Pressure Metalorganic Chemical Vapor Deposition Research Articles

Preparation of ZnO:CeO 2– x thin films by AP-MOCVD: Structural and optical properties

The growth of columnar CeO 2, ZnO and ZnO:CeO 2− x films on quartz and AA6066 aluminum alloy substrates by economic atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) is reported. A novel and efficient combination of metal acetylacetonate precursors as well as mild operating conditions were used in the deposition process. The correlation among crystallinity, surface morphology and optical properties of the as-prepared films was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) and UV–vis spectroscopy. The synthesized films showed different crystallographic orientations depending on the ZnO and CeO 2 lattice mismatch, cerium content and growth rate. The CeO 2 films synthesized in this work showed plate-like compact structures as a result of the growth process typical of CVD. Both pure and ZnO:CeO 2− x films were obtained with a hexagonal structure and highly preferred orientation with the c-axis perpendicular to both substrates under the optimal deposition conditions. The microstructure was modified from dense, short round columns to round structures with cavities (“rose-flower-like” structures) and the typical ZnO morphology by controlling the cerium doping the film and substrate nature. High optical transmittance (>87%) was observed in the pure ZnO films. As for the ZnO:CeO 2− x films, the optical transmission was decreased and the UV absorption increased, which subsequently was affected by an increase in cerium content. This paper assesses the feasibility of using ZnO:CeO 2− x thin films as UV-absorbers in industrial applications.

Photoluminescence, photoacoustic and Raman spectra of zinc oxide films grown by LP-MOCVD using diethylzinc and water as precursors

Polycrystalline zinc oxide (ZnO) films were grown on alkali-free glass substrates by low pressure metalorganic chemical vapor deposition (LP-MOCVD) using diethylzinc (DEZn) and water (H 2O) as precursors. Photoluminescence (PL) spectra of the films were composed of the near-band-edge (NBE) emission at ~ 380 nm and the orange band (OB) emission at ~ 600 nm. Variations of the intensity ratio of the NBE emission to the OB emission ( I NBE/ I OB) and the photon energy of the knee on the photoacoustic spectrum (PA edge) as a function of substrate temperature ( T S) could be divided into two regions at the boundary temperature ( T B). Below T B, the NBE emission exhibited the tail extending to the higher energy than the bandgap of ZnO and the I NBE/ I OB value became smaller with increasing T S. In addition, the PA edge shifted towards lower energies with increasing T S. Concerning the Raman results, these tendencies seem to have some relations with the decrease in concentration of the secondary phase Zn(OH) 2 with increasing T S. Above T B, however, the I NBE/ I OB became larger and the PA edge shifted towards higher energies with increasing T S, which is probably due to the removal of defect levels related to the excess oxygen atoms.

Growth of AlGaSb Compound Semiconductors on GaAs Substrate by Metalorganic Chemical Vapour Deposition

EpitaxialAlxGa1-xSb layers on GaAs substrate have been grown by atmospheric pressure metalorganic chemical vapour deposition using TMAl, TMGa, and TMSb. We report the effect of V/III flux ratio and growth temperature on growth rate, surface morphology, electrical properties, and composition analysis. A growth rate activation energy of 0.73 eV was found. For layers grown on GaAs at 580∘C and 600∘C with a V/III ratio of 3 a high quality surface morphology is typical, with a mirror-like surface and good composition control. It was found that a suitable growth temperature and V/III flux ratio was beneficial for producing good AlGaSb layers. Undoped AlGaSb grown at 580∘C with a V/III flux ratio of 3 at the rate of 3.5 μm/hour shows p-type conductivity with smooth surface morphology and its hole mobility and carrier concentration are equal to 237 cm2/V.s and 4.6 × 1017 cm-3, respectively, at 77 K. The net hole concentration of unintentionally doped AlGaSb was found to be significantly decreased with the increased of aluminium concentration. All samples investigated show oxide layers (Al2O3,Sb2O3, andGa2O5) on their surfaces. In particular the percentage of aluminium-oxide was very high compared with a small percentage of AlSb. Carbon content on the surface was also very high.

