Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Of specified material other than unalloyed aluminum
Reexamination Certificate
2000-03-16
2002-04-02
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified material other than unalloyed aluminum
C257S744000
Reexamination Certificate
active
06365969
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ohmic electrode, suitable to a semiconductor device, having a base material of a p-type group III-V (group 3B-5B) semiconductor, particularly a p-type InP based semiconductor, and a semiconductor device employing the electrode.
2. Description of the Prior Art
In general, a multilayer electrode consisting of Au—Zn metals is employed as an ohmic electrode (hereinafter also referred to simply as an electrode) for semiconductor devices such as a photodiode, a laser diode or a light emitting diode, for example, having a base material of a p-type group III-V semiconductor, particularly an InP based semiconductor. In this electrode, Au is employed for the following reason:
After depositing an Au—Zn metal film, heat treatment is performed in the temperature range of 400 to 450° C. At this time, Au reacts with the InP based semiconductor base material and diffuses into the base material through a native oxide film present on the surface thereof. Consequently, electrical contact is attained between the base material and the Au metal film.
In order to attain electrical contact between the base material and the metal film as described above, a metal reacting with the base material and thermally diffusing in a high temperature range, as described above must be selected as the material forming the metal film. In the electrode, further, Zn is employed for the following reason:
Zn diffuses into the base material due to the aforementioned heating, to form a p-type doped layer of high concentration on the surface of the base material. Thus, holes can tunnel through a Schottky barrier defined on the interface between the metal film and the base material after deposition of the metal film. Consequently, Zn serves as a p-type impurity carrier for the base material.
If the metal film consists of only Au, the surface of the base material will have a low p-type carrier concentration (concentration of about 10
18
cm
−3
in general) although electrical contact between the metal and the base material is attained as described above. Thus, an ohmic electrical contact resistivity (hereinafter simply referred to as contact resistivity) characteristic is not obtained on the contact interface between the metal film and the base material. Therefore, Zn which is a p-type impurity must diffuse into the surface of the base material for improving the carrier concentration on this surface.
Attempts of providing Au/Zn/Au metal electrodes on p-type InP based semiconductor base materials for reducing contact resistivity on the interfaces therebetween are presented in some literature. Sumitomo Denki, No. 141 (September 1992), pp. 100 to 104 presents an exemplary electrode employing an impurity-free InP based semiconductor crystal for a base material. This electrode is formed by depositing fourth and fifth layers consisting of Ti/Au on first to third layers having controlled thicknesses and thereafter alloying the layers by heating the same in a nitrogen atmosphere at a temperature of 435° C. However, this electrode has such a problem that protrusions of the alloyed layers (hereinafter referred to as reaction layers) on the surface of the base material reach an operating layer of a semiconductor device, to result in a failure of the semiconductor device.
For example,
J. Electron. Mater.,
Vol. 20 (1991), p. 237 presents an electrode having a shallow reaction layer and small contact resistivity obtained by additionally stacking a Pd layer on the same system metal layers. Further,
Appl. Phys. Lett.,
Vol. 66 (1995), p. 3310 describes an electrode having a shallow reaction layer and small contact resistivity obtained by forming Ge/Pd/Zn/Pd metal layers, and
Appl. Phys. Lett.,
Vol. 70 (1997), p. 99 describes an electrode having a shallow reaction layer and small contact resistivity obtained by forming Pd/Sb/Zn/Pd metal layers.
Each of these electrodes requires heat treatment at a high temperature exceeding 400° C. In general, however, a p-type electrode and an n-type electrode are simultaneously arranged in the same semiconductor base material wafer, and when heating an Au/Ge/Ni metal film widely employed as the n-type electrode at such a high temperature, for example, the components of this film diffuse into the base material and deeply infiltrate into the base material wafer to disadvantageously deteriorate the essential practical characteristics of the wafer. While an Au layer is provided as the uppermost layer in order to suppress sheet resistance of a part connected to an external device and improve bondability of a connection wire (increase connection strength), diffusion of Au may abnormally progress to deteriorate the characteristics of the semiconductor wafer if the heating temperature in film formation exceeds 400° C. To this end, Japanese Patent Laying-Open No. 3-16176 (1991), for example, describes an attempt providing a stopper layer consisting of Ti or Cr between an Au/Zn/Au electrode and an uppermost layer of Au for suppressing excess diffusion of Au. Also in this case, however, the electrode must be heated at a high temperature of 430 to 450° C.
In order to reduce the heating temperature, transition metals must be employed for the electrode as already proposed by the inventors in Japanese Patent Application No. 10-18843 (1998), for example. The transition metals react with an oxide film formed on the surface of a base material at a lower temperature as compared with Au. When employing these metals, reaction generally progresses at a low temperature of not more than 400° C. As a result, an electrode having a low contact resistance value is obtained. Further, the aforementioned problem in the case of simultaneously arranging a p-type electrode and an n-type electrode can also be solved. In order to perform heat treatment at a relatively low temperature as described above, reactivity between the oxide film provided on the semiconductor base material and the transition metals employed for the electrode is important.
In order to bear no relation to the oxide film provided on the surface of the base material, the oxide film is previously removed in general. In this case, the oxide film is removed by chemical etching, for example, and thereafter sputtering is performed with an ion beam or plasma in a vacuum chamber, for example. Each of Japanese Patent Laying-Open Nos. 62-60218 (1987) and 58-141529 (1983) presents a method applying a molecular beam of Sb or the like onto a compound semiconductor base material for removing an oxide film from the surface of the compound semiconductor base material. In the method of removing the oxide film by chemical etching, an oxide film of about several nm is formed on the surface of the base material exposed to the atmosphere after removal of the oxide film, and hence application of a molecular beam is required in any case. However, the method employing sputtering with an ion beam or plasma is undesirable since charged particles damage the surface of the base material to deteriorate the electric characteristics thereof. Such influence by the charged particles is remarkable particularly when the reaction layer has a small thickness. A similar problem arises also in the Pd/Zn/Pd electrode proposed by the inventors in Japanese Patent Application No. 10-18843. In other words, it is important to control the thickness, the composition etc. of a native oxide film formed on the surface of the base material.
In
J. Vac. Sci. Technology,
Vol.12, No. 3 (1994), p. 1419, Fischer et al. propose a method of preventing oxidation of the surface of the base material after etching. According to this proposal, the base material is dipped in an ammonium solution so that the surface chemically adsorbs sulfur (S) atoms, thereby preventing the surface from oxidation. However, the inventors have confirmed that this method is insufficient for preventing oxidation since an S atom layer is unstable. Further, the solution generates hydrogen sulfide gas or precipitates powder of a polysulfide, leading to a p
Asamizu Hirokuni
Murakami Masanori
Yamaguchi Akira
Clark Sheila V.
Fasse W. F.
Fasse W. G.
Sumitomo Electric Industries Ltd.
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