Carrier Lifetime Degradation Research Articles

Device Modeling of 4H-SiC PIN Photodiodes with Shallow Implanted Al Emitters for VUV Sensor Applications

A numerical model is presented for the simulation of ultraviolet ion-implanted 4H-SiC photodiodes with shallow p- emitter doping profiles. An existing model for SiC pin photodiodes, taken from literature, is modified with a dedicated SiO2-SiC interface layer to account for degradation of carrier mobility and lifetime at the interface. Furthermore, aluminum compensation in 4H-SiC is included and its impact on the spectral response and carrier recombination is analyzed. The simulated spectral response in the wavelength range from 200 to 400 nm is compared to experimental data. While the existing model, taken from literature, fails to predict the performance of VUV photodiodes with a shallow p- emitter, the newly designed model successfully achieves high accuracy, even with a basic modeling approach featuring an abrupt material parameter transition.

Kinetics of Carrier Lifetime Degradation in High‐Temperature 1 MeV Electron‐Irradiated Cz n‐Si Associated with the Formation of Divacancy‐Oxygen Defects

Kinetics of degradation of the nonequilibrium charge carrier lifetime (τ) in Czochralski‐grown (Cz) n‐Si irradiated with 1 MeV electrons at different temperatures in the range from 20 to 285 °C are experimentally and theoretically investigated. It is established that changes in τ are qualitatively and quantitatively determined by the temperature of electron irradiation. Analysis of experimental data using the Shockley‐Read‐Hall (SRH) theory shows that the formation of recombinationally active divacancy‐oxygen (V2O) and vacancy‐oxygen (VO) complexes is the main mechanism of τ degradation in this experiment.

Tracking Hydrogen During Poly-Si/SiOx Contact Fabrication: An Infrared Spectroscopy Analysis of Si–H Bonds Configurations

The hydrogenation step contributing to the high efficiencies (>25%) reached with poly-Si/SiOx passivated contacts solar cells is still poorly understood. In this study, Fourier transform infrared spectroscopy (FTIR) is used to follow the different bonding configurations of H during the fabrication process. The carrier lifetime degradation upon annealing is correlated to an important loss of Si–H bonds, from both the a­‑Si:H film and the SiOx interfaces. The subsequent hydrogenation step results in the formation of a small number of Si–H bonds near the crystalline silicon c-Si/SiOx interface, associated with the low stretching mode (LSM) and correlated to a significant lifetime improvement. These bonds feature a preferential orientation, as shown by polarized measurements.

Improved Surface Passivation of Rear SiNx/AlOx Stacks in p‐Type Boron‐Doped Czochralski Silicon Wafers During Dark Annealing at Elevated Temperatures

Herein, the surface passivation quality of p‐type boron‐doped Czochralski silicon (Cz‐Si) wafers with passivated emitter and rear cell (PERC) structure during 200 °C dark annealing is investigated. During such a treatment, a reduction in the saturation current density (J 0) along with the degradation and regeneration of minority carrier lifetime can only be observed in PERC‐structured wafers that fast fired at high temperatures instead of in unfired wafers, which is possibly correlated with hydrogen injected into the silicon bulk during fast firing. Besides, no notable variations in J 0 can be observed in fast‐fired SiN x /n + /p‐Si/n + /SiN x symmetrical structure wafers during the same treatment, although their minority carrier lifetime undergoes degradation and regeneration cycles, demonstrating that the improved surface passivation comes from the rear surface of PERC structure. The capacitance–voltage measurement on the fabricated rear surface samples with metal‐insulator‐semiconductor (MIS) structure further demonstrates that the improved surface passivation is not related to field‐effect passivation. By comparing the surface state density of rear surface during dark annealing, it can be concluded that the improved rear surface passivation originates from the hydrogenation of surface defects such as dangling bonds, possibly by the outdiffused hydrogen from silicon bulk during such a treatment.

