Oxide Desorption Research Articles

Electrical properties of two-dimensional electron gases grown on cleaned SiGe virtual substrates

The low temperature electrical properties of modulation-doped two-dimensional electron gases (2DEG) in the SiGe system were studied. The effect on the electrical properties of removing the substrate from the growth chamber after the growth of the virtual substrate, chemically cleaning the virtual substrate and then growing the 2DEG on a thin SiGe buffer were investigated. The results demonstrate that the carrier density and mobility decrease as the regrowth interface is moved closer to the 2DEG. Lower temperature desorption of the passivating chemical oxide improves the mobility and carrier density when a regrowth interface is close to the quantum well. The mismatch of Ge content in the virtual substrate and the heterolayers was also shown to produce a reduction in the carrier density and mobility.

The desorption of propylene oxide from oxygen atom and hydroxyl covered Ag(110)

Coadsorbed water decreases the desorption temperature for propylene oxide on the oxygen covered Ag(110) surface. On oxygen atom covered Ag(110), the desorption activation energy of propylene oxide increases by up to 30% with increasing oxygen coverage relative to clean Ag(110). When propylene oxide is desorbed from a hydroxyl covered surface [generated by water adsorption on oxygen atom covered Ag(110)], the desorption temperature and activation energy decrease when compared to the oxygen covered surface. On the oxygen precovered Ag(110) surface, the maximum propylene oxide desorption rate occurs at 208 K with a desorption activation energy of 12.5 kcal/mol, while on the same oxygen precovered surface, coadsorption of water reduces the temperature of the propylene oxide desorption rate maximum to 194 K and the activation energy to 11.6 kcal/mol. In the absence of preadsorbed oxygen, no reaction or significant interaction is observed between coadsorbed water and propylene oxide. The desorption spectra and peak temperatures are the same as observed for adsorption/desorption from clean Ag(110). The water induced reduction in the propylene oxide heat of desorption from O–Ag(110) may increase the propylene oxide desorption rate and limit secondary reactions, leading to the enhanced propylene oxidation selectivity observed when water is added to the reactor feed gases.

Quantitative secondary ion mass spectrometry analysis of SiO 2 desorption during in situ heat cleaning

Analysis of silicon oxide (SiO 2) desorption was performed by a combination of thermal annealing in an ultra high vacuum (UHV), consecutive growth of a thin silicon (Si) layer by molecular beam epitaxy (MBE) and ex situ secondary ion mass spectrometry (SIMS). Together with the Si substrate the Si-MBE layer creates a uniform Si matrix which allows quantitative analysis by SIMS. The protective oxide on as-received wafers stored for a few months turned out to be 0.8–1.6 nm thick. Desorption in a vacuum takes place in a narrow temperature window below 875°C. The main residual contaminations were carbon (C) with a surface concentration of about 10 12 cm −2 and boron (B) with a concentration of 6×10 10–6×10 11 cm −2. Metallic impurities were below the SIMS detection limit.

Atomic hydrogen cleaning of polar III–V semiconductor surfaces

Atomic hydrogen (H*) generated by a simple thermal cracker source has been used to efficiently clean the polar surfaces of several III–V semiconductors at temperatures significantly lower than those normally required for oxide desorption. The process of atomic hydrogen cleaning (AHC) is demonstrated for the preparation of the (001) and (111)A surfaces of InAs, and the InSb(001) and GaSb(001) surfaces. Both the substrate anneal temperature and the required H* dose vary for the different materials, but in all cases clean, well-ordered surfaces can be produced, as determined by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). It is found that both the InAs and GaSb surfaces can be cleaned with H* exposures at a specific temperature (as low as 470 K for GaSb and 700 K for InAs); however, for InSb it is also necessary to supply an Sb 4 flux from a Knudsen cell to avoid decomposition of the surface. High resolution electron energy loss spectroscopy (HREELS) has been used to monitor the surface plasmon excitations in these doped materials to measure the carrier concentration in the near-surface region. It is found that no measurable passivation of donors or acceptors occurs in either the n-type or p-type materials studied. In addition, and in contrast to ion sputtering procedures, no defect induced increase in the carrier concentration occurs as a result of atomic hydrogen cleaning.

