Thallium selenide infrared detector

P. S. Nayar and W. 0. Hamilton

The application of semiconducting thallium selenide for ir detection is described. A responsivity of 106 V/Wand NEP of the order of 10-15 W/I/Hz with a response time of 3 msec can be obtained by operating the de-tector at 1.5 K.

Introduction

Helium cooled bolometers are often the best detectorsfor the far ir. A number of liquid helium cooled semi-conductor bolometers have been developed. 1 6 For thebolometer to be efficient, its operating characteristicshave to be optimized. The bolometer response speeddepends on its thermal mass and thermal conductanceto the helium bath. Some results of a new far ir bo-lometer detector using semiconducting thallium sele-nide as the sensing element were published elsewhere.5 6

In this paper the results of further investigations onimproving its speed are presented. It is shown thatproper design of the optical system can improve theperformance of the detector.

Experimental

The essential features of the experimental arrange-ment are shown in Fig. 1. This arrangement is signifi-cantly different from that reported earlier.5 The lightpipe consists of a polished thin wall stainless steel pipeand a cone made of copper polished inside. Use of thestainless steel considerably minimizes the heat con-ducted from the room temperature end of the light pipeto the liquid helium bath. The bolometer element isp-type thallium selenide. It has dimensions of 3 mmX 1 mm X 0.18 mm. Gold wires of 0.12-mm diam werefirst welded to the ends of the crystal. The ends of thecrystal were also coated with indium. Two copper leadssoldered to the gold wires were heat sunk to two heliumwells in the top flange of the vacuum can. Use of thesetwo helium wells was found to increase the thermalconductance an order of magnitude better than whenthe leads were wound on a pair of copper studs solderedto the flange.5 6 For this reason this design is adoptedhere and in the subsequent investigations. The re-quired thermal conductance was obtained by adjustingthe length and dimensions of the copper leads.

The authors are with Louisiana State University, Department ofPhysics & Astronomy, Baton Rouge, Louisiana 70803.

Received 1 November 1976.

Results and Discussion

The temperature dependence of the resistance of thebolometer between 1.5 K and 4.2 K is presented in Fig.2. This curve could be fitted to the empirical relationof the form

R = R.(T 0/T)A, (1)

where R is the resistance of the bolometer element attemperature T and R0 is the resistance of the elementat temperature T The value of the exponent comesout to be 4.

A detailed analysis of the bolometer characteristicsis given elsewhere. 5 For negligible background radia-tion, compared with the Joule heating produced by aconstant current bias, the maximum responsivity (Smax)is found5 to occur at T/T, = 1.112, where T is the tem-perature of the bolometer when biased at a constantcurrent and To is the temperature of the heat sink.Under these circumstances

ISmaxI = 0.7(R0 /ToG) 1/2, (2)

where G is the thermal conductance between the bo-lometer and the bath and is given by

G = P/(T- T.), (3)

where P is the power dissipated in the bolometer dueto the bias current. The temperature T of the bolom-eter is determined using the curve in Fig. 2 and thecurrent voltage curve given in Fig. 3. The response timer is given by 5

r = 0.7(C/G), (4)

where C is the thermal capacity of the element.The usefulness of a radiation detector is determined

by its ability to detect the minimum value of the signalpower. This is referred to as the noise equivalent power(NEP), defined as the signal power required to make theSNR unity. The total noise power in watts for a cooledbolometer in a unit bandwidth centered around a fre-quence < 1r is given by7

2942 APPLIED OPTICS / Vol. 16, No. 11 / November 1977

Fig. 1. Schematic of the experimental arrangement.

140

120 -

100 -

80 I_

60 -

40 _

20 _

0 1 2 3 4 5

TEMPERATURE (K)

(NEP)2 = 4kT°R + 4kT02G + 8aekaT0

5

Smax2

+ 8rEaTR5 sin2 (0/2) + 2(5)WSmaxc

where k is the Boltzmann constant; is Stefan-Boltzmann constant; e is the emissivity; a is the area ofthe sensing element; TR is the temperature of thebackground viewed through a solid angle; 0, C, a, and,3 are constants, and I is the bias current.

