Improvement Of Crystal Quality Research Articles

Online Measurement Technique for Melt Surface Level of Si Crystal in Growth Furnace of Czochralski Method

We have developed a new instrumentation technique for Si crystal growth process of Czochralski (CZ) method for the purpose of the purpose of the quality improvement. In this paper, we developed the online measurement technique for the melt surface level of Si crystal in growth furnace. Our new technique can measure the melt surface level with high accuracy without the influence of meniscus. The characteristics of this technique are the elimination method of the influence of meniscus. The elimination error of meniscus influence is within +/-0.3 mm. This technique contributes to the improvement of CZ Si crystal quality.

Improvement of crystal quality of AlN and AlGaN epitaxial layers by controlling the strain with the (AlN/GaN) multi-buffer layer

AlN and AlGaN epitaxial layers with Al composition of 0.6–0.8 were grown by low-pressure metal organic vapor phase epitaxy on a (0 0 0 1) 6H–SiC substrate using the (AlN/GaN) multi-buffer layer structure (MBLS). Strain of the grown layer could be controlled by structure of the inserted (AlN/GaN) MBLS. It was found that the crystal quality of the grown layer could be improved with increase of the tensile strain in a-axis (compressive strain in c-axis): full-width at half-maximum (FWHM) of X-ray rocking curves (XRC) of both (0 0 0 2) plane ( ω scan) and (1 0 1¯ 2) plane ( ϕ scan) were decreased from 1530 arcsec to 79 arcsec and 3962 arcsec to 853 arcsec for the AlN template, respectively. FWHM of the ϕ scan XRC was several times larger than that of the ω scan, though the former was roughly proportional to the latter.

Temperature-dependent growth of zinc oxide thin films grown by metal organic chemical vapor deposition

Transparent conductive un-doped zinc oxide (ZnO) thin films are deposited on glass substrate by metal organic chemical vapor deposition and the effect of temperature in the range 393–443 K on the structural, electrical, and optical properties of these films are investigated. X-ray diffraction spectra and scanning electron microscope images indicate that substrate temperature plays a great role on the microstructure of ZnO films and the morphological transition takes place obviously at around 418 K. The morphology of the ZnO films shows sphere-like structure at low temperature (<418 K) and then changes from pyramid-like to rock-like structure at higher temperature (>418 K). The grain of ZnO films grows up with an increase of substrate temperature. Hall measurements indicate that decreased resistivity and increased mobility of ZnO films result from the improvement of grain size and crystal quality. Under the optimal growth condition, the boron-doped ZnO films at 423 K exhibit the lowest resistivity of 1.2×10 −3 Ω cm (its thickness=1000 nm) with a high mobility of 30.4 cm 2/V s and average optical transmittance above 85% in the range 400–900 nm, specially suitable for thin film Si solar cells.

Spherical silicon solar cell with reflector cup fabricated by decompression dropping method

A spherical Si solar cell with a reflector cup was successfully fabricated by a dropping method at decompression state. In the dropping method, melted Si droplets were instilled at decompression state (0.5 × 10 5 Pa) to reduce crystal growth rate, dominating crystal quality such as dislocation density in crystal grains. Spherical Si solar cells were fabricated using the spherical Si crystals with a diameter of 1 mm and then mounted on a reflector cup. The current-voltage measurement of the solar cell shows an energy conversion efficiency of 11.1% (short-circuit current density ( J sc): 24.7 mA·cm −2, open-circuit voltage: 601 mV, fill factor: 74.6%). Minority carrier diffusion length determined by surface photovoltage method was 98 μm. This value can be enhanced by the improvement of crystal quality of spherical Si crystals. These results demonstrate that spherical Si crystals fabricated by the dropping method has a great potential for substrate material of high-efficiency and low-cost solar cells.

Effects of Forced Solution Flow on Protein-Crystal Quality and Growth Process

X-ray diffraction experiments were performed to study the effect of forced solution flow on protein-crystal quality. The tetragonal crystals of hen egg white lysozyme were grown under flow in the rate range of 0 (quiescent condition) to 1000 μm/s. The quality of the crystals grown under flow was superior to that of crystals grown under quiescent conditions. To investigate the mechanism of the quality improvement, we observed the surface morphology of the (110) face using atomic force microscopy (AFM). Under quiescent conditions, the two-dimensional (2D) nuclei were randomly formed over the surface. Under solution flow, a hillock consisting of 2D islands was formed around the facet center. The step density under flow was higher than that under quiescent conditions. The step growth rate decreased with increasing flow rate because of the step-density increase. We found that there was a strong correlation between the crystal quality and the step growth rate. The results suggested that the slow growth decreases the molecular misorientation at the kink site, resulting in an improvement in crystal quality.

