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Electronic and optical studies on Dy2Ti2−Mn x O7 (x = 0.00, 0.05, 0.10, 0.15, & 0.20) have been presented through both, theoretical (density functional theory (DFT) calculations) and experimental (ultraviolet-visible absorption and photoluminescence emission spectroscopy) approaches. DFT calculations were employed considering the local density approximation (LDA) and LDA-1/2 for exchange-correlation interactions. Computed crystallographic parameters and energy band-gap using theoretical formulations are in good agreement with experimental results. The band-gap value obtained through the LDA-1/2 approach indicates insulated ground state of Dy2Ti2−x Mn x O7 (x = 0.00, 0.05, 0.10, 0.15, 0.20) system. Experimentally obtained band gap value reduces from 3.82 eV to 2.45 eV with increase in positive chemical pressure as x increases from 0 to 0.20. Reduction in band gap value is attributed to the fact that there exists a lack of hybridization between the O-2p orbital and Ti-3d orbital, which is well correlated with the crystallographic data. Jahn-Teller effect is likely to be responsible for the presence of a mixed state of Mn (explained using x-ray photoelectron spectroscopy results), resulting in the intermediate Mn state between the valence band and the conduction band with immediate inclusion of Mn at Ti site in Dy2Ti2−x Mn x O7 system.

The metal–semiconductor interface is fundamental to any semiconductor device and the success of advanced technology nodes critically depends upon the minimization of the contact resistance at the interface. In this paper, we calculate the electronic structure of a metal–semiconductor interface (TiGe/Ge contact) within the framework of first-principles density functional theory simulations. We report the modulation of the Schottky barrier height with respect to the different phases of TiGe metal and different crystallographic orientations of Ge substrate. We further compute the ${I}$ – ${V}$ characteristics of the TiGe/Ge contact with nonequilibrium Green’s function formalism, using a two-terminal device configuration. The calculated transmission spectrum allows us to extract the contact resistance at the metal–semiconductor interface. Furthermore, the onset of Ohmic contact for p-doped TiGe/Ge interface is identified by studying the ${I}$ – ${V}$ characteristics as a function of increasing active carrier concentration. We find that a doping concentration of 1e21 is sufficient to transform the Schottky contact into Ohmic and thereby achieve a least possible contact resistance at the interfaces. Our paper thus provides useful physical insights into the nanoscale details of the TiGe/Ge interfaces and can guide further process development to minimize the contact resistance.

Using first-principles density functional theory based transport calculations, we evaluate the grain boundary reflection coefficients in Copper (Cu). We find that the grain boundary reflections can significantly enhance the metal resistivity in Cu interconnects. Later, using the Mayadas-Shatzkes model, we predict the possible enhancement of the metal resistivity due to the grain boundary scattering. These results identify the critical role of grain boundary reflection in polycrystalline Cu. Further, the implications of the grain boundary resistance for shrinking interconnect dimensions are discussed.

This chapter discusses limits of gate dielectric scaling for advanced metal oxide semiconductor field effect transistor (MOSFET). We will review details of hafnium oxide (HfO2) gate oxide and how HfO2 can be modified to hafnium oxynitrides (HfON) and hafnium lanthanum oxynitrides (HfLaON) to increase dielectric constant for continuous equivalent oxide thickness (EOT) scaling. Bilayer hafnium oxide/titanium oxide (HfO2/TiO2) as a higher ‘k’ dielectric option for FinFET gate length (Lg) scaling is discussed. Interfacial layer (IL) scaling technologies for overall EOT scaling are also covered in this chapter. Ab-initio modeling results to evaluate ternary and quaternary gate oxide for higher ‘k’ dielectrics options, and effective metal work functions calculations to optimize and develop new metal gates are discussed. New gate dielectric reliability failure mechanisms due to three dimensional natures of FinFET devices are reviewed. Thermal oxide equivalent quality deposited silicon dioxide (SiO2) using atomic layer deposition (ALD) with post-treatments (e.g., plasma nitridation and anneals) as a high voltage input/output (I/O) gate oxide with silicon-germanium channel (SiGe) results are shown. Finally, we give a brief overview of silicon germanium pFET channel material as a gate dielectric scaling knob, and how it can enable continued EOT scaling without reliability degradation.

