Monochromatic X-ray Photoelectron Spectroscopy Research Articles

Comparison of AlF3 thin films grown by thermal and plasma enhanced atomic layer deposition

Films of aluminum fluoride (AlF3) deposited by thermal and plasma enhanced atomic layer deposition (PEALD) have been compared using in situ multiwavelength ellipsometry (MWE) and monochromatic x-ray photoelectron spectroscopy (XPS). The AlF3 films were grown using cyclic exposures of trimethylaluminum, hydrogen fluoride, and H radicals from a remote H2 inductively coupled plasma. Films were characterized in situ using MWE and XPS for growth rate, film composition, and impurity incorporation. The MWE showed a growth rate of 1.1 and 0.7 Å per cycle, at 100 °C, for thermal and plasma enhanced ALD AlF3 films, respectively. Carbon incorporation was below the XPS detection limit. The plasma enhanced ALD AlF3 film showed the presence of Al-Al chemical states, in the Al 2p scans, suggesting the presence of Al-rich clusters with a concentration of 14%. The Al-rich clusters are thought to originate during the hydrogen plasma step of the PEALD process. The Al-rich clusters were not detected in thermal ALD AlF3 films using the same precursors and substrate temperature.

Water agglomerates on Fe3O4(001)

Determining the structure of water adsorbed on solid surfaces is a notoriously difficult task and pushes the limits of experimental and theoretical techniques. Here, we follow the evolution of water agglomerates on Fe3O4(001); a complex mineral surface relevant in both modern technology and the natural environment. Strong OH-H2O bonds drive the formation of partially dissociated water dimers at low coverage, but a surface reconstruction restricts the density of such species to one per unit cell. The dimers act as an anchor for further water molecules as the coverage increases, leading first to partially dissociated water trimers, and then to a ring-like, hydrogen-bonded network that covers the entire surface. Unraveling this complexity requires the concerted application of several state-of-the-art methods. Quantitative temperature-programmed desorption (TPD) reveals the coverage of stable structures, monochromatic X-ray photoelectron spectroscopy (XPS) shows the extent of partial dissociation, and noncontact atomic force microscopy (AFM) using a CO-functionalized tip provides a direct view of the agglomerate structure. Together, these data provide a stringent test of the minimum-energy configurations determined via a van der Waals density functional theory (DFT)-based genetic search.

Surface sensitivity of four-probe STM resistivity measurements of bulk ZnO correlated to XPS

Multi-probe instruments based on scanning tunnelling microscopy (STM) are becoming increasingly common for their ability to perform nano- to atomic-scale investigations of nanostructures, surfaces and in situ reactions. A common configuration is the four-probe STM often coupled with in situ scanning electron microscopy (SEM) that allows precise positioning of the probes onto surfaces and nanostructures enabling electrical and scanning experiments to be performed on highly localised regions of the sample. In this paper, we assess the sensitivity of four-probe STM for in-line resistivity measurements of the bulk ZnO surface. The measurements allow comparisons to established models that are used to relate light plasma treatments (O and H) of the surfaces to the resistivity measurements. The results are correlated to x-ray photoelectron spectroscopy (XPS) and show that four-probe STM can detect changes in surface and bulk conduction mechanisms that are beyond conventional monochromatic XPS.

Monochromatic X-ray Photoelectron Spectroscopy Study of Three Different Ionic Liquids in Interaction with Lithium-Decorated Copper Surfaces

A comparative study of the chemical reactions of three different ionic liquids prepared by physical vapor deposition on a Li/Cu surface was performed. In this investigation, the ionic liquids 1-butyl-1-methylpyrrolidinium bis[fluorosulfonyl]imide ([Py1,4]FSI), 1-butyl-1-methylpyrrolidinium bis[trifluoromethylsulfonyl]imide ([Py1,4]TFSI), and 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([OMIm]TFSI) were characterized by monochromatic X-ray photoelectron spectroscopy after adsorption on Li/Cu. All spectra prove the formation of LiF and Li2O, which verifies the partial decomposition of the anions. This is found to be much less intense for [Py1,4]TFSI than for [Py1,4]FSI, indicating a more rapid decomposition of the FSI anion in the presence of lithium. In addition, changes of the cation structure, as well as changes of the anion/cation ratio, were observed in the spectra. In summary, cations were most likely displaced from the Li/Cu surface and were partially decomposed, especially in case ...

