Enhancement of surface morphology and optical properties of nanocolumnar ZnO films

Nanocolumnar ZnO films were prepared by electrodeposition (ED) on a glass substrate covered with a conductive layer of thin oxide doped with fluorine (FTO). After deposition the samples were annealed in oxidizing or reducing atmosphere, at temperatures between 100 to 500 C, in order to follow the evolution of optical properties and morphology. The optical properties of these films were studied by means of photoluminescence spectroscopy (PL) and the morphology by scanning electron microscopy (SEM). Films annealed at 300 C exhibit a higher ultraviolet emission peak, originating from exciton transitions. A green band related to deep-level emission centered at 500 nm, shows a drastic increase at 500 C. These results are independent of the annealing atmosphere. An increase of coalescence is also observed after annealing at 500 C. These results are explained taking into account the contribution of different point defects.     

ZnO nanostructured thin films grown by pulsed laser deposition in mixed O2 / Ar background gas

We report on the properties of ZnO nanostructured thin films grown on either bare or gold patterned a-plane sapphire substrates. The pulsed laser deposition technique was used to deposit all the films at a temperature of 700 C in a mixture of oxygen and argon under a total pressure of 35 Pa. SEM surface characterizations typically showed pyramidal nanostructures with hexagonal symmetry and a coverage density strongly dependent on the O₂ partial pressure. For the patterned samples, wall-like structures of nanoneedles were observed. For all samples, x-ray diffraction results confirmed the high crystalline quality of the nanostructures, with the rocking curve widths of the (0002) reflection as low as 0.09. Similarly, photoluminescence results at room temperature testified to the high optical quality of the material.     

The effect on the annealing temperature of Li doped ZnO thin film for a film bulk acoustic resonator

We investigated the material and electrical properties of Li doped ZnO thin film (ZLO) with variation of the annealing temperature. In the 500 C sample, ZLO film showed well defined (002) c-axis orientation and a full width half-maximum property of 0.25. The electrical properties of ZLO thin films showed the excellent specific resistance of 1.5×10¹¹ Ω cm. Finally, the frequency characteristics of the ZLO thin film FBAR, according to the annealing temperature, showed improvement of the return loss from 24.48 to 30.02 dB at a resonant frequency of 1.17 GHz.     

A novel deposition method to grow ZnO nanorods: Spray pyrolysis

ZnO layers were deposited by chemical spray pyrolysis (CSP) using zinc chloride aqueous solutions onto indium tin oxide (ITO) glass substrates at growth temperatures in the region of 400–580 C. The layers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and low-temperature () photoluminescence (PL) measurements. The flat film of ZnO obtained at 400 C evolves to a structured layer by raising the temperature up to 500 C. Deposition around 550 C and above results in a layer comprising well-shaped hexagonal ZnO nanorods with diameter of 100–150 nm and length of up to 1 micron. XRD shows strong c-axis orientation of ZnO being in accordance with the SEM study. Deposition of nanorods was successful using ITO with grain size around 100 nm, whereas on fine-grained ITO (grain size < 50 nm) with smooth surface fat crystals with diameter up to 400 nm and length of about 300 nm were formed. Sharp near band edge (NBE) emission peaks centered at 3.360 and 3.356 eV dominated the PL spectra of ZnO at , originating from the exciton transition bound to neutral donors. PL and XRD results suggest that ZnO rods prepared by spray pyrolysis are of high optical and crystalline quality.     

Study of structural, optical and electrical properties of ZnO and SnO2 thin films

The investigation of structure, optical and electrical properties of tin and zinc oxide films on glass substrates by using magnetron sputtering are carried out. X-ray data show the formation of textured tin oxides film during deposition and its transformation to SnO₂ polycrystalline film at low temperature (200 C) if the concentration of oxygen in the chamber is high (O₂ — 100%, Ar — 0%). Optimal conditions of SnO₂ polycrystalline film deposition (pressure of Ar–O₂ mixture in chamber — 2.7 Pa, concentration of O₂ — 10%) are determined. Low resistivity of as-deposited ZnO film and increasing ZnO crystallite sizes and phase volume at temperatures higher than the melting point of Zn (419.5 C) are explained by formation of conductive Zn and ZnO particle chains and their destruction, respectively.     

