Growth Of Ge Islands Research Articles

The effects of interruption time on the growth of Ge islands prepared by ion beam sputtering deposition

The growth interruption technique was employed to investigate the self-assembled growth of Ge islands prepared by ion beam sputtering deposition. The evolution of island size and density with the interruption time were well studied by atomic force microscopy. During the growth interruption process, Ostwald ripening induces a drastic decrease of the density of islands without capping. The distribution of these islands determines the further growth of Ge islands after depositing the second amount of Ge. For the interruption time below 60 s, the self-limiting growth of islands is significant, leading to a slight increase of island diameter and an obvious enlargement of island height with increasing the time. As the interruption time is more than 60 s, many new smaller islands form after depositing the second amount of Ge. Consequently, the density and the average aspect ratio of Ge islands remain stable. This is indicated that the growth of islands exhibits an equilibrium state between Ostwald ripening and the formation of new smaller islands. With an interruption time of 60 s, both island diameters and heights with an unimodal distribution are observed.

Formation of a Stepped Si(100) Surface and Its Effect on the Growth of Ge Islands

The transition from a two-domain to one-domain surface on a Si(100) substrate is investigated. It is demonstrated using reflection high-energy electron diffraction that at a temperature of 600°C and a deposition rate of 0.652 A/s onto a Si(100) substrate pre-heated to 1000°C and inclined at an angle of 0.35°C to the plane, a series of reflections from the 1 × 2 superstructure completely vanishes at a constant flow of Si. This is attributed to the transition of the surface from monoatomic to diatomic steps. At growth rates lower than 0.652 A/s, the transition from a two-domain to one-domain surface is also observed; with a decrease in the growth rate, the intensity ratio I2 × 1/I1 × 2 decreases and the maximum of the dependences shifts toward lower temperatures. The complete vanishing of the series of superstructural reflections after preliminary annealing at a temperature of 700°C is not observed; this series only vanishes after annealing at 900 and 1000°C. The growth of Ge islands on a Si(100) surface preliminary annealed at a temperature of 800°C is studied. It is shown that the islands tend to nucleate at the step edges. A mechanism of Ge island ordering on the Si(100) surface is proposed.

Формирование ступенчатой поверхности Si(100) и ее влияние на рост островков Ge

AbstractThe transition from a two-domain to one-domain surface on a Si(100) substrate is investigated. It is demonstrated using reflection high-energy electron diffraction that at a temperature of 600°C and a deposition rate of 0.652 Å/s onto a Si(100) substrate pre-heated to 1000°C and inclined at an angle of 0.35°C to the plane, a series of reflections from the 1 × 2 superstructure completely vanishes at a constant flow of Si. This is attributed to the transition of the surface from monoatomic to diatomic steps. At growth rates lower than 0.652 Å/s, the transition from a two-domain to one-domain surface is also observed; with a decrease in the growth rate, the intensity ratio I _2 × 1/ I _1 × 2 decreases and the maximum of the dependences shifts toward lower temperatures. The complete vanishing of the series of superstructural reflections after preliminary annealing at a temperature of 700°C is not observed; this series only vanishes after annealing at 900 and 1000°C. The growth of Ge islands on a Si(100) surface preliminary annealed at a temperature of 800°C is studied. It is shown that the islands tend to nucleate at the step edges. A mechanism of Ge island ordering on the Si(100) surface is proposed.

