ASTM F978-90(1996)e1
(Test Method)Standard Test Method for Characterizing Semiconductor Deep Levels by Transient Capacitance Techniques
Standard Test Method for Characterizing Semiconductor Deep Levels by Transient Capacitance Techniques
SCOPE
1.1 This test method covers three procedures for determining the density, activation energy, and prefactor of the exponential expression for the emission rate of deep-level defect centers in semiconductor depletion regions by transient-capacitance techniques. Procedure A is the conventional, constant voltage, deep-level transient spectroscopy (DLTS) technique in which the temperature is slowly scanned and an exponential capacitance transient is assumed. Procedure B is the conventional DLTS (Procedure A) with corrections for nonexponential transients due to heavy trap doping and incomplete charging of the depletion region. Procedure C is a more precise referee technique that uses a series of isothermal transient measurements and corrects for the same sources of error as Procedure B.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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e1
Designation: F 978 – 90 (Reapproved 1996)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
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Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Characterizing Semiconductor Deep Levels by Transient
Capacitance Techniques
This standard is issued under the fixed designation F 978; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Keywords were added editorially in January 1996.
1. Scope 3. Summary of Test Method
1.1 This test method covers three procedures for determin- 3.1 In this method procedures are given for determining the
ing the density, activation energy, and prefactor of the expo- density, activation energy, and the prefactor of the exponential
nential expression for the emission rate of deep-level defect expression for the emission rate of deep-level defect centers. In
Procedure A (see Fig. 1), the temperature of the diode is slowly
centers in semiconductor depletion regions by transient-
capacitance techniques. Procedure A is the conventional, con- scanned while the bias voltage is repetitively changed. The
stant voltage, deep-level transient spectroscopy (DLTS) tech- high-frequency capacitance transient due to trap emission is
sampled at two successively delayed gate times. The average
nique in which the temperature is slowly scanned and an
difference between these sampled values constitutes the signal
exponential capacitance transient is assumed. Procedure B is
that has a maximum or peak at a temperature that is a function
the conventional DLTS (Procedure A) with corrections for
of the gate times. The time constant associated with the peak
nonexponential transients due to heavy trap doping and incom-
response is fixed by the rate window of the boxcar averager
plete charging of the depletion region. Procedure C is a more
used to sample the transient or by computer simulation of such
precise referee technique that uses a series of isothermal
an instrument. For nonexponential transients, Procedure B adds
transient measurements and corrects for the same sources of
a correction to the calculation of the time constant at the
error as Procedure B.
temperature of the response peak. In Procedure C, the tempera-
1.2 This standard does not purport to address all of the
ture is held constant at each of a series of temperatures and the
safety concerns, if any, associated with its use. It is the
observed capacitance transient is analyzed for its corrected
responsibility of the user of this standard to establish appro-
time constant. An Arrhenius-type semilogarithmic plot of
priate safety and health practices and determine the applica-
normalized emission rate versus reciprocal temperature is
bility of regulatory limitations prior to use.
made in each procedure, and the activation energy and prefac-
tor are calculated from the slope and intercept, respectively.
2. Referenced Documents
The density of the defects is determined from the magnitude of
2.1 ASTM Standards:
the capacitance changes.
E 177 Practice for Use of the Terms Precision and Bias in
3.2 The use of a boxcar averager is assumed in the discus-
ASTM Test Methods
sion of Procedures A and B. However, a lock-in amplifier may
E 178 Practice for Dealing with Outlying Observations
also be used for these procedures, provided that factors which
F 419 Test Method for Determining Carrier Density in
may degrade the results are taken into account. Constant-
Silicon Epitaxial Layers by Capacitance Voltage Measure-
capacitance versions of these procedures are not discussed but
ments on Fabricated Junction or Schottky Diodes
are, of course, suitable for the purposes considered here. The
2.2 Other Standard:
nonexponential corrections covered in this test method are in
MIL-STD-105 Sampling Procedures and Tables for Inspec- general not needed for constant-capacitance measurements as
tion by Attributes the method itself eliminates most of the nonexponentiality.
4. Significance and Use
This test method is under the jurisdiction of ASTM Committee F-1 on
Electronics and is the direct responsibility of Subcommittee F01.06 on Electrical 4.1 Deep-level defect measurement techniques such as iso-
and Optical Measurement.
thermal transient capacitance (ITCAP) (1, 2) and DLTS (3)
Current edition approved June 29, 1990. Published August 1990. Originally
utilize the ability of electrically active defects to trap free
published as F 978 – 86. Last previous edition F 978 – 86.
Annual Book of ASTM Standards, Vol 14.02.
