A series of commercially available pyroelectric detectors made from PVF2, LTO, SBN, and TGS were evaluated in terms of responsivity and detectivity as a function of frequency. The performance of the detectors evaluated was very different, depending upon the manufacturer of the detector, and this dependency was primarily related to the thickness of the various detectors. The best detectors of each material were comparable in performance at frequencies around 10 Hz but differed radically at frequencies above 100 Hz

Crouch, R. K.

Robertson, J. B.

English

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NASA TECHNICAL
MEMORANDUM
NASA TM X -74024
NO
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EVALUATION OF C"ERCIAL PYROELECTRX DETECTORS
t
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Z	 by James B. Robertson and Roger K. Crouch
(bASI-TM-Y-74024) EVILUITICN OF COMMERCI1L
	
N77-21396
FYFOILECTBIC CETECTOES (NASA) 17 p HC
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NASA TM X-74024
4 Title and Subtitle	
f
Evaluation of Commervia2 Pyroelectric Detectors
7, Author{st
James B. Robertson, Roger K. Crouch
9. Pwformirgl orgountion Nam and Address
NASA Langley Research Center
Hampton, VA 23665
12. Sponsoring Awncy Name and Address	 —
National Aeronautic? & Space Administration
Washington, DC 20546
15. Suppwftntary Notes
3C11111110111 No:
& Report Data
April
.._1937
6, Nwfarrn M Orgbrtabort Code
8. Pvtortning OrW.tarnua Repot lbs.
10.whale that No.
11.Contract or Grant No.
13. Type of Report .rtd Period Co.-red
Technical Memorandum
14. ',Nnsnring Agency Code
16. Abstrao
1	 Currently availableoelectric dote tars ar ca ble ..-)f room 'e nature
operation and have detecti ities of 4 x 10 cm Hz l^ p21wand spectral sensitivities
from the visible to greater than 100 um. Because pyroelectrics are thermal
detectors, their spectral response can be made nearly flat over broad spectral
regions by the use of appropriate black surface coating.
1
A series of commercially available pyroelectric detectors made from PVF21	 !
LTO, SBN, and TGS have been evaluated in terms of responsivity and detectivity (D*)
as a function of frequency. It was found that the performance of the detectors
evaluated was very different, depending upon the manufacturer of the detector, a:Id
this dependency was primarily related to the thickness of the various detectors.
The best detectors of each material were comparable in performance at frequencies
around 10 Hz but differed radically at frequencies above 100 Hz.
117 Key Words (Suggested by Author(s))
	
18. Distribution Stateicent
f
IR Detectors, Pyroelectric
Strontium barium niobate 	 Unclassified - Unlimited
Lithium tantalate
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20. Security Clasif. (of this page ► 	 21. No. of Pages
	
