H.J. Möller

Oxygen and Carbon Precipitation in Multicrystalline Solar Silicon

Oxygen and carbon are the main impurities in multicrystalline silicon for photovoltaic applications. Precipitation of oxygen and carbon occurs during crystal growth and solar cell processing. Depending on the thermal conditions and the initial oxygen and carbon content various types of SiO2, SiC precipitates and oxygen related defects are observed and investigated by IR spectroscopy and transmission electron microscopy. Topographic μ-PCD measurements are used to study the minority carrier lifetime in the material locally. It is found that certain types of oxygen defects reduce the lifetime of the bulk and enhance the recombination activity of dislocations. Quantitative measurements of the oxygen precipitation of pre-annealed specimens are carried out to study the oxygen precipitation systematically. A statistical nucleation and growth model using rate equations and a Focker-Planck equation is applied to simulate the precipitation process numerically.

Measurement of the Normalized Recombination Strength of Dislocations in Multicrystalline Silicon Solar Cells

An improved technique is presented to measure the normalized recombination strength Gat dislocations in silicon solar cells that were fabricated of cast grown silicon. G is the number ofrecombinations per unit time, length, and excess carrier density divided by the minority carrierdiffusion coefficient D. The measurement is based on fitting the theoretical correlation betweeninternal quantum efficiency IQE at a single wavelength and dislocation density r to the measureddata. The IQE is measured topographically by the light beam induced current (LBIC) method. Foreach point of the LBIC map a dislocation density is determined by analysing the etched samplesurface with an image recognition programme. The theory for IQE(r) combines Donolatosprediction for L(r) with a calculation of IQE(L) made by the computer programme PC1D. L is thediffusion length of the minority carriers. The programme PC1D takes special properties of the solarcell process into account. The method was applied to solar cells made by a conventional furnaceprocess as well as a rapid thermal process (RTP). In the latter case a correlation between G and theemitter diffusion temperature was found. Finally TEM measurements were made to investigatedislocations with different values of G.

Oxygen and lattice distortions in multicrystalline silicon

Abstract Oxygen is one of the main impurities in multicrystalline silicon for photovoltaic applications. Precipitation of oxygen occurs during crystal growth and solar cell processing. It is shown that dislocations enhance the oxygen precipitation. Depending on the thermal conditions and the initial oxygen content various types of SiO 2 +precipitates and oxygen related defects are observed and investigated by fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy. The large area distribution of oxygen decorated dislocations is studied by scanning infrared microscopy (SIRM). Both inhomogeneous distributions of dislocations and oxygen precipitates occur and can lead to internal stresses. The internal stresses of multicrystalline-silicon wafers are investigated by an optical method using polarized infrared light. The results are compared with the dislocation microstructure and the oxygen distribution in wafers produced by different growth techniques.

The relationship between microstructure and dislocation density distribution in multicrystalline silicon

The investigation of the grain structure is important to understand the origin of dislocations during crystal growth of multicrystalline silicon. This paper studies the dislocation density distribution for different grain orientations that occur during crystal growth. Single grains are analyzed in detail, including their microstructure. The grain orientations are determined by means of the electron backscatter diffraction technique. The obtained information reveals grain orientations, which allow a higher number of active slip planes during crystal growth process. The number of active slip planes during solidification seems to influence the dislocation density in the final crystal.

In situ film thickness and temperature control of molecular beam epitaxy growth by pyrometric interferometry

Abstract In this paper we present an overview as well as latest results of applying pyrometric interferometry (PI) to in situ film thickness and temperature measurements in molecular beam epitaxy facilities. We treat two PI configurations (PI and reflection supported PI) and discuss their different advantages and shortcomings. Physical basics, experimental set-ups as well as evaluation procedures are outlined. Special emphasis is given on long term experience at III–V MBE and the impact of PI on yield, reproducibility, number of required calibration runs and device performance.

Infrared Transmission Investigations of Rod - Like Defects in Multicrystalline Silicon

Parts from a specially grown multicrystalline test ingot showed diffuse absorption regions in the IR transmission. The underlying defects were investigated with IR microscopy and FTIR spectroscopy. The typical features that can be observed in the IR microscope images of wafers from the diffuse absorption regions are long filament-like defects aligned in bundles and having a preferential orientation along the crystallization direction. In FTIR spectroscopy broad absorption bands in the wavenumber range between 750 and 1050 cm can be found in specimens from the same diffuse absorption regions. EDS measurements with a scanning electron microscope yield an increased nitrogen concentration. It is thus suggested that the underlying defects are silicon nitride particles that can grow under certain growth conditions. The preferential orientation indicates that these rod-like defects grow at the liquid-solid interface from the melt during crystallization.

Carbon-Induced Twinning in Multicrystalline Silicon

Multicrystalline EFG ribbon silicon for photovoltaic applications cont ai s high concentrations of carbon, which can exceed the maximum solubility l imit at the melting temperature. In this paper experimental results are presented whi ch show that under certain growth conditions carbon can precipitate in high concentrations together with the formation of twin boundaries. This carbon-induced twining process appears to be a rather ge neral mechanism since it has also been observed in other multicrystalline silicon materia ls as well. A model will be presented that can explain the cooperative twinning and carbon precipitation proces s. The carbon-decorated twin boundaries give also rise to high internal stresses of about 20 – 10 MPa which contributes to the fracture sensitivity of the EFG material.

Investigations on the Behaviour of Carbon during Inductive Melting of Multicrystalline Silicon

The influence of the CO concentration in the gas phase on the distribution of carbon in Bridgman-grown, multicrystalline silicon is studied. The growth experiments were conducted in a high-vacuum induction furnace either under a CO enriched atmosphere or under CO free conditions. Furthermore, thermodynamic calculations in the system silicon/oxygen/carbon were done. In crystal growth under a CO enriched atmosphere a SiC-containing layer is formed on the top surface of the melt in agreement with the calculated phase diagram. In this case, the level of substitutional carbon in the cystal was found to be almost constant, whereas the axial carbon concentration in crystals grown under CO free conditions increases monotonously according to Scheils law.

Structural and electrical properties of silicon epitaxial layers grown by LPE and CVD on identical polycrystalline substrates

We compare structural and electrical properties of polycrystalline Si layers grown by chemical vapour deposition (CVD) and liquid-phase epitaxy (LPE) on multicrystalline, cast silicon substrates with similar grain boundary structures. Time-resolved microwave conductivity shows a higher minority carrier lifetime in LPE than in CVD layers; the calculated diffusion lengths are up to three times the layer thickness for LPE-grown layers. After etching the samples in Secco or Sirtl solution, we measured in the p-type Si epitaxial LPE and CVD layers practically at the same dislocation density as in the same areas of the substrate. Electron-beam-induced current measurements reveal a low recombination strength of grain boundaries and dislocations in the LPE-grown layers compared to those of the CVD layers. Transmission electron microscope investigations indicate that the lower recombination strength at the grain boundaries of the LPE layers is due to a lower density of grain boundary dislocations.

Calculation and experimental characterization of the defect physics in CuInSe2

Abstract Numerical simulations of the defect distribution of CuInSe 2 were carried out as a function of the stoichiometry. The simulations are based on a new calculation of the intrinsic defects in this material. The results of the calculations were compared with earlier electrical and positron lifetime measurements. This leads to the assumption, that the single defects V Se , V Cu , Cu In and the defect pair (2V Cu –In Cu ) occur in the investigated specimens in considerable concentrations.