• Proceedings of the Korea Association of Crystal Growth Conference
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• 1996.06a
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• pp.258-292
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• 1996

In the last year great interest appears to YBCO thin films preparation on different substrate materials. Preparation of epitaxial film is a very difficult problem. There are many requirements to substrate materials that must be fullfilled. Main problems are lattice mismatch (misfit) and similarity of structure. From paper [1] or follows that difference in interatomic distances and angles of substrate and film is mire important problem than similarity of structure. In this work we present interatomic distances and angle relations between substrate materials belonging to ABCO4 group (where A-Sr or Ca, B-rare earth element, C-Al or Ga) of different orientations and YBCO thin films. There are many materials used as substrates for HTsC thin films. ABCO4 group of compounds is characterized by small dielectric constants (it is necessary for microwave applications of HTsC films), absence of twins and small misfit [2]. There most interesting compounds CaNdAlO4, SrLaAlO4 and SrLaGaO4 were investigated. All these compounds are of pseudo-perovskite structure with space group 14/mmm. This structure is very similar to structure of YBCO. SLG substrate has the lowest misfit (0.3%) and dielectric constant. For preparation of then films of substrates of this group of compound plane of <100> orientation are mainly used. Good quality films of <001> orientations are obtained [3]. In this case not only a-a misfit play role, but c-3b misfit is very important too. Sometimes, for preparation of thin films substrates of <001> and <110> orientations were manufactured [3]. Different misfits for different YBCO faces have been analyzed. It has been found that the mismatching factor for (100) face is very similar to that for (001) face so there is possibility of preparation of thin films on both orientations. SrLaAlO4(SLA) and SrLaGaO4(SLG) crystals of general formula ABCO4 have been grown by the Czochralski method. The quality of SLA and SLG crystals strongly depends on axial gradient of temperature and growth and rotation rates. High quality crystals were obtained at axial gradient of temperature near crystal-melt interface lower than 50℃/cm, growth rate 1-3 mm/h and the rotation rate changing from 10-20pm[4]. Strong anisotropy in morphology of SLA and SLG single crystals grown by the Czochralski method is clearly visible. On the basics of our considerations for ABCO4 type of the tetragonal crystals there can appear {001}, {101}, and {110} faces for ionic type model [5]. Morphology of these crystals depend on ionic-covalent character of bonding and crystal growth parameters. Point defects are observed in crystals and they are reflected in color changes (colorless, yellow, green). Point defects are detected in directions perpendicular to oxide planes and are connected with instability of oxygen position in lattice. To investigate facets formations crystals were doped with Cr3+, Er3+, Pr3+, Ba2+. Chromium greater size ion which is substituted for Al3+ clearly induces faceting. There appear easy {110} faces and SLA crystals crack even then the amount of Cr is below 0.3at.% SLG single crystals are not so sensitive to the content of chromium ions. It was also found that if {110} face appears at the beginning of growth process the crystal changes its color on the plane {110} but it happens only on the shoulder part. The projection of {110} face has a great amount of oxygen positions which can be easy defected. Pure and doped SLA and SLG crystals measured by EPR in the<110> direction show more intensive lines than in other directions which allows to suggest that the amount of oxygen defects on the {110} plane is higher. In order to find the origin of colors and their relation with the crystal stability, a set of SLA and SLG crystals were investigated using optical spectroscopy. The colored samples exhibit an absorption band stretching from the UV absorption edge of the crystal, from about 240 nm to about 550 m. In the case of colorless sample, the absorption spectrum consists of a relatively weak band in the UV region. The spectral position and intensities of absorption bands of SLA are typical for imperfection similar to color centers which may be created in most of oxide crystals by UV and X-radiation. It is pointed out that crystal growth process of polycomponent oxide crystals by Czochralski method depends on the preparation of melt and its stoichiometry, orientation of seed, gradient of temperature at crystal-melt interface, parameters of growth (rotation and pulling rate) and control of red-ox atmosphere during seeding and growth (rotation and pulling rate) and control of red-ox atmosphere during seeding and growth. Growth parameters have an influence on the morphology of crystal-melt interface, type and concentration of defects.

