Atomic layer deposition grown composite dielectric oxides and ZnO for transparent electronic applications

In this paper, we report on transparent transistor obtained using laminar structure of two high-k dielectric oxides (hafnium dioxide, HfO₂ and aluminum oxide, Al₂O₃) and zinc oxide (ZnO) layer grown at low temperature (60 °C-100 °C) using Atomic Layer Deposition (ALD) technology. Our research was focused on the optimization of technological parameters for composite layers Al₂O₃/HfO₂/Al₂O₃ for thin film transistor structures with ZnO as a channel and a gate layer. We elaborate on the ALD growth of these oxides, finding that the 100 nm thick layers of HfO₂ and Al₂O₃ exhibit fine surface flatness and required amorphous microstructure. Growth parameters are optimized for the monolayer growth mode and maximum smoothness required for gating.     

Characterization of metal-supported Al2O3 thin films by scanning electrochemical microscopy

In this paper, physiochemical properties of amorphous alumina thin films, grown by the metal organic chemical vapour deposition process on the surface of platinum (Pt/Al₂O₃) and stainless steel (SS/Al₂O₃), were investigated in aqueous media. The study was performed by the use of scanning electrochemical microscopy (SECM), which allowed obtaining information on uniformity, topography and chemical stability/reactivity of the alumina coatings with high spatial resolution. In particular, the effects due to local acid, base and fluoride ions attack on alumina layers of thickness of about 250 nm (in the Pt/Al₂O₃ sample) and 1000 nm (in the SS/Al₂O₃ sample) were investigated. In the acid and base attack, high concentrations of H₂SO₄ and KOH were electrogenerated locally by the use of a 25 μm diameter platinum microelectrode. The latter was also used as SECM tip to monitor the chemical effect on the alumina layers. It was found that, regardless of the thickness of the film, alumina provided good resistance against local attack of concentrated H₂SO₄; instead, the film dissolved when subjected to KOH attack. The dissolution rate depended on several experimental parameters, such as SECM-tip to substrate distance, electrolysis time and alumina film thickness. The alumina layer proved also relatively poor resistance to etching in 0.1 M NaF solutions.     

Improving low pressure chemical vapor deposited zinc oxide contacts for thin film silicon solar cells by using rough glass substrates

Compared to zinc oxide grown (ZnO) on flat glass, rough etched glass substrates decrease the sheet resistance (R_sq) of zinc oxide layers grown on it. We explain this R_sq reduction from a higher thickness and an improved electron mobility for ZnO layers deposited on rough etched glass substrates. When using this etched glass substrate, we also obtain a large variety of surface texture by changing the thickness of the ZnO layer grown on it. This new combination of etched glass and ZnO layer shows improved light trapping potential compared to ZnO films grown on flat glass. With this new approach, Micromorph thin film silicon tandem solar cells with high total current densities (sum of the top and bottom cell current density) of up to 26.8 mA cm^− 2 were fabricated.     

Growth of stoichiometric TiO2 thin films on Au(100) substrates by molecular beam epitaxy

The present study reports on the growth of thin TiO₂ films onto Au(100) single crystals by Ti evaporation in a reactive O₂ atmosphere at two different substrate temperatures: room temperature (RT) and 300 °C. The growth of the oxide films was monitored by means of X-ray photoemission spectroscopy, while the valence and conduction band electronic structure was investigated by UV and inverse photoemission spectroscopy, respectively.The TiO₂ film grows epitaxially on the Au(100) substrate at 300 °C exhibiting the rutile (100) surface. The evolution of the Ti 2p lineshape with the oxide coverage shows the presence of reduced oxide species (characterized by Ti^3 + ions) at the Au(100) interface. A crystalline and stoichiometric TiO₂ oxide is produced at high substrate temperature, while growth at RT gives a measurable concentration of defects. Post growth annealing in ultra-high vacuum of the RT grown film increases this concentration, while subsequent annealing in O₂ atmosphere restores the sample to the as-grown conditions.     

