Mechanisms of the Persistent Photoconductivity Quenching in Pb 1 -x. Snx. 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. Snx. Te(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
Cooperation n M. V. Lomonosov Moscow State University n n Institute of Applied Physics, Kishinev, Moldova n n Ludmila Ryabova Dmitry Dolzhenko Vladimir Chernichkin Andrey Nicorici University of Beer Sheva, Israel n n Vladimir Kasiyan Zinovy Dashevsky n University of Regensburg n n n Sergey Ganichev Sergey Danilov A. F. Ioffe Physical. Technical Institute, St. Petersburg n Vassily Bel’kov
Outline n n n 1. 2. 3. n n n 4. 5. 6. Introduction Undoped lead telluride-based alloys. Effects appearing upon doping. a) b) c) Fermi level pinning effect. Persistent photoconductivity. Theoretical model Terahertz photoconductivity and local metastable states Pb 1 -x. Snx. Te(In)-based terahertz photodetectors. Summary.
Spectrum of the electromagnetic radiation «Terahertz gap»
Terahertz radiation n n In this spectral region both radiophysics methods (at the long-wavelength side) and optical methods (at the shortwavelength side) work not well Consequence: absence of good sources and sensitive detectors of radiation
Areas of application of the Terahertz radiation n n Monitoring of concentration of heavy organic molecules Medical applications (oncology, stomatology) Meteorology Security systems (search and detection of explosives) Infrared astronomy
Medical applications Cancer tissue in the. Terahertz and in the visible spectral range
Security systems A boot with a ceramic knife and a plastic explosive “Semtex” in its sole
Security systems A polyethylene box under a 10 cm layer of sand. Pictures are taken in the Terahertz range
Asteroid danger Maximum of the blackbody radiation spectral density l( m)=3000/T(K) Sun: T=6000 K, l=500 nm Earth: T=300 K, l=10 m Asteroids: T=10 K, l=300 m u=1 THz – Terahertz range!
Terahertz astronomy
Russian Space Missions in Terahertz and Millimeter Ranges n RADIOASTRON n n n Test launch – 21 January 2011 Launch scheduled for July 2011 MILLIMETRON n n 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
Proton-M launcher, L 2 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.
The Space Observatory in the single-dish mode Telescope: Primary mirror diameter 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 (at 1. 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: at 1 THz, NEP = 10 -19 W(s)0. 5, A = 100 m 2, R=3 and 1 h integration 5∙ 10 -9 Jy (1 s)
State of the art sensitive terahertz detectors n n Transition edge sensors Hot electron bolometers Ge(Ga) blocked impurity band detectors Kinetic inductance detectors
Problems (as I see them) n n Very low operating temperature < 150 m. K NEP not better than 4*10 -19 W/Hz 1/2 in the lab and not better than 10 -17 W/Hz 1/2 in real space missions Quite poor dynamic range Problems with arrays
Alternative possibility Doped lead telluride-based alloys
Undoped Lead Telluride-Based Alloys Pb. Te: narrow-gap semiconductor: n 1. Cubic face-centered lattice of the Na. Cl type n 2. Direct gap Eg = 190 me. V at T = 0 K at the L-point of the Brillouin zone n 3. High dielectric constant 103. n 4. Small effective masses m 10 -2 me.
Pb 1 -x. Snx. Te Solid 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
Effects Appearing upon Doping Fermi Level Pinning Effect. Pb. Te(In), NIn > Ni
Consequences n 1. Absolute reproducibility of the sample parameters independently of the growth technique. Therefore low production costs. n 2. Extremely high spatial homogeneity. n 3. High radiation hardness (stable to hard radiation fluxes up to 1017 cm-2)
Fermi Level Pinning in the Pb 1 -x. Snx. Te(In) Alloys.
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')
Photoconductivity Kinetics Long lifetime of the photoexcited electrons is due to a barrier between local and extended electron states – DX-like impurity centers.
Shubnikov – de Haas oscillations induced by illumination
Mixed valence model: picture
Model for long-term relaxation processes Free electron In the conduction band Bound state Of one electron Configurationcoordinate diagram Etot = Eel + Elat = = (Ei- ) n + 2/2 0 (n = 0, 1, 2) – number of localized electrons Bound electron, The lattice is locally deformed
n n E 2 – ground local state; E 1 – metastable local state
Photoconductivity kinetics Fast relaxation is due to transitions to the metastable state, slow relaxation corresponds to transitions to the ground local state
Local metastable states The metastable states are responsible for appearance of a range of strong effects: n n Enhanced diamagnetic response up to 1% of ideal Enhancement of effectic dielectric permittivity up to 105 at Tera. Hertz illumination Giant negative magnetoresistance up to 106 Persistent photoconductivity in the terahertz spectral range
Spectral response n Two approaches n n Low-background: sample screened from the background radiation, low-intensity sources High-background: sample is not screened from the background radiation, highintensity sources
High-background approach n n n Laser wavelengths: 90, 148, 280, 496 m Pulse length: 100 ns Power in a pulse: up to 30 k. W Sample temperature: 4. 2 – 300 K Samples: single crystalline Pb 0. 75 Sn 0. 25 Te(In), polycrystalline Pb. Te(In) films
Fermi Level Pinning in the Pb 1 -x. Snx. Te(In) Alloys. X=0. 25
Photoconductivity kinetics Time profile of a laser pulse and photoconductivity kinetics at different temperatures
Photoconductivity mechanisms n n 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
Dependence of the photoresponse amplitude on the radiation wavelength for Pb 0. 75 Sn 0. 25 Te(In) Considerable photoresponse is observed up the wavelength of 496 m which is more than two times higher than the previous record value of 220 m observed for uniaxially stressed Ge(Ga) Linear extrapolation of the quantum efficiency to the zero photoresponse gives the cut-off energy Еred=0!
