Technical. This collaborative project addresses materials science growth/processing research of InGaAs/GaAsSb multiple quantum wells (MQW) with related investigations aimed toward mid-IR wavelength detector applications. The nature of the band alignment allows tuning of the en-ergy gap by varying layer thickness, strain, and composition. Emphasis is placed on gaining greater understanding of the trap formation in GaInAs/GaAsSb MQWs and correlation of their formation with prototype device performance. The approach involves the use of InP to provide advantages: these include the use of compressive and tensile strained materials for flexible device design options; mature wafer foundry capabilities for processing InP-based structures; the ability to leverage advances in InP-based epi-growth over the past decade; and the ability to leverage fu-ture advances driven by InP electronics. Currently, device performance apppears limited by mid-gap traps in the absorption region; hence this work is focused on providing a more complete un-derstanding of these traps and correlating them with device performance. While the MIR photo-diode test structure used in this work has its merit from device perspectives, the basic under-standing of trap states in GaInAs, GaAsSb, and GaInAs/GaAsSb MQWs will improve our fun-damental understanding of these materials. In turn, this will help to better understand the nature of the Sb-based MQW structure. Additionally, these MQWs also impact other important devices such as heterojunction bipolar transistors and mid-IR semiconductor lasers. Non-Technical. The project addresses fundamental research issues in a topical area of elec-tronic/photonic materials science having technological relevance. Societal benefits of the pro-posed research of these materials are potentially broad since the materials and prototype devices being studied support civilian and military applications including pollution detection, medical di-agnostics, night vision, and missile tracking. At present, the best detectors are based on band-to-band transitions in HgCdTe or quantum-well infrared photodetectors (QWIPs) using III-V com-pound semiconductors. Neither technology is well-suited for operation at or near room-temperature. An important advantage of InGaAs/GaAsSb MQW detectors is the potential for high detectivity at relatively high temperatures (200-300K). Through their participation in state-of-the-art research both graduate and undergraduate students will gain invaluable skills and better understand the connection between materials growth, characterization, device design, and device fabrication. To show the impact of mid-IR photodiodes on applications, the photodiodes devel-oped will be used in a trace-gas monitoring demonstration platform. This platform will be used in outreach programs (Engineering Open House; a summer program called Introduction to Engi-neering (ITE); and ENGR 162 (UVA) and EG EG10111/10112 (Notre Dame) required first year engineering courses) designed to illustrate the societal benefits of Electrical Engineering. The goals of these outreach activities are to (i) educate the public about engineering, (ii) recruit pre-college students to pursue engineering as a career, and (iii) motivate first-year engineering students to remain in the major after their first year.
In this project, a comprehensive experimental investigation of material defects and their electrical effects in the InGaAs/GaAsSb material system has been undertaken. This material system is especially promising for infrared photodetectors operating in the 3-5 µm wavelength range (a technologically relevant spectral region for sensing), as well as for tunnel field effect transistors (TFETs), which are currently under intensive investigation by many researchers for their promise to extend Moore’s law and enable continued advances in ultra-scaled electronics and for power-efficient electronic systems. In this project, we have studied both the native defects in these materials individually, as well as additional defects that are formed at heterointerfaces between these materials. This project has also resulted in the first demonstration of a TFET as an RF detector, and the evaluation of the noise characteristics of this device (since the noise signatures are directly linked with the defect structure in the materials). The outcomes of this project include a catalog of electrically-active defects (traps) identified in both lattice-matched and strained InGaAs, GaAsSb, and their heterostructures. The activation energy, cross-section, and defect characteristics have been identified for each electrically-active defect that was detected in the heterostructures evaluated. The techniques employed for defect detection includes deep level transient spectroscopy, low-frequency noise spectroscopy, and random telegraph noise spectroscopy. This catalog specifically shows the impact of material strain on the cross-section of the identified defects, providing a means for device designers to optimize photodiode and other heterostructure devices. The information obtained is expected to enable device designers to improve the quality of mid-infrared photodiodes based on InGaAs/GaAsSb superlattices, as well as InGaAs/GaAsSb TFETs for future sensing and electronic systems.
