Patent Description:
The reversible optical phase-change materials (ROPCM) present a major variation in the optical and electrical properties (like complex refractive index, n'=n-j * k, where j=(-<NUM>)<NUM>/<NUM> and energy bandgap, Eg) when they are switched from the amorphous phase to the (poly)crystalline phase. The variation of the refractive index (Δn) and the extinction coefficient (Δk), (identified as the real and imaginary part of the complex refractive index) related to structural state (amorphous or polycrystalline) is called optical contrast. The variation of the electrical resistivity as a function of structural state is called electrical contrast. The optical and electrical contrasts in ROPCM materials are basically explained by a change in the nature of atomic bonding configuration as a result of the phase transition. This structural transition of the ROPCM materials from amorphous to crystalline state can be performed either by Joule effect via resistive heating or optically by laser irradiation. Synthetically, the optical contrast due to phase transition is represented by a figure of merit (FOM) which has the expression FOM=Δn/Δk, where n is the refractive index (real part of complex refractive index) and k is the extinction coefficient (imaginary part of complex refractive index). From physics point of view, FOM is related to both variation in the dielectric polarizability of the material (Δn) and the radiation absorption properties (Δk) of the material as a function of structural state. It is well known that both refractive index, n, and extinction coefficient, k, vary with the wavelength of the incident radiation.

The representative type of materials with optical and electrical contrast due to structural phase transition is the chalcogenide family (compounds containing sulfur, selenium or tellurium, but excluding oxygen). At this stage, the best understood and already industrially-exploited chalcogenide material is Ge<NUM>Sb<NUM>Te<NUM> (GST), which can be switched millions of times from amorphous to (poly)crystalline and back to amorphous state, with low energy consumption for performing these fast structural phase transitions, while it does not need any energy for preserving either amorphous or crystalline state. Such structural behavior of GST where the energy is necessary only for doing a transition, but not for maintaining the polycrystalline or the amorphous phase determines the non-volatility of the GST. In the same time, the fact that the constant optical contrast between amorphous and polycrystalline state is preserved after such a great number of repeated transitions demonstrates GST's reversibility. The phase transition proceeds as follows. If, for example, the initial state of the GST alloy after its preparation is amorphous, then by heating it at about <NUM>-<NUM>, it will become polycrystalline. For performing its reversible transition from polycrystalline to amorphous transitions, GST should be further heated to the melting point of about <NUM> followed by a rapid cooling (quenching), so that the atoms have no time to re-arrange in the crystalline state, and thus the amorphous state is obtained, due to this quenching.

These ROPCM materials have an increasing number of photonic applications, in different stages of development. An example of industrial application of GST is its use as optical recording media, which is already a mature commercial technology in the field of high-density optical memories. The working principle is obtained from the major difference in the reflectivity of GST material as a function of amorphous or polycrystalline state, which will thus define the "<NUM>" or "<NUM>" digital states for these optical memories. On the other hand, the electrical contrast of GST, demonstrated by a major decrease in the electrical resistivity of GST, of about five orders of magnitude (from about <NUM><NUM> Q*m to about <NUM>-<NUM> Q*m) when switching from (poly)crystalline to amorphous state, has generated an emerging technology for next generation of electronic non-volatile memories, where the high resistivity versus low resistivity will define the "<NUM>" state versus "<NUM>" states for digital memories (<NPL>).

The controlled reversibility of the optical properties of GST as a result of reversible structural phase transitions and its associated optical contrast in refractive index and extinction coefficient have opened the avenue towards a new field of applications called reconfigurable photonics, where the optical contrast allows reconfiguring of multiple functionalities. For example, considering the optical contrast of GST in the mid-infrared one can obtain a non-volatile reconfigurable mid-infrared band pass filter with a peak of <NUM>% for the narrow band reflection spectrum centered on <NUM> for the amorphous GST and a peak of <NUM> % centered on <NUM> when GST is in the polycrystalline state. In the same time, the reflection spectrum could be tuned in between those values when the crystallization fraction, m, increased from m=<NUM> to m=<NUM>.

Other examples of reconfigurable photonic devices have been described in the literature, e.g. reconfigurable light modulators (Koba, I. High-resolution Spatial Light Modulator for <NUM>-dimensional Holographic Display. <CIT>), optical switches (<NPL>), optical limiters (<NPL>), or polarizing reflectors (<NPL>).

However, despite the successful use of GST in optical memory applications, there are issues of Ge<NUM>Sb<NUM>Te<NUM> related to the high value of the absorption in the telecom wavelength band (<NUM>-<NUM>) µm (k=<NUM> for polycrystalline state and k=<NUM> for amorphous phase) (<NPL>) and also related to the coupling of refractive index contrast (Δn) to the extinction coefficient contrast (Δk), which limits the performances of some applications, as in the case of light modulators (<NPL>. Another major limitation of the GST is its rather low band gap (<NUM> eV in the (poly)crystalline state and <NUM> eV in the amorphous state), <NPL>), which implies an absorption edge of <NUM>, with higher absorption in the visible range, which will limit again many reconfigurable applications in the visible range.

Under these circumstances, there has been an increasing interest in exploring the optical and electronic properties of other ROPCM chalcogenide semiconducting materials focusing in the same time on identifying those with increasing bandgap above <NUM> eV and with high optical contrast, which may also allow optical amplitude and phase modulation in the visible range. Thus, it was shown that by replacing the tellurium with sulfur the bandgap of the chalcogenide obtained increased and reached a value of <NUM> eV for Sb<NUM>S<NUM> in the amorphous state and about <NUM> eV in the (poly)crystalline state.

The interest in the study of wider bandgap semiconducting chalcogenides as OPCM materials was much increased after the recent excellent demonstration of Dong et al. ), who proved that the Sb<NUM>S<NUM> thin films presented reversible, non-volatile phase transitions from amorphous to crystalline state (and back to amorphous) by a specific thermal annealing sequence. Thus, contrary to previous belief from the end of the years <NUM>'s (<NPL>) , where it was said that Sb<NUM>S<NUM> material can be used only for "writing once and reading many times (WORM)", (i.e., in other words, one cannot have reversible structural phase transitions in Sb<NUM>S<NUM> from crystalline state to amorphous state), the recent work of Dong et al. has shown that an amorphous Sb<NUM>S<NUM> can be switched to a (poly)crystalline state if heated at a temperature higher than <NUM>, and then for a back transition to the amorphous state the film has to be heated above its meting temperature (<NUM>) followed by a rapid cooling (quenching), so as the crystallization to be avoided. These results were later confirmed by more systematic results on cyclability of the Sb<NUM>S<NUM> phase transition, where only a limited decay between <NUM> and <NUM>% in reflectivity was found after <NUM> cycles of phase change. However, Delaney et al. show that these results of good optical contrast stability between amorphous and crystalline state, in Sb<NUM>S<NUM> and Sb<NUM>Se<NUM> were obtained only for the case of using a capping layer of the binary composite of ZnS:SiO<NUM>=<NUM>:<NUM> above the chalcogenide semiconductor layer, so that to avoid sulfur or selenium atom loss, and thus to preserve the Sb<NUM>S<NUM> and Sb<NUM>Se<NUM> stoichiometry during thermal/laser annealing. In the absence of the capping layer, the laser annealings for the crystallization process and the amorphization are accompanied by the decrease of the sulphur or selenium concentration, due to loss of these atoms from the film, by the formation of their oxides which evaporate in the ambient. Therefore, the reconfigurable devices disclosed in the present invention will contain this capping layer, which is transparent to the radiation in visible and infrared spectrum, acting in the same time as an anti-reflection coating, due to its intermediate refractive index between the air and semiconducting chalcogenide layer (<NPL>).

The Sb<NUM>S<NUM> films have been already used in the past as active absorbing layer in the research of photosensitive devices, like solar cells (<NPL>), while the emerging field of optical phase change chalcogenide materials has further increased the interest for a systematic investigation of the optical and electronic film properties as a function of preparation route and thermal annealing. Thus, recently, the authors of this invention have used the chemical bath deposition method (CBD) for preparation of the Sb<NUM>S<NUM> films and they also experimentally studied the associated structural, compositional and optical properties were theoretically and (<NPL>, <NPL>, ) performing in the same time a detailed comparison of the optical properties (dielectric function, refractive index, extinction coefficient, optical contrast, FOM) of the Sb<NUM>S<NUM> CBD films with those obtained by other preparation methods, like sputerring (<NPL>, <NPL>, <NPL>), or electrophoretic deposition. Definitely, the optical properties are fingerprinted by the different Sb<NUM>S<NUM> film deposition methods, and this variation can be explained by the nature of each deposition process. For example, it may be intuitively explained that in the case of sputtering deposition process a higher gravimetric density of the film may be obtained in comparison with CBD method, and thus a higher refractive of the film would be expected, which is actually the case. However, the shape of overall dependence of the refractive index, n, and its contrast (Δn) as a function of incident photon energy is similar for different preparation methods. Moreover, even if there are some small differences in the value of the bandgap energy obtained for different Sb<NUM>S<NUM> deposition methods used, which will obviously affect the evolution of the extinction coefficient, k, and its contrast, Δk for incident photon energy close to bandgap, in all cases, the, the k value and its contrast is almost equal to zero in the telecom wavelength range (<NUM>-<NUM>) µm, and this will make even impossible to define a conventional FOM value, which is a major atribute of this wideband chalcogenide semiconductor in the telecom photonic applications. On the other hand, the expected high k value in the visible range (with photon energy higher than the bandgap) may be useful in the visible range for photodetectors and solar cell-based energy harvesting applications, even if the optical contrast of extinction coefficient, Δk may have a lower value (below <NUM> for photon energy equl to <NUM> eV) (<NPL>).

