Dissertations / Theses on the topic 'Magnetron gun'

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.
Includes bibliographical references (leaves 124-139).
This thesis investigates the experimental measurement and theoretical simulation of the effects of azimuthal emission non-uniformity of a 96 kV, 40 amp magnetron injection gun (MIG) used in a gyrotron. The accomplishments of this thesis include: Experimental measurement of the azimuthal emission non-uniformity of the MIG gun; Simulation of the beam quality of the MIG gun using MICHELLE 3-D, the first simulation of a MIG electron beam with azimuthal non-uniformity; Benchmarking the MICHELLE 3-D code to other established gun optics codes; Evaluation of the effects on the velocity spread and pitch factor of azimuthal non-uniformity in the MIG gun, showing that the direct effect on the beam quality is very small; Design, fabrication, and testing in the gyrotron of a capacitive probe system divided into four quadrants to measure azimuthal asymmetries of the electron beam; Use of the capacitive probes to measure low-frequency (100 - 160 MHz) oscillations on the beam, the first measurement of such oscillations in a microsecond pulse length gyrotron; First results on testing a new MIG cathode for emission non-uniformity using a special test chamber built by Calabazas Creek Research.
(cont.) This research will contribute to our understanding of the properties of intense electron beams produced by MIG guns in high-power gyrotrons. MIG's are widely used in gyrotron oscillators and amplifiers for fusion applications to create a beam of gyrating electrons generally operating in the temperature limited regime of emission. Due to this dependence on the temperature of the cathode, variation of the emitter surface temperature will result in inhomogeneous emission. Non-uniform emission is attributed to a deviation in the cathode work function as well. Studies have shown this inhomogeneous beam current density can lead to increased mode competition and velocity spread contributing to an overall decreased efficiency of the gyrotron. This research focuses on the effects on velocity spread and in turn the efficiency of the device from non-uniform current emission. Initially, we measured experimentally the detailed azimuthal non-uniformity profile of an existing 110 GHz gyrotron oscillator at MIT. Using a rotating collector current probe the current density of different emitter angles was extracted. These results agreed fairly well with previous measurements of Anderson et al.
(cont.) This non-uniformity profile was then used with a 3-D simulation code to do the first complete 3-D model from the cathode to the cavity of a MIG. In order to investigate these effects of beam non-uniformity with simulation, we use MICHELLE 3-D developed by SAIC. MICHELLE 3-D has been benchmarked to MICHELLE 2-D and EGUN in the case of a uniform beam. The non-uniform beam measurements are entered into MICHELLE and results are computed at four different azimuthal quadrants of different current densities and for the overall beam, giving special attention to the differences in the beam pitch factor and perpendicular velocity spread. MICHELLE found azimuthal non-uniformity to be a fairly small effect on the overall beam quality. Concurrently with the MICHELLE 3-D simulations, segmented pitch factor probes are implemented to measure the pitch factor in the four azimuthal quadrants. In an attempt to compare with MICHELLE's results, these four capacitive probes measure the induced image charge of different azimuthal sections of the electron beam, enabling an estimation of differences in the pitch factor between quadrants.
(cont.) Unfortunately, the experimental error is found to be quite high (±15%) rendering differences in the pitch factor to be contained within the error boundaries. Though the capacitive probes are found to have too much error for adequate resolution of the pitch factor, they are also used to discover the first observations of low-frequency oscillations in a short pulse MW gyrotron. These frequencies, from 100-160 MHz, are found to be dependent on the beam parameters such as the beam voltage, current, magnetic field, and magnetic compression ratio. The frequency range is remarkably close to the frequency of an electron in the adiabatic trap and the experimental as well as the predicted theoretical oscillation behavior of trapped electrons are discussed. Last, initial progress has been made to test three new cathodes on the Calabazas Creek Research cathode tester. This tester is a dedicated test stand for azimuthal non-uniformity able to obtain a measurement directly at the cathode instead of at the collector end of the device. The setup procedure and results on the first cathode test for the 96 kV, 40 amp gun are reported and future tests are summarized.
by Chad D. Marchewka.
S.M.

In this research, electronic-grade GaN(0001) epilayers and nanorods have been grown onto Al2O3(0001) and Si(111) substrates, respectively, by reactive magnetron sputter epitaxy (MSE) using liquid Ga as a sputtering target. MSE, employing ultra high vacuum conditions, high-purity source materials, and lowenergy ion assisted deposition from substrate biasing, is a scalable method, lending itself to large area GaN synthesis. For the growth of epitaxial GaN films two types of sputtering techniques, direct current (DC) magnetron sputtering and high power impulse magnetron sputtering (HiPIMS) were studied. The GaN epitaxial films grown by DC-MSE directly on to Al2O3(0001) in a mixture of Ar and N2, feature low threading dislocation densities on the order of ≤ 1010 cm-2, as determined by transmission electron microscopy (TEM) and modified Williamson-Hall plots. X-ray rocking curves reveal a narrow full-width at half maximum (FWHM) of 1054 arcsec of the 0002 reflection. A sharp 4 K photoluminescence (PL) peak at 3.474 eV with a FWHM of 6.3 meV is attributed to intrinsic GaN band edge emission. GaN(0001) epitaxial films grown on Al2O3 substrates by HiPIMS deposition in a mixed N2/Ar discharge contain both strained domains and almost relaxed domains in the same epilayers, which was determined by a combination of x-ray diffraction (XRD), TEM, atomic force microscopy (AFM), μ-Raman microscopy, μ-PL, and Cathodoluminescence (CL). The almost fully relaxed domains show superior structural and optical properties evidenced by a rocking curves with full width at half maximum of 885 arc sec and a low temperature band edge luminescence at 3.47 eV with the FWHM of 10 meV. The other domain exhibits a 14 times higher isotropic strain component, which is due to higher densities of point and extended defects, resulting from  bombardment of energetic species during growth. Single-crystal GaN(0001) nanorods have been grown directly on Si(111) substrates by DC-MSE in a pure N2environment. The as-grown GaN nanorods exhibit very high crystal quality from bottom to the top without any stacking faults, as determined by TEM. The crystal quality is found to increase with increasing working pressure. XRD results show that all the rods are highly 0001 oriented. All nanorods exhibit an N-polarity, as determined by convergent beam electron diffraction methods. Sharp and well-resolved 4 K μ-PL peaks at ~3.474 eV with a FWHM ranging from 1.7 meV to 22 meV are attributed to the intrinsic GaN band edge emission and corroborate the exceptional crystal quality of the material. Texture measurements reveal that the rods have random in-plane orientation when grown on Si(111) with its native oxide while they have an inplane epitaxial relationship of GaN[11̅20] // Si[1̅10] when grown on Si(111) without the surface oxide. The best structural and optical properties of the rods were achieved at N2 partial pressures of 15 to 20 mTorr. By diluting the reactive N2 working gas in DC-MSE with Ar, it is possible to achieve favorable growth conditions for high quality GaN nanorods onto Si(111) at a low total pressure of 5 mTorr. With an addition of small amount of Ar (0.5 mTorr), we observe an increase in nanorod aspect ratio from 8 to ~35, a decrease in average diameter from 74 nm to 35 nm, and a 2-fold increase in nanorod density compared to pure N2 conditions. By further dilution, the aspect ratio continuously decreases to 14 while the diameter increases to 60 nm and the nanorod density increases to a maximum of 2.4×109 cm-1. The changes in nanorod morphology upon Ar-dilution of the N2 working gas are explained by a transition from N-rich growth conditions, promoting the diffusion induced nanorods growth mode, to Ga-rich growth conditions, in qualitative agreement with GaN nanorods growth by MBE. At N2 partial pressure of 2.5 mTorr, the Ga-target is close to a non-poisoned state which gives the most perfect crystal quality which is reflected in an exceptionally narrow band edge emission at 3.479 eV with a FWHM of only 1.7 meV. Such structural and optical properties are comparable to rods previously grown at 3 to 4 time higher total working pressures of pure N2.

