Electric field dependent structural and vibrational properties of the Si(100)-H(2 \times 1) surface and its implications for STM induced hydrogen desorption

We report a first principles study of the structure and the vibrational properties of the Si(100)-H(2 \times 1) surface in an electric field. The calculated vibrational parameters are used to model the vibrational modes in the presence of the electric field corresponding to a realistic STM tip-surface geometry. We find that local one-phonon excitations have short lifetimes (10 ps at room temperature) due to incoherent lateral diffusion, while diffusion of local multi-phonon excitations are suppressed due to anharmonic frequency shifts and have much longer lifetimes (10 ns at room temperature). We calculate the implications for current induced desorption of H using a recently developed first principles model of electron inelastic scattering. The calculations show that inelastic scattering events with energy transfer $n \hbar \omega$, where n>1, play an important role in the desorption process.Comment: 10 pages, RevTeX, epsf files. submitted to surface scienc

Construction of transferable spherically-averaged electron potentials

A new scheme for constructing approximate effective electron potentials within density-functional theory is proposed. The scheme consists of calculating the effective potential for a series of reference systems, and then using these potentials to construct the potential of a general system. To make contact to the reference system the neutral-sphere radius of each atom is used. The scheme can simplify calculations with partial wave methods in the atomic-sphere or muffin-tin approximation, since potential parameters can be precalculated and then for a general system obtained through simple interpolation formulas. We have applied the scheme to construct electron potentials of phonons, surfaces, and different crystal structures of silicon and aluminum atoms, and found excellent agreement with the self-consistent effective potential. By using an approximate total electron density obtained from a superposition of atom-based densities, the energy zero of the corresponding effective potential can be found and the energy shifts in the mean potential between inequivalent atoms can therefore be directly estimated. This approach is shown to work well for surfaces and phonons of silicon.Comment: 8 pages (3 uuencoded Postscript figures appended), LaTeX, CAMP-090594-

Temperature suppression of STM-induced desorption of hydrogen on Si(100) surfaces

The temperature dependence of hydrogen (H) desorption from Si(100) H-terminated surfaces by a scanning tunneling microscope (STM) is reported for negative sample bias. It is found that the STM induced H desorption rate ($R$) decreases several orders of magnitude when the substrate temperature is increased from 300 K to 610 K. This is most noticeable at a bias voltage of -7 V where $R$ decreases by a factor of ~200 for a temperature change of 80 K, whilst it only decreases by a factor of ~3 at -5 V upon the same temperature change. The experimental data can be explained by desorption due to vibrational heating by inelastic scattering via a hole resonance. This theory predicts a weak suppression of desorption with increasing temperature due to a decreasing vibrational lifetime, and a strong bias dependent suppression due to a temperature dependent lifetime of the hole resonance.Comment: 5 pages, RevTeX, epsf files. Accepted for surface science letter

Efficient first-principles calculation of phonon assisted photocurrent in large-scale solar cell devices

We present a straightforward and computationally cheap method to obtain the phonon-assisted photocurrent in large-scale devices from first-principles transport calculations. The photocurrent is calculated using nonequilibrium Green's function with light-matter interaction from the first-order Born approximation while electron-phonon coupling (EPC) is included through special thermal displacements (STD). We apply the method to a silicon solar cell device and demonstrate the impact of including EPC in order to properly describe the current due to the indirect band-to-band transitions. The first-principles results are successfully compared to experimental measurements of the temperature and light intensity dependence of the open-circuit voltage of a silicon photovoltaic module. Our calculations illustrate the pivotal role played by EPC in photocurrent modelling to avoid underestimation of the open-circuit voltage, short-circuit current and maximum power. This work represents a recipe for computational characterization of future photovoltaic devices including the combined effects of light-matter interaction, phonon-assisted tunneling and the device potential at finite bias from the level of first-principles simulations