Initial stages of the autocatalytic oxidation of the InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen

The initial stages of oxidation of the In-rich InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen (O₂) were studied using scanning tunneling microscopy (STM) and density functional theory (DFT). It was shown that the O₂ dissociatively chemisorbs along the rows in the [1 1 0] direction on the InAs surface either by displacing the row-edge As atoms or by inserting between In atoms on the rows. The dissociative chemisorption is consistent with being autocatalytic: there is a high tendency to form oxygen chemisorption sites which grow in length along the rows in the [1 1 0] direction at preexisting oxygen chemisorption sites. The most common site size is about 21-24 Å in length at ∼25% ML coverage, representing 2-3 unit cell lengths in the [1 1 0] direction (the length of ∼5-6 In atoms on the row). The autocatalysis was confirmed by modeling the site distribution as non-Poisson. The autocatalysis and the low sticking probability (∼10⁻⁴) of O₂ on the InAs(0 0 1)-(4 × 2)/c(8 × 2) are consistent with activated dissociative chemisorption. The results show that is it critical to protect the InAs surface from oxygen during subsequent atomic layer deposition (ALD) or molecular beam epitaxy (MBE) oxide growth since oxygen will displace As atoms.     

Growth and oxidation of Cr films on the W(1 0 0) surface

The growth and oxidation of Cr films on the W(1 0 0) surface have been studied with low energy electron microscopy (LEEM) and diffraction (LEED). Cr grows in a Stranski-Krastanov (SK) mode above about 550 K and in a kinetically limited layer-by-layer mode at lower temperature. Stress relief in the highly strained pseudomorphic (ps) Cr film appears to be achieved by the formation of (4 × 4) periodic inclusions during the growth of the third layer between 575 and 630 K and by growth morphological instabilities of the third layer at higher temperature. Kinetic or stress-induced roughening is observed at lower temperature. In the SK regime, three-dimensional (3D) Cr islands nucleate after the growth of three Cr layers. 3D island nucleation triggers dewetting of one layer from the surrounding Cr film. Thus, two ps Cr layers are thermodynamically stable. However, one and two layer ps Cr films are unstable during oxidation. 3D clusters, that produce complex diffraction features and are believed to be Cr₂O₃, are formed during oxidation of one Cr layer at elevated temperature, T ? 790 K. The single layer Cr film remains intact during oxidation at T ? 630 K. 3D bulk Cr clusters are formed predominantly during oxidation of two ps Cr layers.     

Core-level shifts of the c(8 × 2)-reconstructed InAs(1 0 0) and InSb(1 0 0) surfaces

We have studied In-stabilized c(8 × 2)-reconstructed InAs(1 0 0) and InSb(1 0 0) semiconductor surfaces, which play a key role in growing improved III–V interfaces for electronics devices, by core-level photoelectron spectroscopy and first-principles calculations. The calculated surface core-level shifts (SCLSs) for the ζ and ζa models, which have been previously established to describe the atomic structures of the III–V(1 0 0)c(8 × 2) surfaces, yield hitherto not reported interpretation for the As 3d, In 4d, and Sb 4d core-level spectra of the III–V(1 0 0)c(8 × 2) surfaces, concerning the number and origins of SCLSs. The fitting analysis of the measured spectra with the calculated ζ and ζa SCLS values shows that the InSb spectra are reproduced by the ζ SCLSs better than by the ζa SCLSs. Interestingly, the ζa fits agree better with the InAs spectra than the ζ fits do, indicating that the ζa model describes the InAs surface better than the InSb surface. These results are in agreement with previous X-ray diffraction data. Furthermore, an introduction of the complete-screening model, which includes both the initial and final state effects, does not improve the fitting of the InSb spectra, proposing the suitability of the initial-state model for the SCLSs of the III–V(1 0 0)c(8 × 2) surfaces. The found SCLSs are discussed with the ab initio on-site charges.     

