Pressure Metalorganic Chemical Vapor Research Articles

The effect of Ag doping on the structure, optical, and electronic properties of ZnO nanostructures deposited by atmospheric pressure MOCVD on Ag/Si substrates

Atmospheric pressure metal–organic chemical vapour deposition was used to synthesize Ag-containing ZnO nanostructures of different morphology on Si substrates coated by Ag. Ag from Ag/Si substrates and Ag from silver acetylacetonate after its decomposition were used as a catalyst for ZnO nanocrystal growth for deposition of ZnO nanostructures with different morphologies. We investigated the relation of the structural parameters and chemical composition probed by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy with the photoluminescence (PL) properties and electron–phonon coupling (EPC) reflected in the multi-phonon Raman spectra. The Raman and PL spectra were studied at different powers of the exciting laser radiation (P exc). The spectral position and width of the phonon Raman peaks and the near bandgap PL band at low P exc are supposed to be determined by the structural quality of the surfaces/boundaries of the crystallites. The intensity of the near-bandgap and defect-related PL and the magnitude of the EPC are additionally affected by the dopant concentration. Because of the large crystallite size (>30 nm, determined from XRD), the effects of phonon or electron confinement are negligible in these nanostructures. The behaviour of the position and width of phonon and PL bands with increasing P exc indicates that the heat dissipation in the film, which is dependent on the nanostructure morphology and Ag content, plays an important role. In addition, the cytotoxicity of ZnO:Ag nanostructures was investigated by using monolayer cell cultures of epithelioid origin Madin-Darby bovine kidney and Madin-Darby canine kidney cells at a MTT assay revealing that the level of silver doping of ZnO nanostructures, their morphology, and geometric dimensions determine their toxic effects.

Ethylene for carbon doping of GaN by atmospheric pressure metal organic chemical vapor deposition

The carbon incorporation properties of GaN from ethylene and trimethylgallium precursors was studied by atmospheric pressure metal organic chemical vapor deposition. It is found that V-III ratio was the main restriction factor for carbon incorporation. Carbon concentration could be regulated effectively one order of magnitude by tuning V-III ratio for C2H4 doped GaN, and would be enhanced at relatively low V-III ratio for self-doped GaN. Low threading dislocation density and atomic flat surface of C2H4 doped GaN films was kept even if carbon doping concentration of 9E18/cm3 compared with UID-GaN and a high breakdown voltage of 2278 V at leakage current of 10µA was obtained in C2H4 doped GaN buffer layer.

The Effect of Ag-Doping on the Cytotoxicity of ZnO Nanostructures Grown on Ag/Si Substrates by APMOCVD

The search and development of new nanostructures and nanomaterials are very important for the progress of nanotechnology and modern microbiology. Due to the unique properties of silver and zinc oxide, these nanoparticles are the optimal basis for creating nanostructures with potential antiviral activity. An important issue in these studies is the establishment of cytotoxicity of these nanoparticles and their composites. Aim. To define the influence of substrate temperature and Ag concentration in ZnO lattice on the microstructure and cytotoxicity of zinc oxide nanostructures. Methods. Pure and Ag-doped ZnO nanostructures were grown on Ag/Si substrates by atmospheric pressure metalorganic chemical vapor deposition method using a mixture of zinc acetylacetonate and silver acetylacetonate powders as a precursor. Argentum thin films were deposited on Si substrates by a thermal evaporation method. MTT-assay was used for the analysis of MDBK and MDCK cell viability in the definition of zinc oxide nanostructure cytotoxicity. Results. Ag-doped zinc oxide nanostructures were grown and characterized by X-ray diff raction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. It was found that Si substrate and pure zinc oxide do not inhibit the cell viability of both epithelial cultures whereas Ag-doped ZnO nanostructures inhibit the cell viability because of all-time exposure in a sample without dilution. The cytotoxic effect was not observed at higher dilutions for Ag-doped zinc oxide nanostructures. Conclusions. The investigation of the effect of Ag-doping on the morphology and cytotoxicity of zinc oxide nanostructures is very important for implementing zinc oxide nanostructures into the current optoelectronics and photocatalysis.

