Conventional Metalorganic Chemical Vapor Deposition Research Articles

Reduced Defect Density in MOCVD-Grown MoS2 by Manipulating the Precursor Phase.

Advancements in the synthesis of large-area, high-quality two-dimensional transition metal dichalcogenides such as MoS2 play a crucial role in the development of future electronic and optoelectronic devices. The presence of defects formed by sulfur vacancies in MoS2 results in low photoluminescence emission and imparts high n-type doping behavior, thus substantially affecting material quality. Herein, we report a new method in which single-phase (liquid) precursors are used for the metal-organic chemical vapor deposition (MOCVD) growth of a MoS2 film. Furthermore, we fabricated a high-performance photodetector (PD) and achieved improved photoresponsivity and faster photoresponse in the spectral range 405-637 nm compared to those of PDs fabricated by the conventional MOCVD method. In addition, the fabricated MoS2 thin film showed a threshold voltage shift in the positive gate bias direction owing to the reduced number of S vacancy defects in the MoS2 lattice. Thus, our method significantly improved the synthesis of monolayer MoS2 and can expand the application scope of high-quality, atomically thin materials in large-scale electronic and optoelectronic devices.

Scalable growth of atomically thin MoS2 layers in a conventional MOCVD system using molybdenum dichloride dioxide as the molybdenum source

To bring atomically thin transition metal dichalcogenides (TMDs) to practical application, a highly reproducible process to grow them over a large scale is indispensable. Here, we develop a carbon-free reproducible route for the scalable growth of high-quality monolayer MoS2 by selecting molybdenum dichloride dioxide (MoO2Cl2) as the Mo source and integrating the precursor with a standard low-pressure cold-wall metalorganic chemical vapor deposition (MOCVD) system. Specifically, combined with H2S as the sulfur source and using catalytic Dragontrail glass (DT-glass) as the main substrate, we investigate the effect of MoO2Cl2 flux, temperature, and deposition time on the growth, confirming MoO2Cl2 with reasonably high vapor pressure is well compatible with the MOCVD system that enables precise control of sources for uniform nucleation and growth of MoS2 over a large area. We successfully demonstrate the growth of carbon-free MoS2 monolayers on DT-glass with decent crystalline, optical, and electrical properties. Additionally, the initial attempts also manifest that the proposed strategy could be generalized for growing ultrathin MoS2 on other technologically important substrates, such as SiO2/Si and quartz, exhibiting superior optical quality and wafer-scale uniformity. This work provides a new avenue for the large-scale production of high-quality MoS2 monolayers and facilitates their use in practical applications.

Investigation of carbon incorporation in laser-assisted MOCVD of GaN

Background carbon (C) impurity incorporation in metalorganic chemical vapor deposition (MOCVD) grown gallium nitride (GaN) represents one of the major issues in further improving GaN vertical power device performance. This work presents a laser-assisted MOCVD (LA-MOCVD) technique to address the high-C issue in MOCVD homoepitaxial GaN under different growth rate (Rg) regimes and studies the correlations between [C] and Rg. [C] in LA-MOCVD GaN is reduced by 50%–90% as compared to the conventional MOCVD GaN for a wide growth rate range between 1 and 16 μm/h. A mass-transport based model is developed to understand the C incorporation at different Rg regimes. The results obtained from the developed model are in good agreement with experimental data. The model further reveals that LA-MOCVD effectively suppresses C incorporation by reducing the active C species in the gas phase. Moreover, high step velocity in step flow growth mode can facilitate C incorporation at fast Rg, exhibiting steeper C increase. The theoretical model indicates that [C] can be suppressed below 1016 cm−3 with a fast growth rate (Rg) of 10 μm/h by utilizing higher power LA-MOCVD and freestanding GaN substrates with larger off-cut angles.

