Dislocation density reduction limited by inclusions in kerfless high‐performance multicrystalline silicon

Kerfless high‐performance multicrystalline silicon is an emerging material for photovoltaic applications that is characterized by having a smaller grains, and general lower average dislocation density than conventional ribbon multicrystalline silicon. Although a significant improvement over state‐of‐the‐art, dislocation reduction at the crystal growth stage is not complete. Here we employ an annealing process previously tested in conventional ingot mc‐Si to reduce dislo‐ cation clusters that remain after crystal growth. A sample is subjected to a 1390 °C annealing process for 24 h and its dislocation density reduction is evaluated. We employ infrared birefringence imaging to observe that despite achieving significant average dislocation density reduction, if inclusions are present in the sample, these may serve to nucleate new dislocations due to thermal strain. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

A deep level transient spectroscopy study on the interface states across grain boundaries in multicrystalline silicon

Degrading the recombination activities of grain boundaries (GBs) is essential to improve the efficiency of multi‐crystalline silicon (mc‐Si) based solar cells. We apply the deep level transient spectroscopy technique to detect interface states at Σ3 and Σ9 GBs in mc‐Si. The density of interface states close to midgap is found comparable for both as‐grown GBs. Gettering or hydrogenation leads to shallower states with a smaller capture cross section and lower density. Recombination activity reduction for Σ3 GBs is stronger than for Σ9 GBs especially after hydrogenation. Both the analysis approach and experimental results could be applied for a specific GB engineering of mc‐Si based solar cells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Impact of the firing temperature profile on light induced degradation of multicrystalline silicon

Light‐ and elevated temperature‐induced degradation in multicrystalline silicon can reduce the efficiency of solar cells significantly. In this work, the influence of the firing process and its temperature profile on the degradation behaviour of neighbouring mc‐Si wafers is analysed. Five profiles with measured high peak temperatures ≥800 °C and varying heating and cooling ramps are examined. With spatially resolved and lifetime calibrated photoluminescence images, normalized defect concentrations N^*_t are calculated to determine the degradation intensity. Wafers that underwent a fast firing process typical for industrial solar cell production show a significantly stronger degradation than samples that were subjected to the same peak temperature but with slower heating and cooling rates. A spatially resolved analysis of the carrier lifetime in the whole wafer shows that the degradation begins in low lifetime areas around dislocation clusters, spreading into good grains after several hours. By the use of optimized ramp‐up and/or ramp‐down rates during the firing even at very high peak temperatures, light and elevated temperature induced degradation can be suppressed. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Relation between the internal stress measured in creep and the stress generated by the dislocation structure in the fcc metals

From the steady-state creep rate data treated as a function of the applied and the measured effective stress and temperature, a phenomenological dependence of the internal stress on the applied stress and temperature was derived. The result determined the expected character of the applied stress- and temperature dependences of the dislocation density, which was considered the microstructure parameter of the internal stress σ _i?=?α MGbρ ^1/2 (σ_i is the internal stress, α the dislocation interaction factor, M the Taylor factor, G the shear modulus, b the Burgers vector length and ρ the dislocation density). A scaling of the expected dislocation density by fitting it to measured dislocation density data yielded reasonable values of the parameter α in the Taylor formula, but the experimental data indicated a weaker applied stress dependence of the measured dislocation density than that of the expected dependence. An admission of an empirical formula fitting the dependence of dislocation density on the applied stress and temperature leads to a suggestion that the parameter α might be dependent on applied stress and temperature.     

