Post-growth Thermal Treatment Research Articles

Improvement of the Conformational Stability of 150 mm 4H SiC Wafers

A method for mitigating loss of conformational stability in 150 mm n-type 4H SiC wafers was investigated. Modifications to the physical vapor transport (PVT) process used to grow the parent bulk crystals, combined with post-growth thermal treatment, were examined as means of reducing the internal stresses hypothesized to promote instability. The magnitude of the stresses was analyzed by mechanically thinning sets of wafers produced from each process to determine the critical thickness of stability loss. The average critical thickness was found to be reduced by 13% via growth cell modification, at a reduced level of thermal treatment relative to a control process, with all wafers becoming unstable greater than 30 μm below the minimum recorded production thickness. Assessment of the spatial uniformity of dislocations indicated that lower conformational stability corresponded to elevated densities of basal plane dislocations (BPDs) and threading edge dislocations (TEDs) at the wafer edge relative to the center.

Growth, structural and vibrational properties of hydrogenated nanocrystalline silicon thin films prepared by radiofrequency magnetron sputtering technique at room temperature

The main objective of this paper is to produce the hydrogen-free and hydrogenated nanocrystalline silicon (nc-Si:H(x)) thin films deposited onto the single-crystalline Si(100) wafer by radio-frequency magnetron sputtering (RF-MS) at a substrate temperature of ∼ 300 K. Sputtering was effectuated by a microwave plasma in a mixture of argon and hydrogen (Ar + (H2(x)) gasses taken at different proportions of hydrogen dilution: x = 0, 2.5, and 5 sccm (standard cubic centimeter per minute). The deposition rates of the nc-Si:H were fully controlled, and the thickness of these films was ∼ 200 and 300 Å. Pt-capping layer was deposited in order to protect the nc-Si:H layer from oxidation. The thickness of the capping layer was fixed at ∼ 5 Å.The effects of the hydrogen flow rate and post-growth thermal treatment on the crystalline structure and morphology of as-grown nanocrystalline silicon thin films were systematically investigated by ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS), atomic force microscopy (AFM), grazing incidence x-ray diffraction (GI-XRD) and micro-Raman spectroscopy methods. From the measured data, the average nanocrystallite sizes, surface morphology and other important characteristics of these thin films were analyzed. The grown nc-Si:H thin films may be used as a semiconducting electrode material for electrochemical proton production, which is of great technological importance, especially for solid-state electrochromic device applications.

Thermal neutron scintillation improvement in Ce:Li6Y(BO3)3 single crystals by thermal treatment

The development of efficient, low-cost, and stable solid-state materials for portable thermal neutron detection is highly expected in order to substitute the currently used 3He and BF3 tank detectors. A few Li-based glasses and halide compounds have emerged as candidates, however, all of them present critical drawbacks for their practical implementations. Ce:Li6Y(BO3)3 is a priori a very promising oxide candidate that however has been disregarded so far due to its negligibly low light yield, caused by a poor crystalline and optical quality. In this study, we demonstrate that a post-growth thermal treatment is the key parameter to drastically reduce the concentration of intrinsic defects and scattering centers that lead to severe non-radiative recombination of excited electrons. Even though this annealing step also involves the oxidation of activator Ce3+ ions to Ce4+, a drastic enhancement of the light yield by ∼600% is achieved independently of the Ce3+ concentration within the considered range. The obtained light yield of 4650 ph n−1 is already close to that of reference Li-glass (commercial GS20 with 6000 ph n−1). An additional improvement can be envisaged upon further optimization of the Ce3+ concentration and the annealing time, so that Ce:Li6Y(BO3)3 reaches a light yield comparable to the state-of-the-art one for thermal neutron detection.

Synthesis of hydrophilic carbon nanotube sponge via post-growth thermal treatment

Clean water is vital for healthy ecosystems, for human life and, in a broader sense, it is directly linked to our socio-economic development. Nevertheless, climate change, pollution and increasing world population will likely make clean water scarcer in the near future. Consequently, it becomes imperative to develop novel materials and more efficient ways of treating waste and contaminated water. Carbon nanotube (CNT) sponges, for example, are excellent in removing oleophilic contaminants; however, due to their super-hydrophobic nature, they are not as efficient when it comes to absorbing water-soluble substances. Here, by means of a scalable method consisting of simply treating CNT sponges at mild temperatures in air, we attach oxygen-containing functional groups to the CNT surface. The functionalized sponge becomes hydrophilic while preserving its micro- and macro-structure and can therefore be used to successfully remove toxic contaminants, such as pesticides, that are dissolved in water. This discovery expands the current range of applications of CNT sponges to those fields in which a hydrophilic character of the sponge is more suitable.

