Growth of Si and Ge nanostructures on Si substrates using ultrathin SiO/sub 2/ technology

Using scanning reflection electron microscopy and a high-temperature scanning tunneling microscopy (STM), we study the growth processes of Si and Ge nanostructures on Si substrates covered with ultrathin SiO/sub 2/ films. Si windows are formed in the ultrathin SiO/sub 2/ films by irradiating focused electron beams used for SREM or field emission electron beams from STM tips before or during heating samples. Ge nanoislands are grown only at the Si window positions by depositing Ge on the samples and by subsequent annealing of them. Moreover, Ge nanoislands about 7 nm in size and ultrahigh density (>10/sup 12//cm/sup 2/) are grown on the ultrathin SiO/sub 2/ films. These nanoislands can be manipulated by STM when they are separated from Si substrates by the ultrathin SiO/sub 2/ films. Si, Ge, Ge/Si and Si/Ge/Si nanoislands can also be grown on the Si windows by selective growth using Si/sub 2/H/sub 6/ and GeH/sub 4/ gases. These nanoislands are found to be stable on the Si windows during high-temperature annealing. These results indicate that ultrathin SiO/sub 2/ technology is useful for growing Si and Ge nanostructures on given areas.     

Resonance raman scattering in Ge nanoislands grown on a Si(111) substrate coated with an ultrathin SiO<Subscript>2</Subscript> layer

Germanium nanoislands formed on a Si (111) surface coated with an ultrathin oxide layer were investigated by Raman spectroscopy. For analysis of the experimental data, the spectra of real islands containing some hundreds of Ge atoms were calculated numerically. The effects of the resonance enhancement of the intensity of Raman scattering in the Ge-nanoisland-SiO₂-Si system and the influence of the lateral sizes of nanoislands on the frequencies of phonons localized in them are discussed.     

Controllable growth of GeSi nanostructures by molecular beam epitaxy

We present an overview on the recent progress achieved on the controllable growth of diverse GeSi alloy nanostructures by molecular beam epitaxy. Prevailing theories for controlled growth of Ge nanostructures on patterned as well as inclined Si surfaces are outlined firstly, followed by reviews on the preferential growth of Ge nanoislands on patterned Si substrates, Ge nanowires and high density nanoislands grown on inclined Si surfaces, and the readily tunable Ge nanostructures on Si nanopillars. Ge nanostructures with controlled geometries, spatial distributions and densities, including two-dimensional ordered nanoislands, three-dimensional ordered quantum dot crystals, ordered nanorings, coupled quantum dot molecules, ordered nanowires and nanopillar alloys, are discussed in detail. A single Ge quantum dot-photonic crystal microcavity coupled optical emission device demonstration fabricated by using the preferentially grown Ge nanoisland technique is also introduced. Finally, we summarize the current technology status with a look at the future development trends and application challenges for controllable growth of Ge nanostructures.     

The correlation between the surface-energy minima and the shape of self-induced SiGe nanoislands

The effects of the composition, size, and thermal expansion coefficient of self-induced Ge and SiGe nanoislands formed on Si on the value of the islands’ total energy are examined. A correlation between the discrete minima in the surface energy of the islands and their shape is considered. The interdiffusion processes that are important at high temperatures of epitaxy are taken into account. The results of calculations are compared with experimental data obtained using atomic-force microscopy.     

Causes of the stability of three-bilayer islands and steps on a Si (111) surface

The initial stages of growth of Ge and Si layers on a singular Si (111) surface result in an unusual morphology of the growth surface if the layers are deposited at a low rate; i.e., triangular islands with a height of as much as three atomic layers are formed. A simulation based on the Monte Carlo method has been used to show that an additional barrier with a height of 0.5–0.6 eV, serving to incorporate atoms into dimerized bonds at the edges of the triangular islands, brings about enhanced growth of the islands in relation to their height and a change in the triangles’ orientation. According to the suggested hypothesis, the increase in the islands’ height and the limitation of their height to three bilayers are due to the effect of the edge dimers, whose orientation changes when the height of a step perpendicular to the \(\langle \bar 1\bar 12\rangle \) direction becomes as large as three bilayers. Scanning tunneling microscopy has been used to detect new special features in the atomic structure of regular three-bilayer steps on a Si (557) surface. The results of an analysis of the images obtained using a scanning tunneling microscope confirm the hypothesis that a row of dimers is formed at the edge of a three-bilayer step.     

Photoluminescence line width of self-assembled Ge(Si) islands arranged between strained Si layers

The effect of variations in the strained Si layer thicknesses, measurement temperature, and optical excitation power on the width of the photoluminescence line produced by self-assembled Ge(Si) nanoislands, which are grown on relaxed SiGe/Si(001) buffer layers and arranged between strained Si layers, is studied. It is shown that the width of the photoluminescence line related to the Ge(Si) islands can be decreased or increased by varying the thickness of strained Si layers lying above and under the islands. A decrease in the width of the photoluminescence line of the Ge(Si) islands to widths comparable with the width of the photoluminescence line of quantum dot (QD) structures based on direct-gap InAs/GaAs semiconductors is attained with consideration of diffusive smearing of the strained Si layer lying above the islands.     

