D. S. Shen

Electrophotographic patterning of thin-film silicon on glass foil

We report the patterning of thin films of amorphous silicon (a-Si:H) using electrophotographically applied toner as the etch mask. Using a conventional xerographic copier, a toner pattern was applied to 0.1 /spl mu/m thick a-Si:H films deposited on /spl sim/50 /spl mu/m thick glass foil. The toner then served as the etch mask for a-Si:H, and as the lift-off material for the patterning of chromium. This technique opens the prospect of roll-to-roll, high-throughput patterning of large-area thin-film circuits on glass substrates.<<ETX>>

Steady state and transient transport in a-Si, Ge : H, F alloys☆

Abstract Steady state and transient transport data are presented for a series of a-Si, Ge : H, F alloys which indicate that, upon introduction of Ge (1) the conduction band tail appears to widen abruptly while the valence band tail is relatively unaffected (2) the number of deep defects increases exponentially with decreasing bandgap. The manner in which the deep defects affect transport indicates that they are negatively charged. A model based on compositional inhomogenieties is proposed to explain these results.

Photocurrent collection in a Schottky barrier on an amorphous silicon‐germanium alloy structure with 1.23 eV optical gap

We report the result of voltage‐dependent photocurrent collection measurements in Pd Schottky barriers on an undoped a‐Si,Ge:H,F multilayer alloy structure with an effective optical gap of 1.23 eV. The hole mobility‐lifetime product (μτ)p, extracted from a fit of the voltage dependence of the photocurrent to the Hecht expression, is 7×10−9 cm2 V−1. Our result is an important indication that it is possible to produce the low‐gap a‐Si,Ge:H,F alloys with the optoelectronic characteristics needed for efficient tandem solar cells.

Amorphous silicon-germanium thin-film photodetector array

The authors report a photodetector array based on an amorphous silicon-germanium alloy thin film. The detector array is designed for detecting light pulses from (Ga,Al)As laser diodes. An experimental 2*2 detector array, addressed by a silicon shift register, works to a clock frequency of 25 MHz and shows good stability.<<ETX>>

Sub‐bandgap optical absorption and light‐induced defects in amorphous silicon

Photothermal deflection spectroscopy (PDS) and the constant photocurrent method (CPM) are used to monitor the changes in density of deep defects associated with the Staebler‐Wronski effect. These values are correlated with the electron spin resonance (ESR) signal, dc photoconductivity, and minority‐carrier μτ values determined from bias‐dependent carrier collection efficiency studies. The density of deep defects determined by subgap absorption grows with light‐soaking time t following the same functional form (t1/3) as the ESR signal. The initial and light‐induced defects have similar carrier recombination cross‐sections.

Properties of amorphous silicon/amorphous silicon‐germanium multilayers

A comprehensive study of multilayer structures made of hydrogenated and fluorinated amorphous silicon and its alloy with germanium, a‐Si:H,F/a‐Si,Ge:H,F, is reported. After briefly describing the deposition process and the chemical composition of the samples, the optical and the electronic properties of the multilayers are concentrated on. Raman scattering spectra suggest mixing over a 1‐nm‐thick interfacial layer. Auger electron spectra combined with sputter profiling and x‐ray linewidth are compatible with this observation. The rise in optical band gap observed as the a‐Si,Ge:H,F well layer thickness is reduced below 5 nm is also compatible with this interfacial mixing. No extra defect states above the detection limit of 2×1010 cm−2 are associated with the interfaces. Electron transport parallel to the layers takes place in the well layers. Perpendicular electron and hole transport is dominated by elastic tunneling through thin (<5 nm) barrier layers, and by thermal emission over thicker barriers.

a-(Si, Ge):H, F Alloys Prepared from SiH4 and GeF4

The properties of a-(Si, Ge):H, F alloys prepared by glow discharge deposition from SiH 4 and GeF 4 are described. The measured IR absorption spectra, sub-gap absorption spectra, dark conduction activation energies, carrier drift mobilities and deep trapping lifetimes of these alloys are similar to those of alloys prepared from SiF 4 , GeF 4 , and H 2 . However, they have over an order of magnitude lower photoconductivity over most of the composition range. Infrared absorption measurements indicate that these alloys have a fluorine content less than 1.5 at.% and a Si-H 2 content that increases with germanium concentration.

Optical and electronic properties of an amorphous silicon‐germanium alloy with a 1.28 eV optical gap

We report the deposition and comprehensive evaluation of a hydrogenated, fluorinated amorphous silicon‐germanium alloy with an optical gap of 1.28 eV. This low‐gap alloy of the a‐Si, Ge system possesses a small midgap defect density (6.5×1016 cm−3), and useful electron (σph/σd=23) and hole (LD=0.13 μm) transport properties. The alloy was grown by radio‐frequency plasma‐enhanced decomposition of SiF4, GeF4, and H2 in a reactor built to ultrahigh‐vacuum specifications.

Carrier transport mechanisms in a-Si:H,F/a-Si,Ge:H,F superlattices

Abstract The dark conductivity σ d , dark conductivity activation energy E a and photoconductivity σ ph of a-Si:H,F/a-Si,Ge:H,F superlattices are studied perpendicular to the plane of the layers. The samples have the same well (d g ) and barrier (d s ) thicknesses but their well composition varies. The perpendicular transport shows an interplay of quantum mechanical tunneling through the barriers and of classical thermal emission over the barrier layer. A model is presented that explains the results quantitatively.

Determination of the D 0/− level in amorphous Si,Ge:H(F) by time‐of‐flight charge collection

We describe and demonstrate a new method for determining the density of states of the doubly occupied D0/− level in amorphous hydrogenated silicon and its alloys. The total charge collected during an electron time‐of‐flight measurement is determined as a function of T. T determines the energy E from which electrons are emitted during the measurement. Variation of T (and E) probes the local density of states. We present results for hydrogenated amorphous silicon before and after light soaking, and for amorphous silicon‐germanium (a‐Si,Ge:H,F) alloys.