GB1596706A

GB1596706A - Liquid crystal light valve

Info

Publication number: GB1596706A
Application number: GB16130/78A
Authority: GB
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1977-05-02
Filing date: 1978-04-24
Publication date: 1981-08-26
Also published as: SE443245B; SE7804733L; JPS53137165A; IL54544A0; DE2818002C2; FR2390012A1; FR2390012B1; DE2818002A1; JPH027051B2

Description

(54) LIQUID CRYSTAL LIGHT VALVE (71) We, HUGHES AIRCRAFT COMPANY, a corporation organized and existing under the laws of the State of Delaware, United States of America, of Centinela and Teale Street, Culver City, State of California, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to a liquid crystal light valve.

A liquid crystal light valve is known from out British Patent Specification 1,492,289 in which a liquid crystal layer is activated in response to an input radiation pattern, such that the liquid crystal layer provides a spatial representation of the input radiation pattern. The input radiation is detected by a photodiode formed as a layer in a semiconductor body disposed adjacent the liquid crystal layer. The photodiode layer produces a spatial distribution of charge carriers in the semiconductor body, the distribution representing an input radiation pattern. This charge distribution activates the liquid crystal layer.

According to the present invention there is provided a liquid crystal light valve comprising a liquid crystal layer, a semiconductor body including a rectifying junction layer, said layers overlying one another, means for introducing a spatial distribution of charge carriers into said semiconductor body, and a d.c. voltage source arranged to reverse bias said rectifying junction layer such as to form in said body a depletion region extending through at least a major part of the thickness of said semiconductor body and in such a manner that the introduced charge carriers are swept across said body whilst being maintained substantially in said spatial distribution, towards said liquid crystal layer to activate the liquid crystal and provide a spatial representation of said distribution.

An advantage of the present invention is that the depletion region formed in the semiconductor body allows the spatial charge carrier distribution to move across the thickness of the semiconductor body under the influence of the electric field set up across the rectifying junction layer, without substantial lateral spreading due to diffusion, thus improving the resolution of the image produced by the liquid crystal layer.

The charge carrier distribution may be produced in response to optical images, Xrays, high energy electrons or anything else that can generate or inject appropriate minority carriers into the semiconductor body.

In one embodiment of the invention the semiconductor body includes a photodiode providing said rectifying junction layer, arranged to produce said spatial distribution of charge carriers in response to an incident spatial radiation pattern.

In another embodiment a charge coupled device is provided for introducing the spatial distribution of charge carriers into the semiconductor body.

A charge coupled device (CCD) input register can be used to accept serial input data, store it and reformat it for subsequent parallel processing. Such an arrangement is useful for many wide bandwidth optical data processing applications. For example, an optical data processing liquid crystal light valve structure that accepts the distribution of charge from the CCD and converts it to an equivalent variation of optical birefringence, can be used to spatially modulate a laser beam.

The invention will be described hereinafter by way of example and with reference to the accompanying drawings wherein: Fig. 1 is a diagrammatic cross sectional view of a liquid crystal light valve structure with a silicon photodiode, constructed in accordance with the present invention, Figs. 2, 3 and 4 show three embodiments of photodiodes for the device of the invention, Fig. 5 is a diagrammatic cross sectional view of another liquid crystal light valve structure constructed in accordance with the present invention, Fig. 6 is an equivalent circuit of a readout structure of the invention, Fig. 7 is a graph of the voltage peak across the liquid crystal in an embodiment of the invention, and Fig. 8 is the liquid crystal response to the voltage of Fig. 7.

Referring now to Fig. 1, the shown direct current (DC) liquid crystal light valve includes a substrate 5 that is transparent to incident light 10. Next to the substrate 5 is a silicon photodiode 12. It has a thin P layer 14, a relatively thick N layer 16 and a PN junction 15. In a test unit of one embodiment, the thin P layer had a thickness of 0.2cm and a resistivity of 0.02s2- cm. This layer Is highly conductive and in addition to being the P-side of a PN junction, it is also used as one electrode of the entire device. The adjacent layer 16 is N type, relatively thicker and highly resistive.

