Browsing by Subject "Quantum well"
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Item Open Access Development of nano hall sensors for high resolution scanning hall probe microscopy(Bilkent University, 2008-09) Dede, MünirScanning Hall Probe Microscopy (SHPM) is a quantitative and non invasive method of local magnetic field measurement for magnetic and uperconducting materials with high spatial and field resolution. Since its demonstration in 1992, it is used widely among the scientific community and has already commercialized. In this thesis, fabrication, characterization and SHPM imaging of different nano-Hall sensors produced from heterostructure semiconductors and Bismuth thin films with effective physical probe sizes ranging between 50nm‐1000nm, in a wide temperature range starting from 4.2K up to 425K is presented. Quartz crystal tuning fork AFM feedback is demonstrated for the first time for SHPM over a large temperature range. Its performance has been analyzed in detail and experiments carried with 1×1μm Hall probes has been successfully shown for a hard disk sample in the temperature range of 4.2K to 425K. Other samples, NdFeB demagnetized magnet, Bi substituted iron garnet and, single crystal BSCCO(2212) High Temperature superconductor were also imaged with this method to show the applicability of the method over a wide range of specimens. By this method, complex production steps proposed in the literature to inspect the non‐conductive samples were avoided. A novel Scanning Hall probe gradiometer has also been developed and a new method to image x, y & z components of the magnetic field on the sample surface has been demonstrated for the first time with 1μm resolution. 3D field distribution of a Hard Disk sample is successfully measured at 77K using this novel approach to prove the concept.Item Open Access Electric field dependence of radiative recombination lifetimes in polar InGaN/GaN quantum heterostructures(IEEE, 2009) Sarı, Emre; Nizamoğlu, Sedat; Lee I.-H.; Baek J.-H.; Demir, Hilmi VolkanWe report on external electric field dependence of recombination lifetimes in polar InGaN/GaN quantum heterostructures. In our study, we apply external electric fields one order of magnitude less than and in opposite direction to the polarization-induced electrostatic fields inside the well layers. Under the increasing external electric field, we observe a decrease in carrier lifetimes (τ) and radiative recombination lifetimes (τr), latter showing a weaker dependence. Our results on τr show an agreement with our transfer matrix method based simulation results and demonstrate Fermi's golden rule in polar InGaN/GaN quantum heterostructures dependent on electric field. For our study, we grew 5 pairs of 2.5 nm thick In0.15Ga 0.85N quantum well and 7.5 nm thick GaN barrier layers in a p-i-n diode architecture using metal-organic chemical vapor deposition (MOCVD) on a c-plane sapphire substrate. Devices with 300 μm × 300 μm mesa size were fabricated using standard photolithography, reactive ion etching and metallization steps. We used indium-tin oxide (ITO) based semi-transparent contacts in top (p-GaN) layer for uniform application of electric field across the well layers. The fabricated devices were diced and mounted on a TO-can for compact testing. © 2009 IEEE.Item Open Access Electronic structure of low dimensional semiconductor systems(Bilkent University, 1992) Gülseren, OğuzRecent progress made in the growth techniques has led to the fabrication of the artificial semiconductor systems of lower dimension. Electrons and holes in these materials have quantization different from those of the three dimensional systems presenting unusual electronic properties and novel device applications. In this work, the important features of the free carriers in semiconductor superlattices are examined, and the electronic structure of some novel 2D semiconductor systems are investigated theoretically. This thesis studies various systems of lower dimensionality such as: the strained Si/Ge superlattices, i-doping. Si (100) surface and the tip-sample interaction in scanning tunneling microscopy (STM) study of this surface, and Wannier-Stark localization in finite length superlattices. The electronic energy structure of pseudomorphic Ge„i/Si„ superlattices is investigated by using the empirical tight binding method. Effects of the band offset, sublattice periodicity and the lateral lattice constant on the transition energies have been investigated. It is found that Ge„i/Si„ superlattices grown on Ge (001) can have a direct band gap, if m + n = 10 and m = 6. However, optical matrix elements for in-plane and perpendicular polarized light are negligible for the transition from the highest valence band to the lowest conduction band state at the center of the superlattice Brillouin zone. The electronic structure of the Si i-layer in germanium is explored by using the Green’s function formalism with layer orbitals. We found two dimensional parabolic subbands near the band edges. This approach is extended to treat the electronic structure of a single quantum well without invoking the periodically repeating models. Quantum well formation in Ge,„Si„ superlattices is also