JP2000012831A - Ohmic electrode and formation thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ohmic electrode of superior heat resistance for III-V group nitride base compound semiconductor, and formation thereof. SOLUTION: This ohmic electrode 3 is formed on an n-type layer 2 made of III-V group nitride compound semiconductor and comprises Al layer 3a, Ti layer 3b, and Au layer 3c laminated in this order. With such a constitution, the thickness of the Al layer 3a does not exceed 200 nm while this ohmic electrode 3 made of Al layer, Ti layer and Au layer laminated in this order on the n-type layer 2 is formed by subjecting it to heat treatment at 400-700 deg.C for 3 minutes to 250 hours in a nitrogen atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ohmic electrode loaded on a semiconductor device as a drive electrode of the device and a method of forming the same, and more particularly, to an n-type layer made of a group III-V nitride compound semiconductor. The present invention relates to an ohmic electrode which is loaded, exhibits excellent ohmic characteristics even at high temperatures, has excellent adhesion to the n-type layer, and has low contact resistance with an Au lead wire, and a method for forming the same.

[0002]

2. Description of the Related Art For example, when manufacturing a field effect transistor (FET) using a GaAs compound semiconductor, a GaAs compound semiconductor is epitaxially grown on a predetermined substrate and includes an n-type layer, an i-type layer, and a p-type layer. A target layer structure is formed, and AuGe / Au, A
depositing an electrode material such as uGeNi / Au, In
Type electrode, and a predetermined portion of the p-type layer
An electrode material such as / Au, Ti / Pt / Au is deposited to form a p-type electrode.

In this case, in order to minimize the occurrence of power loss during the operation of the FET, ohmic contact must be realized between the n-type layer and the n-type electrode and between the p-type layer and the p-type electrode. Becomes a requirement. In particular, it is required between the n-type layer and the n-type electrode. By the way, GaAs-based FE
In the case of T, when the above-described electrode material is deposited on the n-type layer to form an n-type electrode, a Schottky barrier height is formed at the contact interface with the electrode of the n-type layer, and the driving current is reduced. A problem arises in that the flow is not sufficiently smooth. To solve such a problem, for example, N ₂
In order to improve ohmic characteristics, a heat treatment is performed in an atmosphere at a temperature of about 380 ° C. for about 3 minutes. However, when the temperature during the heat treatment is high or the processing time is long even at a low temperature, the deposited metal material thermally diffuses into the n-type layer, and as a result, the electrode And the contact resistance between the n-type layer and the n-type layer is increased, thereby causing a problem that ohmic characteristics are deteriorated.

On the other hand, among III-V group compound semiconductors, G
III-V nitride-based compound semiconductors typified by aN, AlGaN, InGaN, and AlInGaN have a large forbidden band, are direct transition type, and are excellent in high-temperature operation. Light-emitting elements such as light-emitting diodes and laser diodes, light-receiving elements such as photodiodes and phototransistors, as well as bipolar transistors (HBTs), field-effect transistors (FETs), and high mobility transistors (HEMs)
Research and development of electronic devices such as T) are underway.

However, in the case of a group III-V nitride-based compound semiconductor, its quantum chemical properties have not yet been completely elucidated, so that φm <, which is a necessary condition for realizing an ohmic contact state, is not satisfied. At present, search for electrode materials satisfying φs (in the case of n-type electrode) or φm> φs (in the case of p-type electrode) is continued by trial and error.

For example, as a material of an n-type electrode loaded on an n-type layer made of a group III-V nitride compound semiconductor,
In, Al, Ti and the like are known.

[0007] Among these materials, Al is known as a material that exhibits ohmic characteristics with a III-V group nitride-based compound semiconductor without performing the above-described heat treatment after vapor deposition.

However, since Al has a low melting point of about 640 ° C. and is a metal which is easily oxidized, it becomes very difficult to handle it as an electrode after vapor deposition.

For example, when the thickness of the Al electrode is 200 nm or more, if the temperature becomes 600 ° C. or more, bumping of Al and surface oxidation occur, and the electrode surface is roughened, and the ohmic characteristics are degraded. . That is, Al
It is composed of a group III-V nitride compound semiconductor alone.
It is unsuitable for use as a material for an n-type electrode of a device that features high-temperature operation as one feature.

Therefore, conventionally, in order to prevent this surface oxidation of the Al layer, an Al layer formed by further laminating a Ti layer thereon is used.
In reality, the / Ti layer structure is an n-type electrode.