• Nuclear Engineering and Technology
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• v.28 no.2
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• pp.118-128
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• 1996

The lattice parameters of stoichiometric $UO_2$ and $U_{1-y}Er_{y}O_2$ in the range of y=0.01 to y =0.33 were determined with use of X-ray diffraction data. Oxygen potentials have been measured by means of a thermogravimetric method in the range of 1200~$1500^{\circ}C$ and $10^{-14}$ $\leq$ $Po_2$ $\leq$ $10^{-3}$ for pure $UO_2$ and $U_{1-y}Er_{y}O_{2{\pm}x}$ solid solutions with y=0.02, y=0.06 and y=0.20, respectively. Their oxygen partial pressures were maintained by controlling $CO_2$/CO mixture atmosphere, and the $Po_2$ values corresponding to x of $U_{1-y}Er_{y}O_{2{\pm}x}$ solid solutions were measured with an electrolyte oxygen sensor. The lattice parameter decreases linearly with an increase in the erbium content. The change of the lattice parameter can be expressed in a linear equation of y as a($\AA$) =5.4695-0.220y for 0 $\leq$y$\leq$0.33. The experimental coefficient of y -0.220 in $U_{1-y}Er_{y}O_2$ was an intermediate value between the calculated values -0.273 and -0.156 in the case of $U^{5+}$ and $U^{6+}$, respectively. The (equation omitted) has been found to undergo abrupt increase in the range of -360 to -270 kJ/mole for y=0.06 and -320 to -220 H/mole for y=0.20, respectively, in the temperature range of 1200-$1500^{\circ}C$. (equation omitted) increases with erbium content, but the effect of the dopant for x =0.01 is less significant than that for stoichiometry. The oxygen potentials for $UO_2$ and $U_{0.98}Er_{0.02}O_{2+x}$ can be approximately represented by the $U^{5+}$/$U^{4+}$ model but those for y$\geq$ 0.06 in $U_{1-y}Er_{y}O_{2{\pm}x}$ solid solutions cannot be interpreted by the mean uranium valence model.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.57-58
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• 2011

