Juha Toivonen

Observation of defect complexes containing Ga vacancies in GaAsN

Positron annihilation spectroscopy was used to study GaAsN/GaAs epilayers. GaAsN layers were found to contain Ga vacancies in defect complexes. The density of the vacancy complexes increases rapidly to the order of 1018u2009cm−3 with increasing N composition and decreases after annealing at 700u200a°C. The anticorrelation of the vacancy concentration and the integrated photoluminescence intensity suggests that the Ga vacancy complexes act as nonradiative recombination centers.

Ultrasonic enrichment of microspheres for ultrasensitive biomedical analysis in confocal laser-scanning fluorescence detection

An ultrasonic particle concentrator based on a standing-wave hemispherical resonator is combined with confocal laser-scanning fluorescence detection. The goal is to perform ultrasensitive biomedical analysis by concentration of biologically active microspheres. The standing-wave resonator consists of a 4 MHz focusing ultrasonic transducer combined with the optically transparent plastic bottom of a disposable 96-well microplate platform. The ultrasonic particle concentrator collects suspended microspheres into dense, single-layer aggregates at well-defined positions in the sample vessel of the microplate, and the fluorescence from the aggregates is detected by the confocal laser-scanning system. The biochemical properties of the system are investigated using a microsphere-based human thyroid stimulating hormone assay.

High nitrogen composition GaAsN by atmospheric pressure metalorganic vapor-phase epitaxy

Abstract Highly luminescent GaAs 1− x N x alloys were successfully grown by atmospheric pressure metalorganic vapor-phase epitaxy (MOVPE). The nitrogen composition x of as high as 5.6% was obtained using trimethylgallium (TMGa), tertiarybutylarsine (TBAs) and dimethylhydrazine (DMHy) precursors. In-situ and post-growth rapid thermal annealing was performed to enhance the optical quality of the material. Intense low temperature photoluminescence was obtained from GaAsN down to 0.9xa0eV (1.38xa0μm).

GaInNAs quantum well structures for 1.55 μm emission on GaAs by atmospheric pressure metalorganic vapor phase epitaxy

Abstract GaInNAs/GaAs multiple quantum well (MQW) structures for long wavelength emission were grown by atmospheric pressure metalorganic vapor phase epitaxy using trimethylgallium, trimethylindium, tertiarybutylarsine and dimethylhydrazine precursors. The dependence of the N concentration and the emission wavelength on the In concentration was investigated. The longest wavelengths were obtained with In concentrations of around 23%. Post-growth rapid thermal annealing was performed to enhance the optical quality of the material. Low-temperature photoluminescence (PL) down to 0.77xa0eV (1.61xa0μm) was obtained from a Ga 0.74 In 0.26 N 0.03 As 0.97 MQW structure. After annealing the PL wavelength of 1.51xa0μm was obtained at room temperature.

Self-assembled GaIn(N)As quantum dots: Enhanced luminescence at 1.3 μm

Self-assembled GaIn(N)As quantum dots are fabricated on GaAs by atmospheric pressure metalorganic vapor-phase epitaxy using dimethylhydrazine (DMHy) precursor as a nitrogen source. The incorporation of nitrogen into the islands is observed to be negligible. However, the areal density of the islands is increased by up to one order of magnitude compared to that of the respective GaInAs islands. The GaIn(N)As island size can also be controlled by varying the DMHy flow. An enhancement of the room-temperature luminescence at 1.3 μm is observed in the GaIn(N)As samples grown with DMHy.

The electron effective mass at the bottom of the GaNAs conduction band

We determined the electron effective mass, m*e, from the photoluminescence (PL) of GaNxAs1−x/GaAs single quantum wells. The energy of the interband optical transitions is analysed within the frame of the band anti-crossing model (BAC), which accounts for a highly nonparabolic nature of the GaNAs conduction band (CB). From the PL we found that m*e at the bottom of the CB in GaNxAs1−x/GaAs quantum wells (QWs) increases from 0.095m0 to 0.12m0 for x increasing from 0.009 to 0.04. We found that the BAC model for III-N-V QWs predicts a strong increase of m*e as a function of the electronic subband number. We analyse also m*e at the bottom of the bulk GaNAs CB using the linear-muffin-tin-orbital (LMTO) method adjusted by inclusion of external potentials to account properly for energy gaps. The effective mass dependence obtained from LMTO calculations reproduces the experimental results. The nature of the nitrogen related A1 symmetry state, which enters in the BAC model, and similarities of this state with the Ga dangling bond state are discussed.

