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a-Si: H Heterojunction
X-Ray Imaging Sensor Using CdTe/a-Si :H Heterojunction Y. HATANAKA, S. G. MEIKLE, Y. TOMITA and T. TAKABAYASI Research Institute of Electronics, Shizuoka University, Hamamatsu. Japan
INTRODUCTION Hydrogenated amorphous silicon (a-Si :H) having high resistivity and high photoconductivity has been extensively studied for imaging devices such as camera tubes (Imamura et al., 1979; Jones et al., 1985), overlaid solid-state image devices (Tsukada et al., 1981) and linear array input devices (Kanoh et al., 1981). Its small carrier diffusion length is suitable for the high resolution and anti-blooming that are most important for imaging devices. For X-ray imaging, there are sensing systems that use visible light conversion in a phosphorescent screen or direct X-ray photoconduction. Direct photoconduction without a light-optical system has been used in vidicon-type imaging devices, resulting in very high resolving power. Vidicontype camera tubes with photoconductive targets made from materials such as PbO (Heijine et al., 1954; Nishida and Okamoto, 1966), amorphous Se (Cope and Rose, 1966; Smith, 1960; Mithel and Rhoten, 1962), crystalline Si (Chester, 1969; Ashikawa and Takemoto, 1971), amorphous SeAs (Kawamura, 1982) and a-Si:H (Hatanaka et al., 1985) have been investigated. PbO vidicon tubes that are highly X-ray sensitive were found to be the most practical. PbO, however, is severely affected by environment, making mass production of devices difficult. Low-atomic-mass materials such as Si, a-Si :H and Se are poor for X-ray absorption and thus require a thick layer for device fabrication. In this paper, we describe the properties of single-crystal (sc-) and polycrystalline (pc-) CdTe/a-Si :H heterojunction devices constructed in an attempt to exploit the photoconductive properties of CdTe. The visible range optical band gap (1.5 eV) and high absorption coefficiency of CdTe make it ideal for solar cell applications as well as X-ray and y-ray detectors. The most promising use of the CdTe/a-Si :H heterojunction is not only in an X-ray camera tube but also in a large area sensor such as a solid-state panel, because it can function over the long term in the ambient ar. 257 ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS VOL. 74
Copyright 0 1988 Academic Press Limited All rights of reproduction in any form reserved ISBN 0-12-014674-6
258
Y . HATANAKA ET A L
X-RAY ABSORPTION COEFFICIENT OF CDTE AND A-SI:H FILM X-ray absorption coefficients of a-Si :H and pc-CdTe films were measured, with the results as shown in Fig. 1. a-Si :H films and pc-CdTe were deposited on a beryllium (Be) plate 0.5 mm thick and 26mm in diameter by a capacitively coupled glow discharge and by RF sputtering in argon gas respectively. The deposited films covered half the plate. The transmitted values of X-ray flux were measured for a plate with and without the film. The X-ray detector used had no window and consisted of two parallel plate electrodes of area 1 cm2 spaced 1 cm apart and filled with one atmosphere of dry air (Knoll, 1979). The electrodes were biased with a 1 kV DC voltage. The ion currents generated by the X-rays were measured after calibration with a Victorin radiometer (RADOCON model 500). The a-Si:H film has an absorption coefficient of about the same order as the crystalline silicon. Soft X-ray absorption is fairly strong, but hard X-ray absorption is poor. In comparison with the silicon material, CdTe has an absorption coefficient one decade larger. Most significant is the fact that the hard X-ray absorption coefficient is large.
-1 L ld
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c
u
Y
o
m
a
0
Cd Te
k 0
Z
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10 20 30 40 50 X RAY TUBE VOLTAGE(HV1
FIG.1. Absorption coefficient versus X-ray tube voltage.
