Good Structural Quality Research Articles

Synthesis and Characterization of Al<sub>4</sub>SiC<sub>4</sub>: A “New” Wide Band Gap Semiconductor Material

High temperature solution growth of Al4SiC4 single crystals was carried out from the melt of silicon and aluminium pieces in a graphite crucible used as carbon source (typically 1800°C under 1 bar of argon). The obtained crystals, in the mm scale, were then characterized by a variety of techniques including: Raman spectroscopy, transmission electron microscopy techniques, X-ray diffraction and UV-Vis-NIR spectroscopy. A good structural quality was revealed by TEM, ensuring a well resolved Raman spectrum. The UV-Vis transmission spectrum shows an optical gap located around 2-2.5 eV.

Trends in the Development of the Epitaxial Nitride Compounds Technology

The main trends in the development of technology for nitride heterostructures element base of microwave − technology and power electronics, as well as light − emitting diodes have been reviewed. It has been noted that most modern technological focus is the development of nitride heterostructures on silicon substrates. The basic problems of nitride compounds on silicon substrate and the ways of their solution have been discussed. Some results of GaN/Si heterostructures technology development in «Elma − Malachit» JSC have been presented. The AlGaN/GaN/Si heterostructures have been grown by MOCVD. We show that early process stages such as Si − surface treatment and Al pre − deposition are of great importance for the growth of crack − free structures with good structural and surface quality. Meanwhile the surface curvature of the grown structures is influenced mainly by the composition of multilayered transition region between the AlN nucleation layer and the GaN layer. Transistors fabricated on AlGaN/GaN structures grown on Si substrates under optimized conditions demonstrated rather good static characteristics: I d,max = 800 mA/mm, U br > 120 V, g m = = 170 mS/mm. For the further technology development experimental and technological work should be arranged in close coordination with analytical prediction and calculation of properties of the grown material with mathematical modeling methods. This approach will help enhance the efficiency of technology development and deepen scientific views on the processes responsible for the formation of properties of heterostructures.

High-temperature annealing effect of α-Al2O3 (0001) substrates with nominal 0.25° miscut toward the a-plane $${\mathbf{ \left( {11\overline{2} 0} \right)}}$$ 11 2 ¯ 0 on ZnO films grown by MOCVD

The annealing effects of c-plane sapphire (α-Al2O3) substrate with a vicinal-cut angle of α = 0.25° toward the a-plane $$\left( {11\overline{2} 0} \right)$$ on the quality of epitaxial ZnO films grown by metal organic chemical vapor deposition were studied. The atomic steps formed on sapphire substrate surface by annealing at high temperature were analyzed by atomic force microscopy (AFM). The annealing effects of sapphire substrate on the microstructural and optical properties of the ZnO films were examined by high-resolution X-ray diffraction, scanning electron microscopy, AFM and photoluminescence spectroscopy. Experimental results indicate that the film quality is strongly affected by annealing treatment of the sapphire substrate. X-ray diffraction study revealed that ZnO films deposited on c-plane sapphire substrate annealed at T ≥ 1050 °C exhibit a wurtzite phase and have a c-axis orientation. The decrease in FWHM for (0004) and $$\left( {10\overline{1} 4} \right)$$ ZnO peak confirms the improvement of the crystalline quality of ZnO thin film as increasing annealing substrate temperature. Sapphire annealing at 1100 °C for 3 h under oxygen prior to ZnO film growth is the best to achieve ZnO film with good structural and optical quality.

Improved efficiency of electrodeposited p-CuO/n-Cu2O heterojunction solar cell

We report electrodeposition of n-type cuprous oxide (Cu2O) films on p-type CuO films electrodeposited on Ti substrates for forming p-CuO/n-Cu2O heterostructures. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis revealed that the films had good structural quality, with substrates being well-covered by the films. The p-CuO/n-Cu2O heterojunctions exhibited good photovoltaic properties and diode characteristics. The surfaces of Cu2O films subject to ammonium sulfide treatment exhibited enhanced photocurrents. Under AM 1.5 illumniation, the obtained sulfur-treated and annealed Ti/p-CuO/n-Cu2O/Au solar cell structure yielded energy conversion efficiency of 0.64%, with Voc = 220 mV and Jsc = 6.8 mA cm−2.

