Ion implantation and thermal processing have been used to synthesize compoundsemiconductor nanocrystals (SiGe, GaAs, and CdSe) in both SiO2 and (0001) AlO3. Equaldoses of each constituent are implanted seauentially at energies chosen to give an overlap of theprofiles, Subsequent annealing results in precipitation and the formation of compoundnanocrystals. In SiO2 substrates, nanocrystals are nearly spherical and randomly oriented. InAl2O3, nanocrystals exhibit strong orientation both in-plane and along the surface normal.
In recent years, considerable interest has been generated in nanometer size semiconductornanocrystals which have unique properties resulting from quantum confinement in systems ofreduced linear dimensions. Much of this interest occurs because of the possibility for fabricatingoptoelectronic devices and optical amplifiers. The elemental semiconductor nanocrystals Si andGe exhibit room-temperature visible-light emission at energies greater than the bandgap of thebulk semiconductor. Intense light emission is also observed in the visible region from porous Si.Compound semiconductor nanocrystals can have nearly discrete electronic states6 andexhibit very large third-order nonlinear optical properties and fast relaxation times,7 makingthem very interesting for possible applications in optical switching devices. Because of the veryused to rohesize uehcnaoc wsta ine icareriator hese inciude comeroes techno goes are einand II-Vi compound semiconductor nanocrystals in SiO2), plasma decomposition of SiH4 toform Si nanocrystals,I organometallic reaction to form compound semiconductor nanocrystals.
Ion implantation is ideally suited for the fabrication of very high densities of nanocrystals inthe near-surface region of a wide variety of materials and this method has been used to create Sior Ge nanocrystals in SiO2 We are exploring the use of ion implantation to form a widevariety of nanocrystals in several insulating materials. We have demonstrated thatimplantation can be used to synthesize Si and Ge nanocrystals in both SiO and AlO3. In thispaper, we report the synthesis of compound semiconductor nanocrystals in SiO2 and Al2Oz bysequential implantation of the individual constituents of the compound, at energies chosen togive an overlap of the profiles. If the individual constituents are insoluble in the matrix, and ifthey have a chemical affinity for each other, then precipitation and compound formation canoccur during subsequent annealing. We show results for SiGe, GaAs, and CdSe in the twosubstrates. This is the first report, to our knowledge, of the use of sequential implantation toform compound semiconductor nanoparticles. Previously, sequential implantation of metal ionswas used to form ternary metal silicides in silicon. In addition, the alloying of metal ionssequentially implanted into SiO, has been inferred from shifts in the wavelength of the surfaceplasmon resonance.
Fig1
Nanocrystals of GaAs have also been synthesized in Si02 and AlO3 by sequentialimplantation of equal doses of Ga and As, followed by thermal annealing to induce precipitationFigure 2 shows x-ray diffraction results for the case of GaAsand compound formation.nanocrystals in Si02 The 0-20 scan in Fig. 2 shows the expected scattering from amorphousSiO and a relatively strong multiple-scattering Si (200) peak from the underlying Si substrate.In addition, there are strong peaks which are identified as arising from diamond cubic GaAs.The intensities of these reflections are consistent with those for a randomly oriented powder.The width of these peaks provides information on the volume-weighted correlation length (sizeof coherently diffracting domains) which, in this case, is of order 140 A in the direction of thesurface normalTEM results from this same sample show that the GaAs nanoparticles are nearlyspherical and randomly oriented, The average size is -100 A diameter, with a few extending upto ~250 A in diameter. Infrared reflectance measurements on similar samples show a welldefined peak at 280 cm-l which we believe corresponds to the longitudinal optical mode of GaAsin these nanoparticles since it is close to that measured by others for bulk and epitaxial GaAsfilms.
Ion implantation followed by thermal processing has been used to synthesize a variety ofcompound semiconductor nanocrystals in both Si and Al203. Supersaturated solutions ofindividual constituents are created by sequential implantation at energies chosen to give anoverlap of the profiles and at doses chosen to give the desired stoichiometry. If both implantedconstituents are insoluble in the matrix, and if the constituents have a chemical affinity for eachother, then thermal annealing gives rise to precipitation and the foration of compoundsResults are demonstrated for the case of SiGe, GaAs, and CdSe in the two matrices. In SiO2, thenanocrystals are nearly spherical, randomly oriented, and have diameters ranging from 20300 A. In AlzO3, nanocrystals are strongly oriented with the matrix. It should be possible toSynthesize a very broad range of nanocrystals in a wide variety of host materials by using thesequential implantation method.