In this paper, we discuss a technique for selective GeO x -scavenging which creates a GeO x -free interfacial layer (IL) on Si 1−x Ge x substrates. This process reduces interface trap density (N it ) and increases high-field hole mobility in Si 1−x Ge x pFETs. In addition, we identify the existence of electronic defect levels close to the Si 1−x Ge x band edges associated with the Ge surface concentration at the Si 1−x Ge x /IL interface. These electronic defects act as carrier scattering centers severely degrading the channel mobility and modulate the device threshold voltage. By successfully eliminating the GeO x component in the IL and electronic defects states at the Si 1−x Ge x /IL interface, high channel carrier mobility over a wide range of inversion carrier density in compressively-strained Si 1−x Ge x channel pFETs is demonstrated.

Chemical pressure plays a crucial role in determining the electronic properties of the quantum materials. Investigation of electronic structure of Ho2Ge x Ti2-x O7 (x = 2, 1.9, 1.75, 1.5 1, 0.5, 0.25, 0.1 and 0) series has been performed. Pyrochlore and Pyrogermanate, Re2B2O7 (Re = Ho3+, B = Ti4+ and Ge4+; rare earth titanates and germanates), substituted with increasing amount of Ge4+ at the Ti4+ site and vice versa develops structural distortions. Distinct shrinkage effect has been established in the Ho2Ti2O7 matrix upon Ge+4 substitutions at B site, resulting in the modification of band gap value. Band gap of 5.24 eV drastically drops to 3.92 eV with immediate Ti4+ substitution in Ho2Ge2O7. Electronic states of Ho3+ (4f forbidden transitions) had also been identified. We observe favored sub level transition (Specific Stark component) corresponding to5F5 to 5I8 electronic transition for Ho3+ at λ exc. = 450 nm. The upper valence band consisted of O 2p state hybridized with Ho 5p and Ti and Ge 4p states and conduction band primarily formed by Ho 5d state hybridized with Ti 3d and Ge 4d states as obtained from density of states (DOS) calculations. Strong hybridization between Ho 5p1/2 and Ti 3p orbital upon Ti4+ inclusion in Ho2Ge2O7 has been observed through both theoretical studies using LDA-1/2 and UV-Vis, photoluminescence, ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy. The evolution of total DOS of all studied composition shows that valence band edge is more sensitive than conduction band to composition. These results provide chemical pressure as an excellent tool to tailor the band gap and fine tune the intermediate electronic states in Ho2Ge x Ti2-x O7.

We report experimental characterization and modeling of direct and trap-assisted tunneling (TAT) in high-K metal gate (HKMG)-based access transistor and deep trench (DT) capacitor constituting a 32 nm embedded dynamic random access memory (eDRAM) device. This is the first eDRAM technology that has successfully integrated HKMG-based access transistor and DT technology. The experimental results are compared with direct and TAT models implemented in a finite element-based device simulator. While in HKMG-based nFET both TAT, and direct tunneling are present, in the DT capacitor TAT is dominant due to higher interface and bulk traps. We demonstrate, through ab initio simulations, that the bulk and interface traps arise due to oxygen vacancies (Ov) in the bulk HfO2, and SiO2/HfO2 interface and quantitatively compare direct and TAT currents with experimental results.

A novel method to model the effect of local workfunction variation in high-k metal gate nanoscale transistors is proposed. Impact of variability in metal grain granularity on device performance is studied using ab initio density functional theory calculations and device simulations, which show that different metal grain orientations (GOs) can result in large (≥100 mV) variation in metal gate effective work function. Probabilities of occurrence of each GO and the grain size are used to estimate the work-function variations. Full 3-D device simulations are performed to study the effect of metal grain granularity on FinFET and planar MOSFET behavior. Simulated mismatch trends are shown to be in good agreement with the grain diameters and device geometries.

We demonstrate a technique for selective GeO x -scavenging which creates a GeO x -free IL on Si 1−x Ge x substrates. This process reduces N it by >60% to 2e11 and increases high-field mobility at N inv =1e13 cm−2 by ∼1.3× in Si 0.6 Ge 0.4 pFETs with sub-nm EOT.

