Near infrared radiation detection

Abstract

A near-infrared radiation detector comprises: a substrate comprising a semiconductor; a substrate electrode formed in contact with the substrate; an active layer formed over the substrate, the active layer comprising a ferroelectric material; and an active layer electrode formed in contact with the active layer and substantially transparent to near-infrared radiation. The detector is operable without applied external bias so that, when near-infrared radiation is incident on the detector, charge carriers are generated by a photovoltaic effect in the substrate / active layer heterojunction and a temperature gradient is generated in the active layer, providing a pyro-phototronic current signal indicative of the near-infrared radiation incident on the detector. Also disclosed is a corresponding method of detecting near-infrared radiation.

Description

[0001] NEAR INFRARED RADIATION DETECTION

[0002] Field of the Invention

[0003] The present invention relates to near-infrared radiation detectors, method for manufacturing such detectors and to method of operation of such detectors. Of particular, although not exclusive, interest are self-powered near-infrared radiation detectors.

[0004] Background

[0005] Commercial near-infrared (NIR) photodetectors (PDs), suitable for detection in the range of 0.8-3 pm, are based on inorganic single crystal Ge, Si or InGaAs, and use the photovoltaic phenomenon [Refs. 1-3]. They can detect light within the wavelength range of 0.8-3 pm and are deployed for image sensing, optical communications, medical diagnostics, touchless technology, automobile driving, security surveillance and loT sensors [Refs. 4-9]. For instance, in health diagnosis, Si PDs can be used to track vital signs such as the heart rate and the peripheral capillary blood oxygen saturation through the non-invasive photoplethysmography (PPG) technique [Ref. 10]. The infrared detector market size is projected to grow from $542.0 million in 2023 to $867.9 million by 2030, at a CAGR of 7% during the forecast period [Ref. 11].

[0006] There are some disadvantages of current commercial NIR PD materials. First and foremost, they all require power to operate and they need to be used at low temperature to suppress large dark currents. For instance, Si PDs, which are mostly used for PPGs [Ref. 12], need to operate at <20°C and sometimes near cryogenic temperatures [Ref. 13], This increases their size, weight and power, when compared to the desired requirement. They also have a challenging fabrication process as they have complex device architectures and are comprised of single crystalline bulk materials, which increases their cost [Refs. 1-3],

[0007] An alternative to inorganic PDs is organic lead-halide perovskites and chalcogenides of PDs. They have shown promising characteristics such as higher absorption coefficients in the NIR region than Si, simple processing and cooling-free operation. However, low carrier mobility, high exciton binding energy and disordered molecular alignment still limit the OPD performance [Ref. 14], In addition, the use of heavy metals, such as Pb and Cd in metal-halide perovskites (e.g., Pb-Sn perovskites) and chalcogenide quantum dots (QDs) (e.g., PbS and CdTe) restricts their use in commercial electronics [Ref. 2]. Besides that, organic PDs based on ultralow bandgap push-pull polymer TQ-T combined with state-of-the-art nonfullerene acceptors, IEICO-4F and Y6, show promising results for PPG. However, they need voltage (-2 V) to operate [Ref. 15], Organic self-powered PDs are also being proposed for NIR, but their responsivity is still below 0.6 A / W [Refs. 16-19].

[0008] Recently, non-toxic AgBiS? nanocrystals (NCs) and AgAuSe QDs were demonstrated to be promising for PPG. AgBiS2 NCs show promising PD response, such as high responsivity (-150 mA / W), detectivity (-5 x 109Jones), and fast rise and fall times (1.9 ps). However, they still need external voltage (-0.5 V) to operate [Ref. 2]. In the case of AgAuSe QDs, the PDs are self-powered and exhibit a high detectivity of 1.55 x 1010Jones, together with a responsivity of 0.36 mA / W. However, these PDs are slow with rise and fall times >0.5 s and >61 s, respectively, which hinders them from practical applications [Ref. 20].

[0009] Therefore, novel inorganic PDs currently offer most promise. The present inventors consider it surprising that to date little attention has been paid to the development of NIR detectors even though there are important applications needs, as mentioned above.

[0010] Summary of the invention

[0011] In view of the insights of the present inventors in this technical field, it is considered that it would be advantageous to develop simple, self-powered, room temperature operation PDs with high sensitivity, responsivity and detectivity in NIR regions with high wavelength selectivity.

[0012] The present invention has been devised in light of the above considerations.

[0013] Ferroelectric HfO2- and ZrO2-based materials have been explored extensively for local data storage and in-memory computing [Refs. 21-23] as well as for energy storage capacitors [Refs. 24-26]. However, to the knowledge of the present inventors, they have not been explored for NIR detection although antiferroelectric ZrO2 has been shown to work as a pyroelectric sensor, with >2 V non-volatile switching of pyroelectric and non-pyroelectric states demonstrated [Ref. 27],

[0014] While the pyroelectric properties of ferroelectric ZrCh films have been measured [Ref. 28], the use of ferroelectric ZrO2, for example, as a self-powered photodetector has not been demonstrated.

[0015] Accordingly, in a first aspect of the invention, there is provided a near-infrared radiation detector comprising:

[0016] a substrate comprising a semiconductor;

[0017] a substrate electrode formed in contact with the substrate;

[0018] an active layer formed over the substrate, the active layer comprising a ferroelectric material;

[0019] an active layer electrode formed in contact with the active layer and substantially transparent to nearinfrared radiation,

[0020] wherein the detector is operable without applied external bias so that, when near-infrared radiation is incident on the detector, charge carriers are generated by a photovoltaic effect in the substrate / active layer heterojunction and a temperature gradient is generated in the active layer, providing a pyro-phototronic current signal indicative of the near-infrared radiation incident on the detector.

[0021] In a second aspect of the invention, there is provided a method of detecting near-infrared radiation, the method comprising providing a detector comprising:

[0022] a substrate comprising a semiconductor;

[0023] a substrate electrode formed in contact with the substrate;

[0024] an active layer formed over the substrate, the active layer comprising a ferroelectric material;

[0025] an active layer electrode formed in contact with the active layer and substantially transparent to nearinfrared radiation, the method comprising, without applying external bias to the detector, causing near-infrared radiation to be incident on the detector, thereby generating charge carriers by a photovoltaic effect in the substrate and thereby generating a temperature gradient in the active layer, and providing a pyro-phototronic current signal indicative of the near-infrared radiation incident on the detector.

