A deep level transient capacitance spectroscopy (DLTS) system modified for the measurement of transient conductance has been used to observe gamma-ray induced defect centres in the gate junction of 2N4416 Si field effect transistors. The defect concentrations increased linearly wth gamma-dose in the range 50 kGy to 10 x 10 3 kGy (5-1000 Mrad) for the common E(c) - 0.17 eV level and in the range 500 kGy to 10 x 10 3 kGy (50- 1000 Mrad) for the levels E(c) - 0.22 eV and E(c) - 0.44  eV. Another common level a hole trap at E(v) + 0.42 eV was the only minority trap observed. The technique may be useful for measuring gamma-fluxes in situations inaccessible to standard  dosemeters (e.g. flux-mapping)

Pearton, SJ

Tavendale, AJ

Williams, AA

ANSTO Publications Online

AAEC/E504
AUSTRALIAN ATOMIC ENERGY COMMISSION
RESEARCH ESTABLISHMENT
LUCAS HEIGHTS
T R A N S I E N T C O N D U C T A N C E SPECTROSCOPY M E A S U R E M E N T S O F
DEFECT STATES I N y - I R R A D I A T E D n - C H A N N E L S I L I C O N F I E L D
EFFECT T R A N S I S T O R S WITH POSSIBLE y -DOSEMETER A P P L I C A T I O N S
by
S.J. P E A R T O N
A.J. T A V E N D A L E
A.A. W I L L I A M S
December 1980
ISBN 0 642 59703 0
AUSTRALIAN ATOMIC ENERGY COMMISSION
RESEARCH ESTABLISHMENT
LUCAS HEIGHTS
TRANSIENT CONDUCTANCE SPECTROSCOPY MEASUREMENTS OF DEFECT
STATES IN Y-IRRADIATED n-CHANNEL SILICON FIELD EFFECT
TRANSISTORS WITH POSSIBLE Y-DOSEMETER APPLICATIONS
by
S.J. PEARTON*
A.J. TAVENDALE
A.A. WILLIAMS
ABSTRACT
A deep level transient capacitance spectroscopy (DLTS) system, modified
for the measurement of transient conductance, has been used to observe Y-ray
induced defect centres in the gate junction of 2N4416 Si field effect
transistors. The defect concentrations increased linearly with Y-dose in the
range 50 kGy to 10 x 103 kGy (5-1000 Mrad) for the common EC - 0.17 eV level,
and in the range 500 kGy to 10 x 103 kGy (50-1000 Mrad) for the levels E_ -
L*
0.22 eV and E - 0.44 eV. Another common level, a hole trap at E + 0.42 eV,
\s V
was the only minority trap observed. The technique may be useful for
measuring Y-fluxes in situations inaccessible to standard dosemeters (e.g.
flux-mapping).
* AINSE Postgraduate student on attachment to the AAEC from the Department
of Physics, University of Tasmania.
National Library of Australia card number and ISBN 0 642 59703 0
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EMISSION SPECTROSCOPY; VACANCIES; RADIATION EFFECTS; GAMMA RADIATION;
FIELD EFFECT TRANSISTORS; SILICON; DOSEMETERS
CONTENTS
1. INTRODUCTION 1
2. APPARATUS 1
3. EXPERIMENTAL 1
4. RESULTS 2
5. DISCUSSION 4
6. SUMMARY 5
7. ACKNOWLEDGEMENTS 5
8. REFERENCES 5
Figure 1 (a) DLT conductance spectrum for dose of 5 MGy (500 Mrad), 7
gate forward biased to display minority defects
Figure 1 (b) Transient conductance majority defect spectrum from a 8
y-irradiated 2N4416 field effect transistor for dose
of 5 MGy (500 Mrad)
Figure 1 (c) DLT conductance spectrum for dose of 50 kGy (5 Mrad) 8
Figure 2 (a) Relative correlator signal output v. pulse width for 9
three dominant majority levels
Figure 2 (b) Transient conductance change v. pulse amplitude for 9
three dominant majority levels
Figure 3 Dose dependence of relative defect concentration for 10
the three dominant majority levels
Figure 4 Arrhenius plots of the four main defect levels 11
1. INTRODUCTION
The technique of deep transient capacitance spectroscopy (DLTS) [Lang
1974] is standard for analysis of defect levels in semiconductors. By
periodically reducing the reverse bias on a diode, trapping centres are
filled, and their emission is monitored as a function of temperature by
measuring transient changes in the device capacitance. In this way,
individual defect levels may be observed.