Development of arsenic doped Cd(Se,Te) absorbers by MOCVD for thin film solar cells

Recent developments in CdTe solar cell technology have included the incorporation of ternary alloy Cd(Se,Te) in the devices. CdTe absorber band gap grading due to Se alloying contributes to current density enhancement and can result in device performance improvement. Here we report Cd(Se,Te) polycrystalline thin films grown by a chamberless inline atmospheric pressure metal organic chemical vapour deposition technique, with subsequent incorporation in CdTe solar cells. The compositional dependence of the crystal structure and optical properties of Cd(Se,Te) are examined. Selenium graded Cd(Se,Te)/CdTe absorber structure in devices are demonstrated using either a single CdSe layer or CdSe/Cd(Se,Te) bilayer (with or without As doping in the Cd(Se,Te) layer). Cross-sectional TEM/EDS, photoluminescence spectra and secondary ion mass spectroscopy analysis confirmed the formation of a graded Se profile toward the back contact with a diffusion length of ~1.5 μm and revealed back-diffusion of Group V (As) dopants from the CdTe layer into Cd(Se,Te) grains. Due to the strong Se/Te interdiffusion, CdSe in the Se bilayer configuration was unable to form an n-type emitter layer in processed devices. In situ As doping of the Cd(Se,Te) layer benefited the device junction quality with current density reaching 28.3 mA/cm 2 . The results provide useful insights for the optimisation of Cd(Se,Te)/CdTe solar cells. Compositional control of optical band gap in Cd(Se,Te) thin films (0 ≤ x ≤ 1) (left) and the Se profile in finished Cd(Se,Te) devices (right) fabricated by MOCVD. • Cd(Se,Te) thin films (0 ≤ x ≤ 1) deposited with linear compositional control using a chamber-less MOCVD process. • CdSe-only and CdSe/CdSe 0.2 Te 0.8 bilayers utilised for Cd(Se,Te) grading in devices. • In-situ and ex-situ group V (arsenic) doping of the graded Cd(Se,Te) obtained. • Device performance improved for thinner CdSe layers and in-situ doped bilayers. • Good control of Se profile achieved via CdSe and CdSe/CdSe 0.2 Te 0.8 properties.

The Influence of the Preheating Temperature of the (−2 0 1) β-Ga2O3 Substrates on c-Plane GaN Epitaxial Growth

In this paper, we demonstrate the direct epitaxial growth of c-plane GaN on a preheated (−2 0 1) β-Ga2O3 single-crystal substrate with no interlayer or pre-patterning processes by using the atmospheric pressure metalorganic chemical vapor deposition method. The results show that high-temperature preheating (>500 °C) can modify the surface morphology of the substrate so that the crystalline quality of the grown GaN layer can be improved. With higher preheated temperatures, the grown GaN layer reveals smaller FWHM (full width at half-maximum) of the X-ray rocking curve. In addition, we find that the photoluminescence spectra of the GaN layers reveal their narrowest linewidth at a preheated temperature of 800 °C. These results support improvements of crystalline quality and provide optimization of a c-GaN grown epitaxially on the preheated (−2 0 1) β-Ga2O3 substrates for further device fabrication.

Computational fluid dynamics modeling of a new high-pressure chemical vapor deposition reactor design

The design of a new super-atmospheric pressure metal organic chemical vapor deposition (MOCVD) reactor with spatially separated source zones and rotating susceptor is proposed and analyzed using computational fluid dynamics (CFD) techniques to determine fluid transport phenomena and suitability for thin-film synthesis of group III nitrides. This High-Pressure Spatial Chemical Vapor Deposition (HPS-CVD) reactor is designed to permit high-temperature growth of high-indium content group III nitrides. This is made possible by increasing the partial pressure of nitrogen by up to two orders of magnitude in the system thereby increasing decomposition temperatures of the group III nitride thin film. The effects of rector design, chamber height, system pressure, inlet flow rate, and rotational speed are investigated and discussed. Flow instabilities arising from the heated and rotating susceptor traversing separate source zones are minimized. Growth rates of group III nitride materials synthesized in an HPS-CVD reactor are estimated to be enhanced by approximately a factor of 10 compared to an existing super-atmospheric MOCVD reactor due to improved reactor design thereby providing comparable growth rates to current atmospheric MOCVD systems.

Atmospheric pressure metal organic chemical vapor deposition of thin germanium films

The deposition of thin germanium films by atmospheric pressure metal organic chemical vapor deposition at temperatures below 400 °C on substrates such as silicon wafers, float glass, and polyimide (Kapton®) using the diorganogermanes GeH2Cp4M2 and GeH2Cp*2 as molecular precursors is described. The deposition rates and thus the layer thicknesses can be varied by temperature and time to give layers with a thickness in the nanometer range. The homogeneity and roughness of the deposited films were analyzed by means of atomic force microscopy measurements showing the formation of smooth and uniform surfaces with roughnesses of the films in the range of (1 ± 0.15) nm to (4.5 ± 1.5) nm. Films with thicknesses between 50 and 750 nm were deposited and analyzed by Raman spectroscopy, vis–NIR spectroscopy, electron microscopy, energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The as-deposited films are composed of amorphous germanium containing approximately 10% of carbon. Using Kapton® as a substrate highly flexible films were obtained.

Atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) growth of undoped and aluminium-doped ZnO thin film using hot wall reactor

In this contribution, a hot wall reactor via economic atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) was adopted for Un-doped and Al-doped Zinc oxide films were deposited on borosilicate glass and silicon substrates. To avoid the use of an expensive vacuum system, all experiments were realized at atmospheric pressure. The chemical reagents used for this experiment are Zinc acetylacetonate (Zn(acac)2) and aluminium acetylacetonates (Al (acac)2) under atmospheric conditions. The obtained films are characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and Uv-–vis spectrometer, respectively. As results, it is found that The un-doped ZnO films are polycrystalline. However, a significant enhancement in the intensity of the relevant (100) reflection is observed when Zinc oxide films are doped with Al. It is also observed that the Al-doped Zinc oxide films present higher transparency in the visible region and resistivities of 2.55 ohm cm and 1.44 ohm cm for un-doped and Al doped films respectively

Improved Performance of Ge Solar Cell Using Graphene Quantum Dots.

We investigated the effect of graphene quantum dots (GQDs) on performance of single-junction Ge solar cell grown on (100) substrate by low pressure metalorganic chemical vapor deposition (LP-MOCVD). Isobutylgermane (IBuGe) is used as a Ge precursor for the Ge solar cell growth. By employing GQDs, the power conversion efficiency of the Ge solar cell is improved up to 3.90% (Voc of 0.22 V, Jsc of 28.52 mA/cm², and FF of 63.83%) through effective photon management as compared to bare Ge solar cells of 3.24% under AM 1.5G illumination.

Application of ZnO Nanorods Based Whispering Gallery Mode Resonator in Optical Immunosensors

In this research a whispering gallery mode (WGM) resonator based on vertically oriented ZnO nanorods, which were formed on silicon surface (silicon/ZnO-NRs), has been applied in the design of optical immunosensor that was dedicated for the determination of grapevine virus A-type (GVA) proteins. Vertically oriented ZnO-NRs were grown on silicon substrates by atmospheric pressure metal organic chemical vapor deposition (APMOCVD) and the silicon/ZnO-NRs structures formed were characterized by structural and optical methods. Optical characterization demonstrates that silicon/ZnO-NRs-based structures can act as ‘whispering gallery mode’ (WGM) resonator where quasi-whispering gallery modes (quasi-WGMs) are generated. These quasi-WGMs were experimentally observed in the visible and infrared ranges of the photoluminescence spectra. In order to design an immuno-sensing system the anti-GVA antibodies were immobilized on the surface of silicon/ZnO-NRs and in this way silicon/ZnO-NRs/anti-GVA structure was formed. The immobilization of anti-GVA antibodies and then the interaction of silicon/ZnO-NRs/anti-GVA structure with GVA proteins (GVA-antigens) resulted in an opposite shifts of the WGMs peaks in the visible range of the photoluminescence spectra observed as a defect-related photoluminescence emission of ZnO-NRs. Here designed silicon/ZnO-NRs/anti-GVA immuno-sensing structure demonstrates the sensitivity towards GVA-antigens in the concentration range of 1-200 ng/ml. Bioanalytical applicability of the silicon/ZnO-NRs-based structures in the WGMs registration mode is discussed.

Fabrication and studies on Si/InP core-shell nanowire based solar cell using etched Si nanowire arrays

We report Si/InP core-shell nanowire radial heterojunction array based solar cell. Silicon nanowire (SiNW) arrays were fabricated by room temperature metal assisted chemical etching method on a p-type Si (100) wafer using silver nanoparticles whereas n-InP layer, as a shell, was deposited onto the SiNW arrays by atmospheric pressure metal organic chemical vapor deposition to obtain core-shell radial heterojunctions. A 100 nm transparent conductive oxide layer was deposited onto top of n-InP layer by sputtering. Transmission electron microscope images confirm the formation of Si/InP core-shell radial nanowire heterostructure. From the studies of reflectance spectroscopy, higher absorption of visible photons has been found. Current-voltage measurements on the radial core-shell nanowire heterojunction based solar cell have been taken under dark and an AM 1.5 solar radiation at room temperature. The device is found to provide a conversion efficiency of 4.39% with an open circuit voltage of 0.56 V and a short circuit current density 14.26 mA/cm2 under AM 1.5 solar radiation. The core-shell radial heterojunction solar cell on nanowire arrays shows great improvement of the performance in comparison with conventional nanowire based solar cells. Our study provides new insights into the Si/InP core-shell nanowire based heterojunction which can have potential applications in fabricating nanoscale optoelectronic devices on Si platform.

