Light-induced Annealing Research Articles

Photo-induced and Thermal Annealing of Chalcogenide Films for Waveguide Fabrication

We describe the effect of light-induced annealing of As2S3 thin films and its impact on the performance of an optical waveguide. An as-deposited film was subjected to illumination with band-edge light before being used to fabricate sub- micron thick waveguides using photolithography and dry plasma etching. Studies of the film microstructure revealed a difference between the atomic bonds and linked phases between the as-deposited, the thermally-annealed, and the optically-annealed films. Although optical-annealing evolves the structure of the film in a similar way to thermal annealing (i.e., it causes polymerization of the glass network), it can drive the film much closer to bulk state in terms of refractive index and the numbers of wrong bonds. The waveguides showed, however, almost the same propagation losses to those from thermally-annealed material. Nonetheless our results indicate that optical-annealing provides some advantages over thermal annealing for waveguide fabrication.

Light-induced annealing of hole trap states: A new aspect of light-induced changes in hydrogenated amorphous silicon

Details of light-induced annealing of hole trap state in undoped hydrogenated amorphous silicon (a-Si:H) have been studied; it has been found that prolonged illumination significantly reduces the density of hole trap states in the energy range deeper than 0.5eV, and subsequent thermal annealing increases the density of hole trap states and restored the sample to the initial state before the illumination. We can speculate, from the experimental results and discussion in this work, that defect conversion processes are taking place during the long exposure to light; Si dangling bonds are generated from the precursors or latent sites which manifested as hole trap states located between 0.5 and 0.7eV from the top of the valence band.

A Light-Induced Annealing of Silicon Implanted Layers

Since introducing Rapid Thermal Annealing, there has been disagreement among experimental data of ion-implanted and annealed layers. One explanation of these differences is the impact of optical irradiation and its interaction with semiconductor material. Although no plausible explanation has been offered, experimental evidence of “photonic effects” was reported in many works. In this work we estimate energy per atom available during recombination of the excited carriers. It is argued that the localization of energy states inside the band gap in ion-implant damaged material is responsible for “photonic effects.”

Photo-formation of gold nanoparticles: Photoacoustic studies on solid monoliths of Au(III)–chitosan–silica aerogels

Photo-formation of gold nanoparticles in the solid monoliths of Au(III)–chitosan–silica aerogels with different Au/NH 2 molar ratios has been investigated using photoacoustic spectroscopy. Upon exposing to 320 nm UV light, a new absorption feature in the visible region around 525 nm could be seen due to the surface plasmon resonance of gold particles that are generated as a result of UV-induced reduction of Au(III) to Au(0). The plasmon band becomes stronger and shows saturation effects upon increasing the UV exposure time. A blue shift of about 7 nm is also noticed on exposing the sample (Au/NH 2 = 1/5) for 6 h, indicating a slight decrease in the nanoparticle size due to light-induced annealing with increasing the UV exposure time. The PA signals monitored as a function of chopping frequency show ω −1 dependence, implying the thermally thin character of Au(III)–chitosan–silica aerogels.

Investigation of the a-Si:H films by using thermal and light-induced annealing treatment in atomic hydrogen atmosphere in H-W-ECR CVD system

To investigate the stability of hydrogenated amorphous silicon (a-Si:H) films, the thermal and light-induced annealing treatment in an atomic hydrogen atmosphere (TLAH) is carried out by using a new hot-wire-assisted microwave electron-cyclotron-resonance chemical vapour deposition system (H-W-ECR CVD) modified from a conventional microwave electronic cyclotron resonance chemical vapor deposition system (MWECR CVD). In order to compare with the TLAH method, the experiments of thermal annealing and thermal and light-induced annealing are also performed. Meanwhile, for the purpose of analysing the photoconductivity degradation quantitative, the photoconductivity degradation is assumed to obey the extended exponential law: 1/σph=1/σs-(1/σs-1/σ0)exp[−(t/τ)β], where the extended exponential β and the time constant τ are gained by the slope and the intercept of the line according to the linear relationship between ln (−ln ((σs−1−σph−1)/(σs−1−σ0−1))) and ln t, deduced from the extended exponential law; the photoconductivity saturation value σs can be obtained by Gaussian fitting according to the relationship between photoconductivity and light-soaking time in the logarithmic coordinate system. The experimental results show that the TLAH can improve the stability, microstructure and opto-electronic properties of the annealed a-Si:H films, obviously decrease their optical band gaps and remarkably move their photoluminescence spectrum (PL) peaks toward low energies.