Heteroepitaxial ZnO nano hexagons on p-type SiC

ZnO single crystal nanohexagons have been grown heteroepitaxially on p-type Si-face 4H–SiC substrates with 8 ° miscut from [0 0 0 1] by catalyst-free atmospheric pressure metalorganic chemical vapor deposition and characterized by x-ray diffraction, scanning and transmission electron microscopy as well as energy disperse x-ray and cathodoluminescence analyses. The as-grown ZnO nanohexagons have a pillar shape terminated by a and c plane facets, and are aligned along the growth direction with the epitaxial relation [0 0 0 1] ZnO‖[0 0 0 1] 4H–SiC and [ 1 0 1 ¯ 0 ] ZnO ∥ [ 1 0 1 ¯ 0 ] 4 H - SiC . The ZnO nanohexagons demonstrate intense UV emission ( λ NBE=376 nm) and negligible defect-related luminescence.

GaN buffer layer growth by MOCVD using a thermodynamic non-equilibrium model

In this work, a gallium nitride (GaN) buffer layer was grown on a sapphire substrate (α-Al 2O 3) in a horizontal reactor by low pressure metal-organic chemical vapor deposition (LP-MOCVD). Trimethylgallium (TMGa) and ammonia (NH 3) were precursors of gallium and nitrogen, respectively, and hydrogen (H 2) was used as carrier gas. TMGa and NH 3 fluxes were kept constant, with flow rates of 3.36 μmole/min and 0.05 standard liter/min, respectively. The fluence of hydrogen was also kept constant with the flux rate of 4.5 standard liter/min. GaN was deposited at 550 °C and 100 mbar. According to the X-ray diffraction spectra, a buffer layer was formed with a wurtzite structure, which is the stable phase. The thermodynamic affinities were estimated as A 1 = 175.9 kJ/mole and A 2 = 62.88 kJ/mole.

Characterizations of Ga-doped ZnO films on Si (111) prepared by atmospheric pressure metal-organic chemical vapor deposition

Gallium-doped ZnO films were grown on p-Si(111) substrates by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) using diethylzinc and water as reactant gases and triethyl gallium (TEG) as a n-type dopant gas. The structural, electrical, and optical properties of ZnO:Ga films obtained by varying the flow rate of TEG from 0.56to3.35μmol∕min were examined. X-ray diffraction patterns and scanning electron microscopy images indicated that Ga doping plays a role in forming microstructures in ZnO films. A flat surface with a predominant orientation (101) was obtained for the ZnO:Ga film fabricated at a flow rate of TEG=2.79μmol∕min. This film also revealed a lowest resistivity of 4.54×10−4Ωcm, as measured using the van der Pauw method. Moreover, low temperature photoluminescence (PL) emission recorded at 12K demonstrated the Burstein Moss shift of PL line from 3.365to3.403eV and a line broadening from 100to165meV as the TEG flow rate varied from 0.56to2.79μmol∕min. This blueshift behavior of PL spectra from ZnO:Ga films features the degeneracy of semiconductor, which helps to recognize the enhancing of transparency and conductivity of ZnO films fabricated by AP-MOCVD using Ga-doping technique.

Diffusion behaviour of vanadium in GaN thin films studied by secondary ion mass spectrometry

A sandwiched multi-structure made of alternatively undoped and vanadium (V) doped GaN layers was grown on SiN-treated (0 0 0 1) sapphire substrates by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD). The V diffusion in GaN has been investigated in the temperature range of 1000–1100 °C by using secondary ion mass spectroscopy. The diffusion coefficients were obtained through a data-fitting program based on Fick's first law. The results suggest that V diffuses faster near the GaN/sapphire interface. During annealing, V redistributed by both in- and out-diffusion mechanisms. An activation energy of 2.9 ± 0.4 eV is estimated for V diffusion in GaN.

