Reflection High-energy Electron Diffraction Patterns Research Articles

Study of Sb template for heteroepitaxial growth of GaSb thin film on Si(111)substrate

GaSb thin films have been grown on Si(111) substrates by molecular beam epitaxy (MBE), using an Sb template as an initiation layer. The film's crystal properties were investigated by in situ reflection high energy electron diffraction (RHEED), atomic force microscope (AFM), X-ray reciprocal space mapping (RSM) and ϕ-scan X-ray diffraction (XRD). RSMs around Si and GaSb 224 reciprocal lattice points indicated that the GaSb thin film grown on the Sb template is nearly unstrained, in contrast to that grown on an AlSb initiation layer which is under tensile strain. The extra peaks on the ϕ-scan XRD profiles can be assigned to {224} diffraction originating from a sub-domain region that corresponds to the region rotated by 180° with respect to the fundamental domain of epitaxial film. The very weak intensity of the extra peaks suggests that using the Sb template is an effective way to suppress sub-domain generation in the epitaxial film, resulting in a higher quality GaSb thin film on Si(111) substrates. The spotty RHEED patterns observed were analyzed as a superposition of zinc-blende type diffraction spots on the [111]–[2¯11] plane and [111]–[21¯1¯] plane, indicating that the GaSb quantum dots at the initial growth stage are not randomly orientated polycrystalline structures but a two-domain structure with zinc-blende type crystals.

Structure and Electrochemical Stability of Pt-Enriched Ni/Pt(111) Topmost Surface Prepared by Molecular Beam Epitaxy

We investigated the atomic-scale structure and electrochemical stability of a Pt-enriched topmost surface (Pt-enriched Ni/Pt(111)) prepared through monolayer Ni deposition on Pt(111) at 823 K using molecular beam epitaxy. Reflection high-energy electron diffraction patterns and an ultra-high vacuum scanning tunneling microscopic (UHV-STM) image of the Pt-enriched Ni/Pt(111) surface showed that the surface has long-range-ordered six-fold symmetry with atomic-scale corrugations. Although the oxygen reduction reaction (ORR) activity of the as-prepared Pt-enriched surface was eight times higher than that of clean Pt(111), the ORR activity degraded steeply during potential cycles between 0.6 and 1.0 V in O2-saturated 0.1 M HClO4. After 1000 potential cycles, the enhancement factor was estimated to be 2. An STM image collected after the potential cycles showed island-like structures (ca. 5–9 nm in diameter and 0.4–0.8 nm in height). Furthermore, the intensity of the Ni 2p core-level band for the as-prepared sample decreased after the 1000 potential cycles, revealing dissolution of the Ni atoms located at the subsurface layer. This experimental study clearly demonstrates that the underlying-Ni-induced specific surface strains and electronic state of the Pt-enriched topmost surface sorely contribute to the remarkable ORR activity of the Ni/Pt(111) surface.

ZnO/MgO(011)的异质外延与特性

本文着重研究ZnO薄膜在MgO(011)衬底上的生长行为。利用分子束外延法(MBE)在MgO(110)衬底上生长ZnO薄膜，通过原子力显微镜(AFM)和扫描电子显微镜(SEM)表征了其表面形貌，利用X射线光电子能谱学(XPS)对薄膜的组分进行深度分层剥析，并采用透射谱(XL)研究了其光学性质。结果显示，薄膜以三维模式生长，而且薄膜表面平整度受到衬底温度的影响；而所生长的薄膜均显示了ZnO透射谱的特征峰，特征峰的位置几乎不受衬底温度的影响。不同生长时间段的薄膜结构的演变过程也通过原位的反射高能电子衍射(RHEED)和原子力显微镜进行了考察。XPS分层组分分析发现，在ZnO薄膜生长过程中，Zn离子会扩散到MgO(110)衬底之中。 In this work, the growth behaviors of ZnO thin films on MgO (011) substrates were studied. ZnO films were deposited on MgO(110) substrates using oxygen plasma-assisted molecular beam epitaxy (MBE). Their surface mor- phologies were probed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Elemental con- stituents at different depths were also analyzed by X-ray photoelectron spectroscopy (XPS) and the optical properties of the ZnO films were characterized by transmission spectra. The film growth was found to follow a three-dimensional growth mode and the surface morphologies could be monitored by substrate temperature. The characteristic band gap of ZnO was found for all the films and the value was little influenced by the substrate temperature. In addition, the evolu- tion of the structure was examined by capturing in situ reflection high-energy electron diffraction (RHEED) patterns and ex situ AFM for different growth steps. During the growth of ZnO films, the Zn ions were found to diffuse into the MgO(110) substrate.

