Reflection High-energy Electron Diffraction Study Research Articles

RHEED studies during growth of TiN/SiN x/TiN trilayers on MgO(0 0 1)

TiN/SiN x /TiN(0 0 1) trilayers have been deposited on MgO(0 0 1) substrates using ultra-high vacuum based reactive magnetron sputtering and studied by in situ reflection high energy electron diffraction (RHEED). Depositions were carried out at 500 °C and 800 °C, with SiN x layer thicknesses between 3 and 300 Å. Here, we find that SiN x (0 0 1) layers grown at 800 °C exhibit 1 × 4 surface reconstructions along orthogonal 〈1 1 0〉 directions up to a critical thickness of ∼9 Å, where an amorphous phase forms. Growth of TiN overlayers on the reconstructed SiN x (0 0 1) layers yield RHEED patterns indicating the growth of (0 0 1)-oriented epitaxial layers with a 1 × 1 reconstruction. For the case of amorphous SiN x layers the TiN overlayers grow polycrystalline.

Reflection high-energy electron diffraction study of molecular beam epitaxy growth of Pr 2O 3 on Si(0 0 1)

Thin Pr 2O 3 films have been grown by molecular beam epitaxy on Si(0 0 1) and investigated by in situ reflection high-energy electron diffraction (RHEED). The epitaxial growth results in [1 0 1]-oriented films of cubic Pr 2O 3 with low interface and surface roughness, as confirmed by subsequent X-ray diffraction and reflectivity measurements. During nucleation of Pr 2O 3 on double-domain Si(0 0 1) (2×1)-reconstructed surfaces, a diffuse RHEED pattern with weak streaks evolves. Subsequently, a pattern with modulated streaks appears. According to the analysis of the conventional RHEED patterns and the in-plane symmetry of the reciprocal lattice rods obtained by RHEED azimuthal scans, the modulated streaks are attributed to the reflections resulting from two orthogonal [1 0 1]-oriented domains of cubic Pr 2O 3. The [0 1 0] direction of one domain and the [ 1 0 1 ¯ ] of the other are parallel to Si[1 1 0]. The RHEED patterns resulting from good epitaxy deteriorate when the layer thickness reaches 18 nm, due to the formation of hexagonal Pr 2O 3 in the films. The structural features of this phase dominate thereafter. This observation indicates an epitaxial stabilization of the cubic phase at the interface.

Epitaxial growth of AlN on Cu(1 1 1) substrates using pulsed laser deposition

We have succeeded in the epitaxial growth of AlN on Cu(1 1 1) substrates using pulsed laser deposition (PLD) for the first time. We achieved atomically flat surfaces of Cu(1 1 1) by annealing in an ultra high vacuum chamber. In situ X-ray photoelectron spectroscopy and reflection high-energy electron diffraction studies revealed that the native oxide layers on the surfaces had been successfully removed and that the Cu surfaces were reformed by the segregation of antimony atoms, incorporated in the substrates as impurities. We found that AlN (0 0 0 1) grows epitaxially on Cu(1 1 1) at a substrate temperature of as low as 500 °C with in-plane epitaxial relationships of AlN [ 11 2 ¯ 0 ] / / Cu [ 1 1 ¯ 0 ] . Electron backscattering diffraction measurements showed that neither 30° rotational domains nor cubic phase domains exist in the AlN films. Spectroscopic ellipsometry measurements revealed that no interfacial layers exist at the AlN/Cu hetero interfaces within the limits of detection. These results indicate that the PLD low-temperature growth technique permits the preparation of high-quality AlN expitaxial films on single-crystal metal substrates.

Eucentric four-axis ultrahigh vacuum goniometer for reflection high-energy electron diffraction applications

The design and performance of a four-axis low-profile eucentric UHV goniometer for in situ reflection high-energy electron diffraction (RHEED) studies during film deposition is reported. The design provides one translational and three rotational degrees of freedom that are fully independent. Although developed to facilitate high-pressure RHEED during the growth of oxide thin films by pulsed laser deposition, this goniometer design is applicable to other UHV techniques including molecular beam epitaxy. The goniometer requires only a single DN 100 CF flange (6in. o.d., 100mm i.d.), making it suitable for small deposition systems, too. Samples, attached to a resistively heated holder, can be easily transferred on and off of the goniometer without breaking vacuum. The holder accommodates samples up to 10mm×10mm in size and allows them to be heated to 900°C in pure oxygen while being attached to the goniometer. Full eucentric motion of the hot sample is possible with a typical axis precision of <0.1°. Most of the mechanism is located in air, allowing the use of standard materials and lubricants, substantially reducing the in-vacuum mechanics, and increasing the precision, reliability, and robustness of the system.

