ALBERT

Notes: Coherent Si1−xGex alloys and multilayers synthesized by molecular beam epitaxy (MBE) on Si(100) substrates have been characterized by low-temperature photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM). Phonon-resolved transitions originating from excitons bound to shallow impurities were observed in addition to a broad band of intense luminescence. The broad PL band was predominant when the alloy layer thickness was greater than 40–100 A(ring), depending on x and the strain energy density. The strength of the broad PL band was correlated with the areal density (up to ∼109 cm−2) of strain perturbations (local lattice dilation ∼15 A(ring) in diameter) observed in plan-view TEM. Thinner alloy layers exhibited phonon-resolved PL spectra, similar to bulk material, but shifted in energy due to strain and hole quantum confinement. Photoluminescence excitation spectroscopy, external quantum efficiency, time-resolved PL decay, together with the power and temperature dependence of luminescence intensity, have been used to characterize Si1−xGex/Si heterostructures exhibiting both types of PL spectra. The role of MBE growth parameters in determining optical properties was investigated by changing the quantum well thickness and growth temperature. The transition from phonon-resolved, near-band-gap luminescence in thin layers to the broad PL band typical of thick layers is discussed in terms of a strain energy balance model which predicts a "transition thickness'' which decreases with increase in x.

Notes: We have applied the technique of photothermal ionization spectroscopy to the study of a 1-μm-thick p-Ge epilayer, grown by molecular beam epitaxy on a n-Si substrate, 500 μm in thickness. The spectra indicate that in the Ge layer there exists a series of charged acceptor defects with an ionization energy continuum starting at 15 meV, an ionization energy somewhat larger than those of the elemental substitutional acceptors. Our results show that photothermal ionization spectroscopy can be applied very advantageously to epitaxial layers of the Si1−xGex alloys that are Ge-like, i.e., for x≥0.85. For these layers, charged impurity centers and defects have their spectral features well separated from those of the Si substrate.

Notes: A semiempirical kinetic model is presented which maps out the thermal budget for processing of strained layer devices through epitaxial growth and postgrowth anneals. Misfit strain relaxation in Si1−xGex/Si heterostructures by the injection and propagation of a/2 〈110〉 60°-type misfit dislocations has been studied for a range of geometries and dimensions. Strained layer superlattices, Si1−xGex alloy layers, 0〈x〈0.3, and alloy layers with unstrained Si capping layers of thickness 0 to 400 nm were grown by molecular-beam epitaxy on (100) Si substrates and subjected to post-growth thermal cycles. Velocity and nucleation rate data from Nomarski interference microscopy of defect-etched surfaces were correlated with electron beam induced current microscopy transmission electron microscopy and x-ray diffraction results to define Arrhenius relationships for misfit dislocation injection rates and propagation velocities. A unified kinetic model for misfit strain relaxation that incorporates both nucleation and propagation is then developed, which is applicable for all heterostructures and thermal cycles in the low dislocation density regime 〈103 mm−1.Nonuniform strain distribution in graded device heterostructures is considered by defining the effective stress acting on misfit dislocations for an arbitrary geometry. The effective stress was varied from 0 to 750 MPa in Si1−xGex/Si heterostructures by varying both layer dimensions and Ge concentration. Misfit dislocation nucleation rates varied from 10−3 to 103 mm−2 s−1 and misfit extension velocities of 25 nm s−1 to 2 mm s−1 were obtained over the temperature range 450–1000 °C for anneals of duration 5–2000 s. Activation energies, stress exponents, and prefactors in the Arrhenius relations were found to be independent of Ge concentration, effective stress, and heterostructure geometry allowing a comprehensive model to be developed. The onset of strain relaxation during epitaxial growth cycles (the "apparent critical thickness'' or metastability limit) characteristic of molecular-beam epitaxy and chemical vapor deposition was measured and correlated with the simulation of misfit dislocation injection and propagation in typical growth sequences. The kinetic model is also used to define the maximum time-temperature envelope, or thermal budget (t, T), for the misfit dislocation-free processing of Si1−xGex/Si heterostructures subjected to post-growth thermal treatme

Notes: We have measured the diamagnetic shift of band-edge luminescence, no-phonon peak, and phonon sideband from a pseudomorphic, undoped Si0.83Ge0.17/Si quantum well using high-field magnetoluminescence spectroscopy. The quadratic dependence of the diamagnetic shift of the no-phonon luminescence peak suggests that the luminescence originates from excitons. Using a variational calculation, we determine the effective heavy-hole mass and the exciton binding energy to be 0.27m0 and 14.8 meV, respectively, and compare these results with reported values obtained from other measurement techniques. © 1995 American Institute of Physics.

