Mechanisms of the Persistent Photoconductivity Quenching in Pb1-xSnxTe(In)

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
Cooperation

• 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 Belkov

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

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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
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Asteroid danger

• Maximum of the blackbody radiation spectral
  density
• l(mm)3000/T(K)
• Sun T6000 K, l500 nm
• Earth T300 K, l10 mm
• Asteroids T10 K, l300 mm
• u1 THz Terahertz range!

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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

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Proton-M launcher, L2 orbit, 45002100 kg.
SB space buster DM, SM service module, WC
warm cabin, TSEM thermal screens expanding
mast, CC cold cabin, T telescope.
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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 m2, R3 and 1 h integration
510-9 Jy (1 s)
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State of the art sensitive terahertz detectors

• Transition edge sensors
• Hot electron bolometers
• Ge(Ga) blocked impurity band detectors
• Kinetic inductance detectors

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Problems (as I see them)

• Very low operating temperature lt 150 mK
• NEP not better than 410-19 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 ? 10-2 me.

20
Pb1-xSnxTe 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 gt 1016
cm-3
21
Effects Appearing upon Doping
Fermi Level Pinning Effect.
PbTe(In), NIn gt 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')
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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 the
metastable state,slow relaxationcorresponds to
transitions to theground local state
31
Local metastable states

• The metastable states 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 105 at 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 to 30 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.
X0
X0.25
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 wavelength for Pb0.75Sn0.25Te(In)
Considerable photoresponseis observed up the
wavelengthof 496 mm which is more than two
times higherthan the previous recordvalue of
220 mm observed for uniaxially stressed Ge(Ga)
Linear extrapolation of the quantum efficiency to
the zero photoresponsegives the cut-off energy
?red0!
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Kinetics of the terahertz photoresponse in
PbTe(In)
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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
Integrationincreases the signal-to noiseratio
but
It is important to be able toquench fast the
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

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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) ? 2 (900 ms after the pulse)
Measured valuess1 s2 (s2-s1)/s1 as a
function of - radiofrequency in a pulse f (70
MHz-3 GHz) - pulse length Dt (1-64 ms) - power in
a pulse P (up to 70 mW)
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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 of Ds 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
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

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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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Internal modulation

• 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 300200 ?m
• quenching rate 1000 Hz
• lock-in amplifier integration time 1 s (bandwidth
  1Hz)
• NEP 810-17 W/Hz1/2

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Problems

• Possible thermal leaks
• Measurements with a filter
• Question with transients

s
t
Light off
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Usual set up
Input 300 K window
Sample
50 mK cold finger
Input 1.5 K filter
Blackbody
Chopper
T300K
Background power 1.4 10-13 W Fluctuations 7.3
10-18 W/Hz1/2
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Electrical connections
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Blackbody temperature modulation
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Performance at 1.57 K

• 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, Q4)
• area 300200 ?m
• quenching rate 1000 Hz
• blackbody modulation rate 0.3 Hz
• lock-in amplifier integration time 100 s
• Blackbody temperature providing S/N1 Tbb2.7 K
• NEP 610-20 W/Hz1/2 !!!
• WOW!!!

BUT
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Problems

• 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

58
Therefore
No firm conclusion yet
59
Summary

• 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
  quasiFermi level position
• We have demonstrated NEP 610-20 W/Hz1/2 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

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Directions of the future activities

• 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

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• 2-nd International Conference "Terahertz and
  Microwave radiation Generation, Detection and
  Applications
• Moscow, June 20-22
• Tera2012.phys.msu.ru