DLTS has been in use since 1974, based on the innovative development of David Lang. It is a very sensitive method of measuring the characteristics of defects in semiconductor materials. It requires a depletion region that can be changed with applied bias, such as a Schottky diode, p-n junction diode, or MIS structure. The defects in the semiconductor can trap a charge carrier. After filling the traps, the bias is changed to bring the Fermi level below the defect energy level. The emission rate is measured as a change in capacitance, as the defects release their charges. The following sequence shows the process graphically.
Original implementation of DLTS used boxcar integrators to obtain a signal proportional to the difference in capacitance at two different points on the emission transient. Peaks in the signal versus temperature correspond to emission rates, calculated from the gated sample times. Pairs of (e,T) plotted on an Arrhenius plot of the emission rate, corrected for a T2 dependence, are used to obtain the defect energy and capture cross section. The SEMETROL system collects entire transients at temperature steps in one temperature sweep, and fits each transient for multiple exponential components.
The first image shows a capacitance transient as the temperature increases, resulting in faster emission rates. The second image shows a conventional ratewindow analysis, where the peak in the plot corresponds to the temperature where the trap has an emission rate defined by the gating times, t1 and t2. Otherwise, the transients can be fit at each temperature for multiple exponential components. The third image shows results of fitting the transients in an Arrhenius plot. Note the two traps with closely spaced characteristics. These are not detected by conventional ratewindow analysis, but only by fitting the transient data.
