Transcript No Slide Title

Introduction to DLTS
(Deep Level Transient Spectroscopy)
I. Basic Principles
O. Breitenstein
MPI MSP Halle
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
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
intrinsic energy
CB
electron traps
hole traps
VB
Possible electronic processes
electron
capture
electron
emission
CB
electron trap
hole trap
VB
hole
capture
hole
emission
n vn Nc
E t
exp
thermal (electron) emission probability: e n 
g
kT
"emission rate" [s-1]
capture prababilities: Pcn  n  n v n  n c n
cn;p: "capture coefficients [cm3s-1] Pcp  p  p v p  p c p
trap parameters: Et (thermal activation energy), n and p resp. cn and cp
(thermal) emission rate (T): "Arrhenius plot" (fingerprint)
log(en;p)
n vn Nc
E t
en 
exp
g
kT
198* n
E t meV  
 2kT
1000

T
prefactor contains n,
but this parameter is often
exponentially T-dependent!
 E t  E n

vn Nc
n
en 
exp
g
kT
1000/T [K-1]
• prefactor gives not n !
• Et not equilibrium energy !
2. Basic Principles of DLTS
Electron trap in n-type space charge region (Schottky diode)
Wr(Vr)
W0
metal
Vr
RF-capacitance (1 MHz):
investigated
volume
 0 A
 0 e ( N D  N t )
C(Vr ) 
 C 0  C( N t ) 
W
2(V  VD )
capacitance change due to recharging of Nt [cm-3] traps:
C 1 N t

C0 2 N D
net doping concentration, from C/V meas.
basic (equilibrium) capacitance
Sign of C depends on trapped carrier type:
• majority carrier capture: C negative
• minority carrier capture: C positive
Best sensitivity for low doping concentration
DLTS routine (repeating!) :
Vr
reverse
reduced
or forward
reverse
bias
t
0
band
diagram
e
e
e
-
-
-
e
-
e
-
RFcapacitance
t
C
0
t
T-dependence of C-transient
opt. T
low T
high T
Cmeas
t1
t2
t
t1
t2
t
t1
t2
t
DLTS signal = C(t1)-C(t2)
Tpeak
T
Cpeak
If T is slowly varying, at a certain temperature a DLTS peak occures
Different deep levels are leading to different peaks
log(en;p)
en1(T)
en2(T)
e0
T
DLTS
T
T1
C1
T2
C2
t2
ln
t1
peak
e0 
t 2  t1
condition:
peak height C 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)
DLTS measurements at different rate windows
allow one to measure Et
198* n
ln(en)
DLTS
e01
E t meV  
e03
1000

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