Metalorganic Vapor Phase Epitaxy Research Articles

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.

Thickness and Mg doping of graded AlGaN layers: Influence on contact layer's structural and electrical properties for DUV emitters

One of the methods to improve p-doping properties in (Al)GaN-based materials is using a technique called polarization doping. In our study, two sets of p-AlGaN contact layers with different thicknesses of the graded layer were deposited by metalorganic vapor phase epitaxy. The grown samples were investigated by atomic force microscopy, secondary ion mass spectrometry, X-ray diffraction and Hall effect measurements to study the influence of the thickness of the graded Al0.6→0Ga0.4→1N:Mg layer on the structural and the electrical properties of the whole grown structure. Additionally, theoretical calculations for the contact structures with different thicknesses of the p-Al0.6→0Ga0.4→1N graded layer were performed. The obtained results show that the concentration of p-type carriers on the p-GaN/p-AlGaN gradient interface decreases with increasing thickness of the graded layer. Our findings reveal that among the graded structures the highest concentration of holes, 1.2 × 1018 cm−3, was obtained for a structure with 50-nm-thick graded layer, while maintaining low sample resistivity. In addition, it was also shown that non-doping of the graded layer allows to improve electrical properties.

Dielectric function and interband critical points of compressively strained ferroelectric K0.85Na0.15NbO3 thin film with monoclinic and orthorhombic symmetry

The dielectric function and interband critical points of compressively strained ferroelectric K0.85Na0.15NbO3 thin film grown by metal-organic vapor phase epitaxy (MOVPE) are studied in broad spectral and temperature ranges by spectroscopic ellipsometry (SE). The temperature dependence of the measured pseudodielectric functions is strongly affected by a structural phase transition from the monoclinic Mc-phase to the orthorhombic c-phase at about 428 K. Using a parametric optical constant model, the corresponding dielectric functions as well as the interband optical transitions of the film are determined in the spectral range of 0.73–6.00 eV. Standard critical point (SCP) analysis of the 2nd derivatives of the dielectric functions identified three and four critical points for monoclinic and orthorhombic symmetries, respectively. A systematic redshift of the threshold energies with increasing temperatures was observed.

Nonvolatile memory operations using intersubband transitions in GaN/AlN resonant tunneling diodes grown on Si(111) substrates

Nonvolatile memory using intersubband transitions and quantum-well electron accumulation in GaN/AlN resonant tunneling diodes (RTDs) is a promising candidate for high-speed nonvolatile memory operating on a picosecond timescale. This memory has been fabricated on sapphire(0001) substrates to date because of the high affinity between the nitride materials and the substrate. However, the fabrication of this memory on Si(111) substrates is attractive to realize hybrid integration with Si devices and nonvolatile memory and three-dimensional integration such as chip-on-wafer and wafer-on-wafer. In this study, GaN/AlN RTDs are fabricated on a Si(111) substrate using metal-organic vapor phase epitaxy. The large strain caused by the differences in the thermal expansion coefficients and lattice constants between the Si(111) substrate and nitride materials are suppressed by a growth technique based on the insertion of low-temperature-grown AlGaN and thin AlN layers. The GaN/AlN RTDs fabricated on Si(111) substrates show clear GaN/AlN heterointerfaces and a high ON/OFF ratio of >220, which are equivalent to those for devices fabricated on sapphire(0001) substrates. However, the nonvolatile memory characteristics fluctuate by repeated write/erase memory operations. Evaluation of the ON/OFF switching time and endurance characteristics indicates that the instability of the nonvolatile memory characteristics is caused by electron leakage through deep levels in the quantum-well structure. Possible methods for suppressing this are discussed with an aim of realizing high-speed and high-endurance nonvolatile memory.

Investigation of carbon-related complexes in highly C-doped GaN grown by metalorganic vapor phase epitaxy

We investigated the C-related complexes in highly C-doped GaN by electron spin resonance (ESR) spectroscopy, Fourier transform IR spectroscopy (FTIR), and minority carrier transient spectroscopy (MCTS) measurements. In the ESR spectra, two resonances with g values of 2.02 and 2.04 were found to be assigned by (0/−) deep acceptor and (+/0) charge transition levels of carbon substituting for nitrogen site (CN). In the FTIR spectra, two local vibrational modes positioned at 1679 and 1718 cm−1 were confirmed to be associated with tri-carbon complexes of CN–CGa–CN (basal) and CN–CGa–CN (axial), respectively. In the MCTS spectra, we observed the hole trap level of E v + 0.25 ± 0.1 eV associated with the tri-carbon complexes, which are the dominant C-related defects, suggesting that these complexes affect the electronic properties in the highly C-doped GaN.