Submitted by Ziani DE SOUZA SCHIABER (zianisouza@yahoo.com.br) on 2016-05-02T20:43:07Z No. of bitstreams: 1 Tese_Final_Ziani_Schiaber.pdf: 4224142 bytes, checksum: 63114f480403729da0d811c82872c3cc (MD5)
Approved for entry into archive by Felipe Augusto Arakaki (arakaki@reitoria.unesp.br) on 2016-05-04T19:24:06Z (GMT) No. of bitstreams: 1 schiaber_zs_dr_bauru.pdf: 4224142 bytes, checksum: 63114f480403729da0d811c82872c3cc (MD5)
Made available in DSpace on 2016-05-04T19:24:06Z (GMT). No. of bitstreams: 1 schiaber_zs_dr_bauru.pdf: 4224142 bytes, checksum: 63114f480403729da0d811c82872c3cc (MD5) Previous issue date: 2016-04-19
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Nanosestruturas de GaN destacam-se devido à baixa densidade de defeitos e consequentemente alta qualidade estrutural e óptica quando comparadas ao material em forma de filme. O entendimento dos mecanismos de formação de nanofios e nanocolunas de GaN por diferentes técnicas é fundamental do ponto de vista da ciência básica e também para o aprimoramento da fabricação de dispositivos eletrônicos e optoeletrônicos baseados nesse material. Neste trabalho discorre-se sobre a preparação e caracterização de nanofios e nanoestruturas de GaN pelas técnicas de epitaxia por magnetron sputtering e epitaxia por feixe molecular em diferentes tipos de substratos. Pela técnica de epitaxia por magnetron sputtering foram obtidos nanocristais e nanocolunas de GaN, além de uma região com camada compacta. Visando criar uma atmosfera propícia para o crescimento de nanoestruturas de GaN não coalescida, atmosfera de N2 puro e um anteparo, situado entre o alvo e o porta-substratos, foram utilizados. O anteparo causou diferença no fluxo incidente de gálio no substrato, ocasionando a formação de diferentes tipos de estruturas. A caracterização das amostras se deu principalmente através de medidas de microscopia eletrônica de varredura, difração de raios X e espectroscopia de fotoluminescência. As nanocolunas, de 220 nm de altura, foram formadas na região distante 2 mm do centro da sombra geométrica do orifício do anteparo e apresentaram orientação [001] perpendicular ao substrato, comumente encontrada em nanofios de GaN depositados por MBE. Em relação aos nanofios obtidos pela técnica de MBE, investigou-se a possibilidade de controlar a densidade de nanofios através de uma camada de Si sobre o GaN–Ga polar visando inibir a coalescência. Diferentes quantidades de Si foram depositadas e a densidade dos nanofios foi diferenciada significativamente. Os nanofios apresentaram densidade média de 108 nanofios/cm2 com 0,60 nm de espessura da camada de Si. Espessuras menores não resultaram no crescimento de nanofios, porém espessuras superiores causaram uma alta densidade de nanofios de 1010 nanofios/cm2 que permaneceu constante, independentemente do tempo de deposição. Medidas de polo por difração de raios X evidenciaram que os nanofios nuclearam-se orientados e em uma camada cristalina de Si ou SixNy. Experimentos de ataque químico com KOH indicaram a polaridade N para o nanofio e as medidas de difração por feixe convergente confirmaram a polaridade de N para o nanofio e Ga para a buffer layer. Os resultados obtidos neste trabalho permitiram um melhor entendimento da nucleação e dos mecanismos de formação de nanoestruturas de GaN, viabilizando maior controle das características dessas nanoestruturas produzidas.
GaN nanowires and nanocolumns stand out due to the low defect density and high structural and optical quality compared to the corresponding thin films. The understanding of the formation mechanism of the different GaN structures using different techniques is critical to improving the manufacture of the electronic and optoelectronic devices based on this material. This thesis focuses on the preparation and characterization of GaN nanowires and nanostructures. The molecular bem epitaxy (MBE) and magnetron sputtering epitaxy (MSE) were used and different substrates were tested. Concerning GaN nanocrystals and nanocolumns obtained by MSE, optimization of the deposition conditions was necessary in order to produce non-coalesced GaN nanostructures. The best conditions were: pure N2 atmosphere, silicon substrate, and a perforated screen placed between the target and the substrate holder. The later produced differences on the Ga flow to the substrate, inducing the formation of different structures, depending on the position of growth spot. Samples were characterized using scanning electron microscopy, X-ray diffraction and photoluminescence spectroscopy. Nanocolumns were observed, mainly in sites corresponding to a disc of radius 2 mm from the geometric centre of the hole. The columns were oriented with the GaN [001] axis perpendicular to the Si (111) substrate surface, situation which is commonly found in GaN nanowires deposited by MBE. Regarding the nanowires prepared by MBE technique, in order to inhibit coalescence and to investigate the possibility of controlling the numerical density of nanowires, we have used Si cap layers on top of the Ga-polar GaN buffer layer. Different amounts of Si have been deposited, and the density of the nanowires was significantly modified. With Si layer thickness of 0.60 nm, the nanowires had an average density of 108 nanowires/cm2 . Lower thickness did not result in the growth of nanowires, but higher thickness caused a high density of nanowires of 1010 nanowires/cm2 which remained constant regardless of the deposition time. X-ray diffraction pole figures showed that the different nanowires grown up in oriented fashion in a crystalline layer of Si or SixNy. Etching with KOH indicated N polarity for the grown nanowires, in spite of the fact that they were grown using Ga polar GaN buffer layers. Measurements by convergent beam electron diffraction confirmed the N polarity to the nanowire and Ga polarity for the buffer layer. Aspects obtained in this study allowed a better understanding of nucleation and nanostructures formation mechanisms of GaN, enabling greater control of the characteristics of these nanostructures produced.
FAPESP: 2011/22664-2
FAPESP: 2013/25625-3

Made available in DSpace on 2014-06-11T19:30:18Z (GMT). No. of bitstreams: 0 Previous issue date: 2012-02-24Bitstream added on 2014-06-13T18:40:19Z : No. of bitstreams: 1 schiaber_zs_me_bauru.pdf: 1292517 bytes, checksum: a77a9460815db1ad11c8b75efaad7290 (MD5)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Semicondutores de gap largo são materiais de grande interesse devido às suas amplas aplicações tecnológicas. Entre os semicondutores de gap largo se destaca o GaN que apresenta características desejáveis para tais aplicações, como valor de energia de bandgap de 3,4 eV, alta condutividade térmica e alta dureza. As técnicas convencionais para a produção de filmes finos de GaN são a epitaxia por feixe molecular (MBE) e deposição de vapor químico de precursores metalorgânicos (MOVPE), porém tais técnicas possuem um elevado custo. Este trabalho discorre sobre a preparação e caracterização de filmes policristalinos de GaN pela técnica alternativa de RF magnetron sputtering reativo com diferentes temperaturas e tipos de substratos. Analisou-se o efeito da variação destes dois parâmetros sobre estrutura e propriedades ópticas destes filmes. Utilizou-se medidas de difração de raios-X, microscopia de força atômica, transmitância no ultravioleta/visível/infravermelho e espectroscopia de espalhamento Rutherford (RBS). As medidas realizadas reportaram que tanto a temperatura quanto o tipo de substrato influenciaram na textura de orientação, morfologia e propriedades ópticas dos filmes. Medidas de transmitância no infravermelho indicaram a presença de bandas relacionadas à contaminação com higrogênio e oxigênio em filmes depositados em temperaturas de substratos menores que 500ºC. As referidas contaminações são compatíveis com a análise residual da água detectada no sistema de deposições, e não foram observadas em temperaturas maiores de substrato. Os diafratogramas de raios-X revelaram que somente em temperaturas altas (Ts>500ºC) a textura de orientação dos filmes é influenciada pelo substrato utilizado, podendo apresentar indícios de crescimento epitaxial. As medidas...
Wide bandgap semiconductor materials are of great interest due to the broad range of their technological applications. Among the wide bandgap semiconductor GaN stands out due to its desirable characteristics for such aplications as the value of energy bandgap of 3.4 eV, high thermal conductivity and high hardness. Conventional techniques for producing GaN thin films are the molecular beam epitaxy (MBE) and chemical vapor deposition of metalorganinc precursors (MOVPE), nevertheless these are high techniques. This work brings into focus the preparation and characterization of polycrystalline GaN films by the alternative technique of reactive RF magnetron sputtering with different temperatures and substrates. The effects of varying theses two parameters on structured and optical properties of these films were analysed. Therefore, X-ray diffraction, atomic force microscopy, optical transmittance in the ultraviolet/visible/infrared, and Rutherford Backscattering Spectrometry (RBS) were used to characterize the samples. The results show that temperature, substrate type, and substrate orientation influence the texture, morphology and optical properties of the films. The X-ray diffraction patterns revealed that the orientation texture of films is influenced by the substrate used only at high substrate temperature (Ts>500ºC). This evidences a tendency of epitaxial growth. Besides, the atomic force microscopy at temperature above 500ºC showed that the surface morphology is different for amorphous and crystalline substrates. It also became evident that the decrease of deposition rate and bandgap of the films with increasing deposition temperature is possibly due to nitrogen deficiency by the high rate of desorption at these temperatures. In addition, measurements of trasmisttanc in the infrared Fourier Transform indicated the presence... (Complete abstract click electronic access below)

