Yi Wei

Two-dimensional growth of high-quality strontium titanate thin films on Si

Most semiconductor materials such as Si, Ge, and GaAs are subject to oxidation when exposed to oxidants. This results in difficulties in the heterointegration of epitaxial oxides on these semiconductors. Even though certain oxides may be thermodynamically stable when placed in contact with semiconductors, direct epitaxy of these oxides encounters kinetic difficulties due to the loss of epitaxy caused by the formation of an amorphous oxide at the interface. In this article, we address some important issues on the heteroepitaxy of oxides on semiconductors and show a stepped growth method that utilizes the kinetic characteristics of the growth process to suppress the oxidation of the substrate surface and thereby achieve oxide films with a high degree of crystallinity. The epitaxy of high-quality SrTiO3 (STO) thin films directly on Si was achieved. The chemical and structural properties of the STO/Si interface were evaluated in situ using reflection high-energy electron diffraction, x-ray photoelectron spect...

Mechanism of cleaning Si(100) surface using Sr or SrO for the growth of crystalline SrTiO3 films

A method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as-received Si(100) wafer in an ultrahigh vacuum molecular-beam epitaxy system. The substrate is then heated to ∼800u200a°C for about 5 min, the SiO2 is removed to leave behind a Sr- or SrO-terminated ordered Si(100) surface. This Sr- or SrO-terminated Si(100) surface is well suited for the growth of crystalline high-k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle were mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalytic reaction SiO2+Sr(oru2009SrO)→SiO(g)+O+Sr(oru2009SrO). The reaction SiO2+Si→2SiO(g) at the SiO2/Si interface is limited and the pit formation is suppressed. The main roles that Sr or SrO play during the oxide removal process are catalysts promoting SiO formation and passivating the newly exposed Si surface, preventing further etching and the formation of pits in the substrate.A method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as-received Si(100) wafer in an ultrahigh vacuum molecular-beam epitaxy system. The substrate is then heated to ∼800u200a°C for about 5 min, the SiO2 is removed to leave behind a Sr- or SrO-terminated ordered Si(100) surface. This Sr- or SrO-terminated Si(100) surface is well suited for the growth of crystalline high-k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle were mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalytic reaction SiO2+Sr(oru2009SrO)→SiO(g)+O+Sr(oru2009SrO). The reaction SiO2+Si→2SiO(g) at the SiO2/Si interface is limited and the pit formation is suppresse...

Optoelectronic and microstructure attributes of epitaxial SrTiO3 on Si

We have investigated the optoelectronic characteristics of bulk single-crystal SrTiO3 (STO) and epitaxial STO on Si by photoluminescence and cathodoluminescence (CL) techniques. In particular, we have explored to what extent these techniques can offer information about crystal quality. We have complemented these observations with atomic force microscopy, transmission electron microscopy (TEM), and micro-Raman measurements. Panchromatic CL imaging of bulk STO revealed contrast features associated with growth-related striations, extended defects, and mechanical damage. CL imaging of undoped high-resistivity substrates was limited by beam charging effects. The weak nature of the CL signal from epitaxial STO (relative to bulk material) made it very difficult to visualize any features by analog detection. On the other hand, spectrally resolved CL measurements of epitaxial STO using single-photon counting techniques, revealed sensitivity to the defect content and film quality across a 3‐in wafer. Preliminary re...

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.