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Schottky Barrier and the Behaviour of the Optically Addressed Spatial Light Modulator with Metal Mirrors
F. Pérennès, Z. Wu
To cite this version:
F. Pérennès, Z. Wu. Schottky Barrier and the Behaviour of the Optically Addressed Spatial Light Modulator with Metal Mirrors. Journal de Physique III, EDP Sciences, 1997, 7 (2), pp.451-459.
�10.1051/jp3:1997132�. �jpa-00249587�
Schottky Barrier and the Behaviour of the Optically Addressed
Spatial Light Modulator with Metal Mirrors
F. PdrennAs and Z-Y- Wu (*)
Ddpartement Optique (**), #cole Nationale Sup6rieure des T616communications de Bretagne,
BP 832, 29285 Brest Cedex, France
(Received 24 May 1996, accepted 24 October 1996)
PACS 42.30.-d Imaging and optical processing PACS.42.70.-a Optical materials
Abstract. The Schottky barriers formed by the ITO la-SiH/Al and ITO la-Si:H /Pt contact have strong effects on the characteristics of an OASLM, its sensitivity, the maximum working frequency and the recording of images.
R4sum4. La barriAre de Sihottky du contact ITOIa-Si:H/Al ou ITO la-Si H/Pt a des effets
importants sur les caract4ristiques d'OASLM la sensibilit4, le maximum de fr4quence de travail et l'enregistrement d'images.
1. Introduction
The Optically Addressed Spatial Light Modulator (OASLM) is a key component in the optical correlation and processing. It is made of two ITO coated glass electrodes, a photoaddressing layer deposited on one of the electrodes and a liquid crystal layer between the photoconductor
and the other electrode. See Figure I. A voltage is applied to the two electrodes. When
an image is projected onto the photoaddressing layer, the photoconductivity increases in the illuminated areas and the main applied voltage falls across the liquid crystal to make them switch. The liquid crystals layer in dark areas do not receive sufficient voltage and will remain in the original state for some time. The incident image information is therefore transformed
into a corresponding birefringence image in the liquid crystal. The birefringence image may be read out by a light beam either in transmission or reflection. If the reading is by reflection,
it is useful to deposit a pixelated metal mirror or a dielectric mirror on the photoaddressing layer to enhance the reflection. However it is complicated to deposit a dielectric mirror. In
addition this kind of mirror must be very thick (1.3 ~m in the case of Ti02 Si02 multilayer)
and can introduce image lag ill. Therefore the metal mirror is most commonly used, except for
cases requiring high resolution [2]. In the metal mirror case, a metal /semiconductor contact is
formed. Certain metal /semiconductor contacts can be considered as ohmic contacts, but not all. This paper addresses the relationship between the photoconductor/metal contact and the
characteristics of an OASLM.
(*) Author for correspondence (e-mail: zy wuflenst-bretagne.fr) (**) UMR CNRS 1329
© Les (ditions de Physique 1997
452 JOURNAL DE PHYSIQUE III N°2
a-Si
LClayer
LED Phoiodiode
0 Laser i
Laser2 Mask
OASLM
v
Fig 1. The schematic of the measurement of the first diffractive order intensity
2. Experiment
Two kinds of OASLM were made, the first based on an ITO la-Si:H /Al/FLC /ITO, the other an
ITOIa-Si:H/Pt/FLC/ITO a-Si:H is hydrogenated amorphous silicon and is photoconductor.
Its thickness is about 2 ~m. The metals, Al and Pi, were chosen to make the mirror because
they have good reflectivity and form two quite different Schottky barriers on a-Si:H. The metal mirrors were about 1500 I thick and pixelated with 10 x 10 ~ size pixels. The liquid crystal
is ferroelectric (FLC), MERK, SCEI3, and its thickness is about 2 ~m.
To study the effects of metal /semiconductor contacts or Schottky barriers on OASLM char- acteristics, two kinds of diode were made, one ITO la-Si:H/Al, the other ITO la-Si:H/Pt. The former diode area is 0.03 cm~2; the later is 0.07 cm~2. Their V-I characteristics were mea- sured. Except for the non-pixelation of the metal layer, the configuration of each example was
the same as the photoaddressing part of the corresponding OASLM.
Figure I sho~vs the schematics of the measurement of the resolution of an OASLM. A grating image is projected onto the a-Si:H side. The projected image may be a square grating formed by a test chart. It may also be a sinusoidal grating produced by an interference setup. An
expanded laser beam (Laser 2 in Fig. I) reads the birefringence image formed on the FLC side to form a diffraction pattern. The intensity of the first order diffraction and its evolution
with the time are measured by a photodiode and are used to evaluate the behaviour of image recording [3]. This intensity is called diffraction intensity in the present paper. The LED in
Figure I is used to erase the recorded image.
