Effect of atomic layer deposited ultra thin HfO2 and Al2O3 interfacial layers on the performance of dye sensitized solar cells

The objective of this work was to investigate the improvement in performance of dye sensitized solar cells (DSSCs) by depositing ultra thin metal oxides (hafnium oxide (HfO₂) and aluminum oxide (Al₂O₃)) on mesoporous TiO₂ photoelectrode using atomic layer deposition (ALD) method. Different thicknesses of HfO₂ and Al₂O₃ layers (5, 10 and 20 ALD cycles) were deposited on the mesoporous TiO₂ surface prior to dye loading process used for fabrication of DSSCs. It was observed that the ALD deposition of ultrathin oxides significantly improved the performance of DSSCs and that the improvement in the DSSC performance depends on the thickness of the deposited HfO₂ and Al₂O₃ films. Compared to a reference DSSC the incorporation of a HfO₂ layer resulted in 69% improvement (from 4.2 to 7.1%) in the efficiency of the cell and incorporation of Al₂O₃ (20 cycles) resulted in 19% improvement (from 4.2 to 5.0%) in the efficiency of the cell. These results suggest that ultrathin metal oxide layers affect the density and the distribution of interface states at the TiO₂/organic dye and TiO₂/liquid electrolyte interfaces and hence can be utilized to treat these interfaces in DSSCs.     

Low-temperature atomic layer deposition of ZnO thin films: Control of crystallinity and orientation

Low-temperature atomic layer deposition (ALD) processes are intensely looked for to extend the usability of the technique to applications where sensitive substrates such as polymers or biological materials need to be coated by high-quality thin films. A preferred film orientation, on the other hand, is often required to enhance the desired film properties. Here we demonstrate that smooth, crystalline ZnO thin films can be deposited from diethylzinc and water by ALD even at room temperature. The depositions were carried out on Si(100) substrates in the temperature range from 23 to 140 °C. Highly c-axis-oriented films were realized at temperatures below ~ 80 °C. The film crystallinity could be further enhanced by post-deposition annealing under O₂ or N₂ atmosphere at 400-600 °C while keeping the original film orientation intact.     

Combined spectroscopic ellipsometry and attenuated total reflection analyses of Al2O3/HfO2 nanolaminates

The microstructure of thin HfO₂-Al₂O₃ nanolaminate high κ dielectric stacks grown by atomic vapor deposition has been studied by attenuated total reflection spectroscopy (ATR) and 8 eV spectroscopic ellipsometry (SE). The presence of Al₂O₃ below HfO₂ prevents the crystallisation of HfO₂ if an appropriate thickness is used, which depends on the HfO₂ thickness. A thicker Al₂O₃ is required for thicker HfO₂ layers. If crystallisation does occur, we show that the HfO₂ signature in both ATR and 8 eV SE spectra allows the detection of monoclinic crystallites embedded in an amorphous phase.     

High performance self-aligned top-gate ZnO thin film transistors using sputtered Al2O3 gate dielectric

High performance self-aligned top-gate zinc oxide (ZnO) thin film transistors (TFTs) utilizing high-k Al₂O₃ thin film as gate dielectric are developed in this paper. Good quality Al₂O₃ thin film was deposited by reactive DC magnetron sputtering technique using aluminum target in a mixed argon and oxygen ambient at room temperature. The resulting transistor exhibits a field effect mobility of 27 cm²/V s, a threshold voltage of − 0.5 V, a subthreshold swing of 0.12 V/decade and an on/off current ratio of 9 × 10⁶. The proposed top-gate ZnO TFTs in this paper can act as driving devices in the next generation flat panel displays.     

Atomic layer deposited aluminum oxide barrier coatings for packaging materials

Thin aluminum oxide coatings have been deposited at a low temperature of 80 °C on various uncoated papers, polymer-coated papers and boards and plain polymer films using the atomic layer deposition (ALD) technique. The work demonstrates that such ALD-grown Al₂O₃ coatings efficiently enhance the gas-diffusion barrier performance of the studied porous and non-porous materials towards oxygen, water vapor and aromas.     

