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.

Picosecond Photocarrier Lifetimes in Ion-Irradiated Amorphous and Crystalline Silicon

1991 ◽  
Vol 235 ◽  
Author(s):  
P. A. Stolk ◽  
L. Calcagnile ◽  
S. Roorda ◽  
H. B. van Linden ◽  
Van den Heuvell ◽  
...  
Keyword(s):  
Amorphous Silicon ◽  
Liquid Nitrogen ◽  
Crystalline Silicon ◽  
Defect Density ◽  
Liquid Nitrogen Temperature ◽  
Generation Rate ◽  
Structural Defects ◽  
Pump Probe ◽  
Recombination Centers

ABSTRACTCrystalline silicon (c-Si) and structurally relaxed amorphous silicon (a-Si) were implanted with 1 MeV Si+ at liquid nitrogen temperature. The photocarrier lifetime τ in the implanted samples was determined with sub-picosecond resolution through pump-probe reflectivity measurements. At low damage levels (i.e. <1014 ions/cm2), τ decreases with increasing ion dose in both materials, indicating a build up of trapping and recombination centers. The dominant centers in c-Si appear to be related to simple defects. The generation rate of electrically active defects is found to be the same in relaxed a-Si and c-Si, which suggests that the structural defects formed in a-Si strongly resemble the simple defects in c-Si. For ion doses > 1014 /cm2, τ saturates at a level of 0.8 ps for both materials. Strikingly, the saturation sets in far below the dose needed to amorphize (>1015 /cm2). The defect density in a-Si at saturation is estimated to be ≈1.6 at.%.

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.

The Role of Charged Defects In Current Transport Through Hydrogenated Amorphous Silicon Alloys

1998 ◽  
Vol 507 ◽  
Author(s):  
S.P. Lau ◽  
J.M. Shannon ◽  
B.J. Sealy ◽  
J.M. Marshall
Keyword(s):  
Silicon Nitride ◽  
Amorphous Silicon ◽  
Defect Density ◽  
Current Transport ◽  
Dangling Bond ◽  
Silicon Alloys ◽  
Metal Structures ◽  
Bond State ◽  
Donor States ◽  
Charged Defects

ABSTRACTCurrent transport in metal-semiconductor-metal structures based on amorphous silicon alloys has been studied in relation to the density of dangling bond state defects. The density of defects was changed by varying alloy composition or by current stressing. We show that the change of current-voltage characteristics and activation energy with defect density and the onset of Poole-Frenkel conduction with composition require charged defects. It is found that there are more charged defects in amorphous silicon nitride (a-Si1−xNx:H) than in amorphous silicon carbide (a-Si1−xCx:H). In addition, an excess of negatively charged dangling bond defects compared to positively charged dangling bond defects is observed in a-Si1−xNx:H films. This is attributed to the presence of N4+ act as the donor states in silicon nitride. We find that the density of charged dangling bond defects can be higher than 1019cm−3.

Improved Light Soaking Stability of R.F. Sputter Deposited Amorphous Silicon

1991 ◽  
Vol 219 ◽  
Author(s):  
A. Wynveen ◽  
J. Fan ◽  
J. Kakalios ◽  
J. Shinar
Keyword(s):  
Amorphous Silicon ◽  
Hydrogen Content ◽  
Defect Density ◽  
Hydrogenated Amorphous Silicon ◽  
Dark Conductivity ◽  
Initial Defect ◽  
Optical Gap ◽  
Sputter Deposited ◽  
Staebler Wronski Effect ◽  
Hydrogenated Amorphous

ABSTRACTStudies of r.f. sputter deposited hydrogenated amorphous silicon (a-Si:H) find that the light induced decrease in the dark conductivity and photoconductivity (the Staebler-Wronski effect) is reduced when the r.f. power used during deposition is increased. The slower Staebler-Wronski effect is not due to an increase in the initial defect density in the high r.f. power samples, but may result from either the lower hydrogen content or the smaller optical gap found in these films.

