High-temperature Metalorganic Chemical Vapor Deposition Research Articles

Evolution of Dislocations and Strains in AlN Grown by High-Temperature Metal–Organic Chemical Vapor Deposition

Evolution of Dislocations and Strains in AlN Grown by High-Temperature Metal–Organic Chemical Vapor Deposition

Van der Waals Epitaxy of c-Oriented Wurtzite AlGaN on Polycrystalline Mo Substrates for Enhanced Heat Dissipation.

The epitaxy of III-nitrides on metallic substrates is competitive due to the advantages of vertical carrier injection, enhanced heat dissipation, and flexible application in various III-nitride-based devices. However, the serious lattice mismatch, atom diffusion, and interface reaction under the rigorous growth conditions have caused enormous obstacles. Based on the thermal and chemical stability of the graphene layer, we propose the van der Waals epitaxy of c-oriented wurtzite AlGaN on the polycrystalline Mo substrate by high-temperature metal-organic chemical vapor deposition. The insertion of a graphene layer interrupts the chaotic epitaxial relationship between the polycrystalline metal and epilayers, resulting in the single-crystalline orientation along the wurtzite (0002) plane and residual stress release in AlGaN because of the weak van der Waals interaction. We also demonstrate that the epitaxy of AlGaN on Mo metal possesses enhanced heat dissipation ability, in which the epilayer temperature is controlled at only 28.7 °C by the heating of a ∼54 °C hot plate. The heat dissipation enhancement for the present epitaxial structures provides a desirable strategy for the fabrication of efficient ultraviolet devices with excellent stability and lifetime.

Growth of self-assembled nanovoids embedded AlN layer on a low- temperature buffer by metal organic chemical vapor deposition

In this study, we demonstrate a high-quality aluminum nitride (AlN) film with a thickness of 2.5 μm grown on 4H-SiC substrate using a three-dimensional (3D) buffer via high-temperature metal-organic chemical vapor deposition. For the buffer layer, 3D structure morphology without polycrystals was selected by the temperature change from 850 to 1350°C. The main layer was grown on the 3D buffer, and self-assembled nanovoids were formed, enabling the growth of a high-quality AlN film without cracks. However, many cracks occurred in the sample without a buffer. The full width at half maximum of the X-ray rocking curve was 144/368 arcsec for (002)/(102) planes, respectively. Cross-sectional transmission electron microscopy showed nanovoids and a rapid defect reduction effect at the interface of the buffer layer.

Dislocation density control of GaN epitaxial film and its photodetector

At present, GaN-based ultraviolet photodetectors (UV PDs) is confronted with the challenge of high dark current and low responsivity. In this work, we designed and prepared high-performance GaN-based UV PDs on the sapphire substrates by the combination of low-temperature pulsed laser deposition (LT-PLD) and high-temperature metal-organic chemical vapor deposition growth (HT-MOCVD). The as-grown GaN film shows high-crystalline with edge dislocation density of 6.50 × 107 cm−2 and screw dislocation density of 1.92 × 108 cm−2, respectively. Furthermore, the optimized electrode structure has been well designed to enhance the carrier transport characteristics in GaN-based UV PDs, which improves the performance of GaN-based UV PDs accordingly. Based on the high-quality GaN film and the optimized electrode structure, high-performance GaN-based UV PDs are fabricated, which reveals the very high responsivity and small dark current of 1.4 A/W and 4.8 nA@5V, respectively. These high-performance GaN-based UV PDs shed light on the potential application in UV warning.

Extraction efficiency simulation in deep ultraviolet AlGaN light emitting diodes

A ray tracing model is developed to study the light extraction efficiency (LEE) of the nitride wide band gap semiconductor deep ultraviolet light emitting diodes (DUV-LEDs). The basic device structure is flip-chip LED device with dome shape encapsulation. Various device parameters such as reflecting p-metal, absorption coefficient of the AlN layer, sapphire surface roughness etc have been examined. We have found that the transparency of the AlGaN/AlN epitaxial layers as well as the encapsulation material play key roles in improving the LEE and spatial intensity distribution. To verify the transparency of the epi-layers, highly doped n-type Al\(_{0.61}\)Ga\(_{0.39}\)N on AlN template are grown on sapphire substrate by high-temperature metalorganic chemical vapor deposition (HT-MOCVD), then the crystal and optical properties are measured and analyzed. The calculated the absorption coefficient of the AlN is below 10\(^{3}\)cm\(^{-1}\), which paves the way to achieve high extraction efficiency DUV LEDs.

