WO2004097068A2 - Polymorphous materials, method for preparing same, and device comprising same - Google Patents

Abstract

The invention concerns a polymorphous material of formula (I) SiXGeyCZXk: H, wherein: x is such that 0 = x, preferably 0 < x; y is such that 0 = y, preferably 0 < y; z is such that 0 = z, preferably 0 < z; k > 0; - x + y + z > 0; and X is selected among He, C, elements of the third column of the periodic table, such as B, Al, Ga, and In; elements of the fifth column of the periodic table, such as N, P, As, Sb; O; halides, such as F Cl, and Br; Ti; lanthanides and actinides; and mixtures thereof. Preferably, X is C. The invention also concerns a microelectronic, optoelectronic device, sensor or imager comprising said polymorphous material. The invention also concerns a method for preparing said material.

Description

 POLYMORPHIC MATERIALS, PREPARATION METHOD THEREOF, AND DEVICE COMPRISING SAME

DESCRIPTION

The invention relates to polymorphic materials, in particular, the invention relates to polymorphic alloys based on silicon-germanium-carbon. The invention also relates to a process for the preparation of these materials.

The invention also relates to devices comprising these materials.

The technical field of the invention can be generally defined as that of polymorphic materials.

Such materials can in particular find their applications in the fields of micro and optoelectronics, for example in circuits, sensors, electromechanical microsystems ("MEMS") and thin film transistors ("TFT" or "Thin -Film-Transistors ", in English), devices emitting and / or receiving electromagnetic waves, and biochips. Polymorphic silicon and polymorphic silicon-germanium alloy are until now the only polymorphic materials that have been produced.

Polymorphic silicon has made it possible to obtain significant progress in the performance of solar cells, particularly in terms of stability. This material, as described in document [1], comprises monocrystallites or aggregates in a partially ordered amorphous matrix.

However, polymorphic silicon and polymorphic silicon-germanium alloy have a limited range of physical properties, related to the intrinsic properties of these materials. For example, the electrical and optical bandwidths of polymorphic silicon do not extend within a wide range of values that it would be possible to adjust from the processing parameters. However, in many applications, it would be advantageous to be able to modulate the physical properties to obtain a gain in performance. For example, modulation of the "gap" would make it possible to obtain new devices such as photodetectors with quantum wells, metal oxide semiconductor field effect transistors ("OFSETS" or "Metal Oxide Semiconductor Field- Effect Transistor ", in English), heterojunction transistors, and semiconductor lasers.

Modulation of the constraints or the local adjustment of the diffusion coefficients, for example, would make it possible to control the final properties of a device.

More generally, the use of a palette of materials from the same family, the properties of which could be easily varied by changing the composition, for example, would open up great possibilities in terms of performance. There is therefore generally a need for polymorphic materials other than polymorphic silicon and polymorphic silicon-germanium alloys. There is still a need for polymorphic materials which exhibit a wide range of physical properties and not a very restricted range, as is the case of polymorphic silicon and polymorphic silicon-germanium alloys, in order to obtain performance gains in the field of microelectronic, and optoelectronic devices, as well as the field of sensors.

The object of the present invention is to meet, inter alia, the needs listed above. The aim of the present invention is therefore, inter alia, to provide new polymorphic materials which do not have the defects, drawbacks, limitations and disadvantages of polymorphic materials of the prior art and which solve the problems of the materials of the prior art .

This object, and others still, are achieved, in accordance with the invention by a polymorphic material of formula:

If _x GeyC _z X] - '. H (D

in which :

- x is such that 0 <x, preferably 0 <x; - y is such that 0 <y, preferably 0 <y;

- z is such that 0 <z, preferably 0 <z; - k>0;

- x + y + z> 0;

- and X is chosen from H; Hey ; VS ; the elements of the third column of the Periodic Table of the Elements, such as B, Al, Ga, and In; the elements of the fifth column of the Periodic Table of the Elements, such as N, P, As, Sb, and Bi; O; halides, such as F, Cl, and Br; Ti; lanthanides and actinides; and mixtures thereof. Preferably X is C.

Advantageously, x, y, and z are strictly positive and X is the carbon.

The polymorphic materials or alloys according to the invention can be defined in particular as materials or alloys with a polymorphic structure comprising, for example a base Si _x Ge _y : H and a fraction of atom (s) chosen from, for example H , He, C, B, Al, Ga, In, N, P, As, Sb, Bi, O, F, Cl, Br, I and the lanthanides and actinides, such as Pr, Nb, Tb, Er, Yb. The materials or alloys of the invention also include materials of type C: H (with x = y = z = 0 and k = 1).

