The Fermi level is a foundational concept in materials science, holding great importance in understanding the electronic and chemical properties of different structures. For materials with charge-density-free regions, determining the Fermi level position using traditional ab initio periodic calculations is accurate only under neutral supercell conditions. The reason is that the introduction of additional charges within the supercell triggers an automatic distribution of a homogeneous compensating background charge (CBC), tasked with maintaining electrical neutrality and avoiding unphysical interactions between the periodic copies of the supercell, which makes the vacuum potential no longer well-defined as a reliable standard energy reference. Here, by employing the virtual crystal approximation (VCA) implemented within density functional theory (DFT), we develop a crystal-structureindependent method to negatively charge the bulk region enclosed by hydrogen atoms, successfully reducing the effects of spurious electrostatic interactions in the vacuum spacing while ensuring a constant Z-plane-averaged electrostatic potential (PAEP). We test this method on graphene and molybdenum disulfide (MoS2) as atomically thin two-dimensional (2D) systems and demonstrate that this method can achieve reasonable agreement with Fermi level determination techniques such as post-processing DFT numerical techniques and DFT-independent finite-element method simulations.

Crystalline nanowire (NWire) cross-section shapes are known to vary considerably in experiment; an accurate analytic evaluation of NWire cross-sections from a thermodynamic perspective is still missing. Building on previous work, analytic descriptions are given for wurtzite (w) NWire cross-section morphing to arbitrary convex hexagonal shapes in order to evaluate the crystallographic-structural stability of experimental NWire cross-sections. The maximum of the ratio of NWire-internal bonds per NWire atom describes the most stable crystallographic and thermodynamic shape of the NWire as far as internal forces are concerned. This maximum occurs when the first derivative of the above ratio , with presenting respective optimum morphing indices to obtain . The stability evaluation is then carried out by comparing derived from the experimental image over the complete morphing range of the NWire cross-section represented by . This user-friendly analytic approach allows to calculate the optimum morphing indices and thus all follow-on parameters, such as , , the number of interface bonds and the NWire cross-section area . The normalization of analytic results enables a direct comparison of cross-sections with different shape, size, and even material. General results of the analytic decriptions explain the preferred growth of w-NWires with growth vector and six facets by their highest ratio for the same cross-section area of all three NWire types considered. Examples with experimental data demonstrate the versatility and detailed insight the stability evaluation provides to any w-lattice NWire cross-section.

With the shrinking dimensions of semiconductor structures reaching the nanoscale, conventional impurity doping techniques face several challenges due to their statistical nature, temperature dependence, and degradation in efficiency of the doping method. In addition, the cryogenic operation of highly doped transistors is complicated due to carrier freeze-out, which significantly reduces the availability of mobile charges, degrading device performance and inducing noise. Here, an innovative material solution is presented that enables silicon nanowire junctionless transistors without requiring impurity doping within the active semiconductor region. To this end, a SiO2 dielectric shell with deliberate defect engineering surrounding both the channel and the contact regions - known as direct modulation doping─is used to modify the nanoscale transport properties of the silicon. The obtained active carrier densities in the experiment are comparable to highly impurity-doped devices in the range of ∼1018⁡cm-3 and remain stable over a broad temperature range from 400 K down to 77 K. The primary advantage of removing dopants from the channel is evident in the enhanced field-effect mobilities, which increase from 115 to 331 cm2V-1s-1 as temperature decreases. The fabricated nanowire transistors in this work provide a high on/off ratio of ≥106, and a stable on-state performance down to 77 K. Hybrid-density-functional-theory calculations are carried out to show that there are no fundamental roadblocks to employing the method to devices with ultrascaled dimensions. The device architecture is positioned for applications in energy-efficient cryo-electronics and quantum technologies by addressing the limitations associated with conventional impurity doping.

Setting out from previous work, an analytic description of regular wurtzite‐ (w‐) structure nanowires (NWires) is extended by introducing morphing terms to describe arbitrary convex cross sections featuring linear interfaces as encountered in experiment. Add‐on terms to the existing number series of regular cross sections are provided with their respective running indices, yielding the required flexibility for cross section morphing. The main variables are the number of NWire atoms , bonds between NWire atoms and interface bonds as a function of NWire size and shape, complemented by other basic geometric variables such as specific lengths of interface facets, as well as widths, heights, and total area of the cross section. Cross section morphing is demonstrated for the three high symmetry w‐NWires with low‐index faceting frequently occurring in NWire processing. The fundamental insights revealed here offer a universal gauge and thus enable major advancements in data interpretation and understanding of above‐mentioned w‐structure based NWires with arbitrary convex cross sections. As a corroborating example, a precise description of an irregular w‐GaAs/w‐Ge core/shell NWire cross section is given, whereby a radially changing lattice constant can be included.

