DLTS Manual
O. Breitenstein, 05.02.2003
Content:
p.
1. DLTS physical basics
1
2. DLTS hardware
3
2.1. Cryostat and T-measurement
3
2.2. C-meter
4
3. Software
6
3.1. Measurement modes
6
3.2. DLTS correlation
8
3.3. Batch measurements
8
3.4. The baseline option
9
3.5. The user surface
9
3.6. The C-calibration tool
14
4. DLTS measurement
14
4.1. Electrical connections
14
4.2. Performing a DLTS measurement 15
4.3. Data display and evaluation
16
1. DLTS physical basics
Deep Level Transient Spectroscopy (DLTS) was developed in 1974 by D.V. Lang to
investigate energetically "deep" charge trapping levels in semiconductor space charge
structures, which may be either pn junctions or Schottky barriers. It utilizes the fact that the rf
capacitance of the sample (usually measured at 1 MHz under reverse bias) depends on the
charge state of deep levels in the space charge region. In total depletion approximation, the rf
capacitance of a sample having a homogeneous doping concentration is:
C0  A
0 e (N D  N A )
2(Vr  Vd )
(1)
Here A is the sample area, ND-NA is the total net charge density in the space charge layer
(SCL), Vr is the reverse bias, 0 is the permittivity of the semiconductor material, and e is the
electron charge. If the sample is a pn junction, ND-NA refers to the lower doped side of the
junction. Vd is the built-in diffusion voltage of the space charge structure, which is the
crossing point of the extrapolated 1/C2 plot vs Vr with the Vr-axis. If A is measured in units of
mm2, ND-NA in cm-3, C0 in pF, and Vr+Vd in V, eq. (1) leads to the following relation with
adapted units:


N D  N A cm  3  1.41 *1012
C 0 pF2 (Vr  Vd )
 
A mm
2 2
(2)
If charged trapping levels are existent in the SCL, their space charge has to be added to N DNA. Assuming a donor-like trap level of concentration Nt in an n-type sample biased under a
reverse bias Vr, the capacitance change by recharging these levels is:
1
C  A
0 e (N D  N A )
0 e (N D  N A  N t )
Nt
A
 C0
2(Vr  Vd )
2(Vr  Vd )
2(N D  N A )
(3)
The last identity holds approximatively if Nt << ND-NA holds. Then the trap concentration
calculates from the capacitance change C as:
Nt 
2 C
(N D  N A )
C0
(4)
reverse bias
Vr
timp
time
capacitance
high temperature
C0
C
low temperature
t1
t2
time
Fig. 1: DLTS measurement procedure (see text)
Fig. 1 shows the basic experimental procedure of DLTS. The capacitance of the sample is
measured under reverse bias Vr. For performing the DLTS routine, bias pulses are
periodically applied to the sample (hence its reverse bias Vr is reduced or even reversed for a
certain filling pulse width timp), leading to a periodic recharging of the levels in the SCR.
During these pulses the rf capacitance increases, which, however, is not regarded in DLTS.
Immediately after each pulse the rf capacitance changes by C, whereby C is negative for
majority carrier traps (shown in Fig. 1) and positive for minority carrier traps. If a Schottky
diode is used, or if in a pn junction the reverse bias is only reduced by a pulse bias V imp  Vr,
only majority carrier traps are recharged. If during the pulse a pn junction is forward-biased
into injection, also minority carriers may be recharged. The degree of trap filling depends on
the filling pulse width timp and on the capture coefficient of the traps cn;p, which is often
formally expressed as the product of the thermal velocity times the capture cross section of
the trap for the corresponding carrier type. If the pulse width is large enough (in the order of
the pulse repetition period), one speaks from "saturation pulses", hence all deep levels in the
SCR should be filled after the pulse. For sufficiently small pulses only some part of all levels
are filled, hence the signal gets smaller. The pulse width tcapt, for which the signal height has
reduced to 1/e = 0.367 of its maximum value, allows one to measure the capture coefficient cn
(for electrons) or cp (for holes):
c n; p 
n; p
t capt
(5)
2
Here n and p are the free carrier concentrations of electrons and holes, respectively, during the
capture process. Note that (4) and (5) only hold if V is large compared to Vd and close to Vr.
For V < Vr the trap concentration calculates for saturation pulses for homogeneous trap
incorporation as:
Nt 
2 C Vr
(N D  N A )
C 0 V
(6)
Hence, for homogeneous trap distribution the DLTS peak height C is expected to be
proportional to the filling pulse height V. Note that the equilibrium degree of trap filling in
the SCR under reverse bias is zero, hence the levels are ionized. If after a filling pulse traps
are filled and the reverse bias is reestablished, the system is in thermal non-equilibrium and
relaxes into equilibrium by thermally emitting the trapped charges into the corresponding
bands, where they are swept away by the electric field. For isolated point defects this
relaxation is exponential in time, but for extended defects like dislocations, precipitates, and
interface layers the emission transient (just as the trap filling process) may be nonexponential. Therefore all these relations strictly only hold for point defects. This relaxation is
connected with a capacitance transient, which is converted into a measurable transient signal
by a C-meter. The time constant e of the thermal emission is governed by the thermal
emmission rate en;p, which depends on the trap energy Et and on the temperature T:
e n; p 
1 N c; v c n; p
  Et 

exp 

e
g
 kT 
(7)
Here Nc;v is the effective density of states in the conduction band (for electron emission) or
the valence band (for hole emission, respectively), g is the degeneracy factor of the level,
usually assumed to be unity, and kT is the thermal energy. Hence, the thermal emission rate
en;p is the inverse of the emission time constant e. The exponential dependence of the
emission rate on 1/T (the so-calles Arrhenius plot), with the thermal energy Et determining its
slope and the capture coefficient cn;p determining its prefactor, is the signature of each trap,
which is used for their identification.
