The band structure of self-assembled Si/Ge quantum dot structures deposited by molecular beam epitaxy in the Stranski Krastanov growth mode is characterized by optical and electrical spectroscopy. Interband and intraband absorption, photocurrent, photoluminescence, Raman and admittance spectroscopy of structures with quantum dots of about 20 nm lateral size offer insight into the discrete level scheme within the valence band, the optical transitions and the lifetime of localized hole states. The results are discussed with respect to their possible applications in infrared light detection, storage and quantum-logic devices.

We present the first demonstration of spectrally selective charge storage in self-assembled quantum dots that utilizes optical charge generation and readout of the stored charge distribution. Resonant optical excitation charges only a sub-ensemble of the inhomogeneous broadened dots. The optical readout process directly recovers the spectral distribution of stored charges. Wavelength selectivity is demonstrated in a two-color experiment exhibiting charge storage in two independent QD sub-ensembles. Resonant charge storage is observed at least up to 85K depending on excitation energy. Lack of time evolution of the spectral emission indicates charge storage times Δt much longer than 25μs.

Two-dimensionally ordered arrays of Ge islands are realized by molecular beam epitaxy on vicinal Si(001) surfaces with regular ripples. Deposition of a 2.5 nm Si0.55Ge0.45/10 nm Si multilayer on vicinal Si(001) surfaces gives rise to the formation of regular ripples with a typical period of 100 nm, due to step-bunching. The ripples lead to the long-range line-up of the Ge islands along their direction, while the strong repulsive interaction between the dense Ge islands determines their relative arrangement on different step bunches of a ripple. The ordering pattern can be controlled by the Ge coverage as well as the direction of the ripples. The Ge islands show a narrow size distribution with the lateral size limited by the ripple period

In contrast, when deposited directly on well-prepared biatomic-stepped vicinal Si(001) surfaces under the same growth conditions, only weak ordering of Ge islands along the step direction is achieved. No ordering of Ge islands has been observed, when a flat Si(001) surface is employed, where no obvious step-bunching occurs.

The results promise efficient control on the position and size of self-assembled and selfordered Ge islands by the steps prepared on vicinal surfaces.

We have fabricated erbium- and oxygen-doped Si/SiGe waveguide diodes showing the characteristic 1.54 µm electroluminescence (EL) from incorporated Er+3, ions. All samples were grown by molecular beam epitaxy (MBE). The EL from the polished end facet of the waveguide was measured with a confocal microscope revealing a spatially narrow emission. Additional annealing was not necessary to improve the luminescence characteristics. Only a weak temperature dependence is found for the EL intensity between 4K and room temperature.

We report on a systematic study of the growth parameters of erbium-oxygen-doped silicon grown by molecular beam epitaxy. The surface quality of the grown layers was measured in situ by RHEED. The samples were characterized by photoluminescence measurements and SIMS. An Er-O-doped Si light emitting diode grown with the optimized parameters is presented.

Self-organized Ge-dots on (001)-oriented Si-substrates have been studied using two-dimensionally resolved high resolution x-ray diffraction and reflectivity. The degree of the vertical correlation of the dot positions ("stacking") has been derived as well as a lateral ordering of the dots in a (disordered) square array with main axes parallel to ]100] and ]010].

Diffuse x-ray reflection from a SiGe/Si multilayer grown pseudomorphically on slightly miscut Si(OOl) substrates has been studied theoretically and experimentally. In the framework of the Distorted-Wave Born Approximation (DWBA), we demonstrated that the distribution of the diffusely scattered intensity gives conclusive information on both the amount and the in-plane and inter-plane correlation properties of the interface roughness. The best model for the description of the interface-morphology was found to be a combination of a two-level model and a staircase model.

A survey is given about the potential use of Si/SiGe heterostructures for applications in the mid-infrared spectral range. We discuss theoretical foundations and experiments of intersubband absorption in p-type Si/SiGe quantum wells and show that due to the complex valence-band structure, normal-incidence absorption can be observed. On the basis of these quantum wells, mid-infrared detectors were fabricated and characterized in terms of responsivity, dark current and detectivity. In asymmetric, compositionally stepped quantum wells second harmonic generation of CO2 laser radiation has been demonstrated.

We report on band-gap luminescence in short period, strain symmetrized (Si)m(Ge)n superlattices grown on relaxed, step-graded Sil-xGex alloy buffer layers. The dislocation density in the superlattices, which were grown at 500°C using Sb as a surfactant, is reduced by 2-3 orders of magnitude compared with superlattices grown on thin, partly relaxed Sil-xGex buffer layers. Due to the improved quality of the superlattices, well defined band-gap luminescence could be observed which is for a (Si)6(Ge)4 superlattice strongly enhanced compared with a Si0.6Ge0.4 alloy reference sample. The measured band-gap energies compare well with theoretical predictions. To study the influence of interdiffusion of the Si- and Gelayers on the band-gap of the superlattices, the samples were annealed and studied with photoluminescence and Raman spectroscopy. An increasing band-gap and a decreasing luminescence efficiency was found with increasing intermixing of the layers. These experimental results are well described with an interdiffuision model of the layers in conjunction with an effective mass calculation.

