The present invention relates to photosensitive devices, and in particular the use of nanowires or similar semiconductor structures in photosensitive devices.

In the past, InAs and/or InAsSb alloys have attracted broad interest as a promising alternative to the widely used HgCdTe and InSb based detectors in the mid- wavelength infrared (MWIR) regime, due to their unique intrinsic properties. With the in-depth study of their properties, they became ideal candidates for high performance optoelectronics operating at room temperature, both in MWIR and long-wavelength infrared (LWIR) spectral range.

However, due to the large lattice and thermal mismatch between lll-V thin films and available substrates, it continued to be challenging to realise conventional thin-film InAs or InAsSb devices with high performance, thus hindering the fabrication of effective devices using lll-V/Si hybrid structures. The large mismatch between the substrate and the epilayer materials generates dislocations, degrading the device performance. One-dimensional nanowires have been introduced as a promising architecture to overcome the restraints of lattice-match requirements. Owing to their small footprint, nanowires readily release the lattice-mismatch induced strain, which results in dislocation free material. This feature enables epitaxial growth on inexpensive substrates, in particular silicon, offering the promise of integration of lll-V optoelectronics with complementary metal-oxide-semiconductor devices.

Furthermore, nanowires exhibit unique physical properties that are favourable for optoelectronic applications. For instance, nanowires have demonstrated enhanced light absorption, efficient carrier separation and collection, leading to applications in indium arsenide (InAs) nanowire photodetectors, InAs nanowire

phototransistors, InAs field-effect transistors and Indium Phosphide (InP) nanowire solar cells with record efficiency of 13.8%.

However, several challenges still prevent further improvement of nanowire device performance. In particular, their crystalline quality, the presence of surface states, efficient recombination and quantum confinement.

Firstly, it is difficult to control the geometry and density of nanowires, stacking faults, and the variations in stacking that produce mixed phases such as zinc- blende (ZB) and wurtzite (WZ). These difficulties may be controlled through optimisation of growth conditions or the use of impurities/surfactants.

Due to the large surface area-to-volume ratio of nanowires, the large number of surface states significantly degrade their optical and electronic properties and hence impede their use in optoelectronic devices. This problem may be eliminated through engineering a chemically inactive surface to produce surface passivation and hence prevent the formation of surface states. It is an aim of the present invention to provide a photodetector which mitigates or resolves one or more of the above-discussed problems.

According to a first aspect of the present invention, there is provided a

photosensitive device comprising a sensor having at least one sensing element, the at least one sensing element having a core layer and a shell layer of semiconductor materials, the core and shell layers being arranged to form a heterojunction having a type-ll band alignment; a plurality of electrical contacts for applying a voltage across the core layer of the at least one sensing element; and a monitor for measuring the current through the core layer of the at least one sensing element, such that incident radiation is detected by a reduction in the measured current.

The photosensitive device according to the first aspect of the invention is advantageous principally because the use of a heterojunction having a type-ll band alignment to provide a negative photo-response in a core layer of the sensing element provides an alternative photo-sensing configuration that may provide benefits over prior art photosensitive devices, including enabling use at room temperature. The heterojunction may be formed at a boundary of the core layer that is transversely separated from a central longitudinal axis of the core layer. The central axis of the core layer may extend in the direction of current flow through the core layer, and may be centrally positioned with respect to the transverse boundaries of the core layer. In particular, the central axis may correspond to an axis that extends through the cross-sectional centres of the core layer. The heterojunction may be formed by the core and shell layers being joined, at a transverse boundary of the core layer.

The shell layer may extend at least partially around the core layer in at least one transverse plane of the core layer, where the transverse plane may be

perpendicular to the central axis of the core layer. The shell layer may extend around the full circumference of the core layer in at least one transverse plane of the core layer, such that the shell layer surrounds the core layer in that transverse plane. The shell layer may also extend longitudinally along the core layer. The shell layer may extend a greater distance longitudinally along the core layer than its dimension in the transverse plane. The shell layer may surround the core layer in transverse planes along the entire longitudinal extent of the core layer.

