WO2016198830A1

WO2016198830A1 - Switching circuit

Info

Publication number: WO2016198830A1
Application number: PCT/GB2016/051556
Authority: WO
Inventors: Harish Bhaskaran; Peiman HOSSEINI
Original assignee: Oxford University Innovation Limited
Priority date: 2015-06-09
Filing date: 2016-05-27
Publication date: 2016-12-15
Also published as: GB201509992D0; TW201705570A

Abstract

A switching circuit has a threshold device in series with a phase-change material part and comprise a circuit operable as a relaxation oscillator. There is an input for receiving a voltage which is applied to the switching circuit. Application of a suitable voltage can reversibly electrically switch the phase-change material part between two states.

Description

SWITCHING CIRCUIT

The present invention relates to a switching circuit.

There has been considerable research into so-called phase change material (PCM) technology and its use in optoelectronic devices such as ultra-high resolution reflective displays, see-through displays, and force sensors. PCMs include materials that can be electrically switched between more than one phase, and the phases have different optoelectronic properties. Bi-stable PCMs are particularly attractive because after a phase transition has been completed it is not necessary to continuously apply power to maintain the device in the new state.

One key technological challenge of such devices is the so-called "filamentary issue" in which only a tiny portion of material is generally actively involves in a switching cycle, and the surrounding portion of material is unchanged. In practice this means that the devices can be made to work satisfactorily when the portion of material being switched is at the nanoscale. However, there is a difficulty in switching real macro-scale systems, such as a switchable window.

One way to switch a large area is to divide the active PCM area into nanoscale- sized pixels. However, the transition from the crystalline to the amorphous state can require application of a sharp nanosecond duration electrical pulse to re-amorphise the PCM. The problem is that addressing millions of pixels in sequence at very high speed is a significant engineering challenge. The amount of computational power required by the electronic peripheral driving circuitry for the pixels increases drastically with the number of pixels.

The present invention has been devised in view of the above problems.

Accordingly, one aspect of the present invention provides a switching circuit for reversibly electrically switching a phase-change material part between two states, the switching circuit comprising:

a threshold device in series with the phase-change material part and which comprise a circuit operable as a relaxation oscillator; and

an input for receiving a voltage applied to the switching circuit.

Another aspect of the invention provides a device comprising a plurality of switching circuits according to the first aspect of the invention. Further optional features of aspects of the invention are defined in the dependent claims.

Embodiments of the present invention can enable switching of a large area of phase change material without requiring complicated driver circuitry operable at radio frequencies, and without occupying a large area of the periphery of a device employing the switching circuit.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is a circuit diagram of a switching circuit according to an embodiment of the invention;

Fig. 2 is a graph of the current against time characteristic of a relaxation oscillator used in an embodiment of the invention;

Fig. 3 is a schematic cross-section of a structure forming a portion of a circuit according to an embodiment of the invention; and

Fig. 4 is a schematic plan view of a device incorporating embodiments of the invention.

An embodiment of a switching circuit will be described with reference to Fig. 1. As shown in the circuit of Fig. 1, a phase change material (PCM) part is placed in series with an insulator-to-metal transition (IMT) part. The PCM can be, for example, an optoelectronic component such as forming part of a display or non-volatile

memory/storage. The overall circuit is a modified version of a Pearson Anson oscillator, a specific type of relaxation oscillator. The IMT acts as the threshold device for the relaxation oscillator. A capacitance C is in parallel with the PCM and IMT, and a resistance R is between these components and an input 10 for receiving an applied voltage relative to the ground (GND) or other terminal 12.

The PCM has a refractive index that is permanently, yet reversibly, changeable by the application of an appropriate electrical voltage. Such a material undergoes a drastic change in both the real and imaginary refractive index when switched between amorphous and crystalline phases. The PCM is capable of undergoing an electrically induced reversible phase change. It is deposited in the amorphous state. When a suitable voltage is applied, an electronic transition occurs that allows a much greater current to flow, which generates heating and crystallizes the material. The material is now indefinitely stable in the crystalline phase under ambient conditions. To switch back to the amorphous state, a different voltage is applied that melts the material and, if the voltage is removed sufficiently rapidly, the material freezes back into the amorphous phase.

As previously explained, there is a substantial change in the refractive index when the material is switched between amorphous and crystalline phases. The material is stable in either state. This means that the voltage can be entirely removed when the device is in a steady-state (not being switched), so the power consumption of the device is low. Switching can be performed an effectively limitless number of times. The switching speed is also very rapid, typically less than 300 ns.

