Academic literature on the topic 'Two Dimensional Electron Gas System (2DEG)'

Control of dimensionality has proven to be an effective way to manipulate the electronic properties of materials, thereby enabling exotic quantum phenomena, such as superconductivity, quantum Hall effects, and valleytronic effects. Another example is thermoelectricity, which has been theoretically proposed to be favorably controllable by reducing the dimensionality. Here, we verify this proposal by performing a systematic study on a gate-tuned 2D electron gas (2DEG) system formed at the surface of ZnO. Combining state-of-the-art electric-double-layer transistor experiments and realistic tight-binding calculations, we show that, for a wide range of carrier densities, the 2DEG channel comprises a single subband, and its effective thickness can be reduced to ∼ 1 nm at sufficiently high gate biases. We also demonstrate that the thermoelectric performance of the 2DEG region is significantly higher than that of bulk ZnO. Our approach opens up a route to exploit the peculiar behavior of 2DEG electronic states and realize thermoelectric devices with advanced functionalities.

Abstract Using the method developed in a recent paper (2019 Euro. Phys. J. B 92 1–28) we consider 1/f noise in two-dimensional electron gas (2DEG). The electron coherence length of the system is considered as a basic parameter for discretizing the space, inside which the dynamics of electrons is described by quantum mechanics, while for length scales much larger than it the dynamics is semi-classical. For our model, which is based on the Thomas-Fermi–Dirac approximation, there are two control parameters: temperature T and the disorder strength (Δ). Our Monte Carlo studies show that the system exhibits 1/f noise related to the electronic avalanche size, which can serve as a model for describing the experimentally observed flicker noise in 2DEG. The power spectrum of our model scales with the frequency with an exponent in the interval 0.3 < α PS < 0.6. We numerically show that the electronic avalanches are scale-invariant with power-law behaviors in and out of the metal-insulator transition line.

The study of electron transport in low-dimensional systems is of importance, not only from a fundamental point of view, but also for future electronic and spintronic devices. In this context heterostructures containing a two-dimensional electron gas (2DEG) are a key technology. In particular GaAs/AlGaAs heterostructures, with a 2DEG at typically 100 nm below the surface, are widely studied. In order to explore electron transport in such systems, low-resistance ohmic contacts are required that connect the 2DEG to macroscopic measurement leads at the surface. Here we report on designing and measuring a dedicated device for unraveling the various resistance contributions in such contacts, which include pristine 2DEG series resistance, the 2DEG resistance under a contact, the contact resistance itself, and the influence of pressing a bonding wire onto a contact. We also report here a recipe for contacts with very low resistance values that remain below 10 Ω for annealing times between 20 and 350 s, hence providing the flexibility to use this method for materials with different 2DEG depths. The type of heating, temperature ramp rate and gas forming used for annealing is found to strongly influence the annealing process and hence the quality of the resulting contacts.

The monolithic integration structure of the AlGaN/InGaN/GaN−based high electron mobility transistor (HEMT) and light−emitting diode (LED) is attractive in LED lighting and visible light communication (VLC) systems owing to the reduction in parasitic elements by removing metal interconnections. Due to the band−offset and polarization effect, inserting a certain thickness in the InGaN layer into the traditional AlGaN/GaN single heterostructure increases the density of 2DEG to nearly twice the original. At the same time, inserting the InGaN quantum well layer can also improve the luminous efficiency of LED. In this paper, the physical models of two−dimensional electron gas (2DEG) densities and the threshold voltage of AlGaN/InGaN/GaN HEMTs are established and verified with experimental results from the literature. According to the calculation results, the two−dimensional electron gas (2DEG) density in the AlGaN/InGaN/GaN HEMT is 1.47 × 1013 cm−2, and the two−dimensional hole gas (2DHG) density is 0.55 × 1013 cm−2, when Al % = 0.2, In % = 0.1, dAlGaN = 20 nm. In addition, a physical model for the radiative recombination rate in the monolithic integration structure of HEMT−LED is proposed. This work provides a design guideline for AlGaN/InGaN/GaN HEMT and its application in visible light communication systems.

