Introduction to DLTS

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Introduction to DLTS (Deep Level Transient
Spectroscopy) II. Advanced Techniques 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
• Advanced DLTS measurement techniques
• 3. (next time) Our DLTS system
• - Philosophy
• - Hardware
• - User surface

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Recapitulation DLTS routine (repeating!)
reverse
reduced or forward
reverse
Vr
bias
t
0
e-
e-
e-
band diagram
e-
e-
RF- capacitance
t
DC
0
t
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Generation of the DLTS signal
opt. T
low T
high T
"rate window"
If T is slowly varying, at a certain temperature
a DLTS peak occures
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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
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2. Advanced DLTS measurement techniques 2.1.
Possible samples Schottky diodes or pn-junctions
Schottky diode
pn-junction
reverse bias
V 0
xe
forward bias
minority carrier injection
majority carrier flow
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• Schottky diodes
• Standard, easy to prepare, high quality demand !
• Only majority carrier traps visible, even under
  forward bias
• pn-junctions
• reverse bias reduction up to 0V "majority
  carrier pulse"
• forward bias (injection) "minority carrier
  pulse" (MC)
• MC pulse may reveal both minority and majority
  carrier traps
• However, if opposite carrier capture dominates,
  traps may remain uncharged (invisible in DLTS)
  gt basic limitation !
• Asymmetric doping concentration signal from
  lower doped side
• Other sample types
• Grain boundary (anti-serial Schottky diodes) gt
  bonded wafers
• MIS devices
• FETs ("conductivity DLTS")
• point contacts at nanowires ? ...

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• 2.2. Optically excited DLTS (minority carrier
  DLTS, MCDLTS)
• trap filling by optically excited minority
  carriers (hn gt Eg)
• reverse bias remains constant

thermal equilibrium traps emptied (from holes)
measurement hole emission
filling pulse hole capture

• allows investigation of minority carrier traps
  in Schottky diodes

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• 2.3. Optical DLTS (ODLTS)
• trap filling by bias pulses
• continuous irradiation of IR light (lt Eg)
• optical emission additional to thermal emission
• strong dependence on intensity and l

illuminated
dark

• ODLTS allows to measure optical capture cross
  sections sopt(l)
• connection between deep levels electrically
  detected (DLTS) and optically detected
  (absorption)

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2.4. Concentration depth profiling (pulse height
scan)
Vr
t
Vr
t
Vr
t
Vr
t

• linear dependence on Vp homogeneous
  concentration !

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2.5. Measurement of field dependence of enp
(pulse height scan)
Vr
t
Vr

• quantitative evaluation difference spectra
  (DDLTS)
• field depencence indicates charged occupied state

t
Vr
t
Vr
t
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2.6. Measurement of capture cross sections (pulse
width scan)
Vr
t
Vr
t
Vr

• "real" capture cross section
• measurement at different rate windows
  T-dependence of CCS
• injection measurement of minority carrier CCS

t
Vr
t
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2.7. Point defects and extended defects

• all previous considerations referred to isolated
  point defects
• "extended defects" dislocations, grain
  boundaries, precipitates ...
• continuum of states, "broadened states"
• emission probability depends on average
  occupation state
• barrier-controlled capture, depending on
  occupation state

point defect
extended defect
DLTS
low occupation
T
DLTS
high occupation
tc
log(timp)

• extended defects show logarithmic capture
  behaviour

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• Summary
• DLTS on Schottky diodes only reveals majority
  carrier taps
• DLTS on pn junctions also reveals minority
  carrier traps
• Optically excited DLRS (MCDLTS) also reveals
  minority carrier traps in Schottky diodes
• ODLTS reveals optical trap parameters sopt(l)
• There are special DLTS procedures for measuring
• - concentration depth profiles
• - electric field dependence of enp
• - capture cross sections for electrons and holes
• Extended defects are usually characterized by a
  logarithmic capture behaviour and often show
  non-exponential emission (broadened peaks)
• Next time Introduction of our own DLTS system