Atypical grain boundaries in ZnO layers deposited on sapphire by rf magnetron sputtering on (0001) sapphire

The crystalline quality of ZnO is investigated, in thin films prepared by RF magnetron sputtering on c-plane sapphire. The ZnO layers exhibit a columnar growth and the average column diameter depends on the deposition temperature. Along the [] zone axis of sapphire, the diffraction pattern exhibits two zone axis patterns; the central rows belong to the two zones, with the 0002n spots clearly underlined by the overlap. On each part, the two inner rows belong to the [] zone and the next two to the [] zone. Therefore, the two main epitaxial relationships in mismatched growth of wurtzite structures coexist in these layers, and adjacent columns are rotated, with respect to one other, by 90 around the [0001] direction. In cross section observations, the interface between two grains is abrupt, but it is not easy to determine its atomic structure. Observations in planar view show that although the long range rotation between grains is in agreement with the theoretical epitaxial relationships, the local angles oscillate between 27 and 32. This discrepancy is explained by the formation of grain boundaries which are found to follow the coincidence site lattice rules which make them settle into minimal energy configurations.     

Morphological control of ZnO nanostructures on silicon substrates

ZnO nanostructures are grown on Au-catalyzed Si substrates by vapour phase transport between 800 and 1150 C. Nanostructures grown at 800 C are mainly rod-like in structure with diameters of <200 nm. Increasing growth temperature yields combination growth modes with 2D structures (nanowalls/nanosheets) connecting 1D nanorods at intermediate temperatures and a 3D growth mode of foam-like appearance at the highest temperatures. The present work indicates that it may be possible to systematically control the morphology of ZnO nanostructures by varying the growth temperature.     

Effects of annealing temperature on morphologies and optical properties of ZnO nanostructures

The effects of annealing temperature on the morphologies and optical properties of ZnO nanostructures synthesized by sol–gel method were investigated in detail. The SEM results showed that uniform ZnO nanorods formed at 900 C. The PL results showed an ultraviolet emission peak and a relatively broad visible light emission peak for all ZnO nanostructures sintered at different temperature. The increase of the crystal size and decrease of tensile stress resulted in the UV emission peak shifted from 386 to 389 nm when annealing temperature rose from 850 to 1000 C. The growth mechanism of the ZnO nanorods is discussed.     

UV-Raman scattering study of lattice recovery by thermal annealing of Eu -implanted GaN layers

Lattice recovery of Eu-implanted GaN has been studied by means of Raman scattering under UV excitation. GaN epilayers implanted at 300 keV with doses ranging from 2×10¹⁴ to 4×10¹⁵ cm⁻² and subsequently annealed at 1000 C for 20 min show an increasing degree of disorder as the implantation dose increases. Higher temperature annealings up to 1300 C were also investigated in samples having an AlN capping layer. Disorder related modes, observed in samples annealed at 1000 C, disappear at higher annealing temperatures, indicating an incomplete lattice recovery at 1000 C. The Raman scattering spectra show resonant A₁(LO) multiphonon scattering up to sixth order, whose relative intensities depend on the implantation dose. The intensity ratios between multiphonon peaks observed for the highest implantation doses suggest a spread of the resonance, which could be related to a heterogeneous strain distribution, also indicative of incomplete lattice recovery.     

Dielectric/piezoelectric properties and temperature dependence of domain structure evolution in lead free single crystal

(K_0.5Na_0.5)NbO₃ (KNN) single crystals were grown using a high temperature flux method. The dielectric permittivity was measured as a function of temperature for [001]-oriented KNN single crystals. The ferroelectric phase transition temperatures, including the rhombohedral–orthorhombic T_R−O, orthorhombic–tetragonal T_O−T and tetragonal–cubic T_C were found to be located at −149  C, 205 C and 393 C, respectively. The domain structure evolution with an increasing temperature in [001]-oriented KNN single crystal was observed using polarized light microscopy (PLM), where three distinguished changes of the domain structures were found to occur at −150  C, 213 C and 400 C, corresponding to the three phase transition temperatures.     

