Active solid-state devices (e.g. – transistors – solid-state diode – Specified wide band gap semiconductor material other than... – Ii-vi compound
Reexamination Certificate
1992-06-03
2001-07-24
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Specified wide band gap semiconductor material other than...
Ii-vi compound
C257S744000, C257S461000
Reexamination Certificate
active
06265731
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to semiconductor electronic devices, and, more particularly, to an approach for making low-electrical-resistance contacts to such devices.
A semiconductor device requires external connections from the semiconductor elements to wires, leads, or other metallic conductors which connect the semiconductor elements to other parts of a circuit. The semiconductors and the metallic conductors have fundamentally different electronic structures and electronic conduction mechanisms. The result of the different conduction mechanisms is the presence of a contact potential and energy barrier when the semiconductor is placed into contact with the metal. The contact potential adds a contact resistance to the flow of electrons in the circuit, which may be substantial and adversely affect the operation of the semiconductor device and the circuit containing the device. It is therefore usually desirable to reduce the contact resistance between the semiconductor and the metal to as low a value as possible.
An ohmic contact between a semiconductor and a metal is one where the contact resistance is a independent of the direction of current flow and is negligibly small. While ohmic contacts are readily created in a number of instances, in others such contacts are made only with difficulty. It is often difficult to make a satisfactory ohmic or near-ohmic contact to a wide bandgap semiconductor, because metals typically do not have a sufficiently small work function that leads to a small energy barrier.
The II-VI semiconductor materials include those based on ZnSe, ZnTe, and alloys derived from these compounds, often with additional elements such as mercury, cadmium, or magnesium from Group II of the periodic table, manganese from Group VII, or sulfur, selenium, or tellurium from Group VI. Some of these semiconductor materials are of significant commercial interest for use in the preparation of short wavelength diode lasers, as an example. These II-VI semiconductors exhibit relatively wide bandgaps of more than 2 electron volts. It is therefore difficult to form ohmic contacts to p-type semiconductor materials of this family, and in some cases no satisfactory ohmic contact structures are known. In those instances, the full potential of the II-VI semiconductor devices cannot be reached.
There is a need for an approach for achieving ohmic contacts to II-VI semiconductors such as ZnSe, ZnTe, their alloys, and doped structures based upon these materials. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides an approach for making ohmic contacts to p-type, wide bandgap II-VI semiconductor materials such as ZnSe, ZnTe, and their alloys. The ohmic contacts retain an epitaxial relation to the semiconductor, while providing a negligible energy barrier in the metal-semiconductor interface region. The ohmic contacts may be formed with existing equipment, and in a controlled manner.
In accordance with the invention, a device is made with a metal-semiconductor ohmic contact. The device comprises a p-doped layer made from a ZnSe-, or ZnTe-based alloy. There is further provided means for making electrical contact to the p-doped layer, the means including a graded-alloy contact layer in epitaxial contact with the p-doped layer and whose bandgap varies from about that of the p-doped layer adjacent the p-doped layer to about zero at a location remote from the p-doped layer. The graded-alloy contact layer is preferably formed from either a HgZnSSe-based graded-composition alloy where the p-doped layer is a ZnSe-based alloy, or a HgZnSeTe-based graded-composition alloy where the p-doped layer is a ZnTe-based alloy.
In an important application of this approach, an active element is also provided in the device. Such a device comprises an active element including an active layer formed from group II-VI elements, an n-doped layer on one side of the active layer, and a p-doped layer on the other side of the active layer. The p-doped layer is a ZnSe-based alloy, or a ZnTe-based alloy. There is an electrical contact to the n-doped layer. There Is also means for making electrical contact to the p-doped layer, the means including a graded-alloy contact layer in epitaxial contact with the p-doped layer and whose bandgap varies from about that of the p-doped layer adjacent the p-doped layer to about zero at a location remote from the p-doped layer. The graded-alloy contact layer is preferably formed from a HgZnSSe-based graded-composition alloy where the p-doped layer is a ZnSe-based alloy, or a HgZnSeTe-based graded-composition alloy where the p-doped layer is a ZnTe-based alloy.
The graded-composition approach of the invention thus provides mercury-containing complex alloys that can be gradually changed in composition over the thickness of the contact layer to vary the bandgap from that of the II-VI semiconductor to that of the metal (zero). This gradation can be made while maintaining the lattice structure and lattice parameter of the graded-composition alloys unchanged and the same as that of the II-VI semiconductor material, at least near to the interface with the II-VI semiconductor material. An epitaxial relation is thereby retained.
REFERENCES:
patent: 4319069 (1982-03-01), Tyan
patent: 4710786 (1987-12-01), Ovshinsky
patent: 4801984 (1989-01-01), Woodall
patent: 5045897 (1991-09-01), Ahlgren
Best et al, “HgSe, . . . contact for semiconductor . . . ” Appl Phys Lttr Vol 29 #7, pp. 1133-1134, 1976.
Lenzen, Jr. G. H.
Meier Stephen D.
Raytheon Company
Schubert W. C.
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