Chemical vapor deposition and characterization of nitrogen doped TiO 2 thin films on glass substrates

Photocatalytically active, N-doped TiO 2 thin films were prepared by low pressure metalorganic chemical vapor deposition (MOCVD) using titanium tetra-iso-propoxide (TTIP) as a precursor and NH 3 as a reactive doping gas. We present the influence of the growth parameters (temperature, reactive gas phase composition) on the microstructural and physico-chemical characteristics of the films, as deduced from X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and ultra-violet and visible (UV/Vis) spectroscopy analysis. The N-doping level was controlled by the partial pressure ratio R = [NH 3]/[TTIP] at the entrance of the reactor and by the substrate temperature. For R = 2200, the N-doped TiO 2 layers are transparent and exhibit significant visible light photocatalytic activity (PA) in a narrow growth temperature range (375–400 °C). The optimum N-doping level is approximately 0.8 at.%. However, the PA activity of these N-doped films, under UV light radiation, is lower than that of undoped TiO 2 films of comparable thickness.

Characterizations of gallium-doped ZnO films on glass substrate prepared by atmospheric pressure metal-organic chemical vapor deposition

Ga-doped zinc oxide (ZnO:Ga) films were grown on glass substrate by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) using diethylzinc and water as reactant gases and triethyl gallium (TEG) as an n-type dopant gas. The structural, electrical and optical properties of ZnO:Ga films obtained at various flow rates of TEG ranging from 1.5 to 10 sccm were investigated. X-ray diffraction patterns and scanning electron microscopy images indicated that Ga-doping plays an important role in forming microstructures in ZnO films. A smooth surface with a predominant orientation of (101) was obtained for the ZnO:Ga film grown at a flow rate of TEG = 7.5 sccm. Moreover, a lowest resistivity of 3.6 × 10 − 4 Ω cm and a highest mobility of 30.4 cm 2 V − 1 s − 1 were presented by the same sample, as evaluated by Hall measurement. Otherwise, as the flow rate of TEG was increased, the average transmittance of ZnO:Ga films increased from 75% to more than 85% in the wavelength range of 400–800 nm, simultaneously with a blue-shift in the absorption edge. The results obtained suggest that low-resistivity and high-transparency ZnO films can be obtained by AP-MOCVD using Ga-doping sufficiently to make the films grow degenerate and effect the Burstein–Moss shift to raise the band-gap energy from 3.26 to 3.71 eV.

A comparative study of guided modes and random lasing in ZnO nanorod structures

Hexagonal and arrow-headed ZnO nanorod structures have been grown by low pressure chemical vapour deposition (CVD) and atmospheric pressure metal-organic CVD. The technology ensures a high optical quality of the produced nanostructures to act as gain medium for stimulated emission in the ultraviolet spectral region in combination with high quality factor laser resonators. Multiple sharp lasing peaks related to the guided modes were realized from single hexagonal nanorods and arrays of hexagonal ZnO nanorods. A comparative analysis of the variations of lasing spectra from shot to shot of pumping, and the dependence of lasing threshold on the area of pump beam spot on the sample surface in disordered agglomerations of hexagonal nanorods and in layers consisting of arrow-headed nanorods, demonstrate that lasing is determined by the superposition of guided modes in the first case, while random lasing occurs in the second case.

Studies on metal/n-GaAs Schottky barrier diodes: The effects of temperature and carrier concentrations