Metallic AsSb nanoinclusions strongly enriched by Sb in AlGaAsSb metamaterial

We have elucidated the microstructure of Al0.28Ga0.72As0.972Sb0.028 metamaterial containing a developed array of AsSb nanoinclusions. The AlGaAsSb films were grown by low-temperature molecular-beam epitaxy followed by high-temperature postgrowth thermal treatment. The process resulted in an array of self-organized AsSb nanoinclusions with an average diameter of 15 nm (after annealing at 750 °C), which occupied about 0.003 of the volume of the crystalline AlGaAsSb matrix. The nanoinclusions were found to have an A7-type rhombohedral microstructure and contained a very high concentration of antimony that exceeded 90%, whereas the Sb content in the AlGaAsSb matrix was 2.8 at. %. The strong enrichment of the inclusion with Sb resulted from the local thermodynamic equilibrium between the solid AlGaAsSb matrix and the As0.1Sb0.9 inclusions that became liquid at an annealing temperature of 750 °C. The inclusion-matrix orientation relationship was established to be (003)p || {111}m and [2¯110]p || 〈220〉m, similar to that for pure As precipitates in low-temperature grown GaAs despite a rather large difference in particle lattice constants and the c/a ratio. The increased inclusion-matrix lattice mismatch resulted in the generation of (001) prismatic dislocation loops associated with the inclusions bigger than 8.5 nm in diameter. The Burgers vector of the dislocation was determined as b = amatrix[001]. The established microstructural and compositional features create an opportunity to specify plasmonic resonance conditions in the low-temperature grown AlGaAsSb metamaterial.

Postgrowth Control of the Interfacial Oxide Thickness in Semiconductor–Insulator–Semiconductor Heterojunctions

AbstractThe electronic properties of the heterojunction formed by chemical bath deposition of a thorium‐ and oxygen‐doped PbS nanostructured layer on GaAs substrate as a function of postgrowth thermal treatments are studied. A correlation is found between the heterojunction conductance and the duration of thermal treatment in air. In contrast to previous reports on the effect of air annealing on PbS films, where the conductance increased due to oxygen incorporation within the PbS, in this oxygen‐saturated system, the PbS properties are unaffected by exposure to oxygen. Control over the heterojunction conductance is achieved by tuning the interface oxide thickness, enabled by thermal treatment in air, resulting in the elimination of Fermi level pinning caused by interface induced gap states. The ability to control the interfacial insulator thickness post film‐growth is a unique feature of this system.

Features of the Postgrowth Thermal and Electrothermal Treatment of Nominally Pure and Heavily Doped Lithium–Niobate Crystals

It is established that the postgrowth thermal and electrothermal annealing of heavily doped LiNbO3:ZnО and LiNbO3:MgО crystals differ greatly from such processes for nominally pure LiNbO3 crystals of congruent composition. Ways of optimizing the technological regimes of monodomenization for heavily doped lithium–niobate crystals are shown.

Evidence of quantum dot size uniformity in strain-coupled multilayered In(Ga)As/GaAs QDs grown with constant overgrowth percentage