In one embodiment, this layer was 5 mil thick and had a resistivity of 3000cm.

Next to the photodiode is a liquid crystal arrangement 32, comprising the liquid crystal 23, the other electrode 24 of the device, and a series of interface layers 18, 20 for blocking (18) the readout light 30 from the silicon photodiode 14, 16 and for providing a reflective surface (20) for readout. Silicon is photosensitive up to the near infrared. In order to provide light blocking for silicon, a materal with a band gap equal to or smaller than silicon's band gap must be employed. Such a material usually does not have sufficient sheet resistivity to maintain high resolution, if its mobility is within an order of magnitude of the mobility in silicon. It is particularly difficult to find a single phase material that provides adequate light blocking and at the same time high enough sheet resistivity to maintain the required resolution.

Therefore, the device shown in Fig. 1 uses for the layer 18 a two phase material called cermet and consisting of metallic and dielectric components. Such a thin film light blocking cermet layer can be built by imbedding small metal particles in a dielectric layer. The metal particles are insulated from each other by the dielectric.

It is known that many metals, such as Sn, In and Pb tend to form islands, instead of continuous films, when deposited in very thin (200A) films. Such a multilayer structure will have a high sheet resistivity because the metal particles are insulated from each other in the plane of the film.

However, in a direction perpendicular to the film plane, the resisitivity is low because the thin interleaving insulator films allow tunneling or high electric field injection of electrons between metal particles in neighbouring metal island films. Thus, the observed perpendicular resistivity is low with respect to that in the plane of the film and a DC current can then flow through the entire film structure with no transverse spreading.

Next to the layer 18 is a cermet mirror 20.

It consists of high and low index dielectric layers deposited with a small concentration of metal. They provide a large DC conductivity anisotropy between the insulator-like sheet resistance and the low resistance through the film. Hence, the light valve can operate in the reflective mode.

The liquid crystal 23 is disposed adjacent the cermet mirror 20 and between two passivating films 21a and 21b. The thickness of the liquid crystal 23 is determined by spacers 22a and 22b. Next to the passivating film 21b is the conducting electrode 24, which is transparent, and next to the latter is a transparent cover plate 26. A DC source or battery 25 is connected between the electrode 24 and the P layer 14 of the photodiode 12. The voltage of the battery 25 is selected to reverse-bias the PN junction 15 of the photodiode 14, 16 and to establish a depletion region that extends from either side of the junction 15 to the entire silicon body around the junction. When the minority carriers are introduced on the P side of the junction, they will diffuse in this highly conductive region towards the junction. Since the PN junction is reverse biased, it will collect the minority carriers and insert them (from a high output impedance) into the interface layers 18, 20 and the liquid crystal 23. The spatial resolution of the carriers is maintained because in the depleted region the potential is determined by the space charge and not by current flow. Therefore, there is no lateral field in the region.

Fig. 2 shows a relatively thin layer 14, at the input, a relatively thick layer 16, and a PN junction 15 between them. Next to the thick layer 16, there is shown a portion of the liquid crystal arrangement 32. The thin layer 14 can be undepleted of charges during the operation, if its thickness and conductivity permit the incoming signals to reach the depleted side 16 without substantial signal spreading. For example, if radiation is used to produce the signal charges, then the undepleted region must be thinner than the desired resolution.

Alternatively, the undepleted region can be made so that it does not absorb the incoming radiation.

For example, layer 16 can have a resistivity on the order of 10Kg-cm, while the undepleted region 14 may have a resistive in the range between 1 and IOsz- cm. With such a high conductivity layer 14 at the input side, there is no need for an extra electrode on that side for the biasing of the photodiode, since an electrical connection can be made directly through the layer 14.

The layer 16 may be relatively thick and may be of y-type, i.e. a highly resistive Ntype layer which is nearly intrinsic disposed next to the liquid crystal arrangement 32 and followed by the PN junction 15 and by a relatively thin P-type layer 14. Another possibility would be a 7r-type layer 16 (a highly resistive P-type layer which is nearly intrinsic) next to the liquid crystal arrangement 32, followed by the PN junction and an N-type layer 14.