Thermodynamic study on two-step desorption of oxides on InAs(001) surface

Reflection high energy electron diffraction (RHEED) is used to monitor the two-step desorption of oxides on InAs(001) surface in the vacuum chamber, and the high temperature indium-assisted desorption processes of surface oxides under high arsenic pressure and low arsenic pressure are compared. The first step of two-step deoxidation method for InAs substrate is to heat the substrate slowly at high temperature. The second step is high temperature indium beam-assisted desorption of surface oxides. The RHEED patterns of sample at high temperature desorption of oxides show that the high temperature indium beam-assisted desorption of InAs surface oxide method could eventually clear residual oxide that the traditional slow heating method cannot remove. The scanning tunneling microscope images of sample after homogeneous epitaxial growth prove the viability of high-heat indium beam-assisted desorption of InAs surface oxide under high arsenic pressure. Finally, we analyse the mechanism of high-heat indium beam assisted desorption of surface oxides of substrate.

Growth of N-polar GaN Using a CrN buffer layer on (0001) Al2O3 via plasma-assisted molecular beam epitaxy

The growth of N-polar GaN films on (0001) Al2O3 substrates by plasma-assisted molecular beam epitaxy was demonstrated using a CrN buffer layer. Analysis of reflection high energy electron diffraction (RHEED) patterns and a chemical etching method were used in order to confirm the polarity of GaN on CrN. RHEED patterns show the 3×3 pattern characteristic of GaN. Chemical etching significantly changes the GaN surface morphology which implies that the N-polar GaN was grown on the CrN buffer layer. In addition, an improvement in the crystal properties of GaN was achieved using the annealing process for the CrN buffer layers.

Structural, morphological, and band alignment properties of GaAs/Ge/GaAs heterostructures on (100), (110), and (111)A GaAs substrates

Structural, morphological, and band offset properties of GaAs/Ge/GaAs heterostructures grown in situ on (100), (110), and (111)A GaAs substrates using two separate molecular beam epitaxy chambers, connected via vacuum transfer chamber, were investigated. Reflection high energy electron diffraction (RHEED) studies in all cases exhibited a streaky reconstructed surface pattern for Ge. Sharp RHEED patterns from the surface of GaAs on epitaxial Ge/(111)A GaAs and Ge/(110)GaAs demonstrated a superior interface quality than on Ge/(100)GaAs. Atomic force microscopy reveals smooth and uniform morphology with surface roughness of Ge about 0.2–0.3 nm. High-resolution triple axis x-ray rocking curves demonstrate a high-quality Ge epitaxial layer as well as GaAs/Ge/GaAs heterostructures by observing Pendellösung oscillations. Valence band offset, ΔEv, have been derived from x-ray photoelectron spectroscopy (XPS) data on GaAs/Ge/GaAs interfaces for three crystallographic orientations. The ΔEv values for epitaxial GaAs layers grown on Ge and Ge layers grown on (100), (110), and (111)A GaAs substrates are 0.23, 0.26, 0.31 eV (upper GaAs/Ge interface) and 0.42, 0.57, 0.61 eV (bottom Ge/GaAs interface), respectively. Using XPS data obtained from these heterostructures, variations in band discontinuities related to the crystallographic orientation have been observed and established a band offset relation of ΔEV(111)Ga>ΔEV(110)>ΔEV(100)As in both upper and lower interfaces.

Initial stage growth of GexSi1-x layers and Ge quantum dot formation on GexSi1-x surface by MBE.

Critical thicknesses of two-dimensional to three-dimensional growth in GexSi1−x layers were measured as a function of composition for different growth temperatures. In addition to the (2 × 1) superstructure for a Ge film grown on Si(100), the GexSi1−x layers are characterized by the formation of (2 × n) reconstruction. We measured n for all layers of Ge/GexSi1−x/Ge heterosystem using our software with respect to the video recording of reflection high-energy electron diffraction (RHEED) pattern during growth. The n reaches a minimum value of about 8 for clear Ge layer, whereas for GexSi1−x films, n is increased from 8 to 14. The presence of a thin strained film of the GexSi1−x caused not only the changes in critical thicknesses of the transitions, but also affected the properties of the germanium nanocluster array for the top Ge layer. Based on the RHEED data, the hut-like island form, which has not been previously observed by us between the hut and dome islands, has been detected. Data on the growth of Ge/GexSi1−x/Ge heterostructures with the uniform array of islands in the second layer of the Ge film have been received.

