Carrier-induced Defect Research Articles

Carrier-induced formation of electrically active boron-interstitial clusters in irradiated boron-doped silicon

Excess minority carriers create boron-related recombination centers that degrade the efficiency of the non-particle-irradiated silicon solar cells. However, the carrier-induced reactions among the radiation-induced defects are poorly understood for devices exposed to particle radiation. This study investigates the structure, electronic properties, formation and annihilation mechanisms, and diffusion dynamics of the carrier-induced defects in particle-irradiated boron-doped silicon using density-functional modeling and junction spectroscopy. By revisiting the ground-state structures of the boron-di-interstitial clusters (BI2), we find that the calculated acceptor and donor levels of such defects agree well quantitatively with the carrier-induced deep-level transient spectroscopy (DLTS) hole emission signatures at 0.43 and 0.53 eV above the valence band edge (Ev), respectively. We also find that the formation of BI2 is thermally activated by an energy of 0.50 eV, which we explain theoretically by the reduction of the migration barrier of mono-interstitials to 0.53 eV in the presence of excess minority carriers. Moreover, we discover that the BI2 are potentially mobile with a migration barrier of 1.18 eV, contrary to the present understanding.

Carrier-Induced Defect Saturation in Green InGaN LEDs: A Potential Phenomenon to Enhance Efficiency at Higher Wavelength Regime

Non-radiative defects play a deterministic role in regulating the performance of LEDs. Yet, defect saturation in LEDs is relatively unexplored in the literature. Here, we establish the theoretical ...

A four-state kinetic model for the carrier-induced degradation in multicrystalline silicon: Introducing the reservoir state

In this work, we present new insight into the multi-crystalline silicon carrier-induced defect (CID) by performing multiple degradation and regeneration cycles and further investigation on the partial recovery of mc-CID through extended dark annealing (DA). The maximum normalised defect density was found to decay exponentially with the number of cycles, suggesting that the defect precursors were slowly depleted by DA. A four-state kinetic model is proposed by introducing a reservoir state. Simulation results generated by mathematical modelling based on the proposed state diagrams exhibited good agreement with the experimental results. Extended DA on a partially recovered sample combined with simulation results suggests that the capability of defect formation through DA and the existence of the reservoir state proposed herein were the root causes for the partial recovery reported in the literature. Finally, the change in bound hydrogen state is speculated to cause the modulation of mc-CID formation. A qualitative reservoir model based on the interaction between hydrogen molecules (H2), boron-hydrogen pairs (B-H) and free hydrogen (H+, H°) is proposed and further discussed.

Overcoming the Challenges of Hydrogenation in Silicon Solar Cells

The challenges of passivating defects in silicon solar cells using hydrogen atoms are discussed. Atomic hydrogen is naturally incorporated into conventional silicon solar cells through the deposition of hydrogen-containing dielectric layers and the metallisation firing process. The firing process can readily passivate certain structural defects such as grain boundaries. However, the standard hydrogenation processes are ineffective at passivating numerous defects in silicon solar cells. This difficulty can be attributed to the atomic hydrogen naturally occupying low-mobility and low-reactivity charge states, or the thermal dissociation of hydrogen–defect complexes. The concentration of the highly mobile and reactive neutral-charge state of atomic hydrogen can be enhanced using excess carriers generated by light. Additional low-temperature hydrogenation processes implemented after the conventional fast-firing hydrogenation process are shown to improve the passivation of difficult structural defects. For process-induced defects, careful attention must be paid to the process sequence to ensure that a hydrogenation process is included after the defects are introduced into the device. Defects such as oxygen precipitates that form during high-temperature diffusion and oxidation processes can be passivated during the subsequent dielectric deposition and high-temperature firing process. However, for laser-based processes performed after firing, an additional hydrogenation process should be included after the introduction of the defects. Carrier-induced defects are even more challenging to passivate, and advanced hydrogenation methods incorporating minority carrier injection must be used to induce defect formation first, and, second, provide charge state manipulation to enable passivation. Doing so can increase the performance of industrial p-type Czochralski solar cells by 1.1 % absolute when using a new commercially available laser-based advanced hydrogenation tool.

Impact of annealing on the formation and mitigation of carrier-induced defects in multi-crystalline silicon

Carrier-induced degradation (CID) of multi-crystalline silicon (mc-Si) wafers is a major problem currently affecting the photovoltaic industry. A large number of studies investigating this phenomenon have provided important clues regarding the identification of the defect, however, as of yet, none have isolated a specific cause. In this work, we provide further insight into the kinetics of CID in mc-Si by presenting a detailed study of the impact of dark annealing on the formation and subsequent mitigation of the carrier-induced defect. Previous work has shown that such anneals can modulate the kinetics of the defect. Here, we extend that work and demonstrate that dark annealing can result in accelerated defect formation and extended degradation throughout a subsequent light soaking cycle, irrespective to when the dark annealing was applied. It is suggested that dark annealing could release extra defect precursors into mc-Si, which then become recombination active upon illumination. Therefore, the subsequent degradation after dark annealing might not necessary involve a reverse reaction. Through multiple dark anneal and light soak cycles, the extent of degradation in each cycle continues to reduce. A direct reverse reaction (de-stabilisation) alone does not explain this observation. We suggest that this effect could be explained by the presence of a reservoir of defect precursors, which are gradually depleted throughout the dark annealing processes. Finally, it is demonstrated that dark annealing alone could potentially cause a similar degradation to illumination at elevated temperature.

