GaN related compound semiconductor light-emitting device

A layer comprising cobalt (Co) is formed on a p.sup.+ layer by vapor deposition, and layer comprising gold (Au) is formed thereon. The two layers are alloyed by a heat treatment to form a light-transmitting electrode. The light-transmitting electrode therefore has reduced contact resistance and improved light transmission properties, and gives a light-emitting patten which is stable over a long time. Furthermore, since cobalt (Co) is an element having a large work function, satisfactory ohmic properties are obtained.

This application claims foreign priority from Japanese applications Hei.
 8-3344956 filed Nov. 29, 1996; Hei. 9-19748 filed Jan. 17, 1997; and Hei.
 9-47064 filed Feb. 14, 1997, all of which are incorporated herein by
 reference.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a device having a light-transmitting
 electrode and a pad electrode which are formed on a p-type GaN related
 compound semiconductor layer.
 2. Description of the Related Art
 In conventional compound semiconductors, an ohmic contract is obtained by
 depositing metals on the semiconductor surface and heating the metal to
 convert the same to an alloy and to cause metal diffusion into the
 semiconductor, because an ohmic contact is not attainable by the mere
 deposition of metals.
 Even when the p-type GaN related compound semiconductors are subjected to a
 treatment for reducing resistance, e.g., irradiation with electron beams,
 the thus-treated semiconductors still have higher resistivities than
 n-type GaN related compound semiconductors. Consequently, in such p-type
 GaN related compound semiconductors, the p-type layer has almost no
 current flow in lateral directions, and only the part thereof directly
 beneath the electrode emitts light.
 Under these circumstances, a current-diffusing electrode having light
 transmission properties and ohmic properties has been proposed which if
 formed by depositing a nickel (Ni) layer and a gold (Au) layer, each
 having a thickness of several tens of angstroms (.ANG.) and heating the
 metal layers (see Japanese Unexamined Patent Publication No. Hei.
 6-314822).
 However, the electrode formed by depositing nickel (Ni) and gold (Au) each
 having a thickness of several tens of angstroms and heating metal poses a
 problem that the light-emitting pattern quality deteriorates with the
 lapse of time, resulting in an increased driving voltage. However, the
 electrode has satisfactory optical and electrical characteristics in the
 initial stage.
 The reason for the quality deterioration is believed to be as follows.
 Since the nickel (Ni) and gold (Au) deposited layers are extremely thin,
 part of the nickel (Ni) is replaced by gold (Au) during the heat
 treatment, and the nickel (Ni) exposed on the electrode surfaces oxidizes
 to for NiO. When current is caused to flow through the electrode in this
 state, the NiO reacts with the OH.sup.- group of water present in the
 surrounding atmosphere to form a substrate comprising NiOOH, as shown by
 the following scheme (1). Since NiOOH has a poor wettability by gold (Au)
 and by the GaN related compound semiconductor, the NiOOH aggregates. As a
 result, light-emitting pattern quality deteriorates with the lapse of time
 and the contact resistance of the electrode increase. Thus, conventional
 art devices employing the proposed electrode are believed to deteriorate
 in optical and electrical characteristics.
EQU NiO+OH.sup.-.fwdarw.NiOOH+e.sup.- (1)
 Further, since this current-diffusing electrode is thin, a pad electrode
 made of Ni/Au or Au is formed therein for bonding.
 However, the conventional art device described above has insufficient
 adhesion between the pad electrode and the current-diffusing electrode.
 Hence, if the surface of the current-diffusing electrode on which a pad
 electrode is to be formed has been soiled, there is a problem that the
 finally obtained device has problems such as the peeling of the pad
 electrode and a poor light-emitting pattern. In addition, even if the pad
 electrode has satisfactory adhesion to the current-diffusing electrode,
 the light emission occurring in the shade of the bonding pad cannot be
 directly observed, unavoidably resulting in a light emission loss.
 Further, there is still another problem as follows.
 In conventional GaN related compound semiconductors, low-resistivity p-type
 conduction is not attainable by mere doping with a p-type impurity. It has
 hence been proposed to impart p-type low resistance to a GaN related
 compound semiconductor doped with a p-type impurity by irradiating th
 doped semiconductor with electron beams (see Japanese Unexamined Patent
 Publication No. Hei. 2-257679) or by subjecting the doped semiconductor to
 thermal annealing (see Japanese Unexamined Patent Publication No. Hei.
 5-183189). It has also been proposed to conduct the thermal annealing for
 imparting p-type low resistance simultaneously with alloying for forming
 an electrode (see Japanese Unexamined Patent Publication No. Hei.
 8-51235).
 However, in the method using thermal annealing described in Japanese
 Unexamined Patent Publication No. Hei. 5-183189, the heat treatment should
 be conducted at a room temperature not lower than 700.degree. C. in order
 to obtain a saturated low resistivity. Although this kind of semiconductor
 has conventionally employed aluminum as the main electrode material, use
 of a temperature not lower than 700.degree. C. for electrodes alloying
 produces problems, such as the formation of aluminum balls resulting from
 aluminum melting, and impaired surface state, increased contact resistance
 of the electrode, and wire bonding failure.
 Consequently, the heat treatment for electrode alloying should be conducted
 at a relatively low temperature of from 500 to 600.degree. C. It is,
 however, noted that the heat treatment for imparting p-type low resistance
 does not result in a sufficiently low resistivity when conducted at a
 temperature in the range of from 500 to 600.degree. C. It has hence been
 necessary to conduct the heat treatment for imparting p-type low
 resistance and the heat treatment for electrodes alloying as separate
 steps, respectively.
 On the other hand, Japanese Patent Publication No. Hei 8-51235 proposes to
 conduct the impartation of p-type low resistance simultaneously with
 electrode alloying by performing a heat treatment as a temperature of from
 400 to 800.degree. C. However, this method has the following problems. The
 impartation of p-type low resistance is insufficient in the
 low-temperature range where electrode alloying is achieved satisfactorily.
 In the high-temperature region suitable for the sufficient impartation of
 p-type low resistance, electrode alloying cannot be conducted
 satisfactorily, resulting in increased contact resistance and poor ohmic
 properties.
 SUMMARY OF THE INVENTION
 In view of the problems described above, an object of the present invention
 is realized to GaN related compound semiconductor light-emitting device
 which has light transmission properties and ohmic properties and retain a
 stable light-emitting pattern and a constant driving voltage over a long
 period of time, and to realize process for producing the device.
 Another object of the present invention is to impart p-type low resistance
 to a GaN related compound semiconductor through a heat treatment so that a
 saturated low resistivity value can be realized using a lower temperature
 for the treatment.
 Still another object of the present invention is to realize the impartation
 of p-type low resistance at a lower temperature to thereby sufficiently
 impart p-type low resistance and obtain an electrode having low contact
 resistance and satisfactory ohmic properties, even when the heat treatment
 for imparting p-type low resistance and that for electrode alloying are
 conducted as the same step.
 Still another object of the present invention is to improve the adhesion
 between a pad electrode and a current-diffusing electrode to thereby
 prevent the pad electrode from peeling off and, at the same time, to form
 a high-resistivity region under the pad so that current flows in the
 current-diffusing electrode selectively through areas other than that
 under the pad to thereby diminish light emission under the pad and attain
 effective utilization of light emission.
 The above-described problem is eliminated with the light-emitting device of
 the present invention according to a first aspect of the present
 invention. This light-emitting device has a p-type GaN related compound
 semiconductor layer having formed thereon an electrode which transmits
 light to the semiconductor layer and which is a metal layer comprising a
 cobalt (Co) alloy, palladium (Pd), or a palladium (Pd) alloy. Since the
 elements constituting the electrode are unsusceptible to oxidation, not
 only is the electrode prevented from suffering the light-emitting pattern
 change with time caused by electrode oxidation to thereby give a stable
 light-emitting pattern over a long period of time, but also the electrode
 can have reduced contact resistance to thereby enable a constant driving
 voltage over a long period of time. In addition, since cobalt (Co) and
 palladium (Pd) each is an element having a large work function,
 satisfactory ohmic properties are obtained.