The storage and release of energy is an economic cornerstone. In quantum dots (QDs), energy storage is mostly governed by their surfaces, in particular by surface chemistry and faceting. The impact of surface free energy (SFE) through surface faceting has already been studied in QDs. Here, we introduce dominant faceting representing the structural order of the surface. In particular, we propose that realistic QDs attain complicated polyhedral quasi-spherical shapes while keeping the dominance of a certain type of facet. The type of dominant facet determines the rates of surface-related processes. Therefore, by connecting dominant faceting with SFE, trends analogical to bulk material are kept despite the lack of evident microscopic shape control. To demonstrate the applicability of dominant faceting, we synthesize sets of silicon QDs with sizes around 5 nm and classify them based on increasing SFE of the corresponding analytic geometrical models, using a detailed surface chemistry analysis. Total energies released during oxidation of the synthesized QDs reach the theoretical limit, unlike in the reference, "large" (>100 nm) silicon nanoparticles, which release about 15% less energy. Next, we perform a comprehensive experimental study of dehydrogenation and thermal oxidation of the synthesized QDs in the temperature range of 25-1100 °C, identifying SFE as the key factor determining their thermal stability and surface reactivity. In particular, four distinctive stages of energy release were observed with onset temperatures ranging between 140 and 250 °C, ≈500 and 650-700 °C, respectively, for the SFE-differing samples. Finally, the thermal oxidation of the synthesized QDs is completed at lower temperatures with increasing SFE, decreasing from 1065 to 970 °C and being > 150 °C lower in QDs than in the larger reference nanoparticles. Therefore, despite a rich mixture of features, our description based on linking dominant faceting with SFE allows us to fully explain all the observed trends, demonstrating both the potential of SFE-based engineering of energy-storage properties in QDs and the prospects of silicon QDs as an energy-storage material.

Hard entropy limits of impurity doping prevent further miniaturization of low nanoscale silicon-based very large scale integration (VLSI) devices, thereby obstructing the path toward more energy-efficient VLSI designs with higher yield in compute power. As demonstrated here by synchrotron UV photoelectron spectroscopy (UPS) and X-ray absorption spectroscopy in total fluorescence yield mode (XAS-TFY), intrinsic Si at the bottom of the nanoscale (i-nano-Si) turns into strong p- or n-Si by embedding in silicon nitride (Si3N4) or silicon dioxide (SiO2), respectively. The associated Nanoscale Electronic Structure Shift Induced by Anions at Surfaces (NESSIAS) creates a p/n junction in i-nano-Si by the quantum-chemical impact of Si3N4- vs SiO2-coating, providing energy landscapes to accumulate electrons (holes) when SiO2- (Si3N4-) coated, with free charge carriers provided by metallic interconnects. Hybrid density functional theory (h-DFT) calculations demonstrate Si NWire FETs with physical gate lengths down to 3 nm, while the electronic structure remains stable under carrier injection. A mesoscopic band model derived from synchrotron characterization data on ultrathin embedded Si nanowells (NWells), and from h-DFT confirms and further explains the NESSIAS impact to generate p/n homojunctions in i-nano-Si. Presenting a paradigm shift for Si-based VLSI, NESSIAS removes miniaturization limits, achieves faster charge carrier transport with massive reductions of energy demand and associated heat generation as required for ultralow power VLSI, and enables full cryo-functionality for quantum computing.

Corrections to the article by König & Smith [Acta Cryst. (2022), B78, 665–677] are given.

