J. Kudrna

Dominant role of surface states in photoexcited carrier dynamics in CdSe nanocrystalline films prepared by chemical deposition

We report on ultrafast carrier dynamics in nanocrystalline CdSe thin films prepared by chemical solution deposition. The photoluminescence and transient absorption dynamics have spectrally dependent decay faster at shorter wavelengths. The spectral behavior of the decay rates in samples with different nanocrystal sizes and the sensitivity of the dynamics on the surface modification indicate the dominant role of the surface states in the photoluminescence.

Visible photoluminescence and electroluminescence in wide-bandgap hydrogenated amorphous silicon

Abstract The work describes basic photoluminescence (PL) and electroluminescence (EL) properties of wide-bandgap (2.0 eV or greater) hydrogenated amorphous silicon (a-Si: H). Thin films of wide-bandgap (high-hydrogen-content) a-Si: H were prepared by microwave electron cyclotron resonance plasma-enhanced chemical vapour deposition from SiH4 under high dilution with He. The films exhibit spectrally broad (full width at half-maximum, 0.4 eV or greater) visible PL at room temperature, peaked at about 1.5eV. On the basis of measurements of the PL temperature dependence, PL dynamics, infrared absorption spectra, picosecond pump-and-probe experiments and hydrogen thermal desorption spectroscopy the dominant microscopic mechanism of visible PL has been revealed to be the radiative de-excitation of oligosilane (-(SiH2)2-) units or of a specific defect in their close vicinity. EL has been investigated in sandwich p+‒i‒n+ and p+‒p‒n‒n+ structures with (Cr‒Ni)/indium tin oxide contacts. The as-grown structures exhibit good rectifying properties, low conductivity and no EL. After being subjected to a ‘forming’ procedure (application of a sufficiently high voltage), the current flowing through the structures increases abruptly by several orders of magnitude, up to about 10 mA, and the structures begin to emit weak EL at room temperature. The EL occurs in reverse bias only and its external quantum efficiency is about 10−5%. It is concluded that the forming procedure leads to partial crystallization of a-Si: H between the contacts and impact lattice ionization can participate in the light emission.

Optical non-linearity and hysteresis in porous silicon

Abstract We present results of experimental investigations of picosecond (decay of ∼ 200 ps) optical non-linearities and optical hysteresis in porous silicon. We consider the application of porous silicon (PS) for all-optical switching. We discuss the physical origin of the non-linearities observed and we propose a theoretical model describing well the optical absorption non-linearity and hysteresis in PS.

Carrier diffusion in microcrystalline silicon studied by the picosecond laser induced grating technique

We report on the picosecond laser induced grating technique applied to hydrogenated microcrystalline silicon (μc-Si:H) and aimed at studying the photocarrier diffusion coefficient D. We have studied a series of three samples having a distinctly different content of the crystalline phase, and the microstructure and morphology of which are known in detail. Our results show that the coefficient D scales with the degree of crystallinity of the samples, reaching values up to D=9 cm2 s−1 close to crystalline silicon. The obtained carrier lifetime (≈1 ns) is constant in all measured samples. The extracted diffusion lengths are much greater than the dimensions of the microcrystalline grains in the samples. We conclude that small grain boundaries do not limit substantially the carrier diffusion length in microcrystalline silicon.

Ultrafast carrier dynamics in undoped microcrystalline silicon

We have studied ultrafast dynamics of photoexcited carriers in μc-Si:H by pump and probe laser spectroscopy. We have found that the dynamics of photoexcited carriers in μc-Si:H depend on the crystallinity of the material: in the samples with low crystalline fraction, the dynamics have a fast decay and resemble those in a-Si:H, while in the samples with high crystallinity the dynamics are slower and similar to those in c-Si. We have identified an intensity dependent bimolecular recombination in the samples with lower crystalline fraction (coefficient B = 2×10-10 cm3 s-1 for deposition with silane dilution ratio ≈5% at a fixed power of 6 W), and no bimolecular recombination in the samples with high crystallinity.

Negative and positive nonlinear absorption in CdS-doped glasses

. Weinvestigated photodarkend samples, which were exposed to pump light for a long time(about 1 h). Fig.1The optical transmission spectra of the samples studied are shown in Fig. 1. All thesamples are doped by CdS, and different positions of the absorption edge are due tothe different nanocrystal sizes (effect of quantum confinement). We observed negative2

Time-resolved photoluminescence in porous silicon

Abstract We report a detailed study of red photoluminescence decay in porous silicon at temperatures 10–300 K. We concentrate on the slow component (~ 10−4 s) which originates in the recombination of carriers trapped in the surface localized states. Analysis of the experimental data shows that photoluminescence dynamics is dominated by a thermally activated nonradiative process and brings new information on its microscopic origin.

Photoluminescence dynamics of porous silicon : picoseconds to milliseconds

Abstract We present results of the investigation of spectrally resolved photoluminescence decay in porous silicon on the time interval which spans seven orders of magnetude. The luminescence decay can be interpreted in terms of a fast bimolecular recombination of free carriers in the core of silicon nanocrystallites and of a slow recombination of localized carriers in the surface states. The etching time of the samples has an effect on the relative weight of both components and on the dynamics of the slow recombination. We discuss the dependence of relevant parameters on the etching time of the samples.

Hydrogenated amorphous silicon deposited by glow discharge of SiH4 diluted with He : photoluminescence and electroluminescence in the visible region

Abstract Samples of a-Si:H having a wide gap (≥2.1 eV) were prepared using routine rf glow discharge decomposition of silane (SiH4) strongly diluted with He. Also, microwave electron–cyclotron–resonance plasma-enhanced chemical-vapour-deposition was used to prepare wide-gap a-Si:H via decomposition of He diluted SiH4. The wide-gap a-Si:H samples, containing above 30 at.% of hydrogen, exhibit visible photoluminescence at room temperature. From the temperature dependence of photoluminescence intensity and from the observed shift of the maximum with temperature we deduce that the customary model of radiative recombination of carriers localised in tail states is not directly applicable to the case of wide-gap a-Si:H. Instead, we propose that the photoluminescence arises in parallel from tail states and from isolated (extrinsic) centres. Electroluminescence results obtained on a p–i–n structure with wide-gap a-Si:H are reported.

Persistent hole burning and femtosecond pump-probe absorption spectroscopy of green sulphur photosynthetic bacteria antennae

Abstract The excited state relaxation in antenna of green sulphur photosynthetic bacteria was studied at low temperatures both directly, by femtosecond one-colour pump-probe absorption spectroscopy, and indirectly, by persistent spectral hole burning. The transient absorption profiles yielded several exponential components. The fastest decay component (lifetime 250–350 fs) probably reflects an excitonic relaxation within the aggregate. At natural anaerobic conditions, the main component at around 775 nm is characterized by a lifetime of 5.8 ps that agrees with the lifetime of the lowest excited state of bacteriochlorophyll c determined from zero-phonon hole widths ( δ ZPH = 1.85 cm −1 ). We interpret it as the mean time of energy transfer between bacteriochlorophyll c clusters. At aerobic conditions, the excitation decays mainly with a 2.6 and 16 ps lifetime. The observed hole widths are correspondingly broader ( δ ZPH = 4.0 cm − ). This is due to quenching by a hitherto unidentified mechanism protecting photosynthetic apparatus against a singlet-oxygen-induced damage.