The fundamental limit to the sensitivity of the technique is reached for
semi-insulating devices, or for those in which capacitance change with bias is
very small. In this case, transient current or conductance measurements may be
performed. For field-effect transistor (FET) structures, the drain
conductance may be monitored as detrapping from defect states modulates the
channel depletion depth. This report describes observation of the coramon deep
level A- and E-centres in^-irradiated sil icon FETs, using a modified transient
capacitance spectroscopy system.
2. APPARATUS
The capacitance bridge used in the system is a Boonton model 71A L-C
meter and signal processing is by an electronic correlator [Miller et al.
1975]. To convert to the conductance mode, the sample RF signal voltage into
the phase detector of the bridge is interrupted and the phase of the signal
shifted 270° to enable observation of the real component of device impedance.
This is effected by using a Canberra 1457 delay amplifier and a. calibrated
coaxial cable acting as a phase trimmer. The phase is optimised by noting the
maximum response when a resistor is placed across the test terminals (or
alternatively a null response if a capacitance is added). The modifications
to the bridge and setting up of the system are minor [Williams et al. 1980],
so any standard DLTS apparatus without a transient conductance capability may
be converted.
3. EXPERIMENTAL
The experiment involved six groups of three FETs selected from different
batches. One set was used as a control group and the others received doses of
50, 250, 500, 5 x 103 and 10 x 103 kGy (5, 25, 50, 500 and 1000 Mrad),
respectively, at 21 kGy h (2.1 Mrad h ). Irradiations were performed in a
cobalt-60 irradiation facility at ambient temperature, fluxes being calibrated
by the AAEC's Isotope Division. The current-voltage (I-V) characterist ics of
each FET were recorded before irradiation so that each group contained FETs of
similar transconductance (gm)- Transient conductance scans were performed
over the temperature range 77-300 K by locating the FETs in a brass holding
block which was mounted in a continuous flow cryostat (Oxford Instruments CF-
100). The FET gate was pulsed towards or into forward bias to obtain the
majority or minority defect level spectrum. A 500 ys bias reduction pulse
was used, supplied by a Systron-Donner 100A pulse generator. The drain
voltage was 1.35 V; this voltage is not critical as long as the FET is below
pinch-off and above zero volts, since the I-V characteristic is linear in this
region.
4. RESULTS
Figure l(a) shows a typical majority carrier spectrum. The E - 0.17 eV
level is the A-centre, oxygen-vacancy (0-V), and has been studied after
irradiation of silicon by protons [Kimerling et al. 1973], neutrons [Whan
1966] and electrons [Kimerling 1977]. The EC - 0.22 eV level is commonly
assigned to a divacancy (V-V) defect, and the E - 0.44 eV level to(*
phosphorus-vacancy (P-V) [Kimerling 1977]. Apart from the three dominant
defects, there is evidence for levels estimated as EC - 0.19 eV and EC - 0.36
eV respectively. These are probably levels E41 and E6 of Kimerling et al.
[1978]. When our results are compared with those of Kimerling [1977] it can
be inferred that the starting material for the FETs is low oxygen content,
float-zoned, phosphorus-doped silicon.
Figure l(b) shows the appearance of the hole trap E + 0.42 eV when the
gate is forward biased. This is commonly thought to be either a higher charge
state of the A-centre, or an interstitial related defect [Kimerling 1977].
From Figure l(c) it can be seen that at a dose of 50 kGy (5 Mrad), the A-
centre is visible, but it is not until 500 kGy (50 Mrad) that the other levels
