SeGi Yu

Transfer matrix method for interface optical-phonon modes in multiple-interface heterostructure systems

Interactions of carriers with interface optical phonons dominate over other carrier–phonon scatterings in narrow quantum-well structures. Herein, a transfer matrix method is used to establish a formalism for determining the dispersion relations, electrostatic potentials, and Frohlich interaction Hamiltonians of the interface optical phonons for multiple-interface heterostructure systems within the framework of the macroscopic dielectric continuum model. This method facilitates systematic calculations for complex structures where the conventional method is very difficult to implement. Several specific cases are treated to illustrate the advantages of the general formalism.

QUANTIZED ACOUSTIC PHONON MODES IN QUANTUM WIRES AND QUANTUM DOTS

Acoustic phonon modes in isotropic cubic media are derived for a number of quantum‐wire and quantum‐dot geometries of significant interest in nanoelectronics and optoelectronics. In each case, the mode amplitude is determined by requiring that the mode energy be given by that of the properly quantized phonon. For the case of cylindrical quantum wires and quantum dots with rectangular faces, the Hamiltonians for the deformation potential interactions are derived. These quantized acoustic modes and the associated deformation potential Hamiltonians provide a basis for modeling carrier‐acoustic‐phonon interactions in a variety of mesoscopic devices. Our new results supplement previous treatments of related piezoelectric effects in cylindrical quantum wires.

Phonon assisted intersubband transitions in step quantum well structures

We evaluate effects of heterointerfaces on optical phonon modes and phonon assisted electron intersubband transition rates in step quantum well structures for intersubband lasers. Various phonon modes and electron–phonon interaction Hamiltonians, including the interface modes, confined longitudinal-optical modes, and half space modes in the quantum well structures are calculated based on the macroscopic dielectric continuum model and microscopic analysis. The transfer matrix method is used to calculate the interface modes. The intersubband transition rates due to electron–phonon scattering by these phonon modes are evaluated using Fermi’s golden rule, with the electron wave functions obtained by solving the Schrodinger equation for the heterostructures under investigation. Our results show that, compared with the transition rates in the same structures calculated using the bulk phonon modes and the bulk Frohlich interaction Hamiltonian, the electron interface–phonon interactions give significantly larger ...

Acoustic phonon quantization in buried waveguides and resonators

Starting from a classical Hamiltonian for nonhomogeneous elastic media, a procedure is developed for acoustic phonon quantization in resonators as well as linear and planar waveguides. The formalism is illustrated in an example of acoustic phonon modes in a buried cylindrical waveguide. The deformation potential Hamiltonian for electron - acoustic phonon interaction is also obtained.

CONFINED PHONONS AND PHONON-MODE PROPERTIES OF III-V NITRIDES WITH WURTZITE CRYSTAL STRUCTURE

Abstract Stimulated by the recent interest in the use of nitride-based III–V wurtzite structures for optoelectronic and electronic devices, this paper reports on the application of the Loudon model for uniaxial crystals to derive the Frohlich interaction Hamiltonian as well as the electron–optical-phonon scattering rate in wurtzite crystals. This paper also presents experimental analyses of the mode behavior of phonons in wurtzite crystals.

Electron interaction with confined acoustic phonons in cylindrical quantum wires via deformation potential

The effects of phonon confinement on electron–acoustic‐phonon scattering is studied in cylindrical semiconductor quantum wires. In the macroscopic elastic continuum model, the confined‐phonon dispersion relations are obtained for several crystallographic directions with the two cardinal boundary conditions: free‐surface and clamped‐surface boundary conditions. The scattering rates due to the deformation potential interaction are obtained for these confined phonons and are compared with those of bulk‐like phonons for a number of quantum wire materials. The results show that the inclusion of acoustic phonon confinement effects may be crucial for calculating accurate low‐energy electron scattering rates in nanostructures. It is also demonstrated that the scattering rates may be significantly influenced by the direction of phonon propagation, especially for low‐energy electrons. Furthermore, it has been found that there is a scaling rule governing the directional dependence of the scattering rates: the direct...

Tailoring of optical phonon modes in nanoscale semiconductor structures: role of interface-optical phonons in quantum-well lasers

This paper discusses the concept of enhancing semiconductor laser performance through tailoring of scattering rates of confined polar-optical phonons. Studies of optically pumped intersubband scattering in coupled quantum-well lasers have demonstrated that interface-phonon-assisted transitions are important in such structures; furthermore, simple analytical expressions have been derived that indicate the importance of interface-phonon scattering in quantum-well lasers. These calculations reveal that the interface-phonon-assisted transitions are dominant for small quantum well dimensions of approximately 40 As ; such dimensions are typical of novel lasers including both the unipolar quantum cascade laser and the tunneling injection laser. Recent numerical calculations have confirmed these e⁄ects and have extended them to indicate how confined and interface phonons also a⁄ect critical laser properties such as optical gain. The application of confined phonon e⁄ects to intersubband lasers is one of the most important applications of confined-phonon physics to the present time. ( 1999 Elsevier Science B.V. All rights reserved.

Enhanced electroluminescence performances by controlling the position of carbon nanotubes

Alternating current driven powder electroluminescence (EL) devices with single-walled carbon nanotubes (SWCNTs) were fabricated to utilize the field enhancement by controlling the position of SWCNTs. The SWCNT conditions, which could lead to increased EL performance, were optimized by examining the characteristics of EL devices after controlling the length of the SWCNTs, where the SWCNT layer was positioned at the interface between the dielectric and emitting layers. The EL device with a short SWCNT layer for a crushing time of 20 min exhibited the highest EL performance of the samples examined. In addition, SWCNTs with the optimized SWCNT condition were inserted at various interfaces to determine the effective position of the SWCNT layer in the EL device. The highest EL performance, such as a 51% and 65% increase in luminance and efficiency, respectively, was achieved by inserting SWCNTs at the interface between the dielectric and emitting layers together with SWCNTs between the emitting layer and bottom...

Acoustic phonons in rectangular quantum wires: approximate compressional modes and the corresponding deformation potential interactions

The Hamiltonian describing the deformation potential interaction of confined acoustic phonons with carriers is derived by quantizing the appropriate, experimentally verified approximate compressional acoustic phonon modes in a rectangular quantum wire. The scattering rate due to the deformation potential interaction is calculated for a range of quantum wire dimensions.

Impedanc analysis of the organic light emitting displays containing carbon nanotubes

Carbon nanotubes (CNTs) have been widely utilized in the organic light emitting displays (OLEDs). Polymer-based OLEDs with CNT inclusion with the emitting layer, especially for alternating current (AC) driving onews, exhibited higher luminance than OLEDs without CNTs. But the current level of AC OLEDs with CNTs was not clearly reported in other publications suggesting poor current characteristics. This might be caused by the unwanted bypass current paths through CNT networking within the devices. Thus, controlling the properties of CNTs mechanically or chemically is required for high efficiency devices. Here, CNTs were mixed with an emitting layer to be a CNT-polymer composite by varying the concentration of the CNTs. The luminance of the CNT-OLEDs was higher than the reference OLEDs without CNTs. The current could be roughly 30-40% reduced. One of the primary reasons for this high efficiency is considered to be the micro-capacitor effect caused by CNT networking. This CNT effect was examined by the impedance analysis, which could also explain low current mechanisms. Due to AC driving, the capacitive reactance could contribute to the device. Consequently, The CNTs incorporation within OLEDs would improve the efficiency of OLED.