Microprobe AES Combined With Scanning RHEED Microscope

Biaxially texture magnesium oxide (MgO) films have been grown using ion beam assisted deposition (IBAD). IBAD-MgO films can easily to grow in the preferred direction and to form biaxial texture fast by the effect of ion beam assisted current, where the ability of preferred growth is directly controlled by the ion beam assisted current. An in situ reflection high energy electron diffraction (RHEED) has often been used to monitor the texture development of MgO film during the IBAD process. Here, we developed novel numerical analysis methods for RHEED patterns based on gray level. We constructed corresponding gray scale images and binary images from RHEED patterns. The area and brightness of selected diffraction spots, such as (004) diffraction spots, were measured based on the gray scale images. Further, rich information on the whole RHEED binary images, such as high-brightness pixels proportion of gray value higher than 50% gray level, can be obtained. Results from these qualitative analyses of RHEED patterns were coincided to the texture evolution obtained by phi scan rocking curves. This indicates these kinds of analysis methods based on gray level are effective for RHEED patterns.

Growth and surface structure of vanadium oxide on anatase (0 0 1)

Reflection high energy electron diffraction (RHEED) was used to investigate surface roughening during low temperature Si(100) homoepitaxy. The use of RHEED allowed in situ real-time collection of structural information from the growth surface. RHEED patterns were analyzed using a simple kinematic diffraction model which related average surface roughness and average in-plane coherence lengths to the lengths and widths of individual RHEED diffraction features, respectively. These RHEED analyses were quantified by calibrating against cross-section transmission electron microscopy (TEM) analyses of surface roughening. Both the RHEED and TEM analyses revealed similar scaling of surface roughness with deposited thickness, with RHEED analyses resulting in roughness values a factor of ∼2 times lower than those obtained from TEM analyses. RHEED was then used to analyze surface roughening during Si(100) homoepitaxial growth in a range of temperatures, 200–275 °C. Initially, surface roughness increased linearly with deposited thickness at a roughening rate that decreased with increasing growth temperature. At each growth temperature, near the crystalline/amorphous Si phase transition, the rate of surface roughening decreased. This decrease coincided with the formation of facets and twins along Si{111} planes. Surface roughness eventually saturated at a value which followed an Arrhenius relation with temperature Eact∼0.31±0.1 eV. This activation energy agrees well with the activation energy for the crystalline/amorphous Si phase transition, Eact∼0.35 eV, and suggests that limited thickness epitaxy is characterized by this saturation roughness. Once the saturation roughness was reached, no significant changes in surface roughness were detected. In addition, the decay of average in-plane coherence lengths was also temperature dependent. Values of average coherence lengths, at the crystalline/amorphous Si phase transition, also increased with growth temperature. All of these data are consistent with a model that links surface roughening to the formation of critically sized Si{100} facets and the eventual breakdown in crystalline growth.

Recently great progress has been made in surface science due to the development of ultra-high-vacuum (UHV) techniques and related surface-analytical methods like low energy electron diffraction (LEED), Auger electron spectroscopy (AES), UHV reflection high energy electron diffraction (RHEED) and others. Well-defined clean surfaces were prepared and maintained; structure and chemical compositions of the surfaces on the monatomic layer level were detected and analyzed. The information given by surface-analytical instruments, however, was an averaged one over the area covered by the probe electron beams. Although efforts were made to reduce the probe size to get microscopic information in the scanning mode, the area-to-area resolutions so far obtained were poorer than those achieved very recently by a UHV electron microscope with conventional optics. This article reviews results of electron microscopic investigations which revealed microtopography of surface structures and processes on the same monatomic layer level as the surface-analytical instruments both in the transmission (TEM) and the reflection (REM) modes. Useful accounts on recent surface electron microscopy have been given by Howiel) and by Venables2). While Howie and the authors of this contribution concentrated on the conventional electron microscopy, Venables surveyed it from a more general point of view. Particularly for scanning electron microscopy (SEM) in the transmission (STEM) and the reflection (SREM) modes this survey should be referred to. This technique is now in practical use at modest resolution of a few to a few ten nm in combination with microtopographic or microchemical analyses by means of AES and electron energy loss spectroscopy (EELS). Howie’s brief review includes also studies using the transfer method of post-mortem specimens and of environmental cells of high-voltage electron microscopes which are not dealt with in this article.

