Thermal Cleaning Research Articles

シリコンの分子線エピタキシー（ＭＢＥ）プロセスにおける基板のサーマルクリーニング温度の低温化

A modified RCA cleaning method was employed for cleaning silicon substrates before silicon molecular beam epitaxy (MBE) growth. Three kinds of wet chemical processes using the conventional SC1 and SC2 solutions were employed in this study. The type of the solution and/or the dipping time at the final step of the wet chemical process was changed. When the cleaning process was ended with SC 1 solution, thermal cleaning temperature (Ttc) of 800°C was required to obtain a smooth surface of the silicon substrate. On the other hand, when it was ended with SC2 solution, a smooth surface was successfully obtained even at Ttc 690°C. SC2 was a preferable solution at the final step of the modified RCA cleaning method for lowering Ttc, and the roughness became smaller with increasing the dipping time in the final SC2 solution.

Formation of the ion current of a high-temperature femtosecond laser plasma on the target surface containing an impurity layer

The effect of an impurity film lying on the surface of a target in a vacuum of up to 10-5 Torr on the ion acceleration in a plasma, which is formed by a 2×1016-W cm-2 femtosecond laser pulse is studied using the time-of-flight and mass spectrometry techniques. It is shown that, under such conditions, the maximum mean energy per unit charge (8 keV) is gained by protons, whereas the ions constituting the target substance (Si, Ti) acquire an energy per unit charge of <1 keV. The use of a <10 J cm-2 nanosecond laser pulse applied 0.1–100 ms before a femtosecond laser pulse allows the target surface to be efficiently cleaned due to the removal of H-, C-, and O-containing molecules from it. Unlike a continuous thermal heating of a surface, the pulse laser cleaning ensures higher heating temperatures and can be efficiently applied to any solid targets in both the thermal and plasma cleaning modes.

Surface structures of clean and oxidized Nb(100) by LEED, AES, and STM

The surface structures of a Nb(100) single crystal during thermal cleaning in ultrahigh vacuum (UHV) and oxidation in low-pressure oxygen at 300 and 900 K have been studied by combined auger electron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy. The oxygen-induced $(3\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}\mathrm{O},$ $(4\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}\mathrm{O},$ $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{O},$ and clean $(1\ifmmode\times\else\texttimes\fi{}1)$ structures are sequentially observed on the Nb(100) surface at atomic resolution during thermal cleaning in UHV at temperatures from 1970 to 2500 K. At 300 K, the clean Nb(100) surface is sequentially oxidized into the $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{O}$ and $(1\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}\mathrm{O}$ structures and the amorphous oxides of NbO and ${\mathrm{NbO}}_{2}$ in oxygen. At 900 K, the clean Nb(100) surface is sequentially oxidized into the $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{O},$ $(4\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}\mathrm{O},$ and $(3\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}\mathrm{O}$ structures in oxygen. The $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{O}$ and $(1\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}\mathrm{O}$ structures result from oxygen chemisorption and the $(3\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}\mathrm{O}$ and $(4\ifmmode\times\else\texttimes\fi{}1)\ensuremath{-}\mathrm{O}$ structures result from the epitaxial growth of NbO nanocrystals on Nb(100). Atomic models for these oxygen-induced structures are proposed and the atomic-scale oxidation processes of the Nb(100) surface at 300 and 900 K are discussed.

Nigeria: Azara — barytes

In the present work, we report the luminescence data obtained from heavy, barytes loaded, concrete containing many crystalline inclusions, extracted from a shielding block located at CERN. The use of both Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) signals, resulting from a specific trap at about 200 °C, is investigated for retrospective dosimetry purposes. By applying thermal cleaning experiments the TL signal of interest was isolated. Basic TL and OSL properties as thermal and optical stability, repeatability and mainly the linearity of the TL and OSL signals as a function of beta dose were investigated. The implications of all these luminescence properties to retrospective dosimetry are also briefly discussed.

