Metalorganic Vapor Phase Epitaxy Technique Research Articles

FeGa (0/−) acceptor level as a reference energy level in dilute AlxGa1−xN

Laplace deep-level transient spectroscopy and photoluminescence have been used to demonstrate that the FeGa (0/−) acceptor level in dilute AlxGa1−xN (x ≤ 0.063) can be considered as a common reference level as expected for energy levels of transition metals in isovalent semiconductor compounds. Furthermore, the conduction and valence band offsets (ΔEC and ΔEV, respectively) in GaN/AlxGa1−xN heterojunctions, as a function of Al content for samples grown by the metalorganic vapor-phase epitaxy technique on native Ammono-GaN substrates, have been found. The band-offsets determined in this study are ΔEC = 1.17x eV and ΔEV = −0.95x eV over the range of x studied and are in good agreement with other experimental results reported for actual GaN/AlxGa1−xN heterojunctions as well as with the recent theoretical calculations based on hybrid density functional theory. Moreover, we confirmed that the band bowing effect related to compositional dependence in AlxGa1−xN is accommodated practically only in the conduction band as suggested by theoretical calculations.

The effect of FeGa (0/–) level presence on material properties in dilute AlxGa1−xN layers

AlxGa1−xN epilayers are used as the basis of ultraviolet LEDs and detectors. The trap states produced by defects and impurities can play a key role in the device performance. In this work, conventional deep-level transient spectroscopy, photoluminescence (PL), and secondary ion mass spectrometry have been used to characterize a deep-level trap termed as E3 in dilute AlxGa1−xN (x < 0.063) epilayers grown by metal-organic vapor phase epitaxy (MOVPE) on highly conductive ammono-GaN substrates. The AlxGa1−xN epilayers were doped with silicon to about 3 × 1016 cm−3. The electrical and the optical measurements were conducted on Ni/Au Schottky barrier diodes and virgin samples, respectively. First, we observed a general trend that the E3 (FeGa) electron trap concentration significantly changes along the wafers in AlxGa1−xN layers that is fully consistent with previously reported results for GaN materials grown by the MOVPE technique. Second, we report that the activation energies for electron emission for the E1 and E3 traps in dilute AlxGa1−xN exhibit linear variations with Al content. Moreover, low-temperature PL results show a proportional relation between the intensity of the line with its maximum at 1.299 eV and concentration of residual Fe impurity. Finally, we discuss how the presence of defects resulting from Fe contamination may result in degradation of AlxGa1−xN-based devices.

Potential of La-Doped SrTiO3 Thin Films Grown by Metal-Organic Vapor Phase Epitaxy for Thermoelectric Applications.

La-doped SrTiO3 thin films with high structural quality were homoepitaxially grown by the metal-organic vapor phase epitaxy (MOVPE) technique. Thermogravimetric characterization of the metal-organic precursors determines suitable flash evaporator temperatures for transferring the liquid source materials in the gas phase of the reactor chamber. An adjustment of the charge carrier concentration in the films, which is necessary for optimizing the thermoelectric power factor, was performed by introducing a defined amount of the metal-organic compound La(tmhd)3 and tetraglyme to the liquid precursor solution. X-ray diffraction and atomic force microscopy verified the occurrence of the pure perovskite phase exhibiting a high structural quality for all La concentrations. The electrical conductivity of the films obtained from Hall-effect measurements increases linearly with the La concentration in the gas phase, which is attributed to the incorporation of La3+ ions on the Sr2+ perovskite sites by substitution inferred from photoemission spectroscopy. The resulting structural defects were discussed concerning the formation of occasional Ruddlesden-Popper-like defects. The thermoelectric properties determined by Seebeck measurements demonstrate the high potential of SrTiO3 thin films grown by MOVPE for thermoelectric applications.

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.

Optical and structural properties of In-rich InxGa1−xAs epitaxial layers on (1 0 0) InP for SWIR detectors