Dislocation reduction in GaN film using Ga-lean GaN buffer layer and migration enhanced epitaxy

A GaN buffer layer grown under Ga-lean conditions by plasma-assisted molecular beam epitaxy (PAMBE) was used to reduce the dislocation density in a GaN film grown on a sapphire substrate. The Ga-lean buffer, with inclined trench walls on its surface, provided an effective way to bend the propagation direction of dislocations, and it reduced the dislocation density through recombination and annihilation processes. As a result, the edge dislocation density in the GaN film was reduced by approximately two orders of magnitude to 2 × 10⁸ cm^− 2. The rough surface of the Ga-lean buffer was recovered using migration enhanced epitaxy (MEE), a process of alternating deposition cycle of Ga atoms and N₂ radicals, during the PAMBE growth. By combining these two methods, a GaN film with high-crystalline-quality and atomically-flat surface can be achieved by PAMBE on a lattice mismatch substrate.     

Indium Tin Oxide thin film gas sensors for detection of ethanol vapours

Indium Tin Oxide (ITO: In₂O₃ + 17% SnO₂) thin films grown on alumina substrate at 648 K temperatures using direct evaporation method with two gold pads deposited on the top for electrical contacts were exposed to ethanol vapours (200-2500 ppm). The operating temperature of the sensor was optimized. The sensitivity variation of films having different thickness was studied. The sensitivity of the films deposited on Si substrates was studied. The response of the film with MgO catalytic layer on sensitivity and selectivity was observed. A novel approach of depositing thin stimulating layer of various metals/oxides below the ITO film was tried and tested.     

Epitaxial growth and electrical measurement of single crystalline Pb(Zr0.52Ti0.48)O3 thin film on Si(001) for micro-electromechanical systems

We report in this work the epitaxial growth and the electrical characteristics of single crystalline Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin film on SrTiO₃(STO)-buffered Si(001) substrate. The STO buffer layer deposited by molecular beam epitaxy allows a coherent oxide/Si interface leading enhanced PZT crystalline quality. 70 nm-thick PZT (52:48) layer was then grown on STO/Si(001) by sol-gel method. X-ray diffraction demonstrates the single crystalline PZT film on Si substrate in the following epitaxial relationship: [110] PZT (001)//[110] STO (001)//[100] Si (001). The macroscopic electrical measurements show a hysteresis loop with memory window of 2.5 V at ± 7 V sweeping range and current density less than 1 μA/cm² at 750 kV/cm. The artificial domains created by piezoresponse force microscopy with high contrast and non-volatile properties provide further evidence for the excellent piezoelectric properties of the single crystalline PZT thin film.     

Ni-Al-O diffusion barrier layer for high-κ metal-oxide-semiconductor capacitor

Using amorphous Ni-Al-O (a-Ni-Al-O) thin film as the intermediate layer, poly-crystalline Er₂O₃ thin film was grown on a-Ni-Al-O/Si (p-type) via laser molecular beam epitaxy, forming the Er₂O₃/Ni-Al-O gate stack. It was found that the mean dielectric constant of the Er₂O₃/Ni-Al-O gate stack with an equivalent oxide thickness of 1.5 nm is about 17-23, the interfacial states density is about 3.16 × 10¹² cm⁻² and the stack gate leakage current density is as small as 4.1 × 10^− 6 A/cm². Furthermore, The insertion of the Ni-Al-O thin film between the Er₂O₃ gate dielectric and p-Si substrate prevents the oxygen from being out-diffused, which significantly improved the stability of gate stack, showing that the Er₂O₃/Ni-Al-O gate stack thin film could be used as an ideal gate oxide layer for the future Metal Oxide Semiconductor Field Effect Transistors.     

Growth of 3C-SiC on 150-mm Si(100) substrates by alternating supply epitaxy at 1000 °C

To lower deposition temperature and reduce thermal mismatch induced stress, heteroepitaxial growth of single-crystalline 3C-SiC on 150 mm Si wafers was investigated at 1000 °C using alternating supply epitaxy. The growth was performed in a hot-wall low-pressure chemical vapor deposition reactor, with silane and acetylene being employed as precursors. To avoid contamination of Si substrate, the reactor was filled in with oxygen to grow silicon dioxide, and then this thin oxide layer was etched away by silane, followed by a carbonization step performed at 750 °C before the temperature was ramped up to 1000 °C to start the growth of SiC. Microstructure analyses demonstrated that single-crystalline 3C-SiC is epitaxially grown on Si substrate and the film quality is improved as thickness increases. The growth rate varied from 0.44 to 0.76 ± 0.02 nm/cycle by adjusting the supply volume of SiH₄ and C₂H₂. The thickness nonuniformity across wafer was controlled with ± 1%. For a prime grade 150 mm virgin Si(100) wafer, the bow increased from 2.1 to 3.1 μm after 960 nm SiC film was deposited. The SiC films are naturally n type conductivity as characterized by the hot-probe technique.     