Abstract Doping silicon on the nanoscale by the intentional introduction of impurities into the intrinsic semiconductor suffers from effects such as dopant deactivation, random dopant fluctuations, out‐diffusion, and mobility degradation. This paper presents the first experimental proof that doping of silicon nanowires can also be achieved via the purposeful addition of aluminium‐induced acceptor states to the SiO2 shell around a silicon nanowire channel. It is shown that modulation doping lowers the overall resistance of silicon nanowires with nickel silicide Schottky contacts by up to six orders of magnitude. The effect is consistently observed for various channel geometries and systematically studied as a function of Al2O3 content during fabrication. The transfer length method is used to separate the effects on the channel conductivity from that on the barriers. A silicon resistivity is achieved as low as 0.04–0.06 Ω ·cm in the nominal undoped material. In addition, the specific contact resistivity is also strongly influenced by the modulation doping and reduced down to 3.5E‐7 Ω · cm2, which relates to lowering the effective Schottky barrier to 0.09 eV. This alternative doping method has the potential to overcome the issues associated with doping and contact formation on the nanoscale.

High-energy-density lithium-ion batteries (LIBs) are essential to meet the requirements of emerging technologies for advanced power storage and enhanced device performance. The next generation of LIBs will require high-capacity anode materials that move beyond the lithium intercalation chemistry of conventional graphite electrodes. The use of two-dimensional (2D) bilayer structures offers immediate advantages in the development of LIBs. Herein, motivated by the recently synthesized 2D Cairo pentagon nickel diazenide (NiN2) material, we conduct a scrutiny of the intercalation process of lithium atoms in the interlayer gap of NiN2/NiN2 homostructure. Based on density functional theory (DFT), we demonstrate that the diffusion energy barrier of lithium move across the NiN2/NiN2 anode is relatively low, ranging from 0.058 to 0.52 eV, and the corresponding reversible capacity reaches a remarkable value of 499.0927 mA h g-1 per formula unit, surpassing that of graphite (372 mA h g-1). Furthermore, we investigate a 2D van der Waals (vdW) heterostructure composed of pre-strained structures of graphene and NiN2 for use as an anode material in LIBs. It is found that the introduction of graphene leads to improvements in both electrochemical activity and deformation characteristics. The presented results provide theoretical support for the potential of bilayer structures combining NiN2, suggesting them as promising candidates for the development of high-performance anode materials.

Setting out from our recent publication [König & Smith (2021). Acta Cryst. B77, 861], we extend our analytic description of the regular cross sections of zincblende- and diamond-structure nanowires (NWires) by introducing cross section morphing to arbitrary convex shapes featuring linear interfaces as encountered in experiment. To this end, we provide add-on terms to the existing number series with their respective running indices for zinc-blende- (zb-) and diamond-structure NWire cross sections. Such add-on terms to all variables yield the required flexibility for cross section morphing, with main variables presented by the number of NWire atoms NWire(dWire[i]), bonds between NWire atoms Nbnd(dWire[i]) and interface bonds NIF(dWire[i]). Other basic geometric variables, such as the specific length of interface facets, as well as widths, heights and total area of the cross section, are given as well. The cross sections refer to the six high-symmetry zb NWires with low-index faceting frequently occurring in the bottom-up and top-down approaches of NWire processing. The fundamental insights into NWire structures revealed here offer a universal gauge and thus enable major advancements in data interpretation and the understanding of all zb- and diamond-structure-based NWires with arbitrary convex cross sections. We corroborate this statement with an exact description of irregular Si NWire cross sections and irregular InGaAs/GaAs core-shell NWire cross sections, where a radially changing unit-cell parameter can be included.

The present paper studies with experiment and simulation a number of measures that improve current cryogenic MOSFETs to enable the device to be operated at very low supply voltages.

Doping of silicon nanostructures is crucial to understand their properties and to enhance their potential in various fields of application. Herein, SiO2‐embedded Si nanocrystals (quantum dots) ≈3–6 nm in diameter are used as a model system to study the incorporation of B dopants by X‐ray absorption near‐edge spectroscopy (XANES). Such samples represent a model system for ultimately scaled, 3D‐confined Si nanovolumes. The analysis is complemented by real‐space density functional theory to calculate the 1s (K shell) electron binding energies of B in 11 different, thermodynamically stable configurations of the Si/SiOx/SiO2 system. Although no indications for a substitutional B‐acceptor configuration are found, the predominant O coordination of B indicates the preferred B incorporation into the SiO2 matrix and near the Si‐nanocrystal/SiO2 interface, which is inherently incompatible with charge carrier generation by dopants. It is concluded that B doping of ultrasmall Si nanostructures fails due to a lack of B incorporation onto Si lattice sites that cannot be overcome by increasing the B concentration. The inability to efficiently insert B into Si nanovolumes appears to be a boron‐specific fundamental obstacle for electronic doping (e.g., not observed for phosphorus) that adds to the established nanosize effects, namely, increased dopant activation and ionization energies.

