Impurity Incorporation Research Articles

Investigation of carbon incorporation in laser-assisted MOCVD of GaN

Background carbon (C) impurity incorporation in metalorganic chemical vapor deposition (MOCVD) grown gallium nitride (GaN) represents one of the major issues in further improving GaN vertical power device performance. This work presents a laser-assisted MOCVD (LA-MOCVD) technique to address the high-C issue in MOCVD homoepitaxial GaN under different growth rate (Rg) regimes and studies the correlations between [C] and Rg. [C] in LA-MOCVD GaN is reduced by 50%–90% as compared to the conventional MOCVD GaN for a wide growth rate range between 1 and 16 μm/h. A mass-transport based model is developed to understand the C incorporation at different Rg regimes. The results obtained from the developed model are in good agreement with experimental data. The model further reveals that LA-MOCVD effectively suppresses C incorporation by reducing the active C species in the gas phase. Moreover, high step velocity in step flow growth mode can facilitate C incorporation at fast Rg, exhibiting steeper C increase. The theoretical model indicates that [C] can be suppressed below 1016 cm−3 with a fast growth rate (Rg) of 10 μm/h by utilizing higher power LA-MOCVD and freestanding GaN substrates with larger off-cut angles.

Case Studies in the Application of a Workflow-Based Crystallization Design for Optimized Impurity Rejection in Pharmaceutical Development

Crystallization is an important unit operation in pharmaceutical drug substance manufacturing for the isolation of organic products. Impurity control is a recurring critical quality attribute in most commercial pharmaceutical crystallization processes. A systematic workflow that can reveal the incorporation mechanism of various impurities during crystallization and guide us to understand impurity rejection is useful for crystallization process design. Such a workflow was recently published by Urwin et al. Herein, we present three case studies implementing this workflow, covering surface deposition, conglomerate formation, and agglomeration as principal routes for impurity incorporation during crystallization of a drug substance’s active pharmaceutical ingredient and its intermediates. We demonstrate the experimental steps to identify various impurity incorporation mechanisms and discuss the strategy for impurity rejection in each case. We built upon the previous approach to demonstrate a material sparing approach through our case studies. We highlight the value of stepwise dissolution and show the importance of doing it early in the impurity incorporation identification stage. At this stage, we introduce a missing incorporation mechanism in prior work, namely, conglomerate systems. We show how to identify and resolve this mode of incorporation mechanism through stepwise dissolution and solubility-limited rejection maps.

Reaction mechanism and film properties of the atomic layer deposition of ZrO2 thin films with a heteroleptic CpZr(N(CH3)2)3 precursor

The surface reaction of a heteroleptic precursor in the atomic layer deposition (ALD) of ZrO2 has been investigated using atomically thin films for scaled-down semiconductor devices. Heteroleptic precursors have been widely used in ALD. Because different ligands in heteroleptic precursors have different reaction pathways, understanding the surface reaction is crucial. In this study, we investigate the growth mechanisms of thin films using a CpZr(N(CH3)2)3 precursor with different reactants, such as H2O and O2 plasma. Chemical composition, surface roughness, film density, and microstructures were analyzed through electron microscopy and X-ray techniques together with quantum chemical calculations to elucidate the ZrO2 growth mechanism. The results showed that the reaction pathway of Zr–Cp ligand exchange with surface hydroxyl groups is less favorable than the Zr–N(CH3)2 reaction pathway. Furthermore, H2O molecules showed less effective oxidation reactions with Cp ligands than with alkyl amide ligands. In contrast, all the ligands were completely removed, resulting in negligible impurity incorporation, as revealed via X-ray photoelectron spectroscopy, when O2 plasma was used, leading to an improved leakage current (30-fold reduction). We believe that these findings on the surface reactions of heteroleptic ALD precursors will be helpful for developing future scaled-down semiconductor devices.

Amorphous Silicon Films and Nanocolumns Deposited on Sapphire and GaN by DC Sputtering

Sputtering is a deposition technique used to fabricate low‐cost silicon films on crystalline and amorphous substrates. Herein, the deposition of amorphous silicon films by DC sputtering on both sapphire and GaN/sapphire substrates is reported. Films are deposited using argon plasma with a pressure of 0.47 Pa at 30–60 W of DC power and different deposition temperatures from RT to 550 °C. The effect of different deposition conditions is investigated on structural quality, layer morphology, and optical properties of the layers. X‐ray diffraction measurements do not show any peak associated to crystalline silicon, while energy‐dispersive X‐ray demonstrates the presence of silicon in the layers. Silicon films deposited on sapphire show a compact morphology but the formation of silicon columns on GaN. On both substrates, the growth rate increases a factor of 3 with the applied DC power (50–150 nm h−1). Finally, the optical bandgap energy extracted from transmission measurements decreases from 2.40 to 2.10 eV with the DC power, due to the reduction of impurity incorporation. This work offers a low‐cost alternative for the deposition of amorphous compact silicon films and silicon nanocolumns at low temperature, for application in sensing, photonic, electronic, and photovoltaic devices.

