Harry Efstathiadis

Two-dimensional computer modeling of single junction a-Si:H solar cells

A two dimensional physically-based computer simulation of single junction pin amorphous silicon solar cells is presented using Sentaurus, Technology Computer-Aided Design (TCAD). The simulation program solves the Poisson, the continuity, and the current density equations by using a standard procedure for amorphous materials, including the continuous density of state model, Shockley-Read-Hall and Auger recombination mechanisms, and computes the generation function of electron-hole pairs from the optical parameters of each layer. The dependence of these optical parameters with the photon energy has been included, taking into account the doping level, thickness of each layer and their effect on cell efficiency. The simulator is applied to the analysis of a p-i-n single junction a-SiC:H/a-Si:H/a-Si:H solar cell, obtaining results comparable to one dimensional simulation results using AMPS (analysis of microelectronic and photonic structure)-1D. More advanced simulation models for novel solar cell devices such as tandem cells are in progress, with the aim of achieving an optimal design of solar cells based on amorphous materials or micro-/nanocrystalline layers.

Photoluminescence in erbium doped amorphous silicon oxycarbide thin films

Photoluminescence (PL) in Er-doped amorphous silicon oxycarbide (a-SiCxOy:Er) thin films, synthesized via thermal chemical vapor deposition, was investigated for carbon and oxygen concentrations in the range of 0–1.63. Intense room-temperature PL was observed at 1540 nm, with the PL intensity being dependent on the carbon and oxygen content. The strongest PL intensity was detected for a-SiC0.53O0.99:Er when pumped at 496.5 nm, with ∼20 times intensity enhancement as compared to a-SiO2:Er. Broadband excitation in the visible was observed for a-SiC0.53O0.99:Er. Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy analyses suggest that the formation of Si–C–O networks plays an important role in enhancing the Er optical activity in a-SiCxOy:Er films.

Atomic bonding in amorphous carbon alloys: A thermodynamic approach

The free energy model previously developed for the prediction of the bonding in amorphous Si‐based alloys is extended here to amorphous carbon alloys, a‐CxH1−x, containing carbon atoms with sp3 and sp2 hybridization. Predictions have been made for the bonds present in the alloys, with the case of ‘‘chemical’’ ordering at T=0 K corresponding to phase separation into separate C (sp3) and C(sp2) regions. For T≳0 K phase separation is eliminated and there is no evidence for the clustering of graphitic carbon, indicating the importance of the configurational entropy in influencing the bonding in the alloys. Hydrogen atoms are predicted to bond preferentially to C (sp3) atoms for all T. The sp3/sp2 ratio is predicted to increase with increasing H content, as observed experimentally, and also with increasing T due to entropy effects. Predictions have been made for the distribution of bonds in tetrahedral C(sp3)‐ and planar C(sp2)=C(sp2)‐centered units. It is found that essentially no aromatic or graphitic struct...

Biaxially textured constantan alloy (Cu 55 wt%, Ni 44 wt%, Mn 1 wt%) substrates for YBa2Cu3O7−x coated conductors

Commercially available constantan alloy rods (nominal composition Cu55?Ni44?Mn1?wt%) have been thermo-mechanically processed to develop biaxially textured substrates. It was found that the (001) recrystallization cube texture percentage could be increased from 72% to nearly 100% as the annealing temperature of the rolled substrates was increased from 750 to 1200??C. A full width half maximum (FWHM) of 6.5? in (111) phi scans and an FWHM of 4.9? in (100) omega scans were observed in the substrates annealed at 1200??C for 2?h. These substrates were found to have a Curie temperature of 35?K and so were paramagnetic at 77?K and ferromagnetic at 5?K with a saturation magnetization that is 2.5 times less than that of Ni?5?at.%?W substrates. Yield strengths of highly textured constantan substrates were found to be 1.5 times that of textured pure Ni substrates at room temperature.

Improving performance of cryogenic power electronics

Cryogenic Power Electronics (CPE) provides promising benefits for power conditioning system compared to their room-temperature counterparts in terms of reduced size and weight (increased power density), improved efficiency, improved switching speed, and improved reliability. Active devices such as semiconductor switches can exhibit performance improvements such as reduced conduction losses, higher switching speed, reduced diode reverse recovery, greater device gain, higher over-current capability, and increased power levels. Passive devices (inductors, capacitors, interconnects) will also improve by the lowered resistance of their constituent metal conductors or the use of superconductors. This paper aims to review the present status of CPE and to provide an outlook on emerging device technologies that are gaining in interest. Advanced power electronic packaging/interconnect methods that adapt well to cryogenics and help further improve system performance is discussed. Given improved and known device and interconnect properties, the system designer can develop the best circuit topologies for maximum CPE system performance.

