Optical and Electrical Characteristics of ZnO Films Grown on Nitridated Si (1 0 0) Substrate with GaN and ZnO Double Buffer Layers

The optical and electrical characteristics of zinc oxide (ZnO) films grown by molecular-beam epitaxy (MBE) on Si substrates were investigated. ZnO epitaxial layer was successfully grown on nitridated Si(100) substrate initially covered with high-temperature GaN and low-temperature ZnO double buffer layers using MBE. X-ray diffraction and photoluminescence results both indicated that a reasonable quality of ZnO epitaxial layer was obtained. As the CV measurement had indicated, the carrier concentration was reduced virtually in a linear fashion from ZnO surface down to GaN buffer layer. A reduction in electron concentration was caused by the carrier depletion due to the presence of the Schottky barrier of Ni/ZnO. The large density of electron accumulated at the ZnO/GaN interface was due to the large conduction band discontinuity and offset.     

Various one-dimensional GaN nanostructures formed by non-catalytic routes

GaN crystals were nucleated and grown into various one-dimensional forms by the supersayuration of a source gas via non-catalytic routes. Chemical vapor deposition (CVD) was employed for the GaN growth using Ga metal and ammonia gas as sources. The formation of one-dimensional GaN naostructures including nano-columns, nano-cakes, nano-flowers, and nano-bundles was identified using scanning electron microscopy (SEM). X-ray diffraction (XRD) was performed to analyze crystallinity of each nanostructure. The growth mechanism of thick nano-columns was proposed as the epitaxial growth of GaN (002) hexagonal columns with {100} facets from the GaN thick films. The growth of nano-cakes was suggested as the (002) hexagonal plate formation with {100} facets at the first stage followed by the secondary (002) crystal growth from each plate. The growth mechanism of nano-flowers would be the formation of GaN seed particles first and the subsequent nucleation and growth of GaN (101) nanowires with several directions from the seeds. The growth of nano-bundles is the most interesting structure and the formation mechanism was proposed as the first formation of a GaN (002) nanowire and then subsequent nucleation and growth of GaN (002) and (10-1) nanocrystals at the surface of the primary GaN (002) nanowire.     

Growth and characterization of device quality ZnO on Si(111) and c-sapphire using a conventional rf magnetron sputtering

In this article, it is shown that high quality ZnO films were grown on Si(111) and Al₂O₃(0001) substrates using a conventional rf magnetron sputtering. High-resolution X-ray diffractometry (HR-XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and photoluminescence (PL) investigations clearly confirmed that the ZnO films grown on Al₂O₃ (0001) at substrate temperatures above 650^∘C are single crystal as well as high optical quality. It is also estimated in both cases grown on Si and Al₂O₃ that an introduction of template pre-grown at 500^∘C can induce a homogeneous interface and improvement of emission characteristic by relaxing the strain caused by large lattice and thermal mismatch between the film and substrate and by reducing defect density in interface region.     

Growth of flower-like patterns of TiO2 nanorods over FTO substrate

ABSTRACT

Nanostructures such as nanoparticles, nanowires, nanorods, nanotubes have attracted great interest with the understanding of shape and size dependence in the electronic and sensing properties. This work demonstrates growth and characterization of one-dimensional nanorods of titanium-dioxide (TiO₂) on the fluorine doped tin oxide (FTO) substrate by using hydrothermal method. Field effect scanning electron microscopy shows flower-like morphology, and energy-dispersive X-ray spectroscopy confirms the presence of Ti and O. The X-ray diffraction peaks agree well with the JCPDS data. Initially, the 1D nanorods are formed and high concentration of precursor assists evolution of the short branches to form flower-like structures.     

A new facile synthesis of ultra fine magnesium oxide nanowires and optical properties

One-dimensional metal oxides nanowires like magnesium oxide (MgO) play an important role in several nanotechnological applications. MgO nanowires were synthesized for the first time via hydrothermal route using magnesium acetate and urea at 180?°C for 2?h. The synthesized MgO nanowires were characterized by means of X-ray diffraction (XRD), Fourier transformer Infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), transmission scanning electron microscopy (TEM) and thermal gravimetric analysis (TGA). The obtained results indicated that the MgO nanowires show a predominant well- crystalline cubic face-centered with an average diameter of 10?nm and an average length of 40???m. The optical band gap energy of the sample was found to be 3.45?eV. The sample indicates a broadband PL emission in UV region and exhibits a good photoluminescence behavior for photonic devices applications. It is evaluated that this novel synthetic method is very useful and serves as a facile, direct preparation mild chemical method giving mass production of one dimensional MgO nanowires.     

