Method for producing semiconductor particles

#This invention relates to techniques for producing nanometrically-sized materials, and more particularly relates to techniques for producing nanometric crystalline semiconducting material particles.

#Semiconductor particles, and particularly semiconductor particles having a size in the nanometer regime, i.e., in the range of 1-1000 nanometers, are rapidly gaining interest for their quantum confinement effects, and notably for their luminescence properties. Nanometric particles of the semiconductor silicon are of particular interest for their capability to luminescence in the wavelength regime of visible light. Such silicon luminescence has been reported by, among others, DiMaria et al., in "Electroluminescence Studies in Silicon Dioxide Films Containing Tiny Silicon Islands," J. Appl. Phys., Vol. 56, No. 2, pp. 401-416, July 1984, and has been investigated for luminescence properties induced by both electrical and optical stimuli.

#Following this interest in semiconductor particle properties, many techniques have been proposed for producing such nanometrically-sized particles. For example, wet grinding and micromilling of a bulk semiconductor sample has been suggested by, among others, Juen et al., in "Technology and Photoluminescence of GaAs Micro- and Nanocrystallites," Superlatt. and Microstr., Vol. 11, No. 2, pp. 181-184, 1992. Here a block of semiconductor material is ground in water to produce semiconductor particulates. The minimum size of particulates produced by such a grinding technique or other mechanical attrition processes is fundamentally limited by energy considerations to a size much larger than nanometers, however, for typical semiconductor materials of interest. Indeed, the minimum particulate diameter, d.sub.m, that can be produced by such a mechanical technique is given as: ##EQU1## where Y is the yield strength of the material being mechanically processed, E is the Young's modulus of the material being mechanically processed, and R is the fracture strength of the material being processed. For a given material, particle sizes less than the value of d.sub.m are plastically deformed, rather than cracked apart, by a mechanical attrition process like grinding. As a result, particles of typical semiconductor materials, such as silicon, can be mechanically ground to an average size that is no less than on the order of 1 .mu.m this size is larger than the nanometric semiconductor particle size now widely desired.

#FIG. 1 is a schematic drawing of an anodic cell for producing semiconductor particles in accordance with the invention.

#Referring to FIG. 1, there is shown an apparatus for processing bulk semiconductor material to produce nanometrically-sized semiconductor particles in accordance with the invention. In overview, the apparatus is employed as an electrochemical, i.e., anodic, cell 10 in which a bulk semiconductor starting material, e.g., a semiconductor wafer 12, and a conducting material piece 14 are employed as the anode and cathode of the cell, respectively. A power supply 15 is connected between the semiconductor wafer 12 and the conducting piece 14 to bias the semiconductor wafer 12 to a desired bias voltage with respect to the conducting piece 14 such that the wafer is anodically etched.

#In operation, in brief, the semiconductor wafer and conducting material piece are both immersed, or at least partially submerged, in an electrolytic solution 16 consisting of an etchant and possibly a solvent, as is conventional for anodic etch systems, and further including a surfactant that is attracted to the semiconductor material. Under appropriate electrochemical cell conditions, given in detail below, the semiconductor bulk material 12 is anodically oxidized and etched at the wafer surface 18. During this anodic etch process, nanoparticles 20 of the semiconductor material are produced. The surfactant is electrically and/or chemically attracted to the nanoparticles and as a result, the surfactant effectively encapsulates each nanoparticle with a surface layer 22 of the surfactant in the electrolytic solution. Such encapsulated nanoparticles do not further etch and do not agglomerate in the solution. The nanoparticles can be separated out of the solution using conventional techniques. The size of the nanoparticles produced in accordance with the invention is controlled insitu by the etching process parameters. This etching process produces a relatively large volume of nanoparticles from a given bulk semiconductor and can do so in a relatively short time.

#Considering now the electrochemical cell set up in more detail, the bulk semiconductor feed material can consist of a wafer 12, e.g., a conventional semiconductor wafer such as a 4-inch wafer. The bulk semiconductor feed material can alternatively consist of a piece of a wafer or a chunk, e.g., a cube or block, of bulk semiconductor material. This semiconductor feed material can be crystalline, polycrystalline, or amorphous. The semiconductor feed material can also be provided as a crystalline epitaxial layer of the semiconductor of interest. Similarly, the semiconductor feed material can be provided as a polycrystalline layer of the semiconductor of interest. Suitable example epitaxial and deposition processes for producing such crystalline and polycrystalline layers are described by Ghandhi in "VLSI Fabrication Principles," John Wiley & Sons, Inc., New York, 1983. In general, there are no limitations to the geometry of a piece of bulk feed semiconducting material to be processed in accordance with the invention save that the piece correspond to the type of semiconductor particles desired and that the piece is of sufficient size to make adequate electrical contact to the piece. The bulk feed material can consist of a piece of e.g., silicon, germanium, silicon carbide, or other semiconductor, can be n-type, p-type, or compensated, and can be amorphous, polycrystalline, or single crystalline of any crystallographic orientation.