VOL.I0

NR 6

DECEMBER

1955

Philips Research Reports OF N.V. PHILIPS'

EDITED BY THE RESEARCH GLOEILAMPENFABRIEKEN,

R279

LABORATORY EINDHOVEN, NETHERLANDS

Philips Res. Rep. 1 0, 401~424, 1955

SOLID-STATE IMAGE INTENSIFIERS by G. DIEMER,

H. A. KLASENS and J. G. van SANTEN

621.383.2: 621.3.032.36 Summary A theory is given of the characteristics of solid-state image-intensifying screens consisting of a photoconducting and an electroluminescent layer, including the influence of positive internal feedback and negative electrical feedback. The possibilities of brightness 'as well as contrast' amplification are discussed. With intermittent irradiation it is possible to increase the amplification factor by storage, due to the decay, or by triggering of a feedback amplifying-screen. In the latter case the gradation need not be lost. The dimensioning of the parameters with regard to specific applications (e.g. radar, X-ray images, television) is discussed. Preliminary experimental results are in agreement with theory. With X-rays a brightness-amplification factor of 30 was obtained.with respect to a normal fluorescent X-ray screen. Résumé L'on expose la théorie des caractëristiques des écrans intensificateurs d'image à l'état solide se composant d'une couche photoconductrice et d'une couche électro-luminescente, en y ajoutant l'influence d'une réaction interne et d'une contre-rëaction ëlectrique, Les possibilités d'amplification de luminance, comme celles d'amplification de contraste, sont discutées. Avec une irradiation intermittente, il est possible d'augmenter le coefficient d'amplification grace à l'accumulation provoquée par l'affaiblissement, ou par Ie déclenchement d'un êcran amplificateur à réaction. Dans ce dernier cas, la gradation ne devra pas en souffrir. L'on discute de l'ordre de grandeur des paramètres par rapport à leurs applications spécifiques (notamment radar, images radioscopiques, tëlëvision), Les résultats obtenus lors des expériences préliminaires sont conformes à la thëorie. Avec les rayons X, un coefficient d'amplification de luminance de 30 a été obtenu par rapport à un écran normal fluorescent pour rayons X. Zusammenfassung Der Artikel gibt eine Theorie der Eigenschaften von FestkörperBildverstärkersehirmen, die aus einer photoleitenden und einer elektrolumineszierenden Schicht bestehen, sowie des Einflusses von positiver innerer Rückkopplung und negativer elektrischer Rückkopplung. Es werden die Möglichkeiten sowohl von Helligkeits- als auch von Kontrastverstärkung diskutiert. Bei intermittierender Bestrahlung ist es möglich, den Verstärkungsfaktor zu vergröBern, entweder durch Speicherwirkung im Zusammenhang mit dem Abklingen oder durch den Rückkopplungseffekt des Verstärkersehirmes. Im letzteren Fall braucht die Gradation nicht verlorenzugehen. Die Dimensionierung der Parameter wird im Hinblick auf ibre jeweilige spezielle Anwendung (z.B. Radar, Röntgenbilder, Fernsehen) diskutiert. Vorläufige experi-

402

G. DIEMER,

H. A. KLASENS nnd J. G. 'van SANTEN

mentelle Ergebnisse stimmen mit der Theorie -überein. Mit Röntgenstràhlen wurde ein Vetstärkungsfaktor für die Helligkeit erzielt gleich dem 30fachen in bezug auf einen normalen Röntgen-Lëuchtschirm.