The core layer may have the form of a wire, for example with a length that is significantly greater than its diameter, eg at least 10 times greater, at least 20 times greater, at least 30 times greater or at least 40 times greater. The core layer may have a substantially uniform cross-sectional diameter along its central longitudinal axis. The cross-sectional shape of the core layer may be circular or elliptical, or may be polygonal in shape. The shell layer may have a substantially uniform thickness, extending from the outer boundary of the core layer, and hence may have a similar exterior shape to the core layer. However, where the sensing element is formed with at least its diameter less than 1 μιη, ie in the nanometre range, and hence is a so-called nanowire, the core layer may have a polygonal shape, eg a hexagonal prism shape, and the shell layer may extend outwardly from the boundary with the core layer, with a cylindrical exterior shape. The core layer and the shell layer may be approximately concentric. The length of the core layer and/or the shell layer may be in the range of 0.1 -10 μιη, or 0.5 - 5 μιη, or 1 - 3 μιη, eg approximately 2 μιη. The cross-sectional diameter of the core layer may be less than 1 μιη, ie in the nanometre range, such as in the range of 1 - 100 nm, or 5 - 75 nm, or 15 - 55 nm, eg approximately 30-40 nm. The thickness of the shell layer may also be less than 1 μιη, ie in the nanometre range, such as in the range of 0.5 - 50 nm, or 1 - 35 nm, or 2 - 20 nm, eg approximately 5-10 nm.

Where the sensing element is formed with at least its diameter less than 1 μιη, ie in the nanometre range, the sensing element may be a so-called nanowire. The density of the nanowire may be of the magnitude 10 ⁸ cnr ² .

According to a second aspect of the present invention, there is provided a photosensitive device comprising a sensor having at least one nanowire, the at least one nanowire having a core layer and a shell layer of semiconductor materials; a plurality of electrical contacts for applying a voltage across the core layer of the at least one sensing element; and a monitor for measuring the current through the core layer of the at least one nanowire, such that incident radiation is detected by a reduction in the measured current.

According to a third aspect of the present invention, there is provided a nanowire for use in a photosensitive device, the nanowire comprising a core layer and a shell layer of semiconductor materials, the core and shell layers being arranged to form a heterojunction having a type-ll band alignment.

The core and shell layers may have a negligible lattice mismatch, eg through appropriate selection of the semiconductor materials. The core and shell layers may have a lattice mismatch of less than 5%, or of less than 2%.

This may enable there to be no dislocations generated between the layers, thus improving the performance of the device. The core and shell layers may have different conduction and valence band energies, and may have different band gaps, in order to form a heterojunction having a type-ll band alignment. Where the current in the core layer is carried by electrons, the heterojunction having a type-ll band alignment may be configured to cause the electrons generated by the incident radiation to be confined in the core layer, in the region of the boundary between the core layer and the shell layer. In addition, the holes generated by the incident radiation may be confined in the shell layer, in the region of the boundary between the core layer and the shell layer.

The valence band of the core layer may have a lower energy level than the valence band of the shell layer. The conduction band of the core layer may have a lower energy level than the conduction band of the shell layer. The valence band of the shell layer may have a lower energy level than the conduction band of the core layer.

This band alignment may also comprise band bending, including the conduction band of the core layer reducing in energy as distance from the central longitudinal axis of the core layer increases. The conduction band of the core layer may have an energy greater than the fermi level in the region of the central longitudinal axis of the core layer, and may have an energy substantially equal to or less than the fermi level in the region of the boundary between the core layer and the shell layer. This configuration facilitates confinement of electrons in the core layer, in the region of the boundary between the core layer and the shell layer.

The band bending may include the valence band of the shell layer reducing in energy as distance from the central longitudinal axis of the core layer increases, with all energies being less than the fermi level. This configuration facilitates confinement of positive holes in the shell layer, in the region of the boundary between the core layer and the shell layer. Where the current in the core layer is carried by positive holes, the heterojunction having a type-ll band alignment may be configured to cause the holes generated by the incident radiation to be confined in the core layer, in the region of the boundary between the core layer and the shell layer. In addition, the electrons generated by the incident radiation may be confined in the shell layer, in the region of the boundary between the core layer and the shell layer.

The valence band of the core layer may have a higher energy level than the valence band of the shell layer. The conduction band of the core layer may have a higher energy level than the conduction band of the shell layer. The valence band of the shell layer may have a higher energy level than the conduction band of the core layer.

