Among the semiconductor materials commonly used in infrared avalanche photodiode (APD) applications, indium arsenide (InAs) has drawn considerable research attention. In the short-wave infrared wavelength, InAs-based APDs provide high detection efficiency attributed to their remarkable direct bandgap of 0.34 eV, which correspond to approximately 3.55 µm cut-off wavelengths at room temperatures. Additionally, their high spectral response makes InAs APD a promising detector for remote sensing of common atmospheric gases.
HgCdTe is the most known material for fabricating IR detectors with desirable performance. However, it faces several drawbacks that limit its application: difficulty growing HgCdTe detectors, using costly CdTe to reduce crystal defects, cryogenic operation temperature requirements and environmental concerns. Most of these limitations also apply to InSb photodetectors. To overcome these issues, extended InGaAs, also capable of detecting in the short-wave infrared, have been proposed. Despite its perceived benefits over other IR detector fabrication materials, extended InGaAs fail to provide satisfactory avalanche gain. Thus, enhancing their sensitivity requires complex separate-absorption-multiplication structures.
Most InAs APDs reported in the literature are based on mesa topology, which is not ideal for commercial fabrication because leakage current level is mainly dependent on the surface conditions of the device. Mesa topology experiences many other limitations, including the anisotropic property of the commonly used wet etch as well as the required fill factor and high device yields. To this end, overcoming these challenges has been the main focus of current research in this direction. Although many studies have concentrated on planar InAs photodetectors, information about the uniformity of the associated arrays is still missing. Consequently, there are limited attempts to develop large planar InAs avalanche photodiodes, despite their practical implications.
Herein, PhD candidate Tarick Osman, Dr. Leh Woon Lim, Professor Jo Shien Ng and Professor Chee Hing Tan from The University of Sheffield in England proposed a new method for fabricating 128-pixel linear arrays of InAs planar avalanche photodiodes. This fabrication process utilized selective area implantation of Beryllium ions into epitaxially-grown InAs wafers. The resulting linear arrays were characterized to elucidate more on their properties. The avalanche gain measurements and forward and reverse currents were assessed at varying temperatures in the range of 300 – 150K. Their research work is currently published in the journal, Optics Express.
The research team reported good pixel uniformity in terms of avalanche gain, responsivity and external quantum efficiency. At 1550 nm and 2004 nm wavelengths, the room temperature responsivity values were, respectively, 0.49 ± 0.017 and 0.89 ± 0.024 A/W. The pixels achieved a maximum avalanche gain of 22.5 ± 1.18 and a dark current density of 0.68 ± 0.48 A/cm2 at 200K at -15 V reverse bias. The planar APDs exhibited uniform dark currents at temperatures 250 K and 300K. While the dark currents reported in the present study were slightly higher than that of InAs mesa APDs reported in previous studies, it was comparable to previously reported InAs planar APDs. Non-uniformity in the dark current was caused by surface leakage currents, especially at lower temperatures.
In summary, this is the first study ever to fabricate planar linear array of InAs APDs using ion implantation. The study findings demonstrated the possibility of fabricating arrays of planar APDs with improved capabilities in terms of avalanche gains and responsivities. Since the dark currents were largely non-uniform at lower temperatures, further studies were recommended to improve uniformity at low temperatures and assess crosstalk among planar InAs APD pixels. Professor Chee Hing Tan, the lead and corresponding author told Advances in Engineering that the fabricated InAs APDs hold potential applications as low-cost alternative short- and medium-wave infrared detectors.
Osman, T., Lim, L. W., Ng, J. S., & Tan, C. H. (2022). Fabrication of infrared linear arrays of InAs planar avalanche photodiodes. Optics Express, 30(12), 21758-21763.