Gallium arsenide molecular beam epitaxy: Low temperature and surfactant-mediated

The reflection high-energy electron diffraction (RHEED) specular spot intensity oscillations that were obtained during low-temperature regime and surfactant mediated regime of molecular beam epitaxial (MBE) growth of GaAs is studied and explained using modified stochastic model and a rate equation model, respectively; The dynamics of the physisorbed As layer were introduced into the stochastic model by including the thermally activated processes of chemisorption into and evaporation out of the As physisorbed state. Increased scattering of the RHEED beam due to the higher physisorbed As coverage at 2:1 leads to a factor of 5 decrease in the steady-state amplitude of the RHEED oscillations compared to the 1:1 case. These results are in excellent agreement with the experimental results. A factor in maintaining this growth mode is that arsenic stays in the physisorbed state with lifetimes in the range of 10{dollar}\sp{-3}{dollar} to 10{dollar}\sp{-5}{dollar} seconds and incorporates only when an appropriate configuration of Ga atoms forms on the surface; Beating in the reflection high energy electron diffraction (RHEED) intensity oscillations were observed during molecular beam epitaxial (MBE) growth of GaAs with Sn as a surfactant. A rate equation model of growth was developed to explain this phenomenon by assuming that the GaAs covered by the Sn grows at a faster rate compared to the GaAs not covered by Sn. Assuming that the electron beams reflected from the Sn covered surface and the rest of the surface are incoherent, the results of the dependence of the RHEED oscillations on Sn submonolayer coverages for various Sn coverages were obtained and compared with experimental data and the qualitative agreement is very good. (Abstract shortened by UMI.)

Elastic relaxation during 2D epitaxial growth: a study of in-plane lattice spacing oscillations

The purpose of this paper is to report some new experimental and theoretical results about the analysis of in-plane lattice spacing oscillations during two-dimensional (2D) homo and hetero epitaxial growth. The physical origin of these oscillations comes from the finite size of the strained islands. The 2D islands may thus relax by their edges, leading to in-plane lattice spacing oscillations during the birth and spread of these islands. On the one hand, we formulate the problem of elastic relaxation of a coherent 2D epitaxial deposits by using the concept of point forces and demonstrate that the mean deformation in the islands exhibits an oscillatory behaviour. On the other hand, we calculate the intensity diffracted by such coherently deposited 2D islands by using a mean model of a pile-up of weakly deformed layers. The amplitude of in-plane lattice spacing oscillations is found to depend linearly on the misfit and roughly linearly on the nucleation density. We show that the nucleation density may be approximated from the full-width at half maximum of the diffracted rods at half coverages. The predicted dependence of the in-plane lattice spacing oscillations amplitude with the nucleation density is thus experimentally verified on V/Fe(001), Mn/Fe(001), Ni/Fe(001), Co/Cu(001) and V/V(001).Comment: 39 pages, 10 figure

Reflection High-Energy Electron Diffraction Studies of Indium Phosphide (100) and Growth on Indium and Indium Nitride on Silicon (100)