259
X-RAY IMAGING SENSOR
SINGLE-CRYSTAL CDTE/A-SI :H HETEROJUNCTION The sc-CdTe/a-Si:H heterojunction consisted of a high-resistivity (1 031O5 R cm) n-type CdTe substrate onto which p-type (boron-doped) a-Si :H was deposited. Depositions of a-Si:H were performed at 250°C in an R F capacitively coupled system with SiH4and B2H6 as source gases. Immediately prior to deposition the CdTe substrates were etched in 1 % and 0.1 % bromine in methanol solutions for 2 minutes in each. Contacts were provided by evaporated indium and gold for the CdTe and a-Si :H respectively. Figure 2 shows the current density versus voltage (Z-V) characteristics of a device with the a-Si:H layer doped using a B2H6/SiH4ratio of Forward current showed a steep rise with increased applied voltage and a diode factor that was larger than the value between 1 and 2 which had been expected. The large diode factor is considered to be a result of the high bulk resistivity of the CdTe and of space-charge effects in the a-Si: H film. The reverse current was cm-2) and showed saturation characteristics with an activation low ( energy of approximately 0.8 eV. This is considered to be a recombinationgeneration current at the interface, since neither JO nor E, showed a strong dependence on the a-Si :H doping level.
0
280
4.O 6 .O VOLTAGE ( volt 1
8 .O
FIG.2. I- V characteristics of an sc-CdTe/a-Si: H heterojunction. Open circles show forward current, solid circles dark current and solid triangle X-ray response.
260
Y . HATANAKA ET A L .
B I A S VOLTAGE 0 A 0
500
o v
1.0 V
10.0
v
6 00 7 00 8 00 WAVELENGTH ( n m )
FIG.3. Spectral response of an sc-CdTe/a-Si:H heterojunction.
In Fig. 2 are also shown the data obtained when X-rays of 100 R min-’, 40 kV were used. The photoexcited current showed a three-decade increase,
corresponding to several microamperes in reverse bias saturation current. Since the X-ray absorption coefficient for CdTe is approximately ten times larger than that for a-Si :H, it is considered that absorption in CdTe governs the sensitivity characteristics. Figure 3 shows the spectral photoresponse when light was shown through the a-Si:H layer. A wide spectral response down to 1.5 eV indicates that a depletion region forms in the sc-CdTe and photoexcited carriers are efficiently injected into the a-Si :H film.
POLYCRYSTALLINE CDTE/A-SI:H HETEROJUNCTION The sc-CdTe described above is difficult to use in imaging devices because it is difficult to make a high-quality thin film. For imaging devices, we used a polycrystalline CdTe film prepared by the R F sputtering method. The pcCdTe film was prepared by sputtering of a CdTe polycrystalline target (purity 99.999%), 100 mm in diameter. The deposition conditions were: a substrate
26 1
X-RAY IMAGING SENSOR
C(220) C(311)
z - 0 20
60
C (511)
80
20(deg) FIG.4. X-ray diffraction spectrum of pc-CdTe prepared by the RF sputtering method.
temperature of 310"C, argon gas pressure of 0.06Torr and R F power of 150 W. The a-Si :H film was prepared by capacitively coupled glow discharge deposition. Conditions were approximately the same as for the case of the scCdTe/a-Si :H heterojunction. Figure 4 shows the X-ray diffraction pattern of a typical CdTe film deposited by the sputtering technique. The structure of this film was of the zinc blende type with a preferential orientation of the (1 1 1) planes parallel to the substrate. From FWHM (full width at half-maximum) of the (1 11) peak, grain size was estimated to be about 1000 A, which was smaller than that for films grown by the HWVE (hot wall vacuum evaporation) technique (Mimura et al., 1985). Electrical resistivity was about 1O'O l2 cm and was one order higher than that for the HWVE films. Electrical properties of pc-CdTe were measured with coplanar electrodes. Figure 5 shows the dependence of the light and dark current characteristics of the pc-CdTe on the sputtering substrate temperatures. Dark electrical resistivity and photoconductivity were highest at a temperature of 340°C. At temperatures higher than 340"C, a multi-random-axis domain grew in the films and the dark resistivity fell rapidly. The heterojunction characteristics for sandwich cells such as ITO/CdTe/Au (sample I), and ITO/n+a-Si:H, undoped a-Si : H/CdTe/Au (sample 2) were investigated. Figure 6 shows the I- V characteristics. Solid triangles represent sample 1; open circles and solid circles represent forward and reverse current of sample 2, respectively. The pc-CdTe/a-Si :H heterojunction shows diode behaviour, although the ratio of forward to reverse current of this junction is less than that for the sc-CdTe/a-Si :H heterojunction.