Fabricating GaN-based LEDs on (−2 0 1) β-Ga2O3 substrate via non-continuous/continuous growth between low-temperature undoped-GaN and high-temperature undoped-GaN in atmospheric pressure metal-organic chemical vapor deposition

This study demonstrates two approaches to the growth of GaN-based LEDs on (−2 0 1)-oriented β-Ga2O3 single crystal substrates using metal-organic CVD under atmospheric pressure. One approach induces non-continuous growth between low-temperature undoped-GaN (u-GaN) and high-temperature u-GaN, whereas the other approach induces continuous growth. We observed the following reduction in the FWHM of X-ray diffraction rocking curves: GaN (0 0 2) on (−2 0 1) β-Ga2O3 substrate (from 464 to 342 arcsec) and GaN (1 0 2) (from 886 to 493 arcsec). An LED with six pairs of InGaN/GaN multiple quantum wells was successfully fabricated on the (−2 0 1) β-Ga2O3 single crystal substrate.

Structural and morphological characterization of the Cd-rich region in Cd1-xZnxO thin films grown by atmospheric pressure metal organic chemical vapour deposition

We have analysed the growth, morphological and structural characterization of Cd1-xZnxO thin films grown on r-sapphire substrates by atmospheric pressure metal organic chemical vapour deposition, mainly focusing on the Cd-rich rock-salt phase for its promising optical and technological applications. The evolution of the surface morphology and crystalline properties as a function of Zn content has been studied by means of high resolution x-ray diffraction and electron microscopy techniques. Monocrystalline (002) single-phase cubic films were obtained with Zn contents up to 10.4%, and with a low density of dislocations as a consequence of the optimized crystal growth process. Particular attention has been paid to the alloy characterization for compositions when cubic and wurtzite phases coexist.

Crystal Phase Control of ε-Ga2O3 Fabricated using by Metal-Organic Chemical Vapor Deposition

e-Ga2O3 thin films were grown on (0001) sapphire, (0001) GaN, and a low-temperature buffer layer at different temperatures and flow rates of bubbled H2O (oxygen source) using trimethylgallium and H2O as precursors by using atmospheric-pressure metal-organic chemical vapor deposition. Due to the atmospheric pressure conditions, most of the Ga2O3 thin films were not grown in a pure e-phase, but contain a small portion of the β-phase. The crystal structure, crystal quality, phase ratio, and surface morphology were analyzed by using X-ray diffraction, rocking curve measurements, and field-emission scanning electron microscopy. A certain minimum H2O flow rate was required to form e-Ga2O3 thin films, and the optimal growth temperature for e-Ga2O3 was 650°C. The β-phase fraction of the mixed-phase (e + β) thin films was dominant at temperatures higher than 650 °C. The crystallinities and phase compositions of the thin films changed with the flow rate of H2O. Nearly single-crystalline Ga2O3 thin films were successfully grown on GaN and sapphire, but not on a low-temperature buffer layer. Hexagonally shaped Ga2O3 islands become aligned in an ordered direction that correlated to the substrate’s orientation during the initial stage of the growth and coalesced to complete a two-dimensional layer as the growth went on.

Исследование однородности состава по толщине слоев GaInAsP, полученных на подложках InP методом газофазной эпитаксии

GaInPAs/InP heterostructures grown by low pressure (0.1 bar, 600 oC) metal-organic chemical vapor phase deposition were investigated. The thicknesses of grown GaInAsP layers were about 1 µm. For the epitaxial layers Ga<sub>1-x</sub>In<sub>x</sub>P<sub>1-y</sub>As<sub>y)</sub> with average compositions of x = 0.77 – 0.87 and y = 0.07 – 0.42 the variation of V group elements content y with the epilayer depth were revealed, weher the compositions of V-group elements were changed up to Δy = 0.1 atomic fractions in V group elements sublattice. In most cases, y change occurs in a GaInAsP region up to 200 nm thick adjacent to the InP. In some cases, y changes throughout the whole GaInPAs layer thickness. Fo the epitaxial layers with a satisfactory crystal perfection the less was the mismatch between the substrate and the GaInPAs epitaxial layer, the smaller was the value of Δy. For GaInPAs layers characterized by a low degree of crystal perfection and a high lattice mismatch between GaInAsP and InP layers, the value of Δy was about zero. These data let us suggest that the incorporation of atoms of the V group in the epitaxial layer strongly depends on elastic deformation of the growing monolayer, that is mismatched with the underlying crystal surface.

