G. S. Cargill

Enhanced room-temperature luminescence efficiency through carrier localization in AlxGa1−xN alloys

AlGaN samples grown by plasma-assisted molecular-beam epitaxy on sapphire (0001) substrates, with 20%–50% Al content and without the use of indium, show intense room-temperature photoluminescence that is significantly redshifted, 200–400meV, from band edge. This intense emission is characterized by a long room-temperature lifetime (∼375ps) comparable to that seen in low defect density (∼108cm−2) GaN. Room-temperature monochromatic cathodoluminescence images at the redshifted peak reveal spatially nonuniform emission similar to that observed in In(Al)GaN alloys and attributed to compositional inhomogeneity. These observations suggest that spatial localization enhances the luminescence efficiency despite the high defect density (>1010cm−2) of the films by inhibiting movement of carriers to nonradiative sites.

Ohmic contacts to plasma etched n-Al0.58Ga0.42N

Plasma etching is required to expose n-AlxGa1−xN layers for bottom-emitting ultraviolet light emitting diodes grown on sapphire. However, etching can increase the difficulty of forming Ohmic contacts. X-ray photoelectron spectroscopy and cathodoluminescence reveal how the semiconductor changes with etching and help explain why it becomes more difficult to form an Ohmic contact. A V∕Al∕V∕Au metallization has been investigated for Ohmic contacts to n-Al0.58Ga0.42N etched with a BCl3∕Cl2∕Ar chemistry. Increased V thickness and higher annealing temperatures were required to obtain a specific contact resistance of 4.7×10−4Ωcm2 for etched n-Al0.58Ga0.42N compared to optimized contacts on unetched films.

Strain evolution in Al conductor lines during electromigration

Monochromatic and white beam synchrotron x rays were used to study the deviatoric strains and full elastic strains in passivated Al conductor lines with near-bamboo structures during electromigration (EM) at 190 °C. A strong strain gradient formed in the upstream part of the Al lines. Strains along the downstream part of the lines were smaller and more scattered. Numerical analysis using the Eshelby model and finite element method (FEM) calculations suggest that the moving of atoms during EM in these near-bamboo Al lines is dominated by top and/or bottom interface diffusion, which differs from the reported results for nonbamboo, polycrystalline Al conductor lines, where EM is mainly along the grain boundaries. Local strain measurements and FEM calculations indicate that the EM flux is also nonuniform across the width of the conductor line because of stronger mechanical constraint by the passivation layer near the edges of the line. Plastic deformation is observed during EM by changes in the Laue diffracti...

Electromigration of copper in Al(0.25 at. % Cu) conductor lines

Electromigration and diffusion of Cu have been investigated for polycrystalline Al(0.25 at. % Cu) conductor lines. In situ measurements of the evolution of Cu concentration profiles along 200 μm long, 10 μm wide conductor lines with 1.5 μm thick SiO2 passivation during electromigration have been obtained by synchrotron-based white x-ray microbeam fluorescence. The apparent effective charge ZCu* of Cu in Al(Cu) has been found to be −8.6±1.0. The evolution of Cu concentration profiles can be manipulated by controlling the direction and magnitude of the current flow at different temperatures. The effective grain boundary diffusivity DCueff has been determined by fitting the time dependent experimental Cu concentration profiles. The results show Arrhenius behavior of DCueff=D0 exp(−Q/kT) for T=275–325 °C with D0=10−(2.3±1.6) cm2/s and Q=0.76±0.19 eV.

Relationship between copper concentration and stress during electromigration in an Al(0.25 at. % Cu) conductor line

Synchrotron-based x-ray microbeam fluorescence and diffraction have been used for in situ measurements of Cu concentration and biaxial stress in a 200-μm-long, 10-μm-wide Al(0.25 at. % Cu) conductor line with 1.5-μm-thick SiO2 passivation during electromigration. Measurements over 48 h with T=300 °C and j=1.5×105 A/cm2 show that a stress gradient of 3 MPa/μm develops over the upstream 130 μm of line length where Cu concentration drops below 0.15 at. %, and a 10-μm-long void develops at the cathode end of the line, but little change in stress occurs over the downstream 70 μm of line length where Cu concentration remains above 0.15 at. %. These experimental results have been reproduced by a finite element model in which the downstream Cu transport is accompanied by a counter flow of Al in the upstream direction, and downstream Al motion is blocked where the local Cu concentration is above ∼0.15 at. %. Defect mediated coupling between Al and Cu diffusive flows, e.g., Cu–vacancy binding, is proposed as the ca...