[0026] In a third aspect of the invention, there is provided a method of manufacturing a near-infrared radiation detector according to the first aspect, wherein the active layer is formed by deposition of a precursor layer over the substrate, the precursor layer then being subjected to a heat treatment process to stabilise a ferroelectric phase in the active layer.

[0027] In a fourth aspect of the invention, there is provided a sensor apparatus, the sensor apparatus including a light source configured to generate at least near-infrared radiation and a detector according to the first aspect.

[0028] Optional features of the present invention will now be set out. These can be applied singly or in any combination to any aspect of the invention, unless the context demands otherwise.

[0029] The device may be configured and / or operated so that, at initial incidence of the near-infrared radiation on the detector, an electric field Epv generated due to the photovoltaic effect and an electric field Epyrogenerated due to the pyroelectric effect are not opposed to each other. In some embodiments, the electric field Epv generated due to the photovoltaic effect and the electric field Epyro generated due to the pyroelectric effect may be substantially aligned with each other, e.g. having the same direction. This has the effect that, at least during the initial incidence and shortly thereafter, these electric field components sum to provide a larger overall electric field and therefore a greater signal indicative of the near-infrared radiation. The device may further be configured and / or operated so that, at the end of incidence of the near-infrared radiation on the detector, the electric field Epv generated due to the photovoltaic effect and the electric field Epyrogenerated due to the pyroelectric effect may be in opposite directions to each other.

[0030] In some embodiments, the active layer is poled. This has the advantage of providing the active layer with a net depolarisation field. In some embodiments, this has the effect of increasing the response of the device to near-infrared radiation. At initial incidence of the near-infrared radiation on the detector, an electric field Epv generated due to the photovoltaic effect, an electric field Epyrogenerated due to the pyroelectric effect and the electric field EdPdue to the net depolarisation field are in the same direction. Note that the electric field EdPdue to the net depolarisation field is in the opposite direction to the ferroelectric polarisation of the active layer.

[0031] In some embodiments, the substrate comprises silicon, such as single crystal silicon. This may be (100) oriented single crystal silicon. The silicon may be p-type silicon. A SiOx layer may be formed between the silicon and the active layer. The thickness of the SiOx layer may be at least 0.1 nm, or at least 0.5 nm, or at least 1 nm, or at least 1.5 nm, or at least 2 nm. The thickness of the SiOx layer is typically not more than 10 nm, or not more than 8 nm, or not more than 6 nm, or not more than 5 nm, or not more than 4 nm, or not more than 3 nm.

[0032] In some embodiments, the active layer is formed from a material based on HfxZri-xC, wherein 0 < x < 1. It is possible that the active layer may include dopants and / or incidental impurities. The value for x may be 0 or may be more than 0. x may be at least 0.1, at least 0.2, at least 0.3, at least 0.4, at least 0.5, at least 0.6, at least 0.7, at least 0.8, at least 0.9. In some embodiments, x may be 1. X may be at most 1, at most 0.9, at most 0.8, at most 0.7, at most 0.6, at most 0.5, at most 0.4, at most 0.3, at most 0.2, or at most 0.1. Suitable ranges for x will be apparent from combinations of these end point limits. For example, in some embodiments, 0 < x < 0.5.

[0033] In some embodiments, the active layer may comprise a polar orthorhombic phase. The active layer may be epitaxial with respect to the substrate. In the case where the substrate is (100) Si, the [11-1] direction of the active layer orthorhombic phase may be substantially parallel to the

[0200] direction of the Si substrate.

[0034] The substrate electrode may be formed in contact (typically ohmic contact) with the back plane of the substrate. The substrate electrode may for example be formed from Al.

[0035] The active layer electrode may be formed of ln-Sn-0 (ITO), for example.

[0036] The thickness of the active layer may be in the range 1-400 nm. In some embodiments, the thickness of the active layer may be at least 2 nm, at least 4 nm, at least 6 nm, at least 8 nm, or at least 10 nm. In some embodiments, the thickness of the active layer may be at most 350 nm, at most 300 nm, at most 250 nm, at most 200 nm, at most 150 nm, at most 100 nm, at most 80 nm, at most 60 nm, at most 40 nm, at most 20 nm, at most 15 nm, or at most 10 nm. For example, using sputtering or atomic layer deposition techniques, an active layer of thickness 1-40 nm is readily achievable. For a thicker active layer within the ranges identified above, a chemical solution deposition technique can be used.

[0037] According to embodiments of the present invention, it is therefore possible for the device to be operated without external power. The response of the device to incident near-infrared radiation can be sufficient to generate a detectable signal using conventional and low cost electronics. Furthermore, the devices can be CMOS compatible, which provides suitable routes for large scale and low cost manufacture.

[0038] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0039] Summary of the Figures

[0040] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figs 1-3 show ferroelectric and electrical characterization of a Si / SiOx / ZrC / ITO device according to an embodiments of the invention.

[0041] Fig. 1 shows a polarization-electric field hysteresis loop, the inset showing the device structure for the ferroelectric and electrical characterization.

[0042] Fig. 2 shows current- voltage curves for the AI / Si / SiOx / ZrCh / ITO device in the dark and under illumination with different light sources. The insets correspond to the enlarged region around the origin.

[0043] Fig. 3 shows current-time response for the AI / Si / SIOx / ZrO2 / ITO device with a laser illumination of 1064 nm with 50 milisecond pulses and 50 milisecond intervals.

[0044] Figs 4-6 show experimental and simulated transient response of the AI / SI / SiOx / ZrO2 / ITO device with 1064 nm laser illumination.

[0045] Fig. 4 shows an enlarged region around the transient response shown in Fig. 3. The inset shows the current on a log scale as a function of time to better distinguish the photovoltaic current (IPV) from the dark current (loark).

[0046] Fig. 5 shows the relative temperature (solid black) of the pyroelectric layer at r=0, and laser pulse power density (solid red) as function of time on the top left and a schematic representation of the structure with an in-depth temperature gradient distribution across the ZrCh layer in the bottom left. On the bottom right is shown the temperature variation rate (dT / dt) and on the top right is shown the pyroelectric current through the external resistor as a function of the time.