Bias Temperature Instability (BTI) is a tremendous reliability concern for deeply scaled CMOS technologies, and is the limiting mechanism for further inversion layer thickness (Tinv) scaling for future nodes [1]. Replacement Metal Gate technologies are of particular concern, since the gate stack is not exposed to the high temperature source/drain anneals. We have identified four strategies for reducing BTI in Replacement metal gate technologies: Rapid Thermal Anneal (RTA) optimization, optimization of the HfO2 layer thickness, introducing a gate dielectric dopant for NBTI reduction, and effective Work Function tuning. Judiciously combining these four techniques enable further Tinv scaling for 10 nm and below.

We evaluate the impact of band-to-band tunneling (BTBT) on the characteristics of n-channel junctionless transistors (JLTs). A JLT that has a heavily doped channel, which is fully depleted in the OFF state, results in a significant band overlap between the channel and drain regions. This overlap leads to a large BTBT of electrons from the channel to the drain in n-channel JLTs. This BTBT leads to a nonnegligible increase in the OFF-state leakage current, which needs to be understood and alleviated. In the case of n-channel JLTs, tunneling of electrons from the valence band of the channel to the conduction band of the drain leaves behind holes in the channel, which would raise the channel potential. This triggers a parasitic bipolar junction transistor formed by the source, channel, and drain regions induced in a JLT in the OFF state. Tunneling current is observed to be a strong function of the silicon body thickness and doping of a JLT. We present guidelines to optimize the device for high ON-to-OFF current ratio. Finally, we compare the OFF-state leakage of bulk JLTs with that of silicon-on-insulator JLTs.

We propose a modified structure of tunnel field-effect transistor (TFET), called the sandwich tunnel barrier FET (STBFET). STBFET has a large tunneling cross-sectional area with a tunneling distance of ~2 nm. An orientation-dependent nonlocal band-to-band tunneling (BTBT) model was employed to investigate the device characteristics. The feasibility of the STBFET realization using a complementary metal-oxide-semiconductor-compatible process flow has been shown using advanced process calibration with Monte Carlo implantation. STBFET gives a high ION, exceeding 1 mA/μm at IOFF of 0.1 pA/μm with a subthreshold swing below 40 mV/dec. The device also shows better static and dynamic performances for sub-1-V operations. STBFET shows a very good drain current saturation, which is investigated using an ab initio physics-based BTBT model. Furthermore, the simulated ION improvement is validated through analytical calculations. We have also investigated the physical root cause of the large voltage overshoot of TFET inverters. The previously reported impact of Miller capacitance is shown to be of lower importance; the space-charge buildup and its relaxation at the channel drain junction are shown to be the dominant effect of large voltage overshoot of TFETs. The STBFET are shown to have negligible voltage overshoots compared with conventional TFETs.

The efficient and successful realization of low-power semiconductor devices demands, among other things, the ability to quantitatively model and minimize myriad leakage phenomena. We report herein a general physical model to quantitatively compute crystallographic-orientation-dependent gate-induced drain leakage (GIDL), and its numerical implementation in a continuum-based device simulator. This simulation model has been successfully compared with relevant experimental data derived from heavily doped vertical diodes and 45-nm silicon-based CMOS devices. Also, the process optimization of next-generation 32-nm low-power devices has been discussed in the context of GIDL.

The Mg K-edge and Zn K- and L3-edge X-ray absorption near edge spectra of Mg and Zn porphyrins in the ground state and low-lying optically excited states are calculated. Also computed are X-ray absorption near edge spectra of Fe(II) spin crossover compound in its ground and low-lying optically excited states, motivated by a recent experiment (J. Phys. Chem. A 2006, 110, 38). The calculated absorption spectra of optically excited states can be used to simulate ultrafast optical pump/X-ray probe experiments.

We identified unique device width and length dependencies of Vt for FinFET Replacement Metal Gate (RMG). Choice of fill metal is important to obtain a flat Vt-Wdes trend. We proposed a new model for Vt-Lg roll-up for High-k last RMG and demonstrated a flat Vt-Lg trend via optimization of High-k and MOL films based on the understanding.

The discovery of efficient photoluminescence (PL) in porous and nanocrystalline silicon has generated considerable interest over the last decade. The present work makes the following simple proposition: the dependence of the PL efficiency on size is non-monotonic and hence there exists an optimum size for PL efficiency in nanocrystalline silicon. We present simple phenomenological formulae which delineate the dependence of the efficiency on size. Besides the existence of an optimum size they also indicate that there is a nanometer range over which the efficiency is substantial. This assertion is not made on the basis of a theoretical calculation but by a wide study and careful analyses of extant literature, largely experimental. We present two sets of compelling reasons for our assertion, one empirical, and the other semi-empirical. We indicate how the assertion of the optimum size may be verified by experimental studies.

