Studies

Home
Studies
Defended Dissertations
Defended Dissertations in 2012

Defended Dissertations in 2012

Andrius Bičiūnas

Author: Andrius Bičiūnas

Dissertation title: Semiconductor materials for components of optoelectronic terahertz systems activated by femtosecond 1 µm wavelenght lasers pulses

Fields of science: Physical sciences, Physics (02P), Semiconductor physics (P 265)

Scientific supervisor: Prof. Habil. Dr. Arūnas Krotkus

Defence of the dissertation: 2012-10-26

Summary

Main goal

The main goal of this dissertation was to develop and to investigate semiconductor materials for terahertz (THz) pulse emitters to be used in Terahertz time-domain spectroscopy (THz-TDS) systems using a 1 µm wavelength femtosecond laser radiation.

The objectives of the study

To compare the efficiency of various semiconductor surface THz emitters photoexcited by femtosecond 1 |im wavelength laser pulses.
To explore the physical mechanisms of terahertz generation from narrow gap semiconductors (InAs, Ge, InSb) surfaces.
To produce and to investigate ultrafast photoconductors from low-temperature grown (LTG) GaBiAs layers and to adapt them for THz pulse generation.
To explore the THz pulse generation and detection using a 1 µm femtosecond laser radiation and photoconductors manufactured from LTG GaAs.

The novelty of the study

THz pulse emission from the surfaces of various semiconductors illuminated by 1 µm wavelength femtosecond laser pulses have been studied systematically.
Azimuthal angle dependences of terahertz pulse emission from low symmetry, (112)-cut InSb samples were measured for the first time. It has been shown that both nonlinear optical effects (optical rectification and electric field induced optical rectification) are necessary to take into account when explaining the THz signal generation from the surfaces of this material.
New LTG GaBiAs, LTG GaAs emitters and LTG GaBiAs detectors for THz-TDS systems based on femtosecond 1 µm wavelength laser have been developed and THz pulse emission from GaBiAs and LTG GaAs emitters have been thoroughly investigated.
Using the THz-TDS system based on photoconductive antennae manufactured from GaBiAs layers and activated by low average power, 1030 nm femtosecond Yd-doped fiber laser record high optical-to-THz power conversion efficiency ~5 x10-4 was obtained.

Conclusion

In conclusion, photoconductive antenna manufactured from as-grown and moderately annealed LTG GaAs layers activated by 1030 nm wavelength femtosecond duration laser pulses were used for terahertz pulse generation and detection. An optical pump - terahertz probe technique was used to study the ultrafast photoconductivity in these layers. It has been shown that this photoconductivity caused by the electrons excited from deep AsGa donor levels is the largest for LTG GaAs annealed at the temperature of about 420 °C. In the as-grown material it is lower because of the lower electron mobility, whereas higher annealing temperatures are leading to a significant reduction of the AsGa defects.

As GaAs has a much larger energy bandgap than other semiconductors used for manufacturing optoelectronic THz components sensitive to 1 µm wavelength laser radiation, the investigated THz emitters could be biased to relatively large voltages. Because of their high optical-to-THz radiation conversion efficiencies and reliability, the THz emitters made from moderately annealed LTG GaAs are preferable choice for THz-TDS systems activated with femtosecond near-infrared lasers. On the other hand, the sensitivity of the pulsed THz detectors made from this material is by an order of magnitude worse than for GaBiAs detectors - the best detecting devices for THz-TDS systems using the lasers of the near-IR spectral range.

Česlav Paškevič

Author: Česlav Paškevič

Dissertation title: Investigation of electric properties of semiconductor heterostructures for microwaves elctronics

Fields of science: Physical sciences, Physics (02P), Semiconductor physics (P 265)

Scientific supervisor: Prof. Habil. Dr. Steponas Ašmontas

Defence of the dissertation: 2012-09-13

Summary

Object of the research

The objects of investigations are semiconductor AlGaAs/GaAs, AlGaAs/InGaAs/AlGaAs and InAlAs/InGaAs/InAlAs heterostructures used for microwave electronics.

Aim of the research

The aim of this research is to investigate electrical properties of the lowdimensional semiconductor heterojunctions in the microwave electric field, as well as to suggest ways to increase the sensitivity of microwave sensors and operating speeds of field-effect transistors using peculiarities of electrical properties of low-dimensional semiconductor heterojunctions.