III-V족 화합물 반도체의 일종인 InSb는 77 K에서 0.23 eV의 작은 밴드 갭을 가지며 높은 전하 이동도를 가지고 있기 때문에 대기권에서 전자파 흡수가 일어나지 않는 3~5 ${\mu}m$범위의 장파장 적외선 감지가 가능하여 중적외선 감지 소자로 이용되고 있다. 하지만 InSb는 밴드 갭이 매우 작기 때문에, 소자 제작시 누설전류에 의한 소자 특성의 저하가 문제시 되고 있다. 또한 다른 화합물 반도체에 비해 녹는점이 낮고, 휘발성이 강한 5족 원소인 Sb의 승화로 기판의 화학양론적 조성비(stoichiometry)가 변하기 쉬워, 계면특성 저하의 원인이 된다. 따라서 우수한 특성을 가지는 적외선 소자의 구현을 위해서, 저온에서 계면 특성이 우수한 고품질의 절연막 증착 연구가 필수적이다. 본 연구에서는 InSb 기판 위에 $SiO_2$, $Si_3N_4$의 절연막 형성시 증착온도의 변화에 따른 계면 트랩 밀도를 분석하였다. $SiO_2$, $Si_3N_4$ 절연막은 플라즈마 화학 기상 증착법(PECVD)을 이용하여 n형 InSb 기판 위에 증착하였으며, 증착온도를 $120^{\circ}C$부터 $240^{\circ}C$까지 변화시켰다. Metal oxide semiconductor(MOS) 구조 제작을 통하여, 커패시턴스-전압(C-V)분석을 진행하였으며, 절연막과 InSb 사이의 계면 트랩 밀도를 Terman method를 이용하여 계산하였다[1]. 또한, $SiO_2$와 $Si_3N_4$의 XPS 분석과 TOF-SIMS 분석을 통하여 계면 트랩 밀도의 원인을 밝혀 보았다. $120{\sim}240^{\circ}C$ 온도 범위에서 계면 트랩 밀도는 $Si_3N_4$의 경우 $2.4{\sim}4.9{\times}10^{12}cm^{-2}eV^{-1}$, $SiO_2$의 경우 $7.1{\sim}7.3{\times}10^{11}cm^{-2}eV^{-1}$ 값을 나타냈고, 두 절연막 모두 증착 온도가 증가할수록 계면 트랩 밀도가 증가하는 경향을 보였다. 그러나 모든 샘플에서 $Si_3N_4$의 경우, flat band voltage가 음의 전압으로 이동한 반면, $SiO_2$의 경우, 양의 전압으로 이동하는 것을 확인할 수 있었다. 계면 트랩 밀도 증가의 원인을 확인하기 위해서, oxide를 $120^{\circ}C$, $240^{\circ}C$에서 증착시킨 샘플을 XPS 분석을 통하여 깊이에 따른 성분분석을 하였고, 그 결과, $240^{\circ}C$에서 증착된 샘플에서 계면에서 $In_2O_3$와 $Sb_2O_3$ 피크의 증가를 확인하였다. 이는 계면에서 oxide양이 증가함을 의미하며, 이렇게 생성된 oxide는 계면 트랩으로 작용하므로, 계면 특성을 저하시키는 원인으로 작용함을 알 수 있었다. Nitride 절연막을 증착시킨 샘플은 TOF-SIMS 분석을 통해, 계면에서의 성분 분석을 하였고, 그 결과, $240^{\circ}C$에서 증착된 샘플에서 In-N, Sb-N, Si-N 결합의 감소를 확인하였다. 이렇게 분해된 결합들의 dangling 결합이 늘어 계면 트랩으로 작용하므로, 계면 특성을 저하시키는 원인으로 작용함을 알 수 있었다. 최종적으로, 소자특성을 확인 하기 위하여 계면 트랩 밀도가 가장 낮게 측정된 $200^{\circ}C$ 조건에서 $SiO_2$ 절연막을 증착하여 InSb 적외선 소자를 제작하였다. 전류-전압(I-V) 분석 결과 -0.1 V에서 16 nA의 누설 전류 값을 보였으며, $2.6{\times}10^3{\Omega}cm^2$의 RoA(zero bias resistance area)를 얻을 수 있었다. 절연막 증착조건의 최적화를 통하여, InSb 적외선 소자의 특성이 개선됨을 확인할 수 있었다.

• Korean Journal of Crystallography
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• v.6 no.2
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• pp.125-133
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• 1995

The crystal structure of dehydrated, partially Co(II)-exchanged zeolite X, stoichiometry Co2+Na+-X (Co41+Na10Si100Al92O384) per unit cell, has been determined from three-dimensional X-ray diffraction data gathered by counter methods. The structure was solved and refined in the cubic space group Fd3:α=24.544(1)Å at 21(1)℃. The crystal was prepared by ion exchange in a flowing stream using a solution 0.025 M each in Co(NO3)2 and Co(O2CCH3)2. The crystal was then dehydrated at 380℃ and 2×10-6 Torr for two days. The structure was refined to the final error indices, R1=0.059 and R2=0.046 with 211 reflections for which I > 3σ(I). Co2+ ions and Na+ ions are located at the four different crystallographic sites. Co2+ ions are located at two different sites of high occupancies. Sixteen Co2+ ions are located at the center of the double six-ring (site I; Co-O = 2.21(1)Å, O-Co-O = 90.0(4)°) and twenty-five Co2+ ions are located at site II in the supercage. Twenty-five Co2+ ions are recessed 0.09Å into the supercage from its three oxygen plane (Co-O = 2.05(1)Å, O-Co-O = 119.8(7)°). Na+ ions are located at two different sites of occupandies. Seven Na+ ions are located at site II in the supercage (Na-O = 2.29(1)Å, O-Na-O = 102(1)°). Three Na+ ions are statistically distribyted over site III, a 48-fold equipoint in the supercages on twofold axes (Na-O = 2.59(10)Å, O-Na-O = 69.0(3)°). Seven Na+ ions are recessed 1.02Å into the supercage from the three oxygen plane. It appears that Co2+ ions prefer sites I and II in order, and that Na+ ions occupy the remaining sites, II and III.

