Grouped nanostructured units system forming a metamaterial

Abstract

This invention concerns a grouped nanostructured unit system forming a metamaterial within the silicon and the manufacturing process to arrange them therein in an optimal manner. The nanostructured units are grouped and conditioned in an optimal arrangement inside the silicon material. The process comprises the modification of the elementary crystal unit together with the stress field, the electric field and a heavy impurity doping in order to form a superlattice of nanostructured units grouped in an optimal arrangement so as to improve the efficiency of the light-to-electricity conversion by means of efficient use of the kinetic energy of hot electrons and efficient collection of all electrons generated within the converter.

Claims

1. A method for forming and conditioning an elementary superlattice structure within an all-silicon light-to-electricity converter, comprising a crystalline silicon emitter mass delimited by a PN junction and a front exposure face operatively associated therewith, so as to achieve enhanced light-to-electricity conversion properties when said elementary superlattice structure, disposed within said silicon crystalline emitter mass, is illuminated with solar light illumination, the method comprising: (1) providing the crystalline silicon emitter mass comprising a heavily n-doped crystalline silicon (c-Si) emitter mass comprising elementary crystalline silicon units (ECSUs), with a density doping between 10.sup.18 and 10.sup.21 atom cm.sup.3 up from said PN junction to said front exposure face of said converter, wherein said elementary crystalline silicon units (ECSUs) comprise a multiplicity of silicon atoms, and said heavily n-doped crystalline silicon (c-Si) emitter mass defines a part of a semi-conductor wafer, wherein said heavily n-doped crystalline silicon (c-Si) emitter mass comprises at least one buried amorphized silicon (a-Si) insertion formed at a predetermined depth within said crystalline silicon (c-Si) emitter mass, between upper and lower portions of said heavily n-doped crystalline Silicon (c-Si) emitter mass; (2) performing a first annealing by subjecting said amorphized silicon (a-Si) insertion and said heavily n-doped crystalline silicon (c-Si) emitter mass to annealing temperatures within a range of approximately 500 to approximately 550 C. in order to create dilatation forces and a built-in electric field between said amorphized silicon (a-Si) insertion and said upper and lower portions of said heavily n-doped crystalline silicon (c-Si) emitter mass, so that, on each side of said amorphized silicon insertion (a-Si), a strained crystalline silicon nanolayer (<c-Si), defined as an upper strained crystalline silicon (<c-Si>) transition wrapping nanolayer and a lower strained crystalline silicon (<c-Si>) transition wrapping nanolayer, respectively, is formed, wherein each of the upper and lower strained crystalline silicon (<c-Si>) transition wrapping nanolayers is delimited by a respective pair of nanomembranes, the respective pair of nanomembranes comprising a heterointerface crystalline silicon/amorphized silicon (<c-Si>/a-Si) nanomembrane and a heterointerface strained crystalline silicon/crystalline silicon (<c-Si>/c-Si>) nanomembrane to cover or wrap around the respective elementary crystalline silicon units (ECSUs) in each respective strained crystalline silicon (<c-Si>) transition wrapping nanolayer, so that a crystalline phase of said strained crystalline silicon (<c-Si>) transition wrapping nanolayers is reordered and said elementary crystalline silicon units (ECSUs) are settled and trapped within said strained crystalline silicon (<c-Si>) transition wrapping nanolayers, wherein said elementary crystalline silicon units (ECSUs) disposed within said upper and lower strained crystalline silicon (<c-Si>) transition wrapping nanolayers are subjected to tensile forces resulting from said dilatation forces, said built-in electric field, and said n-type doping, so as to transform said elementary crystalline silicon units (ECSUs) into elementary nanoscale units called SEGTONS and distributing said SEGTONS throughout said upper and lower strained crystalline silicon (<c-Si>) transition wrapping nanolayers, thereby forming the uniformized, silicon-modified elementary superlattice structure of a metamaterial called SEG-MATTER nanolayer; and (3) performing a second annealing by subjecting said SEGTONS to annealing temperatures in a range of from approximately 350 C. to approximately 450 C. so as to provide said SEGTONS with a permanent double negative charge state; wherein said SEGTONS are configured so that absorption of light illumination generates warm/hot electrons which exhibit high kinetic energy and which collide and interact with said SEGTONS, thereby effectively generating additional electron populations, so as to achieve enhanced light-to-electricity conversion properties.

