Landau level spectroscopy in current solid state physics

We report on oscillatory magnetoabsorption experiments on several indium nitride (InN) epilayers under a pulsed magnetic field up to 56 T. This optical technique is uniquely suited to probe the bulk electronic structure in the vicinity of the fundamental band edge by Landau level spectroscopy. We discuss these results in light of two recent studies on the InN band structure by high-field magnetotransport [Goiran et al., Appl. Phys. Lett. 96, 052117 (2010)] and magneto-photoluminescence [Pettinari et al., Phys. Rev. B 79, 165207 (2009)]. In particular, we evidence a strong nonparabolicity of the conduction band and demonstrate that the electron effective mass ${m}_{e\ensuremath{\perp}0}^{*}=0.055{m}_{0}$ measured by Shubnikov--de Haas oscillations is confirmed by the magneto-optical experiments.

Semiconductors in the shape of nanostructures, including quantum dots (0-dimension) and nanoplatelets (2-dimension) demonstrate unique electronic and optoelectronic properties because of the quantum mechanism related to excitons confined in low dimensions. Such confinement can provide intriguing properties like high capability of photo-activated carriers, band gap tunability, splendid photoluminescence (PL) yield, narrow emission bandwidth, high absorption efficiency of light, fast fluorescence lifetime, etc. These properties attract great attention due to their high potential in applications like solar cells, light-emitting diodes (LEDs), lasers, photodetectors, catalysts, etc. Novel semiconductors including MoS2, MXene and perovskite have received increasing research interest in recent years. Unlike traditional semiconductor materials like Si, Ge, II-VI and III-V materials, the novel ones usually have low-dimensional form natively, which have emerged as very promising candidates for advanced electronic and optoelectronic devices. Among the various novel semiconductor materials, this thesis focuses on two types of them, which are MXene Sc2CO2 and cesium lead halide perovskite CsPbBr3. The two materials have some features in common. Both of them are direct band gap materials, which makes them available for optical applications. And they can be synthesized directly into two-dimensional structure, which o ers the opportunity to learn the influence of one-dimensional confinement. To extend the study into another dimension, the quantum dots of CsPbBr3 are also studied. The first part of the thesis focuses on the study of Sc2COx which is a type of semi-conductive MXene. The material is synthesized for the first time by using magnetron sputtering. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are performed to determine the composition of the sample. The band gap of Sc2COx sample is determined by ultraviolet-visible (UV-VIS) range absorption and photoluminescence (PL) spectrum. The Tauc plot indicates that Sc2COx is a direct band gap material. The high sensitivity of scandium to oxygen makes the sample contain both scandium carbide and scandium oxide. Thermal annealing introduces more oxidation into the sample. The structure of MXene will be broken when the annealing temperature is higher than 600C. The second part of the thesis focuses on the study of perovskite CsPbBr3 in forms of nanoplatelets and quantum dots. For CsPbBr3 nanoplatelets, the effect of thickness of cesium lead halide perovskite CsPbBr3 nanoplatelets (NPLs) on their electronic structure and optical properties are investigated using an 8-band k · p model which is based on effective-mass envelope function theory with exciton binding energy consideration. We first reported the CsPbBr3 nanoplatelets' band structure and optical gain with exciton effect. As the thickness of NPLs decreases, their band gap increases and the band mixing is more obvious which influences the transition matrix element (TME). The optical gain of CsPbBr3 nanoplatelets is calculated by taking into account TME, Fermi factor, injected carrier density and thickness. A blue shift of peak position in optical gain can be observed as the thickness of NPLs decreases. For any given NPL with a certain thickness, there is a slight blue shift in the peak position of optical gain as the carrier density increases because of band filling effect. The maximum optical gain of thinner NPLs needs higher carrier density to reach saturation. To obtain high differential gain, the carrier density in thick NPLs has to maintain in a small range. Experimental work is carried out and the results agree with our theoretical results very well. For quantum dot, the size effect on the electronic structure and optical properties of cubic CsPbBr3 perovskite quantum dots are investigated by using an 8-band k · p model which is also based on effective-mass envelope function theory with exciton binding energy consideration. Quantum dots with smaller sizes have larger band gaps due to quantum confinement. The transition matrix element (TME) is also influenced by the size. The optical gain of such material is calculated by considering TME, Fermi factor, injected carrier density, quantum dot size, and dephasing rate. A higher density of the injected carrier is needed for smaller quantum dots to get a positive optical gain. The peak of optical gain has a blue shift as the size of a quantum dot decreases. When the carrier density increases for a quantum dot with a certain size, a blue shift in emission peak position can be observed due to the band filling effect. Smaller quantum dots have higher differential optical gain at carrier densities within a certain range. In a word, two direct band gap semiconductor materials, Sc2COx and CsPbBr3 are studied via theoretical and experimental approaches. Their band-structure and optical properties are presented, followed by the analysis of their potential in applications.

