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

Photodetectors

Kelsey PoineauResearch Advisor: Sid Ghosh

Infrared Detection Any object at non-zero

temperature emits heat (electromagnetic radiation)

Use infrared wavelengths because they have good transmittance through the atmosphere

Motivation Detection of mid- and long-wavelength infrared

radiation is important in many industries

InAs/GaSb type-II superlattice materials have potential to outperform existing detectors Limited by poor surface quality

Military Biomedical Space

How to Detect Infrared Radiation

Object

IR Radiation

IR Detector

Detector OutputElectrical

Optical

Magnetic

Solid State Material

Semiconductor

Photogeneratedelectrons can be usedas the detector output

If Eph>EG, photons can be absorbed and create free electrons in conduction band

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 (SiO2, 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 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

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 SiH4 (silane) - 500 sccm NH3 (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 Si3N4 passivation layers

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.

References

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?