Self-assembled InAs island formation on GaAs (1 1 0) by metalorganic vapor phase epitaxy

Formation of self-assembled InAs 3D islands on GaAs (1 1 0) substrate by metal organic vapor phase epitaxy has been investigated. Relatively uniform InAs islands with an average areal density of 10⁹ cm⁻²are formed at 400 ° C using a thin InGaAs strain reducing (SR) layer. No island formation is observed without the SR layer. Island growth on GaAs (1 1 0) is found to require a significantly lower growth temperature compared to the more conventional growth on GaAs (1 0 0) substrates. In addition, the island height is observed to depend only weakly on the growth temperature and to be almost independent of the V/III ratio and growth rate. Low-temperature photoluminescence at 1.22 eV is obtained from the overgrown islands.     

Anisotropy of electron structure at InAs(1 1 1) surfaces by laser pump-and-probe photoemission spectroscopy

The electronic structure and the electron dynamics of the clean InAs(1 1 1)A 2 × 2 and the InAs(1 1 1)B 1 × 1 surfaces have been studied by laser pump-and-probe photoemission spectroscopy. Normally unpopulated electron states above the valence band maximum (VBM) are filled on the InAs(1 1 1)A surface due to the conduction band pinning above the Fermi level (E_F). Accompanied by the downward band banding alignment, a charge accumulation layer is confined to the surface region creating a two dimensional electron gas (2DEG). The decay of the photoexcited carriers above the conduction band minimum (CBM) is originated by bulk states affected by the presence of the surface. No occupied states were found on the InAs(1 1 1)B 1 × 1 surface. This fact is suggested to be due to the surface stabilisation by the charge removal from the surface into the bulk. The weak photoemission intensity above the VBM on the (1 1 1)B surface is attributed to electron states trapped by surface defects. The fast decay of the photoexcited electron states on the (1 1 1)A and the (1 1 1)B surfaces was found to be τ_1 1 1 A ? 5 ps and τ_1 1 1 B ?  4 ps, respectively. We suggest the diffusion of the hot electrons into the bulk is the decay mechanism.     

InAs/GaAs(0 0 1) wetting layer formation observed in situ by concurrent MBE and STM

The growth of InAs on GaAs(0 0 1) has attracted great interest and investigation over the past few decades primarily due to the opto-electronic properties of the self-assembled quantum dot (QD) arrays formed. Scanning tunnelling microscopy (STM) has been extensively employed to investigate the complicated and spontaneous mechanism of QD growth via molecular beam epitaxy (MBE). In order to access surface dynamics, MBE and STM must be performed concurrently. The system herein combines MBE functionality into an STM instrument in order to investigate wetting layer formation dynamically. The GaAs(0 0 1)-(2 × 4) starting surface undergoes 2D island growth, reconstruction change-induced roughness and re-entrant 3D island formation prior to the Stranski-Krastanow transition point.     

Diffusion-limited electrodeposition of ultrathin Au films on Pt(1 1 1)

The electrodeposition of Au on Pt(1 1 1) from electrolytes containing μM concentrations of was studied by in situ scanning tunneling microscopy. Under these conditions the Au flux is limited by diffusion in the electrolyte over a wide potential range, which allows to assess the effect of the electrochemical environment on the growth kinetics. Similar to gas phase metal deposition Au film growth proceeds via nucleation and lateral growth of Au monolayer islands, with the saturation island density strongly depending on the deposition potential and on the anion species in the electrolyte. For deposition in H₂SO₄ solution the saturation island density continuously increases with increasing potential between −0.2 and 0.5 V (SCE), whereas in Cl-containing H₂SO₄ it first decreases and then increases again. Following nucleation and growth theories this behavior can be attributed to potential-induced changes of the Au surface mobility, caused by changes in the density and structure of coadsorbed sulfate/bisulfate and chloride adlayers. Under conditions of high Au surface mobility multilayer growth proceeds via a typical Stranski-Krastanov growth mode, with layer-by-layer growth of a pseudomorphic Au film up to 2 ML and 3D growth of structurally relaxed islands at higher coverage, indicating thermodynamic control under these conditions.     

