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Register a new userElectronic Structure Shift of Deep Nanoscale Silicon by SiO$_2$- vs. Si$_3$N$_4$-Embedding as Alternative to Impurity Doping
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Conventional impurity doping of deep nanoscale silicon (dns-Si) used in ultra large scale integration (ULSI) faces serious challenges below the 14 nm technology node. We report on a new fundamental effect in theory and experiment, namely the electronic structure of dns-Si experiencing energy offsets of ca. 1 eV as a function of SiO$_2$- vs. Si$_3$N$_4$-embedding with a few monolayers (MLs). An interface charge transfer (ICT) from dns-Si specific to the anion type of the dielectric is at the core of this effect and arguably nested in quantum-chemical properties of oxygen (O) and nitrogen (N) vs. Si. We investigate the size up to which this energy offset defines the electronic structure of dns-Si by density functional theory (DFT), considering interface orientation, embedding layer thickness, and approximants featuring two Si nanocrystals (NCs); one embedded in SiO$_2$ and the other in Si$_3$N$_4$. Working with synchrotron ultraviolet photoelectron spectroscopy (UPS), we use SiO$_2$- vs. Si$_3$N$_4$-embedded Si nanowells (NWells) to obtain their energy of the top valence band states. These results confirm our theoretical findings and gauge an analytic model for projecting maximum dns-Si sizes for NCs, nanowires (NWires) and NWells where the energy offset reaches full scale, yielding to a clear preference for electrons or holes as majority carriers in dns-Si. Our findings can replace impurity doping for n/p-type dns-Si as used in ultra-low power electronics and ULSI, eliminating dopant-related issues such as inelastic carrier scattering, thermal ionization, clustering, out-diffusion and defect generation. As far as majority carrier preference is concerned, the elimination of those issues effectively shifts the lower size limit of Si-based ULSI devices to the crystalization limit of Si of ca. 1.5 nm and enables them to work also under cryogenic conditions.
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To quantify charge transport through molecular junctions fabricated using the conducting probe atomic force microscopy (CP-AFM) platform, information on the number of molecules $N$ per junction is absolutely necessary. $N$ can be currently obtained only via contact mechanics, and the Youngs modulus $E$ of the self-assembled monolayer (SAM) utilized in the key quantity for this approach. The experimental determination of $E$ for SAMs of CP-AFM junctions fabricated using oligophenylene dithiols (OPDn, $1 leq n leq 4$) and gold electrodes turned out to be too challenging. Recent measurements (Z. Xie et al, J. Am. Chem. Soc. 139 (2017) 5696) merely succeeded to provide a low bound estimate ($E approx 58,$GPa). It is this state of affairs that motivated the present theoretical investigation. Our microscopic calculations yield values $E approx 240 pm 6,$GPa for the OPDn SAMs of the aforementioned experimental study, which are larger than those of steel ($ E approx 180 - 200,$GPa) and silicon ($E approx 130 - 185,$GPa). The fact that the presently computed $E$ is much larger than the aforementioned experimental lower bound explain why experimentally measuring $E$ of OPDn SAMs is so challenging. Having $E approx 337 pm 8,$GPa, OPDn SAMs with herringbone arrangement adsorbed on fcc (111)Au are even stiffer than Si$_3$N$_4$ ($E approx 160 - 290,$GPa).
