WO2018224807A1

WO2018224807A1 - Phase change switching

Info

Publication number: WO2018224807A1
Application number: PCT/GB2018/051473
Authority: WO
Inventors: Nathan YOUNGBLOOD; Carlos A. RIOS OCAMPO; Harish Bhaskaran
Original assignee: Oxford University Innovation Limited
Priority date: 2017-06-05
Filing date: 2018-05-31
Publication date: 2018-12-13
Also published as: GB201708925D0

Abstract

A device (200) comprising: a PCM region (213), a first electrode (211), a second electrode (212), and an insulating region (215) disposed between the PCM region (213) and the first electrode (211) to prevent direct electrical contact between the first electrode (211) and PCM region (213), wherein a capacitor structure is formed by the first electrode (211), second electrode (212), PCM region (213) and insulating region (215). A controller is disclosed, configured to: control the phase of the PCM region (213) by applying an alternating voltage across the capacitor structure so as to at least partially change the phase of the PCM region (213) by causing an AC current to flow through the PCM region (213); and/or read the state of the PCM region (213) by applying an alternating voltage across the capacitor structure so as to detect the phase of the PCM region (213) from the impedance of the capacitor structure.

Description

PHASE CHANGE SWITCHING

Technical Field

The invention relates generally to a method and apparatus for changing and monitoring the phase of a phase change material. Background to the Invention

Phase-change materials (PCMs), have been the subject of intense research and development over the last decade, for example in the context of electronic memories. PCMs may exhibit a high contrast in their electrical and optical properties between a crystalline and an amorphous phase. In particular, PCMs (e.g. chalcogenide-based PCMs, such as GST) may have the ability to switch between these two states in response to appropriate heat stimuli (resulting in crystallization) or melt-quenching processes (resulting in amorphization). The phase transition occurs at a crystallisation temperature, Tc. Initially, at temperatures below Tc the PCM may be in the amorphous state. When heated to temperatures above Tc the PCM transitions to the crystalline state, and remains in the crystalline state when it is cooled back to below Tc. The PCM can be "reset" back to the amorphous state by heating it above the melting point, Tm, and rapidly cooling it back to below Tc.

These PCMs (which include tellurides and antimonides) can be switched on a sub- nanosecond timescale with high reproducibility, which enables ultra-fast operation over switching cycles up to 10¹² times using current-generation materials. New and improved PCM materials, such as the so-called phase-change super-lattice materials, are expected to deliver even better performance in the future.

In addition to a change in electrical conductivity, many PCMs show significant change in refractive index (optical reflectance/transmission) in the visible and even larger changes in the near-infrared wavelength regime. In particular, the amorphous state may have a low electrical conductivity and low reflectance, and the crystalline state may have a relatively high conductivity and high reflectance.

Switching of PCMs is presently typically carried out electrically, either by resistive heating of a heating element in contact with the PCM, or by passing a current directly through the PCM. The former method requires a heating element to be patterned, in contact with the PCM material, which may add cost and device complexity. The latter method may be referred to as resistive switching of a PCM, and has the advantage of simplicity.

When a voltage difference is applied between a pair of electrodes in contact with the PCM (for instance, in a vertical structure, above and below the a PCM layer), a current will flow through the initially amorphous PCM. The effect of the current flow is to heat the PCM in the region of current flow (Joule heating). In some circumstances, the current will tend to be restricted to a small region or filament of PCM that defines the least resistive path between the electrodes. Once this filament of PCM has transitioned to the less resistive crystalline state, it is difficult to cause further Joule heating of the remaining amorphous region, because the filament or region of crystalline PCM between the electrodes effectively provides a short circuit past the remaining amorphous material. In devices with relatively large regions of PCM, which require high speed and relatively uniform switching of PCM, such as some types of optical device, this filamentation between electrodes can be a problem. Similarly, detecting the phase of a region of PCM using electrodes in contact with the PCM can be misleading when only some regions of the PCM have undergone a phase transition. It may be difficult to infer that phase of a relatively large area of PCM from the resistance between two electrodes in contact therewith, because a short circuit comprising crystalline material will tend to dominate the overall resistance/conductance of the PCM.

Summary of the Invention

According to a first aspect, there is provided a device comprising: a PCM region, a first electrode, a second electrode, and an insulating region disposed between the PCM region and the first electrode to prevent direct electrical contact between the first electrode and PCM region, wherein a capacitor structure is formed by the first electrode, second electrode, PCM region and insulating region.