are clearly defined.
Cross sections were measured from the dependence of correlator output
signal on trap filling time. The results correlate closely with those of
Kimerling [1977]. Figure 2(a) shows the reduction in relative correlator
output signal as a function of bias reduction pulse width (during which time
the traps are filled).
The conductivity, g, of a semiconductor in which electrons are the
majority carriers, is given by
9 = % n (1)
where e = electronic charge, y = electron mobility, and n = net free carrier
density (= Ng-N,, where these are the total donor and acceptor concentrations,
respectively). A slight increase An («n) in the free carrier density leads
to a slight increase, Ag, in the conductivity of the sample. In this
technique the thermal detrapping of electrons from defects provides such an
increase, i.e.
An = NT
where NT is the concentration of the particular defect level. For the
condition Ag/g «1,
NT = f n (2)
This equation was used to estimate trap concentrations by measuring Ag^, the
magnitude of the transient conductance change by pulsing to zero bias and g.,
the drain conductance of the FET at the peak temperature. Figure 2(b) shows
the dependence of Ag/g on the bias pulse amplitude; the linearity of the plot
indicates that the defects are uniformly distributed through the region.
Figure 3 shows a plot of relative defect concentration versus dose
received. The slopes are
EC - 0.17 eV 1.2 x 10"5 kGy"1
(1.2 x 10~4 Mrad"1)
Ec - 0.22 eV 7.23 x 10"7 kGy"1
(7.23 x 10"6 Mrad"1)
Ec - 0.44 eV 9.48 x 10"7 kGy"1
(9.48 x 10"6 Mrad"1)
Absolute defect concentrations require a knowledge of n, which may be obtained
from a measurement of the FET pinch-off voltage and a knowledge of the channel
depth. It appears that the ultimate sensitivity of this system is
approximately 10 of the net background doping density, which is similar to
the capacitance mode operation. Figure 4 shows the Arrhenius plot of the four
main levels, these data leading to the activation energies of the traps.
5. DISCUSSION
The homogeneity of the FETs (results within any dose group were within 15
per cent of each other, aided by preselection on the basis of similar gm*
(approx. ± 10 per cent), combined with the linearity of defect concentration
with dose suggests that, at least in the range approx. 500 kGy to approx. 10
x 10 kGy (approx. 50 to approx. 1000 Mrad), they may be useful as dosemeters.
They may be particularly useful in situations where conventional monitoring is
difficult, for example where there is a demand for physically small, readily
available, mass produced monitors (e.g. flux-mapping). It is intended to use
them at the AAEC Research Establishment as a secondary check for flux
measurements when irradiating other semiconductor materials for DLTS studies.
For general application, of course, a DLTS system must be available, but
as the technique is now standard, 2N4416 FETs are cheap (and widely available)
and the levels monitored are well characterised, this may be a simple and
novel means of Y-flux measurement.
It may be possible to construct a 'sensitised' FET structure dedicated to
flux measurement using DLT conductance spectroscopy. For example, a highly
oxygen-doped silicon FET would give improved sensitivity of the 0-V level. If
it were possible to increase the oxygen content by two orders of magnitude,
this would allow measurement of fluxes in the 25 kGy (2.5 Mrad) region, the
approximate legal minimum for the Y-ray induced sterilisation of medical
items, and the 500 Gy to 5 kGy (50 krad - 0.5 Mrad) dose range used in food
processing. Defect introduction rates vary with temperature, particle type
and energy [Kimerling et al. 1978], so a prior calibration would be needed
