Interfacial roughness and magnetoresistance in [formula omitted] multilayers

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.

ABSTRACTAn organic molecular beam epitaxy (OMBE) system has been designed and constructed with in situ reflection high-energy electron diffraction (RHEED) specifically for the deposition of organic molefelar layers under ultrahigh vacuum (UHV), the order of 10–10 torr. The system is equipped with a portable UHV chamber which allows easy transfer of the OMBE film samples to a separate UHV scanning tunneling microscopy (STM) system. A structural investigation of heteroepitaxially grown organic ultrathin layers of copper phthalocyanine (CuPc) on inorganic substrates was carried out by the combined UHV system from less than a monolayer of CuPc.

Multilayer thin film structures for reflecting soft x-rays are now being fabricated in a number of laboratories. However, understanding of. the optical properties of these structures is presently limited by lack of knowledge of the microstructure of the layers, as well as of the details of the interfaces. In this paper we present results from our studies of multilayers grown by molecular beam epitaxy (MBE), characterized in situ by reflection high energy electron diffraction (RHEED), low energy electron diffraction (LEED), Auger, and x-ray photoelectron spectroscopy (XPS), and characterized ex situ by scanning tunneling microscopy (STM), transmission electron microscopy (TEM), x-ray diffraction, and Rutherford back scattering (RBS). In the case of Mo/Si multilayers, we observe the formation of an amorphous interfacial silicide, which can have a positive effect on the performance of these evaporated multilayer mirrors. We observe a contraction in the period of these multilayers as the deposition temperature is raised from 50 °C to 250 °C, corresponding to an increase in the thickness of the interfacial silicide. This contraction indicates that the silicide is more dense than the average atomic density of its components. We also discuss Ag/B and Pd/B multilayers, which have very similar theoretical performance. However, due to differences in the multilayer structures formed, the actual performance of multilayers made from these materials is radically different. The structural differences originate from different growth modes for Ag and Pd on B.

Sc2O3 films had been found to be epitaxially grown on Si (111) substrate with different thickness. With electron beam evaporated from a high-purity Sc2O3 source in UHV (a molecular beam epitaxy (MBE) approach), we got good crystallinity Sc2O3 film on silicon substrate. The in-situ reflection high energy electron diffraction (RHEED) was used to monitor the surface of the sample during the growth process. Structural and morphological studies were examined bysingle x-ray diffraction(XRD), and high-resolution transmission electron microscopy(HRTEM). The film thickness has been studied using low-angle x-ray diffraction (XRR) and cross-sectional TEM. The Sc2O3 films grown in our MBE system have a cubic phase with a very uniform thickness and a highly structural perfection, even for films as thin as 1.5 nm. Moreover ,without capping amorphous silicon to protect the Sc2O3 film, there has no interfacial layer exist in the interface. The film normal was found to be , which is well aligned with the Si substrate normal. From the phi scan the in plane relation was rotated by 60 degree, and was proved by electron diffraction. To describe the periodical dislocation in the interface, we quoted Bollmann O-lattice theory, and estimated of the spacing between dislocations. We are surprised with large lattice mismatch (up to 9.3%) Sc2O3 still can form one domain crystalline film.

Interest in the study of magnetic/non-magnetic multilayered structures took a giant leap since Grünberg and his group established that the interlayer exchange coupling (IEC) is a function of the non-magnetic spacer width. This interest was further fuelled by the discovery of the phenomenal Giant Magnetoresistance (GMR) effect. In fact, in 2007 Albert Fert and Peter Grünberg were awarded the Nobel Prize in Physics for their contribution to the discovery of GMR. GMR is the key property that is being used in the read-head of the present day computer hard drive as it requires a high sensitivity in the detection of magnetic field. The recent increase in demand for device miniaturization encouraged researchers to look for GMR in nanoscale multilayered structures. In this context, one dimensional(1-D) multilayerd nanowire structure has shown tremendous promise as a viable candidate for ultra sensitive read head sensors. In fact, the phenomenal giant magnetoresistance(GMR) effect, which is the novel feature of the currently used multilayered thin film, has already been observed in multilayered nanowire systems at ambient temperature. Geometrical confinement of the supper lattice along the 2-dimensions (2-D) to construct the 1-D multilayered nanowire prohibits the minimization of magnetic interaction- offering a rich variety of magnetic properties in nanowire that can be exploited for novel functionality. In addition, introduction of non-magnetic spacer between the magnetic layers presents additional advantage in controlling magnetic properties via tuning the interlayer magnetic interaction. Despite of a large volume of theoretical works devoted towards the understanding of GMR and IEC in super lattice structures, limited theoretical calculations are reported in 1-D multilayered systems. Thus to gauge their potential application in new generation magneto-electronic devices, in this thesis, I have discussed the usage of first principles density functional theory (DFT) in predicting the equilibrium structure, stability as well as electronic and magnetic properties of one dimensional multilayered nanowires. Particularly, I have focused on the electronic and magnetic properties of Fe/Pt multilayered nanowire structures and the role of non-magnetic Pt spacer in modulating the magnetic properties of the wire. It is found that the average magnetic moment per atom in the nanowire increases monotonically with an ~1/(N(Fe)) dependance, where N(Fe) is the number of iron layers in the nanowire. A simple model based upon the interfacial structure is given to explain the 1/(N(Fe)) trend in magnetic moment obtained from the first principle calculations. A new mechanism, based upon spin flip with in the

