Session V27: Invited Session: Flexible and Rolled Up Semiconductor Nanomembranes

Making Novel Materials Using Strain in Nanomembranes

The controlled introduction of strain in materials offers an important degree of freedom for fundamental studies of materials as well as advanced device engineering. Strain in a crystalline solid modifies the lattice constants and reduces the crystal symmetry. Because strain energy is proportional to thickness, a free-standing crystalline thin sheet, which we call a nanomembrane (NM), can be strained to a greater degree that a bulk material with the same surface area. I show the use of nanomembrane strain engineering to make defect-free single crystals that cannot be grown any other way, and materials with strain symmetries that they do not have naturally, in both cases alloys of Si and Ge. Strain in NMs causes significant shifts in energy band edges, splitting of degenerate states, and changes in effective masses. These effects can be used to produce a desired band offset between different materials, to increase carrier mobility, and to change relative energy positions of valleys. In the latter respect, through the use of NMs it has recently become possible, using tensile strain, to make Ge direct-bandgap and light emitting at room temperature. Periodic local stress can produce strain superlattices and thus single-element heterojunctions. Work performed with the Roberto Paiella, Mark Eriksson, Feng Liu, and Irena Knezevic research groups.   

Strain Superlattice: A Combination of Strain Induced Self-Assembly and Strain Engineered Band Structure

Semiconductor nanomembrane affords a novel 2D platform for nanoscience and nanotechnology, especially for strain engineering of nanoelectronics. Strain is well known for band engineering to improve the performance of Si devices. The evidence that the band gap of Si changes significantly with strain suggests that by alternating regions of strained and unstrained Si one creates a single-element hetero-strain-junction electronic superlattice (SL), with the carrier confinement defined by strain rather than by the chemical differences in conventional SLs. Using first-principles calculations, we map out the electronic phase diagram of a 1D pure-silicon strain SL. It exhibits a high level of phase tunability, e.g., tuning from type I to type II. Our theory rationalizes a recent observation of a strain SL in a Si nanowire and provides general guidance for the fabrication of single-element strain SLs. The low-dimensional nanoscale strain SLs extend the concept of SL to a single element that exists in different structural states. It can be made in 1D nanowires or 2D nanomembranes by nanoscale self-assembly or by nanopatterning. It expands the application of strain engineering to new territories, by combining strain induced self-assembly with strain engineered band structure.   

Nanomembrane photonics for Si photonic integration and flexible optoelectronics

Crystalline semiconductor nanomembranes (NMs) offer unprecedented opportunities for unique electronic and photonic devices for vertically stacked high density photonic/electronic integration, high performance flexible electronics, and adaptive flexible/conformal photonics. Research progresses have been made in the areas of optical filters/modulators, spectral selective IR photodetectors, flexible LEDs, solar cells, and novel light sources, based on quantum dots, Fano resonance photonic crystal cavities, and heterogeneous integration of III-V/Si material systems. The potentials and prospects of nanomembrane photonics will also be discussed, for a wide range of applications, in the areas of hyper-spectral imaging and gas sensing (lab-on-a-chip), high capacity data network and optical computing (WDM-on-a-chip), high performance flexible inorganic displays, solid state lighting, and photovoltaic solar cells, etc.