N. A. El-Masry

Room temperature ferromagnetic properties of (Ga, Mn)N

Dilute magnetic semiconductor GaN with a Curie temperature above room temperature has been achieved by manganese doping. By varying the growth and annealing conditions of Mn-doped GaN we have identified Curie temperatures in the range of 228–370 K. These Mn-doped GaN films have ferromagnetic behavior with hysteresis curves showing a coercivity of 100–500 Oe. Structure characterization by x-ray diffraction and transmission electron microscopy indicated that the ferromagnetic properties are not a result of secondary magnetic phases.

PHASE SEPARATION IN INGAN GROWN BY METALORGANIC CHEMICAL VAPOR DEPOSITION

We report on phase separation in thick InGaN films with up to 50% InN grown by metalorganic chemical vapor deposition from 690 to 780 °C. InGaN films with thicknesses of 0.5 μm were analyzed by θ–2θ x-ray diffraction, transmission electron microscopy (TEM), and selected area diffraction (SAD). Single phase InGaN was obtained for the as-grown films with <28% InN. However, for films with higher than 28% InN, the samples showed a spinodally decomposed microstructure as confirmed by TEM and extra spots in SAD patterns that corresponded to multiphase InGaN.

MOCVD of Bi2Te3, Sb2Te3 and their superlattice structures for thin-film thermoelectric applications

The characteristics of metalorganic chemical vapor deposition (MOCVD) of Bi2Te3, Sb2Te3 and their superlattice structures are discussed in this paper. We have grown c-oriented films on both hexagonal sapphire and fcc GaAs substrates, with specular morphology and occasional stacking faults. Single crystallinity was confirmed by X-ray diffraction and low-energy electron diffraction (LEED). The stoichiometry (Bi:Te = 2:3, Sb:Te = 2:3) of the films were confirmed by X-ray photo-emission spectroscopy (XPS) and Rutherford back-scattering. We have also attempted to grow short-period (∼ 10 to 80 A) superlattice structures in the Bi2Te3Sb2Te3 materials system. X-ray diffraction data indicating the quality of these layered structures is presented. The advantages offered by the nature of chemical bonding in these materials, along the growth direction, for obtaining abrupt interfaces is discussed. The electrical transport properties of the MOCVD-grown p-type Bi2Te3Sb2Te3 structures and other thermoelectric properties including thermal conductivity and Seebeck coefficient are discussed. The initial results on the performance parameter known as figure-of-merit of the superlattice structures, measured parallel to the plane of the superlattice interfaces, are significantly higher than in conventional bulk materials. These initial results suggest a significant potential for MOCVD-based materials technology for high-performance, thin-film, thermoelectric refrigeration.

VIOLET/BLUE EMISSION FROM EPITAXIAL CERIUM OXIDE FILMS ON SILICON SUBSTRATES

Violet/blue photoluminescence was observed from epitaxial cerium oxide films on silicon substrates. The films were deposited on silicon (111) substrates under ultrahigh vacuum conditions using pulsed laser ablation of a cerium oxide target and treated by rapid thermal annealing in argon. High resolution transmission electron microscopy and x-ray diffraction measurements indicated the formation of a single crystal cerium oxide phase Ce6O11 different from CeO2 in the annealed films. The emission might be due to charge transfer transitions from the 4f band to the valence band of the oxide.

Atomic layer epitaxy of III‐V binary compounds

Atomic layer epitaxy (ALE) of III‐V semiconductors is reported for the first time using metalorganic and hydride sources. This is achieved by using a new growth chamber and susceptor design which incorporates a shuttering mechanism to allow successive exposure to streams of gases from the two sources. Also, most of the gaseous boundary layer is sheared off after exposure to the gas streams. GaAs and AlAs deposited by ALE are single crystal and show good optical properties.