Influencing the morphological stability of MOVPE-grown β-Ga2O3 films by O2/Ga ratio

The morphology evolution of (100) β-Ga2O3 films grown by metalorganic vapor phase epitaxy (MOVPE) is studied by atomic force microscopy (AFM). In particular, when grown under a high O2/Ga ratio (O2/Ga = 1250) and above a thickness of 350 nm, these films exhibit striking morphological instabilities, including step meandering and bunching, which contribute significantly to surface roughening. Transmission electronic microscopy (TEM) measurements reveal a morphological transition on the growing surface with a coexistence of step-flow and step-bunching growth modes resulting from these instabilities. In contrast, growth conditions with low O2/Ga ratios (O2/Ga = 350) effectively suppress meandering and bunching instabilities, resulting in films with excellent electrical properties. These observations are rationalized through the Burton-Cabrera-Frank (BCF) theory and Bales-Zangwill (BZ) instability (Gibbs-Thompson effect and Mullins-Sekerka instability) , thus bridging theoretical models and the experimental results.

Reduction in density of interface traps determined by C-V analysis in III-nitride-based MOSHFET structure

An in situ metal-organic chemical vapor phase epitaxy is used to grow a complete AlGaN/GaN metal oxide semiconductor heterojunction field effect transistor (MOSHFET) structure, gated by a gallium oxide (Ga2O3) layer; we observed reduction in the interfacial trap density compared to its version wherein the Ga2O3 was grown ex situ, after breaking the vacuum, all else being the same. A remarkable decrease in the interfacial charge density for in situ MOSHFET structures in the range of 70%–88% for 10–30 nm oxide layer thickness and improvements in other electrical parameters required for high-performing devices were observed.

Low‐Pressure, Modified Halide Vapor‐Phase Epitaxy for Chemically Pure GaN Epilayers

The halide vapor‐phase epitaxy (HVPE) technique has been extensively used for the production of freestanding GaN crystals for GaN wafer manufacturing, but has had renewed interest in recent years as a technique for producing high‐quality GaN epilayers at high growth rates and without the issues of carbon incorporation which complicate the growth of GaN using metal–organic vapor phase epitaxy. Herein, thick epilayers grown by HVPE using a low‐pressure modified quartz reactor with controlled H2 injection are presented which exhibit surface root‐mean‐square roughnesses <0.5 nm with low background doping concentrations <2E15 cm−3 and room‐temperature mobilities >1200 cm2 V−1 s−1. Such films are considered to be strong contenders for the production of lightly doped GaN drift layers for vertical power electronics devices.

Review on ultrahigh growth rate GaAs solar cells by metalorganic vapor-phase epitaxy

The aim of this review paper is to summarize a decade of research focused on enhancing metalorganic vapor-phase epitaxy (MOVPE) growth rates of GaAs, driven by the imperative for most cost-effective and energy-efficient III–V compounds’ production. While MOVPE is renowned for producing high-quality devices, it has been constrained by production cost. For example, MOVPE was traditionally thought to have moderate growth rates that limit the throughput of the cost-intensive reactors. Recent research endeavors, however, have demonstrated ultrafast growth rates, exceeding 280 μm/h, with a remarkable group III precursor utilization efficiency of over 50%. It is worth noting that even with increased growth rates, the surface quality remains unaffected in terms of roughness and morphology. Nonetheless, optoelectronic properties, such as minority carrier lifetime, deteriorate for both p- and n-doped materials under constant growth conditions. This is attributed to an increase in the defect density of arsenic antisites, particularly EL2 and HM1 defects, as revealed by deep-level transient spectroscopy investigations. Some of these losses can be mitigated by optimizing growth conditions, such as elevating the temperature and reducing the V/III ratio. The latter not only restores some of the material quality but also increases the growth rate and reduces precursor consumption. Still, fully recovering the original reference lifetimes remains a challenge. Solar cell results indicate that structures with predominantly n-type absorbers are less affected by reduced minority carrier lifetimes. A remarkable 24.5% efficiency was achieved in a GaAs single-junction solar cell grown at 120 μm/h, representing less than 1 min of growth time for the absorber layers.