Surface Control of ZrB2 (0001) Substrate for Molecular-Beam Epitaxy of GaN

ABSTRACTZirconium diboride (ZrB2) is a promising lattice-matched substrate for GaN-based materials. A key issue to realize high-quality heteroepitaxial growth is preparation of the substrate surface. The ZrB2 surface was studied by x-ray photoemission spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED). XPS results indicated the presence of both ZrO2 and ZrB2 on the as-received substrate surface. Thermal cleaning at 1000°C in ultra-high vacuum, Ar+ ion sputtering, and wet chemical treatments were examined as surface preparation methods. After treatment with HF acid, the O peak intensity was much reduced. The combination of HF treatment and thermal cleaning resulted in sharp and intense RHEED from the ZrB2 surface. GaN grown on the surface by molecular-beam epitaxy exhibited intense photoluminescence, suggesting that this treatment is effective to obtain high-quality GaN on ZrB2 substrates.

Correlation of surface chemistry of GaAs substrates with growth mode and stacking fault density in ZnSe epilayers

We report on the correlation of surface chemistry of GaAs substrates with growth mode and stacking fault density in ZnSe epilayers. High quality ZnSe epilayers with low stacking fault density have been directly grown on commercial epiready GaAs (001) substrates without GaAs buffer layer growth. It is found that proper pregrowth treatments of epiready GaAs (001) substrates to obtain clean surfaces are crucial to two-dimensional layer-by-layer growth and suppression of generation of stacking faults. The surface oxide layers of epiready GaAs substrates before and after chemical etching using a NH4OH-based solution are carefully characterized by angle-resolved x-ray photoelectron spectroscopy. The surface oxide layer is composed of Ga2O3 and As2O3. The chemical etching is found to reduce not only the thickness of the oxide layers but also the ratio of Ga2O3 to As2O3 to about half of that before etching. A clean GaAs (001) surface typically characterized by a (4×1) reconstruction is obtained in the present case after thermal cleaning. Prior to ZnSe growth Zn preexposure is carried out. Reflection high-energy electron diffraction intensity oscillations with more than 50 periods are observed even from the beginning of ZnSe growth on such cleaned GaAs substrates. High quality ZnSe layers with a stacking fault density of low-105 cm−2 range are grown.

Palaeomagnetic and rock magnetic studies on the dykes of Goa, west coast of Indian Precambrian Shield

Abstract Detailed alternating field (AF) and thermal demagnetization techniques on 120 samples belonging to 10 dykes of the Goa region along the west coast of the Indian Precambrian Shield, have yielded characteristic remanent magnetization (ChRM) directions from 9 dykes of which 6 dykes exhibited a reverse magnetic polarity direction (D=150°, I=44.30°, k=58, α95=5.8, N=6 dykes), while 3 dykes exhibited normal polarity D=351°, I=−50°, k=21, α95=14.9, N=3 dykes) direction. The remaining 1 dyke has shown stable direction, but with a strong overprint of the present earth field (PEF) direction, which could not be removed by either AF or thermal cleaning techniques. A Palaeomagnetic pole, computed by averaging the VGPs of 9 dykes is situated at 41.2°N 78.1°W (N=9 dykes; A95=9.81°), which is close to that of the Deccan super pole (36.9°N, 78.8°W), suggesting that the dykes in the Goa region are related to the Deccan basaltic eruptions. Recently published 40 Ar / 39 Ar dating on the normal polarity dykes of the studied area have assigned the ages as 62.8±0.2 Ma. These results demonstrate that the Deccan magnetism continued to affect western India for at least 1–2 Ma after the peak Deccan eruption during the K–T boundary. On the basis of published geological and geochemical information along with our Palaeomagnetic results, these intrusive bodies could be attributed as the feeders for the uppermost formations of Wai subgroup of the Western Ghat Deccan basalts, or to the major geodynamic events such as the opening of the Arabian Sea and the rifting of the Seychelles–Mascarene oceanic plateau, which occurred soon after the cessation of the Deccan volcanism. Rock magnetic experiments such as the Lowrie–Fuller test along with low-temperature magnetic susceptibility and isothermal remanent magnetization (IRM) measurements have identified single domain (SD) type titano-magnetite as the major characteristic remanent magnetization carrying magnetic mineral in the dyke samples.