Nanoscale interfacial engineering to grow Ge on Si as virtual substrates and subsequent integration of GaAs

We have demonstrated the scalability of a process previously dubbed as Ge “touchdown” on Si to substantially reduce threading dislocations below 10⁷/cm² in a Ge film grown on a 2 inch-diameter chemically oxidized Si substrate. This study also elucidates the overall mechanism of the touchdown process. The 1.4 nm thick chemical oxide is first formed by immersing Si substrates in a solution of H₂O₂ and H₂SO₄. Subsequent exposure to Ge flux creates 3 to 7 nm-diameter voids in the oxide at a density greater than 10¹¹/cm². Comparison of data taken from many previous studies and ours shows an exponential dependence between oxide thickness and inverse temperature of void formation. Additionally, exposure to a Ge or Si atom flux decreases the temperature at which voids begin to form in the oxide. These results strongly suggest that Ge actively participates in the reaction with SiO₂ in the void formation process. Once voids are created in the oxide under a Ge flux, Ge islands selectively nucleate within the void openings on the newly exposed Si. Island nucleation and growth then compete with the void growth reaction. At substrate temperatures between 823 and 1053 K, nanometer size Ge islands that nucleate within the voids continue to grow and coalesce into a continuous film over the remaining oxide. Coalescence of the Ge islands is believed to result in the creation of stacking faults in the Ge film at a density of 5 × 10⁷/cm². Additionally, coalescence results in films of 3 µm thickness having a root-mean-square roughness of 8 to 10 nm. We have found that polishing the films with dilute H₂O₂ results in roughness values below 0.5 nm. However, stacking faults originating at the Ge-SiO₂ interface and terminating at the Ge surface are polished at a slightly reduced rate, and show up as 1 to 2 nm raised lines on the polished Ge surface. These lines are then transferred into the subsequent growth morphology of GaAs deposited by metal-organic chemical vapor deposition. Room temperature photoluminescence shows that films of GaAs grown on Ge-on-oxidized Si have an intensity that is 20 to 25% compared to the intensity from GaAs grown on commercial Ge or GaAs substrates. Cathodoluminescence shows that nonradiative defects occur in the GaAs that spatially correspond to the stacking faults terminating at the Ge surface. The exact nature of these nonradiative defects in the GaAs is unknown, however, GaAs grown on annealed samples of Ge-on-oxidized Si, whereby annealing removes the stacking faults, have photoluminescence intensity that is comparable to GaAs grown on a GaAs substrate.     

Metal-oxide-semiconductor characteristics of thermally grown nitrided SiO2 thin film on 4H-SiC in various N2O ambient

Metal-oxide-semiconductor characteristics of thermally grown nitrided SiO₂ (9-11.5 nm) on n-type 4H-SiC at 1175 °C in various N₂O ambient (1, 10, and 50% N₂O mixed with 99, 90, and 50% of high purity N₂ gas, respectively) have been investigated. The chemical composition of oxide-semiconductor interface has been evaluated by X-ray photoelectron spectroscopy. The interfacial layer consists of either silicon oxynitride, silicon nitride, and/or silicon oxide phases that may be segregated or mixed in a single layer. Depending on the percentage of N₂O being used, the stoichiometry may vary accordingly. The lowest leakage current density is recorded for thin film oxide grown in10% N₂O and it is limited to an applied electric field of not more than 7 MV/cm. This is attributed to the lowest density value of deep oxide trap in this sample if compared with others. The highest dielectric breakdown field has been obtained for thin film oxide grown in 50% N₂O as this sample is having the lowest interface trap density and negative effective oxide charge. The origin of these charges is explained in the text.     

TEM characterization of super-hydrophilic Ni-Ti thin film

A super-hydrophilic Ni-Ti thin film is evaluated using cross-sectional transmission electron microscopy (TEM). Following sputter deposition and annealing, the Ni-Ti thin films are chemically treated with buffered oxide etchant, nitric acid and 30% hydrogen peroxide solution to produce a 100-nm-thick surface oxide layer. The fabricated Ni-Ti thin films show a 10- to 15-nm TiO₂ thick layer on its surface with a ~ 67° wetting angle. For the surface treated sample, a porous oxide 100 nm TiO layer is found with a < 5°. It is believed that TiOOH(H₂O)_n complex forms on the surface to produce the observed super-hydrophilic behavior.     

