Synthesis and Isolation of an Acyclic Tridentate Bis(pyridine)carbodicarbene and Studies on Its Structural Implications and Reactivities

The simple synthetic development of acyclic pincer bis(pyridine)carbodicarbene is depicted herein. Presented is the first isolated structural pincer carbodicarbene with a C‐C‐C angle of 143°, larger than the monodentate framework. More importantly, theoretical analysis showed that this carbodicarbene embodies a more allene‐like character. Palladium complexes supported by this pincer ligand are active catalysts for Heck–Mizoroki and Suzuki–Miyaura coupling reactions.     

Order parameters in a-Si systems

We have used Raman scattering to characterize the change in structural order for three types of a-Si materials: fluorinated, a-Si:F:H; hydrogenated, a-Si:H; and sputtered, a-Si (sput) under heat-treatment. The narrower TO linewidth, the higher TO-phonon frequency, the lower TA-phonon frequency, and the lower “crystallization temperature”, all indicate an increase in the local order in the amorphous phase for a-SiH, and particularly for a-Si:F:H. This improved structural order is consistent with the reduction of electronic gap-states.     

Nonthermal pulsed laser annealing of Si; plasma annealing

In the accompanying paper we have given evidence that pulsed laser annealing of Si does not involve normal thermal melting and recrystallization. Here we argue the importance of the electron-hole plasma produced by the laser to the annealing process.     

Adsorption and dissociation of Si2H6 on Ge(001)2 × 1

Adsorption and thermally-induced dissociation of disilane (Si₂H₆) on clean Ge(001)2 × 1 surfaces have been investigated using a combination of Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), reflection high-energy electron diffraction (RHEED), and scanning tunneling microscopy (STM). With initial Si₂H₆ exposure at room temperature, the Si surface coverage increased monotonically, the EELS surface dangling bond peak intensities continuously decreased, and the intensity of half-order RHEED diffraction rods decreased. The low-coverage Si₂H₆ sticking probability at 300 K on Ge(001) was found to be 0.5 while the saturation coverage was 0.5 ML. A new EELS feature, GSH, involving Si-H and Ge-H bond states was observed at Si₂H₆ exposures φ 3.4 × 10¹³ cm⁻². In contrast to Si₂H₆ -saturated Si(001), the saturated Ge(001) surface significant fraction of dimerized bonds. Adsorbed overlayers were highly disordered with the primary species on saturated surfaces being SiH₂, GeH, and undissociated SiH₃· Si₂H₆-saturated Ge(001)2 × 1 substrates were annealed for l min at temperatures T_a between 425 and 825 K. Admolecules were mobile at T_a = 545 K giving rise to significant ordering in one-dimensional chains. By T_a = 605 K, essentially all of the admolecules were captured into coarsened islands. Dangling-bond EELS peaks reappeared by 625 K and the intensities of the half-order RHEED diffraction rods increased. Ge segregation to the surface, which began at T_a 625 K, occurred rapidly at T_a 675 K. All H was desorbed by 725 K.     

Longitudinal dielectric constant for quantum wells

We have calculated the longitudinal dielectric constant for a GaAs-AlGaAs quantum well structure. In particular, the total contribution to the transitions is taken as a sum of separate contributions of the regions around the Λ, X, and L symmetry points of the Brillouin zone. Since the electron- and hole-state are quantized in a well formed by potential barriers of the hetrojunction, the static dielectric constant is significantly reduced. For a well width of 2 nm and barriers formed by Al_.35Ga_.65As, the reduction is almost 20%.     

Phenomena in silicon nanostructure devices

In nanostructures, whenever the electron mean-free-path exceeds the appropriate dimensions of the device structure, quantum natures may dictate the physical properties of devices. Among many important issues, some are selected in this work, whereas others, such as the reduction of dielectric constant, the increased binding energy of dopants, etc., are discussed briefly with references for further considerations. In the past several years, resonant tunneling via nanoscale silicon particles imbedded in an oxide matrix has shown striking similarity to the so-called soft breakdown (SBD), an important current subject in devices with ultrathin oxide gates. The relevance in applying results discussed here to SBD is discussed. A Si/O superlattice, a particular form of a new type of superlattice, semiconductor-atomic superlattice (SAS), is fully discussed. This Si/O superlattice can be used in silicon quantum and light-emitting devices. A diode structure with green electroluminescence has been life-tested for more than one year without degradation. High-resolution TEM shows defect density below 10⁹/cm². Preliminary calculation shows that the Si/O complexes result in a barrier height of 0.9 eV for silicon, sufficient for an epitaxially grown SOI, which is potentially far better than the SOI using buried oxide implantation followed by high temperature anneal. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000