30-07-2012, 01:25 PM
Deep-level transient spectroscopy
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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 D. V. Lang (David Vern Lang of Bell Laboratories) in 1974.[1] US Patent[2] was awarded to Lang in 1975.
Typical conventional DLTS spectra
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 300K or above). The equipment reference frequency is the voltage pulse repetition rate. In the conventional DLTS method this frequency multiplied by some constant (depending on the hardware used) is called the “rate window”. When during the sample temperature variation the emission rate of carriers from some defect equals to the rate window one obtains in the spectrum a peak. By setting up different rate windows in subsequent DLTS spectra measurements one obtains different temperatures at which some particular peak appears. Having a set of the emission rate and corresponding temperature pairs one can make an Arrhenius plot which allows for the deduction of defect activation energy for the thermal emission process. Usually this energy (sometimes called the defect energy level) together with the plot intersect value are defect parameters used for its identification or analysis.
Recently, DLTS has been used to study quantum dots.[3][4][5]
MCTS and minority-carrier DLTS
For the Schottky diodes, majority carrier traps are observed by the application of a reverse bias pulse, while minority carrier traps can be observed when the reverse bias voltage pulses are replaced with light pulses with the photon energy from the above semiconductor bandgap spectral range.[6][7] This method is called Minority Carrier Transient Spectroscopy (MCTS). The minority carrier traps can be also observed for the p-n junctions by application of forward bias pulses which inject minority carriers into the space charge region.[8] In DLTS plots the minority carrier spectra usually are depicted with an opposite sign of amplitude in respect to the majority carrier trap spectra.