J. H. G. Owen

Self-assembled nanowires on semiconductor surfaces

A number of different families of nanowires which self-assemble on semiconductor surfaces have been identified in recent years. They are particularly interesting from the standpoint of nanoelectronics, which seeks non-lithographic ways of creating interconnects at the nm scale (though possibly for carrying signal rather than current), as well as from the standpoint of traditional materials science and surface science. We survey these families and consider their physical and electronic structure, as well as their formation and reactivity. Particular attention is paid to rare earth nanowires and the Bi nanoline, both of which self-assemble on Si(001).

Identification of the Si(001) missing dimer defect structure by low bias voltage STM and LDA modelling

Abstract A JEOL scanning tunneling microscope (STM) has been used to image the clean silicon (001) surface at low sample bias voltages (around −0.4 V). At this bias, many dimer vacancies are highlighted by a bright feature on the neighbouring dimers. On other defects, this situation is reversed; the area around the defect becomes dark at low voltages. In both cases, at higher bias voltages (around −0.8 V), this contrast disappears. For a number of proposed structures of the single dimer vacancy, ab initio calculations of charge density as a function of energy have been used to simulate STM images. These images show significant bias voltage dependence, and the low bias voltage images differ markedly between the structures modelled. On this basis, we identify the rebonded structure with the bright defect, and the non-rebonded structure with the dark defect.

Gas-source growth of group IV semiconductors: I. Si(001) nucleation mechanisms

The initial stages of gas-source growth of Si(001) using disilane have been investigated using a combination of elevated-temperature STM and atomistic modelling. The reaction pathway from the initial adsorption of disilane fragments up to the nucleation of short strings of epitaxial dimers is discussed. By the use of our STM to study disilane at the temperatures of interest, and atomistic modelling to calculate structural stability and significant activation barriers, we are able to propose a complete description of the mechanisms which underlie gas-source growth.

In situ observation of gas-source molecular beam epitaxy of silicon and germanium on Si (001)

We have observed the development of the surfaces during gas-source growth of silicon and germanium in an elevated temperature ultrahigh vacuum scanning tunneling microscopy (STM), with near-atomic resolution under a range of temperature and flux, which are the two dominant parameters, and applied atomistic modeling to the structures seen by STM to enable us to give confident interpretation of the results. A key role in the growth of silicon and germanium on Si(001) from disilane and germane, respectively, is played by the surface hydrogen. The growth of germanium follows a similar path to that of silicon for the first few monolayers, after which the strain becomes relieved by periodic trenches, and eventually by a combination of faceted pits and clusters, both of which nucleate heterogeneously at surface defects. Understanding these processes is crucial to controlling the self-assembled Ge/Si quantum structures.

Gas-source growth of group IV semiconductors: II. Growth regimes and the effect of hydrogen

Abstract The crucial difference between gas-source molecular beam epitaxy (MBE) and conventional MBE is the presence of hydrogen on the growth surface. The amount and behaviour of the hydrogen are controlled by a combination of temperature and disilane flux. In situ observations under growth conditions are essential for an accurate understanding of non-equilibrium growth phenomena such as nucleation and coarsening, because once the flux has been cut off the surface material will redistribute itself. We have found that not only does surface hydrogen block silicon diffusion, but also hydrogen saturation of the substrate step edges blocks step-flow growth so that island growth predominates below 700 K, even at low fluxes. The denuded zones seen in MBE are not observed. Above 700 K, the adsorption barrier at step edges is overcome, and a transition from island growth to step-flow growth is observed as the flux is varied.

A proposed structure of the nucleus for gas-source epitaxial growth of silicon

Abstract A novel structure has been observed by scanning tunneling microscopy (STM) on the Si(001) surface after exposure to disilane between 400 and 600 K. The feature is a bright square, with dark lines running across it forming a cross. The proposed structure, a ring of four silicon atoms bonded together and connected by one back-bond per atom to the underlying silicon dimers, has been modelled using tight-binding and density functional theory (DFT) calculations. This ring has been found to be energetically stable with respect to isolated ad-dimers. As it is the first feature to form from disilane fragments with increasing temperature, and its local bonding configuration is very similar to the rebonded B-type step edge which is known to be the favoured adsorption site for epitaxial growth, it may play a crucial role as the nucleus of the new epitaxial layer during gas-source growth of silicon.

Short range and long range strain fields of Bi nanoline

Abstract The practical realisation of nanoscale devices requires the development of practical nanofabrication techniques. The Bi nanolines which self-assemble on Si(001) are promising templates for atomic scale wires, and also have a fascinating subsurface reconstruction. Elastic interactions are often responsible for the limited dimensionality of epitaxial nanoscale structures. The present work examines the elastic strain field around the Bi nanoline in terms of its interaction with other surface features. A short range tensile strain around the Bi nanoline may be identified by the effect on the electronic structure of the neighbouring dimers. A longer range elastic interaction is exhibited in a repulsive interaction between the nanoline and defects and steps. Atomic resolution variable bias scanning tunnelling microscopy (STM) and first principles electronic structure calculations have been used to elucidate these effects, and excellent agreement has been found between experimental observations and theoretical results.

Elevated-Temperature STM Study of Ge and Si Growth on Si(001) From GeH4 and Si2H6

In this study we have investigated the mechanism of Si and Ge growth from gaseous precursors, at intermediate temperatures, in elevated-temperature scanning tunnelling microscope (STM). Based on our observations, we suggest a plausible mechanism for the growth of Ge and Si from GeH4 and Si2H6.