In situ surface and interface study of crystalline (3×1)-O on InAs

The oxidation behavior of de-capped InAs (100) exposed to O2 gas at different temperatures is investigated in situ with high resolution of monochromatic x-ray photoelectron spectroscopy and low energy electron diffraction. The oxide chemical states and structure change dramatically with the substrate temperature. A (3 × 1) crystalline oxide layer on InAs is generated in a temperature range of 290–330 °C with a coexistence of In2O and As2O3. The stability of the crystalline oxide upon the atomic layer deposition (ALD) of HfO2 is studied as well. It is found that the generated (3 × 1) crystalline oxide is stable upon ALD HfO2 growth at 100 °C.

(Invited) Excellent Wetting Behavior of Yttria on 2D Materials

A high quality yttrium oxide (yttria, Y2O3) dielectric has been grown on different carbon derivatives materials (carbon nanotubes, exfoliated graphene, chemical vapor deposition graphene).1-5 This opens the possibility to use such dielectric for the fabrication of graphene devices.4-5 In this talk, the good wettability of Y2O3 on graphene is demonstrated using various surface science methods. Furthermore, the favorable wetting behavior of yttria is exploited as an effective seeding layer for atomic layer deposition of Al2O3 on graphene.6 Moreover, we investigated the interface between Y2O3 and the transition metal dichalcogenide MoS2 as well as on highly ordered pyrolytic graphite (HOPG). The growth of the yttria film was monitored in-situ by monochromatic X-ray photoelectron spectroscopy and scanning tunneling microscopy (XPS and STM). Depositing yttrium metal results in a chemical interaction between the adlayer and the substrate. However, no indication of chemical interaction was detected for the Y2O3 growth, formed by evaporating yttrium in background O2 environment. This is due to the initial formation of Y-O bonds, before the weak Y-MoS2 and Y-C interaction occurs. The STM results indicate the formation of a uniform dielectric film on both MoS2 and HOPG surfaces, confirming the previous findings on graphene. In conclusions, our results reveal that the underlying MoS2and graphene remains intact following yttria seed deposition. Photoemission measurements of the graphene/oxide contacts indicate n-type doping of graphene with different doping levels caused by the charge transfer at the interfaces. This work was supported in part by the Southwest Academy on Nanoelectronics sponsored by the Nanoelectronic Research Initiative. -- [1] R. Addou, A. Dahal, and M. Batzill Growth of a Two-Dimensional Dielectric Monolayer on Quasi-Freestanding Graphene. Nature Nanotechnol. 8, 41-45 (2013). [2] A. Dahal, H. Coy-Diaz H, R. Addou, J. Lallo, E. Sutter, and M. Batzill Preparation and Characterization of Ni(111)/Graphene/Y2O3(111) Heterostructures. J. Appl. Phys. 113, 194305 (2013). [3] A. Dahal, R. Addou, H. Coy-Diaz, J. Lallo, and M. Batzill. Charge Doping of Graphene in Metal/Graphene/Dielectric Sandwich Structure Evaluated by C-1s Core Level Photoemission Spectroscopy. APL Mater. 1, 042107 (2013). [4] H. Xu, Z. Zhang, Z. Wang, S. Wang, X. Liang, and L. M. Peng Quantum capacitance limited vertical scaling of graphene field- effect transistor. ACS Nano 5, 2340–2347 (2011). [5] H. Xu, Z. Zhang, H. Xu, Z. Wang, S. Wang, and L. M. Peng Top-gated graphene field-effect transistors with high normalized transconductance and designable Dirac point voltage. ACS Nano 5, 5031–5037 (2011). [6] A. Dahal, R. Addou, A. Azcatl, H. Coy-Diaz, N. Lu, X. Peng, F. de Dios, J. Kim, M. J. Kim, R. M. Wallace, and M. Batzill. Seeding Atomic Layer Deposition of Alumina on Graphene with Yttria. ACS Appl. Mater. Interfaces 7, 2082-2087 (2015).

Surface state modulation through wet chemical treatment as a route to controlling the electrical properties of ZnO nanowire arrays investigated with XPS

ZnO is a wide bandgap semiconductor that has many potential applications including solar cell electrodes, transparent thin film transistors and gas/biological sensors. Since the surfaces of ZnO materials have no amorphous or oxidised layers, they are very environmentally sensitive, making control of their semiconductor properties challenging. In particular, the electronic properties of ZnO nanostructures are dominated by surface effects while surface conduction layers have been observed in thin films and bulk crystals. Therefore, the ability to use the ZnO materials in a controlled way depends on the development of simple techniques to modulate their surface electronic properties. Here, we use monochromatic x-ray photoelectron spectroscopy (XPS) to investigate the use of different wet chemical treatments (EtOH, H2O2) to control the electronic properties of ZnO nanowires by modulating the surface depletion region. The valence band and core level XPS spectra are used to explore the relationship between the surface chemistry of the nanowires and the surface band bending.

