SiGe Nanostructures Research Articles

Self‐assembled Si and SiGe nanostructures: New growth concepts and structural analysis

AbstractSelf‐organization during strained‐layer heteroepitaxy has been a field of intense research in the last few years. Especially the Si/SiGe heterosystem is considered a model system for strain‐driven Stranski–Krastanow growth. In this review article, we concentrate on ordered Stranski–Krastanow growth of Ge and SiGe islands on prepatterned Si(001) substrates, which provide preferential nucleation sites. Perfectly ordered SiGe dot arrays in two and three dimensions are demonstrated, and the basic mechanisms for preferential nucleation in geometrically well‐defined etch pits are discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of Si and Ge low-loss spectra to interpret the Ge contrast in EFTEM images of Si 1− x Ge x nanostructures

Recently, an EFTEM imaging method, exploiting the inelastically scattered electrons in the 60–90 eV energy range, was proposed to visualise Ge in SiGe alloys [Pantel, R., Jullian, S., Delille, D., Dutartre, D., Chantre, A., Kermarrec, O., Campidelli, Y., Kwakman, L.F.T.Z., 2003. Inelastic electron scattering observation using Energy Filtered Transmission Electron Microscopy for silicon-germanium nanostructures imaging. Micron 34, 239–247]. This method was proven to be highly more efficient in terms of noise, drift and exposure time than the imaging of the weak and delayed ionization GeL 2,3 edge at 1236 eV. However, the physical phenomenon behind this Ge contrast was not clearly identified. In this work, we explain the origin of this Ge contrast, by comparing in details EELS low-loss spectra (<100 eV) recorded from pure Si and Ge crystals. High resolved low-loss experiments are performed using analytical Field Emission Gun Transmission Electron Microscopes fitted or not with a monochromator. Low-loss spectra (LLS) are then deconvoluted from elastic/quasi-elastic and plural scattering effects. The deconvolution procedure is established from Si spectra recorded with the monochromated machine. The absence of second plasmon and the measurement of a band gap (1.12 eV) on the Si single scattering distribution (SSD) spectrum allowed us to control the accuracy of the deconvolution procedure at high and low energy and to state that it could be reliably applied to Ge spectra. We show that the Ge-M 4,5 ionisation edge located at 29 eV, which is shadowed by the high second plasmon in the unprocessed Ge spectrum, can be clearly separated in the single scattering spectrum. We also show that the front edge of Ge-M 4,5 is rather sharp which generates a high intensity post edge tail on several tens of eV. Due to this tail, the Si and Ge EELS signals in the 60 to 100 eV energy window are very different and the monitoring of this signal gives information about the Ge concentration inside SiGe alloys. It is now evident that the EFTEM imaging technique proposed to quantify Ge (90 eV/60 eV image ratio) in Si–Ge nanostructures is valid and is a relevant way of exploiting the Ge-M 4-5 ionisation edge.

Strain-induced ordering of small Ge islands in clusters at the surface of multilayered Si–Ge nanostructures

Classical molecular-dynamics simulations based on the Tersoff potential are used to compute at the atomic level the strain-induced potential well generated at the surface of the capping layer by a buried, three-dimensional Ge island on Si(001). A simple model is outlined in order to predict the configurational arrangement for the nucleation of small Ge islands in such a potential well. The theoretical predictions are compared with atomic force microscope images of multilayered SiGe nanostructures grown by chemical vapor deposition. The cluster configuration is shown to be strongly dependent on the capping layer thickness, and to closely mimic the behavior predicted by the model.

Quantitative analysis of electric force microscopy: The role of sample geometry

Quantitative electric force microscopy (EFM) is usually restricted to flat samples, because vertical sample topography traditionally makes quantitative interpretation of EFM data difficult. Many important samples, including self-assembled nanostructures, possess interesting nanoscale electrical properties in addition to complex topography. Here we present techniques for analysis of EFM images of such samples, using voltage modulated EFM augmented by three-dimensional simulations. We demonstrate the effectiveness of these techniques in analyzing EFM images of self-assembled SiGe nanostructures on insulator, report measured dielectric properties, and discuss the limitations sample topography places on quantitative measurement.

Properties of self-organized SiGe nanostructures formed by ion implantation

Properties of self-organized SiGe quantum dots formed for the first time by ion implantation of Ge ions into Si are studied using Auger electron spectroscopy, atomic-force microscopy, and scanning electron microscopy. It is found that a spatially correlated distribution of Ge atoms is observed in Si layers implanted with Ge ions after subsequent annealing of these layers. As a result, nanometer-sized regions enriched with germanium are formed; germanium concentration in these regions is 10–12% higher than that in the surrounding matrix of the SiGe solid solution. Optical properties of the layers with SiGe quantum dots were studied using Raman scattering and photoluminescence. An intense photoluminescence peak is observed in the wavelength region of 1.54–1.58 µm at room temperature.