Atomic layer deposition of MnO using Bis(ethylcyclopentadienyl)manganese and H2O

Manganese oxide (MnO) atomic layer deposition (ALD) was accomplished using sequential exposures of bis(ethylcyclopentadienyl)manganese (Mn(CpEt)₂) and H₂O. Rutherford backscattering analysis revealed a nearly 1:1 atomic ratio for Mn:O in the MnO ALD films. X-ray diffraction determined that the films were crystalline and consistent with the cubic phase of MnO. Quartz crystal microbalance (QCM) measurements monitored the mass deposition rate during MnO ALD and verified self-limiting reactions for each reactant. Extremely efficient reactions were observed that required reactant exposures of only 3 × 10⁴ L (1 L = 1.33 × 10^− 4 Pa s). X-ray reflectivity (XRR) studies were used to confirm the QCM measurements and determine the film density and film thicknesses. The MnO ALD film density was 5.23 g/cm³. The growth per cycle was investigated from 100-300 °C. The largest MnO ALD growth per cycle was 1.2 Å/cycle at 100 °C and the growth per cycle decreased at higher temperatures. Transmission electron microscopy images observed the conformality of MnO films on ZrO₂ nanoparticles and confirmed the growth per cycle observed by the XRR studies. Fourier transform infrared spectroscopy was used to study the -CpEt? and -OH? surface species during MnO ALD and also monitored the bulk vibrational modes of the growing MnO films. The results allowed a growth mechanism to be established for MnO ALD using Mn(CpEt)₂ and H₂O. Only 54% of the Mn sites are observed to retain the -CpEt? surface species after the Mn(CpEt)₂ exposure. Efficient MnO ALD using Mn(CpEt)₂ and H₂O should be useful for a variety of applications where metal oxides are required that can easily change their oxidation states.     

ZnO thin films prepared by atomic layer deposition and rf sputtering as an active layer for thin film transistor

Recently, the application of ZnO thin films as an active channel layer of transparent thin film transistor (TFT) has become of great interest. In this study, we deposited ZnO thin films by atomic layer deposition (ALD) from diethyl Zn (DEZ) as a metal precursor and water as a reactant at growth temperatures between 100 and 250 °C. At typical growth conditions, pure ZnO thin films were obtained without any detectable carbon contamination. For comparison of key film properties including microstructure and chemical and electrical properties, ZnO films were also prepared by rf sputtering at room temperature. The microstructure analyses by X-ray diffraction have shown that both of the ALD and sputtered ZnO thin films have (002) preferred orientation. At low growth temperature T_s ≤ 125 °C, ALD ZnO films have high resistivity (> 10 Ω cm) with small mobility (< 3 cm²/V s), while the ones prepared at higher temperature have lower resistivity (< 0.02 Ω cm) with higher mobility (> 15 cm²/V s). Meanwhile, sputtered ZnO films have much higher resistivity than ALD ZnO at most of the growth conditions studied. Based upon the experimental results, the electrical properties of ZnO thin films depending on the growth conditions for application as an active channel layer of TFT were discussed focusing on the comparisons between ALD and sputtering.     

Electrical and chemical analysis of zinc oxide interfaces with high dielectric constant barium tantalate and aluminum oxide in metal-insulator-semiconductor structures fabricated at Low temperatures