Carrier Lifetime in 60Co Gamma and 1 MeV Electron‐Irradiated Tin‐Doped n‐Type Czochralski Silicon: Conditions for Improving Radiation Hardness

Herein, the results that determine some conditions for increasing the radiation hardness of 60Co gamma or 1 MeV electron‐irradiated tin‐doped n‐type Czochralski silicon (Cz n‐Si:Sn) are presented. These conditions are determined from the analysis of the formation kinetics of dominant radiation defects (namely, VO and SnV complexes) and the recombination of charge carriers through the electronic levels of these defects in samples with different concentrations of phosphorus and tin. It is shown that low‐resistivity Cz n‐Si with P doping levels >5 × 1014 cm−3 containing in addition [Sn] ≈1017–1019 cm−3 has a radiation hardening potential. In this material, the radiation degradation of carrier lifetime is several times smaller compared with Sn‐free n‐Si, whereas the conductivity compensation is negligible for both materials. The reduction of the lifetime degradation rate is due to a reduction of VO concentration in low‐resistivity Cz n‐Si:Sn.

Carrier Lifetime Degradation and Regeneration in Gallium- and Boron-Doped Monocrystalline Silicon Materials

In this article, carrier lifetime degradation phenomena on fired gallium-doped Czochralski-grown silicon (Cz-Si:Ga) and boron-doped float-zone silicon (FZ-Si:B) are observed. We examine lifetime degradation and regeneration as a function of illumination intensity and temperature and observe qualitatively similar degradation effects in both material classes, which are triggered by a fast-firing high-temperature step. Charge carrier injection, e.g., through illumination, is required to activate the defects responsible for degradation. The extent of degradation increases with increasing temperature, which is untypical for degradation effects reported before. Despite different degradation time constants are measured for Cz-Si:Ga and FZ-Si:B, the activation energies are for both materials in the narrow range (0.58 ±0.04) <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eV</b> . The extracted activation energy is quite different compared with other degradation effects in silicon, suggesting a novel defect formation mechanism. Since the lifetime degradation is triggered by the fast-firing of the silicon wafers during the presence of a hydrogen-rich dielectric at the surface, the involvement of hydrogen in the defect reaction is very likely. During prolonged illumination at elevated temperature (135 °C), we observe a permanent regeneration of the lifetime, whereas at temperatures close to room temperature (36 °C), the defect deactivation is only temporary.

Influence of Dopant Elements on Degradation Phenomena in B‐ and Ga‐Doped Czochralski‐Grown Silicon

The response of lifetime samples made from boron‐ and gallium‐doped Czochralski‐grown silicon from the same producer to light‐ and elevated temperature‐induced degradation (LeTID) conditions (varying illumination at 75 °C), to dark anneals (DAs) at 175 °C, and the temporary recovery (TR) reaction under different conditions is investigated. It is found that Ga‐doped samples behave very differently than their B‐doped counterparts: while the carrier lifetime remains at a high level if an illumination equivalent to 1 sun at 75 °C is applied, strong carrier lifetime degradation occurs at low light intensities. A capture cross‐sectional ratio in degraded Ga‐doped samples of ~26 is found, which is typical for the LeTID defect. TR of this degraded state is observed on Ga‐doped samples when the illumination intensity is increased at 75 °C and when samples are illuminated at 25 °C with intermediate intensity. During DA of B‐doped samples, a bulk‐related degradation and a subsequent surface‐related degradation are observed. In contrast, degradation of Ga‐doped samples during DA only occurs on long timescales, and its cause is not clear, yet. It is concluded that the specific dopant species plays an active role both during LeTID and for surface‐related degradation—possibly as a result of differences in the acceptor–hydrogen pair properties.

Radiation Resistance of High-Voltage Silicon and 4H-SiC Power p-i-n Diodes

The different effect of displacement damage produced by neutron irradiation on the static characteristics of 4.5-kV silicon and 4H silicon carbide (SiC) p-i-n power diodes is explained using deep level transient spectroscopy (DLTS), ${C}$ – ${V}$ profiling, and open-circuit voltage decay (OCVD) measurements. The number of introduced defects in SiC is higher, also the degradation of carrier lifetime and carrier removal proceeds more swiftly in SiC than those in silicon. However, smaller dimensions and a higher doping level of the n-base of the SiC diode compensate for these negative effects. As a result, the SiC p-i-n diode exhibits substantially higher resistance to neutron irradiation at higher fluences when the diode loses its ON-state carrier modulation capability. SiC also shows a negligible effect of irradiation on leakage current due to the wider bandgap. One may assume a better reliability of SiC bipolar devices over the silicon in a high neutron radiation environment.