In the case of this electrode structure, the adhesion to the n-type layer is good up to a temperature around 400 ° C., and the Ti
The contact resistance with, for example, an Au lead wire connected to the layer is also small. However, when the temperature is higher than that, specifically, when the temperature is higher than 550 ° C., surface discoloration starts to occur. The characteristics are significantly deteriorated.

This phenomenon is based on the fact that Ti is oxidized and converted into an insulator such as TiO ₂ .

[0013]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems in the Al / Ti layer structure n-type electrode loaded on the n-type layer made of a group III-V nitride compound semiconductor. Even if the temperature is higher than 600 ° C., n
The present invention provides an ohmic electrode which does not deteriorate the ohmic characteristics between the mold layer and the adhesiveness with the n-type layer and has a low contact resistance with the Au lead wire, and a method of forming the ohmic electrode on the n-type layer. Aim.

[0014]

In order to achieve the above-mentioned object, according to the present invention, an Al layer, a Ti layer and an Au layer are formed on an n-type layer made of a group III-V nitride compound semiconductor. And an ohmic electrode having a thickness of less than 200 nm, wherein the thickness of the Al layer is less than 200 nm.

Further, in the present invention, an Al layer, a Ti layer, and an Au layer are laminated in this order on an n-type layer made of a III-V nitride-based compound semiconductor. A method of loading an ohmic electrode (hereinafter, referred to as a first formation method) characterized by performing a heat treatment at 400 to 700 ° C. for 3 minutes to 250 hours, and an n-type nitride III-V compound semiconductor Al layer and T on the layer
After laminating the i-layer in this order, a heat treatment is performed under a reducing gas atmosphere at a temperature of 300 to 600 ° C. for 1 minute to 250 hours, and then an Au layer is laminated on the Ti layer after the heat treatment. (Hereinafter, referred to as a second formation method).

[0016]

FIG. 1 shows an example of a layer structure of a device on which an ohmic electrode of the present invention is formed. In FIG. 1, for example, such as GaN, on an Si substrate 1 via an insulating layer and a buffer layer (both not shown)
For example, an n-type GaN layer 2 is formed by epitaxially growing a group III-V nitride compound semiconductor and doping it with an n-type impurity such as Si.

Then, at a predetermined portion of the n-type GaN layer 2,
An ohmic electrode 3 of the present invention to be described later is formed, and the other surface is covered with an insulating film 4 such as a SiO ₂ film.

Here, the ohmic electrode 3 has a three-layer structure (Al / Ti / Au) in which an Al layer 3a, a Ti layer 3b, and an Au layer 3c are stacked in this order.

The Al layer 3a is made of n-type GaN
This is a layer for making ohmic contact with the layer 2, and its thickness is preferably smaller than 200 nm. 20 thickness
If the thickness is greater than 0 nm, when the temperature is higher than 600 ° C., the bumping phenomenon and oxidation of the Al layer are advanced, and the ohmic characteristics are deteriorated, and at the same time, the surface of the electrode 3 is roughened.
This is because the contact resistance with the lead wire also increases.

The Ti layer 3b is a layer provided to ensure the adhesion between the Al layer 3a and the Au layer 3c and to prevent the Al layer 3a from being oxidized.
Preferably it is ~ 150 nm. If the thickness is less than 50 nm, the antioxidant effect and adhesion to the Al layer 3a will be poor, and if the thickness is more than 150 nm,
This is because a problem of increasing the contact resistance in the vertical direction is caused.

The Au layer 3c serves to prevent oxidation of the Ti layer 3b and to reduce the contact resistance with the Au lead wire connected to the Au layer 3b.
Preferably, it is 0 nm. If the thickness is more than 50 nm, the amount of use is increased, which is economically disadvantageous.

In the present invention, the ohmic electrode 3 is formed on the n-type layer 2 as follows.

First, prior to the formation of the electrodes, predetermined epitaxially grown layers are sequentially laminated on the substrate 1 using a group III-V nitride compound semiconductor, and the outermost layer is an n-type layer 2. After the structure is formed, ordinary photolithography and etching are performed on the laminated structure to expose a portion of the surface of the n-type layer 2 where the ohmic electrode 3 is to be formed.

The ohmic electrode 3 of the present invention is formed on the exposed n-type layer 2 as follows.
First, the first loading method will be described.

First, for example, using a vacuum evaporation apparatus, n
On the mold layer 2, Al, Ti, and Au are sequentially deposited to form a laminated structure of an Al layer 3a, a Ti layer 3b, and an Au layer 3c having a predetermined thickness.