Zinc oxide (ZnO) was incorporated into metal-insulator-semiconductor (MIS) structures featuring high dielectric constant (high-κ) barium tantalate (BaTa₂O₆)or alumina (Al₂O₃)as the insulator, and the structures were electrically evaluated for potential applications in transparent thin film transistors. The ZnO films were deposited by radio-frequency magnetron sputtering at 100 °C whereas the dielectric films were deposited by the same method at room temperature. The leakage currents of both the BaTa₂O₆ and Al₂O₃ structures were on the order of 10⁻⁷A/cm². The trap density and trapped charge concentration at the BaTa₂O₆/ZnO interface were determined to be 6.18 × 10¹¹ eV⁻¹ cm⁻²and 5.82 × 10¹¹ cm⁻² from conductance-voltage and capacitance-voltage measurements. At the Al₂O₃/ZnO interface the trap density and trapped charge were more than an order of magnitude smaller at 1.09 × 10¹⁰ eV⁻¹ cm⁻²and 1.04 × 10¹⁰ cm⁻² respectively. The BaTa₂O₆ structures had significantly larger frequency dispersions due to the larger number of interface traps. Chemical analysis using X-ray photoelectron spectroscopy with depth profiling indicates that acceptor type defects associated with a deficiency of oxygen are related to the observed electron trapping in the BaTa₂O₆MIS structure. Overall, the results indicate that Al₂O₃ would be better suited for transparent thin film transistors deposited at low temperature or without substrate heating.     

HfO2 gate insulator formed by atomic layer deposition for thin-film-transistors

We have investigated the effects of annealing temperature on the physical and electrical properties of the HfO₂ film deposited by an atomic layer deposition (ALD) method for high-k gate oxides in thin-film-transistors (TFTs). The ALD deposition of HfO₂ directly on the Si substrate at 300 °C results in the formation of thin HfSi_xO_y interfacial layer between Si and HfO₂. The subsequent low temperature N₂-annealing of HfO₂ films (i.e., 300 °C) using a rapid thermal processor (RTP) improves the overall electrical characteristics of HfSi_xO_y-HfO₂ films. Based on the current work, we suggest that HfO₂ film deposited by the ALD method is suitable for high-k gate oxides in TFTs, which have to be fabricated at low temperature.     

Atomic layer deposition on polymer based flexible packaging materials: Growth characteristics and diffusion barrier properties

One of the most promising areas for the industrial application of atomic layer deposition (ALD) is for gas barrier layers on polymers. In this work, a packaging material system with improved diffusion barrier properties has been developed and studied by applying ALD on flexible polymer based packaging materials. Nanometer scale metal oxide films have been applied to polymer-coated papers and their diffusion barrier properties have been studied by means of water vapor and oxygen transmission rates. The materials for the study were constructed in two stages: the paper was firstly extrusion coated with polymer film, which was then followed by the ALD deposition of oxide layer. The polymers used as extrusion coatings were polypropylene, low and high density polyethylene, polylactide and polyethylene terephthalate. Water vapor transmission rates (WVTRs) were measured according to method SCAN-P 22:68 and oxygen transmission rates (O₂TRs) according to a standard ASTM D 3985. According to the results a 10 nm oxide layer already decreased the oxygen transmission by a factor of 10 compared to uncoated material. WVTR with 40 nm ALD layer was better than the level currently required for most common dry flexible packaging applications. When the oxide layer thickness was increased to 100 nm and above, the measured WVTRs were limited by the measurement set up. Using an ALD layer allowed the polymer thickness on flexible packaging materials to be reduced. Once the ALD layer was 40 nm thick, WVTRs and O₂TRs were no longer dependent on polymer layer thickness. Thus, nanometer scale ALD oxide layers have shown their feasibility as high quality diffusion barriers on flexible packaging materials.     

Atomic layer deposition of Ru films from bis(2,5-dimethylpyrrolyl)ruthenium and oxygen

Ru thin films were grown on hydrogen terminated Si, SiO₂, Al₂O₃, HfO₂, and TiO₂ surfaces by atomic layer deposition from bis(2,5-dimethylpyrrolyl)ruthenium precursor and oxygen. The 4-20 nm thick films on these surfaces consisted of nanocrystalline hexagonal metallic ruthenium, regardless of the deposition temperature. At the lowest temperatures examined, 250-255 °C, the growth of the Ru films was favored on silicon, compared to the growth on Al₂O₃, TiO₂ and HfO₂. At higher temperatures the nucleation and growth of Ru became enhanced in particular on HfO₂, compared to the process on silicon. At 320-325 °C, no growth occurred on Si-H and SiO₂-covered silicon. Resistivity values down to 18 μΩ·cm were obtained for ca. 10 nm thick Ru films.     