Light-Soaking Effects on the Open-Circuit Voltage of a-Si:H Solar Cells

AbstractWe present measurements on the decline of the open-circuit voltage VOC in a-Si:H solar cells during extended illumination (light-soaking) at 295 K. We used a near-infrared laser that was nearly uniformly absorbed in the intrinsic layer of the cell. At the highest photogeneration rate (about 2x1021 cm-3), a noticeable decline (0.01 V) occurred within about 10 minutes; VOC stabilized at 0.04 V below its initial value after about 200 hours. We found that both the kinetics and the magnitudes of VOC are reasonably consistent with the predictions of a calculation combining a bandtail+defect picture for recombination and a hydrogen-collision model for defect generation. The version of the hydrogen-collision model that we used assumes that only bandtail recombination drives the hydrogen collision processes. Within this picture, the crossover between bandtail and defect recombination occurs on the same timescale as the “light-induced annealing” process that accounts for stabilization of the optoelectronic properties for long lightsoaking times.

Insights into the mechanisms of light-induced degradation from studies of defects in low Ge fraction a-Si,Ge:H alloys

The modulated photocurrent (MPC) method was used to study deep defect creation and annealing in low Ge fraction a-Si,Ge:H alloys (2–10 at.% Ge). These measurements reveal two distinct bands of deep defects in these samples which are identified as neutral Si and neutral Ge dangling bonds. Upon thermal annealing in the dark from a strongly light degraded state, these two defects are found to decrease in a manner which indicates a direct competition between the annealing of the Si and Ge dangling bonds, and therefore implies a global reconfiguration mechanism. Comparing purely thermal annealing with light-induced annealing indicates that the relative anneal rate for the two types of defects is different. This therefore tends to rule out models in which the rate limiting step in the annealing process comes from the release of the mediating entity (hydrogen? strain?) from a remote site.

Light-induced Annealing of Deep Defects in Low Ge Fraction a-Si,Ge:H Alloys: Further Insights into the Fundamentals of Light-induced Degradation

ABSTRACTModulated photocurrent (MPC) was employed to study metastable deep defect creation and annealing in low Ge fraction a-Si,Ge:H alloys (Ge fractions below 10at.%). These MPC spectra reveal two distinct bands of deep defects identified as neutral Si and neutral Ge dangling bonds. Upon heating in the dark from a strongly light degraded state, these are reduced in direct competition to each other, therefore implying a global reconfiguration mechanism. We have extended our studies to compare purely thermal annealing with light-induced annealing in these alloys. We have found that the relative anneal rate for the two types of defects differs from the case of purely thermal annealing. This therefore tends to rule out models in which the rate limiting step during annealing comes from the release of the mediating entity from a remote site.

Changes in hydrogenated amorphous silicon upon extensive light-soaking at elevated temperature

ABSTRACTPreviously a through the substrate ellipsomtery technique was used to study the high temperature dynamics of light induced reversible changes in amorphous silicon thin films [1]. Since this technique was based on above gap optical changes it is sensitive to the structural aspects of the light induced effects, differently from the below-gap absorption techniques which detect dangling-bond defect states [2-3]. It was found that high intensity light soaking at an elevated temperature causes surprising large, reversible, changes [1]. By comparing these optical changes with the changes in dangling bond concentrations probed by electronic and below gap methods, a fuller picture of temperature dependent light-induced defect creation and annealing dynamics emerges. A high temperature high intensity light soaking method is developed which reduces saturation times, decreases the saturated dangling bond density, as well as decreases the annealing activation energy. These results are discussed in terms of the coupling between network disorder and its relaxation with respect to defect concentrations at high temperature.

Electron Spin Resonance Study of Light-Induced Annealing of Dangling Bonds in Glow Discharge Hydrogenated Amorphous Silicon: Deconvolution of Electron Spin Resonance Spectra

We analyze the electron spin resonance (ESR) spectra of dangling bonds in a-Si:H samples before and after strong illumination which gives rise to the phenomenon of light-induced annealing of dangling bonds. The spectra are deconvoluted into two components, that due to normal dangling bonds and that due to H-related dangling bonds. Both before and after prolonged illumination, the normal dangling bonds constitute about 60% of the total spin density, while the H-related dangling bonds constitute about 40%. However, the photoannealed densities of the two types of dangling bonds are found to be different after strong illumination. In addition, we analyze the ESR spectra of dangling bonds observed after weak illumination.

