Monolayer contact doping of silicon surfaces and nanowires using organophosphorus compounds

Monolayer Doping of Germanium with Arsenic: A New Chemical Route to Achieve Optimal Dopant Activation

Reported here is a new chemical route for the wet chemical functionalization of germanium (Ge), whereby arsanilic acid is covalently bound to a chlorine (Cl)-terminated surface. This new route is used to deliver high concentrations of arsenic (As) dopants to Ge, via monolayer doping (MLD). Doping, or the introduction of Group III or Group V impurity atoms into the crystal lattice of Group IV semiconductors, is essential to allow control over the electronic properties of the material to enable transistor devices to be switched on and off. MLD is a diffusion-based method for the introduction of these impurity atoms via surface-bound molecules, which offers a nondestructive alternative to ion implantation, the current industry doping standard, making it suitable for sub-10 nm structures. Ge, given its higher carrier mobilities, is a leading candidate to replace Si as the channel material in future devices. Combining the new chemical route with the existing MLD process yields active carrier concentrations of dopants (>1 × 10¹⁹ atoms/cm³) that rival those of ion implantation. It is shown that the dose of dopant delivered to Ge is also controllable by changing the size of the precursor molecule. X-ray photoelectron spectroscopy (XPS) data and density functional theory (DFT) calculations support the formation of a covalent bond between the arsanilic acid and the Cl-terminated Ge surface. Atomic force microscopy (AFM) indicates that the integrity of the surface is maintained throughout the chemical procedure, and electrochemical capacitance voltage (ECV) data shows a carrier concentration of 1.9 × 10¹⁹ atoms/cm³ corroborated by sheet resistance measurements.

Nanoscale Junction Formation by Gas-Phase Monolayer Doping

A major challenge in transistor technology scaling is the formation of controlled ultrashallow junctions with nanometer-scale thickness and high spatial uniformity. Monolayer doping (MLD) is an efficient method to form such nanoscale junctions, where the self-limiting nature of semiconductor surfaces is utilized to form adsorbed monolayers of dopant-containing molecules followed by rapid thermal annealing (RTA) to diffuse the dopants to a desired depth. Unlike ion implantation, the process does not induce crystal damage, thus making it highly attractive for nanoscale transistor processing. To date, reported MLD processes have relied on solution processing for monolayer formation. Gas-phase processing, however, benefits from higher intra- and interwafer uniformity and conformal coverage of 3D structures and is more desirable for manufacturing. In this regard, we report a new approach for MLD in silicon and germanium using gas-phase monolayer formation. We call this technology gas-phase monolayer doping (GP-MLD). This method relies on sequential pulse-purge cycles of gas-phase dopant-containing molecules to form a boron- or phosphorus-containing monolayer on a target semiconductor surface. Here, we show the feasibility of our approach through the formation of ultrashallow B- and P-doped junctions on Si and Ge surfaces. The mechanism of adsorption is characterized using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Sub-5 nm junction depths with high dopant dose are obtained as characterized by secondary ion mass spectrometry and sheet resistance measurements. Additionally, we demonstrate that area selectivity can be achieved via lithographic patterning of the monolayer dopants before the diffusion step. The results demonstrate the versatility of the GP-MLD approach for formation of controlled and ultrashallow junctions.

Direct Dopant Patterning by a Remote Monolayer Doping Enabled by a Monolayer Fragmentation Study

The development of new doping methods extending beyond the traditional and well-established techniques is desired to match the rapid advances made in semiconductor (SC)-processing methods and nanostructure synthesis in numerous emerging applications, including the doping of 3D architectures. To address this, monolayer doping (MLD) and monolayer contact doping methods have been introduced recently. The MLD methods enable separation of the doping process of nanostructures from the synthesis step; hence, it is termed ex situ doping. Here, we present a new ex situ MLD method termed remote MLD (R-MLD). The noncontact doping method is based on the thermal fragmentation of dopant-containing monolayers and evaporation processes taking place during annealing of the uncapped monolayer dopant source positioned in proximity, however, without making physical contact with the target SC surface. We present a two-step annealing procedure that allows the study of the dopant monolayer fragmentation and evaporation stages and quantification of the doping levels obtained during each step. We demonstrate the application of R-MLD for achieving a large-scale direct patterning of silicon substrates with sharp doping profiles. The direct dopant patterning is obtained without applying lithographic processing steps to the target substrate. The noncontact doping process, monolayer decomposition, and fragment evaporation were studied using thermogravimetric analysis coupled with mass spectrometry and sheet resistance measurements. The doped patterns were characterized using scanning electron microscopy, scanning capacitance microscopy, and time-of-flight secondary ion mass spectroscopy.