Photoelectric solid-state devices and the perception of images in the infra-red

264 PHLlPS TECHNCAL REEW. OLUME 30 Photoelectric solid-state devices and the perception of images in the infra-red F. Desvignes, J. Revuz and R. Zeida ntroduetion t is generally realized nowadays that-the observation of images in that part of the infra-red region of the spectrum in which objects at normal temperatures radiate strongly, namely from 3 to 30 microns, is of considerably practical interest. This region can be used for military applications, some of which are fairly obvious, as well as for numerous scientific, medical and technical purposes.. For many years now, this problem has been investigated at LEP in a number of different ways. The problem is very different from that of image perception in the ultra-violet, visible and even near-infra-red regions (up to 2 (lm) so that it will be useful to briefly recall here its essential points. At ordinary temperatures, bodies emit approximately 150 watts per square metre and per steradian (or 10 22 photons per m 2 sr s). Less than 210000 of this power - or fewer than 3 photons per million - is found in the wavelength region lower than 3.2 microns; if the threshold is raised to 5.5 microns, then one obtains 2 % of the power and 0.2 % of the photons, and with a threshold at 12 microns these figures become approximately 25 % and 15%. These wavelengths have not been chosen arbitrarily: they correspond to the thresholds of the photoelectric materials indium arsenide (3.2 (lm), indium antimonide (5.5 (lm), mercury-doped germanium and cadmiummercury telluride (both 12 (lm). t should be recalled that the atmosphere has pronounced absorption bands between 2.6 and 2.8 {lm, between 5.5 and 7 {lm and beyond 14 {lm. Another important point is that in an almost isothermal enclosure, such as a closed room, the contrasts in the infra-red scene are very low. The relative emission difference between twobodies at different temperatures is at a maximum if they are "black bodies" and depends only on the difference between their respective temperatures. f the total emitted energy is considered, this relative difference is 1.3 % for a temperature difference of 1 C. When it is recalled that the minimum luminance difference perceptible to the eye (in the visible spec- F. Desvignés, ngenieur E.S.O., ngenieur C.N.A.M., and J. Revuz, L. ès Sc., are with Laboratotres d'electronique et de Physique -Appliquëël Limeil-Brëvannes (al-de-marne), France. F. Desvignes is aso a Professor at the Ecole Superieure d'optique. R. Zeida, D. ès Sc. Appl., formerly with these Laboratories, is now with the Faculdad de Ciencias, Universidad de Buenos Aires. trum) is 1-2 %, and realized that for the majority of applications it is desirable, for example, to perceive temperature differences of O.l C, it will be appreciated that the instruments required must have a differential sensitivity which is 10 to 20 times better than that of the eye. There is a further difficulty caused by the fact that the image to be observed must be produced by an infra-red optical system whose relative aperture and transparency are rather limited and that, unless special precautions are taken, the low-contrast image provided by the objective will be swamped by the ambient radiation. The precautions in question consist in protecting the photoelectric body by nonradiating (i.e. absorbing and cooled) screens. The most natural course - which is also the most ambitious - is to try to use in the infra-red the solutions which have proved best in the visible region ofthe spectrum, i.e. to produce vidicons; successful attempts along these lines are described in another article in this issue [11. So far these attempts have led only to the production of tubes whose photoelectric threshold does not exceed 2.5 {lmand which are not capable of detecting bodies whose temperature is lower than 400 OK. Theory and practice show in fact that with a normal television standard the target must supply 10 10 photoelectrons per second or 1 na to give a detectable signal, and 10 12 photoelectrons per second or 0.1 {la to give an acceptable picture with 40 or 50 levels of contrast. With a quantum efficiency equal to 1 and an optical system whose aperture is equal to f2 (cf. [21), calculation shows that this last condition can only be satisfied for a scene at normal' temperatures if the threshold is beyond 3 microns. On the other hand, there is a drawback in choosing a threshold which is too high, for the signal is then so large that there is a risk of it exceeding the capacity of the target to store information (maximum current: several tenths of a microampere). f the threshold is at 5.5' {lm, the radiation at ordinary temperatures contains 500 times more useful photons than at 3.0 {lm. n order to preventthis excess it is necessary to use a cold diaphragm with an aperture equal to 140, or to use a target whose quantum efficiency is only 21000. These things are not impossible to achieve but technical difficulties arise when it is necessary to detect very