RHEED and STM studies of the pseudo-tenfold surface of theξ′−Al77.5Pd19Mn3.5approximant crystal

The pseudo-tenfold surface of the ${\ensuremath{\xi}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Al}}_{77.5}{\mathrm{Pd}}_{19}{\mathrm{Mn}}_{3.5}$ crystal, an approximant of the icosahedral $\mathrm{Al}--\mathrm{Pd}--\mathrm{Mn}$ quasicrystal, is investigated by reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy. The observed RHEED patterns of the surface after sputtering are found to be consistent with those of a simple cubic lattice with $(1\overline{1}0)$ surface plane. The [001] and [110] axes of the surface plane are oriented along the principal low-index axes of the bulk. The RHEED patterns of the sputter-annealed surface consist of diffraction streaks with periodic spacings expected for the bulk truncated surface. The surface prepared under different preparation methods is found to exhibit different step-height distribution and terrace morphology. A longer annealing yields a high density of shallow pentagonal pits on terraces, separated predominantly by $0.80\text{\ensuremath{-}}\mathrm{nm}$ high steps and occasionally by double steps. In contrast, the surface prepared with shorter annealing time exhibits highly perfect terraces with $0.80\text{\ensuremath{-}}\mathrm{nm}$-high steps and additional unusual steps of heights close to $0.40\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. All step heights observed for both preparation methods are consistent with interlayer spacings of the bulk model.

Pulsed laser deposition of epitaxial Al-doped ZnO film on sapphire with GaN buffer layer

Al-doped ZnO (ZnO:Al) films with thickness in the range of 0.5–0.9 μm were grown epitaxially on epi-GaN/sapphire (0001) by pulsed laser deposition (PLD; XeCl, λ=308 nm). The growth parameters such as substrate temperature, oxygen pressure and pulse repetition rate were established in a sequential manner to obtain highly epitaxial ZnO:Al film. The best films were obtained at substrate temperature of 400 °C, oxygen pressure of 1 mTorr and pulse repetition rate of 5 Hz. Reflection high-energy electron diffraction (RHEED) and low temperature photoluminescence (PL) studies confirm the high quality epitaxial nature of the film with near match and stacking order between ZnO and GaN.

Morphological instability of GaAs (7 1 1)A: A transition between (1 0 0) and (5 1 1) terraces

We report on the use of reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) study that indicates that the GaAs (7 1 1)A is right at the transition between vicinal GaAs (1 0 0) and vicinal GaAs (5 1 1)A surfaces and that a variation of the As overpressure switches the surface morphology between the two vicinal surfaces. The steps on the vicinal (1 0 0) surface have a width of 1.5 nm creating a staircase surface with excellent possibilities for growth of quantum wells. As-rich conditions can be described by vicinal (5 1 1)A surfaces with a width of 3.5 nm. This surface could find applications as a template for quantum wire growth. The observation suggests that the transition between these two morphologies is understandable based on the increase in surface energy of a vicinal (1 0 0) surface as the step separation approaches the dimer reconstructed separation.

RHEED study of CdTe and HgCdTe thin films grown on Si by pulse laser deposition

Solar cell technology develops into two directions: (a) design of new architecture of devices, such as tandem cells, MIS inversion layers, and (b) searching new photovoltaic materials, such as amorphous silicon, copper indium diselenide etc. Nowadays, CdTe/CdS is a very promising system for solar cells. In the paper, we study the properties of patterned Si surfaces as a substrate for CdTe layers. Two kinds of such surfaces were used: pyramid-like and plate-like. Such surfaces have particular properties for layer growth. The first surface is faceted by {111} planes, and the second one consists of crystallographic planes of high Miller indices. By pulsed laser deposition method, the layers of CdTe, as well as Hg 0.8Cd 0.2 Te, were grown on the structured Si surfaces. The third substrate-type (001) Si with flat surfaces was used also for comparison. The reflection high-energy electron diffraction (RHEED) patterns were obtained and interpreted for layers of various thickness. Comparing the diffraction patterns obtained from layers deposited on various Si substrates, one can conclude that the flat substrate surface is promising for polycrystalline films growth.

In situ IR and RHEED study of carbon monoxide surfactant assisted growth of fcc-Fe on Cu(1 0 0)