This band alignment may also comprise band bending, including the valence band of the core layer increasing in energy as distance from the central longitudinal axis of the core layer increases, with all energies being less than the fermi level. This configuration facilitates confinement of holes in the core layer, in the region of the boundary between the core layer and the shell layer. The band bending may also include the conduction band of the shell layer reducing in energy as distance from the central longitudinal axis of the core layer decreases. The conduction band of the core layer may have an energy greater than the fermi level in the region of the exterior surface of the shell layer, and may have an energy substantially equal to or less than the fermi level in the region of the boundary between the core layer and the shell layer. This configuration facilitates confinement of electrons in the shell layer, in the region of the boundary between the core layer and the shell layer.

These arrangements of band energy levels provide the function that upon receiving incident radiation of a wavelength that generates electron-hole pairs in the sensing element, the respective electron and hole of the pair are separated either side of the heterojunction. The separation of electron-hole pairs at the heterojunction may cause charge carriers, eg electrons, in the core layer of the sensing element to be electrically attracted away from the central axis of the core layer and towards the

heterojunction. This has been found to reduce the mobility of the charge carriers, eg electrons. Hence, on receiving incident radiation, the conductivity of the core layer may be reduced, and hence the resistance of the core layer may be increased. A negative photocurrent is therefore generated.

It has been found that the magnitude of the photocurrent is larger than in the prior art, which provides an increased Signal-to-Noise Ratio (SNR) of the photosensitive device.

The core and shell layers may comprise different semiconductors. The core layer and/or the shell layer may comprise a lll-V semiconductor. The core layer may comprise indium arsenide. The core layer may comprise InAs, or InAsSb or

InGaAs. The shell layer may comprise a semiconductor having a high affinity for oxygen. The shell layer may comprise aluminium antimonide. The shell layer may comprise AlSb, or AIGaSb, or AllnSb. As a result of the shell layer comprising a semiconductor having a high affinity for oxygen, the shell layer may comprise a combination of the semiconductor and the oxidised semiconductor, where the oxidised semiconductor may form an outermost layer of the sensing element. The presence of the oxidised

semiconductor may provide a natural surface passivation layer.

This may enable the presence of surface states to be prevented, or at least dramatically reduced, which reduces the magnitude of the dark current flowing through the at least one sensing element, ie the current that is measured when the sensing element is illuminated by background radiation, and not by the radiation being detected.

The at least one sensing element may additionally comprise a surface passivation layer. The surface passivation layer may comprise a thin layer of GaSb, or AIGaSb, or AllnSb. This may enable the surface states to be even further reduced, thus further reducing the magnitude of the dark current flowing through the at least one sensing element. The dark current flowing through the at least one sensing element may be reduced by up to two orders of magnitude relative to the prior art. This provides the advantage that the Signal-to-Noise Ratio (SNR) of the sensor is increased.

The core and shell layers may comprise materials that allow tuning of the band gap and/or conduction and valence band energies. This provides the advantage that the electrical behaviour of the sensing element may be specifically

engineered, eg to provide detection of a particular wavelength or range of wavelengths. The at least one sensing element may be configured to have an internal quantum efficiency (IQE) of more than 0.5% at room temperature. The photosensitive device may therefore be capable of operating at room temperature without the requirement of device cooling. This may enable the photosensitive device to be able to detect incident radiation at higher temperatures. Standard photosensitive devices have a much lower IQE at room temperature, therefore requiring the sensing element to be cooled in order to be able to detect incident radiation. The invention herein may enable the

requirement for such cooling to be negated.

The sensor may include a substrate on which the sensing element is formed. The substrate may be of the shape and dimensions of readily available commercial substrates. The at least one sensing element may be grown on the substrate. The substrate may be conductive so that the electrical contacts can apply a voltage across the core layer of the at least one sensing element via the substrate. Where the at least one sensing element is a nanowire, the nanowire may be grown substantially perpendicularly on a surface of the substrate. Where the at least one sensing element is a nanowire, this may enable the at least one sensing element to overcome the restraints of lattice-match requirements, allowing the at least one sensing element to be grown onto a readily available, cheap, commercial substrate. The substrate may therefore be silicon, eg a silicon wafer.

