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A brief guide to pyroelectric detectorsS. G. Porter aa Plessey Optoelectronics & Microwave Ltd., Towcester, Northants, NN127JN, EnglandPublished online: 07 Feb 2011.

To cite this article: S. G. Porter (1981): A brief guide to pyroelectric detectors, Ferroelectrics, 33:1,193-206

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Ferroelectrics. 1981. Vol. 33, pp. 193-206 001 5-0 19318 1/3301-0193/$06.50/0

9 1981 Gordon and Breach, Science Publishers, Inc. Printed in the United States of America

A BRIEF GUIDE TO PYROELECTRIC DETECTORS

S. G. PORTER

Plessey Optoelectronics & Microwave Ltd., Towester. Northants. "12 7JN England

(Rercived August 4. 1980. in / i t i d fortn h'owtnhu 10, 19x0)

The principle5 of operation of ii pyoclectric detector are sumniarizcd and responsivity. noisc. noisc equivalent power and detcctivit! are derived. The relationship hetween responsivity and f'requrnc! is discussed i i b are thc various wurccs ol noise.

There lollowa a discussion of the relative merits of the five principal pyroelrctric materials in commoii use: Trigly- cine Sulphate. Lithium Tantalate. Strontium Barium Niobatc. Pyroclecrric Ceramics and Polyvinylidene Fluoride. Fac- tors intlucncing t h ~ choice ot material are outlined and sunimariz.ed in performance curvcs lor three sizes of detector.

Mention is made o f a varicty of 'detector t>pe\ currently ;iLailable and sonic o l thc current applications. A very hriel summary of possible future developments is included.

I . THE PYROELECTRIC EFFECT

A pyroelectric material is one which possesses an inherent electrical polarization. the magnitude of which is a function of temperature.'-' Most pyroe- lectrics are also ferroelectric, which means that t h e direction of their polarization can be reversed by the application of a suitable clectric field, and their- polarization reduces to zero at some temper- ature known as the Curie temperature, Tr, by analog) with ferromagnetism.' The dependence of polarization on temperature is typically of the form illustrated in Figure 1. The gradient of this curve. dP/dT, at a particular temperature. T, is the pyroelectric coefficient, which will be denoted by p .

The strange effects produced when the mineral tourmaline is heated have been known for many

hundreds of years, and the pyroelectric properties of Rochelle salt were studied early in the nine- teenth century,' but it is only in the last twenty o r thirty years that pyroelectric infrared detectors have been developed.

The importance of the pyroelectric eI'fect in infra- red detection was becoming obvious about ten years ago,h and a widely acclaimed review of pyro- electric detectors was published in 1970 by Putky.' At this time the only pyroelectric material of sig- nificant value was triglycine sulphate.

A considerable amount of research and devel- opment has been devoted to pyroelectric detectors during recent years, covering all aspects of mate- rials, device fabrication, and applications. Putley therefore published an update to his review article in 1977.'

2. THE PYROELECTRIC DETECTOR

A simple pyroelectric detector consists of a slice o f pyroelectric material with metal electrodes on opposite faces, the material being oriented such that its polar axis is perpendicular to the elec- troded faces. Generally a ferroelectric consists o f a large number of separate domains with differing directions of polarization, so that the net effect over the whole slice is zero. Before use, therefore,

I \ ~ these domains must be reoriented by the applica- tion of an electric field so that all become parallel to one another (or as near parallel as possible, in the case of a ceramic). This is usually done a t an

Temperature (TI TC f:IG[:RE I T ~ ~ ~ ~ ~ ; ~ ~ ~ ~ ~ depenc~enc. of p o l a r i r a t i o n o f fe r ro- clectric..

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194 S. G .

-

PORTER

-

elevated temperature so that the coercive field is reduced.

Even across a correctly poled detector, there will generally be no observable voltage. This is because its internal polarization is balanced by a surface charge which accumulates via various leakage paths between the two faces. For this rea- son, the pyroelectric detector can only be used in an a.c. mode and at a frequency high enough for this electrical leakage to be ineffective. In other words, the pyroelectric detector can only be used to detect changes in irradiance.

When the detector is heated by incident radia- tion, the polarization changes by an amount deter- mined by the temperature change and the pyro- electric coefficient of the material. This change in polarization appears as a charge on the capacitor formed by the pyroelectric with its two electrodes. Typically this charge is of the order of lo-’’ cou- lombs on a capacitance of the order of 10pF.

Pyroelectric

3. AMPLIFIERS

OVO

5% c R

0 ov

Figure 2(b). Here the signal current flows through the amplifier feedback resistor, RF, which can now be much larger since the time constant is now RFC divided by the open loop gain of the amplifier.

I t is generally more difficult to produce current amplifiers which give a signal t o noise ratio as good as the simple voltage amplifier of Figure 2(a). Stray capacitances across the feedback resistor, RF, also present problems in obtaining the desired frequency response.

4. RESPONSIVITY

When a pyroelectric detector is exposed to radia- tion which is modulated at an angular frequency w, the temperature of the detector will be modu- lated at this frequency by an amount which de- pends on the fraction of the incident radiation ab- sorbed, 7 (the emissivity of the surface), and the heat capacity, H , of the detector. This tempera- ture modulation will also depend on the thermal conductance GT, coupling the detector to its en- capsulation, which may be considered as a heat sink at constant temperature.