(Invited) Atomically Controlled Processing for Si and Ge CVD Epitaxial Growth

Atomically controlled processing has become indispensable for the fabrication of Si-based ultrasmall devices and heterodevices for ultralarge scale integration. Our concept of atomically controlled processing is based on atomic-order surface reaction control [1-3]. The final goal is the generalization of atomic-order surface reaction processes and the creation of new properties in Si-based ultimate small structures. In the stage of nanometer-scale device fabrication, preparing atomic-order controlled surfaces and interfaces and the introduction of Ge into the Si process are required. In this work, we describe thesegregation of dopants to the surface and surface roughness generation observed during in-situ doping and their suppression by applying an atomic layer doping approach. Furthermore results of nm-order thick Ge epitaxial growth on Si(100) using an ultraclean low-pressure CVD system are presented. Figure 1 shows the P depth profile in a Si epitaxial film. Even after switching off the PH3 gas supply, P atoms are incorporated into the Si layer during Si epitaxial growth. In the steady state, it is estimated that P concentration saturates at around 3x1019cm-3 and P coverage on the Si surface is about 0.1 monolayer (6.8x1013cm-2). These values are calculated from the PH3 partial pressure and Si growth rate shown in Fig. 1 based on the surface reaction model shown in Ref. 3. The P incorporation rate, that is the product of the Si growth rate (0.8nm/min) and P concentration (about 1x1019cm-3), is about 8x1011cm-2/min. If it is assumed that the desorption of P atoms on the Si surface can be neglected [4], 6.8x1013cm-2 P atoms on the surface are incorporated into the Si epitaxial layer with 8x1011cm-2/min even after the PH3 supply has been stopped. The increase of surface roughness from RMS 0.1nm to 0.8nm during in-situ doping can be explained by the suppression of the SiH4reaction caused by the P atoms which are present on the surface. Suppression of P segregation and surface roughening are achieved using P atomic layer doping in Si. This includes the optimization of the capping Si layer deposition conditions and the P atomic amount [5] as shown in Fig. 2. This may result from low temperature Si epitaxial growth on P atoms with high growth rate. That could be demonstrated for P atomic layer doping during Ge epitaxial growth too [6]. In a second part of the paper results for nm-order-thick Ge high quality epitaxial growth on Si (100) are described. Because pure Ge and Si oxide are formed on Si substrate even at 300oC by the reaction between vaporized Ge oxide and Si substrate [7], the suppression of Ge oxide vaporization in the CVD reactor is very important. It was proposed that Ge island growth in an initial growth stage is caused by the suppression of adsorption and/or decomposition of GeH4 on the H-terminated Si surface [8]. Even on the initially H-free Si surface, Si atoms are H-terminated by the decomposition of GeH4. In order to decrease the hydrogen desorption temperature from the Si site, nm-thick Si1-xGex buffer layer on Si (100) was proposed [9,10]. The suppression of surface roughening was improved by nm-order-thick Ge epitaxial growth on nm-order-thick Si1-xGex/Si(100). The surface segregation of Si through nm-order-thick Ge layer from the substrate was observed at Ge growth temperature of 350oC. These results demonstrate the capability of atomically controlled processing for nm-order thick Si and Ge CVD epitaxial growth for ultralarge scale integration. This work is partially supported by the JSPS Core-to-Core Program, “International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration”.

Atomic structure and composition distribution in wetting layers and islands of germanium grown on silicon (001) substrates

We present a comprehensive structural investigation of the Ge wetting layer (WL) and island growth on Si(001) substrates by a combination of AFM, high resolution transmission electron microscopy and the energy-differential coherent Bragg rod analysis (COBRA) x-ray method. By considering the influence of the initial Si surface morphology on the deposited Ge, these techniques provide quantitative information on the Ge content and its distribution, in particular within the WL which plays a crucial role in the formation of epitaxial nanostructures. In the WL, the Ge content was found to be above 80% for our growth conditions. Furthermore, from the digital analysis of high-resolution transmission electron microscope images, quantitative information on the strain relaxation is obtained, which complements the COBRA analysis of the Ge distribution and content in these nanostructures.

Direct molecular dynamics simulation of Ge deposition on amorphous SiO2 at experimentally relevant conditions

Molecular dynamics simulations were used to study Ge island nucleation and growth on amorphous SiO2 substrates. This process is relevant in selective epitaxial growth of Ge on Si, for which SiO2 is often used as a template mask. The islanding process was studied over a wide range of temperatures and fluxes, using a recently proposed empirical potential model for the Si–SiO2–Ge system. The simulations provide an excellent quantitative picture of the Ge islanding and compare well with detailed experimental measurements. These quantitative comparisons were enabled by an analytical rate model as a bridge between simulations and experiments despite the fact that deposition fluxes accessible in simulations and experiments are necessarily different by many orders of magnitude. In particular, the simulations led to accurate predictions of the critical island size and the scaling of island density as a function of temperature. The overall approach used here should be useful not just for future studies in this particular system, but also for molecular simulations of deposition in other materials.

