Electrically Detected Magnetic Resonance
Encyclopedia
Electrically Detected Magnetic Resonance (EDMR) is a materials characterisation
technique that improves upon electron spin resonance. It involves measuring the change in electrical resistance
of a sample when exposed to certain microwave
frequencies. It can be used to identify very small numbers (down to a few hundred atoms) of impurities in semiconductors.
s of the electron
s occupying the donor and acceptor in the direction of the magnetic field
. To study the donor, we apply a microwave
pulse ("γ" in the diagram) at a resonant frequency of the donor. This flips the spin of the electron on the donor. The donor electron can then decay to the acceptor energy state (it was forbidden from doing that before it was flipped due to the Pauli exclusion principle
) and from there to the valence band, where it recombines
with a hole. With more recombination, there will be fewer conduction electrons in the conduction band
and a corresponding increase in the resistance, which can be directly measured. Above-bandgap light is used throughout the experiment to ensure that there are lots of electrons in the conduction band.
By scanning the frequency of the microwave pulse, we can find which frequencies are resonant, and with knowledge of the strength of the magnetic field, we can identify the donor's energy levels from the resonant frequency and knowledge of the Zeeman effect
. The donor's energy levels act as a 'fingerprint' by which we can identify the donor and its local electronic environment. By changing the frequency slightly, we can study the acceptor instead.
. Measurements of less than 100 donors and theoretical analyses of such a measurement have been published, relying on the Pb interface defect
to act as the acceptor.
Characterization (materials science)
Characterization, when used in materials science, refers to the use of external techniques to probe into the internal structure and properties of a material...
technique that improves upon electron spin resonance. It involves measuring the change in electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
of a sample when exposed to certain microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
frequencies. It can be used to identify very small numbers (down to a few hundred atoms) of impurities in semiconductors.
Outline of technique
To perform a pulsed EDMR experiment, the system is first initialised by placing it in a magnetic field. This orients the spinSpin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
s of 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...
s occupying the donor and acceptor in the direction of the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. To study the donor, we apply a microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
pulse ("γ" in the diagram) at a resonant frequency of the donor. This flips the spin of the electron on the donor. The donor electron can then decay to the acceptor energy state (it was forbidden from doing that before it was flipped due to the Pauli exclusion principle
Pauli exclusion principle
The Pauli exclusion principle is the quantum mechanical principle that no two identical fermions may occupy the same quantum state simultaneously. A more rigorous statement is that the total wave function for two identical fermions is anti-symmetric with respect to exchange of the particles...
) and from there to the valence band, where it recombines
Carrier generation and recombination
In the solid state physics of semiconductors, carrier generation and recombination are processes by which mobile charge carriers are created and eliminated. Carrier generation and recombination processes are fundamental to the operation of many optoelectronic semiconductor devices, such as...
with a hole. With more recombination, there will be fewer conduction electrons in the conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
and a corresponding increase in the resistance, which can be directly measured. Above-bandgap light is used throughout the experiment to ensure that there are lots of electrons in the conduction band.
By scanning the frequency of the microwave pulse, we can find which frequencies are resonant, and with knowledge of the strength of the magnetic field, we can identify the donor's energy levels from the resonant frequency and knowledge of the Zeeman effect
Zeeman effect
The Zeeman effect is the splitting of a spectral line into several components in the presence of a static magnetic field. It is analogous to the Stark effect, the splitting of a spectral line into several components in the presence of an electric field...
. The donor's energy levels act as a 'fingerprint' by which we can identify the donor and its local electronic environment. By changing the frequency slightly, we can study the acceptor instead.
Recent developments
EDMR has been demonstrated on a single electron from a quantum dotQuantum dot
A quantum dot is a portion of matter whose excitons are confined in all three spatial dimensions. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. They were discovered at the beginning of the 1980s by Alexei...
. Measurements of less than 100 donors and theoretical analyses of such a measurement have been published, relying on the Pb interface defect
Carrier scattering
Defect types include atom vacancies, adatoms, steps, and kinks which occur most frequently at surfaces due to finite material size causing crystal discontinuity. What all types of defects have in common, whether they be surface or bulk, is that they produce dangling bonds which have specific...
to act as the acceptor.