Metalorganic Vapor Phase Epitaxy Research Articles

Enhancement of two-dimensional electron gas mobility using strain reduced AlGaN barriers grown by metalorganic vapor phase epitaxy under nitrogen atmosphere

We demonstrated high-electron-mobility transistors (HEMTs) with enhanced two-dimensional electron gas (2DEG) mobility using a low-strain AlGaN barrier grown by metalorganic vapor phase epitaxy under a nitrogen atmosphere. We investigated the effects of the growth temperature under a nitrogen atmosphere on the electrical properties of AlGaN-HEMT structures, focusing on 2DEG mobility. At growth temperatures below 855 °C, the 2DEG mobility decreased with decreasing growth temperature owing to an increase in the threading dislocation density. However, at growth temperatures above 855 °C, the 2DEG mobility decreased with increasing growth temperature. This finding was attributed to the compressive strain in the GaN channel, which increased with increasing growth temperature owing to the increased tensile strain in the AlGaN barriers. We concluded that temperatures around 855 °C are suitable for AlGaN barrier growth under nitrogen atmosphere. Finally, we achieved the highest 2DEG mobility of 2182 cm2 V−1 s−1 with a low sheet resistance of 406 Ω sq−1. using an Al0.41Ga0.59N barrier.

In-situ and ex-situ face-to-face annealing of epitaxial AlN

AlN films have been deposited on c-plane sapphire substrates by metalorganic-vapor-phase-epitaxy (MOVPE). The changes in the film structure have been investigated by applying different annealing processes which are ex-situ rapid thermal annealing (RTA) and in-situ process after the nucleation-layer (NL). The AlN nucleation-layer grown on sapphire has been annealed face-to-face with ex-situ (RTA) process for 3 min and with in-situ process for 3 h, then pulsed-atomic-layer-epitaxy AlN film has been grown at a high temperature. The samples have been characterized by high-resolution X-ray diffraction, atomic force microscopy, Ultraviolet–visible spectrometry, and Raman scattering to examine the structural properties, surface morphology, and optical properties. The sample annealed with the ex-situ (RTA) process, where rapid diffusion took place, has reached larger grain sizes and the dislocation density has decreased as the grain boundary decreased. Although better crystal quality has been obtained with the ex-situ (RTA) process, it has been observed that the surface roughness of the sample annealed with the ex-situ (RTA) process is higher than that of the sample annealed with the in-situ process. Considering the results, a schematic prediction of the growth process after face-to-face annealing has been proposed. Experimental findings have shown that different annealing processes after growing the AlN-NL have a great effect on the properties of the AlN.

Structural characterization of ELO-GaN film on mask-stripe patterned GaAs (0 0 1) substrate grown by metalorganic vapor phase epitaxy

GaN films were epitaxial laterally overgrown on the [110]-stripe-patterned GaAs (001) substrate by MOVPE. AlGaN intermediate layers and low-temperature GaN buffer layer were inserted in order to prevent structural defects. The epitaxial lateral overgrowth of GaN results in a trapezoidal shape with flat facets. Structural phases and defects distribution were studied by TEM. Selective-area diffraction pattern indicates the mixed phase of GaN and a high density of stacking faults at nucleation. However, with the deposition of dielectric SiNx masks, a lateral overgrowth resulted in nearly free of defects, and the cubic phase transformed into hexagonal despite a cubic template. Moreover, dark-field TEM images were used to demonstrate the structural phase transformation.

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

The influence of TMGa pre-flow time and amount as surfactant on the structural and optical properties of AlN epilayer

AIN is used as a template layer for deep UV (DUV) emitter and detector applications, because of its wide bandgap and high thermal conductivity. In this study, trimethylgallium (TMGa) source is used as surfactant to improve crystal quality and decrease dislocation density (DD) of AlN layers grown on sapphire (Al2O3) substrate surfaces by Metal Organic Vapor Phase Epitaxy (MOVPE) system. TMGa pre-flow time and pre-flow amount that TMGa pre-flow to the nucleation stage are the subjects of two distinct optimization studies. The structural and optical properties of grown AIN are examined by a high resolution X-ray diffractometer (HR-XRD), Raman spectroscopy, and UV–Vis–NIR spectrophotometer, respectively. TMGa pre-flow time and TMGa pre-flow amount determined to obtain high crystal quality AlN epilayers are 2 s and 0.446 × 10−5 mol/min, respectively. HR-XRD investigation of these growths yields FWHM values of 159/2718 arcsec and 201/1550 arcsec for the ω (002) and ω (102) scans, respectively.

