Optimization of Graded Buffer Layers for Metamorphic Semiconductor Devices

Metamorphic semiconductor devices such as high electron mobility transistors (HEMTs), light-emitting diodes (LEDs), laser diodes, and solar cells are grown on mismatched substrates and typically exhibit a high degree of lattice relaxation. In order to minimize the incorporation of threading defects it is common to use a linearly-graded buffer layer to accommodate the mismatch between the substrate and device layers. However, some work has suggested that buffer layers with non-linear grading could offer superior performance in terms of limiting the surface density of threading defects. In this work, we have compared S-graded buffer layers with different orders and thicknesses. To do so we calculated the expected surface threading dislocation density for each buffer design assuming a GaAs (001) substrate. The threading dislocation densities were calculated using the LMD model, in which the coefficient for second-order annihilation and coalescence reactions between threading dislocations is considered to be equal to the length of misfit dislocations.

Investigation of Direct Water Photoelectrolysis Process Using III-N Structures

GaN, GaN/AlGaN and GaN/InGaN-based structures were used to study water photoelectrolysis in KOH-based electrolyte, measurement of current-potential characteristics, investigation of electrode corrosion and for hydrogen generation. The corrosion process of p-n AlGaN/GaN structure starts in the p-layers, spreads via vertical channels associated with threading defects, and continues laterally along the n-layers, where large local hollows and voids were observed. The H2 production rate of 0.3-0.6 ml/cm2×h was measured for n-GaN structure.

Growth temperature dependence of crystal symmetry in Nb-doped BaTiO3 thin films

Growth temperature effects on the microstructure of Nb -doped BaTiO 3 thin films of the composition BaTi 0.98 Nb 0.02 O 3 are studied using X-ray diffraction and transmission electron microscopy (TEM). Reciprocal space maps and electron diffraction patterns show that the a-axis lattice parameter increases and the c-axis parameter decreases with increasing growth temperature, indicating a decrease of tetragonality. Bright-field TEM images show low and high densities of threading defects in films grown at low and high temperatures, respectively. The observations are discussed in terms of a hindering of the cubic-to-tetragonal phase transition by a high defect density and a high unit cell volume.

Grazing Incidence X-ray Topographic Studies of Threading Dislocations in Hydrothermal Grown ZnO Single Crystal Substrates

ABSTRACTZnO single crystal substrates grown by the hydrothermal method have been characterized by grazing incidence X-ray topography using both monochromaticand whitesynchrotron X ray beams.11$\bar 2$4 reflection wasrecorded from the (0001) wafers and the different contrast patterns produced by different threading defects were noted. To uniquely identify the Burgers vectors of these threading dislocation defects, we use raytracingsimulation to compare with observed defect contrast. Our studies showed that threading screw dislocations are not commonly observed.Most threading edge dislocationshavetheBurgers vector of1⁄3[2$\bar 1$$\bar 1$0] or1⁄3[12$\bar 2$10]and a density of 2.88×104/cm2.

Germanium Adsorption on SrTiO3 (001) 2×1 Surface: A Density Functional Theory Study

Integrating germanium on Si is one of the major challenges of epitaxial growth and presents important applicative interest. Recently, SrTiO3 was adopted as a buffer layer to accommodate the mismatch between Ge and Si. Germanium can take its bulk lattice parameter as soon as the growth begins without threading defects on SrTiO3 surface. However, the details of Ge adsorption on SrTiO3 surface are not clear. In present work, the electronic structures of Ge deposited on the SrTiO3 (001) 2×1 Double Layer (DL) TiO2 surfaces were investigated by means of density functional theory calculations. Several stable adsorption sites are identified. It is found that the germanium adsorption shows site selectivity and causes noticeable surface distortion. The charge transfer from germanium atom to surface contributes to the formation of strong Ge-O bondings and surface metallization.

Characterization of Dislocations and Micropipes in 4H n+ SiC Substrates

Etching of 4H-SiC wafers in molten KOH as a method for micropipe and dislocation density analysis was investigated. The obtained results were correlated with those of the synchrotron white beam x-ray topography. Heavily nitrogen-doped SiC shows a significantly different etching behavior in comparison with the low-doped material. This complicates identification of different types of threading defects. In particular, it is difficult to separate Threading Screw Dislocations (TSD) from Threading Edge Dislocations (TED). Depending on the level of doping and thermal history of the crystal, some of the etch pits emerging due to the 1c screw dislocations can be as large as those due to the micropipes.

Nanorods as a precursor for high quality GaN layers

ABSTRACTThe density of threading dislocations in GaN/(0001)sapphire films grown by molecular beam epitaxy can be reduced to about 108 cm−2 by growing an intermediate nanorod layer. This paper examines the growth of the nanorods and proposes that threading defects in the overlayer arise either through grain boundaries formed when nanorods coalesce, or through the propagation of dislocation dipoles seen during nanorod growth. Results showing that the latter often terminate or develop into voids during growth are discussed.