Reflection high-energy electron diffraction evaluation of thermal deoxidation of chemically cleaned Si, SiGe, and Ge layers for solid-source molecular beam epitaxy

The authors present a study on the thermal evolution of the reflection high-energy electron diffraction pattern of chemically cleaned (001)-oriented Si, Ge, and SiGe surfaces, associating observed changes in the reconstructions with the desorption of known residual contaminants for Si and Ge surfaces. The implications of residual oxides prior to epitaxy on stacking fault densities in the grown films are presented. Further evidence for the two-phase nature of oxides on SiGe surfaces is provided, demonstrating that it is necessary to heat a SiGe surface up to the thermal deoxidation temperature of a Si surface to obtain stacking fault-free growth.

Understanding the Evolution of CdTe Buffer Layer Surfaces on ZnTe/Si(211) and GaAs(211)B During Cyclic Annealing

We present the results of a detailed study of the changes that occur on CdTe buffer layer surfaces grown on ZnTe/Si(211) and GaAs(211)B during the routine thermal cyclic annealing (TCA) process. Observations indicate that CdTe buffer layer surfaces are Te saturated when the TCA is performed under Te overpressure. In the absence of Te flux during the TCA step, the CdTe surface loses CdTe congruently and the typical CdTe nanowires show the presence of nodules on their surfaces. The observed changes in reflection high-energy electron diffraction patterns during TCA are explained in terms of surface chemistry and topography observations. Overall, the Te overpressure is necessary to maintain a smoother and pristine surface to continue the molecular beam epitaxy (MBE) growth.

RHEED monitoring of elastic stresses during MBE growth of group III nitride heterostructures

Results of statistical analysis of the patterns of reflection high-energy electron diffraction (RHEED) measured during molecular beam epitaxy (MBE) of heterostructures of wide-bandgap (Al, Ga, In)N compounds are presented, which can be used for determining the lateral lattice constant (a) of the epitaxial layers. It is established that the error of determination can vary from a minimum of 0.17% up to several percent, depending on the contrast of RHEED patterns, which is determined by the stoichiometry of MBE growth. It is demonstrated that the RHEED data can reveal relaxation of elastic stresses during the growth of short-period {GaN(4 nm)/AlN(6nm)}30/AlN superlattices.

Three-Dimensional Reciprocal-Lattice Analysis Using Azimuth-Scan Reflection High-Energy Electron Diffraction: Determination of Complex Crystal Orientations of Al Grains on Si(111) Surface

We have applied a three-dimensional (3D) reciprocal-lattice analysis method using a typical reflection high-energy electron diffraction (RHEED) system – all RHEED patterns in scanning sample-surface azimuth are converted into 3D reciprocal-lattice space. This analysis method can determine complex crystal orientations of nanoclusters, islands, and grains with multiple domains, which are difficult to obtain from a small number of non-converted two-dimensional RHEED patterns. For an Al-deposited Si(111) surface followed by annealing, we successfully determined new crystal orientations of Al grains: Al(001), Al(012) and Al(011) ∥ Si(111) with Al[100] ∥ Si<01̄1>. The typical acquisition time of 3D RHEED patterns is 10–20 min, which is shorter than that by a standard X-ray diffraction system with φ and ω scans for 3D reciprocal-lattice mapping. This is one of the advantages of this analysis method, in addition to the convenient observation of in situ vacuum-fabricated nanocrystals on substrate surfaces with high sensitivity.

Surface segregation of InGaAs films by the evolution of reflection high-energy electron diffraction patterns

Surface segregation is studied via the evolution of reflection high-energy electron diffraction (RHEED) patterns under different values of As4 BEP for InGaAs films. When the As4 BEP is set to be zero, the RHEED pattern keeps a 4×3/(n × 3) structure with increasing temperature, and surface segregation takes place until 470 °C. The RHEED pattern develops into a metal-rich (4 × 2) structure as temperature increases to 495 °C. The reason for this is that surface segregation makes the In inside the InGaAs film climb to its surface. With the temperature increasing up to 515 °C, the RHEED pattern turns into a GaAs(2 × 4) structure due to In desorption. While the As4 BEP comes up to a specific value (1.33 × 10-4 Pa−1.33 × 10-3 Pa), the surface temperature can delay the segregation and desorption. We find that As4 BEP has a big influence on surface desorption, while surface segregation is more strongly dependent on temperature than surface desorption.

