Jerald A. Hallmark

The (3×2) phase of Ba adsorption on Si(001)-2×1

Abstract The initial stages and surface structures of the (3×2) phase of Ba adsorption on an Si(100)-2×1 surface held at 900°C have been studied by low-energy electron diffraction, Auger electron spectroscopy, and ultra-high vacuum scanning tunneling microscopy (STM). At low coverages ( 1 6 ML) , the Ba atoms form atomic chains across the Si dimer rows by occupying valley bridge sites, as well as on fourfold sites by replacing Si dimers and exhibiting a local (3×2) phase, with the 3×-phase along the Si dimer row direction. Two different configurations for the (3×2) phase, namely, mono- and dimer-Ba models, are proposed based on the STM images. Below a coverage of 1 6 ML , the (3×2) phase is formed by single Ba atoms at fourfold sites by replacing original Si dimers. For a Ba coverage of 1 3 ML, the (3×2) phase is formed by buckled Ba dimers, as revealed by high-resolution STM images.

Anisotype-gate self-aligned p-channel heterostructure field-effect transistors

A new self-aligned p-channel HFET structure was evaluated for application to complementary HFET (CHFET) circuits. The AlGaAs/InGaAs HFET structure uses an anisotype graded N+ InGaAs/GaAs semiconductor gate to enhance the barrier height of the FET, resulting in a significant reduction in gate leakage current at low voltages. The anisotype PFET also uses a non-alloyed graded InGaAs/GaAs ohmic contacts that are stable to temperature as high as 550 degrees C. With AlGaAs composition of x=0.3, and a thin AlAs spacer of 60AA, leakage current was reduced by a factor of about 1000 at gate voltage of 1 V, when compared to AlGaAs/InGaAs HIGFET of aluminum content x=0.75. The anisotype PFET maintains high device transconductance, typically 50 mS/mm for 1.3 x 10 mu m PFETs, high reverse breakdown voltages, 9-10 V, and low capacitance. Microwave S-parameter characterization resulted in F/sub t/ of 5 GHz for a 1*50 mu m PFET. >