Pre-patterned Si Research Articles

Unlocking the monolithic integration scenario: optical coupling between GaSb diode lasers epitaxially grown on patterned Si substrates and passive SiN waveguides

Silicon (Si) photonics has recently emerged as a key enabling technology in many application fields thanks to the mature Si process technology, the large silicon wafer size, and promising Si optical properties. The monolithic integration by direct epitaxy of III–V lasers and Si photonic devices on the same Si substrate has been considered for decades as the main obstacle to the realization of dense photonics chips. Despite considerable progress in the last decade, only discrete III–V lasers grown on bare Si wafers have been reported, whatever the wavelength and laser technology. Here we demonstrate the first semiconductor laser grown on a patterned Si photonics platform with light coupled into a waveguide. A mid-IR GaSb-based diode laser was directly grown on a pre-patterned Si photonics wafer equipped with SiN waveguides clad by SiO2. Growth and device fabrication challenges, arising from the template architecture, were overcome to demonstrate more than 10 mW outpower of emitted light in continuous wave operation at room temperature. In addition, around 10% of the light was coupled into the SiN waveguides, in good agreement with theoretical calculations for this butt-coupling configuration. This work lift an important building block and it paves the way for future low-cost, large-scale, fully integrated photonic chips.

III–V Microwires with Reversed Ridge Waveguides Selectively Grown on Pre-Patterned Si Substrates

To construct functional photonic integrated circuits, an efficient and compact laser is expected to be incorporated into the complementary metal–oxide–semiconductor platform. Monolithic integration of III–V lasers on pre-patterned Si substrates by the aspect ratio trapping method is a promising solution. Here, microwires with reversed ridge waveguides (RRWs) on pre-patterned Si substrates were reported. By metal–organic chemical vapor deposition, high-quality InP microwires with RRWs were obtained, and InGaAs/InGaAsP multi-quantum-well structures with InGaAsP separate confinement hetero structure (SCH–MQW) were successfully achieved. The SCH–MQW structure was buried in the InP microwire, which was beneficial for transferring the heat generated in the active region. The micron size also contributes to the efficiency of thermal diffusion. Further, simulation results showed that the metal absorption loss could be less than 4 dB/cm by properly controlling the contact area between metal electrodes and microwires. This proposed structure opens up an alternative pathway for electrically driven III–V lasers seamlessly interfaced with Si-photonics.

Spin-wave nonreciprocity and formation of lateral standing spin waves in CoFeB/Ta/NiFe meander-shaped films

Studying the spin-wave (SW) propagation in 3D periodic structures opens new possibilities for joining functional units placed on the different layers of the magnonic circuitry. In the path toward 3D magnonics, the main challenge is the fabrication of large-scale 3D magnetic structures with nanometric precision control of geometry and material composition. In this work, we study the dependence on the Ta spacer thickness of the magnonic band structure, measured by Brillouin light scattering spectroscopy, of CoFeB/Ta/NiFe meander-shaped bilayers fabricated on pre-patterned Si substrate with thickness steps of 50 nm. Both propagating and stationary SW modes are observed. While the frequency of the dispersive mode slightly depends on the Ta spacer thickness, the frequency position of the three stationary modes in the lowest frequency range of the spectra significantly increases by increasing the Ta thickness. Micromagnetic calculations indicate that each of the three stationary modes is composed of a doublet of modes whose frequency separation, within each doublet, increases by increasing the mode frequency. The origin of this frequency separation is ascribed to the dynamic dipolar coupling between the magnetic layers that generate a significant frequency nonreciprocity of counterpropagating SWs. For these reasons, the investigated structures offer potential application as the nonreciprocal versatile interconnections performing the frequency selective regimes of signal propagation in magnonic circuits.