(Invited) Vertical GaN Diodes: Effect of Growth Parameters, Indium Surfactants, and Device Development for High-Power Electronics

GaN has achieved widespread interest in high-power and high-speed electronics due to its wide-bandgap (WBG), large breakdown field, and high mobility compared to Si and SiC. There is also continued interest in the growth and development of GaN vertical devices owing to their advantageous properties of high current capability from backside ohmic, better field management, scaling feasibility, and enhanced thermal transport, which is not feasible with the lateral device topology. To realize the predicted high breakdown voltage and low on-resistance from vertical devices, high-quality drift layers with low unintentional doping and low compensating defects are fundamentally required. Hence, our research aims to pursue a comprehensive investigation of growth and device development with vertical GaN structures, building from the optimization of thick GaN homoepitaxy with high growth rate but low impurities, improving surface morphology using Indium as a surfactant as well as realizing punch-through field-plate p-n diodes to maximize the average electric field in high-voltage operation.Toward the goal of high-quality GaN epitaxy growth, we performed systematic studies of growth rate effects on the drift layer doping using ammonia molecular beam epitaxy (NH3 MBE) and plasma-assisted molecular beam epitaxy (PA MBE) [1]. The NH3 MBE homoepitaxy GaN demonstrated an unintentional doping (UID) of ~1015 cm-3, significantly lower than that of the conventional metalorganic chemical vapor deposition (MOCVD) reports, which can be potentially contributed by clean growth environment of MBE.We further focused on improving surface morphology using Indium as a surfactant. Through a combination of varying V/III ratios, In flux and growth temperatures, an optimal condition for surface morphology, characterized by atomic force microscopy, was achieved for fast growth rates (on the order of 1 µm/hr and beam equivalent pressures on the order of 5 × 10-7 Torr). However, excessive indium causes the surface morphology to degrade, potentially due to the enhancement of the Ga desorption from the surface as a result of the reaction of indium with ammonia for high indium fluxes. The indium surfactant can also reduce silicon impurities as revealed from the Secondary ion mass spectrometry (SIMS) (Fig. 2).Subsequently, we developed vertical p-n GaN power devices using the NH3 MBE-grown high-quality epitaxy [2]. The vertical p-n GaN diode consisted (from bottom up): ~0.25 µm of n+ GaN ([Si]: 1×1019 cm-3) buffer, ~4 µm UID GaN drift layer for n-type region, ~0.4 µm p+GaN ([Mg]: 3×1019 cm-3) for p-type region, and ~10 nm p++ GaN cap ([Mg]: 3×1020 cm-3) to support the anode ohmic contact. Circular p-n diodes of 80-100 µm diameter were fabricated with Pd/Pt anode and a backside Ti/ Au ohmic contact. The field management was achieved by field-plate of 15 µm formed with Ti/Au metals on top of a field-plate dielectric of a Al2O3 (26 nm)/ Si3N4 (205 nm) stack (Fig. 3).The p-n GaN diodes exhibited an outstanding forward J-V behavior with a low specific on-resistance (Ron,sp) of 0.28 mΩ-cm2 and a minimum ideality factor of 1.36 (Fig. 4), which are among the best achieved in vertical homoepitaxy GaN p-n diodes [2-4]. The best diodes also exhibited a breakdown voltage of > 1 kV (equipment limit 1 kV). These reverse-voltage properties imply a punch-through characteristics with peak electric field (EC) >2.6 MV/cm, as revealed by Silvaco simulation (Fig. 5). The combination of >1 kV breakdown voltage and on-resistance of 0.28 mΩ-cm2 represents one of the best performances of the p-n GaN diodes, achieved with a significantly downscaled (~4 um) drift layer compared to the MOCVD GaN p-n diodes of ≥ 8 um (Fig. 6). Work is ongoing to develop NHE MBE p-n diodes with indium surfactant-assisted drift layer growth which will be promising to further improve the surface morphology, reduce leakage, and enhance breakdown performance of the vertical devices.