Influence of deposition parameters on physico-chemical and optical properties of sputtered PbTiO3 thin films

Abstract

Stoichiometric thin films of lead titanate (PbTiO₃) have been grown “in situ” without postannealing on (0001) sapphire substrates by rf magnetron sputtering technique. X-ray diffraction scans have revealed that as-grown films consist of the perovskite phase and are polycrystalline with a high (111) orientation. The structure, the microstructure and the optical properties have been studied as a function of the process parameters i.e., substrate temperature, gas pressure and the target composition, in particular the lead content. We report the dependence of the deposition conditions on the optical constants. The optimum experimental conditions (100m Torr and 600°C) to produce thin films with high transparency and refractive indices (n = 2.61 at 633 nm), similar to values for bulk materials, are given in this paper.     

Characterization of hetero-epitaxial PZT capacitor

Abstract

A hetero-epitaxial Au/PbZr_0.48Ti_0.52O₃(PZT)/SrRuO₃(SRO) capacitor was fabricated on a single crystal SrTiO₃ (STO) substrate by pulsed laser deposition. An SRO buffer layer (a nucleation layer) was formed at the SRO/STO interface to ensure the highly epitaxial growth of the PZT and SRO films. An X-ray diffraction measurement revealed that the (00l) planes of the PZT and SRO grew parallel to the substrate surface. A transition layer of ～ 5 nm thickness was observed at the SRO/STO interface by high-resolution transmission electron microscopy (HR-TEM). This transition layer corresponds to the nucleation layer intentionally grown at the interface. Remanent polarization of the capacitor was 32.1 μC/cm² due to the good epitaxial growth of the films.     

Influence of Growth Temperature on Surface Morphologies of GaN Crystals Grown on Dot-Patterned Substrate by Hydride Vapor Phase Epitaxy

This paper studies the influence of growth temperatures in the range 825 to 1050_°C on the surface morphologies of GaN crystals grown on a SiO₂ dot-patterned substrate using Epitaxy Lateral Overgrowth (ELO) and Hydride Vapor Phase Epitaxy (HVPE) techniques. A lower growth temperature of 850_°C prompts the formation of GaN hexagonal pyramidal crystals with a higher fraction of {1 ${\bar 1}$ 01}}(0001) facet areas than those grown at high temperatures (>1000°C). In a subsequent coalescent (or lateral growth) process, a high temperature of 1050°C is applied to the original GaN hexagonal pyramidal crystals, and the morphologies of the GaN layers are inspected. It is established that the original {1 ${\bar 1}$ 01} faceted morphology of the hexagonal pyramids changes to an irregularly-shaped surface comprising {1 ${\bar 1}$ 01}, {11 ${\overline 2}$ 2} and high index facets, and that the nature of the surface morphology is influenced by the growth time and the application (or not) of Ga precursor support. Hence, the results show that the coalescence and planarization of the GaN layer can be controlled through an appropriate specification of the process parameters. At low temperatures in the region of 850°C, high index facets are observed on the tops of a small percentage of the hexagonal GaN columnar crystals. It is proposed that this phenomenon is caused by a reduction in the surface diffusion length of the precursors, e.g. NH₃ and GaCl, at lower temperature, which in turn, reduces the probability of desorption and increases the lifetime.     

Influence of the growth temperature on the epitaxial quality of PbTiO3 films deposited in-situ by sputtering

Abstract

Thin films of lead titanate were prepared in-situ on SrTiO₃ substrates using radio-frequency magnetron sputter deposition. The epitaxial quality of the films has been studied as a function of the substrate temperature. Stoichiometric films have been obtained in the temperature range [550°C, 600°C]. The films deposited in the equilibrium zone have a high degree of c-axis oriented epitaxial crystalline structure as shown by X-ray diffraction in the 2θ/θ, θ, and ? scans configuration as well as by electron channeling pattern. The optimum conditions for growing epitaxial PbTiO₃ layers were determined. The crystallinity of films deposited at 550°C is suprior to those deposited at 600°C. The PbTiO₃ films grown at 550°C have a rocking curve full width at half maximum (FWHM) of 0.2°; Normaski optical and atomic force microscopy show that the surface is apparently free of grain boundaries and very smooth. The refractive index of these films has been evaluated from transmission spectra; it is very close to the bulk material value.     