Metal/Semiconductor Schottky diodes were fabricated to study the effect of temperature and carrier concentrations on diode parameters, such as ideality factor and barrier heights. The diodes were formed on the epitaxial layers of metal organic chemical vapor deposition (MOCVD) grown n-GaAs with metals such as Al, Pd, and Zn–Pd deposited onto n-GaAs by thermal evaporation technique. Trimethyl gallium and AsH3 were used as Ga and As precursors, respectively, to grow GaAs on semi-insulating GaAs substrates at 600 °C and H2S was used for n-type doping in a horizontal reactor atmospheric pressure MOCVD system. The Schottky diodes were characterized by forward bias current-voltage measurements in the temperature range 130–300 K and capacitance-voltage measurement at room temperature and diode parameters such as ideality factor and barrier height have been evaluated. It is found that the Schottky barrier height decreases with decrease in temperature while the ideality factor increases. It is also observed that the barrier height increases linearly with the applied forward bias voltage and the rate of change of barrier height with voltage increases for higher carrier concentration of the semiconductor. The carrier concentration of n-GaAs was chosen in the regime 1×1016–8.2×1016 cm−3 so that the depletion region extends inside the semiconductor and the diode can be used as a III–V photovoltaic device.

Growth behaviour of InGaN/GaN self‐assembled quantum dots with different growth conditions in horizontal MOCVD

AbstractInGaN/GaN self‐assembled quantum dots (QDs) were grown on (0001) sapphire substrates by low pressure metalorganic chemical vapor deposition. The growth behavior of the InGaN/GaN QDs was examined under different experimental conditions. The InGaN/GaN QDs were formed in Stranski‐Krastanow growth mode by the continuous supply of metalorganic (MO) sources, whereas they were formed in the Volmer‐Weber growth mode by the periodic interruption of the MO sources. In addition, a quantum ring (QRs) structure was confirmed during the formation of InGaN QDs. The In content in the InGaN QDs appeared to increase with decreasing temperature. The photoluminescence spectrum (PL) showed peaks for the GaN buffer layer and the InGaN QDs layer at 362.2 nm (3.41 eV) and between 420–460 nm (2.96–2.70 eV), respectively. The composition of InGaN QDs was estimated from the PL peak position to be In0.12‐0.15Ga0.88‐0.85N. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface morphology and optical properties of ZnO epilayers grown on Si(1 1 1) by metal organic chemical vapor deposition

Heteroepitaxial ZnO epilayers were grown on Si(1 1 1) substrates using a vertical geometry atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) system. The growth temperature was varied from 550 °C to 650 °C in steps of 25 °C. The ZnO growth rate and surface morphology were strong functions of the growth temperature and ranged from ∼0.16 μm/h to 1.36 μm/h. The surface morphology of the ZnO films changed from granular to sharp tips as the growth temperature increased. The effect of buffer thickness was also examined, and was found to have a strong effect on the optical properties of the ZnO. An optimized growth condition for ZnO epilayers was found at 625 °C, producing a FWHM in the room temperature photoluminescence (PL) spectrum of 4.5 nm and a preferred growth orientation along the (0 0 2) direction. Transmission electron microscopy images and selected area diffraction patterns showed excellent crystalline quality of both the buffer and ZnO overlayer. When non-optimized growth temperatures were employed, post-growth annealing was found to greatly enhance the ratio of band-edge to deep level emission.

Al x Ga1-x N solar-blind photodetectors grown by low pressure MOCVD

Al x Ga1-x N ternary alloys are very attractive materials for application to ultraviolet (UV) photodetection. In this work, high Al content Al x Ga1-x N films are grown on sapphire substrate by low pressure metalorganic chemical vapor deposition (MOCVD). The Al content in the Al x Ga1-x N epilayer is estimated to be 54% by high resolution X-ray diffraction (HRXRD) and Vegard’s law. The full width at half maximum (FWHM) of the rocking curve for the Al0.54Ga0.46N (0002) is about 597 arcsec. According to the transmittance measurement result, our sample is suitable for fabricating solar-blind photodetectors. The observed Fabry-Perot fringes in the transmission region indicate that high optical quality is obtained. Solarblind metal-semiconductor-metal (MSM) photodetectors based on the MOCVD-grown Al0.54Ga0.46N film are fabricated and tested. The detector has a low dark current of about 31 pA under a bias voltage of 5 V. An UV/visible contrast of about four orders of magnitude is observed and responsivity increases with increments of the bias voltage.

