In this paper we (i) describe a model for the stress distribution across a wafer during chemical‐mechanical polishing, which is solved using I‐DEAS (a commercial software package) and (ii) summarize the predicted effects of carrier film and pad compressibility on polishing nonuniformity. Results indicate that (i) the Von Mises stress correlates with polishing nonuniformity, while the normal stress does not correlate with the nonuniformity and (ii) CMP uniformity improves with decreasing polishing pad and carrier film compressibility.

Recent discoveries of degradation mechanisms in organic light emitting diodes (OLEDs) are reviewed. One common observation regarding the OLED reliability is the gradual increase of the diode driving voltage under forward bias. This trend reverses when the bias polarity is reversed, and the initial driving voltage can be recovered almost completely. The mobile ions are proposed to explain the phenomenon. By solving a system of transient equations governing the mobile ion motion under an external field, we obtained the transient mobile ion distributions and their contribution to the driving voltage. We found that the mobile ion model with reasonable assumptions could very well explain the experimental results. Another advancement in the field is the utilization of a single organic alloy layer to improve the device lifetime. A typical organic alloy layer consists of a mixture of hole and electron transport materials. Significant lifetime improvement over heterojunction structures is achieved with comparable efficiency. The significant improvement in the device reliability is attributed to the elimination of the heterojunction interface and the minimization of the formation of Alq3 cations. The high efficiency is achieved due to the balanced carrier injection, transport and confined recombination in the device.

High-entropy alloys (HEAs), which have been intensely studied due to their excellent mechanical properties, generally refer to alloys with multiple equimolar or nearly equimolar elements.According to this definition, Si-Ge-Sn alloys with equal or comparable concentrations of the three Group IV elements belong to the category of HEAs. As a result, the equimolar elements of Si-Ge-Sn alloys likely cause their atomic structures to exhibit the same core effects of metallic HEAs such as lattice distortion. Here we apply density functional theory (DFT) calculations to show that the SiGeSn HEA indeed exhibits a large local distortion effect. Unlike metallic HEAs, our Monte Carlo and DFT calculations show that the SiGeSn HEA exhibits no chemical short-range order due to the similar electronegativity of the constituent elements, thereby increasing the configurational entropy of the SiGeSn HEA. Hybrid density functional calculations show that the SiGeSn HEA remains semiconducting with a band gap of 0.38 eV, promising for economical and compatible mid-infrared optoelectronics applications. We then study the energetics of neutral single Si, Ge, and Sn vacancies and (expectedly) find wide distributions of vacancy formation energies, similar to those found in metallic HEAs. However, we also find anomalously small lower bounds (e.g., 0.04 eV for a Si vacancy) in the energy distributions, which arise from the bond reformation near the vacancy. Such small vacancy formation energies and their associated bond reformations retain the semiconducting behavior of the SiGeSn HEA, which may be a signature feature of a semiconducting HEA that differentiates from metallic HEAs.

Experimental and theoretical results are presented on the lifetime of organic light emitting diodes (OLED's) for active matrix display applications. DC aging tests on the OLED's show that the driving voltage increases under forward bias and then reverses its trend when the bias polarity is reversed, which reproduce our previous tests under AC conditions. Furthermore, the voltage seems to be able to relax slowly toward its initial value when the device bias is reset to zero after a long forward bias stress. The mobile ions are proposed to be the origin of the observed voltage shifts. By solving a system of transient equations governing the mobile ion motion under an external field, we obtained the transient mobile ion distributions and their contribution to the driving voltage. Several cases (uniform initial ion distribution, nonuniform initial distribution, and constant source) were studied. We found that the mobile ion model with reasonable assumptions could very well explain the experimental results. Furthermore, by comparison between the data and simulation, the possibility of the initial mobile ion sources can be narrowed.

The donor and exciton states in ultra high-quality heteroepitaxial GaN grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) on sapphire substrates are investigated using low temperature photoluminescence (PL), reflectance, magnetospectroscopy in fields up to 12 T, and resonant electronic Raman scattering (RERS). The A free exciton is confirmed to have a binding energy of about 26.4 meV, independent of strain in the material. Bound n=2 exciton peaks are distinguished in the PL spectrum by their thermalization and sample dependence. The Si donor is shown to have a binding energy of about 21 meV using Si-doped HVPE samples grown at Epitronics. Up to five additional residual donor species are observed when comparing various HVPE and MOCVD samples. Pronounced temperaturedependence of the two-electron satellites is observed, suggesting the existence of unresolved excited rotator states of the neutral donor-bound exciton. Highly resolved magnetic splitting patterns are observed in the two-electron satellites. A nonperturbative theory of these donor splittings is developed, including anisotropy. Resonant electronic Raman scattering from residual donors is reported, and yields improved linewidths compared to PL.