JP2008085151A - Nitride semiconductor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor element that has low contact resistance not only with a nitride semiconductor layer but with a pad electrode, and is tremendously excellent in adhesiveness and mechanical strength. <P>SOLUTION: The nitride semiconductor element has a laminated semiconductor layer with a primary conductive semiconductor layer, an active layer, and a secondary conductive semiconductor layer laminated in order and an electrode formed on the top surface of the secondary conductive semiconductor layer, wherein the electrode has a first metal layer, a second metal layer, and a third metal layer laminated in order at least from the laminated semiconductor layer side, the first and third metal layers are metal layers containing the same material, the first metal layer has a density higher than the third metal layer's, and the second metal layer contains a material different from that of the first and the third metal layers. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a nitride semiconductor device formed using a semiconductor layer in which nitride semiconductors are stacked, and a method for manufacturing the same. Applications include laser diodes, high-intensity LEDs, light receiving elements, and other electronic devices such as FETs that can be driven with a large current.

A nitride semiconductor element formed by stacking nitride semiconductors is expected as a light emitting element that emits light in a wide wavelength range from the ultraviolet region to blue to further green. Among them, the laser diode is attracting attention as a light source for optical disks, as well as a light source used for medical equipment, processing equipment, and optical fiber communication.

  The nitride semiconductor element is not limited to a laser diode, and has a structure in which a nitride semiconductor is stacked on a sapphire substrate or a GaN substrate, and an electrode is formed on the stacked body. The laminated nitride semiconductor is provided with at least a contact layer in ohmic contact with the electrode.

  In the nitride semiconductor device described above, an electrode that is in ohmic contact with the contact layer in the semiconductor layer is extremely important in order to ensure stable operation. For such an electrode, a metal single layer film, a multilayer film, or an alloy having a large work function is mainly used. For example, an electrode made of a multilayer film such as Ni / Au has been used. If the multilayer film of the electrode is Ni / Au, Ni is the lower layer and Au is the upper layer. The material before the “/” constitutes the lower layer, and the material after the “/” constitutes the upper layer. The same applies to the following.

Japanese Patent Laid-Open No. 10-135515 JP 2003-59860 A

The nitride semiconductor device disclosed in Patent Document 1 described above has the following problems. When Ni / Au is used as an electrode, a heat treatment (annealing) step is required after the electrode is formed. Here, the heat treatment means holding for a certain period of time in a high temperature atmosphere, and it is difficult to obtain ohmic characteristics unless an electrode such as Ni / Au is heat treated. Therefore, when such an electrode material is used, heat treatment is an essential process.

An electrode such as Ni / Au is subjected to a heat treatment process to obtain ohmic characteristics, thereby reversing the distribution in the depth direction between the Ni layer as the lower layer and the Au layer as the upper layer. As a result, the Ni layer, which is the lower layer, moves from the upper Au layer and is exposed to the outermost surface, thereby forming an oxide layer. This increases the contact resistance at the interface with the pad electrode formed on this electrode, and decreases the adhesion with the pad electrode.

Patent Document 2 discloses a configuration in which an electrode includes a Ni layer and an Au layer. Further, it is disclosed that an Ag layer is formed between the Ni layer and the Au layer in order to prevent the Ni layer from being oxidized due to mutual diffusion of the Ni layer and the Au layer. However, the electrode disclosed here is not an electrode formed on the semiconductor layer but an electrode formed on the back surface of the substrate. Moreover, there is no suggestion that it can be applied to nitride semiconductors. In the electrode having such a configuration, contact resistance reduction with a nitride semiconductor and stable life characteristics cannot be expected.

Accordingly, in view of the above problems, the present invention provides a nitride semiconductor device that has a low contact resistance with a pad electrode as well as a contact resistance with a nitride semiconductor layer and is extremely excellent in adhesion and mechanical strength. With the goal.

The nitride semiconductor device of the present invention includes a laminated semiconductor layer in which a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer are laminated in order, an electrode formed on an upper surface of the second conductivity type semiconductor layer, The electrode has a first metal layer, a second metal layer, and a third metal layer sequentially stacked from at least the stacked semiconductor layer side, and the first metal layer and the first metal layer The three metal layers are metal layers containing the same material, and the first metal layer has a higher density than the third metal layer, and the second metal layer includes the first metal layer and the third metal layer. It contains a material different from that of the metal layer.

  The third metal layer in the nitride semiconductor element is preferably a barrier layer.

  The first metal layer in the nitride semiconductor element is preferably thicker than the third metal layer.

The density of the first metal layer in the nitride semiconductor element is preferably 10 g / cm ³ or more and 30 g / cm ³ or less.

