Silicon Nanomembranes with Hybrid Crystal Orientations and Strain States.

Methods to integrate different crystal orientations, strain states, and compositions of semiconductors in planar and preferably flexible configurations may enable nontraditional sensing-, stimulating-, or communication-device applications. We combine crystalline-silicon nanomembranes, patterning, membrane transfer, and epitaxial growth to demonstrate planar arrays of different orientations and strain states of Si in a single membrane, which is then readily transferable to other substrates, including flexible supports. As examples, regions of Si(001) and Si(110) or strained Si(110) are combined to form a multicomponent, single substrate with high-quality narrow interfaces. We perform extensive structural characterization of all interfaces and measure charge-carrier mobilities in different regions of a 2D quilt. The method is readily extendable to include varying compositions or different classes of materials.

Single-Crystalline SrRuO3 Nanomembranes: A Platform for Flexible Oxide Electronics.

The field of oxide electronics has benefited from the wide spectrum of functionalities available to the ABO3 perovskites, and researchers are now employing defect engineering in single crystalline heterostructures to tailor properties. However, bulk oxide single crystals are not conducive to many types of applications, particularly those requiring mechanical flexibility. Here, we demonstrate the realization of an all-oxide, single-crystalline nanomembrane heterostructure. With a surface-to-volume ratio of 2 × 10(7), the nanomembranes are fully flexible and can be readily transferred to other materials for handling purposes or for new materials integration schemes. Using in situ synchrotron X-ray scattering, we find that the nanomembranes can bond to other host substrates near room temperature and demonstrate coupling between surface reactivity and electromechanical properties in ferroelectric nanomembrane systems. The synthesis technique described here represents a significant advancement in materials integration and provides a new platform for the development of flexible oxide electronics.

Electronic Transport Properties of Epitaxial Si/SiGe Heterostructures Grown on Single-Crystal SiGe Nanomembranes.

To assess possible improvements in the electronic performance of two-dimensional electron gases (2DEGs) in silicon, SiGe/Si/SiGe heterostructures are grown on fully elastically relaxed single-crystal SiGe nanomembranes produced through a strain engineering approach. This procedure eliminates the formation of dislocations in the heterostructure. Top-gated Hall bar devices are fabricated to enable magnetoresistivity and Hall effect measurements. Both Shubnikov-de Haas oscillations and the quantum Hall effect are observed at low temperatures, demonstrating the formation of high-quality 2DEGs. Values of charge carrier mobility as a function of carrier density extracted from these measurements are at least as high or higher than those obtained from companion measurements made on heterostructures grown on conventional strain graded substrates. In all samples, impurity scattering appears to limit the mobility.

Strain imaging of nanoscale semiconductor heterostructures with x-ray Bragg projection ptychography.

We report the imaging of nanoscale distributions of lattice strain and rotation in complementary components of lithographically engineered epitaxial thin film semiconductor heterostructures using synchrotron x-ray Bragg projection ptychography (BPP). We introduce a new analysis method that enables lattice rotation and out-of-plane strain to be determined independently from a single BPP phase reconstruction, and we apply it to two laterally adjacent, multiaxially stressed materials in a prototype channel device. These results quantitatively agree with mechanical modeling and demonstrate the ability of BPP to map out-of-plane lattice dilatation, a parameter critical to the performance of electronic materials.

Fast flexible electronics with strained silicon nanomembranes.

Fast flexible electronics operating at radio frequencies (>1 GHz) are more attractive than traditional flexible electronics because of their versatile capabilities, dramatic power savings when operating at reduced speed and broader spectrum of applications. Transferrable single-crystalline Si nanomembranes (SiNMs) are preferred to other materials for flexible electronics owing to their unique advantages. Further improvement of Si-based device speed implies significant technical and economic advantages. While the mobility of bulk Si can be enhanced using strain techniques, implementing these techniques into transferrable single-crystalline SiNMs has been challenging and not demonstrated. The past approach presents severe challenges to achieve effective doping and desired material topology. Here we demonstrate the combination of strained- NM-compatible doping techniques with self-sustained-strain sharing by applying a strain-sharing scheme between Si and SiGe multiple epitaxial layers, to create strained print-transferrable SiNMs. We demonstrate a new speed record of Si-based flexible electronics without using aggressively scaled critical device dimensions.

