JP2004221493A - Electrode structure for nitride semiconductor and method for manufacturing same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for an n-type nitride semiconductor which forms a good ohmic electrode having an excellent aging characteristic and a high mechanical strength. <P>SOLUTION: An Ti layer, an Al layer, a Mo layer, a Pt layer, and an Au layer are sequentially laminated on the surface of an n-type GaN contact layer (102) to form an electrode structure (112). When compared with a prior art n-type electrode of a laminated structure of the Ti layer, the Al layer, the Mo layer and the Au layer; the insertion of the Pt layer enables suppression of delamination of the Au layer. In addition, since the diffusion of the Au layer into the contact layer side can be nearly completely suppressed, the characteristic of the electrode can be prevented from being deteriorated with time. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an n-type ohmic electrode in a group III nitride semiconductor device such as a semiconductor laser diode.
[0002]
[Prior art]
In _x Ga _y Al _z N (provided that x + y + z = 1,0 ≦ x, y, z ≦ 1) III nitride compound semiconductor represented by have a larger energy band gap and high thermal stability, the light emitting element It is expected as a material system that can be applied to various applications, including high-temperature devices. Especially, as a light emitting diode (Light Emitting Diode; LED), several cd class devices are practically used in a blue to green wavelength range. Further, as a light source for next-generation high-density optical discs, a laser diode (Laser Diode; LD) using the same material system is about to be put into practical use.
[0003]
As an n-type ohmic electrode in such a group III nitride semiconductor device, Al electrodes and Al / Ti electrodes have been developed. Al electrodes have problems such as difficulty in obtaining sufficient ohmic characteristics and deterioration of electrodes due to a heat treatment step after electrode formation. However, Ti is used for contact with n-type GaN to form an Al / Ti electrode. By doing so, these problems were solved to some extent (Japanese Patent Application Laid-Open No. 7-45867).
[0004]
When an electrode is actually mounted on a device, steps such as joining a power supply line to the electrode and mounting the electrode surface on a substrate such as a Cu block are required. It is preferable to dispose an Au layer. As such an electrode structure, an Au / Pt / Al / Ti electrode in which Au and a metal serving as a barrier layer are arranged above the Al / Ti electrode to suppress the diffusion of Au (Japanese Patent Application Laid-Open No. H10-200161). ) And an Au / Mo / Al / Ti electrode (JP-A-2002-134822) in which the defect is improved.
[0005]
[Patent Document 1] Japanese Patent Application Laid-Open No. 7-45867 [Patent Document 2] Japanese Patent Application Laid-Open No. 10-200161 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-134822
[Problems to be solved by the invention]
However, in the Au / Pt / Al / Ti electrode, Pt has a material property that gas is easily adsorbed, so that the adhesion to the Al layer is poor compared to other interfaces. There is a problem that electrode peeling easily occurs when a high-density current is injected. If such an electrode is mounted on, for example, an LD element, the oscillation threshold value is deteriorated and the operating voltage is deteriorated with time.
[0007]
In order to overcome the drawbacks of the Au / Pt / Al / Ti electrode, the present applicant proposes an Au / Mo / Al / Ti electrode proposed in Japanese Patent Application Laid-Open No. 2002-134822. The adhesion strength of the interface between the layers is lower than that of the other interfaces, and the Au layer peels off when a power supply line is bonded to the surface of the Au layer or when the surface of the Au layer is mounted on a substrate such as a Cu block. It has been found. This is because the Mo layer has a high barrier effect against Au, so that the Au layer provided for bonding by being alloyed with the solder material (blow material) at the time of mounting selectively alloys only with the solder material. This is considered to be due to the fact that the adhesion between Au / Mo becomes insufficient.
[0008]
The present invention has been made in view of such a problem, and has a low-resistance electrode structure for a group III nitride semiconductor containing an n-type impurity, in which the adhesion strength at each interface is high, and electrode peeling is prevented. It aims to provide something that does not occur.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, in the present invention, the electrode structure for a group III nitride semiconductor containing an n-type impurity is a first layer containing at least one of Hf, Ti, Sc, Y, La and Zr. And a second layer containing at least one of Mo, Cr, W, Zr, and Ta, a third layer consisting of at least one of Pt, Cr, and Zr, and containing one of Au and Al. The fourth layer is configured to include the fourth layer in this order from the nitride semiconductor side.