Recently, Delaney et al (<NPL>) have demonstrated that the antimony selenide ( Sb<NUM>Se<NUM>) is another (binary) chalcogenide which entered the family of the reversible optical phase change materials (ROPCM), showing an excellent stability of the optical contrast of the refractive index (Δn=<NUM>) in the telecom C band at <NUM> after more than <NUM> optical transitions between amorphous state and polycrystalline state. The phase transition from amorphous to (poly)crystalline state occured at about <NUM> and it was performed by laser radiation with wavelength of <NUM>, with a power of about <NUM> mW applied for <NUM>. The amorphization was made by a <NUM> laser pulse, with laser power density of <NUM> W/cm<NUM>, for about <NUM> ns. Like in the case of Sb<NUM>S<NUM>, the conventional figure of merit (FOM=Δn/Δk)) of Sb<NUM>Se<NUM> cannot even be defined, as the extinction coefficient is equal to zero in the telecom wavelength range, for both amorphous and crystalline state. These low absorption losses can be benefic for a large family of wave-guided programmable integrated photonic circuits. The bandgap energy of Sb<NUM>Se<NUM> depends on the deposition method and structural phase, and it can be in the range of (<NUM>-<NUM>) eV, which means that this material can work as an excellent visible light absorber, and this explains its use in the solar cell and visible light photodetector. Based on the latest discovery related to reversibility of the structural phase change in Sb<NUM>Se<NUM>, the present inventions demonstrates that this material can be used for the realization of the reconfigurable plasmonic photodetectos, too.

The present invention is disclosing radically-new concepts of reconfigurable photonic devices, like reconfigurable plasmonic photodetectors for telecom wavelength band (<NUM>-<NUM>) µm based on semiconducting chalcogenide like Sb<NUM>S<NUM>, Sb<NUM>Se<NUM> and their ternary composites like Sb<NUM>SxSe<NUM>-x. As a background of this invention, the fundamentals of the surface plasmon enhanced photonic devices will be briefly described, firstly, followed by the prior art in this field of plasmonic photoconductive optoelectronic devices, and intelectual property review.

In any plasmonic device, the interaction between the free-space or guided-wave light and the solid-state matter may generate surface plasmon polaritons (SPP) at the interface between a metal and a dielectric, which should be understood simultaneously as decaying light surface waves propagating along that interface, as well as collective oscillations of the surface electrons of the metal resonating with the incoming light. However, as the momentum of the surface plasmon polaritons (ℏ*kSPP where ℏ is the reduced Planck constant and kspp is the propagation wave vector of the SPP) is higher than that of the incident light (ℏ* ko; where ko=ω/c=2π/λ is the magnitude of the wave vector of incident light in free space), the SPP generation is possible only in the presence of a topological defect like a metal grating whose momentum (ℏ* kΛ) is added to the momentum of incident light for matching the SPP momentum. For our invention, a metal grating consisting of a multitude of parallel metal digits with a certain periodicity will be used for momentum conservation law. The grating momentum is defined as a vector with the direction perpendicular to the parallel-oriented metal digits and a magnitude equal to kλ=<NUM>*π/Λ. On the other hand, this momentum matching is possible only for transversal magnetic (TM) incident light, characterized by its magnetic field vector perpendicular on the propagation direction and the electric field vector (EL) located in the incidence plan (p-polarized light). Therefore, upfront, one can notice that the generation of SPP is selective to the propagation mode of the light. If the incident light is doing an angle θ with the direction normal to the grating surface, then the momentum conservation law becomes as written below: <MAT> where m is an integer number corresponding to the diffraction order of the indent light. The explicit formula of the propagation constant, kspp, of the SPP's is written below: <MAT>.

In equation [<NUM>], kSPP is a real number (the real part of the SPP propagation wavevector), εm is the real part of dielectric permittivity of the metal (which is a negative number) and εd is the (real part) dielectric permittivity of the dielectric material in contact with the metal, both parameters being wavelength dependent. From equation [<NUM>] it is very easy to note that kSPP is a real number only if the summation of εm and εd is a negative number! (<NPL>). Under these considerations, and assuming that the SPP propagation wave vector, the vector of the electric field of the light projected on the device surface and the grating vector are all colinear, then the explicit scalar formula of the momentum conservation becomes: <MAT>.

From equation [<NUM>] one can calculate the value of the grating period (Λ) which will assure the momentum conservation and the generation of the surface plasmon polaritons at the interface between a metal and a dielectric, when the electric field vector is parallel with the grating vector. In the most general case, when there is a polarization angle, δ≠<NUM>, between the electric field vector of p-polarized light (EL) and the grating vector, the equation [<NUM>] for momentum matching becomes: <MAT>.

Therefore, from equation [<NUM>] one can understand that the generation of SPP is selective to light propagation mode (TM only will produce SPP's), wavelength of incident light, polarization angle, and incidence angle, and this is one of the fingerprints of the plasmonic devices.

It is important to note that in the case of incident light perpendicular to the surface of plasmonic device one obtains sin(θ)=<NUM>, and thus only the grating momentum will be used for momentum matching and the associated generation of the surface plasmon polariton. In this later case of θ=<NUM>, according to equation [<NUM>], the polarization angle, δ, does not affect the resonance wavelength and grating period value, but the projected value of electric field along the grating vector will reduce the component of electric field on that direction by multiplying it with cos(δ). Therefore, the resonance short-circuit photocurrent (Isc) of a grating-based plasmonic photodetector, which is proportional to the square of the electric field of p-polarized light becomes (intensity of the light) becomes: (M.

From equation [<NUM>] one can notice that for δ=<NUM>°, the enhanced plasmonic component of photocurrent is cancelled, even if the p-polarized light is used, and thus the polarization angle selectivity of the plasmonic photocurrent is proven.

The above selectivity properties of the plasmonic devices can be benefic for some photonic applications where narrowband response is needed, while for the case of the wideband response applications this is may create more challenges. In this invention, the authors present innovative solutions for increasing the bandwidth of the plasmonic devices and for minimizing the limitations introduced by the polarization angle of the incident light with respect to the grating vector.

Another important basic aspect of the plasmonic devices is that depending of the period of the metal grating, the surface plasmon generation at the interface between a metal and a solid-state isolator, or metal-air, or even between a metal and semiconductor (when fulfilling the above requirement that the summation of the dielectric permittivity of the two materials is a negative number) can be performed. Therefore, SPP generation can be obtained at each of the two interfaces of a metal (either upper interface between metal and air or at the lower interface between the metal and the solid-state dielectric or semiconductor), but grating period is different for these two cases.

By doing calculation of the grating period for each type of interface (metal-air or metal-dielectric) it is easily obtained that the period of grating Λ for the metal-solid state dielectric is much smaller with respect to metal-air interface, due to differences in the dielectric permittivity, εd, of air versus that of the solid-state material. Actually, in the case that lεml>>εd, from equation [<NUM>] one can obtain that the grating period is proportional to (<NUM>/εd), which explains the much lower value of Λ (deep sub-wavelength device) for the SPP generation at the metal-solid state dielectric interface with respect to SPP generation at the metal-air interface.

The analytical modeling of the SPP generation in the plasmonic photodetectors disclosed in this invention is based on equation [<NUM>], and it is aiming at the calculation of the period of the grating as a function of the incident wavelength, while the values of the dielectric permittivity of the metal, εm and of dielectric, εd, at each incident wavelength are taken from the literature data. As an example, one can consult the site https://refractiveindex. info/?shelf=3d&book=metals&page=aluminium for an exhaustive description of the dispersion of the optical properties of different types of materials.

The interest for plasmonic devices is coming from the "extraordinary optical transmission" of the light through such sub-wavelength spacing regions of the grating (<NPL>), beyond the limit imposed by diffraction, as initially considered by Bethe theory (<NPL>). Such extraordinary transmission due to SPP generation and propagation through small slits means that for the case of this invention the flux of photons through the interdigit spacing is higher than the incident flux per (metal free) area unit, i.e., the efficiency of light transmission is higher than <NUM>. Such an increased light transmission is driving an enhanced evanescent electromagnetic field perpendicular on the interface between the metal and dielectric and which is decaying in both media, as it was experimentally measured by scanning near-field optical microscopy (SNOM). It is this evanescent field enhancement at the interface which will be exploited in the present invention for the increase of the electric field at the interface between metal and semiconductor in the chalcogenide semiconducting devices for the increase of photocurrents and spectral responsivity in the metal-semiconductor-metal (MSM) photodetectors.

