Abstract:
A method for forming a nano-textured surface on a substrate is disclosed. An illustrative embodiment of the present invention comprises dispensing of a nanoparticle ink of nanoparticles and solvent onto the surface of a substrate, distributing the ink to form substantially uniform, liquid nascent layer of the ink, and enabling the solvent to evaporate from the nanoparticle ink thereby inducing the nanoparticles to assemble into an texture layer. Methods in accordance with the present invention enable rapid formation of large-area substrates having a nano-textured surface. Embodiments of the present invention are well suited for texturing substrates using high-speed, large scale, roll-to-roll coating equipment, such as that used in office product, film coating, and flexible packaging applications. Further, embodiments of the present invention are well suited for use with rigid or flexible substrates.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with Government support under DE-FG36-08GOI8004 awarded by The United States Department of Energy. The Government has certain rights in the invention. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/320,695, filed Apr. 2, 2010, entitled “Fast and Scalable Printing of Large Area Monolayer Particles for Nanotexturing Applications,” which is incorporated herein by reference. 
     Further, the underlying concepts, but not necessarily the language, of U.S. patent application Ser. No. 12/909,064, filed Oct. 21, 2010 is incorporated by reference. 
     If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case. 
     FIELD OF THE INVENTION 
     The present invention relates to semiconductor devices in general, and, more particularly, to optoelectronic semiconductor devices. 
     BACKGROUND OF THE INVENTION 
     Nanotexturing the surface of a substrate can enhance many physical and chemical functions of the substrate as well as devices formed on the nanotextured surface. It has been demonstrated, for example, that a substrate surface can be made superhydrophobic (i.e., water repelling) or superhydrophylic (i.e., water attractive) by texturing the surface with nano-scale elements. Further, it has recently been demonstrated that texturing a surface with micro- or nano-fiber arrays that mimic gecko foot-hair can create an adhesive surface. 
     For optoelectronic devices, such as solar cells, lasers, photodetectors, optical modulators, light emitting diodes, and the like, substrates having a surface textured with nanowires, microwires, nanocones, nanodomes, and nanopillars have been shown to improve device performance by providing effective broadband antireflection and light-trapping characteristics both at the surface of the devices as well as within constituent layers. 
     To date, nanotextured surfaces have been produced using many different processes, such as electron-beam lithography, random chemical etching, vapor-liquid solid growth of nanowires or nanopillars, Langmuir-Blodgett deposition, spin coating, and dip coating. While these methods may be suitable for fundamental studies, they do not readily scale to commercially viable production. Typically production fabrication requires the ability to rapidly deposit layers over large area substrates with low-cost. Further, it is desirable in many applications that deposition processes be compatible with the use of flexible substrates. 
     A fast, inexpensive method for producing a nanotextured surface on any of a variety of large-area substrates, therefore, is highly desirable. 
     SUMMARY OF THE INVENTION 
     The present invention enables large-area substrates having a textured surface. Embodiments of the present invention are well suited for texturing substrates using high-speed, large scale, roll-to-roll coating equipment, such as that used in office product, film coating, and flexible packaging applications. Further, embodiments of the present invention are well suited for use with rigid or flexible substrates. 
     Prior-art approaches to forming textured substrates require relatively complicated and expensive equipment, such as would typically be used for integrated circuit fabrication. In contrast, the present invention is compatible with low-cost manufacturing equipment, such as high-speed material transfer and film coating systems. 
     In some embodiments, nano-scale particles are mixed with a solvent comprising ethanol and poly-4-vinylphenol to form a nanoparticle ink. In some embodiments, the nano-scale particles are spheres of silica. The nanoparticle ink is dispensed onto the top surface of a substrate and spread, via a wire-wound rod, to form a layer of wet ink having a substantially uniform thickness. The solvent in this layer of ink is then evaporated, which leaves behind a monolayer of nano-particles on the top surface of the substrate. 
     In some embodiments, the concentration of nano-particles in the nanoparticle ink is controlled to enable the formation of multi-layer nanoparticle arrays on the top surface of a substrate. 
     In some embodiments, at least one property of the nanoparticles, such as particle size and/or concentration is controlled. In some embodiments, at least one property of the solvent, such as viscosity, evaporation rate, and/or contact angle, is controlled to control physical characteristics of the resultant textured surface. 
     An embodiment of the present invention comprises a method for forming a textured surface on a substrate, the method comprising: dispensing a first material on a first surface of the substrate, wherein the first material comprises nanoparticles and a first solvent; establishing a relative motion between the substrate and a tool that is physically separated from the substrate by a first barrier, wherein the relative motion between the tool and the substrate distributes first material substantially completely over the first surface and forms a first layer having a substantially uniform thickness; and enabling the removal of the first solvent from the first layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic drawing of a solar cell structure having a nano-textured surface in accordance with an illustrative embodiment of the present invention. 
