of merit for TGS and SBN detectors. Since GMO does not exhibit a di- electric anomaly, it can be used as a threshold detector by heating through the transition ...
Journal of Electronic Mater/sls, Vol. I , No. 3, 1972
PYROELECTRIC DETECTION PROPERTIES OF GADOLINIUM MOLYBDATE
(GMO)
F. G. U l l m a n , B. N. G a n g u l y , a n d J. R. Z e i d l e r
The pyroelectric detection properties of gadolinium molybdate (GMO) c r y s t a l s h a v e b e e n s t u d i e d n e a r a n d a t i t s 1 5 9 ° C f e r r o e l e c t r i c transition temperature. Responsivity and detectivity figures of merit are calculated from measurements of pyroelectric currents induced by w h i t e l i g h t i r r a d i a t i o n a n d a r e c o m p a r e d w i t h r o o m temperature f i g u r e s of merit for TGS and SBN detectors. Since GMO does not exhibit a die l e c t r i c a n o m a l y , it c a n b e u s e d a s a t h r e s h o l d d e t e c t o r b y h e a t i n g through the transition temperature from a pre-selected temperature i n c r e m e n t b e l o w t h e t r a n s i t i o n . Voltage-sensltlve p y r o e l e c t r i c c u r rents at the transition, found previously, permit voltage control of the threshold.
F. G. U l l m a n is P r o f e s s o r o f E l e c t r i c a l E n g i n e e r i n g a n d B. N. G a n g u l y a n d J. R. Z e l d l e r a r e G r a d u a t e R e s e a r c h A s s i s t a n t s i n t h e D e p a r t m e n t o f Physics, University of Nebraska, Lincoln, Nebraska 68508.
425
Journal of Electro~e Mater/ab, V o l . / , N o . 3 , 1 9 7 ~ Introduction Radiation detectors utilizing the pyroelectric effect in ferroelectric materials have been developed in the past decade. The output of t h e s e d e v i c e s i s d e r i v e d f r o m t h e c h a n g e i n s p o n t a n e o u s p o l a r i z a t i o n of a ferroelectric material from the heating induced by incident radiation. Such detectors have fast response, low noise characteristics, and exhibit uniform response over wide spectral regions. The history of this development and the properties of currently used detectors have been rev i e w e d i n a r e c e n t a r t i c l e b y B e e r m a n (i). S i n c e (dPs/dT), t h e s l o p e o f t h e p o l a r i z a t i o n vs. t e m p e r a t u r e c u r v e ( p y r o e l e c t r i c c o e f f i c i e n t , %) i n c r e a s e s r a p i d l y n e a r t h e f e r r o e l e c t r i c transition temperature, T , one might expect a pyroelectric detector to b e m o s t s e n s i t i v e a t t ~ p e r a t u r e s c l o s e to T . H o w e v e r , for a s i m p l e d e t e c t o r c o n s i s t i n g of a f e r r o e l e c t r i c c r y s t a l ~ n s e r i e s w i t h a l o a d r e s i s t o r , the r e s p o n s i v l t y ( d e f i n e d a s t h e c h a n g e i n v o l t a g e a c r o s s the load resistor per unit intensity of the incident radiation) depends on t h e r a t i o o f (dP /dT) t o t h e c a p a c i t a n c e o f t h e c r y s t a l (2). Since in most ferroelectr~c materials the dielectric constant becomes very large a s T ÷ T , t h e i r r e s p o n s e d e c r e a s e s a s T is a p p r o a c h e d and, i n f a c t , t h e r e s p ~ n s i v i t i e s of T G S a n d S B N d e t e c t oc r s g o t h r o u g h a m a x i m u m w e l l b e l o w Tc (I). R e c e n t l y , g a d o l i n i u m m o l y b d a t e (GMO), o n e o f a g r o u p o f f e r r o e l e c tric r a r e - e a r t h m o l y b d a t e s , w a s s h o w n n o t t o e x h i b i t a d i e l e c t r i c a n o m a l y o n h e a t i n g t h r o u g h its 1 5 9 ° C t r a n s i t i o n t e m p e r a t u r e (3). T h i s u n u s u a l p r o p e r t y s u g g e s t e d t h e p o s s i b i l l t y o f u s i n g G M O c l o s e to i t s t r a n s i t i o n t e m p e r a t u r e for p y r o e l e c t r i c d e t e c t i o n s i n c e i t w o u l d n o t b e l o a d e d b y a dielectric anomaly as are other better k n o w n materials. Further, s i n c e t h e f e r r o e l e c t r i c t r a