CMPL SEMICON
National Institute of Physics
University of the Philippines - Diliman
An anti-reflection coating (ARC) is an optical coating that aids the reduction of the substrate's reflectance. This increases the efficiency as less light is lost in the process. Reflection due to index of refraction and the thickness of the film that produces interference are two causes that directly influence the effect of ARC. Most wide bandgap semiconductor possesses this ARC property, which can be applied to photovoltaic cells (solar cells). Some examples of prominent ARC include Zinc Oxide, Aluminum Nitride and Silicon Dioxide.
Anti Reflection Coatings
Zinc Oxide (ZnO) exhibits a lot of interesting properties such as wide and direct band gap (3.37 eV) and high exciton binding energy (60 meV) . It became prominent in the field of semiconductor research and has been a topic of various studies. Because of the characteristics ZnO possesses, it can be used as an Anti-Reflection Coating (ARC) by depositing it on the silicon substrate to further decrease the reflectivity of the cell. Zinc Oxide, naturally a hexagonal wurtzite crystalline solid, has a high luminous transmittance that allows photons to enter it.
As light enters the solar cell, Zinc Oxide acts like a one-way mirror as a result of the difference between Silicon and Zinc Oxide's refractive index. This difference allows the light to bounce back to the substrate. Zinc Oxide then traps the light in between these two layers hence increasing the absorption of the solar cell. The Zinc Oxide thick film was also successful in lowering the reflectance of the samples. All of the samples showed a significant drop in reflectance compared to the controlled variable. Samples A, B, and C observed the same trend. As the wavelength of the light increased, the reflectance also increased, while Sample D exhibited an opposite trend. However, all of the samples have lowered the reflectance.
Aluminum nitride (AlN) was deposited on textured silicon for antireflection coating (ARC) applications. Aluminum Nitride is a wide bandgap semiconductor with a bandgap of 6.0 eV. AlN was grown using RF magnetron sputtering at a low power of 70 watts from an Al target with argon as sputtering gas and nitrogen as the reacting gas. As shown on the figure on the left, he reflectivity dropped when the substrate was textured and then depositen with an ARC such as AlN.
Naturally, silicon dioxide (SiO2) exists with a layer called the native oxide on silicon (Si) formed through air or wet chemicals even at room temperature. The native oxide layer is industrially used as surface passivation layer. When characterized, it shows incomplete SiO2 structures with Si-Si and Si-H bonding which is far more different from thermally grown SiO2. Among many materials, SiO2 can be used in surface modification. It can be used in high reflective coating. High reflective coating makes use of materials with different indices of refraction n; SiO2 has a low index of refraction. SiO2 has a high melting temperature which contributes to its chemical stability and is therefore used as an insulator.It can be used as a piezoelectric material shifting mechanical to electrical energy and vice versa. Because of this, it may be used in signal transmission.
The figure on the right shows the reflectance of SiO2 grown via thermal oxidation. The oxide layer was allowed to form on the Si substrate in an annealing furnace at 100 0C to 374 0C. In wet oxidation, water is added to increase the rate of oxidation. Water then becomes a source of the oxides aside from the free oxides in the atmosphere. Water in an Erlenmeyer flask was heated to push the oxides escaping from the flask to the diffusion tube in the annealing furnace. Changing the temperature of furnace within the range of temperatures for thermal oxidation does not change the rate of oxidation of Si.