Effect of the Growth Interruption on the Surface Morphology and Crystalline Quality of MOCVD-Grown h-BN

Hexagonal boron nitride (h-BN) is one promising material class for applications in DUV optoelectronics due to the layered structure and ultra-wide bandgap. The synthesis of h-BN with smooth surface morphology and high quality on dielectric substrates is the key to construct efficient functional devices thereon. In this study, we reported wafer-scale h-BN on c-plane sapphire substrates by metal organic chemical vapor deposition utilizing the flow modulation epitaxy (FME) with growth interruptions. The effect of the growth interruption location within FME on the surface morphology and crystalline quality of h-BN films was systematically investigated. The interruption after the TEB injection could promote the mobility of B adatoms, and the interruption after the NH3 injection could further relieve the passivation of N terminal growth fronts and mitigate the parasitic gas-phase reaction between growth precursors. By simultaneously employing interruptions after TEB and NH3 injections, the growth rate of h-BN increased significantly from 0.16 nm/min to 4.76 nm/min, and the surface roughness of 2-nm-thick h-BN was reduced to 0.587 nm. In addition, h-BN grown with an interruption solely after the NH3 injection presented the best crystallinity because the relatively slow growth rate reduced the possibility of impurity incorporation.

Off-centering of the Fe atom at the A site in SrTiO3: calculations and experiment

The influence of annealing temperature and deviation from stoichiometry on the incorporation of the Fe impurity into the A and B sites of the perovskite structure is studied using XAFS spectroscopy and X-ray diffraction. It is shown that at high annealing temperature about 30% of the iron atoms enter the A sites in SrTiO3. The obtained results agree with the results of first-principles calculations, according to which the Fe2+ ion at the A site exhibits strong off-centering (the displacement of ∼1 Å). The mechanism of magnetoelectric interaction for magnetic off-center ions is discussed.

Effect of Growth Temperature and Atmosphere Exposure Time on Impurity Incorporation in Sputtered Mg, Al, and Ca Thin Films.

Impurities can be incorporated during thin film deposition, but also can originate from atmosphere exposure. As impurities can strongly affect the composition-structure-property relations in magnetron sputter deposited thin films, it is important to distinguish between both incorporation channels. Therefore, the impurity incorporation by atmosphere exposure into sputtered Mg, Al, and Ca thin films is systematically studied by a variation of the deposition temperatures and atmosphere exposure times. Deposition temperature variation results in morphological modifications explained by considering surface and bulk diffusion as well as grain boundary motion and evaporation. The film morphologies exhibiting the lowest oxygen concentrations, as measured by energy dispersive X-ray spectroscopy, are obtained at a homologous temperature of 0.4 for both Mg and Al thin films. For Ca, preventing atmosphere exposure is essential to hinder impurity incorporation: By comparing the impurity concentration in Al-capped and uncapped thin films, it is demonstrated that Ca thin films are locally protected by Al-capping, while Mg (and Al) form native passivation layers. Furthermore, it can be learned that the capping (or self-passivation) efficiency in terms of hindering further oxidation of the films in atmosphere is strongly dependent on the underlying morphology, which in turn is defined by the growth temperature.

Local density changes and carbonate rotation enable Ba incorporation in amorphous calcium carbonate.

Incorporation of a Ba impurity in amorphous calcium carbonate (ACC) is shown with ab initio molecular dynamics simulations to have a long-range effect on its atomic-level structure and to be energetically favoured relative to incorporation in crystalline calcium carbonate polymorphs. The ability of carbonate ions to rotate and of ACC to undergo local density changes explain ACC's propensity for incorporating divalent metal impurities with a wide range of ionic radii. These findings provide an atomic-level basis for understanding the significant effects of low concentrations of impurities on the structure of ACC.

First-principles study of indium nitride monolayers doped with alkaline earth metals.