Formation of GeSn alloy on Si(100) by low-temperature molecular beam epitaxy

GeSn alloys grown on Si(100) by the low-temperature (100 °C) molecular beam epitaxy are studied using scanning tunneling microscopy and Raman spectroscopy. It is found that the effect of Sn as a surfactant modifies substantially the low-temperature growth mechanism of Ge on Si. Instead of the formation of small Ge islands surrounded by amorphous Ge, in the presence of Sn, the growth of pure Ge islands appears via the Stranski-Krastanov growth mode, and a partially relaxed Ge1−xSnx alloy layer with the high Sn-fraction up to 40 at. % is formed in the area between them. It is shown that the observed growth mode induced by high surface mobility of Sn and the large strain of the pseudomorphic state of Ge to Si ensures the minimum elastic-strain energy of the structure.

Si/Ge intermixing during Ge Stranski-Krastanov growth.

The Stranski–Krastanov growth of Ge islands on Si(001) has been widely studied. The morphology changes of Ge islands during growth, from nucleation to hut/island formation and growth, followed by hut-to-dome island transformation and dislocation nucleation of domes, have been well described, even at the atomic scale, using techniques such as scanning tunneling microscopy and transmission electron microscopy. Although it is known that these islands do not consist of pure Ge (due to Si/Ge intermixing), the composition of the Ge islands is not precisely known. In the present work, atom probe tomography was used to study the composition of buried dome islands at the atomic scale, in the three-dimensional space. The core of the island was shown to contain about 55 atom % Ge, while the Ge composition surrounding this core decreases rapidly in all directions in the islands to reach a Ge concentration of about 15 atom %. The Ge distribution in the islands follows a cylindrical symmetry and Ge segregation is observed only in the {113} facets of the islands. The Ge composition of the wetting layer is not homogeneous, varying from 5 to 30 atom %.

Surface morphology of Ge layers epitaxially grown on bare and oxidized Si(001) and Si(111) substrates

The surface morphology of Ge layers grown by Ge deposition on the Si(001) and Si(111) surfaces covered with ultrathin SiO2 films is studied with scanning tunneling microscopy. It is essentially different from that obtained by Ge deposition on the bare Si surfaces, which are performed in similar conditions for comparison. The SiO2 film is partly decomposed at temperatures above 430°C, leading to the formation of bare Si nanoareas which serve for the epitaxial Ge island nucleation and growth with respect to the Si substrate. At the same time, we found that the SiO2 film residuals prevent the Ge–Si intermixing providing pure Ge islands formation. They appear to be different in shape from the islands grown on the bare surfaces, which contain a significant amount of Si due to the strain-induced Ge–Si intermixing. The possibility of pure Ge islands formation may lead to the Ge/Si nanostructures fabrication with modified optoelectronic characteristics.

Growth of Ge islands on SrTiO3 (001) 2 × 1 reconstructed surface: Epitaxial relationship and effect of the temperature

The structural properties of Ge islands grown by molecular beam epitaxy on SrTiO3 (001) substrates are investigated. We report on the effect of the temperature on the epitaxial relationship and morphology of the Ge islands. By combining X-ray diffraction measurements and atomic force microscopy we evidence a correlation between the island size and shape, the structural properties of the interface and the sample texture. In particular, we show that the growth temperature affects the nature of the interface between the semiconductor and the perovskite oxide and thus the island orientation, that evolves from fully (001) oriented to a mixture of (111) and (001) oriented islands.

Effects of boron dopants of Si (001) substrates on formation of Ge layers by sputter epitaxy method

The formation of Ge layers on boron-doped Si (001) substrates by our sputter epitaxy method has been investigated. The surface morphology of Ge layers grown on Si substrates depends on the substrate resistance, and flat Ge layers are obtained on Si substrates with 0.015 Ω cm resistivity. Highly boron-doped Si substrates cause a transition in the dislocation structure from complex dislocations with 60° dislocation glide planes to 90° pure-edge dislocations, resulting in the formation of flat Ge layers. Furthermore, we have found that the surface morphology of the Ge layers improves with increasing Ge layer thickness. Ge atoms migrating on the deposited Ge layers tend to position themselves at the reactive sites, where the reactivity is related to the number of bonding contacts between the Ge atom and the surface. This modifies the surface morphology, resulting in a flatter surface. Boron dopants together with the sputter epitaxy method effectively suppress the growth of Ge islands and result in the formation of flat Ge layers.

