new type of sensitive infrared and submillimeter radiation photodetectors dmitry khokhlov moscow...
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New Type of Sensitive Infrared and Submillimeter Radiation Photodetectors
Dmitry Khokhlov
Moscow State University, Moscow, Russia
Acknowledgements N.B.Brandt B.A.Akimov L.I.Ryabova S.N.Chesnokov I.I.Ivanchik D.E.Dolzhenko
D.Watson J.Pipher
A.Nikorich E. Slyn’ko
B.Volkov K.Kikoin
A.Artamkin A.Kozhanov N.Matrosov
Outline
1. Undoped lead telluride-based alloys. 2. Effects appearing upon doping. a) Fermi level pinning effect. b) Persistent photoconductivity. 3. Theoretical models 4. Pb1-xSnxTe(In)-based infrared photodetectors. a) Radiometric parameters. b) Comparison with Si(Sb) and Ge(Ga). c) Spectral response. d) “Continuous” focal plane array. 5. Summary.
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
Doping with In, Ga, Tl
Ev
Ec
190 meV
130 meV
70 meV
Tl
In
Ga
70 meV
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)
Shubnikov – de Haas Magnetoresistance Oscillations in
PbTe(In)
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.
Model of the mixed valence
2In2+In3+ + In+
neutr. donor accept.
Еion(left) = Е(1) + Е(1)
Еion(right) = Е(2) + Е(1)
Е(2) = Е(1) + UUsually U>0“Negative-U” center: U<0
In3+
In+
In2+
e-
e-
e-
In3+In3+
In3+In3+
e-
In2+
Model for long-term processes
Configuration-coordinate diagramEtot = Eel + Elat =
= (Ei-)n +2/20
(n = 0,1,2)
E2 – ground local state;
E1 – metastable local state
Alternative explanation
Ec
Ev
s2p1
s1p2
s0p1pl2 s1p2 s2p1 s0p3
s0p1pl
2
One local level pl is formed for N >> 1 (up to 103 - 104) s0p3 centers
2 In2+ In+ + In3+
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 sQuantum efficiency of the photodetector may be increased up to 102 in some special regime of the microwave quenching.
<< I
3I2I
1
I3
I2
I1
t
quenching pulses
Pb1-xSnxTe(In)-Based
Infrared Photodetectors Single photodetector operating in the regime of the periodical accumulation and successive fast quenching of the photosignal, quantum efficiency stimulation regime.
operating temperature 4.2 K; wavelenghth 18 m (defined by the filter); operating rate 3 Hz; area 300*200 m; current sensitivity > 107 A/W; minimal power detected < 10-16 W (sensitivity
of the measuring electronics was only 10-7 A).
Experimental setup 1 - 300 K or 77 K
blackbody; 2 - 300 K window; 3 - 77 K filter; 4 - 4.2 filter and a
stop aperture; 5 - cold filter on the
filter wheel; 6 - filter wheel; 7 - sample; 8 - liquid helium can.
Comparison with Si(Sb) and Ge(Ga)
-0,5 0,0 0,5 1,0 1,5 2,0 2,5
0,0
5,0x10-3
1,0x10-2
1,5x10-2
2,0x10-2
amplifier overload
77
300
I, A
V, V
= 14 m; state of the art Si(Sb) BIB
Pb1-xSnxTe(In) photodetector: dark current at the minimal possible bias of 40 mV (red point)
Kinetics of Response of the Pb1-xSnxTe(In) Photodetector at = 90 m and 116 m, at 40 mV Bias
0 10 20 30 40 50 60 70 800,0
2,0x10-2
4,0x10-2
6,0x10-2
8,0x10-2
1,0x10-1
1,2x10-1
1,4x10-1
1,6x10-1
77 K, 89 m
77 K, 115 m300 K, 89 m
300 K, 115 m
I, A
t, s
Pb1-xSnxTe(In): SI103A/W
State of the art Ge(Ga): SI = (3.3-3.5) A/W
Extremely important:
E=(90, 116)m < Ea
Photoresponse is due to excitation from metastable excited local states.The cutoff wavelength red may be much higher than 220 m.
Ea E
ground
Emetastable
Ec
Ev
Photoresponse at 176 m and 241 m
Strong photoresponse at both 176 and 241 m
= 241 m is higher than red = 220 m observed for Ge(Ga) 0 200 400 600 800
0,0
1,0x10-2
2,0x10-2
3,0x10-2
4,0x10-2
5,0x10-2
6,0x10-2
Cur
rent
, A
Time, s
241 m
176 m
Focal-Plane “Continuous” Array.
Local infrared illumination leads to local generation of photoexcited free electrons.
Spatial characteristic scale < 100 m
radiation "spot"photodetector
h
Ideas of Readout
n rows
m columns
1.Contact readout
Columns and rows are insulated at the intersections
Disadvantages:
Sophisticated multiplexing;
n+m electrical leads
Ideas of Readout
Incident radiation
Scanning beamof electrons
Semitransparentelectrode
Active Pb1-xSnxTe(In)layer
2. Electron beam readout
Disadvantages:
Difficulties with high vacuum and low temperatures;
Not clear how electrons will affect conductivity;
Heated cathode needs to be screened from the sample
Ideas of Readout
3. Laser beam readout
1 - semitransparent electrodes
2 - active Pb1-xSnxTe(In) layer
3 - fluoride buffer layer, 4 – wide-gap
semiconductor layer, 5 - short-wavelength laser 6 - incident infrared
radiation flux 1 12 3 4
56
A
Summary
Pb1-xSnxTe(In) photodetectors have a number of advantageous features that allow them to compete successfully with the existing analogs:
Internal accumulation of the incident radiation flux,
Possibility of effective fast quenching of an accumulated signal
Microwave stimulation of the quantum efficiency up to 102
Possibility of realization of a "continuous" focal-plane array
Possibility of application of a new readout technique
High radiation hardness
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