Low-Temperature and Ammonia-Free Epitaxy of the GaN/AlGaN/GaN Heterostructure

Wide band-gap semiconductors are very attractive because of their broad applications as electronics and optoelectronics materials, GaN-based materials being by far the most promising. For the production of such nitride-based optical and power devices, metal–organic chemical vapor deposition (MOCVD) is routinely used. However, this has disadvantages, such as the large consumption of ammonia gas and the need for a high growth temperature. To go beyond such a limit, in this study we successfully developed a remote plasma MOCVD (RP-MOCVD) approach for the epitaxial growth of high-quality GaN/AlGaN heterostructures on 4H-SiC substrates. Our RP-MOCVD has the advantages of a lower growth temperature (750 °C) compared to the conventional MOCVD route and the use of a remote N2/H2 plasma instead of ammonia for nitrides growth, generating in situ the NHx (x = 0–3) species needed for the growth. As assessed by structural, morphological, optical, and electrical characterization, the proposed strategy provides an overall cost-effective and green approach for high-quality GaN/AlGaN heteroepitaxy, suitable for high electron mobility transistors (HEMT) technology.

(Invited) Vertical GaN Devices for High-Power Electronics

With the demand of high-power and high-speed electronics, GaN has achieved widespread interest owing to its wide-bandgap (WBG), large breakdown field, and high mobility that can provide superior performance compared to the Si and SiC that are limited by inherent material properties. In power electronics, GaN is a potential candidate due to its high-quality p-n junctions and availability of native substrates to prevent dislocation-assisted leakage, particularly for vertical devices. For high-power electronics, these vertical devices are more advantageous compared to their lateral counterparts due to their high current capability from large-area backside ohmic, high breakdown voltage from confined electric field within WBG material, scaling feasibility, and better thermal management. Moreover, the punch-through vertical device can extract the full potential of the WBG semiconductors by maximizing the average electric field over the fully depleted drift layer.For high-performance vertical device, high-quality drift layers with low controlled background doping and low compensating defects are essential to obtain the expected high breakdown voltage and low on-resistance. Hence, we have focused on both materials and device development for GaN power devices. Toward the first goal of high-quality GaN epitaxy, we optimized the growth condition for controlled low unintentionally doped (UID) and low impurity drift layer from ammonia molecular beam epitaxy (NH3 MBE). For this, we performed a systematic study of growth rate effects on the drift layer doping for a series of growth rates from 0.37 µm/hr to 1.68 µm/hr in both sapphire and free-standing GaN substrates (Fig. 1). A low background UID doping of ~1015 cm-3 was achieved with a growth rate 1.4 µm/hr on the free-standing GaN substrate. This UID background doping is one of the lowest reported for GaN homoepitaxy [1-3]. The potential of NH3 MBE-growth for low-doped epitaxy is recognized from our work, contributed from the cleaner growth environment of MBE, compared to the conventional metal-organic chemical vapor deposition (MOCVD).The next step of our work involved developing high-performance power devices with these high-quality epitaxies. Our vertical p-n GaN diode consisted of a ~4 µm UID GaN drift layer on ~0.25 µm of n+ GaN ([Si]: 1×1019 cm-3) buffer for the n-type region. The p-type region included a 10 nm p++ GaN cap ([Mg]: 3×1020 cm-3) to support the anode ohmic contact followed by a 0.4 µm p+GaN ([Mg]: 3×1019 cm-3) to form the p-n junction.The sample was fabricated with circular diodes of 80-100 µm diameter Pd/Pt Anode and a backside Ti/ Au Cathode. The field management was achieved by a combination of ~1.4 µm dry etch mesa isolation with a sidewall angle of ~55° and field plate (Fig. 2). For the field-plate dielectric, a stack of atomic layer deposition of Al2O3 (26 nm) and plasma-enhanced chemical vapor deposition of Si3N4 (205 nm) at 300 °C was used. Finally, a Ti/Pt metal stack of 15 µm length was patterned to form the field-plate.Capacitance-Voltage characteristics performed in these p-n diodes showed a doping of ~3×1015 cm-3 in the n- drift layer (Fig. 3a). Secondary ion mass spectrometry (SIMS) detected Si~2×1015 cm-3 and O ~2×1016 cm-3 respectively across the n-layer (Fig. 3b), likely the source of UID doping, accounting for their possible partial ionization. The compensating carbon impurities was less than the SIMS detection limit, certifying the high-quality NH3 MBE-growth. The forward J-V characteristics (Fig. 4) revealed the leakage current below the detection limit (~1.6 pA) for V<2 V. At higher forward biases (V=2.87 V) in the diffusion current region, the ideality factor reaches to a minimum of 1.36. The extracted specific on-resistance (Ron,sp) of 0.28 mΩ-cm2 and the minimum ideality factor of 1.36 are among the best achieved in vertical homoepitaxy GaN p-n diodes [1-3]. The highest breakdown voltage of these diodes were > 1 kV (equipment limit 1 kV) whereas some diodes exhibited breakdown at -890 V (Fig. 5). Silvaco simulation showed a punch-through breakdown of the diodes with peak electric field (EC) >2.6 MV/cm. Remarkably, this breakdown performance with NH3 MBE GaN of only ~4 um channel is comparable with the best available MOCVD GaN p-n diodes with a significantly thicker drift layer of ≥ 8 um (Fig. 6). Our work thus demonstrates the potential of high-performance power devices by NH3 MBE, benefitted from high-quality epitaxy, controlled low doping, and fast growth rates, that will be promising for high-voltage devices.