Annual Book of ASTM Standards, Vol 10.05.
4 5
Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700 The boldface numbers in parentheses refer to the list of references at the end of
Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS. this test method.
F 978
+
FIG. 1 Schematic of Biased n p Diode and Waveforms Associated with Repetitively Changing the Bias and Analyzing the Resulting
Capacitance Transient
carriers and to re-emit them by thermal emission. related to the densities of the defects present. The interest in
Theoretically, the emission rate e for electrons is given by the measurement of deep levels in semiconductors stems from the
n
following equation: following two related aspects:
4.3.1 Detection, identification, and control of unwanted
e 5s v N exp ~2DG/kT!
n n t c
native or process-induced impurities or defects; and
where:
4.3.2 Characterization and control of impurities specifically
s 5 capture cross section of the defect for an
n introduced for lifetime or other parameter control.
electron,
v 5 thermal velocity of the electron,
t 5. Interferences
N 5 density-of-states in the conduction band,
c
5.1 Temperature errors will significantly reduce the
DG 5 Gibbs free energy (a function of temperature) of
accuracy of emission-rate measurements and, therefore, reduce
the defect,
the accuracy of the energy determination. Temperature
k 5 Boltzmann constant, and
inaccuracies that vary in magnitude with temperature are even
T 5 absolute temperature.
more significant.
4.2 A form commonly used for emission rate,
5.2 Nonexponentiality of the capacitance transient interferes
e 5 BT exp(−DE/kT), where B is assumed to be independent
n
with the characterization technique. Tests for
of temperature, is obtained by using DG5DE − TDS, where
nonexponentiality are given in 10.1. Causes of
DE is the activation energy (the enthalpy to be more exact
nonexponentiality are as follows:
which is the energy of the trap below the conduction band) and
5.2.1 The density of the deep-level defects is not small
DS is the change in entropy (4). For the equivalence of BT to
compared to the net shallow dopant density. Procedures B and
s v N exp(DS/k), one assumes s and DS to have no
n t c n
1/2
C correct for this interference.
dependence on temperature, a T dependence for v , and a
t
3/2
5.2.2 Trap charging does not take place throughout the
T dependence for N . For DG to equal DE, DS 5 0 (that is,
c
depletion region at moderate (or higher) levels of trap density
no change between the initial and final state degeneracy or
relative to net shallow dopant density. Procedures B and C
lattice relaxation associated with the transition).
correct for this interference.
4.3 An analogous expression can be written for the whole
emission rate. Analysis of the measured thermal emission rate 5.2.3 The junction is not sufficiently abrupt.
in the depletion layer of a test device as a function of 5.2.4 The onset of free carriers at the edge of the depletion
temperature leads to activation energies and effective capture region is not sufficiently abrupt (that is, the approximation of
cross sections of the defects present. The magnitude of the complete depletion is not valid). Procedures B and C help
capacitance changes associated with the emission can be correct for this.
F 978
5.2.5 The emission rate varies with electric field intensity to attain temperature accuracy of 0.1 K at temperatures much
(for example, Poole-Frankel effect). below or much above room temperature.
5.2.6 The observed emission is the sum of emissions from 6.9 Thermometry System, capable of determining the diode
two or more closely spaced and unresolved defect centers. temperature with a precision of 0.1 K and an accuracy of 0.5 K
5.2.7 The response time of the capacitance meter, the for Procedures A and B and a precision of 0.02 K and an
recording system, or the test specimen (high resistance) is not accuracy of 0.1 K for Procedure C.
negligible compared to the transient time constant.
7. Sampling
5.3 Temperature dependencies of the capture cross section
7.1 These procedures are nondestructive and are suitable for
and the entropy change will introduce error in the proposed
use on 100 % inspection. If a sampling basis is employed, the
analysis.
method of sampling shall be agreed upon by the parties to the
6. Apparatus
test and shall be in accordance with acceptable statistical
procedures (see MIL-STD-105).
6.1 Capacitance Bridge or Meter, using a high-frequency
test signal and capable of measuring from 1 to 100 pF full scale
8. Test Specimen
with an accuracy as defined in Practice E 177 of 60.5 %
8.1 The procedures of this test method require that the deep
(1s %). Its response time should be much less than the smallest
levels to be characterized shall be in a depletable region of a
time constant to be measured. The instrument shall be capable
semiconductor such as in a p-n junction diode or a Schottky
of sustaining external dc bias of about 650 V and have offset
diode. It is desira ble to use a peripheral guard ring suitably
provisions for compensating or nulling out the external
biased to isolate the depletion region of the device from surface
capacitance of the specimen holder, connecting cables, steady-
states (see 10.2.1).
state capacitance, etc. A provision for blocking out the large
9. Calibration and Standardization
capacitance during the fill pulse is desirable. The capacitance
measurement system used for determining the DLTS peaks in
9.1 Measure the standard capacitances to determine that the
Procedures A and B does not need to be direct reading but must
capacitance bridge or meter is within specifications.
have an output that is sufficiently linear and a response time
9.2 Verify time calibration by use of a calibrated time-mark
that is sufficiently fast to give undistorted peaks.
generator or an interval timer for the recorder system. (For
6.2 Standard Capacitances, of accuracy 0.25 % or better
Procedure C only.)