1 22 P•^cr
Unclassified
	
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15	 $3.50
For sale by the National Technical Information Service, Springfield, Virginia 22161
thermal detectors, their spectral response can be made nearly flat over broad
spectral regions by the use of appropriate black surface coatings.
A series of comercia.11y available pyroelectric detectors made from PVF2
ILTO, SBN, and TGS have been evaluated in terms of responsivity and detectivity
(D*) as a function of frequency.
	 It was found that the performance of the
detectors evaluated was very different, depending upou the manufacturer of the
detector, and this dependency was primarily related to the thickness of the
various detectors. 	 The best detectors of each material were comparable in
performance at frequencies around 10 Hz but differed radically at frequencies
above 100 Hz.
INTRODUCTION
Currently available pyroelectric detectors F-re capable of room temperature
8
operation and have detectivities of 4 x 10	 cm H;./w and spectral sensitivities
from the visible to greater than 100 M.
	 Because pyroelectrics are thermal
detectors, their spectral response can be made nearly flat over broad spectral
L
regions by the use of appropriate black surface coatings.
NASA's commitment to air pollution monitoring and thermal mapping of the
earth requires photodetection in the 3 W to 20 W spectral region.
	 If pyro-
electric detectors can be used in these applications in place of cooled photo-
conduCtive and photovoltaic detectors, a large savings in volume, weight, and
power can be realized.
The purpose of this report is to give to potential users of pyroelectric
detectors a brief introduction to the operation and characteristics of pyro-
electric detectors and to present performance data on pyroelectric detectors
which are currently available on the market. All of the detectors evaluated
for this report were purchased prior to March 1976.
TECHNICAL BACKGROUND
A pyroelectric detector is a thermal-to-electrical transducer. By
absorbing incident light and converting it to heat, the pyroelectric detector
becomes an optical-to-electrical transducer.
Pyroelectric materials are insulators with a spontaneous polarization
that changes with temperature. A detector is made from the material by cutting
the material into wafers with the plane of the wafer normal to the polar axis
and making electrical contact to the flat sides. The detector is essentially
a capacitor with the pyroelectric material between the plates.
Radiation incident upon the detector is absorbed by the detector causing
an increase in temperature. The change in temperature causes a change in the
polarization of the pyroelectric material and, thereby, a change in the amount
of charge attracted to its surface. The magnitude of the change of polarization
per unit change of temperature is called the pyroelectric coefficient, p. If
the incident radiation is chopped, the detector becomes an AC generator, and
the charge flowing to and from the surface via an external circuit can be
measured.
The impedance of the pyroelectric detectors at normal operating frequencies
is very high (1013
 ohm), and for practical applications, a field effect tran-
sistor (FED) must be used to lower the impedance. Typical circuits are shown
2
a
s
sin figure 1. Since practical use of the pyroelectric requires the FET, mean-
ingi'al performance data must include the -performance of the FET. For the
remainder of this paper, the word "detector" shall refer to the pyroelectric
chip and any integrated impedance-converting circuit. The pyroelectric chip
with its two surface contacts will be referred to as "detector element."
There are many pyroelectric materials from which detectors may be made.
Only four of these, however, have survived the competition and made it to the
marketplace. These are:
(1)Triglycine sulfate (TGS) --- Triglycine sulfate was the material
used in the first marketed detector. It is difficult to handle and is bygro-
scopic, but produces detectors with the highest detectivity.
If the triglycine sulfate is made with deuterium atoms in place
of the hydrogen atoms, the resulting material, called deuteratea triglycine
sulfate (D TGS), has the advantage of a higher Curie temperature than TGS.
(2)Lithium tantalate, (LTO) --- Lithi= tantalite is inexpensive,
easy to handle, nonhygroscopic, but not as sensitive as TGS.
(3) Strontium barium niobate (SBN) --- Strontium barium niobate is
nonhygroscopic and is more difficult to process than LTO; detectivity values
just below TGS are possible.
(4)Polyvin;;lidene fluoride (PVF2 ) --- rolyvinylidene fluoride is
flexible plastic film, nonhygroscopic. It is not competitive fo- small area
detectors, but is the best available material for large area (-1 cm dia.)
detectors.
3
4
rEXPEPIMMUAL
Detectors from nine different manufacturers were evaluated. The detectors 	 4
with their pertinent design information are listed in Table 1. FET impedance-
converter circuits were added to all detectors not so supplied by the manufac-
turer. All detectors except the PVF 2
 were operated in the voltage mode
(fig. la ). The ?FV2 detector was operated in the current mode (fig. lb ) being
supplied this way with the circuit within the detector housing.
The signal voltage and noise voltage of each detector was measured as a
function of chopping frequency over the frequency range of 2 Hz to 500 Hz.
From these data, responsivities (R) and detectivities (D e ) were calculated.
Equipment.- A schematic of the experimental setup is shown in figure 2.
The radiation source was a 500 0
 K, conical cavity, black-body. Tempera-
ture of the black body was monitored with a thermocouple which was connected
to an ice-point reference. The radiation was chopped with a 23 cm diameter
wheel. Typical source aperture was 0.300 em 2 ; typical source-to-detector
distance was 15 cm; and typical chopper form factor (ref. 1) was 0.422, giving
a typical flux density of 6.3 x 10 5 watt/cm. 2.
Signals from the detector were amplified by a Quan Tech 206C preamp and
measured with a Quan Tech 304A wave analyzer set to a 1 IIz bandwidth.
Detector noise was measured with the Quan Tech 304A set to 1 Hz bandwidth
and a 100 second integration time. The chopper motor was stopped, and the
radiation was blocked during the noise measurements.
The equations used are listed below.
ff xaxT4XA
Ii =	 s
nd2
4
R=HAd
D*=SNP
r H 
VV
where:
H = irradiance, watt cm-2
ff = chopper form factor (ref. 1)
Q = Stefan-Boltzmann Constant, 5.67 x 107"" W cm 2
T = black-body temperature, °K
As = source arirture, cm 
d = source-to-detector distance, cm
R = responsivity, volt/watt
S = signa;.l voltage, volt
Ad = active area of detector, cm2
D*
 = detectivity, cm Hz 1/2 watt-1
N = noise voltage, volt
Af = amplifier bandwidth, Hz
5
s
A
W
RMULTS
Figure 3 is a plot of D * versus frequency of one each of the nine
different brands of detectors. Where more than one detector from a given
manufacturer was tested, a curve representative of the mean performance was
chosen. Figure 3a presents curves of the best performing detectors of each
material; LTO, DTGS, PVF and SBN. It should be noted that the PVF2'	 2 was
operated with a built-in operational amplifier in the current mode.
Figure 3b presents D* versus frequency of all of the LTO detectors. The
curves in 3a and 3b are the same as in figure 3; they are shown separately to
aid in comparison. Fijrirze 4 presents the noise voltage versus frequency of
one d-ttector of each material.
DISCUSSION
On figure 3 several observations can be made: the performance of all
detect ,)rs peak at 20 Hz or below. D* values are within one order of magnitude
at 10 Hz and spread to 2.5 orders of magnitude at 500 Hz. There is a larger
difference between different manufacturers using the same material (LTO) than
between detectors of different material.
In fiC;ure 3b, the large differences in performance of different models
of LTO detectors can be seen. The active areas of the detector chips range
from 0.00785 cm 2 to 0.0314 cm 2 but show no correlation with performance.
Three of the detectors have windows and two do not, but no effect is noticed
here. The use of windows decreases the responsivity of the detector by
reducing the amc-,unt of energy reaching the detector chip, however the windows
decrease noise by reducing airborne acoustics, by reducing thermal fluctuations
due to air currents, and protecting by the detector from moisture and dirt.
6
4
4The differences in performance are taken to be due to wafer thickness,
wafer mounting, and choice of FET's; with thickness being the primary factor.
The thicknesses of the lithium tantalite chips of figure 3b are:
#5 - 12.5 um, #3 - 8.5 Wa, N2 - 20 Wu, Nk - 25 Pm, Nl - TO Um.
The noise voltage versus frequency curves for one detector of each
material are shown in figure k. Again, it must be noted that the PVF2
detector is operated in the current mode. The noise of the PVF 2 detector is
dominated by the noise contribution from the amplifier current noise. The
curve should not be used to compare PVF2 to other materials but is given as
an example of noise from a current-mode-operated detector.
Is
-	
^3
RMFMWCEB
1. Hudson, R. D., Jr.: Infrared System Engineering. John Wiley and Sons
19699
 pp. 323 -327.
L
8
PVF2 A. D. Little B100/off .785 10 KRS-5
DIGS Barnes Engineering T-300 .20 10 KRS-5
SBNM1 Harshav Chemical PV3 .04 250 None
SBN#2 Honeywell LK191 .0026 20 Ge (coated)
LTONl Eltec 404 .0314 70 Ge
LTON2 Laser Presision kT-2210 .00784 20 IR-II
LTOM3 Martin-Marietta Laboratory .01 8.5 None
device
LT0#4 Molectron P1-71 .00785 25 None
Lm0 5 Servo 1401 .01 12.5 KRS-5
9
^11-
detector
element
signal
detector
element s i(7nal
(a) Voltage mode
RFB 1010
(b) current mode
Fifrure 1. Detector circuit., used for determination of D*
SOURCE APERTURE	 STOP
DETECTOR
i
PRE MD"	 WAVE ANALYZER
BLACK BODY
CHOPPER \
Figure 2. Experimental setup for evaluation of detectors
0
f -T	 I - . 	 .
4-
1,	
t
t
y 1I 1 ^r ,	 1 !
ld
DIGS or
.1772 '
LTO#3 - ' _ '	 '
1
S BN#2
! LT
^
-
`
L44
--
- 1
LTO#1
t LTO#5: t:-
' - - -
_
1
_
yT, F,uT _ —
-- LTO#2
_ I LTO#2
_
FVF?
1 a
- -'- • -  - t-
LTOA
!	 f a r-	 1}--- +II
1 i
_
LTO#1
DIGS
i	
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- —
ri•.+^.	 I
., -,	 SBl
10
	
