Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru

46 J.M. Elzerman et al.
waveform generator (Sony Tektronix AWS520), which can generate more complicated
pulses with a rise time of 1.5 ns. With the cables described above, the
fastest pulse flank we can transmit to the sample is about 200 ps. Microwave
signals are transmitted with about 10 dB loss at 50 GHz.
Special care needs to be given to the connection from the coaxial cable to
the chip, in order to minimize reflections. The sample holder we use, has an
SMA connector that can be connected to the 2.4 mm coaxial cable. At the
other end, the pin of the SMA connector sticks through a small hole in the
chip carrier. This allows it to be soldered to a metal pad on the chip carrier,
from which we can then bond to the chip. This sample holder is used to apply
pulses or microwave signals to a gate electrode.
1.8 Sample Stability
A severe experimental difficulty that is not related to the measurement setup,
but to the sample itself, is the problem of “charge switching”. It shows up in
measurements as fluctuations in the position of a Coulomb peak, or as sudden
jumps in the QPC-current that are not related to charging or discharging of a
nearby quantum dot. Generally, these switches are attributed to (deep) traps
in the donor layer that capture or release an electron close to the quantum
dot [37]. This well-known but poorly understood phenomenon is a manifestation
of 1/f noise in semiconductors, which causes the electrostatic potential
landscape in the 2DEG to fluctuate.
The strength of the fluctuations can differ enormously. In some samples,
switching occurs on a time scale of seconds, making only the most trivial
measurements possible, whereas in other samples, no switches are visible on
a time scale of hours. It is not clear what exactly determines the stability. It
certainly depends on the heterostructure, as some wafers are clearly better
than others. A number of growth parameters could be important, such as the
Al concentration in the AlGaAs, the doping density and method (modulation
doping or delta doping), the thickness of the spacer layer between the n-
AlGaAs and GaAs, the depth of the 2DEG below the surface, a possible
surface layer, and many more. We have recently started a collaboration with
the group of Professor Wegscheider in Regensburg to grow and characterize
heterostructures in which some of these parameters are systematically varied.
In this way we hope to find out what makes certain heterostructures stable.
Even for the same heterostructure, some samples are more quiet than
others. The reasons for this are not clear. There are reports that stability is
improved if the sample is cooled down slowly, while applying a positive voltage
(about +280 mV) on all gates that are going to be used in the experiment.
This procedure effectively “freezes in” a negative charge around the gates,
such that less negative gate voltages are sufficient to define the quantum dot
at low temperatures. Most samples described in this work have been cooled
down from room temperature to 4 K slowly (in one to two days) with all gates