 The metal layer comprising a cobalt (Co) alloy may be formed from one
 member selected from the group consisting of a two-layer structure
 comprising a first metal layer made of cobalt (Co) and a second layer made
 of pure gold (Au) formed on the first metal layer, a two-layer structure
 comprising a first metal layer made of gold (Au) and a second metal layer
 made of cobalt (Co) formed on the first metal layer, and an alloy layer
 made of cobalt (Co) and gold (Au), by alloying the one member through a
 heat treatment. This metal layer is free from the problem in electrodes
 made of cobalt (Co) alone that the light-emitting pattern changes with the
 lapse of time because of the susceptibility of cobalt (Co) to oxidation.
 Specifically, the electrode formed by heating a two-layer structure
 comprising a layer made of cobalt (Co) and a layer made of gold (Au) or by
 heating a layer of an alloy of cobalt (Co) with gold (Au) is prevented
 from undergoing cobalt (Co) oxidation, has reduced contact resistance,
 enables a stable light-emitting pattern over a long period of time, and
 has excellent light transmission properties.
 An electrode which has a reduced contact resistance, enables a stable
 light-emitting pattern over a long period of time, and has an excellent
 light transmission properties is also obtained from a three-layer
 structure comprising a first metal layer of cobalt (Co), a second layer
 made of a group II element formed on the first metal layer, and a third
 metal layer made of gold (Au) formed on the second metal layer, by
 alloying the three-layer structure through heat treatment, or obtained
 from a two-layer structure comprising a first metal layer made of cobalt
 (Co) and a second metal layer made of an alloy of palladium (Pd) with
 platinum (Pt) formed on the first metal layer, by alloying the two-layer
 structure through a heat treatment. Effective examples of the group II
 element include beryllium (Be), magnesium (Mg), calcium (Ca), strontium
 (Sr), barium (Ba), zinc (Zn), and cadmium (Cd).
 The metal layer comprising a palladium (Pd) alloy may be formed from either
 a two-layer structure comprising a first metal layer made of palladium
 (Pd) and a second metal layer made of gold (Au) formed on the first metal
 layer, or a two-layer structure comprising a first metal layer made of
 gold (Au) and a second metal layer made of palladium (Pd) formed on the
 first metal layer, by alloying the two-layer structure through a heat
 treatment. Thus, an electrode is obtained which has reduced contact
 resistance, enables a stable light-emitting pattern over a long period of
 time, and has excellent light transmission properties.
 An electrode which has reduced contact resistance, enables a stable
 light-emitting pattern over a long period of time, and has excellent light
 transmission properties is obtained also from a layer made of an alloy of
 palladium (Pd) with platinum (Pt) by alloying the layer through a heat
 treatment.
 A metal layer may be formed on a p-type GaN related compound semiconductor
 layer through a heat treatment conducted at a temperature from 400 to
 700.degree. C. The metal layer formed can be a satisfactorily alloyed
 layer. Thus, an electrode having stable light-emitting properties and
 stable electrical characteristics can be obtained.
 A metal layer having reduced contact resistance can be formed through a
 heat treatment conducted under low-vacuum conditions. The term "low-vacuum
 conditions" used herein means a pressure of 10 Torr or lower.
 A metal layer having reduced contact resistance can be formed through a
 heat treatment without reducing light-emitting pattern quality, by
 conducting the heat treatment in an atmosphere comprising at least oxygen
 (O.sub.2) or a gas containing oxygen (O), or by conducting the heat
 treatment in an inert gas atmosphere. The term "atmosphere comprising
 oxygen (O.sub.2)" as used herein include 100% oxygen (O.sub.2). The term
 "gas containing oxygen (O)" means CO, CO.sub.2, etc. Effective examples of
 the inert gas contemplated by the present invention include nitrogen
 (N.sub.2), helium (He), neon (Ne), argon (Ar), and krypton (Kr).
 Further, the above-described problem is eliminated with the process for
 producing a p-type GaN related compound semiconductor of the present
 invention according to a second aspect of the present invention. This
 process for producing a p-type GaN related compound semiconductor
 comprises subjecting a GaN related compound semiconductor doped with a
 p-type impurity to a heat treatment in a gas comprising at least oxygen.
 Further, the above described problem is eliminated by the process for
 producing a p-type GaN related compound semiconductor having a p-type GaN
 related compound semiconductor layer and an electrode according to a third
 aspect of the present invention. This process for producing a GaN related
 compound semiconductor device having a p-type GaN related compound
 semiconductor layer and an electrode comprises: forming a layer of a GaN
 related compound semiconductor doped with p-type impurity; forming an
 electrode on the GaN related compound semiconductor layer; and subjecting
 the GaN related compound semiconductor layer having the electrode formed
 thereon to a heat treatment in a gas comprising at least oxygen.
 Furthermore, the above-described problem is eliminated by the process for
 producing GaN related compound semiconductor having a p-type GaN related
 compound semiconductor layer, an n-type GaN related compound semiconductor
 layer, and two electrodes respectively for these layers according to a
 fourth aspect of the present invention. This process for producing a GaN
 related compound semiconductor device having a p-type GaN related compound
 semiconductor layer, an n-type GaN related compound semiconductor layer,
 and two electrodes respectively for these layers comprises:
 forming a first electrode on the GaN related compound semiconductor layer
 doped with p-type impurity, and forming a second electrode on the n-type
 GaN related compound semiconductor; and subjecting the resultant structure
 to a heat treatment in a gas comprising at least oxygen.
 The term "GaN related compound semiconductor" means a compound which is
 based on GaN and contains one or more group III elements, e.g., In and Al,
 by which part of the gallium has been replaced. An example of the GaN
 related compound semiconductor is a four-element compound represented by
 the general formula (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y n
 (O.ltoreq.x.ltoreq.l, O.ltoreq.y.ltoreq.l).
 The gas comprising oxygen used in each of the processes according to the
 present invention may be at least one member selected from O.sub.2,
 O.sub.3, CO, CO.sub.2, NO, N.sub.2 O, NO.sub.2, and H.sub.2 O or a mixed
 gas comprising two or more of these members. The gas comprising oxygen may
 also be a mixed gas comprising at least one of O.sub.2, O.sub.3, CO,
 CO.sub.2, NO, N.sub.2 O, NO.sub.2, and H.sub.2 O and one or more inert
 gases, or be mixed gas comprising a mixture of two or more of O.sub.2,
 O.sub.3, CO, CO.sub.2, NO, N.sub.2 O, NO.sub.2, and H.sub.2 O and one or
 more inert gases. In short, the gas comprising oxygen means a gas
 containing oxygen atoms or a gas of molecules containing oxygen atoms.
 The pressure of the atmosphere in which the heat treatment is conducted is
 not particularly limited as long as the GaN related compound semiconductor
 is not pyrolyzed at the temperature used for the heat treatment. In the
 case of where O.sub.2 gas alone is used as the gas comprising oxygen, the
 gas may be introduced at a pressure higher than the decomposition pressure
 for the GaN related compound semiconductor. In the case where a mixture of
 O.sub.2 with an inert gas is used, the pressure of the whole mixed gas is
 regulated to a value higher than the decomposition pressure for the GaN
 related compound semiconductor. In this case, an O.sub.2 gas proportion
 not smaller than about 10.sup.-4 based on the whole mixed gas is
 sufficient. In short, and extremely small amount of oxygen suffices to the
 gas comprising oxygen for the reason which will be given later. There is
 no particular upper limit on the amount of the gas comprising oxygen
 introduced from the standpoints of the impartation of p-type low
 resistance and electrode alloying. Any high pressure is usable as long as
 production is possible.
 The most preferred range of the temperature for the heat treatment is from
 500 to 600.degree. C. As will be described later, a p-type GaN related
 compound semiconductor having a completely saturated resistivity can be
 obtained at temperatures not lower than 500.degree. C. At the temperatures
 not higher than 600.degree. C., the alloying treatment of an electrode can
 be conducted satisfactorily.
 Preferred temperature ranges are from 450 to 650.degree. C., from 400 to
 600.degree. C., and from 400 to 700.degree. C. The lower the temperature,
 the higher the p-type resistivity. The higher the temperature, the poorer
 the electrode properties and the higher the possibility for thermal
 deterioration of crystals.
