- Email: [email protected]
Correlation of photoelectric properties of a-Si:H image sensor with compositional distribution of Si3N4 blocking interface
Journal of Non-CrystallineSolids 59 & 60 (1983) 1227-1230 North-HollandPublishingCompany
1227
C O R R E L A T I O N O F P H O T O E L E C T R I C P R O P E R T I E S O F a-Si:H I M A G E S E N S O R W I T H C O M P O S I T I O N A L DISTRIBUTION O F Si3N 4 B L O C K I N G I N T E R F A C E
S.KANEKO, F.OKUMURA, H.UCHIDA, U.KAJIWARA AND T.SAITO
M.KANAMORI, M.SAKAMOTO, T.ITANO,
Microelectronics Research Labs., NEC Corporation, Kawasaki, 213, Japan A long linear image sensor using a-Si:H has been fabricated for compact facsimile equipment. In this photosensor, Si3N 4 and p-a-Si:H blocking layers were inserted between photosensitive i-a-Si:H and electrodes to decrease a dark current. An undesireble thick transition layer, which contains nitrogen, was observed between Si3N4 bloking layer and photosensitive i-a-Si:H by SIMS measurement. This transition layer was able to be thinned by decreasing the a-Si:H deposition rate and photoelectric properties were remarkably improved.
I. INTRODUCTION A long linear image sensor is an attractive device for compact facsimile equipment, because this sensor does not need high magnification lens system which requires considerable space in the equipment. 1,2 In a-Si:H application to this sensor, a-Si:H is sandwiched between a transparent electrode and a metal electrode. In this case, both contracts must be blocking contacts to obtain small dark current and short photoresponse time. In electrophotography and vidicons, Shimizu et al 3 have used a blocking layer, such as thin Si3N4 layer or doped a-Si:H layers, between the a-Si:H and an electrode to obtain a blocking contact. Good photoelectric properties have been obtained using the Si3N4 layer. This paper reports a long a-Si:H linear image sensor, using Si3N4 and p-type a-Si:H (pa-Si:H) blocking layers, and photoelectric properties correlation for this sensor with the Si3N4 blocking interface compositional depth distribution. 2. BASIC S T R U C T U R E
A N D C O M P O S I T I O N A L D E P T H DISTRIBUTION
Figure I shows a basic structure in which a photosensitive intrinsica-Si:H (i-a-Si:H)is sandwiched between an indium tin oxide (ITO) transparent electrode and an AI electrode.
In order to make blocking contact, Si3N4 and p-a-Si:H layers are inserted between the i-aSi:H layer and electrodes.
These Si3N4 and a-Si:H layers were produced by capacitive
coupled rf glow discharge technique. Deposition temperature was 210 "C and rf power was 0.047 w/cm ,~ 0.097 w/cm. Deposition rate for a-Si:H was S0 '~ I SOA/min. A SiH4/NH3/N 2 = 3/4/60 gaseous mixture was used to obtain transparent and highly resistive (greater than 1015.Q-cm) Si3N4 films.
The i- and p-a-Si:H layers were deposited using a B2H6/SiH4
gaseous mixture of 3.5 ppm and 200~ S00ppm, respectively. Resistivity values for these layers are ~,5X I 010 Q.-cm for the i-a-Si:H layer and 108 ~- I 010 ~ - c m for the p-a-Si:H layer, 10022-3093/83/0000-0000/$03.00
~1983 North-Holland/Physical Society of Japan
S. Kaneko et al. / Photoelectric properties o f a-Si.'lt image sensor
1228
42Si N* i - a - Si:H
Jf
Va
b. /: jwi
c
I q ;i
.d
Light
1
v
,
(a)// .....
AI
•
\
c
Glass
p-a-Si : H 2000~
Si~l~ 3OO A
..--(~
2.5/=m
2.5
2.7
2.9
Depth (~um)
Figure I
~asic structure of photosensor
Figure 2 Nitrogen depth distribution at the interface between Si3N 4 and i-a-Si:H layers
respectively. Since these Si3N4, i- and p-a-Si:H layers are produced continuously in the same apparatus, a transition layer, which contains residual nitrogen for Si3N4 formation, is formed unintentionally between the Si3N 4 and the i-a-Si:H layers. This transition layer can be thined by decreasing the a-Si:H deposition rate at the i-a-Si:H formation.
Figure 2
shows nitrogen depth distribution at an interface between Si3N 4 and i-a-Si:H layers measured by SIMS. In this figure, distribution (a) corresponds to IS0 A/min deposition rate for a-Si:H and indicates an about IS00 A thick transition layer. distribution (b) indicates a less than 500 A thick transition layer.
