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Improvements in the detectivity of infrared pyroelectric detectors
Volume
1, number
5
OPTICS COMMUNICATIONS
IMPROVEMENTS
IN THE
DETECTIVITY
PYROELECTRIC
November/December
1969
OF INFRARED
DETECTORS
Armand HADNI University of Nancy, France Received
17 September
1969
We have constructed a pyroelectric detector with a 100 m TGS plate and a silicon window to check a simple theory leading to the calculation of the detectivity, and to the definition of a figure of merit for pyroelectric materials used in the construction of infrared detectors. We have measured a NEP (7 mm2, 0.1 mm, 320°K, 5 cps, 500°K) = 10-9 Watt CPS-~‘~, in good accordance with calculations assuming 30% absorption in the target. A figure of merit is defined as M = hi(c~)-I(enT)+2 where h cl and E” are respectively the pyroelectric coefficient, the specific heat per unit volume, and the imagmary part of the dielectric constant. Using this figure of merit, triglycine selenate appears as one of the best materials with a detectivity three times higher than for TGS. It would lead to the background limit at room temperature (D* FJ 3 X lOlo W-l cm cp&*) for a 1 pm thick plate. The NEP when optimized by the choice of a high enough bias resistor is shown to increase as the square root of frequency.
1. INTRODUCTION After some pioneering work [l], several papers have recently appeared [Z-6] on pyroelectric detectors where it has been seen that they were in a special class of thermal detectors where a physical property is proportional to the first derivative of the change of temperature versus time, while for common thermal detectors a physical property is used the change of it is only proportional to the variation of temperature. In a recent paper [7] we have shown that for usual frequencies w > l/7: r’being the thermal time constant of the pyroelectric detector (about 2 set for a detector made of a pyroelectric target stuck on a mylar film in vacua), the Noise Equivalent Power can be written 4o’ d”rA”2 (kT)‘/z (NEP)~~& H =
The resistivity depends on both temperature and frequency, and it is useful to write its expression in terms of the imaginary part E” of the relative dielectric constant which is generally frequency independent up to 106 cps: y. = (E,E”O)‘l. “Then (NEp)opt I or II = or
a7
a7 (B*)Opt I Or n =4(kco)i12di~2
X
x c’(TE”)~‘~ x
x J/Z
(Ii
A
(2)
C’(E”T)iftWi12
In this expression all parameters, h(T) and excepted, are independent. The factor M(T) X/C’(E”T)~‘~, is only temperature dependent appears as a figure of merit of the material the detection of thermal radiation.
2.
where I or II refer to pyroelectric detectors with a target surface either perpendicular or parallel to the pyroelectric direction, opt means that the detector has been optimized by the choice of a load resistor of higher resistance than the crystal, c’ is the specific heat per unit volume, d the thickness, and A the area of the pyroelectric plate, r is the transmission coefficient of the detector window, a is the percentage of radiation energy absorbed by the target, A = dF’S/dT is the pyroelectric coefficient of the material, and r. its resistivity.
4(k~o)i’zd”2A”2
l” (T) =
and for
TCiS DETECTORS
To check formula (l), we have made a pyroelectric type I detector with a selected triglycine sulphate plate, for which ln (320OK) = 0.60 and A (32O’K) = 0.14 ,uCb deg-l cmm2. Much larger values of en have been found in a number of samples. The other parameters of the detector are: d = 10m4m, A = 7 X 10e6 m2 (circular area of 3 mm diameter), c = 1.04 J gm-l or c’ = 1.6 X lo6 J mm3 (specific mass p. X 1.6 gm cmm3), T = 0.5 (silicon windown); we have assumed Q! = 0.6. 251
Volume
1, number
5
OPTICS COMMUNICATIONS
NEP (7 mm2, 0.1 mm, 320°K, 5 cps, 5OOOK) = 10mgWatt cps-“’
,
D* (0.1 mm, 320°K, 5 cps,
(3)
500°K)obs
Figure
= 0.26 x 10’ cm CPS*‘~Watt-’
.
