Chemical Vapor Epitaxy Research Articles

On the polarity determination and polarity inversion in nitrogen-polar group III-nitride layers grown on SiC

We investigate the interfaces and polarity domains at the atomic scale in epitaxial AlN and GaN/AlN grown by hot-wall metal organic chemical vapor epitaxy on the carbon face of SiC. X-ray diffraction, potassium hydroxide (KOH) wet chemical etching, and scanning transmission electron microscopy combined provide an in-depth understanding of polarity evolution with the film thickness, which is crucial to optimize growth. The AlN grown in a 3D mode is found to exhibit N-polar pyramid-type structures at the AlN–SiC interface. However, a mixed N-polar and Al-polar region with Al-polarity domination along with inverted pyramid-type structures evolve with increasing film thickness. We identify inclined inversion domain boundaries and propose that incorporation of oxygen on the ⟨40–41⟩ facets of the N-polar pyramids causes the polarity inversion. We find that mixed-polar AlN is common and easily etched and remains undetected by solely relying on KOH etching. Atomic scale electron microscopy is, therefore, needed to accurately determine the polarity. The polarity of GaN grown on mixed-polar AlN is further shown to undergo complex evolution with the film thickness, which is discussed in the light of growth mechanisms and polarity determination methods.

Controllable Epitaxial Growth of Large-Area MoS2 /WS2 Vertical Heterostructures by Confined-Space Chemical Vapor Deposition.

The controllable large-area growth of single-crystal vertical heterostructures based on 2D transition metal dichalcogenides (TMDs) remains a challenge. Here, large-area vertical MoS2 /WS2 heterostructures are synthesized using single-step confined-space chemical vapor epitaxy. The heterostructures can evolve into two different kinds by switching the H2 flow on and off: MoS2 /WS2 heterostructures with multiple WS2 domains can be achieved without introducing the H2 flow due to the numerous nucleation centers on the bottom MoS2 monolayer during the transition stage between the MoS2 and WS2 monolayer growth. In contrast, isolated MoS2 /WS2 heterostructures with single WS2 domain can be obtained with introducing the H2 flow due to the reduced nucleation centers on the bottom MoS2 monolayer arising from the hydrogen etching effect. Both the two kinds of the vertical MoS2 /WS2 heterostructures feature high quality. The photodetectors based on the isolated MoS2 /WS2 heterostructures exhibit a high responsivity of 68 mA W-1 and a short response time of 35ms. This single-step chemical vapor epitaxy can be used to synthesize vertical MoS2 /WS2 heterostructures with high production efficiency. The new epitaxial growth approach may open new pathways to fabricate large-area heterostructures made of different 2D TMDs monolayers of interest to electronics, optoelectronics, and other applications.

Semiconductor Nanostructures for Modern Electronics

Modern electronics is based on semiconductor nanostructures in practically all main parts: from microprocessor circuits and memory elements to high frequency and light-emitting devices, sensors and photovoltaic cells. Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) with ultimately low gate length in the order of tens of nanometers and less is nowadays one of the basic elements of microprocessors and modern electron memory chips. Principally new physical peculiarities of semiconductor nanostructures are related to quantum effects like tunneling of charge carriers, controlled changing of energy band structure, quantization of energy spectrum of a charge carrier and a pronounced spin-related phenomena. Superposition of quantum states and formation of entangled states of photons offers new opportunities for the realization of quantum bits, development of nanoscale systems for quantum cryptography and quantum computing. Advanced growth techniques such as molecular beam epitaxy and chemical vapour epitaxy, atomic layer deposition as well as optical, electron and probe nanolithography for nanostructure fabrication have been widely used. Nanostructure characterization is performed using nanometer resolution tools including high-resolution, reflection and scanning electron microscopy as well as scanning tunneling and atomic force microscopy. Quantum properties of semiconductor nanostructures have been evaluated from precise electrical and optical measurements. Modern concepts of various semiconductor devices in electronics and photonics including single-photon emitters, memory elements, photodetectors and highly sensitive biosensors are developed very intensively. The perspectives of nanostructured materials for the creation of a new generation of universal memory and neuromorphic computing elements are under lively discussion. This paper is devoted to a brief description of current achievements in the investigation and modeling of single-electron and single-photon phenomena in semiconductor nanostructures, as well as in the fabrication of a new generation of elements for micro-, nano, optoelectronics and quantum devices.

