Thermal Cleaning Research Articles

Photoreflectance study of the substrate-film interface of GaAs homoepitaxial structures with different in situ substrate surface cleaning processes

The substrate-film interface of GaAs homoepitaxial structures prepared by molecular beam epitaxy, employing three different in situ treatments for the substrate surface preparation: (1) cleaning by hydrogen radicals (H*), (2) exposure to trisdimethylaminoarsine (TDMAAs), and (3) the usual thermal cleaning under an arsenic flux have been studied. The concentrations of interfacial residual impurities of C and O were measured by secondary ion mass spectroscopy (SIMS). These impurities pin the Fermi level producing an interfacial electric field, whose strength was evaluated by photoreflectance spectroscopy. For semi-insulating substrates, the usual thermal cleaning process resulted in very high concentrations of C (2×1019 atoms/cm3) and O (1.3×1018 atoms/cm3) at the interface, producing a large electric field (1.3×106 V/m). The impurities were drastically diminished to below the SIMS detection limit by using the H* cleaning, and as a result the electric field was effectively reduced. On the other hand, we observed higher concentrations of impurities, and larger interfacial electric fields on Si-doped substrates.

Altanlagen – Sanierung nach TA Luft durch Kombination von Verbrennung und Absorption

AbstractA waste‐gas cleaning system was constructed for a group of drug production plants. The chemical – synthetic processes taking place there are characterized by inevitable emissions, mainly of vapours of organic solvents. Their concentrations vary within a wide range and at any given moment in principle remain unknown. This paper describes the initial situation and the requirements for the waste gas cleaning process, including the calculation of the gas volume flow capacity. The polluting substances were divided into two groups; for critical substances only thermal oxidation was found to be adequate, whereas for non‐critical, water‐soluble material, water‐fed absorption columns were chosen; on one hand to avoid full exhaustion of the capacity of the thermal waste‐gas system and on the other hand to offer additional possibilities in the case that this system should be out of action. The criteria for this decision as well as for the combination of one central with several decentralised cleaning units are discussed in detail. The equipment of the waste‐gas emitting plants for feeding the cleaning units and the technical details thereof are described as well as the safety design. At last, experience gaine during preparation of the systems and considerations concerning further use of the thermal cleaning system are discussed.

Atomic force microscopy study of [formula omitted] heteroepitaxy processes by metalorganic vapour phase epitaxy

We investigate the initial growth of ZnSe on (001) GaAs in metalorganic vapour phase epitaxy using atomic force microscopy, X-ray diffraction, and photoluminescence. GaAs surfaces become atomically flat after thermal cleaning at a temperature of 620–750°C. The property of ZnSe films is critically dependent on the GaAs surface preparation and the flow rate ratio of Se to Zn. The simultaneous As flow during thermal cleaning is effective to grow high-quality ZnSe films. The growth processes at the initial stages of heteroepitaxy of ZnSe on GaAs are dominated by the two-dimensional growth mode for Zn-rich growth conditions, while those are dominated by the three-dimensional growth mode as the Se flow rate increased. The optimum condition for the ZnSe optical quality is different from that for the initial two-dimensional growth. We demonstrate that the 2-step growth is effective to grow atomically flat ZnSe films with better optical quality.

Successful Surface Passivation of Air-Exposed AlGaAs by a Silicon Interface Control Layer-Based Technique

In order to make the silicon interface control layer (Si ICL)-based surface passivation technique applicable to air-exposed AlGaAs surfaces, various surface treatments were systematically studied. The treatments investigated include UHV thermal cleaning, ( NH4)2 Sx treatment, HCl treatment and H2 plasma treatment. Chemical status and quality of the treated surfaces were evaluated by in-situ and ex-situ X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) measurements. A strong correlation was found between the band-edge PL intensity and the amount of surface oxide components, in particular Al-oxides. Marked reduction of the amount of oxide components and enhancement of PL intensity were realizable by a combination of HCl surface treatment and Si ICL formation.