Influence of ion beam bombardment on characteristic of InGaN/GaN single quantum well grown by metal–organic chemical vapor deposition

The influence of ion beam bombardment on sapphire substrate was investigated on the electrical and optical characteristics of Indium–Gallium–Nitride/Gallium–Nitride (InGaN/GaN) single quantum well (SQW) structure. Ion bombardment of N⁺, He⁺, H⁺ ions were made on single crystal substrate of sapphire with dose of 1?×?10^14–17 ions/cm². The InGaN/GaN SQW was fabricated on the ion beam bombarded sapphire substrate in two-flow Metal Organic Chemical Vapor Deposition (MOCVD) equipment. The thickness of InGaN/GaN SQW was about 20 nm and the composition of InGaN/GaN SQW was found to be In_0.1Ga_0.9N. In PL spectra, it is found that InGaN/GaN SQW was emitted from 441.1 to 446.6 nm (2.8–2.7 eV). The highest mobility value of 118 cm²/V–S and the lowest carrier concentration of 3.41?×?10¹⁷/cm² was found for N of 10¹⁶ ions/cm² ion beam bombarded sample. The optimal condition for InGaN/GaN SQW on sapphire substrate of ion beam bombardment was deduced to be N⁺ ion dose of 10¹⁶ ions/cm².     

A Freestanding GaN/HfO $_{bf 2}$ Membrane Grown by Molecular Beam Epitaxy for GaN–Si Hybrid MEMS

Combination of GaN light source and Si-microelectromechanical systems (MEMSs) is a promising hybrid structure for optical MEMS. As one of GaN-Si hybrid structures, a freestanding GaN/HfO₂ membrane was fabricated on Si substrate. Unlike conventional GaN membrane on Si substrate, the fabricated membrane had a tensile stress by using the HfO₂ layer. Therefore, the GaN/HfO₂ membrane was flat enough to be useful for several MEMS. The GaN crystal was grown by molecular beam epitaxy on the HfO₂ layer deposited on Si substrate. The surface of the HfO₂ layer was nitrified before GaN crystal growth, and thus, a part of HfO₂ surface was changed to HfN, the lattice of which matched well to that of GaN. The characteristics of the GaN crystal grown on the nitrified HfO₂ layer were also investigated.     

Tailoring the structural and optical properties of RF magnetron sputtered ZnO/graphene thin films by annealing temperature

Abstract

ZnO and ZnO/Graphene thin films were deposited on Cu substrate using a low pressure chemical vapor deposition (LPCVD) and the magnetron sputtering method. The impacts of graphene layer growth and annealing temperature on the optical properties ZnO and ZnO/Graphene thin films were investigated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), X-ray photoelectron spectroscopic (XPS), and photoluminescence (PL) measurements respectively. XRD and SEM results reveal that all the thin films preferred the crystalline [001] orientation along the c-axis direction, which were vertical grown on substrate surface. By comparing the results and analysis of their structure, morphology, chemical bonding and optical property, it is proved that using Graphene as a buffer layer can improve the crystal quality of ZnO thin films. For the annealed ZnO/graphene nanostructures, the area ratio of UV and visible emission region of ZnO/graphene thin films increase with increasing the annealing temperature, reaches a maximum at 500?°C and then starts decreasing with further increase in annealing temperature, which indicating that the controllable ZnO/Graphene thin films have the higher crystallization quality at the annealing temperature of 500?°C. Our results demonstrate that for high quality ZnO/graphene thin films deposition, decreasing the defect concentration should be preferable to simply applying the proper annealing temperature, which might have promising applications for various UV photodetectors devices.     