Tight-binding versus effective mass approximation calculation of electronic structures of semiconductor nanocrystals and nanowires

1D topological systems for next-generation electronics

Tunneling spectroscopy of surface and bulk Landau levels in Si(0 0 1) electron accumulation layers

New multicomponent equiatomic rare earth oxides (ME-REOs) containing 3-7 rare earth elements (Ce, Gd, La, Nd, Pr, Sm and Y) in equiatomic proportions are synthesized using nebulized spray pyrolysis. All the systems crystallized as a phase pure fluorite type (Fm3[combining macron]m) structure in spite of the high chemical complexity. A nominal increase in the lattice parameter compared to CeO2 is observed in all ME-REOs. X-ray photoelectron spectroscopy performed on the ME-REOs confirmed that all the constituent rare earth elements are present in the 3+ oxidation state, except for Ce and Pr which are present in 4+ and in a mixed (3+/4+) oxidation state, respectively. The presence of Ce4+ contributes substantially to the observed stability of the single phase structure. These new oxide systems have narrow direct band gaps in the range of 1.95-2.14 eV and indirect band gaps in the range of 1.40-1.64 eV, enabling light absorption over the entire visible spectral range. Furthermore, the oxygen vacancy concentration rapidly increases and then saturates with the number of rare earth elements that are incorporated into the ME-REOs. The lowering of the band gap is found to be closely related to the presence of multivalent Pr. Interestingly, the band gap values are relatively invariant with respect to the composition or thermal treatments. Considering the high level of oxygen vacancies present and the observed low band gap values, these new material systems can be of importance where the presence of oxygen vacancies is essential or in applications where a narrow band gap is desirable.

Exploring the structural, dynamical, mechanical, electronic, magnetic, and optical properties of Ta2AlN, Ti2AlN & Ti2GaN MAX phase compounds: First-principles study

Using multiple-scattering theory, we studied the transmission properties of a slab of composite material that have sonic band gaps due to local resonances. Thin slabs of such material have transmission properties that are apparently different from conventional band gap material. For example, there can be transmission peaks in the frequency range inside the bulk sonic band gap. If the slab thickness is changed, we found that the top of band gap shifts while the bottom of band gap, being pinned by the resonance frequency, does not. By changing the slab thickness, the “effective band gap” may be narrowed or broadened, depending on the filling fraction of the locally resonant units. In order to provide an intuitive understanding of the phenomena, we constructed a simple model to understand the phenomena by comparing its transmission and band structure with that of the locally resonant sonic materials.

Electronic properties of graphene (carbon) nanoribbons are studied and compared to those of carbon nanotubes. The nanoribbons are found to have qualitatively similar electron band structure which depends on chirality but with a significantly narrower band gap. The low- and high-field mobilities of the nanoribbons are evaluated and found to be higher than those of carbon nanotubes for the same unit cell but lower at matched band gap or carrier concentration. Due to the inverse relationship between mobility and band gap, it is concluded that graphene nanoribbons operated as field-effect transistors must have band gaps <0.5eV to achieve mobilities significantly higher than those of silicon and thus may be better suited for low power applications.

