published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Broadband light emitters operation, which covers multiple wavelengths of the electromagnetic spectrum, has been established as an indispensable element to the human kind, continuously advancing the living standard by serving as sources in important multi-disciplinary field applications such as biomedical imaging and sensing, general lighting and internet and mobile phone connectivity. In general, most commercial broadband light sources relies on complex systems for broadband light generation which are bulky, and energy hungry. Recent demonstration of ultra-broadband emission from semiconductor light sources in the form of superluminescent light emitting diodes (SLDs) has paved way in realization of broadband emitters on a completely novel platform, which offered compactness, cost effectiveness, and comparatively energy efficient, and are already serving as a key component in medical imaging systems. The low power-bandwidth product is inherent in SLDs operating in the amplified spontaneous emission regime. A quantum leap in the advancement of broadband emitters, in which high power and large bandwidth (in tens of nm) are in demand. Recently, the birth of a new class of broadband semiconductor laser diode (LDs) producing multiple wavelength light in stimulated emission regime was demonstrated. This very recent manifestation of a high power-bandwidth-product semiconductor broadband LDs relies on interband optical transitions via quantum confined dot/dash nanostructures and exploiting the natural inhomogeneity of the self-assembled growth technology. This concept is highly interesting and extending the broad spectrum of stimulated emission by novel device design forms the central focus of this dissertation. In this work, a simple rate equation numerical technique for modeling InAs/InP quantum dash laser incorporating the properties of inhomogeneous broadening effect on lasing spectra was developed and discussed, followed by a comprehensive experimental analysis of a novel epitaxial structure design. The layered structure is based on chirping the barrier layer thickness of the over grown quantum dash layer, in a multi-stack quantum dash/barrier active region, with the aim of inducing additional inhomogeneity. Based on material-structure and device characterization, enhanced lasing-emission bandwidth is achieved from the narrow (2 u m)ridge-waveguide LDs as a result of the formation of multiple ensembles of quantum dashes that are electronically different, in addition to improved device performance. Moreover, realization of SLDs from this device structure demonstrated extra-ordinary emission bandwidth covering the entire international telecommunication union (O- to U-) bands. This accomplishment is a collective emission from quantum wells and quantum dashes of the device active region. All these results lead to a step forward in the eventual realization of more than 150 nm lasing bandwidth from a single semiconductor laser diode.

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Disertacijoje nagrinėjama, kaip padidinti susiaurintų spinduliuotės jutiklių jautrį. Taip pat disertacijoje siekiama ištirti mikrobangų spinduliuotės poveikį susiaurintiems puslaidininkiniams dariniams ir atskleisti stebimų efektų fizinę prigimtį bei nustatyti bandinių struktūros įtaką detektuojamo signalo dydžiui. Darbe sprendžiami tokie pagrindiniai uždaviniai: tiriamos įvairialyčių susiaurintų puslaidininkinių darinių savybės, priklausančios nuo darinių sluoksnių kokybės ir puslaidininkinių medžiagų parametrų bei analizuojamos savybės, priklausančios nuo stipriai legiruoto puslaidininkinio sluoksnio laidumo, nuo skiriamojo sluoksnio storio ir nuo sklendės pobūdžio metalizacijos. Siekiant užsibrėžto tikslo, buvo gaminami ir tiriami susiaurinti skirtingi įvairialyčiai dariniai (AlGaAs/GaAs, AlGaAs/InGaAs/GaAs) ir n-GaAs dariniai. Disertaciją sudaro penki skyriai, kurių paskutinis – rezultatų apibendrinimas. Pirmajame (įvadiniame) skyriuje nagrinėjamas problemos aktualumas, formuluojamas darbo tikslas bei uždaviniai, aprašomas mokslinis darbo naujumas, pristatomi autoriaus pranešimai, disertacijos struktūra. Antrasis skyrius skirtas literatūros apžvalgai. Jame apžvelgiami elektromagnetinės spinduliuotės detektavimo principai, aptariamos šiluminės ir bigradientinės elektrovaros susidarymo priežastys, AlGaAs/GaAs įvairialytė sandūra, selektyvusis legiravimas bei puslaidininkinių prietaisų fizikinės galimybės. Trečiajame skyriuje pateikta eksperimento tyrimo metodika. Išsamiai... [toliau žr. visą tekstą] / The thesis presents the investigation on how to increase the sensitivity of the narrowed sensors of radiation. Also the thesis also deals with the attempts to analyze the influence of the microwave radiation on to the narrowed semiconductor formations and to reveal the physical nature of the observed effects as well as to determine the influence of structure of the samples on the detected signal magnitude. The work solves the following major tasks: the characteristics of the narrowed semiconductor heterostructures depending on the quality of the modulation layers and on the parameters of the semiconductor materials as well as the characteristics, depending on the selectively doped structure, on the conductivity of the highly doped semiconductor layer, and on the thickness of the separating layer, and the type of metallization of the gate. In order to achieve the goal there were produced and investigated narrowed different heterostructures (AlGaAs/GaAs, AlGaAs/InGaAs/GaAs) and n-GaAs structures. The thesis consists of four chapters, the final one is the generalization of the results. The first chapter (introductory) deals with the actuality of the problem, the aim and the tasks are stated, the novelty of the scientific research is described, the reports of the author are presented together with the publications, and the structure of the thesis. The second chapter is assigned to the review of the literature. It presents the principals of electromagnetic radiation detection... [to full text]

