Chapter 5 Image Pickup Tubes

SEMICONDUCTORS AND SEMIMETALS. VOL. 21, PART D

a

CHAPTER 5

Image Pickup Tubes Suchio Ishioka CENTRALRESEARCH LABORATORY HITACHI, LTD. TOKYO. JAPAN I. 11. 111. IV. v . VI. VII.

INTRODUCTION

....................

a-si :H IMAGE PICKUP T U B E . . . . . . . . . . . . . . PROPERTIES O F a-si :H . . . . . . . . . . . . . . . . .

BLOCKING CONTACT STRUCTURE OF THE PHOTOCONDUCTIVE TARGET. . . . . . . . . . . . . . IMPURITY DOPING O F a-Si :H . . . . . . . . . . . . . . CHARACTERISTICS OF a-si :H IMAGE PICKUP TUBES. . . . Applications for a-Si :H TARGET.. . . . . . . . . . . . REFERENCES .....................

75 76 78 80 82 83 86 87

I. Introduction

When we consider the three well-known applications of amorphous selenium, in solar cells, electrophotography (Mort and Pai, 1976), and the image pickup tubes (Weimer and Cope, 1951 ), it is not surprising that the material that has shown such gre romise for practical photovoltaic cells should also be useful in the other two applications (Rose, 1979). Similarly, since hydrogenated amorphous silicon (a-Si :H) ha s shown itself useful in solar cells, it ought to be regarded as a candidate for the other two uses. However, certain additional properties that go beyond the needs of energy conversion are required if a-Si :H is to be used for image processing. That is why much more time is needed for the development of imaging applications than for solar cell applications. The first attempt to realize such imaging device uses was made by Imamura et aI. ( 1979),and a vidicon-type image pickup tube was proposed. Especially in the field of color television cameras, there have been great expectationsthat this device will lead to the realization of high-performance imagers. This section focuses on a-Si :H image pickup tubes and presents a-Si :H property requirements and fabrication and impurity doping techniques, as well as the structure of photoconductive targets. Pickup tube characteristics that have been attained and some of their applications will be described. 75 Copynght 0 1984 by Academic Press, Inc. All nghts of reproduction in any form reserved. ISBN 0-12-752150-X

t

76

SACHIO ISHIOKA

11. a-Si :H Image Pickup Tube

Figure 1a shows the fundamental structure and operation mechanism for the image pickup tube. Figure l b is the equivalent circuit. The photoconductive plate in a pickup tube is called a target. The resistivity of the photoconductor in the target must be high enough to be subdivided into an array of picture elements having resistivity rp and capacitance cp. In ordinary image pickup tube operation, the transparent electrode is biased positively with respect to the cathode. When the low-velocity scanning beam lands on the surface of the photoconductive target, the surface is charged negatively and br ought to cathode potential. Since the effectiveresistance of the photoconductor is much higher than

the electron-beam resistance R,,the surface potential of the photoconductor can be regarded as being almost the same as that o f the cathode. When the photocondhctor is illuminated, rp is decreased by the photogenerated carriers, and the surface potential is increased. This change is accumulated and stored during the scanning period. At the next electron-beam scanning,

Electron Beam

/ Pho toconducf or Signal Output@

1

Cathode

(a) CP

(b)

FIG.1. (a) The fundamental structure and operation mechanism of the image pickup tube. (b) The equivalent circuit of the image pickup tube.

5.