 The first electrode desirably comprises a metal layer which comprises a
 cobalt (Co) alloy, palladium (Pd), or a palladium (Pd) alloy and has light
 transmission properties and ohmic properties. This metal layer comprising
 a cobalt (Co) alloy is a layer formed from a two-layer structure
 comprising a first metal layer made of cobalt (Co) and a second metal
 layer made of pure gold (Au) formed on the first metal layer, from a
 two-layer structure comprising a first metal layer made of gold (Au) and a
 second metal layer made of cobalt (Co) formed on the first metal layer, or
 from a layer of an alloy of cobalt (Co) with gold (Au), by alloying the
 same through a heat treatment. Alternatively, the metal layer comprising a
 cobalt (Co) alloy is a layer formed from a three-layer structure
 comprising a first metal layer made of cobalt (Co), a second metal layer
 made of a group II element formed on the first metal layer, and a third
 metal layer made of gold (Au) formed on the second metal layer, by
 alloying the three-layer structure through a heat treatment. The metal
 layer comprising a palladium (Pd) alloy is a layer formed from a two-layer
 structure comprising a first metal layer made of palladium (Pd) and a
 second metal layer made of gold (Au) formed on the first metal layer or
 from a two-layer structure comprising a first palladium (Pd) formed on the
 first metal layer, by alloying the two-layer structure through a heat
 treatment.
 The first electrode can be a layer formed by alloying, through a heat
 treatment, a two-layer structure comprising a first metal layer made of
 nickel (Ni) and a second metal layer made of gold (Au) formed thereon.
 The above-described materials of the first electrode have been selected so
 as to result in satisfactory properties with respect to contact resistance
 with p-type GaN related compound semiconductors, light-emitting pattern,
 property change with time, junction strength, and ohmic properties.
 The second electrode desirably comprises aluminum, (Al) or an aluminum
 alloy. These electrodes materials have been selected from the standpoints
 of contact resistance with n-type GaN related compound semiconductors and
 ohmic properties.
 In the process according to the second aspect of the present invention, a
 gas comprising oxygen is used as the surrounding gas for the heat
 treatment. As a result, it has become possible to use a lower temperature
 for obtaining a GaN related compound semiconductor having p-type low
 resistance. As will be described later, use of temperatures not lower than
 500.degree. C. resulted in a saturated low value of resistivity. The
 resistivity began to decrease at around 400.degree. C. At 450.degree. C.,
 the resistivity was about a half of that at 400.degree. C.
 In the processes according to the third and fourth aspects of the present
 invention, a saturated low resistivity suitable for practical use is
 obtained at lower temperatures as described above. Consequently, the heat
 treatment for imparting p-type low resistance and the heat treatment for
 electrode alloying can be carried out as the same step. As a result,
 processes for device production can be simplified. In addition, since heat
 treatment can be conducted at a low temperature, thermal deterioration of
 devices can be alleviated.
 With respect to the fact that the heat treatment in a gas comprising oxygen
 is effective in imparting low resistance at lower temperatures, the
 following explanation is given by the present inventors. A GaN related
 compound semiconductor cannot be made to have a p-type low resistance by
 merely doping the same with a p-type impurity , e.g., magnesium. This is
 because the atoms of the p-type impurity are bonded to hydrogen atoms and,
 hence, do not function as an accepter. It is therefore thought that upon
 the removal of the hydrogen atoms bonded to the atoms of the p-type
 impurity, the impurity comes to function as an accepter. When a heat
 treatment is conducted in a gas comprising oxygen, the separation of the
 impurity atoms from the hydrogen atoms is thought to be catalyzed by the
 oxygen. As a result, semiconductor devices having a reduced resistivity
 are obtained at lower temperatures.
 Still further, the above-described problem is eliminated with the GaN
 related compound semiconductor device having a p-type GaN related compound
 semiconductor according to a fifth aspect of the present invention. This
 GaN related compound semiconductor device having a p-type GaN related
 compound semiconductor comprises: a current-diffusing electrode having
 light transmission properties which has been formed on the p-type GaN
 related compound semiconductor and a pad electrode for bonding which has
 been formed on the current-diffusing electrode and contains at least one
 metal reactive with nitrogen. The device further includes a
 high-resistivity region on the p-type GaN related compound semiconductor
 in its part located under the pad electrode, the high-resistivity region
 having been formed through an alloying treatment by the reaction of the
 metal with the p-type GaN related compound semiconductor.
 According to this device, a current-diffusing electrode having light
 transmission properties is formed on a p-type GaN related compound
 semiconductor, and a pad electrode containing at least one metal reactive
 with nitrogen is formed thereon.
 In an alloying treatment, the metal reactive with nitrogen which is
 contained in the pad electrode reacts with the p-type GaN related compound
 semiconductor. As a result, the adhesion between the pad electrode and the
 current-diffusing electrode as well as between the pad electrode and
 p-type GaN surface is improved and the pad electrode can be prevented from
 peeling off. The reaction between the metal reactive with nitrogen
 contained in the pad electrode and the GaN related compound semiconductor
 further produces an effect that since the reaction generates nitrogen
 holes within part of the GaN related compound semiconductor, the donor
 attributable to these holes in that part compensates for an accepter to
 thereby form a high-resistivity region in that part of the semiconductor.
 Consequently, current flows from the pad electrode not downward but in
 lateral directions along the current-diffusing electrodes. Since the
 region of a pad electrode originally has a large thickness and no light
 transmission properties, it is virtually impossible to take light out of
 the device through the pad electrode or to cause external light to strike
 on the semiconductor through the pad electrode. According to the present
 invention, only the part where light is effectively utilizable can have an
 improved current density and, as a result, the efficiency of
 electricity-to-light conversion or light-to-electricity conversion is
 improved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention will be explained below by reference to embodiments
 thereof. However, the invention should not be construed as being limited
 to the following embodiments.
 1st Embodiment
 FIG. 1 is a sectional view diagrammatically illustrating the structure of a
 light-emitting device 100 having a GaN related compound semiconductor
 formed over a sapphire substrate 1. This light-emitting device comprises a
 buffer layer 2 comprising AlN formed on the sapphire substrate 1 and a
 silicon (Si)-doped n-type CaN layer 3 (n.sup.+ layer) formed on the buffer
 layer 2. The light-emitting device further comprises a silicon (Si)-doped
 n-type Al.sub.0.1 Ga.sub.0.9 N layer 4 (n layer) having a thickness of 0.5
 .mu.m formed on the n.sup.+ layer 3, an In.sub.0.2 Ga.sub.0.8 N layer 5
 (active layer) having a thickness of 0.4 .mu.m formed on the n layer 4,
 and a magnesium (Mg)-doped p-type Al.sub.0.1 Ga.sub.0.9 N layer 6 (p
 layer) formed on the active layer 5. A p-type GaN layer 7 (p.sup.+ layer)
 heavily doped with magnesium (Mg) has been formed on the p layer 6. A
 light-transmitting electrode 8A has been formed on the p.sup.+ layer 7 by
 metal vapor deposition, while an electrode 8B has been formed on the
 n.sup.+ layer 3. The light-transmitting electrode 8A is constituted of
 cobalt (Co) bonding to the p.sup.+ layer 7 and of a metallic element,
 e.g., gold (Au), bonding to the cobalt (Co) (the metallic element will be
 described later). The electrode 8B is constituted of aluminum (Al) or an
 aluminum alloy.
 A process for producing the light-transmitting electrode 8A of this
 light-emitting device 100 is explained next.
 The layers ranging from the buffer layer 2 to the p.sup.+ layer 7 are
 formed by metal-organic vapor-phase epitaxy (MOVPE). The gases which can
 be used are ammonia (NH.sub.3), carrier gases (H.sub.2, N.sub.2),
 trimethylgallium (Ga(CH.sub.3).sub.3) (hereinafter referred to as "TMG"),
 trimethylaluminum (Al(CH.sub.3).sub.3) (hereinafter referred to as "TMA"),
 silane (SiH.sub.4), cyclopentadienylmagnesium (Mg(C.sub.5 H.sub.5).sub.2)
 (hereinafter referred to as "CP.sub.2 Mg"), and trimethylindium
 (In(CH.sub.3).sub.3) (hereinafter referred to as "TMI"). And a mask layer
 (SiO.sub.2 or the like) is formed on the p.sup.+ layer 7, and the
 predetermined area of the mask layer is removed. Those parts of the
 p.sup.+ layer 7, p layer 6, active layer 5, and n layer 4 which are
 uncovered by the resultant mask are removed by reactive ion etching with a
 gas containing chlorine to expose a surface of the n.sup.+ layer. The mask
 is then removed. Subsequently, the light-transmitting electrode 8A is
 formed by conducting the following procedure.