Unrolling the evolution kinetics of ordered SiGe islands via Ge surface diffusion

During molecular beam epitaxial growth of Ge islands on Si(001) substrates containing pit-patterned areas, an efficient Ge surface diffusion from unpatterned to pit-patterned substrate regions results in position-dependent island growth rates. We exploit this effect to map out the evolution of strictly ordered Ge islands from prepyramids via pyramids and transition domes into domes by atomic force microscopy. A one-dimensional diffusion model is established which allows a quantitative determination of the Ge surface diffusion constant and the rates at which Ge atoms are incorporated into the growing islands. We find evidence that during the shape transitions from nonfaceted islands to ${105}$-faceted pyramids and from pyramids to domes, Ge is incorporated at higher rates than during the growth of pure pyramids or domes. From these data it follows that a Ge atom passes on average several thousand pits before it is incorporated into one. Within our model we show that secondary-island nucleation in the substrate regions between the pits is a consequence of a mismatch between Ge incorporation and deposition rates, which results in a growing wetting layer between the ordered island sites, the thickness of which eventually exceeds the critical one for spontaneous island nucleation. We experimentally demonstrate that by lowering the Ge deposition rate, perfectly ordered islands with large pit periods of around 1 $\ensuremath{\mu}$m can be grown, in agreement with the model predictions.

Investigation of the peculiarities of Ge island growth on Si (100) under MBE conditions

The results of investigating the features of Ge-island formation on Si (100) under conditions of molecular-beam epitaxy is presented. Nanosized structures at heteroboundaries (islets and quantum dots) are attracting great interest due to new useful properties and broad prospects of their application.

Recipes for the fabrication of strictly ordered Ge islands on pit-patterned Si(001) substrates

We identify the most important parameters for the growth of ordered SiGe islands on pit-patterned Si(001) substrates. From a multi-dimensional parameter space we link individual contributions to isolate their influence on ordered island growth. This includes the influences of: the pit size, pit depth and pit period on the Si buffer layer and subsequent Ge growth; the pit sidewall inclination on Ge island growth; the amount of Ge on island morphologies as well as the influences of the pit-size homogeneity, the pit period, the Ge growth temperature and rate on island formation. We highlight that the initial pit shape and pit size in combination with the growth conditions of the Si buffer layer should be adjusted to provide suitable preconditions for the growth of Ge islands with the desired size, composition and nucleation position. Furthermore, we demonstrate that the wetting layer between pits can play the role of a stabilizer that inhibits shape transformations of ordered islands. Thus, dislocation formation within islands can be delayed, uniform arrays of one island type can be fabricated and secondary island nucleation between pits can be impeded. These findings allow us to fabricate perfectly ordered and homogeneous Ge islands on one and the same sample, even if the pit period is varied from a few hundred nanometres to several micrometres.

Nucleation and growth kinetics during metal-induced layer exchange crystallization of Ge thin films at low temperatures

The kinetics of Al-catalyzed layer exchange crystallization of amorphous germanium (Ge) thin films at low temperatures is reported. Observation of Ge mass transport from an underlying amorphous Ge layer to the Al film surface through an interposed sub-nanometer GeOx interfacial layer allows independent measurement of the areal density and average area of crystalline Ge islands formed on the film surface. We show that bias-voltage stressing of the interfacial layer can be used to control the areal density of nucleated Ge islands. Based on experimental observations, the Johnson-Mehl-Avrami-Kolmogorov phase transformation theory is used to model nanoscale nucleation and growth of Ge islands in two dimensions. Ge island nucleation kinetics follows an exponentially decaying nucleation rate with time. Ge island growth kinetics switches from linear growth at a constant growth velocity to diffusion-limited growth as the growth front advances. The transition point between these two regimes depends on the Ge nucleation site density and the annealing temperature. Knowledge of the kinetics of low-temperature crystallization is important in achieving textured polycrystalline Ge thin films with large grains for applications in large-area electronics and solar energy conversion.