In DLTS filling pulses are applied periodically to the sample, leading to periodical
capacitance transients (see Fig. 1). The basic idea of DLTS is to convert the capacitance
transients into the DLTS-signal by "correlating" them on-line. The simplest kind of DLTS
correlation is the 2-point correlation, where the capacitance is measured at two times t1 and t2
after the end of each filling pulse, and the difference between these two values is displayed:
DLTS  aC(t 2 ) - C(t1)
(8)
Hence, the DLTS signal is scaled in units of capacitance (usually pF). Other kinds of
correlation are providing an especially good signal-to-noise ration (exponential correlation) or
a better energy resolution (high resolution correlation) than 2-point correlation (see Sect. 3.2).
Usually the DLTS measurement starts at low temperature, the temperature is slowly ramped
up, and the DLTS signal is recorded. During the T-ramp the relaxation time constants of all
levels in the sample are gradually increasing from very large values to very low ones
according to (7). As long as T is too low for significant thermal emission until t2, the
difference (8) is zero (see "low temperature" trace in Fig. 1). If T is so high that the relaxation
is already over at t1, the difference (8) is also zero (see "high temperature" trace in Fig. 1).
Only if the emission time constant (or the emission rate, respectively) of one level falls into
3
the so-called "rate window" given by the selection of t1 and t2, a DLTS peak appears. For 2point correlation we obtain the following condition for the DLTS peak to appear:
t
ln 2
t1
e n; p 
t 2  t1
(9)
The "rate window", which according to (7) is the inverse of the relaxation time constant e,
has the unit of s-1. For 2-point correlation the corresponding time constant is always lying
between t1 and t2. The prefactor in (8) is chosen so that (for exponential transients) the DLTS
peak height directly allows to read the capacitance change C, which according to (4) and (6)
allows one to measure the trap concentration.
2. DLTS hardware
2.1. Cryostat and T-measurement
We are using a simple Cu-block bath cryostate shown in Fig. 2, which allows us to measure
from liquid nitrogen temperature up to 400 K. The sample has to be mounted e.g. using silver
paste and a tiny wire on so-called TO-4 holders (Fig. 3), which have to be inserted into the
Cu-block and fixed with 4 screws. The Cu-block represents one electrical contact, the second
contact has to be soldered to the corresponding leg of the sample holder after inserting it. The
temperature measurement appears by a small Pt resistance thermometer, which is fixed at a
thermally equivalent site at the opposite side of the Cu-block. This thermometer has not to be
removed! If the samples are mounted on smaller TO-18 holders, a Cu fitting ring can be used
for mounting them.
The cryostate is connected with a Raith temperature controller, which is actually designed
for heating and cooling by controlling a nitrogen valve. Hence, this controller may also
control flow cryostats. In the small bottom LED display the set (aimed) temperature in °C is
shown, whereas in the large upper display the actual temperature is shown in °C. The up and
down buttons allow to change the set temperature manually. However, in a DLTS experiment
the whole T-control is performed by the computer via RS 232 interface. Note that the
regulation steepness of this controller is set to a low value in order to avoid T-oscillations
during ramping. Therefore there is usually a difference between set and actual temperature,
which is no problem for DLTS since here only the actual temperature is measured. The Tcontroller displays its activity by a "heating / cooling" LED line. An "on / off" switch allows
to switch off the heating activity of the controller. This switch has to be used only in case of
emergency, hence in any case of malfunction of the T-control system. As a rule, during all
phases of a DLTS investigation this swith has to be "on" and the computer controls the
complete T-ramping process (see Sect. 3.5.).
For performing a measurement, the dewar has to be approximately half-filled with liquid
nitrogen (LN). For cooling down the sample, the whole Cu-block has to be immersed into the
liquid nitrogen of the dewar by using the height adjustment means. If the starting temperature
is reached, the Cu-block has to be raised, until only the down-hanging Cu-wire is immersed in
the LN for residual cooling. Then the whole T-ramping is controlled by the DLTS software.
The ramp parameters can be set in the "settings" menu (see Sect. 3.5.).
There is a special option which allows one to change the internal parameters of the Tcontroller and to test its function. This is the "comport / test ET818" option in the "options"
menu (see Sect. 3.5). Don't use this option without contacting Dr. O. Breitenstein.
4
2.2 C-meter
The preamplifier of the C-meter has to be attached to the cryostat as shown in Fig. 2 for an ntype sample. The sample sockets are the two lying opposite to the connecting cable. The
outermost socket carries the positive bias. The residual two sockets are for externally
connecting the sample e.g. to an I-V measurement setup. For investigating a p-type sample,
the sample polarity has to be reversed, hence the preamplifier has to be connected in opposite
direction, since the C-meter provides only positive bias polarity.
The C-meter allows an electronic compensation of the basic capacitance C as well as of the
conductance G of the sample. There is the possibility to compensate the sample capacitance
and conductance either manually or automatically. There are three possible compensation
ranges (500 pF, 100 pF, and 20 pF), which can be selected manually. The lower ranges
provide a better signal-to-noise ratio and a more exact C0-reading, but they cannot be used if
the basic capacitance C0 is too large. The sensitivity ranges 50, 5, 5e-1, 5e-2, and 5e-3 pF/V
can be set independently on the compensation range. In standard operation mode an rf-signal
of 100 mV pk-pk (30 mV rms) is applied for the rf capacitance measurement. To improve the
sensitivity, an rf-signal of 1 V pk-pk (300 mV rms) may be applied by pressing the "1V HF
Empf./10" button, whereby the sensitivity improves by a factor of 10. The instrument allows
one to set the measurement bias and the pulse bias from 0 to 15 V either manually, or to feed
them in externally via sockets at the rear panel. The pulse bias is defined here as the filling
pulse height V (see Chapter 1), which is applied at the other side of the sample than the bias.