 A photoresist 9 is evenly applied to the surface. That part of the
 photoresist 9 which corresponds to the area where the electrode is to be
 formed on the p.sup.+ layer 7 is removed by photolithography to form a
 window part 9A as shown in FIG. 2.
 Using a vapor deposition apparatus, cobalt (Co) is deposited in a thickness
 of 40 .ANG. on the exposed p.sup.+ layer 7 under a high vacuum on the
 order of 10.sup.-6 Torr or less to form a first metal layer 81 as shown in
 FIG. 2.
 Gold (Au) is then deposited on the first metal layer 81 in a thickness of
 60 .ANG. to form a second metal layer 82 as shown in FIG. 2.
 Subsequently, the sample is taken out of the vapor deposition apparatus.
 The cobalt and gold deposited on the photoresist 9 are removed by the
 lift-off method to form an electrode 8A which transmits light to the
 p.sup.+ layer 7.
 In the case where a bonding electrode pad 20 is to be formed on part of the
 light-transmitting electrode 8A, a photoresist is applied evenly, and that
 part of the photoresist which corresponds to the pad formation part is
 removed to form a window. Subsequently, a film of an alloy of cobalt (Co)
 or nickel (Ni) with gold (Au), aluminum (Al) or both is formed by vapor
 deposition in a thickness of about 1.5 .mu.m. The film alloy of cobalt or
 nickel with gold, aluminum, or both which has been vapor-deposited on the
 photoresist is removed by the lift-off method as mentioned above to
 thereby form an electrode pad 20.
 Thereafter, the atmosphere surrounding the sample is evacuated with a
 vacuum pump, and a mixed gas of N.sub.2 and O.sub.2 (1%) is introduced
 into the deposition apparatus to adjust the internal pressure to
 atmospheric pressure. The temperature of this atmosphere surrounding the
 sample is elevated to about 550.degree. C. to heat the sample for about 3
 minutes. Thus, the first metal layer 81 and the second metal layer 82 are
 alloyed.
 This heat treatment can be conducted under the following conditions. A
 surrounding gas containing one or more of N.sub.2, He, O.sub.2, Ne, Ar,
 and Kr is utilizable. Any pressure ranging from vacuum to pressures higher
 than atmospheric pressure can be used. The partial pressure of N.sub.2,
 He, O.sub.2, Ne, Ar, and Kr in the surrounding gas is from 0.01 to 1 atm.
 The heating may be conducted with the surrounding gas enclosed in the
 apparatus or while circulating the same through the apparatus.
 As a result of the heat treatment after the deposition of cobalt (Co) and
 gold (Au), part of the gold (Au) constituting the second metal layer 82
 formed on the first metal layer 81 made of cobalt (Co) is diffused through
 the first metal layer 81 on the p.sup.+ layer 7 to thereby form a good
 contact with GaN contained in the p.sup.+ layer 7.
 When a current of 20 mA was caused to flow through the thus-formed
 light-transmitting electrode 8A, a driving voltage of 3.6 V was obtained.
 It was thus ascertained that the contact resistance was sufficiently low.
 The surface of the p.sup.+ layer 7 was evenly covered with the thus-formed
 light-transmitting electrode 8A, which had a satisfactory surface state.
 Since the light-transmitting electrode 8A is formed from a two-layer
 structure comprising the first metal layer 81 made of cobalt (Co) and the
 second metal layer 82 formed thereon, the cobalt (Co) is inhibited from
 oxidizing. As a result, the change of light-emitting pattern, decrease of
 light transmission properties, and increase of contact resistance all
 caused by cobalt (Co) oxidation can be prevented. In addition, since the
 light-transmitting electrode 8A is made of an alloy containing cobalt
 (Co), which has a large work function, satisfactory ohmic properties are
 obtained. This electrode 8A was tested by exposing the same to a
 high-temperature and high-humidity atmosphere for a prolonged period of
 time. As a result, the electrode was capable of stably maintaining the
 initial light-emitting pattern and driving voltage even after 1,000-hour
 exposure.
 Besides the conditions used above for alloying the first metal layer 81
 made of cobalt (Co) and the second metal layer 82 made of gold (Au), the
 following two sets of conditions were also used for this embodiment. One
 set of conditions was that the atmosphere surrounding the sample was
 evacuated with a vacuum pump to form a low-vacuum state and the
 temperature of this atmosphere surrounding the sample was elevated to
 about 550.degree. C. to heat the sample for about 3 minutes to thereby
 alloy the first and second metal layers 81 and 82. The other set of
 conditions was that the atmosphere surrounding the sample was evacuated to
 vacuum, subsequently N.sub.2 was introduced at a rate of 3 liter/min to
 adjust the internal pressure to atmospheric pressure, and then the
 temperature of this atmosphere surrounding the sample was elevated to
 about 550.degree. C. to heat the sample for about 3 minutes to thereby
 alloy the first and second metal layers 81 and 82. The driving voltage of
 each device obtained was measured. The results obtained are shown in FIG.
 3 under Case No. 1.
 The three sets of atmospheric conditions described above were used for the
 alloying of: an electrode precursor comprising a first metal layer 81 made
 of gold (Au) and a second metal layer 82 made of cobalt (Co) (Case No. 2);
 an electrode precursor having only of a first metal layer 81 comprising an
 alloy of cobalt (Co) with gold (Au) (Case No. 3); a three-layer precursor
 for light-transmitting electrode which was constituted by a first metal
 layer 81 made of cobalt (Co), a second metal layer 82 made of magnesium
 (Mg), and a third metal layer made of gold (Au) formed on the second metal
 layer (Case No. 4); and an electrode precursor comprising of a first metal
 layer 81 made of cobalt (Co) and a second metal layer 82 made of an alloy
 of palladium (Pd) with platinum (Pt) (Case No. 5). The driving voltage of
 each device obtained was measured. The results obtained are shown in FIG.
 3.
 The evaluation results given in FIG. 3 are based on the driving voltage
 measured when a current of 20 mA was caused to flow through the
 light-transmitting electrode 8A. In FIG. 3, .largecircle. indicates that
 the driving voltage was lower than 4 V, and x indicates that the driving
 voltage was not lower than 5 V. In FIG. 3, the numeral in the parentheses
 for each metal layer indicates film thickness (.ANG.).
 All the device samples described above were subjected to a 1,000-hour
 continuous driving test in a high-temperature high-humidity atmosphere.
 The device samples indicated by .largecircle. each had the same driving
 voltage and light-emitting pattern as the initial ones even after the
 1,000-hour driving test, and retained optically and electrically stable
 properties over a long period of time.
 In Case No. 1 shown in FIG. 3, a device having a driving voltage not lower
 than 5 V was measured at a current of 20 mA. Hence, increased contact
 resistance was obtained when alloying was conducted in N.sub.2 (alone) in
 the absence of O.sub.2. Through alloying under low-vacuum conditions, a
 device having a driving voltage lower than 4 V, and hence reduced contact
 resistance, was obtained. In the case where a light-transmitting electrode
 8A is formed from a two-layer structure comprising a first metal layer 81
 made of cobalt (Co) and a second metal layer 82 made of gold (Au) as in
 Case No. 1, a light-emitting pattern which is stable over a long period of
 time and a low driving voltage is obtained by alloying the two-layer
 structure either in the atmosphere containing O.sub.2 or under the
 low-vacuum conditions.
 As in Case No. 2 shown in FIG. 3, a first metal layer 81 made of gold (Au)
 having a thickness of 40 .ANG. may be formed before a second metal layer
 82 made of cobalt (Co) is formed thereon with a thickness of 60 .ANG.. A
 light-emitting pattern which is stable over a long period of time and a
 low driving voltage are obtained in Case No. 2 by conducting alloying
 either in the atmosphere containing O.sub.2 or under the low-vacuum
 conditions, as in Case No. 1.
