Reaction of BCl3 with H- and Cl-terminated Si(1 0 0) as a pathway for selective, monolayer doping through wet chemistry

Tailoring amorphous boron nitride for high-performance two-dimensional electronics

Two-dimensional (2D) materials have garnered significant attention in recent years due to their atomically thin structure and unique electronic and optoelectronic properties. To harness their full potential for applications in next-generation electronics and photonics, precise control over the dielectric environment surrounding the 2D material is critical. The lack of nucleation sites on 2D surfaces to form thin, uniform dielectric layers often leads to interfacial defects that degrade the device performance, posing a major roadblock in the realization of 2D-based devices. Here, we demonstrate a wafer-scale, low-temperature process (<250 °C) using atomic layer deposition (ALD) for the synthesis of uniform, conformal amorphous boron nitride (aBN) thin films. ALD deposition temperatures between 125 and 250 °C result in stoichiometric films with high oxidative stability, yielding a dielectric strength of 8.2 MV/cm. Utilizing a seed-free ALD approach, we form uniform aBN dielectric layers on 2D surfaces and fabricate multiple quantum well structures of aBN/MoS2 and aBN-encapsulated double-gated monolayer (ML) MoS2 field-effect transistors to evaluate the impact of aBN dielectric environment on MoS2 optoelectronic and electronic properties. Our work in scalable aBN dielectric integration paves a way towards realizing the theoretical performance of 2D materials for next-generation electronics.

Enhancing the reactivity of Si(100)-Cl toward PBr3 by charging Si dangling bonds

The interaction of the PBr3 molecule with Si dangling bonds (DBs) on a chlorinated Si(100) surface was studied. The DBs were charged in a scanning tunneling microscope (STM) and then exposed to PBr3 directly in the STM chamber. Uncharged DBs rarely react with molecules. On the contrary, almost all positively charged DBs were filled with molecule fragments. As a result of the PBr3 interaction with the positively charged DB, the molecule dissociated into PBr2 and Br with the formation of a Si-Br bond and PBr2 desorption. These findings show that charged DBs significantly modify the reactivity of the surface toward PBr3. Additionally, we calculated PH3 adsorption on a Si(100)-2 × 1-H surface with DBs and found that the DB charge also has a significant impact. As a result, we demonstrated that the positively charged DB with a doubly unoccupied state enhances the adsorption of molecules with a lone pair of electrons.

The Effect of Surface Terminations on the Initial Stages of TiO2 Deposition on Functionalized Silicon

As atomic layer deposition (ALD) emerges as a method to fabricate architectures with atomic precision, emphasis is placed on understanding surface reactions and nucleation mechanisms. ALD of titanium dioxide with TiCl₄ and water has been used to investigate deposition processes in general, but the effect of surface termination on the initial TiO₂ nucleation lacks needed mechanistic insights. This work examines the adsorption of TiCl₄ on Cl-, H-, and HO- terminated Si(100) and Si(111) surfaces to elucidate the general role of different surface structures and defect types in manipulating surface reactivity of growth and non-growth substrates. The surface sites and their role in the initial stages of deposition are examined by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Density functional theory (DFT) computations of the local functionalized silicon surfaces suggest oxygen-containing defects are primary drivers of selectivity loss on these surfaces.

Ultradoping Boron on Si(100) via Solvothermal Chemistry*

Ultradoping introduces unprecedented dopant levels into Si, which transforms its electronic behavior and enables its use as a next-generation electronic material. Commercialization of ultradoping is currently limited by gas-phase ultra-high vacuum requirements. Solvothermal chemistry is amenable to scale-up. However, an integral part of ultradoping is a direct chemical bond between dopants and Si, and solvothermal dopant-Si surface reactions are not well-developed. This work provides the first quantified demonstration of achieving ultradoping concentrations of boron (∼1e14 cm² ) by using a solvothermal process. Surface characterizations indicate the catalyst cross-reacted, which led to multiple surface products and caused ambiguity in experimental confirmation of direct surface attachment. Density functional theory computations elucidate that the reaction results in direct B-Si surface bonds. This proof-of-principle work lays groundwork for emerging solvothermal ultradoping processes.

