Metalorganic Vapor Phase Epitaxy Growth Research Articles

Optimization of the growth of GaN epitaxial layers in an indigenously developed MOVPE system

Growth of GaN epitaxial layers is optimized in an indigenously developed nitride metal organic vapour phase epitaxy (MOVPE) system. The motivation of present work is to develop a simple MOVPE reactor which is though affordable but can deliver GaN epilayers of desired quality. The design and fabrication methodology of nitride MOVPE system is briefly discussed. MOVPE growth of GaN epilayers is carried out through a two-step growth process where influence of the growth parameters of low temperature buffer layer on the crystalline properties of high temperature GaN epilayer is discussed in detail. Optimum values of the growth temperature, annealing duration and thickness of GaN buffer layer are achieved. High resolution X-ray diffraction measurements (HRXRD) confirmed that the crystalline quality of GaN epitaxial layers is reasonable. A good surface morphology of the optimized sample is strongly corroborated by the results obtained from photoluminescence (PL) and HRXRD techniques. Usefulness of a systematic Williamson-Hall analysis in the optimization of growth of GaN epitaxial layers is demonstrated where a high value of lateral coherence length of GaN buffer layer is found to be a key parameter. Such an optimization process leads to the good crystalline quality of GaN epitaxial layers with low dislocation density. It is found that the high temperature GaN epilayers grown on the optimized GaN buffer layer are compressively stressed which is also revealed by the PL measurements.

Crystalline quality improvement of face-to-face annealed MOVPE-grown AlN on vicinal sapphire substrate with sputtered nucleation layer

Face-to-face annealed metalorganic vapor phase epitaxy (MOVPE)- grown AlN templates with sputtered AlN nucleation layers (FFA MOSp-AlN templates) were fabricated on vicinal sapphire substrates. The sputtered thin AlN layer supplies AlN seeds that are well aligned to the c-axis. The MOVPE growth with a relatively thick AlN enlarges crystal grains with a low concentration of impurities. Face-to-face annealing at high temperatures reduces the twist component of AlN films due to recrystallization, and AlN films with low threading dislocation densities are formed. The full width at half maximum (FWHM) of the (10−12) X-ray rocking curves (XRCs) of AlN templates decreased as the thickness of the MOVPE-grown AlN layer increased. The FWHMs for the (0002) and (10−12) XRCs of the 300-nm-thick AlN templates with an off-cut angle of 0.6° were 9 and 337 arcsec, respectively. The concentrations of O, Si, and C impurities in the FFA MOSp-AlN templates were lower than those in the face-to-face annealed sputtered AlN templates. Si doped Al0.7Ga0.3N films, which are typically used as n-type AlGaN layers for UVC LEDs, were grown on the FFA MOSp-AlN templates with various off-cut angles. The AlGaN films with smooth flat surfaces were obtained using the FFA MOSp-AlN template with larger off-cut angles due to the suppression of the large hillock formation.

MOVPE Growth of AlInP–InGaP Distributed Bragg Reflectors

This article discusses metal–organic vapor phase epitaxy growth of all-phosphide (AlInP–InGaP) distributed Bragg reflectors (DBRs) and their applications for thin flexible multijunction solar cell devices. DBRs with a peak reflectance of 884 nm—close to the GaAs bandgap energy—were grown. This all-phosphide DBR was monolithically integrated with a dual-junction (DJ) InGaP/GaAs solar cell. The DJ cell with a DBR exhibited a 3% increase in short-circuit current density ( J sc), in comparison with the DJ without a DBR. Both open-circuit voltage ( V oc) and fill factor ( FF) values of the solar cell did not change with the DBR integration.