The temperature difference, 0, between the de- tector a n d the heat sink is related to the incident radiant power, W , by the equation:

If the incident radiation can be expressed in the form W = Woeim‘, then equation (1) has the solution:

FIGURE 2 Alternative amplifier arrangements

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GIJIDE TO PYROEI.F.CII’RIC DETECTORS I95

4.1. Current responsivity

The pyroelectric charge generated, q, is given by:

q = P A 8 (3)

where p is the pyroelectric coefficient and A is the electrode area of the detector. The current responsivity is defined as:

4 where i =- dt

Thus

(4)

H . GT

where T~ = - is the thermal time constant.

The current responsivity as a function of fre-

At frequencies which are high compared with quency is thus of the form shown in Figure 3.

‘ / T T , equation 5 may be reduced to:

V P ai= - sd

where s is the heat capacity per unit volume (vol- ume specific heat) and d is the thickness of the detector.

4.2. Voltage responsivity

I f the detector is connected to a high impedance amplifier, such as that shown in Figure 2(a), then the observed signal is equal to the voltage pro-

/-- I I

I

TT

* log w 1 we-

F I G U R E 3 Frequency dependence of current rcsponsivity.

duced by the charge, q. The detector may be rep- resented a s a capacitor, a current generator, and a shunt conductance, as shown in Figure 4.

The voltage generated is therefore given by:

(7) i

G E + jwC V =

and the voltage responsivity is defined as:

8” = I w l V

giving:

c . G E

where = - is the electrical time constant.

Again, this simplifies at frequencies which are high compared with ‘ / T T and ’/TE to give:

where t o is the permittivity of free space and E , is the relative permittivity of the pyroelectric ma- terial.

Equation 10 shows that, a t high frequencies, the voltage responsivity of a pyroelectric detector is inversely proportional t o frequency. At low fre- quencies this is modified by the electrical and thermal time constants, as in equation 9, so that the true frequency response is of the form shown in Figure 5.

Typically T T is within the range 0.01 seconds to 10 seconds. TE, however, can be anywhere between

seconds and 100 seconds, depending on the sizes of the detector capacitance and the shunt resistor.

FIGURE 4 Equivalent circuit of simple detector

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196 S. ti. PORTER

log n v

I I I I I I I I * 0.1 w . 1 log w

TT TE

FlGUREl 5 Frcqucnc! dependence uf voltage responsivit?.

As mentioned in Section 3, at frequencies below T T the output voltage is that produced by the py- roelectric current flowing through the resistor R . This is equivalent t o using a current amplifier, the responsivity being equal t o RIRi.

In practical detectors there is usually a n ampli- fier whose input impedance must be included in Figure 4. In the case of a JFET this impedance may be considered a s a capacitance, CA, with a parallel resistance, R A . In practice RA is large compared with the shunt resistor, R , and can be ignored. but Ca is not always small compared with the detector capacitance C and T E must be written as:

c + CA TE = 7

More rigorous analyses of pyroelectric detectors have been performed,'-'' taking into account the effects of mounting techniques and black coat- ings. The above treatment, however, is adequate for the majority of applications.

5. NOISE

The usefulness of a detector is usually assessed in terms of the minimum detectable incident power. This is a function of both the responsivity and the noise generated in the detector and its amplifier. Therefore a n analysis of noise sources is necessary for a full understanding of pyroelectric detectors.

There are three major noise sources in a simple pyroelectric detector with a shunt r e ~ i s t o r , ~ as shown in Figure 6. The following discussion re- lates to noise in unit bandwidth.

5.1. Thermal Noise

Thermal noise, AWr. in the detector occurs ac-

F I G t J R E 6 shunt resistor.

Noisc equivalent circuit lor siniplc dctector wit11

cording to the relation:"

3. W r = (4kT2Gr)' ' (12)

Which gives a noise current:'

5.2 Dielectric Noise

The pyroelectric detector is a capacitance C with a dielectric loss tan 6 giving a n equivalent conduc- tance WC tan 6.'' The noise voltage generated by this conductance is given by the $tandard cxpres- sion for Johnson noise in a r e s i ~ t o r : ~ '

WC tan 6

giving a n equivalent noise current of:

(14)

in = (4kTwC tan 6)' ' (15)

5.3 Resistor Noise

In similar manner, the shunt resistor, R, gives a current noise:

5.4 Amplifier Noise

The noise produced by a n amplifier. such as a n FET, may be represented by two noise generators a t the amplifier input: e,, the equivalent input volt- age noise. and i,, the equivalent input current noise."

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GUIDE TO PYROELECIRIC DETECTORS

The noise equivalent circuit of detector and amplifier is thus as shown in Figure 7.

FIGt!RE 7 Noise equivalent circuit of dctector with ampli l ier .

Herr the detector and amplifier input impedances are represented by Zd and Z, respectively.

The amplifier voltage noise may be represented by a n equivalent current noise a t the amplifier input:

. e , , = - Zd

5.5 Total Equivalent Input Noise

The total equivalent input noise current is now given by:

ii = i++ i X + iR + if + is (18) In order t o obtain the appropriate noise voltage

t o compare with the signal voltage of equation 9, this noise current must be converted into an equiv- alent noise voltage across the parallel combina- tion of detector impedance and amplifier input impedance. The total mean square noise voltage may then be written as:

v f = v; + v; + Y', + vz + vs (19)

where:

+ w?R?C? 1 2

1 + w-Ti. and: v c = e u ( , , ) (24)

In deriving these equations it has been assumed that tan S < 1 and that the amplifier input imped- ance is a capacitance, CA, so that T E is given by equation 1 1 . If C,A < C, then equation 24 reduces to li, = e,.

