High intense UV-luminescence of nanocrystalline ZnO thin films prepared by thermal oxidation of ZnS thin films

High quality zinc oxide (ZnO) films were obtained by thermal oxidation of high quality ZnS films. The ZnS films were deposited on a Si substrate by a low-pressure metalorganic chemical vapor deposition technique. X-ray diffraction spectra indicate that high quality ZnO films possessing a polycrystalline hexagonal wurtzite structure with preferred orientation of (0 0 2) were obtained. A fourth order LO Raman scattering was observed in the films. In photoluminescence (PL) measurements, a strong PL with a full-width at half-maximum of 10 nm around 380 nm was obtained for the samples annealed at 900°C at room temperature. The maximum PL intensity ratio of the UV emission to the deep-level emission is 28 at room temperature, providing evidence of the high quality of the nanocrystalline ZnO films.     

Hexagonal AlN films grown on nominal and off-axis Si(0 0 1) substrates

Nucleation and growth of wurtzite AlN layers on nominal and off-axis Si(0 0 1) substrates by plasma-assisted molecular beam epitaxy is reported. The nucleation and the growth dynamics have been studied in situ by reflection high-energy electron diffraction. For the films grown on the nominal Si(0 0 1) surface, cross-sectional transmission electron microscopy and X-ray diffraction investigations revealed a two-domain film structure (AlN¹ and AlN²) with an epitaxial orientation relationship of [0 0 0 1]_AlN || [0 0 1]_Si and AlN¹ || AlN² || [1 1 0]_Si. The epitaxial growth of single crystalline wurtzite AlN thin films has been achieved on off-axis Si(0 0 1) substrates with an epitaxial orientation relationship of [0 0 0 1]_AlN parallel to the surface normal and 0 1 1 0_AlN || [1 1 0]_Si.     

Determination of the electrical properties of 2.5 nm thick silicon-based dielectric films: thermally grown SiOx

Ultra-thin (<5 nm thick) thermal oxide and oxynitride films with different compositions are candidates for complementary metal/oxide/semiconductor technology in ultra-large-scale integration (ULSI) applications. The latter are expected to offer the best compromise between nitrides and oxides. The aim of this work is to measure the electrical properties of a leaky 2.5 nm thick thermally grown oxide film using the high frequency capacitance–voltage (HF C(V)) measurements. The cleanliness and the surface roughness of the Si(1 0 0) surface were measured prior to in situ oxidation by means of, respectively, Auger electron spectroscopy (AES) and atomic force microscopy (AFM). The physical–chemical properties of the thermal oxide film were measured by AES (film thickness, composition), Fourier transform infrared spectroscopy (FTIR) (composition, vibration modes), cross-sectional transmission electron microscopy (TEM) (film thickness, homogeneity) and electron energy loss spectroscopy (EELS) (gap width determination). The results are compared to those obtained for the native oxide film and a chemical oxide film. The latter was first grown on the silicon substrate to prevent contamination and surface disorder after flash heating in vacuum prior to oxide growth. The substrate Si(1 0 0) surface cleaned in ultra-high vacuum (UHV) was then oxidized in a 10⁻³ mbar oxygen (O₂) gas pressure at 900°C to get the 2.5 nm thick oxide film. The grafting of a self-assembled insulating monolayer (SAM) of organic molecules on the grown oxide film permits us to obtain analysable capacitance as a function of voltage data. Indeed, this monolayer made up of octadecyltrichlorosilane molecules leads to a reduction of the leakage current through the aluminium/self-assembled monolayer/oxide/silicon heterostructure. The resulting current as a function of voltage data were compared to those of a standard 2.5 nm thick oxide film. The method proposed here to extract the electrical parameters of the thermal oxide film is demonstrated to be valid. We show mainly that the reduction of the leakage current through the aluminium/self-assembled monolayer/thermal oxide/silicon heterostructure is seven orders of magnitude bigger than in the case of the native oxide film.     

