Nanostructured Amorphous Silicon on Metal Electrodes: Electrical and Optical Properties

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Nanostructured Amorphous Silicon on Metal Electrodes: Electrical and Optical Properties George A. Hernandez, Daniel Martinez, Stephen Patenaude, Michael C. Hamilton1* 1 Electrical and Computer Engineering Department, Auburn University, 200 Broun Hall, Auburn University, AL 36849, U.S.A. *Corresponding Author E-mail: [email protected], Tel: 334-844-1879 ABSTRACT We present two distinct methods to nanostructure the surface of amorphous silicon to produce unique, nanoscale surface features. One method is a dry etch process that employs a modified Bosch1 process on an advanced silicon etcher to produce needle-like features of amorphous silicon. Likewise, we also investigated metal-assisted wet chemical etching2 as an alternative method to nanostructure the amorphous silicon to produce porous-like features. The resulting surface topography leads to an optically black appearance over patterned or large areas. This is a result of the interspacing between each needle and pore that leads to a high optical absorption. Thus, we designate it as black amorphous silicon (b-a-Si). We have deposited and formed regions of b-a-Si on variety of insulating films and metal electrodes, including chrome and titanium. In this study, we characterize the electrical and optical properties of as-deposited amorphous silicon and nanostructured amorphous silicon. INTRODUCTION The fields of optoelectronics, thin film devices and photovoltaics have been widely impacted by the use of silicon in its various forms, whether they are crystalline, amorphous or nanostructured. For instance, amorphous silicon (a-Si) can be deposited over large areas with low processing temperatures on diverse substrates such as glass and plastic. This ease of processability has led to the development of large area amorphous silicon thin film transistorbased circuits for flat panel displays3,4, photovoltaics5, and optoelectronics6. The unique optical and structural properties of nanostructured silicon also suggest a range of applications in these fields. For example, black silicon has interspaced features that lead to high optical absorbance advantageous in solar cell applications7 and may exhibit needle-like structures that can interlock to provide a viable packaging mechanism8. Due to the demonstrated impact of amorphous silicon in these applications and promise of usefulness of black-Si, we study two distinct methods for nanostructuring amorphous silicon over large areas. One method is a dry etch process using an advanced silicon etcher (ASE) with a modified deep reactive ion etch (DRIE) Bosch process, which is referred to as a black silicon method9. By varying the etchant and passivation times, we are able to nanostructure amorphous silicon surface to obtain needles with varying geometries. In addition, we also studied metalassisted wet chemical etching as an additional method to nanostructure the surface with porouslike features. The resulting surface morphology leads to a high absorption of light, resulting in a visibly black appearance. Thus, we refer to our nanostr