We present two methods to access reciprocal space with reflection high-energy electron diffraction (RHEED) during substrate rotation. The extraction of an arbitrary number of still frames from a continuously changing RHEED pattern is realized by triggering the substrate rotation and it allows analysis of quasistatic RHEED patterns that are updated every revolution. At the same time, the intensity along a line parallel to the shadow edge can be used to reconstruct a planar cut through the reciprocal lattice similar to a low-energy electron diffraction pattern. This RHEED pattern directly reveals the symmetry of the surface reconstruction and its changes during the deposition process.

The Si(001) surface subjected to different treatments in ultrahigh vacuum molecular beam epitaxy chamber for SiO$_2$ film decomposition has been in situ investigated by reflected high energy electron diffraction (RHEED) and high resolution scanning tunnelling microscopy (STM). A transition between (2 x 1) and (4 x 4) RHEED patterns was observed. The (4 x 4) pattern arose at T <~600C during sample posttreatment cooling. The reconstruction was observed to be reversible. The c(8 x 8) structure was revealed by STM at room temperature on the same samples. The (4 x 4) patterns have been evidenced to be a manifestation of the c(8 x 8) surface structure in RHEED. The phase transition appearance has been found to depend on thermal treatment conditions and sample cooling rate.

A scanning tunnelling microscopy study of the deposition of Si on GaAs(001); Implications for Si δ-doping

Epitaxial growth of CoO films on semiconductor and metal substrates by constructing a complex heterostructure

Reflection high energy electron diffraction study of CoSi2/Si multilayer structures

Epitaxy of Al films deposited on GaN has been studied using reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), x-ray diffraction, and ion channeling techniques. Al (111) films have been found to grow epitaxially on GaN (0001) surfaces with Al 〈21̄1̄〉‖GaN〈21̄1̄0〉. For growth at 15 and 150 °C with a deposition rate of 0.26 Å/s, the epitaxial quality of the film was poor initially, as evidenced by the observation of diffuse RHEED patterns. After a few monolayers, a sharp and streaky RHEED pattern develops and is maintained during subsequent deposition, indicating an improvement in epitaxial quality with a two-dimensional growth mode. AFM studies indicate that the initial GaN surface quality is a significant factor in achieving epitaxial growth, and that the size of Al epitaxial islands increases substantially for higher growth temperatures. X-ray diffraction and ion channeling results confirm the epitaxial nature of the Al films in spite of a significant lattice mismatch of 10.2%.

A combination of the surface diagnostic techniques Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED), and secondary ion mass spectroscopy (SIMS) was used in order to get more detailed information on basic processes which lead to the formation of high quality monocrystalline GaAs and Al x Ga1−x As films by molecular beam epitaxy (MBE) under ultra-high vacuum conditions. The formation and changes of reconstructed surface structures on (100) GaAs as a function of growth parameters were observedduring growth by RHEED. AES was used to determine the relative ratio of Ga/As on the surface for different reconstructed structures, to investigate the impurity contamination on substrate surfaces and grown films, and to study the surface segregation of Sn in MBE GaAs during doping. Finally, intentional and unintentional impurities incorporated during the growth of GaAs and Al x Ga1−x As by MBE were detected by the SIMS technique immediately after growth within the reaction chamber.

Summary form only given. CdTe has been considered as an ideal material for solar cells and has been employed in HgCdTe alloys to elaborate infrared photodetectors. However, when few monolayers of CdTe are deposited within ZnTe, the fundamental optical transitions of CdTe/ZnTe quantum wells (QWs) can be designed to be in the 1.5 - 2.3 eV (infrared-green) range at room temperature. In order to consider possible device applications it is necessary a systematic study of the optical and structural properties of ultra-thin quantum wells (UTQWs) of CdTe and the determination of the most favorable growth parameters. In this work we present an investigation by low temperature photoluminescence (PL) and room temperature photoreflectance (PR) of UTQWs of CdTe grown by atomic layer epitaxy (ALE). The epitaxial growth was carried out in a Riber 32P system with Cd, Zn and Te elemental solid sources. Before the deposition of the CdTe QWs a 0.5 /spl mu/m thick ZnTe buffer layer was grown onto the GaAs[100] substrates. The ALE growth of CdTe UTQWs was performed by alternate exposure of the substrate surface to individual fluxes of Cd and Te. After each flux exposure, a 4 sec dead time (flux interruption) was applied to allow re-evaporation and migration of the atomic species. The ZnTe barriers were deposited by molecular beam epitaxy (MBE). The samples were prepared at a substrate temperatures (T/sub s/) of 270/spl deg/C and 290/spl deg/C. They consisted of five CdTe UTQWs made with four and eight ALE-cycles. Under our growth conditions a /spl sim/0.5 ML coverage per cycle is obtained, variations are observed as a function of temperature. This means that the QWs are nominally 2 and 4 ML thick. The growth was monitored in-situ by reflection high energy electron diffraction (RHEED). Information about the kinetics of growth was obtained through the analysis of the temporal evolution of some features of the RHEED pattern of the (2/spl times/1)-Te reconstruction along the [110] azimuth. The UTQWs, grown at 270/spl deg/C presented very intense and sharp PL emission, while those grown at 290/spl deg/ presented peak structures attributed to thickness fluctuations of the QWs due to a chemical interaction that results in the substitution of Cd atoms by Zn atoms in the near-surface region during the ALE growth; this interaction has been clearly demonstrated by Auger spectroscopy. No evidence of a 3-D growth was observed from in-situ RHEED experiments. Room temperature PR spectra presented optical transitions originating in the CdTe UTQWs, the ZnTe barriers and the GaAs substrate.

