Brenda Long

Organo-arsenic Molecular Layers on Silicon for High-Density Doping

This article describes for the first time the controlled monolayer doping (MLD) of bulk and nanostructured crystalline silicon with As at concentrations approaching 2 × 10(20) atoms cm(-3). Characterization of doped structures after the MLD process confirmed that they remained defect- and damage-free, with no indication of increased roughness or a change in morphology. Electrical characterization of the doped substrates and nanowire test structures allowed determination of resistivity, sheet resistance, and active doping levels. Extremely high As-doped Si substrates and nanowire devices could be obtained and controlled using specific capping and annealing steps. Significantly, the As-doped nanowires exhibited resistances several orders of magnitude lower than the predoped materials.

NiGe Contacts and Junction Architectures for P and As Doped Germanium Devices

In this paper, the contact resistivity of NiGe on n-doped Ge is extracted. Although phosphorus is the slowest n-type dopant in terms of diffusion in Ge, the corresponding contact resistivity data for this dopant are sparse. Contact resistivity dependence on implant dose will be determined, as well as a comparison of phosphorus- and arsenic-doped Ge layers. The impact of high contact resistance is evaluated for future technology n-type metal-oxide-semiconductor germanium devices.

Access resistance reduction in Ge nanowires and substrates based on non-destructive gas-source dopant in-diffusion

To maintain semiconductor device scaling, in recent years industry has been forced to move from planar to non-planar device architectures. This alone has created the need to develop a radically new, non-destructive method for doping. Doping alters the electrical properties of a semiconductor, related to the access resistance. Low access resistance is necessary for high performance technology and reduced power consumption. In this work the authors reduced access resistance in top–down patterned Ge nanowires and Ge substrates by a non-destructive dopant in-diffusion process. Furthermore, an innovative electrical characterisation methodology is developed for nanowire and fin-based test structures to extract important parameters that are related to access resistance such as nanowire resistivity, sheet resistance, and active doping levels. Phosphine or arsine was flowed in a Metalorganic Vapour Phase Epitaxy reactor over heated Ge samples in the range of 650–700 °C. Dopants were incorporated and activated in this single step. No Ge growth accompanied this process. Active doping levels were determined by electrochemical capacitance–voltage free carrier profiling to be in the range of 1019 cm−3. The nanowires were patterned in an array of widths from 20–1000 nm. Cross-sectional Transmission Electron Microscopy of the doped nanowires showed minimal crystal damage. Electrical characterisation of the Ge nanowires was performed to contrast doping activation in thin-body structures with that in bulk substrates. Despite the high As dose incorporation on unpatterned samples, the nanowire analysis determined that the P-based process was the better choice for scaled features.

Molecular Layer Doping: Non-destructive doping of silicon and germanium

This work describes a non-destructive method to introduce impurity atoms into silicon (Si) and germanium (Ge) using Molecular Layer Doping (MLD). Molecules containing dopant atoms (arsenic) were designed, synthesized and chemically bound in self-limiting monolayers to the semiconductor surface. Subsequent annealing enabled diffusion of the dopant atom into the substrate. Material characterization included assessment of surface analysis (AFM) and impurity and carrier concentrations (ECV). Record carrier concentration levels of arsenic (As) in Si (~5×1020 atoms/cm3) by diffusion doping have been achieved, and to the best of our knowledge this work is the first demonstration of doping Ge by MLD. Furthermore due to the ever increasing surface to bulk ratio of future devices (FinFets, MugFETs, nanowire-FETS) surface packing spacing requirements of MLD dopant molecules is becoming more relaxed. It is estimated that a molecular spacing of 2 nm and 3 nm is required to achieve doping concentration of 1020 atoms/cm3 in a 5 nm wide fin and 5 nm diameter nanowire respectively. From a molecular perspective this is readily achievable.

Diagnosis of phosphorus monolayer doping in silicon based on nanowire electrical characterisation

The advent of high surface-to-volume ratio devices has necessitated a revised approach to parameter extraction and process evaluation in field-effect transistor technologies. In this work, active d...