As a result of this extraordinary optical transmission, the light absorption inside the photonic device is increased around the incident wavelength for which the SPP can be generated, and this result can be experimentally proven by the simultaneous narrowband decrease in the spectrum of the reflected light from the surface of the device, accompanied by an increase of the "resonant absorption" in the plasmonic device at that "resonance" wavelength, which is finally followed by the increase of the photocurrent and spectral responsivity, as it is in the case of photodetectors (<NPL>, <NPL>).

The numerical modeling and simulation of the subwavelength plasmonic devices can offer a much more complete description of the SPP generation process and photonic device operation with respect to analytical modelling described above. Thus, for a specific layout of the structure, with known optical properties of materials, geometry and thickness of the metal digits of the grating, the commercial simulation tools are providing not only the value of the grating periodicity, but also the distribution of the enhanced evanescent electric and magnetic field at the interface between upper metal-air interface and lower metal-dielectric interface, the resonant wavelength for absorption and reflection of the incident light, as well as the effect of the metal thickness on these phenomena. The absorption results described in the present invention are obtained by the Rigorous Coupled Wave Analysis (RCWA) method, which is a semi-analytical method in computational electromagnetics that is typically employed to solve diffraction of a field by a given periodic grating structure. The fields are represented by a sum of spatial harmonics, since RCWA is a Fourier-space method. In a uniform layer the spatial harmonics correspond to the well-known plane waves. The method is based on Floquet's theorem, which states that the solutions of periodic differential equations can be expanded in Floquet functions.

As the present invention is disclosing a radically new concept of reconfigurable plasmonic photodetectors, based on phase-change Sb<NUM>S<NUM> chalcogenide semiconductor and interdigitated metal-semiconductor-metal (MSM) hot electrons Schottky devices for telecom wavelengths (<NUM>-<NUM>) µm, in this paragraph, the background of the hot electrons Schottky devices is briefly described. Hot electrons plasmonic Schottky photodetectors for telecom wavelength band are well known in the art. For example, in the reference (<NPL>), a surface plasmon enhanced hot electrons photodetector based on Schottky contact between gold and p-type InP is demonstrated. The direct bandgap energy of InP is equal to Eg=<NUM> eV, which means that absorption edge (λedge=h*c/Eg=<NUM>/Eg) of this semiconductor is equal to <NUM>/<NUM> =<NUM>. Therefore, the incident IR light with wavelengths higher than <NUM> cannot be absorbed by this semiconductor, as the energy of photons is smaller than the bandgap, and this energy is not enough for the transition of electrons from the valence band to the conduction band. Under these conditions the near-infrared (NIR) radiation is absorbed only by the metal, which is transferring this energy to the electrons located at and below the Fermi level. Most of these electrons will lose their energy by scattering with metal network, including electron-electron scattering. Only those energetic ("hot") electrons located near the metal-semiconductor interface, within a distance equal or lower than the mean-free path of electrons in metal, and which have an energy higher than the barrier height between the metal and semiconductor (hot electrons) may surmount the barrier (φB) and enter the conduction band of the semiconductor. By simple calculations, one can prove that for the case of telecom wavelengths incident on a photodetector, a photocurrent will be generated by hot electrons emitted from the metal only if the barrier height is lower than <NUM> eV. Therefore, such Schottky energy barriers can have heights below <NUM> eV depending in the ideal case on the Fermi energy of the metal and semiconductor for telecom bands. The enhanced (plasmonic) electric field in the semiconductor at the interface between metal and semiconductor, in the depletion region of the Schottky contact will accelerate those hot electrons toward the opposite electrode (on the semiconductor) and generate the photocurrent. This process is called internal photoemission (IPE). From this IPE qualitative description one can understand that only a small fraction of the absorbed light in the metal can participate to the increase of the photodetector current above the value of the (dark) current of the detector. The above plasmonic Schottky (Au-InP) detector was obtained by using a periodically corrugated InP substrate, which was then covered with a thin metal gold. Thus, the surface plasmons generated at the surface of the corrugated Au metallization will create an enhanced resonant absorption at certain wavelengths, incidence and polarization angles and diffraction orders (m=<NUM>, <NUM>,<NUM>) for which momentum matching occurs. In addition, an increased evanescent electric field in the semiconductor will be present, which will further accelerate the hot electrons and reduce their transit time between the two electrodes. All these effects together will increase the responsivity of the plasmonic detector with respect to a conventional one, but, overall, this IPE-based detector has a quantum efficiency of less than <NUM>%, due to this low efficiency of hot electrons generation. For the increase of the IPE efficiency, some methods have been invented, as it will be shown in the next paragraphs. The above work is also experimentally proving the wavelength and incidence angle selectivity of the plasmonic detectors, as discussed in the previous paragraphs. The theory of MSM Schottky devices is described in detail in the work of Sze et al (<NPL>. Briefly, the device consists of two back-to-back Schottky contacts on the same surface of substrate material, while the two metal-semiconductor junctions are made using the same type of metal. The major advantage of this structure is that it is obtained with a simple planar fabrication process, as the two electrodes are deposited and patterned simultaneously. As it will be shown in the next paragraphs, the interdigitated MSM Schottky consists of two interdigitated comb electrodes, where each comb electrode has a multitude of digits, which are interpenetrated by the digits of the second comb electrode. This MSM structure has been successfully applied for detection of long wavelength in optical communication, due to its low capacitance per area unit (<NPL>). Such a structure will be also used in the present invention, as the interdigitated electrodes can play a dual role, as both Schottky contacts of the MSM structure and metal optical grating for the surface plasmon polariton generation at the interface of metal with a semiconductor or a dielectric (air included).

As the present invention is also describing a radically new concept of reconfigurable plasmonic photodetector for visible range based on an interdigitated plasmonic MSM Schottky contact on chalcogenides phase change materials like Sb<NUM>S<NUM>, Sb<NUM>Se<NUM> and its ternary nanocomposite Sb<NUM>SxSe<NUM>-x semiconductor, some basic aspects of these photonic devices are described here. The Sb<NUM>S<NUM> chalcogenide semiconductor has a bandgap energy equal to about <NUM> eV in the amorphous state and <NUM> eV in the (poly)crystalline state. This means that the absorption edge is equal to λedge-am= <NUM>/<NUM>=<NUM> for the amorphous phase and λedge-crys=<NUM>/<NUM>=<NUM>. Similarly, the Sb<NUM>Se<NUM> chalcogenide semiconductor, with the bandgap of (<NUM>-<NUM>,<NUM>) eV will have the absorption edge at about <NUM>, in the polycrystalline state and about <NUM> in the amorphous state, (<NPL>) , respectively, covering even better the light absorption in the visible range. Therefore, for photons with energy higher than the above bandgaps, the absorption of the visible light is possible from the bulk of these semiconductor, by band-to-band electron transition, i.e., from valence band to conduction band. Thus, a pair electron-hole is generated, with the hole in the valence band, as a free charge carrier and electron as a free charge in the conduction band. Once these charge carriers are generated inside the depletion region of the MSM Schottky, they are separated by the electric field existing in the depletion region of the metal-semiconductor contact and then electrons go the positive biased electrode and holes to the negative biased electrode determining thus the photocurrent. Unlike the conventional detectors for visible light based on Sb<NUM>S<NUM> (<NPL>) or Sb<NUM>Se<NUM> absorber, as described above, the plasmonic MSM interdigitated Schottky photodetector from the present invention benefits of the surface plasmon light waves and enhanced evanescent electrical fields in the depletion region, which may increase the photocurrent by about one order magnitude.

As revealed in this and the next paragraphs, considering the specific features of the surface plasmon polaritons generated at the interface between a metal and a dielectric/semiconductor or even air, with main focus on enhanced evanescent electric fields at that interface, a continuously increasing number of photoconductive plasmonic devices for different photonic applications are emerging. Thus, in the <CIT>, it is disclosed an optoelectronic device consisting of two interdigitated metal comb electrodes, which are used as both electrodes between which an electrical voltage can be applied, and as a periodic optical grating for coupling the external optical wave to a local wave on the substrate. The local wave could be either a surface plasmon evanescent wave, due to the surface plasmon generation on the periodic grating, or a total internal reflection guided wave. The author is claiming multiple applications for this device, like optical modulators or photodetectors. In the case of the modulator, when an external optical wave is incident on the biased interdigitated structure, a reflected wave is modulated by the changes of the refractive index of the substrate due to the applied voltage between the two interdigitated electrodes. In the case of the photodetector, this electrooptical device is used for the sensing of the electric (photo)current between the two biased interdigitated electrodes, in the presence of the external optical wave incident on the structure. The author mentions both types of metal-semiconductor contacts, either ohmic or Schottky, but he does not claim the hot electron Schottky detector. In addition, he does not claim the family of semiconducting chalcogenides for its use as a substrate for photodetectors, and, finally, the concept of reconfigurable plasmonic photodetector performed on phase change semiconductor material was not disclosed by anybody in the prior art, from the best of our knowledge.