In the present preferred embodiment, the PCM is Ge2Sb₂Te5 (GST).

The IMT part is formed of a material that transitions from a lower conductivity state ('insulator') to a much higher conductivity state ('metal') when heated. In the preferred embodiments the IMT is vanadium dioxide (V0₂) or more generally VO_x

(depending on stoichiometry). The V0₂ is monostable in nature: at temperatures below 65C it is in a first crystalline state (monoclinic); when heated above 65C it transitions to a second crystalline state (rutile), but it reverts back to the first state as soon as the heat source is removed.

VOx compounds are merely one example of suitable IMT materials for these embodiments. Any of the so-called "Mott memristors" (materials that undergo an insulator-to-metal transition, IMT, at some specific temperature) are suitable, for example NbO_x.

The operation of this circuit will now be described with the PCM starting in its crystalline (low resistance) state. A DC voltage is applied to the input 10. In this embodiment, the DC voltage is in the region of 5 volts, such as 4.7 volts. Current begins to flow through the resistance R and charges up the capacitance C. In this embodiment the value of the resistance R is 50k ohms, and the capacitance C is lpF.

As the voltage on the capacitance increases, and the current through the IMT increases, at some point the IMT makes the transition to the "metal" state and the electrical conductivity abruptly increases by a very large factor. The current through the IMT suddenly increases producing a current spike until the capacitor C has been discharged, at which point the IMT reverts back to the "insulator" state passing a low quiescent current. If the PCM were not present, then this process would cyclically repeat for as long as the DC voltage were applied to the input 10. Fig. 2 shows a graph of current through the IMT as a function of time, and shows the oscillating behaviour. The period of the oscillations is governed by the values of R and C (mentioned above). As can be seen, this is a self-resonating circuit, and when the PCM is in its crystalline (lower resistance) state the IMT will fire periodic pulses with a duration of the order of 50 nanoseconds (in this embodiment). Also in this embodiment, the resistance of the PCM is of the order of 15k ohms.

However, each current pulse re-amorphises part of the PCM until a point is reached when the series resistance of the PCM is too high, such that the oscillation condition is not met because there is insufficient current through the IMT to cause it to transition to the "metal" state. At this stage, the PCM is now in the amorphous phase with a resistance of the order of 100 times higher than the resistance in the crystalline stage (such as 1500k ohms in this embodiment). Negligible current now flows through the whole circuit, and because the PCM is bi-stable, the input voltage can be removed and the PCM will remain in the amorphous state. The entire switching process takes a few, or even just one, of the 50 nanosecond pulses. The switching process is passive and does not require any control because the circuit self-resonates until switching is complete and then the current returns to close to zero.

To switch the PCM such that it transitions back from the amorphous state to the crystalline state, a different voltage (higher or lower) is applied to the input 10 such that the oscillation criteria are not met. In this embodiment, the voltage can be, for example, 8 volts DC. This switches the IMT (but without oscillation) and recrystallizes the PCM once again.

Figure 3 shows a schematic cross-section of a structure for implementing the switching circuit of Fig. 1. A PCM part is deposited on an IMT part which are themselves sandwiched between top and bottom transparent (or semi-transparent) conductors 20, 22. The conductors 20, 22 are made of a transparent, electrically conductive, material such as indium tin oxide (ITO). In this embodiment, the device is encapsulated by a silica (S1O2) layer 24 which both protects the structure from oxidation and also provides the parallel capacitance needed for the oscillator circuit, as shown schematically in the inset of Figure 3. Other dielectric materials can be used in place of the silica to provide the capacitance, for example, silicon nitride, hafnia or alumina. Examples of ranges of thicknesses of the layers for the structure of Fig. 3 and for other embodiments are as follows:

PCM (e.g. GST) from 5 to 50 nm;

threshold device (e.g. IMT) from 10 to 50 nm;

conductors 20, 22 (e.g. ITO) from 0 to 300 nm.

These ranges are, of course, purely examples, and can be used in isolation from each other, e.g. with one layer within its range defined above, and the other layers not necessarily limited to their range.

In some embodiments, a back-reflector layer 26 is provided to act as a mirror, and can be formed of, for example, platinum, aluminium, silver and so forth. By selecting the thickness of the IMT and/or the bottom conductor 22, the optical contrast/apparent color difference of the structure when the PCM is switched between two states can be tuned or enhanced. In other embodiments, either or both of the bottom conductor 22 and mirror 26 can be omitted, provided there is a means for electrically contacting the IMT part.

