Conventional Growth Method Research Articles

Effects in B–doped KDP crystals irradiated with neutrons of large spectra energy

AbstractThe results obtained for the r63 electro‐optic coefficient of B‐doped and undoped KDP (KH2PO4) crystals irradiated with neutrons (including thermalized neutrons) produced by scattering of 30 Mev cyclotron protons on a target of Ta201, are presented and compared to those obtained for non‐irradiated doped and undoped crystals. The B‐doped (H3BO3, Na2B4O7 and Li2B4O7) crystals were obtained by the conventional growth method by temperature decrease with 1 wt % dopant concentration in solution. The thermal neutron flux was around ϕ = 1. 1010 n/cm2 s. Pulses of ∼15 μs long, in damped oscillatory mode (V= 8 kV, τ=1.95 μs) were used for the electro‐optic measurements. A Pockels cell, a photomultiplier, a He‐Ne laser (λ=632.8 nm, 5 mW, linearly polarized) and a Tk 720 A oscilloscope complete the experimental setup. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Important research targets to be explored for β-FeSi 2 device making

To place β-FeSi 2 as a 3rd generation Kankyo (Environmentally Friendly) Semiconductor after GaAs, one should demonstrate its superior features by fabricating practical devices. One has to prepare high-quality β-FeSi 2 films with (1) precisely controlled Fe/Si ratio (1:2), (2) flat and cracks-free surface, (3) pinhole-free surface and interfaces, (4) flat interfaces, (5) well-controlled electron or hole concentration with residual carrier concentration as low as ∼10 16 cm −3. One should accordingly explore novel thin film manufacturing technologies by considering specific properties of constituent Fe and Si atoms. Conventional growth methods used for III–V and II–VI compound semiconductor films are not suited for β-FeSi 2. Here we summarize the current status of film preparation technologies and describe their advantages and drawbacks. To explore the possibility of β-FeSi 2 for low cost and high conversion efficiency solar cells, high quality β-FeSi 2 films have been formed on Si substrates by molecular beam epitaxy (MBE) and sputtering methods. The critical feature about the device structure is an optimized thin β-FeSi 2 template buffer layer on Si(111) substrate. The template served as a substrate for epitaxial growth of single crystal β-FeSi 2 film and restrains the Fe diffusion into Si at β-FeSi 2/Si interface. For n-β-FeSi 2/p-Si structure under air mass 1.5, an energy conversion efficiency of 3.7% was obtained, showing that β-FeSi 2 is practically a promising semiconductor for making solar cells.

Important research targets to be explored for $beta;-FeSi2 device making

To place β-FeSi2 as a 3rd generation Kankyo (Environmentally Friendly) Semiconductor after GaAs, one should demonstrate its superior features by fabricating practical devices. One has to prepare high-quality β-FeSi2 films with (1) precisely controlled Fe/Si ratio (1:2), (2) flat and cracks-free surface, (3) pinhole-free surface and interfaces, (4) flat interfaces, (5) well-controlled electron or hole concentration with residual carrier concentration as low as ∼10 16 cm−3. One should accordingly explore novel thin film manufacturing technologies by considering specific properties of constituent Fe and Si atoms. Conventional growth methods used for III–V and II–VI compound semiconductor films are not suited for β-FeSi2. Here we summarize the current status of film preparation technologies and describe their advantages and drawbacks. To explore the possibility of β-FeSi2 for low cost and high conversion efficiency solar cells, high quality β-FeSi2 films have been formed on Si substrates by molecular beam epitaxy (MBE) and sputtering methods. The critical feature about the device structure is an optimized thin β-FeSi2 template buffer layer on Si(111) substrate. The template served as a substrate for epitaxial growth of single crystal β-FeSi2 film and restrains the Fe diffusion into Si at β-FeSi2/Si interface. For n-β-FeSi2/p-Si structure under air mass 1.5, an energy conversion efficiency of 3.7% was obtained, showing that β-FeSi2 is practically a promising semiconductor for making solar cells.

