Single-Electron Occupation in Quantum Dot Arrays at Selectable Plunger Gate Voltage.

The small footprint of semiconductor qubits is favorable for scalable quantum computing. However, their size also makes them sensitive to their local environment and variations in the gate structure. Currently, each device requires tailored gate voltages to confine a single charge per quantum dot, clearly challenging scalability. Here, we tune these gate voltages and equalize them solely through the temporary application of stress voltages. In a double quantum dot, we reach a stable (1,1) charge state at identical and predetermined plunger gate voltage and for various interdot couplings. Applying our findings, we tune a 2 × 2 quadruple quantum dot such that the (1,1,1,1) charge state is reached when all plunger gates are set to 1 V. The ability to define required gate voltages may relax requirements on control electronics and operations for spin qubit devices, providing means to advance quantum hardware.

Electrical Control of Uniformity in Quantum Dot Devices.

Highly uniform quantum systems are essential for the practical implementation of scalable quantum processors. While quantum dot spin qubits based on semiconductor technology are a promising platform for large-scale quantum computing, their small size makes them particularly sensitive to their local environment. Here, we present a method to electrically obtain a high degree of uniformity in the intrinsic potential landscape using hysteretic shifts of the gate voltage characteristics. We demonstrate the tuning of pinch-off voltages in quantum dot devices over hundreds of millivolts that then remain stable at least for hours. Applying our method, we homogenize the pinch-off voltages of the plunger gates in a linear array for four quantum dots, reducing the spread in pinch-off voltages by one order of magnitude. This work provides a new tool for the tuning of quantum dot devices and offers new perspectives for the implementation of scalable spin qubit arrays.

InSbAs Two-Dimensional Electron Gases as a Platform for Topological Superconductivity.

Topological superconductivity can be engineered in semiconductors with strong spin-orbit interaction coupled to a superconductor. Experimental advances in this field have often been triggered by the development of new hybrid material systems. Among these, two-dimensional electron gases (2DEGs) are of particular interest due to their inherent design flexibility and scalability. Here, we discuss results on a 2D platform based on a ternary 2DEG (InSbAs) coupled to in situ grown aluminum. The spin-orbit coupling in these 2DEGs can be tuned with the As concentration, reaching values up to 400 meV Å, thus exceeding typical values measured in its binary constituents. In addition to a large Landé g-factor of ∼55 (comparable to that of InSb), we show that the clean superconductor-semiconductor interface leads to a hard induced superconducting gap. Using this new platform, we demonstrate the basic operation of phase-controllable Josephson junctions, superconducting islands, and quasi-1D systems, prototypical device geometries used to study Majorana zero modes.

Influence of High-κ Dielectrics Integration on ALD-Based MoS2 Field-Effect Transistor Performance.

The integration of high-κ dielectrics on MoS2 field-effect transistors (FETs) is essential for the realization of MoS2 in ultrascaled nanoelectronic devices and circuits. Most studies covering this topic are based on exfoliated MoS2 flakes or chemical vapor deposition (CVD) grown MoS2 films, whereas other techniques, such as atomic layer deposition (ALD), are also gaining attention for the growth of MoS2 in recent years. In this work, we grow large-area MoS2 by means of plasma-enhanced (PE-)ALD and evaluate the influence of high-κ dielectrics on the properties of ALD-based MoS2 FETs through electrical characterization combined with surface-chemical and high-resolution scanning transmission electron microscopy (HR-STEM) analyses. We grow HfO x , AlO x , or both by means of PE-ALD or thermal ALD on our fabricated devices and show that, in addition to the dielectric constant, three other major parameters related to the processing of the dielectrics can simultaneously affect the MoS2 FET electrical characteristics and govern its doping. These parameters are the stoichiometry of the dielectric, its carbon impurity content, and the degree to which the MoS2 surface oxidizes upon the dielectric growth. When grown at 100 °C, our HfO x films are oxygen-vacant whereas our AlO x films are oxygen-rich. In addition, carbon impurities are incorporated into the dielectrics at low deposition temperatures, being one of the likely causes of the MoS2 FET overall n-type performance in all of the studied cases. Our investigations also reveal that PE-ALD of HfO x or AlO x oxidizes the MoS2 surface, whereas thermal ALD AlO x leaves MoS2 almost intact. In this respect, if thermal ALD AlO x of proper thickness is grown between MoS2 and HfO x , it can reduce the degree to which the MoS2 surface oxidizes by HfO x and meanwhile improve the total dielectric constant, altogether leading to the most optimal electrical performance in ALD-based MoS2 FETs.

Atomic insights into the oxygen incorporation in atomic layer deposited conductive nitrides and its mitigation by energetic ions.

Oxygen is often detected as impurity in metal and metal nitride films prepared by atomic layer deposition (ALD) and its presence has profound and adverse effects on the material properties. In this work, we present the case study of HfNx films prepared by plasma-assisted ALD by alternating exposures of CpHf(NMe2)3 and H2 plasma. First, we identify the primary source of O contamination in the film. Specifically, we find that the extent of O incorporation in HfNx films is determined by the flux of background H2O/O2 residual gases reaching the HfNx surface during the ALD process and leads to the formation of Hf-O bonds. Then, we report on the decrease in the concentration of Hf-O bonds in the film upon application of an external radiofrequency (rf) substrate bias during the H2 plasma step. The experimental work is accompanied by first principles calculations to gain insights into the O incorporation and its mitigation upon the impingement of energetic ions on the surface. Specifically, we find that the dissociative binding of H2O on a bare HfN surface is highly favored, resulting in surface Hf-OH groups and concomitant increase in the oxidation state of Hf. We also show that energetic cations (H+, H2+ and H3+) lead to the dissociation of surface Hf-OH bonds, H2O formation, and its subsequent desorption from the surface. The latter is followed by reduction of the Hf oxidation state, presumably by H˙ radicals. The atomic-level understanding obtained in this work on O incorporation and its abstraction are expected to be crucial to prevent O impurities in the HfNx films and contribute to the fabrication of other technologically relevant low resistivity ALD-grown transition metal nitride films.