Study of the effects of atomic hydrogen exposure on structure and morphology of semiconductor surfaces is important for fundamental properties and applications. In this dissertation, the electron yield of a hydrogen-cleaned indium phosphide (InP) surface was measured and correlated to the development of the surface morphology, which was monitored by in situ reflection high-energy electron diffraction (RHEED). Atomic hydrogen treatment produced a clean, well-ordered, and (2x4)-reconstructed InP(100) surface. The quantum efficiency, after activation to negative electron affinity, and the secondary electron emission were shown to increase with hydrogen cleaning time. RHEED patterns of low-index InP(100) surface were modified by the step structure and resulted in splitting of the specular beam at the out-of-phase diffraction condition. Quantitative RHEED showed reduction in the average terrace width and a decrease of the adatom-vacancy density with hydrogen exposure. This suggests that atomic hydrogen etching occurs preferentially at terrace edges, and thermal diffusion on the surface causes changes in the terrace edge morphology, which result in the observed decrease in the average terrace width. The results show that the decrease in the surface disorder, measured from the RHEED intensity-to-background ratio, correlated with the increased quantum efficiency. The growth of group-III metals on Si surfaces has become an attractive area of research because of its scientific importance and great potential in technological applications. In this work, the growth dynamics, structure, and morphology of indium (In) on a vicinal Si(100)-(2×1) surface by femtosecond pulsed laser deposition (fsPLD) were studied using in situ RHEED and ex situ atomic force microscopy. Indium was found to grow on Si(100) by the Stranski-Krastanove mode. At room temperature, the initial growth formed strained two-dimensional (2D) layers in the In(2×1) structure followed by growth of three-dimensional islands. During the 2D growth, the surface diffusion coefficient of deposited In on the In(2×1) layer was estimated to be in the order of 10−14 cm2/s, from recovery of the RHEED intensity. This was attributed to surface diffusion of In clusters by step flow mode. The results suggest that fsPLD of In removed the reconstruction of the Si(100)-(2×1) surface in the early growth and resulted in the initial In(2x1) structure. Next, growth of In on Si(100)-(2×1) was studied at temperature of 350–420°C and showed formation of In(4×3) structure. The growth stages, probed by RHEED intensity relaxation, proceed in a two-step process, formation of small In clusters and surface diffusion to the terrace step edges with activation energy of 1.4±0.2 eV and diffusion rate constant of 1.0±0.1x1011 s −1. The terrace width dynamics and the related surface processes were studied during growth of the In(4×3) phase with increase in film coverage. Finally, the fsPLD was used to grow nitride films of InN on Si(100) substrates. A buffer layer of In was grown on Si(100) by fsPLD prior to growth of InN and different nitridation procedures were used

Low-temperature molecular beam epitaxy of gallium arsenide Antisite incorporation and Rheed oscillations: A theoretical study

Surface dynamics dominate the incorporation of charged and neutral antisite arsenic and the temporal variation of reflection high energy electron diffraction (RHEED) intensity in the low temperature (LT) molecular beam epitaxy (MBE) of (100) gallium arsenide (GaAs). A comprehensive rate equation model is proposed based on the presence and dynamics of a physisorbed arsenic (PA) riding the growth surface which dictates the incorporation and concentration of antisites and the RHEED oscillations (ROs) behavior. The dependence of antisite concentrations on growth parameters can be explained based on the saturation of the PA layer coverage at a monolayer and the competing rate processes such as the incorporation into and evaporation of antisite arsenic from the crystalline surface. The RHEED intensity is computed based on kinematical theory of electron diffraction with different interplanar distances for the PA layer (2.48A) and the crystal (1.41A). At temperatures and beam equivalent pressures (BEP)s when the surface coverage is 0.5, the resulting RHEED reflection contributions from both surfaces covered by the PA layer and the crystal interfere destructively to result in no ROs

Delayed crystallization of ultrathin Gd2O3 layers on Si(111) observed by in situ X-ray diffraction

We studied the early stages of Gd2O3 epitaxy on Si(111) in real time by synchrotron-based, high-resolution X-ray diffraction and by reflection high-energy electron diffraction. A comparison between model calculations and the measured X-ray scattering, and the change of reflection high-energy electron diffraction patterns both indicate that the growth begins without forming a three-dimensional crystalline film. The cubic bixbyite structure of Gd2O3 appears only after a few monolayers of deposition

Surface Dynamics of Silicon Low-Index Surfaces Studied by Reflection High-Energy Electron Diffraction

Surface morphology during the growth of Si on Si(111)-(7x7) by femtosecond pulsed laser deposition (fsPLD) is studied using reflection high-energy electron diffraction (RHEED) at different temperatures. The growth of Si on Si(111) has received considerable attention as a model system of homoepitaxy. PLD is a deposition technique that uses much more energetic species (atoms and ions) compared to other physical vapor deposition (PVD), such as in molecular beam epitaxy. In this work, in situ reflection high energy electron diffraction (RHEED) was used to study the dynamics of PLD of Si on Si(111)-(7x7). Epitaxial growth of Si/Si(111)-(7x7) at temperatures as low as 210°C was observed. For this substrate temperature, no change in RHEED patterns after growth, and only reduction in intensity during deposition was observed. Surface Debye temperature of the topmost layer of the Si(111)-7x7 is measured by using RHEED. The diffraction intensity is distorted by the thermal vibration amplitude of atoms on the topmost layer of the surface. Influence of Si deposition on the temperature of Si(111) to (7x7) phase transition is also studied. The phase transition showed that Si deposition lowers the transition temperature. A Ti-sapphire laser (100 fs, 800 nm, 1 kHz) was used to ablate a Si target on Si(111)-(1x1) during quenching from high temperature. The RHEED intensity was observed as the substrate was exposed to the Si plume and the Si(111) substrate was quenched. The RHEED patterns showed a shift in the transition temperature from 840°C without the plume to 820°C with the plume. With laser fluence below the damage threshold, laser enhanced epitaxial growth shows a great improvement in deposit Si on Si(111)-7x7 at low temperature (room temperature)

