M.P. Ray

A hyperthermal energy ion beamline for probing hot electron chemistry at surfaces

An ultrahigh vacuum ion beamline and chamber have been assembled to produce hyperthermal (<400 eV) energy ions for studying hot electron chemistry at surfaces. The specific design requirements for this modified instrument were chosen to enable the exposure of a metal-oxide-semiconductor (MOS) device to monoenergtic, well-collimated beams of alkali ions while monitoring both the scattered beam flux and the device characteristics. Our goal is to explore the role that hot electrons injected toward the MOS device surface play in the neutralization of scattered ions. To illustrate the functionality of our system, we present energy-resolved spectra for Na+, K+, and Cs+ ions scattered from the surface of a Ag(001) single crystal for a range of incident energies. In addition, we show MOS device current-voltage characteristics measured in situ in a new rapid-turnaround load lock and sample translation stage.

Towards hot electron mediated charge exchange in hyperthermal energy ion–surface interactions

We have made Na (+) and He (+) ions incident on the surface of solid state tunnel junctions and measured the energy loss due to atomic displacement and electronic excitations. Each tunnel junction consists of an ultrathin film metal-oxide-semiconductor device which can be biased to create a band of hot electrons useful for driving chemical reactions at surfaces. Using the binary collision approximation and a nonadiabatic model that takes into account the time-varying nature of the ion-surface interaction, the energy loss of the ions is reproduced. The energy loss for Na (+) ions incident on the devices shows that the primary energy loss mechanism is the atomic displacement of Au atoms in the thin film of the metal-oxide-semiconductor device. We propose that neutral particle detection of the scattered flux from a biased device could be a route to hot electron mediated charge exchange.

Compact deposition system for device-based ultrathin crystalline film growth

The study of hot electron excitation at surfaces requires the deposition of ultrathin metal films. To probe the role of particle bombardment in such film excitations, homogeneous, atomically ordered, and relatively defect free thin films must be deposited in the same ultrahigh vacuum system where they will be studied. With these constraints in mind, the authors designed a compact deposition chamber that allows for in situ growth and analysis of metal layers, which are only a few monolayers. This deposition chamber is attached to the commercial variable temperature scanning tunneling microscope (STM) and has an internal volume of 500cm3. The target substrates for deposition are compatible with the STM design and are held in place in a specially designed clamping slot that enables low temperature growth. They used the custom built chamber to deposit Ag top layers with thicknesses between 8 and 15nm on Si(100). Electronic and morphological characteristics of the prototype Ag∕n-Si(100) devices are presented.

Vacancy island creation and coalescence using automated scanning tunneling microscopy.

We demonstrate that scanning tunneling microscope tip-surface crash events can be utilized as an efficient means for the creation of predefined island configurations for diffusion studies. Using this method, islands of varying size can be created and placed in close proximity, increasing the probability of initiating and observing coalescence events. Data obtained from crash initiated events on a Ag(111) surface are presented. Relaxation time exponents extracted from these data confirm that our method gives results consistent with previous, sputter-obtained island coalescence studies. We also describe an instrument-control routine developed for these measurements that utilizes commercial imaging and off-the-shelf automation software to automate the tracking of islands or other features by the microscope.