Novel InGaPBi single crystal grown by molecular beam epitaxy

InGaPBi crystalline thin films with up to 2.1% bismuth concentration have been grown on GaAs substrates by molecular beam epitaxy. Rutherford backscattering spectrometry confirms that the majority of Bi atoms are located at substitutional lattice sites. The films exhibit good surface, structural, and interface quality, and their strains can be tuned from tensile to compressive by increasing the Bi content. InBi LO and GaBi LO vibrational modes in Raman spectroscopy were observed, and their intensities increased with Bi concentration. A weak photoluminescence signal was observed at 1.78 eV at room temperature for the sample with a Bi content of 0.5%.

Strain compensated CdSe/ZnSe/ZnCdMgSe quantum wells as building blocks for near to mid-IR intersubband devices

In order to increase the conduction band offset of the ZnCdMgSe-based material system we studied the incorporation of strained CdSe layers to obtain deeper quantum wells for shorter wavelength intersubband transitions than those obtained in lattice-matched structures. Five CdSe/ZnSe/ZnCdMgSe multi-quantum wells (QW) samples grown by molecular beam epitaxy are studied in detail by transmission electron microscopy (TEM), X-ray diffraction (XRD), cw-photoluminescence (PL), and Fourier Transform Infrared (FTIR) absorption experiments. TEM and XRD results confirmed good structural quality of the samples. All the multi-QW PL energies were below the ZnCdSe lattice-matched to InP alloy bandgap (2.1eV), which serves as first evidence of having achieved deeper quantum wells. FTIR absorptions from 3.83 to 2.56μm were measured, shorter than those achieved by the lattice matched system. Simulations based on these results predict that absorptions as low as 2.18μm can be obtained with these materials.

Alterações na qualidade física de um latossolo amarelo sob pastagem

Indicadores de qualidade física do solo representam uma importante ferramenta no estudo das alterações que ocorrem no solo em função de ações de uso e manejo. Objetivou-se com este estudo utilizar o intervalo hídrico ótimo (IHO) e o índice-S (declividade da curva de retenção de água no solo no ponto de inflexão) para quantificar as alterações físicas do solo promovidas pelo sistema de pastagem, tendo como referência um solo sob mata. O estudo foi realizado em um Latossolo Amarelo distroférrico típico sob mata nativa e pastagem, onde foram coletadas amostras deformadas e indeformadas nas profundidades de 0-5 e 25-30 cm, utilizadas para determinação do carbono orgânico do solo e análises físicas. Os valores de índice-S, com exceção do solo sob mata (0-5 cm), estiveram abaixo do limite sugerido para separar solos com boa e pobre qualidade estrutural. O IHO foi fortemente reduzido no solo sob pastagem, chegando a atingir valor zero. O principal fator limitante para a disponibilidade de água foi a resistência à penetração, que exceto em mata (0-5 cm), atuou como limite inferior do IHO em todas as áreas e profundidades avaliadas. Os indicadores de qualidade física do solo, IHO e índice-S, apresentaram conformidade quanto à avaliação da qualidade física dos solos estudados e indicaram as alterações físicas promovidas no solo sob sistema de pastagem.

Isolation and in silico analysis of Fe-superoxide dismutase in the cyanobacterium Nostoc commune

