Further, the material for the transparent electrode  45  is not limited to ITO, but may be any substance that is transparent with respect to light with a wavelength of about 470 nm, and tin oxide or zinc oxide may thus be used. 
   Furthermore, as in the second embodiment, if the diode is formed so that the MQW active layer  13  emits light having a wavelength in the ultraviolet region, the use of tin-oxide-added gallium oxide in forming the transparent electrode  45  makes the transparent electrode  45  transparent with respect to the wavelength of the emitted light. 
   Next, after the transparent electrode  45  has been grown, the transparent electrode  45  is subjected to an annealing process performed at a temperature of about 500° C. Through the annealing process, part of the silicon introduced into the ITO is diffused into the first semiconductor layer  12  through the interface between the transparent electrode  45  and the first semiconductor layer  12 . This results in a decrease in the value of resistance in the first semiconductor layer  12  where the first semiconductor layer  12  is near the interface with the transparent electrode  45 , thus leading to the formation of the transparent electrode  45  with small contact resistance with respect to the first semiconductor layer  12 . 
   Then, as shown in  FIG. 10C , bonding pads  16  for wire bonding are selectively formed on the transparent electrodes  45 . The semiconductor multilayer structure is then divided into chips each about 300 μm square, thereby obtaining blue light emitting diodes. 
   As described above, in the blue light emitting diode having the N-up structure in accordance with the third embodiment, silicon, that is, the impurity with which the n-type first semiconductor layer  12  has been doped, is introduced beforehand into the transparent electrode  45  (n-type electrode), and then diffused into the first semiconductor layer  12  through an annealing process. This allows the transparent electrode  45  to have small contact resistance with respect to the first semiconductor layer  12 , enabling the operating voltage to be decreased. 
   In addition, the substrate  11  made of sapphire is removed, and the plated layer  43  of gold is formed covering the p-type electrode  41  formed on the p-type second semiconductor layer  14 . Therefore, when the plated layer  43  is mounted onto a submount, for example, a device having excellent heat-dispersion characteristics is obtained. 
   As a first modified example of the third embodiment, instead of introducing a dopant that determines the conductivity type of the gallium nitride into the transparent material (ITO) that forms the transparent electrode  45 , a metal element which tends to adsorb (bind to) hydrogen atoms, e.g., nickel (Ni), palladium (Pd), or platinum (Pt), may be introduced into the transparent material. 
   Further, as a second modified example, as shown in  FIG. 11 , the p-type electrode  41  made of platinum may be formed to be a transparent p-type electrode  41 A made of transparent material, and a multilayer film (reflecting film)  46  made of dielectrics or semiconductors may be formed to the side of the transparent p-type electrode  41 A opposite to the p-type second semiconductor layer  14 . 
   In this case, it is also preferable that magnesium, which is the acceptor impurity in the second semiconductor layer  14 , be introduced into the transparent p-type electrode  41 A. Further, the optical reflectance of the multilayer film  46  is preferably 70% or higher. 
   (Fourth Embodiment) 
   Hereinafter, a fourth embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIG. 12  illustrates a cross-sectional structure of a blue light emitting diode in accordance with the fourth embodiment of the present invention. In  FIG. 12 , the same members as those shown in  FIGS. 1A and 1B  are identified by the same reference numerals and the description thereof will be omitted herein. 
   In the blue light emitting diode in accordance with the fourth embodiment, a passivation film  51  made of magnesium-added silicon oxide (SiO 2 ) is formed so as to cover the upper surface of a second semiconductor layer  14  except the region where a transparent electrode  15  is formed, and to cover the respective exposed lateral faces of the second semiconductor layer  14 , MQW active layer  13 , and first semiconductor layer  12 . 
   As described above, the lateral sides of the blue light emitting diode of the fourth embodiment are covered by the passivation film  51 , which prevents current leakage due to the solder material flowing into the lateral sides of the semiconductor multilayer structure when the diode is mounted onto a submount, for example. 
   In addition, magnesium, which is the impurity element serving as a dopant in the p-type second semiconductor layer  14 , is introduced into the transparent electrode  15  and the passivation film  51 . Therefore, contact resistance between the transparent electrode  15  and the second semiconductor layer  14  is allowed to be reduced, thereby enabling low-voltage operation. 
   It should be noted that the impurity element to introduce into the transparent electrode  15  is not limited to magnesium, but zinc, beryllium, or any other dopant that makes the conductivity type of gallium nitride be p-type, may be used. 
