Abstract
This paper attempts to identify key factor(s) for highly reflective silver (Ag) coatings. Investigation was made over the crystal orientation and surface roughness, using several types of surfaces, including electroplated Ag polycrystal films, physical vapour deposited polycrystal Ag films, and single crystal Ag foils with different crystal orientations. In each type of the surfaces, surface roughness was varied so that, for different combinations of crystal orientation, roughness would elucidate the key factors towards highly reflective Ag coatings. It is found that surface roughness plays a critical role in determining the reflectance, while the crystal orientation has negligible effect. The mean reflectance and one-way ANOVA analysis indicate that the single crystal Ag foils with three orientations performed statistically the same in the same roughness group at significance levelα= 0.05. Moreover, correlation between the surface reflectance and surface roughness has been proposed for the benefit of coating design. Refection data obtained from the polycrystalline silver samples are used to verify the accuracy of the proposed correlations. It was observed that the development surface area ratio,Sdr, is a better roughness indicator in predicting the reflectance of polycrystalline silver films than the arithmetic average roughness,Ra.
Highlights
In recent years, high brightness light emitting diodes (HB LEDs) which emit white light have been expanding their markets to replace incandescent and halogen bulbs for general illuminations applications because of their higher efficiencies and are more pleasing to the eyes [1]
When the microscopic roughness is much smaller than the wavelength, the light is reflected specularly from the surface
It is found that a decrease in specular reflectance is associated with an increase in the surface roughness of single crystals, regardless of the fact that it is expressed either in Ra or in Sdr
Summary
High brightness light emitting diodes (HB LEDs) which emit white light have been expanding their markets to replace incandescent and halogen bulbs for general illuminations applications because of their higher efficiencies and are more pleasing to the eyes [1]. An LED consists of lead frame, in which a light emitting semiconductor chip is positioned, and epoxy encapsulation. Since the light emitting chip is placed on the lead frame, the lead frame must have excellent electrical, thermal, and reflective properties [2]. High light extraction efficiency is one of the critical performance indicators for HB LEDs. Due to the large contrast in the refractive indices among the different materials used, certain amount of the emitted photons within the LED chip is trapped by its internal reflection. Improving the light extraction efficiency is vital for the development of HB LEDs. One of the methods is to reduce the light scattering by depositing a highly reflective silver (Ag) thin film on the lead frame surface [3,4,5,6,7,8]. The Ag layer serves the purpose of enhancing the solderability and heat dissipation of the lead frame
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