Daniel S. Marshall

Photoemission from the Sr/Si(001) interface

The growth of Sr on n-type Si(001) was studied in detail for coverages between 0 and 1 monolayer (ML) using core level photoemission spectroscopy. In a similar manner, the Sr saturation coverage was studied in the 600–925 °C temperature range. Data analysis was carried out by a method that allows accurate determination of the band-bending shifts. Using this method it is possible to pinpoint the formation and destruction of chemical species from bungled core level photoemission data without needing to know details of the chemical composition of the spectra. Through this analysis it was established that the interaction between Sr and Si breaks down the binding energy difference between upward and downward Si dimer atoms. In addition, it was found that the saturation coverage exhibits a clear plateau at 1 ML around 650 °C, and a slope change at 1/3 ML around 850 °C. The surface band bending suffers a discontinuous increase as the Sr coverage surpasses 12 ML and as the low energy electron diffraction symmetry...

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.

Epitaxial Oxide Films on Silicon: Growth, Modeling and Device Properties

Using molecular beam epitaxy, thin films perovskite-type oxide Sr x Ba 1−x TiO 3 (0≤×≤1) has been grown epitaxially on Si(001) substrates. Reflection high energy electron diffraction measurements and X-ray diffraction analysis indicate that high quality heteroepitaxy on Si takes place with Sr x Ba 1−x TiO 3 (001)//Si(001) and Sr x Ba 1−x TiO 3 [010]//Si[10]. Extensive atomic simulations have been carried out to understand the initial growth mechanism of the oxide layers on silicon. SrTiO 3 layers grown directly on Si have been used as the gate dielectric for the fabrication of MOSFET devices. By varying the growth conditions the thickness of the amorphous interfacial silicon oxide layer formed during the growth of the oxide layers has been engineered to minimize the device short channel effects. An effective oxide thickness 3 dielectric film with interface state density around 6.4 × 10 10 cm −2 e V-1 , and the inversion layer carrier mobilities of 220 cm 2 V −1 s −1 and 62 cm 2 V −l s −1 for NMOS and PMOS devices, respectively. The gate leakage in these devices is 2 orders of magnitude smaller than a comparable SiO 2 gate dielectric MOSFET.

Epitaxial BaTiO3 films on silicon for MFSFET applications

Abstract Recently, we have grown epitaxial BaTiO3 films directly on a Si (001) substrate using molecular beam epitaxy (MBE). The films have been characterized both physically (RHEED, XRD, AFM, SE) and electrically (CV & IV). The films show streaky RHEED patterns and sharp X-ray diffraction patterns, indicating epitaxial BTO growth. The leakage current is below 1E-8 A/cm2 at 2V. The CV results show good interface properties. The films exhibit a 0.5V hysteresis in the CV curves.

Barium adsorption on Si(100)-(2×1) at room temperature: a bi-polar scanning tunneling microscopy study

Abstract The initial stages of barium adsorption on Si(100)-(2×1) at room temperature has been studied by ultrahigh vacuum scanning tunneling microscopy (STM) under both positive and negative sample-bias imaging conditions. Two distinct adsorption sites have been identified by the high-resolution STM images. It was found that, with the substrate held at room temperature, barium atoms can occupy both valley-bridge sites in the trough between silicon dimers and silicon-vacancy sites. It is possible to image the barium atoms at valley-bridge sites with both negative and positive sample bias, revealing filled and empty surface states, respectively. For barium atoms adsorbed at vacancy sites, however, it is only possible to obtain filled-state images, and imaging with positive sample bias will induce the removal of the atom, possibly transferred to the tip, revealing a missing silicon dimer below.

Progress in Epitaxial Oxides on Semiconductors

In this paper, we review the recent progress in the area of epitaxial oxides on semiconductors at Motorola Labs. Critical issues such as surface preparation, initial nucleation and growth behaviors of SrTiO 3 (STO) thin film epitaxy on Si(001) are addressed. Using a systematic approach, high-quality epitaxial STO films are successfully grown on semiconductor substrates such as Si, silicon-on-insulator (SOI) and Ge. Amorphous interfacial layer between the epitaxial STO and the semiconductor can be eliminated or tailored by controlling oxide growth process and parameters. STO-based metal-oxide-semiconductor (MOS) capacitors and transistors are fabricated and tested, in order to explore the potential of STO as high-k gate dielectrics for future generation CMOS transistor technology. In addition, high-quality STO epitaxial films are utilized as thin buffer layers for fabricating integrated oxide heterostructures on semiconductors. Various perovskite oxide films such as SrZrO 3 , LaAlO 3 and Pb(Zr,Ti)O 3 are deposited epitaxially on STO-buffered Si(001) for potential high-k gate dielectrics and surface-acoustic-wave (SAW) device applications.

Development of integrated heterostructures on silicon by MBE

The semiconductor industry is facing the challenge of scaling of the gate dielectric of Si CMOS devices, which are continually being made smaller. Presently SiO/sub 2/ is being used, but at thickness below 20/spl Aring/, it suffers from high tunneling leakage current and reliability problems. Alternative high-k materials to replace SiO/sub 2/ need to be developed as soon as possible. The alkaline earth oxides such as barium strontium titanate (Ba/sub x/Sr/sub 1-x/TiO/sub 3/) have a substantially higher dielectric constant and are ideal candidates for gate dielectrics. Because of the higher dielectric constant a physically thicker layer can yield an equivalent oxide thickness of <20/spl Aring/, thereby eliminating the leakage problems experienced with ultra-thin SiO/sub 2/. These oxides also exhibit ferroelectric behavior and their use as the gate dielectric on Si can be exploited in the realization of a single transistor memory element. These types of oxides also have a number of functionalities which when combined with other types of semiconductors will enable the development of novel device applications. Molecular beam epitaxy can be used for the deposition of oxide based epitaxial layers both for Si device applications and integration of GaAs devices with silicon. The potential for increased functionality and integration of devices based on III-V semiconductors, crystalline oxides and silicon make this an attractive and promising technology.

Defect Formation in Boron Carbide-An ab-initio Electronic Structure Study

The electronic structure of a crystalline boron carbide has an energy forbidden gap of ∼ 3 eV and is hence a good insulator. But, on the other hand, the electrical conductivity of boron carbide is measurable. It is therefore believed that the defects formation in boron carbide is responsible for its electrical conductivity and a theory of hopping conduction of bipolaron through localized defects were developed, accordingly. Although the bipolaron electrical conductivity model does not rely on any specific type of defect, the bipolaron formation in boron carbide is believed to be a defective CBB intraicosahedral chain in connection with an B 11 C icosahedron. The current study examined the existing theory of bipolaron electrical conductivity by performing a systematical study on the formation energies of the defects in boron carbide using a state-of-the-art ab-initio electronic structure method. The studied defects cover a) stoichiometric variations of carbon concentration, b) missing boron atoms, and c) distribution of carbon atoms in the materials. It is found that the ground state of a fully carbonated boron carbide consists of B 11 C icosahedra connected by CBC intraicosahedral chains, i.e. consistent with the reported structural model of B 4 C . When carbon concentration is reduced, however, the population of CBC chain is found to be intact, while the population of B 11 C icosahedron is reduced by the replacements of B 12 icosahedron. This observation is fundamentally different from the existing model of boron-rich boron carbide. The localized states associated with missing boron atoms are identified and the electrical conductivity through these localized defects states is studied.