2. The method according to claim 1, comprising: before the first annealing, implanting ions through an implantation mask to form the at least one buried amorphized silicon (a-Si) insertion at the predetermined depth within said crystalline silicon (c-Si) emitter mass.

3. The method according to claim 2, wherein: the ion implanting is performed said heavily n-doped crystalline silicon (c-Si) emitter mass close to said front face at a dose of 510.sup.14 ions cm.sup.3.

4. The method according to claim 1, wherein: each of said nanomembranes has a thickness in a range of 3 to 10 nm.

5. The method according to claim 1, wherein: said metamaterial called SEG-MATTER is organized from building blocks which are ordered within the elementary superlattice structure as a network, due to said built-in electric field that is induced by said dilatation forces at heterointerface crystalline-silicon/amorphized silicon (c-Si/a-Si) transition zones said delimiting said metamaterial called SEG-MATTER, said elementary nanoscale units called SEGTONS forming at least one nanolayer or two superposed nanolayers so as to be close to an absorption zone of energetic photons.

6. The method according to claim 1, wherein: heterointerface crystalline-silicon/amorphized silicon (c-Si/a-Si) transition zones delimiting said metamaterial called SEG-MATTER comprises divacancies having a density of about 10.sup.20 cm.sup.3.

7. The method according to claim 1, further comprising: unidirectionally extracting secondary electrons, after their collisional generation outside of said metamaterial called SEG-MATTER via a conduction band, achieved by their injection into said emitter mass so as to avoid recapture of kicked out equilibrium electrons by maintaining their inability to return to their starting points by electric screening, wherein said localized electron injection and extraction is performed across each of said nanomembranes delimiting each of said strained crystalline silicon (<c-Si>) wrapping nanolayers, in accordance with specific electron conduction through a nanoscale system.

8. The method according to claim 1, wherein: heterointerface crystalline-silicon/amorphized silicon (c-Si/a-Si) transition zones delimiting said metamaterial called SEG-MATTER comprise divacancies which are trapped by said dilatation forces within said strained crystalline silicon (<c-Si>) wrapping nanolayers.

9. The method according to claim 1, wherein: said strained crystalline silicon (<c-Si>) transition wrapping nanolayers are homogenized.

10. The method according to claim 1, further comprising: performing heavy n-type doping of said silicon crystalline emitter mass (c-Si) by low-temperature thermal diffusion at T<1000 C.; pre-conditioning a surface portion of said wafer after said diffusion process so as to allow proper control of said implanted ions; using 10-200 keV ion beam irradiation leading to said buried amorphization of said amorphized silicon (a-Si) insertion within said crystalline silicon (c-Si) emitter mass; and performing final processing of said wafer by AR coating, electronic passivation, and metallization.

11. The method according to claim 1, wherein: said at least one buried amorphized silicon (a-Si) insertion within said crystalline silicon (c-Si) emitter mass is achieved by using ion beam irradiation.

12. The method according to claim 1, wherein: said forming and conditioning of the elementary superlattice structure within said all-silicon light-to-electricity converter allows the creation of a predetermined set of electron energy levels that is useful in the low-energy generation of secondary electrons adapted to the solar spectrum.

Description

9. BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph which shows the comparison of phosphorous profiles (.sup.31P) diffused at 850 C. and measured by SIMS;

(2) FIG. 2 is a graph which shows the comparison of doubly implanted phosphorous profiles (.sup.31P), implantation energies;

(3) FIG. 3 is a schematic view in cross section TEM image of the planar multi-interface substructure;

(4) FIG. 4 is 3D BSE (Backscattered Electron Microscopy) image of the buried substructure showing three different Si phases: crystalline, amorphized and metamaterial;

(5) FIG. 5 is a general schematic view of a monolayer superlattice of an example of the ordered superlattice of segtons in their closest environment;

(6) FIG. 6 shows schematic representations of examples of buried discontinuous amorphizations;

(7) FIG. 7 is a representation which shows a comparison of the optical microscopy of an implantation through a mask with the FIB (Focused Ion Beam) microscopy of a test device showing directly a buried discontinuous substructure;

(8) FIG. 8 is a X-TEM image of an example of a buried amorphized nanostructure after a relatively long thermal treatment;

(9) FIGS. 9 and 10 are two X-TEM images of two different geometries of amorphized nanostructures buried within the crystalline Si.

