Special University Ph. D. Oral Examination

Xian Liu xianliu at stanford.edu
Mon Apr 7 14:31:47 PDT 2003

>Special University Ph. D. Oral Examination
>Arsenic-Doped Silicon by Molecular Beam Epitaxy
>Xian Liu
>Department of Materials Science and Engineering
>Stanford University
>Thursday, April 10th, 2003; 3pm
>Center for Integrated Systems Extension (CISX) Auditorium
>(Refreshments will be served at 2:45pm)
>As MOSFETs scale to the deep-submicron regime, the need for ultra-shallow 
>junctions and modulation-doped channel structures has brought an 
>increasing demand for silicon epitaxial layers with abrupt doping 
>profiles. For these devices, arsenic is an attractive N-type dopant 
>because of its high solubility and low diffusion rate, but suffers from 
>severe surface segregation during epitaxy, making high-concentration 
>incorporation with abrupt transitions difficult.
>This talk describes arsenic surface segregation and incorporation during 
>Si molecular beam epitaxy (MBE) using a unique combination of solid and 
>gas sources. Using disilane gas for silicon and dimer molecules for 
>arsenic sources, it is shown that relatively high substrate temperatures 
>are needed to activate surface reactions during growth. Surface 
>segregation of arsenic under these conditions is investigated and a new 
>segregation energy model is proposed based on surface 2-D islanding of 
>arsenic. Replacing disilane with an elemental silicon source, on the other 
>hand, eliminates surface reaction steps and enables deposition at lower 
>temperatures, where surface segregation becomes kinetically suppressed. 
>Under these conditions extremely high arsenic concentrations can be 
>achieved with relatively low surface coverage. In this work, we 
>demonstrated Si (100) epilayers with As concentrations up to 4 x 1021 cm-3 
>and doping transitions better than 3 and 2 nm/dec at the start and end of 
>arsenic-doped growth, respectively. Electrical properties of heavily doped 
>as-grown and annealed materials are discussed and correlated to 
>atomic-scale defects. While electrical properties in thicker epilayers are 
>limited by bulk values, confining dopants to a thin sheet a few nanometers 
>thick leads to significant improvements in both dopant activation and 
>carrier mobility. The former is correlated to suppressed arsenic 
>clustering and the latter to quantum confinement. Effects of doping layer 
>thickness and spacing are also discussed.

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