Cyanobacteria are known to endure various stress conditions due to the inbuilt potential for oxidative stress alleviation owing to the presence of an array of antioxidants. The present study shows that Antarctic cyanobacterium Nostoc commune possesses two antioxidative enzymes viz., superoxide dismutase (SOD) and catalase that jointly cope with environmental stresses prevailing at its natural habitat. Native-PAGE analysis illustrates the presence of a single prominent isoform recognized as Fe-SOD and three distinct isoforms of catalase. The protein sequence of Fe-SOD in N. commune retrieved from NCBI protein sequence database was used for in silico analysis. 3D structure of N. commune was predicted by comparative modeling using MODELLER 9v11. Further, this model was validated for its quality by Ramachandran plot, ERRAT, Verify 3D and ProSA-web which revealed good structure quality of the model. Multiple sequence alignment showed high conservation in N and C-terminal domain regions along with all metal binding positions in Fe-SOD which were also found to be highly conserved in all 28 cyanobacterial species under study, including N. commune. In silico prediction of isoelectric point and molecular weight of Fe-SOD was found to be 5.48 and 22,342.98Da respectively. The phylogenetic tree revealed that among 28 cyanobacterial species, Fe-SOD in N. commune was the closest evolutionary homolog of Fe-SOD in Nostoc punctiforme as evident by strong bootstrap value. Thus, N. commune may serve as a good biological model for studies related to survival of life under extreme conditions prevailing at the Antarctic region. Moreover cyanobacteria may be exploited for biochemical and biotechnological applications of enzymatic antioxidants.

Semipolar (202¯1) GaN laser diodes operating at 388 nm grown by plasma-assisted molecular beam epitaxy

Laser diodes (LDs) were grown by plasma-assisted molecular beam epitaxy on semipolar (202¯1) GaN substrates. Metal-rich conditions provided smooth surface morphology and good structural quality as confirmed by atomic force microscopy and transmission electron microscopy studies. No stacking faults or any linear defects were formed during the growth in the InGaN/InGaN multiquantum well active region. Devices were processed with ridge-waveguide along the [1¯21¯0] direction. Mirrors were cleaved or fabricated by focused ion beam (FIB) processing. LDs operated at 388.2 nm with the threshold current density 13.2 kA/cm2 and the threshold voltage 10.8 V. The device with mirrors fabricated by FIB processing reached practically the same threshold current and slope efficiency as the one with cleaved mirrors. The authors present the beneficial role of the InGaN optical confinement layers in semipolar LDs, which can be optimized to improve the threshold current of these devices.

Structural and optical characterizations of InPBi thin films grown by molecular beam epitaxy

InPBi thin films have been grown on InP by gas source molecular beam epitaxy. A maximum Bi composition of 2.4% is determined by Rutherford backscattering spectrometry. X-ray diffraction measurements show good structural quality for Bi composition up to 1.4% and a partially relaxed structure for higher Bi contents. The bandgap was measured by optical absorption, and the bandgap reduction caused by the Bi incorporation was estimated to be about 56 meV/Bi%. Strong and broad photoluminescence signals were observed at room temperature for samples with xBi < 2.4%. The PL peak position varies from 1.4 to 1.9 μm, far below the measured InPBi bandgap.

Double-Position-Boundaries Free 3C-SiC Epitaxial Layers Grown on On-Axis 4H-SiC

High quality double-position-boundaries free 3C-SiC epilayers have been successfully grown on on-axis (0001) 4H-SiC by chemical vapor deposition at optimized conditions as observed with optical microscopy and X-ray diffraction. The effect of the growth parameters, including temperature, C/Si ratio, ramp-up condition, Si/H2 ratio, N2 addition and pressure, on the quality of the grown layers is investigated. Different techniques, including microscopic and spectroscopic techniques, are used to characterize the epilayers. High resolution X-ray diffraction shows 2θ-ω curve with full width at half maximum of only 16 arcsec for the (111) reflection detected from a 35 μm thick 3C-SiC layer, showing the good structural quality of the layer. Reciprocal space maps confirm the absence of double-position-boundaries in a large depth of the layers. Low temperature photoluminescence measurement shows clear near-bandgap emission with sharp and single peaks, which further verifies the high quality of the epilayers.

Laser molecular beam epitaxy of ZnO thin films and heterostructures

We report on the growth of epitaxial ZnO thin films and ZnO-based heterostructures on sapphire substrates by laser molecular beam epitaxy (MBE). We first discuss some recent developments in laser-MBE such as flexible ultraviolet laser beam optics, infrared laser heating systems or the use of atomic oxygen and nitrogen sources, and describe the technical realization of our advanced laser-MBE system. Then we describe the optimization of the deposition parameters for ZnO films such as laser fluence and substrate temperature and the use of buffer layers. The detailed structural characterization by x-ray analysis and transmission electron microscopy shows that epitaxial ZnO thin films with high structural quality can be achieved, as demonstrated by a small out-of-plane and in-plane mosaic spread as well as the absence of rotational domains. We also demonstrate the heteroepitaxial growth of ZnO-based multilayers as a prerequisite for spin transport experiments and the realization of spintronic devices. As an example, we show that TiN/Co/ZnO/Ni/Au multilayer stacks can be grown on (0 0 0 1)-oriented sapphire with good structural quality of all layers and well defined in-plane epitaxial relations.

