P. L. Ryder

INCORPORATION OF INDIUM DURING MOLECULAR BEAM EPITAXY OF INGAN

We report on the incorporation of In during growth of InxGa1−xN by molecular beam epitaxy under varying In/Ga flux ratios and with different film thicknesses. The incorporation efficiency studied by energy dispersive x-ray microanalysis, high-resolution x-ray diffraction and photoluminescence spectroscopy is strongly affected by the chosen fluxes of Ga and N and is limited by the excess of nitrogen compared to gallium. Furthermore, thick films exhibit a decrease of the In content in growth direction. The behavior can be explained by considering the different stabilities of the two binary compounds InN and GaN.

Magnesium segregation and the formation of pyramidal defects in p-GaN

Magnesium doping of GaN was found to generate extended defects with a pyramidal shape. Transmission electron micrographs of layers with different doping levels typically showed a defect-free region at the start of doping and a modulation of the defect density in the subsequent film. We developed a rate equation model based on the segregation of Mg to explain the formation process of these defects. The model explains the dependence of the defect-free thickness on the doping level and yields a criterion to avoid the defect formation. Hall measurements show a significant reduction of the free hole concentration for samples grown at doping levels beyond defect formation.

Compositional inhomogeneities in InGaN studied by transmission electron microscopy and spatially resolved cathodoluminescence

The structural and optical properties of InGaN epilayers grown by different molecular beam epitaxy (MBE) techniques were studied with high spatial resolution. Mappings of the local emission wavelength obtained by cathodoluminescence (CL) spectroscopy indicate lateral and spectral inhomogeneities in the luminescence of InGaN epilayers grown with continuous In and Ga fluxes. These results agree well with variations in the chemical composition in the lateral and in the growth direction seen in mappings of the local composition which were obtained by energy-dispersive X-ray (EDX) microanalysis in cross-sectional transmission electron microscopy (TEM). Possible origins of these variations are discussed. In comparison, epilayers grown with alternating deposition of (In, Ga)N and (Ga)N are more homogeneous.

Influence of buffer layers on the structural properties of molecular beam epitaxy grown GaN layers

Abstract The influence of low temperature buffer layers on the structural characteristics of GaN grown by molecular beam epitaxy on sapphire (0001) substrates was investigated. Layers grown on GaN and AlN buffers were studied by high-resolution X-ray diffraction and transmission electron microscopy (TEM). For both buffer materials, the variation of the buffer parameters, like their thickness and growth temperature, is reflected in a clear change of the GaN (0002) rocking curve width. For strongly decreased as well as for increased Bragg reflection width a deterioration of optical and electrical properties of GaN layers grown on buffers with respect to reference samples without buffer layers was observed. Moreover, layers grown on thin GaN buffer layers show extremely narrow ω scans and layer thickness interferences in 2θ/ω direction, while TEM reveals a high defect density throughout the entire layer. Therefore, not only the width of the rocking curves but also their shape has to be considered for the estimation of the defect densities by X-ray diffraction.

Comparative study of molecular beam and migration-enhanced epitaxy of ZnCdSe quantum wells: influence on interface and composition fluctuations

Optical and structural properties of ZnCdSe quantum wells (QWs) grown by either migration-enhanced epitaxy (MEE) or molecular beam epitaxy (MBE) have been compared. Firstly the QWs are grown by depositing Zn and Cd at the same time during MEE. These samples exhibit significantly lower Cd concentrations than samples grown by MBE with similar Cd fluxes. Compared to MBE grown samples with similar low Cd concentrations neither photoluminescence (PL) nor high-resolution transmission electron microscopy (HRTEM) reveal significant differences in the optical and structural properties, respectively. Secondly the QWs are grown as a digital alloy by depositing CdSe and ZnSe by turns. In this case typical PL spectra show line widths as narrow as 16 meV which means a reduction of about 10 meV compared to typical values from MBE grown samples. By means of HRTEM this could be correlated to improved interfaces and a better composition homogeneity.

Pyramidal defect formation in view of magnesium segregation

The formation process of pyramidal defects in Mg doped GaN grown by metal organic vapor phase epitaxy (MOVPE) was studied by means of transmission electron microscopy (TEM). The formation process is ascribed to the segregation of Mg on the surface during growth. A segregation model explains the onset of defect formation. The model yields a low incorporation coefficient of only 1% per grown bilayer from surface states into the bulk material. The observed modulation of the defect density is explained by an efficient reduction of Mg on the surface by the formation of the pyramidal defects. Layers optimized on the basis of this model are demonstrated to be free of pyramidal defects.

Correlations between Structural, Electrical and Optical Properties of GaN Layers Grown by Molecular Beam Epitaxy

Screw and edge dislocation densities were estimated from X-ray diffraction profiles of GaN layers grown by molecular beam epitaxy. The results were confirmed by transmission electron microscopy. The layers had varying thicknesses or were deposited on differently nitridated sapphire substrates. It was found that the low temperature Hall mobility correlates with the screw dislocation density whereas the full width at half maximum of the donor bound exciton emission line changes similarly as the edge dislocation density.

The Relationship Between the Newcomb-Benford Law and the Distribution of Rational Numbers

The Newcomb-Benford law, also known as Benford’s law or the first-digit law, applies to many tabulated sets of real-world data. It states that the probability that the first significant digit is n, (n ∈ {1,2,3,4,5,6,7,8,9}) is given by log(1+ 1/n). The law has been verified empirically with widely differing data sets. In the present paper it is shown that it does not necessarily follow from the requirement of scale invariance alone, as has been claimed. This condition is necessary, but not sufficient. In addition, it is necessary to consider the properties of certain finite subsets of the set of rational numbers.

Mg related Defect Formation during MOVPE Growth of GaN based Films studied by Transmission Electron Microscopy

P-type active incorporation of magnesium into the gallium nitride lattice is still a challenge for the realization of wide band gap light emitters. This work presents a detailed microstructural study of phenomena connected with the problem of Mg doping of films grown by metal organic vapor phase epitaxy. Transmission Electron Microscopy both in conventional and in high resolution mode are used to obtain a better understanding of the Mg related defect formation processes. It is shown that the defect formation never occurs instantaneously at the beginning of the doping but a defect free zone always precedes the onset of the defect formation. This effect cannot solely be addressed to the memory effect of the reaction chamber since it is found that the defect density oscillates during growth. A more detailed investigation reveals that segregation of Mg plays a crucial role in the formation of defects. The observations emphasize the fact that a critical surface concentration is necessary for the defects to form. Beside the known processes of inversion boundary formation it is found that small nitrogen polar GaN grains in GaN films are gradually increasing on the expense of Ga polar grains if a sufficiently high Mg coverage and film thickness is realized. This process is found to occur in alternating steps along the {1123} and {0001}planes.

Thermal Expansion of Glass-Forming Zr-based Alloys in the Melt, the Undercooled Liquid and the Different Solid States

The thermal expansion coefficients of glass-forming Zr-based alloys were measured in the melt, the undercooled liquid and the glassy/crystalline state. Due to the high reactivity of the liquid material the experiments were performed containerlessly in an electrostatic levitator. We used an optical method where the samples were imaged with a high-resolution CCD- camera and the volume of the samples was evaluated by digital image processing. The coefficients of thermal expansion in the liquid and in the solid state could be determined from the volume versus temperature curves. The results can be compared with measurements in the electromagnetic levitation facility TEMPUS performed under microgravity conditions in the mission MSL-1 and ground based DMA-measurements. The thermal expansion data can be interpreted in terms of the free volume model.