Structural and optical properties of Zn(Mg,Cd)O alloy films grown by remote-plasma-enhanced MOCVD

We report the structural and optical properties of wurtzite-structure Zn(Mg,Cd)O ternary alloys. Wurtzite (0 0 0 1) Zn_1−xCd_xO and Mg_yZn_1−yO films were grown on (11–20) sapphire substrates using remote-plasma-enhanced metalorganic chemical vapor deposition. The large bowing parameters of Zn_1−xCd_xO and Mg_yZn_1−yO ternary alloys are 3.0 and 3.5, respectively, which reflects the large difference of each binary’s electronegativity. We have analyzed the broadening of photoluminescence (PL) in Zn(Mg,Cd)O alloys on alloy content by taking into account the statistical alloy fluctuation and the localization of the exciton, and have clarified that the localization of the exciton strongly affects to PL full-width at half-maximum (FWHM) in Zn(Mg,Cd)O alloys. The alloy broadenings in steady-state PL of Zn(Mg,Cd)O alloys are in good agreement with the calculated tendency by the theoretical model based on the statistical alloy fluctuation, while PL FWHM of Zn_1−xCd_xO is three times larger than the calculated results. Moreover, as another way to confirm alloy broadening, we also have done time-resolved PL measurements and derived the localized depth of the exciton in ZnO-based system, indicating a good agreement with the tendency of PL FWHM broadening.     

Morphological control of MgxZn1−xO layers grown on Ga:ZnO/glass substrates for photovoltaics

Mg_xZn_1?xO has been used in various photovoltaic cells because its energy bandgap can be tailored by controlling the Mg composition in this ternary compound. The Mg_xZn_1?xO layers with different surface morphologies including two-dimensional (2-D) films and one-dimensional (1-D) nanostructures are preferred for conventional p–n junction solar cells and polymer–inorganic hybrid solar cells, respectively. The Mg_xZn_1?xO layers are sequentially grown on Ga-doped ZnO (GZO) transparent conductive electrode using metalorganic chemical vapor deposition (MOCVD). The effect of the buffer layers on Mg_xZn_1?xO surface morphology is investigated. It is observed that Mg_xZn_1?xO deposited at ～500 °C on a low-temperature (～250 °C) ZnO buffer layer is in the form of 2-D dense and smooth films, whereas, on a high-temperature (～520 °C) ZnO buffer layer is in the form of 1-D nanostructures. Based on the structure characterization results, a growth mechanism in terms of nucleation and texturing is proposed to explain the buffer layer effect.     

Local structural,magnetic, and optical properties of Zn1−xFexO thin films

High quality Zn_1−xFe_xO thin films were deposited on α-sapphire$\alpha \text{-sapphire}$ substrates by RF magnetron sputtering. X-ray absorption fine structure measurements showed that the chemical valence of Fe ions in the films was a mixture of 2+ and 3+ states, and Fe ions substituted mainly for the Zn sites in the films. DC-magnetization measurements revealed ferromagnetic properties from 5 to 300 K. The photoluminescence measurements at 15 K showed a sharp main transition peak at 3.35 eV along with a broad impurity peak at 2.45 eV. The structural and magnetization analyses of the Zn_1−xFe_xO films strongly suggested that the ferromagnetism was the intrinsic properties of the films.     

Solar-blind 4.55 eV band gap Mg0.55Zn0.45O components fabricated using quasi-homo buffers

A route for synthesizing high Mg content single-phase wurtzite MgZnO films having band gaps in the solar-blind region is demonstrated by employing molecular beam epitaxy on Al₂O₃ substrates. Importantly, a low Mg content “quasi-homo” buffer, Mg_0.17Zn_0.83O, was applied to accommodate a host of structural discrepancies and therefore, avoiding phase separation in a high Mg content film, Mg_0.55Zn_0.45O, as proved by X-ray diffraction. The Mg fraction in the overgrown single-phase epilayer, Mg_0.55Zn_0.45O, was confirmed by Rutherford backscattering spectrometry.     