The materials or alloys according to the invention are new and have never been described in the prior art where the only known polymorphic materials are polymorphic silicon and the polymorphic silicon-germanium alloy.

Furthermore, the preparation of the polymorphic materials or alloys according to the invention which comprise elements other than Si and Ge was absolutely not obvious and could in no case be deduced from the preparation of the polymorphic silicon-germanium alloy. Indeed, the comparison of the physical properties of Si and Ge shows that, if it were obvious to obtain polymorphic Ge if polymorphic Si was obtained, the preparation of a polymorphic alloy including other elements than Si and Ge n 'was nothing obvious and went against a number of prejudices in this area of technology. Indeed, the most stable crystalline phase under standard conditions of temperature and pressure for silicon and germanium is the diamond structure (Fd3m), and the atomic rays of Si and Ge are close (0.146 nm and 0.152 nm, respectively) . The covalent rays are also close (0.11 nm and 0.122 nm) and the electronegativities in the Pauling sense are close (1.9 and 2.01). In addition, the silicon pressure-temperature phase diagram is similar to that of germanium. This similarity in fundamental structural properties explains why the silicon-germanium alloy constitutes a model of solid substitution solution. The phase diagram of the silicon-germanium alloy [2] then has a characteristic appearance given in FIG. 1.

This diagram shows that it is possible to produce Siι- _x Ge _x alloys without any particular difficulty in the whole range of concentrations (0 <x <1). This great similarity in terms of basic properties makes it possible to affirm that if silicon is arranged in a particular microstructure, then it is possible to obtain a similar microstructure with the silicon-germanium alloy. The interatomic potentials used in the literature to calculate the properties of Si and Ge confirm this state of affairs. Consequently, if it is possible to obtain silicon of polymorphic structure, then it is possible to obtain a silicon-germanium alloy of polymorphic structure. This path has therefore been explored in the literature [3].

On the other hand, the phase diagrams involving elements other than Si and Ge show that it is not easy to obtain a solid substitution solution. Consequently, the differences in fundamental physical parameters, in structure, and in phase diagrams make it possible to affirm that it was not at all obvious to obtain the polymorphic materials or alloys according to the invention with elements other than Si and Ge.

This is clearly demonstrated by the phase diagrams and the crystallographic parameters of alloys of elements in the fourth column of the Periodic Table of the Elements, such as SiC and GeC, by the phase diagrams and the fundamental crystalline parameters of alloys d elements of the fourth with those of the fifth column, such as AsGe and AsSi, or with those of the third column, such as BSi, GeB, as well as by the phase diagrams of alloys, such as SiO, GeTi and STi . The materials of the invention go against widespread prejudices in this field of technology, based on the data mentioned above and according to which it would have been impossible to prepare polymorphic materials or alloys with elements other than Si and Ge.

According to the invention, it is demonstrated, precisely, that polymorphic materials can be prepared from elements other than Si and Ge; these materials being stable, isolable, and capable of being characterized. Therefore, the invention triumphs over a widely held prejudice in this field of technology.

The polymorphic materials of the invention provide a solution to the problems posed by the polymorphic materials of the prior art and meet all of the needs mentioned above.

In particular, the materials according to the invention have physical properties which lie within a wide range of values and which can be easily modulated, adjusted to meet the specific applications targeted.

For example, the electrical and optical bandwidths of the materials according to the invention extend over a very wide range of easily adjustable values.

Therefore, significant performance gains are obtained in devices incorporating these materials compared to devices incorporating polymorphic materials of the art. such as polymorphic silicon and polymorphic silicon-germanium alloy.

The material according to the invention contains hydrogen, the overall content (that is to say hydrogen, and hydrogen present in X) of hydrogen in the material is from 10 to 30% in atomic fraction.

The material according to the invention may contain helium, the helium content in the material is then generally from 0 to 10%, more preferably from 0 to 5% in atomic fraction.

The material according to the invention may contain carbon, the carbon content is preferably from 0 to 100%.

The material according to the invention may contain one or more elements from the third column of the periodic table, the content of element (s) in the third column of the periodic table is preferably from 0 to 5%.

This or these elements are chosen, for example, from B, Al, Ga, In.

The material according to the invention may contain one or more elements of the fifth column of the periodic table, the content of element (s) of the fifth column of the periodic table is preferably from 0 to 5%.

This or these elements are chosen, for example, from N, P, As, Sb.