Metal-organic vapor phase epitaxy of the quaternary metastable alloy In1–xGaxAs1−yBiy and its kinetics of growth

The metal–organic vapor phase epitaxy (MOVPE) of the quaternary alloy In1-xGaxAs1-yBiy has been explored on InAs and InP substrates to determine the group III and group V incorporation kinetics and their interdependences during low temperature growth. Under the growth conditions investigated, Ga did not incorporate into the epitaxial InAs1-yBiy on the InAs substrate. In1-xGaxAs1-yBiy epilayers could be grown pseudomorphically on the InP substrate with 0.37 ≤ x ≤ 0.59 and y ≤ 0.022. Strong interactions between the Ga and In incorporation processes were observed in the In1-xGaxAs1-yBiy growth, which were attributed to the precursor gas-phase alkyl-exchange reactions and the formation of Ga-CH3 on the growth front. Bi incorporation is independent of the group III composition and the strain state of the In1-xGaxAs1-yBiy epilayers on the InP substrate. Comparisons between the In1-xGaxAs1-yBiy and the GaAs1-yBiy growth also suggest an absence of Bi surface site-blocking on the In-terminated surface. This study demonstrates the successful MOVPE growth of In1-xGaxAs1-yBiy on the InP substrate and reveals the influences of the gas-phase and surface species interactions on the quaternary alloy compositional control during the low temperature MOVPE growth.

Density Functional Theory Study of the Gas Phase and Surface Reaction Kinetics for the MOVPE Growth of GaAs1-yBiy.

The kinetics of chemical reactions occurring during the metal-organic vapor phase epitaxy (MOVPE) of GaAs1-yBiy have been studied using density functional theory (DFT). GaAs1-yBiy is a metastable semiconductor alloy that has potential applications in high-performance long-wavelength emitters. Its growth is complicated by the low solubility of Bi within the GaAs lattice, which leads to phase segregation under conventional III-V semiconductor growth conditions. In this study, the thermochemical and kinetic parameters of the gas-phase pyrolysis and surface reactions occurring in the MOVPE growth of GaAs1-yBiy from trimethyl bismuth, tertiary butyl arsine, and triethyl gallium are calculated from first-principles electronic structure and vibrational mode calculations. These calculations indicate that the pyrolysis products AsH2 and Bi(CH3)2 are the principle sources for the deposition of their respective metallic elements. The surface-adsorbed methyl species and their interaction with the gas-phase pyrolysis products lead to the self-limiting growth described within this model. The calculated thermochemical and kinetic values provide initial parameters for the development of a microkinetic model of GaAs1-yBiy deposition.

MOVPE growth of AlN films on nano-patterned sapphire substrates with annealed sputtered AlN

AlN films with low-dislocation density and flat surfaces were grown by metalorganic vapor phase epitaxy (MOVPE) on nano-patterned sapphire substrates (NPSSs) with face-to-face annealed sputtered AlN (FFA Sp-AlN) as an initial layer. Dislocation annihilation mechanisms and coalescences of AlN layers were studied by transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray rocking curve (XRC) measurements. From cross-sectional TEM images, the coalescence thickness was obtained to be 1.2 µm. This thickness is thinner than that for MOVPE-grown AlN films on NPSS without FFA Sp-AlN. AFM images revealed that atomically flat surfaces can be obtained from the samples with thicknesses above 3 μm. From XRC full width at half maximum values, the screw- and edge-dislocation densities of the 5-μm-thick AlN film on FFA Sp-AlN/NPSS were calculated to be 4 × 107 cm−2 and 6 × 108 cm−2, respectively. This edge-dislocation density, i.e., the dominant threading dislocation density, is approximately the same to that of AlN films grown on FFA Sp-AlN/flat-sapphire substrates (FSSs). These results show promise that FFA Sp-AlN is applicable not only for FSSs but also for NPSSs as the method for growing high-crystalline-quality AlN film.