Volatile trinuclear heterometallic beta-diketonates: Structure and thermal properties related to the chemical vapor deposition of composite thin films

A series of new trinuclear molecular complexes [PbL12{Co(hfa)2}2] (1), [PbL12{Ni(hfa)2}2] (2), [PbL22{Co(hfa)2}2] (3), [PbL22{Ni(hfa)2}2] (4), [PbL32{Co(hfa)2}2] (5), [PbL32{Ni(hfa)2}2] (6), where L = methoxy-substituted β-diketonate RCOCHCOC(CH3)2OCH3, L1: R = CH3, L2: R = CF3, L3: R = C(CH3)3; hfa = hexafluoroacetylacetonate, were synthesized. Crystal structures were determined by single crystal X-ray analysis. Volatility and thermal stability of the obtained compounds were studied by thermal gravimetric analysis, vacuum sublimation and by mass transfer in helium flow. It was established that all the obtained heterometallic complexes are volatile and can be sublimed in vacuum without dissociation into monometallic complexes. Compounds 3–6 were examined as SSP (single source precursors) in conventional low pressure MOCVD (Metalorganic Chemical Vapor Deposition) of inorganic films on Si(100) substrates. Influence of precursor composition on their structure and thermal properties as well as on the composition of the deposited inorganic films are discussed.

Reduced dislocation density and residual tension in AlN grown on SiC by metalorganic chemical vapor deposition

Crack-free AlN films with threading dislocation density (TDD) below 109 cm−2 are needed for deep-UV optoelectronics. This is typically achieved using pulsed lateral overgrowth or very thick buffer layers (&amp;gt;10 μm), a costly and time-consuming approach. A method for conventional metalorganic chemical vapor deposition growth of AlN/SiC films below 3 μm with greatly improved quality is presented. Focusing on substrate pretreatment before growth, we reduce average film stress from 0.9 GPa (tension) to −1.1 GPa (compression) and eliminate cracking. Next, with optimized growth conditions during initial deposition, AlN films with x-ray rocking curve widths of 123 arc-sec (0002) and 304 arc-sec (202¯1) are developed, and TDD is confirmed via plan view transmission electron microscopy (TEM) to be 2 × 108 cm−2. Film stress measurements including x-ray 2θ-ω, reciprocal space mapping, and curvature depict compressively stressed growth of AlN on 4H-SiC due to lattice mismatch. The thermal expansion coefficient mismatch between AlN and SiC is measured to be Δα=αAlN−αSiC=1.13×10−6 °C−1 and is found to be constant between room temperature and 1400 °C. TEM confirms the existence of dense misfit dislocation (MD) networks consistent with MD formation near SiC step edges and low MD density regions attributed to nearly coherent AlN growth on SiC terraces. These low-TDD, crack-free AlN/SiC buffers provide a platform for deep-UV optoelectronics and ultrawide bandgap electronics.

