A method of preparing a film of an inorganic metal halide perovskite compound, a semiconductor structure comprising said film and an optoelectronic device comprising said semiconductor structure

Abstract

The present invention relates to a method of preparing a film of an inorganic metal halide perovskite (IMHP) compound comprising the steps of: (a) providing a substrate having a surface layer (buffer layer) comprising an amorphous oxide of an element selected among those of the groups 13, 14 and 15 of the IUPAC periodic table; (b) sublimating a solid precursor comprising the IMHP compound to form a vapor stream; (c) depositing said vapor stream on said buffer layer to form said film. The invention also relates to a semiconductor structure comprising the IMHP film and an optoelectronic device comprising said semiconductor structure.

Description

[0001] A METHOD OF PREPARING A FILM OF AN INORGANIC METALHALIDE PEROVSKITE COMPOUND, A SEMICONDUCTOR STRUCTURECOMPRISING SAID FILM AND AN OPTOELECTRONIC DEVICE COMPRISING SAID SEMICONDUCTOR STRUCTURE ------ Technical field The present invention relates to a method ofpreparing a film of an inorganic metal halide perovskitecompound. The present invention also relates to asemiconductor structure comprising said film and an optoelectronic device comprising said semiconductor structure. Prior art Aperovskite compound has the general formula ABX3,where A is a cation with a large ionic radius, B is a metal cation, and X is an anion. The crystal structure of a perovskite can be displayed as an AX12cuboctahedronthat shares its edges with a BX6 octahedron. Such acrystal structure is the same as that of CaTiO3.Deviations from the ABX3 stoichiometry can be obtainedwhen the A and B cation sites become partially or fully vacant (vacancy-ordered perovskites), or when they are replaced by a combination of other cations (with different valences but with an overall neutral charge balance), forming double or quadruple perovskites. Metal Halide Perovskites (MHP) are a subgroup ofperovskite compounds, characterized in that they havehalides as X anions. In the case of perovskites having an ABX3 stoichiometry the B cations are divalent (e.g. Pb2+, or Sn2+) and the A cations are large monovalent alkali metals (e.g. cesium) or small organic cations like methylammonium (MA) or formamidinium (FA). When the perovskite contains inorganic and organic cations, thecompound is named hybrid organic-inorganic perovskite(HOIP). When the perovskite contains only inorganiccations, the compound is named inorganic metal halideperovskite (IMHP) or all-inorganic halide perovskite.Perovskite compounds are attracting increasinginterest as semiconductors in the field of optoelectronic applications owing to their remarkableoptical and electrical properties. In this field,perovskite compounds in the form of single-crystal thinfilms are especially desired as they are suitable forbeing integrated in optoelectronic devices (e.g photovoltaic cells, light-emitting diodes, lasers,etc.). Compared with perovskite polycrystalline filmsand nanocrystals, single-crystal perovskites thin films are substantially free of grain-boundaries and possess lower trap-state densities, higher carrier mobilities as well as longer diffusion lengths, which are responsible of the superior optoelectronic performance. Among perovskite materials, IMHP compounds, such asfor example CsPbBr3, are particularly preferred as theyexhibit higher thermal and chemical stability compared to hybrid organic-inorganic counterparts, the latter being more prone to structural degradation and chemicalinstability. For example, CsPbBr3 exhibits adecomposition temperature of approximately 577 °C, which is far beyond the temperatures that can be sustained by hybrid perovskites. The preparation of an IMHP compound in form of a thin film, however, is very challenging. A wide range ofsynthesis techniques is available nowadays, such assolution-based growth, vapor-phase epitaxial growth andtop-down (slicing, mechanical exfoliation) methods.Vapor-phase epitaxial growth is one of the most powerfultechniques as it leads to single-crystal films havingcontrollable thicknesses (as low as a monolayer and upto several hundreds of micrometers) and high phasepurity. In vapor-phase epitaxy (VPE), the perovskitecompound to be grown or its constituent elements are vaporized by evaporation, sublimation, sputtering, laserablation or other process from a source at high localtemperatures and subsequently transported in the form ofa vapor stream toward a substrate, kept at propertemperature, on the surface of which they react and growinto a single-crystal thin film.A major drawback of the VPE lies in the fact thatthe lattice constants and expansion coefficients of thegrown perovskite film should match, as much as possible,those of the substrate. This constraint severely limitsthe practical application of perovskite films as itrestricts the number of substrates on which perovskite thin films can be grown. In the state of the art, to overcome this drawback SrTiO3 (STO) is often used as substrate for the epitaxialgrowth of IMHP compounds because of the similar latticeconstants. For example, J. Chien et al. J. Am. Chem.Soc. 2017, 139, 13525−13532, discloses the growth oflarge-area continuous film of CsPbBr3on an STO(100) substrate by means of VPE at elevated temperature. Further substrates that have been demonstrated suitablefor the epitaxial growth of IMHP compounds aremonocrystalline alkali halides as disclosed in Wang etal., Sol. RRL, 2019, 1800294. In view of the above-described state of the art,there remains a need for alternative and possiblyimproved method of preparing a thin film of an IMHPcompound. Summary of the invention Applicants have thus faced the problem of providinga method