Application of InAs/AlSb materials system for development of short-wavelength quantum cascade lasers is explored. Molecular beam epitaxy (MBE) technology allowing to grow multiperiodical unstrained InAs/AlSb heterostructures with roughness of 1-2 monolayers is developed. It is demonstrated that InAs/AlSb materials system is well-suitable for development of short-wavelength quantum cascade lasers operating below 4 µm wavelength. Lasers containing plasmon-enhanced waveguides as well as the short period InAs/AlSb superlattices as waveguides were designed, MBE-grown and studied. The effect of waveguide properties on the device parameters is revealed. Usage of these waveguides and innovations in laser active region introducing “funnel” injector allowed one to reach operation temperature 420 K at the emission wavelength of 3.3 µm. The obtained optical peak power exceeded 1 W per facet. The room temperature operation has been obtained at wavelength below 3 µm. As for wavelength range, applying the new active region design strategy and the short period InAs/AlSb superlattice spacers InAs based quantum cascade lasers emitting at the wavelengths as short as 2.63 µm were developed, which is today the shortest emission wavelength of the operation of semiconductor lasers based on the intersubband transitions. / Disertaciniame darbe nagrinėjamas InAs/AlSb medžiagų sistemos panaudojimas trumpabangių tarppajuostinių lazerių kūrimui. Buvo išplėtota molekulinių pluoštelių epitaksijos technologija, leidžianti auginti daugiaperiodines neįtemptas InAs/AlSb heterosandūras su mažu 1-2 atominių sluoksnių šiurkštumu. Buvo parodyta, jog InAs/AlSb medžiagų sistema yra tinkama kurti trumpabangiams kvantiniams kaskadiniams lazeriams, veikiantiems žemiau 4 µm bangos ilgio ribos. Buvo ištirtas kvantinių kaskadinių lazerių, turinčių tiek plazmoninius bangolaidžius su stipriai legiruotais InAs apdariniais sluoksniais, tiek ir mažo periodo InAs/AlSb supergardelių bangolaidžius, veikimas bei jų įtaka prietaiso parametrams. Šie sprendimai dėl bangolaidžių bei tolimesni aktyviosios terpės patobulinimai, naudojant piltuvėlio formos injektorių, leido sukurti didelio našumo prietaisus, galinčius veikti iki 420 K temperatūros, esant 3,3 µm bangos ilgio emisijai, ir pasiekti maksimalią optinę galią siekiančią 1 W kambario temperatūroje. Šios inovacijos leido sukurti ir InAs/AlSb kvantinį kaskadinį lazerį, emituojantį ~2,6 µm bangos ilgio spinduliuotę  šiai dienai tai yra trumpiausią bangos ilgį spinduliuojantis tokio tipo prietaisas pasaulyje.

Disertaciniame darbe nagrinėjamas InAs/AlSb medžiagų sistemos panaudojimas trumpabangių tarppajuostinių lazerių kūrimui. Buvo išplėtota molekulinių pluoštelių epitaksijos technologija, leidžianti auginti daugiaperiodines neįtemptas InAs/AlSb heterosandūras su mažu 1-2 atominių sluoksnių šiurkštumu. Buvo parodyta, jog InAs/AlSb medžiagų sistema yra tinkama kurti trumpabangiams kvantiniams kaskadiniams lazeriams, veikiantiems žemiau 4 µm bangos ilgio ribos. Buvo ištirtas kvantinių kaskadinių lazerių, turinčių tiek plazmoninius bangolaidžius su stipriai legiruotais InAs apdariniais sluoksniais, tiek ir mažo periodo InAs/AlSb supergardelių bangolaidžius, veikimas bei jų įtaka prietaiso parametrams. Šie sprendimai dėl bangolaidžių bei tolimesni aktyviosios terpės patobulinimai, naudojant piltuvėlio formos injektorių, leido sukurti didelio našumo prietaisus, galinčius veikti iki 420 K temperatūros, esant 3,3 µm bangos ilgio emisijai, ir pasiekti maksimalią optinę galią siekiančią 1 W kambario temperatūroje. Šios inovacijos leido sukurti ir InAs/AlSb kvantinį kaskadinį lazerį, emituojantį ~2,6 µm bangos ilgio spinduliuotę  šiai dienai tai yra trumpiausią bangos ilgį spinduliuojantis tokio tipo prietaisas pasaulyje. / Application of InAs/AlSb materials system for development of short-wavelength quantum cascade lasers is explored. Molecular beam epitaxy (MBE) technology allowing to grow multiperiodical unstrained InAs/AlSb heterostructures with roughness of 1-2 monolayers is developed. It is demonstrated that InAs/AlSb materials system is well-suitable for development of short-wavelength quantum cascade lasers operating below 4 µm wavelength. Lasers containing plasmon-enhanced waveguides as well as the short period InAs/AlSb superlattices as waveguides were designed, MBE-grown and studied. The effect of waveguide properties on the device parameters is revealed. Usage of these waveguides and innovations in laser active region introducing “funnel” injector allowed one to reach operation temperature 420 K at the emission wavelength of 3.3 µm. The obtained optical peak power exceeded 1 W per facet. The room temperature operation has been obtained at wavelength below 3 µm. As for wavelength range, applying the new active region design strategy and the short period InAs/AlSb superlattice spacers InAs based quantum cascade lasers emitting at the wavelengths as short as 2.63 µm were developed, which is today the shortest emission wavelength of the operation of semiconductor lasers based on the intersubband transitions.

