Design of Solar Blind Photodetectors for Communication with Blue Signal (λ=441nm) in Space. Part III

Abstract

Commercial lasers operating in red, green and blue lights combined with three photodetectors, each sensitive to selected wavelengths (colors) present basics of long distance wireless optical communication system. Current study (Part III) presents design and operation of photo-detector which is completely blind to solar spectra while working explicitly with blue signal. High frequency of blue signal can carry more information. In addition, high frequency of optical signal is not impacted by electromagnetic storms, which often interrupt radio communication. The overall structure of our solar blind photodetector consists of two sections fabricated from InGaN (Indium Gallium Nitride) hetero-structure- a filter and a double barrier tunneling diode. The topmost 0.8μm thick section with E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> (Energy Band Gap)=2.81eV acts as a filter and as a p-i-n solar cell biasing the photodiode. The filter absorbs all photons with wavelengths shorter than 441nm. The photodiode which comprises the lower section operates as double barrier tunneling photodiode with E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> =2.76eV. It consists of lightly n-type doped 3nm wide quantum well housed between two lightly doped barriers of 10nm thickness. The 0.12μm topmost and bottom region of the photodiode are doped with p- and n-type impurities to the order of 1.2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . The illuminated cross-section area of the device is 1mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> λ= (0.441μm-0.449μm). Similar to the design of red and green solar blind photodetectors described in Parts I and II [4-5], the cutoff filter on the top of the structure operates as a solar cell providing voltages needed to operate the double barrier photodetector. The major part of the system is the double barrier tunneling diode [6-7] designed to detect photons of blue light and thus indium gallium nitride has been chosen as one of the main materials in epitaxial heterostructure. Presence of differential window discrimination within the visible spectrum in our design makes our photodetector completely solar blind. This element makes the operation of our photodetector very different from the devices published in recent years [8-9].