Principal Investigator P V Smith Project g38

Physics Department, Machine VP

University of Newcastle

Co-Investigators M W Radny, P C L Stephenson and A J Dyson

Physics Department, University of Newcastle


Chemisorption and Etching on Silicon Surfaces

The main aim of this project is to study theoretically the chemisorption of different adsorbates on silicon surfaces and the associated desorption and etching processses. Understanding these processes is very important because of the crucial role played by silicon surfaces in most present-day electronic devices. Chemisorption studies on semiconducting surfaces are essential in helping us grow thin metallic or semiconducting films on semiconducting substrates and in determining the extent to which such surfaces can be modified for specific applications. The presence of adsorbates on the surfaces of semiconductors can also significantly alter the wear characteristics of such surfaces and their susceptibility to corrosion, reaction rates and usefulness as catalytic agents. Theoretical chemisorption studies are an essential precursor to any real understanding of these important industrial processes. Understanding the etching of silicon surfaces is also of immense fundamental and technological importance. This is due primarily to the direct application of these processes in the fabrication of semiconducting devices and the manufacture of patterned silicon substrates for the very-large-scale integrated (VLSI) circuits which underlie so much of modern day electronics.


What are the basic questions addressed?

Two main topics are currently being addressed within the broad context of the above project. These are:

(i) The etching of silicon surfaces by atomic fluorine and chlorine, and the important role of hydrogen in these etching processes.

(ii) The chemisorption of boron onto silicon and its subsequent role in the adsorption of alkali metals onto semiconductor surfaces.

The specific questions we are seeking to answer are:

(i) What are the chemisorption sites and equilibrium configurations for different exposures of atomic fluorine and chlorine on the (001) and (111) surfaces of silicon?

(ii) How do the species SiFx and SiClx (1<x<4) desorb from the various Si(001) and Si(111) surfaces? That is, what are the actual mechanisms whereby etching of these surfaces occurs?

(iii) How are the etching rates dependent on temperature, exposure and the presence of hydrogen?

(iv)What are the main chemisorption sites and equilibrium structures appropriate to the adsorption of boron onto the (001) and (111) surfaces of silicon?

(v) Why is the chemisorption behaviour of boron on these surfaces so different from that of the other Group III elements?

What are the results to date and the future of the work?

(i) We have determined the equilibrium topologies for the Si(001):F and Si(001):Cl systems up to exposures of 1.0 monolayer (ML). These calculations are now being extended beyond monolayer coverage in order to study the formation and subsequent desorption of the various SiFx and SiClx (1<x<4) species on the Si(001) surface. This should enable us to delineate the actual mechanisms whereby this surface is etched by atomc fluorine and chlorine.

(ii) Reliable ab initio calculations of the minimum energy structures arising from the chemisorption of up to five fluorine or chlorine atoms onto the adatom and restatom sites of the Si(111)7x7 surface have been completed and shown to be in good agreement with our earlier semiempirical results. These calculations have also enabled accurate binding energies for the various SiClx and SiFx (1<x<4) species on the Si(111)7x7 surface to be determined.

(iii) Calculations of the equilibrium topologies and binding energies of the various SiHx complexes which result from the chemisorption of atomic hydrogen near the adatom and restatom sites of the Si(111)7x7 surface have also been performed to complement our earlier studies of the interaction of hydrogen with the (001)2x1 and (111)2x1 surfaces of silicon.

(iv) A molecular dynamics (MD) code suitable for studying chemisorption/desorption processes on semiconductor surfaces has been developed, together with a new empirical potential for silicon. These will now be employed in the performance of detailed molecular dynamics simulations of the etching of silicon surfaces by atomic fluorine and chlorine. Such studies will enable us to study these industrially important processes dynamically (that is, as a function of time), at a variety of different temperatures and exposures. Initial work will be focussed on obtaining results consistent with the above-mentioned ab initio calculations on the Si-F and Si-Cl systems. A Si-H potential has also been developed within our group for performing MD simulations of the interaction of hydrogen with silicon surfaces. Our longer term plan is to combine these two initiatives into a comprehensive study of both Si-F-H and Si-Cl-H systems to simulate the etching of silicon surfaces by atomic fluorine and chlorine in the presence of hydrogen.