         FIG. 2  depicts operations of a method for forming a solar cell in accordance with the illustrative embodiment of the present invention. 
         FIG. 3  depicts a portion of a substrate comprising a texture layer in accordance with the illustrative embodiment of the present invention. 
         FIG. 4  depicts sub-operations suitable for use in operation  201  in accordance with the illustrate embodiment of the present invention. 
         FIG. 5A  depicts a schematic drawing of substrate  104  after mixture  502  has been dispensed on surface  120 . 
         FIGS. 5B-D  depict substrate  104  before, during, and after, respectively, the spreading of mixture  502  on surface  120  by tool  506 . 
         FIG. 5E  depicts substrate  104  after the nanoparticles  302  of nascent layer  512  have assembled into texture layer  106 . 
         FIG. 6A  depicts a schematic drawing of a cross-sectional view of a tool in accordance with the illustrative embodiment of the present invention. 
         FIG. 6B  depicts a schematic drawing of a cross-sectional view of a tool in accordance with a first alternative embodiment of the present invention. 
         FIG. 6C  depicts a schematic drawing of a cross-sectional view of a tool in accordance with a second alternative embodiment of the present invention. 
         FIG. 7A  depicts a scanning electron microscope image of a texture layer in accordance with the illustrative embodiment of the present invention. 
         FIG. 7B  depicts a scanning electron microscope image of a texture layer formed using a first nanoparticle ink not in accordance with the present invention. 
         FIG. 7C  depicts a scanning electron microscope image of a texture layer formed using a second nanoparticle ink not in accordance with the present invention. 
         FIGS. 8A-C  depict scanning electron microscope images of texture layers formed from nanoparticle inks having different nanoparticle concentrations. 
         FIG. 9  depicts a cross-sectional view of region  118  of completed solar cell  100 , in accordance with the illustrative embodiment of the present invention. 
         FIG. 10A  depicts a scanning electron microscope image of the top surface of a semiconductor layer structure formed on a nano-textured substrate. 
         FIG. 10B  depicts schematic drawing of a cross-sectional view of a semiconductor layer structure formed on a nano-textured substrate. 
         FIG. 10C  depicts a schematic drawing of a cross-sectional view of structure  1000  including a back-side reflection layer. 
         FIG. 10D  depicts a schematic drawing of a cross-sectional view of layer structure  1006  formed directly on planar substrate  1002 . 
         FIG. 11A  depicts measured light absorption in structures  1000 ,  1016 , and  1020  over the wavelength range from 400 nm to 800 nm. 
         FIG. 11B  depicts measured total absorption for structures  1000 ,  1016 , and  1020  integrated over the Air Mass 1.5 solar spectrum over the wavelength range from 400 nm to 800 nm. 
         FIGS. 12A-D  depict scanning electron microscope images of structure  1006  disposed on texture layers comprising nanoparticles of different diameters. 
         FIG. 13A  depicts measured light absorption for samples  1200 ,  1202 ,  1204 ,  1206  and  1020  over the wavelength range from 400 nm to 800 nm. 
         FIG. 13B  depicts measured total absorption for structures  1020 ,  1200 ,  1202 ,  1204 , and  1206  integrated over the Air Mass 1.5 solar spectrum over the wavelength range from 400 nm to 800 nm. 
     
    
    
     DETAILED DESCRIPTION 
     The following terms are defined for use in this Specification, including the appended claims:
         Disposed on is defined as meaning “exists on” an underlying material or layer. This layer may comprise intermediate layers. For example, if a material is described to be “disposed on a substrate,” this can mean that either (1) the material is in intimate contact with the substrate; or (2) the material is in contact with one or more interposing layers that reside on the substrate.   Nanoparticle is defined as a particle whose largest dimension is smaller than one micron. Nanoparticles can have any suitable generalized shape, including spherical, facetted, rectangular, square, and irregular.       

       FIG. 1  depicts a schematic drawing of a solar cell structure having a nano-textured surface in accordance with an illustrative embodiment of the present invention. Solar cell  100  comprises substrate  104 , texture layer  106 , back reflector  108 , bottom electrode  110 , optically active layer  112 , and top electrode  114 . Solar cell  100  is suitable for providing electrical energy when illuminated by sunlight  102 . 