Metalorganic vapor phase epitaxy growth and characterization of quaternary (Ga,In)(As,Bi) on GaAs substrates

The incorporation of dilute amounts of Bi into the host lattice of a III/V semiconductor has a strong influence on its electronic properties. The bandgap is strongly redshifted which makes these materials interesting for application in the near- to mid-infrared regime. Furthermore, the spin-orbit splitting is increased resulting in suppression of hot-hole producing Auger recombination, which makes the fabrication of highly efficient optical devices feasible. However, for ternary Ga(As,Bi) grown using metalorganic vapor phase epitaxy (MOVPE), it has proven difficult to achieve the desired composition of the ternary material. Therefore, the additional incorporation of indium (In) into Ga(As,Bi), which should induce a further redshift of the bandgap, is investigated and summarized in this paper. For deposition of quaternary (Ga,In)(As,Bi), two different low temperature growth techniques using MOVPE are conducted. The strain and photoluminescence peak positions of the samples are correlated to estimate the composition of the (Ga,In)(As,Bi) layers. It was found that the trimethylindium and tertiarybutylarsine supplies need to be carefully adjusted to grow high quality bulk materials and that the incorporation of indium is inversely related to the amount of incorporated Bi.

Controlling the orientations of directional sputtered non- and semi-polar GaN/AlN layers

We have studied the impact on the surface orientation of different sputtered Al layers followed by AlN layers sputtered on m-plane sapphire. These initial layers were then overgrown by AlN and followed by GaN using metal-organic vapor phase epitaxy (MOVPE). By increasing sputtering time of the initial Al layer from 0 s to 15 s, we obtained single phase (10-10), (10-13) and (10-14) GaN/AlN layers. The thickness of the initial Al layer was estimated by optical transmission measurements to be about 0.5–1 nm for the (10-13) orientation, and >1 nm for the (10-14) orientation. After MOVPE growth, no trace of metallic Al was found by transmission electron microscopy, indicating that this layer was fully converted to AlN.

Impact of intermediate high temperature annealing on the properties of AlN/sapphire templates grown by metalorganic vapor phase epitaxy

In this work, we investigate AlN/sapphire templates grown by metalorganic vapor phase epitaxy (MOVPE) subjected to an intermediate high temperature annealing (HTA) step at different AlN film thicknesses. To avoid cracking of the MOVPE layers during HTA at 1700 °C these layers have to be grown under low incorporation of tensile strain. For each sample the growth is stopped at a certain thickness between 230 nm and 1.2 μm followed by HTA. X-ray rocking curve FWHM of 0002 and 10–12 reflections lead to an estimation of threading dislocation densities (TDD) as low as cm−2 for the thickest AlN layers of 880 nm and 1.2 μm after annealing. For all layer thicknesses MOVPE growth is subsequently continued to a total AlN layer thickness of 1.5 μm to reach for a smooth surface. The change of the AlN strain state after HTA leads to an increased wafer curvature at room temperature and compressive strain in the subsequently grown AlN layers increasing the wafer bow at room temperature. Hence, to serve as a base layer, e.g. for ultraviolet light emitting diodes, a trade-off between low TDD, surface smoothness and wafer bow has to be found.

Influence of polymerization among Al- and Ga-containing molecules on growth rate and Al content in AlGaN

AlGaN metalorganic vapor-phase epitaxial growth simulation in a TMAl/TMGa/NH3/H2 system was studied to explain its growth rate and Al content. We proposed three kinds of Al- and Ga-containing polymers in this system. By using those polymers, we got good agreements in pressure, TMAl/(TMAl+TMGa) inlet ratio, and temperature dependences of AlGaN growth rate and its Al content between simulations and experiments, even at pressures higher than 40 kPa. Our results showed that the formation of these polymers is enhanced under higher pressures. The simulation considering those polymers could explain the linearity in the TMAl/(TMAl+TMGa) inlet ratio dependences of growth rate and Al content at lower pressure and their non-linearity at higher pressure.