Orientador: José Humberto Dias da Silva
Banca: Mario Antonio Bica de Moraes
Banca: Jose Roberto Ribeiro Bortoleto
Programa de Pós Graduação em Ciência e Tecnologia de Materiais, PosMat, tem carater institucional e integra as atividades de pequisa em materiais de diversos campi
Resumo: Semicondutores de gap largo são materiais de grande interesse devido às suas amplas aplicações tecnológicas. Entre os semicondutores de gap largo se destaca o GaN que apresenta características desejáveis para tais aplicações, como valor de energia de bandgap de 3,4 eV, alta condutividade térmica e alta dureza. As técnicas convencionais para a produção de filmes finos de GaN são a epitaxia por feixe molecular (MBE) e deposição de vapor químico de precursores metalorgânicos (MOVPE), porém tais técnicas possuem um elevado custo. Este trabalho discorre sobre a preparação e caracterização de filmes policristalinos de GaN pela técnica alternativa de RF magnetron sputtering reativo com diferentes temperaturas e tipos de substratos. Analisou-se o efeito da variação destes dois parâmetros sobre estrutura e propriedades ópticas destes filmes. Utilizou-se medidas de difração de raios-X, microscopia de força atômica, transmitância no ultravioleta/visível/infravermelho e espectroscopia de espalhamento Rutherford (RBS). As medidas realizadas reportaram que tanto a temperatura quanto o tipo de substrato influenciaram na textura de orientação, morfologia e propriedades ópticas dos filmes. Medidas de transmitância no infravermelho indicaram a presença de bandas relacionadas à contaminação com higrogênio e oxigênio em filmes depositados em temperaturas de substratos menores que 500ºC. As referidas contaminações são compatíveis com a análise residual da água detectada no sistema de deposições, e não foram observadas em temperaturas maiores de substrato. Os diafratogramas de raios-X revelaram que somente em temperaturas altas (Ts>500ºC) a textura de orientação dos filmes é influenciada pelo substrato utilizado, podendo apresentar indícios de crescimento epitaxial. As medidas... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: Wide bandgap semiconductor materials are of great interest due to the broad range of their technological applications. Among the wide bandgap semiconductor GaN stands out due to its desirable characteristics for such aplications as the value of energy bandgap of 3.4 eV, high thermal conductivity and high hardness. Conventional techniques for producing GaN thin films are the molecular beam epitaxy (MBE) and chemical vapor deposition of metalorganinc precursors (MOVPE), nevertheless these are high techniques. This work brings into focus the preparation and characterization of polycrystalline GaN films by the alternative technique of reactive RF magnetron sputtering with different temperatures and substrates. The effects of varying theses two parameters on structured and optical properties of these films were analysed. Therefore, X-ray diffraction, atomic force microscopy, optical transmittance in the ultraviolet/visible/infrared, and Rutherford Backscattering Spectrometry (RBS) were used to characterize the samples. The results show that temperature, substrate type, and substrate orientation influence the texture, morphology and optical properties of the films. The X-ray diffraction patterns revealed that the orientation texture of films is influenced by the substrate used only at high substrate temperature (Ts>500ºC). This evidences a tendency of epitaxial growth. Besides, the atomic force microscopy at temperature above 500ºC showed that the surface morphology is different for amorphous and crystalline substrates. It also became evident that the decrease of deposition rate and bandgap of the films with increasing deposition temperature is possibly due to nitrogen deficiency by the high rate of desorption at these temperatures. In addition, measurements of trasmisttanc in the infrared Fourier Transform indicated the presence... (Complete abstract click electronic access below)
Mestre

Made available in DSpace on 2015-03-03T11:52:52Z (GMT). No. of bitstreams: 0 Previous issue date: 2014-05-27Bitstream added on 2015-03-03T12:06:56Z : No. of bitstreams: 1 000808135.pdf: 2777565 bytes, checksum: e57516b1335c820bcfe63c7183d37b8b (MD5)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
O nitreto de Gálio (GaN) têm recebido grande atração nos últimos anos devido a sua possível aplicação em semicondutor magnético diluído (DMS) pela incorporação de íons como o Manganês (Mn). No entanto, a preparação destes tipos de amostras tem sido conseguida nos últimos anos usando Epitaxia por Feixe Molecular (Molecular Beam Epitaxy, MBE) e Deposição de Vapor Químico com precursoresMetalorgânicos (Metal Organic Chemical Vapor Deposition, MOCVD), que são técnicas de crescimento muito caras e necessitam de condições especiais de substrato, tal como alta temperatura de crescimento. Uma alternativa é a utilização de técnicas de crescimento como o Sputerring. A vantagem desta técnica de sputerring é o baixo custo e a possibilidade de crescimento de filme em temperatura relativamente baixa. Neste trabalho, foram realizadas medidas de fotoluminescência, espectroscopia Raman e espectroscopia no infravermelho nos filmes de Ga1-xMnxN e GaN obtidos por RF Magnetron Sputtering Reativo. Os espectros de fotoluminescência proporcionaram o entendimento da concentração máxima de Mn incorporado nos filmes de Ga1-xMnxN. Ainda, os dados revelaram a presença de emissões de fotoluminescência em aproximadamente 3,31 eV atribuída à incorporação de Mn, em aproximadamente 3,35 eV atribuída à contaminação por Hidrogênio e em 3,36 eV atribuída a éxciton ligado a falta de empilhamento. Os dados de fotoluminescência são consistentes com os dados de espectroscopia Raman que mostram o efeito da tensão sobre os modos vibracionais com o aumento de concentração de Mn. Isto ocorre devido à diferença de raio iônico do Mn em relação ao Ga que gera tensão na estrutura do cristal. Estes resultados proporcionaram um melhor entendimento do processo de crescimento de filmes de GaN e Ga1-xMnxN por RF Magnetron Sputtering Reativo
Galium nitride (GaN) has gained an unprecedented attention in the last years due to their possible application in dilute magnetic semiconductor (DMS) by incorporation of ions like Mn. However, the preparation of these samples is very complicated and has been achieved only in the past few years by using Molecular Beam Epitaxy and Metal Organic Chemical Vapor Deposition, which are very expensive techniques and require special condition like high temperature of growth. One alternative route is to use growth techniques like reactive magnetron sputtering. The advantage of sputtering technique is the low cost and the possibility to grow film at relatively low temperature. In this work, we perform measurements of photoluminescence, Raman spectroscopy and infrared spectroscopy in Ga1-xMnxN and GaN films obtained by RF Reactive Magnetron Sputtering. The photoluminescence spectra have provided he understanding of the maximum Mn concentration in Ga1-xMnxN films. In addition, the data revealed the presence of photoluminescence emission, around 3.31 eV assigned by incorporation of Mn, around 3.35eV assigned by hydrogen and 3.36eV assigned by exciton bound to stacking faults. The photoluminescence data is consistent with Raman spectroscopy data that show the tension effect at the vibrational models with increasing Mn concentration. This is due to the difference in ionic radius of Mn relative to Ga that generates tensions in the crystal lattice. These allow understanding the growth process of GaN and Ga1-xMnxN films by RF Reactive Magnetron Sputtering