<p>Li-ion batteries are not only a technology for the future, they are indeed already the technology of choice for today’s mobile phones, laptops and cordless power tools. Their ability to provide high energy densities inexpensively and in a way which conforms to modern environmental standards is constantly opening up new markets for these batteries. To be able to maintain this trend, it is imperative that all issues which relate safety to performance be studied in the greatest detail. The surface chemistry of the electrode-electrolyte interfaces is intrinsically crucial to Li-ion battery performance and safety. Unfortunately, the reactions occurring at these interfaces are still poorly understood. The aim of this thesis is therefore to increase our understanding of the surface chemistries and stability phenomena at the electrode-electrolyte interfaces for three novel Li-ion battery electrode materials.</p><p>Photoelectron spectroscopy has been used to study the surface chemistry of the anode material AlSb and the cathode materials LiFePO₄ and Li₂FeSiO₄. The cathode materials were both carbon-coated to improve inter-particle contact. The surface chemistry of these electrodes has been investigated in relation to their electrochemical performance and X-ray diffraction obtained structural results. Surface film formation and degradation reactions are also discussed.</p><p>For AlSb, it has been shown that most of the surface layer deposition occurs between 0.50 and 0.01 V <i>vs.</i> Li°/Li⁺ and that cycling performance improves when the lower cut-off potential of 0.50 V is used instead of 0.01 V. For both LiFePO₄ and Li₂FeSiO₄, the surface layer has been found to be very thin and does not provide complete surface coverage. Li₂CO₃ was also found on the surface of Li₂FeSiO₄ on exposure to air; this was found to disappear from the surface in a PC-based electrolyte. These results combine to give the promise of good long-term cycling with increased performance and safety for all three electrode materials studied.</p>

Li-ion batteries are not only a technology for the future, they are indeed already the technology of choice for today’s mobile phones, laptops and cordless power tools. Their ability to provide high energy densities inexpensively and in a way which conforms to modern environmental standards is constantly opening up new markets for these batteries. To be able to maintain this trend, it is imperative that all issues which relate safety to performance be studied in the greatest detail. The surface chemistry of the electrode-electrolyte interfaces is intrinsically crucial to Li-ion battery performance and safety. Unfortunately, the reactions occurring at these interfaces are still poorly understood. The aim of this thesis is therefore to increase our understanding of the surface chemistries and stability phenomena at the electrode-electrolyte interfaces for three novel Li-ion battery electrode materials. Photoelectron spectroscopy has been used to study the surface chemistry of the anode material AlSb and the cathode materials LiFePO4 and Li2FeSiO4. The cathode materials were both carbon-coated to improve inter-particle contact. The surface chemistry of these electrodes has been investigated in relation to their electrochemical performance and X-ray diffraction obtained structural results. Surface film formation and degradation reactions are also discussed. For AlSb, it has been shown that most of the surface layer deposition occurs between 0.50 and 0.01 V vs. Li°/Li+ and that cycling performance improves when the lower cut-off potential of 0.50 V is used instead of 0.01 V. For both LiFePO4 and Li2FeSiO4, the surface layer has been found to be very thin and does not provide complete surface coverage. Li2CO3 was also found on the surface of Li2FeSiO4 on exposure to air; this was found to disappear from the surface in a PC-based electrolyte. These results combine to give the promise of good long-term cycling with increased performance and safety for all three electrode materials studied.

In this work, the electronic band structure of III (In, Al, Ga) - V (N, As, Sb) compounds and their ternary alloys have been investigated by density functional theory (DFT) within generalized gradient approximation (GGA) and empirical tight binding (ETB) calculations, respectively. The present DFT-GGA calculations have shown direct band gap structures in zinc-blende phase for InN, InAs, InSb, GaN, and GaAs. However, indirect band gap structures have been obtained for cubic AlN, AlSb and AlAs com- pounds / here, the conduction band minima of both AlN and AlAs are located at X symmetry point, while that of AlSb is at a position lying along Gamma- X direction. An important part of this work consists of ETB calculations which have been parameterized for sp3d2 basis and nearest neighbor interactions to study the band gap bowing of III(In / Al)- V(N / As / Sb) ternary alloys. This ETB model provides a satisfactory electronic properties of alloys within reasonable calculation time compared to the calculations of DFT. Since the present ETB energy parameters reproduce successfully the band structures of the compounds at &iexcl / and X symme- try points, they are considered reliable for the band gap bowing calculations of the ternary alloys. In the present work, the band gap engineering of InNxAs1&iexcl / x, InNxSb1&iexcl / x, InAsxSb1&iexcl / x, Al1&iexcl / xInxN, Al1&iexcl / xInxSb and Al1&iexcl / xInxAs alloys has been studied for total range of constituents (0 &lt / x &lt / 1). The downward band gap bowing seems the largest in InNxAs1&iexcl / x alloys among the alloys considered in this work. A metallic character of InNxAs1&iexcl / x, InNxSb1&iexcl / x and InAsxSb1&iexcl / x has been ob- tained in the present calculations for certain concentration range of constituents (N / As) as predicted in the literature. Even for a small amount of contents (x), a decrease of the electronic e&reg / ective mass around &iexcl / symmetry point appears for InNxAs1-x, InNxSb1-x and InAsxSb1-x alloys manifesting itself by an increase of the band curvature. The calculated cross over from indirect to direct band gap of ternary Al alloys has been found to be consistent with the measurements. As a last summary, the determinations of the band gaps of alloys as a function of contents, the concentration range of con- stituents leading to metallic character of the alloys, the change of the electronic effective mass around the Brillioun zone center (Gamma) as a function of alloy contents, the cross over from indirect to direct band gap of the alloys which are direct on one end, indirect on the other end, are main achievements in this work, indispensable for the development of mate- rials leading to new modern circuit components.