We intend to study the magneto-transport properties of two dimensional electron-hole systems in the condition of ultra-low temperature and high magnetic field by SdH measurement and QHE measurement. In these samples, the SdH oscillations show a 2DEG behavior. We observed that the sample shows negative persistent photoconductivity effect at low temperature and high field under illumination. We want to compare with results before 4 years, so we change more sets of measurement. And we want to know if these samples still show the same effect after 4 years.

We discussed the electronic properties in AlxGa1-xN/GaN heterostructures. There are six different samples of AlxGa1-xN/GaN we prepared for this experiment, three of them are undoped AlxGa1-xN/GaN with different x values which is 0.17, 0.29 and 0.33, respectively. The others are Fe-doped AlxGa1-xN/GaN which the x value is 0.18, 0.19 and 0.21 for each sample. Comparing these two types¡¦ materials¡¦ results, we tried to confirm the impurity¡¦s characteristic in our samples. From the Hall measurement results, carrier concentration of Fe-doped AlxGa1-xN/GaN were pretty lower than undoped AlxGa1-xN/GaN, and the mobility at 4.2 K are almost two times to undoped AlxGa1-xN/GaN. At the same time, we performed Shubnikov-de Hass measurement, and two subbands of the 2DEG were populated on the samples for x value is 0.29 and 0.33. The energy separation for first two subbands is 109meV. We also observed obvious beat pattern in the SdH oscillations due to the spin splitting on the samples and the greatest spin-splitting energy is 5.96meV in our measurement. Furthermore, we observed evident PPC effect on the samples of Fe-doped AlxGa1-xN/GaN, the carrier concentrations increased at least 23% after illumination. Meanwhile the samples of undoped AlxGa1-xN/GaN can just produce 10.7% increment at most.

We have studied the electronic properties of InxGa1-xAs/ In0.52Al0.48As quantum wells by using Shubnickove-de Hass (SdH) measurement. The indium composition (x) of well layers was varied from 0.5 to 0.56 whit different structures, such as sample A is simply ¡§In0.53Ga0.47As¡¨, sample B is a step-well like¡§In0.56Ga0.44As/In0.53Ga0.47As/In0.5Ga0.47As¡¨,sample C is linearly graded well is a opposite way¡§In0.56Ga0.44As down to In0.5Ga0.5As, and sample D is linearly graded well ¡§In0.5Ga0.5As up to In0.56Ga0.44As.¡¨ It was found that the two SdH oscillations beat each other due to the population of the lowest two subbands in these samples. In order to investigate the electronic properties of the two subbands, we have done the Ven der Pauw Hall measurement . From SdH and Hall measurement, we are able to determine the individual mobility and carrier concentrations for two-subband-populated samples.

The electrical conductivity of InN, group III-V semiconductor, is measured by four point measurement at low temperatures and high magnetic fields. From Resistance Vs temperature measurements (done in the absence of magnetic field) there is a transition from semiconducting state to superconducting state at 2.5K. This superconducting state disappears when the measurements are repeated but at a magnetic field of 0.1 Tesla. Mover the Hall voltage is not proportional to the magnetic field.

This thesis presents the design and theoretical modeling of an aluminum gallium arsenide/gallium arsenide heterostructure from which suspended nanoscale mechanical resonators with embedded two-dimensional electron gas (2DEG) can be made. The mechanical characteristics of the resonator and the piezoelectric actuation scheme are investigated using finite-element modeling. For a 836 nm-long, 250 nm-wide and 164 nm-thick beam with gold electrodes on top, out-of-plane flexural vibrations are verified to be piezoelectrically excited at the beam's fundamental frequency of 925.6 MHz. Fabrication recipes for the making of ohmic contacts to the 2DEG, Hall bars and suspended structures are developed using the designed crystal structure. Electrical properties of the 2DEG are evaluated in both large, unsuspended structures as well as in sub-micron size suspended structures. It is found that the 2DEG has a reasonable electron density of 7.04E11 cm^-2 and electron mobility of 1.72E5 cm^2/(V s). / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2007-09-28 10:13:56.094

We haved studied the magneto-transport properties of two dimensional electron gas (2DEG) in Si £_-doped In0.22Ga0.78As/AlGaAs single quantum wells ( QWs ) by using Shubnikov-de Haas ( SdH ) measurements . From the SdH measurement , we can clearly observe the SdH oscillations and obtain the SdH frequencies. It indicates the 2DEG in these QWs was confirmed . We also obtain the deep level binding energies¡G 104.4 meV and 9.6 meV for sample 1 and 50.2meV for sample2 by T-dependent Van der Pauw Hall effect measure- ment at magnetic field 0.3T. The difference of these two samples was the In0.1Ga0.9As layer of sample 2 was inserted between In0.22Ga0.78As well and the GaAs spacers . So in this paper , we tried to propose a model to interpret the deep-level traps in the QWs and studied the effect of In0.1Ga0.9As inserted-layer on the In0.22Ga0.78As/GaAs Single Quantum Wells.