3. Results and Discussion
3.I. V-I CHARACTERISTICS. The V-I characteristics of ITOIa-Si.H/Al and ITOIa-
Si:H/Pt are shown in Figure 2. The sign "-" or "+" is used to indicate the bias condi- tion, for example, -ITOIa-Si.H/Al+.
For -ITOIa-Si:H/Al+ in the dark, the current increases slowly with the applied voltage.
Even at V
= 20 V, the current is less than 1.3 x 10~~ A cm~2. When the bias is reversed, the current rapidly increases for Vi < 0.5 V. Then -it varies very slowly for 20 > VI > 0.5 V.
The current is always less than 4 x 10~~ A cm~2. Looking at the graphs more closely, one
IF IA) 10 mW cm~2
decade '
V>0 ~~~~
20.00
~) 2.000/div (V)
IF (A) 10 mW cm~2
/dlv lldB
V<0
0.0000
~~
20.00
b) 2.000/div (V)
,~
Fig. 2 l~-I characteristics: a) +ITOIa-Si:H/Al-, b) -ITOIa-Si:H/Al+, c) +ITOIa-Si:H/Pt-, d) -ITOIa-Si:H/Pt+.
can see that the current even decreases slightly in the range of 0 5 < )V) < 12 V. It is
believed that the applied voltage modifies the a-Si:H/Al barrier and carriers are injected into a-Si:H [4]. Under illumination, for -ITO la-Si:H/Al+, the current is saturated when V > 6 V.
The saturated current is proportional to the photoconductivity. When the bias is reversed, the effect of voltage on the barrier is also observed and is same as in the dark. But this happens
in a lower voltage range. The current is saturated when IV > 12 V. The saturation current is determined by the photoconductivity. But it should be noted that there is a fundamental
difference in the quantum efficiency between the direct and the reverse polarization. For the
reverse polarization under an 0.I mW cm~~ illumination of at 1= 0.632 ~m and V
= 14 V,
the photocurrent is 3 x 10~~ A cm~~ giving a quantum efficiency 7. There must therefore be the contribution of the secondary photocurrent However when Al is positively biased,
454 JOURNAL DE PHYSIQUE III N°2
IF (A) ~~-2
jo-4
V>0
~~~~
10~9
0.0000 w
~) 2.000/div (V)
IF (A) 10 mW cm'2
decade V<0 ~~~~
v~
20.00
d) 2.000/div (V)
Fig. 2. (Continued.)
the photocurrent is 3 x 10~~ A cm~~. The quantum efficiency is only 0.7, indicating that just primary photocurrent is present. In both cases, the ratio Ip~oto » Idark, the injection current is always negligible. Under a 0.I mW cm~~ illumination, Iphoto/Idark " 10~ for the forward
bias and Iphoto/Idark = 10~ for the reverse bias. The ratio Iphoto/Idark of an optoelectronic device is an important parameter, it represents the SIN ratio at the output. For an OASLM,
it determines the contrast. In the present case, it determines the diffraction intensity.
As to -ITOIa-Si:H/Pt+ in the dark, it shows rectifying V-I characteristics. The for-
ward current increases exponentially when V < 0.2 V. When V > 0 2 V, the forward cur-
rent continues to increase but more slowly with the applied voltage and may be as high as
1.5 x 10~~ A cm~~ at V
= 14 V. Under illumination, the forward current does not show saturation because of this injected current. For a 0.I mW cm~~ illumination, the ratio
o.oi
- Pt+8V
if ~'"~ ~~ ~8V /~
) 0.001 ° AL+8V , ~ '
~ -- Al -8V ~'
H
,
W'
( ~'
/ '
( 10~~ ~
(
10~
0.001 0,01 0.I
l/Io
Fig. 3. Comparison of photocurrents for the two configurations. lo
= 8 mW cm~~,
= 632 nm
Iphoto/Idark
" 5 at l~
= 14 V, and the injection current is no longer negligible compared
to the photocurrent. Idark may even become higher than Iphoto when l~ > 17 V. Under reverse bias, the current shows saturation. The saturated current is illumination dependent as in the ITO la-Si:H /Al case. In the dark, the current is as low as 1.5 x10~~ A cm~~. Iphoto/Idark
" 10~
for the illumination of 0.I mW cm~~.
Figure 3 shows the photocurrents under the forward and the reversed biases for the Al mirror
case. The + ITOIa-Si:H/Al- is clearly different from the other three cases.