The material according to the invention can contain oxygen, the oxygen content is preferably from 0 to 15%. The material according to the invention may contain one or more halide (s) preferably chosen from Cl, Br, F. The content of halide (s) is preferably from 0 to 8%. The material according to the invention may contain one or more rare earth (s), that is to say lanthanides and actinides.

This or these rare earths are for example chosen from Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Th.

Preferred rare earths are Pr, Nd, Tb, Er, Yb.

The content of rare earth (s) is preferably from 0 to 10%. The material according to the invention may contain titanium. The titanium content is preferably 0% to 5%.

The effects and advantages of incorporating the various elements listed above into the material of the invention are described below.

The invention further relates to a device comprising the polymorphic material, as described above.

This device can be, for example, a microelectronic device, an optoelectronic device or a sensor, in particular a photosensitive sensor for example in the range of visible wavelengths, that is to say from 400 to 800 nm. These sensors comprise a layer possibly composed of several sublayers of a photosensitive material converting photons into electrical charges. One or more of these layers or sub-layers can (wind) consist (s) of the polymorphic material according to the invention. These sensors can be in an isolated form or in the form of a set of sensors which together constitute an imager. Among these sensors or imagers, mention may in particular be made of sensors or imagers with PIN or PIN diodes. Whatever the type of device in which the material according to the invention is used, thanks to the intrinsic properties of the latter, the device has very significant performance gains, compared with devices which use a polymorphic material. of the prior art, such as polymorphic silicon or a polymorphic silicon-germanium alloy without the addition of other elements.

The invention further relates to a process for preparing the polymorphic material of formula (I) above by a plasma assisted chemical vapor deposition technique (“Plasma Enhanced Chemical Vapor Deposition” or PECVD), in which precursors of the constituent elements of the material are sent into a plasma created in a vacuum enclosure at a temperature less than or equal to 400 ° C, preferably from 100 to 400 ° C, more preferably from 200 to 350 ° C, subject to whereby said precursors dissociate and form the material which is generally deposited on a substrate. Preferably, the plasma is a radio frequency (RF) plasma. The pressure in the enclosure can be easily adjusted by a person skilled in the art in this field of technology.

Preferably, the substrate is chosen from glass or silicon substrates.

The invention will now be described in detail in the description which follows, made with reference to the accompanying drawings, in which:

- Figure 1 is the phase diagram of the silicon-germanium alloy; FIG. 2 is a schematic representation of the structure of a polymorphic material, such as polymorphic silicon or the polymorphic material according to the invention; - Figures 3A and 3B respectively represent a high resolution electron microscope image (HRTEM) of a polymorphic material of the invention and its spatial Fourier transform equivalent to an electron diffraction spectrum; - Figure 4 shows the spectrum of hydrogen exodiffusion of the material according to one invention; FIG. 5 represents curves of the infrared absorption spectrum of the material according to the invention.

The invention relates to polymorphic materials or alloys.

A polymorphic material or alloy, as in the case of polymorphic silicon or of an alloy or polymorphic material according to the invention can be defined as having a low density of state of defects in the middle of the forbidden band and a product mobility of the carriers by high lifetime of said carriers [4].

In what follows, we indicate how it may be possible on examination to recognize a polymorphic material from this same amorphous material.

The structure of a polymorphic material such as polymorphic silicon or the material according to the invention is shown diagrammatically in FIG. 2.

The polymorphic material comprises a partially ordered matrix 100 in which nanoaggregates or nanocrystallites 101, 102, etc., n, n being greater than 102, represented by black spots of variable shape and size, are incorporated. By nanocrystallites or nanoaggregates is generally meant crystallites or aggregates which have a nanometric size, that is to say preferably less than or equal to 20 nm, more preferably less than or equal to 5 nm. Microscopy measurements make it possible to show that the matrix containing the nanocrystals presents an order at medium distance, between the second and the sixth atomic neighbor. The nanostructure of the polymorphic material can also be characterized in particular by infrared absorption, microscopy, in particular high resolution transmission electronic microscopy, or Raman spectroscopy.

The nanostructure can also be characterized by a spectrum of hydrogen hexodiffusion clearly distinct from that of the amorphous material. To distinguish a polymorphic material from the amorphous material of the same composition, we can therefore examine the microstructure of the material by high resolution transmission electron microscopy. In Figure 3, there is shown schematically the images of a polymorphic material according to one invention, obtained by electron microscopy in high resolution transmission.