In-rich InxGa1−xAs epitaxial layers were grown on InP (100) substrates by a metalorganic vapor phase epitaxy (MOVPE) technique. The effect of Indium (In) composition on the crystalline quality and optical properties are investigated. High resolution X-ray diffraction (HR-XRD) measurement and Raman scattering spectrum are used to evaluate the crystalline quality, the residual strain and dislocation density property. The number of dislocations in the epitaxial layers is found to increase by increasing the Indium content in order to release the stresses due to the epitaxial clamping. Photoluminescence (PL) measurement is used to characterize the optical properties. At 10 K, PL measurements show that the InGaAs band gap redshifts with the indium content. Moreover, the asymmetry at the low-energy side of the PL peak has been attributed to the presence of localized excitons. In all samples, a blue shift of PL peaks is evidenced by increasing the excitation power density, which is in line with the presence of carrier’s localization and non-idealities in this system. Moreover, the temperature-dependence of the PL peak energy displays an unusual red-blue-red shift (S-shaped) behavior when raising the temperature. These observations can be related to the inhomogeneous distribution of indium which gives rise to the appearance of dislocations and other defects which serve as traps for charge carriers. Interestingly, those highly In-content InxGa1−xAs epitaxial layers show PL emission located between 1637 and 1811 nm (depending on In content) and thus might be suitable for in the design of novel heterostructure devices such as short wave infrared (SWIR) detectors.

Strain distribution in wrinkled hBN films

Sapphire-supported hBN films of different thicknesses are grown using metalorganic vapour phase epitaxy technique by following a flow modulation scheme. Though these wafer-scale films are potential candidates for real device applications, they exhibit wrinkling. The wrinkles are a key signature of strain distribution in the films. We utilized Raman imaging to study the residual strain distribution in the wrinkled hBN films. An increase in the overall compressive strain in the films with an increase in the layer thickness is observed and explained. An empirical relation is proposed for estimating the wrinkle mediated strain relaxation from the morphology of the films. Furthermore, we show that the residual strain can be partially released by the delamination of the films.

Features of carrier confinement in InAsSb(P) alloys and quantum wells for room-temperature operating mid-infrared emitters

In this communication, we focus on the optical properties of InAsSbP/InAsSb/InAsSbP single quantum well structures grown on InAs substrates using the metal-organic vapor phase epitaxy (MOVPE) technique. The obtained Fourier transformed photoluminescence spectra allowed us to perform an advanced analysis of the temperature evolution of the measured spectra, which revealed radiative recombination at room temperature in the mid-infrared spectral range (∼3.5 μm) associated with localized states in the quantum well. These results suggest that improvements in the quality of the InAsSbP/InAsSb heterointerfaces could open the way to new types of heterostructures based on nanoscale systems for the fabrication of cheap mid-infrared lasers and light-emitting diodes, which are demanded by the optical gas sensing market.

Strong exciton–photon coupling and polariton lasing in GaN microrod

We report on the studies of strong exciton photon coupling and polariton lasing at room temperature from single GaN microrods grown by metal-organic vapor phase epitaxy technique. Emission spectra were recorded at room temperature by exciting GaN microrods with pulsed Nd:YAG laser of 355 nm wavelength at different excitation power density. Clear mode formation corresponding to Whispering gallery modes in GaN microrod cavity has been observed. Strong exciton–photon coupling as well as large Rabi splitting energy of maximum 400 ± 40 meV has been observed from the microrods. This is attributed to the formation of new quasi-particles named polaritons with much lighter mass than free electron mass. Among those polariton modes, one of the modes increases super linearly with the increase in excitation power density which is an indication of polariton lasing.

Raman Spectra of Thick Epitaxial GaN Layers Formed on SiC by the Sublimation Sandwich Method

The Raman spectra of thick (~100 μm and more) GaN layers grown on crystalline SiC substrates by the sublimation sandwich method are studied. Good agreement between the spectra of the SiC substrates used in the study and published data indicates that the measurements made in the study are reliable. The minimum difference between the results of the measurements and published evidence for GaN layers means that the layers grown by the sublimation sandwich method in the study compare well with those fabricated by the metalorganic vapor phase epitaxy (MOVPE) or chloride-hydride vapor phase epitaxy (CHVPE) techniques.

Спектры рамановского рассеяния толстых эпитаксиальных слоев GaN на SiC, полученных сублимационным сандвич-методом

AbstractThe Raman spectra of thick (~100 μm and more) GaN layers grown on crystalline SiC substrates by the sublimation sandwich method are studied. Good agreement between the spectra of the SiC substrates used in the study and published data indicates that the measurements made in the study are reliable. The minimum difference between the results of the measurements and published evidence for GaN layers means that the layers grown by the sublimation sandwich method in the study compare well with those fabricated by the metalorganic vapor phase epitaxy (MOVPE) or chloride-hydride vapor phase epitaxy (CHVPE) techniques.

Obtaining solid solutions of InGaAsPInGaAsP solid solutions in the spinoidal decomposition region

Technology for growing layers of InGaAsP solid solutions with Eg ∼⃒ 1.0 - 1.2eV on InP substrates in the spinodal decomposition region by the metalorganic vapour phase epitaxy (MOVPE) technique has been developed. Results of investigation of the obtained solid solution layers by the methods of photoluminescence, reflectance anisotropy spectroscopy and fractal analysis are presented.