The density of the third metal layer in the nitride semiconductor element is preferably 3 g / cm ³ or more and 15 g / cm ³ or less.

  The first metal layer and the third metal layer in the nitride semiconductor element are preferably formed of a layer containing at least gold or an alloy thereof.

  The second metal layer in the nitride semiconductor element is preferably composed of a layer containing at least a platinum group element or an alloy thereof.

The nitride semiconductor device of the present invention includes a laminated semiconductor layer in which a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer are laminated in order, an electrode formed on an upper surface of the second conductivity type semiconductor layer, The electrode includes a metal layer that prevents oxidation of the nitride semiconductor layer and other metal layers from at least the laminated semiconductor layer side, and a barrier metal layer that prevents precipitation of the other layers. The metal layer (hereinafter sometimes referred to as a protective metal layer) that is laminated in order and prevents oxidation is a metal layer containing the same material, and the protective metal layer is a barrier layer. The density is higher than that of the metal layer.

  The nitride semiconductor element is a laser diode in which a stripe-shaped ridge is formed in the second conductivity type semiconductor layer, and the electrode is formed on the upper surface of the ridge.

  As described above, according to the present invention, not only the contact resistance with the nitride semiconductor layer but also the contact resistance with the pad electrode is low even when driven with a large current, and the adhesion and mechanical strength are extremely low. An excellent nitride semiconductor device can be provided.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view schematically showing an electrode structure of a semiconductor element using a nitride semiconductor according to this embodiment. The present invention is not limited to the structure of the nitride semiconductor device shown in the embodiments described below.

(electrode)
The electrode in the present invention may be provided on both the p-side electrode and the n-side electrode, or either one may be provided. Therefore, the nitride semiconductor layer T forming the contact interface s with the electrode is not limited to the p-side contact layer but may be an n-side contact layer.

As shown in FIG. 1, the electrode has a multilayer structure on the nitride semiconductor layer T and has a laminated structure of at least three layers. The three layers shown here are metal layers each having an individual function. The first metal layer M1 has an antioxidant function and a protective function for other metal layers that are in ohmic contact with the nitride semiconductor layer T. The protective function is a function that prevents oxidation of the nitride semiconductor layer T. The material of the first metal layer M1 is made of Au, Pt, Rh, Pd, Ir, Ru, or the like. Among these, gold (Au) or an alloy thereof is particularly preferable because it has good ohmic properties (contact resistance) and adhesion with the nitride semiconductor layer.
The thickness of the first metal layer M1 is 100 to 5000 mm, preferably 500 to 2000 mm. By being this film thickness, there exists an antioxidant effect of another metal layer. If the thickness of the first metal layer M1 is out of the above range, the ohmic characteristics are impaired, which is not preferable. Further, the contact resistance value of the interface s increases. Furthermore the density of the first metal layer ^{M1, 10g / cm 3 ~30g / cm} 3, preferably from ^{12g / cm 3 ~20g / cm 3} . If the density of the first metal layer is in this range, better ohmic properties are exhibited.
However, it is difficult to configure an electrode having excellent ohmic properties and adhesion between both the nitride semiconductor layer and the pad electrode by using only the first metal layer M1 and the second metal layer M2. That is, if the electrodes are only the first metal layer M1 and the second metal layer M2, the generation of an oxide film formed on the electrode surface cannot be suppressed, the contact resistance with the pad electrode is increased, and the contact with the pad electrode is increased. Adhesion will be reduced. Therefore, in the present invention, an electrode having a third metal layer on the first metal layer M1 and the second metal layer M2 is used.

  The first metal layer M1 is a metal layer containing the same material as the third metal layer M3 to be laminated later. Accordingly, if the adhesion between the first metal layer M1 and the second metal layer M2 is good, the adhesion between the second metal layer M2 and the third metal layer M3 is also good.

Further, the density of the first metal layer M1 is set higher than the density of the third metal layer M3. The first metal layer M1 and the third metal layer M3 are metal layers containing the same material, and the density of the first metal layer M1 is made higher than the density of the third metal layer M3. An interface is formed between the layers. The interface formed by this density difference has a barrier effect that captures other metals to be reversed and further reduces the metal to be reversed upward.

