A successful observation and analysis of the Zeeman effect on the near 1.54 μm photoluminescence spectrum in Er-doped crystalline MBE-grown silicon are reported. A clearly resolved splitting of 5 major spectral components was observed in magnetic fields up to 5.5 T. Based on the analysis of the data the symmetry of the dominant optically active center was conclusively established as orthorhombic I (C2v), with g‼≈18.4 and g⊥≈0 in the ground state. The fact that g⊥≈0 explains why EPR detection of Er-related optically active centers in silicon may be difficult. Preferential generation of a single type of an optically active Er-related center in MBE growth confirmed in this study is essential for photonic applications of Si:Er.

Design, fabrication and performance of a microspectrometer for operation in the infrared spectral range between 1 and 8 μm and fabricated in silicon are presented. The opto-electrical system is composed of two bonded silicon wafers, which have been subjected to microelectronic process compatible micromachining to enable co-integration of the optical components (an aluminum based grating, an optical path in crystalline silicon and an array of integrated polysilicon thermocouples) with readout circuits in silicon. The FWHM is smaller than 0.5 μm at λ= 5 μm. The performance is compared to alternatives and directions for improving the selectivity are given.

ptical microcavity structures containing Si nanocrystals are fabricated by plasma enhanced chemical vapour deposition (PECVD) of SiO2, Si3N4 and SiOx layers. The nanocrystals are formed within Si-rich oxide layers (SiOx) by precipitation and growth, and the microcavity structures defined by two parallel distributed Bragg mirrors (DBM) made from either alternate SiO2/Si3N4 layers or alternate SiO2/SiOx layers. In the latter case, Si nanocrystal layers form part of the DBM structure thereby providing a distributed emission source. The optical emission from these and related structures are examined and compared with that from isolated nanocrystal layers.

The aim of this work is to investigate the structural, electronic and optical properties of hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations. Recent results about ab-initio calculations of Stokes shift as a function of the cluster dimension and of optical gain will be presented here. A structural model that can be linked to the four level scheme recently invoked to explain the experimental outcomes relative to the observed optical gain in Si-nc embedded in a SiO2 matrix will be suggested too.

The photoluminescence band around 0.78 eV that is sometimes seen in strained layer SiGe samples or deformed silicon containing a high density of dislocations has been attributed to the presence of oxygen complexes. In this study we have prepared a set of Si0.9Ge0.1 samples by MBE which have then been implanted with iron, erbium, or oxygen in order to study the effect of implanted impurities on photoluminescence in the technologically important region around 1.5 microns.

Following implantation with oxygen, two luminescence bands appear around 0.85 eV and 0.78 eV, respectively. However, these bands are not present in either the unimplanted sample or those subject to Er or Fe implantation. The correlation between oxygen doping and the appearance of these bands supports the conjecture that they are associated with oxygen complexes.

The Nd-nanocluster Si (nc-Si) coupling strength and its effect in excitation/de-excitation of Nd3+ luminescence in Nd-doped silicon-rich silicon oxide (SRSO) is investigated. Nd-doped SRSO thin films, which consist of nc-Si embedded inside a SiO2 matrix, were prepared by electron-cyclotron-resonance plasma enhanced chemical vapor deposition (ECR-PECVD) of SiH4 and O2 with co-sputtering of Nd and subsequent anneal at 950 °C. Efficient Nd3+ luminescence with moderate temperature quenching is observed. Based on the temperature dependence of Nd3+ luminescence lifetime, a coupling strength between nc-Si and Nd that is strong enough to result in efficient excitation of Nd3+ via quantum confined excitons while weak enough to result in a small back-transfer rate is identified as the key to Nd3+ luminescence.

One of the open questions in semiconductor physics is the origin of the small splittings of the excited states of bound excitons in silicon. A free electron laser as a tunable source of the mid-infrared radiation (MIR) can be used to investigate such splittings of the excited states of optical centers created by transition metal dopants in silicon. In the current study, the photoluminescence from silver and copper doped silicon is investigated by two color spectroscopy in the visible and the MIR. It is shown the PL due recombination of exciton bound to Ag and Cu is quenched upon application of the MIR beam. The time-resolved photoluminescence measurements and the quenching effects of these bands are presented. By scanning the wavelength of the free-electron laser ionization spectra of relevant traps involved in photoluminescence are obtained. The formation and dissociation of the bound excitons, and the small splittings of the effective-mass excited states are discussed. The applied experimental method allows correlation of DLTS data on trapping centers to specific channels of radiative recombination. It can be applied for spectroscopic analysis in materials science of semicondutors.