262
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FIG.5. Dependence of substrate temperature on the pc-CdTe prepared by the RF sputtering method.
IMACMGDEVICES For an applications experiment, an X-ray imaging pick-up tube was fabricated. The target configuration was as shown in Fig. 7. For the substrate plate illumination window, a Be plate 0.5 mm thick and 26 mm in diameter was used for X-ray imaging and borosilicate glass 2 mm thick and 26 mm in diameter was used for visible-light imaging. The n+ a/Si :H and undoped aSi :H on the substrate plate served as blocking layers against holes from the substrate electrode. The pc-CdTe layer was grown to be intrinsic and was intended as a generation layer for the X-ray photocarriers. A weak p-type aSi :H 1 pm layer was deposited on the pc-CdTe. The a-Si :H and pc-CdTe layers formed the heterojunction. Holes injected from CdTe were carried to the a-Si :H surface without lateral diffusion. The 500 A layer of Sb2S3 was Torr. This layer functioned as deposited under vacuum at a pressure of protection against electron injection from the electron beam. Figure 8 shows the resolution chart reproduced by the glass-face-plate
100
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f
FIG.6. pc-CdTe/a-Si: H heterojunction characteristics.
X RAY
Be(0.5mm)
u
/
L
SbZS3( sood 1
FIG.7. Configuration of pc-CdTe/a-Si: H heterojunction target.
264
Y. HATANAKA ET A L
FIG.8. Image of visible-light resolution chart.
vidicon. High resolution of over 800 TV lines was obtained. Sensitivity to light input was about 200 nA lx-' as shown in Fig. 9. For X-ray imaging, pc-CdTE/a-Si :H heterojunction vidicons with a Be face plate were constructed as shown in Fig. 7. Target signal current characteristics are shown in Fig. 10. The CdTe/a-Si: H heterojunction target is
-
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z
a
cl
+
Z
w
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0 I-
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1 I L L U M I N ANCE X-RAY
10
RADIATION
10 (Lx) 100 (Rlmin)
I
FIG.9. Light (triangles) and X-ray (circles) conversion characteristics.
26 5
X-RAY IMAGING SENSOR
0
5
10
TARGET
15
20
25
VOLTAGE (V)
FIG.10. Target signal current versus target voltage.
one decade higher in X-ray sensitivity than the a-Si: H target. This observation agrees with the absorption coefficient data shown in Fig. 1. Figure 9 also shows that the signal current versus X-ray intensity had a power dependence of 0.65. Here the X-ray induced current was about 300 nA for 100 R min-', 40 kV soft X-rays. Measurements were performed in a standard TV system with a field time of 16 ms. Figure 11 shows the X-ray monitor image of an IC package. Clearly resolved gold bonding wire images of an IC package were observed, though they included some imperfections in the form of white spots. After-image lag was quite large. These problems can possibly be overcome by improvement of the film deposition process.
266
Y . HATANAKA ET A L .
FIG. 1 I . Reproduced image of IC package observed using X-ray radiation.
