The aim of the proposed effort is to develop a high sensitivity, mid-infrared sensor called QDAP (Quantum Dot Avalanche Photodiode), based on intersubband transitions in nanoscale QDs in conjunction with avalanche multiplication. Intersubband QD sensors are perceived as a promising technology for mid infrared regime since they are based on a mature GaAs technology, are sensitive to normal incidence radiation, exhibit large quantum confined stark effect that can be exploited for hyperspectral imaging, and have lower dark currents than their quantum well counterparts. However, the low quantum efficiency of QD detectors has limited their operating temperature to 70-80K. State of the art photonic detectors are based on the narrow bandgap mercury cadmium telluride (MCT) material, which offer higher single pixel performance at the same operating temperature. However, non-uniformity issues associated with native defects have limited the progress of MCT-based focal plane arrays. Presently all mid infrared photonic detectors operate at cryogenic temperatures (4-100K) and have complicated cooling requirements that include multi-stage sterling coolers. A high-sensitivity mid infrared sensor operating at temperatures achievable by the relatively inexpensive Peltier coolers (150-250K) would represent a major technological leap and lead to a significant reduction in the cost and complexity of infrared sensors and imaging systems. Based on our preliminary calculations, we estimate that the operating temperature of QDAPs would be about 100K higher than that of conventional QD detectors.
Intellectual Merit: In this proposal, a novel device called QDAP is proposed which is expected to have a higher sensitivity and improved performance over conventional QD detectors. In the QDAP, an intersubband quantum dot detector is coupled with an avalanche photodiode (APD) through a tunnel barrier. While the tunnel barrier reduces the dark current more effectively than the photocurrent in the QD section of the device, the APD provides the necessary photocurrent gain required to increase in the signal-to-noise ratio (SNR). In particular, the APD provides the large gain necessary to overcome the readout noise and achieve a significantly enhanced shot-noise-limited SNR, which comes at the slight expense of the avalanche excess noise. With this novel combination, we can achieve a higher sensitivity at the same temperature (D* responsivity) or have a comparable performance at higher operational temperatures. Moreover, the APD could be operated in the linear mode or in the Geiger mode (single photon counting mode). Photon-counting systems are regarded as the ultimate in photon-sensing techniques from a sensitivity perspective. Such sensors would be extremely useful for sensing ultralow-level images and signals in many scientific and engineering fields stretching from microscopy and medical imaging to astronomy and astrophysics, where the photon flux is very limited. Presently there are no single photon detectors available for the mid infrared regime. The PI`s group has recently reported three color QD detectors operating in the mid wave infrared (MWIR, p ~ 4m), long wave infrared and very long wave infrared. These designs would be incorporated into the sensor proposed in this project to realize multi-color detectors with high sensitivity. Theoretical modeling of the proposed structures has revealed that the coupling of the QD detectors with both homostructure and heterostructure APDs is possible, thereby providing the opportunity for optimizing the APD for either linear- or Geiger-mode operation through bandgap engineering of the APDs multiplication region. This project draws from the complementary expertise of the PIs who have an established record of active collaboration. The self- assembled InAs/(In,Ga)As quantum-dot APDs will be designed, optimized, grown, fabricated, and characterized by the PIs at the University of New Mexico (UNM). Testing of the Geiger-mode operation of the devices will be undertaken in collaboration with researchers at the University of Sheffield (support letter attached).
Broader Impacts: The PIs have a commitment both to classroom education and to involving undergraduates in theoretical and laboratory research. Last year, the PIs involved four undergraduates (including two minority students and two students sponsored by the NSF REU program) in their research activities. This tradition will continue in this project with the additional participation of local high school students. Additionally, results from this project will impact two courses that have already been developed by the PIs in optoelectronic devices and optical communication. These courses are intended for launch on Web-CT platforms and be offered at multiple institutions simultaneously. In addition, through our interaction with our international collaborators at the Univesidad de Concepcion, Chile, our students will have an opportunity to receive additional practical training at the TIGO observatory facility in Chile, in which near infrared Geiger-mode APDs are used for satellite tracking. The project would lead to a broader dissemination of knowledge in forums such as the New Mexico Nano-Science Alliance and NSF's National Nanotechnology Infrastructure Network (NNIN) program, of which UNM is a team member.
