Mechanisms of the Persistent Photoconductivity Quenching

in Pb1-xSnxTe(In)

V.I. Chernichkin, D.E. Dolzhenko, L.I. Ryabova, D.R.

KhokhlovM.V. Lomonosov Moscow State

University

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.

KhokhlovM.V. Lomonosov Moscow State

University

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 Bel’kov

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.

Spectrum of the electromagnetic radiation

«Terahertz gap»

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

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

Medical applications

Cancer tissue in theTerahertz 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 dangerMaximum of the blackbody radiation spectral density(m)=3000/T(K)

Sun: T=6000 K, =500 nmEarth: T=300 K, =10 mAsteroids: T=10 K, =300 m =1 THz – Terahertz range!

Terahertz astronomy

10-20

10-19

10-18

10-17

100

101

102

103

104

105

106

1 10

Bac

kgro

und

Pho

ton

Noi

se N

EP

(W/H

z1/2 )

Background photon arrival rate (1/s)

Frequency (THz)

Planck HFI

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

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.

The Space Observatory in the single-dish mode

Telescope: Primary mirror diameter 12 m, surface RMS accuracy 10 m, 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 = 3Medium resolution spectrometers: wavelength ranges 0.03-0.1 mm, and 0.1-0.8 mm spectral resolution R = 1000High 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, R=3 and 1 h integration 5∙10-9 Jy (1 )

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

detectors Kinetic inductance detectors

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

Alternative possibility

Doped lead telluride-based alloys

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.

Pb1-xSnxTe Solid Solutions:

0 0.1 0.2 0.3 0.4

-100

0

100

200

Ev

Ec

E, meV

x

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.

7 1018 cm-3

NIn

p

n

PbTe(In), NIn > Ni

Consequences1. 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)

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

0,0 0,1 0,2 0,3 0,4

-100

-50

0

50

100

150

200

semiinsulating state

p-type metal

n-type metal

Ev

Ec

EF

E, meV

x

Persistent Photoconductivity

0 5 10 15 20 2510-2

10-1

100

101

102

103

104

105

3'

2'

32

4'

41'

1

R, Ohm

100/T, K-1

Temperature dependence of the sample resistance Rmeasured 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

> 104 s

Ordinaryphotoconductor

Persistent photoconductor

tradiationoff

radiation on

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

TeTe

Te

In2+

Te

-e

TeTe

Te

In3+

Te-e

TeTe

Te

In+

Te

-e

Model for long-term relaxation processes

TeTe

Te

In

Te

eh

Configuration-coordinate diagramEtot = Eel + Elat = = (Ei-)n +2/20

(n = 0,1,2) – number of localized electrons

Bound electron,The lattice is locally deformed

Bound state Of one electron

Free electronIn the conduction band

E2 – ground local

state; E1 – metastable

local state

Photoconductivity kinetics

> 104 s

Ordinaryphotoconductor

Persistent photoconductor

tradiationoff

radiation on

Fast relaxationis due to transitionsto the metastable state,slow relaxationcorresponds to transitions to theground local state

Local metastable statesThe 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

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

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

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

0,0 0,1 0,2 0,3 0,4

-100

-50

0

50

100

150

200

semiinsulating state

p-type metal

n-type metal

Ev

Ec

EF

E, meV

x

Photoconductivity kinetics

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

0 1 2 3 4 5 6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

RS

RSRL

hRL

>>

/

*102

t (s)

shape of the laser pulse (arb.un.)

4.2 K

25 K

= 280 m

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

Dependence of the photoresponse amplitude on the radiation wavelength for Pb0.75Sn0.25Te(In)

0 20 40 60 80 100 1200

1

2

3

4

5

6

/

0*102

(cm-1)

496m

280m148m

90m

Considerable photoresponseis observed up the wavelengthof 496 m which is more than two times higherthan the previous recordvalue of 220 m observed for uniaxially stressed Ge(Ga)

Linear extrapolation of the quantum efficiency to the zero photoresponsegives the cut-off energy Еred=0!