Element doping has been widely employed to modify the ground state properties of two-dimensional (2D) materials. In this work, the effects of doping with alkaline earth metals (AEMs) on the structural, electronic, and magnetic properties of indium nitride (InN) monolayers are investigated using first-principles calculations based on density functional theory. In a graphene-like honeycomb structure, the InN monolayer possesses good dynamical and thermal stability, and exhibits an indirect gap semiconductor character with a band gap of 0.37 (1.48) eV as determined by using the PBE(HSE06) functional. A single In vacancy leads to the emergence of a magnetic semiconductor character, where magnetic properties with a large total magnetic moment of 3.00 μB are produced mainly by the N atoms closest to the defect site. The incorporation of AEMs impurities causes local structural distortion due to the difference in atomic size, where Mg and Ca doping processes are energetically most favorable. Half-metallicity is induced by the partial occupancy of the N-2p orbital, which is a consequence of having one valence electron less. In these cases, the total magnetic moment of 1.00 μB mainly originates from N atoms neighboring the dopants. Further increasing the doping level preserves the half-metallic character, where N atoms play a key role on the magnetism of the highly doped systems. Results presented herein suggest the In replacement by AEMs impurities is an effective approach to make prospective spintronic 2D materials from InN monolayers.

Структурные исследования кристаллов ниобата лития, легированных церием

In this research the structural characteristics of LiNbO3 : Ce (0.13–2.15 mol. %) samples were studied.The concentration dependences of the unit cell parameters of LiNbO3 : Ce crystals were considered. The mechanism of impurity incorporation and the location of intrinsic defects in a LiNbO3 : Ce crystal (0.43 mol. %) were established. Cerium occupied the vacant lithium position in the lithium niobate lattice.

Heavily Doped Zinc Oxide with Plasma Frequencies in the Telecommunication Wavelength Range

Heavy and hyper doping of ZnO by a combination of gallium (Ga) ion implantation using a focused ion beam (FIB) system and post‐implantation laser annealing is demonstrated. Ion implantation allows for the incorporation of impurities with nearly arbitrary concentrations, and the laser‐annealing process enables dopant activation close to or beyond the solid‐solubility limit of Ga in ZnO. Heavily doped ZnO:Ga with free‐carrier concentrations of ≈1021 cm−3, resulting in a plasma wavelength of 1.02 μm, which is substantially shorter than the telecommunication wavelength of 1.55 μm is demonstrated. Thus, this approach enables the control of the plasma frequency of ZnO from the far infrared down to 1.02 μm, thus, providing a promising plasmonic material for applications in this regime.

A Review of Recent Progress in β‐Ga2O3 Epitaxial Growth: Effect of Substrate Orientation and Precursors in Metal–Organic Chemical Vapor Deposition

Gallium oxide (Ga2O3) is a highly promising ultrawide‐bandgap semiconductor for power electronics that emerged about a decade ago. Epitaxial growth Ga2O3 at the small scale is demonstrated. In order to develop scalable manufacturing of high‐performance epitaxial structures, in‐depth understanding of the fundamental growth processes, control parameters, and mechanism is imperative. This review discusses the recent progress in epitaxial growth of β‐Ga2O3 films and highlights challenges in obtaining high growth rate, low defects, and high carrier mobilities. Compared with the other epitaxy methods, metal–organic chemical vapor deposition (MOCVD) offers a wider growth window and precursor selection option, to minimize the tradeoff between crystal quality and growth rate. Growth rate is inversely proportional to temperature, within a certain temperature window, because of the unavoidable premature gas‐phase reactions and desorption of the highly volatile gallium suboxide (Ga2O) at elevated temperatures. Growth defects, background impurity incorporation, and carrier mobilities can be affected by the choice of MOCVD precursors, nucleation, and adsorption/desorption/diffusion of adatoms on substrate surfaces of different orientations, including the effect of growing on cleavage and noncleavage planes. This review summarizes the current status of the epitaxial growth of β‐Ga2O3 and analyzes the major factors that enhance mobility and reduce background doping concentration. The insights gained help advance the manufacturability of device‐grade epitaxial thin films.

Morphology evolution and cathodoluminescence properties of non-polar aluminum nitride microwires

The aluminum nitride (AlN) one-dimensional microwires (UWs) are promising candidates for the achievement of highly efficient optoelectronic devices. Here, we report the successful synthetization of large-scale nonpolar AlN UWs without any catalysts using physical vapor transport (PVT). The growth direction of high-quality UWs is along m-orientation. The morphology of AlN UWs was examined by scanning electron microscopy (SEM), and they show a rectangular cross-section with a polyhedral tip. The tip of UW undergoes four morphological evolution driven by growth kinetics and free surface energy minimization. The unique cathodoluminescence (CL) patterns were found in the UW. Both sides of UW exhibit strong violet luminescence (345 nm). In contrast, other violet luminescence (320 nm) and blue luminescence (450 nm) were observed in the center of the UW. The evolution of the morphology of UW crystal suggests that the defect-related luminescence pattern may be related to the difference in impurity incorporation efficiency on a- and m-crystalline planes. The photoluminescence (PL) spectra were obtained and the Fabry–P é rot cavity mode in the microwire was demonstrated. The emission anisotropy of AlN may also contribute to the CL luminescence pattern of the microwires.