A detailed theoretical study is presented for spontaneous optical emission from a two-dimensional electron gas (2DEG) in the presence of a unidirectional spatially periodic modulation at low temperatures. The momentum- and energy-balance equations for electron–photon interactions in the device system are solved self-consistently using the Boltzmann equation, from which we can obtain the frequency and the intensity of the electromagnetic radiation generated. The results obtained indicate that: (1) in an electrically modulated 2DEG at low temperatures, a strong electromagnetic radiation emission up to W/cm2 can be generated by applying a small d.c. electric field; (2) the radiation emission is generated indirectly in the occupied subbands through electronic transitions around the Fermi level; (3) the frequency of the radiation generated is at about 0.1 THz; (4) in the low frequency regime, spontaneous multiphoton emissions can be observed; and (5) this type of optical emission depends strongly on the sample parameters such as the electron density of the 2DEG and the modulation length.

ABSTRACTIntegrating oxide heterostructures on silicon has the potential to leverage the multifunctionalities of oxide systems into semiconductor device technology. We present the growth and characterization of two-dimensional electron gas (2DEG) oxide systems LaTiO3/SrTiO3 (LTO/STO) and GdTiO3/SrTiO3 (GTO/STO) on Si(001). We show interface-based conductivity in the oxide films and measure high electron densities ranging from ∼9 × 1013 cm-2 interface-1 in GTO/STO/Si to ∼9 × 1014 cm-2 interface-1 in LTO/STO/Si. We attribute the higher measured carrier density in the LTO/STO films to a higher concentration of interface-bound oxygen vacancies arising from a lower oxygen partial pressure during growth. These vacancies donate conduction electrons and result in an increased measured carrier density. The integration of such 2DEG oxide systems with silicon provides a bridge between the diverse electronic properties of oxide systems and the established semiconductor platform and points toward new devices and functionalities.

Abstract KTaO3 has become a new material of attraction for exploring various exotic properties at its surface and interfaces. To explore the substrate effect of KTaO3, we have studied a new heterostructure made by KTaO3 and SrTiO3. KTaO3 is a 5d polar perovskite with the complementary planes of KO− and TaO 2 + , whereas SrTiO3 is a 3d non-polar perovskite with alternative stacking SrO0 and TiO 2 0 . By taking the epitaxial layers of SrTiO3 on top of KTaO3, we have created two different interfaces along (0 0 1), which are KO − / TiO 2 0 and TaO 2 + / SrO 0 . We have studied these two interfaces by using density functional theory. In KO − / TiO 2 0 interface, the signature of two-dimensional hole gas (2DHG) has been found. Though the interface turns to insulator when epitaxial layers reach beyond 4.5 u.c. The other one, i.e., TaO 2 + / SrO 0 interface, shows the typical behaviour of two-dimensional electron gas (2DEG), only at the epitaxial layer 2.5 uc. The presence of both types of carriers in a single system for different kinds of interfaces has great use in various device applications.

Abstract The unusual observation of the Kondo effect in the two-dimensional electron gas (2DEG) of magnetically undoped AlGaN/GaN heterostructures is reported. The temperature-dependent zero-field resistivity data exhibits an upturn below 120 K, while the standard low-temperature weak localization and then weak antilocalization behaviour is revealed at T → 0. Magnetic transport investigations of the system are performed in the temperature range of 0.1–300 K and at magnetic fields up to 8 T, applied perpendicularly to the 2DEG plane. The experimental data are analyzed in terms of the multichannel Kondo model for d _0 magnetic materials and weak localization theory taking into account the spin-orbit interaction.