Carbon contamination free Ge(100) surface cleaning for MBE

A carbon free wet treatment for Ge(100) surface has been proposed. This treatment consists of three steps: (1) several cycles of surface oxidation with H 2O 2 solution and the oxide etching with HCl solution, (2) the formation of protective oxide by NH 4OH/H 2O 2/H 2O treatment and (3) thermal desorption of the protective oxide in ultra high vacuum. An atomic force microscope image of the Ge(100) surface after step (1) indicates the surfaces is very smooth. After step (2), it is confirmed that the protective oxide contains no carbon contamination by Auger electron spectroscopy measurement. A reflection high energy electron diffraction pattern indicates that the surface, after step (3), has only atomic-scale steps.

Molecular beam epitaxy growth and characterizations of AlGaAsSb/AlAsSb Bragg reflectors on InP

The molecular beam epitaxy growth of A1AsSb/A1 0.04Ga 0.96AsSb distributed Bragg reflectors lattice-matched to InP is studied. The fabrication and characterization of two such reflectors are reported. Fine structural and optical properties of these samples are investigated using double crystal X-ray diffraction, scanning electron microscopy and reflectivity measurements. Main device performances are a 260 nm wide stop-band centred at 1.55 μm with a maximum reflectivity of 90% for the first 8.5 pairs Bragg reflector while the second 15.5 pairs reflector have allowed an improved maximum reflectivity exceeding 96% at 1.6 μm with a stop-band of 220 nm. As a major result, a new type of surface defects is observed on the A1GaAsSb/A1AsSb Bragg reflector surface. Planar and cross-sectional microscopic observation has revealed that defects are initiated from the reflector-substrate interface. The formation of these defects are discussed and it has been related to InAs islands that appear during the in situ InP-substrate oxide desorption procedure.

Preservation of rectangular-patterned InP gratings overgrown by gas source molecular beam epitaxy

The ability to preserve a grating profile during epitaxial overgrowth is vital to the design and operation of devices such as planar waveguide-coupled Bragg-resonant filters. Intentional preservation of rectangular-patterned InP gratings during gas source molecular beam epitaxial overgrowth has been accomplished. The use of a low temperature atomic hydrogen-assisted oxide removal technique alleviates the dependence on elevated temperatures for desorption of the native oxide, and is crucial to the preservation of the patterned profile. Confirmation of the overgrown grating fidelity is demonstrated via scanning electron microscopy and triple axis x-ray diffractometry.

X-ray and UV photoelectron spectroscopy of oxide desorption from InP under As4 and/or Sb4 overpressures: exchange reaction As ⇔ Sb on InP surfaces

Abstract We report here on the oxide desorption of InP substrates in a molecular beam epitaxy (MBE) chamber, under As 4 and/or Sb 4 overpressures. In situ characterizations including reflection high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) are used. We show that even if the deoxidation is feasible using one of the different beam pressures, the less critical temperature control leading to a good surface quality is obtained under (As 4 + Sb 4 ) combined pressures. The oxide desorption is complete at a substrate temperature of 530°C far below the In-stabilized surface. In this case, XPS measurements show the formation of about 2 ML thick InAs x Sb 1 − x layer on top of InP. An annealing of such a surface under As 4 (Sb 4 ) up to 480°C led to the formation of ∼2 ML thick pure InAs (InSb) layer on top of InP.

Integration issues for 850 nm optical modulators on Si electronics by direct epitaxy

We report on the various strategies for integrating III–V semiconductors to very large scale integrated Si logic along with their strengths and limitations. We also detail the monolithic integration method involving direct epitaxial growth where several material science issues have arisen. They require fundamental investigation to further advance implementation. Those issues are (i) the need for a highly ordered array of bilayer steps as an initial Si surface condition for heteroepitaxy, (ii) lower Si oxide desorption temperature, e.g., via electron cyclotron resonance plasma treatments to preserve the bilayer step ordering, and (iii) thinner strain relief layers to reduce the III–V on Si topology mismatch.