In-situ prepared WSe2/Si 2D-3D vertical heterojunction for high performance self-driven photodetector

Two-dimensional (2D) transition metal chalcogenides (TMDs) have shown tremendous feasibility as building blocks for the development of high-performance optoelectronic devices owing to their distinct electrical and optical properties. However, the relatively narrow sensing range as well as the complex fabrication technique impede their technological applications. Here, we demonstrate the mixed-dimensional van der Waals (vdW) WSe2/Si 2D-3D vertical heterojunction by in-situ fabrication of WSe2 multilayer on pre-patterned Si, for broadband and fast-speed photodetection. Thanks to the novel high-quality vertical p-n heterojunction, the as-fabricated WSe2/Si photodetector shows an excellent rectifying characteristic and a prominent photovoltaic effect, making the device capable of light detection in self-driven mode. Additionally, the device reveals remarkable performance in terms of a high specific detectivity of ∼8.79 × 1013 Jones, a large responsivity of ∼294 mA/W, and a fast response time of 4.1 μs. Significantly, the device shows high sensitivity to a wide spectra (200–1550 nm) owing to the production of a type-II band structure of the WSe2/Si vertical heterojunction. The mechanism of photo-generated carriers separation and transfer in the heterojunction is analyzed by KPFM. Our work offers a potential route to the development of unique 2D-3D heterojunction for optoelectronic devices and system applications.

Scanning x-ray microscopy: A sub-100 nm probe toward strain and composition in seeded horizontal Ge(110) nanowires

We have spatially investigated lattice spacing, twist, and bending in individual laterally (110)-oriented Ge nanowires (NWs) on pre-patterned Si(001) substrates. A combination of synchrotron-based scanning x-ray diffraction microscopy with an x-ray focus size of 50 nm and numerical finite element calculations on the elastic strain reveals a three-dimensional relaxation scenario, which becomes particularly complex next to NW nucleation points. Despite a lattice mismatch of 4.2%, lattice compliance is preserved, since strain can effectively be released close to the seeding window. Areas in the NWs other than that appear fully relaxed. The resulting NW twist, i.e., lattice rotations around the growth axis, amounts to less than 0.1°.

Lamellar Orientation of a Block Copolymer via an Electron-Beam Induced Polarity Switch in a Nitrophenyl Self-Assembled Monolayer or Si Etching Treatments

Directed self-assembly (DSA) was investigated on self-assembled monolayers (SAMs) chemically modified by electron beam (EB) irradiation, which is composed of 6-(4-nitrophenoxy) hexane-1-thiol (NPHT). Irradiating a NPHT by EB could successfully induce the orientation and selective patterning of block copolymer domains. We clarified that spatially-selective lamellar orientations of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) could be achieved by a change of an underlying SAM. The change of an underlying SAM is composed of the transition of an NO2 group to an NH2 group, which is induced by EB. The modification in the polarity of different regions of the SAM with EB lithography controlled the lamellar orientation of PS-b-PMMA. The reduction of the NPHT SAM plays an important role in the orientation of block copolymer. This method might significantly simplify block copolymer DSA processes when it is compared to the conventional DSA process. By investigating the lamellae orientation with EB, it is clarified that only suitable annealing temperatures and irradiation doses lead to the vertical orientation. We also fabricated pre-patterned Si substrates by EB lithographic patterning and reactive ion etching (RIE). DSA onto such pre-patterned Si substrates was proven to be successful for subdivision of the lithographic patterns into line and space patterns.

Spatially-resolved luminescence and crystal structure of single core–shell nanowires measured in the as-grown geometry

We report on the direct correlation between the structural and optical properties of single, as-grown core-multi-shell GaAs/In0.15Ga0.85As/GaAs/AlAs/GaAs nanowires. Fabricated by molecular beam epitaxy on a pre-patterned Si(111) substrate, on a row of well separated nucleation sites, it was possible to access individual nanowires in the as-grown geometry. The polytype distribution along the growth axis of the nanowires was revealed by synchrotron-based nanoprobe x-ray diffraction techniques monitoring the axial 111 Bragg reflection. For the same nanowires, the spatially-resolved emission properties were obtained by cathodoluminescence hyperspectral linescans in a scanning electron microscope. Correlating both measurements, we reveal a blueshift of the shell quantum well emission energy combined with an increased emission intensity for segments exhibiting a mixed structure of alternating wurtzite and zincblende stacking compared with the pure crystal polytypes. The presence of this mixed structure was independently confirmed by cross-sectional transmission electron microscopy.