     It should be noted that although the illustrative embodiment comprises a semiconductor device that is a solar cell, the present invention is applicable to other semiconductor devices, such as light-emitting diodes, fuel cells, lasers, optical modulators, thermionics, thermal photovoltaic and photodetectors. It should be further noted that, in embodiments of the present invention directed toward optical applications, the wavelength range of interest is dependent upon the application. Device-specific characteristics, such as wavelength sensitivity, nano-texture periodicity, material composition, etc., are also based on the intended application. As a result, material properties, material characteristics, and physical dimensions provided for elements of solar cell  100  are based on solar cell applications and are merely exemplary. One skilled in the art will recognize that these parameters, among others, can be different for embodiments of the present invention intended for different applications. 
     Sunlight  102  spans a very broad spectral range from approximately 300 nm to approximately 2000 nm. For practical purposes, however, the spectral range of interest for solar cell technology is typically approximately 300 nm to approximately 1200 nm. It should be noted that a significant portion of this spectral range is above the bandgap wavelength of a typical solar cell structure. For example, for an amorphous silicon-based solar cell, which has a bandgap wavelength of approximately 700 nm, the spectral range of interest typically includes wavelengths from approximately 300 nm to approximately 800 nm. 
       FIG. 2  depicts operations of a method for forming a solar cell in accordance with the illustrative embodiment of the present invention. Method  200  begins with operation  201 , wherein texture layer  106  is formed on surface  120  of substrate  104 . Method  200  is described herein with continuing reference to  FIG. 1 . 
       FIG. 3  depicts a portion of a substrate comprising a texture layer in accordance with the illustrative embodiment of the present invention. Texture layer  106  comprises a monolayer of nanoparticles  302 , which are substantially uniformly distributed on surface  120 . In some embodiments, texture layer  106  comprises a plurality of layers of nanoparticles  302 . 
     Substrate  104  is a soda-lime glass substrate that is substantially transparent in the wavelength range of interest. In some embodiments, substrate  104  is a rigid substrate comprising a different material, such as a semiconductor, ceramic, glass, metal, dielectric, and the like. In some embodiments, substrate  104  is a flexible substrate comprising a suitable material, such as polymers, polyethylene, polyethylene terephthalate, ferropaper, carbon-impregnated paper, parylene-N, and the like. 
     Nanoparticles  302  silica particles having substantially spherical shape and an average diameter within the range of approximately 100 nanometers (nm) to approximately 600 nm. In the illustrative embodiment, nanoparticles  302  have a diameter of approximately 400 nm. In some embodiments, nanoparticles  302  comprise a different material, such as a dielectric, metal, polymer, and the like. Further, in some embodiments, nanoparticles  302  have a different shape and/or size. 
       FIG. 4  depicts sub-operations suitable for use in operation  201  in accordance with the illustrate embodiment of the present invention. Operation  201  is described herein with continuing reference to  FIGS. 1 and 3 , and with additional reference to  FIGS. 5A-E . Operation  201  begins with sub-operation  401 , wherein mixture  502  is formed. 
     Mixture  502  is prepared by mixing nanoparticles  302  at a concentration of approximately 50 grams/liter (g/l) in solvent  504 . In some embodiments, the nanoparticles and solvent form a colloidal solution. Solvent  504  comprises PVPh at a concentration of approximately 0.2% by weight in ethanol. In some embodiments, the concentration of nanoparticles in mixture  502  is within the range of approximately 10 g/l to approximately 400 g/l. In some embodiments, the concentration of PVPh is within the range of approximately 0.2% to approximately 5%. 
     In some embodiments, mixture  502  is prepared by mixing nanoparticles a solvent that is a mixture of a different suitable alcohol and liquid polymer or monomer. Alcohols suitable for use in mixture  502  include, without limitation, ethanol, methanol, polypropanol, isopropanol, and the like. Liquid polymers suitable for use in mixture  502  include, without limitation, poly-4-vinylphenol (PVPh), 2-pyrrolidone, polyvinylpolypyrrolidone, and the like. 
     It is an aspect of the present invention that the internal properties of mixture  502  significantly affect the characteristics of texture layer  106 . Specifically, contact angle, evaporation rate, viscosity, and nanoparticle concentration of mixture  502  control the quality and structure of texture layer  106 . As a result, in sub-operation  401 , control of the concentration of nanoparticles and PVPh in the alcohol that constitutes the bulk of mixture  502  enables control of the internal properties of mixture  502  and, therefore, the characteristics of texture layer  106 . The impact that size, type, nanoparticle concentration and liquid polymer concentration in mixture  502  have on the properties of a texture layer is discussed below and with respect to  FIGS. 7A-B  and  8 A-C. 
     At sub-operation  402 , mixture  502  is dispensed onto surface  120  using a conventional liquid dispensing technique. 
       FIG. 5A  depicts a schematic drawing of substrate  104  after mixture  502  has been dispensed on surface  120 . 
     At sub-operation  403 , tool  506  distributes mixture  502  on surface  120  to form nascent layer  512 . 