n s i t i o n of G M O i s f i r s t o r d e r , a l a r g e curr e n t s p i k e is o b t a i n e d o n h e a t i n g t h r o u g h T_ w h i c h c o u l d b e e x p l o i t e d for s w i t c h i n g a p p l i c a t i o n s a n d t h r e s h o l d d e ~ e c t l o n . t O f c o u r s e , any f i r s t o r d e r f e r r o e l e c t r i c c a n b e u s e d i n this w a y , i n p r i n c i p l e . H o w e v e r , t h e a p p l i e d e l e c t r i c f i e l d , n e e d e d in r e p e t i t i v e m e a s u r e m e n t s to k e e p t h e s a m p l e f u l l y p o l a r i z e d o n c o o l i n g b a c k t h r o u g h t h e transition, l a r g e l y o r w h o l l y d e s t r o y s t h e f i r s t o r d e r c h a r a c t e r o f the t r a n s i t i o n i n m o s t m a t e r i a l s w h e r e a s i n G M O , the f i r s t o r d e r c h a r a c t e r is, i f anything, enhanced by the applied field. I n this p a p e r , the p r o p e r t i e s o f GMO, o p e r a t e d as a " n o r m a l " and a s a " t h r e s h o l d " d e t e c t o r a r e d e s c r i b e d . N o a t t e m p t i s m a d e to s u g g e s t s p e c i f i c a p p l i c a t i o n s n o r w e r e a n y a t t e m p t s m a d e to o p t i m i z e f o r m a x i m u m sensitivity. To estimate the relative merits of GMO detectors, we have m e a s u r e d t h e p a r a m e t e r s that d e t e r m i n e t h e r e s p o n s i v l t y a n d d e t e c t i v l t y f i g u r e s of m e r i t , R M a n d D ~ (i). These figures of merit depend only on material properties and not on detector configurations or circuit
t B y t h r e s h o l d d e t e c t i o n , w e m e a n s e t t i n g the s t e a d y - s t a t e t e m p e r a t u r e a t some v a l u e b e l o w T c so that a f i x e d m i n i m u m a m o u n t of r a d i a t i o n h e a t i n g , t h e " t h r e s h o l d " , i s r e q u i r e d t o t r i g g e r t h e l a r g e r e s p o n s e a t Tc.
426
Journal of Electron/c Mater/al~ Vol. I , No. S, 197~ p a r a m e t e r s a n d t h u s , a r e c o n v e n i e n t for c o m p a r i n g d e t e c t o r m a t e r i a l s . W e h a v e r e p o r t e d o n t h e p y r o e l e c t r l c e f f e c t i n G M O p r e v i o u s l y (4); t h e s a l i e n t p o i n t s o f that w o r k a r e i n Fig. i. These data were obtained o n p o l e d c r y s t a l s , b i a s e d to a s t e a d y - s t a t e t e m p e r a t u r e J u s t b e l o w Tc and then slowl~ heated at .05°C/sec. Two distinct transitions separated by about l.S°CT were observed. The height of the second peak was found to increase in proportion to the decrease in the first with increasing applied field, and at fields greater than the coercive field (about 5 kv/cm), the lower temperature peak had essentially vanished. This l a s t p r o p e r t y s u g g e s t s t h a t i n the t h r e s h o l d d e t e c t i o n m o d e t h e t h r e s hold could be voltage-controlled. This effect has been observed in many samples of different size and shape and obtained from different sources a n d is thus a n i n t r i n s i c p r o p e r t y o f GMO. Experimental Measurements of the detection properties of GMO were made on poled crystals. (Samples were kept poled by applying an appropriate voltage on cooling back through the transition.) The front face of the sample was fully electroded with silver paint and a second overlayer of absorbing gold resinate (Hanovia Liquid Gold, 24%). The sample was mounted on a polished silica plate with a thin layer of silver paint; thus, the back face was also fully electroded. The samples were fully illuminated wlgh w h i t e l i g h t f r o m a v a r i a b l e i n t e n s i t y , t u n g s t e n , p r o j e c t i o n l a m p that was chopped at repetition rates ranging from 0.05 to 30 pulses per second. S a m p l e t h i c k n e s s e s r a n g e d f r o m 0.5 to 1 . 