for preparing a thin film of an IMHP compoundcapable of overcoming, or at least, ameliorating thedrawbacks of the known methods and materials of the stateof the art. Particularly, a scope of the present invention is toprovide a method that allows the vapor-phase epitaxialgrowth of a thin film of an IMHP compound on a widevariety of substrates other than the conventional STOsubstrate or alkali halide substrate.The method should lead to the preparation of a thin film of an IMHP compound having high-purity phase (i.e.a phase formed by a single crystal material substantiallyfree of defects), a controllable thickness (e.g. up to300 nm or more) and a large surface area (i.e. acontinuous film with low density of grain boundaries). Applicants have now found that the above scopes and others, that will more clearly result from the following disclosure, are solved by growing an IMHP film through vapor-phase evaporation of a powder precursor of theIMHP compound on a substrate comprising a superficiallayer of an amorphous oxide (hereinafter “bufferlayer”), such as a layer of amorphous SiO2.In one embodiment, the amorphous oxide is obtainedby oxidizing a surface of a substrate that is made of aselected element in its elemental state (e.g. Si, Ga, Ge, etc.), for example by exposing the substrate to a stream of gaseous oxygen. The powder precursor comprises the same IMHPcompound that it is desired to grow as epitaxial film onthe substrate buffer layer, the IMHP precursor compoundbeing either in monocrystalline or polycrystalline form.It has been observed that an amorphous oxide layeroverlying the surface of the substrate allows theepitaxial growth of a continuous film of the IMHPcompound that has high purity, low density of grainboundaries and controllable thickness (within a range of5 nm to 500 nm) overcoming therefore the lattice mismatchproblems observed in the state of the art. Withoutwishing to be bound to any theory, it is believed thatthe amorphous oxide layer (e.g. SiO2) decouples the surface atoms of the element forming the substrate (e.g.Si) from those of the IMHP film (e.g. CsPbBr3) thusfavouring the growth of the latter in the form of acontinuous film of exceptional quality. By means of abuffer layer made of an amorphous oxide it therefore becomes possible to prepare IMHP films in a veryefficient way on many different substrates foroptoelectronic applications. According to a first aspect, the present invention relates therefore to a method of preparing a film of an inorganic metal halide perovskite (IMHP) compound comprising the steps of: (a) providing a substrate having a surface layer (buffer layer) comprising an amorphous oxide of an element selected among those of the groups 13, 14 and 15 of the IUPAC periodic table; (b) sublimating a solid precursor comprising an IMHP compound to form a vapor stream; (c) depositing said vapor stream on said buffer layer to form said film. According to a preferred embodiment, the present invention relates to a method of preparing a film of an inorganic metal halide perovskite (IMHP) compound comprising the steps of: (a) providing a substrate having a surface layer (buffer layer) comprising an amorphous oxide of an element selected among those of the group 14 of the IUPAC periodic table; (b) sublimating a solid precursor comprising the IMHP compound to form a vapor stream; (c) depositing said vapor stream on said buffer layer to form said film, wherein said sublimation and deposition are carriedout through molecular beam epitaxy, preferably in ultra-high vacuum (UHV). According to a second aspect, the present inventionrelates to a semiconductor structure comprising:- a substrate;- a buffer layer overlying said substrate, said buffer layer comprising an amorphous oxide of an element selected among those of the groups 13, 14 and 15 of the IUPAC periodic table; -a film of an inorganic metal halide perovskiteoverlying said buffer layer. According to a preferred embodiment, the presentinvention relates to a semiconductor structurecomprising: -a substrate;- a buffer layer overlying said substrate, saidbuffer layer comprising an amorphous oxide of an element selected among those of the group 14 of the IUPAC periodic table; -a film of an inorganic metal halide perovskiteoverlying said buffer layer. In an even more preferred embodiment, said film is obtained by the method according to the first aspect of the invention. A third object of the present invention relates toan optoelectronic device comprising the semiconductorstructure according to the second aspect. The method of the present invention is suitable to prepare a thin film of an IMHP compound supported on a substrate without any particular restriction on thecomposition of the grown IMHP compound.In one embodiment, the IMHP compound has a general formula (i) ABX3, wherein: -A is a monovalent cation,- B is a divalent cation different from A,- X is a monovalent anion. Preferably, in the formula ABX3: -A is selected from: Li+, Na+, K+, Rb+, Cs+ andcombinations thereof; preferably, it is Cs+;- B is selected from: Pb2+, Sn2+, Ge2+, divalenttransition metal and combinations thereof; preferably,it is Pb2+; -X is selected from: chloride, bromide and iodide,preferably it is Br-. In one embodiment, the IMHP film has a general formula (ii) ABX’3-nX”n, wherein: -A is a monovalent cation,- B is a divalent cation different from A,- X’ is a monovalent anion;- X” is a monovalent anion different from X’;- n is higher than 0 and lower than 3, for examplewithin the range of 0.01 and 2.99. Preferably, in the formula ABX’3-nX”n: -A is selected from: Li+, Na+, K+, Rb+, Cs+ andcombinations thereof; -B is selected from: Pb2+, Sn2+, Ge2+, divalenttransition metal and combinations thereof;- X’ and X” are each selected from: chloride, bromideand iodide, with