JPH098351A - Electrode for n-type nitride semiconductor layer - Google Patents

Abstract

PURPOSE: To obtain a stabilized and deterioration resistant electrode which can provide good ohmic contact with an n-type nitride semiconductor layer on which a wire can be bonded firmly thereto. CONSTITUTION: The ohmic electrode to be formed on an n-type nitride semiconductor layer has double layer structure where titanium and/or gold is laminated on indium or tin on the side touching the n-type nitride semiconductor layer or double layer structure where chromium and/or niobium is laminated on titanium on the side touching the n-type nitride semiconductor layer.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a nitride semiconductor (In _X Al _Y Ga) used for light emitting devices such as LEDs and LDs, photodiodes, light receiving devices such as solar cells, and the like.
_1-XY N, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1), and particularly to an ohmic electrode formed on an n-type nitride semiconductor layer required for those devices.

[0002]

2. Description of the Related Art Nitride semiconductors have a band gap of 1.9.
Since eV to 6.0 eV, it is known that it can be used as a material for light emitting devices and light receiving devices in the ultraviolet to red region, and recently, high-luminance blue LE using this material.
D, blue-green LED has been put to practical use, and already full color LE
It is used in practical applications such as D displays and signal lights.

FIG. 9 shows the structure of a conventional LED element. Basically, on the sapphire substrate 51, an n-type contact layer 52 made of n-type GaN, an n _- type cladding layer 53 made of n _- type Al _X Ga _{1 -X} N, and an activity made of In _Y Ga _{1 -Y} N. Layer 54 and p-type cladding layer 5 made of p _- type Al _x Ga _{1 -x} N
5 and a p-type contact layer 56 made of p-type GaN are stacked, and a positive electrode 57 containing Ni and Au is formed on almost the entire surface of the p-type contact layer 56 and is etched. N-type contact layer 52 exposed by
A negative electrode 58 containing Ti and Al is formed on the. The positive electrode 57 and the negative electrode 58 of this LED element are connected to external leads by wire bonding.

In general, a nitride semiconductor device including an LED element requires a favorable ohmic contact between the compound semiconductor layer and the electrode in order to lower the forward voltage. Also in the LED device having the above structure, ohmic contact is obtained with the positive electrode 57 containing Au and Ni with the p-type contact layer 56, and ohmic contact with the negative electrode 58 containing Ti and Al with the n-type contact layer 52. Has been obtained.

[0005]

Although the negative electrode containing Ti and Al has excellent ohmic properties, it has a slightly unstable element with respect to the adhesion to the ball and is still insufficient. I was not satisfied. Especially outdoors L
Considering use under harsh conditions such as ED and laser diode where the temperature difference is large, the electrode contains Al. Therefore, the deterioration of Al causes the ball to peel off from the electrode.
The ED may not light up. In addition, nitride semiconductors are often grown on an insulating substrate, and the device has a flip-chip type in which two types of electrodes are taken out from the same side of the nitride semiconductor layer. Therefore, when connecting to an external lead electrode by wire bonding, it is necessary to prevent deterioration of the electrode due to oxidation or the like and increase the adhesive force between the electrode and the ball to further enhance the reliability of the light emitting element.

Therefore, the present invention has been made in view of the above circumstances, and provides an electrode which is capable of obtaining a preferable ohmic contact with an n-type nitride semiconductor layer and which can be firmly adhered to a wire and is stable and resistant to alteration. To provide.

[0007]

[Means for Solving the Problems] We have previously disclosed Japanese Patent Laid-Open No. 5-2.
In Japanese Patent No. 916211, Al, Cr, Ti and I are used as preferable ohmic electrodes formed on an n-type nitride semiconductor.
It has been disclosed therein that at least one metal of n is shown, especially Cr based, ie on the side in contact with the n-type layer, which gives a preferred ohmic contact. Next, as a result of detailed experiments on the stability of the electrode and the adhesiveness with other metals, we have completed the present invention. The electrode of the n-type nitride semiconductor layer of the present invention has two types of modes. The first mode is an ohmic electrode formed on the n-type nitride semiconductor layer, wherein the electrode is an n-type nitride semiconductor. The side in contact with the layer is made of In or Sn, and Ti is formed on it.
And / or at least a two-layer structure in which Au is laminated.