The <CIT>, is disclosing a Schottky barrier photodetector with a metal strip embedded near the surface of silicon semiconductor. This strip is acting in the same time as a contact to silicon and a plasmonic wave guide at the interface between the metal and silicon. Due to a specific geometry of the metal strip with its width equal to the thickness, the authors are claiming a polarization-independent plasmonic photodetector and a possible increase of the quantum efficiency, which is an advantage considering the multiple selectivity of the plasmonic devices. This patent is also claiming an increased efficiency of the photodetector, due to an increased number of interfaces between a metal and a semiconductor, which can increase the total active area of the device. The concept is developed in the silicon bipolar technology, while the authors claim its applicability to other semiconductor materials from IV, III-V, II-VI column of the Mendeleev periodic table. A detailed evaluation of the types of the semiconductor materials claimed in the patent reveals the fact that it does not include the family of the V-VI chalcogenide semiconductors, like Sb<NUM>S<NUM> , Sb2Se3, Sb<NUM>SxSe<NUM>-x semiconductor, as a potential active material for light absorption. In addition, there is no mention about a possible plasmonic photodetector based on phase change materials (PCM).

The <CIT>, is disclosing a metal-semiconductor-metal (MSM) plasmonic photodetector aiming the increase of the quantum efficiency. Each metal electrode of the device contains a plasmonic lens performed by doing a periodic corrugation in the metal layer. Such a corrugation can be obtained by etching superficial parallel trenches with a certain periodicity in the metal film, or by superficially etching the metal so that to obtain circular trenches around a central point. The plasmonic lentils have the role to concentrate the light on the lens, increase the evanescent electric field in the semiconductor and thus to increase the number of free carriers in the semiconductor, the speed of the carriers and determine an increase of the photocurrent, without reducing the bandwidth due to length of the lens. The patented device does not contain interpenetrating metal fingers, as the two plasmonic lenses are separated one from the other, and it does not consider phase change chalcogenide materials or reconfigurability, to mention only a few differences with respect to the present invention.

The <CIT>, discloses a photoconductive plasmonic device working as either THz detector or emitter. The system consists of an active semiconductor layer which is able to absorb light at the specified wavelength of the optical pump input signal incident on the detector area, two plasmonic comb electrodes which are not interdigitated one to the other, and an antenna. Each of the plasmonic comb electrodes contains a metal grating made of parallel and periodic digits. Specific to this invention is the fact that there is a small distance between the two comb plasmonic electrodes, where the light is freely absorbed, but where the surface plasmons may still enhance the light concentrations and the evanescent electric field, which will further increase the transit time of the photogenerated carriers. Like in the previously described patents, in this patent the digits of the comb electrodes with their periodic arrangement on the surface of semiconductor will make possible the generation of surface plasmon waves and the associated advantages. This patent extends the family of the existing plasmonic photonic devices, and allow fabrication of high-power THz emitter/detectors, but does not overlap with the specific claims of our invention related to IR and visible plasmonic interdigitated photodetectors on chalcogenide semiconductor, as it will be very clear from the claims of our invention, where for the first-time reconfigurable photodetectors are disclosed.

The <CIT> patent is disclosing an IR photodetector based on the contact between a metal and a semiconductor, where the semiconductor is surrounding the metal strip on at least three sides. The purpose of the patent is to improve the efficiency of the radiation coupling between an optical fiber and photodetector, up to a level of (<NUM>-<NUM>) %. This result is targeted by means of an extended surface area of the metal-semiconductor (MS) interface, due to those multiple MS contacts. Unlike the invention disclosed in <CIT> patent, where the plasmonic metal strip was embedded in the bulk of the semiconductor, this Schottky photodetector is located at the surface of the semiconductor and describes explicitly that the principle of operation in the telecom band (<NUM>-<NUM>) is based on internal photoemission (IPE) of hot electrons from the metal, the semiconductor being transparent at this wavelength band. As can be observed from the claims and the entire content of the <CIT> patent, only well-known semiconductor material like Si is used, which is transparent to NIR telecom wavelengths. Therefore, it is easy to notice the conceptual differences at the level of design and materials used within our patent with respect to this patent, not to mention the reconfigurable feature of our IR photodetector based on phase change materials, which has never been disclosed before, after our best knowledge.

The <CIT> patent describes a dynamically reconfigurable close-loop vision system containing complex computing units for imaging data processing, as well as a reconfigurable photodetector array containing a multitude of optical devices able to detect light in a wide wavelength spectrum from ultraviolet to near infrared. The term "reconfigurable" here means the ability of the system to collect the relevant pixel data from the fix structure of the photodetector array, by varying the temporal and spatial parameters of the human or computer-controlled optical data collection system. Therefore, the innovative step would be to go from computer-controlled reconfigurable optical system to device-level reconfigurability, and this will be disclosed in the following paragraphs of our invention.

The present invention is disclosing reconfigurable photodetectors, a radically-new concept of surface plasmon enhanced metal-semiconductor-metal (MSM) interdigitated Schottky photonic photodetectors, where the semiconductor is from the family of chalcogenide phase change materials like Sb<NUM>S<NUM>, Sb<NUM>Se<NUM>, and their ternary composites Sb<NUM>SxSe<NUM>-x, which presents similar wavelength-dependent optical contrast for the complex refractive index, when they switch repeatedly from the amorphous state to (poly)crystalline state. In certain embodiments, the reconfigurable plasmonic interdigitated MSM Schottky hot electron-based photodetector is used for the detection of the IR telecom band (<NUM>-<NUM>) µm. The novelty of the invention comes from the device-level reconfigurability of the present plasmonic photodetector, which, for example, is able to detect the telecom "C band" (<NUM>-<NUM>) µm when the device has the Sb<NUM>S<NUM> (or Sb<NUM>Se<NUM>, or Sb<NUM>SxSe<NUM>-x) film in the (poly)crystalline state, and the "O band" (<NUM>-<NUM>) µm when the same device (obviously, no change of the grating period) has the Sb<NUM>S<NUM> (or Sb<NUM>Se<NUM> or Sb<NUM>SxSe<NUM>-x) switched in the amorphous state. Similarly, a reconfigurable plasmonic interdigitated MSM hot electron Schottky photodetector is able to detect the telecom "U band" (<NUM>-<NUM>) µm when the device has the Sb<NUM>S<NUM> (or Sb<NUM>Se<NUM>) film in the (poly)crystalline state and the "E band" (<NUM> - <NUM>) µm, when the same device has switched the Sb<NUM>S<NUM> (or Sb<NUM>Se<NUM> or Sb<NUM>SxSe<NUM>-x) film in the amorphous state. The repeatable (poly)crystalline-amorphous-(poly)crystalline phase transition can be optically performed as described in the background. The wavelength selectivity of the plasmonic devices is further supporting the filter-type functionality of the IR detector. Thus, it is for the first time when the same device can be optically tuned to perform hopping between two telecom wavelength bands, and this change to be operational in a nanosecond time scale. In addition, our invention will show that by an original design of the plasmonic photodetector, containing multiple photodetectors connected in parallel, a wideband, multi-polarization IR photodetector can be also obtained.

In another embodiment, a reconfigurable plasmonic interdigitated MSM Schottky photodetectors for the visible spectrum is disclosed. In this case, the visible incident light with the energy of photons higher than the bandgap energy of Sb<NUM>S<NUM> and Sb<NUM>Se<NUM> and their ternary nanocomposites Sb<NUM>SxSe<NUM>-x will be absorbed by the bulk of chalcogenide semiconductor and generate a photocurrent. The figure of merit (FOM) in the visible range is much much smaller than in the IR wavelength band, due to absorption of the photons with their energy higher than the bandgap of these chalcogenides. The optical contrast in the refractive index between the crystalline state and amorphous state of Sb<NUM>S<NUM> is enough for obtaining a reconfigurable plasmonic photodetector, which, for example will detect the orange color in the crystalline state and the green color in the amorphous state for the case of Sb<NUM>S<NUM> as an active absorber. Similarly, as an example, the reconfigurable plasmonic photodetector based on Sb<NUM>Se<NUM> as absorber is able to detect orange color (<NUM>) in the polycystalline state and blue color (<NUM>) in the amorphous state. Considering the inherent wavelength selectivity of the plasmonic photodetector, the reconfigurable plasmonic photodetectors will represent a major functionality increase of these photonic devices.

