Deep-level transient spectroscopy

Deep Level Transient Spectroscopy (DLTS) is a unique and powerful tool for the study of electrically active defects (known as traps) in semiconductors. DLTS can be used in one of two modes of operation; with Schottky diodes or with p-n junctions. With Schottky diodes, majority carrier traps are observed by the application of a reverse bias pulse, while minority carrier traps can be observed by the application of a forward bias pulse^[1].

The technique works by observing the capacitance transient associated with the change in depletion region width as the diode returns to equilibrium from an initial non-equilibrium state. The capacitance transient is measured as a function of temperature (usually in the range from liquid nitrogen temperature to room temperature 300K or above). By using a double box-car averaging technique peaks at a particular emission rate are found as a function of temperature. By looking for emissions at different rates and monitoring the temperature of the associated peak, an Arrhenius plot allows for the deduction of a trap's activation energy.

It was pioneered by D. V. Lang (David Vern Lang of Bell Laboratories) in 1974^[2].

US Patent 3,859,595 was awarded to Lang in 1975.

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DLTS

Laplace DLTS

There is an extension to DLTS known as a high resolution Laplace transform DLTS (LDLTS). Laplace DLTS is an isothermal technique in which the capacitance transient is averaged at a fixed temperature. It provides a spectral plot of a processed capacitance signal against emission rate rather than against temperature ^[3][4] . The main advantage of Laplace DLTS in comparison to conventional DLTS is the increase in energy resolution.

Laplace DLTS in combination with uniaxial stress results in a splitting of the defect energy level. Assuming a random distribution of defects in non-equivalent orientations, the number of split lines and their intensity ratios reflect the symmetry class^[5] of the initial charge state of the given defect^[6].

Concerning LDLTS on MOS capacitors, the polarization zone is the region between depletion and weak inversion where G-V curves yield a peak of interface defects. The pulses are low voltage (< 0.2 V) and long time (> 0.4 ms) to fill all interface defects at same energy trap. Based on this, the Laplace transform DLTS on MOS capacitors could define the distribution of interface defects between oxide and semiconductors versus energy trap and cross section pattern.

CCDLTS

Transient capacitance based DLTS theory involves some sort of approximation such that it is not so good when the trap density is high. Constant capacitance DLTS (CCDLTS) removes this approximation such that it is more accurate when the trap density is high according to Johnson et al. published in 1979^[7]. Constant capacitance implies constant depletion region width. An analysis of the CCDLTS system using feedback theory was provided by Lau and Lam in 1982^[8].

I-DLTS

Before the invention of DLTS, thermally stimulated current (TSC) spectroscopy was a popular technique to study traps in semiconductors. Nowadays, for traps in Schottky diodes or pn junctions, DLTS is the standard method to study traps. However, there is an important shortcoming for DLTS: it cannot be used for an insulating material while TSC can be applied to such a situation. (Note: an insulator can be considered as a very large bandgap semiconductor.) More advanced modifications of TSC have been applied to study traps in ultrathin high-k dielectric thin films^[9][10][11]. In addition, the standard transient capacitance based DLTS method may not be very good for the study of traps in the i-region of a p-i-n diode while the transient current based DLTS (I-DLTS) may be more useful.

See also

• Carrier generation and recombination
• Bandgap
• Effective mass
• Semiconductor device
• Vacancy (chemistry)
• Capacitance voltage profiling
• Lau Wai Shing
• high-k dielectric

References

1. ^ V.P. Markevich et al, "Vacancy–group-V-impurity atom pairs in Ge crystals doped with P, As, Sb, and Bi ", Phys. Rev. B 70 (2004) 235213.
2. ^ D.V. Lang, "Deep-level transient spectroscopy: A new method to characterize traps in semiconductors", J. Appl. Phys., vol. 45, no. 7, pp. 3023-3032, July 1974.
3. ^ L. Dobaczewski, P. Kaczor, I.D. Hawkins and A.R. Peaker, "Laplace transform deep-level transient spectroscopic studies of defects in semiconductos", J. Appl. Phys., vol. 76, no. 1, pp. 194-198, 1 July 1994.
4. ^ http://www.laplacedlts.eu
5. ^ http://www.phys.ncl.ac.uk/staff/njpg/symmetry/index.html
6. ^ L. Dobaczewski, A. R. Peaker, and K. Bonde Nielsen, "Laplace-transform deep-level spectroscopy: The technique and its applications to the study of point defects in semiconductors", J. Appl. Phys., vol. 96, no. 9, pp. 4689-4728, January 2004.
7. ^ N.M. Johnson, D.J. Bartelink, R.B. Gold and J.F. Gibbons, "Constant-capacitance DLTS measurement of defect-density profiles in semiconductos", J. Appl. Phys., vol. 50, no. 7, pp. 4828-4833, July 1979.
8. ^ W.S. Lau and Y.W. Lam, "Analysis of and some design considerations for the constant capacitance DLTS system", International Journal of Electronics, vol. 52, no. 4, pp. 369-379, 1982.
9. ^ W.S. Lau, L. Zhong, A. Lee, C.H. See, T. Han, N.P. Sandler and T.C. Chong, "Detection of defect states responsible for leakage current in ultrathin tantalum pentoxide (Ta2O5) films by zero-bias thermally stimulated current spectroscopy", Appl. Phys. Lett., vol. 71, no. 4, pp. 500-502, 28 July 1997.
10. ^ W.S. Lau, K.F. Wong, T. Han and N.P. Sandler, "Application of zero-temperature-gradient zero-bias thermally stimulated current spectroscopy to ultrathin high-dielectric-constant insulator film characterization", Appl. Phys. Lett., vol. 88, article number 172906, 2006.
11. ^ W.S. Lau, "Similarity between the first ionized state of the oxygen vacancy double donor in tantalum oxide and the first ionized state of the cadmium vacancy double acceptor in cadmium sulfide", Appl. Phys. Lett., vol. 90, article number 222904, 2007.