IMAGE PICKUP TUBES

77

the surface is recharged to the cathode potential. This additional beam current landing on the target corresponds to the light intensity at each picture element and is detected through the load resistance R as a signal current. This storage type of operation can produce signals at low light levels approximately 1O5 times larger than would nonstorage operation, in which each element in the equivalent circuit switches only when the beam scans the element in question. If cpis too small, the potential of the photoconductor surface rises to that of the transparent electrode within the electron-beam scanning period, and the signal storage procedure is stopped at that point. Therefore a time constant

cprp= eeop > Tf

(1)

is needed, where Tfis a scanning frame period (8sec). When cpis too large, on the other hand, it takes a few frames to reac! out the whole stored charge, since the signal-reading time constant is the product of cpand R,. Consequently, the decay lag characteristics become poor. From these points ofview, the total capacitanceofthe photoconductor for the imaging target should be around 1GOO pF. The photoconductor resistivity should then be large to satisfy Eq. (1). For instance, in the case of an a-Si :H target 26 mrn in diameter and 5 pm thick, a resistivity of loL4R cm is required. High resistivity is important for atchieving high resolution. In summary, to apply a-Si :H to image pickup tubes, high photoconductivity and resistivity high enough for long storage of image signals are required simultaneously. Although a-Si :H is believed to have the potential to fulfill these conditions, the resistivity of a 3 :H is not high enough by itself. Furthermore, as was explained before, the illuminated side of the target is positively biased in the normal image pickup tube operation. Accordingly, photogenerated holes, which generally have a very small p7, become very important ( p is drift mobility and 7 is camer lifetime). Additional requirements that are vital from the point of view of image pickup tube application are listed below. (1) Low dark current. Dark current, the signal current when there is no illumination, not only defines the minimum detectable limit of the signal level, but also causes shading phenomena and instability during temperature changes. The dark current level needs to be reduced to less than 1-2 nA. ( 2 ) Photoresponse properties. Photorespoiise properties are very important properties for imaging devices, especially in "broadcasting use. The amount of decay lag can be defined as the ratio of the residual signal current

78

SACHIO ISHIOKA

at the third field (50 ms) to the constant photosignal current. The lag must be less than a few percentage points. (3) Spectral photoconductivity. Each image device has a desirable spectral photosensitivity relevant to its purpose. For color television cameras, in pzlhicular, this range of photosensitivity should cover the whole visible spectrum and extend very little into the infrared region. Trials of a-Si :H fabrication and improvement of target structure will be discussed in the next section in the context of the requirements that have just been described. 111. Properties of a-Si :H As was explained in several chapters of Volume 21A, many a-Si:H preparation methods are known. In this section, sputtering will be discussed with the aim of clarifying a-Si :H properties (Imamura et al., 1980), and it will be seen that electrical and optical properties can be controlled over a considerablerange by manipulating reaction parameters (Paul et al., 1976). The sputtering system details are described in Volume 21A, Chapter 4, by Moustakas. A polycrystalline silicon plate of purity 5 -9's was sputtered in a mixed atmosphere of argon and hydrogen. Figure 2 shows the relation between the H, partial pressure of the reactive gas atmosphere and optical band-gap values (,Yipt). Optical band gaps are calculated from plots of (ahv)'/2versus

0

I

I

1

20

40

60

1

80 H 2 Gas Concentration ( % I

100

FIG.2. Relation between the partial pressureof the sputtering gas and optical band gap e p ' .

Tsub = 110°C (A), 160°C (O),and 250°C (0).

5.

79

IMAGE PICKUP TUISES

FIG.3. Absorption spectra for a-Si:H having various optical gaps E;P1 (ev): -,

-.-,

1.9; -..-,

1.8;

_______ , 1.7;

*

. ., 1.6.

2.0;

hv. These results show that a-Si: H with an E;Pt of 1.9-2.0 eV and high resistivity are obtained when the H, ratio of the mixed sputtering gas is 40% or more. When the substrate temperature is low, E;pt also becomes large. The resulting photosensitivity, however, is not good enough for image pickup tubes. Hydrogenated amorphons silicon prepared by glow discharge with low substrate temperature also shows poor sensitivity. The substrate temperature during the sputtering reaction should be 200-250°C. The absorption spectra for 2-pm-thick a-Si : H films having various E;Pt are shown in Fig. 3. Since the spectral shape of the absorption of a-Si :H coincides with that for the photoconductivity of the film, the E;Pt of a-Si :H for a color imager must be around 1.9-2.0 eV. Typical deposition parameters for image pickup tube a-Si :H are listed in Table I. Samples of a-Si :H prepared under these conditions have an E;Pt of 1.9-2.0 eV and contain 10- 15% of bonded hydrogen atoms. Infrared absorption measurement of a-Si :H provides information about the Si - H bonding configuration (see Volume 21l3, Chap. 4 by Zanzucchi).

