Charge compensation and Ce3+ formation in trivalent doping of the CeO2(110) surface: The key role of dopant ionic radius

In this paper, we use density functional theory corrected for on-site Coulomb interactions (DFT + U) and hybrid DFT (HSE06 functional) to study the defects formed when the ceria (110) surface is doped with a series of trivalent dopants, namely, Al3+, Sc3+, Y3+, and In 3+. Using the hybrid DFT HSE06 exchange-correlation functional as a benchmark, we show that doping the (110) surface with a single trivalent ion leads to formation of a localized MCe / + O O • (M = the 3+ dopant), O- hole state, confirming the description found with DFT + U. We use DFT + U to investigate the energetics of dopant compensation through formation of the 2MCe ′ +VO ̈ defect, that is, compensation of two dopants with an oxygen vacancy. In conjunction with earlier work on La-doped CeO2, we find that the stability of the compensating anion vacancy depends on the dopant ionic radius. For Al3+, which has the smallest ionic radius, and Sc3+ and In3+, with intermediate ionic radii, formation of a compensating oxygen vacancy is stable. On the other hand, the Y3+ dopant, with an ionic radius close to that of Ce4+, shows a positive anion vacancy formation energy, as does La3+, which is larger than Ce4+ (J. Phys.: Condens. Matter 2010, 20, 135004). When considering the resulting electronic structure, in Al3+ doping, oxygen hole compensation is found. However, Sc 3+, In3+, and Y3+ show the formation of a reduced Ce3+ cation and an uncompensated oxygen hole, similar to La3+. These results suggest that the ionic radius of trivalent dopants strongly influences the final defect formed when doping ceria with 3+ cations. In light of these findings, experimental investigations of these systems will be welcome.

Nanoporous InP: anodic formation and growth mechanism in aqueous electrolytes

Porous layers can be formed electrochemically on (100) oriented n-InP substrates in aqueous KOH. A nanoporous layer is obtained underneath a dense near-surface layer and the pores appear to propagate from holes through the near-surface layer. In the early stages of the anodization transmission electron microscopy (TEM) clearly shows individual porous domains that appear to have a square-based pyramidal shape. Each domain appears to develop from an individual surface pit which forms a channel through this near-surface layer. We suggest that the pyramidal structure arises as a result of preferential pore propagation along the <100> directions. AFM measurements show that the density of surface pits increases with time. Each of these pits acts as a source for a pyramidal porous domain. When the domains grow, the current density increases correspondingly. Eventually the domains meet, forming a continuous porous layer, the interface between the porous and bulk InP becomes relatively flat and its total effective surface area decreases resulting in a decrease in the current density. Current-time curves at constant potential exhibit a peak and porous layers are observed to form beneath the electrode surface. The density of pits formed on the surface increases with time and approaches a plateau value. Porous layers are also observed in highly doped InP but are not observed in wafers with doping densities below ~5 × 1017 cm-3. Numerical models of this process have been developed invoking a mechanism of directional selectivity of pore growth preferentially along the <100> lattice directions. Manipulation of the parameters controlling these curves shows that the fall-off in current is controlled by the rate of diffusion of electrolyte through the pore structure with the final decline in current being caused by the termination of growth at the pore tips through the formation of passivating films or some other irreversible modification of the pore tips.

Hybrid density functional theory description of N- and C-doping of NiO

The large intrinsic bandgap of NiO hinders its potential application as a photocatalyst under visible-light irradiation. In this study, we have performed first-principles screened exchange hybrid density functional theory with the HSE06 functional calculations of N- and C-doped NiO to investigate the effect of doping on the electronic structure of NiO. C-doping at an oxygen site induces gap states due to the dopant, the positions of which suggest that the top of the valence band is made up primarily of C 2p-derived states with some Ni 3d contributions, and the lowest-energy empty state is in the middle of the gap. This leads to an effective bandgap of 1.7 eV, which is of potential interest for photocatalytic applications. N-doping induces comparatively little dopant-Ni 3d interactions, but results in similar positions of dopant-induced states, i.e., the top of the valence band is made up of dopant 2p states and the lowest unoccupied state is the empty gap state derived from the dopant, leading to bandgap narrowing. With the hybrid density functional theory (DFT) results available, we discuss issues with the DFT corrected for on-site Coulomb description of these systems.

