Infrared and Terahertz Modulation Characteristics of n-GeBi/p-Si Photodiodes

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In this paper, germanium-bismuth (Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> , x = 0-0.32) is grown by low-temperature molecular beam epitaxy. Because Bi is an element belonging to group V, GeBi films show inherent n-type doping properties compared with GeSn ones. Inherent n-type Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> films with a doping concentration of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> -2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> are epitaxially deposited on p-type Si substrates to form p-n junctions. Current-voltage measurements show that the dark current density of the diodes can approach 0.32 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The influence of Bi concentration on the infrared (IR) and terahertz (THz) transmittance of the films is investigated. Near-IR (1-2 μm) and mid/far-IR (2.6-10 μm) responsivities of the films are 0.65 and 0.032 A/W, respectively. The THz wave transmittance is tuned by -6%-8% by tailoring the bias voltage. A modulation depth of -12% is obtained for a Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.78</sub> Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.22</sub> /p-Si diode. The dynamic modulation characteristics of n-Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> / p-Si diodes are further investigated using a 340-GHz carrier. The experimental maximum THz wave modulation speed is up to 2 MHz. The present results demonstrate that n-GeBi/ p-Si diodes are promising for both mid/far-IR photodetectors and broadband high-speed THz wave modulators.