Tyrimų tikslas. Ištirti kietakūnio apšvietimo, pagrįsto šviesą emituojančių diodų technologija, panaudojimo galimybes žalumyninių daržovių, želmenų ir daigintų sėklų vidinės kokybės rodikliams valdyti. Mokslinis naujumas. Pirmą kartą nustatyta, kad raudona 638 nm šviesa ir jos derinys su raudona 669 nm, mėlyna 447 nm ir tolimąja raudona 731 nm šviesa paskatina nitratų redukcijos procesus salotose ir kitose žalumyninėse daržovėse. Nustatyta, kad reikšmingam teigiamam efektui žalumyninių daržovių vidinei kokybei pasiekti pakanka jas švitinti didelio tankio fotosintetiškai aktyvios raudonos 638 nm spinduliuotės srautu ~72 h prieš planuojamą derliaus nuėmimą. Pirmą kartą įvertintas kietakūnio apšvietimo spektro efektas skirtingų lietuviškų javų želmenų, lapinių ridikėlių ir daigintų sėklų antioksidacinėms savybėms. Nustatytas teigiamas raudonų 638, 669 nm, mėlynos 447 nm ir tolimosios raudonos 731 nm bangų ilgių deriniui papildomų žalios 518 nm ir geltonos 595 nm šviesos efektas natūraliai aukštomis bioaktyvių medžiagų koncentracijomis audiniuose išsiskiriančių žalumyninių daržovių ir želmenų antioksidacinėms savybėms. Darbo praktinė svarba. Įvertintos originalios konstrukcijos kietakūnio apšvietimo įrenginių taikymo žalumyninių daržovių vidinei kokybei gerinti galimybės. Remiantis darbe nustatytais dėsningumais, parengtas ir patentuotas žalingų nitratų kiekio augaluose sumažinimo, apšvitinant kietakūnio šviestuvo sukuriamu šviesos srautu, metodas ir įrenginys. Atliktų tyrimų... [toliau žr. visą tekstą] / The aim of the research was to investigate the usability of the solid state lighting, based on light emitting diode technology, for the management of nutritional quality indices in green vegetables and sprouts. Scientific originality. It was determined for the first time, that red 638 nm light and its combination with blue 447 nm, red 669 nm and far red 731 nm light promoted nitrate reduction processes in lettuce and other green vegetables. It is enough to irradiate green vegetables with the high flux of photosyntheticaly active 638 nm red light for ~72 h before harvesting for the pronounced positive effect on their internal quality. The effect of the solid-state lighting spectra on antioxidant properties of Lithuanian cereal greens, leafy radish and sprouted seeds was evaluated for the first time. The positive effect of green 518 nm and yellow 595 nm light, supplemental for the red 638, 669 nm light, blue 447 nm and far red 731 nm light on the antioxidant properties of green vegetables and sprouts, naturally containing higher concentrations of bioactive compounds, was determined. Practical value of the work. Solid-state lighting application possibilities for the improvement of green vegetable internal quality were evaluated. According to defined trends, the method and apparatus for the reduction of harmful nitrates in plants, when irradiating plants with the light flux generated by semiconductor lighting unit, was designed and patented. The complex investigations and... [to full text]