(10) FIG. 9 an image with discontinuous amorphization and

(11) FIG. 10 an image with amorphized layer with local circular valley system;

(12) FIG. 11 is a schematic representation of an example of a nanomembrane system. Drawing not to scale;

(13) FIG. 12 is a cross section view of possible insertion places of the seg-matter nanolayers;

(14) FIG. 13 is a multigraph image for the comparison of stair-like theoretical orders of electron multiplication;

(15) FIG. 14 is a multigraph image for the comparison of the photovoltaic impact of the seg-matter;

(16) FIG. 15 is a curve which illustrates the additional current from photon energy;

(17) FIG. 16A is an organigram of the different steps of a process to create seg-matter nanolayers; and

(18) FIG. 16B is a continuation of FIG. 16A.

10. THE INVENTION AND ITS MANUFACTURING PROCESS

(19) The description of the parallel protection bearing on elementary units called segtons is incorporated herewith for useful further explanation if needed and by way of reference.

(20) To become useful in the light-to-electricity conversion the silicon must undergo a complex transformation, which will lead to the harmful, randomly and sparsely distributed structural defects onto the elementary units called segtons of the ordered superlattice that forms a metamaterial called seg-matter. The most important aspect concerns the nature, the density and the number of defect points or divacancies being well positioned in the converter space.

(21) The preferred method of seg-matter manufacture is based on reservoirs of divacancies that are amorphized insertions within the emitter. These reservoirs realize divacancies into a nanolayer wrapping around each amorphization during suitable processing. The nanolayer grafted with divacancies and immersed in various physical fields present at the a-Si/c-Si heterointerface is transformed into a metamaterial called the seg-matter for secondary electron generation matter.

(22) Seg-matter results from a local conditioning of the semiconductor crystal lattice by intense physical fields working together, such as stress fields, electric fields, and heaving impurity doping, whereby ordered structural defects are able to form a metamaterial nanolayer.

(23) This nanoscale transformation may be realized close or around a crystalline or amorphous hereointerface, preferentially a-Si/c-Si heterointerface, due to a suitable thermodynamic treatment.

(24) By the analogy, the arrangement of seg-matter within the host material c-Si can be compared to the location of silicon nanocrystals within the dielectric. In both cases a set of electron energy levels is artificially tuned to efficient interaction with the solar spectrum.

(25) Seg-matter has to be placed preferentially within the emitter nearby to the front face of the converter in close connection with converted light wavelength. A method of nanoscale transformation being able to fulfill several required conditions is based on a local amorphization of a previously crystalline semiconductor using an ion beam and a thermodynamic processing. This processing allows a good self-organization in an ordered superlattice due to the built-in strain field that is induced by dilatation forces at the c-Si/a-Si interface. There are, at least, two possible methods of implementation: two step processing: previous n-type doping (phosphorous) that is next followed by an ion implantation up to local or buried amorphization (P, Si), single ion implantation up to local or buried amorphization using exclusively doping ions (P).

(26) Both mentioned methods lead to locally heavily doped material and allow, by the consequence, a unipolar conduction involving simultaneously the impurity and the conduction bands. The choice depends on the fine adjustment to the converted spectrum.

(27) In the first step, the amorphization produces buried insertions with rough or rugged a-Si/c-Si heterointerfaces and small a-Si inclusions in c-Si and c-Si inclusions in a-Si. The subsequent annealing cycle, preferentially at about 500-550 C. takes the form of a solid state epitaxy and leads to clear separation of both silicon phases, crystalline and amorphized, as well as to sharp planar a-Si/c-Si heterointerfaces. The same annealing cycle leads to the creation of more or less planar-parallel nanolayers that are smoothly wrapping around each amorphized insertion. The said nanolayers of 3-5-10 nm thick are uniform, without any faults and inequalities and contains numerous and ordered segtons forming together the seg-matter which is silicon based MTM.