Optical pump-probe measurements were performed on planar slab waveguides containing silicon nanocrystals in an attempt to measure optical gain from photo-excited silicon nanocrystals. Two experiments were performed, one with a continuous-wave probe beam and a pulsed pump beam, giving a time resolution of approximately 25 ns, and the other with a pulsed pump and probe beam, giving a time resolution of approximately 10 ps. In both cases the intensity of the probe beam was found to be attenuated by the pump beam, with the attenuation increasing monotonically with increasing pump power. Time-resolved measurements using the first experimental arrangement showed that the probe signal recovered its initial intensity on a time scale of 45-70 μs, a value comparable to the exciton lifetime in Si nanocrystals. These data are shown to be consistent with an induced absorption process such as confined carrier absorption. No evidence for optical gain was observed.

We have fabricated the first CMOS process compatible high-responsivity Ge p-i-n diodes for 1.55 μm wavelengths. The thermal expansion mismatch between Ge epilayers and Si substrates was used to engineer tensile strain upon cooling from the growth temperature. This 0.2% tensile strain results in a lowering of the direct transition energy in Ge by 30 meV and extends the responsivity curve to near 1.6μm.

Integration of photodetectors with high responsitivity in the near infrared (1.3-1.55 μm) on standard Si electronic circuits is important for a variety of applications in the field of on-chip, local area and long haul optical communications. In this work we report on a detailed structural and optical characterization of epitaxial Si1-xGex films and Si1-xGex /Si multilayers grown by chemical vapor deposition on (100) Si wafers. Cross-sectional transmission electron microscopy analyses show that metastable strained Si1-xGex films of few nanometer with x>40% can be deposited at low growth temperature and pressure. Absorption measurements on these films demonstrate the extension of the photo-response to 1.55 μm. Using these films as active layers Schottky integrated photodetectors have been fabricated.

We have performed a comparative study of oxide- and nonoxide-passivated silicon nanocrystals to probe the role of the silicon/oxygen interface in low coverage, non-interacting silicon nanocrystal systems. Ensembles of Si nanocrystals characterized by a narrow distribution and diameters of 2–5 nm were synthesized by ion implantation into SiO2 films followed by a high-temperature anneal in Ar. The nanocrystals were removed from the SiO2 film matrix and deposited on Si substrates using a chemical etch in HF, leaving a hydrogen-terminated surface. A natural oxide layer grows on these surfaces in air. We characterized the morphology of the samples with atomic force microscopy (AFM) and the spectroscopic properties with photoluminescence (PL) and X-Ray photoelectron spectroscopy. We found that the PL energy of Si nanocrystals can be shifted by particle size reduction and hydrogen or oxygen termination. Further, PL peak energy shifts upon etching and oxidation were consistent with the model of Wolkin et al. that proposes that for very small radii, a silicon-oxygen double bond will produce deep interface states which red shift the luminescence.

Optical transmission measurements were carried out on Pb1-xSrxSe samples with different Sr compositions (x ranging from 0 to 1) grown by molecular beam epitaxy (MBE). Refractive indices were calculated at room temperature and at 77K by fitting the transmittance data. The bandgap energies were determined by fitting the absorption curve with direct and indirect energy bandgap transitions. A distinct bandgap inversion from the direct to the indirect transition is observed at x ∼ 0.20 as the Sr composition increases. This is the first observation of such transition in the Pb1-xSrxSe material system. Both direct and indirect bandgap energies were determined by fitting the experimental results.

PbSe and lead-alkaline-earth-chalcogenide materials Pb1-xSrxSe have attracted considerable attention in optoelectronic applications especially in the mid-infrared (mid-IR) lasers and mid-IR/ultraviolet (UV) detectors.1 The bandgap energies and the refractive indices of this material system vary widely with the change of Strontium composition. Such properties have been used to grow quantum wells and diffraction Bragg reflectors (DBR) for vertical cavity surface emitting lasers2 (VCSELS) and detectors. Yet only a narrow range of the whole material system has been explored and the basic electronic and optical properties are widely unknown for Pb1-xSrxSe with high Sr compositions. The primary aim of this research is to determine, experimentally, the electronic and optical properties of these ternary compounds for different compositions. PbSe which is a narrow bandgap material, have been studied widely and are know to have a direct narrow bandgap of 0.3eV at L point.3 SrSe on the other hand is a wide bandgap material which have an indirect bandgap between X-Γ.4 As the Sr composition in the ternary compound Pb1-xSrxSe increases, its electronic structure changes from a narrow direct bandgap material to a wide indirect bandgap material. However, the composition at which the material becomes indirect was unknown, even though it is an important parameter in designing optoelectronics devices. The other properties of this material system on which we have investigated upon are the refractive index and absorption coefficient, which were largely unknown.

