Semetrol-Thermal Admittance Spectroscopy

TAS

Thermal admittance spectroscopy (TAS) measures the capacitance and conductance as a function of frequency and temperature. TAS is a steady state measurement, in contrast to the non-equilibrium measurements made in DLTS. Also, where DLTS is used for doped semiconductors, TAS may also be used for low-doped or high resistivity material.

At a fixed temperature, as the frequency of the test signal is reduced, deeper traps are able to respond and contribute to the capacitance and conductance. Similarly, as the temperature increases, the emission rate of a defect increases and the trap can contribute to the measured property at higher frequencies. At a threshold in frequency or temperature, the deep level appears as a step in the capacitance, or a peak in the conductance. The temperature of the inflection point of each plot of capacitance versus temperature at a fixed frequency is used along with the frequency, as one point in an Arrhenius plot. The energy of the defects can be obtained from plotting pairs of frequency and temperature points in an Arrhenius plot of emission rate (frequency of measurement) versus 1/kT.

A screen-shot of the user interface for data collection is shown below, along with representative results.

TAS/TASDataAcqUI.jpg

Select the imaginary or real part of the impedance or admittance for recording using the data acquisition program. The frequency ranges from 20Hz to 1MHz. Collecting admittance or impedance data over a wide range of frequencies results in emission rate measurement over several orders of magnitude, and very accurate energy measurement in turn.

After all of the data is collected, the analysis consists of finding the steps in capacitance, or peaks in conductance from a plot of the capacitance or conductance measured over temperature, at each frequency. The pairs of coordinates of frequency and temperature are then used to generate the Arrhenius plot and measure the energy of the traps.

TAS/TAS3D-G.jpg

TAS/GaN_TAS_Spectra_500_100.JPG

Two peaks are easily visible in the plot to the left. The spectra are generated from the conductance and shown at two different frequencies.

Energy measurements are very accurate with TAS, generated from emission rate measurements over several orders of magnitude.

TAS/TASArrResults.jpg

TAS data acquisition is performed easily, and the analysis program facilitates efficient extraction of defect characteristics with the Semetrol TAS system. TAS is a method that is complementary to the DLTS method. TAS is applicable to low-doped semiconductors, where DLTS is generally applicable to moderately doped material.