[0010]
The second layer located on the first layer serves as a first barrier layer when the metal (Au or Al) of the fourth layer located thereon further diffuses in the direction of the nitride semiconductor. Has a function. Mo is a high melting point metal, and its linear thermal expansion coefficient is almost close to that in the direction parallel to the C-plane of a typical GaN semiconductor of group III nitride, and is a metal material suitable as a barrier layer. The present applicant has confirmed that there are Cr, W, Zr and Ta as metal materials capable of providing this function, and these metals can be used as the second layer in addition to Mo. These metals forming the second layer do not have to be a single substance, and are equivalent to the Mo layer if at least one metal among Mo, Cr, W, Zr, and Ta is contained. Can play a role.
[0011]
In addition, the third layer located on the second layer has a function as a second barrier layer when the metal of the fourth layer has diffused, and the third layer has a function as the second layer and the fourth layer. Has the effect of increasing the adhesiveness between them. As described above, Pt is essentially a material that easily causes gas adsorption, and if it is formed directly on the upper surface of Al or the like, electrode peeling is likely to occur. However, the adhesion of Pt to Mo is higher than that of Al. It is considered that this is because Mo is less likely to cause gas adsorption than Al, and since parameters such as the melting point of Pt are closer to Mo than Al, the stress generated at the electrode interface immediately after film formation is reduced.
[0012]
Pt forms a compound with Au, and easily forms a Au / (Pt-Au compound) / Pt laminated structure. This Pt-Au compound is very stable thermally, and once formed, it is difficult for Au and Pt to react across the Pt-Au compound layer. That is, the formation of the Mo / Au interface having poor adhesion during mounting is suppressed. The present inventors have confirmed that there are Cr and Zr as metal materials having a similar role in addition to Pt, and these can also be used as the third layer. The metal constituting the third layer does not have to be a single substance. If at least one metal among Pt, Cr, and Zr is contained, the metal plays a role equivalent to that of the Pt layer. it can.
[0013]
The fourth layer located on the third layer is a bonding layer used for bonding a power supply line when mounting an electrode structure on a device or for mounting an electrode surface on a substrate such as a Cu block. is there. At the time of mounting or the like, strong adhesion is enabled by easily alloying with a solder material or a power supply line (Au-based or Al-based wire). Either Au or Al or a compound of Au and Al may be used as the metal material that plays such a role. In addition, when Ni or Pd is contained in this layer, the adhesion strength to the power supply line and the Cu block is further increased, which is preferable in the manufacturing process.
[0014]
Hf forming the first layer located on the group III nitride semiconductor has a function of promoting an interface reaction between the electrode metal and the semiconductor layer, and is necessary for the electrode structure to have low resistance ohmic properties. In addition, there is an effect of improving the adhesion strength between the electrode metal formed above and the semiconductor layer. The present inventor has confirmed that Ti, Sc, Y, La, and Zr are metal materials having a similar role in addition to Hf, and these can also be used as the first layer. These metals constituting the first layer do not have to be a single substance, and are equivalent to the Hf layer as long as at least one metal among Hf, Ti, Sc, Y, La, and Zr is contained. Can play a role.
[0015]
The first layer may include Al. Al is useful for giving the electrode structure a low resistance ohmic property.
[0016]
In manufacturing the above electrode structure, a first layer, a second layer, a third layer, and a fourth layer are stacked in this order on a group III nitride semiconductor, and the first layer is formed. After that, heat treatment may be performed at a temperature in the range of 300 to 900 ° C. before forming the second layer. This heat treatment is performed as an annealing step for the laminated structure containing Mo to exhibit low-resistance ohmic characteristics with the group III nitride semiconductor. The heat treatment is performed in a vacuum or in an atmosphere of an inert gas such as N ₂ or Ar.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, two embodiments in which the electrode structure and the manufacturing method of the present invention are applied to an LD element will be described with reference to the drawings. Note that these embodiments are merely preferred examples, and the present invention is not limited to these.
[0018]
<First embodiment>
First, as shown in FIG. 1, a GaN buffer layer 101, an n-type GaN contact layer 102, an n-type AlGaN cladding layer 103, an n-type GaN light guide layer 104, and a sapphire substrate 100 having a C-plane crystal plane. The InGaN multiple quantum well active layer 105, the p-type GaN light guide layer 106, the p-type AlGaN layer 107, and the p-type GaN contact layer 108 are sequentially epitaxially grown by a metal organic chemical vapor deposition (MOCVD) method to form a group III nitride semiconductor stack. Create the structure.
