Surface Chemical Analysis Using Reflection Electron Energy Loss Spectrometry During

Molecular Beam Epitaxy

Author: Wong, Selmer Siu Man

Year: 1996

Degree: Dissertation (Ph.D.)

Advisor: Atwater, Harry Albert

Committee Members: Atwater, Harry Albert; Goodstein, David L.; Scherer, Axel; Tombrello, Thomas A.

Option: Physics

DOI: 10.7907/7817-ae48

Abstract

This thesis demonstrates for the first time the versatility of a recently developed surface chemical analysis technique, reflection electron energy loss spectrometry (REELS) of RHEED electrons, for gleaning quantitative surface structural and compositional information during molecular beam epitaxial (MBE) growth. In REELS, a component of the scattered RHEED beam is allowed to pass through a small aperture in the RHEED screen and is energy analyzed. The energy losses of the incident electrons are characteristic of inner-shell core electron ionizations of the target material.

We have employed REELS to determine the chemical composition of GexSi1-x alloys, by using empirical cross sections and by comparing the signal intensities of the Ge L2,3 and Si L2,3 core loss intensities. Compositional analysis by REELS was found to agree well with compositional determinations made by Rutherford backscattering spectrometry.

Fabrication of device-quality epitaxial semiconductors with no defects requires that the starting substrate be atomically clean. Thus it is desirable to directly determine in situ that a substrate surface is clean on the atomic level prior to epitaxial growth. We have performed REELS measurements of the carbon K edge on HF /H2O-dipped substrates prebaked at 200°C for one hour and found that physisorbed hydrocarbons desorb from the surface during this prebake, rendering the surface clean and ready for growth. We have also performed the first time- dependent measurements on hydrocarbon desorption from the Si(100) surface during a low-temperature substrate cleaning.

The ability to determine local order structural and chemical information is useful in learning about the evolution of thin film microstructure. The REELS spectrum contains extended-edge energy loss fine structure (EXELFS) which can provide local order information in condensed matter. We have performed EXELFS analysis on thin monolayers of Sn adsorbed on hydrogen-terminated Si(100) and discovered a (3 x 1) Sn-induced reconstruction of the Si surface. REELS EXELFS analysis of dilute Sn Ge alloys has provided insight on the local surface structure of this material, which, combined with long-range structure information, demonstrate the ability for surface structure determination of reconstructions and adsorbed films.

Lastly, we have successfully designed and demonstrated the viability of a parallel-detection REELS spectrometer, which is capable of very high data rate, high dynamic range, superior sensitivity, and resistance to mechanical and electromagnetic noise, while affording data acquisition times 50 times to several thousand times smaller than has previously been achieved with serial-detection REELS. Parallel REELS has great potential to be a novel diagnostic tool for real-time monitoring of complex dynamic processes in the MBE environment.

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