J.G. Zhu

Controlling the size, structure and orientation of semiconductor nanocrystals using metastable phase recrystallization

Materials engineering at the nanometre scale should provide smaller technological devices than are currently available,. In particular, research on semiconductor nanostructures with size-dependent optical and electronic properties is motivated by potential applications which include quantum-dot lasers and high-speed nonlinear optical switches,. Here we describe an approach for controlling the size, orientation and lattice structure of semiconductor nanocrystals embedded in a transparent matrix. We form nanocrystalline precipitates by implanting ions of the semiconductor into a single-crystal alumina substrate and applying thermal annealing. Control over the microstructure of the nanocrystals is achieved using substrate amorphization and recrystallization. In essence, the substrate microstructure is manipulated using ion beams to induce changes in impurity solubility, crystal symmetry and cation bonding, which exert a profound influence on the microstructure of the embedded precipitates—a concept familiar in metallurgy. This approach can be extended to exercise control over virtually any type of precipitate (such as metals, insulators or magnetic clusters) as well as epitaxial thin films.

GaAs nanocrystals formed by sequential ion implantation

Sequential ion implantation of As and Ga into SiO2 and α‐Al2O3 followed by thermal annealing has been used to form zinc‐blende GaAs nanocrystals in these two matrices. In SiO2, the nanocrystals are nearly spherical and randomly oriented, with diameters less than 15 nm. In Al2O3, the nanocrystals are three dimensionally aligned with respect to the crystal lattice. Infrared reflectance measurements show evidence for surface phonon modes in the GaAs nanocrystals in these matrices.

Electronic and vibrational spectra of InP quantum dots formed by sequential ion implantation

Sequential ion implantation of indium and phosphorus into silica combined with controlled thermal annealing has proven to be a novel and effective technique to fabricate InP quantum dots in dielectric hosts. This technique has been applied to synthesize other III–V and II–VI quantum dots. Due to the unique bimodal distribution of the indium in a silica host and a stoichiometric mismatch between the indium and phosphorus concentration profiles, it is believed that two different‐sized InP quantum dots were fabricated. More importantly, we have shown that the infrared reflectance technique is a very effective method to identify the species in the dielectric host and is also a powerful tool to investigate surface phonon modes.

Atomic force microscopy, electronic and vibrational spectroscopy of Au colloids formed by ion implantation in muscovite mica

Abstract Au was implanted into the (001) surface of muscovite mica at an energy of 1.1 MeV and at doses of 1, 3, 6, and 10 × 10 16 ions/cm 2 . Absorption spectra of the as-implanted samples revealed a peak at 2.28 eV (545 nm) which is attributed to the surface plasmon resonance of Au colloids. The infrared reflectance measurements show a decreasing reflectivity with increasing ion dose in the Si-O stretching region (900–1200 cm −1 ). A new peak is observed at 967 cm −1 , increases with the ion dose and is attributed to a Si-O dangling bond. Atomic force microscopy images of freshly cleaved samples implanted with 6 and 10 × 10 16 ions/cm 2 indicated metal colloids with diameters between 0.9–1.5 nm. AEM images of the annealed samples showed irregularly shaped structures with a topology that results from the fusion of smaller colloids.

Compound semiconductor nanocrystals formed by sequential ion implantation

Ion implantation and thermal processing have been used to synthesize compound semiconductor nanocrystals (SiGe, GaAs, and CdSe) in both SiO{sub 2} and (0001) Al{sub 2}O{sub 3}. Equal doses of each constituent are implanted sequentially at energies chosen to give an overlap of the profiles. Subsequent annealing results in precipitation and the formation of compound nanocrystals. In SiO{sub 2} substrates, nanocrystals are nearly spherical and randomly oriented. In Al{sub 2}O{sub 3}, nanocrystals exhibit strong orientation both in-plane and along the surface normal.

Optical response from the ultraviolet to the far infrared and atomic force microscopy of Au implanted in CaF2

Gold was implanted into randomly oriented single‐crystal CaF2 hosts at doses of 1, 3, 6, and 10×1016 ions/cm2. The modifications of the host resulting from the ion implantation were characterized by measuring the optical response in the 0.0062–6.70 eV range and the changes in surface topography were investigated by atomic force microscopy (AFM). The implantation depth profiles were obtained by Rutherford backscattering measurements. In the as‐implanted samples, a peak is observed at 2.33 eV in the electronic spectra which we attribute to the surface plasmon resonance of gold nanocrystals. The 1.24–0.099 eV range shows no significant difference compared to the spectra of the virgin host. However, the far‐infrared spectra show a splitting in the longitudinal optical phonon and a decrease in the reflectivity of the transverse optical phonon as the ion dose increases. The surface topography changes from one where the scratch and dig features of the virgin substrate are clearly revealed in the constant force A...

Ion beam synthesis of nanocrystals and quantum dots in optical materials

High-dose ion implantation has been used to synthesize a wide range of nanocrystals and quantum dots, and these structures can be encapsulated in a number of host materials using this technique.

Effects of ion beam mixing on the formation of SiGe nanocrystals by ion implantation

Nanocrystals of SiGe alloy have been formed inside a SiO/sub 2/ matrix by the ion implantation technique. It is demonstrated that the sequence of implantation of Si and Ge ions affects the nanocrystal formation significantly. This is explained by the ion-beam mixing effect during sequential implantation. The size distributions of the SiGe nanocrystals can also be controlled by annealing conditions.

Synthesis, Optical Properties, and Microstructure of Semiconductor Nanocrystals Formed by Ion Implantation

High-dose ion implantation, followed by annealing, has been shown to provide a versatile technique for creating semiconductor nanocrystals encapsulated in the surface region of a substrate material. The authors have successfully formed nanocrystalline precipitates from groups IV (Si, Ge, SiGe), III-V (GaAs, InAs, GaP, InP, GaN), and II-VI (CdS, CdSe, CdS{sub x}Se{sub 1{minus}x}, CdTe, ZnS, ZnSe) in fused silica, Al{sub 2}O{sub 3} and Si substrates. Representative examples will be presented in order to illustrate the synthesis, microstructure, and optical properties of the nanostructured composite systems. The optical spectra reveal blue-shifts in good agreement with theoretical estimates of size-dependent quantum-confinement energies of electrons and holes. When formed in crystalline substrates, the nanocrystal lattice structure and orientation can be reproducibly controlled by adjusting the implantation conditions.

Carbon implanted in optical grade fused silica: annealing effects in reducing and oxidizing atmospheres

Carbon is implanted into fused silica with doses of 1, 3, 6, 10X10{sup 16} ions/cm{sup 2}. Infrared spectroscopy identified the formation of CO and CO{sub 2} molecules in the implanted glasses. Relations among concentrations of carbon dioxide, carbon monoxide, and carbon doses are established by the infrared measurements. Annealing under different atmospheres have dramatic effects on CO and CO{sub 2} concentrations.