Site-Controlled Uniform Ge/Si Hut Wires with Electrically Tunable Spin-Orbit Coupling.

Semiconductor nanowires have been playing a crucial role in the development of nanoscale devices for the realization of spin qubits, Majorana fermions, single photon emitters, nanoprocessors, etc. The monolithic growth of site-controlled nanowires is a prerequisite toward the next generation of devices that will require addressability and scalability. Here, combining top-down nanofabrication and bottom-up self-assembly, the growth of Ge wires on prepatterned Si (001) substrates with controllable position, distance, length, and structure is reported. This is achieved by a novel growth process that uses a SiGe strain-relaxation template and can be potentially generalized to other material combinations. Transport measurements show an electrically tunable spin-orbit coupling, with a spin-orbit length similar to that of III-V materials. Also, charge sensing between quantum dots in closely spaced wires is observed, which underlines their potential for the realization of advanced quantum devices. The reported results open a path toward scalable qubit devices using nanowires on silicon.

A suspended graphene-based optical interferometric surface stress sensor for selective biomolecular detection.

Graphene-based sensors are of great interest in research due to their high specific surface area and high electron mobility that make them suitable for numerous advanced applications. In this paper, selective molecular detection using an antigen–antibody reaction on suspended graphene with a cavity-sealing structure was demonstrated. The suspended graphene sealed nanocavities in a pre-patterned Si substrate, which increased robustness and allowed the use of wet chemical processes for surface functionalization of the suspended graphene to achieve selective molecular binding. The selectivity was evaluated by nanomechanical deflection induced by molecular adsorption on the suspended graphene, resulting in spectral shifts in the optical interference between the suspended graphene and Si substrate. The chemically functionalized suspended graphene enables the analysis of intermolecular interactions and molecular kinetics by colorimetry using optical interference.

Low temperature selective growth of GaN single crystals on pre-patterned Si substrates

Low temperature selective growth of GaN single crystals on pre-patterned Si substrates

Low temperature selective growth of GaN single crystals on pre-patterned Si substrates

We report on a hybrid method for fabrication of arrays of GaN nanocrystals by low-temperature UHV selective growth on pre-patterned silicon substrates covered by native oxide. Patterning of the substrates was performed by using a gallium focused ion beam (FIB). To get GaN nanocrystals at specific positions, Ga droplets were created at FIB patterned sites by evaporation of Ga atoms at 280 °C substrate temperature first, and then modified by their post-nitridation using an ultra-low energy (50 eV) nitrogen ion–beam at a sample temperature of 200 °C. To get larger arrays of GaN nanocrystals (≈150 nm and 200 nm in diameter), such a sequential process was repeated in several cycles at slightly modified operation conditions. The quality of the nanocrystals was checked by measurement of their photoluminescence properties which proved the occurrence of the peak of a band edge emission at around 367 nm (3.38 eV).

Nucleation of Three-Dimensional Ge Islands on a Patterned Si(100) Surface

The nucleation of three-dimentional Ge islands formed on a pre-patterned Si substrate with an array of round pits is studied. It is found that the Ge islands nucleate within pits with pointed bottoms and along the perimeters of pits with flat bottoms. This effect is determined by the difference between the distributions of elastic strains at the Ge/Si interface for differently shaped pit bottoms. The results of the simulation of growth show that, in the case of pits with pointed bottoms, the most relaxed regions are at the centers of the pit bottoms and the nucleation of islands takes place just in these regions. At the same time, in the case of pits with flat bottoms, the most relaxed regions are shifted from the pit bottoms to the pit edges, resulting in the nucleation of islands along the pit perimeters.

