Streaky Reflection High-energy Electron Diffraction Patterns Research Articles

Self-Smoothening of an Ion-Beam-Sputtered Ag (100) Surface: Evolution of Surface Morphology Using RHEED

The development of the surface morphology and rough-to-smooth transition of the ion beam-sputtered Ag (100) surface was studied using reflection high-energy electron diffraction (RHEED) at two different temperatures. After sputtering, self-evolution of the Ag (100) surface morphology was continuously monitored by the passage from spotty to streaky RHEED patterns. This approach allowed us to follow the qualitative time evolution of the surface morphology of the Ag (100) surface, which underwent a self-transition from a rough-to-smooth surface (3D rough surface to 2D-like smooth surface) within 120 minutes at room temperature (RT) and about 60 minutes at 323 K temperature. Once the saturation in smoothness was reached, no significant changes in surface morphology with time were observed. At temperatures about 323 K, the achieved smoothness value was about 12 pct more than that observed at RT. The surface smoothing process at RT is understood in terms of self-diffusion, nucleation of randomly distributed defects and vacancies, whereas increased surface diffusion due to an increase in temperature leads to a smoother surface.

Stoichiometry dependence of the optical and minority-carrier lifetime behaviors of CdTe epitaxial films: A low-temperature and time-resolved photoluminescence study

Cadmium telluride (CdTe) epitaxial films (EFs) were grown on near-lattice-matched Cd0.96Zn0.04Te (CZT) substrates by molecular beam epitaxy at different ambients to achieve Cd-rich samples with extra Cd molecular flux or Te-rich samples with extra Te molecular flux. The evolution of epitaxial growth was in situ monitored by reflection high-energy electron diffraction (RHEED). A two-dimensional growth mode was indicated by the streaky RHEED patterns. Crystal structures of the CdTe EFs were characterised by X-ray diffraction (XRD). XRD data suggested that the crystal quality of the CdTe EFs was improved by controlling the Cd and Te flux ratio. Low-temperature photoluminescence (PL) spectra were carried out in these CdTe EFs. The typical characteristic peak at ∼1.552eV denoted as the bound-to-free transition was only found in CdTe samples grown under an extra Cd flux, and Cd vacancy-related defects were absent in the Cd-rich EFs, confirming the Cd-rich or Te-rich states of the epitaxial CdTe films. Finally, minority-carrier lifetime was prolonged in Cd-rich CdTe EFs as supported by time-resolved photoluminescence (TRPL) measurement.

Perpendicular Magnetic Anisotropy and Spin Glass-like Behavior in Molecular Beam Epitaxy Grown Chromium Telluride Thin Films

Reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), vibrating sample magnetometry, and other physical property measurements are used to investigate the structure, morphology, magnetic, and magnetotransport properties of (001)-oriented Cr2Te3 thin films grown on Al2O3(0001) and Si(111)-(7×7) surfaces by molecular beam epitaxy. Streaky RHEED patterns indicate flat smooth film growth on both substrates. STM studies show the hexagonal arrangements of surface atoms. Determination of the lattice parameter from the atomically resolved STM image is consistent with the bulk crystal structures. Magnetic measurements show the film is ferromagnetic, having a Curie temperature of about 180 K, and a spin glass-like behavior was observed below 35 K. Magnetotransport measurements show the metallic nature of the film with a perpendicular magnetic anisotropy along the c-axis.

Investigation of CuGaSe2/CuInSe2 double heterojunction interfaces grown by molecular beam epitaxy.

In-situ reflection high-energy electron diffraction (RHEED) observation and X-ray diffraction measurements were performed on heterojunction interfaces of CuGaSe2/CnInSe2/CuGaSe2 grown on GaAs (001) using migration-enhanced epitaxy. The streaky RHEED pattern and persistent RHEED intensity oscillations caused by the alternate deposition of migration-enhanced epitaxy sequence are observed and the growths of smooth surfaces are confirmed. RHEED observation results also confirmed constituent material interdiffusion at the heterointerface. Cross-sectional transmission electron microscopy showed a flat and abrupt heterointerface when the substrate temperature is as low as 400 °C. These have been confirmed even by X-ray diffraction and photoluminescence measurements.