Complete Elimination of Multi-angle Rotation Domains in ZnO Epilayers Grown on (0001) Sapphire Substrate

ZnO films were grown on (0001) sapphire substrates by using rf plasma-assisted MBE. The rotation domains at 21.8° were firstly observed together with the well-known 30° rotation domains and main domains in the ZnO epilayer. These multi-angle rotation domains were completely eliminated by the surface modification of the sapphire substrate using Ga pre-exposure after the pretreatment of plasma-excited oxygen, resulting in a drastic improvement of the crystal quality and optical property of ZnO epilayer. The origin of the multi-angle rotation domains was investigated by thermal cleaning and the atomic-hydrogen pretreatment of sapphire substrate. The mechanism of the elimination of the rotation domains by Ga pre-exposure was also discussed.

Exergy analysis of methanol from the sewage sludge process

This paper concerns a new method of sewage sludge treatment that contributes more than traditional methods to the sustainable technology by achieving a higher rational efficiency of sludge processing. This is obtained by preserving the chemical exergy present in the sludge and transforming it into a chemical one––methanol. The proposed method combines a sludge gasification process and a modified methanol plant. The sludge gasification produces a synthetic gas mainly containing CO and H 2, which gas is next used as a reactant to make methanol. The plant, with a capacity of 50 000 tons (dry) solids per year, is simulated by a computer model using the flow-sheeting program Aspen Plus (Aspen Technology). The exergy analysis is performed for various operating conditions and the optimal values of these conditions are found. The irreversibilities of different plant segments: thermal dryer, gasifier, gas cleaning, compressors, methanol reactor, distillation column and purge gas combustion are evaluated. It is shown that the rational efficiency of the overall process is much higher than that of traditional plants of thermal sludge treatment.

Molecular-beam epitaxial growth of insulating AlN on surface-controlled 6H–SiC substrate by HCl gas etching

Insulating AlN layers were grown on surface-controlled 6H–SiC subtrates by molecular-beam epitaxy (MBE) using elemental Al and rf plasma-excited nitrogen (N*). HCl gas etching was introduced as an effective pretreatment method of substrate for MBE growth of AlN. 6H–SiC substrates pretreated by HCl gas etching had no surface polishing scratches and an atomically flat surface. In addition, evident (∛×∛)R30° surface reconstruction was observed even before thermal cleaning. AlN layers grown on this substrate had no defects related to surface polishing scratches and excellent insulating characteristics.

The Structural and Optical Characterization of MBE-ZnSe Layer Grown on High-Quality VGF-ZnTe Substrate

We have studied ZnSe films grown on ZnTe (001) substrate by using MBE method. The surface reconstruction of ZnSe is investigated systematically by RHEED. The Zn beam irradiation during thermal cleaning was done to suppress the desorption of Zn atoms from ZnTe substrate. The 3D growth occurs at initial growth stage. After 5 min of growth, streak patterns indicated that 2D growth was achieved in the ZnSe/ZnTe heteroepitaxial growth. The structural and optical properties of the ZnSe layers are characterized by photoluminescence and X-ray diffraction measurements. Four emission peaks were observed, and then a band gap energy increase of about 3 meV was confirmed. Lattice relaxation took place in the layers with thickness below 0.1 μm, and the layers suffered from the compressive strain due to the difference in the thermal expansion coefficients.