Structural properties of ZnS/GaAs epilayers grown by atomic-layer epitaxy

ZnS epilayers were grown on (1 0 0) semi-insulating GaAs substrates using an atmospheric pressure metal-organic chemical vapor deposition (CVD) technique under the atomic-layer epitaxy (ALE) mode. Atomic force microscopy (AFM) and photoluminescence (PL) measurements were carried out to find the effect of the II–VI ratio of the 30-nm thick ZnS epilayers and to investigate the thickness-dependent characteristics of ZnS epilayers with the thicknesses of 30 and 100 nm. While the II–VI ratio-dependent ZnS quality is consistent regardless of the measurement, the thickness-dependent epilayer quality is quite contrary depending on the measurement. This difference demonstrates the non-uniform distribution of the strain–relaxation in the ZnS epilayer along the depth.

ZnO films grown by MOCVD on GaAs substrates: Effects of a Zn buffer deposition on interface, structural and morphological properties

Integration of ZnO with the well-developed GaAs technology presents several aspects that need to be previously analyzed and considered. The large lattice mismatch between ZnO and GaAs and its different crystallographic structure lead to many structural defects. In addition, their potential chemical reactivity is another source of complexity and an academic challenge. Recently some interesting contributions on this subject have been carried out by Liu and co-workers. As an additional step to the knowledge of the ZnO/GaAs heterostructure, we have deepened on the study of the morphology and orientation of ZnO thin films grown by atmospheric pressure metal-organic chemical vapour deposition (AP-MOCVD) on GaAs(1 0 0) and GaAs(1 1 1)A substrates with and without a Zn buffer pre-deposition on them. The analysis has been made as a function of growth temperature and precursors ratio. Structural, morphological and compositional characterizations have been made by X-ray diffraction (XRD), high-resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDX) and X-ray photoemission spectroscopy (XPS). ZnO layers present an out-of-plane (0 0 0 1) preferred orientation, while the in-plane orientation has a random distribution. The layers are constituted by large tilted columnar grains with a top angle of around 55° indicating that the planes which constitute the conic heads are the {1 0 1¯ 1} ones. ZnO films grown after a previous Zn deposition exhibit a less compact morphology. In some cases and depending on growth conditions, interfacial processes with crystallization of extrinsic phases have been observed by XRD, revealing the presence of a body centred tetragonal phase of Zn 3As 2. In order to get an insight into these interfacial effects, EDX on cross-sectional views of the interface has been carried out. Strain measurements indicate a tensile nature of the biaxial stress, which is reduced by a factor two when the Zn buffer is used.

Growth and characterization of InAsSb alloys by metalorganic chemical vapour deposition

Low pressure metalorganic chemical vapour deposition (LP-MOCVD) growth and characteristics of InAsSb on (100) GaSb substrates are investigated. Mirror-like surfaces with a minimum lattice mismatch are obtained. The samples are studied by photoluminescence spectra, and the output is 3.17μm in wavelength. The surface of InAsSb epilayer shows that its morphological feature is dependent on buffer layer. With an InAs buffer layer used, the best surface is obtained. The InAsSb film shows to be of n-type conduction with an electron concentration of 8.52 × 1016 cm−3.

Carrier localization degree of In0.2Ga0.8N/GaN multiple quantum wells grown on vicinal sapphire substrates

