S.‐Tong Lee

Synthesis of diamond from methane and nitrogen mixture

We have found that diamond can be synthesized from a mixture of CH4 and N2 without adding any H2. This new synthesis is sharply different from the common practice of diamond growth by chemical vapor deposition, which uses a hydrogen‐rich mixture of CH4 and H2. In this new approach, nitrogen becomes an active component of microwave plasma leading to diamond growth. Nitrogen participates in abstraction of hydrogen from the diamond surface. We hypothesize that formation of HCN is an indication of hydrogen abstraction that allows diamond to grow from CH4+N2 mixtures. As a consequence of surface processes, the crystal structure of the grown diamond is distorted. The sequence of tetrahedral layers is mixed (cubic and hexagonal) and it suffers from turbostatic disorder. Diamond films were characterized by x‐ray diffraction, Auger electron spectroscopy, x‐ray photoelectron spectroscopy, and Raman spectroscopy.

Outdiffusion and diffusion mechanism of oxygen in silicon

The outdiffusion profiles of oxygen in Czochralski Si(111) within the temperature range 700–1160 °C and for three processing conditions (nitrogen atmosphere, steam oxidation, and phosphorus indiffusion) were measured by secondary ion mass spectrometry. The diffusivity and solubility of oxygen in Si were determined by fitting these profiles to a simple diffusion model. Oxygen diffusivity shows little or no dependence on processing conditions and can be expressed as D=0.14 exp(−2.53 eV/kT) cm2 s−1 for the temperature range studied. Our observations show that point defects in Si have little effect on oxygen diffusion and demonstrate that oxygen diffuses primarily via an interstitial mechanism. Oxygen solubility was the largest during steam oxidation.

Heteroepitaxy of carbon on copper by high-temperature ion implantation

The recently reported carbon‐ion‐implantation‐outdiffusion method [J. F. Prins and H. L. Gaigher, Mater. Res. Soc. Sym. Proc. (to be published, 1991)] of growing epitaxial diamond layers on copper was carefully examined. X‐ray diffraction, Raman scattering, and transmission electron diffraction characterization of films prepared by implanting 200 keV carbon ions into (100), (110), (111), and (210) copper, held at temperatures of 850–1000 °C, showed that the films were invariably highly oriented crystalline graphite. No evidence has been found to support the claim that diamond was formed by this implantation‐outdiffusion method.

Characterization of proton exchange lithium niobate waveguides

Proton exchanged samples of LiNbO3 have been profiled by micro‐Raman spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering channeling, and by x‐ray diffraction (XRD). Following proton exchange (PE) there are two different phases in addition to pure LiNbO3 detected by XRD. After successive annealing steps the outermost phase disappears and an interfacial region forms progressively between PE and LiNbO3. Specific vibrational bands are correlated to electro‐optic and nonlinear optical properties of the system, and the recovery of these properties upon annealing is correlated to chemical bonding changes.

Laser processing of carbon‐implanted Cu, Ni, and Co crystals: An attempt to grow diamond films

Carbon films have been prepared by laser‐pulse treatment of single crystals of copper, nickel, and cobalt, which have been previously implanted at room temperature with 50 keV carbon ions to a fluence of 1×1018 cm−2. Characterization using Raman scattering, Auger spectroscopy, and transmission electron microscopy showed that the films consisted of amorphous carbon and microcrystalline graphite but not diamond. In addition, an appreciable amount of substrate material was found present embedded in the carbon films.

Disordering of Si‐implanted GaAs‐AlGaAs superlattices by rapid thermal annealing

We have studied the disordering phenomenon in GaAs‐AlGaAs superlattices induced by Si implantation followed by rapid thermal annealing. Disordering has been detected in superlattices implanted with 220 keV Si+ at doses ≥1×1015 cm−2 and annealed at 1050 °C for 10 s. The amount of disordering saturates with time after 10 s annealing, whence the lattice damage caused by the implantation is predominantly annealed out and little Si diffusion detected. The transient disordering is attributed to defect‐induced layer intermixing occurring during the annealing of the implantation damage. The defect‐induced disordering has been simulated by solving two coupled diffusion equations for aluminum and vacancies, and good qualitative agreement with experimental results has been obtained.

Enhanced and wafer‐dependent oxygen diffusion in CZ‐Si at 500–700 °C

Oxygen diffusivity in silicon in the intermediate temperature range 500–650 °C has, for the first time, been directly measured. Oxygen diffusion in this temperature range is enhanced relative to the normal diffusion of interstitial oxygen. The degree of enhancement increases with decreasing temperature, with the enhancement factor reaching >102 at ≤550 °C. The diffusivity at 550–750 °C is found to vary among Si wafers by as much as 10×. Oxygen diffusivity generally decreases with increasing annealing time but does not vary proportionally with oxygen concentration as expected from a molecular oxygen model.

Raman scattering study of lattice disorder in 1‐MeV Si‐implanted GaAs

We have used Raman scattering to study the lattice disorder created by the implantation of 1‐MeV Si ions into GaAs. Using the change in the longitudinal optical (LO) phonon‐line position as the signature for lattice damage, combined with chemical etching for controlled layer removal, we monitored the evolution of the disorder depth profile as a function of implantation dose. The shape of the depth profile of the disorder agrees with the theoretical simulation trim for doses of 1×1014 cm−2 or lower. For higher doses a saturation is observed in the amount of residual disorder. This saturation is a manifestation of dynamic annealing occurring during the high‐energy implantations, which we attribute to enhanced defect mobility, induced by the transfer of energy to the lattice, in atomic collision cascade processes. In order to correlate the spectral features in the Raman spectra with structural changes in the ion‐implanted samples, we characterized the implantation‐induced lattice damage using ion‐channeling ...

Enhanced oxygen diffusion in silicon at thermal donor formation temperature

From the outdiffusion profiles of oxygen in Czochralski silicon at 500 °C measured with secondary ion mass spectrometry (SIMS), oxygen diffusivities of 2.5–4.0×10−14 cm2 s−1 are deduced, which are enhanced nearly four orders of magnitude relative to the normal diffusivity. Diffusion of implanted oxygen‐18 in float‐zone silicon at 400–525 °C yields SIMS profiles consisting of an exponential decay of oxygen concentration. The exponential profiles of oxygen‐18 above background (1014 cm−3) as deep as 4–16 μm also show direct evidence of enhanced long‐range oxygen diffusion. The results of enhanced oxygen diffusion can be explained with a fast‐diffusing species, which is in dynamical equilibrium with the interstitial oxygen.

Void formation and inhibition of layer intermixing in ion‐impIanted GaAs/AlGaAs superlattices

Voids have been found in the near‐surface region of GaAs/AlGaAs superlattices in a transmission electron microscopy study. The superlattices were Si‐ or Al‐implanted and subsequently either furnace or rapid thermally annealed. Concurrent with the presence of voids is an inhibition of superlattice layer intermixing enhancement in the near‐surface region. This inhibition does not occur in the deeper region of the samples where voids are not found. The voids can form via condensation of the Ga and As vacancies produced by the implantation process. We suggest that voids can depress dopant activation, suppress dopant diffusion, and inhibit the superlattice layer intermixing enhancement.