Substrates Used for Scanning Probe Microscopy

university wafer substrates

What Gallium Arsenide Substrate Should be ued for Canning Probe Microscopy?

A postdoc requeste a quote for the following:

Id like to purchase some semi-insulating GaAs; preferably a 2" diameter wafer polished flat enough for scanning probe microscopy measurements. Pleas e let me know what you have in stock, what the specs are for these wafers, and what the pricing would be. Also, do you supply LT-GaAs? If so, Id like to know what you have in stock.

Reference #252628 for specs and pricing.

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What is Scanning Probe Microscopy?

SPM (Scanning Probe Microscopy) is a set of related imaging methods in which variations in the interaction force between a probe and a sample surface are used to generate image contrast.

SPMs are incredibly sensitive and can measure forces on the order of those required to break physical and chemical bonds. Under optimal conditions, they can produce atomic-scale resolution.

What are the Key Terms Associated With Scanning Probe Microscopy?

The following terms are often used:

  1. Scanning Probe Microscopy (SPM)
  2. Nanoscale
  3. Imaging
  4. Analysis
  5. Scanning Tunneling Microscopy (STM)
  6. Atomic Force Microscopy (AFM)
  7. Magnetic Force Microscopy (MFM)
  8. Materials Science
  9. Biology
  10. Electronics

What is a SPM?

Scanning Probe Microscopy (SPM) is a family of surface measurement techniques that can image atomic-scale structures and processes. Unlike electron microscopes, which must cut a sample to measure its structure, SPM can do so without damaging the specimen. In addition, these imaging techniques operate in a vacuum, which makes them more suitable for studying surfaces.

SPM imaging is generally performed with a tip that can be mounted on a force-sensing cantilever. The cantilever is manipulated to apply attractive and repulsive forces on the sample, and at each point the tip's position on the surface is recorded. This information is then sent to a computer where an image of the surface is formed.

The cantilever consists of a rigid wire or rod, with a sharp tip attached to one end. It can be fabricated from several materials, including silicon nitride for ambient SPM and tungsten for ultrahigh vacuum SPM.

Another important part of an SPM is the 'probe' which is what senses its proximity to the sample. The probe can be a single atom or multiple atoms, depending on the technique being used. For example, in atomic force microscopy, a single-atom tip is used, while cold-atom SPM uses a trapped, ultracold gas of atoms as the probe instead of a standard AFM tip.

A probe can also be a wire, in which case it is insulated from the sample. In the case of STM, this is usually done with a platinum/iridium wire for ambient operation or a tungsten wire for UHV.

Other types of SPM use conducting tips. These are a conductive, thin-walled tube that is placed on top of the sample. In this case, the current flow between the tip and the sample can be measured in terms of potential difference and ion conductivity. These techniques are particularly useful for studying biological interfaces, where both topography and chemical signals can be acquired simultaneously.

Another type of SPM is the scanning ion conductance microscope (SICM). This instrument uses an electrolyte-filled nanopipette to generate a steady-state ion current that can be used to probe both topography and chemical signals from the sample. This is a highly sensitive and robust method for obtaining spatially resolved atomic-scale features and physical properties of solid-state and biophysical interfaces.

What is an AFM?

Scanning probe microscopy, or AFM, is a microscope that uses a cantilever with a sharp tip (probe) to scan the surface of samples. AFM can be used in a variety of applications, including physics, chemistry, cell biology, materials science, and many other fields.

The cantilever is typically made of silicon or silicon nitride with a radius of curvature on the order of nanometers. It consists of a sharp tip that protrudes from a holder chip, which is attached to the scanning head of an AFM.

When the cantilever is brought close to a sample, it bends as the force between the tip and the surface increases. This deflection of the cantilever is then measured by a laser beam and a detector. This bending is correlated to the contour of the sample and is the key to imaging with AFM.

AFM can be operated in several different modes, and a particular mode may be selected for the type of property being studied. The most common mode is called contact mode, which involves a constant force between the tip and the surface of the sample. This mode is useful for obtaining topographic images of the surface without too much noise.