Study of the Structural and Emission Properties of Ge(Si) Quantum Dots Ordered on the Si(001) Surface

A method for creation of Ge/Si structures with space-arranged nanoislands by heteroepitaxy on the pre-patterned Si(001) substrates with a square grid of the etched pits is developed. The influence of depth and inter-pit spacing on the nucleation and growth of Ge(Si) nanoislands is studied. It is shown, that the nanoislands are formed either inside pits or at their periphery depending on the pit depth. It is found that the size of the nanoislands grown inside the pits goes up with the increase of the inter-pit distance from 1 to 4 μm. The pronounced photoluminescence signal related with the space-arranged arrays of quantum dots with a period of 1 μm is observed in the range of energies from 0.9 to 1.0 eV.

Исследование структурных и излучательных свойств Ge(Si) квантовых точек, упорядоченных на поверхности Si(001)

AbstractA method for creation of Ge/Si structures with space-arranged nanoislands by heteroepitaxy on the pre-patterned Si(001) substrates with a square grid of the etched pits is developed. The influence of depth and inter-pit spacing on the nucleation and growth of Ge(Si) nanoislands is studied. It is shown, that the nanoislands are formed either inside pits or at their periphery depending on the pit depth. It is found that the size of the nanoislands grown inside the pits goes up with the increase of the inter-pit distance from 1 to 4 μm. The pronounced photoluminescence signal related with the space-arranged arrays of quantum dots with a period of 1 μm is observed in the range of energies from 0.9 to 1.0 eV.

Зарождение трехмерных островков Ge на структурированной поверхности Si(100)

AbstractThe nucleation of three-dimentional Ge islands formed on a pre-patterned Si substrate with an array of round pits is studied. It is found that the Ge islands nucleate within pits with pointed bottoms and along the perimeters of pits with flat bottoms. This effect is determined by the difference between the distributions of elastic strains at the Ge/Si interface for differently shaped pit bottoms. The results of the simulation of growth show that, in the case of pits with pointed bottoms, the most relaxed regions are at the centers of the pit bottoms and the nucleation of islands takes place just in these regions. At the same time, in the case of pits with flat bottoms, the most relaxed regions are shifted from the pit bottoms to the pit edges, resulting in the nucleation of islands along the pit perimeters.

Anisotropic relaxation behavior of InGaAs/GaAs selectively grown in narrow trenches on (001) Si substrates

Selective area growth of InGaAs inside highly confined trenches on a pre-patterned (001) Si substrate has the potential of achieving a high III-V crystal quality due to high aspect ratio trapping for improved device functionalities in Si microelectronics. If the trench width is in the range of the hetero-layer thickness, the relaxation mechanism of the mismatched III-V layer is no longer isotropic, which has a strong impact on the device fabrication and performance if not controlled well. The hetero-epitaxial nucleation of InxGa1-xAs on Si can be simplified by using a binary nucleation buffer such as GaAs. A pronounced anisotropy in strain release was observed for the growth of InxGa1-xAs on a fully relaxed GaAs buffer with a (001) surface inside 20 and 100 nm wide trenches, exploring the full composition range from GaAs to InAs. Perpendicular to the trench orientation (direction of high confinement), the strain release in InxGa1-xAs is very efficiently caused by elastic relaxation without defect formation, although a small compressive force is still induced by the trench side walls. In contrast, the strain release along the trenches is governed by plastic relaxation once the vertical film thickness has clearly exceeded the critical layer thickness. On the other hand, the monolithic deposition of mismatched InxGa1-xAs directly into a V-shaped trench bottom with {111} Si planes leads instantly to a pronounced nucleation of misfit dislocations along the {111} Si/III-V interfaces. In this case, elastic relaxation no longer plays a role as the strain release is ensured by plastic relaxation in both directions. Hence, using a ternary seed layer facilitates the integration of InxGa1-xAs covering the full composition range.

