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Mechanisms of the Persistent Photoconductivity Quenching in Pb 1-x Sn x Te(In)

Mechanisms of the Persistent Photoconductivity Quenching in Pb 1-x Sn x Te(In). V.I. Chernichkin, D.E. Dolzhenko, L.I. Ryabova, D.R. Khokhlov M.V. Lomonosov Moscow State University. Unusual Impurity States in Pb 1-x Sn x Te(In) and on a Way to the Passive Terahertz Imager.

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Mechanisms of the Persistent Photoconductivity Quenching in Pb 1-x Sn x Te(In)

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  1. Mechanisms of the Persistent Photoconductivity Quenching in Pb1-xSnxTe(In) V.I. Chernichkin, D.E. Dolzhenko, L.I. Ryabova, D.R. Khokhlov M.V. Lomonosov Moscow State University

  2. Unusual Impurity States in Pb1-xSnxTe(In)and on a Way to the Passive Terahertz Imager V.I. Chernichkin, D.E. Dolzhenko, L.I. Ryabova, D.R. Khokhlov M.V. Lomonosov Moscow State University

  3. M.V. Lomonosov Moscow State University Ludmila Ryabova Dmitry Dolzhenko Vladimir Chernichkin Institute of Applied Physics, Kishinev, Moldova Andrey Nicorici University of Beer Sheva, Israel Vladimir Kasiyan Zinovy Dashevsky University of Regensburg Sergey Ganichev Sergey Danilov A.F. Ioffe Physical-Technical Institute, St-Petersburg Vassily Bel’kov Cooperation

  4. Outline • 1. Introduction • 2. Undoped lead telluride-based alloys. • 3. Effects appearing upon doping. • a) Fermi level pinning effect. • b) Persistent photoconductivity. • c) Theoretical model • 4. Terahertz photoconductivity and local metastable states • 5. Pb1-xSnxTe(In)-based terahertz photodetectors. • 6. Summary.

  5. Spectrum of the electromagnetic radiation «Terahertz gap»

  6. Terahertz radiation • In this spectral region both radiophysics methods (at the long-wavelength side) and optical methods (at the short-wavelength side) work not well • Consequence: absence of good sources and sensitive detectors of radiation

  7. Areas of application of the Terahertz radiation • Monitoring of concentration of heavy organic molecules • Medical applications (oncology, stomatology) • Meteorology • Security systems (search and detection of explosives) • Infrared astronomy

  8. Medical applications Cancer tissue in theTerahertz and in the visible spectral range

  9. Security systems A boot with a ceramic knife and a plastic explosive “Semtex” in its sole

  10. Security systems A polyethylene box under a 10 cm layer of sand. Pictures are taken in the Terahertz range

  11. Asteroid danger Maximum of the blackbody radiation spectral density l(mm)=3000/T(K) Sun: T=6000 K, l=500 nm Earth: T=300 K, l=10 mm Asteroids: T=10 K, l=300 mm u=1 THz – Terahertz range!

  12. Terahertz astronomy

  13. Russian Space Missions in Terahertz and Millimeter Ranges • RADIOASTRON • Test launch – 21 January 2011 • Launch scheduled for July 2011 • MILLIMETRON • Launch scheduled for 2017-2018 • The project is accepted by the Russian Space Agency • Supported by the German Space Agency • Pending support from the European Space Agency

  14. Proton-M launcher, L2 orbit, 4500+2100 kg. SB – space buster DM, SM – service module,WC – warm cabin, TS&EM – thermal screens & expanding mast, CC – cold cabin, T – telescope.

  15. The Space Observatory in the single-dish mode Telescope: Primary mirrordiameter 12 m, surface RMS accuracy 10 mm, diffraction beam 4’’ and field of view 4.5’ at 1.5 THz. Bolometer arrays: wavelength ranges 0.2-0.4 mm, and 0.7-1.4 mm HPBW beam (at1.5 THz)4'' Low resolution spectropolarimeter: wavelength range 0.02-0.8 mm spectral resolution R = 3 Medium resolution spectrometers: wavelength ranges 0.03-0.1 mm, and 0.1-0.8 mm spectral resolution R = 1000 High resolution spectrometer: wavelength ranges 0.05 – 0.3 mm spectral resolution R =106 Bolometric sensitivity:at1 THz, NEP = 10-19 W(s)0.5, A = 100 m2, R=3 and 1 h integration 5∙10-9Jy (1 s)

  16. State of the art sensitive terahertz detectors • Transition edge sensors • Hot electron bolometers • Ge(Ga) blocked impurity band detectors • Kinetic inductance detectors

  17. Problems (as I see them) • Very low operating temperature < 150 mK • NEP not better than 4*10-19 W/Hz1/2 in the lab and not better than 10-17 W/Hz1/2 in real space missions • Quite poor dynamic range • Problems with arrays

  18. Alternative possibility Doped lead telluride-based alloys

  19. Undoped Lead Telluride-Based Alloys PbTe: narrow-gap semiconductor: • 1. Cubic face-centered lattice of the NaCl type • 2. Direct gap Eg = 190 meV at T = 0 K at the L-point of the Brillouin zone • 3. High dielectric constant  103. • 4. Small effective masses m 10-2me.