Fig. 3 shows a different configuration, namely, a relatively thin layer 14 next to the liquid crystal arrangement 32. In this case, both sides of the diode 14, 16 must be depleted of all mobile carriers because the.

incoming signals will impinge on the relatively thick and resistive layer 16 first.

An undepleted and conductive layer 17 is added next to the resistive layer 16 in order to provide an ohmic contact for the biasing of the photodiode 14, 16.

In the configuration of Fig. 3 the liquid crystal arrangement 32 is followed by a relatively thin P-type layer 14, a PNjunction 15, a relatively thick resistive type (R-type) layer 16, and an N-type ohmic contact layer 17. Alternatively, the liquid crystal arrangement 32 may be followed by a thin N-type layer 14, a PN junction 15, a relatively thick type layer 16, and a P-type conductive contact layer 17.

Fig. 4 shows a configuration which is similar to that of Fig. 3, except that the PN junction 15 is located close to the middle of the photodiode 14, 16. Here again both sides of the photodiode 14, 16 have to be depleted of all mobile charge carriers during the operation of the device. The liquid crystal arrangement 32 is followed by a type layer 14, the PN junction 15, an type layer 16, and an N-type ohmic contact layer 17. Alternatively, the liquid crystal arrangement 32 may be followed by an Rtype layer 14, a BN junction 15, a type layer 16, and a P-type conductive ohmic contact layer 17.

Referring now to Fig. 5, there is shown a CCD-liquid crystal light valve of the present invention. It includes a glass substrate 80 on -whtlch there is a SiO2 insulating layer 82 inside which are CCD electrodes 84, followed by a high resistivity silicon semiconductor substrate 88 with a conductive epitaxial layer 86 adjacent the insulating layer 82. The thin epitaxial layer 86 forms a CCD channel. This layer 86 has a thickness in the range of 525,um and has the same conductivity type as the semiconductor substrate layer 88. On the opposite side of the semiconductor layer 88 is a PN junction 90 and another semiconductor layer 92 of opposite conductivity type. This is followed by two interface layers 94, 96, namely, a light blocking layer 94 and a dielectric mirror 96.

The interface layers 94, 96 are followed by a liquid crystal 98, a transparent electrode 100, and a glass plate 102. A DC source (not shown) is connected between the conductive epitaxial layer 86 and the electrode 100 so as to cause reverse biasing of the PN junction 90 and depletion of mobile charge carriers in the semiconductor layers 88 and 92. This depletion region extends to only a very shallow portion of the epitaxial layer 86 adjacent the semiconductor substrate layer 88, leaving almost the entire epitaxial layer 86 undepleted of its mobile charge carriers.

Therefore, when in response to information signals the CCD electrodes 84 introduce charge carriers into the liquid crystal valve, the charge is stored in potential wells or CCD buckets within the undepleted portion of the epitaxial layer 86 adjacent the SiO2 insulating layer 82 and is kept there by a clock voltage on the CCD electrodes 84.

When the clock voltage goes to zero, then the stored charge falls into the charge depletion region and is swept therethrough and through the interface layers 94 and 96 to the liquid crystal layer 98 and activates the latter.

Thus, thirst charge carriers representing information signals are brought by the CCD electrodes 84 to the epitaxial semiconductor layer 86. Then during the reading time (charge transfer time) the CCD clock signals are driven to zero or close to zero.

For example, for an N-channel CCD the clock signals will have to be driven to zero or slightly negative. The minority charge carriers stored in the CCD buckets in the epitaxial layer 86 diffuse toward the depleted substrate 88 and are electric field guided towards the PN junction 90. The PN junction 90 is reverse biased and, therefore, it collects the minority carriers and injects them through the interface layers 94, 96 into the liquid crystal 98. The entire structure is like a common base transistor. The PN junction is like the collector junction, the undepleted region is like the base, and the CCD is like the emitter that injects charge into the base.

