Deep-level transient spectroscopy

Deep-level transient spectroscopy (DLTS) is an experimental tool for studying electrically active defects (known as charge carrier traps) in semiconductors. DLTS establishes fundamental defect parameters and measures their concentration in the material. Some of the parameters are considered as defect "finger prints" used for their identifications and analysis. DLTS investigates defects present in a space charge (depletion) region of a simple electronic device. The most commonly used are Schottky diodes or p-n junctions. In the measurement process the steady-state diode reverse polarization voltage is disturbed by a voltage pulse. This voltage pulse reduces the electric field in the space charge region and allows free carriers from the semiconductor bulk to penetrate this region and recharge the defects causing their non-equilibrium charge state. After the pulse, when the voltage returns to its steady-state value, the defects start to emit trapped carriers due to the thermal emission process. The technique observes the device space charge region capacitance where the defect charge state recovery causes the capacitance transient. The voltage pulse followed by the defect charge state recovery are cycled allowing an application of different signal processing methods for defect recharging process analysis. The DLTS technique has a higher sensitivity than almost any other semiconductor diagnostic technique. For example, in silicon it can detect impurities and defects at a concentration of one part in 1012 of the material host atoms. This feature together with a technical simplicity of its design made it very popular in research labs and semiconductor material production factories. The DLTS technique was pioneered by David Vern Lang at Bell Laboratories in 1974. A US Patent was awarded to Lang in 1975. In conventional DLTS the capacitance transients are investigated by using a lock-in amplifier or double box-car averaging technique when the sample temperature is slowly varied (usually in a range from liquid nitrogen temperature to room temperature 300 K or above).