Metalorganic Vapor Phase Epitaxy Research Articles

Perspectives on the Development of Metalorganic Vapor Phase Epitaxy for III-V Optoelectronic Devices

The mainstream production of optoelectronic and electronic devices based on epitaxial III-V compound semiconductor materials largely employs metalorganic vapor phase epitaxy, a process first demonstrated for devices in the 1970’s. For this special issue in honor of Prof. P. Daniel Dapkus, this paper first briefly reviews his early contributions that spearheaded the interest of MOVPE as a III-V device epitaxy process. Next, the paper discusses MOVPE reactor development in the 1980’s, which at the time was one of the key challenges in establishing MOVPE technology as a reliable epitaxy manufacturing process. The approach combined physical modeling and numerical simulations of reactor processes followed by materials and device demonstrations. Highly uniform performance metrics of AlGaAs/AlGaAs quantum-well diode lasers and two-dimensional monolithic surface-emitting lasers were demonstrated with excellent run-to-run reproducibility. This foundational approach advanced efforts to standardize reactors and subsequent scaling for high-throughput production.

Metal organic vapor phase epitaxy of high-indium-composition InGaN quantum dots towards red micro-LEDs

Micro-scale light-emitting diodes (micro-LEDs) are regarded as the next generation display technology. Compared to blue and green ones, InGaN-based red micro-LEDs require higher indium composition in their active region, which is quite challenging for material growth. Here, high-indium-composition InGaN quantum dots (QDs) with a density of 3 × 1010 cm-2 are self-assembly grown by metal-organic vapor phase epitaxy (MOVPE) based on a precursor-alternate-admittance method. The growth mechanism is systematically studied, and consequently a 613-nm red QDs sample with an internal quantum efficiency (IQE) of 12% is demonstrated. Furthermore, when micro-LEDs based on these red InGaN QDs with a chip size of 1-20 µm are fabricated, an electroluminescence blueshift to yellow and green is observed. The 20-µm and 1-µm micro-LEDs show 4.92% and 1.78% external quantum efficiency (EQE) at 0.3 and 20 A/cm2, respectively. By introducing multiple quantum wells (MQWs) pre-strained layer beneath the QD layers, a 10-µm micro-LED with 638 nm emission wavelength is demonstrated, with a price of reduced EQE to 0.03% at 10 A/cm2.

Si doping mechanism in MOVPE-grown (100) β-Ga2O3 films

A Langmuir adsorption model of the Si incorporation mechanism into metalorganic vapor-phase epitaxy grown (100) β-Ga2O3 thin films is proposed in terms of the competitive surface adsorption process between Si and Ga atoms. The outcome of the model can describe the major feature of the doping process and indicate a growth rate-dependent doping behavior, which is validated experimentally and further generalized to different growth conditions and different substrate orientations.

Understanding the Anisotropy in the Electrical Conductivity of CuPtB-type Ordered GaInP Thin Films by Combining In Situ TEM Biasing and First Principles Calculations.

In this work, the effect of CuPtB ordering on the optoelectronic properties of Ga0.5In0.5P is studied by combining in situ transmission electron microscopy measurements and density functional theory (DFT) calculations. GaInP layers were grown by metal organic vapor phase epitaxy with a CuPtB single-variant-induced ordering due to the intentional misorientation of the Ge(001) substrate. Moreover, the degree of order was controlled using Sb as the surfactant without changing other growth parameters. The presence of antiphase ordered domain boundaries (APDBs) between the ordered domains is studied as a function of the order parameter. The in situ electrical measurements on a set of samples with controlled degree of order evidence a clear anisotropic electrical conductivity at the nanoscale between the [110] and [1–10] orientations, which is discussed in terms of the presence of APDBs as a function of the degree of order. Additionally, DFT calculations allow to determine the differences in the optoelectronic properties of the compound with and without ordering through the determination of the dielectric function. Finally, the anisotropy of the electrical conductivity for the ordered case is also discussed in terms of the effective mass calculated from the band structure on specific k-paths. By comparing the experimental measurements and the theoretical calculations, two factors have been presented as the main contributors of the electric conductivity anisotropy of CuPtB-type ordered GaInP thin films: antiphase boundaries that separate domains with uniform order (APDBs) and the anisotropy of the effective mass due to the alternating of In/Ga rich planes.

Influence of interface states on built-in electric field and diamagnetic-Landau energy shifts in asymmetric modulation-doped InGaAs/GaAs QWs

The impact of interface defect states on the recombination and transport properties of charges in asymmetric modulation-doped InGaAs/GaAs quantum wells (QWs) is investigated. Three sets of high-mobility InGaAs QW structures are systematically designed and grown by the metal-organic vapor phase epitaxy technique to probe the effect of carrier localization on the electro-optical processes. In these structures, a built-in electric field drifts electrons and holes towards the opposite hetero-junctions of the QW, where their capture/recapture processes are assessed by temperature-dependent photoreflectance, photoluminescence, and photoconductivity measurements. The strength of the electric field in the structures is estimated from the Franz Keldysh oscillations observed in the photoreflectance spectra. The effects of the charge carrier localization at the interfaces lead to a reduction of the net electric field at a low temperature. Given this, the magnetic field is used to re-distribute the charge carriers and help in suppressing the effect of interface defect states, which results in a simultaneous increase in luminescence and photoconductivity signals. The in-plane confinement of charge carriers in QW by the applied magnetic field is therefore used to compensate the localization effects caused due to the built-in electric field. Subsequently, it is proposed that under the presence of large interface defect states, a magnetic field-driven diamagnetic-Landau shift can be used to estimate the fundamental parameters of charge carriers from the magneto-photoconductivity spectra instead of magneto-photoluminescence spectra. The present investigation would be beneficial for the development of high mobility optoelectronic and spin photonic devices in the field of nano-technology.

Improving the performance of GaInP solar cells through rapid thermal annealing and delta doping

We show that the effect of rapid thermal annealing (RTA) on carrier lifetime in GaInP grown by molecular beam epitaxy depends strongly on both doping type and density, and that these disparities must be accounted for to realize high-performance GaInP solar cells. Although the photoluminescence intensity and lifetime of lightly doped p- and n-GaInP improved greatly with RTA, heavily doped n + -GaInP showed sharp degradation upon RTA, preventing the realization of GaInP front-junction solar cells with low emitter sheet resistance. Since a low series resistance is important to achieve high fill factor (FF), we designed a front-junction cell utilizing a thin, lightly doped n-type emitter with delta doping to enhance conductivity, attaining an open-circuit voltage (V OC ) of 1.40 V and FF of 86%. We then designed rear-heterojunction solar cells to further leverage the relatively long lifetime of lightly n-doped GaInP (∼19 ns). With the help of delta doping in the n-AlInP window to improve surface passivation, we attained a V OC of 1.42 V, similar to cells grown by metal-organic vapor phase epitaxy. • Rapid thermal annealing greatly improves luminescence of lightly doped n- and p-GaInP but degrades heavily doped n + -GaInP. • We demonstrate front-junction cells with V OC = 1.40 V, the highest value for cells grown by molecular beam epitaxy (MBE). • We present the first rear-junction GaInP solar cells grown by MBE with V OC of 1.42 V; delta doping the window improves surface passivation.

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.