Equation 19 gives the noise in unit bandwidth at a frequency W , and is expressed in units of' V' Hz-', Y, is therefore in units of V Hz-' '.

An indication of the relative magnitudes of these various sources of noise for a typical detec- to r is given in Figure 5. It has been assumed that both the thermal and electrical time constants are longer than one second.

I n nearly all practical detectors the thermal noise, v,, is insignificant and is often ignorcd in calculations.

5.6 Environmental Noise

When analyzing the performance of a detector it is important t o include a consideration of the en- vironment in which it is to be used. There are var- ious forms of external stimulation which can pro- duce unwanted electrical outputs from the detec- tor, and these must be considered a s additional sources of noise.

5.6. I Microphony

In certain applications a major limitation to the usefulness of pyroelectric detectors is that they are

Frequency ( Hz F.'ICiIJRE 8 cal dctcctor .

Relative miigiiitudes of iioisc Lol tagcs 111 ii I y -

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198 S. G . PORTER

microphonic; electrical outputs are produced by mechanical vibration or acoustic noise. This mi- crophonic signal may dominate all other noise sources if the detector is in a high vibration envi- ronment.

The basic course of microphony is the piezoe- lectric nature of pyroelectric materials, meaning that a change in polarization is produced by a mechanical strain a s well as by a change in temperature. Piezoelectric coefficients d o vary from material to material, but the corresponding differences in microphony are much less than those associated with the details of the detector design. Alterations to the mounting and packag- ing of a detector may change the microphony coefficients by as much as a factor of one hundred.

The detailed analysis of microphony is beyond the scope of this paper, but i t may be noted that in general lower microphony is obtained by mak- ing the mounting of the pyroelcctric less rigid. Single point supports, which are often used to give good thermal isolation, also give low micro- phony. Further reductions in microphony have been achieved by using compensated detectors,34 as described in section 11.2.

5.6.2 Electromagnetic Interference

Electromagnetic interference is a source of un- wanted signals in electrically noisy environments. Detectors designed for use a t low frequencies, having a very high input impedance preamplifer, require careful screening. This is generally achieved by using electrically conducting windows of ger- manium or silicon which are connected to the earthed metal can. Alternative precautions may be necessary if the required spectral response dictates that a nonconducting window is used; in general the field of view should not be greater than that which is necded for the application.

5.6.3 Thermally Induced Transients

If a pyroelectric detector is subjected to changes in ambient temperature fast pulses are sometimes observed, superimposed on the normal pyroelec- tric response. These pulses, known as thermally induced transients (TITS) occur in a random fash- ion, but their number and amplitude increase with increasing rate of change of temperature. The origin of this effect is not yet understood, al- though suggestions have been made that it is a

domain switching phenomenon analogous to the Barkhausen effect in ferromagnetics. The explana- tion is obviously more subtle than this, however, since it occurs in nominally single domain crystals when the temperature, not the applied field, is changed. There is some evidence that the effect is less marked in polycrystalline materials than in single crystals.

6. NOISE EQUIVALENT POWER

The sensitivity of a particular detector is usually expressed in terms of its noise equivalent power, or NEP.” This is the incident power which is re- quired to produce a signal equal t o the r.m.s. noise voltage, i.e.:

V n NEP = - R V

The units of NEP are normally W Hz-l”, it being specified for a particular frequency and unit bandwidth. Occasionally, however, the broadband NEP, expressed in W, is quoted, this being the ratio of broadband noise to responsivity at a spec- ified frequency.

7. NOISE EQUIVALENT IRRADIANCE

It is sometimes more useful to consider the noise equivalent irradiance (NEI). l9 This enables one to compare the ability of different sized devices to detect a given irradiance o r incident power den- sity. The NEI is defined by:

and the units are Wm-2 o r Wm-’ Hz-’;’ according to whether it is related to broadband noise or noise in unit bandwidth a t a specific frequency.

8. DETECT1 VITY

It is often considered that it is aesthetically more pleasing to have a “figure of merit” which in- creases, rather than decreases, in magnitude as the detector improves in performance. For this reason the reciprocal of NEP, defined as the detectivity D, is employed.”’

A further refinement is the specific detectivity,

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GUIDE TO PYROELECTRIC DETECTORS I99

D*, which is defined as:"

A L 2

NEP D* = -

and is commonly expressed in units of cm- Hz' 'W-' (although mHzi'*W-' would be more consistent with present day standards).

The major reason for defining D* in terms of the square root of the detector area is that when dielectric noise is dominant the NEP is propor- tional to A ' ' so that D* is independent of area and can be used to compare detectors of different areas. As Figure 5 shows, however, dielectric noise is not always dominant, particularly in the case of small area detectors operated at very low or very high frequencies. D* must therefore be used with caution.

9. PRYOELECTRIC MATERIALS

The five most commonly used pyroelectric mate- rials are triglycine sulphate (TGS)," lithium tan- talate (LT)," " strontium barium niobate (SBN)," polyvinylidene fluoride (PVDF)"." and ceramic materials based on lead zirconate (PZ).2x,29 Some of the relevant properties of these materials are listed in Table I ; typical values at 20°C are given, there being some variation in the reported values.