Strain coupled multi-layer quantum dot (QD) structures are limited due to their non-uniform dot size distribution, as-grown defects and dislocations. Even with these limitations, they are subject of interest because of high optical and device efficiency. In this work, we propose a modified growth strategy in which monolayer (ML) deposition of QDs is varied based on reflection high energy electron diffraction (RHEED) pattern to achieve defect free structure with uniform dot size distribution. The overgrowth percentage, i.e. monolayer deposition above critical thickness (minimum thickness required for transition from two-dimensional (2D) to three-dimensional (3D) confined structures) was kept constant in modified strategy. To verify this claim, we have compared it with conventional strain coupled multilayer structure and demonstrated improved optical performance and device characteristics. Photoluminescence spectroscopy exhibited monomodal QD distribution in strain-coupled multi-stacked heterostructure grown by the proposed strategy. Power dependent photoluminescence (PL) measurement confirms existence of single ground state peak. The proposed strategy yielded a defect free structure and better carrier capture rate compared to conventional multilayer QDs. The spectral response observed from proposed quantum dot infrared photodetector (QDIP) exhibited multiple peaks in mid-IR region (3.92µm and 4.54µm), which is one of the essential requirements for fabricating infrared detectors or focal plane arrays. Post-growth thermal annealing treatment at various temperatures (650–800°C) were performed to verify the change in activation energy, ground-state emission peak, and thermal stability. Negligible change in the ground-state PL emission peak (< 2%) and thermal activation energy (< 2%) were observed up to 750°C. The full width at half maximum (FWHM) values of the ground-state peak of the annealed samples exhibited a similar trend. High-temperature thermal stability was achieved probably due to high strain field hindering material interdiffusion and improved quantum confinement realized from uniform QD sizes. The modified growth strategy used in this paper is helpful not only for improved optical characteristics, but also for useful device performances, such as a night vision camera in defense areas, cancer detector in medical applications and fabrication of thermal imagers; focal plane arrays.

Kinetic path towards the passivation of threading dislocations in GaN by oxygen impurities

Defect tolerance can be critically important for optoelectronics. GaN, specifically, tolerates a relatively large concentration of threading dislocations, but the physical origin of this tolerance remains a mystery. First-principles calculations reveal the removal of deep-level states from edge dislocations by oxygen passivation. This removal is, however, not a thermodynamic ground state but kinetically driven. Oxygen incorporation during growth can be harmful; it becomes beneficial if introduced in the cooling-down phase or post-growth thermal treatment at a significantly lower temperature. Our findings extend first-principles defect study to the nonequilibrium regime where low-diffusion-barrier defects affect electronic behavior of semiconductors in unexpected fashion.

Growth of large sized two-dimensional MoS2 flakes in aqueous solution.

A large lateral size and low dimensions are prerequisites for next generation electronics. Since the first single layer MoS2 transistor reported by Kis's group in 2011, layered transition metal dichalcogenides (TMDs) have been demonstrated to be the ideal candidate for next generation electronics. However, the development of large scale and low cost growth techniques is a crucial step towards TMDs' inclusion in modern electronics and photoelectronics. In this work we develop a cheap, wet chemical, and environment friendly deposition process for two dimensional MoS2 flakes with extended size. For our deposition process, ammonium tetrathiomolybdate (ATTM) dissolved in deionized water was used as precursor solution and was deposited on a SiO2/Si substrate through a Langmuir-Blodgett like deposition process. To our knowledge, this is the first time MoS2 flakes have been grown in an aqueous solution. Large-sized MoS2 flakes exceeding 150 μm in lateral size were obtained after thermal decomposition. Thicknesses ranging from a monolayer to 5 monolayers were confirmed by AFM and Raman spectroscopy. Further investigations revealed that the quality of the produced flakes strongly depends on the post growth thermal treatment and its atmosphere. This simple and nontoxic deposition method is suitable for the preparation of large (hybrid) transition metal dichalcogenide nanostructures for applications in next generation electronics.

Large-area zinc oxide nanorod arrays templated by nanoimprint lithography: control of morphologies and optical properties

Vertically aligned, highly ordered, large area arrays of nanostructures are important building blocks for multifunctional devices. Here, ZnO nanorod arrays are selectively synthesized on Si substrates by a solution method within patterns created by nanoimprint lithography. The growth modes of two dimensional nucleation-driven wedding cakes and screw dislocation-driven spirals are inferred to determine the top end morphologies of the nanorods. Sub-bandgap photoluminescence of the nanorods is greatly enhanced by the manipulation of the hydrogen donors via a post-growth thermal treatment. Lasing behavior is facilitated in the nanorods with faceted top ends formed from wedding cakes growth mode. This work demonstrates the control of morphologies of oxide nanostructures in a large scale and the optimization of the optical performance.