Anisotropic phonon properties and effective electron mass in α-Ga2O3

The ordinary and extraordinary infrared dielectric functions of α-Ga2O3 thin films with the corundum structure were investigated. The films were grown by mist chemical vapor epitaxy on (101¯0) sapphire substrates in (101¯0) surface orientation. They were doped by the donor tin with the resulting free-electron concentrations between 1017 and 1019 cm−3. Structural characterization revealed the absence of strain and the high crystallographic quality of the samples. It allows one to resolve all 7 Raman modes. 5 out of 6 infrared active transverse optical phonon modes lie in the investigated wave number range >250 cm−1, and are unambiguously identified by spectroscopic ellipsometry. For the highest doped sample, the free-carrier contribution (plasmon) to the infrared spectra allowed accurate determination of effective electron masses. They are m⊥*=(0.297±0.010)m0 (perpendicular to [0001]) and m||*=(0.316±0.007)m0 (parallel to [0001]) for an electron concentration of n = 1.1 × 1019 cm−3.

Micro-Raman Spectroscopy in Self-Catalyzed Indium Phosphide Nanostructures: Morphology and Substrate Effects.

We report the effect of morphology and substrate of self-catalyzed indium phosphide (InP) nanostructures on phonon vibration modes. Using liquid indium as a catalyst, we grew self-catalyzed InP nanocones and nanopillars on single crystal substrates of InP(111)B, Si(111), and Si(100) via metal-organic chemical vapor epitaxy. Due to crystal symmetry breaking in one-dimensional nanostructure, longitudinal-optical (LO) and transverse-optical (TO) phonon modes are clearly resolved with the strong anisotropic behavior. Broadening and downshift of LO phonon modes are found to be sensitive to the morphology (i.e., aspect ratio and surface-to-volume ratio) and crystal structure (i.e., Wurtzite and Zinc Blende) of the as-grown nanostructures. This work demonstrated that Raman spectroscopy provides statistical insights on the quality of as-grown nanostructures (i.e., growth orientation, crystal structures, and the presence of structural defects) without destroying samples.

Transition metal dichalcogenides bilayer single crystals by reverse-flow chemical vapor epitaxy

Epitaxial growth of atomically thin two-dimensional crystals such as transition metal dichalcogenides remains challenging, especially for producing large-size transition metal dichalcogenides bilayer crystals featuring high density of states, carrier mobility and stability at room temperature. Here we achieve in epitaxial growth of the second monolayer from the first monolayer by reverse-flow chemical vapor epitaxy and produce high-quality, large-size transition metal dichalcogenides bilayer crystals with high yield, control, and reliability. Customized temperature profiles and reverse gas flow help activate the first layer without introducing new nucleation centers leading to near-defect-free epitaxial growth of the second layer from the existing nucleation centers. A series of bilayer crystals including MoS2 and WS2, ternary Mo1−xWxS2 and quaternary Mo1−xWxS2(1−y)Se2y are synthesized with variable structural configurations and tunable electronic and optical properties. The robust, potentially universal approach for the synthesis of large-size transition metal dichalcogenides bilayer single crystals is highly-promising for fundamental studies and technological applications.

Фотоприемники с активной областью InGaAs и метаморфным буферным слоем InGaP, выращенные на подложках GaAs

AbstractThe results of studying the reverse dark currents of photodiodes for a wavelength of 1.06 μm grown on a GaAs substrate with the help of an InGaP metamorphic buffer layer are presented. The metalorganic chemical vapor epitaxy (MOCVD) growth technology of InGaP metamorphic buffer layers with a stepwise composition variation is developed. Photodiodes with a photosensitive area of 1 mm in diameter and radiation input through the substrate are fabricated. The photodiode dark current at room temperature and reverse bias of –3 V was 50 nA. It is assumed that the bulk component of the dark current is caused by the tunneling mechanism through the trap levels.