The second metal layer M2 is formed on the first metal layer M1 and functions not only as a function separation layer between the first metal layer M1 and the third metal layer M3 but also as a lower metal layer. It has an effect as a cap layer that suppresses decomposition of the layer. The material of the second metal layer M2 is Ni, Co, Fe, Cu, W, Mo, Ti, Ta, Ag, Al, Cr, Pt, Pd, Ph, Ir, Ru, Os, V, Hf, Rh, It is made of Zr and is made of a material different from that of the first metal layer M1 and the third metal layer M3. Among them, it is preferable that the second metal layer contains nickel (Ni) or an alloy thereof because the effect of promoting p-type formation with a nitride semiconductor and ohmic resistance can be reduced.
The film thickness of the second metal layer M2 is 10 to 500 mm, preferably 50 to 200 mm. When the second metal layer M2 is in this film thickness range, the decomposition of other metal layers is suppressed, and further, ohmic stability is obtained. Furthermore the density of the second metal layer M2 ^{is, 3g / cm 3 ~20g / cm} 3, preferably ^{5g / cm 3 ~10g / cm 3} . When the density of the second metal layer M2 is within this range, the decomposition of other metal layers can be suppressed, and further, ohmic stability can be obtained.

  The third metal layer M3 is formed on the second metal layer, and contains the same material as the first metal layer M1 as described above. Further, the density of the third metal layer M3 is lower than the density of the first metal layer. The density of the third metal layer M3 is preferably less than or equal to ½ of the density of the first metal layer M1. This prevents other metal layers from being deposited on the upper surface, and maintains good contact resistance between the electrode and the pad electrode. More preferably, the density of the third metal layer M3 is 1/3 or less of the density of the first metal layer M1. This makes it possible to stabilize the adhesion with the pad electrode in accordance with the above effect. When the other metal layer is exposed on the surface, an oxide film is formed and the adhesion with the pad electrode is lowered, but such a problem is solved.

Here, the film thickness range of the third metal layer M3 is 100 to 1500 mm, preferably 300 to 800 mm. The material of the third metal layer M3 contains the same material as that of the first metal layer M1. The third metal layer M3 is preferably made of a platinum group element or gold, and more preferably made of any one of Au, Rh, and Pt.

FIG. 2 shows a configuration in which the electrode has a metal layer Mb formed as the uppermost layer. Here, the metal layer Mb formed on the third metal layer M3 is a contact layer with the pad electrode. Since this metal layer Mb is exposed to the outside air, it tends to be an oxide film. For this reason, the metal layer Mb is made of a metal that is difficult to oxidize, and further, it is necessary to reduce stress during annealing in order to prevent peeling, and therefore the metal layer Mb is preferably a thin film rather than a thick film. A preferable film thickness range of the metal layer Mb is 5 to 50 mm, preferably 5 to 20 mm. This metal layer Mb is made of the same material as that of the second metal layer M2 when the second metal layer M2 which is the lower metal layer is formed so as to be reversed upward.

In the electrode in which the third metal layer M3 does not exist, the ratio of the second metal layer M2 and the uppermost metal layer Mb becoming an integral layer is also improved. Even if the third metal layer is provided, the metal exposed to the surface by being inverted from the metal layer lower than the third metal layer cannot be completely suppressed. The volume ratio of the second metal layer to the total volume of the second metal layer and the uppermost metal layer Mb is 20% or more, preferably 30% or more, more preferably 50%. This can be done. By using the electrode including the third metal layer as a configuration, the ratio of the oxide film formed on the surface can be reduced as compared with an electrode not including the third metal layer.
Even if the first metal layer and the third metal layer are made of the same material, and the first metal layer M1, the second metal layer M2, and the third metal layer M3 are formed in this order, the first metal layer M1 and the third metal layer are formed in this order. When the density of the metal layer M3 is the same, or when the density of the third metal layer M3 is higher than the density of the first metal layer M1, the second metal layer and the third metal layer are reversed and the second metal layer M3 is inverted. Two metal layers are exposed on the surface. At this time, the volume ratio of the second metal layer to the total volume of the second metal layer and the uppermost metal layer Mb is less than 20%, and 80% or more of the second metal layer M2 is exposed on the surface. . This makes it difficult to achieve the effects of the present invention.

The total film thickness constituting the electrode is 200 ° C. to 10000 ° C., preferably 300 ° C. to 5000 ° C., including the multilayer structure of the three layers. By setting the total film thickness to 300 ° C. to 5000 ° C., the sheet resistance can be lowered.

By performing heat treatment after forming the electrode, good ohmic contact with the nitride semiconductor layer T can be obtained, and contact resistance between the nitride semiconductor layer and the electrode can be reduced. As heat processing temperature, the range of 200 to 1200 degreeC is preferable, More preferably, it is the range of 300 to 900 degreeC.

As the heat treatment conditions other than the above, the atmosphere gas is an atmosphere containing oxygen and / or nitrogen. Therefore, heat treatment in an atmosphere containing an inert gas such as Ar or atmospheric conditions is also possible.