Light-induced annealing of dangling bonds in He-diluted glow discharge a-Si:H films

We report the results of a study of light-induced annealing of dangling bonds, as well as their light-induced creation in He-diluted glow discharge a-Si:H with a large amount of hydrogen at room temperature (RT) and at 75°C, by using an electron spin resonance (ESR) technique. Under illumination 0.7 W/cm2 at room temperature, dangling bonds are created by prolonged illumination, and their density tends to saturate. On the other hand, under strong illumination (1.6 W/cm2) at room temperature, dangling bonds are initially created and then photoannealed after 5 min of illumination. These results for He-diluted glow discharge samples are discussed and compared with those for standard and H2-diluted glow discharge samples.

Metastable hydrogen atom trapping in hydrogenated amorphous silicon films: a microscopic model for metastable defect creation

Abstract In hydrogenated amorphous silicon (a-Si: H) films, the increase in the metastable defect density under high-intensity illumination is usually described by an empirical two-parameter stretched-exponential (SE) time dependence (characteristic time τSE and dispersion parameter β). In this study, a clearly different (one-parameter) analytic function is obtained from a microscopic model based on the formation and trapping of metastable hydrogen (MSH) atoms. In this microscopic model deduced from experimental observations, assuming that MSH atoms are the only mobile species, only three elemental chemical reactions are thus significant; MSH are produced from doubly hydrogenated (Si-H H-Si) configurations and trapped at either broken bonds or Si-H bonds, corresponding respectively to light-induced annealing and light-induced creation of defects. Competition between trapping sites results in a saturation of the defect density N(t) at a steady-state value N SS. An implicit analytic function is obtained for the continuous-wave illumination time dependence of the metastable defect density; a one-parameter fit of this analytical function to experimental data is generally good, indicating that the use of a statistical distribution of trap energies is not necessary. A comparison of the empirical SE parameters with the microscopic ‘MSH model’ shows that these parameters are strongly related to the steady-state value N SS.

Metastable hydrogen atom trapping in hydrogenated amorphous silicon films: A microscopic model for metastable defect creation

Abstract In hydrogenated amorphous silicon (a-Si: H) films, the increase in the metastable defect density under high-intensity illumination is usually described by an empirical two-parameter stretched-exponential (SE) time dependence (characteristic time τSE and dispersion parameter β). In this study, a clearly different (one-parameter) analytic function is obtained from a microscopic model based on the formation and trapping of metastable hydrogen (MSH) atoms. In this microscopic model deduced from experimental observations, assuming that MSH atoms are the only mobile species, only three elemental chemical reactions are thus significant; MSH are produced from doubly hydrogenated (Si-H H-Si) configurations and trapped at either broken bonds or Si-H bonds, corresponding respectively to light-induced annealing and light-induced creation of defects. Competition between trapping sites results in a saturation of the defect density N(t) at a steady-state value N SS. An implicit analytic function is obtained for...

Kinetics of Light-Induced Deffect Formation and Annealing in Hydrogenated Amorphous Silicon Alloyed with Sulfur

ABSTRACTThe creation and annealing kinetics of light-induced defects in a-SiSx:H are studied by ESR and LESR measurements. The dispersion of defect creation after prolonged illumination with white light is greater for a-SiSx:H than that for undoped a-Si:H. In addition, the saturated value of the dark spin density is slightly lower for a-SiSx:H than that for a-Si:H. The annealing behavior can be fitted with a Gaussian distribution of annealing activation energies as is the case for undoped a-Si:H. The incorporation of sulfur decreases the peak energy and increases the width of the distribution of activation energies. Light-soaking does not change the low temperature LESR spectrum and LESR spin density.

Light-Induced Annealing of Dangling Bonds in a-Si:H

We have investigated the kinetics of light-induced defect (dangling bond) creation and annealing processes in a-Si:H containing a large amount of hydrogen at 300 K and 77 K using the ESR technique. We have obtained direct evidence for the light-induced annealing of dangling bonds at 300 K. A model, in which nonradiative recombination of electrons and holes at hydrogen-related dangling bonds is taken into account, is presented to interpret the experimental results.