Atomic-force-microscopy visualization of GeSi buried nanoislands on crystal cleavages in silicon structures

The topographic characteristics of cleaved surfaces in Ge/Si structures containing buried layers of GeSi nanoislands were investigated by atomic-force microscopy in atmospheric air. The relaxation of elastic stresses in islands and in adjacent regions of the Si matrix on the free cleaved surface was shown to result in local topographic features on the cleavages of the structures. It was found that the islands can appear on the cleaved surface in the form of two types of topographic features: as hillocks, if the cleavage plane directly crosses an island elastically compressed in the Si matrix, or as a pit, if the cleavage plane crosses the Si-matrix region adjacent to the island and subjected to tensile stress. The investigations performed showed the potential of the new method of studying buried nanoislands for revealing their presence; estimating their sizes, size distribution, and interaction effects in multilayer structures; and also for revealing strains associated with them.     

Silicon-germanium nanostructures with quantum dots: Formation mechanisms and electrical properties

The generally accepted notions about the formation mechanisms for germanium islands with nanometer-scale sizes in a Ge-on-Si system are reviewed on the basis of analysis of recent publications. The presence of elastic strains in the epilayers and in the three-dimensional Ge islands on Si is a key factor that not only initiates a morphological transition from a planar film to an island-containing film (the Stranski-Krastanov mechanism) but also influences the subsequent stages of the islands’ evolution, including their shape, size, and spatial distribution. In many cases, this factor modifies appreciably the classical mechanisms of phase-formation and their sequence up to the quasi-equilibrium coexistence of three-dimensional Ge nanoislands at the surface of the Si substrate. The methods for improving the degree of the ordering of nanoislands to attain the smallest possible sizes and large density of areal distribution of these islands are discussed. The published data on optical absorption in the multilayered Ge-Si systems with quantum dots are considered; these data are indicative of an anomalously large cross section of intraband absorption, which makes this class of nanostructures promising for the development of photodetectors of the infrared region of the spectrum. The results of original studies of electrical and optical properties of heterostructures that involve Ge quantum dots and are synthesized by molecular-beam epitaxy on the Si substrates are reported.     

Comparative analysis of radiation effects on the electroluminescence of Si and SiGe/Si(001) heterostructures with self-assembled Islands

The effect of neutron radiation on the electroluminescence of the Si p-i-n diode containing a multilayered Ge/Si heterostructure with self-assembled nanoislands is studied. In comparison with bulk Si, the diodes containing Ge(Si) nanoislands exhibit a higher radiation hardness of the electroluminescence signal, which is attributed to spatial localization of charge carriers in the Ge/Si nanostructures. The spatial localization of charge carriers impedes their diffusion to radiation defects followed by nonradiative recombination at the defects. The results show the possibilities of using Ge/Si heterostructures with self-assembled nanoislands for the development of optoelectronic devices resistant to radiation.     

The nucleation of coherent semiconductor islands during the Stranski-Krastanov growth induced by elastic strains

The Ge island growth on the Si(100) and Si(111) surfaces was investigated through spectral ellipsometry in real time. It is found that both cases correspond to Stranski-Krastanov growth; i.e., a Ge wetting layer is initially formed, and only then do the islands of the new phase grow on the surface of this layer. However, the island nucleation on the (100) surface is accompanied by a substantial decrease in the wetting layer thickness, whereas, on the (111) surface, the islands nucleate and grow on the wetting layer of constant thickness. The Ge atoms on the (100) surface transfer from the wetting layer to islands, thus substantially decreasing the elastic energy of the system, but increasing the surface energy. For this reason, it is concluded that, in this case, it is the elastic energy which represents the fundamental driving force of the island nucleation. Thermodynamic and kinetic theories of island nucleation from the wetting layer under the effect of elastic energy are developed. A new notion of overstress is introduced by analogy with supersaturation and overcooling. The time evolution of the wetting layer thickness, the nucleation rate, and the island surface density of the new phase is described. The theoretical results are compared to experimental data obtained through ellipsometric simulation, and it is found that the theory and experiment are in good agreement.     

Study of initial stages of Pb growth on the Si (7710) surface by the method of scanning tunneling spectroscopy

Lead island growth on the Si (7710) surface containing steps three interplanar spacings d ₍₁₁₁₎ in height and on the Si (111) singular surface was studied by scanning tunneling microscopy at room temperature. It is shown that triple steps control the shape and orientation of lead islands formed on the Si (7710) surface. It was found that preliminary storage of a silicon sample in a vacuum chamber affects the size and density of growing islands. According to the Auger electron spectroscopy data, this effect is caused by oxygen adsorption on the substrate surface from the residual atmosphere of the vacuum chamber.     

Control of island formation on silicon surfaces using ultra-high-vacuum scanning electron microscopy

In situ scanning electron microscopy has been used to control Au island formation on a patterned Si(111) surface with a periodic array of atomic-step bunches and holes. Liquid phase Au-Si islands were observed to redistribute on the patterned surface by annealing. The islands accumulate at a particular position of the step bunch in each pattern unit. This phenomenon is interpreted in terms of the energetic stability of a droplet on a patterned surface.     