The capacitor structure may comprise an electrode that comprises the first electrode, and a further electrode that comprises the PCM region and the second electrode. The electrode and further electrode of the capacitor structure may be separated by a gap that is defined by the insulating region. The device may comprise a substrate, on which the PCM region, first electrode and insulating region are disposed.

The device may further comprise a controller for controlling the phase of the PCM region by applying an alternating voltage across the capacitor structure so as to at least partially change the phase of the PCM region by causing an AC current to flow through the PCM region.

The term AC current is not limited to a sinusoidal waveform, and is intended to encompass any form of alternating current (e.g. including a square wave, sawtooth, triangle wave, chirp, etc).

The device may further comprise a controller for readout of the state of the PCM region by applying an alternating voltage across the capacitor structure so as to detect the phase of the PCM region from the impedance of the capacitor structure.

The same controller may be operable to readout and switch the PCM region.

The controller may comprise electronic circuitry, which may be driven by a microprocessor or microcontroller. Some or all of the circuitry of the controller may be provided integrated on the substrate.

The device may further comprise a second electrode, which may be in direct (galvanic and/or ohmic) contact with the PCM region. In some embodiments, a further insulating region may be provided between the second electrode and the PCM region, and the second electrode may not be in galvanic electrical contact with the PCM region.

The controller may be connected to the first and second electrode.

The insulating region may be a dielectric material region.

The term "light" is used in this specification in a non-restrictive sense, to refer generally to any form of electromagnetic radiation. The first and second electrode may be respectively disposed below and above the phase change material region. The second electrode may be in electrical contact with a lower surface of the phase change material region, and the insulating region may be in contact with an upper surface of the phase change material region.

The device may comprise a mirror region/layer, arranged to reflect light through the PCM region so as to increase the absorbance of incident light by the detector.

The device may be configured to act as a resonant optical cavity so as to maximise the absorbance of a selected wavelength of light by the detector.

The selected wavelength may be within the range 400nm to 700nm.

The first and second electrode may comprise an at least partially transparent conducting material.

The first and second electrode may comprise indium tin oxide, graphene, multi-layer graphene, graphite, gold, or PEDOT. The phase change material may comprise a compound or alloy of a combination of elements selected from the following list of combinations: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AglnSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb. The device may comprise a plurality of capacitor structures, each comprising a PCM region, a first electrode, and an insulating region disposed between the PCM region and the first electrode to prevent direct electrical contact between the first electrode and PCM region, each capacitor structure being formed by the respective first electrode, PCM region and insulating region. A second electrode may be provided for each capacitor structure. Each second electrode may be directly electrically connected to its respective PCM region. The first electrode or second electrode of each structure may be a common electrode.

Each region may comprise part of a corresponding material layer. The material layers may be patterned to define the regions and electrodes. The device may comprise a photonic waveguide, and the PCM region may be evanescently coupled to the photonic waveguide, so that the transmission, reflection or absorption properties of the waveguide are controlled by the phase of the PCM region.

The device may further comprise an inductor, connected in series or parallel with the capacitor structure, so as to form an LC resonator comprising the PCM region. The controller may be configured to determine the phase of the PCM region from a resonant frequency of the LC resonator.

According to a second aspect, there is provided a display comprising a device according to the first aspect, in which an alternating current through the PCM region is used to control the refractive index of the PCM region by at least partially changing its phase.

According to a third aspect, there is provided a method of controlling or detecting the phase of a PCM region in a device according to the first aspect, comprising applying an alternating current through the capacitor structure.

The method may comprise at least partially changing the phase of the PCM region from an amorphous to a crystalline state. The method may comprise at least partially changing the phase of the PCM region from a crystalline to an amorphous state. The method may comprise determining a state (phase) of the PCM region from the impedance of the capacitor structure. The method may comprise determining a resonant frequency of an electrical resonator comprising the capacitor structure.

The method may be performed using an apparatus according to the first aspect, including any of the optional features thereof, in any combination.

More generally, features which are described in the context of separate aspects and embodiments of the invention may be used together and/or be interchangeable. Similarly, where features are, for brevity, described in the context of a single embodiment, these may also be provided separately or in any suitable sub- combination. Features described in connection with the device may have corresponding features definable with respect to the method(s) and these embodiments are specifically envisaged.