Next, the processing object is taken out of the apparatus and N ₂
It is transferred to an atmosphere furnace where heat treatment is performed. Note that N
_{In the 2-} atmosphere furnace, in addition to flowing (flowing) N ₂ during or after the treatment, it is preferable to sufficiently perform N ₂ substitution at room temperature prior to operation.

However, if the processing temperature is too low, good ohmic characteristics cannot be obtained, and even if the processing time is shortened and the processing is performed for a long time, good ohmic characteristics cannot be obtained.

In order to obtain good ohmic characteristics, the processing temperature is set to 400 to 700 ° C. and the processing time is set to 3 hours.
It is necessary to set the time between minutes and 250 hours.

The appropriate value of the processing time varies depending on the processing temperature and the electrical characteristics of the semiconductor layer.
If the processing temperature is high, the processing time tends to be short.

Next, in a second forming method, when the Al layer 3a and the Ti layer 3b are formed on the n-type layer 2, a heat treatment is performed in a reducing atmosphere, and then the heat treatment is performed on the Ti layer after the treatment. This is a method for forming an Au layer.

As the reducing atmosphere, for example, an H ₂ gas atmosphere, a CO gas atmosphere, or the like is a preferred example.

At this time, the processing temperature is set at 300 to 600 ° C., and the processing time is set at 1 minute to 250 hours.

If these conditions are not satisfied at the same time, the finally obtained electrode will not exhibit good ohmic characteristics, as in the case of the above-described first forming method.

In this method, the problem of nitridation of the Ti layer and the problem of oxidation of the Ti layer do not occur as in the case of the first method, and Au is laminated thereon. As a result, excellent ohmic characteristics can be realized even at a high temperature.

[0035]

Examples 1 and 2 and Comparative Examples 1 to 4 Various electrode materials were vapor-deposited under the following conditions on the above-mentioned n-type electrode formation portion of the substrate in a state where the n-type electrode can be formed. In this case, the n-type layer 2 was an n-type GaN layer.

The pressure in the vacuum deposition apparatus was set to 1 × 10 ⁻⁵ Pa, first, Al was deposited to a thickness of 20 nm, and after recovering the degree of vacuum, Ti was deposited to a thickness of 150 nm. After depositing Au to a thickness of 20 nm, a heat treatment was performed at a temperature of 400 ° C. for 200 hours in an N ₂ atmosphere furnace.
An ohmic electrode having a l / Ti / Au structure was formed on the n-type GaN layer. This is designated as Example electrode 1. A deposited
Al / Ti / Au in the same manner as in Example electrode 1 except that the thicknesses of l and l were 100 nm and 300 nm, respectively.
An ohmic electrode having the structure was formed on the n-type GaN layer.
The former is referred to as Example electrode 2 and the latter as Comparative example electrode 1. The pressure in the vacuum evaporation apparatus was set to 1 × 10 ⁻⁵ Pa and the thickness of Al was 20 nm.
An ohmic electrode having only an Al layer was formed on the n-type GaN layer by vapor deposition. This is designated as Comparative Example Electrode 2.

Ti was deposited to a thickness of 150 nm at a pressure in the vacuum deposition apparatus of 1 × 10 ⁻⁵ Pa to form an ohmic electrode having only a Ti layer on the n-type GaN layer. This is referred to as Comparative Example Electrode 3
And

The pressure in the vacuum deposition apparatus was set to 1 × 10 ⁻⁵ Pa, first, Al was deposited to a thickness of 20 nm, and after recovering the degree of vacuum, Ti was deposited to a thickness of 150 nm to form an ohmic electrode having an Al / Ti structure of n. Formed on the GaN layer. This is designated as Comparative Example Electrode 4. In each case, after the end of vapor deposition,
Lift-off was performed with a mixture of a stripping solution containing dichlorobenzene heated at a temperature of 90 to 120 ° C. and acetone to form an electrode pattern.

The above six types of samples were put into an electric furnace, the electrode surface was observed while increasing the furnace temperature, and after taking out the samples, a forward bias voltage (V) was applied between the two ohmic electrodes. The current (I) flowing between the two electrodes was measured to check the ohmic characteristics. The results were as follows.

(1) Example In the case of electrode 1, the furnace temperature was 60
Even at around 0 ° C., the adhesion to the n-type GaN layer was good, and no surface roughness was observed. The ohmic characteristics when heat-treated at a temperature of 600 ° C. showed perfect linearity as shown in FIG.