A second aspect of the present invention is an ohmic electrode formed on an n-type nitride semiconductor layer, wherein the electrode is made of Ti on the side in contact with the n-type nitride semiconductor layer, and In and It is characterized by having at least a two-layer structure in which / or Sn is laminated. In the first and second aspects of the present invention, the two-layer structure means In, Ti and S.
It goes without saying that two kinds of metals such as n and Ti and In and Au are directly contacted and stacked, and two kinds of metals may be stacked separately to have a structure of two or more layers,
For example, in the first embodiment, Ti and Au may be laminated on In via another metal.

In the first and second aspects of the present invention, a metal thin film of In, Sn, and Ti formed on the side in contact with the n-type layer (hereinafter, the metal thin film formed on the side in direct contact with the n-type layer is Thin film), the easier it is to obtain ohmic contact with the n-type layer, preferably 10 angstrom to 50 angstrom.
The film thickness is adjusted to 0 angstrom, more preferably 10 angstrom to 300 angstrom.

Next, Ti and / or / formed on the first thin film
Or a thin film of Au laminated, or In and / or Sn
The thickness of the metal thin film (hereinafter, the metal thin film formed on the first thin film is referred to as the second thin film) is not particularly limited, but is thicker than the first thin film formed previously. If formed, for example, with a film thickness of about 20 angstroms to 2 μm, an n-type layer and preferable ohmic properties can be obtained, and an electrode stable to heat can be obtained. N of the present invention
Although some electrodes can obtain ohmic properties without being annealed after laminating the second thin film, more preferable ohmic properties can be obtained by annealing at 400 ° C. or higher. The reason is as follows.

Generally, a nitride semiconductor has an n-type property because nitrogen vacancies are formed in the crystal in a non-doped state.
Further, it is known that more preferable n-type is obtained by adding n-type impurities such as Si and Ge during the growth. further,
The gallium nitride compound semiconductor is a metal organic vapor phase epitaxy method (M
OCVD, MOVPE), hydride vapor phase epitaxy (H
It is grown using a vapor phase growth method such as DCVD). In the vapor phase growth method, for example, trimethylgallium as a gallium source, a compound containing a hydrogen atom such as ammonia or hydrazine as a nitrogen source, or a gas such as H ₂ is used as a carrier gas. These gases containing hydrogen atoms are thermally decomposed during the growth of the gallium nitride-based compound semiconductor and taken into the crystal, and combine with nitrogen vacancies or n-type dopants such as Si and Ge to inhibit the action as a donor. are doing. Therefore, it is considered that by annealing at 400 ° C. or higher, the hydrogen atoms that have entered the crystal can be expelled, and the n-type dopant is activated to increase the electron carrier concentration and facilitate ohmic contact with the electrode. The action of hydrogen by annealing is similar to that described in Japanese Patent Application Laid-Open No. 5-183189, which was previously filed by the present inventors. This publication shows that a gallium nitride-based compound semiconductor doped with a p-type dopant is annealed at 400 ° C. or higher. The resistivity gradually began to drop and was about 70
It shows that the resistivity becomes constant at 0 ° C. or higher. When this is applied to the n-type layer of the present application, hydrogen starts to escape at 400 ° C. or higher and the resistivity decreases. However, unlike the p-type layer, the n-type layer does not show a sharp decrease in resistivity, and has a resistivity of about ½ at 600 ° C. or higher, and has a substantially constant resistivity at an annealing temperature higher than that. The upper limit of the annealing temperature is not particularly limited, but is the temperature at which the nitride semiconductor decomposes, 1
It is preferably performed at 200 ° C. or lower. Although the two-layer structure and the three-layer structure electrodes are alloyed by annealing to be in a completely integrated state, it is defined in the claims of the present application to include alloyed and integrated electrodes.

Next, in the first and second aspects of the present invention, the uppermost layer of the n-electrode is made of Au. As a specific combination, for example, In-Au, In-Ti-Au, In-Sn-Ti-A in the first embodiment.
u, Sn-Au, Sn-Ti-Au, Sn-In-Ti
-A metal thin film can be formed like Au. On the other hand, in the second aspect, Ti-In-Au, Ti-Sn-Au, Ti-
By combining a metal thin film such as In-Sn-Au, n
It is possible to obtain an electrode having a strong ohmic force with respect to a wire as well as obtaining a preferable ohmic contact with the mold layer. The film thickness of Au formed in the uppermost layer is not particularly limited, but it is desirable that the film is formed to be thicker than the first thin film like the second thin film, for example, 20 angstrom to 10 μm.
It is desirable to form the film with the thickness before and after. It is possible to stack these metal thin films to form an electrode by using a vapor phase film forming apparatus such as vapor deposition or sputtering.

