A. G. Taboada

Unexpected Dominance of Vertical Dislocations in High-Misfit Ge/Si(001) Films and Their Elimination by Deep Substrate Patterning

An innovative strategy in dislocation analysis, based on comparison between continuous and tessellated film, demonstrates that vertical dislocations, extending straight up to the surface, easily dominate in thick Ge layers on Si(001) substrates. The complete elimination of dislocations is achieved by growing self-aligned and self-limited Ge microcrystals with fully faceted growth fronts, as demonstrated by AFM extensive etch-pit counts.

Perfect crystals grown from imperfect interfaces

The fabrication of advanced devices increasingly requires materials with different properties to be combined in the form of monolithic heterostructures. In practice this means growing epitaxial semiconductor layers on substrates often greatly differing in lattice parameters and thermal expansion coefficients. With increasing layer thickness the relaxation of misfit and thermal strains may cause dislocations, substrate bowing and even layer cracking. Minimizing these drawbacks is therefore essential for heterostructures based on thick layers to be of any use for device fabrication. Here we prove by scanning X-ray nanodiffraction that mismatched Ge crystals epitaxially grown on deeply patterned Si substrates evolve into perfect structures away from the heavily dislocated interface. We show that relaxing thermal and misfit strains result just in lattice bending and tiny crystal tilts. We may thus expect a new concept in which continuous layers are replaced by quasi-continuous crystal arrays to lead to dramatically improved physical properties.

Strain relaxation of GaAs/Ge crystals on patterned Si substrates

We report on the mask-less integration of GaAs crystals several microns in size on patterned Si substrates by metal organic vapor phase epitaxy. The lattice parameter mismatch is bridged by first growing 2-μm-tall intermediate Ge mesas on 8-μm-tall Si pillars by low-energy plasma enhanced chemical vapor deposition. We investigate the morphological evolution of the GaAs crystals towards full pyramids exhibiting energetically stable {111} facets with decreasing Si pillar size. The release of the strain induced by the mismatch of thermal expansion coefficients in the GaAs crystals has been studied by X-ray diffraction and photoluminescence measurements. The strain release mechanism is discussed within the framework of linear elasticity theory by Finite Element Method simulations, based on realistic geometries extracted from scanning electron microscopy images.

GaAs/Ge crystals grown on Si substrates patterned down to the micron scale

Monolithic integration of III-V compounds into high density Si integrated circuits is a key technological challenge for the next generation of optoelectronic devices. In this work, we report on the metal organic vapor phase epitaxy growth of strain-free GaAs crystals on Si substrates patterned down to the micron scale. The differences in thermal expansion coefficient and lattice parameter are adapted by a 2-μm-thick intermediate Ge layer grown by low-energy plasma enhanced chemical vapor deposition. The GaAs crystals evolve during growth towards a pyramidal shape, with lateral facets composed of {111} planes and an apex formed by {137} and (001) surfaces. The influence of the anisotropic GaAs growth kinetics on the final morphology is highlighted by means of scanning and transmission electron microscopy measurements. The effect of the Si pattern geometry, substrate orientation, and crystal aspect ratio on the GaAs structural properties was investigated by means of high resolution X-ray diffraction. The th...

Onset of vertical threading dislocations in Si1-xGex/Si (001) at a critical Ge concentration

We show that the Ge concentration in Si1−xGex alloys grown under strong out-of-equilibrium conditions determines the character of the population of threading dislocations (TDs). Above a critical value x ∼ 0.25 vertical TDs dominate over the common slanted ones. This is demonstrated by exploiting a statistically relevant analysis of TD orientation in micrometer-sized Si1−xGex crystals, deposited on deeply patterned Si(001) substrates. Experiments involving an abrupt change of composition in the middle of the crystals clarify the role of misfit-strain versus chemical composition in favoring the vertical orientation of TDs. A scheme invoking vacancy-mediated climb mechanism is proposed to rationalize the observed behavior.

Three-dimensional Ge/SiGe multiple quantum wells deposited on Si(001) and Si(111) patterned substrates

In this work we address three-dimensional heterojunctions,ndemonstrating that photoluminescence from defect-free, Ge/SiGenmultiple quantum well (MQW) micro-crystals grown on deeplynpatterned Si(001) and Si(111) substrates exhibit similarnradiative intensity and analogous spectral shape.

