Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming multiple superposed electrolytic coatings
Reexamination Certificate
1999-02-08
2001-05-08
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming multiple superposed electrolytic coatings
C205S171000, C205S157000, C205S159000, C205S162000, C205S915000
Reexamination Certificate
active
06228243
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention is directed generally toward methods to synthesize crystalline films, superlattices and multilayered devices based on metallic or semiconductor compounds or alloys, in electrolyte media. More specifically, this invention is directed toward a method for the electrochemical synthesis of molecular layers of compounds or alloys on non-crystalline substrates.
Epitaxial semiconductor films and superlattices are currently sought for a wide range of device applications. Superlattice films can be designed with specific physical, electronic or optical properties. Material utilization, cost and environmental impacts are important concerns during superlattice manufacture. Presently such materials are synthesized with expensive, high temperature vacuum methods, molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE). ALE and MBE methods allow the fabrication of advanced material structures of very thin oriented layers of metallic alloys or semiconductor films but economics and crystalline substrate requirements limit their use as research tools or for small, highly sophisticated expensive devices such as, quantum-well lasers.
Chemical bath deposition (CBD) has been employed for low cost, large area applications; it produces low quality semiconductor films. Electrodeposition offers a simple, low-cost and large area alternative to the vacuum or CBD techniques, in terms of the required capital equipment, power needs and material. It eliminates environmental and safety hazards associated with toxic vapor phase reactants and large volumes of chemical waste. These advantages have been explored by numerous researchers with limited success. Kroger et al first reported the electrodeposition of semiconductor compounds. U.S. Pat. No. 4,426,194 describes the approach. Unfortunately stringent material quality requirements for opto-electronic devices have excluded electrodeposition as an acceptable method for synthesis of semiconductor compounds. Recent works use electrodeposition to produce mainly precursor films as described in U.S. Pat. No. 5,730,832. These films require further vacuum processing to improve stoichiometry and grain size. Thus, economic and scale up barriers of the vacuum steps remain.
Conventional electrodeposition tends to produce small grains and non-stoichiometric films that are unsuitable for devices. The deposition is controlled by the mass transport rate of at least one metal, co-depositing to form a compound. This causes three-dimensional nucleation, hence small grain films. U.S. Pat. No. 5,320,736 describes an electrochemical method for atomic layer epitaxy for the deposition of semiconductors, comprising alternating electrodeposition of atomic layers of selected pairs of elements using underpotential deposition. This method eliminates the mass transport dependence and can produce atomically ordered layer of a compound. It uses a separate solution to deposit each of the component elements, constituting the compound. The use of two or more electrolytes to synthesize one compound evidently requires an elaborate deposition apparatus, a rinse cycle between deposition of each fraction of the monolayer, large quantities of electrolyte and very long (several hours/&mgr;m) deposition period time. Thus, the method is impractical for manufacturing large devices, for example photovoltaic modules; such devices need low-cost, large-area, high-throughput deposition. For many compounds, this approach leads to the re-dissolution of one metal during deposition of the next. Nevertheless, the method provides evidence for the electrochemical epitaxial semiconductor formation.
BRIEF SUMMARY OF THE INVENTION
Accordingly, the object of this invention to provide a new method, ‘Electrochemical Molecular Epitaxy’ (EME), that is much simpler and more effective than the prior art for the synthesis of crystalline semiconductor films and superlattices and metallic alloys. The EME method is a low-cost electrochemical analogue to MBE, for the synthesis of compound semiconductor superlattices. It combines the cost, scale-up and environmental advantages of electrodeposition with the atomic level control MBE. It aims to synthesize highly ordered, stoichiometric films with a self-regulating, computerized electrodeposition method.
It is another objective to extend this invention to provide a deposition apparatus that: (a) is simple, inexpensive to construct and operate; (b) reduces costs, energy usage, material waste and environmental concerns; (c) allows rapid synthesis of ordered, stoichiometric films or superlattices; (d) is sufficiently versatile for fabrication of quantum well lasers or large-scale PV modules; (e) permits in-situ analysis of electrolyte composition and reaction products; and (f) lowers the level of potential contamination during deposition.
It is a further objective of this invention to devise an integrated system to fabricate multi-layered, multi-component thin-film devices by sequential deposition of each semiconducting film to produce the semiconductor device.
It is yet another objective of this invention to eliminate the need for crystalline substrates to produce epitaxial films by depositing textured semiconducting or metallic films on non-crystalline substrates. This objective in turn would eliminate the high temperature/pressure crystal growth equipment, the substrate size constraints; and the crystal slicing, polishing and pretreatment steps.
To achieve the foregoing and other objectives and advantages in accordance with the purpose of the present invention, as embodied and broadly described herein, the process of this invention includes a sequence of computer controlled flow, equilibration and underpotential electrodeposition steps to construct a semiconductor superlattice, molecular layer at a time, using a specially designed diffusion-layer cell. Each cycle deposits an ordered, stoichiometric molecular layer of a semiconductor compound from a single electrolyte medium containing its constituent elements. Self-limiting reactions and the free energy gain (&Dgr;G°) due to compound formation facilitate the formation of a stoichiometric molecular layer when the substrate is exposed at underpotential to a thin quiescent electrolyte layer containing a predetermined ratio of the constituent ions. The deposition potential and the electrolyte composition can be adjusted to limit the layer growth to a single molecular layer of the compound. The quiescent deposition from a thin solution layer eliminates the mass transport dependence. The small solution volume excludes surface-active impurities from the substrate surface to ensure deposit purity. Computer controlled switching of solutions further permits the successive deposition of different semiconductor films to produce a multi-component thin-film device or a multiple quantum well structure. Thus, similar to MBE, the EME method allows molecular level control, electrical doping control and facilitates the transition from the fabricating the substrate to a complete electro-optic device which may contain quantum well structures. Unlike MBE, the EME approach offers all the advantages of prior art electrochemical synthesis techniques. It incorporates environmental benefits, amenability to large-area scale-up, irregular shapes and flexible substrates and low-temperature epitaxial growth without inter-diffusion. much lower cost, practical (much faster) deposition rates, and low material and energy requirements.
The EME method offer many advantages over the prior electrodeposition art, including the method described in U.S. Pat. No. 5,320,736. The use of a single multi-ionic electrolyte for the deposition of one compound eliminates cross contamination problems and rinse cycles, greatly simplifies the deposition apparatus design and reduces the time by orders of magnitude. It alleviates the formation of flow pattern that tend to result from alternate electrolyte deposition. Since EME uses one single solution and equilibrated quiescent deposition to form successive layers of a compound, it avoids residu
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