Beyond Néel’s theories: thermal demagnetization of narrow-band partial thermoremanent magnetizations

Partial thermoremanent magnetization (pTRM) was imparted over a narrow temperature interval, T=370–350°C, to a suite of crushed and annealed natural magnetite samples, ranging in grain size from ≈1 to 125–150 μm and in domain state from small pseudo-single-domain (PSD) to multidomain (MD). In this way, effectively a single blocking temperature, T B, of pTRM was activated. Stepwise thermal demagnetization did not erase the pTRMs sharply at T B, as for single-domain (SD) grains. Demagnetization began well below 350°C and continued above 370°C, with a median unblocking temperature, T UB, close to 360°C. The largest grains deviated most from SD behavior. Their pTRM demagnetized over the entire interval from room temperature to the Curie point, in accordance with predictions for MD grains. In terms of the unblocking temperature distribution f( T UB) or slope d M/d T of the thermal demagnetization curve, SD grains have a sharp spectrum, T UB= T B; MD grains were observed to have a broad, roughly symmetrical spectrum centered on T B; and intermediate size grains in the PSD range had non-Gaussian spectra, combining a central peak near T B with broad tails above and below T B. In this respect, PSD grains display a superposition of SD and MD remanences, not a blend between the two. Practical implications of these observations are that Thellier’s law of reciprocity ( T UB= T B) will be increasingly violated as grain size increases in the PSD range. The low- T UB part of f( T UB) produces anomalously large demagnetization of NRM in low-temperature heating steps of Thellier-type paleointensity determinations and a sagging shape of the Arai plot. The high- T UB part of f( T UB) results in undemagnetized remanence at and above T B in thermal demagnetization. Among pre-treatments designed to make remanence more SD-like in subsequent thermal cleaning, alternating field (AF) pre-cleaning sharpened f( T UB) more effectively than low-temperature demagnetization for the 20 μm sample.

Nanostructure formation on Si (111) surface assisted by synchrotron radiation illumination

Abstract The surface structures after the synchrotron radiation (SR) stimulated removal of native oxide on Si (111) exactly oriented and 4° misoriented surfaces were investigated by scanning tunneling microscopy. The exactly oriented surface showed large regions of atomically flat Si (111)-7×7 structure, and was characterized by the formation of single bilayer steps nicely registered to the underlying crystal structure, clearly different from the disordered step edge obtained by the usual high temperature thermal cleaning. The 4° misoriented sample showed nearly uniformly spaced step bunches and terraces terminated by 7×7 unit cells in both SR assisted and thermal cleanings.

Conduction-type control of Ge films grown on (NH 4) 2S-treated GaAs by molecular beam epitaxy

We have performed epitaxial growth of Ge films on (NH 4) 2S-treated GaAs (0 0 1) substrates under various growth temperatures using molecular beam epitaxy. We confirmed that this sulfur passivation is quite effective for preventing the oxidation of GaAs surface. Thus, the Ge films were grown epitaxially on GaAs substrate without thermal cleaning. The electric properties of Ge films were investigated using Hall measurement and it was found that the conduction type of Ge films can be controlled by growth temperature. The Ga–S bond is thought to be the key for conduction type control, although the details are not identified yet.

Field emission current cleaning and annealing of microfabricated cold cathodes

Thermal cleaning and annealing by the extracted field emission current has been used to achieve and maintain stable emission characteristics with microfabricated cold field emission cathodes. Our studies show that the cathode tip surface can be smoothed, recrystallized, and at least partially cleaned of surface adsorbates. The result is a significant decrease in emission current noise and recovery of the initial current–voltage characteristics of the cathode following surface contamination.

Origin of the exponential distribution of traps in glass

A recent rigorous analysis of the broad thermoluminescence (TL) peak of a Brown microcline has shown it to be due to an exponential distribution of traps. Incidentally, from the statistical point of view one expects the traps to follow a Gaussian distribution. In order to elucidate the origin of the exponential trap distribution we have analyzed a set of glow peaks of a light green glass recorded under varying conditions of trap filling. The sample upon γ-rays irradiation (dose=7×103 Gy) exhibits a broad TL peak around 191 °C. The broad peak becomes narrower once a thermal cleaning technique is applied to the irradiated specimen. Application of computerized glow curve deconvolution reveals that the broad glow curve is due to the presence of a Gaussian distribution of traps. However, in thermally cleaned glow curves the distribution of traps is found to be exponential. This shows that the exponential distribution of traps results from the deeper tail portion of the Gaussian distribution.