Main tasks of the work

To investigate interaction of microwave electric field with semiconductor AlGaAs/GaAs and AlGaAs/InGaAs/AlGaAs nanostructures
To measure current-voltage characteristics of semiconductor AlGaAs/GaAs. AlGaAs/InGaAs/AlGaAs and InAlAs/InGaAs/InAlAs nanostructures in high electric fields.
To determine electron drift velocity and mobility dependence on electric fieId strength in semiconductor AlGaAs/GaAs, AlGaAs/InGaAs/AlGaAs and InAlAs/InGaAs/InAlAs nanostructures.
To investigate possibilities (and conditions) for the increase of the electron drift velocity in the semiconductor AlGaAs/GaAs, AlGaAs/InGaAs/AlGaAi InAlAs/InGaAs/InAlAs quantum wells in strong electric fields.

Scientific novelty

It is established experimentally that electron drift velocities in the heterojunction InAlAs/InGaAs/InAlAs structures can reach 5x 107 cm/s value; in the GaAs quantum wells 2D electron drift velocity can reach the value up to 1.6xl07 cm/s.

There was determined that electron scattering can be confined by the insertion of phonon walls into the heterojunction quantum well. This approach allows us to obtain higher electron drift velocities.

General conclusions

It was found experimentally that voltage-power sensitivity of asymmetrically shaped modulation-doped AlGaAs/GaAs microwave diodes increases up to 3 orders when the active area of the diode was gate-like metalized.

Voltage-power sensitivity of the microwave diodes made on the base of Al0.28 Ga0.72As/In0.15Ga0.85As/GaAs is twice as higher comparing to the Al0.25Ga0.75As/GaAs diode.

It was also noticed that in AlGaAs/GaAs/AlGaAs heterojunction, drift velocity monotonically saturated at high (8-15 kV/cm) electric fields.

Experimentally it was found that saturated electron drift velocity in the AlGaAs/InGaAs/AlGaAs structures is 1.6x 107 cm/s, and it is higher than that in the bulk GaAs (1 x 107 cm/s).

Experimental investigation has shown that insertion of the InAs phonon barriers into the quantum well nanostructures increases the saturation value of the electron drift velocity.

Also it is shown that electron drift velocity in the AlGaAs/GaAs/AlGaAs structures with InAs phonon barriers is higher (2.1 xl07 cm/s) compared to that of AlGaAs/GaAs/AlGaAs without InAs phonon barriers in the GaAs layer (1.6xl07cm/s).

Electron saturated drift velocity in In0.52Al0.48As/In0.53Ga0.48As/In0.52Al0.48As heterojunction with InAs phonon barriers increases to the 5x107cm/s, depending on the doping and insertions in the quantum well of the structure.

Eglė Kazlauskienė

Author: Eglė Kazlauskienė

Dissertation title: Sorption of metal complex dyes onto ion exchange resins

Fields of science: Physical sciences, Chemistry (03P)

Scientific supervisor: Dr. Danutė Kaušpėdienė

Defence of the dissertation: 2012-01-27

Summary

The main aim of the present work was to investigate absorption regularity metal complex Lanasyl Navy M-DNL and Acid Blue 249 (copper (II) phthalocyanine) dyes on synthetic ion exchangers under static conditions and evaluate facility of ion exchangers to removal dyes from wastewaters by dynamic conditions.

The objectives of the research are the following:

To screen most potential ion exchangers depending dyes sorption capacity, dye distribution coefficient and ion exchanger regeneration.
To determine and compare anion exchangers sorption capacity dependence from anionic matrix structures, type and functional groups for metal complex dyes.
To investigate sorption mechanism, equilibrium and kinetics parameters in dependence from solution pH, concentration, temperatures and interaction time for metal complex dyes on anion exchangers.
To estimate suitability theoretical isotherm and kinetics models for analysis of experimental data.
To investigate metal complex dyes sorption/desorption under dynamic condition, according environmental standards. Estimating suitability theoretical Wolborska, Bohart-Adams and Juang models for this process.

Scientific novelty and practical value of the work.

Determine the interaction mechanism between harmful chromium and copper complex dyes and weak/strong base anion exchangers, also the measurement of the sorption equilibrium and kinetics parameters. The sorption kinetics investigations are supplemented with optic microscopy, determination of metal complex dyes removal mechanism in static and dynamic conditions. The metal complex dyes removal possibility using Macronet sorbent are not investigated at all.