N-type doped germanium contact resistance extraction and evaluation for advanced devices

The authors extract contact resistivity of NiGe layers on phosphorus-doped and arsenic-doped germanium, using the Transfer Length Method. It is shown experimentally that higher implant dose yields lower contact resistivity. Furthermore phosphorus is a better choice of dopant in terms of contact resistance and sheet resistance at low activation anneal temperatures, such as 500 °C. The impact of high contact resistance is evaluated for 22 nm technology NMOS germanium devices and beyond.

Oxide removal and stabilization of bismuth thin films through chemically bound thiol layers

Bismuth has been identified as a material of interest for electronic applications due to its extremely high electron mobility and quantum confinement effects observed at nanoscale dimensions. However, it is also the case that Bi nanostructures are readily oxidised in ambient air, necessitating additional capping steps to prevent surface re-oxidation, thus limiting the processing potential of this material. This article describes an oxide removal and surface stabilization method performed on molecular beam epitaxy (MBE) grown bismuth thin-films using ambient air wet-chemistry. Alkanethiol molecules were used to dissolve the readily formed bismuth oxides through a catalytic reaction; the bare surface was then reacted with the free thiols to form an organic layer which showed resistance to complete reoxidation for up to 10 days.

AsH3 gas-phase ex situ doping 3D silicon structures

Dopant incorporation in Si can be done in situ during epitaxial growth, or ex situ for localised material modification from a variety of sources including ion, solid, liquid, or gas. Gas-phase doping has the advantage that it does not require a thin film deposition, it is more effective at entering tight spaces than a liquid, and it is less damaging and more conformal than a beam-line ion implant. In this work, we apply arsine (AsH3) gas at approximately atmospheric pressures in order to n-type dope three-dimensional (3D) Si device structures. It was observed that the gas-phase doping can be either corrosive or gentle to thin-body Si depending on the process conditions. Initial doping processes caused damage to the Si due to etching, but after process optimisation, the structural integrity of the Si nanostructures could be maintained successfully. Moreover, it was noted that evaluating doping processes entirely on planar Si surfaces can be misleading: processes which appear promising initially may not be ...

Phosphorus monolayer doping (MLD) of silicon on insulator (SOI) substrates

This paper details the application of phosphorus monolayer doping of silicon on insulator substrates. There have been no previous publications dedicated to the topic of MLD on SOI, which allows for the impact of reduced substrate dimensions to be probed. The doping was done through functionalization of the substrates with chemically bound allyldiphenylphosphine dopant molecules. Following functionalization, the samples were capped and annealed to enable the diffusion of dopant atoms into the substrate and their activation. Electrical and material characterisation was carried out to determine the impact of MLD on surface quality and activation results produced by the process. MLD has proven to be highly applicable to SOI substrates producing doping levels in excess of 1 × 1019 cm−3 with minimal impact on surface quality. Hall effect data proved that reducing SOI dimensions from 66 to 13 nm lead to an increase in carrier concentration values due to the reduced volume available to the dopant for diffusion. Dopant trapping was found at both Si–SiO2 interfaces and will be problematic when attempting to reach doping levels achieved by rival techniques.

Surface Functionalization Strategies for Monolayer Doping

Due to limitations of ion-beam implantation for thin-body and 3D device geometries, techniques that allow for strict control over dopant diffusion are required. Advanced and conformal doping technologies are key for the continued scaling of semiconductor device past sub-10-nm dimensions. Monolayer doping (MLD) has been shown to satisfy the requirements for conformal and controllable doping on many materials ranging from devices fabricated from silicon and germanium to emerging replacement materials such as III–V compounds. Despite the enormous progress in the last decade, challenges still remain, especially with regard to suitable single-atom characterization techniques, surface roughness characterization, and investigation of the role of carbon. This article concisely summarizes the monolayer-doping technique and its application to dope silicon-, germanium-, and III–V-based materials and nanostructures to obtain shallow diffusion depths coupled with high-carrier concentrations.