The sensor may comprise a single sensing element or a plurality of sensing elements. Where the sensor comprises a plurality of sensing elements, the sensing elements may be arranged in an array. The array may comprise 2-1000 nanowires, or 20-500 nanowires, or 50-300 nanowires. The array may have a density of 1 -50 nanowires/μιτι ² , or 3-30 nanowires/μιη ² , or 10-20 nanowires/μιη ² . The core layers of the sensing elements may be electrically connected to the plurality of electrical contacts, and may be connected in parallel. The sensing elements may be electrically connected to the plurality of electrical contacts, such that a voltage may be applied across each of the core layers.

The electrical contacts may be in direct physical contact with the core layer of the sensing element(s), or may be electrically connected to the core layer of the sensing element(s) via an intermediate material. Where the sensor comprises a substrate, and the substrate is electrically conductive, one of the electrical contacts may be disposed on the substrate. In this arrangement, the other electrical contact may be disposed at the end of the sensing element(s) remote from the substrate, eg with direct physical and electrical contact with the core layer of the sensing element(s). The electrical contacts may be gold and/or aluminium electrical contacts, for example. The electrical contacts may be transparent to incident radiation.

The photosensitive device may comprise a controller. The controller may control the application of a voltage across the core layer of the sensing element(s). The controller may receive an output from the sensing element(s). The controller may include the monitor for measuring the current through the core layer of the at least one sensing element, such that incident radiation is detected by a reduction in the measured current. The controller may comprise a microprocessor. The photosensitive device may be a photodetector. In this embodiment, the controller may provide an output indicating detection of incident radiation to a user. The output may comprise a count or measure of magnitude of the incident radiation, and/or may comprise positional information, eg in respect of an array of sensors. This output may be provided via an electrical output, or via an integral display.

The detector may comprise a plurality of sensors. The plurality of sensors may be arranged in an array, eg a rectangular or hexagonal array. Each of the sensors may configured to provide to a pixel of the positional data, eg such that the array of sensors correspond to the pixels of a display on which the output is provided. The detector may be used in a photographic and/or videographic camera. The camera may be for real-time viewing, ie night vision goggles. The camera may be for capturing or recording images. The detector may be a visible light photon counter. The detector may be an infrared photodetector.

The photosensitive device may comprise a switch. The switch may be triggered to switch between an OFF state and an ON state in response to incident radiation. The switch may be a component of an apparatus. The apparatus may therefore effect a function in response to incident radiation. The switch may comprise a transistor or a phototransistor. The switch may comprise a diode or a photodiode.

According to a fourth aspect of the present invention, there is a method of manufacturing the photosensitive device defined above, the method comprising the steps of:

(a) forming a sensor having at least one sensing element, wherein the at least one sensing element comprises a core layer and a shell layer of semiconductor materials arranged to form a heterojunction having a type-ll band alignment;

(b) forming a plurality of electrical contacts for applying a voltage across the core layer of the at least one sensing element; and

(c) providing a monitor for measuring the current through the core layer of the at least one sensing element, such that incident radiation is detectable by a reduction in the measured current.

The core layer may be formed by depositing a first material onto a substrate, the first material forming favourable nucleation sites for growth of a core layer of semiconductor material; opening molecular beam epitaxy shutters comprising the semiconductor materials of the core layer; and maintaining the shutters open for a time period sufficient for a core layer of semiconductor material to grow onto the substrate. The shell layer may be formed by depositing a shell layer directly around the core layer by molecular beam epitaxy.

According to a fifth aspect of the present invention, there is provided a method of detecting incident radiation on the photosensitive device of claim 1 , the method comprising determining a reference current, wherein the reference current is the current present in the core layer of the at least one sensing element when there is no incident radiation; monitoring the current through the core layer of the at least one sensing element; comparing the measured current to the reference current; and detecting incident radiation when the measured current is less than the reference current by at least a predetermined threshold.

Practicable embodiments of the invention will now be described in further detail, with reference to the accompanying drawings, of which: Figure 1 is a perspective view of a substrate and nanowire sensing elements that form part of a photosensitive device according to the present invention;

Figure 2a is a fragmentary view of the configuration of a nanowire of the photosensitive device;

Figure 2b is an end view of the nanowire of Figure 2a; Figure 3 is an energy band diagram of the nanowire of Figures 2a and 2b, through the diametric cross-section C-D of Figure 2a, prior to illumination;

Figure 4 is an energy band diagram of the nanowire of Figures 2a and 2b, through the diametric cross-section C-D of Figure 2a, after illumination;

Figure 5 is a graph showing the /ds - ds characteristics of a field effect transistor comprising a core-shell nanowire; Figure 6 is a schematic cross-sectional view of a method of growing core-shell nanowires onto a silicon substrate;

Figures 7(a)-(b) are temperature dependent photoluminescence spectra of prior art InAs nanowires and InAs/AISb nanowires according to the invention; and

Figures 8(a)-(e) are photoluminescence spectra of prior art InAs nanowires and InAs/AISb nanowires according to the invention at selected temperatures.

