Crystals for energy conversion and storage

Resume : GaN crystals of high structural quality are very much required for expanding applications in full color light sources and high power -high frequency electronics. However due to its extreme melting conditions GaN cannot be grown from stoichiometric GaN liquid. New melting data coming from very high pressure (up to 10.0GPa) and temperature (up to 3400K) experiments will be discussed in the context of theoretical simulations of GaN melting. GaN bulk crystals of high quality are therefore grown at pressures much lower than the one expected for melting. Sophisticated approaches (A-DEEP, VAS) based on HVPE on foreign substrates have been developed for obtaining free standing GaN wafers with quality and size sufficient for laser applications. The HVPE is at present, the only method supplying GaN substrates for industry. Its main advantage is high growth rate exceeding 100 m/h. Real bulk GaN crystals of very high quality are grown by ammonothermal method at moderate pressures of 0.1-0.3GPa and low temperatures of about 400-600oC. Development of this technology is limited by discouragingly low grow rate of about 1 m/h. This can be improved by increasing both pressure and temperature of the process. Higher rate of about 20 m/h can be also achieved in Na-flux system where pressures lower than 100MPa and temperature of about 850oC are used. The existing methods and their current state will be discussed. A new approach to GaN bulk crystallization based on growth by HVPE on Ammono-GaN seeds will be also presented. It will be shown that thick (d>2mm) GaN crystals with quality as good as the quality of the seeds can be grown with a rate exceeding 200 m/h. These studies are crucial for establishing physical limitations of real bulk GaN crystallization process by HVPE.

Resume : Adsorption of several molecular species, pertinent for crystal growth of the semiconductors at the polar surfaces of GaN, SiC and ZnO were investigated by DFT calculations. The investigated cases include adsorption of ammonia and hydrogen at polar GaN(0001) surface, hydrogen, silicon and carbon at SiC(0001) surface and zinc and oxygen at ZnO(0001) surface. The results of DFT calculations confirm recently obtained predictions that charge transfer between surface and the bulk of semiconductor may affect the adsorption energy. The process may change the adsorption energy, depending on the availability of empty states at the surface and pinning of Fermi level at the surface. In case of the nonpinned Fermi level, the adsorption energy depends on the doping in the bulk. Crystal growth from the vapor is reviewed showing that the adsorption of growing species leads to the increase of the adsorbate to the point where Fermi level is unpinned. Thus majority of the growth processes occurs at this condition, so that the adsorption depends on the doping in the bulk. This mechanism explains the dependence of the growth and doping on the Fermi level in the bulk. These predictions is verified by thermodynamic analysis of the growth of GaN, SiC and ZnO with application of DFT data.

Resume : Wafers cut from aluminium nitride (AlN) bulk crystals are most promising substrates for devices based on high Al content AlGaN epitaxial layers, due to their chemical stability, low thermal and lattice mismatch, and compressive strain to AlGaN layers. While AlN substrates are now available commercially, serious technological challenges still prevent mass production of AlN substrates having an industrially relevant diameter and defect density at reasonable cost. In this talk, we will present our status and progress in homoepitaxial AlN bulk crystal growth. AlN crystals are grown on N-polar basal plane AlN seeds prepared from spontaneously nucleated freestanding AlN crystals. The excellent crystal structural quality of the spontaneously nucleated AlN crystals is inherited in subsequent homoepitaxial bulk growth. In order to preserve the seed quality during bulk growth and to provide for single-crystalline diameter enlargement, seed backside evaporation, crystal cracking, and parasitic nucleation adjacent to the seed have to be prevented. These and other technological challenges are addressed in the presentation. Optical properties with corresponding impurity issues of the substrates are discussed. Finally, we show that proper surface preparation results in a smooth morphology of AlN layers grown by MOVPE on substrates sliced from the AlN crystals. Lasing of optically pumped AlGaN/AlN laser structures demonstrate the quality of the obtained substrates.

Resume : Silicon carbide has been thoroughly studied in various crystal growth processes. The common method for SiC epitaxial growth is based on chemical vapor deposition that is typically carried out at 1500-1600 degrees for research in transistor applications. There are benefits in high temperature growth while also challenges appear. Interestingly, we have explored a high temperature sublimation epitaxy approach at 1800-1900 degrees for growth of fluorescent silicon for white LED layers with potential in general lighting. SiC doped with certain elements act as a monolithic rare earth metal free light converter and produces a pure white light. We have also initiated growth of cubic SiC, which doped with boron fits nicely in to the model of intermediate bandgap solar cell theory. The quality of cubic SiC has been an obstacle in use of this polytype. We have shown that the cubic SiC can reach similar quality as produced in commercial hexagonal polytypes. This may open a new research area in SiC, in particular for studies of the photovoltaic properties. In addition, graphene is an emerging material. Our high temperature epitaxial graphene process carried out at 2000 degrees, which is more than 300 degrees higher than in other methods, has shown an outstanding quality. Potentially, it could be a contact material on photovoltaic cubic SiC. We describe the challenges with high temperature crystal growth of fluorescent and photovoltaic SiC, as well as graphene on SiC.

