Changes In Minority Carrier Lifetime Research Articles

Temperature dependent behavior of sub-monolayer quantum dot based solar cell

This work investigates the temperature dependence of the performance of In(Ga)As-based solar cells made from sub-monolayer (SML) quantum dots (QDs), quantum wells (QWs), and Stranski-Krastanov QDs (SK-QDs). Well-defined sub-bandgap peaks are observed in external quantum efficiency (EQE) spectra for all samples. Apparently, changes in minority carrier lifetime affect both the sub-bandgap and the above bandgap EQE. Lateral confinement of carriers in the SMLs and QDs also affects the temperature dependence of the EQE. Finally, a “U” shaped curve was observed from the temperature dependence of the short-circuit current (ISC) from these nanostructure-based solar cells. This is due to the competition between the decreasing bandgap and the increasing carrier loss to recombination within the nanostructures as the temperature increases. In contrast, the solar cell's open-circuit voltage and power efficiency decrease monotonically with temperature.

Comparison of phosphorus gettering effect in faceted dendrite and small grain of multicrystalline silicon wafers grown by floating cast method

We attempt to clarify the effect of phosphorus diffusion gettering (PDG) on the differences in microstructures of multicrystalline silicon (mc-Si) wafers. From the results of the floating cast method, the obtained microstructure of mc-Si was determined to contain unique microstructures, consisting of large faceted dendrites and small grains with low and high dislocation densities. The PDG efficiency was evaluated through the change in minority carrier lifetime. As a result, a stronger positive PDG effect was found for large faceted dendrites with low dislocation density, attributed to a small number of defects that can act as recombination centers of carriers. Lifetime improvement was also found in dislocation regions possibly owing to the redistribution of interstitial metals. These results suggest the importance of controlling the microstructure of mc-Si, which could strongly affect the PDG efficiency and existence of incorporated impurities.

The relaxation behaviour of supersaturated iron in single-crystal silicon at 500 to 750 °C

Iron-related defects cause major problems in silicon for both microelectronic devices and photovoltaics. Iron contamination can occur during high temperature processing or, particularly in the case of low-cost photovoltaics, from the feedstock. In many situations, silicon is cooled too rapidly for the establishment of equilibrium, and so the bulk iron concentration exceeds the solubility value. We have investigated the relaxation of supersaturated bulk iron to the equilibrium solubility in single-crystal silicon. Bulk iron concentrations are measured by analysing the change in minority carrier lifetime that occurs when iron-boron pairs are dissociated. High-purity silicon is rubbed with iron and annealed at 750 °C for 24 h. This process creates an iron silicide phase on the rubbed surface and allows the equilibrium solubility of ∼2 × 1012 cm−3 to be established. Samples are then annealed at lower temperatures (500 to 700 °C) for a range of times. The rate of decay in iron concentration depends upon whether a silicide was formed on one side or two sides, with the kinetics in excellent agreement with iron diffusion to one or both surfaces, respectively. Even for the highest supersaturation (∼2000 times the solubility), the pre-existence of a silicide on one surface means there is insufficient driving force for nucleation of a silicide on the other surface. Relaxation experiments were also performed on contaminated samples for which the iron silicide source at the surface was removed after contamination. The iron concentration decays substantially more slowly in these specimens. The kinetics can be explained by relaxation to bulk voids.

Changes in Minority Carrier Lifetime of Hydrogen-Terminated Si Surfaces in Dry- and Wet-Air

Minority carrier lifetime of HF-treated Si(100) surfaces decreases in wet-air more rapidly than in dry-air. XPS measurements show that OH and O are mainly included in Si by keeping in wet-air and dry-air, respectively. OH cannot form network, and thus backbonded OH generates Si dangling bonds. On the other hand, backbonded O can form Si-O-Si networks, leading to the higher lifetime due to lower defect density. When the atmosphere is switched from dry-air to wet-air, the lifetime greatly decreases, and the workfunction increases. These changes are attributed to dominant formation of backbonded OH.