Enhanced thermal stability of NiSi films on Si(1 1 1) substrates by a thin Pt interlayer

A thin interlayer of Pt can greatly enhance the thermal stability of NiSi films formed by rapid thermal annealing (RTA) on Si(1 1 1) substrates, as was revealed by X-ray diffraction (XRD) data and sheet resistance measurement. High-resolution transmission electron microscopy (HRTEM) reveals a well-defined interface between the Ni(Pt)Si film and the Si(1 1 1) substrate for the Ni/Pt/Si sample annealed at 640°C. The orientation relationship in this sample determined by selected area electron diffraction (SAED) was NiSi(1 0 0)||Si(1 1 1) and NiSi[0   0]||Si[0 1  ]. With the increase of temperature, the texture of NiSi films transform from NiSi(1 0 0)||Si(1 1 1) to NiSi(0 0 1)||Si(1 1 1). The reduction in the interfacial energy due to the formation of the (1 0 0) textured NiSi film is proposed as a possible reason for the improved thermal stability of NiSi and the transition in NiSi texture during high-temperature annealing. Detailed study on the XRD data combined with Auger electron spectra (AES) indicates PtSi and NiSi form a solid solution following Vegard's law, which adjusts the lattice constant ratio c/b to and may account for the texture of NiSi(1 0 0)||Si(1 1 1).     

Epitaxial variations of Ni films grown on MgO(0 0 1)

Epitaxial Ni films were deposited on (0 0 1)MgO by DC magnetron sputtering under ultra-high vacuum conditions for studies involving magnetic-multilayer applications. The deposition temperatures of the Ni films studied in this work were 100 and 400°C. Examination by transmission electron microscopy (TEM) and electron diffraction revealed that the film deposited at the lower temperature was predominately Ni[0 0 1]MgO[0 0 1] and Ni(0 1 0)MgO(0 1 0) oriented and smooth, as expected. However, the higher temperature films were predominately of the Ni MgO[0 0 1] and Ni MgO(1 0 0) orientation and facetted. The orientation has been confirmed by X-ray diffraction, where this orientation was observed to be four-fold degenerate. For each of these four orientations there also existed a twin orientation, reflected about the MgO(1 0 0) planes, giving eight possible orientations for the Ni crystallites on MgO. This epitaxial relationship was studied by dark-field TEM and electron diffraction. Because these films were polycrystalline and hence produced many diffraction spots from both the Ni and MgO with similar lattice spacings, electron diffraction patterns of the films were indexed using an electron diffraction image processing (EDIP) technique. In this technique, the polycrystalline electron diffraction pattern was converted into a graph, with the x-axis displaying lattice spacings and the y-axis, integrated intensity.     

Epitaxial growth of ZnO thin films on Si substrates by PLD technique

Epitaxial ZnO thin films have been grown on Si(1 1 1) substrates at temperatures between 550 and 700 °C with an oxygen pressure of 60 Pa by pulsed laser deposition (PLD). A ZnO thin film deposited at 500 °C in no-oxygen ambient was used as a buffer layer for the ZnO growth. In situ reflection high-energy electron diffraction (RHEED) observations show that ZnO thin films directly deposited on Si are of a polycrystalline structure, and the crystallinity is deteriorated with an increase of substrate temperature as reflected by the evolution of RHEED patterns from the mixture of spots and rings to single rings. In contrast, the ZnO films grown on a homo-buffer layer exhibit aligned spotty patterns indicating an epitaxial growth. Among the ZnO thin films with a buffer layer, the film grown at 650 °C shows the best structural quality and the strongest ultraviolet (UV) emission with a full-width at half-maximum (FWHM) of 86 meV. It is found that the ZnO film with a buffer layer has better crystallinity than the film without the buffer layer at the same substrate temperature, while the film without the buffer layer shows a more intense UV emission. Possible reasons and preventive methods are suggested to obtain highly optical quality films.     

Growth of AlN films on SiC substrates by RF-MBE and RF-MEE

Epitaxial AlN films have been grown on SiC substrate by molecular beam epitaxy (MBE) and migration-enhanced epitaxy (MEE) using radio frequency (RF) plasma-excited nitrogen. In the RF-MBE growth, the growth rates have been found to be almost constant and the crystal quality improved with increasing the substrate temperature up to 850°C. Further increases of substrate temperature decreased the growth rate and degraded the crystal quality. Using the optimum substrate temperature of 850°C and optimizing the shutter open time, smooth AlN films with atomic force microscope roughness as low as 0.2 nm have been grown by RF-MEE growth.     