Giant pyroelectric coefficient determined from the frequency dependence of the pyroelectric signal generated by epitaxial Pb(Zr0.2Ti0.8)O3 layers grown on single crystal SrTiO3 substrates

Epitaxial Pb(Zr0.2Ti0.8)O3 layers of good structural quality were grown on single crystal SrTiO3 substrates. The pyroelectric coefficient was estimated from the signal generated by the ferroelectric film working as a pyroelectric detector in the voltage mode, without pre-poling procedure. The obtained value is as high as 1.9 × 10−3 C/m2 K. The large value is attributed to the presence of 90° ferroelectric domains and to the compressive misfit strain, leading to an enhanced ferroelectric polarization.

Influence of Al-flux on the growth of AlN/GaN/AlN films on Si (1 1 1) substrate by MBE

The effect of the Al flux on the crystal quality of AlN/GaN/AlN heterostructures grown on Si (111) substrates by solid source molecular beam epitaxy, using a 13.56MHz RF nitrogen source, was investigated. The thickness of 69.94nm is obtained for good growth conditions giving the comparable FWHM of the XRD-rocking curve of 0.46° (27.6arcmin) when compared with other samples and previous reports. We found that the AlN sample grown under low Al-flux has produced a good structural quality and low compressive strain value compared to the sample grown under high Al-fluxes.

PFG NMR diffusion measurements of CH4 and CO2 through large ZSM-58-crystals

Large crystals of zeolite ZSM-58 having the DDR-topology have been synthesised. The crystals are diamond shaped and 100–160μm long. Basic characterisation involving X-ray diffraction and N2-porosimetry proved good structure quality and microporosity of the sample. 1H and 13C PFG NMR self-diffusion studies of single gas and binary 1:1 mixtures with CO2 and CH4 show that – in agreement with the topology of the ZSM-58 – the intracrystalline molecular diffusion occurs in a two dimensionally interconnected micropore space. The self-diffusion coefficients of the smaller carbon dioxide exceed those of the larger methane molecules by about two orders of magnitude, which is clearly the effect of increased diffusional resistance and shape selectivity within the narrow 8-ring window system of the ZSM-58 micropores. Carbon dioxide diffusion is rather constant and methane diffusion increases with increasing total loading. For methane this indicates that the narrow windows act as barriers which are easier passed if additional interactions with adsorbed molecules are present. These trends for the loading dependences in both the single-component and the binary mixture diffusion studies confirm earlier predictions by kinetic MC and MD computer simulations. However, at low loadings the predicted values for methane self-diffusion are significantly smaller than the experimental NMR data.

Structural, optical, electrical and resistive switching properties of ZnO thin films deposited by thermal and plasma-enhanced atomic layer deposition

In this study, we report on a systematical investigation on the crystal structure, optical and electrical properties of ZnO thin films grown by thermal and remote plasma-enhanced atomic layer deposition (Thermal ALD and PEALD) and their applications in resistive switching devices. The conductivity of ZnO films grown by Thermal ALD at 200°C is ∼169S/cm, demonstrating a good potential for the applications in transparent conducting films. It is possible to deposit ZnO films with good structural quality and few defects at lower temperatures by PEALD. The Al/PEALD-ZnO/Pt devices show good resistive switching properties, while the devices using Thermal ALD ZnO films failed to show any resistive switching behavior, but a perfect Ohmic behavior. The thickness ZnO active layer has a strong effect on the device properties. When the thickness of ZnO film is ∼23nm, the high state-resistance to low state-resistance ratio maintains at larger than 103, while the current compliance for safe operation is ∼1mA much smaller than those for devices with thick active layers. The results have demonstrated the PEALD grown ZnO films have the excellent properties for the applications in high-density 3D resistive random access memory.

