Doping of particulate semiconductor materials

DOPING OF PARTICULATE SEMICONDUCTOR MATERIALS

The present invention relates to a method for doping particulate semiconductor material. In particular, the present invention relates to doping of semiconductor nanoparticles and is generally applicable to doping of particles having a wide size range.

Semiconductor nanoparticles with characteristic sizes ranging from several nanometers to several hundreds of nanometers are a form of material that has been extensively studied, and their size influence determines the properties of bulk materials. In general, depending on a particular material and its application, three different size-related phenomena can change the electronic, optical, thermal and mechanical properties of nanoparticles:

One. different structures and compositions compared to known bulk phases;

2. higher surface-to-volume ratio of particles compared to bulk phase, governing the surface state and process 0; and

3. Quantum confinement effect when the size of an object is less than or equal to the wavelength and coherence length of the fundamental excitation (electronic state, light wavelength, or phonon excitation).

In some cases, controlling the doping level of semiconductor particles, semiconductor layers, or composite semiconductor materials is a particular problem. In known processes, already doped bulk materials are used to subdivide these materials into small particle sizes. Another possibility is to incorporate dopant atoms into the nanomaterial during the bottom-up synthesis of nanoscale clusters. In all cases, although the doping type (n or p) is generally maintained on the nanoscale, the electrical properties of the particles, and their compositions, can differ significantly from the bulk prototype and can be difficult to control.

It is an object of the present invention to provide an alternative method for doping particulate semiconductor particles.

<b><u>Summary of the invention</u></b>

According to a first aspect of the present invention, there is provided a method for doping a semiconductor material comprising mixing a quantity of particulate semiconductor material with an ionic salt or a formulation of an ionic salt.

"Doping" means changing the carrier concentration and/or type of a semiconductor material.

The ionic salt or formulation of the ionic salt may include one or more metal halides.

Alternatively, the ionic salt or formulation of the ionic salt may comprise one or more alkali halide, rare earth halide, or transition metal halide; one or more alkali metal salts; one or more rare earth salts; one or more transition metal salts; or sulfates, carbonates, nitrates or similar anionic complexes.

Ionic salts or formulations of ionic salts may include compounds comprising metal cation and anionic groups.

The method may comprise adding to the mass of particulate semiconductor material a preparation of an ionic salt with an excess of cationic species achieved by adding a base of the corresponding salt.

The cationic species may be alkali metals, rare earth metals, transition metals, or other positively charged metal ions.

For example, the salt may be sodium chloride (NaCl), and the base may be sodium hydroxide (NaOH).

As another example, the salt is magnesium chloride (MgCl<sb>2</sb>), and the base is magnesium hydroxide (Mg(OH)<sb>2</sb>) can be

The method may comprise adding to the mass of particulate semiconductor material a preparation of an ionic salt with an excess of anionic species achieved by adding an acid of the corresponding salt.

In this case, the anionic species may be a halogen, sulfate, carbonate, nitrate or other negatively charged species or complex.

For example, the salt may be sodium chloride (NaCl), and the acid may be hydrochloric acid (HCl).

As another example, the salt is magnesium chloride (MgCl<sb>2</sb>), and the acid may be hydrochloric acid (HCl).

The particulate semiconductor material may include a group IV element; binary, ternary or quaternary compound semiconductors; oxide; or a chalcogenide semiconductor material.

In a preferred embodiment, the particulate semiconductor material comprises silicon.

The particulate semiconductor material may include an intrinsic material such as intrinsic silicon.

Alternatively, the particulate semiconductor material may include an n-type material such as metallic grade silicon.

The particle size of the particulate semiconductor material may range from 1 nm to 100 μm.

Preferably, the particle size of the particulate semiconductor material is in the range from 10 nm to 1000 nm, very preferably from 50 nm to 500 nm.

In one embodiment, the particulate semiconductor material may comprise intrinsic silicon nanopowders having a nominal average particle size of 60 nm.

In another embodiment, the particulate semiconductor material may include metallic grade silicon nanopowders having an average particle size of 200 nm.