Geometric linewidth and the impact of thermal processing on the stress regimes induced by electroless copper metallization for Si integrated circuit interconnect technology

Mechanical strains and stresses are a major concern in the development of copper-based on-chip metallization. Synchrotron x-ray topography (SXRT), micro-Raman spectroscopy, finite element modeling (FEM), and atomic force microscopy (AFM) have been used to examine the strain fields imposed by electroless Cu metallization on the underlying Si. As expected, we have observed enhanced strain regions close to the metal line edges. These strain fields tend to zero at annealing temperatures approaching 200 ° C, and thereafter the magnitudes of the strain fields at 300 ° C and 400 ° C are much higher, implying a return to a higher strain regime. Although the strain transition point is slightly different from the SXRT result, the FEM results confirm the existence of a zero-strain transition point as a function of thermal anneal. We have also examined the generated stress in Si as a function of Cu linewidth L. We have found that the stress sXX due to the electroless copper metallization is empirically related to the Cu linewidth in terms of an exponential distribution. For Cu linewidths less than 20 mm, the stress magnitudes increased with decreasing Cu linewidth due to the thermal stress in the absence of self-annealing, whereas the stress decreased with increasing linewidths in the range of 60‐ 100 mm due to a relief of the thermal stress possibly via the self-annealing effect. This self-annealing phenomenon was observed using AFM. It is observed that the stresses in the Si shifted to a compressive state after annealing at 400 ° C.

Pumping of Quantum Dots with Surface Acoustic Waves

Surface acoustic waves on a semiconductor quantum well structure are accompanied by strong lateral piezo-electric fields. Those fields can ionize photo-generated excitons and trap the fragments, electron-hole pairs, in the moving lateral potential wells to the wave. We show that self-assembled quantum dots present in the structure can act as very efficient recombination centers and demonstrate the concept of a periodically pumped quantum dot.

Two-Photon Excited Fluorescence Energy Transfer: A Study Based on Oligonucleotide Rulers

The use of two-photon excitation of fluorescence for detection of fluorescence resonance energy transfer (FRET) was studied for a selected fluorescent donor–acceptor pair. A method based on labeled DNA was developed for controlling the distance between the donor and the acceptor molecules. The method consists of hybridization of fluorescent oligonucleotides to a complementary single-stranded target DNA. As the efficiency of FRET is strongly distance dependent, energy transfer does not occur unless the fluorescent oligonucleotides and the target DNA are hybridized. A high degree of DNA hybridization and an excellent FRET efficiency were verified with one-photon excited fluorescence studies. Excitation spectra of fluorophores are usually wider in case of two-photon excitation than in the case of one-photon excitation [1]. This makes the selective excitation of donor difficult and might cause errors in detection of FRET with two-photon excited fluorescence. Different techniques to analyze the FRET efficiency from two-photon excited fluorescence data are discussed. The quenching of the donor fluorescence intensity turned to be the most consistent way to detect the FRET efficiency. The two-photon excited FRET is shown to give a good response to the distance between the donor and the acceptor molecules.

Time-Resolved Fluorescence Immunoassay for C-Reactive Protein Using Colloidal Semiconducting Nanoparticles

Besides the typical short-lived fluorescence with decay times in the nanosecond range, colloidal II/VI semiconductor nanoparticles dispersed in buffer also possess a long-lived fluorescence component with decay times in the microsecond range. Here, the signal intensity of the long-lived luminescence at microsecond range is shown to increase 1,000-fold for CdTe nanoparticles in PBS buffer. This long-lived fluorescence can be conveniently employed for time-gated fluorescence detection, which allows for improved signal-to-noise ratio and thus the use of low concentrations of nanoparticles. The detection principle is demonstrated with a time-resolved fluorescence immunoassay for the detection of C-reactive protein (CRP) using CdSe-ZnS nanoparticles and green light excitation.