Large-area far ultraviolet-C emission of Al0.73Ga0.27N/AlN multiple quantum wells using carbon nanotube based cold cathode electron-beam pumping

In this study, we demonstrated a far ultraviolet-C (far-UVC) emitter using Al0.73Ga0.27N/AlN multiple quantum wells (MQWs) by carbon nanotube based cold cathode electron-beam (C-beam) excitation. The Al0.73Ga0.27N/AlN MQW structure was grown on AlN/sapphire by high-temperature metal-organic chemical vapor deposition. The large-area far-UVC emission (276 mm2) was investigated using C-beam, as a function of anode voltage (VA) and anode current (IA), and the near-band-edge emission of Al0.73Ga0.27N/AlN MQWs was observed at a peak wavelength of 233 nm, with a VA of 4 kV, an IA of 0.5 mA. The results suggest that the large-area C-beam pumped far-UVC emitter could be a promising sterilization light source.

Epitaxial growth of deep ultraviolet light emitting diodes with two-step n-AlGaN layer

In this study, the effect of a two-step n-AlGaN layer grown via high temperature metal organic chemical vapor deposition (HT-MOCVD) was investigated. The crystal and electrical characteristics of n-AlGaN over Al composition 60% are crucial for the high performance deep ultraviolet light emitting diode (DUV LED). The improvement in the crystal quality of the n-AlGaN is proved by inserting Al 83% n-AlGaN between Al 65% n-AlGaN and AlN using atomic force microscope, photoluminescence, high resolution X-ray diffraction and scanning transmission electron microscopy. The measurement of the Hall effect demonstrated the enhanced electrical properties of the n-AlGaN layer with two-step. Finally, we demonstrated a DUV LED with conventional and two-step n-AlGaN layers using HT-MOCVD. The optical power of the DUV LED with two-step n-AlGaN increased by a factor of three as compared to those of the DUV LED with conventional n-AlGaN.

Gas-Phase Chemical Reaction Mechanism in the Growth of AlN during High-Temperature MOCVD: A Thermodynamic Study.

We presented a comprehensive thermodynamic study of the gas-phase chemical reaction mechanism of the AlN growth by high-temperature metal-organic chemical vapor deposition, investigating the addition reactions, pyrolysis reactions, and polymerization of amide DMANH2 and subsequent CH4 elimination reaction. Based on the quantum chemistry calculations of the density functional theory, the main gas-phase species in different temperature ranges were predicted thermodynamically by comparing the enthalpy difference and free energy change before and after the reactions. When T > 1000 °C, it was found that MMAl, (MMAlNH)2, and (MMAlNH)3 are the three most probable end gas products, which will be the main precursors of surface reactions. Also, in high temperatures, the final product of the parasitic reactions is mainly (DMA1NH2)2 and (DMAlNH2)3, which are easy to decompose into small molecules and likely to be the sources of AlN nanoparticles.

AlGaN-based ultraviolet light-emitting diode on high-temperature annealed sputtered AlN template

We demonstrate 297.5-nm AlGaN-based ultraviolet (UV) light-emitting diodes (LEDs) grown on a high-temperature annealed (HTA) sputtered AlN template upon sapphire substrate. After HTA at 1600 °C, full width at half maximum values of (0002) and (101¯2) planes of the 200-nm sputtered AlN template are significantly improved from 120.7 to 2794.0 arcsec to 82.4 and 352.6 arcsec, respectively, showing comparable threading dislocation densities with the 2-μm AlN template grown by high-temperature metal organic chemical vapor deposition (MOCVD). Therefore, typical AlN template grown by MOCVD is not necessary in our study. A UV LED grown on this HTA sputtered AlN template reaches light output power of 9.83 mW at 100 mA and external quantum efficiency of 2.77% at 30 mA. Our result indicates that the HTA sputtered AlN template is able to replace the commonly used high-temperature MOCVD AlN templates and thus decrease the growth complexity and cost of AlGaN-based UV LEDs.

Effect on optical, structural and electrical properties by the AlGaN/AlGaN multi quantum wells with different well and barrier thicknesses

In this study, we investigated the effect of well and barrier thicknesses on the optical, structural and electrical properties of multi quantum wells structures and deep-ultraviolet light emitting diodes (DUV-LEDs) through simulation and experiments. DUV-LEDs with different well and barrier thicknesses were simulated to understand the polarization effect and carrier transport characteristics. All the structures were grown on a sapphire substrate using high-temperature metal-organic chemical vapor deposition. The photoluminescence, X-ray diffraction and electroluminescence results showed different optimal well and barrier thicknesses. The photoluminescence and structural properties were superior at well and barrier thickness of 3 nm and 15 nm, respectively, which attributed to the polarization effect, interface and crystal quality. The electroluminescence properties were affected by the carrier transport as well as the crystal quality, showing the highest intensity at well and barrier thickness of 3 nm and 9 nm.