Visualization of precipitation induced crystallographic shear planes as one-dimensional structures on surfaces: an STM and RHEED study on TiO 2(110)

Giant Magnetoresistance (GMR) head technology is one of the latest advancement in hard disk drive (HDD) storage industry. The GMR head superlattice structure consists of alternating layers of extremely thin metallic ferromagnet and paramagnet films. A large decrease in the resistivity from antiparallel to parallel alignment of the film magnetizations can be observed, known as giant magnetoresistance (GMR) effect (Baibich et al., 1988; Binasch et al., 1989). The GMR effect is generally due to the spin dependent electron bulk and interfacial scattering in the GMR multilayer structures (Zhang et al., 1992). However, in order to understand the nature of the spin-dependent electron scattering mechanism responsible for the GMR effect, both electrical and thermal transport properties of such multilayer structures must be measured and understood. It is suggested that the thermal transport property measurements in GMR can be used to judge whether the scattering processes responsible for the GMR have elastic and/or inelastic components (Shi et al., 1996). Moreover, the GMR effect is anticipated to have a thermal counterpart, known as giant magnetothermal resistance (GMTR) effect in which the thermal conductivity shows a ‘giant’ change under magnetic field.

Single-layer FeSe film grown on SrTiO3(001) surface (STO surface) by molecular beam epitaxy has aroused a great research boom ever since the discovery of its huge superconductive energy gap which indicates a possible critical temperature (Tc) higher than the liquid nitrogen temperature. The interface enhanced superconductivity with a Tc above 100 K is revealed in an in situ electrical transport measurement by using a four-point probe installed in a scanning tunneling microscope (STM). Consequent research interest in multi-layer FeSe films grown on STO surface is also increasing. The quality of thick FeSe film, however, has not been well studied yet in previous studies, although it is related to the sample properties including superconductivity. Here, reflection high-energy electron diffraction (RHEED) is used to monitor the growths of multi-layer FeSe thin films on STO surface under different growth conditions. Combing the RHEED results with STM observations taken at various FeSe coverages, we find that the intensity evolution of the RHEED pattern in the early growth stage can be well explained by the step density model but not by the widely known facet model. The intensity evolution of the FeSe(02) diffraction streak exhibits a single-peak oscillation in the growing of the first layer of FeSe. As the oscillation does not depend on the grazing angle of the high-energy electron beam, the FeSe(02) diffraction streak is very suitable for calibrating the FeSe growth rate. In contrast, the intensity of the specular spot exhibits different evolution pattern when the grazing angle of electron beam is changed. It is found in STM observations that only at an appropriate substrate temperature and a growth rate can the high-quality multi-layer FeSe films be grown on STO substrates. If the growth temperature is too high, the FeSe molecules nucleate into islands so that FeSe films with various thickness values eventually come into being on the STO surface. If the growth temperature is too low, a different phase of FeSe film is formed. The optimal growth temperature is in a range from 400 ℃ to 430 ℃, within which a two-layer FeSe film grown at a low rate (0.15 layer/min) coveres the whole STO surface with a negligible number of small FeSe islands. In contrast, a larger growth rate is necessary for growing thicker FeSe film. This is because FeSe islands tend to come into form at steps when the growth rate is too low, which is more distinct in a thicker FeSe film. An STM image of 80-layer FeSe film grown under an optimal condition, i.e., the substrate temperature of 420 ℃ and the growth rate of 2.3 layer/min, shows that it is in a perfect layer-by-layer growth mode. These experimental results are useful for growing high-quality multi-layer FeSe films on STO substrates, which could be critical for studying their physical properties and relevant physical phenomena.

