Cryogenic Scanning Probe Microscopy
The goal of this project is to use Scanning Probe Microscopy (SPM)
to study electron-electron phenomena in GaAs 2D electron heterostructures.
This project is based on Prof. Gleb Finkelstein
previous work
.
We have created a low temperature Scanning
Probe Microscope (SPM). This is a purely electronic SPM based on the "Besoke
beetle" design. It is designed to operate in our He3 and dilution refrigerators.
Scanning probe microscopes can be purchased commercially but not with
the low temperature capabilities or adaptability required by these projects.
There are two central parameters that limit all possible SPM designs: the
amount of space it can occupy and the amount of heat it can generate (or
import) into the refrigerator. The constructed SPM is designed to fit into
our He3 cryostat (0.3K, 9-11T). Thus it has a bore of 1.5".
The scanning of the tip is made possible by mounting it on a piezotube.
By cutting the tube metallization into four quadrants, the tube it can be
flexed in any direction by application of the proper voltage combination.
The coarse approach of the tip to the sample requires movement of a distance
of millimeters, necessary for sample insertion, to a distance of nanometers,
necessary for scanning. This is a stopping distance five orders of magnitude
less that the distance that need to be traversed. Mechanical course positioning
systems are simpler and more reliable but cannot be used at very low temperatures.
Such a design would require heat-carrying metal screws that extend down
to the microscope. The cooling power of He3 is not great enough to cope
with this heat load. The alternative is to use a piezotube based electronic
approach mechanism.
In our first design we tried using a commercially purchased coarse approach
mechanism. This device turned out to be too sensitive to external vibration.
In our final design we chose a relatively simple method of approach, the
so-called "Besoke beetle" design. [4] This design is based on inertial slider
movement. The sample holder rests atop three piezotubes capped by sapphire
balls. Application of a saw-tooth voltage repeatedly jerks the piezotubes
quickly enough to overcome the static friction and then returns them to
their initial position slowly enough to carry the sample holder along. This
causes a net lateral movement of the sample holder. In the Besoke design
the sample holder has 3 additional shallow ‘ramps’ (4 degrees) on its bottom.
Each ramp occupies 120 degrees of the circular holder. As we apply a proper
voltage combination to the three piezotubes, they move back and forth along
the ramp and the sample moves up and down bringing the tip in and out of
scanning range. This design has the advantage that it can be used for coarse
positioning in all three directions.
Between the 3 sample-supporting piezotubes is a fourth piezotube, the
scan tube. The tip and cryogenic amplifier are mounted atop this tube. The
four tubes are placed in this close arrangement so that the design is mostly
thermally compensated. When the SPM is in STM approach mode after each
step is taken along the ramps, the central piezotube is extended to see
if it can detect a tunneling current. This process can be repeated many
times a second, using very fast electronics that can be purchased commercially,
allowing our tip and sample to approach quickly while preventing a crash.
A major feature of the microscope we have constructed is its adaptability.
Once the course of study outlined below has been completed, the microscope
can easily be converted to do a number of other forms of SPM. These include
superconducting tip STM and tuning fork Atomic Force Microscope (AFM).
[5] More probable is that the knowledge we gained building this microscope
will be used for the construction of SPM’s that can be used by other group
members to carry out these other projects in parallel.
Signal Detection
As mentioned earlier we will be using SCM for measurements as our 2D
electron gas is beneath the surface and thus not available for tunneling.
SCM requires high levels of amplification and high signal-to-noise ratios.
To facilitate this we have mounted a cryogenic amplifier on the tip of
the SPM. [6] This current sensitive preamplifier is build around a high
electron mobility transistor (HMET). Since the operation of this device
does not rely on terminally activated charge carriers there is no carrier
freeze out. The transistor used has been chosen for its low input capacitance
and excellent low temperature performance. The power dissipation of the
amplifier can be kept to 5microWatt making it compatible with all the cryogenic
systems we intend to use.
Sample construction
I will create the structures for study in GaAs. The structure of interest
has to be defined by separating it from the electron gas in the rest of
the sample. This can be done two ways. The first way is by etching away
a trough of a depth greater than the GaAs interface around the area we wish
to study. The second way is to deplete the area of the 2 D Electron Gas
(2DEG) around the structure. This can be done with gating. This method is
typically used for creating a quantum dot. Both types of patterns will be
created by e-beam lithography.
Scanned Images
Here are two typical images from the testing stage of the microscope.
The first shows a gold surface made by evaporating gold onto a flat glass
slide. The second show the atom plateaus in mica. Attaching a layer of gold
to the mica allows this image to be taken. The scales on these images have
not been calibrated.
Back to the nanoscience group.