-

TABLE I PREPARATIONCONDITIONS OF a-Si :H Parameter Total pressure Hydrogen gas concentration Deposition rate Substrate temperature

Value 1- 5

x

10-3 TO^

40% or mgre

- 3 A sec-'

200-250°C

80

an SACHIO ISHIOKA

4 0 'A

30 Y o

20%

I

10

1

1900

I

I

,

I

,

I

,

2000 2100 2200 Wave Number ( c m ' )

FIG.4. Infrared absorption spectra of a-Si :H as a function of the H, ratio of sputtering gas at Si- H stretching mode.

Figure 4 shows the infrared absorption spectra for a-Si :H at the Si - H stretching mode frequencies with the H2partial pressure in the sputtering gas as a parameter. As can be seen in Fig. 4, infrared absorption peaks at 2000 cm-I (monohydride) and 2100 cm-I (dihydride) are both observed under low H, gas ratio sputteringconditions. At an H, ratio of 4096, the 2 100 cm-* peaks becomes dominant. These results suggest that a-Si :H of dihydride configuration provides the better characteristics as a photoconductor for the image pickup tube. The result that the dihydride configuration a-Si :H has a large E;Pt is consistent with previous theoretical calculations(Allan and Joannopoulos, 1980).It is interesting that this situation seems to be quite different from results reported for an a-Si :H solar cell, where a monohydride configuration is believed to be favorable. IV. Blocking Contact Structure of the Photoconductive Target

If the electrodes for the photoconductivetargets are ohmic-type contacts, the dark current is at least comparable to the value determined by the bulk resistivity of the a-Si :H itself. In addition, carriers continue to flow into a-Si :H even after the illumination is stopped, until the carriers trapped in a-Si :H are neutralized. Thus, it is hard to obtain a low dark current and a high photoresponse.

5.

IMAGE PICKUP TUE:ES

81

On the other hand, if the target electrodes are blocking-type contacts; that is, if carrier injection into the photoconductive target is suppressed while photogenerated carriers in the a-Si :H are allowed to be drawn out at the electrode, only a primary photoinduced current contributes to the signal current from the target (Rose, 1963). Such cont.actsare effective in reducing the dark current and they obtain a rapid photoresponse, thus satisfying the requirements listed in Part 11. The conductivity of a-Si :H can also be changed through impurity doping. As a consequence, a blocking structure can easily be incorporated in an n-i-p diode structure (Shimizu et al., 1980).By using such photodiodes as the target, and under reversed-biasoperation, dark current levels of less than a few nanoamperes can be obtained. However in such a structure, n- and player resistivity drops to such a low value that high resolution is difficult to attain. Moreover, it is difficult to separate hole-electron pairs generated inside these layers, thus resulting in decreased photosensitivity. To avoid these problems, other types of blocking contacts have been proposed (Oda et al., 1981, Hatanaka et al., 1982; Ishioka et al., 1983). In these cases, a wide-gap transparent thin film such as SiO, or Sipllx,is used as a hole-blockinglayer in place of the n layer. A. chalcogenide, such as Sb2S3 or As,Se,,,Te,,,, or an oxide such as CeO, thin film, is then used instead of the p layer. Figure 5 illustrates an example of the schematic structure for an a-Si :H target. On a glass substrate, a tin oxide transparent electrode layer is depositedby CVD. On this layer, a very thin Sic), ,as a hole-blocking layer, a photoconductive a-Si :H layer, and an electron-blocking Sb2S3layer are added, in that order. The thickness of the a-Si H layer is 2-4 pm. The current - voltage characteristics of this SiO,/a-Si :H/Sb,S3 target are shown in Fig. 6. The SiO, thickness was chosen to be around 20 nm. The Sb2S3electron-blocking layer is a porous film about 60 nm thick, chosen with the aim of preventing secondary electron emission during the electronbeam scanning. The blocking layers remarkably suppress the dark current to less than 1 nA, up to 3 X lo5 V cm-'. which is satisfactory for television -Si:H

s:,*s3 / Electron

+-

Beam

FIG.5. Schematic structure of the a-Si: H photoconductive target.