Tunable Bandgap GaInAsP Solar Cells With 18.7% Photoconversion Efficiency Synthesized by Low-Cost and High-Growth Rate Hydride Vapor Phase Epitaxy

As market-dominant Si solar cell technology approaches its practical efficiency limit of 27.1%, a key challenge for the photovoltaic industry is to search for a low-cost ~1.7 eV top cell that can enable cost-competitive tandems with solar-to-electricity conversion efficiency exceeding 30%. III-V semiconductors offer tunable bandgap and unparalleled efficiencies for tandem devices. However, their high manufacturing cost has been the biggest impediment for market entry. Hydride vapor phase epitaxy (HVPE) has recently reemerged as a promising low-cost alternative to incumbent metalorganic chemical vapor deposition (MOCVD) for III-V solar cells. Here, we show the first demonstration of ~1.7 eV GaInAsP solar cells that achieve nearly 19% photoconversion efficiency synthesized by HVPE. The unprecedented growth rate of ~42 μm/h achieved via HVPE (~10× faster than conventional MOCVD) allowed the device layers to be deposited in less than seven minutes. Nearly abrupt and chemically distinct heterointerfaces were attained in spite of high-growth rates, which are known to exacerbate phase separation in GaInAsP alloys. Demonstration of ideality factor of n= 1 at max power point highlight the advancements in growth optimization and device design. The results presented here highlight a promising path toward tunable bandgap, high-growth rate, low-cost, and high-efficiency GaInAsP devices for direct solar-to-electricity and solar-to-hydrogen conversion.

Low-temperature MOCVD deposition of Bi2Te3 thin films using Et2BiTeEt as single source precursor

Et2BiTeEt was used as single source precursor for the deposition of Bi2Te3 thin films on Si(1 0 0) substrates by metal organic chemical vapor deposition (MOCVD) at very low substrate temperatures. Stoichiometric and crystalline Bi2Te3 films were grown at 230 °C, which is approximately 100 °C lower compared to conventional MOCVD processes using one metal organic precursors for each element. The Bi2Te3 films were characterized using scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. The elemental composition of the films, which was determined by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, was found to be strongly dependent of the substrate temperature.

Induction-heating MOCVD reactor with significantly improved heating efficiency and reduced harmful magnetic coupling

In a conventional induction-heating III-nitride metalorganic chemical vapor deposition (MOCVD) reactor, the induction coil is outside the chamber. Therefore, the magnetic field does not couple with the susceptor well, leading to compromised heating efficiency and harmful coupling with the gas inlet and thus possible overheating. Hence, the gas inlet has to be at a minimum distance away from the susceptor. Because of the elongated flow path, premature reactions can be more severe, particularly between Al- and B-containing precursors and NH3. Here, we propose a structure that can significantly improve the heating efficiency and allow the gas inlet to be closer to the susceptor. Specifically, the induction coil is designed to surround the vertical cylinder of a T-shaped susceptor comprising the cylinder and a top horizontal plate holding the wafer substrate within the reactor. Therefore, the cylinder coupled most magnetic field to serve as the thermal source for the plate. Furthermore, the plate can block and thus significantly reduce the uncoupled magnetic field above the susceptor, thereby allowing the gas inlet to be closer. The results show approximately 140% and 2.6 times increase in the heating and susceptor coupling efficiencies, respectively, as well as a 90% reduction in the harmful magnetic flux on the gas inlet.

Studies of deep level centers determining the diffusion length in epitaxial layers and crystals of undoped n-GaN

A wide variety of parameters were measured for undoped n-GaN grown by hydride vapor phase epitaxy and compared to n-GaN films grown by conventional and lateral overgrowth metalorganic chemical vapor deposition. The parameters included deep level electron and hole trap spectra, microcathodoluminescence, electron beam induced current, diffusion length, and electron capture cross section from the dependence of the low temperature persistent photocapacitance on forward bias injection pulse duration. The results show a prominent role of electron traps with levels near Ec-0.56 eV in limiting the lifetime and diffusion length values in all these materials.

