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.

Metastability of Light-Induced Defects in Very Low Density of Gap States α- Si1-αCα:H Alloys

1992 ◽  
Vol 258 ◽  
Author(s):  
M. Sebastiani ◽  
P. Fiorini ◽  
F. Alvarez ◽  
F. Pozzilli ◽  
O. Pulci ◽  
...  
Keyword(s):  
Defect Formation ◽  
Defect Density ◽  
Light Exposure ◽  
Bond Breaking ◽  
Silicon Carbon ◽  
Gap States ◽  
Model Conversion ◽  
Induced Defects ◽  
Kinetics Of ◽  
Actual Occupation

ABSTRACTWe have prepared silicon carbon alloys with Tauc's gap of 2.1 eV, low defect density (≃ 3–1015cm-3) and large photoconductivity (αPhoto/αdark=105 in AM 1.5 illumination). On these samples light soaking induces a large number of metastable gap defects which are annealed out at 250 °C. We have studied the kinetics of defect formation varying the duration of light exposure and the light intensity. The experimental data are consistent with a bond breaking model (conversion of tail weak bonds into dangling bonds), provided that the actual occupation of tail states is taken into account.

Isolation of Temperature Effects on the Kinetics of Light Induced Defect Generation in a-Si:H

1992 ◽  
Vol 258 ◽  
Author(s):  
L. Benatar ◽  
M. Grimbergen ◽  
A. Fahrenbruch ◽  
A. Lopez-Otero ◽  
D. Redfield ◽  
...  
Keyword(s):  
Defect Formation ◽  
Defect Density ◽  
Dark Conductivity ◽  
Stretched Exponential ◽  
Defect Generation ◽  
Thermally Activated ◽  
A Value ◽  
Effects Of Temperature ◽  
Kinetics Of

ABSTRACTData are presented here that show the effects of temperature on the kinetics of metastable defect formation in undoped a-Si:H over the range 45°-110°C. CPM (Constant Photocurrent Method), photoconductivity, and dark conductivity measurements were made and provide independent checks of the defect generation behavior. A stretched exponential description of defect formation as a function of time was used to fit the CPM defect density data. The stretched exponential time constant, τSE, is thermally activated with an apparent activation energy of 1 eV, a value that agrees well with data for defect anneal and solar cell degradation. The data indicate that thermal terms are not negligible for temperatures as low as 45°C, and therefore should be included in any model of the kinetics of defect formation. The role of adistribution of anneal energies and the regimes of dominance of thermal and optical rate terms are discussed in the context of the model.

Saturation behavior of the Light-Induced Defect Density in Hydrogenated Amorphous Silicon

1990 ◽  
Vol 192 ◽  
Author(s):  
H. R. Park ◽  
J. Z. Liu ◽  
P. Roca i Cabarrocas ◽  
A. Maruyama ◽  
M. Isomura ◽  
...  
Keyword(s):  
Amorphous Silicon ◽  
Room Temperature ◽  
Urbach Energy ◽  
Defect Density ◽  
Generation Rate ◽  
Carrier Generation ◽  
Defect Generation ◽  
Ion Laser ◽  
Hydrogenated Amorphous ◽  
Defect Densities

ABSTRACTUsing a Kr ion laser (λ = 647.1 nm) to produce a carrier generation rate G of 3 × 1020 cm−3s−1, we have saturated the light-induced defect generation in hydrogenated (and fluorinated) amorphous silicon (a-Si:H(F)), within a few hours near room temperature. While the defect generation rate scales roughly with 1/G2, the saturation defect densities Ns,sat are essentially independent of G. The saturation is not due to thermal annealing. We have further measured Ns,sat m 37 a-Si:H(F) films grown in six different reactors under different conditions. The results show that Ns,sat lies between 5 × 1016 and 2 × 1017 cm−3, that Ns,sat drops with decreasing optical gap and hydrogen content, and that Ns,sat is not correlated with the initial defect density or with the Urbach energy.