Abstract A GaN-based metal–semiconductor–metal varactor with a two-dimensional electron gas (2DEG) layer is proposed and fabricated. The capacitance variation of this fabricated varactor biased at different external voltages is studied and measured, and the frequency-dependent capacitance and resistance of the varactor are simulated by a corresponding empirical formula. A high-frequency protective filter is further constructed and placed under a large pulsed-current injection in a malicious electromagnetic interference immunity test. The results show that the proposed GaN-based module can reduce the large pulsed current to an acceptably small level. Thus, the GaN-based 2DEG varactor is an attractive candidate for applications designed to protect the upcoming 5G high-frequency system from risks such as electrostatic discharge, lightning, and electromagnetic pulses.

We study the sweep speed dependence of electron injection voltage in Si-Nano-Dots (Si-NDs) floating gate MOS Capacitor by using our collective tunneling model, which models the tunneling between two-dimensional electron gas (2DEG) and the Si-NDs. We clarify the sweep speed dependence of electron injection energy with a numerical calculation based on our collective tunneling model, that we developed to emulate the experiment in this system, and obtained a new insight into the origin of sweep speed dependence. We revealed that our model can reproduce the sweep speed dependence of electron tunneling. This insight is useful for designing future nano-electronic devices.

Modulation doped heterostructures have revolutionized the operation of field effect devices by increasing the speed of operation. One of the factors that affects the speed of operation of these devices is the mobility of the carriers, which is intrinsic to the material used. Mobility of electrons in silicon based devices has improved drastically over the years, reaching as high as 50.000cm2/Vs at 4.2K and 2600cm2/Vs at room temperature. However, the mobility of holes in p-type silicon devices still remains comparatively lesser than the electron mobility because of large effective masses and complicated valence band structure involved. Germanium is known to have the largest hole mobility of all the known semiconductors and is considered most suitable to fabricate high speed p-type devices. Moreover, it is also possible to integrate germanium and its alloy (Si1_zGex ) into the existing silicon technology. With the use of sophisticated growth techniques it has been possible to grow epitaxial layers of silicon and germanium on Si1_zGex alloy layers grown on silicon substrates. In tills thesis we investigate in detail the electrical properties of p-type germanium and n-type silicon thin films grown by these techniques. It is important to do a comparative study of transport in these two systems not only to understand the physics involved but also to study their compatibility in complementary field effect devices (cMODFET). The studies reported in this thesis lay emphasis both on the low and high field transport properties of these systems. We report experimental data for the maximum room temperature mobility of holes achieved m germanium thin films grown on Si1_zGex layers that is comparable to the mobility of electrons in silicon films. We also report experiments performed to study the high field degradation of carrier mobility due to "carrier heating" in these systems. We also report studies on the effect of lattice heating on mobility of carriers as a function of applied electric field. To understand the physics behind the observed phenomenon, we model our data based on the existing theories for low and high field transport. We report complete numerical calculations based on these theories to explain the observed qualitative difference in the transport properties of p-type germanium and ii-type silicon systems. The consistency between the experimental data and theoretical modeling reported in this work is very satisfactory.

Modulation doped heterostructures have revolutionized the operation of field effect devices by increasing the speed of operation. One of the factors that affects the speed of operation of these devices is the mobility of the carriers, which is intrinsic to the material used. Mobility of electrons in silicon based devices has improved drastically over the years, reaching as high as 50.000cm2/Vs at 4.2K and 2600cm2/Vs at room temperature. However, the mobility of holes in p-type silicon devices still remains comparatively lesser than the electron mobility because of large effective masses and complicated valence band structure involved. Germanium is known to have the largest hole mobility of all the known semiconductors and is considered most suitable to fabricate high speed p-type devices. Moreover, it is also possible to integrate germanium and its alloy (Si1_zGex ) into the existing silicon technology. With the use of sophisticated growth techniques it has been possible to grow epitaxial layers of silicon and germanium on Si1_zGex alloy layers grown on silicon substrates. In tills thesis we investigate in detail the electrical properties of p-type germanium and n-type silicon thin films grown by these techniques. It is important to do a comparative study of transport in these two systems not only to understand the physics involved but also to study their compatibility in complementary field effect devices (cMODFET). The studies reported in this thesis lay emphasis both on the low and high field transport properties of these systems. We report experimental data for the maximum room temperature mobility of holes achieved m germanium thin films grown on Si1_zGex layers that is comparable to the mobility of electrons in silicon films. We also report experiments performed to study the high field degradation of carrier mobility due to "carrier heating" in these systems. We also report studies on the effect of lattice heating on mobility of carriers as a function of applied electric field. To understand the physics behind the observed phenomenon, we model our data based on the existing theories for low and high field transport. We report complete numerical calculations based on these theories to explain the observed qualitative difference in the transport properties of p-type germanium and ii-type silicon systems. The consistency between the experimental data and theoretical modeling reported in this work is very satisfactory.