SUMMARY
The visible-range optical band-gap and high absorption coefficient of CdTe make it ideal for solar cells as well as X-ray and y-ray detectors. The most promising use of the CdTe/a-Si :H heterojunction is not only in X-ray camera tubes but also in large-area sensors such as solid-state panels. The X-ray absorption coefficient of CdTe was measured as about 3000 cm-I, one decade larger than that for c-Si or a-Si :H. A sc-CdTe/a-Si:H heterojunction consisting of an n-type single-crystal CdTe substrate and p-type (B-doped) aSi: H prepared by the glow discharge method exhibited good diode behaviour with a low reverse bias saturation current of a few nA cm-*. The wide spectral response down to 1.5 eV indicates that a depletion region forms in the sc-CdTe and photoexcited carriers are injected efficiently into the a-Si :H film. A pcCdTe/a-Si :H heterojunction consisting of a CdTe film prepared by RF sputtering and glow discharge a-Si:H also showed clear diode characteristics and was used in a vidicon type tube. This device showed resolution greater than 800 TV lines and a sensitivity of 300 nA per 100 R per minute, i.e. one decade higher than that of a-Si:H based devices. ACKNOWLEDGEMENTS This work was in part supported by the Research Development Corporation of Japan. The authors wish to thank Dr. R. Nishida for his useful advice on the X-ray sensor and Mr. T. Kawai
X-RAY IMAGING SENSOR
267
of Hamamatsu Photonics Co. for providing the Be face-plate and facilities for tube assembly, and finally, Dr. K. Hirata of Nippon Mining Co. for providing the single-crystal CdTe wafers.
REFERENCES Ashikawa, M. and Takemoto, I. (1971). J. Inst. Telev. Eng. Jpn. 20, 715 Chester, A. N. (1969). Bell Sysf.Tech. J . 48, 345 Cope, A. D. and Rose, A. (1966). J. Appl. Phys. 25, 192 Hatanaka, Y., Zeng Bai Chuang and Mimura, H. (1985). Jpn J . Appl. Phys. 24, L129 Heijine, L., Schagen, P. and Bruining, H. (1954). Philip Tech. Rev. 16, 23 Imamura, I., Takasaki, Y.,Kusano, C., Hirai, T. andMaruyama, E. (1979). Appl. Phys. Lerr. 35, 349-35 1 Jones, B. L., Burrage, J. and Holtom, R. (1985). In “Adv. E.E.P.” Vol. 64B, pp. 437-445 Kanoh, Y., Usui, S., Sawada, A. and Kiuchi, M. (1981). In ”Tech. Dig. Int. Electron Devices Meet.”, 313 Kawamura, T. (1982). In “Proc. Nat. Conv. Inst. Telev. Eng. Jpn”, pp. 3-7 Knoll, G. F. (1979). In “Radiation Detection and Measurement”, p. 123. Wiley, New York Mimura, H., Kajiyama, S., Nogami, M. and Hatanaka, Y. (1985). Jpn J . Appl. Phys. 24, L717 Mithel. J. P. and Rhoten, M. L. (1962). SMPTEJ. 71,444 Nishida, R. and Okamoto, S. (1966). J . Insf. Telev. Eng. Jpn 20, 192 Smith, C. W. (1960). In “Adv. E.E.P.” Vol 12. pp. 345-361 Tsukada, T., Baji, T., Shimomoto, Y.,Sasano, A., Tanaka,,Y.,Maruyama, E., Takasaki, Y., Koike, N. and Akiyama, T. (1981). In “Tech. Dig. Int. Electron Devices Meet.”, 479
INTRODUCTION Hydrogenated amorphous silicon (a-Si :H) having high resistivity and high photoconductivity has been extensively studied for imaging devices such as camera tubes (Imamura et al., 1979; Jones et al., 1985), overlaid solid-state image devices (Tsukada et al., 1981) and linear array input devices (Kanoh et al., 1981). Its small carrier diffusion length is suitable for the high resolution and anti-blooming that are most important for imaging devices. For X-ray imaging, there are sensing systems that use visible light conversion in a phosphorescent screen or direct X-ray photoconduction. Direct photoconduction without a light-optical system has been used in vidicon-type imaging devices, resulting in very high resolving power. Vidicontype camera tubes with photoconductive targets made from materials such as PbO (Heijine et al., 1954; Nishida and Okamoto, 1966), amorphous Se (Cope and Rose, 1966; Smith, 1960; Mithel and Rhoten, 1962), crystalline Si (Chester, 1969; Ashikawa and Takemoto, 1971), amorphous SeAs (Kawamura, 1982) and a-Si:H (Hatanaka et al., 1985) have been investigated. PbO vidicon tubes that are highly X-ray sensitive were found to