Influence of crystallinity of as-deposited Ge film on formation of quantum dot in carbon-mediated solid-phase epitaxy

The influence of crystallinity of as-deposited Ge films on Ge quantum dot (QD) formation via carbon (C)-mediated solid-phase epitaxy (SPE) was investigated. The samples were fabricated by solid-source molecular beam epitaxy (MBE). Ge/C/Si structure was formed by sequential deposition of C and Ge at deposition temperature (T_D) of 150–400 °C, and it was heat-treated in the MBE chamber at 650 °C. In the case of amorphous or a mixture of amorphous and nano-crystalline Ge film grown for T_D ≤250 °C, density of QDs increased with increasing T_D due to the increase of C-Ge bonds in Ge layer. Ge QDs with diameter of 9.2±2.1 nm were formed in the highest density of 8.3×10¹¹ cm⁻² for T_D =250 °C. On the contrary, in the case of polycrystalline Ge film for T_D ≥300 °C, density of QDs decreased slightly. This is because C incorporation into Ge layer during SPE was suppressed due to the as-crystallized columnar grains. These results suggest that as-deposited Ge film in a mixture of amorphous and nano-crystalline state is suitable to form small and dense Ge QDs via C-mediated SPE.     

Influence of annealing temperature on the structural and optical properties of nanocrystalline zirconium oxide

Nanocrystalline zirconium oxide powder was prepared by sol-gel method using zirconyl chloride octahydrate (ZrOCl₂·8H₂O) and ethylenediaminetetraacitic acid (EDTA) in ammonium hydroxide (NH₄OH) solution. The as-synthesized complex product was annealed at 650 °C, 750 °C and 850 °C for 2 h to get fine ZrO₂ powder. These samples were further analyzed by Scanning electron microscopy (SEM), X- ray diffraction (XRD), Energy-dispersive X- ray spectroscopy (EDX), UV-vis analysis, Fourier transform infrared (FT-IR) spectroscopy, Photoluminescence spectroscopy (PL) and Raman Spectroscopy to study their structural and optical properties. The structural studies revealed that nanocrystalline ZrO₂ powder exhibits monoclinic phase with variation in crystallite size with annealing temperature. The UV–vis absorption band edge of ZrO₂ decreases from 514 nm to 451 nm as annealing temperature rises from 650 °C to 750 °C. It seems that the drastic reduction in band gap energy may be one of the novel unexpected characteristics of ZrO₂. The FTIR analyses further confirmed the formation of nanocrystalline monoclinic ZrO₂. PL analysis revealed the novel emission peaks at 305 and 565 nm. The Raman spectroscopy confirmed the transformation of amorphous zirconium hydroxide to m-ZrO₂ with increase in temperature from 650 °C to 850 °C.     

Structural and optoelectronic properties of nanostructured TiO2 thin films with annealing

About 480 nm thick titanium oxide (TiO₂) thin films have been deposited by electron beam evaporation followed by annealing in air at 300–600 °C with a step of 100 °C for a period of 2 h. Optical, electrical and structural properties are studied as a function of annealing temperature. All the films are crystalline (having tetragonal anatase structure) with small amount of amorphous phase. Crystallinity of the films improves with annealing at elevated temperatures. XRD and FESEM results suggest that the films are composed of nanoparticles of 25–35 nm. Raman analysis and optical measurements suggest quantum confinement effects since Raman peaks of the as-deposited films are blue-shifted as compared to those for bulk TiO₂ Optical band gap energy of the as-deposited TiO₂ film is 3.24 eV, which decreases to about 3.09 eV after annealing at 600 °C. Refractive index of the as-deposited TiO₂ film is 2.26, which increases to about 2.32 after annealing at 600 °C. However the films annealed at 500 °C present peculiar behavior as their band gap increases to the highest value of 3.27 eV whereas refractive index, RMS roughness and dc-resistance illustrate a drop as compared to all other films. Illumination to sunlight decreases the dc-resistance of the as-deposited and annealed films as compared to dark measurements possibly due to charge carrier enhancement by photon absorption.     

Effects of Cu/In ratio and annealing temperature on physical properties of dip-coated CuInS2 thin films

CuInS₂ thin films were prepared by sol–gel dip-coating method on glass substrates using 0.75, 1 and 1.25 ratios of Cu/In in the solution. The prepared films were annealed at 380 °C, 420 °C and 460 °C for 30 min under argon environment. The structural, optical, morphological and composition properties of those were investigated by X-ray diffraction (XRD), UV–vis transmittance spectroscopy and scanning electron microscopy with an energy dispersive X-ray spectrometer. The XRD results showed that the films exhibit polycrystalline tetragonal CuInS₂ phase with (112) orientation. According to the EDX results the Cu/In ratios of the films were respectively 0.65, 0.92 and 1.35 for the Cu/In ratios of 0.75, 1 and 1.25 in the solutions. The optical band gap was found to be between 1.30 eV and 1.43 eV, depending on Cu/In ratio.     

