III nitride compound semiconductor element an electrode...

Details III nitride compound semiconductor element an electrode... III nitride compound semiconductor element an electrode...

2003-02-19

2004-10-19

Kang, Donghee (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Combined with electrical contact or lead

Of specified material other than unalloyed aluminum

C257S081000, C257S094000, C257S103000, C257S744000, C438S602000, C438S604000

Reexamination Certificate

active

06806571

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a device including an electrode which has low contact resistance to a p-type Group III nitride compound semiconductor, and which does not occur any chemical reaction such as oxidation as time passes. The present invention also relates to a method for forming the electrode. As used herein, the term “Group III nitride compound semiconductor” refers to a semiconductor generally represented by the following formula: Al
x
Ga
y
In
1-x-y
N (0≦x≦1, 0≦y≦1, 0≦x+y≦1), and examples thereof include binary semiconductors such as AlN, GaN, and InN; ternary semiconductors such as Al
x
Ga
1-x
N, Al
x
In
1-x
N, and Ga
x
In
1-x
N (in each case, 0<x<1); and quaternary semiconductors represented by the following formula: Al
x
Ga
y
In
1-x-y
N (0<x<1, 0<y<1, 0<x+y<1). Unless otherwise specified, in the present specification, the term “Group III nitride compound semiconductor” also includes p-type or n-type Group III nitride compound semiconductors doped with an impurity.
BACKGROUND ART
Group III nitride compound semiconductors have direct transition and whose emission spectrum can be changed over a wide range from UV to red when used in a device such as a light-emitting device. Therefore Group III nitride compound semiconductor have been used in light-emitting devices such as a light-emitting diodes (LEDs) and laser diodes (LDs). In addition, since a Group III nitride compound semiconductor has a wide band gap, a device employing the semiconductor is considered to be operated reliably at high temperature, as compared with a device employing a semiconductor other than a Group III nitride compound semiconductor. Therefore, applying group III nitride compound semiconductors to electron devices including an FET, have been developed. Moreover, because arsenic (As) is not contained in Group III nitride compound semiconductors as a main constitution element, application of the semiconductors to the various semiconductor devices has been expected from the environmental viewpoint.
In general, when a metallic layer is merely formed on the surface of a compound semiconductor, ohmic contact between the metallic layer and the compound semiconductor fails to be obtained. Therefore, conventionally, the ohmic contact can be obtained by thermal treatment of the sample to diffuse the metal in the semiconductor. In the case of a p-type Group III nitride compound semiconductor, a resistivity of the p-type Group III nitride semiconductor is not reduce to the same level of that of n-type semiconductor, even the sample is taken heat treatment process or electron beam irradiation process. Therefore, the current does not spread in a lateral direction in the p-type layer, but flows just below electrade. Accordingly light is emitted merely from a portion directly beneath the electrode. To solve this problem, there has been proposed a current-diffusing electrode which is formed by laminating a nickel (Ni) layer (thickness: some hundreds Å) and a gold (Au) layer (thickness: some hundreds Å) and performing heat treatment thereafter, which exhibits light transmittance and ohmic characteristics (Japanese Patent Application Laid-Open (kokai) No. 6-314822). However, this electrode of two-layer structure including a nickel (Ni) layer and a gold (Au) layer has a contact resistivity as high as 2×10
−3
&OHgr;cm
2
when the electrode contacts with a p-type Group III nitride compound semiconductor but the resistivity is still high. Therefore, a Group III nitride compound semiconductor device having this electrode still has a high operation voltage.
DISCLOSURE OF THE INVENTION
The present inventors have previously applied for a patent regarding an invention related to a p-electrode including a titanium (Ti) layer and a tantalum (Ta) layer (Japanese Patent Application No. 10-202697). This p-electrode is superior to the aforementioned electrode of two-layer structure including a nickel (Ni) layer and a gold (Au) layer in terms of initial contact resistivity, but there still remains room for improvement of the p-electrode. That is, when the p-electrode including a titanium (Ti) layer and a tantalum (Ta) layer is exposed to air for one week, the contact resistance of the electrode increases by a factor of about 1,000. The reason for the increase in contact resistance is thought to be as follows: the two-metallic-layer electrode is oxidized by oxygen and moisture contained in air; or a metal nitride is formed by nitrogen contained in a Group III nitride compound semiconductor which is in contact with the electrode. As a result, the ohmic contact can not keep low contact resistance.
The present invention has been accomplished in order to solve the aforementioned problems. An object of the present invention is to provide an electrode which has low contact resistance to a p-type Group III nitride compound semiconductor, and which does not occur any chemical reaction such as oxidation as time passes.
In order to solve the aforementioned problems, a first feature of the invention can be employed. Through use of this feature, a member selected from the group consisting a titanium nitride (TiN
x
) electrode, a tantalum nitride (TaN
x
) electrode, and a tantalum titanium nitride (Ta
y
Ti
1-y
N
z
) electrode is formed on a p-type Group III nitride compound semiconductor. The titanium nitride (TiN
x
) electrode, tantalum nitride (TaN
x
) electrode, or tantalum titanium nitride (Ta
y
Ti
1-y
N
z
) electrode formed on the p-type Group III nitride compound semiconductor is not oxidized by oxygen or moisture contained in air, and is not chemically reacted with nitrogen (N) atoms contained in the Group III nitride compound semiconductor which is in contact with the electrode. Therefore, the characteristics of the titanium nitride (TiN
x
) electrode, tantalum nitride (TaN
x
) electrode, and tantalum titanium nitride (Ta
y
Ti
1-y
N
z
) electrode do not vary as time passes. A reduction of the contact resistance greatly contributes to suppression of generating heat in a semiconductor device, and improving the life time of the device.
A second feature of the invention includes heat treatment of a member selected from the group consisting the tantalum nitride (TaN
x
) electrode, titanium nitride (TiN
x
) electrode, and tantalum titanium nitride (Ta
y
Ti
1-y
N
z
) electrode at 700 to 1,000° C. after deposition of the electrode. Because the tantalum nitride (TaN
x
) electrode, titanium nitride (TiN
x
) electrode, or tantalum titanium nitride (Ta
y
Ti
1-y
N
z
) electrode is alloyed with the Group III nitride compound semiconductor which is in contact with the electrode through this heat treatment, the value of the contact resistance can be further reduced.
A third feature of the invention is a method for forming a p-electrode of a device including a p-type Group III nitride compound semiconductor, the method comprising forming the p-electrode made of a member selected from the group consisting tantalum nitride (TaN
x
), titanium nitride (TiN
x
), and tantalum titanium nitride (Ta
y
Ti
1-y
N
z
) by sputtering. Such metal nitride electrode can be formed readily by sputtering.
A fourth feature of the invention includes reactive sputtering by use of a mixing gas of nitrogen and a rare gas. By employment of a mixing gas of nitrogen and a rare gas, nitrogen atoms for forming a metal nitride can be readily generated, and thus such a metal nitride electrode can be formed more readily.
A fifth feature of the invention includes heat treatment at 700 to 1,000° C. after sputtering. Through heat treatment, the metal nitride electrode is alloyed with the Group III nitride compound semiconductor which is in contact with the electrode, and therefore, contact resistance can be further reduced.
The present invention can be carried out with reference to the following description.
A metal nitride electrode is preferably formed of electrically conductive titanium nitride (TiN) or tantalum nitride (TaN). The electrode may be formed of zirconium nitride (ZrN), niobiu

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