Weak localization

Weak localization is a physical effect which occurs in disordered electronic systems at very low temperatures. The effect manifests itself as a positive correction to the resistivity of a metal or semiconductor. The name emphasizes the fact that weak localization is a precursor of Anderson localization, which occurs at strong disorder. General principle The effect is quantum-mechanical in nature and has the following origin: In a disordered electronic system, the electron motion is diffusive rather than ballistic. That is, an electron does not move along a straight line, but experiences a series of random scatterings off impurities which results in a random walk. The resistivity of the system is related to the probability of an electron to propagate between two given points in space. Classical physics assumes that the total probability is just the sum of the probabilities of the paths connecting the two points. However quantum mechanics tells us that to find the total probab

Dimensional crossover in the quantum transport behaviour of the natural topological insulator Aleksite

Marko Burghard, Pascal Gehring, Alexander Ronald Hoyer, Klaus Kern, Kristina Vaklinova

Three-dimensional topological insulators comprise topologically protected surface states displaying massless, Dirac-like linear dispersion with spin-momentum locking. Electrical conduction through such surface states has been documented to manifest itself in a two-dimensional character of the angle-dependent magnetotransport behavior. Here, we explore the size-dependent electronic properties of nanostructures made of the lead-containing mineral Aleksite, a naturally occurring topological insulator. Compared to its sister compound Kawazulite, a member of the wellstudied Tetradymite crystal class, the crystal structure of Aleksite is distinguished by its lack of any counterpart within the group of synthetic topological insulators. Low temperature Hall measurements on thin Aleksite nanosheets reveal a significant carrier mobility on the order of 1000 cm(2)/(Vs), and a high carrier density of n = 3.9 x 10(25) m(-3). Importantly, for Aleksite nanoribbons with a width below 150 nm, a 1D weak antilocalization effect along with 1D universal conductance fluctuations emerges, which transforms into 2D behavior for larger ribbon widths

Nature Publishing Group2015

Ultrafast carrier dynamics in III-nitride nanostructures and LED quantum efficiency

Wei Liu

Since the seminal report about the first Candela-class-brightness InGaN blue light-emitting diodes (LEDs) by Shuji Nakamura et al. in 1994, III-nitride semiconductors have been one of the most important platforms for optoelectronic devices. The achievement in III-nitride LEDs has been awarded by the physics Nobel Prize in 2014. Despite the success of blue LED technology, the quantum efficiency is still limited as LEDs towards high power, green-red colors, and micrometer dimensions. The interaction of carrier recombination dynamics with alloy disorder, dislocations and point defects has not yet been fully understood, which is essential for tackling these issues. Through the use of nanoscopic and ultrafast spectroscopy techniques, the work of this thesis is dedicated to studying carrier recombination dynamics and its relation to the quantum efficiency of III-nitride LEDs, as summarized as follows: (1) We directly probe exciton dynamics around an isolated single dislocation, by using time-resolved cathodoluminescence (TR-CL). The exciton diffusion length and effective area of the dislocation are deduced by modeling the CL decays across the dislocation, which reveals the intrinsic properties of dislocations as NRCs. The type of dislocation is confirmed by observing the energy shift of the exciton induced by local strain fields. Meanwhile, we demonstrate that the key role of the underlayer (UL) is to trap non-radiative point defects. Low-kV CL measurements provide strong evidences of annihilation of carriers by point defects at the nanometer scale and the change of diffusion length under different defect densities. (2) We establish a comprehensive model to describe the carrier dynamics in m-plane InGaN/GaN QWs taking into account the presence of excitons, residual doping, and phase-space filling. Based on picosecond time-resolved photoluminescence (TR-PL) and modeling, we estimate the amount of excitons in QWs at room temperature, and explain the interplay between excitonic population and electron-hole (e-h) plasma. Beyond the e-h plasma scenario, our study provides insight into the physical origins of radiative recombination in LEDs. (3) By using high injection TR-PL at cryogenic temperatures, we demonstrate evidence that carrier localization in the InGaN/GaN QWs plays a crucial role in intensifying the Auger process, which is linked to the relaxation of momentum conservation by alloy disorder. Furthermore, we study the impact of different densities of point defects on efficiency droop in InGaN/GaN QWs at 300 K. This study suggests an extra efficiency loss, which is likely related to a defect-assisted Auger process. We derive a linear relation between Shockley-Read-Hall (SRH) and defect-assisted Auger recombination, which implies SRH-type defects can be the scattering centers for Auger recombination. (4) We study highly efficient single InGaN/GaN core-shell microwires, in order to evaluate their potential for micro-/nano- LEDs and mitigating the efficiency droop. We decompose radiative and non-radiative lifetimes of carriers in the sidewall m-plane QW along the wire by spatial mapping of TR-CL from 4 to 300 K, which provides nanoscopic insight on the carrier dynamics in the presence of growth induced gradient of In content. By using high injection TR-PL at low temperature, we observe significant variation of Auger recombination in the active layer, which can be attributed to a different degree of alloy disorder.

EPFL2019

Ultrafast carrier dynamics in III-nitride nanostructures and LED quantum efficiency

Wei Liu

EPFL2019

Relevant scale of structural disorder for carrier localization in YBa2Cu3O6.9

Dimensional crossover in the quantum transport behaviour of the natural topological insulator Aleksite

Ultrafast carrier dynamics in III-nitride nanostructures and LED quantum efficiency

Relevant scale of structural disorder for carrier localization in YBa2Cu3O6.9

Dimensional crossover in the quantum transport behaviour of the natural topological insulator Aleksite

Ultrafast carrier dynamics in III-nitride nanostructures and LED quantum efficiency