States in the Gap of Improved a-Ge:H Studied by Photomodulation Spectroscopy

1991 ◽  
Vol 219 ◽  
Author(s):  
L. Chen ◽  
J. Tauc ◽  
D. Pang ◽  
W. A. Turner ◽  
W. Paul
Keyword(s):  
Glow Discharge ◽  
Valence Band ◽  
Density Of States ◽  
Pump Beam ◽  
Defect Density ◽  
Dangling Bond ◽  
Band Tail ◽  
Valence Bands

ABSTRACTThe photomodulation spectra of a-Ge:H of average photoelectronic quality(ημπ = 1 × 10-10cm2/V) and of improved quality (ημπ = 3 × 10-7cm2/V), produced under different plasma conditions in an r.f. diode reactor by glow discharge, were measured at 80K and are analyzed in analogy with earlier studies of a-Si:H. The spectra of the poorer material are dominated by transitions between dangling bond states and the conduction and valence bands. By contrast, the spectra of the better material require contributions of transitions from the band tail states, indicating that the reduced defect density has resulted in pump-beam induced quasi-Fermi levels reaching near the conduction and valence band edges. A very acceptable fit between plausible density-of-states distributions and the experimental spectra has been found.

Effect of Deposition-Induced Annealable Defects on Light-Induced Defect Generation in a-Si:H

1993 ◽  
Vol 297 ◽  
Author(s):  
Jong-Hwan Yoon
Keyword(s):  
Amorphous Silicon ◽  
Defect Density ◽  
Hydrogenated Amorphous Silicon ◽  
Generation Rate ◽  
Defect Generation ◽  
Saturated State ◽  
Microscopic Models ◽  
Hydrogenated Amorphous ◽  
Substrate Temperatures

In this paper we present a method to determine the annealable defect density(ΔNann) present in hydrogenated amorphous silicon(a-Si:H). The effects of the annealable defects on the light-induced defect generation rate, saturated defect density (Nsat) and the change of defect density in the light-induced saturated state(ΔNsat) have been studied. Annealable defect density was varied by depositing samples at various substrate temperatures or by post-growth anneals of samples grown at low substrate temperatures. It is found that the generation rate, N satand ΔNsat are well correlated with ΔNann. In particular, the ΔNsat is found to follow a relation ΔNsat ≈ ΔNann. These results suggest that defect-related microscopic models are appropriate for light-induced metastability.

Radiation-induced Defects in Nonradioactive Natural Minerals: Mineralogical and Environmental Significance

2003 ◽  
Vol 792 ◽  
Author(s):  
Georges Calas ◽  
Thierry Allard ◽  
Etienne Balan ◽  
Guillaume Morin ◽  
Stéphanie Sorieul
Keyword(s):  
Clay Minerals ◽  
Defect Formation ◽  
High Specific Surface Area ◽  
Radiation Background ◽  
Geochemical Parameters ◽  
Wide Range ◽  
Natural Analogues ◽  
Radiation Induced ◽  
Electronic Defects ◽  
Induced Defects

ABSTRACTNatural short-lived radionuclides generate electronic defects in minerals, such as trapped electrons and positive holes, often associated with element impurities, which act as final traps over geological periods. Two main examples will be illustrated. The first example will concern the point defects, which are observed in clay minerals. The high specific surface area makes clay minerals sensitive to the geochemical radiation background and provides a record of the past occurrence of radionuclides in geological systems. In kaolinite, three types of hole-centers are trapped by oxygen atoms linked to Si- or Al-sites. An experimental dosimetry gives the paleodose, which can be used either to assess mean past U-concentration or for kaolinite dating, depending on the available geochemical parameters. The detection of past migrations of radioelements in natural analogues may be used in the safety assessment of radioactive waste disposals. The second example will concern the role of mineral impurities in defect formation and stabilization. Natural fluorites (CaF2) exhibit hole-and electron-centers trapped on several rare earths and oxygen impurities, often present at the ppm level, which are responsible for the wide range of coloration observed in natural fluorites. Ca colloids may form under severe irradiation and give rise to a characteristic absorption. The thermal stability of radiation-induced defects gives constraints on the evolution of fluorites as a function of temperature and time. Other minerals, such as apatite, confirm the importance of impurities in stabilizing radiation-induced defects over geological periods.

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.

