1. (Deep Level Transient Spectroscopy)
Introduction to DLTS
(Deep Level Transient Spectroscopy)
I. Basic Principles
O. Breitenstein
MPI MSP Halle

2. 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

3. 1. Application field of DLTS
"Deep levels" = energy states in semiconductor band gap, > 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 CB VB
electron traps
hole traps
intrinsic energy

4. Possible electronic processes
capture
electron
emission CB
electron trap
hole trap VB
hole
capture
hole
emission
thermal (electron) emission probability:
"emission rate" [s-1]
capture prababilities:
cn;p: "capture coefficients [cm3s-1]
trap parameters: Et (thermal activation energy), sn and sp resp. cn and cp

5. (thermal) emission rate (T): "Arrhenius plot" (fingerprint)
log(en;p)
prefactor contains sn,
but this parameter is often
exponentially T-dependent!
1000/T [K-1]
prefactor gives not sn !
Et not equilibrium energy !

6. 2. Basic Principles of DLTS
Electron trap in n-type space charge region (Schottky diode) Vr W0
Wr(Vr)
investigated volume
metal
RF-capacitance (1 MHz):

7. 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

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

9. T-dependence of C-transient
low T
opt. T
high T t1 t2 t t1 t2 t
DCmeas t1 t2 t T
Tpeak
DCpeak
DLTS signal = C(t1)-C(t2)
If T is slowly varying, at a certain temperature a DLTS peak occures

10. Different deep levels are leading to different peaks
DC1
DC2 e0
en1(T)
en2(T)
log(en;p)
DLTS
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)

11. 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