The top conductor 20 and bottom conductor 22 are led out (through appropriate resistance R) to suitable contacts for the voltage input and ground for electrically driving the switching circuit, such as illustrated in Fig. 1.

The whole structure illustrated in Fig. 3 can be provided on a substrate (not shown) such as a semiconductor wafer, quartz (S1O2), glass, or a flexible substrate such as a polymer film e.g. mylar. The structure could be provided on items such as glasses, windows or transparent display panels. The layers can be deposited using sputtering, which can be performed at a relatively low temperature of 100 Celsius. The layers can also be patterned as required, using conventional techniques known from lithography, or other techniques e.g. from printing. Additional layers may also be provided for the device as necessary, depending on the application.

The sequence of the PCM and the IMT (or other threshold device) in the circuit of Fig. 1 and Fig. 3 can, of course, be reversed because they are simply connected in series. The polarity of the voltage applied to the terminals 10, 12 can also be either way round, and either terminal can be at the ground potential.

Fig. 4 illustrates a schematic plan of a device incorporating a plurality of switching circuits embodying the invention. The electrical topography is that of a crossbar-type device. Highly conductive (e.g. metal) rails 10, 12 are provided around the periphery of the device, which act as the voltage input. Vertical and horizontal conductors 20, 22 are provided which act as the top and bottom conductors where they intersect as shown in the cross-section of Fig. 3. Each intersection is effectively a PCM pixel 40, so the device comprises a 2D array of pixels. However, all of the pixels are in parallel with each other and can be driven simultaneously simply by applying an appropriate voltage between the conductive rails 10, 12. In fact, in some embodiments, the patterning of individual pixels is not required, which makes fabrication simpler and cheaper. Effectively the whole area is a single giant pixel. Alternatively, many more pixels can be provided than are shown schematically in Fig. 4. In one embodiment, the dimensions of each pixel can be in the range of from approximately 100 nm x 100 nm to 300 nm x 300 nm, though, of course, need not be square.

A controller and/or other circuitry (not shown) can be provided to apply the required switching voltages, and can be integrated onto a substrate with the device of Fig. 4, or can be provided as separate dedicated circuitry.

Any PCM device could use the switching circuit and arrangements described above, including commercial solid state memories, and switchable windows (also referred to as smart windows, smart glass or switchable glass). A switchable window comprises glazing in which the light transmission properties can be altered by switching the state of the PCM. This alters the refractive index of the PCM, and can be used to change the transmittance of the glazing as a function of the wavelength of light, for example, by employing interference effects. If a bi-stable PCM is used, then power only need be applied when actually switching the device, and no power is consumed in the steady state. A switchable widow can be fabricated which is at least several cm in size along each edge, and multiple such devices can be put together in a mosaic to make a bigger window.

The preceding embodiments have been described with reference to GST

(Ge2Sb₂Te5) as the PCM, but this is not essential to the invention, and many other suitable materials are available, either separately or in combination, including compounds or alloys of the combinations of elements selected from the following list: GeSbTe, GeTe, GeSb, GaSb, AglnSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb. It is also understood that various stoichiometric forms of these materials are possible; for example Ge_xSbyTe_z; and another suitable material is Ag3ln₄Sb76 ei7 (also known as AIST). Furthermore, the material can comprise one or more dopants, such as C or N.

In general, the term PCM encompasses any solid material that undergoes a change in refractive index (real and/or imaginary part) when an electrical signal is applied. The change can be permanent (though reversible) at normal operating temperatures (i.e. bi-stable), or can be transitory.

Although the embodiments described herein mention that the PCM layer is switchable between two states such as crystalline and amorphous phases, the transformation could be between any two solid phases, including, but not limited to: crystalline to another crystalline or quasi-crystalline phase or vice-versa; amorphous to crystalline or quasi-crystalline/semi-ordered or vice versa, and all forms in between.

Embodiments are also not limited to just two states.

A further enhanced embodiment is that the PCM does not have to be switched simply between a fully crystalline and a fully amorphous state. A mixture of phases can be achieved, such as 20% crystalline, 40% crystalline etc. Partial crystallization is achieved by simply limiting the maximum current allowed during a switching event

(e.g. using a variable resistor in series with the input as the resistance R). The resulting effective refractive index of the material is somewhere between the two extremes of fully crystalline and fully amorphous depending on the degree of partial crystallization. Typically between 4 and 8 distinct mixed phases are achievable, but with appropriate control, can be much higher, such as 128 values, and effectively a continuum of refractive index values can be achieved, corresponding to tracing a path through color space.

