Photo-induced conductance fluctuations in mesoscopic Ge/Si systems with quantum dots N.P. Stepina,...
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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 ACADEM OF SCIENCE o o o o Outline: Experimental data and discussion Summary Motivation Samples preparation and structure characterization
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
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
Slide 2
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
Slide 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. -Persistant photocondactance
is observed after several hours of relaxation.
Slide 4
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.
Slide 5
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.
Slide 6
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
Slide 7
Photoconductance fluctuations in mesoscopic structures
Photoconductance kinetics for meso- (b) and macroscopic (a)
samples. =1.5 m
Slide 8
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.
Slide 9
Effect of different structure size and geometry on
photoconductance kinetics Photoconductance kinetics for samples
with different size and geometry. 2D-short Quasi-1D
Slide 10
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).
Slide 11
Dependence of counts on light intensity Linear dependence of
counts on light intensity as expected for a single-photon
process
Slide 12
Pulse excitation Every pulse causes step-like change in the
conductance = =1.55 m = 0.9 m = 0.9 m Illumination pulse
Slide 13
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%
Slide 14
Increase of the detecting temperature Structures on
SOI-substrate SOI Si
Slide 15
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?
Slide 16
Mesoscopic scale at different temperatures connection criterion
ES : ~1.34 for 2D L K (4K)~0.3-1.1 m, 77K- 15-55 nm????
Slide 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%.