Orientador: Américo Sheitiro Tabata
Banca: Virgilio de Carvalho dos Anjos
Banca: Alexandre Levine
Banca: Luis Vicente de Andrade Scalvi
Banca: Dayse Iara dos Santos
Resumo: O nitreto de Gálio (GaN) têm recebido grande atração nos últimos anos devido a sua possível aplicação em semicondutor magnético diluído (DMS) pela incorporação de íons como o Manganês (Mn). No entanto, a preparação destes tipos de amostras tem sido conseguida nos últimos anos usando Epitaxia por Feixe Molecular ("Molecular Beam Epitaxy", MBE) e Deposição de Vapor Químico com precursoresMetalorgânicos ("Metal Organic Chemical Vapor Deposition", MOCVD), que são técnicas de crescimento muito caras e necessitam de condições especiais de substrato, tal como alta temperatura de crescimento. Uma alternativa é a utilização de técnicas de crescimento como o "Sputerring". A vantagem desta técnica de "sputerring" é o baixo custo e a possibilidade de crescimento de filme em temperatura relativamente baixa. Neste trabalho, foram realizadas medidas de fotoluminescência, espectroscopia Raman e espectroscopia no infravermelho nos filmes de Ga1-xMnxN e GaN obtidos por RF Magnetron Sputtering Reativo. Os espectros de fotoluminescência proporcionaram o entendimento da concentração máxima de Mn incorporado nos filmes de Ga1-xMnxN. Ainda, os dados revelaram a presença de emissões de fotoluminescência em aproximadamente 3,31 eV atribuída à incorporação de Mn, em aproximadamente 3,35 eV atribuída à contaminação por Hidrogênio e em 3,36 eV atribuída a éxciton ligado a falta de empilhamento. Os dados de fotoluminescência são consistentes com os dados de espectroscopia Raman que mostram o efeito da tensão sobre os modos vibracionais com o aumento de concentração de Mn. Isto ocorre devido à diferença de raio iônico do Mn em relação ao Ga que gera tensão na estrutura do cristal. Estes resultados proporcionaram um melhor entendimento do processo de crescimento de filmes de GaN e Ga1-xMnxN por RF Magnetron Sputtering Reativo
Abstract: Galium nitride (GaN) has gained an unprecedented attention in the last years due to their possible application in dilute magnetic semiconductor (DMS) by incorporation of ions like Mn. However, the preparation of these samples is very complicated and has been achieved only in the past few years by using Molecular Beam Epitaxy and Metal Organic Chemical Vapor Deposition, which are very expensive techniques and require special condition like high temperature of growth. One alternative route is to use growth techniques like reactive magnetron sputtering. The advantage of sputtering technique is the low cost and the possibility to grow film at relatively low temperature. In this work, we perform measurements of photoluminescence, Raman spectroscopy and infrared spectroscopy in Ga1-xMnxN and GaN films obtained by RF Reactive Magnetron Sputtering. The photoluminescence spectra have provided he understanding of the maximum Mn concentration in Ga1-xMnxN films. In addition, the data revealed the presence of photoluminescence emission, around 3.31 eV assigned by incorporation of Mn, around 3.35eV assigned by hydrogen and 3.36eV assigned by exciton bound to stacking faults. The photoluminescence data is consistent with Raman spectroscopy data that show the tension effect at the vibrational models with increasing Mn concentration. This is due to the difference in ionic radius of Mn relative to Ga that generates tensions in the crystal lattice. These allow understanding the growth process of GaN and Ga1-xMnxN films by RF Reactive Magnetron Sputtering
Doutor

碩士
國立臺灣大學
物理研究所
101
This thesis is about the design and theoretical simulation of a type of electron-optical system—single-anode magnetron injection guns ( MIGs ), which operate in the temperature limited regime of emission. The first half of this thesis discusses some physical principles applied for MIGs, and for the second half, I’ll present a MIG design example, and simulation results for the MIG being used in NTU applied electrodynamics lab, together with a modified MIG structure based on it to minimize the velocity spread. Theoretical results were obtained by employing a 2-l/2 dimension electron gun code ( EGUN ).

碩士
國立中山大學
物理學系
87
In this thesis the polycrystalline gallium nitride ( GaN ) films were achieved by magnetron rf sputtering. The X-ray diffraction ( XRD ), scanning electron microscope ( SEM ), electron probe microscope analysis ( EPMA ) and photoluminescence measurement ( PL ) had been used to investigate these GaN films. The growth rate was 0.35 μm/h in average. It was found that the GaN films had the preferential orientation at [10 (bar1) 0] direction. The ratio of Ga to N was 1.33 in average. The oxygen was also observed by EPMA that might come from previous sputtered materials remained in the chamber during sputtering. The PL measurement indicated the photon energy was 3.4 ±0.07 eV at 64 K and 3.2 eV at R.T.

碩士
大葉大學
電機工程研究所
87
The characteristics of gallium nitride (GaN) thin films deposited by r.f. reactive magnetron sputtering are studied. The deposition parameters are constant during sputtering (e.g. the r.f. power, deposition pressure, substrate temperature) besides the nitrogen partial pressure of sputtering atmosphere. The effects of films deposited at different nitrogen content of sputtering gases are discussed. When the films are deposited at higher nitrogen content, the deposition rate is lower, the surface morphologies of films become more flat and the resistivity is higher. Contrarily, when the films are deposited at lower nitrogen content, the deposition rate increases. However, the surface morphologies of films become rough and the resistivity becomes low. Persistent photoconductivity (PPC) behavior was observed in the sputtered GaN films. The films deposited at higher nitrogen partial pressure present the more obvious PPC. For example, the films deposited at 20% nitrogen content of sputtering gases present at a level 60% higher than the equilibrium dark current for over one hour after removing the photoexcitation. The “erase” method for PPC behavior by heating samples to the different temperatures was experimented. The PPC can be erased at the appropriate temperature. Although the PPC has deleterious effects on both optic and electric device, the PPC behavior is similar to the optical memory effect, i.e., “writing” with photoexcitation, and it can be “erased” by appropriate thermal energy. In this thesis, the possible traps distributions are measured by current－temperature and thermally stimulated current measurement. The activation energy and the possible trap levels are calculated. The descriptions of PPC processes and thermal effects on PPC are discussed. The possible trap levels distribution is proposed.

碩士
國立中山大學
物理學系研究所
88
ABSTRACT In this thesis we deposit GaN films by magnetron rf sputtering with changes substrate temperature. The electron probe microscope analysis ( EPMA ), scanning electron microscope ( SEM ), photoluminescence measurement ( PL ) and X-ray diffraction ( XRD ) had been used to investigate these GaN films. We find GaN films crystalline quality deposit at low temperature is better then deposit at high temperature. From EPMA analysis we know higher substrate temperature lower oxygen amount of film. The ratio of Ga to N is 1.18 ~ 1.83 in average. The growth rate is about 0.30 μm/h ~ 0.35 μm/h in average. Thus changes substrate temperature do not influence growth rate obviously. From SEM and EDS analysis we find the roughness magnitude of films growth on sapphire substrate was smaller than the films growth on silicon substrate. We also find lower substrate temperature the roughness magnitude of films larger on silicon substrate.

