Photoluminescence from GeSn nano-heterostructures.

We investigate the distribution of Sn in GeSn nano-heteroepitaxial clusters deposited at temperatures well exceeding the eutectic temperature of the GeSn system. The 600 °C molecular beam epitaxy on Si-patterned substrates results in the selective growth of GeSn nano-clusters having a 1.4 ± 0.5 at% Sn content. These nano-clusters feature Sn droplets on their faceted surfaces. The subsequent deposition of a thin Ge cap layer induced the incorporation of the Sn atoms segregated on the surface in a thin layer wetting the nano-dots surface with 8 ± 0.5 at% Sn. The presence of this wetting layer is associated with a relatively strong photoluminescence emission that we attribute to the direct recombination occurring in the GeSn nano-dots outer region.

Advanced GeSn/SiGeSn Group IV Heterostructure Lasers.

Growth and characterization of advanced group IV semiconductor materials with CMOS-compatible applications are demonstrated, both in photonics. The investigated GeSn/SiGeSn heterostructures combine direct bandgap GeSn active layers with indirect gap ternary SiGeSn claddings, a design proven its worth already decades ago in the III-V material system. Different types of double heterostructures and multi-quantum wells (MQWs) are epitaxially grown with varying well thicknesses and barriers. The retaining high material quality of those complex structures is probed by advanced characterization methods, such as atom probe tomography and dark-field electron holography to extract composition parameters and strain, used further for band structure calculations. Special emphasis is put on the impact of carrier confinement and quantization effects, evaluated by photoluminescence and validated by theoretical calculations. As shown, particularly MQW heterostructures promise the highest potential for efficient next generation complementary metal-oxide-semiconductor (CMOS)-compatible group IV lasers.

A self-ordered, body-centered tetragonal superlattice of SiGe nanodot growth by reduced pressure CVD.

Self-ordered three-dimensional body-centered tetragonal (BCT) SiGe nanodot structures are fabricated by depositing SiGe/Si superlattice layer stacks using reduced pressure chemical vapor deposition. For high enough Ge content in the island (>30%) and deposition temperature of the Si spacer layers (T > 700 °C), we observe the formation of an ordered array with islands arranged in staggered position in adjacent layers. The in plane periodicity of the islands can be selected by a suitable choice of the annealing temperature before the Si spacer layer growth and of the SiGe dot volume, while only a weak influence of the Ge concentration is observed. Phase-field simulations are used to clarify the driving force determining the observed BCT ordering, shedding light on the competition between heteroepitaxial strain and surface-energy minimization in the presence of a non-negligible surface roughness.

Structural and optical characterization of GaAs nano-crystals selectively grown on Si nano-tips by MOVPE.

We present the nanoheteroepitaxial growth of gallium arsenide (GaAs) on nano-patterned silicon (Si) (001) substrates fabricated using a CMOS technology compatible process. The selective growth of GaAs nano-crystals (NCs) was achieved at 570 °C by MOVPE. A detailed structure and defect characterization study of the grown nano-heterostructures was performed using scanning transmission electron microscopy, x-ray diffraction, micro-Raman, and micro-photoluminescence (µ-PL) spectroscopy. The results show single-crystalline, nearly relaxed GaAs NCs on top of slightly, by the SiO2-mask compressively strained Si nano-tips (NTs). Given the limited contact area, GaAs/Si nanostructures benefit from limited intermixing in contrast to planar GaAs films on Si. Even though a few growth defects (e.g. stacking faults, micro/nano-twins, etc) especially located at the GaAs/Si interface region were detected, the nanoheterostructures show intensive light emission, as investigated by µ-PL spectroscopy. Achieving well-ordered high quality GaAs NCs on Si NTs may provide opportunities for superior electronic, photonic, or photovoltaic device performances integrated on the silicon technology platform.

Bi-modal nanoheteroepitaxy of GaAs on Si by metal organic vapor phase epitaxy.

Nano-heteroepitaxial growth of GaAs on Si(001) by metal organic vapor phase epitaxy was investigated to study emerging materials phenomena on the nano-scale of III-V/Si interaction. Arrays of Si nano-tips (NTs) embedded in a SiO2 matrix were used as substrates. The NTs had top Si openings of 50-90 nm serving as seeds for the selective growth of GaAs nano-crystals (NCs). The structural and morphological properties were investigated by high resolution scanning electron microscopy, atomic force microscopy, electron backscatter diffraction, x-ray diffraction, and high resolution scanning transmission electron microscopy. The GaAs growth led to epitaxial NCs featuring a bi-modal distribution of size and morphology. NCs of small size exhibited high structural quality and well-defined {111}-{100} faceting. Larger clusters had less regular shapes and contained twins. The present work shows that the growth of high quality GaAs NCs on Si NTs is feasible and can provide an alternate way to the integration of compound semiconductors with Si micro- and opto-electronics technology.