A comprehensive study of defects in gallium oxide by density functional theory

Ultrawide bandgap gallium oxide (Ga2O3) is a promising material for power semiconductor devices and deep ultraviolet (UV) solar-blind photodetectors. Understanding the properties of point defects in Ga2O3 is necessary to realize better-performing devices. A comprehensive study based on density functional theory (DFT), using the generalized gradient approximation (GGA): Perdew-Burke-Ernzerhof (PBE) exchange–correlation functional, of point defects in corundum (α), monoclinic (β), and orthorhombic (ε) phases of Ga2O3 is presented. The point defects include vacancies, interstitials, antisites, and extrinsic impurities in various phases of Ga2O3. Defect formation energies, charge transition energy levels, and defect concentrations variation with temperature are listed and presented under both gallium-rich (Ga-rich) and oxygen-rich (O-rich) growth conditions. The formation energy diagrams predict that the Ga2O3 phases favor the formation of Ga and O vacancies and the incorporation of extrinsic impurities. The calculations also show that the charge transition levels are deep inside the bandgap regardless of the Ga2O3 phase and growth environment. The impacts of temperature on the intrinsic point defects are analyzed by probing the vibrational modes of β-Ga2O3 thin films grown in a metal–organic chemical vapor deposition (MOCVD) system at 700 °C temperature and annealed at 1100 °C in an O-rich environment.

Crystal Chemistry of Titanates and Zirconates of Rare Earths—Possible Matrices for Actinide Isolation

Titanates and zirconates of light rare earth elements (REE): REE2TiO5, REE2Ti2O7, REE4Ti9O24, and REE2Zr2O7, are of interest as matrices for isolating the REE actinide fraction of high-level waste from the reprocessing of irradiated nuclear fuel. Data on the incorporation of impurities (Zr, U, Ca) into Nd and La titanates are examined. They display limited isomorphism toward these elements, including by the reaction 2REE3+ ↔ Ca2+ + U4+, which is common for minerals and their synthetic analogues. The reasons for the low solubility of Zr and U in Nd titanates and the role of the crystal chemical factor in the choice of crystalline matrices for the immobilization of the REE actinide fraction are considered.

Numerical simulation on the impurity photovoltaic (IPV) effect in c-Si wafer-based dual-heterojunction solar cell

The impurity photovoltaic effect (IPV) with thallium (TI) impurity has theoretically been investigated in crystalline silicon (c-Si) wafer reigned dual-heterojunction (DH) solar cell that ensures an extended absorption of longer wavelength light with zinc selenide (ZnSe) window and aluminium antimonide (AlSb) back surface layers. The results imply that the incorporation of impurity enhances the photocurrent while the dual-heterojunction ensures the higher V OC of the proposed solar cell structure. The pristine Si (150 µm wafer) solar cell exhibits a power conversion efficiency (PCE) of 35.04% with J SC = 38.85 mA/cm 2 and V OC = 1.05 V, respectively. On the contrary, the introduction of Tl impurity within the bandgap of absorber enhances the PCE to 44.47%, with a J SC = 48.95 mA/cm 2 , V OC = 1.063 V and FF= 85.46%, respectively. This augmentations of the V OC and J SC and hence PCE is originated from the DH structure and longer wavelength light absorption due to impurity-assisted two-step photon upconversion phenomena in the solar cell. • IPV effect in an n + -p-p + Si DH solar cell with Tl impurity has been simulated. • ZnSe and AlSb have been used as window and BSF layers, respectively. • The optimized metal contacts Ti (cathode) and Ni (anode) has been established. • A maximum J SC of 48.95 mA/cm 2 and PCE= 44.47% with V OC = 1.063 V has been achieved. • n + - ZnSe /p- Si /p + - AlSb DH solar cell with Tl impurity shows potential in future.