Origin of carrier lifetime degradation in floating-zone silicon during a high-temperature process for insulated gate bipolar transistor

We report on the relationship between carrier lifetime degradation of floating-zone silicon (FZ Si) and the guard ring formation process for silicon integrated gate bipolar transistor (Si-IGBT). A clear carrier lifetime degradation was observed through the guard ring oxidation and annealing processes for Si-IGBT and showed interstitial oxygen (Oi) concentration dependence, which was obtained by Fourier transform infrared absorption spectroscopy. Based on the carrier lifetime measurements through the step etching of the FZ Si substrate, it has been suggested that the carrier lifetime degradation is not only caused by the Oi itself near the FZ Si surface region but also the other defects induced by the Oi injection. Diffused interstitial Si atoms kicked-out by the Oi into the FZ Si substrate, which has a longer diffusion length than Oi, can be considered to be the origin of the carrier lifetime degradation.

Influence of Tabula Rasa on Process- and Light-Induced Degradation of Solar Cells Fabricated From Czochralski Silicon

Monocrystalline Si solar cells are fabricated from Czochralski (Cz) Si, which contains 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> oxygen atoms. Cz Si undergoes degradation during high-temperature thermal processing steps, such as dopant diffusion to form the p-n junction. This degradation in the bulk minority carrier lifetime can be related to the formation of oxygen precipitates. We found that a high-temperature annealing process known as tabula rasa (TR) not only mitigates process-induced degradation via oxygen precipitate nuclei dissolution, but also modifies subsequent light-induced degradation. We report on the bulk carrier lifetime of n- and p-type Cz Si after TR, which homogenizes the interstitial oxygen in the bulk Si to its monoatomic form in either an N2 or O2 environment. A control sample, which was not subjected to a TR processing step, experienced severe process-induced degradation during a boron emitter thermal budget as oxygen precipitates were formed in the Si bulk. These precipitates could be imaged using photoluminescence. Additionally, samples that underwent a TR processing step prior to the boron emitter thermal budget show efficient gettering of metallic impurities compared to the control sample, which showed a decline in the implied open-circuit voltage after the gettering step. Furthermore, modification of the interstitial oxygen bonding by TR had a strong effect on the light-induced degradation kinetics. Instead of a typically observed monotonic decrease, minority carrier lifetime increases first, followed by a nonmonotonic decrease over a ~20 h period. We conclude that by modifying the interstitial oxygen bonding via TR pretreatment, p-type Cz Si wafers become substantially resistant to harsh solar cell processes and strongly modified light-induced degradation, which would open alternative ways to mitigate this degradation mechanism.

The Role of Grown-In Defects in Silicon Minority Carrier Lifetime Degradation During Thermal Treatment in Epitaxial Growth Chambers

Severe silicon lifetime degradation was found after its high-temperature treatment in III–V material growth chambers for the fabrication of III–V/Si multijunction solar cells. Further improvement of the cell efficiency requires insights into the root cause of such lifetime degradation and how to protect the silicon lifetime accordingly. While the exact origins of such degradation remain largely unknown, most published work concluded that extrinsic impurities that diffuse into the silicon bulk during the thermal treatment are the sole reason. In this article, we show that while not necessarily present in every float-zone silicon wafer, grown-in defects that can be thermally activated is also a key mechanism behind the observed silicon lifetime degradation during anneal in our molecular beam epitaxy chamber. As such, annealing of the silicon wafer in the furnace at 1000 °C to annihilate the grown-in defects, together with the deposition of a SiNX diffusion barrier to prevent the extrinsic impurities from diffusing into the silicon bulk, are both required to preserve the silicon lifetime throughout the III–V material growth steps.

Investigation of ITO magnetron deposition effect on carrier lifetime degradation in silicon wafer

The effect of ITO magnetron deposition on carrier lifetime in silicon was demonstrated. The method of 2D photoluminescence decay time measurements revealed the lifetime degradation in silicon after magnetron deposition of thin ITO film which is used for top contact in modern solar cells.

Reliability Characterization and Modeling of High Speed Ge Photodetectors

Understanding the origin and protocols to induce performance degradations of silicon photonics high speed photodetector represent a major issue for the qualification of the reliability of these devices. Using advanced characterization technics, it is shown that the dark current, the photonic current and the cut-off frequency of the photodiode can be degraded during voltage stress of 106 s, which could ultimately induce some significant device performance drift and failure. An explanation of these degradations is presented based on both electrical characterization and device modelling. The observed degradations of both dark current and responsivity can indeed be modeled by a single carrier lifetime degradation, attributed to an increase of the surface recombination rate, impacting an unexpected large contribution of carrier diffusion in the photocurrent. The results obtained with this model are experimentally confirmed by extracting the activation energy of the dark current, before and after stress. The improved physical understanding of the degradation is expected to lead to shorter test protocols for silicon photonics devices.