Incorporation of pervasive impurities on HVPE GaN growth directions

High crystallinity thick films with low free carrier concentration (≤1×1015/cm3) and low compensation are required for many GaN-based electronic device applications. It has been demonstrated that low pressure chemical vapor and molecular beam epitaxy techniques can reproducibility deposit homoepitaxial films with low residual impurity concentrations. However, their typical slow growth rates prevent their utilization for thick film growth. Presently, hydride vapor phase epitaxy is the sole method that can deposit films with residual impurity concentrations ≤5×1016/cm3 at hundreds of microns per hour growth rate. It is crucial to verify if this method can reproducibly deliver thick free-standing GaN films of high crystalline quality with exceptionally low and uniform free carrier concentration. X-ray diffraction, Raman scattering, and low temperature photoluminescence experiments were carried out on a number of samples prepared by dicing a free-standing wafer into several pieces perpendicular and parallel to the major growth directions; namely, c-plane {0001}, a-plane {11−20}, and m-plane {1−100}. SIMS depth profiles were employed to identify and quantify the concentration of the pervasive impurities. Spatial maps of a Raman line sensitive to free-carrier concentration were measured to determine the spatial distribution of the net impurity concentration. The reduced concentration of un-compensated shallow donors was also verified by low temperature electron paramagnetic resonance.

How Predictive Are Reactor-Scale Simulations of Wbg Semiconductor Epitaxy? a Sensibility Analysis of SiC and Nitride Epitaxy Models

Process modeling can be the perfect complement to process development: in addition to offering a cost-effective way of exploring the parameter space of a given process or new reactor configurations, models are able to provide information regarding details of surface and gas phase chemistry that is not readily available experimentally. This information can be extremely useful in helping us rationalize trends in yield, epitaxial quality and defect concentration, or why a particular set of experimental conditions leads to the highest quality materials. In the case of physical vapor and chemical vapor epitaxy, the fundamental aspects of reactor-scale simulations are well established. However, it is our knowledge of homogeneous and heterogeneous processes what limits the predictive nature of our models. Every reactor rate and transport coefficient comes with an associated error that propagates into the model solution. In addition to this, the complexity of the overall kinetics of the growth process complicates our ability to identify rate limiting processes exclusively based on experimental trends of growth rates as a function of experimental parameters. Consequently, knowing how sensitive our model is to the different assumptions will help us both better understand the margin of error of model predictions and identify the key steps in the growth process. In this talk I will summarize the sensitivity analysis studies that we have carried out on the heterogeneous and homogeneous processes during SiC, GaN, and AlN epitaxy. Using datasets compiled from different models available in the literature, we will compare the predicted results in both 0.5D and simplified 1D and 2D models that will allow both local and non-local sensitivity studies. The advantage of 0.5D models is that they can be used as a first order approximation to simulate epitaxy on planetary systems where wafer to wafer variability is lower than in-wafer variability. We will use these models to identify both the key steps and the main sources of error as a function of the experimental conditions. Finally, we will study the impact that reduction on complexity has on model predictions.

Twinning in GaAs nanowires on patterned GaAs(111)B

We have studied twinning in GaAs nanowires grown via holes in a silica mask deposited on GaAs(111)B substrates by metal‐organic chemical vapour epitaxy without catalysts. Twins perpendicular to the growth direction form in the nanowires, their {111}‐type side facets leading to corrugation of their {110} side walls. The top facets are almost atomically smooth. Aberration corrected annular dark‐field scanning transmission electron microscopy reveals all twins are rotational, commencing with layers of Ga and finishing with As atoms. Energy‐loss spectroscopic profiling has shown no significant changes in the band‐gap or bulk plasmon energy at those twin boundaries, and the observed reduction of the interface plasmon energy by ∼0.13 eV is close to the detection limit of the technique, reflecting the very low energetic electronic changes related to twin formation.

Growth of ZnO bulk crystals: A review

A critical review is proposed of the different techniques of bulk growth of ZnO crystals for their use as a substrate in the homoepitaxial growth of this attractive compound. The crystals are assessed from their structural and electrical properties and from the structural properties and purity of homoepitaxial films grown on them by various techniques such as plasma-assisted molecular beam epitaxy, pulsed laser deposition, magnetron sputtering, chemical vapor deposition, metalorganic chemical vapor epitaxy, liquid phase epitaxy.