Purolite A847 weak base anion exchanger and nonfunctionalized Macronet MN 200 are able to remove the metal complex dyes from aqua solutions. Possibility to use the sorbents for multifold removal Cr, Cu and organics impurities from water would fulfill the requirements of the environmental standards and water recycling.

Conclusions

Screening with chromium-complex dye (acid brown NKM, Lanasyn Navy M- DNL) solutions wide range sorbents, according to the physical-chemical characteristics of the sorbents and established dyes removal parameters (sorption capacity, distribution coefficient and sorbent regeneration) shows that the polyacrylic, gel type, weakly basic anion exchanger Purolite A 847 (A 845); polystyrene, macroporous, strongly basic anion exchanger Purolite A 500 PS and nonfunctionalized Macronet MN 200 are the most promising sorbents for the metal complex dye removal.
The sorption of dyes Navy and CuPc proceeded: 1) on the weak basic Purolite A 847(A 845) and strong basis Purolite A 500PS anion exchangers according to the ion exchange mechanism in the presence of additional hydrogen, coloumbic or weak Van der Waals forces; 2) on the nonfunctionalized sorbent Macronet MN 200 according to the physical sorption mechanism including diffusion process following formation hydrogen bonds or weak Van der Waals forces. The mechanism of sorption was confirmed by analyzing the recorded of FT-IR spectra of sorbents, dye unloaded and loaded.
The sorption capacity for anion exchangers and MN 200 strongly depends on solution pH, with higher values at acidic pH. With an increase temperature from 293 to 333 K in the solution the dye Navy sorption capacity on both anion exchanger and on MN 200 increases, whereas the dye CuPc decreases.
The experimental data showed a good correlation with Langmuir (correlation coefficients R2 are from 0.9317 till 0.9789) and Freundlich (correlation coefficients R2 are from 0.8430 till 0.9848) theoretical isotherm models for all system Navy-sorbent. Whereas, the data of the systems CuPc-sorbent satisfies only Freundlich mathematical model (correlation coefficients R2 are from 0.9995 till 1).
Pseudo-second order kinetic model (the correlation coefficients R2 are from
0.9811 till 0.9959 for anion exchangers and from 0.9914 till 0.9998 for nonfunctionalized MN 200) is suitable to determination of equilibrium sorption capacity and rate constant according to the dye Navy and dye CuPc. The Weber-Moris intraparticle diffusion model was used for calculation the intraparticle diffusion coefficient (kI) and for indirect evaluation of the influence of boundary layer, which was formed about the sorbent bead. Determinate boundary layer thickness for sorption dye Navy on both anion exchangers was biggest neither for dye CuPc sorption, and it was lesser for sorption dye Navy on MN 200 neither for dye CuPc sorption. Thermodynamic parameter was determination for all system dye-sorbent.
High metal complex dyes removal efficiency (Cr 36 - 100 %, Cu 86-94 %) was obtained for dye Navy interaction with A 847 (A 845) at initial solution pH 2 and with Macronet MN 200 (only at low initial dye concentration (<3.08 mg/l) in solution) and can satisfy the environmental requirement for chromium concentration less than 0.1 mg/l and COD less than 150 mg/l 02.
The mass transfer in the system anion exchanger and one-component dye solution depended on the concentration of the sulphonate groups in the chemical structure of dye molecule and was lower for the dye with four-sulpho groups (C.I. Acid Blue 249) than that for the dye containing two-sulphonate groups (Lanasyn Navy M-DNL).
Several models were applied to the experimental data obtained from dynamic studies performed on fixed bed columns to predict the breakthrough curves and to determine the column kinetic parameters. The simulation of the whole breakthrough curve is effective with Bohart-Adams and Juang models for sorption dye Navy and dye CuPc onto Purolite A 847 and with the Bohard-Adams for sorption dye Navy onto nonionic MN 200. A linear relationship between ln(c/c0) and the time at a given bed height and flow rate was obtained in the initial region of effluent concentration (c<0.5c0), suggesting that this segment of the breakthrough curve fit Wolborska model, and allows estimating the kinetic mass transfer coefficient in the fixed bed.
The column capacity for Purolite A 845 and MN 200 was found to be higher than the batch capacity. It is possible to regenerate the anion exchanger quantitatively (70%) recovering the dye with mixture of 4% NaOH and ethanol (1:1). Whereas MN 200 regeneration with methanol is perfect (100 %).

