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

Motivation Ge V дырки Si High density of QDs(~4×10 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

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  m

Correlation radius L~1-4  m In mesoscopic samples (size smaller than L), 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.

The structures under study Channel size ~ nm G=GiG=Gi R=RiR=Ri

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 electromete r Source meter sample preamplifier R C rуrу Laser λ=1.55, 0.9  m W=1mW

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

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 Mechanism Redistribution of the carriers between different QDs under illumination new potential landscape new conductive path providing a step like 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.

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

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, nm channel width).

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

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

Structures on SOI-substrate SOI Si ~160 нм

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

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

Conclusion The samples with channel size nm show the mesoscopic behavior in conductance. The method of analyzing of counts number at different fluctuation amplitude was proposed. 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 nm. 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. Increase of the temperature leads to decrease of the scale for mesoscopic behavior.