Use of n-i-p-i-n a-Si:H structure for bistable optically addressed spatial light modulator

O -CR i UDS

Journal of Non-Crystalline Solids 137&138 (1991) 1325-1328 North-Holland

USE OF N-I-P-I-N a-SI:H STRUCTURE FOR BISTABLE OPTICALLY ADDRESSED SPATIAL LIGHT MODULATOR J.B.CHEVRIER 1, P.CAMBON 2, R.C.CHITTICK 3, B.EQUER 1 1 Laboratoire de Physique des Interfaces et des Couches Minces,UPR 258 du CNRS, Ecole Polytechnique, 91128 PALAISEAU, FRANCE. 2 Ecole Nationale Sup~rieure des T~l~communications de Bretagne, BP 832, 29285 BREST CEDEX, FRANCE. 3 S.T.C. Technology LTD, London road, HARLOW, ESSEX CM17 9NA, ENGLAND. We present the association of n-i-p-i-n a-Si:H structure and ferroelectric liquid crystal to realize a bistable optically addressed spatial light modulator. We demonstrated a good output power contrast ratio, and a high processing speed. The choice of a such amorphous silicon structure is discussed, as well as the influence of its p doped layer.

1. INTRODUCTION Light valves or optically addressed spatial light modulators (OASLM) play an important role in coherent or i n c o h e r e n t light processing applications. They are based on a liquid crystal (LC) layers, the control signal is either electrical (as in LC flat display) or optical. In the last case, a photoconductor is used to address the device. Main applications are : T.V. projection display, computer data displays. The second category includes: spatial filtering, optical computing, wavelength shifter (for example white light to IR converters), incoherent to coherent light converters, and interconnection networks. For many applications, spatial light modulators (SLM) must satisfy the requirements of : fast frame rates, high contrast ratio and resolution. In a first stage, the LC was associated with a crystalline photoconductor (cadmium sulfide or silicon), silicon has a good spectral bandwidth and relatively high speed, but because of its large thickness, its spatial resolution was low. Ashley1, 2 proposed to associate an amorphous silicon photoconductor layer. The aSi:H, as a photosensor, has been demonstrated to have many advantages over crystalline photosensors: good spatial resolution, possibility to be deposited in a thin and uniform layer, at low 0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V.

temperature. The device speed was then photoconductor limited. Moreover, the LC needs a bias of 20V to be switched. A simple a-Si:H intrinsic layer must have an important thickness in order to reduce the leakage current and avoid breakdown. OASLM can be used to control a light beam either by reflexion or by transmission. In this case, a compromise between the quantum efficiency (writing) and the transparency (reading) oblige to deposit only thin semiconductor layer. Moddel 3 proposed to associate a bistable ferroelectric liquid crystal (FLC) with an amorphous silicon p-i-n structure. But FLC or other LC need a zero average polarization. So the photosensor must be alternatively, positively and negatively, polarized. Therefore, a symmetrical structure is preferable. So, a new device was proposed 4,5, which has the potential to achieve such requirements: a hydrogenated amorphous silicon symmetrical n-i-pi-n structure to optically address a bistable FLC based OASLM. In this paper, we discuss the effect of inserting a p doped electron blocking layer in an electron conducting n-i-n structure. We present the results on a n-i-p-i-n structure with a high breakdown voltage, low leakage current, and a fast switching time.

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J.B. Chevrier et aL / Use of n-i-p-i-n a-Si:H structure

2. DEVICE TECHNOLOGY Figure 1 shows the prototype device structure. The a-Si:H deposition has been performed by PECVD technology in the ARCAM reactor6 on 5 x 5 cm 2 Glass (Hoya touched polished borosilicate) covered with 80 W/square inch ITO (Donnely mirrors). WRITE SIDE

a-Si:H). A quick calculation shows that: if T=20ms (for example, TV projection) then L>>301~m. For optical computing, a short switching time (time necessary to shift the polarization into the crystal after irradiation) is the second requirement that we need. At T=100~s (for example, light valves), then we must have L>>0.51~m, say, for example, L=5~m. As the absorption depth, in the visible, is 0.51~m in a-Si:H, a large part of the material would be not irradiated and behaves as a serial resistance. The switching time will be dominated by the RC product, where R is the resistance of the non irradiated part, and C the capacity of the crystal, in this case RC=lms. In all cases RC will be larger than the dielectric relaxation time ('Crel) of a-Si:H : 1:rel=l ms. We see that the use of a simple intrinsic a-Si:H used as PC is not possible to make a fast light valve, therefore a thin (<0.5~m) and blocking