   In addition, instead of the MQW active layer  13 , a single-quantum well (SQW) active layer of gallium nitride having a thickness of about 20 nm may be provided. 
   Hereinafter, referring to the accompanying drawings, it will be described how to fabricate a blue light emitting diode having the above-mentioned structure. 
     FIGS. 13A through 13D  and  FIGS. 14A through 14C  are cross-sectional views illustrating sequential process steps for fabricating a blue light emitting diode in accordance with the fourth embodiment of the present invention. 
   First, as shown in  FIG. 13A , a low-temperature buffer layer (not shown) is grown on a substrate  11  made of sapphire by a MOVPE process. The substrate  11  is about 5.1 cm in diameter, and the plane orientation of the principal surface thereof is a (0001) plane. In the MOVPE process, trimethylgallium is used as a gallium source, ammonia is used as a nitrogen source, and hydrogen is used as a carrier gas, while the growth temperature is set at about 500° C. The low-temperature buffer layer buffers a lattice mismatch between the sapphire and a first semiconductor layer  12 , for example, grown on the sapphire. Subsequently, while mono-silane, which is a source material containing silicon as a donor impurity, is introduced, and with the growth temperature being set at about 1030° C., the first semiconductor layer  12  made of n-type gallium nitride having a thickness of about 4 μm is grown on the low-temperature buffer layer. Then, the supply of the mono-silane is stopped, and a barrier layer made of gallium nitride with a thickness of about 7 nm is grown on the first semiconductor layer  12 . The carrier gas is then changed to nitrogen (N 2 ), and at the same time the growth temperature is lowered to about 800° C., and while trimethylindium as an indium source is also supplied, a well layer is grown on the barrier layer. The well layer has a thickness of about 3 nm, and is made of indium gallium nitride, in which indium proportion is 30%. The barrier layer and the well layer are grown alternately in three pairs, thereby forming a MQW active layer  13 . Subsequently, cyclopentadienyl magnesium, which is a source material containing magnesium as an acceptor impurity, is introduced into the respective source gases of trimethylgallium and ammonia, and a second semiconductor layer  14  made of p-type gallium nitride having a thickness of about 0.8 μm is grown on the MQW active layer  13 . After the second semiconductor layer  14  has been grown, the second semiconductor layer  14  is subjected to an annealing process performed for 20 minutes in a nitrogen ambient at a temperature of about 750° C. Through the annealing process, the p-type dopant introduced into the second semiconductor layer  14  is activated, which further decreases the resistance of the second semiconductor layer  14 . 
   Then, as shown in  FIG. 13B , the second semiconductor layer  14 , the MQW active layer  13 , and upper portions of the first semiconductor layer  12  are selectively removed by dry etching, such as RIE using, e.g., chlorine as an etching gas, or ICP etching, thereby forming n-type electrode formation regions  12   a  in the first semiconductor layer  12 . 
   Next, as shown in  FIG. 13C , a passivation film  51  made of magnesium-added silicon oxide is deposited to a thickness of about 300 nm on the entire surface of the second semiconductor layers  14  as well as on the n-type electrode formation regions  12   a  by a sputtering process, for example. Subsequently, the passivation film  51  is subjected to an annealing process performed at a temperature of about 500° C., so that the magnesium introduced into the passivation film  51  is diffused from the passivation film  51  across the interfaces into the upper portions of the second semiconductor layers  14 , thereby decreasing the resistance value of the second semiconductor layers  14  in the vicinity of the interfaces with the passivation film  51 . It should be noted that in order to introduce magnesium into the passivation film  51 , if a sputtering process is employed, magnesium may be mixed into a target material, and if a sol-gel method is adopted, magnesium may be mixed into a source solution as an organic compound. 
   Next, as shown in  FIG. 13D , transparent-electrode formation portions in the passivation film  51  on the second semiconductor layer  14 , and portions of the passivation film  51  located on n-type electrode formation regions  12   a  are selectively removed by dry etching. 
