Metastable Defect Formation by Hydrogen Relocation and Rebonding

1995 ◽  
Vol 377 ◽  
Author(s):  
Qiming Li ◽  
R. Biswas
Keyword(s):  
Defect Formation ◽  
Defect Density ◽  
Tight Binding ◽  
Rate Equations ◽  
Dangling Bond ◽  
Defect Formation Energy ◽  
Fundamental Process ◽  
Induced Defects ◽  
Metastable Defect ◽  
Density Rate

ABSTRACTMolecular dynamics with the tight-binding approach are utilized to examine the fundamental process of dangling bond creation via the rebonding of H from Si-H bonds to weak Si-Si bonds. The defect formation energy is found to strongly correlate with the bond-length of the weak Si-Si bond, indicating that the distribution of weak Si-Si bonds controls the total defect density. Rate equations for thermally generated and light-induced defects are developed and utilized to calculate the equilibrium and saturated defect density. The results agree well with experimental data.

The Effect of Post-Deuteration on Metastability in a-Si:H

1993 ◽  
Vol 297 ◽  
Author(s):  
N.H. Nickel ◽  
W.B. Jackson ◽  
C.C. Tsai
Keyword(s):  
Defect Formation ◽  
Chemical Potential ◽  
Defect Density ◽  
Hydrogenated Amorphous Silicon ◽  
Dangling Bond ◽  
Deuterium Plasma ◽  
Bond Density ◽  
Intense Light ◽  
Hydrogenated Amorphous ◽  
Metastable Defect

Hydrogenated amorphous silicon films were deuterated through a sequence of 1h exposures to a remote deuterium plasma at 350°C. The concentration profiles of hydrogen and deuterium were determined by SIMS at various times during the exposure sequence. The defect density in state A, after deuteration and after illumination with white light were determined using CPM measurements following each 1h exposure sequence. We find that post-deuteration does not alter the defect density in state A, change the Urbach edge, nor significantly alter metastable defect formation. Intense light soaking increases the defect density by about 5 × 10l6cm−3 independent of the total H + D concentration. These results suggest that D always enters the sample in pairs pinning the hydrogen chemical potential which supports the idea of a negative U system for hydrogen and deuterium. Despite an increase of Si-H bonds by as much as 3 × 1021cm−3, the annealed dangling bond density and the weak Si-Si bond density did not change.This suggests that the density of weak Si-Si bonds as well as the dangling bond density is determined by equilibration with strong Si-Si bonds through the interchange of H. The implications of these results for H bonding will be discussed.

New experiments on the relationship between light-induced defects and photoconductivity degradation

2001 ◽  
Vol 664 ◽  
Author(s):  
Stephan Heck ◽  
Howard M. Branz
Keyword(s):  
Activation Energy ◽  
Defect Density ◽  
Activation Energies ◽  
Dangling Bond ◽  
Fast Recovery ◽  
Slow Recovery ◽  
Degradation Model ◽  
Induced Defects ◽  
The Relationship ◽  
Hydrogenated Amorphous

ABSTRACTWe report experimental results that help settle apparent inconsistencies in earlier work on photoconductivity and light-induced defects in hydrogenated amorphous silicon (a-Si:H) and point toward a new understanding of this subject. After observing that light-induced photoconductivity degradation anneals out at much lower T than the light-induced increase in deep defect density, Han and Fritzsche[1] suggested that two kinds of defects are created during illumination of a-Si:H. In this view, one kind of defect degrades the photoconductivity and the other increases defect sub-bandgap optical absorption. However, the light-induced degradation model of Stutzmann et al.[2] assumes that photoconductivity is inversely proportional to the dangling-bond defect density. We observe two kinds of defects that are distinguished by their annealing activation energies, but because their densities remain in strict linear proportion during their creation, the two kinds of defects cannot be completely independent.In our measurements of photoconductivity and defect absorption (constant photocurrent method) during 25°C light soaking and during a series of isochronal anneals between 25 < T < 190°C, we find that the absorption measured with E ≤1.1 eV, first increases during annealing, then exhibits the usual absorption decrease found for deeper defects. The maximum in this absorption at E ≤1.1eV occurs simultaneously with a transition from fast to slow recovery of photoconductivity. The absorption for E ≤1.1eV shows two distinct annealing activation energies: the signal rises with about 0.87 eV and falls with about 1.15 eV. The 0.87 eV activation energy roughly equals the activation energy for the dominant, fast, recovery of photoconductivity. The 1.15 eV activation energy roughly equals the single activation energy for annealing of the light-induced dangling bond absorption.