be the most practical. PbO, however, is severely affected by environment, making mass production of devices difficult. Low-atomic-mass materials such as Si, a-Si :H and Se are poor for X-ray absorption and thus require a thick layer for device fabrication. In this paper, we describe the properties of single-crystal (sc-) and polycrystalline (pc-) CdTe/a-Si :H heterojunction devices constructed in an attempt to exploit the photoconductive properties of CdTe. The visible range optical band gap (1.5 eV) and high absorption coefficiency of CdTe make it ideal for solar cell applications as well as X-ray and y-ray detectors. The most promising use of the CdTe/a-Si :H heterojunction is not only in an X-ray camera tube but also in a large area sensor such as a solid-state panel, because it can function over the long term in the ambient ar. 257 ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS VOL. 74
Copyright 0 1988 Academic Press Limited All rights of reproduction in any form reserved ISBN 0-12-014674-6
258
Y . HATANAKA ET A L
X-RAY ABSORPTION COEFFICIENT OF CDTE AND A-SI:H FILM X-ray absorption coefficients of a-Si :H and pc-CdTe films were measured, with the results as shown in Fig. 1. a-Si :H films and pc-CdTe were deposited on a beryllium (Be) plate 0.5 mm thick and 26mm in diameter by a capacitively coupled glow discharge and by RF sputtering in argon gas respectively. The deposited films covered half the plate. The transmitted values of X-ray flux were measured for a plate with and without the film. The X-ray detector used had no window and consisted of two parallel plate electrodes of area 1 cm2 spaced 1 cm apart and filled with one atmosphere of dry air (Knoll, 1979). The electrodes were biased with a 1 kV DC voltage. The ion currents generated by the X-rays were measured after calibration with a Victorin radiometer (RADOCON model 500). The a-Si:H film has an absorption coefficient of about the same order as the crystalline silicon. Soft X-ray absorption is fairly strong, but hard X-ray absorption is poor. In comparison with the silicon material, CdTe has an absorption coefficient one decade larger. Most significant is the fact that the hard X-ray absorption coefficient is large.
-1 L ld
'E
c
u
Y
o
m
a
0
Cd Te
k 0
Z
a-s;:H
10 20 30 40 50 X RAY TUBE VOLTAGE(HV1
FIG.1. Absorption coefficient versus X-ray tube voltage.
259
X-RAY IMAGING SENSOR
SINGLE-CRYSTAL CDTE/A-SI :H HETEROJUNCTION The sc-CdTe/a-Si:H heterojunction consisted of a high-resistivity (1 031O5 R cm) n-type CdTe substrate onto which p-type (boron-doped) a-Si :H was deposited. Depositions of a-Si:H were performed at 250°C in an R F capacitively coupled system with SiH4and B2H6 as source gases. Immediately prior to deposition the CdTe substrates were etched in 1 % and 0.1 % bromine in methanol solutions for 2 minutes in each. Contacts were provided by evaporated indium and gold for the CdTe and a-Si :H respectively. Figure 2 shows the current density versus voltage (Z-V) characteristics of a device with the a-Si:H layer doped using a B2H6/SiH4ratio of Forward current showed a steep rise with increased applied voltage and a diode factor that was larger than the value between 1 and 2 which had been expected. The large diode factor is considered to be a result of the high bulk resistivity of the CdTe and of space-charge effects in the a-Si: H film. The reverse current was cm-2) and showed saturation characteristics with an activation low ( energy of approximately 0.8 eV. This is considered to be a recombinationgeneration current at the interface, since neither JO nor E, showed a strong dependence on the a-Si :H doping level.
0
280
4.O 6 .O VOLTAGE ( volt 1
8 .O
FIG.2. I- V characteristics of an sc-CdTe/a-Si: H heterojunction. Open circles show forward current, solid circles dark current and solid triangle X-ray response.
260
Y . HATANAKA ET A L .
B I A S VOLTAGE 0 A 0
500
o v
1.0 V
10.0
v
6 00 7 00 8 00 WAVELENGTH ( n m )
FIG.3. Spectral response of an sc-CdTe/a-Si:H heterojunction.
