Strained Superlattice Structures Research Articles

Reconfiguration of Amorphous Complex Oxides: A Route to a Broad Range of Assembly Phenomena, Hybrid Materials, and Novel Functionalities.

Reconfiguration of amorphous complex oxides provides a readily controllable source of stress that can be leveraged in nanoscale assembly to access a broad range of 3D geometries and hybrid materials. An amorphous SrTiO3 layer on a Si:B/Si1- x Gex :B heterostructure is reconfigured at the atomic scale upon heating, exhibiting a change in volume of ≈2% and accompanying biaxial stress. The Si:B/Si1- x Gex :B bilayer is fabricated by molecular beam epitaxy, followed by sputter deposition of SrTiO3 at room temperature. The processes yield a hybrid oxide/semiconductor nanomembrane. Upon release from the substrate, the nanomembrane rolls up and has a curvature determined by the stress in the epitaxially grown Si:B/Si1- x Gex :B heterostructure. Heating to 600°C leads to a decrease of the radius of curvature consistent with the development of a large compressive biaxial stress during the reconfiguration of SrTiO3 . The control of stresses via post-deposition processing provides a new route to the assembly of complex-oxide-based heterostructures in 3D geometry. The reconfiguration of metastable mechanical stressors enables i) synthesis of various types of strained superlattice structures that cannot be fabricated by direct growth and ii) technologies based on strain engineering of complex oxides via highly scalable lithographic processes and on large-area semiconductor substrates.

Evaluation of GaAsSb/AlGaAs strained superlattice photocathodes

GaAs-class strained superlattice (SSL) photocathodes can provide electron beams with electron spin polarization (ESP) exceeding the theoretical maximum 50% of bulk GaAs. In this paper, we describe the evaluation of a SSL structure composed of GaAsSb/AlGaAs and grown on a GaAs substrate. Theoretical analysis and numerical calculations show GaAsSb/AlGaAs SSL structures have the largest heavy-hole and light-hole energy splitting of all existing GaAs-class SSL structures, which should lead to the highest initial ESP. Five GaAsSb/AlGaAs SSL photocathode samples with different constituent species concentrations, number of layer pairs, and layer thicknesses were fabricated and evaluated. The highest ESP was ∼77% obtained from a photocathode based on the GaAsSb0.15/Al0.38GaAs (1.55/4.1 nm ×15 layer pairs) SSL structure.

Signature of optical absorption in highly strained and partially relaxed InP/GaAs type-II quantum well superlattice structures

Signature of optical absorption in highly strained and partially relaxed InP/GaAs type-II quantum well superlattice structures is clearly observed at room temperature. Strong coupling effects between the wavefunctions of confined carriers in the neighbouring quantum wells of InP/GaAs type-II superlattice structures are observed in electroreflectance (ER) spectra. Only ER measurements provide clear signatures of the superlattice feature for partially relaxed superlattice structures. However, both ER and photoreflectance (PR) measurements provide the signature of superlattice features for highly strained superlattice structures. A possible explanation is given in terms of the trapping of photogenerated carriers at defect/trap levels generated by the partial relaxation or due to the presence of oscillatory features in PR spectra. ER being a direct modulation technique, where one applies an external electric field to modulate the built-in electric field, is therefore preferred over PR. Increase in the surface photovoltage magnitude for energies lower than the band edge position of GaAs buffer layer confirms that the absorption takes place between the electronic minibands formed in the type-II superlattice structures.

Structure Analysis of SrTiO3/BaTiO3 Strained Superlattice Films Prepared by Atomic-Layer Metalorganic Chemical Vapor Deposition

Ferroelectric films of SrTiO3/BaTiO3 with strained superlattice structures, prepared by atomic layer metalorganic chemical vapor deposition, are investigated by the X-ray diffraction spectroscopy, transmission electron microscopy and secondary ion mass spectrometry. Stress is evident in a strained superlattice grown heteroepitaxially. In order to relax the stress in the superlattice film, dislocations are formed as well as atomic displacement which results in enhanced dielectric properties.

