Formation of atomically ordered and chemically selective Si-O-Ti monolayer on Si0.5Ge0.5(110) for a MIS structure via H2O2(g) functionalization.

Si0.5Ge0.5(110) surfaces were passivated and functionalized using atomic H, hydrogen peroxide (H2O2), and either tetrakis(dimethylamino)titanium (TDMAT) or titanium tetrachloride (TiCl4) and studied in situ with multiple spectroscopic techniques. To passivate the dangling bonds, atomic H and H2O2(g) were utilized and scanning tunneling spectroscopy (STS) demonstrated unpinning of the surface Fermi level. The H2O2(g) could also be used to functionalize the surface for metal atomic layer deposition. After subsequent TDMAT or TiCl4 dosing followed by a post-deposition annealing, scanning tunneling microscopy demonstrated that a thermally stable and well-ordered monolayer of TiOx was deposited on Si0.5Ge0.5(110), and X-ray photoelectron spectroscopy verified that the interfaces only contained Si-O-Ti bonds and a complete absence of GeOx. STS measurements confirmed a TiOx monolayer without mid-gap and conduction band edge states, which should be an ideal ultrathin insulating layer in a metal-insulator-semiconductor structure. Regardless of the Ti precursors, the final Ti density and electronic structure were identical since the Ti bonding is limited by the high coordination of Ti to O.

Passivation of InGaAs(001)-(2 × 4) by Self-Limiting Chemical Vapor Deposition of a Silicon Hydride Control Layer.

A saturated Si-Hx seed layer for gate oxide or contact conductor ALD has been deposited via two separate self-limiting and saturating CVD processes on InGaAs(001)-(2 × 4) at substrate temperatures of 250 and 350 °C. For the first self-limiting process, a single silicon precursor, Si3H8, was dosed at a substrate temperature of 250 °C, and XPS results show the deposited silicon hydride layer saturated at about 4 monolayers of silicon coverage with hydrogen termination. STS results show the surface Fermi level remains unpinned following the deposition of the saturated silicon hydride layer, indicating the InGaAs surface dangling bonds are electrically passivated by Si-Hx. For the second self-limiting process, Si2Cl6 was dosed at a substrate temperature of 350 °C, and XPS results show the deposited silicon chloride layer saturated at about 2.5 monolayers of silicon coverage with chlorine termination. Atomic hydrogen produced by a thermal gas cracker was subsequently dosed at 350 °C to remove the Si-Cl termination by replacing with Si-H termination as confirmed by XPS, and STS results confirm the saturated Si-Hx bilayer leaves the InGaAs(001)-(2 × 4) surface Fermi level unpinned. Density function theory modeling of silicon hydride surface passivation shows an Si-Hx monolayer can remove all the dangling bonds and leave a charge balanced surface on InGaAs.

Tuning the reactivity of semiconductor surfaces by functionalization with amines of different basicity.

Surface functionalization of semiconductors has been the backbone of the newest developments in microelectronics, energy conversion, sensing device design, and many other fields of science and technology. Over a decade ago, the notion of viewing the surface itself as a chemical reagent in surface reactions was introduced, and adding a variety of new functionalities to the semiconductor surface has become a target of research for many groups. The electronic effects on the substrate have been considered as an important consequence of chemical modification. In this work, we shift the focus to the electronic properties of the functional groups attached to the surface and their role on subsequent reactivity. We investigate surface functionalization of clean Si(100)-2 × 1 and Ge(100)-2 × 1 surfaces with amines as a way to modify their reactivity and to fine tune this reactivity by considering the basicity of the attached functionality. The reactivity of silicon and germanium surfaces modified with ethylamine (CH(3)CH(2)NH(2)) and aniline (C(6)H(5)NH(2)) is predicted using density functional theory calculations of proton attachment to the nitrogen of the adsorbed amine to differ with respect to a nucleophilic attack of the surface species. These predictions are then tested using a model metalorganic reagent, tetrakis(dimethylamido)titanium (((CH(3))(2)N)(4)Ti, TDMAT), which undergoes a transamination reaction with sufficiently nucleophilic amines, and the reactivity tests confirm trends consistent with predicted basicities. The identity of the underlying semiconductor surface has a profound effect on the outcome of this reaction, and results comparing silicon and germanium are discussed.

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.

Periodic trends in organic functionalization of group IV semiconductor surfaces.