Resume : SiC single crystals have become widely used as substrates for power electronic devices like diodes and electronic switches. Today, 4 inch and 6 inch wafer diameters are commercially available which are processed from vapor grown crystals. The state of the art physical vapor transport (PVT) method may be called mature. Nevertheless, low defect density and uniform doping are still topics which can be further improved by current research and development of more sophisticated processes and process control. The aim of the paper is to review the PVT growth method as it is applied today. Special emphasis will be put on in-situ growth monitoring tools based on 2D and 3D x-ray imaging as well as x-ray diffraction. These techniques allow a precise determination of the crystal and source material evolution. In addition the strain that causes defect formation during growth can be visualized qualitatively, giving rise to growth process optimization strategies. Another topic will be the processing of highly conductive p-type 4H-SiC which is of particular interest for electronic switches like power MOSEFETs, HBTs and related devices.

Resume : We are going to discuss the recent progress as well as the view of GCL on the PV silicon technology development. Our focus will be on the upstream of the solar business chain, e.g., the polysilicon feedstock and PV silicon wafers. The high performance mono-like and multicrystalline silicon wafers developed via directional solidification by GCL and its competitors will be discussed extensively from the fundamentals to the market positioning. The product roadmap and the technology route of GCL will be presented as well.

Resume : For many materials the preparation of high-quality single crystals by traditional techniques is especially challenging. Examples include group IV materials where the elemental constituents have greatly differing melting points and/or vapor pressures, when the desired compound is thermodynamically metastable, or where growth with participation of the melt is generally not possible. A variety of synthetic techniques have been successfully employed in the preparation of group IV compounds however these approaches are typically not suitable for new or metastable phases. As such, conventional crystal growth techniques are here generally inapplicable. New crystal growth techniques and apparatus are therefore essential in investigating the intrinsic and fundamental properties of new and novel materials. I will present new crystal-growth techniques that allow for crystal-growth of group IV materials, including silicon nanocrystals, inorganic materials with clathrate-hydrate crystal structures and other group IV “open-framework” materials, and therefore the ability to perform fundamental investigations into their intrinsic physical properties, in many cases for the first time. Several new compositions that have not previously been investigated due to the formidable task of synthesizing single crystals have been were obtained. The ability to grow single-crystals allows for a fundamental investigation into the properties of different group IV elements in novel crystal structures and bonding schemes. The intellectual merit of this investigation is very closely tied with the new and novel structure types and corresponding novel physical properties they exhibit, and aims to develop important fundamental research towards advances for energy conversion and storage applications.

Resume : Rare earth ion (REI)-doped III-nitride has been very attractive for high luminescence efficiency of next generation photonics. In this work, Eu-doped GaN (GaN:Eu) films are grown by plasma assisted molecular beam epitaxy with different growth modes, 3-dimentional (3D) and step-flow/2D, and the Eu ions incorporated into the GaN are controlled by Eu beam equivalent pressure. First, the growth modes of GaN:Eu films are investigated with different III/V ratios under a constant Eu beam pressure. Growth rate, RHEED pattern, and AFM image reveal that the growth mode changes from 3D to step-flow/2D when the III/V ratio exceeds 1. When the growth mode transfers from 3D to step-flow/2D, the Eu content in GaN film is abruptly decreased from 0.75 to 0.02 at.%, indicating that Eu ions are not well incorporated into the film. But, changing the Eu beam pressure, it is found that the GaN:Eu films are grown with constant group V condition (3D growth mode) and the Eu content strongly depends on the ratio between Eu and Ga beam pressures. While the Eu concentration is below 1 at.% the surface of GaN:Eu film is relatively flat and island-like grains are observed due to slightly V-rich condition. However, if the Eu concentration exceeds 1% the surface morphology seems to be abruptly rough and the island-like grains become precipitates composed of high concentrated Eu and Ga. This result means that Eu atoms segregate along the growth direction from the bottom to the top surface to reduce the strain energy, resulting in precipitates contained the high concentrated Eu. The PL efficiency is also strongly sensitive to the E concentration within 1 at.%. We will discuss photoluminescence measured with the GaN:Eu films grown the different growth modes and crystallinities.