2 ram; t h e m e a s u r e m e n t s s h o w n i n the following Figures were made on the 1.2 mm sample. In all cases, the l i g h t w a s n o r m a l l y i n c i d e n t o n the e l e c t r o d e d f r o n t f a c e w h i c h w a s c u t n o r m a l to t h e p o l a r " c " a x i s . The performance of GMO as a "normal" pyroelectrlc detector was determined in a straightforward manner by measuring the current a few degrees below T . A typical measurement at 151@C with an applied field o f 4 3 0 0 v / c m a n ~ a 13 p p s r e p e t i t i o n r a t e i s s h o w n i n Fig. 2. A t t h i s t e m p e r a t u r e , the r e s p o n s e i s c l o s e t o l i n e a r f o r s m a l l t h e r m a l e x c u r s i o n s ; in this case, the temperature change during a light pulse was about 0.05oC. The figures of merit for this case are shown, along with others discussed b e l o w , i n T a b l e I. S i g n i f i c a n t l y d i f f e r e n t r e s u l t s a r e o b t a i n e d if t h e s t e a d y - s t a t e t e m p e r a t u r e i s h e l d c l o s e e n o u g h to T f o r t h e r a d i a t i o n t o h e a t the sample through the transition. For a~plled fields much smaller than the c o e r c i v e f i e l d , t w o c u r r e n t s p i k e s , c o r r e s p o n d i n g t o t h e two p e a k s i n Fig. i, a r e o b t a i n e d a s s h o w n i n Fig. 3a. (The p e a k s a p p e a r c l o s e t o g e t h e r b e c a u s e o f t h e time b a s e u s e d i n this m e a s u r e m e n t ; t h e y are, h o w e v e r , s e p a r a t e d b y 1 . 8 ° C so t h e i r s e p a r a t i o n o n a t i m e s c a l e d e p e n d s on the light intensity.) With sufficient applied field, the lower peak d i s a p p e a r s a n d t h e u p p e r p e a k i s e n h a n c e d a s s h o w n in Fig. 3 b for a n
tThese
t e m p e r a t u r e m e a s u r e m e n t s w e r e l i m i t e d t o a p r e c i s i o n of _+ 0 . 2 5 0 C .
427
Journal ol Electronic Mate~ads,VoL | , No. S, 1972
applied field of 4,333 v/om. Note that in both figures, a pulse of opposite polarity is obtained during the dark portion of the period, corresponding to the current obtained at T on cooling. These cycles can be repeated indefinitely if the crysta~ is kept poled and thermal drift is eliminated. Also, the small oscillations in Fig. 3a are caused by "ringing" from piezoelectric effects since the thin silver paint mounting appears to have been too elastic to cause significant clamping. The ringing is absent in Fig. 4, since in GMO, an applied field along the polar axis is equivalent to a stress in the basal plane which reduces the "ringing". Although these measurements were made at very slow chopping rates for convenience, the response at T can be seen at higher rates. A measurement at 26 pps is shown in Zig. 4. (Several light pulses are required to heat the sample to T to obtain the single pulse shown in the Figure.) c Calculations and Results The responsivity and detectivity figures of merit are defined as dPs/dT RM
Cpeg
D~
(c°ul/cm2de~ K) (Joule/gm degK)(gm/cm 3)
,
dPs/dT
(ohm-cm)~coul/cm2de~K)
~CpgT
(Joule/gin degK) (Em/cm3) (degK) ½
(la)
(ib)
where g is the density, c_ the specific heat at constant pressure, the real part of the diel~ctrlc constant, p the resistivity, and (dP_/dT) the slope of the polarization vs. temperature curve. R, is rel~ted to the detector response and D~ is inversely proportlona~ to the noise equivalent power, and is thus determined by the dominant noise source in the system. Equation (ib) applies in the low frequency range where Nyqulst noise is dominant. Known from the literature, are the specific heat (5)~ density (6), and dielectric constant (7) of GMO. To estimate ~ . a n d D~, (dP /dT) .M s and p are determined from our measurements. In the pyroe~ectrlc current measurements, i
P
-
(dPs/dT) (dT/dt)A
(2)
where A is the illuminated area of the sample and T and t are temperature and time, respectively. Hence, = (dPs/dT) =
ip/A(dT/dt)
(3)
Since i is measured and A is known, the heating rate, (dT/dt), at the temperature of interest is the only other quantity that needs to be determined. We have measured our heating rates in two ways. First, the sample temperature was set a known amount below T and the time required c to heat through T was determined from the oscilloscope trace of the c
