High sensitivity measurement of implanted As in the presence of Ge in GexSi1−x/Si layered alloys using trace element accelerator mass spectrometry

This article discusses high sensitivity measurement of implanted As in the presence of Ge in Ge(x)Si(1-x)/Si layered alloys using trace element accelerator mass spectrometry

Elastic strain engineering for unprecedented materials properties

“Smaller is stronger.” Nanostructured materials such as thin films, nanowires, nanoparticles, bulk nanocomposites, and atomic sheets can withstand non-hydrostatic (e.g., tensile or shear) stresses up to a significant fraction of their ideal strength without inelastic relaxation by plasticity or fracture. Large elastic strains, up to ∼10%, can be generated by epitaxy or by external loading on small-volume or bulk-scale nanomaterials and can be spatially homogeneous or inhomogeneous. This leads to new possibilities for tuning the physical and chemical properties of a material, such as electronic, optical, magnetic, phononic, and catalytic properties, by varying the six-dimensional elastic strain as continuous variables. By controlling the elastic strain field statically or dynamically, a much larger parameter space opens up for optimizing the functional properties of materials, which gives new meaning to Richard Feynman’s 1959 statement, “there’s plenty of room at the bottom.”National Science Foundation (U.S.) (DMR-1240933)National Science Foundation (U.S.) (DMR-1120901

Study of simulations of double graded InGaN solar cell structures

The performances of various configurations of InGaN solar cells are compared using nextnano software. Here we compare a flat base graded wall GaN/InGaN structure, with an InxGa1-xN well with sharp GaN contact layers, and an InxGa1-xN structure with InxGa1-xN contact layers, i.e. a homojunction. The doping in the graded structures are the result of polarization doping at each edge (10 nm from each side) due to the graded structure, while the well structures are intentionally doped at each edge (10 nm from each side) equal to the doping concentration in the graded structure. The solar cells are characterized by their open-circuit voltage, V_oc, short circuit current, I_sc, solar efficiency, and energy band diagram. The results indicate that an increase in I_sc and efficiency results from increasing both the fixed and the maximum indium compositions, while the V_oc decreases. The maximum efficiency is obtained for the InGaN well with 60% In.Comment: 14 pages, 12 figure

GaAs on Si epitaxy by aspect ratio trapping: analysis and reduction of defects propagating along the trench direction

The Aspect Ratio Trapping technique has been extensively evaluated for improving the quality of III-V heteroepitaxial films grown on Si, due to the potential for terminating defects at the sidewalls of SiO2 patterned trenches that enclose the growth region. However, defects propagating along the trench direction cannot be effectively confined with this technique. We studied the effect of the trench bottom geometry on the density of defects of GaAs fins, grown by metal-organic chemical vapor deposition on 300 mm Si (001) wafers inside narrow (<90 nm wide) trenches. Plan view and cross sectional Scanning Electron Microscopy and Transmission Electron Microscopy, together with High Resolution X-Ray Diffraction, were used to evaluate the crystal quality of GaAs. The prevalent defects that reach the top surface of GaAs fins are {111} twin planes propagating along the trench direction. The lowest density of twin planes, 8 108 cm 2, was achieved on “V” shaped bottom trenches, where GaAs nucleation occurs only on {111} Si planes, minimizing the interfacial energy and preventing the formation of antiphase boundaries

Hot Phonon and Carrier Relaxation in Si(100) Determined by Transient Extreme Ultraviolet Spectroscopy

The thermalization of hot carriers and phonons gives direct insight into the scattering processes that mediate electrical and thermal transport. Obtaining the scattering rates for both hot carriers and phonons currently requires multiple measurements with incommensurate timescales. Here, transient extreme-ultraviolet (XUV) spectroscopy on the silicon 2p core level at 100 eV is used to measure hot carrier and phonon thermalization in Si(100) from tens of femtoseconds to 200 ps following photoexcitation of the indirect transition to the {\Delta} valley at 800 nm. The ground state XUV spectrum is first theoretically predicted using a combination of a single plasmon pole model and the Bethe-Salpeter equation (BSE) with density functional theory (DFT). The excited state spectrum is predicted by incorporating the electronic effects of photo-induced state-filling, broadening, and band-gap renormalization into the ground state XUV spectrum. A time-dependent lattice deformation and expansion is also required to describe the excited state spectrum. The kinetics of these structural components match the kinetics of phonons excited from the electron-phonon and phonon-phonon scattering processes following photoexcitation. Separating the contributions of electronic and structural effects on the transient XUV spectra allows the carrier population, the population of phonons involved in inter- and intra-valley electron-phonon scattering, and the population of phonons involved in phonon-phonon scattering to be quantified as a function of delay time

Study of Thin GaN/InGaN/GaN double graded structures for Future photovoltaic application

Indium gallium nitride (In_x Ga_(1-x) N) materials have displayed great potential for photovoltaic and optoelectronic devices due to their optical and electrical properties. Properties such as direct bandgap, strong bandgap absorption, thermal stability and high radiation resistance qualify them as great materials for photovoltaic devices. The tunable bandgap which absorbs the whole solar spectrum is the most significant feature which became attractive for scientists. The bandgap for these materials varies from 0.7 eV for InN to 3.4 eV for GaN covering from infrared to ultraviolet. In_x Ga_(1-x) N wurtzite crystal is grown on GaN buffer layer by Molecular Beam Epitaxy (MBE). Epitaxial growth of high quality In_x Ga_(1-x) N material, however, creates great challenges due to lattice mismatch between InN and GaN (up to 11%). This might be the actual reason of partially and fully strain at the interface relating to growth condition which affect optical properties of the materials. Therefore, studying solar cell parameters for different indium compositions (low to high) in the material is significant. In this work, graded composition In_x Ga_(1-x) N (44 nm ramping up followed by 44 nm ramping down) were grown on GaN/sapphire template. The growth was done at different indium compositions (low to high) in plasma-assisted MBE. Additionally, optical and structural characterizations of the materials were done. The results showed that by increasing indium composition, the composition was not linearly graded as expected and was accompanied by strain relaxation along the growth direction. In other words, for low indium composition, the results showed fully strained. However, for high indium composition partially strain relaxation was seen. The optical respond of three samples was studied with photoluminescence. For the first: to study the source of each peak in aspect of either exciton or different kinds of defect states. Second, peaks related to ground state transition. Furthermore, nextnano3 and nextnano+ software were used to simulate optical properties of 100 nm graded structures such as the band structure, ground state wave-function position as well as determine the optical transition probabilities among ground state hole and electrons as well as solar cell parameters for different structures under different strained conditions. Simulation continued for higher alloys (20% to 90%) under strain and (20%-100%) under relaxed condition. An equation like Vegard’s law was created to predict the energy bandgap under strain for different indium compositions. The simulation was performed for 100 nm -graded structure to find the optimum xmax for both conditions for maximum solar efficiency. In addition, the performance of graded structure in a Flat Base Graded (FBG) was studied to compare with Square well and Homojunction structure