Resume : A three-dimensional local model was used to study the axial oxygen concentration distribution in the silicon crystal during the Czochralski growth under a transverse uniform magnetic field. The flow, temperature, and oxygen concentration distributions inside the furnace are calculated for different crystal lengths. The results show that at the initial stage the average oxygen concentration at the melt-crystal interface decreases as the length of the growth crystal increases. When the growth length of the crystal further increases, it reaches the minimum value and then increases continuously. This trend is in consistence with the experimental one. The variation of the axial oxygen concentration with the growth length of the silicon crystal is related to the melt depth of the crucible, the flow structure inside the melt, the crucible temperature, and the argon flow speed along the free surface. In order to improve the axial non-uniform of oxygen concentration, the heater position is adjusted.

Resume : Activated ZnSe crystals are high-efficient scintillators and they are already being applied to the radiation detecting. Often activation (increasing the scintillation efficiency) is provided by doping with rare-earth elements atoms. In such case, the study of defect structure transformation induced by Er-doping and mechanisms of the Er-stimulated radiative recombination in ZnSe single crystals are important tasks. In the present work, we present an investigation of the changes in luminescence and structural properties of ZnSe single crystals caused by Er-doping. Optical methods of low-temperature photoluminescence, Raman spectroscopy and IR-spectroscopy were used for the investigation. Our investigations were done on the ZnSe:Er crystals grown by Bridgman method. It was obtained that Er dopant atoms of the concentration in the solid phase of about 10-3 wt. % leads to the substantial disordering of initial crystalline structure and decrease of mean-free-path of optical and acoustical phonons. Mentioned processes manifest in the substantial decrease of the amplitudes of corresponding vibrational modes and increase of their full width on half maximums in the first-order Raman spectra of Er-doped ZnSe crystals. Also, Er-doping stimulates an appearance of additional absorbance bands in the IR transmittance spectra. However, Er-doping leads to the substantial increase (by factor more than 100 times) in the efficiency of luminescence.

Resume : Monocrystal matrices of high gap oxides are finding increasing applications as host of luminescent ions, typically Rare Earths (RE)s. For the time being RE doped oxyorthosilicates, aluminium perovskites and garnets (RE2SiO5 – REAlO3 – RE3Al5O12) are widely used as high efficient and fast scintillators for g ray detection. On the other hand shallow or deep intra-gap energy levels, due to stoichiometric deviation or impurities non intentionally added un the crystals, play a counteractive role giving rise to slower scintillation decay time, reduced light yield and afterglow. These unwelcome outcomes could be used to engineering new devices for optical memory storage. In this work we show how it is possible a selective read of the information stored in the crystal as a function of the excitation wavelength and the deep of the traps energy levels. In this sense experimental results of thermo- and radio- luminescence are presented. Theoretically the role of the band gap and the location in energy of the levels due to the RE dopants and to the defects, are discussed. The feasibility in a near future, of new promising transparent displays is, also, discussed.

Resume : Compound semiconductor nanowires (NWs) grown on Si substrates have attracted attention because of their potential for the novel device application such as opt-electronic integrated circuits (OEIC). To apply NWs to actual devices, it is necessary to control precisely the structure such as a growth position on the substrate and a size. In this study, we reported a selective area growth of GaAs NWs on patterned Si (001) substrates. The substrate surface consisted of flat (001) terraces and straight grooves covered with {111} facets lying along [110] directions. They were sized in several micrometers. The surfaces were prepared using wet etching processes and the substrates were cleaned by HF solution before loading to a growth chamber. GaAs were evaporated by a conventional molecular beam epitaxy technique on the substrates without any catalysts. NWs with a few hundred nm diameter and a few μm length were formed only on the {111} facets at the grooves, while thin films were formed on the terraces. The NWs grew along <111> directions and were covered by {110} facets. At the top of NWs, droplets expected from the self-catalytic growth were not observed, but the enclosing regions were also covered by facets. The NWs consisted of zincblend and wurtzite structures with a lot of phase boundaries. These demonstrated that both GaAs layer and NWs could be grown on Si substrates simultaneously using the patterned surface of the substrate. These structures allow us to realize OEIC devices.