Comparison of Current and Light Induced Defects in a-Si:H

1993 ◽  
Vol 297 ◽  
Author(s):  
R.A. Street ◽  
W.B. Jackson ◽  
M. Hack
Keyword(s):  
Numerical Modelling ◽  
Reverse Bias ◽  
Defect Density ◽  
Reverse Current ◽  
Bias Current ◽  
Spatial Profile ◽  
Forward Current ◽  
Defect Creation ◽  
Induced Defects ◽  
Metastable Defect

Metastable defect creation by illumination and by a forward current in p-i-n devices are compared using CPM and reverse current measurements of the defect density. The data show that the same defects are formed by the two mechanisms, but with different spatial profiles. Numerical modelling shows how the spatial profile influences the reverse bias current.

Metastable Defects in Tritiated Amorphous Silicon

2007 ◽  
Vol 989 ◽  
Author(s):  
Tong Ju ◽  
Janica Whitaker ◽  
Stefan Zukotynski ◽  
Nazir Kherani ◽  
P. Craig Taylor ◽  
...  
Keyword(s):  
Amorphous Silicon ◽  
Liquid Nitrogen ◽  
Beta Decay ◽  
Defect Density ◽  
Average Energy ◽  
Liquid Nitrogen Temperature ◽  
Beta Particle ◽  
Dangling Bond ◽  
Staebler Wronski Effect ◽  
Induced Defects

AbstractThe appearance of optically or electrically induced defects in hydrogenated amorphous silicon (a-Si:H), especially those that contribute to the Staebler-Wronski effect, has been the topic of numerous studies, yet the mechanism of defect creation and annealing is far from clarified. We have been observing the growth of defects caused by tritium decay in tritiated a Si-H instead of inducing defects optically. Tritium decays to 3He, emitting a beta particle (average energy of 5.7 keV) and an antineutrino. This reaction has a half âlife of 12.5 years. In these 7 at.% tritium-doped a-Si:H samples each beta decay will create a defect by converting a bonded tritium to an interstitial helium, leaving behind a silicon dangling bond. We use ESR (electron spin resonance) and PDS( photothermal deflection spectroscopy) to track the defects. First we annealed these samples, and then we used ESR to determine the initial defect density around 1016 to 1017 /cm3 , which is mostly a surface spin density. After that we have kept the samples in liquid nitrogen for almost two years. During the two years we have used ESR to track the defect densities of the samples. The defect density increases without saturation to a value of 3x1019/cm3 after two years, a number smaller than one would expect if each tritium decay were to create a silicon dangling bond (2x1020/cm3). This result suggests that there might be either an annealing process that remains at liquid nitrogen temperature, or tritium decay in clustered phase not producing a dangling bond due to bond reconstruction and emission of the hydrogen previously paired to Si-bonded tritium atom. After storage in liquid nitrogen for two years, we have annealed the samples. We have stepwise annealed one sample at temperatures up to 200°C, where all of the defects from beta decay are annealed out, and reconstructed the annealing energy distribution. The second sample, which was grown at 150°C, has been isothermally annealing at 300 K for several months. The defects remain well above their saturation value at 300 K, and the shape of decay suggests some interaction between the defects.

Saturation of Metastable-Defect Density in a-Si:H

1990 ◽  
Vol 192 ◽  
Author(s):  
David Redfield ◽  
Richard H. Bube
Keyword(s):  
Steady State ◽  
Defect Density ◽  
Atomic Model ◽  
Transient Responses ◽  
Induced Defects ◽  
Metastable Defect

ABSTRACTThe existence of saturation (or steady state) in the density of light-induced defects in amorphous Si:H is shown to have major importance for the interpretation of the nature and origin of these defects. First, a number of characteristics of the steady-state and transient responses to light and temperature are described and contrasted. These lead to the conclusion that the saturation value is the only useful criterion of the number of defects in these materials. We then describe a new atomic model for defects, unifying both dopant-induced and light-induced defects. This model invokes foreign atoms in defects, and saturation reflects the limitation imposed by the numbers of such atoms. Many other observed properties of defects are explained by this model.

An Alternative Model for the Kinetics of Light-Induced Defects in A-Si:H

1991 ◽  
Vol 219 ◽  
Author(s):  
Paulo V. Santos ◽  
W. B. Jackson ◽  
R. A. Street
Keyword(s):  
Time Dependence ◽  
Valence Band ◽  
Defect Formation ◽  
Defect Density ◽  
Generation Rate ◽  
Band Tail ◽  
Defect Generation ◽  
Wide Range ◽  
Induced Defects ◽  
Kinetics Of

ABSTRACTThe kinetics of light-induced defect generation in a-Si:H was investigated over a wide range of illumination intensities and temperatures. The defect density around 1016cm-3 exhibits a power-law time dependence Ns ∼ G2εfε with ε = 0.2 to 0.3, where G is the photo-carrier generation rate. A model for the kinetics of defect generation is proposed based on the existence of an exponential distribution of defect formation energies in the amorphous network, associated with the valence band tail states. The model reproduces the observed time dependence of the defect density with an exponent e determined by the exponential width of the valence band tail. The temperature dependence of the defect generation rate is well-reproduced by the model, which provides a connection between the Stabler-Wronski effect and the weak-bond model.