.1969, No. 8910 SOLD-STATE DECES FOR NFRA-RED PERCEPTON 265 weakcontrastscorrespondingto température differences of several tenths of a degree in the vicinity of 20 oe. H can be shown that these difficulties are reduced if thresholds lower than 5.5 flm are used, because the contrast is then slightly better. With these considerations in mind we undertook a detailed study of the technology of P-N junctions, first of nsb (threshold about 5.5 flm) and then of nas (threshold about 3.2 flm); our aim was not to produce photodiodes but camera tubes of the type described in referencel-l, with a target consisting of a mosaic of P-N junctions. As will be seen, the results obtained are perhaps too good for the production of photodiodes but not quite good enough for the targets of camera tubes. An alternative and well-known method of observing infra-red images uses a single detector with a small sensitive surface and a high response speed. By sweeping the two-dimensional infra-red image past this detector in two directions in accordance with a certain scanning code by opto-mechanical means, the image is analysed dot by dot. Systems ofthis type are described in this issue [3] and possess the great advantage that they completely eliminate those faults in the final image which are due to local variations in the target sensitivity. However, they are restricted in their space- and timeresolving power by the high speed which they impose on one of the two mechanical movements required for scanning. We now believe that a solution halfway between the two preceding methods may prevail in the next few years. This solution consists of using a linear array of single-element detectors (a "cell-strip") in a system. with only one mechanical scanning movement. ts advantages over the two methods described above are threefold, namely, the sole remaining mechanical movevement can be relatively slow, the problem of storing the photoelectric information in the target no longer occurs in certain cases, and local sensitivity variations can be compensated in a relatively simple way. The "cellstrip" ofvery high quality indium-antimonide photodiodes, developed and produced by LEP, is, like the nas and nsb photodiodes themselves, a by-product of the work done during research on television camera tubes. While it is of advantage in the case of television camera tubes to use a photoelectric target whose threshold is not too far into the infra-red (situated at about 5 urn), is it preferable with single-cell detector systems to have a detector whose threshold is farther in the infra-red since it is better to receive as large a signal as possible. The designers have still not decided whether to choose detectors which are sensitive up to 12 flmor up to 5.5 flm, the relatively modest benefit given by the first (minimum detectable temperanire difference 3 to 4 times smaller) being offset by.more complicated cryogenic problems and therefore increased cost. n. any case, the lowerthe temperatures ofthe scenes to be observed, the greater the importance of the larger wavelengths. Conditions to be satisfied for the target ofthe camera tube The charge-storage effect used in television camera tubes whose target consists of a mosaic of P-N junctions requires these junctions to possess very definite properties (see article [1]). These properties are the following:. a) very low dark current, b) break-down voltage as high as possible, ' c) high capacitance per unit area. The theory of the P-N junctions, borne out by experiments made here, underlines the importance of certain electronic and geometric parameters, affecting the production of junctions with the required properties. :rhus: ) The dark current can be considered as consisting of two terms: one depending on bulk effects in the semiconductor, the other on the surface properties. The current due to bulk effects will be small if the lifetime of the minority carriers is high,' if the thickness of the depletion layer is small and if the concentrations of the dopants are sufficiently high. The, contribution due to surface effects will be relatively small if the number of impurity ions and atoms per unit area that act as recombination centres is small.. Moreover, the crystal quality of the semiconductor can be responsible for a high dark current; in fact, defects such as dislocations, grain boundaries, vacancies, etc. are the origin of local break-downs in the neighbourhood of a junction. 2) A high break-down voltage and a thick depletion layer are related to a low concentration of impurities in the most lightly doped zone. ndeed, the electric field existing there must not exceed a value which is characteristic of each semiconductor material and for a given break-down voltage the field will be less for greater thickness. 3) High capacitances permit the storage of considerable charges for a given voltage change. The capacitance of the junction is linked to the thickness d of the depletion layer by the equation: C ex: d-ln, in whichn equals 2 or 3 depending on.the type of junction (abrupt or gradual). (J M. Berth and J.-J. Brissot, Targets for infra-red television camera tubes, page 270 of this issue.. (2] C. Hily, Objective lenses for infra-red image formation, page 290 ofthis issue. (3J M. Jatteau, nfra-red therrnography equipment for medical applications, page.278 of this issue.. '