In Fig. 2 are also shown the data obtained when X-rays of 100 R min-’, 40 kV were used. The photoexcited current showed a three-decade increase,
corresponding to several microamperes in reverse bias saturation current. Since the X-ray absorption coefficient for CdTe is approximately ten times larger than that for a-Si :H, it is considered that absorption in CdTe governs the sensitivity characteristics. Figure 3 shows the spectral photoresponse when light was shown through the a-Si:H layer. A wide spectral response down to 1.5 eV indicates that a depletion region forms in the sc-CdTe and photoexcited carriers are efficiently injected into the a-Si :H film.
POLYCRYSTALLINE CDTE/A-SI:H HETEROJUNCTION The sc-CdTe described above is difficult to use in imaging devices because it is difficult to make a high-quality thin film. For imaging devices, we used a polycrystalline CdTe film prepared by the R F sputtering method. The pcCdTe film was prepared by sputtering of a CdTe polycrystalline target (purity 99.999%), 100 mm in diameter. The deposition conditions were: a substrate
26 1
X-RAY IMAGING SENSOR
C(220) C(311)
z - 0 20
60
C (511)
80
20(deg) FIG.4. X-ray diffraction spectrum of pc-CdTe prepared by the RF sputtering method.
temperature of 310"C, argon gas pressure of 0.06Torr and R F power of 150 W. The a-Si :H film was prepared by capacitively coupled glow discharge deposition. Conditions were approximately the same as for the case of the scCdTe/a-Si :H heterojunction. Figure 4 shows the X-ray diffraction pattern of a typical CdTe film deposited by the sputtering technique. The structure of this film was of the zinc blende type with a preferential orientation of the (1 1 1) planes parallel to the substrate. From FWHM (full width at half-maximum) of the (1 11) peak, grain size was estimated to be about 1000 A, which was smaller than that for films grown by the HWVE (hot wall vacuum evaporation) technique (Mimura et al., 1985). Electrical resistivity was about 1O'O l2 cm and was one order higher than that for the HWVE films. Electrical properties of pc-CdTe were measured with coplanar electrodes. Figure 5 shows the dependence of the light and dark current characteristics of the pc-CdTe on the sputtering substrate temperatures. Dark electrical resistivity and photoconductivity were highest at a temperature of 340°C. At temperatures higher than 340"C, a multi-random-axis domain grew in the films and the dark resistivity fell rapidly. The heterojunction characteristics for sandwich cells such as ITO/CdTe/Au (sample I), and ITO/n+a-Si:H, undoped a-Si : H/CdTe/Au (sample 2) were investigated. Figure 6 shows the I- V characteristics. Solid triangles represent sample 1; open circles and solid circles represent forward and reverse current of sample 2, respectively. The pc-CdTe/a-Si :H heterojunction shows diode behaviour, although the ratio of forward to reverse current of this junction is less than that for the sc-CdTe/a-Si :H heterojunction.
262
h
a C
v
Y. HATANAKA ET A L .
1
I
I
I
I
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d
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I
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I
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I
I
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/
//
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n
a c
/
v
0.4 IZ a CT
3 0
0.2&
a
Q
D
0
10 20 30 APPLIED VOLTAGE ( V )
0
FIG.5. Dependence of substrate temperature on the pc-CdTe prepared by the RF sputtering method.
IMACMGDEVICES For an applications experiment, an X-ray imaging pick-up tube was fabricated. The target configuration was as shown in Fig. 7. For the substrate plate illumination window, a Be plate 0.5 mm thick and 26 mm in diameter was used for X-ray imaging and borosilicate glass 2 mm thick and 26 mm in diameter was used for visible-light imaging. The n+ a/Si :H and undoped aSi :H on the substrate plate served as blocking layers against holes from the substrate electrode. The pc-CdTe layer was grown to be intrinsic and was intended as a generation layer for the X-ray photocarriers. A weak p-type aSi :H 1 pm layer was deposited on the pc-CdTe. The a-Si :H and pc-CdTe layers formed the heterojunction. Holes injected from CdTe were carried to the a-Si :H surface without lateral diffusion. The 500 A layer of Sb2S3 was Torr. This layer functioned as deposited under vacuum at a pressure of protection against electron injection from the electron beam. Figure 8 shows the resolution chart reproduced by the glass-face-plate
100
-a -
70
I
+7z/ d.