Structure and electrical properties of Mg‐doped ZnO nanoparticles

Mg_xZn_1‐xO (x=0.01‐0.3) nanoparticles were synthesized by the sol‐gel technique using solutions of Mg and Zn based organometalic compounds. The electrical properties of Mg doped zinc oxide (ZnO) were studied within wide temperature range from 300 to 500 K under the N₂ gas flow (flow rate: 20 sccm) and in the frequency range from 40 Hz to 1 MHz for ac electrical measurements. The dc conductivities and the activation energies were found to be in the range of 10^‐9‐10^‐6 S/cm at the room temperature and 0.26‐0.86 eV respectively depending on doping rate of these samples. The ac conductivity was well represented by the power law Aω^s. The conduction mechanism for all doped ZnO could be related to correlated barrier hopping (CBH) model. The complex impedance plots (Nyquist plot) showed the data points lying on a single semicircle, implying the response originated from a single capacitive element corresponding to the nanoparticle grains. The crystal structures of the Mg_xZn_1‐xO nanoparticles were characterized using X‐ray diffraction. The calculated average particle sizes values of Zn_1‐xMg_xO samples are found between 29.72 and 22.43 nm using the Sherrer equation. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and optical properties of Mg‐doped ZnO nanoparticles by polyacrylamide method

Mg‐doped ZnO (Mg_xZn_1‐xO) nanoparticles with precise stoichiometry are synthesized through polyacrylamide polymer method. Calcination of the polymer precursor at 650 °C gives particles of the homogeneous solid solution of the (Mg_xZn_1‐xO) system in the composition range (x < 0.15). ZnO doping with Mg causes shrinkage of lattice parameter c. The synthesized Mg_xZn_1‐xO nanoparticles are typically with the diameter of 70–85 nm. Blue shift of band gap with the Mg‐content is demonstrated, and photoluminescence (PL) from ZnO has been found to be tunable in a wide range from green to blue through Mg doping. The blue‐related PL therefore appeared to be caused by energetic shifts of the valence band and/or the conduction band of ZnO. Mg_xZn_1‐xO nanoparticles synthesized by polyacrylamide‐gel method after modified by polyethylene glycol surfactant have a remarkable improvement of stability in the ethanol solvent, indicating that these MZO nanoparticles could be considered as the candidate for the application of solution–processed technologies for optoelectronics at ambient temperature conditions.     

Influence of metal organic and inorganic precursors on spray pyrolyzed ceramic MgO (2 0 0) thin films for epitaxial over layers

The MgO (2 0 0) surface is widely used as a substrate for epitaxial growth of superconducting and ferro-electric films. Highly oriented, single crystalline, extremely flat and transparent MgO films have been successfully deposited on quartz substrates by the chemical spray pyrolysis technique using economically viable metal organic and inorganic precursors under optimized conditions at the substrate temperature of 600 °C. Thermal analysis (TGA/DTA) in the temperature range 30-600 °C with the heating rate of 10 °C/min revealed the decomposition behavior of the precursors and confirmed the suitable substrate temperature range for film processing. The heat of reaction, ΔH due to decomposition of metal organic precursor contributed additional heat energy to the substrate for better crystallization. The intensity of the (2 0 0) peak in X-ray diffraction (XRD) measurements and the smooth surface profiles revealed the dependency of precursor on film formation. The compositional purity and the metal-oxide bond formation were tested for all the films. UV-Vis-NIR optical absorption in the 200-1500 nm range revealed an optical transmittance above 80% and the absorption edge at about 238 nm corresponding to an optical band gap E_g = 5.25 eV. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) micrographs of MgO films confirmed better crystallinity with larger grain size (0.85 μm) and reduced surface roughness (26 nm), respectively.     

A study on structural formation and optical property of wide band-gap Be0.2Zn0.8O layers grown by RF magnetron co-sputtering system

Wide band-gap BeZnO layers were grown on Al₂O₃ (0 0 0 1) substrate using radio-frequency magnetron co-sputtering. The rate of Be_xZn_1−xO crystallized as a hexagonal structure was x=0.2. From the X-ray photoelectron spectroscopy measurement, the O–Zn bonds relating the crystal structure and the Be–O bonds related to the deviation of the stoichiometry in the BeZnO layer were caught at 530.4 and 531.7 eV in the O 1s spectrum, respectively. Thus, the observance on the Be 1s peak of 113.2 eV associated with the bonding Be–O indicates that the sputtered Be atoms are substituted for the host-lattice site in ZnO. This Be–O bonding shows a relatively low intense and broadening spectrum caused by large fluctuation of Be content in the BeZnO layer. From the photoluminescence and transmittance measurement, the free exciton and the neutral donor-bound exciton (D⁰, X) emissions were observed at 3.7692 and 3.7313 eV, respectively, and an average transmittance rate over 95% was achieved in a wide ultraviolet (UV)–visible region. Also, the binding energy for the (D⁰, X) emission was extracted to be 37.9 meV. Through the wide band-gap material BeZnO, we may open some possibilities for fabricating a ZnO-based UV light-emitting diode to be utilized as a barrier layer comprised of the ZnO/BeZnO quantum well structure and/or an UV light emitting material itself.     

