Impact of iron on the room temperature luminescence efficiency of oxygen-containing precipitates in silicon

Oxygen precipitation in silicon has been associated with a weak room temperature sub-bandgap luminescence emission at around 1600 nm. We show that the additional presence of iron impurities enhances this emission by an order of magnitude and results in a red shift of the peak luminescence by approximately 45 nm. We not only observe an increase in the luminescence emission with iron contamination level but also with the density and size of the oxide precipitates. Moreover, we provide evidence that the sub-bandgap luminescence emission increases proportionally with the concentration of iron segregated to oxide precipitates after high temperature (>700 °C) annealing and thus allows evaluation of the gettering efficiency of oxygen-containing precipitates. Annealing of iron-contaminated samples at low temperatures (550 °C) results in a considerable reduction in the interstitial iron concentration without changing the sub-bandgap luminescence, indicating that the sink to which iron diffuses depends upon temperature

Dynamic photoluminescence lifetime imaging for injectiondependent lifetime measurements

We investigate the impact of an injection-dependent carrier lifetime in crystalline silicon on dynamic photoluminescence lifetime imaging (dynamic PLI). Although the dynamic lifetime approach is a technique that evaluates the time-dependence of a quantity proportional to the excess carrier density, it is only weakly influenced by the injection-level dependence of the lifetime. The reason for the little impact is the fact that the evaluation of dynamic PLI measurements does not only involve the decay of the carrier density, as it is common for photoconductance decay measurements, but also the increase of the carrier density directly after switching on the excitation source. In this contribution, we present injection-dependent lifetime measurements that are acquired with the camera-based dynamic PLI technique. We find that the deviation of the actual steady-state carrier lifetime from the lifetime obtained with dynamic PLI is less than 20 % for a wide range of measurement conditions.State of Lower SaxonyGerman Federal Ministry for the Environment, Nature Conservation, and Nuclear Safet

Electronic properties of iron-boron pairs in crystalline silicon by temperature- and injection-level-dependent lifetime measurements

Iron-boron pairs in crystalline silicon are studied by measuring the recombination lifetime as a function of injection density, doping concentration, and temperature. The characteristic crossover point of the injection-level-dependent carrier lifetime curves measured before and after optical dissociation of the iron-boron pairs is analyzed to determine the energy level as well as the electron- and hole-capture cross sections of the acceptor level of iron-boron pairs, assuming known recombination parameters for interstitialiron. The doping concentration dependence of the crossover point gives an electron-capture cross section of (1.4±0.2)×10¯¹⁴cm², while the temperature dependence results in a hole-capture cross section in the range from 0.5×10¯¹⁵to2.5×10¯¹⁵cm² and an energy level of (0.26±0.02)eV below the conduction-band edge

Layer selective laser ablation for local contacts to thin emitters

High efficiency solar cells require high generation and low recombination rates. High bulk lifetime, well passivated surfaces, and lowly doped thin emitters allow for low recombination rates. Thin passivated emitters should be contacted locally in order to avoid excessive contact recombination. This is common practice for front junction solar cells but is also advantageous for back junction cells. We analyze a novel layer selective laser ablation process. From a passivating stack composed of 70 nm silicon nitride that we deposit on top of 35 nm of amorphous silicon we selectively ablate the silicon nitride layer. Transmission electron microscopy investigations confirm the full ablation of the silicon nitride layer. After the ablation process, a 17 nm-thick amorphous silicon layer remains on the substrate. The crystalline silicon substrate shows no dislocations after the process. Evaporating aluminum on top of the locally ablated nitride layers forms local contacts of the aluminum to the silicon

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

Generation and annihilation of boron–oxygen-related recombination centers in compensated p- and n-type silicon

The impact of boron–oxygen-related recombination centers as well as their defect kinetics have been intensely studied in boron-doped oxygen-rich p-type crystalline silicon. Experimental data for the defect in simultaneously boron- and phosphorus-doped compensated p- and n-type silicon, however, is sparse. In this study, we present time-resolved carrier lifetime measurements on Czochralski-grown silicon (Cz-Si) doped with both boron and phosphorus under illumination at 30 °C(defectgeneration) as well as at 200 °C in the dark (defect annihilation).Funding was provided by the State of Lower Saxony. D.M. is supported by an Australian Research Council QEII Fellowship

Impact of dopant compensation on the deactivation of boron-oxygen recombination centers in crystalline silicon

The boron-oxygen recombination center responsible for the light-induced degradation of Czochralski silicon solar cells can be permanently deactivated by illumination at elevated temperature. In this study, we examine the impact of dopant compensation on the deactivation process. The experimental results show that the deactivation rate depends inversely on the total boron concentration instead of the net doping concentration, suggesting that boron is directly involved in the deactivation process. A linear dependence of the activation energy on the total boron concentration further supports this conclusion.Funding was provided by the State of Lower Saxony and the DAAD/Go8 Australia Germany Joint Research Cooperation Scheme

Fundamental consideration of junction formation strategies for phosphorus-doped emitters with J0e < 10 fA/cm2

This work shows the potential of further optimization of phosphorus-doped emitters in p-type silicon solar cells. We investigate the impact of different combinations of phosphorus doping profiles and surface passivation qualities on the saturation current density J0e by considering boundary conditions based on published experimental data. Our simulation study shows that there are two possible ways to achieve J0e values below 10 fA/cm2. One is the reduction of the electrically active phosphorus concentration nsurf at the surface beneath 2×1019 cm-3 and simultaneously reducing the surface recombination velocity Sp to below 103 cm/s. The other contrarily increases nsurf to values of up to 1×1021 cm-3 while ensuring full activation of all phosphorus dopants. In the latter case, J0e values below 10 fA/cm2 seem possible, even for Sp = 107 cm/s which is equal to the thermal velocity

Uncertainty of the coefficient of band-to-band absorption of crystalline silicon at near-infrared wavelengths

We present data of the coefficient of band-to-band absorption of crystalline silicon at 295 K in the wavelength range from 950 to 1350 nm and analyze its uncertainty. The data is obtained from measurements of reflectance and transmittance as well as spectrally resolved photoluminescence measurements and spectral response measurements. A rigorous measurement uncertainty analysis based on an extensive characterization of our setups is carried out. We determine relative uncertainties of 4% at 1000 nm, increasing to 22% at 1200 nm and 160% at 1300 nm, and show that all methods yield comparable results. © 2014 AIP Publishing LLC

Recombination at local aluminum-alloyed silicon solar cell base contacts by dynamic infrared lifetime mapping

The application of local aluminum (Al)-alloyed contacts to the p-type base of silicon solar cells reduces minority charge carrier recombination due to the formation of a local back surface field (LBSF). We study the recombination properties and formation of base contacts, which are realized by local laser ablation of a dielectric stack (laser contact opening - LCO) and subsequent full area screen printing of Al paste. Based on charge carrier lifetime measurements using the camera-based and calibration-free dynamic infrared lifetime mapping (ILM) technique, we determine contact recombination velocities at the contacts as low as Scont = 65 cm/s on 200 Ωcm float-zone silicon (FZ-Si) and corresponding reverse saturation current densities of J0,cont = 900 fA/cm2 on 1.5 Ωcm FZ-Si. As a result we show that local contact geometries with point contact radii r &gt; 100 μm and line contact widths a &gt; 80 μm are appropriate for lowest contact recombination employing local Al alloyed contacts. Furthermore, complete and high quality laser ablation of the dielectric stack is necessary for the formation of a sufficiently thick LBSF