C
F-
z
i
20
/ I
/
I
0
a Q
50
Y
[II
3
I
u
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/
z W
I
Q
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W rl)
53 30
0
a W >
U
W
ot
O,
1
2 3 4 5 APPLIED VOLTAGE ( V )
f
FIG.6. pc-CdTe/a-Si: H heterojunction characteristics.
X RAY
Be(0.5mm)
u
/
L
SbZS3( sood 1
FIG.7. Configuration of pc-CdTe/a-Si: H heterojunction target.
264
Y. HATANAKA ET A L
FIG.8. Image of visible-light resolution chart.
vidicon. High resolution of over 800 TV lines was obtained. Sensitivity to light input was about 200 nA lx-' as shown in Fig. 9. For X-ray imaging, pc-CdTE/a-Si :H heterojunction vidicons with a Be face plate were constructed as shown in Fig. 7. Target signal current characteristics are shown in Fig. 10. The CdTe/a-Si: H heterojunction target is
-
N
E,
z
a
cl
+
Z
w
= 5 10- 7 L)
0 I-
0
I
a
1o *
L
1 I L L U M I N ANCE X-RAY
10
RADIATION
10 (Lx) 100 (Rlmin)
I
FIG.9. Light (triangles) and X-ray (circles) conversion characteristics.
26 5
X-RAY IMAGING SENSOR
0
5
10
TARGET
15
20
25
VOLTAGE (V)
FIG.10. Target signal current versus target voltage.
one decade higher in X-ray sensitivity than the a-Si: H target. This observation agrees with the absorption coefficient data shown in Fig. 1. Figure 9 also shows that the signal current versus X-ray intensity had a power dependence of 0.65. Here the X-ray induced current was about 300 nA for 100 R min-', 40 kV soft X-rays. Measurements were performed in a standard TV system with a field time of 16 ms. Figure 11 shows the X-ray monitor image of an IC package. Clearly resolved gold bonding wire images of an IC package were observed, though they included some imperfections in the form of white spots. After-image lag was quite large. These problems can possibly be overcome by improvement of the film deposition process.
266
Y . HATANAKA ET A L .
FIG. 1 I . Reproduced image of IC package observed using X-ray radiation.
SUMMARY
The visible-range optical band-gap and high absorption coefficient of CdTe make it ideal for solar cells as well as X-ray and y-ray detectors. The most promising use of the CdTe/a-Si :H heterojunction is not only in X-ray camera tubes but also in large-area sensors such as solid-state panels. The X-ray absorption coefficient of CdTe was measured as about 3000 cm-I, one decade larger than that for c-Si or a-Si :H. A sc-CdTe/a-Si:H heterojunction consisting of an n-type single-crystal CdTe substrate and p-type (B-doped) aSi: H prepared by the glow discharge method exhibited good diode behaviour with a low reverse bias saturation current of a few nA cm-*. The wide spectral response down to 1.5 eV indicates that a depletion region forms in the sc-CdTe and photoexcited carriers are injected efficiently into the a-Si :H film. A pcCdTe/a-Si :H heterojunction consisting of a CdTe film prepared by RF sputtering and glow discharge a-Si:H also showed clear diode characteristics and was used in a vidicon type tube. This device showed resolution greater than 800 TV lines and a sensitivity of 300 nA per 100 R per minute, i.e. one decade higher than that of a-Si:H based devices. ACKNOWLEDGEMENTS This work was in part supported by the Research Development Corporation of Japan. The authors wish to thank Dr. R. Nishida for his useful advice on the X-ray sensor and Mr. T. Kawai
X-RAY IMAGING SENSOR
267
of Hamamatsu Photonics Co. for providing the Be face-plate and facilities for tube assembly, and finally, Dr. K. Hirata of Nippon Mining Co. for providing the single-crystal CdTe wafers.
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