Modifying germanium nanowires for future devices: an in situ TEM study

Germanium was of great interest in the 1950’s when it was used for the first transistor device. However, due to the water soluble and unstable oxide it was surpassed by silicon. Today, as device dimensions are shrinking the silicon oxide is no longer suitable due to gate leakage and other low-κ dielectrics such as Al2O3 and HfO2 are being used. Germanium (Ge) is a promising material to replace or integrate with silicon (Si) to continue the trend of Moore’s law. Germanium has better intrinsic mobilities than silicon and is also silicon fab compatible so it would be an ideal material choice to integrate into silicon-based technologies. The progression towards nanoelectronics requires a lot of in depth studies. Dynamic TEM studies allow observations of reactions to allow a better understanding of mechanisms and how an external stimulus may affect a material/structure. This thesis details in situ TEM experiments to investigate some essential processes for germanium nanowire (NW) integration into nanoelectronic devices; i.e. doping and Ohmic contact formation. Chapter 1 reviews recent advances in dynamic TEM studies on semiconductor (namely silicon and germanium) nanostructures. The areas included are nanowire/crystal growth, germanide/silicide formation, irradiation, electrical biasing, batteries and strain. Chapter 2 details the study of ion irradiation and the damage incurred in germanium nanowires. An experimental set-up is described to allow for concurrent observation in the TEM of a nanowire following sequential ion implantation steps. Grown nanowires were deposited on a FIB labelled SiN membrane grid which facilitated HRTEM imaging and facile navigation to a specific nanowire. Cross sections of irradiated nanowires were also performed to evaluate the damage across the nanowire diameter. Experiments were conducted at 30 kV and 5 kV ion energies to study the effect of beam energy on nanowires of varied diameters. The results on nanowires were also compared to the damage profile in bulk germanium with both 30 kV and 5 kV ion beam energies. Chapter 3 extends the work from chapter 2 whereby nanowires are annealed post ion irradiation. In situ thermal annealing experiments were conducted to observe the recrystallization of the nanowires. A method to promote solid phase epitaxial growth is investigated by irradiating only small areas of a nanowire to maintain a seed from which the epitaxial growth can initiate. It was also found that strain in the nanowire greatly effects defect formation and random nucleation and growth. To obtain full recovery of the crystal structure of a nanowire, a stable support which reduces strain in the nanowire is essential as well as containing a seed from which solid phase epitaxial growth can initiate. Chapter 4 details the study of nickel germanide formation in germanium nanostructures. Rows of EBL (electron beam lithography) defined Ni-capped germanium nanopillars were extracted in FIB cross sections and annealed in situ to observe the germanide formation. Chapter 5 summarizes the key conclusions of each chapter and discusses an outlook on the future of germanium nanowire studies to facilitate their future incorporation into nanodevices.

Chemical approaches for doping nanodevice architectures

Advanced doping technologies are key for the continued scaling of semiconductor devices and the maintenance of device performance beyond the 14 nm technology node. Due to limitations of conventional ion-beam implantation with thin body and 3D device geometries, techniques which allow precise control over dopant diffusion and concentration, in addition to excellent conformality on 3D device surfaces, are required. Spin-on doping has shown promise as a conventional technique for doping new materials, particularly through application with other dopant methods, but may not be suitable for conformal doping of nanostructures. Additionally, residues remain after most spin-on-doping processes which are often difficult to remove. In-situ doping of nanostructures is especially common for bottom-up grown nanostructures but problems associated with concentration gradients and morphology changes are commonly experienced. Monolayer doping (MLD) has been shown to satisfy the requirements for extended defect-free, conformal and controllable doping on many materials ranging from traditional silicon and germanium devices to emerging replacement materials such as III-V compounds but challenges still remain, especially with regard to metrology and surface chemistry at such small feature sizes. This article summarises and critically assesses developments over the last number of years regarding the application of gas and solution phase techniques to dope silicon-, germanium- and III-V-based materials and nanostructures to obtain shallow diffusion depths coupled with high carrier concentrations and abrupt junctions.

Organic functionalisation, doping and characterisation of semiconductor surfaces for future CMOS device applications

Organic Functionalisation, Doping and Characterisation of Semiconductor Surfaces for Future CMOS Device Applications Semiconductor materials have long been the driving force for the advancement of technology since their inception in the mid-20th century. Traditionally, micro-electronic devices based upon these materials have scaled down in size and doubled in transistor density in accordance with the well-known Moore’s law, enabling consumer products with outstanding computational power at lower costs and with smaller footprints. According to the International Technology Roadmap for Semiconductors (ITRS), the scaling of metal-oxide-semiconductor field-effect transistors (MOSFETs) is proceeding at a rapid pace and will reach sub-10 nm dimensions in the coming years. This scaling presents many challenges, not only in terms of metrology but also in terms of the material preparation especially with respect to doping, leading to the moniker “More-than-Moore”. Current transistor technologies are based on the use of semiconductor junctions formed by the introduction of dopant atoms into the material using various methodologies and at device sizes below 10 nm, high concentration gradients become a necessity. Doping, the controlled and purposeful addition of impurities to a semiconductor, is one of the most important steps in the material preparation with uniform and confined doping to form ultra-shallow junctions at source and drain extension regions being one of the key enablers for the continued scaling of devices. Monolayer doping has shown promise to satisfy the need to conformally dope at such small feature sizes. Monolayer doping (MLD) has been shown to satisfy the requirements for extended defect-free, conformal and controllable doping on many materials ranging from the traditional silicon and germanium devices to emerging replacement materials such as III-V compounds This thesis aims to investigate the potential of monolayer doping to complement or replace conventional doping technologies currently in use in CMOS fabrication facilities across the world.

Low valence cation doping of bulk Cr2O3: Charge compensation and oxygen vacancy formation

The different oxidation states of chromium allow its bulk oxide form to be reducible, facilitating the oxygen vacancy formation process, which is a key property in applications such as catalysis. Similar to other useful oxides such as TiO2, and CeO2, the effect of substitutional metal dopants in bulk Cr2O3 and its effect on the electronic structure and oxygen vacancy formation are of interest, particularly in enhancing the latter. In this paper, density functional theory (DFT) calculations with a Hubbard + U correction (DFT+U) applied to the Cr 3d and O 2p states, are carried out on pure and metal-doped bulk Cr2O3 to examine the effect of doping on the electronic and geometric structure. The role of dopants in enhancing the reducibility of Cr2O3 is examined to promote oxygen vacancy formation. The dopants are Mg, Cu, Ni, and Zn, which have a formal +2 oxidation state in their bulk oxides. Given this difference in host and, dopant oxidation states, we show that to predict the correct ground state two metal dopants charge compensated with an oxygen vacancy are required. The second oxygen atom removed is termed "the active" oxygen vacancy and it is the energy required to remove this atom that is related to the reduction process. In all cases, we find that substitutional doping improves the oxygen vacancy formation of bulk Cr2O3 by lowering the energy cost.