We carry out detailed studies of charged (D+) and neutral (D0) impurity states of helium-like donors in spherical semiconductor quantum dots using both the density functional-effective mass and tight-binding formalisms. We find the two sets of results to be consistent with each other down to very small size quantum dots. We vary the shape of the confining potential, taking it to be square, harmonic and triangular. Independent of the confining potential, we observe a shallow to deep transition of the donor level as the dot size is reduced. However, on further reduction of the size the donor level tends to become shallow again. This suggests that carrier 'freeze out' is a distinct possibility for both D+ and D0. Further, optical gaps also show similar behaviour with the size of the dot.

Semiconductor quantum dots (QDs) exhibit shell structures, very similar to atoms. Termed as ’artificial atoms’ by some, they are much larger (1 100 nm) than real atoms. One can study a variety of manyelectron effects in them, which are otherwise difficult to observe in a real atom. We have treated these effects within the local density approximation (LDA) and the Harbola-Sahni (HS) scheme. HS is free of the self-interaction error of the LDA. Our calculations have been performed in a three-dimensional quantum dot. We have carried out a study of the size and shape dependence of the level spacing. Scaling laws for the Hubbard ‘U’ are established.

Ability to understand and model the performance limits of nanowire transistors is the key to design of next generation devices. Here, we report studies on high-mobility junction-less gate-all-around nanowire field effect transistor with carrier mobility reaching 2000 cm2/V.s at room temperature. Temperature-dependent transport measurements reveal activated transport at low temperatures due to surface donors, while at room temperature the transport shows a diffusive behavior. From the conductivity data, the extracted value of sound velocity in InAs nanowires is found to be an order less than the bulk. This low sound velocity is attributed to the extended crystal defects that ubiquitously appear in these nanowires. Analyzing the temperature-dependent mobility data, we identify the key scattering mechanisms limiting the carrier transport in these nanowires. Finally, using these scattering models, we perform drift-diffusion based transport simulations of a nanowire field-effect transistor and compare the device performances with experimental measurements. Our device modeling provides insight into performance limits of InAs nanowire transistors and can be used as a predictive methodology for nanowire-based integrated circuits., 22 pages, 5 Figures, Nano Letters

This paper summarizes the ab-initio electronic and phonon band structures and the temperature-dependent carrier transport in layered Ti2AlC. It is found that the cylindrical Fermi surface is the origin of the anisotropic carrier effective mass (infinite effective mass along the c axis), which leads to a strong anisotropic (insulator along the c axis and metallic along the layer) carrier transport in these films. Using electronic and phonon band structure calculations, we have developed an analytical model for the carrier-phonon interaction and the in-plane carrier conductivity originating from the strong inter-valley (s → d) scattering in Ti2AlC. The density functional theory is used to calculate the average deformation potential corresponding to the acoustic phonon vibrations. The calculated deformation potential is in good agreement with the extracted deformation potential from the transport data available in the literature. The extracted deformation potential will be useful for predicting the transport...

In modern integrated circuits (ICs), billions of transistors are connected to each other via thin metal layers (e.g. copper, cobalt, etc) known as interconnects. At elevated process temperatures, inter-diffusion of atomic species can occur among these metal layers, causing sub-optimal performance of interconnects, which may lead to the failure of an IC. Thus, typically a thin barrier metal layer is used to prevent the inter-diffusion of atomic species within interconnects. For ICs with sub-10 nm transistors (10 nm technology node), the design rule (thickness scaling) demands the thinnest possible barrier layer. Therefore, here we investigate the critical thickness of a titanium–nitride (TiN) barrier that can prevent the cobalt diffusion using multi-scale modeling and simulations. First, we compute the Co diffusion barrier in crystalline and amorphous TiN with the nudged elastic band method within first-principles density functional theory simulations. Later, using the calculated activation energy barriers, we quantify the Co diffusion length in the TiN metal layer with the help of kinetic Monte Carlo simulations. Such a multi-scale modelling approach yields an exact critical thickness of the metal layer sufficient to prevent the Co diffusion in IC interconnects. We obtain a diffusion length of a maximum of 2 nm for a typical process of thermal annealing at 400 °C for 30 min. Our study thus provides useful physical insights for the Co diffusion in the TiN layer and further quantifies the critical thickness (~2 nm) to which the metal barrier layer can be thinned down for sub-10 nm ICs.