100
	
1000
U
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I	 f
C,0
r-1 .
Co
r4
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0
r"I
moPPlrrG FREQUM, ICY (HZ)
JR.
Firzure 3. Performance curves of one detector of each manufacturer.
-t T 1 1 tr
i T t L
1. t t 1 II
LTO
PVF
2
SEN
TTI
..-t .
i f
1
ITT
1 I
,—
}
4t
^ Ih`
.1 _
^-
_.	 1 1
LTO
jo
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a.
• 111 .. ..	 1}... i ,.... t+:
FVF2
-1
f
Will I
11
L
Ad
1
7WILLI T
}
L, ri i 'i i
J., LidmhL
_
I i
_
_	 _ i_i_II iIiil:.
1
1?T	 7 1	 iL t
DTGS
I ,[^r^
O^
0
P'I
00
rl
N
x
Uv
A
ti
0
1
	
10
	
100
	
1000
CHOPPING FREQUENCY (H7)
Figure 3a. Performance curves of the best detectors of each material.
..^.^
7H
Cc
a
Nx
U^J
A
N
r4
0
1
J^: l- I _
_ }
HEIT y LI I j'
._
h	 t
y r
^ I `
t
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_
r
_ LTD# 5^ ^ : r ij.
r^ I . 7j }
— LTO##4
-^-
J
+
_ I
LTO##1
1
—
' I
0110
0
NN
+ TO #2
1 Y
I
I
` I
It
"II ; '^1
++
^
1
I
10
	
	
100
	
1000
CHOPPING FREQUENCY (HZ)
E
	 L
Figure 3b. Performance curves, one of each lithium tantalate manufacturer
H
a0
w
HO
z i
CH
H
H
C
1	 10	 100	 1000
CHOPPING FREQUENCY (H?)
Figure 4. Noise curves, one of each material
W


Evaluation of commercial pyroelectric detectors

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