博士
國立臺灣科技大學
材料科學與工程系
103
High thermal conductivity, high electron mobility, high electron saturation velocity, and large band gap of nitride based materials have attracted many research interests in recent years. The GaN and InGaN materials have brought promising future for the application of electronic devices such as metal–oxide–semiconductor field effect transistors (MOSFETs), hetero junction field-effect transistors (HJ-FETs), Schottky diodes, p–n junction diodes, laser diodes, light emitting diodes (LEDs) etc. However, high-quality GaN, InGaN films and III-V nitride semiconductors for optoelectronic and electronic devices often have been grown on sapphire and several other semiconductor substrates by using metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) above 800 oC. The developments of III-V nitride materials and their devices made by using low-cost methods at low processing temperatures are very important in fabricating electronic devices. In this study, all diodes based on GaN and InGaN materials will be made at the low temperature with the cost-effective and reactive radio-frequency (RF) sputtering technique. The GaN and its alloy films were characterized by FE-SEM, EDS, XRD, TEM, AFM and Hall measurement at the room temperature. The electrical characterizations of diodes were determined by I–V and C–V measurements. The characteristics of our diodes can be successfully explained with the thermionic-emission (TE) model. Cheungs' and Norde methods were used to determine all electrical parameters of Schottky and p–n junction diodes. For the diode devices based on n–GaN and n–InGaN, the 500 oC-annealed n–GaN MS (metal–semiconductor) and MOS (metal–oxide–semiconductor) Schottky diodes showed the smallest leakage currents of 1.02�e10-8 and 1.86�e10-9 A, respectively, at -1V. The highest SBHs for n–GaN MS and MOS Schottky diodes were calculated to be 0.79 and 0.81 eV, respectively, by the Cheungs’ method, and 0.91 and 0.94 eV by the Norde method. The n–GaN MOS diode showed a high series resistance of 84.4 k��, as compared to 27.9 k�� for the n–GaN MS diode. In addition, n–InGaN MS and MOS Schottky diodes were studied before and after annealing at 400 oC. The 400 oC-annealed samples displayed the leakage current of 3.86�e10-6 (MS) and 1.42�e10-7 A (MOS). The SBH of n–InGaN MOS diode increased from 0.69 eV (I–V), 0.77 eV (Norde) to 0.82 eV (C–V) after annealing at 400 oC. By C–V measurement for n–InGaN MOS diode, the carrier concentration was found to be 4.48�e1017 cm-3 for the as-deposited and 2.41�e1017 cm-3 for the annealed samples. The n–InGaN MOS diode had a small series resistance of 911 ��, as compared to 84.4 k�� for the n–GaN MOS diode. For the homo junction diodes, four different diodes based on n– and p–GaN, InGaN, and AlInGaN materials were made by reactive RF sputtering technique. All homo junction diodes were designed with the electrode configuration of the top–bottom mode on Pt/TiO2/Si(100) substrates. They can be presented as n�{GaN/p�{GaN (Mg-doped-GaN), n–GaN/p�{InGaN (Mg-doped In0.05Ga0.95N), n–InGaN/p–GaN (Mg-doped GaN), and n–AlInGaN/p–GaN diodes. Among of them, the good performance had been showed in the n–InGaN/p–GaN diode. The variations of electrical properties of this diode with the wide bias range of [-20; 20] V were measured at 25-150 oC. The diode tested at 25 oC showed the turn-on voltage of 2.1 V, the leakage currents of 4.7�e10-7 A at -5 V and 1.04�e10-5 A at -20 V, the breakdown voltage above ~20 V, the barrier height of 0.60 eV, and the ideality factor of 5.9. By increasing the testing temperature, the barrier height increased from 0.60 eV (I–V) and 0.69 eV (Norde) at 25 oC to 0.73 eV and 0.85 eV at 150 oC, respectively. The series resistance decreased from 1925 Ω (25 oC) to 131 Ω (150 oC). For the ideality factor, it decreased from 5.9 to 4.8 by the I–V tests and from 6.4 to 5.1 by the Cheungs’ method. For studying the hetero junction diodes, all diodes based on GaN, InGaN, and Si wafer were made by using RF sputtering technique. All hetero junction diodes were designed on n�{ and p�{Si wafers by the electrode configuration of the top-top mode. They can be listed as n–InxGa1-xN/p–Si (x = 0, 0.15, and 0.4), Mg-doped p–InxGa1-xN/n–Si (with x = 0 and 0.05), and Zn-doped p–GaN/n–Si diode. The best diode was the Zn-doped p–GaN/n–Si, which were made to confirm the strong p�{type behavior by Zn doping. The lowest leakage current of ~1.65�e10-9 A at -5 V and the breakdown voltage above 20 V were found at 25 oC. The series resistance Rs and ideality factor decreased from 539 Ω to 234 Ω and 6.5 to 4.2, respectively, with testing temperature changed from 25 to 125 oC. The barrier height increased from 0.64 eV at 25 oC to 0.77 eV at 150 oC.

碩士
國立臺北科技大學
光電技術研究所
92
This dissertation investigates the growth and characteristics of GaNxP1-x alloys on GaN by RF reactive magnetron sputtering. Before depositing the GaNP samples, a 2-μm-thick GaN buffer layer was growth on (0001) sapphire substrates by MOCVD. The characteristics of GaNxP1-x alloys are discussed in detail under varying deposition conditions such as different of nitrogen flow rate percentage, RF power, and growth temperature. Then we measured the band gap energy of samples and compared them with two of theories currently. By analyzing the measurement data, the direct band gap energy of GaNxP1-x alloys would change with varying content ratio of nitrogen and phosphorous. And we speculated the band gap energy of deposited samples tally with the prediction by Miyoshi et al.. Besides, the crystal performance of GaNP alloys will be improved when the temperature up toward to 600ºC by rapid temperature annealing process.

碩士
國立成功大學
光電科學與工程學系
100
In this experiment, transparent and conductive indium tin oxide (ITO) film were deposited on p-GaN and sapphire by the rf magnetron sputter system. Investigation of ohmic contact between ITO on p-type GaN determined by the transmission line model (TLM), and discussed optoelectronic characteristics of ITO on Sapphire. We had compared the ITO films on nitride-based light emitting diodes by sputtering and electron beam evaporation. 　We had demonstrated the ITO film as ohmic contacts on p-type GaN by sputtering. The ITO contact on p-GaN was an ohmic property after heat treatment process at an alloying temperature of 600℃ for 5min in nitrogen ambient. The lowest specific contact resistance of ITO contact on p-GaN was 1.2×10-2 Ω-cm2. And the ITO films showed high transmittance（〉90%）in the visible wavelength range. GaN-based LEDs with Sputter ITO and E-beam ITO contact layers were fabricated for the optoelectronic characteristics study. The forward voltage was 3.23 V and 3.33 V for the LEDs with Sputter ITO and E-beam ITO contact layer under 20 mA current injection, respectively. The forward voltage of the LED with E-beam ITO contact layer was higher 0.1 V than the LED with Sputter ITO contact under 20 mA current injection. The series resistance was 10.29 Ω and 13.73 Ω for the LEDs with Sputter ITO and E-beam ITO contact layer, respectively. The LED with Sputter ITO had lower sheet resistance and better current spreading than with E-beam ITO contact layer. The output power was 27.13 mW and 22.80 mW for the LEDs with Sputter ITO and E-beam ITO contact layer under 300 mA current injection, respectively. However, the light output power of the LED with Sputter ITO contact layer was larger 19% than the LED with E-beam ITO contact under 300 mA current injection. 　These results revealed that the ITO deposited by the rf magnetron sputter system was better than electron beam evaporation system for on LED fabrication process.

碩士
國立中山大學
物理學系研究所
97
In this thesis, we study the magnetro-transport properties of Fe-doped AlxGa1-xN/GaN heterostructure with different Al content by Shubnikov-de Haas measurements. On the samples KTH640(x=0.294)、KTH643(x=0.344)、KTH642(x=0.390)、KTH644 (x=0.397), we find that the electron have occupied the lowest two subbands, and the energy separation for each sample before illumination is 98 meV、107 meV、111 meV、119 meV. On the samples KTH641(x= 0.216)、KTH640(x= 0.294)、KTH642(x= 0.390)、KTH644(x= 0.394), we observe the electrons have spin splitting phenomenon, and the highest spin splitting energy separation’s value in our experiments is 10.6 meV. For all six samples have persistent photoconductivity effect’s behavior after illumination.