[0047] Fig. 6 shows a schematic representation of the pyro-phototronic effect in the AI / Si / SIOx / ZrO2 / ITO device for a complete chopper period. Steps 1, 2, 3 and 4 are related to each step of the transient response shown in Fig. 4. Epv corresponds to the built-in electric field formed at the junction between ZrO2 and Si, while Epyro corresponds to the pyroelectric field generated through the temperature change rate in ZrC>2. Ipyro+ v corresponds to the current generated through the pyroelectric and photovoltaic effects.

[0048] Fig. 7 shows / -t curves for the AI / Si / SiOx / ZrO2 / ITO device measured after different poling voltages for light of 1064 nm wavelength at a fixed chopper frequency of 10 Hz and a fixed power density of 407 mW / cm2. Fig. 7(a): from 0 V up to +12 V. Fig. 7(b): from +12 V down to 0 V.

[0049] Fig. 8 shows the variation of responsivity (R), detectivity (D*), and sensitivity (S) as a function of poling voltage.

[0050] Fig. 9 shows a schematic representation of the ferro-pyro-phototronic effect in the AI / Si / SiOx / ZrC / ITO device after poling and when the laser is turned on. EPV corresponds to the built-in electric field formed at the junction between ZrO2 and Si, Epyrocorresponds to the pyroelectric field generated through the temperature change rate in ZrO2 and EdPis the depolarization field in ZrO2. The ferroelectric polarization is also denoted as P in the ZrO2 and lFE+pyro+pVcorresponds to the current generated through the ferroelectric, pyroelectric and photovoltaic effects.

[0051] Fig. 10 shows / -t curves for the AI / Si / SiOx / ZrOz / ITO device measured with a light-emitting diode (wavelength of 940 nm) after poling, under zero bias, at a fixed power density of 1.4 mW / cm2and with a fixed chopper frequency varying from 10 up to 400 Hz.

[0052] Fig. 11 shows the variation of responsivity (R), detectivity (D*), and sensitivity (S) as a function of pulse repetition rate.

[0053] Fig. 12 shows a typical transient response of the device with 940 nm light-emitting diode at a fixed power density of 1.4 mW / cm2and with a fixed chopper frequency of 10 Hz..

[0054] Fig. 13 shows the stability of the transient response with 940 nm light-emitting diode at a fixed power density of 1.4 mW / cm2and with a fixed chopper frequency of 10 Hz.

[0055] Fig. 14 shows the l-t response for the AI / Si / SiOx / ZrO2 / ITO devices with the NIR signal provided by a TV remote control.

[0056] Fig. 15 shows a magnified region of the l-t response shown in Fig. 14. The photodetector parameters are also given.

[0057] Fig. 16 shows the l-t response for a Si commercial photodiode. The photodetector parameters are also given.

[0058] Fig. 17 shows a simulated schematic view of simulation domain with material layers. P1: ZrO2 surface under ITO electrode.

[0059] Fig. 18 shows an X-ray diffraction pattern over a narrow range to observe the orthorhombic (111) peak from ZrO2.

[0060] Fig. 19 shows nanoscale structural characterization of the Si / SiOx / ZrO2 structure. Fig. 19(a) shows a cross-section TEM image and Fig. 19(b) shows the associated SAED pattern of the ZrO2 thin film grown on top of a Si(100) wafer. Fig. 19(c) is a HRTEM image of a boundary between two adjacent o-ZrO2 grains close to BzrO2 = [-110]oorientation. Fig. 19(d) shows a structural model of an 0-ZrO2 grain viewed along the BzrO2 = [-110]odirection.

[0061] Fig. 20(a) is a HRTEM image exhibiting the curvature of the {111 }0crystal planes within the ZrO2 layer. Fig. 20(b) is a HRTEM image of a boundary between two adjacent 0-ZrO2 grains with a 4° misalignment of the {111 }o planes. Fig. 21 shows the emission spectrum for the TV remote control with a LED.

[0062] Fig. 22 shows a schematic view of an array of two light sources (typically LEDs) emitting at 650nm and 940nm, respectively, with an opposed detector according to an embodiment of the invention and a subject’s finger interposed between them.

[0063] Fig. 23 shows a typical absorption-time response from the detector of Fig. 22.

[0064] Figs. 24-27 show measured responses of current against time for different light wavelengths and for different compositions of the active layer, all in a self-powered configuration.

[0065] Detailed Description of the Invention

[0066] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0067] Self-powered near-infrared (NIR) photodetectors that are useful for example for surveillance systems, sensing in loT electronics, facial recognition, health monitoring, optical communication networks, night vision and biomedical imaging. However, the current silicon commercial detectors need external power to operate and cooling to suppress large dark currents, which means the overall device size is relatively large. Set out in this disclosure is a new class of high performance, self-powered NIR photodetectors. Embodiments of the devices can be fully CMOS-compatible. Embodiments of the devices are based on AI / Si / SiOx / ZrO2 / ITO. At room temperature, under 940 nm wavelength illumination (1.4 mW / cm2power density, 10 Hz repetition rate), and without any power applied, fast rise and fall times of ~2 and ~4 ps, respectively are achieved, along with responsivity, detectivity and sensitivity values of up to ~3.4 A / W, 5.8x1010Jones and 4.2x103, respectively, far exceeding all other emerging self-powered systems. The combined ferroelectric, pyroelectric and photovoltaic functionalities are proven to underlie the strong performance. Furthermore, dual-band NIR detection is shown to be possible for different NIR wavelengths, proof-of-concept feasibility being demonstrated for smart identification of NIR targets.

[0068] In this work, using 5 nm ZrO2 films grown by sputtering on Si, we show very strong NIR detection capability. The performance is shown to arise from the pyroelectric effect, combined with the ferroelectric effect in ZrCh, and the photovoltaic (PV) effect in Si (ferro-pyro-phototronic effect). We demonstrate that optical stimuli of 940 and 1064 nm wavelength can be converted to electrical stimuli without requiring an external power supply, producing efficient NIR PDs. Overall, we have created a simple CMOS-compatible device for self-powered NIR PDs than enhances the performance of current Si PDs.

