S. Ostapenko

Thermal activation of excitons in asymmetric InAs dots-in-a-well InxGa1−xAs∕GaAs structures

Photoluminescence, its temperature dependence, and photoluminescence excitation spectra of InAs quantum dots embedded in asymmetric InxGa1−xAs∕GaAs quantum wells [dots in a well (DWELL)] have been investigated as a function of the indium content x (x=0.10–0.25) in the capping InxGa1−xAs layer. The asymmetric DWELL structures were created with the aim to investigate the influence of different barrier values at the quantum dot (QD)/quantum well interface on the photoluminescence thermal quenching process. The set of rate equations for the two stage model for the capture and thermal escape of excitons in QDs are solved to analyze the nature of thermal activation energies for the QD photoluminescence quenching process. The two stage model for exciton thermal activation was confirmed experimentally in the investigated QD structures as well. The localization of nonradiative defects in InAs∕InGaAs DWELL structures is discussed on the base of comparison of theoretical and numerically calculated (fitting) results.

Resonance ultrasonic vibrations for crack detection in photovoltaic silicon wafers

The resonance ultrasonic vibrations (RUV) technique is adapted for non-destructive crack detection in full-size silicon wafers for solar cells. The RUV methodology relies on deviation of the frequency response curve of a wafer, ultrasonically stimulated via vacuum coupled piezoelectric transducer, with a periphery crack versus regular non-cracked wafers as detected by a periphery mounted acoustic probe. Crack detection is illustrated on a set of cast wafers. We performed vibration mode identification on square-shaped production-grade Si wafers and confirmed by finite element analyses. The modelling was accomplished for the different modes of the resonance vibrations of a wafer with a periphery crack to assess the sensitivity of the RUV method relative to crack length and crack location.

Crack detection and analyses using resonance ultrasonic vibrations in full-size crystalline silicon wafers

An experimental approach for fast crack detection and length determination in full-size solar-grade crystalline silicon wafers using a resonance ultrasonic vibrations (RUV) technique is presented. The RUV method is based on excitation of the longitudinal ultrasonic vibrations in full-size wafers. Using an external piezoelectric transducer combined with a high sensitivity ultrasonic probe and computer controlled data acquisition system, real-time frequency response analysis can be accomplished. On a set of identical crystalline Si wafers with artificially introduced periphery cracks, it was demonstrated that the crack results in a frequency shift in a selected RUV peak to a lower frequency and increases the resonance peak bandwidth. Both characteristics were found to increase with the length of the crack. The frequency shift and bandwidth increase serve as reliable indicators of the crack appearance in silicon wafers and are suitable for mechanical quality control and fast wafer inspection.

Scanning room-temperature photoluminescence in polycrystalline silicon

Photoluminescence (PL) mapping was performed on polycrystalline silicon wafers at room temperature. Two PL bands are observed: (1) a band-to-band emission with a maximum at 1.09 eV, and (2) a deep “defect” luminescence at about 0.8 eV. PL mapping of 10 cm×10 cm wafers revealed inhomogeneity of the band-to-band PL intensity which could be correlated to the distribution of minority carrier diffusion length in the wafer bulk. We have also observed that the intensity of the 0.8 eV band is strongest along those grain boundaries where the band-to-band PL is suppressed as well as minority carrier diffusion length. The origin of the 0.8 eV luminescence band is discussed.

Photoluminescence spectroscopy of bioconjugated CdSe∕ZnS quantum dots

The authors performed scanning photoluminescence (PL) spectroscopy on CdSe∕ZnS core/shell quantum dots (QDs) bioconjugated to Interleukin 10 (IL10) antibody. The influence of IL10 on the QD photoluminescence spectra was explored on samples dried on solid substrates at various temperatures. A “blue” up to 15nm spectral shift of the PL maximum was observed on the bioconjugated QDs. The spectral shift is strongly increased after samples annealing above room temperature. A mechanism of the observed effect is attributed to changes in the QD electronic energy levels caused by local electric or stress field or chemical reactions which occurred on the QD surface.