Hence, during the pulse the difference between "bias" and "pulse bias" is applied to the
sample. For "pulse bias" > "bias" the sample is driven into forward bias during the pulse
("injection pulse"). The bias effectively lying on the sample can be directly monitored at a
BNC socket at the front. The instrument allows one to superimpose a certain bias modulation
signal to the sample bias, which may be advantageous for certain scaling procedures. This
bias modulation signal may be attenuated by a factor up to 1000. There are two general
operation modes of this instrument, which are "manual control" and "external control" from
the computer. In the manual control mode, a pulse trigger signal (TTL-level) may be applied
to the pulse trigger input socket at the front panel, and the delta C signal may be fed out at the
front or at the corresponding rear socket. In the "external control" mode, the pulse trigger is
fed in, together with other control signals, at the "computer control" connector at the rear. In
this mode a number of control means at the front are disabled (see table below).
Fig. 4 shows the front plate of the C-meter. All buttons are lighting, if they are active. The
buttons, displays, and sockets at the front plate have the following meaning:
Netz
mains switch
Steuer. ext.
selecting external (computer) control mode
Vorspannung
bias, displayed in V
Impulsspannung
pulse bias, displayed in V
Mod. ein
superimposing the modulation bias, inactive in external control mode
Faktor
the attenuation factor of the bias modulation
Modulation
input socket for feeding in the modulation bias
Imp. aus
disabling the bias pulse for performing a baseline measurement, inactive
in external control mode
Vorsp. ext.
external bias setting; bias and pulse bias are fed in at the rear sockets
5
Probe ext.
sample is connected to "ext" sockets at the preamplifier, e.g. for I-V
measurements. Inactive in external control mode.
Vorsp. Ausg.
output socket for sample bias, connect with oscilloscope
Impuls Trigger
input socket for TTL pulse trigger in manual control mode, inactive in
external control mode
delta C (display)
capacitance deviation signal, displayed from -10 V to 10 V
Delta G (display) conductance deviation signal, displayed from -10 V to 10 V
C-Kompensation display and potentiometer for adjusting the capacitance compensation,
potentiometer is inactive in "auto compensation" mode. The signal (0 ...
10 V) is accessible at the rear.
G-Kompensation display and potentiometer for adjusting the conductance compensation,
potentiometer is inactive in "auto compensation" mode. The signal (0 ...
10 V) is accessible at the rear.
1V HF Empf./10
selecting an applied rf-signal of 1 V pk-pk (300 mV eff.) instead of 100
mV pk-pk (30 mV eff.). This measure increases the sensitivity (= reduces
the sensitivity factor) by a factor of 10.
auto Komp.
selecting "auto compensation" mode, inactive in external control mode
Empfindlichkeit
sensitivity selection from 50 to 5e-3 pF/V, referred to delta C output
Komp. bereich
compensation range selection, governs maximum allowed basic
capacitance
delta C (socket)
output socket for delta C (capacitance change) signal, also present at rear.
Connect with oscilloscope.
delta G (socket)
output socket for delta G (conductance change) signal, also present at
rear
The sockets at the rear have the following meaning:
Computer
connector with computer interface
Vorverstärker
connector with the preamplifier
Netz
mains connector
Delta G Ausg.
output delta G (conductance change) -10 V ... 10 V
G-Komp. Ausg.
output conductance compensation signal, 0 ... 10 V
Delta C Ausg.
output delta C (capacitance change) -10 V ... 10 V, connect with "ADC
0"
C-Komp. Ausg.
output capacitance compensation signal, 0 ... 10 V, connect with "ADC
2"
Impulssp. Ausg.
output internal pulse bias, 0 ... 15 V
Impulssp. Ext.
input external pulse bias, 0 ... 10 V, connect with "DAC 2"
Vorsp. Ausg.
output internal bias, 0 ... 15 V
Vorsp. Ext.
input external bias, 0 ... 10 V, connect with "DAC 0"
6
3. Software
3.1. Measurement modes
This system measures and stores complete capacitance transients together with the
temperature and the basic capacitance values as the primary data file (see Sect. 4.3). Its
computer interface card contains two ADCs (ADC 0 and ADC 2) and two DACs (DAC 0 and
DAC 2), a digital signal processor (DSP) and a timer, which organizes and carries out the
DLTS routine. After a pulse trigger has been given out, ADC 0 is reading the capacitance
change (delta C) values at a rate of 100 kHz (every 10 microseconds) until close to the next
bias pulse. The DSP averages these deltaC values in certain time intervals up to the next bias
pulse, depending on the choice of the measurement parameters, leading to the C-transient
data. The special feature of our system is that no measured data are thrown away, hence all
measured transient data are used for the DLTS correlation (except for a blanking time at the
beginning of the transient, where a parasitic C-transient may occure, see Sect. 3.4.). There are
two timing modes, which are the linear equidistant timing and the logarithmic equidistant
timing mode. Figs. 5 and 6 schematically demonstrate the different timing sequences in these
linear and logarithmic modes. In both modes after an initial blanking period the data are
averaged over certain data periods, whereby the nominal measure times (to which the data
points are displayed on the transient) are always lying in the center of each data period. The
first (minimum) measure time is called tmin and plays a dominant role in defining the time
scale of the measured transient.
bias pulse trigger:
0 20µs
ADC measurements:
1
4
data periods: 1
8
0 20µs
12
2
16
3
20
4
24
5
28
6
32
4
8
7
12
1
16
2
compensation trigger:
0 30µs
Fig. 5: Timing diagram of the beginning of a measurement in linear mode.
Assumed parameters: timp= 20 µs, tmin = 60 µs, points = 7
Note that the ADC digitizes always at the end of its internal averaging period of 10 µs, hence
it measures the analog signal before the digitizing event. According to Fig. 5 the measurement
starts 40 µs after the filling pulse, hence the first 4 digitized ADC measurements are skipped.