(4)
E”
NEP (7 mm2, 0.1 mm, 320°K, 5 cps, 500°K)calc Watt c~s-“~ .
k(/.LCB deg-lcmm2) ~(E”)-llz
(5)
The (NEP),,lc is only 2 times less than the (NEP),bs. This can be considered as a good agreement since we have assumed a net absorption o! = 0.6, which is certainly not true (the bulk reflectivity is higher than 50% from 2000 to 500 cm ml [8]). To get result (5) exactly, we should have to suppose LY= 30%~.It thus appears that formula (1) or (2) is a good approximation. To get a better detectivity we might cancel the window (7 = 1) and make an appropriate coating for the wavelengths of use to increase the absorbtivity (Y up to 60% and thus the detectivity up to 109 cm c~s’/~ Watt-l. Then we could reduce d down to 0.11 microns to increase D* up to the ideal background limited detectivity at 320’K [ll], &mit N 3 X lOlo Watt-l cm CPS’~~. Nevertheless, up to now, we have not got any improvement by reducing d to 25 microns. For laser ap-
Figure
of merit
&f(T) = X(c’)-’
46
48
49
50
50.5
0.16
0.49
1
2
3.4
3.6
0.02
0.13
0.20
0.32
0.60
0.05
0.18
0.20
0.23
0.34
Table 2 gives E“, 1, c’ and M(T) for various pyroelectric materials. There is a factor of three in favor of TGS (300°K) over SrHaNb206 (333OK) [6], and TGSe (295’K) is about three times better than TGS (320’K) as we have seen in a preliminary experiment [3]. It thus appears that with T = 1, o = 0.6, we could achieve the ideal background limited detectivity of TGSe at room temperature with a 1.2 pm thick plate, which is still too small a thickness to be feasible. 4. DEPENDENCEOFTHENEPONFREQUENCY The dependence of the NEP on frequency as
Table 2 materials
X( /Xb
20
3. OTHER PYROELECTRIC MATERIALS
(E"T)-"~ pyroelectric E”
deg-l
used for detection
cme2)
c’(J cmv3)
of thermal
radiations
M(T)XTilz
TGS (3OO’K)
0.16
0.020
1.8
0.030
TGS (320OK)
0.60
0.140
1.8
0.100
TGS, TGSe (60%) (314’K)
5.6
0.130
1.9 (?)
0.030
TGSe (295OK)
1.1
0.650
1.9 (?)
0.310
SrO.66BaO.33Nb206 Ti03Ba
(333’K)
(333OK)
TiO3Ba (381’K) GASH (300’K) N(CH3)4. HgC13 (293OK)
252
Table 1 of merit M’(T) = x(E”)-iia of a rrgoodlt TGS crystal versus temperature T(‘C)
From formula (1) we get:
= 5 X lo-lo
25
0.110
2
300
0.070
2
(7)
2
(7)
0.300 0.007 0.250
270
1969
plications a small target is sufficient [9] and A can be sized down to 0.25 mm2, hence a total gain of 5.3 x 2 = 10.6 if the window is cancelled out, leading to a NEP (0.25 mm2, 0.1 mm, 300°K, 5 cps, 500’8) M lo-lo Watt cps-112, a value which is only attainable with good Golay detectors. Table 1 gives E” and A versus T for TGS and shows that the figure of merit for this material increases from 20 to 46OC, but from 46 to 49OC it does not appear very sensitive to temperature.
The source of radiation is a black body at 500’K giving a maximum flux of 0.5 x 10-v Watt on the detector. The flux is chop ed at a frequency of 5 cps (w = 30 rad set- ‘1). With a load resistor of 1012 Ohm made of a metal layer in vacuum and a field effect transistor preamplifier, we get a signal-to-noise ratio equal to 50 for a one second time constant, hence a
or a detectivity
November/December
10
0.011
0.02
0.001
1
(7)
0.02
0.070
2
(7)
0.003
Volume 1, number 5
OPTICS COMMUNICATIONS
proportional to WlJ2 (formula (1)) seems to be right at least for frequencies as high as 20 000 cps where the detectivity of a gold doped germanium is very approximately [ 111 D* (10 I.L,80’K)
= 0.3 x lOa cm CPS~‘~Watt-l. Hence a NEP (7 mm 2, 80°K, 20000 cps, 10 IL) = lOwa Watt. At this frequency, assuming a w1’2 law, the NEP of the pyroelectric detector would be increased by a factor of 63 to reach 6 x 10e8 Watt CPS-‘/~. We have seen [9] that its NEP was five times larger than for a gold doped germanium and this is a check of the w112law expressed in formula (1).
