III-V -puolijohteiden sinkkidiffuusioseostus uusia optoelektroniikan sovelluksia varten

Modern high-power light-emitting diodes (LEDs) employ nanoscale structures composed of multiple layers of different III-V compound semiconductors. The same basic types of double heterojunctions (DHJs) have been used in high-efficiency LEDs for decades, and these structure have enabled LEDs to reach higher luminous efficacies than any competing lighting technology. However, issues with current crowding are limiting further improvements in efficiency. One novel method of current injection that could solve some of these problems is diffusion-driven current transport (DDCT), which involves using diffusion currents to drive charge carriers into the active area of the LED. This thesis aims to investigate the viability of Zn diffusion doping as a fabrication technique for realizing GaAs-based DDCT LED structures. A spin-on glass process for Zn diffusion doping is developed and samples are fabricated using this method. An alternative contact scheme involving metal contacts directly deposited on the sample with no diffusion doped areas is also tested. Characterization of the fabricated samples is performed using current-voltage (IV), capacitance-voltage (CV) and Hall effect measurements. Visual inspections of surface and coating quality were performed using scanning electron microscopy (SEM). Light emission from the samples was observed using an infrared microscope camera system. Successful p-type doping of intrinsic GaAs samples was achieved but attempts to convert n-type samples into p-type were unsuccessful due to insufficient density of diffused dopants. Light emission from the alternative contact structure was observed, which indicates successful hole injection. This suggests that direct deposition of metal contacts could be a viable option for realizing the first DDCT devices.Nykyaikaisissa hohtodiodeissa (LED, engl. lightemitting diode) käytetään useista eri III-V -puolijohteista koostuvia nanomittakaavan kerrosrakenteita, joilla pyritään kasvattamaan niiden hyötysuhdetta. Korkean hyötysuhteen LEDien pohjana on jo vuosikymmenien ajan ollut samantyyppinen kaksoisheterorakenne, ja sillä on saavutettu merkittävästi muita valaistustekniikoita korkeampia hyötysuhteita. Virransyötön ongelmat rajoittavat kuitenkin nykyisillä rakenteilla saavutettavaa suorituskykyä. Näiden ongelmien ratkaisemiseksi on esitetty uudenlaista, diffuusiovirtoja hyödyntävää virransyöttömenetelmää (DDCT, engl. diffusion driven current transport) LEDeihin. Tämän diplomityön tavoite on tutkia mahdollisuutta käyttää sinkkidiffuusioseostusta galliumarsenidiin (GaAs) pohjautuvien DDCT LED -rakenteiden valmistuksessa. Tätä tarkoitusta varten työn osana kehitettiin näytteen pinnalle levitettävään piidioksidikerrokseen perustuva seostusmenetelmä. Tällä menetelmällä valmistettiin diffuusioseostettuja näytteitä. Lisäksi tehtiin näytteitä, joihin ei valmistettu diffuusioseostettuja alueita, vaan metallikontaktit höyrystettiin suoraan n-tyypin näytteen pinnalle. Näiden näytteiden tarkoitus oli tutkia vaihtoehtoista aukkojen syöttötapaa rakenteeseen. Näytteitä karakterisoitiin virtajännite-(IV), kapasitanssi–jännite- (CV) sekä Hall-mittauksilla. Lisäksi näytteitä tarkasteltiin pyyhkäisyelektronimikroskoopilla (SEM, engl. scanning electron microscope) pinnan laadun havainnoimiseksi. Valon emissiota havainnoitiin infrapunakameralla. Työn menetelmällä onnistuttiin seostamaan intrisiikkisten GaAs-näytteiden pintaan p-tyypin johtava kerros. Yritykset muuttaa n-tyypin näytteen pinta p-tyypin puolijohteeksi sen sijaan eivät onnistuneet, sillä seostustiheys oli liian matala. Vaihtoehtoisesta metallikontaktirakenteesta havaittiin valon emissiota, mikä osoittaa aukkojen injektoinnin rakenteeseen onnistuneen. Näin ollen suorat metallikontaktit voisivat olla lupaava tapa toteuttaa ensimmäiset DDCT-laitteet

A Numerical Analysis of Heterojunction Bipolar Devices.

Numerical simulation plays an important role in the design, analysis and fabrication of semiconductor devices. In this work, a computer program is developed to obtain a one-dimensional steady-state constant temperature current-voltage characteristics of diodes and bipolar transistors fabricated from materials having position dependent material properties such as band-gap, electron affinity, permittivity and the density of states functions. The general formulation of the problem allows for an unambiguous choice of reference potential. The modular form of the program allows for the choice of appropriate recombination processes for each of the materials used in the structure. The program can adjust the step sizes automatically during the calculations. This reduces the convergence problem significantly and increases the application of the program to a wider variety of device structures and bias voltages. The automatic step selection process was found to take up an excessive amount of the computer CPU time. Hence, an alternate step selection process was also employed that retains many of the benefits of the variable step size selection but requires considerably less CPU time. A finite-difference method through quasi-linearization technique is employed to numerically solve the three second-order non-linear partial differential equations describing the behavior of semiconductor devices. The computer program can handle a large variation in the device size and has no restrictions in the impurity doping profile other than the Boltzmann approximation. The program is applied to a variety of homo and heterostructure diodes and bipolar transistors. The individual electron and hole current densities are computed with position in the device along with carrier densities and potentials. Structures with abrupt and graded heterostructure interfaces are considered. The results obtained from this program compare well with those of others reported in the literature