The data for the first C-value are the average over ADC values No. 5 through 8, the second
data point is the average over ADC values No. 9 through 12, and so on. So here t min lies in the
middle of data period 1 after the 6th ADC measurement, hence tmin = t1 = 60 µs holds, t2 =
100 µs, t3 = 140 µs and so on. So the time base of the linear measurement (distance between
successive measurements) is always 2/3 tmin. After ADC measurement No. 32 one complete
transient is measured as 7 data points. Then the so-called compensation trigger is given to the
C-meter for a duration of 1/10th of the data acquisition period. Note that in this external
(computer) control mode of the C-meter the automatic C-compensation works only during the
action of this compensation trigger, but not in the measurement periods. This prevents any
distortion of the transient shape, even for slow transients. After the end of the compensation
trigger the whole sequence starts again with the next filling pulse, with the measured data
7
being averaged by the DSP with the corresponding data of the first transient and so on. Only
after the programmed integration time tint , being typically in the order of seconds, the whole
procedure stops, and the seven data values averaged over the whole integration period are
submitted to the host computer.
bias pulse trigger:
0 20µs
ADC measurements:
4
8
data periods:
1
0 20µs
16
2
32
3
4
8
1
16
2
3
compensation trigger:
0 30µs
Fig. 6: Timing diagram of the beginning of a measurement in logarithmic mode.
Assumed parameters: timp= 20 µs, tmin = 60 µs, points = 3
In the logarithmic mode the procedure is very similar, except that the duration of the
averaging periods in each transient doubles from data point to data point. Hence, the data for
long times after the pulse are recorded with a lower time resolution. As for Fig. 5, in the case
of Fig. 6 the first data point is the average over ADC measurement 5 through 8 (representing
the mean measure time of tmin = t1 = 60 µs), but the second data point is the average over
ADC value 9 through 16 (representing the mean measure time of t2 = 120 µs), and t3 = 240µs.
The advantage of the logarithmic mode is that a wide time constant range can be measured
with a low number of data points without loosing time resolution for the fast transients.
3.2. DLTS correlation
As mentioned at the beginning of Sect. 3.1, only the delta C data points are stored by the
program. From these values the program calculates the DLTS signal e.g. using the 2-point
correlation. In this program we have chosen a modified 2-point correlation procedure. Instead
of measuring the capacitance in 2 small time windows, we measure it averaged over 2
sucessive time periods, the first one beginning after a delay time after the end of the relaxation
pulse, and the second one beginning immediately after the first one. The second time period
has the double length as this, just as shown in the sketch of the logarithmic timing diagram in
Fig. 6. Since we need two C-data for defining one rate window in analogy to to (9), from the
transient in Fig. 6 we may calculate DLTS data for two rate windows: one using the
difference C(t1) - C(t2), and one using the difference C(t2) - C(t3). If the data are recorded in
the linear mode according to Fig. 5, we need 15 points (which are combined in groups, just as
in Fig. 6) for getting two DLTS values. Due to this philosophy the rate windows coming from
one transient are always differing from each other by a factor of two. The exact values of the
rate windows can be set both in linear and logarithmic mode by choosing t min. Due to the
above mentioned differences between the definition of the times in these modes (in linear
moder at the beginning of each averaging period, and in logarithmic mode in the middle of
it), the same tmin formally leads to different emission rates in both modes. Nevertheless, both
procedures work equivalent, hence for the same emission rates in both modes the DLTS peak
temperatures coincide. Note also that the timebase is 100 kHz, hence the smallest value of
Tmin is 10 µs (0.01 ms). In practice tmin should be chosen always larger because of the
8
inevitable parasitic relaxation of the C-meter after the filling pulse. In practise, starting from
tmin = 50 µs a meaningful measurement is possible (see also Sect. 3.4.). The more data points
are recorded, the more rate windows can be calculated from each recorded transient. Note,
however, that in this case the sognal-to-noise ratio of the highest rate windows (using only the
first data points) are strongly degraded with respect to the low rate windows, since they are
averaging over a considerably lower number of primary data points.
Except the 2-point correlation two other correlation procedures are implemented into this
system, which are the exponential and the high resolution correlation. Both correlations need
at least 7 data points in the linear and 3 points in the logarithmic mode. If there are more
points, also here several rate windows differing by a factor of 2 can be calculated from each
transient. The exponential correlation has the advantage that it produces the best signal-tonoise ratio. However, it has the disadvantage that its CLTS peaks are broader than for 2-point
correlation, so the energy resolution is lower. The high-resolution correlation, on the other
hand, produces sharper DLTS peaks than the 2-point correlation, but its signal-to-noise ratio
is considerably worse. The advantage of this DLTS system is that any kind of correlation may
be selected after the measurement. The selection of the different DLTS correlations can be
made in the display menu (see Sect. 3.5.).
3.3. Batch measurements
This system allows to perform so-called batch measurement, hence to automatically vary the
parameters of one DLTS variable during the measurement. This is organized within one Tramp, hence at every temperature several DLTS measurements are performed sucessively
with all programmed batch parameters. Possible batch variables are: tmin, timp, bias, and pulse
bias. The list of batch variables can be edited in the settings menu (see Sect. 3.5.). In a batch
measurement the T-ramp automatically slows down so that also here the same temperature
accuracy is obtained as in a single DLTS measurement.
3.4. The baseline option
Note that during the filling pulse the capacitance dramatically changes, hence the C-meter is
usually overloaded. Though it was tried here to reduce this effect, also this C-meter shows
some spurious (parasitic) relaxation of the delta C signal after each filling pulse, which does
not represent a real relaxation of the sample capacitance. In the DLTS spectrum, such a
parasitic transient is visibble as a baseline shift of the spectrum, which is strongest for large
rate windows. Fortunately, this parasitic relaxation at least partly also appears, if no pulses are
applied to the sample, hence if actually no capacitance transient should appear. This is used in
the "baseline" measurement option. If the "baseline" option is selected, at the beginning of the
measurement (at the deepest temperature) the system performs exactly 7 DLTS measuremets
without applying a pulse (baseline measurement). The average value of this "baseline"
transients is automatically subtracted from each transient which is measured applying bias
pulses in the following. The averaging over 7 measurements is done in order to prevent the
introduction of additional noise by this precedure. If a batch measurement is programmed, the
baseline measurement is performed with all programmed batch parameters, hence this
procedure may take a while. This baseline option helps to remove the parasitic transient at
low time and restores the baseline of the DLTS signal. Unfortunately, only some part of the
parasitic transient is removed, hence even by using the baseline option, measurements for tmin
< 50 µs may be partly corrupted for the largest rate windows.