次世代超電導線材の開発と特性評価 磁場中高Ｊｃ線材を目指したＲＥＢａ２Ｃｕ３Ｏｙ薄膜の作製

We developed a low-temperature growth (LTG) technique as a novel deposition process to obtain RE1+xBa2-xCu3Oy (REBCO) thin films with high superconducting properties. SmBCO films fabricated using the LTG-technique (LTG-SmBCO) showed complete c-axis orientations at a low substrate temperature (Ts). This Ts is about 100°C lower than the Ts when fabricating c-axis oriented SmBCO films using the conventional vapor growth method. Moreover, the critical current density (Jc) of the LTG-SmBCO in magnetic fields at 77 K has been improved reaching a high value of 0.17 MA/cm2 at 5 T (B//c). The magnetic field angular dependence of the Jc at 77 K in LTG-SmBCO had a smaller anisotropy than that in PLD-SmBCO. From the microstructure observation, we found that the LTG-SmBCO showed a compositional fluctuation in Sm/Ba. We assume that the high Jc of LTG-SmBCO was affected by the solid solution acting as three-dimensional pinning center. From the viewpoint of diffusion in the solid phase, we speculated that this solid solution was generated by the low Ts.

Crystal growth of photovoltaic polycrystalline Si1-xGex by die-casting growth

ABSTRACTCoin-shaped multicrystalline Si1-xGex crystals were grown using a Brigdman method combined with die-casting growth. Si1-xGex alloy is known as a candidate material for producing Auger generation, which creates more than one electron/hole pair per absorbed photon. Since Si1-xGex alloy shows a complete series of solid solutions, precipitating crystals with a certain composition of silicon or germanium by conventional selective growth methods is burdensome. Using die-casting combined with Bridgman growth brought about Si1-xGex precipitation in a form completely different from that predicted by the Si-Ge phase diagram. By combining this growth with subsequent heat treatment of the precipitated Si1-xGex sample, Si1-xGex (x= 0.5 ± 3 %) could be obtained. Indirect band-gap energy was estimated by measuring room-temperature optical absorption coefficient of the grown samples.

Realization of Low Dislocation GaN/Sapphire Wafers by 3-Step Metalorganic Vapor Phase Epitaxial Growth with Island Induced Dislocation Control

A novel dislocation reducing mechanism was successfully introduced into the conventional 2-step metalorganic vapor phase epitaxial (MOVPE) growth method of GaN/sapphire wafer by inserting an additional intermediate-temperature (IT)-GaN buffer between the low-temperature (LT)-GaN buffer and the main high-temperature (HT)-GaN layer. During the growth of the IT-GaN buffer, high-density islands with faceted slopes were formed. Vertically propagating dislocations from the GaN/sapphire interface were bent at the faceted slopes so as to gather at the island/island boundaries, where dislocation loops were formed efficiently. Due to such an island induced dislocation control mechanism, the dislocation density was reduced markedly from the 109 cm-2 range to the mid-108 cm-2 range during the growth of the IT-GaN buffer. As a result, the GaN/sapphire wafers with low dislocation density of 3×108 cm-2 were successfully grown by such a 3-step MOVPE method, whereas high-density dislocations (1×109 cm-2) remained in the final GaN/sapphire wafers grown by the conventional 2-step growth method without dislocation control.

3C-SiC thin epilayer formation at low temperature using ion beams

3C-SiC heteroepitaxy by means of direct irradiation of isotopically mass-selected 28Si and 12C ion beams onto Si substrates has been performed. The surface structural evolution during ion irradiation was monitored by reflection high-energy electron diffraction (RHEED). The process temperatures of both carbonization and ∼15nm-thick thin SiC layer formation are ∼670°C which is quite low as compared to other conventional film growth methods using gas species providing nearly equilibrium processes. The results suggest that a far equilibrium and subplantation film growth process provided by low-energy ion beams contributes to the low temperature SiC epilayer formation.