All‐optical modulation based on silicon quantum dot doped SiOx:Si‐QD waveguide

All‐optical modulation based on silicon quantum dot doped SiO_x:Si‐QD waveguide is demonstrated. By shrinking the Si‐QD size from 4.3 nm to 1.7 nm in SiO_x matrix (SiO_x:Si‐QD) waveguide, the free‐carrier absorption (FCA) cross section of the Si‐QD is decreased to 8 × 10⁻¹⁸ cm² by enlarging the electron/hole effective masses, which shortens the PL and Auger lifetime to 83 ns and 16.5 ps, respectively. The FCA loss is conversely increased from 0.03 cm⁻¹ to 1.5 cm⁻¹ with the Si‐QD size enlarged from 1.7 nm to 4.3 nm due to the enhanced FCA cross section and the increased free‐carrier density in large Si‐QDs. Both the FCA and free‐carrier relaxation processes of Si‐QDs are shortened as the radiative recombination rate is enlarged by electron–hole momentum overlapping under strong quantum confinement effect. The all‐optical return‐to‐zero on‐off keying (RZ‐OOK) modulation is performed by using the SiO_x:Si‐QD waveguides, providing the transmission bit rate of the inversed RZ‐OOK data stream conversion from 0.2 to 2 Mbit/s by shrinking the Si‐QD size from 4.3 to 1.7 nm.     

Microstructural identification of Cu in solar cells sensitive to light‐induced degradation

Light‐induced degradation (mc‐LID or LeTID) can lead to a severe efficiency loss in multi‐crystalline solar cells. The underlying mechanism clearly distinguishes from known mechanisms as B‐O‐LID and Fe‐B‐LID. Various defect models have been suggested for mc‐LID mainly based on metal impurities, including Cu which is known to cause light‐induced degradation. We investigate mc‐LID sensitive PERC cells that show an efficiency degradation of 15%_rel. The weaker degradation of the grain boundaries (GBs) typical for mc‐LID is identified and further investigated from front and rear side with respect to recombination activities. The combination of local electrical measurements (LBIC), target preparation (REM, FIB) and element analysis (EDX, TEM) unveil Cu‐containing precipitates at the rear side of the solar cells. They accumulate at grain boundaries and at the rear surface of the Si‐bulk material where the passivation stack is damaged. We conclude that Cu originates from the cell material and discuss its relation to mc‐LID.

LBIC mapping (EQE at fixed wavelength) of a degraded mc‐Si PERC cell from front and rear side results in qualitatively different appearance of GBs.     

Plasma Diagnostics Using K‐Line Emission Profiles of Silicon

Modifications of K‐line profiles due to a warm dense plasma environment are a suitable tool for plasma diagnostics. We focus on Si K_α emissions due to an electron transfer from 2P to 1S shell. Besides 2P fine structure effects we also consider the influence of excited and higher ionized emitters. Generally spoken, a plasma of medium temperature and high density (warm dense matter) is created from bulk Si the greater part of atoms is ionized. The high energy of K_α x‐rays is necessary to penetrate and investigate the Si sample. The plasma effect influences the many‐particle system resulting in an energy shift due to electron‐ion and electron‐electron interaction. In our work we focus on pure Si using LS coupling. Non‐perturbative wave functions are calculated as well as ionization energies, binding energies and relevant emission energies using the chemical ab initio code Gaussian 03. The plasma effect is considered within a perturbative approach to the Hamiltonian. Using Roothaan‐Hartree‐Fock wave functions we calculate the screening effect within an ion‐sphere model. The different excitation and ionization probabilities of the electronic L‐shell and M‐shell lead to a charge state distribution. Using this distribution and a Lorentz profile convolution with a Gaussian instrument function we calculate spectral line profiles depending on the plasma parameters. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Si–Si optical phonon behavior in localized Si clusters of Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> alloy nanocrystals

Raman spectra acquired from Si _x Ge_1−x -nanocrystal-embedded SiO₂ films show dependence of the Si–Si optical phonon frequency on Si content. The frequency upshifts, and peak intensity increases as the silicon concentration increases. For a given Si content, the frequency remains unchanged with annealing temperature. Spectral analysis and density functional theory calculation reveal that the optical Si–Si phonon is related to the formation of localized Si clusters surrounded by Si/Ge atomic layers in the Si _x Ge_1−x nanocrystals and the intensity enhancement arises from the larger cluster size. The synergetic effect of surface tensile stress and phonon confinement determines the Si–Si optical phonon behavior.     