Integration of MgO on Si(001) Using SrO and SrTiO3 Buffer Layers by Molecular Beam Epitaxy

Epitaxial MgO was deposited onto Si(001) substrates by molecular beam epitaxy using elemental metallic sources and molecular oxygen at temperatures from 150 to 400C. To facilitate epitaxy through misfit strain relaxation, epitaxial MgO layers were grown on SrO and SrTiO₃ buffer layers deposited on Si(001) substrates. The structure of the epitaxial layers was determined by X-ray diffraction, reflection high-energy electron diffraction and transmission electron microscopy. The observed orientation for the MgO/SrO/Si multilayer is cube-on-cube. The X-ray rocking curve full width half maximum of the MgO on SrO buffer layers was 2.2. SrTiO₃ buffer layers grown by recrystallization were epitaxial and exhibited improved morphology relative to those grown at a fixed growth temperature. X-ray analysis of a 5.2 nm recrystallized SrTiO₃ film indicates a fully relaxed and phase pure film. The observed orientation of MgO using SrTiO₃ buffer layers is MgO[100]SrTiO₃[100]Si[110].     

Characterization of Ba(Zr0.05 Ti0.95)O3 thin film prepared by sol-gel process

Ba(Zr_0.05Ti_0.95)O₃ (BZT) thin film (∼330 nm) was grown on Pt/Ti/SiO₂/Si(100) substrate by a simple sol-gel process. The microstructure and the surface morphology of BZT thin film were studied by X-ray diffraction and atomic force microscopy. The optical properties of BZT thin film were obtained by spectroscopic ellipsometry. The optical bandgap was found to be 3.74 eV of direct-transition type. Ferroelectric and dielectric properties of BZT thin film were also discussed. The electrical measurements were conducted on BZT films in metal-ferroelectric-metal (MFM) capacitor configuration. The results showed the film exhibited good ferroelectrity with remanent polarization and coercive electric field of 3.54 μC/cm² and 95.5 kV/cm, respectively. At 10 kHz, the dielectric constant and dielectric loss of the film are 201 and 0.029, respectively.     

Effects of Mg dopant on the degradation of InGaN multiple quantum wells in AlInGaN-based light emitting devices

We investigated the effects of Mg dopant on the degradation of AlInGaN-based light emitting diodes (LEDs) and laser diodes (LDs) with InGaN multi-quantum wells (MQWs). Photoluminescence (PL) intensity of InGaN MQWs was significantly decreased with increasing the Mg intentional doping process in InGaN active region, indicating that Mg dopant could degrade the optical quality of InGaN MQWs. From secondary ion mass spectroscopy (SIMS) analysis of AlInGaN-based LDs grown on GaN/sapphire and GaN substrate with different dislocation densities, we found that Mg concentration of LD on GaN/sapphire was higher than that of LD on GaN substrate at the InGaN MQWs regions. Additionally, we observed that Mg atoms were significantly diffused from p-type layer to InGaN MQWs region in the LD structure after aging evaluation. From these results, we could conclude that Mg diffusion along threading dislocations is one of the major gradual degradation mechanisms of AlInGaN-based LD/LEDs during the device operation under high voltage condition.     

Preparation of hafnium oxide thin films by sol–gel method

Hafnium oxide (HfO₂) films were grown on SiO₂/Si substrates by a sol–gel method, and their crystalline structure, microstructure and electrical properties were investigated. X-ray diffraction analysis indicated that the monoclinic HfO₂ films could be obtained by annealing at 500 °C. A transmission electron microscopy (TEM) image showed that the films were grown as a spherulite grain structure with a mean grain size of approximately 15 nm. The dielectric constant of the HfO₂ films of 300 nm was approximately 21.6, and the current–voltage measurements showed that the leakage current density of the HfO₂ films was approximately 1.14?×?10^?5 A/cm² at an applied electric field of 100 kV/cm. The sol–gel method-fabricated HfO₂ films are concluded to be feasible for MEMS applications, such as capacitive-type MEMS switches.     

