Publications

Year: 2007

Femtosecond carrier dynamics of In[sub x]Ga[sub 1 - x]N thin films grown on GaN (0001): Effect of carrier-defect scattering
Lioudakis E, Othonos A, Iliopoulos E, Tsagaraki K, Georgakilas A

Journal of Applied Physics, DOI: 10.1063/1.2786610Download
Ultrafast carrier dynamics in ultrathin InxGa1−xN ternary alloys were investigated in detail, using femtosecond transient differential optical absorption measurements. Measurements were performed with probing wavelengths above and below the band edge of the materials. Furthermore, we performed a structural characterization by x-ray diffraction reciprocal space maps and we found that the alloys with the higher InN mole fraction (x = 0.89 and x = 0.43) present large lattice mismatch-strain relaxation whereas the lower InN content alloy was fully coherent with the underlying GaN (0001) layer. Our analysis showed that the observed nonradiative carrier dynamics was strongly related with the carrier-defect scattering of the materials. Our measurements in conjunction with the numerical analysis showed that when we excite these alloys with ultrashort laser pulses the background carrier concentration for In-rich InGaN samples participates to the photoexcited carrier relaxation process via carrier momentum scattering. For the higher InN content alloys (x = 0.89 and 0.43) the relaxation times were short (0.4–1.4 ps) whereas for the full strained alloy (x = 0.07) a slower nonradiative relaxation time ( ∼ 25 ps) was observed. Finally, the energy loss rate of this material (2.05±0.10 meV/fs) as well as the optical phonon lifetime (44±2 fs) were extracted.

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Ultrafast carrier dynamics on conjugated poly(3-hexylthiophene)/[6,6]-phenylC[sub 61]-butyric acid methyl ester composites
Lioudakis E, Othonos A, Alexandrou I, Hayashi Y

Applied Physics Letters, DOI: 10.1063/1.2785120Download
The authors have studied ultrafast carrier dynamics on poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester (PCBM) composites up to 50% PCBM concentration. They have resolved the exciton intraband relaxation of composites as well as the subsequent electron/polaron relaxation of dissociated excitons. The observed fast time constant of intraband relaxation is found to be 0.5–1.5 ps, whereas the second long-live relaxation (0.5–1 ns) is strongly fullerene related. A wavelength dependent ultrafast study is performed giving fundamental information on the nonradiative exciton relaxation, exciton dissociation, and electron relaxation of PCBM-related states.

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Ultrafast transient photoinduced absorption in silicon nanocrystals: Coupling of oxygen-related states to quantized sublevels
Lioudakis E, Othonos A, Nassiopoulou AG

Applied Physics Letters, DOI: 10.1063/1.2728756Download
The authors have studied transient photoinduced absorption in single monolayers of oxidized silicon nanocrystals. Transient photoinduced absorption measurements along with optical absorption and photoluminescence (PL) emission reveal that the light-absorption process takes place in defects related to strong PL emission, suggesting that the photoexcited carriers are in oxygen-related interface states. They have time-resolved ultrafast relaxation paths in oxygen-related states and quantized sublevels, which have important implications in the understanding of fundamental optical properties for this system.

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Influence of grain size on ultrafast carrier dynamics in thin nanocrystalline silicon films
Lioudakis E, Othonos A, Nassiopoulou AG, Lioutas CB, Frangis N

Applied Physics Letters, DOI: 10.1063/1.2738383Download
The ultrafast carrier dynamics in thin nanocrystalline silicon films (10 and 20 nm thick) on quartz over a broad spectral range using optical pumping at a moderate fluence of 2.5 mJ/cm2 were studied. The films were composed of randomly oriented silicon nanocrystals of various sizes and shapes. The authors probed a fast and a slow relaxation mechanism. The slow decay ( ∼ 300 ps) was attributed to bulk nanocrystal states similar to those of bulk silicon, while the faster one ( ∼ 3 ps) was attributed to surface states at grain boundaries, more dominant in the smaller nanocrystals due to their larger surface/volume ratio.