1. Introduetion Intensification of images visible or latent in the form of X-rays, ultraviolet, infrared or cathode rays is an important problem for which several solutions have already been given or indicated. One solution is the conversion of a "radiation image" into an "electronic image" by means of a photo-emitter, accelerating the electrons in vacuum and converting the electronic image into a visible image by means of a fluorescent screen 1). Intensifying tubes using this principle have already been made for X-ray and infrared images. To avoid the vacuum tubes suggestions have been given to accelerate the photo-electrons in a solid 2) 3). However, both experiment *) and theoretical considerations show that when a solid is placed in a very high electric field it is not possible to accelerate electrons to .velocities of more than a few electron volts, i.e., the order of magnitude of the energy gap between the filled band and the conduction band in insulators. At ,this "energy barrier" the accelerated electrons lose their kinetic energy by , impact with valency electrons, which are transferred to the conduction band. An avalanche starts and dielectric breakdown takes place. A moderate acceleration is therefore only useful if it is confined to a very thin layer as is e.g. the case with electroluminescence 4). The principle of any kind of amplification is an easy way of power control. We have seen that it is not possible to use the acceleration of electrons in a solid, but in section 2 we shall show that photoconductivity * *) does provide a power control which can compete very well with the devices using photo-emission and acceleration of 'the photo-eléctrons in vacuum. With photoconductors relatively small voltages are used; thus for transferring the power control into "a control of light intensity, we have to use an "output system" which converts an electric power of low voltage into light. In fig. 1 a possible arrangement is given of a photoconducting layer (1) and an output layer (2), sandwiched between transparent electrodes (3) and (4), which are connected to asource of voltage V. Fig. 2 shows the equivalent electrical circuit; the impedance Zl is a function of the incident radiation BI' hence the photoconductor acts as a kind of trigger to the power supply> Thë successful development of electroluminescence in recent *) We have tried to eject electrons from a germanium surface by putting a large reverse voltage (ilOvolts) across a pon junction lying just beneath this surface. The result was negative due to the physicallimitations mentioned. **) Throughout this ~ticle we shall use the term photoconductor for a solid the impedance of which is sensitive to any kind of radiation; the radiation need not be visible and may be either electromagnetic or corpuscular. /

.-------------~----~-~----_

..-,--.

SOLID-STATE

403

IMAGE INTENSIFIERS

, BI

r---=LIJJ_3 ! E~:rhiCkneSS t

dl

E~;JhiCkness ! ! !·l ' B2

d2

8550Q

Fig. 1. Series connection of a photoconductive layer (I) and a light-output layer (2). The layer 5 is discussed in sections 6 and 7.

years has provided us with an attractive solution for the output system5-lO). The direct stimulation of electroluminescenee by u.v. radiation and X-rays, as fust described by Destriau 11) 12), gives another possibility of an. image intensifier consisting of a single electroluminescent layer 13). In this article we shall confine ourselves to analysing the properties of image intensifiers in which a separate photoconductive layer is incorporated. For the sake of brevity we suggest the term amplificons as the generic name for this type of amplifiers.

v 85509

Fig. 2. Equivalent electrical circuit of the arrangement of fig. 1. Zl = impedance of the photoconducting layer per unit area, Z2 = the same , of the electrolumincscent layer.

2. Comparison of power control by photoconductivity tion of photo-electrons in vacuo

with that by aceelera-

We shall denote the "vacuum case" by (a) and the "photoconductivity case" by ((3). With (a) as well as with ((3) the incident radiation Blliberates· a number of electronic charges, ïla and ?'J (J respectively per energy unit of BI *). We now have to investigate how much energy W can he transported by these electrons during the time they are available. In' both cases we shall consider a surface of unit area. (a) Here we have (1) *) For the sake of simplicity in this section we assume 1]aand

,.

1]{J

toohe independent of Bl'

404

G. DIEMER,

H. A. KLASENS

and J. G. van SAJ.'ITEN

where e = electronic charge and Va is the potential difference by which the electrons are accelerated. (f3) During the life time LP the field strength El in the layer 1 gives rise tb a displacement of the photo-e!ectrons ("Schuhweg")

Lt thus to an energy transport

= /hl

El Lp;

(2)

(due to 'YJpBl electrons)

W p , eElLt 'YJpBl

= 'YJpe

#L pEi BI'

(3)

In photoconductors like CdS LP need not to decrease if El ~s increased to such values that zl exceeds dl' due to the well-known phenomenon of replenishment 'of electrons at the cathode. Comparing eq. (3) with eq. (1)

'YJP

Wp

we have

#1L pEi

(4)

-=-.

Wa

'YJa

Va

For a well-prepared CdS layer we found that the product /hlLP can have a' value of the order of 10-4 m2/V, whilst 'YJp~0·4 (eVtl• The efficiency 'YJa of the hest photo-emitters is ah out ten times smaller, so we may put 'YJp/'YJa= 10.For a photoconductor having a high dark-impedance, El may he increased up to ahout 3.106 V/m; with the vacuum device there are certain practical limitations to the maximum value of V which may he put equal to 105 volts. Inserting these values into eq. (4) we get the ratio of the transported energies under optimal conditions

Wp) (Wa

.

