X-ray Reflectivity Spectroscopy Research Articles

Effect of electronic transitions on near edge optical properties of off-stoichiometric boron carbide thin films

In the present study, soft x-ray optical properties of off-stoichiometric boron carbide thin films are investigated, and the structure and chemical composition of the film is analyzed using angle dependent x-ray reflectivity and x-ray photoelectron spectroscopy techniques. Energy dependent soft x-ray reflectivity measured at a fixed grazing angle of 1.5° is used to determine the optical constants in the boron K edge region by applying the Kramers–Kronig technique. The measured optical constants show near edge fine features corresponding to σ* and π* resonances. The electronic transitions corresponding to σ* resonance cause a 40%–75% increase in the delta value in the above boron K edge region. The π* transitions corresponding to off-stoichiometric nature of the boron carbide are observed in the absorption spectra near ∼192.7 eV. Details of the measured soft x-ray optical properties of the off-stoichiometric boron carbide thin film are discussed.

Impact of the Density and Oxygen Concentration of Initial Amorphous Carbon on Layer Exchange of Multilayer Graphene

Layer exchange (LE) of amorphous carbon (a-C) is a promising method for fabricating high-quality multilayer graphene (MLG) on insulating substrates. We investigated the effects of initial a-C properties on the growth temperature and crystal quality of LE-MLG by controlling the a-C sputtering pressure (Pa-C). X-ray reflectivity and X-ray photoelectron spectroscopy revealed that the a-C density decreased and the O concentration in the a-C film increased with increasing Pa-C. LE occurred at the lowest temperature of 600 °C, reflecting a lower density and O concentration of a-C. Raman spectroscopy results showed that the crystal quality of LE-MLG improved with increasing Pa-C. Based on our results, we developed guidelines for the a-C properties in the LE technique to reduce the growth temperature and improve the crystal quality of MLG.

Low-Temperature Self-Limiting Growth of Cubic Boron Nitride Via Hollow-Cathode Plasma-Enhanced Atomic Layer Deposition

The two main polymorphs of boron nitride are hexagonal (h-BN) and cubic (c-BN) phases and both show attractive material properties to be employed under extreme environments. h-BN is also known as “white graphene” due to the matching hexagonal crystal symmetry and shows a layered structure which provides advantages as a lubricant material and as gate dielectrics for two-dimensional (2D) transistor channels. On the other hand, c-BN is analogous to another important elemental carbon material, diamond. While the mechanical strength of c-BN is lower than diamond, the thermal and chemical stability is even superior. Both h-BN and c-BN films are therefore strong candidates for applications under harsh ambient, including high-temperature, high-pressure, and corroding factors.BN films and coatings are conventionally grown using high-temperature CVD methods with typical substrate temperatures over 1000 oC. To reduce the deposition temperatures of this material while preserving the attractive crystal properties, alternative lower temperature physical and chemical vapor deposition strategies have been offered in the literature. Plasma-enhanced atomic layer deposition (PEALD) provides significant advantages over other methods such as precision thickness control, ultimate conformality, while reducing the growth temperature substantially. In our previous work, we had demonstrated the crystalline synthesis of h-BN films at substrate temperatures as low as 350 oC, while no growth was observed at lower substrate temperatures.In this work, we aim to demonstrate the feasibility of growing c-BN at substrate temperatures reaching 200 oC via a customized PEALD reactor featuring a large-diameter capacitively-coupled hollow-cathode plasma source with a relatively short source-to-substrate distance. We used Si(100), sapphire, and glass as the substrates to deposit the films. The growth temperature is kept between 150 and 250 °C, 10 sccm of N2 flow is used as a carrier gas and Ar-purging gas flow was kept at 50 sccm. The triethylboron (TEB) precursor and various nitrogen plasma chemistries including different compositions of N2, H2, and Ar gases are utilized. The optimized growth condition is determined from in-situ monitored saturation studies, where the growth-per-cycle (GPC) parameter is noted for different precursor pulse time (15 and 30 ms), rf-plasma power (100 W to 250 W) and plasma-gas chemistries (N2-only, N2/H2, Ar/N2, Ar/N2/H2 at varying ratios). While the 200 °C as-grown BN films exhibited nanocrystalline c-BN with relatively weak crystal peaks as verified by XRD measurements, vacuum annealing at 5 mTorr N2 flow and 700 °C transformed the film structure into single-phase polycrystalline c-BN. The spectroscopic ellipsometer, x-ray reflectivity (XRR), and x-ray photoelectron spectroscopy (XPS) studies will be presented as well as the influence of post-deposition vacuum annealing to further improve the crystalline structure of c-BN films. Basic electrical properties of the grown BN films will be characterized using metal-insulator-semiconductor (MIS) diode temperature-dependent current-voltage (I-V) measurements.A specific effort will be devoted to enhance the as-grown film crystal properties via implementing novel atomic layer plasma annealing processes, which showed its potential for the ultrawide bandgap Ga2O3 alloy. Another research question to be investigated is whether there is a possibility of tuning the material phase from cubic to hexagonal by carefully adjusting growth and in-situ Ar-plasma annealing conditions.

