Photo-induced conductance fluctuations in mesoscopic Ge /Si systems with quantum dots

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

2. Motivation 2s 4p Ge Si High density of QDs(~4×1011cm-2)allows to observe hopping among tunnel-coupled QDs holes V Strong non-monotonic dependence of VRH on number of holes in QDs is the characteristic feature of QD system. (Yakimov) To change the hole filling factor it is possible to change the conductance of the system

3. 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. -Persistantphotocondactance is observed after several hours of relaxation.

4. Motivation Correlation radius LK In mesoscopic samples (size smaller than LK), 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 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.

5. The structures under study G=Gi R=Ri Channel size~70-500 nm 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.

6. Experimental setup Source meter Laser λ=1.55, 0.9mW=1mW sample preamplifier 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. Si SI rу C R electrometer

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

8. INSTITUTE OF SEMICONDUCTOR PHYSICS, SIBERIAN BRANCH OF THE RUSSIAN ACADEMY OF SCIENCE Motivation Illumination with  =1.55 m = 0.9m Interband illumination 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.

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

10. The method of experimental fluctuation treatment G=(G2-G1)/G1 – discrimination level G2 G1 Number of counts with different fluctuation amplitude in dark and under illumination (1-70, 2-100, 3-150, 4-200 nm channel width).

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

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

13. Problems: Decisions: • Low efficiency • Low temperature • many-layers QD structure • Bragg mirrors • SOI-substrate Output laser power (for 1.5 m) PL=2,65×10-7W Power on sample PО=PL×2πr2S/(λ2l2)=1,87×10-16W Number of incident photons (λ=1,5 мкм) n=PL/(hc/λ)=1416 s-1 Absorption coefficient in QDs k=8×10-4 Number of absorbed photons per pulse nabs=k×n~10 Internal efficiency~10%

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

15. Comparison between low and high temperature measurements 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?

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

17. 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%.