       FIG. 6A  depicts a schematic drawing of a cross-sectional view of a tool in accordance with the illustrative embodiment of the present invention. Tool  506  is a conventional wire-wound rod, which comprises rod  508  and barrier  510 . 
     Rod  508  is a substantially rigid rod. Typically, rod  508  has a diameter within the range of approximately 3 millimeters (mm) to approximately 40 mm. It will be clear to one skilled in the art, however, after reading this specification, how to specify, make, and use alternative embodiments of the present invention wherein rod  508  has any suitable diameter. 
     Barrier  510  is a wire that is wound around rod with a substantially uniform spacing, P, between individual windings to form a plurality of grooves  602 . The thickness of barrier  510  is equal to the thickness of the wire, h 1 , which is typically within the range of approximately 0.05 mm to approximately 2.5 mm. In the illustrative embodiment, h 1  is equal to approximately 0.23 mm and P is also substantially equal to 0.23 mm. It will be clear to one skilled in the art, however, after reading this specification, how to specify, make, and use alternative embodiments of the present invention wherein barrier  510  has any suitable diameter. 
       FIGS. 5B-D  depict substrate  104  before, during, and after, respectively, the spreading of mixture  502  on surface  120  by tool  506 . Mixture  502  is distributed on surface  120  by establishing a relative motion between tool  506  and substrate  104 , along the x-direction, and enabling tool  506  to pass through the mixture. As tool  506  passes through the mixture, the tool sweeps some of the mixture from surface  120  leaving behind a volume of solution equal to the aggregate volume of grooves  602  of barrier  508 . This remaining volume of mixture  502  forms nascent layer  512  as a substantially uniform film having a thickness of t 1 . The thickness, t 1 , of nascent layer  512  is based on the values of P 1  and h 1 . In the illustrative embodiment, for example, a wire-wound rod wherein both P 1  and h 1  are equal to 0.23 mm yields a nascent layer  512  having a thickness of approximately 20.57 microns. 
     In some embodiments, the relative motion is established by tool  506  while substrate  104  remains stationary. In some embodiments, substrate  104  is moved while tool  506  is stationary. In some embodiments, both rod  508  and substrate  104  are moved. Suitable coating systems known in the prior art include roll-to-roll transfer systems, film emulsion coating systems, wire-wound rod coating systems, and doctor-blade systems, among others. 
       FIG. 6B  depicts a schematic drawing of a cross-sectional view of a tool in accordance with a first alternative embodiment of the present invention. Tool  604  comprises rod  508  and barrier  606 . Tool  604  is analogous to tool  506 . Barrier  606  comprises a plurality of shoulders  608 , which collectively define a plurality of grooves  610 . Barrier  606  is analogous to barrier  510  and grooves  610  are analogous to grooves  602 . The thickness, t 1 , of a nascent layer formed using tool  604  is based on the values of P 2  and h 2 . 
       FIG. 6C  depicts a schematic drawing of a cross-sectional view of a tool in accordance with a second alternative embodiment of the present invention. Tool  612  is a doctor-blade system that comprises blade  614  and barrier  616 . Barrier  616  comprises a frame that defines a separation substantially equal to t 1  between blade  614  and surface  120 . In operation, blade  614  passes through mixture  502 , while the blade is in contact with barrier  616 , which screens mixture  502  from surface  120  except for the material located in the volume defined by surface  120 , blade  614 , and barrier  616 . This results in the formation of nascent layer  512  with a substantially uniform thickness equal to t 1 . 
     Returning now to operation  201 , at sub-operation  404 , the assembly of nanoparticles  302  into texture layer  512  is enabled by the removal of solvent  504  from nascent layer  512 . As a result, the nanoparticles in nascent layer  512  assemble into texture layer  106 . In some embodiments, the temperature of substrate  104  is controlled to control the rate of evaporation of solvent  504  from nascent layer  512 . 
     In some embodiments, sub-operation  404  comprises heating substrate  104  to increase the rate of evaporation solvent  504 . In some embodiments, substrate  104  is maintained substantially at room temperature to enable solvent  504  to evaporate from nascent layer  512 . In some embodiments, substrate  104  is cooled below room temperature to retard the rate of evaporation of solvent  504  from nascent layer  512 . 
       FIG. 5E  depicts substrate  104  after the nanoparticles  302  of nascent layer  512  have assembled into texture layer  106 . 