Brief Description of Drawings In order that the invention can be well understood, embodiments will be discussed below by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of resistive switching in a PCM region, illustrating filament formation in the PCM; Figure 2 is a schematic diagram of a device according to an embodiment, in which control over phase of the PCM is achieved as a result of a displacement current in capacitor comprising the PCM as part of an electrode thereof;

Figure 3 is a schematic diagram of the device of Figure 2, alongside an equivalent circuit for the device; Figure 4 is a schematic diagram of an element of a reflective colour display according to an embodiment;

Figure 5 is a simulation result showing temperature distribution after 10 microseconds of heating the PCM material in the embodiment of Figure 4 using a displacement current, and Figure 6 is a device according to an embodiment comprising a photonic waveguide in which the properties of the waveguide can be adjusted by varying the phase of the PCM region using a displacement current through the capacitor structure.

Detailed Description

Figure 1 shows an arrangement for resistive switching of a PCM region 1 13, comprising a first electrode 1 1 1 , second electrode 1 12 and DC voltage source 105. The PCM is GST (Ge₂Sb₂Te₅) and the first and second electrode are ITO, which is substantially transparent to visible light. The first electrode 1 1 1 is in direct electrical contact with a first side of the PCM region 1 13, and the second electrode 1 12 is in direct electrical contact with a second side of the PCM region 1 13. The DC voltage source 105 is connected between the first and second electrode 1 1 1 , 1 12 and operable to cause a voltage difference therebetween. A voltage difference between the first and second electrode 1 1 1 , 1 12 will result in current flowing through the PCM region 1 13. The current through the relatively resistive PCM region 1 13 causes Joule heating, which may result in a phase change of the PCM region 1 13. As described above, in practice the current will cause a filament 102 of the PCM region 1 13 to change phase (e.g. from an amorphous to a crystalline state). The resistance of the PCM may be lower after it has changed state (e.g. in the crystalline state) with the result that any such filament 102 will effectively "short-circuit" the first and second electrode 1 1 1 , 1 12, preventing further Joule heating of the remaining PCM. Any phase transition in the remaining PCM must rely on thermal diffusion from Joule heating in the filament 102, which limits the speed and uniformity with which the PCM region 1 13 can be switched. This filamentation may limit the area of PCM that can be switched, and may drive a requirement for micro heaters in order to switch large areas of PCM.

Figure 2 is an example embodiment of a device 200, comprising: a PCM region 213, first electrode 21 1 , insulator 215, second electrode 212 and voltage source 205. The device 200 may comprise a layer stack deposited onto a substrate (not shown). A layer/region of PCM 213 is sandwiched between the first electrode 21 1 and second electrode 212.

In contrast with the arrangement of Figure 1 , the first electrode 21 1 is not in galvanic electrical contact with the PCM region 213, so a DC voltage applied between the first electrode 21 1 and second electrode 212 will not result in current flowing through the PCM region 213. Instead the first electrode 21 1 is insulated from the PCM region 213 by an electrical insulator/dielectric 215, which is disposed between the first electrode 21 1 and the PCM region 213. The effect of the insulator 215 between the first electrode 21 1 and the PCM region 213 is to produce a capacitor structure from the first electrode 21 1 , insulator 215, second electrode 212 and the PCM region 213 (which is not an electrical insulator, and which may be a conductor or semiconductor). In this embodiment the second electrode 212 is in electrical contact with the PCM region 213, and one of the electrodes of the capacitor structure comprises the second electrode 212, but in some embodiments it may be possible to omit the second electrode 212. An advantage of the second electrode 212 is that it improves the uniformity of the electric field 202 applied across the PCM region 213. In this example the higher electrical conductivity of the first and second electrode 21 1 , 212 (than the PCM) improves the uniformity of the electrical field 202 over the PCM region 213.

The voltage source 205 is configured to apply an AC voltage across the capacitor structure, in this example via the first and second electrode 21 1 , 212. Figure 3 illustrates the same device arrangement as Figure 2, alongside an equivalent circuit 250, in which the resistance of the first electrode 21 1 is represented as a resistor R_ITO between the voltage source 205 and a first electrode of capacitor C. The resistance of the second electrode 212 and the PCM region 213 are represented by Rrro and R_GST, in series between the voltage source 205 and a second electrode of capacitor C. An AC voltage at the voltage source 205 will result in an alternating current flowing through the circuit. Only AC current can flow across the capacitor, in accordance with Maxwell' s equations. As the capacitor charges and discharges, power is dissipated by the resistive elements in the circuit. The power dissipated can be approximated by the following formula:

Where P_rms is the RMS power dissipated, V_rms is the RMS voltage of the AC signal, R_esr is the equivalent series resistance, f is the frequency of the AC signal, and C is the capacitance of the device. The effective resistance will be dominated by the resistance of the PCM region 213, because the resistivity of the PCM will be higher than the electrodes (especially when the PCM is in the amorphous state).