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Ultrafast carrier dynamics in band edge and broad deep defect emission ZnSe nanowires
Othonos A, Lioudakis E, Philipose U, Ruda HE

Applied Physics Letters, DOI: 10.1063/1.2825290Download
Ultrafast carrier dynamics of ZnSe nanowires grown under different growth conditions have been studied. Transient absorption measurements reveal the dependence of the competing effects of state filling and photoinduced absorption on the probed energy states. The relaxation of the photogenerated carriers occupying defect states in the stoichiometric and Se-rich samples are single exponentials with time constants of 3–4 ps. State filling is the main contribution for probe energies below 1.85 eV in the Zn-rich grown sample. This ultrafast carrier dynamics study provides an important insight into the role that intrinsic point defects play in the observed photoluminescence from ZnSe nanowires.

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Year: 2006

Ellipsometric analysis of ion-implanted polycrystalline silicon films before and after annealing
Lioudakis E, Nassiopoulou A, Othonos A

Thin Solid Films, DOI: DOI: 10.1016/j.tsf.2005.08.339Download
A study of arsenic ion-implanted polycrystalline silicon films before and after annealing at various temperatures has been performed using spectroscopic ellipsometry in the ultraviolet to the visible spectral region. Using the Bruggeman effective medium approximation, an optical/structural model is presented for all the annealed samples explaining the measurements. Ellipsometric measurements reveal important structural changes as a function of annealing temperature which provide an interesting inside into the annealing kinetics of ion-implanted polycrystalline silicon films. This work also demonstrates the importance of spectroscopic ellipsometry in determining non-destructively the dielectric functions in materials that have undergone complex processing.

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Femtosecond carrier dynamics in implanted and highly annealed polycrystalline silicon
Lioudakis E, Nassiopoulou AG, Othonos A

Semiconductor Science and Technology, DOI: Download
We have studied the ultrafast optical response of highly implanted and highly annealed polycrystalline silicon films using 400 nm ultrashort amplified pulses with fluence ranging between 8 mJ cm −2 to 56 mJ cm −2 . Transient reflection measurements reveal differences both in the short and long temporal behaviour between the implanted non-annealed and annealed samples. Important contributing factors to the dynamics of the non-annealed sample are the carrier recombination centres and traps induced by ion implantation. In contrast to the non-annealed sample, the Auger recombination process is a key factor in the dynamics in the first few picoseconds for the sample annealed at 1100 °C. A model based on two coupled differential equations has been employed to investigate in detail the carrier dynamics in these systems. Parameters including carrier trapping times, diffusion coefficients and the Auger coefficient have been extracted.

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Optical and structural properties of implanted Si wafers: the effects of implantation energy and subsequent isochronal annealing temperature
Lioudakis E, Christofides C, Othonos A

Semiconductor Science and Technology, DOI: 10.1088/0268-1242/21/8/013Download
We have studied the influence of implantation energy and subsequent isochronal annealing temperature on the optical and structural properties of implanted Si wafers employing a multiwavelength spectroscopic ellipsometer. A temperature-dependent multilayer optical model is used to explain the ellipsometric data for all implantation energies (20 to 180 keV) and annealing temperatures (300 to 1100 °C) of this work. This work completely characterizes the structural and optical properties of these implanted samples via the pseudodielectric functions and the integrated damage depth profile. For the highest implantation sample self-annealing phenomena have appeared, reducing the integrated damage depth profile. Finally, the dynamics of isochronal annealing temperature on the integrated damage depth profile of these wafers exhibit an abrupt drop in the transition temperature where a long-range ordering is obtained and pseudodielectric functions approach the crystallinity shapes.

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Femtosecond time-resolved study in In[sub x]Ga[sub 1 - x]N (0001) ultrathin epilayers: Effects of high indium mole fraction and thickness of the films
Lioudakis E, Othonos A, Dimakis E, Georgakilas A

Applied Physics Letters, DOI: 10.1063/1.2405413Download
In view of promising full-solar-spectrum photovoltaic systems based on InxGa1−xN ternary alloys, femtosecond time-resolved study in ultrathin epilayers was employed in order to extract the fundamental properties of material. Two different thicknesses of epilayers were employed with relative high indium mole fractions. State filling effect at various probing energy states has been observed for both epilayers. Saturation of state filling as well as enhanced photoinduced absorption occurred at higher probing wavelengths. Furthermore, coherent acoustic phonon oscillations were also observed for both ultrathin epilayers with a thickness dependent oscillation frequency. Finally, absorption band edge of these alloys has been determined.