before the FET is used as a dosemeter. The FETs would also need a fairly
strict (say ± 10 per cent) constancy of the oxygen, and possibly phosphorus
(or As or Sb) content. Operation as a dosemeter at elevated temperatures would
be no problem as the levels of interest do not begin to anneal out until at
least 120°C [Evwaraye 1977; Kimerling 1977].
* Note that preselection on the basis of gm (related to v/rf) is essential as
the doping density of Texas Instruments 2N4416 JFETs extends over the
range 5.3 x 1015 - i.o x 1016 cm-3.
6. SUMMARY
The common and well-characterised A- and E-centres have been observed in
the gate junction of Y-irradiated n-channel silicon FETs using a modified DLTS
system to enable transient conductance measurements. The possibility exists
of using commercial FETs as dosemeters for the flux range 50 kGy to 10 x 10
kGy (5-1000 Mrad). A dedicated FET structure, suitably doped for more
sensitive defect detection, could prove useful in the technologically
important flux ranges 500 Gy to 25 kGy (50 krad to 2.5 Mrad).
7. ACKNOWLEDGEMENTS
Dr E.M. Lawson kindly arranged the Y-irradiation performed by Mr J. Gray
of the AAEC's Isotope Division. The authors thank Texas Instruments for
supplying information on the doping density of the silicon used in the
manufacture of the FETs. S.J. Pearton acknowledges the support of an
Australian Institute of Nuclear Science and Engineering (AINSE) postgraduate
studentship.
8. REFERENCES
Evwaraye, A.O. [1977] - J. Appl. Phys., 48(5)1840.
Kimerling, L.C. [1977] - Radiation effects in semiconductors. Inst.Physics
Conf. Series No. 31:222.
Kimerling, L.C., Gibson, W.M. and Blood, P. [1978] - Int. Conf. on Defects in
Semiconductors, Nice, France. Inst. Physics Conf. Series No.
46:273.
Lang, D.V. [1974] - J. Appl. Phys., 45:3023.
Miller, G.L., Ramirez, O.V. and Robinson, D.A.H. [1975] - J. Appl. Phys.,
46(16)2638.
Whan, R.E. [1966] - J. Appl. Phys., 37(9)3378.
•/iTIiams, A.A., Pearton, S.J. and Alexiev, D. [1980] - AAEC unpublished
report.
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o:O
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85 95 110
77
ISO 175 200 220 240
SAMPLE TEMPERATURE (K)
260 280. 300
FIGURE l(a) DLT CONDUCTANCE SPECTRUM FOR DOSE OF 5 MGy (500 Mrad), GATE
FORWARD BIASED TO DISPLAY MINORITY DEFECTS
in
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SAMPLE TEMPERATURE (K)
260 280 300
FIGURE 1 (b) TRANSIENT CONDUCTANCE MAJORITY DEFECT SPECTRUM FROM A 7- IRRADIATED
2N4416 FIELD EFFECT T R A N S I S T O R FOR DOSE OF 5 MGy (500 Mrad)
(c) DLT CONDUCTANCE SPECTRUM FOR DOSE OF 50 kGy (5 Mrad)
c
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= Peak 218.
- Peak 128
20 4O 6O SO 10O 12O 14O 16O 18O
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FIGURE 2 (a) RELATIVE CORRELATOR SIGNAL OUTPUT v. PULSE WIDTH
FOR THREE DOMINANT MAJORITY LEVELS
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All linear : NT W - n(x)
as expected because the
y-damage should be
uniform
O O'2 O-4 O-6 O-8 1 V2
V p ( V )
FIGURE 2 (b) TRANSIENT CONDUCTANCE CHANGE v. PULSE AMPLITUDE
FOR T H R E E DOMINANT MAJORITY LEVELS
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FIGURE 4. ARRHENIUS PLOTS OF THE FOUR MAIN DEFECT LEVELS
Printed by the AAEC Research Establishment


Transient conductance spectroscopy measurements of defect states in y-irradiated n-channel silicon field effect transistors with possible y-dosemeter applications.

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Transient conductance spectroscopy measurements of defect states in y-irradiated n-channel silicon field effect transistors with possible y-dosemeter applications.

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