the proviso that X is different fromX”. In a preferred embodiment, the IMHP film is a perovskite of formula ABX3 selected from: CsPbBr3,CsPbI3, CsPbCl3, CsSnI3, CsSnBr3, CsSnCl3, CsGeI3,CsGeCl3, CsGeBr3; more preferably it is CsPbBr3.In a preferred embodiment, the IMHP film is aperovskite of formula ABX’3-nX”nselected from: CsPbBr3-nCln, CsSnBr3-nCln, CsGeBr3-nCln CsPbBr3-nIn, CsSnBr3-nIn,CsSnBr3-nCln, CsGeBr3-nIn, wherein n is higher than 0 and lower than 3, for example within the range of 0.01 and 2.99. The CsPbBr3perovskite exhibits three differentstructural phases, including, the cubic Pm3m, thetetragonal P4 / mbm and the orthorhombic Pbnm, in whichthe Pb atom is located in the center of the octahedraformed by the six Br atoms, whereas the Cs atoms arelocated in the three-dimensional framework cavities. Atambient temperature the CsPbBr3 perovskite is monoclinically distorted. It crystallizes in theorthorhombic (Pnma) phase, adopting the distortedperovskite structure, with a = 8.2440(6) Å, b =8.1982(8), and c = 11.7351(11) Å. The {PbBr 4−6} octahedraare tilted with respect to the orthogonal geometry ofthe ideal cubic perovskite structure. Two successivephase transitions of CsPbBr3 occur at 88 °C and 130 °C,transforming the crystal structure to tetragonal(P4 / mbm) and cubic (Pm-3m), respectively, as second andfirst order phase transitions, respectively.In another embodiment, the IMHP compound has a general formula (iii) A2BX6, wherein: -A is a monovalent cation,- B is a tetravalent cation different from A,- X is a monovalent anion.Preferably, in the formula A2BX6: -A is selected from: Li+, Na+, K+, Rb+, Cs+ andcombinations thereof; -B is selected from: Pb4+, Sn4+, tetravalenttransition metal (e.g. Pt4+, Pd4+) and combinationsthereof. -X is selected from: chloride, bromide, iodide andcombinations thereof. In a preferred embodiment, the IMHP compound is aperovskite of formula A2BX6 selected from: Cs2PbBr6,Cs2SnBr6, Cs2PtBr6, Cs2PdBr6; Cs2PbI6, Cs2SnI6, Cs2PtI6, Cs2PdI6more preferably it is Cs2PbBr6. In another embodiment, the IMHP compound has a general formula (iv) AB2X5, wherein: -A is a monovalent cation,- B is a divalent cation different from A,- X is a monovalent anion.Preferably, in the formula AB2X5: -A is selected from: Li+, Na+, K+, Rb+, Cs+ andcombinations thereof; -B is selected from: Pb2+, Sn2+, Ge2+, divalenttransition metal and combinations thereof;- X is selected from: chloride, bromide, iodide andcombinations thereof. In a preferred embodiment, the IMHP compound is a perovskite of formula AB2X5selected from: CsPb2Br5,CsPb2I5, CsSn2I5, CsSn2Br5, CsSn2Cl5, CsGe2I5, CsGe2Cl5,CsGe2Br3. Preferably, the IMPH film grown on the amorphous oxide has a thickness within the range of from 5 to 500 nm, more preferably from 10 to 300 nm. The substrate serves the function of supporting thebuffer layer on to which the IMHP film is grown. In one embodiment, the substrate is made of an element selected among those of the groups 13, 14 and 15of the IUPAC periodic table (also commonly named asgroups III-A, IV-A and V-A), the element being in its elemental state. In an embodiment, the element is selected among those of the group 14. In an embodiment, it is selected among the isoelectronic elements of group 14, wherein forisoelectronic is intended that the elements have thesame number of valence electrons and the same structure. Preferably, the element of the substrate is selected from: Si, Ge, Sn, Pb, Al, Ga, In, Tl, As, Sb or Bi, more preferably is selected from Si, Ge, Ga, In or As. In one embodiment, the substrate is made of asingle-crystal material, for example single-crystal Si (or Ge, Ga, In, etc.) in its elemental state. In one embodiment, the substrate is made of an oxide, preferably an amorphous oxide. When the substrate is an amorphous oxide, its surface may serve as buffer layer for the growth of the IMHP film; in that case the IMHP film can be grown directly on the substrate,providing an additional amorphous oxide layer beingoptional. In one embodiment, the substrate is made of an oxide, preferably an amorphous oxide, of an element selected among those of the groups 13, 14 and 15 of the IUPAC periodic table (also commonly named as groups III- A, IV-A and V-A). The amorphous oxides forming the substrate may be an oxide of the type generally present in anoptoelectronic device, e.g. a photovoltaic cell.Examples of oxides suitable for being used as asubstrate are: Tin-doped Indium Oxide, called Indium TinOxide (ITO); Fluorine-doped Tin Oxide (FTO); Titaniumdioxide (TiO2), Aluminum-doped Zinc Oxide (AZO), IndiumTin Oxide (IZO), Indium Gallium Tin Oxide (IGZO) andTungsten-doped Indium Oxide (IWO). In one embodiment, which is illustrated in more details hereinafter, the substrate is made of one of the above listed elements in its elemental state and issubjected to an oxidation reaction to form a superficialbuffer layer comprising the amorphous oxide, e.g. SiO2 (GeO2, Ga2O3, In2O3, …). Preferably, the amorphous oxide buffer layer is formed by oxidizing a (111) surface of the single-crystal element (e.g. Si(111)). In one embodiment, the crystal surface is preferably a reconstructed surface, for example a 7x7 reconstructed surface (e.g. Si(111)7x7). As known, surface reconstruction is a process by which atoms at the surface of a crystal assume a different structure than that of the bulk. The crystal structure of the IMHP film deposited onthe buffer layer depends mainly on two parameters: 1)the flux rate (atoms / s) of the vapor stream in theevaporation cell, which can be tuned by varying thetemperature of the cell; 2) the temperature of thesubstrate. By tuning one or more of the aforementioned parameters, it is possible to grow IMHP films having different crystalline structure (exhibiting own preferential