博士
國立臺灣大學
物理學研究所
87
This thesis concerns with the studies on the optical and electrical properties of GaN-based semiconductors and the magnetic-field induced phase transition in the p-SiGe/Si heterostructures. Photoluminescence, photoluminescence excitation, photoconductivity, and magneto-transport measurements are carried out to study the physical properties of the GaN-based semiconductors, including GaN epifilms, InxGa1-xN epiflims, InxGa1-xN/GaN multiquantum wells and AlxGa1-xN/GaN heterostructures. On the other hand, the quantum phase transitions in p-SiGe/Si heterostructures are carefully analyzed with the results of the magnet-transport measurements using several low temperature techniques. These results are presented in the following parts: 1. Optical quenching of the photoconductivity in n-type GaN: Results of optical quenching of photoconductivity measurements in undoped n-type and Se-doped GaN epitaxial thin films are presented. The spectral distribution of quenching phenomena shows a broad band centered around 1.26 eV. Transient changes in photoconductivity on application or removal of the quenching radiation are shown to exhibit a metastable behavior. The results reveal that the origin of the optical quenching phenomena is closely related to the defects corresponding to the persistent photoconductivity(PPC) effects and the yellow luminescence band observed in most n-type GaN. In additions, this result indicates that these defects can have multiple charge states. It is found that the quenching ratio increases with increasing Se doping concentration. We point out that the origin of the defects that are responsible for the optical quenching can be attributed to nitrogen antisite and/ or Ga vacancy. 2. Effects of alloy potential fluctuations in InGaN epitaxial films: Results of photoluminescence and photoconductivity measurements in InxGa1-xN epitaxial films are presented. The photoluminescence peak energy and intensity show several anomalous behaviors. The peak energy changes with temperature exhibiting inverted S shape dependence, where it decreases, then increases with increasing temperature in the range 40-100 K, and finallly decreases with increasing temperature. The intensity shows a temperature dependence similar to that of amorphous semiconductors and disordered superlattices. A blue shift of the photoluminescence energy with increasing excitation intensity is observed. A large Stokes shift between the photoluminescence peak position and the band edge transition energy is found; it decreases with decreasing indium content. A persistent photoconductivity effect has been detected up to room temperature with a stretched-exponential function for its decay rate. All these observations can be explained in a consistent way by alloy potential fluctuations, and these clearly indicate the existence of compositional fluctuations. These two related effects thus appear to constitute the mechanism for the widely observed localized excitons in InGaN-based devices. 3. Optical characteristics and persistent photoconductivity of undoped InGaN/GaN multiquantum wells: We have investigated the decay kinetics of persistent photoconductivity effect in the undoped InGaN/GaN MQWs and a localization depth of the localized energy states caused by In composition fluctuation in the InGaN well layers is determined. Compared with the results of complementary absorption and photoluminescence measurements, it is found that the quantum confined Stark effect due to piezoelectric field and alloy potential fluctuations both exist in the InGaN/GaN MQWs. And these two effects are responsible for the photoluminescence Stokes-like shift in the InGaN well layers. We point out that the piezoelectric field and the alloy potential fluctuations can significantly influence the optical properties of the InGaN-based materials and should be taken into account for the design and fabrication of group-III nitride based heterointerface devices. 4. Two-dimensional electron gas and persistent photoconductivity in AlGaN/GaN heterostructure: We present results of electrical and optical measurements in AlGaN/GaN heterostructure. The presence of a two-dimensional electron gas(2DEG) at the high quality AlGaN/GaN heterointerface is confirmed by Shubnikov-de Haas measurement, which shows well resolved magnetoresistance oscillations starting in field below three Tesla at 1.3 K. From the temperature dependence of the oscillation amplitude, the obtained effective mass (0.24±0.02) m0 is in excellent agreement with the value of cyclotron resonance measurements in 2D systems, but larger than the values of theoretical and experimental results in GaN bulk films. We point out that the effective mass enhancement in 2D systems is due to the effects of band nonparabolicity and wavefunction penetration into the barrier material. The results of photoconductivity measurements reveal that persistent photoconductivity(PPC) does exist in the AlGaN/GaN heterostructure, and that the PPC behavior of AlGaN/GaN heterojunction is quite different from that of the GaN epitaxial thin films. A possible mechanism is presented to interpret the observed PPC effect. 5. Magnetic-field induced anomalous phase transitions in p-SiGe/Si heterostructures: We present the low-temperature magneto-transport measurements on a two-dimensional hole gas in the p-type modulation doped SiGe alloy confined by Si barrier. We observe a magnetic-field-induced transition which occurs at high magnetic fields from an integer quantum Hall phase to a Hall insulator phase and finally reenters to a υ = 1 integer quantum Hall phase. This Hall insulator is centered at υ = 1.5, which is unanticipated by the global phase diagram. Scaling analysis and universal phenomena are found to be different at the two transition points. These results are attributed to the unusual energy level scheme in p-SiGe.