Size control of InP nanowires by in situ annealing and its application to the formation of InAsP quantum dots

We carried out in situ annealing of InP nanowires (NWs) in a metal-organic vapor phase epitaxial (MOVPE) growth reactor to control and reduce the tip size of InP NWs. InP NWs were grown by selective-area (SA) MOVPE on partially masked (111)A InP substrates, and annealing was successively applied in tertiarybutylphosphine (TBP) ambient. Initially, the InP NWs had a hexagonal cross-section with {112¯} facets vertical to the substrates; they became tapered, and the edges were rounded by annealing. By appropriately selecting the annealing temperature and initial NW diameter, the tip size of the NW was reduced and NWs with a tip size of 20 nm were successfully formed. Subsequently, a thin InAsP layer was grown on the annealed NWs and their photoluminescence was investigated at low temperatures. The characterization results indicated the formation of InAsP quantum dots (QDs) emitting in the telecom band. Our approach is useful for reducing the size of the NWs and for the controlled formation of InAsP QDs embedded in InP NWs in photonic devices compatible with telecom bands.

Kilovolt-class β-Ga2O3 MOSFETs on 1-in. bulk substrates

We report on the demonstration of β-Ga2O3 MOSFETs fabricated on 1-in. bulk substrates using metalorganic vapor phase epitaxy (MOVPE) with disilane (Si2H6) as the silicon precursor. Sheet charge uniformity of the as-grown films was measured via Hall and ranged from 5.9–6.7 × 1012 cm−2 with a uniform Hall mobility of 125–129 cm2/V s across the sample. MOSFET devices with a source-drain length of 5.1 μm were measured across the wafer and had a minimum on-resistance (RON) of 47.87 Ω mm with a maximum on-current (ION) of 165 mA/mm. For these same devices, the on-current (ION) and pinch-off voltage (VP) uniformity across the wafer were 137 ± 12 mA/mm and −27.3 ± 7.3 V, respectively. Devices showed low reverse leakage current until catastrophic breakdown occurred, with measured breakdown voltages (VBR) of up to 2.15 kV. This work provides valuable insights into understanding the growth, fabrication, and characterization processes for β-Ga2O3 FETs on full wafer-scale substrates. It also projects the promise of developing lateral β-Ga2O3 FETs with high current-carrying capabilities and breakdown voltages, especially on substrates of 1 in. or larger.

Strain modulation of epitaxial h-BN on sapphire: the role of wrinkle formation for large-area two-dimensional materials.

Strain built-in electronic and optoelectronic devices can influence their properties and lifetime. This effect is particularly significant at the interface between two-dimensional materials and substrates. One such material is epitaxial hexagonal boron nitride (h-BN), which is grown at temperatures often exceeding 1000 °C. Due to the high growth temperature, h-BN based devices operating at room temperature can be strongly affected by strain generated during cooling due to the differences in lattice thermal expansion of h-BN and the substrate. Here, we present results of temperature-dependent Raman studies of the in-plane E2ghighphonon mode in the temperature range of 300-1100 K measured for h-BN grown by metalorganic vapor phase epitaxy. We observe a change, by an order of magnitude, in the rate of the temperature-induced frequency shift for temperatures below 900 K, indicating a strong reduction of the effective h-BN/substrate interaction. We attribute this behavior to the creation of h-BN wrinkles which results in strain relaxation. This interpretation is supported by the observation that no change of layer/substrate interaction and no wrinkles are observed for delaminated h-BN films transferred onto silicon. Our findings demonstrate that wrinkle formation is an inherent process for two-dimensional materials on foreign substrates that has to be understood to allow for the successful engineering of devices based on epitaxially grown van der Waals heterostructures.

