Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
2002-01-24
2003-06-03
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
C438S604000, C438S796000
Reexamination Certificate
active
06573117
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device having a light-transmitting electrode and a pad electrode which are formed on a p-type GaN related compound semiconductor layer.
2. Description of the Related Art
In conventional compound semiconductors, an ohmic contact is obtained by depositing metals on the semiconductor surface and heating the metals to convert the same to an alloy and to cause metal diffusion into the semiconductor, because an ohmic contact is not obtainable by the mere deposition of metals.
Even when p-type GaN related compound semiconductors are subjected to a treatment for reducing resistance, e.g., irradiation with electron beams, the thus-treated semiconductors still have higher resistivities than n-type GaN related compound semiconductors. Consequently, in such p-type GaN related compound semiconductors, the p-type layer has almost no current flow in lateral directions, and only the part thereof directly beneath the electrode emitts light.
Under these circumstances, a current-diffusing electrode having light transmission properties and ohmic properties has been proposed which is formed by depositing a nickel (Ni) layer and a gold (Au) layer, each having a thickness of several tens of angstroms, (Å) and heating the metal layers (see Japanese Unexamined Patent Publication No. Hei. 6-314822).
However, the electrode formed by depositing nickel (Ni) and gold (Au) each having a thickness of several tens of angstroms and heating the metals poses a problem that the light-emitting pattern quality deteriorates with the lapse of time, resulting in an increased driving voltage. However, the electrode has satisfactory optical and electrical characteristics in the initial stage.
The reason for the quality deterioration is believed to be as follows. Since the nickel (Ni) and gold (Au) deposited layers are extremely thin, part of the nickel (Ni) is replaced by gold (Au) during the heat treatment, and the nickel (Ni) exposed on the electrode surface oxidizes to form NiO. When current is caused to flow through the electrode in this state, the NiO reacts with an OH
−
group of water present in the surrounding atmosphere to form a substance comprising NiOOH, as shown by the following scheme (1). Since NiOOH has poor wettability by gold (Au) and by the GaN related compound semiconductor, the NiOOH aggregates. As a result, light-emitting pattern quality deteriorates with the lapse of time and the contact resistance of the electrode increases. Thus, conventional art devices employing the proposed electrode are believed to deteriorate in optical and electrical characteristics.
NiO+OH
−
−NiOOH+e
−
(1)
Further, since this current-diffusing electrode is thin, a pad electrode made of Ni/Au or Au is formed thereon for bonding.
However, the conventional art device described above has insufficient adhesion between the pad electrode and the current-diffusing electrode. Hence, if the surface of the current-diffusing electrode on which a pad electrode is to be formed has been soiled, there is a problem that the finally obtained device has problems such as the peeling of the pad electrode and a poor light-emitting pattern. In addition, even if the pad electrode has satisfactory adhesion to the current-diffusing electrode, the light emission occurring in the shade of the bonding pad cannot be directly observed, unavoidably resulting in a light emission loss.
Further, there is still another problem as follows.
In conventional GaN related compound semiconductors, low-resistivity p-type conduction is not obtainable by mere doping with a p-type impurity. It has hence been proposed to impart p-type low resistance to a GaN related compound semiconductor doped with a p-type impurity by irradiating the doped semiconductor with electron beams (see Japanese Unexamined Patent Publication No. Hei. 2-257679) or by subjecting the doped semiconductor to thermal annealing (see Japanese Unexamined Patent Publication No. Hei. 5-183189). It has also been proposed to conduct the thermal annealing for imparting p-type low resistance simultaneously with alloying for forming an electrode (see Japanese Unexamined Patent Publication No. Hei. 8-51235).
However, in the method using thermal annealing described in Japanese Unexamined Patent Publication No. Hei. 5-183189, the heat treatment should be conducted at a temperature not lower than 700° C. in order to obtain a saturated low resistivity. Although this kind of semiconductor has conventionally employed aluminum as the main electrode material, use of a temperature not lower than 700° C. for electrode alloying produces problems, such as the formation of aluminum balls resulting from aluminum melting, an impaired surface state, increased contact resistance of the electrode, and wire bonding failure.
Consequently, the heat treatment for electrode alloying should be conducted at a relatively low temperature of from 500 to 600° C. It is, however, noted that the heat treatment for imparting p-type low resistance does not result in a sufficiently low resistivity when conducted at a temperature in the range of from 500 to 600° C. It has hence been necessary to conduct the heat treatment for imparting p-type low resistance and the heat treatment for electrode alloying as separate steps, respectively.
On the other hand, Japanese Patent Publication No. Hei. 8-51235 proposes to conduct the impartation of p-type low resistance simultaneously with electrode alloying by performing a heat treatment at a temperature of from 400 to 800° C. However, this method has the following problems. The impartation of p-type low resistance is insufficient in the low-temperature range where electrode alloying is achieved satisfactorily. In the high-temperature region suitable for the sufficient impartation of p-type low resistance, electrode alloying cannot be conducted satisfactorily, resulting in increased contact resistance and poor ohmic properties.
SUMMARY OF THE INVENTION
In view of the problems described above, an object of the present invention is to realize a GaN related compound semiconductor light-emitting device which has light transmission properties and ohmic properties and retains a stable light-emitting pattern and a constant driving voltage over a long period of time, and to realize processes for producing the device.
Another object of the present invention is to impart p-type low resistance to a GaN related compound semiconductor through a heat treatment so that a saturated low resistivity value can be realized using a lower temperature for the treatment.
Still another object of the present invention is to realize the impartation of p-type low resistance at a lower temperature to thereby sufficiently impart p-type low resistance and obtain an electrode having low contact resistance and satisfactory ohmic properties, even when the heat treatment for imparting p-type low resistance and that for electrode alloying are conducted as the same step.
Still another object of the present invention is to improve the adhesion between a pad electrode and a current-diffusing electrode to thereby prevent the pad electrode from peeling off and, at the same time, to form a high-resistivity region under the pad so that current flows in the current-diffusing electrode selectively through areas other than that under the pad to thereby diminish light emission under the pad and attain effective utilization of light emission.
The above-described problem is eliminated with the light-emitting device of the present invention according to a first aspect of the present invention. This light-emitting device has a p-type GaN related compound semiconductor layer having formed thereon an electrode which transmits light to the semiconductor layer and which is a metal layer comprising a cobalt (Co) alloy, palladium (Pd), or a palladium (Pd) alloy. Since the elements constituting the electrode are unsusceptible to oxidation, not only is the electrode prevented from suffering the light-emitting pattern change
Ito Jun
Koide Yasuo
Murakami Masanori
Noiri Shizuyo
Shibata Naoki
Mulpuri Savitri
Toyoda Gosei Co,., Ltd.
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