280 nm Emission Deep UV LEDs with Integrated Fresnel Microlenses
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0916-DD05-09
280 nm Emission Deep UV LEDs with Integrated Fresnel Microlenses Mikhail E Gaevski, Maxim Shatalov, Shuai Wu, and Asif M Khan University of South Carolina, Columbia, 29208
ABSTRACT We report on the integration of flowable oxide-based Fresnel microlenses with 280 nm AlGaN multiple quantum well light-emitting diodes (LEDs). The lenses were fabricated on the back side of the LED sapphire substrates using direct electron beam writing. Ten concentric rings with different widths and variable thicknesses were designed for a 360o phase correction. Within each ring the thickness was varied in five steps to approximate a linear profile. The width of each thickness step varied from 100 nm to several microns. The outer diameter of the lens was 65 µm. A focal distance of 68 µm was measured for the fabricated microlenses. At the focal plane a FWHM of intensity profile as small as 14 µm was measured for lenses integrated with 30 µm diameter UV LEDs. The maximum intensity at the focal plane exceeded the background radiation by a factor of 50. A comparison of the LED performance before and after the lens fabrication did not reveal any degradation of the integral efficiency of the devices. These results demonstrate the feasibility of using flowable oxide Fresnel microlenses in optical systems based on micro-pixel deep UV AlGaN LEDs. INTRODUCTION III-nitride deep UV light-emitting devices are promising light sources for biodetection, white lighting, micro-lithography, and water and air purification systems.[1-4] However, for these applications the emitted beam divergence is a serious issue. For many applications focused beam spots are needed to increase the spatial resolution and power density. Similarly, collimation helps in reducing the cross talk in micro-pixel devices.[5] Khizar et al. reported the enhanced efficiency of UV light-emitting diodes (LEDs) by using micro-patterned sapphire substrates.[6] Diamond and polymer microlenses were also reported for visible and near-UV LEDs to collimate the light output especially in micro-pixel design devices, which ultimately can be used to create micro-sensors and micro-displays.[7,8] However, the currently available potential collimation solutions, such as the use of polymer optical elements, are difficult to implement in the deep UV emission wavelength range due to the strong UV light absorption in the majority of polymers. Deep etching of UV transparent crystalline materials like sapphire or quartz results in lenses with rough surfaces, which are unacceptable for short wavelength optics. Fresnel lenses provide low losses and high flexibility for deep UV applications. It was shown that flowable oxide material based on hydrogen silsesquioxane (HSQ) can be successfully used for optical elements designed for extremely short wavelength applications.[9] In this paper we report on the integration of HSQ Fresnel microlenses into AlGaN-based 280 nm light-emitting diodes (LEDs).
Figure 1. SEM image of Fresnel lens. Scale bar on the insert is 1 µm. EXPERIMENTAL DETAILS Lenses were
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