Improved properties of Pt-HfO2 gate insulator-ZnO semiconductor thin film structure by annealing of ZnO layer

Metal-insulator-semiconductor capacitors were fabricated with sputtered ZnO and atomic layer deposited HfO₂ as the semiconductor and gate dielectric layers, respectively. From the capacitance-voltage measurements, it was confirmed that pre-deposition annealing of the sputtered ZnO layer at 300 °C in air greatly decreased the interfacial trap density (∼ 2 × 10¹² cm^− 2 eV^− 1). X-ray photoelectron spectroscopy showed a decrease in the OH bonds adsorbed on the ZnO surface after pre-deposition annealing, which improved the interface property. A very small capacitance equivalent thickness of 1.3 nm was achieved, which decreased the operation voltage (< 5 V) of the device significantly.     

Effect of alumina doping on structural, electrical, and optical properties of sputtered ZnO thin films

A systematic study of the influence of alumina (Al₂O₃) doping on the optical, electrical, and structural characteristics of sputtered ZnO thin films is reported in this study. The ZnO thin films were prepared on 1737F Corning glass substrates by R.F. magnetron sputtering from a ZnO target mixed with Al₂O₃ of 0-4 wt.%. X-ray diffraction (XRD) analysis demonstrates that the ZnO thin films with Al₂O₃ of 0-4 wt.% have a highly (002) preferred orientation with only one intense diffraction peak with a full width at half maximum (FWHM) less than 0.5°. The electrical properties of the Al₂O₃-doped ZnO thin films appear to be strongly dependent on the Al₂O₃ concentration. The resistivity of the films decreases from 74 Ω·cm to 2.2 × 10^− 3 Ω·cm as the Al₂O₃ content increases from 0 to 4 wt.%. The optical transmittance of the Al₂O₃-doped ZnO thin films is studied as a function of wavelength in the range 200-800 nm. It exhibits high transparency in the visible-NIR wavelength region with some interference fringes and sharp ultraviolet absorption edges. The optical bandgap of the Al₂O₃-doped ZnO thin films show a short-wavelength shift with increasing of Al₂O₃ content.     

High performance metal-insulator-metal capacitors with atomic vapor deposited HfO2 dielectrics

Thin HfO₂ films were grown as high-k dielectrics for Metal-Insulator-Metal applications by Atomic Vapor Deposition on 8 inch TiN/Si substrates using pure tetrakis(ethylmethylamido)hafnium precursor. Influence of deposition temperature (320-400 °C) and process pressure (2-10 mbar) on the structural and electrical properties of HfO₂ was investigated. X-ray diffraction analysis showed that HfO₂ layers, grown at 320 °C were amorphous, while at 400 °C the films crystallized in cubic phase. Electrical properties, such as capacitance density, capacitance-voltage linearity, dielectric constant, leakage current density and breakdown voltage are also affected by the deposition temperature. Finally, TiN/HfO₂/TiN stacks, integrated in the Back-End-of-Line process, possess 3 times higher capacitance density compared to standard TiN/Si₃N₄/TiN capacitors. Good step coverage (> 90%) is achieved on structured wafers with aspect ratio of 2 when HfO₂ layers are deposited at 320 °C and 4 mbar.     

