Michelle M. Paquette

The local physical structure of amorphous hydrogenated boron carbide: insights from magic angle spinning solid-state NMR spectroscopy

Magic angle spinning solid-state nuclear magnetic resonance spectroscopy techniques are applied to the elucidation of the local physical structure of an intermediate product in the plasma-enhanced chemical vapour deposition of thin-film amorphous hydrogenated boron carbide (B(x)C:H(y)) from an orthocarborane precursor. Experimental chemical shifts are compared with theoretical shift predictions from ab initio calculations of model molecular compounds to assign atomic chemical environments, while Lee-Goldburg cross-polarization and heteronuclear recoupling experiments are used to confirm atomic connectivities. A model for the B(x)C:H(y) intermediate is proposed wherein the solid is dominated by predominantly hydrogenated carborane icosahedra that are lightly cross-linked via nonhydrogenated intraicosahedral B atoms, either directly through B-B bonds or through extraicosahedral hydrocarbon chains. While there is no clear evidence for extraicosahedral B aside from boron oxides, ∼40% of the C is found to exist as extraicosahedral hydrocarbon species that are intimately bound within the icosahedral network rather than in segregated phases.

The influence of hydrogen on the chemical, mechanical, optical/electronic, and electrical transport properties of amorphous hydrogenated boron carbide

Because of its high electrical resistivity, low dielectric constant (κ), high thermal neutron capture cross section, and robust chemical, thermal, and mechanical properties, amorphous hydrogenated boron carbide (a-BxC:Hy) has garnered interest as a material for low-κ dielectric and solid-state neutron detection applications. Herein, we investigate the relationships between chemical structure (atomic concentration B, C, H, and O), physical/mechanical properties (density, porosity, hardness, and Youngs modulus), electronic structure [band gap, Urbach energy (EU), and Tauc parameter (B1/2)], optical/dielectric properties (frequency-dependent dielectric constant), and electrical transport properties (resistivity and leakage current) through the analysis of a large series of a-BxC:Hy thin films grown by plasma-enhanced chemical vapor deposition from ortho-carborane. The resulting films exhibit a wide range of properties including H concentration from 10% to 45%, density from 0.9 to 2.3 g/cm3, Youngs modulus ...

Valence and conduction band offsets at amorphous hexagonal boron nitride interfaces with silicon network dielectrics

To facilitate the design of heterostructure devices employing hexagonal/sp2 boron nitride, x-ray photoelectron spectroscopy has been utilized in conjunction with prior reflection electron energy loss spectroscopy measurements to determine the valence and conduction band offsets (VBOs and CBOs) present at interfaces formed between amorphous hydrogenated sp2 boron nitride (a-BN:H) and various low- and high-dielectric-constant (k) amorphous hydrogenated silicon network dielectric materials (a-SiX:H, X = O, N, C). For a-BN:H interfaces formed with wide-band-gap a-SiO2 and low-k a-SiOC:H materials (Eg ≅ 8.2−8.8 eV), a type I band alignment was observed where the a-BN:H band gap (Eg = 5.5 ± 0.2 eV) was bracketed by a relatively large VBO and CBO of ∼1.9 and 1.2 eV, respectively. Similarly, a type I alignment was observed between a-BN:H and high-k a-SiC:H where the a-SiC:H band gap (Eg = 2.6 ± 0.2 eV) was bracketed by a-BN:H with VBO and CBO of 1.0 ± 0.1 and 1.9 ± 0.2 eV, respectively. The addition of O or N to ...

The electronic and chemical structure of the a-B3CO0.5:Hy-to-metal interface from photoemission spectroscopy: implications for Schottky barrier heights

The electronic and chemical structure of the metal-to-semiconductor interface was studied by photoemission spectroscopy for evaporated Cr, Ti, Al and Cu overlayers on sputter-cleaned as-deposited and thermally treated thin films of amorphous hydrogenated boron carbide (a-B(x)C:H(y)) grown by plasma-enhanced chemical vapor deposition. The films were found to contain ~10% oxygen in the bulk and to have approximate bulk stoichiometries of a-B(3)CO(0.5):H(y). Measured work functions of 4.7/4.5 eV and valence band maxima to Fermi level energy gaps of 0.80/0.66 eV for the films (as-deposited/thermally treated) led to predicted Schottky barrier heights of 1.0/0.7 eV for Cr, 1.2/0.9 eV for Ti, 1.2/0.9 eV for Al, and 0.9/0.6 eV for Cu. The Cr interface was found to contain a thick partial metal oxide layer, dominated by the wide-bandgap semiconductor Cr(2)O(3), expected to lead to an increased Schottky barrier at the junction and the formation of a space-charge region in the a-B(3)CO(0.5):H (y) layer. Analysis of the Ti interface revealed a thick layer of metal oxide, comprising metallic TiO and Ti (2)O (3), expected to decrease the barrier height. A thinner, insulating Al(2)O(3) layer was observed at the Al-to-a-B(3)CO(0.5):H(y) interface, expected to lead to tunnel junction behavior. Finally, no metal oxides or other new chemical species were evident at the Cu-to-a-B(3)CO(0.5):H(y) interface in either the core level or valence band photoemission spectra, wherein characteristic metallic Cu features were observed at very thin overlayer coverages. These results highlight the importance of thin-film bulk oxygen content on the metal-to-semiconductor junction character as well as the use of Cu as a potential Ohmic contact material for amorphous hydrogenated boron carbide semiconductor devices such as high-efficiency direct-conversion solid-state neutron detectors.

