Change In Surface Reconstruction Research Articles

Identification of the Segregation Kinetics of Ultrathin GaAsSb/GaAs Films Using AlAs Markers.

For optoelectronic devices from the near to the far infrared, the advantages of using ultrathin III-Sb layers as quantum wells or in superlattices are well known. However, these alloys suffer from severe surface segregation problems, so that the actual profiles are very different from the nominal ones. Here, by inserting AlAs markers within the structure, state-of-the-art transmission electron microscopy techniques were used to precisely monitor the incorporation/segregation of Sb in ultrathin GaAsSb films (from 1 to 20 monolayers (MLs)). Our rigorous analysis allows us to apply the most successful model for describing the segregation of III-Sb alloys (three-layer kinetic model) in an unprecedented way, limiting the number of parameters to be fitted. The simulation results show that the segregation energy is not constant throughout the growth (which is not considered in any segregation model) but has an exponential decay from 0.18 eV to converge asymptotically towards 0.05 eV. This explains why the Sb profiles follow a sigmoidal growth model curve with an initial lag in Sb incorporation of 5 MLs and would be consistent with a progressive change in surface reconstruction as the floating layer is enriched.

Influence of UDMHy on GaAs (0 0 1) surface reconstruction before and during growth of Ga(NAs) by MOVPE

III–V semiconductors containing small amounts of Nitrogen (“dilute nitrides”) are very promising material systems for optoelectronic applications. There are many studies about growth characterization of Ga(NAs) in metalorganic vapor phase epitaxy (MOVPE), but very little is known about the nitridation process on the GaAs (0 0 1) surface and the influence of the surface reconstruction on the Nitrogen (N) incorporation. Therefore, we investigated GaAs (0 0 1) surfaces under different tertiarybutylarsine (TBAs) and 1,1-dimethylhydrazine (UDMHy) ambient conditions in MOVPE. For in situ surface characterization reflectance anisotropy spectroscopy (RAS) was used. Under sufficient TBAs stabilization the surface forms an As rich c(4 × 4)β surface reconstruction. This changes towards a more Ga rich (2 × 6)/(6 × 6)-like reconstruction if additional N is supplied. Therefore, UDMHy seems to enhance the As desorption from the surface or the surface reconstruction changes due to incorporated N. This conversion occurs rapidly within 5 s when UDMHy is supplied and was observed in a temperature range from 450 °C to 600 °C. The measured RAS spectra are then compared to the RAS spectra obtained during Ga(NAs) growth. These also exhibit a (2 × 6)/(6 × 6)-like surface reconstruction which seems to be necessary to incorporate a sufficient amount of N into the GaAs crystal.

Influence of anisotropic Si(111)-(4 × 1)-In surface on growth of nanoscale In islands

Reflectance anisotropy spectroscopy (RAS) and scanning tunneling microscopy (STM) are used to study the growth of indium (In) on the anisotropic Si(111)−(4 × 1)−In surface at room temperature. RAS shows that epitaxial growth of In is accompanied by the disappearance of the surface optical anisotropy at 1.9 eV which is the fingerprint for the (4 × 1)-In surface reconstruction and the appearance of a large minimum at 1.4 eV which is at the same energy as interband transitions observed on bulk crystalline In. Subsequent spectra taken on the surface, over 3 h, show that this minimum decreases and eventually disappears along with the reappearance of the original RAS signature of the (4 × 1)-In surface. STM of this surface shows elongated, anisotropic In crystal islands on top of a (4 × 1)-In reconstructed surface. Upon annealing the surface to 720 K, the surface reconstruction changes with STM showing regions covered with a phase that resembles the (√7 × √3)-In reconstruction and RAS showing a large positive anisotropy at about 1.5 eV. The epitaxial In islands now show a hexagonal shape, unlike on the (4 × 1) surface. Thus, the growth morphology of the islands is shown to be dependent on the initial surface reconstruction. The authors attribute these findings to Ostwald ripening of the In islands mediated by diffusion, which is dependent on the structurally different In wetting layers on the Si substrate.