The observation of these images shows that in the polymorphic material according to the invention, one can distinguish crystals of a few nanometers in diameter. These nanocrystals appear in the photograph as regions in which we notice parallel lines between them. Therefore, these nanocrystals have been represented in FIG. 3A by regions within which appear dotted lines parallel to each other. In other words, in the polymorphic material according to the invention, a distinction is made between nanocrystals which appear as a periodic network of tasks, characteristic of a periodic arrangement of atoms (crystal).

It is not excluded that this crystal is imperfect, it may for example have gaps, stacking faults or males, that is to say defects usually encountered in crystals. In the least ordered cases, nanocrystals are replaced by nanoaggregates. Between these regions is a partially ordered matrix. The spatial Fourier transform or the electron diffraction spectrum of the image of the. partially ordered area of Figure 3A is shown in Figure 3B and gives characteristic concentric circles. The spatial Fourier transform makes it possible to highlight an order at medium distance, which materializes by the presence of rings surrounding a central axis. In the case of standard amorphous material, one can distinguish two rings and possibly very planarly a third ring. For a polymorphic material according to the invention, one succeeds in distinguishing four or more rings. This fact is clear from Figure 3B, where there are white and black rings.

In addition, these rings have a stronger intensity and a thinner width than in the case of a standard amorphous material. This means that the atoms are distributed more periodically than in an amorphous structure.

The details of this analysis are presented in reference [5], in the case of polymorphic silicon and the principle is the same for any other polymorphic material and in particular for the polymorphic material according to the invention.

Likewise, the peaks obtained in X-ray diffraction or in Raman spectrometry are narrower in polymorphic structure than in amorphous structure: this still translates a more important arrangement.

Another means of distinguishing the polymorphic nature of the material according to the invention consists in producing a spectrum of hydrogen exodiffusion for this material as a function of the temperature, as shown in FIG. 4. This hydrogen exodiffusion spectrum is defined by the curves representing the partial pressure of hydrogen in millibars (PH2) _r as a function of the temperature of the material in ° C. The manner of producing such spectra is well known to a person skilled in the art in this field of technology.

In a simplified manner, it can be indicated that the partial pressure of hydrogen leaving the material is measured as a function of the annealing temperature. The hydrogen is bound to the material in different atomic configurations, each of which has a different binding energy. Each connection configuration therefore corresponds to a hydrogen release curve as a function of temperature, in the form of a bell curve having a peak. The spectrum of standard amorphous silicon has the shape represented by the curve a. It has only a peak between 500 and 600 ° C, associated with the hydrogen uniformly distributed in the amorphous matrix. Curves b, c, d and e respectively represent hydrogen release curves each corresponding to a specific hydrogen bonding configuration. When the measurement of the exodiffusion of the polymorphic material according to the invention is obtained, the curve f which corresponds to the result of the different hydrogen bonding configurations which exist in the polymorphic material. The shape of the curve f thus characterizes the incorporation of hydrogen on the surface of aggregates and nanocrystals and in the matrix presenting an order at medium distance. Compared to the exodiffusion spectrum of standard amorphous silicon (curve a), it can be seen that the spectrum of the polymorphic material according to the invention comprises additional peaks between 350 ° C. and 450 ° C. (in particular 400 ° C.) which correspond to hydrogen bonds in the material, characteristics of the polymorphic nanostructure. This “signature” therefore constitutes a complementary means of identifying the polymorphic material of the invention.

Another means of recognizing the polymorphic material according to the invention with respect to amorphous silicon is infrared spectroscopy. FIG. 5 represents the infrared absorption spectrum in the zone whose wave number expressed in cm ^"1 is between 1900 and 2200. The absorption, in arbitrary unit, is plotted on the ordinate and the wave number is plotted on the abscissa. The curve d represents the experimental result of the absorption measurement. The curves a, b, c represent respectively, the curves obtained by a deconvolution calculation which can be done since the different elementary absorption peaks This deconvolution of the experimental spectrum highlights for the polymorphic material the presence of an additional peak p between 2030 and 2050 cm ^-1 compared to the amorphous silicon. This peak corresponds to the curve b and translates a specific bond of l hydrogen in a polymorphic structure The position of the peak depends on the conditions of production of the polymorphic material. The polymorphic material according to the invention which can be characterized by one or more of the means described above corresponds to the formula (I) given above:

SiχGeC _z X _κ : H

where x, y, z, k and X have already been defined above. Depending on the additional element (s) X incorporated into the material according to the invention, and their proportions, and whether or not carbon is incorporated, particular advantages and microstructures can be obtained.