Tyrimų tikslas. Ištirti kietakūnio apšvietimo, pagrįsto šviesą emituojančių diodų technologija, panaudojimo galimybes žalumyninių daržovių, želmenų ir daigintų sėklų vidinės kokybės rodikliams valdyti. Mokslinis naujumas. Pirmą kartą nustatyta, kad raudona 638 nm šviesa ir jos derinys su raudona 669 nm, mėlyna 447 nm ir tolimąja raudona 731 nm šviesa paskatina nitratų redukcijos procesus salotose ir kitose žalumyninėse daržovėse. Nustatyta, kad reikšmingam teigiamam efektui žalumyninių daržovių vidinei kokybei pasiekti pakanka jas švitinti didelio tankio fotosintetiškai aktyvios raudonos 638 nm spinduliuotės srautu ~72 h prieš planuojamą derliaus nuėmimą. Pirmą kartą įvertintas kietakūnio apšvietimo spektro efektas skirtingų lietuviškų javų želmenų, lapinių ridikėlių ir daigintų sėklų antioksidacinėms savybėms. Nustatytas teigiamas raudonų 638, 669 nm, mėlynos 447 nm ir tolimosios raudonos 731 nm bangų ilgių deriniui papildomų žalios 518 nm ir geltonos 595 nm šviesos efektas natūraliai aukštomis bioaktyvių medžiagų koncentracijomis audiniuose išsiskiriančių žalumyninių daržovių ir želmenų antioksidacinėms savybėms. Darbo praktinė svarba. Įvertintos originalios konstrukcijos kietakūnio apšvietimo įrenginių taikymo žalumyninių daržovių vidinei kokybei gerinti galimybės. Remiantis darbe nustatytais dėsningumais, parengtas ir patentuotas žalingų nitratų kiekio augaluose sumažinimo, apšvitinant kietakūnio šviestuvo sukuriamu šviesos srautu, metodas ir įrenginys. Atliktų tyrimų... [toliau žr. visą tekstą] / The aim of the research was to investigate the usability of the solid state lighting, based on light emitting diode technology, for the management of nutritional quality indices in green vegetables and sprouts. Scientific originality. It was determined for the first time, that red 638 nm light and its combination with blue 447 nm, red 669 nm and far red 731 nm light promoted nitrate reduction processes in lettuce and other green vegetables. It is enough to irradiate green vegetables with the high flux of photosyntheticaly active 638 nm red light for ~72 h before harvesting for the pronounced positive effect on their internal quality. The effect of the solid-state lighting spectra on antioxidant properties of Lithuanian cereal greens, leafy radish and sprouted seeds was evaluated for the first time. The positive effect of green 518 nm and yellow 595 nm light, supplemental for the red 638, 669 nm light, blue 447 nm and far red 731 nm light on the antioxidant properties of green vegetables and sprouts, naturally containing higher concentrations of bioactive compounds, was determined. Practical value of the work. Solid-state lighting application possibilities for the improvement of green vegetable internal quality were evaluated. According to defined trends, the method and apparatus for the reduction of harmful nitrates in plants, when irradiating plants with the light flux generated by semiconductor lighting unit, was designed and patented. The complex investigations and... [to full text]

One dimensional (1-D), zinc oxide (ZnO) and copper (II) oxide (CuO), nanostructures have great potential for applications in the fields of optoelectronic and sensor devices. Research on nanostructures is a fascinating field that has evolved during the last few years especially after the utilization of the hydrothermal growth method. Using this method variety of nanostructures can be grown from solutions, it is a cheap, easy, and environment friendly approach. These nanostructures can be synthesized on various conventional and nonconventional substrates such as silicon, plastic, fabrics and paper etc. The primary purpose of the work presented in this thesis is to realize controllable growth of ZnO, CuO and nanohybrid ZnO/CuO nanostructures and to process and develop white light emitting diodes and sensor devices from the corresponding nanostructures. The first part of the thesis deals with ZnO nanostructures grown under different hydrothermal conditions in order to gain a better understanding of the growth. Possible parameters affecting the growth such as the pH, the growth temperature, the growth time, and the precursors  concentration which can alter the morphology of the nanostructures were investigated (paper 1). Utilizing the advantage of the low temperature for growth we synthesized ZnO nanostructures on different substrates, specifically on flexible substrates, which are likely to be integrated with flexible organic substrates for future foldable and disposable electronics (paper 2, 3). In the second part of the thesis, using the results and findings from the growth of ZnO nanostructures, it was possible to successfully implement ZnO nanostructures for white light emitting diodes (LEDs) on different flexible substrates (paper 4, 5). In paper 4 we realized a ZnO/polymer LED grown on a paper substrate. In paper 5 we extended the idea to print the ZnO nanorods/polymer hybrid LEDs with potential application to large area flexible displays. In the last part of the thesis, CuO and nanohybrid ZnO/CuO nanostructures were utilized to fabricate Ag+ detection and humidity sensors. In paper 6 we reported Ag+ selective electrochemical sensor based on the use of functionalized CuO nanopetals. To combine the advantages of both oxides nanostructures and to improve the performance we fabricated a pn-heterojuction using intrinsic n-ZnO nanorods and p-CuO nanostructures which were then utilized as an efficient humidity sensor (paper 7).