Infrared reflection absorption spectroscopy (IRRAS) and reflection high-energy electron diffraction (RHEED) have been applied to the in situ characterization of fcc-Fe epitaxial growth on Cu(1 0 0) substrates under very low carbon monoxide (CO) pressures. When CO is not admitted into the deposition chamber, the layer-by-layer epitaxial growth up to 14 monolayer (ML) thickness takes place on the substrate at room temperature. The layer-by-layer growth extends up to 18 ML at 2 × 10 −10 Torr of CO, for which no IR absorption features due to CO adsorption on the growing film surface can be observed. In contrast, layer-by-layer growth becomes unstable for deposition under 1 × 10 −8 Torr CO. In this case, an IR band due to CO adsorption appears at 1950 cm −1 only in the early stages of deposition. For deposition at 90 K, on the other hand, no layer-by-layer growth occurs on clean Cu(1 0 0). However, pre-adsorption of CO on Cu(1 0 0) followed by Fe deposition without CO admission into the chamber brings about the layer-by-layer growth up to 8 ML. The Fe deposition leads to an intensity reduction of the band due to adsorbed CO on the substrate with the emergence of the band ascribable to CO on the deposited Fe, indicating that the pre-adsorbed CO molecules on Cu(1 0 0) migrated in part to the deposited Fe surface. Proceeding Fe deposition results in an intensity reduction of the band due to CO on the Fe. In contrast, the layer-by-layer growth hardly takes place when 2 × 10 −10 Torr CO was admitted in the growth atmosphere. These observations indicate that the molecular adsorption of surfactant CO at the growing film surface as well as the dissociation of the adsorbed CO is a key process to promote the layer-by-layer growth of fcc-Fe.

Time-resolved core-level photoelectron spectroscopy and reflection high-energy electron diffraction study of surface phase transition on GaAs(001)

Time-resolved core-level photoelectron spectroscopy and reflection high-energy electron diffraction study of surface phase transition on GaAs(001)

Time-resolved core-level photoelectron spectroscopy and reflection high-energy electron diffraction study of surface phase transition on GaAs(0 0 1)

The surface phase transition on GaAs(001) was investigated in real time by core-level photoelectron spectroscopy and reflection high-energy electron diffraction (RHEED). The time dependence of the relative surface band bending was obtained from the deconvolution of core-level photoelectron spectra. We found that band bending on the 4 × 2 surface increases 0.06eV from that of the 2 × 4 surface and that the time dependence of relative surface band bending is in good agreement with that of core-level photoelectron intensities but not with the RHEED intensities. This indicates that the long-range ordering of surface superstructures and microscopic surface morphology does not influence surface band bending, whereas the termination atoms, As or Ga, does.

RHEED study of Pd film growth on Al 2O 3 (111)/NiAl (110)

The palladium (Pd) layer growth on well-ordered γ-Al 2O 3 (111) layer has been studied by means of reflection high energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). Epitaxial Al 2O 3 layer of the thickness 7–8 Å was prepared by the oxidation of single-crystalline NiAl (110) surface performed at 1070 K and 1200 L of O 2 under pO 2=6.6×10 −6 Pa. The stepwise Pd deposition has been performed at a substrate temperature of 300 K. Depending on the amount of deposited Pd, two different growth modes were detected. The formation of polycrystalline phase has been found at low Pd coverage, whereas the appearance of preferentially oriented Pd islands has been detected at Pd coverage 0.5 ML. Two populations of Pd islands having (111) epitaxial plane parallel to (111) Al 2O 3 surface were found.

Incommensurability imposed by the low-temperature phase transition at the (0 0 1) surface of SrTiO 3

The reflection high-energy electron diffraction (RHEED) study of the SrTiO 3 (0 0 1) surface structure in the temperature region from 6 to 300 K shows the “anomalous” growth of the surface lattice parameters under the temperature decrease. This behavior is ascribed to the specific character of the low-temperature phase transition connected with the rotation of the metal-centered oxygen octahedra around the normal to the (0 0 1) surface. RHEED intensities analysis based on R-factor minimization shows that the best-fit between the experiment and calculations is reached for oxygen octahedra rotations which are essentially different for two (0 0 1) surface terminations SrO and TiO 2. Octahedra rotations were assumed to give rise to an increase of the surface lattice parameters resulting in the fact that incommensurability between the bulk and surface parameters inevitably arises. The histogram method applied to the set of lattice parameter values shows the discrete character of the distribution obtained.

RHEED study of Pd/Al bimetallic thin film growth on NaCl (0 0 1) substrate

Pd/Al bimetallic thin films were grown by molecular beam epitaxy on single-crystalline NaCl (0 0 1) surface. Substrate and deposit crystallographic structures were investigated by reflection high-energy electron diffraction (RHEED). An evolution of the intensity of diffraction spots gave information on the formation of a bimetallic system. The formation of bimetallic alloy in early stages of the growth was studied in dependence on the succession of deposited materials. It was discussed in terms of evolution of the diffraction spot intensity and Pd/Al lattice parameter during the growth. The three-dimensional growth of Pd/Al bimetallic clusters has been observed. The oriented Pd/Al phase exhibited (1 0 0) epitaxial plane parallel to the substrate surface.