Gallium nitride (GaN) is nowadays one of the most important semiconductors, especially from the point of view of high-temperature and high-frequency applications in the fields of high-speed electronics, optoelectronics and blue, violet as well as ultraviolet optics. Concerning basic research, however, focused on fundamental phenomena that can be explored in III-V semiconductor structures, it still remains in the background of gallium-arsenide-based systems. It is particularly true for collective excitations of a two-dimensional electron gas (2DEG). An important class of these excitations are charge-density oscillations, usually referred to as plasma oscillations or plasma waves in order to emphasize the fact that at sufficiently low temperature, a 2DEG of high density acquires all the features which underlie the definition of plasma - one of the four fundamental states of matter. In the presence of an external magnetic field (B), plasma waves become a powerful tool to probe the non-local properties of a 2DEG, which to the best knowledge of the author have not yet been systematically investigated in GaN-based systems. The main goal of the following dissertation was therefore to at least partially fill this gap by analysing the influence of the non-local effects on the spectrum of two-dimensional magnetoplasma excitations in GaN/AlGaN heterostructures. The samples under study were large-area (about 1.6mm × 1.6mm) field-effect transistors (FETs) with grating-gate electrodes of different periodicity (L) and width of an individual metallic gate’s stripe (W). We have considered four transistor structures with L ranging from 2.0µm to 4.0µm and a separation between the adjacent gate’s fingers (S = L − W) equal either to 0.35µm or to 0.85µm. A primary role of the gratings was to ensure an optical coupling between the 2DEG magnetoplasmons and the incoming electromagnetic radiation. When electrically polarized, they additionally served to control the concentration of a 2DEG in the FET’s channel. We also had at our disposal a reference sample of exactly the same dimensions as grating FETs, but with no gate electrode deposited on its top surface. All the samples were processed on a high-quality GaN/AlGaN heterostructure grown Ga-face up on a sapphire substrate by Migration-Enhanced Metal-Organic Chemical Vapor Deposition technique. Each of them contained a quasi-2DEG of areal density N_0 ∼ 10^13 cm^{−2}, originating from both the spontaneous and piezoelectric polarization as well as doping of AlGaN barrier with silicon donors to approximately 2 × 10^18 cm^{−3} . The electrical-transport properties of our structures were determined from two-terminal measurements of the magnetic-field dependence of the channel resistance, R(B), recorded at 4.3–8K for a given polarization of the gate electrode (V g 6 0) and the B field up to 10T applied perpendicularly to the plane of a 2DEG. Both orientations of the magnetic field (parallel and anti-parallel to the heterostructure’s growth direction) were considered in order to correctly extract the diagonal part of the magnetoresistivity tensor from the experimental data. An analysis of the Shubnikov-de Haas oscillations, developing in a high-field part of R(B) traces, enabled us to estimate the 2DEG density (N) as a function of V_g as well as the so-called electron quantum relaxation time, τ_q , for V_g = 0. We found out that in FETs with a larger slit between the adjacent grating-gate’s fingers, an application of V_g < 0 resulted in creating a lateral modulation of N, whereas for a narrower slit, each section of the gate controlled N in the entire period of the structure. A minimal distance, required to observe two populations of carriers with clearly different densities, was initially a bit surprising to us, since it exceeded a typical range resulting from the electric-field-fringing effect and diffusion by almost four times. Following the conclusions from independent experiments performed by other researchers (scanning capacitance microscopy, UV-light-induced persistent photocurrent measurements) we now attribute this phenomenon to charging the surface states on top of the AlGaN barrier (proven to be of acceptor-like type), which can extend the physical dimensions of the source of electrostatic potential by even a 0.5µm. Such a mechanism was shown to be especially efficient in devices that exhibit substantial gate-leakage currents, what fits very well the structures studied in this dissertation. A low-magnetic-field part of the properly symmetrized magnetoresistance data helped us to estimate an N-dependent mobility and momentum relaxation time, τ_t . An unexpectedly high τ_t /τ_q ratio at V_g = 0, varying from sample to sample from 25 to almost 40 and partially originating from the inhomogeneity of the 2DEG layer, which is hard to avoid in large-area crystals, also confirmed the presence of surface states occupied by electrons and acting as a source of the long-range scattering potential. In order to explain the general shape of R(B) curves recorded on the grating-gate structures we propose a model based on the gate-leakage-current-enhanced geometri- cal magnetoresistance - an effect, which to the best knowledge of the author, has not yet been reported in the literature. We show, that when accurately analysed (with the aid of a conformal mapping description of the current-path distortion), it can give a very deep insight into the measured system, offering, for instance, the possibility of determining the spatial distribution of the gate-leakage current density along the FET’s channel. In our case, estimating the shape of this distribution turned out to be very important for understanding a surprisingly weak susceptibility to the gate voltage that was exhibited by magnetoplasmon modes in most of the samples under investigation. The optical part of our research was based on two complementary magnetotrans- mission experiments. In the first one, transmission measurements covering the spec- tral range from 0.6THz to 15THz were carried out at 1.8K with the use of a Fourier transform infrared spectrometer coupled with the magnetic field up to 13T. Spectra were recorded for different values of B and floating gate potential. In the second experiment, an optically pumped far-infrared laser was utilized to shine a 2.52THz radiation on a sample kept at 4.3–8K. Due to the constant energy of excitation, a transmission signal was registered either as a function of B up to 10T with V g set to a given value or V g down to -5V, but for B = const. As in the case of magneto- transport characterization, also in both optical experiments the B field was directed perpendicularly to the plane of a 2DEG. Clear evidence of magnetoplasmon resonances were found in transmission spec- tra collected for all the samples under study. The quality of experimental data improved, if necessary, by means of a numerical transformation known as Fourier Self-Deconvolution, enabled us to construct the B-field dispersion diagrams of the recorded modes of magnetoplasma oscillations. When analysed in such diagrams, the experimental points denoting the minima in respective transmission curves turned out to represent either a so-called upper-hybrid mode (UHM), which originates from plasma waves that exist also for B = 0, or the modes discovered on a theoretical basis by I. B. Bernstein, which appear only in magnetized plasmas of frequency- and wave-vector-dependent conductivity. Up to the best knowledge of the author, it is the first time, when observation of these modes is reported for the GaN/AlGaN-based structures. In plasma physics, Bernstein modes (BMs) are recognized as a direct probe of the non-local properties of the medium through which they propagate at frequencies close to the harmonics of the cyclotron resonance frequency. Since the energy of any given BM increases with the magnetic field faster than the energy of the UHM, there are values of B at which these two modes should become degenerate. Such a scenario is not, however, allowed by the symmetry rules and what happens instead is the mode-mixing, which in the B-field dispersion diagram appears as an anti-crossing (or repulsion) between curves corresponding to the UHM and a given BM. Apart from these interaction regions, the amplitudes of BMs are very small, so that extracting them from experimental data becomes a challenging task. Further- more, the anti-crossings get smaller as the order of BM increases. In consequence, the undeniable traces of only the first two BMs excited in solid state plasmas have been reported so far for high-quality GaAs/AlGaAs samples (concerning the exper- iments performed as a function of the magnetic field for a given value of the 2DEG density). In spite of much worse parameters of a 2DEG, we have obtained the same results in the GaN/AlGaN structures, what demonstrates their potential for com- peting with the GaAs-based systems also in the field of basic research in condensed matter physics. Moreover, thanks to rich spectra, composed of many magnetoplas- mon modes (up to 10th harmonics of the fundamental frequency corresponding to the longest wave vector allowed by the grating coupler) we were able to examine the existing semi-classical theory of magnetoplasma excitations to the extent, which was unattainable in the previous studies carried on with the help of GaAs/AlGaAs sam- ples. The organisation of the thesis is the following. It is composed of two parts entitled ’Theoretical considerations’ and ’Experimental results’. Each of them contains two chapters. In Chapter 1, we systematically develop the description of two-dimensional magnetoplasma oscillations, starting from the case of a free electron gas and progres- sively moving towards more and more complex semiconductor structures. In Chapter 2, the fundamentals of relevant magnetotransport phenomena, occurring both at low and high magnetic fields, are briefly reminded with a special emphasis placed on collecting all the expressions used to process and interpret the experimental data presented in the second part of this dissertation. Chapter 3 contains a discussion of general properties of the grating-couplers as well as provides the details of grating- gate GaN/AlGaN transistor structures under investigation. In Chapter 4, the ex- perimental techniques and the results of our magnetotransport and magnetooptical measurements are widely described and interpreted with the aid of theoretical con- cepts introduced in Chapters 1 and 2. Finally, we briefly summarize the thesis listing once again the most important results it announces.