Low-temperature plasma-enhanced atomic layer deposition of 2-D MoS2: large area, thickness control and tuneable morphology.

Low-temperature controllable synthesis of monolayer-to-multilayer thick MoS2 with tuneable morphology is demonstrated by using plasma enhanced atomic layer deposition (PEALD). The characteristic self-limiting ALD growth with a growth-per-cycle of 0.1 nm per cycle and digital thickness control down to a monolayer are observed with excellent wafer scale uniformity. The as-deposited films are found to be polycrystalline in nature showing the signature Raman and photoluminescence signals for the mono-to-few layered regime. Furthermore, a transformation in film morphology from in-plane to out-of-plane orientation of the 2-dimensional layers as a function of growth temperature is observed. An extensive study based on high-resolution transmission electron microscopy is presented to unravel the nucleation mechanism of MoS2 on SiO2/Si substrates at 450 °C. In addition, a model elucidating the film morphology transformation (at 450 °C) is hypothesized. Finally, the out-of-plane oriented films are demonstrated to outperform the in-plane oriented films in the hydrogen evolution reaction for water splitting applications.

Low-Temperature Plasma-Assisted Atomic-Layer-Deposited SnO2 as an Electron Transport Layer in Planar Perovskite Solar Cells.

In this work, we present an extensive characterization of plasma-assisted atomic-layer-deposited SnO2 layers, with the aim of identifying key material properties of SnO2 to serve as an efficient electron transport layer in perovskite solar cells (PSCs). Electrically resistive SnO2 films are fabricated at 50 °C, while a SnO2 film with a low electrical resistivity of 1.8 × 10-3 Ω cm, a carrier density of 9.6 × 1019 cm-3, and a high mobility of 36.0 cm2/V s is deposited at 200 °C. Ultraviolet photoelectron spectroscopy indicates a conduction band offset of ∼0.69 eV at the 50 °C SnO2/Cs0.05(MA0.17FA0.83)0.95Pb(I2.7Br0.3) interface. In contrast, a negligible conduction band offset is found between the 200 °C SnO2 and the perovskite. Surprisingly, comparable initial power conversion efficiencies (PCEs) of 17.5 and 17.8% are demonstrated for the champion cells using 15 nm thick SnO2 deposited at 50 and 200 °C, respectively. The latter gains in fill factor but loses in open-circuit voltage. Markedly, PSCs using the 200 °C compact SnO2 retain their initial performance at the maximum power point over 16 h under continuous one-sun illumination in inert atmosphere. Instead, the cell with the 50 °C SnO2 shows a decrease in PCE of approximately 50%.

Tuning Material Properties of Oxides and Nitrides by Substrate Biasing during Plasma-Enhanced Atomic Layer Deposition on Planar and 3D Substrate Topographies.

Oxide and nitride thin-films of Ti, Hf, and Si serve numerous applications owing to the diverse range of their material properties. It is therefore imperative to have proper control over these properties during materials processing. Ion-surface interactions during plasma processing techniques can influence the properties of a growing film. In this work, we investigated the effects of controlling ion characteristics (energy, dose) on the properties of the aforementioned materials during plasma-enhanced atomic layer deposition (PEALD) on planar and 3D substrate topographies. We used a 200 mm remote PEALD system equipped with substrate biasing to control the energy and dose of ions by varying the magnitude and duration of the applied bias, respectively, during plasma exposure. Implementing substrate biasing in these forms enhanced PEALD process capability by providing two additional parameters for tuning a wide range of material properties. Below the regimes of ion-induced degradation, enhancing ion energies with substrate biasing during PEALD increased the refractive index and mass density of TiO x and HfO x and enabled control over their crystalline properties. PEALD of these oxides with substrate biasing at 150 °C led to the formation of crystalline material at the low temperature, which would otherwise yield amorphous films for deposition without biasing. Enhanced ion energies drastically reduced the resistivity of conductive TiN x and HfN x films. Furthermore, biasing during PEALD enabled the residual stress of these materials to be altered from tensile to compressive. The properties of SiO x were slightly improved whereas those of SiN x were degraded as a function of substrate biasing. PEALD on 3D trench nanostructures with biasing induced differing film properties at different regions of the 3D substrate. On the basis of the results presented herein, prospects afforded by the implementation of this technique during PEALD, such as enabling new routes for topographically selective deposition on 3D substrates, are discussed.

Controlled release from protein particles encapsulated by molecular layer deposition.

Molecular layer deposition (MLD) was used to coat micron-sized protein particles in a fluidized bed reactor. Our results show that the dissolution rate of particles coated via MLD rapidly decreases with the increase in number of coating cycles, while the uncoated particles dissolve instantaneously.