Fast and Slow Stages of Lifetime Degradation by Boron–Oxygen Centers in Crystalline Silicon

A conflict between previous and recently published data on the two‐stage light‐induced degradation (LID) of carrier lifetime in boron‐doped oxygen‐containing crystalline silicon is addressed. The previous experiments showed the activation of two boron–oxygen recombination centers with strongly differing recombination properties for the fast and slow stages of LID, whereas more recent studies found only a single center for both stages. To resolve this controversy, the historic silicon samples of these previous examinations are re‐examined in this study after more than one decade. It is found that, in the historic samples, the fast stage can be either described by two different centers or a mixture of the two, depending on the duration of previous dark annealing. A possible solution is suggested based on the involvement of different activating impurities in the boron–oxygen defect. In dark‐annealed samples, the defect consisting of boron, oxygen, and the activation impurity is present in two latent configurations, which reconfigure during LID at a fast and a slow stage. In the examined historic silicon samples, which did not undergo a gettering pretreatment, a significant concentration of an additional boron–oxygen defect with a different kind of activating impurity attached exists. The historic and modern results are thus reconciled.

Application of the Concept of Lifetime‐Equivalent Defect Density in Defect Systems Comprising a Multitude of Defect Species

The specific injection dependence of either excess carrier lifetime or lifetime‐equivalent defect density generally allows for the identification of a single defect species and the analysis of its properties. However, the presence of a multitude of defect species noticeably complicates this endeavor. A temporally different activation/deactivation dynamic of different defect species, as encountered in the context of light‐induced degradation phenomena in crystalline silicon solar cells, in combination with the distinct injection dependence of specific defect species can help to distinguish the involved defect species. Within this simulation‐based contribution, it is shown how injection‐dependent information can be used to reveal the presence of a second defect species in the context of light‐induced degradation and to distinguish clearly whether the formation of deep‐level defects in the bulk or a deterioration in the surface passivation quality is responsible for an observed degradation of effective excess carrier lifetime.

Strain-Energy Release in Bent Semiconductor Nanowires Occurring by Polygonization or Nanocrack Formation.

Strain engineering of semiconductors is used to modulate carrier mobility, tune the energy bandgap, and drive growth of self-assembled nanostructures. Understanding strain-energy relaxation mechanisms including phase transformations, dislocation nucleation and migration, and fracturing is essential to both exploit this degree of freedom and avoid degradation of carrier lifetime and mobility, particularly in prestrained electronic devices and flexible electronics that undergo large changes in strain during operation. Raman spectroscopy, high-resolution transmission electron microscopy, and electron diffraction are utilized to identify strain-energy release mechanisms of bent diamond-cubic silicon (Si) and zinc-blende GaAs nanowires, which were elastically strained to >6% at room temperature and then annealed at an elevated temperature to activate relaxation mechanisms. High-temperature annealing of bent Si-nanowires leads to the nucleation, glide, and climb of dislocations, which align themselves to form grain boundaries, thereby reducing the strain energy. Herein, Si nanowires are reported to undergo polygonization, which is the formation of polygonal-shaped grains separated by grain boundaries consisting of aligned edge dislocations. Furthermore, strain is shown to drive dopant diffusion. In contrast to the behavior of Si, GaAs nanowires release strain energy by forming nanocracks in regions of tensile strain due to the weakening of As-bonds. These insights into the relaxation behavior of highly strained crystals can inform the design of nanoelectronic devices and provide guidance on mitigating degradation.