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Probing carrier dynamics in implanted and annealed polycrystalline silicon thin films using white light
Lioudakis E, Othonos A, Nassiopoulou AG

Applied Physics Letters, DOI: 10.1063/1.2200745Download
Polycrystalline silicon thin film samples implanted and annealed at various temperatures have been studied using ultrafast laser pulse excitation. Nondegenerate pump-probe technique has been utilized to investigate carrier dynamics in the highly implanted samples at a relatively small fluence. A model based on two coupled differential equations has been used to fit the experimental data, giving a simple but adequate picture of the dynamics of this system. Basic sample parameters such as carrier trapping times, diffusion coefficient, and penetration depths have been extracted, providing a dependence on the annealing temperature for the samples under investigation

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Ultrafast carrier dynamics in InxGa1 - xN (0001) epilayers: Effects of high fluence excitation
Lioudakis E, Othonos A, Dimakis E, Iliopoulos E, Georgakilas A

Applied Physics Letters, DOI: 10.1063/1.2190456Download
Ultrafast carrier dynamics in InxGa1−xN (0001) epilayers were investigated, using femtosecond transient differential transmission and reflection measurements for x = 0.07, 0.15, and 0.33, over a fluence range of 1–12 mJ/cm2. Stimulated emission as well as band gap renormalization play a crucial role in the dynamics of the photogenerated carriers. Threshold fluence leading to saturation of the differential reflectivity and transmission signals related to the In mole fraction has been observed, which is attributed to band gap renormalization, Auger process, and carrier recombination through In-rich nanoclusters. Furthermore, coherent acoustic phonon oscillations have also been observed in the In0.15Ga0.85N at high fluence excitation

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Study of the annealing kinetic effect and implantation energy on phosphorus-implanted silicon wafers using spectroscopic ellipsometry
Lioudakis E, Christofides C, Othonos A

Journal of Applied Physics, DOI: 10.1063/1.2207688Download
In this work, we have studied the changes in the optical properties on crystalline silicon implanted wafers (1×1013–1×1016 P+/cm2) using an extensive ellipsometric analysis. The effects of implantation energy (20–180 KeV) and subsequent isochronical annealing temperature (300–1100 °C) on the electronic band structure of material are investigated. The evolution of pseudodielectric functions is studied using a temperature dependent multilayer model for each implantation dose and energy. The temperature evolution of integrated damage depth profile for each wafer is presented depicting the amorphous/crystalline transition temperatures. Finally, the critical implantation dose and energy of crystalline to amorphous silicon phase are given.

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Fibre Bragg Gratings
Othonos A, Kalli K, Pureur D, Mugnier A

, DOI: Download
Today optical fibres plus related components are synonymous with the word “telecommunication”. In addition to applications in telecommunications, optical fibres are also utilized in the rapidly growing field of fibre sensors. Despite the improvements in optical fibre manufacturing and advancements in the field in general, basic optical components such as mirrors, wavelength filters, and partial reflectors have been a challenge to integrate with fibre optics. Recently, however, all this has changed with the ability to alter the core index of refraction in a single mode optical fibre by optical absorption of UV light

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Year: 2005

Effects of Ge concentration, boron co-doping, and hydrogenation on fiber Bragg grating characteristics
Konstantaki M, Tamiolakis G, Argyris A, Othonos A, Ikiades A

, DOI: 10.1002/mop.20572Download
In this paper, we compare photosensitivity, refractive-index modulation depth, spectral bandwidth, and excess loss through the inscription of fiber Bragg gratings in unloaded and hydrogen-loaded fibers with low- or high-germanium concentration and boron co-doping. Gratings in hydrogenated boron co-doped fibers possess the highest reflectivity, but with wider bandwidth and strong excess loss