266 PHLlPS TECHNCAL REEW OLUME 30 4) The photoelectric sensitivity of the junction is of pa,! 77 K, less importancein the problem which concerns us, since it is possible to make do with very low values if the 0.5 photoelectric threshold is situated appreciably beyond»e«: --nas r 3 [Lm. Thus the detection qualities of the junction will -1.5-1 -0.5 0 _., Q5 not be a vital point in a pilot study. -. -- -- re=" When all these points are taken into account it is -0.5 seen that the above important parameters must satisfy conditions which are sometimes contradictory. n particular, the concentration of impurities in the semicon- -pa ductor will have to be sufficiently high to increase the Fig. 1. Current-voltage characteristic of a P-N junction in nsb at a temperature of 77 ok. The dashed curve applies to rnas at 'capacitance value and decrease the intensity of the this temperature. dark current without, however, reducing the break-, down voltage of the junction to excessively ow values or influencing too strongly the life-time of the minority 1.5mA nas carriers. The choice of the parameters mentioned with - compounds is further restricted at the present time because it is not possible to obtain sufficiently pure 1.0 crystals (indium antimonide: minimum concentration of majority carriers 10 14 cm-3; indium arsenide: 300 0 K 2 X 10 16 cm-3). Technological problems raised by the making of!nsb and nas junctions Numerous technological problems are encountered at various stages when making P-N junctions of - compounds. They start with the sawing, cutting and polishing operations. nsb and nas are very fragile materials, so it was necessary to develop special mechanical methods to avoid disturbing the crystal structure of the semiconductor excessively. The various cleaning and etching processes which follow the mechanical operations demand particularcare; chemical agents were chosen after numerous tests designed to find the best results. n spite of all the precautions taken, additional cleaning is necessary as a final operation for nas in order to obtain the surface condition which is indispensable for the success of subsequent operations; a mechanical polishing operation with chemically active substances has been found suitable. The diffusion process used to obtain the two regions of different conductivity on either side of the junction must prevent both the loss by diffusion of the most volatile component and the introduetion of impurities into interstitial sites in the crystal structure of the atom. These problems are not completely understood but lengthyexperimentation has made it possible to apply the method of diffusion from the vapour phase using the elements zinc and cadmium and to define operational conditions such that junctions made from the diffused wafers have optimum characteristics. The óhmic contacts with the N andpregions are made using different.alloys one of whose constituents s~ould preferably be indium. Special precautions are taken -1.5-1 -a5 o 19~K]7 K 0.5 nas ~1.0 0.5-0.5-1.5mA 10flA 196 K )77 0 K -1.5-1 -a5 01J O.5 t- - ~ i.-> -,10{.lA )Ó:~~> Fig. 2. a) Current-voltage characteristics of a P-N junction in nas at three different 'temperatures. b) The curves for 77 ok and 196 "K on a scalemore similar to the one of fig. 1. a b

1969, No. 8910 SOLD-STATE DECES FOR NFRA-RED PERCEPTON 267 Fig. 3. Photoelectric characteristics of a P-N junction in nsb and a P-N junction in nas, both at a temperature of 77 K~ for irradiation by a black body at 300 ok (half-aperture angle of the cone of radiation 45 ). to ensure that contact with the diffused layer is sharply localized. This means that only the area of the joint must be submitted to the necessary heating. n order to limit the junction area to the required value and to remove (by chemical means) most of the surface defects of the crystal in the region where the junction reaches the free surface, an etching operation is needed. The process has been worked out in detail and leads to almost complete elimination of the contaminated zone. Since the junction in this state is particularly sensitive to the environment, it must be immediately inserted in an air-tight enclosure serving also as a cryostat. This set-up is necessary for the use of detectors which have to operate at temperatures very much lower than' ambient. To facilitate the diode assembly operations and also to improve the diode's reliability, surfacepassivation research, chiefly concerned with nas junctions, was done at LEP and enabled a suitable method to be adopted. Characteristics obtained ndium antimonide and indium arsenide behave differently in one important respect: while indium-arsenide P-N junctions can be used at ambient temperatures, indium-antimonide P-N junctions can only operate at temperatures below -150 C. Consequently, the results reported below refer to nsb junctions cooled to 77 "K and to nas junctions kept at the following three temperatures: 300 ok, 196 -x and 77 -x. The change of the current-voltage characteristics of the nas P-N junctions with temperature has clearly shown the importance of etching and of passivation of the junction, and also has given an indication of the kind of residual vapours present in the cryostatenclosure. The current-voltage curve of an nsb' junction is shown in fig. 1, while fig. 2 shows three curves for an nas junction. The maximum reverse voltage (breakdown voltage) is of the order of 4 to 5 volts in all four cases. For a given range of voltages corresponding to reverse-polarization operation of the diodes, the capacitance is inversely proportional to the cube root of Fig, 4, Spectral response curves of a P-N junction in nsb at 77 ok and in n.as at 77 ok, 196 ok and 300 "K. The photocurrent per unit of power received is plotted in amperes per watt. fla -100m -50 0 50 l00m..