Ramūnas Nedzinskas

Author: Ramūnas Nedzinskas

Dissertation title: Modulated reflectance and photoluminescence spectroscopy of epitaxial InGaAs quantum dot structures

Fields of science: Physical sciences, Physics (02P), Semiconductor physics (P 265)

Scientific supervisor: Prof. Habil. Dr. Gintaras Valušis

Defence of the dissertation: 2012-09-24

Summary

Epitaxial InGaAs quantum dot (QD) structures is the topic of this doctoral dissertation. The QD structures are studied using modulated reflectance and photoluminescence spectroscopy techniques. Such nanostructures are the key-ingredients in many novel photonics devices, operating in infrared and terahertz spectral range. Hence, a comprehensive knowledge of optical properties and electronic energy spectrum of these QD structures is essential in order to define their optimal design parameters and favorable growth conditions.

The doctoral thesis is organized as follows. Motivation, main aims of the work, scientific novelty, statements for defence and list of publications together with a list of conference reports are given in the Introduction. Then, Chapter 1 presents the principles and examples of modulation spectroscopy, a very sensitive tool for optical characterization, followed by a brief introduction to molecular beam epitaxy and Stranski-Krastanow self-assembling growth mode for realization of QD nanostructures. Moreover, an extensive literature overview, regarding main achievements of QD-based nanostructures investigated, can be found here. In Chapter 2, details of QD samples studied with the relevant structure schemes, bandstructure diagrams, growth protocols and TEM images followed by experimental set-up of spectroscopic techniques employed are given. Chapters 3 and 4 covers original results of the dissertation. In particular, Chapter 3 contains comprehensive investigation of electronic structure and optical properties of InAs/GaAs/AlAs dots-in-a-superlattice, InAs/InGaAs/GaAs/AlAs dots-in-a-well and InGaAs quantum rod (QR) structures. Moreover, an intriguing optical anisotropy properties of InGaAs QRs are revealed and discussed therein. Chapter 4 is devoted to original analytical approach, based upon variable separation approximation, for energy spectrum calculations of cylindrical self-assembling QDs and QRs. Finally, Chapter 5 underlines the main results and presents conclusions of the work.

Paulius Gečys

Author: Paulius Gečys

Dissertation title: Ultrashort pulsed laser processing of thin-films for solar cells

Fields of science: Technological sciences, Material engineering (08T), Laser technology (T 165)

Scientific supervisor: Dr. Gediminas Račiukaitis

Defence of the dissertation: 2012-09-20

Summary

Present PhD thesis is the experimental and theoretical analysis of thin layer ablation processes for photovoltaic devices. The picosecond (10 ps) and femtosecond (300 fs) lasers were used in the ablation and scribing experiments. Flexible CIGS solar cell samples with different top-contact structures were investigated. Laser scribing was performed using different wavelengths including: 1575 nm, 1064 nm, 775 nm, 532 nm, 355 nm and 266 nm. Laser scribed areas were investigated with optical and Scanning electron microscopes together with EDS (X-ray energy dispersion) and Raman spectrometers. Electrical characterization was obtained with LIT (Lock-in thermography), LBIC (Laser beam induced current measurements) techniques. Efficiency of the solar cells after laser scribing was also investigated.

Experimental work was supported by modeling and simulation of energy coupling and dissipation inside the layers. Selectiveness of the ablation process is defined by optical and mechanical properties of the materials, and selection of the laser wavelength facilitated control of the structuring process.

The 1064 nm wavelength was found optimal for the CIGS solar cell scribing in terms of quality and process speed. It is very positive result for industrial applications as the cost and system complexity are decreased.

The solar cell efficiency test revealed minor degradation in photo-electrical efficiency after the laser scribing was applied to the solar cell samples. Lock-in thermography measurements did not revealed any internal shunt formation during laser scribing with picosecond pulse duration.

Picosecond lasers with fundamental harmonics and high repetition rates can be used to accomplish efficient and fast scribing process which is able to fit the demands for industrial solar cell scribing applications.