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Ellipsometry on optically thin palladium films on silicon-based substrate: effects of low concentration of hydrogen
Lioudakis EE, Othonos A

Optical Engineering, DOI: 10.1117/1.1840955Download
Optically thin palladium films evaporated on silicon substrates are investigated following exposure to low concentrations of hydrogen gas in nitrogen using spectroscopic ellipsometry. Changes in the parameters tan Psi and cos Delta are observed for concentrations as low as 0.01% hydrogen in nitrogen. A nonlinear behavior of the change in the ellipsometry parameters as a function of hydrogen concentration is demonstrated, with saturation occurring at a flow of 0.05% hydrogen in nitrogen.

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Prism-based ultrafast pulse-shaping apparatus

Optical Engineering, DOI: 10.1117/1.1873432Download
We report on a computer-controlled pulse-shaper setup with high-efficiency throughput, based on dispersive prisms, as an alternative to the conventional grating ultrafast pulse-shaping apparatus. A detailed description of the experimental apparatus and operation of this system is given. The advantages and disadvantages of this configuration are discussed. Experimental implementation of the prism pulse shaper in recovering nearly bandwidth-limited laser pulses from chirped ultrafast laser pulses is demonstrated using both closed-loop and open-loop operation

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Ultrafast carrier dynamics in highly implanted and annealed polycrystalline silicon films
Lioudakis E, Nassiopoulou AG, Othonos A

Journal of Physics: Conference Series, DOI: Download
We have studied the ultrafast optical response of highly implanted and annealed polycrystalline silicon films. One-micron thin polycrystalline silicon samples-on-quartz implanted with As ions at a high dose of 2 × 10 16 cm −2 at 100 keV and annealed at various temperatures have been investigated using ultrafast pulses. Frequency doubled amplified femtosecond pulses at 400 nm have been used in a pump-probe configuration to measure the temporal reflective response from the polysilicon samples. A super-continuum of ultrafast laser pulses were generated and used in probing the samples at various wavelengths in the visible part of the spectrum. Transient reflection measurements reveal negative and positive contributions to the change in the index of refraction of the samples attributed to an increase in the carrier density and lattice temperature. Ion induced defects and subsequent annealing are key contributing factors in the dynamic behavior of these samples.

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Year: 2004

Fine art painting characterization by spectroscopic ellipsometry: preliminary measurements on varnish layers
Christofides C, Castellon B, Othonos A, Polikreti K, De Deyne C

Thin Solid Films, DOI: DOI: 10.1016/j.tsf.2004.02.033Download
The long-term conservation of the original state of paintings requires high quality multi-spectral digital image technologies. CRISATEL Program addresses fine art professionals as primary end-users base. The analysis by spectroscopic ellipsometry of the varnish on fine art paintings will allow the curator to correct the effect of aged or glossy varnish on the digital images and to simulate the original state of the masterpiece through this superficial layer. Sets of varnish samples have been measured first at the University of Cyprus using a SOPRA GES5 Spectroscopic Ellipsometer and then at SOPRA facilities with a Fourier transform infrared spectroscopic ellipsometer. The capability to measure a varnish layer (thickness and optical index) on glass and to distinguish synthetic from natural varnishes has been proved. The goal of this paper is to review the first results obtained in the characterization of varnishes for fine art paintings, and to present some problems that are still open and can represent a challenging research field for the future.

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Year: 2003

Photomodulated thermoreflectance investigation at elevated temperatures: plasma versus thermal effect
Christofides C, Othonos A, Loizidou E

Applied Physics Letters, DOI: 10.1063/1.1541935Download
Photomodulated thermoreflectance measurements were performed at elevated temperatures (294 to 623 K), on crystalline silicon lightly doped with boron. The temperature dependence is qualitatively and quantitatively discussed. The “competition” between thermal and plasma contribution, as a function of temperature, is one of the main subjects of this letter.