Investigation of a nanostructured GaP/MoS2 p-n heterojunction photodiode

We report on the properties of a GaP/MoS2 heterojunction prepared on a nanocone (NC)-structured GaP substrate and a planar GaP substrate. The nanocone-structured GaP substrate was prepared by the growth of GaP NCs at gold seeds on a ⟨111⟩B GaP substrate at 650 °C by metal organic vapor phase epitaxy. At this growth temperature, most NCs exhibited a hexagonal symmetry with six heavily facetted sides that contained numerous facets, ledges, and edges with a large surface area. A thin Mo layer was deposited on both types of GaP substrates by direct current magnetron sputtering. The Mo layer was then sulfurated at 700 °C and turned into a MoS2 layer. Electrical and optical characterization gave evidence that a PN heterojunction formed between GaP and MoS2 during the sulfuration process. The spectral response measurement showed two separate regions between 400 and 550 nm linked with the generation of carriers in GaP and between 550 and 1100 nm associated with the generation of carriers in the MoS2 layer. The planar GaP/MoS2 heterojunction generated a lower photocurrent compared with the GaP/MoS2 heterojunction that formed on the nanocone-structured GaP substrate. The results support theoretical assumptions that edge rich substrates can help to increase the quality of deposited 2D materials.

Models for Impurity Incorporation during Vapor-Phase Epitaxy

Impurity incorporation during vapor-phase epitaxy on stepped surfaces was modeled by classifying rate-limiting processes into i) surface diffusion, ii) step kinetics, and iii) segregation. Examples were shown for i) desorption-limited Al incorporation during chemical vapor deposition (CVD) of (0001) SiC, ii) preferential desorption of C atoms from kinks during CVD of Al-doped (000-1) SiC, and iii) segregation-limited C incorporation during metalorganic vapor-phase epitaxy of (0001), (000-1), and (10-10) GaN.

Control of Morphology and Substrate Etching in InAs/InP Droplet Epitaxy Quantum Dots for Single and Entangled Photon Emitters.

We present a detailed atomic-resolution study of morphology and substrate etching mechanism in InAs/InP droplet epitaxy quantum dots (QDs) grown by metal–organic vapor phase epitaxy via cross-sectional scanning tunneling microscopy (X-STM). Two different etching processes are observed depending on the crystallization temperature: local drilling and long-range etching. In local drilling occurring at temperatures of ≤500 °C, the In droplet locally liquefies the InP underneath and the P atoms can easily diffuse out of the droplet to the edges. During crystallization, the As atoms diffuse into the droplet and crystallize at the solid–liquid interface, forming an InAs etch pit underneath the QD. In long-range etching, occurring at higher temperatures of >500 °C, the InP layer is destabilized and the In atoms from the surroundings migrate toward the droplet. The P atoms can easily escape from the surface into the vacuum, forming trenches around the QD. We show for the first time the formation of trenches and long-range etching in InAs/InP QDs with atomic resolution. Both etching processes can be suppressed by growing a thin layer of InGaAs prior to the droplet deposition. The QD composition is estimated by finite element modeling in combination with X-STM. The change in the morphology of QDs due to etching can strongly influence the fine structure splitting. Therefore, the current atomic-resolution study sheds light on the morphology and etching behavior as a function of crystallization temperature and provides a valuable insight into the formation of InAs/InP droplet epitaxy QDs which have potential applications in quantum information technologies.

