Strained Sil-xGex quantum wells and multi-quantum wells, synthesized by solid source ebeam evaporated MBE on Si(100) substrates have been studied by low temperature photoluminescence (PL) spectroscopy. Phonon resolved transitions originating from excitons bound to shallow impurities in Sil-xGex layers were observed over the temperature range 2K to 100K and used to characterize Sil-xGex/Si heterostructures. Thin Sil-xGex quantum wells exhibited phonon-resolved PL spectra, similar to bulk material, but shifted in energy due to strain, quantum well width and Ge fraction. In single quantum wells confinement shifts up to ∼200 meV were observed (1.2 nm wells with x = 0.38) and NP linewidths down to 1.37 meV were obtained. The confinement shifts were modeled by hole confinement in Sil-xGex wells. An annealing study was performed to investigate the role of Si-Ge interdiffusion on luminescence. Both the geometrical shape and optical emission of the quantum well were found to significantly change through intermixing. In addition to near edge luminescence a broad band of intense luminescence was obtained from several Sil-xGex/Si heterostructures. Some layers exhibited both types of PL spectra. However, the broad PL band (peak energy ∼120meV below the strained bandgap) was predominant when the alloy layer thickness was greater than 2 - 10nm, depending on x, growth temperature, and substrate surface preparation. The strength of the broad PL band was correlated with the areal density of strain perturbations (∼109cm−2 per quantum well corresponding to a spacing of 300nm in the plane of the well; local lattice dilation ∼ 1.5 nm in diameter) observed in plan-view TEM. The first few wells of MQW exhibited only band edge luminescence as was revealed by etching off the upper MQW periods. In addition, post growth anneals at temperatures in the range 700°C to 1100°C were found to enhance band edge luminescence, while the broad luminescence band decayed to zero intensity. Interdiffusion at these these temperatures has been shown to dramatically change the QW shape and consequently interfacial asperities would be expected to disappear, consequently only shallow phonon resolved luminescence is observed in PL after annealing. The influence of PL measurement parameters such as excitation power density and PL sample temperature on the relative strengths of band edge versus broad band luminescence were also consistent with the presence of exciton traps at sites of reduced bandgap.

We have observed photoluminescence from strained SiGe quantum well layers at energies approximately equal to twice the SiGe band-gap energy. This luminescence is caused by the simultaneous recombination of two electron hole pairs yielding a single photon. Detection of luminescence at twice the band-gap has been previously used in Si to observe luminescence originating from electron-hole droplets, biexcitons, bound multiexciton complexes and polyexcitons. Time resolved spectra at twice the band-gap have been obtained from our SiGe samples prepared by molecular beam epitaxy (MIRE) as well as rapid thermal chemical vapor deposition (RTCVD). This new luminescence clearly distinguishes multiexciton or dense e-h plasma processes from single exciton processes such as bound excitons, free excitons or localized excitons, which are difficult to separate in the usual nearinfrared luminescence.

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.

The optoelectronic properties of SimGen strained layer superlattices (SLS's) depend strongly on the structural perfection. We used double crystal and triple axis x-ray diffractometry to characterize the structural properties of short period Si9Ge6 SLS's grown on about lμm thick step-graded SiGe alloy buffers. As grown SLS's and samples annealed subsequently at 550°C, 650°C and 780°C for 60 mmn were investigated. Precise strain data were extracted from two-dimensional reciprocal space maps around (004) and (224) reciprocal lattice points. These data were used as refined input parameters for the dynamical simulation of the integrated intensity along the qll[004] direction. Annealing causes interdiffusion as indicated by the decreasing superlattice (SL)-satellite peak intensities and by the change of the Si/Ge thickness ratio. However, the full width at half maximum of the SL satellite peaks does not change significantly with annealing up to 650°C. The in-plane SL lattice constant in both samples is increased only slighty by annealing (< 9×10−3 Å). Consequently the interface intermixing due to interdiffusion is the main cause for the shift of the luminescence energy to higher values in these annealed samples.

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.

In Si0.88Ge0.12/Si strained layers misfit dislocations formed during growth in small pads are generated at a significantly higher critical thickness than on extended areas, while pads of lateral size of 10 μm or smaller show no evidence of misfit dislocations at all. The SiGe layers investigated were selectively grown on patterned substrates with pad sizes from 2 μm to 1 cm. An elastic relaxation model was used to calculate the pad size dependence of the critical thickness. The main hypothesis of the model is that the density of misfit dislocations is solely affected by the elastic relaxation at the edges of small epitaxial areas. This equilibrium model is able to explain the observed absence of misfit dislocations on small pads, however it predicts a critical thickness for finite sizes much lower than the observed one.