Recently, interest in superconductor-semiconductor interfaces was renewed by the search for non-Abelian states. One of the possible platform is proximity induce superconductivity into an 1D semiconductor system with strong spin orbit (SO) interaction, such system is predicted to support Majorana excitation. Another candidate is superconductivity coupled to the edge of fractional quantum Hall state, in such system, higher order of non-Abelian statistics is predicted. With such non-Abelian states, topological quantum computing can be realized. In this thesis, I will discuss the approach made by us to investigate such system.

L'électrodynamique quantique en cavité, c'est-à-dire l'étude du couplage lumière-matière en géométries confinées, a permis d'observer, grâce à des cavités de plus en plus performantes, le régime de couplage fort lumière-matière.
Dans ce régime, le temps de vie d'un photon est plus long que le temps caractéristique de l'interaction avec la matière ; un seul photon subit donc plusieurs cycles d'absorption et de réémission avant de s'échapper de la cavité.
Les premières expériences dans ce régime, effectuées avec des atomes dans des cavités supraconductrices, ont été suivies par des réalisations en matière condensée, utilisant des excitons dans des microcavités planaires, des boites de Cooper couplées à des résonateurs unidimensionnels ou bien des transitions intersousbandes dans des puits quantiques dopés, couplées à un mode de microcavité. Le couplage fort dans ce dernier système donne naissance à des excitations mixtes, moitié lumière et moitié matière, nommées polaritons intersousbandes.
Ma thèse s'attache à plusieurs aspects de la physique de ces excitations, qui se caractérisent par la force extrême du couplage, qui a poussé les chercheurs à introduire le terme couplage ultra-fort.

Dans la première partie de ma thèse, après avoir donné un aperçu général des différents concepts théoriques engagés, j'étudie les conséquences de ce couplage ultra-fort en présence d'une modulation externe appliquée au système. Je montre, en utilisant une théorie de Langevin quantique, qu'une radiation peut être émise à partir du vide, effet qui rappelle de près l'effet Casimir dynamique. L'intensité de cette radiation est assez forte pour pouvoir être mesurée et je reporte ici les résultats de deux expériences préliminaires menées en vue de l'observation d'un tel effet, auxquelles j'ai participé pour la partie théorique.

J'étudie ensuite la manière dont le couplage fort lumière-matière peut influencer le transport électronique et les expériences d'électroluminescence. Dans ce but j'ai développé des méthodes analytiques et numériques que j'ai exploitées pour montrer qu'il est possible d'augmenter grandement l'efficacité quantique des LEDs basées sur des transitions intersousbandes. J'ai aussi donné une première preuve d'extension de l'effet Purcell au régime de couplage fort.
Enfin, dans ma dernière partie, j'ai développé la théorie du scattering stimulé entre polaritons intersousbandes dû au couplage avec des phonons optiques. Je montre que ce mécanisme peut être exploité afin d'obtenir des lasers sans inversion de population avec un seuil extrêmement bas.