(28) One of the best techniques being able to assume numerous requirements concerning segton shaping is the transition of silicon phase around an amorphized-crystalline heterointerface, i.e., a controlled recrystallization of the previously amorphized crystalline material. The amorphized phase contains displaced atoms with a memory of their previous position in the crystalline network. By a suitable recrystallization in which the energy necessary to the transformation is relatively low, a part of displaced atoms stay at their new places while the phase becomes crystalline with numerous point defects. The recrystallized material has a dominant crystalline behavior but contains numerous displaced atoms that are distributed rather uniformly. The controlled recrystallization has to respect several conditions concerning the rate of recrystallization by solid state epitaxy, temperature, delay to conserve numerous displaced atoms in their new positions and to avoid additional harmful structural damages resulting, for example, from too violent atom movements.

(29) This is the only one processing that is able to satisfy all the mentioned above requirements simultaneously, namely the ion implantation and a well-controlled, subsequent or real-time, thermal treatment. This processing allows a good localization of the transformed material from the absorption viewpoint.

(30) The a-Si/c-Si heterointerface gathers numerous divacancies to the density of about 10.sup.20 cm.sup.3, which are trapped within the dilatation from tensile strain field. This tensile field plays, at least, four roles: i) reduces divacancy recombination, ii) reduces divacancy mobility allowing their trapping in a well-defined volume space, iii) allows divacancy conservation at unusually high temperatures of 500-550 C. during the device manufacture processing and iv) orders divacancy distribution into a self-ordered superlattice network.

(31) The same a-Si/c-Si heterointerface provides the seg-mater nanolayer in the well-directed built-in LH-like electric field, resulting from the transition from the lightly to the heavily doped zone, which extract just-liberated secondary electrons outside the seg-matter nanolayer.

(32) The ion implantation and subsequent processing allows fulfillment of other requirements as the permanent double negative charge state, the spatial disposition, the electron transport etc. . . . All these transformations lead to a metamaterial build from segtons. The segton double negative charge state allows a low-energy electron transition and/or release between the divacancy or segton energy level in the upper half of the indirect Si band gap and the conduction band. The local concentration of doping impurity, n-type semiconductor, has to be large enough to charge and recharge all numerous divacances. The technically useful double negative charge state has to be instantaneously renewable just after an electron emission or extraction. All mentioned above requirements transform the divacancy, which is a single physical object, into segton which is a divacancy with its suitable environment, i.e., technical object that furnishes suitable energy levels occupied by weakly bonded electrons.

(33) In general, the fabrication or manufacture and conditioning of segtons and segmatter allow simultaneously:

(34) Ion implantation allows a reorganization of atoms in the crystalline lattice and/or in the amorphous or amorphized phase; the structural transformation leading to a new material phase results from an accumulation of point defects concentrated or contained in a specific nanospace or nanolayer, for example, the density and the internal energy of the new phase is lower than of its crystalline counterpart being placed somewhere between crystalline and amorphized.
Doping profile, n-type and dense enough, may be obtained, for example, in two ways, by the diffusion of doping ions or by the suitable doping ion implantation. The specific high density of the doping profile concerns mainly the buried substructure and its near neighborhood.
Material modulation and structural transformations lead to a new phase of well-known old material, being useful in soft light-matter interactions. One can start, for example, from its crystalline phase by its local deep transformation leading to a specific modulation of atom positions in crystalline lattice. Such an atom scale processing, results, for example, in shifting some atom population from their equilibrium sites in the crystal unit cell into metastable sites that are usually unoccupied. The new metastable atom distribution has to concern a large enough atom density. The required processing has to allow a local energy deposition as, for example, the irradiation by a more or less focalized energy beam. The operation is able to shift a numerous atom population occupying a specific volume and assumes in this way a necessarily profound material modulation.
Example of material modulation leading to the creation of seg-matter by an ion irradiation being realized in several stages: 1) the initial n-type doping profile, e.g., phosphorous, by the so-called low-temperature thermal diffusion at T<1000 C. or another method such as a doping implantation in the homogeneously, lightly or moderately, p-doped wafer, e.g., pre-doped by boron, 2) pre conditioning of the wafer surface after the diffusion process to allow a good control of the implanted volume, 3) 10-200 keV ion beam irradiation leading to a buried amorphization, possible, for example, by two ways, by a self Si implantation or by P implantation, 4) constitution of excellent c-Si/a-Si heterointerfaces by the thermal annealing cycle, for example at 500 C., 5) constitution of MTM nanolayers by the thermal annealing cycle, for example at 350-450 C., 6) activation of segtons, 7) device final processing steps: AR coating, electronic passivation, metallizations.