Fabrication and properties of ZnO:Cu and ZnO:Ag thin films

Thin films of ZnS and ZnO:Cu were grown by an original metal–organic chemical vapour deposition (MOCVD) method under atmospheric pressure onto glass substrates. Pulse photo-assisted rapid thermal annealing of ZnO:Cu films in ambient air and at the temperature of 700–800 C was used instead of the common long-duration annealing in a vacuum furnace. ZnO:Ag thin films were prepared by oxidation and Ag doping of ZnS films. At first a closed space sublimation technique was used for Ag doping of ZnO films. The oxidation and Ag doping were carried out by a new non-vacuum method at a temperature >500 C. Crystal quality and optical properties were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL). It was found that the doped films have a higher degree of crystallinity than undoped films. The spectra of as-deposited ZnO:Cu films contained the bands typical for copper, i.e. the green band and the yellow band. After pulse annealing at high temperature the 410 and 435 nm photoluminescent peaks were observed. This allows changing of the emission colour from blue to white. Flat-top ZnO:Ag films were obtained with the surface roughness of 7 nm. These samples show a strong ultraviolet (UV) emission at room temperature. The 385 nm photoluminescent peak obtained is assigned to the exciton–exciton emission.     

Study of the MOCVD growth of ZnO on GaAs substrates: Influence of the molar ratio of the precursors on structural and morphological properties

ZnO thin films were grown by metal-organic chemical vapour deposition (MOCVD) on GaAs(100) and GaAs(111)A substrates. The growth experiments were performed at temperatures ranging from 290 to 500 C and atmospheric pressure. Diethylzinc (DEZn) and tertiary butanol (tBuOH) were used as Zn and O precursors, respectively. The crystallinity of the grown films was studied by X-Ray Diffraction (XRD) and the thickness and morphology were investigated by Scanning Electron Microscopy (SEM). The influence of substrate orientation and molar ratio of the precursors on the crystalline orientation and morphology of the ZnO grown films was analysed.     

Effects of linear birefringence inside sensing head upon bulk glass current sensors’ sensitivity

The effects of the linear birefringence inside a bulk glass current sensing element and the incident polarizing angle upon the performance of a bulk glass optical current sensor are derived and analyzed theoretically. The investigation results show that the linear birefringence will modify the scale factor of the system with a sample function; it can also affect the extent of the influence of the incident polarizing angle, at the same time. When the incident polarizing angle has some special values such as 0, 45, or 90, its effect on the system will be zero. These results might provide some useful reference to the researchers and designers of bulk glass optical current sensors.     

Atomic layer deposition and characterization of Ga-doped ZnO thin films

Low-resistivity n-type ZnO thin films were grown by atomic layer deposition (ALD) using diethylzinc (DEZ) and H₂O as Zn and O precursors. ZnO thin films were grown on c-plane sapphire (c- Al₂O₃) substrates at 300 C. For undoped ZnO thin films, it was found that the intensity of ZnO () reflection peak increased and the electron concentration increased from 6.8×10¹⁸ to 1.1×10²⁰ cm⁻³ with the increase of DEZ flow rate, which indicates the increase of O vacancies () and/or Zn interstitials (Zn_i). Ga-doping was performed under Zn-rich growth conditions using triethylgallium (TEG) as Ga precursor. The resistivity of 8.0×10⁻⁴ Ω cm was achieved at the TEG flow rate of 0.24 μmol/min.     

Thermal annealing of ZnO substrates

Zinc oxide crystals were grown by Chemical Vapor Transport using Contactless Crystal Growth technique. After X-ray examination the 8 off-axis oriented slices were polished using alumina powder followed by a mixture of oxides. The mechanical polishing was followed by chemo-mechanical polishing using colloidal silica in water and supplemented by thermal annealing in an air atmosphere. The range of temperature was between 770 and 1070 C, time of annealing ranged up to 160 h. The quality of surfaces was studied using atomic force microscopy. A wide spectrum of surface morphology was observed. The morphology was dependent on the annealing conditions and additionally on the quality of chemo-mechanical polishing and crystallographic orientation of the surfaces. It was found possible to obtain the lowest RMS surface roughness factor in extremely different annealing conditions. The best annealing procedure for surface improvement was investigated.     