READ SIDE

FIGURE 1 Structure of the light valve using a n-i-p-i-n aSi:H structure with a bistable ferroelectric liquid crystal The plasma conditions are: 150°C substrate temperature, 40mT pressure and a growing rate of 0.7 A/s. The n layer is doped with PH3 (2% diluted in H2) and the p layer with TMB (2% diluted in H2). The typical thickness of the layers are: 150 A for the n doped one, 3500 A for both i layers. A 800A p doped layer was chosen for the prototype component, while thickness from 50 to 4000A are used for the p doped layer study (see below). AlumiNum dots (800gm 2) provide a reflective l a y e r usefull for testing. 3. PHOTOSENSOR STRUCTURE The simplest technique to make light valves is to use a simple photoconductor (PC) layer. But it can be seen that the PC needs to satisfy several contradictory requirements. First, it must have a low dark leakage current. It must be sufficiently low not to have the time to charge the capacity of the crystal and to switch this one. Let T be the time between two writings and L the thickness of the PC (intrinsic

structure is necessary. Moreover, a thin structure allows a better spatial resolution. Simple n-i-n structure can be considered. They have been used in other optoelectronic devices 12. Inserting a p layer as an electron blocking layer, gives an extra degree of freedom. The n-i-p-i-n structure, besides its other applications 8-9, meets the required specifications. In first approximation, the structure can be seen as two head to foot diodes. One behaves like an reverse polarized photodetector, the other one as a very small resistance. As the writing beam comes to the same side, the two diodes have two different spectral sensitivities. The first (on beam side) is more sensitive to the blue light, the second one to the red light. Different means can be used to guarantee the same responsivity. In our case the working wavelength (633nm He-Ne laser) corresponds to a rather similar sensitivity of the two diodes. This optical disymmetry can be turned in an advantage, by using the device for colour identification 10. The symmetry can also be reached by changing one of the i layer thickness for given wavelength.

ZB. Chevrier et aL /Use of n-i-p-i-n a-Si:H structure

In fact, the electrical behaviour is more complicated than two head to foot diodes. The dark conductivity of the n-i-p-i-n structure is mainly controled by the minority carriers in the p doped layer. Moreover, our structure is not really electrically symmetrical : i-p and p-i interfaces are not the same. In order to study the electron transport, we deposited a series of samples, keeping the n and i layer thickness fixed, and changing the p doped layer ones (50,&, 125,&,, 250,&,, 500,&,, 1000,&,, 2000A and 4000,&,). 4. P DOPED STUDY We first study the ohmic conductivity of the structure using a low polarization of 10mV and varying the substrate temperature of the sample between -80°C to 180°C. The conductivity versus thickness L shows two completely different behaviours depending on whether L>250,&, (fig. 2) or L<250/&, (fig. 3). 10 .5 10 -e

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agreement with the time of flight experiments 11. We confirm here, that a thick p doped layer (2000,&,) is a good mean to have low leakage current in p-i-n sensors 7. The current activation energy increases at high temperature (>100°C), and for larger p layer thickness. The Arrhenius plot indicates that, at least two conduction mechanisms are implied, and that localized states electron conduction in the p layer is dominating at room temperature. Figure 3 shows the current in the structures for thickness _< 250A. The current increases with the p doped layer thickness. In this case: I=10 exp(L/Z2) {18ooo ! 10

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FIGURE 2 Current in n-i-p-i-n structure as function of the player thickness (>250,&,), for different temperatures, at low polarization (10mV) We find that the current has the form: I=i0exp-(L/X1) Where l0 is a constant, activated with temperature, its activation energy is Eal=0.2 eV. ;L1 can be considered as an effective electron diffusion length through the i-p-istructure. Its value, at room temperature is Xl=1600,&,, and it increases almost linearly, with kT. The corresponding effective life time at room temperature is 10-8 s, in a good

Except for the highest temperatures (100°C and 180°C), where it is nearly constant, I0 is again temperature activated with Ea2=Eal=0.2eV and X2=100,&,. Notice that it's possible to have a good electron blocking structure with a very thin p doped layer (50,&,). At the high polarization required for OASLM operation, only thick p layer devices have a large enough breakdown voltage. 5. RESULTS ON OASLM Figure 4 shows the results obtained on some of best OASLMS. The upper trace represents the periodic driving pulses 20V peak. Lower traces represent, superimposed, the 100 ms width light pulses (with an amplitude of 25mW/cm 2) and the optical response of FLC. Two frames are represented, the first one without writing.

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].B. Chevrier et aL / Use o f n - i - p - i - n a-Si:H structure

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ratio. The p layer thickness is a key parameter in the n-i-p-i-n b e h a v i o u r . There is a large room for improvements in optimizing the other parameters, for example to increase the photosensitivity and to find the best compromise between transparency and quantum efficiency to allow cascability. Furthermore, it remains to better understand electronic transport in structures with very thin p doped layers.

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FIGURE 4 Recorded oscillograms for the test OASLM device. Upper part: applied voltage. Lower part: writing light level (doted ligne), reading light level (solid ligne) The first light pulse is the erasure one's. It is applied to the device, together with the +/- erasure electrical driving pulses, at each frame. This subcycle forces always the device to the reflective state. The figure clearly shows such a correct erasure process. On the second subcycle, no writing is attempted, the same electrical pulses, (but with opposite phase) are applied and the FLC shows no commutation but a small transient. This transient is probably due to the capacitive coupling of the driving pulse to the liquid crystal through the a-Si:H structure. In the second frame, after an erasure subcycle, a writing light pulse is applied with the +/- writing electrical driving pulses (writing subcycle). The oscillogram clearly shows the correct switching of the FLC on the second frame. The optical pulse is then effective in storing optical information in the FLC (last subcycle of the second frame). An excellent power contrast ratio of 10 db measured with calibrated optical densities is displayed. 6. CONCLUSION We have designed a fast OASLM using a n-i-p-in a-Si:H structure, with an equivalent processing speed _> 3 Mbit/s and with a good power contrast

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