Temperature dependence of the first-order Raman scattering in silicon nanowires

Temperature dependence of the first-order Raman scattering in silicon nanowires was studied using an amended phonon confinement model. The experimental band position and linewidth both showed a better agreement with the calculated data if the temperature effect was considered. Furthermore, a slight improvement in crystal quality was observed.

X‐ray microdiffraction imaging investigations of wing tilt in epitaxially overgrown GaN

AbstractThe crystalline quality in epitaxially laterally overgrown (ELO) GaN and the amount of wing tilt is characterized on a local basis, with high spatial and angular resolution. A method of full‐field X‐ray microdiffraction imaging, termed rocking curve imaging, is used to record simultaneously a large set of local X‐ray diffraction profiles originating from sample surface areas of micrometer size. x –ω maps of diffracted intensity allow to quantify the amount of wing tilt in individual lateral ELO periods as well as to monitor the fluctuations of tilt between adjacent periods. Automated shape analysis of the full set of local rocking curves provides a means to quantitatively characterize the local crystalline perfection of GaN. The ELO window and wing regions can be clearly separated; comparison indicates an average improvement of crystal quality by a factor 3–4 due to the lateral overgrowth process. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Ultraviolet Lasing of Sol–Gel-Derived Zinc Oxide Polycrystalline Films

The effect of post-annealing on sol–gel-derived ZnO films has been investigated. For these films, structural investigations including analyses of surface morphology and microstructures were carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Also, optical properties were determined by photoluminescence analysis, ellipsometry and optical pumping measurement. The XRD results indicate that the (002) peak will predominate with increasing annealing temperature. SEM images show that grain size increased with annealing temperature. Moreover, photoluminescence spectra revealed the enhancement in UV emission intensity with annealing temperature, which was attributed to an improvement in crystal quality. Room-temperature ultraviolet random lasing action was observed in the ZnO films. The threshold intensity for the lasing was estimated to be ∼70 kW/cm2. Furthermore, it was found that the formation of random laser action is affected by post-annealing, which is associated to the presence of a high-gain medium and efficient light scattering.

Ultraviolet electroluminescence of ZnO based heterojunction light emitting diode

AbstractZnO/GaN p‐i‐n heterojunctions light emitting diodes were fabricated by plasma‐assisted molecular beam epitaxy. We make use of high resistivity of nitrogen doped ZnO to fabricate n‐ZnO/i‐ZnO/p‐GaN heterojunction light emitting diode. The emission of i‐ZnO was obtained due to the limitation effect of i‐ZnO on electrons and holes. Moreover, n‐ZnO/i‐MgO/p‐GaN heterojunction light emitting diode was also fabricated. The limitation effect on electrons is increased in this heterojunction. The bright ultraviolet electroluminescence at 382 nm originating from the ZnO layer was observed in the room temperature electroluminescence spectrum. We hope to realize the stimulated emission of ZnO using these heterojunctions by the improvement of crystal quality and the optimization of device structure. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Improvement of crystal quality by time controlled annealing

Improvement of crystal quality by time controlled annealing

Investigation of the annealing effects on the structural and optical properties of sputtered ZnO thin films

Various effects of annealing on the optical and structural properties of sputtered ZnO layers have been investigated. ZnO layers were sputtered on Si (1 1 1) substrates and annealed at 700–900 °C under H 2O atmosphere. Optical and structural properties along with surface morphologies were estimated by using photoluminescence (PL), X-ray diffraction (XRD), and atomic force microscopy (AFM), respectively. Improvement of crystal quality was observed up to 800 °C, however, it deteriorated at higher annealing temperature due to the enhanced interdiffusion of ZnO layer and Si substrate. Transmission electron microscopy (TEM) and Auger electron spectroscopy (AES) were used to characterize the microstructure of ZnO layers. An amorphous interfacial layer is found owing to strong interdiffusion at the interface, and the reduction of the layer thickness as a result of interdiffusion is observed, which is responsible for the degradation of ZnO layer.

