There are many different types of atomic force microscopy (AFM) in use today. These range from different imaging modes that use the principles of AFM only, to other methods which build on the principles of AFM and couple them with principles from other analysis techniques. The latter is usually for measuring a specific property (or properties) within certain types of materials. In this article, we look at one of these variations known as scanning spreading resistance microscopy (SSRM), which has found a lot of use in measuring the resistance of a material and for the deduction of a material’s charge carrier concentration.
Even though there are many different types of AFM, there are specific characteristics and principles that remain the same, regardless of the variation, operational mode, or imaging mode. The critical difference between the AFM instrument that most people know, and the lesser-known variations, is all in the tip, and how this tip interacts with the sample. Often, this difference in the tip-sample interaction yields information that it is possible with that specific variation and is the reason why there are many different ‘subset techniques’ of AFM.
What is Scanning Spreading Resistance Microscopy (SSRM)
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Scanning Spreading Resistance Microscopy (SSRM) is a variation of AFM that employs a conductive tip in contact mode to measure the resistance of a surface in combination with a high force. Other AFM variations use conductive tip, but they rely on measuring the actual conductivity and not the resistance of a material. Additionally, SSRM specifically employs a force to minimize the noise in the measurements.
Like many AFM variations, there are elements of traditional AFM principles coupled with the principles of other techniques. For SSRM, determining the relative position of the cantilever beam to build a topographic map is the same as conventional AFM, however, the interaction between the tip and the sample uses principles from spreading resistance profiling (SRP) methods.
It is a technique that has found a lot of use in measuring the charge carrier concentrations in doped semiconductors and 2D materials, as well as providing cross-sectional measurements of a material’s surface. So, while SSRM has a small number of specific applications, it can be a very useful and powerful tool in these specialist areas.
Additionally, the use of doped semiconductors has significantly grown in recent years, so SSRM can also be used on a wide range of devices that employ these materials, such as photodetectors, light-emitting diodes (LEDs), diode lasers, as well as for any new technologies that come out which use 2D materials.
How SSRM Works
SSRM uses a combination of AFM and SRP principles. From an AFM point of view, the cantilever moves in a contact mode and the localized position of resistance is determined in the same way as AFM – through the deflection of a laser beam off the back of the cantilever beam onto a position-sensitive photodiode (PSPD).
From a tip to sample interaction point of view, it is based off SRP methods. However, most SRP methods generally use a two-probe system, whereas SSRM only requires a single tip attached to the cantilever beam. There is another variant of AFM know as tunneling AFM (TUNA) which uses similar principles, but SSRM has been developed to extend the range of the current between the tip and the sample so that the surface resistance can be measured. SSRM also uses the same principles as conductive AFM (C-AFM), with the only difference being that it scans a cross-sectional area of the surface instead of just generally scanning the surface.
A direct current (DC) is passed through the conductive tip so that it acts like an electrode as it scans the surface of the material. As it scans the surface, SSRM measures the current by referencing it to an internal resistor. Once the current is known, a logarithmic amplifier is used to deduce the local resistance value (which is then positionally located by conventional AFM methods mentioned above).
The resistance is calculated in Ohm-meters, which is the resistance multiplied by the cross-sectional area being imaged, divided by the length of the conducting path. By keeping the force at a constant setpoint, both a topographic map and current images can be generated simultaneously, and this can be used to correlate the local topography with the measured electrical properties directly. In SSRM, the contact force is set to above the minimum required value, and this helps to minimize the noise. If a higher force cannot be used, then the voltage bias is increased.
There are a couple of different modes that can be employed when performing SSRM. These are known as internal SSRM, and external SSRM modes and they relate to the type of current amplifier used. Internal SSRM refers to a mode that uses a current amplifier with a fixed gain, whereas external SSRM is a mode which uses an external low noise current amplifier with a variable gain. External SSRM is a more versatile mode as it enables the current to be changed by altering the gain of the amplifier.
Sources and Further Reading