Structure and optical characterization of photochemically prepared ZrO2 thin films doped with erbium and europium

Zirconium oxide thin films loaded with 10, 30 and 50 mol% lanthanide ions (Er or Eu) have been successfully prepared by direct UV (254 nm) irradiation of amorphous films of β-diketonate complexes on Si(1 0 0) substrates, followed by a post annealing treatment process. The resultant films were characterized by X-ray photoelectron spectroscopy and Atomic Force Microscopy. The results showed that the stoichiometry of the resulting films were in relative agreement with the composition of the precursor films. The effects of annealing as well as the lanthanide ion loading on the photoluminescence (PL) emission intensity were investigated, finding that thermal treatment decreases surface roughness as well as PL emission intensity.     

Epitaxial growth of MgO films on Si(1 1 1) by metal organic chemical vapor deposition

Epitaxial MgO films were grown on Si(1 1 1) substrates at 800°C using methylmagnesium tert-butoxide (MeMgO^tBu) as a single precursor under high-vacuum conditions (5×10⁻⁶ Torr). The crystalline structure, morphology, and chemical composition of the deposited films were investigated by X-ray diffraction, X-ray pole figure analysis, scanning electron microscopy, and X-ray photoelectron spectroscopy. The results show that epitaxial MgO films with correct stoichiometry can be deposited on Si(1 1 1) at 800°C. The single precursor methylmagnesium tert-butoxide has been found suitable for the epitaxial growth of MgO on Si(1 1 1) substrates.     

Effect of the N/Al ratio of AlN buffer on the crystal properties and stress state of GaN film grown on Si(1 1 1) substrate

The effect of the N/Al ratio of AlN buffers on the optical and crystal quality of GaN films, grown by metalorganic chemical vapor deposition on Si(1 1 1) substrates, has been investigated. By optimizing the N/Al ratio during the AlN buffer, the threading dislocation density and the tensile stress have been decreased. High-resolution X-ray diffraction exhibited a (0 0 0 2) full-width at half-maximum as low as 396 acrsec. The variations of the tensile stress existing in the GaN films were approved by the redshifts of the donor bound exiton peaks in the low-temperature photoluminescence measurement at 77 K.     

Solvothermal growth of ZnO

The growth of ZnO single crystals and crystalline films by solvothermal techniques is reviewed. Largest ZnO crystals of 3 inch in diameter are grown by a high-pressure medium-temperature hydrothermal process employing alkaline-metal mineralizer for solubility enhancement. Structural, thermal, optical and electrical properties, impurities and annealing effects as well as machining are discussed. Poly- and single-crystalline ZnO films are fabricated from aqueous and non-aqueous solutions on a variety of substrates like glass, (100) silicon, -Al₂O₃, Mg₂AlO₄, ScAlMgO₄, ZnO and even some plastics at temperatures as low as 50 °C and ambient air conditions. Film thickness from a few nanometers up to some tens of micrometers is achieved. Lateral epitaxial overgrowth of thick ZnO films on Mg₂AlO₄ from aqueous solution at 90 °C was recently developed. The best crystallinity with a full-width half-maximum from the (0002) reflection of 26 arcsec has been obtained by liquid phase epitaxy employing alkaline-metal chlorides as solvent. Doping behavior (Cu, Ga, In, Ge) and the formation of solid solutions with MgO and CdO are reported. Photoluminescence and radioluminescence are discussed.     

