|
|
Facilities
STM
 Air
Scanning Probe Microscopy is a simple, elegant, very powerful technique
to investigate the topography of a wide variety of samples. We have used
it in collaboration with a wide assortment of groups to assess the growth
of various semiconductor crystals. Our Topometrix Explorer scanning probe
microscope (SPM) operates in air or within an controlled air environment.
It can operate as a scanning tunneling microscope (STM) or as and atomic
force microscope (AFM). We use it primarily as an AFM, which allows us
to look at non-conductive and conductive samples. We have found that non-contact
operation is the most versatile mode in which to operate. In this mode,
the tip is not in hard contact with the sample, i.e not in the repulsive
portion of the force vs. separation curve describing a tip-sample interaction.
Rather we are in the attractive or "van der Waals" portion of
the force vs. separation curve. This non-contact operation is achieved
by driving the cantilever, on which the tip is mounted, at one of its
resonance frequencies and detecting the amplitude of the response of the
tip at this frequency. Extremely small forces alter the cantilever resonance,
which in turn reduces its amplitude. This change in amplitude is easily
measured so one can easily measure the small attractive forces between
the tip and sample. Topographic images are made in the usual way by scanning
the tip across the sample and maintaining the change in amplitude,caused
by the tip-sample force, constant. This non-contact operation is crucial
for soft materials such as polymers and still very useful for softer semiconductors
such as InSb and PbSe.
 In
our measurements we typically use high-aspect ratio Si or silicon nitride
tips on high-resonance frequency tips. The radius of curvature of the
tip itself is typically in the 10 to 50 nm range, while the angle of the
tip is greater than about 50 o. Typically our lateral resolution is in
the 20 nm range, while our vertical resolution is often below 1 nm.We
routinely measure angles on samples up to 40 to 50o.
We have used such AFM measurements to look a wide variety of samples
in order to investigate a variety of important issues. These include:
-
The affect of surface roughness on InSb Quantum Well Systems, such
as high-mobility electron layers, which display mobility anisotropy,
or multi-quantum wells which show sharp exciton features.
-
Surface morphology of BaF2/CaF2 Buffer Layers for Growth of PbSe
on Si for Lift-Off applications.
-
Surface morphology of LPE grown of crack-free PbSe layers on Si(100)
using MBE-grown PbSe/BaF2/CaF2 buffer layers.
-
Eu:CaF2 layers on p-Si(100) grown using Molecular Beam Epitaxy as
materials for Si-based optoelectronics.
-
Surface roughness of images chemically etched polycrystalline CdTe
solar cell layers.
|