Comparison of the structural and optic al properties of amorphous silicon thin films prepared by direct current, bipolar pulse, and high-power impulse magnetron sputtering methods

This paper presents a comparative study of amorphous silicon (a-Si) thin films deposited by three different techniques: direct current magnetron sputtering (DCMS), bipolar pulse magnetron sputtering (bipolar pulse MS) and high-power impulse magnetron sputtering (HiPIMS). The structurafl properties of the a-Si films with a thickness of ∼112 nm were characterized by X-ray reflectivity and Raman spectroscopy. The surface morphology, optical and mechanical properties of a-Si films with a thickness of ∼400 nm were investigated using a field emission secondary electron microscope, an atomic force microscope, spectroscopic ellipsometry and nanoindentation. It was found that for the same time-averaged discharge power, the deposition rate of the HiPIMS method was lower compared to DCMS and bipolar pulse MS. The a-Si film prepared by HiPIMS method had the lowest root mean square roughness of 0.65 nm and the highest density of 2.21 g/cm3. These results are consistent with the highest refractive index (400–700 nm) of 3.66 and a hardness of 9.09±0.19 GPa in the film, indicating that the HiPIMS method produces denser films than the DCMS and bipolar pulse MS methods. The amorphous silicon thin films fabricated by HiPIMS have the potential to improve the structural and mechanical properties for high performance thin film semiconductor applications.

Effect of surface oxidation on soft x-ray optical properties of ion beam sputter deposited amorphous AlN thin film

In the present study, structural and compositional analyses of reactive ion beam sputter deposited aluminum nitride (AlN) thin film of thickness 100 Å are carried out using x-ray reflectivity and x-ray photoelectron spectroscopy techniques. Soft x-ray optical response of the film is derived from energy dependent soft x-ray reflectivity measurements performed in photon energy region of 380–1700 eV. Optical constants (δ and β) obtained from the reflectivity spectra show features corresponding to absorption edges of the constituent elements. Observed fine features in the β profile are further confirmed from x-ray absorption (XAS) measurements carried out in the total electron yield mode. The measured XAS spectra are correlated with electronic and compositional properties of the AlN film. The effects of surface oxidation on soft x-ray optical properties of the AlN thin film are discussed.

Oligothiophene Phosphonic Acids for Self-Assembled Monolayer Field-Effect Transistors.

