S. Aljishi

Photothermal and photoconductive determination of surface and bulk defect densities in amorphous silicon films

The sub‐band‐gap optical absorption spectra of high‐quality hydrogenated amorphous silicon (a‐Si:H) films are shown to be dominated by surface and interface state absorption when measured by photothermal deflection spectroscopy (PDS), while spectra determined using the constant photocurrent method (CPM) are not. For bulk defect states (both as‐deposited and light‐induced), the integrated subgap absorption is approximately twice as large for PDS as for CPM. Similarly, the conversion factor relating integrated subgap absorption with neutral dangling bond density is twice as large for CPM as PDS. This factor of 2 results from CPM seeing only transitions from below midgap into the conduction band while PDS sees transitions from the valence band into states above midgap as well.

Kinetics of the generation and annealing of deep defects and recombination centers in amorphous silicon

The number of deep defects (as measured by sub‐band‐gap optical absorption) and the number of recombination centers (as measured by steady‐state photoconductivity) in high quality undoped hydrogenated amorphous silicon are monitored as the film undergoes light‐induced defect generation and thermal annealing (the Staebler–Wronski effect). The kinetics of the growth in the density of deep defects agree well with the kinetics of spin centers reported by Stutzmann, Jackson, and Tsai. [Appl. Phys. Lett. 45, 1075 (1984)]. The density of recombination centers is directly related, though not simply proportional to, the density of deep defects; as samples are annealed, the recombination center density drops much more quickly than the total defect density. This behavior is shown to arise if the distribution of defect energy levels corresponds to a distribution of defect annealing energies.

Properties of a‐Si,Ge:H,F alloys prepared by rf glow discharge in an ultrahigh vacuum reactor

a‐Si1−xGex: H,F alloys with the Ge concentration x ranging from 0 to 1 have been prepared by the rf glow discharge decomposition of SiF4, GeF4, and H2 gas mixtures. An ultrahigh vacuum deposition system has been designed and constructed for the preparation of these alloys. The stainless‐steel deposition chamber has Cu gasket‐sealed flanges, is turbomolecular pumped, and reaches a base pressure below 10−7 Torr (10−5 Pa). This deposition system incorporates a load lock which permits short pump‐down cycles and which may reduce impurity contamination of the films. The system is described in detail. Compositional, structural, optical, and electronic properties of alloy films made with this system are reported.

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.

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.

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.