Field and photo-field electron emission from self-assembled Ge–Si nanostructures with quantum dots

Monolayer and multilayer Ge nanocluster structures were prepared on Si(1 0 0) using molecular beam epitaxy. The cluster size was ⩽10 nm and cluster density was ∼10 10 cm −2. A stable field electron emission was obtained from these structures, showing current peaks in the current–voltage characteristics, which may be attributed to the resonant electron tunneling via the energy levels of the nanocluster potential well. For cluster multilayers, the current–voltage curves also showed current peaks with a complex shape. The cluster multilayer structures had a considerable temperature sensitivity, as well as photosensitivity, in the wavelength range from 0.4 to 10 μm.

Inelastic electron scattering observation using energy filtered transmission electron microscopy for silicon–germanium nanostructures imaging

This paper presents a new technique using energy filtered TEM (EFTEM) for inelastic electron scattering contrast imaging of Germanium distribution in Si–SiGe nanostructures. Comparing electron energy loss spectra (EELS) obtained in both SiGe and Si single crystals, we found a spectrum area strongly sensitive to the presence of Ge in the range [50–100eV]. In this energy loss window, EELS spectrum shows a smooth steeply shaped background strongly depending on Ge concentration. Germanium mapping inside SiGe can thus be performed through imaging of the EELS background slope variation, obtained by processing the ratio of two energy filtered TEM images, respectively, acquired at 90 and 60eV. This technique gives contrasted images strongly similar to those obtained using STEM Z-contrast, but presenting some advantages: elastic interaction (diffraction) is eliminated, and contrast is insensitive to polycrystalline grains orientation or specimen thickness. Moreover, since the extracted signal is a spectral signature (inelastic energy loss) we demonstrate that it can be used for observation and quantification of Ge concentration depth profile of SiGe buried layers.

Absorption measurement of strained SiGe nanostructures deposited by UHV-CVD

In order to obtain a low band gap photocell based on the widely spread silicon technology, e.g. for thermophotovoltaics, SiGe nanostructures can be introduced into a monocrystalline silicon photocell. Beforehand, it is necessary to know the absorption coefficient of the SiGe quantum wells. On a silicon (1 0 0) substrate multiple Si/SiGe quantum well structures were grown by UHV-CVD. The Ge concentration and the well width were used as growth parameters. To obtain significant absorption, the experiment was set up to allow for 200 internal reflections. The total reflection of the light results in a standing electromagnetic wave. The absorption coefficient was obtained from the experimental data taking the geometry and the electric field distribution in the absorbing layer into account. The influence of well width and germanium content on the absorption was investigated with the goal of maximizing the absorption for photons with energies below the band gap energy of silicon. The measurement results are compared with a theoretical model, which takes the band structure of strained SiGe including confinement effects into account.

Field electron emission from Ge-Si nanostructures with quantum dots

Self-assembled arrays of Ge-Si clusters with sizes of ∼ 10 nm and a density of ∼ 1010 cm−2 have been grown by molecular beam epitaxy. Stable steady-state field electron emission from such clusters has been observed and studied. The emission is characterized by resonance current peaks, which are explained by the quantization of the electron energy in nanoclusters. The estimation of the ground level energy from their emission measurements coincides with estimates obtained by other methods.

SiGe nanostructures: new insights into growth processes

During the last decade, Si/Si1−xGex heterostructures have emerged as aviable system for use in CMOS technology with the recent industrialproduction of heterojunction bipolar transistor-based integrated circuits.However, many key problems have to be solved to further expand thecapabilities of this system to other more attractive devices. This papergives a comprehensive review of the progress achieved during the last fewyears in the understanding of some fundamental growth mechanisms. Thediscrepancies between classical theories (in the framework of continuum elasticity)and experimental results are also specially addressed. In particular, themajor role played by kinetics in the morphological evolution of layersis particularly emphasized. Starting from the unexpected differences inSi1−xGex morphological evolution when deposited on (001) and on (111), ourreview then focuses on: (1) the strain control and adjustment (from fullystrained to fully relaxed 2D and 3D nanostructures)—in particular, someoriginal examples of local CBED stress measurements are presented; (2) thenucleation, growth, and self-assembly processes, using self-patterned templatelayers and surfactant-mediated growth; (3) the doping processes (usingB for type p and Sb for type n) and the limitations induced by dopantredistribution during and after growth due to diffusion, segregation, anddesorption. The final section will briefly address some relevant opticalproperties of Si1−xGex strained layers using special growth processes.