Kinetics of the terahertz photoresponse in Pb. Te(In)
Equ E, me. V 100 Equ Eq. F 80 E, me. V 60 40 EF 60 20 40 0 20 -20 0 Equ Ec Pb. Te(In) Eq. F Ec EF -40 Pb 0. 75 Sn 0. 25 Te(In)
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 quasi. Fermi level position which may be tuned by photoexcitation.
Low-background approach Integration increases the signal-to noise ratio but It is important to be able to quench fast the persistent photoconductivity
Quenching of the Persistent Photoconductivity n n 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
Mechanism of the radiofrequency quenching: experimental Illumination at the wavelength 200 m We have measured conductivity at the point 1 (100 s after the pulse) и 2 (900 ms after the pulse) Measured values: s 1 s 2 (s 2 -s 1)/s 1 as a function of - radiofrequency in a pulse f (70 MHz-3 GHz) - pulse length t (1 -64 s) - power in a pulse P (up to 70 m. W)
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
Dependence of s on the radiofrequency f 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
Dependence of the radiofrequency corresponding to the maximal signal on the radiofrequency pulse length As the quenching pulse length increases, the radiofrequency corresponding to the signal maximum saturates. The saturation level increases with increasing power in a pulse.
Dependence of the relative signal amplitude on the pulse length The relative signal amplitude may reach 40%!
Conclusions of the quenching features n n 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”
Operation of an “integrating” photodetector Options: 1. 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
Low-temperature insert 3 4 6 2 1 1 2 6 5 1 2 3 4 5 6 Blackbody Thermal shield 1 Thermal shield 2 Thermal shield 3 Stop aperture Sample holder
Internal modulation n n n Single photodetector operating in the regime of the periodical accumulation and successive fast quenching of the photosignal. operating temperature 4. 2 K; wavelenghth below 1100 m (defined by the stop aperture diameter); area 300*200 m; quenching rate 1000 Hz; lock-in amplifier integration time 1 s (bandwidth 1 Hz); NEP = 8*10 -17 W/Hz 1/2
Problems n n t Light off Possible thermal leaks Measurements with a filter Question with transients
Usual set up Input 300 K window Sample 50 m. K cold finger Input 1. 5 K filter Blackbody Chopper T=300 K Background power 1. 4 * 10 -13 W Fluctuations 7. 3 * 10 -18 W/Hz 1/2
Electrical connections
Blackbody temperature modulation
Performance at 1. 57 K n n n n n Single photodetector operating in the regime of the periodical accumulation and successive fast quenching of the photosignal. operating temperature 1. 57 K; wavelenghth 350 m (defined by the filter, Q=4); area 300*200 m; quenching rate 1000 Hz; blackbody modulation rate 0. 3 Hz; lock-in amplifier integration time 100 s; Blackbody temperature providing S/N=1 Tbb=2. 7 K NEP ~ 6× 10 -20 W/Hz 1/2 !!! WOW!!! BUT
Problems n n n No control on the signal form Possible thermal leaks Possible radiation leaks Possible influence of the off-band transmission of the filter Possible cross-talks of the blackbody heater and the measurement circuit
Therefore No firm conclusion yet
Summary n n We have observed a new type of semiconductor local states which are linked not to a definite position in the energy spectrum, but to the quasi. Fermi level position We have demonstrated NEP = 6× 10 -20 W/Hz 1/2 for a single photodetector operating in the regime of the periodical accumulation and successive fast quenching of the photosignal, with the operating temperature 1. 57 K at the wavelength of 350 μm HOWEVER further tests are needed to confirm this
Directions of the future activities n n n n Measurements of the photon noise Single photon counting? Why not Development of the portable readout electronics Development of linear arrays and full-scale arrays Development of tunable terahertz filters Development of a system for passive terahertz vision in medical applications Investigation for possibilities of application in space missions
n n n 2 -nd International Conference "Terahertz and Microwave radiation: Generation, Detection and Applications“ Moscow, June 20 -22 Tera 2012. phys. msu. ru
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