SiGe Superlattices Research Articles

Comparison of Growth and Strain Relaxation of Si/Ge Superlattices Under Compressive and Tensile Strain Field

ABSTRACTOptimization of growth parameters of short period Si/Ge superlattices (SLs) has been achieved via in situ low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) measurements during homo- and heteroepitaxy on Si (001) and Ge (001) substrates. Transmission electron microscopy (TEM) reveals that pseudomorphic SimGe12-m (m = 9 and 3 for growth on Si and Ge, respectively) SLs with extended planar layering can be prepared almost defect-free by a modified molecular beam epitaxy (MBE) technique. Whereas the SLs on Ge can be deposited at a constant substrate temperature, high-quality growth on Si demands for temperature variations of more than 100°C within one superlattice period. Strain relaxation of these SLs with increasing number of periods has been directly compared by means of TEM. For the compressively strained structures grown on Si we found misfit dislocations of the type 60° (a/2)<110>. Under opposite strain conditions i.e. for growth on Ge, strain relief occurs only by microtwin formation through successive glide of 90° (a/6)<211> Shockley partial dislocations. This is consistent with a calculation of the activation energy for both cases based on a homogeneous dislocation nucleation model.

Microstructural Characterization of SiGe Heterolayers

ABSTRACTSiGe is of interest for use in heterojunction-bipolar transistors, infrared detectors and field-effect transistors. In this study, graded SiGe heterolayers grown on Si, and heterolayers grown on SIMOX by CVD, were characterized using TEM. The graded-heterolayers consisted of ten layers of Si1-xGex on substrate silicon. Misfit dislocations were present at interfaces in the bottom 4–5 layers of the heterostructure. This conforms with predictions from qualitative strain-energy considerations. The greatest density of misfit dislocations was present at the Si1-xGex interface between the bottom two layers of the heterostructure. Dislocations were observed to extend out of the interface and up into the heterolayer structure. The defects were found to interact with interfaces in the structure and finally cease extending upwards towards the surface of the wafer. In addition to graded heterolayers, SiGe heterolayers grown on SIMOX were also investigated. The structures consisted of epi-silicon grown on a Si/Si1-xGex superlattice which was in turn grown on a Si/SiO2 (SIMOX) structure. The behavior of defects in the layers was of interest. TEM characterization showed a large density of extended-defects present in the layers. Dislocations were observed to originate at the SIMOX oxide/Si interface, propagate up through the SiGe superlattice and into the epi-Si layer. Some dislocations were found to interact with the SiGe superlattice and cease propagating up towards the top of the wafer. SiGe superlattices with a higher concentration of Ge are more effective in reducing defect propagation towards the surface of the wafer.

Wave Function Engineering of Linear and Non-Linear Optical Response in Si-Ge Quantum Well Structures and Superlattices

ABSTRACTWe show that the infinite Si-Ge superlattices predicted to exhibit strong optical transitions across the fundamental gap and between conduction subbands also give rise to significant optical nonlinearities. Efficient optical transitions can also be achieved in finite-length structures grown on Si or Ge substrates. However, the zone folding argument does not apply and the electronic structure strongly reflects the finite length of such systems.

Optical and structural studies of relaxation in [formula omitted] strained-layer superlattices

Asymmetrically and symmetrically strained Si SiGe superlattices were grown on Si(001) by Molecular Beam Epitaxy (MBE) and studied as a function of thermal treatments. X-ray diffraction was used to monitor the extent of interdiffusion with annealing time and temperature and results indicate that initially the interdiffusion is very rapid and non-linear, but later a steady-state regime is attained. Raman spectroscopy has shown that in the very early heating stages, strain relaxation occurs predominantly by interdiffusion, and Ge diffusion can be enhanced by strain. The results also indicate that a significant contribution to the interdiffusion occurs by an interstitial-type mechanism.

QUANTUM WELLS IN OPTOELECTRONICS: Linear and nonlinear optical properties of direct gap Si-Ge superlattices

Optical properties of Si-Ge superlattices grown on virtual substrates are investigated with a view to engineering optimum characteristics. The oscillator strength and transition energy, both across the fundamental gap and between the conduction sub-bands, are examined as a function of the choice of substrate and well/barrier width. In the former case, the transition energy is shown to be tunable in the range 0.6–1.0 eV which covers the technologically important optical fibre ‘window’ at 1.55 μm. Intersub-band transitions occur at wavelengths in the far infrared. A strong nonlinear optical response can also be achieved via a novel virtual excitation mechanism. Growth parameters are presented for systems in which this response is maximised while keeping the absorption to a minimum.