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.

Two-dimensional growth of SrTiO3 thin films on (001) MgO substrates using pulsed laser deposition and reflection high energy electron diffraction

Two-dimensional epitaxial growth of SrTiO3 is achieved on MgO, a substrate presenting a large lattice mismatch of 7.9%. In situ reflection high energy electron diffraction (RHEED) is used to monitor and control the growth. An interval deposition technique is implemented to force the two-dimensional growth. It is necessary to deposit a single atomic layer of TiO2 before SrTiO3 to prevent the formation of high energy SrO/MgO interfaces and so allow the film to completely wet the substrate surface. Defect formation is inhibited. The resulting film exhibits a similar surface to the substrate and shows a streaky RHEED pattern.

GaN on Si with nm-thick single-crystal Sc 2O 3 as a template using molecular beam epitaxy

The epitaxial growth of GaN on Si (1 1 1) substrates has been performed using plasma-assisted molecular beam epitaxy with a thin (∼19.3 nm) single-crystal layer of Sc 2O 3 as a template/buffer layer. The structural properties and in situ epitaxial growth were studied using reflection high energy electron diffraction (RHEED), high-resolution transmission electron microscopy, and high-resolution X-ray diffraction. An orientation relationship of GaN(0 0 0 2)∥Sc 2O 3(1 1 1)∥Si(1 1 1) and GaN[1 0 1¯ 0]∥Sc 2O 3[4 2¯ 2¯] off 60° with Si[4 2¯ 2¯] was determined. The excellent growth of Sc 2O 3 on Si and GaN on Sc 2O 3 was achieved, as observed from streaky and bright RHEED patterns. The Sc 2O 3 template serves as an effective barrier layer preventing diffusion of Si and Ga during the GaN growth at high temperatures under nitrogen plasma. No cracking was observed in the GaN layer with thickness around 0.2 μm when inspected with an optical microscope.

Effects of oxygen pressure on the microstructure of LaNiO3 conductive thin film monitored byin situreflection high energy diffraction

LaNiO3 (LNO) conductive thin films with different thicknesses were deposited on SrTiO3 (100) substrates in different oxygen pressures. Effects of oxygen pressure on microstructure of LNO conductive film have been studied by in situ reflection high energy electron diffraction (RHEED) and ex situ x-ray photoelectron spectroscopy (XPS). In the relatively low oxygen pressure, LNO film displays spotty RHEED pattern. When the thickness increases up to a critical value, about 30nm, the spotty RHEED pattern gradually changes to streaky pattern, and the RHEED oscillation curve appears. The streaky RHEED pattern of LNO film deposited in the relatively high oxygen pressure can be observed at the initial growth. With pumping the oxygen pressure to a relatively low value, the RHEED pattern gradually changes to spotty one. When the oxygen pressure is increased back to a high value, the RHEED pattern changes to streaky one again. This RHEED pattern transformation induced by the oxygen pressure is reversible. Ex situ XPS results indicate that the element Ni of LNO film deposited in the relatively low oxygen pressure with a thickness below the critical value exists in the form as Ni2+ while as Ni3+ in the relatively high oxygen pressure. When the thickness increases to the critical value, the chemical valence of element Ni in the top layers of LNO film deposited in the relatively low oxygen pressure is +3. A mechanism of effects of oxygen pressure on microstructure of LaNiO3 conductive thin film is given. This sensitivity to oxygen for LNO thin film can be used in sensor devices.