Preferential ordering of self-assembled Ge islands on focused ion-beam patterned Si(100)

We report the growth of Ge islands on Si (001) substrates with lithographically defined two-dimensionally periodic pits using focused ion-beam patterning and molecular beam epitaxy. The formation of circularly ordered Ge islands has been achieved by means of nonuniform strain field around the periphery of the holes due to ion bombardment. Lateral ordering of the Ge islands have been controlled by both the pit size and pit separation. Preferential growth at the pit sites has also been achieved by using appropriate pattern shape and size.

In situ x-ray study of the formation of defects in Ge islands on Si(001)

Plastic relaxation and formation of defects is a crucial issue in the epitaxial growth of nanoparticles and thin films. Here, we report an in situ non-destructive method based on x-ray diffuse scattering close to forbidden reflections to study the formation of defects during the growth of Ge islands on Si(001). The dependence of in-plane spacing between interfacial dislocations, defect size, and density is analyzed as a function of Ge-deposit. It is found that the introduction of additional defects is preferred to relieve strain during growth.

A Combined <I>In Situ</I> Grazing Incidence Small Angle X-Ray Scattering and Grazing Incidence X-Ray Diffraction Study of the Growth of Ge Islands on Pit-Patterned Si(001) Substrates

The growth of Ge islands on a pit-patterned Si(001) template is investigated in situ, combining grazing incidence diffraction, multiple wavelength anomalous diffraction, and small angle scattering. This allows monitoring in situ the detailed structural and morphological evolutions of the pits, of the wetting-layer and of the nucleated islands on the pit-patterned Si(001) substrate. It is shown that after Si regrowth, the Si substrate displays {107} and {1 1 11} facets. During the very first stages of Ge growth, the preliminary facets of the Si substrate are energetically unfavourable, and the pit facets break up into a rather complex pattern of {10n} and {11m} facets with n > 7 and m > 11. At 5 and 6 ML, intensity rods from {105} and {113}-type facets appear in the GISAXS images revealing the formation of pyramids and domes, respectively. The degree of ordering, the shape, strain and composition of the islands are characterized during the growth process to provide a detailed evolution of their structure and morphology.

Enhanced Ge/Si(001) island areal density and self-organization due to P predeposition

The predeposition of P, with coverages θP ranging from 0 to 1 ML, on Si(001) significantly increases both the areal density and spatial self-organization of Ge islands grown by gas-source molecular beam epitaxy from hydride precursors. The Ge island density ρGe initially increases with θP, reaching a maximum of 1.4 × 1010 cm−2 at θP = 0.7 ML, a factor of four times higher than on bare Si(001) under the same deposition conditions, before decreasing at higher P coverages. The increase in ρGe(θP) is due to a corresponding decrease in Ge adatom mean free paths resulting from passivation of surface dangling bonds by adsorbed pentavalent P atoms which, in addition, leads to surface roughening and, therefore, higher Ge coverages at constant Ge2H6 dose. As θP (and hence, ρGe) increases, so does the degree of Ge island ordering along 〈100〉 directions due to the anisotropic strain field surrounding individual islands. Similar results are obtained for Ge island growth on P−doped Si(001) layers where strong P surface segregation provides partial monolayer coverage prior to Ge deposition.

Strained single-crystal GOI (Ge on Insulator) arrays by rapid-melting growth from Si (1 1 1) micro-seeds

Liquid-phase epitaxial growth of Ge islands on insulator (GOI) using Ni-imprint-induced Si (1 1 1) micro-crystal seeds (∼1 μm ϕ) is proposed. As a result, single-crystalline GOI (1 1 1) structures with large area (∼10 μm ϕ) are realized. The transmission electron microscopy observations reveal no dislocation or stacking fault in the laterally grown regions. Moreover, the Raman measurements show that the tensile strain (∼0.2%) which enhances the carrier mobility is induced in the growth regions. This new method can be employed to realize the multi-functional SiGe large scale integrated circuits.