Light‐induced degradation of PECVD aluminium oxide passivated silicon solar cells

Light‐induced degradation (LID) has been identified to be a critical issue for solar cells processed on boron‐doped silicon substrates. Typically, Czochralski‐grown silicon (Cz‐Si) has been reported to suffer from stronger LID than block‐cast multicrystalline silicon (mc‐Si) due to higher oxygen concentrations. This work investigates LID under conditions practically relevant under module operation on different cell types. It is shown that aluminium oxide (AlOx) passivated mc‐Si solar cells degrade more than a reference aluminium back surface field mc‐Si cell and, remarkably, an AlOx passivated Cz‐Si solar cell. The defect which is activated by illumination is shown to be doubtful a sole bulk effect while the AlOx passivation might play a certain role. This work may contribute to a re‐evaluation of the suitability of boron‐doped Cz‐ and mc‐Si for solar cells with very high efficiencies. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Silicon nanoparticle charge trapping memory cell

A charge trapping memory with 2 nm silicon nanoparticles (Si NPs) is demonstrated. A zinc oxide (ZnO) active layer is deposited by atomic layer deposition (ALD), preceded by Al₂O₃ which acts as the gate, blocking and tunneling oxide. Spin coating technique is used to deposit Si NPs across the sample between Al₂O₃ steps. The Si nanoparticle memory exhibits a threshold voltage (V_t) shift of 2.9 V at a negative programming voltage of –10 V indicating that holes are emitted from channel to charge trapping layer. The negligible measured V_t shift without the nanoparticles and the good re‐ tention of charges (>10 years) with Si NPs confirm that the Si NPs act as deep energy states within the bandgap of the Al₂O₃ layer. In order to determine the mechanism for hole emission, we study the effect of the electric field across the tunnel oxide on the magnitude and trend of the V_t shift. The V_t shift is only achieved at electric fields above 1 MV/cm. This high field indicates that tunneling is the main mechanism. More specifically, phonon‐assisted tunneling (PAT) dominates at electric fields between 1.2 MV/cm < E < 2.1 MV/cm, while Fowler–Nordheim tunneling leads at higher fields (E > 2.1 MV/cm). (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of partially strain-relaxed Si1-yCy epilayers on (100)Si

1-y C_y epilayers were grown by MBE on (100) Si single-crystal substrates either directly on a dislocation-free or on a highly dislocated Si buffer layer. The orientation of the epilayers and their strain status were measured by double-crystal X-ray diffraction. Cross sections were prepared for TEM investigations. Epitaxial layers of about 130 nm thickness and carbon contents up to [%at.]1.38 grown on top of dislocation-free 1-μm-thick Si buffer layers were fully strained. In TEM bright field images, no dislocations were found. In order to introduce a high dislocation density in the Si buffer layer, the native oxide on the substrate was only partially removed prior to growing the Si buffer. A Si_1-yC_y film grown on top of that highly dislocated buffer layer showed a partial stress relaxation (a_∥=5.429 Å<a_si=5.431 Å). The large FWHM of transverse rocking scans through the Bragg reflection corresponding to the epilayer indicates a high defect density. TEM cross-section micrographs showed an extension of threading dislocations from the Si buffer layer into the Si_1-yC_y layer. Received: 22 April 1998/Accepted: 22 April 1998     

Heavily doped Si:P emitters of crystalline Si solar cells: recombination due to phosphorus precipitation