Light-Induced Metastable Defects or Light-Induced Metastable H Atoms in a-Si:H Films?

ABSTRACTIn hydrogenated amorphous silicon (a-Si:H) films, the increase of the metastable defect density under high-intensity illumination is usually described by an empirical two-parameter stretched-exponential time dependence (characteristic time τSE and dispersion parameter β). In this study, a clearly different (one-parameter) analytic function is obtained from a microscopic model based on the formation of metastable H (MSH) atoms in a-Si:H films. Assuming that MSH atoms are the only mobile species, only three chemical reactions are significant : MSH are produced from doubly hydrogenated (SiH HSi) configurations and trapped either at broken bonds or Si-H bonds, corresponding respectively to light-induced annealing (LIA) and light-induced creation (LIC) of defects. Competition between trapping sites results in a saturation of N(t) at a steady-state value Nss. A one-parameter fit of this analytical function to experimental data is generally good, indicating that the use of a statistical distribution of trap energies is not necessary.

Influence of Light-Soaking Temperature on the Distribution of Thermal-Annealing Activation Energies for Photocreated Dangling Bonds in Hydrogenated Amorphous Silicon

Influence of light-soaking temperature on the formation of photocreated neutral dangling bonds (DBs) in hydrogenated amorphous silicon is investigated by light soaking at 77 K, room temperature (RT) and 90°C. Distributions of thermal annealing activation energies for the DBs photocreated at the three temperatures are obtained. The effect of light-soaking temperature on the increase rate of DBs and on the distribution of thermal annealing activation energies can be self-consistently fitted using two different rate equations. One contains a photocreation term, a thermal annealing term and a light-induced annealing term. The other has a photocreation term and a thermal annealing term modified by the light-induced annealing effect. The former rate equation can explain the very high density of photocreated DBs, while the latter can explain the recent result of light-induced annealing.

Are both thermal and light-induced annealing of metastable defects in a-Si:H driven by electrons?

Results of a search for a unifying rate law for the annealing of metastable defects in hydrogenated amorphous silicon (a-Si:H) are reported. The hypothesis that defect annealing by both heating or illumination is driven by the density of free electrons was tested. This hypothesis is formulated via the rate equation − dN dt = An 1 α Nf(T) , where N is the defect density, t is the time, A is a constant, n is the free electron density and f( T) is a function of temperature derived from a distribution of annealing energies. The model fits two sets of data, with light-intensity and electrical conductivity as the independent variables, reasonably well, with 1 α ranging from 0.39 to 0.76, but not the third set, where the temperature was varied.

Seasonal variations in amorphous silicon solar module outputs and thin film characteristics

Hydrogenated amorphous silicon (a-Si:H) solar modules exposed to outdoor conditions exhibit, over a long-time scale, an efficiency pattern which improves during summer months and decreases in winter time. The variations are usually attributed to two main mechanisms: (a) thermal annealing effects enhanced by summer month temperatures, which might partly offset the efficiency decrease caused by light-induced changes in the amorphous silicon material (known as the Staebler-Wronski effect), and (b) seasonal spectral variations in the solar radiation reaching the earth's surface, which are quite marked in the wavelength region in which amorphous silicon solar cells respond. While both factors might contribute to a more severe performance degradation in winter, we show, by exposing both commercial a-Si : H solar modules and thin films to the same AM 1.0 spectrum while keeping them at temperatures corresponding to extreme summer and winter operating cell temperatures, that the second effect might be the major factor in the overall seasonal efficiency changes. Light-induced annealing, enhanced by the higher radiation levels to which modules are exposed during summer months, might be playing a role as well, adding extra complexity to the effect.

Pulsed-Light-Induced Metastable Defect Creation in Hydrogenated Amorphous Silicon

ABSTRACTWe report the results of a study of metastable defect creation by pulsed light soaking in undoped hydrogenated amorphous silicon (a-Si:H). An illumination time dependence of the defect density, a saturated defect density, and light-induced annealing under pulsed laser light have been studied. Measurements show approximately a t1/2 time-dependence of the defect creation, which is independent of light intensity. It is observed that the saturation value of the defect density is about one order of magnitude higher than by cw illumination in device quality films. It has been suggested that these results would be due to the difference in the light-induced defect annealing rate between cw and pulsed lights, in which it is found that the light-induced annealing rate by pulsed light is lower than by cw light.