Single-Crystalline Elastically Relaxed SiGe Nanomembranes: Substrates for Epitaxial Growth of Defect-Free Strained-Si/SiGe Heterostructures

The strain engineering of silicon-germanium alloys plays a pivotal role in advanced Group IV opto- and nanoelectronics. Strain induced in Si, Ge, and Si1-yGey via epitaxial growth on relaxed Si1-xGex, has enabled enhanced electronic transport and created charge confinement in heterostructures. A current application of strained-Si/relaxed-SiGe heterostructures, Si-Ge based quantum information processing, involves the fabrication of 2-dimensional electron gas layers (2DEGs) that can be patterned and gated to confine individual electrons into quantum qubits with long spin relaxation times [1]. The performance of such qubit devices is quite sensitive to changes in the electrostatic potential and thus high-quality materials are essential. The crystalline quality of the initial substrate, in the conventional case plastically relaxed SiGe, largely dictates the structural quality of the heterostructures grown on top. Conventional methods for creating relaxed SiGe substrates involve step- graded heteroepitaxial growth on Si substrates and relaxation of the alloy via dislocations. The density of defects that reach the top relaxed SiGe layer can be limited though various techniques [2], but at minimum strain inhomogeneities and mosaic structure created by the dislocations remain. The nonuniformity in the starting substrate influences the epitaxy of heterostructures grown on top, including the strained- Si quantum well. We demonstrate the fabrication of SiGe nanomembranes (NM): fully elastically relaxed, smooth, single- crystalline sheets of SiGe alloy. A thin SiGe layer (less than the kinetic critical thickness for dislocation formation) is grown on a silicon-on-insulator (SOI) substrate with molecular beam epitaxy, fol-lowed by a Si capping layer with thickness similar to that of the Si template layer of the SOI (Figure 1A). The SiO2 layer of the SOI is selectively etched away, leaving the Si/SiGe/Si trilayer heterostructure free to strain share (Figure 1B). The Si layers of the trilayer are then selectively etched away, leaving a fully elastically relaxed SiGe NM (Figure 1C). These SiGe NMs are then transferred to new handling sub-strates and bonded (Figure 1D). The strain states of the SiGe NMs are measured throughout the fabrication process with Raman spectroscopy (Figure 2A). Initially, the SiGe is fully strained to the Si lattice constant, and relaxes to the bulk SiGe lattice constant only after release from the original growth substrate and removal of the surrounding Si layers. The elastically relaxed SiGe NMs are viable substrates for growth of new defect-free materials. To create an improved strained-Si 2DEG, we grow lattice matched SiGe on a transferred SiGe NM, followed by ~10 nm of strained Si and capped with another layer of SiGe. Raman spectroscopy on the heter-ostructure confirms that the Si is strained to the SiGe lattice constant and the additional SiGe is lattice matched to the original SiGe NM (Figure 2B). We compare the strain uniformity and mosaic structure of strained-Si/SiGe heterostructures grown on SiGe NM substrates with those grown on SiGe substrates relaxed via dislocations (the conventional way). Figure 3 shows x-ray diffraction reciprocal- space maps for two such structures, illustrating the vast improvement in the average mosaic structure on the NM grown 2DEG. We will also present comparisons of local strain variations (~1 μm range) based on micro-Raman spectroscopy, and local mosaic tilt (~200 nm range or less) within heterostructures grown on the two types of substrates based on synchrotron x- ray nanodiffraction measurements