Title of dissertation: GROWTH OF Zn POLAR BeMgZnO/ZnO HETEROSTRUCTURE WITH TWO DIMENSIONAL ELECTRON GAS (2DEG) AND FABRICATION OF SILVER SCHOTTKY DIODE ON BeMgZnO/ZnO HETEROSTRUCTURE By Md Barkat Ullah, Ph.D A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical and Computer Engineering at Virginia Commonwealth University. Virginia Commonwealth University,2017 Major Director: Dr. Hadis Morkoç, Professor, Electrical and Computer Engineering This thesis focuses on growth of Zn polar BeMgZnO/ZnO heterostructure on GaN/sapphire template with two dimensional electron gas (2DEG) for the application of UV photodetector/emitter and high speed electronics. The motivation of using BeMgZnO as a barrier layer originates from the need to reach plasmon-LO phonon resonance in order to obtain minimum longitudinal optical (LO) phonon lifetime. Presence of 2DEG was realized in BeMgZnO/ZnO heterostructure only when the Zn polarity was achieved during the nucleation growth of ZnO on GaN/sapphire template. It was found that, polarity of ZnO on (0001) GaN/sapphire template can be controlled by the oxygen to Zn ratio used during the nucleation growth. To obtain high structural and optical quality of BeMgZnO quaternary alloy, growth kinetics of BeMgZnO layer has been studied at the temperature range from 450°C-500°C. We have achieved the growth of single crystal Be.03Mg00.15ZnO alloy at 500 °C, more than 100°C higher compared to what reported in literature, on the (0001) GaN/sapphire template through the control of Zn/(Be+Mg) flux ratio. We have also observed a thermodynamic limitation of Mg incorporation into the wurtzite BeMgZnO alloy where the excess Mg adatom accumulated in the growing surface as a MgO rich cluster. Two dimensional electron gas with high (1.2×1013cm-2) sheet carrier density was achieved at the Be0.03Mg0.41ZnO/ZnO interface through strain engineering by incorporating Be into MgZnO ternary alloy. To obtain the similar sheet carrier density it would require above 60% of Mg in MgZnO/ZnO heterostructure with reduced structural quality. A systematic comparison of sheet carrier density has been made with the already reported results from Zn polar MgZnO/ZnO heterostructure as well as with the theoretical calculation. Silver Schottky diode on Be0.02Mg0.26ZnO/ZnO heterostructure with barrier height 1.07 eV and ideality factor 1.22 was obtained with 8 order of rectification ratio. The temperature-dependent electrical characteristics were studied by using temperature dependent current-voltage (I-V) measurements. Richardson constant value of 34.8 Acm-2K-2 was found experimentally which was close to the theoretical value of 36 Acm-2K-2 known for Be0.02Mg0.26ZnO alloy.