studied by using different number of ^-layers. Subband structure is observed by changing the height of the Si quantum well. The confinement of acoustical modes within 2DEG due to only the electronphonon interaction is proposed. The confined modes split out from the bulk phonons, if the 2DEG is created by means of modulation doping. This occurs even if the lattice has uniform parameters. The effect is more pronounced when the wave vector q of the modes increases and is maximum a,t q = 2kp {kp is the Fermi wave vector). In the case of several electron sheets the additional features of the confinement effect appear. Green’s function method is also applied to treat the modifications of electronic state density in STM. The tip-sample interaction in STM study of Si (100) surface is explored by calculating the Gieen’s function within the empirical tight binding method. Both of the proposed reconstruction models, buckled and symmetrical dimer model, is investigated. A dip occurs in the change of density of states of surface atoms at the energy of surface states for small tip-sample distances, and it decreases with increasing tip-sample separation. Although, in-plane tip position (above the up- or down-surface atom) affects the surface atoms differently in buckled dimer model, it influences the surface atoms symmetrically in symmetric dimer model. Recent experimental studies revealed the significant information on the Wannier-Stark localization. Following these experimental results, the WannierStark ladder is investigated by carrying out numerical calculations on a multiple quantum well structure under an applied electric field. The variation of the Wannier-Stark ladder energies and localization of the corresponding wave II function are examined for a wide range of applied electric field. Our results show that Wannier-Stark ladder do exist for finite but periodic system which consists of a large number of quantum well having multi-miniband structure. It is found that the miniband states are localized in the well regions with the applied electric field, while the continuum states preserve their extended character. Energies of the well states show a linear shift with the electric field except the small field values in which a nonlinear shift is resulted. Multiband calculations show that there is a mixing between the different band states although they are localized in different well regions.Item Open Access Enhanced optical characteristics of light emitting diodes by surface plasmon of Ag nanostructures(SPIE, 2011) Jang L.-W.; Ju J.-W.; Jeon J.-W.; Jeon, D.-W.; Choi J.-H.; Lee, S.-J.; Jeon, S.-R.; Baek J.-H.; Sarı, Emre; Demir, Hilmi Volkan; Yoon H.-D.; Hwang, S.-M.; Lee I.-H.We investigated the surface plasmon coupling behavior in InGaN/GaN multiple quantum wells at 460 nm by employing Ag nanostructures on the top of a roughened p-type GaN. After the growth of a blue light emitting diode structure, the p-GaN layer was roughened by inductive coupled plasma etching and the Ag nanostructures were formed on it. This structure showed a drastic enhancement in photoluminescence and electroluminescence intensity and the degree of enhancement was found to depend on the morphology of Ag nanostructures. From the time-resolved photoluminescence measurement a faster decay rate for the Ag-coated structure was observed. The calculated Purcell enhancement factor indicated that the improved luminescence intensity was attributed to the energy transfer from electron-hole pair recombination in the quantum well to electron vibrations of surface plasmon at the Ag-coated surface of the roughened p-GaN. © 2011 SPIE.Item Open Access Formation of quantum structures on a single nanotube by modulating hydrogen adsorption(American Physical Society, 2003) Gülseren, O.; Yildirim, T.; Çıracı, SalimUsing first-principles density functional calculations we showed that quantum structures can be generated on a single carbon nanotube by modulating the adsorption of hydrogen atoms. The band gap of the hydrogen-free zone of the tube widens in the adjacent hydrogen covered zone. The sudden variation of the band gap leads to band offsets at the conduction- and valence-band edges. At the end, the band gap of the whole system is modulated along the axis of the tube, which generates quantum wells or quantum dots. Specific electronic states are confined in these quantum wells. The type and radius of the nanotube and the extent and sequence of hydrogen-free and hydrogen-covered zones can provide several options to design a desired optoelectronic nanodevice.Item Open Access Gain measurements via spontaneous emission in quantum well semiconductor lasers(Bilkent University, 1996) Azfar, Talal111 this work ail analysis of gain in single c|iianliini well last'rs as a riinction of sonu' of llu'ir o|)(M’alioiial paranu'l('rs is earrii'd oiil. I^'irsl, a 1 li('or('liea.l inodi'l of gain is |)resent('d. 