High charge collection efficiency detector based on plasma purified high-quality diamond

Diamond radiation detector is an ideal choice for the next-generation radiation detection in harsh environments, due to its small size, high irradiation resistance and fast response. However, high-purity single crystal diamond is hard to be prepared at the high growth rate due to the inevitable impurity incorporation. In this paper, based on the high-quality high-temperature/high-pressure (HPHT) single crystal diamond substrate, detector-grade single crystal diamonds material with high purity and low defect density were epitaxially grown at growth rate of about 4 μm/h by using plasma environment purification technology. It showed that the full width at half maximum of X-ray rocking curve of high-quality single crystal diamond could reach 0.007°, there was no nitrogen related impurity in PL spectrum, and the substitutional nitrogen content estimated by UV absorption spectrum was less than 10 ppb. After metallization and packaging process, the diamond detectors were prepared and the detector response under irradiation with 241 Am α-particles was realized. For the sample with fewer dislocations, the charge collection efficiency and energy resolution of this detector were 86.8 % and 7.85 % for electrons, and 96.7 % and 2.48 % for holes. The product of mobility and lifetime of electrons and holes were 1.7 × 10 −4 cm 2 V −1 and 3.7 × 10 −4 cm 2 V −1 , respectively. The high performances of the detector indicate that the current diamond detector can meet the application requirements of commercial diamond detectors. • Detector-grade diamond was prepared by plasma environment purification technology. • The high-purity single crystal diamond was grown at growth rate of about 4 μm/h. • The diamond detector with charge collection efficiency of 96.7 % was realized. • The products of mobility and lifetime of electrons and holes were 1.7 × 10 −4 and 3.7 × 10 −4 cm 2 V −1 .

Luminescence technique for studying the growth of AgInS2 quantum dots

Although nanoparticle production techniques are well-known, getting nanoparticles with specific characteristics that enable their application as biosensors is an entirely other problem. Many issues occur as a result of employing the method for producing repeatable and time-stable nanostructures. We created AgInS2 nanoparticles as colloidal quantum dots in a variety of methods to test the efficiency of the synthesis process on the optical characteristics of the nanoparticles, as well as their size, composition, absorption, and luminescence spectra. The capillary electrophoresis (CE) approach for AgInS2 production was employed, with modifications in solvent and temperature, to get nanocrystal (NC) particles. The researchers discovered that Ag accumulation in the InS lattice promotes deformation which leads to structural defects. Consequently, the direction of a nanoparticle light band may now be changed. The features of mixed AgInS2 nanoparticles have been examined with respect to different fabrication procedures, surface stability, and metal impurity incorporation. One band dominates in the luminescence spectra of AgxIn1–xS2 nanoparticles. The relationship between the stoichiometric ratio, luminescence amplitude, line width, and the maximum wavelength is investigated. The average size of the received nanocrystals was determined using dynamic light scattering studies. The computed nanoparticle diameter range has an average particle size of 3–3.5 nm.

Plasma-Assisted Growth of Two-Dimensional Ga<sub>2</sub>O<sub>3</sub>/ Gas Heterophases on Liquid Alloy Substrate for Nanoelectronic Applications

Ultra-low-level incorporation of trace impurities and dopants into two-dimensional (2D) semiconductors is a challenging step towards the development of functional electronic devices based on 2D materials. Herein, the incorporation of sulfur atoms into 2D Ga2O3 surface oxide film of eutectic gallium-indium alloy (EGaIn) is achieved through plasma-enhanced metal-catalyst dissociation of H2S gas on EGaIn substrate. This process led to the growth of GaS crystalline nanodomains inside of amorphous 2D Ga2O3 sublayer films. Consequently, 2D lateral heterophases were developed between amorphous Ga2O3 and crystalline GaS nanodomains. The materials characterization revealed the alteration of Photoluminescence (PL) characteristics and change of valence band maximum (VBM) of functionalized 2D films. The comprehensive studies by conductive atomic force microscopy (c-AFM) showed considerable enhancement of conductivity of 2D Ga2O3/GaS materials (300 times improvement) compared with that of 2D Ga2O3 film. This technique has a great potential for fabrication of 2D metal oxide devices with tunable electronic characteristics similar to nanojunction memristors and transistors.

Properties and ballistic tests of strong B4C-TiB2 composites densified by gas pressure sintering

B4C-TiB2 composites with classical 75/25 vol ratio were sintered by pressureless sintering with and without gas pressure application in the final stage of densification, using a novel prototypal furnace. A small fraction of WC was introduced through high energy milling of the starting powders with WC-Co spheres. High energy milling facilitated the densification thanks to incorporation of WC impurities acting as sintering aid, and size reduction of the starting powders. Strength, stiffness and toughness of the ceramic densified at 2050 °C via gas pressure sintering were even better than hot pressed composites at 1900 °C. Depth of Penetration tests on plates with 3–5 mm thickness demonstrated that the gas pressure sintered material had a superior performance compared to the hot pressed one. This work also revealed that hardness was not the property spotting the best ballistic performance.