Variations of microstructures and electrical properties of Bi4Ti3O12/SrTiO3/(La0.5, Sr0.5)CoO3/MgO epitaxial thin films by annealing

Epitaxial thin films of a heterostructure with Bi₄Ti₃O₁₂(BIT)/SrTiO₃(ST) were successfully grown with a bottom electrode consisting of La_0.5Sr_0.5CoO₃(LSCO) on MgO(001) substrates using pulsed laser deposition. The grown BIT and ST (001) planes were parallel to the growth surface with the orientation relationship of BIT <110>//ST <010>. In the as-deposited film, the BIT (001) plane appeared to expand to relieve a lattice mismatch with the ST (001) plane. However, annealing for 20-40 min induced the BIT (001) plane to contract horizontally with its c-axis expanding, which was associated with a local perturbation in the layer stacking of the BIT structure. This structural distortion was reduced in the film annealed for 1 h, with restoration of the periodicity of the layer stacking. Correspondingly, the dielectric constant of the as-deposited film was increased from 292 to 411 by annealing for 1 h. In parallel, the film was paraelectric but became more ferroelectric, with the remanent polarization and the coercive field changing from 0.1 μC/cm² and 14 kV/cm to 1.7 μC/cm² and 69 kV/cm, respectively.     

Temperature effects on the growth and electrical properties of Er2O3 films on Ge substrates

Er₂O₃ films were grown on Ge (001) substrates at different temperatures by molecular beam epitaxy using metallic Er and molecular oxygen sources with otherwise identical conditions. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were used to characterize the microstructures and compositions of the films. The film deposited at room temperature is found to be composed of an Er₂O₃ layer and an ErGe_xO_y interface layer with a thickness of 5.5 nm; the film grown at 300 °C has a mixed structure of Er₂O₃ and ErGe_xO_y and the thickness was found to be reduced to 2.2 nm; the film grown at 450 °C becomes much rougher with voids formed underneath the film, having a mixed structure of three compounds of Er₂O₃, GeO and ErGe_xO_y. The growth mechanisms of the films at different temperatures are suggested. Current images obtained by tunneling atomic force microscopy show that the film grown at 450 °C has much more leaky spots than those grown at RT and 300 °C, which may arise from the formation of volatile GeO in the film.     

Growth of oriented Ag nanocrystals on air-oxidized Si surfaces: An in-situ reflection high energy electron diffraction study

Growth of Ag nanoislands on air-oxidized Si(001), (111) and (110) surfaces has been investigated by reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and cross-sectional transmission electron microscopy. We have shown that the oriented nanocrystalline Ag, similar to the epitaxial growth of Ag on clean Si surfaces, can be grown on oxide-covered Si surfaces. A thin oxide layer (~ 2-3 nm thick) is formed on ultra-high vacuum (UHV)-cleaned Si surfaces via exposure of the clean reconstructed surface to air. Deposition of Ag was carried out under UHV at different substrate temperatures and monitored by RHEED. RHEED results reveal that Ag deposition at room temperature leads to the growth of randomly oriented Ag islands while, in spite of the presence of the oxide layer between Ag islands and Si, preferred orientations with an epitaxial relationship with the substrate evolve when Ag is deposited at higher substrate temperatures. STM images of the oxidized surfaces, prior to Ag deposition, apparently do not show any order. However, Fourier transforms of STM images show the presence of a short range order on the oxidized surface following the unit cells of the underlying reconstructed Si surface. It is intriguing that Ag nanoislands follow an epitaxial orientational relationship with the substrate in spite of the presence of a 2-3 nm thick oxide layer between Ag and Si. Apparently, the short range order existing on the oxide surface influences the orientation of the Ag nanoislands.     

Local modification of NaCl thin films on Cu(111) under different bias voltages

The local modification of NaCl thin films on Cu(111) under different bias voltages is investigated using a scanning tunneling microscope (STM) at room temperature. We find that the type of modification of NaCl thin films is dependent on sample bias voltage (V_s). Defects in a triple-layer-thick NaCl film are destabilized and repelled away from the region beneath an STM tip by applying V_s in the range of 0.3 V ≤ |V_s| ≤ 0.5 V. When V_s is larger than + 1.2 V or smaller than − 4.0 V, the removal of NaCl films takes place and a bare Cu surface appears. In this case, the removed NaCl is transferred to the STM tip and can be supplied back to the surface from the STM tip. The redeposition of NaCl enables not only the reformation of single-crystalline NaCl films on a bare Cu surface but also the formation of additional NaCl films on a clean NaCl film surface.     