Deposition of zinc oxide thin films for application in bulk acoustic wave resonator

High quality piezoelectric zinc oxide (ZnO) thin films were deposited on (1 0 0)-oriented silicon substrate by reactive rf magnetron sputtering for bulk acoustic wave resonator. In order to improve the ZnO thin films quality, structural and electrical characteristics have been compared before and after annealing in helium (He) by X-ray diffraction and reflection coefficient S 11 measurements. Scanning electron microscopy (SEM) was used to study the crystallographic structure. A previous study has shown that a substrate temperature of 100 °C, a distance between the target and the substrate of 70 mm, and a pressure of 3.35 × 10 −3 Torr in argon and oxygen mixed gas atmosphere, are the optimum conditions to sputter ZnO thin films with good homogeneity and a high degree of crystallinity. These films exhibit an electrical resistivity higher than 10 10 Ω cm and an energy band gap of 3.3 eV at room temperature. X-ray diffraction measurements have shown that ZnO films are highly c-axis-oriented with a full width at half maximum (FWHM) below 0.5°. The decrease of the FWHM after annealing treatment has shown the crystal quality improvement. A growth of c-axis (0 0 2)-oriented ZnO films allows predominant longitudinal wave propagation. Bulk acoustic wave (BAW) resonators have been fabricated by stacking different layers of Al/ZnO/Pt on a silicon substrate which could be used for the fabrication of humidity sensors based on the quartz microbalance principle.

Surfactant and impurity properties of antimony on GaAs and GaAs 1 −xN x on GaAs [100] by solid source molecular beam epitaxy

Low temperature (4.5 K) photoluminescence measurements and two-dimensional [115] high resolution X-ray diffractometry rocking curves of antimony doped III–V–N on GaAs grown by solid source molecular beam epitaxy, show a possible non-radiative recombination defect known as the Sb Ga heteroantisite and another Sb-related defect peak at 1017 nm (∼1.22 eV). The elimination of these defects can be a measure of the improvement in crystal quality of GaAsN:Sb. We find that Sb behaves as an impurity and competes with N for As sites until the surfactant effect commences at 1.733 × 10 − 6 Pa. The Sb latency effect which results in a graded Sb composition at the interface was found by secondary ion mass spectroscopy measurements.

Pulse electrodeposition and electrochemical quartz crystal microbalance techniques for high perpendicular magnetic anisotropy cobalt nanowire arrays

This research is focused on the development of pulse electrodeposition techniques to fabricate a high-density array of vertically oriented, high-magnetic anisotropy cobalt nanowires using a porous polymer film template. This type of array is a competitive candidate for future perpendicular magnetic media capable of storage densities exceeding 1Terabit∕in.2 The polymer template, derived from a self-assembling P(S-b-MMA) diblock copolymer film, provides precise control over the nanowire diameter (15nm) and interwire spacing (24nm), whereas nanowire length (typically 50to1000nm) is controlled accurately with the aid of real-time electrochemical quartz crystal monitoring. Pulse and pulse-reversed electrodeposition techniques, as compared to dc, are shown to significantly enhance the perpendicular magnetic anisotropy of the magnetic nanowire array and ultimately result in coercivity as large as 2.7kOe at 300K. Magnetic and structural characterizations suggest that these properties arise from an improved degree of magnetocrystalline anisotropy (due to c-axis oriented crystal growth and improvements in crystal quality) that strongly supplements the basic shape anisotropy of the nanowires. Low temperature magnetometry is used to investigate exchange bias effects due to the incorporation of CoO antiferromagnetic impurities during the electrodeposition process and subsequent Co oxidation in air.

Improvement of crystal quality of GaInNAs films grown by atomic hydrogen-assisted RF-MBE

The effect of post-growth annealing and atomic H irradiation in molecular beam epitaxy (MBE) of Ga(In)NAs on vacancy-type defects was studied by positron annihilation technique. We observed a reduction of vacancy-type defects after post-growth anneal at 600 °C for 5 min in a flowing N 2 ambient. However, irradiation of atomic H during growth showed no apparent effect on vacancy-type defects. The role of atomic H irradiation in MBE is known to enhance two-dimensional growth mode, which results in overall improvement of material quality. However, we found that supply of excessive amounts of atomic H would result in degradation of GaInNAs films. Under optimized conditions of ∼1% cracking ratio and 0.6 sccm H 2 flow rate, a surface roughness of 0.27 nm or ∼1 monolayer (ML) was successfully obtained.