In another embodiment, a method for the fabrication of these IR reconfigurable plasmonic interdigitated MSM Schottky hot electron photodetector is presented. Thus, in the first step the chalcogenide film (Sb<NUM>S<NUM> or Sb<NUM>Se<NUM> or Sb<NUM>SxSe<NUM>-x), is deposited on a dielectric substrate and then the plasmonic metal electrodes are deposited and patterned above the chalcogenide semiconductor, where the metal deposition may be preceded by an ultrathin barrier layer, e.g., like Si<NUM>N<NUM> (<NUM>-<NUM> thick) located in-between the metal like aluminum and chalcogenides. In the case of gold metallization, a titanium barrier layer of <NUM>-<NUM> can be deposited before gold deposition and patterning. Finally, above the entire structure an anti-reflection, capping layer of ZnS (<NUM>%)-SiO<NUM>(<NUM>%) composite with a thickness of <NUM>-<NUM> is deposited over the plasmonic device for avoiding the sulfur or selenium loss during crystallization and amorphization anneals and thus preserving the stoichiometry of the chalcogenide film during optical/thermal treatments. On-chip reconfigurability is performed by thermal or laser annealing of the chalcogenide so that a phase transition from amorphous phase to polycrystalline phase takes place. For the phase transition of each of the chalcogenides from above from the polycrystalline phase to amorphous phase an ultrarapid laser annealing of the device to bring the OPCM material to a specific temperature, above melting point of that material followed by the very rapid cooling of the device is done.

In another embodiment, a method for the fabrication of IR reconfigurable plasmonic interdigitated MSM Schottky photodetector is started by depositing the metal directly on the isolating substrate (Si/SiO<NUM>, Al<NUM>O<NUM>, glass), eventually preceded by the deposition of a barrier or adherence layer, followed by metal layer patterning for creating the interdigitated metal electrodes and grating. Then, the conformal deposition of the chalcogenide semiconductor is made, and finally the protection, anti-reflection coating of ZnS(<NUM>%)-SiO<NUM>(<NUM>%) is deposited. The highest quantum efficiency of the IR reconfigurable plasmonic interdigitated MSM Schottky hot electron photodetectors can be obtained when the plasmonic metal electrodes are fully embedded in the chalcogenide Sb<NUM>S<NUM> semiconductor, due to the fact that Schottky contacts are formed at all four interfaces between the metal and semiconductor.

In another embodiment, different methods for the design and fabrication of the reconfigurable plasmonic interdigitated metal-semiconductor-metal Schottky detectors for visible light are disclosed. Due to the light absorption in the entire depletion region of the chalcogenide film, the quantum efficiency of these reconfigurable photodetectors for visible range is much higher than that of the IR reconfigurable photodetectors, which are based on internal photoelectron emission (IPE).

The embodiments of the present disclosure will be described only as an example, by reference to <FIG> of the accompanying drawings in which:.

The following description should be read and understood with reference to the above drawings. These drawings are not necessarily at scale and are not intended to limit the scope of the invention. Although different examples of the design of the novel reconfigurable plasmonic photodetectors are illustrated in those figures, the people skilled in the art will recognize that the provided examples have many suitable alternatives that may be utilized.

<FIG> shows a schematic perspective-view of an interdigitated metal-semiconductor metal (MSM) plasmonic Schottky photodetector (<NUM>), which is defined as two-electrode optoelectronic device with both electrodes placed on the same surface of semiconductor substrate (<NUM>), where the planar metallization of the device, <NUM>, in direct contact with the semiconductor <NUM> has dual role, i.e., both electric role as metal electrode for extracting the photocurrent from the device, and optic role, as grating, for the generation of the surface plasmon at the interface between metal and air or metal-substrate when the momentum matching condition described in the background of this invention. The first collecting metal electrode, <NUM>, is provided with a number of digits, <NUM> positioned perpendicularly on the first collecting electrode, <NUM>, and the second collecting electrode, <NUM>, is provided with a number of digits, <NUM>, positioned perpendicularly on the second collecting electrode. These fingers, <NUM> and <NUM>, are interpenetrated, without touching one another, as shown in <FIG>. These collecting electrodes, <NUM> and <NUM> are connected to a dc power supply, <NUM>. In the same circuit a microampere meter can be added for photocurrent measurement. Together, the first collecting electrode, <NUM>, and the second collecting electrode, <NUM>, together with their digits, <NUM>, and <NUM> respectively, define also an interdigitated grating with a certain periodicity of the digits and spacing between them. The external radiation source, <NUM>, is placed above the grating, and the incident radiation is characterized by wavelength, incidence angle with respect to the direction normal on the photodetector surface and polarization angle of the electric field EL, <NUM>, with respect to the grating vector, kΛ, <NUM>, as defined in the background, and shown in <FIG>. The substrate, <NUM>, of the photodetector, <NUM>, electrically isolated from the semiconducting layer,<NUM>, can be a silicon wafer covered with a SiO<NUM> layer of thickness in the range <NUM>-<NUM>, or sapphire, or glass wafer, but not restricted to these examples.

<FIG> illustrates in the upper section (a) a detailed top-view of the interdigitated MSM Schottky photodetector and in the lower section (b) a cross-section view of the same photodetector. Thus, in <FIG>, where the reference numbers have the same significance as in <FIG>, it is easy to notice that the period of the metal interdigitated grating is equal with the sum between the width ,W, of the fingers <NUM> or <NUM> and the space ,S, between two adjacent fingers, <NUM> and <NUM>. The grating with the period, P= Λ =W+S, is optically characterized as a vector, by the grating vector, <NUM>, denoted by kΛ, with the magnitude, kΛ =2π/Λ, and the direction perpendicular to the length of the digits, as shown in <FIG>. In <FIG> the electric field vector, EL, <NUM>, is parallel to the grating vector, kΛ, <NUM>, and perpendicular to the length of the digits, the light being p-polarized. For this vectorial configuration, the polarization angle, δ, between the incident light and the grating is equal to zero, and therefore according to the equation [<NUM>] the generated plasmonic photocurrent is maximum, only for this case of EL ∥ kΛ. In <FIG>, one can notice that the metal fingers, <NUM>, and <NUM> make a direct contact on the surface of the chalcogenide semiconductor, <NUM>, which could be Sb<NUM>S<NUM>, Sb<NUM>Se<NUM>, or their ternary composite, as a solid solution, Sb<NUM>SxSe<NUM>-x. In addition, above the entire surface of the device, the capping layer, <NUM>, of ZnS(<NUM>%)-SiO<NUM>(<NUM>%) with a thickness of <NUM>-<NUM> is deposited. This layer is preserving the stoichiometry of the chalcogenide semiconductor during optical or thermal annealing for crystallization and amorphization, and it is also an anti-reflection coating, with a refractive index higher than that of the air and smaller than that of the chalcogenide.

For the realization of a plasmonic interdigitated metal-semiconductor-metal photodetector from <FIG>, the collecting electrodes, <NUM> and <NUM> and their digits <NUM> and <NUM>, should have the real part of the dielectric constant with a negative value in the optical spectrum of interest, which should be lower than the negative value of the dielectric constant, n, of the material from the interface. Such plasmonic metals are gold, silver, aluminum, or even gallium. A key aspect of this invention is that the entire design of the photodetector is performed for surface plasmon polaritons generation at the interface between metal (<NUM>, <NUM>, <NUM>, <NUM>) and the chalcogenide semiconductor,!, so that the enhanced evanescent electric field in semiconductor due to extraordinary light transmission through the interdigit spacings, S, to be maximum in the semiconductor, <NUM>, near the interface between metal and semiconductor, in its depletion region.

The height of the Schottky barrier between metal and semiconductor depends on the work function of metal, the electron affinity in the semiconductor, the type of semiconductor (n-type or p-type), and the density of the surface states at the interface between metal and semiconductor, in the energy bandgap of the semiconductor,!. If this density of states is very high, the barrier height will be very weakly dependent or even not dependent on the metal work function.

<FIG> show the dependence of the refractive index, n, and the extinction coefficient, k, as a function of incident wavelength in the domain <NUM>-<NUM>, covering the ultraviolet, visible and near infrared (NIR) spectrum for the case of chemically bath deposited (CBD) Sb<NUM>S<NUM> films (<FIG>) and physically vapor deposited (PVD) Sb<NUM>Se<NUM> films (<FIG>). In <FIG>, the plots <NUM> and <NUM> show the variation of the refractive index, n, and extinction coefficient, k, of polycrystalline CBD Sb<NUM>S<NUM> film as a function wavelength, respectively, while the plots <NUM> and <NUM> show the refractive index and extinction coefficient for the amorphous CBD Sb<NUM>S<NUM> film, respectively. The crystallization temperature of the CBD Sb<NUM>S<NUM> was about <NUM>, the onset of the crystallization process starting at about <NUM>. The CBD deposition mechanism, as an electroless process, consists of dissolving and dissociation of the SbCl<NUM> and Na<NUM>S<NUM>O<NUM> precursors followed by the adsorption and reaction of cations (Sb+) and anions (S-) of these reagents for nucleating and growing the film of Sb<NUM>S<NUM>. Such a chemical deposition mechanism is determining a rather porous film, which may explain why the refractive indexes for CBD Sb<NUM>S<NUM> are systematically lower than the same values of the Sb<NUM>S<NUM> films obtained by (PVD) sputtering process where energetic Sb and S atoms are rather "implanted" in the film, making denser films, as described in the cited literature from background of this invention. In <FIG>, the Plots <NUM> and <NUM> of extinction coefficient, k, confirm absorption edge at about <NUM> (bandgap of about <NUM> eV) for the amorphous Sb<NUM>S<NUM> film and an absorption edge of about <NUM> (<NUM> eV) for the (poly)crystalline film, respectively. From these two plots it is obvious that both amorphous and polycrystalline CBD Sb<NUM>S<NUM> films are not absorbing radiation in the NIR spectrum, and this explains the use of such material in specific applications where transparency of the film is needed. In the same time the classical FOM is practically equal to infinity for the telecom wavelengths (<NUM>-<NUM>). In <FIG>, the Plots <NUM>-<NUM> show the variation of the refractive index, n, and extinction coefficient, k, for the sputtering-deposited Sb<NUM>Se<NUM> films. Rather similar type of dependencies of n and k with wavelength can be easily noticed for these with films with respect to Sb<NUM>S<NUM> films. The differences consist in higher values of the refractive index in Sb<NUM>Se<NUM> due to the sputtering process, as explained above, and the red shift of the absorption edges of the (poly)crystalline and amorphous Sb<NUM>Se<NUM> films, due to the lower bandgap of these films. Actually, from absorption edges of the Plots <NUM> and <NUM> of extinction coefficient, one can easily calculate the bandgap of Sb<NUM>Se<NUM> amorphous film (<NUM> eV) and (poly)crystalline film (<NUM> eV). Again, an excellent transparency of the Sb<NUM>Se<NUM> film to the telecom wavelengths is obtained. As demonstrated in the background of this invention, the above values of the refractive index are used for the calculation of the grating period which will match the momentum law for surface plasmon generation. The values of the refractive index of the plasmonic metals (which have negative sign in the optical spectrum) are taken from excellent database, as described above.

