Submonolayer Deposition Research Articles

Strong charge-carrier localization in InAs/GaAs submonolayer stacks prepared by Sb-assisted metalorganic vapor-phase epitaxy

Stacks of InAs/GaAs submonolayer depositions forming small In-rich islands are interesting for efficient optoelectronic applications due to the high areal density of localization centers and their fast carrier relaxation. The electronic confinement of charge carriers in InAs/GaAs submonolayers can be influenced by adding Sb during growth. Eight-band $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ simulations show that electrons and holes experience a different localization depending on where the Sb is incorporated into the submonolayer stacks. Samples grown with metalorganic vapor-phase epitaxy show sharp interfaces between InAs(Sb)/GaAs submonolayers and the surrounding matrix material in transmission electron microscopy and x-ray diffraction measurements. Cross-sectional scanning tunneling microscopy shows the formation of In-rich agglomerations as well as a slight clustering of Sb atoms. Temperature-dependent photoluminescence and spatially resolved cathodoluminescence show evidence for strong electronic confinement whose depth is controlled by the Sb amount.

Surface structure of manganese gallium quantum height islands on wurtzite $${\mathbf {GaN}}{\mathbf{(000}}{\bar{\mathbf{1}}})$$ studied by scanning tunneling microscopy

Submonolayer deposition of manganese on gallium-rich, nitrogen polar $$\hbox {GaN}(000\bar{1})$$ surface using radio-frequency nitrogen plasma molecular beam epitaxy leads to the spontaneous formation of manganese gallium into two distinct quantum height islands, 5-layer and 6-layer islands. Atomically resolved scanning tunneling microscopy reveals the atomically flat but unstable 5-layer island surface and the 6-layer island surface with relatively stable row structures. We propose possible surface models for these islands’ surfaces and discuss the clear structural differences explained with strains and partial relaxations. It is found that the 5-layer islands form under lateral strains and a relaxation process leading to non-uniform alternating strains results in the more energetically favorable row structures on the 6-layer island.

Interface formation for a ferromagnetic/antiferromagnetic bilayer system studied by scanning tunneling microscopy and first-principles theory

The initial stages of interface formation for a real-world ferromagnet/antiferromagnet bi-layer system (iron/manganese nitride) are investigated down to the atomic scale using a combination of molecular beam epitaxy, in situ scanning tunneling microscopy, and first-principles theoretical calculations. Submonolayer deposition of iron onto manganese nitride nanopyramid surfaces results in an unexpected yet well-ordered structural and magnetic arrangement. It is shown that although the island structures seen in scanning tunneling microscopy images are of single monolayer height, their chemical composition, based on Auger electron spectroscopy, conductance map imaging, and theoretical models, does not consist of iron. It is found theoretically that models that consider iron on the surface of manganese nitride are highly unfavorable. Instead, models with iron atoms incorporated into specific subsurface layers are most stable, in excellent agreement with Auger spectroscopy measurements. Calculations also reveal the magnetic alignment of iron with the manganese nitride layers.

Submonolayer deposition on glassy carbon electrode for anodic stripping voltammetry: An ultra sensitive method for antimony in tap water

Atomic layers of antimony were electrodeposited onto glassy carbon electrode in overpotential region. The antimony deposition was characterized by chronocoulometry and scanning electron microscopy. Based on the excellent electrochemical properties of the glassy carbon/antimony system, a simple method was developed for rapid and reliable determination of ultra-trace Sb(V) by square-wave anodic stripping voltammetry. The key operational parameters of influencing the electroanalytical response of Sb, such as deposition potential, deposition time, and composition of the measurement solution, were investigated. Under the optimized condition, the method provided a linear calibration ranging from 1 to 16pgL−1 with a respective correlation coefficient of 0.9906. The detection limit was 1.64fmolL−1, which is the lowest ever reported for an electroanalytical technique and one of the lowest analytical methods for Sb(V) determination. The relative standard deviation for a Sb(V) solution (5pgL−1) was 3.2% for eight successive assays. Finally, the method was applied to the determination of antimony in tap water and compared with inductively coupled plasma mass spectrometry technique.

Experimental and numerical study of submonolayer sputter deposition of polystyrene fragments on silver for the storing matter technique.