Semiconducting self-assembled monolayers (SAMs) represent highly relevant components for the fabrication of organic thin-film electronics because they enable the precise formation of active π-conjugates in terms of orientation and layer thickness. In this work, we demonstrate self-assembled monolayer field-effect transistors (SAMFETs) composed of phosphonic acid oligomers of 3-hexylthiophene (oligothiophenes-OT) with systematic variations of thiophene repeating units (5, 10, and 20). The devices exhibit stable lateral charge transport with increased mobility as a function of thiophene unit counts. Importantly, our work reveals the packing and intermolecular order of varied-chain-length SAMs at the molecular scale via X-ray reflectivity (XRR) and quantitative X-ray photoelectron spectroscopy (XPS). Short oligomers (OT5-PA and OT10-PA) arrange almost perpendicular to the substrate, forming highly ordered SAMs, whereas the long-chain OT20-PA exhibits a folded structure. By tuning the molecular order in the monolayers via the SAM substitution reaction, the OT20-PA devices show a tripling in mobility.

Converting molecular layer deposited alucone films into Al2O3/alucone hybrid multilayers by plasma densification.

Alucones are one of the best-known films in the Molecular Layer Deposition (MLD) field. In this work, we prove that alucone/Al2O3 nanolaminate synthesis can be successfully performed by alternating alucone MLD growth with static O2 plasma exposures. Upon plasma treatment, only the top part of the alucone is densified into Al2O3, while the rest of the film remains relatively unaltered. X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy (XPS) depth profiling show that the process yields a bilayer structure, which remains stable in air. Fourier-transform infrared spectroscopy (FTIR) measurements show that Al2O3 features are generated after plasma treatment, while the original alucone features remain, confirming that plasma treatment results in a bilayer structure. Also, an intermediate carboxylate is created in the interface. Calculations of Al atom density during plasma exposure point towards a partial loss of Al atoms during plasma treatment, in addition to the removal of the glycerol backbone. The effect of different process parameters has been studied. Densification at the highest temperature possible (200 °C) has the best alucone preservation without hindering its thermal stability. In addition, operating at the lowest plasma power is found the most beneficial for the film, but there is a threshold that must be surpassed to achieve successful densification. About 70% of the original alucone film thickness can be expected to remain after densification, but thicker films may result in more diffuse interfaces. Additionally, this process has also been successfully performed in multilayers, showing real potential for encapsulation applications.

Thickness dependent magnetic properties of ferromagnetic films (Fe, Co) interfaced with Ta

In the present work, magnetic properties of ferromagnetic films viz. Co and Fe interfaced with Ta have been studied using magneto-optical Kerr effect (MOKE), X-ray reflectivity (XRR) and soft X-ray absorption spectroscopy (SXAS) on wedge shaped samples. Thickness of ferromagnetic film at different points of wedge sample as determined from XRR were employed to study the evolution of magnetic properties with increasing film thickness using MOKE measurements. Co film exhibits a uniaxial magnetic anisotropy on Ta which varies as a function of increasing film thickness, while no thickness dependent magnetic anisotropy has been observed for Fe film grown on Ta. The variation of coercivity with increasing film thickness is in agreement with the observed variation of magnetic anisotropy. SXAS measurements qualitatively support alloying / intermixing at interface. Thicknesses of interfacial magnetic dead layer as seen from variation of Kerr intensity with film thickness is relatively higher for Co as compared to Fe, while interfaced with Ta.

Immobilization of Ethynyl-π-Extended Electron Acceptors with Amino-Terminated SAMs by Catalyst-Free Click Reaction.

Surface modification of SiO2 using a catalyst-free quantitative reaction between an amine and an ethynyl-π-extended naphthalenediimide was investigated. A post-reaction method, in which the catalyst-free reaction was performed at the surface after the formation of amino-terminated self-assembled monolayers (SAMs), resulted in dense, uniform modification of the SiO2 surface with the naphthalenediimide molecules. Both X-ray reflectivity and angle-resolved X-ray photoemission spectroscopy showed consistent results for the layer thickness and density. In contrast, a pre-reaction method, in which an amino-silane and the ethynyl-π-extended naphthalenediimide reacted first and then formed a SAM, afforded a sparse SAM on the SiO2 surface, probably due to the steric hindrance of the naphthalenediimide moieties. The in situ decoration of the SiO2 surface by a catalyst-free quantitative reaction offers a facile route for modifying surface properties with various π-conjugated molecules suitable for many applications.