REFERENCES [l] C.Rossetti, Compt. Rend. Acad. Sci. (Paris) 231 (1950) 126; J.Cooper, Rev. Sci. Instr. 33 (1962) 92;
November/December
1969
R.A.Hanel, J. Opt. Sot. Am. 51 (1961) 220. I21 A.Hadni, J. Phys. 24 (1963) 694. [3] A.Ha&i, Y.Henninger, R.Thomas, P.Vergnat and B.Wyncke, J. Phys. 26 (1965) 345. (41 K.Taksmi, J. Appl. Phys. (Japan) 34 (1965) 930; G.A.Burdick and R. T.Arnold, J. Appl. Phys. 37 (1966) 3223; J.H.Ludlow, W.H.Mitchell, E.H.PutleyandN. Shaw, J. Sci. Instr. 44 (1967) 694. [ 51 R. W . Astheimer and F .Schwarz, Appl. Opt. 7 (1968) 1687. [S] A.M.Glass, Appl. Phys. Letters 13 (1968) 147. [7] A.Hadni, R.Thomas and J.Perrin, Compt. Rend. Acad. Sci. (Paris) 40 (1969) 2740. [8] A.Hadni, D.Grandjean, F.Brehat, J.Claudel, X. Gerbaux, P . Strimer , R . H .Thomas, F .Vermillard and R.Thomas, J. Phys. 30 (1969) 377. (91 A. Hadni, R. Thomas and J. Perrin, J. Appl. Phys. 268 (1969) 325. [lo] E. Leiba, Compt. Rend. Acad. Sci. (Paris) 268 (1969) 31. [ll] P.W.Kruse et al., Elements of infrared technology (John Wiley, New York, 1962) p. 427.
253
1, number
5
OPTICS COMMUNICATIONS
IMPROVEMENTS
IN THE
DETECTIVITY
PYROELECTRIC
November/December
1969
OF INFRARED
DETECTORS
Armand HADNI University of Nancy, France Received
17 September
1969
We have constructed a pyroelectric detector with a 100 m TGS plate and a silicon window to check a simple theory leading to the calculation of the detectivity, and to the definition of a figure of merit for pyroelectric materials used in the construction of infrared detectors. We have measured a NEP (7 mm2, 0.1 mm, 320°K, 5 cps, 500°K) = 10-9 Watt CPS-~‘~, in good accordance with calculations assuming 30% absorption in the target. A figure of merit is defined as M = hi(c~)-I(enT)+2 where h cl and E” are respectively the pyroelectric coefficient, the specific heat per unit volume, and the imagmary part of the dielectric constant. Using this figure of merit, triglycine selenate appears as one of the best materials with a detectivity three times higher than for TGS. It would lead to the background limit at room temperature (D* FJ 3 X lOlo W-l cm cp&*) for a 1 pm thick plate. The NEP when optimized by the choice of a high enough bias resistor is shown to increase as the square root of frequency.
1. INTRODUCTION After some pioneering work [l], several papers have recently appeared [Z-6] on pyroelectric detectors where it has been seen that they were in a special class of thermal detectors where a physical property is proportional to the first derivative of the change of temperature versus time, while for common thermal detectors a physical property is used the change of it is only proportional to the variation of temperature. In a recent paper [7] we have shown that for usual frequencies w > l/7: r’being the thermal time constant of the pyroelectric detector (about 2 set for a detector made of a pyroelectric target stuck on a mylar film in vacua), the Noise Equivalent Power can be written 4o’ d”rA”2 (kT)‘/z (NEP)~~& H =
The resistivity depends on both temperature and frequency, and it is useful to write its expression in terms of the imaginary part E” of the relative dielectric constant which is generally frequency independent up to 106 cps: y. = (E,E”O)‘l. “Then (NEp)opt I or II = or
a7
a7 (B*)Opt I Or n =4(kco)i12di~2
X
x c’(TE”)~‘~ x
x J/Z
(Ii
A
(2)
C’(E”T)iftWi12
In this expression all parameters, h(T) and excepted, are independent. The factor M(T) X/C’(E”T)~‘~, is only temperature dependent appears as a figure of merit of the material the detection of thermal radiation.