Direct determination of the band offset in atomic layer deposited ZnO/hydrogenated amorphous silicon heterojunctions from X-ray photoelectron spectroscopy valence band spectra

The chemical composition and band alignment at the heterointerface between atomic layer deposition-grown zinc oxide (ZnO) and hydrogenated amorphous silicon (a-Si:H) is investigated using monochromatized X-ray photoelectron spectroscopy. A new approach for obtaining the valence band offset ΔEV is developed, which consists in fitting the valence band (VB) spectrum obtained for a-Si:H with a thin ZnO overlayer as the sum of experimentally obtained VB spectra of a bulk a-Si:H film and a thick ZnO film. This approach allows obtaining ΔEV = 2.71 ± 0.15 eV with a minimum of assumptions, and also yields information on the change in band bending of both substrate and ZnO film. The band offset results are compared to values obtained using the usual approach of comparing valence band edge-to-core level energy differences, ΔEB,CL − ΔEB,VB. Furthermore, a theoretical value for the VB offset is calculated from the concept of charge neutrality level line-up, using literature data for the charge neutrality levels and the experimentally determined ZnO/a-Si:H interface dipole. The thus obtained value of ΔEVCNL = 2.65 ± 0.3 eV agrees well with the experimental ΔEV.

Investigation of arsenic and antimony capping layers, and half cycle reactions during atomic layer deposition of Al2O3 on GaSb(100)

In-situ monochromatic x-ray photoelectron spectroscopy, low energy electron diffraction, ion scattering spectroscopy, and transmission electron microscopy are used to examine the GaSb(100) surfaces grown by molecular beam epitaxy after thermal desorption of a protective As or Sb layer and subsequent atomic layer deposition (ALD) of Al2O3. An antimony protective layer is found to be more favorable compared to an arsenic capping layer as it prevents As alloys from forming with the GaSb substrate. The evolution of oxide free GaSb/Al2O3 interface is investigated by “half-cycle” ALD reactions of trimethyl aluminum and deionized water.

Surface and interfacial reaction study of InAs(100)-crystalline oxide interface

A crystalline oxide film on InAs(100) is investigated with in situ monochromatic x-ray photoelectron spectroscopy and low energy electron diffraction before and after in situ deposition of Al2O3 by atomic layer deposition (ALD) as well as upon air exposure. The oxidation process leads to arsenic and indium trivalent oxidation state formation. The grown epitaxial oxide-InAs interface is stable upon ALD reactor exposure; however, trimethyl aluminum decreases oxidation states resulting in an unreconstructed surface. An increase in oxide concentration is also observed upon air exposure suggesting the crystalline oxide surface is unstable.

Solution-processed-MoO3 hole extraction layer on oxygen plasma-treated indium tin oxide in organic photovoltaics

Effects of oxygen plasma (O2 plasma) treatment of ITO on the characteristics of solution-processed molybdenum oxide (MoO3) hole extraction layer in bulk hetero-junction organic photovoltaics (OPVs) are studied. The chemical composition of O2 plasma-treated ITO was determined using monochromatic X-ray photoelectron spectroscopy (XPS). The valence band energies were investigated by ultraviolet photoemission spectroscopy (UPS) measurements. XPS and UPS measurements reveal that O2 plasma treatment of bare ITO film was found to incorporate the polar surface species such as (O2)2−, resulting in an increase of both workfunction from 4.62 to 5.05eV and polar surface energy from 27 to 38mN/m. The high work function results in efficient hole transport at the ITO/MoO3 interfaces. The highly polar surface is readily available for uniform coating of MoO3 on ITO. Electrical conductivity of oxidized ITO changes four orders of magnitude from 2.4×10–2 to 4.08×102S/cm, depending on O2 plasma pressure conditions. Thus, the ITO/MoO3 interface dominates the series resistance of OPVs fabricated with poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The presence of (O2)2− states in the ITO/MoO3 interface in OPVs is suggested to play a significant role in controlling the device lifetime as well as the efficiency of OPV.

