Hydrogen Ion Beam Smoothening of Oxygen Roughened Ge(001) Surfaces
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HYDROGEN ION BEAM SMOOTHENING OF OXYGEN ROUGHENED Ge(001) SURFACES K.M. Horn, E. Chason, J.Y. Tsao, and S.T. Picraux Sandia National Laboratories, Albuquerque, NM 87185
ABSTRACT A smooth Ge(001) surface can be severely roughened by chemical etching with oxygen gas and then returned to its original smoothness by exposure to hydrogen. The rate of recovery of the surface depends strongly on temperature, as well as on whether the hydrogen is introduced as a low energy ion beam or simply as a gas. The roughness induced in the surface by etching is "locked-in" after removal of the oxygen gas through pinning of ledges by residual contamination. This pinning prevents the surface from smoothening thermally. The introduction of hydrogen to the surface promotes a chemical reaction which frees the pinned sites and allows thermal smoothening to proceed. INTRODUCTION The presence of hydrogen has long been thought beneficial to molecular beam epitaxy (MBE) of GaAs, as well as to other processes such as the plasma deposition of diamond-like films. Calawa has reported that the dominant effect of the introduction of hydrogen during MBE growth of GaAs is to reduce the incorporation of oxygen and carbon in the epitaxial layer [1]. Bozack, et.al. have found that passivating Si(110) dangling bonds with hydrogen before exposing to propylene prevents its adsorption [2], while introducing hydrogen after the Si has been exposed to propylene increases the silicon-propylene chemical reaction [3]. It is therefore not unreasonable to expect that the application of hydrogen ion beams to surfaces can induce chemical reactions that result in a change of the surface morphology. In this work we have studied an oxygen-hydrogen surface chemical reaction which leads to smoothening of a surface which has been severely roughened by oxygen etching. The use of a low energy hydrogen beam, and even hydrogen gas, to promote this reaction results in the creation of a surface suitable for epitaxial growth, without the need of a high temperature anneal and further growth. EXPERIMENT The measurements were per ormed in an MBE growth chamber with a base pressure of lxlO- Torr. Controlled amounts of oxygen can be admitted to the chamber through a leak valve and monitored with both an ion gauge and a residual gas analyzer. Hydrogen is admitted to the chamber via a Pd leak valve. Through use of a standard electron-impact-ionization sputter gun, hydrogen ion beams of 100 to 1000 eV can be directed onto
Mat. Res. Soc. Symp. Proc. Vol. 131. c1989 Materials Research Society
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the sample surface. The Ge(001)±l° substrate used in these pad polishes in measurements was prepared by sequential situ surface The initial in and methanol. Br:methanol preparation consisted of two repetitions of a 750 C anneal for 20 minutes followed by growth of a 2000 angstrom buffer layer at 500 C. Our principle diagnostic tool is reflection high-energy electron diffraction (RHEED). Using a 15 keV electron beam, incident at a glancing angle, diffraction patterns are observed on a phosphor
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