82

SACHIO ISHIOKA

7 Photocurrent (10 lux 1

without B l o c k i n g

A’

Sb2Sj

?u

i

;\

I

rn I i

n s 1 10

20

30

40

50

60

Target Voltage ( V 1

70

FIG.6. The current - voltage characteristics of a-Si :H blocking contact structure target.

camera use. The photocurrent reaches the saturation level, which suggests that photogenerated camers in the a-Si :H are effectively drawn out through the Si02layer. The situation is the same when the SiOzthickness is increased to 40 nm. V. Impurity Doping of a-Si :H

An undoped a-Si :H film is thought to have n-type characteristics, while holes travel a longer distance than electrons in a photoconductive target. Therefore, a ptype a-Si :H is preferable for the present purpose. Figure 7 shows the photocurrent -voltage characteristics for a-Si :H targets that have various boron-doping levels (Ishioka et al., 1983). Boron doping was carried out by mixing diborane (BzH2)into the sputtering gas. With imaging devices, the dopant amount must not be excessive, for fear of degradingthe resolution. The amount of dopant is designated in terms of the diborane gas-mixing ratio. However, when the incident light is 600 nm, photocaniers are generated

5.

:

1o4 lo5 Electric Field ( Vcm')

83

IMA.GE PICKUP TUBES

10 -91

I 1 1 1 1

I

I

I

I

I

1 1 1 1

I

I

,I

1 o4 105 Electric Field ( V c d )

FIG. 7. Photocurrent-voltage characteristics for a-Si : H targets having various doping levels. The wavelength of incident light is (a) 425 nm and (b) 600 nm. Doping levels: 0 , O ppm; A, 0.5 ppm; D,5 ppm; 0, 10 ppm; A,50 ppm.

throughout the whole layer of a-Si :H. Therefore, it can be seen in Fig. 7b that both holes and electrons contribute to the photocurrent. The photocurrent of the a-Si :H target increases with increasing target voltage until it reaches the saturation level at voltage V,. This V, value can be seen to be proportional to (puz)-* of a-Si from the results of analysis of primary photocurrent - voltage characteristics (Oda et id., 1983). Therefore a-Si :H with a low V,is supposed to have a large carrier mobility and a low trap density. As shown in Fig. 7a decreases monotonically as the content of B2H6increases up to 50 ppm. The V, controlling the electron current, on the contrary, decreases at a low doping level but increases again when the content Of B2H6is 50 ppm. These results indicate that a low doping of B,H6, up to a 10-ppm level, serves to decrease the trap density. Hydrogenated amorphous silicon produced by rf sputtering is thought to have a smaller vpz value, but this problem can be improved by a light doping of boron. The same effects are observed in glow-discharge-produced a-Si :H films. The dark current is reported as being at a minimum at the point where B2H6/SiH,is 10 ppm (Oda et a/., 1981).

v8

VI. Characteristics of a-Si :H Image Pickup Tubes Image pickup tubes are operated at an applied voltage high enough to draw out the saturated level of the photocurrent as shown in Fig. 7. In this region, the photocurrent is determined by thle number of electron-hole

84

SACHIO ISHIOKA I

0.01

1

I

1

I

1

I

600 700 800 Wavelength ( n m 1 FIG.8. Spectral photosensitivity of the sputtering-produceda-Si : H target. The target structure is Si02 (1 5 nm)/a-Si :H (2 pm)/Sb2S3(60 nm). 400

500

-4-

J

10

-

-

yi a

I

2. . .--

2 a t

-7 10

0 .-

T

-

C

0,

-

L

3

u

-8

-

0 -

I

I

I

I

I

I ,

, I

I

I

t

FIG. 9. Relation between the signal current of the a-Si : H target and the incident light intensity for white (O),green (O), red (0),and blue (A)light.