[0069] Materials and methods

[0070] ZrO2 films were grown by ion-beam sputtering (IBS) onto p-type (100) Si / SiOx substrates (Si-Mat). The conditions for the growth of the ZrO2 films are given in a previous work [Ref. 29]. After the growth, a rapid thermal annealing (RTA) process was performed in N2 at T = 700 °C for 60 s, in order to stabilize the polar o-phase. Then, top ITO electrodes using a 90 wt% ln2O3-10 wt% SnO target (Kurt J. Lesker, 99.99%) were grown by IBS on the surface of the ZrO2 films. The ITO electrodes were deposited at a temperature of 100 °C and the gas pressure inside the chamber was kept constant at 3.4 x 10-4 mbar. A gas flow of 7.7 ml / min of Ar+ 0.3 ml / min of O2 was introduced into the ion beam gun where the atoms were ionized using a 50 W RF source. Bottom aluminium (Al) electrodes were attached to the Si wafer backside by electric spark to ensure an ohmic contact.

[0071] The structural characterization of the deposited layers was performed using X-ray diffraction (XRD), which was carried out in a Bruker D8 Advance DaVinci (Germany) diffractometer at room-temperature over the 2G range = 24-38°, in a Bragg-Brentano configuration using a nickel filtered CuKa radiation (A = 1.5406 A). A cross-section thin lamella forTEM investigations has been prepared by tripod polishing and ion milling on a Gatan PIPS installation. The details for transmission electron microscopy (TEM) are given in a previous work [Ref. 29],

[0072] Ferroelectric hysteresis loops (P-E) were measured at room temperature with a modified Sawyer-Tower circuit using a triangular signal of 1.0 kHz.

[0073] I-V curves were measured using Keithley 2601 System Source Meter in the dark and under illumination with different light sources (405, 650, 940 and 1064 nm).

[0074] Fortesting the NIR sensing capabilities, first the / -f curves were measured using a current amplifier Keithley 428 and National Instrument I / O card PCI-6251 programmed in Labview. The illumination of the AI / Si / SiOx / ZrO2 / ITO devices was provided by a semiconductor laser with a wavelength of 1064 nm and a light-emitting diode (LED) with a wavelength of 940 nm. The power was controlled with a TTL signal, while the pulse repetition rate was varied from 10 upto 400 Hz. The poling was done by applying a voltage (range from 0 up to 12 V) to the bottom Al electrode using a Keithley 2601 A source meter for 10 s. Then, a standard TV remote control was utilized, operating with the RC5 communication protocol commonly found in infrared remote controls. The remote was equipped with an IR LED emitting light with a wavelength of 932 nm at a maximum intensity and a signal frequency of 36 kHz. Spectrum measurements were conducted using an iHR 320 spectrometer and a Symphony II CCD camera by Horiba. Frequency measurements were done using a typical silicon photodiode BPW34, in reverse bias. The signal was observed on an oscilloscope. Notably, both measurements were taken directly from the emitted signal of the remote. The pyroelectric response of the ITO / ZrCh / Si multilayer device was simulated with the COMSOL Multiphysics software. The details of the simulation are given further below.

[0075] Results and discussion

[0076] After performing the growth and RTA, the samples were structurally characterized by XRD and TEM, as further discussed below. The detailed analysis confirms the presence of the polar orthorhombic phase in 5 nm thick ZrO2 films. A uniform layer of SiOx with a thickness of ca. 2.5 nm was observed at the interface between the ZrO2 layer and the Si substrate (lighter contrast in Fig. 19(a)). To confirm the presence of ferroelectricity in the ZrO2 films, we measured the room temperature polarization-electric field (P-E) hysteresis loops of the AI / Si / SiOx / ZrO2 / ITO film capacitor devices. A typical P-E loop is shown in Fig. 1 and it is possible to confirm the ferroelectric nature of the ZrC film. The device structure is shown in inset of Fig. 1. The average values of Ps, Pr, and Ecfor the AI / Si / SiOx / ZrO2 / ITO film capacitor are 1.2 pC cm-2, 0.8 pC cm-2and 1.3 MV cm-1, respectively. The asymmetric shape of the P-E loops is typical of a metal-ferroelectric-insulator-semiconductor (MFIS) structure [Refs. 30-32]. In addition, the Ps and Prvalues in HfO2-ZrO2 films in a MFIS structure are usually lowerthan the ones found in a metal-ferroelectric-metal configuration [Ref. 31].

[0077] To evaluate the sensitivity of the AI / Si / SiOx / ZrO2 / ITO film capacitor to light, we performed l-V curves in the dark and then under different light conditions of different wavelengths. Figure 2 shows the l-V curves. In dark conditions, it is possible to observe a diode-like behaviour, which can indicate the formation of a Schottky-like junction [Ref. 33]. In addition, it is possible to observe a strong light effect for negative bias for all the different laser wavelengths. The magnified l-V curves around the origin are shown in the inset of Fig. 2 for the different laser lights. It is possible to observe that for NIR wavelengths, the PV effect is higher. This proof of carrier generation in the Si substrate (bandgap around 1.12 eV) [Ref. 34], the electrons and holes are then separated by the junction. In addition, the AI / Si / SiOx / ZrO2 / ITO film capacitor shows selective responses to NIR signals and therefore is capable of avoiding the interference of visible light.

[0078] To further explore the PV effect from the Si substrate, we investigated the transient response characteristics of the AI / Si / SiOx / ZrO2 / ITO device, under zero bias, with a 940 nm LED and with 1064 nm laser illuminations. Figure 3 shows the l-t response for the AI / Si / SiOx / ZrC / ITO device, under zero bias, with 1064 nm laser illumination, at a fixed power density of 407 mW / cm2and pulse repetition rate of 10 Hz. From the l-t curves shown in Fig. 2, one can observe one positive sharp current peak when the light is turned on and one negative current peak when the light is turned off with stable characteristics. To observe this behaviour in more detail, Fig. 4 shows an enlarged region around one positive and one negative current peak. Also, the inset of the same figure shows the transient response after a fast current peak, allowing us to distinguish Idark from IPV. This transient response is usually attributed to a light-induced surface temperature change in time, which induces a potential difference between the top and bottom of the ZrC film, the so-called pyro-phototronic effect [Refs. 33,35,36],

[0079] When NIR light is applied, Si absorbs the light and carriers are generated and separated through this field (EPV), which leads to the PV effect. In addition, the pulsed light heats the ZrO2 layer, leading to the appearance of a pyroelectric induced electric field (Epyro) in this layer, with the same direction of the built-in electric field (EPV) formed at the junction between ZrO2 and Si. The temperature change rate generates a pyroelectric field (Epyro) amplifying the current as long as the temperature evolution remains. This stage is labelled as 1 in Fig. 4 and for better understanding the electric fields across the devices together with the generated current (lpyro+pv) are shown in Fig. 6. Once the temperature stabilizes, Epyrodisappears, as shown in the schematic representation of the electric field across the devices in step 2 of Figs. 4 and 6, only a small current from the PV effect is observed, which is clearly visible in the inset of Fig. 4. When the laser is turned off (stage 3) a reversed temperature gradient in ZrO? occurs, which causes Epyroto be reversed across the ZrO2 film and leads to a reverse current spike (lpyro) flowing in the reverse direction, as shown in the schematic diagram of Figs. 4 and 6. When the temperature stabilizes in ZrO2 film and reaches the room temperature, the current returns back to Idark (stage 4 in Figs. 4 and 6).