Temperature dependent thermal conductivity of Si/SiC amorphous multilayer films

The cross-plane thermal conductivity of 22 nm period Si/SiC amorphous multilayer films deposited by magnetron sputtering and measured using a differential 3ω method was found to decrease from 2.0 W/mK at 300 K to 1.1 W/mK at 80 K. Structural disorder in each of the constituent layers of the amorphous multilayer films was confirmed by high resolution transmission electron microscopy. Estimations of the relative contributions of interface and intrinsic layer thermal resistance based on microscopic phonon transport models indicate that mean free path reductions induced by the structural disorder within the multilayer films are responsible for the observed experimental trends.The cross-plane thermal conductivity of 22 nm period Si/SiC amorphous multilayer films deposited by magnetron sputtering and measured using a differential 3ω method was found to decrease from 2.0 W/mK at 300 K to 1.1 W/mK at 80 K. Structural disorder in each of the constituent layers of the amorphous multilayer films was confirmed by high resolution transmission electron microscopy. Estimations of the relative contributions of interface and intrinsic layer thermal resistance based on microscopic phonon transport models indicate that mean free path reductions induced by the structural disorder within the multilayer films are responsible for the observed experimental trends.

TiO2 nanotubes: interdependence of substrate grain orientation and growth rate.

Highly ordered arrays of TiO2 nanotubes can be produced by self-organized anodic growth. It is desirable to identify key parameters playing a role in the maximization of the surface area, growth rate, and nanotube lengths. In this work, the role of the crystallographic orientation of the underlying Ti substrate on the growth rate of anodic self-organized TiO2 nanotubes in viscous organic electrolytes in the presence of small amounts of fluorides is studied. A systematic analysis of cross sections of the nanotubular oxide films on differently oriented substrate grains was conducted by a combination of electron backscatter diffraction and scanning electron microscopy. The characterization allows for a correlation between TiO2 nanotube lengths and diameters and crystallographic parameters of the underlying Ti metal substrate, such as planar surface densities. It is found that the growth rate of TiO2 nanotubes gradually increases with the decreasing planar atomic density of the titanium substrate. Anodic TiO2 nanotubes with the highest aspect ratio form on Ti(-151) [which is close to Ti(010)], whereas nanotube formation is completely inhibited on Ti(001). In the thin compact oxide on Ti(001), the electron donor concentration and electronic conductivity are higher, which leads to a competition between oxide growth and other electrochemical oxidation reactions, such as the oxygen evolution reaction, upon anodic polarization. At grain boundaries between oxide films on Ti(hk0), where nanotubes grow, and Ti(001), where thin compact oxide films are formed, the length of nanotubes decreases most likely because of lateral electron migration from TiO2 on Ti(001) to TiO2 on Ti(hk0).

Numerical Thermal Simulation of Cryogenic Power Modules Under Liquid Nitrogen Cooling

Understanding the thermal performance of power modules under liquid nitrogen cooling is important for the design of cryogenic power electronic systems. When the power device is conducting electrical current, heat is generated due to Joule heating. The heat needs to be efficiently dissipated to the ambient in order to keep the temperature of the device within the allowable range; on the other hand, it would be advantageous to boost the current levels in the power devices to the highest possible level. Projecting the junction temperature of the power module during cryogenic operation is a crucial step in designing the system. In this paper, we present the thermal simulations of two different types of power metal-oxide semiconductor field effect transistor modules used to build a cryogenic inverter under liquid nitrogen pool cooling and discussed their implications on the design of the system.

Highly Transparent Conductive Electrode with Ultra-Low HAZE by Grain Boundary Modification of Aqueous Solution Fabricated Alumina-Doped Zinc Oxide Nanocrystals

Commercial production of transparent conducting oxide (TCO) polycrystalline films requires high electrical conductivity with minimal degradation in optical transparency. Aqueous solution deposited TCO films would reduce production costs of TCO films but suffer from low electrical mobility, which severely degrades both electrical conductivity and optical transparency in the visible spectrum. Here, we demonstrated that grain boundary modification by ultra-violet laser crystallization (UVLC) of solution deposited aluminium-doped zinc oxide (AZO) nanocrystals results in high Hall mobility, with a corresponding dramatic improvement in AZO electrical conductance. The AZO films after laser irradiation exhibit electrical mobility up to 18.1 cm2 V−1 s−1 with corresponding electrical resistivity and sheet resistances as low as 1 × 10−3 Ω cm and 75 Ω/sq, respectively. The high mobility also enabled a high transmittance (T) of 88%-96% at 550 nm for the UVLC films. In addition, HAZE measurement shows AZO film scatteri...

On the limits to Ti incorporation into Si using pulsed laser melting

Fabrication of p-Si(111) layers with Ti levels well above the solid solubility limit was achieved via ion implantation of 15 keV 48Ti+ at doses of 1012 to 1016 cm−2 followed by pulsed laser melting using a Nd:YAG laser (FWHM = 6 ns) operating at 355 nm. All implanted layers were examined using cross-sectional transmission electron microscopy, and only the 1016 cm−2 Ti implant dose showed evidence of Ti clustering in a microstructure with a pattern of Ti-rich zones. The liquid phase diffusivity and diffusive velocity of Ti in Si were estimated to be 9 × 10−4 cm2/s and (2 ± 0.5) × 104 m/s, respectively. Using these results the morphological stability limit for planar resolidification of Si:Ti was evaluated, and the results indicate that attaining sufficient concentrations of Ti in Si to reach the nominal Mott transition in morphologically stable plane-front solidification should occur only for velocities so high as to exceed the speed limits for crystalline regrowth in Si(111).