High-performance barrier using a dual-layer inorganic/organic hybrid thin-film encapsulation for organic light-emitting diodes

We report an inorganic/organic hybrid barrier that combines the alternating deposition of a layer of ZrO₂ using low temperature atomic layer deposition and a 16-μm-thick layer of UV-curable NOA63 epoxy using spin-coating. The effective water vapor transmission rates of single ZrO₂ film was improved by adding solution epoxy from 3.03 × 10⁻³ g/m² day to 1.27 × 10⁻⁴ g/m² day in the hybrid NOA63/ZrO₂/NOA63/ZrO₂ films at 20 °C and a relative humidity of 60%. In consequence, the organic light-emitting diodes encapsulated with inorganic/organic hybrid barriers were undamaged by environmental oxygen and moisture and their luminance decay time improved by a considerable extent.     

Low-temperature formation of self-assembled Ge quantum dots on Si(100) under high carbon mediation via solid-phase epitaxy

CMOS-compatible low-temperature formation of self-assembled Ge quantum dots (QDs) by carbon (C) mediation via a solid-phase epitaxy (SPE) has been demonstrated. The samples were prepared by a solid-source molecular beam epitaxy (MBE) system. C and Ge were successively deposited on Si(100) at 200 °C and Ge/C/Si heterostructure was annealed in the MBE chamber. Sparse Volmer-Weber mode Ge dots without a wetting layer were formed for C coverage (θ_C) of 0.25 and 0.5 ML by lowering SPE temperature (T_S) to 450 °C, but small and dense Stranski-Krastanov (SK)-mode Ge QDs with the wetting layer were obtained with increasing C coverage of 0.75 ML even at 450 °C. From the investigation of SPE temperature effect on Ge QD formation for θ_C of 0.75 ML, SK-mode Ge QDs of about 10 nm in diameter and of about 4.5×10¹¹ cm⁻² in density were formed at T_S≥400 °C. The wetting layer of SK-mode QDs was almost constant 0.2-nm thick at T_S≥450 °C. Measurements of chemical binding states of C in Ge QDs and at Ge/Si interface revealed that a large amount of C–Ge bonds were formed in the wetting layer for high C coverage, and the formation of C–Ge bonds, together with the formation of C–Si bonds, enabled the low-temperature formation of small and dense Ge QDs. These results suggest that the C-mediated solid-phase epitaxy is effective to form small and dense SK-mode QDs at low temperature.     

Epitaxial growth of Ge thin film on Si (001) by DC magnetron sputtering

We have demonstrated that sub-10 nm-thick heteroepitaxial Ge films on Si (001) having smooth surfaces can be obtained by DC magnetron sputtering. Ge films grown at 350 °C preserve the smooth surfaces with a roughness root mean square (RMS) of 0.39 nm, whereas, the Ge films grown at 500 °C show significant roughness with an island-like morphology. In samples grown at 350 °C, it is confirmed that the Ge films are grown epitaxially by cross-section transmission electron microscopy (TEM) and X-ray diffraction (XRD) rocking curve measurements. Rapid thermal annealing (RTA) at 720 °C is effective in improving the crystalline quality and the degradation in the roughness is negligible. Raman spectra and an XRD reciprocal space map reveal that the epitaxial Ge grown at 350 °C show an in-plane compressive strain and that the strain continues to remain after a 720 °C RTA.     

Influence of annealing temperature on the phase composition and optical properties of CuInS2 films

We studied the growth of CuInS₂ thin films by single-source evaporation of CuInS₂ powder in a high-vacuum system with a base pressure of 10^?3 Pa. After evaporation, the films were annealed in a sulfur atmosphere at temperatures from 200 to 500 °C for 1 h. XRD curves and Raman spectra of the films demonstrated that chalcopyrite CuInS₂ was the major crystalline phase. The morphology of Cu_xS exhibited a star-like structure, which we report for the first time. The phase composition and optical properties of our polycrystalline thin films were effectively modified by annealing in S. For films annealed at 200 and 350 °C, a secondary CuIn₁₁S₁₇ phase appeared, which may be related to solid-state reaction in the S atmosphere. This secondary CuIn₁₁S₁₇ phase has not been widely reported in previous studies. After annealing at 500 °C, only a chalcopyrite phase was detected, with bandgap energy of 1.46 eV, which is nearly identical to the optimal bandgap energy (1.5 eV) of single-crystal CuInS₂. This indicates that the composition of the CuInS₂ film annealed at 500 °C was nearly stoichiometric. The bandgap of the samples first increased and then decreased with increasing annealing temperature, which may be attributed to an increase in grain size, the secondary CuIn₁₁S₁₇ phase, and deviation from stoichiometry.     

