Scanning voltage microscopy
Encyclopedia
Scanning voltage microscopy (SVM) -- sometimes also called nanopotentiometry -- is a scientific experimental technique based on atomic force microscopy. A conductive probe, usually only a few nanometers wide at the tip, is placed in full contact with an operational electronic
or optoelectronic sample. By connecting the probe to a high-impedance
voltmeter
and rastering over the sample's surface, a map of the electric potential
can be acquired. SVM is generally nondestructive to the sample although some damage may occur to the sample or the probe if the pressure required to maintain good electrical contact is too high. If the input impedance of the voltmeter is sufficiently large, the SVM probe should not perturb the operation of the operational sample.
SVM is particularly well suited to analyzing microelectronic
devices (such as transistors or diodes) or quantum
electronic devices (such as quantum well
diode lasers) directly because nanometer spatial resolution is possible. SVM can also be used to verify theoretical simulation of complex electronic devices.
For example, the potential profile across the quantum well structure of a diode laser can be mapped and analyzed; such a profile could indicate the electron
and hole
distributions where light is generated and could lead to improved laser designs.
In a similar technique, Scanning gate microscopy
(SGM), the probe is oscillated at some natural frequency some fixed distance above the sample with an applied voltage relative to the sample. The image is constructed from the X,Y position of the probe and the conductance of the sample, with no significant current passing through the probe, which acts as a local gate. The image is interpreted as a map of the sample's sensitivity to gate voltage. A lock-in amplifier
aids noise reduction by filtering through only the amplitude oscillations that match the probe's vibration frequency. Applications include imaging defect sites in carbon nanotube
s and doping profiles in nanowires.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
or optoelectronic sample. By connecting the probe to a high-impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
voltmeter
Voltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...
and rastering over the sample's surface, a map of the electric potential
Electric potential
In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...
can be acquired. SVM is generally nondestructive to the sample although some damage may occur to the sample or the probe if the pressure required to maintain good electrical contact is too high. If the input impedance of the voltmeter is sufficiently large, the SVM probe should not perturb the operation of the operational sample.
SVM is particularly well suited to analyzing microelectronic
Microelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...
devices (such as transistors or diodes) or quantum
Quantum
In physics, a quantum is the minimum amount of any physical entity involved in an interaction. Behind this, one finds the fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization". This means that the magnitude can take on only certain discrete...
electronic devices (such as quantum well
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...
diode lasers) directly because nanometer spatial resolution is possible. SVM can also be used to verify theoretical simulation of complex electronic devices.
For example, the potential profile across the quantum well structure of a diode laser can be mapped and analyzed; such a profile could indicate the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
and hole
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
distributions where light is generated and could lead to improved laser designs.
In a similar technique, Scanning gate microscopy
Scanning gate microscopy
Scanning gate microscopy is a scanning probe microscopy technique with an electrically conductive tip used as a movable gate that couples capacitively to the sample and probes electrical transport on the nanometer scale. Typical samples are mesoscopic devices, often based on semiconductor...
(SGM), the probe is oscillated at some natural frequency some fixed distance above the sample with an applied voltage relative to the sample. The image is constructed from the X,Y position of the probe and the conductance of the sample, with no significant current passing through the probe, which acts as a local gate. The image is interpreted as a map of the sample's sensitivity to gate voltage. A lock-in amplifier
Lock-in amplifier
A lock-in amplifier is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment . It is essentially a homodyne with an extremely low pass filter...
aids noise reduction by filtering through only the amplitude oscillations that match the probe's vibration frequency. Applications include imaging defect sites in carbon nanotube
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...
s and doping profiles in nanowires.