Self-organized quantum wires on patterned GaAs(311)A and on unpatterned GaAs(100)

In der vorgelegten Arbeit wurden zwei Arten von Quantendrahtstrukturen untersucht, die mittels Molekularstrahlepitaxie (MBE) hergestellt wurden. Erstens ist dies eine laterale Quantendrahtstruktur, die sich entlang einer Mesakante durch selektives Wachstum auf strukturierten GaAs (311)A-Substraten ausbildet. Zunächst wurden vertikal gestapelte Quantendrähte mit starker elektronischer Kopplung realisiert. Weiterhin wurden, unter Nutzung des amphoteren Einbaus von Si, p-i-n-Leuchtdioden mit einem einzelnen Quantendraht in der aktiven Zone hergestellt, die sich durch selektive Ladungsträgerinjektion in die Quantendrähte auszeichnen. Die Leuchtdioden wurden weitergehend mittels Mikrophotolumineszenz(µ-PL), Kathodolumineszenz (CL) und Elektronenstrahl-induziertem Strom (EBIC) charakterisiert. Zur Erklärung der selektiven Elektrolumineszenz (EL) wurde ein Modell, basierend auf der lateralen Diffusion von Elektronen und Löchern, vorgeschlagen. Für verspannte Systeme wurde der Einfluss von atomarem Wasserstoff auf das Wachstum von (In,Ga)As auf GaAs (311)A und die Bildung von lateralen Quantendrähten untersucht. Atomarer Wasserstoff spielt dabei die Rolle eines Surfaktanden und unterdrückt deutlich die Bildung von dreidimensionalen Inseln. Zweitens wurde das Wachstum von verspannten (In,Ga)As-Schichten auf GaAs (100) untersucht. Es zeigte sich, dass die dreidimensionale Inselbildung durch die Wachstumskinetik bestimmt ist, und ein Übergang von symmetrischen zu asymmetrisch verlängerten Inseln bei Erhöhung der Wachstumstemperatur auftritt. Dieser Prozess wird durch das Zusammenspiel von Oberflächen- und Verspannungsenergie bestimmt, wobei die experimentellen Befunde in guter Übereinstimmung mit den theoretischen Arbeiten von Tersoff und Tromp sind. Ausgehend von asymmetrischen (In,Ga)As-Inseln wurden selbstorganisierte Quantendrähte hergestellt, deren Homogenität und Länge sich durch Wachstum einer Vielschichtstruktur deutlich erhöhen. Strukturell wurden die (In,Ga)As-Quantendrähte mittels Rasterkraftmikroskopie (AFM), Röntgendiffraktometrie (XRD) und Transmissionselektronenmikroskopie (TEM) untersucht. Der laterale Ladungsträgereinschluss in den Quantendrähten zeigte sich deutlich in polarisationsabhängigen Photolumineszenz- und Magnetophotolumineszenzmessungen.The present work focuses on two types of quantum wire structures which were grown by molecular beam epitaxy (MBE). First, the sidewall quantum wires based on the selective growth on mesa stripe patterned GaAs(311)A are studied. Single stacked sidewall quantum wires with strong electronic coupling have been fabricated. p-i-n type LEDs of the quantum wires employing the amphoteric Si incorporation for p- and n-type doping on GaAs(311)A have been fabricated. Strong selective carrier injection into the quantum wires is observed in electroluminescence (EL) measurements. The samples are characterized by micro-photoluminescence (µ-PL), cathodoluminescence (CL), as well as electron beam induced current (EBIC) measurements. To account for the highly selective EL, a model is proposed, which is based on the lateral diffusion of electrons and holes resulting in self-enhanced carrier injection into the quantum wires. Atomic hydrogen effects in the growth of (In,Ga)As on GaAs(311)A and its application to the sidewall quantum wire are investigated. It is found that atomic hydrogen suppresses island formation. Atomic hydrogen delays the relaxation by islanding thus playing the role of a surfactant. Second, the growth of (In,Ga)As layers on GaAs(100) is investigated showing that the formation of coherent 3D islands is a kinetically limited process. The transition from square-shaped islands to elongated islands is observed by changing the growth temperature for the growth of (In,Ga)As single layers. The elongation of the islands is a tradeoff between the surface free energy and the strain energy. A quantitative comparison between the experimental results and the theoretical work done by Tersoff and Tromp shows a good agreement. Self-organized quantum wires based on elongated discolation-free islands have been fabricated. The uniformity of the quantum wires is greatly improved by a superlattice growth scheme which also makes the wires much longer. The structural characterization of the quantum wires is performed by atomic force microscopy (AFM), x-ray diffractometry (XRD), and transmission electron microscopy (TEM). The lateral carrier confinement in the quantum wires is confirmed by polarization dependent PL and magneto-PL measurements