[0019]
Subsequently, as shown in FIG. 2, after a dry etching mask 109 is formed on this GaN-based semiconductor laminated structure, a portion not covered with the dry etching mask 109 is n-reactive by reactive ion etching (RIE). The mesa structure is formed by digging down to the type GaN contact layer 102. Then, the dry etching mask 109 is completely removed, and a stripe pattern of the insulating film 110 is formed on the mesa.
[0020]
Next, as shown in FIG. 3, a p-type electrode 111 made of Au / Mo / Pd is formed on the mesa, and Au / Pt / Mo / Al is formed on the n-type GaN contact layer 102 on the bottom of the mesa. An n-type electrode 112 made of / Ti is formed. FIG. 4 schematically shows a cross section of the n-type electrode 112. Here, Ti (112a) and Al (112b) are the first layer in the present invention, Mo (112c) is the second layer, Pt (112d) is the third layer, and Au (112e) is the fourth layer. It is.
[0021]
In forming the n-type electrode 112, after forming an Al / Ti laminated structure, the whole element structure is heat-treated at about 450 ° C. for 3 minutes in an N ₂ atmosphere, and then the Au / Pt / Mo laminated structure is formed. The step of forming on the Al layer is taken.
[0022]
The lamination film thickness of each electrode metal is 200 nm for Au, 15 nm for Mo, and 15 nm for Pd for the p-type electrode 111, and 200 nm for Au, 15 nm for Pt, 30 nm for Mo, and 150 nm for Al for the n-type electrode. Ti is 30 nm.
[0023]
When an Au wire was bonded as a power supply line to the n-type electrode of the LD element of the present embodiment manufactured as described above, the Au wire was compared with a conventional case using an Au / Mo / Al / Ti electrode as the n-type electrode. As a result, the frequency of occurrence of defects in which the Au layer peels off during bonding can be greatly reduced. In addition, when a constant output drive test was performed by injecting a current into the LD element, it was confirmed that the number of cases in which the operating voltage was increased due to deterioration of the n-type electrode and the device was defective was reduced.
[0024]
<Second embodiment>
First, as shown in FIG. 5, an n-type GaN buffer layer 201, an n-type AlGaN cladding layer 202, an n-type GaN light guide layer 203, an InGaN The multiple quantum well active layer 204, the p-type GaN optical guide layer 205, the p-type AlGaN layer 206, and the p-type GaN contact layer 207 are sequentially epitaxially grown by MOCVD to produce a group III nitride semiconductor multilayer structure.
[0025]
Subsequently, as shown in FIG. 6, a stripe pattern of the insulating film 208 is formed on the upper surface of the p-type GaN contact layer 207. Next, a p-type electrode 209 of Au / Mo / Pd is formed on the p-type GaN contact layer 207, and an n-type electrode of Au / Pt / Mo / Al / Hf is formed on the back surface of the n-type GaN substrate 200. An electrode 210 is formed. FIG. 7 schematically shows a cross section of the n-type electrode 210. Here, Hf (210a) and Al (210b) are the first layer in the present invention, Mo (210c) is the second layer, Pt (210d) is the third layer, and Au (210e) is the fourth layer. It is.
[0026]
In forming the n-type electrode 210, after forming an Al / Hf laminated structure, the entire element structure is heat-treated at 500 ° C. for 10 minutes in a vacuum atmosphere of 5 × 10 ⁻⁴ Pa, and then Au / Pt / Mo. A step of forming a laminated structure on the Al layer is taken.
[0027]
The layer thickness of each electrode metal is 200 nm for Au, 15 nm for Mo, and 15 nm for Pd for the p-type electrode 209, and 200 nm for Au, 15 nm for Pt, 30 nm for Mo, and 150 nm for Al for the n-type electrode 210. , Hf are 10 nm.
[0028]
In this embodiment, an n-type GaN substrate is used as a substrate on which a nitride semiconductor layer is epitaxially grown. For this reason, unlike the first embodiment, the p-type electrode and the n-type electrode can be formed so as to face the upper and lower sides of the element structure.
[0029]
Further, the LD element of the present embodiment was heat-mounted on a Cu block serving both for power supply and heat dissipation with the n-type electrode 210 facing downward (as the mount surface side). On the Cu block surface, an AuSn alloy film was formed as a solder material for improving the adhesion.