Plasma-Assisted Molecular Beam Epitaxy of InXGa1-XN Films on C-plane Sapphire Substrates

We report plasma-assisted molecular beam epitaxy of <TEX>$In_XGa_{1-X}N$</TEX> films on c-plane sapphire substrates. Prior to the growth of <TEX>$In_XGa_{1-X}N$</TEX> films, GaN film was grown on the nitride c-plane sapphire substrate by two-dimensional (2D) growth mode. For the growth of GaN, Ga flux of <TEX>$3.7{\times}10^{-8}$</TEX> torr as a beam equivalent pressure (BEP) and a plasma power of 150 W with a nitrogen flow rate of 0.76 sccm were fixed. The growth of 2D GaN growth was confirmed by <TEX>$in-situ$</TEX> reflection high-energy electron diffraction (RHEED) by observing a streaky RHEED pattern with a strong specular spot. InN films showed lower growth rates even with the same growth conditions (same growth temperature, same plasma condition, and same BEP value of III element) than those of GaN films. It was observed that the growth rate of GaN is 1.7 times higher than that of InN, which is probably caused by the higher vapor pressure of In. For the growth of <TEX>$In_xGa_{1-x}N$</TEX> films with different In compositions, total III-element flux (Ga plus In BEPs) was set to <TEX>$3.7{\times}10^{-8}$</TEX> torr, which was the BEP value for the 2D growth of GaN. The In compositions of the <TEX>$In_xGa_{1-x}N$</TEX> films were determined to be 28, 41, 45, and 53% based on the peak position of (0002) reflection in x-ray <TEX>${\theta}-2{\theta}$</TEX> measurements. The growth of <TEX>$In_xGa_{1-x}N$</TEX> films did not show a streaky RHEED pattern but showed spotty patterns with weak streaky lines. This means that the net sticking coefficients of In and Ga, considered based on the growth rates of GaN and InN, are not the only factor governing the growth mode; another factor such as migration velocity should be considered. The sample with an In composition of 41% showed the lowest full width at half maximum value of 0.20 degree from the x-ray (0002) omega rocking curve measurements and the lowest root mean square roughness value of 0.71 nm.

Delayed crystallization of ultrathin Gd2O3 layers on Si(111) observed by in situ X-ray diffraction

We studied the early stages of Gd2O3 epitaxy on Si(111) in real time by synchrotron-based, high-resolution X-ray diffraction and by reflection high-energy electron diffraction. A comparison between model calculations and the measured X-ray scattering, and the change of reflection high-energy electron diffraction patterns both indicate that the growth begins without forming a three-dimensional crystalline film. The cubic bixbyite structure of Gd2O3 appears only after a few monolayers of deposition.

Interface magnetism of iron grown on sulfur and hydrogen passivated GaAs(001)

Sulfur (S) and hydrogen (H) atom passivated GaAs(001) templates were used for deposition of ultrathin crystalline Fe films using molecular beam epitaxy, where the Fe thickness ranged from 10 to 45 atomic layers. Reflection high-energy electron diffraction patterns showed that the S- and H-passivated surfaces had no and very weak (1 × 2) superlattice reconstructions, respectively. This indicates that these GaAs(001) templates have a square-like symmetry. Magnetic anisotropies were investigated using the in-plane angular dependence of ferromagnetic resonance at 36 GHz. The in-plane cubic and uniaxial anisotropies and perpendicular uniaxial field were described by bulk and interface contributions, indicating that the Fe films have a high lattice coherence. The magnetic properties of the Fe films were compared to those grown on more commonly used GaAs(001) templates having a (4 × 6) reconstruction with an As-rich in-plane uniaxial symmetry. The Fe films grown on S-passivated templates exhibited unique magnetic properties caused by a decreased lattice spacing compared to the bulk Fe.

Wurtzite ZnO (001) films grown on cubic MgO (001) with bulk-like opto-electronic properties

We report the growth of ZnO (001) wurtzite thin films with bulk-like opto-electronic properties on MgO (001) cubic substrates using plasma-assisted molecular beam epitaxy. In situ reflection high-energy electron diffraction patterns and ex situ high resolution transmission electron microscopy images indicate that the structure transition from the cubic MgO substrates to the hexagonal films involves 6 ZnO variants that have the same structure but different orientations. This work demonstrates the possibility of integrating wurtzite ZnO films and functional cubic substrates while maintaining their bulk-like properties.

