Number Of Lattice Periods Research Articles

Influence of surface roughness on a change in the growth mode from two-dimensional to three-dimensional for strained SiGe heterostructures

The influence of the surface microroughness on the critical thickness for the two-dimensional growth of strained SiGe structures on Si(001) and Ge(001) substrates is investigated. A decrease in the critical thickness for the two-dimensional growth of Ge films with increasing number of lattice periods or a decrease in the thickness of Si spacer layers is found for Ge/Si lattices grown on Si(001) substrates. This change is related to an increase in the surface roughness with the accumulation of elastic energy in compressed structures. A comparative study of the growth of SiGe structures on Si(001) and Ge(001) substrates shows that the critical thickness for the two-dimensional growth of tensile-strained layers is much larger than for compressed layers in a wide range of SiGe-layer compositions at an identical (in magnitude) lattice mismatch between the film and substrate.

Theoretical optimization of transverse waveguiding in oxide-confined VCSELs with internal photonic crystals

We systematically investigate transverse optical modes in oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with photonic crystal (PC) structures embedded within the cavity of the VCSEL. We consider the interplay between the effective refractive index step of the oxide and the semiconductor and the oxide aperture size. The PC lattice period a, the diameter of the holes d, and the number of lattice periods around the central defect are considered. We evaluate confinement factors for the various transverse modes and consider their spatial extent in the transverse plane which we relate to the output power. Results suggest that the power of the single-mode photonic crystal vertical-cavity surface-emitting lasers (PC-VCSELs) is limited by the smaller radius of the fundamental mode of the PC-VCSEL compared to the VCSEL with no PC. We discuss various designs aimed to optimize power into the single-fundamental mode and to discriminate from power going into other modes.

Characterization of InGaN/GaN Multiple Quantum Well Structures. Application to LEDs

InGaN/GaN Multi Quantum Well (MQW) structures were grown by MOCVD. For this study, various thicknesses of InGaN/GaN periods were investigated (from 90 to 130 Å) as well as the number of lattice periods (from 1 to 10). Transmission Electron Microscopy (TEM) revealed that the well/barrier interfaces are sharp. The (0002) High Resolution X-ray Diffraction (HRXRD) spectra show multiple satellite peaks due to interferences on well/barrier interfaces. We observed a very accurate correlation between the thickness deduced from X-ray spectra and TEM. All these observations prove that the InGaN/GaN interfaces are of high quality and well defined. The photoluminescence (300 K PL) peak intensity was shown to increase when the number of wells increases. Light Emitting Device (LED) structures have been grown following the above-mentioned model of MQW structures. Electroluminescence (EL) was performed on the devices and the results are in accordance with the material structures involved.

Simultaneous Dynamic Compensation of Stopbands and Multipurpose Location of Correction Lenses in the CERN PS Booster

Three 3rd order stopbands, amongst them the structural 3Qv = 16 (= number of lattice periods), are compensated simultaneously for the first 100 msec of the acceleration cycle. The location of the correction multipoles was optimized for all relevant resonance harmonics. The ultimate QH-QV working area still being uncertain, all stopbands up to order four as well as 3rd order crossing point were considered when choosing the locations of the new sextupoles and octupoles, eight per type and ring. A solution providing ideal locations for pairs of lenses applying to almost all harmonics was found. Experiments dealing with compensation of 3QV = 16 are described. With simultaneous dynamic correction of 2QH + QV = 14, QH + 2QV 15 and 3QV = 16, high intensity beams of a transverse density hitherto unknown in the PS Booster were obtained.