[0030]
Unlike the wire bonding as in the first embodiment, when the entire surface of the n-type electrode is heated and mounted on the Cu block, heat is directly transmitted to the electrode itself. Therefore, defects such as peeling of the Au layer and diffusion of the Au layer in the direction of the contact layer are likely to occur. In order to suppress these problems, it is necessary to precisely control the temperature and time during mounting.
[0031]
However, the Au / Pt / Mo / Al / Hf electrode, which is the n-type electrode of the present invention, has significantly improved adhesion strength of the Au layer as compared with the conventional n-type electrode, Au / Mo / Al / Ti electrode. Therefore, the occurrence of Au layer peeling failure can be suppressed, and the Pt layer functions as a barrier layer for preventing diffusion of the Au layer in addition to the Mo layer. It has almost never occurred.
[0032]
When the Au / Mo / Al / Ti electrode was mounted with AuSn solder and the peeled chip was observed, Mo was exposed on the peeled surface. In other words, the soldering material and Au are sufficiently alloyed at the time of mounting, but the alloying reaction does not proceed between Mo and the soldering material or between Mo and Au, so that it is considered that peeling has occurred at the surface portion of Mo. Can be
[0033]
The effect of improving the adhesion due to the Au / Pt / Mo / Al / Hf configuration is the same when a constant output drive test is performed by injecting current into the LD element, and the operating voltage increases due to deterioration of the n-type electrode. It was confirmed that the occurrence of defects could be greatly reduced.
[0034]
In this embodiment, since the n-type GaN substrate is used as the substrate, the crystallinity of the epitaxial layer is improved by reducing the defect density and the like as compared with the LD device using the sapphire substrate described in the first embodiment. Thus, the characteristics as an LD element are improved.
[0035]
In the n-type electrodes of the first and second embodiments, Ti or Hf is used as the metal (first layer) that contacts the n-type contact layer. In addition, Sc, Y, La, and Zr are used. It has been confirmed that the electrode structure of the present invention has the same effect as a simple substance or an alloy containing one or more of these. Examination of an appropriate film thickness for these materials revealed that the thickness should be in the range of 1 to 100 nm.
[0036]
Further, in the n-type electrode of each embodiment, the Mo layer is used as the metal layer (second layer) formed above the Ti layer or the Hf layer, but other than Cr, W, Zr, and Ta alone. Or an alloy containing at least one of these, it was confirmed that the electrode structure of the present invention had the same effect. Examination of an appropriate film thickness for these materials also revealed that it was only necessary to be in the range of 1 to 200 nm.
[0037]
Further, in the n-type electrode of each embodiment, the Pt layer is used as the metal layer (third layer) formed above the Mo layer, but in addition, Cr or Zr alone, or one or more of these. It was confirmed that an alloy containing the same has the same effect as the electrode structure of the present invention. Examination of an appropriate film thickness for these materials also revealed that it was only necessary to be in the range of 1 to 200 nm.
[0038]
In the n-type electrode of each embodiment, Au is used as the metal (fourth layer) formed on the outermost side of the electrode structure. However, other than this, Al may be used, or Au and Al may be used. May be used. When Ni or Pd was contained in the outermost layer, it was confirmed that the bonding strength with the solder material and the power supply line on the Cu block was further increased.
[0039]
In each embodiment, the heat treatment after the formation of the electrodes is performed in an N ₂ atmosphere or a vacuum atmosphere, but may be performed in an Ar atmosphere. When employing a vacuum atmosphere, the pressure may be 1.5 Pa or less. The heat treatment temperature varies depending on what kind of metal is used as the metal contacting the n-type contact layer, but can be selected in the range of 300 to 900 ° C. It was also found that the heat treatment time was preferably selected in the range of 1 to 120 minutes.
[0040]
In each embodiment, GaN is selected as the material of the n-type contact layer, but this is due to the LD element structure to which the electrode of the present invention is applied, and the present invention is not limited to this. Even if the contact layer is made of n-type InGaN or AlGaN, or n-type InGaAlN that is a quaternary mixed crystal, the effect can be obtained.
[0041]
Further, as a solder material for mounting, in addition to the AuSn-based alloy described in each embodiment, an AuGe-based alloy, an AuSi-based alloy, a SnAg-based alloy, a SnAgCu-based alloy, an In, an InAl-based alloy, an InAlGe-based alloy, Materials such as an InAg-based alloy and an AlSi-based alloy can also be used.