Reversible and non-reversible photo-enhanced luminescence in CdSe/ZnS quantum dots

We report on optical enhancement of the 655 nm photoluminescence (PL) band intensity in the core-shell CdSe/ZnS nano-particles (quantum dots) under a laser illumination. Kinetic curves and 80 K-to-room temperature dependences of the PL intensity reveal two parallel processes: a reversible enhancement when the PL intensity is recovered after the laser illumination is turned off, and a non-reversible permanent increase of the PL output. Experimental data evidence that the PL enhancement is attributed to the light-activated increase of the energy barrier for photo-generated carriers to escape quantum dot levels. A trap recharging and/or photo-chemical bond restructure play a possible role in the luminescence photo-enhancement.

Scanning photoluminescence spectroscopy in InAs∕InGaAs quantum-dot structures

Spatially-resolved photoluminescence (PL) spectroscopy was performed at different temperatures on self-assembled InAs quantum dots embedded into MBE-grown In0.15Ga0.85As∕GaAs multiquantum-well heterostructures. Strong inhomogeneity of the PL intensity is observed by mapping samples with different In∕Ga composition of the InxGa1−xAs capping layers (0.1⩽x⩽0.2). Two different behaviors in the quantum-dot PL maps are observed: (1) a reduction of the PL intensity is accompanied by a gradual “blue” shift of the luminescence maximum at 300K and “red” shift at 80K, and (2) PL intensity variation occurs at a stable peak position of the PL maximum. Two separate mechanisms are suggested to account for the observed intensity variation of the quantum-dot luminescence.

Spatially resolved defect diagnostics in multicrystalline silicon for solar cells

Abstract Scanning room temperature photoluminescence (PL) spectroscopy was applied to cast multicrystalline Si to assess the electronic properties of high-quality solar-grade material. The intensity of band-to-band emission with the maximum at 1.09 eV positively correlates with minority carrier lifetime measured concurrently using the laser-microwave reflection technique. A point-by-point mapping revealed the linear dependence of the band-to-band PL intensity and lifetime across entire multicrystalline-Si wafers. We have also found at room temperature an intense ‘defect’ PL band with the maximum at about 0.8 eV in wafer regions with a low band-to-band emission and degraded lifetime. The PL mapping of the 0.8 eV band intensity revealed a linkage to areas of a high dislocation density. Dislocation topography was obtained independently using light scattering technique and mapping the dislocation D-lines at 77 K. PL spectroscopy down to 4.2 K was performed at areas with high and low ‘defect’ band intensity. The origin of the 0.8 eV band is discussed in a connection with dislocations in multicrystalline Si. We demonstrate advantages of scanning room temperature PL spectroscopy for in-line diagnostics of Si-based materials.

BAND-TAIL PHOTOLUMINESCENCE IN POLYCRYSTALLINE SILICON THIN FILMS

We have found a new photoluminescence (PL) band with a maximum at 0.9 eV and a halfwidth of 0.1 eV at 4.2 K in polycrystalline Si thin films deposited on glass at 625 °C. The PL band strongly shifts toward low energy with increasing the temperature (1.3 meV/K) and toward high energy with increasing the excitation intensity. Hydrogenation of polycrystalline Si enhances the PL intensity by factor of 3 to 5. The luminescence characteristics are consistent with radiative recombination of electrons and holes trapped in tail states of the conduction and the valence band, respectively. Excellent agreement is achieved between the 0.9 eV band shape and theoretical calculations based on a band‐tail recombination. It is also argued that a corresponding luminescence spectroscopy provides a new possibility for band‐tail diagnostics in polycrystalline Si thin films.

Raman-scattering and structure investigations on porous SiC layers

Raman scattering spectroscopy, scanning electron microscopy, and scanning acoustic microscopy were studied on porous SiC layers prepared by different technological routes and subjected to reactive ion treatment. The Raman spectra revealed a number of features specific for nanocrystallite materials, which can be used for characterization and diagnostics of porous SiC layers for technological applications.