Metalorganic vapor-phase epitaxial growth simulation to realize high-quality and high-In-content InGaN alloys

We studied chemical reactions in the metalorganic vapor-phase epitaxial growth of InGaN from the TMGa/TMIn/NH3/H2/N2 system. It was found that the InGaN growth has three main pathways: pyrolysis of TMGa:NH3 adducts; pyrolysis of TMIn; and pyrolysis of NH3 molecules. The InGaN simulations indicated good agreement with the experiments in temperature-dependences of growth rate and composition by considering atomic In re-evaporation. The simulation result showed that our thinner flow channel method could produce more condensed NH2 and In-atom concentrations compared with conventional technology. Therefore, we could raise InGaN growth temperature with the same In content by the thinner flow channel. Since the higher growth temperature can realize the higher quality of InGaN, we have succeeded to fabricate InGaN LEDs in the range from green to deep red by using the 5-mm flow channel.

Thermodynamic approach of AlGaN MOVPE growth at atmospheric pressure

AlxGa1-xN epilayers were grown on GaN/sapphire substrate by metalorganic vapor-phase epitaxy (MOVPE) at atmospheric pressure. Different trimethylaluminum (TMA) flow rates were used in order to vary the solid aluminum (Al) molar fraction. In situ laser reflectometry shows that the higher the TMA flow, the lower the growth rate. The Al molar fraction was determined by high-resolution X-ray diffraction and energy-dispersive X-ray spectroscopy. In order to explain the evolution of the growth rate and Al fraction, we have developed a complete thermodynamic model in which all possible parasitic reactions between TMA and ammonia (NH3) have been considered. The experimental results are explained by adding a correction factor γ in the definition of the theoretical Al molar fraction. This factor is related to parasitic reactions. Two principal adducts AlCH3.NH and (AlCH3.NH)3 were predicted to form in a temperature range from 300 to 1000 K with a maximum equilibrium partial pressure at 600 K for AlCH3.NH species. These adducts undergo a thermal decomposition and disappear for temperatures above 700 K and 1000 K for (AlCH3.NH)3 and AlCH3.NH, respectively. We conclude that the parasitic reactions would be avoided or minimized if TMA and NH3 are mixed in a reactor region where the temperature is above 1000 K. Thus, the growth rate and Al incorporation would be better controlled in a MOVPE reactor designed such a way that the mixing of TMA and NH3 flows takes place at the latest, in the hottest region.

AlN and AlN/Al2O3 seed layers from atomic layer deposition for epitaxial growth of AlN on sapphire

The annealing of amorphous AlN and AlN/Al2O3 seed layers from atomic layer deposition (ALD) on sapphire substrates and their application as starting layers for metal-organic vapor phase epitaxial growth on sapphire was investigated. During annealing in hydrogen, the amorphous ALD layers become crystalline with epitaxial relation to the underlying sapphire substrate. In contrast to the pure AlN ALD seed layers, mixed layers containing Al2O3 help to avoid the formation of polycrystalline material. Additionally, such mixed ALD seeds support void formation at the AlN/sapphire interface resulting in the formation of a smooth AlN surface. This void formation can be seen in situ during AlN growth in the reflectivity at 405 nm. The tilt and twist component of the AlN grains could be decreased by increasing the annealing time at 1290 °C from 1.5 to 40 min.

Heteroepitaxial growth of α-, β-, γ- and κ-Ga2O3 phases by metalorganic vapor phase epitaxy