Effect of Bi doping on structural,morphological, optical and ethanol vapor response properties of SnO2 nanoparticles

The gas sensing behavior of thick films of Bi doped SnO₂ has been investigated towards ethanol vapor. The screen printing technique was used to prepare the thick films. The films were sintered at 650 °C for 2 h. The structural, surface morphological, optical and gas sensing properties of undoped and Bi doped SnO₂ thick films have been studied. X-ray diffraction and Raman spectroscopy confirmed that the films consisted exclusively of tetragonal tin oxide, without any impurity phases. FE-SEM studies revealed the formation of highly porous microstructure with grain size in few tens of nanometers. From the optical studies, the band gap was found to be decreased with bismuth doping (3.96 eV for undoped, 3.83 eV, 3.71 eV and 3.6 eV for 1 mol%, 2 mol% and 3 mol% Bi, respectively). The 3 mol % Bi doped SnO₂ thick films exhibited the highest sensitivity to 100 ppm of ethanol vapor at 300 °C. The effect of microstructure on sensitivity, response time and recovery time of the sensor was studied and discussed.     

Crystallization properties of Sb-rich GeSbTe alloys by in-situ morphological and electrical analysis

Phase Change Memory (PCM) operation relies on the reversible transition between two stable states (amorphous and crystalline) of a chalcogenide material, mainly of composition Ge₂Sb₂Te₅ (GST). In Wall type PCM cells, cycling endurance induces a gradual change of the cell electrical parameters caused by variations in the chemical composition of the active volume. The region closer to the GST-heater contact area, becomes more Sb rich and Ge depleted. The new alloy has usually different thermal characteristics for the phase transitions that influence the electrical behavior of the cell. In this study we analyze the morphological, structural and electrical properties of two Sb-rich non-stoichiometric alloys: Ge₁₄Sb₃₅Te₅₁ and Ge₁₄Sb₄₉Te₃₇, at their amorphous and crystalline phase. Experiments have been performed in non-patterned blanket films and, to simulate the device size, in amorphous regions of 20 nm, 50 nm and 100 nm diameter respectively. The amorphous Ge₁₄Sb₃₅Te₅₁ film crystallizes in the meta-stable face centered cubic structure at 150 °C and in the rhombohedral phase at 175 °C, behavior characteristic of the Ge₁Sb₂Te₄ composition. The average grain size is of about 100 nm after an annealing at 400 °C. The Ge₁₄Sb₄₉Te₃₇ film crystallizes only in the hexagonal phase, with an average grain size of about 60 nm after annealing at 400 °C. The X-ray fluorescence analysis shows a non uniform distribution of the constituent atoms and in particular a Ge signal decrement and a Sb enrichment at grain boundaries. The in situ annealing of amorphous nano-areas (RESET state under a thermal stress) indicates a fast re-crystallization speed for Ge₁₄Sb₃₅Te₅₁, 80 pm/s at 90 °C, and a lower speed for Ge₁₄Sb₄₉Te₃₇, at 130 °C a grain growth velocity of 50 pm/s has been measured. The different behavior of the two alloys is discussed in terms of structural vacancies filling by the Sb atoms in excess and by their segregation at grain boundaries. The influence of the obtained results on the device characteristics is discussed.     

Substrate temperature effect on structural,optical and electrical properties of vacuum evaporated SnO2 thin films

Tin oxide (SnO₂) thin films were deposited on glass substrates by thermal evaporation at different substrate temperatures. Increasing substrate temperature (T_s) from 250 to 450 °C reduced resistivity of SnO₂ thin films from 18×10⁻⁴ to 4×10⁻⁴▒ Ω ▒cm. Further increase of temperature up to 550 °C had no effect on the resistivity. For films prepared at 450 °C, high transparency (91.5%) over the visible wavelength region of spectrum was obtained. Refractive index and porosity of the layers were also calculated. A direct band gap at different substrate temperatures is in the range of 3.55−3.77 eV. X-ray diffraction (XRD) results suggested that all films were amorphous in structure at lower substrate temperatures, while crystalline SnO₂ films were obtained at higher temperatures. Scanning electron microscopy images showed that the grain size and crystallinity of films depend on the substrate temperature. SnO₂ films prepared at 550 °C have a very smooth surface with an RMS roughness of 0.38 nm.     