We study the electronic and structural properties of substitutional impurities of graphenelike nanoporous materials C$_2$N, $tg$-, and $hg$-C$_3$N$_4$ by means of density functional theory calculations. We consider four types of impurities; boron substitution on carbon sites (B(C)), carbon substitution on nitrogen sites (C(N)), nitrogen substitution on carbon sites (N(C)), and sulfur substitution on nitrogen sites (S(N)). From cohesive energy calculations, we find that the C(N) and B(C) substitutions are the most energetically favorable and induce small bond modifications in the vicinity of the impurity, while the S(N) induces strong lattice distortions. Though all of the studied impurities induce defect levels inside the band gap of these materials, their electronic properties are poles apart depending on the behavior of the impurity as an acceptor or a donor. It is also observed that acceptor (donor) wavefunctions are composed only of $sigma$ ($pi$) orbitals from the impurity itself and/or neighboring sites. Consequently, acceptor wavefunctions are directed towards the pores and donor wavefunctions are more extended throughout the neighboring atoms, a property that could further be explored to modify the interaction between these materials and adsorbates. Moreover, impurity properties display a strong site sensitivity and ground state binding energies ranging from $0.03$ to $1.13$ eV, thus offering an interesting route for tuning the optical properties of these materials. Finally, spin-polarized calculations reveal that all impurity configurations have a magnetic ground state that rises from the spin splitting of the impurity levels. In a few configurations, more than one impurity level can be found inside the gap and two of them could potentially be explored as two-level systems for single-photon emission, following similar proposals recently made on defect complexes on TMDCs.
We investigate the structural and quantum transport properties of isotopically enriched $^{28}$Si/$^{28}$SiO$_2$ stacks deposited on 300 mm Si wafers in an industrial CMOS fab. Highly uniform films are obtained with an isotopic purity greater than 99.92%. Hall-bar transistors with an equivalent oxide thickness of 17 nm are fabricated in an academic cleanroom. A critical density for conduction of $1.75times10^{11}$ cm$^{-2}$ and a peak mobility of 9800 cm$^2$/Vs are measured at a temperature of 1.7 K. The $^{28}$Si/$^{28}$SiO$_2$ interface is characterized by a roughness of $Delta=0.4$ nm and a correlation length of $Lambda=3.4$ nm. An upper bound for valley splitting energy of 480 $mu$eV is estimated at an effective electric field of 9.5 MV/m. These results support the use of wafer-scale $^{28}$Si/$^{28}$SiO$_2$ as a promising material platform to manufacture industrial spin qubits.
Metal contacts to two-dimensional (2D) semiconductors are ubiquitous in modern electronic and optoelectronic devices. Such contacts are, however, often plagued by strong Fermi level pinning (FLP) effect which reduces the tunability of the Schottky barrier height (SBH) and degrades the performance of 2D-semiconductor-based devices. In this work, we show that monolayer MoSi$_2$N$_4$ and WSi$_2$N$_4$ - a recently synthesized 2D material class with exceptional mechanical and electronic properties - exhibit strongly suppressed FLP and wide-range tunable SBH when contacted by metals. An exceptionally large SBH slope parameter of S=0.7 is obtained, which outperform the vast majority of other 2D semiconductors. Such surprising behavior arises from the unique morphology of MoSi$_2$N$_4$ and WSi$_2$N$_4$. The outlying Si-N layer forms a native atomic layer that protects the semiconducting inner-core from the perturbance of metal contacts, thus suppressing the FLP. Our findings reveal the potential of MoSi$_2$N$_4$ and WSi$_2$N$_4$ monolayers as a novel 2D material platform for designing high-performance and energy-efficient 2D nanodevices.
We report the growth, structural and magnetic properties of the less studied Eu-oxide phase, Eu$_3$O$_4$, thin films grown on a Si/SiO$_2$ substrate and Si/SiO$_2$/graphene using molecular beam epitaxy. The X-ray diffraction scans show that highly-textured crystalline Eu$_3$O$_4$(001) films are grown on both substrates, whereas the film deposited on graphene has a better crystallinity than that grown on the Si/SiO$_2$ substrate. The SQUID measurements show that both films have a Curie temperature of about 5.5 K, with a magnetic moment of 0.0032 emu/g at 2 K. The mixed-valency of the Eu cations has been confirmed by the qualitative analysis of the depth-profile X-ray photoelectron spectroscopy measurements with the Eu$^{2+}$ : Eu$^{3+}$ ratio of 28 : 72. However, surprisingly, our films show no metamagnetic behaviour as reported for the bulk and powder form. Furthermore, the Raman spectroscopy scans show that the growth of the Eu$_3$O$_4$ thin films has no damaging effect on the underlayer graphene sheet. Therefore, the graphene layer is expected to retain its properties.
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