9. I Triglycme Sulphate

TGS i \ a colorless water soluble crystal whlch is grown from solution; its chemical formula is (NH2 CH2COOH)3H2S0.t. It is hygroscopic and rather fragile, so that special precautions are necessary when i t is being processed, and its lower Curie temperature IS a major disadvantage, particularly

PYROELECTRIC MATERIALS 1 I 7 -7 - - 1

P Z L T ' TGS PVDF I SBN I

rP 110 4Cm2K 1 3 '5 1.8 2.8 0 3 , 6 5 7

Er t 250 54 1 38 F0-13.. , t - -1

*. +- ~ * __ --i ~. -- &-

tan 6 0,005 0.0031 0 .01 0.03 i - -- - ' . _

S 2.6 3.3 2.3 ~ 2.4 2-3 c --f - 4-- 4 1 106Jm-3K')

Tc 200 1620 I 49 lzl00 116

P ' 10e-1011 1013 ' 1013 1014 ' 1010 ~

TABLt I

p-

t - - _._ - _ _ 1 "c

i nml - - 1 L - - l - -

for detectors which are required to meet military specifications. In spite of these problems, how- ever, TGS remains the favorite material for appli- cations where sensitivity is of prime importance; i t offers the highest detectivity for all but relatively small detectors operating at low frequencies.

A number of variants on pure TGS have been d e ~ e l o p e d . ~ ~ ~ ' ~ ~ ~ ' These are generally intended to overcome the problem of the low Curie tempera- ture. Alanine doped TGS has a permanent bias to its polarization, so that repoling is not necessary following a temperature excursion above the Curie temperature. Deuterated TGS has a Curie temperature of 60"C, and can also be alanine doped. Triglycine fluorobcryllate has a Curie temperature of 74°C. but presents the additional problems associated with handling a water soluble beryllium compound, both in production and in discarded detectors.

9.2 Lithium Tantalate

Lithium tantalate, LiTaO,, gives inferior per- formance to TGS, due to its lower pyroelectric coefficient and slightly higher relative perinittiv- i ty . I t has the advantages, however, of a very high Curie temperature and insolubility in water. Detec- tors made of this material have responsivities which are independent of temperature over a wide range.

Good single crystals of Lithium tantalate can be produced by the Czochralski technique. Carc is required in processing because it is rather fragile in t h i n sections.

9.3 Strontium Barium Niobatc

Strontium Barium Niobate is, in fact, a family name for a range of solid solutions defined by the formula Srl-,Ba,Nbz06, in which x can bc varied from 0.25 to 0.75. The figures given in Table I are for x = 0.52; this composition gives good 'all round' performance, offering detectivities a factor of two worse than TGS for large detectors. but twice as good as TGS for small detectors operat- ing at low frequencies. Even better performancc of small detectors can be obtained by decreasing the value of x, but this also produces a reduction in Curie temperature and inferior performance of' large area detectors.

Thus SBN offers a range of properties t o suit a variety of detector sizes and applications. Like l i th- ium tantalate. it is produced by the Czochralski

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200 S. G . PORTER

technique, Gut good quality single crystals large enough for pyroelectric detector production are relatively difficult t o grow.

9.4 Pyroelecrric Ceramic

There is a vast range of ceramic materials which consist of solid solutions of Lead Zirconate, Lead Titanate, and a variety of- similar oxides. These have been developed over a period of many years to satisfy a variety of ferroelectric, piezoelectric, electro-optic and pyroelectric requirements. I n general the most useful pyroelectric materials may be described as modified Lead Zirconate (PZ), and the figures listed in Table 1 are for a typical PZ composition.32

The performance of PZ is generally comparable with, or better than. lithium tantalate, except in the case of large area detectors.

One of the major advantages of a ceramic over a single crystal is that i t is relatively easy to pro- duce large blocks of uniform material. This is generally achieved by the hot-pressing technique.

These blocks can be cut, lapped, polished, and diced by processes very similar to those employed i n the manufacture of semiconductor devices. There is no need to consider orientation during this processing since the ceramic can be poled in any desircd direction by the application of a suit- able electric field at an elevated temperature.

Another important advantage of PZ is that its electrical conductivity can be controlled by small modifications to the composition,2' without seri- ously affecting any of the other properties. Resis- tivities covering the range 10'IZm to 10"Rm can readily be obtained.

This electrical conductivity means that FET bias resistors may be omitted from detectors, giv- ing a very significant advantage in the manufac- ture of arrays of small elements, and a reduction in the cost of all detectors. The conductivity of the ceramic increases with increasing temperature, compensating for the increase in leakage current of the FET.

9.5 Polyvinylidene Fluoride

PVDF is a plastic film very similar in appearance to polyethylene. I t is a valuable pyroelectric mate- rial when large area devices are required. PVDF is often used for laser pulse energy monitors, where a large detector is necessary to ensure that the whole of the laser beam is collected.

The performance of PVDF as a detector is infe- rior to the other four materials listed here, except f-or very large detectors operating at high frequen- cies. It is, however, an obvious candidate for very low cost detectors since i t is readily available in large thin sheets which do not require the slicing, lapping and polishing processes necessary with other materials. It does, however, present its own handling problems in small areas.

10. CHOICE O F MATERIAL

The choice of material for a pyroelectric detector is not always a straightforward matter; i t depends on the size of the required detector and the in- tended frequency of operation. Environmental conditions (particularly temperature range), max- imum incident power and cost must also be considered.