428
Journal of Electronic Mater/aM, Vol. I, No. 3, 1971 pyroelectric current, giving values of dT/dt of about 0.3°C/see. A t y p i c a l m e a s u r e m e n t i s s h o w n i n Fig. 5, (The h e a t i n g r a t e s i n Fig. 3 a n d Fig. 4 a r e d i f f e r e n t . )
T h e s a m p l e r e s i s t i v i t y m u s t b e k n o w n to c a l c u l a t e D~. R e s i s t a n c e m e a s u r e m e n t s w e r e m a d e a t dc, 5 . 4 H z a n d 54 H z a t a s a m p l e t e m p e r a t u r e o f 1 5 9 ° C . F o r c o m p a r i s o n w i t h t h e d a t a i n r e f e r e n c e i, t h e v a l u e o f t h e resistivity at i0 Hz was estimated from these data to be 6.0 x I0 I0 ohm-cm compared to a d c value of 4.5 x 1012 . Assuming the same reduction a t o t h e r t e m p e r a t u r e s (ac m e a s u r e m e n t s w e r e m a d e a t 1 5 9 ° C o n l y ) g i v e s the values shown in Table I for 143 and 151°C. Table I Calculated Figures of Merit for GMO Compared with Corresponding P u b l i s h e d F i g u r e s (I) f o r T G S a n d S B N a t I 0 H z
D"~ (i) dP s
(2)
coul
dT cm2degK o(ohm-cm)
143°C
GMO l ~ C
6.7xi0 -II
1.2x10-I0
5.8xi0-I0
3.8xi0-I0
3.1x10-II
1.6xi0-5
2.0x10-5
6.7xi0-5
l.lx10-3
9.8xi0-5
1.4x10-9
2.5xi0-9
4.9Xi0 -8
3.0x10-8
l.lxl0-7
2.3xi0 II
1.2x1011
6.0x10I0
159°C
TGS 25°C
.
.
SBN 25°C
.
.
.
.
(1) B e e r m a n ' s D * v a l u e s h a v e b e e n m o d i f i e d to i n c l u d e t h e t e m p e r a t u r e , a s d e f i n e d i n t h i s p a p e r i n e q u a t i o n lb. (2) V a l u e s o f d P /dT, m e a s u r e d b e l o w t h e t r a n s i t i o n a r e i n g o o d a g r e e m e n t s w i t h the s l o p e o f t h e P vs. T c u r v e i n r e f e r e n c e 7 a t t h e s a m e t e m p e r a s ture. As can be seen, the relatively large pyroelectric coefficients at the transition, the low dielectric constant, and the high resistivity of GMO make its detection properties comparable in magnitude to the best known materials for room temperature detection. Discussion and Conclusions These results show that R~ for temperatures b e l o w Tc is roughly an o r d e r o f m a g n i t u d e s m a l l e r t h a ~ t h e m a x i m u m v a l u e s f o r T G S a n d SBN. T h i s s u g g e s t s t h a t G M O is n o t l i k e l y t o r e p l a c e t h e s e b e t t e r k n o w n materials in pyroelectric detectors. However, the rare-earth molybdates have smaller saturation polarization than most ferroelectrics. Cons e q u e n t l y , i f m a t e r i a l s l l k e G M O , i.e. w i t h n o d i e l e c t r i c a n o m a l y , b u t with larger polarization can be found, the construction of detectors more sensitive than those utilizing TGS and SBN will be feasible. I n the t h r e s h o l d m o d e of o p e r a t i o n , ~ f o r G M O i s c o m p a r a b l e t o T G S a n d SBN. The values shown in Table I are for measurements with an applied field, w h i c h gives the largest response. It is i m p o r t a n t t o
429
Journal of Electronic Materlal~ Vol. I, No. 3, 1972
note, in this connection, that no measurable shift or broadening of t h e . transition by applied fields and no thermal hysteresis have been observed in GMO (4). Consequently, GMO detectors could be operated with varying applied field without changes in operating characteristics due to the field. In the threshold mode, a large enough field to keep the crystal poled would have to be applied. Acknowledgements This work was carried out in the Electrical Materials Laboratory of the Department of Electrical Engineering. The authors are indebted to M. Hess for calling their attention to the use of the pyroelectric effect for practical radiation detection. Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant AFOSR-70-1926. The United States Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation hereon. References
i.