Figure 1 illustrates a silicon substrate and nanowire sensing elements of a photosensitive device according to the invention. The silicon substrate may be a conventional commercial substrate. The substrate may include a silicon dioxide top surface. The silicon substrate has an array of nanowires grown thereon. Each of the nanowires grown thereon comprises a core layer and a shell layer.

Alternatively, the silicon substrate may only have a single nanowire grown thereon.

In this example, the core layer of the at least one nanowire comprises indium arsenide (InAs) and the shell layer of the at least one nanowire comprises aluminium antimonide (AlSb). Indium arsenide is a semiconductor having a narrow band gap and a high electron mobility. Aluminium antimonide is a lll-V

semiconductor having a much lower electron mobility and a larger band gap. The difference in band gaps between the InAs core layer and the AlSb shell layer therefore produce a weak type II (staggered gap) band alignment at the

heterojunction. It will be appreciated that other semiconductors could be used for either the core layer or the shell layer, provided that the core-shell layer

heterojunction has a type II (staggered gap) band alignment.

Figures 2a and 2b illustrate the configuration of the at least one InAs-AISb core- shell nanowire described in relation to the photosensitive of Figure 1 . Figure 2a illustrates the core-shell configuration from a perspective view of the at least one nanowire. Figure 2b shows the core-shell configuration from the view of the end of the at least one nanowire. The core layer may be approximately cylindrically shaped, having an approximately circular cross-section. The core layer may have an approximately hexagonal cross section. The shell layer has a cylindrically shaped outer surface, and a hollow interior in which the core layer is housed, so that the shell layer completely encapsulates the core layer. The core layer and the shell layer are approximately concentric. Optionally, the at least one core-shell nanowire may also comprise an additional passivation layer deposited onto the surface of the core-shell nanowire. The additional passivation layer may comprise a thin layer of GaSb.

The at least one core-shell nanowire has a uniform diameter along its entire length. The diameter of the at least one core-shell nanowire is of the magnitude of 40-50nm, where the diameter of the InAs core layer is approximately 30-40nm and the shell layer thickness is approximately 5-10 nm. The density of the at least one nanowire is of the magnitude 10 ⁸ cnr ² . The length of the at least one core-shell nanowire is significantly larger than the diameter of the at least one core-shell nanowire. The length of the at least one core-shell nanowire is approximately 30-60 times larger than the diameter of the at least one core-shell nanowire. The length of the at least one core-shell nanowire is of the magnitude of 2 μιη.

The InAs core layer and the AlSb shell layer have a 1 .18% lattice mismatch.

Experimental imaging suggests that this lattice mismatch is overcome completely, and that no dislocations are seen in the AlSb shell layer, indicating that it coherently strains to the InAs core layer. Additionally, the cylindrical shape of the at least one nanowire provides the at least one nanowire with a large area-to- volume ratio. This provides more flexible strain relaxation mechanisms, which prevents the likelihood of interfacial dislocations between the InAs core layer and the AlSb shell layer, and thus reduces any negative effects of the lattice mismatch between the core layer and the shell layer.

Figure 3 shows a band diagram of the core-shell nanowire of Figure 2a through the cross-section C-D in Figures 2a and 2b prior to illumination, where the x-axis represents the physical position across the nanowire, and the y-axis represents the energy of the electronic band structure. No photo-generated electron-hole pairs are present at this stage because no photons of a wavelength corresponding to the energy of the band gap of the InAs core have been absorbed, and hence no electrons within the InAs core have been excited to transition from the valence band to the conduction band.

InAs has a band gap of approximately 0.145-0.360eV and AlSb has a band gap of approximately 1 .35 eV. This difference in band gaps creates a weak type II (staggered gap) band alignment at the heterojunction occurring at the cylindrical interface between the AlSb shell layer and the InAs core layer. The band alignment has a valence band offset of approximately 0.18eV.