Investigation on the origin of luminescence quenching in N-polar (In,Ga)N multiple quantum wells

The growth of N-polar (In,Ga)N structures by plasma-assisted molecular beam epitaxy is studied. (In,Ga)N multiple quantum well samples with atomically smooth surface were grown and their good structural quality was confirmed by x-ray diffraction, scanning transmission electron microscopy, and defect selective etching. The In incorporation was higher in the N-polar than in the Ga-polar oriented crystal, consistent with previous reports. However, despite the good morphological and structural properties of these samples, no photoluminescence signal from the (In,Ga)N wells was detected. In contrast, a thick N-polar (In,Ga)N layer exhibited a broad peak at 620 nm in good agreement with the In content determined by x-ray diffraction. The potential source of the luminescence quenching in the N-polar (In,Ga)N multiple quantum wells is discussed and attributed either to a strong nonradiative recombination channel at the surface promoted by the electric field or to the high concentration of point defects at the interfaces of the quantum well structures.

Growth mechanisms in semipolar [formula omitted] and nonpolar m-plane [formula omitted] AlGaN/GaN structures grown by PAMBE under N-rich conditions

The growth of multiquantum well (MQW) AlGaN/GaN structures was carried out by plasma assisted molecular beam epitaxy simultaneously on semipolar (202¯1), m-plane (101¯0) and c-plane (0001) GaN substrates under nitrogen-rich (N-rich) conditions in order to investigate the growth mechanisms on different crystal surfaces. A smooth surface with atomic steps was found only for the semipolar (202¯1) structure as opposite to m-plane and c-plane surfaces which were rough and covered by islands. Transmission electron microscopy (TEM) studies of the semipolar MQW structure showed uniform AlGaN layers with sharp interfaces and no extended defect formation, which proves good structural quality. At higher resolution, on the surface and at the interfaces of the semipolar MQWs, {10-11} and {10-10} nano-facets were found. In contrast to the semipolar case, the TEM studies of the m-plane structure showed no clear periodicity and very uneven distribution of Al. The AlN was formed on island top surfaces and AlGaN grew only on island slopes that were tilted from the m-plane. The growth rate was slightly smaller for the semipolar structure and much smaller for the m-plane one, as compared to the c-plane MQW structure. This can be attributed to Ga losses from semipolar and m-plane surfaces, in contrast to c-plane where no Ga losses were observed. The mechanisms for the Ga losses under N-rich conditions are discussed.

Electrochemical growth and characterization of CdTe nanorod arrays

The straight one-dimensional CdTe nanorod arrays on indium tin oxide coated glass substrate with high purity have been synthesized using a rapid and convenient electrodeposition method. The electrodeposited thin films were studied and investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV–vis absorption spectroscopy. The structural and optical studies revealed that films are possess sphalerite phase. The device based on the aligned array-on-ITO configuration demonstrates excellent photoresponse to the visible light, which is ascribed to the large absorption coefficient of the material and also suggests good electronic structure quality of the nanorods.

Electrodeposition of a novel CdTe array on Ni foils and photoelectrochemical performance

A novel perpendicularly aligned CdTe array with cauliflower-like morphology was fabricated on a Ni substrate using a simple electrochemical synthesis method. The samples were characterized by XRD, EDX and FESEM, and the results demonstrate that a high-purity, uniform cauliflower-like CdTe array was obtained. Rod lengths can reach to 3.0 μm, with nanoparticles stacking along the rod growth direction, keeping the diameter at 1.5 μm. A schematic diagram of the possible growth process and evolution from CdTe seed nuclei to the cauliflower-like CdTe array was proposed. The device based on the aligned array-on-Ni configuration demonstrates excellent photoelectrical properties, which is ascribed to the large absorption coefficient of the material and also suggests good electronic structure quality of the cauliflower-like rods. The technique described is applicable to fabricate highly aligned cauliflower-like semiconductor arrays on other conductive substrates.