orientation, having different perovskite formula). It has been observed, for example, that for a given perovskite precursor having a certain crystalline structure and a given flux rate, films having different crystalline structures can be achieved by applyingdifferent substrate temperatures. For example, theformation of films of perovskite compounds of formulaABX3 is favored at relatively low temperatures (about110 °C – 150°C), whereas the formation of films ofperovskite compounds of formula AB2X5is favored atrelatively higher temperatures (about 220 °C – 270°C).The person skilled in the art may determine the mostappropriate conditions for obtaining the desired crystalline structure and orientation of the IMHP compound by routine experimentation. The buffer layer comprises an amorphous oxide of an element selected among those of the groups 13, 14 and 15 of the IUPAC periodic table of the elements. Preferably, the amorphous oxide is an amorphous oxide of an element selected from: Si, Ge, Sn, Pb, Al, Ga, In, Tl, As, Sb or Bi, more preferably from Si, Ge, Ga, In or As. The amorphous oxide layer supported on the substrate can be prepared according to known preparation techniques, such as those used in the field of manufacturing of optoelectronic devices. Preferably, the buffer layer has a thickness of atleast 1 nm or more and up to 30 nm or less. Preferably,the buffer layer has a thickness of from 1 to 20 nm, preferably from 2 to 10 nm. The thickness of the IMPH film and the buffer layer may be determined by means of a quartz microbalanceduring their growth or by scanning transmission electronmicroscopy (STEM). According to the present invention, the growth ofthe IMHP film is carried out by vapor-phase epitaxy onthe substrate comprising the superficial buffer layer.Vapor-phase epitaxy comprises evaporating a solidprecursor (i.e. sublimating) comprising the IMHP compound that has to be grown on the buffer layer of the substrate so as to form a vapor stream and depositing the vapor stream on the buffer layer so as to epitaxially grow the IMHP film. The steps of sublimating and depositing of themethod can be carried out through a variety of processes, preferably through one of the following: molecular beamepitaxy (MBE), thermal evaporation, sputtering (e.g.magnetron sputtering), pulsed laser deposition andphysical vapor deposition, preferably by molecular beam epitaxy. In a preferred embodiment, sublimating and / ordepositing are carried out under ultra-high vacuum (UHV) conditions. For ultra-high vacuum is intended a vacuum characterized by a pressure lower than about 1.0x10-8mbar. Preferably, the pressure is lower than 1.0x10-9mbar. For example, the pressure can be about 8.0x10-10mbar. It has been experimentally observed thatsublimating the IMHP precursor leads to the formation ofa vapor stream containing IMHP moieties having the samestoichiometry of the IMHP precursor, i.e. the vaporized precursor species are not decomposed. This is deemedadvantageous as it favors the epitaxial growth of adefect-free continuous film on the buffer layer. In one preferred embodiment, the amorphous oxide ofthe buffer layer is obtained by oxidizing a substratemade of the selected element in its elemental state. According to this embodiment, the aforementioned step (a) of the method comprises: (a1) providing a substrate made of an element selected among those of the groups 13, 14 and 15 of the IUPAC periodic table, preferably of the group 14, said element being in its elemental state; (a2) oxidizing a surface of said substrate to obtain said buffer layer comprising said amorphous oxide of said element. Preferably, the step (a2) of oxidizing the surfaceof the substrate comprises exposing the surface to gaseous oxygen. The oxidation of the substrate is carried out by exposing it to a stream of gaseous oxygen until the amorphous oxide layer having the desired thickness is obtained. The thickness of the buffer layer can be controlled by tuning the pressure of the gaseous oxygen, the time of exposure of the substrate to oxygen and the temperature of the substrate. Preferably, the substrate is exposed to 0.5 – 5.0·104Langmuir (L), for example 1.0 – 2.0·104 L (1 L = 1·10-6mbar·1s) of the gaseous oxygen stream.Preferably the exposure is carried out keeping thesubstrate at a temperature of 25 °C - 200 °C, morepreferably at 25 °C – 40°C.The deposition of the vapor stream comprising the precursor species to form the thin film on the buffer layer supported on the substrate may be carried out maintaining the substrate and the buffer layer attemperatures that may vary within a wide range.Preferably, during the deposition step the substrate comprising the buffer layer is kept at a temperature within the range of from 50 to 300°C, more preferably of from 100 to 200°C. Preferably, the vapor stream comprising the IMHP precursor species is fluxed, for example in a Knudsencell, on the substrate comprising the buffer layer at arate of from 0.05 to 25 Å / s, more preferably from 0.5 to10 Å / s, even more preferably from 0.1 to 1 Å / s, for example from 0.1 to 0.5 Å / s. The method of the present invention can be carried out using conventional apparatuses that are well known to the person skilled in the art in the field of the epitaxial growth of thin films. The operating conditions illustrated in the present description, such as those for the oxidation of the substrate, sublimation of the powder precursor anddepositing of the IMHP film, are especially suitable forcarrying out the method of the present invention bymolecular beam epitaxy and may vary when othersublimation and deposition techniques such as thermalevaporation, sputtering and pulsed laser deposition areused. Suitable conditions for carrying out the methodusing other techniques, however, can be