Stephen McDonnell

GaAs interfacial self-cleaning by atomic layer deposition

The reduction and removal of surface oxides from GaAs substrates by atomic layer deposition (ALD) of Al2O3 and HfO2 are studied using in situ monochromatic x-ray photoelectron spectroscopy. Using the combination of in situ deposition and analysis techniques, the interfacial “self-cleaning” is shown to be oxidation state dependent as well as metal organic precursor dependent. Thermodynamics, charge balance, and oxygen coordination drive the removal of certain species of surface oxides while allowing others to remain. These factors suggest proper selection of surface treatments and ALD precursors can result in selective interfacial bonding arrangements.

Frequency dispersion reduction and bond conversion on n-type GaAs by in situ surface oxide removal and passivation

The method of surface preparation on n-type GaAs, even with the presence of an amorphous-Si interfacial passivation layer, is shown to be a critical step in the removal of accumulation capacitance frequency dispersion. In situ deposition and analysis techniques were used to study different surface preparations, including NH4OH, Si-flux, and atomic hydrogen exposures, as well as Si passivation depositions prior to in situ atomic layer deposition of Al2O3. As–O bonding was removed and a bond conversion process with Si deposition is observed. The accumulation capacitance frequency dispersion was removed only when a Si interlayer and a specific surface clean were combined.

Indium stability on InGaAs during atomic H surface cleaning

Atomic H exposure of a GaAs surface at 390°C is a relatively simple method for removing the native oxides without altering the surface stoichiometry. In-situ reflection high energy electron diffraction and angle-resolved x-ray photoelectron spectroscopy have been used to show that this procedure applied to In0.2Ga0.8As effectively removes the native oxides resulting in an atomically clean surface. However, the bulk InGaAs stoichiometry is not preserved from this treatment. The In:Ga ratio from the substrate is found to decrease by 33%. The implications for high-mobility channel applications are discussed as the carrier mobility increases nearly linearly with the In content.

HfO2 on UV–O3 exposed transition metal dichalcogenides: interfacial reactions study

The surface chemistry of MoS2, WSe2 and MoSe2 upon ultraviolet (UV)?O3 exposure was studied in situ by x-ray photoelectron spectroscopy (XPS). Differences in reactivity of these transition metal dichalcogenides (TMDs) towards oxidation during UV?O3 were observed and correlated with density functional theory calculations. Also, sequential HfO2 depositions were performed by atomic layer deposition (ALD) while the interfacial reactions were monitored by XPS. It is found that the surface oxides generated on MoSe2 and WSe2 during UV?O3 exposure were reduced by the ALD process (?self-cleaning effect?). The effectiveness of the oxide reduction on these TMDs is discussed and correlated with the HfO2 film uniformity.

ZnO films grown by pulsed-laser deposition on soda lime glass substrates for the ultraviolet inactivation of Staphylococcus epidermidis biofilms

Abstract We found that a ZnO film of 2 μm thickness which was laser-deposited at room temperature onto a plain soda lime glass substrate, exhibits notable antibacterial activity against a biofilm of Staphylococcus epidermidis when back-illuminated by a UVA light source with a peak emission wavelength of about 365 nm. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-visible absorption spectroscopy, Raman spectroscopy and x-ray photoemission spectroscopy (XPS) were used to characterize the ZnO films before and after the interactions with the biofilm and the ultraviolet light, respectively. The as-deposited film was highly textured with the wurtzite (0002) in-plane orientation (c-axis perpendicular to ZnO surface) and had a surface rms roughness of 49.7 nm. In the as-deposited film, the Zn to O ratio was 1 to 0.95. After the UV and biofilm treatments, the ZnO film surface had become rougher (rms roughness 68.1 nm) and presented uniform micron-sized pitting randomly distributed, while the zinc to oxygen ratio had become 1 to 2.2. In this case, both the UV-visible and Raman spectra pointed to degradation of the structural quality of the material. On the strength of these data, we propose a model for the mediation of the bactericidal activity in which the photogeneration of highly oxidizing species and the presence of active surface defect sites both play an important role. This study is of particular interest for the acute problem of disinfection of pathogenic biofilms which form on medical device/implant surfaces.