Plastic substrate with gas barrier layer and transparent conductive oxide thin film for flexible displays

A novel plastic substrate for flexible displays was developed. The substrate consisted of a polycarbonate (PC) base film coated with a gas barrier layer and a transparent conductive thin film. PC with ultra-low intrinsic birefringence and high temperature dimensional stability was developed for the base film. The retardation of the PC base film was less than 1 nm at a wavelength of 550 nm (film thickness, 120 µm). Even at 180 °C, the elastic modulus was 2 GPa, and thermal shrinkage was less than 0.01%. The surface roughness of the PC base film was less than 0.5 nm. A silicon oxide (SiO_x) gas barrier layer was deposited on the PC base film by a roll-to-roll DC magnetron reactive sputtering method. The water vapor transmission rate of the SiO_x film was less than 0.05 g/m²/day at 40 °C and 100% relative humidity (RH), and the permeation of oxygen was less than 0.5 cc/m² day atm at 40 °C and 90% RH. As the transparent conductive thin film, amorphous indium zinc oxide was deposited on the SiO_x by sputtering. The transmittance was 87% and the resistivity was 3.5 × 10^− 4 ohm cm.     

X-ray microdiffraction from α-Ti0.04Fe1.96O3 (0001) epitaxial film grown over α-Cr2O3 buffer layer boundary

Ti-doped hematite (α-Ti_0.04Fe_1.96O₃) film grown over patterned α-Cr₂O₃ buffer layer on α-Al₂O₃(0001) substrate was characterized with synchrotron X-ray microdiffraction. The film was grown by oxygen plasma assisted molecular beam epitaxy method. The film growth mode was correlated to buffer layer boundary and Ti concentration variation. Epitaxial α-Ti_0.04Fe_1.96O₃ film was formed on bare substrate adjacent to the buffer layer. The epitaxial film was connected laterally to a strain-relaxed epitaxial α-Ti_0.04Fe_1.96O₃ film grown on the buffer layer. On bare α-Al₂O₃ substrate with diminished Ti concentration only a small portion of α-Ti_xFe_1 − xO₃ film was epitaxial either as coherent to the substrate or strain-relaxed form.     

Electrical resistivity of Ti-Zn mixed oxide thin films deposited by atomic layer deposition

Ti-Zn mixed oxide thin films, with thickness less than 50 nm, were grown with atomic layer deposition (ALD) technique at low temperature (90 °C) varying the composition. ALD is a powerful chemical technique to deposit thin films with thickness of few atomic layers. ALD oxide material growth is achieved by dosing sequentially the metal precursor and the oxidizing agent. Thanks to ALD nature of layer by layer growth it was possible to realize mixed metal, Ti and Zn, oxide thin films with controlled composition, simply by changing the number of cycles of each metal oxide layer. Structural and electrical properties of the prepared thin films were studied as a function of their composition. Synchrotron radiation X-ray diffraction technique was used to follow thin film crystallization during sample annealing, performed in situ. It was observed that the onset temperature of crystallization raises with Ti content, and sample structure was Zn₂TiO4 phase. Electrical resistivity measurements were performed on crystalline samples, annealed at 600 °C, revealing an increase in resistivity with Ti content.     

Metalorganic chemical vapor deposition of β-FeSi2 on β-FeSi2 seed crystals formed on Si substrates

We have fabricated a β-FeSi₂ film by metalorganic chemical vapor deposition on a Si(001) substrate with β-FeSi₂ seed crystals grown by molecular beam epitaxy, and investigated the crystallinity, surface morphology and temperature dependence of photoresponse properties of the β-FeSi₂ film. The surface of the grown β-FeSi₂ film was atomically flat, and step-and-terrace structure was clearly observed. Multi-domain structure of β-FeSi₂ whose average size was approximately 200 nm however was revealed. The photoresponse was obtained in an infrared light region (~ 0.95 eV) at temperatures below 200 K. The external quantum efficiency reached a maximum, being as large as 25% at 100 K when a bias voltage was 2.0 V.