In static secondary ion mass spectrometry (SIMS), quantification and high ionization probabilities are difficult to obtain. The Storing Matter technique has been developed to circumvent these issues and has already been applied to deposit inorganic and organic samples. For organic samples, the effect of fragmentation during sputter deposition and changing coverage on time-of-flight (TOF)-SIMS mass spectra has not been investigated. In this work, polystyrene (PS) was sputter deposited on silver using an argon ion beam in order to investigate these parameters and to get a better control of the whole process. For this purpose, we introduce a multitechnique characterization approach for the submonolayer deposition of PS. Experimental methods (TOF-SIMS, X-ray photoelectron spectroscopy (XPS)) were used in combination with simulations (density functional theory (DFT) calculations) in order to obtain information about the molecular and structural changes and the interactions of organic matter with the metal surface. Alterations of the PS surface and PS sputter deposit as a function of surface coverage and Ar(+) ion fluence are addressed. A major finding is that this approach can be used to identify surface reactions between different fragments on the collector surface. Indeed, in the dynamic regime, the ratio of large to small fragments is increasing although the fragmentation during the sputter deposition should lead to increasingly smaller fragments. Hence, for Storing Matter, the coverage on the collector must be kept low in order to minimize the reactions between fragments and to preserve the information on the original sample.

Photocatalyzed and photosensitized conversion of organic dyes on porous and non-porous air–solid surfaces: Kinetic models reconsidered

The use of organic dyes to assay photocatalyst activity in air–solid systems has been explored frequently in recent years. We earlier proposed a two step kinetic model for photcatalyzed conversion of dyes in sub-monolayer and multi-layer deposits on titania films and powders. The present paper reconsiders both our own work and that of other researchers to propose more fundamental kinetic models which include the influence of illumination profiles within porous films and powder layers. The new models are shown to rationalize results for dye-photocatalyst data from multiple laboratories published over the last two decades. This outcome indicates that different kinetic analyses must be used to obtain rate constants in non-porous, uniformly illuminated systems (e.g., self-cleaning glasses) vs. porous systems such as thick macro- and meso-porous layers and powders, which exhibit illumination intensity variation with depth.

Growth dynamics in supersonic molecular beam deposition of pentacene sub-monolayers on SiO2

Studying highly energetic pentacene impinging on a surface, we demonstrated that the perpendicular component of the momentum drives the dynamics of molecule-molecule interactions and hence the island nucleation process, while the parallel component governs the dynamics of the interactions between the surface and the molecule and therefore determines the sticking coefficient and the island fractality.

Iron on GaN(0001) pseudo-1 × 1 (1+112) investigated by scanning tunneling microscopy and first-principles theory

We have investigated sub-monolayer iron deposition on atomically smooth GaN(0001) pseudo-1 × 1 (1+112). The iron is deposited at a substrate temperature of 360 °C, upon which reflection high energy electron diffraction shows a transformation to a 3×3-R30° pattern. After cooling to room temperature, the pattern transforms to a 6 × 6, and scanning tunneling microscopy reveals 6 × 6 reconstructed regions decorating the GaN step edges. First-principles theoretical calculations have been carried out for a range of possible structural models, one of the best being a Ga dimer model consisting of 2/9 monolayer of Fe incorporated into 7/3 monolayer of Ga in a relaxed but distorted structure.

Structure and Reactivity of Sub-Monolayer Pt on CeO2 Surface from First Principles Thermodynamics

