Crystalline thin-film photovoltaic structures

This application claims priority to and the benefit of, and incorporates herein by reference in its entirety, U.S. Provisional Patent Application No. 60/984,796, which was filed on Nov. 2, 2007.

In various embodiments, the present invention relates to photovoltaic structures and devices, and in particular to thin-film photovoltaics.

Both the alternative-energy and flat-panel display markets have a need for high-quality, flexible substrates on which to produce highly crystalline semiconductor thin films.

The current solar cell (i.e., photovoltaic) market relies on technology that has been essentially unchanged for decades. Over 90% of the market is served by crystalline silicon (Si), either single-crystal or polycrystalline, with average conversion efficiencies of 12-20%. The costs of crystalline Si devices are high due to high-cost production methods and high demand for the raw materials in competition with the semiconductor electronics industry. Si devices must also be quite thick to achieve these efficiencies, consuming significant quantities of material. The remaining 10% of the market is served largely by thin-film structures based on amorphous Si, CdTe, or copper-indium-gallium-selenide (“CIGS”) that are cheaper to produce but have energy conversion efficiencies below 10%. Amorphous Si efficiencies also degrade with time.

Higher conversion efficiencies, over 30%, have been demonstrated for thin film multi-junction devices based on ITT-V semiconductors such as GaAs. However, their production costs are very high since these devices are most advantageously grown on single-crystal germanium (Ge) or GaAs wafers costing over $10,000 per square meter.

Emerging low-cost photovoltaic technologies include ribbon-grown Si, polymeric/organic films, and nanotechnology-based approaches. None of these new solutions fully addresses the market needs for increased production volume, increased efficiency, and lower cost per watt generated.

A useful substrate for the growth of high efficiency semiconductor films (e.g., III-V semiconductor films) preferably enables the growth of low-defect films (similar to those formed on single-crystal wafers) but at much lower cost and with higher area. Flexibility is a useful attribute. The substrate is also preferably chemically compatible with both the semiconductor material and with the semiconductor process environment. These demanding attributes restrict the number of materials that may effectively be used for this application.

The ability to produce polycrystalline metals with crystallographic orientation (e.g., “biaxial texture”) approaching single-crystal quality in pure metals has been known since the 1940\'s. Practical applications of such texture control have included the production of aluminum sheet with textures that enhance the production of cans. Most commercial uses of sheet materials avoid texture, however, because the properties of sheet materials are more isotropic in its absence.

Face-centered cubic (fcc) metals, some body-centered cubic (bcc) metals, and some alloys based on fcc metals may be useful as substrate materials, as they may be biaxially textured using well-known rolling-deformation and annealing processes. A well-known texture in fcc metals and alloys is the “cube texture,” in which the c-axis of each of the substrate grains is substantially perpendicular to the substrate surface, and the a-axes align primarily along a length direction. Under controlled rolling and annealing processes, these deformation-textured metals may possess biaxial texture approaching that of single crystals.

Nickel (Ni) is one fcc metal that may be made into thin foils with a well-defined cube texture using a rolling and annealing process. Prior work has shown that oxide intermediate layers may be deposited on a biaxially textured Ni surface using conditions under which nickel oxide is not stable, but where the intermediate layer (for example, CeO₂ or Y₂O₃) is stable, allowing the oxide to inherit the texture of the underlying Ni foil, i.e., form epitaxially thereon. The high-purity Ni required to achieve good biaxial texture is expensive and Ni is mechanically weak following the typical annealing heat treatment used to form the cube texture.

For these reasons, Ni alloys and other alloys have been developed to make stronger, non-magnetic biaxially textured foils. These alloys often contain alloying elements such as tungsten (W), molybdenum (Mo), vanadium (V), or chromium (Cr) in small controlled amounts. Relatively pure copper (Cu) may also be processed to produce a high-quality cube texture. Commercial grades of Cu with relatively low oxygen content and relatively low content of substitutional and interstitial elements have been of particular utility. In addition, prior work has shown that a wide range of Ni—Cu alloys may also be processed to produce high-quality cube textures.

Epitaxial films of other materials such as metals, oxides and nitrides can be grown on the biaxially textured foil. As used herein, “epitaxial” means that the crystallographic orientation of the deposited film is derived from and directly related to the crystallographic orientation of the underlying template.

Unfortunately, the existing deformation-textured foil approach is frequently not commercially viable for the deposition of semiconducting films necessary for high-performance optical and photovoltaic devices, e.g., Si, Ge, GaAs, InP, and related alloys and compounds. One promising approach utilizing Cu or Cu—Ni alloy textured foils is described in U.S. Patent Application Publication No. 2007/0044832A1, the entire disclosure of which is incorporated by reference herein. However, such foils may be incompatible with conventional semiconductor processes. Both Cu and Ni form arsenide and silicide phases when exposed to As- or Si-containing gases at typical processing temperatures above approximately 350° C. The formation of these phases causes a significant increase in volume, embrittles the foils, and renders the foils largely unusable for subsequent processing and for most applications.

Thus, while considerable progress has been made in the use of biaxially textured foils for superconductor applications, there is a need for processes and structures for non-superconductor materials and applications such as optical devices, optoelectronic devices, and photovoltaics.

The foregoing limitations of conventional thin-film photovoltaic platforms and fabrication processes are herein addressed by removing a highly textured template layer after a buffer layer has “inherited” a texture therefrom but before the formation of semiconductor layers. The template layer is thereby utilized to create a texture suitable for the subsequent fabrication of high-quality semiconductor materials and devices, but is removed before exposure to processes tending to embrittle the template layer.

In one aspect, embodiments of the invention feature a method for forming a semiconductor device. The method includes providing a textured template, forming a buffer layer over the textured template, forming a substrate layer over the buffer layer, removing the textured template (thereby exposing a surface of the buffer layer), and forming a semiconductor layer over the exposed surface of the buffer layer. During formation of the buffer layer, the buffer layer may inherit the texture of the textured template (such that the texture is revealed on the exposed surface). A sacrificial layer, which may include or consist essentially of Pd, may be formed over the textured template prior to forming the buffer layer. During formation of the sacrificial layer, the sacrificial layer may inherit the texture of the textured template. The sacrificial layer may be removed during removal of the textured template.

The method may include forming an insulator layer over the buffer layer prior to forming the substrate layer. The insulator layer may include or consist essentially of an oxide, and/or may be amorphous. A diffusion barrier, which may be substantially free of the texture of the textured template, may be formed over the buffer layer prior to forming the substrate layer. The diffusion barrier may include or consist essentially of a metal or metal alloy such as W or Re, and/or may include or consist essentially of an oxide or a nitride. The diffusion barrier may include or consist essentially of W.