1. Tin Based Absorbers for Infrared Detection, Part 1
Presented By: Justin Markunas

2. 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

3. Current Technology Advantages: Disadvantages:
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)

4. 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

5. 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

6. Key Issues
Gray tin has a 0eV bandgap
13°C Phase Transition

7. Bandgap Adjustment Quantum size effect Results from quantitative model
Confinement of electrons and holes changes the electronic structure
Thin film can be roughly defined as 1-D quantum square well:
Results from quantitative model
Peak Bandgap: .43eV
Absorption edge > 2.9mm
Drop in peak due to increased role of surface structure on electronic properties

8. 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

9. a-Sn Grown on CdTe by MBE
CdTe lattice constant: 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

10. a-Sn Grown on CdTe by MBE
Results:
Substrate orientation: both (100) and (110) provided best results
Substrate temperature: increased temperature improved stability ( °C is optimal)
Growth rate: slower rate improves stability (.1-.5 mm/s)
Total film thickness: thicker films decreased stability ( 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

11. a-Sn/CdTe Superlattices
CdTe Buffer ~250Å
CdTe Substrate (110)
a-Sn 50Å
CdTe 50Å
a-Sn/CdTe superlattices were grown and their properties were monitored by RHEED
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

12. 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

13. References
A. Rogalski, “Infrared Detectors: Status and Trends,” Progress in Quantum Electronics, vol. 27, pp , 2003.
S. Groves and W. Paul, “Band Structure of Gray Tin,” Physical Review Letters, vol. 11(5), pp , Sep
F. Vnuk, A. DeMonte, and R.W. Smith, “The effect of pressure on the semiconductor-to-metal transition temperature in tin and in dilute Sn-Ge alloys,” J. Appl. Phys., vol. 55(12), pp , Jun
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 , Jun
R.F.C. Farrow, “The stabilization of metastable phases by epitaxy,” J. Vac. Sci. Technol. B, vol. 1(2), pp , Apr.-Jun
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 , May 1989.
H. Höchst, D.W. Niles, and I.H. Calderon, “Interface and growth studies of a-Sn/CdTe(110) superlattices,” J. Vac. Sci. Technol. B, vol. 6(4), pp , Jul.-Aug