Transition metal catalysts supported on metal-oxide surfaces are promising materials for usage in heterogeneous catalysis due to strong metal-support interaction, which affects the intrinsic activity at the surface, and results in enhanced dispersion of metal atoms in some cases, thus promoting stability. One of the most widely investigated such systems are Pt/CeO2 systems, which have been found to be highly active toward the water gas shift reaction [1]. Experimental and theoretical studies point to electron and oxygen transfer between adsorbed Pt atoms and CeO2 support as the reason for the enhanced activity in such systems [2, 3]. The effect of this metal-support interaction, however, is far from clear in context of the redox reactions that occur in DMFCs and PEMFCs. Especially, the role of CeO2, well known for its oxygen storage capacity, remains yet to be investigated in co-catalysis. To this end, understanding and characterizing the dispersion of Pt on the CeO2 surface is of utmost importance. Stability and activity of a few Pt layers on rutile metal-oxides has been investigated computationally, yielding insights into the geometric and electronic influences of the support on the catalyst [4]. Sub-monolayer deposition of catalyst atoms opens up the support surface to direct engagement with the reactants and is well worth investigating. In this study, first principles DFT calculations are used in combination with rigorous statistical mechanics methods to find stable phases of Pt atoms dispersed on the CeO2 (111) surface as a function of their chemical potential. Employing the Cluster Expansion formalism in tandem with the Metropolis Monte Carlo Method to account for the configurational contribution to the system’s free energy [5], the important interactions dictating the surface morphology are identified. In agreement with experiments [2], a remarkable transfer of oxygen from CeO2 to the Pt atoms is observed, and a tendency of Pt atoms to aggregate at low surface coverage is also seen. Further, using a linear Gibbs energy relation, the energetics of various reaction steps in a 2 e- oxygen reduction (ORR) process of the Pt/CeO2 system are computed for several instances of oxygen co-adsorption and are compared to those of a model Pt (111) system as a function of the electrode potential. Additionally, the effect of support on the dissolution potential of Pt atoms is also evaluated to understand the stabilizing effect of the support, which has been observed experimentally [6]. Enhancement in ORR kinetics for Pt/CeO2 systems has been observed experimentally, but such studies are few [7]. Two mechanisms are proposed in this study to explain such a behavior: (1) the preferential adsorption of OH on only Pt in contrast to the co-adsorption of O on both Pt and CeO2, and (2) the activation of O-O bond breaking due to the oxygen starved CeO2 surface. In principle, such systems with the possibility for the support material to take part in the reaction along with the catalyst atoms can open up new channels that facilitate ORR.

Beyond the Young-Laplace model for cluster growth during dewetting of thin films: Effective coarsening exponents and the role of long range dewetting interactions

Long range dewetting forces acting across thin films, such as the fundamental van der Waals interactions, may drive the formation of large clusters (tall multilayer islands) and pits, observed in thin films of diverse materials such as polymers, liquid crystals, and metals. In this study we further develop the methodology of the nonequilibrium statistical mechanics of thin films coarsening within continuum interface dynamics model incorporating long range dewetting interactions. The theoretical test bench model considered here is a generalization of the classical Mullins model for the dynamics of solid film surfaces. By analytic arguments and simulations of the model, we study the coarsening growth laws of clusters formed in thin films due to the dewetting interactions. The ultimate cluster growth scaling laws at long times are strongly universal: Short and long range dewetting interactions yield the same coarsening exponents. However, long range dewetting interactions, such as the van der Waals forces, introduce a distinct long lasting early time scaling behavior characterized by a slow growth of the cluster height/lateral size aspect ratio (i.e., a time-dependent Young angle) and by effective coarsening exponents that depend on cluster size. In this study, we develop a theory capable of analytically calculating these effective size-dependent coarsening exponents characterizing the cluster growth in the early time regime. Such a pronounced early time scaling behavior has been indeed seen in experiments; however, its physical origin has remained elusive to this date. Our theory attributes these observed phenomena to ubiquitous long range dewetting interactions acting across thin solid and liquid films. Our results are also applicable to cluster growth in initially very thin fluid films, formed by depositing a few monolayers or by a submonolayer deposition. Under this condition, the dominant coarsening mechanism is diffusive intercluster mass transport while the cluster coalescence plays a minor role, both in solid and in fluid films.

Carrier dynamics in InAs/GaAs submonolayer stacks coupled to Stranski-Krastanov quantum dots

Time-resolved photoluminescence experiments on a coupled system of Stranski-Krastanov quantum dots (SK QDs) overgrown by a stack of submonolayer (SML) depositions reveal an acceleration of the carrier dynamics in the SML stack mediated by distance-dependent transfer processes. The coupling between an SML stack and an SK QD layer increases with decreasing spacer thickness $d$, which separates the two different nanostructure systems. This control parameter $d$ as well as the optical transition energies of the SML stack can be adjusted via the growth process. The observed photoluminescence dynamics of the combined SK QD and SML system is well described by a rate equation model which includes both pure zero-dimensional and zero-dimensional\char21{}two-dimensional states.