     The coverage of mixture  502  on surface  120  and the rate at which solvent  504  is removed from nascent layer  512  have significant impact on the characteristics of texture layer  106 . A high-quality texture layer results, for example, when mixture  502  substantially completely wets the substrate and evaporates from surface  120  at a rate suitable for enabling the nanoparticles to assemble as desired. Control over the internal properties of solvent  504  (e.g., contact angle, evaporation rate, and viscosity), therefore, plays a critical role in obtaining a satisfactory texture layer  106 . In some embodiments of the present invention, the concentration of liquid polymer in solvent  504  is controlled to control the internal properties of the solvent. 
     For example, in the illustrative embodiment, solvent  504  comprises 0.2% (by weight) of PVPh mixed in ethanol. Once mixture  502  is spread evenly by tool  506 , solvent  504  begins to evaporate, beginning with that portion of nascent layer  512  formed first. At this concentration of PVPh, mixture  502  wets semiconductor, as well as polymer-based substrates, substantially completely. For embodiments wherein solvent  504  comprises PVPh and ethanol, preferred PVPh concentration is within the range of approximately 0.1% to approximately 0.5% (by weight), and preferably 0.2% (by weight). It should be noted, however, concentrations of PVPh up to 10% (by weight) are characterized by a contact angle below approximately 20 degrees, are typically below 5 degrees, and are, in some concentrations, close to zero. PVPh is merely one example of a liquid polymer that, when added to solvent  504 , decreases its evaporation rate and increases its viscosity. 
     Further, the evaporation rate and viscosity of solvent  504  directly impact the manner in which the nanoparticles assembly on surface  120  to form texture layer  106 . During the evaporation of solvent  504  from nascent layer  512 , the solvent thins to a liquid layer approximately equal to the diameter of nanoparticles  302 . As this occurs, it is desirable that the solvent forms a continuous meniscus between the nanoparticles. This meniscus induces a capillary force that drives the nanoparticles together, thereby nucleating a thin film assembly. This nucleate grows from the convective flux of nanoparticles towards the drying front of the wet film. 
       FIG. 7A  depicts a scanning electron microscope image of a texture layer in accordance with the illustrative embodiment of the present invention. Texture layer  700  was formed using a nanoparticle ink analogous to mixture  502 . The nanoparticle ink contained  400  nm-diameter silica nanoparticles at a concentration of 50 g/l in a solvent of ethanol and 0.2% of PVPh (by weight). 
       FIG. 7B  depicts a scanning electron microscope image of a texture layer formed using a first nanoparticle ink not in accordance with the present invention. Texture layer  702  is a nanoparticle layer formed using a nanoparticle ink comprising ethanol without a liquid polymer (i.e., pure ethanol). 
     Pure ethanol has an evaporation rate (at room temperature) of approximately 164 micrograms/second and a viscosity of approximately 1.07 centipoise. As a result, pure ethanol is too volatile and has a viscosity that is too low to enable formation of a high-quality texture layer. Instead, as it dries, the resultant nascent layer will separate into individual droplets during operation  404  leaving a poorly assembled texture layer. 
       FIG. 7C  depicts a scanning electron microscope image of a texture layer formed using a second nanoparticle ink not in accordance with the present invention. Texture layer  704  is a nanoparticle layer formed using a nanoparticle ink comprising ethanol mixed in even proportions with ethylene glycol. 
     Ethylene glycol has a much lower vapor pressure (0.06 mmHg at 20° C.) than that of ethanol (44 mmHg at 20° C.). As a result, a 1:1 mixture of ethanol and ethylene glycol has an evaporation rate (at room temperature) of less than 10 micrograms/second, which is significantly lower than the evaporation rate of pure ethanol. In addition, a 1:1 mixture of ethanol and ethylene glycol has a viscosity of approximately 6.89 centipoise, more than six times that of pure ethanol. Unfortunately, the different vapor pressures of ethanol and ethylene glycol result in non-uniform drying of a nascent layer comprising such a solvent. Further, non-uniform drying leads to a change in the contact angle of the nanoparticle ink, which leads to improper assembly of the nanoparticles in texture layer  704 . 
     By controlling (1) the evaporation rate of solvent  504  to be within the range of approximately 70 micrograms/second to approximately 130 micrograms per second, and (2) the viscosity of solvent  504  to be within the range of approximately 1.08 centipoise to approximately 4.06 centipoise, and (3) the contact angle to be below 5 degrees, the assembly of nanoparticles  302  can be controlled to form a well-ordered texture layer. 
     It is another aspect of the present invention that the concentration and size of nanoparticles  302  directly impact the structure of texture layer  106 . As nanoparticle concentration increases, the number of nanoparticle layers in texture layer  106  also increases. For example, using the same solvent and deposition method, texture layers having different number of nanoparticle layers can be formed simply by changing the concentration of nanoparticles dispersed in the solvent. 
       FIGS. 8A-C  depict scanning electron microscope images of texture layers formed from nanoparticle inks having different nanoparticle concentrations. 