Effects of the low temperature grown buffer layer thickness on the growth of GaAs on Si by MBE

The effect of initial low temperature grown buffer (LT buffer) layer thickness on GaAs on Si grown by conventional two-step growth method for molecular beam epitaxy has been investigated. The grown layers have been characterized by photoluminescence (PL), double crystal X-ray diffraction (XRD) and atomic force microscopy (AFM). It is indicated that the crystalline quality and the root-mean-square (RMS) roughness of thick GaAs layer on Si depends on the thickness of initial low temperature grown nucleation buffer layer. The present investigation confirms that the initial layer should be sufficiently thick to obtain good quality smooth layers of GaAs on Si substrate. The FWHM of heavy-hole band emission was as low as 4.8 meV for the samples with initial nucleation layer of around 200 nm. The AFM study revealed that the RMS roughness of the thick GaAs layer on Si is large for the samples with thin initial nucleation layer. Also, the annealed LT buffer layers indicate a similar RMS roughness tendency instead of almost constant RMS roughness for the as-grown LT buffer layers. They suggest that the macroscopic surface morphologies and the film quality of the stressed heteroepitaxial layer are dominated during the heating steps of the initial LT buffer layer.

Biomimetic Crystallization of Calcium Carbonate Spherules with Controlled Surface Structures and Sizes by Double-Hydrophilic Block Copolymers

Big CaCO3 spherules with controlled surface structures and sizes ranging from several hundreds of nanometers and micrometers can be easily fabricated through a slow gas–liquid diffusion reaction at room temperature by using double-hydrophilic block copolymers (DHBCs) as crystal modifiers. The influence of the DHBCs with different functionalities such as carboxyl, partially phosphated, and phosphorylated groups on the crystallization and structure of calcite formation was investigated. The morphology evolution process and the early stage of the big spherical superstructures were followed. Such big spherules with complex surface structure made of calcite rhombohedra are not easily produced by conventional solution growth methods, and furthermore show high potential for chromatographic purposes due to their exposition of multiple calcite faces and the huge particle sizes suitable for chromatographic column packings.

On the morphology of films grown by droplet-assisted molecular beam epitaxy

Atomic Force microscopy was used to investigate the morphology of GaAs(001) layers deposited by molecular beam epitaxy using a non conventional growth method that involves the supply of a few monolayers of gallium (the arsenic cell is shuttered) followed by the annealing of the surface under an arsenic ux (the gallium cell is shuttered). When the silicon shutter is opened and closed together with the gallium one, this particular growth mode allows the silicon atoms to be incorporated into the arsenic sites and p-type GaAs(001) layers can be obtained. Several sets of samples were grown and analyzed in order to better understand the microscopic growth mechanisms of this kind of layers and minimize the number and size of the structural defects that are characteristic of this peculiar technique.

High-quality crystal growth of organic nonlinear optical crystal DAST

Methods for growing high-quality nonlinear optical 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystals have been developed. In conventional growth methods, it was difficult to control the position of spontaneous nucleation and the nucleation period. Difficulty in the nucleation and growth positioning was effectively solved by the slope nucleation method (SNM), and the nucleation period could be controlled by the laser irradiation method (LIM). In this study, we could obtain a high-quality DAST crystal by combining SNM and LIM. This crystal had a full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve of 18.7 arcsec, compared with 50–130 arcsec FWHM for conventional crystals.

Growth and Physical Characterization of La-Doped Bi 4 Ti 3 O 12 Crystals

The crystals of La-doped Bi 4 Ti 3 O 12 were grown by a conventional flux growth method using Bi 2 O 3 , TiO 2 and La 2 O 3 . The quantitative analysis of x-ray photoelectron spectra and change of band gap make an estimate of La sites in the La doped Bi 4 Ti 3 O 12 crystals. All of the La ions doped in the Bi 4 Ti 3 O 12 crystals are well substituted with Bi ions. The remanent polarization, P r and the coercive field, E c were 3.2, 5.2 and 0.3 w C/cm 2 , and 5.6, 7.6 and 0.6 kV/cm for Bi 4.3 Ti 3 O 12.4 , Bi 3.65 La 0.47 Ti 3 O 12.17 and Bi 2.76 La 1.67 Ti 3 O 12.65 crystals, respectively. However, Bi 2.76 La 1.67 Ti 3 O 12.65 crystal seems to be not saturated.