According to a second aspect of the present invention, there is provided a printable composition comprising a particulate semiconductor material doped according to a method as defined above, a binder and a solvent.

The printable composition may comprise a particulate semiconductor material doped with an ionic salt or formulation of an ionic salt prior to admixture with a binder and/or solvent.

Alternatively, the particulate semiconductor material may comprise particulate semiconductor material mixed with a binder and/or solvent prior to addition of the ionic salt or formulation of the ionic salt.

The binder may be cellulose acetate butyrate (CAB), and the solvent may be chloroform, acetone or thinner.

Alternatively, the binder may be a polyester or an autopolymerising ester (monomer) and the solvent may be an alcohol, acetone or thinner.

When the solvent is an alcohol, the solvent may be ethanol.

According to a third aspect of the present invention there is provided a semiconductor composite comprising a mixture of a binder and particulate semiconductor material doped according to the method defined above.

According to a fourth aspect of the present invention, there is provided an electrical contact comprising a substrate, first and second layers of semiconductor material deposited in contact with each other on the substrate, and respective electrical contacts consisting of the first and second layers, each The first and second layers comprise a printable composition as defined above, wherein one of the first and second layers has an n-type property and the other has a p-type property between these layers. A semiconductor device in which a pn junction is formed is provided.

1A and 1B are schematic diagrams showing the chemical mechanism of the doping process according to the present invention.

2a and 2b show NaCl and MgCl, respectively.<sb>2</sb>It is a graph showing the change of the Hall coefficient of the nanoparticle silicon ink doped with

3A and 3B are transmission electron micrographs and corresponding elemental maps of silicon nanopowders doped with 10 wt% NaCl, respectively.

4A and 4B are schematic side and top views, respectively, of a prototype diode structure according to the present invention.

FIG. 5 is a graph showing current-voltage characteristics for an example of the diode structure of FIG. 4 .

FIELD OF THE INVENTION The present invention relates generally to methods of doping particulate semiconductor materials, and more particularly to nanoparticles prepared therefrom, as well as compositions and composite materials. This is achieved by adding an ionic salt, or a formulation of a different ionic salt, as a dopant to the bulk of the particulate material or particle.

The term "salt" refers to an alkali halide such as NaCl, a rare earth halide such as MgCl<sb>2</sb>It is to be construed broadly enough to include any substance generally referred to as a salt, comprising:

The salt may be added to the semiconductor material by first dissolving it in water or other suitable solvent, mixing with the particulate semiconductor material, and then drying. Alternatively, the dopant salt may be mixed directly into the composition of the particulate semiconductor, binder and/or solvent.

In the method of the present invention, doped semiconductor particles are generally produced by doping performed on the semiconductor particles themselves and not on the bulk semiconductor material of which the particles are composed, as is usually the case in conventional semiconductor technology. In typical semiconductor materials, for example, p-type doping usually occurs by adding a trivalent atom that accepts electrons, and thus becomes anionized as it is incorporated into the crystal structure. In other situations, the opposite happens. In a chemical reaction, the same atom will be cationized by donating one or more electrons.

Because the entire particle is doped, the method of the present invention is thus suitable for all size scales from a few nanometers to tens or hundreds of microns, and for mono-element semiconductors such as silicon, binary and ternary compounds, chalcogenides and oxides. It can be applied to all particulate semiconductors composed of any suitable material comprising: Furthermore, the method of the present invention can be applied to particles in any structural phase, whether crystalline, amorphous, or a mixture of the two.

Addition of an ionic salt to a large amount of semiconductor particles causes the salt to adsorb on the surface of each particle, or, more rarely, to absorb the particles into ions, changing the free carrier concentration of the total particle, thus changing its electrical properties as well as its electrical properties. It affects the properties of composite materials containing many of these particles. The addition of ionic salts can be used to modify the carrier density as well as change the particulate semiconductor from n-type to p-type and vice versa. This puts much lower tolerances for the purity and electrical properties of the materials used to create the particles. In particular, metallic grade silicon, typically n-type, can be modified without pre-purification to create pn junctions and other similar semiconductor device structures.