Next, in the electrode of the present invention, n
The surface of the type nitride semiconductor layer is etched. Since the nitride semiconductor is often grown on the insulating substrate as described above, the n-type layer rarely appears on the outermost surface. That is, an n-type layer is often formed on a substrate, and a light-emitting layer and a p-type layer are laminated on the n-type layer, which is often an element structure. Therefore, as shown in FIG. 9, the n-type layer is etched to expose its surface. Compared to the surface of the n-type layer immediately after growth, the surface of the n-type layer after etching has a preferable ohmic property because the nitride semiconductor surface is invaded by the gas, liquid, etc. used for etching to cause etching damage. Hard to get. However, the electrode of the present invention can obtain a preferable ohmic contact even with an n-type layer damaged by etching.

As a concrete etching means, either wet etching or dry etching is used. For example, in wet etching, a mixed acid of phosphoric acid and sulfuric acid,
Means such as reactive ion etching and ion milling can be used in dry etching, CF ₄ , SF ₆ , CCl ₄ , SiCl _{4 and} other gases are used in reactive ion etching, and Ar, N _{2 and the} like in ion milling. Inert gas can be used. There are many other devices for dry etching, and the type of gas used differs depending on each device.

The above-mentioned electrodes and preferable ohmic properties can be obtained for all n-type nitride semiconductors.
An n-type nitride semiconductor doped with a donor impurity such as Ge is preferable, and among them, an n-type In _X Ga _1-X N (0 ≦ X ≦ 1) doped with a donor impurity such as Si or Ge exhibits a preferable ohmic property. .

[0016]

1 to 7 are current-voltage straight lines showing the ohmic characteristics of the n-electrode of the present invention. In these figures, a Si-doped n-type GaN layer is grown to 5 μm on the surface of a sapphire substrate and then etched to a thickness of 4 μm to damage the surface, and the metal shown in the figure is sequentially deposited on the n-type layer. Then, the state of not annealing, 400 ℃, 500 ℃, 6
The current-voltage characteristics between the electrodes when they are respectively annealed at 00 ° C. are shown for comparison. In FIG.
03 μm and Au were laminated in this order in the order of 0.3 μm.
0.03 μm, Ti 0.03 μm, Au 0.3 μm, FIG. 3 is Sn 0.03 μm, Au 0.3 μm, and FIG. 4 is Sn.
0.03 μm, Ti 0.03 μm, Au 0.3 μm, and FIG. 5 shows Ti 0.03 μm, Sn 0.03 μm, Au 0.3
μm, FIG. 6 shows Ti 0.03 μm, Sn 0.03 μm, T
i 0.03 μm, Au 0.3 μm, FIG. 7 shows Ti 0.03
μm, In0.03 μm, Ti0.03 μm, Au0.
It shows an electrode in which a film having a film thickness of 3 μm is sequentially laminated.

1 to 4 show an electrode according to the first embodiment of the present invention, and FIG. 1 shows an electrode made of In-Au. As shown in this figure, ohmic properties are not exhibited in the non-annealed state, but ohmic properties begin to be exhibited by annealing at 400 ° C. or higher, and preferable ohmic properties are exhibited at 600 ° C. FIG. 2 shows In-Ti-Au.
The electrode is composed of In, and by stacking Ti on In, a preferable ohmic property is shown even in a non-annealed state. By annealing, the contact resistance is lowered and a more preferable ohmic property is obtained. Figure 3 is S
An electrode made of n-Au is shown, and this electrode is also In-.
Similar to Au, it does not exhibit ohmic properties in the non-annealed state, but it exhibits favorable ohmic properties by annealing. FIG. 4 shows an electrode made of Sn-Ti-Au. By stacking Ti on Sn, this electrode also shows a preferable ohmic property in a non-annealed state, and an electrode having a low contact resistance can be obtained by annealing.