Texture of molybdenum thin films on SrTiO3(1 0 0): A RHEED study

The structure of ultrathin Mo films on SrTiO3(100) was studied by in situ reflection high-energy electron diffraction (RHEED). A different structure was observed for films less than 20 Å thick than for thicker films. These films were epitaxial and had a metastable structure. Thicker films had the dimensions of equilibrium bcc Mo(110). Relaxation processes transformed the metastable Mo into bcc Mo, resulting in the following orientation relationships between Mo and SrTiO3: (110)[001]bccMo∥(100)[001]SrTiO3 and (110)[111]bccMo∥(100)[011]SrTiO3. The formation of such specific orientations is related to transformations via the Bain and Needle Path, respectively.

Reflection high-energy electron diffraction study of surface morphology of vicinal Au(1 1 0) throughout the (2 × 1) to (1 × 1) phase transition

The vicinal Au(1 1 0) surface morphology throughout the (2 × 1) to (1 × 1) phase transition is studied by quantitative reflection high-energy electron diffraction. As the surface is heated from 300 K, the average terrace width remains unchanged up to 635 ± 10 K. Above that temperature, the average terrace width increases and at 683 K has a value (34 ± 10)% more than at 300 K. At higher temperatures, the average terrace width decreases. The average string length at step terraces on Au(1 1 0) remains basically unchanged up to ∼650 K and decreases sharply at higher temperatures indicative of a step-induced roughening transition. Thermal-induced adatom/vacancy generation on terraces is shown to increase significantly above ∼680 K. At room temperature, the spacing between the topmost two layers of the Au(1 1 0) surface is contracted by 0.31 ± 0.03 Å or ∼22% of the bulk-terminated structure.

RHEED study of irradiation-induced surface damage processes in Si and α-Al 2O 3

Surface damaging processes of crystalline silicon and alumina (α-Al 2O 3) by gas ion (H +, He +, Ar +, 0.5–5 keV) irradiation at a moderate temperature range (300–450 K) were examined by reflection high-energy electron diffraction (RHEED). In order to explain the changes in RHEED intensities, a phenomenological kinetic equation was extended to take a dynamical interaction between hydrogen and target atoms into account. Apparent activation energies for simultaneous recovery and the cross-sections for damage production were derived and compared for different implant species by fitting the kinetic equation to the experimental data. The obtained physical parameters showed that the surface damage by inert gas irradiation could be understood simply by the dpa (displacement per atom) rates due to nuclear stopping. In case of H + irradiation, on the other hand, electronic stopping and chemical interaction between hydrogen and the matrix atoms significantly affected the damaging rate and thermal recovery. In particular, hydrogen released from a weakly bound state in the Al 2O 3 matrix suppressed the thermal recovery at the moderate temperatures.

STM and RHEED studies on low-temperature growth of GaAs( [formula omitted

By using scanning tunneling microscopy and reflection high-energy electron diffraction (RHEED), kinetics during the low temperature (LT) growth of GaAs as well as anomaly in RHEED intensity oscillation (RO) has been studied. At the intermediate temperature in the range of 300–500 °C, the growing surface roughens, while at the still LT, with decreasing the island size, the surface roughening settles down to some extent, which renders reentrant behavior of the RO. In the reentrant LT region, the RO shows temporal frequency shift. Dynamical calculations reproduce the observed features qualitatively, confirming that the anomaly is largely concerned with the multilayered morphology evolved during the growth.

RHEED study of modified barrier for c-axis oriented YBaCuO trilayer junctions

We have studied a formation process of the modified barrier for YBaCuO trilayer junctions by using reflection high-energy electron diffraction (RHEED). c-axis oriented YBaCuO films were prepared on (1 0 0) oriented MgO substrates by using a co-evaporation method with pure O 3 vapor. The modified barriers were formed by an Ar plasma bombardment with an acceleration voltage of 1 kV and by a subsequent annealing with various gas conditions. The film surface exhibited a hallo pattern after Ar plasma bombardment at room temperature. As the film was annealed at 400, 600 and 710 °C under the vacuum condition, the RHEED pattern was drastically changed to weak spots, clear spots with a streak pattern, and a spotty pattern. The film annealed in the ozone vapor condition, on the other hand, showed a streak pattern.

Reflection high-energy electron diffraction studies of epitaxial oxide seed-layer growth on rolling-assisted biaxially textured substrate Ni(001): The role of surface structure and chemistry

We present a study of the {100}〈100〉 biaxially textured Ni(001) surface and seed-layer growth using in situ reflection high-energy electron diffraction and Auger electron spectroscopy. Our observations are consistent with formation of a c(2×2) two-dimensional superstructure due to the surface segregation of sulfur contained in the metal. We show that this superstructure can have a dramatic effect on the heteroepitaxial growth of oxide seed layers. In particular, the surface superstructure promotes the (200) epitaxial oxide growth of Y2O3-stabilized ZrO2, which is necessary for the development of high-Jc superconducting films for coated conductors.