F. Bozso

Atom‐resolved surface chemistry: The early steps of Si(111)‐7×7 oxidation

The early stages of Si(111)‐7×7 oxidation using scanning tunneling microscopy and scanning tunneling and photoemission spectroscopies have been studied. It has been found that there are at least two different oxygen‐containing sites being formed. Their different surface site distributions and behavior as a function of O2‐ exposure show them to be two distinct early products. By correlating the spectroscopic results and the results of theoretical calculations one is able to identify one of these products as involving an O atom tying up an adatom dangling bond with a second O atom inserted in one of the adatom back bonds, while the other involves a single O atom inserted in an adatom back bond. The preference of these products for the faulted half of the 7×7 unit cell and for corner‐adatom sites is explained in terms of a site‐dependent sticking coefficient involving a process analogous to the gas‐phase ‘‘harpooning’’ processes. It is shown that the majority of the resulting molecular precursors involve O2 ...

The reaction of Si(100) 2×1 with NO and NH3: The role of surface dangling bonds

We present the results of a detailed study of the reactions of Si(100) 2×1 with NO and NH3. We use several electron spectroscopies and ion scattering to study the reactivity of the surface and to determine the nature of the rate limiting steps of the above reactions over a wide temperature range (90–1200 K). We find that the Si(100) 2×1 surface is quite reactive and can dissociate NO or NH3 even at 90 K. The reactions are, however, self‐limiting. The released O or H atoms tie up the surface dangling bonds and passivate the surface. The N atoms, on the other hand, occupy mostly subsurface sites. Sustained reactivity and thin film growth occurs only at temperatures sufficiently high enough to desorb the surface oxygen or hydrogen and regenerate the surface dangling bonds. We find, however, that electron‐stimulated desorption of the hydrogen can be used to nonthermally regenerate the surface dangling bonds and allow silicon nitride thin film growth even at 90 K. Because of the crucial role the top layer play...

Adsorption of boron on Si(111): Physics, chemistry, and atomic‐scale electronic devices

We have used scanning tunneling microscopy, atom‐resolved tunneling spectroscopy, and photoemission to investigate the interaction of B with Si(111). Using decaborane as the source of B, we have followed the structural and electronic modifications of the surface as a function of the annealing temperature. In the stable B/Si(111)‐ 7/8 × 7/8 surface, B occupies a novel configuration where it substitutes for a Si atom in the 3rd atomic layer directly below a Si adatom. Because of a Si‐to‐B charge transfer, the top Si adatom layer has no occupied dangling‐bond states and is insulating. As a result, the chemical properties of Si adatoms on the B/Si(111)‐ 7/8 × 7/8 surface are very different from those of the adatoms on the Si(111)‐7×7 surface. We find evidence for doping effects on chemistry that involve short‐range direct dopant‐reactive site interactions. Finally, we report on the electrical characteristics of localized defect sites on the B‐doped Si surface. We found that I–V curves over such sites may show...

A scanning tunneling microscopy study of the reaction of Si(001)‐(2×1) with NH3

We have performed a study of a semiconductor surface chemical reaction at the atomic level using a scanning tunneling microscope (STM). The ‘‘topographic’’ structure and electronic density of states are measured before and after reacting Si(001)‐(2×1) with NH3. While both clean and NH3‐dosed surfaces exhibit a (2×1) symmetry, STM images reveal changes in the spatial distribution of occupied electronic states which allow us to distinguish reacted and unreacted Si(001) dimers. Using tunneling spectroscopy, the occupied and unoccupied surface states of the clean and reacted surface are identified. The results are interpreted in terms of a Si(001)‐(2×1)H monohydride resulting from dissociative adsorption of NH3.

Electron-induced chemical vapor deposition by reactions induced in adsorbed molecular layers

We demonstrate the feasibility of electron‐induced chemical vapor deposition of thin films using low‐energy electrons to induce reactions in adsorbed molecular layers. Amorphous hydrogenated silicon, silicon dioxide, silicon oxynitride, and silicon nitride films have been deposited by establishing adsorbed Si2H6, Si2H6‐O2, Si2H6‐NO, and Si2H6‐NH3 layers at 100 K and using 300–1000 eV electron beams.

Electronic Excitation-Induced Surface Chemistry and Electron-Beam-Assisted Chemical Vapor Deposition

Selective area deposition of thin films and surface structures with precise control over their composition is possible in UHV by using low energy electron beams to induce electronic excitations in adsorbed molecular layers. Upon electron impact, adsorbed/co-adsorbed molecules decompose into reactive species, resulting in film growth. The composition of the film reflects that of the adsorbed molecular layer, which at cryogenic temperatures can sensitively be controlled by the partial pressure of the reactant gases. We present results of detailed studies of adsorption, thermal and electron-beam-induced dissociation of disilane and ammonia on silicon. We show that by proper choice of temperature, gas phase composition and electron beam, amorphous silicon, silicon nitride, oxide, silicon oxinitride films can be grown with nearly monolayer thickness resolution.

Thermal and Non-thermal Activation of Si(100) Surface Nitridation

There is currently great interest in understanding the surface chemistry and thin film growth mechanisms of technologically important semiconductors. One intriguing finding of many studies in this area is that unusually high temperatures are required for sustained reactivity and film growth. Thus, for example, silicon nitride film growth using the reaction of Si with NH3 is carried out at temperatures of l000–1300K.(1 ,2) Intuitively one would expect that the presence of surface dangling bonds should give to semiconductor surfaces a free-radical-like reactivity characterized by low activation barriers. The high process temperatures are often undesirable because they can also enhance the rates of unwanted processes such as dopant diffusion. For these reasons non-thermal means of reaction are actively being sought.(3) Essential for the success of such efforts is to understand the mechanism of the reaction, particularly the nature of the rate-limiting step.