Effects of bias on cathodoluminescence in ZnCdSe quantum well light emitting diodes

Bias voltages applied to Zn0.24Cd0.76Se quantum well light emitting diodes (QW-LEDs) affect both the intensity and wavelength of room temperature cathodoluminescence (CL). These effects have been studied experimentally and theoretically to advance understanding of the CL and optoelectronic behavior of these devices. QW CL intensity and photon energy are increased by forward bias, and they are decreased by reverse bias, with an exponential dependence of CL intensity on bias voltage from −1 to +1 V and little dependence from 1.5 to 2.5 V. The p-n junction current and electroluminescence increase rapidly for forward bias greater than 2.34 V, the calculated built-in potential. The bias dependence of QW CL intensity is little affected when electron beam currents change by ∼300 times, from 0.1 to 29 nA with 10 kV beam voltage and ∼1 μm2 irradiated area. The QW CL intensity increases sublinearly with beam current. Small hysteresis effects are seen in bias-dependent CL intensity for low beam currents. The effects...

The Astrolabe Craftsmen of Lahore and Early Brass Metallurgy

Summary A study of the metallurgy and manufacturing techniques of a group of eight astrolabes (seven from Lahore, one attributed to India) using non-destructive methods has produced the earliest evidence for systematic use of high-zinc (α + β) brass. To produce this alloy, the brass industry supplying the Lahore instrument makers must have co-melted metallic copper and zinc. This brass-making technology was previously believed to have been developed on an industrial scale in the nineteenth century in Europe. This work hypothesizes that this technology was used in Lahore on an industrial scale as early as ad 1601. In addition, this work hypothesizes that the α + β brass alloy was used specifically for its ease in manufacturing the thin sheet brass required for astrolabe-component manufacture.

Electron Beam Bombardment Induced Decrease of Cathodoluminescence Intensity from GaN Not Caused by Absorption in Buildup of Carbon Contamination

Monochromatic CL imaging, CL spectra, WDS spectra, and EDS spectra and imaging demonstrate that electron beam bombardment of LEO-GaN causes decrease of near band edge cathodoluminescence intensity that cannot be attributed to absorption in a growing carbon contamination layer. An alternative explanation is needed, such as generation of defects, or charge injection and buildup of internal electric fields, caused by electron beam bombardment.

Investigation of Thermal Stress Variability Due to Microstructure in Thin Aluminum Films

An X-ray microbeam study and a polycrystal finite element model of a 1010 m 2 section of a 1 m thick polycrystalline aluminum film on a silicon substrate are used to investigate the effect of microstructure on thermal stress variability. In the X-ray microbeam study, the grain orientations and deviatoric elastic strain field are measured at the subgrain level in the film during and after two thermal cycles. A finite element model of the observed grain structure is created and modeled with an elastoviscoplastic crystal constitutive model that incorporates film thickness and grain size effects as well as dislocation entanglement hardening. The experimental and simulation results are compared at both the film and subgrain scales. While the experiment and model agree fairly well at the film level, the experimental results show much greater elastic strain variability than the simulations. In considering the grain size effect, the experiment and model both predict a similar Hall‐Petch coefficient, which is consistent with literature data on free standing aluminum thin films. DOI: 10.1115/1.4002212

Thermal and Electromigration‐Induced Strains in Polycrystalline Films and Conductor Lines: X‐ray Microbeam Measurements and Analysis

X‐ray microbeam measurements of thermal and electromigration‐induced strains have been made at NSLS using white‐beam energy dispersive x‐ray diffraction, averaging over many grains, and at APS using white‐beam Laue x‐ray diffraction, from single grains. Grain‐by‐grain deviatoric strain measurements in Al films show wide variation in behavior for different grains in the films. Room temperature relaxation of residual strains was observed to occur at different rates for Al films with different bonding layers and substrates. X‐ray microbeam measurements of strain development during electromigration for Cu and Al conductor lines show that strain gradients do not develop in the copper lines under conditions similar to those for which large strain gradients have been seen for Al lines.