2.
where I or II refer to pyroelectric detectors with a target surface either perpendicular or parallel to the pyroelectric direction, opt means that the detector has been optimized by the choice of a load resistor of higher resistance than the crystal, c’ is the specific heat per unit volume, d the thickness, and A the area of the pyroelectric plate, r is the transmission coefficient of the detector window, a is the percentage of radiation energy absorbed by the target, A = dF’S/dT is the pyroelectric coefficient of the material, and r. its resistivity.
4(k~o)i’zd”2A”2
l” (T) =
and for
TCiS DETECTORS
To check formula (l), we have made a pyroelectric type I detector with a selected triglycine sulphate plate, for which ln (320OK) = 0.60 and A (32O’K) = 0.14 ,uCb deg-l cmm2. Much larger values of en have been found in a number of samples. The other parameters of the detector are: d = 10m4m, A = 7 X 10e6 m2 (circular area of 3 mm diameter), c = 1.04 J gm-l or c’ = 1.6 X lo6 J mm3 (specific mass p. X 1.6 gm cmm3), T = 0.5 (silicon windown); we have assumed Q! = 0.6. 251
Volume
1, number
5
OPTICS COMMUNICATIONS
NEP (7 mm2, 0.1 mm, 320°K, 5 cps, 5OOOK) = 10mgWatt cps-“’
,
D* (0.1 mm, 320°K, 5 cps,
(3)
500°K)obs
Figure
= 0.26 x 10’ cm CPS*‘~Watt-’
.
(4)
E”
NEP (7 mm2, 0.1 mm, 320°K, 5 cps, 500°K)calc Watt c~s-“~ .
k(/.LCB deg-lcmm2) ~(E”)-llz
(5)
The (NEP),,lc is only 2 times less than the (NEP),bs. This can be considered as a good agreement since we have assumed a net absorption o! = 0.6, which is certainly not true (the bulk reflectivity is higher than 50% from 2000 to 500 cm ml [8]). To get result (5) exactly, we should have to suppose LY= 30%~.It thus appears that formula (1) or (2) is a good approximation. To get a better detectivity we might cancel the window (7 = 1) and make an appropriate coating for the wavelengths of use to increase the absorbtivity (Y up to 60% and thus the detectivity up to 109 cm c~s’/~ Watt-l. Then we could reduce d down to 0.11 microns to increase D* up to the ideal background limited detectivity at 320’K [ll], &mit N 3 X lOlo Watt-l cm CPS’~~. Nevertheless, up to now, we have not got any improvement by reducing d to 25 microns. For laser ap-
Figure
of merit
&f(T) = X(c’)-’
46
48
49
50
50.5
0.16
0.49
1
2
3.4
3.6
0.02
0.13
0.20
0.32
0.60
0.05
0.18
0.20
0.23
0.34
Table 2 gives E“, 1, c’ and M(T) for various pyroelectric materials. There is a factor of three in favor of TGS (300°K) over SrHaNb206 (333OK) [6], and TGSe (295’K) is about three times better than TGS (320’K) as we have seen in a preliminary experiment [3]. It thus appears that with T = 1, o = 0.6, we could achieve the ideal background limited detectivity of TGSe at room temperature with a 1.2 pm thick plate, which is still too small a thickness to be feasible. 4. DEPENDENCEOFTHENEPONFREQUENCY The dependence of the NEP on frequency as
Table 2 materials
X( /Xb
20
3. OTHER PYROELECTRIC MATERIALS
(E"T)-"~ pyroelectric E”
deg-l
used for detection
cme2)
c’(J cmv3)
of thermal
radiations
M(T)XTilz
TGS (3OO’K)
0.16
0.020
1.8
0.030
TGS (320OK)
0.60
0.140
1.8
0.100
TGS, TGSe (60%) (314’K)
5.6
0.130
1.9 (?)