The expected band bending occurring at the heterojunction is also illustrated. At the outer edges of the InAs core layer, the conduction band bends towards a lower energy, whilst at the inner edges of the AlSb shell layer, the valence band bends towards a higher energy. This band bending brings the energy level of the conduction band at the outer edge of the InAs core layer and the energy level of the valence band at the inner edge of the AlSb shell layer closer together. The band bending occurs naturally as a result of the nature of the InAs-AISb heterojunction. However, in alternative foreseeable core-shell nanowire material combinations, such as InAsSb-AIGaSb nanowires, it may be possible to tune the band bending according to the composition of the core and shell layers. Figure 4 shows a band diagram of the same cross-section as Figure 3, after illumination by photons of a wavelength corresponding to the energy of the band gap of the InAs core layer. Here, the x-axis still represents the physical position across the at least one nanowire, and the y-axis still represents the energy of the electronic band structure at the interface. Figure 4 illustrates the location of the electrons and the holes generated as a result of the absorption of photons of a wavelength corresponding to the energy of the band gap of the InAs core, when the at least one core-shell nanowire has been illuminated. This figure also shows the emission of photons, which occur on recombination of the electron-hole pairs.

The electron-hole pairs are naturally separated at the core-shell interface due to the type II band alignment, which also ensures that the excited electrons remain in the InAs core layer, whilst the holes are trapped in the AlSb shell layer, at the interface between the core and shell layers.

The shell region comprises a mixture of AlSb and oxidised AlSb. This oxidation of some of the AlSb occurs naturally due to the high affinity of AlSb for oxygen, making it highly susceptible to oxidation. As a result of the oxidation occurring naturally, the oxidation of the shell layer is not uniform, and the shell layer therefore comprises both AlSb and oxidised AlSb. This mixture in the shell layer traps electrons energised by the illumination (commonly known as "hot electrons"). At the same time, due to the type II band alignment at the heterojunction, the remaining AlSb in the shell layer also traps the holes of the electron-hole pairs that were generated in the InAs core layer, reducing the number of photo-generated holes in the core layer.

Hence, when the at least one core-shell nanowire is illuminated and the photo- generated holes are swept into the AlSb regions in the shell layer, the electrons remaining in the core layer are also electrically attracted towards the interface between the core layer and the shell layer. As electrons move from the centre of the core layer to the interface between the core layer and the shell layer, the electrons' mobility reduces significantly. Due to this reduction in electron mobility, the conductivity of the core layer also reduces significantly, and the current measured as a result of the illumination is reduced.

In use, a voltage bias is applied to the at least one nanowire via electrical contacts. Where the at least one nanowire comprises a single nanowire, the voltage bias is applied via the two contacts deposited at opposite ends of the nanowire. Where the at least one nanowire comprises an array of nanowires, the voltage bias is applied via the contact layer deposited on top of the array of nanowires so as to contact the exposed ends of the nanowires.

Dark current represents noise for a photosensitive device, as it is a current that is present as a result of generation by means other than photons intended to be detected by the device. When illuminated, a different current will be apparent, the different current having been induced by the presence of the radiation to be detected, and therefore known as the light current. The difference between the light current and the dark current is known as the photocurrent /PC = /Light- /Dark. In use, the photocurrent is measured.

Figure 5 illustrates the /ds - ¼s characteristics of a field effect transistor (FET) comprising a single InAs core-AISb shell nanowire, wherein Ids represents the drain-source current of the FET, ds represents the drain-source voltage of the FET, and V _g represents the gate voltage provided for effective tuning of the drain current. The results shown in Figure 5 were measured in atmospheric conditions, ie at room temperature and not in a vacuum. Measurements were first taken for the core-shell nanowire FET in an unilluminated state, to obtain dark current measurements. Measurements were then taken when the core-shell nanowire FET is illuminated by a green laser of 532 nm at power 8 mW/mm ² .

Figure 5 shows that the core-shell nanowire FET demonstrates a dark current of 2.8x10 ^"8 A under a ¼s of 0.1 V. This value is significantly reduced compared with prior art nanowire devices. The dark current demonstrated by the core-shell nanowire FET may be as much as two orders of magnitude lower than in prior art nanowires.