Kinetics of the terahertz photoresponse in PbTe(In)

-1 0 1 2 3 4

0.0

0.2

0.4

0 1 2 3 4

-0,5

0,0

0,5

1,0

= 280 m100*

0

t, s

0=4*10-4 Ohm-10=3.3*10-4 Ohm-1

after 200 laser pulses passstationary state

T=4.2 K

= 148 m

= 90 m

0

t, s

EF

Ec

EqF

0

20

40

60

80

100

E, meV

Equ

E, meV

Ec

EF

EqF

Equ

0

20

40

60

-20

-40

PbTe(In) Pb0.75Sn0.25Te(In)

EqF

Equ

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.

Low-background approach

> 104 s

Ordinaryphotoconductor

Persistent photoconductor

tradiationoff

radiation on

Integrationincreases the signal-to noiseratio

butIt is important to be able toquench fast the persistentphotoconductivity

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 samplesf = 250 MHz, P = 0.9 W, t = 10 s

Mechanism of the radiofrequency quenching: experimental

0 200 400 600 800 1000

0

10

20

30

40

50

P, m

W

t, s

t=100 s t=900 spoint 1 point 2

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:1 2 (2-1)/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 mW)

Dependence of the “quenching level” of the radiofrequency

0 500 1000 1500 2000 2500 3000 3500

0,0

0,5

1,0

1,5

2,0

2,5

P16mW32 mW79 mW

1, mS

f, MHz

T=32 s Quenching is more effective at low frequencies.

The quenching efficiency rises with increasing power in a pulse

Dependence of on the radiofrequency f

0 500 1000 1500 2000 2500 3000

0

5

10

15

20

25

T=32 s

S

f, MHz

12 15 19

P16mW32 mW79 mW

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

0 10 20 30 40 50

200

400

600

800

1000

P16mW32 mW79 mW

f (

max

), M

hz

T, s

As the quenching pulse length increases, the radiofrequencycorresponding to the signal maximumsaturates.

The saturation level increaseswith increasing power in a pulse.

Dependence of the relative signal amplitude on the pulse length

0 10 20 30 40 500

10

20

30

40

T, s

P16mW32 mW79 mW

m

ax, / 1, %

The relative signal amplitude may reach 40%!

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”

<< I

3I2I

1

I3

I2

I1

tquenching pulses

Operation of an “integrating” photodetector

Options:1. Internal modulationRadiation intensity is constant,registration of the signal usinga lock-in amplifier at the frequency of quenching2. External modulationModulation of the radiation intensity, registration of the signal using a lock-in amplifier at the frequency of modulation

Low-temperature insert

1 Blackbody2 Thermal shield13 Thermal shield24 Thermal shield35 Stop aperture6 Sample holder6

6

5

2

2

3 4

1

1

Internal modulationSingle 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 1Hz); NEP = 8*10-17 W/Hz1/2

Problems

Possible thermal leaks

Measurements with a filter

Question with transients

tLight off

Usual set up

Blackbody

Chopper T=300K

Input 300 K window

Input 1.5 K filter

Sample

50 mKcold finger

Background power 1.4 * 10-13 WFluctuations 7.3 * 10-18 W/Hz1/2

Electrical connections

Lock-inamplifier

DC-RFfilter

Oscilloscope

RFgenerator

Drivingpulsegenerator

Thermocouple

BlackbodyHeater wires

input

output

K=1000

RL=1k

U=0.1V

Rs

Blackbody temperature modulation

0 5000 10000 150001,0

1,5

2,0

2,5

3,0

T, K

t, ms

Performance at 1.57 KSingle 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/Hz1/2 !!! WOW!!!

BUT

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

Therefore

No firm conclusion yet

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 = 6×10-20 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 μmHOWEVERfurther tests are needed to confirm this

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

2-nd International Conference "Terahertz and Microwave radiation: Generation, Detection and Applications“

Moscow, June 20-22 Tera2012.phys.msu.ru