Integration of GaN thin films to SiO2–Si(100) substrates by laser lift-off and wafer fusion method

A process methodology has been adopted to bond GaN thin films to Si(100) substrates using the combination of laser lift-off and direct wafer fusion. Using optimum excimer laser conditions, 2–10 μm of GaN is lifted-off from sapphire. The lifted-off thin film is cleared from gallium residual and then suitably treated in a hydrofluoric, nitric and acetic acid mixture to render the surface hydrophilic. This treatment provides van der Waals bonds to immediately contact bond with SiO₂–Si(100) substrate at room temperature. The bonds are further strengthened by a high temperature annealing at 650 C for 2 h. The structural and mechanical characteristics of the bonded structure reveal uniform and high quality bonding. The optical characteristics of the transferred bonded film on SiO₂–Si(100) substrate exhibit similar properties to that of GaN on sapphire. In a similar manner, high-brightness blue LEDs were transferred from sapphire to SiO₂–Si(100) substrate with no deterioration in the electrical and optical performance of the device.     

ZnO films grown by laser ablation with and without oxygen CVD

In the present work we have studied the properties of zinc oxide (ZnO) thin films grown by laser ablation of ZnO targets under different substrate temperature and background oxygen conditions. The ZnO layers were deposited with a Pulsed Laser Deposition (PLD) system on pre-nitrided (0001) sapphire (Al₂O₃), using the base line of a Nd:YAG laser at 1064 nm. The films were characterized by different structural and optical methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), optical transmission spectroscopy, and steady-state photoluminescence (PL). XRD analysis with rocking curves and θ–2θ scans indicates preferential growth along the c-axis direction with a full width at half maximum (FWHM) smaller than 1.5. Low-temperature photoluminescence (PL) showed strong excitonic emission near 3.36 eV between 9 and 65 K.     

Radiation source dependence of device performance degradation for 4H-SiC MESFETs

The impact of radiation source dependence on the device performance degradation of 4H-SiC MESFETs (Metal Schottky Field Effect Transistors), which have been irradiated at room temperature with 2 MeV electrons and 20 MeV protons, is studied. No performance degradation is observed by 1×10¹⁵ e/cm² and 5×10¹¹ p/cm², while a slight increase of the linear drain current together with a decrease of the threshold voltage are noticed above higher fluence. The damage coefficient for protons is about three orders of magnitude larger than that for electrons. The radiation source dependence of the device performance degradation is attributed to the difference of mass and the possibility of nuclear collision for the formation of lattice defects. Based on thermal annealing results of electron-irradiated MESFETs, it is found that the recovery of the drain current characteristics principally takes place from 100 C, and that the drain current recovers to the pre-rad value after 200 C annealing. On the other hand, capacitance and induced deep levels do not recover by 200 C annealing. It is concluded that the degradation of the drain current is mainly sensitive to the radiation-induced decrease of the Schottky barrier height at the gate contact.     

Al Schottky contact on p-GaSe

A new Schottky diode, Al/p-GaSe, was presented in this study. It shows an effective barrier height of 0.96 eV with an ideality factor of 1.24 over five decades and a reverse leakage current density of 4.12×10⁻⁷ A/cm² at −2 V after rapid thermal annealing at 400 C for 30 s. The generation–recombination effect of the Schottky diode was decreased as the annealing temperature was increased. The formation of Al_1.33Se₂ was observed by X-ray diffraction analysis after the diode was annealed at 400 C for 30 s. Owing to the grains’ growth, the surface morphology of the 400 C-annealed diode was rougher than that of the unannealed diode, which was observed both by the AFM and the SEM analysis.     

Vapour phase transport growth of ZnO layers and nanostructures

We have developed a novel advanced VPT set-up. ZnO layers and nanorods were grown employing a specially designed horizontal vapour transport system with elemental sources at relatively low temperatures without catalysis. We employed 6N elemental Zn carried by nitrogen and 99.995% oxygen as reactants. Sapphire, SiC, bulk ZnO and ZnO epitaxial layers were implemented as substrates for ZnO growth. Growth temperatures ranged from 500 to 900 C. Reactor pressures were from 5 mbar to atmospheric pressure. We employed x-ray diffractometry, optical microscopy, scanning electron microscopy and atomic force microscopy to investigate the obtained ZnO samples and the influence of different growth parameters on the ZnO homo- and heteroepitaxial growth and to optimise the set of growth parameters either for both epitaxial layers and nanostructures. We also show that the quality of the VPT grown ZnO epitaxial layers on sapphire can be even higher (evaluated from FWHM of the XRD rocking curves) than the MBE grown ones used as epiwafers for VPT growth. High quality ZnO layers with extremely narrow FWHM of the (0002) rocking curve of 38″ are fabricated employing our VPT approach.