 As in Case No. 3 shown in FIG. 3, a first metal layer 81 may be formed by
 simultaneously vapor-depositing gold (Au) and cobalt (Co) with a thickness
 of 100 .ANG.. A light-emitting pattern which is stable over a long period
 of time and a low driving voltage are obtained in Case No. 3 by conducting
 alloying either in the atmosphere containing O.sub.2 or under the
 low-vacuum conditions, as in Cases Nos. 1 and 2.
 As in Case No. 4 shown in FIG. 3, a light-transmitting electrode 8A having
 a three-layer structure may be formed by forming an electrode precursor
 comprising a first metal layer 81 made of cobalt (Co) having a thickness
 of 20 .ANG., a second metal layer 82 formed thereon which is made of
 magnesium (Mg) having a thickness of 20 .ANG., and a 60 .ANG.-thick layer
 of gold (Au) formed on the second metal layer 82. In Case No. 4, any of
 the atmosphere containing O.sub.2, the low-vacuum conditions, and the
 N.sub.2 atmosphere can be used for obtaining a light-emitting pattern
 which is stable over a long period of time and a low driving voltage.
 As in Case No. 5 shown in FIG. 3, palladium (Pd) and platinum (Pt) may be
 simultaneously vapor-deposited as a second metal layer 82 with a thickness
 of 80 .ANG. on a first metal layer 81 made of cobalt (Co) having a
 thickness of 40 .ANG.. A light-emitting pattern which is stable over a
 long period of time and a low driving voltage can be obtained in Case No.
 5 by conducting alloying either under the low-vacuum conditions or in the
 N.sub.2 atmosphere.
 As described above, the light-transmitting electrode 8A may be formed from
 a two-layer structure comprising a first metal layer 81 made of cobalt
 (Co) and a second metal layer 82 formed thereon, or from a two-layer
 structure comprising a first metal layer 81 made of gold (Au) and a second
 metal layer 82 made of cobalt (Co) formed thereon, or from a single-layer
 structure comprising a first metal layer 81 made of a gold (Au)-cobalt
 (Co) alloy.
 Although magnesium (Mg) was used as the material of a constituent metal
 layer in Case No. 4 in the embodiment described above, other group II
 elements may be used, such as beryllium (Be), calcium (Ca), strontium
 (Sr), barium (Ba), zinc (Zn), and cadmium (Cd).
 2nd Embodiment
 In contrast to the first embodiment described above, in which cobalt (Co)
 was used as the first metal layer 81 or second metal layer 82, this
 embodiment is characterized by employing a light-transmitting electrode 8A
 which is made of palladium (Pd) alone or a palladium (Pd) alloy, and
 contains no cobalt (Co).
 The semiconductor devices used have the same constitution as in the first
 embodiment, except the composition of the light-transmitting electrode 8A.
 FIG. 4 shows the relationship between the composition of each of the
 precursors for the light-transmitting electrode 8A and the driving voltage
 as measured when a current of 20 mA was caused to flow through the
 light-transmitting electrode 8A after the precursor was alloyed under each
 of the same sets of conditions as those used for the first embodiment. The
 ratings used in FIG. 4 have the following meanings: .largecircle.
 indicates that the driving voltage was lower than 4 V; .DELTA. indicates
 that the driving voltage was 4 V or higher but below 5 V; and x indicates
 that the driving voltage was not lower than 5 V. The device samples
 indicated by .largecircle. or .DELTA. each had the same driving voltage
 and light-emitting pattern as the initial ones even after 1,000-hour
 driving test, and retained optically stable properties over a long period
 of time.
 As in Case No. 1, a light-emitting pattern which is stable over a long
 period of time and a driving voltage as low as below 4 V were obtained by
 forming an electrode precursor comprising a first metal layer 81 made of
 40 .ANG.-thick palladium (Pd) formed on the p.sup.+ layer 7 and a second
 metal layer 82 made of 60 .ANG.-thick gold (Au) formed on the first metal
 layer 81, and then alloying the precursor under any of the three sets of
 conditions. Thus, the same effects as in the first embodiment could be
 obtained. In addition, since the light-transmitting electrode 8A was made
 of an alloy of palladium (Pd), which has a large work function,
 satisfactory ohmic properties were obtained as in the first embodiment.
 As in Case No. 2, a light-emitting pattern which was stable over a long
 period of time and a low driving voltage were obtained by forming an
 electrode precursor comprising a first metal layer 81 made of 40
 .ANG.-thick gold (Au) formed on the p.sup.+ layer 7 and a second metal
 layer 82 made of 60 .ANG.-thick palladium (Pd) formed on the first metal
 layer 81, and then alloying the precursor under any of the three sets of
 conditions. Thus, the same effects as in Case No. 1 were obtained.
 In Cases Nos. 1 and 2, two-layer structures were used for forming
 light-transmitting electrode 8A. In contrast thereto, a 100 .ANG.-thick
 single-layer structure was formed by simultaneously vapor-depositing
 palladium (Pd) and platinum (Pt) and was alloyed under low-vacuum
 conditions to form a light-transmitting electrode 8A, as in Case No. 3,
 whereby a light-emitting pattern which was stable over a long period of
 time and a low driving voltage were obtained.
 Furthermore, as in Case No. 4, a light-transmitting electrode 8A was formed
 by forming a 100 .ANG.-thick single-layer structure made of palladium (Pd)
 and alloying the structure under low-vacuum conditions, whereby a
 light-emitting pattern which was stable over a long period of time and a
 low driving voltage were obtained. A driving voltage of from 4 to 5 V was
 obtained when the single-layer structure was alloyed in an N.sub.2
 atmosphere.
 As described above, by forming a light-transmitting electrode 8A made of an
 alloy of palladium (Pd) with gold (Au) or platinum (Pt) or made of
 palladium (Pd) alone, a light-emitting pattern which was stable over a
 long period of time and a low driving voltage were obtained and the same
 effects as in the first embodiment could be obtained.
 Although the temperature of the atmospheres used for alloying for producing
 the embodiments described above was regulated to about 550.degree. C.,
 usable alloying temperatures are not limited thereto. The heat treatment
 is desirably conducted at a temperature in the range of from 400 to
 700.degree. C. This is because heat treatments conducted at temperatures
 lower than 400.degree. C. result in electrodes not showing ohmic
 properties, while heat treatments conducted at temperatures higher than
 700.degree. C. result in electrodes having increased contact resistance
 and an impaired surface morphology.
 The light-emitting devices 100 shown above as embodiments of the invention
 each had a structure containing an active layer 5 consisting of a single
 layer of In.sub.0.2 Ga.sub.0.8 N. However, the light-emitting device of
 the invention may have a light-emitting layer which is made of a mixed
 crystal comprising four or three elements in any proportion, e.g.,
 AlInGaN, or has a multi-quantum well structure consisting, e.g., of
 In.sub.0.2 Ga.sub.0.8 N/GaN or a single-quantum well structure.
 In producing the embodiments described above, an atmosphere containing 1%
 O.sub.2 was used as an oxygen-containing atmosphere. However, a 100%
 O.sub.2 atmosphere or an atmosphere containing a gas such as CO or
 CO.sub.2 may be used.
 The total thickness of the light-transmitting electrode 8A, including the
 first metal layer 81 and the second metal layer 82, is preferably not
 larger than 200 .ANG. from the standpoint of obtaining light transmission
 properties. It is more preferably in the range of from 15 to 200 .ANG.
 from the standpoints of adhesion and light transmission properties.
 As shown above, the present invention brings about the following effects.
 By forming a metal layer comprising a cobalt (Co) alloy, palladium (Pd),
 or a palladium (Pd) alloy as a light-transmitting electrode on a surface
 of a semiconductor comprising a p-type GaN related compound, not only can
 the electrode be inhibited from oxidizing to thereby prevent the electrode
 from suffering a decrease in light transmission properties, but the
 electrode can also have reduced contact resistance to thereby enable a
 light-emitting pattern which is stable over a long period of time and a
 low driving voltage.
 3rd Embodiment
 The present invention will be explained below by reference to FIGS. 5 to 9.