   Subsequently, as shown in  FIG. 14A , ITO, into which magnesium, that is, the impurity element serving as the p-type dopant in the second semiconductor layer  14 , has been introduced, is selectively grown to a thickness of about 100 nm by a sputtering process or a PLD method, for example, on the exposed faces of the second semiconductor layers  14 , thereby forming transparent electrodes  15 . Thereafter, the transparent electrodes  15  are also subjected to an annealing process performed at a temperature of about 500° C. Through the annealing process, the magnesium introduced into the transparent electrodes  15  is further diffused from the transparent electrodes  15  across the interfaces into the upper portions of the second semiconductor layers  14 , thereby further decreasing the value of resistance in the second semiconductor layers  14  where the second semiconductor layers  14  are near the interfaces with the transparent electrodes  15 . This results in the formation of the transparent electrodes  15  having further reduced contact resistance, on the p-type second semiconductor layers  14 . 
   It should be noted that the impurity element to introduce into the passivation film  51  and the transparent electrodes  15  is not limited to magnesium, but may be a dopant, such as zinc, that makes gallium nitride develop p-type conductivity. 
   Then, as shown in  FIG. 14B , bonding pads  16  for wire bonding are selectively formed on the respective transparent electrodes  15 . Subsequently, titanium and gold are sequentially grown on the n-type electrode formation regions  12   a  in the first semiconductor layer  12 , thereby forming n-type ohmic electrodes  17 . 
   Next, as shown in  FIG. 14C , the substrate  11  is divided into chips each 300 μm square, thereby obtaining blue light emitting diodes. 
   As described above, in the blue-light-emitting-diode fabrication method in accordance with the fourth embodiment, since magnesium, which is the p-type dopant in the second semiconductor layer  14  in contact with the transparent electrode  15 , is introduced into the passivation film  51  and the transparent electrode  15 , the magnesium is diffused into the second semiconductor layer  14  through the interfaces with those members by the annealing process performed after the deposition of the passivation film  51  and by the annealing process performed after the formation of the transparent electrode  15  formed after the removal of the passivation film  51 . As a result, the value of resistance in the second semiconductor layer  14  where the second semiconductor layer  14  is in the vicinity of the interface with the transparent electrode  15  decreases significantly, such that contact resistance between the transparent electrode  15  and the second semiconductor layer  14  is reduced, enabling operation at low operating voltage. 
   In addition, since the passivation film  51  covers the semiconductor multilayer structure laterally, current leakage, caused by the solder material flowing into the lateral sides of the semiconductor multilayer structure during mounting process, is prevented, therefore resulting in an increase in yield. 
   As a first modified example of the fourth embodiment, instead of introducing a dopant that determines the conductivity type of the gallium nitride into the passivation film  51  and the transparent material (ITO) that forms the transparent electrode  15 , a metal element which tends to adsorb (bind to) hydrogen atoms, e.g., nickel (Ni), palladium (Pd), or platinum (Pt), may be introduced into at least one of the passivation film  51  and the transparent material. 
   Further, the effects of the present invention are obtained by introducing an impurity that serves as a dopant in the nitride semiconductor, or a metal element that tends to adsorb hydrogen atoms, into just one of the passivation film  51  and the transparent electrode  15 . 
   (Fifth Embodiment) 
   Hereinafter, a fifth embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIG. 15  illustrates a cross-sectional structure of a blue-light surface-emitting laser device in accordance with the fifth embodiment of the present invention. In  FIG. 15 , the same members as those shown in  FIGS. 1A and 1B  are identified by the same reference numerals and the description thereof will be omitted herein. 
   As shown in  FIG. 15 , between a substrate  11  made of, e.g., sapphire and a first semiconductor layer  12  made of n-type gallium nitride, a first DBR (distributed bragg reflect) mirror  61  is formed by alternately stacking aluminum gallium nitride and gallium nitride one upon the other. Further, a second DBR mirror  65  made of dielectrics is formed in an optical waveguide portion on a transparent electrode  15  that has been doped with magnesium, for example. 
   Moreover, a passivation film  51  made of, e.g., magnesium-added silicon oxide is formed on the end portion of a second semiconductor layer  14 , thereby forming a current confinement structure for confining operating current supplied from the transparent electrode  15 . 
   In this embodiment, a MQW active layer  63  made of indium gallium nitride may have a SQW structure. 
   By the above-mentioned structure, contact resistance between the transparent electrode  15  and the p-type second semiconductor layer  14  is decreased as in the first embodiment, which enables the blue-light surface-emitting laser device to operate at low voltage. 
   In the fifth embodiment, a p-type dopant such as magnesium may be introduced into at least one of the transparent electrode  15  and the passivation film  51 , but introducing the p-type dopant into both allows the effects of the present invention to be attained more notably. 
