Naomi Nandakumar

Low-Absorbing and Thermally Stable Industrial Silicon Nitride Films With Very Low Surface Recombination

Amorphous silicon nitride has become the state-of-the-art antireflection coating for silicon solar cells. Optimization of silicon nitride films requires consideration of both the films optical and electrical properties. It is commonly assumed that silicon-rich silicon nitride films (films with high refractive index) provide better surface passivation, compared to that obtained by films with lower indices. However, silicon-rich films are usually very absorptive in the short (and even medium) wavelength range. Development of low absorption silicon nitride films, that provide good surface passivation, is therefore highly valuable. In this study we compare nine different industrial silicon nitride films, all with similarly low refractive index of 2.09 ± 0.01 measured at 633 nm. We demonstrate that these films exhibit very different electrical, chemical, and optical properties despite their similar refractive index values and correlate these differences with the specific deposition conditions. As a result of this investigation, we have developed industrial thermally stable low-absorbing silicon nitride films that provide excellent surface passivation, with surface saturation current density of 7 fA/cm2 on both n- and p-type wafers. We demonstrate that the developed low absorption films provide surface passivation with equal quality to that obtained by industrial silicon-rich silicon nitride films.

Should the refractive index at 633 nm be used to characterize silicon nitride films

The refractive index at 633 nm is often used to characterize silicon nitride films. Besides providing information about the reflection at this particular wavelength, it is frequently used to indicate additional information regarding the films absorption and even regarding its surface passivation quality. In this study, we compare nine different silicon nitride films, all with a similar refractive index at 633 nm (2.09±0.01). We demonstrate that these films exhibit very different electrical, chemical and optical properties despite their similar refractive index values. As a result of this investigation, we have developed industrial low-absorption silicon nitride films that provide excellent surface passivation, with saturation current density of 7 fA/cm2 on both n- and p-type wafers. This surface passivation quality is equal to that obtained by industrial silicon-rich silicon nitride films. All the films developed in this study were fabricated using industrial equipment and are thermally stable.

Resistive Intrinsic ZnO Films Deposited by Ultrafast Spatial ALD for PV Applications

Spatial atomic layer deposition (ALD) of intrinsic zinc oxide (i-ZnO) films is scaled up from the laboratory to the industrial level, and the film properties are investigated in detail. A high growth rate of 35 nm/min is achieved. The deposited films are transparent and have an unusually high resistivity of about 100 Ω · cm. This is attributed to the extremely short precursor exposure and purge duration of spatial ALD (~8 ms), as compared with temporal ALD (~1-10 s). The growth of highly crystalline and nearly stoichiometric i-ZnO films is achieved. This makes these i-ZnO layers ideal for applications as insulating window layers in Cu(In,Ga)Se2 solar cells.

Surface passivation investigation on ultra-thin atomic layer deposited aluminum oxide layers for their potential application to form tunnel layer passivated contacts

The surface passivation performance of atomic layer deposited ultra-thin aluminium oxide layers with different thickness in the tunnel layer regime, i.e., ranging from one atomic cycle (~0.13 nm) to 11 atomic cycles (~1.5 nm) on n-type silicon wafers is studied. The effect of thickness and thermal activation on passivation performance is investigated with corona-voltage metrology to measure the interface defect density D it(E) and the total interface charge Q tot. Furthermore, the bonding configuration variation of the AlO x films under various post-deposition thermal activation conditions is analyzed by Fourier transform infrared spectroscopy. Additionally, poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) is used as capping layer on ultra-thin AlO x tunneling layers to further reduce the surface recombination current density to values as low as 42 fA/cm2. This work is a useful reference for using ultra-thin ALD AlO x layers as tunnel layers in order to form hole selective passivated contacts for silicon solar cells.

Impacts of light illumination on monocrystalline silicon surfaces passivated by atomic layer deposited Al2O3 capped with plasma-enhanced chemical vapor deposited SiN x

In this work, we investigate the impact of light illumination on crystalline silicon surfaces passivated with inline atomic layer deposited aluminum oxide capped with plasma-enhanced chemical vapor deposited silicon nitride. It is found that, for dedicated n-type lifetime samples under illumination, there is no light induced degradation (LID) but enhanced passivation. The lifetime increase happened with a much faster speed compared to the lifetime decay during dark storage, resulting in the overall lifetime enhancement for actual field application scenarios (sunshine during the day and darkness during the night). In addition, it was found that the lifetime enhancement is spectrally dependent and mainly associated with the visible part of the solar spectrum. Hence, it has negligible impact for such interfaces applied on the rear of the solar cells, for example p-type aluminum local back surface field (Al-LBSF) cells.

Application of non-contact corona-Kelvin metrology for characterization of PV dielectrics on textured surfaces

Non-contact corona-Kelvin metrology has been extensively used in the IC industry for over 20 years and has more recently been utilized in the photovoltaic (PV) industry for dielectric characterization. Application to leaky low temperature PV dielectrics required various enhancements to the technique, such as an accelerated time-resolved charge-measure cycle to mitigate leakage and longer wavelength illumination to eliminate photo-induced leakage observed in silicon-rich silicon nitrides (SiNx). These enhancements allowed extraction of important passivation properties such as interface state density (Dit) spectra and total dielectric charge (Qtot) for a wide variety of PV dielectrics. However to this point there has been very little published on the application of corona-Kelvin metrology to the characterization of PV dielectrics on textured surfaces. The ability to measure quickly and accurately on passivated, textured surfaces with minimal impact of leakage is a very important and unique advantage of corona-Kelvin metrology over traditional metal-oxide-semiconductor (MOS) C-V measurements. In this work we present application of the corona-Kelvin technique to the characterization of SiNx and aluminum oxide (Al2O3) dielectrics on textured substrates. Due to the increased surface area of the textured surface a correction factor must be applied to the areal corona charge dose used in the technique. Using planar and textured surfaces passivated with SiNx and Al2O3 deposited under the same conditions, we empirically determine this surface area correction factor for a standard alkaline texture etch which allows accurate determination of passivation properties such as Dit and Qtot on textured surfaces.

Excellent surface passivation of silicon at low cost: Atomic layer deposited aluminium oxide from solar grade TMA

In this work, we investigated the surface passivation performance of thermal atomic layer deposited Al2O3 films using two different grades of trimethylaluminum (TMA). All films were grown on the InPassion Lab tool from SoLayTec. We demonstrate that the surface passivation quality is not compromised by the higher impurity concentration in the cheaper solar grade TMA. Excellent passivation on both p- and n-type silicon surfaces was obtained in a wide process window for samples deposited using both grades of TMA. Remarkably, even a better passivation quality was obtained by the solar grade TMA, especially on n-type samples. It is therefore demonstrated that excellent surface passivation by ALD Al2O3 films can be realized using a lower cost precursor.

C-Si surface passivation by aluminum oxide studied with electron energy loss spectroscopy

In this work the mechanism of c-Si surface passivation by Al₂O₃ films is studied in detail by means of spatially resolved electron energy loss spectroscopy (EELS). The bonding configuration of Al and O is studied in as-deposited and annealed Al₂O₃ films grown on c-Si substrates by plasma-assisted and thermal atomic layer deposition (ALD). The ratio of tetrahedrally and octahedrally coordinated Al is found to increase after annealing, especially for the plasmaassisted ALD sample. The increase is strongest close to the c-Si/Al₂O₃ interface and thus these results strongly support tetrahedrally coordinated Al as the origin for the negative fixed charge in Al₂O₃.