Combinatorial survey of fluorinated plasma etching in the silicon-oxygen-carbon-nitrogen-hydrogen system

New multipass optical lithography patterning methods needed to print features for future <10 nm technologies will demand an increasingly complex combination of hardmasks, antireflection coatings, spacers, and etch stopping materials with distinct yet complementary properties. Finding the right mix, however, has proven particularly challenging given that the materials most commonly used are drawn from a limited set of Si- and C-based solids comprising Si, C, O, N, and H. To understand and maximize the limits of this composition space, the authors have performed a combinatorial survey of the dry etch characteristics for the Si-C-O-N-H system with the goal of understanding material composition–etch interactions and identifying material subclasses with distinct etch properties. Over 50 thin films sampling Si-C-O-N-H composition space were surveyed using two fluorinated etches commonly utilized for selective patterning of SiO2 and a-SiN:H dielectrics (CHF3 and CF4/O2, respectively). It was found that the incor...

Boron and high-k dielectrics: Possible fourth etch stop colors for multipattern optical lithography processing

In a companion article, the etch characteristics of materials within the Si-C-O-N-H system were surveyed using two common fluorinated plasma etches used to etch SiO2 interlayer dielectrics and SiN:H etch stop layers (CHF3 and CF4/O2, respectively) with the goal of identifying new materials or “colors” to assist in the simplification of advanced multipass optical lithography. In this study, the authors investigate additional materials outside the traditional Si-C-O-N-H phase diagram with the hope of identifying potential third or fourth color pattern-assist materials. The specific materials investigated include a series of high-k dielectrics commonly used in the industry (Al2O3, AlN, and HfO2) and boron-based solids (a-B:H, a-BxN:H, a-BxP:H, and a-BxC:H) that have been previously identified as potential hard mask, polish stop, and/or low-k dielectric materials. The high-k dielectrics were all found to exhibit low to unmeasureable etch rates in both fluorinated etches. In contrast, the boron-based solids al...

Steady-state space-charge-limited current analysis of mobility with negative electric field dependence

We revisit the theory of steady-state space-charge-limited current (SS-SCLC) enhanced by Frenkel emission originally addressed by Murgatroyd using analytical rather than numerical integration to obtain an exact solution. For the first time, the analysis is also extended to the case of mobility exhibiting negative field dependence, generally observed in disordered materials at lower electric field. For the case of positive electric field dependence, we confirm that Murgatroyds original solution is close to the exact solution for low and moderately high electric fields. At a very high field, the exact solution is consistent with the analytical solution given by Barbe. For the case of negative electric field dependence, the fit expression retains the same functional form as Murgatroyds solution, however with a different exponential factor of −0.801. The development of SS-SCLC theory for the case of negative field dependence is an important step in the generalization of this analysis technique to the investigation of complex materials such as disordered organic semiconductors.We revisit the theory of steady-state space-charge-limited current (SS-SCLC) enhanced by Frenkel emission originally addressed by Murgatroyd using analytical rather than numerical integration to obtain an exact solution. For the first time, the analysis is also extended to the case of mobility exhibiting negative field dependence, generally observed in disordered materials at lower electric field. For the case of positive electric field dependence, we confirm that Murgatroyds original solution is close to the exact solution for low and moderately high electric fields. At a very high field, the exact solution is consistent with the analytical solution given by Barbe. For the case of negative electric field dependence, the fit expression retains the same functional form as Murgatroyds solution, however with a different exponential factor of −0.801. The development of SS-SCLC theory for the case of negative field dependence is an important step in the generalization of this analysis technique to the invest...

Nanoscale Structure-Property Relationship in Amorphous Hydrogenated Boron Carbide for Low-k Dielectric Applications

One of the main challenges in scaling of future integrated circuits is the need for new low dielectric constant (low-k) materials that can substantially reduce the resistance-capacitance delay at the system level. The new low-k material must also overcome the fatal falloff in the mechanical properties, known as the low-k death curve, that is typically associated with the low density of the material required to achieve low k values [1]. Amorphous hydrogenated boron carbide (a-BC:H) combines low k with other advantages, including chemical, thermal, electrical, and mechanical reliability, and therefore has recently gained attention as a high-performing low-k material alternative to conventional SiO2 and SiOC:H/SiCO:H. Our a-BC:H films are synthesized using plasma-enhanced chemical vapor deposition of molecular ortho-carborane, which converts into an amorphous polymer upon deposition [2]. The method enables a high degree of variation in structure and composition (a-BxC:Hy), which allows for tuning the properties of the material by controlling its nanoscale structure. For example, controlling the degree of medium range order (MRO) arising from the connection or gathering of icosahedral BxC orthocarborane clusters (Fig. 1a) can enable tuning the dielectric constant and other important properties of the material with high flexibility. Understanding the MRO structure of a-BC:H is therefore crucial, but the characterization of MRO has been difficult because conventional imaging or diffraction methods are insensitive to such ordering due to its small size and elusive nature.