Elshaer, A: Molecular Beam Epitaxy Growth and Characerizatio

spinel layer is formed at about 700C, it remains stable at least up to 1000C, and even it is extended in thickness. By recording and analyzing RHEED intensity oscillations, the growth kinetics has been investigated. Flat surface morphology and layer-by-layer growth has been achieved. The stoichiometry has been deduced by analyzing the growth rate as a function of Zn and O fluxes for various growth temperatures. It is found that the sticking coefficient of oxygen radicals is less dependent on the substrate temperature than that of Zn. The stoichiometric condition shifts to larger Zn flux at higher growth temperature. The kink rZnO values determine the activated O-flux supplied by the RF plasma source at TS=500C, 400W and a given O2-flow rate. It equals 0.5±0.05 Ås-1 per sccm. Absolute ?Zn values versus TS, defined as ?Zn=rZnO(T)/rZnO(max), where rZnO(max) is recalculated from the Zn flux measured by a quartz monitor, using Zn/ZnO molar mass and density ratios. Ex-situ characterization of the grown ZnO layers indicate that the surface morphology and crystal quality of the ZnO films grown on sapphire by MBE using either oxygen plasma cell or H2O2 as an oxidant can be extensively improved by using an HT MgO buffer. ZnO layers reveal strong variation of surface morphology versus the O/Zn flux ratio. The most flat surface morphology of ZnO is obtained when the ratio is within the 0.7-1 range. The growth under O-rich conditions leads to formation of hexagonal pyramids and at higher O/Zn ratios to a 3D growth with the top layer formed by perfectly c-oriented columnar structures of 50-100 nm in a diameter. It was also possible to recover the initial 3D growth mode to the 2D one by employing the Zn-rich growth conditions at O/Zn=0.4-0.6. Structural characterizations by high resolution X-ray diffraction (HR-XRD) and transmission electron microscopy (TEM) indicate a dramatic reduction in defect density in the ZnO epilayers grown with an HT MgO buffer. By using TEM, it was found that the dominant extending defects are edge, screw and mixed-type dislocations along c-axis. The main defects were threading dislocations. This is resulted from the well controlled layer-by-layer growth, since only the edge-type dislocation is able to accommodate the lattice mismatch, while the screw type dislocation forms much related to the initial nucleation environment.The microstructure of ZnO epilayers has been studied by HR-XRD. The full width at half maximum of the (0002) reflection, 0.007 degree, is much smaller than that of the (10-10) reflection, 0.27 degree revealing the micro-twist dominates the mosaicity, while micro-tilt is much less important.This pronounced difference of the rocking curve widths between the (0002) and (1010) reflections strongly indicates that the density of pure edge threading dislocations is greater than that of pure screw dislocations. Optical characterizations reveal that exciton plays an important role in ZnO. At room temperature free exciton recombinations dominate the photoluminescence. The ZnO epilayers reveal well resolved low temperature PL excitonic spectra with a dominant bound exciton line (3.355 eV) possessing a 2 meV half-width and a peak of free A exciton at 3.374 eV. The low-energy tail extending from the excitonic emission peaks due to the lattice deformation is significantly reduced, which allows the observation of two electron satellites and LO-phonons replicas of free and bound excitons. Variation of growth stoichiometry from O-rich to Zn-rich results in the pronounced quench of the acceptor-bound part of the excitonic band, as well as the strong intensity redistribution of donor-bound lines which seems to be attributed to a change in the point defect density. Temperature dependence of PL spectra between 6K and room temperature every 30 K under the same excitation conditions was performed. Slowly decreases coming at 300K to about one third of the intensity at 6K. This corresponds to the activation of non-radiative channels in the capturing and recombination processes. This result was confirm by decay time measurements. PL mapping of 2 inch ZnO epilayer shows high lateral homogeneity from PL intensity distribution and PL FWHM distribution. Hall-effect measurements and Electrochemical profiling (ECV) were used to characterize the electrical properties of ZnO samples. Hall-effect measurements indicated n-type behavior with carrier concentration of 2.0x1016 cm-3 and mobility of approximately 96 cm2/Vs. ECV profile versus depth measured for the top 2.5 ?m thick sample gives surface carrier concentration is 2.0x1016 cm-3 increasing to a maximum value of 1.0x1018 cm-3 the semiconductor/substrate interface. P-n heterojuntions and mesa structures comprising MBE n-ZnO layers and CVD p-4H-SiC laser were manufactured and investigated. Electrical properties of the mesa diodes have been studied with Hall measurements, and current-voltage measurements (I-V). I-V measurements of the device show good rectifying behavior, from which a turn-on voltage of about 2 V was obtained. With the excitation of O and N gas mixture in a single plasma cell, followed by the sample annealing procedure. P-type ZnO:N layers with a net hole concentration 3x1017 cm-3 using was measured. The combination of low growth temperature, slightly O-rich conditions and post-growth annealing is shown to be effective way to obtain p-doping. Further efforts are necessary to improve structural quality of the low-temperature p-type ZnO:N films. Optical properties of ZnO based II-VI heterostructures and quantum structures have also been studied. The surface roughness of ZnxMg1?xO was as low as 0.7 nm. The optical band gap and photoluminescence peak can be turned to larger energy with the same high crystallinity and without significant change in the lattice constant. The prominent PL peaks related to the SQW show a systematic blueshift with decreasing well width, which is consistent with the quantum size effect. The SQW-related emission peaks exhibit an S-shaped (redshift-blueshiftredshift) behaviour with increasing temperature, which is in contrast with that ascribed to band gap shrinkage (redshift). The observed behavior is discussed in terms of localization at lateral interface potential fluctuations. For T >70 K the integrated PL intensity is thermally activated with activation energies much less than the band offsets. It is argued that the dominant mechanism leading to the quenching of the ZnO SQW-related PL is due to the thermionic emission of excitons out of the lateral potential minima caused by potential fluctuations, such as interface fluctuations by 1 ML. Stimulated emission has been achieved at room temperature in a separate confinement double heterostructure having a 3 nm wide SQW as an active region. It has been found that a critical parameter for the lasing is the inhomogeneous broadening of both QW and barrier emission bands. MBE process for ZnO has been developed where high quality ZnO epilayers and heterostructures can be grown by molecular beam epitaxy on sapphire substrate. For nitrogen doping of ZnO, Oxygen and nitrogen were activated in the single plasma cell. No reproducible and reliable experimental results on the achievement of p-type conductivity achieved. Stimulated emission has been achieved at room temperature.