Effect of the interface on the local structure of Ge–Si nanostructures

We first discuss the limitations of Raman scattering as applied to Ge/Si nanostructures. We further summarize our recent efforts to investigate the local structure of various Ge nanostructures, namely, Ge quantum dots grown by molecular beam epitaxy (MBE) on bare Si(100), on Si(111) with a 0.3 nm SiO2 coverage, and nanocrystals embedded in SiO2, by x-ray absorption fine structure spectroscopy. In particular, the MBE growth of Ge dots on bare Si(100) has been studied as a function of the growth conditions: in most cases strong alloying with Si takes place. Ge nanoislands on Si(111) with SiO2 coverage, on the other hand, may retain the local structure of bulk Ge and be very stable against oxidation. The Ge nanocrystals embedded in SiO2 possess the structure of relaxed bulk Ge without any Ge–Si bonding. The latter two kinds of Ge nanostructures possess visible photoluminescence.

Silicon Epitaxial Regrowth Passivation of SiGe Nanostructures Pattered by AFM Oxidation

ABSTRACTSiGe quantum devices were demonstrated by AFM oxidation and selective wet etching with features size down to 50 nm. To passivate the devices and eliminate the interface states between Si/SiO2, low temperature regrowth of epitaxial silicon over strained SiGe has been tested. The silicon regrowth on Si0.8Ge0.2 was done by rapid thermal chemical vapor deposition (RTCVD) at 700 °C using a hydrogen pre-cleaning process at 800 °C and 10 torr. SIMS analysis and photoluminescence (PL) of strained SiGe capped with epitaxial regrown silicon show a clean interface. Nano-gaps between doped SiGe filled and overgrown with epitaxial silicon show an electrical insulating property at 4.2 K.

Ordering and Electronic Properties of Self-Assembled Si/Ge Quantum Dots

We have combined Ge island nucleation during molecular beam epitaxy in the Stranski-Krastanow growth mode with step bunching in Si/SiGe multilayers on vicinal Si substrates in order to realize self-ordered arrays of nanostructures. Surface steps and the local strain fields surrounding strain-relaxed SiGe nanostructures influence the island nucleation and result in spatially ordered stable arrays of Ge dots. The controlled formation of vertically correlated stacks of wires, dots or dots on wires by such self-ordering processes offer novel functionality for future applications like infrared detectors and memories. The fundamental electronic and optical properties of self-assembled Ge dots embedded in Si are analyzed by photoluminescence, admittance and intra valence band photocurrent spectroscopy.

SiGe nanostructures by selective epitaxy and self-assembling

There is increasing interest in Si-based optoelectronics using Si 1− x Ge x nanostructures due to the possibility of their integration with the Si technology. To overcome the problem of the indirect character of SiGe one is looking for solutions to improve the carrier confinement and collection efficiency by realizing structures involving quantum-size effects. Several fabrication strategies for semiconductor nanostructures have been proposed. One possible approach involves selective epitaxy to fill-in the small holes in patterned substrates. To realize the lateral confinement below 100 nm the patterned substrates are made either by e-beam lithography or by optical lithography. In the latter case, the sub-100 nm confinement is realized using the development of facets. Another approach for nanostructures is based on self-assembling which leads to island formation in highly lattice-mismatched layers. These items will be discussed for SiGe as well as device applications such as light emitting diodes with Ge islands.

Fabrication of semiconductor nanostructures by nanoindentation of photoresist layers using atomic force microscopy

Fabrication of semiconductor nanostructures by atomic force microscopy (AFM) is investigated. In our processing scheme, ultrathin photoresist layers are mechanically modified by an AFM tip with a subsequent pattern transfer by reactive ion or wet chemical etching. In combination with conventional optical lithography, this method allows the selective modification of micrometer scale patterns. Several parameters influence the size and shape of the nanostructures as well as the reliability of the process: the shape and the material of the tip, the thickness and hardness of the resist layer, the force applied to the tip, and the corrections of the scanner nonlinearities. Pattern transfer by reactive ion etching enforces a compromise between etch depth and resolution: Thinner resist layers allow the fabrication of smaller structures but inhibit a deep pattern transfer due to the limited resistance of the photoresist to sputtering. Application of our mask-based lithographic process for the fabrication of Si, SiGe, and GaAs nanostructures is demonstrated.