Optimization of growth parameters for direct band gap SiGe superlattices

The criteria affecting the direct/indirect nature of the lowest energy transition across the band gap of a SiGe superlattice are discussed, and a prescription for obtaining a direct gap system is given. A systematic study of the optical properties of Si nGe n superlattices as a function of period and substrate is performed in order to optimize the energy and transition strength. Specific structural parameters are presented for two systems in which the lowest energy transition is direct and of a magnitude large enough to be of practical use. The range of energies for which direct transitions are possible is found to cover an almost continuous spectrum from 50 meV to around 900 meV, the lowest energy transitions being between conduction subbands. Of particular interest to optical fibre applications is the prediction that, with a suitably chosen substrate, the Si 5Ge 5 superlattice has a direct optical band gap corresponding to a wavelength of 1.55 μm.

Calculations of phonon spectra in III–V and SiGe superlattices: A tool for structural characterization

This paper reviews some of the basic concepts of phonons in semiconductor superlattices with emphasis on the aspects which are of relevance for characterization.

Raman scattering from SiGe superlattices with disordered interface layers

Raman spectra of realistic SiGe superlattices with disordered interface layers are calculated. The superlattice structure is simulated by a great cluster of atoms. The Raman spectrum is calculated using short-range interaction models and applying the recursion method. The relaxation of quantum effects in the presence of disordered interface layers and the appearance of alloy contributions to the Raman spectra are discussed for a series of SiGe superlattices with increasing width of the disordered interface region.

Large optical nonlinearities in SiGe superlattices

A nonlinear optical response in a SiGe superlattice can be achieved by means of a multi-photon virtual excitation process. The nonlinearity can be greatly enhanced by engineering the energy levels and transition probabilities across the band gap and between higher lying conduction subbands. In this paper we discuss how such an enhancement can be realized in a SiGe superlattice and predict structural parameters for systems in which these conditions are fulfilled, resulting in a ‘direct’ gap superlattice structure with a potentially large value of χ (3). This virtual excitation mechanism is particularly attractive in that the excitation energies are below the absorption edge. Consequently, the strong dissipative effects associated with band filling processes do not occur.

E-Beam electroreflectance of short-period SiGe superlattices

Electron-beam electroreflectance (EBER) has been used to investigate electronic and optical properties of short-period SiGe superlattices grown by molecular beam epitaxy on (001)Si. The advantage of the EBER method over conventional ER techniques is that no contact has to be made to the sample. The results are compared with those obtained from conventional Schottky barrier electroreflectance (SBER) measurements performed on samples taken from the same wafer. The two ER techniques give results which are in good agreement. The EBER and SBER experimental results on these SiGe superlattice samples closely match theoretically predicted transitions obtained from full-scale empirical pseudopotential calculations.

Performance and processing line integration of a silicon molecular beam epitaxy system

Abstract Silicon molecular beam epitaxy (MBE) has been shown to be a promising technology for ultralow temperature epitaxy. Its capability to deposit strained Si-Ge layers and superlattices has received much attention. In this paper, the design, performance tests and processing line integration of a silicon MBE system capable of handling waters of 150 mm diameter is described. Several parameters, such as particle density, deposition uniformity, background doping, etch pit density and metallic contamination, have been carefully monitored during the first year of operation. These parameters are important acceptance criteria for the integration of silicon MBE into a standard processing line. The deposition uniformity across 140 mm was within 2%. Extensive measurements on the particulates identified the major source to be the first handling of wafers outside the cassette. Levels below 100 cm -2 are found. The background doping has been reduced from around 10 16 to 10 15 cm -3 . Strong reductions have been observed for the metallic contaminants tantalum and chromium. The system has been used as part of a standard processing line for the fabrication of thin, low temperature epitaxial layers. Two examples, epitaxial emitters for bipolar transistors, and p-i-n diodes, are given.

Matrix effects in sims depth profiles of SiGe superlattices

Abstract The dependence of the secondary ion yield of Ge and Si atoms in Si1-xGex layers, where x varies from 0.005 to 0.75, has been measured for incident sputtering beams of O+2 and Cs+ of varying energy. For the O+2 beam, the positive ion yield per atom was constant for Ge concentrations 5%, whereas the negative ion yield decreased significantly.

Confined phonons in strained short-period (001) Si/Ge superlattices

Confined and quasi-confined phonons have been studied in strained Si/Ge superlattices (SLs) with different strain distributions and periods by inelastic light (Raman) scattering. Additional excitations have been found in strained Si/Ge SLs with thin layers (2–6 monolayers) of silicon and germanium. The observed strain effects have been compared with uniaxial stress experiments of bulk silicon and germanium, and the confinement effects have been compared with the phonon branches of bulk silicon and germanium. The additional excitations are assigned to higher-order confined modes and to localized modes caused by SiGe ordering in 〈111〉 direction along the interfaces.