Selective Epitaxy of InP on Si and Rectification in Graphene/InP/Si Hybrid Structure.

The epitaxial integration of highly heterogeneous material systems with silicon (Si) is a central topic in (opto-)electronics owing to device applications. InP could open new avenues for the realization of novel devices such as high-mobility transistors in next-generation CMOS or efficient lasers in Si photonics circuitry. However, the InP/Si heteroepitaxy is highly challenging due to the lattice (∼8%), thermal expansion mismatch (∼84%), and the different lattice symmetries. Here, we demonstrate the growth of InP nanocrystals showing high structural quality and excellent optoelectronic properties on Si. Our CMOS-compatible innovative approach exploits the selective epitaxy of InP nanocrystals on Si nanometric seeds obtained by the opening of lattice-arranged Si nanotips embedded in a SiO2 matrix. A graphene/InP/Si-tip heterostructure was realized on obtained materials, revealing rectifying behavior and promising photodetection. This work presents a significant advance toward the monolithic integration of graphene/III-V based hybrid devices onto the mainstream Si technology platform.

Reduced-Pressure Chemical Vapor Deposition Growth of Isolated Ge Crystals and Suspended Layers on Micrometric Si Pillars.

In this work, we demonstrate the growth of Ge crystals and suspended continuous layers on Si(001) substrates deeply patterned in high aspect-ratio pillars. The material deposition was carried out in a commercial reduced-pressure chemical vapor deposition reactor, thus extending the "vertical-heteroepitaxy" technique developed by using the peculiar low-energy plasma-enhanced chemical vapor deposition reactor, to widely available epitaxial tools. The growth process was thoroughly analyzed, from the formation of small initial seeds to the final coalescence into a continuous suspended layer, by means of scanning and transmission electron microscopy, X-ray diffraction, and µ-Raman spectroscopy. The preoxidation of the Si pillar sidewalls and the addition of hydrochloric gas in the reactants proved to be key to achieve highly selective Ge growth on the pillars top only, which, in turn, is needed to promote the formation of a continuous Ge layer. Thanks to continuum growth models, we were able to single out the different roles played by thermodynamics and kinetics in the deposition dynamics. We believe that our findings will open the way to the low-cost realization of tens of micrometers thick heteroepitaxial layer (e.g., Ge, SiC, and GaAs) on Si having high crystal quality.

Material insights of HfO2-based integrated 1-transistor-1-resistor resistive random access memory devices processed by batch atomic layer deposition.

With the continuous scaling of resistive random access memory (RRAM) devices, in-depth understanding of the physical mechanism and the material issues, particularly by directly studying integrated cells, become more and more important to further improve the device performances. In this work, HfO2-based integrated 1-transistor-1-resistor (1T1R) RRAM devices were processed in a standard 0.25 µm complementary-metal-oxide-semiconductor (CMOS) process line, using a batch atomic layer deposition (ALD) tool, which is particularly designed for mass production. We demonstrate a systematic study on TiN/Ti/HfO2/TiN/Si RRAM devices to correlate key material factors (nano-crystallites and carbon impurities) with the filament type resistive switching (RS) behaviours. The augmentation of the nano-crystallites density in the film increases the forming voltage of devices and its variation. Carbon residues in HfO2 films turn out to be an even more significant factor strongly impacting the RS behaviour. A relatively higher deposition temperature of 300 °C dramatically reduces the residual carbon concentration, thus leading to enhanced RS performances of devices, including lower power consumption, better endurance and higher reliability. Such thorough understanding on physical mechanism of RS and the correlation between material and device performances will facilitate the realization of high density and reliable embedded RRAM devices with low power consumption.

Dislocation-free Ge Nano-crystals via Pattern Independent Selective Ge Heteroepitaxy on Si Nano-Tip Wafers.