3.2. I'-I CHARACTERISTICS AND OASLM IMAGE RECORDING. When the two OASLMS,
ITO la-Si:H/Pt /FLC /ITO and ITO la-Si:H/Al/FLC /ITO, record an image, they show different
behaviour. The Pt-OASLM can record images only if it is biased as +ITOIa-Si:H/Pt/FLC/
ITO-. When the bias is reversed, one observes an imageless uniform white cell. However the Al-OASLM can record images whatever the polarization. Of course, the image recorded under reversed bias is the negative of the forward bias. The above phenomena can be explained by
the V-I characteristics described in the previous paragraph. When a voltage is applied to an
OASLM, the voltage drops across the liquid crystal can be calculated by the equivalent circuit and is written as:
t
VLcit)
= jRslph + Vamp)
I ~ij
, T = Rshic~_sj + CLC) il)
where the symbols are defined as in Figure 4. The maximum current which does not make
liquid crystals switch is therefore:
Va-si(Ca-si + CLC) In 1 ~~
fin "
~~~~~ ~ ~~~~
(2) where VLC is the threshold voltage above which the liquid crystal starts switch. In our case, VLC = I V. §_sj is the voltage drops aiross a-Si.
456 JOURNAL DE PHYSIQUE III N°2
Rs
~a-Si
R~
Fig 4. Equivalent circuit of an OASLM.
The cell capacitance consists of two components, the geometry capacitance and the spon-
taneous polarization of the LC. The first is about 1.5 nF cm~2, for
eFLc " 3.5, d
= 2 ~m.
The second is about 6 nF cm~2 if the spontaneous polarization is 30 nC cm~2 and the ap- plied voltage to LC is 10 V. But the spontaneous polarization of the LC is not taken into account because the liquid crystals are not switched. When the cell is polarized as ITO la-
Si:H/Pt/FLC/ITO+ and is kept in the dark, fin
= 2 x 10~~ A cm~2, as indicated by the V-I characteristics. So even when the OASLM is not illuminated, the current is injected by the
applied voltage and is strong enough to make the liquid crystals switch. The current is injected every~v.here. As consequence, the OASLM records an uniform image. However, if the polar-
ization is reversed, the injection current is only 3.5 x 10~~ A cm~2. The liquid crystals in the dark could not switch by the application of a voltage alone. One has to increase the injection
of current by illumination. Only in the illuminated areas is there enough injected current (of
course the illumination should be strong enough) and only there the LC can switch. In this
way an image is recorded. The same augment can explain why ITO la-Si:H/Al/FLC /ITO can
record images for both polarization directions.
It should be noted that the record image can only persist for a given time, after which it will
disappear. If the OASLM is not bistable and we need a persistent image, we have to repeat the
erase-registration cycle. For the illuminated areas, the injected current is strong and the liquid crystals there are switched rapidly. But this does not mean that the liquid crystals in dark
areas will remain unswitched for ever. As indicated by equation (2), the current required to
make the liquid crystals switch is time dependent. As the injected current in the dark areas is 1.5 -3.5 x10~~ A cm~2, the voltage applied to LC is less than I V as long as t < 1.2 x 10~2
s. But
if the voltage is applied for more than 1.2 x 10~2 s, the injection of current makes the voltage drop over the LC greater than the threshold I V, thus the LC may switch in the dark areas.
The difference in birefringence between the illuminated and the dark areas will disappear. And therefore the image will disappear. In our present case, the diffraction intensity will drop to zero after a certain time. See Figure 5.
3.3. l~-I CHARACTERISTICS AND OASLM SENSITIVITY. It seems appropriate to define the OASLM sensitivity as the minimum illumination required to make the liquid crystals switch. Actually it is not so simple. Three parameters influence the image recording, the
illumination, the amplitude and the duration the applied voltage. Any one of them can be
compensated by the others. To register an image, one can illuminate the OASLM weakly
but apply a relatively high voltage. Or one can equhlly apply a lower voltage with a longer
50
~ sqr 101ps/mm
) 40 ,'
( '
---~- sin 101p/mm
I ',
.@ 30 ',
~ j
#
? 20 j
11 ,
fi 10 ',
g ,,
0
"'
0 0.0002 0.0004 0.0006 0.0008 0.001
t (sec)
Fig. 5. -Diffraction intensity for square and sinusoidal gratings The spatial frequency is 101psmm~~
2
')~ l ~ Al ° Pt g a
j g o
j 0.8 ~°
fl ,
I 0.6 ,'o
5 ,o
1 0.4 .'°
i~[ , , oo
o 0.2
, ~°
~ G~~~
(.01 0.I 10
Illumination (mW/cm~2)
Fig. 6. Comparison of the diffraction intensities of ITOIa-Si.H/Al/FLC/ITO, ITOIa-Si.H/
Pt/FLC/ITO as functions of the writing intensity A sinusoidal grating was formed on the two OASLMS. The applied voltage was 20 V, the impulsion lasted 820 ~ts
duration (there is of course some limit to this kind of compensation). Therefore to calibrate the sensitivity of OASLM, the best method is to compare OASLMS under the same conditions.
