Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
2001-10-05
2004-04-20
Cole, Elizabeth M. (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S312200, C428S307300, C428S307700, C428S310500, C136S261000, C136S258000, C423S324000, C423S325000, C423S341000, C423S345000, C423S347000, C423S348000
Reexamination Certificate
active
06723421
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor materials with novel properties and internal structures. In particular, the invention relates to a non-single crystalline semiconductor, and a process for making same, comprised of coordinatively irregular structures, each of which has distorted chemical bonding, reduced dimensionality, and a state of structural order distinct from the amorphous and single crystalline forms of the semiconductor.
BACKGROUND OF THE INVENTION
Semiconductor materials have had a tremendous impact on the quality of life and economic progress across the world over the past four decades. Semiconductor materials are integral to modern electronic devices such as transistors, diodes, LEDs, and lasers. These and related devices are responsible for the advent of the information age with all of its promise for today and tomorrow.
First generation electronic devices were based primarily on crystalline silicon. Single crystalline silicon, especially when doped to produce n-type and p-type material, was and continues to be successfully used in a variety of electronic devices. The efficacy of crystalline silicon is due to its high charge carrier mobility and suitability for high speed electronics applications.
The primary limitations of single crystalline silicon for photovoltaic applications are its indirect bandgap and the inability to produce it in large areas in a continuous manufacturing process. The indirect bandgap of crystalline silicon has two important deleterious consequences. First, optical transitions from the valence band to the conduction band of crystalline silicon occur with weak intensity and second, crystalline silicon is unable to emit light. As a result, crystalline silicon is impractical for many optical and photonic applications.
Although single crystalline silicon can be prepared with a high degree of purity and with well-controlled spatial distributions of n and p type dopants, its preparation is slow and not amenable to high speed manufacturing processes. Consequently, the preparation of single crystalline silicon is expensive and cost considerations limit its range of applications.
The need for an optically efficient semiconductor material for laser, LED, solar energy and photovoltaic applications has motivated much research over the years. Direct gap III-V materials such as GaAs and InP exhibit strong optical transitions and have been shown to be effective light emitting and absorbing materials. These materials, however, are only practically useful in the single crystalline state and are subject to many of the same processing and cost constraints associated with crystalline silicon. Since III-V materials and alloys are compound semiconductors, their preparation is further complicated by the need for a proper stoichiometric ratio of two or more elements. The need for uniform chemical composition imposes additional restrictions on the preparation and processing of III-V materials and alloys. Consequently, III-V materials and alloys are currently limited to niche applications.
Amorphous silicon has emerged as the leading material for large scale solar energy and photovoltaic applications. Amorphous silicon is an unusual material in that although it is silicon based, it possesses a direct bandgap and therefore exhibits high absorption efficiency. Since the bandgap of amorphous silicon occurs in the visible part of the spectrum, it has been demonstrated to be an effective material for solar cells and other photovoltaic devices capable of being powered by the sun. The amorphous nature of amorphous silicon precludes the need to establish the structural regularity associated with single crystalline silicon. As a result, the growth rate of amorphous silicon is much faster than that for single crystalline silicon and amorphous silicon can be prepared on a large scale by a variety of deposition techniques in a rapid, continuous and cost-effective manner.
S. R. Ovshinsky recognized that the disordered nature of amorphous materials provides new opportunities for tailoring electronic properties. S. R. Ovshinsky believed that crystalline solids are restrictive in terms of their properties because of the limited number of structures available and the limited flexibility in achieving new chemical compositions. These limitations are inherent to the unforgiving periodic and ordered structural requirements of crystalline solids. By embracing the structural disorder of the amorphous state, S. R. Ovshinsky argued, it becomes possible to achieve new structures and new compositions with new electronic properties.
S. R. Ovshinsky further showed that it was possible to prepare materials that included features of both the amorphous and single crystalline forms of the composition. These materials typically comprise an amorphous or crystalline matrix that contains regions of ordered clusters or aggregations of atoms with a degree of order intermediate between the highly ordered single crystalline form and the highly disordered amorphous form. The presence of the clusters or intermediate range order aggregations imparts unusual electronic properties to the material and has motivated further development of this fundamentally new class of materials. This seminal concept of achieving new materials with superior electronic properties by varying the degree of order through atomic engineering has been described in, for example, “Amorphous and Disordered Materials—The Basis of New Industries”, S. R. Ovshinsky, Materials Research Society Symposium Proceedings, Vol. 554, pp. 399-412, 1999; “Heterogeneity in Hydrogenated Silicon: Evidence for Intermediately Ordered Chainlike Objects”, D. V. Tsu et al., Physical Review B, Vol. 63, pp. 125338: 1-9, 2001; “Semiconductor with Ordered Clusters”, S. R. Ovshinsky et al., U.S. Pat. No. 6,087,580; and “Semiconductor having Large Volume Fraction of Intermediate Range Order Material”, S. R. Ovshinsky et al., U.S. Pat. No. 5,103,284.
From the viewpoint of solar energy applications, amorphous silicon is not an optimal material because it does not absorb the full range of photon energies present in the solar spectrum. Since amorphous silicon has a bandgap energy of approximately 1.8 eV, it is only capable of efficiently absorbing light with photon energies greater than about 1.8 eV. (The UV and higher energy visible portions of the solar spectrum.) The solar spectrum, however, contains a significant amount of light with photon energies less than 1.8 eV. (The lower energy visible and infrared portions of the solar spectrum.) As a result, solar energy devices incorporating only amorphous silicon capture only a limited fraction of the total energy available from sunlight.
In order to increase the amount of sunlight collected, practical solar energy devices are normally based on multilayer structures comprised of amorphous silicon to capture the high photon energy portion of the solar spectrum and an alloy of amorphous silicon with a bandgap narrowing element to capture the low photon energy portion of the solar spectrum. A bandgap narrowing element is an element that leads to a reduction in the bandgap energy and consequently an increased absorption of lower photon energy light. Bandgap narrowing elements preferably lower the bandgap while maintaining a direct bandgap so that the resulting alloy material retains the high absorption strength characteristic of amorphous silicon. Germanium is the most commonly used bandgap narrowing element. Alloys of silicon and germanium are capable of strongly absorbing the low energy visible and infrared portions of the solar spectrum and lead to substantial improvements in sunlight-to-electricity conversion efficiency when incorporated into solar energy devices.
Although the incorporation of germanium in solar energy devices improves device performance, its inclusion has two disadvantages. First, the most common source of germanium, germane gas (GeH
4
), is expensive and not widely available. Second, incorporation of germanium adds complexity to the process used to manufacture sola
Ovshinsky Stanford R.
Pashmakov Boil
Tsu David V.
Bray Kevin L.
Cole Elizabeth M.
Energy Conversion Devices Inc.
Siskind Marvin S.
Vo Hai
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