In order to obtain an indication of the relative merits of different materials specific detectivities have been calculated using equations 9, 11, 19 to 25 and 27 with the values listed in Table 1. This has been done for the frequency range 0.1 H z to 10k Hz and for three detector areas: 100 mm', 1 mm2 and 0.01 mm2. A thickness of 30 pm has been as- sumed and the values of and R have been taken as 1.0 and 5 X l O " l 2 respectively. The tempera- ture is taken as 290 K.

The thermal conductance, GT, is assumed to be proportional to the detector area, i.e., GT = gA, a n d g has been assigned the value 1.8 X lo3 WK-'m-2. This value accords with that which is found in typical detectors mounted on alumina substrates.

The F E T model is similar to a Texas Instruments B F 800. This has a typical current noise of 3 X 10-lh A Hz-'" and a voltage noise which is approx- imated by the formula:

w

The input capacitance is approximately 1 pF. The results of these calculations are shown in

Figures 9 to 11. The situation is quite straightfor- ward in the case of the 100 mm' detector (Figure 9), with TGS, LT, SBN, and PZ appearing neatly in order at all frequencies. PVDF, however, fits the pattern only a t low frequencies, becoming rel- atively better as the frequency is increased. Com- parison with the other areas suggests that PVDF will be better still for even larger detectors.

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GUIDE TO PYROEI.EC7‘RIC DFTECTORS 201

In the case of small detectors (0.01 mm’), Fig- ure 1 I , the top place is taken by SBN with PZ and T G S taking second place a t low a n d high fre- quencies respectively.

For detectors of the most commonly occurring areas. around 1 mm’, Figure 10 shows that the sit- uation is quite complicated. with n o one material being best for all situations.

It must be emphasized that these results can only be taken as a guide. The picture would be a little different if a n alternative F E T were used; for example larger detectors often perform better with larger FETs. Changes also occur if the thickness is changed and if different values of R and g are used.

As a general rule it can be said that the major- it? of applications can be satisfied by the ceramic PZ. This offers reasonable performance together with cost. Its advantages are best displayed in production line environments, where large numbers of devices need to be produced economically, and in the production of arrays of small elements, where the elimination of bias resistors is of gen- eral benefit.

11 . DEVICES

As alreadq explained a pyroelectric detcctor con- sists essentially o f a chip of pyroelectric material with 11 pair of electrodes, connected to a suitable high impedance amplifier. such as a junction field effect transistor ( JFET) . A suitable biss current is provided for the J F E T by a high value resistor o r the inherent conductivity of the pyroelectric mate- rial. The performance of the detector is affected by a variety of factors including the pyroelectric material used, the dimensions of the detector chip, the method of mounting this chip, the value of the bias resistor. and the type of J F E T used. Since the detector is usually encapsulated in some way. con- sideration must also be given t o the inlrared transmission of the window and the nature of the enclosed atmosphere.

11.1 Single Element Detectors

Consideration must be given t o both responsivity m d noise. At high modulation frequencies the respon- sivity is generally inversely proportional t o fre- quency, but a t low frequencies the responsivity drops below this due t o the electrical a n d thermal time constants of the circuit (section 4.2). Thus

1 10 lo2 lo3 lo4 Frequency (Hz)

I-IGURE 9 100 mm’ detccton

I-requcnc\ dependence of specific dctccilvity tor

.ILL 1 .L L 1 A w l ULLd L , L d L J , L I I1 / , 111

lO0.1 1 10 lo2 103 104 Frequency (Hz)

FlGlJRE 10 Frequencb depentlencc of \PCCIIIL ~ C I C L ~ I \ I I \

for I mm’ detector.,

logF A = 0.01 mm2

61_LLi;LUL-uLYlli! I I L , U L ...l.LllL,,LL-, i .:.A ’O0.1 1 10 102 103 104

Frequency (Hz) FIGtJRE 1 I lor 0.01 mm’ detectors.

Frequency dependcncc ol‘ specific detcctivity

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202 S. G. PORTER

for low frequency work the detector must be well insulated from its enclosure, and the bias resistor must be as high as possible. The major source of noise depends on many factors (see Figure 8). Generally, at low frequencies the J F E T current noise and bias resistor noise are most significant, at higher frequencies the dielectric noise is impor- tant and at very high frequencies the J F E T volt- age noise must be considered.

In many cases (but not always) an.improvement in performance can be obtained by making the pyroelectric chip thinner. This gives a reduction in dielectric, resistor, and current noise without af- fecting the responsivity (except a t low frequen- cies). Considerable effort has therefore been de- voted recently to making very thin section pyroelectric detectors.

Detectors of lithium tantalate have been made only 2 pm thick by employing ion-beam milling techniques," and detectivities approaching 2 X lo9 cm Hz' 'W- ' at 10 Hz are claimed.

11.2 Compensated Detectors

In a stable environment thermal noise is small compared with the four noise sources mentioned above. If , however, a detector is used in an envi- ronment where the ambient temperature is fluc- tuating o r the background temperature which the detector is viewing is not stable, then large un- wanted 'noise' signals will be produced. In order to overcome this problem, compensated detector configurations have been developed. ''

Compensation is obtained by connecting two oppositely polarized detector elements either in series or in parallel (the two elements must be of equal area if connected in parallel) but only ex- posing one element t o the infrared radiation to be detectcd. One method of achieving this is illus- tratcd in Figure 12.