A theoretical analysis of the current-voltage characteristics of solar cells

The following topics are discussed: (1) dark current-voltage characteristics of solar cells; (2) high efficiency silicon solar cells; (3) short circuit current density as a function of temperature and the radiation intensity; (4) Keldysh-Franz effects and silicon solar cells; (5) thin silicon solar cells; (6) optimum solar cell designs for concentrated sunlight; (7) nonuniform illumination effects of a solar cell; and (8) high-low junction emitter solar cells

Modeling, design and fabrication of thin-film microcrystalline silicon solar cells

The modeling, design and fabrication of low-cost thin-film microcrystalline silicon(µcSi) solar cells is studied in this thesis. The cell, considered in this investigation, utilizes low-cost glass as the substrate and microcrystalline Si (µc-Si) as the active layer. A comprehensive refractive index (n) and extinction coefficient (k) model of silicon as function of doping, temperature and wavelength is developed to assist the optical design of the cell. In order to obtain acceptable short circuit current density (Jsc) from the cell, it is found that the thickness of the silicon thin film should be more than 10µm. To get the best light trapping effect, the surface of the cell should be double-side or frontside textured. The density of the texture pits should be as high as possible and the bottom angle of the texture pits should be as small as possible. However, the depth of the texture pits does not have too much influence on the overall performance of the cell. A model and corresponding software are developed to investigate the electronic properties of the material /device built using low cost µc-Si. This model divides the defect regions inside the material into different categories according to the defect levels in them. Therefore, this model is able to deal with various kinds of defects and defect clusters. The software uses finite element method to solve time-dependent continuity equations with different boundary conditions to get the carrier distribution inside the materials and hence the I-V characteristics of the devices. It is found that, to get satisfying thin film µc-Si thin.film cell, the grain size of the film should be about 10µm, and the surface recombination velocities at the grain boundaries should be less than 1000cm/s. The requirement on the minority carrier lifetime is not rigid because of better tolerance of thin-film solar cells to this property. Some critical fabrication steps in making such a thin film solar cell are also investigated. An Al-involved crystallization / grain enhancement procedure using optical processing is used to get large-grain µc-Si thin films. This process involves both a-Si/Al reaction and Al diffusion inside Si. It can produce µc-Si at temperatures lower than the softening point of low-cost glass within a much shorter duration compared with other crystallization / grain enhancement techniques. Crystallization of Si film can start at temperatures as low as 200°C when Al is involved. However, to get strong crystallization and grain enhancement, the processing temperature should be more than 450°C. At temperature around 500°C, the crystallization becomes much stronger. The local melting at the Si-Al interface may cause this crystallization

Physical and technological principles of particle detecting instruments in satellites

Abstract. Measuring different properties of particles in space has applications in space and atmospheric science. It is important to choose the right type of instrument for the mission at hand as different detectors have very different properties. Some things that need to be taken into account in satellites are cost, size, type of particles that need to be measured and what properties should be measured, for example mass or energy or just count of the particles. To achieve accurate or more complex measurements careful study of physical effects and theory is required in addition to rigorous empirical testing and calibration of the detectors used. This thesis is going to give brief introduction to some of the methods used for particle detection and some basic physical principles related to them. Commonly used detectors will also be introduced in addition to some example instruments and filtering techniques to support the theory

Enhancement of thermoelectric properties by energy filtering: Theoretical potential and experimental reality in nanostructured ZnSb

Energy filtering has been suggested by many authors as a means to improve thermoelectric properties. The idea is to filter away low-energy charge carriers in order to increase Seebeck coefficient without compromising electronic conductivity. This concept was investigated in the present paper for a specific material (ZnSb) by a combination of first-principles atomic-scale calculations, Boltzmann transport theory, and experimental studies of the same system. The potential of filtering in this material was first quantified, and it was as an example found that the power factor could be enhanced by an order of magnitude when the filter barrier height was 0.5~eV. Measured values of the Hall carrier concentration in bulk ZnSb were then used to calibrate the transport calculations, and nanostructured ZnSb with average grain size around 70~nm was processed to achieve filtering as suggested previously in the literature. Various scattering mechanisms were employed in the transport calculations and compared with the measured transport properties in nanostructured ZnSb as a function of temperature. Reasonable correspondence between theory and experiment could be achieved when a combination of constant lifetime scattering and energy filtering with a 0.25~eV barrier was employed. However, the difference between bulk and nanostructured samples was not sufficient to justify the introduction of an energy filtering mechanism. The reasons for this and possibilities to achieve filtering were discussed in the paper

Heterojunctions of CdTe on Si

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Strong Doping of the n-Optical Confinement Layer for Increasing Output Power of High- Power Pulsed Laser Diodes in the Eye Safe Wavelength Range

Abstract—An analytical model for internal optical losses at high power in a 1.5 μm laser diode with strong n-doping in the n-side of the optical confinement layer is created. The model includes intervalence band absorption by holes supplied by both current flow and two-photon absorption, as well as the direct two-photon absorption effect. The resulting losses are compared with those in an identical structure with a weakly doped waveguide, and shown to be substantially lower, resulting in a significant improvement in the output power and efficiency in the structure with a strongly doped waveguid