(2) Example In the case of the electrode 2, the temperature was 600
At ℃, slight surface roughness was observed. Then, as shown in FIG. 3, the ohmic characteristics at that time showed almost good linearity, although it was slightly inferior to the linearity in the case of the example electrode 1.

(3) In the case of the comparative example electrode 1, no discoloration or the like of the surface occurred even when the heating temperature was around 600 ° C., but the ohmic characteristics at that time were, as shown in FIG. Does not show sex. That is, this Al
In the case of the / Ti / Au electrode, the ohmic characteristics disappeared at a temperature around 650 ° C.

(4) In the case of the comparative example electrode 2, discoloration of the surface started at a heating temperature of around 400 ° C. The ohmic characteristics at that time do not show linearity as shown in FIG. That is, in the case of this Al electrode, the ohmic characteristics disappear from a temperature around 400 ° C.

(5) In the case of the comparative example electrode 3, discoloration of the surface started around a heating temperature of 550 ° C. The ohmic characteristics at that time do not show linearity as shown in FIG. That is, in the case of this Ti electrode, the ohmic characteristics disappear from around the temperature of 550 ° C.

(6) In the case of the comparative example electrode 4, discoloration of the surface started at a heating temperature of around 600 ° C. The ohmic characteristics at that time do not show linearity as shown in FIG. That is, in the case of the Al / Ti electrode, the ohmic characteristics disappear from a temperature around 600 ° C.

The following is clear from the above results.
That is, the ohmic electrode should have a three-layer structure of Al / Ti / Au.
The thickness of the l layer should be set to less than 200 nm.

Example 3, Comparative Examples 5 and 6 Various electrode materials were deposited under the following conditions on the n-type electrode forming portion of the substrate in a state where the n-type electrode could be formed. In this case, the n-type layer 2 was an n-type GaN layer.

The pressure in the vacuum deposition apparatus was set to 1 × 10 ⁻⁵ Pa, first, Al was deposited to a thickness of 20 nm, and after recovering the degree of vacuum, Ti was deposited to a thickness of 150 nm. Au was deposited to a thickness of 20 nm, and an ohmic electrode having an Al / Ti / Au structure was formed on the n-type GaN layer to obtain a sample.

Next, this sample was set in an N ₂ atmosphere furnace in which N ₂ substitution was performed at room temperature for 10 minutes, and heat-treated at 400 ° C., 500 ° C., and 600 ° C. while flowing N ₂ gas. Was done.

By changing the processing time at each processing temperature,
The state of the electrode surface at that time was observed.

When the processing temperature was 400 ° C., no surface roughness was observed even when the processing time exceeded 300 hours, but there were some places where the ohmic characteristics were partially deteriorated. When the treatment time was from 180 to 240 hours, the surface was not rough, and the ohmic characteristics of the treated sample for 240 hours showed excellent linearity as shown in FIG.

When the treatment temperature was 500 ° C., the surface was not rough and the ohmic characteristics were good until the treatment time was about 10 hours. However, when the treatment time was 20 hours or more, surface roughness due to slight volatilization of Ti began to be recognized, and the ohmic characteristics at that time also had significantly reduced linearity as shown in FIG. .

When the treatment temperature was 600 ° C., the surface roughness was small and the ohmic characteristics were good until the treatment time was about 40 minutes. However, when the treatment time was 1 hour and 20 minutes or more, the surface became rough due to the apparent volatilization of Ti, and at that time, the current stopped flowing as shown in FIG.

From the above, when heat treatment is performed on an electrode having a three-layer structure of Al / Ti / Au in an N ₂ atmosphere, the treatment temperature is 400 to 700 ° C. and the treatment time is 3 minutes to 3 minutes. It turns out that 250 hours should be set.

Example 4 and Comparative Example 7 Various electrode materials were vapor-deposited under the following conditions on the n-type electrode forming portion of the substrate in a state where the n-type electrode can be formed. In this case, the n-type layer 2 was an n-type GaN layer.

The pressure in the vacuum deposition apparatus connected to a hydrogen chamber equipped with a heater was set to 1 × 10 ⁻⁵ Pa,
1 was deposited to a thickness of 20 nm, and after recovering the degree of vacuum, Ti was deposited to a thickness of 150 nm. Then, the wafer was transferred to a hydrogen chamber and left in hydrogen gas for 10 minutes, and then heated to 300 ° C. and left for 3 minutes. After cooling to room temperature,
It was returned to the 10 ^-5 Pa vacuum deposition apparatus, where Au was deposited to a thickness of 20 mm.
nm to form an ohmic electrode of Al / Ti / Au structure with n
Formed on the GaN layer. This is designated as Example electrode 4.