On the contrary,
In this case, deposition
rate for a-Si:H was SO A/min. 3. PHOTOELECTRIC PROPERTIES Figure 3 shows I-V characteristics for this sensor with positively biased ITO electrode. Monochromatic
light
from
a tungsten
lamp was used
in this
measurement.
I-V
characteristics for the sensors with only one blocking layer are also shown in this figure. The photosensors without one of the two blocking lasyers show large dark current.
On the
other hand, the dark current for the sensor with both blocking layers is sufficiently small. Phatocurrent under 35 Ix illumination satrurates at 4V applied voltage.
These facts
indicate that the Si3N 4 blocking layer rejects hole injection from the ITO electrode and the
S. Kaneko et al. I Photoelectric properties o f a-Si.'H image sensor
1229
Photo-current
]
10 -7
570nm 35
,ix
~/ Va = 5V
/
E E i0 -ll
1.0
~' r+" Da;~r'ent "~
f f,,,+',;.
I0 -e
o
0.5
//,-#1"
-p I
I0-1°
8
(3 a~
400
io-il
Wave .--o
i" 0
I
I I0
5
600
~
800
length (nm)
. . . . . ~o . . . . D a r k current
..o---I O -12
500
I 15
__
Voltage (V]
Figure 3 I-V characteristics for photosensors with and without blocking layers, o , with Si3N 4 and p-a-Si:H blocking layers; , without Si3N4blocking layer; x , without p-a-Si:H blocking layer
Figure 4 Spectral sensitivity for the photosensor. Applied voltage isSV. Photosensor with thin transition layer - - - Phofosensor with thick transition layer
p-a-Si:H layer rejects electron injection from the AI electrode, while the photocarriers pass through these blocking layers. Figure 4 shows spectral sensitivity for the photosensors. The applied voltage for the photosensar was S V.
The photosensor with the thin transition
layer shows high
photosensitivity throughout the visible light region. In the case of the photosensor with thick transition layer, photosensitivity was decreased in the shorter wave length region. It has been reported that nitrogen doping in a-Si:H decreases resistivity and shows 107~108~-cm resistivity 4,5,6
In this experiment, this notrogen doping effect was
confirmed and 109 ,~ 10 I0 _O.-cm resistivity a-Si:H was obtained using gaseous mixture of B2H 6 / Sill 4 = 3.Sppm and N2/SiH4= I ,~ 4. In this sensor, resistivity for the transition layer will decreases by this nitrogen doping effect, resulting in small electric field in the transition layer. This small electric field decreases collection efficiency for photoearriers, especially, in the shorter wavelength and for the thick transition layer. 4. LINER IMAGE SENSOR A 960 element, 8 elements/mm linear photosensor array was fabricated.
Figure S
S. Kaneko et al. / Photoelectric properties o f a-Si.'H image sensor
1230
Boron doped p-o-Sl(H) 2000A
AI electrode
200 prn I:" ;{
8 elements /ram
B( ISI SI
O. "-~00~m
I
I
Gloss ,ubstrom
Llght
lOOjum ~
I"
Figure S Linear photosensor array (a) Cross sectional view (b) Microphotographic plane view shows the structure,
The photosensor array was fabricated on a glass substrate with Cr
light shield layer and SiO2 insulative layer. The Cr light shield layer has a 100 um light window. The effective photosensitive area for one element is the overlap portion between light window and the AI individual electrode (100pm X 100ym). Over all photosensor array length is 120 mm. A contact linear image sensor was fabricated. 2 The sensor consists of a pair of yellow-green LEDs for light source, a rod lens array for a compact optical guide and a photosensor array connected to MOS operated with 200 kHz clock frequency. obtained using a thermal printer.
FET switches.
The linear image sensor was
A satisfactory reproduced image has been
ACNOWLEDGEMENTS The authors would like to thank Dr. T. Kamejima and A. Jitsukawa for technical support in the SIMS measurement. They also wauld like to thank Drs. IK. Ayaki, H.Shiraki, H.Katoh and T.Ohkubo for their continuous encouragement.
REFERENCES (I)
K.Komiya et al. IEDM Tech. Dig. (1981) 309.
(2)
S.Kaneko et oh IEDM Tech. Dig. (1982) 328.
(3)
l.Shimizu et al. J. de Physique 42 Suppl. 10 (1981) C4-1123.
(4)
J.Baixeras et al. Philos. Mag. B 37 (1978) 403.