Origin of the parasitic luminescence of 235 nm UVC LEDs grown on different AlN templates

AlN layers annealed at high temperatures offer low threading dislocation densities of mid 108 cm−2 and are therefore increasingly used as base layers in ultraviolet (UV) light emitting diode (LED) heterostructure growth. These LEDs, just like those grown on conventional metalorganic vapor phase epitaxy (MOVPE) AlN templates, often suffer from long-wavelength parasitic luminescence. In this work, luminescence properties of far-UVC LED heterostructures grown on MOVPE-AlN/sapphire templates and high-temperature annealed AlN/sapphire templates are compared. To investigate the origin of parasitic emission with high spatial resolution, cross section scanning transmission electron microscopy was combined with cathodoluminescence measurements. As a result, the main origin of the parasitic luminescence band centered at 3.5 eV (354 nm) for the heterostructure grown on annealed AlN is assigned to point defects related to oxygen in the AlN template layer. The defect band centered at 3.0 eV (413 nm) for the heterostructure grown on MOVPE-AlN was found to be related to self-compensating VAl-Si point defect complexes in the n-AlGaN layer and oxygen incorporation close to the AlN/sapphire interface. The results also suggest that the type of AlN template determines the kind of parasitic luminescence from the n-AlGaN layer.

Defects in layered boron nitride grown by Metal Organic Vapor Phase Epitaxy: luminescence and positron annihilation studies

Defects in two-dimensional boron nitride (BN) are candidates for a manifold of applications, for example, as single-photon emitters or optically addressable spin defects. However, the topic has proven to be complex and despite extensive research efforts, knowledge on midgap defects and their signature in photoluminescence (PL) remains fragmentary. Mass production of high-quality layered BN by Metal Organic Vapor Phase Epitaxy (MOVPE) is possible and effective, thus understanding the conditions of formation and identifying defects that arise in the material grown by this method is extremely important. Here, we present an analysis of the influence of the MOVPE growth conditions of layered BN on the occurrence of specific structural defects. PL spectra in the visible range were analysed involving the Huang-Rhys theory implemented in the fitting procedure for polycrystalline samples grown under various conditions. Such an approach allowed us to separate bands from individual entities. Two bands at 1.57 eV and 2.6 eV with phonon replicas were observed. The three additional bands are characterized by a broad emission centered at 1.38 eV, 1.9 eV and 2.24 eV. Further properties, including an analysis of the surface morphology, the impact of annealing and laser bleaching on the PL spectra and Positron Annihilation Spectroscopy (PAS) were considered to draw conclusions about the nature of the examined defects. The PAS analysis correlated with the PL of the BN layers allows us to classify the studied samples in terms of the concentrations of boron vacancies and their complexes. Based on the obtained results, we propose that the rich family of PL bands observed in various sp2-BN layers is associated with different defect complexes build of boron vacancies (VB), boron antisite defects (BN), substitutional carbon defects (CB, CN) and their donor-acceptor pairs. The main value of this paper is the transfer of knowledge gained about defects and the corresponding luminescent bands to characterize high-quality continuous epitaxial sp2-BN layers. The presented results constitute an important step toward the deterministic manipulation of defect concentrations in two-dimensional boron nitride, which opens up new pathways for future optoelectronic device application.

Ab initio study for adsorption behavior on AlN(0001) surface with steps and kinks during metal-organic vapor-phase epitaxy

We present our systematic theoretical study by performing ab initio calculations to clarify the behavior of adsorption for constituent atoms such as Al and N on a vicinal AlN(0001) surface with step edges and kinks during metal-organic vapor-phase epitaxy (MOVPE). The calculations reveal that the surface reconstruction affects the adsorption of Al and N adatoms near the kinks and step edges. Furthermore, we find the incorporation of an Al adatom at the kink and that of N adatoms not only at the kink but also in the terrace regions. The calculated results give some insights for an atomic-scale understanding of the step-flow growth during the MOVPE growth of AlN.