This reduced dark current is thought to be due to the surface passivation induced by the presence of the AlSb shell layer, because AlSb has a natural affinity for oxygen. In a standard nanowire, electron surface states result in a large dark current. In the described at least one core-shell nanowire, because the shell layer produces effective surface passivation, the shell layer prevents (or at least dramatically reduces) the presence of surface states, resulting in a much smaller dark current.

It can also be seen in Figure 5 that the core-shall nanowire FET demonstrates a negative photocurrent. This differs from standard nanowire devices, which demonstrate a positive photocurrent. The photocurrent is the parameter to be determined by the core-shell nanowire FET, ie the signal. It is an aim of any photosensitive device to maximise the magnitude of the measured photocurrent, thus improving the photosensitive device's signal-to-noise ratio (SNR), which is defined as the ratio of photocurrent : dark current. The negative photocurrent arises as a result of the anomalous photocurrent behaviour exhibited by the core-shell nanowire FET, ie upon illumination, the light current (/ught) decreases to a very low level compared with the dark current, which results in a negative photocurrent. In Figure 5, for V _gs = 0 V and Vds = 0.1 V, the light current is approximately 3.4x10 ^"9 A, resulting in a negative photocurrent of -24.6 nA.

This observed anomalous photocurrent, and hence the negative photocurrent, occurs as a result of the type II band gap alignment causing a reduction in the electron mobility of the electrons in the core layer as they are attracted from the centre of the core layer to the interface between the core layer and the shell layer.

In use, the photosensitive device monitors the photocurrent of the at least one nanowire. The photosensitive device measures the current through the at least one nanowire at any moment in time and compares the measured current to a dark current baseline, thus determining a photocurrent. When the monitored photocurrent is measured to be below a predetermined threshold, it is determined that illumination has been detected. The predetermined threshold may be OA, ie the photocurrent has become negative because a light current is present that is of a lower magnitude than the dark current baseline. Alternatively, the predetermined threshold may be a negative value such as -5nA, -10nA, -15nA or -20nA, so as to account for fluctuations in the dark current and/or error in measurement accuracy. Where the at least one nanowire comprises an array of nanowires, each of the nanowires generates a photocurrent under illumination, contributing to a total light current of the array. This total current is compared to a baseline dark current that is taken for the whole of the array, and a total photocurrent is calculated. The predetermined threshold for a photosensitive device having an array of nanowires will therefore be dependent on the number of nanowires in the array.

By creating a significantly reduced light current, the magnitude of the photocurrent is significantly increased. This strongly negative photocurrent (ie the signal), along with the significant reduction in dark current (ie the noise), produces a much- improved SNR. The InAs-AISb core-shell nanowire FET exhibits a SNR of 88% for Vg _S = 0 V and Vds = 0.1 V. This is significantly larger than the SNR of a standard InAs-only nanowire device, which is approximately 13%.

Figure 6 illustrates a method of growing the at least one core-shell nanowire. The at least one core-shell nanowire is grown on a bare Si substrate using molecular beam epitaxy (MBE). Prior to growth of the at least one nanowire, the Si substrate is chemically cleaned. This may be done, for example, by placing the substrate in 12% aqueous hydrofluoric acid solution for 30 seconds. The substrate is then immediately loaded into a MBE apparatus to prevent re-oxidation and is annealed at high temperature for a specific period of time. The high temperature may be, for example, between 500°C and 600°C. The high temperaitire may preferably be 580 °C. The specific period of time may be 4 hours. After the silicon substrate has been annealed, at step A indium (In) droplets are deposited on the substrate. These indium droplets act as favourable nucleation sites to trigger growth of the at least one InAs core layer. Optimal growth conditions are then provided so that the at least one InAs core layer is efficient and its growth rate can be determined and/or controlled. For example, the substrate temperature is raised to a temperature in the range of 420-470 °C, the Ast beam equivalent pressure (BEP) is set to approximately 1 -10x10 ^"6 mbar, and the resultant InAs nominal growth rate is 0.1 μιτι/h. At step B, the molecular beam epitaxy indium (In) and arsenide (As) shutters are then opened simultaneously for a period of time that allows the at least one core layer to be long enough for device fabrication. Where the InAs nominal growth rate is 0.1 μιτι/h, this period of time may be two hours, so as to form at least one core layer of length 0.2 μιη. As can be seen in step C, the at least one core layer grows substantially vertically from the silicon substrate, so that the core layer is upstanding from the substrate.