'riuMi two diderent nudliods of gain iiK'asiir('in('nt, wliieli iis(' the spontaneous emission IVom the iaec't and the nnamplified s[)ontan('ons ('mission from the top of the ridg(', arc' discussed. Indirication procc'sses of lasers to racilitate tJie colh'clion of unampliiic'd spontaiK'ous ('mission are ch'tailed. R('S|)onse of the gain sp('ctrum to chang('s in inject('d cnrr('nt (h'lisity and temperature are measur('d and iinderstood in terms of hand Idling, hand gap renormalization and tenpx'rature d('pendenc(' of tlu' handgap. (!ain sainration above threshold is v('ri(ied and s|)atial variations in spontaiu'ons ('mission in lh(' longitiidnal and hiteral directions are observed.Item Open Access Impurity-free quantum well intermixing for high-power laser diodes(Bilkent University, 2015-08) Kahraman, AbdullahThe demand for ever higher powers and efficiencies from semiconductor lasers, continues. State-of-the-art high power lasers require not only sophisticated designs but also complex fabrication technologies to push the boundaries. A major obstacle to ever higher powers is catastrophic optical mirror damage that occurs at the mirrors of the cavity. Among several approaches to increase the threshold for damage, local manipulation of the band gap near the mirrors stands out, as it eliminates reabsorption. The structure of modern lasers employing quantum wells surrounded by large band gap and low index claddings gives the opportunity in intermix the quantum well and increase the effective band gap close to cavity edges during fabrication. The research presented in this thesis reports the results of Impurity-Free Vacancy Disordering (IFVD) of GaAs quantum wells in high power laser diode structures that leads to blue shifting of the effective band gap. In contrast with previous work, this study concentrates on actual large optical cavity (LOC) high power laser diode structures where the waveguide and cladding layers are thick. Using selective area QWI can be extremely beneficial in terms of enhancing catastrophic optical mirror damage (COMD) threshold, spatial mode instability, propagation losses and overheating which are the main limitations to fabricate HPLDs. In the course of the fabrication of HPLDs, the last and most problematic step is to manage QWI. IFVD was realized by capping the crystal surface with a sputtered dielectric layer of SiO2 to enhance intermixing and thermally evaporated SrF2 to prevent intermixing for selected parts of the laser cavity. Disordering the layers takes place by diffusion of Ga atoms from GaAs QW into sputtered SiO2 layer during rapid thermal annealing (RTA), leaving Ga vacancies in QW. It allows the Ga vacancy defects free to move AlxGa1 the photoluminescence peak. Relative composition in the layers that make up the laser structure was measured with X-ray photoelectron spectroscopy in conjunction with depth proling. A blue shift of 65 nm (154 meV) was achieved, in parallel with both Ga and Al diffusion in the laser structure.Item Open Access Radiative Dark-Bright Instability and Critical Casimir Effect in DQW Exciton Condensates(Elsevier, 2011) Hakioglu, T.; Özgün E.It is already well known that radiative interband interaction in the excitonic normal liquid in semiconducting double quantum wells is responsible for a negligible splitting between the energies of the dark and bright excitons enabling us to consider a four fold spin degeneracy. This has also lead many workers to naively consider the same degeneracy in studying the condensate. On the other hand, the non-perturbative aspects of this interaction in the condensed phase, e.g. its consequences on the order parameter and the dark-bright mixture in the ground state have not been explored. In this work, we demonstrate that the ground state concentrations of the dark and the bright exciton condensates are dramatically different beyond a sharp interband coupling threshold where the contribution of the bright component in the ground state vanishes. This shows that the effect of the radiative interband interaction on the condensate is nonperturbative. We also observe in the free energy a discontinuous derivative with respect to the layer separation at the entrance to the condensed phase, indicating a strong critical Casimir force. An estimate of its strength shows that it is measurable. Measuring the Casimir force is challenging, but at the same time it has a conclusive power about the presence of the long sought for condensed phase.Item Open Access Silicon-germanium multi-quantum wells for extended functionality and lower cost integration(IEEE, 2010) Onbasli, M.C.; Yesilyurt, Alper; Yu H.Y.; Nayfeh, A.M.; Okyay, Ali KemalSilicon-Germanium quantum wells were grown in p-i-n layers using a recently developed epitaxial technique. Nanostructural characterization (TEM, XPS, photoluminescence) indicates low-dislocation density, high quality films. Solar cells made of these layers have low leakage current. ©2010 IEEE.Item Open Access White light generating nonradiative energy transfer directly from epitaxial quantum wells to colloidal nanocrystal quantum dots(Optical Society of America, 2009) Nizamoğlu, Sedat; Sarı, Emre; Baek J.-H.; Lee I.-H.; Demir, Hilmi VolkanWe present white light generating nonradiative Förster resonance energy transfer at a rate of (2ns)-1 directly from epitaxial InGaN/GaN quantum wells to CdSe/ZnS heteronanocrystals in their close proximity at chromaticity-coordinates (x,y)=(0.42,0.39) and correlated-color-temperature CCT=3135K. ©2009 Optical Society of America.