Band Engineering for High Gain Digital III-V Avalanche Photodiodes

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Nontechnical:Avalanche photodiodes (APDs) are widely used to convert optical data into electrical signal and can be used in a wide range of applications. These include optical fiber communication, medical spectroscopy, infrared sensing, and applications needing high gains and bandwidth such as quantum information networks. APDs provide very high sensitivity through internal gain and carrier multiplication in a region of high electric field in the device. This gain, however, comes at the cost of increased shot noise. The noise arises due to the pair creation by separate electron and hole currents that couple through a complex feedback mechanism. One way to limit the noise while preserving the gain is to constrain one type of carrier to a narrow bandwidth so that overall carrier multiplication stays primarily unipolar. This proposal focuses on band engineering of digital III-V heterostructures that can create minigaps selectively inside one band, thereby constraining the corresponding carrier energy and reducing shot noise. The PIs will undertake a detailed computational and experimental study of the materials physics and interface chemistry in APDs based on digital III-V alloys with high aluminum content. The successful fulfillment of the proposal will expand the materials genome of stable III-V alloys and set design rules for high gain photodiodes. These "designer" APDs will have ten to twenty times lower noise in the critical telecommunications spectrum of wavelengths from three to five microns and be capable of single photon detection. Investigators will incorporate results into upcoming courses and books. They will also recruit students and teachers from underrepresented groups through a variety of programs and fellowships. Results will be presented at conferences and meetings with industry and government members through affiliated cooperative research centers. They will also be incorporated into demonstrations at annual Montessori events for children.Technical:The proposal will lead to the fundamental study of band formation and orbital chemistry in III-V alloys, towards controlling carrier ionization coefficients, excess noise and overall optoelectronic device design and noise reduction of Avalanche Photo Diodes (APDs). The proposal will involve the development of multi-scale computational models for electronic and optical transport of III-V alloy APDs and creation of design rules for low noise APDs with high gain. Experiments will focus on fabrication and characterization of digital and random III-V APDs, with varying material composition, orientation, temperature, periodicity, and roughness profile, and measurements of their electronic and optical properties to confirm predictions from the theoretical study and isolate the effect of competing mechanisms. Armed with these results, a Separate Absorption Charge Multiplication (SACM) APD will be fabricated to combine the different alloying needs for absorption vs multiplication, as well as high-speed Modified Uni-Traveling Carrier (MUTC) photodiodes to explore carrier saturation velocity. The ultimate goal will be to show operation at 25 and 50 Gigabit per second, enabling high performance 100 and 400 Gigabits per second coarse Wavelet Division Multiplexing Ethernet. Improving the performance of APDs for photon detection will open up their use in a wide range of commercial, military and research applications.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.