Optimization of high power AlGaInP laser diodes at 626 nm

The rapid advancement of 9Be+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{9}\\hbox {Be}^{+}$$\\end{document} ion-based quantum computing is creating a high demand for scalable and specialized laser sources. For this purpose, laser radiation at 313 nm is necessary which is generated by frequency doubling of 626 nm laser emission. Laser sources in this spectral region lack simplicity and need to be miniaturized. We carry out a systematic improvement of room temperature semiconductor laser structures emitting at 626 nm. They are based on a GaInP single quantum well embedded in AlGaInP grown on GaAs. We first investigate the structure theoretically and determine its limits and optima. We select the three most promising quantum well combinations, which are grown by metal-organic vapor-phase epitaxy. Furthermore, we fabricate broad-area lasers with dimensions of 1600 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu$$\\end{document}m ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document} 100 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu$$\\end{document}m. All structures show laser operation around 626 nm. We are able to achieve a threshold current density of 721 A/cm2, a slope of 0.384 mW/mA and total maximum output power of 708 mW under pulsed excitation. Based on these results, we believe that quantum computing focusing on beryllium ions will highly benefit from these results.

Structural and Photoelectronic Properties of κ-Ga2O3 Thin Films Grown on Polycrystalline Diamond Substrates.

Orthorhombic κ-Ga2O3 thin films were grown for the first time on polycrystalline diamond free-standing substrates by metal-organic vapor phase epitaxy at a temperature of 650 °C. Structural, morphological, electrical, and photoelectronic properties of the obtained heterostructures were evaluated by optical microscopy, X-ray diffraction, current-voltage measurements, and spectral photoconductivity, respectively. Results show that a very slow cooling, performed at low pressure (100 mbar) under a controlled He flow soon after the growth process, is mandatory to improve the quality of the κ-Ga2O3 epitaxial thin film, ensuring a good adhesion to the diamond substrate, an optimal morphology, and a lower density of electrically active defects. This paves the way for the future development of novel hybrid architectures for UV and ionizing radiation detection, exploiting the unique features of gallium oxide and diamond as wide-bandgap semiconductors.

In-situ spectral reflectance investigation of hetero-epitaxially grown β-Ga2O3 thin films on c-plane Al2O3 via MOVPE process

Metalorganic vapor-phase epitaxy of β-Ga2O3/c-plane Al2O3 heterostructures was monitored in-situ by spectral reflectance in different wavelengths. The reflectance spectrum was analysed as a function of the growth time and the incident wavelength to estimate the growth rate and the refractive index at the growth temperatures. The obtained values are validated by ex-situ methods such as secondary ion mass spectrum measurement and spectroscopic ellipsometry. A theoretical simulation of the reflectance spectrum was carried out by combining a transfer matrix method with a multilayer model, and a good agreement with the experimental results is presented.

(Invited) Mulitjunction III-V Semiconductors for Photo-Electrochemical Hydrogen Production: Recent Progress in Efficiency, Durability, and Cost

Photo-electrochemical (PEC) hydrogen production is a route to carbon-free fuel that combines light absorption and water electrolysis into one device. Challenging targets for solar-to-hydrogen (STH) efficiency, durability, and semiconductor absorber costs must be realized for this approach to be economically competitive with other hydrogen production pathways. Research on PEC hydrogen production at the National Renewable Energy Laboratory (NREL) has focused on III-V semiconductor absorber materials because their tunable bandgaps, high photon conversion efficiencies, and ability to make multi-junction structures have resulted in the highest STH efficiencies yet demonstrated. This talk will focus on recent NREL results showing progress toward PEC hydrogen production efficiency, durability, and cost targets. Specifically, we have been able to improve theoretically achievable STH efficiency through bandgap engineering in multijunction devices. Photon management strategies to close the gap between theoretical and measured STH efficiency will also be discussed. Durability remains a significant challenge for III-V materials in a PEC environment. We will describe modifications to the semiconductor surface and the electrolyte constitution that can make the photoabsorber less susceptible to photocorrosion during operation. Finally, we will present preliminary results on III-V PEC materials synthesized via hydride vapor phase epitaxy, an alternative route to high-efficiency photoelectrodes that could potentially have much lower costs than the metalorganic vapor phase epitaxy method traditionally used. These improvements have yielded III-V PEC devices that are close to the STH efficiency target (17% vs. 25%) while the demonstrated durability and cost are still considerably far from their target values.

Impacts of off-angle and off-direction on surface morphology of GaN grown by metalorganic vapor phase epitaxy on (0001) GaN substrate

Impacts of off-angle and off-direction on surface morphology of GaN grown by metalorganic vapor phase epitaxy on (0001) GaN substrate