5.

IMAGE PICKUP TL'BES

85

pairs generated in the a-Si :H. Consequentl)., the carrier generation efficiency can be evaluated if the number of photclns absorbed in the photoconductor is known. Figure 8 shows the spectral photosensitivity of an a-Si :H produced by sputtering (Ishioka et al., 1983). It covers the whole visible range, shows a high quantum efficiency, and extends little into the infrared region. Thus, it is favorable for color imaging. The a-Si :H target produced by glow-discharge CVD, on the other hand, has a narrow band gap of 1.6- 1.8 eV, which results in high infrared sensitivity. Figure 9 showsthe relation between the signal current of the a-Si :H target and the intensity of the incident light. The signal current is proportional to the light intensity ( y = l), and this provides a great advantage in balancing color signal elements. The resolution pattern taken from a sputtered a-Si :H target pickup tube is shown in Fig. 10. The horizontal resolution is not affected by boron doping and is more than 600 television lines fix a tube 18 mm in diameter. This value is the same as for chalcogenide pickup tubes, which are used in the broadcasting industry (Maruyama et al., 1974). The decay lag for an undoped a-Si :H target with scanning area of 8.8 X 6.6 mm is about lo%, at 50 msec after turning off the light of a 0.2-nA signal intensity. On the other hand, the decay lag is drastically reduced to less than 4% by a light doping with boron. This value is satisfactory for industrial or

FIG. 10. Resolution pattern reproduced by the a-Si: H image pickup tube.

86

SACHIO ISHIOKA

home-use cameras. Finally, a-Si :H image pickup tubes have no anomalous effects, such as after-image burning or blooming under strong illumination. VII. Applications for a-Si :H Target

The properties of an a-Si :H image pickup tube with a blocking structure are summarized here: (1) Spectral photosensitivity is high and favorable for color imaging. Sensitivity in the green region is more than twice as large as that for conventional vidicon tubes. (2) Resolution is high, and almost no burning or blooming can be seen at any applied voltage. This is a big advantage over solid state imagers. (3) An a-Si :H film is stable up to 100°Cor more. This is also favorable for cameras used for outdoor service.

These properties indicate that a-Si :H image pickup tubes are quite useful over a wide area of applications. The TV camera for an x-ray image intensifier system is an example of such an application (Iamura er al., 1982; Ishikawa et al., 1982). The peak wavelength of the light emitted from the fluorescent plate of the intensifier is around 520 nm, which matches well the a-Si :H photosensitivity spectrum. The image resolution is measured to be 20% better than in conventional intensifier tubes. Furthermore, by applying a color-filter-integratedsubstrate to the a-Si :H target, a single-tube color-imaging camera was successfully fabricated (Ishioka et al., 1983). Stripe-shaped organic filters of cyan and yellow color combinations are employed, and signals can be obtained using a frequency division multiplex method. Figure 11 shows a color image taken by this camera system (here reproduced in black and white). The objects were illuminated at only 50 lux, but a bright image was still obtained. Hydrogenated amorphous silicon is thought to be very promising, not only for image pickup tubes but also for solid state imagers such as “twostory” area sensors (Tsukada et al., 1979). That is, an a-Si : H photoconductive layer is placed on top of the Si scanner. Many advantages have been reported for the a-Si :H “two-story” sensor over conventional solid state imagers (Baji et al., 1982). Although a-Si :H has been proved to be a very good photoconductor for image pickup tubes, some problems remain to be solved. As has already been mentioned, the decay lag in a-Si :H tubes is presently 4%, and it is necessary to reduce this lag, especially in order to apply the technology to broadcasting use. The decay lag as determined by the capacitance of the

5.