determined by aperson skilled in the art through routine experiments onthe basis of the teaching of the present invention. The method of the present invention allows to manufacture single-crystal, low-defect density IMHPfilms having high thickness and large lateral size. Thesecharacteristics make the IHMP according to the presentinvention suitable to be effectively exploited in the manufacturing of optoelectronic devices. Indeed, many optoelectronic devices include oxide layers having an amorphous structure onto which the IMHP films can bedeposited according to the method of the presentinvention. The thin IMHP film of the present invention may beincorporated substantially in any type of optoelectronicdevices without particular limitations. Preferred examples of optoelectronic devices that may incorporate an IMHP film according to the present invention are:photodetectors, field-effect transistors, photodiodes,UV photodetectors, photovoltaic cells, light emittingdiodes (LED), photoconductors, laser devices.The present disclosure, in at least one of theaforementioned aspects, can be implemented according toone or more of the above-illustrated embodiments,optionally combined together.The following examples are provided only forillustrative purposes of the present invention and must not be understood as limiting the scope of protection defined by the appended claims. Detailed description of the preferred embodiments Further characteristics will be apparent from thedetailed description given hereinafter with reference tothe accompanying drawings, in which: Figure 1 shows RHEED patterns of a comparative example. Figure 2 shows a XRD pattern of a CsPbBr3 film grown according to the method of the present invention. Figure 3 shows a HAADF image of the CsPbBr3film prepared according to the method of the invention. Figure 4 shows a HAADF image of figure 2 (a), thecorresponding High Resolution HAADF STEM images (b) and the atomic model of the orthorhombic CsPbBr3(c). Figure 5 shows High Resolution HAADF STEM images of the cross section of the CsPbBr3film prepared accordingto the method of the invention (a) with the correspondingHAADF images and EDX elemental maps (b).Figure 6 shows SEM image of the CsPbBr3film grown on SiO2 / Si according to the example of the present invention. Figure 7 shows Auger Electron Spectroscopy (AES) spectra of the surface of Si(111)7×7 after oxidation byexposure to gaseous oxygen.The following examples are provided to further illustrate the invention. EXAMPLES 1. Preparation of the materials CsPbBr3 thin films have been grown on an elementalSi substrate by means of molecular beam epitaxy (MBE)under ultra-high vacuum (UHV) conditions (ca. 8.0x10-10mbar) following the procedure described hereinafter.The Si(111) surface of the substrate was cleaned ina chamber of the UHV-MBE apparatus by applying severalflashes, at about 1150 °C, until the Si(111)7×7 reflection high electron energy diffraction (RHEED) pattern of clean silicon was obtained. In the samples according to the invention, theCsPbBr3 films were grown on the Si(111)7×7 surface afterits oxidation to form a buffer layer of amorphous SiO2.For comparison purposes, CsPbBr3 films were also growndirectly on a non-oxidized Si(111)7×7 substrate. Oxidation of the substrate was carried out byexposing the Si(111)7×7 surface to 1.2 x 104 L of O2 atambient temperature in the chamber of the UHV-MBEapparatus with the formation of a 1.5 ^ 2 nm nm-thicklayer of amorphous SiO2.The MBE growth of the CsPbBr3 films was carried outkeeping the substrate at 130 °C and using a vapor stream of ~0.1 Å / s from a Knudsen cell (RIBER) loaded with astoichiometric CsPbBr3 powder as precursor (purchasedfrom TCI CAS RN:15243-48-0, used without any furtherpurification). 2. Characterization methods In situ mass spectra of the vapor stream were collected during the CsPbBr6films growth using a SRS RGA-300 AMU instrument. In situ Auger Electron Spectroscopy (AES) spectra were collected on the deposited films at the normal incidence with a primary electron beam, EP = 3.0 KeV. AES data were acquired in the first derivative mode with a PHY 255G double-pass cylindrical mirror analyzer equipped with a coaxial electron gun, with an energy resolution of 0.5 eV. The thickness of the deposited CsPbBr3films weremeasured by means of a quartz microbalance.X-ray diffraction (XRD) and photoluminescence (PL)measurements on the deposited perovskite films wereperformed ex situ by means of a Rigaku SmartLabdiffractometer, working in (ν – 2ν) Bragg–Brentanogeometry, equipped with a Cu source (Kα1 = 1,54060 Å)and a D / teX Ultra 250 silicon strips detector. XRDspectra (5°– 60°) were acquired in a single scan with astep of 0.05°, ~0.2 s / pt, and a horizontal slit of 5 mm, whereas PL spectra were obtained using a Jobin–Yvon– Horiba micro-Raman system (LabRAM ARAMIS) equipped with a Cobolt Fandango solid-state laser (514 nm) as excitation source. 3. Results 3.1 Characterization of the CsPbBr3precursor X-ray diffraction pattern (not shown) of the CsPbBr3powder used as precursor, carried out ex situ in aconventional XRD diffractometer, showed the orthorhombicsymmetry, Pbnm (62) space group, of the material withthe characteristic distortion of the octahedra. The clear splitting of the (110) and (220) peaks in the diffraction pattern, indicated the orthorhombic phase ofthe polycrystalline CsPbBr3 powder at ambienttemperature. Additionally, no PbBr2 phase, typicallyexhibiting XRD peaks at 14.37°, 18.58°, 21.63°, 22.04°and 23.69° corresponding to the lattice planes (101),(002), (011), (102) and (111)32, respectively, wasdetected, confirming the high purity of the CsPbBr3powder precursor. 