Investigation of interfacial oxidation control using sacrificial metallic Al and La passivation layers on InGaAs

The ability of metallic Al and La interlayers to control the oxidation of InGaAs substrates is examined by monochromatic x-ray photoelectron spectroscopy (XPS) and compared to the interfacial chemistry of atomic layer deposition (ALD) of Al2O3 directly on InGaAs surfaces. Al and La layers were deposited by electron-beam and effusion cell evaporators, respectively, on In0.53Ga0.47As samples with and without native oxides present. It was found that both metals are extremely efficient at scavenging oxygen from III–V native oxides, which are removed below XPS detection limits prior to ALD growth. However, metallic Ga/In/As species are simultaneously observed to form at the semiconductor–metal interface. Upon introduction of the samples to the ALD chamber, these metal bonds are seen to oxidize, leading to Ga/In–O bond growth that cannot be controlled by subsequent trimethyl-aluminum (TMA) exposures. Deposition on an oxide-free InGaAs surface results in both La and Al atoms displacing group III atoms near the surface of the semiconductor. The displaced substrate atoms tend to partially oxidize and leave both metallic and III–V oxide species trapped below the interlayers where they cannot be “cleaned-up” by TMA. For both Al and La layers the level of Ga–O bonding detected at the interface appears larger then that seen following ALD directly on a clean surface.

Surface composition, work function, and electrochemical characteristics of gallium-doped zinc oxide

Gallium-doped zinc oxide (GZO) possesses the electric conductivity, thermal stability, and earth abundance to be a promising transparent conductive oxide replacement for indium tin oxide electrodes in a number of molecular electronic devices, including organic solar cells and organic light emitting diodes. The surface chemistry of GZO is complex and dominated by the hydrolysis chemistry of ZnO, which influences the work function via charge transfer and band bending caused by adsorbates. A comprehensive characterization of the surface chemical composition and electrochemical properties of GZO electrodes is presented, using both solution and surface adsorbed redox probe molecules. The GZO surface is characterized using monochromatic X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy after the following pretreatments: (i) hydriodic acid etch, (ii) potassium hydroxide etch, (iii) RF oxygen plasma etching, and (iv) high-vacuum argon-ion sputtering. The O 1s spectra for the GZO electrodes have contributions from the stoichiometric oxide lattice, defects within the lattice, hydroxylated species, and carbonaceous impurities, with relative near-surface compositions varying with pretreatment. Solution etching procedures result in an increase of the work function and ionization potential of the GZO electrode, but yield different near surface Zn:Ga atomic ratios, which significantly influence charge transfer rates for a chemisorbed probe molecule. The near surface chemical composition is shown to be the dominant factor in controlling surface work function and significantly influences the rate of electron transfer to both solution and tethered probe molecules.

Interfacial oxide re-growth in thin film metal oxide III-V semiconductor systems

The Al2O3/GaAs and HfO2/GaAs interfaces after atomic layer deposition are studied using in situ monochromatic x-ray photoelectron spectroscopy. Samples are deliberately exposed to atmospheric conditions and interfacial oxide re-growth is observed. The extent of this re-growth is found to depend on the dielectric material and the exposure temperature. Comparisons with previous studies show that ex situ characterization can result in misleading conclusions about the interface reactions occurring during the metal oxide deposition process.

Evidence for near-Surface NiOOH Species in Solution-Processed NiOx Selective Interlayer Materials: Impact on Energetics and the Performance of Polymer Bulk Heterojunction Photovoltaics

The characterization and implementation of solution-processed, wide bandgap nickel oxide (NiOx) hole-selective interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) are discussed. The surface electrical properties and charge selectivity of these thin films are strongly dependent upon the surface chemistry, band edge energies, and midgap state concentrations, as dictated by the ambient conditions and film pretreatments. Surface states were correlated with standards for nickel oxide, hydroxide, and oxyhydroxide components, as determined using monochromatic X-ray photoelectron spectroscopy. Ultraviolet and inverse photoemission spectroscopy measurements show changes in the surface chemistries directly impact the valence band energies. O2-plasma treatment of the as-deposited NiOx films was found to introduce the dipolar surface species nickel oxyhydroxide (NiOOH), rather than the p-dopant Ni2O3, resulting in an increase of the electrical band gap energy for the near-surface region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking properties of the as-deposited and O2-plasma treated NiOx films are compared using both electron-only and BHJ devices. O2-plasma-treated NiOx interlayers produce electron-only devices with lower leakage current and increased turn on voltages. The differences in behavior of the different pretreated interlayers appears to arise from differences in local density of states that comprise the valence band of the NiOx interlayers and changes to the band gap energy, which influence their hole-selectivity. The presence of NiOOH states in these NiOx films and the resultant chemical reactions at the oxide/organic interfaces in OPVs is predicted to play a significant role in controlling OPV device efficiency and lifetime.