Selective growth of fully relaxed GeSn nano-islands by nanoheteroepitaxy on patterned Si(001)

In this letter, we explore in detail the potential of nanoheteroepitaxy to controllably fabricate high quality GeSn nano-structures and to further improve the crystallinity of GeSn alloys directly grown on Si(001). The GeSn was grown by molecular beam epitaxy at relatively high temperatures up to 750 °C on pre-patterned Si nano-pillars embedded in a SiO2 matrix. The best compromise between selective GeSn growth and homogenous Sn incorporation of 1.4% was achieved at a growth temperature of 600 °C. X-ray diffraction measurements confirmed that our growth approach results in both fully relaxed GeSn nano-islands and negligible Si interdiffusion into the core of the nanostructures. Detailed transmission electron microscopy characterizations show that only the small GeSn/Si interface area reveals defects, such as stacking faults. Importantly, the main part of the GeSn islands is defect-free and of high crystalline quality. The latter was further demonstrated by photoluminescence measurements where a clear redshift of the direct ΓC-ΓV transition was observed with increasing Sn content.

(Invited) III-V Selective Area Growth and Epitaxial Functional Oxides on Si: From Electronic to Photonic Devices

Driven by fabrication cost reduction and device performance improvement, the Silicon semiconductor industry continues its never-ending pursuit of new approaches for fabricating integrated circuits. In this context, the monolithic integration of III-V semiconductors epitaxially grown on Si substrate have been attracting much attention as building blocks for next-generation electronics and photonics due to their potential intrinsic properties. Direct heteroepitaxy of III-V compound semiconductors on Si has traditionally represented a formidable challenge, due to the high material mismatch (lattice parameter, polarity, thermal coefficient, ...) between Si and III-V semiconductors generating high density of defect density during the epitaxial process. To overcome this, selective area growth of III-Vs in a pre-patterned Si substrate by Selective Area Metal-Organic Vapor Phase Epitaxy allow the possibility to obtain high quality and reduced defect density III-V based active layers onto standard Si(001) substrate. Using this approach we integrate III-V materials monolithically on Si while focusing on both ultimate trench widths (W < 20 nm) scaling as well as relaxed trench dimension in function of the targeted application. We report here on the heteroepitaxy of InP buffer on initiated from a <111> V-groove Si surface and discuss the unique relaxation mechanism of such compound with respect to Si substrate. We derived a fundamental understanding and theoretical modeling of the growth mechanisms in STI trenches as well as the determining role of the InP nucleation layer. The subsequent growth approach of InGaAs active layer, used for both nFET or Laser applications, is further studied. Finally, III-V based electronic and photonic devices realized using selective area growth technique are presented and benchmarked to the current state of the art.

Optical properties of individual site-controlled Ge quantum dots

We report photoluminescence (PL) experiments on individual SiGe quantum dots (QDs) that were epitaxially grown in a site-controlled fashion on pre-patterned Si(001) substrates. We demonstrate that the PL line-widths of single QDs decrease with excitation power to about 16 meV, a value that is much narrower than any of the previously reported PL signals in the SiGe/Si heterosystem. At low temperatures, the PL-intensity becomes limited by a 25 meV high potential-barrier between the QDs and the surrounding Ge wetting layer (WL). This barrier impedes QD filling from the WL which collects and traps most of the optically excited holes in this type-II heterosystem.

Linear chains of Ge/Si quantum dots grown on a prepatterned surface formed by ion irradiation

The growth of Ge nanoclusters on a prepatterned Si (100) surface formed by imprint lithography in combination with subsequent irradiation with Ge+ ions is studied. The prepatterned surface presents a system of parallel 10-nm-wide grooves repeating with a period of 180 nm. Irradiation of the substrate was conducted at two temperatures, room temperature (cold irradiation) and 400°C (hot irradiation). It is shown that, during epitaxy (550–700°C), the residual radiation defects located in the bulk under the grooves suppress the nucleation of Ge nanoclusters in the grooves. In the case of prepatterned substrates, from which imperfect regions are completely removed, nanoclusters grow in the grooves.