Free standing c-GaN films grown by low-pressure metalorganic chemical vapor deposition on GaP (100) substrates

Gallium nitride (GaN) films were synthesized by low pressure metalorganic chemical vapor deposition on GaP (100) substrates. Samples were grown using three-step growth method, which includes a) a nitridation process of the GaP surface, b) the growth of a GaN buffer layer and c) the growth of the high-temperature epilayer. X-ray diffraction, Raman spectroscopy and FESEM and HRTEM were used to study the changes in the surface, which suggests that the cubic phase nucleation takes place during nitridation process and remained at the top surface, which results in preferential growth of c-GaN.

Surface morphological properties of CdxZn(1-x)S thin films deposited by low-cost atmospheric pressure metal organic chemical vapour deposition technique (AP-MOCVD)

The CdxZn(1-x)S films have been deposited by low cost atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD) technique and the resulting morphological properties were successfully evaluated for solar cell applications. All morphological properties presented here were investigated by Field-Emission Scanning Microscopy (FESEM) and Energy Dispersive X-Ray (EDX) spectroscopy. It has been observed that the film thickness and grain size have been greatly influenced by the molar ratio of cadmium and zinc, deposition temperature and time. The average grain size of Cd0.5Zn0.5S films (98.16 nm thick) was obtained with deposition temperature and time of 440°C and 2.5 minutes, respectively.

A study on film formation mechanisms and precursor/solvent compatibility in PP-MOCVD of Al2O3 films using aluminum tri-sec-butoxide with hexane and toluene

Pulsed pressure metal-organic chemical vapor deposition (PP-MOCVD) has been used to deposit aluminum oxide films using aluminum tri-sec-butoxide (ASB) as a precursor and hexane and toluene as solvents at 475°C. Comparisons between solvent compatibility with ASB have been made. In addition to the solvents, different precursor/solvent solution concentrations (0.125mol% and 0.5mol%) and a shield have been used to investigate film formation mechanisms. This was done by studying the influence of these parameters on film properties (morphology and deposition rate). A droplet vaporization numerical model has been used to support explanations given for the dependence of properties on the process parameters. The results suggest that film formation might take place by four mechanisms including vapor phase deposition, Leidenfrost aerosol formation, homogeneous particle formation and liquid droplet impingement. The results also suggest that hexane has a better compatibility with ASB and is a better solvent than toluene.

Influence of Growth Temperature on the Characteristics of Single-Junction p–i–n InGaP Solar Cells.

Single-junction p–i–n InGaP solar cells are grown at various temperatures from 620 to 700 °C by low pressure metalorganic chemical vapor deposition on GaAs (001) substrates. The short circuit current density of the p–i–n InGaP solar cells increases by up to 38.8% when the growth temperature is reduced from 700 to 620 °C, while the open circuit voltage and fill factor show relatively small changes. The external quantum efficiency, especially, in the wavelength regime below 500 nm, is improved for the p–i–n InGaP solar cells grown at lower temperatures. The improvement might be attributed to the reduced absorption loss of the photons in the n-InGaP emitter region. The highest conversion efficiency of 11.01% is attributed from the p–i–n InGaP solar cell grown at 640 °C. Electron mobility and concentration of undoped InGaP layers are investigated as a function of the growth temperature and correlated with the p–i–n InGaP solar cell performance.

Highly efficient epitaxial Ge solar cells grown on GaAs (001) substrates by MOCVD using isobutylgermane

Single-junction Ge solar cells have been epitaxially grown on GaAs (001) substrates by a low pressure metalorganic chemical vapor deposition using isobutylgermane. Various growth conditions have been studied to reduce the high doping level of over 1×1019cm−3 of the p-type Ge epitaxial layer, which is detrimental to the minority carrier collection efficiency. By increasing the growth rate and employing a discontinuous doping technique, the p-type doping level of the epitaxial Ge layer has been lowered to 2×1018cm−3, and the hole mobility increased from 44 to 162cm2/Vs. A high power conversion efficiency of 6.72% can be achieved under AM1.5G illumination by the epitaxial Ge solar cell with the lowered doping level in the p-type Ge base region. The spectral response of the epitaxial Ge solar cell with a 5µm thick base layer is good enough to use for InGaP/GaAs/Ge triple-junction solar cells.