Incorporation of hydrogen

Polymorphic silicon already has a significant hydrogen content which is typically of the order of 16%. The modification of the hydrogen fraction modifies the optical "gap". Hydrogen passive dangling bonds and therefore plays an essential role in the electrical properties and stability of the material. An excess of hydrogen, for example greater than 40%, can contribute to making the material porous or mechanically unstable, and therefore the optimum of the overall hydrogen content is generally in the range of 10% to 30% d hydrogen in atomic fraction. Incorporation of helium

The incorporation of helium, generally at a content of between 0 and 10% atomic fraction, during deposition for example by PECVD makes it possible to improve the density of the material, to decrease the density of states of the devices and also contributes to better adhesion of the layers. Incorporation of up to 5% He can lead to a significant improvement in the compactness of the layers for a given deposition process.

Incorporation of carbon

The incorporation of carbon in polymorphic silicon, at a content ranging from 0 to 100%, brings effects which depend on the atomic arrangement in the vicinity of C.

At a very low carbon content, the atoms are preferentially distributed in the non-crystalline phase of the polymorphic material. Nanocrystallites of pure silicon are then obtained in a matrix comprising the hydrogenated carbonaceous Sic alloy with low carbon content (typically less than 5%). At higher carbon content, nanocrystallites can incorporate a few atoms of C. We then obtain nanocrystallites of Si _p C _q : H in a partially ordered phase, with carbon contents of less than 5% (q <5%, and p can be any). These materials are designated by the terms of polymorphic carbonaceous silicon.

For the highest C contents, generally close to 100%, hydrogenated amorphous carbon is obtained in the non-crystalline phase.

At high temperature, namely generally greater than 200 ° C. and high plasma energy, namely generally greater than 20 m / cm ² , it is also possible to obtain silicon carbide (SiC) crystallites in the non-crystalline phase, as shown by the molecular dynamics calculations and the presence of peaks located between 750 and 850 cm ^-1 in infrared spectrometry and in Raman spectrometry. The non-crystalline phase (matrix) is then composed of a-Si _x C _z : H. This material x is designated by the terms polymorphic silicon carbide.

Ultimately, in all cases, the polymorphic structure is preserved, but the nanocrystallites are made of Si, carbonaceous silicon (SipCq: H) or silicon carbide SiC, and the non-crystallized phase is made of hydrogenated silicon (a-Si: H), in hydrogenated carbonaceous silicon (a-Si _x C _z : H) or in hydrogenated amorphous carbon (aC: H), or in partially ordered amorphous silicon carbide (a-SiC: H).

In the case where the crystallized phase consists of diamond carbon and the non-crystalline phase is made of hydrogenated amorphous carbon, polymorphous carbon (or polymorphic diamond) is obtained. The advantage of incorporating carbon is the increased bandwidth prohibited and improving the temperature resistance. The polymorphic silicon carbide makes it possible to obtain a temperature resistance of the components up to more than 300 ° C. Polymorphic diamond makes it possible in particular to obtain a high heat conductivity and a high mechanical hardness. Monocrystallites of Si _x Ge _y : H in an Si _y C _z : H matrix constitute quantum dots which luminesce at room temperature. This material therefore makes it possible to produce LEDs or lasers for example. The size of the nanocrystallites makes it possible to choose the light emission color by quantum confinement effect.

Incorporation of elements from the third column of the periodic table

By creating states located in the forbidden band, the incorporation of elements from the third column of the periodic table allows p-type doping of the polymorphic alloys of the fourth column of the periodic table.

Said preferred elements of the third column which are incorporated are chosen from B, Al, and Ga.

For example, in the case of polymorphic silicon doped with boron, a more electrically conductive material is obtained. The optimal doping should not generally exceed 5%, otherwise the excess fault generates poor electrical properties, and in particular a significant increase in the density of deep states.

Incorporation of elements from the fifth column of the periodic table

By creating states located in the forbidden band, the incorporation of elements from the fifth column of the periodic table allows n-type doping of the polymorphic alloys of the fourth column of the periodic table.

The preferred element (s) of the fifth column of the classification which are incorporated are chosen from P, As, and Sb.

For example, in the case of polymorphic silicon doped with phosphorus, a more electrically conductive material is obtained. The optimal doping should not generally exceed 5%, otherwise the excess fault generates bad electrical properties and in particular a significant increase in the density of deep states.

