Abstract:
The invention relates to a method of doping semiconductor material. Essentially, the method comprises mixing a quantity of particulate semiconductor material with an ionic salt or a preparation of ionic salts. Preferably, the particulate semiconductor material comprises nanoparticles with a size in the range 1 nm to 100 μm. Most preferably, the particle size is in the range from 50 nm to 500 nm. Preferred semiconductor materials are intrinsic and metallurgical grade silicon. The invention extends to a printable composition comprising the doped semiconductor material as well as a binder and a solvent. The invention also extends to a semiconductor device formed from layers of the printable composition having p and n type properties.

Description:
BACKGROUND OF THE INVENTION 
     THIS invention relates to a method of doping particulate semiconductor material. In particular, the invention relates to the doping of semiconductor nanoparticles, but has general applicability to the doping of particles with a wide range of sizes. 
     Semiconductor nanoparticles, with a characteristic size of a few nanometres up to several hundred nanometres, are a widely studied type of material, in which size effects dominate over properties of the bulk material. In general, depending on the specific material and its application, three different size-related phenomena may change the electronic, optical, thermal and mechanical properties of such nanoparticles:
     1. a different structure and composition compared to the known bulk phases;   2. the higher surface to volume ratio of the particles compared to the bulk phase, which causes surface states and processes to dominate; and   3. quantum confinement effects when the size of the object is similar to or smaller than the wavelength and coherence length of a fundamental excitation (electronic state, optical wavelength or phonon excitation).   

     A specific problem concerns the control of the doping level of either the semiconducting particles, the semiconducting layer, or the composite semiconducting material, as the case may be. In known processes, use has been made of previously doped bulk material which is comminuted to a small particle size. Another possibility is to incorporate dopant atoms into the nanomaterial during bottom-up synthesis of nanoscale clusters. In all cases, even though the doping type (n or p) is normally maintained on the nanoscale, the electrical characteristics of the particles, and compositions thereof, may differ significantly from the bulk prototype, and be difficult to control. 
     It is an object of the invention to provide an alternative method of doping particulate semiconductor material. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a method of doping semiconductor material, the method comprising mixing a quantity of particulate semiconductor material with an ionic salt or a preparation of ionic salts. 
     By “doping” is meant changing the carrier concentration and/or type of the semiconductor material. 
     The ionic salt or preparation of ionic salts may comprise one or more metal halides. 
     Alternatively, the ionic salt or preparation of ionic salts comprises one or more alkali halides, rare earth halides, or transition metal halides; one or more alkali metal salts; one or more rare earth salts; one or more transition metal salts; or sulphates, carbonates, nitrates or similar anionic complexes. The ionic salt or preparation of ionic salts may comprise a compound comprising a metal cation and anion group. 
     The method may comprise the addition of a preparation of ionic salts with an excess of a cationic species, achieved by the addition of the base of the corresponding salt, to a quantity of particulate semiconductor material. 
     The cationic species may be an alkali metal, rare earth metal, transition metal or other positively charged metal ion. 
     For example, the salt may be sodium chloride (NaCl) and the base may be sodium hydroxide (NaOH). 
     By way of another example, the salt may be magnesium chloride (MgCl 2 ) and the base may be magnesium hydroxide (Mg(OH) 2 ). 
     The method may comprise the addition of a preparation of ionic salts with an excess of an anionic species, achieved by the addition of the acid of the corresponding salt, to a quantity of particulate semiconductor material. 
     In this case, the anionic species may be a halogen, sulphate, 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). 
     In another example, the salt may be magnesium chloride (MgCl 2 ) and the acid may be hydrochloric acid (HCl). 
     The particulate semiconductor material may comprise a group IV element; a binary, ternary or quaternary compound semiconductor; an oxide; or a chalcogenide semiconductor material. 
     In a preferred embodiment, the particulate semiconductor material comprises silicon. 
     The particulate semiconductor material may comprise intrinsic material such as intrinsic silicon. 
     Alternatively, the particulate semiconductor material may comprise n-type material such as metallurgical grade silicon. 
     The particulate semiconductor material may have a particle size in the range 1 nm to 100 μm. 