Auger investigation of group III nitride films grown by molecular beam epitaxy

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.

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.

Surface micro-structure analysis is very important for surface study in material science. Observations of surface atomic steps and reconstructed structures have been made using several techniques: reflection high energy electron microscopy (RHEEM), low energy electron reflection microscopy (LEERM) and low energy electron diffraction microscopy (LEEDM).In the present experiment, observations of surface micro-structures have been made using a scanning type reflection high energy electron diffraction (RHEED) microscopy. This technique has certain advantages of easy combinations with multiple surface analyzing techniques such as scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS).A schematic diagram of the scanning RHEED microscope combined with the microprobe AES is shown in Fig. 1. RHEED patterns are observed on the fluorescent screen through a viewing port. To observe the micro-structure (scanning RHEED image or dark field image), a particular diffraction spot is selected by means of the other small fluorescent screen with an aperture.

The formation of the interface between GaAs(001) and Al was studied by reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). All growth was performed by molecular beam epitaxy. Two surface reconstructions defined the native surface: c(2×8) and (4×6). In the studied temperature interval, 268 to 673 K, two temperature ranges were found. Above 550 K no nucleation was observed. At lower temperatures the growth of the first monolayer (ML) was detected as a linear (temperature independent) increase of the Auger signal with coverage. In the coverage range between 1 ML and the point of nucleation the Auger signal became more nonlinear. Both the Al and Ga Auger energies shifted toward metallic states. This shift, however, decreased with substrate temperature. The event of nucleation as observed by RHEED had a clear temperature dependence only when starting the growth from the c(2×8) surface. Evidence was found by both AES and electrical characterization that increasing temperature caused an intermixing at the interface. Electrical characterization of the interface was made by capacitance-voltage (C-V) and current-voltage (I-V) techniques. Averaging over a large number of diodes gave the Schottky-barrier height φb ≊0.75 eV for both techniques. No dependence on the initial surface reconstruction was found but excess Ga gave a larger barrier while excess As typically gave a lower barrier. Annealing the interface usually increased the Schottky barrier and increased the ideality factor above ∼1.05.

Ex situ characterization techniques in molecular beam epitaxy (MBE) have inherent limitations, such as being prone to sample contamination and unstable surfaces during sample transfer from the MBE chamber. In recent years, the need for improved accuracy and reliability in measurement has driven the increasing adoption of in situ characterization techniques. These techniques, such as reflection high-energy electron diffraction, scanning tunneling microscopy, and X-ray photoelectron spectroscopy, allow direct observation of film growth processes in real time without exposing the sample to air, hence offering insights into the growth mechanisms of epitaxial films with controlled properties. By combining multiple in situ characterization techniques with MBE, researchers can better understand film growth processes, realizing novel materials with customized properties and extensive applications. This review aims to overview the benefits and achievements of in situ characterization techniques in MBE and their applications for material science research. In addition, through further analysis of these techniques regarding their challenges and potential solutions, particularly highlighting the assistance of machine learning to correlate in situ characterization with other material information, we hope to provide a guideline for future efforts in the development of novel monitoring and control schemes for MBE growth processes with improved material properties.

RHEED (reflection high energy electron diffraction) is a technique widely used in molecular beam epitaxy (MBE) and for the characterization of surfaces in other contexts. Despite its widespread use, however, the interpretation of RHEED has largely been limited to the use of positions of the reflections with no attempt made to use the intensities in the patterns.The reasons for this are related to the complexity of RHEED computations. There are several methods which can be used to calculate RHEED intensities from flat surfaces. However, in MBE (and other crystal growth methods) where RHEED is most often used, the surfaces are rough. Of the dynamical methods for calculating RHEED intensities from flat surfaces, only the modified Cowley-Moodie multislice method with the edge patching algorithm can be applied to rough surfaces without making severe approximations. This method is computationally intensive because the ensemble average over the statistical parameters of the surface cannot be done before the dynamical calculations.