High-temperature Metal Organic Chemical Vapor 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.

Heteroepitaxial AlN growth on c-plane sapphire substrates by ammonia-free high temperature metalorganic chemical vapor deposition

AlN films were hetero-epitaxially grown on c-plane sapphire substrates by ammonia-free high temperature metalorganic chemical vapor deposition (AFHT-MOCVD). The dependence of the AlN growth rate on the flow rate of source gases (H2, N2 and trimethylaluminum (TMA)) and the growth temperature were systematically investigated. The maximum AlN growth rate up to ∼4.0 μm/hour was obtained at the present growth conditions. Based on the experimental results, the possible mechanisms in the AFHT-MOCVD growth concerning the gas reactions are qualitatively discussed. The as-grown AlN epilayers were characterized by high-resolution X-ray diffraction (HRXRD), scanning electron microscope (SEM) and scanning transmission electron microscope (STEM). The narrow full width at half maximum (FWHM) values of symmetric (0002) and asymmetric (10–12) AlN diffractions are obtained ((0002): 211 arcsec, (10–12): 325 arcsec from a ∼3 μm thick AlN epilayer), showing the potential of the growth technique for the high quality AlN growth.

Behaviours of the lattice-polarity inversion in AlN growth on c-Al2O3 (0001) substrates by ammonia-free high temperature metalorganic chemical vapor deposition

A BF-STEM image and inserted magnified HR-HAADF-STEM images showing the different lattice-polarity of AlN at each position.

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.

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.

Deep-Ultraviolet AlGaN/AlN Core-Shell Multiple Quantum Wells on AlN Nanorods via Lithography-Free Method

We report deep ultraviolet (UVC) emitting core-shell-type AlGaN/AlN multiple quantum wells (MQWs) on the AlN nanorods which are prepared by catalyst/lithography free process. The MQWs are grown on AlN nanorods on a sapphire substrate by polarity-selective epitaxy and etching (PSEE) using high-temperature metal organic chemical vapor deposition. The AlN nanorods prepared through PSEE have a low dislocation density because edge dislocations are bent toward neighboring N-polar AlN domains. The core–shell-type MQWs grown on AlN nanorods have three crystallographic orientations, and the final shape of the grown structure is explained by a ball-and-stick model. The photoluminescence (PL) intensity of MQWs grown on AlN nanorods is approximately 40 times higher than that of MQWs simultaneously grown on a planar structure. This result can be explained by increased internal quantum efficiency, large active volume, and increase in light extraction efficiency based on the examination in this study. Among those effects, the increase of active volume on AlN nanorods is considered to be the main reason for the enhancement of the PL intensity.

Improved carrier injection of AlGaN-based deep ultraviolet light emitting diodes with graded superlattice electron blocking layers.

A DUV-LED with a graded superlattice electron blocking layer (GSL-EBL) is demonstrated to show improved carrier injection into the multi-quantum well region. The structures of modified EBLs are designed via simulation. The simulation results show the carrier behavior mechanism of DUV-LEDs with a single EBL (S-EBL), graded EBL (G-EBL), and GSL-EBL. The variation in the energy band diagram around the EBL region indicates that the introduction of GSL-EBL is very effective in enhancing carrier injection. Besides, all DUV-LEDs emitting at 280 nm are grown in the high temperature metal organic chemical deposition system. It is confirmed that the optical power of the DUV-LED with the GSL-EBL is significantly higher than that of the DUV-LED with the S-EBL and G-EBL.

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.

Status of Growth of Group III-Nitride Heterostructures for Deep Ultraviolet Light-Emitting Diodes

We overview recent progress in growth aspects of group III-nitride heterostructures for deep ultraviolet (DUV) light-emitting diodes (LEDs), with particular emphasis on the growth approaches for attaining high-quality AlN and high Al-molar fraction AlGaN. The discussion commences with the introduction of the current status of group III-nitride DUV LEDs and the remaining challenges. This segues into discussion of LED designs enabling high device performance followed by the review of advances in the methods for the growth of bulk single crystal AlN intended as a native substrate together with a discussion of its UV transparency. It should be stated, however, that due to the high-cost of bulk AlN substrates at the time of writing, the growth of DUV LEDs on foreign substrates such as sapphire still dominates the field. On the deposition front, the heteroepitaxial growth approaches incorporate high-temperature metal organic chemical vapor deposition (MOCVD) and pulsed-flow growth, a variant of MOCVD, with the overarching goal of enhancing adatom surface mobility, and thus epitaxial lateral overgrowth which culminates in minimization the effect of lattice- and thermal-mismatches. This is followed by addressing the benefits of pseudomorphic growth of strained high Al-molar fraction AlGaN on AlN. Finally, methods utilized to enhance both p- and n-type conductivity of high Al-molar fraction AlGaN are reviewed.

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.

High Quality Al-Polar AIN Growth on (0001) Sapphire Using Polarity-Selective Thermal Etching by High Temperature Metalorganic Chemical Vapor Deposition.

In this study, we suggest a polarity-selective in-situ thermal etching and re-growth process for the fabrication of high quality Al terminated AIN epilayers by high temperature metalorganic chemical vapor deposition. Mixed-polar AIN layers grown on a thin (5 nm) buffer layer at a high temperature (950 degrees C) exhibited high crystalline quality. Surface morphologies of in-situ thermally etched AIN layers depended on the grain size and distance between grains. Increasing the initial grain size and diminishing the space between grains increased etching depth and width. During re-growth, threading dislocations were bent and annihilated in the vicinity of voids, which were formed by lateral growth of Al-polar AIN regions after thermal etching. Finally, a high quality Al-polar AIN template, as verified by an aqueous KOH solution, was successfully fabricated.

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.