• Korean Journal of Materials Research
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• v.4 no.7
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• pp.759-766
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• 1994

Our investigation aimed to reduce the leakage current of $Ta_2O_5$ thin film capacitor by layering SnOz thin film layer under Ta thin film, thereby supplying extra oxygen ions from the $SnO_{2}$ underlayer to enhance the stoichiometry of $Ta_2O_5$ during the oxidation of Ta thin film. Tantalum was evaporated by e-beam or sputtered on p-Si wafers with various deposition temperatures and was oxidized by dry--oxygen at the temperatures between $500^{\circ}C$ and $900^{\circ}C$. Aluminum top and bottom electrodes were formed to make Al/$Ta_2O_5$/p-Si/Al or $Al/Ta_2O_5/SnO_2$p-Si/AI MIS type capacitors. LCR meter and pico-ammeter were used to measure the dielectric constants and leakage currents of the prepared thm film capacitors. XRD, AES and ESCA were employed to confirm the crystallization of the thin f~lm and the compositions of the films. Dielectric constant of $Ta_2O_5$ thin film capacitor with $SnO_{2}$ underlayer was found to be about 200, which is about 10 times higher than that of $Ta_2O_5$ thin film capacitor without $SnO_{2}$ underlayer. In addition, higher oxidation temperatures increased the dielectric constants and reduced the leakage current. Higher deposition temperature generally gave lower leakage current. $Ta_2O_5/SnO_2$ capacitor deposited at $200^{\circ}C$ and oxidized at $800^{\circ}C$ showed significantly lower leakage current, $10^{-7}A/\textrm{cm}^2$ at $4 \times 10^{5}$V/cm, compared to the one without $SnO_{2}$ underlayer. XRD showed that $Ta_2O_5$ thin film was crystallized above $700^{\circ}C$. AES and ESCA showed that initially the $SnO_{2}$, underlayer supplied oxygen ions to oxidize the Ta layer, however, Sn also diffused into the Ta thin film layer to form a new $Ta_xSn_YO_Z$ , ternary oxide layer after all.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.13 no.3
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• pp.178-189
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• 2008

The distribution of inorganic nutrients and their remineralization ratio in the divergence zone ($7^{\circ}{\sim}10.5^{\circ}N$) of the northeastern Pacific were investigated from July 2003 to July 2007. A divergence zone along the boundary of the North Equatorial Counter Current (NECC) and North Equatorial Current (NEC) at $10^{\circ}N$ was observed in July 2007 when the La Nina event and divergence-related upwelling was strong. The mean depth of oligotrophic surface mixed layer in the divergence zone was 46, 61, and 30 m in July 2003, August 2005, and July 2007, respectively. Below the surface mixed layer, a nutricline was clearly observed. The depth integrated value of nitrate including nitrite (DIVn) in the upper layer($0{\sim}100$ m depth) ranged from 5.51 to 21.71 $gN/m^2$(mean 12.82 $gN/m^2$) in July 2003, from 5.62 to 8.46 $gN/m^2$ (mean 7.15 $gN/m^2$) in August 2005, and from 8.98 to 27.80 $gN/m^2$(mean 21.12 $gN/m^2$) in July 2007. The maximum DIVn was observed at the divergence zone. The distributions of phosphate(DIVp) and silicate(DIVsi) were similar to that of DIVn and the DIVn/DIVsi ratio was $0.87{\pm}0.11$ in the upper layer. The limiting nutrient for phytoplankton growth in the study area was identified as nitrogen(N/P ratio=14.6). The nitrate (including nitrite) concentrations were lower in the region mainly affected by NEC than in the region affected by NECC. The study area of low silicate concentrations was also considered to be Si-limiting environment. The remineralization ratios of nutrients were $P/N/-O_2=1/14.6{\pm}1.1/100.4{\pm}8.8(23.44{\leq}Sigma-{\theta}{\leq}26.38)$ in the study area. These ratios suggested remineralization process in the surface layer of divergence zone.