(v) The equilibrium structures corresponding to the chemisorption of boron onto the Si(111)!3x!3R30o surface have been determined from ab initio Hartree-Fock-DFT calculations. These calculations complement our earlier calculations of the chemisorption of boron onto the Si(001)2x1 surface and highlight the tendency of boron atoms to chemisorb below the surface. Calculations of the chemisorption of hydrogen onto the Si(111):B surface have also been completed for comparison with the clean surface hydrogen chemisorption results. These calculations will now be extended to the alkali metals in an endeavour to elucidate the important role of boron in the adsorption of alkali metals on silicon surfaces.

What computational techniques are used and why is a supercomputer required?

Experience has shown that in order to gain an adequate understanding of these semiconductor chemisorption/desorption systems it is necessary to employ a number of different and complementary techniques. Our general approach is thus to first employ a semiempirical technique, which is quite fast computationally, to delineate the various possible minimum energy chemisorption sites. More rigorous geometry optimisation calculations will then be conducted using the ab-initio GAUSSIAN'94 or CRYSTAL'92 Hartree-Fock (HF) programs or the local density functional (LDF) program from the Fritz-Haber Institute in Berlin. These programs allow for both cluster and periodic calculations as well as the incorporation of correlation effects. Electronic structure determinations are performed using either CRYSTAL'92 or the Berlin LDF code. At present, the bulk of these calculations are being conducted on our own workstation network which includes three IBM RISC6000 machines and two DEC-ALPHA's. To obtain sufficiently accurate results to discriminate between different possible structures or reaction pathways, however, it is often necessary to employ very sophisticated basis sets (HF) or very large numbers of plane waves (LDF). Such calculations are beyond the capacity of our workstations and demand the capabilities of a supercomputer. Our approach to date has thus been to obtain the best HF geometries possible on our own workstation network, and then use these structures as the starting point for more accurate calculations on the VP2200. This has worked very successfully and has provided the final definitive results for at least three separate publications during the last six months.

Publications

An ab-initio Hartree-Fock study of the B/Si(001)2x1 adsorption system, M.W. Radny and P.V. Smith, . Vacuum 46, 419 (1995).

Semiempirical calculations of fluorine chemisorption on the Si(111)7x7 surface, P.L. Cao and P.V. Smith, , Jour.Phys.:Condensed Matter 7, 7113 (1995).

Semiempirical calculations of the chemisorption of chlorine on the Si(111)7x7 surface, P.V. Smith, and P.L Cao, Jour.Phys.:Condensed Matter 7, 7125 (1995).

A modified Stillinger-Weber potential for modelling silicon surfaces, P.C.L. Stephenson, M.W. Radny and P.V. Smith, accepted by Surface Science.

An ab initio Hartree-Fock-DFT cluster study of the F/Si(111)7x7 adsorption system, M.W. Radny, P.V. Smith, and P.L. Cao, accepted by Surface Science.

An extension of the Brenner interatomic potential to Si-C-H systems, A.J. Dyson and P.V. Smith, accepted by Surface Science.

An ab initio Hartree-Fock/Density Functional Study of the cluster simulated Si(111)7x7:Cl adsorption system, M.W Radny P.V Smith and P.L. Cao, submitted to Surface Science.

High exposure hydrogen chemisorption on the Si(111)7x7 surface: a semiempirical cluster study, J-Z Que, M.W. Radny and P.V. Smith, submitted to J. Phys.:Condensed Matter.

Hartree-Fock-DFT cluster calculations of the Si(111)7x7:H system, J-Z. Que, M.W. Radny and P.V.Smith in preparation for Surface Science.

Boron, hydrogen and silicon adatoms on the Si(111)!3!3R30o surface: an ab initio Hartree-Fock and DFT cluster study, S. Wang, M.W Radny and P.V. Smith, in preparation for Surface Science.

Ab initio Hartree-Fock and DFT cluster study of the chemisorption of hydrogen adsorption on the B/Si(111)!3x!3R30o surface, S Wang, M.W. Radny and P.V. Smith, in preparation for Surface Science.