High Performance 2G-HTS Wire Using a Novel MOCVD System

We present the development of a novel Metal Organic Chemical Vapor Deposition (MOCVD) system aimed at producing high performance 2G-HTS wire in the ARPA-E GRIDS program on Superconducting Magnetic Energy Storage. A susceptor-free MOCVD system with non contact optical temperature monitoring and highly laminar parallel flow in a confined volume has been developed to address the issues of a-axis grain formation and low precursor conversion efficiency in conventional MOCVD designs. For the first time, near-1000 A/12 mm critical current tapes have been achieved by MOCVD in single pass deposition of 1.8-μm REBCO (RE = rare earth) on IBAD-MgO/LMO substrates. In addition, completely a-axis free films have been grown to thickness up to 4 μm, while the precursor conversion efficiency has been increased by 40%. The system and REBCO film characteristics, as well as further development aims will be presented.

Hydrothermally Grown Nonpolar a-Plane ZnO and Its Applications

There has been great interest in ZnO as promising alternative light emitting material. It has wide direct bandgap of 3.37 eV and large exciton binding energy of 60 meV. A diverse of growth methods have been developed for ZnO epitaxial layer growth such as laser-assisted plasma enhanced chemical vapor deposition (PECVD), metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and hydrothermal synthesis. Among them, hydrothermal growth is the cost-effective, environmentally friendly, and low thermal budget requiring process compared to conventional MOCVD. Most of hydrothermally grown ZnOs are in the shapes of nanorod or nanowire along c-axis due to fast growth rate of c-plane. Commercially available LEDs are based on GaN related material system, which is grown along the polar c-axis of wurtzite crystal structure. In this crystal, there exists spontaneous and piezoelectric polarization inducing internal electric fields which result in spatial separation of electron and hole wave functions. This quantum confined stark effect (QCSE) causes reduction in radiative recombination efficiency under high current injection. As one of solution, extensive efforts on nonpolar a-plane GaN are exerted. Nonpolar a-plane ZnO with high crystalline quality was successfully grown on a-plane GaN tempelate by simple, low-cost, and mass producible hydrothermal method. Synthesized a-plane ZnO thin film was studied with SEM, TEM, XRD, AFM, and PL analyses. In this presentation, growth of hydrothermal ZnO and its applications will be introduced.

Growth of InN and In-Rich InGaN Layers on GaN Templates by Pulsed Metalorganic Chemical Vapor Deposition

InN and In-rich InGaN layers have been grown on GaN templates using the pulsed metalorganic chemical vapor deposition (MOCVD) technique and compared with analogous layers grown by conventional MOCVD. Structural investigations were performed using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Surface morphology was studied using atomic force microscopy. Electrical and optical properties were studied using Hall measurements and photoluminescence (PL) spectroscopy. All layers were free of metal droplets. InN grown using pulsed MOCVD showed fairly low background electron concentration (n e = 8 × 1018 cm−3 to 9 × 1018 cm−3) and high electron mobility (μ e = 644 cm2 V−1 s−1). Structural studies revealed increase of size (from 100 nm to 500 nm) and decrease of density of InN islands at higher growth temperatures. For In-rich InGaN layers (In content 68% and 80%) the density of islands was similar to that in InN, while the diameter varied from 50 nm to 150 nm. Inhomogeneities of In and Ga distribution in the layers resulting in broadened XRD lines and PL bands are discussed.

Optimization of ion-atomic beam source for deposition of GaN ultrathin films.

We describe the optimization and application of an ion-atomic beam source for ion-beam-assisted deposition of ultrathin films in ultrahigh vacuum. The device combines an effusion cell and electron-impact ion beam source to produce ultra-low energy (20-200 eV) ion beams and thermal atomic beams simultaneously. The source was equipped with a focusing system of electrostatic electrodes increasing the maximum nitrogen ion current density in the beam of a diameter of ≈15 mm by one order of magnitude (j ≈ 1000 nA/cm(2)). Hence, a successful growth of GaN ultrathin films on Si(111) 7 × 7 substrate surfaces at reasonable times and temperatures significantly lower (RT, 300 °C) than in conventional metalorganic chemical vapor deposition technologies (≈1000 °C) was achieved. The chemical composition of these films was characterized in situ by X-ray Photoelectron Spectroscopy and morphology ex situ using Scanning Electron Microscopy. It has been shown that the morphology of GaN layers strongly depends on the relative Ga-N bond concentration in the layers.