Memristive Behavior Enabled by Amorphous–Crystalline 2D Oxide Heterostructure

The emergence of memristive behavior in amorphous–crystalline 2D oxide heterostructures, which are synthesized by atomic layer deposition (ALD) of a few-nanometer amorphous Al₂O₃ layers onto atomically thin single-crystalline ZnO nanosheets, is demonstrated. The conduction mechanism is identified based on classic oxygen vacancy conductive channels. ZnO nanosheets provide a 2D host for oxygen vacancies, while the amorphous Al₂O₃ facilitates the generation and stabilization of the oxygen vacancies. The conduction mechanism in the high-resistance state follows Poole–Frenkel emission, and in the the low-resistance state is fitted by the Mott–Gurney law. From the slope of the fitting curve, the mobility in the low-resistance state is estimated to be ≈2400 cm² V⁻¹ s⁻¹, which is the highest value reported in semiconductor oxides. When annealed at high temperature to eliminate oxygen vacancies, Al is doped into the ZnO nanosheet, and the memristive behavior disappears, further confirming the oxygen vacancies as being responsible for the memristive behavior. The 2D heterointerface offers opportunities for new design of high-performance memristor devices.     

Properties of atomic layer deposited HfO2 thin films

The growth, composition and morphology of HfO₂ films that have been deposited by atomic layer deposition (ALD) are examined in this article. The films are deposited using two different ALD chemistries: i) tetrakis ethylmethyl amino hafnium and H₂O at 250° and ii) tetrakis dimethyl amino hafnium and H₂O at 275 °C. The growth rates are 1.2 Å/cycle and 1.0 Å/cycle respectively. The main impurities detected both by X-ray Photoelectron Spectroscopy and Fourier transform infrared spectroscopy (FTIR) are bonded carbon (~ 3 at.%) and both bulk and terminal OH species that are partially desorbed after high temperature inert anneals up to 900 °C. Atomic Force Microscopy reveals increasing surface roughness as a function of increasing film thickness. X-ray diffraction shows that the morphology of the as-deposited films is thickness dependent; films with thickness around 30 nm for both processes are amorphous while ~ 70 nm films show the existence of crystallites. These results are correlated with FTIR measurements in the far IR region where the HfO₂ peaks are found to provide an easy and reliable technique for the determination of the crystallinity of relatively thick HfO₂ films. The index of refraction for all films is very close to that for bulk crystalline HfO₂.     

Growth and structure of heteroepitaxial thin films of homologous compounds RAO3(MO)m by reactive solid-phase epitaxy: Applicability to a variety of materials and epitaxial template layers

A reactive solid-phase epitaxy (R-SPE) method combines deposition of a thick amorphous or polycrystalline layer with a desired chemical composition and post-deposition solid-phase epitaxial growth. The solid-phase epitaxial growth is invoked by thermal annealing with an assistance of a sacrificial layer working as an epitaxial template. Thereby it enables us to grow high-quality epitaxial films of complex oxides whose epitaxial films are not grown by conventional high-temperature growth techniques. It was reported that 2-nm-thick ZnO layers worked as template for growing InGaO₃(ZnO)_m (m = integer) epitaxial films. The present study extended the R-SPE technique to growth of various complex oxides with chemical compositions of RAO₃(MO)_m and to use of various epitaxial template layers. We found that mono oxide epitaxial layers such as In₂O₃ and Ga₂O₃ work as template layers as well. Alternatively, a ZnO epitaxial layer is also applicable to ZnO-free compounds. The films obtained were grown heteroepitaxially on YSZ(111) and single-crystalline when the fabrication conditions are optimized.     

X-ray mirrors on flexible polymer substrates fabricated by atomic layer deposition

Atomic layer deposition (ALD) techniques were used to fabricate W/Al₂O₃ superlattices with high X-ray reflectivity on flexible Kapton® polyimide and polyethylene naphthalate (PEN) polymer substrates. Reflectivities of 78% and 74% at λ = 1.54 Å were measured for 6-bilayer W/Al₂O₃ superlattices on Kapton® polyimide and PEN, respectively. These excellent X-ray reflectivities are attributed to precise bilayer thicknesses and ultrasmooth interfaces obtained by ALD and smoothing of the initial polymer surface by an Al₂O₃ ALD layer. The conformal ALD film growth also produces correlated roughness that enhances the reflectivity. These W/Al₂O₃ superlattices on flexible polymers should be useful for ultralight and adjustable radius of curvature X-ray mirrors.     