The spatial resolution of the charges in the overall structure is maintained because: a. In the undepleted region (which is the epitaxial layer) the lateral field is negligible and charge moves by diffusion. Therefore, the thickness of the depleted region should be lower than the required resolution (i.e. 5 to 25,us).

b. In the depleted region the potential is determined by the space charge and not by the current flow. Therefore, there is no lateral field in this region either. Moreover, the spreading of the charge in this region is much lower, because of field focusing.

The interface layers are a cermet light blocking layer 94 and a cermet mirror 96.

In Fig. 6 there is shown an equivalent common base transistor circuit for the transfer mechanism for the CCD charge from one side of a silicon wafer to the other.

The undepleted, grounded, epitaxial layer 86 is represented as the base of the transistor, and the thick depleted collection junction 88, 90, 92 corresponds to the collection junction in the equivalent circuit.

As soon as the CCD clock electrode bias goes to zero the stored minority carriers diffuse through the undepleted epitaxial layer and are collected by the collecting junction, in much the same manner as in the common base transistor. The two interface layers 94, 96 are represented by two RC circuits. For example, the cermet light blocking layer 94 can be represented by a resistor 104 in parallel with a capacitor 106.

Similarly, the cermet mirror 96 can be represented by resistor 108 in parallel with capacitor 110, and the liquid crystal 98 by resistor 112 in parallel with capacitor 114.

By way of illustration, Table 1 provides some typical values for the resistance and capacitance of the different layers represented in the equivalent circuit of Fig.

6.

TABLE 1 Designation Value Cermet light blocking layer 104 30cm2 106 l5nF/cm2 Cermet mirror 108 6KQcm2 110 lOnF/cm2 Liquid crystal 112 1.5cm2 114 3nF/cm2 Bias voltage 116 may be in the range of 50 to 100 volts. In Fig. 7 there is shown the required voltage across the liquid crystal. In Fig. 8 there is shown the liquid crystal response to the voltage of Fig. 7.

The main purpose of the epitaxial layer 86 of Fig. 5 is to shield the CCD circuitry from the readout structure. However, it is not an indispensable layer. The CCD structure may be disposed directly on the high resistivity semiconductor layer 88, in which case the charges released from the CCD buckets are guided by electric fields towards the other side of the semiconductor layer 88 and the liquid crystal 98 due to collapsing CCD clocks.

WHAT WE CLAIM IS: 1. A liquid crystal light valve comprising a liquid crystal layer, a semiconductor body including a rectifying junction layer, said layers overlying one another, means for introducing a spatial distribution of charge carriers into said semiconductor body, and a d.c. voltage source arranged to reverse bias said rectifying junction layer such as to form in said body a depletion region extending through at least a major part of the thickness of said semiconductor body and in such a manner that the introduced charge carriers are swept across said body whilst being maintained substantially in said spatial distribution, towards said liquid crystal layer to activate the liquid crystal and provide a spatial representation of said distribution.

2. A light valve according to claim 1 wherein said semiconductor body includes a photodiode providing said rectifying junction layer, arranged to produce said spatial distribution of charge carriers in response to an incident spatial radiation pattern.

3. A light valve according to claim 1 including a charge coupled device for introducing said spatial distribution of charge carriers into the semiconductor body.

4. A light valve according to claim 3 wherein said charge coupled device includes a conductive epitaxial layer on the semiconductor body, and an insulating layer including therein a plurality of charge transfer electrodes.

5. A light valve according to any preceding claim wherein the rectifying junction layer is formed by first and second different conductivity type layers of the semiconductor body, one of said layers having a higher conductivity than'the other

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

CCD is like the emitter that injects charge into the base.

The spatial resolution of the charges in the overall structure is maintained because: a. In the undepleted region (which is the epitaxial layer) the lateral field is negligible and charge moves by diffusion. Therefore, the thickness of the depleted region should be lower than the required resolution (i.e. 5 to 25,us).

b. In the depleted region the potential is determined by the space charge and not by the current flow. Therefore, there is no lateral field in this region either. Moreover, the spreading of the charge in this region is much lower, because of field focusing.