Increasing the quantum efficiency of InAs/GaAs QD arrays for solar cells grown by MOVPE without using strain‐balance technology

AbstractResearch into the formation of InAs quantum dots (QDs) in GaAs using the metalorganic vapor phase epitaxy technique is presented. This technique is deemed to be cheaper than the more often used and studied molecular beam epitaxy. The best conditions for obtaining a high photoluminescence response, indicating a good material quality, have been found among a wide range of possibilities. Solar cells with an excellent quantum efficiency have been obtained, with a sub‐bandgap photo‐response of 0.07 mA/cm2 per QD layer, the highest achieved so far with the InAs/GaAs system, proving the potential of this technology to be able to increase the efficiency of lattice‐matched multi‐junction solar cells and contributing to a better understanding of QD technology toward the achievement of practical intermediate‐band solar cells. Copyright © 2016 John Wiley & Sons, Ltd.

Demonstration of Sn-seeded GaSb homo- and GaAs–GaSb heterostructural nanowires

The particle-assisted epitaxial growth of antimonide-based nanowires has mainly been realized using gold as the seed material. However, the Au-seeded epitaxial growth of antimonide-based nanowires such as GaSb nanowires presents several challenges such as for example direct nucleation issues and crystal structure tuning. Therefore, it is of great importance to understand the role of seed material choice and properties in the growth behavior of antimonide-based nanowires to obtain a deeper understanding and a better control on their formation processes. In this report, we have investigated the epitaxial growth of GaSb and GaAs–GaSb nanowires using in situ-formed tin seeds by means of metalorganic vapor phase epitaxy technique. This comprehensive report covers the growth of in situ-formed tin seeds and Sn-seeded GaSb nanowires on both GaAs and GaSb (111)B substrates, as well as GaAs–GaSb nanowires on GaAs (111)B substrates. The growth behavior and structural properties of the obtained GaSb nanowires are further investigated and compared with the Au-seeded counterparts. The results provided by this study demonstrate that Sn is a promising seed material for the growth of GaSb nanowires.

Photovoltaic converters of concentrated sunlight, based on InGaAsP(1.0 eV)/InP heterostructures

Results obtained in the development of a metal-organic vapor-phase epitaxy (MOVPE) technology and in studies of the electrical parameters of photovoltaic converters based on heterostructures in the InP/InGaAsP system for the spectral range 0.95–1.2 μm are presented. A MOVPE technique is developed for obtaining and doping InGaAsP solid solutions around the spinodal dissociation region with a band-gap width of about 1.0 eV. Photovoltaic converters of 1000-× concentrated sunlight are fabricated.

GaN/AlN superlattice high electron mobility transistor heterostructures on GaN/Si(111)

In this work we report the growth of AlGaN/GaN heterostructures on silicon substrates using quasi AlGaN barrier layer formed by 6×(AlN/GaN) superlattice (SL) stack with nominal Al content of 35%. This is the first attempt of implementation of quasi AlGaN barrier layer in AlGaN/GaN heterostructure, originally proposed by Kawakami, on the Si(111) substrates by metalorganic vapor phase epitaxy (MOVPE) technique. The electrical properties of high‐electron mobility transistors (HEMT) type SL structures and conventional Al0.28Ga0.72N/GaN heterostructures grown on sapphire and silicon substrates were investigated and compared. The obtained results showed deterioration of channel resistivity in the heterostructure with SLs (1430 Ω/sq) in comparison to the alloy Al0.28Ga0.72N/GaN heterostructure (751 Ω/sq). Additional, surface and chemical structure characterizations were done by atomic force microscopy (AFM) and secondary ion mass spectrometry (SIMS) techniques. The roughness parameter Ra, estimated based on AFM scans of investigated structures was between 0.5 and 1 nm. The SIMS depth profiles of the alloy AlGaN and SL quasi AlGaN barrier did not show any significant differences between the samples deposited on sapphire and silicon substrates. The room temperature photoluminescence spectra of HEMT type SL structures revealed peaks corresponding to GaN and AlGaN band transitions, which indicates lack of sharp interfaces between AlN and GaN layers and thus formation of AlGaN transition regions in AlN/GaN SL stack.