Structural change of selenium-treated GaAs(001) surface observed by STM

We have controlled the surface structures of selenium (Se) treated GaAs(001) using a Se molecular beam in a molecular-beam epitaxy (MBE) system and obtained a variety of surface reconstructions. The detailed surface structures were observed in-situ using a high-performance scanning tunneling microscope (STM) system. After producing Ga-stabilized GaAs(001) (4 × 6) reconstructions by thermal cleaning, Se was evaporated slowly onto the surfaces. As the Se deposition time was increased, the surface structure could be changed from (4 × 6) to (4 × 3) to (2 × 3) and finally to the (2 × 1) reconstruction. All surfaces were well ordered and STM images were obtained successfully for all of these reconstructions. The (4 × 3) structure was observed to be similar to (2 × 3), however, dimers might be buckled in alternate directions together with the nearest neighbors in the [1 1 0] direction. The (2 × 3) structure consisted of elliptical protrusions in the dimer rows forming an additional periodicity along the dimer row direction. For the (2 × 1) surface, simple dimers row structures were observed. These structural changes were thought to be caused by irregularities in the SeSe or SeGa dimers and Ga vacancies under the surface. We found that Se coverage played an important role as well as heat treatment in this phase change.

Thickness of the [formula omitted] interface and composition of silicon oxide thin films: effect of wafer cleaning procedures

We determined the areal density of Si atoms constituting the oxide-silicon interface and the stoichiometry of ultra-thin silicon oxide films, thermally grown on Si(001) in dry 18O 2 atmospheres, using the channeling of α-particles along the 〈001〉 axis of the Si substrates associated with grazing angle detection of the scattered particles. The amount of 18O atoms in the films was determined independently using the 18O(p,α) 15N nuclear reaction at 730 keV. The Si wafers were submitted to different cleaning procedures before oxidation in 18O 2, namely: standard RCA cleaning, HF etching followed by a rinse in ethanol and rapid thermal cleaning (RTC) under high vacuum. The stoichiometry of all oxide films having thicknesses between 2 and 13 nm could be fitted assuming a ratio O Si = 2 , that is, the films were constituted by silicon dioxide. By comparing the results for samples cleaned in different ways, however, we noticed a pronounced change in the number of atoms in the non-registered Si layers at the SiO 2 Si interface and so in the thickness of these interfaces.

Dipolar relaxations in a side-chain polyacrylate liquid crystal. A study by thermally stimulated currents

The dipolar relaxation mechanisms present in a side-chain liquid crystalline polyacrylate were studied in a wide temperature range covering the glassy state, the glass transition region and the liquid crystalline phase. The complex relaxation processes observed in these different temperature regions were analysed in detail using the technique of thermal cleaning. The significance of the so-called compensation behaviour, which is frequently considered as a feature of the glass transition relaxation in polymeric materials, is discussed.

Defect reduction in ZnSe grown by molecular beam epitaxy on GaAs substrates cleaned using atomic hydrogen

Atomic hydrogen is demonstrated to effectively clean GaAs substrates for subsequent growth of ZnSe by molecular beam epitaxy. Optical fluorescence microscopy is shown to be a useful technique to image nonradiative defects related to stacking faults. While the density of stacking faults in ZnSe films grown using conventional thermal cleaning is greater than 107 cm−2, stacking fault densities lower than 104 cm−2 are obtained using atomic hydrogen cleaning. Low-temperature photoluminescence spectra of undoped ZnSe are dominated by excitonic transitions for the low defect density samples in contrast to the high level of defect-related emission from high defect density samples.

Cleaning of GaN surfaces

The work described in this paper is part of a systematic study of surface cleaning and ohmic contact strategies for GaN. The goal of this investigation was to determine the most effective methods of wet chemical and thermal desorption cleaning for the removal of oxygen (O) and carbon (C) prior to metallization. Hydrochloric (HC1) and hydrofluoric (HF) acid-based cleaning treatments were compared, and thermal desorption as a function of temperature was characterized by sequential heating under ultra high vacuum (UHV) conditions. Auger electron spectroscopy (AES) analysis was used to monitor the presence of surface O and C throughout the study. For the removal of surface oxide, HCl-based solutions were found to be most effective; under as-cleaned, air-exposed conditions, HC1:DI H2O (1:1) solution resulted in the lowest levels of residual O and C. However, HF-based solutions resulted in more effective thermal desorption of C from the surfaces. In contrast to the results typically observed in the thermal desorption cleaning of GaAs, complete removal of oxygen and carbon from airexposed GaN surfaces was not seen using vacuum heating alone, even to temperatures where GaN decomposition occurs (>800-900°C). The results of this study indicate that the presence of oxygen and carbon on the GaN surface is persistent even to high temperatures, and that further in-situ cleaning methods must be added to obtain spectroscopically clean GaN surfaces.