<FIG> shows the results of the numerical simulations of the absorption of the telecom wavelengths (<NUM>-<NUM>) µm in the plasmonic interdigitated metal-semiconductor-metal (MSM) Schottky hot electron photodetector, <NUM>, described in <FIG>, where the metal (<NUM>, <NUM>, <NUM>, <NUM>) is aluminum and the semiconductor (<NUM>) is amorphous Sb<NUM>S<NUM>. As described in the background and the above <FIG>, the metallization (the first collecting electrode, <NUM>, the second collecting electrode, <NUM>, as well as the digits <NUM> and <NUM>) consisting of two interpenetrated combs has a dual role, as device contacts and grating for surface plasmon polaritons (SPP) generation. As described in <FIG>, in this wavelength band, the chalcogenide semiconductor is transparent (extinction coefficient is equal to zero), and therefore the telecom wavelengths can be absorbed only in the metal (aluminum, here) by the so-called internal photoemission (IPE), as explained with more details in the background section of this invention. Briefly, for Schottky barrier lower than <NUM> eV, the telecom wavelengths absorbed in the aluminum, near interface with amorphous Sb<NUM>S<NUM> will provide enough energy to the (hot) electrons located at the Fermi level, or below, to surmount the energy barrier between Al and amorphous Sb<NUM>S<NUM> and enter the semiconductor. The enhanced evanescent electric field due to surface plasmons will accelerate the hot electrons to the opposite electrode. Thus, these hot electrons will create the photocurrent of the plasmonic detector, which is about one order of magnitude higher than in the conventional (non-plasmonic) photodetectors due to an enhanced (resonant) absorption and enhanced electric field. It is well-known by those skilled in the art that in the case of plasmonic detectors, the peak in the spectral absorption coincides with the peak in the detector photocurrent. The absorption results from <FIG>, obtained by the Rigorous Coupled Wave Analysis (RCWA) method, are evidencing a resonant absorption of the telecom wavelengths with the absorption peak shifting to red as a function of aluminum thickness increase. From Plot <NUM>, one can notice that for an aluminum thickness equal to <NUM>, the peak of the resonant absorption is located in the telecom O band (<NUM>-<NUM>) µm. The simulation results from <FIG> were obtained for normal incidence radiation, diffraction order=<NUM> and an aluminum grating with a periodicity P=W+S=<NUM>, where the width of the metal digits, <NUM> and <NUM> is equal to <NUM>, and the space between two subsequent digits, S, is equal to <NUM>. The well-known wavelength selectivity of the plasmonic devices can be proved also here by the narrow bandwidth of the spectral absorption, which will trigger a narrow band of the spectral responsivity of the photodetector, and this will be the case for the following exemplary results from below. In addition, the lower value of the grating period (<NUM>. <NUM>) with respect to incident telecom wavelength (<NUM>-<NUM>) µm proves the subwavelength character of the photonic device, which will be obvious for the rest of the results to be described, here. Finally, the capping layer, <NUM>, from <FIG> is deposited above the photodetector and it is transparent to the light from ultraviolet to mid infrared. This layer is acting as an anti-reflection coating and as a mechanical barrier against sulfur atoms loss during thermal or laser annealing.

<FIG> shows the results of the numerical simulations of the absorption of the telecom wavelengths in the plasmonic interdigitated MSM Schottky hot electron photodetector, <NUM> described in <FIG> where the metal (<NUM>, <NUM>, <NUM>, <NUM>) is aluminum and the semiconductor (<NUM>) is (poly)crystalline Sb<NUM>S<NUM> film. The plasmonic detector, <NUM>, operates as described above. The only difference between the results presented in <FIG> and <FIG> is due to the phase transition of the Sb<NUM>S<NUM> from the amorphous state to the (poly)crystalline state plasmonic photodetector, <NUM>, a process which can be done either by thermal (resistive annealing) or laser annealing, as described in the background of this invention. From <FIG>, one can notice that for an aluminum thickness equal to <NUM>, the peak absorption is located in the <NUM> telecom C band (<NUM>-<NUM>) µm. The simulation results from <FIG> were obtained for normal incidence radiation, diffraction order=<NUM> and an aluminum grating with a periodicity P=W+S=<NUM>, where the width of the metal digits, <NUM> and <NUM> is equal to <NUM>, and the space between two subsequent digits, S, is equal to <NUM>.

<FIG> shows a superposition of the resonant absorption peaks presented in <FIG> and <FIG>, which is demonstrating very clearly the IR reconfigurability of the plasmonic interdigitated MSM hot electron Schottky photodetector, <NUM>, from <FIG>, when the phase change material Sb<NUM>S<NUM> is switched between amorphous state to the (poly)crystalline state. Thus, the photodetector, <NUM> is detecting the telecom O band (Plot <NUM>) in the amorphous state and the telecom C band (Plot <NUM>) in the polycrystalline state. Unlike the prior art, where the reconfigurability was possible at the system level, by varying the temporal and spatial parameters of computer-controlled optical systems for reading the data from an array of photodetectors, here, for the first time, the reconfigurability was obtained at the device level, by reversibly switching the active PCM Sb<NUM>S<NUM> from the amorphous to the polycrystalline state for the same photodetector.

<FIG> shows the results of the numerical simulations of the absorption of the telecom wavelengths in the reconfigurable plasmonic interdigitated MSM Schottky hot electron photodetector, <NUM> described in <FIG> where the metal (<NUM>, <NUM>, <NUM>, <NUM>) is aluminum and the semiconductor (<NUM>) is amorphous Sb<NUM>S<NUM>. In <FIG> one can notice that for an aluminum metal thickness of <NUM>, a resonant absorption of telecom E band (<NUM>-<NUM>) µm (Plot <NUM>) can be obtained. These results are associated to an aluminum grating equal with period equal to P= <NUM> (W=<NUM>, S=<NUM>), normal incidence radiation and first diffraction order, m=<NUM>.

<FIG> shows the results of the numerical simulations of the absorption of the telecom wavelengths in the reconfigurable plasmonic interdigitated MSM Schottky hot electron photodetector, <NUM> described in <FIG> where the metal (<NUM>, <NUM>, <NUM>, <NUM>) is aluminum and the semiconductor (<NUM>) is (poly)crystalline Sb<NUM>S<NUM>. In <FIG> one can see that for an aluminum metal thickness of <NUM>, a resonant absorption of telecom U band (<NUM>-<NUM>) µm (Plot <NUM>) can be obtained. These results are associated to an aluminum grating equal with period equal to P= <NUM> (W=<NUM>, S=<NUM>), normal incidence radiation and first diffraction order, m=<NUM>.

<FIG> shows a superposition of the resonant absorption peaks of telecom wavelength presented in <FIG> and <FIG>, which is demonstrating very clearly the IR reconfigurability of the plasmonic interdigitated MSM hot electron Schottky photodetector, <NUM>, from <FIG>, when the phase change material Sb<NUM>S<NUM> is switched from amorphous state to the (poly)crystalline state. Thus, the photodetector, <NUM> is detecting the telecom O band (Plot <NUM>) in the amorphous state and the telecom U band (Plot <NUM>) in the polycrystalline state. These results are associated to an aluminum grating equal with period equal to P= <NUM> (W=<NUM>, S=<NUM>), normal incidence radiation and first diffraction order, m=<NUM>.