B-Doped δ-Layers and Nanowires from Area-Selective Deposition of BCl3 on Si(100)

Atomically precise, δ-doped structures forming electronic devices in Si have been routinely fabricated in recent years by using depassivation lithography in a scanning tunneling microscope (STM). While H-based precursor/monatomic resist chemistries for incorporation of donor atoms have dominated these efforts, the use of halogen-based chemistries offers a promising path toward atomic-scale manufacturing of acceptor-based devices. Here, B-doped δ-layers were fabricated in Si(100) by using BCl₃ as an acceptor dopant precursor in ultrahigh vacuum. Additionally, we demonstrate compatibility of BCl₃ with both H and Cl monatomic resists to achieve area-selective deposition on Si. In comparison to bare Si, BCl₃ adsorption selectivity ratios for H- and Cl-passivated Si were determined by secondary ion mass spectrometry depth profiling (SIMS) to be 310(10):1 and 1529(5):1, respectively. STM imaging revealed that BCl₃ adsorbed readily on bare Si at room temperature, with SIMS measurements indicating a peak B concentration greater than 1.2(1) × 10²¹ cm^-3 with a total areal dose of 1.85(1) × 10¹⁴ cm^-2 resulting from a 30 langmuir BCl₃ dose at 150 °C. In addition, SIMS showed a δ-layer thickness of ∼0.5 nm. Hall bar measurements of a similar sample were performed at 3.0 K, revealing a sheet resistance of ρ_□ = 1.9099(4) kΩ □^-1, a hole carrier concentration of p = 1.90(2) × 10¹⁴ cm^-2, and a hole mobility of μ = 38.0(4) cm² V^-1 s^-1 without performing an incorporation anneal. Finally, 15 nm wide B δ-doped nanowires were fabricated from BCl₃ and were found to exhibit ohmic conduction. This validates the use of BCl₃ as a dopant precursor for atomic-precision fabrication of acceptor-doped devices in Si and enables development of simultaneous n- and p-type doped bipolar devices.

The stability of Cl-, Br-, and I-passivated Si(100)-(2 × 1) in ambient environments for atomically-precise pattern preservation

Atomic precision advanced manufacturing (APAM) leverages the highly reactive nature of Si dangling bonds relative to H- or Cl-passivated Si to selectively adsorb precursor molecules into lithographically defined areas with sub-nanometer resolution. Due to the high reactivity of dangling bonds, this process is confined to ultra-high vacuum (UHV) environments, which currently limits its commercialization and broad-based appeal. In this work, we explore the use of halogen adatoms to preserve APAM-derived lithographic patterns outside of UHV to enable facile transfer into real-world commercial processes. Specifically, we examine the stability of H-, Cl-, Br-, and I-passivated Si(100) in inert N₂and ambient environments. Characterization with scanning tunneling microscopy and x-ray photoelectron spectroscopy (XPS) confirmed that each of the fully passivated surfaces were resistant to oxidation in 1 atm of N₂for up to 44 h. Varying levels of surface degradation and contamination were observed upon exposure to the laboratory ambient environment. Characterization byex situXPS after ambient exposures ranging from 15 min to 8 h indicated the Br- and I-passivated Si surfaces were highly resistant to degradation, while Cl-passivated Si showed signs of oxidation within minutes of ambient exposure. As a proof-of-principle demonstration of pattern preservation, a H-passivated Si sample patterned and passivated with independent Cl, Br, I, and bare Si regions was shown to maintain its integrity in all but the bare Si region post-exposure to an N₂environment. The successful demonstration of the preservation of APAM patterns outside of UHV environments opens new possibilities for transporting atomically-precise devices outside of UHV for integrating with non-UHV processes, such as other chemistries and commercial semiconductor device processes.

Fabrication of a Nanoporous Silica Hydrogel by Cross-Linking of SiO2-H3BO3-Hexadecyltrimethoxysilane for Excellent Adsorption of Azo Dyes from Wastewater

This study reports a novel cross-linking approach to fabricate the hydrothermally neutralized silica hydrogel of SiO₂-H₃BO₃-hexadecyltrimethoxysilane by grafting alkylsilane groups onto the nanoporous silica. The synthesized silica hydrogel possessed a large specific surface area of 51.3 m²g^-1 and showed excellent dye adsorption capability of cationic dyes in neutral (pH 7) and alkaline (pH 9) medium from wastewater. The colloidal electrokinetic potential analysis revealed that the outstanding adsorption efficiency of cationic dyes over anionic dyes strongly relies on the surface charge of the hydrogels. Moreover, the hydrophobic interactions between the dye molecules and the hydrogels were studied, and it was found that the dye adsorption performance can be tuned by altering the concentration of hydrophobic reagents of the hydrogel. The dye adsorption mechanism was established, and the kinetic study suggested that the adsorption is a pseudo-second-order reaction. Adsorption isotherms at various equilibrium conditions fitted well with the Langmuir isotherm. Therefore, this strongly supports the promising and practical application of the prepared silica hydrogel. The recyclability of the hydrogel was studied, and it showed 90% adsorption efficiency by the regenerated gel up to 6 cycles, which has a high potential in wastewater treatment.