The integration of dislocation-free Ge nano-islands was realized via selective molecular beam epitaxy on Si nano-tip patterned substrates. The Si-tip wafers feature a rectangular array of nanometer sized Si tips with (001) facet exposed among a SiO2 matrix. These wafers were fabricated by complementary metal-oxide-semiconductor (CMOS) compatible nanotechnology. Calculations based on nucleation theory predict that the selective growth occurs close to thermodynamic equilibrium, where condensation of Ge adatoms on SiO2 is disfavored due to the extremely short re-evaporation time and diffusion length. The growth selectivity is ensured by the desorption-limited growth regime leading to the observed pattern independence, i.e. the absence of loading effect commonly encountered in chemical vapor deposition. The growth condition of high temperature and low deposition rate is responsible for the observed high crystalline quality of the Ge islands which is also associated with negligible Si-Ge intermixing owing to geometric hindrance by the Si nano-tip approach. Single island as well as area-averaged characterization methods demonstrate that Ge islands are dislocation-free and heteroepitaxial strain is fully relaxed. Such well-ordered high quality Ge islands present a step towards the achievement of materials suitable for optical applications.

Photodetection in Hybrid Single-Layer Graphene/Fully Coherent Germanium Island Nanostructures Selectively Grown on Silicon Nanotip Patterns.

Dislocation networks are one of the most principle sources deteriorating the performances of devices based on lattice-mismatched heteroepitaxial systems. We demonstrate here a technique enabling fully coherent germanium (Ge) islands selectively grown on nanotip-patterned Si(001) substrates. The silicon (Si)-tip-patterned substrate, fabricated by complementary metal oxide semiconductor compatible nanotechnology, features ∼50-nm-wide Si areas emerging from a SiO2 matrix and arranged in an ordered lattice. Molecular beam epitaxy growths result in Ge nanoislands with high selectivity and having homogeneous shape and size. The ∼850 °C growth temperature required for ensuring selective growth has been shown to lead to the formation of Ge islands of high crystalline quality without extensive Si intermixing (with 91 atom % Ge). Nanotip-patterned wafers result in geometric, kinetic-diffusion-barrier intermixing hindrance, confining the major intermixing to the pedestal region of Ge islands, where kinetic diffusion barriers are, however, high. Theoretical calculations suggest that the thin Si/Ge layer at the interface plays, nevertheless, a significant role in realizing our fully coherent Ge nanoislands free from extended defects especially dislocations. Single-layer graphene/Ge/Si-tip Schottky junctions were fabricated, and thanks to the absence of extended defects in Ge islands, they demonstrate high-performance photodetection characteristics with responsivity of ∼45 mA W(-1) and an Ion/Ioff ratio of ∼10(3).

Structural Mapping of Functional Ge Layers Grown on Graded SiGe Buffers for sub-10 nm CMOS Applications Using Advanced X-ray Nanodiffraction.

We report a detailed advanced materials characterization study on 40 nm thick strained germanium (Ge) layers integrated on 300 mm Si(001) wafers via strain-relaxed silicon-germanium (SiGe) buffer layers. Fast-scanning X-ray microscopy is used to directly image structural inhomogeneities, lattice tilt, thickness, and strain of a functional Ge layer down to the sub-micrometer scale with a real space step size of 750 µm. The structural study shows that the metastable Ge layer, pseudomorphically grown on Si(0.3)Ge(0.7), exhibits an average compressive biaxial strain of -1.27%. By applying a scan area of 100 × 100 µm(2), we observe microfluctuations of strain, lattice tilt, and thickness of ca. ±0.03%, ±0.05°, and ±0.8 nm, respectively. This study confirms the high materials homogeneity of the compressively strained Ge layer realized by the step-graded SiGe buffer approach on 300 mm Si wafers. This presents thus a promising materials science approach for advanced sub-10 nm complementary metal oxide-semiconductor applications based on strain-engineered Ge transistors to outperform current Si channel technologies.

High-resolution characterization of the forbidden Si 200 and Si 222 reflections.

The occurrence of the basis-forbidden Si 200 and Si 222 reflections in specular X-ray diffraction ω-2Θ scans is investigated in detail as a function of the in-plane sample orientation Φ. This is done for two different diffractometer types with low and high angular divergence perpendicular to the diffraction plane. It is shown that the reflections appear for well defined conditions as a result of multiple diffraction, and not only do the obtained peaks vary in intensity but additional features like shoulders or even subpeaks may occur within a 2Θ range of about ±2.5°. This has important consequences for the detection and verification of layer peaks in the corresponding angular range.

Imaging Structure and Composition Homogeneity of 300 mm SiGe Virtual Substrates for Advanced CMOS Applications by Scanning X-ray Diffraction Microscopy.