Direct-bandgap light-emitting germanium in tensilely strained nanomembranes.

Silicon, germanium, and related alloys, which provide the leading materials platform of electronics, are extremely inefficient light emitters because of the indirect nature of their fundamental energy bandgap. This basic materials property has so far hindered the development of group-IV photonic active devices, including diode lasers, thereby significantly limiting our ability to integrate electronic and photonic functionalities at the chip level. Here we show that Ge nanomembranes (i.e., single-crystal sheets no more than a few tens of nanometers thick) can be used to overcome this materials limitation. Theoretical studies have predicted that tensile strain in Ge lowers the direct energy bandgap relative to the indirect one. We demonstrate that mechanically stressed nanomembranes allow for the introduction of sufficient biaxial tensile strain to transform Ge into a direct-bandgap material with strongly enhanced light-emission efficiency, capable of supporting population inversion as required for providing optical gain.

Defect-free single-crystal SiGe: a new material from nanomembrane strain engineering.

Many important materials cannot be grown as single crystals in bulk form because strain destroys long-range crystallinity. Among them, alloys of group IV semiconductors, specifically SiGe alloys, have significant technological value. Using nanomembrane strain engineering methods, we demonstrate the fabrication of fully elastically relaxed Si(1-x)Ge(x) nanomembranes (NMs) for use as growth substrates for new materials. To do so, we grow defect-free, uniformly and elastically strained SiGe layers on Si substrates and release the SiGe layers to allow them to relax this strain completely as free-standing NMs. These SiGe NMs are transferred to new hosts and bonded there. We confirm the high structural quality of these new materials and demonstrate their use as substrates for technologically relevant epitaxial films by growing strained-Si layers and thick, lattice-matched SiGe alloy layers on them.

Symmetry in strain engineering of nanomembranes: making new strained materials.

Strain in a material changes the lattice constant and thereby creates a material with new properties relative to the unstrained, but chemically identical, material. The ability to alter the strain (its magnitude, direction, extent, periodicity, symmetry, and nature) allows tunability of these new properties. A recent development, crystalline nanomembranes, offers a powerful platform for using and tuning strain to create materials that have unique properties, not achievable in bulk materials or with conventional processes. Nanomembranes, because of their thinness, enable elastic strain sharing, a process that introduces large amounts of strain and unique strain distributions in single-crystal materials, without exposing the material to the formation of extended defects. We provide here prescriptions for making new strained materials using crystal symmetry as the driver: we calculate the strain distributions in flat nanomembranes for two-fold and four-fold elastically symmetric materials. We show that we can controllably tune the amount of strain and the asymmetry of the strain distribution in elastically isotropic and anisotropic materials uniformly over large areas. We perform the experimental demonstration with a trilayer Si(110)/Si((1-x))Ge(x)(110)/Si(110) nanomembrane: an elastically two-fold symmetric system in which we can transfer strain that is biaxially isotropic. We are thus able to make uniformly strained materials that cannot be made any other way.

Si/Ge junctions formed by nanomembrane bonding.

We demonstrate the feasibility of fabricating heterojunctions of semiconductors with high mismatches in lattice constant and coefficient of thermal expansion by employing nanomembrane bonding. We investigate the structure of and electrical transport across the interface of a Si/Ge bilayer formed by direct, low-temperature hydrophobic bonding of a 200 nm thick monocrystalline Si(001) membrane to a bulk Ge(001) wafer. The membrane bond has an extremely high quality, with an interfacial region of ∼1 nm. No fracture or delamination is observed for temperature changes greater than 350 °C, despite the approximately 2:1 ratio of thermal-expansion coefficients. Both the Si and the Ge maintain a high degree of crystallinity. The junction is highly conductive. The nonlinear transport behavior is fit with a tunneling model, and the bonding behavior is explained with nanomembrane mechanics.