PowerPoint PresentationJ. Moers, D. Gruetzmacher, {2}
1Institute 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
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Outline:
Motivation
Ge
V
holes
Si
High density of QDs(~4×1011cm-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
Motivation
-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.
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
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.





G=Gi
R=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.
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
SI
Si
and macroscopic (a) samples.
Interband illumination
= 0.9m
new potential landscape new
conductive path providing change
Changing of the hole numbers in QD
under illumination
Effect of different structure size and geometry
on photoconductance kinetics
2D-short
Quasi-1D
G=(G2-G1)/G1 – discrimination level
G1
G2
Number of counts with different fluctuation amplitude in dark and under illumination
(1-70, 2-100, 3-150, 4-200 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
=1.55 m
= 0.9m
Illumination pulse
PL=2,65×10-7 W
Number of incident photons (λ=1,5 )
n=PL/(hc/λ)=1416 s-1
Absorption coefficient in QDs
nabs=k×n~10
Structures on SOI-substrate
temperature measurements
-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
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 1.55m wavelength illumination is about of 10-20%.
i
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V
E
C
E
F
E
5
10
15
20
25
10
-7
10
-6
10
-5
10
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10
-3
10
-2
10
-1
Conductance (
e
2