Advanced semiconductor heterostructures are at the very heart of many modern technologies, including aggressively scaled complementary metal oxide semiconductor transistors for high performance computing and laser diodes for low power solid state lighting applications. The control of structural and compositional homogeneity of these semiconductor heterostructures is the key to success to further develop these state-of-the-art technologies. In this article, we report on the lateral distribution of tilt, composition, and strain across step-graded SiGe strain relaxed buffer layers on 300 mm Si(001) wafers treated with and without chemical-mechanical polishing. By using the advanced synchrotron based scanning X-ray diffraction microscopy technique K-Map together with micro-Raman spectroscopy and Atomic Force Microscopy, we are able to establish a partial correlation between real space morphology and structural properties of the sample resolved at the micrometer scale. In particular, we demonstrate that the lattice plane bending of the commonly observed cross-hatch pattern is caused by dislocations. Our results show a strong local correlation between the strain field and composition distribution, indicating that the adatom surface diffusion during growth is driven by strain field fluctuations induced by the underlying dislocation network. Finally, it is revealed that a superficial chemical-mechanical polishing of cross-hatched surfaces does not lead to any significant change of tilt, composition, and strain variation compared to that of as-grown samples.

Oxygen vacancy induced room temperature ferromagnetism in Pr-doped CeO2 thin films on silicon.

Integration of functional oxides on Si substrates could open a pathway to integrate diverse devices on Si-based technology. Oxygen vacancies (Vo(··)) can strongly affect solid state properties of oxides, including the room temperature ferromagnetism (RTFM) in diluted magnetic oxides. Here, we report a systematical study on the RTFM of oxygen vacancy engineered (by Pr(3+) doping) CeO2 epitaxial thin films on Si substrates. High quality, mixed single crystalline Ce1-xPrxO2-δ (x = 0-1) solid solution films were obtained. The Ce ions in CeO2 with a fluorite structure show a Ce(4+)-dominant valence state in all films. The local crystal structures of the films were analyzed in detail. Pr doping creates both Vo(··) and PrO8-complex defects in CeO2 and their relative concentrations vary with the Pr-doping level. The RTFM properties of the films reveal a strong dependence on the relative Vo(··) concentration. The RTFM in the films initially increases with higher Pr-doping levels due to the increase of the F(+) center (Vo(··) with one occupied electron) concentration and completely disappears when x > 0.2, where the magnetic polaron concentration is considered to decline below the percolation threshold, thus long-range FM order can no longer be established. We thus demonstrate the possibility to directly grow RTFM Pr-doped CeO2 films on Si substrates, which can be an interesting candidate for potential magneto-optic or spintronic device applications.

Engineering of the chemical reactivity of the Ti/HfO2 interface for RRAM: experiment and theory.

The Ti/HfO2 interface plays a major role for resistance switching performances. However, clear interface engineering strategies to achieve reliable and reproducible switching have been poorly investigated. For this purpose, we present a comprehensive study of the Ti/HfO2 interface by a combined experimental-theoretical approach. Based on the use of oxygen-isotope marked Hf*O2, the oxygen scavenging capability of the Ti layer is clearly proven. More importantly, in line with ab initio theory, the combined HAXPES-Tof-SIMS study of the thin films deposited by MBE clearly establishes a strong impact of the HfO2 thin film morphology on the Ti/HfO2 interface reactivity. Low-temperature deposition is thus seen as a RRAM processing compatible way to establish the critical amount of oxygen vacancies to achieve reproducible and reliable resistance switching performances.

X-ray characterization of Ge dots epitaxially grown on nanostructured Si islands on silicon-on-insulator substrates.

On the way to integrate lattice mismatched semiconductors on Si(001), the Ge/Si heterosystem was used as a case study for the concept of compliant substrate effects that offer the vision to be able to integrate defect-free alternative semiconductor structures on Si. Ge nanoclusters were selectively grown by chemical vapour deposition on Si nano-islands on silicon-on-insulator (SOI) substrates. The strain states of Ge clusters and Si islands were measured by grazing-incidence diffraction using a laboratory-based X-ray diffraction technique. A tensile strain of up to 0.5% was detected in the Si islands after direct Ge deposition. Using a thin (∼10 nm) SiGe buffer layer between Si and Ge the tensile strain increases to 1.8%. Transmission electron microscopy studies confirm the absence of a regular grid of misfit dislocations in such structures. This clear experimental evidence for the compliance of Si nano-islands on SOI substrates opens a new integration concept that is not only limited to Ge but also extendable to semiconductors like III-V and II-VI materials.