Although this example device employs a vertical structure, in which the first electrode is an upper electrode, and the second electrode is lower electrode, and the PCM layer/region is sandwiched between the lower and upper electrode, other embodiments may employ a lateral structure, comprising electrodes configured to pass current through the PCM region laterally. Such lateral electrodes may be patterned from a single layer, and/or may be in electrical contact with only one side of the PCM region. In other embodiments lateral electrodes may be in contact with both sides of the PCM region.

The device 200 may comprise a further encapsulation layer (not shown), which may comprise an oxide or polymer, for example, to protect the device from degradation (e.g. due to oxidation, or diffusion of species which may affect the device performance).

The PCM may be the well-studied germanium antimony tellurite (GST) compound or alloy, Ge₂Sb₂Te₅, because of its proven chemical and solid state stability down to nanoscale dimensions and potential for device miniaturisation. GST has Tc ~ 100 degrees C and Tm ~ 600 degrees C.

In other embodiments, the PCM 213 may be or comprise material comprising a compound or alloy of a combination of elements selected from the following list of combinations: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AglnSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb. The PCM may be doped with any element (e.g. C, Ni, Ce, Si etc).

The PCM layer/region 213 may have a thickness in the range 10 nm to 50 nm. In other embodiments, the PCM layer 213 may have a thickness in the range 10 nm to 20 nm, 20 nm to 30 nm, 30 nm to 40 nm, or 40 nm to 50 nm. The first electrode 21 1 and the second electrode 212 comprise an electrically conductive material. In an embodiment, the electrodes 21 1 , 212 may have an electrical conductivity greater than lxlO³ S/cm. In other embodiments, the electrodes 21 1 , 212 may have an electrical conductivity greater than lxlO² S/cm or 10 S/cm.

The first and/or second electrodes 21 1 , 212 may be at least partially transparent in the visible spectrum. In an embodiment, the first and second electrodes have a transmission of at least 50% in the visible spectrum (e.g. 400 nm to 700 nm), or an average transmission of at least 50%. In other embodiments, the electrodes 21 1 , 212 have a minimum transmission of at least 60% , 70% , 80% or 90%.

The first and/or second electrode 21 1 , 212 may be or comprise a metal, metal alloy, semi-metal, semi-metal alloy, semiconductor, semiconductor compound, oxide or polymer. Suitable materials for the upper and lower electrode may include, but are not limited to: indium tin oxide (ITO), graphene, multi-layer graphene, graphite, gold, PEDOT (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate).

The first and second electrode 21 1 , 212 may not comprise the same material or the same electrical/optical properties. In the exemplary embodiments described herein, ITO is used due to its ease of fabrication and well controlled optical and electronic properties.

The first and/or second electrode 21 1 , 212 may have a thickness in the range 10 nm to 50 nm. The thickness of the first electrode 21 1 and second electrode 212 may not be equal. In other embodiments, the first and/or second electrode 21 1 , 212 may have a thickness in the range 10 nm to 20 nm, 20 nm to 30 nm, 30 nm to 40 nm, or 40 nm to 50 nm.

The insulating layer 215 may be a dielectric, and may comprise any suitable material such as nitride, silicon oxide, aluminium oxide, hafnium oxide etc. The insulating layer may be selected based on its refractive index (in optical applications), its electrical properties (e.g. a high κ dielectric) and/or based on convenience of manufacture.

The device 200 may comprise a number of partially transparent thin films stacked on top of each other (optionally on a mirror layer). The device may be designed to provide a maximum (enhanced) optical absorption in the PCM layer/region 213 for specific wavelengths in the visible spectrum, and a minimum optical absorption in the PCM layer/region 213 for the rest of the visible spectrum. This can be achieved by exploiting thin film interference effects which can be modelled using well known techniques, for example using commercially available software such as COMSOL Multiphysics. The device 200 may therefore be configured to be wavelength selective, such that it appears to take on a particular colour, depending on the phase of the PCM. A partial phase transition may be used to control the apparent colour of the device with a multi-bit resolution (e.g. 4 bit or 8 bit etc).