Virginija Kepenienė

Author: Virginija Kepenienė

Dissertation title: Investigation of peculiarities of electroless copper plating systems using hydroxycarboxylic acids as Cu(II) ligands

Fields of science: Physical sciences, Chemistry (03P)

Scientific supervisor: Prof. Habil. Dr. Eugenijus Norkus

Defence of the dissertation: 2012-05-30

Summary

The aim of the work was to investigate peculiarities of formaldehyde containing alkaline electroless copper deposition systems using environment friendly hydroxycarboxylic acids as Cu(II) ligands.

The main tasks of the work were as follows:

To investigate or define more precisely equilibria in alkaline formaldehyde containing electroless copper solutions using citric acid and D-, L- and DL-isomers of tartaric acid as Cu(II) ligands.
To estimate the stability of electroless copper deposition solutions, to investigate influence of pH and concentration of ligand on the rate of electroless copper reduction and characteristics of coatings deposited using citric acid as Cu(II) ligand.
To investigate influence of temperature, pH and the nature of ligand on the rate of electroless copper reduction and characteristics of coatings deposited using isomers of D-, L- and DL-tartaric acid as Cu(II) ligand.
To perform the measurements of the real surface area of electrochemically and electrolessly deposited copper coatings.
To investigate the parameters of anodic oxidation of formaldehyde on electrolessly deposited copper coatings obtained from solutions containing D-, L- and DL-tartrate as Cu(II) ligand.

Conclusions

When investigating Cu(II) complex formation with citrate and D-, L- and DL- tartrate in alkaline solutions by means of pH-metric titration and spectrophotometry, it was found that Cu(II) ions promote deprotonation of the -OH groups of citric and tartaric acids, forming stable complexes with the mentioned ligands.
The possibility of using of citric acid as Cu(II) ligand in electroless copper plating systems is shown: the autocatalytic reduction of Cu(II) by formaldehyde from solutions containing trisodium citrate as the ligand begins at pH above 12, accelerates with a further increase in pH, reaches a maximum value at pH 12.75 (ca. 3 µm/h at 20 °C), then slows down at higher pH values.
The real surface area of copper coatings electrolessly deposited from trisodium citrate-containing solutions depends on solution pH and rises with increase in solution alkalinity. The roughness factor (Rf) of copper coatings obtained could reach 28.
The possibility of using of all investigated isomers of tartaric acid, namely D- tartrate, L-tartrate and and racemic mixture of DL-tartrate, as Cu(II) ligands in electroless copper plating systems is shown. The autocatalytic reduction of Cu(II) by formaldehyde from solutions containing mentioned ligands begins at pH about 12.
Under equal conditions at pH 12.0 - 13.0 the kinetics of electroless copper deposition from solutions containing D-, L- and DL-tartrate as Cu(II) ligands depends: a) on nature of the ligand - the lowest rates were observed for D-tartrate, whereas the highest - for DL-tartrate; b) on solution pH - the maximum deposition rate was found at solution pH 13.0 for D- and L-tartrate, whereas in the case of DL tartrate the maximum rate was achieved at pH 12.5; c) on temperature - in the case of all used isomers of tartaric acid the electroless copper deposition rate increased with the increment of temperature.
The mixed potential of copper electrode (Emix) in solutions for electroless copper deposition depends on the nature of the ligand used. Using trisodium citrate relatively negative Emix values were measured (ca. -520 - -630 mV), whereas in the case of tartrate isomers comparatively more positive values were observed (ca. -300 - -430 mV).
The real surface area of copper coatings electrolessly deposited from tartrate isomers containing solutions (pH 12.0 - 13.0; 10 - 30 ° C) depends on the conditions of electroless copper deposition process - under conditions of lower alkalinity and lower temperature the coatings with more developed surfaces were obtained, whereas the increase in solution pH and increment of temperature result formation of smoother copper coatings.
The rate of formaldehyde anodic oxidation depends on the following factors: a) solution pH - with the increase in solutions alkalinity (from 12.0 to 13.0), the rate of HCHO oxidation increases on all investigated surfaces of copper coatings; b) conditions of the formation of copper coatings - the highest rate of HCHO oxidation was determined on the surfaces electrolessly obtained at 10 °C and pH 13.0; c) nature of the ligand - the highest rate of HCHO oxidation was found on copper surfaces electrolessly deposited from solutions containing DL-tartrate.