De nombreux oxydes de métaux de transition (TMOs) possèdent des propriétés physiques complexes (ferroélectricité, magnétisme, supraconductivité à haute Tc ou magnétorésistance colossale). Les différents degrés de liberté (le réseau, la charge, le spin ou l'ordre orbitalaire) interagissent pour donner des phases différentes, très proches en énergie, qui vont former une grande variété d'états fondamentaux accessibles. La possibilité de fabriquer des hétérostructures de TMOs a encore accru la complexité de ces systèmes, de nouveaux phénomènes apparaissant aux interfaces. Un exemple typique est le gaz d'électrons bidimensionnel (2DEG) créé à l'interface entre deux oxydes isolants, LaAlO3 et SrTiO3, qui montre une transition métal-isolant, du magnétisme ou de la supraconductivité (contrôlée par une tension de grille). Le point de départ de cette thèse a été la découverte d'un 2DEG similaire à la surface nue de SrTiO3 fracturée sous vide, rendant possible l'étude de sa structure électronique par photoémission angulaire.Dans cette thèse, l'étude de surfaces préparées, plutôt que de petites facettes fracturées, a permis l'obtention de données spectroscopiques possédant des largeurs de raie proches des valeurs intrinsèques. Il est alors possible d'étudier les effets à N corps comme la renormalisation de la self-énergie due à l'interaction électron-phonon.Ces recherches sur la structure électronique du 2DEG à la surface de SrTiO3 ont pris un tour nouveau lorsqu'une texture de spin complexe y a été mesurée par photoémission résolue en spin. Nous présentons des résultats qui contredisent ces conclusions et nous discutons des raisons pouvant expliquer ce désaccord.Une des motivations de cette thèse était de savoir si la structure électronique et les propriétés du 2DEG pouvaient être contrôlées. L'étude du 2DEG sur des surfaces (110) et (111) de SrTiO3 révèle que sa structure de bandes (ordre orbitalaire, symétrie de la surface de Fermi, masses effectives) peut être ajustée en confinant les électrons sur des surfaces de différentes orientations du même matériau.Un succès majeure est la mise en évidence de 2DEGs à la surface de nombreux autres TMOs (TiO2-anatase, CaTiO3, BaTiO3) ou d'oxydes plus simples utilisés dans les applications (ZnO). Dans tous ces oxydes, nous avons identifié les lacunes en oxygène comme étant à l'origine de la création des 2DEGs.Dans l'anatase, ou d'autres TMOs en configuration électronique initiale d0, les lacunes en oxygène produisent à la fois des électrons localisés ou itinérants (le 2DEG). Il peut être subtile de prévoir quel est le cas est le plus favorable énergétiquement comme le démontre l'étude de deux polymorphes de TiO2, anatase et rutile. Dans CaTiO3, l’octaèdre formé par les atomes d'oxygène autour du Ti est incliné. Cette rupture de symétrie provoque un mélange des orbitales d et modifie le 2DEG. Dans BaTiO3, la création d'un 2DEG entraîne la coexistence de deux phénomènes normalement incompatibles, la ferroélectricité et la métallicité, dans deux zones spatialement distinctes du même matériau. Ce travail démontre qu'un 2DEG existe aussi à la surface de ZnO qui est, contrairement aux oxydes à base de Ti, plutôt un semiconducteur conventionnel, le caractère des orbitales pour les électrons itinérants étant alors de type s et non de type d.Le principal résultat est la mise au point d'une méthode simple et versatile pour la création de 2DEGs en évaporant de l'aluminium sur des surfaces d'oxydes. Une réaction d'oxydo-réduction entre le métal et l'oxyde permet de créer un 2DEG à l'interface entre le métal oxydé et l'oxyde réduit. Dans cette thèse, les 2DEGs ont été étudiés uniquement par photoémission sous ultra-vide. Cette méthode ouvre la possibilité d'étudier ces 2DEGs dans des conditions de pression ambiante en utilisant, par exemple, des techniques de transport, un pas important vers la production de masse et à bas coûts de 2DEGs dans les oxydes pour de futures applications
Many transition metal oxides (TMOs) show complex physics, ranging from ferroelectricity to magnetism, high-Tc superconductivity and colossal magnetoresistance. The existence of a variety of ground states often occurs as different degrees of freedom (e.g. lattice, charge, spin, orbital) interact to form different competing phases which are quite similar in energy. The capability to epitaxially grow heterostructures of TMOs increased the complexity even more as new phenomena can emerge at the interface. One typical example is the two-dimensional electron system (2DES) at the interface of two insulating oxides, namely LaAlO3/SrTiO3, which shows metal-to-insulator transitions, magnetism or gate-tunable superconductivity. The origin of this thesis was the discovery of a similar 2DES at the bare surface of SrTiO3 fractured in vacuum, making it possible to study its electronic structure by angle-resolved photoemission spectroscopy (ARPES).In this thesis, the study of well-prepared surfaces, instead of small fractured facets, results in spectroscopic data showing line widths approaching the intrinsic value. This approach allows a detailed analysis of many-body phenomena like the renormalization of the self-energy due to electron-phonon interaction.Additionally, the understanding of the electronic structure of the 2DES at the surface of SrTiO3(001) was given an additional turn by the surprising discovery of a complex spin texture measured by spin-ARPES. In this thesis data is presented which contradicts these conclusions and discusses possible reasons for the discrepancy.One major motivation of this thesis was the question if and how the electronic structure and the properties of the 2DES can be changed or controlled. In this context, the study of 2DESs at (110) and (111) surface revealed that the electronic band structure of the 2DES (orbital ordering, symmetry of the Fermi surface, effective masses) can be tuned by confining the electrons at different surface orientations of the same material, namely SrTiO3.A major achievement of this thesis is the generalization of the existence of a 2DES in SrTiO3 to many other surfaces and interfaces of TMOs (TiO2 anatase, CaTiO3, BaTiO3) and even simpler oxides already used in modern applications (ZnO). In all these oxides, we identify oxygen vacancies as the origin for the creation of the 2DESs.In anatase and other doped d0 TMOs, both localized and itinerant electrons (2DES) can exist due to oxygen vacancies. Which of the two cases is energetically favorable depends on subtle differences as demonstrated by studying two polymorphs of the same material (anatase and rutile).In CaTiO3, the oxygen octahedron around the Ti ion is slightly tilted. This symmetry breaking results in the mixing of different d-orbitals demonstrating again why and how the electronic structure of the 2DES can be altered.In BaTiO3, the creation of a 2DES results in the coexistence of the two, usually mutual exclusive, phenomena of ferroelectricity and metallicity in the same material by spatially separating the two.Moreover, this work demonstrates that the 2DES also exists in ZnO which is - compared to the Ti-based oxides - rather a conventional semiconductor as the orbital character of the itinerant electrons is of s and not d-type.The main result of this thesis is the demonstration of a simple and versatile technique for the creation of 2DESs by evaporating Al on oxide surfaces. A redox reaction between metal and oxide results in a 2DES at the interface of the oxidized metal and the reduced oxide. In this thesis the study of such interfacial 2DESs was limited to photoemission studies in ultra high vacuum. However, this technique opens up the possibility to study 2DESs in functional oxides in ambient conditions by e.g. transport techniques, and might be an important step towards cost-efficient mass production of 2DESs in oxides for future applications