 Many samples having the structure shown in FIG. 5 were prepared. Each
 sample was constituted by a sapphire substrate 1 and, formed thereon in
 this order, an AlN buffer layer 2 having a thickness of 50 nm, an n-GaN
 layer 103 made of a silicon (Si)-doped GaN having a thickness of about 4.0
 .mu.m, an electron concentration of 2.times.10.sup.18 /cm.sup.3, and a
 silicon concentration of 4.times.10.sup.18 /cm.sup.3, and a p-GaN layer
 104 having a magnesium (Mg) concentration of 5.times.10.sup.19 /cm.sup.3.
 These samples were produced by MOVPE, like the aforementioned
 light-emitting devices 100.
 First, a single-crystal sapphire substrate 1 having, as the main surface, a
 surface which had been cleaned by organic washing and heat treatment was
 mounted on a susceptor placed in the reaction chamber of an MOVPE
 apparatus. The sapphire substrate 1 was baked at 1,100.degree. C. while
 passing H.sub.2 through the reaction chamber at a rate of 2 liter/min for
 about 30 minutes at ordinary pressure.
 After the temperature of the substrate 1 was lowered to 400.degree. C.,
 H.sub.2, NH.sub.3, and TMA were fed for about 1.5 minutes at rates of 20
 liter/min, 10 liter/min, and 1.8.times.10.sup.-5 mol/min, respectively, to
 form an AlN buffer layer 2 in a thickness of about 50 nm.
 Subsequently, while keeping the temperature of the sapphire substrate 1 at
 1,150.degree. C., H.sub.2, NH.sub.3, TMG, and silane diluted with H.sub.2
 gas to 0.86 ppm were fed for 40 minutes at rates of 20 liter/min, 10
 liter/min, 1.7.times.10.sup.-4 mol/min, and 20.times.10.sup.-8 mol/min,
 respectively, to form an n-GaN layer 103 having a thickness of about 4.0
 .mu.m, an electron concentration of 2.times.10.sup.18 /cm.sup.3, and a
 silicon concentration of 4.times.10.sup.18 /cm.sup.3.
 Thereafter, while keeping the temperature of the sapphire substrate 1 at
 1,100.degree. C., either N.sub.2 or H.sub.2, NH.sub.3, TMG, and CP.sub.2
 Mg were fed for 40 minutes at rates of 10 liter/min, 10 liter/min,
 1.7.times.10.sup.-4 mol/min, and 2.times.10.sup.-5 mol/min, respectively,
 to form a p-GaN layer 104 having a thickness of about 4.0 .mu.m and a
 magnesium (Mg) concentration of 5.times.10.sup.19 /cm.sup.3.
 Many samples thus prepared were subjected to a 20-minute heat treatment at
 various temperatures in a 1-atm oxygen gas atmosphere (only of O.sub.2).
 Needle electrodes were set up on each of the thus-treated p-GaN layers 104
 to measure the current which flowed upon application of a voltage of 8 V,
 and the relationship between this current value and the heat treatment
 temperature used was determined. On the other hand, for the purpose of
 comparison, semiconductor samples were subjected to the same heat
 treatment as the above, except that 1-atm nitrogen gas (only of N.sub.2)
 was used as the atmosphere for the heat treatment as in conventional
 processes, and the relationship between current value and heat treatment
 temperature was determined in the same manner. The values of resistivity
 were calculated, and the relationships between heat treatment temperature
 and resistivity are shown in FIG. 6.
 The following features can be understood from FIG. 6. 1) Both the heat
 treatment in oxygen atmosphere and the heat treatment in nitrogen
 atmosphere result in a decrease in resistivity [(resistivity before heat
 treatment)/(resistivity after heat treatment)] of 10.sup.4. Namely, there
 is no difference in the saturated resistivity value between the two kinds
 of heat treatments. 2) The saturated low resistivity value is obtained by
 a treatment at lower temperatures in the oxygen atmosphere than in the
 nitrogen atmosphere. 3) The heat treatment in oxygen atmosphere results in
 a more abrupt change in resistivity with changing heat treatment
 temperature than the heat treatment in nitrogen atmosphere. 4) The heat
 treatment in oxygen atmosphere at 500.degree. C. results in a saturated
 low resistivity value, whereas the heat treatment in nitrogen atmosphere
 at 500.degree. C. results in a resistivity change as small as about 10.
 Namely, the resistivity resulting from the heat treatment in oxygen
 atmosphere at 500.degree. C. is lower by 10.sup.3 than that resulting from
 the heat treatment in nitrogen atmosphere at 500.degree. C. 5) At
 400.degree. C., both the heat treatment in an oxygen atmosphere and that
 in a nitrogen atmosphere result in almost no decrease in resistivity. At
 temperatures higher than 400.degree. C., the heat treatments are effective
 in reducing resistivity.
 In summary, in the oxygen atmosphere, heating at temperatures not lower
 than 400.degree. C. is effective in lowering resistivity. The heat
 treatment is desirably conducted at a temperature not lower than
 500.degree. C., because this treatment provides the completely saturated
 low resistivity value.
 The relationship between the pressure of oxygen gas and resistivity was
 then determined. At a temperature of 800.degree. C., semiconductor samples
 were heat-treated at various pressures of oxygen gas. Needle electrodes
 were set up on each of the thus-treated p-GaN layers 104 to measure the
 current which flowed upon application of a voltage of 8 V, and the
 relationship between this current value and oxygen gas pressure was
 determined. The results obtained are shown in FIG. 7.
 The following characteristics are understood from these results. 1)
 Resistivity drops abruptly in the oxygen gas pressure range of about from
 3 to 30 Pa. 2) The heat treatment at oxygen gas pressures not lower than
 about 100 Pa results in a saturated low resistivity value.
 It is understood from the above that oxygen contributes to the effective
 reduction of resistivity. Oxygen gas pressures of at least 3 Pa are
 effective in reducing resistivity. The oxygen gas atmosphere preferably
 has an oxygen pressure of 30 Pa or higher, more preferably 100 Pa.
 Subsequently, semiconductor samples were heat-treated at a temperature of
 600.degree. C. in mixed gas atmospheres (1 atm) containing oxygen gas and
 nitrogen gas to determine the change of resistivity with changing partial
 oxygen gas pressure in the same manner as the above. For the purpose of
 comparison, the change of resistivity with changing pressure in heat
 treatment in oxygen gas alone was determined. The results obtained are
 shown in FIG. 8. The results show that at partial oxygen gas pressures not
 lower than about 10 Pa, low resistivity are obtained. The results further
 show that the resistivity is saturated at pressures not lower than 30 Pa,
 ideally not lower than 100 Pa. To sum up, in the case of using a mixed gas
 containing oxygen gas and one or more other gases, the partial pressure of
 oxygen gas effective in reducing resistivity is 10 Pa or higher,
 preferably 30 Pa or higher, more preferably 100 Pa or higher.
 A layer of a magnesium-doped GaN related compound semiconductor represented
 by (Al.sub.x Ga.sub.1-x).sub.y In.sub.l-y N (0.ltoreq.x,y.ltoreq.1) gave
 the same results with respect to all the properties described above. It is
 thought that oxygen serves to remove the hydrogen atoms bonded to
 magnesium and to thereby activate the magnesium atoms. Consequently,
 besides pure oxygen gas, any gas containing oxygen (O) atoms capable of
 bonding to hydrogen atoms bonded to magnesium, e.g., a mixed gas
 containing oxygen and an inert gas, may be used to produce the same
 effect.
 An explanation is given below on a process for producing a light-emitting
 device 100 using the above-described method for imparting p-type low
 resistance by reference to FIG. 9. FIG. 9 is a sectional view
 diagrammatically illustrating the structure of a light-emitting device 100
 having a GaN related compound semiconductor formed over a sapphire
 substrate 1. This light-emitting device 100 has the substantially same
 structure as the aforementioned embodiments. However, in this embodiment,
 a clad layer 114 made of silicon (Si)-doped n-type GaN is formed on the
 high-carrier-concentration n.sup.+ layer 3.
 Further, on the clad layer 114 has been formed a light emitting layer 115
 having a multi-quantum well structure (MOW) comprising barrier layers 151
 made of GaN each having a thickness of 35 .ANG. and well layers 152 made
 of In.sub.0.20 Ga.sub.0.80 N each having a thickness of 35 .ANG.. The
 number of the barrier layers 151 is six, while the number of the well
 layers 152 is five. On the light-emitting layer 115 has been formed a clad
 layer 116 made of p-type Al.sub.0.15 Ga.sub.0.85 N. A contact layer 117
 made of p-type GaN is formed on the clad layer 116.