  FIG. 3 shows a configuration in which the metal layer Ma is formed in the lowermost layer where the electrode is a contact interface with the nitride semiconductor layer T. The material of the lowermost metal layer Ma is a metal selected from the group consisting of Ni, Pt, Rh, Ru, Ir, and Pd or an alloy thereof. A metal layer Ma, a first metal layer M1, and a third metal layer M3 are stacked in this order on the nitride semiconductor layer T to form an electrode, and then heat treatment is performed, so that the metal layer Ma is inverted and deposited on the top. To do. The metal layer trapped between the first metal layer M1 and the third metal layer M3 forms the second metal layer M2. Further, the metal layer that could not be captured by the third metal layer M3 is deposited above the third metal layer M3 to form the metal layer Mb. Here, if the first metal layer M1, the second metal layer M2, and the third metal layer M3 satisfy the above-described relationship, the ratio of the metal layer Mb deposited above the third metal layer M3 is reduced. It is possible to achieve the effects of the present invention.

By making the electrode a multilayer film of three or more layers, the stress can be relaxed as compared with a case where the film thickness is increased by a single composition layer. In particular, when forming a laser diode having a striped ridge, an electrode (for example, a p-side electrode) formed on the ridge is formed in a very narrow region, and the load on the ridge is large due to the film quality. Although it depends, it is possible to obtain highly reliable laser characteristics by relaxing the stress applied to the ridge as a multilayer structure.

In addition to the electrode in contact with the nitride semiconductor layer T, a pad electrode is provided on the electrode as an extraction electrode for bonding a wire. When an insulating substrate is used, since the p-side electrode and the n-side electrode are provided on the same surface side, a pad electrode is provided on both of them. Further, by forming a metallized layer for connection with an external electrode or the like instead of a wire on the pad electrode, it can also be used face down.

As the electrode film forming apparatus in the present invention, a sputtering apparatus, a vapor deposition apparatus, and other known film forming apparatuses can be used. For example, as a condition for forming the metal layer Ma using a sputtering apparatus, the initial vacuum degree is set to 5.0 × 10 ⁻⁴ Pa or more. The film forming pressure is 0.5 × 10 ⁻¹ Pa or higher. Next, the first metal layer M1 and the third metal layer M3 are formed using a sputtering apparatus. The first metal layer M1 is preferably formed continuously from the metal layer Ma. The film forming pressure is 0.5 × 10 ⁻¹ Pa. Next, the third metal layer M3 is formed. As a condition for forming the third metal layer M3, first, the initial vacuum is set to 8.0 × 10 ⁻⁴ Pa or more. Thereafter, the film forming pressure is set to 0.5 × 10 ⁻¹ Pa or more. By forming each metal layer under the above conditions, the density of the first metal layer and the third metal layer can be formed in a desired range.

Moreover, the measuring method of the density of the metal layer in this invention is measured using the X-ray interferometry using the film thickness composition measuring apparatus made from TECHNOS.

  The electrode material of the pad electrode includes Ni, Co, Fe, Cr, Ti, Cu, Rh, Au, Al, Mo, Ru, W, Zr, Mo, Ta, Pt, Ag, and oxides and nitrides thereof. These monolayers, alloys, or multilayer films can be used. Since the uppermost layer connects a wire or the like, it is preferable to use Au in consideration of adhesion to a generally used wire material (for example, Au). In order to prevent this Au from diffusing, a material having a relatively high melting point that functions as a diffusion preventing layer is preferably used for the lower layer.

Further, the nitride semiconductor layer T in the present invention is not limited to the p-side contact layer but may be an n-side contact layer. When the nitride semiconductor layer T is a p-side contact layer, it is composed of p-type In _x Al _y Ga _1-xy N (x ≦ 0, y ≦ 0, x + y ≦ 1). Mg is used for the p-type impurity. The p-side contact layer has a thickness of 100 mm or more. Moreover, it is preferable to set it as 1000 or less. When the nitride semiconductor element has an n-side contact layer and the n-side contact layer is the nitride semiconductor layer T, the n-side contact layer is an n-type Al _x Ga _1-x. N (0 ≦ x <1). Si, Ge, O, or the like is used as the n-type impurity.

  As an example of the nitride semiconductor device of this embodiment, an n-side nitride semiconductor is used as a nitride semiconductor layer on a first main surface of a substrate 100 having a first main surface and a second main surface as shown in FIG. A layer 110, an active layer 120, and a p-side nitride semiconductor layer 130 are sequentially stacked, and the electrode 150 is formed on the p-side nitride semiconductor layer 130. The p-side nitride semiconductor layer 130 includes a striped ridge portion, a p-side electrode 150 thereon, and a p-side pad electrode 170 thereon, and an n-electrode on the second main surface of the substrate 100. 180 is a semiconductor laser element having a counter electrode structure including 180. The electrode structure of the present invention is employed for the p-side electrode 150. Further, as shown in FIG. 8, the substrate may be configured such that both electrodes are formed on the same surface of the substrate regardless of conductivity or insulation.