Referring to Figure 4, reflective display element 201 is shown, comprising a device 200. The device comprises a similar arrangement of layers to that described in relation to Figures 2 and 3, deposited onto a substrate 220. In addition to the first electrode 21 1 , insulating region/layer 215, PCM region/layer 213 and second electrode 212, the device 200 further comprises the substrate 220 and a mirror layer 216. In this example the mirror layer 216 is positioned between the first electrode 21 1 and the substrate 220, but the mirror may be positioned elsewhere.

The mirror layer 216 may have a reflectivity greater than 90% in the visible spectrum. In another embodiment, the mirror layer 216 may have a reflectivity greater than 85%, 80% or 75% in the visible spectrum. The mirror layer may be or comprise a material which does not absorb light in the wavelength range of interest. In an embodiment, the mirror 216 layer may be or comprise a metal layer. For example, the mirror layer 216 may be or comprise aluminium, or platinum and titanium (e.g. a Ti adhesion layer with a Pt reflector on top).

The substrate 220 may be substantially opaque and absorbing in the visible spectrum. Alternatively, substrate 220 may be substantially optically transparent in the visible spectrum. Where the substrate 220 is substantially transparent in the visible spectrum, the mirror layer 216 may be positioned to allow the PCM region/layer 213 to be illuminated from the back side (through the substrate 220). For instance, the mirror layer 216 may be positioned above the second electrode 212.

The device 200 may be fabricated using a number of processing steps known in the art. The PCM region/layer 213 and/or electrodes 21 1 , 212 may be deposited with any appropriate technique, for instance using physical or chemical methods, such as thermal evaporation, electron beam evaporation, sputtering, chemical vapour deposition, atomic layer deposition, etc. , depending on the materials required.

The first and second electrode 21 1 , 212 may be configured to connect to a controller (e.g. voltage source 205), which is operable to control the phase of the PCM region/layer 213 by applying an alternating voltage across the capacitor structure. The controller may be configured to determine a state of the PCM region/layer, based on the impedance of a circuit comprising the capacitor (that includes the PCM). The resistivity of the PCM material may vary with state, resulting in a change in the effective resistance of the overall circuit (e.g. as shown in Figure 3). The impedance of the PCM may be more representative of the state of a broad region of PCM, because it cannot be dominated in the same way by a narrow conducting path between the electrodes (e.g. a filament of a particular phase). The voltage source 205 may be a programmable AC voltage source. The controller may comprise a current detector, for monitoring the amount of current flowing through the PCM region/layer 213 as a result of the applied AC voltage. The AC current or voltage may be measured using known phase sensitive detection techniques, such a lock-in amplifier.

The PCM layer 213 may have different optical properties in the crystalline state (compared with in the amorphous state), which may result in a change in the wavelength absorbed by the detector 200 following a phase change. A display device similar to that disclosed in EP3087430 may be formed from a device or devices according to an embodiment. The state/phase of an array of PCM regions may be controlled by passing AC current through an array of capacitors, each capacitor comprising a PCM region through which current passes when a displacement current passes through the capacitor. Such an arrangement may avoid a significant difficulty with previous display devices, in which it may be difficult to achieve reliable, rapid and uniform switching.

A further application for devices according to an embodiment is in glazing with selectable optical properties. The tint, or the extent to which a glazing unit is transparent to IR may be selectable based on the phase of a PCM disposed on the glass of the glazing unit. Controlling the phase of the PCM according to the present disclosure makes large area control significantly more straightforward.

Figure 5 shows a result of a coupled electro-thermal simulation of the device structure shown in Figure 4, in which the rate of Joule heating resulting from an AC voltage between the first electrode 21 1 and second electrode 212 is simulated, and then the heat flow and temperature resulting from this heating is calculated. The substrate is 10 micron thick silicon, with a 300 nm silicon oxide layer at the surface. The simulation is a transient simulation, with the temperature distribution shown taken at t= 10 microseconds, at which time the temperature distribution has essentially reached equilibrium. The simulation parameters include a sine wave AC voltage at a frequency of 1GHz with a maximum voltage of 5V applied to the top surface of the second electrode 212, and a temperature boundary condition of room temperature applied to the bottom of the substrate. A line 230 indicating the crystallisation temperature threshold is plotted on the simulation result. The simulation indicates that for a 1 micron wide second electrode, a radius of approximately 6 microns of the PCM layer has successfully been heated to above the crystalline transition temperature. It is therefore feasible to switch large regions of PCM with a second electrode 212 that has a relatively small amount of surface coverage (i.e. a grid electrode can be used to provide uniform PCM switching). Figure 6 illustrates a further example application for a device 200 according to an embodiment. In this example the PCM region 213 is coupled to a photonic waveguide 240 (e.g. evanescently), for example, as described in WO2017/046590. The state of the PCM region 213 affects at least one of the reflection, transmission and absorption properties of the waveguide, which may be used to switch optical signals, store/readout information etc.