Azotek galu (GaN) jest obecnie jednym z najważniejszych półprzewodników, zwłaszcza z punktu widzenia wysokotemperaturowych i wysokoczęstotliwościowych zastosowań w dziedzinach takich jak szybka elektronika, optoelektronika oraz optyka światła niebieskiego, fioletowego, a także ultrafioletu. Biorąc pod uwagę badania podstawowe, skoncentrowane na fundamentalnych zjawiskach, do jakich dostęp dają struktury półprzewodnikowe z rodziny III-V, wciąż pozostaje jednak w cieniu ukła- dów opartych na arsenku galu. Dotyczy to w szczególności kolektywnych wzbudzeń dwuwymiarowego gazu elektronowego (z ang. two-dimensional electron gas, 2DEG). Ważną klasą tych wzbudzeń są oscylacje gęstości ładunku, nazywane zwykle oscyla- cjami plazmowymi lub falami plazmowymi dla podkreślenia faktu, że ochłodzony do odpowiednio niskiej temperatury 2DEG o dużej gęstości zyskuje wszystkie cechy, ja- kie są uznawane za charakterystyczne dla plazmy - jednego z czterech podstawowych stanów skupienia materii. W obecności zewnętrznego pola magnetycznego (B), fa- le plazmowe stają się potężnym narzędziem w próbkowaniu nielokalnych właściwości 2DEG, które wedle wiedzy autora niniejszej pracy, nie były jeszcze systematycznie ba- dane w układach opartych na GaN. Stąd też, podstawowym celem tej rozprawy było przynajmniej częściowe wypełnienie powyższej luki poprzez przeanalizowanie wpływu efektów nielokalnych na widmo wzbudzeń poddanej wpływowi pola magnetycznego dwuwymiarowej plazmy elektronowej w heterostrukturach GaN/AlGaN. Badane próbki miały postać tranzystorów polowych o dużej powierzchni (1,6mm × 1,6mm), wyposażonych w bramki o konstrukcji siatek dyfrakcyjnych o różnym okresie (L) i szerokości (W) tworzących je pasków metalu. Rozważono cztery struktury o okre- sach z przedziału 2,0µm – 4,0µm i odstępie pomiędzy sąsiadującymi ze sobą paskami siatki, S = L − W, równym 0,35µm lub 0,85µm. Podstawową rolą siatek było zapew- nienie sprzężenia optycznego pomiędzy wzbudzeniami magnetoplazmowymi w 2DEG a padającym na próbkę promieniowaniem elektromagnetycznym. Spolaryzowane elek- trycznie, służyły one również do kontroli koncentracji 2DEG w kanale tranzystora. Poza strukturami ze sprzęgaczami siatkowymi mieliśmy także do dyspozycji próbkę referencyjną o dokładnie tych samych, co wyżej wymienione, wymiarach geometrycz- nych, ale nieposiadającą bramki. Wszystkie próbki zostały wytworzone na wysokiej jakości heterostrukturze GaN/AlGaN wyhodowanej stroną galową do góry na podło- żu szafirowym za pomocą techniki MEMOCVD R ⃝ (z ang. Migration-Enhanced Metal- Organic Chemical Vapor Deposition). Każda z nich zawierała kwazi-dwuwymiarowy gaz elektronowy o gęstości powierzchniowej N 0 ∼ 10 13 cm − 2 , pochodzący zarówno ze spontanicznej i piezoelektrycznej polaryzacji, jak i domieszkowania bariery AlGaN- owej atomami krzemu (donorami) na poziomie 2 × 10 18 cm − 3 . Właściwości transportowe naszych struktur zostały określone na podstawie dwu- kontaktowych pomiarów zależności oporu kanału tranzystora od pola magnetycznego, R(B), przeprowadzonych w temperaturze 4,3K – 8K dla zadanej polaryzacji elek- trycznej bramki (V g 6 0) oraz pola B (do 10T) skierowanego prostopadle do płasz- czyzny 2DEG. W celu prawidłowego wyznaczenia diagonalnych składowych tensora magnetooporności, podczas ich wykonywania, wzięto pod uwagę oba zwroty pseudo- wektora indukcji magnetycznej (równoległy i antyrównoległy do kierunku wzrostu he- terostruktury). Analiza oscylacji Shubnikov’a-de Haas’a rozwijających się w wysoko- polowej części krzywych R(B) umożliwiła nam określenie koncentracji 2DEG (N) jako funkcji V g jak również tak zwanego kwantowego czasu relaksacji, τ q , dla V g = 0. Zaobserwowaliśmy, że w próbkach charakteryzujących się większą szczeliną pomiędzy sąsiadującymi ze sobą paskami bramki, przyłożenie do sprzęgacza siatkowego napię- cia V g < 0 skutkowało wytworzeniem przestrzennej modulacji N, podczas gdy w tych z węższymi szczelinami, każdy segment bramki kontrolował N na długości całego okresu struktury. Minimalna odległość wymagana do pojawienia się dwóch populacji nośników ładunku o wyraźnie różnych koncentracjach była dla nas początkowo za- skoczeniem, albowiem przekraczała prawie czterokrotnie typowy dystans, jaki wynika z dyfuzji i efektu wnikania pola elektrycznego do przestrzeni poza konturem bramki (z ang. electric-field-fringing effect). Posiłkując się wnioskami płynącymi z niezależ- nych eksperymentów przeprowadzonych przez innych badaczy, opartych na skaningo- wej mikroskopii pojemnościowej oraz pomiarach fotoprądu indukowanego promienio- waniem UV, przypisujemy obecnie to zjawisko ładowaniu stanów powierzchniowych w barierze AlGaN-owej (zidentyfikowanych jako stany typu akceptorowego), które może prowadzić do rozszerzenia fizycznych wymiarów źródła potencjału elektrosta- tycznego nawet o 0,5µm. Jak pokazano, taki mechanizm powinien być szczególnie efektywny w przypadku urządzeń charakteryzujących się znaczącymi prądami upływ- ności bramki, co bardzo dobrze odpowiada strukturom badanym w ramach niniejszej rozprawy. Nisko-polowa część wyników magnetotransportowych poddanych odpowiedniej sy- metryzacji pomogła nam w oszacowaniu zależącej od koncentracji ruchliwości elektro- nów oraz czasu relaksacji pędowej, τ t . Nieoczekiwanie wysoka wartość stosunku τ t /τ q dla V g = 0, wynosząca dla różnych próbek od 25 do prawie 40 i związana częściowo z niejednorodnością warstwy 2DEG, której trudno uniknąć w przypadku kryszta- łów o dużej powierzchni, również wskazała na obecność stanów powierzchniowych obsadzonych elektronami i działających jako źródło daleko-zasięgowego potencjału rozpraszającego. W celu wyjaśnienia ogólnego kształtu krzywych R(B) otrzymanych na próbkach ze sprzęgaczami siatkowymi proponujemy w niniejszej pracy model oparty na zjawi- sku spotęgowanego prądem upływności bramki geometrycznego magnetooporu, które wedle naszej wiedzy nie było do tej pory opublikowane. Jak pokazujemy, podda- ny skrupulatnej analizie bazującej na opisie zakrzywienia ścieżek prądowych metodą mapowania konforemnego, efekt ten może dać głęboki wgląd we właściwości badane- go układu, umożliwiając na przykład wyznaczenie przestrzennego rozkładu gęstości prądu upływności bramki wzdłuż kanału tranzystora polowego. Określenie kształtu tego rozkładu okazało się być w naszym przypadku niezwykle ważne dla zrozumienia zadziwiająco słabej podatności modów magnetoplazmowych na napięcie przyłożone do bramki, jaką zaobserwowaliśmy na większości próbek będących przedmiotem ba- dań tej rozprawy. Optyczna część naszych pomiarów była oparta na dwóch typach uzupełniających się eksperymentów magnetotransmisyjnych. W ramach pierwszego z nich, badania obejmujące zakres spektralny od 0,6THz do 15THz zostały przeprowadzone w tem- peraturze 1,8K z wykorzystaniem spektrometru Fourierowskiego sprzężonego z polem magnetycznym do 13T. Dostarczyły one widm zarejestrowanych dla różnych warto- ści B i niespolaryzowanej intencjonalnie bramki. W przypadku drugiego eksperymen- tu, źródłem światła o częstotliwości 2,52THz kierowanego na próbkę utrzymywaną w temperaturze 4,3K – 8K był pompowany optycznie laser dalekiej podczerwieni. Ze względu na stałą energię pobudzenia, sygnał transmisyjny był mierzony albo w funkcji B do 10T przy zadanej wartości napięcia przyłożonego do bramki, albo też w zależno- ści od V g do -5V dla B = const. Tak jak w trakcie wykonywania badań magnetotran- sportowych, w obu eksperymentach optycznych pole B było skierowane prostopadle do płaszczyzny 2DEG. Wyraźne ślady wzbudzeń magnetoplazmowych zostały zaobserwowane w widmach transmisyjnych wszystkich próbek, na których zostały przeprowadzone pomiary. Wy- soka jakość danych doświadczalnych, uzyskana między innymi dzięki zastosowaniu transformacji numerycznej znanej jako auto-dekonwolucja Fourierowska (z ang. Fo- urier Self-Deconvolution), pozwoliła na skonstruowanie diagramów dyspersyjnych za- rejestrowanych modów oscylacji magnetoplazmowych. Po przeprowadzonej z ich po- mocą analizie, okazało się, że punkty eksperymentalne oznaczające lokalne minima odpowiednich krzywych transmisyjnych, reprezentują bądź tzw. górne mody hybry- dowe (z ang. upper-hybrid modes, UHM) powstające z fal plazmowych istniejących również w granicy B → 0, bądź też mody odkryte na gruncie teoretycznym przez I. B. Bernsteina, które pojawiają się wyłącznie w poddanej wpływowi pola magne- tycznego plazmie o przewodności zależącej tak od częstotliwości, jak i od wektora falowego. Wedle naszej wiedzy, nikomu przed nami nie udało się zaobserwować mo- dów Bernsteina (BM) w strukturach półprzewodnikowych opartych na GaN/AlGaN. W fizyce plazmy, mody te są uznawane za bezpośredni próbnik nielokalnych właści- wości ośrodka, w którym propagują się z częstotliwościami bliskimi harmonicznym częstotliwościom rezonansu cyklotronowego. Ponieważ energia dowolnie wybranego BM rośnie z polem magnetycznym szybciej aniżeli energia UHM, dla pewnej warto- ści B mody te powinny stać się zdegenerowane. Taki scenariusz wykluczają jednak względy symetrii, za sprawą których, w rzeczywistości dochodzi między nimi do silne- go oddziaływania, które na wspomnianych wyżej diagramach dyspersyjnych objawia się w postaci niedozwolonych przecięć (lub odpychania) pomiędzy krzywymi odpo- wiadającymi UHM i danemu BM. Poza obszarami oddziaływania, amplituda modów Bernsteina bardzo szybko osiąga na tyle małe wartości, że wyekstrahowanie ich z da- nych doświadczalnych staje się niezwykle trudnym zadaniem. Co więcej, mierzona w jednostkach energii odległość pomiędzy krzywymi dyspersyjnymi w granicach nie- dozwolonych przecięć maleje ze wzrostem rzędu BM. W konsekwencji, nie budzące żadnych wątpliwości dowody istnienia uzyskano do tej pory jedynie dla dwóch pierw- szych modów Bernsteina zaobserwowanych na próbkach wykonanych z wysokiej ja- kości heterostruktur GaAs/AlGaAs (mamy w tym przypadku na myśli eksperymenty przeprowadzone w funkcji pola magnetycznego dla zadanej gęstości powierzchnio- wej 2DEG). Pomimo znacznie gorszych parametrów 2DEG, otrzymaliśmy dokładnie te same rezultaty na strukturach GaN/AlGaN, co wyraźnie pokazuje ich potencjał w konkurowaniu z układami opartymi na GaAs również w dziedzinie badań podstawo- wych materii skondensowanej. Warto zwrócić także uwagę na fakt, że dzięki bogatym widmom złożonym z wielu rezonansów magnetoplazmowych (o częstościach docho- dzących włącznie do 10 harmonicznej fundamentalnej częstości plazmowej określo- nej przez najdłuższy wektor falowy odpowiadający okresowi sprzęgacza siatkowego) byliśmy w stanie dokonać weryfikacji istniejącej teorii półklasycznej, która opisuje wzbudzenie magnetoplazmowe, w stopniu, jaki nie był osiągalny we wcześniejszych badaniach podjętych z pomocą struktur GaAs/AlGaAs. Układ niniejszej rozprawy przedstawia się następująco. Jest ona złożona z dwóch części zatytułowanych „Rozważania teoretyczne” i „Wyniki eksperymentalne”. Każda z nich obejmuje dwa rozdziały. W rozdziale pierwszym, konstruujemy systematycz- nie opis oscylacji dwuwymiarowej plazmy poddanej wpływowi pola magnetycznego, poczynając od przypadku gazu swobodnych elektronów i przechodząc stopniowo do coraz bardziej złożonych struktur półprzewodnikowych. W rozdziale drugim, przypo- minamy krótko podstawy wybranych zjawisk magnetotransportowych występujących zarówno w niskich jak i wysokich polach magnetycznych. Naszym głównym celem jest w tym przypadku zebranie wszystkich wyrażeń, jakie są potrzebne do zrozu- mienia szczegółów opracowania i interpretacji danych eksperymentalnych zaprezen- towanych w drugiej części pracy. Rozdział trzeci zawiera dyskusję na temat ogólnych właściwości sprzęgaczy siatkowych, jak również dostarcza wyczerpujących informacji odnośnie badanych próbek GaN/AlGaN. W rozdziale czwartym opisujemy szeroko zarówno techniki doświadczalne jak i wyniki pomiarów magnetotransportowych i ma- gnetooptycznych, które są następnie poddane interpretacji z wykorzystaniem modeli teoretycznych rozwiniętych w rozdziałach pierwszym i drugim. Pracę zamyka krótkie podsumowanie wszystkich najważniejszych rezultatów, które zostały w niej ogłoszone.