Tin Based Absorbers for Infrared Detection, Part 1

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 • 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 • 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: 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

10. 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

11. CdTe 50Å a-Sn 50Å CdTe 50Å a-Sn 50Å CdTe 50Å a-Sn 50Å CdTe Buffer ~250Å CdTe Substrate (110) a-Sn/CdTe Superlattices • 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. 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-to-metal 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 a-Sn/CdTe(110) superlattices,” J. Vac. Sci. Technol. B, vol. 6(4), pp. 1219-1223, Jul.-Aug. 1988.