In this research, we have grown epitaxial layers of Pb1-xSrxSe by molecular-beam epitaxy (MBE), on <111> BaF2 substrate, with nine different Sr compositions (x) ranging from zero to one. The crystal growth is characterized by high-resolution X-ray diffraction measurements and the Sr composition is determined by assuming that the material follows the Vegard's law. Transmissions, in the wavelength range mid-IR to UV, were measured at different temperatures. The refractive indices and absorption coefficients of Pb1-xSrxSe with different Strontium composition were determined. Bandgap energies of all different compositions are calculated by fitting the absorption coefficients to theoretical models of either direct or indirect bandgap. A distinct bandgap inversion from the direct to the indirect is observed at x ∼ 0.20. While the 3.81eV direct bandgap of SrSe calculated from our experiments agreed closely with that from reported photoluminescence measurements, the lowest bandgap of SrSe is observed to be ∼ 1.82 eV, contradicting previously published results.

Compound sources of PbSe and elemental sources of Sr and Se were used to grow epitaxial layer of Pb1-xSrxSe on freshly cleaved <111>BaF2 on a custom made MBE chamber. The substrate temperature varied from 360°C to 450°C for growths with different Sr composition and the epitaxial layers were grown to a thickness in the range of 2μm to 4μm for different samples. Details of the growth are described elsewhere.5 X-ray diffraction measurements were performed on Philips high resolution X-ray diffraction spectrometer (HRXRD) having four crystal Ge (220) monochromator. The Sr composition was determined by considering that lattice constant of Pb1-xSrxSe increases linearly with increase in Sr composition. Figure 1 shows the composition vs lattice constant graph, and insert on it is a typical XRD curve of Pb1-xSrxSe grown on BaF2<111>.

Optical transmittance of the epitaxial layers, were measured at different temperatures by Fourier transform infrared (FTIR) transmission spectroscopy, using deuterated triglycine sulfate (DTGS) detector and globar source for the mid-IR region, Silicon detector and Tungsten source for nearinfrared (NIR) and visible regions, and Xenon-arc source and Gallium-Phosphide (GaP) detector for the UV region of the spectrum. The transmission spectra are shown in figure 2. The refractive index and the absorption coefficient of the epitaxial layers have been calculated by the method described elsewhere.6 The approximate refractive index for each composition is calculated first from the odd interference peaks of the no absorption region of the transmission spectra (long wavelength regions that are well below the energy bandgap). The approximate values were then in turn used in the equation of transmittance6 and matched with the experimental curve by iterative method to calculate the true values of refractive index, absorption coefficient and thickness of the epilayer.

T(n2, k2, λ, d, n1, n3)– Texp = 0,6

It may be noted that absorption coefficient and the refractive index of the substrate, whose thickness is in the range of 0.6 to 0.8mm, and which might affect the interference peak, have been measured and used in the calculation to eliminate the error introduced by the substrate. The refractive index at 4μm (non-absorbing region for all compositions of Pb1-xSrxSe) calculated from the samples at room temperature and at 77K is plotted in figure 3.

Metal/PS/c-Si structures with porous silicon (PS) layers of 55-75% porosity were fabricated on moderately doped p- and n-type c-Si substrates and were thermally oxidized at 400-960°C. The effect of oxidation on the photoluminescence and the transport of charge carriers in these structures were studied. We demonstrated that trap-filled space charge limited current (SCLC) is the dominant transport mechanism at large forward bias. The analysis of the current – voltage characteristics in the SCLC region allowed us to determine the oxidation dependence of the effective thickness of the trap-rich tissue isthmuses, in which space charge is mostly accumulated. The exponential dependence of the ohmic conductance on the thickness of SiOX tissue is explained by tunneling of carriers through potential barriers formed by the tissue surrounding silicon crystallites.

The local environment of light emitting silicon nanocrystals (Si-nc) embedded in amorphous SiO2 has been studied by x-ray absorption spectroscopy (XAS) and by ab-initio total energy calculations. Si-nc have been formed by PECVD deposition of SiOx with different Si content (from 35 to 42 at.%) and thermal annealing at high temperature (1250 °C). The comparison between total electron yield (TEY) and photoluminescence yield (PLY) spectra has allowed the identification of a modified region of SiO2 (about 1 nm thick) surrounding the Si-nc, which participates to the light emission of Si-nc. Total energy calculations, within the density functional theory, clearly show that Si-nc are surrounded by a cap-shell of stressed SiO2 with a thickness of about 1 nm. The optoelectronic properties show the appearance of localized states not only in the Si-nc core region but also in the modified SiO2 region.