IMAGE PICKUP TUBES

87

FIG. 1 1. Color image (reproduced here in black and white) reproduced by a-Si :H singletube camera.

photoconductive layer is calculated as being about 2%. The difference is thought to be caused by a trapping level inside the a-Si :H layer. Thus, a-Si :H fabrication techniques should continue to be improved to decrease the density of harmful states in the a-Si :H layer. To achieve high performance of a-Si :H image pickup tubes, a new operation method with a different target structure has been proposed (Kusano et al., 1983). Regarding device reliability, the stability of a-Si :H under such conditions as high light intensity, high temperature, or operation in a vacuum should be confirmed. During fabrication, in addition, decreasing the deposition temperatures to 150°C or less would also be useful if a-Si :H were deposited directly on a filter-integratingsubstrate. REFERENCES Allan, D. C., and Joannopoulos, J. D. (1980). Phys. Rev.Lett. 44,43. Baji, T., Yamamoto, H., Matsumaru, H., Koike, N., Akiyama, T., %no, A., and Tsukada, T. (1982). Jpn. J. Appl. Phys. 21-1,269. Hatanaka, Y., Yamagishi, K., So, H., and Ando, T. (1982 I. Proc. Meet. TVEng., 1982 p. 69 (in Japanese). Imamura, Y., Ataka, S.,Takasaki, Y., Kusano, C., Hirai, T., and Maruyama, E. (1979). Appl. Phys. Lett. 35, 349. Imamura, Y., Ataka, S., Takasaki, Y., Kusano, C., Ishioka, S.,Hirai, T.,and Maruyama, E. (1980). Jpn. J. Appl. Phys. 19-1, 573.

88

SACHIO ISHIOKA

Imamura, Y., Hirai, T., Maruyama, A., and Nobutoki, S. (1982). Proc. Meet. TVEng., 1982 p. 7 1 (in Japanese). Ishikawa, K., Ikeda, M., and Okabe, K. (1982). Proc. Meet. Jpn. Radiol. Technol.38th, Tokyo I982 p. 299 (in Japanese). Ishioka, S., Imamura, Y., Takasaki, Y., Kusano, C., Hirai, T., and Nobutoki, S. (1983). Jpn. J. Appl. Phys. 22-1,46 1. Kusano, C., Ishioka, S., Imamura, Y., Takasaki, Y. Shimonoto, Y., Hirai, T., and Maruyama, E. (1983). Int. Electron Device Meet of Tech. Dig., 509. Maruyama, E., Hirai, T., Goto, N., Isozaki, Y., and Sidara, K. (1974). Proc. Int. Conf: Amorphous and Liquid Sernicond., 5th. p. 58 1. Taylor and Francis, London. Mort, J., and Pai, D. M. (1976). In “Photoconductivity and Related Phenomena,” p. 421. Amer. Elsevier, New York. Oda, S., Tomita, H., and Shimizu, I. (1983). In “Amorphous Semiconductor Technologiesand Devices” (Y. Hamakawa ed.), p. 113. Ohmsha, Tokyo. Oda, S., Saito, K., Tomita, H., Shimizu, I., and Inoue, E. (1981). J. Appl. Phys. 52,7275. Paul, W., Lewis, A. J., Connell, G. A. N., and Moustakas, T. D. (1976). Solid State Cornmun. 20,969.

Rose, A. ( 1963). “Concepts in Photoconductivity and Allied Problems.” Wiley (Interscience), New York. Rose, A. (1979). Phys. Today 12,20. Shimizu, I., Oda, S.,Saito, K., and Inoue, E. (1980). J. Appl. Phys. 51,6422. Tsukada, T., Baji, T., Yamamoto, H., Hirai, T., Maruyama, E., Ohba, S., Koika, N., Ando. H., and Akiyama, T. ( 1 979). Tech. Dig. -Int. Electron Devices Meet. p 134. Weimer, P. K.,andCope,A. D.(1951). RCA Rev. 12,314.