Embodiment 1
6 is a cross-sectional view showing the nitride semiconductor laser device of this embodiment, FIG. 4A is a partially enlarged view showing an example of a ridge structure and an electrode structure, and FIG. 4B is a cross-sectional view of FIG. FIG. The ridge can be formed by etching a part of the p-side nitride semiconductor layer 130, whereby an effective refractive index type waveguide can be formed. In addition, a first insulating film 140 is formed from the side surface of the ridge and from the ridge to the continuous p-side nitride semiconductor layer. On the top surface of the ridge and the top surface of the first insulating film 140, the p-side electrode 150 having the multilayer structure of the metal layers (M1, M2, M3) described above is provided. A p-side pad electrode 170 is provided on the p-side electrode 150 so as to cover the p-side electrode. Further, a buried laser element in which a semiconductor layer is regrown on the ridge surface after forming the ridge, a laser element having a current confinement structure having a current confinement layer having an opening in a stacked semiconductor, an n-side semiconductor layer, and / or Alternatively, a DFB laser having a diffraction grating in the p-side semiconductor layer may be used.

The p-side electrode 150 is formed in a region that covers the first insulating film 140, and the second insulating film 160 is formed so as to cover a part of the p-side electrode 150 that is separated from other than the ridge. Yes. The p-side pad electrode 170 is formed over the p-side electrode 120 and the second insulating film 160.

Further, in order to set the stripe direction of the ridge as the resonator direction, the pair of resonator surfaces provided on the end face can be formed by cleavage or etching. In the case of forming by cleavage, it is necessary that the substrate and the semiconductor layer have cleavage properties, and an excellent mirror surface can be easily obtained by utilizing the cleavage properties.

  As the material of the insulating film, at least one of oxides containing at least one element selected from the group consisting of Si, Ti, Al, V, Zr, Nb, Hf, Ta, SiN, BN, SiC, AlN, AlGaN It is desirable to use one type, and among these, it is particularly preferable to use oxides of Si, Al, Zr, Hf, and Nb, BN, AlN, and AlGaN.

  The film thickness of the insulating film is specifically in the range of 10 to 10,000 mm, preferably in the range of 100 to 5,000 mm. This is because if it is 10 mm or less, it is difficult to ensure sufficient insulation during formation of the electrode 120, and if it is 10000 mm or more, the uniformity of the protective film is lost and a good insulating film is not obtained. is there. Moreover, by being in the preferred range, a uniform film having a good refractive index difference between the ridge and the ridge can be formed on the side surface of the ridge.

The second insulating film is preferably provided so as to continue to the side end faces of the p-side nitride semiconductor layer and the active layer exposed by etching. A preferable material is an oxide containing at least one element selected from the group consisting of Si, Ti, V, Zr, Nb, Hf, and Ta, and formed of at least one of SiN, BN, SiC, AlN, and AlGaN. Among them, particularly preferable materials include single layer films and multilayer films such as SiO ₂ , Al ₂ O ₃ , ZrO ₂ , and TiO ₂ . Note that the second insulating film can be omitted.

Embodiment 2
FIG. 7 is a cross-sectional view showing another embodiment of the present invention. In the present embodiment, similarly to the first embodiment, a stacked semiconductor layer in which an n-side nitride semiconductor layer 110, an active layer 120, and a p-side nitride semiconductor layer 130 are sequentially stacked on a substrate 100 is configured. In the laser element, the p-side nitride semiconductor layer is provided with a striped ridge, and the p-side electrode 150 is formed only on the ridge. The p-side electrode 150 has a structure in which a metal layer Ma, a first metal layer M1, a second metal layer M2, a third metal layer M3, and a metal layer Mb are stacked in this order from at least the stacked semiconductor layer side.
In order to form a p-side electrode having substantially the same width as the width on the ridge, a p-side electrode having a desired ridge width is formed on a flat wafer, and the semiconductor layer is etched using the p-side electrode as a mask. A p-side electrode having the same width as the ridge is formed on the ridge. In order to etch the semiconductor layer using such a self-alignment method, it is preferable to perform dry etching mainly using a chlorine-based etching gas.