Pendeoepitaxy of GaAs and In 0.15Ga 0.85As using laterally oxidized GaAs/Al 0.96Ga 0.04As templates

A technique is presented to achieve pendeoepitaxy of GaAs and In 0.15Ga 0.85As. GaAs/Al 0.96Ga 0.04As templates were formed by etching patterns into the surface and oxidizing the exposed Al 0.96Ga 0.04As to form AlO x , which acts as a mask material. Experiments were performed for the homoepitaxial GaAs growth while varying the growth rate, AsH 3 partial pressure, and the growth temperature to find conditions that produce near vertical sidewalls and smooth surfaces. In 0.15Ga 0.85As heteroepitaxy was performed to evaluate if any improvement in crystal quality could be achieved for growth on the engineered template compared to planar heteroepitaxy.

Ultrahigh‐Quality Silicon Carbide Single Crystals.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Irradiation-induced improvement in crystal quality of epitaxial Ag∕Si(111) films

It has been found that 0.5MeV Si+ irradiation at −150°C greatly improves the crystal quality of epitaxially grown Ag films on Si(111) substrates. The improvements include the decrease in the population of twinning grains and the decrease in mosaic spread in the films. To clarify the mechanism of the irradiation-induced improvement in crystal quality (IIICQ), polycrystalline Ag films with [111] preferred orientation were also irradiated at −150°C. Grain growth in a lateral direction was clearly observed in such Ag films using x-ray diffraction (XRD) analysis. It is evident that the atomic rearrangements occur at grain boundaries due to low-temperature irradiation. On irradiation with 0.5MeV Si ions at −150°C the cross section for the grain growth, estimated by XRD analysis, is about 1.8×10−16cm2, very close to that achieved with IIICQ (1.9×10−16cm2) estimated by Rutherford backscattering spectroscopy/channeling analysis. This result indicates that the mechanism of the IIICQ for the epitaxial Ag∕Si(111) films is very similar to that of the ion bombardment enhanced grain growth.

Effect of annealing on the electrical and optical properties of electron beam evaporated ZnO thin films

Zinc oxide thin films have been grown on (100)-oriented silicon substrate at a temperature of 100 °C by reactive e-beam evaporation. Structural, electrical and optical characteristics have been compared before and after annealing in air by measurements of X-ray diffraction, real and imaginary parts of the dielectric coefficient, refractive index and electrical resistivity. X-ray diffraction measurements have shown that ZnO films are highly c-axis-oriented with a full width at half maximum (FWMH) lower than 0.5°. The electrical resistivity increases from 10−2 Ω cm to reach a value about 109 Ω cm after annealing at 750 °C. The FWHM decreases after annealing treatment, which proves the crystal quality improvement. Ellipsometer measurements show the improvement of the refractive index and the real dielectric coefficient after annealing treatment at 750 °C of the ZnO films evaporated by electron beam. Atomic force microscopy shows that the surfaces of the electron beam evaporated ZnO are relatively smooth. Finally, a comparative study on structural and optical properties of the electron beam evaporated ZnO and the rf magnetron deposited one is discussed.

Ultrahigh-quality silicon carbide single crystals.

Silicon carbide (SiC) has a range of useful physical, mechanical and electronic properties that make it a promising material for next-generation electronic devices. Careful consideration of the thermal conditions in which SiC [0001] is grown has resulted in improvements in crystal diameter and quality: the quantity of macroscopic defects such as hollow core dislocations (micropipes), inclusions, small-angle boundaries and long-range lattice warp has been reduced. But some macroscopic defects (about 1-10 cm(-2)) and a large density of elementary dislocations (approximately 10(4) cm(-2)), such as edge, basal plane and screw dislocations, remain within the crystal, and have so far prevented the realization of high-efficiency, reliable electronic devices in SiC (refs 12-16). Here we report a method, inspired by the dislocation structure of SiC grown perpendicular to the c-axis (a-face growth), to reduce the number of dislocations in SiC single crystals by two to three orders of magnitude, rendering them virtually dislocation-free. These substrates will promote the development of high-power SiC devices and reduce energy losses of the resulting electrical systems.