     Preferably, the particulate semiconductor material has a particle size in the range from 10 nm to 1000 nm, and most preferably in the range from 50 nm to 500 nm. 
     In one embodiment, the particulate semiconductor material comprises nominally intrinsic silicon nanopowder with a mean particle size of 60 nm. 
     In another embodiment, the particulate semiconductor material comprises metallurgical grade silicon nanopowder with a mean particle size of 200 nm. 
     According to a second aspect of the invention there is provided a printable composition comprising particulate semiconductor material doped according to the method defined above, a binder, and a solvent. 
     The printable composition may comprise particulate semiconductor material that has been doped with an ionic salt or a preparation of the salt prior to mixing with the binder and/or solvent. 
     Alternatively, the particulate semiconductor material may comprise particulate semiconductor material that has been mixed with the binder and/or solvent prior to addition of an ionic salt or a preparation of the salt. The binder may be cellulose acetate butyrate (CAB), and the solvent may be chloroform, acetone or thinners. 
     Alternatively, the binder may be a polyester or autopolymerising ester (monomer) and the solvent may be an alcohol, acetone or thinners. 
     In the case where the solvent is an alcohol, it may be ethanol. 
     According to a third aspect of the invention there is provided a semiconducting composite composed of an admixture of particulate semiconductor material doped according to the method defined above, and a binder. 
     According to a fourth aspect of the invention there is provided a semiconductor device comprising a substrate, first and second layers of semiconducting material deposited on the substrate in contact with one another, and respective electrical contacts made to the first and second layers, wherein each of the first and second layers comprises a printable composition as defined above, one of the first and second layers having n-type properties and the other of the first and second layers having p-type properties so that a p-n junction is defined between the layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1(   a ) and  1 ( b ) are schematic diagrams illustrating the chemical mechanism of a doping process according to the invention; 
         FIGS. 2(   a ) and  2 ( b ) are graphs illustrating changes in the Hall coefficient of nanoparticulate silicon inks doped with NaCl and MgCl 2 , respectively; 
         FIGS. 3(   a ) and  3 ( b ) are a transmission electron micrograph and a corresponding elemental map, respectively, of silicon nanopowder doped with 10% NaCl by weight; 
         FIGS. 4(   a ) and  4 ( b ) are schematic side and top views, respectively, of a prototype diode structure according to the invention; and 
         FIG. 5  is a graph showing the current-voltage characteristics of an example of the diode structure of  FIG. 4 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The invention concerns a method of doping particulate semiconductor material in general, and nanoparticles in particular, as well as compositions and composite materials made from them. This is achieved by the addition of an ionic salt, or a preparation of different ionic salts, to a quantity of the particulate material or particles as a dopant. 
     The term “salt” should be interpreted sufficiently broadly to include any material generally referred to as a salt, including an alkali halide, e.g. NaCl, a rare earth halide, e.g. MgCl 2 , or including any complex anion group, such as sulphates, nitrates, carbonates, acetates or the like. 
     The salt may be added to the semiconducting material by first dissolving it in water, or another suitable solvent, mixing with the particulate semiconductor material and then drying. Alternatively, the dopant salt may be directly mixed into a composition consisting of the particulate semiconductor, a binder and/or a solvent. 
     In the method of the invention, doped semiconducting particles are produced by doping that is effected on the semiconducting particles themselves, rather than on bulk semiconducting material, of which the particles are constituted, as would normally be the case in conventional semiconductor technology. In conventional semiconductor materials, p-type doping, for example, occurs typically by the addition of a trivalent atom which accepts an electron, thus becoming negatively ionised, because of its incorporation into the crystal structure. In other circumstances the opposite occurs. In a chemical reaction, the same atom will become positively ionised by donating one or more electrons. 
     Because the whole particle is doped, the method of the invention is therefore applicable to all particulate semiconductors, over all size scales from nanometres to tens, or hundreds, of microns, and composed of any suitable material including elemental semiconductors such as silicon, binary and tertiary compounds, chalcogenides and oxides. Also the method is applicable to particles in any structural phase, whether crystalline, amorphous, or a mixture of both. 
