Claudiu V. Falub

Scaling Hetero-Epitaxy from Layers to Three-Dimensional Crystals

Laying It on Thick The growth of one layered material onto a second lies at the heart of many electronic devices. However, if there is a lattice mismatch between the two materials, strains develop in the overgrowth material leading to bowing and cracking. Falub et al. (p. 1330; see the cover) patterned Si substrates into a series of pillars onto which they grew a germanium layer. The germanium initially coated the top of each silicon pillar but then widened as the layer thickened, leading to thick, crack-free germanium films. A space-filling array of self-limited three-dimensional epitaxial crystals averts wafer bowing, layer cracking, and dislocation propagation. Quantum structures made from epitaxial semiconductor layers have revolutionized our understanding of low-dimensional systems and are used for ultrafast transistors, semiconductor lasers, and detectors. Strain induced by different lattice parameters and thermal properties offers additional degrees of freedom for tailoring materials, but often at the expense of dislocation generation, wafer bowing, and cracks. We eliminated these drawbacks by fast, low-temperature epitaxial growth of Ge and SiGe crystals onto micrometer-scale tall pillars etched into Si(001) substrates. Faceted crystals were shown to be strain- and defect-free by x-ray diffraction, electron microscopy, and defect etching. They formed space-filling arrays up to tens of micrometers in height by a mechanism of self-limited lateral growth. The mechanism is explained by reduced surface diffusion and flux shielding by nearest-neighbor crystals.

Interface-roughness-induced broadening of intersubband electroluminescence in p-SiGe and n-GaInAs∕AlInAs quantum-cascade structures

The effect of intrasubband interface roughness scattering on intersubband transition linewidths in double-quantum-well and quantum-cascade (QC) structures is studied. In n-GaInAs∕AlInAs structures, the calculated ratios between the linewidths of the spatially vertical and diagonal transitions agree with the experimental values. In p-Si∕Si0.2Ge0.8 QC structures, the experimentally observed linewidth is a factor of 4–7 smaller than the predicted value. However, by assuming a vertical interface correlation between adjacent interfaces separated by less than ∼1.5nm, the theory reproduces the experiment. Transmission electron microscopy of the SiGe QC sample reveals this vertical correlation, supporting the model.

Tribological behavior of DLC-coated articulating joint implants

Coatings from diamond-like carbon (DLC) have been proven to be an excellent choice for wear reduction in many technical applications. However, for successful adaption to the total joint replacement field, layer performance, stability and adhesion in realistic physiological setups are quintessential and these aspects have not been consistently researched. In our teams efforts to develop long-term stable DLC implant coatings, test results gained from a simplified linear spinal simulator setup are presented. It is shown that metal-on-metal (MoM) pairs perform well up to 7 million loading cycles, after which they start to generate wear volumes in excess of 20 times those of DLC-coated implants. This is attributed to the roughening observed on unprotected metal surfaces. Furthermore, we illustrate that in contrast to DLC-on-DLC, MoM tribopairs require protein-containing media to establish low-friction conditions. Finally, results of defect monitoring during testing are presented, showing catastrophic failure of layers whose interfaces are too weak with respect to the stress-corrosion-cracking mechanism encountered in vivo.

Retrospective lifetime estimation of failed and explanted diamond-like carbon coated hip joint balls

Diamond-like carbon (DLC) coatings are known to have extremely low wear in many technical applications. The application of DLC as a coating has aimed at lowering wear and to preventing wear particle-induced osteolysis in artificial hip joints. In a medical study femoral heads coated with diamond-like amorphous carbon, a subgroup of DLC, articulating against polyethylene cups were implanted between 1993 and 1995. Within 8.5 years about half of the hip joints had to be revised due to aseptic loosening. The explanted femoral heads showed many spots of local coating delamination. Several of these explanted coated TiAlV femoral heads have been analyzed to investigate the reason for this failure. Raman analysis and X-ray photoelectron spectroscopy (XPS) depth profiling showed that the coating consists of diamond-like amorphous carbon, several Si-doped layers and an adhesion-promoting Si interlayer. Focused ion beam (FIB) transverse cuts revealed that the delamination of the coatings is caused by in vivo corrosion of the Si interlayer. Using a delamination test set-up dissolution of the silicon adhesion-promoting interlayer at a speed of more than 100 μm year(-1) was measured in vitro in solutions containing proteins. Although proteins are not directly involved in the corrosion reactions, they can block existing small cracks and crevices under the coating, hindering the exchange of liquid. This results in a build-up of crevice corrosion conditions in the crack, causing a slow dissolution of the Si interlayer.

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.

Hall mobility of narrow Si0.2Ge0.8–Si quantum wells on Si0.5Ge0.5 relaxed buffer substrates

We studied in-plane transport of a two-dimensional hole gas in modulation-doped p-Si0.2Ge0.8 quantum wells (QWs) on Si0.5Ge0.5 relaxed buffer substrates with thicknesses L between 2.5 and 7 nm. We found that interface roughness scattering limits the low-temperature mobility μ of the samples with L between 2.5 and 4.5 nm. The interface roughness parameters were evaluated by fitting the experiment with the calculated μ limited by interface roughness scattering. We found that the obtained parameters were consistent with the values estimated from x-ray reflectivity and the transmission electron micrograph of the samples. When L is increased from 4.5 to 7 nm, μ increases only gradually and the highest μ of 0.44 m2/V s was observed for 7-nm-thick QWs. The scattering by defects, interface charge, and strain fluctuation are discussed as possible additional mobility-limiting mechanisms.

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.

Monolithic integration of optical grade GaAs on Si (001) substrates deeply patterned at a micron scale

Dense arrays of micrometric crystals, with areal filling up to 93%, are obtained by depositing GaAs in a mask-less molecular beam epitaxy process onto Si substrates. The substrates are patterned into tall, micron sized pillars. Faceted high aspect ratio GaAs crystals are achieved by tuning the Ga adatom for short surface diffusion lengths. The crystals exhibit bulk-like optical quality due to defect termination at the sidewalls. Simultaneously, the thermal strain induced by different thermal expansion parameters of GaAs and Si is fully relieved. This opens the route to thick film applications without crack formation and wafer bowing.

SiGe quantum well infrared photodetectors on pseudosubstrate

In the SiGe system, freedom in the design of quantum well (QW) devices is constrained by the 4.2% lattice mismatch between silicon and germanium. The substitution of the Si substrate by a SiGe pseudosubstrate customized to the respective QW structure’s requirements enables the growth of a p-type SiGe QW infrared photodetector featuring interfaces between pure Si and SiGe layers of ultrahigh Ge content for a full exploitation of the band offset between the two materials. Our presented device realizes design concepts for narrowing the spectral response and reducing the noise gain made feasible by the utilization of a Si0.5Ge0.5 pseudosubstrate.