S. X. Liu

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.

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.

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.

PHASE SEPARATION AND ORDERING COEXISTING IN INXGA1-XN GROWN BY METAL ORGANIC CHEMICAL VAPOR DEPOSITION

We have recently reported the occurrence of phase separation in InxGa1−xN samples with x>0.25. Theoretical studies have suggested that InxGa1−xN can phase-separate asymmetrically into a low InN% phase and an ordered high InN% phase. In this letter, we report on the existence of simultaneous phase separation and ordering of InxGa1−xN samples with x>0.25. In these samples, phase separation was detected by both transmission electron microscopy selected area diffraction (TEM-SAD) and x-ray diffraction. Ordering was detected by both imaging and TEM-SAD.

Optical band gap dependence on composition and thickness of InxGa1−xN (0<x<0.25) grown on GaN

Band gap measurements have been carried out in strained and relaxed InxGa1−xN epilayers with x<0.25. Values of x were determined from x-ray diffraction of relaxed films. The lowest energy absorption threshold, measured by transmittance, was found to occur at the same energy as the peak of the photoluminescence spectrum. Bowing parameters for both strained and relaxed films were determined to be 3.42 and 4.11 eV, respectively. The dependence of the band gap shift, ΔEg, on strain is presented.

A Model for Indium Incorporation in the Growth of InGaN Films

The development of high quality indium based III-nitride compounds is lagging behind the corresponding aluminum and gallium based compounds. Potential problems confronting the growth of epitaxial and double heterostructure InGaN will be discussed. A mass balance model is presented describing the competing reaction pathways occurring during the growth of indium containing compounds. Atomic layer epitaxy and metalorganic chemical vapor deposition grown InGaN films will be used to explain this model. Also, the growth parameters leading to the attainment of high InN percentages, reduced indium metal formation, and improved structural and optical properties of indium containing nitrides will be discussed.

Mn-prelayer effects on the epitaxial growth of MnSb on (111)B GaAs by pulsed laser deposition

Abstract Ferromagnetic thin films of MnSb have been grown epitaxially on (111)B GaAs substrate by pulsed laser deposition. The epitaxial growth was achieved by either depositing Mn/Sb multilayers or direct ablation of a Mn 50 Sb 50 alloy target. The epitaxial growth occurs with (0001) of the MnSb film parallel to (111) of GaAs, with [0111] of MnSb 4° off [200] of the GaAs substrate, as determined from selected area diffraction (SAD). The easy magnetization in these films lies in the basal plane with coercivities, which vary from 40 to 110 Oe measured by Vibrating Sample Magnetometry (VSM). A Mn prelayer was important for high quality MnSb epitaxial growth with the c -axis perpendicular to the substrate surface. Films with an Sb prelayer or without Mn as a prelayer resulted in polycrystalline growth of MnSb. The mechanism of how the Mn prelayer facilitates the MnSb epitaxial growth is discussed in terms of two models.

Stacked InGaN/AlGaN Double Heterostructures

InGaN ternary alloys can be the basis for light emission from the near UV to the red region of the electromagnetic spectrum. When InGaN/AIGaN double heterostructures emitting different colors are stacked in a single structure, simultaneous emission of different wavelengths will be achieved. If the color and the intensity of emission for each well are adjusted properly, tailored emission spectra, including white light, will be feasible. We demonstrate this concept with two wells emitting at different wavelengths that are stacked between AIGaN barrier layers. The emitted PL spectra for the stacked structure is found to be the superposition of the emission from the individual double heterostructures that were grown separately.