Time resolved microwave conductivity
Time resolved microwave conductivity (TRMC) is an experimental technique used to evaluate the electronic properties of semiconductors.
The technique works by photo-generating electrons and holes in a semiconductor, allowing these charge carriers to move under a microwave field, and detecting the resulting changes in the electric field.
TRMC systems cannot be purchased as a single unit, and are generally "home-built" from individual components.
One advantage of TRMC over alternative techniques is that it does not require direct physical contact to the material.
The technique was later refined to study semiconductors by Kunst and Beck at the Hahn Meitner Institute in Berlin.
[4] Delft remains a significant center for TRMC,[5] however the technique is now used at a number of institutions around the world, notably the National Renewable Energy Laboratory[6] and Kyoto University.
[7] The experiment relies upon the interaction between optically-generated charge carriers and microwave frequency electromagnetic radiation.
The oscillating current is incident on an antenna, resulting in the emission of microwaves of the same frequency.
Because they can transmit microwaves with lower loss than cables,[9] metallic waveguides are often used to form the circuit.
The sample to be studied is placed at a maximum of the electric field component of the standing wave.
Because metals act as cavity walls,[9] the sample needs to have a relatively low free carrier concentration in the dark to be measurable.
TRMC is hence best suited to the study of intrinsic or lightly doped semiconductors.
Electrons and hole are generated by illuminating the sample with above band gap optical photons.
The photo-generated charge carriers move under the influence of the electric field component of the standing wave, resulting in a change in intensity of microwaves that leave the cavity.
The intensity of microwaves out of the cavity is measured as a function of time using an appropriate detector and an oscilloscope.
Knowledge of the properties of the cavity can be used[8] to evaluate photoconductance from changes in microwave intensity.
is the quality factor of the external coupling, which is generally adjusted by iris.
, is defined as follows: The photo-generated charge carriers reduce the quality of the cavity,
Furthermore, dissipation factor of the cavity is mainly determined by the conductivity of the inside space including the sample.
, of the cavity contents is proportional to relative changes in microwave intensity:[2] Here
is the background (unperturbed) microwave power measured coming out of the cavity and
When the sample is inserted into dry cavity, only vacuum permittivity should be used because most of the inside space is filled by air.
depends on whether the cavity is in the under-coupled (lower) or over-coupled (upper) regime.
If the thickness of the sample is sufficiently thin (below several μm), the electric field to photo-generated carriers would be uniform.
is linearly proportional to the thickness, only the fractional absorbance of the semiconductor (between 0 and 1) should be additionally measured to determine the TRMC figure of merit
(e.g. using ultraviolet–visible spectroscopy): Knowledge of charge carrier mobility in semiconductors is important for understanding the electronic and materials properties of a system.
TRMC has been used to study electron and hole dynamics in hydrogenated amorphous silicon,[14] organic semiconductors,[15] metal halide perovskites,[16] metal oxides,[17] dye sensitized systems,[18] quantum dots,[19] carbon nanotubes,[20] chalcogenides,[21] metal organic frameworks,[22] and the interfaces between various systems.
The technique is hence well suited to the study of solar absorbers, but not to wide bandgap semiconductors such as metal oxides.
As a time-resolved technique, TRMC also provides information on the timescale of carrier recombination in solar cells.
Unlike time resolved photoluminescence measurements, TRMC is not sensitive to the lifetime of excitons.