     Texture layer  800  ( FIG. 8A ) was formed using a nanoparticle ink analogous to mixture  502 . The nanoparticle ink contained  400  nm-diameter silica nanoparticles at a concentration of 50 g/l in a solvent of ethanol and 0.2% of PVPh (by weight). 
     Texture layer  802  ( FIG. 8B ) was formed using a nanoparticle ink containing silica nanoparticles having a diameter of approximately 400 nm at a concentration of 100 g/l in a solvent comprising ethanol and 0.2% of PVPh (by weight). 
     Texture layer  804  ( FIG. 8C ) was formed using a nanoparticle ink containing silica nanoparticles having a diameter of approximately 400 nm at a concentration of 200 g/l in a solvent comprising ethanol and 0.2% of PVPh (by weight). 
     For nanoparticle concentrations of 50, 100, and 200 g/l, the number of nanoparticle layers was proportional at one, two, and four layers, respectively. It should be noted that the uniformity of the multi-layered films is comparable to that of the monolayer film and that this concentration dependence is found for semiconductor and polymer-based substrates. 
     The ability to form texture layers having different numbers of layers affords embodiments of the present invention with advantages over the prior art in different applications. 
     In some embodiments, sub-operation  404  is followed by an optional oxygen plasma treatment (or equivalent) to ensure complete removal of solvent  504  from nascent layer  512 . 
     Returning now to method  200 , at operation  202 , back reflector  108  is formed on texture layer  106  using conventional metal deposition techniques. Back reflector  108  is a layer of silver having a thickness of approximately 100 nm. Back-reflector  108  is substantially conformal with texture layer  106 . Silver provides high reflectivity for light having a wavelength within the range of interest for solar cell  100 . In some embodiments, back-reflector  108  comprises a reflective layer other than silver. It will be clear to one skilled in the art, after reading this specification, how to specify, make, and use back-reflector  108 . 
     At operation  203 , bottom electrode  110  is formed on back reflector  108 . Bottom electrode  110  is deposited on back-reflector  106  using conventional deposition techniques. Bottom electrode  110  is a layer of transparent conductive oxide having a thickness of approximately 80 nm. Bottom electrode  110  is substantially conformal with back reflector  108 . Materials suitable for use in bottom electrode  110  include, without limitation, indium-tin oxide, zinc-oxide, aluminum-zinc-oxide, and the like. It will be clear to one skilled in the art, after reading this specification, how to specify, make, and use bottom electrode  110 . 
     At operation  204 , semiconductor layer  112  is formed on bottom electrode  110 . Semiconductor layer  112  is a composite layer comprising a plurality of hydrogenated amorphous silicon layers that collectively define a p-i-n solar cell. All of the layers that collectively define semiconductor layer  112  are conformally deposited on bottom electrode  110  using conventional deposition techniques. In some embodiments, semiconductor layer  112  comprises one or more semiconductor layers suitable for a semiconductor device other than a solar cell. 
     At operation  205 , top electrode  114  is formed on semiconductor layer  112  using conventional deposition techniques. Top electrode  114  is substantially conformal with semiconductor layer  112 . Top electrode  114  is analogous to bottom electrode  108  and has a thickness of approximately 80 nm. Electrodes  108  and  114  enable electrical connectivity to semiconductor layer  112 . 
     Top electrode  114  also functions as an anti-reflection layer for semiconductor layer  112 . In the illustrative embodiment, top electrode  114  comprises indium-tin-oxide (ITO), which has a refractive index suitable for an anti-reflection layer for semiconductor layer  112 , which has a refractive index of approximately 4. 
       FIG. 9  depicts a cross-sectional view of region  118  of completed solar cell  100 , in accordance with the illustrative embodiment of the present invention. Since each of layers  108 ,  110 ,  112 , and  114  are substantially conformal, the nano-texture of texture layer  106  propagates through these layers such that surface  116  is a nano-textured surface as well. Providing nano-texture to a semiconductor layer can reduce its reflectivity and increase its absorption as compared to a comparable planar semiconductor layer. 
     In some embodiments, an increased absorption in semiconductor layer  112 , due to its nano-textured nature, obviates the configuration of top electrode  114  as an anti-reflection layer. Further, in some embodiments, an increased absorption in semiconductor layer  112  enables the use of co-planar electrical contacts disposed beneath semiconductor layer  112  and top electrode  114  is, therefore, not included in the device structure. 
     As discussed in detail in U.S. patent application Ser. No. 12/909,064, substrates comprising a texture layer of nanoparticles (i.e., nano-textured substrates) can improve the performance of semiconductor devices formed on them—particularly optoelectronic semiconductor devices. 