When the ridge is formed using the self-alignment method, the upper surface of the p-side electrode is exposed to a chlorine-based gas during etching of the semiconductor layer, a fluorine-based gas during etching of the SiO ₂ film, or the like. Therefore, not oxide but chloride or fluoride is formed. However, even if the platinum group element layer reacts with the chlorinated gas or the fluorinated gas, the reaction is limited to the vicinity of the surface. Therefore, as in the case of heat treatment, the inside of the layer is easily held with the same composition as that during film formation. If the compound with chlorine or fluorine is stable and exhibits insulating properties, an interface resistance will be generated between the pad electrode, and in such a case, no compound is generated inside the layer by washing the surface. By exposing the region and forming the pad electrode on the exposed portion, the ohmic property can be made difficult to be impaired.
In the second embodiment, since the width of the p-side electrode is further narrowed, the load on the ridge increases depending on the film quality, but the stress on the ridge is reduced by using a p-side electrode as a multilayer structure. Thus, laser characteristics with excellent reliability can be obtained.

  Hereinafter, a nitride semiconductor laser device will be described as an example, but the present invention is not limited to the following example, and it goes without saying that various modifications based on the technical idea of the present invention are possible. .

[Example 1]
As the substrate, a wafer-like GaN substrate 100 having a C surface as a main surface is used. The substrate is not particularly limited to this, and a GaN substrate having an R plane, an A plane, and an M plane as main surfaces is used as necessary.

(N-side nitride semiconductor layer 110)
Next, the GaN substrate is transferred to the MOCVD apparatus. The atmosphere temperature in the furnace is set to 1050 ° C., and TMA (trimethylaluminum), TMG (trimethylgallium), and ammonia are used as source gases, and an n-type cladding layer made of undoped Al _0.04 Ga _0.96 N is formed in thickness. Grow at 2.0 μm.

Next, TMG and ammonia are used as source gases at substantially the same temperature as the n-type cladding layer, and an n-type light guide layer made of undoped GaN is grown to a thickness of 0.19 μm. This layer may be doped with n-type impurities.

(Active layer 120)
Next, the temperature is set to 800 ° C., TMI (trimethylindium), TMG, and ammonia are used as raw materials, silane gas is used as an impurity gas, and a barrier layer made of Si-doped In _0.02 Ga _0.98 N is formed at 140 ° C. Grow with film thickness. Subsequently, the silane gas is stopped and a well layer made of undoped In _0.1 Ga _0.9 N is grown to a thickness of 80 mm. This operation is repeated twice. Finally, a barrier layer made of Si-doped In _0.02 Ga _0.98 N is grown to a film thickness of 140 mm, and an active layer having a total quantum film structure (MQW) of 580 mm is formed. Grow.

(P-side nitride semiconductor layer 130)
At a similar temperature, a p-type electron confinement layer made of Mg-doped Al _0.25 Ga _0.75 N is grown to a thickness of 100 mm in an N ₂ atmosphere or an H ₂ atmosphere.

Next, the temperature is set to 1050 ° C., TMG and ammonia are used as source gases, and a p-type light guide layer made of undoped GaN is grown to a thickness of 0.13 μm.

Subsequently, an A layer made of undoped Al _0.08 Ga _0.92 N is grown to a thickness of 80 mm, and a B layer made of Mg-doped GaN is grown to a thickness of 80 mm. This is repeated 28 times, and the A layer and the B layer are alternately laminated to grow a p-type cladding layer made of a multilayer film (superlattice structure) having a total film thickness of 0.45 μm.

Finally, a p-type contact layer made of Mg-doped GaN is grown on the p-type cladding layer at 1050 ° C. to a thickness of 150 mm. The p-type contact layer can be composed of p-type In _x Al _y Ga _1-xy N (x ≦ 0, y ≦ 0, x + y ≦ 1). The most favorable ohmic contact with the electrode is obtained. After the completion of the reaction, the wafer is annealed at 700 ° C. in a nitrogen atmosphere in the reaction vessel to further reduce the resistance of the p-type layer.

After the nitride semiconductor is grown on the GaN substrate to form a laminated structure as described above, the wafer is taken out of the reaction vessel, and a protective film made of SiO ₂ is formed on the surface of the uppermost p-type contact layer. Etching with Cl ₂ gas using RIE (reactive ion etching) to expose the surface of the n-type cladding layer. At this time, a W-shaped groove may be formed in the vicinity of the end surface of the light emission side. This step can be omitted.

Next, in order to form a striped waveguide region, a protective film made of Si oxide (mainly SiO ₂ ) is formed to a thickness of 0.5 μm on almost the entire surface of the uppermost p-type contact layer by a CVD apparatus. After the formation, a mask having a predetermined shape is formed on the protective film by a photolithography technique, and a protective film made of striped Si oxide is formed by etching using CHF ₃ gas by an RIE apparatus. Using this Si oxide protective film as a mask, the semiconductor layer is etched using Cl ₂ gas and SiCl ₄ gas to form a ridge stripe above the active layer. At this time, the width of the ridge is set to 1.6 μm.