Comparison of MOCVD and MBE Regrowth for CAVET Fabrication

In this paper, we demonstrate the fabrication of current aperture vertical electron transistors (CAVET) realized with two different epitaxial growth methods. Templates with a p-GaN current blocking layer (CBL) were deposited by metal organic chemical vapor deposition (MOCVD). Channel and barrier layers were then regrown by either molecular beam epitaxy (MBE) or MOCVD. Scanning electron microscope (SEM) images and atomic force microscope (AFM) height profiles are used to identify the different regrowth mechanisms. We show that an AlN interlayer below the channel layer was able to reduce Mg diffusion during the high temperature MOCVD regrowth process. For the low-temperature MBE regrowth, Mg diffusion was successfully suppressed. CAVET were realized on the various samples. The devices suffer from high leakage currents, thus further regrowth optimization is needed.

The defect evolution in homoepitaxial AlN layers grown by high-temperature metal–organic chemical vapor deposition

The defect evolution in homoepitaxial AlN grown by high-temperature metal–organic chemical vapor deposition on AlN/sapphire templates was studied.

Point defect reduction in MOCVD (Al)GaN by chemical potential control and a comprehensive model of C incorporation in GaN

A theoretical framework that provides a quantitative relationship between point defect formation energies and growth process parameters is presented. It enables systematic point defect reduction by chemical potential control in metalorganic chemical vapor deposition (MOCVD) of III-nitrides. Experimental corroboration is provided by a case study of C incorporation in GaN. The theoretical model is shown to be successful in providing quantitative predictions of CN defect incorporation in GaN as a function of growth parameters and provides valuable insights into boundary phases and other impurity chemical reactions. The metal supersaturation is found to be the primary factor in determining the chemical potential of III/N and consequently incorporation or formation of point defects which involves exchange of III or N atoms with the reservoir. The framework is general and may be extended to other defect systems in (Al)GaN. The utility of equilibrium formalism typically employed in density functional theory in predicting defect incorporation in non-equilibrium and high temperature MOCVD growth is confirmed. Furthermore, the proposed theoretical framework may be used to determine optimal growth conditions to achieve minimum compensation within any given constraints such as growth rate, crystal quality, and other practical system limitations.

Efficiency Improvement of Deep‐Ultraviolet Light Emitting Diodes with Gradient Electron Blocking Layers

In this paper, a gradient electron blocking layer (GEBL) is introduced to improve efficiency of deep‐ultraviolet light‐emitting diodes (DUV‐LEDs). Various structures of DUV‐LEDs are simulated to determine the energy band diagram variation and carrier injection mechanism resulting from the insertion of the GEBL. The simulation results show the improved electron and hole transport behavior in AlGaN multi quantum wells (MQWs). Especially, the injection efficiency of holes is improved with the increasing number of GEBL steps, which lead to the enhancement of internal quantum efficiency (IQE). The DUV‐LED structures with GEBL are grown in a high‐temperature metal‐organic chemical vapor deposition (HT‐MOCVD) system. Electroluminescence (EL) spectra show that the emission intensity at a wavelength of 280 nm from a DUV‐LED with a 12‐steps EBL is about 2.3 times that from a DUV‐LED with a single EBL.

GaN Epitaxial Layer Grown with Conductive Al(x)Ga(1-x)N Buffer Layer on SiC Substrate Using Metal Organic Chemical Vapor Deposition.

This study investigated GaN epitaxial layer growth with a conductive Al(x)Ga(1-x)N buffer layer on n-type 4H-SiC by high-temperature metalorganic chemical vapor deposition (HT-MOCVD). The Al composition of the Al(x)Ga(1-x)N buffer was varied from 0% to 100%. In terms of the crystal quality of the GaN layer, 79% Al was the optimal composition of the Al(x)Ga(1-x)N buffer layer in our experiment. A vertical conductive structure was fabricated to measure the current voltage (I-V) characteristics as a function of Al composition, and the I-V curves showed that the resistance increased with increasing Al concentration of the Al(x)Ga(1-x)N buffer layer.