Contamination of silicon by iron at temperatures below 800 °C

AbstractIron‐related defects are deleterious in silicon‐based integrated circuits and photovoltaics, ruining devices and acting as strong recombination centres. Unless great care is taken, iron contamination will result from high temperature processing and so it is essential to understand the degree to which this can occur. Iron solubility data above ∼800 °C have been summarised by Istratov et al. (Appl. Phys. A 69, 13 (1999)), but many processes are performed at lower temperatures for which solubility data are scarce. We have studied iron contamination below ∼800 °C. Iron concentrations in intention‐ ally contaminated air‐annealed Czochralski silicon samples were determined from the change in minority carrier lifetime due to photodissociation of FeB pairs measured by quasi‐steady‐state photoconductance. In the ∼600 to 800 °C temperature range the iron concentration was found to vary according to $ 1.3 \times 10^{21} \; {\rm exp} \;\left (‐ { {1.8\;{\rm eV} } \over {kT} } \right)\;{\rm cm}^{‐3}. It is therefore the case that significantly more iron can dissolve in silicon at these temperatures than extrapolation of higher temperature data suggests, with the enhancement being by a factor of >20 at 600 °C. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-Temperature Boron Gettering for Improving the Carrier Lifetime in Fe-Contaminated Bifacial Silicon Solar Cells with n+pp+ Back-Surface-Field Structure

Gettering kinetics of Fe contaminant by doped boron during low-temperature annealing is discussed to improve the minority carrier lifetime in bifacial silicon solar cells with an n+pp+ back surface field (BSF) structure composed of a boron-doped p-base and a boron diffused p+ layer. A model for Fe-gettering by boron is introduced and computer simulations are carried out for the change in minority carrier lifetime along the thermal process in cell fabrication. Lifetime behavior shows good consistency with experimental results when “Fe-behavior parameters” and proper boundary conditions of the initial Fe concentration being higher than the solubility limit at the gettering temperature are taken into account. As a consequence, low-temperature boron gettering employed after boron diffusion for BSF fabrication is found to markedly improve the carrier lifetime cooperating with the phosphorous gettering associated with the pn junction formation, and can recover the initial high lifetimes before cell fabrication. Additionally, a certain condition of short-time heat treatment at higher temperature is found for firing which does not deteriorate the recovered lifetimes.

Characteristics and performance of silicon solar cells between low-and high-level injection—Part II: Results of the study

Applying the extension of the transport velocity transformation method described in Part I of this paper [1], the I-V characteristics of several types of solar-cell structures have been studied in the range between the validity of the low-level or of the high-level injection assumptions. The structures investigated were a more heavily doped "wide-base diode," a lowly doped "narrow-base diode," and a structure with a high-low junction in its base. Various effects have been seen to dominate in different ranges of the I-V characteristic, causing observable changes in its slope. Depending on the particular structure, the major effects are: changes in the majority-carrier distribution, ohmic effects and changes in conductivity modulation, changes in minority-carrier lifetime, and reduction of the high-low junction barrier height, both of the latter resulting from increasing carrier concentrations. In addition, the influence of the impurity concentration in the more lowly doped layer of the base in solar cells with a high-low junction, on the conversion efficiency and on its dependence on the light intensity was investigated for optical concentration ratios up to 1000. In these studies, the doping range of 5E15-5E17/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> was seen to yield a broad efficiency maximum, with the higher doping more favorable except for its limitation by the onset of the heavy doping effects. It has also been seen that the collection efficiency is increased or decreased by some of the effects investigated. In consequence, the collection efficiency can no longer be rigorously considered as independent of the light intensity. In some cases, this dependency occurs even when the low-level injection condition is still fulfilled.

A comparison of Nd:YAG fundamental and second-harmonic Q-switched laser-beam lifetime doping in single-crystal silicon

Silicon devices including bipolar transistors, junction diodes, and MOS capacitors were scanned by a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -switched Nd:YAG (1.06 µm) and frequency-doubled Nd:YAG (0.53 µm) radiations under various conditions. The electrical characteristics of these devices were measured before and after scanning and again after thermal annealing. The data includes transistor gain versus laser power; junction diode leakage current versus junction depth; MOS <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C-V</tex> lifetime versus laser power and the effects of subsequent thermal anneals on all of these. The results are that bulk minority-carrier lifetime decreases of several orders of magnitude will be produced by either of these radiations at peak power levels below those which will produce any visible surface damage. The changes in minority-carrier lifetime are stable for post scanning thermal anneals up to 400°C and are almost completely removed from an 800°C anneal. The depths within which minority-carrier lifetime changes significantly are 0.7 and 1.8 µm for 0.53- and 1.06-µm wavelength laser radiations, respectively. The results indicate that the recombination centers produced by the scanning are point defects and their density decreases exponentially with the distance into the silicon. The average power thresholds for point defect production (for both 0.53- and 1.06-µm wavelengths) were determined and are observed to increase with increased laser wavelength and pulse width. Potential applications in silicon devices and integrated circuits such as selective lifetime doping, β trimming, and selective-link making without passivation damage are possible.