In the example of Figure 6, the equivalent circuit for the device includes an inductor L, which may be provide in the controller or readout/drive circuit. An inductor may be included in any embodiment, and is not restricted to (or necessary for) applications comprising a photonic waveguide. The inclusion of an inductor produces an electrical resonator, having a resonant frequency that varies in response to a change in the effective capacitance or the effective inductance of the circuit. At least one of the effective capacitance and effective inductance may be affected by a change in phase of the PCM region/layer, and the state of the PCM may thereby be detected from the resonant frequency of the resonant LC circuit.

From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art, and which may be used instead of, or in addition to, features already described herein.

Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

For the sake of completeness, it is also stated that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, and any reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

1. A device comprising: a PCM region, a first electrode, a second electrode, and an insulating region disposed between the PCM region and the first electrode to prevent direct electrical contact between the first electrode and PCM region, wherein a capacitor structure is formed by the first electrode, second electrode, PCM region and insulating region; and

a controller configured to :

control the phase of the PCM region by applying an alternating voltage across the capacitor structure so as to at least partially change the phase of the PCM region by causing an AC current to flow through the PCM region; and/or

read the state of the PCM region by applying an alternating voltage across the capacitor structure so as to detect the phase of the PCM region from the impedance of the capacitor structure.

2. The device of claim 1 wherein the controller is connected to the first and second electrode.

3. The device of any preceding claim, wherein the first and second electrode are respectively disposed below and above the PCM region, the second electrode in electrical contact with a lower surface of the phase change material region, and the insulating region may be in contact with an upper surface of the phase change material region.

4. The device of any preceding claim, further comprising a mirror region, arranged to reflect light through the PCM region so as to increase the absorbance of incident light by the PCM region or modify optical reflection from the device.

5. The device of any preceding claim, wherein the device is configured to act as a resonant optical cavity so as to maximise the absorbance of a selected wavelength of light by the device.

6. The device of claim 5, wherein the selected wavelength is within the range 400nm to 700nm.

7. The device of any preceding claim, wherein the first and second electrode comprise an at least partially transparent conducting material.

8. The device of claim 7, wherein the first and second electrode comprise indium tin oxide, graphene, multi-layer graphene, graphite, gold, or PEDOT.

9. The device of any preceding claim, wherein the PCM comprises a compound or alloy of a combination of elements selected from the following list of combinations: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AglnSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb.

10. The device of any preceding claim, comprising a plurality of capacitor structures, each comprising a PCM region, a first electrode, a second electrode in contact with the PCM region, and an insulating region disposed between the PCM region and the first electrode to prevent direct electrical contact between the first electrode and PCM region, each capacitor structure being formed by the respective first electrode, second electrode, PCM region and insulating region.

1 1. The device of claim 10, wherein each second electrode is directly electrically connected to its respective PCM region.

12. The device of claim 10, wherein the first electrode or second electrode of each structure is a common electrode.

13. The device of any preceding claim, further comprising a photonic waveguide, wherein the PCM region is evanescently coupled to the photonic waveguide, so that the transmission, reflection or absorption properties of the waveguide are controlled by the phase of the PCM region.

14. The device of any preceding claim, wherein the device further comprises an inductor, connected in series with the capacitor structure, so as to form an LC resonator comprising the PCM region.

15. The device of any claim 14, wherein the controller is configured to determine the phase of the PCM region from a resonant frequency of the LC resonator.

16. A display comprising a device according to any preceding claim, configured to control refractive index of the PCM region by at least partially changing the phase of the PCM region using an AC current through the capacitor structure, wherein the apparent colour of a display region comprising the PCM region is adjustable by changing the phase of the PCM region.

17. A method of controlling or detecting the phase of the PCM region in a device according to any of claims 1 to 15 , comprising applying an alternating current through the capacitor structure.

18. The method of claim 17, wherein controlling the phase of the PCM region comprises at least partially changing the phase of the PCM region from an amorphous to a crystalline state.

19. The method of claim 17 or 18, wherein detecting the phase of the PCM region comprises determining a phase of the PCM region from the impedance of the capacitor structure.

20. The method of claim 17, wherein detecting the phase of the PCM region comprises determining a resonant frequency of an electrical resonator comprising the capacitor structure.