Ultrathin magnetic metallic structures provide a variety of systems in which unique magnetic properties can be engineered. The investigation of magnetic anisotropies in ultrathin structures has brought exciting results to the basic studies of magnetism in systems with reduced dimensionality and to the engineering of new magnetic materials. Molecular-beam epitaxy (MBE) techniques turned out to be particularly useful in the formation of new stable and metastable structures whose sharply defined interfaces, lattice relaxations, and lattice reconstructions have resulted in a wide range of interesting magnetic properties. In this presentation the present authors’ recent studies of single and multilayered structures composed of bcc Fe(001), bcc Cu(001), lattice-expanded Pd(001), fcc Co(001), and fcc Cu(001) will be summarized. Besides presenting their interesting magnetic properties the following techniques will be highlighted: reflection high-energy electron diffraction (RHEED) and ferromagnetic resonance (FMR). RHEED patterns and RHEED intensity oscillations were used to monitor the MBE growths. It will be shown that RHEED studies provide an effective way to characterize the quality of growth which can then be correlated with the observed magnetic properties. It will be shown that the FMR technique is extremely useful in determining all of the basic magnetic parameters. The role of interfaces, lattice relaxations, and proximity effects on the in-plane fourfold and perpendicular uniaxial anisotropies will be discussed.

Homoepitaxial, GaN films on both c-plane surfaces of bulk GaN crystals were examined using reflection high-energy electron diffraction (RHEED). Differences in the RHEED pattern, time development of the RHEED intensity, and surface reconstructions were observed. The substrate surfaces were prepared either by mechanical polishing [GaN(0001)A] or by chemo-mechanically polishing [GaN(0001̄)B]. Then films were grown by molecular beam epitaxy; Ga was provide by a Knudsen cell and nitrogen from NH3. On the B surface, the Ga rich reconstructions reported by Smith and co-workers [Phys. Rev. Lett. 79, 3934 (1997)] were observed. On the A surface, a (2×2) reconstruction was observed. Both reconstructions were much sharper than those seen on GaN films grown on sapphire. RHEED measurements of the specular intensity vs time showed that two different surface terminations could be maintained on the B surface, one of which is a stable, gallided surface, while the other is a nitrided surface, which is unstable in vacuum. If the nitrided surface is heated in vacuum it changes to the gallided surface in several minutes at 800 °C. Only one termination was detected on the A surface. The results are complemented by desorption mass spectroscopy measurements, and the resulting surfaces were then investigated using atomic force microscopy and scanning tunneling microscopy. We were able to distinguish the two surface terminations on the B surface, and a unique annealing process under NH3 will be documented. Preliminary investigation of the A surface revealed decorated step edges. The results were compared to films grown on sapphire with different nucleation layers, which can be grown to yield either polarity.

SrTiO3 surface was monitored in-situ by reflection high-energy electron diffraction(RHEED) during annealing and homoepitaxial growth in laser molecular beam epitaxy(LMBE). By analyzing RHEED pattern, we show the oscillation behavior of in-plane lattice constant and full-width at half maximum (FWHM) of diffraction streaks; the former is due to the interface between the annealed reconstruction surface and the growing film, and the origin of the latter is related to the relaxation of 2D islands by their edges. In addition, the phase shift of RHEED intensity oscillation was observed, due to plasma influence on the incident electron beam.