Comparison of Electron Beam-Induced and Light-Induced Defect Saturation in a-Si:H

1992 ◽  
Vol 258 ◽  
Author(s):  
M. Grimbergen ◽  
A. Lopez-Otero ◽  
A. Fahrenbruch ◽  
L. Benatar ◽  
D. Redfield ◽  
...  
Keyword(s):  
Electron Beam ◽  
Electron Irradiation ◽  
Defect Density ◽  
Dark Conductivity ◽  
High Light Intensity ◽  
Generation Rate ◽  
Temperature Dependent ◽  
Photon Irradiation ◽  
Temperature Ranges ◽  
Induced Defects

ABSTRACTGeneration, saturation, and annealing characteristics of metastable defects formed by electron beam irradiation at 20 keV and photon irradiation at 1.9 eV have been compared. Saturation density reached by electron irradiation is temperature independent over the range 225 K to 300 K, although a small activation energy of the generation rate may be present. This differs from observed temperature dependent light-induced saturation from 330 K to 470 K, although differences are expected because of the separate temperature ranges and dissimilar carrier excitation rates. The electron beam-induced saturated defect density is about 5 times larger than for light-induced saturation at 350 K and high light intensity (generation rate ≈ 1022cm-3s-1). Defects formed by electron irradiation anneal at 300 K with a stretched exponential time constant three orders of magnitude smaller than for light-induced defects. After electron irradiation, dark conductivity relaxes faster than photoconductivity. Once the dark Fermi level becomes constant during defect density relaxation, photoconductivity is inversely proportional to the defect density.

Metastability Under High-Intensity Light of Device-Quality He-Diluted, H2-Diluted and Standard a-Si:H Films Deposited Between 50°C and 350°C

1994 ◽  
Vol 336 ◽  
Author(s):  
P. Morin ◽  
P. Roca i Cabarrocas
Keyword(s):  
High Intensity ◽  
Defect Density ◽  
Red Light ◽  
Rf Power ◽  
Wide Range ◽  
Order Of Magnitude ◽  
Kinetics Of ◽  
Defect Densities ◽  
High Intensity Light

ABSTRACTWe report the results of a study of the metastability under illumination by high intensity red light of device quality a-Si:H thin films deposited using a wide range of deposition conditions. The process variables included substrate temperature, pressure, rf power, and dilution of silane by He or H2. In-situ Monitoring of the sample conductivity and defect density during light-soaking provides the kinetics of the degradation of the electronic properties of the films. We observe equilibration of the photoconductivity and of the defect density. The characteristic time of equilibration τse of the defect density varies by more than an order of magnitude, dividing the samples into two groups: one group with a τse on the order of 103 seconds, the other with a τse on the order of 104 seconds. Low steady state defect densities combined with high ημτ products are observed for “standard” a-Si:H deposited between 100°C and 250°C and He-diluted films deposited above 250°C.

Magnetic Resonance Probes Of Band Tail States And Defects In Tetrahedrally Coordinated Amorphous Semiconductors

2002 ◽  
Vol 715 ◽  
Author(s):  
P. C. Taylor
Keyword(s):  
Amorphous Silicon ◽  
Hydrogenated Amorphous Silicon ◽  
Amorphous Semiconductors ◽  
Dangling Bonds ◽  
Band Tail ◽  
Wide Range ◽  
Recombination Processes ◽  
Staebler Wronski Effect ◽  
Hydrogenated Amorphous ◽  
Kinetics Of

AbstractRecent electron spin resonance (ESR) results relating to (1) recombination processes for optically excited electrons and holes in tetrahedrally coordinated amorphous semiconductors and (2) kinetics of metastable defects (dangling bonds associated with the Staebler-Wronski effect) in hydrogenated amorphous silicon (a-Si:H). With regard to recombination processes, ESR measurements have been performed over a wide range of excitation intensities (nW/cm2 to W/cm2) on hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous germanium (a-Ge:H). The kinetics can be studied down to carrier densities as low as 1014 cm-3. The longtime decay curves show that at large carrier separation (1) the random distribution of optically excited electrons and holes is subject to the condition of charge neutrality, and (2) the decays are universal and independent of the densities of localized, band-tail states. With regard to the metastable defects in a-Si:H, the kinetics of the production and thermal annealing of silicon dangling bonds have been measured at temperatures between 25 and 480 K using ESR. Below about 150 K the measurement of the dangling bonds is masked by long-lived, band tail carriers that accumulate with time. The production rate for silicon dangling bonds decreases with decreasing temperature and is nearly temperature independent below approximately 100 K. Defects created by 10 hours of irradiation below 100 K anneal almost completely at 300 K. In a- Ge:H, the first measurements of optically induced, metastable germanium dangling bonds have been made.

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