Indium diffusion through high-k dielectrics in high-k/InP stacks

Evidence of indium diffusion through high-k dielectric (Al2O3 and HfO2) films grown on InP (100) by atomic layer deposition is observed by angle resolved X-ray photoelectron spectroscopy and low energy ion scattering spectroscopy. The analysis establishes that In-out diffusion occurs and results in the formation of a POx rich interface.

Photoemission studies of the interface formation of ultrathin MgO dielectric layers on the oxidised Si(111) surface

Magnesium oxide (MgO) has been proposed as a medium-k dielectric material for integration into advanced transistor fabrication. In this study, soft x-ray synchrotron radiation based photoemission and conventional x-ray photoelectron spectroscopy have been used to characterise the evolution of the MgO/Si(111) interface as an ultrathin film is been grown. The MgO film was grown by thermally depositing magnesium in an oxygen partial pressure at room temperature. The Mg2p peak profile indicates that the magnesium exists in a single chemical state throughout the deposition sequence with a binding energy indicative of MgO. The initial submonolayer deposition resulted in the Si2p peak shifting 0.3eV towards higher binding energy consistent with electron transfer between the deposited magnesium and the substrate. This is accompanied by an increase in the intensity of the Si oxidation states indicating growth of the interfacial oxide thickness. As the MgO film thickness increases, the Fermi level moves back to within 0.1eV of its original midgap position. Subsequent XPS measurements show that the saturation thickness of the interfacial silicon oxide layer is less than 0.7nm. The valence band offset has been measured and the conduction band offset has been deduced by estimating the bandgap of the thin dielectric film. The valence band offset between the thin MgO layer and the silicon substrate is 4.1eV. The MgO workfunction was 2.8eV and assuming an electron affinity value of 0.85eV, the bandgap of this film is 6.72eV, smaller than that reported for bulk MgO. The calculated conduction band offset is 1.5eV which should be a sufficiently high barrier to minimise leakage currents.

Low-κ organic layer as a top gate dielectric for graphene field effect transistors

We demonstrate the characteristics of dual gated graphene field effect transistors using a thin layer (∼7u2009nm) of parylene-C as a top-gate dielectric. Our devices exhibit good dielectric properties with minimal doping, low leakage current (∼10−6 A/cm2 at ±2u2009V), and a dielectric constant of ∼2.1. Additionally, Raman spectroscopy did not reveal any process induced defects after dielectric deposition. Electrical characterization performed in air showed a carrier mobility of ∼5050u2009cm2/Vs with hysteresis less than 30u2009mV during top gate operation (−2.5u2009V to 2.5u2009V) which indicates that parylene and its interface with graphene does not have a significant amount of trapped charges.

In situ study of the role of substrate temperature during atomic layer deposition of HfO2 on InP

The dependence of the “self cleaning” effect of the substrate oxides on substrate temperature during atomic layer deposition (ALD) of HfO2 on various chemically treated and native oxide InP (100) substrates is investigated using in situ X-ray photoelectron spectroscopy. The removal of In-oxide is found to be more efficient at higher ALD temperatures. The P oxidation states on native oxide and acid etched samples are seen to change, with the total P-oxide concentration remaining constant, after 10 cycles of ALD HfO2 at different temperatures. An (NH4)2u2009S treatment is seen to effectively remove native oxides and passivate the InP surfaces independent of substrate temperature studied (200u2009°C, 250u2009°C and 300u2009°C) before and after the ALD process. Density functional theory modeling provides insight into the mechanism of the changes in the P-oxide chemical states.

High resolution photoemission study of SiOx/Si(111) interface disruption following in situ HfO2 deposition

We report on an in situ high resolution core level photoemission study of the early stages of interface formation between an ultrathin SiOx layer (∼0.3u2002nm) grown on the atomically clean Si(111) surface and a HfO2 dielectric layer. Si 2p core level spectra acquired at 130 eV photon energy reveal evidence of a chemically shifted component on the lower binding energy side of the substrate peak which is attributed to interface defect states resulting from the incorporation of silicon atoms from the substrate into the interfacial oxide at room temperature. This evidence of Si/SiOx interface disruption would be expected to increase charge carrier scattering mechanisms in the silicon and contribute to the generally observed mobility degradation in high-k stacks with ultrathin silicon oxide interface layers.