Self-assembling SiGe and SiGeC nanostructures for light emitters and tunneling diodes

We report structural and photoluminescence (PL) investigations of self-assembling typically 100 nm lateral size Ge islands and 10 nm size C-induced Ge quantum dots in Si. The different recombination pathways are discussed for single and stacked dot layers. The aim is to evaluate different ways of using self-assembling quantum structures for Si-based emitters in the wave length range of 1.3 to 1.55 μm. The pre-growth of a small fraction of a monolayer C on Si(100) introduces nucleation centers for the subsequent formation of Ge islands which results in the formation of a high density of extremely small 10 nm size quantum dots. PL-studies indicate a spatially indirect radiative recombination mechanism with the no-phonon line strongly dominating. For larger stacked Ge islands with 13 nm thin Si spacer layers, we observe a significantly enhanced Ge dot-related PL signal up to room temperature at 1.55 μm wave length. This is attributed to a spatially indirect transition between heavy holes confined within the compressively strained Ge dots and two-fold degenerated Δ state electrons in the tensile strained Si spacer layers between the stacked Ge dots. In the second part we present the preparation and I–V characteristics of Si/SiGe/Si p+/i/n+ Esaki diodes. The incorporation of a maximum amount of Ge within the i-zone increases the interband tunneling probability and thus, gives rise to an increased peak current density of about 3 kA/cm2 and a peak to valley current ratio of 4.2 at room temperature. The further increase of Ge content by incorporation of self assembling Ge islands within the intrinsic cone of the diode is discussed.

Step bunching and correlated SiGe nanostructures on Si(113)

We have studied the step bunching properties of Si and SiGe layers on vicinal Si(113) substrates and the self-organized formation and spatial correlation of SiGe wire and island structures induced by such surface steps. The parameters for MBE growth have been varied systematically in order to investigate the self-assembling and self-ordering mechanisms appearing in growth of strained SiGe layers on vicinal surfaces. The nanostructures are analyzed by atomic force microscopy (AFM), transmission electron microscopy (TEM) and micro Raman spectroscopy. Si(113) substrates have a low surface energy which is comparable to Si(001) but show a much stronger tendency for bunching of steps on vicinal surfaces. The Si(113) surface reveals step bunches which are mainly multiples of a 4-atomic step unit. Periodic arrays of SiGe wires form by strain-mediated self-organization during deposition of a SiGe/Si multilayer. The correlation of wires in different layers is inclined with respect to the growth direction. This is well described by a model calculation assuming the Si surface steps to locate at the maxima of local strain. Ge islands nucleate preferentially at the step edges of vicinal Si(113) substrates and form ordered rows of islands. No Ge islands but wire-like Ge accumulations are observed at the step edges of the step bunched SiGe/Si multilayer buffers.

Self-organized periodic arrays of SiGe wires and Ge islands on vicinal Si substrates

We have investigated the properties of SiGe nanostructures fabricated by applying self-assembling and self-ordering processes during molecular beam epitaxy on vicinal Si surfaces without any lithographic patterning. Experimental parameters like growth temperature, orientation of the substrate surface versus (001) or (113), SiGe/Si multilayer buffer parameters, Ge content and Ge coverage have been varied systematically. The structures were analyzed by atomic force microscopy and transmission electron microscopy. During SiGe/Si multilayer deposition, surface steps on vicinal Si can develop to very regular, periodically arranged step bunches that serve as templates for the formation of SiGe wires. The spatial correlation of wires in different layers is either vertical or inclined, depending on sample surface, Ge content and multilayer period. Ge deposition on such a surface with periodical steps and strain domains results in nucleation of regular Ge wires, individual pearl strings of Ge islands, or two-dimensional close-packed arrays of Ge islands, depending on the buffer parameters. The experimental results demonstrate that different pathways of strain relaxation in SiGe nanostructures can be chosen by adequate growth parameters in order to control the nucleation of islands or wires, their lateral and vertical correlation, their size, and their shape.

Self-organization of nanostructures on Si wafers using surface structure control

The final goal of nanofabrication on Si wafers is the high-density integration of atomically controlled structures on the wafer scale. This paper describes a scenario for nanointegration through the control of the self-organization processes on the Si surfaces. It is demonstrated here that atomic steps on Si(111) surfaces can be regularly rearranged by using a patterning-assisted control and that Ge quantum-dot networks can be self-ordered by controlling the surface structures. Interface reaction controls to equip Si Ge nanostructures with buried oxide and silicide layers required for device applications are presented. These techniques form the basis of a new approach for nanostructure integration on the wafer scale.

Growth of epitaxial SiGe nanostructures at low temperature on Si(100) using hot-wire assisted gas source molecular beam epitaxy

Ge/Si and Si 1− x Ge x nanostructures were grown on Si(100) at 350 °C substrate temperature with a significant growth rate of about 10 Å min −1 using hot-wire decomposition of disilane and germane in an ultrahigh vacuum environment. A quartz crystal microbalance (QCM) was used to monitor growth rates and to study the influence of feed gas pressures. In-situ X-ray photoelectron spectroscopy measurements allowed us to calibrate alloy stoichiometry monitored by QCM. Ex-situ high-resolution transmission electron microscopy observations demonstrated the 2D epitaxial growth of partly relaxed nanolayers. These promising results may be closely related to a surfactant-like role of atomic hydrogen, which appears as a beneficial sub-product of the hot-wire decomposition process of hydride sources. © 1997 Elsevier Science S.A.