DC and AC NBTI in deep EOT scaled HKMG p-MOSFETs with different IL (scaled to sub 2A) are measured by UF-MSM method with 10μs delay. A model with interface trap generation (ΔV IT-IL ) at Si/IL interface, hole trapping (ΔV HT ) in IL bulk and trap generation (ΔV IT-HK ) linked to H passivated Oxygen vacancy (Ov-H) defects in IL/HK interfacial transition layer has been proposed. The existence of Ov defects and their energy levels are verified using DFT simulation. The model can successfully predict V T shift (ΔV T ) during and after DC stress, dependence on pulse duty cycle (PDC) and frequency (f) for AC stress, and gate insulator process dependence with consistent set of parameters. Impact of EOT scaling on DC and AC NBTI is studied, and end-of-life degradation has been estimated.

Independent Trap Generation (TG) monitors such as DCIV and SILC have been used during NBTI, PBTI (and TDDB) stress in differently processed HKMG devices. TG from DCIV for NBTI is attributed to Si/IL and IL/HK interfaces; TG from DCIV for PBTI to IL/HK interface but at similar energy location as NBTI. TG from DCIV shows similar stress bias (V G,STR ), time (t STR ) and temperature (T) dependence for NBTI and PBTI, while TG for PBTI from SILC shows very different dependence as it likely scans TG at different spatial and energetic locations. TG contribution to V T shift (ΔV T ) is compared to ΔV T from ultra-fast measurements. A compact model is used to predict overall BTI ΔV T considering uncorrelated contributions from independently measured TG and trapping (TP) in pre-existing and generated bulk traps. Impact of IL scaling on BTI and its underlying subcomponents are studied. Physical origins of different TG and TP processes have been identified using Density Functional Theory (DFT) simulations.

Numerical modeling is used to explain the origin of the large ON/OFF ratios, ultra-low leakage, and high ON current densities exhibited by BEOL-friendly Access Devices (AD) based on Cu-containing MIEC materials [1-5]. Motion of large populations of copper ions and vacancies leads to exponential increases in hole current, with a turn-ON voltage that depends on material bandgap. Device simulations match experimental observations as a function of temperature, electrode aspect-ratio, thickness, and device CD.

In the present paper charging and discharging transient currents in polyvinyl formal (PVF) were measured as a function of temperatures (40–80°C), poling fields (90 ×103−9.0 × 104 V/cm) and electrode combinations (Al-Al, Au-Al, Zn-Al, Bi-Al, Cu-Al and Ag-Al). The current-time characteristics have different values of slope lying between 0.42–0.56 and 1.42–1.63. The polarization is considered to be due to dipolar reorientation associated with structural motions and space charge relaxations due to trapping of injected charge carriers in energetically distributed traps.

An appreciable electric current is observed in a system consisting of a polyvinyl formal (PVF) film in a sandwich configuration, in the temperature range 30–110°C. The maximum value of the current during first heating is found to be of the order of 10−10 A and its thermograms exhibit one transition (i.e. current peak) at around 60°C. The position of the current peak in thermal spectrum shifts with the heating rate. A temperature dependence of the open circuit voltage is also observed. The activation energy of the process responsible for the current is determined. The magnitude of the current is more in the case of dissimilar electrode systems. It is proposed that the electric current arising from unpolarized metal-polymer-metal system is a water activated phenomenon, which is influenced by the transitional changes of the polymer.

The charge-transport behaviour in pure and chloranil (Chl) doped ethyl cellulose (EC) system has been studied by measuring the dependence of current on field, temperature, electrode material and dopant concentration. The role of doping molecular concentration in the polymer matrix and modification in the conduction characteristics are studied. Lowering of the activation energy due to doping was observed. The current was found to increase with an increase in the chloranil concentration. An explanation for this has been attempted on the basis of formation of molecular aggregates between chloranil molecules and ethoxy groups of ethyl cellulose. It is suggested that chloranil occupies interstitial positions between the polymer chains and assists in carrier transportation by reducing the hopping barriers. The current-voltage characteristics of different samples are analyzed using space charge limited current theory and quantitative information about the transport parameters is derived. The values of effective drift mobility and trapped charge carrier concentration which result in the build up of space charge have been calculated.