Demonstration of Acceptor-Like Traps at Positive Polarization Interfaces in Ga-Polar P-type (AlGaN/AlN)/GaN Superlattices

The shortcomings with acceptors in p-type III-nitride semiconductors have resulted in not many efforts being presented on III-nitride based p-channel electronic devices (here, field effect transistors (FETs)). The polarization effects in III-nitride superlattices (SLs) lead to the periodic oscillation of the energy bands, exhibiting enhanced ionization of the deep acceptors (Mg in this study), and hence their use in III-nitride semiconductor-based light-emitting diodes (LEDs) and p-channel FETs is beneficial. This study experimentally demonstrates the presence of acceptor-like traps at the positive polarization interfaces acting as the primary source of holes in Ga-polar p-type uniformly doped (AlGaN/AlN)/GaN SLs with limited Mg doping. The observed concentration of holes exceeding that of the dopants incorporated into the samples during growth can be attributed to the ionization of acceptor-like traps, located at 0.8 eV above the valence band of GaN, at positive polarization interfaces. All samples were grown using the metal organic vapor phase epitaxy (MOVPE) technique, and the materials’ characterization was carried out using X-ray diffraction and Hall effect measurements. The hole concentrations experimentally measured are juxtaposed with the calculated value of hole concentrations from FETIS®, and the measured trends in mobility are explained using the amplitude of separation of the two-dimensional hole gas in the systems from the positive polarization interfaces.

Machine learning supported analysis of MOVPE grown β-Ga2O3 thin films on sapphire

In this work, we demonstrate a machine learning approach, Random Forest, for the β-Ga2O3 growth rate prediction in the metal–organic vapor phase epitaxy (MOVPE) by analyzing the growth process of β-Ga2O3 on sapphire optically. The proposed model can assess the complex non-linear dependencies among the growth parameters and optimize them for the optimal growth rate. The model based on the process parameters (e.g., precursor concentration, chamber pressure, and push gas) provides high predictive power, reaching the coefficient of determination (R2) of 0.95 and 0.92 for the training and testing sets. The outcome of the model is applicable to both homoepitaxial and heteroepitaxial processes and on different substrate orientations.

Optical and nano-mechanical characterization of c-axis oriented AlN film

This paper reports the temperature effects on the optical properties of metalorganic vapour-phase epitaxy (MOCVD) grown c-axis oriented AlN epilayer thin film studied by in-situ high-temperature spectroscopic ellipsometry. The crystal structure and the quality of the grown AlN epilayer film are analyzed using X-ray Diffraction and rocking curve techniques, respectively. Modelling of the ellipsometric data revealed that the uniaxial anisotropic refractive indices of the c-axis oriented film in the directions n‖ and n⊥ increased from 2.50 to 2.59 and 2.32 to 2.37, respectively with the increase in temperature from 223 to 573 K. The thermo-optic coefficients were evaluated to be around 10−5. Nano-mechanical characterization of this film showed an average hardness of 19.4 GPa at ambient temperature, which is higher than a-axis oriented AlN film. The average surface free energy of the synthesized film as evaluated from contact angle measurements is reported to be around 36.22 ± 0.64 mN/m. These results are highly relevant for a better understanding of c-axis oriented AlN-based materials in high-temperature ultraviolet optical devices.

Microstructural analysis of N-polar InGaN directly grown on a ScAlMgO4(0001) substrate

We report the characterization of a N-polar InGaN layer deposited by metalorganic vapor-phase epitaxy on a ScAlMgO4(0001) (SAM) substrate without a low-temperature buffer layer. The InGaN layer was tensile-strained, and its stoichiometry corresponded to In0.13Ga0.87N. We also present the microstructural observation of the InGaN/SAM interface via integrated differential phase contrast-scanning transmission electron microscopy. The results show that the interface between N-polar InGaN and SAM occurs between the O atoms of the O–Sc SAM surface and the (Ga,In) atoms of InGaN.