Anodization-induced ZrOx/AlOx stacked films for low-temperature, solution-processed indium oxide thin-film transistors

In this study, anodization is introduced to induce dehydroxylation and condensation of solution-processed ZrOx films as well as growth of AlOx films. The anodization-induced ZrOx/AlOx stacked films are systematically studied in structure, surface morphology, density, chemical composition, dielectric and electrical properties. X-ray diffraction and atomic force microscopy reveal that anodization-induced ZrOx/AlOx stacked films exhibit amorphous structure and smooth surface roughness (<1 nm). X-ray reflectivity and x-ray photoelectron spectroscopy reveal that the solution-processed ZrOx films exhibit a higher density and a lower content of hydroxyl groups after anodization, suggesting the anodization can induce dehydroxylation and condensation for the ZrOx films. The synergistic effect of collision inducing dissociation and the self-heating from anodization of Al film is proposed to explain the dehydroxylation of the ZrOx films. Dielectric and electrical measurements illustrate that the anodization-induced ZrOx/AlOx stacked films exhibit a lower leakage current, a higher breakdown voltage and a slighter capacitance dispersion on frequency comparing with that of solution-processed ZrOx films without anodization. To verify the possible applications of anodization-induced ZrOx/AlOx stacked films as the gate dielectric in metal-oxide (MO) thin-film transistors (TFTs), they were integrated in low-temperature, directly photopatternable InOx TFTs. Remarkably, with a low process temperature of 200 °C, the InOx TFTs based on the optimized ZrOx/AlOx dielectrics exhibit an average mobility of 8.94 cm2 V−1 s−1, an I on/I off of large than 105 and negligible hysteresis in transfer curves. These results demonstrate the potential application of anodization-induced solution-processed MO dielectric films for low-temperature TFTs.

Study of reactively sputtered nickel nitride thin films

Nickel nitride (Ni–N) thin film samples were deposited using reactive magnetron sputtering process utilizing the different partial flow of N2 (RN2). They were characterized using x-ray reflectivity (XRR), x-ray diffraction (XRD) and x-ray absorption near-edge spectroscopy (XANES) taken at N K-edge and Ni L-edges. From XRR measurements, we find that the deposition rate and the density of Ni–N films decrease due to successively progression in RN2, signifying that Ni–N alloys and compounds are forming both at Ni target surface and also within the thin film samples. The crystal structure obtained from XRD measurements suggest an evolution of different Ni–N compounds given by: Ni, Ni(N), Ni4N, Ni3N, and Ni2N with a gradual rise in RN2. XANES measurements further confirm these phases, in agreement with XRD results. Polarized neutron reflectivity measurements were performed to probe the magnetization, and it was found Ni–N thin films become non-magnetic even when N incorporation increases beyond few at.%. Overall growth behavior of Ni–N samples has been compared with that of rather well-known Fe–N and Co–N systems, yielding similarities and differences among them.

GLAD Magnetron Sputtered Ultra-Thin Copper Oxide Films for Gas-Sensing Application

Copper oxide (CuO) ultra-thin films were obtained using magnetron sputtering technology with glancing angle deposition technique (GLAD) in a reactive mode by sputtering copper target in pure argon. The substrate tilt angle varied from 45 to 85° and 0°, and the sample rotation at a speed of 20 rpm was stabilized by the GLAD manipulator. After deposition, the films were annealed at 400 °C/4 h in air. The CuO ultra-thin film structure, morphology, and optical properties were assessed by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), X-ray reflectivity (XRR), and optical spectroscopy. The thickness of the films was measured post-process using a profilometer. The obtained copper oxide structures were also investigated as gas-sensitive materials after exposure to acetone in the sub-ppm range. After deposition, gas-sensing measurements were performed at 300, 350, and 400 °C and 50% relative humidity (RH) level. We found that the sensitivity of the device is related to the thickness of CuO thin films, whereas the best results are obtained with an 8 nm thick sample.