Defects in electron-irradiated, gallium-doped silicon

Defect processes were studied in electron-irradiated, Ga-doped silicon using deep level transient spectroscopy. Evidence for the production of a Ga-related defect state is presented. Changes in minority-carrier lifetime are correlated with recovery and growth of defect states in annealing studies of Ga-doped silicon.

Correlation of proton damage of silicon with neutron and electron damage

Abstract The damage introduced in a semiconductor is made evident by a change in minority-carrier lifetime. Irradiation by electrons produces relatively low-energy silicon-atom primary recoils which, in turn, produce isolated defect centers. Neutron irradiation produces high-energy recoils which create clusters of damage, as first discussed by Gossick. In the case of proton irradiation, both high–and low-energy recoils are produced. This physical process is used to derive a quantitative means of extrapolating proton damage from electron and neutron data. This is done by using cross-section data or theory to calculate the number of recoil atoms produced with energy E p. If E p < 20 keV, the defects are “electron-like,” N E; otherwise, they are “neutron-like,” N N. The change in reciprocal lifetime per unit fluence (cm−2) is equal to k E N E plus k N N N, where k E (= 5.4 × 10−9 cm2/sec) and k E (= 10−5 cm2/sec) are found from appropriate experiments reported in the literature. This rule is used to calculat...

Effect of neutron irradiation on GaAs1−xPx electroluminescent diodes

Light-emitting diodes fabricated from the GaAs1−xPx alloy system undergo degradation upon neutron irradiation. It has been found that the smallest change in efficiency occurs when the composition of the alloy is between x = 0.30 and x = 0.75. The change in minority-carrier lifetime, however, is approximately constant throughout the entire alloy range. It is suggested that the relative insensitivity to neutron irradiation of the efficiency in the alloy range x = 0.30–0.75 is connected with the inherent disorder already present within the GaAs1−xPx alloy system.

Activation Energy of Defects in Neutron Irradiated Silicon

Annealing studies of defects introduced in Si by a fast neutron flux have been made, using planar transistor samples to follow changes in minority carrier lifetime. An activation energy of 1.2 ± 0.1 eV was obtained from measurements of annealing over the range 100°C to 156°C.

Correlation of Electron-Induced Changes in Transistor Gain with Components of Recombination Current

After low-exposure dosage of electron and/or gamma radiation, anomalous permanent changes in transistor gain have been observed which cannot be attributed to changes in base minority-carrier lifetime or to temporary surface effects. A study of these permanent effects has been performed using base current component analysis similar to that applied to neutron damage by Goben. By this method it was possible to observe the buildup with dose of individual components of base recombination current. These components can be related to damage in the base-emitter space-charge region and in the base regions of the devices. It was possible to correlate the buildup and/ or saturation of the components with structure observed in the curves of anomalous gain degradation. Furthermore, analysis of damage in the base region revealed two components which appear to be related to changes in the surface recombination velocity and to changes in the minority-carrier lifetime, respectively.

Radiative recombination in germanium as an indicator of radiation damage

It is shown that the radiative recombination of electron-hole pairs from a semiconductor can be used as a monitor of minority-carrier lifetime. By observing the change in the radiative signal under electron-bombardment from n- and p-type germanium for a wide range of majority carrier concentrations, it is shown that a single type of recombination center can account for the minority-carrier lifetime. This level is located at either 0·21 eV from the conduction band or 0·28 eV from the valence band, and has a capture probability for minority holes 24 times that for minority electrons. This technique can be used for the study of radiation damage in other semiconductors where electron-hole radiative recombination has been observed. A prime attribute of the method is that changes in minority-carrier lifetime can be detected in material whose initial lifetime can be of the order of 10 −10 sec.