The detailed mechanism of the doping process by the addition of ionic salts is still under investigation. Currently, there are two possible mechanisms shown in FIG. 1 . The simplest scenario is that ions are absorbed or adsorbed by the particles, and then the particles are neutralized. Thereafter, positively charged cations will accept electrons from the particle, resulting in overall p-type doping. Similarly, negatively charged anions will donate electrons, which in turn will lead to n-type doping.

In Figure 1a, free (positive) cations C+ are adsorbed or absorbed by semiconductor particles 10 containing a total of n electrons and p holes. The cations are neutralized by removing electrons from the semiconductor particles. The neutral species C° may remain attached to the particles 10 .

In other cases, if an ion is adsorbed, but maintains its state of charge so that the charge is conserved, the particle must change its overall charge to the opposite of that of the ion. For this to happen, the ions will have to be adsorbed onto the surface of the particle. In this case, if the anion is adsorbed, the particle will lose electrons. In a liquid environment, this will most likely happen either by neutralizing a positively charged species such as a cation, or by ionizing a neutral species in solution.

In Figure 1b, free (negative) anion A<sp>-</sp>Silver is adsorbed or absorbed by the semiconductor particle 10 containing a total of n electrons and p holes. cation C<sp>+</sp>In order to retain its charge state and remain neutral as a whole, electrons must be removed by neutralization of free cations or ionization of neutral species.

In other circumstances, doping can be controlled by buffering the salt solution with an acid corresponding to the anion, such as HCl for NaCl, or with a base corresponding to the cation, for example NaOH for NaCl. Buffering with hydrochloric acid (HCl) is a chlorine ion (Cl<sp>-</sp>), but buffering with sodium hydroxide (NaOH)<sp>+</sp>) to change the Na:Cl ratio of the solution.

The doping method described above was performed on semiconductor nanoparticulate inks prepared from commercially available silicon nanopowders with an average size of 60 nm from MTI Crystals Corp. Printed layers prepared using inks containing the Si nanopowders specified above and different inert binders are shallow n-type as shown by Hall effect measurements, even if the supplied powder is nominally intrinsic. conductivity was shown. In a third example, the powder used was milled to an average size of 200 nm from 2305 grade metallic silicon supplied by Silicon Smelters (Pty) Ltd, South Africa.

<b><u>Example</u></b><b><u> 1: p-type doping with NaCl</u></b>

In a first embodiment of doping semiconductor nanoparticles according to the present invention, sodium chloride is added to the powder in an amount to vary the doping level and doping type of the nanoparticulate ink prepared from the nominally intrinsic commercially available nanopowder described above. It was added in various proportions by weight for use. The amount of binder was kept constant at an amount corresponding to 8% by volume of the undoped composition. Autopolymerized esters were used as binders and ethanol was used as solvents.

Figure 2a shows the behavior of the Hall effect inversely proportional to the actual carrier concentration, depending on the amount of NaCl added to the mixture. At 2 wt% NaCl, the printed layer remained n-type, but had a slightly lower carrier concentration and a more negative Hall coefficient than the undoped material. When 2% to 4% NaCl was added, the material changed from n-type to p-type semiconductor. When 4% and more NaCl was added, the Hall coefficient was positive and decreased with increasing concentration. At 10%, the Hall coefficient was very low, but still showed a positive value.

From the data above, it can be deduced that the amount of NaCl added to the mixture should be in the range of approximately 0 to 10% by weight [approximately 0 to 5% by mole].

Elemental maps using 120keV scanning transmission electron microscopy (STEM) clearly show that chlorine is adsorbed to the nanoparticles and not to the surrounding matrix of binder material. 3a and 3b show elemental maps for chlorine and corresponding photomicrographs for powders doped with 10% NaCl. The position of the chlorine clearly corresponds to the position of the particle.