X-ray reflectivity and X-ray photoelectron spectroscopy studies on reactively sputtered $$\hbox {Nb}_{2}\hbox {O}_{5}$$-based thin-film devices

The reactively sputtered $$\hbox {Nb}_{2}\hbox {O}_{5}$$ -based metal–oxide–semiconductor devices deposited under different $$\hbox {O}_{2}/\hbox {Ar}$$ ratios have been studied using laboratory-based X-ray reflectivity (XRR) and synchrotron-based X-ray photoelectron spectroscopy techniques. The $$I-V$$ characteristics reveal a sharp transition in the magnitude of current ( $$\sim$$ up to five orders) signifying a switching phenomenon between high-resistance and low-resistance states. The electron density profile, derived from XRR data, shows a dependence of the switching properties on the film porosity. Further, the non-lattice oxygen in the film also plays a role in determining the on/off ratio of the devices. Therefore, besides the presence of non-lattice oxygen, film porosity also influences the resistive switching behavior of the $$\hbox {Nb}_{2}\hbox {O}_{5}$$ -based devices.

Compositionally tuned magnetron co-sputtered PtxNi100-x alloy as a cathode catalyst for proton exchange membrane fuel cells

In this study, PtxNi100-x (0 ≤ x ≤ 100) alloy catalysts were prepared using magnetron co-sputtering in order to reveal the correlation between their composition and catalytic properties as a cathode in proton exchange membrane fuel cells (PEMFCs). Fine power adjustment on magnetrons allowed to deposit alloys with precise composition and at the same time with identical thickness of 10 nm and similar morphologies.The powerful surface characterization techniques such as atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy (XPS) were applied to thoroughly investigate the catalytic layers. The desired composition of the films was confirmed by EDX and XRD results. All deposited layers showed similar morphologies with vertical and horizontal roughness of ~0.35 nm and ~6 nm, respectively. XRD confirmed the alloy nature of the films with one crystalline phase of the fcc PtNi.The PtxNi100-x alloys showed significant enhancement of specific power (SP) comparing to the pure Pt in PEMFC. Particularly, the Pt25Ni75 sample exhibited the highest SP of 24 kW/g(Pt). This catalyst showed a 2- and almost 10-fold improvement in SP with respect to the Pt film and commercial nanopowder Pt catalyst, respectively.

Leakage current lowering and film densification of ZrO2 high-k gate dielectrics by layer-by-layer, in-situ atomic layer hydrogen bombardment

A layer-by-layer, in-situ H2 plasma treatment in each cycle of atomic layer deposition, referred to as “atomic layer hydrogen bombardment” (ALHB), is applied to improve electrical properties of ZrO2 high-k gate dielectrics. The H2 plasma bombardment facilitates the adatom migration due to energy delivery to each as-deposited monolayer from the H2 plasma. In addition, the H2 plasma treatment contributes to the removal of precursor ligands for the release of steric hindrance. Hence the ALHB treatment leads to film densification and suppression of oxygen vacancies of ZrO2, as evidenced by X-ray reflectivity and X-ray photoelectron spectroscopy characterizations. As a result, ~90% decrease of gate leakage current is achieved in the ZrO2 high-k gate dielectrics with capacitance equivalent thicknesses of ~1.3 nm and ~0.6 nm in metal-insulator-semiconductor and metal-insulator-metal capacitors, respectively. The results manifest that the ALHB treatment is a promising technique to enhance dielectric and electrical characteristics of nanoscale thin films, for further progress of advanced devices such as sensors, solar cells, memories, and nanoelectronics.