[0080] To confirm that a pyroelectric effect is indeed in operation, the pyroelectric response of a multilayered device is simulated with the COMSOL Multiphysics software to gain deep understanding on the pyro-phototronic effect. The details are given below. In top left of Fig. 5 is plotted the temporal evolution of the temperature of the pyroelectric ZrO2 layer at the axis relative to the initial layer temperature. After an initial fast rise of the temperature at t = 1 ms, after the light pulse is turned ON, the temperature continues to increase at a lower rate before stabilizing at t = 10 ms. The maximum temperature change of the pyroelectric layer relative to ambient temperature is about 14 mK. At the bottom left of Fig. 5 is shown a schematic representation of the structure with the simulated in-depth temperature profile across the ZrO2 layer. The maximum temperature difference between the top and bottom surface of ZrO2 is 0.01 mK. On the right side of Fig. 5 is shown the temperature variation rate (dT / dt) (bottom) and pyroelectric current (top) through the external resistor as a function of time. The temperature variation rate is very high at t = 1 ms, consistent with the fast rise of the laser pulse, followed by a sharp decay due to heat flow dissipation at Si substrate with high thermal conductivity and thermal capacity. The pyroelectric current profile differs from the dT / dt, due to influence of the external resistance and capacitance introduced by ZrO2 pyroelectric layer, but the shape and duration of the current peak is in good agreement with the one measured experimentally (inset of Fig. 4) with a decay time of about 100 ps.

[0081] So far, we did not consider the role of the ferroelectric polarization of the ZrC>2 film in the PD performance of the AI / Si / SiOx / ZrO2 / ITO device. To do this, we measured transient response characteristics after the application of a poling voltage. Figs. 7(a) and (b) show the l-t curves after applying a poling voltage.

[0082] From Fig. 7(a), one can conclude that the pyro-phototronic response is controlled by the poling voltage when this voltage is increased from 0 up to 12 V. Without any poling voltage, the PD performance is attributed to the pyro-phototronic effect and after applying a poling voltage, the l-t response of the device changes significantly. However, when the poling voltage is decreased from 12 V up to 0 V, no significant change in the pyro-phototronic response is observed (Fig. 7(b)). The poling voltage effect on the PD performance is further demonstrated through the estimation of the PD parameters, such as responsivity (R), detectivity (D*), and sensitivity (S) as follows [Refs. 35,36]:

[0083]

[0084] where lught and loark are the short-circuit currents with and without illumination, respectively. P and A are the effective illumination power and area on the PD, respectively, and q represents the electron charge. Fig. 8 displays R, D* and S as a function of poling voltage. It is possible to observe a butterfly shape similar to the one observed in the P-E loop shown in Fig. 1. The maximum R, D*and S are achieved after poling the device at 12 V. The maximum R, D*and S are ~16.5 mA / W, 2.8x108Jones and 5.9x103, respectively, which are around 15% higher than the values estimated without any poling. Therefore, it is possible to conclude that the l-t response as a function of the poling voltage is controlled through the ferroelectricity in the ZrO? layer arises from coupling the ferroelectric, pyroelectric, and photovoltaic effects in the structure, the so-called ferro-pyro-phototronic effect [Refs. 37,38].

[0085] To aid with the understanding of the phenomenon, the generated electric fields across the device when the light is turned on together with the generated current due to the ferro-pyro-phototronic effect (jFe+Pyro+PV) are shown in Fig. 9. The ferro-pyro-phototronic effect can be understood as follows: with an increase in the poling voltage, an additional electric field (EdP) associated with the depolarization field in ZrO2 emerges in the same direction of EPyroand EPV, which is opposite to the ferroelectric polarization, further amplifying the current until its saturation, as illustrated in Fig. 7(a). This leads to the further enhancement of the PD performance, as shown in Fig. 8. By decreasing the poling voltage the PD performance remains the same, as shown in Fig. 8, since the ZrCh is already polarized, confirming the role of the ferroelectric polarization.

[0086] To further investigate the PD performance of the AI / Si / SiOx / ZrO / ITO device, we have measured the l-t response, under zero bias, but after the poling voltage procedure, with 940 nm LED, at a fixed power density of 1.4 mW / cm2and pulse repetition rate varying from 10 up to 400 Hz. Figure 10 shows the pulse repetition rate effect on the l-t response of the AI / Si / SIOx / ZrO2 / ITO device.

[0087] Similar to what was observed in p-Si / n-ZnO nanowires heterojunction [Ref. 39], it is possible to observe a strong decrease on the current response as the pulse repetition rate increases. This is due to the fact that the pulse repetition rate is higher than the resonance frequency, and therefore the pyroelectric current decreases due to a decrease in the electric field in the depletion zone [Refs. 39,40]. Figure 11 shows the evolution of R, D* and S as a function of pulse repetition rate. It is possible to observe a decrease of R, D*and S with increasing the pulse repetition rate. The maximum R, D*and S are ~3.4 A / W, 5.8x101DJones and 4.2x103, respectively. The significant higher NIR PD performance when compared the two different wavelengths is a consequence of a significant lower power density used in the case of the use of a LED, while by keeping a significant current generation through the ferro-pyro-phototronic effect.

[0088] Figure 12 displays the transient characteristic of the device in response to an ON / OFF cycle under LED irradiation measured with a power density of 1.4 mW / cm2and with a repetition frequency of 10 Hz. The rise time (rr) and the fall time ( f) were estimated and are equal to ~2 and ~4 ps, respectively.

[0089] Furthermore, the stability of the l-t response of the device was also evaluated and is shown in Fig. 13. It can be observed that the PD shows a stable response up to 100 cycles, without a significant change in the performance.

