On the Effect of Surface Texture and Nanoscale Surface Oxides on the Optical and Mechanical Properties Of Silicon Single Crystals and Mems Thin Films.
ABSTRACT
This dissertation presents the combined results of analytical, computational and experimental study of the effects of surface texture and nanoscale surface oxides on the optical and mechanical properties of silicon single crystals and MEMS thin films.
The first part of this work is focused on the analytical modeling of the reflectance of flat and textured silicon substrates. The model was used to study the reflection behavior of textured silicon surfaces under non-normal incidence.
By characterizing the incident light and facets of the silicon wafer with vector geometry, dot products and Phong’s reflection model were used to determine the reflection angles between incident light rays and pyramidal facets.
The possible optical interactions are considered for a wide range of pyramidal geometries and light incidence angles that are relevant to the exposure of textured silicon surfaces to incident sunlight. Furthermore, the model was used to investigate the possibility of secondary reflection, for the full range of incidence angles to the substrate.
The textured silicon surfaces were found to reduce the reflection angles more effectively than flat substrates at lower angles of incidence. Secondary reflection was also found to be experienced or guaranteed, for all pyramid heights, when the angle of incidence to the substrate was less than 19.4°.
The predictions are validated with experimental measurements of reflectance from (001)-textured silicon surfaces.
Secondly, the results of an experimental study of the effects of surface texture on the optical and light trapping properties of silicon wafers are presented.
Surface texture is controlled by anisotropy etching with potassium hydroxide (KOH) and isopropyl alcohol (IPA) solutions. The effects of KOH/IPA etching on the uniform distribution of pyramidal textures on the (001)-oriented silicon wafer are elucidated.
The effects of etchant concentration and KOH/IPA ratio are also explored, along with the effects of etching duration and temperature. The resulting surface morphologies are then characterized with atomic force microscopy and scanning electron microscopy.
The optical reflectance associated with the different surface textures are also determined using surface profilometry and Ultra-Violet (UV)-visible spectroscopy.
The results show that IPA concentration has the strongest effect on the surface roughness of (001)-single crystal crystals at temperatures up to 80oC. Above this value, evidence of temperature-induced cracking was revealed on the silicon substrate.
The best volume concentration ratio of KOH: IPA is also found to be 2:4. In an effort to ascertain the effectiveness of the texturing process for light trapping scheme, simple 1 x 1 cm2 solar cell devices with the structure of ITO/p-Si/Al were fabricated on both textured and flat silicon wafers.
Finally, analytical and computational models for the study of fatigue crack nucleation in silicon micro-electro-mechanical systems (MEMS) structures are presented.
Finite element modeling is used to study the role of stress-assisted dissolution on the formation and growth of cracks in the topical SiO2 layer on the silicon MEMS structures. The possible formation and growth of cracks by such dissolution is elucidated and compared with measured surface profiles from prior work using Atomic Force Microscopy (AFM).
The experimental results are also compared with predictions from linear perturbation analysis of the stability of surface topology that evolves during stress-assisted dissolution of the silica layer. The implications of the results are then discussed for the development of micro- pyramids for light trapping and improved photo-conversion in silicon solar cells and the design of reliable Si-MEMS structures.
INTRODUCTION
Background
Silicon (Si) is one of the most abundant of elements and an essential ingredient of a large number of minerals that make up the Earth’s surface [1]. It was first isolated in 1824 by the Swedish chemist Jöns Jacob Berzelius who also discovered Cerium, Selenium and Thorium [2, 3].
Crystalline silicon was first produced in 1854 by Henri Deville using electrolysis [3]. Since that time, it has been the most important element used in the semiconductor industry [4].
Without a supply of this important element, it would be impossible to make the range of high quality computers, calculators, cell phones, radios, etc.
It is, therefore, a key element that modern industrial societies rely upon. Silicon is used in various ways in microelectronics such as computer chips, with one example being the metal-oxide-semiconductor field effect transistor (MOSFET), the basic switch in modern electronics and computing [4, 5].
It is also used in photoelectric cells in solar panels to convert solar energy to electrical energy for use in both terrestrial and space applications such as space-crafts, satellites, electronic watches and pocket-sized solar calculators, transistors, lighting and the provision of other sources of energy [4, 5].
Today’s silicon research spans the full range from idealized models to realistic models across different industries [5]. The solar and micro-electro-mechanical systems (MEMS) industries are few among many other that have benefited from intensive research on silicon and its applications [5-7].
In any case, silicon research has improved the performance of several microelectronic products [4-8].
1 Importance and Prospects of Silicon-based Solar Cells.
Silicon can be used to make Photovoltaics (PVs). Photovoltaics are devices that convert solar energy directly into electrical energy [9].
They are promising, renewable, almost unlimited, and environmentally friendly [9]. The development of photovoltaics is imperative today because of the gradual depletion of the earth’s fossil fuels.
The increased use of photovoltaics could also result in a reduction in the emission of heat trapping “greenhouse gases” [10] that are emitted by traditional power plants.
Currently, silicon-based solar cells are the most widely used commercial photovoltaics in the world [11]. The PV market, dominated by crystalline silicon, has grown by more than 20% per year on average over the past 15 years.
However, PVs faces the problem of profitability, as they must compete with traditional sources and methods of energy conversion [12].
To become competitive, PV modules must achieve stable conversion efficiencies more than fifteen percent; have lifetime of at least thirty years, and cost about $US 0.4 per peak Watt or less [13].
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