Transient discharging currents and steady-state conduction in solution-grown ethyl cellulose (EC)–poly(methyl methacrylate) (PMMA) blends measured as a function of temperature (30–80 °C) and field strength (10–100 kV cm−1) are reported. Transient currents are found to follow the Curie–VonSchweidler law, characterized by different slopes in short- and long-time regions, having different decay constant values lying between 0.75–0.99, and 1.68–1.95. The corresponding activation energies are found to increase with time of measurement of discharge current. Isochronal characteristics (ie current versus temperature plots at constant times) constructed from the data seem to reveal a broad peak observed at 60 °C. The dependence of dark current at different temperatures (30–80 °C) in a metal (1)–EC–PMMA blend–metal (1)/(2) system on the applied voltage in the range 10–100 kV cm−1 has also been studied; the current is found to be strongly temperature dependent. Dipole polarization and space charge resulting from trapping of injected charge carriers in energetically distributed traps and induced dipoles created because of the piling up of charge carriers at the phase boundary of the heterogeneous components of the blend are considered to account for the observed transient currents. The results of current–voltage measurement on blends are interpreted to show that the low-field steady-state conduction is ohmic in nature, and in high fields the charge carriers are generated by field-assisted lowering of coulombic barriers at the traps and are conducted through the bulk of the material by a hopping process between the localized states by a Jonscher–Ansari Poole–Frenkel mechanism. The modified P–F barrier is calculated to be 1.89 × 10−19 J (1.18 eV), 1.92 × 10−19 J (1.20 eV) and 1.95 × 10−19 J (1.22 eV) for P1, P2 and P3 blends, respectively. © 2000 Society of Chemical Industry

Thermally stimulated depolarization current (TSDC) studies have been performed on solution grown cellulose acetate films doped with different concentrations of acrylic acid (AA) prepared at the poling temperatures (40–75°C) with poling fields (10–50 kV/cm). The TSDC spectra of pure and AA doped CA films reveal two relaxation peaks at 80°C and 180 ±2°C, having activation energies centred around 0.25 and 0.55 eV. The phenomena of the existence of these current maxima have been analyzed and discussed in terms of the molecular motion of the polar side groups and release of the remaining part of the frozen dipoles by their cooperative motion with adjoining segments of the main polymer chain. The peak currents, released charge and activation energies associated with the peaks are affected by AA doping. The effect of doping with acrylic acid on the discharge current indicates the formation of molecular aggregates

Current-voltage (I–V) characteristics of pure polyvinyl formal (PVF) were investigated at different fields, range 5–100 kV/cm, as a function of temperature, range 313–363 K. It was observed that while at low fields (up to 25 kV/cm), the conduction was Ohm’s law-dependent at high fields (beyond 25 kV/cm), the conduction was Poole-Frenkel (P-F) mechanism-dependent. An attempt was made to identify the nature of the current by comparing its observed dependence on temperature, electric field and electrode materials with their respective characteristic features of the existing theories of electrical conduction. The current showed a strong dependence on temperature. To identify the possible mechanism of conduction, current versus square root of field characteristics were drawn with aluminium, silver, copper and gold as upper electrodes and Al as the lower electrodes. The observed characteristic suggested that the charge carriers were generated by the field-assisted lowering of coulombic barriers at the traps, and were subsequently conducted through the bulk of the material by a hopping process between the localized states by a Jonscher-Ansari-modified P-F mechanism. The calculated value of the modified P-F barrier was ⋍ 1·94×10−19 J (1·21 eV).

Continued scaling of semiconductor devices in logic and memory applications requires the introduction of high-k (HK) dielectrics to enhance the capacitance density while maintaining a low leakage. Of particular concern in DRAM memory applications is the so called `dielectric relaxation current', which is significantly enhanced with HfO2 dielectrics as compared to SiO2. In this paper, it is shown that these relaxation currents arise from electron trapping/detrapping by/from oxygen vacancy defects in the HfO2 dielectric from/to the contact electrodes. This understanding is utilized to minimize `dielectric relaxation currents' by optimizing the defect structure in the HK dielectric. By creating trap-free dielectric films with N or La doping, we show that substantial reductions in the relaxation currents can be achieved. For the first time, it is demonstrated that a HK dielectric stack formed as a result of these treatments can have reduced relaxation currents similar to SiO2, thus providing a pathway to solving a fundamental paradigm associated with HK dielectrics that has persisted for several years.

