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Photo-induced conductance fluctuations in mesoscopic Ge/Si systems with quantum dots N.P. Stepina, A.V. Dvurechenskii, A.I. Nikiforov {1} J. Moers, D. Gruetzmacher, {2} 1 Institute of Semiconductor Physics, Novosibirsk, Russia 2 Institute of Bio- and Nanosystems, Forschungszentrum Julich, Germany INSTITUTE OF SEMICONDUCTOR PHYSICS, SIBERIAN BRANCH OF THE RUSSIAN ACADEMY OF SCIENCE o o o o Outline: Experimental data and discussion Summary Motivation Samples preparation and structure characterization

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Motivation Ge V holes Si High density of QDs(~410 11 cm -2 ) allows to observe hopping among tunnel-coupled QDs To change the hole filling factor it is possible to change the conductance of the system Strong non-monotonic dependence of VRH on number of holes in QDs is the characteristic feature of QD system. (Yakimov) 2s 4p

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INSTITUTE OF SEMICONDUCTOR PHYSICS, SIBERIAN BRANCH OF THE RUSSIAN ACADEMY OF SCIENCE Motivation Photoconductance in macroscopic samples Results: -Both positive and negative photoeffect are observed in QD samples. -Kinetics of photoconductance is anomalously slow. -Persistant photocondactance is observed after several hours of relaxation.

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Correlation radius L K In mesoscopic samples (size smaller than L K ), there is no self-averaging among different realization of the current paths One can observe the physical processes corresponding to the unit events of network transformation As conductance depends on the particular realization of the potential, the illumination should provoke the conductance fluctuations Motivation The aim of this work is to show the possibility to observe the photo-stimulated conductance switchings under single photon absorption in mesoscopic structures with quantum dots.

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The structures under study Channel size ~70-500 nm G=GiG=Gi R=RiR=Ri We present the experimental results of photo-induced conductance fluctuations in nanometer size QDs structures with different width and length of conductance channels under small flux of infrared illumination.

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Source meter: Keithley 6430 Electrometer: Keithley 6514 Pre-amplifier on the basis of INA116 chip for differential measurement of voltage GUARDING around of the signal wires for preventing of leakage current and shunting of parasitic capacitance. Experimental setup R C rr electrometer Source meter sample preamplifier Laser =1.55, 0.9 m W=1mW SI Si

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Photoconductance fluctuations in mesoscopic structures Photoconductance kinetics for meso- (b) and macroscopic (a) samples. =1.5 m

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INSTITUTE OF SEMICONDUCTOR PHYSICS, SIBERIAN BRANCH OF THE RUSSIAN ACADEMY OF SCIENCE Interband illumination = 0.9 m = 0.9 m = Illumination with =1.55 m Motivation Redistribution of the carriers between different QDs inder illumination new potential landscape new conductive path providing change of the conductance with time. Changing of the hole numbers in QD under illumination New conductive path providing change of the conductance with time.

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Effect of different structure size and geometry on photoconductance kinetics Photoconductance kinetics for samples with different size and geometry. 2D-short Quasi-1D

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The method of experimental fluctuation treatment G=(G 2 -G 1 )/G 1 discrimination level G1G1 G2G2 Number of counts with different fluctuation amplitude in dark and under illumination (1-70, 2-100, 3-150, 4-200 nm channel width).

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Dependence of counts on light intensity Linear dependence of counts on light intensity as expected for a single-photon process

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Pulse excitation Every pulse causes step-like change in the conductance = =1.55 m = 0.9 m = 0.9 m Illumination pulse

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Problems: Low efficiency Low temperature Decisions: many-layers QD structure Bragg mirrors SOI-substrate Output laser power (for 1.5 m) P L =2,6510 -7 W Power on sample P =P L 2r 2 S/( 2 l 2 )=1,8710 -16 W Number of incident photons (=1,5 ) n=P L /(hc/)=1416 s -1 Absorption coefficient in QDs k=810 -4 Number of absorbed photons per pulse n abs =kn~10 Internal efficiency ~10%

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Increase of the detecting temperature Structures on SOI-substrate SOI Si

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Comparison between low and high temperature measurements 4.2K Size~150 -Decrease of the correlation radius with increase of the temperature? -Decrease of the depletion range with increase of the doping?

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Mesoscopic scale at different temperatures connection criterion ES : ~1.34 for 2D L K (4K)~0.3-1.1 m, 77K- 15-55 nm????

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Conclusion The samples with channel size 70-200 nm show the mesoscopic behavior in conductance at 4.2K. In QDs grown on SOI substrate, the temperature of the transition from band to hopping transport increase from 25 to 100K. Increase of the temperature up to 77K significantly decrease the characteristic value of the mesoscopic scale. It was shown that the dark noise does not exceed 10% value of fluctuation amplitude. Under illumination giant (up to 70%) step-like switching of the conductance was observed in mesoscopic samples with channel size 70-200 nm at 4.2K. Single-photon mode operation is indicated by the linear dependence of the frequency of photo-induced fluctuations on the light intensity and the step-like response of conductance on the pulse excitation. The number of counts is linearly changes with light intensity as it expected for single-photon process. The internal efficiency of detection at 1.55 m wavelength illumination is about of 10-20%.