Transition between amorphous and crystalline phases of SiC deposited on Si substrate using H3SiCH3

This paper presents a study of the transition between amorphous and crystalline phases of SiC films deposited on Si(1 0 0) substrate using H₃SiCH₃ as a single precursor by a conventional low-pressure chemical vapor deposition method in a hot-wall reactor. The microstructure of SiC, characterized by X-ray diffraction and high-resolution transmission electron microscopy, is found to vary with substrate temperature and H₃SiCH₃ pressure. The grain size decreases with increasing MS pressure at a given temperature and also decreases with reducing temperature at a given MS pressure. The deposition rates are exponentially dependent on the substrate temperature with the activation energy of around 2.6 eV. The hydrogen compositional concentration in the deposited SiC films, determined by secondary ion mass spectrometry depth profiling, is only 2.9% in the nanocrystalline SiC but more than 10% in the amorphous SiC, decreasing greatly with increasing deposition temperature. No hydride bonds are detected by Fourier transform infrared spectroscopy measurements. The chemical order of the deposited SiC films improves with increasing deposition temperature.     

Preparation and characterization of ZnO-TiO2 films obtained by sol-gel method

The sol-gel route has been applied to obtain ZnO-TiO₂ thin films. For comparison, pure TiO₂ and ZnO films are also prepared from the corresponding solutions. The films are deposited by a spin-coated method on silicon and glass substrates. Their structural and vibrational properties have been studied as a function of the annealing temperatures (400-750 °C). Pure ZnO films crystallize in a wurtzite modification at a relatively low temperature of 400 °C, whereas the mixed oxide films show predominantly amorphous structure at this temperature. XRD analysis shows that by increasing the annealing temperatures, the sol-gel Zn/Ti oxide films reveal a certain degree of crystallization and their structures are found to be mixtures of wurtzite ZnO, Zn₂TiO₄, anatase TiO₂ and amorphous fraction. The XRD analysis presumes that Zn₂TiO₄ becomes a favored phase at the highest annealing temperature of 750 °C. The obtained thin films are uniform with no visual defects. The optical properties of ZnO-TiO₂ films have been compared with those of single component films (ZnO and TiO₂). The mixed oxide films present a high transparency with a slight decrease by increasing the annealing temperature.     

Preparation of single-crystalline ZnO films on ZnO-buffered a-plane sapphire by chemical bath deposition

High-quality zinc oxide (ZnO) films were successfully grown on ZnO-buffered a-plane sapphire (Al₂O₃ (1 1 2¯ 0)) substrates by controlling temperature for lateral growth using chemical bath deposition (CBD) at a low temperature of 60 °C. X-ray diffraction analysis and transmission electron microscopy micrographs showed that the ZnO films had a single-crystalline wurtzite structure with c-axis orientation. Rocking curves (ω-scans) of the (0 0 0 2) reflections showed a narrow peak with full width at half maximum value of 0.50° for the ZnO film. A reciprocal space map indicated that the lattice parameters of the ZnO film (a=0.3250 nm and c=0.5207 nm) were very close to those of the wurtzite-type ZnO. The ZnO film on the ZnO-buffered Al₂O₃ (1 1 2¯ 0) substrate exhibited n-type conduction, with a carrier concentration of 1.9×10¹⁹ cm⁻³ and high carrier mobility of 22.6 cm² V⁻¹ s⁻¹.     