High temperature phase transitions in NaNbO3 epitaxial films grown under tensile lattice strain

We have investigated high temperature phase transitions in NaNbO3 thin films epitaxially grown under tensile lattice strain on (110) DyScO3 substrates using metal-organic vapor phase epitaxy. At room temperature, a very regular stripe domain pattern consisting of the monoclinic a1a2 ferroelectric phase was observed. Temperature-dependent studies of the refractive index and the optical bandgap as well as in situ high-resolution x-ray diffraction measurements prove a ferroelectric–ferroelectric phase transition in the range between 250 and 300 °C. The experimental results strongly suggest that the high-temperature phase exhibits a distorted orthorhombic a1/a2 crystal symmetry, with the electric polarization vector lying exclusively in the plane. A second phase transition was observed at about 500 °C, which presumably signifies the transition to the paraelectric phase. Both phase transitions show a pronounced temperature-dependent hysteresis, indicating first-order phase transitions.

Reduction of parasitic reaction in high-temperature AlN growth by jet stream gas flow metal–organic vapor phase epitaxy

AlGaN-based deep ultraviolet light-emitting diodes (LEDs) have a wide range of applications such as medical diagnostics, gas sensing, and water sterilization. Metal–organic vapor phase epitaxy (MOVPE) method is used for the growth of all-in-one structures, including doped layer and thin multilayers, using metal–organic and gas source raw materials for semiconductor devices. For AlN growth with high crystalline quality, high temperature is necessary to promote the surface migration of Al atoms and Al-free radicals. However, increase in temperature generates parasitic gas-phase prereactions such as adduct formation. In this work, AlN growth at 1500 °C by a stable vapor phase reaction has been achieved by jet stream gas flow metal–organic vapor phase epitaxy. The AlN growth rate increases with gas flow velocity and saturates at ~ 10 m/s at room temperature. Moreover, it is constant at an ammonia flow rate at a V/III ratio from 50 to 220. These results demonstrate the reduction in adduct formation, which is a typical issue with the vapor phase reaction between triethylaluminum and ammonia. The developed method provides the in-plane uniformity of AlN thickness within 5%, a low concentration of unintentionally doped impurities, smooth surface, and decrease in dislocation density because of the suppression of parasitic reactions.

Point defect localization and cathodoluminescence emission in undoped ε-Ga2O3

In this study, experimental and theoretical investigations have been performed on nominally undoped ϵ-Ga2O3 films deposited on (0001)-Al2O3 substrates by metal-organic vapor phase epitaxy using different O and Ga precursor ratios. Hydrogen and helium were used as carrier gas. Low-temperature cathodoluminescence (CL) broad emissions extending over the range 1.5–3.4 eV were deconvoluted in five peaks, whose position, integrated intensity, and full width at half maximum were investigated in the temperatures range 80 K–300 K. A non-monotonic behavior of the extracted CL peaks is observed, which is attributed to localization phenomena connected with families of point defects. The behavior of two main luminescence emissions with temperature has been simulated using the localized state ensemble model. The derived parameters agree with the experimental observations and provide a new interpretation of micro-and macroscale disorder inside ϵ-Ga2O3 and related potential fluctuations.

Study of Size, Shape, and Etch pit formation in InAs/InP Droplet Epitaxy Quantum Dots

We investigated metal-organic vapor phase epitaxy grown droplet epitaxy (DE) and Stranski–Krastanov (SK) InAs/InP quantum dots (QDs) by cross-sectional scanning tunneling microscopy (X-STM). We present an atomic-scale comparison of structural characteristics of QDs grown by both growth methods proving that the DE yields more uniform and shape-symmetric QDs. Both DE and SKQDs are found to be truncated pyramid-shaped with a large and sharp top facet. We report the formation of localized etch pits for the first time in InAs/InP DEQDs with atomic resolution. We discuss the droplet etching mechanism in detail to understand the formation of etch pits underneath the DEQDs. A summary of the effect of etch pit size and position on fine structure splitting (FSS) is provided via the k · p theory. Finite element (FE) simulations are performed to fit the experimental outward relaxation and lattice constant profiles of the cleaved QDs. The composition of QDs is estimated to be pure InAs obtained by combining both FE simulations and X-STM results. The preferential formation of {136} and {122} side facets was observed for the DEQDs. The formation of a DE wetting layer from As-P surface exchange is compared with the standard SKQDs wetting layer. The detailed structural characterization performed in this work provides valuable feedback for further growth optimization to obtain QDs with even lower FSS for applications in quantum technology.