In this study, the authors investigate the impact of radical oxygen plasma on nitrided and annealed atomic layer deposited (ALD) SiO2 as a thick gate oxide (1.65–3 V) with a high-k/metal gate transistor. Time-dependent-dielectric-breakdown voltage, secondary ion mass spectroscopy (SIMS), and x-ray photoelectron spectroscopy (XPS) studies were conducted, and the results are discussed for nitrided and annealed ALD SiO2 with and without radical oxygen plasma exposure. Atomistic material simulations were performed to understand the reactions between oxygen radicals and silicon oxynitride (SiON). Our key findings from SIMS and XPS show that radical oxygen plasma exposure led to a 34% nitrogen loss from the thick SiON gate oxide and damaged the gate oxide, resulting in a 1 V reduction in the dielectric breakdown voltage. Atomistic material simulation results also show that atomic oxygen can react with Si-(ON)-Si to form Si–O–Si bonds and mobilize NO into the interstitial space. Similarly, when O atoms and O2 mo...

The impact of the gate insulator process on interlayer (IL) hole traps in IL/high-K dual-layer p-MOSFET gate-stack is studied by physical and electrical measurements along with atomistic simulations. Processes that lead to higher concentrations of Hf and N in IL, measured by angle-resolved X-ray photoelectron spectroscopy, result in higher IL hole traps measured by flicker noise in prestress and verified by atomistic simulations. The influence of these process induced preexisting IL hole traps on parametric degradation of p-MOSFETs during Negative bias temperature instability (NBTI) stress is studied. The mechanism responsible for superior NBTI of thermal IL stack, having lower Hf and N content in the IL as compared with Chem-Ox IL stack, is explained.

By performing density-functional effective-mass-theory calculations on spherical quantum dots, we show the phenomena of spin blockade effects. It is observed that the spin-blockade depends on the shape of the confinement potential, size of the dot, and depth of the potential.

The x-ray absorption near-edge spectra (XANES) and fluorescence spectra of molecules in the ground state and optically excited states are computed using time-dependent density functional theory and time-dependent Hartree-Fock theory. The calculated XANES spectra of optically excited methanol, benzonitrile, hydrogen sulphide, and titanium tetrachloride and the fluorescence spectra of optically excited methanol can be used to simulate ultrafast optical pump/x-ray probe experiments.

The role of many-body effects in quantum dots is of both academic and practical interest. We study the electron-electron interaction within a simplified spherical quantum dot using the local spin-density approximation. We experiment with a variety of confining potentials (triangular, harmonic, square well, etc.) and with a varying number of electrons $(N=2$ to 20). We carry out a detailed study of the scaling behavior of the ``Hubbard $U\mathrm{''}$ potential, which is a measure of the capacitive energy, with quantum dot size R $(U\ensuremath{\sim}{1/R}^{\ensuremath{\beta}}).$ We find that the scaling exponent $\ensuremath{\beta}$ is $\ensuremath{\approx}1/2$ for harmonic confinement and equal to 1 for the square-well confinement. The dependence of the scaling exponents on the confining potential and the number of electrons N is elucidated. We also examine the relative importance of Coulomb, exchange, and correlation terms in the Hubbard U potential and find that correlation plays a relatively more important role at a larger size. We provide a partial explanation for the value of the exponent in the Appendix.

We carry out a detailed investigation of neutral ($D^0$) and charged ($D^-$) impurity states of hydrogen-like donors in spherical semiconductor quantum dots. The investigation is carried out within the effective mass theory (EMT). We take recourse to local density approximation (LDA) and the Harbola-Sahni (HS) schemes for treating many-body effects. We experiment with a variety of confining potentials: square, harmonic and triangular. We observe that the donor level undergoes shallow to deep transition as the dot radius ($R$) is reduced. On further reduction of the dot radius it becomes shallow again. We term this non-monotonic behaviour \textbf{SHADES}. This suggests the possibility of carrier {\textbf{\textit{``freeze out''}}} for both $D^0$ and $D^-$. Further, our study of the optical gaps also reveals a {\textbf{SHADES}} transition., Comment: 19 pages, 8 figures, Revised Version