 A process for producing this light-emitting device 100 is explained next
 together with the steps which are not explained in the first embodiment.
 The light-emitting device 100 was produced by MOVPE. The gases used were
 ammonia (NH.sub.3), carrier gases (H.sub.2, N.sub.2), TMG, TMA, TMI,
 silane, and CP.sub.2 Mg.
 First, a single-crystal sapphire substrate 1 having, as the main surface, a
 surface which ad been cleaned by organic washing and heat treatment was
 mounted on a susceptor placed in the reaction chamber of an MOVPE
 apparatus. The sapphire substrate 1 was baked at 1,100.degree. C. while
 passing H.sub.2 through the reaction chamber at a rate of 2 liter/min for
 about 30 minutes at ordinary pressure.
 After the temperature of the substrate 1 was lowered to 400.degree. C.,
 H.sub.2, NH.sub.3, and TMA were fed for about 1 minute at rates of 20
 liter/min, 10 liter/min, and 1.8.times.10.sup.-5 mol/min, respectively, to
 form an AlN buffer layer 2 in a thickness of about 25 nm.
 Subsequently, while keeping the temperature of the sapphire substrate 1 at
 1,150.degree. C., H.sub.2, NH.sub.3, TMG, and silane diluted with H.sub.2
 gas to 0.86 ppm were fed for 40 minutes at rates of 20 liter/min, 10
 liter/min, 1.7.times.10.sup.-4 mol/min, and 20.times.10.sup.-8 mol/min,
 respectively, to form a high-carrier-concentration n.sup.+ layer 3 made of
 GaN and having a thickness of about 4.0 .mu.m, an electron concentration
 of 2.times.10.sup.18 /cm.sup.3, and a silicon concentration of
 4.times.10.sup.18 /cm.sup.3.
 Thereafter, while keeping the temperature of the sapphire substrate 1 at
 1,150.degree. C., either N.sub.2 or H.sub.2, NH.sub.3, TMG, TMA, and
 silane diluted with H.sub.2 gas to 0.86 ppm were fed for 60 minutes at
 rates of 10 liter/min, 10 liter/min, 1.12.times.10.sup.-4 mol/min,
 0.47.times.10.sup.-4 mol/min, and 5.times.10.sup.-9 mol/min, respectively,
 to form a clad layer 114 made of GaN and having a thickness of about 0.5
 .mu.m, an electron concentration of 1.times.10.sup.18 /cm.sup.3, and a
 silicon concentration of 2.times.10.sup.18 /cm.sup.3.
 Subsequent to the formation of the clad layer 114, either N.sub.2 or
 H.sub.2, NH.sub.3, and TMG were fed for 1 minute at rates of 20 liter/min,
 10 liter/min, and 2.0.times.10.sup.-4 mol/min, respectively, to form a
 barrier layer 151 made of GaN and having a thickness of about 35 .ANG..
 Subsequently, TMG and TMI were fed for 1 minute at rates of
 7.2.times.10.sup.-3 mol/min and 0.19.times.10.sup.-4 mol/min,
 respectively, while feeding either N.sub.2 or H.sub.2 and NH.sub.3 at
 constant rates to thereby form a well layer 152 made of In.sub.0.20
 Ga.sub.0.80 N and having a thickness of about 35 .ANG.. Under the same
 conditions as the above, five barrier layers 151 in total were formed
 alternately with five well layers 152 in total. Furthermore, a barrier
 layer 151 made of GaN was formed thereon. Thus, a light-emitting layer 115
 of the 5-cycle MQW structure was formed.
 Thereafter, while keeping the temperature of the sapphire substrate 1 at
 1,100.degree. C., either N.sub.2 or H.sub.2, NH.sub.3, TMG, TMA, and
 CP.sub.2 Mg were fed for 3 minutes at rates of 10 liter/min, 10 liter/min,
 1.0.times.10.sup.-4 mol/min, 1.0.times.10.sup.-4 mol/min, and
 2.times.10.sup.-5 mol/min, respectively, to form a clad layer 116 made of
 magnesium (Mg)-doped p-type Al.sub.0.15 Ga.sub.0.85 N and having a
 thickness of about 50 nm and a magnesium (Mg) concentration of
 5.times.10.sup.19 /cm.sup.3.
 Subsequently, while keeping the temperature of the sapphire substrate 1 at
 1,100.degree. C., either N.sub.2 or H.sub.2, NH.sub.3, TMG, and CP.sub.2
 Mg were fed for 30 seconds at rates of 20 liter/min, 10 liter/min,
 1.12.times.10.sup.-4 mol/min, and 2.times.20.sup.-5 mol/min, respectively,
 to form a contact layer 117 made of magnesium (Mg)-doped p-type GaN and
 having a thickness of about 100 nm and a magnesium (Mg) concentration of
 5.times.10.sup.19 /cm.sup.3.
 In this embodiment, a window is formed in a predetermined region of the
 photoresist by photolithography on the contact layer 117 through the
 aforementioned steps in the first embodiment. Under a high vacuum on the
 order of 10.sup.-6 Torr or below, vanadium (V) and aluminum (Al) are
 vapor-deposited in thicknesses of 200 .ANG. and 1.8 .mu.m, respectively.
 The photoresist and the SiO.sub.2 mask are then removed.
 Subsequently, a photoresist 9 is evenly applied to the surface. That part
 of the photoresist 9 which corresponds to the area where the electrode is
 to be formed on the contact layer 117 is removed by photolithography to
 form a window part 9A as shown in FIG. 2.
 Using a vapor deposition apparatus, a first metal layer 81 made of cobalt
 (Co) is formed in a thickness of 15 .ANG. on the exposed contact layer 117
 under a high vacuum on the order of 10.sup.-6 Torr or below, and a second
 metal layer 82 made of gold (Au) is then formed in a thickness of 60 .ANG.
 on the first metal layer 81.
 Subsequently, the electrode 8A and electrode pad 20 are formed by the same
 processes as in the first embodiment.
 Thereafter, the atmosphere surrounding the sample is evacuated with a
 vacuum pump, and O.sub.2 gas is introduced into the deposition apparatus
 to adjust the internal pressure to 100 Pa. The temperature of this
 atmosphere surrounding the same is elevated to about 550.degree. C. to
 heat the sample for about 3 minutes. Thus, p-type low resistance is
 imparted to the contact layer 117 and clad layer 116 and, at the same
 time, the alloying of the contact layer 117, first metal layer 81, and
 second metal layer 82 and the alloying of the electrode 8B and n.sup.+
 layer 3 are conducted.
 As a result of this heat treatment, the resistivity of the contact layer
 117 and that of the clad layer 116 became 1 .OMEGA.cm and 0.71 .OMEGA.cm,
 respectively. The most preferred range of the temperature for this heat
 treatment is from 500 to 600.degree. C. As long as the heat treatment is
 conducted at a temperature in this range, the p-type layer reaches a
 sufficiently low saturated resistivity value and the electrodes 8A and 8B
 are alloyed most satisfactorily. As a result, not only can the contact
 resistance of electrodes or the sheet resistivity of the current-diffusing
 electrode be reduced and ohmic properties improved, but also the
 light-transmitting electrode 8A is prevented from oxidizing, whereby the
 finally obtained light-emitting device can be free from an uneven
 light-emitting pattern and undergo no change in light-emitting pattern
 with the lapse of time. The heat treatment can be conducted at a
 temperature of from 450 to 650.degree. C., and can be conducted even in
 the range of from 400 to 700.degree. C. in some cases. Heat treatment was
 further conducted in an atmosphere containing a mixture of N.sub.2 gas and
 1% O.sub.2 gas and having a partial O.sub.2 gas pressure of 100 Pa. As a
 result, the same effects as the above were obtained. All of the gases
 enumerated above which are used as the surrounding gas for heat treatment
 with regard to the impartation of p-type low resistance are also effective
 in the alloying of the electrodes 8A and 8B described in the first
 embodiment. Consequently, besides pure oxygen gas, a mixed gas can be
 utilized which contains O.sub.2 and at least one of N.sub.2, He, Ne, Ar,
 and Kr. Any pressure and any partial O.sub.2 pressure within the
 aforementioned optimal ranges for the impartation of p-type low resistance
 are utilizable.