Efficient kinematical simulation of reflection high-energy electron diffraction streak patterns for crystal surfaces

An efficient program with a user-friendly graphical interface has been developed for simulating reflection high-energy electron diffraction patterns obtained on crystal surfaces. The calculations are based on a kinematical approach which considers single electron scattering events at the surface. This time-efficient approach is in most cases sensitive enough for distinguishing different structural models, even if they differ very subtly. The results are presented in realistic two-dimensional density plots which can be directly compared to experimental observations. This program provides a useful tool in studying different structural models for crystal surfaces. Program summaryProgram title: RHEEDsimCatalogue identifier: AEJC_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEJC_v1_0.htmlProgram obtainable from: CPC Program Library, Queenʼs University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 2752No. of bytes in distributed program, including test data, etc.: 367 205Distribution format: tar.gzProgramming language: Matlab (version > 7.6.0)Computer: Personal ComputersOperating system: Windows with Matlab environmentRAM: Greater than 1 MBClassification: 7.4Nature of problem: Reflection high-energy electron diffraction (RHEED) has been widely used in studying crystal surface structures all over the world, especially in combination with ultra-high vacuum molecular beam epitaxy systems. In addition to determining the surface smoothness, RHEED is also a very useful tool in studying surface reconstructions, which are often encountered at the growth surfaces of semiconductors and alloys. While the positions of the fractional streaks can be used to determine the basic information of the surface supercells, the intensity modulations on the fractional streaks provide further insights on the details within these unit cells. Kinematic approach is an efficient method for simulating the RHEED patterns based on various surface structural models, which can help unraveling the surface atomic structure.Solution method: The kinematic approach utilized here assumes single scattering events. Furthermore, the Ewald sphere is approximated into a planar surface for computing the streak intensities (which are most relevant to real experiments). Structure factors are calculated based on a given input of atomic species and their coordinates, with user modifiable form factors. In addition to the intensity modulation within the surface plane, additional modulations extending into the z direction is also taken into consideration, resulting in realistic density maps of the RHEED streaks, which can be directly compared to experimental observations.Unusual features: The main program RHEEDsim.m calls several local subprograms for certain computational tasks. As a result, all programs should be extracted into a single folder, and that folder should be set as the main directory in Matlab.Running time: The computing time is computer and user parameter dependent, but typically ranges from few seconds to few minutes.

Densitometric analysis of calus mineralization in nonunion defect of rabbit radius filled with free autologus omental graft

Using the spot profile analysis, reflection high-energy electron diffraction (RHEED) can be advantageously applied for defect structure evaluation. Two types of GaAs(001) surfaces has been studied that are prepared by molecular beam epitaxy (MOE) under As-rich growth conditions, surfaces as they develop during growth on slightly misoriented GaAs(001) substrates, and those observed after growth on substrates without intended misorientation. RHEED patterns for intensity profile measurements by photodensitometry were photographically recorded using an in situ system (15 kV) and an ex-situ apparatus (50 kV), respectively. This chapter shows a typical pattern with clearly resolved slashes taken from a misoriented Ge(001) surface. Similar patterns evidencing the development of ordered arrays of steps one monolayer GaAs in height were obtained also for GaAs films grown on misoriented GaAs(001) substrates. For small misorientations and less well-ordered step arrays, however, the clear diffraction features are diminishing. The profiles for the singular GaAs(001) surface are also discussed, which show a sharp central spike and a broadened part. Their relative contributions change periodically with K⊥. This can be understood within the framework of a model of irregular step arrays with only a few levels included in step formation.

In situ atomic layer deposition and synchrotron-radiation photoemission study of Al 2O 3 on pristine n-GaAs(0 0 1)-4 × 6 surface

This work presents the in situ reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and synchrotron-radiation photoemission studies for the morphological and interfacial chemical characterization of in situ atomic layer deposited (ALD) Al 2O 3 on pristine molecular beam epitaxy (MBE) grown Ga-rich n-GaAs (1 0 0)-4 × 6 surface. Both the RHEED pattern and STM image demonstrated that the first cycle of ALD-Al 2O 3 process reacted immediately with the GaAs surface. As revealed by in situ synchrotron-radiation photoemission studies, two types of surface As atoms that have excess in charge in the clean surface served as reaction sites with TMA. Two oxidized states were then induced in the As 3d core-level spectra with chemical shifts of +660 meV and +1.03 eV, respectively.

Perpendicular ferrimagnetism in strained Mn2As film

Ferrimagnetic Mn2As thin films with perpendicular magnetic anisotropy were successfully grown on Si(100) by molecular-beam epitaxy. From the reflection high-energy electron diffraction and X-ray diffraction patterns, the orientation of the Mn2As film on Si was along the c-axis in the tetragonal crystal structure. Mn2As film exhibited ferrimagnetic ordering at temperatures greater than 300 K, which differs from antiferromagnetic or paramagnetic behaviors in the bulk form. The magnetic moment of Mn2As determined by saturated magnetization was 0.51 µB per unit cell.