     The addition of an ionic salt to a quantity of semiconducting particles leads to adsorption of the salt onto the surface of each particle or, more rarely, absorption into it of an ion, which changes the free carrier concentration of the whole particle, thus influencing its electronic properties, as well as those of a composite material containing many such particles. The addition of an ionic salt can be used not only to modify the carrier density, but also to change the particulate semiconductor from n-type to p-type and vice versa. This allows much lower tolerances on the purity and electrical properties of the material used to produce the particles. In particular, metallurgical grade silicon, which is generally n-type, can be modified without prior refinement to produce p-n junctions and other similar semiconductor device structures. 
     The detailed mechanisms of the doping process caused by the addition of an ionic salt are still being investigated. At present there are two possible mechanisms, shown in  FIG. 1 . The simplest scenario is absorption or adsorption of an ion, by a particle which is then neutralised. A positively charged cation will then accept an electron from the particle, resulting in overall p-type doping. Similarly a negatively charged anion would donate an electron, leading to overall n-type doping. 
     In  FIG. 1(   a ) a free (positive) cation C+ is adsorbed or absorbed by a semiconducting particle  10  containing a total of n electrons and p holes. The cation is neutralised by removing an electron from the semiconducting particle. The neutral species C° may remain attached to the particle  10 . 
     In the second case, if the ion is adsorbed, but maintains its charge state, for charge to be conserved the particle has to change its overall charge by the opposite of that of the ion. For this to occur, the ion will most likely be adsorbed onto the surface of the particle. In this case if an anion is adsorbed, the particle will lose an electron. In a liquid environment, this will most probably occur by neutralisation of a positively charged species, such as a cation, or by the ionisation of a neutral species in solution. 
     In  FIG. 1(   b ), a free (negative) anion A −  is adsorbed or absorbed by a semiconducting particle  10  containing a total of n electrons and p holes. The cation C+ retains its charge state, and to maintain overall neutrality, an electron has to be removed by neutralisation of a free cation or ionisation of a neutral species. 
     In either situation the doping may be adjusted by buffering the salt solution with the acid corresponding to the anion, e.g. HCl in the case of NaCl, or the base corresponding to the cation, e.g. NaOH in the case of NaCl. Buffering with hydrochloric acid (HCl) adds chlorine ions (Cl − ) while buffering with sodium hydroxide (NaOH) adds sodium ions (Na + ), thus changing the Na:Cl ratio of the solution. 
     The above described method of doping was performed on a semiconducting nanoparticulate ink prepared from a commercially available silicon nanopowder, of mean size 60 nm, from MTI Crystals Corp. Printed layers, produced using an ink comprising the above specified Si nanopowder and different inert binders, all showed light n-type conductivity as seen by Hall effect measurements, although the powder as supplied is nominally intrinsic. In the third example, the powder used was milled to 200 nm mean size from 2305 grade metallurgical silicon provided by Silicon Smelters (Pty) Ltd, South Africa. 
     EXAMPLE 1 
     P-Type Doping with NaCl 
     In the first example of doping semiconductor nanoparticles according to the invention, sodium chloride was used to change the doping level and doping type of the nanoparticulate ink, produced from the nominally intrinsic commercially obtained nanopowder specified above, by its addition in various proportions by weight relative to the amount of powder. The quantity of binder was kept constant, at an amount equivalent to 8% by volume of the undoped composition. An autopolymerising ester was used as a binder, with ethanol being used as a solvent. 
       FIG. 2(   a ) shows the behaviour of the Hall coefficient, which is inversely proportional to the net carrier concentration, as a function of the amount of NaCl added to the mixture. At 2% by weight of NaCl, the printed layer remains n-type, but has a slightly lower carrier concentration, and a more negative Hall coefficient than the undoped material. Between 2% and 4% of NaCl added, the material has changed from an n-type to a p-type semiconductor. For 4% and more NaCl added, the Hall coefficient is positive, and decreases with increasing concentration. At 10% the Hall coefficient is very low, but remains positive. 
     It can be inferred from the above data 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 molar %]. 
     Elemental mapping, using a 120 keV scanning transmission electron microscope (STEM) shows clearly that chlorine is adsorbed onto the nanoparticles, and is not taken up in the surrounding matrix of binder material.  FIGS. 3(   a ) and  3 ( b ) show the elemental map for chlorine and the corresponding micrograph for a powder doped with 10% NaCl. The position of the chlorine clearly corresponds to the position of the particles. 
