Eda Goldenberg

Hollow cathode plasma-assisted atomic layer deposition of crystalline AlN, GaN and AlxGa1−xN thin films at low temperatures

The authors report on the use of hollow cathode plasma for low-temperature plasma-assisted atomic layer deposition (PA-ALD) of crystalline AlN, GaN and AlxGa1−xN thin films with low impurity concentrations. Depositions were carried out at 200 °C using trimethylmetal precursors and NH3 or N2/H2 plasma. X-ray photoelectron spectroscopy showed the presence of 2.5–3 at.% O in AlN and 1.5–1.7 at.% O in GaN films deposited using NH3 and N2/H2 plasma, respectively. No C impurities were detected within the films. Secondary ion mass spectroscopy analyses performed on the films deposited using NH3 plasma revealed the presence of O, C (both <1 at.%), and H impurities. GIXRD patterns indicated polycrystalline thin films with wurtzite crystal structure. Hollow cathode PA-ALD parameters were optimized for AlN and GaN thin films using N2/H2 plasma. Trimethylmetal and N2/H2 saturation curves evidenced the self-limiting growth of AlN and GaN at 200 °C. AlN exhibited linear growth with a growth per cycle (GPC) of ∼1.0 A. For GaN, the GPC decreased with the increasing number of deposition cycles, indicating substrate-enhanced growth. The GPC calculated from a 900-cycle GaN deposition was 0.22 A. Ellipsometric spectra of the samples were modeled using the Cauchy dispersion function, from which the refractive indices of 59.2 nm thick AlN and 20.1 nm thick GaN thin films were determined to be 1.94 and 2.17 at 632 nm, respectively. Spectral transmission measurements of AlN, GaN and AlxGa1−xN thin films grown on double side polished sapphire substrates revealed near-ideal visible transparency with minimal absorption. Optical band edge values of the AlxGa1−xN films shifted to lower wavelengths with the increasing Al content, indicating the tunability of band edge values with the alloy composition.

Low-temperature grown wurtzite InxGa1−xN thin films via hollow cathode plasma-assisted atomic layer deposition

Herein, we report on atomic layer deposition of ternary InxGa1−xN alloys with different indium contents using a remotely integrated hollow cathode plasma source. Depositions were carried out at 200 °C using organometallic Ga and In precursors along with N2/H2 and N2 plasma, respectively. The effect of In content on structural, optical, and morphological properties of InxGa1−xN thin films was investigated. Grazing incidence X-ray diffraction showed that all InxGa1−xN thin films were polycrystalline with a hexagonal wurtzite structure. X-ray photoelectron spectroscopy depicted the peaks of In, Ga, and N in bulk of the film and revealed the presence of relatively low impurity contents. In contents of different InxGa1−xN thin films were determined by energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. Transmission electron microscopy also confirmed the polycrystalline structure of InxGa1−xN thin films, and elemental mapping further revealed the uniform distribution of In and Ga within the bulk of InxGa1−xN films. Higher In concentrations resulted in an increase of refractive indices of ternary alloys from 2.28 to 2.42 at a wavelength of 650 nm. The optical band edge of InxGa1−xN films red-shifted with increasing In content, confirming the tunability of the band edge with alloy composition. Photoluminescence measurements exhibited broad spectral features with an In concentration dependent wavelength shift and atomic force microscopy revealed low surface roughness of InxGa1−xN films with a slight increase proportional to In content.

Fabrication of flexible polymer–GaN core–shell nanofibers by the combination of electrospinning and hollow cathode plasma-assisted atomic layer deposition

Here we demonstrate the combination of electrospinning and hollow cathode plasma-assisted atomic layer deposition (HCPA-ALD) processes by fabricating flexible polymer–GaN organic–inorganic core–shell nanofibers at a processing temperature much lower than that needed for the preparation of conventional GaN ceramic nanofibers. Polymer–GaN organic–inorganic core–shell nanofibers fabricated by the HCPA-ALD of GaN on electrospun polymeric (nylon 6,6) nanofibers at 200 °C were characterized in detail using electron microscopy, energy dispersive X-ray analysis, selected area electron diffraction, X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence measurements, and dynamic mechanical analysis. Although transmission electron microscopy studies indicated that the process parameters should be further optimized for obtaining ultimate uniformity and conformality on these high surface area 3D substrates, the HCPA-ALD process resulted in a ∼28 nm thick polycrystalline wurtzite GaN layer on polymeric nanofibers of an average fiber diameter of ∼70 nm. Having a flexible polymeric core and low processing temperature, these core–shell semiconducting nanofibers might have the potential to substitute brittle ceramic GaN nanofibers, which have already been shown to be high performance materials for various electronic and optoelectronic applications.