Surface structure analysis of Eu Zintl template on Ge(001)

The surface reconstructions and surface core level shifts of 0.25 monolayer (ML), 0.5 ML and 1.0 ML of europium (Eu) on the Ge(001) surface are investigated experimentally and theoretically, using molecular beam epitaxy (MBE), reflection high-energy electron diffraction (RHEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Starting with a clean Ge(001) surface we find that the surface reconstruction changes from the initial (2 × 1) to a (3 × 2), (8 × 1) and a final (3 × 1) pattern with increasing Eu coverage of 0.25 ML, 0.5 ML and 1.0 ML, respectively. XPS scans demonstrate that the Ge 3d core level shifts to higher binding energies with increasing Eu coverage. We find that Eu is in the 2+ oxidation state, transferring its charge to the Ge surface, resulting in the disappearance of the asymmetry of the dimer tilt at a coverage of 0.5 ML of Eu. From density functional calculations we find that the Ge dimers flatten out at 0.5 ML Eu coverage and partially break apart at 1.0 ML Eu.

Surface diffusion measurements of In on InGaAs enabled by droplet epitaxy

Surface diffusion is a critical parameter for non-equilibrium growth techniques such as molecular beam epitaxy. However, very little is known about diffusion rates of individual cations in a mixed cation material. Using droplet epitaxy as the growth technique, we isolate the diffusivity prefactor (D0) and activation energy (EA) of indium on the surface of In0.53Ga0.47As/InP(100). We report two regimes of indium diffusivity under As2-rich conditions: above and below the droplet deposition temperature of 300 °C, corresponding to a change in surface reconstruction. We also discuss methods of extracting the indium diffusion parameters on metal-rich surfaces using droplet epitaxy and nucleation theory. The obtained diffusion parameters are compared to previous work in the literature and could be employed to optimize growth conditions for non-equilibrium crystal growth.

Surface reconstructions in molecular beam epitaxy of SrTiO3

We show that reflection high-energy electron diffraction (RHEED) can be used as a highly sensitive tool to track surface and resulting film stoichiometry in adsorption-limited molecular beam epitaxy of (001) SrTiO3 thin films. Even under growth conditions that yield films with a lattice parameter that is identical to that of stoichiometric bulk crystals within the detection limit of high-resolution x-ray diffraction (XRD), changes in surface reconstruction occur from (1 × 1) to (2 × 1) to c(4 × 4) as the equivalent beam pressure of the Ti metalorganic source is increased. These surface reconstructions are correlated with a shift from mixed SrO/TiO2 termination to pure TiO2 termination. The crossover to TiO2 surface termination is also apparent in a phase shift in RHEED oscillations observed at the beginning of growth. Comparison with prior results for carrier mobilities of doped films shows that the best films are grown under conditions of a TiO2-saturated surface [c(4 × 4) reconstruction] within the XRD growth window.

Dynamic grazing incidence fast atom diffraction during molecular beam epitaxial growth of GaAs

A Grazing Incidence Fast Atom Diffraction (GIFAD) system has been mounted on a commercial molecular beam epitaxy chamber and used to monitor GaAs growth in real-time. In contrast to the conventionally used Reflection High Energy Electron Diffraction, all the GIFAD diffraction orders oscillate in phase, with the change in intensity related to diffuse scattering at step edges. We show that the scattered intensity integrated over the Laue circle is a robust method to monitor the periodic change in surface roughness during layer-by-layer growth, with oscillation phase and amplitude independent of incidence angle and crystal orientation. When there is a change in surface reconstruction at the start of growth, GIFAD intensity oscillations show that there is a corresponding delay in the onset of layer-by-layer growth. In addition, changes in the relative intensity of different diffraction orders have been observed during growth showing that GIFAD has the potential to provide insight into the preferential adatom attachment sites on the surface reconstruction during growth.

Surface reconstruction and optical absorption changes for Ge nanoclusters grown on chemically oxidized Si(1 0 0) surfaces

Germanium (Ge) nanoclusters are grown by a molecular-beam epitaxy technique on the chemically oxidized Si(1 0 0) surface at 700 °C. X-ray diffraction and photocurrent spectroscopy demonstrate that the nanoclusters have the local structure of body-centred-tetragonal Ge, exhibiting an optical adsorption edge at 0.48 eV at 50 K. Deposition of silicon on the surface with Ge nanoclusters leads to surface reconstruction and formation of polycrystalline diamond-like Si coverage, while nanoclusters' core becomes tetragonal SiGe alloy. The intrinsic absorption edge is shifted to 0.73 eV due to Si–Ge intermixing. Possible mechanisms for nanoclusters growth are discussed.