Coherent Hole Transport in Selective Area Grown Ge Nanowire Networks.

Holes in germanium nanowires have emerged as a realistic platform for quantum computing based on spin qubit logic. On top of the large spin–orbit coupling that allows fast qubit operation, nanowire geometry and orientation can be tuned to cancel out charge noise and hyperfine interaction. Here, we demonstrate a scalable approach to synthesize and organize Ge nanowires on silicon (100)-oriented substrates. Germanium nanowire networks are obtained by selectively growing on nanopatterned slits in a metalorganic vapor phase epitaxy system. Low-temperature electronic transport measurements are performed on nanowire Hall bar devices revealing high hole doping of ∼1018 cm–3 and mean free path of ∼10 nm. Quantum diffusive transport phenomena, universal conductance fluctuations, and weak antilocalization are revealed through magneto transport measurements yielding a coherence and a spin–orbit length of the order of 100 and 10 nm, respectively.

Room-temperature polariton lasing in GaN microrods with large Rabi splitting.

Room-temperature polariton lasing is achieved in GaN microrods grown by metal-organic vapor phase epitaxy. We demonstrate a large Rabi splitting (Ω = 2g0) up to 162 meV, exceeding the results from both the state-of-the-art nitride-based planar microcavities and previously reported GaN microrods. An ultra-low threshold of 1.8 kW/cm2 is observed by power-dependent photoluminescence spectra, with the linewidth down to 1.31 meV and the blue shift up to 17.8 meV. This large Rabi splitting distinguishes our coherent light emission from a conventional photon lasing, which strongly supports the preparation of coherent light sources in integrated optical circuits and the study of exciting phenomena in macroscopic quantum states.

In situ control of indium incorporation in (AlGa)1−xInxP layers

Metalorganic vapor phase epitaxy growth of red emitting laser structures based on (AlGa)1-xInxP (AlGaInP) compounds requires thorough control of In incorporation during growth of AlGaInP layers which heavily depends on growth parameters and reactor state. We established an in situ method to determine the In incorporation in AlGaInP layers during growth without any prior calibration based on ex situ characterization. This requires to model the lattice constants of substrate and growing layer and feed it with the in situ measured wafer temperature, growth rate and wafer curvature. The model makes use of the Stoney equation and Vegard’s law to calculate the In content of AlGaInP layers grown on GaAs substrates. Correlation of in situ results to ex situ results from X-ray diffraction confirmed the accuracy of our method. Our in situ method allowed us to study the influence of dopants on growth of AlGaInP layers. At high temperature Si doping from Si2H6 showed no influence on the In incorporation during growth, however, Zn doping from DMZn showed strong impact on In incorporation and growth rate. In particular, increasing the DMZn/III ratio during AlGaInP growth linearly decreases In incorporation and growth rate. Accordingly, the presence of Zn on the growth surface hinders In incorporation and causes enhanced In re-evaporation from the surface at high temperatures.

263 nm wavelength UV-C LED on face-to-face annealed sputter-deposited AlN with low screw- and mixed-type dislocation densities

Regarding deep-ultraviolet optical device applications, face-to-face annealed sputter-deposited AlN (FFA Sp-AlN) is a promising alternative to the conventional metalorganic vapor phase epitaxy (MOVPE)-prepared AlN templates on sapphire substrates. However, FFA Sp-AlN tends to exhibit AlGaN growth-related hillock generation and surface morphology deterioration. In this study, we optimized the sputter-deposition conditions for AlN and MOVPE growth conditions for AlGaN to respectively reduce hillock density and size. After confirming AlGaN surface-flattening, we fabricated 263 nm wavelength UV-C LEDs on the FFA Sp-AlN and achieved maximum external quantum efficiencies of approximately 4.9% and 8.0% without and with silicone encapsulation, respectively.