  20. Pb1-xSnxTeSolid Solutions: Origin of free carriers: deviation from stoikhiometry  10-3. As-grown alloys: n,p  1018-1019 cm-3 Long-term annealing: n,p > 1016 cm-3

  21. Effects Appearing upon Doping Fermi Level Pinning Effect. PbTe(In), NIn > Ni

  22. Consequences • 1. Absolute reproducibility of the sample parameters independently of the growth technique. Therefore low production costs. • 2. Extremely high spatial homogeneity. • 3. High radiation hardness (stable to hard radiation fluxes up to 1017 cm-2)

  23. Fermi Level Pinning in the Pb1-xSnxTe(In) Alloys.

  24. Persistent Photoconductivity Temperature dependence of the sample resistance R measured in darkness (1-4) and under infrared illumination (1'-4') in alloys with x = 0.22 (1, 1'), 0.26 (2, 2'), 0.27 (3, 3') and 0.29 (4, 4')

  25. Photoconductivity Kinetics Long lifetime of the photoexcited electrons is due to a barrier between local and extended electron states – DX-like impurity centers.

  26. Shubnikov – de Haas oscillations induced by illumination

  27. Mixed valence model: picture

  28. Model for long-term relaxation processes Configuration-coordinate diagram Etot = Eel + Elat = = (Ei-)n +2/20 (n = 0,1,2) – number of localized electrons Free electron In the conduction band Bound state Of one electron Bound electron, The lattice is locally deformed

  29. E2 – ground local state; • E1 – metastable local state

  30. Photoconductivity kinetics Fast relaxationis due to transitionsto themetastablestate,slow relaxationcorresponds to transitions to theground local state

  31. Local metastable states Themetastablestates are responsible for appearance of a range of strong effects: • Enhanced diamagnetic response up to 1% of ideal • Enhancement of effectic dielectric permittivity up to 105at TeraHertz illumination • Giant negative magnetoresistance up to 106 • Persistent photoconductivity in the terahertz spectral range

  32. Spectral response • Two approaches • Low-background: sample screened from the background radiation, low-intensity sources • High-background: sample is not screened from the background radiation, high-intensity sources

  33. High-background approach • Laser wavelengths: 90, 148, 280, 496 m • Pulse length: 100 ns • Power in a pulse: up to30 kW • Sample temperature: 4.2 – 300 K • Samples: single crystalline Pb0.75Sn0.25Te(In), polycrystalline PbTe(In) films

  34. Fermi Level Pinning in the Pb1-xSnxTe(In) Alloys. X=0 X=0.25

  35. Photoconductivity kinetics Time profile of a laser pulse and photoconductivity kinetics at different temperatures

  36. Photoconductivity mechanisms • Negative photoconductivity: electron gas heating, change in electron mobility • Positive photoconductivity: generation of non-equilibrium electrons from metastable impurity states, change in free electron concentration

  37. Dependence of the photoresponse amplitude on the radiation wavelengthfor Pb0.75Sn0.25Te(In) Considerable photoresponseis observed up the wavelengthof 496 mmwhich is more thantwo times higherthan the previous recordvalue of 220 mmobserved for uniaxially stressed Ge(Ga) Linear extrapolation of the quantum efficiency to the zero photoresponsegives the cut-off energy Еred=0!

  38. Kinetics of the terahertz photoresponse in PbTe(In)

  39. Equ EqF E, meV E, meV EqF 100 60 Equ 80 40 EF Equ 60 20 EqF 40 Ec 0 -20 EF 20 0 -40 Ec Pb0.75Sn0.25Te(In) PbTe(In)

  40. New type of local states in semiconductors A new type of semiconductor local states which are linked not to a definite position in the energy spectrum, but to the quasiFermi level position which may be tuned by photoexcitation.

  41. Low-background approach Integrationincreasesthe signal-to noiseratio but It is importantto be able toquench fastthe persistent photoconductivity

  42. Quenching of the Persistent Photoconductivity 1. Thermal quenching: heating to 25 K and cooling down: too slow process. 2. Microwave quenching: application of microwave pulses to the samples f = 250 MHz, P = 0.9 W, t = 10 s

  43. Mechanism of the radiofrequency quenching: experimental Illumination at the wavelength 200 mm We have measured conductivity at the point 1 (100 ms after the pulse) и 2 (900 msafter the pulse) Measured values:s1 s2 (s2-s1)/s1 as a function of - radiofrequency in a pulsef (70 MHz-3 GHz) - pulse lengthDt (1-64 ms) - power in a pulse P (up to 70 mW)

  44. Dependence of the “quenching level” of the radiofrequency Quenching is more effective at low frequencies. The quenching efficiency rises with increasing power in a pulse

  45. Dependence ofDson the radiofrequencyf Too effective quenching at low frequencies leads to the photoresponse decrease! The photoresponse decreases at high frequencies, too. There exists an optimal in the radiofrequency region of quenching

  46. Dependence of the radiofrequency corresponding to the maximal signal on the radiofrequency pulse length As the quenching pulse length increases, the radiofrequencycorresponding to the signal maximumsaturates. The saturation level increaseswith increasing power in a pulse.

  47. Dependence of the relative signal amplitude on the pulse length The relative signal amplitude may reach 40%!

  48. Conclusions of the quenching features • The thermal mechanism of quenching is excluded • The mechanism related to the electron gas heating is likely • As the radiofrequency decreases, the power in a pulse or the pulse length increase, the quenching efficiancy rises • At the same time it is easy to destroy the “photosensitive state” of a sample if the quenching pulse is “too effective”

  49. Operation of an “integrating” photodetector • Options: • Internal modulation • Radiation intensity is constant, • registration of the signal using • a lock-in amplifier • at the frequency of quenching • 2. External modulation • Modulation of the radiation • intensity, registration of the • signal using a lock-in amplifier • at the frequency of modulation

  50. Low-temperature insert 3 4 6 2 1 1 1 Blackbody 2 Thermal shield1 3 Thermal shield2 4 Thermal shield3 5 Stop aperture 6 Sample holder 2 6 5

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