In situ reflection anisotropy spectroscopy analysis of heteroepitaxial GaP films grown on Si(100)

In situ reflection anisotropy spectroscopy (RAS)/reflection difference spectroscopy was applied as a quantitative probe of antiphase domains in heteroepitaxial films deposited on Si(100). The in situ probe was deduced from the spectroscopic signature of the P-rich, homoepitaxial GaP(100) surface and its well-established atomic reconstruction via a comparative investigation using RAS (homoepitaxial versus heteroepitaxial). For that, we determined changes in temperature, surface reconstruction, atomic order, and excess phosphorus on the surface of the homoepitaxial GaP(100) samples to specifically change the RA spectra in terms of shape and intensity. According to the presence of antiphase disorder a linear reduction in the RAS signal occurred. In addition, RA spectra of the heteroepitaxially prepared GaP/Si(100) films contained characteristic deviations from RA spectra of homoepitaxial GaP(100). They originated from reflections at the additional GaP/Si(100) heterointerface. Simple interference affecting the normalization of the RAS signal was found as a major source of the deviations and, thus, corresponding corrections were applied in the RA spectra. At photon energies around 3.2 eV, the correction even amplified the difference between the spectra of GaP/Si(100) and GaP(100) samples. This indicated an additional optical anisotropy induced by the interface reflection which peaked in the range of the determined spectral position.

Direct formation of InAs quantum dots grown on InP (001) by solid-source molecular beam epitaxy

We have developed a growth process that leads to the direct formation of self-assembled InAs quantum dots on InP(001) by solid-source molecular beam epitaxy avoiding the previous formation of quantum wires usually obtained by this technique. The process consists of a periodically alternated deposition of In and As correlated with InAs(4×2)↔(2×4) surface reconstruction changes. Based on the results obtained by in situ characterization techniques, we propose that the quantum dots formation is possible due to the nucleation of In droplets over the InAs(4×2) surface during the In deposition step and their subsequent crystallization under the As step.

An effect of As flux on GaAs/AlAs quantum wells: A combined photoluminescence and reflection high-energy electron diffraction study

Photoluminescence (PL) measurements and in situ reflection high-energy electron diffraction (RHEED) observations were used to optimize GaAs/AlAs multi-quantum-well (MQW) structures grown by molecular beam epitaxy (MBE) under different As fluxes. PL peaks were identified with computer simulations, intensity behavior as a function of temperature, and with previous results in literature. The room temperature PL intensity from our MQW samples first increases with the As flux and then decreases as the flux is increased. Combining our RHEED and PL observations, we propose that an optimum As flux for growth of GaAs/AlAs structures is close to the special As flux at which the GaAs(1 0 0) surface reconstruction changes from ( 2 × 4 ) to ( 4 × 2 ). The reasons for the findings are discussed.

Interfaces between [formula omitted]- [formula omitted] and silicon

As the epitaxy of crystalline LaAlO 3 has not been realized yet, we investigated the use of a γ - Al 2 O 3 buffer layer between the high- κ and the substrate. We firstly studied the structural matching of γ - Al 2 O 3 (0 0 1) with a Si(0 0 1)- p ( 2 × 1 ) reconstructed surface. According to experimental data and computations in the density functional theory framework, we found stable interfaces between γ - Al 2 O 3 and Si which encounters surface reconstruction changes. These interfaces satisfy the criterion of an insulating buffer layer.

Ag-induced atomic structures on the Si(110) surface

We report the results of STM investigation of the initial stage of Ag adsorption on an Si(110) surface. At 0.21 ML Ag coverage, the size and orientation of the unit cell correspond to the parameters of a 16 × 2 unit cell of clean Si(110) surface. With increasing of the Ag coverage up to 0.42 ML, the type of surface reconstruction changes to a 4 × 1-Si(110)-Ag structure.

Temperature-Driven Change in the Unstable Growth Mode on Patterned GaAs(001)

We observe a dramatic change in the unstable growth mode during GaAs molecular beam epitaxy on patterned GaAs(001) as the temperature is lowered through approximately 540 degrees C, roughly coincident with the preroughening temperature. Observations of the As2 flux dependence, however, rule out thermodynamic preroughening as driving the growth mode change. Similar observations rule out the change in surface reconstruction as the cause. Instead, we find evidence that the change in the unstable growth mode can be explained by a competition between the decreased adatom collection rate on small terraces and a small anisotropic barrier to adatom diffusion downward across step bunches.