[0090] Table 1 shows a comparison between the performance of our AI / Si / SiOx / ZrOs / ITO device compared to relevant inorganic NIR PDs reported in the literature. It is possible to conclude that our AI / Si / SiOx / ZrO2 / ITO device exhibits state-of-the art performance in terms of the combination of R, D* rr and rf, when compared to other devices based on different materials that are currently being investigated. Compared to Au / MoSe2 / Au PDs which show the highest R values, our AI / Si / SiOx / ZrO2 / ITO devices have R values more than doubled, with the advantage of being self-powered. In addition, the D* of our device is two orders of magnitude higher, with the response time several orders of magnitude higher. Compared to self-powered Ag / Si / AbOs / ZnO / ITO PDs, our device shows two orders of magnitude lower D*. We note that the area normalization of the D* is not always useful for characterizing a ridge detector [Ref. 41], Furthermore, the R of our devices is 8 times higher and the response times are at least 5 times faster. Advantageously, the strong performance is achieved without the need of any external voltage.

[0091] Table 1. Comparison of the R, D* rr and rf obtained in this work with the ones presented in the literature for inorganic NIR PDs, both powered and self-powered.

[0092] Power External

[0093] Wavelength R

[0094] Device density voltage D* (Jones) rr(gs) tj (gs) Ref.

[0095] (nm) (mA / W)

[0096] (mW / cm2) (V)

[0097] TiN / IGZO / C60: LiF / PTB7-Th / IEICO- 927 943 -3.5 405 1.9 * 10130.6 1.5

[0042] 4F / MoOx / ITO

[0098] Au / MoSe2 / Au - 1064 5 1500 2.23 * 10s1.2xl071.4xl07

[0043] ITO / ZnO / N2200 / PPhTQ: COTIC- 37.7 1400 -0.2 18.7 4.19 * 10“ 7.6 7.7

[0044] 4F / MoOa / Ag

[0099] AgNWs / HWSiOz / Si 4 940 0 68xl0~3- 60 20xl03

[0033] Cu / Si / ZnO / ITO 4.8 1064 0 220 - 15 21

[0045] Ag / Si / ALOr / ZnO / ITO 8.4 * 10‘2940 0 410 3.7 * 101215 21

[0046] ITO / ZnO / AgBiS2 / PTB7 / MoOx / Ag 940 -0.5 -150 ~5 * 10’ 1.9 1.9 [2]

[0100] This Al / Si / SiOx / ZrO / ITO 1.4 940 0 3400 5.8x10'° 2 4

[0101] work

[0102] The NIR sensing capabilities of our device were tested by using a TV remote control LED with a maximum emission for a wavelength of 932 nm (see emission spectrum in Fig. 21). The power of the LED is ~2 mW and the chopper frequency is 36 kHz. The TV remote control was fixed in position to point to the AI / Si / SiOx / ZrCh / ITO device. The l-t response of a control AI / Si / SiOx / / ITO device (no ZrCh) was measured, and it was observed that there was no response to the TV remote control. Fig. 14 shows the l-t response of the AI / Si / SiOx / ZrCb / ITO device (ZrCh included). During this test, it was possible to observe that different buttons on the TV remote control exhibit different signals, as suggested by the different blinking of the LED of the TV remote control. Thus, the PD performance of the AI / SI / SiOx / ZrO2 / ITO device was evaluated considering the light emitted by the TV remote control.

[0103] Figure 15 shows a magnified region of Fig. 14. The PD parameters were also estimated from the l-t response and R, D*, rr and if, are equal to 0.13 A / W, 3.0x108Jones, ~2 and ~4 ps, respectively. For comparison, the V-t response of a Si photodiode is given in Fig. 16. According to the manufacturer [Refs.

[0104] 47,48] of the Si photodiode, R is equal to 0.62 A / W, while rr and rfare equal to 0.1 ps. The D*was estimated to be 1.8x107Jones. It is possible to conclude that the AI / Si / SiOx / ZrO2 / ITO device exhibits a performance comparable to the commercial Si photodiode, but with higher D* higher (and R higher at 10Hz) by 16 (5), and advantageously there is no power required as for Si. Thus, even though R (at high chopper frequencies of 36 kHz), rrand rf are lower than for Si, these parameters are not important for PPGs, since the heart rate frequency is much lower, nor for autonomous loT sensors used in mobile weather stations where the response time of the sensors are in the ms range [Ref. 49]. Thus our AI / Si / SiOx / ZrO2 / ITO devices represent an important advance towards self-powered wireless loT sensors, medical diagnosis and for specialist military applications where external power is either not available (or not available over a long duration).

[0105] Accordingly, CMOS compatible AI / Si / SiOx / ZrO2 / ITO photodetector devices have been fabricated and demonstrated to show high performance self-powered near-infrared sensing (940-1064 nm wavelengths) via the ferro-pyro-phototronic effect. The responsivity, detectivity, and sensitivity of ~ 3.4 A / W, 5.8x101° Jones and 4.2x103, under the illumination of a light- emitting diode (wavelength of 940 nm), at a fixed power density of 1.4 mW / cm2and pulse repetition rate of 10 Hz is demonstrated. Ultrafast rise and fall times of ~2 and ~4 ps, respectively, were also demonstrated. Clear NIR sensing capabilities of the device were demonstrated, outperforming commercial Si photodiodes in terms of responsivity and detectivity, but advantageously without the need of an external voltage.

[0106] Computational simulation

[0107] The pyroelectric response of the ITO / ZrO2 / SiOx / Si multilayer stack was simulated with the COMSOL Multiphysics software. A 2D axisymmetric domain, shown in Fig. 17, was used in the simulations.

[0108] The heat transport in the multilayer stack was described by solving the nonlinear inhomogeneous heat conduction equation,

[0109]

[0110] where T(r,z,t) is the temperature, p is the density, cthe specific heat, k the thermal conductivity of the material. The laser power is applied at the top ITO electrode surface as a boundary condition

[0111]

[0112] with Po the laser power density, R ITO electrode reflection coefficient, and q(t) is a step pulse temporal profile with 1 ps rise time, described by

[0113]

[0114] The initial temperature of the multilayer stack was set in the computational model to the room temperature:

[0115] T(r, Z,t=0) = Tarnb (S4)

[0116] where Tamt the ambient temperature, 293,15 K. For the Si substrate bottom surface a fixed temperature was considered:

[0117] T(r, Z=0, t) = Tamb

[0118] (S5)

[0119] while for r=0 the axial symmetry boundary condition was considered:

[0120]

[0121] For the remaining surfaces the boundary conditions consider radiation heat losses to the ambient using the Stefan-Boltzmann law:

[0122]

[0123] where n is the surface normal vector, E is the surface emissivity, a is the Stefan-Boltzmann constant. We also assume a perfect thermal contact between the layers.