GaN Lateral Overgrowth by Hydride Vapor Phase Epitaxy through Nanometer-Size Channels Fabricated with Nanoimprint Lithography

Epitaxial lateral overgrowth (ELO) has been used for reducing the dislocation density to grow high-quality GaN crystals. In conventional ELO, micrometer-size channels formed on a mask material such as SiO2, where GaN growth starts, are generally used. In the present study, ELO through nanometer-size (50–80 nm) channels is investigated to improve the dislocation reduction ability. Channels are fabricated using nanoimprint lithography and dry etching. We demonstrate for the first time successful hydride vapor phase epitaxy (HVPE)-based GaN ELO growth through nanochannels. In the growth interface, distinct facet structures appear and coalescence between neighboring facets proceeds. The surface of a 20-µm-thick GaN layer becomes flat by the valleys between facet structures being buried. The dislocation density is shown to decrease to approximately 5×107 cm-2 for a 20-µm-thick GaN layer on sapphire. Photoluminescence measurements show a decay time of over 3 times longer than that of a conventional metalorganic chemical vapor deposition (MOCVD) template.

Design of a three-layer hot-wall horizontal flow MOCVD reactor

A new three-layer hot-wall horizontal flow metal-organic chemical vapor deposition (MOCVD) reactor is proposed. When the susceptor is heated, the temperature of the wall over the susceptor also increases to the same temperature. Furthermore, the flowing speed of the top layer is also increased by up to four times that of the bottom layer. Both methods effectively decrease the convection and make most of the metal organic (MO) gas and the reactive gas distribute at the bottom surface of the reactor. By selecting appropriate shapes, sizes, nozzles array, and heating area of the walls, the source gases are kept in a laminar flow state. Results of the numeric simulation indicate that the nitrogen is a good carrier to reduce the diffusion among the precursors before arriving at the substrate, which leads to the reduction of pre-reaction. To get a good comparison with the conventional MOCVD horizontal reactor, the two-layer horizontal MOCVD reactor is also investigated. The results indicate that a two-layer reactor cannot control the gas flow effectively when its size and shape are the same as that of the three-layer reactor, so that the concentration distributions of the source gases in the susceptor surface are much more uniform in the new design than those in the conventional one.

Effect of the nanostructure on room temperature ferromagnetism and resistivity of undoped ZnO thin films grown by chemical vapor deposition

Undoped zinc oxide thin films having various types of morphology and nanostructure were deposited by metal organic chemical vapor deposition (MOCVD) on single-crystalline substrates. Water-assisted MOCVD process at low temperatures (300–500°C) was applied along with conventional MOCVD in oxygen-containing atmosphere at 500 and 600°C. The strong correlation between room-temperature ferromagnetism of the films, their electrical properties and morphology at nano-scale was demonstrated.

Digitally Alloyed Modulated Precursor Flow Epitaxial Growth of Ternary AlGaN with Binary AlN and GaN Sub-Layers and Observation of Compositional Inhomogeneity

We report the growth of ternary aluminum gallium nitride (AlGaN) layers on AlN/sapphire template/substrates by digitally alloyed modulated precursor flow epitaxial growth (DA-MPEG), which combined an MPEG AlN sub-layer with a conventional metalorganic chemical vapor deposition (MOCVD)-grown GaN sub-layer. The overall composition in DA-MPEG Al x Ga1−x N was controlled by adjustment of the growth time (i.e., the thickness) of the GaN sub-layer. As the GaN sub-layer growth time increased, the Al composition in AlGaN decreased to 50%, but the surface morphology of the AlGaN layer became rough, and a three-dimensional structure with islands appeared for the DA-MPEG AlGaN with relatively thick GaN sub-layers, possibly resulting from the Ga adatom surface migration behavior and/or the strain built up from lattice mismatch between AlN and GaN sub-layers with increasing GaN sub-layer growth time. Through strain analysis by high-resolution x-ray diffraction, reciprocal space mapping, and scanning transmission electron microscopy, it was found that there was compositional inhomogeneity in the DA-MPEG AlGaN with AlN and GaN binary sub-layers for the case of the layer with relatively thick GaN sub-layers.