Thermal desorption of Ge native oxides and loss of Ge from the surface

The authors have identified oxidation and desorption processes of Ge native oxide by chemical bonding states measured by X-ray photoemission spectroscopy. Ge oxidation occurs at the temperatures of 450–500 °C in an oxidizing ambient. Ge desorption in nitrogen ambient is observed at the temperatures of 500–550 °C, which is higher than the oxidation temperature by 50 °C. Combined oxidation and desorption processes proceed subsequently and cause a loss of Ge from the surface when Ge is annealed in oxidizing ambient at a temperature higher than desorption temperature. The surface loss is avoided when Ge is annealed with SiO₂ cap layer in an identical annealing condition.     

Properties of In2O3 films obtained by thermal oxidation of sprayed In2S3

In₂S₃ thin films were grown by the chemical spray pyrolysis (CSP) method using indium chloride and thiourea as precursors at a molar ratio of S:In=2.5. The deposition was carried out at 350 °C on quartz substrates. The film thickness is about 1 µm. The films were then annealed for 2 h at 550, 600, 650 and 700 °C in oxygen flow. This process allows the transformation of nanocrystal In₂O₃ from In₂S₃ and the reaction is complete at 600 °C. X-ray diffraction spectra show that In₂O₃ films are polycrystalline with a cubic phase and preferentially oriented towards (222). The film grain size increases from 19 to 25 nm and RMS values increase from 9 to 30 nm. In₂O₃ films exhibit transparency over 70–85% in the visible and infrared regions due to the thickness and crystalline properties of the films. The optical band gap is found to vary in the range 3.87–3.95 eV for direct transitions. Hall effect measurements at room temperature show that resistivity is decreased from 117 to 27 Ω cm. A carrier concentration of 1×10¹⁶ cm^?3 and mobility of about 117 cm² V^?1 s^?1 are obtained at 700 °C.     

Photoluminescence of phosphorous doped Ge on Si (100)

Photoluminescence (PL) of selectively grown phosphorus (P) doped germanium (Ge) is investigated. 350–600 nm thick P-doped Ge is grown on 100 nm thick P-doped Ge buffer layer, which is annealed at 800 °C before the main part of Ge deposition. In the case of Ge deposited at 325 °C, approximately two times higher PL intensity is observed by P doping of ~3.2×10¹⁹ cm⁻³. Further increase of PL intensity by a factor of 1.5 is observed by increasing the growth temperature from 325 °C to 400 °C due to improved crystal quality. Varying PH₃ partial pressure at 400 °C, red shift of the PL occurred with increasing P concentration due to higher bandgap narrowing. With increasing P concentration up to ~1.4×10¹⁹ cm⁻³ at 400 °C the PL peak intensity increases by filling electrons into the L valley and decreases due to enhanced point defect concentration and degraded crystallinity. By post-annealing at 500–800 °C, the PL intensity is further increased by a factor of 2.5 because of increased active P concentration and improved crystal quality. Reduced direct bandgap energy by introducing tensile strain is also observed.     

Effect of annealing on structural,optical and electrical properties of pulse electrodeposited tin sulfide films

Polycrystalline tin sulfide (SnS) thin films were grown on conducting glass substrates by pulse electrodeposition. The effect of annealing on the physical properties such as structure, morphology, optical, and opto-electronic properties were evaluated to understand the effect of post-deposition treatment for SnS films. Annealing at temperatures higher than 250 °°C resulted in the formation of SnS₂ as a second phase, however, no significant grain growth or morphological changes were observed for films after annealing at 350 °C. A small change in band gap of 0.1 eV observed for films annealed at 350 °C was interpreted as due to the formation of SnS₂ rather than due to morphological changes. This interpretation was supported by X-ray diffractometry, scanning electron microscopy, and Raman spectral data. The electric conduction in the films is controlled by three shallow trap levels with activation energies 0.1, 0.05, and 0.03 eV. The trap with energy 0.03 eV disappeared after annealing at higher temperature, however, the other two traps were unaffected by annealing.     