The electronic properties, namely, the band structures, the band gaps, and the electron effective masses of hydrogen-passivated InAs nanowires grown in 〈100〉 , 〈110〉, and 〈111〉 crystallographic directions are studied using sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> s* orbital-basis tight-binding model. We then parameterize the band gaps and electron effective masses to facilitate device simulation and to study the orientation-dependent performance of n-channel InAs nanowire transistors using a top-of-the-barrier model. The 〈111〉 and 〈110〉 wire transistors have better performance metrics. The quantum-confinement effect is largest in the 〈100〉 wire, which results in a higher band gap and a heavier effective mass for relatively smaller diameter wires. The consequence is lower current, higher density of states, higher quantum capacitance, and longer delay in the 〈100〉 wire transistors. The 〈110〉 and 〈111〉 wires have a very similar quantum-confinement effect, even for the smaller diameters, which results in similar band gaps, similar effective masses, and similar performance metrics.

Acoustic metamaterial consisting of an array Helmholtz resonators has been revealed to have the band gap phenomenon which implies a great potential for noise reduction. However, the application of the conventional acoustic metamaterial is often limited by the narrow band gap. In this paper, an acoustic metamaterial containing a series of membrane-coupled Helmholtz resonators is proposed to produce mult iple band gaps in the low-frequency regime for achieving broadband noise reduction. First, a theoretical model is developed to describe the dynamic of the membrane-coupled Helmholtz resonator system. The membrane is assumed to be loaded with a small mass at the centre for tuning its natural frequencies, thus the resonances of the coupled system and the band gaps of the proposed acoustic metamaterial. Subsequently, based on the theory of acoustic wave propagation together with the derived theoretical model for the membrane-coupled Helmholtz resonator system, a theoretical analysis of the proposed acoustic metamaterial is performed. Using the Bloch’s theorem, the dispersion relation of the proposed acoustic metamaterial is derived. Multiple band gaps are observed in the band structure. A corresponding finite element model of the proposed metamaterial is built to confirm the predictions from the theoretical analysis.

Accurate models of carrier transport are essential for describing the electronic properties of semiconductor materials. To the best of our knowledge, the current models following the framework of the Boltzmann transport equation (BTE) either rely heavily on experimental data (i.e., semi-empirical), or utilize simplifying assumptions, such as the constant relaxation time approximation (BTE-cRTA). While these models offer valuable physical insights and accurate calculations of transport properties in some cases, they often lack sufficient accuracy -- particularly in capturing the correct trends with temperature and carrier concentration. We present here a general transport model for calculating low-field electrical drift mobility and Seebeck coefficient of n-type semiconductors, by explicitly considering all relevant physical phenomena (i.e. elastic and inelastic scattering mechanisms). We first rewrite expressions for the rates of elastic scattering mechanisms, in terms of ab initio properties, such as the band structure, density of states, and polar optical phonon frequency. We then solve the linear BTE to obtain the perturbation to the electron distribution -- resulting from the dominant scattering mechanisms -- and use this to calculate the overall mobility and Seebeck coefficient. Using our model, we accurately calculate electrical transport properties of the compound n-type semiconductors, GaAs and InN, over various ranges of temperature and carrier concentration. Our fully predictive model provides high accuracy when compared to experimental measurements on both GaAs and InN, and vastly outperforms both semi-empirical models and the BTE-cRTA. Therefore, we assert that this approach represents a first step towards a fully ab initio carrier transport model that is valid in all compound semiconductors.

Band-gap narrwing in tin-doped indium oxide films

Absolute band structure determination on naturally occurring rutile with complex chemistry: Implications for mineral photocatalysis on both Earth and Mars

Spectra of edge photoluminescence (PL) at 300 K have been studied in a set of Czochralski-grown Te-doped GaAs single crystals with a free carrier density of n 0=1017–1019 cm−3. The carrier density dependences of the chemical potential and band gap narrowing are obtained by analyzing the PL spectral line profiles. The dependence of the effective mass of electrons at the bottom of the conduction band on their density, m 0 * (n 0), is calculated. It is shown that the nonmonotonic m 0 * (n 0) dependence correlates with data on electron scattering in the material under study and results from the ordering of impurity complexes.