Sil-xGex, undoped strained layers with a pure Si-capping layer were grown epitaxially by UHV CVD on Si (100) substrates. The samples were subjected to thermal treatments corresponding to typical deviceprocessing. Effects connected with thermally induced relaxations were studied by Raman scattering, XRD, photoluminescence (PL) and defect etching. The thickness of the layers was determined from X-ray reflectance measurement. Strain values for the as-grown and relaxed samples were extracted from Raman scattering, XRD measurements and defect etching and correlated. Sil-xGex-layer related peaks were observed in PL spectra of several samples. An explanation why those peaks are not observed in all the samples is suggested.

Interband optical transitions are observed in a series of strain-adjusted, short-period Si/Ge superlattices by means of photocurrent spectroscopy, infrared absorption, photo (PL)- and electroluminescence (EL). The onsets of the interband absorption in the energy range of 0.7 - 0.9 eV are in good agreement with the observed PL and EL. Bandgap-related EL is observed in mesa diodes at room temperature, whereas the PL disappears at about 40K. In samples, annealed at growth temperatures (550°C) and higher, a systematic shift of the bandgap is observed which is discussed in terms of a process involving interdiffusion of the Si and Ge atoms. Photocurrent measurements at low temperatures support the model from PL studies suggesting that the photogenerated electrons are immobile in the SLS at low temperatures and have to be thermally ionized from shallow levels.

A new, fast, intrasubband 1.55 μm electrooptic modulator in the SiGe/Si/CaF2-on-Si steppedquantum- well system is proposed and analyzed. At electric fields of ±8 V/μm, resonant 1-3 conduction intrasubband absorption is predicted to give 18 dB of optical extinction for narrow-linewidth transitions. We expect this field effect modulator to have microwave response, plus compatibility with Si-based optoelectronic integration.

Well-resolved band-edge exciton photoluminescence (PL) has been observed in strained Si1-xGex. heterostructures grown on Si(100) by rapid thermal chemical vapor deposition. The luminescence is due to shallow-impurity bound excitons at low temperatures (under 20K) and at higher temperatures is due to free excitons or electron-hole plasmas, depending on the pump power. The luminescence can also be electrically pumped, with both the electroluminescence and PL persisting above room temperature in samples with a sufficient bandgap offset. Loss of carrier confinement and subsequent non-radiative recombination outside the Si1-xGex. is found to be the reason for reduced PL and EL at high temperature.

Well-resolved band edge photoluminescence spectra were obtained from SiGe quantum wells of various widths. In addition to the usual shallow bound exciton features, we observed a highly efficient deeper luminescence process, under conditions of low excitation density, in thick SiGe quantum wells. This luminescence band can be attributed to excitons localized by fluctuations in alloy concentration. The binding energy of the localized exciton feature is found to decrease with decreasing well width. In the thinnest quantum well samples only a single luminescence feature is observed at all power levels, while in several other thin quantum well samples having very sharp lines the localized exciton feature appears at higher energy than the bound exciton. Despite these changes in the spectra, the localized exciton luminescence could be identified in all cases by its characteristic intensity saturation at low excitation power density, as well as its slow decay time (∼ 1 ms). The mechanism behind the changes in the localized exciton luminescence may originate from limiting the exciton motion to two dimensions in thin wells, which at low temperatures would hinder migration to the lowest energy alloy fluctuation centers.

Sil-xGex/Si(100) single quantum well samples were grown by MBE with x varying from 18.5% to 36%, well widths Lz from 1.1 nm to 175 nm, and growth temperatures To in the range from 350°C to 750°C. We studied the photoluminescence (PL) properties in detail using Fourier transform spectroscopy. In most of the samples, the dominant and phononresolved SiGe band edge PL exhibits exclusively free exciton luminescence even at sample temperatures as low as 2 K. From the absence of bound excitons we conclude that the background concentration of shallow impurities is low in our SiGe layers. Variation of the growth temperature of the Si cap layer strongly influences the efficiency of the SiGe band edge PL at higher temperatures, whereas the PL efficiency is less affected at T=4 K. This indicates a reduction of the concentration of non-radiative defects in the cap layer when it is grown at higher temperatures. A broad PL band 150 meV below the SiGe bandgap, supposed to be typical for MBE-grown material, is weakly observed in only a few samples. The strength of the broad PL band does not unambiguously depend on the strain energy density in the SiGe layer as recently suggested.