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.

scholarly journals Impurity-Defect Complexes in Hydrogenated Amorphous Silicon

1990 ◽  
Vol 209 ◽  
Author(s):  
Lin H. Yang ◽  
C. Y. Fong ◽  
Carol S. Nichols
Keyword(s):  
Amorphous Silicon ◽  
Defect Density ◽  
Hydrogenated Amorphous Silicon ◽  
Dangling Bond ◽  
Defect Complexes ◽  
Amorphous Network ◽  
Pseudopotential Calculations ◽  
Impurity Defect ◽  
Supercell Model ◽  
Hydrogenated Amorphous

ABSTRACTThe two most outstanding features observed for dopants in hydrogenated amorphous silicon (a-Si:H) - a shift in the Fermi level accompanied by an increase in the defect density and an absence of degenerate doping - have previously been postulated to stem from the formation of substitutional dopant-dangling bond complexes. Using firstprinciples self-consistent pseudopotential calculations in conjunction with a supercell model for the amorphous network and the ability of network relaxation from the first-principles results, we have studied the electronic and structural properties of substitutional fourfoldcoordinated phosphorus and boron at the second neighbor position to a dangling bond defect. We demonstrate that such impurity-defect complexes can account for the general features observed experimentally in doped a-Si:H.

scholarly journals Material Parameters in a Thick Hydrogenated Amorphous Silicon Detector and their Effect on Signal Collection

1989 ◽  
Vol 149 ◽  
Author(s):  
S. Qureshi ◽  
V. Perez-Mendez ◽  
S. N. Kaplan ◽  
I. Fujieda ◽  
G. Cho
Keyword(s):  
Amorphous Silicon ◽  
Defect Density ◽  
Hydrogenated Amorphous Silicon ◽  
Silicon Detector ◽  
Dangling Bond ◽  
Material Parameters ◽  
Bond Density ◽  
Transient Photoconductivity ◽  
Signal Collection ◽  
Hydrogenated Amorphous

ABSTRACTTransient photoconductivity and ESR measurements were done to relate the ionized dangling bond density and the spin density of thick hydrogenated amorphous silicon (a-Si:H) detectors. We found that only a fraction (∼30–35%) of the total defect density as measured by ESR is ionized when the detector is biased into deep depletion. The measurements on annealed samples also show that this fraction is about 0.3. An explanation based on the shift of the Fermi energy is given. The measurements show that the time dependence of relaxation is a stretched exponential.

Contributions of D0 and non-D0 Gap States to the Kinetics of Light Induced Degradation of Amorphous Silicon under 1 sun Illumination

2001 ◽  
Vol 664 ◽  
Author(s):  
J. Pearce ◽  
X. Niu ◽  
R. Koval ◽  
G. Ganguly ◽  
D. Carlson ◽  
...  
Keyword(s):  
Amorphous Silicon ◽  
Degradation Kinetics ◽  
Hydrogen Dilution ◽  
Spin Resonance ◽  
The Creation ◽  
Staebler Wronski Effect ◽  
Gap States ◽  
Induced Changes ◽  
Induced Defects ◽  
Hydrogenated Amorphous

ABSTRACTLight induced changes to 1 sun degraded steady state (DSS) have been investigated on hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells and corresponding films fabricated with and without hydrogen dilution of silane. Striking similarities are found for the degradation kinetics, between the electron mobility lifetime (μτ) products and the corresponding fill factors (FF). These correlations that exist for both intrinsic materials at temperatures between 25 and 100°C, are present for the DSS as well as in the kinetics, which exihibit distinctly different dependence on temperature. No such correlations are present between μτ, FF and densities of D0 defects, measured with subgap absorption α(E) at 1.2eV, and electron spin resonance (ESR). The creation of non-D0 defects is also clearly indicated by the temperature dependence of the kinetics and the changes in the shape of α(E) with the results suggesting the presence of more than one mechanism for the creation of light induced defects associated with the Staebler-Wronski effect (SWE).

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.

Dispersive Hydrogen Motion and Creation of Light-Induced Defects in Hydrogenated Amorphous Silicon

1989 ◽  
Vol 149 ◽  
Author(s):  
W. B. Jackson
Keyword(s):  
Kinetic Equation ◽  
Temperature Dependence ◽  
Amorphous Silicon ◽  
Hydrogen Diffusion ◽  
Defect Density ◽  
Hydrogenated Amorphous Silicon ◽  
Illumination Time ◽  
Induced Defects ◽  
Hydrogenated Amorphous ◽  
Creation Rate

ABSTRACTThis paper investigates the application of the dispersive hydrogen diffusion defect kinetic equation for the generation of light-induced defects. Self-limited monomolecular carrier defect generation by dispersive motion can explain the observed t1/3 and the G0.6 dependence where t is the illumination time and G is the illumination intensity as well as the equilibrium defect density as a function of temperature. However, the temperature dependence of the creation rate and compatibility with current degradation experiments remain unresolved problems.

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