Matthew J. Beck

Dehydrogenation of defects and hot-electron degradation in GaN high-electron-mobility transistors

Degradation mechanisms limiting the electrical reliability of GaN high-electron-mobility transistors (HEMTs) are generally attributed to defect generation by hot-electrons but specific mechanisms for such processes have not been identified. Here we give a model for the generation of active defects by the release of hydrogen atoms that passivate pre-exisiting defects. We report first-principles density-functional calculations of several candidate point defects and their interaction with hydrogen in GaN, under different growth conditions. Candidate precursor point defects in device quality GaN are identified by correlating previously observed trap levels with calculated optical levels. We propose dehydrogenation of point defects as a generic physical mechanism for defect generation in HEMTs under hot-electron stress when the degradation is not spontaneously reversible. Dehydrogenation of point defects explains (1) observed hot electron stress transconductance degradation, (2) increase in yellow luminescence...

Diffusivity control in molecule-on-metal systems using electric fields

The development of methods for controlling the motion and arrangement of molecules adsorbed on a metal surface would provide a powerful tool for the design of molecular electronic devices. Recently, metal phthalocyanines (MPc) have been extensively considered for use in such devices. Here we show that applied electric fields can be used to turn off the diffusivity of iron phthalocyanine (FePc) on Au(111) at fixed temperature, demonstrating a practical and direct method for controlling and potentially patterning FePc layers. Using scanning tunneling microscopy, we show that the diffusivity of FePc on Au(111) is a strong function of temperature and that applied electric fields can be used to retard or enhance molecular diffusion at fixed temperature. Using spin-dependent density-functional calculations, we then explore the origin of this effect, showing that applied fields modify both the molecule-surface binding energies and the molecular diffusion barriers through an interaction with the dipolar Fe-Au adsorption bond. On the basis of these results FePc on Au(111) is a promising candidate system for the development of adaptive molecular device structures.

Nanoceria: factors affecting its pro- and anti-oxidant properties

Nanoceria redox properties are affected by particle size, particle shape, surface chemistry, and other factors, such as additives that coat the surface, local pH, and ligands that can participate in redox reactions. Each CeO2 crystal facet has a different chemistry, surface energy, and surface reactivity. Unlike nanocerias industrial catalytic applications, biological and environment exposures are characterized by high water activity values and relatively high oxygen activity values. Electrochemical data show that oxygen levels, pH, and redox species affect its phase equilibria for solution and dissolution. However, not much is known about how the many and varied redox ligands in environmental and biological systems might affect nanocerias redox behaviour, the effects of coated surfaces on redox rates and mechanisms, and whether the ceria solid phase undergoes dissolution at physiologically relevant pH and oxygen levels. Research that could answer these questions would improve our understanding of the links between nanocerias redox performance and its morphology and environmental conditions in the local milieu.

Impact of Proton Irradiation-Induced Bulk Defects on Gate-Lag in GaN HEMTs

The relationship between proton-induced defects and gate-lag in GaN high-electron mobility transistors (HEMTs) is examined using simulations and experiments. Surface traps are primarily responsible for the pre-irradiation gate-lag. Experimental data and detailed two-dimensional device simulations demonstrate that bulk traps increase the amount of observed gate-lag after irradiation to high-proton fluences.

Atomic Displacement Effects in Single-Event Gate Rupture

Swift heavy ion (SHI) damage, including single-event gate rupture (SEGR), radiation-induced soft breakdown (RISB), and long-term reliability degradation (LTRD), plays an important role in limiting device lifetime and reliability. However, the atomic-scale physical origins of these phenomena have not been elucidated. In this work, we explain the underlying physical processes responsible for SHI-induced effects in oxides, providing a direct link between atomic motion and macroscopic electrical effects. SRIM 2008 calculations show that SHIs produce low-energy atomic recoils in SiO2. Using parameter-free quantum mechanical calculations, we probe the atomic-scale dynamics of the resulting low-energy atomic displacements. We show that low-energy displacements in SiO2 produce pockets containing high densities of network defects, and that these defects generate electronic states throughout the SiO2 band gap. These spatially correlated defect states represent a low-resistivity ldquoconducting piperdquo through SiO2 layers, and provide an atomistic mechanism for the formation of electrically-active damage that does not rely on thermal spike effects. In the case of SEGR, the conducting pipe allows energy stored on the gate capacitance to be discharged into the oxide, resulting in the permanent damage observed experimentally. The persistence of defects resulting from SHI-induced atomic displacements provides a physical explanation for percolation models of LTRD and RISB.

