Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Passivating of surface
Reexamination Certificate
1998-10-29
2001-05-08
Elms, Richard (Department: 2824)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Passivating of surface
C438S026000, C438S762000, C438S767000, 43, 43, C257S098000
Reexamination Certificate
active
06228672
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
REFERENCE TO A “MICROFICHE APPENDIX”
Not applicable.
BACKGROUND OF THE INVENTION
This invention relates to methods for passivating the surfaces of semiconductors. More particularly, this invention relates to methods for stabilizing such passivated surfaces from the deleterious effect of oxidation for compound semiconductors.
It has been demonstrated for most III-V compound semiconductors that sulfidation of the surface results in improved electronic properties through the reduction of mid-gap surface states that promote rapid surface carrier recombination. Treatment with selenium has also been investigated and has been shown to sometimes work somewhat better than sulfur. Calculations for tellurium on GaAs have suggested that tellurium, in the chalcogen family with sulfur and selenium, would not have the same effect on mid-gap state as S and Se and has not been used for this purpose. High surface recombination velocities and/or Fermi-level pinning due to a high density of mid-gap surface states (>
10
12
/cm
2
) have diminished the performance of heterojunction bipolar transistors (HBTs) and delayed the realization of metal-insulator-semiconductor (MIS) devices in III-V compound semiconductors. Reaction of the GaAs (or other III-V compound semiconductor) surface with sulfur or its compounds produces a dramatic decrease in the interface states responsible for surface recombination and Fermi-level pinning. This sulfidation has been accomplished in a number of ways including immersion in aqueous Na
2
S and/or NH4Sx solutions, anodic sulfidation, treatments with polythiols, exposure to H
2
S that has been thermally or plasma activated, and photosulfidation. The following references are of interest in these areas and are incorporated by reference herein in their entirety: C. J. Sandroff, R. N. Nottenburg, J. C. Bischoff, and R. Bhat, Appl. PHys. Lett. 51, 33 (1987) (treatment of HBTs with Na
2
S, instability of the S-treated surface); E. Yablonovich, C. J. Sandroff, R. Bhat, and T. Gmitter Appl. Phys. Lett. 52, 439 (1988)(treatment with NH
4
S, Li
2
S, Na
2
S): J. F. Fan, H. Oigawa, and Y. Nannichi, Jpn.J. Appl. Phys. 27, L1331 (1988) (treatment with (NH
4
)
2
S
x
); S. Shikata, H. Okada, and H. Hayashi, J. Appl. Phys. 69, 2717 (1991)((NH
4
)
2
S
x
suppression of emitter size effect on beta of HBTs; and J. S. Herman and F. L. Terry, J. Vac. Sci. Technol. A11, 1094 (1993) (treatment with H
2
S plasma). A preferred process is the photosulfidation process taught in U.S. Pat. No. 5,451,542 that utilizes UV photodissociation of sulfur vapor to form reactive S species that react with a semiconductor surface that has had the native oxide removed.
Unfortunately, the major problem with all these approaches has been the instability of the sulfided surface with respect to oxidation when exposed to air. Selenided surfaces are somewhat better but suffer from oxidation effects as well. As these surfaces re-oxidize, the density of mid-gap states rapidly returns to its original level. There have been at least two attempts to passivate these sulfidated surfaces against re-oxidation: a glow discharge in sulfur vapor with the GaAs heated to 400° C., X.Hou, X. Chen, Z. Li, X. Ding, and X. Wang. Appl. Phys. Lett. 69, 1429 (1996), and immersion in S
2
Cl
2
/CCl
4
, X. Cao, X. Hou, X. Chen, Z. Li,R. Su, X. Ding, and X. Wang, Appl. Phys. Lett. 70, 747 (1997). Despite their air stability, these processes are limited for actual device applications by either the high temperatures employed or the over-etching of GaAs by S
2
Cl
2
, which necessitates in-situ measurement of current gain to terminate the etch process at maximum device performance.
There remains an unmet need in the art for a process that easily provides for a stable passivation of a low-surface-state-density surface of a III-V semiconductor that is resistant to the deleterious effects of re-oxidation in air.
BRIEF SUMMARY OF THE INVENTION
The method of this invention is based upon the reaction of a pre-chalcogenided surface with a metal ion in solution to form a new metal-chalcogenided surface that retains the good surface electronic properties of the III-V-S surface while being more oxygen resistant than a “bare” chalcogenided surface. As will be explained more fully below, the scope of the invention is intended to encompass compound semiconductor surfaces that have been treated with sulfur, selenium or tellurium. These are the members of the chalcogen family, other than oxygen. For simplicity, most of the discussion that follows will be described in the context of a sulfidated surface. When ‘chalcogen’ or ‘chalcogenided’ are used, use of sulfur, selenium or tellurium should be understood.
The basic steps are as follows. First, the native oxide on the III-V surface is removed if necessary. Then, the non-oxidized surface is chalcogenated. When sulfur is the chalcogen employed, the surface may be sulfidated by any of the above mentioned processes, but preferably by the UV photodissociation process taught in U.S. Pat. No. 5,451,542. It is most preferable to deposit essentially a monolayer of chalcogen on the surface to replace deleterious surface electronic states with more electronically desirable ones, but less than a monolayer or multiple monolayers may also be deposited (any of which will be taken herein to mean a “near monolayer”). The final step is to immerse the chalcogenated semiconductor in a solution of a metallic (or a nonmetallic) element that can form a relatively insoluble sulfide, selenide, and/or telluride. This group of cations includes but is not restricted to the alkali metals, the alkaline earth metals, the transition metals, the lanthanides, the actinides, and the group IIB metals such as Zn and Cd. A number of other metals should also work due to their ability to assume a tetrahedral configuration coupled with their atomic radius to provide a good lattice match. The anion in solution is conveniently a sulfate, chloride or fluoride, but other anions can also be employed because the anions may not be incorporated into the final stabilized surface near monolayer. The solution preferably employs water as the solvent, but use of another solvent, such as an alcohol or other solvent that can dissolve suitable quantities of metal salts and promote formation of relatively insoluble metal sulfides, selenides, and/or tellurides, can be employed as well.
REFERENCES:
patent: 5451542 (1995-09-01), Ashby
patent: 5799028 (1998-08-01), Geels et al.
patent: 5933705 (1999-08-01), Geels et al.
Cone Gregory A.
Elms Richard
Luu Pho
Sandia Corporation
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