(1s %) at the measurement frequency. One capacitor shall be in
9.3 Verify temperature calibration. Comparison with a
the range from 1 to 10 pF and another in the range from 10 to
calibrated platinum resistance thermometer under isothermal
100 pF.
conditions is preferred. An alternative check is to use a
6.3 Pulser, with controllable repetition rate capable of
well-characterized diode, lightly doped with platinum or
changing from one bias voltage adjustable within the range of
another deep level (DE and B known), measure the emission
at least + 10 to − 10 V to another bias voltage in the same
rate e 5 1/t, and calculate iteratively: T 5 11604.5D
n
range. Switching time shall be much less than the smallest time
E/(lnt+2 lnT+lnB).
constant to be measured and overshoot and undershoot shall be
10. Procedure
less than 1 % of pulse amplitude under operating conditions.
10.1 Tests for Nonexponentiality of the Capacitance
6.4 Boxcar Averager, or equivalent instrument (needed for
Transients—Use one or more of the following techniques:
Procedures A and B only) to process the capacitance transient.
10.1.1 Thurber et al Technique (5)—Choose a convenient
Desirable features are two separately controllable gate delay
−1 −1
value of rate window t , for example (500 μs) , and choose
times with adjustable sampling time.
a sequence of values of t /t , for example, 2, 5, 10, 20, 50.
6.5 Interval Timer, (needed for Procedures A and B only) 2 1
Calculate for each value of t /t as follows:
capable of measuring gate delay times of a few microseconds 2 1
to several seconds with an accuracy of 0.1 % (1s %).
t 5t·ln ~t /t !/~t /t 2 1!
1 2 1 2 1
6.6 Recorder System, capable of digitally or continuously
then:
measuring and recording the following:
t 5 t ~t /t !
2 1 2 1
6.6.1 Capacitance as a function of time or temperature, or
6.6.2 The average difference in capacitance at two gate 10.1.1.1 Perform 10.2.1 and 10.2.2 of Procedure A using the
delay times as a function of temperature. gate times t and t calculated here. Compare values of T from
1 2 m
6.7 Oscilloscope, capable of observing the pulser output, the each run. If T is constant, the transient is exponential and the
m
capacitance transient, and boxcar or other output to be recorded corrections of Procedure B are not needed; consequently
(not required but extremely helpful). follow Procedure A. If T changes with changes in the t /t
m 2 1
6.8 Cryostat, containing a specimen holder capable of ratio, apply corrections by following Procedure B or use
maintaining a selectable temperature or of ramping the
Procedure C.
temperature up or down at a controlled rate. For silicon, the 10.1.2 Manglesdorf Test (6)—For a single capacitance
temperature range is usually between cryogenic and room transient, recorded at constant temperature, plot capacitance at
temperature, and for gallium arsenide, the range is from time t against capacitance at a delayed time t + Dt for several
cryogenic or room temperature to higher temperatures Dt values ranging from about 0.5 t to a few t . Calculate the
depending upon the energy levels of interest. In Procedure C, slope m of the linear regression line fitted to the data points as
a radiation shield surrounding the specimen holder is necessary follows:
F 978
10.2.4 Make Arrhenius plot (see 11.6.1).
m 5 nZ 2 X ·Y !/ nX 2 X !
~ ~
1 1 1 2 1
10.2.5 Calculate and record activation energy of the defect
where:
level and its standard deviation, DE 6 s (see 11.6.2 to
DE
n 5 number of points,
11.6.4).
X 5(C (t),
10.2.6 Calculate and record prefactor of the exponential and
X 5([C (t)] ,
its standard deviation, B 6 s (see 11.6.5).
B
Y 5(C (t + Dt), and
10.2.7 To the extent possible, allow specimen to reach
Z 5([C(t)·C (t + Dt)].
equilibrium at a temperature that will permit the recording of a
10.1.2.1 Calculatet5 −Dt/ln(m). If t does not change as Dt
capacitance transie
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