Evidence of an identical firing-activated carrier-induced defect in monocrystalline and multicrystalline silicon

While progress has been made in understanding the behaviour of the recently identified carrier-induced degradation mechanism in p-type multicrystalline silicon solar cells, little is currently known about the root cause of the defect or its possible existence in other materials. In this work, we present evidence suggesting that the defect also exists in Czochralski grown monocrystalline silicon wafers. For both mono- and multicrystalline silicon we demonstrate: 1) the presence of a degradation and recovery of bulk minority carrier lifetime induced by either illuminated or dark annealing; 2) a modulation in the magnitude of degradation by varying the firing conditions; and 3) capture cross-section ratios of 39.4 ± 4.9 and 33.4 ± 1.5 in monocrystalline and multicrystalline silicon, respectively. The results indicate that the recently identified degradation mechanism does not only occur in multicrystalline silicon from illuminated annealing at elevated temperatures, but it is also induced by dark annealing at elevated temperatures, and that the degradation can occur in Czochralski grown silicon.

Recombination parameters of lifetime-limiting carrier-induced defects in multicrystalline silicon for solar cells

In p-type multicrystalline silicon solar cells, carrier-induced degradation (CID) can cause up to 10% relative reduction in conversion efficiency. Although, a great concern has been drawn on this degradation in the photovoltaic community, the nature of this degradation is still yet unknown. In this contribution, the recombination parameters of the responsible defect causing this degradation are extracted via temperature and injection dependent lifetime spectroscopy. Three wafers from three different ingots were processed into cell precursor and lifetime structures for the study. Similar defect recombination parameters were obtained for all samples. Two candidates for the defect energy level were identified: Et − Ei = −(0.32 ± 0.05) eV or Et − Ei = (0.21 ± 0.05) eV in the lower and upper bandgap halves, respectively. The capture cross section ratios were found to be k = 56 ± 23 or k = 49 ± 21 for the lower and upper bandgap halves, respectively. Contrary to previous studies, these parameters have been extracted for the responsible defect of CID, without making assumptions regarding the defect energy level. The result allows to model and to predict the impact of this defect on the solar cell efficiency.

Rapid passivation of carrier-induced defects in p-type multi-crystalline silicon

A slow forming carrier-induced degradation effect has previously been reported for p-type multi-crystalline silicon (mc-Si) solar cells and has been observed to be most severe in passivated emitter and rear contact (PERC) designs. The as yet undetermined defect (or defects) responsible for this degradation is typically described as being light-induced. However, in this work we confirm that a directly equivalent degradation also occurs when subjecting a cell to current injection, thus the effect can therefore be more accurately described as being carrier-induced. The defect can take years to form under normal operating conditions, but acceleration of this formation and an apparent subsequent passivation through the use of high intensity illumination at elevated temperatures has recently been demonstrated. In this work we further investigate this approach, analyzing the effects of temperature and time on degradation, regeneration, and resulting stability, as well as the variations in treatment response for mc-Si materials from two different manufacturers. Susceptibility to carrier-induced degradation after 225h of light soaking at 70°C is shown to be reduced by 80% using a rapid, 10s treatment with an illumination intensity of 44.8kw/m2 at 200°C. Stability is shown to further improve by extending treatment time but is reduced with increasing treatment temperature.

Acceleration and mitigation of carrier-induced degradation in p-type multi-crystalline silicon

Recently, a new carrier-induced defect has been reported in multi-crystalline silicon (mc-Si), and has been shown to be particularly detrimental to the performance of passivated emitter and rear contact (PERC) cells. Under normal conditions, this defect can take years to fully form. This Letter reports on the accelerated formation and subsequent passivation of this carrier-induced defect through the use of high illumination intensity and elevated temperatures resulting in passivation within minutes. The process was tested on industrial mc-Si PERC solar cells, where degradation after a 100 hour stability test was suppressed to only 0.1% absolute compared to 2.1% for non-treated cells. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Implications of Accelerated Recombination-Active Defect Complex Formation for Mitigating Carrier-Induced Degradation in Silicon

A three-state model is used to explore the influence of the accelerated formation of recombination-active defect complexes on the mitigation of carrier-induced degradation in p-type silicon containing boron and oxygen. Defect formation is observed to be a critical factor for the speed at which carrier-induced degradation can be mitigated. Defect formation also plays a critical role in determining the effectiveness of mitigation at elevated temperatures. It is observed that under conventional conditions, at a processing temperature of $\text{200}\;^\circ{\text{C}}$ , approximately $170\;\text{s}$ are required to form and passivate $99\%$ of possible defects. The experimentally demonstrated improved effectiveness of carrier-induced defect passivation with a process time of $10\;\text{s}$ at temperatures over $\text{300}\;^\circ {\text{C}}$ is consistent with a substantial acceleration of defect formation.

