Dr. John O’Connell

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Biography

Dr. John O’Connell graduated with a BSc. in Chemistry in 2010. He furthered his education by undertaking an M.Sc. in Inorganic Chemistry from 2010 to 2011 which was awarded as of January 2012. The work undertaken for the M.Sc. involved the synthesis of novel nitrogen analogues of beta-diketonates as precursors for Atomic Layer Deposition. John’s Ph.D research investigated the functionalisation, doping and characterisation of semiconductor surfaces.  He defended his thesis entitled “Organic Functionalisation, Doping and Characterisation of Semiconductor Surfaces for Future CMOS Device Applications” and was awarded his Ph.D in July 2016.

Current Interests

John’s work focuses on the modification of semiconductor surfaces using organic molecules.. The speed limit of current silicon technology is rapidly being approached, and novel materials are required to overcome these limitations. To this end, John’s work involves the functionalisation of semiconductor surfaces with dopant-containing molecules and investigating on the surfaces and electrical properties with a wide-range of analytical techniques.  This work has been extensively reviewed in a Topical Review invited by IOP Nanotechnology (Nanotechnology 2016 10.1088/0957-4484/27/34/342002)  Additionally, John is also working on process optimisation for large-scale preparation of two-dimensional materials.

Organo-arsenic monoalyers on Si for high density doping

Arsenic is a suitable candidate for heavy and ultra-shallow n-doping of silicon.  We have shown controlled monolayer doping (MLD) of bulk and nanostructured crystalline silicon with As at concentrations approaching 2 × 1020 atoms cm–3. Characterisation 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 characterisation 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. (ACS Applied Materials and Interfaces 2015 10.1021/acsami.5b03768)

Clockwise from top left. Oxide-free Si is functionalised with triallylarsine via thermally-induced hydrosilylation. The sample is then capped and thermally annealed to induce dopant diffusion. Next, a 20 nm Si on SOI test structure which was doped using the procedure shown in the schematic. Finally, SIMS profile showing increasing concentration of dopant with increased anneal temperature and diffusivity data extracted from SIMS data.

Clockwise from top left. Oxide-free Si is functionalised with triallylarsine via thermally-induced hydrosilylation. The sample is then capped and thermally annealed to induce dopant diffusion. Next, a 20 nm Si on SOI test structure which was doped using the procedure shown in the schematic. Finally, SIMS profile showing increasing concentration of dopant with increased anneal temperature and diffusivity data extracted from SIMS data.

Monolayer doping of Si with improved oxidation resistance

While the approach outlined above obtained excellent doping profiles, the resistance towards oxidation by ambient conditions was not as expected.  Therefore an alternative strategy is required.  Click-chemistry offers a route that is much more resistant to oxidation than traditional MLD reactions.

The functionalisation of planar silicon with arsenic- and phosphorus-based azides was investigated. Covalently bonded and well-ordered alkyne-terminated monolayers were prepared from a range of commercially available dialkyne precursors using a well-known thermal hydrosilylation mechanism to form an acetylene-terminated monolayer. The terminal acetylene moieties were further functionalised through the application of copper-catalysed azide–alkyne cycloaddition (CuAAC) reactions between dopant-containing azides and the terminal acetylene groups. The introduction of dopant molecules via this method does not require harsh conditions typically employed in traditional monolayer doping approaches, enabling greater surface coverage with improved resistance toward reoxidation. X-ray photoelectron spectroscopy studies showed successful dialkyne incorporation with minimal Si surface oxidation, and monitoring of the C 1s and N 1s core-level spectra showed successful azide–alkyne cycloaddition. Electrochemical capacitance–voltage measurements showed effective diffusion of the activated dopant atoms into the Si substrates . (ACS Applied Materials and Interfaces 2016, 10.1021/acsami.5b11731)

Oxide-free Si is firstly functionalised with well-ordered and tightly packed dialkyne molecules which passivate the surface to prevent oxygen and water ingress and also terminate with reactive acetylene groups on to which dopant-containing azides can be "clicked" using alkyne-azide click chemistry.

    Oxide-free Si is firstly functionalised with well-ordered and tightly packed dialkyne molecules which passivate the surface to prevent oxygen and water ingress and also terminate with reactive acetylene groups on to which dopant-containing azides can be “clicked” using alkyne-azide click chemistry.

Monolayer doping of ternary III-V compounds

InGaAs is a potential future channel material for complementary metal-oxide semiconductor (CMOS) applications due to its direct band gap and high electron mobility. With device feature sizes perennially decreasing and a move from SiO2-based gate dielectric strategies ongoing, new methods for passivating and doping of InGaAs based materials will become more important if the material is to become integrated in future technology nodes. Metal-oxide-semiconductor field-effect transistors (MOSFETs) based on InGaAs will allow continued scaling through a reduction in operation voltage and device footprints without compromising performance.

Si and Sn are typical dopants of choice for n-type doping of InGaAs.  Doping of InGaAs conventionally takes place either in-situ by introduction of a dopant-containing gas during epilayer growth, by ion-implantation post-growth, or in the case of a device such as a MOSFET, selective epitaxy on each side of the gate of in-situ doped source/drain materials using the channel material as a seed layer. John is currently working on MLD of InGaAs in addition to functionalisation of Ge with Sn-containing moieties.

We have succesfully carried out both Sn, and Si+S codoping of epitaxial InGaAs layers recently. (ACS Omega 2017, 10.1021/acsomega.7b00204)

General schematic for the InGaAs MLD process: (a) an oxide-free InGaAs surface was functionalized with 3-mercaptotriethoxysilane (MPTES) or (b) allyltributylstannane (ATBS). (c) The functionalized substrates were capped with SiO2 and annealed in a rapid thermal anneal furnace to cause in-diffusion of the dopant atoms to yield (d) doped InGaAs substrates.

 Two-dimensional materials.
There is currently great interest in the synthesis and physical properties of black phosphorus, due to the emergence of phosphorene; a graphene analogous 2D lattice of phosphorous.  Black phosphorus is known to be the highest density allotrope of phosphorous with the lowest reactivity.  Unlike white or red phosphorus, black phosphorus is chemically stable and can sustain temperatures up to 400°C in air without spontaneously igniting. Black phosphorus is a promising channel material for transistors since it has a high carrier mobility (electrons can move quickly through it). This gives BP the ability to operate at lower voltages while also increasing performance, which translates to greatly reduced power consumption. John is currently investigating efficient routes to synthesise large amounts of black phosphorus, while retaining high crystal quality.