Incorporation of oxygen

The incorporation of oxygen into a polymorphic material creates localized centers which generate changes in the crystal field. This modification of the crystal field creates local disturbances which modify the stresses and can make an ion optically active. For example, the Er ³⁺ ion is not optically active if it is located in a perfect octahedral cavity, but if this cavity is deformed by the vicinity of oxygen, then the erbium ion becomes optically active and luminescent. The incorporation of oxygen into polymorphic alloys therefore has the effect of creating localized states which generate local distortions of the crystal field. These constraints at the nanoscopic scale can significantly modify the electrooptical properties of materials. The oxygen fraction should not generally exceed 15% in order not to degrade the polymorphic structure.

Incorporation of halides

The incorporation of halides (Cl, Br, F, I) has the effect of reducing the average free path of the carriers in polymorphic alloys. This effect is particularly useful in fast photodetectors of the Metal-Polymorphic-Metal devices type, because the reduction in the average free path makes it possible to increase the operating speed of these devices. To preserve the polymorphic nature of the material, it is generally not necessary to incorporate more than 8% of halides. The preferred halides are F, Cl, Br.

Incorporation of rare earths

The main advantage of incorporating one or more rare earths (actinides and lanthanides) resides in the luminescence properties which result therefrom.

These rare earths are chosen from Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Th, preferably from Pr, Nd, Tb, Er, Yb. For example, the incorporation of Er ³⁺ in polymorphic silicon makes it possible to obtain photoluminescence at room temperature because the presence of nanocrystallites in the vicinity of erbium locally modifies the crystalline field and makes this ion optically active, therefore luminescent. The incorporation of rare earths in polymorphic alloys therefore opens the way to a new generation of optoelectronic compounds on silicon produced at low temperature (deposition temperature generally below 350 ° C), therefore compatible with most of the CMOS reading circuits. standards. To preserve the polymorphic structure of the material, the fraction of rare earths must not exceed 10%.

Incorporation of titanium

The incorporation of titanium at more than 0 to 5% in the polymorphic alloys according to the invention has the effect of reducing the diffusion coefficient within the material. This element therefore makes it possible for example to reduce the diffusion of hydrogen during heat treatments.

The polymorphic materials according to the invention can be prepared by any process known to man of the trade in this technical field. All the materials according to the invention however have the advantage of being able to be produced by processes operating at low temperature, that is to say generally at a temperature less than or equal to 400 ° C., preferably less than or equal to 350 ° C, which allows materials to be deposited directly on various circuits without degrading them. This low deposition temperature therefore offers great flexibility of use in many circuits and sensors. A preferred operating process at low temperature is the plasma assisted chemical vapor deposition process (PECVD or “Plasma Enhanced Chemical Vapor Deposition”, in English). This technique also offers the advantage of low cost and makes it possible to obtain homogeneous deposits over large areas, namely for example of the order of m ² , and has the reliability and robustness of an industrial technology. .

The preparation of the polymorphic alloys according to the invention by PECVD is carried out at low temperature, namely less than or equal to 400 ° C, for example at from 100 ° C to 400 ° C preferably at from 200 to 350 ° C under conditions close to the formation of powders. The range of pressures in the enclosure is very wide. Experiments have been carried out for example between 0.1 Pa and 10 ³ Pa.

In the PECVD technique, precursors of the species to be deposited, to be formed, are dissociated in the plasma and lead, on deposition, to the desired species generally on a substrate. The deposits are usually made on glass plates or in silicon. However, other materials can be envisaged, the only constraint being good adhesion of the deposit to the substrate. Materials like Al, Cu, Cr, AlCu, TiN, ... give satisfactory results. The silicon element is introduced in the form of a gas, generally silane, possibly mixed with other gases (He, H ₂ , Ar). In general, polymorphic silicon is obtained by adjusting the technological parameters of the plasma from which the decomposition is made: pressure, dilution of the gases and power of the radio frequency, which result in the formation of aggregates and nanocrystals in the plasma. We can refer to this subject in reference [5]. The idea is usually to be in the vicinity of a powder formation regime. Exploration of the total deposition pressure and the relative flow rates of the chemical species generally reveals a transition in the deposition mechanisms, linked to the appearance of a powder regime in which the dissociated species recombine within the plasma and become deposit in the layer in formation. The deposition speed then undergoes a sudden increase. Measuring the deposition rate therefore constitutes a means of monitoring the appearance of the powder formation regime. The second harmonic measurement of the plasma polarization voltage also makes it possible to control the formation of polymorphic materials.

The polymorphic material is formed from the incorporation of aggregates and nanocrystals which give it its specific properties. In general, additional doping elements are introduced in gaseous form.