3.2 Growth of CsPbBr3 thin film on a non-oxidizedSi substrate (comparative material)The deposition of CsPbBr3on the non-oxidized Si substrate led to the growth of a thin film (~10 nm-thick) with a preferential growth orientation of CsPbBr3(Pnmaspace group, with a = 8.2440 Å, b = 8.1982, c = 11.7351Å), on Si(111)7×7 (space group Fd3m, with aSi = b = c =5.43 Å), having a large lattice mismatch factor of 7% [f= (1−aoverlayer / asubstrate)], for the planes (001)CsPbBr3 being parallel to (111)Siwith lattice constant aCsPbBr3much higher than aSi. Despite such a large difference in lattice constants, one-unit cell of CsPbBr3could match with two-unit cells of Si in a x2, as shown on the RHEED patternof Fig. 1. In Figures 1a and 1b the dashed and straightarrows indicate the bulk Si integer streaks order (00),(0-1), (01) and the fractional streaks order of the ×2reconstructed CsPbBr3 film in reciprocal space; theprimary energy (Ep) is 12 KeV; the Ep incident beam wasparallel to the [11-2] orientation of silicon, while thefractional streaks order (0-1 / 7), and (01 / 7), stemmingfrom the ×7 Si reconstruction are marked.The RHEED pattern of the Si(111)7×7 surface (Fig.1a) shows the six order streaks (-1 / 7, 1 / 7) of the cleanreconstructed Si surface, whereas the RHEED pattern ofthe overlying thin film (ca. 10 nm-thick) of CsPbBr3(Fig. 1b) shows the half order streak (-1 / 2,1 / 2) inducedby the film growth. The poor sharpness and spotty-like presence of the streaks (-1 / 2,1 / 2) of the CsPbBr3film, which arepossibly explained by the hypothesized largeincommensurate relation with a large lattice mismatch factor, indicates that when the perovskite is grown directly on the Si substrate the coalescence of CsPbBr3 islands occurs instead of the formation of a continuous film. 3.3 Growth of CsPbBr3 thin film on an oxidized Sisubstrate (material according to the invention)A CsPbBr3 film (300 nm-thick) was grown on a bufferlayer of amorphous SiO2 formed upon oxidation of theSi(111)7x7 substrate. The composition of the amorphousoxide formed on the buffer layer was confirmed by theLVV SiO2and KLL O transitions observed in the AESspectra shown in Fig. 7, wherein: (a) is the LVVtransition of Si (89,4 eV) and SiO2(76.39 eV), while(b) is the KLL of O (503.4 eV).The XRD pattern of the CsPbBr3 film preparedaccording to the invention is shown in Fig. 2a. Thispattern features a main peak at 2^ values of 15.195° andtwo minor peaks at 2^ values of 30.700° and 28.435°. Thepeaks at 15.195° and 30.700° are associated to theCsPbBr3 perovskite film. The peak at 28.435° is the (111)reflection of Si(111) substrate.To detail the structure of the perovskite peaks,two enlarged regions of the XRD spectrum around 15.195°and 30.700° are reported as Fig. 2b and 2c. Both peakshave an asymmetric shape, related to a multicomponentcontribution. A least square fitting procedure, by using the convolution of Gaussian and Lorentzian functions,has been applied to reveal the position andcharacteristics of the peak components. Both peaks showfour components. The peaks at 15.187° ± 0.02° and 15.203°± 0.02° of Fig. 2b are attributed to the (002) and (110)reflections of the orthorhombic CsPbBr3 perovskite phase(Pnma (62) space group), whereas the peaks at 15.225°±0.02° and 15.240° ± 0.02° are peak replica due to the CuK^2 x-ray. The peaks at 30.688° ± 0.02°and 30.704° ±0.02° are attributed to the (004) and (220) reflectionsof the orthorhombic CsPbBr3 perovskite phase (Pnma (62)space group), whereas the peaks at 30.767°± 0.02° and30.783° ± 0.02° are peak replica due to the Cu K^2 X-ray(λ = 1.54439 Å).Notably, in agreement with the augmented first derivative, as a function of ^, from the Bragg’s law,the replica at higher 2^^ angle (Fig. 2c) are well shiftedfrom their main peaks, while it is just a shoulder atlower angle (Fig. 2b). The intensities of each componenthave been fitted under the constrain that the replica ofthe Cu K^2x-ray line have a natural half intensity withrespect to the main Cu K^1 X-ray line (λ = 1.54056 Å),and, furthermore, their angle positions imposed by the Cu K^2wavelength. It is worth noting that, for the four components of each peak similar parameters (Gaussian width, σG, and Lorentzian width, ΓL), were used:σ(002),(110) = 0.02 (002),(110) replica (002),(110)G 0°; Γ L = 0.020°; σ G= 0.025°; Γ replica(002),(110) = 0.025° (004),(220)L ); σ G = 0.032°;Γ (004),(220) replica (004), (220)L = 0.032°; σ G = 0.032°; Γ replica(004), (220) L= 0.032°. The Δ (°) between the main(002) and (110), (004) and (220) components, as well astheir replica is of 0.016°. On the other hand, the shift between the main component (002) and its replica is 0.038°, whereas that of (004) and its replica, is of 0.079°, as expected, and similar to that found for the (111) reflection peak of 0.070° from the bare Si(111) substrate. By applying the Bragg’s law (n·λ = 2·d·sin^), whereλ(Cu K^1) = 1.54056 Å and n=1 for the (110) and n = 2 for (002), the orthorhombic CsPbBr3lattice constant values aCsPbBr3= 8.235 Å and cCsPbBr3= 11.658 Å. This points to a preferential orientation of the type (002)CsPbBr3parallel to (111) Si planes, similar to that obtained for the growth of a few monolayers of CsPbBr3on clean Si (111)7×7 surface. Moreover, the total (002)(110) main peak (Fig. 2a)shows a full width at half maximum (FWHM) of 0.06°, whichdrops down to 0.035° by separating the (002) and (110), and their Cu K^2X-ray replica components after thefitting procedure. The value of 0.035° is close to thatreported in the literature for the (111) XRD of a Si(111)single crystal substrate (0.025° - XRD pattern notshown). To the best of the inventors’ knowledge, theobserved FWHM value of 0.06° is the lowest value reportedso far in the literature for XRD of a CsPbBr3 material.The high intensity of the peaks (002)(110), (Fig.2), optimized with respect to the (111) Si substrate,denotes a preferential growth along the

