Surface Diffusion Length Of Adatoms Research Articles

Why is it difficult to grow spontaneous ZnO nanowires using molecular beam epitaxy?

Surface diffusion is known to be of prime importance in the growth of semiconductor nanowires. In this work, we used ZnMgO layers as markers to analyze the growth mechanisms and kinetics during the deposition of ZnMgO/ZnO multilayered shells by molecular beam epitaxy on previously grown ZnO nanowire cores (so called core–shell heterostructures). Specifically, the influence of the O2 flow sent into the plasma cell on the adatom surface mobility was investigated. By carefully measuring the growth rate on the lateral facets as well as on the top of the nanowires, it is concluded that the surface diffusion length of adatoms, within the used MBE growth conditions, is very low. Such poor surface mobility explains why so few works can be found related to the spontaneous growth (without catalyst) of ZnO nanowires by MBE, contrary to other deposition techniques.

Lateral patterning of multilayer InAs/GaAs(001) quantum dot structures by in vacuo focused ion beam

We report on the effects of patterning and layering on multilayer InAs/GaAs(001) quantum dot structures laterally ordered using an in vacuo focused ion beam. The patterned hole size and lateral pattern spacing affected the quantum dot size and the fidelity of the quantum dots with respect to the lateral patterns. 100% pattern fidelity was retained after six layers of dots for a 9.0 ms focused ion beam dwell time and 2.0 µm lateral pattern spacing. Analysis of the change in quantum dot size as a function of pattern spacing provided a means of estimating the maximum average adatom surface diffusion length to be approximately 500 nm, and demonstrated the ability to alter the wetting layer thickness via pattern spacing. Increasing the number of layers from six to 26 resulted in mound formation, which destroyed the pattern fidelity at close pattern spacings and led to a bimodal quantum dot size distribution as measured by atomic force microscopy. The bimodal size distribution also affected the optical properties of the dots, causing a split quantum dot photoluminescence peak where the separation between the split peaks increased with increasing pattern spacing.

Influence of the substrate geometrical parameters on microcrystalline silicon growth for thin-film solar cells

The effect of substrate morphology on the growth and electrical properties of single-junction microcrystalline silicon cells is investigated. A large variety of V-shaped and U-shaped substrates are characterized by scanning electron microscopy (SEM) and the growth of thin-film microcrystalline silicon (μc-Si:H) devices is observed by cross-sectional transmission electron microscopy (TEM). It is shown that enhanced electrical properties of solar cells are obtained when U-shaped substrates are used and the effect is universal, i.e. independent of the substrate or feature size. U-shaped substrates prevent the formation of two dimensional “cracks”, which are identified as zones of porous material, from propagating throughout the active part of the solar cell. A numerical growth simulation program reproduces satisfactorily these experimental observations. According to these simulations, shadowing effect due to surface morphology and low adatom surface diffusion length are responsible for the formation of cracks in μc-Si:H material.

Flux Pinning Characteristics of ${\rm Sm}_{1+x}{\rm Ba}_{2-x}{\rm Cu}_{3}{\rm O}_{y}$ Films With the Additional $c$-Axis Correlated Pinning Centers

It is known that columnar defects comprised of self-organized BaZrO3 (BZO) nanorods within REBa2Cu3Oy (REBCO) films are attractive as c-axis correlated pinning centers. On the other hand, we have fabricated high-Jc Sm1+xBa2-xCu3Oy (SmBCO) films including nanosized low-Tc phases by using the low-temperature growth (LTG) technique. In this study, BZO nanorods were added to the LTG-SmBCO film for a further enhancement of the magnetic flux pinning. Additionally, we also deposited a conventional PLD-SmBCO film including BZO nanorods and discussed differences of the flux pinning properties and microstructures between LTG- and PLD-SmBCO films. In a cross-sectional transmission electron microscopy (TEM) image of the BZO doped PLD-SmBCO film, we could see the self-organized BZO nanorods were about 10 nm in diameter, extending along the c-axis of the film. In contrast, in the BZO doped LTG-SmBCO film, the high density BZO nanorods with smaller diameters tended to be short columns and tilted against the c-axis direction. These facts can be attributed to a suppression of the surface diffusion length of adatoms and/or an increase of nucleation frequency due to the low substrate temperature during the growth of the BZO doped LTG-SmBCO film. Furthermore, we concluded that differences of superconducting properties between the BZO doped PLD- and LTG-SmBCO films might be attributed to the difference in the number density of BZO nanorods which generated lattice strain around them.