The use of a source of non-ionized atoms (such as by evaporation or spraying) leads to a lower incorporation of dopants, and the pressures required to place in molecular mode are hardly compatible with the standard PECVD deposition conditions. For post-deposition doping, it is possible to use implantation doping followed by annealing at a temperature close to the deposition temperature to obtain the desired polymorphic structure. The initial implantation must be weak enough not to completely amorphize the material. Monte-Carlo implantation calculations (like the TRIM code) make it possible to predict the energies and doses of elements to be implanted so as not to completely amorphize the polymorphic material. The implantation therefore constitutes an alternative route for doping polymorphic materials. In the following, we detail only the in situ doping channels using gas sources.

Method for incorporating silicon

The silicon is preferably incorporated via silane (SiH ₄ ) or disilane (Si ₂ H ₆ ), optionally mixed with other gases (He, H2, Ar). The literature specifies the properties obtained in the case of polymorphic silicon as a function of the deposition parameters. Germanium incorporation process

Germanium is incorporated by means of germane (GeH ₄ ) [3].

Method for incorporating hydrogen

The production of polymorphic alloys is obtained by PECVD in the presence of hydrogen [3]. The element H is supplied by gas, by means of H ₂ gas injected into the radiofrequency plasma. The best electrooptical performance is obtained for H ₂ contents in the plasma greater than 35%. The best polymorphic materials have a hydrogen content of between 10 and 30%.

Helium incorporation process

The element He is brought in by gas, by means of helium gas injected into the radiofrequency plasma. The fraction of helium incorporated in the material depends on operating conditions such as partial pressure in the plasma, temperature, plasma excitation. Overall, optimal performance is obtained for He contents of less than 5%.

Method of incorporating carbon

Carbon is typically supplied by methane (CH ₄ ) or trimethylborane (B (CH ₃ ) ₃ ) injected in the plasma. There are many other organic gases which may be suitable, such as acetylene (C2H ₂ ). Methylsilane, dimethylsilane or trimethylsilane may be used to simultaneously incorporate C and Si. The use of more branched gases, such as ethane, butane or propane, creates a high probability of retaining long, branched carbon chains in the final material. The gas content chosen in the plasma obviously depends on the fraction of carbon desired in the polymorphic material. In the case of polymorphic carbonaceous silicon, the fraction of carbonaceous gas is generally from 0.1 to 50%. For carbonaceous silicon carbide, the content will be chosen to ensure a silicon to carbon ratio close to 1. The use of carrier gas already comprising Si-C bonds is the most favorable (such as methylsilane, for example).

Methods of incorporating elements from the third and fifth columns

The incorporation of boron is preferably carried out using diborane. To incorporate B and C simultaneously, it is possible to use trimethylbore or triethylbore. To incorporate B and Cl simultaneously, it is possible to use boron trichloride (BCI ₃ ). The disadvantage of this type of compound is the etching action linked to chlorine. The incorporation of aluminum is carried out by means of trimethylaluminium or triethylaluminium. Of even, the incorporation of Ga is generally carried out by means of trimethylgallium or triethylgallium.

The incorporation of As, Sb, Bi can be carried out by means of trimethylarsenic, trimethylantimony or trimethylbismuth. These species also include carbon, so the polymorphic material will also ultimately contain carbon. For the incorporation of arsenic, we therefore favor the use of arsine AsH ₄ . For the incorporation of phosphorus, we will preferentially use phosphine

(PH ₄ ). For nitrogen, we preferentially use

NH ₃ . It is also possible to use N ₂ , but the dissociation of the molecule is energetically more difficult than for NH ₃ .

Method of incorporating oxygen

The incorporation of oxygen is preferably done with the gas 0 ₂ . The O ₂ content must be low enough (ie generally less than 5%) to avoid explosive reactions with hydrogen.

Process for incorporating halides

The incorporation of Cl, Br, I and F can be carried out for example by means of elementary gases

CI2, Br ₂ , I2 and F2. Other candidates are possible, such as gases combining silicon with halides (SiCl ₄ , SiBr ₄ , SiF ₆ , ...). Method for incorporating rare earths

Rare earths (lanthanides and actinides) can be incorporated using two different methods.

Method 1

The rare earths are preferably incorporated in ionic form. An ion source is pointed at the PECVD deposition plasma in order to incorporate the species into the growing surface, without disturbing the plasma too much.

Method 2

The rare earths can be incorporated in gaseous form.

In this case, for example to incorporate Er, it is also possible to use trimethylcyclopentadienylEr or triisopropylcyclopentadienylEr.