Solution-phase chemical syntheses are presented for the fabrication of Ge quantum structures, including 0-dimensional dots, 1-dimensional wires, and 2-dimensional platelets. The syntheses, which are based on the reduction of chlorogermanes and organochlorogermanes, are carried out in two steps. Step I, the nucleation step, is done at ambient temperature with a liquid metal reducing agent (NaK) and under the influence of sonication. Step 2, crystallization, is carried out at elevated temperatures and pressures in a sealed bomb. Separation of these two steps is shown to be critical for obtaining size and shape control over the product crystallite.

Si nanocrystallites have been prepared by ultrasonicating thin sections of porous silicon. The materials produced from 20 and 49 wt % HF are characterized and compared. Samples were characterized by optical absorption and photoluminescence spectroscopy, and HRTEM.

Thin films of CaF2 containing layers of Si and Ge nanocrystallites were grown epitaxially on Si( 111) substrates by MBE while varying the substrate temperature. The sticking coefficient of both Si and Ge to CaF2 are less than unity at the temperatures studied. Evidence that chemical reactions are minimal between Si and CaF2 leading to the formation of volatile species such as SiFx is presented and it is surmised that the dominant mechanism for re-evaporation is thermal desorption. Both Si and Ge sticking coefficients vary exponentially with 1/T and activation energies are determined. A cluster growth model describing the evolution of the amount of Si in clusters over time is given. Solving in the limit of low cluster coverage, a solution that varies linearly with time and exponentially with 1/T is obtained. Weak room temperature photoluminescence from Si nanocrystallites in CaF2 is seen, however, it is unclear whether the luminescence is coming from the Si nanocrystallites or the CaF2. Second harmonic generation is observed from samples containing single layers of Ge in CaF2.

Stokes settling in viscous gels was used as a method for classifying mixed powders of silicon crystallites and amorphous silicon; Quantum-size materials derived from porous silicon were encapsulated and trapped in the viscous gels. After complete gelation the matrix retained size gradations developed during Stokes settling, thus yielding a new variant of functionally gradient materials (FGM). Production and rheological properties of the sol and preparation of specimens for centrifuging are discussed. The relative position of particles with various sizes are quantitatively interpreted in relation to the increase in gel viscosity with time.

Light emitting nanocrystalline silicon has been prepared by a completely dry processing which uses standard silicon technology. This enables us to prepare compact films on various substrates and to control the crystallite size. Dependence of the photoluminescence intensity and its peak energy on the crystallite size is reported and compared with current theoretical models.

A novel step—like and pinning behavior of photoluminescence peak energy connected with changes in the concentration of HIF and current density were for the first time observed for p— type porous silicon. Based on a theoretical calculation of the electron structure of the silicon quantum wire it is argued that these behaviors can be explained in terms of a novel formation mechanism model of porous silicon.

A series of three p-n junctions consisting of ten monolayer strain adjusted SinGem superlattices (Si6Ge4, Si5Ge5, Si4Ge6) has been characterized with respect to Current-Voltage, Capacitance-Voltage and Short Circuit Photocurrent measurements. The samples were grown by MBE with an Antimony surfactant resulting in improved interface sharpness. Superlattice interband transitions were measured by Short Circuit Current spectroscopy and bandgap energies were determined by a fitting procedure. The temperature dependence of the junction capacitance and of the Short Circuit Current indicate a potential barrier for electrons at the superlattice-buffer interface that impedes electron transport from the p-n junction at lower temperatures.

In an attempt to substantiate our previous findings of boron deactivation and/or donor complex formation due to high-dose Ge and C implantation, SiGe and SiGeC layers were fabricated and characterized. Cross-sectional transmission electron microscopy indicated that the SiGe layer with peak Ge concentration of 5 at% was strained; whereas, for higher concentrations, stacking faults were observed from the surface to the projected range of Ge as a result of strain relaxation. Results of spreading resistance profiling were found to be consistent with the model of dopant deactivation due to Ge implantation and subsequent solid phase epitaxial growth of the amorphous layer. Furthermore, for unstrained SiGe layers (Ge peak concentration ≥7 at%), formation of donor complexes is indicated. Preliminary photoluminescence results correlate with the spreading resistance profiling results and indicate shallow donor complex formation.