Growth and Physical Characterization of La-Doped Bi 4 Ti 3 O 12 Crystals

The crystals of La-doped Bi 4 Ti 3 O 12 were grown by a conventional flux growth method using Bi 2 O 3 , TiO 2 and La 2 O 3 . The quantitative analysis of x-ray photoelectron spectra and change of band gap make an estimate of La sites in the La doped Bi 4 Ti 3 O 12 crystals. All of the La ions doped in the Bi 4 Ti 3 O 12 crystals are well substituted with Bi ions. The remanent polarization, P r and the coercive field, E c were 3.2, 5.2 and 0.3 w C/cm 2 , and 5.6, 7.6 and 0.6 kV/cm for Bi 4.3 Ti 3 O 12.4 , Bi 3.65 La 0.47 Ti 3 O 12.17 and Bi 2.76 La 1.67 Ti 3 O 12.65 crystals, respectively. However, Bi 2.76 La 1.67 Ti 3 O 12.65 crystal seems to be not saturated.

High temperature growth of InN on GaP(1 1 1)B substrate using a new two-step growth method

High temperature growth of InN is studied on GaP(1 1 1)B substrate using a new two-step growth method by metal organic vapor phase epitaxy. A single crystalline InN film with an excellent surface morphology can be grown on GaP(1 1 1)B at high temperature (600–650°C) using a low temperature InN buffer layer. The low temperature InN buffer layer is grown at 450°C, and then the temperature is raised to the epitaxial growth temperature (600–650°C) while continuing the growth. It is found to be a very effective growth technique to obtain high temperature growth of InN using a low temperature buffer layer. The low temperature InN buffer layer was also needed to suppress the substrate surface nitridation during the growth at such high temperature. The differences of this new two-step growth method from the conventional two-step growth method as well as their influences on the grown InN films are also discussed in details.

Transmission Electron Microscope Analysis of Microstructures in GaN Grown on (111)A and (111)B of GaAs by Metalorganic Hydrogen Chloride Vapor-Phase Epitaxy

Cross-sectional transmission electron microscope (TEM) observation has been performed for GaN grown simultaneously on (111)A and (111)B of a GaAs substrate by metalorganic hydrogen chloride vapor-phase epitaxy. A low-temperature (LT) buffer layer of GaN about 85 nm thick was deposited at 550°C, and then a thick layer of GaN (HT-GaN) was grown at 920°C. On GaAs(111)A, the LT-buffer layer has a fine columnar structure and its stacking order is of 2H (wurtzite) or a mixture of 2H and 3C (zincblende). On the LT-layer, the HT-GaN was grown in the wurtzite structure with Ga-polarity. In the HT-GaN, a columnar structure is formed on the interface of GaN/LT-GaN, and a good quality GaN is grown in the upper region. These characteristics in microstructure are very similar to those in GaN grown on α-Al 2 O 3 by the conventional two-step growth method. On GaAs(111)B, the LT-buffer layer has a disordered stacking sequence. A block structure is formed on the interface of GaN/LT-GaN, and the HT-GaN has a fairly good quality. The HT-GaN has Ga-polarity in spite of the fact that the substrate has As-polarity.