[0124] The dynamic pyroelectric response current of multilayer stack can be expressed as:

[0125]

[0126] where p' is the component of the pyroelectric coefficient vector p orthogonal to the electrode surface of area A and 7 is the temperature of the pyroelectric film.

[0127] The simulation was performed using the finite element method and a mapped mesh was used for domain discretization with a minimum mesh size of 1 nm along z and 100 pm along r. The simulations were performed for a single step laser pulse starting at 1 ms. A transient analysis was done from 0.999 ms to 10 ms, using the implicit Backward differentiation formula (BDF) method with variable time steps selected from 0.1 - 100 s. The resulting linear system of equations was solved using the PARDISO solver. The pyroelectric response was simulated using COMSOL pyroelectric Multiphysics interface and considering that the pyroelectric stack is connected to an external load resistor of 100Q, representing electrode contacts and input resistance of an eventual current amplifier used in measurements.

[0128] The materials thermophysical properties were expressed as a function of temperature to reflect more realistic physical phenomena. Since the laser beam wavelength used is 1064 nm, the materials optical properties corresponding to 1064 nm were used for simulating the temperature distribution across the multilayer stack. The full list of parameters used in the simulations is given in the Tables S1 and S2.

[0129] Table S1. Laser parameters and simulation domain dimensions

[0130] Name Description Value Unit Tau Laser pulse rise time IE-6 s W Laser Power density 0.4 W / cm2A Wavelength 1.064 E-6 m Lsi Thickness Silicon substrate 2.75 E-4 m LW2 Thickness ITO eletrode 1.2 E-8 m LZro2 Thickness Zirconia layer 5.0 E-9 m Lsiox Thickness S1O2 layer 2.5 E-9 m Lwldth Sample radius 3.73E-3 m Lgpot Electrode radius 1.91 E-3 m Table S2. Materials parameters used in simulation. [Refs, column refers to references with “S” prefix.] Name Description value Unit Ref. k Thermal conductivity k(T) W / (m K) 1 c Heat capacity c(T) J / (kg K) 2 Zirconia p Density p (T) Kg / m33 p Pyroeletric coefficient 3.5E-5 C / nr24 R Refletance 0.15 - 5 s Surface emissivity 0.7 - 6 k Thermal conductivity 4 W / (m K) 7 c Heat capacity 341 J / (kgK) 7 p Density 7120 Kg / m37 R Refletance 5.8E-2 8 e Surface emissivity 0.3 9 k Thermal conductivity k(T) W / (mK) 10 Silicon c Heat capacity c(T) J / (kg K) 11 p Density p (T) Kg / m312 e Surface emissivity 0.71 - 13 k Thermal conductivity k(T) SiOx c Heat capacity c(T) p Density p (T) e Surface emissivity 0.74

[0131]

[0132] Structural characterization of the Si / SiOx / ZrO2 structure

[0133] Figure 18 shows the x-ray diffraction pattern of the p-Si / SiOx / ZrO2 structure. One significant diffraction peak centred at 20 = 30.35° (d = 2.93 A) is observed which can be attributed to the presence of the orthorhombic phase (111) planes [Ref. S18].

[0134] In order to unravel the phases that exist in the ZrO2 layer, TEM measurements were performed. The cross-section TEM image in Fig. 19(a) displays the ZrO2 thin film with a thickness of 5 nm grown on top of a Si(100) substrate. A uniform layer of SiOx with a thickness of ca. 2.5 nm can be also observed at the interface between the ZrO2 layer and the Si substrate (lighter contrast in Fig. 19(a)).

[0135] The selected-area electron diffraction (SAED) pattern in Fig. 19(b) provides information about the crystallographic status of the ZrO2 layer with respect to the Si substrate in the [01 -1 ]si projection. Hence, the SAED pattern contains the diffraction spots from the Si substrate (denoted with “Si” subscript) as well as a number of doubled or elongated spots stemming from the ZrC>2 layer. A careful measurement of the diffraction pattern based on the Si pattern taken as calibration reference points out the formation of the o-ZrO2 phase (“o” subscript in Fig. 19(b)) [Ref. S19]. The strong 11-10spots indicates a well defined texture of the ZrC>2 film around the [11-1 ]0direction oriented nearly parallel to the

[0200] si direction (the normal to the substrate surface). The doubling of the 11 -10spot and the presence of elongated 1310and 022ospots reveals a certain local mosaicity of the ZrO2 film, here observed next to the [2-11]0zone axis. Therefore, although the presence of the SiOxlayer prevents a direct crystallographic connection between the coating and the substrate, the local orientation of the ZrO? thin film with respect to the Si substrate can be approximated as:

[0136] [11 - 1 ]o ||

[0200] si and [2-11 Jo || [01 - 1 ]si

[0137] A better understanding of the low-scale crystallographic information regarding the ZrO2 growth arises from cross-section HR-TEM imaging. Micrographs acquired in several locations along the interface reveal the locally varying orientation of the crystal grains within the 0-ZrO2 layer, while the crystal {111 }0planes remain almost parallel to the interface. The HR-TEM image shown in Fig. 19(c) reveals the boundary, marked by the arrow, between two ZrC>2 grains. Inside each grain, although not perfectly aligned with respect to the electron beam, two families of lattice planes may be observed. The angle between the two families of lattice planes is 71.5° which, corroborated with the measured distances between lattice fringes, allowing us to identify the (111)0and (11-1 )0lattice planes of the 0-ZrO2 structure. In addition, the two grains show an almost mirrored orientation, as depicted by protractors.