Since environmental changes affect both ele- ments alike, the signals from the two elements cancel each other, and no output is observed. Greater efficiency can be obtained where a mov- ing target is viewed by arranging the system so that the image moves from one element to the other.

11.3 Multi-element Arrays

For certain applications, such as thermal imaging or laser beam profile monitoring, i t is advantage- ous to use a detector consisting of a n array of small element^.'^-^' It is possible to define photo-

- Radiation -

Screen

r- -D- Amplifier

FIGURE 12 Compensated detector.

lithographically an array in which each element is as small as 100 pm square." If electrically con- ducting pyroelectric ceramic is used then FET chips can be mounted directly onto the slice of pyroe- lectric. The production of an array of this size would be much more difficult with a nonconduct- ing pyroelectric requiring separate F E T bias resis- tors which are typically 1.3 mm X 0.6 mm. Apart from the problem of fitting in the resistors, the array performance would be degraded by the in- creased stray capacitance and microphony due to bonding wires.

11.4 The Pyroelectric Vidicon

One application for pyroelectrics which has re- ceived considerable attention in recent years is the pyroelectric v i d i ~ o n . ~ ~ ~ ~ ' * ~ " This consists of a vidicon tube with a pyroelectric target and germanium faceplate.

The target consists of a disc of pyroelectric material, with a transparent electrode on the front surface. An infrared lens produces a thermal image on this target and the resulting charge dis- tribution is read off the back surface by the scan- ning electron beam. Good thermal images are now attainable using vidicons with T G S targets.

The major factor limiting the resolution achiev- able with pyroelectric vidicons is thermal diffu- sion within the target.4' This causes the thermal resolution to degrade rapidly as the spatial fre- quency increases. Fo r this reason reticulated targets are being developed for use in pyroelectric

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GIJIDF TO PYROELECTRIC DtzTECI ORS 203

vidicons."." Present vidicons achieve 0.2"C reso- lution in a n image consisting of 100 TV lines, whereas reticulated targets should produce similar resolution in images with 200 TV lines or more.

12. APPLICATIONS

There are two characteristics of pyroelectric detec- tors which are of particular significance when considering possible applications. The first is that pyroelectric detectors respond only t o changes in incident radiation. Once thermal and electrical equilibrium have been achieved, a constant inci- dent flux produces n o change in the temperature of' the pyroelectric and therefore n o electrical out- put. Thus pyroelectric detectors are useful for de- tecting small changes in a relatively large back- ground level of incident energy.

The second characteristic of note is that a pyro- electric detector can be used t o detect radiation of any wavelength. The only qualification is that a t least some of the incident radiation must be ab- sorbed by the detector. Pyroelectric detectors a re m a t commonly used in the infrared region of the spectrum, where it is possible t o achieve respon- sivities which a re independent of wavelength from 0.3 pm to beyond 25 pm. Pyroelectric detectors have been used for both microwaves and X-rays.

12.1 Intruder Alarms

The largest market for pyroelectric detectors a t the present time is intruder alarms, in which in- dustry many thousands of detectors are consumed each week.

In some ways this is an ideal application for pyro- electrics. In the absence of an intruder the interior of an unoccupied building presents a fairly con- stant thermal scene. When a n intruder enters however. he will normally be at a different temperature to his surroundings and will produce ;I change in the flux of infrared radiation incident on a detector viewing the scene.

I n order t o improve the efficiency of the sys- tems. the detector is usually arranged so that it views the scene via a faceted mirror which divides the area t o be protected into a number of zones (Figure 13). As an intruder moves through these zones a n alternating signal is produced by the de- tector, usually in the frequency range 0.1 Hz t o 10 Hz where the pyroelectric detector works very well.

FIGURE 13 Simplc intruder illariii

intruder alarms normally operate in the infrared wavelength range 8 pm-14 pm, around the emis- sion peak a t 10 pm for 300 K black bodies. By using a filter which blocks all radiation a t wave- lengths shorter than 6 or 7 pm it is possible t o make the detector insensitive to radiation trans- mitted through windows, e.g., sunlight.

For outdoor use, single beam systems are avail- able which can detect a man moving 100 meters away. These use a parabolic mirror or infrared lens giving a field of view of approximately two meters a t 100 meters range. Outdoor systems present the problem of sometimes rapidly varying ambient temperature, for example in windy condi- tions. If a simple single element detector is used. these temperature variations can produce false alarms. It is therefore common practice to use a compensated detector arrangement in which two elements are connected in opposition so that thermal drift signals are cancelled out , but only one element is exposed to incoming radiation from a particular point in the scene.

12.2 Fire Alarm5

Pyroelectric detectors are also used in fire alarms. although not in such large numbers as intruder alarms. For detecting fires it is normal to operate at shorter wavelengths than those used for in- t ruder alarms. typically a round 4 pm. They are normally designed to give a n alarm only when they sense a varying signal corresponding to the typical flicker frequency of flames, usually within the range 5 Hz t o 40 Hz. In this way alarms caused by the sun shining through windows can be avoided.

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204 S. G . PORTER

12.3 Pollution Monitoring and Gas Analysis

A common method of detecting the presence of a particular gas is to look for its absorption spectra. For example, COZ gas absorbs 4.3 pm radiation very strongly so that small amounts of COZ can be detected by viewing an infrared source through a sample of the gas and a 4.3 p m narrow band fil- ter. The intensity of radiation detected is an in- verse measure of the concentration of COZ pres- ent. Other gases have different absorption lines so that selective monitors can easily be produced.