Effects of spontaneous and piezoelectric polarization fields on the electronic and optical properties in GaN/AlN quantum dots: multimillion‐atom sp3d5s* tight‐binding simulations

GaN quantum dots (QDs) grown on AlN substrates are strong candidates for UV and near‐infrared applications. The wurtzite crystal symmetry in these dots induces internal fields arising from the following: (i) crystal atomicity;(ii) strained active region; (iii) piezoelectricity; and (iv) spontaneous polarization (pyroelectricity). Accurate modeling of electronic and optical properties of these QDs must capture the interplay of these atomistic and long‐range fields and the size quantization on an equal footing. In this work, single‐particle electronic structure and interband optical transition rates of a GaN/AlN QD grown along the c‐axis are studied using a coupled molecular mechanics‐atomistic 20‐band sp³d⁵s^* tight‐binding (VFF‐TB) framework. To calculate piezoelectricity, a recently reported model that takes into account both the linear and the nonlinear dependence of polarization on the strain tensors has been employed. The simulated GaN/AlN dot is realistically sized (containing ~3 million atoms) and of hexagonal disk shape having height and base length of 4.1 and 17 nm, respectively. It is found that, in contrast to the well‐studied InN/GaN systems, the pyroelectric potential in GaN/AlN dot is larger than the piezoelectric counterpart, and the effects of piezoelectric and pyroelectric fields add up. The internal fields result in a large redshift in the electronic states near the Brillouin zone center (known as quantum confined Stark effect), pronounced non‐degeneracy in the excited states, strongly suppressed optical transition, and anisotropic emission spectra. Copyright © 2014 John Wiley & Sons, Ltd.     

Controlled growth of silicon nanowires on silicon surfaces

This paper reports on silicon nanowire growth on oxidized silicon substrates using different approaches for gold catalyst deposition. The gold coated surfaces and the resulting nanowires were characterized using scanning electron microscopy. The gold catalysts were made up of gold nanoparticles (50 nm diameter), which were either dispersed or spotted at different concentrations using a robot, or were formed on a patterned Si/SiO₂ substrate by metal evaporation (63 nm diameter). The subsequent silicon nanowire growth was accomplished by CVD decomposition of silane gas (SiH₄) at high temperature (400–500^°C) in a vapor-liquid-solid (VLS) process. Under these conditions, a high density of silicon nanowires (SiNWs) was achieved on the oxidized silicon surfaces, but the distribution of the nanowires was found to be inhomogeneous in the case of the gold nanoparticles. Such result is attributed to the aggregation of the nanoparticles during the growth process. Alternatively, when gold nanodot catalysts were lithographically patterned on the surface, the nanowires were obtained in the patterned regions.     

Electrical and optical properties of fluorine-doped tin oxide (SnOx:F) thin films deposited on PET by using ECR–MOCVD

The electrical, optical, structural and chemical bonding properties of fluorine-doped tin oxide (SnO_x:F) films deposited on a plastic substrate prepared by Electron Cyclotron Resonance–Metal Organic Chemical Vapor Deposition (ECR–MOCVD) were investigated with special attention to the process parameters such as the H₂/TMT mole ratio, deposition time and amount of fluorine-doping. The four point probe method, UV visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic emission spectroscopy (AES), X-Ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the films. Based on our experimental results, the characteristics of the SnO_x:F thin films were significantly affected by the process parameters mentioned above. The amount of fluorine doping was found to be one of the major parameters affecting the surface resistivity, however its excess doping into SnO₂ lead to a sharp increase in the surface resistivity. The average transmittance decreased with increasing film thickness. The lowest electrical resistivity of 5.0?×?10^?3 Ω^.cm and highest optical transmittance of 90% in the visible wavelength range from 380 to700 nm were observed at an H₂/TMT mole ratio of 1.25, fluorine-doping amount of 1.3 wt.%, and deposition time of 30 min. From the XRD analysis, we found that the SnO_x:F films were oriented along the (2 1 1) plane with a tetragonal and polycrystalline structure having the lattice constants, a?=?0.4749 and c?=?0.3198 nm.     

MOCVD PBZRxti1−xo3 thin films on platinized silicon wafers and srtio3 crystals

Abstract

PbZr_xTi_1-xO₃ (PZT) thin films were grown on 6” platinized silicon substrates (Pt / Si) and SrTiO₃ (STO) crystals by Metal-Organic Chemical Vapor Deposition (MOCVD) as a function of the Zr / (Zr+Ti) ratio in the gas phase. Morphology, optical properties, and crystal structure were investigated by scanning electron microscopy, atomic force microscopy, ellipsometry, and X-ray diffraction. The morphology, structure, and optical properties of the polycrystalline and epitaxial films were compared. The determination of the refractive index by ellipsometry (from 550 nm to 2000 nm) was not sensible for the films grown on (Pt / Si) but successful for the films grown on SrTiO₃.