- nas -1-2 -3-4 JnSb -5 ~ -BpA the voltage applied. Typical capacitance values per unit area at zero voltage are 7 nffcm 2 for nsb diodes and 12 nffcm 2 for nas junctions, The photoelectric current-voltage' curves obt~ined when the photodiode was held at 77 OK and exposed' to the radiation of a black body at 300 OK are given in fig. 3 for nsb and for nas. The spectral sensitivity curves for nsb and nas junctions' (the latter at different temperatures) are reproduced in fig.'4. l8aw.s l4 1.2 0.8 o.s 0,4 0.2 nsb 1,. 77 K ~9S0K nas ('{OooK h ~ ~. ~ '... '0 o 2 3 4 5 _À Spm

268 PHLlPS TECHNCAL REEW OLUME 30 E " " s...,..." Al Refr ~, -ec ~, Fig. 5. A photodiode (P) of inas or nsb is mounted in a vacuumtight enclosure that also functions as a cryostat. The photodiode is mounted in a molybdenum holder M whose rear surface is in direct contact with the cooling fluid Refr. The radiation to be detected reaches the photodiode through a sapphire window W. A aluminium coating, functioning as a radiation shield. S soldering flanges of Dilver P. E external connections for the photodiode. The specific detectivity was calculated from measurements performed using radiation from a black body at 500 "K modulated at 500 Hz. The following results were obtained: 45 x 108 W -1 cm HZ12 for n Sb (77 K), 8 X 108 W -1 cm HZ12 for lnas (300 OK), 50 X 108 W-1 cm HZ12 for lnas (77 OK). Fig. 5 shows a section through the photodetector assembly. Linear arrays of photodiodes (cell-strips) Fig. 6. Linear array of 40 P-N diffused photodiodes made with a mesa technology on a single crystal of indium antimonide (a single wafer 20 mm long). Each mesa is 400 x 650 flm, the distance between centres is 500 flm. The responsivity of individual photodiodes differs less than 15 % from the mean value. 1.5f1A 1 0.5 A few remarks should be made about the design of the linear arrays of photodiodes mentioned in the foregoing. n one case, which is presented here as an example, a row of 40 P-N junctions is spread out over a distance of 20 mm (fig. 6) and each of the junctions is connected individually to an external circuit. The individual photodiodes have a sensitive area of 0.4 xo.4 mm and the mean current-voltage characteristic with ambient illumination is shown in fig. 7. The envelope, of glass and metal, incl udes an internal cooling device and a crown of 40 insulating passages as shown in fig. 8. Fig. 9 is a photograph of the device shown diagrammatically in fig. 8. Under the window can be seen the cold diaphragm with an oval aperture and behind this is mounted the cell-strip with 40 indium antimonide photodiodes; the electrodes are visible around the circumference; on the left, the tube in which a Joule-Thomson expansion valve is inserted to produce liquid nitrogen from compressed nitrogen at ambient temperature. f these P-N junctions are illuminated by a body at room temperature and the short-circuit current is measured, differences not exceeding 15 % of the mean current value will be observed between the diodes. -200m -100 0 1 100m -0.5-1f1.A Fig. 7. Photoelectric characteristic of one of the photodiodes of the strip, at a température of 77 ok, when irradiated by the surroundings at 300 ok.

1969, No. 8910 SOLD-STATE DECES FOR NFRA-RED PERCEPTON 269 Fig. 8. Cross-section of a pick-up tube with a linear array of nsb photodiodes. The infra-red radiation R arrives at the strip P via a sapphire window W. M molybdenum sheet, which carries the photodiode strip and is in thermal contact with the chamber A, cooled by liquid nitrogen (Refr). The window is mounted in a cup K of Dilver P. D cooled diaphragm. E disc carrying the 40 connections for the 40 photodiodes in the strip. 0 one of the output electrodes. W Fig. 9. The pick-up tube. The ring of 40 connectors for the 40 photodiodes of the strip can be seen through the window, and so can the cooled diaphragm, with the strip behind it. Summary. Pick-up tubes are available nowadays for optical images formed by visible or ultra-violet radiation, but there is no really satisfactory device for scanning images formed by thermal radiation at ordinary temperatures. The various aspects of the problem and the requirements which it imposes are examined. The article describes the work which has been done in attempting to solve the problem by using the photoelectric properties of junctions made of monocrystalline indium antimonide and indium arsenide; a brief description is given of the techniques developed and the results obtained. A method is discussed in detail in which a linear array of a number of [nsb photodiodes (say 40) is used. n this case mechanical scanning in only one direction is required; one version of a pick-up tube based on this principle and made in the LEP is described.