We report on detailed luminescence studies of MBE grown Si/Si1-xGex quantum well structures. Both well width and composition is varied over a wide range. Bandgap photoluminescence is observed for all samples grown at elevated temperatures. The measured bandgap energies are in good agreement with subband calculations based on effective mass approximation and taking into account the segregation of Ge atoms during growth. Diffusion is found to limit quantum well (QW) growth with Ge-contents above 35% at high temperatures. The photoluminescence signals are detected up to about 100K and can be attributed to interband transitions of free excitons. We also present investigations of the exciton binding energy as a function of well width and composition. The observed shift of the exciton binding energy is compared with results of a variational calculation. A distinct onset in photocurrent and electroluminescence up to 200 K are observed in quantum well diodes.

P-i-n doped short-period SimGen strained layer superlattices (SLS) are grown on (100) silicon substrates by low temperature molecular beam epitaxy (300C°<∼Tg<∼400C°). The SLS's are grown with period lengths around 10 monolayers (ML) to a thickness of 250nm on a rather thin (50nm) homogeneous Si1−ybGeyb alloy buffer layer serving as strain symmetrizing substrate. Photoluminescence at T=5K is observed for various SimGen SLS samples, the strongest signal was found for a Si5 Ge5 SLS. Samples with identical SLS's but different buffer layer composition and thicknesses are grown to study the influence of strain on the PL. Electroluminescence (EL) at the same energy range is observed from mounted SimGen SLS mesa and waveguide diodes up to T=130K – for the first time reported in strain symmetrized short-period SimGen SLS. The intensity and peak positon of the EL signal was found to be dependent on the injected electrical power.

Short period Si/Ge superlattices have been grown on Ge (001) and Si (001) substrates by molecular beam epitaxy. The optical properties of the superlattices have been studied with photoreflectance. (PR) and resonant Raman scattering (RRS). With PR we are able to observe new, structural induced transitions for all superlattices which are related to E0-and E1-like gaps. The analysis of PR spectra is complicated by an optical etalon effect if the samples are sufficently thick. The E1-like transitions in the range between 1.9eV and 2.7eV are also studied with RRS. Due to the confinement of the optical phonons in the Ge and Si layers RRS is able to probe the bandstructure in each layer seperately. Localized electronic states in the Ge layers can be observed with RRS for a Si4Ge18 superlattice and are compared with PR measurements.

EXTENDED ABSTRACT: We investigated resonant tunneling of holes in Si/Si1−xGex double barrier resonant tunneling (DBRT) diodes grown by MBE [1]. The aim of this study was an unambiguous identification of the quantum states involved In the two dominating resonances (Fig. 1), which have been observed by several groups [2],[3], [4]. For this purpose we calculated the transmission probabilities as a function of the well width by means of a transfer matrix method in a one-particle model [5]. The respective effective masses of heavy-holes (hh) and light-holes (lh) in the strained SiGe layers were calculated following Refs. (6) and [7], with appropriate strain corrections added according to Ref. (8); hh/lh mixing [9] was Introduced a posteriori. The calculated resonance positions are in reasonable agreement with experiments (Fig. 2), if the resonance at lower energy is attributed to a hh channel in the well, and the second resonance to a lh channel. Since only hh states are occupied in the emitter cladding layer at the temperatures (≈1.5K) studied, hh/lh coverslon due to the loss in rotational symmetry is involved in the second resonance.

The performance of future microelectronic circuits will be strongly enhanced by the monolithic integration of superlattice devices with convenventional integrated circuits ontop of a silicon substrate. With this material concept the mismatch between the superlattice materials and the silicon substrate has to be accommodated. SiGe/Si is a model system for the study of mismatch effects because of similar chemistry and well pronounced strain effects.

Growth of SiGe/Si strained layer superlattices (SLS) by molecular beam epitaxy is reported. Adjustment of strain within the superlattice influences strongly the stability and band ordering of the SLS. SiGe/Si superlattices with strain symmetrization exhibit good thermodynamic stability and band ordering of type II. The concept of strain symmetrization by a thin homogeneous buffer layer is explained. Design rules for a virtual substrate consisting of the actual substrate with the thin buffer layer on it are given.

Growth of Si/SiGe superlattices on Si substrates by molecular beam epitaxy (MBE) is described. Strain symmetrization in the superlattice is achieved with an incommensurate SiGe buffer layer. The concept of strainsymmetrization is explained and properties of buffer and strained layer superlattices are investigated. A twodimensional electron gas with enhanced room temperature mobility and folded phonon modes within the reduced onedimensional Brillouin zone are observed. An n-channel Si/SiGe MODFET demonstrates the device applications of this material concept.