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Radiation Induced effects

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This thesis presents a new simulation model for DC fuses used to protect semiconductor devices using PSCAD /EMTDC. The basic construction and operation of fuses is discussed, highlighting the difference between the operations of AC and DC fuses. The melting and arcing models of the fuse are implemented separately. The modeling concept is explained in detail and the model is validated with experimental results. / text

Materials have always had a large impact on society over the different ages. Piezoelectric materials are the often ‘invisible’ materials which find widespread use, unknown to the general public by large. Mobile electronics, automotive systems, medical and industrial systems are few of the key areas where ‘piezoelectricity’ is indispensable. The parking sensor of our car uses the effect and even the echo to image an unborn baby in a womb requires the exploitation of the piezoelectric effect. The work presented in this thesis investigates the piezoelectric effect in semiconductors, namely in III V, III N and II VI materials to have a better understanding and design potential applications in light emitting diodes (LEDs) and other electronic devices. The current work focuses on the non-linear behaviour in the strain of the piezo effect, which is manifested by the generation of electric field under crystal deformation. Previous works have already confirmed the reports of the existence of non-linear piezoelectric effects in zincblende III V semiconductors. Here, the same semiempirical approach using Density Functional Theory has been utilized to investigate the strain dependent elastic and dielectric properties of wurtzite III N materials. While we report the strong non-linear strain induced piezoelectric behaviour with second order coefficients, all spontaneous polarization terms are substantially smaller than the previously proposed values. We show that, unlike existing models, our calculated piezoelectric coefficients and nonlinear model provide a close match to the internal piezoelectric fields of quantum well and superlattice structures. Also, pressure dependence of the piezoelectric field in InGaN based LEDs predicts a significant improvement of the spontaneous emission rate can be achieved as a result of a reduction of the internal field. The LED devices using the proposed structures including a metamorphic layer under the active region of the device are expected to increase their light output power by up to 10%. We also explored the impact of the non-linear piezo effect in nanowires and present a further theoretical computational study of single photon sources optimization in InGaN based wurtzite single quantum dots. We observed the light emission can be made by those single photon sources covering the entire visible spectrum through suitable change in the alloy composition.

Orientador: Antonio Manoel Mansanares / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-09-26T13:55:51Z (GMT). No. of bitstreams: 1 Freitas_LauraRamosde_D.pdf: 4490607 bytes, checksum: c7b66729395ef47ae8f2ab3bb212544e (MD5) Previous issue date: 2005 / Resumo: A Microscopia Fototérmica de Reflexão vem sendo utilizada na investigação de dispositivos micro e opto-eletrônicos em operação, devido ao seu caráter não destrutivo e por não requerer contato com a superfície da amostra. Esta técnica se baseia na dependência da refletância da amostra com a temperatura, com o campo elétrico local, bem como com a densidade de portadores livres, que são, por sua vez, afetados por defeitos. Este fato torna esta técnica muito adequada para investigar defeitos em processos de fabricação e envelhecimento destas estruturas. Neste trabalho, apresentamos um estudo experimental e teórico sobre a resposta fototérmica em função do comprimento de onda do feixe de prova, para estruturas micro-eletrônicas estratificadas. As amostras consistiram basicamente de trilhas condutoras de silício policristalino de diversos chips. As medidas de termo-refletância foram realizadas na faixa de comprimento de onda de 450nm até 750nm, sendo que as trilhas foram alimentadas sempre com corrente modulada, com três montagens experimentais diferentes. Um padrão oscilatório é observado na região espectral em que a camada superior é transparente. Essas oscilações são causadas pelas múltiplas reflexões nas interfaces. Utilizando um modelo termo-óptico, mostramos que as constantes (n e k), que dependem do comprimento de onda, assim como suas derivadas com relação à temperatura (dn/dT e dk/dT), influenciam fortemente o sinal de termo-refletância. A espessura óptica das camadas, principalmente determinadas pela parte real dos índices de refração, define o período de oscilação. Por outro lado, a parte imaginária estabelece o comprimento de onda em que as oscilações começam. Abaixo de um certo comprimento de onda, a luz de prova não penetra no material e a refletância da camada superficial domina o sinal / Abstract: Photothermal microscopy has been used as a suitable technique for the investigation of micro- and opto-electronic devices in operating cycle, because of its non-contact and non-destructive character. This technique is based on the dependence of the sample reflectance with temperature and with local electric field, as well as with free carrier density, which are in their turn disturbed by defects. This fact makes this technique very useful for investigating defects in fabrication and aging processes of such structures. In the present work, we report an experimental and theoretical study of the thermoreflectance response as a function of the probe wavelength for layered microelectronics structures. The investigated samples consisted of polycrystalline silicon conducting tracks from various chips. Thermore²ectance measurements were carried out in the wavelength range from 450 to 750 nm with the tracks biased in modulated regime, with three diÿerent experimental setups. An oscillating pattern is observed in the spectral region where the upper layer is transparent. Such oscillations are due to the interference resulting from the multiple reflections at the interfaces. Using a thermo-optical model, we show that the optical constants (n and k) of the materials, which are wavelength dependents, as well as their temperature derivatives (dn/dT and dk/dT), strongly in²uence the thermoreflectance signal. The optical thicknesses of the layers, mainly determined by the real part of the refractive indexes, de½ne the period of oscillation. On the other hand, the imaginary part of the refractive indexes establishes the cutoÿ wavelength of the oscillations. Below this cutoÿ wavelength, the probe light does not penetrate the material, and the upper surface reflectance dominates the signal / Doutorado / Física da Matéria Condensada / Doutor em Ciências

GaN RF switches are widely used in today’s communication systems. With digital communications getting more and more popular nowadays, the need for improving the performance of involved RF switches is inevitable. Designing low ON-state resistance GaN switches are exceedingly important to improve the switch insertion loss, isolation and power loss. Moreover, considerations need to be taken into account to improve the switching speed of the involved GaN HEMTs. In this dissertation, a new GaN HEMT structure called “Tunnel MOS Heterostructure FET (TMOSHFET)” is introduced which has lower ON-state resistance and faster switching speed compared to conventional AlGaN/GaN HEMTs. In the switch ON process, the channel of this device is charged up by electron tunneling from a layer underneath the channel as opposed to typical AlGaN/GaN HEMTs in which electron injection from the source is charging up the channel. The tunneling nature of this process together with the shorter travel distance of electrons in TMOSHFET provide for a faster switching speed. In order to understand the tunneling mechanisms in TMOSHFET, the fabrication of AlGaN/GaN Schottky Barrier Diodes (SBDs) with various AlGaN thicknesses is demonstrated on Si (111) substrate. The impacts of SF6 dry etching on the trap density and trap state energy of AlGaN surface are investigated using the GP/w- w method. Various tunneling mechanisms at different biases are then characterized in samples and compared with each other. To improve the source and drain resistances in TMOSHFET, a model is generated to optimize the 2DEG density and electric field in AlGaN/GaN heterostructure based on Al mole fraction, AlGaN thickness and the thickness of SiN passivation layer and it is experimentally verified by non-contact Hall 2DEG density measurements. The spontaneous and piezoelectric polarizations together with strain relaxation have been implemented into the model, taking into account the annealing effects. From the experimental data on obtained parameters, the operation and device parameterization of the TMOSHFET is outlined and design considerations to improve the device R_(ON)-V_(BR) figure of merit are discussed.