   Hereinafter, referring to the accompanying drawings, it will be described how to fabricate a blue-light surface-emitting laser device having the above-mentioned structure. 
     FIGS. 16A through 16C  and  FIGS. 17A through 17C  are cross-sectional views illustrating sequential process steps for fabricating the blue-light surface-emitting laser device in accordance with the fifth embodiment of the present invention. 
   First, as shown in  FIG. 16A , a low-temperature buffer layer (not shown) is grown on a substrate  11  made of sapphire by a MOVPE process. The substrate  11  is about 5.1 cm in diameter, and the plane orientation of the principal surface thereof is a (0001) plane. In the MOVPE process, trimethylgallium is used as a gallium source, ammonia is used as a nitrogen source, and hydrogen is used as a carrier gas, while the growth temperature is set at about 500° C. The low-temperature buffer layer buffers a lattice mismatch between the sapphire and a first DBR mirror  61 , for example, grown on the sapphire. Subsequently, with the growth temperature being set at about 1030° C., a first layer made of aluminum gallium nitride and a second layer made of gallium nitride are stacked alternately one upon the other, on the low-temperature buffer layer, thereby forming the first DBR mirror  61 . In this process step, in growing the first layers, trimethylaluminum as an aluminum source is added to the source material. The first DBR mirror  61  is formed so as to have 99% or higher reflectance with respect to the wavelength of light emitted from a MQW active layer  63 . Thereafter, mono-silane, which is a source material containing silicon as a donor impurity, is introduced, and a first semiconductor layer  12  made of n-type gallium nitride is grown. Then, the supply of the mono-silane is stopped, and a barrier layer made of gallium nitride is grown on the first semiconductor layer  12 . The carrier gas is then changed to nitrogen (N 2 ), and at the same time the growth temperature is lowered to about 800° C., and while trimethylindium (TMI) as an indium source is also supplied, a well layer made of indium gallium nitride is grown on the barrier layer. The barrier layer and the well layer are grown, e.g., alternately in three pairs, thereby forming the MQW active layer  63 . The MQW active layer  63  generates blue light having a wavelength of about 470 nm. Subsequently, cyclopentadienyl magnesium, which is a source material containing magnesium as an acceptor impurity, is introduced into the respective source gases of trimethylgallium and ammonia, and a second semiconductor layer  14  made of gallium nitride is grown on the MQW active layer  63 . After the second semiconductor layer  14  has been grown, the second semiconductor layer  14  is subjected to an annealing process performed for 20 minutes in a nitrogen ambient at a temperature of about 750° C. Through the annealing process, the p-type dopant introduced into the second semiconductor layer  14  is activated, which further decreases the resistance of the second semiconductor layer  14 . 
   Then, as shown in  FIG. 16B , the second semiconductor layer  14 , the MQW active layer  63 , and upper portions of the first semiconductor layer  12  are selectively removed by dry etching, such as RIE using, e.g., chlorine as an etching gas, or ICP etching, thereby forming an n-type electrode formation region  12   a  in the first semiconductor layer  12 . 
   Next, a passivation film  51  made of magnesium-added silicon oxide is deposited to a thickness of about 300 nm on the entire surface of the second semiconductor layer  14  as well as on the n-type electrode formation region  12   a  by a sputtering process, for example. The passivation film  51  is then subjected to an annealing process performed at a temperature of about 500° C., so that the magnesium introduced into the passivation film  51  is diffused from the passivation film  51  across the interface into the upper portion of the second semiconductor layer  14 , thereby lowering the resistance value of the second semiconductor layer  14  in the vicinity of the interface with the passivation film  51 . Thereafter, a transparent-electrode formation portion of the passivation film  51  on the second semiconductor layer  14 , and a portion of the passivation film  51  located on the n-type electrode formation region  12   a  are selectively removed by dry etching, thereby resulting in the state shown in  FIG. 16C . 
   Subsequently, as shown in  FIG. 17A , ITO, into which magnesium, i.e., the impurity element serving as the p-type dopant in the second semiconductor layer  14 , has been introduced, is selectively grown on the exposed face of the second semiconductor layer  14  by a sputtering process or a PLD method, for example, so as to cover the passivation film  51 , thereby forming a transparent electrode  15 . Thereafter, the transparent electrode  15  is also subjected to an annealing process performed at a temperature of about 500° C. Through the annealing process, the magnesium introduced into the transparent electrode  15  is further diffused from the transparent electrode  15  through the interface into the upper portion of the second semiconductor layer  14 . This further reduces the value of resistance in the second semiconductor layer  14  near the interface with the transparent electrode  15 . As a result, it is possible to form the transparent electrode  15  having further reduced contact resistance, on the p-type second semiconductor layer  14 . 