Comparison of the self-cleaning effects and electrical characteristics of BeO and Al2O3 deposited as an interface passivation layer on GaAs MOS devices

Beryllium oxide (BeO) is a promising dielectric because of its high energy bandgap (10.6 eV) and short Be and O atom bonds and its excellent electrical insulating characteristics and high thermal stability. In a previous study, the authors showed that BeO grown by atomic layer deposition (ALD) as a gate dielectric on Si and GaAs substrates has excellent electrical and physical characteristics. In this work, we used monochromatic x-ray photoelectron spectroscopy (XPS) and electrical analysis to compare the ability of ALD BeO and Al2O3 to reduce the surface oxide on GaAs substrates. High resolution XPS shows that the BeO reduced surface oxide more efficiently than Al2O3 and that the capacitance-voltage characteristics correspond with the XPS results. In addition, ALD BeO exhibits less interfacial oxide growth after post-deposition annealing and a more efficient suppression of the leakage current

In-situ characterization of Ga2O passivation of In0.53Ga0.47As prior to high-k dielectric atomic layer deposition

Ga2O interfacial passivation layers (IPLs) on In0.53Ga0.47As are investigated using in-situ monochromatic x-ray photoelectron spectroscopy. The oxide is entirely composed of Ga2O when deposited with an effusion cell temperature of 1500 °C and substrate temperature of 425 °C. The growth on In0.53Ga0.47As reveals slight chemical modification of the surface. The Ga2O behavior and ability to protect the III-V surface are observed following Al2O3 deposition by atomic layer deposition following each precursor pulse. Al2O3 growth by trimethyl-Al (TMA) and water reveals that the IPL undergoes the “clean-up” effect following TMA exposures causing As-As bonding formation resulting in a high interface state density.

In situ X-ray photoelectron spectroscopy characterization of Al 2O 3/GaSb interface evolution

GaSb(0 0 1) was treated with (NH 4) 2S x and the evolution of the interfacial chemistry was investigated, in situ, with monochromatic X-ray photoelectron spectroscopy (XPS), following heat treatment and exposure to trimethylaluminum (TMA) and deionized water (DIW) in an atomic layer deposition reactor. Elemental Sb (Sb–Sb bonding) as well as Sb 3+ and Sb 5+ chemical states were initially observed at the native oxide/GaSb interface, yet these diminished below the XPS detection limit after heating to 300 °C. No evidence of Ga–Ga bonding was observed whereas the Ga 1+/Ga–S chemical state was robust and persisted after heat treatment and exposure to TMA/DIW at 300 °C.

An in situ examination of atomic layer deposited alumina/InAs(100) interfaces

Undoped InAs(100) wafers were either passivated with sulfur from a (NH4)2Sx solution or etched with NH4OH and then characterized with monochromatic x-ray photoelectron spectroscopy (XPS) before and after in situ deposition of Al2O3 by atomic layer deposition. Sulfur passivation minimized oxidation. Trimethyl aluminum (TMA) exposure reduced trivalent indium and arsenic oxidation states. The In1+ chemical state persisted while elemental arsenic remained at the Al2O3/InAs interface prior to TMA exposure and possibly a mixture of As–As and As–Al bonds were present afterwards. The In 3d5/2 peak line shape from bulk InAs differed from previous XPS experiments on epitaxial InxGa1−xAs.

Characterization of the “clean-up” of the oxidized Ge(100) surface by atomic layer deposition

While the “clean-up” effect on III-V substrates has recently been well documented interfacial reactions during atomic layer deposition (ALD) on Ge substrates are not fully explored. The “clean-up” of Ge oxides is studied by interrupting the ALD process following individual precursor pulses for in situ monochromatic x-ray photoelectron spectroscopy analysis. Germanium oxides are found to be reduced by TMA and water, while an interfacial GeON layer is only affected by the initial TMA pulse. Oxide free germanium surfaces behave analogously to a surface with initial native oxides since they are oxidized measurably prior to the first TMA pulse due to residual oxidants in a commercial ALD chamber.