Transmission Electron Microscope Analysis of Microstructures in GaN Grown on (111)A and (111)B of GaAs by Metalorganic Hydrogen Chloride Vapor-Phase Epitaxy

Cross-sectional transmission electron microscope (TEM) observation has been performed for GaN grown simultaneously on (111)A and (111)B of a GaAs substrate by metalorganic hydrogen chloride vapor-phase epitaxy. A low-temperature (LT) buffer layer of GaN about 85 nm thick was deposited at 550°C, and then a thick layer of GaN (HT-GaN) was grown at 920°C. On GaAs(111)A, the LT-buffer layer has a fine columnar structure and its stacking order is of 2H (wurtzite) or a mixture of 2H and 3C (zincblende). On the LT-layer, the HT-GaN was grown in the wurtzite structure with Ga-polarity. In the HT-GaN, a columnar structure is formed on the interface of GaN/LT-GaN, and a good quality GaN is grown in the upper region. These characteristics in microstructure are very similar to those in GaN grown on α-Al 2 O 3 by the conventional two-step growth method. On GaAs(111)B, the LT-buffer layer has a disordered stacking sequence. A block structure is formed on the interface of GaN/LT-GaN, and the HT-GaN has a fairly good quality. The HT-GaN has Ga-polarity in spite of the fact that the substrate has As-polarity.

Electron-beam-assisted evaporation of epitaxial CeO2 thin films on Si substrates

Electron-beam-assisted evaporation is a way to lower the growth temperature and improve crystalline quality of CeO2(110) layers on Si(100) substrates. The electron-beam-assisted evaporation system is constructed utilizing an electron-beam-irradiation system with a suppressor electrode around the sample holder. The suppressor bias condition is optimized as a function of acceleration energy of assisting electrons. The epitaxial growth quality depends on the assisting electron-beam energy. Optimum electron energy is experimentally determined to be around 360 eV, wherein the epitaxial temperature is lowered to 710 °C, i.e., temperature lowering of more than 100 °C compared with the conventional growth method.

Properties and self-organization of CdSe:S quantum islands grown with a cadmium sulfide compound source

We demonstrate a new technique to grow high-quality CdSe quantum films and islands with a very small sulfur contamination by using a cadmium sulfide compound source as Cd supply and additional Se flux. By monitoring the lattice constant with reflection high-energy electron diffraction, it is shown that the sulfur is almost completely substituted by Se and CdSe with a contamination below 5% sulfur is formed. The quantum structures obtained by the new method are generally of higher quality than those obtained by more conventional growth methods using elemental sources, even if migration enhanced methods were employed. With a brief growth interruption or post-growth annealing step the initially smooth CdSe layer can be reorganized into islands. The duration of this step as well as the initial amount of deposition allows a rather good control over the island formation. A strongly enhanced growth rate is observed for the first few monolayers of the ZnSe capping layer, which indicates a partial dissolution of the islands in the ZnSe growth front and Cd segregation.

Molecular layer epitaxy

Molecular layer epitaxy (MLE) of GaAs related compounds and Si with SiO2 deposition has been developed to realize the tera-hertz operating devices. At sufficiently low process temperature, lower than in the conventional growth method, device quality epitaxial layers were achieved by MLE. In GaAs, 100 Å scale static induction transistors are fabricated with MLE. These transistors operated in a mixed ballistic-tunneling mode or in the pure tunneling mode. Basic research on MLE in the field of surface science and material science is performed, from the point of view of device application of this technology.

Rapid growth of deuterated L-Alanine-doped Triglycine Sulfate crystal and its pyroelectric property

The higher supersaturation was achieved by means of the over-heating of growth solution. This method was applied to watersoluble crystal, Deuterated L-A lanine-doped TGS (dLATGS). The dLATGS crystalis a pyroelectric material for the infrared detector which, in general, exhibits one of the highest pyroelectric response of all such materials. The over-heating enlarges the metastable zone during temperature-reducing growing process as a result of eliminating the initial clusters in the mother solution.A dLATGS crystal (9×2×7cm) was grown with an average rate of 2×2×0.6mm/day (24g/day) in direction of a, -b and c axe s, respectively. This growth rate was several times larger than that of the conventional growth method. The internal bias shown in D-E curves depends on the growth rate of L-Alanine doped crystal of which an uniformity was enough fine to use as an electronic device.The distribution coefficient of L-Alanine molecule increased with increasing the growth rate.