Introduction to DLTS

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Introduction to DLTS (Deep Level Transient
Spectroscopy) I. Basic Principles O.
Breitenstein MPI MSP Halle
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• Outline
• 1. Basic principles
• Application field of DLTS
• Principles of DLTS
• Basic measurement techniques
• 2. Advanced techniques and application
• Advanced measurement techniques
• Our DLTS system - Philosophy
• - Hardware
• - User surface

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1. Application field of DLTS

• "Deep levels" energy states in semiconductor
  band gap, gt 100 meV binding energy (otherwise
  "shallow levels")
• Usually caused by isolated point defects, but
  also extended defects generate DLs
• Terminology acceptors (charge state / 0),
  donors (0 / -), also double acceptors ( / /
  0), double donors (0 / - / --), amphoteric (- / 0
  / ) etc. Charge state governs capture cross
  sections to electrons and holes, but not position
  in gap !
• Upper gap half electron traps, lower gap half
  hole traps

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Possible electronic processes
electron capture
electron emission
CB
electron trap
hole trap
VB
hole capture
hole emission
thermal (electron) emission probability
"emission rate" s-1
capture prababilities
cnp "capture coefficients cm3s-1
trap parameters Et (thermal activation energy),
sn and sp resp. cn and cp
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(thermal) emission rate (T) "Arrhenius plot"
(fingerprint)
log(enp)
prefactor contains sn, but this parameter is
often exponentially T-dependent!
1000/T K-1

• prefactor gives not sn !
• Et not equilibrium energy !

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2. Basic Principles of DLTS
Electron trap in n-type space charge region
(Schottky diode)
RF-capacitance (1 MHz)
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capacitance change due to recharging of Nt
cm-3 traps
net doping concentration, from C/V meas.
basic (equilibrium) capacitance

• Sign of DC depends on trapped carrier type
• majority carrier capture DC negative
• minority carrier capture DC positive
• Best sensitivity for low doping concentration

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DLTS routine (repeating!)
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T-dependence of C-transient
opt. T
low T
high T
If T is slowly varying, at a certain temperature
a DLTS peak occures
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Different deep levels are leading to different
peaks
peak condition
peak height DC is proportional to trap
concentration
By choosing t1 and t2 a "rate window" s-1 is
selected, in which the emission rate has to fall
for a DLTS peak to appear (D.V. Lang 1974)
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DLTS measurements at different rate windows allow
one to measure Et
This "Arrhenius plot" allows an identification of
a deep level defect