A scanning tunneling microscopy study of water on Si(111)-(7 × 7): adsorption and oxide desorption

Scanning tunneling microscopy and temperature-programmed desorption have been used to investigate the chemistry of water on Si(111)-(7 × 7) substrates which were misoriented 2° toward the [1̄10] direction. Upon room temperature exposure to water, the adatoms of the (7 × 7) unit cell are still evident even after high exposures, implying that major modifications of the substrate do not occur. At high coverages, the distribution of reacted adatoms shifts from one controlled by dissociative adsorption across the adatom-rest atom pair to a statistical distribution based on the availability of dangling bonds. Desorption of the oxide layer which remains after water adsorption and the desorption of hydrogen have also been characterized. The oxide desorption occurs along well-defined wavefronts which originate at step edges and advance in directions consistent with the underlying substrate symmetry, primarily the [1̄1̄2] direction (i.e. the wave vector points in the [1̄1̄2] direction). In regions of the surface where the oxide has desorbed, the (7 × 7) unit cell can be seen clearly. Vacancies resulting from the loss of surface silicon atoms (via the etching) coalesce into islands in the clean regions of the terraces, but unlike desorption of oxide layers from Si(100), the desorption does not occur from the boundaries of these vacancy islands.

Novel Ga2O3 (Ga2O3) passivation techniques to produce low Dit oxide-GaAs interfaces

Abstract Molecular beam epitaxy (MBE) has been extended to fabricate heterostructures of Ga 2 O 3 (Gd 2 O 3 )GaAs. Two processes were used: (1) in situ approach in which the oxide molecules were deposited on freshly prepared GaAs (1 0 0), and (2) ex situ approach which comprises thermal desorption of native oxides of GaAs and subsequent Ga 2 O 3 (Gd 2 O 3 ) film deposition on GaAs (1 0 0) wafers all under ultra-high vacuum. A low interface recombination velocity S of 9000 cm/s equivalent to an interface state density D it in the upper 10 10 cm − 2 eV − 1 range has been inferred for the ex situ processed samples. In comparison, an interface recombination velocity of 4000–5000 cm/s and an interface state density D it in the lower 10 10 cm − 2 eV − 1 range were obtained for the in situ processed samples. The ex situ technique provides excellent passivation for GaAs wafers which may have been exposed to room air and/or processing environments during fabrication of devices such as FETs, HBTs, etc.

The growth of AlGaAsInGaAs quantum-well structures by molecular beam epitaxy: observation of critical interdependent effects utilizing the design of experiments approach

The traditional approach to determine relationships between growth conditions and material properties has rested on the standard experimental approach - varying one parameter while holding all others constant. This technique does not effectively allow the observation of important interactions in complicated multilayer structures. Critical interdependent effects have been observed in the growth of AlGaAs/InGaAs quantum-well structures by molecular beam epitaxy (MBE). It is shown that statistical experimental design is an effective method for optimizing complex multilayer structures quickly. This technique is very useful for the optimization of processes with a large number of interdependent parameters and allows for the clear visualization and separation of complex interwoven effects. In the present work, we show for the first time the importance of the oxide desorption process for the optimal growth of AlGaAs-containing structures. The choice of an optimized oxide desorption process can lead to a decrease in the interfacial oxygen by almost two orders of magnitude.

Real-time monitoring of RHEED using machine vision techniques

Abstract A RHEED system has been developed that allows real-time monitoring of RHEED information throughout a multilayer growth run with rotation. The machine vision system consists of high-speed image capture hardware coupled with digital signal processing software that allows the real-time extraction/analysis of the RHEED intensity and width signals from the noise induced by substrate rotation. This system has been used to investigate the oxide desorption process on GaAs substrates, along with the specular spot intensity and width variation during the growth of a set of InGaAs AlGaAs single quantum well structures with systematically varied process parameters. A strong correlation of the specular spot intensity with growth parameters has been observed. It is also shown that the observed specular spot intensity can be used to predict the quality of the InGaAs quantum well structures.

A Ga 2O 3 passivation technique compatible with GaAs device processing

Electronic and structural properties of ex-situ fabricated Ga 2O 3GaAs interfaces have been investigated. The fabrication comprises thermal desorption of native oxide and subsequent Ga 2O 3 film deposition on (100) GaAs wafers in ultra-high vacuum. Interfacial x-ray photoelectron spectroscopy revealed the absence of As x O y indicating thermodynamic stability of the oxide-GaAs interface. A low interface recombination velocity S of 9000 cm/s equivalent to an interface state density D it in the upper 10 10 cm −2 eV −1 range has been inferred. The technique provides excellent passivation for GaAs wafers or surfaces exposed to room air and/or processing environments during fabrication of devices such as FETs, HBTs, etc.