Impurity doping in silicon (Si) ultra-large-scale integration is one of the key challenges which prevent further device miniaturization. Using ultraviolet photoelectron spectroscopy and X-ray absorption spectroscopy in the total fluorescence yield mode, we show that the lowest unoccupied and highest occupied electronic states of ≤3 nm thick SiO2-coated Si nanowells shift by up to 0.2 eV below the conduction band and ca. 0.7 eV below the valence band edge of bulk silicon, respectively. This nanoscale electronic structure shift induced by anions at surfaces (NESSIAS) provides the means for low-nanoscale intrinsic Si (i-Si) to be flooded by electrons from an external (bigger, metallic) reservoir, thereby getting highly electron- (n-) conductive. While our findings deviate from the behavior commonly believed to govern the properties of silicon nanowells, they are further confirmed by the fundamental energy gap as per nanowell thickness when compared against published experimental data. Supporting our findings further with hybrid density functional theory calculations, we show that other group IV semiconductors (diamond, Ge) do respond to the NESSIAS effect in accord with Si. We predict adequate nanowire cross-sections (X-sections) from experimental nanowell data with a recently established crystallographic analysis, paving the way to undoped ultrasmall silicon electronic devices with significantly reduced gate lengths, using complementary metal-oxide-semiconductor-compatible materials.

General photoluminescence design rules for interstitial transition-metal-doped silicon nanocrystals are derived; Zn shows excellent properties for medical imaging and plasmonic microwave excitation to exactly eliminate marked cells.

Float‐zone (FZ) silicon often has grown‐in defects that are thermally activated in a broad temperature window (≈300–800 °C). These defects cause efficient electron‐hole pair recombination, which deteriorates the bulk minority carrier lifetime and thereby possible photovoltaic conversion efficiencies. Little is known so far about these defects which are possibly Si‐vacancy/nitrogen‐related (VxNy). Herein, it is shown that the defect activation takes place on sub‐second timescales, as does the destruction of the defects at higher temperatures. Complete defect annihilation, however, is not achieved until nitrogen impurities are effused from the wafer, as confirmed by secondary ion mass spectrometry. Hydrogenation experiments reveal the temporary and only partial passivation of recombination centers. In combination with deep‐level transient spectroscopy, at least two possible defect states are revealed, only one of which interacts with H. With the help of density functional theory V1N1‐centers, which induce Si dangling bonds (DBs), are proposed as one possible defect candidate. Such DBs can be passivated by H. The associated formation energy, as well as their sensitivity to light‐induced free carriers, is consistent with the experimental results. These results are anticipated to contribute to a deeper understanding of bulk‐Si defects, which are pivotal for the mitigation of solar cell degradation processes.

A p+-i-n+ self-cooled light-emitting diode with type-II band offset is numerically simulated in one-dimension to examine the underlying cooling/heating mechanisms. The Peltier effect is confirmed to be the dominant cooling mechanism under forward bias, even when the carriers are injected without an energy barrier. Meanwhile, Joule heating in the active layer is identified as the main heating mechanism for bandgaps below 0.52 eV under an ultra-low forward bias. In contrast to non-radiative recombination, electroluminescence itself is found to be a cooling mechanism, producing most photons above the bandgap of the active layer. However, this effect only becomes noticeable under an ultra-low bias in very small bandgap materials. While it is desirable to inject more carriers to leverage larger band offsets for a higher cooling power, Joule heating limits the maximum cooling power achievable. With small band offsets (<0.21 eV), a reverse bias instead of a forward bias may become the best cooling condition, where non-radiative generation processes are discovered to be the dominant cooling mechanisms.