Study of the Molecule Adsorption Process during the Molecular Doping

Molecular Doping (MD) involves the deposition of molecules, containing the dopant atoms and dissolved in liquid solutions, over the surface of a semiconductor before the drive-in step. The control on the characteristics of the final doped samples resides on the in-depth study of the molecule behaviour once deposited. It is already known that the molecules form a self-assembled monolayer over the surface of the sample, but little is known about the role and behaviour of possible multiple layers that could be deposited on it after extended deposition times. In this work, we investigate the molecular surface coverage over time of diethyl-propyl phosphonate on silicon, by employing high-resolution morphological and electrical characterization, and examine the effects of the post-deposition surface treatments on it. We present these data together with density functional theory simulations of the molecules–substrate system and electrical measurements of the doped samples. The results allow us to recognise a difference in the bonding types involved in the formation of the molecular layers and how these influence the final doping profile of the samples. This will improve the control on the electrical properties of MD-based devices, allowing for a finer tuning of their performance.

Reactivity of the Si(100)-2 × 1-Cl surface with respect to PH3, PCl3, and BCl3: comparison with PH3on Si(100)-2 × 1-H

Despite the interest in a chlorine monolayer on Si(100) as an alternative to hydrogen resist for atomic-precision doping, little is known about its interaction with dopant-containing molecules. We used the density functional theory to evaluate whether a chlorine monolayer on Si(100) is suitable as a resist for PH₃, PCl₃, and BCl₃molecules. We calculated reaction pathways for PH₃, PCl₃, and BCl₃adsorption on a bare and Cl-terminated Si(100)-2 × 1 surface, as well as for PH₃adsorption on H-terminated Si(100)-2 × 1, which is widely used in current technologies for atomically precise doping of Si(100) with phosphorus. It was found that the Si(100)-2 × 1-Cl surface has a higher reactivity toward phosphine than Si(100)-2 × 1-H, and, therefore, unpatterned areas are less protected from undesirable incorporation of PH₃fragments. On the contrary, the resistance of the Si(100)-2 × 1-Cl surface against the chlorine-containing molecules turned out to be very high. Several factors influencing reactivity are discussed. The results reveal that phosphorus and boron trichlorides are well-suited for doping a patterned Cl-resist by donors and acceptors, respectively.

Solution Chemistry to Control Boron-Containing Monolayers on Silicon: Reactions of Boric Acid and 4-Fluorophenylboronic Acid with H- and Cl-terminated Si(100)

The reactions of boric acid and 4-fluorophenylboronic acid with H- and Cl-terminated Si(100) surfaces in solution were investigated. X-ray photoelectron spectroscopy (XPS) studies reveal that both molecules react preferentially with Cl-Si(100) and not with H-Si(100) at identical conditions. On Cl-Si(100), the reactions introduce boron onto the surface, forming a Si-O-B structure. The quantification of boron surface coverage demonstrates that the 4-fluorophenylboronic acid leads to ∼2.8 times higher boron coverage compared to that of boric acid on Cl-Si(100). Consistent with these observations, density functional theory studies show that the reaction of boric acid and 4-fluorophenylboronic acid is more favorable with the Cl- versus H-terminated surface and that on Cl-Si(100) the reaction with 4-fluorophenylboronic acid is ∼55.3 kJ/mol more thermodynamically favorable than the reaction with boric acid. The computational studies were also used to demonstrate the propensity of the overall approach to form high-coverage monolayers on these surfaces, with implications for selective-area boron-based monolayer doping.

Reaction of Hydrazine with Solution- and Vacuum-Prepared Selectively Terminated Si(100) Surfaces: Pathways to the Formation of Direct Si-N Bonds

The reactivity of liquid hydrazine (N₂H₄) with respect to H-, Cl-, and Br-terminated Si(100) surfaces was investigated to uncover the principles of nitrogen incorporation into the interface. This process has important implications in a wide variety of applications, including semiconductor surface passivation and functionalization, nitride growth, and many others. The use of hydrazine as a precursor allows for reactions that exclude carbon and oxygen, the primary sources of contamination in processing. In this work, the reactivity of N₂H₄ with H- and Cl-terminated surfaces prepared by traditional solvent-based methods and with a Br-terminated Si(100) prepared in ultrahigh vacuum was compared. The reactions were studied with X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy, and the observations were supported by computational investigations. The H-terminated surface led to the highest level of nitrogen incorporation; however, the process proceeds with increasing surface roughness, suggesting possible etching or replacement reactions. In the case of Cl-terminated (predominantly dichloride) and Br-terminated (monobromide) surfaces, the amount of nitrogen incorporation on both surfaces after the reaction with hydrazine was very similar despite the differences in preparation, initial structure, and chemical composition. Density functional theory was used to propose the possible surface structures and to analyze surface reactivity.