9
3.5. The user surface
After starting the computer, he asks for the password for the user "guest", which is
"weinberg2". Then every time under "extended" you have to select the server "MPI-FS1" in
order to be connected with our LAN. In Windows'95, after opening the "DLTS" program, the
software asks you for defining a new project. As a default file it proposes
"C:\sem\aktuell.dat", which may be confirmed by pressing "save". However, this is a scratch
file, which may be used after each starting. If you press "save", he asks to "overwrite" the old
"aktuell.dat" file. If you do so and this measurement should have been successful, this
"aktuell.dat" file should be stored by "save as" additionally under another name in another
directory afterwards, in order to prevent overwriting your file after the next start of the
program. As a rule, you should define at the beginning your own file name and your directory
and press "save". Then you produce a file on the disk which contains the header with all
parameters, but no measured data yet. The proposed measurement parameters are always that
of the last measurement, after which the program has been closed last time. During the
measurement, the file is contiuously completed with the measured data, so that even after a
computer crash all your data should be on the harddisk. Note that the harddisk is divided into
many logical disks, each having a capacity of 1 GB. It is advisable that every user uses its
own logical disk and only this one and transfers old data files via the LAN to another storage.
Fig. 5: User surface
Fig. 5 shows the user surface with some project loaded. As usual in Windows programs, in
the "file" menu you may choose between defining a "new project", "load" a saved project,
"save" your project, if you have made any changes, "save as" for making a duplicate under a
new name or in another directory, "print" the DLTS curves together with the measurement
10
information on a printer, "preview" the printed image, choose the "printer settings", or "close"
the program. In the "export" option you may export the "dlts" data, the "transient" data, or the
"C0" data as ASCII files. All these files are containing the header information with all
measurement parameters and the remarks (see Sect. 4.3.). As usual in Windows, all these and
other selections can also be made py pressing the underlined character keys. In the "options"
menu a "calibration tool" cor calibrating the C0-reading may be accessed (see Sect. 3.6.), the
parameter "settings" can be checked (see below), and the temoerature controller may be
programmed and tested in the "comport / test ET818" option (see Sect. 2.1.). The upper left
graph contains the last measured capacitance transient "dC[pF]". As default, the scaling in this
graph is automatic, but the automatic scaling can also be disabled and the scaling limits can be
set manually. In the upper right field a comment can be written. You may clear this field by
pressing the "clear" button on top of it. Please note that for technical reasons the "enter"
command ( can be used to write here, but cannot be reproduced in this field after reloading
the file. So, try to avoid to use this character here and navigate with the mouse or with the
arrow keys. The lower graph contains the DLTS curve(s) as a function of temperature. If the
"C0[pF]" button at the right is activated, also the measured temperature dependence of the
basic capacitance is displayed and scaled at the right. Below the lower graph a number of softkeys allow to select the basic activities of the program. They have the following meaning:
"adjust"
Pressing this button brings the system into the DLTS routine, but without saving
any measured data and without ramping the temperature. However, the actual
temoperature is read and displayed both in °C and in K in the blue T-display field
at the lower right. Moreover, in this mode the "set temperature [°C]" field in the
lower right corner is filled with the measured temperatures, and these values are
transmitted to the T-controller. Moreover the "on" field besides the T display is
disabled to prevent heating during the cooling procedure. If the measurement shall
start at an intermediate temperature between liquid nitrogen and room
temperature, it is advisable to activate the "on" field besides the "set temperature
[°C]" field at the desired temperature. In this case the system stops reading the
actual temperature, it keeps the last value as the "set" value, and it stabilizes this
temperature. In the "adjust" mode you may change the "display" options (see
below), but for changing the measurement parameters (see below) you have to
leave this mode by pressing "stop". You also may leave the "adjust" mode by
directly pressing "measure", leading to the beginning of the DLTS measurement.
"measure" This starts the DLTS measurement. The temperature in this starting moment is
taken as the real starting temperature, and the T-ramping is performed up to the
programmed "Tend" temperature. After reaching the end temperature, the
measurement is complete, and the system displays "overstep T end". Then the
system writes 26°C into the "set temperature [°C]" field and approaches this
temperature, since it assumes that you want to exchange the sample. If you don't
want to go to room temperature, you may disable the "on" field or manually set
another temperature in the "set temperature [°C]" field. After the measurement
you still may complete the comment field and save the file again under the same
name either with the small "save" button above the comments field or via "file >
save". If you press "stop" during the measurement, the system also assumes that
the measurement is over, hence it approaches room temperature. If you want to
restart the measurement, than disable "on" here, go to the desired starting
temperature, and press "measure" again. Then he asks you to overwrite this
(previous) file, which you may or may not quit.
"stop"
This is to leave the "adjust" or the "measure" mode (see above). If "stop" is
pressed in the "measure" mode, 26°C is written into the "set temperature [°C]"
field, but in the "adjust" mode it is not.