0.030
TGSe (295OK)
1.1
0.650
1.9 (?)
0.310
SrO.66BaO.33Nb206 Ti03Ba
(333’K)
(333OK)
TiO3Ba (381’K) GASH (300’K) N(CH3)4. HgC13 (293OK)
252
Table 1 of merit M’(T) = x(E”)-iia of a rrgoodlt TGS crystal versus temperature T(‘C)
From formula (1) we get:
= 5 X lo-lo
25
0.110
2
300
0.070
2
(7)
2
(7)
0.300 0.007 0.250
270
1969
plications a small target is sufficient [9] and A can be sized down to 0.25 mm2, hence a total gain of 5.3 x 2 = 10.6 if the window is cancelled out, leading to a NEP (0.25 mm2, 0.1 mm, 300°K, 5 cps, 500’8) M lo-lo Watt cps-112, a value which is only attainable with good Golay detectors. Table 1 gives E” and A versus T for TGS and shows that the figure of merit for this material increases from 20 to 46OC, but from 46 to 49OC it does not appear very sensitive to temperature.
The source of radiation is a black body at 500’K giving a maximum flux of 0.5 x 10-v Watt on the detector. The flux is chop ed at a frequency of 5 cps (w = 30 rad set- ‘1). With a load resistor of 1012 Ohm made of a metal layer in vacuum and a field effect transistor preamplifier, we get a signal-to-noise ratio equal to 50 for a one second time constant, hence a
or a detectivity
November/December
10
0.011
0.02
0.001
1
(7)
0.02
0.070
2
(7)
0.003
Volume 1, number 5
OPTICS COMMUNICATIONS
proportional to WlJ2 (formula (1)) seems to be right at least for frequencies as high as 20 000 cps where the detectivity of a gold doped germanium is very approximately [ 111 D* (10 I.L,80’K)
= 0.3 x lOa cm CPS~‘~Watt-l. Hence a NEP (7 mm 2, 80°K, 20000 cps, 10 IL) = lOwa Watt. At this frequency, assuming a w1’2 law, the NEP of the pyroelectric detector would be increased by a factor of 63 to reach 6 x 10e8 Watt CPS-‘/~. We have seen [9] that its NEP was five times larger than for a gold doped germanium and this is a check of the w112law expressed in formula (1).
REFERENCES [l] C.Rossetti, Compt. Rend. Acad. Sci. (Paris) 231 (1950) 126; J.Cooper, Rev. Sci. Instr. 33 (1962) 92;
November/December
1969
R.A.Hanel, J. Opt. Sot. Am. 51 (1961) 220. I21 A.Hadni, J. Phys. 24 (1963) 694. [3] A.Ha&i, Y.Henninger, R.Thomas, P.Vergnat and B.Wyncke, J. Phys. 26 (1965) 345. (41 K.Taksmi, J. Appl. Phys. (Japan) 34 (1965) 930; G.A.Burdick and R. T.Arnold, J. Appl. Phys. 37 (1966) 3223; J.H.Ludlow, W.H.Mitchell, E.H.PutleyandN. Shaw, J. Sci. Instr. 44 (1967) 694. [ 51 R. W . Astheimer and F .Schwarz, Appl. Opt. 7 (1968) 1687. [S] A.M.Glass, Appl. Phys. Letters 13 (1968) 147. [7] A.Hadni, R.Thomas and J.Perrin, Compt. Rend. Acad. Sci. (Paris) 40 (1969) 2740. [8] A.Hadni, D.Grandjean, F.Brehat, J.Claudel, X. Gerbaux, P . Strimer , R . H .Thomas, F .Vermillard and R.Thomas, J. Phys. 30 (1969) 377. (91 A. Hadni, R. Thomas and J. Perrin, J. Appl. Phys. 268 (1969) 325. [lo] E. Leiba, Compt. Rend. Acad. Sci. (Paris) 268 (1969) 31. [ll] P.W.Kruse et al., Elements of infrared technology (John Wiley, New York, 1962) p. 427.
253