Improved MOVPE (metalorganic vapor phase epitaxy) growth conditions for the different Ga2O3 phases are reported. The influence of the substrate material, the growth conditions and the variation of precursors on the phase formation was investigated. Trimethylgallium (TMG) and N2O were used as common precursors for gallium and oxygen, respectively. In addition, water was supplied for the growth of κ-Ga2O3. The influence of additional triethylboron (TEB) and dimethylhydrazin (DMHy) on the formation of the α-phase is discussed. Growth conditions could be determined, where α-, β-, γ- and κ-Ga2O3 is epitaxially deposited. The κ-phase was epitaxially grown on c-, a- and r-plane Al2O3, (1 1 1) MgO, (1 1 1) MgAl2O4 and on a (0 0 0 1) Al2O3/(0 0 0 1) GaN heterostructure. Successful MOVPE growth of γ-Ga2O3 with defect spinel structure has been achieved on (0 0 0 1) Al2O3, (1 1 1) and (1 0 0) MgAl2O4, (1 0 0) MgO as well as on (0 0 0 1) Al2O3/(0 0 0 1) GaN. Boron addition stabilizes the MOVPE growth of the α-phase on c-, a- and r-plane Al2O3. The various Ga2O3 phases were identified by X-ray diffraction (XRD). The epitaxial relationships between the Ga2O3 phases and the different substrates were determined.Optical transmission spectra measured in the spectral range from 200 to 2000 nm show a well-defined absorption edge at about 4.9 eV for layers of β-, γ- and κ-Ga2O3 and 5.3 eV for layers of α-Ga2O3 grown on c-plane sapphire.

Fabrication of AlN templates on SiC substrates by sputtering-deposition and high-temperature annealing

High-quality AlN templates fabricated by sputtering-deposition and post-deposition high-temperature annealing have great potential for deep ultraviolet light-emitting device applications. In this work, we fabricated AlN films on 6H-SiC substrates by sputtering and face-to-face annealing and characterized the structural quality of the AlN films before and after annealing. As reported in previous studies, to accomplish high-quality AlN films on SiC substrates using conventional methods, such as molecular beam epitaxy or metalorganic vapor phase epitaxy (MOVPE), it is important to grow the AlN on the SiC coherently. However, in this work, although the annealed AlN films were fully relaxed from the SiC substrates, or even had tensile strain, the AlN films indicated high crystallinity. The X-ray rocking curve full width at half maximum (XRC-FWHM) values of the 200-nm-thick annealed AlN film were 17 and 246 arcsec for the AlN (0002) and (10–12) diffraction, respectively. Though the annealed AlN film indicated rough surfaces with bunched step structures, the surface morphology was remarkably improved by MOVPE growth and clear atomic step-and-terrace structures were formed. The XRC-FWHM values of the MOVPE-grown AlN were 90 and 239 arcsec for the AlN (0002) and (10–12) diffraction, respectively.

Wafer-scale MOVPE growth and characterization of highly ordered h-BN on patterned sapphire substrates

Layered h-BN were grown on sub-micron size dome patterned sapphire substrates (PSS) by metal organic vapor phase epitaxy (MOVPE) to study the effects of substrate orientations, strain and subsequent thermal stress evolution. Electron micrograph images on the surface of the c-plane sapphire in-between the dome structures show typical wrinkled morphology of layered h-BN, similar to the reference sample surface confirming the high quality h-BN growth. However, the h-BN morphology on the domes exhibited pattern like folding which were drastically different from the h-BN on the c-plane sapphire. Detailed TEM studies show that the h -BN is layered and covers the dome patterns of the sapphire substrates like a “blanket of snow” continuously with localized strain variations induced turbostratic phase insertions. In addition, the local variation in optical emission properties of layered materials, which are dramatically affected by the rippled morphology were also investigated using depth resolved cathodoluminescence. The intensity variation of the 320 nm peak confirms that the BN on dome structures have more strain related defects than the c-plane h-BN. Thus, these results give better insight on the origin of turbostratic BN grains and local control over the structural and optical emission characteristics.

Absolute surface energies of semipolar planes of AlN during metalorganic vapor phase epitaxy growth

We propose a new approach for calculating absolute surface energies of semipolar planes of AlN under metalorganic vapor phase epitaxy (MOVPE) growth conditions. The calculations using wedge-shape geometries demonstrate that the stable surface orientation depends on Al chemical potential. The calculated absolute surface energies of semipolar AlN112¯2 and 11¯01 are lower than that of N-polar AlN0001¯ surface under Al-rich condition. On the other hand, the surface energy of N-polar AlN0001¯ is lower than those semipolar surfaces under N-rich condition. The results on the orientation dependence of surface stability could provide valuable insights for the epitaxial growth conditions of AlN along semipolar orientations during the MOVPE growth.