Structural and optical characterization of GaNAs layers grown by molecular beam epitaxy

We have grown GaNxAs1−x layers by molecular beam epitaxy on GaAs(100) substrates using a radio frequency plasma nitrogen source and solid sources for Ga and As. Employing reflection high-energy electron diffraction (RHEED), the GaNAs growth mode was in situ monitored. A three dimensional (3D) growth mode was obtained at the low growth temperature of 420°C. At higher temperatures streaky RHEED patterns were observed during all the GaNAs deposition, indicating a two dimensional (2D) growth mode. The structural and optical properties of the GaNAs layers were studied by employing high-resolution x-ray diffraction, atomic force microscopy, Raman scattering, and spectroscopic ellipsometry. The films grown in a 3D mode presented high density of crystal defects, degraded structural properties, and broad optical transitions. In contrast, GaNAs layers grown in a 2D mode are pseudomorphic with high crystal quality. The properties of samples with a high N concentration were improved by first growing a GaNAs layer wit...

Growth Temperature Dependence of SrTiO3 Thin Films by Molecular Beam Epitaxy

The effect of Sr deposition on the chemically formed SiO2 layer of Si(001) substrates and consequently the growth of SrTiO3 thin films on the Si(001)-Sr(2×1) surface have been studied using reflection high-energy electron diffraction (RHEED), X-ray diffraction and atomic force microscopy. After Sr deposition on the chemically formed SiO2/Si surface, a stable and well ordered Si(001)-Sr(2×1) surface is formed. The SrTiO3 film grown on the reconstructed Si(001)-Sr(2×1) surface at 350°C is amorphous. The sharp, streaky RHEED patterns and the strong STO (002) diffraction peaks and the smooth surface with root mean square roughness of approximately 4 Å suggest that high-quality SrTiO3 films are fabricated at temperatures (400–500°C) using molecular oxygen in molecular beam epitaxy.

Single-crystalline rocksalt CdO layers grown on GaAs (001) substrates by metalorganic molecular-beam epitaxy

In this letter, we report the growth of single-crystalline rocksalt CdO layers on (001) GaAs substrates using ZnS buffer layers. The growth processes of CdO layers were studied by reflection high-energy electron diffraction (RHEED), and the grown CdO layers were evaluated with atomic force microscopy (AFM), and x-ray diffraction (XRD) measurements. After an initial growth delay, the formation of polycrystalline CdO was observed in RHEED measurements during the initial growth of very thin CdO layers. With the increase of the CdO layer thicknesses, streaky RHEED patterns were observed, which indicate the formation of single-crystalline cubic-phase CdO layers. Surface morphology of the CdO layers observed by AFM was atomically flat with root-mean-square roughness of ∼1 nm. The crystalline structures were elucidated from XRD measurements by the determination of the lattice constant to be 4.686±0.001 Å, indicating the single-phase rocksalt CdO structure.

Growth mode of AlN epitaxial layers on 6H-SiC by plasma assisted molecular beam epitaxy

AlN layers were grown on 3.5° off 6H-SiC substrate by plasma-assisted molecular beam epitaxy (MBE). A study of the growth mechanism has been carried out by varying the growth time and the III/V ratio, at a constant growth temperature of 800°C. The layers were characterized by means of Reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction (XRD). It is shown that nitrogen-rich conditions always give rise to a three-dimensional growth with spotty RHEED patterns and rough surface. In Al-rich conditions, the growth is much more two dimensional like with streaky RHEED patterns. By AFM, step bunching growth is observed. The size of the steps depends on the thickness of the layer as well as on the III/V ratio. All the samples grown in Al-rich conditions showed better crystalline quality than those grown in N-rich conditions.