To incorporate ytterbium, fluorinated Yb-beta-diketonate from Yb (“Yb-fluorinated beta-diketnate”) or trisisopropylcyclopentadienylYb should be used. For neodymium, trimethylcyclopentadienylNd can be used. Method for incorporating titanium

Titanium is incorporated for example in the form of TiCl. The invention will now be described with reference to the following example, given by way of illustration and not limitation.

Example

Obtaining the polymorphic material:

pm-Sio, 495Geo, 95Co, oι 'H

comprising SiGe crystallites in a partially ordered hydrogenated matrix of Sio, 9sGeo, 495Co, oι •

49.5 sccm of SiH ₄ , 49.5 sccm of GeH ₄ , 1 sccm of CH ₄ and 400 sccm of H _{2 are} sent into a radio frequency plasma (13.56 MHz) in a chamber at 200 ° C. By varying the total pressure around 1,800 mTorr, the deposition rate shows a sudden increase near this threshold. The material obtained under these conditions has a polymorphic structure. The various characterization methods presented above make it possible to identify the partially ordered matrix and the nanocrystallites containing Si and Ge. REFERENCES

[1] G. VIERA et al., J. Appl. Phys. 90, 8, 4272 (2001) [2] R. W. OLESINSKI and G. J. ABBASCHIAN,

Bull. Alloy Phase Diagrams 5 (2), Apr. 1984

[3] M. E. GUEUNIER, J. P. KLEIDER, R. BRUGGEMANN, S. LEBIB and P. ROCA i CABARROCAS, J. Appl. Phys. 92, 9, 4959 (2002) [4] R. MEAUDRE et al., J. Appl. Phys. 86, 2,

946 (1999)

[5] A. FONTCUBERTA et al. , J. No Cryst. Ground. 299-302, 284 (2002).

Claims

1. Polymorphic material of formula (I): If _x Ge _y C _z X _k : H (I) in which : - x is such that 0 <x, preferably 0 <x; - y is such that 0 <y, preferably 0 <y; - z is such that 0 <z, preferably 0 <z; - k> 0; - x + y + z> 0; - and X is chosen from He; VS ; the elements of the third column of the Periodic Table of the Elements, such as B, Al, Ga, and In; the elements of the fifth column of the Periodic Table of the Elements, such as N, P, As, Sb, and Bi; O; halides, such as F, Cl, and Br; Ti; lanthanides and actinides; and mixtures thereof. 2. Material according to claim 1, in which x, y and z are> 0 and X is carbon. 3. Material according to any one of the preceding claims, in which the overall hydrogen content is from 10 to 30% by atomic fraction. 4. Material according to any one of the preceding claims which contains helium, and in which the helium content is from 0 to 10%, preferably from 0 to 5%. 5. Material according to any one of the preceding claims which contains carbon, and in which the carbon content is from 0 to 100%. 6. Material according to any one of the preceding claims which contains one or more elements of the third column of the periodic table, and in which the content of the element (s) of the third column of the periodic table is 0 at 5 %. 7. Material according to claims 1 to 6 which contains one or more elements of the fifth column of the periodic table, and in which the content of the element (s) of the fifth column of the periodic table is from 0 to 5 %. 8. Material according to any one of the preceding claims which contains oxygen, and in which the oxygen content is from 0 to 15%. 9. Material according to any one of the preceding claims, which contains one or more halides, and in which the content of the halide (s) is from 0 to 8%. 10. Material according to any one of the preceding claims, which contains one or more lanthanides and actinides (rare earths) and in which the content of the lanthanide and actinides (rare earths) is from 0 to 10%. 11. The material as claimed in claim 10, in which the rare earth or rare earths are chosen from Ce Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu and Th, preferably from Pr, Nd, Tb, Er, and Yb. 12. Material according to any one of the preceding claims which contains titanium and in which the titanium content is from 0 to 5%. 13. Device comprising the polymorphic material according to any one of the preceding claims. 14. Device according to claim 13, which is a microelectronic device, an optoelectronic device, a sensor or an imager. 15. A process for preparing the material according to any one of claims 1 to 12, by a plasma assisted chemical vapor deposition technique (“Plasma Enhanced Chemical Vapor Deposition” or PECVD, in which we send precursors of the constituent elements of the material in a plasma created in a vacuum enclosure at a temperature less than or equal to 400 ° C, preferably from 100 to 400 ° C, more preferably from 200 to 350 ° C, whereby said precursors dissociate and form the material. 16. The method of claim 15, wherein the material is deposited on a substrate. 17. The method of claim 15 or 16, wherein the plasma is a radio frequency (RF) plasma. 18. Method according to any one of claims 14 to 17, in which the substrate is chosen from glass and silicon substrates.