Light-Soaking Effects on Photoconductivity in a-Si:H Thin Films

1997 ◽  
Vol 467 ◽  
Author(s):  
E. Morgado ◽  
M. Rebelo da Silva ◽  
R. T. Henriques
Keyword(s):  
Thin Films ◽  
Defect Density ◽  
Light Exposure ◽  
Dangling Bond ◽  
Photothermal Deflection Spectroscopy ◽  
Spin Resonance ◽  
Photo Conductivity ◽  
Photothermal Deflection ◽  
Gap States ◽  
Induced Defects

ABSTRACTMetastable defects have been created by light exposure in thin films of a-Si:H. The samples have been characterized by Photothermal Deflection Spectroscopy, Electron Spin Resonance, dark- and photo-conductivity. The experimental results are consistent with numerical calculations with a recombination model involving band tails and one class of correlated dangling-bond states. The effects of light-soaking on the ligh intensity and defect density dependences of photoconductivity are reproduced by the calculations. The model allows to explain the experimental trends by changes in the electronic occupation of the gap states produced by light-induced defects.

Coupled Effects of Light and Temperature on Degradation of a-Si:H

1991 ◽  
Vol 219 ◽  
Author(s):  
Lisa E. Benatar ◽  
Michael Grimbergen ◽  
David Redfeeld ◽  
Richard H. Bube
Keyword(s):  
Steady State ◽  
Defect Formation ◽  
Defect Density ◽  
Hydrogenated Amorphous Silicon ◽  
Gradual Change ◽  
State Behavior ◽  
Excitation Rate ◽  
Barrier Heights ◽  
Hydrogenated Amorphous ◽  
Metastable Defect

ABSTRACTThe effects of excitation rate and temperature on the kinetics and steady-state behavior of metastable defect formation in hydrogenated amorphous silicon (a-Si:H) have been studied. The dependences on temperature of the lifetime, τ, and stretching parameter, β, from a stretched exponential description of the kinetics were measured for one sample. We do not see a linear dependence of β on temperature over die entire temperature range studied (270K–370K), and τ increases monotonically with decreasing temperature. Steady-state results show defect density to be dependent on bodi temperature and excitation rate over the ranges measured (from 395K to 470K and from 6 × 1020 to 2 × 1022 s-l cm-3). The gradual change in temperature dependence is explained by a distribution of barrier heights between the ground and metastable states.

Relaxation of Electron Beam-Induced Metastable Defects in a-Si:II

1993 ◽  
Vol 297 ◽  
Author(s):  
M. Grimbergen ◽  
R. Mcconville ◽  
D. Redfield ◽  
R.H. Bube
Keyword(s):  
Activation Energy ◽  
Electron Beam ◽  
Amorphous Silicon ◽  
Time Constant ◽  
Electron Irradiation ◽  
Defect Density ◽  
Initial Density ◽  
Induced Defects ◽  
Metastable Defect

Relaxation of the metastable defect density in undoped amorphous silicon is observed after keV electron irradiation. The time constant for relaxation has an activation energy close to 1 eV, similar to that for light-induced defects. Relaxation appears to follow two or more stages. A large initial density relaxes rapidly, followed by slower relaxation more characteristic of light-induced defects. Separation of these components allows for a better comparison of e-beam and light-induced saturation defect density.

Mechanisms for Metastability in Hydrogenated Amorphous Silicon

2000 ◽  
Vol 609 ◽  
Author(s):  
R. Biswas ◽  
Y.-P. Li ◽  
B.C. Pan
Keyword(s):  
Amorphous Silicon ◽  
Structural Changes ◽  
Defect Formation ◽  
Hydrogenated Amorphous Silicon ◽  
Dangling Bond ◽  
Dangling Bonds ◽  
New Model ◽  
Gap States ◽  
Hydrogenated Amorphous ◽  
Metastable Defect

ABSTRACTWe propose metastabilities in amorphous silicon fall into two classes. One class is the local changes of structure affecting a macroscopic fraction of sites. The other class is the metastable generation of dangling bonds with mid-gap states. The local metastability is explained by a new metastable state formed when H is flipped to the backside of the Si-H bond at monohydride sites. The dipole moment of this H-flip defect is larger and increases the infrared absorption. This H-flip defect accounts for large structural changes observed on light soaking including larger absorption and volume dilation. We propose a new model for the generation of metastable dangling bonds. The new ‘silicon network rebonding model’ involves breaking of weak silicon bonds and formation of isolated dangling bonds, through rebonding of the silicon network. Hydrogen motion is not involved in metastable defect formation. Defect formation proceeds by breaking weak silicon bonds and formation of dangling bond-floating bond pairs. The floating bonds migrate through the network and annihilate, producing isolated dangling bonds. This new model provides a new platform for understanding the atomistic origins of lightinduced degradation.

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