Another commonly used mode is tapping mode, which involves oscillating the cantilever. This causes the cantilever to touch the surface of the sample intermittently, and this is most effective for soft samples. This oscillating action causes the cantilever to swell and contract when it touches the sample, but this can also cause the surface to be distorted.

When AFM is used in ambient conditions, a liquid meniscus layer develops between the tip and the sample. This interferes with the ability to measure short-range forces between the tip and the sample in contact mode. This problem was solved by the development of dynamic contact mode, or AC mode for short, which allows the tip to move quickly between the surface and the sample when measuring contact forces. This mode has become the most popular AFM method for ambient conditions and in liquids.

What is a STM?

Scanning Probe Microscopy is a technique that allows researchers to map a conductive sample's surface atom by atom with ultra-high resolution, without the use of electron beams or light. This technique has been around for nearly forty years and has shown insights into matter at the atomic level, while revealing a wealth of information on the structure of materials that is not accessible to optical or electron microscopy techniques.

To start, the STM tip is connected to a voltage supply and brought close enough to the sample that it produces a measurable tunneling current between the tip and the surface (see below). When the separation between tip and sample is small enough, this current is amplified and converted into an electronic output signal of order one volt. This makes it possible to record the tunneling current over time as the tip is scanned over the surface.

The STM's most important advantage is its ability to produce high-resolution images. This is achieved through its extreme separation sensitivity and the fact that it uses piezoelectric materials to control the height of the probe above the surface.

Another key aspect of STM is the fact that it can map both topography and electronic properties at very detailed levels. It is a very versatile tool for this purpose and has been used for a range of applications across the sciences.

For example, it has been used to reveal the morphology of crystals such as boron nitride and copper oxide, and to characterize the structure of biological surfaces like plant leaves in a saline solution. STM has also been used to observe the formation of atomic-scale quantum corrals on metals such as copper and cobalt, where wave-like features emerge due to the confinement of electrons.

Depending on the application, there are different scan modes that can be used in STM. A common mode is called constant current and involves a feedback loop that allows the tip to maintain a consistent height while the tunneling current is recorded over time. This type of imaging mode can often be used to detect striped features in the raster image, because of the way the feedback can oscillate.

What is a Scanning Probe Microscope (SNOM)?

A scanning probe microscope (also called a SNOM) is a type of microscopy that involves moving the probe tip along a sample. It does this by sensing the distance between the probe tip and the surface of the sample. This information is then used to create an image of the sample.

Several types of SPM equipment are available, including atomic force microscopy (AFM), scanning tunneling microscopy (STM), and others. Each type of equipment has a specific set of features and capabilities, so the right equipment for your project will depend on what you’re trying to do.

The main components of a scanning probe microscope are a tunable cantilever tip and photodiodes. The cantilever tip can be adjusted to different angles, depending on the desired imaging resolution. This makes it possible to study surfaces that are difficult or impossible to access with an AFM or STM.

Scanning probe microscopy has been developed for studying a variety of materials, including metals, semiconductors, ceramics, polymers, and composites. It is useful for determining the chemical composition and structure of material, measuring its density and surface area, and comparing its properties to those of other materials.

SPM is particularly suitable for obtaining detailed images of small objects, and it is commonly used to obtain atomic-scale details in thin films. It is also an excellent tool for examining nanoscale patterns, such as DNA structures.

One of the most important aspects of SPM is its high resolution. This is due to the fact that the probe can be moved within several nanometers of the surface of the sample. In addition, the probe can also be moved with a high speed. This allows for fast and precise measurements.

Another advantage of SPM is that it can be used to examine a variety of samples, even ones that are buried in solid or liquid. In contrast, other types of microscopy require the use of a sample that can be held in place.

AFM and STM imaging techniques can be performed with a wide range of materials, but the most common probes are made from a glass fiber that has been fabricated using the heating and pulling method or by forming it with a fluorescence imaging beam milling (FIB). The resulting aperture is then coated with 200-nm thick aluminum. This helps to confine the light, preventing it from escaping through pinholes in the coating.

Video: Learning about Scanning Probe Microscopy