The observation of voltage-tunable plasmon resonances in the terahertz range in two dimensional electron gas (2-deg) of a high electron mobility transistor (HEMT) fabricated from the InGaAs/InP and AlGaN/GaN materials systems is reported. The devices were fabricated from a commercial HEMT wafer by depositing source and drain contacts using standard photolithography process and a semi-transparent gate contact that consisted of a 0.5 [micro]m period transmission grating formed by electron-beam lithography. Narrow-band resonant absorption of THz radiation was observed in transmission in the frequency range 10-100 cm-1. The resonance frequency depends on the gate voltage-tuned sheet-charge density of the 2deg. The fundamental and higher resonant harmonics were observed to shift towards lower frequencies with the implementation of negative gate bias. The theory of interaction of sub millimeter waves with 2deg through corrugated structure on top has been applied to calculate and understand the phenomena of resonant plasmon excitations. The observed separation of resonance fundamental from its harmonics and their shift with gate bias follows theory, although the absolute frequencies are lower by about a factor of 2-3 in InGaAs/InP system. However, calculated values match much better with AlGaN/GaN system.

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In dieser Arbeit werden das Wachstum und die Charakterisierung der Heterostruktur eines FeFET auf der Grundlage von BaSnO3, LaInO3 und (K,Na)NbO3 Schichten untersucht. Für jedes Material wurden die Wachstumsbedingungen bestimmt und im Hinblick auf die strukturellen und elektrischen Eigenschaften optimiert. Epitaktische BaSnO3 Filme, die auf SrTiO3 Substraten gewachsen sind, weisen eine hohe Dichte an Versetzungen auf, die ihre elektrischen Eigenschaften beeinträchtigen. Die Verwendung von NdScO3 Substraten und Einführung einer SrSnO3 Pufferschicht verbesserten die strukturellen und elektrischen Eigenschaften der BaSnO3 Schichten. Dies ermöglichte schließlich Untersuchungen an der LaInO3/BaSnO3 Grenzfläche. Schon eine geringe La-Dotierung der BaSnO3 Schicht von 0,3 % führte zur Bildung eines 2DEG nach der Grenzflächenbildung und damit zum Einschluss von Elektronen an der Grenzfläche. Dies konnte durch C-V, Van-der-Pauw und Hall-Effekt-Messungen eindeutig nachgewiesen werden. Eine deutliche Verbesserung der strukturellen und elektrischen Eigenschaften der BaSnO3 Schichten wurde durch die Verwendung von LaInO3:Ba Substraten erreicht. Diese sind gitterangepasst an BaSnO3, sodass zum ersten Mal vollständig verspannte Schichten ohne Versetzungen gewachsen werden konnten. Strukturelle und elektrische Eigenschaften von (K,Na)NbO3 Schichten wurden auf SrRuO3/DyScO3 und SrTiO3:Nb-Substraten untersucht. Auf diese Weise wurden der Einfluss der Gitterdehnung auf die kritische Schichtdicke und die Prozesse der plastischen Relaxation des Gitters bestimmt. Die elektrische Charakterisierung ergab einen hohen Leckstrom, der durch strukturelle Defekte verursacht wird. Die gesamte FeFET Heterostruktur wurde auf LaInO3:Ba Substraten gewachsen und untersucht. BaSnO3 und LaInO3 Schichten wuchsen kohärent, während (K,Na)NbO3 Schichten eine plastische Gitterrelaxation aufwiesen. Das führte zur Bildung von Strukturdefekten und zu einer Verschlechterung der ferroelektrischen Eigenschaften. / In this thesis, the design, growth and characterisation of the heterostructure of a FeFET based on BaSnO3, LaInO3 and (K,Na)NbO3 thin films are investigated. For each material, the growth conditions were determined and optimised with respect to their structural and electrical properties. Epitaxial BaSnO3 thin films grown on SrTiO3 substrates exhibit a high density of threading dislocations, which degrade their electrical properties. The use of NdScO3 substrates and the introduction of a SrSnO3 buffer layer improved the structural and electrical properties of the BaSnO3 thin films. This finally allowed investigations on the LaInO3/BaSnO3 heterointerface. Even a low La doping of the BaSnO3 layer of 0.3 % led to the formation of a 2DEG after interface formation and thus to the confinement of electrons at the interface. This could be clearly demonstrated by C-V, Van-der-Pauw and Hall effect measurements. A significant improvement of the structural and electrical properties of the BaSnO3 thin films was achieved by using LaInO3:Ba substrates. These are lattice-matched to BaSnO3 so that, for the first time, fully strained thin films could be grown without dislocations. Structural and electrical properties of (K,Na)NbO3 thin films were investigated on SrRuO3/DyScO3 and SrTiO3:Nb substrates. In this way, the influence of lattice strain on the critical film thickness and plastic lattice relaxation were determined. Their electrical characterisation revealed a high leakage current caused by structural defects. Therefore, the entire FeFET heterostructure was grown and investigated on LaInO3:Ba substrates. The BaSnO3 and LaInO3 thin films were grown coherently, while the (K,Na)NbO3 thin films exhibited plastic lattice relaxation. This led to the formation of structural defects and consequently to a deterioration of their ferroelectric properties.

Orientador: GustavoMichel Mendoza La Torre / Dissertação (mestrado) - Universidade Federal do ABC. Programa de Pós-Graduação em Física, 2012