Fabrication of AIN Nano-Structures Using Polarity Control by High Temperature Metalorganic Chemical Vapor Deposition.

This study investigates the crystallographic polarity transition of AIN layers grown by high temperature metalorganic chemical vapor deposition (HT-MOCVD), with varying trimethylaluminum (TMAI) pre-flow rates. AIN layers grown without TMAI pre-flow had a mixed polarity, consisting of Al- and N-polarity, and exhibited a rough surface. With an increasing rate of TMAI pre-flow, the AIN layer was changed to an Al-polarity, with a smooth surface morphology. Finally, AIN nano-pillars and nano-rods of Al-polarity were fabricated by etching a mixed polarity AIN layer using an aqueous KOH solution.

AlN Nanostructures Fabricated on a Vicinal Sapphire (0001) Substrate

We report a novel and facile method for the fabrication of various AlN nanostructures with Al polarity using polarity control and selective etching without a mask or metal catalyst. To investigate the polarity transitions of the AlN layers obtained with different growth parameters, AlN layers were grown by high-temperature metalorganic chemical vapor deposition with varying growth temperatures and trimethylaluminum (TMAl) preflow rates. The growth of Al-polar AlN was clearly supported by a lower growth temperatures and higher TMAl preflow rates. Transmission electron microscopy showed that the threading dislocations (TDs) generated at the AlN–sapphire interface were bent toward the boundary of the N-polar grain because of the three-dimensional growth mode of the mixed-polarity AlN layer. Finally, defect-free nanopillars, nanorods, nanofurrows, and nanowalls were fabricated by etching mixed-polarity AlN layers with an aqueous KOH solution.

Influence of the growth temperature of AlN nucleation layer on AlN template grown by high-temperature MOCVD

We studied the influence of the growth temperature of AlN nucleation layer (TNL) on the AlN template grown by high-temperature metal-organic chemical vapor deposition (HT-MOCVD). The AlN templates were characterized by high-resolution X-ray diffractometer, atomic force microscopy and room-temperature Raman scattering spectrometer. The results revealed that the TNL had a direct influence on the quality of the AlN template. By optimizing the TNL at 950°C, we obtained a high-quality AlN template with the full width at half maxima for the (0002) and (10–12) planes of 90″ and 612″, respectively. The AlN template also presented atomic level step with a root mean square (RMS) roughness of 0.133nm. In addition, it performed excellent single crystallographic orientation along the c-axis. The growth evolution of AlN nucleation layer at different TNL was also explained in detail.

A comparative study of effects of SiNx deposition method on AlGaN/GaN heterostructure field-effect transistors

The effects of thin SiNx deposition on AlGaN/GaN heterostructure field-effect transistors were systematically studied by comparing their electrical and device characteristics. Two aspects of the thin SiNx film deposition were investigated: (i) the increase in two-dimensional electron gas (2DEG) density at the heterointerface and (ii) its capability as a gate insulating layer. Three different SiNx deposition methods were studied: catalytic chemical vapor deposition (Cat-CVD), metalorganic chemical vapor deposition (MOCVD), and plasma enhanced chemical vapor deposition (PECVD). A large increase in 2DEG density was obtained after SiNx deposition for all methods. The devices with MOCVD SiNx gate insulator showed a larger gate leakage current than those with the Cat-CVD and PECVD SiNx, implying that a thinning of the AlGaN surface barrier occurred due to Si diffusion into the AlGaN barrier during the high-temperature MOCVD process.

Selective Synthesis of Hollow and Filled Fullerene-like (IF) WS2 Nanoparticles via Metal–Organic Chemical Vapor Deposition

The synthesis of WS2 onion-like nanoparticles by means of a high-temperature metal–organic chemical vapor deposition (MOCVD) process starting from W(CO)6 and elemental sulfur is reported. The reaction can be carried out as a single-step reaction or in a two-step process where the intermediate products, amorphous WS2 nanoparticles, formed through the high-temperature reaction of tungsten and sulfur in the initial phase of the reaction, are isolated and converted into onion-type WS2 nanoparticles in a separate annealing step. Analysis of the reaction product using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) combined with energy dispersive X-ray spectroscopy (EDX) allowed to optimize the reaction in such a way that filled onion-like structures are formed in a one-step reaction, whereas hollow onion-like structures are obtained by the two-step procedure. A model could be devised that allows us to rationalize the different outcome of the reactions. The MOCVD approach therefore allows a selective synthesis of open and filled fullerene-like chalcogenide nanoparticles.