Structural characterization of epitaxial cobalt-chromium superlattices.

A series of epitaxial Co-Cr superlattices has been grown by molecular beam epitaxy and characterized with x-ray diffraction and in-situ RHEED. Using some x-ray techniques newly developed in my laboratory I have been able to provide complete descriptions of both the growth process and an analysis of epitaxial phase transitions. The superlattices are epitaxially ordered such that the in-plane crystallographic orientations of the individual layers are preserved during the growth process as indicated by the RHEED studies. This epitaxial ordering is confirmed (for Cr layers up to 6A) by in-plane x-ray scattering measurements. We have accomplished this work with only standard laboratory x-ray equipment. While synchrotron radiation offers several advantages, e.g., for higher resolution measurements, the techniques developed here are convenient for a laboratory setting which combines growth and structural characterization. X-ray scans along 101l are used to distinguish hcp and fcc stacking sequences in both Co and Cr layers. We find that the Co layers and the first 2-3 monolayers of Cr are stacked according to an hcp sequence. The data clearly show that we were able to synthesize a new close-packed metastable phase of Cr up to a layer thickness of 6A, providing an example of epitaxial stabilization of a structure which does not occur in the bulk. In addition, we showed that beyond this thickness there is an abrupt structural transition to bcc (110) Cr in the classical Nishiyama-Wasserman and Kurdjumov-Sachs orientations.Ph.D.PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103573/1/9332050.pdfDescription of 9332050.pdf : Restricted to UM users only

GROWTH TEMPERATURE EFFECTS ON SURFACE MORPHOLOGY OF EPITAXIAL GAAS LAYERS GROWN BY MOCVD

Heteroepitaxial growths of GaAs layers on Ge substrates have received a significant attentiondue to their closely matched lattice constants and thermal expansion coefficients. This study isfirst part of InxGa1-xP/InyGa1-yAs/Ge high efficiency tandem solar cell project which issupported by TÜBİTAK. In this study epitaxial GaAs thin films grown by MetalorganicChemical Vapor Deposition (MOCVD) on Ge [100] substrates with a 60 off orientationtowards the nearest [111] direction. Root mean square roughness (rms) and surfacemorphology of epitaxial GaAs layers were investigated by contact-mode Park System XE-100Atomic force microscopy (AFM). Also growth temperature effect on surface morphology ofGaAs layers was investigated.</p

Influences of thickness and temperature of low temperature GaAs buffer layer on two-step MOVPE grown GaAs/Ge heterostructures

We investigate influence of GaAs buffer layer (BL) growth parameters such as temperature and thickness on the structural, morphological, crystalline and optical quality of metal organic vapor phase epitaxy (MOVPE) grown heterostructures of GaAs on Ge. It was found that the optimal BL conditions significantly decrease the effects of anti-phase boundaries (APBs) even when grown on offcut Ge substrate by two-step growth technique with AsH3 pre-flow to promote double atomic step formation. It is observed that as the growth temperature increases, the growth rate of the GaAs BL increases, too. Improvement on the structural quality is observed up to BLs temperature of 535 °C, then it decreases. On the other hand, as the different thick BLs, 12, 25, 75 nm are considered, the epilayer grown on the 25 nm thick BL has shown the lowest full width at half maximum (FWHM) value, large photoluminescence peak intensity and internal quantum efficiency (IQE)

Structural and electrical properties of nitrogen-doped ZnO thin films

ZnO and nitrogen-doped ZnO thin films were prepared onto glass substrate by pulsed filtered cathodic vacuum arc deposition (PFCVAD) system at room temperature. The influence of doping on the structural, electrical and optical properties of thin films was investigated. XRD studies were carried out to analyze and compare the structural quality of undoped and nitrogen-doped ZnO films. Raman measurement was performed to study the doping effects in the ZnO. The optical transmittances of all films are studied as a function of wavelength using UV-VIS-NIR spectrophotometer. The optical band gap values of the films are found using absorbance spectrums. The electrical studies for the films are carried out by using Hall-effect measurements. (C) 2014 Elsevier B.V. All rights reserved

Optical and structural properties of In-rich InxGa1−xAs epitaxial layers on (1 0 0) InP for SWIR detectors

International audienceIn-rich InxGa1−xAs epitaxial layers were grown on InP (1 0 0) substrates by a metalorganic vapor phase epitaxy (MOVPE) technique. The effect of Indium (In) composition on the crystalline quality and optical properties are investigated. High resolution X-ray diffraction (HR-XRD) measurement and Raman scattering spectrum are used to evaluate the crystalline quality, the residual strain and dislocation density property. The number of dislocations in the epitaxial layers is found to increase by increasing the Indium content in order to release the stresses due to the epitaxial clamping. Photoluminescence (PL) measurement is used to characterize the optical properties. At 10 K, PL measurements show that the InGaAs band gap redshifts with the indium content. Moreover, the asymmetry at the low-energy side of the PL peak has been attributed to the presence of localized excitons. In all samples, a blue shift of PL peaks is evidenced by increasing the excitation power density, which is in line with the presence of carrier’s localization and non-idealities in this system. Moreover, the temperature-dependence of the PL peak energy displays an unusual red-blue-red shift (S-shaped) behavior when raising the temperature. These observations can be related to the inhomogeneous distribution of indium which gives rise to the appearance of dislocations and other defects which serve as traps for charge carriers. Interestingly, those highly In-content InxGa1−xAs epitaxial layers show PL emission located between 1637 and 1811 nm (depending on In content) and thus might be suitable for in the design of novel heterostructure devices such as short wave infrared (SWIR) detectors