Nucleus and spiral growth mechanisms of nitride semiconductors in metalorganic vapor phase epitaxy

Nucleus and spiral growth mechanisms of GaN and InN are investigated using the selective-area metalorganic vapor phase epitaxy (SA-MOVPE) technique on GaN bulk substrates. Nucleus growth of GaN occurs within selective areas having no screw-type dislocations, while spiral growth occurs within selective areas having screw-type dislocations. These growth modes are simultaneously observed on a single substrate in a single growth run. The nucleus and spiral growths of GaN result in the formation of step-free surfaces and growth spirals, respectively, wherein the interstep distance of growth spirals enables us to estimate the degree of surface supersaturation (σ). The σ dependences of nucleus and spiral growth rates of GaN are experimentally investigated. We found that these dependences are well explained by the classical crystal growth theories advocated by Burton, Cabrera, and Frank. We also investigate nucleation of InN using step-free GaN surfaces as an ideal platform.

Controlling morphology and optical properties of self-catalyzed, mask-free GaN rods and nanorods by metal-organic vapor phase epitaxy

A simple self-catalyzed and mask-free approach will be presented to grow GaN rods and nanorods based on the metal-organic vapor phase epitaxy technique. The growth parameter dependent adjustment of the morphology of the structures will be discussed. Rods and nanorods with diameters reaching from a few μm down to 100 nm, heights up to 48 μm, and densities up to 8⋅107 cm–2 are all vertically aligned with respect to the sample surface and exhibiting a hexagonal shape with smooth sidewall facets. Optical properties of GaN nanorods were determined using cathodoluminescence. It will be shown that the optical properties can be improved just by reducing the Ga precursor flow. Furthermore, for regular hexagonal shaped rods and nanorods, whispering gallery modes with quality factors up to 500 were observed by cathodoluminescence pointing out high morphological quality of the structures. Structural investigations using transmission electron microscopy show that larger GaN nanorods (diameter > 500 nm) contain threading dislocations in the bottom part and vertical inversion domain boundaries, which separate a Ga-polar core from a N-polar shell. In contrast, small GaN nanorods (∼200 nm) are largely free of such extended defects. Finally, evidence for a self-catalyzed, Ga-induced vapor-liquid-solid growth will be discussed.

Proposal and Analysis of Distributed Reflector-Laser Diode Integrated with an Electroabsorption Modulator

A novel integrated laser, that is, a distributed reflector laser diode integrated with an electroabsorption modulator, is proposed to improve the output efficiency, single-mode stability, and chirp. The proposed laser can be realized using the selective metalorganic vapor phase epitaxy technique (that is, control of the width of the insulating mask), and its fabrication process is almost the same as the conventional electroabsorption modulated laser (EML) process except for the asymmetric coupling coefficient structure along the cavity. For our analysis, an accurate time-domain transfer-matrix-based laser model is developed. Based on this model, we perform steady-state and large-signal analyses. The performances of the proposed laser, such as the output power, extinction ratio, and chirp, are compared with those of the EML. Under 10-Gbps NRZ modulation, we can obtain a 30% higher output power and about 50% lower chirp than the conventional EML. In particular, the simulation results show that the chirp provided by the proposed laser can appear to have a longer wavelength side at the leading edge of the pulse and a shorter wavelength side at the falling edge.

RBS/Channeling and TEM Study of Damage Buildup in Ion Bombarded GaN

A systematic study on structural defect buildup in 320 keV Ar-ion bombarded GaN epitaxial layers has been reported, by varying ion fluences ranged from 5 × 10 to 1 × 10 at./cm. 1 μm thick GaN epitaxial layers were grown on sapphire substrates using the metal-organic vapor phase epitaxy technique. Rutherford backscattering/channeling with 1.7 MeVHe beam was applied for analysis. As a complementary method high resolution transmission electron microscopy has been used. The later has revealed the presence of extended defects like dislocations, faulted loops and stacking faults. New version of the Monte Carlo simulation code McChasy has been developed that makes it possible to analyze such defects on the basis of the bent channel model. Damage accumulation curves for two distinct types of defects, i.e. randomly displaced atoms and extended defects (i.e. bent channel) have been determined. They were evaluated in the frame of the multistep damage accumulation model, allowing numerical parameterization of defect transformations occurring upon ion bombardment. Displaced atoms buildup is a three-step process for GaN, whereas extended defect buildup is always a two-step process.

Growth temperature influence on the GaN nanowires grown by MOVPE technique

GaN nanowires (NWs) were successfully grown by Vapor-Liquid-Solid (VLS) growth mechanism on GaN template using metal-organic vapor phase epitaxy (MOVPE) with diameters ranging from 20 to 200 nm and length up to few microns. The characterization by scanning electron microscopy (SEM) reveals an optimum growth temperature at 790°C and X-ray diffraction (XRD) investigations indicates oriented crystallinity of grown NWs.