Decisive role of Au layer on the oriented growth of ZnO nanorod arrays via a simple aqueous solution method

Vertically aligned arrays of ZnO nanorod were synthesized on the Au/SiO₂/Si(1 0 0) substrate by a simple aqueous solution growth process, without pre-prepared ZnO seed layer. For comparison, glass and SiO₂/Si were also used as substrates, and the results show that the Au layer plays a decisive role in orienting the growth of the ZnO nanorod. The effects of other growth parameters, including Zn²⁺ concentration and growth time, on morphology, density, and orientation of the ZnO nanostructure were also studied and with longer reaction time, a new structure namely ZnO nanotip was obtained. Moreover, the growth mechanism of ZnO nanorod arrays grown on the Au/SiO₂/Si substrate was proposed.     

Surface morphology,structural and optical properties of polar and non-polar ZnO thin films: A comparative study

The polar and non-polar ZnO thin films were fabricated on cubic MgO (1 1 1) and (0 0 1) substrates by plasma-assisted molecular beam epitaxy. Based on X-ray diffraction analysis, the ZnO thin films grown on MgO (1 1 1) and (1 0 0) substrates exhibit the polar c-plane and non-polar m-plane orientation, respectively. Comparing with the c-plane ZnO film, the non-polar m-plane ZnO film shows cross-hatched stripes-like morphology, lower surface roughness and slower growth rate. However, low-temperature photoluminescence measurement indicates the m-plane ZnO film has a stronger 3.31 eV emission, which is considered to be related to stacking faults. Meanwhile, stronger band tails absorbance of the m-plane ZnO film is observed in optical absorption spectrum.     

Electrical properties and deep traps spectra in undoped M-plane GaN films prepared by standard MOCVD and by selective lateral overgrowth

Electrical properties, deep traps spectra and structural performance were studied for m-GaN films grown on m-SiC substrates by standard metalorganic chemical vapor deposition (MOCVD) and by MOCVD with lateral overgrowth (ELO) or sidewall lateral overgrowth (SELO). Standard MOCVD m-GaN films with a very high dislocation density over 10⁹ cm⁻² are semi-insulating n-type with the Fermi level pinned near E_c−0.7 eV when grown at high V/III ratio. For lower V/III they become more highly conducting, with the electrical properties still dominated by a high density (∼10¹⁶ cm⁻³) of E_c−0.6 eV electron traps. Lateral overgrowth that reduces the dislocation density by several orders of magnitude results in a marked increase in the uncompensated shallow donor density (∼10¹⁵ cm⁻³) and a substantial decrease of the density of major electron traps at E_c−0.25 and E_c−0.6 eV (down to about 10¹⁴ cm⁻³). Possible explanations are briefly discussed.     

Low-temperature/high-temperature AlN superlattice buffer layers for high-quality AlxGa1−xN on Si (1 1 1)

We present MOVPE-grown, high-quality Al_xGa_1−x N layers with Al content up to x=0.65 on Si (1 1 1) substrates. Crack-free layers with smooth surface and low defect density are obtained with optimized AlN-based seeding and buffer layers. High-temperature AlN seeding layers and (low temperature (LT)/high temperature (HT)) AlN-based superlattices (SLs) as buffer layers are efficient in reducing the dislocation density and in-plane residual strain. The crystalline quality of Al_xGa_1−xN was characterized by high-resolution X-ray diffraction (XRD). With optimized AlN-based seeding and SL buffer layers, best ω-FWHMs of the (0 0 0 2) reflection of 540 and 1400 arcsec for the (1 0 1¯ 0) reflection were achieved for a ∼1-μm-thick Al_0.1Ga_0.9N layer and 1010 and 1560 arcsec for the (0 0 0 2) and (1 0 1¯ 0) reflection of a ∼500-nm-thick Al_0.65Ga_0.35N layer. AFM and FE-SEM measurements were used to study the surface morphology and TEM cross-section measurements to determine the dislocation behaviour. With a high crystalline quality and good optical properties, Al_xGa_1−x N layers can be applied to grow electronic and optoelectronic device structures on silicon substrates in further investigations.     