Deposition behaviour of metal impurities in acidic cleaning solutions and their impact on effective minority carrier lifetime in n-type silicon solar cells

Further increasing the conversion efficiency of silicon solar cells by advanced solar cell concepts like SHJ (Silicon Hetero-Junction), PERT (Passivated Emitter Rear Totally diffused), IBC (Interdigitated Back-Contact), requires a highly controlled silicon surface in terms of metal impurity density. The goal of this study is investigating the deposition property of different metals in acidic cleaning solutions and consequently determining the impact of these impurities on the effective minority lifetime of the crystalline silicon solar cell material. The deposition behaviour of impurities is mathematically described with an adsorption isotherm theory. Once attached at the silicon surface, the metal impurity can diffuse into the silicon bulk material during a high temperature process step like diffusion, passivation or contact annealing (firing).First, we have studied the adsorption isotherm behaviour of prominent metal impurities (Fe, Cu, Ti, Co, Cr and Zn) in acidic liquid onto a silicon surface. Moreover, the impact of metal impurities on effective minority carrier lifetime in commercially available n-type Cz-silicon (3–5 Ω cm, 239 cm2) is determined. From controlled contamination experiments we can extract an exchange volume, that provides the minimum amount on wet chemical cleaning solution that is required to clean a silicon surface from a given high metal impurity level to a specified low metal impurity level. This exchange volume plays a crucial role by defining feed and bleed recipes on wet chemical cleaning tanks in solar cell manufacturing. A second parameter extracted is the effective minority carrier lifetime degradation of the metal impurities. After deposition on the surface, the impurities are driven into the silicon bulk by high temperature processes. Depending on the lifetime degradation capacity, a relation of effective minority carrier lifetime versus surface contamination level could be identified for each metal impurity. We found Co, Fe and Cu having a strong impact on effective minority carrier lifetime degradation, e.g. 2e10/cm2 Fe surface concentration can reduce the effective minority carrier lifetime by 75%, while Ti, Zn and Cr are more moderate and only high surface concentrations above 1e12/cm2 lead to effective minority carrier lifetime degradation of more than 20% compared to the non-contaminated wafers.

Dependence of the Bulk Electrical Properties of Multisilicon on the Grain Misorientation Parameters

The recombination activity of intragrain defects in multicrystalline silicon is investigated by the electron or laser beam induced current methods. The interrelation of the grain orientation with the character of the distribution of intragrain defects (dislocations and impurity inclusions) and their recombination activity is revealed. The defect grain structure is investigated using various etching procedures to reveal the defects. It is shown that the defect density and distribution in the grains depend on their orientation relative the growth axis. Therefore, it is intragrain defects and impurities that are to a large degree responsible for degradation of the nonequilibrium carrier lifetime when compared with grain boundaries.

Зависимость объемных электрофизических свойств мультикремния от параметров разориентации зерен

AbstractThe recombination activity of intragrain defects in multicrystalline silicon is investigated by the electron or laser beam induced current methods. The interrelation of the grain orientation with the character of the distribution of intragrain defects (dislocations and impurity inclusions) and their recombination activity is revealed. The defect grain structure is investigated using various etching procedures to reveal the defects. It is shown that the defect density and distribution in the grains depend on their orientation relative the growth axis. Therefore, it is intragrain defects and impurities that are to a large degree responsible for degradation of the nonequilibrium carrier lifetime when compared with grain boundaries.

Damage to passivation contact in silicon heterojunction solar cells by ITO sputtering under various plasma excitation modes

Analysis of damage to passivation contact in heterojunction with intrinsic thin layer (HIT) solar cells by indium tin oxide (ITO) sputtering under various plasma excitation modes such as radio frequency (RF), direct current (DC), RF-superimposed DC, pulsed DC is presented. A significant degradation of effective minority carrier lifetime of the precursor cells at both low and high-level injections after the ITO deposition by sputtering technique was observed regardless of plasma excitation modes. We found that the main reason for this degradation was mainly due to the ion flux in the sputtering plasma rather than the ion kinetic energy. On the basis of the experimental and simulated results, we attributed the degradation of surface passivation quality mainly to the damage of the amorphous-silicon/crystalline-silicon interfaces rather than that of the intrinsic hydrogenated amorphous silicon layers. This degradation could be significantly reduced by the ITO deposition at a high temperature in which a trade-off between degradation and curing step was expected to occur. The cell performance of the HIT solar cells using the ITO films deposited by various plasma excitation modes at 180 °C was subsequently discussed. The ITO films deposited by the RF-superimposed DC mode exhibited a high potential as excellent opto-electrical properties and free sputter-induced damage electrodes for HIT solar cell applications. To our best knowledge, this is the first time that the ITO films deposited by the RF-superimpose DC mode are used as electrodes in the HIT solar cells.