Toward durable Al-InSb hybrid heterostructures via epitaxy of 2ML interfacial InAs screening layers

The large Land\'{e} g-factor, high spin-orbit coupling, and low effective mass of the two-dimensional electron gas in InSb quantum wells combined with proximal superconductivity may realize a scalable platform for topological quantum computation. Aluminum thin films directly deposited on top of InSb planar structures result in the formation of a reactive AlInSb layer at the interface. This interlayer progressively consumes the whole Al film, resulting in a disordered AlInSb layer after few months at room temperature. We report on a heterostructure design that results in a significant increase of the durability of these hybrid Al-InSb heterostructures with the preservation of a pure Al film and sharp superconductor-semiconductor interface for more than one year. Two monolayers of epitaxial InAs at the superconductor-semiconductor interface prevent interfacial reactivity as evidenced by X-ray reflectivity and energy dispersive spectroscopy measurements. Structural characterizations of the Al films by transmission electron microscopy reveal the presence of tens of nanometers wide grains predominantly oriented with Al(110) parallel to InSb(001).

Effects of ion irradiation on the structural and electrical properties of HfO2/SiON/Si p-metal oxide semiconductor capacitors

The influence of 120 MeV Ag ion irradiation on the structural and electrical properties of HfO2 (3 nm)/SiON (1 nm)/Si (n-type), grown by atomic layer deposition, has been investigated. X-ray Reflectivity and X-ray Photoelectron Spectroscopy measurements suggested the formation of a mixed interlayer of HfSiON above a critical fluence of 1 × 1012 ions/cm2. The observed irradiation induced changes in the leakage current have been attributed to the defects and structural changes caused by ion irradiation. The influence of various quantum tunneling mechanisms on the leakage current has been investigated as a function of ion fluence. The structural changes, defects dynamics and the consequent effects on leakage current have been explained within the framework of ion induced annealing, creation of defects and intermixing effects.

Investigation of the buried planar interfaces in multi-layered inverted organic solar cells using x-ray reflectivity and impedance spectroscopy

The hole and electron extracting interlayers in the organic solar cells (OSCs) play an important role in high performing devices. The present work focuses on an investigation of Zinc oxide/bulk heterojunction (ZnO/BHJ) and BHJ/MoOx (Molybdenum oxide) buried planar interfaces in inverted OSC devices using the optical contrast in various layers along with the electrical measurements. The x-ray reflectivity (XRR) analysis demonstrates the formation of additional intermixing layers at the interfaces of ZnO/BHJ and BHJ/MoOx. Our results indicate infusion of PC71BM into ZnO layer up to ~4 nm which smoothen the ZnO/BHJ interface. In contrast, thermally evaporated MoOx molecules diffuse into PTB7-Th dominant upper layers of BHJ active layer resulting in an intermixed layer at the interface of MoOx/BHJ. The high recombination resistance (~5 kΩ cm2) and electron lifetime (~70 μs), obtained from the impedance spectroscopy (IS), support such vertical segregation of PTB7-Th and PC71BM in the active layer. The OSC devices, processed in ambient condition, exhibit high power conversion efficiency of 6.4%. We consider our results have great significance to understand the structure of buried planar interfaces at interlayers and their correlation with the electrical parameters representing various interfacial mechanisms of OSCs.