Effect of Growth Pressure and Gas-Phase Chemistry on the Optical Quality of InGaN/GaN Multi-Quantum Wells

ABSTRACTBlue light-emitting diodes (LED's), utilizing InGaN-based multi-quantum well (MQW) active regions deposited by organometallic chemical vapor epitaxy (OMVPE), are one of the fundamental building-blocks for current solid-state lighting applications. Studies [1,2] have previously been conducted to explore the optical and physical properties of the active MQW's over a variety of different OMVPE growth conditions. However, the conclusions of these papers have often been contradictory, possibly due to a limited data set or lack of understanding of the fundamental fluid dynamics and gas-phase chemistry that occurs during the deposition process.Multi-quantum well structures grown over a range of pressures from typical low-pressure production processes at 200 Torr, up to near-atmospheric growth conditions at 700 Torr, have been investigated in this study. At all growth pressures, clear trends of gas-phase chemical reactions are observed for increased gas residence times (lower gas speeds from the injector flange and lower rotation rates) and increased V/III ratios (higher NH3 flows).Confocal microscopy, excitation-dependent PL (PLE), and time-resolved photo-luminescence (TRPL) have been employed on these MQW structures to investigate the carrier lifetime characteristics. Confocal emission images show spatially-separated bright and dark regions. The bright regions are red-shifted in wavelength relative to the dark regions, suggesting microscopic spatial localization of high indium content regions. As the growth pressure and gas residence times are reduced, a larger difference in band-gap between bright and dark regions, longer lifetimes, and higher average PL intensities can be obtained, indicating that higher optical quality material can be realized. Optimized MQW's grown at high pressure exhibit higher PLE slope intensities and IQE characteristics than lower pressure samples. Results on simple LED structures indicate that the improvement in MQW optical quality at high pressures translates to higher output power at a 110 A/cm2 injection current density.

Ferromagnetic GaAs Structures with Single Mn Delta-Layer Fabricated Using Laser Deposition

The new technique combining metal-organic chemical vapor epitaxy with laser ablation of solid targets was used for fabrication of ferromagnetic GaAs structures with single Mn delta-doped layer. The structures demonstrated anomalous Hall effect, planar Hall effect, negative and anisotropic magnetoresistance in temperature range of 10-35 K. In GaAs structures with only single Mn delta-layer (without additional 2D hole gas channel or quantum well) ferromagnetism was observed for the first time.

Structural and optical studies of thick freestanding GaN films deposited by Hydride vapor phase epitaxy

Thick freestanding films or bulk GaN substrates with very low background impurity levels (≤1×1015/cm3) and high crystalline quality are required for a number of electronic device applications. Low pressure chemical vapor and molecular beam epitaxy techniques can systematically deposit films with low residual impurity concentrations. However, their typical slow growth rate prevents their utilization for substrate growth. The hydride vapor phase epitaxy deposition technique can achieve hundreds of microns per hour growth rate, but these films have typically high free carrier concentration (≥3×1017/cm3). It is crucial to verify if this method can reproducibly deliver thick freestanding GaN films of high crystalline quality with exceptionally low free carrier concentration. Low temperature photoluminescence and room temperature Raman scattering experiments carried out on a number of samples indicate that they have high crystalline quality and uncommonly low donor background levels. Reduced concentration of uncompensated shallow donors verified by low temperature electron paramagnetic resonance was confirmed by detailed high sensitive SIMS analyses. In addition, it was verified by X-ray diffraction analysis that relatively low dislocation densities can be achieved.

Indium incorporation into InGaN and InAlN layers grown by metalorganic vapor phase epitaxy

Experimental data on indium incorporation in InGaN and InAlN layers grown by metalorganic chemical vapor epitaxy (MOVPE) on bulk GaN substrates are presented and discussed. For the step-flow growth mode, realized for InGaN layers grown at relatively high temperatures (around 800°C), incorporation of indium increases with growth rate, and similarly, with a decrease in GaN substrate misorientation. Both dependences are explained by a higher velocity of flowing steps incorporating the indium atoms. For InAlN layers, three-dimensional nucleation takes place, and thus no significant changes of indium incorporation versus either growth rate or GaN substrate misorientation were observed.