Influence of oxygen/argon ratio on structural,electrical and optical properties of Ag-doped ZnO thin films

Ag-doped ZnO (ZnO:Ag) thin films were deposited on quartz substrates by radio frequency magnetron sputtering technique. The influence of oxygen/argon ratio on structural, electrical and optical properties of ZnO:Ag films has been investigated. ZnO:Ag films gradually transform from n-type into p-type conductivity with increasing oxygen/argon ratio. X-ray photoelectron spectroscopy measurement indicates that Ag substitutes Zn site (Ag_Zn) in the ZnO:Ag films, acting as acceptor, and being responsible for the formation of p-type conductivity. The presence of p-type ZnO:Ag under O-rich condition is attributed to the depression of the donor defects and low formation energy of Ag_Zn acceptor. The I–V curve of the p-ZnO:Ag/n-ZnO homojunction shows a rectification characteristic with a turn-on voltage of ∼7 V.     

Epitaxial NiO (1 0 0) and NiO (1 1 1) films grown by atomic layer deposition

Epitaxial NiO (1 1 1) and NiO (1 0 0) films have been grown by atomic layer deposition on both MgO (1 0 0) and α-Al₂O₃ (0 0 l) substrates at temperatures as low as 200 °C by using bis(2,2,6,6-tetramethyl-3,5-heptanedionato)Ni(II) and water as precursors. The films grown on the MgO (1 0 0) substrate show the expected cube on cube growth while the NiO (1 1 1) films grow with a twin rotated 180° on the α-Al₂O₃ (0 0 l) substrate surface. The films had columnar microstructures on both substrate types. The single grains were running throughout the whole film thickness and were significantly smaller in the direction parallel to the surface. Thin NiO (1 1 1) films can be grown with high crystal quality with a FWHM of 0.02–0.05° in the rocking curve measurements.     

Growth characteristics of ultra long double-ended acicular ZnO synthesized by a hydrothermal process

Double-ended acicular ZnO structure can be synthesized via a hydrothermal process with tetramethylammonium hydroxide and zinc acetate as precursors and polyvinyl alcohol (PVA) as a structure-directing agent. The as-prepared ZnO products show the well crystalline wurtzite structure with growth direction along [0 0 0 1]. For the first time, PVA is found to be employed as a reservoir of Zn²⁺ ions in the present study, and can control the concentration of Zn²⁺ in reaction solution, and the acicular morphology can be formed at the two ends of the 1-D ZnO structure, due to the effect of secondary growth that occurs as the sufficient concentration of Zn²⁺ ions chelated by PVA releasing to the reaction solution. Furthermore, the size of the 1-D ZnO structure can be tuned by different amounts of PVA addition.     

Metal organic vapour phase epitaxy of MgO films grown on c-plane Sapphire

Metal organic vapour phase epitaxy (MOVPE) has been used to epitaxially grow MgO films on c-plane sapphire substrates. Bismethylcyclopentadienyl magnesium (MCP₂Mg) and nitrous oxide (N₂O) were used as the magnesium and the oxygen source, respectively, with nitrogen (N₂) as the carrier gas. The dependence of the growth rate on the partial pressure of magnesium and on the growth temperature was investigated. The growth rate increases with the magnesium partial pressure. The morphological and structural properties of MgO films were investigated using atomic force microscopy and X-ray diffraction. The structural properties are strongly dependent on the growth temperature in the range 400–800 °C. (1 1 1)-oriented MgO layers are observed at growth temperatures above 600 °C whereas no diffraction peak is found at lower growth temperatures. The atomic force microscopy (AFM) images reveal a smooth surface morphology.     

Fabrication and thermal evolution of nanoparticles in SiO2 by Zn ion implantation

SiO₂ samples were implanted with 45 keV Zn ions at doses ranging from 5×10¹⁵ to 1.0×10¹⁷ ions/cm², and were then subjected to furnace annealing at different temperatures. Several techniques, such as ultra-violet–visible spectroscopy (UV–vis), grazing incidence X-ray diffraction spectroscopy (GXRD) and atomic force microscopy (AFM), have been used to investigate formation of nanoparticles and their thermal evolution. Our results clearly show that Zn nanoparticles could be effectively formed in SiO₂ at doses higher than 5×10¹⁶ ions/cm². The subsequent thermal annealing at oxygen ambient could induce the growth of Zn nanoparticles at intermediate annealing temperature range. While at temperature above 600 °C, Zn nanoparticles could be transformed into ZnO, or even Zn₂SiO₄ nanoparticles. The results have been tentatively discussed in combination with Zn diffusion and migration obtained by Rutherford backscattering spectroscopy (RBS) measurements.