Scaling in reversible submonolayer deposition

The scaling of island and monomer density, capture zone distributions (CZDs), and island size distributions (ISDs) in reversible submonolayer growth was studied using the Clarke-Vvedensky model. An approach based on rate-equation results for irreversible aggregation (IA) models is extended to predict several scaling regimes in square and triangular lattices, in agreement with simulation results. Consistently with previous works, a regime I with fractal islands is observed at low temperatures, corresponding to IA with critical island size i=1, and a crossover to a second regime appears as the temperature is increased to \epsilon R^{2/3} ~ 1, where \epsilon is the single bond detachment probability and R is the diffusion-to-deposition ratio. In the square (triangular) lattice, a regime with scaling similar to IA with i=3 (i=2) is observed after that crossover. In the triangular lattice, a subsequent crossover to an IA regime with i=3 is observed, which is explained by the recurrence properties of random walks in two dimensional lattices, which is beyond the mean-field approaches. At high temperatures, a crossover to a fully reversible regime is observed, characterized by a large density of small islands, a small density of very large islands, and total island and monomer densities increasing with temperature, in contrast to IA models. CZDs and ISDs with Gaussian right tails appear in all regimes for R ~ 10^7 or larger, including the fully reversible regime, where the CZDs are bimodal. This shows that the Pimpinelli-Einstein (PE) approach for IA explains the main mechanisms for the large islands to compete for free adatom aggregation in the reversible model, and may be the reason for its successful application to a variety of materials and growth conditions.

Combined in-situ photoemission spectroscopy and density functional theory of the Sr Zintl template for oxide heteroepitaxy on Si(001)

Half-monolayer Sr on Si(001) is a Zintl template necessary for epitaxial growth of SrTiO3 on Si(001). The authors investigate the reconstruction in the atomic and electronic structure of Si(001) induced by sub-monolayer Sr deposition using in-situ x-ray/ultraviolet photoemission spectroscopy and density functional theory. Sub-monolayer Sr is deposited on Si(001) using molecular beam epitaxy and the structural evolution of the surface is monitored using reflection high-energy electron diffraction. Experimentally, the authors find that the ionization energy of Si(001) decreases as a function of Sr coverage from 4.82 eV for pure Si(001) to 3.97 eV for half-monolayer Sr on Si(001) due to charge transfer from Sr to Si. They calculate the ionization energy for sub-monolayer Sr on Si(001) by considering several atomistic models and find good agreement with experiment. The authors clearly establish the Zintl character of the template by measuring the surface core level shifts of Si(001) and half-monolayer Sr/Si(001). They analyze and interpret the results of these measurements using first-principles final state theory.

Influence of the patch field on work function measurements based on the secondary electron emission

A work function study based on the onset shift (i.e., following low energy cut-off) of secondary electron spectra has been used for the last four decades to monitor the deposition and adsorption in real time, measure the dipole momentum and polarizability of the surface layer, and determine the lateral distribution of the work function. In this work, we show that the onset shift depends on both the coverage of adsorbed species that change the work function and the size of low work function patches. Additionally, the extraction field, which is always applied in these measurements, may also influence the onset shift. Numerical calculations of the potential distributions above different non-uniform surfaces were performed in order to quantitatively determine each of these influences. Depending on the patch size, we define three measurement regimes in which the onset position is related to either the surface average of the work function (small patches), the minimum local work function (large patches), or a value in-between (intermediate size patches). Experimental data have corroborated these findings and demonstrated that manipulating the extraction field intensity enables transition between the measurement regimes. Typical misinterpretations due to neglecting the patch size contribution and the surface non-uniformity to the onset shift are analyzed. Additionally, possible application of work function study for determination of the growth mode in the case of submonolayer deposition was discussed.

Drastic change in electronic structure of AlGaN under Ba adsorption

AbstractA synchrotron‐radiation photoemission spectroscopy is applied to the clean and Ba‐deposited AlGaN(0001) surfaces to investigate the interface electronic structure relevant to both the surface states and an electron accumulation layer and to clarify their origins. Valence‐band photoemission and Ga 3d, Al 2p, Ba 4d core level spectra of AlxGa1‐xN (x=0.16, x=0.42) have been studied under the step‐by‐step Ba submonolayer deposition. The clean n ‐Al0.16Ga0.84N surface exhibits the surface states at energies of 2.8 eV, 5.5 eV, and 7.8 eV with respect to the valence band maximum, while the states for the clean Al0.42Ga0.58N surface are at higher energies of 3.9 eV, 5.6 eV, and 8.2 eV. Drastic change in the valence band spectra is revealed and it is showed that narrowing of valence band and suppression of surface states occur with increasing Ba coverage. The appearance of a new photoemission peak at the Fermi level clearly indicates the creation of the accumulation layer on the n ‐Al0.16Ga0.84N surface under the Ba adsorption. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical anisotropy of InGaAs quantum dots