(35) Transformations: phase transformation (ion implantation): crystalline.fwdarw.amorphized.fwdarw.crystalline with locally concentrated point defects, processed further into segtons, c-Si/a-Si interface (ion implantation): creation.fwdarw.smoothing.fwdarw.seg-matter, local homogenization of amorphized and crystalline phases (annealing cycle): dissolution of inclusions, nanomembranes delimiting seg-matter: a-Si/c-Si.fwdarw.a-Si/<c-Si>energy barrier, a high offset in the valence band and <c-Si>/a-Sichange of the electron transport mode, conditioning or processing of segtons (annealing cycle): activation of heavy doping,

(36) TABLE-US-00001 TABLE Basis of improved photoconversion Object Operation converter main goal of the nanostructuration: incorporation of new efficient conversion mechanisms into conventional rather Si converters - an efficient low-energy secondary generation introduction of appropriate facilities into the device emitter (appropriate depth and fine structure design) design of substructute(s) for a low-energy secondary generation - mapping, architecture, superposition, . . . amorphizing appropriate distribution of amorphized nanoobjects with ion suitable initial sizes: insertions/substructures providing the implantation converter in fine in reservoirs of segtons and seg-matter appropriate processing - one beam or more beams, energies, doses, angle(s) of the incidence, species, the order, looking for interactions, conditions (for example, target temperature), shaping with respect of further processing (eventual superposition of successive treatments), balance of overall processing, previous and further processing steps, specific conditions structure the cycle(s) of thermal treatment has(ve): at first place, to figure the two neighboring areas on both sides of the c-Si/a-Si heterointerface to order their extreme mismatch just after the amorphization to dissolve or restructure inclusions of opposite phases (a- Si in c-Si and c-Si in a-Si) to release the crystalline <c-Si> phase stuffed with numerous point defects and to uniform the distribution of the crystalline <c-Si> phase that has surround homogeneously the amorphized objects to homogenize the distribution of segtons inside the crystalline <c-Si> phase (it can be denser close to the <a- Si>/<c-Si> interface and rarer close to the <c-Si>/c-Si interface to well-form, smooth and stabilize c-Si/a-Si interfaces to become uniform, aligned and stabilized due to the dilatation strain (especially on the c-Si side) n-type previously on or simultaneously with amorphization doping activation of the heavy n-type doping (better ionization rate due to ion implantation) electric permanent doubly negative charge state even under light charge illumination state extremely fast dynamics of segton recharging after a secondary generation electron free vertical transport through the emitter allowing an transport optimal carrier collection advantageous weighting between the transport mechanisms of conduction and impurity bands charac- optical: intermediate characterization (for example terization reflectivity) using testifying reference samples or dedicated wafer areas (without implantation masks or with dedicated scanned paths) electronic: I(V) curves

11. EXPLANATION BY MEANS OF THE FIGURES

(37) Hereunder is a brief description of the invention with the help of the appended figures.

(38) FIG. 1 is a graph which shows the comparison of phosphorous profiles (.sup.31P) diffused at 850 C. and measured by SIMS after ion-amorphization and related processing.

(39) FIG. 2 is a graph which shows the comparison of doubly implanted phosphorous profiles (.sup.31P), implantation energies: 15 keV and from the range lying between 100-250 keV, measured by SIMS after ion-amorphization and related processing. The continuous line shows the implanted phosphorous profile (.sup.31P) resulting from the Monte-Carlo simulation SRIM code for 180 keV implantation energy.

(40) FIG. 3 is a schematic view in cross section TEM image of the planar multi-interface substructure realized by controlled recrystallization of the amorphized phase, buried by ion-implantation and post-implantation processing within a crystalline Si; details are explained in the right graph illustrating 2D nanoscale sub-structure.

(41) FIG. 4 is an image: 3D BSE (Backscattered Electron Microscopy) image of the buried substructure showing three different Si phases: crystalline, amorphized and metamaterial.