FIGS. 5 to 7 show an electric power source according to the second embodiment of the present invention.
Fig. 5 shows electrodes, and Fig. 5 shows electrodes made of Ti-Sn-Au.
It is. This electrode is preferable even in the non-annealed state.
It exhibits a high resistance and the contact resistance is reduced by annealing.
Furthermore, a preferable electrode can be obtained. FIG. 6 shows Ti-Sn-Ti.
Although it is an electrode made of -Au, this electrode is also a non-aluminum.
Shows ohmic property in neal state, and further annealing
A preferred electrode is obtained. FIG. 7 shows Ti-In-Ti-A.
Although the electrode is made of u, this electrode is non-annealed.
Does not have ohmic properties, but is better
A good ohmic is obtained. In addition, conventional electrodes
In some cases, a mirror-uniform surface, such as that after nitride semiconductor growth, is used.
Ohmic resistance is obtained, and conversely, etching of nitride semiconductor is performed.
There are things that you can't get ohmic for
However, the electrode of the present invention does not correspond to the surface of the etched n-type layer.
Even so, a very favorable ohmic contact is obtained. What
This graph shows that for Si-doped n-type GaN
However, Si-doped In _XGa_1-XAbout N (X ≠ 0)
However, it was confirmed that the same tendency was obtained.

Next, in order to investigate the adhesive strength between the electrode of the present invention and the ball, the following test was conducted in comparison with the conventional electrode. FIG. 8 is a sectional view of an electrode showing the test method, and the test method is as follows.

First, on the etched n-type layer 21, 100 n-electrodes each having a combination of the metal thin films shown in FIGS. 1 to 7 are formed with a size of 120 μmφ and annealed at 600 ° C. Then, the n-electrode 22 was formed. After forming the n-electrode 22, 60 as a forced oxidation test
The electrode surface is left to stand in a high temperature and high humidity chamber at 80 ° C and 80% RH for one day to oxidize the electrode surface.
Wire bonding 4 to 100 μmφ ball 2
The gold wire 24 was connected by forming 3. afterwards,
As shown in FIG. 4, the ball 23 is scratched horizontally from the side of the ball 23 with the blade 25 so that the ball is n-electrode 2
Evaluation was made by applying a load to the blade 25 until the ball peeled from No. 2 or the ball collapsed without peeling. However, the film thickness of the metal thin film in each electrode was the same as the above film thickness. Table 1 shows the results. In Table 1, the numerical value at each load shows the number of balls peeled from the electrode out of 100, and the one that is crushed without peeling is described as "crushed".

[0021]

[Table 1]

As shown in Table 1, in the electrode containing only Ti--Al, all the balls were peeled off under a load of up to 30 g due to the oxidation of Al, whereas the electrode of the present invention contained 30 g. It withstands the above loads sufficiently and shows very strong adhesive strength without the balls peeling. This is A
It is shown that the electrode formed before the lamination of u is not easily altered.

[0023]

【Example】

[Example 1] On a sapphire substrate of 2 inches φ, Ga
N buffer layer, Si-doped n-type GaN contact layer, S
i-doped n-type GaAlN cladding layer, non-doped InG
aN active layer, Mg-doped p-type GaAlN cladding layer, M
A wafer having a double hetero structure in which a g-doped p-type GaN contact layer is sequentially stacked is prepared.

Next, part of the p-type GaN contact layer of the wafer is etched in the depth direction to expose the n-type GaN contact layer layer on the surface so that one chip has a sectional structure as shown in FIG. Let After applying a mask of a predetermined shape on the n-type GaN layer, In was added to 0.05 μm and Ti was added.
Of 0.05 μm and Au of 1.0 μm in thickness are sequentially deposited to form an n-electrode.

After the n-electrode is formed, the mask is removed, the mask is formed again on the surface of the nitride semiconductor, and then the p-type Ga is formed.
Ni on the surface of the N contact layer as a p electrode 0.1 μm
Then, Au is vapor-deposited with a film thickness of 0.5 μm.

After forming the p-electrode, the mask is removed, the wafer is placed in an annealing apparatus, and the wafer is placed in an inert gas atmosphere at 60.
Anneal for 5 minutes at 0 ° C. After annealing, the current-voltage characteristics between the n-electrodes were measured with a wafer prober,
The ohmic characteristics as shown at 600 ° C. in FIG. 2 were obtained.

Next, this wafer was separated into chips by a conventional method, and 15,000 chips were obtained from a 2-inch φ wafer. A light emitting chip made of the nitride semiconductor thus obtained is die-bonded and mounted on a lead frame, and then gold wires are connected to each electrode by a wire bonder, and then the whole is molded with an epoxy resin to form an LED element. And

This LED element has a forward current If of 20 mA.
, The forward voltage is 3.5 V, and
100 pieces were randomly sampled from D element, and a continuous repeated test of lighting at room temperature for 12 hours and lighting at 60 ° C. and 80% RH in a high temperature and high humidity tank for 12 hours was performed 50 times. None of the LEDs turned off.

