Deep Level Transient Spectroscopy (DLTS) ECE 658 Deep Level Transient Spectroscopy (DLTS) Polina Burmistrova

Motivation: Pioneering Work on Depth Measurement ECE 658 Measurement of the depth of defect-related traps in the semiconductors webphysics.davidson.edu/faculty/thg/talks-posters/nc-aapt-02.ppt

Deep Level Transient Spectroscopy (DLTS) What is DLTS? ECE 658 Deep Level Transient Spectroscopy (DLTS) is an experimental tool for studying electrically active defects (a.k.a. charged carrier traps) in semiconductors. enables to establish fundamental defect parameters (some considered as defect “finger prints”) and measure their concentration in the material. has a higher sensitivity than almost any other semiconductor diagnostic technique. features simple technical design. Note: DLTS technique was pioneered by David Vern Lang of Bell Laboratories in 1974 http://en.wikipedia.org/wiki/DLTS

How It Works? ECE 658 semiconductor junction (p-n diode or Schottky diode) is a subject to the voltage pulse at different temperatures capacitance transients are monitored and spectrum is generated which exhibits peaks for each deep levels the height of the peak is proportional to the trap density; sign distinguishes minority from majority traps; position of the peak determine the fundamental parameters governing thermal emission and capture (activation energy and cross section) Application of the method has led to the discovery of new phenomena and has provided a unique tool for the understanding of materials processing for semiconductor devices.

The Basics ECE 658 Pulsed Bias Capacitance Transient (Schottky diode) nT – density of traps occupied by e pT – density of traps occupied by h NT – totaldensity of traps NT = nT + pT capture dominates emission dominates Steady-state

DLTS Theory ECE 658 Capacitance transient at different temperatures Carrier escape Low Temp High Temp time Change in Capacitance kT Low Temperature slow - kT fast High Temperature -

DLTS Theory ECE 658 Ability to set the emission rate window – measurement equipment responses ONLY when it sees a transient with the rate within this window. Use boxcar averager or the lock-in amplifier (capacitance decay waveform corrupted by noise – extraction of the signal in an automated manner) temperature dependence DLTS – correlation technique (input signal multiplied by reference signal, the weighing function, and the product filtered by linear filter) Correlation output:

DLTS Theory ECE 658 Lock-in amplifier Box-car avareging A lock-in amplifier (a.k.a. phase-sensitive detector) is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment (S/N ratio can be -60 dB or even less). It is essentially a homodyne with an extremely low pass filter (making it very narrow band). Box-car avareging A boxcar function (simple step function) is any function which is zero over the entire real line except for a single interval where it is equal to a constant, A. In terms of the uniform distribution: where f(a,b;x) is the uniform distribution of x for the interval [a, b].

DLTS Theory – Instrument Control Loop ECE 658 Computer w/ LabVIEW Temperature Controller Circuit + Cap. Meter Oscilloscope Set temperature T Wait for T to stabilize Send bias pulse Measure capacitance C Obtain C vs. time graph from scope Next T Cryostat and SAMPLE 1&2 3 4 5

DLTS Methods ECE 658 Conventional DLTS Double-Correlation DLTS (D-DLTS) Correlation DLTS Isothermal DLTS Computer DLTS Laplace DLTS (L-DLTS) Interface Trapped Charge DLTS Optical and Scanning DLTS Photoinduced Current Transient Spectroscopy (PITS or PICTS) Scanning DLTS (S-DLTS)

Conventional DLTS ECE 658 capacitance transients are investigated by using a lock-in amplifier or box-car averaging technique when the sample temperature is slowly varied (from liquid nitrogen to room temperature 300K or above). Capacitance difference: using weight function: Series of plots generated by changing rate window (t1 and t2)

Conventional DLTS ECE 658 By setting up different rate windows in subsequent DLTS spectra measurements on 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.

Double-Correlation DLTS (D-DLTS) ECE 658 uses of pulses of different amplitude sets window within the scr (all traps are well above the Fermi level; capacitance transient is due to emission only) traps near Fermi level are excluded

Constant Capacitance and Correlation DLTS ECE 658 Constant Capacitance DLTS (CC-DLTS) capacitance held constant by dynamically varying the applied voltage during the transient through s feedback path valid for high trap concentration NT>0.1ND limitations – slow circuit response due to the feedback Correlation DLTS (not widely applied) based on optimum filter theory (optimum weighing function of the unknown signal corrupted by the white noise has the form of the noise-free signal itself) multiply capacitance by a repetitive decaying exponential generated with a RC function generator and integrating the product higher signal/noise ratio

Isothermal and Computer DLTS ECE 658 Isothermal DLTS sample temperature held constant and the sample time is varied requires fast capacitance meters Computer DLTS capacitance waveform is digitized and stored electronically for further data management various signal processing functions can be applied has algorithm allowing to separate close spacing peaks

Laplace DLTS ECE 658 isothermal technique high resolution technique emission rate obtained by numerical methods (inverse Laplace transformation) allow to separate states with very small emission rates Note: 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 of the given defect.

Interface Trapped Charge DLTS ECE 658 Same instrumentation as DLTS, but different data interpretation Measurement are independent of surface potential MOS-C For and where then Very sensitive – 109 cm-2eV-1 range

Optical and Scanning DLTS ECE 658 Photoinduced Current Transient Spectroscopy (PITS or PICTS) DLTS cannot be used for insulating materials or very large bandgap semiconductors. For insulating materials it is difficult or impossible to produce a device having a space region which width could be changed by the external voltage bias. Measure current transient Rapid drop due to ehp recombination Photocurrent transient Analyzed by DTLS Photo pulse Slow decay due to carries emission - Not suited for trap density determination; reliability of the data falls as trap energy approaches intrinsic Fermi level

Optical and Scanning DLTS ECE 658 Scanning DLTS (S-DLTS) SIDLTS SEM electron beam as the excitation source high spatial resolution, but very small DLTS signal quantitative measurements difficult, but impurity distribution can be mapped by scanning the sample area J.S.Laird et.al, J. Phys. D: Appl. Phys. 39 (2006) 1342–1351

Precautions ECE 658 Leakage Current more strong DLTS peak amplitude decrease error in trap energy and cross-section Series Resistance DLTS signal reversal Instrumentation precise temperature control, thermocouple location, etc. Incomplete Trap Filling usually edges can be ignored, but sometimes can introduce appreciable error Blackbody Radiation optical emission in addition to the thermal emission lead to erroneous activation energy

Example ECE 658

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Defect Type (A-Center) ECE 658 A-nitrogen center the most common defect in natural diamonds. consists of a neutral nearest-neighbor pair of nitrogen atoms substituting for the carbon atoms. produces UV absorption threshold at ~4 eV (310 nm, i.e. invisible to eye) and thus causes no coloration. is diamagnetic, but if ionized by UV light or deep acceptors, it produces an electron paramagnetic resonance spectrum W24, whose analysis unambiguously proves the N=N structure. - shows an IR absorption spectrum with no sharp features, which is distinctly different from that of the C or B centers.

ECE 658 Questions? Thank You !