The measurement of diffraction intensity is considered to be good method for evaluating the
sensitivity.
Figure 6 shows the diffraction intensities of the two OASL~IS versus illumination intensity.
Obviously, +ITO la-Si:H/Al/FLC/ITO- demonstrates a higher sensitivity. This is consistent with the fact that + ITO la-Si:H/Al- has a higher quantum efficiency than ITO la-Si.H/Al+
or +ITOIa-Si:H/Pt+, as discussed in the preceding paragraph. ITOIa-Si.H/Pt/FLC/ITO
458 JOURNAL DE PHYSIQUE III N°2
1,1
i~
j 0.9 ,
c §
.@8 °.8 ~
( 0 7 &
0.6 ---~- A10.8mW
i -.-.~... Al 8mW 4
° ~
-a Pt 0.8mW .
fl o 4 -.--.o-.- Pt 8mW I
b '
0.3
10 100 1000 10000
Frequency
Fig 7. The variation of diffraction intensity of OASLMS ITOIa-Si:H/Al/FLC/ITO and ITOIa-
Si:H/Pt/FLC /ITO as a function of working frequency. Illumination was 0.8 mW cm ~~; applied voltage
was +20 V; and the a-Si.H 2 ~tm thick.
reaches the maximum diffracted intensity when the illumination is I mW cm~2; then it shows
a decrease with increasing illumination. +ITOIa-Si:H/Al/ FLC/ITO- reaches the maximum at I mW cm~2 too; but its diffraction intensity then remains unchanged for increasing illumi-
nation.
3.4. WORKING RATE. One of the main advantages of FLC is its rapid switching. The
typical switching time is about 100 ~s The highest working frequency of an OASLM is determined by the least time necessary to charge the LC capacitance, which is naturally related
to the photoconductivity and the applied voltage. We use the diffraction intensity as a function of frequency to evaluate the working rate of OASLM. This is based on the idea that if the
OASLM cannot follow the rhythm of writing-erasing, the diffraction efficiency will decrease.
The most rapid rhythm that it can follow represents its limit of working frequency. Because of the mutual-compensation effect discussed in the previous paragraph, the measurement is
carried out under a given illumination and applied voltage.
Figure 7 shows that the diffraction intensity of ITOIa-Si:H /Al/FLC /ITO starts to decrease at 600 Hz, while for ITOIa-Si:H/Pt/FLC/ITO it does so at 1000 Hz. Both cells were illumi- nated at 0.8 mW cm~2 under an applied voltage of 2 V. For an illumination up to 8 mW cm~2,
the working frequency may be as high as 3 4000 Hz, and there is no difference between the two OASLMS. As shown by the V-I characteristics, ITO la-Si:H/Pt has a built-in field, which aids in collecting the photocarriers. While in ITOIa-Si:H/Al, there is no visible built-in field.
To collect the photocarriers, the illumination should be higher. Under the conditions: +20 V and 8 mW cm~2, the a-Si:H is completely depleted for the two OASLMS, they show no differ-
ence in working frequency.
Conclusions
1. The Schottky barriers formed at the interfaces of ITOIa-Si.H/Al and ITOIa-Si.H/Pt have strong effects on the characteristics of an OASLM, its sensitivity, the maximum working fre-
quency and the recording of images.
2. V-I characteristics indicate the existence of the secondary photocurrent in ITOIa-Si:H/Al configuration.
3. Generally speaking, the working frequency of ITOIa-Si:H/Pt/FLC/ITO is higher than
ITOIa-Si:H/Al/FLC/ITO. But the working frequency is a function of illumination intensity.
Under strong illumination, the two OASLMS show no difference in working frequency.
References
ill Fukushima S., Kurokawa T. and Ohno M., Ferroelectric spatial light modulator achieving bipolar image operation and cascadability, Appt. Optics 31 (1992) 6859-6868.
[2] PArennAs F., Crossland W.A., Koslowski D. and Wu Z.Y., New reflective layer technolo- gies for fast ferroelectric liquid crystal optically addressed spatial light modulators, Ferro-
electrics181 (1996) 129-137.
[3] PArennAs F. and Wu Z-Y-, The dominant factor of the resolution of ferroelectric liquid crystal OASLM, to be published in Appt. Optics.
[4] See for example Rose A., Concepts in Photoconductivity and Allied Problems il. Wiley,
New York, 1963).