Epitaxial ferromagnetic MnAs thin films grown on Si (001): The effect of substrate annealing

We have studied two different types of epitaxial ferromagnetic MnAs thin films on Si (001) substrates grown by molecular beam epitaxy. When the Si substrates were annealed at a relatively high temperature (∼900 °C) and then MnAs was grown, we obtained epitaxial MnAs films with twofold crystal symmetry (type I). In contrast, when the thermal cleaning of the Si substrate was done at a lower temperature (∼600 °C), epitaxial MnAs thin films had fourfold crystal symmetry (type II). The growth plane in both types of MnAs thin films was the (1̄101) of the hexagonal MnAs. The type I MnAs films are single domain with strong magnetic anisotropy, whereas the type II MnAs films are double domain with the lack of strong magnetic anisotropy.

MBE growth of ZnCdSe and MgZnCdSe alloys on InP substrates with a GaInAs buffer-layer

Molecular beam epitaxy growth of ZnCdSe and MgZnCdSe systems on (001) InP substrates were investigated. These films were grown on a lattice-matched GaInAs buffer-layer which was ex-situ grown on (001) InP by metalorganic vapor phase epitaxy. During the thermal cleaning process prior to II–VI film growth, group-V molecular beams were not used. By introducing this buffer-layer and lowering the initial growth temperature, the surface morphology of ZnCdSe films changed from rough to specular and the full width at half maximum (FWHM) of the double-crystal (004) X-ray rocking curves of the films decreased from more than 400 arcseconds to 60 arcseconds. The ZnCdSe film showed a very sharp excitonic emission in the 5 K photoluminescence spectra with a FWHM as narrow as 5.5 meV. No emission through deep levels was observed. High-quality MgZnCdSe films were also obtained using a GaInAs buffer-layer.

AFM and XPS characterization of the Si(111) surface after thermal treatment

Several phases of the thermal cleaning of crystalline Si(111) have been studied using AFM and XPS. The use of a sample exposed to a high thermal gradient has allowed us to investigate various surface morphologies on the same sample: formation of voids (native oxide decomposition), islands (pinning centers), rough area, flat and clean surface. These observations provide a novel description of the different steps involved in the thermal cleaning of a crystalline surface.

A geomagnetic event recorded in loess deposits of the Tönchesberg (Germany): identification of the Blake magnetic polarity episode

SUMMARY High-resolution palaeomagnetic studies on a collection of nearly 350 specimens. taken from a 1.30m thick section of an Upper Pleistocene loess profile from the Tonchesberg (Germany), are described. The Blake magnetic polarity episode is recorded in a layer above the partly eroded palaeosol of the Eemian interglacial. It is identified by its stratigraphic position and thermoluminescence data (=90.0110.0 kyr: Zoller, Conard & Hahn 1991; Frechen 1991, 1992). Magnetite and haematite contribute to the natural remanent magnetization (NRM). Directional behaviour during thermal and alternating-field cleaning suggests that magnetite carries a primary, (post-)depositional remanent magnetization (pDRM) and haematite a secondary, chemical remanent magnetization (CRM) of early diagenetic origin.

Role of the substrate deoxidation process in the growth of strained InAs/InP heterostructures

We have investigated the role of the arsenic flux used during the substrate deoxidation process in the MBE (molecular beam epitaxy) growth of strained InAs/InP heterostructures. Two different experiments were performed: (i) thermal cleaning of the InP wafer under an As flux at different exposure times and (ii) the growth of very thin InAs layers (3–9 ML). The samples grown were characterized by Raman spectroscopy and selected area X-ray photoelectron spectroscopy. The results obtained demonstrated the formation of an InAs x P 1− x sublayer at the interface of the InAs/InP system. The annealing of InP under an As flux promotes not only As → P substitution on the surface, but also the subsequent diffusion of As atoms into the deeper subsurface region of InP.

Palaeomagnetic results from southern Albania and their significance for the geodynamic evolution of the Dinarides, Albanides and Hellenides

Mesozoic and Cenozoic rocks were studied in three tectonic units in southern Albania. The area belongs to the Ionian Zone, Outer Albanides. Rock magnetic investigations, comprising alternating field and thermal cleaning, stepwise acquisition of isothermal remanent magnetization and hysteresis experiments were carried out. They established magnetite and titanomagnetite as the main carriers of the natural remanent magnetization. In the high field behaviour, haematite and goethite can also be seen. Several fold tests on the site and on a regional scale indicate that the directions are characteristic. These directions prove a general clockwise rotation of about 40° since the Tortonian. Using all available palaeomagnetic data from Albania, individual movements of the different tectonic units within the Inner and Outer Albanides can be deduced. This is also valid for the Albanian Alps, which are understood to be the southernmost part of the Dinarides.