Optical characteristics of nanocrystalline AlxGa1−xN thin films deposited by hollow cathode plasma-assisted atomic layer deposition

Gallium nitride (GaN), aluminum nitride (AlN), and AlxGa1−xN films have been deposited by hollow cathode plasma-assisted atomic layer deposition at 200 °C on c-plane sapphire and Si substrates. The dependence of film structure, absorption edge, and refractive index on postdeposition annealing were examined by x-ray diffraction, spectrophotometry, and spectroscopic ellipsometry measurements, respectively. Well-adhered, uniform, and polycrystalline wurtzite (hexagonal) GaN, AlN, and AlxGa1−xN films were prepared at low deposition temperature. As revealed by the x-ray diffraction analyses, crystallite sizes of the films were between 11.7 and 25.2 nm. The crystallite size of as-deposited GaN film increased from 11.7 to 12.1 and 14.4 nm when the annealing duration increased from 30 min to 2 h (800 °C). For all films, the average optical transmission was ∼85% in the visible (VIS) and near infrared spectrum. The refractive indices of AlN and AlxGa1−xN were lower compared to GaN thin films. The refractive index o...

Postdeposition annealing on RF-sputtered SrTiO3 thin films

Understanding of structural, optical, and electrical properties of thin films are very important for a reliable device performance. In the present work, the effect of postdeposition annealing on stoichiometric SrTiO3 (STO) thin films grown by radio frequency magnetron sputtering at room temperature on p-type Si (100) and quartz substrates were studied. Highly transparent and well adhered thin films were obtained in visible and near infrared regions. As-deposited films were amorphous, while nanocrystalline and polycrystalline phases of the STO thin films formed as a function of annealing temperature. Films annealed at 300 °C showed nanocrystallinity with some amorphous phase. Crystallization started after 15 min annealing at 700 °C, and further improved for films annealed at 800 °C. However, crystallinity reduced for films which were annealed at 900 °C. The optical and electrical properties of STO thin films affected by postdeposition annealing at 800 °C: Eg values decreased from 4.50 to 4.18 eV, n(λ) valu...

Zno nanostructures via hydrothermal synthesis on atomic layer deposited seed-layers

The original results of two different types of ZnO nanostructures grown via hydrothermal synthesis on ZnO seed-layers coated by atomic layer deposition process on Si substrates were presented. Scanning electron microscopy and X-ray diffractometry were used for the analysis of resulting nanostructured ZnO samples. The influence of annealing on crystal properties of the ZnO nanostructures was shown. It was ascertained that solution composition had a significant influence on the morphology of nanostructures and post-growth annealing modified the crystal properties of nanostructures.

Hollow-cathode plasma-assisted atomic layer deposition: A novel route for low-temperature synthesis of crystalline III-nitride thin films and nanostructures

Hollow cathode plasma-assisted atomic layer deposition is a promising technique for obtaining III-nitride thin films with low impurity concentrations at low temperatures. Here we report our efforts on the development of HCPA-ALD processes for III-nitrides together with the properties of resulting thin films and nanostructures. The content will further include nylon 6,6/GaN core/shell and BN/AlN bishell hollow nanofibers, proof-of-concept thin film transistors and UV photodetectors fabricated using HCPA-ALD-grown GaN layers, as well as early results for InN thin films deposited by HCPA-ALD technique.

Template-assisted synthesis of III-nitride and metal-oxide nano-heterostructures using low-temperature atomic layer deposition for energy, sensing, and catalysis applications (Presentation Recording)

Recent experimental research efforts on developing functional nanostructured III-nitride and metal-oxide materials via low-temperature atomic layer deposition (ALD) will be reviewed. Ultimate conformality, a unique propoerty of ALD process, is utilized to fabricate core-shell and hollow tubular nanostructures on various nano-templates including electrospun nanofibrous polymers, self-assembled peptide nanofibers, metallic nanowires, and multi-wall carbon nanotubes (MWCNTs). III-nitride and metal-oxide coatings were deposited on these nano-templates via thermal and plasma-enhanced ALD processes with thickness values ranging from a few mono-layers to 40 nm. Metal-oxide materials studied include ZnO, TiO2, HfO2, ZrO2, and Al2O3. Standard ALD growth recipes were modified so that precursor molecules have enough time to diffuse and penetrate within the layers/pores of the nano-template material. As a result, uniform and conformal coatings on high-surface area nano-templates were demonstrated. Substrate temperatures were kept below 200C and within the self-limiting ALD window, so that temperature-sensitive template materials preserved their integrity III-nitride coatings were applied to similar nano-templates via plasma-enhanced ALD (PEALD) technique. AlN, GaN, and InN thin-film coating recipes were optimized to achieve self-limiting growth with deposition temperatures as low as 100C. BN growth took place only for >350C, in which precursor decomposition occured and therefore growth proceeded in CVD regime. III-nitride core-shell and hollow tubular single and multi-layered nanostructures were fabricated. The resulting metal-oxide and III-nitride core-shell and hollow nano-tubular structures were used for photocatalysis, dye sensitized solar cell (DSSC), energy storage and chemical sensing applications. Significantly enhanced catalysis, solar efficiency, charge capacity and sensitivity performance are reported. Moreover, core-shell metal-oxide and III-nitride materials showed promise to be used in applications where flexibility is critical like functional membranes, textile and flexible electronic applications.