1. (Deep Level Transient Spectroscopy) II. Advanced Techniques
Introduction to DLTS
(Deep Level Transient Spectroscopy)
II. Advanced Techniques
O. Breitenstein
MPI MSP Halle

2. 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

3. Recapitulation: DLTS routine (repeating!) :
reverse
reverse
reduced
or forward Vr
bias t e- e- e-
band
diagram e- e-
RF-
capacitance t DC t

4. Generation of the DLTS signal
low T
opt. T
high T t1 t2 t t1 t2 t
DCmeas t1 t2 t
"rate window": T
Tpeak
DCpeak
DLTS signal = C(t1)-C(t2)
If T is slowly varying, at a certain temperature a DLTS peak occures

5. DLTS measurements at different rate windows allow one to measure Et
ln(en)
1000/T
DLTS
This "Arrhenius plot" allows an identification of a deep level defect

6. 2. Advanced DLTS measurement techniques
2.1. Possible samples: Schottky diodes or pn-junctions
Schottky diode
pn-junction e-
reverse
bias e- h+ e- h+ e-
V = 0 xe e- h+ xh e-
forward
bias
majority carrier flow
minority carrier injection

7. Schottky diodes: pn-junctions: Other sample types:
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 ? ...

8. 2.2. Optically excited DLTS (minority carrier DLTS, MCDLTS)
trap filling by optically excited minority carriers (hn > Eg)
reverse bias remains constant e- e- h+ e- h+
thermal equilibrium
traps emptied
(from holes)
filling pulse
hole capture
measurement
hole emission
allows investigation of minority carrier traps in Schottky diodes

9. trap filling by bias pulses
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
dark
illuminated T
Tpeakdark T
Tpeakillumin.
ODLTS allows to measure optical capture cross sections sopt(l)
connection between deep levels electrically detected (DLTS) and optically detected (absorption)

10. 2.4. Concentration depth profiling (pulse height scan) Vr
Tpeak e- t e- Vr T
Tpeak t Vr e- T
Tpeak t T
Tpeak Vr e- t
linear dependence on Vp: homogeneous concentration !

11. 2.5. Measurement of field dependence of en;p (pulse height scan) Vr
Tpeak1 t Vr T
Tpeak2
quantitative evaluation: difference spectra (DDLTS)
field depencence indicates charged occupied state t Vr T
Tpeak3 t Vr T
Tpeak4 t

12. 2.6. Measurement of capture cross sections (pulse width scan)Vr T
Tpeak t Vr T
Tpeak t T
Tpeak Vr
"real" capture cross section
measurement at different rate windows: T-dependence of CCS
injection: measurement of minority carrier CCS t Vr T
Tpeak t

13. 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
e - T
DLTS
e -
high
occupation tc
log(timp)
extended defects show logarithmic capture behaviour

14. 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