The initiation of GaN growth by molecular beam epitaxy on GaN composite substrates

We report on an investigation of the initial stages of growth by molecular beam epitaxy (MBE) of GaN layers on composite substrates consisting of hydride vapour phase epitaxial (HVPE) GaN films on SiC substrates. In situ reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) were used to monitor the surface, reconstruction of the GaN and to measure the structure of the grown surface, respectively. A 1×3 streaky RHEED pattern was observed on the HVPE GaN layers from room temperatures up to 750°C. This corresponds to a (√3×√3)R30° reconstruction of the GaN surface. At a substrate temperature of 600–650°C, growth by MBE of GaN layers on the HVPE GaN substrate leads to a change of the RHEED pattern from 1×3 to hexagonal 2×2 during the first few minutes of epitaxy. RHEED oscillations were observed at the beginning of growth at 400°C. MBE growth at higher temperatures ∼730–750°C significantly enhances the flatness of the surface of the GaN layer. In this case, RHEED patterns were more intense and streaky than those seen for GaN layers grown at lower temperatures. The surface of the MBE GaN layers grown at high temperatures were atomically flat within the sensitivity of our AFM system.

Properties of Epitaxial SrTiO3 Thin Films Grown on Silicon by Molecular Beam Epitaxy

ABSTRACTSingle crystalline perovskite oxides such as SrTiO3 (STO) are highly desirable for future generation ULSI applications. Over the past three decades, development of crystalline oxides on silicon has been a great technological challenge as an amorphous silicon oxide layer forms readily on the Si surface when exposed to oxygen preventing the intended oxide heteroepitaxy on Si substrate. Recently, we have successfully grown epitaxial STO thin films on Si(001) surface by using molecular beam epitaxy (MBE) method. Properties of the STO films on Si have been characterized using a variety of techniques including in-situ reflection high energy electron diffraction (RHEED), ex-situ X-ray diffraction (XRD), spectroscopic ellipsometry (SE), Auger electron spectroscopy (AES) and atomic force microscopy (AFM). The STO films grown on Si(001) substrate show bright and streaky RHEED patterns indicating coherent two-dimensional epitaxial oxide film growth with its unit cell rotated 450 with respect to the underlying Si unit cell. RHEED and XRD data confirm the single crystalline nature and (001) orientation of the STO films. An X-ray pole figure indicates the in-plane orientation relationship as STO[100]//Si[110] and STO(001)// Si(001). The STO surface is atomically smooth with AFM rms roughness of 1.2 AÅ. The leakage current density is measured to be in the low 10−9 A/cm2 range at 1 V, after a brief post-growth anneal in O2. An interface state density Dit = 4.6 × 1011 eV−1 cm−2 is inferred from the high-frequency and quasi-static C-V characteristics. The effective oxide thickness for a 200 Å STO film is around 30 Å and is not sensitive to post-growth anneal in O2 at 500-700°C. These STO films are also robust against forming gas anneal. Finally, STO MOSFET structures have been fabricated and tested. An extrinsic carrier mobility value of 66 cm2 V−11 s−1 is obtained for an STO PMOS device with a 2 μm effective gate length.

Surface smoothing induced by epitaxial Si capping of rough and strained Ge or Si 1− xGe x morphologies: a RHEED and TEM study

The same Si/Ge/Si or Si/Si 1− x Ge x /Si structures grown at 400°C on Si(0 0 1) are compared, either in real time by reflection high-energy electron diffraction (RHEED) or on the final product by transmission electron microscopy (TEM). This allows us to follow interface morphology variations during Si re-growth upon Ge containing layers of various Ge thicknesses or alloy x fractions. As shown by the passage from spotty to streaky RHEED patterns and by specular beam intensity oscillation evolutions, the surfaces roughen systematically during strained Ge or Si 1− x Ge x (SiGe) growth and smooth rapidly during subsequent growth of 4 to 6 Si monolayers, at least in the elastic hut-clustering islanding range with {1 0 5} facets. With the help of TEM examinations, a coherent picture may be proposed for these surface smoothing observations: (i) A dominant mechanism in form of a quick Si surface diffusion occurring initially on the Ge-strained surfaces. It ensures a heteregeneous Si accumulation towards the places of minimized misfit, i.e., in the troughs of the Ge or SiGe morphologies. (ii) A slower Ge diffusion (as occuring on Si) depleting the emerging island crests and contributing to an overall Ge surface termination (Ge surface segregation) and to a complementary island smoothing. The latter mechanism, only important at low growth kinetics, favours the formation of alloyed interfaces as a by-product of the island smoothing and lateral intermixing. At high Si growth kinetics the former mechanism prevails leading to better preserved island morphologies and final interfaces appearing chemically more abrupt but less flat.