Surface poisoning in the nucleation and growth of palladium atomic layer deposition with Pd(hfac)2 and formalin

Palladium (Pd) atomic layer deposition (ALD) can be performed with Pd(hfac)₂ (hfac = hexafluoroacetyl-acetone) and formalin as the reactants. For Pd ALD on oxide surfaces, the nucleation of Pd ALD has been observed to require between 20 and 100 ALD cycles. To understand the long nucleation periods, this study explored the surface reactions occurring during Pd ALD nucleation and growth on hydroxylated Al₂O₃ substrates. In situ Fourier transform infrared (FTIR) spectroscopy on high surface area nanopowders was used to observe the surface species. The adsorption of Pd(hfac)₂ on hydroxylated Al₂O₃ substrates was found to yield both Pd(hfac)* and Al(hfac)* surface species. The identity of the Al(hfac)* species was confirmed by separate FTIR studies of hfacH adsorption on the hydroxylated Al₂O₃ substrates. Isothermal loss of the Al(hfac)* species revealed second-order kinetics at 448-523 K with an activation barrier of E_d = 39.4 kcal/mol. The lack of correlation between Al(hfac)* and AlOH* species during the loss of Al(hfac)* species suggested that the Al(hfac)* species may desorb as Al(hfac)₃. After Pd(hfac)₂ exposure and the subsequent formalin exposure on hydroxylated Al₂O₃ substrates, only hfac ligands from Pd(hfac)* species were removed from the surface. In addition, the formalin exposure added formate species. The Al(hfac)* species was identified as the cause of the long nucleation period because Al(hfac)* behaves as a site blocker. The surface poisoning by Al(hfac)* species was corroborated by adsorbing hfacH prior to the Pd(hfac)₂ exposures. The amount of Pd(hfac)* species after Pd(hfac)₂ exposures decreased progressively versus the previous hfacH exposure. Pd ALD occurred gradually during the subsequent Pd ALD cycles as the Al(hfac)* species were slowly removed from the Al₂O₃ surface. Ex situ transmission electron microscopy analysis revealed Pd nanoclusters that grew in size and dispersion with increasing number of Pd ALD cycles. These nanoclusters eventually coalesced to form a continuous Pd ALD film. Surface poisoning by the hfac ligands may help to explain the nucleation difficulties for metal ALD on oxide substrates using β-diketonate reactants.     

Metal nanocrystal memory with sol-gel derived HfO2 high-κ tunnel oxide

An all-solution processed metal-oxide-semiconductor (MOS) capacitor structure containing gold (Au) nanoparticles (NPs) within HfO₂ high-κ oxide was fabricated. The ultra-thin (~ 10 nm) HfO₂ high-κ tunnel oxide layer was prepared by sol-gel process and showed good electrical properties, which were critical to superior memory property of the MOS structure. Au NPs with particle size of about 3.3 nm were synthesized by chemical reduction method and then self-assembled onto HfO₂ tunnel oxide. Finally, a Si/HfO₂/Au NPs/HfO₂ memory structure was constructed after the substrate had been covered with a sol-gel-derived HfO₂ control oxide layer (~ 13 nm). By utilizing high-quality HfO₂ as tunnel oxide, the MOS structure containing Au NPs showed memory effect even at a low voltage of ± 3 V. Although its memory window was only 0.8 V by a swapping voltage between ± 5 V, the MOS showed desirable retention characteristics. Therefore, we have fabricated nanocrystal memory device with sol-gel derived HfO₂ high-k tunnel oxide which are attractive for low operation voltage non-volatile memory applications.