In-Situ X-Ray Observations of Sn Electrodeposition into Anodized Alumina

Aluminum is used widely for various products and application because of its characteristics of high strength, low density and good corrosion resistance. When aluminum is exposed to air a native oxide forms on its surface and protects it from corrosion[1,2]. The native oxide can by an electrochemical method called anodization be grown thicker to give a better corrosion protection of the aluminum[3]. By using acidic electrolytes for the anodization the grown oxide will become porous, these pores can become ordered under certain anodization conditions[4]. The grown Nano-Porous Anodic Aluminum Oxide (NP-AAO) can be used as a template for growing nano wires and can be colored by deposition of metals into the pores. We have followed in-situ the electrochemical deposition of Sn into the NP-AAO by Grazing Transmission Small Angle X-Ray Scattering (GTSAXS), X-Ray Fluorescence (XRF) and X-Ray Absorption Near Edge Structure (XANES). A two-step anodization method was used to create the ordered NP-AAO, this is achieved by anodizing the sample in acidic electrolytes in two anodizing steps with a chemical etch between the steps. The second anodization step was followed in-situ with GTSAXS[5]. To achieve a better deposition into the pores of the oxide, either a barrier layer thinning or a pore widening step was used after the second anodization. These methods will decrease the barrier layer at the bottom of the pores by a controlled down stepping of the anodization potential and make the pores wider trough a chemical etch. Both methods were followed continuously by GTSAXS. The electrochemical deposition of Sn was achieved using an alternating voltage and an electrolyte containing tin ions. We followed the deposition continuously with GTSAXS, while XRF and XANES was measured at intervals during the deposition. Our GTSAXS measurements shows an increase in intensity during deposition, indicating deposition into the pores of the aluminum oxide. From our XRF measurements an increase of deposited Sn could be detected as the XRF signal at the sample increased over time. We used XANES for determining the chemical state of the deposited Sn. Ex-situ FIB-SEM measurements shows the pores filled with Sn, confirming the in-situ measurements. The sample also had a change in color confirming the Sn presence inside the NP-AAO. References L.P.H. Jeurgens, W.G. Sloof, F.D. Tichelaar, C.G. Borsboom, E.J. Mittemeijer, Determination of thickness and composition of aluminium-oxide overlayers on aluminium substrates, Appl. Surf. Sci. 144, 11 (1999)J. Evertsson et al, The thickness of native oxides on aluminum alloys and single crystals, Appl.Surf. Sci. 349, 826 (2015)F. Bertram et al, In situ anodization of aluminum surfaces studied by x-ray reflectivity and electrochemical impedance spectroscopy, J. Appl. Phys. 116, 034902, (2014)W. Lee. and S.-J. Park, Porous Anodic Aluminum Oxide: Anodization and Templated Synthesis of Functional Nanostructures, Chem. Rev. 114, 7487 (2014)Nikolay A. Vinogradov, Gary S. Harlow, Francesco Carlà, Jonas Evertsson, Lisa Rullik, Weronica Linpé, Roberto Felici, and Edvin Lundgren, Observation of Pore Growth and Self-Organization in Anodic Alumina by Time-Resolved X-ray Scattering, ACS Applied Nano Materials, Article ASAP

Low-temperature fabrication of sputtered high-k HfO2 gate dielectric for flexible a-IGZO thin film transistors

In this work, low temperature fabrication of a sputtered high-k HfO2 gate dielectric for flexible a-IGZO thin film transistors (TFTs) on polyimide substrates was investigated. The effects of Ar-pressure during the sputtering process and then especially the post-annealing treatments at low temperature (≤200 °C) for HfO2 on reducing the density of defects in the bulk and on the surface were systematically studied. X-ray reflectivity, UV-vis and X-ray photoelectron spectroscopy, and micro-wave photoconductivity decay measurements were carried out and indicated that the high quality of optimized HfO2 film and its high dielectric properties contributed to the low concentration of structural defects and shallow localized defects such as oxygen vacancies. As a result, the well-structured HfO2 gate dielectric exhibited a high density of 9.7 g/cm3, a high dielectric constant of 28.5, a wide optical bandgap of 4.75 eV, and relatively low leakage current. The corresponding flexible a-IGZO TFT on polyimide exhibited an optimal device performance with a saturation mobility of 10.3 cm2 V−1 s−1, an Ion/Ioff ratio of 4.3 × 107, a SS value of 0.28 V dec−1, and a threshold voltage (Vth) of 1.1 V, as well as favorable stability under NBS/PBS gate bias and bending stress.