[Embodiment 2] In Embodiment 1, the n-type Ga is used.
The electrode formed on the surface of the N contact layer is made of Ti0.05
An LED device was obtained in the same manner except that the film thicknesses of μm, Sn were 0.05 μm and Au was 1 μm in this order. These L
At the measurement stage of the wafer prober, the ED element has obtained ohmic contact as shown at 600 ° C. in FIG. 5, and shows the performance of Vf3.5V at If20mA.
Also, in the continuous repeated test of the LED, none of the LEDs turned off due to the peeling of the ball of the n-electrode.

[Embodiment 3] The wafer used in Embodiment 1 is the same as the wafer used in Embodiment 1 except that the n-type AlGaN layer clad layer is removed and the n-type GaN contact layer is replaced with a Si-doped n-type InGaN contact layer. To produce an LED element. Of course, the n-type electrode is this n-type InGaN.
It is needless to say that the band gap of the contact layer formed on the etched surface of the contact layer is larger than that of the active layer. As a result, these LED elements obtained the same ohmic contact as in Example 1 at the measurement stage with a wafer prober, exhibited Vf3.5V performance at If20 mA, and were also subjected to continuous LED repeated tests. ,
There was no non-lighting due to the peeling of the n-electrode ball.

[0031]

As described above, since the n-electrode of the present invention can obtain a very favorable ohmic characteristic with the n-type nitride semiconductor layer, a light emitting device such as an LED device or an LD device having a low forward voltage can be obtained. be able to. Furthermore, even when the light emitting device is used under severe conditions, the electrodes are unlikely to deteriorate, so that it is possible to provide a device that is resistant to peeling and excellent in reliability. Furthermore, not only the light emitting element but also the light receiving element, F
It is also applicable to any electronic device having an n-type nitride semiconductor such as ET and transistor.

Although the ohmic property of the n-electrode and the adhesive property at the time of wire bonding have been described in the present specification, the n-electrode of the present invention is not limited to a light emitting device connected to an external lead electrode by wire bonding, and may be, for example, a lead frame. It is also applicable to a device in which the n-electrode and the n-electrode are directly connected via a conductive material such as silver paste.

[Brief description of drawings]

FIG. 1 is a graph showing current-voltage characteristics of an n-electrode according to an example of the present invention.

FIG. 2 is a graph showing current-voltage characteristics of an n-electrode according to an example of the present invention.

FIG. 3 is a graph showing current-voltage characteristics of an n-electrode according to an example of the present invention.

FIG. 4 is a graph showing current-voltage characteristics of an n-electrode according to an example of the present invention.

FIG. 5 is a graph showing current-voltage characteristics of an n-electrode according to an example of the present invention.

FIG. 6 is a graph showing current-voltage characteristics of an n-electrode according to an example of the present invention.

FIG. 7 is a graph showing current-voltage characteristics of an n-electrode according to an example of the present invention.

FIG. 8 is a schematic cross-sectional view of an electrode showing a method for testing an electrode according to an example of the present invention.

FIG. 9 is a schematic cross-sectional view showing the structure of a nitride semiconductor light emitting device.

[Explanation of symbols] 21 ... N-type layer 22 ... N electrode 23 ... Ball 24 ... Gold wire 25 ... Blade

Claims (4)

[Claims] 1. An ohmic electrode formed on an n-type nitride semiconductor layer, wherein the electrode is made of indium or tin on the side in contact with the n-type nitride semiconductor layer, and titanium and / or gold is laminated thereon. An electrode of an n-type nitride semiconductor layer, which has at least a two-layer structure. 2. An ohmic electrode formed on an n-type nitride semiconductor layer, wherein the electrode is made of titanium on the side in contact with the n-type nitride semiconductor layer, and at least two layers are formed by laminating indium and / or tin thereon. An electrode of an n-type nitride semiconductor layer having a layered structure. 3. The electrode of the n-type nitride semiconductor layer according to claim 1, wherein gold is laminated on the uppermost layer of the electrode. 4. The surface of the n-type nitride semiconductor layer is etched, and the surface of the n-type nitride semiconductor layer is etched.
An electrode of the n-type nitride semiconductor layer according to 1.