Transcript Introduction to DLTS - Max Planck Institute of

Introduction to DLTS
(Deep Level Transient Spectroscopy)
III. Our DLTS System
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. 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
Advanced techniques
• 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
• There are special DLTS procedures for measuring:
- concentration depth profiles (Vimp scan)
- electric field dependence of en;p (Vimp scan)
- capture cross sections for electrons and holes (timp scan)
• Extended defects are usually characterized by a logarithmic
capture behavior and often show non-exponential emission
(broadened DLTS peaks)
Philosophy of our DLTS system
1. We don’t save DLTS data but transient data
• Conventional approach: On-line conversion of transient data to
DLTS data, saving DLTS(T) (1-dimensional vector of data).
Advantage: Small file sizes. Disadvantage: No flexibility with
respect to different correlation techniques (see below)
• Our approach: C(t, T) is saved as a 2-dimensional data file
Advantage: Flexible DLTS correlation. Disadvantage: Larger file
sizes (see below).
2. We have both linear and logarithmic time scale at choice
• Linear time scale: time resolution for large times is the same as
immediately after the filling pulse. May be advantageous for
software-based multiexponential transient deconvolution
tn = n tmin
0 tmin
tn
• Logarithmic time scale (base 2, also 1.1 possible): Time
resolution proportional to elapsed time, drastic savings in file size
averaging over differently sized periods!
tn = (2n-1) tmin
0 tmin
tn
3. We have three different kinds of DLTS correlation at choice
3.1. Modified 2-Point correlation: DLTS = C(t1)-C(t2)
Mathematical formulation:
t
t
t
3
3
2
1
1
DLT S
C( t )dt 
C( t )dt   C( t )K( t )dt


t 2  t1 t
t3  t2 t
t
1
2
1
C(t)
K(t)
t1
t2
t3
good compromise between resolution and sensitivity, many rate windows
3.2. Exponential correlation: High sensitivity, but less resolution
K(t)
t
DLTS
DC(0)
2-point
T
3.3. High resolution correlation: Low sensitivity
K(t)
t
DLTS
DC(0)
2-point
T
Hardware
• C-Meter working at 1 MHz, made at our electronic workshop
• applied HF signal: 100 mV (pk-pk) or 1 V (pk-pk) at choice
• electronic C- and G- (conductivity) compensation
• manual or automatic compensation
• sample bias range: 0 ... 15 V
• pulse bias range: 0 ... 15 V. If pulse bias > bias: injection !
• preamplifier separated, connected with main unit by 1 m cable
• computer controlled via 2x16 bit ADC / DAC interface card
•T control unit, controlled via RS232 by computer
• linear or exponential T-ramp at choice, speed adjustable
C Meter and T controller
DLTS system wiring scheme
ADC 0
bias
ext.
ADC 2
ADC 2
DAC 0
pulse bias Delta C
ext.
out
C-compens computer preamp.
out
Computer
rear side
C-meter
front side
delta C
bias out
RS
232
"Trig."
INPUT CH.1 INPUT CH.2
EXT.TRIG.
preamplifier
Tcontroller
Probe
extern
Oscilloscope
- Probe
+ Probe
Cryostat
sample
2 different cryostats
at choice:
1. Bath cryostat
• only for samples
mounted on TO5
transistor holders
• manually immersing in
liquid nitrogen (cool
down), measurement after
lifting above LN2 level,
quick measurement
• not optimum for very
slow T-ramps or constant
T measurements
2. Evaporator cryostat
• for samples mounted on TO5 or TO18 holders or bare samples
• fully automatic cooling down and heating up (software controlled)
• slower measurement, larger LN2 consumption
Software
Made by MSC Technik Halle (http://www.msc-technik.de/)
“settings” menu:
“display” menu
What this system can do:
• DLTS measurements from 78 K to 400 K
• sample capacitance < 500 pF
• sample parallel resistance > 500 W
• bias and pulse bias range: 0 ... 15 V
• samples mounted on transistor holders or as raw chips
• linear and logarithmic sampling (to base 2 or 1.1)
• rate window range from < 1 s-1 to 104 s-1
• monitoring and storage of C0(T) (basic capacitance)
• sensitivity < 10-4pF for 0.1V HF (pk-pk), < 10-5pF for 1 V HF
• “batch” measurements for bias, pulse bias, tmin, and timp
• display of up to 10 DLTS traces
• export of C transients, C0(T) and DLTS traces as ASCII files
• system is available in room B.2.05, to be used only after
personal introduction by O.B. !
Plans for the future:
• Establishment of Minority Carrier DLTS (optical excitation)
• DLTS peak evaluation software (parameter fitting etc.)
3 ppt Files of this introduction and the DLTS operation manual
are available on-line