Low stacking-fault density in ZnSe epilayers directly grown on epi-ready GaAs substrates without GaAs buffer layers

We report a remarkably low stacking-fault density in ZnSe epilayers directly grown on commercial epi-ready GaAs (001) substrates without GaAs buffer layer growth. It is found that proper pregrowth treatments on epi-ready GaAs (001) substrates to obtain clean surfaces are crucial for two-dimensional layer-by-layer growth and suppression of stacking fault generation. Chemical etching using a NH4OH-based solution is found to reduce not only the thickness of the oxide layers but also the ratio of Ga2O3 to As2O3 to about half of that before etching. A clean GaAs (001) surface characterized by a (4×1) reconstruction in the present case is obtained after thermal cleaning followed by Zn pre-exposure. Reflection high-energy electron diffraction intensity oscillations with more than 50 periods are observed even from the very beginning of ZnSe growth on GaAs substrates cleaned as such. The stacking fault density in such a ZnSe layer is in the low-105 cm−2 range.

Built-in electric fields in GaAs/GaAs structures with different in situ substrate treatments

We have prepared GaAs substrates prior to molecular beam epitaxial growth by the following in situ treatments: (1) The usual thermal cleaning under an arsenic flux, (2) cleaning by hydrogen radicals (H*), and (3) exposure to trisdimethylaminoarsine (TDMAAs). The effects of these treatments on the optical properties and built-in electric fields in GaAs/GaAs structures were studied. In order to investigate the effects of the substrate type on the properties of the GaAs epilayers, undoped semi-insulating (SI) GaAs (100) and Si-doped n+-GaAs(100) substrates were used. Reflection high-energy electron diffraction during the growth, and atomic force microscopy in air showed that the smoothest surface morphology was obtained for the layer grown on a H*-cleaned SI substrate at 570 °C. For Si-doped substrates the smoothest layer was obtained on a TDMAAs-treated substrate. The concentrations of interfacial residual impurities of C and O were measured by secondary ion mass spectroscopy (SIMS). For SI substrates, the ...

Fabrication and Characterization of Carbon Nanotube Field Emitters

AbstractWe report an effective procedure for fabricating carbon nanotube emitters by directly synthesizing carbon nanotubes on an electrochemically sharpened tungsten tip. The nanotubes adhere very well to the tip of tungsten without any painting materials. Thermal cleaning of the tungsten tip under applied electric field reduced the number of nanotubes formed on the tip resulting in a single nanotube emitter. Electron field emission properties were investigated by employing a field emission microscope with a base pressure ~ 1 × 10-9 Torr. The emission images with respect to the applied field and time were obtained. Different emission images consisting of one to four lobes at different applied fields were observed. The characteristic of the emission current vs. applied voltage was analyzed. Applied potentials up to 3000 V were tested. The estimated field on the emitter was on the order of several tens of volts per nanometer. Our investigation suggests that at lower fields, the I-V characteristic of the nanotube emitter follows Fowler-Nordheim (F-N) emission behavior. At higher applied field, current saturation was observed.

混合ビームによるＳｉ基板上へのＧａＡｓ薄膜成長におけるイオンの役割

Thin GaAs films were grown on Si (001) substrates by Ga and As beams, consisting of low-energy ions and neutral molecules, without thermal cleaning of substrate surfaces and buffer layers. The energy of the ions in the mixed beams had to be higher than 60eV to grow single-crystalline GaAs films on the Si substrates, In this case, the surface of the Si substrates was amorphized by collisions of ions in the mixed beams. The amorphous Si layer was inserted between Si substrates and GaAs films and served as a buffer layer reliving strain due to the large mismatch and the difference in linear expansion between GaAs films and Si substrates.