Growth of c-GaN on carbonized Si(1 0 0) surfaces

Growth of GaN thin films on carbonized Si(100) was investigated by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The carbonized Si(100) surface was formed by reacting with C2H2 at a substrate temperature of about 800°C. AFM observation shows a mesa structure with a height and width of about 26 and 600nm, respectively, at the carbonized conditions of about 800°C and C2H2 pressure of 3×10−6Torr for 30min. SiC may be formed at the carbonized surface because the chemical shift of the 2p level of Si in XPS spectra is just the same as that for bulk SiC. After the growth of a 0.8μm thick GaN on the carbonized Si, which is prepared by the above reaction conditions, streaky RHEED patterns and diffraction from cubic-GaN in XRD were observed. These results suggest the high potentiality of carbonized Si as a substrate for GaN growth.

The use of atomic hydrogen for low temperature oxide removal from HgCdTe

Reflection high energy electron diffraction (RHEED) patterns of HgCdTe surfaces etched with bromine methanol are diffuse with a faint ring pattern indicative of an overlayer consisting of a mixture of oxides and amorphous Te. Exposure to an atomic hydrogen flux results in a RHEED pattern indicative of a high quality, two-dimensional surface. Atomic force microscopy (AFM) measurements indicate a rms surface roughness less than 1 nm. CdTe grown on this surface at 80°C maintains the streaky RHEED pattern and smooth surface as indicated by AFM. X-ray photoelectron spectroscopy measurements indicate that the etched surfaces contain both an oxide layer and a metallic Te overlayer which were removed by continued exposure to atomic hydrogen. Further exposure results in significant HgTe depletion, which appears to be a near-surface phenomenon. Preliminary device results indicate that use of atomic hydrogen is a viable approach for low temperature cleaning of etched HgCdTe surfaces.

Diamond films epitaxially grown by step-flow mode

Homoepitaxial diamond films were grown by the step-flow mode. The obtained films grown by this mode were characterized using atomic force microscopy, reflection high-energy electron diffraction (RHEED), cathodoluminescence (CL), and junction properties of Al-Schottky contacts. It is found that the surfaces of the films are covered with macroscopic steps running parallel to the [1 1 0] direction. The narrow and streaky RHEED patterns show the surfaces of the films are flat with 2 × 1 and 1 × 2 double-domain structure. From the CL spectra, strong excitonic luminescence is clearly observed in the temperature range between 120 and 300 K. The absence of the band-A emission in the CL spectra suggests a low density of dislocation in the films. The current-voltage characteristics of Al-Schottky contacts on the as-deposited diamond films show excellent junction properties with the ideality factor of 1.1. These results indicate immense potential for diamond as a semiconducting material.

Surface stability of MBE-grown Nd 1.85Ce 0.15CuO 4 thin films in air investigated by RHEED, XPS, and UPS

This paper reports on the stability of a Nd 1.85Ce 0.15CuO 4 (NCCO) surface investigated by reflection high-energy electron diffraction (RHEED) and photoemission spectroscopy. Experiments were performed on an “intrinsic” surface prepared by molecular beam epitaxy (MBE). Ex situ observation of the time evolution of the MBE-grown NCCO surface showed that it is substantially more stable than previously reported, in that the sharp streaky RHEED patterns are maintained and the above 531 eV component in the O 1s X-ray photoelectron spectra develops only slightly even after exposure to air for two weeks.

Epitaxy of Al films on GaN studied by reflection high-energy electron diffraction and atomic force microscopy

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%.