High-quality ZnO thin films prepared by two-step thermal oxidation of the metallic Zn

In this paper, we report the preparation of nanocrystalline ZnO thin films on Si (1 0 0) substrates using a simple method, in which a resistive thermal evaporation of Zn and a two-step annealing process were employed. The aim of the first annealing step in an oxygen ambient at 300°C for 2 h is to form ZnO layers on the surface of the Zn films to prevent the diffusion of the metallic Zn from the films during the high-temperature annealing process. To obtain high-quality ZnO films, a high-temperature annealing step was performed at temperature in the range of 600–900°C. The effects of the annealing temperature on the photoluminescence (PL) and orientation of ZnO nanocrystalline thin films were studied. A very strong near-band-edge emission around 375 nm with a full-width at half-maximum of 105 meV and a relatively weak emission around 510 nm related to deep-level defects were observed, which indicated that high-quality ZnO films have been obtained.     

Low-temperature synthesis of C60 thin films by ionized cluster beam deposition technique

C₆₀ films on Si(1 1 1) have been grown at low substrate temperature of 100°C by ionized cluster beam deposition technique. Fourier transform infrared (FTIR) transmission spectroscopy, Raman measurements and X-ray diffraction are employed to investigate the structure of deposited films. The results show that the acceleration voltage plays an important role in the growth of the films. As the acceleration voltage is moderate (100 V), a pristine C₆₀ film with face-centered-cubic (fcc) crystal structure can be grown. Further increase of the acceleration voltage leads to the formation of amorphous carbon (a-C) in the grown films. When the acceleration voltage is increased to 600 V, the deposited film has a complete amorphous carbon (a-C) structure.     

Structure and thermal stability of MOCVD ZrO2 films on Si (1 0 0)

The structure and thermal stability of ZrO₂ films grown on Si (1 0 0) substrates by metalorganic chemical vapor deposition have been studied by high-resolution transmission electron microscopy, selected area electron diffraction and X-ray energy dispersive spectroscopy. As-deposited films consist of tetragonal ZrO₂ nanocrystallites and an amorphous Zr silicate interfacial layer. After annealing at 850°C, some monoclinic phase is formed, and the grain size is increased. Annealing a 6 nm thick film at 850°C in O₂ revealed that the growth of the interfacial layer is at the expense of the ZrO₂ layer. In a 3.0 nm thick Zr silicate interfacial layer, there is a 0.9 nm Zr-free SiO₂ region right above the Si substrate. These observations suggest that oxygen reacted with the Si substrate to grow SiO₂, and SiO₂ reacted with ZrO₂ to form a Zr silicate interfacial layer during the deposition and annealing. Oxygen diffusion through the tetragonal ZrO₂ phase was found to be relatively easier than through the monoclinic phase.     

HREM investigations of intermediate ZnO and ZnO/Y–ZrO2 layers in Y–ZrO2 bicrystals and ZnO films grown on Y–ZrO2 bicrystal substrates

The structure of yttrium-stabilized ZrO₂ (YSZ) bicrystals with ZnO and ZnO/YSZ/ZnO/YSZ/ZnO intermediate layers, as well as ZnO films grown on YSZ bicrystal (1 1 0)/90° substrates, has been investigated by means of high-resolution electron microscopy (HREM) and microanalysis. All bicrystals were produced by the solid-phase intergrowth (SPI) method. The internal ZnO film in the bicrystal formed at the SPI temperature of 1400°C consisted of domains with two symmetrical orientations: , and , . A bicrystal with a ZnO/YSZ/ZnO/YSZ/ZnO internal film was formed at the temperature of 1200°C. There was no mixing of ZnO and YSZ films and no traces of any solid-phase reactions were observed. Grains in all internal ZnO films and ZnO films grown on the bicrystal substrates had numerous stacking faults. It was found that SPI does not influence the density and structure of these defects. Orientational relationships between YSZ and ZnO in all samples were determined. The ZnO films grown on (1 1 0)/90° bicrystal substrates inherited the grain boundary (GB) from the substrate. Its structure and geometry is determined by four variants of ZnO grain growth.     

Effect of gallium incorporation on the physical properties of ZnO films grown by spray pyrolysis

Gallium-doped zinc oxide thin films were deposited by the spray pyrolysis technique onto Corning 7059 glass substrates at a temperature of 350°C using a precursor solution of zinc acetate in isopropyl alcohol. The films were prepared using different gallium concentrations keeping the other deposition parameters such as air and solution flow rates and solution concentration constant. The variations of the structural, electrical and optical properties with the doping concentration were investigated. X-ray diffraction data showed that the films were polycrystalline with the (0 0 2) preferred orientation. The texture coefficient and grain size were evaluated for different doping concentrations. The films with 5.0 at% gallium had a resistivity of 1.5×10⁻³ Ω cm and a transmittance of 85% with an energy band gap of 3.35 eV.     