(42) FIG. 5 is a general schematic view of a monolayer superlattice of an example of the ordered superlattice of segtons with their closest environment i.e., suitable and dense distribution of segtons resulting from structural defects conditioned by physical fields and forming the seg-matter nanolayer.

(43) FIG. 6 is a schematic representation of examples of buried discontinuous amorphizations; motives realized across an implantation mask by optical microscopy. The remaining crystalline structure appears as the dark line of the width varying between 2 and 10 m from one image to another. The border band of different gray intensity at right upper corners represents the SiO.sub.2, frame layer used at this stage of processing.

(44) FIG. 7 is a representation which shows a comparison of the optical microscopy of an implantation through a mask with the FIB (Focused Ion Beam) microscopy of a test device showing directly a buried discontinuous substructure.

(45) FIG. 8 is a X-TEM image of an example of a buried amorphized nanostructure after a relatively long thermal treatment, a-Si total thickness is of about 5 nm. The good crystallinity in the recrystallised zones is well visible.

(46) FIGS. 9 and 10 are two X-TEM images of two different geometries of amorphized nanostructures buried within the crystalline Si. FIG. 9 is an image with discontinuous amorphization and FIG. 10 an image with amorphized layer with local circular valley system.

(47) FIG. 11 is a schematic representation of an example of a nanomembrane system. The thickness of the seg-matter <c-Si> nanolayer is determined on the crystalline side by the built-in strain coming from a local mechanical field from a dilatation induced by or resulting from the recrystallization cycle by solid phase epitaxy. Drawing not to scale.

(48) FIG. 12 is a cross sectional view of possible insertion places of the seg-matter nanolayers.

(49) FIG. 13 is a multigraph image for the comparison of stair-like theoretical orders of electron multiplication determined on the basis of three mechanisms: i) impact ionization of the silicon lattice, ii) exciton multiplicity with the specific energy corresponding to the silicon band gap and iii) low-energy impact ionization due to segtons.

(50) FIG. 14 is a multigraph image for the comparison of the photovoltaic impact of the seg-matter (experimental carrier generation yield, diamonds) with corresponding effects in bulk silicon and with values of quantum yield for nanocrystals with smooth and rough interfaces [D. Timmerman, J. Valenta, K. Dohnalova, W. D. A. M. de Boer, T. Gregorkiewicz, Step-like enhancement of luminescence quantum yield of silicon nanocrystals, Nature Nanotechnology 6, 710-713 (2011)].

(51) FIG. 15 is a curve which illustrates the additional current from photon energy.

(52) FIG. 16A is an organigram of the different steps of a process to create seg-matter nanolayers positioned within the light-to-electricity converter according to this invention; and

(53) FIG. 16B is a continuation of FIG. 16A.

12. DETAILED EXAMPLE

(54) A detailed example is hereunder described with the help of FIG. 12 according to the following steps. Pre-treatment of a Si wafer with the preferential (100) crystalline orientation and moderate p-type doping (bore) First operations at the rear face of the converteradditional p-type doping forming a BSF Operations at the front face of the converter: conditioning before implantation sequence: etching and masking n-type doping by diffusion from a surface doping source to create an emitter and a photogenerator internal membrane such a PN junction, impurity doping density between 10.sup.18 to 10.sup.21 atoms cm.sup.3 Formation of segtons: buried amorphization through implantation masks with P ions using pre-determined implantation energy and dose, for example from the range between 50 to 100 keV and of about 510.sup.14 ions cm.sup.3, thermal treatment cycle shaping uniformed segtons and placing-ordering them in the same conditions within the stressed nanolayer Shaping of segtons and grouping them in a nanolayer, called seg-matter: shaping of the buried amorphization with its wrapping <c-Si> nanolayers stuffed with segtons to obtain a new Si phase that represents a metamaterial called seg-matter during a solid state epitaxy annealing cycle with temperatures varying between near room temperatures and 500 C. Segton and sag-matter conditioning by specific annealing at 400-500 C. during 5-30 min leading to a good, near uniform distribution of segtons, total activation of segtons, permanent doubly negative electric charge-state, advantageous exposition of the sag-matter to the incident photons Final converter operations: electronic passivation, metallization and light trapping.