Strain and crystal defects in thin AlN/GaN structures on (0001) SiC

High-resolution x-ray diffraction was used to compare strain relaxation and defect populations in thin GaN/AlN heterostructures (total thickness ≈480 nm) grown on (0001) SiC using metalorganic chemical vapor deposition (MOCVD) and hydride vapor epitaxy (HVPE) techniques. The results of high-resolution x-ray diffraction measurements (rocking curves and reciprocal space mapping) were corroborated using transmission electron microscopy. Differently grown films exhibited dissimilar strain relaxation and defect populations that were related to specific growth conditions. In the MOCVD films, grown under lower deposition rates, the elastic strain in the AlN and GaN layers was fully relaxed at the initial stages of the epitaxial growth yielding nearly similar densities of threading dislocation segments (TDS) in layer volumes. Additional, “secondary” elastic stresses in these layers were attributed to the excess of point defects. In the HVPE films, grown under higher (five to ten times) deposition rates, these layers were over relaxed and the density of TDS in the GaN layer was an order of magnitude larger than that in AlN. The MOCVD-grown sample was devoid of planar defects whereas the HVPE film contains significant densities of stacking faults in both GaN and AlN layers. Formation of “secondary” extended defects was interpreted in terms of creation and structural transformation of point defects during epitaxial growth. Differences in strain levels, types, and defect populations/distributions for the two heterostructures were attributed to the different growth rates for MOCVD and HVPE.

Epitaxial CdTe Rods on Au/Si Islands from a Molecular Compound

Formation of unusual CdTe rods on Si/SiO(2) and gold coated Si/SiO(2) surfaces is reported from chemical vapor deposition of Cd[(TeP(i)Pr(2))(2)N](2).

Utilizing the interface adsorption of nitrogen for the growth of high-quality GaInAsN/GaAs quantum wells by metal organic chemical vapor deposition for near infrared applications

We have investigated the composition and optical properties of GaInAsN/GaAs single quantum wells grown using metal organic chemical vapor epitaxy at 500 °C. Using time-of-flight secondary ion mass spectrometry and photoluminescence spectroscopy, we have shown the presence of a 1–2 nm thick nitrogen-rich interfacial layer at the first interface grown. The inhomogeneous asymmetric distribution of nitrogen atoms along the growth direction is attributed to the dominance of surface kinetics, nonlinear dependence of N incorporation on In content, and the strain gradient effect on the effective diffusion of N. We have utilized this finding to grow high quality quantum wells.

Homoepitaxial growth and characterization of ZnO(0001) thin films grown by metalorganic chemical vapor epitaxy

AbstractWe study the properties of Zn‐face ZnO(00.1) layers grown on the Zn‐face of high quality ZnO(00.1) single crystal wafers by metalorganic chemical vapour epitaxy. The layers are grown directly on the ZnO substrates without any treatment of the substrate before starting the growth. The growth temperature is varied between 600–1050 °C while the reactor pressure is kept at 50 Torr. Atomic force microscopy reveals monolayer steps with a step length of 170 nm. The root‐mean‐square roughness is 0.1 nm over a 20×20 μm2 area. A two‐dimensional step morphology is only seen for growth temperatures above 950 °C. Layers grown below 950 °C exhibit very rough three dimensional surface morphologies. The full width at half maximum for x‐ray rocking curves is 22 arcsec and 36 arcsec across the (00.2) and the (20.1) reflections, respectively. This is comparable to the rocking curve widths of the as‐received substrates. The peaks of the ZnO epitaxial thin film and the substrate are indistinguishable in reciprocal space maps. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Step-flow growth of ZnO(0 0 0 1) on GaN(0 0 0 1) by metalorganic chemical vapor epitaxy

We studied the properties of 500–900 nm thick ZnO(0 0 0 1) layers grown by metalorganic chemical vapor deposition on GaN(0 0 0 1) templates. A two-step growth process was employed where a buffer layer was first grown at 350 – 400 ∘ C . The temperature was then ramped up to 750 – 950 ∘ C for the growth of the epitaxial layer. Atomic force micrographs over a 20 × 20 μ m 2 area revealed locally straight steps, indicating step-flow growth. The root-mean-square roughness was less than 2.6 nm. Hexagonal pits were also observed with a pit density of 7.5 × 10 5 – 2.1 × 10 8 cm - 2 . The full width at half maximums of ω -scans across the symmetric (0 0 0 2) and the ( 2 0 2 ¯ 1 ) reflections were as narrow as 0 . 091 ∘ and 0 . 159 ∘ , respectively. Reciprocal space maps showed that the ZnO films were fully relaxed. Transmission electron microscopy revealed that the defects were mainly of mixed or pure edge dislocation type. No pure screw dislocations or stacking faults were observed.