Research on nanomaterials has become increasingly popular because of their unique physical, chemical, optical and catalytic properties compared to their bulk counterparts. Therefore, many efforts have been made to synthesize multidimensional nanostructures for new and efficient nanodevices. Among those materials, zinc oxide (ZnO), has gained substantial attention owing to many outstanding properties. ZnO besides its wide bandgap of 3.34 eV exhibits a relatively large exciton binding energy (60 meV) at room temperature which is attractive for optoelectronic applications. Likewise, cupric oxide (CuO), having a narrow band gap of 1.2 eV and a variety of chemo-physical properties that are attractive in many fields. Moreover, composite nanostructures of these two oxides (CuO/ZnO) may pave the way for various new applications. This thesis can be divided into three parts concerning the synthesis, characterization and applications of ZnO, CuO and their composite nanostructures. In the first part the synthesis, characterization and the fabrication of ZnO nanorods based hybrid light emitting diodes (LEDs) are discussed. The low temperature chemical growth method was used to synthesize ZnO nanorods on different substrates, specifically on flexible non-crystalline substrates. Hybrid LEDs based on ZnO nanorods combined with p-type polymers were fabricated at low temperature to examine the advantage of both materials. A single and blended light emissive polymers layer was studied for controlling the quality of the emitted white light. The second part deals with the synthesis of CuO nanostructures (NSs) which were then used to fabricate pH sensors and exploit these NSs as a catalyst for degradation of organic dyes. The fabricated pH sensor exhibited a linear response and good potential stability. Furthermore, the catalytic properties of petals and flowers like CuO NSs in the degradation of organic dyes were studied. The results showed that the catalytic reactivity of the CuO is strongly depending on its shape. In the third part, an attempt to combine the advantages of both ZnO and CuO NSs was performed by developing a two-step chemical growth method to synthesize the composite NSs. The synthesized CuO/ZnO composite NSs revealed an extended light absorption and enhanced defect related visible emission.

No description available.

This thesis describes the fabrication and characterization of GaN homojunction visible-blind ultraviolet (UV) p-i-n avalanche photodiodes (APDs) grown by metalorganic chemical vapor deposition (MOCVD) on free-standing bulk GaN substrates. The objective of this research is to develop GaN UV p-i-n APDs with high linear-mode avalanche gains and the Geiger-mode operation for single photon detection. Low noise, high responsivity, and high detectivity are also required for fabricated APDs used as photodiodes in the photovoltaic mode (zero bias) and the photoconductive mode (low reverse bias). High material defect density and immature fabrication technology have hampered the development of III-nitride APDs in the past. In this thesis, sidewall leakage reduction methods have been developed to achieve significant improvement in dark current density, noise performance, and photo detection performance. A record linear-mode avalanche gain > 10⁵ for GaN APDs was demonstrated at λ = 360 nm. The first Geiger-mode deep UV (DUV) APD using front-illuminated homojunction p-i-n diode structure on a free-standing bulk GaN substrate was also measured with single photo detection efficiency (SPDE) of 1.0 % and dark count probability (DCP) of 0.03 at 265 nm. The performance of fabricated homojunction GaN p-i-n photodiodes was also evaluated in the photoconductive mode as well as the photovoltaic mode. For an 80-µm-diameter device biased at - 20 V (in the photoconductive mode) the dark current density is lower than 40 pA/cm² which is the lowest value achieved for any III-nitride photodiode so far. Its responsivity is 0.140 A/W at 360 nm with an ultraviolet-visible rejection ratio of 8×10³. The room-temperature noise equivalent power is 4.27×10 ⁻¹⁷ W-Hz-[superscript 0.5] and the detectivity D* is 1.66×10¹⁴ cm-Hz[superscript 0.5]-W ⁻¹ at - 20 V. The minimum detectable optical power is as low as 100 fW. They are among the best values reported for reverse-biased GaN p-i-n photodiodes to date.