Notes: We present a physical model to study the interface states in p-n heterostructures at different ambient temperatures. Field-assisted emission of such states is considered as the source for the linear increase in charge concentration at the p-n interface with the applied reverse voltage. The high frequency capacitance-voltage technique is used to study the charging and discharging of interface states in an MBE-made sample at different temperatures and different biases. The experimental results show good agreement with the prediction of our model. © 1995 American Institute of Physics.

Notes: Electroluminescence has been observed from Si1−xGex/Si p-n heterostructures grown by molecular beam epitaxy and fabricated into mesa diodes. The luminescence from each sample was observed at temperatures up to 80 K with diodes forward biased at current densities up to 50 A/cm2. For x=0.18 and x=0.25, broad (∼80 meV) electroluminescence peaks were observed at 890 and 860 meV, respectively. These energies as well as the peak shapes and quantum efficiencies (∼1%) were the same as those from corresponding photoluminescence spectra.

Notes: Temperature-dependent photoluminescence (PL) measurements have been used to characterize 5-μm-thick Si(100) epitaxial layers doped in situ during molecular beam epitaxial growth with low-energy (100, 500, and 1000 eV) 11 B+ ions at growth temperatures of 500, 650, and 800 °C. Moderate doping (NB ∼1017 cm−3) yielded PL features comprised of both sharp and broad peaks in the boron bound exciton (B-BE) region. At 4.2 K a broad B-BE feature near 1086 meV dominated, although the sharp transverse optical phonon-assisted B-BE peak (B1TO ) at 1092.5 meV was resolvable for NB〈1017 cm−3. Increasing the PL sample temperature above 4.2 K caused a rapid decay of the broad B-BE peak intensity, thus permitting comparison of B1TO intensity for a range of ion energies and growth temperatures. At 10 K, a bulk-like spectrum containing a sharp B1TO peak with weaker multiexciton peaks B2TO and B3TO was observed for the film growth at the highest temperature and lowest ion energy (800 °C and 100 eV). However, the intensity of the B1TO peak decreased with decreasing growth temperature (constant ion energy) and with increasing ion energy (constant growth temperature). Samples grown at the lowest temperature (500 °C) displayed very different PL spectra with much weaker line emission, a rising PL background, and additional lines near 1040 meV due to ion-induced residual lattice defects. Quenching of B1TO and the other sharp B-BE peaks was accompanied by an increase in the N1 peak at 745.7 meV.

Notes: Double-crystal x-ray diffraction has been successfully used to study dimensional and compositional variations in Si1−x Gex /Si superlattices. For most samples studied an adequate agreement between theoretical and experimental diffraction profiles is obtained by using a theoretical model (kinematical) which assumes uniform superlattices with abrupt interfaces. In some samples, however, the superlattice-related diffraction peaks reveal an asymmetric broadening which is shown by comparison with dynamical diffraction simulations to be due to a grading in layer compositions and thicknesses throughout the superlattice stack. Detailed analysis of the diffraction data identified a gradual drift of 1.25% in the Ge flux during molecular beam epitaxy (MBE) growth. The MBE system parameters were modified to compensate for this effect and allow the growth of more structurally perfect strained-layer superlattices.

Notes: A fully adjustable self-limiter and optical clamp in a geometry suitable for use in a fiber optics context is demonstrated. Edge-coupled light into a silicon thin film on a doped silicon substrate lowers the refractive index of the guiding film and eventually brings it to the cutoff condition. Furthermore, by varying the amount of light incident on the nonlinear substrate and the nonlinear film waveguide, a fully adjustable limiting action is obtained. It is shown that absorption effects are not important at power levels compatible with fiber optics, making the limiter highly damage resistant.

Notes: Coevaporation of B2O3 during silicon molecular beam epitaxy has been used to prepare heavily doped superlattices (pipi's). Full activation up to 3×1020 cm−3 (100 times the solid solubility limit) was obtained at growth temperatures below 700 °C. Significant boron redistribution has been observed into the undoped layers when the dopant level in the intentionally doped layers exceeds the solid solubility limit and the growth temperature is greater than 700 °C. Oxygen was not incorporated into the lattice for growth temperatures above 700 °C when using B2O3 as the source of boron, a Si growth rate for 0.5 nm s−1, and a B2O3 arrival rate of ∼2×1013 cm−2 s−1.