Modeling and analysis of smoothly diffused vertical cavity surface emitting lasers

Inter-diffusion in a 850 nm AlGaAs/GaAs top-emitting oxide-confined vertical-cavity surface-emitting laser (VCSEL) has been modeled and analyzed. Some important VCSEL parameters such as, the threshold gain, relative confinement factor, and the effective cavity length have been derived as a function of diffusion length and were found to reliably describe intermixing in VCSELs. It has been shown that inter-diffusion in VCSELs during typical molecular beam epitaxy (MBE) and metal-organic vapor-phase deposition (MOCVD) growth conditions is negligible and has no effect on a various range of VCSEL parameters. The model reveled that the VCSEL reflectivity spectra remains roughly unchanged for diffusion lengths of up to 16 nm, however, it is associated with a very small resonance shift and a decrease in the band width. The VCSEL threshold gain was found to increase noticeably at small diffusion lengths. The error-function solution to the diffusion equation as presented in the model could be adapted as a new compositional grading scheme in VCSELs. This newly proposed compositional grading scheme could be controlled during VCSELs growth and/or by post-growth thermal treatment.

Polymorphic Phase-Dependent Optical and Electrical Properties of a Diketopyrrolopyrrole-Based Small Molecule.

Four different polymorphic conformations of diethyl 5,5'-[5,5'-[2,5-bis(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diyl]bis(thiophene-5,2-diyl)]difuran-2-carboxylate (DPP-(CF)2), namely, DPP-(CF)2-α, DPP-(CF)2-β, DPP-(CF)2-γ, and DPP-(CF)2-ω, were identified from X-ray diffraction analysis conducted on their thin films and single crystals. Highly crystalline and well-textured thin films of these four polymorphs were successfully prepared via postgrowth solvent vapor and thermal annealing treatments to investigate the polymorphic phase-dependent optical and electrical properties of DPP-(CF)2. Interestingly, during the phase transition from DPP-(CF)2-α to DPP-(CF)2-ω, the optical band gap decreases from 1.75 to 1.5 eV because of the enhanced π-π interaction between the neighboring molecules. Except for DPP-(CF)2-γ, the other three phases show ambipolar charge transport. Although DPP-(CF)2-β and DPP-(CF)2-γ exhibit a similar way of packing, a small increment in the π-π-stacking distance (0.006 Å) and twist conformation of the grafted electron-donating moieties of DPP-(CF)2-γ are found to reduce its hole mobility.

Effect of annealing on the compositional modulation of InAlAsSb

The effect of a post-growth thermal treatment in two different heterostructures with InAlAsSb as the top layer grown by molecular beam epitaxy lattice-matched to InP, have been studied by diffraction contrast transmission electron microscopy (TEM).11TEM: transmission electron microscopy This novel top cell layer material with application in ultra-high efficiency solar cells were grown on (001) InP substrate with or without an InGaAs buffer layer. Initial photoluminescence (PL)22PL: photoluminescence measurements revealed deviations from their predicted bandgap, suggesting non-random atomic distribution of the quaternary layer. Then, a thermal annealing was performed at different temperatures and times. The effect on the structure of the InAlAsSb active layer caused by the new arrangement of layers and the post-growth annealing treatments has been reported. Our results show that the small compositional fluctuations of the as-grown heterostructures disappear after being annealed, and the bandgap energy correspondingly increases towards the predicted value.

Radiative mechanism and surface modification of four visible deep level defect states in ZnO nanorods.

Visible luminescence from ZnO nanorods (NRs) is attracting large scientific interest for light emission and sensing applications. We study visible luminescent defects in ZnO NRs as a function of post growth thermal treatments, and find four distinct visible deep level defect states (VDLSs): blue (2.52 eV), green (2.23 eV), orange (2.03 eV), and red (1.92 eV). Photoluminescence (PL) studies reveal a distinct modification in the UV (3.25 eV) emission intensity and a shift in the visible spectra after annealing. Annealing at 600 °C in Ar (Ar600) and O2 (O600) causes a blue and red-shift in the visible emission band, respectively. All samples demonstrate orange emission from the core of the NR, with an additional surface related green, blue, and red emission in the As-Prep, Ar600, and O600 samples, respectively. From PL excitation (PLE) measurements we determine the onset energy for population of the various VDLSs, and relate it to the presence of an Urbach tail below the conduction band due to a presence of ionized Zni or Zni complexes. We measured an onset energy of 3.25 eV for the as prepared sample. The onset energy red-shifts in the annealed samples by about 0.05 to 0.1 eV indicating a change in the defect structure, which we relate to the shift in the visible emission. We then used X-ray photoemission spectroscopy (XPS), and elastic recoil detection analysis (ERDA) to understand changes in the surface structure, and H content, respectively. The results of the XPS and ERDA analysis explain how the chemical states are modified due to annealing. We summarize our results by correlating our VDLSs with specific intrinsic defect states to build a model for PL emission in ZnO NRs. These results are important for understanding how to control defect related visible emission for sensing and electroluminescence applications.