Comparing the results from <FIG> and <FIG>, it becomes obvious that by tunning the period of the aluminum grating from <NUM> to <NUM>, by changing the width W value of the fingers, <NUM> and <NUM>, (see <FIG>), the plasmonic photodetector can perform either the hopping from telecom C band to O band at P=<NUM>, or the hopping from telecom U band to O band at P= <NUM>, when switches from polycrystalline state to amorphous state of the Sb<NUM>S<NUM> semiconductor.

<FIG> shows an excellent agreement between the analytical (small dots) and the numerical simulation results (big dots) of the period of aluminum grating as a function of resonant incident IR wavelength for the case of plasmonic interdigitated MSM hot electron Schottky photodetector, <NUM>, from <FIG> for the case of aluminum metallization, <NUM>, <NUM>, <NUM>, <NUM> deposited on Sb<NUM>S<NUM>, when the semiconductor, <NUM>, is in the (poly)crystalline state. In both theoretical cases the simulation was performed for the case of normal incidence of telecom wavelengths and the first order of diffraction, m=<NUM>. The two big dots from <FIG>, obtained by numerical simulation, provide the values of the grating periods for which the IR photodetector, <NUM>, from <FIG> are detecting the telecom C or U band, as described in <FIG> and <FIG>, respectively, when the Sb<NUM>S<NUM> semiconductor, <NUM>, is in the (poly)crystalline state. The analytical simulation consists in the calculation of the period of grating by using the equation [<NUM>] based on matching of the momentum of incident photons with that of the grating for the case of the incidence angle equal to zero. By using the refractive index of the chemically bath deposited polycrystalline Sb<NUM>S<NUM> from <FIG> and the refractive index of the aluminum (https://refractiveindex. info/?shelf=3d&book=metals&page=aluminium), it was possible to find analytically the period of the grating, P=W+S, for which the momentum matching condition from equation [<NUM>] occurs at the interface between metal and Sb<NUM>S<NUM>. Thus, the generated surface plasmons polaritons will provide enhanced absorption in the metal near interface (resonant absorption) and enhanced electric field in the same region, finally enhancing the photocurrent of the detector. The added value of the numerical approach is coming from the fact that it indicates the thickness of the metal for fabrication technology and the spatial distribution of the enhanced evanescent electric field.

<FIG> shows also an excellent agreement between the analytical (small dots) and numerical simulation results (big dots) of the period of aluminum grating as a function of resonant incident IR wavelength for the case of plasmonic interdigitated MSM hot electron Schottky photodetector, <NUM>, from <FIG> for the case of aluminum metallization, <NUM>, <NUM>, <NUM>, <NUM> deposited on Sb<NUM>S<NUM>, when the semiconductor, <NUM>, is in the amorphous state. In this case, the two big dots from <FIG>, obtained by numerical simulation, provide the values of the grating periods for which the IR photodetector, <NUM>, from <FIG> is detecting the telecom O or E band, as described in <FIG> and <FIG>, respectively, when the Sb<NUM>S<NUM> semiconductor, <NUM>, is in the amorphous state.

The radically new concept of reconfigurable telecom wavelength photodetector,<NUM>, from <FIG>, based on interdigitated plasmonic MSM, Al/Sb<NUM>S<NUM>, hot electron Schottky device, obtained when the PCM Sb<NUM>S<NUM> material was reversibly switched from the amorphous to crystalline state, and which was proven by the numerical simulation, as shown in <FIG> and <FIG>, can be also proven by the analytical modelling, as shown in <FIG> and <FIG> where the numerical results are perfectly interpolated between the analytical results of grating period versus wavelength. For example, the telecom wavelength photodetector, <NUM>, with the grating period equal to <NUM> is detecting the telecom C band when Sb<NUM>S<NUM> is in the polycrystalline state, as shown in <FIG>, and the same detector (with grating period of <NUM>) is detecting the telecom O band when the Sb<NUM>S<NUM> is in the amorphous state, as shown in <FIG>.

<FIG> shows the numerical simulation results of the resonant absorption of the reconfigurable interdigitated plasmonic MSM, Al/Sb<NUM>S<NUM>, Schottky photodetector, <NUM>, from <FIG>, as a function of the incident visible light. The results were obtained for the case of normal incidence of light, first order of diffraction, m=<NUM>, aluminum thickness equal to <NUM>, aluminum grating period, P, equal to <NUM>, metal digits (<NUM>,<NUM>) width, W, equal to <NUM> and the interdigit spacing, S, equal to <NUM>. To better understand these results, it is necessary to consider the results presented in <FIG> where it is shown that the extinction coefficient for amorphous and (poly)crystalline Sb<NUM>S<NUM> film is high in the visible range. This means that the plasmonic interdigitated MSM Al/Sb<NUM>S<NUM> Schottky photodetector, <NUM>, from <FIG> is absorbing the visible light by the semiconductor. Therefore, the photons with energy higher than the bandgap will generate electron-hole pairs, which are then separated by the enhanced electric field in the depletion region of the Schottky contact, and finally generate the photocurrent. From the numerical simulation of the plasmonic photodetector, <NUM>, with the above geometry of the metal grating, it is obtained that a resonant absorption centered on orange color (<NUM>) is obtained when the Sb<NUM>S<NUM> absorber is in the polycrystalline state (<FIG>, Plot <NUM>) and a resonant absorption centered on green color (<NUM>) when the Sb<NUM>S<NUM> absorber is in the amorphous state (Plot <NUM>). Thus, it is for the first time when a reconfigurable plasmonic photodetector for the visible spectrum is disclosed, which is able to detect either a color or the other depending on the amorphous or (poly)crystalline state of the PCM Sb<NUM>S<NUM>, in this case.

<FIG> makes a comparison between the analytical (small dots) simulation and the numerical simulation results (big dots) of the period of aluminum grating as a function of resonant visible wavelengths for the case of reconfigurable plasmonic interdigitated MSM Schottky photodetector, <NUM>, from <FIG>, for the case of aluminum metallization <NUM>, <NUM>, <NUM>, <NUM> deposited on Sb<NUM>S<NUM> absorber (<NUM>), when this absorber is either in (poly)crystalline state (<FIG>) or in amorphous state (<FIG>). As mentioned before, the numerical simulation (big dots) brings a wealth of information in terms of metal thickness optimum for resonant absorption at a certain incident wavelength, as well as the profiling of the evanescent electric field at the interface between metal and semiconductor.

<FIG> and <FIG> show the results of analytical modeling and simulation of the metal grating period as a function of the incident IR wavelength (mainly the telecom band) for the case of the plasmonic interdigitated MSM hot electron Schottky Al/Sb<NUM>Se<NUM> photodetector, <NUM>, from <FIG>, when the PCM Sb<NUM>Se<NUM> chalcogenide semiconductor is either in the (poly)crystalline state (<FIG>) or in the amorphous state (<FIG>). The same analytical simulation methodology as described for the case of plasmonic Al/Sb<NUM>S<NUM> Schottky photodetector was used also for the plasmonic Al/Sb<NUM>Se<NUM> Schottky photodetector with the specification that for this situation the refractive index of PCM Sb<NUM>Se<NUM> from <FIG> was applied. The reconfigurability of this plasmonic Al/Sb<NUM>Se<NUM> Schottky photodetector, <NUM>, can be again analytically demonstrated. For example, the detection of the C band (<NUM>) is performed when the PCM Sb<NUM>Se<NUM> layer of the photodetector, <NUM>, is in the polycrystalline state, for a period of metal grating equal to <NUM>. Now, if the Sb<NUM>Se<NUM> semiconductor of photodetector, <NUM>, is switched in the amorphous state, for the period equal to <NUM>, one can find an incident wavelength equal to <NUM> (telecom O band). Similar examples of hopping from a shorter telecom wavelength (<NUM>) to a longer telecom wavelength (<NUM>) can be seen when the PCM Sb<NUM>Se<NUM> is performing a phase transition from the amorphous to polycrystalline a phase.

<FIG> and <FIG> show the results of analytical modeling and simulation of the metal grating period as a function of the incident visible wavelengths for the case of the plasmonic interdigitated MSM hot electron Schottky Al/Sb<NUM>Se<NUM> photodetector, <NUM>, from <FIG>, when the PCM Sb<NUM>Se<NUM> chalcogenide semiconductor is either in the (poly)crystalline state (<FIG>) or in the amorphous state (<FIG>). The same analytical simulation methodology as described for the case Al/Sb<NUM>S<NUM> Schottky contact was used for Al/Sb<NUM>Se<NUM> Schottky contact, with the specification that for this situation the refractive index of PCM Sb<NUM>Se<NUM> from <FIG> was applied. The reconfigurability of this plasmonic Al/Sb<NUM>Se<NUM> Schottky photodetector, <NUM>, can be again analytically demonstrated. For example, the detection of the red color (<NUM>) is performed with the PCM Sb<NUM>Se<NUM> layer of the photodetector, <NUM>, in the polycrystalline state, for a period of metal grating equal to <NUM> nm. Now, if the Sb<NUM>Se<NUM> semiconductor of photodetector, <NUM>, is switched in the amorphous state, for the period equal to <NUM>, one can find a plasmonic incident wavelength equal to <NUM> (cyan color). Such similar examples of hopping from longer wavelengths of visible light to shorter wavelengths of visible light can be identified when the PCM Sb<NUM>Se<NUM> is performing a phase transition from the polycrystalline a phase to the amorphous one.