Self-Assembly and Characterization of Germanium Quantum Dots on Silicon by Pulsed Laser Deposition

Self-assembled Ge quantum dots (QD) are grown on Si(100)-(2×1) by pulsed laser deposition (PLD). In situ reflection-high energy electron diffraction (RHEED) and post-deposition atomic force microscopy (AFM) are used to study the growth dynamics and morphology of the QDs. Several films of different thicknesses were grown at a substrate temperature of 400°C using a Q-switched Nd:YAG laser (λ = 1064 mu, 40 ns pulse width, 23 J/cm2 fluence, and 10 Hz repetition rate). At low film thicknesses, but clusters that are faceted by different planes, depending on their height, are observed after the completion of the wetting layer. With increasing film thickness, the size of the clusters grows, and they gradually lose their facetation and become more rounded. With further thickness increase, the shape of these clusters becomes dome-like with some pyramids observed among the majority of domes. The effect of the laser fluence on the morphology of the grown clusters was studied. The cluster density was found to increase dramatically while the average cluster size decreased with the increase in the laser fluence. For a laser fluence of 70 J/cm2, dome-shaped clusters that are smaller than the large huts formed at 23 J/cm2 were observed. At a substrate temperature of 150°C, misoriented three-dimensional (3D) clusters formed producing only a RHEED background. At 400 and 500°C, huts and a lower density of domes formed, respectively. Above 600°C, 3D clusters formed on top of a discontinuous textured layer. As an application, pulsed laser deposition is used to fabricate multilayered Ge quantum-dot photodetector on Si(100). Forty successive Ge quantum dot layers, each covered with a thin Si layer, were deposited. Deposition and growth are monitored by in situ reflection-high energy electron diffraction and the morphology is further studied by ex situ atomic force microscopy. The difference in the current values in dark and illumination conditions was used to measure the device sensitivity to radiation. Spectral responsivity measurements reveal a peak around 2 μm, with responsivity that increases three orders of magnitude as bias increases from 0.5 to 3.5 V. The effects of laser-induced electronic excitations on the self-assembly of Ge quantum dots on Si(100)-2×1 grown by pulsed laser deposition are also studied. Electronic excitations, due to laser irradiation of the Si substrate and the Ge film during growth, are shown to decrease the roughness of films grown at a substrate temperature of ∼120°C. At this temperature, the grown films are nonepitaxial. However, electronic excitation results in the formation of an epitaxial wetting layer and crystalline Ge quantum dots at ∼260°C, a temperature at which no crystalline quantum dots form without excitation under the same deposition conditions. Finally, the very early stages of formation of Ge but clusters on Si(100) has been studied by UHV STM. Growth starts by the formation of a very low density of asymmetric huts with high aspect ratios. Further deposition results in a higher density of clusters characterized by their narrow size and height distributions. These clusters are almost of the same lateral size as those deposited at lower thicknesses