Showing papers by "Mark S. Hybertsen published in 1998"

Interface structure between silicon and its oxide by first-principles molecular dynamics

Abstract: The requirement for increasingly thin (<50 A) insulating oxide layers in silicon-based electronic devices highlights the importance of characterizing the Si–SiO2 interface structure at the atomic scale. Such a characterization relies to a large extent on an understanding of the atomic-scale mechanisms that govern the oxidation process. The widely used Deal–Grove model invokes a two-step process in which oxygen first diffuses through the amorphous oxide network before attacking the silicon substrate, resulting in the formation of new oxide at the buried interface1. But it remains unclear how such a process can yield the observed near-perfect interface2,3,4,5,6,7,8,9,10,11,12. Here we use first-principles molecular dynamics13,14,15 to generate a model interface structure by simulating the oxidation of three silicon layers. The resulting structure reveals an unexpected excess of silicon atoms at the interface, yet shows no bonding defects. Changes in the bonding network near the interface occur during the simulation via transient exchange events wherein oxygen atoms are momentarily bonded to three silicon atoms — this mechanism enables the interface to evolve without leaving dangling bonds.

Process for fabricating an optical waveguide

Abstract: A process for fabricating a waveguide with a desired tapered profile is disclosed. The waveguide has a first section with a first height and a second section with a second height. The first height is greater than the second height. The waveguide height tapers from the first height to the second height. The waveguide is a compound semiconductor material and is formed using selective area growth. In selective area growth, a dielectric mask is formed on a substrate. The dimensions of the dielectric mask are selected to provide a waveguide with the desired dimensions. The compound semiconductor material is deposited on the substrate using chemical vapor deposition. The dielectric mask affects the rate at which the compound material is deposited in areas of the substrate proximate to the mask. Therefore, the profile of the waveguide formed using the selected mask dimensions is modeled and compared with the desired profile. If modeled profile is not acceptably similar to the desired profile, the dimensions of the mask are modified. The profile of the waveguide formed using the modified mask dimensions is again modeled, and the modeled waveguide profile is compared with the desired waveguide profile. This process is repeated until the modeled profile is sufficiently similar to the desired profile. After the mask dimensions are selected, the mask is formed on the substrate, and the compound semiconductor waveguide is formed on the substrate using selective area growth.

Advances in measurements of physical parameters of semiconductor lasers

Abstract: We present a summary of the advances in characterization techniques allowing comprehensive study of physical processes in semiconductor lasers. The studies of the electrical characteristics and optical emission below threshold allow to measure the optical gain, linewidth enhancement factor, transparency wavelength, optical loss and carrier life-time. Some other parameters, such as leakage current and wavelength chirp, can only be deduced from the above threshold measurements. Measurements of the carrier temperature and carrier heating in semiconductor lasers allow to obtain important information about the devices performance at high injection current densities. Taken together, all these measurements provide critical experimental feedback in the laser design process. They also furnish essential information to guide our understanding of the microscopic physical processes determining the laser performance and our efforts to simulate those processes.

Core-level shifts in Si(001)-SiO2 systems: The value of first-principle investigations

Abstract: A first-principle approach allows the study of relaxed structural models for surfaces and interfaces This is a powerful tool for the study of the local bonding The utility of the first-principle theory is substantially extended through the calculation of core-level shifts Such results can be used in conjunction with measured photoemission spectra to make progress in understanding the local atomic structure at interfaces We review the quantitative comparison of the calculated core level shifts with experiment for a series of molecules We then describe results for the Si(001)-SiO2 interface

Role of nonequilibrium carrier distributions in multiple quantum well InGaAsP-based lasers

Abstract: A microscopic model for the operation of multi-quantum well laser diodes is described. It includes bulk transport of carriers modeled by drift-diffusion equations, confined carriers in the quantum wells modeled by the Schroedinger equation, photon modes modeled by a Helmholtz equation and couplings described by rate equations. Application of this model shows that the carrier distribution in the active layer of the laser can not be described by quasi-equilibrium conditions. One consequence is the substantially non-uniform distribution of carriers among the quantum wells when the laser is biased above threshold. Another consequence is the observation of photoluminescence in wide area devices under short circuit conditions.

Wavelength stability, performance, and future trends for electroabsorption-modulated sources

Abstract: The guiding principles used in the design and fabrication of the Lucent Technologies electroabsorption (EA)-modulated device are discussed. They were designed to generate detailed understanding of modulator and laser quantum well design as they are linked through selective area growth (SAG) oxide pad geometry, and, using appropriate characterization techniques, ensure the equivalence of SAG and non-SAG crystal quality.

Comprehensive simulation of quantum well lasers

Abstract: We demonstrate a simulation tool which treats a full two dimensional cross section of a semiconductor laser diode with multiple quantum wells in the active region. The free carrier transport, the bound quantum well populations, the capture of carriers into the quantum wells, the gain and spontaneous emission, the transverse optical mode and the photon mode population are all treated in a fully coupled and self-consistent solution for each bias of the laser diode. The simulations are illustrated for an EMBH laser structure with seven quantum wells in the active region.

Microscopic simulation of InGaAsP diode laser performance

Abstract: We have developed a microscopic model for a semiconductor diode laser which includes the physical transport of carriers, the quantum processes associated with the quantum wells (capture and gain) and the photon modes, all self consistently as a function of the device bias. Detailed comparison to experiment for electrical and optical properties shows the reliability of the model. Simulation shows that in a conventional multi-quantum well active layer device, the carrier distribution among the wells can be quite non-uniform. We propose that this is the fundamental explanation for the dependence of high speed operation on p-doping.

Analysis of the photoreflectance spectra of GaInAsP/GaInAsP multi-quantum well structures

Abstract: In the analysis of the photoreflectance spectra of GaInAsP/GaInAsP mqw structures a transition from the barrier has been observed. Structures with different compositions, thicknesses and strains of barrier all exhibited a barrier transition at an energy close to the expected barrier energy. This is the first time that that the energy of a barrier in an mqw has been observed.

Fabrication and modeling of optical interconnects using selective area growth metalorganic chemical vapor deposition

Abstract: A computational model for selective area growth has been verified using a comprehensive suite of experimental measurements: atomic force microscopy, optical interference microscopy, microphotoluminescence, and micro-Xray diffraction The model then allows for constructive engineering of the material thickness and composition through manipulation of the oxide mask used in selective area growth This can be a fundamental input to the design of optical interconnects and integrated photonic devices