The reconfigurable plasmonic interdigitated photodetectors for the telecom wavelengths or the visible spectrum described above are wavelength-selective, as an intrinsic property of the resonant coupling of the incident light to the surface plasmon polaritons from the interface of a metal with a dielectric or semiconductor. This may create a (too) narrow bandwidth for some applications. For example, this may become obvious, if a comparison is made with the commercial InxGa<NUM>-xAs-based IR photodetectors, which are detecting a larger wavelength band, like from <NUM> to <NUM>. To address this possible limitation, in <FIG> an example of a wideband reconfigurable plasmonic photodetector is disclosed, which consists of three individual plasmonic photodetector connected electrically in parallel. Each of the three photodetectors, from <FIG>, has its own grating period, P1, P2 and P3, and therefore each of them will be wavelength-selective with its own bandwidth, but working together, in parallel, they will enlarge the bandwidth by the superposition of their individual bandwidths. This is one of the major advantages of the interdigitated plasmonic photodetectors, built with two collecting electrodes, <NUM>, <NUM> from <FIG>. This approach is valid for any wavelength band including the (IR) telecom wavelength plasmonic interdigitated photodetectors and the visible light photodetectors.

The reconfigurable plasmonic interdigitated photodetectors are polarization angle-selective, as demonstrated in the background of this invention. This means that the photocurrent is maximum only for the case of the electric field vector (EL) of the p-polarized light parallel to the grating vector, KΛ (according to equation [<NUM>] from background). In <FIG> an innovative reconfigurable, multi-polarization, multiband, plasmonic interdigitated photodetector is disclosed, where for a certain direction of the electric field vector, EL, <NUM>, three polarization angles are shown with the three grating vectors, KΛ1, KΛ2, KΛ3, and the total photocurrent is the sum of these three components, with the highest contribution coming from the grating KΛ2 For normal incidence of the light to the surface of the photodetector, the three photodetectors electrically connected in parallel, KΛ1, KΛ2, KΛ3 have the absorption peak at the same wavelength, if the they have the same grating period, KΛ1 = KΛ2 = KΛ3. However, if the p-polarized light is making an incident angle, θ ≠ <NUM> with the normal to the device surface, then, according to the equation [<NUM>], the three photodetectors KΛ1, KΛ2, KΛ3 will have different resonant absorptions (corresponding to different wavelengths), even if they have the same period of grating, KΛ1 = KΛ2 = KΛ3. Thus, a reconfigurable, multi-band, multi-polarization photodetector can be obtained and disclosed here. This novel concept can be applied for either the telecom band detection, mimicking the large bandwidth of the commercial InGaAs detectors, or for wideband visible light detection by means of plasmonic interdigitated detectors electrically connected in parallel between two collecting electrodes, <NUM> and <NUM> in <FIG>, where each metal grating with its period is tuned to a portion of the wider detection spectrum.

As demonstrated in the background of this invention, the quantum efficiency of the IR hot electron Schottky detectors is very low, below <NUM>%, due to the limited number of hot electrons which have the chance to surmount the energy barrier created at the interface between a metal and a semiconductor. Under these considerations, in <FIG>, an innovative reconfigurable plasmonic interdigitated MSM Schottky hot electron detector is shown where three interfaces metal-semiconductor are formed between the plasmonic metal digits, <NUM>, <NUM>, and the semiconductor PCM chalcogenide, <NUM>. Thus, the photocurrent will increase proportionally with the number of interfaces and the associated areas. As it is obvious from this figure, in this case, the plasmonic metal (Al, Au, Ga) is deposited first on the dielectric substrate, <NUM>, (Si/SiO<NUM>, sapphire, glass) followed by the deposition of the PCM chalcogenide, <NUM>, (Sb<NUM>S<NUM>, Sb<NUM>Se<NUM>, Sb<NUM>SxSe<NUM>-x) with the thickness of <NUM>-<NUM>, and followed by the deposition of the capping layer,<NUM> (ZnS (<NUM>%-SiO<NUM>(<NUM>%)) with the thickness of <NUM>-<NUM>. Local etching of the capping layer will be finally performed for contacting the bond pads located on the collecting electrodes, <NUM>,<NUM>, not shown in <FIG>. In the case of the gold deposition on the substrate, firstly, a titanium adherence layer with a thickness of <NUM>-<NUM> is deposited on the substrate, <NUM>.

<FIG> shows a cross section through a further improved reconfigurable, plasmonic, interdigitated MSM Schottky hot electron photodetector, where the plasmonic metal (<NUM>,<NUM>) is fully embedded in the PCM chalcogenide semiconductor, <NUM>, and where, the number of metal-semiconductor interfaces is equal to four. This technological configuration is maximizing the quantum efficiency of the internal photoemission (IPE) of hot electrons from the plasmonic metal, and thus a maximum photocurrent and spectral responsivity can be obtained.

The fabrication technology of the plasmonic hot electron photodetector with maximum photocurrent and spectral responsivity is obvious from <FIG>, as follows. The first step is the chemical cleaning of the substrate, <NUM>, consisting in degreasing the substrate by ethanol cleaning followed by a superficial etching of the substrate surface in diluted HF-deionized water solution. Then, the deposition of a first layer of PCM chalcogenide layer, <NUM> (Sb<NUM>S<NUM>, Sb<NUM>Se<NUM>, Sb<NUM>SxSe<NUM>-x) on freshly cleaned substrate, <NUM>, is performed. The deposition methods for the PCM chalcogenide semiconductors from above can be : chemical bath deposition, physical vapor deposition (sputtering, electron gun evaporation), atomic layer deposition, pulsed layer deposition. Immediately, a plasmonic metal (<NUM>,<NUM>) (Au/Ti, or Al or Ga) is deposited on the first layer of PCM chalcogenide semiconductor. In the case of Au layer, a titanium adherence (and barrier) layer with thickness of <NUM>-<NUM> is deposited before gold deposition. In the case of Al layer, a Si<NUM>N<NUM> barrier layer with thickness of <NUM>-<NUM> may be deposited before Al deposition The patterning of the metal layer (photolithographic process followed by metal etching /removal) will create the interdigitated MSM geometry of the plasmonic reconfigurable MSM Schottky hot electron photodetector. For gold metallization the lift-off technology can be also used, where a photoresist mask is first created on the substrate. The metal layer is deposited on this photoresist mask. Th photoresist mask is design so that the gold portion which should be removed is deposited on the top of the photoresist mask. For gold patterning, the gold layer deposited on the photoresist is finally removed together with the photoresist substrate. After the plasmonic metal is deposited and patterned (<NUM>,<NUM>), a second layer of chalcogenide semiconductor,!, (Sb<NUM>S<NUM>, Sb<NUM>Se<NUM>, Sb<NUM>SxSe<NUM>-x) is deposited above the patterned plasmonic metal (<NUM>,<NUM>). Finally, the capping layer, (ZnS(<NUM>%)-SiO<NUM>(<NUM>%)), <NUM>, is deposited on the entire processed substrate, followed by a photolithographic and etching process for removing locally the second layer of chalcogenide substrate and contact the bonding pad of the metallization.

The fabrication technology of the reconfigurable plasmonic interdigitated MSM Schottky detector for visible light spectrum can be performed following either the schematic device presentation from <FIG>, where a single PCM semiconductor chalcogenide layer, <NUM>, is deposited on the substrate, <NUM>, followed by the patterned metal (<NUM>, <NUM>, <NUM>, <NUM>) and the capping layer (ZnS(<NUM>%)-SiO<NUM>(<NUM>%)), or the schematic cross section of the device presented in <FIG>, where a plasmonic metal (Al, Au/Ti, Ga) is deposited and patterned first on the substrate, <NUM>, followed by the PCM chalcogenide semiconductor, <NUM>, (Sb<NUM>S<NUM>, Sb<NUM>Se<NUM>, Sb<NUM>SxSe<NUM>-x) and capping layer deposition. The thickness of the PCM chalcogenide semiconductor is in the range of <NUM>-<NUM>, or even higher depending on application requirement.

Claim 1:
A reconfigurable plasmonic interdigitated metal-semiconductor-metal Schottky photodetector (<NUM>) comprising:
a first collecting plasmonic metal electrode (<NUM>) containing a multitude of periodic metal digits (<NUM>), digits, parallel one to the other,
a second collecting plasmonic metal electrode (<NUM>) containing a multitude of periodic metal digits (<NUM>), parallel one to the other and interpenetrated with the digits of the first collecting electrode, the network of all these metal digits forming an optical grating with a certain periodicity,
a phase change chalcogenide semiconductor layer (<NUM>) in contact with the metal,
a capping layer (<NUM>) covering the top surface of photodetector, acting as a mechanica barrier against chalcogenide atoms loss during thermal and laser annealing, and an anti-reflection coating.