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Photomodulated thermoreflectance detection of hydrogen at elevated temperatures: a detection limit
Othonos A, Christofides C

Applied Physics Letters, DOI: 10.1063/1.1543254Download
Laser photomodulated thermoreflectance has been used as a means of detecting, at elevated temperatures, low concentration of hydrogen using an optically thin film of palladium. Data indicate that concentrations as low as a few parts per billion can easily be detected at 100 °C. A semi-quantitative interpretation of the photothermal signal has been achieved using a Langmurian isothermic model.

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High-temperature photomodulated thermoreflectance measurements on phosphorus implanted and annealed silicon wafers
Othonos A, Christofides C, Loizidou E

Journal of Applied Physics, DOI: 10.1063/1.1621723Download
Photomodulated thermoreflectance measurements between 300 and 650 K on phosphorus implanted and annealed silicon wafers are reported. The change of the photothermal amplitude and phase as a function of temperature is discussed. Several measurements have been performed on silicon wafers annealed at various temperatures in the range of 300 to 1100 °C. The activation energy of the local annealing process was also estimated to be 0.17 eV

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Year: 2002

Influence of temperature and modulation frequency on the thermal activation coupling term in laser photothermal theory
Christofides C, Othonos A, Loizidou E

Journal of Applied Physics, DOI: 10.1063/1.1484232Download
A quantitative analysis of the influence of temperature and modulation frequency on the thermal activation coupling term in laser photothermal theory is performed. Until now it was taken for granted that the coupling term is negligible only in the case of “relatively low” temperatures and generally when the equilibrium free-carrier density n0 satisfies the Sablicov's, Vasil'ev, and Sandomirskii inequality. In this work an extensive computational study of this inequality in the temperature range of 300–1000 K was performed and a precise “map” is given concerning the violation of the inequality under various conditions including modulation frequency (0.1–106 Hz) and doping concentration (intrinsic to 1020 cm−3). Some experimental photomodulated measurements have been performed in order to test the validity of the “map

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Characterization of reflectivity inversion, alpha- and beta-phase transitions and nanostructure formation in hydrogen activated thin Pd films on silicon based substrates
Kalli K, Othonos A, Christofides C

Journal of Applied Physics, DOI: 10.1063/1.1417992Download
Optically thin palladium metal films evaporated on different silicon based substrates are investigated following exposure to different concentrations of hydrogen gas in air. Laser modulated reflectance off the palladium surface of silicon oxide and silicon nitride substrates is used to recover information regarding the reflectivity inversion and α/β-phases of the palladium complex after both first and multiple gas cycling. Atomic force microscopy confirms the formation of metal nanostructures following exposure to hydrogen of the optically thin palladium films.

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Ultrafast dynamics in phosphorus-implanted silicon wafers: The effects of annealing
Othonos A, Christofides C

Phys. Rev. B, DOI: 10.1103/PhysRevB.66.085206Download
Carrier relaxation in phosphorus-implanted silicon wafers (1016P+/cm2) annealed at different temperatures ranging from 350 to 1100 °C is investigated by near-infrared ultrafast time-resolved reflectivity measurements. A kinetic model based on four couple differential equations describing the carrier density, electron, hole, and lattice temperatures is used to evaluate the expected changes in the time-resolved reflectivity. This model was used to fit the experimental data having the trap recombination time constant and the optical absorption coefficient as fitting parameters. Our measurements reveal a complicated recovery of the ion-implanted silicon samples to crystallinity following annealing with carrier lifetimes ranging from

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Spatial dependence of ultrafast carrier recombination centers of phosphorus-implanted and annealed silicon wafers
Othonos A, Christofides C

Applied Physics Letters, DOI: 10.1063/1.1497723Download
In this letter, the spatial dependence of the carrier recombination centers induced in phosphorus-implanted and annealed silicon wafers have been examined. Ultrafast time-resolved reflectivity measurements of a set of phosphorus-implanted annealed silicon wafers (1016 P+/cm2) as a function of position on the wafer have been carried out, and an x–y map of the carrier lifetime for each of the samples has been obtained. Measurements reveal distinct features of the distribution of carrier recombination centers for the nonannealed and annealed samples between 350 °C and 1100 °C in an area of 36×36 μm2 with resolution better than 3 μm. The presence of islands of clusters in ion-implanted and annealed samples is also discussed in this letter

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