If the infrared source is modulated at a suitable frequency e.g., 30 Hz, then pyroelectric detectors can be used to monitor the transmitted energy, and hence the gas concentration. The appropriate filter can be incorporated in place of the detector window. It is common practice to simultaneously rnonitor a reference channel with none of the rele- vant gas present. A ratio measurement can then be made, eliminating errors due to variations in source output.

12.4 Radiometers

In principle it is very simple to make a radiometer using a pyroelectric detector. All that is required is a lens or mirror to define the field of view and a chopper to modulate the incoming radiation. There are, however, a variety of problems requir- ing careful consideration, some of which a re men- tioned below.

Since the detector measures the difference in radiation coming from the chopper and the scene, it is important that the temperature of the chopper is carefully monitored.

If i t is required to measure the temperature of a source, then its emissivity must be known at the wavelength being monitored (usually around 10 pm).

The detector will give the same amplitude out- put for a given temperature difference whether the source is hotter or colder than the chopper, so that a phase sensitive detection system is required to remove this ambiguity.

12.5 Thermal Imaging

It is very useful in certain circumstances to be able to produce a visible image from infrared radia- tion. For example, it is possible to produce recog- nizable pictures of human beings in the dark by means of their emitted infrared radiation. As is mentioned in section 10.4, the pyroelectric vidicon

allows images to be produced with thermal reso- lutions of the order of 0.2”C. Alternative ap- proaches include optical scanning, by flapping mirrors for example, of a scene by a linear array of detectors.

12.6 Laser Detectors

The use of lasers is rapidly becoming more wide- spread, and the demand for suitable detectors is increasing. Pyroelectric detectors are particularly suitable for COz, HCN and other lasers which produce long wavelength radiation which cannot be detected by silicon diodes.

I n principle any pyroelectric detector can be used to detect a CW laser beam, provided that it is modulated a t a suitable frequency. Care must be exercised, however, to ensure that the detector and its following amplifier are not destroyed by too much incident power.

For pulsed lasers special pyroelectric detectors have been and rise times faster than Ins have been demon~t ra t ed .~ ‘ These detectors generally consist simply of a pyroelectric chip shunted by a 50 ohm resistor. The voltage across the resistor is then a measure of the current re- sponse, and gives a faithful reproduction of the laser pulse shape within the limits set by the elec- trical time constant of the circuit. In order to de- crease this time constant and to improve the power handling capacity of the detector, edge electrode detectors are usually used. These consist of a thin slice of pyroelectric with electrodes on opposite edges instead of opposite faces. The ca- pacitance of this device is much lower than that of the face electrode type, giving a shorter electrical time constant, and the tendency for the laser pulse to evaporate the front electrode is overcome,

Two problems peculiar to pulsed laser detection are the piezoelectric effects in the pyroelectric, which often produce a “ringing” effect,“ and elec- tromagnetic interference from the discharge which produces the laser pulse. The latter effect often necessitates the use of separate screened rooms for the laser and the detector, with the laser beam passing through a small hole between the two.

13. FUTURE DEVELOPMENTS

The discovery of any new pyroelectric materials offering dramatically improved performance is unlikely, although marginal improvements may be obtained by the use of different compositions or

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GUIDE TO PYROELECTRIC DETfiCTORS 205

dopants. Any significant improvement in per- formance must therefore be obtained by improv- ing the detector construction.

Thc use of ion beam milling techniques has made possible thc production of detectors as thin its 2 l n i and there is little doubt that this process will be exploited more extensively in order to ob- tain devices with the highest possible detectivities.

I n order to realize the full potential of these de- vices. however, it will be necessary to give careful consideration to the other components, especially the FET. For example, it may be that a M O S F E T would give better performance than a J F E T , in certain circumstances a t very low frequencies.

Ion beam milling has also been used to reticu- late vidicon targets with a network of fine grooves (see Scction 11.4). This reduces the lateral thermal diffusivity and so increases the resolution of the device. Similar ion beam milling techniques may be applied to pyroelectric arrays. Isolation of the separate elements by narrow grooves through the material will decrease the crosstalk between adja- cent elements, particularly a t low frequencies.

The availability of a n electrically conducting pyroelectric material offers the possibility of mak- ing two dimensional arrays for applications such as thermal imaging. The major problem is making contact to all the elements in order to read out the signal. F o r this reason there is much interest at the present time in combining pyroelectric arrays with charge coupled device^.^^^^^^^'

Charge coupled imaging devices are already available for visible wavelengths, and it is a logi- cal step to develop similar devices for the infrared region. Hybrid devices, consisting of a two dimen- sional pj roelectric array bonded directly on top of a CCD multiplexer. have been predicted to give a thermal imaging performance better than pyroe- lectric vidicons and possibly approaching that of present systems using cooled detectors.") There are. of course. problems to be overcome in con- necting the two components together, both mechan- ically a n d electrically, and it is likely to be a few years before a pyroelectric CCD camera is available.

The use of pyroelectrics for detecting high speed laser pulses is likely t o increase as the use of lasers continues t o increase. This will lead to the development of better and faster detectors with more attention being paid t o the elimination of piezoelectric interference. Attention must also be given to the development of matching low noise amplifiers. with bandwidths of 1 G Hz or more.

