Optimization of Passivation for Mid and Long Wavelength InAs/GaSb Superlattice Photodetectors

Kelsey Poineau Research Advisor: Sid Ghosh

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Infrared Detection

Any object at non-zero temperature emits heat (electromagnetic radiation)  Use infrared wavelengths because they have good transmittance through the atmosphere

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Motivation

Detection of mid- and long-wavelength infrared radiation is important in many industries Military Biomedical Space  InAs/GaSb type-II superlattice materials have potential to outperform existing detectors  Limited by poor surface quality

How to Detect Infrared Radiation

IR Radiation IR Detector Detector Output Electrical Optical Magnetic Object Solid State Material Semiconductor If E ph >E G , photons can be absorbed and create free electrons in conduction band Photogenerated electrons can be used as the detector output

p-i-n

detectors

 IR photons absorbed in the depletion region generate an electron-hole pair; the electric field sweeps the electron to the n-side and hole to the p-side  Ideally, no current so when an incoming photon creates an electron-hole pair it is detected

Problem

  Surface leakage considerably limits LWIR device performance  Native Oxides  Charged ions  Interfacial traps Surface passivation provides a viable solution  Passivating layer over semiconductor surfaces prevents current flow in oxide and terminates unsatisfied bonds III-V Semiconductor Wafers

Project Goal

 Comparative study of passivants (SiO 2 , SiN, ZnS)  ZnS degrades over time  Stacked passivation  Investigated to enhance long term stability of interface between passivation layer and InAs/GaSb substrate 

ZnS/Silicon nitride

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ZnS/Silicon oxide

 Compared on basis of electrical properties and device performance

Work to date

 Stacked passivation  Unable to achieve good electrical insulation  Considering alternatives:

SiN thin films

 Advantages  High quality dielectric  Hard and strong  High resistivity  Low porosity  Disadvantages  Effects of surface leakage in SiN>ZnS  Possess high mechanical strain

Laying the groundwork

 Strain may increase surface leakage and degrade passivation qualities  Passivate with multiple Si/N ratios to study electrical characteristics  Plasma-enhanced Chemical Vapor Deposition (PECVD)  Vary gas flow rates of silane and ammonia

Low-stress SiN films

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Change mechanical properties of SiN films

French, J. P., and P. M. Sarro. "Optimization of a low-stress silicon nitride process for surface-micromachining applications." Sensors and Actuators A 58 (1997): 149-57

Preliminary Results

 PECVD Parameters  Flow Rates     SiH 4 NH 3 (silane) - 500 sccm (ammonia) - 70 sccm Chamber Pressure - 650 mtorr Temperature - 300 °C  RF power - 20 W  Time - 15 mins  Ellipsometer Data  Thickness - 265 nm  Refractive Index - 1.95

Summary

 Analysis of surface states is key to finding and understanding improved processing leading to increased performance in devices  Could not examine effectiveness of stacked passivation in preventing ZnS degradation over time  Expect low stress (silicon-rich) silicon nitride films will improve device performance compared to stiochometric Si 3 N 4 passivation layers

References

     French, J. P., and P. M. Sarro. "Optimization of a low-stress silicon nitride process for surface-micromachining applications." Sensors and Actuators A 58 (1997): 149-57.

Pierret, Robert F.

Semiconductor Device Fundamentals

. N.p.: Addison-Wesley Company, Inc, 1996. Print. Prineas, J. P., Mikhail Maiorov, and C. Cao. "Processes Limiting the Performance of InAs/GaSb Superlattice Mid-Infrared PIN Mesa Photodiodes." Proceedings of SPIE, the international Society for Optical Engineering 6119 (2006).

Saraswat. "Integrated Circuit Isolation Technologies." Http://www.leb.eei.uni erlangen.de/winterakademie/2008/courses/course3_material/backEnd /Isolation_notes.pdf.

Streetman, Ben G., and Sanjay Kumar Banerjee.

Solid State Electronic Devices

. 6th ed. Upper Saddle River, New Jersey: Pearson Prentice Hall, 2006. Print.

Acknowledgements

Special thanks to my advisor Professor Sid Ghosh and Koushik Banerjee. This project was funded by the National Science Foundation and the Department of Defense from the EEC-NSF Grant # 0755115. Additional financial support was awarded by the National Science Foundation from the CMMI-NSF Grant # 0925425.

Questions?