Effect of irradiation on the luminescence properties of low-dimensional SiGe/Si(001) heterostructures

This study is concerned with the effect of irradiation on the luminescence properties of low-dimensional Si/Ge heterostructures with different degrees of spatial localization of charge carriers. It is shown that the radiation stability of Si/Ge heterostructures is improved with increasing efficiency of localization of charge carriers in the structures. The spatial localization of charge carriers in the SiGe nanostructures decreases the probability of nonradiative recombination of charge carriers at radiation defects produced in the Si matrix. It is demonstrated that, among the structures explored in the study, the highest radiation stability of luminescence properties is inherent in the multilayered structures containing self-assembled Ge(Si) nanoislands, in which the most efficient spatial localization of charge carriers is attained. In this case, the localization is three- and two-dimensional, correspondingly, for holes in the islands and for electrons in the Si layers that separate neighboring layers containing the islands.     

Si/Ge异质结构的生长与特性

<正> 近年来,由Ge,Si这两种晶格失配(4.2％)材料组成的Ge/Si异质结构与Ge/Si应变超晶格,由于共具有重要的科学与技术价值,受到了人们高度重视。分子束外延(MBE)技术已成功地用于生长Ge/Si异质结构和超薄Ge/Si超晶格以及GexSi1-x/Si超晶格。化学汽相淀积技术(CVD)与MBE相比,除了设备简单、价格便宜外,对于Ge,Si外延来说,还便于与现有的硅大规模集成工艺技术相结合,使之更富有实用性。近期已有报道用CVD技术生长GexSi1-x/Si异质结构。     

Interaction of self-assembled Ge islands and adjacent Si layers grown on unpatterned and patterned Si(001) substrates

For use in electronic devices, self-assembled Ge islands formed on Si(001) must be covered with an additional Si layer. Chemically vapor deposited Si layers initially grow very rapidly over Ge islands because of the catalytic effect of Ge on the reaction of the Si-containing gas. The edges of the Si features covering Ge “pyramids” are rotated by 45° with respect to the edges of the Ge pyramids because of the different mechanisms orienting the Ge islands and the Si features. When multiple layers of islands are formed, the in-plane ordering of the Ge islands depends on the thickness of the Si interlayer separating the island layers. When selective Si is grown on a patterned Si wafer to form the underlying structure for the Ge islands, the position of the islands is influenced by the detailed shape of the Si near the edges, which in turn depends on the thickness of the selectively deposited Si, the pattern size, and the amount of surrounding oxide.     

Enhancement of the Luminescence Signal from Self-Assembled Ge(Si) Nanoislands due to Interaction with the Modes of Two-Dimensional Photonic Crystals

Semiconductors - The results of studies of the luminescence properties of two-dimensional photonic crystals formed on the basis of silicon structures with self-assembled Ge(Si) nanoislands are...     

Investigation of the local density of states in self-assembled GeSi/Si(001) nanoislands by combined scanning tunneling and atomic-force microscopy

The local density of states in self-assembled GeSi/Si(001) nanoislands is investigated for the first time by combined tunneling and atomic-force scanning microscopy. Current images and tunneling spectra of individual GeSi/Si(001) islands are obtained. These measurements yield the spatial and energy distributions of the local density of states in GeSi islands, respectively. The tunneling spectroscopy data demonstrate that uncapped Ge_0.3Si_0.7/Si(001) islands behave as type-I heterostructures.     

Suppression of dome-shaped clusters during molecular beam epitaxy of Ge on Si(100)

Morphological properties of Ge nanoscale island arrays formed on the Si(100) surface during molecular beam epitaxy are studied using reflection high-energy electron diffraction and atomic-force microscopy. It is shown that codeposition of Sb and Ge significantly increases the density of final island arrays and suppresses the formation of dome-shaped clusters at substrate temperatures of 550–600°C. The results are discussed in the context of the kinetic model of formation of elastically strained islands in heteroepitaxial systems with a lattice mismatch.     

Study of the I–V characteristics of nanostructured Pd films on a Si substrate after vacuum annealing

The I–V characteristics of nanostructured Pd films on a Si substrate are investigated. The nanostructures (nanoislands) are formed by the vacuum annealing of continuous ultrathin Pd films sputtered onto a substrate. The shape of the I–V characteristics of the investigated Si substrate-Pd film system is shown to be heavily dependent on the degree of film nanostructuring. The surface morphology of the films is studied using scanning electron microscopy.     

Electroluminescence from MIS silicon-based light emitters with arrays of self-assembled Ge(Si) nanoislands

We report on the electroluminescence from silicon-based metal–insulator–semiconductor (MIS) diodes with arrays of self-assembled Ge(Si) nanoislands. Aluminum oxide (Al₂O₃) is used as an insulator material in the MIS contact. Variations in the electroluminescence spectra caused by changing the metal work function are examined. The intense electroluminescence from Ge(Si) nanoislands localized at a distance of 50 nm from the insulator–semiconductor interface is observed at room temperature. The emission spectrum is found to be controlled by choosing the design of the semiconductor structure and the barrier height for injected carriers.