(S) H.Kurata et al. Jpn. J. Appl. Phys. 20 (1981) L81 I. (6) H.Watanabe et al. Jph. J. Appl. Phys. 21 (1982) L341.
1227
C O R R E L A T I O N O F P H O T O E L E C T R I C P R O P E R T I E S O F a-Si:H I M A G E S E N S O R W I T H C O M P O S I T I O N A L DISTRIBUTION O F Si3N 4 B L O C K I N G I N T E R F A C E
S.KANEKO, F.OKUMURA, H.UCHIDA, U.KAJIWARA AND T.SAITO
M.KANAMORI, M.SAKAMOTO, T.ITANO,
Microelectronics Research Labs., NEC Corporation, Kawasaki, 213, Japan A long linear image sensor using a-Si:H has been fabricated for compact facsimile equipment. In this photosensor, Si3N 4 and p-a-Si:H blocking layers were inserted between photosensitive i-a-Si:H and electrodes to decrease a dark current. An undesireble thick transition layer, which contains nitrogen, was observed between Si3N4 bloking layer and photosensitive i-a-Si:H by SIMS measurement. This transition layer was able to be thinned by decreasing the a-Si:H deposition rate and photoelectric properties were remarkably improved.
I. INTRODUCTION A long linear image sensor is an attractive device for compact facsimile equipment, because this sensor does not need high magnification lens system which requires considerable space in the equipment. 1,2 In a-Si:H application to this sensor, a-Si:H is sandwiched between a transparent electrode and a metal electrode. In this case, both contracts must be blocking contacts to obtain small dark current and short photoresponse time. In electrophotography and vidicons, Shimizu et al 3 have used a blocking layer, such as thin Si3N4 layer or doped a-Si:H layers, between the a-Si:H and an electrode to obtain a blocking contact. Good photoelectric properties have been obtained using the Si3N4 layer. This paper reports a long a-Si:H linear image sensor, using Si3N4 and p-type a-Si:H (pa-Si:H) blocking layers, and photoelectric properties correlation for this sensor with the Si3N4 blocking interface compositional depth distribution. 2. BASIC S T R U C T U R E
A N D C O M P O S I T I O N A L D E P T H DISTRIBUTION
Figure I shows a basic structure in which a photosensitive intrinsica-Si:H (i-a-Si:H)is sandwiched between an indium tin oxide (ITO) transparent electrode and an AI electrode.
In order to make blocking contact, Si3N4 and p-a-Si:H layers are inserted between the i-aSi:H layer and electrodes.
These Si3N4 and a-Si:H layers were produced by capacitive
coupled rf glow discharge technique. Deposition temperature was 210 "C and rf power was 0.047 w/cm ,~ 0.097 w/cm. Deposition rate for a-Si:H was S0 '~ I SOA/min. A SiH4/NH3/N 2 = 3/4/60 gaseous mixture was used to obtain transparent and highly resistive (greater than 1015.Q-cm) Si3N4 films.
The i- and p-a-Si:H layers were deposited using a B2H6/SiH4
gaseous mixture of 3.5 ppm and 200~ S00ppm, respectively. Resistivity values for these layers are ~,5X I 010 Q.-cm for the i-a-Si:H layer and 108 ~- I 010 ~ - c m for the p-a-Si:H layer, 10022-3093/83/0000-0000/$03.00
~1983 North-Holland/Physical Society of Japan
S. Kaneko et al. / Photoelectric properties o f a-Si.'lt image sensor
1228
42Si N* i - a - Si:H
Jf
Va
b. /: jwi
c
I q ;i
.d
Light
1
v
,
(a)// .....
AI
•
\
c
Glass
p-a-Si : H 2000~
Si~l~ 3OO A
..--(~
2.5/=m
2.5
2.7
2.9
Depth (~um)
Figure I
~asic structure of photosensor
Figure 2 Nitrogen depth distribution at the interface between Si3N 4 and i-a-Si:H layers
respectively. Since these Si3N4, i- and p-a-Si:H layers are produced continuously in the same apparatus, a transition layer, which contains residual nitrogen for Si3N4 formation, is formed unintentionally between the Si3N 4 and the i-a-Si:H layers. This transition layer can be thined by decreasing the a-Si:H deposition rate at the i-a-Si:H formation.
Figure 2
shows nitrogen depth distribution at an interface between Si3N 4 and i-a-Si:H layers measured by SIMS. In this figure, distribution (a) corresponds to IS0 A/min deposition rate for a-Si:H and indicates an about IS00 A thick transition layer. distribution (b) indicates a less than 500 A thick transition layer.