Surface Reconstruction of Pt/Si(001)

Platinum-induced surface reconstructions on Si(001) were investigated by scanning tunneling microscopy. The Si(001) surface shows c(4×6)+c(4×2)-type reconstruction after Pt hot deposition at 750 °C. The c(4×2) reconstruction is formed by regular arrangement of Pt ad-atoms at the hollow sites between two Si dimers on Si(001). The c(4×6) structure is formed by long-range ordering of Si dimers superimposed on the c(4×2) reconstruction. The surface reconstruction changes with the local Pt coverage and missing dimer density. An atomic model for c(4×6)+c(4×2) is proposed based on the observed c(4×4) surface reconstruction, a new surface reconstruction, which is observed following a high-temperature anneal at 900 °C. The c(4×4) surface reconstruction is regular and ordered in both the dimer-row direction and the direction perpendicular to the dimer rows. The reconstruction is formed by the regular arrangement of Si dimers and Pt ad-atoms sitting on top of the hollow sites surrounded by two dimers. An atomic model for this new reconstruction is proposed.

Growth of InAs quantum dots on shallow spherically shaped craters prepared on GaAs (0 0 1) substrates: an extended set of vicinal surfaces

Atomic force microscopy (AFM, Nanoscope IIIA) is used to study InAs quantum-dot structures grown by molecular-beam epitaxy (MBE) on a shallow spherically shaped crater (also called a dimple) with the continuous polar misorientation and azimuthal orientation prepared on GaAs (001) substrate by a Model 656 Dimple Grinder. It is shown that the step bunches become larger after the growth, suggesting that the Schwoebel barrier, which causes the step bunching, may be decreased by the change of surface reconstruction after the introduction of small amount of In atoms to GaAs surfaces in the initial stage of growth. The number n of the InAs quantum dots per 3×3μm2 area decreases with increase in polar misorientation angles α but does not depend on the azimuthal orientation angle. These results are plotted for the polar misorientation angles along the [1̄10], [110] and [100] directions. The reduction in quantum dots with an increase in polar misorientation angle is due to the suppression of lattice-mismatched strain. It is observed that the quantum dots tend to form on the humps and valleys.

Preparation of different BeTe surface reconstructions by decapping and thermal treatment

We report on the application of protective capping layers for nonultrahigh vacuum (UHV) transport of BeTe layers. It allows the subsequent recovery of the clean BeTe(100) surface by thermal decapping in UHV. For capping tellurium/selenium double layers with a thickness of about 0.2 μm are employed. After decapping by annealing which results in the desorption of Se at 150 °C and of Te at 260 °C, the low-energy electron diffraction pattern confirms the reestablishment of (2×1) surface reconstruction of BeTe(100). Subsequent annealing to 520 °C results in (3×1) reconstruction. In situ monitoring by Raman spectroscopy and reflection anisotropy spectroscopy is used to control the decapping process and the subsequent annealing-induced change in surface reconstruction.

Real-time surface composition and roughness analysis in MBE using RHEED-induced X-ray fluorescence

Using a compact, retrofittable energy-dispersive detector, we measure the RHEED-induced fluorescence signal at grazing exit during molecular beam epitaxy. First results show sub-monolayer sensitivity to all relevant elements down to Al with a time resolution of 1 s. The method is sensitive enough to detect surface reconstruction changes through relative changes in the As signal. In addition, the background signal shows a variation similar to the surface roughness and independent of the chemical composition.

Surface effects on shape, self-organization and photoluminescence of InAs islands grown on InAlAs/InP(001)

InAs nanostructures were grown on In0.52Al0.48As alloy lattice matched on InP(001) substrates by molecular beam epitaxy using specific growth parameters in order to improve island self-organization. We show how the change in InAs surface reconstruction via growth temperature from (2×4) to (2×1) and/or the use of InAlAs initial buffer surface treatments improve the island shape homogeneity (either as quantum wires or as quantum dots). Differences in island shape and in carrier confinement are shown by atomic force microscopy and by photoluminescence measurements, respectively. We point out that such shape amendments induce drastic improvements to island size distribution and discernible changes in photoluminescence properties, in particular concerning polarization.

Physisorption-Induced Surface Reconstruction and Morphology Changes: Adsorption of Glycine on the Au(110) 1 × 2 Surface

Although it is well-known that chemisorption may change the reconstruction or even the morphology of metal surfaces, one would not expect to see similar effects induced by physisorption. In this paper, we report, however, exactly such effects induced by physisorption of glycine on the Au(110) 1 × 2 surface. Specifically, at low coverage the adsorbates can change the 1 × 2 reconstructed surface to a well-ordered 1 × 3, whereas further adsorption can even dramatically modify the surface morphology until becoming completely faceted. As byproducts, two new reconstructions of clean Au(110), i.e., 1 × 1 and 1 × 3, have been found, with the former being metastable at or below room temperature while the latter at higher temperatures not beyond 170 °C. The fast and reversible room-temperature local transition between the two along with its possible mechanism will also be discussed.