11
"display"
This opens a window for defining which DLTS curves are displayed in the DLTS
graph. Note that depending on the number of measure points DLTS curves to
several rate windows may be displayed for each stored transient. Moreover, in a
batch measurement for each rate window measurements to several batch
parameters are present. The "display" window contains a one- or two-dimensional
array of fields, a typical example being shown in Fig. 6. At the top there is the
display of the rate window parameter, and for batch measurements at the left there
is the display of the batch parameter. If the mouse arrow hits one of these fields,
the corresponding parameters are displayed. As default, only the upper left field is
coloured and the others are grey. Clicking with the left mouse button in any field
changes its colour from grey to another colour. Multiple clicking changes the
colour. Clicking with the right mouse button turns the colour back to grey, which
means that this parameter set is not displayed. Note that the maximum number of
displayed DLTS curves is 10. The displayed colours represent the colours of the
DLTS curves in the graphic display. Additionally, the colour codes and their
parameters are displayed in the "DLTS curves" field above the DLTS display. In
the upper right corner of the "display" window one can choose between 2-point
correlation (default), exponential correlation, and high resolution correlation. The
different rate windows for the different kinds of correlation are automatically
actualized.
"settings" This opens a window to define the DLTS measurement parameters. Most
parameters are only allowed to be changed after a new project has been defined
and the measurement has not started yet. The different parameters have the
following meaning:
"mode"
Here the sampling mode can be selected between "linear" and
"logarithmic" averaging (see Sect. 3.1.). Since for our DLTScorrelations the results are completely equivalent, but the file size
is lower in "logartihmic" mode, the logarithmic mode generally
should be preferred. Only for detailed transient shape evaluations
the linear mode has to be used.
"integration time tint"
Integration time for each single DLTS measurement point. Note
that at least one transient has to be measured. So, if tmin and the
number of points are chosen so that a single transient gets longer
than tint, tint gets automatically ingreased so that at least one
transient fits within the integration time.
"start after pulse tmin"
Time of the first measurement (see Sect. 3.2.)
"pulse trigger timp"
Duration of the filling pulse (see Chapter 1)
"points"
Number of data points stored in each transient (see Sect. 3.2.)
"tmax"
Maximum time of the transient. This number cannot be entered, it
is displayed only for your information.
"delay time"
If "bias" is used as the batch parameter, after changing the bias
the C-meter needs some time to relax with the automatic
compensation until the next measurement can start. Here this
delay time can be set, 1 s is a typical value. In all other cases no
delay time is necessary.
"number of samples"
This displays how many transients are averaged within one
integration period. This number cannot be entered, it is displayed
only for your information.
12
"sensitivity"
In this menu the sensitivity range of the C-meter in pf/V has to be
chosen to have a correct DLTS scaling. Contrary to the other
parameters, this value is allowed to be corrected after the
measurement.
"compensation range"
In this menu the C-compensation range (20 pF, 100 pf, or 500 pF)
has to be selected to have a correct C0 scaling. Also this value is
allowed to be corrected after the measurement.
"baseline corrected"
If this button is activated, at the beginning of the DLTS
measurement the baseline transient is measured for each batch
parameter. These baselines are automatically subtracted from
each measured transient.
"bias"
Here the reverse bias has to be entered. This setting works only if
at the C-meter "Vorsp. ext." is activated (see Sect. 2.2.)
"pulse bias"
Here the desired pulse bias is entered. Also this setting works
only if at the C-meter "Vorsp. ext." is activated (see Sect. 2.2.)
Note that the sample bias during the pulse, which can be
monitored at the "Vorsp. Ausg." socket of the C-meter, is the
difference between "bias" and "pulse bias".
"batch variable"
Menu to select a batch variable. A batch variabe can be one of
"none" (single measurement), "t(imp)", "t(min)", "bias", and
"pulse bias".
"edit batch file"
After pressing this button, the batch variable file may be edited.
Take care that the "number of parameters" agrees with the
parameter list. Don't change the style of this file, don't insert any
spaces.
"Tstart"
Here the intended starting temperature can be entered either in °C
or in K. This is only for setting the minimum temperature in the
DLTS display and for calculating the expected measure time. The
real starting temperature may be above or below this value. After
a measurement has started, the actual temperature gets the new
starting temperature Tstart.
"lin / exp"
Here it can be decided, whether a linear or an exponential T-ramp
will be used. In an exponential T-ramp the slope of the ramp is
proportional to the absolute temperature. Hence, we have a
steeper T-ramp and a lower T-resolution. This is no disadvantage,
since at high temperatures the DLTS peaks are broader anyway.
But the advantage is that the total measure time reduces, therefore
exponential T-ramping should be preferred.
"delta T / delta T0"
Here the temperature resolution (temperature step between two
points in each DLTS curve) can be set. It is no use to go below
0.1 K, since this is the resolution of the T-measurement.
Formally, you even may select 0 K, then your whole
measurement will be at constant temperature. For exponential
ramping, this is the smallest T-step at the lowest temperature, at
higher temperature delta T increases. Note that the systematic Terror coming from the not exactly thermally equivalent positions
of the thermometer and the sample is also proportional to delta T.
So for arrhenius plot measurement a low delta T and a larger
13
integration time tint should be used. This parameter is allowed to
be changed even during a measurement.
"Tend"
Here the end temperature is inserted, which is the upper T-scaling
limit of the DLTS curves and that temperature, where the DLTS
measurement automatically stops. If a measurement is stopped
manually before Tend, the last measured T-value will be taken as
Tend. Also this parameter is allowed to be changed during a
measurement.
"actual time"
Here during the measurement the actual time left after starting the
measurement is displayed.
"calculated time"
After inserting the DLTS parameters and the intended ramp
parameters, the program calculates and displays here the expected
total measure time in hrs:min:sec.
As usual for Windows programs, pressing "ok" takes and actualizes the selected parameters,
and pressing "calcel" leaves this menu without actualizing any parameter.
3.6. The C-calibration tool
The varactor diodes used in the C-meter for compensating the basic capacitance C0 show a
non-linear dependence of their capacitance on their reverse bias, which corresponds to the "C
Komp. Ausg." signal, see Sect. 2.2. The DLTS software contains a non-linear digital
calibration tool to convert this voltage into the basic capacitance, which has to be performed
separately in the three C0-compensation ranges 20 pF, 100 pF, and 500 pF. A teach-in
procedure allows to adjust the scaling parameters to the caracteristics of the varactor diodes.