The measured saturation current density J_0e of heavily phosphorus‐doped emitters of crystalline Si solar cells is analysed by means of sophisticated numerical device modelling. It is concluded that Shockley–Read–Hall (SRH) recombination exceeds Auger recombination significantly; it is caused by inactive phosphorus. This explains the large discrepancies between measured and simulated J_0e values, observed persist‐ently over the last two decades in industrially fabricated Si solar cells. As a consequence, the heavily phosphorus‐diffused emitters still bear a significant potential to contribute to higher Si solar cell efficiency levels, if the amount of inactive phosphorus can be reduced. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

X‐ray excited optical luminescence from crystalline silicon

Synchrotron based X‐ray excited optical luminescence (XEOL) has been measured with many direct bandgap semiconductors. We present XEOL measurements on crystalline silicon (Si), obtained despite of its indirect bandgap and the consequently low luminescence efficiency. Spectra of monocrystalline and multicrystalline (mc) Si at room temperature are compared to theoretical spectra. A possible application in the synchrotron‐based research on mc‐Si is exemplified by combining XEOL, X‐ray fluorescence (XRF) spectroscopy, photoluminescence (PL) spectroscopy, and microscope images of grain boundaries. This approach can be utilized to investigate the recombination activity of metal precipitates, to analyze areas of different lifetimes on mc‐Si samples and to correlate additional material parameters to XRF measurements. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interface state density and barrier height improvement in ammonium sulfide treated Al2O3/Si interfaces

The HF treatment removes the native oxide and lays behind the dangling bonds over the Si surface which causes the increment in density of interface traps (D_it) through the direct deposition of high-k dielectric on Si. Here, we propose the facile method for reduction of interface traps and improvement in barrier height with the (NH₄)₂S treatment on Al₂O₃/Si interfaces, which can be used as the base for the non-volatile memory device. The AFM was used to optimize the treatment time and surface properties, while XPS measurements were carried out to study the interface and extract the barrier height (Φ_B). The short period of 20 s treatment shows the improvement in the barrier height (1.02 eV), while the one order reduction in the D_it (0.84 × 10¹² cm²/eV) of sulfur passivated Al/Al₂O₃/Si MOS device. The results indicate the favorable passivation of the dangling bonds over the Si surfaces covered by sulfur atoms.     

Micro‐Raman analysis of GaAs Schottky barrier solar cell

Gallium arsenide (GaAs) cells have been in the race with silicon single‐crystal cells for the highest efficiency photovoltaic devices. The annealed, irradiated Schottky barrier (SB) solar cells were characterised using micro‐Raman spectroscopy at three different regions: namely, at the (1) ohmic contact region, (2) unirradiated region and (3) irradiated region. We also present a micro‐Raman study of the damage process in annealed GaAs SB solar cells bombarded by high‐energy ions. A Gaussian line shape was fitted to the Raman spectra of the longitudinal optical phonon A₁(LO), and parameters such as intensity, full width at half maximum (FWHM) and the area under the peak were obtained for the different annealing temperatures. Biaxial stress (σ), carrier concentration (n), depletion length (L_d), dislocation velocity (ν) and life time of the first‐order optical phonon (τ) of the A₁(LO) mode of the irradiated region of the samples annealed at different temperatures were calculated. Copyright © 2010 John Wiley & Sons, Ltd.     

Ion implantation into amorphous Si layers to form carrier‐selective contacts for Si solar cells

This paper reports our findings on the boron and phosphorus doping of very thin amorphous silicon layers by low energy ion implantation. These doped layers are implemented into a so‐called tunnel oxide passivated contact structure for Si solar cells. They act as carrier‐selective contacts and, thereby, lead to a significant reduction of the cell's recombination current. In this paper we address the influence of ion energy and ion dose in conjunction with the obligatory high‐temperature anneal needed for the realization of the passivation quality of the carrier‐selective contacts. The good results on the phosphorus‐doped (implied V_oc = 725 mV) and boron‐doped passivated contacts (iV_oc = 694 mV) open a promising route to a simplified interdigitated back contact (IBC) solar cell featuring passivated contacts. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

On the observation of electron–hole liquid luminescence under low excitation in Al2O3‐passivated c‐Si wafers