The first term in Eq. (5) is the Johnson noise associ-ated with the bolometer resistance. The second termis the phonon noise power due to the statistical fluctu-ation of the bolometer bath temperature. The thirdterm arises from the random fluctuations in the emis-sion of the sensing element. The fourth term arisesfrom the statistical fluctuations in the background ra-diation. The last term arises from the possible contri-bution due to current noise and crystal imperfections.This term is appreciable at large current bias. Theimportance of the last term in Eq. (5) was checked byvarying the bias current up to a maximum of a few mi-croamperes. No significant change in NEP was ob-served, indicating that the last term is extremely small.Since the bias current at the operating point is of theorder of 10-7 A, the last term in Eq. (5) is neglected.

For an ideal bolometer the responsivity is very largeand the thermal conductance is very small. Underthese circumstances only the third and fourth terms aresignificant. In practice, however, the thermal con-ductance is never negligible; and, hence, the second termis larger than the third and fourth when To is compa-rable with TR. When TO << TR, the third term van-ishes; and the contribution from the background ra-diation becomes important. Thus, in the absence of thethird term, the first two terms constitute the inherentNEP of the bolometer, which is characteristic of theelement and its configuration. This can be evaluatedseparately using Eq. (2),

(NEP)bol 4T.(kG)'/ 2 . (6)

Fig. 2. Temperature

12

'oH-

a0

dependence of the resistance of thebolometer.

0 10 20 30 40 50 60

VOLTAGE (mV)

Fig. 3. Current voltage curve of the bolometer at a bath temperatureof 1.5 K.

For a given NEP, Eq. (6) gives a suitable value of G.The area and thickness could be varied to obtain theminimum response time. The responsivity and re-sponse time constant was measured experimentally ina similar manner described in the earlier paper.5

Typical values of the responsivity and other parametersare given in Table I. The major improvement in thepresent case is the lowering of the response time to 3msec instead of 8 msec reported earlier.6 The NEP ismaintained constant. The reduction in response timeis achieved mainly due to the better heat sink. It is tobe mentioned that no correction for the transmissionof the radiation through the stainless steel pipe is ap-plied. The window and electronics used, in the presentcase, are the same as that reported earlier.5' 6 It is worthmentioning that the response time could be lowered toabout 200 Asec if a chip of about 10 Am thick were used.However, at present no data about the absorption to thefar ir are available beyond 24 gm.8 Attempts are beingmade to measure the absorption using a lamellar gratingFourier transform spectrometer of our own design. It

November 1977 / Vol. 16, No. 11 / APPLIED OPTICS 2943

Ijo00

C),z

I-V)w)

I I I I

l I I I

is to be stressed that a proper design of the system canimprove the performance of the detector considera-bly.

Table I. Characteristics of the Bolometer at a Bath Temperature of 1.5 K

TO 1.5KRO, 137 KgRL (series resistance) 10 MgA 4G 2.4 X 10-8 W/KSmax (calc.) 1.4 X 106 V/WSmax (meas.) 1.2 X 106 V/WNEP (calc.) 3.47 X 10-15 W/IVHzNEP (meas.) 7.3 X 10-15 W/VHzr (meas.) 3 msec

This work was supported by the Air Force Office ofScientific Research under grant AFOSR71-2054.

References1. F. J. Low, J. Opt. Soc. Am. 51, 1300 (1961).2. S. R. Zwerdling, R. A. Smith, and J. P. Theriault, Infrared Phys.

8, 271 (1968).3. H. D. Drew and A. J. Sievers, Appl. Opt. 8, 2067 (1969).4. R. Bachman, H. Kirsch, and G. H. Geballe, Rev. Sci. Instrum. 41,

547 (1970).5. P. S. Nayar, Infrared Phys. 14, 31 (1974).6. P. S. Nayar and W. 0. Hamilton, J. Opt. Soc. Am. 65, 831

(1975).7. F. J. Low and A. R. Hoffman, Appl. Opt. 2, 649 (1963).8. G. A. Akhundov and T. G. Kerimova, Phys. Status Solidi 16, K15

(1966).

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2944 APPLIED OPTICS / Vol. 16, No. 11 / November 1977