Browsing by Subject "Light emitting diodes"
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Item Design of miniature flow through optical absorbance detectors(Texas Tech University, 1998-05) Bellamy, Harvey S.Several novel flow through optical absorbance detectors have been developed which combine small size, low power requirements, simplicity of manufacture, low cost, and unique capabilities. The employment of optical fibers to connect the light emitting and detecting elements to the flow through cell enable separation of the cell and the detector electronics, allowing cell placement in hazardous or inaccessible locations without jeopardizing operator safety. Two spectrometer designs are also presented which include the above advantages and allow visible wavelength spectra to be obtained for fluids passing through the cell without flow interruption. This is achieved without the use of any moving parts.Item GaAs-based apertured vertical-cavity surface-emitting lasers and microcavity light emitting diodes(2003-12) Chen, Hao, 1969-; Deppe, Dennis G.Item Multicolor colloidal quantum dot based inorganic light emitting diode on silicon : design, fabrication and biomedical applications(2010-12) Gopal, Ashwini; Neikirk, Dean P., 1957-; Zhang, Xiaojing, Ph. D.; Dodabalapur, Ananth; Yu, Edward T.; Becker, Michael F.; Bank, Seth R.Controlled patterning of light emitting diodes on semiconductors enables a vast variety of applications such as structured illumination, large-area flexible displays, integrated optoelectronic systems and micro-total analysis systems for real time biomedical screening. We have demonstrated a series of techniques of creating quantum-based (QD) patterned inorganic light emitting devices at room temperature on silicon (Si) substrate. In particular: (I) A combination of QDs self-assembly and microcontact printing techniques were developed to form the light emission monolayer. We expand the self-assembly method with the traditional Langmuir-Schaeffer technique to rapidly deposit monolayers of core: shell quantum dots on flat substrates. A uniform film of QDs self-assembled on water was transferred using hydrophobic polydimethylsiloxane stamps with various nano/micro-scale patterns, and was subsequently stamped. A metal oxide electron transport layer was co-sputtered onto the QDs. The structure was completed by an e-beam evaporating thin metal cathode. Multicolor light emission was observed on application of voltage across the device. (II) We also demonstrate the photolithographic patterning capability of a metal cathode for top emitting QDLEDs on Si substrates. Lithographic patterning technique enables site-controlled patterning and controlled feature size of the electrode with greater accuracy. The stability of inorganic silicon materials and metal oxide based diode structure offers excellent advantages to the device, with no significant damage observed during the patterning and etching steps. Efficient electrical excitation of QDs was demonstrated by both the methods described above. The technique was translated to create localized QD-based light sources for two applications: (1) Three-dimensional scanning probe tip structures for near field imaging. Combined topographic and optical images were acquired using this new class of “self-illuminating” probe in commercial NSOM. The emission wavelength can be tuned through quantum-size effect of QDs. (2) Multispectral excitation sources integrated with microfluidic channels for tumor cell analyses. We were able to detect the variation of sub-cellular features, such as the nucleus-to-cytoplasm ratio, to quantify the absorption at different wavelength upon the near-field illumination of individual tumor cells towards the determination of cancer developmental stage.Item Theoretical study of GaAs-based quantum dot lasers and microcavity light emitting diodes(2004) Huang, Hua; Deppe, Dennis G.Quantum dots are semiconductor nanostructures that act as artificial atoms by confining electrons and holes in three-dimensions. Self-organized QDs sit “on top of” a wetting layer, so that the QDs’ zero-dimensional levels are electronically coupled to the wetting layer’s 2-D density of levels. A theoretical model is presented that is capable of describing both nonequilibrium carrier distributions in the QD zero-dimensional levels at low temperature, as well as quasiequilibrium distribution for higher temperature operation. Due to the larger thermalization rate out of higher energy QDs of the ensemble, rearrangement of the carrier distribution could occur, which produces asymmetric gain spectrum, and leads to the novel behavior that the threshold current density can in fact decrease with increasing temperature for a QD laser, and thus show a negative characteristic temperature coefficient. The narrowing of the full-wdith half-maximum (FWHM) of the spontaneous emission spectrum could occur due to this gain spectrum narrowing. At room temperature the carrier transport is so fast that the most important characteristics of QD lasers can be analyzed using quasi-equilibrium solutions to the rate equations. The closely spaced hole levels result in a thermal smearing of the hole population among many hole states, and cause a large fraction of injected holes that do not occupy the QDs’ ground states, so that injected carriers can be wasted by not optically coupling to the lasing mode. P-doping is proposed to build in excess holes and overcome the thermal effect of the close hole spacings. High characteristic temperature coefficient is predicted for these p-doped QD lasers, large modulation bandwidth with zero or negative chirp is also predicted. According to Fermi’s golden rule, the spontaneous emission probability of an active emitter is given by the electronic transition probability from the excited state to the ground state times the photon density of states. Therefore, by tailoring the surrounding environment of an active emitter, both the spontaneous emission rate and the direction of emission will be altered. We study a QD VCSEL with very small mode volume, and show that the modulation bandwidth has a strong dependence on the mode volume. The pulse response is also strongly mode volume sensitive, and at small mode volume the relaxation oscillation is greatly suppressed, even allowing multi-gigabit data transmission. In planar microcavity LEDs enhanced efficiency and narrower spectral linewidth are achieved through the Purcell effect. Quantum dots are useful to obtain the necessary electronic confinement to very small apertures and can provide a short spontaneous lifetime. To fully take advantage of the enhanced mode coupling provided by the microcavity, it is important to electronically confine the carriers to small optical mode volume, which can be achieved in apertured QD-MCLEDs. We study the performance of an apertured QD MCLED, and find that very high efficiency apertured QD-MCLED can be achieved with small mode volume and narrow QD inhomogeneous linewidth, while only a moderate cavity quality factor Q is needed.