[0001] cperovskite axis. Going straight out from these (002)(110) optimized film signals, by slightly tiltingthe sample keeping the XRD beam and detector conditionsunchanged, their intensity is reduced, showing, indeed,a more (111) pronounced substrate Si peak (XRD pattern not shown). The sharpness of the XRD peaks of Fig. 2 indicates that the perovskite film is characterized bya very low dislocation and defect density and substantialabsence of residual stress. In Fig. 3 the high-angle annular dark-field (HAADF)STEM image of the cross-section of the CsPbBr3filmprepared according to the invention is shown. In thefigure the following layers are identified (from thebottom to the top): silicon substrate, CsPbBr3 film, C / Ptfilm deposited during the Focus Ion Beam (FIB)preparation of the sample. Owing to the low magnificationof the image, the amorphous SiO2 layer is not visible in Fig. 3. The cross-section of the CsPbBr3film is uniform. The film thickness as determined on the STEM images isabout 300 nm, confirming the thickness value estimatedby quartz microbalance during the MBE growth. The circled areas correspond to nanowire-like structures located onthe film surface (also visible in the SEM images of Fig.6). High-resolution HAADF STEM images of the same filmare shown in Fig. 4a. The vertical dashed linecorresponds to the smaller dotted circle of Fig. 3. Itseparates two large domains indicated with A and B,having lateral size of at least 1 μm. In Fig. 4b thecorresponding atomic resolution HAADF and DifferentialPhase Contrast (DPC) STEM images aligned to the zoneaxis of the CsPbBr3 HAADF of Fig. 4a are shown. They areassigned to the