The interface layers are a cermet light blocking layer 94 and a cermet mirror 96.

In Fig. 6 there is shown an equivalent common base transistor circuit for the transfer mechanism for the CCD charge from one side of a silicon wafer to the other.

The undepleted, grounded, epitaxial layer 86 is represented as the base of the transistor, and the thick depleted collection junction 88, 90, 92 corresponds to the collection junction in the equivalent circuit.

As soon as the CCD clock electrode bias goes to zero the stored minority carriers diffuse through the undepleted epitaxial layer and are collected by the collecting junction, in much the same manner as in the common base transistor. The two interface layers 94, 96 are represented by two RC circuits. For example, the cermet light blocking layer 94 can be represented by a resistor 104 in parallel with a capacitor 106.

Similarly, the cermet mirror 96 can be represented by resistor 108 in parallel with capacitor 110, and the liquid crystal 98 by resistor 112 in parallel with capacitor 114.

By way of illustration, Table 1 provides some typical values for the resistance and capacitance of the different layers represented in the equivalent circuit of Fig.

6.

TABLE 1 Designation Value Cermet light blocking layer 104 30cm2

106 l5nF/cm2 Cermet mirror 108 6KQcm2

110 lOnF/cm2 Liquid crystal 112 1.5cm2

114 3nF/cm2 Bias voltage 116 may be in the range of 50 to 100 volts. In Fig. 7 there is shown the required voltage across the liquid crystal. In Fig. 8 there is shown the liquid crystal response to the voltage of Fig. 7.

The main purpose of the epitaxial layer 86 of Fig. 5 is to shield the CCD circuitry from the readout structure. However, it is not an indispensable layer. The CCD structure may be disposed directly on the high resistivity semiconductor layer 88, in which case the charges released from the CCD buckets are guided by electric fields towards the other side of the semiconductor layer 88 and the liquid crystal 98 due to collapsing CCD clocks.

WHAT WE CLAIM IS: 1. A liquid crystal light valve comprising a liquid crystal layer, a semiconductor body including a rectifying junction layer, said layers overlying one another, means for introducing a spatial distribution of charge carriers into said semiconductor body, and a d.c. voltage source arranged to reverse bias said rectifying junction layer such as to form in said body a depletion region extending through at least a major part of the thickness of said semiconductor body and in such a manner that the introduced charge carriers are swept across said body whilst being maintained substantially in said spatial distribution, towards said liquid crystal layer to activate the liquid crystal and provide a spatial representation of said distribution.
2. A light valve according to claim 1 wherein said semiconductor body includes a photodiode providing said rectifying junction layer, arranged to produce said spatial distribution of charge carriers in response to an incident spatial radiation pattern.
3. A light valve according to claim 1 including a charge coupled device for introducing said spatial distribution of charge carriers into the semiconductor body.
4. A light valve according to claim 3 wherein said charge coupled device includes a conductive epitaxial layer on the semiconductor body, and an insulating layer including therein a plurality of charge transfer electrodes.
5. A light valve according to any preceding claim wherein the rectifying junction layer is formed by first and second different conductivity type layers of the semiconductor body, one of said layers having a higher conductivity than'the other

and constituting an electrode of the device, said higher conductivity layer being connected to said d.c. source so as to reverse-bias the rectifying junction layer.
6. A light valve according to any preceding claim including between said semiconductor body and the liquid crystal layer, interface layers comprising a reflective layer and a light blocking layer, said reflective layer enabling the liquid crystal to operate in a reflective mode and said light blocking layer shielding the semiconductor body from light incident upon the liquid crystal.
7. A light valve according to any preceding claim wherein said depletion region extends the entire thickness of said semiconductor body.
8. A light valve substantially as herein described with reference to any of Figures 1 to 4 of the accompanying drawings.
9. A light valve substantially as herein described with reference to Figure 5 of the accompanying drawings.