Introduction to DLTS
(Deep Level Transient Spectroscopy)
II. Advanced Techniques
O. Breitenstein
MPI MSP Halle
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
Recapitulation: DLTS routine (repeating!) :
Vr
reverse
reduced
or forward
reverse
bias
t
0
band
diagram
e
e
e
-
-
-
e
-
e
-
RFcapacitance
t
DC
0
t
Generation of the DLTS signal
opt. T
low T
high T
DCmeas
t1
t2
t
t1
t2
t
t1
t2
t
t2
ln
t1
"rate window": e n;p peak s 1 
t 2  t1
DLTS signal = C(t1)-C(t2)
 
Tpeak
T
DCpeak
If T is slowly varying, at a certain temperature a DLTS peak occures
DLTS measurements at different rate windows
allow one to measure Et
198 * n
ln(en)
DLTS
e01
E t meV  
e03
1000
D
T
 2kT
e02
e03
e02
e01
T1 T2 T3
T
T3
T2 T1 1000/T
This "Arrhenius plot" allows an identification of a deep level defect
2. Advanced DLTS measurement techniques
2.1. Possible samples: Schottky diodes or pn-junctions
Schottky diode
e-
pn-junction
reverse
bias
h+
e-
e-
e-
V=0
e-
e-
majority carrier flow
forward
bias
e-
e-
h+
e-
e-
h+
h+
h+
xe
xh
minority carrier injection
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) => basic limitation !
• Asymmetric doping concentration: signal from lower doped side
Other sample types:
• Grain boundary (anti-serial Schottky diodes) => bonded wafers
• MIS devices
• FETs ("conductivity DLTS")
• point contacts at nanowires ? ...
2.2. Optically excited DLTS (minority carrier DLTS, MCDLTS)
• trap filling by optically excited minority carriers (hn > Eg)
• reverse bias remains constant
e-
h+
eh+
h+
eh+
h+
thermal equilibrium
traps emptied
(from holes)
filling pulse
hole capture
measurement
hole emission
• allows investigation of minority carrier traps in Schottky diodes
2.3. Optical DLTS (ODLTS)
• trap filling by bias pulses
• continuous irradiation of IR light (< Eg)
• optical emission additional to thermal emission
• strong dependence on intensity and l
illuminated
dark
Tpeak
Tpeakillumin.
dark
T
T
• ODLTS allows to measure optical capture cross sections sopt(l)
• connection between deep levels electrically detected (DLTS)
and optically detected (absorption)
2.4. Concentration depth profiling (pulse height scan)
Vr
Tpeak
T
e-
Vr
t
e-
Tpeak
T
t
Vr
e-
Tpeak
T
t
Vr
e-
Tpeak
T
t
• linear dependence on Vp: homogeneous concentration !
2.5. Measurement of field dependence of en;p (pulse height scan)
Vr
1
Tpeak
T
Vr
t
Tpeak2
T
t
Vr
Tpeak3
T
t
Vr
Tpeak4
T
t
• quantitative evaluation:
difference spectra (DDLTS)
• field depencence indicates
charged occupied state
2.6. Measurement of capture cross sections (pulse width scan)
Vr
Tpeak
T
t
Vr
Tpeak

  t imp  


DC( t imp )  DC 1  exp


t
c



T
tc 
t
Tpeak
Vr
T
Vr
t
Tpeak
t
1
1

cn n sn vn n
• "real" capture cross section
• measurement at different
rate windows: T-dependence
T of CCS
• injection: measurement of
minority carrier CCS
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
extended
DLTS
defect
defect
e
e
low
occupation
-
high
occupation
e-
-
e-
DLTS
T
tc
log(timp)
• extended defects show logarithmic capture behaviour
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 en;p
- 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