= rnax

9.108

10--~10 105

5

.

(4a)

Hence from the point of view of power control. the photoconductor may he a much more efficient device than the vacuum tuhe using photo-emission. However, in the latter case high-energy electrons are available which make the light-output system in general more efficient than a low-voltage' output system such as an electroluminescent layer. The ratio of the overall amplification factors may therefore he much less than 105• 3. Some properties of electroluminescence

and photoconductivity

In order to he ahle to go into more details in the following we shall mainly discuss the properties of an amplificon according to fig. 1, operated with an a.c. voltage V = Vo cos cot *), and to hegin with we will recollect some properties of electroluminescence and photoconductivity, a knowledge of which is necessary for calculating the 'amplificon characteristics. The emittance B2 of an electroluminescent layer as a function of voltage V2 = V20 cos wt across it is described hy 15) *) Concerning the possibility of d.c. operation, see at the end of section 6.

SOLID.STATE

IMAGE INTENSIFIERS

405 (5)

B2eo and A heing constants ch~racteristic of the electroluminescent layer involved; for not too large values of o: (e.g. w = 10-1 per period.

• range for I B2 .'

+ wB2ao -with w :10

__

-I....... f,B,

w Fig. 5. General theoretical amplificon characteristic for various values of the capacitance ratio p; Ym = maximum of contrast amplification. , It must be pointed out that a variation of the parameter p by changing the capacity Cl also implies a variation of 11'

A more simplified calculation has already been carried out by other authors 9) neglecting the capacitance of the photoconductive layer and assuming a power law for the dependence of the emittance of the electrolumines cent layer on the voltage across it. *) Up to. now no,quantitative

noise measurements are available. We have, however, the impression that with CdS layers the noise output-level may be rather low, due to the rather slow decay of CdS photo-emission, giving rise to a small bandwidth with regard to fluctuations,

85512

4-

410

G. DIEMER, H. A. KLASENS and J. G. van SANT~N

5. Numerical values for a CdS-ZnS combination To get an impression of the quantitative values of various characteristic . properties the following values, for the various parameters have been inserted: for a sintered CdS layer (activated by Cu and Ga) with input light

82max

----w-

1

I, It/ / , ,/ ,, / .(82::;;")

P-~il::: P=!&r; p:I _/

,

I

r

I' 1£=1 / ,, I L,:

/P=IOÓ

a

I

I

/pdO

r 0·1

I

0'2D-3D-5

2 3 5

10

....:I.: A

--

5.IO-l 5.J1-S

5.10- 7

-, 1\ "-, '.' - - -, ",,:10

'"

p'=IO

8

i;1

'=T

pdOO

")=10 b

,,\p:lOO

,,,=2

,,=2 v

pdO

p;;?t' , I

0·1 0,20,30,5

__

2 3 5 ... .!û.'/2 A

10 855/3

Fig. 6. Theoretical numerical values for a combination of a sintered CdS photoconductor and a (ZnS-Cu, AI) electroluminescent layer versus operating voltage Vo, with capacitance ratio p and average dielectric constant "81 of the layer 1 as parameters, B2CC has heen taken as unity. (a) Minimum and maximum B2-values, (b) Bl-values corresponding to maximum B2-values.

SOLID-STATE

411

IMAGE INTENSIFIERS

in the long-wavelength region matched to the thickness of the layer as indicated above: mobility # = 2.10-3 m2fV sec, electron lifetime '"C = 10--2 sec, 81 = 10, go ~ 0 and electron equivalent q = 1918 electrons per watt sec. For a green electroluminescent layer (Zns activated by Cu and Al) suspended in urea-formaldehyde with a thickness of 50 !L: '82 = 5, g2 = 0, A = 60 Vi and B2 = Llumen/m''. (1 watt of electroluminescent radiation corresponds to 500 lumens.)

1/ 10'

r: 10'

\

1/\

\

pdO

/

\

I1

\p=1

,

I~='a \ \

, \

0·1 ()o20-30·5

f'. 1

1\ ~

2 3

5

_...Y2.~

__

10

A

105

,

Gim ::

10~ p=1

l,=10,-