Solid State Phenomena Vols. 205-206

Abstract: Grain boundaries and dislocations are major crystallographic defects in multicrystalline Si materials for solar cells. Heavily dislocated grains are detrimental to the photovoltaic performance. This paper attempts to clarify the origin of inhomogeneous defect distribution in multicrystalline Si. The impacts of crystal orientation and grain boundary were investigated. The crystal orientation gives an important geometrical effect in the possibility of initiating slip in a grain when subjected to stress. The presence of grain boundary can also affect dislocation distribution depending on boundary character.

Abstract: We report on 0.93 eV luminescence observed in multicrystalline silicon. The spectral line is close to the well known D3 one, but its properties are different. The new feature shows a remarkable intensity at room temperature, exceeding the intensity of the band to band radiative transition. Moreover, it appears as a single line in the entire temperature range 10-300K, in contrast to the D3, which is usually accompanied by D4. Cathodoluminescence (CL) and electron beam induced current (EBIC) micrographs revealed that the centers causing 0.93 eV emission are irregularly distributed along certain grain boundaries. Electron backscattering diffraction examination showed that the 0.93 eV luminescence appears at grain boundaries characterized by a lattice rotation around a <344> axis. The EBIC contrast at those irregularities indicates strong total recombination. Based on an analysis of the temperature dependence of the CL intensity and the EBIC contrast we obtained an activation energy of about 120 meV.

Abstract: To get the optimized condition and ideal furnace structure, we have performed seed cast growth of mono-crystalline Si by using unidirectional solidification furnace. More than 20 ingots of 10 cm diameter and 10 cm height were grown under different growth conditions. The quality of ingots was characterized by using Fourier transform infrared spectroscopy (FTIR), infrared microscopy, scanning infrared polariscope (SIRP), X-ray topography, etc. We have realized reduction of carbon, residual strain and extended defects, which may contribute the increase of solar cell efficiency.

Abstract: The residual strain distribution in cast-grown mono-like Si ingots is analyzed. The effect of the crucible during solidification and the influence of different cooling rates is described. To clarify in which process steps residual strain accumulates, several Si ingots were grown in a laboratory scale furnace (100mm) using different cooling conditions after completion of the solidification. For the cooling, two different cooling rates were distinguished: fast cooling (12deg/min) and slow cooling (5deg/min). It was found that changes in cooling gradients greatly influence the amount of residual strain. The results show that slow cooling in any temperature range leads to strain reduction. The greatest reduction could be found when the temperature gradient was changed to slow cooling in the high temperature region.

Abstract: Photoconductance measurements have been one of the most common ways to measure the lifetime in silicon for over 60 years. Since 1985, the most common method for doing calibrated lifetime measurements is using an eddy-current sensor to monitor photoconductance as a function of time and illumination, providing data that can be interpreted in terms of carrier density and hence lifetime. Here we present recent extensions to this measurement technique that have generalized the method. Bulk lifetime measurements on industrial samples are presented. The information available from the effects of grain boundaries on eddy-current measurements are summarized. Recent applications for the use of these instruments for measurement of mobility in compensated materials are also described.

Abstract: This work presents recent advances in the characterisation of carrier recombination and impurities at Fraunhofer ISE. The role of iron contamination during crystallisation is analysed in more detail. Numerical simulations and comparisons to experimental data are presented which demonstrate the impact of iron from the crucible and crucible coating and show the in-diffusion of iron into the silicon melt as well as into the solid silicon during crystal cooling. Measurements of spatially resolved carrier lifetime and interstitial iron concentration on wafers after phosphorus diffusion gettering are used as input for cell efficiency modelling which reveals the specific and quantitative role of iron on cell parameters in multicrystalline silicon. A new photoluminescence based method is presented which quantitatively determines the interstitial iron concentration in finished solar cells. We finally present advances in defect characterisation with sub-micrometre resolution: We show recent progress in micro photoluminescence spectroscopy for the quantitative measurement of interstitial chromium with high spatial resolution. A further development of this setup will be discussed: By combining the principle of Light Beam Induced Current (LBIC) or voltage (LBIV) and the highly localized illumination, images of carrier recombination at local defects are presented which feature a, compared to EBIC, higher signal-to-noise ratio.

Abstract: Photoluminescence imaging techniques have recently been extended to silicon bricks for early production quality control and electronic characterisation in photovoltaics and microelectronics. This contribution reviews the state of the art of this new method which is fundamentally based on spectral luminescence analyses. We present highly resolved bulk lifetime images that can be rapidly extracted from the side faces of directionally solidified or Czochralski grown silicon bricks. It is discussed how detailed physical modelling and experimental verification give good confidence of the best practice measurement errors. It is also demonstrated that bulk lifetime imaging can further be used for doping and interstitial iron concentration imaging. Additionally, we show that full spectrum measurements allow verification of the luminescence modelling and are, when fitted to the theory, another accurate method of extracting the absolute bulk lifetime.

Abstract: Recently introduced techniques for whole wafer mapping and imaging create new possibilities for root cause analysis of emitter passivation defects. Inline compatible PL imaging identifies such defects as localized regions with increased emitter saturation current and reduced implied open circuit voltage. Advanced offline evaluation of defective areas can be then performed with multiparameter noncontact measurements capable to establish the role of surface recombination, the interface trap density, or the dielectric charge that controls the field-effect passivation. The relevant novel metrologies are discussed and are illustrated using examples of advanced silicon passivation by dielectric films and by a-Si heterojunction structures.

Abstract: To investigate transition metal precipitates in Si, synchrotron based measurements, like micro x-ray fluorescence (μXRF) or detailed transmission electron microscopy (TEM) studies, are usually necessary. Transition metals are among the most detrimental defects in multi-crystalline (mc) silicon material for solar cell applications, due to their impact on minority charge carrier lifetime and possible shunt formation. We present another possibility to investigate transition metal precipitates by 3-dimensional focused ion beam (3D-FIB) cutting using a combined scanning electron microscope (SEM) SEM-FIB-system. This method is able to detect transition metal precipitates down to 5 nm in radius and provides additional information about the 3D shape, size and spatial distribution of the precipitates.

Abstract: A comparative study of multicrystalline Si based solar cells and plastically deformed single crystalline Si by the EBIC, LBIC and XBIC methods as well as a computer simulation were carried out. The XBIC measurements were realized on a laboratory X-ray source. Simulations of LBIC and XBIC contrast values for grain boundaries, dislocations and spherical precipitates were carried out for different diffusion length and beam diameter values. It is shown by a computer simulation that the LBIC and XBIC contrast of two-dimensional defects in the crystals with a large enough diffusion length can be a few times higher than that in the EBIC mode, i.e. these methods in recent multicrystalline Si structures allow to reveal grain boundaries with the lower recombination strength. The contrast of dislocations perpendicular to the surface can be comparable in all three methods. The XBIC and LBIC contrast of precipitates usually is essentially smaller than that in the EBIC mode and could approach it in the structures with the small diffusion length only. Experimental data confirming the results of simulations are presented.