We report on low‐temperature photoluminescence (PL) from aluminum oxide (Al₂O₃)‐passivated c‐Si wafers, which surprisingly exhibits clear signature of the formation of the so‐called electron–hole liquid (EHL), despite the use of excitation powers for which the condensed phase is not usually observed in bulk Si. The elevated incident photon densities achieved with our micro‐PL setup together with the relatively long exciton lifetimes associated with a good quality, indirect band‐gap semiconductor such as our float‐zone c‐Si, are considered the key aspects promoting photogenerated carrier densities above threshold. Interestingly, we observe a good correlation between the intensity of the EHL feature in PL spectra and the passivation performance of the Al₂O₃ layer annealed at different temperatures. The change in the extension of the sub‐surface space‐charge region that results from the balance between the induced fixed charge in the Al₂O₃ and the defect states at the alumina/Si interface is at the origin of the observed correlation. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Excellent Si surface passivation by low temperature SiO2 using an ultrathin Al2O3 capping film

It is demonstrated that the application of an ultrathin aluminum oxide (Al₂O₃) capping film can improve the level of silicon surface passivation obtained by low‐temperature synthesized SiO₂ profoundly. For such stacks, a very high level of surface passivation was achieved after annealing, with S_eff < 2 cm/s for 3.5 Ω cm n‐type c‐Si. This can be attributed primarily to a low interface defect density (D_it < 10¹¹ eV^–1 cm^–2). Consequently, the Al₂O₃ capping layer induced a high level of chemical passivation at the Si/SiO₂ interface. Moreover, the stacks showed an exceptional stability during high‐temperature firing processes and therefore provide a low temperature (≤400 °C) alternative to thermally‐grown SiO₂. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Demolding temperature in thermal nanoimprint lithography

In thermal nanoimprint lithography, temperature is one of the most important process parameters. Temperature is not only important for the flow of resist during molding but also for demolding, the process by which the imprint stamp is removed from the molded resist/substrate. This is because thermal stress and friction and adhesion forces generated at the stamp/resist interface and the mechanical strength of the resist are all dependent on temperature. In this paper, we demonstrate via both experimentation and numerical simulation that an optimal temperature (T _d) leading to minimal deformation of molded resist exists for demolding. The ease of demolding was directly accessed by measuring demolding force at different T _d for a Si stamp/PMMA/Si substrate system of 4-in.-diameter using a mechanical tester. Numerically, the demolding process for a simple two-dimensional model of a Si stamp/poly(methyl methacrylate) (PMMA) resist/Si substrate system was simulated using a finite-element method for different T _d, assuming viscoelasticity of the PMMA resist and temperature dependence of friction coefficients at the stamp/PMMA interface. We found that a temperature leading to the minimum in both the demolding force and the normalized stress vs. T _d curves exists below the glass transition temperature of the PMMA resist, from which the optimal T _d was derived.     

Assessment of the illumination dependency of Al2O3 and SiO2 rear‐passivated p‐type silicon solar cells

This Letter discusses an important difference between positively charged SiO₂ and negatively charged Al₂O₃ rear‐passivated p‐type Si solar cells: their illumination level dependency. For positively charged SiO₂ rear‐passivated p‐type Si solar cells, a loss in short circuit current (J_SC) and open circuit voltage (V_OC) as a function of illumination level is mainly caused by parasitic shunting and a decrease in surface recombination, respectively. Hence, the relative loss in cell conversion efficiency, J_SC, and V_OC as a function of the illumination level for SiO₂ compared to Al₂O₃ rear‐passivated p‐type Si solar cells has been measured and discussed. Subsequently, an exponential decay fit of the loss in cell efficiency is applied in order to estimate the difference in the energy output for both cell types in three different territories: Belgium (EU), Seattle and Austin (US). The observed trends in the difference in energy output between both cells, as a function of time of the year and region, are as expected and discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)