Growth and alloy formation of ultrathin Ni films on Co/Pt(1 1 1)

Low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were used to study the growth and the structural evolution of Ni/Co/Pt(1 1 1) following high-temperature annealing. From the oscillation of the specular beam of the LEED and Auger uptake curve, we concluded that the growth mode of thin Ni films on 1 ML Co/Pt(1 1 1) is at least 2 ML layer-by-layer growth before three-dimensional island growth begins. The alloy formation of Ni/1 ML Co/Pt(1 1 1) was analyzed by AES. The temperature for the intermixing of Ni and Co layers in the upper interface without diffusing into the bulk of Pt is independent of the thickness of Ni when a Co buffer is one atomic monolayer. After the temperature was increased, formations of Ni-Co-Pt alloy, Ni-Pt alloy and Co-Pt alloy were observed. The temperature required for the Ni-Co intermixing layer to diffuse into Pt bulk increases with the thickness of Ni. The interlayer distance as a function of annealing temperature for 1 ML Ni/1 ML Co/Pt(1 1 1) was calculated from the I-V LEED. The evolution of LEED patterns was also observed at different annealing temperatures.     

Growth mode and electronic structure of Au nano-clusters on NiO(0 0 1) and TiO2(1 1 0)

The growth mode and electronic structure of Au nano-clusters grown on NiO and TiO₂ were analyzed by reflection high-energy electron diffraction, a field-emission type scanning electron microscope, medium energy ion scattering and photoelectron spectroscopy. Au was deposited on clean NiO(0 0 1)-1 × 1 and TiO₂(1 1 0)-1 × 1 surfaces at room temperature with a Knudsen cell at a rate of 0.25-0.35 ML/min (1 ML = 1.39 × 10¹⁵ atoms/cm²:Au(1 1 1)). Initially two-dimensional (2D) islands with thickness of one Au-atom layer grow epitaxially on NiO(0 0 1) and then neighboring 2D-islands link each other to form three-dimensional (3D)-islands with the c-axis oriented to the [1 1 1] direction. The critical size to form 3D-islands is estimated to be about 5 nm². The shape of the 3D-islands is well approximated by a partial sphere with a diameter d and height h ranging from 2.0 to 11.8 nm and from 0.95 to 4.2 nm, respectively for Au coverage from 0.13 to 4.6 ML. The valence band spectra show that the Au/NiO and Au/TiO₂ surfaces have metallic characters for Au coverage above 0.9 ML. We observed Au 4f spectra and found no binding energy shift for Au/NiO but significant higher binding energy shifts for Au/TiO₂ due to an electron charge transfer from Au to TiO₂. The work function of Au/NiO(0 0 1) gradually increases with increase in Au coverage from 4.4 eV (NiO(0 0 1)) to 5.36 eV (Au(1 1 1)). In contrast, a small Au deposition(0.15 to 1.5 ML) on TiO₂(1 1 0) leads to reduction of the work function, which is correlated with an electron charge transfer from Au to TiO₂ substrate.     

Atomic and electronic structures of thin NaCl films grown on a Ge(0 0 1) surface

Density functional theory (DFT) with LDA and GGA have been employed to study the interface and thin film properties of NaCl on a Ge(0 0 1) surface. The atomic and electronic structures of thin NaCl films from one to ten monolayers were analyzed. The layer adsorption energies show that a quasi-crystalline (0 0 1) fcc NaCl film is built up via a layer-by-layer growth mode with NaCl thickness above 2 ML. Simulated STM images show a well-resolved (1 × 1) NaCl atomic structure for sample bias voltage V_s < −2.5 V and the bright protrusions should be assigned to the Cl⁻ ions of the NaCl film. The Ge substrate dimer is reserved and buckled like a clean Ge(0 0 1)-p(2 × 2) surface as the result of weak interface interaction between the dangling bonds coming from valence π states of the Ge substrate and the 3p states of the interfacial Cl⁻ ion. These results are consistent with the experiments of STM, LEED and EELS.     

Growth of In nanocrystallite arrays on the Si(1 0 0)-c(4 × 12)-Al surface

Using scanning tunneling microscopy, growth of In nanoisland arrays on the Si(1 0 0)-c(4 × 12)-Al surface has been studied for In coverage up to 1.1 ML and substrate temperature from room temperature to 150 °C. In comparison to the case of In deposition onto the clean Si(1 0 0) surface or Si(1 0 0)4 × 3-In reconstruction, the In growth mode is changed by the c(4 × 12)-Al reconstruction from the 2D growth to 3D growth, thus displaying a vivid example of the Volmer-Weber growth mode. Possible crystal structure of the grown In nanoislands is discussed.     