Optical properties of perfect and imperfect SiGe superlattices

We report fresh pseudopotential calculations of the electronic structure and optical properties of SiGe superlattices. We assess the effect of the finite character of heterostructures, and of the breakdown of the translational symmetry in the interface plane due to interdiffusion and defects. We show that intersubband optical transitions can be used as a useful alternative to band gap luminescence for interface characterization. We also evaluate the key electronic and optical properties of related systems, e.g. Si Si x Sn 1− x and Ge Ge x Sn 1− x .

SiGe strained layer superlattices

High quality SiGe strained layer superlattices can be achieved by low temperature molecular beam epitaxy. Strain distribution and Brillouin zone folding effects have a large influence on the band structure. Photoluminescence from strain-symmetrized Si 6Ge 4 superlattices shows the first evidence for the achievement of a quasi-direct fundamental energy gap. Raman spectroscopy is used as a versatile tool to characterize short-period SiGe superlattices.

Chemical ordering and boundary structure in crystalline SiGe superlattices

Strained-layer superlattices of crystalline silicon and germanium, grown by molecular beam epitaxy on Si(100) have been studied by high resolution transmission electron microscopy (200 and 300 kV) and selected area electron diffraction techniques, to investigate the atomic boundary structure and to obtain information on chemical order in this material. Two types of specimens were investigated: 1. (1)specimens with asymmetrically strained superlattices (cell parameter of germanium totally adapted to that of silicon), with germanium layers having a thickness of two to six monolayers and a superlattice period of 4–6 nm, grown on a silicon buffer at 460–470°C; 2. (2)specimens with symmetrically strained superlattices, with germanium layers having a thickness of three to five monolayers and a lattice period smaller than 1.5 nm, grown at 400–420°C on an Si x Ge 1− x buffer with the same composition as the superlattice. The transmission electron micrographs show the persistence of pseudomorphic growth of the superlattice for both types of specimens. The layer boundaries are sharp in the atomic size range, the interface being defined to within one monolayer of silicon or germanium. Selected area electron diffraction patterns contain weak additional reflections which prove the existence of domains causing a doubling of the periodicity in the [111] direction. This doubling is explained by chemical ordering of germanium and silicon atoms in the germanium layers, in analogy with similar effects in Si x Ge 1− x alloys. An additional strong streaking observed parallel to the direction of growth is found to be a result of inhomogeneities in the lattice period and of lattice distortions. The two additional diffraction effects are observed in both types of specimens.

Empirical potential-based Si-Ge interatomic potential and its application to superlattice stability.

A new empirical interatomic potential for Si-Ge alloy system is proposed based on the Khor--Das Sarma universal interatomic potential for tetrahedrally bonded materials. We modify the Khor--Das Sarma potential and check its validity by calculating the elastic constants of Si and Ge using the homogeneous-deformation method. The feasibility for applying this potential to binary systems is tested by investigating pressure-induced phase transitions associated with isotropic compression and tetragonal distortion. The new empirical potentials for Si, Ge, and Si-Ge are used to calculate thermodynamic properties such as lattice parameters, bond lengths, and excess energies for Si-Ge superlattices. The calculated results are shown to be in good agreement with available experimental data. We also calculate the phonon dispersion for bulk Si. The newly obtained empirical potentials for Si, Ge, and Si-Ge should be useful for investigating Si-Ge binary systems.

Chemical ordering and boundary structure in strained-layer Si-Ge superlattices.

The interface structure of strained-layer Si-Ge superlattices on Si(100) is investigated by selected-area electron-diffraction techniques and high-resolution transmission electron microscopy. Three types of strained-layer Si-Ge superlattices characterized by different strain distributions are compared. Diffraction evidence is presented for the existence of boundary layers at the interfaces consisting of chemically ordered domains with a bilayer stacking of Si and Ge along the 〈111〉 directions. This phenomenon is independent of the strain distribution between Si and Ge, i.e., of the choice of the substrate lattice parameter.

Strain Adjustment in Si / SiGe Superlattices by a SiGe Alloy Buffer Layer

Strain distribution in a strained layer superlattice on Si can be adjusted by epitaxial insertion of a strain relieved homogeneous buffer layer between the substrate and the superlattice. We report on the strain relief in alloy buffer layers with as a function of layer thickness. The layers are grown by molecular beam epitaxy on (100) Si substrates at 450°C. X‐ray diffraction, transmission electron microscopy, and Rutherford backscattering are applied to the film analysis. Above a critical thickness, i.e., the onset of strain relief, in a first range a strong decrease of film strain by formation of misfit dislocations is observed followed by a saturation‐like branch with only a weak strain thickness dependency. A residual strain of about 1/3 of the lattice mismatch is found at a thickness of five times the critical one.

Computer simulation of Si and Ge adatoms and thin layers on Si substrates

The molecular dynamics method is used to determine minimum energy configurations of Si and Ge adatoms on a Si surface. Results for thin layers of Ge on Si substrates and Si-Ge superlattices are also presented.