REACTION AND REGROWTH CONTROL OF CEO2 ON SI(111) SURFACE FOR THE SILICON-ON-INSULATOR STRUCTURE

The interface structure of CeO2/Si(111) grown by laser ablation in ultrahigh vacuum was investigated by, reflection high‐energy electron diffraction (RHEED), high resolution transmission electron microscopy (HRTEM), and Auger electron spectroscopy (AES). The deposited film was single‐crystalline CeO2. However, during the deposition, a reaction between CeO2 and Si occurred at the interface, that resulted in the formation of an oxygen deficient amorphous CeO2−x and SiO2 layer. Post annealing in oxygen atmosphere caused the regrowth of crystalline CeO2 from CeO2−x and increased of the SiO2 thickness. The modified structure of CeO2/SiO2/Si is expected to be useful as a silicon‐on‐insulator structure since it maintains the desirable SiO2/Si interface followed by a single‐crystal insulating film lattice‐matched to Si.

Electrical characteristics of epitaxial CeO2 on Si(111)

Electrical properties of epitaxial CeO2 thin films on silicon (111) substrates grown in ultrahigh vacuum were studied, varying growth conditions and ex situ thermal treatments. Characterization using reflection high‐energy electron diffraction and high resolution transmission electron microscopy reveal that while the ceramic layers have a good single‐crystal structure, a dual amorphous layer of CeOx and SiO2 forms at the CeO2/Si interface. This structure has undesirable electrical properties, however, utilizing a post‐anneal in dry oxygen, the α‐CeOx layer was removed and the SiO2 amorphous layer was made thicker. This newly developed structure benefits from the SiO2/Si interface, having Dit=6×1011 cm−2, and Qf=5×1011 cm−2. The structure exhibits a high capacitance due to the large dielectric constant of CeO2, has electrical properties comparable with those of other reported gate insulators on Si, and has an epitaxial oxide lattice matched to Si.

Effect of hydrogen on the indium incorporation in InGaN epitaxial films

The InN percent in metalorganic chemical vapor deposition (MOCVD) and atomic layer epitaxy (ALE) grown InGaN was found to be significantly influenced by the amount of hydrogen flowing into the reactor. The temperature ranges for this study are 710–780 °C for MOCVD, and 650–700 °C for ALE. For a given set of growth conditions, an increase of up to 25% InN in InGaN, as determined by x-ray diffraction, can be achieved by reducing the hydrogen flow from 100 to 0 sccm. Additionally, the hydrogen produced from the decomposition of ammonia does not seem to change the InN percent in the films, indicating that the ammonia decomposition rate is less than 0.1%. The phenomenon of having hydrogen control the indium incorporation was not reported in the growth of any other III–V compound previously studied.

Low-temperature organometallic epitaxy and its application to superlattice structures in thermoelectrics

We describe a simple, yet phenomenologically very different, low-temperature modification to the conventional metal–organic chemical vapor deposition. It has been applied to the epitaxy of hexagonal-phased Bi2Te3/Sb2Te3 superlattices on zinc-blende GaAs substrates. The modification enables a two-dimensional, layer-by-layer, epitaxy instead of a three-dimensional islanded growth. Therefore, this approach is of generic importance to the epitaxy of many electronic and magnetic materials and their superlattices. High-resolution transmission electron microscopy studies indicate that the interface between the GaAs substrate and Bi2Te3 film is qualitatively defect free and that periodic structures are formed in the Bi2Te3/Sb2Te3 superlattices, with one of the individual layers as small as 10 A. Such ultra-short-period superlattices offer significantly higher carrier mobilities than their respective solid-solution alloys, apparently due to the elimination of alloy scattering and the minimal effects of random inte...

Determination of the critical layer thickness in the InGaN/GaN heterostructures

We present an approach to determine the critical layer thickness in the InxGa1−xN/GaN heterostructure based on the observed change in the photoluminescence emission as the InxGa1−xN film thickness increases. From the photoluminescence data, we identify the critical layer thickness as the thickness where a transition occurs from the strained to unstrained condition, which is accompanied by the appearance of deep level emission and a drop in band edge photoluminescence intensity. The optical data that indicate the onset of critical layer thickness, was also confirmed by the changes in InxGa1−xN surface morphology with thickness, and is consistent with x-ray diffraction measurements.