Growth and thermal evolution of submonolayer Pt films on Ru(0 0 0 1) studied by STM

The growth of submonolayer Pt on Ru(0 0 0 1) has been studied with scanning tunneling microscopy. We focus on the island evolution depending on Pt coverage θ_Pt, growth temperature T_G and post-growth annealing temperature T_A. Dendritic trigonal Pt islands with atomically rough borders are observed at room temperature and moderate deposition rates of about 5 × 10⁻⁴ ML/s. Two types of orientation, rotated by 180° and strongly influenced by minute amounts of oxygen are observed which is ascribed to nucleation starting at either hcp or fcc hollow sites. The preference for fcc sites changes to hcp in the presence of about one percent of oxygen. At lower growth temperatures Pt islands show a more fractal shape. Generally, atomically rough island borders smooth down at elevated growth temperatures higher than 300 K, or equivalent annealing temperatures. Dendritic Pt islands, for example, transform into compact, almost hexagonal islands, indicating similar step energies of A- and B-type of steps. Depending on the Pt coverage the thermal evolution differs somewhat: While regular islands on Ru(0 0 0 1) are formed at low coverages, vacancy islands are observed close to completion of the Pt layer.     

The Fe/ZnO(0 0 0 1) interface: Formation and thermal stability

The room temperature growth mode and the interface reaction of Fe films on single crystalline ZnO(0 0 0 1) substrates prepared in ultra high vacuum (UHV) has been investigated by means of X-ray photoelectron and Auger electron spectroscopy (XPS, AES), low energy electron diffraction (LEED) and low energy ion scattering spectroscopy (LEIS). The results show that Fe grows in the pseudo layer-by-layer mode. At ambient temperature the deposited Fe film reduces the underlying ZnO single crystal resulting in FeO at the interface and metallic Zn, which partially diffuses into the remaining Fe overlayer. Annealing leads to a stepwise oxidation of the Fe to FeO (670 K) and Fe₂O₃ (820 K). The Fe₂O₃ mixes with the substrate resulting in two (1 1 1) oriented textures of a spinel phase found by electron backscatter diffraction analysis (EBSD). Fe-based spin-injection may play a vital role for ZnO-based spintronic devices.     

The transition from surface to bulk oxide growth on Pt(1 0 0): Precursor-mediated kinetics

We investigated the kinetics governing the transition from surface (2D) to bulk (3D) oxide growth on Pt(1 0 0) in ultrahigh vacuum as a function of the surface temperature and the incident flux of an oxygen atom beam. For the incident fluxes examined, the bulk oxide formation rate increases linearly with incident flux (Φ_O) as the oxygen coverage increases to about 1.7 ML (monolayer) and depends only weakly on the surface temperature in the limit of low surface temperature (T_S < 475 K). In contrast, in the high temperature limit (T_S > 525 K), the bulk oxide formation rate increases with for oxygen coverages as high as 1.6 ML, and decreases with increasing surface temperature. We show that the measured kinetics is quantitatively reproduced by a model which assumes that O atoms adsorb on top of the 2D oxide, and that this species acts as a precursor that can either associatively desorb or react with the 2D oxide to form a 3D oxide particle. According to the model, the observed change in the flux and surface temperature dependence of the oxidation rate is due to a change in the rate-controlling steps for bulk oxide formation from reaction at low temperature to precursor desorption at high temperature. From analysis of flux-dependent uptake data, we estimate that the formation rate of a bulk oxide nucleus has a fourth-order dependence on the precursor coverage, which implies a critical configuration for oxide nucleus formation requiring four precursor O atoms. Considering the similarities in the development of surface oxides on various transition metals, the precursor-mediated transition to bulk oxide growth reported here may be a general feature in the oxidation of late transition metal surfaces.     