Resume : ZnS nanorods can be used in energy conversion devices such as piezoelectric energy generators and in solar cells as a matrix for increased surface area. It has been recently shown that ZnS nanorods can be grown by simple and inexpensive spray pyrolysis technique [1]. Here we present a study on ZnS nanorods formation with respect to the solution concentration and feeding rate, substrate (glass, TCO/glass) and solvent types. ZnS layers were grown at Ts=500-550 °C using ZnCl2 and thiocarbamide at molar ratios of 1:3. SEM, XRD, EDS, UV-VIS were applied to characterize the ZnS layers. According to XRD, ZnS layers are composed of single phase highly c-axis oriented ZnS elongated crystals. According to UV-VIS, the Eg of ZnS is 3.7 eV independent of the deposition conditions, corresponding to wurtzite ZnS. Dimensions of the ZnS rods are mainly controlled by the deposition temperature, precursor concentration, solvent and substrate type. For instance, rods with d=80 nm and L=400 nm were obtained using ZnCl2 with c=0.1 mol/l; whereas rods with d=ca. 60 nm and L= 200 nm were produced from ZnCl2 with c=0.05 mol/l. Generally, the ZnS nanorods deposited onto TCO substrates had more uniform sizes and smaller diameters, compared to those grown on bare glass. [1] Dedova T.; Krunks M.; Gromyko I.; Mikli V.; Sildos I.; Utt K., Unt T. (2014) Physica Status Solidi (a), in press.

Resume : The GaTe-CdTe composite was obtained by treatment at 400-420 °C of GaTe thin layers for 2-24 hours. The GaTe thin films were obtained by vapor deposition on glass at 300-350°C. From the analysis of X-ray diffraction Cu Kα was determined that both primary and composite layers consist of GaTe submicron crystals and a mixture of GaTe and CdTe crystallites. The intensity of reflexes at 2Θ = 39.2, that correspond to diffraction of (220) planes in CdTe, increase with the duration of treatment yielding an high concentration of CdTe in the composite. The absorption spectra and photoluminescence at temperatures from 78 to 300 K at the edge of the fundamental absorption band of the composite layers of GaTe and GaTe-CdTe are investigated. The edge of absorption band of GaTe thin layers shifts to higher energies by 0.1 eV from the edge of absorption band of mono-crystalline plates of GaTe, which is 1.65 eV at normal temperature. Also the absorption of light in the range of 1.3-1.6 eV is reduced. This is caused by structural defects in GaTe polycrystalline layers. The treatment of GaTe in Cd vapor, increases absorbance in this spectral region together with the shift to low energies of the edge of fundamental band. Annealing of GaTe thin films in Cd vapor at 420 °C for 24 hours shifts the edge absorption band to lower energies by 0.2 eV, compared with width of the band gap of GaTe crystal plates. The concentration of the components in the composite depends on the duration of the treatment.

Resume : PT (PbTiO3) and PZT (PbTixZr1-xO3) thin films have recently attracted considerable attention because of the unique physical properties of the material, and especially of ferroelectric thin film. That gives them a high potential for applications such as ferroelectric random access memory (FRAM), pyroelectric infrared detectors, piezoelectric properties and other. The main problems of fabrication of such thin films are high temperatures (500-700oC) and stoichiometry, which leads to mixtures of different oxide’s phases (PbO, TiO2 and PbTiO3). Pb are very volatile at high temperatures, therefore many methods use ex-situ or post annealing methods. But thin films, formed by many ex-situ technologies are nonquality (porous, grained and rough). Our proposed method is to form PbTiO3 thin films by deposition of multilayers (0.1-1 nm) of single oxides, sputtered by reactive magnetron deposition method on thermal heated (500-700oC) substrate (in-situ). Substrates were platinum coated silicon. Structure changes, phase composition of as deposited thin films were measured and analyzed using X-ray diffraction method. The surface morphology was investigated by SEM microscopy. Hysteresis loop was measured and dielectric properties were analyzed by impedance spectroscopy method. The results show that single nanocrystal perovskite phase forms at 600oC temperatures when different magnetron deposition rates are optimized to control stoichiometry.

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Resume : Addressing the stability challenges in silicon-based anode materials is necessary for achieving high capacity lithium-ion batteries. Recent studies have shown that surface coatings favorably influence the cycling performance of such materials. Despite the potential benefits, little knowledge is available on the impact of surface coatings on the lithiation behavior and the reverse. This work details on the effect of conformal metallic Ni coatings on the lithiation behavior of crystalline Si nanopillars. The fabricated composites have different morphological parameters: Ni shell thicknesses of 40 nm, 80 nm and 120 nm with a Si core diameter of 170 nm, 330 nm and 480 nm. Pristine nanopillars display anisotropic swelling along <110> directions and fracture sites along <100> directions consistent with previous literature reports. The Ni coated structures exhibit a different swelling behavior with two distinct fracture regimes. For Ni thickness lower than 80 nm, the anisotropic behavior is preserved but fracture sites are present along <110> directions. With thicker coatings, an anisotropy reduction is observed together with a single randomly oriented fracture. A combined thermodynamic - mechanical model corroborated these observations. The proposed theoretical framework can be extended to other types of coatings or nanostructures and provide guidelines for tailoring electrode materials to overcome the drawbacks around Si-based anodes [G. Sandu et al., submitted].

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