Paul F. Miceli

Optical and structural properties of ZnO films deposited on GaAs by pulsed laser deposition

ZnO films were synthesized on GaAs substrates at different growth conditions by pulse laser deposition. High-purity (99.999%) oxygen was used as the ambient gas. The pressure of the ambient oxygen gas for ZnO film growth was varied from 20 to 50 mTorr, and the growth temperature from 300 to 450 °C. ZnO films showed very strong bound exciton peaks located between 3.37 and 3.35 eV. The full width at half maximum of the bound exciton peak is less than 5 meV. These results indicate ZnO films on GaAs substrates can be used for optical devices such as light-emitting diodes. The other significant properties of textured ZnO films on GaAs substrates are described.

X-ray refractive index: a tool to determine the average composition in multilayer structures

We present a novel and simple method to determine the average composition of multilayers and superlattices by measuring the x‐ray refractive index. Since these modulated structures exhibit Bragg reflections at small angles, by using a triple axis x‐ray spectrometer we have accurately determined the peak shifts due to refraction in GaAs/AlxGa1−xAs and Nb/Ta superlattices. Knowledge of the refractive index provides the average fractional composition of the periodic structure since the refractive index is a superposition of the refractive indices of the atomic constituents. We also present a critical discussion of the method and compare the values of the average fractional composition obtained in this manner to the values obtained from the lattice parameter change in the GaAs/AlxGa1−xAs superlattices due to the Al.

Comparative study of textured and epitaxial ZnO films

Abstract ZnO films were synthesized by pulsed laser deposition (PLD) on GaAs and α-Al 2 O 3 substrates. The properties of ZnO films on GaAs and α-Al 2 O 3 have been investigated to determine the differences between epitaxial and textured ZnO films. ZnO films on GaAs show very strong emission features associated with exciton transitions as do ZnO films on α-Al 2 O 3 , while the crystalline structural qualities for ZnO films on α-Al 2 O 3 are much better than those for ZnO films on GaAs. The properties of ZnO films are studied by comparing highly oriented, textured ZnO films on GaAs with epitaxial ZnO films on α-Al 2 O 3 synthesized along the c -axis.

X‐ray scattering study of lattice relaxation in ErAs epitaxial layers on GaAs

We present the results of a high‐resolution x‐ray scattering study of the lattice relaxation in [001]ErAs epitaxial layers grown on [001]GaAs. Three thickness regimes are found. ErAs is pseudomorphic on GaAs for thicknesses below 70 A and between 70 and 300 A lattice relaxation is observed concomitant with an increase of the in‐plane mosaic due to the formation of misfit dislocations. Above 300 A, the out‐of‐plane transverse scattering from the ErAs lattice planes is no longer specular and further relaxation appears to be related to the out‐of‐plane mosaic. The ratio of elastic constants, C12/C11, is measured to be 0.126. Thin‐film interference oscillations are observed and modeled, finding that for 140 A of ErAs the interface fluctuations are ∼2.5 monolayers. ErAs/GaAs is an ideal system for x‐ray scattering studies of lattice relaxation and structure in epitaxial layers.

ZnSe and ZnO film growth by pulsed-laser deposition

ZnSe and ZnO films have been deposited on (001) GaAs substrates under different pressures by pulsed-laser deposition (PLD) with a 193 nm laser beam. The ambient pressures were changed from 8 X 10 -6 to 5 X 10 -2 Torr with high-purity argon gas for ZnSe and oxygen gas for ZnO. X-ray diffraction (XRD) measurement was performed on these samples. The FWHMs of X-ray theta-rocking curves for the (004) peaks of ZnSe films were less than 0.5°. X-ray data show that high-quality ZnO films can be also synthesized by PLD.

Specular and diffuse reflectivity from thin films containing misfit dislocations

Abstract We discuss the effects of misfit dislocations on the specular and diffuse scattering observed in X-ray reflectivity at the Bragg reflections of thin films. Using experimental results from ErAs/GaAs(001), it is shown that an interfacial displacement-difference correlation function can be used to model the scattering from misfit dislocations. We find that the diffuse scattering is correlation-length-limited for weak disorder whereas it has a rotational character for strong disorder. It is suggested that the correlation length arises from the elastic image field of a misfit dislocation.

A novel growth mode of alkane films on a SiO2 surface

Abstract Synchrotron X-ray specular scattering measurements confirm microscopically a structural model recently inferred by very-high-resolution ellipsometry of a solid dotriacontane ( n -C 32 H 66 or C32) film formed by adsorption from solution onto a SiO 2 surface. Sequentially, one or two layers adsorb on the SiO 2 surface with the long-axis of the C32 molecules oriented parallel to the interface followed by a C32 monolayer with the long-axis perpendicular to it. Finally, preferentially oriented bulk particles nucleate having two different crystal structures. This growth model differs from that found previously for shorter alkanes deposited from the vapor phase onto solid surfaces.

Orientation dependence of homoepitaxy: An in situ X-ray scattering study of Ag

Abstract We have investigated the homoepitaxial growth of Ag(111) and Ag(100) using X-ray specular reflectivity measured near the Bragg positions. At a growth temperature of 200 K, we observe that the root-mean-square roughness, σ, evolves as a power law in the deposited thickness for both crystal orientations. However, the growth kinetics of these surfaces are vastly different, leading to exponents of β = 0.5 ± 0.05 for Ag(111) and β = 0.28 ± 0.03 for Ag(100). The differences in growth are attributed to the diffusion barrier at step ledges.

Anomalous lattice expansion of metal-hydrogen thin films

We report x-ray diffraction experiments investigating the lattice expansion due to hydrogen in Nb/Ta superlattices and thin Nb films. For low hydrogen concentrations (c{sub H}{lt}0.09 H/metal), the lattice expands only normal to the plane of the film due to the epitaxial constraints imposed by the substrate. At higher concentrations, corresponding to strains on the order of the critical strains expected for the formation of misfit dislocations, we find a structural instability which leads to the formation of two metal-H phases, one of which has a lattice expansion in the plane of the film. These results differ significantly from the expected phase diagram and properties of the bulk metal-H systems.

Wetting-layer transformation for Pb nanocrystals grown on Si(111)

We present the results of in situ x-ray scattering experiments that investigate the growth of Pb nanocrystalline islands on Si(111). It is conclusively shown that the Pb nanocrystals do not reside on top of a Pb wetting layer. The nucleating Pb nanocrystals transform the highly disordered Pb wetting layer beneath the islands into well-ordered fcc Pb. The surface then consists of fcc Pb islands directly on top of the Si surface with the disordered wetting layer occupying the region between the islands. As the Pb nanocrystals coalesce at higher coverage we observe increasing disorder that is consistent with misfit strain relaxation. These results have important implications for predicting stable Pb island heights.