Polarization studies of InGaAs/GaAs quantum dots (QDs) synthesized in the submonolayer deposition mode (SMLQDs) on a singular GaAs (100) surface are carried out using photoluminescence spectroscopy. The influence of the effective In content in InGaAs SMLQDs and the effect of a wide-gap AlGaAs matrix on the optical anisotropy of the QDs are investigated. The highest degree (>15%) of optical anisotropy between the [011] and [0 $$ \bar 1 $$ 1] directions in the emission corresponding to the ground state of InGaAs/GaAs SMLQDs is observed for an effective In content of ∼40%. The use of a wide-gap AlGaAs matrix resulted in an increase in the optical anisotropy of InGaAs SMLQDs by a factor of 1.5. It is found that vertical stacking of In(Ga)As/AlGaAs SMLQDs in the vertical-coupling mode (with spacer-layer thicknesses of 5–10 nm) leads to a further increase in the degree of optical anisotropy, which becomes as high as 25% on average. According to the data of transmission electron microscopy, the optical anisotropy of the ground-state photo-luminescence is predominantly caused by the anisotropy of the lateral dimensions of QDs in the [011] and [0 $$ \bar 1 $$ 1] directions.

Distributional fixed-point equations for island nucleation in one dimension: A retrospective approach for capture-zone scaling

The distributions of interisland gaps and captures zones for islands nucleated on a one-dimensional substrate during submonolayer deposition are considered using a novel retrospective view. This provides an alternative perspective on why scaling occurs in this continuously evolving system. Distributional fixed-point equations for the gaps are derived both with and without a mean-field approximation for nearest neighbor gap-size correlation. Solutions to the equations show that correct consideration of fragmentation bias justifies the mean-field approach, which can be extended to provide closed-from equations for the capture zones. Our results compare favorably to Monte Carlo data for both point and extended islands using a range of critical island size i=0,1,2,3. We also find satisfactory agreement with theoretical models based on more traditional fragmentation theory approaches.

Ge/SrTiO3(001): Correlation between interface chemistry and crystallographic orientation

In this work, the desorption of a submonolayer deposit of Ge on SrTiO3(001) is studied by reflection high energy electron diffraction. The results are compared to those of a previous experiment done using soft x-ray photoelectron spectroscopy. Combining these techniques allows for correlating interface chemistry and crystal orientation, and for bringing clarifying elements concerning the competition between (111) and (001) crystal orientation typical for the semiconductor/perovskite epitaxial systems. Despite poor interface matching, (111)-oriented islands are stabilized at the expense of (001)-oriented islands due to the relatively low energy of their free facets. Such “surface energy driven” crystallographic orientation of the deposit is enhanced by the low adhesion energy characteristic of the Ge/SrTiO3 system.

Atomic structure of closely stacked InAs submonolayer depositions in GaAs

The atomic structure of multiple buried InAs depositions with a nominal thickness below one monolayer, separated by thin GaAs spacer layers, was investigated using Cs-corrected high-resolution transmission electron microscopy. InAs composition maps were obtained with sub-nanometer resolution by local evaluation of the {200}-Fourier coefficients of the lattice images. A strong segregation behavior of the InAs depositions is found, which leads to significant intermixing with the spacer layers. The segregation coefficient R≈0.7 is found to be independent of the spacer thickness, even for thin spacers with a thickness near the segregation length of about 3 monolayers.

InGaN/GaN heterostructures grown by submonolayer deposition

InGaN/(Al,Ga)N heterostructures containing ultrathin InGaN layers, grown by submonolayer deposition are studied. It is shown that significant phase separation with the formation of local In-enriched regions ∼3–4 nm in height and ∼5–8 nm in lateral size is observed in InGaN layers in the case of InGaN and GaN growth by cyclic deposition to effective thicknesses of less than one monolayer. The effect of growth interruption in a hydrogen-containing atmosphere during submonolayer growth on the structural and optical properties of InGaN/(Al,Ga)N heterostructures is studied. It is shown that these interruptions stimulate phase separation. It is also shown that the formation of In-enriched regions can be controlled by varying the effective InGaN and GaN thicknesses in the submonolayer deposition cycles.