 As a result of the heat treatment after the deposition of cobalt (Co) and
 gold (Au), part of the gold (Au) constituting the second metal layer 82
 formed on the first metal layer 81 made of cobalt (Co) is diffused through
 the first metal layer 81 into the contact layer 117 to thereby form a goof
 contact with GaN contained in the contact layer 117.
 It was ascertained that the light-emitting device 100 in this embodiment
 designates sufficiently low contact resistance and the stability with
 respect to the 1,000 hour continuous driving test just like the
 aforementioned embodiments.
 Although magnesium (Mg) was used in a metal layer described above, it may
 be replaced by another group II element such as, e.g., beryllium (Be),
 calcium (Ca), strontium (Sr), barium (Ga), zinc (Zn), or cadmium (Cd).
 Further, it is possible to apply other structures or other elements for the
 light-transmitting electrode 8A, the first metal layer 81, the second
 metal layer 82, the light-emitting layer 115 as described in the
 aforementioned embodiments.
 4th Embodiment
 The fourth embodiment of the present invention will be explained below by
 reference to FIGS. 10 and 11.
 FIG. 10 is a sectional view diagrammatically illustrating the constitution
 of a light-emitting device 100 having a GaN related compound semiconductor
 formed over a sapphire substrate 1. The light-emitting device 100 was
 produced by MOVPE as in the aforementioned embodiments.
 This light-emitting device 100 has the substantially same structure as the
 third embodiment. However, on a part of the electrode 8A, a pad electrode
 20 has been formed comprising a first metal layer 201 about 300 .ANG.
 thick made of vanadium (V) and a second metal layer 202 having a two-layer
 structure comprising a cobalt layer about 1,000 .ANG. thick and a gold
 layer about 1.5 .mu.m thick. The process for forming this electrode pad 20
 is as follows.
 A vanadium film about 300 .ANG. thick is deposited on a part of this
 electrode 8A to form a first metal layer 201. On the first metal layer 201
 are successively deposited a cobalt film about 1,000 .ANG. thick and a
 gold film about 1.5 .mu.m thick to form a second metal layer 202. Thus,
 the electrode pad 20 is formed.
 After the formation of the electrodes 8A, 8B, and pad 20, p-type low
 resistance is imparted to the contact layer 117 and clad layer 116 and, at
 the same time, the alloying of the contact layer 117, metal layers 81 and
 82, first metal layer 201, and second metal layer 202 was conducted
 simultaneously with the alloying of the electrode 8B and n.sup.+ layer 3
 by the same process as described in the previous embodiments.
 As shown in the embodiment described above, the first metal layer 201 of
 the electrode pad 20, which layer is bonded to the electrode 8A, is
 constituted of vanadium, which is reactive with nitrogen. Consequently, in
 an alloying treatment, the vanadium reacts with GaN of the contact layer
 117 to improve the adhesion between the electrode pad 20 and the electrode
 8A, whereby the electrode pad 20 can be prevented from peeling off.
 Furthermore, as a result of the reaction of vanadium with GaN of the
 contact layer 117, nitrogen holes generate within the contact layer 117.
 Since the donor attributable to these holes compensates for an acceptor to
 result in a reduced hole concentration, a high-resistivity region 171 is
 formed under the electrode pad 20 around the junction of the contact layer
 117 with the electrode 8A, as shown in FIG. 11. Due to the formation of
 this high-resistivity region 171, current flows from the electrode pad 20
 not downward but in lateral directions along the electrode 8A. The
 electrode pad 20 is a thick part which has no light transmission
 properties and through which light cannot generally pass. By thus causing
 the current which has passed through the electrode pad 20 to flow along
 the electrode 8A, through which light can pass, the electrode 8A has an
 increased current density and an improved luminance can be obtained.
 In the embodiment described above, vanadium was used as the material of the
 first metal layer 201. However, use of a chromium (Cr) film about 300
 .ANG. thick as the first metal layer 201 was also found to be effective,
 like the vanadium film, in obtaining tenacious adhesion and in causing
 current to flow from the electrode pad 20 not downward but selectively
 along the electrode 8A.
 Although chromium or vanadium was used for the first metal layer 201 in the
 above embodiment, the layer 201 may be constituted of at least one of
 chromium, vanadium, titanium (Ti), niobium (Nb), tantalum (Ta), and
 zirconium (Zr). Although cobalt and gold were used for the second metal
 layer 202, this layer may be constituted of at least one of cobalt,
 nickel, aluminum, and gold. It is also possible to use two or more of
 these materials to form an electrode pad 20 of a single-layer structure by
 simultaneous vapor deposition.
 The electrode 8A may further contain palladium or a palladium alloy. As
 long as these materials are used, the electrode 8A may have a single-layer
 structure of a multilayer structure comprising three or more layers as the
 aforementioned embodiments.
 In the embodiment described above, the heating for alloying was conducted
 at a temperature of 550.degree. C. However, temperatures in the range of
 from 400 to 700.degree. C. are usable.
 In the embodiment described above, the heat treatment was conducted in an
 O.sub.2 gas atmosphere (as described in the first and third embodiments).
 However, the atmosphere for heat treatment may consist of at least one
 member selected from O.sub.2, O.sub.3, CO, CO.sub.2, NO, N.sub.2 O,
 NO.sub.2, and H.sub.2 O or a mixed gas containing two or more of these.
 The atmosphere for heat treatment may also be a mixed gas containing at
 least one of O.sub.2, O.sub.3, CO, CO.sub.2, NO, N.sub.2 O, NO.sub.2, and
 H.sub.2 O and one or more inert gases, or be a mixed gas containing a
 mixture of two or more of O.sub.2, O.sub.3, CO, CO.sub.2, NO, N.sub.2 O,
 NO.sub.2, and H.sub.2 O and one or more inert gases. In short, the
 atmosphere for heat treatment may be any gas containing either oxygen
 atoms or molecules having oxygen atoms. In the heat treatment, the
 hydrogen atoms bonded to atoms of a p-type impurity in the contact layer
 117 are heated in a gas comprising oxygen and are thereby separated from
 the p-type impurity atoms. As a result, the contact layer 117 can have
 lower resistance.
 In the embodiment described above, alloying was conducted in an O.sub.2 gas
 atmosphere having a pressure of 3 Pa. However, the pressure of the
 atmosphere for heat treatment is not particularly limited as long as the
 GaN related compound semiconductor is not pyrolyzed at the temperature
 used for the heat treatment. In the case where O.sub.2 gas alone is used
 as a gas comprising oxygen, the gas may be introduced at a pressure higher
 than the decomposition pressure for the GaN related compound
 semiconductor. In the case where a mixture of O.sub.2 with an inert gas is
 used, the pressure of the whole mixed gas is regulated to a value higher
 than the decomposition pressure for the GaN related compound
 semiconductor. In this case, an O.sub.2 gas proportion not smaller than
 about 10.sup.-6 based on the whole mixed gas is sufficient. For example,
 when heat treatment was conducted in an atmosphere consisting of N.sub.2
 gas containing 1% O.sub.2 gas and having a partial O.sub.2 gas pressure of
 100 Pa, the same effects as the above were obtained. There is no
 particular upper limit on the introduction amount of the gas comprising
 oxygen from the standpoints of the impartation of p-type low resistance
 and electrode alloying. Any high pressure is usable as long as production
 is possible.
 As shown above, this embodiment provides the following effects. By forming
 a current-diffusing electrode combining light transmission properties and
 ohmic properties on a p-type GaN related compound semiconductor and
 further forming thereon a electrode pad containing a metal reactive with
 nitrogen, not only can the electrode pad be prevented from peeling off,
 but also the current-diffusing electrode can have an increased current
 density and an improved luminance.
 The present invention described above relates to light-emitting diodes
 having a light-transmitting electrode and an electrode pad. However, the
 present invention is applicable also to the production of laser diodes
 (LD), light-receiving devices, and other electronic devices expected to
 employ GaN related compound semiconductor devices, such as, e.g.,
 high-temperature devices and power devices.