High-electron-mobility transistor (HEMT) is a promising device for power applications because of their high breakdown voltage, high electron mobility in two-dimensional electron gas (2DEG) area, fast switching capability, high-temperature operating capabilities, compatibility with standard electronic circuits, and low production cost. In contrast to the gate in metal–oxide–semiconductor field-effect transistor (MOSFET), which extends from source to drain, the gate in HEMT splits the device into two main sections: field-effect (section under the gate) and resistive (section outside the gate). Resistances of the 2DEG outside the gate sections are constant and modelled by fixed resistors. However, the 2DEG resistance under the gate section is dependent to the gate voltage, which can be modelled by channel resistance of a field-effect transistor (FET). Since these resistances depend on the mobility of electrons in the 2DEG, it is important to separate the electron mobility in the resistive and field-effect sections. Therefore, existence of the resistive section in the HEMT structure leads to requiring new methods for the HEMT mobility measurement. Also, since there is no model for the HEMT in SPICE, novel models are required for the SPICE simulation of the HEMT. In order to solve these issues:
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Engineering
Science, Environment, Engineering and Technology
Full Text

博士
國立清華大學
物理系
104
We present a study of electric instability in GaAs/AlGaAs hetero-structure two-dimensional electron gas system under high magnetic fields. As the applied dc current exceeds a threshold value Ith, the longitudinal voltage Vxx drops, fluctuates and exhibits electric instability. The observed instability occurs only in well-separated low-lying Landau levels with a filling factor ν ≦ 2 at relatively high lattice temperatures. We find that Ith increases with increasing magnetic field B and the lattice temperature TL. In contrast, electric instability becomes more pronounced at higher B, but gradually diminishes with increasing TL. The onset of electric instability has been predicted by two phenomenological theories, one is the theory of Andreev et al. [25], and the other is the theory of Kurosawa et al. [ 1]. Data analysis suggests that longitudinal resistance Rxx is suppressed by increasing I and exhibits negative differential resistivity (NDR) when I = Ith, where NDR is viewed as a precursor signal of electric instability. The electric instability is caused by domain growth. The electric instability is actuated by the suppression of Rxx with increasing I, which can be understood in terms of the capability of the spectral diffusion of electrons and electron transfer to higher levels via inelastic inter-Landau levels scattering within the limit of one-occupied Landau level.