Heterointegration of InGaAs/GaAs quantum wells on micro-patterned Si substrates

InGaAs/GaAs quantum wells (QWs) grown on μ-patterned Ge/Si substrates by metal organic vapor phase epitaxy are investigated by electron microscopy and spatially resolved photoluminescence (PL) spectroscopy. The lattice parameter mismatch of GaAs and Si is overcome by a Ge buffer layer grown by low-energy plasma enhanced chemical vapor deposition. The GaAs crystals form truncated pyramids whose shape is strongly affected by the geometry of the underlying pattern consisting of 8 μm deep and 3–50 μm wide square Si pillars. Comparing the measured PL energies with calculations performed in the effective mass approximation reveals that the QW emission energies are significantly influenced by the GaAs morphology. It is shown that the geometry favors indium diffusion during growth from the inclined facets towards the top (001) facet. The Si pillar-size dependent release of thermally induced strain observed in the PL measurements is confirmed by X-ray diffraction.

X-Ray Nano-Diffraction on Epitaxial Crystals

The concept of growing epitaxial Ge and SiGe crystals onto tallnSi pillars may provide a means for solving the problemsnassociated with lattice parameter and thermal expansionncoefficient mismatch, i.e., dislocations, wafer bowing andncracks. For carefully tuned epitaxial growth conditions thenlateral expansion of crystals stops once nearest neighbors getnsufficiently close. We have carried out scanningnnano-diffraction experiments at the ID01 beam-line of thenEuropean Synchrotron Radiation Facility (ESRF) in Grenoble onnthe resulting space-filling arrays of micron-sized crystals tonassess their structural properties and crystal quality. Elasticnrelaxation of the thermal strain causes lattice bending closento the Si interface, while the dislocation network isnresponsible for minute tilts of the crystals as a whole. Tonexclude any interference from nearest neighbors, individual Gencrystals were isolated first by chemical etching followed bynmicro-manipulation inside a scanning electron microscope. Thisnpermitted us to scan an X-ray beam, focused to a spot a fewnhundreds of nm in size, along the height of a single crystalnand to record three-dimensional reciprocal space maps at chosennheights. The resolution limited width of the scattered X-raynbeams reveals that the epitaxial structures evolve into perfectnsingle crystals sufficiently far away from the heavilyndislocated interface.

Three dimensional heteroepitaxy: A new path for monolithically integrating mismatched materials with silicon

In the quest for a Ge x-ray detector mono-lithically integrated onto a Si-CMOS chip we developed a novel method for combining dissimilar materials that may provide a solution to the main problems of heteroepitaxy, e.g. high threading dislocation densities, wafer bowing and cracks. It consists of replacing the conventional continuous layers by space-filling arrays of strain- and defect-free Ge crystals, the width, height and shape of which are controlled by tuning epitaxial growth onto micrometer-sized features deeply etched into Si-substrates. Heterojunctions formed between the Ge-crystals and the Si-substrate exhibit the required rectifying diode behavior with low dark currents (<;1 mA/cm2).

Space-Filling Arrays of Three-Dimensional Epitaxial Ge and Si1-xGex Crystals

In this paper, a Si substrate is patterned into high-aspect ratio pillars or ridges by conventional photolithography and deep reactive ion etching, to expose limited areas on which to grow Ge and Si1-xGex layers. Subsequent epitaxial growth by low-energy plasma-enhanced chemical vapor deposition (LEPECVD) takes advantage both of geometric shielding of the growth species arriving at the patterned substrate surface, as well as of growth parameters (e.g. low temperature, high growth rate) designed to limit the surface diffusion length, thus favoring vertical over lateral growth. This results in a uniform space-filling array of three-dimensional epitaxial crystals. The crystalline quality, tilt and strain of the Ge and Si1-xGex crystals are investigated by high resolution X-ray diffraction (XRD) with reciprocal space mapping (RSM) around the Si(004) and Si(224) reflections. The results provided evidence for the nearly perfect crystal structure of the epitaxial material, and showed the crystals grown on the Si pillars to be strain-free. Synchrotron submicron diffraction experiments performed with a focused (~300×500 nm) X-ray beam revealed tilted small tilt of epitaxial Ge crystals with respect to the Si pillars. Faceted crystals with height, size and shape tunable over a wide range by growth and substrate parameters, are shown to be defect-free by transmission electron microscopy and defect etching. The electrical properties of p-i-n heterojunctions between the epitaxial crystals and the Si-substrate and the interplay between surface and volume effects were investigated by in-situ SEM conductivity experiments. The measured I-V characteristics showed clear diode behavior with dark currents of the order of 10-4 A/cm2.