Effect of Pb adatom flux rate on adlayer coverage for Stranski-Krastanov growth mode on Si(1 1 1)7 × 7 surface

Lead (Pb) has been a prototypical system to study diffusion and reconstruction of silicon surfaces. However, there is a discrepancy in literature regarding the critical coverage at which island formation takes place in the Stranski-Krastanov (S-K) mode. We address this issue by studying the initial stages of evolution of the Pb/Si(1 1 1)7 × 7 system by careful experiments in ultra-high vacuum with in situ characterization by auger electron spectroscopy, electron energy loss spectroscopy and low-energy electron diffraction. We have adsorbed Pb onto clean Si(1 1 1 )7 × 7 surface with sub-monolayer control at different flux rates of 0.05 ML/min, 0.14 ML/min and 0.22 ML/min, at room temperature. The results clearly show that the coverage of the Pb adlayer before the onset of 3D Pb islands in the S-K mode depends on the flux rates. LEED results show the persistence of the (7 × 7) substrate reconstruction until the onset of the island formation, while EELS results do not show any intermixing at the interface. This suggests that the flux rates influence the kinetics of growth and the passivation of dangling bonds to result in the observed rate-dependent adlayer coverages.     

InAs wetting layer evolution on GaAs(0 0 1)

The evolution of two-dimensional (2D) strained InAs wetting layers on GaAs(0 0 1), grown at different temperatures by molecular beam epitaxy, was studied by in situ high-resolution scanning tunneling microscopy. At low growth temperature (400 °C), the substrate exhibits a well-defined GaAs(0 0 1)-c(4 × 4) structure. For a disorientation of 0.7°, InAs grows in the step-flow mode and forms an unalloyed wetting layer mainly along steps, but also in part on the terrace. The wetting layer displays some local c(4 × 6) reconstruction, for which a model is proposed. 1.2 monolayer (ML) InAs deposition induces the formation of 3D islands. At a higher temperature (460 °C), the wetting layer is obviously alloyed even at low InAs coverage. The critical thickness of the wetting layer for the 2D-to-3D transition is shifted to 1.50 ML in this case presumably since the strain is reduced by alloying.     

Bimodal growth of manganese silicide on Si(1 0 0)

Two different growth modes of manganese silicide are observed on Si(1 0 0) with scanning tunneling microscopy. 1.0 and 1.5 monolayer Mn are deposited at room temperature on the Si(1 0 0)-(2 × 1) substrate. The as-grown Mn film is unstructured. Annealing temperatures between room temperature and 450 °C lead to small unstructured clusters of Mn or Mn_xSi_y. Upon annealing at 450 °C and 480 °C, Mn reacts chemically with the Si substrate and forms silicide islands. The dimer rows of the substrate become visible again. Two distinct island shapes are found and identified as MnSi and Mn₅Si₃.     

Ca induced reconstructions of the Si(0 0 1) surface

The growth behavior of Ca on Si(0 0 1) has been studied with scanning tunneling microscopy and low energy electron diffraction. During the growth of the first atomic layer at elevated temperature, Ca induces several different ordered surface reconstructions. In order of ascending metal content, they are: a 2 × n (n = 3, 4, 5) phase that has limited long range order, a 2 × 6 striped phase, and a 1 × 3 phase. The 1 × 3 phase covers the entire surface at and beyond the point where Ca silicide island growth starts.     

Oxidation mechanism of hydrogen-terminated Ge(1 0 0) surface

Control of the surface chemistry to prepare a robust termination on the Ge surface is crucial for the development of high-end Ge devices. In this study, oxidation of a H-terminated Ge surface was studied in air ambient and H₂O using a multiple internal reflection Fourier transform infrared spectroscopy (MIR FT-IR) technique. Ge surface treated in less diluted HF exhibited a stronger Ge-H peak intensity, and the surface was easily oxidized in the air ambient. Therefore, it is believed that the treatment of the Ge surface in highly diluted HF solution has an advantage in suppressing the oxidation of Ge in the air ambient. For the oxidation of Ge(1 0 0) surface in air ambient, the Ge surface is attacked by oxidizing agents to break Ge-H and Ge-Ge bonds, and the transition GeO_x layer is first formed, followed by a layer-by-layer GeO₂ formation with the increase in exposure time. When the H-terminated Ge surface was treated in H₂O, GeO_x was mainly formed, the thickness of the oxide layer was not changed with an increase in treatment time, and the Ge surface was maintained in a suboxide state, which exhibits a different oxidation mechanism from that in air ambient.     

RHEED studies during growth of TiN/SiNx/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.