[0120] and

[0110] zone axes of the CsPbBr3.Note that, the sample was tilted each time to align ontozone axes for areas A and B. Therefore, the

[0120] ,

[0110] CsPbBr3 and

[0110] Si are not perfectly parallel to eachother. The misorientation is as large as a few degrees.In Fig. 4c a common c axis

[0001] orientation along thegrowth direction for the grains in A and B areas isshown, which is in full agreement with XRD defined filmtexture. The atomic model of the orthorhombic CsPbBr3 atthe given zone axes is also shown in Fig. 4c (the spheresrepresent: Cs atoms (light grey), Pb atoms (dark grey)and Br atoms (smallest dark grey spheres).The high-resolution HAADF STEM image of the crosssection of the CsPbBr3 film tilted to the

[0110] CsPbBr3zone axis of area B, which also includes the Sisubstrate, (Fig. 5a), together with a HAADF image, andPb, Cs, Br, O, Si EDX elemental maps of a similar region(Fig. 5b) are shown. In addition to CsPbBr3 perovskitefilm, the presence of the amorphous buffer layer of SiO2between the Si substrate and the perovskite film, witha thickness of about 1.5 ^ 2 nm, is observed. It isevident that the edges of this layer are quite sharp onboth the Si and perovskite sides.SEM images of the CsPbBr3film grown according to the present invention are shown in Fig. 6. The filmsurface is smooth with the step-like features, having acharacteristic periodicity of ~ 1 μm. In addition, long,narrow and very bright elements (nanowire-like structures) are present on the film surface. Thesenanowire-like structures, which correspond to thosevisible in the cross-sectional of fig. 3b (dottedcircles), have a lateral dimension of about 100 nm anda length up to several hundreds nm.The PL spectrum (not shown) collected at ambienttemperature of the CsPbBr3 film (laser wavelength of =514 nm, and 0.01% neutral attenuation (ND) level filter) show an intense peak located at 522.9 nm, with a verynarrow FWHM = 2.0 nm, derived by a least-square fittingprocedure. Such a narrow PL peak confirms that theperovskite structure is substantially free of defects.To the best of the inventors’ knowledge, in the state ofthe art no similar sharpness of the PL peak has been observed for this type of perovskites.

Claims

CLAIMS 1. A method of preparing a film of an inorganic metal halide perovskite (IMHP) compound comprising the steps of: (a) providing a substrate having a surface layer(buffer layer) comprising an amorphous oxide of anelement selected among those of the group 14 of the IUPACperiodic table; (b) sublimating a solid precursor comprising the IMHP compound to form a vapor stream; (c) depositing said vapor stream on said buffer layerto form said film, wherein said sublimation and deposition are carriedout through molecular beam epitaxy in ultra-high vacuum(UHV).

2. The method according to claim 1, wherein said step (a) comprises: (a1) providing a substrate made of an elementselected among those of the group 14 of the IUPACperiodic table, said element being in its elemental state; (a2) oxidizing at least one surface of said substrateto obtain said buffer layer comprising said amorphous oxide of said element.

3. The method according to any one of claims 1 or 2,wherein said amorphous oxide is an amorphous oxide of an element selected from: Si, Ge, Sn, and Pb, preferably Si.

4. The method according to any one of claims 2 to 3, wherein said element in its elemental state of saidsubstrate is selected from: Si, Ge, Sn, and Pb, preferably Si.

5. The method according to any one of claims 1 to 4, wherein said IMHP compound has general formula: ABX3, wherein: -A is a monovalent cation,- B is a divalent cation different from A,- X is a monovalent anion.

6. The method according to any one of claims 1 to 5,wherein said IMHP compound has general formula ABX3, wherein: -A is selected from: Li+, Na+, K+, Rb+, Cs+,preferably Cs+; -B is selected from: Pb2+, Sn2+, Ge2+, divalenttransition metals, preferably Pb2+;- X is selected from: chloride (Cl-), bromide (Br-),iodide (I-) and combinations thereof, preferably Br-.

7. The method according to any one of claims 1 to 6, wherein said IMHP is CsPbBr3.

8. The method according to any one of claims 1 to 7, wherein said buffer layer has a thickness within the range of from 1 to 20 nm, preferably from 2 to 10 nm.

9. The method according to any one of claims 1 to 8, wherein said film of the IMPH compound has a thickness within the range of from 5 to 500 nm, preferably from 10 to 300 nm.

10. The method according to any one of claims 1 to9, wherein during the depositing step (c) said substrate is kept at a temperature within the range of from 50 to 300°C, preferably of from 100 to 200°C.

11. A semiconductor structure comprising:- a substrate;- a buffer layer overlying said substrate, saidbuffer layer comprising an amorphous oxide of an element selected among those of the group 14 of the IUPAC periodic table; -a film comprising an inorganic metal halideperovskite overlying said buffer layer.

12. The semiconductor structure according to claim 11, wherein said film is obtained by the method of any one of claims 1-10.

13. An optoelectronic device comprising thesemiconductor structure according to claim 11 or 12.

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