New Type of Sensitive Infrared and Submillimeter Radiation Photodetectors

1. New Type of Sensitive Infrared and Submillimeter Radiation Photodetectors Dmitry Khokhlov Moscow State University, Moscow, Russia

2. 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 Acknowledgements

3. 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.

4. 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.

5. 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

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

7. Doping withIn, Ga, Tl

8. 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)

9. Shubnikov – de Haas Magnetoresistance Oscillations in PbTe(In)

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

11. 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')

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

13. 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) + U UsuallyU>0 “Negative-U”center:U<0

14. Model for long-term processes Configuration-coordinate diagram Etot = Eel + Elat = = (Ei-)n +2/20 (n = 0,1,2)

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

16. Alternative explanation 2 In2+In+ + In3+ s1p2s2p1s0p3  s0p1pl2 One local level plis formed forN >> 1 (up to 103 - 104) s0p3 centers

17. 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 Quantum efficiency of the photodetector may be increased up to  102 in some special regime of the microwave quenching.

19. 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).

20. 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.

21. Comparison with Si(Sb) and Ge(Ga) l= 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)

22. Kinetics of Response of thePb1-xSnxTe(In) Photodetector at = 90 m and 116 m, at 40 mV Bias Pb1-xSnxTe(In): SI103A/W State of the art Ge(Ga): SI = (3.3-3.5) A/W

23. 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.

24. 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)

25. Focal-Plane “Continuous” Array. Local infrared illumination leads to local generation of photoexcited free electrons. Spatial characteristic scale < 100 m

26. Ideas of Readout • Contact readout • Columns and rows are • insulated at the intersections • Disadvantages: • Sophisticated multiplexing; • n+m electrical leads nrows mcolumns

27. Ideas of Readout 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 Incident radiation Scanning beam of electrons Semitransparent electrode Active Pb1-xSnxTe(In) layer

28. 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

29. 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