Paleomagnetism of the Carboniferous Tepuel Group, central Patagonia, Argentina

Samples of the Carboniferous glaciogenic sequences of the Tepuel Group, western central Patagonia, Argentina, were collected from 23 sites in two localities, Sierra de Tepuel (43.7°S, 70.5°W) and Las Salinas (43.2°S, 70.3°W). Most were fine sandstones to siltstones of late Visean to Westphalian age. A characteristic remanent magnetization was determined in most sites by alternating field and/or thermal cleaning. At Sierra de Tepuel all samples were partially overprinted by a secondary magnetization apparently related to Mesozoic magmatic activity in this area. However, site mean directions of the characteristic remanence from both localities passed a tilt test, indicating a pretectonic (pre‐Early Permian?) magnetization. Isothermal remanent magnetization experiments showed magnetite as the main and often the only magnetic carrier, while thin section analyses indicated a detrital origin for this mineral, suggesting a depositional or postdepositional magnetization. The paleomagnetic pole for the Tepuel Group (Namurian‐Westphalian) at 316.1°E, 31.7°S (number of sites N = 16, minor and major semi‐axes of confidence δp = 15.0°, δm = 16.0°), is considered a key pole for the Middle Carboniferous of South America and Gondwana, confirming the apparent polar wander path for the supercontinent for which the pole moved from central to southern Africa in the Early Carboniferous, followed by a rapid motion from southern Africa to Antarctica during the late Westphalian. The agreement of the Tepuel pole with this Gondwanan path confirms that Patagonia was already part of South America by the Middle Carboniferous.

Characterization of YBa 2Cu 3O 7−x by electron-stimulated desorption

In the present work, the YBa 2Cu 3O 7−x surface and its reactivity with the residual vacuum at room temperature has been characterized using electron-stimulated desorption, ESD, and Auger electron spectroscopy, AES. The samples were prepared by sintering component oxides under pressure. The AES spectra revealed C and Cl as the most usual contaminants. ESD experiments show, in addition to the expected O + surface ions, H +, F +, and Cl +. H + surface ions are more intense than the expected O + surface ions. Thermal cleaning of the sample is rather difficult due to the high porosity of the samples yielding extremely high gas loads. The intensity of the hydrogen species yielding H + are in equilibrium with the H 2 pressure in the system up to a pressure of 10 −9 mbar, beyond which the uptake remains constant.

Surface structures of InP and InAs thermally cleaned in an arsenic flux

The surface structures of InP and InAs thermally cleaned in an arsenic flux were investigated using an ultrahigh vacuum scanning tunneling microscope multichamber system equipped with a molecular beam epitaxy facility. The InP surfaces thus treated at 510 °C showed In-stabilized (4×2) reconstructions which depended on the time of thermal cleaning. The surface structure of this substrate after thermal cleaning for 3 min comprised one In dimer and three missing dimers. This structure was the same as that of InAs treated under the same conditions. These In-stabilized (4×2) structures formed on both InP and InAs substrates changed to an As-stabilized (2×4) structure when the substrate temperature was lowered to below 480 °C under the same arsenic pressure.

Spontaneous Extinction of Twin Surface in GaAs on Si(100) Grown by One-Step Low-Pressure Metal-Organic Chemical Vapor Deposition

In heteroepitaxy of GaAs layers on Si(100) substrates by one-step low-pressure metal-organic chemical vapor deposition (LP-MOCVD), spontaneous extinction of twin crystals was confirmed. From the observation by reflection high-energy electron diffraction (RHEED), the {111} twin surface was found to coexist in the initial stage, and this twin surface was extinguished as deposition proceeded under constant growth temperature. The thickness required to extinguish the twin surface was dependent on both whether or not thermal cleaning was carried out prior to deposition and off-orientation angle toward the Si<011> direction. These suggest that the extinction mechanism of twin crystals is included in the heteroepitaxial growth of GaAs layers. Scanning electron microscopy (SEM) revealed that one-step heteroepitaxy by LP-MOCVD proceeds with embedding of the facet planes with high Miller index.

Auto-doping of Ga in ZnSe/GaAs layers grown at low temperatures by post-heated molecular beam epitaxy

The donor species in Li acceptor-doped ZnSe epilayers grown on GaAs substrate are determined by means of the secondary ion mass spectroscopy (SIMS). The compositional depth profile by the SIMS points out that a large amount of Ga atoms are incorporated into the ZnSe epilayer regardless of doping. In doped epilayers, the Ga atoms are concentrated at the region where the Li atoms are doped. We conclude that Ga atoms originating from the droplets due to the thermal cleaning of the substrate are most likely the major donor species.