Bandlike and localized states of extended defects in n-type In0.53Ga0.47As

In0.53Ga0.47As p + n diodes with different densities of extended defects have been analyzed by detailed structural and electrical characterization. The defects have been introduced during Metal-Organic Vapor Phase Epitaxy (MOVPE) growth by using a lattice-mismatched layer on a semi-insulating InP or GaAs substrate. The residual strain and indium content in the n-type In0.53Ga0.47As layer have been determined by high-resolution X-ray diffraction, showing nearly zero strain and a fixed indium ratio of 0.53. The deep levels in the layer have been characterized by Deep Level Transient Spectroscopy. The mean value of electron traps at 0.17 ± 0.03 eV below the conduction band minimum EC is assigned to the “localized” states of α 60° misfit dislocations; another broad electron trap with mean activation energies between EC− 0.17 ± 0.01 and 0.39 ± 0.04 eV, is identified as threading dislocation segments with “band-like” states. A high variation of the pre-exponential factor KT by 7 orders of magnitude is found for the latter when changing the filling pulse time, which can be explained by the coexistence of acceptor-like and donor-like states in the core of split dislocations in III-V materials. Furthermore, two hole traps at EV+ 0.42 ± 0.01 and EV+ 0.26 ± 0.13 eV are related to the double acceptor of the Ga(In) vacancy (VGa/In3-/2-) and 60° β misfit dislocations, respectively. Finally, the dislocation climbing mechanism and the evolution of the antisite defects AsGa/In are discussed for n-type In0.53Ga0.47As.

Metalorganic vapor phase epitaxial growth of AlGaN directly on reactive-ion etching-treated GaN surfaces to prepare AlGaN/GaN heterostructures with high electron mobility (∼1500 cm2 V−1 s−1): Impacts of reactive-ion etching-damaged layer removal

In this paper, we report the impact of the HCl treatment of reactive-ion-etching-treated GaN (RIE-treated GaN) surfaces in fabricating AlGaN/GaN structures by regrowing an AlGaN layer directly on the RIE-treated GaN surfaces. By dipping RIE-treated GaN surfaces into a hot HCl solution, a 10–30-nm-thick surface layer, which corresponded to the damaged layer, was etched off. After the annealing of the HCl-treated GaN in NH3 + H2 atmosphere, a 30-nm-thick Al30Ga70N layer was grown on it. The HCl treatment resulted in a decrease in dotlike defect density observed on the AlGaN/GaN surfaces and a marked improvement of electrical data of AlGaN/GaN structures. A clear dependence of electron mobility on sheet carrier concentration was observed, and the highest electron mobility obtained was increased to ∼1500 cm2 V−1 s−1. High-electron-mobility transistors (HEMTs) with excellent performances were successfully fabricated by employing the HCl treatment and metal–insulator–semiconductor (MIS) gate structures.

GaAsSb layer thickness dependence of arsenic incorporation on InAs/GaAsSb superlattice on InAs substrate grown by metalorganic vapor phase epitaxy for mid-infrared device

We investigated the gas flow sequence to realize a short-period and lattice-matched InAs/GaAsSb superlattice on an InAs substrate by metalorganic vapor phase epitaxy (MOVPE) growth for applications in mid-infrared photonic devices. The arsenic composition of GaAsSb for lattice matching was adjusted by adding AsH3 flow to residual arsenic incorporation. The growth conditions for lattice-matched superlattices at a period of 5 nm were found. The superlattices with period thicknesses of 1 to 9 nm were successfully confirmed by the photoluminescence (PL) peak in the range from 2.4 to 5 µm depending on layer thickness at 20 K. The emission wavelength can be controlled by changing the layer thickness of superlattices. The results led to the improvement of InAs/GaAsSb type II superlattice growth.