     EXAMPLE 2 
     p-Type and n-Type Doping with MgCl 2    
     In the second example of doping according to the invention, magnesium chloride was used to change the doping level and doping type of the nanoparticulate ink, produced from the nominally intrinsic commercially obtained nanopowder specified above, by its addition in various proportions by weight relative to the amount of powder. The quantity of binder was kept constant, at an amount equivalent to 8% by volume of the undoped composition. 
       FIG. 2(   b ) shows the behaviour of the Hall coefficient, which is inversely proportional to the net carrier concentration, as a function of the amount of MgCl 2  added to the mixture. For 4% doping, it was not possible to observe a Hall voltage, indicating that the carrier concentration is zero and the Hall coefficient is undefined. This suggests that the initial doping occurs by trapping or elimination of conduction electrons from the n-type particles. 
     For additions of MgCl 2  between 4% and 16% by weight the Hall coefficient is positive, but decreases with increasing concentration, indicating p-type conductivity. For 20% MgCl 2 , the conductivity becomes n-type again. In this case, however, it appears that the Hall coefficient passes smoothly through zero, which could be interpreted as a change in net doping type caused by competition between two effects. This could either be co-adsorption of anion or cation species, or adsorption of one type of ion in two different charge states. It can be seen that the amount of MgCl 2  added to the mixture should be in the range of approximately 0 to 20 weight % [0-3.3 molar %]. 
     EXAMPLE 3 
     P-N Junction with NaCl Doped Metallurgical Grade Silicon 
     The abovementioned 2305 grade metallurgical silicon was milled for 180 minutes in an orbital pulveriser and was used for both p and n layers in a diode structure. The average particle size obtained under these milling conditions was 200 nm, with a large size distribution. 
     400 mg of the powder was used in its initial state to produce an n-type ink. To produce the p-type ink, 400 mg of the powder was mixed with a solution of NaCl in deionised water, so that the proportion of NaCl was 6% by weight of the dry silicon. After mixing the solution was oven dried for three days at 60° C. to evaporate the water. 
     Inks were produced from both doped and undoped powders by mixing the powder with 200 μl of the same self-polymerising binder described above, and approximately 400 μl of the same commercial lacquer thinner to adjust the viscosity. 
       FIGS. 4(   a ) and  4 ( b ) show, schematically, a prototype diode fabricated as a test structure to demonstrate the rectifying properties of a p-n junction obtainable using the above described powders. The diode structure was formed on a substrate  10  comprising 100 micron polyester film. The substrate was cut from a sheet of standard Xerox photocopy transparency material. A first conductor  12  was stencil printed on the substrate using Dupont 5000 silver contact material, to define a contact layer 20 microns thick and 7 mm wide. A layer  14  of p-type ink, approximately 11 mm square and 50 microns thick, was drop cast onto the silver bottom contact  12 . After further drying for a day under ambient conditions, an n-type layer  16  was formed by drop casting n-type ink onto the centre of the p-type layer  14  to form a layer approximately 60 microns thick and 7 mm square. After further drying, a thick top contact layer  18  was painted onto the centre of the p-type layer  16  to allow a contact wire  20  to be applied to the device. 
       FIG. 5  shows the current/voltage characteristics of the device described above. The rectifying behaviour of the p-n junction formed between the two printed silicon layers  14  and  16  is clearly shown, with an onset voltage of approximately 1V. 
     Potential applications of semiconducting nanoparticles produced by the method of the invention include their use in electroluminescent materials and dye sensitised solar cells (DSC); organic and inorganic semiconductor inks, printed semiconductor layers and printed devices. Depending on the application, single particles may be dispersed randomly in a matrix (quantum dots, OLEDs, DSC cells, organic semiconducting inks), may be regularly arranged (photonic arrays), or may form an interconnecting structure (inorganic semiconducting inks). The latter may be a close packed structure, random network or a fractal agglomeration of different sized clusters. In some applications, where a single layer or multilayer structure is required, the size restriction may be relaxed so that larger semiconducting particles on the micron or even sub-millimetre scale form the semiconducting component of the composite material or printable composition.