Extraordinary growth of C60 on a GaAs(001) As-rich 2×4 surface

We have systematically investigated, by using scanning tunneling microscopy, the adsorption and film growth of C60 on the various GaAs(001) surface phases prepared by molecular-beam epitaxy. For most phases, the C60 overlayer exhibits the usual close-packed fcc(111) configuration with its lattice constant close to that of the bulk C60 crystal. However, in the case of C60 on the As-rich 2×4 substrate, the epitaxial growth is found to be quite different and unique; C60 film takes its (110) crystalline axis; the C60 overlayer is highly strained with a lattice expansion of ∼13%, and this structure is very stable at least up to 10 ML. We will address the underlying formation mechanism of this new structure in terms of a charge transfer from the As-dangling bonds to C60s and a site-specific C60-substrate interaction, as confirmed by molecular dynamic simulations. The present system provides a unique opportunity to study fullerene and/or noble-gas related two-dimensional phenomena, and demonstrates a potential for fabrication of novel fullerene-based devices, such as strained superlattice structures.

Growth and structural properties of [formula omitted] intentionally disordered superlattice structures grown by metalorganic vapor phase epitaxy

We present an X-ray diffraction (XRD) study of symmetrically strained superlattice structures with intentionally fluctuating individual layer thicknesses. Samples with periodic, aperiodic (random) and graded fluctuations have been realized by metalorganic vapor phase epitaxy (MOVPE). The precise structural properties of the disordered multilayer structures have been determined by high resolution XRD in combination with both kinematical as well as dynamical XRD theory. Periodic modulation of the compressive-strained (GaIn)As layers leads to peak splitting on the “tensile-strained” part of the XRD spectrum, while periodic variation of the tensile-strained Ga(PAs) layers results in splitting on the “compressive-strained” part. Aperiodic variation causes a broadening of the respective peaks. The study demonstrates the power of XRD to quantify fluctuations of layer thicknesses in the growth direction down to the monolayer level.

Misfit accommodation at epitaxial interfaces

The papers presented in this special volume address the critical issues of the mechanisms by which epitaxial films accommodate the interfacial misfit through strain relaxation. In this concluding paper, we wish to highlight some of the important concepts associated with the misfit accommodation at epitaxial interfaces. Towards the goal of achieving defect free epitaxial layers, efforts are continuously made to understand the mechanisms of generation of misfit dislocations. Thermodynamic equilibrium considerations are important in this respect. However, nucleation and growth is considered to be the limiting step for the misfit dislocation generation in the semiconductor heterostructures. Strained superlattice structures have been used to reduce the propagation of threading dislocations. Similarly, patterned substrates are also employed to reduce the propagating dislocation density in the epilayers. These different ideas are used to produce defect free epitaxial structures for device construction. The present state of our understanding of dislocation nucleation and generation are critically examined in this paper and suggestions for future work are given in this paper.

Ion mixing in amorphous and crystalline Si/Ge layered structures

Thermal interdiffusion in Si/Ge layered structures shows large variations in diffusivity between Si/Ge amorphous multilayer films and symmetrically strained superlattice structures. Amorphous layered structures exhibit much higher diffusivities and much lower activation energy for interdiffusion. We have used Rutherford backscattering (RBS) and transmission electron microscopy (TEM) to (1) investigate the mixing behavior in Ge/Si structures under ion mixing condition, (2) study the dependence of mixing on the microstructure of the multilayered films (amorphous layers vs. crystalline layers) and (3) study the effects of electrically active ions (e.g. A.s + ions) on ion mixing behavior. We found that the ion mixing efficiency of the Si/Ge system shows two regimes upon ion irradiation: a thermally activated regime where the efficiency of mixing increases exponentially with the irradiation temperature and a nonthermal regime where the amount of mixing is independent of the irradiation temperature. The critical temperature for the onset of the thermally activated regime is found to be around 200°C. Comparison between thermal interdiffusion and ion-induced intermixing in the Si/Ge system is also made.

Analyses of metalorganic chemical-vapor-deposition-grown AlxGa1−xAs/GaAs strained superlattice structures by backscattering spectrometry and x-ray rocking curves

Backscattering spectrometry with channeling and x-ray rocking curves have been employed to analyze metalorganic chemical-vapor-deposition-grown AlxGa1−xAs/GaAs strained superlattice structures in significant detail. Both techniques complement each other in the precise determination of composition, thickness, and strain in the individual layers of the superlattices. In addition, the sensitivity of the two techniques allows quantitative measurements of transition regions at the interfaces of various layers. Such fine probing into thin layered superlattice structures provides essential feedback in controlling their growth.