Ground and adiabatic excited state properties of silicon nanocrystals (NCs) of 11 Å size with complete OH- or NH2- termination were calculated by Dirk König et al. (see article no. 1800336) by TD-DFT within the B3LYP/6-31 + G(d) formalism as function of interface bond modification, replacing two OH (-NH2) groups by O (NH) having a double- (=) or bridgebond (>). The increase in optical activity is considerable for =O and notable for >O modifications, resulting in 26-fold and 2-fold increase of absorption rate, respectively, over nominal OH-termination. The situation is not as pronounced for NH2 termination, with absorption rate increasing 6- and 2-fold for =NH and >NH, respectively, over nominal NH2-termination. The findings affect energies of the absorption edge, yielding a 10% decrease (7% increase) for =O (>O) over nominal OH-termination, while a mere 2% decrease was found for both, =NH and >NH, over nominal NH2-termination. These findings show that the impact of bond modifications of Si-NCs in SiO2 cannot be ignored, while the consequences for Si-NCs embedded in Si3N4 are tolerable. – Articles 1800296 and 1800336 in this issue have been dedicated to Dietrich R. T. Zahn on the occasion of his 60th birthday (cf. also the February 2019 issue of Phys. Status Solidi B, Vol. 256, issue 2).

Al-O monolayers embedded in ultrathin SiO2 were shown previously to contain Al-induced acceptor states, which capture electrons from adjacent silicon wafers and generate a negative fixed charge that enables efficient Si-surface passivation. Here, we show that this surface passivation is just in part attributed to field-effect passivation, since the electrically active interface trap density D-it itself at the Si/SiO2 interface is reduced by the presence of the acceptor states. For sufficiently thin tunnel-SiO2 films between the Si-surface and the Al-O monolayers, D-it is reduced by more than one order of magnitude. This is attributed to an interface defect deactivation mechanism that involves the discharge of the singly-occupied dangling bonds (P-b0 defects) into the acceptor states, so that Shockley-Read-Hall-recombination is drastically reduced. We demonstrate that the combined electronic and field-effect passivation allows for minority carrier lifetimes in excess of 1 ms on n-type Si and that additional H-2-passivation is not able to improve that lifetime significantly. Published under license by AIP Publishing.

Semiconductor nanowires (NWires) experience stress and charge transfer by their environment and impurity atoms. In return, the environment of NWires experiences a NWire stress response which may lead to propagated strain and change in shape and size of NWire cross sections. We deduce geometrical number series for zinc-blende- (zb-) and diamond-lattice NWires of diameter $d_{\mathrm{Wire}}$ to obtain the number of NWire atoms $N_{\mathrm{Wire}}(d_{\mathrm{Wire}}[i])$, bonds between NWire atoms $N\mathrm{_{bnd}}(d_{\mathrm{Wire}}[i])$ and interface bonds $N\mathrm{_{IF}}(d_{\mathrm{Wire}}[i])$ for six high symmetry zb NWires with low-index faceting frequently ocurring in bottom-up and top-down approaches of NWire processing. Along with these primary parameters, we present specific length of interface facets, cross section widths and heights as well as the cross section area. The fundamental insights into NWire structures revealed here offer a universal gauge and thus enable major advancements in data interpretation and understanding of all zb- and diamond-lattice based NWires. We underpin this statement with results from the literature on cross-section images from III-V core-shell NWire growth and on Si-NWires undergoing self-limiting oxidation and etching. The massive breakdown of impurity doping due to self-purification is shown to occur for both, Si NWires and Si nanocrystals (NCs) for a ratio of $N\mathrm{_{bnd}}/N_{\mathrm{Wire}}=1.94\pm0.01$ using published experimental data.

The authors model fully hydroxyl‐ (OH‐) and amino‐ (NH2‐) terminated silicon nanocrystals (Si‐NCs) by time‐dependent density functional theory (TD‐DFT), and replace OH or NH2 groups by respective double‐ (=) or bridge‐bonded (>) groups  >/ = O or >/ = NH. Investigating ground state (GS) gaps and interface charge transfers (ICTs) from Si‐NCs to anion groups, the authors show the impact of >/ = O and >/ = NH. Excited state (ES) calculations yielded transition energies Etrans, oscillator strengths fosc and transition rates . The exciton binding energy R* increases with ICT modulation in particular for >/ = O. Increase of is high for =O and comparatively low for >O which correlates with increased (decreased) ionisation of =O (>O), as compared to nominal OH termination. Findings are also met by >/=NH on Si‐NCs, though the authors find the results there to be less apparent which is arguably originating from the specific anionic nature of N. As a result, Si‐NCs with >O and in particular =O bonds show significantly increased optical activity, but also higher R* values. The latter hampers exciton dissociation, hence carrier transport, and results in an increased redshift in photoluminescence (PL). These statements apply also to Si3N4‐embedded Si‐NCs, though the differences there are less articulate.