With the SiO ₂ mask formed, a first insulating film 140 made of ZrO ₂ is formed on the surface of the p-type semiconductor layer with a thickness of about 600 mm. After forming the first insulating film, the wafer is heat-treated at 600 ° C. After the heat treatment, the substrate is immersed in a buffered solution to dissolve and remove SiO ₂ formed on the upper surface of the ridge stripe, and ZrO ₂ on the p-type contact layer is removed together with SiO ₂ by a lift-off method. Thereby, the upper surface of the ridge is exposed and the side surface of the ridge is covered with ZrO ₂ .

Next, the p-side electrode 150 is formed on the p-type contact layer. First, Ni is formed as a metal layer Ma with a film thickness of 100 mm using a sputtering apparatus. The film forming conditions at this time are such that the initial vacuum is 5.0 × 10 ⁻⁴ Pa or more. Next, the first metal layer M1 and the third metal layer M3 are formed using a sputtering apparatus. The first metal layer M1 is formed of Au with a thickness of 1000 mm. The third metal layer M3 is formed of Au with a thickness of 500 mm. The first metal layer M1 is formed by continuously forming Ni after forming the metal layer Ma. The film forming pressure at this time is 0.5 × 10 ⁻¹ Pa or more. Next, the condition for forming the third metal layer M3 is that the initial degree of vacuum is 8.0 × 10 ⁻⁴ Pa or more. Thereafter, the film forming pressure is set to 0.5 × 10 ⁻¹ Pa or more.
Thereafter, heat treatment is performed at 600 ° C. in an annealing furnace apparatus in an atmosphere in which the oxygen concentration is 0.5 to 10% with respect to the N 2 atmosphere. As a result, the p-side electrode can have a laminated structure having desired characteristics.

As described above, in the p-side electrode 150, the metal layer Ma, which is the lowermost layer, is made of Ni, and the film thickness is 10 mm. On top of that, the first metal layer M1 is made of Au, the density is 15 g / cm ³ , and the film thickness is 1000 Å. On top of that, the second metal layer M2 is made of Ni and has a thickness of 80 mm. On top of that, the third metal layer M3 is made of Au, the density is 5 g / cm ³ , and the film thickness is 500 mm. Further, the uppermost metal layer Mb is made of Ni and has a thickness of 10 mm.

Next, SiO ₂ is formed on the side surface of the laser element as a second insulating film. Further, a p-pad electrode is formed on the p-electrode in the order of Ni—Ti—Au with a thickness of 1000 to 1000 to 8000. Next, after adjusting the GaN substrate to have a film thickness of about 85 μm, an n-electrode having a thickness of 100 mm, 2000 mm, and 3000 mm is formed on the back surface of the substrate in the order of V-Pt-Au.

  Next, braking is performed from the nitride semiconductor layer side, and the bar shape is obtained by cleaving. The cleavage plane of the nitride semiconductor layer is the M plane (11-00 plane) of the nitride semiconductor, and this plane is the resonator plane.

A dielectric film is provided on the light emitting side end face of the bar-shaped nitride semiconductor formed as described above. A dielectric film such as ZrO ₂ , Nb ₂ O ₅ , Al ₂ O ₃ , or TiO ₂ is formed with a thickness of 150 nm on the light emitting side end face.
Next, after forming a dielectric film made of Al _x O _y on the light reflection side end face, a reflection mirror made of SiO ₂ and ZrO ₂ is formed.

Thereafter, the bar-shaped semiconductor is chipped to form a rectangular nitride semiconductor laser element. The resonator length is 600 μm and the chip width is 200 μm. As described above, the obtained nitride semiconductor laser device has low electrode contact resistance, good adhesion, and a COD level of 800 mW or more. The Kink power is 400 mW. As a result of conducting a life test (Tc = 70 ° C., CW output of 100 mW), a result of 5000 hours or more can be obtained. In addition, the nitride semiconductor laser element in this example can continuously oscillate at a threshold current density of 3.5 kA / cm ² at room temperature and a high output of 150 mW when driven at CW and an oscillation wavelength of 405 nm.
In this embodiment, Vf is 3.89V. In the case where the metal layer M3 is not formed, Vf is 3.96, and the reduction rate of Vf is about 5% or more compared to this. Further, the adhesion with the pad electrode is also improved.
The adhesion is improved according to the contact area between the P-side electrode and the p-pad electrode. In this embodiment, the ratio of the oxide film formed on the surface of the electrode can be reduced to 1/10 or less as compared with the case where the metal layer M3 is not formed.
These characteristic improvements can improve the contact resistance between the electrode and the pad electrode not only in the laser diode but also in the LED, and can improve the yield even when the FaceDown structure is adopted.

