Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2001-04-24
2001-12-04
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S046000, C438S602000, C438S603000, C438S605000, C438S606000
Reexamination Certificate
active
06326294
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating an ohmic metal electrode for use in nitride compound semiconductor devices and, more particularly, to a method of fabricating a p-type ohmic metal electrode that uses Ru and RuO
x
as the cover layer. The present invention also relates to a method of fabricating an n-type ohmic metal layer that uses Ru as the diffusion barrier layer. Here, a nitride compound semiconductor refers to Al
x
In
y
Ga
1−(y+x)
N, where 0 ≦x≦1, 0≦y≦1, and x+y≦1.
2. Description of the Prior Art
Fabrication of ohmic contacts is one of the techniques that are essential in fabricating opto-electronic devices such as nitride-base blue/green and white light emitting diodes (LED's), ultraviolet (UV) detectors, and laser diodes (LD's) and electronic devices such as high electron mobility transistors (HEMT's), metal-insulator-semiconductor field effect transistors (MISFET's), and heterostructure bipolar transistors (HBT's).
Conventionally, p-type ohmic metal electrodes for use in nitride-base optical devices and electronic devices employed Au, such as in Ni/Au, as the cover layer. However, various pollutants such as oxygen in the air penetrate through the Au cover layer to reach the underlying contact layer, thereby forming an oxide pollutant layer at the junction of the contact layer and the nitride semiconductor. Thus, the structural and thermal stability of the p-type ohmic metal electrode experiences degradation during the heating process, and the electrical, thermal, and optical characteristics are deteriorated.
In the case of n-type ohmic metal electrodes, an ohmic metal electrode with a Ti contact layer has been conventionally used. However, unnecessary interaction occurred between the contact layer and the nitride semiconductor during the heating process, and thus the thermal characteristics was not good despite good electrical characteristics. In order to overcome this disadvantage, diffusion barrier layers employing Ni or Pt between Ti/Al and Au, such as in Ti/Al/Ni/Au or Ti/Al/Pt/Au, were used. However, the results were not satisfactory.
In particular, in case of Au-base p-type ohmic metal electrodes, the optical output characteristics were significantly degraded because the photons generated in the optical devices were absorbed into the Au.
One of the most important techniques that need to be developed in fabricating laser diodes having a nitride compound semiconductor substrate is to develop an ohmic metal electrode having low resistance, high light transmittance, and thermal stability. However, there has been neither prior art nor any published research regarding Au-free p-type ohmic metal electrodes.
S. J. Pearton, et al. of the U.S.A. published their research regarding fabricating an ohmic contact layer on p-type GaN by using W and WSi in the American Institute of Physics in 1998 (Applied Physics Letters 73, 942, 1998). The object of the research was to develop an ohmic contact layer having thermal stability and good electrical characteristics. However, according to the publication, thermal stability was good, but specific contact resistance determinative of electrical characteristics was 10
−2
&OHgr;cm
2
, which is worse than 10
−4
&OHgr;cm
2
, the minimum acceptable value for optical devices.
Murakami, et al. published their research regarding Ta/Ti contact system in the American Institute of Physics in 1999 (Applied Physics Letters 74, 275, 1999). According to the publication, the specific contact resistance was 10
−5
&OHgr;cm
2
, which was very good, but there was a problem of instability at room temperature.
There were also reports on various researches regarding ohmic metal electrodes having other types of structures, but there has been no research resulting in superior characteristics sufficient for use in actual devices.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a method of fabricating a p-type ohmic metal electrode that can solve the above-mentioned problems by using Ru and RuO
x
, as the cover layer.
It is another object of the present invention to provide a method of fabricating an n-type ohmic metal electrode that can solve the above-mentioned problems by using Ru as the diffusion barrier layer.
To this end, the method of fabricating an ohmic metal electrode according to one embodiment of the present invention comprises the steps of sequentially forming a contact layer and a Ru layer on a p-type nitride compound semiconductor, and carrying out a heating process in air, argon, nitrogen, and/or oxygen atmosphere. Here, the heating process in air, argon, nitrogen, and/or oxygen atmosphere is carried out in order to form a RuO
x
, layer on the surface of the Ru layer.
The method of fabricating an ohmic metal electrode according to another embodiment of the present invention comprises the steps of sequentially forming a contact layer and a Ru layer on a p-type nitride compound semiconductor, exposing the Ru layer to air, argon, nitrogen, and/or oxygen atmosphere, and carrying out a heating process. Here, the step of exposing the Ru layer to oxygen atmosphere is carried out in order to form a native RuO
x
layer on the surface of the Ru layer. The heating process is preferably carried out in air, argon, nitrogen, and/or oxygen atmosphere, but the air, argon, nitrogen, and/or oxygen atmosphere is not absolutely necessary since a native RuO
x
layer has already been formed before heating process.
The method of fabricating an ohmic metal electrode according to still another embodiment of the present invention comprises the steps of sequentially forming a contact layer, a metallic layer, and a RuO
x
layer on a p-type nitride compound semiconductor, and carrying out a heating process. The RuO
x
layer may be formed by sputtering, chemical vapor deposition, or molecular beam epitaxy, and there is no absolute need to form the metallic layer with Ru because the RuO
x
layer can be formed regardless of the type of the underlying layer.
In each of the above embodiments according to the present invention, the contact layer is comprised of a single-layer structure comprising Pt, Ti, Cr, Pd, Ni, Ta, W, or Mo, or a multi-layer structure comprising a combination of the single layer structures. When the contact layer is comprised of Pt, Pd, or Ni, the heating process is preferably carried out in a temperature range of 300~700° C.
The method of fabricating an ohmic metal electrode according to still another embodiment of the present invention comprises the steps of sequentially forming a contact layer, a Ru layer, and a conductive layer on an n-type nitride compound semiconductor, and carrying out a heating process.
Here, the contact layer is comprised of a single-layer structure comprising Ti, Cr, Mo, Nb, Ta, or W, or a multi-layer structure comprising a combination of the single layer structures. When the contact layer is comprised of Ti, the heating process is preferably carried out in a temperature range of 300~800° C.
REFERENCES:
patent: 6100174 (2000-08-01), Takatani
Applied Physics Letters, vol. 73, No. 7, Aug. 17, 1998, “Thermal Stability of W and WSIxcontacts on p-GaN,” pp. 942-944.
Applied Physics Letters, vol. 74, No. 2, Jan. 11, 1999, “Low-resistance Ta/Ti Ohmic contacts for p-type GaN,” pp. 275-277.
Jang Ja Soon
Park Seong Ju
Seong Tae Yeon
Foley & Lardner
Kwangju Institute of Science and Technology
Niebling John F.
Zarneke David A
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