Y.M. Li

In situ determination of dielectric functions and optical gap of ultrathin amorphous silicon by real time spectroscopic ellipsometry

We have developed techniques to determine the near‐infrared to near‐ultraviolet dielectric function and optical gap of ultrathin amorphous silicon [a‐Si:(H)] using real‐time spectroscopic ellipsometry during preparation and processing. The techniques have been applied to ∼50 A a‐Si:H films prepared by plasma‐enhanced chemical vapor deposition, and to ∼250 A pure a‐Si chemically modified by atomic H exposure. For the latter, the time evolution of the bonded H content can be estimated along with the evolution of the gap.

Effects of microstructure on transport properties of undoped hydrogenated amorphous silicon films

Electronic transport properties have been investigated in undoped hydrogenated amorphous silicon (a‐Si:H) materials whose microstructure and void fraction are changed by deposition temperature (Ts). The hydrogen content in these materials decreases from 15 to 5 at.u2009% and the void fraction by 14% as Ts is raised from 200 to 350u2009°C. The photo and dark conductivities are measured from 40 to 190u2009°C and extended state electron mobilities are derived from a self‐consistent analysis. The room temperature mobilities are found to increase from 0.8 to 30 cm2/Vu2009s and become less temperature dependent as Ts increases. These temperature activated mobilities explain the Meyer–Neldel rule [Z. Tech. Phys. 18, 588 (1937)] in a‐Si:H materials whose dark conductivity activation energies are greater than 0.4 eV where it cannot be explained by the statistical shift of the Fermi level.

Optical Properties and Structure of Microcrystalline Silicon

The optical properties of thin film microcrystalline silicon (μc-Si:H) prepared by plasma-enhanced chemical vapor deposition (PECVD) have been studied by real time spectroscopie ellipsometry in the nucleation regime as isolated crystalline particles increase in size. A simple geometric model of nucleation allows us to remove the dominant effect of voids and extract the dielectric functions of the crystallites themselves. We find that the results can be understood in terms of a classical size effect whereby limitations on the electron mean free path by scattering at crystallite surfaces control the absorption onset from 2.0 to 3.0 eV. Finally, we describe how well-ordered, continuous 15 A c-Si films can be prepared on metal substrates.

Effect of surface recombination on the spectral dependence of photocurrent in intrinsic hydrogenated amorphous silicon films

The spectral dependence of the photocurrent in intrinsic hydrogenated amorphous silicon (a‐Si:H) films was measured in the annealed and light‐soaked states. The photocurrents were modeled using numerical analysis which quantified the recombination in both the bulk as well as at the film surface and glass substrate interface. The results on films, having thicknesses between 1 and 3 μm, are consistent with thickness independent bulk transport properties and surface and substrate interface recombination velocities of (3–6)×104 and (1–2)×106 cm/s, respectively.

Effects of Interfaces on the a-Si:H Schottky Barrier Characteristics

A detailed study of hydrogenated amorphous silicon (a-Si:H) surfaces during etching and subsequent low temperature (T≤200°C) oxidation was carried out using in-situ and spectroscopic ellipsometry (SE). The microstructural information from SE is correlated with the properties of the corresponding evaporated Pd Schottky barrier structures. Oxide layers thinner than ∼ 10 A have little effect on the diode characteristics and electron surface recombination velocities. This is consistent with a porous structure for these oxides. Evidence is found from growth kinetics that higher density, compact oxide growth occurs for greater thicknesses. These oxides have a large effect on electron transport and surface recombination velocities consistent with the formation of metal-insulator-semiconductor structures.

Photocarrier Transport and Recombination in Amorphous Silicon

The effect of microstructure in undoped a-Si:H films on carrier transport, recombination, densities of midgap states and solar cell characteristics has been investigated. Extended state mobilities of electrons were obtained from photo and dark conductivity measurements between 40° C and 190° C and the gap states characterized using Dual Beam Photoconductivity. In these films the estimated room temperature electron mobilities increase from about 1 to 30 cm 2 /V sec as the dihydride concentrations and void volume fractions decrease. It is found that the carrier mobility-lifetime products are not solely determined by the dangling bond states. The effects of changes in the mobilities and midgap states on p-i-n homojunction solar cell characteristics are presented and discussed.

Optoelectronic Properties of Plasma CVD a-Si:H Modified by Filament-Generated Atomic H

We have studied a-Si:H prepared by alternating plasma deposition with atomic H treatments performed with a heated W filament. Real time spectroscopie ellipsometry provides the evolution of film thickness, optical gap, and a measure of the fraction of Si-Si bonds broken in the near-surface (200 A) during H-exposure of single films. This information guided us to the desired parameters for the H-treatments. Here, we concentrate on a weak hydrogenation regime characterized by minimal etching, a higher H content by 2 at.%, and a larger optical gap by 0.02 eV for the growth/hydrogenation structures in comparison to continuously deposited control samples. This new material has shown an improvement in the defect density in the light-soaked state in comparison to the control samples. This may result from stabilization of the Si structure due to an increase in the H chemical potential in the a-Si:H.

Real time spectroscopic ellipsometry determination of the evolution of amorphous semiconductor optical functions, bandgap, and microstructure

A unique real time spectroscopic ellipsometer has been applied to determine the microstructure and optical properties of plasma enhanced CVD (PECVD) and sputtered amorphous semiconductors. With this instrument, the limitations of single wavelength ellipsometry have been overcome, and the dielectric functions and optical gap (corrected for surface effects) have been obtained from real time observations on films as thin as 20 A. A wealth of information on the microstructural evolution at the monolayer scale is also deduced; e.g. the mass thickness evolution, and the nucleation density and cluster thickness when nuclei make contact to form the first full-coverage monolayer. The coalescence process first noted in earlier single wavelength ellipsometry studies of optimum PECVD a-Si:H has also been completely quantified.

The Staebler-Wronski Effect and the Thermal Equilibration of Defect and Carrier Concentrations

Light induced degradation of intrinsic Amorphous silicon (a-Si:H) is investigated as a function of temperature. Previous work described an equilibrium framework for the high temperature behavior of dangling bonds defects (DB) 11]; and the temperature dependence of the annealed state photo, σ PH , and dark, σ D , conductivities of a series of intrinsic a-Si:H Materials deposited over a range of substrate temperatures, 200°C s D and σ PH , decrease compared to the annealed state while the ratio, σ D /σ PH remains unchanged. Relationships between the ratio [DB + ]/[DB] and the Fermi level are derived from the equilibrium framework.

Investigation of intrinsic defect states in hydrogenated amorphous silicon films using steady-state photoconductivity and sub-bandgap absorption

The density, distribution and nature of intrinsic gap states in a-Si:H films deposited over a wide range of substrate temperatures have been investigated using steady-state photoconductivities and sub-bandgap absorption. The sub-bandgap absorption was obtained using dual beam photoconductivity (DBP) measurements and the results were analyzed using a detailed numerical model based on Rose-Simmons-Taylor statistics. This model takes into account the effects of mobility, recombination velocity, position of Fermi level, and the presence of sensitizing states. It is found that there is a large effect of sensitizing states and position of Fermi level on both electron mobility-lifetime products and sub-bandgap photoconductivities used to obtain information about gap states in different films.<<ETX>>