Electroluminescence Afterglow from Indium Tin Oxide/Si-RichSiO2/p-Si Structure

Indium tin oxide/Si-rich SiO2/p-Si structured devicesare fabricated to study the electroluminescence (EL) of the Si-richSiO2 (SRO) material. The obvious peaks at ~1050 nmand ~1260 nm in the EL are ascribed to localized statetransitions of amorphous Si (α-Si) clusters. The EL afterglowassociated with α-Si clusters is observed from this structure at roomtemperature, while the afterglow is absent in the case of opticalpumping. It is believed that carrier-induced defects act as trapcentres in the α-Si clusters, resulting in the EL afterglow. Thephenomenon of the EL afterglow indicates the limits of EL performanceand electrical modulation of the SRO material with a larger fraction ofα-Si clusters.

The Role of Silicon Interstitials in the Formation of Boron-Oxygen Defects in Crystalline Silicon

Oxygen-rich crystalline silicon materials doped with boron are plagued by the presence of a well-known carrier-induced defect, usually triggered by illumination. Despite its importance in photovoltaic materials, the chemical make-up of the defect remains unclear. In this paper we examine whether the presence of excess silicon self-interstitials, introduced by ion-implantation, affects the formation of the defects under illumination. The results reveal that there is no discernible change in the carrier-induced defect concentration, although there is evidence for other defects caused by interactions between interstitials and oxygen. The insensitivity of the carrier-induced defect formation to the presence of silicon interstitials suggests that neither interstitials themselves, nor species heavily affected by their presence (such as interstitial boron), are likely to be involved in the defect structure, consistent with recent theoretical modelling.

Threshold voltage instability of amorphous silicon thin-film transistors under constant current stress

We investigate the time-dependent shift in the threshold voltage of amorphous silicon thin-film transistor stressed with constant drain current. We observe a nonsaturating power-law time dependence, which is in contrast to the conventional stretched exponential that saturates at prolonged stress time. The result is consistent with the carrier-induced defect creation model and corroborates the nonlinear dependence of the rate of defect creation on the band-tail carrier density.

Microscopic Model for Creation And Removal of Metastable Dangling Bonds in a-Si:H

AbstractWe present a new microscopic model for metastable Si dangling bond defect creation in hydrogenated amorphous silicon, which is applicable to both light-induced and carrier-induced defect creation. The key feature of our model is that hydrogen is always in the tedrahedral-like site, which is strongly bound in amorphous silicon, and never in the bond-centered site. Breaking of Si-Si bonds and successive stabilisation by bond-switching of nearby hydrogen from doubly hydrogenated Si-Si bonds (SiHHSi) results in two hydrogen-stabilised dangling bonds (SiHD). Since hydrogen is in the Td site in all configurations, this defect creation reaction is consistent with ESR, NMR, hydrogenation experiments and the mechanical stress dependence of defect creation.

A coupled study by floating-gate and charge-pumping techniques of hot carrier-induced defects in submicrometer LDD n-MOSFET's

The creation of defects by hot-carrier effect in submicrometer (0.85- mu m) LDD n-MOSFETs is analyzed by the floating-gate and the charge-pumping techniques. It is emphasized that the floating-gate technique is an attractive tool for characterizing the oxide traps located in the drain-gate overlap region, near the oxide spacer of the LDD structures. This work gives new insight into the creation of acceptorlike oxide traps which are electrically active only after electron injection phases. These defects are generated in the whole stress gate bias range (from V/sub d//8 to V/sub d/) by hot-hole and/or hot-electron injections, and their generation rates (10/sup -9/ and 10/sup -2/ for electron and hole injections, respectively) are one decade greater than for the interface state generation. Two-dimensional simulations show that they are mainly responsible for the I/sub d/-V/sub g/ degradation of the LDD MOSFETs, and that the trap concentrations deduced from charge-pumping experiments are consistent with the I/sub d/-V/sub g/ degradation. >

Bias stress-induced instabilities in amorphous silicon nitride/hydrogenated amorphous silicon structures: Is the ‘‘carrier-induced defect creation’’ model correct?

The effects of positive and negative bias stress on hydrogenated amorphous silicon nitride/hydrogenated amorphous silicon (a-SiNx:H/a-Si:H) structures are investigated as a function of stress time, and stress temperature. It is shown that bias stress induces a parallel shift of the capacitance voltage (C-V) characteristics. The direction of the C-V shift depends on the sign of the applied stress voltage, while the magnitude of the C-V shift depends on stress time and temperature in a manner which is identical to that observed in a-Si:H thin-film transistors. In addition, it is shown that positive bias stress increases the number of localized states in the a-Si:H mobility gap, but negative bias stress does not. However, the observed increase cannot account for the corresponding C-V shift. These results lead us to conclude that the C-V shift is not induced by dangling bond defects in a-Si:H but rather by carrier trapping in the insulator.