In this work, we have grown In0.2Ga0.8N/GaN multiple quantum well (MQWs) epitaxial structure on vicinal sapphire substrates by low pressure metal-organic chemical vapor deposition and investigated the relationship between carrier localization degree and vicinal angles of sapphire substrates. The optical analysis confirmed that the In0.2Ga0.8N/GaN MQWs grown on 0.2°-off sapphire substrate exhibited the smallest carrier localization degree and more ordered In0.2Ga0.8N/GaN MQW structure. In addition, mechanisms for carrier localization in In0.2Ga0.8N/GaN MQWs grown on vicinal substrate were discussed based on the results obtained from the power and temperature dependent photoluminescence measurements. The Raman spectrum showing the in-plane compressive stress of the GaN epitaxial structures grown on vicinal sapphire substrates revealed the relation between the dislocation density and the carrier localization degree in MQWs. From transmission electron microscopy images, the threading dislocation density (TDD) of In0.2Ga0.8N/GaN MQWs grown on 0.2° vicinal sapphire substrate at the bottom of n-GaN layer was about 9.4×108 cm−2 and reduced to 3.0×108 cm−2 at the top of n-GaN layer. We also obtained the TDD of 5.6×107 cm−2 in the MQW region and only 1.0×107 cm−2 in the p-GaN region. Based on the results mentioned above, 0.2°-off substrate can offer In0.2Ga0.8N/GaN MQW blue light-emitting diode structures with benefits, such as high crystal quality, low defects, and small carrier localization degree.

Suitability of Atmospheric-pressure MOCVD CdTe Solar Cells for Inline Production Scale

AbstractCdTe, with a direct band gap of 1.45 eV is well suited to the terrestrial AM1.5 solar irradiance and currently makes up half of the thin film (TF) photovoltaic (PV) market. There are 4 main factors that determine final cost of PV modules: the conversion efficiency, materials amount per unit area of module, production yield, and the economy of scale. It is therefore valuable to investigate alternative and/or innovative deposition techniques and processes which have the potential to impact on these factors. Metal organic chemical vapour deposition (MOCVD) is a powerful technique offering increased process repeatability, achieving a high level of control of materials characteristics. Recent improvements in CdTe devices using atmospheric–pressure (AP) MOCVD have led to efficiencies of 13.3 % using 2 μm absorber layers and 11 % with a 1 μm absorber layer [11, 12]. These results were achieved: by extending the optical band gap of the window layer, using a ternary alloy (Cd0.9Zn0.1S), with intentional p-type doping of the CdTe layers with As, and the use of an in situ (dry) deposited CdCl2 layer and anneal. All layers, except the front and back contact, are grown by a sequential MOCVD process. Furthermore this is a dry process without the need for any etch treatment. The inherent design of the horizontal MOCVD laboratory chambers do not lend themselves well to large scale production. However, the CSER group has designed and built an experimental inline reaction chamber to evaluate AP-MOCVD as an inline production process. Discussion is made based on kinetically limited growth and molar supply models to assess the suitability of the MOCVD process to deposit fast enough for an inline process. The AP-MOCVD inline reactor uses a showerhead to deliver the precursors onto a moving 5 × 7.5 cm2 substrate and preliminary results for deposited layers are given. From these preliminary results it has been extrapolated that a 1 μm thick CdTe layer can be deposited on substrates moving at 60 cm/min.

Planar GaAs Nanowires on GaAs (100) Substrates: Self-Aligned, Nearly Twin-Defect Free, and Transfer-Printable

We report the controlled growth of planar GaAs semiconductor nanowires on (100) GaAs substrates using atmospheric pressure metalorganic chemical vapor deposition with Au as catalyst. These nanowires with uniform diameters are self-aligned in <110> direction in the plane of (100). The dependence of planar nanowire morphology and growth rate as a function of growth temperature provides insights into the growth mechanism and identified an ideal growth window of 470 +/- 10 degrees C for the formation of such planar geometry. Transmission electron microscopy images reveal clear epitaxial relationship with the substrate along the nanowire axial direction, and the reduction of twinning defect density by about 3 orders of magnitude compared to <111> III-V semiconductor nanowires. In addition, using the concept of sacrificial layers and elevation of Au catalyst modulated by growth condition, we demonstrate for the first time a large area direct transfer process for nanowires formed by a bottom-up approach that can maintain both the position and alignment. The planar geometry and extremely low level of crystal imperfection along with the transferability could potentially lead to highly integrated III-V nanoelectronic and nanophotonic devices on silicon and flexible substrates.