[0138] The structural model of o-ZrO2 viewed along the [-110]ozone axis (Fig. 19(d)) displays the same angle of 71.5° between the (111)0and (11 -1)0lattice planes. The (11-1)0lattice planes in both grains run almost parallel to the interface, with a misalignment of 18.3° between the two grains. Other examples regarding the crystallographic orientation of the ZrO2 thin film are shown in Figs. 20(a) and (b). In both cases, the {111 }0crystal planes are oriented almost parallel to the substrate. However, in the first case (Fig. 20(a)), they are locally bent with and a 12.8° rotation of the {111 }0planes on one side and the other of the vertical arrow, corresponding to a curvature radius of ca. 150 nm. In the other example (Fig. 20(b)) the grain boundary marked with the vertical arrow separates two grains viewed along a direction close to Bzro2 = [-110]o. The {111 }0planes are almost parallel to the interface, with a local misalignment of 4° between the two imaged crystal grains. In conclusion, the ZrO2 layer is continuous and fully crystallized in the orthorhombic structure (S. G. Pca2i). The film is strongly textured around the [11-1 ]0direction oriented in average perpendicular to the Si substrate. Correspondingly, the (11-1 )0planes are in average oriented parallel to the interface, showing local intragrain bending or misorientations from grain to grain up to 12° with respect to the interface.

[0139] Devices using active layer of HfxZr1-xO2

[0140] In the work reported above, the active layer of the device is a ZrO2 layer. Further work has been carried out on devices using this composition but also on devices incorporating Hf in the active layer. This work has considered the effectiveness of the devices as self-powered photodetectors based on HfxZr1-xO2(HZO) for health diagnosis.

[0141] Fig. 22 shows a schematic view of an array of two light sources (typically LEDs) emitting at 650nm and 940nm, respectively. A detector is arranged opposite the light sources, the detector being according to an embodiment of the invention. The detector has a substrate comprising a semiconductor, a substrate electrode formed in contact with the substrate and an active layer formed over the substrate. An active layer electrode formed in contact with the active layer and is substantially transparent to visible and near- infrared radiation. Based on the proposed mechanisms described above, the detector is operable without applied external bias so that, when near-infrared radiation is incident on the detector, charge carriers are generated by a photovoltaic effect in the substrate / active layer heterojunction and a temperature gradient is generated in the active layer, providing a pyro-phototronic current signal indicative of the near-infrared radiation incident on the detector.

[0142] A typical absorption-time response from the detector is shown in Fig. 23, consisting of DC components due to absorption by venous blood and other tissues. A time-variable component is due to absorption by arterial blood, which in turn depends on blood oxygen concentration.

[0143] Figs. 24-27 show measured responses of current against time for different light wavelengths and for different compositions of the active layer, all in a self-powered configuration. Fig. 26 shows the results for a ZrO2 active layer with 650 nm light. This can be compared with Fig. 27, which shows the results for a ZrOs active layer with 940 nm NIR light.

[0144] Fig. 24 shows the results for a Hfo sZro 5O2 active layer with 650 nm light.

[0145] Fig. 25 shows the results for a Hfo aZro 7O2 active layer with 650 nm light.

[0146] These results show that it is feasible to have the active layer formed of compositions over than ZrO2 layer, with active layer compositions of Hfo sZro 502 being readily demonstrated.

[0147] ***

[0148] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0149] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0150] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0151] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0152] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

[0153] References

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Claims

Claims:

1. A near-infrared radiation detector comprising:a substrate comprising a semiconductor;a substrate electrode formed in contact with the substrate;an active layer formed over the substrate, the active layer comprising a ferroelectric material;an active layer electrode formed in contact with the active layer and substantially transparent to nearinfrared radiation,wherein the detector is operable without applied external bias so that, when near-infrared radiation is incident on the detector, charge carriers are generated by a photovoltaic effect in the substrate / active layer heterojunction and a temperature gradient is generated in the active layer, providing a pyro-phototronic current signal indicative of the near-infrared radiation incident on the detector.

2. A near-infrared radiation detector according to claim 1 wherein, at initial incidence of the nearinfrared radiation on the detector, an electric field Epv generated due to the photovoltaic effect and an electric field Epyro generated due to the pyroelectric effect are in the same direction.

3. A near-infrared radiation detector according to claim 2 wherein, at the end of incidence of the near-infrared radiation on the detector, the electric field Epv generated due to the photovoltaic effect and the electric field Epyrogenerated due to the pyroelectric effect are in opposite directions to each other.

4. A near-infrared radiation detector according to any one of claims 1 to 3 wherein the active layer is poled to provide the active layer with a net depolarisation field.

5. A near-infrared radiation detector according to claim 4 wherein, at initial incidence of the nearinfrared radiation on the detector, an electric field Epv generated due to the photovoltaic effect, an electric field Epyro generated due to the pyroelectric effect and the electric field EdPdue to the net depolarisation field are in the same direction.

6. A near-infrared radiation detector according to any one of claims 1 to 5 wherein the substrate comprises single crystal silicon.

7. A near-infrared radiation detector according to claim 6 wherein the silicon is p-type silicon.

8. A near-infrared radiation detector according to claim 6 or claim 7 wherein a SiOx layer is formed between the silicon and the active layer.

9. A near-infrared radiation detector according to any one of claims 1 to 8 wherein the active layer is formed from a material based on HfxZr1-xO2, wherein 0 < x < 1.

10. A near-infrared radiation detector according to claim 9 wherein 0 < x < 0.5.

11. A near-infrared radiation detector according to any one of claims 1 to 10 wherein the active layer comprises a polar orthorhombic phase.

12. A near-infrared radiation detector according to any one of claims 1 to 11 wherein the thickness of the active layer is in the range 1-400nm.

13. A near-infrared radiation detector according to any one of claims 1 to 12 wherein the thickness of the SIOx layer is in the range 0.1-10nm.

14. A sensor apparatus including a light source configured to generate at least near-infrared radiation and a detector according to any one of claims 1 to 13.

15. A method of manufacturing a near-infrared radiation detector according to any one of claims 1 to 13 wherein the active layer is formed by deposition of a precursor layer over the substrate, the precursor layer then being subjected to a heat treatment process to stabilise a ferroelectric phase in the active layer.

16. A method of detecting near-infrared radiation, the method comprising providing a detector comprising:a substrate comprising a semiconductor;a substrate electrode formed in contact with the substrate;an active layer formed over the substrate, the active layer comprising a ferroelectric material;an active layer electrode formed in contact with the active layer and substantially transparent to nearinfrared radiation,the method comprising, without applying external bias to the detector, causing near-infrared radiation to be incident on the detector, thereby generating charge carriers by a photovoltaic effect in the substrate and thereby generating a temperature gradient in the active layer, and providing a pyro-phototronic current signal indicative of the near-infrared radiation incident on the detector.

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