AC conductivity,dielectric relaxation and modulus behavior of Sb2S2O new kermesite alloy for optoelectronic applications

In this work, we present some physical properties of Sb₂S₂O thin films obtained through heat treatment of Sb₂S₃ thin films under an atmospheric pressure at 350 °C. The electrical conductivity, dielectric properties and relaxation model of these thin films were studied using impedance spectroscopy technique in the frequency range from 5 Hz to 13 MHz at various temperatures from 350 °C to 425 °C. Besides, the frequency and temperature dependence of the complex impedance, AC conductivity and complex electric modulus has been investigated.     

Structural and electrical properties of ultra-thin high-k ZrO2 film on nitride passivated Ge(100) prepared by PEALD

The study of ultrathin ZrO₂ films grown on surface passivated germanium substrates by plasma enhanced Atomic Layer Deposition (PEALD) has been carried out. Nitride passivation has been used to form an interfacial layer between ZrO₂/Ge. The ultra-thin ZrO₂ film deposited with thickness of ~5.75 nm and refractive index of ~2.05 as observed through ellipsometry. The Ge3d, Zr3d, N1s and O1s are XPS core level spectra's confirm the formation of GeON and ZrO₂ ultra-thin films. The AFM results show the roughness of deposited films as low as 0.3 nm. The effect of post metallization annealing (PMA) on electrical properties of Au/Cr/ZrO₂/GeON/Ge capacitors has been investigated. The improvement in k value (~38) and an EOT value (~0.5 nm) after PMA on Ge/GeON/ZrO₂ stack has been observed. The flat band voltage and hysteresis of post metallization annealed devices has been reduced as compared to that of without PMA GeON/ZrO₂ stack.     

Antimony sulfide (Sb2S3) thin films by pulse electrodeposition: Effect of thermal treatment on structural,optical and electrical properties

Antimony sulfide films have been deposited by pulse electrodeposition on Fluorine doped SnO₂ coated glass substrates from aqueous solutions containing SbCl₃ and Na₂S₂O₃. The crystalline structure of the films was characterized by X-ray diffraction, Raman spectroscopy and TEM analysis. The deposited films were amorphous and upon annealing in nitrogen/sulfur atmosphere at 250 °C for 30 min, the films started to become crystalline with X-ray diffraction pattern matching that of stibnite, Sb₂S₃, (JCPDS 6-0474). AFM images revealed that Sb₂S₃ films have uniformly distributed grains on the surface and the grain agglomeration occurs with annealing. The optical band gap calculated from the transmittance and the reflectance studies were 2.2 and 1.65 eV for as deposited and 300 °C annealed films, respectively. The annealed films were photosensitive and exhibited photo-to-dark current ratio of two orders of magnitude at 1 kW/m² tungsten halogen radiation.     

Ultra-low-energy ion-beam-synthesis of Ge nanocrystals in thin ALD Al2O3 layers for memory applications

Structural and electrical properties of ALD-grown 5 and 7 nm-thick Al₂O₃ layers before and after implantation of Ge ions (1 keV, 0.5–1 × 10¹⁶ cm^?2) and thermal annealing at temperatures in the 700–1050 °C range are reported. Transmission Electron Microscopy reveals the development of a 1 nm-thick SiO₂-rich layer at the Al₂O₃/Si substrate interface as well as the formation of Ge nanocrystals with a mean diameter of ～5 nm inside the implanted Al₂O₃ layers after annealing at 800 °C for 20 min. Electrical measurements performed on metal–insulator–semiconductor capacitors using Ge-implanted and annealed Al₂O₃ layers reveal charge storage at low-electric fields mainly due to location of the Ge nanocrystals at a tunnelling distance from the substrate and their spatial dispersion inside the Al₂O₃ layers.     

Ge-rich SiGe thin film deposition by co-sputtering in in-situ and ex-situ solid phase crystallization for photovoltaic applications

This study investigates the properties of high Ge content silicon-germanium thin films in the non-hydrogenated state (Ge-rich SiGe) deposited on glass by RF magnetron co-sputtering in both in-situ and ex-situ solid phase crystallization (SPC) at various temperatures, such as RT to 550 °C. The structural and optical characteristics of SiGe films have been explored systematically by optimizing growth temperature. Atomic composition of films was determined by EDX, which showed up to 77 at% of Ge. Structural properties were characterized by XRD, which revealed all samples to be in amorphous nature. The results from Raman and UV–VIS–IR transmittance measurements showed that the properties of amorphous Si_0.23Ge_0.77 films improved at 450 °C in both in-situ and ex-situ SPC processes. In addition, EDX exposed an advantage of in-situ process over ex-situ due to the incorporation of oxygen during ex-situ thermal annealing. Possible deposition at low substrate temperature as found here suggests that these Si_0.23Ge_0.77 films have a substantial potential to be used in thin film Si-based solar cells.