     Nano-textured substrates have been shown to improve light absorption in semiconductor thin-films by reducing reflection and increasing light scattering within the semiconductor material. The anti-reflection effect has already been demonstrated by several nano-textured structures, such as solar cells and silica sphere monolayers. It has been demonstrated that nano-textured substrates based on dielectric nanoparticles having diameters comparable to the wavelength of incident light exhibit a strong Mie scattering effect. This effect can be used for increasing the light path length and ultimately absorption. Further, light scattering effects have also found particular use in plasmonic systems, wherein the nanoparticles comprise metal. 
     The texture of texture layer  106  propagates upward through the layer structure to the top surface of the solar cell (i.e., surface  116 ). As a result, surface  116  has a topography characterized by dome-shaped regions (nano-domes). In some embodiments, these nano-domes are periodic with a periodicity that is less than or comparable to the wavelengths of light within the spectral range of interest. In some embodiments, the dome-shaped regions have size and/or periodicity larger than the wavelengths of light within the spectral range of interest. Further, in some embodiments, the dome-shaped regions are arranged in an aperiodic manner (e.g., random or semi-random) in at least one dimension. 
     For optoelectronic devices, in particular, the nano-textured nature of surface  116  affords embodiments of the present invention with several advantages, including:
         i. improved light absorption over a wavelength band of interest; or   ii. improved light coupling into semiconductor layer  112 ; or   iii. reduced reflectivity over the wavelength band of interest; or   iv. any combination of i, ii, and iii.       

       FIG. 10A  depicts a scanning electron microscope image of the top surface of a semiconductor layer structure formed on a nano-textured substrate. 
       FIG. 10B  depicts schematic drawing of a cross-sectional view of a semiconductor layer structure formed on a nano-textured substrate. 
     Structure  1000  comprises layer structure  1006 , which is disposed on texture layer  1004 , which is disposed on substrate  1002 . 
     Substrate  1002  is a conventional substantially transparent soda-lime glass substrate. 
     Texture layer  1004  is a close-packed monolayer of silica nanoparticles having a diameter of approximately 400 nm. Texture layer  1004  is formed on surface  1012  of substrate  1002  in accordance with the present invention. 
     Layer structure  1006  comprises bottom contact layer  1008 , semiconductor layer  1010 , and top contact layer  1012 . 
     Bottom contact layer  1008  is a layer of ITO having a thickness of approximately 80 nm. 
     Semiconductor layer  1010  is a layer of hydrogenated amorphous silicon having a thickness of approximately 280 nm. 
     Top contact layer  1012  is a layer of ITO having a thickness of approximately 80 nm. 
     Bottom contact layer  1008  and top contact layer  1012  are substantially transparent for light in the wavelength range of 400 nm to 800 nm. As a result, top contact layer  1012  transmits approximately 89% of light in this wavelength range incident on structure  1000  to semiconductor layer  1010 . ITO and hydrogenated amorphous silicon have a large dielectric contrast, however, which enables bottom contact layer  1008  and top contact layer  1012  to serve as confining layers for light once it is within semiconductor layer  1010 . 
       FIG. 10C  depicts a schematic drawing of a cross-sectional view of structure  1000  including a back-side reflection layer. Structure  1016  comprises structure  1000  and reflection layer  1018 . 
     Reflection layer  1018  is a layer of silver having a thickness of approximately 100 nm. Reflection layer  1018  is disposed on back surface  1014  of substrate  1002 . 
       FIG. 10D  depicts a schematic drawing of a cross-sectional view of layer structure  1006  formed directly on planar substrate  1002 . 
     Structure  1020  does not include texture layer  1004 . As a result, each of the layers of structure  1006  is a conventional planar layer. Structure  1020  includes reflection layer  1018  disposed on back surface  1014  of substrate  1002 . 
       FIG. 11A  depicts measured light absorption in structures  1000 ,  1016 , and  1020  over the wavelength range from 400 nm to 800 nm. Plot  1100  comprises: trace  1102 , which corresponds to the absorption of a structure  1000 ; trace  1104 , which corresponds to the absorption of a structure  1016 ; and trace  1106 , which corresponds to the absorption of a structure  1020 . 
     Comparing traces  1102  and  1104  with trace  1106 , the nano-textured layers demonstrate an enhanced absorption of approximately 40% and 68%, respectively, compared with the planar layers. This improvement is attributed to an increase in the effective anti-reflection characteristics of their top surfaces and an increase in the scattering of light within their respective semiconductor layers  1010 . Both of these effects derive from the nano-texture in layers  1008 ,  1010 , and  1012 . 