   Then, as shown in  FIG. 17B , a second DBR mirror  65  is formed by stacking a plurality of dielectric layers having different refraction indexes, on an optical waveguide portion of the transparent electrode  15 , that is, on a portion of the transparent electrode  15  which is in contact with the second semiconductor layer  14 . 
   Next, as shown in  FIG. 17C , a bonding pad  16  for wire bonding is selectively formed on the transparent electrode  15  where the transparent electrode  15  is located on the passivation film  51 . Subsequently, titanium and gold are sequentially grown on the n-type electrode formation region  12   a  in the first semiconductor layer  12 , thereby forming an n-type ohmic electrode  17 . 
   As described above, in the method for fabricating a blue-light surface-emitting laser device in accordance with the fifth embodiment, the passivation film  51  and the transparent electrode  15  are doped with magnesium that is the p-type dopant in the p-type second semiconductor layer  14  in contact with the transparent electrode  15 . Therefore, the magnesium is diffused into the second semiconductor layer  14  across the interfaces with the passivation film  51  and the transparent electrode  15  by the respective annealing processes performed after the deposition of the passivation film  51  and after the formation of the transparent electrode  15  formed after the removal of the passivation film  51 . This significantly reduces the value of resistance in the second semiconductor layer  14  where the second semiconductor layer  14  is near the interface with the transparent electrode  15 , so that contact resistance between the transparent electrode  15  and the second semiconductor layer  14  is decreased, which enables operation at low operating voltage. 
   As a first modified example of the fifth embodiment, instead of introducing a dopant that determines the conductivity type of the gallium nitride into the passivation film  51  and the transparent material (ITO) that forms the transparent electrode  15 , a metal element which tends to adsorb (bind to) hydrogen atoms, e.g., nickel (Ni), palladium (Pd), or platinum (Pt), may be introduced into at least one of the passivation film  51  and the transparent material. 
   Further, the effects of the present invention are obtained by introducing an impurity that serves as a dopant in the nitride semiconductor, or a metal element that tends to adsorb hydrogen atoms, into just one of the passivation film  51  and the transparent electrode  15 . 
   Moreover, as a second modified example, the substrate  11  may be removed as in the third embodiment, and an n-type ohmic electrode may be formed on the face of the first DBR mirror  61  opposite to the first semiconductor layer  12 . Further, in the case of removing the substrate  11 , the first DBR mirror  61  may be etched so that part of the first semiconductor layer  12  is exposed, and an n-type ohmic electrode may be formed on the exposed portion. Furthermore, in the case of removing the substrate  11 , instead of forming the first DBR mirror  61  made of the nitride semiconductors in the process steps for growing the semiconductor multilayer structure, a first DBR mirror  61  made of dielectric materials instead of the nitride semiconductors, may be formed on the face of the first semiconductor layer  12  opposite to the MQW active layer  63 , after the substrate  11  has been removed. 
   In a case in which the first and second DBR mirrors  61  and  65  are formed out of dielectric materials, among silicon oxide (SiO 2 ), silicon nitride (Si 3 N 4 ), niobium oxide (Nb 2 O 5 ), hafnium oxide (HfO 2 ), titanium oxide (TiO 2 ) and tantalum oxide (Ta 2 O 5 ), at least two substances having different refraction indexes may be selected as the dielectric materials. 
   Further, in the first through fifth embodiments, although the principal surface of the substrate  11  is not processed at all, a mask for selective growth may be formed on the substrate  11 , or steps may be created in the upper portion of the substrate  11 , so that a selective epitaxial lateral over growth (ELOG) may be performed. 
   Furthermore, in the foregoing embodiments, the material for the substrate is not limited to sapphire, but silicon carbide (SiC), gallium arsenide (GaAs), zinc oxide (ZnO), spinel, or silicon (Si), for example, may be used. 
   Moreover, in the foregoing embodiments, although light emitting diodes and surface-emitting laser devices are described as surface-emitting nitride semiconductor light-emitting devices, any semiconductor light emitting devices, in which a transparent electrode is provided on a nitride semiconductor, produce the effects of the present invention.