H. P. Beerman, "Improvement in the Pyroelectric Infrared Radiation Detector," Ferroelectrics, Vol. 2, 1971, pp. 123-128.
2.
J. Cooper, "Minimum Detectable Power of a Pyroelectric Thermal Receiver," Review of Scientific Instruments, Vol. 33, 1962, pp. 9295.
3.
L. E. Cross, A. Fouskova, and S. E. Cummins, "Gadolinium Molybdate, A New Type of Ferroelectric Crystal," Physical Review Letters, Vol. 21, 1968, pp. 812-814.
4.
F. G. Ullman, B. N. Ganguly and J. R. Hardy, "Pyroelectric Effect in Gadolinium Molybdate (@MO)," Ferroelectrics, Vol. 2, 1971, pp. 303-306.
5.
A. Fouskova, "The Specific Heat of Gd2(MoO4)3," Journal of the Physical Society of Japan, Vol. 27, 1969, p. 1699.
6.
K. Aizu, A. Kumada, H. Yumato, and S. Ashida, "Simultaneous Ferroelectricity and Ferroelasticity of Gd2(MoO4)3," Journal of the Physical Society of Japan, Vol. 27, 1969, p. 511.
7.
S. E. C u m i n s , "Electrical, Optical, and Mechanical Behavior of Ferroelectric Gd2(MoO4)3," Ferroelectrics, Vol. i, 1970, pp. 11-17.
430
Journal of Electron/c Mater/als, Vol. I, No. 3, 107~
71 6
61
5
51
(/)
4-
=E:- 4 :3
0 ,,ll 1 .
2
Ji - ~ 2 E3
i-
I
I
Z
w rr O rr
I
I
I
154
156
158
~r'
160
I
154
I
I
156 158
,I
160
(~
rr
I-- 6 _
6-
LIJ
--I i,i 5 _
5-
0 rr" >{L
4-
4 _ 5
-
C
5
2
2 I
/
0
I I t 154 156 158 160
0
I
154
I
156
I
158 160
T E M P E R A T U R E (*C) Fig.
i.
Pyroeleetric Current vs. Temperature of Poled Crystal. a. z e r o a p p l i e d f i e l d b. 4 , 7 5 0 v / e m e. 5,100 v/cm d. 7,900 v/em 431
I
Journal of Electronic Materials, Vol. I , No. 3, 1972
Fig. 2. Pyroelectrlc Response With Zero Applied Field at 151°C vertlcal-200 ~v/dlv, horlzontal-100 msec/dlv, chopping rate-13 pps, light on, light off as indicated
i ... !
Fig.
3a.
Pyroelectrlc Response at Tc With 525 v/cm Applied. vertlcal-2 mv/dlv, horlzontal-5 sec/dlv, chopping rate-0.06 pps, heating rate -%l.6°K/se~, light on, light off as indicated 432
Journal of Electronic Materials, Vol. 1, No. 3, 1972
Fig.
3b.
Pyroelectric Response at T With 4333 v/cm Applied. vertical-lOmv/div, horizontal-5 sec/div, chopping rate-0.05 pps, heating rate-%l.6°K/sec, light on, light off as indicated
Fig, 4. Pyroelectric Response at Tc With Zero Field Applied. vertical-500 ~v/div, horizontal-50 msec/div, chopping rate-26 pps 433
Journal of Electronic Materials, Vol. I, No. S, 1@'/2
Fig.
5.
P y r o e l e c t r i e R e s p o n s e M e a s u r e d F r o m (T - 0 . 2 5 ) 0 K to T to determine heating rate as described in text. c
434
