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 (
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.[1] A US Patent was awarded to Lang in 1975.[2]
DLTS methods
Conventional DLTS
In conventional DLTS the capacitance transients are investigated by using a lock-in amplifier[3] 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). 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". During the temperature scan, peaks appear when the emission rate of carriers from some defect equals the rate window. 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 intercept value are defect parameters used for its identification or analysis. On samples with low free carrier density conductance transients have also been used for a DLTS analysis.[4]
In addition to the conventional temperature scan DLTS, in which the temperature is swept while pulsing the device at a constant frequency, the temperature can be kept constant and sweep the pulsing frequency. This technique is called the frequency scan DLTS.[3] In theory the frequency and temperature scan DLTS should yield same results. Frequency scan DLTS is specifically useful when an aggressive change in temperature might damage the device. An example when frequency scan is shown to be useful is for studying modern MOS devices with thin and sensitive gate oxides.[3]
DLTS has been used to study
MCTS and minority-carrier DLTS
For Schottky diodes,
Laplace DLTS
There is an extension to DLTS known as a high resolution
Laplace DLTS in combination with uniaxial
Application of LDLTS to
Constant-capacitance DLTS
In general, the analysis of the capacitance transients in the DLTS measurements assumes that the concentration of investigated traps is much smaller than the material doping concentration. In cases when this assumption is not fulfilled then the constant capacitance DLTS (CCDLTS) method is used for more accurate determination of the trap concentration.[17] When the defects recharge and their concentration is high then the width of the device space region varies making the analysis of the capacitance transient inaccurate. The additional electronic circuitry maintaining the total device capacitance constant by varying the device bias voltage helps to keep the depletion region width constant. As a result, the varying device voltage reflects the defect recharge process. An analysis of the CCDLTS system using feedback theory was provided by Lau and Lam in 1982.[18]
I-DLTS and PITS
There is an important shortcoming for DLTS: it cannot be used for insulating materials. (Note: an insulator can be considered as a
See also
- Carrier generation and recombination
- Bandgap
- Effective mass
- Schottky diode
- Frenkel defect
- Schottky defect
- Semiconductor device
- Vacancy (chemistry)
- Capacitance voltage profiling
- High-k dielectric
References
- ISSN 0021-8979.
- ^ [1], "Method for measuring traps in semiconductors", issued 1973-12-06
- ^ S2CID 5895479.
- ISSN 0003-6951.
- ISSN 1098-0121.
- ISSN 0021-8979.
- ISSN 0003-6951.
- ISSN 1754-5706.
- S2CID 58578989.
- ISSN 0013-5194.
- ISSN 0021-8979.
- ISSN 1098-0121.
- ^ ISSN 0021-8979.
- ^ Laplace transform Deep Level Transient Spectroscopy
- ^ Point Group Symmetry
- ISSN 0003-6951.
- ISSN 0021-8979.
- ISSN 0020-7217.
- ISSN 0003-6951.