On the contrary,
In this case, deposition
rate for a-Si:H was SO A/min. 3. PHOTOELECTRIC PROPERTIES Figure 3 shows I-V characteristics for this sensor with positively biased ITO electrode. Monochromatic
light
from
a tungsten
lamp was used
in this
measurement.
I-V
characteristics for the sensors with only one blocking layer are also shown in this figure. The photosensors without one of the two blocking lasyers show large dark current.
On the
other hand, the dark current for the sensor with both blocking layers is sufficiently small. Phatocurrent under 35 Ix illumination satrurates at 4V applied voltage.
These facts
indicate that the Si3N 4 blocking layer rejects hole injection from the ITO electrode and the
S. Kaneko et al. I Photoelectric properties o f a-Si.'H image sensor
1229
Photo-current
]
10 -7
570nm 35
,ix
~/ Va = 5V
/
E E i0 -ll
1.0
~' r+" Da;~r'ent "~
f f,,,+',;.
I0 -e
o
0.5
//,-#1"
-p I
I0-1°
8
(3 a~
400
io-il
Wave .--o
i" 0
I
I I0
5
600
~
800
length (nm)
. . . . . ~o . . . . D a r k current
..o---I O -12
500
I 15
__
Voltage (V]
Figure 3 I-V characteristics for photosensors with and without blocking layers, o , with Si3N 4 and p-a-Si:H blocking layers; , without Si3N4blocking layer; x , without p-a-Si:H blocking layer
Figure 4 Spectral sensitivity for the photosensor. Applied voltage isSV. Photosensor with thin transition layer - - - Phofosensor with thick transition layer
p-a-Si:H layer rejects electron injection from the AI electrode, while the photocarriers pass through these blocking layers. Figure 4 shows spectral sensitivity for the photosensors. The applied voltage for the photosensar was S V.
The photosensor with the thin transition
layer shows high
photosensitivity throughout the visible light region. In the case of the photosensor with thick transition layer, photosensitivity was decreased in the shorter wave length region. It has been reported that nitrogen doping in a-Si:H decreases resistivity and shows 107~108~-cm resistivity 4,5,6
In this experiment, this notrogen doping effect was
confirmed and 109 ,~ 10 I0 _O.-cm resistivity a-Si:H was obtained using gaseous mixture of B2H 6 / Sill 4 = 3.Sppm and N2/SiH4= I ,~ 4. In this sensor, resistivity for the transition layer will decreases by this nitrogen doping effect, resulting in small electric field in the transition layer. This small electric field decreases collection efficiency for photoearriers, especially, in the shorter wavelength and for the thick transition layer. 4. LINER IMAGE SENSOR A 960 element, 8 elements/mm linear photosensor array was fabricated.
Figure S
S. Kaneko et al. / Photoelectric properties o f a-Si.'H image sensor
1230
Boron doped p-o-Sl(H) 2000A
AI electrode
200 prn I:" ;{
8 elements /ram
B( ISI SI
O. "-~00~m
I
I
Gloss ,ubstrom
Llght
lOOjum ~
I"
Figure S Linear photosensor array (a) Cross sectional view (b) Microphotographic plane view shows the structure,
The photosensor array was fabricated on a glass substrate with Cr
light shield layer and SiO2 insulative layer. The Cr light shield layer has a 100 um light window. The effective photosensitive area for one element is the overlap portion between light window and the AI individual electrode (100pm X 100ym). Over all photosensor array length is 120 mm. A contact linear image sensor was fabricated. 2 The sensor consists of a pair of yellow-green LEDs for light source, a rod lens array for a compact optical guide and a photosensor array connected to MOS operated with 200 kHz clock frequency. obtained using a thermal printer.
FET switches.
The linear image sensor was
A satisfactory reproduced image has been
ACNOWLEDGEMENTS The authors would like to thank Dr. T. Kamejima and A. Jitsukawa for technical support in the SIMS measurement. They also wauld like to thank Drs. IK. Ayaki, H.Shiraki, H.Katoh and T.Ohkubo for their continuous encouragement.
REFERENCES (I)
K.Komiya et al. IEDM Tech. Dig. (1981) 309.
(2)
S.Kaneko et oh IEDM Tech. Dig. (1982) 328.
(3)
l.Shimizu et al. J. de Physique 42 Suppl. 10 (1981) C4-1123.
(4)
J.Baixeras et al. Philos. Mag. B 37 (1978) 403.
(S) H.Kurata et al. Jpn. J. Appl. Phys. 20 (1981) L81 I. (6) H.Watanabe et al. Jph. J. Appl. Phys. 21 (1982) L341.