Surface kinetics and subplantation phenomena affecting the texture, morphology, stress, and growth evolution of titanium nitride films

We present a thorough study of the microstructure, texture, intrinsic stress, surface, and interface morphology of transition metal nitride (mainly TiN but also CrN) films grown on Si by reactive sputter deposition, with emphasis to the mechanisms of adatom migration on the surface and subplantation of energetic species. In order to study the effects of adatom mobility and the subplantation probability we vary the ion energy and growth temperature. For the experimental part of this work we used nondestructive, statistically reliable x-ray techniques (diffraction, reflectivity, scattering). The x-ray results are compared and correlated with supporting data of in situ spectroscopic ellipsometry as well as Monte Carlo simulations of the irradiation effects and surface diffusion of adatoms. We found that the texture and the surface and interface morphology are sensitive to the mechanism of dissipation of the impinging ions. If the energy is enough to overcome the subplantation threshold (∼50eV), then the films are highly compressed and exhibit ultrasmooth surfaces and rough interfaces. In this case, the texture of the films is not affected much by the ion energy, since the energy is dissipated in the bulk and contributes less to the surface mobility of adatoms. On the other hand, when the ion energy is below the subplantation threshold the texture of the films strongly depends on the ion energy and flux, the interfaces are atomically sharp and the surface morphology depends on the mobility and surface diffusion length of adatoms. However, in both cases these effects are dominant at the homogeneous growth. At the initial stages of nucleation and island growth the differences in the growth due to irradiation conditions are not pronounced and the thermodynamics of wetting of TiN on Si are prevailing factors.

Scanning Probe Microscopy: Present Status and New Development. Reul-Space Observation of MBE Growth Processes on GaAs(111) A Substrates.

Using a scanning electron microscope-molecular beam epitaxy system, detailed near-equilibrium growth processes of island nucleation, coalescence, and step motion are clearly observed for the growth of GaAs on (111)A substrates. These observations allow the quantitative analysis of the growth processes on the basis of the BCF theory that provides the standard model for crystal growth. As an example, the Ga adatom surface diffusion length is directly measured from the dependence of measured step velocity on the Ga arrival rate. The presence of denuded zone in the distribution of two-dimensional nuclei is clearly confirmed, quantitativelly showing good agreement with the diffusion length obtained from the step velocity.

Imaging of layer by layer growth processes during molecular beam epitaxy of GaAs on (111)A substrates by scanning electron microscopy

When we use a scanning electron microscope-molecular beam epitaxy system with a GaAs (111)A substrate that dramatically improves the flatness of the growing surface, it makes possible detailed observations of the near-equilibrium growth processes of island nucleation, coalescence, and step motion. These observations allow the quantitative analysis of the growth processes based on the standard model of crystal growth. As an example, the Ga adatom surface diffusion length is directly determined from the dependence of measured step velocity on the Ga arrival rate.

Growth mechanisms of III–V compounds by atomic hydrogen-assisted epitaxy

Epitaxial layers of InP and InGaAsP have been grown on (1 0 0) InP substrates by gas source molecular beam epitaxy while simultaneously exposed to an atomic hydrogen flux produced by a thermal cracker. Transmission electron microscopy and photoluminescence studies indicate improved structural and optical properties of the InGaAsP layers, while Hall effect measurements indicate no degradation in the electrical properties, as compared to layers grown by conventional epitaxy without hydrogen. This improvement is attributed to a reduction in lateral composition modulation (LCM), which develops at the surface of the InGaAsP layers during growth due to the existence of a miscibility gap. A detailed atomistic model, including surface reconstruction effects based on reflection high energy electron diffraction observations, is developed to explain the growth processes occurring on H-exposed (1 0 0) III–V surfaces. A simple rate equation model is used to understand the reduction in LCM in terms of a decreased surface diffusion length of adatoms in the presence of H.

Direct Observations of the Strain-Limited Island Growth of Sn-Doped GaAs(100)

We have used scanning tunneling microscopy (STM) in ultrahigh vacuum and atomic force microscopy (AFM) in air to investigate the microscopic mechanisms of Sn surface segregation during the molecular beam epitaxial growth of GaAs and AlAs(100). Submonolayer amounts of Sn segregate to the surface during growth and strongly modify the growth kinetics. This is indicated by both extra-ordinary reflection high energy electron diffraction (RHEED) measurements, and the STM and AFM images of rapidly quenched growth fronts. At the high surface coverages of 0.1–0.6 monolayers of Sn, studied in this work, neither step bunching nor three-dimensional (3D) growth of GaAs(100), was observed. Instead, STM and RHEED measurements indicated a significantly enhanced layer-by-layer growth of GaAs with increasing surface coverage of Sn. STM snapshots of the initial stages of GaAs growth revealed 2D islands which contained a higher-than-equilibrium bulk concentration of Sn, in Ga-substitutional sites, of up to 50%. Other directly observed Sn effects which are presented in this work include the removal of GaAs(100) island growth anisotropy and the formation of 2D islands with a relatively narrow distribution of size and separation. The completion of the top layers is shown to proceed by the coalescence of these islands before any significant nucleation of the next layer islands. This effect is used to explain the Sn enhancement of the layer-by-layer growth which was indicated in our RHEED and scanning probe observations. A model is presented for Sn segregation which explains these results based on an island-size-dependent, strain-driven, oscillatory Sn occupation of Ga-substitutional sites and surface interstitial sites on top GaAs(100) layers during growth. This model, which introduces a strain-limiting mechanism for the size and shape of the 2D islands, can also explain the observed enhancement of postgrowth surface recovery, as well as a delayed onset in increasing adatom surface diffusion length with increasing Sn coverage. The main conclusion is that, if impurity incorporation results in significant strain, then in addition to step climbing by surface impurities, the exchange of incorporated impurities with native species in top layers can be an important path for impurity segregation during expitaxial growth.

