High-gain, low-noise avalanche photodiodes grown by molecular beam epitaxy
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Avalanche photodiodes (APDs) are important components in short-wave and mid-wave infrared detection systems (imaging, laser radar, communications, etc.) because their internal gain can improve receiver sensitivity and enables the detection of weak photon fluxes. In the mid-infrared, HgCdTe APDs represent the current state-of-the-art; at liquid nitrogen temperatures, advanced devices offer excellent low noise characteristics, multiplication gains of > 1000, and excellent dark currents. However, challenges associated with the growth and fabrication of II-VI compounds has motivated the search for alternative APD materials. InAs APDs, which offer a cutoff wavelength of 3-3.5 μm, have recently been found to exhibit gain and noise characteristics similar to HgCdTe APDs. These characteristics, in addition to the inherent compositional uniformity, stability at temperatures in excess of 500 °C, and potential availability from commercial III-V foundries, make InAs APDs attractive for a variety of applications in the short-wave and mid-wave infrared, including free-space optical communications, chalcogenide fiber-optics, gas detection and monitoring, thermal imaging, and 3D laser detection and ranging (LIDAR). However, the performance of state-of-the-art InAs APDs has thus-far been limited by excessive dark current and unintentional background doping concentrations, which restrict the practical operating temperatures and limit the achievable multiplication gain. In this dissertation, we describe advances made in the design, growth and fabrication of low-noise InAs APDs that resulted in reduced dark currents and background doping, culminating in record high multiplication gains in excess of 300 while maintaining extremely low-noise operation. In addition, we explore one promising approach for extending the cutoff wavelength of these devices, namely the incorporation of dilute amounts of bismuth (Bi) into InAs, resulting in the highly-mismatched alloy InAsBi. Finally, we report, for the first time, on the development of novel AlInAsSb staircase APDs, which are predicted to provide further enhanced gain and noise characteristics over bulk InAs APDs through application of the staircase APD concept, first proposed by Capasso et al.