[0042]
In each of the embodiments, the LD element structure on the p-type electrode side is not directly related to the present invention, and therefore, the simplest electrode stripe type structure is selected as an example. It is needless to say that the same effects as those of the embodiments can be obtained by applying the n-type electrode of the present invention even in the case of the mold structure.
[0043]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the favorable ohmic electrode with high mechanical strength which does not produce electrode peeling at the time of bonding or mounting with respect to an n-type group III nitride semiconductor can be provided. Further, not only the mechanical strength but also the diffusion of Au in the electrode member can be suppressed, so that the deterioration of the ohmic characteristics of the electrode can be prevented. As a result, for example, in a light emitting device such as an LD or LED, electrode defects occurring during electrode bonding or mounting are suppressed, thereby improving the manufacturing yield and suppressing an increase in operating voltage due to electrode deterioration over time during energization. Can contribute to longer life.
[Brief description of the drawings]
FIG. 1 is a sectional view schematically showing a manufacturing process of a nitride semiconductor according to a first embodiment.
FIG. 2 is a cross-sectional view schematically showing a manufacturing process of the nitride semiconductor according to the first embodiment.
FIG. 3 is a cross-sectional view schematically showing a structure of an LD device made of a group III nitride semiconductor according to the first embodiment.
FIG. 4 is a sectional view schematically showing an n-type electrode according to the first embodiment.
FIG. 5 is a cross-sectional view schematically showing a manufacturing process of the nitride semiconductor according to the second embodiment.
FIG. 6 is a cross-sectional view schematically showing a structure of an LD device made of a group III nitride semiconductor according to a second embodiment.
FIG. 7 is a sectional view schematically showing an n-type electrode according to the second embodiment.
[Explanation of symbols]
REFERENCE SIGNS LIST 100 sapphire substrate 101 GaN buffer layer 102 n-type GaN contact layer 103 n-type AlGaN cladding layer 104 n-type GaN light guide layer 105 InGaN multiple quantum well active layer 106 p-type GaN light guide layer 107 p-type AlGaN layer 108 p-type GaN contact Layer 109 dry etching mask 110 insulating film 111 p-type electrode 112 n-type electrodes 112a, 112b first layer 112c second layer 112d third layer 112e fourth layer 200 n-type GaN substrate 201 n-type GaN buffer layer 202 n-type AlGaN cladding layer 203 n-type GaN light guide layer 204 InGaN multiple quantum well active layer 205 p-type GaN light guide layer 206 p-type AlGaN layer 207 p-type GaN contact layer 208 insulating film 209 p-type electrode 210 n-type electrode 210 a 210b First layer 210c Second layer 210d Third layer 210e Fourth layer

Claims (7)

An electrode structure for a group III nitride semiconductor containing an n-type impurity,
A first layer including at least one of Hf, Ti, Sc, Y, La, and Zr;
A second layer containing at least one of Mo, Cr, W, Zr, and Ta;
A third layer made of at least one of Pt, Cr and Zr;
An electrode structure comprising a fourth layer containing any of Au and Al in this order from the nitride semiconductor side. The electrode structure according to claim 1, wherein the first layer further contains Al. The first layer includes a layer containing at least one of Hf, Ti, Sc, Y, La and Zr and located on the nitride semiconductor side, and a layer made of Al and located on the second layer side. The electrode structure according to claim 1, wherein: The electrode structure according to claim 1, wherein the fourth layer further contains Ni. The electrode structure according to claim 1, wherein the fourth layer further contains Pd. A method of manufacturing an electrode structure for a group III nitride semiconductor including an n-type impurity,
Forming a first layer containing at least one of Hf, Ti, Sc, Y, La and Zr on the nitride semiconductor;
Forming a second layer including at least one of Mo, Cr, W, Zr, and Ta on the first layer;
Forming a third layer made of at least one of Pt, Cr and Zr on the second layer;
Forming a fourth layer containing any of Au and Al on the third layer;
A method for manufacturing an electrode structure, comprising a step of performing a heat treatment at 300 ° C. or more and 900 ° C. or less after forming the first layer and before forming the second layer. Manufacturing method of the electrode structure according to claim 6, the step of performing the heat treatment, and performing 1.5Pa or less vacuum atmosphere or an N ₂ or in an atmosphere of Ar.

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