The preceding embodiments use an IMT part as the threshold device.

Alternative threshold devices can be used. Another embodiment uses an ovonic threshold switching part as the threshold device, and a further embodiment uses a threshold vacuum switch as the threshold device. Examples of materials for an ovonic threshold switching part include: chalcogenide materials; AsTeGeSi and AsTeGeSiN based materials. An example of a threshold vacuum switch comprises a stack comprising: a W/TiN bottom electrode; TiN/vacuum top electrode with vacuum gap of ~1 nm (established by a SiN sacrificial layer), and WO_x as a switching layer, prepared by electrochemical oxidation. In general, the threshold device can undergo a voltage-induced or current- induced transition from a starting state that is electrically resistive to a more conductive state when a threshold condition is met, but the material is mono-stable in that it spontaneously reverts back to the stable starting state when the voltage/current is removed.

To switch the PCM such that it transitions back from the amorphous state to the crystalline state, further embodiments of the invention do not necessarily pass current through the PCM itself. Instead, a heater can be arranged to heat the PCM above the crystallization temperature, e.g. 140C for GST; then on ceasing the heating, the PCM re-crystallizes. In one embodiment, the heater can be one or more strips of transparent conductor, such as ITO, through which is passed a current to raise the substrate temperature by joule (ohmic) heating, to quickly switch the PCM with no special control needed. In an embodiment such as shown in Fig. 4, one can use the existing conductors 20 and/or 22 as heaters, and simply pass a sufficiently high current from conductive rail 10 to conductive rail 10 and/or from conductive rail 12 to conductive rail 12.

Although some of the embodiments use ITO as the preferred material for the transparent electrodes, this is merely an example, and other suitable materials can be used, such as carbon nanotubes, or a thin layer of metal, such as silver.

With embodiments of the present invention, no significant electronic circuitry is required around the periphery of the device, so the complexity is greatly reduced and the scalability is much easier and more cost effective.

Claims

1. A switching circuit for reversibly electrically switching a phase-change material part between two states, the switching circuit comprising:

an input for receiving an voltage applied to the switching circuit.

2. A switching circuit according to claim 1, wherein the threshold device is capable of undergoing a voltage-induced mono-stable transition from a resistive state to a conductive state to act as the threshold device of the relaxation oscillator.

3. A switching circuit according to claim 1 or 2, wherein the circuit is arranged such that application of a first voltage to the input causes the phase-change material part to switch from a first state to a second state, the second state having a higher resistance than the first state.

4. A switching circuit according to claim 3, wherein application of a second voltage, different from the first voltage, to the input causes the phase-change material part to switch from the second state to the first state.

5. A switching circuit according to claim 3, further comprising a heater, arranged to heat the phase-change material part above a predetermined temperature when the phase-change material is in the second state, and then to stop heating to cause the phase- change material part to switch from the second state to the first state.

6. A switching circuit according to any preceding claim, wherein the threshold device comprises one of: an insulator-to-metal transition part; an ovonic threshold switching part; and a threshold vacuum switch.

7. A switching circuit according to claim 4, wherein the insulator-to-metal transition part comprises at least one of: VO_x, NbO_x.

8. A switching circuit according to any preceding claim, wherein the phase-change material comprises a compound or alloy of a combination of elements selected from the following list of combinations: GeSbTe, GeTe, GeSb, GaSb, AglnSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb.

9. A switching circuit according to claim 8, wherein the phase-change material comprises a mixture of compounds or alloys of combinations of elements from said list.

10. A switching circuit according to any preceding claim, wherein the phase-change material comprises Ge2Sb₂Te5.

11. A switching circuit according to any preceding claim, further comprising an electrical capacitance in parallel with the series-connected phase-change material part and threshold device.

12. A switching circuit according to claim 11, wherein the electrical capacitance is provided by a layer of dielectric material, such as silica, silicon nitride, hafnia, or alumina.

13. A device comprising a plurality of switching circuits, each switching circuit according to any one of the preceding claims, and each switching circuit including a phase-change material part, wherein the phase change material parts of the plurality of switching circuits are arranged in a two-dimensional array.

14. A device according to claim 13, wherein the inputs of the plurality of switching circuits are connected in common, in parallel.

15. A switchable window comprising a switching circuit according to any one of claims 1 to 12, or comprising a device according to claim 13 or 14.