Thermal desorption of oxides on Si(100): a case study for the scanning photoelectron microscope at MAX-LAB

A scanning photoelectron microscope, utilizing a focused beam of monochromatized photons in the energy range from 15 to 150 eV, has been used to study the thermal desorption of oxide layers on Si(100). The instrument can provide high-resolution photoelectron spectra from selected parts of the surface as well as images showing the lateral distribution (on a micrometer scale) of elements in different chemical states by monitoring the photoemission intensity of chemically shifted core levels. Both native (10-15 A) and thick (200-400 A) oxides have been studied. The desorption (at 840°C) of the native oxide proceeds through a phase of irregular (on a micrometer scale) and diminishing areas of dioxide until a clean surface is obtained. For the thick oxide, annealing to 1100°C creates circular voids in the oxide layer which grow linearly in diameter with annealing time. The surface in these voids mainly consists of clean silicon but a small amount of remaining SiO2 is observed. This remaining dioxide most probably consists of small clusters or particles. For both types of oxide, we find, during and after desorption, a surface-shifted component in the Si 2p core level spectra indicating that at least parts of the surface have an ordered structure which most probably is a 2 × 1 reconstruction. © 1997 Elsevier Science B.V. (Less)

Anomalous Desorption of Copper Oxide Observed by In Situ Transmission Electron Microscopy

Abstract: Here we present our observations of an anomalous desorption of copper oxides from a thin copper film during annealing as observed in an in situ transmission electron microscope. The epitaxial cuprous oxide that formed in air was observed to disappear when the specimen was annealed at 300°C and 10−8 torr. However, removal of the cuprous oxide that formed during oxidation in the microscope required both heating at 300°C and simultaneous exposure to methanol vapor.

Semiconductor substrate cleaning and surface morphology in molecular beam epitaxy

In-situ laser light scattering shows that the surface morphology of GaAs substrates during the initial stages of homoepitaxial growth is a sensitive indicator of substrate cleanliness. Oxide removal by atomic-hydrogen etching has no effect on the morphology of polished (100) GaAs substrates, while thermal oxide desorption roughens the surface. Carbon contamination of the surface causes roughening during subsequent film growth. Secondary ion mass spectrometry and photoemission spectroscopy show that atomichydrogen etching reduces the carbon contamination on the substrate but does not remove silicon oxide. Synchrotron radiation photoemission measurements show that some as-received substrates are contaminated with a thin uniform layer of SiO 2.

Perturbative treatments of photo-stimulated desorption and dissociation on metal surfaces induced by single and multiple electronic transitions

Photo-stimulated desorption and dissociation of small molecules adsorbed on metals are investigated in both the DIET and DIMET limits. A theoretical model is developed based on a perturbative treatment of substrate-mediated transitions. In this model, the system evolves according to the time-dependent Schrödinger equation except for transitions between the neutral and ionic states, which are induced by a series of delta pulses delayed by a pre-specified time (τ). Two examples are studied: the photo-stimulated substrate-mediated desorption of nitric oxide and the dissociative electron attachment of methyl bromide. Translational energy distributions and product yields in both the DIET and DIMET limits are calculated and compared. The influence of the time delay between the excitation and deexcitation is explored.

Heterogeneous nucleation of coherently strained islands during epitaxial growth of Ge on Si(110)

We report on the molecular beam epitaxial growth of Ge on Si(110) surfaces. High temperature cleaning (oxide desorption) results in the formation of shallow faceted pits distributed randomly on the Si(110) surface. Deposition of Ge at temperatures between 600 and 725 °C leads to preferential nucleation along the pit edges forming elongated islands, which subsequently grow to compact three-dimensional coherent islands. The observation of strained islands in close proximity to each other offers insights into their nucleation behavior and strain relaxation. Our observations suggest heterogeneous nucleation as a possible method for fabricating assemblies of quantum dots.

The growth of AlGaAs–InGaAs quantum-well structures by molecular beam epitaxy: Observation of critical interdependent effects

Critical interdependent effects have been observed in the growth of AlGaAs/InGaAs quantum-well structures by molecular beam epitaxy. It is shown that statistical experimental design is an effective method for quickly optimizing complex device structures. This technique is very useful for the optimization of processes with a large number of interdependent parameters, and allows for the clear visualization and separation of complex interwoven effects. In the present work, we show the importance of the oxide desorption process for the optimal growth of AlGaAs-containing structures.