Organic functionalization of group IV semiconductor surfaces provides a means to precisely control the interfacial properties of some of the most technologically important electronic materials in use today. The 2 x 1 reconstructed group IV (100) surfaces in ultrahigh vacuum, in particular, have a well-defined surface that allows adsorbate-surface interactions to be studied in detail. Surface dimers containing a strong sigma- and weak pi-bond form upon reconstruction of the group IV (100) surfaces, imparting a rich surface reactivity, which allows useful analogies to be made between reactions at the surface and those in classic organic chemistry. To date, most studies have focused on single substrates and a limited number of adsorbate functional groups. In this Account, we bring together experimental and theoretical results from several studies to investigate broader trends in thermodynamics and kinetics of organic molecules reacted with group IV (100)-2 x 1 surfaces. By rationalizing these trends in terms of simple periodic properties, we aim to provide guidelines by which to understand the chemical origin of the observed trends and predict how related molecules or functionalities will react. Results of experimental and theoretical studies are used to show that relative electronegativities and orbital overlap correlate well with surface-adsorbate covalent bond strength, while orbital overlap together with donor electronegativity and acceptor electron affinity correlate with surface-adsorbate dative bond strength. Using such simple properties as predictive tools is limited, of course, but theoretical calculations fill in some of the gaps. The predictive power inherent in periodic trends may be put to use in designing molecules for applications where controlled attachment of organic molecules to semiconductor surfaces is needed. Organic functionalization may facilitate the semiconductor industry's transition from traditional silicon-based architectures to other materials, such as germanium, that offer better electrical properties. Potential applications also exist in other fields ranging from organic and molecular electronics, where control of interfacial properties may allow coupling of traditional semiconductor technology with such developing technologies, to biosensors and nanoscale lithography, where the functionality imparted to the surface may be used directly. Knowledge of thermodynamic and kinetic trends and the fundamental basis of these trends may enable effective development of new functionalization strategies for such applications.

Disulfide passivation of the Ge(100)-2 × 1 surface.

Understanding the bonding of sulfur at the germanium surface is important to developing good passivation routes for germanium-based electronic devices. The adsorption behavior of ethyl disulfide (EDS) and 1,8-naphthalene disulfide (NDS) at the Ge(100)-2 × 1 surface has been studied under ultrahigh vacuum conditions to investigate both their fundamental reactivity and their effectiveness as passivants of this surface. X-ray photoelectron spectroscopy, multiple internal reflection-infrared spectroscopy, and density functional theory results indicate that both molecules adsorb via S-S dissociation at room temperature. Upon exposure to ambient air, the thiolate adlayer remains intact for both EDS- and NDS-functionalized surfaces, indicating the stability of this surface attachment. Although both systems resist oxidation compared to the bare Ge(100)-2 × 1 surface, the Ge substrate is significantly oxidized in all cases (17-57% relative to the control), with the NDS-passivated surface undergoing up to two times more oxidation than the EDS-passivated surface at the longest air exposure times studied. The difference in passivation capability is attributed to the difference in surface coverage on Ge(100)-2 × 1, where EDS adsorption leads to a saturation coverage 17% higher than that for NDS/Ge(100)-2 × 1.

Sulfur versus oxygen reactivity of organic molecules at the Ge(100)-2 x 1 surface.

The adsorption behavior of sulfur- versus oxygen-containing organic molecules, including ethanol, ethanethiol, diethyl ether, and diethyl sulfide, at the Ge(100)-2 x 1 surface was investigated using a combination of multiple internal reflection infrared (MIR-IR) spectroscopy and density functional theory (DFT). The results show that ethanol and ethanethiol both adsorb via Ch-H dissociation at 310 K, where Ch (chalcogen) is either S or O. DFT calculations indicate that S-H dissociation is both kinetically and thermodynamically favored over O-H dissociation. IR spectra of diethyl ether and diethyl sulfide reveal that both molecules adsorb via dative bonding through the heteroatom for temperatures up to approximately 255 and 335 K, respectively, and reversibly desorb at higher temperatures. From these desorption temperatures, the S-Ge dative bond of a sulfide is calculated to be 5.9 kcal/mol stronger than the O-Ge dative bond of an ether, a trend consistent with results from DFT calculations. Moreover, for all of the molecules studied, SGe dative bonds are found to be stronger than O-Ge dative bonds, with the magnitude of the difference increasing with substitution of bulkier groups on the Ch atom of the adsorbate. Calculations on diethyl selenide show that the Se-Ge dative bond is slightly stronger than the S-Ge dative bond.