At step D, an AlSb shell layer is deposited directly around the at least one InAs core layer. The shell layer is deposited at a reduced growth temperature and a lower growth rate. The reduced growth temperature may be approximately 250- 400 °C. The lower growth rate may be approximately Q 1 -0.3 monolayer/second. This may be deposited for a period of approximately one hour. Optionally, a thin layer of GaSb may also be deposited onto the surface of the core-shell nanowire as an additional passivation layer.

Finally, contacts are formed on the substrate. For single nanowire photosensitive devices, two contacts are deposited on either side of the single core-shell nanowire, so that a voltage bias may be applied across the nanowire. For photosensitive devices having an array of core-shell nanowires, poly(methyl methacrylate) (PMMA) is used to coat the array of nanowires. The PMMA is then etched back to expose the ends of the top of each nanowire. Next, a contact layer is deposited to contact the exposed ends of all of the nanowires. The contact layer may comprise a gold (Au) contact layer or an indium tin oxide (ITO) contact layer. The contact layer is a thin layer. Lastly, a bonding layer is formed. The bonding layer is formed on top of the contact layer. The bonding layer has a greater thickness than the contact layer. The bonding layer may comprise a gold bonding layer.

In order to elucidate the optical properties of the core-shell nanowire ensembles, photoluminescence (PL) measurements were performed on samples of standard InAs nanowires and InAs/AISb core-shell nanowires at various temperatures and excitation powers, and on different positions of the samples. Temperature dependent PL spectra of standard InAs nanowires and InAs/AISb core-shell nanowires are shown in Figures 7(a) and 7(b), respectively. At 15 K both samples demonstrate a very strong PL signal, however the peak energy is below the free exciton transition energy in bulk InAs at low temperatures (~ 0.415 eV). The standard InAs nanowires show an asymmetric PL band with a dominant peak centred at 0.410 eV and an extra peak at the low energy side (-0.380 eV), while the InAs/AISb nanowires exhibit a quite symmetric PL band with a peak position at 0.391 eV. It is believed that the dominant emission originates from band-to-band transition while the redshift is related to the band bending in the core InAs. With increase of temperature, the emission peak is redshifted due to band gap shrinkage. More importantly, the PL emission persists up to room temperature.

In order to evaluate the emission efficiency, the PL spectra at four selected temperatures for the two samples are directly compared in Figures 8(a)-(d). It shows that the PL intensity is slightly stronger for InAs nanowires at 15 K, while the emission of InAs/AISb core-shell nanowires is getting stronger with further increase in temperature. This behaviour is further illustrated in Figure 8(e) which directly compares the integrated intensity of PL signal for standard InAs nanowires and the core-shell nanowires described herein. It should be noted that assessing optical quality through this direct comparison of PL intensity for nanowires could be controversial due to several factors such as different density and/or different absorption coefficients of different nanowires. Consequently, internal quantum efficiency (IQE) is introduced to evaluate the emission efficiency of the nanowires. IQE is defined as the ratio of PL intensity obtained at a given temperature to the integrated PL intensity, at 15K which the IQE is assumed to be 100%. As depicted in the inset in Figure 8(e), the IQE of the InAs/AISb core-shell nanowires is approximately two times stronger than that of prior art InAs nanowires at room temperature. This implies that the AlSb shell layer enhances the PL emission, which is attributed to surface passivation and quantum confinement induced by the AlSb shell layer. A similar behaviour was observed for the measurements performed at other different positions on each sample. The observation of room temperature PL from InAs and InAs/AISb core-shell nanowires indicates that the mechanism of thermal quenching of PL in the core- shell nanowires has been significantly suppressed. Furthermore, the use of an AlSb shell layer provides efficient surface passivation and quantum confinement which both improve the emission efficiency. It should be noted that blue-shift of peak energy induced by size-related quantum confinement is not observed in the core-shell nanowires. This is due to the larger diameter of the core-shell nanowires compared with the diameter of standard nanowires. On the contrary, the core-shell nanowires demonstrate PL emission with a lower energy than that of standard InAs nanowires. The core-shell nanowires obtained at optimal growth conditions also demonstrate a high quality with small phase segments, hence the type II alignment effect is negligible.