<b><u>Example</u></b><b><u> 2: </u></b><b><u>MgCl</u></b><b><u><sb>2</sb></u></b><b><u>p-type and n-type doping into</u></b>

In a second embodiment of doping according to the present invention, magnesium chloride is added to the powder in various proportions to the amount in order to vary the doping level and doping type of the nanoparticulate ink prepared from the nominally intrinsic commercially available nanopowders described above. It was used by adding the weight of . The amount of binder was kept constant at an amount corresponding to 8% by volume of the undoped composition.

Figure 2b shows MgCl added to the mixture.<sb>2</sb>It shows the behavior of the Hall effect that is inversely proportional to the actual carrier concentration according to the amount of . For 4% doping, it was impossible to observe the Hall voltage, suggesting that the carrier concentration is zero and the Hall coefficient is indeterminate. This suggests that the initial doping occurs by trapping or removing conductive electrons from the n-type particles.

4% to 16% by weight MgCl<sb>2</sb>For the addition of , the Hall coefficient showed a positive value and decreased with increasing concentration. 20% MgCl<sb>2</sb>, the conductivity again became n-type. However, in this case, the Hall coefficient was found to pass through zero smoothly, which can be interpreted as the actual doping type is changed by the contention between the two effects. This may be the simultaneous adsorption of anionic or cationic species, or it may be the adsorption of either ion type in two different charge states. MgCl added to the mixture<sb>2</sb>It can be seen that the amount of should be in the range of approximately 0 to 20% by weight [approximately 0 to 3.3% by mole].

<b><u>Example</u></b><b><u> 3: NaCl </u></b><b><u>doped</u></b><b><u> PN junction by metal grade silicon</u></b>

The aforementioned 2305 grade metallic silicon was milled in an orbital pulveriser for 180 minutes and used for both p and n layers in the diode structure. The average particle size obtained under these milling conditions was 200 nm with a large particle size distribution.

400 mg of powder was used in its initial state to prepare n-type ink. To prepare the p-type ink, 400 mg of the powder was mixed with a solution of NaCl in deionized water so that the proportion of NaCl was 6% by weight of dry silicon. After mixing, the solution was dried in an oven at 60° C. for 3 days to evaporate water.

From both doped and undoped powders, inks were prepared by mixing the powder with 200 μl of the same self-curing binder described above and approximately 400 μl of the same commercial lacquer thinner to adjust viscosity.

Figures 4a and 4b schematically show prototype diodes fabricated as test structures demonstrating the corrective properties of pn junctions that can be obtained using the powders described above. A diode structure was formed on a substrate 10 comprising a 100 micron polyester film. The substrate was cut from a sheet of standard Xerox prototype transparent material. A contact layer 20 having a thickness of 20 microns and a width of 7 mm was formed by stencil printing the first conductor 12 onto a substrate using Dupont 5000 silver contact material. about 11mm<sp>2</sp> and a 50 micron thick layer of p-type ink 14 was drop cast to the bottom silver contact 12 . After further drying under ambient conditions for one day, an n-type layer 16 was formed by drop casting n-type ink in the center of the p-type layer 14 to be approximately 60 microns thick and 7 mm thick.<sp>2</sp>layer was formed. After further drying, a thick top contact layer 18 was painted in the center of the p-type layer 16 to allow a contact wire 20 to be applied to the device.

Figure 5 shows the current/voltage characteristics of the device described above. The corrective behavior of the pn junction formed between the two printed silicon layers 14 and 16 is clearly seen with an onset voltage of approximately 1V.

Possible applications of semiconductor nanoparticles prepared by the method of the present invention include electroluminescent materials and dye-sensitized solar cells (DSCs); organic and inorganic semiconductor inks, printed semiconductor layers and use in printing devices. Depending on the application, single particles may be randomly dispersed in a matrix (quantum dots, OLED, DSC cells, organic semiconductor ink), arranged regularly (photonic array), or interconnecting structures ( inorganic semiconductor ink). The latter may be a fractal agglomeration of clusters of different sizes, a random network, or a close packed structure. In some applications, where single-layer or multi-layer structures are desired, size restrictions can be relaxed so that large semiconductor particles on the micron scale or even sub-millimeter scale can form the semiconductor components of composite materials or printable compositions.