     It is noteworthy that both nano-textured structures (i.e., structures  1000  and  1016 ) showed the same enhancement between 400 and 550 nm, which suggests the incoming light in this wavelength range was absorbed in a single pass through the structure and the enhancement comes from reduced reflection. In the wavelength range beyond 550 nm, however, the light absorption in structure  1016  is greater than for structure  1000 . It is concluded that the long-wavelength light that is not absorbed in a single pass through semiconductor layer  1010  is reflected back into the layer by reflection layer  1018 . 
     Path-length enhancement is seen as particularly strong for wavelengths beyond 720 nm, where the absorption length in hydrogenated amorphous silicon is greater than 10 μm. The addition of reflection layer  1018  increased the absorption between 750 and 800 nm by 70%, for example. 
       FIG. 11B  depicts measured total absorption for structures  1000 ,  1016 , and  1020  integrated over the Air Mass 1.5 solar spectrum over the wavelength range from 400 nm to 800 nm. It is clear from plot  1108  that each of nano-textured structures  1000  and  1018  exhibits higher absorption compared to planar structure  1020 . Structure  1018  absorbs approximately 81% and structure  1000  absorbs approximately 73% compared to absorption of only 57% for planar structure  1020 . As a result, a nano-textured structure is well suited for use as a light-trapping template in photovoltaic applications. 
     It is another aspect of the present invention that control of the size of nanoparticles  302  enables control over the light absorption characteristics of a nano-textured semiconductor device. This is of particular benefit for photovoltaic applications. 
       FIGS. 12A-D  depict scanning electron microscope images of structure  1006  disposed on texture layers comprising nanoparticles of different diameters. 
     Sample  1200  comprises structure  1006  formed on a monolayer texture layer comprising spherical silica nanoparticles having a diameter of approximately 100 nm. 
     Sample  1202  comprises structure  1006  formed on a monolayer texture layer comprising spherical silica nanoparticles having a diameter of approximately 220 nm. 
     Sample  1204  comprises structure  1006  formed on a monolayer texture layer comprising spherical silica nanoparticles having a diameter of approximately 400 nm. 
     Sample  1206  comprises structure  1006  formed on a monolayer texture layer comprising spherical silica nanoparticles having a diameter of approximately 600 nm. 
     The texture of the surface of nano-textured samples grown on spherical nanoparticles is characterized by an arrangement of “dome” structures. The physical characteristics of these dome structures reflect the size of the nanoparticles on which they are formed.  FIGS. 12A-D  show that the shape of structures fabricated on small nanoparticles is less dome-shaped and approaches a nearly flat surface. As the size of the nanoparticles increases, the dome-like structure of the top surface becomes more pronounced. 
     The dome-like characteristic of a surface affords embodiments of the present invention significant advantages—particularly optoelectronic device embodiments. Light incident on a textured surface sees a gradual change of refractive index from air to the absorber layer. This gradual change results from the increasing cross-sectional diameter of the nanoparticles as the light propagates through the structure. As a result, a nano-textured surface has an effective refractive index that is between that of air and that of the top layer, which reduces the reflectivity of the top surface of the nano-textured structure. In other words, the nano-textured surface provides an enhanced anti-reflection effect. This enhanced anti-reflection effect is more pronounced for structures formed on larger nanoparticles since nanoparticles having diameters well below the wavelength of incident light are less effective at reducing reflection from the top surface. 
     Further, larger nanoparticles induce more pronounced dome shapes on the top surface of a formed device structure. Pronounced dome shapes contribute to light trapping through Mie scattering. 
       FIG. 13A  depicts measured light absorption for samples  1200 ,  1202 ,  1204 ,  1206  and  1020  over the wavelength range from 400 nm to 800 nm. 
     Plot  1300  comprises: trace  1302 , which corresponds to the absorption of sample  1020 ; trace  1304 , which corresponds to the absorption of sample  1200 ; trace  1306 , which corresponds to the absorption of sample  1202 ; trace  1308 , which corresponds to the absorption of sample  1204 ; and trace  1310 , which corresponds to the absorption of sample  1206 . Plot  1300  shows that structures with smaller nanoparticle diameters (i.e., samples  1200  and  1202 ) exhibit lower absorption than structures with larger nanoparticle diameters at nearly every wavelength. This is primarily due to higher reflection from their less-textured surfaces. Samples  1204  and  1206 , in contrast, exhibited significant enhancement, as compared to samples  1200 ,  1202 , and  1020 , over the wavelength range of 550˜670 nm, and 670˜800 nm, respectively. 
       FIG. 13B  depicts measured total absorption for structures  1020 ,  1200 ,  1202 ,  1204 , and  1206  integrated over the Air Mass 1.5 solar spectrum over the wavelength range from 400 nm to 800 nm. It is clear from plot  1312  that absorption scales with nanoparticle size. 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.