Two-step thermal process in tellurium vapor for tellurium inclusion annealing in high resistivity CdZnTe crystals

Exploitation of CdZnTe crystals for X-ray imaging detectors is limited by the incorporation of tellurium inclusions during growth. Over the years, several post-growth thermal treatments have been proposed for the removal of tellurium inclusions, often compromising the high electrical resistivity of the material. Moreover, main attention is usually paid to large inclusions, whose direct connection with the deterioration of detector properties has been demonstrated, while the effect of thermal annealing on small inclusions is usually neglected.In this article, the effect of different thermal annealing process on the concentration of inclusions down to 1μm size is studied. A two-step process in tellurium vapors has been shown to be effective to anneal large tellurium inclusions and to preserve the high resistivity of the samples as well.

Ordering of low-density Ge quantum dot on patterned Si substrate

We present a study on the growth of a low-density ordered Ge quantum dot (QD) on a nanohole patterned Si (0 0 1) substrate with periods in the order of microns by molecular beam epitaxy. Ordered Ge QDs with different periods were realized, the largest period being 15 µm. From the height-profile evolution of the QD with Ge deposition, it was found that the nanohole filling started at the initial Ge deposition, indicating an immediate nucleation and growth of QDs inside the nanoholes. Such a phenomenon is attributed to a lower surface chemical potential (SCP) inside the nanoholes, which is supported by calculated results on SCP evolution with growth and verified by the observation that the Ge adatoms around the nanoholes exhibited a higher probability of being incorporated into the nanoholes. In this scenario, low density ordered Ge QDs with any large periods could be achieved. The optical properties of the Ge QDs showed a remarkable improvement after a post-growth rapid thermal annealing treatment.

Effects of strain on the optoelectronic properties of annealed InGaAs/GaAs self-assembled quantum dots

The effect of the lattice-mismatch strain and of the charge carrier confinement profile, on the optical properties of thermally annealed self-assembled InxGa1−xAs/GaAs quantum dots (QDs), is theoretically analyzed by using a recently developed 40-band k.p model. First, to evaluate the composition and size of QDs as a function of thermal annealing conditions, we model the In/Ga interdiffusion by a Fickian diffusion. Second, we investigate the decrease of the strain effects on the carrier confinement potentials with annealing by solving the Schrödinger equation separately for electrons and holes. It is clearly found that the strain strongly modifies the QD potential profile, leading to a different electron and hole energy distribution. Finally, we carry on a comparison between theoretical calculations and photoluminescence (PL) experimental results performed in thermal annealed samples. A good agreement is obtained for the energy blueshift and the linewidth narrowing of the PL spectra measured on annealed QD ensemble. These results prove the relevance of the present approach to describe the optoelectronic properties of the nanostructures through the post-growth thermal annealing treatment.

Dramatic changes in the electronic structure upon transition to the collapsed tetragonal phase in CaFe2As2

We use angle-resolved photoemission spectroscopy and density functional theory calculations to study the electronic structure of CaFe${}_{2}$As${}_{2}$ in the collapsed tetragonal (CT) phase. This unusual phase of iron arsenic high-temperature superconductors was hard to measure as it exists only under pressure. By inducing internal strain, via the postgrowth thermal treatment of single crystals, we were able to stabilize the CT phase at ambient pressure. We find significant differences in the Fermi surface topology and band dispersion data from the more common orthorhombic-antiferromagnetic or tetragonal-paramagnetic phases, consistent with electronic structure calculations. The top of the hole bands sinks below the Fermi level, which destroys the nesting present in parent phases. The absence of nesting in this phase, along with an apparent loss of Fe magnetic moment, are now clearly experimentally correlated with the lack of superconductivity in this phase.

LGSO:Ce scintillation crystal optimization by thermal treatment

Impact of post-growth thermal treatment parameters (temperature, duration, atmosphere composition) on optical, luminescent, and scintillation characteristics of Lu2xGd2−2xSiO5:Ce LGSO:Ce has been analyzed. Absorption, X-ray luminescence spectra, thermostimulated luminescence curves, as well as light yield and energy resolution, have been measured before and after thermal treatment in different regimes. Scintillator optimization by thermal treatment allows to improve the energy resolution up to 6.7% at 662keV.