The Role of Atomic Displacements in Ion-Induced Dielectric Breakdown

Irradiation of electronic devices with heavy ions causes a range of device degradation and failure modes, many of which are characterized and/or triggered by enhanced leakage current through dielectric layers. These damage modes include single-event dielectric rupture (SEDR), long-term reliability degradation (LTRD), and radiation-induced soft breakdown (RISB), and they play a major role in limiting device lifetime and reliability in space applications. The LET-induced transient carrier plasma that is generated along the incident ion path has traditionally been understood as the physical effect ultimately leading to damage in dielectric layers. However, in a recent study we showed that nontrivial densities of atomic displacements are directly generated by incident heavy ions. Here, we report multiscale calculations of the effects of ion-induced atomic displacements on the current-voltage (I-V) characteristics of SiO2 layers. We use both parameter-free quantum mechanical calculations and 3D percolation theory calculations based on Mott defect-to-defect tunneling. We show that ion-induced atomic displacements produce both transient and static low-resistivity paths through SiO2 layers. The calculated I-V characteristics of damaged SiO2 layers agree quantitatively with experimental data and are shown to depend on both the spatial distribution of displacement-induced defects and the distribution of defect energy levels in the SiO2 energy gap.

Quantum Mechanical Description of Displacement Damage Formation

Atomic-scale processes during displacement damage formation have been previously studied using molecular dynamics (MD) calculations and empirical potentials. Low-energy displacements (1 keV) are characterized by a high cross-section for producing secondary knock-on atoms and damage clusters, and determine the threshold displacement energy (an important parameter in NIEL calculations). Here we report first-principles, parameter-free quantum mechanical calculations of the dynamics of low-energy displacement damage events. We find that isolated defects formed by direct displacements result from damage events of les100 eV. For higher energy events, the initial defect profile, which subsequently undergoes thermal annealing to give rise to a final stable defect profile, is the result of the relaxation and recrystallization of an appreciable volume of significantly disordered and locally heated crystal surrounding the primary knock-on atom displacement trajectory.

Atomic-Scale Mechanisms for Low-NIEL Dopant-Type Dependent Damage in Si

While calculated non-ionizing energy loss (NIEL) generally correlates well to first order with radiation-induced displacement damage rates, it does not account for some well-known differences in damage rates for n- and p-type Si. Here we show that the magnitude of these differences, DeltaKn-p, correlates closely with the fraction of total displacement damage due to low-energy primary knock-on atom (PKA) recoils. The primary products of these displacement damage events, with PKA recoils <~ 2 keV, are close vacancy-interstitial pairs, or Frenkel Pairs (FPs). Based on previous studies of vacancy-dopant complex stabilities in Si, and new parameter-free quantum mechanical calculations of FP properties, details of the stable defect profiles arising from low-energy PKA recoil events are shown to give rise to non-zero values of DeltaKn-p

Toward Tuning the Surface Functionalization of Small Ceria Nanoparticles

Understanding and controlling the performance of ceria nanoparticle (CNP) catalysts requires knowledge of the detailed structure and property of CNP surfaces and any attached functional groups. Here we report thermogravimetric analysis results showing that hydrothermally synthesized ∼30 nm CNPs are decorated with 12.9 hydroxyl groups per nm(2) of CNP surface. Quantum mechanical calculations of the density and distribution of bound surface groups imply a scaling relationship for surface group density that balances formal charges in the functionalized CNP system. Computational results for CNPs with only hydroxyl surface groups yield a predicted density of bound hydroxyl groups for ∼30 nm CNPs that is ∼33% higher than measured densities. Quantitative agreement between predicted and measured hydroxyl surface densities is achieved when calculations consider CNPs with both -OH and -Ox surface groups. For this more general treatment of CNP surface functionalizations, quantum mechanical calculations predict a range of stable surface group configurations that depend on the chemical potentials of O and H, and demonstrate the potential to tune CNP surface functionalizations by varying temperature and/or partial pressures of O2 and H2O.

Doping-Type Dependence of Damage in Silicon Diodes Exposed to X-Ray, Proton, and He

Different amounts of degradation for n-Si and p-Si are observed after X-ray, H+, and He+ irradiations. Recombination lifetime and forward I-V measurements made on abrupt-junction diodes are compared to theory. Ionizing damage and displacement damage associated with surface and bulk trapping mechanisms, respectively, compete with each other and lead to different behaviors according to the doping type of the silicon on the lightly doped side of the junction. Surface effects are dominant in the n+/p diodes compared to the p+/n diodes; bulk trapping prevails in the n-Si compared to p-Si. Independently of ion type or fluence, the lifetime damage factor due to irradiation is worse in the p-Si than in the n-Si by a factor of 2-3 times.