Samuel Chen

Surface reconstruction limited mechanism of molecular‐beam epitaxial growth of AlGaAs on (111)B face

We propose a model which gives a fundamental limiting factor for the growth of epilayers on (111)B face. Our model is that the As‐trimer structure of As‐stabilized (111)B surface with the (2×2) reconstruction disturbs the incorporation of group III atoms into lattice sites. This model gives the explanation to most of the reported properties in the growth of GaAs and AlGaAs on (111)B substrates. This model has been verified by comparing the growth rate of GaAs layers grown by molecular‐beam epitaxy over mesa‐shaped substrates with (111)A and (111)B sidewalls using As4 and As2. Moreover, the cause of microtwins found in transmission electron microscopy images of AlGaAs grown on (111)B GaAs at a low Ts can be at least partly explained by this model.

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.

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.

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 ...

Microstructure of 110 K superconducting Bi-Sr-Ca-Cu-O thin films grown on (100)MgO by metallo-organic decomposition

Abstract 1-, 2-, 4- and 6-layer superconducting Bi-Sr-Ca-Cu-O (BSCCO) thin films have been prepared on (100) MgO substrate by metallo-organic decomposition. The 4- and 6-layer films reach zero resistance state at T >100 K. X-ray diffraction shows the oxide films to consist of three phases identifiable by their unique c -axis values of 24, 30 and 38 A. These films show a high degree of orientation in which the c -axis of the grains are aligned perpendicular to the substrate surface. In the 2-layer film, transmission electron microscopy indicates the oriented grain growth to be primarily either (130)BSCCO(100)MgO or (100)BSCCO (100)MgO. Ion channeling measurements using He ++ at energies ranging between 1 to 4.5 MeV, show that the polycrystalline grains in the film have a highly oriented mosaic structure, and among all the films, the 2-layer system exhibits the highest degree of alignment with (100)MgO.

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.

Microstructure of epitaxially oriented superconducting YBa2Cu3O7−x films grown on (100) MgO by metalorganic decomposition

The microstructure in epitaxially oriented thin films of YBa2Cu3O7−x grown on (100) MgO by metalorganic decomposition has been studied by transmission electron microscopy and ion channeling. The as‐prepared films consisted of single‐crystal platelets lying flat on the MgO surface. The majority of the crystallites showed perfect alignment of their c axis with the [100] axis of MgO, while some crystallites were found to have a misorientation of up to 7.5°. Images of the interfacial regions showed good epitaxial growth to within one lattice spacing of the MgO substrate. He++ channeling measurements as a function of energy from 1 to 4.5 MeV indicated a 0.51° spread in crystallite orientation. Extrapolation of the channeling measurements to the limit of zero crystallite spread gave a minimum yield of 0.20 for bulk YBa2Cu3O7−x , which is much larger than the value reported for single crystals. The large backscattering yield is attributed to the grain boundaries in the film. A relatively strain‐free interface wa...

Layer intermixing in 1 MeV implanted GaAs/AlGaAs superlattices

Layer intermixing in MeV Si‐implanted GaAs/AlGaAs superlattices (SLs) with doses between 3×1015 and 1×1016 /cm2 has been examined by transmission electron microscopy and secondary‐ion mass spectrometry. After either rapid thermal annealing at 1050 °C for 10 s or furnace annealing at 850 °C for 3 h, all the SLs showed a highly crystalline, defect‐free zone in the near‐surface region followed by a band of secondary defects, with the maximum density located about 1 μm below the surface. A totally mixed region, within the secondary defect band, occurred only in the SL implanted to 1×1016 Si/cm2 and annealed at 850 °C for 3 h. At lower doses or under rapid thermal annealing, only slight Al/Ga interdiffusion was observed, primarily in the layers that contained the high density of dislocation defects. For either annealing condition, the Si concentration profiles showed only slight broadening and they correlated with the distribution of secondary defects as well as with the depth of the intermixed layer. The effe...

Annealing of Si‐implanted GaAs studied using variable‐energy positrons

Modification of GaAs by Si+‐ion implantation is an important process for selective doping of the material. Defects caused by the implantation process often lead to incomplete electrical activation, and annealing procedures are used to recover the crystal quality. Results are presented of variable‐energy positron (VEP) and cross‐sectional transmission electron microscopy (XTEM) studies of a series of GaAs samples implanted with moderate to high fluences of 3×1013, 3×1014, and 1×1015 Si+ ions cm−2. Samples were irradiated at room temperature, and studied both before and after thermal annealing for one hour at 850 °C. In all cases XTEM results show a high density of small extrinsic dislocations after implantation, and VEP shows high concentrations of point (vacancy type) defects. Annealing leads to a decrease in the point‐defect concentration in the lowest‐fluence sample, but both XTEM and VEP confirm the formation of macroscopic (i.e., ≳20 A diameter) voids following annealing. These data are discussed in t...