REFERENCES

I . D. Brcwster, Edinburgh. J. Scc.. 1, (1x24). 2. W. G. Cady, Piezoelectricity. 699, McGraw Hill. Net+

3. .I. C. Burfoot, Ferrorlrctrics. 49, van Nostrand. London

4. I,'. P. Jona and G. Shirane. Frrroe/eetric. Cry.stal\, 10.

5 . S. B. Lang. Fcrroclectrics, 7, 231 (1974). 6. H. P. Beerman. Am. Ceram. So<. Bull.. 46, 737 (1967). 7. E. H. Putlcy. Seniiconductors and Semimetul.r. 5, 259 Aca-

8. E. H. Putley. Semiconductors and Semimetals. 12, 441

9. A. van der Zcil. .J. Appl. Phys.. 44, 546 (1973).

York (1946).

(1967).

Macmillan, New York (1962).

demic Press. New York ( 1970).

Academic Press. New York (1977).

10. R. M. Logan and K . Moore. Infrared P/ij,.yics. 13, 37

11. R. M. Logan. Infrared Phpics. 13, 91 (1973). 12. W. R. Blrvin and J . Geist. Appl. 0ptic.l. 13, 1171 (1974). 13. R. 1. Peterson. ci. W. Day. P. M. Gru/t.n\ky. a n d K. .I.

Phelan. Jr.. ./. AppI. P/rj-s.. 45, 3296 (1974). 14. S. T. Liu and D. Long. Proe. /EEE, 66, 14 (1978). 15. R . Clark Joncs. Advan. Electran., 5. 1 (1953). 16. B. 1. Bleaney and B. Bleaney. l~lcctrici/j. und .b'a,qneri.sm.

236, Clarendon Press. Oxford (1965). 17. J. B. h h n s o n . Phys. Rev.. 32, 97 (1928). 18. G . 1 . Deboo and c'. N . Burrous. I n r c y y a t d C'rrc.uir\ and

Semiconductor Dcvice.~: Theor}' and A p p l i ~ ~ t i ( ~ ~ ~ , McC;r;i\\- Hill 48 (1971).

19. I-. G. Mundic. SPIE. 132, 36 (197X). 20. R. C. Jones. Acivunces in Electronic.$. 5, I Academic. NCH

York (1953). 21. R. C. Joncs. Proc. Inst. Radio Engrs.. 47, 1495 (19591. 22. H. P. Bcerman. 1erroeIrctric.s. 2, 123 (1971). 23. A. M. Glass. Pl7w Rev., 172, 564 (196X). 24. C. B. Round! and R. I.. Hqer. .I. Appl. 1'lij.s.. 44, 929

25. A. M. Glass. .I. Appl, Ph,vs., 40, 4699 (1969). 26. R. J. Phelan. Jr.. R. J . Mahler. and A. R. Cook. ;Ippl.

Phys. Lett., 19, 337 (1971). 27. P. Buchman. Fcrroeleerric.s, 5, 39 (1973). 28. R. J. Mahlrr. R. J. Phelan. Jr.. and A. K. Cook. Infrared

29. S. G. Porter. D. Appleby. and F. W. Aingcr. / ' e r r o d e - trics, 11, 351 (1976).

30. E. 1. Kcce, K . L. B)e. P. W. Whipps. and A. D. A i i n i ~ .

Ferroelectrics. 3, 39 ( 197 1 ). 31. G . Baker. D. E. Charlton. and P. .I. Lock. 7 h ~ Radio and

Ektron ic Engineer. 42, 260 ( 1972). 32. The Plessey Co. Ltd.. British Patent 1514472 (1978). 33. N. E. Bycr and S. E. Stokowski. Martin Marietra Luhor-

34. N. E. Byer. S. E. Stokawski, and J . D. Vcnablss. Appl .

35. S. G. Porter. The Radio and Electronic Enxinerr. 49, 504

36. C. B. Round?. Applied Optics. 18, 943 (1979). 37. D. W. G . Byatt. Electronicsand Pouer. Ma! 1979 351. 38. R . B. Holeman and W. M. Wrciithall. .I. /%i,.s. I): Appl.

39. M. F. Tompsrtt . lEEE Trans. I;Iectron. I ~ T . . 18, 1070

40. E. H . Putlcy. R. Watton. and J. H . Ludlow. I-iworIectric.\.

( 1973).

( 1973).

Phys.. 12, 57 (1972).

atories Trchnical Report 76-30 ( 1976).

f'hys. Lett . . 27, 639 (1975).

(1979).

Phys., 4. 1898 (1971).

(1971).

3, 263 (1972).

Dow

nloa

ded

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Uni

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ity o

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7:38

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206 S. G. P O R T E R

41. R. Watton. Ferroelectrics. 10, 91 (1976). 46. C. B. Roundy, R. L. Byer, D. W. Phillion. and D. .I. 42. R . Watton, Infrared Physics, 18, 73 (1978). 43. R. Watton, D. Burgess. and P. Nelson. Infrared Physics.

19, 683 (1979). 44. J . Cooper, J. Sci. Instrum.. 39, 467 (1962).

Kuiienga, 0p;ic Communications, 10, 374 (1974). 47. D. E. Marshall, SPIE, 132, 110 (1978). 48, c. B. Roundy, ,nfrared,,hyhysics, 19, 507 (1979). 49. S. Iwasa, J. Gelpcy, and K. Hartnett. Ferroelecirics. 27, 9

( 1980). 45. A. M. Glass, AppL fhys. Letr.. 13, 147 (1968). 50. E. H. Putley, Private Communication.

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