Tin Based Absorbers for Infrared
Detection, Part 1
Presented By: Justin Markunas
IR Detection Introduction
Applications:
•Military: night vision, IR target detection
•Space: weather forecasting, astronomy
•Industrial: quality control, failure analysis
Atmospheric absorption breaks IR
spectrum into several bands:
•SWIR: 1.4-3mm
•MWIR: 3-5 mm
•LWIR: 8-12 mm
•VLWIR: >12 mm
Current Technology
Epitaxially grown Hg(1-x)CdxTe on lattice
matched Cd(1-y)ZnyTe
•x-value adjusts bandgap from 0 eV (x=0) to
1.56 eV (x=1)
Two color photovoltaic pixel arrays are
currently being produced
•Capable of 40mm pitch
•Backside illumination is common
(Cd(1-y)ZnyTe bandgap > 1.56eV)
Advantages:
•High detectivity
•Able to sense the entire IR spectrum
•Fast detectors due to large carrier
mobilities.
Disadvantages:
•High cost
•Difficult to process
•Require cooling to operate well
(especially LWIR)
Competing Technologies
Microbolometers
•Use materials with high thermal coefficient of
resistance that are heated by incident radiation
•No cooling requirements
•Slow
Quantum Well Infrared Photodetector
(QWIP) Arrays
•III-V superlattices absorb IR with intraband
processes
•Fabricated by standard growth and processing
•Absorption strength maximized at 45° angle
Others (past and present)
•Hg1-xCdxSi/CdTe/Si
•PtSi/Si Schottky barrier diodes
•Extrinsic Si and Ge photoconductors
•Lead Salts (PbSnTe)
•Quantum dot infrared photodetectors
Basic Properties of Tin
Two allotropes of Tin:
White Tin (b-Phase)
Gray Tin (a-Phase)
•Tetragonal structure
•Metallic form of tin
•Cubic Structure
•Semimetallic with 0 eV direct bandgap
•Extremely brittle
Phase Transition Occurs around
13°C
•Occurs spontaneously over time
Melting Point ~ 232° C
Lattice Constant (a-Phase): 6.49Å
Key Issues
•Gray tin has a 0eV bandgap
•13°C Phase Transition
Bandgap Adjustment
Quantum size effect
•Confinement of electrons and holes changes the
electronic structure
•Thin film can be roughly defined as 1-D quantum square
well:
 2  n 



2m  L 
2
Results from quantitative model
•Peak Bandgap: .43eV
•Absorption edge > 2.9mm
•Drop in peak due to increased role of
surface structure on electronic properties
Growth of Metastable a-Sn
Delaying the phase transition
•Pseudomorphic epitaxial growth raises transition
temperature
Key requirement for pseudomorphic growth
•Epilayer must be thinner than some critical thickness
•Critical thickness is inversely proportional to
substrate/epilayer mismatch
a-Sn Grown on CdTe by MBE
CdTe lattice constant: 6.482 Å (mismatch < .1%)
Growth Parameters adjusted for optimal stability:
•Substrate orientation
•Substrate temperature
•Growth rate
•Total film thickness
Determination of Stability:
•Sample placed on hotplate under a
microscope
•Phase change is readily observable
•Reproducible to ±1° C
a-Sn Grown on CdTe by MBE
Results:
•Substrate orientation: both (100) and (110) provided best results
•Substrate temperature: increased temperature improved stability (100-150
°C is optimal)
•Growth rate: slower rate improves stability (.1-.5 mm/s)
•Total film thickness: thicker films decreased stability (750-1000 Å can be
achieved)
•High substrate quality is critical
•Highest temperature achieved before transformation: 107 °C
Key Issue:
•Stability is important, but IR absorption is critical
•need ~2-12 mm of Sn for sufficient absorption
•requires Sn/CdTe superlattices to maintain quantum size effects
a-Sn/CdTe Superlattices
a-Sn/CdTe superlattices were grown and their
properties were monitored by RHEED
CdTe 50Å
a-Sn 50Å
•Growth occurred at 100 °C
Results:
•Stable superlattices were grown for several periods
•After 10 periods, quality degraded substantially
•Partly due to nonideal CdTe growth conditions
CdTe 50Å
a-Sn 50Å
CdTe 50Å
a-Sn 50Å
CdTe Buffer ~250Å
CdTe Substrate (110)
Conclusions
•Thickness required for good absorption not achieved
•Quality of CdTe substrates appears to be a problem
•Similar experiments performed with InSb (a = 6.48 Å)
showed comparable results
References
A. Rogalski, “Infrared Detectors: Status and Trends,” Progress in Quantum Electronics,
vol. 27, pp. 59-210, 2003.
S. Groves and W. Paul, “Band Structure of Gray Tin,” Physical Review Letters, vol.
11(5), pp. 194-196, Sep. 1963.
F. Vnuk, A. DeMonte, and R.W. Smith, “The effect of pressure on the semiconductor-tometal transition temperature in tin and in dilute Sn-Ge alloys,” J. Appl. Phys., vol.
55(12), pp. 4171-4176, Jun. 1984.
B.I. Craig and B.J. Garrison, “Theoretical examination of the quantum-size effect in thin
grey-tin films,” Physical Review B, vol. 33(12), pp. 8130-8135, Jun. 1986.
R.F.C. Farrow, “The stabilization of metastable phases by epitaxy,” J. Vac. Sci. Technol.
B, vol. 1(2), pp. 222-228, Apr.-Jun. 1983.
J.L. Reno, “Effect of growth conditions on the stability of a-Sn grown on CdTe by
molecular beam epitaxy,” Appl. Phys. Lett., vol. 54(22), pp. 2207-2209, May 1989.
H. Höchst, D.W. Niles, and I.H. Calderon, “Interface and growth studies of aSn/CdTe(110) superlattices,” J. Vac. Sci. Technol. B, vol. 6(4), pp. 1219-1223, Jul.-Aug.
1988.