Z E. Smith

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.

Carrier lifetime model for the optical degradation of amorphous silicon solar cells

The light‐induced performance degradation of amorphous silicon solar cells is described well by a model in which the carrier lifetimes are determined by the dangling bond density. Degradation will be slower in solar cells operating at lower excess carrier concentrations. This is documented with a comparison of degradation data for cells at open circuit versus load, and for single versus cascade cells. At sufficiently long times, the efficiency will decrease at approximately the same rate for all cases, with an offset in time between the individual cases which can be calculated.

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.

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.

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.

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.

Equilibrium between band tails and dangling bonds in a-Si:H

Abstract The electronic states within the mobility gap of undoped hydrogenated amorphous silicon ( a -Si:H) are divided into two groups—shallow (associated with distorted by still tetrahedrally bonded atoms) and deep (associated with under- or over-coordinated atoms). We propose that the densities of these two groups of defect states are intimately related; the number of deep defects is shown explicitly to depend on the sharpness of the band-tail (shallow defect) distribution. If the band-tail states themselves have been allowed to equilibrate (for a given amount of topological disorder) then a simple relation exists between the mean bond-angle deviation (off the ideal tetrahedral angle) and the sharpness of the valence band tail.

Measurements of Light-Induced Degradation in A-Si, Ge:H, F Alloys,

The effects of illumination on the optical and electronic properties of narrow gap hydrogenated and fluorinated amorphous Si-Ge (a-Si 1-x Ge x :H, F) alloys have been evaluated. A series of alloys with optical gaps ranging from 1.30 eV to 1.64 eV has been light soaked at ∼1 sun intensity for 354 hours. Measurements of sub-gap absorption, photo- and dark conductivities and dark conductivity activation energy were made on alloys in the annealed and the light-soaked states. The results indicate that samples with optical gaps ≳ 1.4 eV degrade significantly. The 1.3 eV sample shows no degradation in its optical or electronic properties except for a factor of 5 increase in the dark conductivity.

Light‐induced defect generation and thermal healing in amorphous silicon‐germanium alloys

The electronic and optical properties of a‐Si,Ge:H,F alloys are investigated for the initial (as‐grown) state as well as after annealing, light soaking, and subsequent annealing. The optical absorption spectra were determined from optical transmission, the constant photocurrent technique and photothermal deflection spectroscopy. The photoconductivity and the temperature dependence of the dark conductivity were measured. A post‐deposition anneal was found to improve the measured properties for some of the alloys. The properties of the high gap, Eopt≳1.4 eV, and the low gap, Eopt<1.4 eV, alloys change differently with light soaking. For the high gap alloys, there is an increase in the subgap absorption, a decrease in the photoconductivity and the dark conductivity and an increase in the activation energy with light exposure. The low gap alloys show increasing subgap absorption with light soaking, however, the photoconductivity remains constant and the dark conductivity increases and its activation energy de...

Stress and microstructural effects on equilibrium and non‐equilibrium defects in amorphous silicon

Experimental evidence is presented that both the initial and light‐induced deep defects in high‐quality undoped a‐Si:H are generated by the same mechanism. A material in which it is easier to generate defects will therefore exhibit both more initial defects and a higher susceptibility to light‐induced degradation. These effects tend to balance each other in such a way as to make the absolute increase in the number of states added by a given light‐soak be about the same over a wide range of film quality. Thus the relative change in the number of states is greater for higher‐quality (lower initial defect‐density) material. Evidence is presented tohat the defect‐generating mechanism is related t strained bonds. Thus microstructural profiles with associated strain profiles can lead to profiles in both initial defect density and light‐induced susceptibility.