Using first-principles simulations, we have computed incorporation energies and diffusion barriers of ammonia, the nitrogen molecule and atomic nitrogen in monoclinic hafnia (m-HfO2). Our calculations show that ammonia is likely to dissociate into an NH2 molecular unit, whereas the nitrogen molecule remains as a molecule either in the interstitial space or at an oxygen lattice site. The lowest energy pathway for the diffusion of atomic nitrogen interstitials consists of the hopping of the nitrogen interstitial between neighboring three-coordinated lattice oxygen atoms that share a single Hf atom, and the barrier for such hops is determined by a switching mechanism. The substitutional nitrogen atom shows a preference for diffusion through the doubly positive oxygen vacancy-mediated mechanism. Furthermore, we have investigated the impact of nitrogen atoms on the diffusion barriers of oxygen and hydrogen interstitials in m-HfO2. Our results show that nitrogen incorporation has a significant impact on the barriers for oxygen and hydrogen diffusion: nitrogen atoms attract oxygen and hydrogen interstitials diffusing in the vicinity, thereby slowing down (reducing) their diffusion (diffusion length).

Semiconductor quantum dots (QDs) came to be identified as ``artificial atoms'' on account of sophisticated and careful experiments on single-electron and Coulomb blockade effects in the last decade. Earlier works on quantum confinement and excitonic effects modeled QDs by a square-well potential with a finite or infinite barrier. On the other hand, Coulomb blockade calculations are generally performed using parabolic confinement. However, there is a growing realization that these shapes of the potential may not represent the correct picture. In this paper we study the many electron effects in QDs within the local-density approximation and the Harbola-Sahni scheme. The latter incorporates the exchange effects essentially exactly. Further, we investigate the effect of the shape of the potential on these systems. The shapes considered are quasitriangular, quasiharmonic, and varying degrees of quasisquare. We examine the level spacing, level degeneracy, and the scaling of the electron-electron interaction with size. We find that the scaling laws have a complex dependence on the size and shape of the potential. Finally, we compare our calculations with experiments on a CdSe quantum dot.

Electronic screening is a many-body phenomenon that strongly depends on the electron-electron interaction in an electron gas. Here, relaxing the assumption of “infinite and homogeneous background,” we show that the dielectric environment surrounding the electron gas alters the electron-electron interaction leading to substantial re-normalization of static and dynamic response of the electron gas. The re-normalization of static screening function by dielectric environment impacts experimentally measurable mobility as shown for Si-inversion layer for illustration. Results are analytical and can be used for accurate prediction of transport quantity for any two dimensional gas system with inhomogeneous dielectric environment.

We investigate the physical properties of a portion of the gate stack of an ultra-scaled complementary metal-oxide-semiconductor (CMOS) device. The effects of point defects, such as oxygen vacancy, oxygen, and aluminum interstitials at the HfO2/TiN interface, on the effective work function of TiN are explored using density functional theory. We compute the diffusion barriers of such point defects in the bulk TiN and across the HfO2/TiN interface. Diffusion of these point defects across the HfO2/TiN interface occurs during the device integration process. This results in variation of the effective work function and hence in the threshold voltage variation in the devices. Further, we simulate the effects of varying the HfO2/TiN interface stoichiometry on the effective work function modulation in these extremely-scaled CMOS devices. Our results show that the interface rich in nitrogen gives higher effective work function, whereas the interface rich in titanium gives lower effective work function, compared to a stoichiometric HfO2/TiN interface. This theoretical prediction is confirmed by the experiment, demonstrating over 700 meV modulation in the effective work function.

Using first principles calculations, we have studied the dielectric properties of crystalline α- and β-phase silicon germanium nitrides and silicon carbon nitrides, A3−ξBξN4 (A = Si, B = Ge or C, ξ=0,1,2,3). In silicon germanium nitrides, both the high-frequency and static dielectric constants increase monotonically with increasing germanium concentration, providing a straightforward way to tune the dielectric constant of these materials. In the case of silicon carbon nitrides, the high-frequency dielectric constant increases monotonically with increasing carbon concentration, but a more complex trend is observed for the static dielectric constant, which can be understood in terms of competition between changes in the unit-cell volume and the average oscillator strength. The computed static dielectric constants of C3N4, Si3N4, and Ge3N4 are 7.13, 7.69, and 9.74, respectively.