Once this scaling has been performed, it is not allowed to be changed anymore. Please,
contact Dr. Breitenstein before using this option and making any changes in this adjustment.
After selecting "calibration tool" in the "options" menu there is the possibility to change
the scaling of the basic capacitance (C0) measurement procedure. The C0-measurement is only
possible if the bridge is working in the automatic C-compensation mode, hence if its basic
capacitance is always automatically compensated. Then the C-compensation signal is a
unique measure of the basic capacitance, however, with a non-linear relation. This relation
can be teached-in in this option by using a capacitance decade as the sample. Connect the
decade instead of the cryostat to the preamplifier and operate the bridge in manual control
mode in the desired sensitivity range (5e-1 pF/V) in automatic compensation mode. In the
"calibration tool" window there is a list of capacitances and voltages. With "back" and
"forward" you may move from line to line, set the capacitance value at the C-decade standing
in the list, press "measure" and, after the bridge has relaxed, press "stop". This has to be done
for every capacitance value on the list and independently for every C-compensation range.
Note that only voltage readings between 0 and 10 V are allowed.
4. DLTS measurement
4.1. Electrical connections
Make sure that the computer, the C-meter, the oscilloscope, and the T-controller are switched
on. The oscilloscope is triggered externally by the pulse trigger coming from the computer
("Trig.", coming from the computer cable to the C-meter). As a rule it displays the
capacitance transient and the effective bias lying at the sample. Fig. 7 shows the complete
wiring scheme of the system, not considering the mains connectors and the computer
components.
14
ADC 0
ADC 2
bias
ext.
ADC 2
DAC 0
pulse bias Delta C C-compens computer preamp.
ext.
out
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
Fig. 7: Wiring scheme of the DLTS setup
4.2. Performing the DLTS measurement
After switching on the mains on the C-meter, the instrument falls into the mode "manual
control, compensation range 500 pF, sensitivity 50 pF/V, auto compensation off, internal bias
setting". First, the optimum C-measurement conditions have to be chosen at the C-meter in
manual control mode. First adjust the desired reverse bias, select auto C-compensation by
pressing the "Auto Komp." button, try to use lower C-compensation ranges, and stepwise
increase the sensitivity up to 5e-1 pF/V, which is the optimum range. Only for very large
transients (at any temperature delta C outside the display range of the LED lines) a lower
sensitivity (5 or 50 pF/V) has to be selected. The higher sensitivity ranges (5e-2 and 5e-3
pF/V) are only electronic amplifications of the 5e-1 pF/V range and don't improve the signalto-noise ration seriously. In tuned state both delta C and delta G should be zero (middle
position of the LED lines). If the delta C reading remains at the right, a higher Ccompensation range must be cosen. If delta G remains at the right, the electrical losses are
larger than they can be compensated by the bridge. Then the sensitivity has to be reduced until
the delta G reading is in the linear range again, in order to avoid overload of the bridge.
If the bridge is correctly tuned, it can be switched to the computer control mode by
pressing the "Steuer. ext." button at the C-meter. In the DLTS program you have to check in
the "settings" whether the correct bridge parameters ("sensitivity", "compensation range",
15
"bias", "pulse bias") are chosen. If a batch measurement with either "bias" or "pulse bias" as
the batch parameter has to be performed, the "Vorsp. ext." button at the C-meter has to be
activated. In all other cases these biases may also be set internally at the C-meter. However,
also then it is useful to insert the selected values in the "settings" field in order to have a
complete documentation of the measurement parameters. All other parameters have to be
selected according to the intended measurement.
Now the measurement can start. After pressing "adjust", the DLTS routine runs
continouously and can be observed at the oscilloscope. First you have to lower the sample
temperature to the intended Tstart by immersing the cryostat into the liquid nitrogen. Take care
that during cooling the regulation "on" field at the T-display is not activated, otherwise you
heat against the cooling. It is useful to lower the temperature in the "adjust" mode, since only
there the actual temperature is contimuously read and displayed in the blue T-display field. At
the desired minimum temperature you may switch directly to the measurement by clicking to
"measure" or go to the "stop" mode. If the measurement has to start at an intermediate
temperature between room temperature and liquid nitrogen temperature, you may stabilize
any intermediate temperature in the "adjust" mode by activating the regulation "on" field.
Then the "adjust" mode continuoues running, but "set temperature" is not actualized but kept
constant and gets stabilized. You may also go in the "stop" mode, directly write Tmin into the
"set temperature" field, and activate the regulation "on" field. After starting the measurement
by pressing "start", first the baseline measurements are performed, if "baseline" is enabled
(see Sect. 3.4), and then the measurement starts. If regulation "on" was not enabled manually
before, it gets enabled automatically with the first regular DLTS-measurement. The
temperature setting is automatically incremented according to the ramp settings, and the
DLTS curves according to the "display" selection are plotted. The measurement ends after the
programmed Tmax is reached (messeage: "T-overstep"), or if "stop" is pressed. Then it is not
allowed to measure again or to go into the "adjust" mode, in order not to overwrite the old
measurement. If you try to do that, he asks "overwrite? y/n", then you may repete the
measurement. You may also overwrite an old measurement by selecting the old file name in
the "file => new project" menu.
Note that it is allowed at any time both in the "adjust" and in the "measure" mode to look
into the "settings" window for checking the measurement parameters. Of course, in these
modes (while the DLTS routine is running) it is not allowed to change any measurement
parameters. However, the T-ramping parameters as well as the "sensitivity" and the
"compensation range" selections are allowed to be changed even then. Also the display option
(see Sect. 3.5) is allowed to be changed in the "adjust" and "measure" mode. Then a messeage
appears: "wait for DSP: ok", which has to be responded. If the DSP is ready with the running
measurement, the "display" option is allowed to be changed. After quitting "display" the
DLTS-measurement continuoues. Note, however, that choosing this option during the
measurement stops the DLTS routine for this time. Hence, since the T-ramping cannot be
stopped so quickly, this leads to the appearance of a larger T-step in the DLTS spectrum.