The electronic structure of infinite periodic systems (crystals) is treated with band structure theory, replacing molecular orbitals by crystal orbitals. The chapter starts out by introducing the electron gas and definitions of the Fermi momentum, the Fermi energy, and the density of states (DOS). A periodic linear combination of atomic orbitals (LCAO) type treatment of an infinite periodic system is facilitated by the construction of Bloch functions. The notions of energy band and band gap are discussed with band structure concepts, using the approximations made in Huckel theory (chapter 12). One, two, and three-dimensional crystal lattices and the associated reciprocal lattices are introduced. The band structures of sodium metal, boron nitride, silicon, and graphite, are discussed as examples of metals, insulators, semi-conductors, and semi-metals, respectively. The chapter concludes with a brief discussion of the projected DOS and measures to determine bonding or antibonding interactions between atoms in a crystal.

Coherent optical spectroscopy has emerged as an important experimental tool in the study of carrier dynamics in semiconductor quantum wells (QWs). Numerous studies of exciton dynamics have shown that the coherent nonlinear interaction in intrinsic QWs could be described in most cases by a simple two level system model. A more complex behavior is expected in modulation doped (MD) structures, where the interaction between the photo-excited electron-hole pairs and the two-dimensional electron gas (2DEG) is important.

Abstract We measured AlGaN/GaN heterostructure using scanning nonlinear dielectric microscopy (SNDM) [1], which can measure both carrier and polarization profile in AlGaN/GaN heterostructure. As a result, GaN spontaneous polarization and AlGaN polarization which is sum of spontaneous polarization and piezoelectric polarization were clearly distinguished. Two dimensional electron gas (2DEG) was observed at the AlGaN/GaN interface. This results show that SNDM is useful method for evaluation of 2DEG profile and polarization profile in AlGaN/GaN heterostructure.

Epilayer design for the MODFET is frequently performed using the depletion approximation [1,2]. The work of authors such as Ponse et. al. and Stern and Das Sarma have provided tools by which electron distributions and capacitances can be calculated without this assumption [3,4]. They demonstrate that for two dimensional electron gas (2DEG) sheet concentrations significantly below the saturation saturation value, free and bound electrons are present in the supply layer. Because these electrons must be modulated with the 2DEG electrons, the gate capacitance is increased while the transconductance is decreased [5]. The unity current gain frequency, fT, is thus decreased.

In analogy to the asymmetric transport of electricity in a familiar electrical diode, a thermal rectifier transports heat more favorably in one direction than in the reverse direction. One approach to thermal rectification is asymmetric scattering of phonons and/or electrons, similar to suggestions in the literature for a sawtooth nanowire [1] or 2-dimensional electron gas with triangular scatterers [2]. To model the asymmetric heat transport in such nanostructures, we have used phonon ray-tracing, focusing on characteristic lengths that are small compared to the mean free path of phonons in bulk. To calculate the heat transfer we use a transmission-based (Landauer-Buttiker) method. The system geometry is described by a four-dimensional transfer function that depends on the position and angle of phonon emission and absorption from each of two contacts. At small temperature gradients, the phonon distribution function is very close to the usual isotropic equilibrium (Bose-Einstein) distribution, and there is no thermal rectification. In contrast, at large temperature gradients, the anisotropy in the phonon distribution function becomes significant, and the resulting heat flux vs. temperature curve (analogous to I-V curve of a diode) reveals large thermal rectification.