Source Impurity Contributions and Polarity Inverting in the Unintentional Doping of Al_<i>x</i>In_1–<i>x</i>As_<i>y</i>Sb_1–<i>y</i> Digital Alloys Grown via Molecular Beam Epitaxy

Molecular beam epitaxy grown AlxIn1–xAsySb1–y digital alloys have shown great promise as a materials platform for infrared photodetection, including the demonstration of several state-of-the-art III–V avalanche photodiodes. A primary obstacle facing further sensitivity improvements to AlInAsSb digital alloy devices is the inconsistent unintentional doping; here we report the first observations of variations in both the polarity and magnitude of the unintentional doping of these materials, and demonstrate outgassing procedures to reach serviceable and stable unintentional doping levels routinely after campaign onset. Through a series of experiments conducted over the course of multiple molecular beam epitaxy campaigns, the aluminum source material purity is shown to be a primary contributor of the unintentional doping. Group-V source outgassing was found to more weakly impact the unintentional background concentration. A transition from p-type to n-type polarity was observed as the aluminum source was progressively outgassed, as well as an order of magnitude reduction in the background carrier concentration. Results obtained from secondary ion mass spectroscopy and other techniques suggest that oxygen- and carbon-related impurities may be the origin of this p-type doping. After group-V outgassing, dark currents in simple pin-type homojunctions were reduced by several orders of magnitude near reverse bias breakdown, underscoring the importance of these studies to device performance.