Role of atomic hydrogen in argon plasma-assisted epitaxy of InGaAsP/InP

Epitaxial layers of InP and InGaAsP have been grown on (100) InP substrates by gas source molecular beam epitaxy over the temperature range 400–480 °C while simultaneously exposed to an Ar plasma stream produced by electron cyclotron resonance (ECR). Transmission electron microscopy, x-ray diffraction, and photoluminescence studies indicate improved structural and optical properties of the InGaAsP layers as compared to layers grown by conventional epitaxy without plasma. This improvement is attributed to a reduction in lateral composition modulation (LCM), which develops at the surface during growth due to the existence of a miscibility gap. Comparison of these results with that achieved by an independent thermal hydrogen cracker suggests that the reduced LCM results from molecular hydrogen, produced from the cracking of the group V hydride sources, backflowing into the ECR chamber and resulting in a flux of atomic hydrogen toward the growth front. Atomic hydrogen exposure of the growing surface may then result in surfactant-mediated epitaxy, thereby, reducing the adatom surface diffusion length and, hence, the LCM. Atomic hydrogen, therefore, appears to be the sole actor in reducing the LCM, while the effects of the plasma itself are negligible.

Kinetics of evaporation of beaded thin films

Theoretical description of the time-dependence of the effective thickness of a beaded thin film, h eff, and the average radius of beads, R̄, is given supposing evaporation. Besides the evaporation and surface diffusion the role of the boundary reaction of exchange of atoms between beads and absorbed surface gas layer is also taken into account. The basic mechanism of the kinetics is not the direct evaporation of atoms from the surface of beads but the desorption of adatoms from the surface of the substrate during their “time of life” τ. Thus the basic parameters of the theory are the surface diffusion length of adatoms λ s = (D sτ) 1 2 , surface diffusion coefficient of adatoms, D s, and kinetic coefficient β s, which determines the velocity of exchange of adatoms between the beads and the surface layer. Analytical expressions for the time dependence of h eff and R̄ are given for different regimes of diminution (i.e. for surface diffusion and surface reaction controls) of individual particles and of an ensemble of overlapping diffusional fields as well. Using the example of a Cu-beaded film on Mo(110) and Mo(100) surfaces it is illustrated, that by measuring the kinetics of the effective thickness in the case of reaction control, we can determine the value β' sλ s 2/ D' s, where β' s = β s c s0 and D' s = D s c s0 are the surface mass transport coefficients and c s0 is the equilibrium surface concentration. Furthermore, from the saturation vapor pressure we can get the ratio D' s/λ s 2, and thus β' s can be calculated.

ＩＩＩ‐Ｖ族化合物半導体成長のμ‐ＲＨＥＥＤ実時間観測

Scanning microprobe reflection high-energy electron diffraction (μ-RHEED) reveals microscopic surface features during MBE growth. The Ga droplet formation and annihilation were observed in GaAs growth with alternative source supply. The droplet movement was also observed on a vicinal plane of a GaAs (111) substrate. InAs hillock formation was observed on high temperature substrates, and the real-time observation showed that the hillocks originated in the droplets on the surface. The surface diffusion length of adatoms was estimated from the distribution of growth rates near the edge of a mesa stripe on the substrate. The obtained values of the diffusion length were much longer than the averaged distance between atomic steps on the surface. This means that the incorporation ratio of the adatom into an atomic step is much less than unity.

Effect of substrate surface roughness on the columnar growth of Cu films

Vapor-deposited films prepared under low adatom mobility conditions commonly exhibit columnar structures. We present the scanning electron microscopy study of Cu films with a thickness around 3.7 μm deposited on rough Al2O3 ceramic substrates and smooth SiO2 substrates. The Cu films were deposited using the partially ionized beam deposition technique under the same deposition conditions. The films deposited on the rough substrates have a typical columnar structure while those deposited on the smooth substrates are noncolumnar. The complete difference in physical structures is due to the difference in the substrate surface roughness. The results indicate that in any structure zone model for the physical structure of the film the substrate surface roughness is an important parameter and cannot be neglected. It is reasoned that the adatom surface diffusion length, which is determined by the deposition conditions, serves as a critical measure for the length scale of surface roughness. At the initial stage of the film growth, the columnar structure develops only for the roughness with a length scale longer than adatom surface diffusion length.