On the other hand, an electrode having the same structure as that of the embodiment electrode 4 except that the hydrogen gas treatment was not performed was replaced with n-type GaN.
Formed on the layer. This is designated as Comparative Example Electrode 7. These were put into an electric furnace to raise the furnace temperature, and the electrode surface was observed and the ohmic characteristics were examined. The results are as follows. In the case of Example electrode 4, discoloration of the electrode surface was observed when the furnace temperature was around 750 ° C. However, even at 950 ° C., the ohmic characteristics showed fairly good linearity as shown in FIG.

On the other hand, in the case of the comparative example electrode 7, the discoloration of the electrode surface also starts around 750 ° C., and the ohmic characteristic at the temperature of 850 ° C. has poor linearity as shown in FIG. Had become.

As is clear from the above results, Al / T
When an electrode having an i / Au structure is formed, if the hydrogen gas treatment is performed at the time when the Al / Ti structure is formed, it is understood that the finally obtained electrode has an ohmic characteristic that is unlikely to cause thermal deterioration. .

[0060]

As is clear from the above description, according to the present invention, the adhesiveness is excellent on a III-V nitride-based compound semiconductor represented by GaN, and even at high temperatures. An ohmic electrode exhibiting ohmic characteristics can be formed. Since the uppermost layer of the ohmic electrode is an Au layer, the contact resistance with the Au lead wire is small.
Therefore, the industrial value of the ohmic electrode of the present invention is great as an electrode for various semiconductor devices composed of a group III-V nitride compound semiconductor.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing an example of a device on which an ohmic electrode of the present invention is formed.

FIG. 2 is a graph showing ohmic characteristics of Example electrode 1.

FIG. 3 is a graph showing ohmic characteristics of an example electrode 2.

FIG. 4 is a graph showing ohmic characteristics of a comparative example electrode 1;

FIG. 5 is a graph showing ohmic characteristics of a comparative example electrode 2;

FIG. 6 is a graph showing ohmic characteristics of a comparative example electrode 3;

FIG. 7 is a graph showing ohmic characteristics of a comparative example electrode 4;

FIG. 8 is a graph showing ohmic characteristics of Example electrode 3.

FIG. 9 is a graph showing ohmic characteristics of a comparative example electrode 5;

FIG. 10 is a graph showing ohmic characteristics of a comparative example electrode 6;

FIG. 11 is a graph showing ohmic characteristics of Example electrode 4.

FIG. 12 is a graph showing ohmic characteristics of a comparative example electrode 7;

[Explanation of symbols]

REFERENCE SIGNS LIST 1 substrate 2 n-type layer (n-type III-V nitride compound semiconductor layer) 3 ohmic electrode 3 a Al layer 3 b Ti layer 3 c Au layer 4 insulating film

Continued on the front page F term (reference) 4K029 AA04 BA03 BA05 BA17 BB02 BD02 CA01 EA01 GA01 GA05 HA00 4M104 AA01 AA04 AA07 BB02 CC01 DD07 DD34 DD68 DD78 DD79 FF13 FF22 GG04 GG06 GG11 GG12 HH08 HH15 HH20

Claims (5)

[Claims] 1. An ohmic electrode formed on an n-type layer made of a group III-V nitride compound semiconductor, wherein an Al layer, a Ti layer, and an Au layer are laminated in this order. 2. The ohmic electrode according to claim 1, wherein said Al layer has a thickness of less than 200 nm. 3. An Al layer, a Ti layer, and an Au layer are laminated in this order on an n-type layer made of a group III-V nitride compound semiconductor, and then heated to a temperature of 400 to 7 in a nitrogen atmosphere.
A method for forming an ohmic electrode, comprising performing heat treatment at 00 ° C. for 3 minutes to 250 hours. 4. An Al layer and a Ti layer are laminated in this order on an n-type layer made of a group III-V nitride-based compound semiconductor, and then, at a temperature of 300 to 600 in a reducing gas atmosphere.
A method for forming an ohmic electrode, comprising: performing heat treatment at 1 ° C. for 1 minute to 250 hours, and then laminating an Au layer on the Ti layer after the heat treatment. 5. The method according to claim 4, wherein the reducing gas atmosphere is a hydrogen gas atmosphere.