Moreover, if the C-meter is running in the externally controlled bias mode ("Vorsp. ext."), the
measurement gets disturbed by an unavoidable fluctuation of the bias in the "settings" mode.
Therefore the activation of the "settings" option should be avoided in the "measure" mode, if
possible.
4.3 Data display and evaluation
The lower "DLTS" graph allows one to display up to 10 different DLTS curves in different
colours as a function of temperature. It can be selected in the "display" option (see Sect. 3.5)
which DLTS curves are displayed. In this window, by clicking with the left mouse button in
one of the fields, the DLTS curve with the parameters being automatically indicated in the
16
parameter display fields can be selected. By multiple clicking in this field, the colour of the
curves can be selected individually. By clicking there with the right mouse button, the
selection of a curve can be cancelled. In the small window at the upper right of the "display"
window, one of the three possible DLTS correlation methods ("2-point", "exponential", "high
res") can be selected. The parameters of the selected DLTS curves and the colour coding are
displayed in the "DLTS curves" window above the DLTS graph. All axis of this graph, as
well as of the upper "C-transient" graph, can be scaled both automatically by pressing "auto"
or manually by inserting the desired scaling limits in the corresponding data fields. If the "C0"
button at the right of the DLTS graph is activated, the temperature dependent basic
capacitance is also displayed in the DLTS graph with the scaling limits (in pF) indicated at the
right. If a batch measurement was performed, the window "select" allows one to choose to
which batch parameter the displayed "C0" plot belongs. Note that the "auto scale" option for
C0 regards the C0 measurement belonging to all batch parameters, so the C0 scaling does not
change by switching from one to another batch parameter. This allows a better comparison of
the influence of the different batch parameters on C0. If one clicks with the right mouse
button at a certain temperature within the DLTS graph, the upper "dC[pF]" C-transient graph
is actualized with the transient measured at this very temperature. In case of a batch
measurement, also here the "select" window allows one to select the batch parameter, to
which the C-transient to be displayed will belong. The "comments" field at the upper right can
be cleared by pressing the "clear" button above and actualized also after a measurement. Note
that the navigation in this field should be performed by using the mouse or the arrow keys, but
not by using the "enter" command. In the "settings" window also the "sensitivity" and the
"compensation range" are still allowed to be changed and saved for the case that they have not
been correctly inserted before the measurement. These new parameters can be actualized in
the file by pressing the "save" button above the "comments" field or by using "file => save".
As usual in Windows, the "file => save as" option allows one to make a copy of the whole
project under another name in another directory.
In the "file" menu there are the commands "print" (Strg+P), "preview" and "printer
settings", which have the usual meaning. The command "print" allows one to print out the
currently selected DLTS curves on the standard printer. This printout can be viewed before
using "preview". It contains the file name, all measurement parameter, the comment, and the
actual DLTS-curves. The "file" menu also contains the "export" option, which allows one to
export the data of the currently selected "dlts"-curve, the "transient", or the "C0" plot as an
ASCII file. These files contain one header lines containing all measurement parameters and
the actual data in form of a table, which can directly imported e.g. into EXCEL, Origin, or
any other software for further display and evaluation. The exported "dlts" ASCII files contain
always pairs of "temperature"- and "DLTS data"-rows for every DLTS plot in the sequence
they had been selected in the "DLTS" software. For batch-measurements, the exported "C0"data file contains the "C0"-data for all batch parameters together with their individual
temperatures, but the "transient" data file contains only the transient selected in the "DLTS"
software for a certain temperature and a certain batch-parameter, if there is any.
Also the original saved data files (estension .dat) of the DLTS software are ASCII files,
which can be viewed in an editor. It is advisable to set the column width in the editor to 15
characters in order to see header elements in the correct position. These files are widely selfexplaining. They contain 4 header lines: The upper one containing the coments within one
line, one containing the names of the DLTS variables, and one containing the values of these
variables, and one containing the description of the actual data file. The actual data structure
is obvious from the following cutout of a measured data file (here shown for the case of a
"t(imp)"-batch measurement):
17
Nr
0.000000
1.000000
2.000000
3.000000
0.000000
1.000000
2.000000
3.000000
0.000000
|
t_imp[ms]
0.010000
0.100000
1.000000
10.000000
0.010000
0.100000
1.000000
10.000000
0.010000
|
Temp[K]
200.949997
200.750000
200.649994
200.549988
200.449997
200.349991
200.250000
200.149994
200.049988
|
CNull
0.943937
0.944853
0.946074
0.946379
0.945463
0.923490
0.913114
0.911283
0.907010
|
Point1
-0.001951
-0.001349
-0.001419
-0.003800
-0.097924
-0.048727
-0.020535
-0.014782
-0.004366
|
Point2
-0.002197
-0.000945
-0.001241
-0.002944
-0.097713
-0.048220
-0.021059
-0.015099
-0.004352
|
Point3 ...
-0.002039
-0.001077
-0.001204
-0.002793
-0.097384
-0.047816
-0.021046
-0.016042
-0.004491
|
The first row contains the current number of the batch variable, the second its real value, the
third the measured temperature (in K), and the forth the measured basic capacitance. The
following columns contain the measured data points of the C-transient. Note that CNull and
the data points are given in units of V measured at the ADC, and not in physically relevant
units (pF). The transient data can be converted into pF by multiplying them with the
"sensitivity" value given in pF/V, which is contant in the header. The basic capacitance can
only be retrieved using the (non-linear!) basic capacitance scaling procedure, which is
implemented in the DLTS software. If these data are needed, they have to be saved separately
in the DLTS software using the "file => export" option.
18