Synthesis and characterization of type-II textured tungsten disulfide thin films by vdWR process with Pb interfacial layer as texture promoter

The growth of type-II textured tungsten disulfide (WS₂) thin films by solid state reaction between the spray deposited WO₃ and gaseous sulfur vapors with Pb interfacial layer has been studied. X-ray diffraction (XRD) technique is used to measure the degree of preferred orientation ‘S’ and texture of WS₂ films. Scanning electron microscopy (SEM) and transmission electron microscopy techniques have been used to examine the microstructure and morphology. The electronic structure and chemical composition were studied using X-ray photoelectron spectroscopy (XPS). The use of Pb interfacial layer for the promotion of type-II texture in WS₂ thin films is successfully demonstrated. The presence of (0 0 3 l), (where l=1, 2, 3, …) family of planes in the XRD pattern indicates the strong type-II texture of WS₂ thin films. The crystallites exhibit rhombohedral (3R) structure. The large value of ‘S’ (1086) prompts the high degree of preferred orientation as well. The stratum of crystallites with their basal plane parallel to the substrate surface is seen in the SEM image. The EDS and XPS analyses confirm the tungsten to sulfur atomic ratio as 1:1.75. We purport that Pb interfacial layer enhances type-II texture of WS₂ thin films greatly.     

Surface and interfacial structure of epitaxial SrTiO3 thin films on (0 0 1) Si grown by molecular beam epitaxy

Effects of relaxation of interfacial misfit strain and non-stoichiometry on surface morphology and surface and interfacial structures of epitaxial SrTiO₃ (STO) thin films on (0 0 1) Si during initial growth by molecular beam epitaxy (MBE) were investigated. In situ reflection high-energy electron diffraction (RHEED) in combination with X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) techniques were employed. Relaxation of the interfacial misfit strain between STO and Si as measured by in situ RHEED indicates initial growth is not pseudomorphic, and the interfacial misfit strain is relaxed during and immediately after the first monolayer (ML) deposition. The interfacial strain up to 15 ML results from thermal mismatch strain rather than lattice mismatch strain. Stoichiometry of STO affects not only surface morphology but interfacial structure. We have identified a nanoscale Sr₄Ti₃O₁₀ second phase at the STO/Si interface in a Sr-rich film.     

High-density silicon and silicon nitride cones

High-density cone-shaped silicon and silicon nitride have been synthesized on Si(1 0 0) substrates via plasma-assisted hot-filament chemical vapor deposition using a gas mixture of nitrogen, hydrogen and methane. Aligned silicon cones containing 3–10 at% C and N have been formed with less than 1 h growth. Further growth can lead to the increase of cone size and density, as well as to the formation of polycrystalline silicon nitride films on the tip and surface. The formation of these materials is thought to be due to the remodification of Si substrates under the effect of plasma and active C and N species. Different nucleation and growth styles were obtained under different growth conditions and reactive gas mixtures.     

Homoepitaxial growth of 6H–SiC thin films by metal-organic chemical vapor deposition using bis-trimethylsilylmethane precursor

Homoepitaxial silicon carbide (SiC) films were grown on 3.5° off-oriented (0 0 0 1) 6H–SiC by metal-organic chemical vapor deposition (MOCVD) using bis-trimethylsilylmethane (BTMSM, C₇H₂₀Si₂). A pronounced effect of the growth conditions such as source flow rate and growth temperature on the polytype formation and structural imperfection of the epilayer was observed. The growth behavior was explained by a step controlled epitaxy model. It was demonstrated by high-resolution X-ray diffractometry and transmission electron microscopy that high-quality 6H–SiC thin films were successfully grown at the optimized growth condition of substrate temperature 1440°C with the carrier gas flow rate of 10 sccm.