[Example 2]
In Example 2, the p-side electrode is formed in the order of Ni—Pt—Ni—Pt—Ni (10 （-1000Å-80Å-500Å-10Å). The same procedure as in Example 1 is performed except that the p-side electrode is formed of such a material. In the nitride semiconductor laser device thus obtained, no electrode peeling is confirmed, and the same effect as in Example 1 can be expected. Further, it can continuously oscillate at an oscillation wavelength of 405 nm at a threshold current density of 2.0 kA / cm ² and a high output of 65 mW at room temperature.

[Example 3]
In Example 3, the p-side electrode is formed in the order of the lowermost layer, the first metal layer, the second metal layer, the third metal layer, and the uppermost layer. Specifically, the p-side electrode is formed of Ni—Au—Ni—Au—Ni (10Å-1000Å-80Å-500Å-10Å). Other steps are performed in the same manner as in Example 1 to obtain the nitride semiconductor laser device of the present invention. The nitride semiconductor laser device obtained as described above is capable of continuous oscillation with a threshold current density of 2.0 kA / cm ² at room temperature and an oscillation wavelength of 405 nm at a high output of 65 mW, with no electrode peeling confirmed. .

  The nitride semiconductor device of the present invention is not limited to a laser diode (laser device) capable of being driven by a large current, but can be used for an electronic device such as a high-brightness LED, a light-receiving device, or an FET. In particular, the laser diode can be used for optical disc applications, optical communication systems, printing presses, exposure applications, measurements, and the like.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 100 ... Substrate, 110 ... N-side nitride semiconductor layer, 120 ... Active layer, 130 ... P-side nitride semiconductor layer, 140 ... First insulating film, 150 ... P-side electrode, 160 ... Second insulating film, 170 ... p-side pad electrode, 180 ... n-side electrode, 190 ... n-side pad electrode

It is sectional drawing which shows an example of the electrode structure which concerns on this invention. It is sectional drawing which shows an example of the electrode structure which concerns on this invention. It is sectional drawing which shows an example of the electrode structure which concerns on this invention. It is the elements on larger scale which show an example of the ridge structure and electrode structure which concern on this invention. It is the elements on larger scale which show an example of the ridge structure and electrode structure which concern on this invention. 1 is a schematic cross-sectional view of a nitride semiconductor laser element according to an embodiment of the present invention. 1 is a schematic cross-sectional view of a nitride semiconductor laser element according to an embodiment of the present invention. 1 is a schematic cross-sectional view of a nitride semiconductor laser element according to an embodiment of the present invention.

Claims (9)

In a nitride semiconductor device comprising: a stacked semiconductor layer in which a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer are sequentially stacked; and an electrode formed on an upper surface of the second conductivity type semiconductor layer.
The electrode has a first metal layer, a second metal layer, and a third metal layer laminated in order from at least the laminated semiconductor layer side,
The first metal layer and the third metal layer are metal layers containing the same material, and the first metal layer is higher in density than the third metal layer,
The nitride semiconductor device, wherein the second metal layer contains a material different from that of the first metal layer and the third metal layer. The nitride semiconductor device according to claim 1, wherein the first metal layer is thicker than the third metal layer. ^3. The nitride semiconductor device according to claim 1, wherein a density of the first metal layer is 10 g / cm ³ or more and 30 g / cm ³ or less. 3. The nitride semiconductor device according to claim 1, wherein a density of the third metal layer is 3 g / cm ³ or more and 15 g / cm ³ or less. 5. The nitride semiconductor device according to claim 1, wherein the first metal layer and the third metal layer are formed of a layer containing at least gold or an alloy thereof. The nitride semiconductor device according to claim 1, wherein the second metal layer is formed of a layer containing at least a platinum group element or an alloy thereof. The nitride semiconductor device according to claim 1, wherein the third metal layer is a barrier layer. In a nitride semiconductor device comprising: a stacked semiconductor layer in which a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer are sequentially stacked; and an electrode formed on an upper surface of the second conductivity type semiconductor layer.
The electrode is formed by sequentially laminating a metal layer having an antioxidant function from at least the laminated semiconductor layer side, and a barrier metal layer for preventing precipitation of other layers,
The metal layer having an antioxidant function and the barrier metal layer are metal layers containing the same material, and the metal layer having the antioxidant function is higher in density than the barrier metal layer, Nitride semiconductor device. 9. The nitridation according to claim 1, wherein the nitride semiconductor element is a laser diode in which a striped ridge is formed in the second conductivity type semiconductor layer, and the electrode is formed on an upper surface of the ridge. Semiconductor device.

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