High-quality Surface Passivation Research Articles

Si surface passivation by Al2O3 thin films deposited using a low thermal budget atomic layer deposition process

High-quality surface passivation of crystalline Si is achieved using 10 nm thick Al2O3 films fabricated by thermal atomic layer deposition at 100 °C. After a 5 min post deposition annealing at 200 °C, the effective carrier lifetime is 1 ms, indicating a functional level of surface passivation. The interplay between the chemical and the field effect passivation is investigated monitoring the density of interface traps and the amount of fixed charges with conductance-voltage and capacitance-voltage techniques. The physical mechanisms underlying the surface passivation are described. The combination of low processing temperatures, thin layers, and good passivation properties facilitate a technology for low-temperature solar cells.

Inductively coupled plasma chemical vapour deposited AlOx/SiNy layer stacks for applications in high-efficiency industrial-type silicon solar cells

Passivated emitter and rear cells (PERC) are considered to be the next generation of industrial-type screen-printed silicon solar cells. Deposition methods for rear passivation layers have to meet both the high-throughput and low-cost requirements of the PV industry in combination with high-quality surface passivation properties. In this paper, we evaluate and optimise a novel deposition technique for AlOx passivation layers by applying an inductively coupled plasma (ICP) plasma-enhanced chemical vapour deposition (PECVD) process. The ICP AlOx deposition process enables high deposition rates up to 5nm/s as well as excellent surface recombination velocities below 10cm/s after firing. A fixed negative charge of −4×1012cm−2 is measured for ICP AlOx single layers with an interface state density of 11.0×1011eV−1cm−2 at midgap position. When applied to PERC solar cells the ICP AlOx layer is capped with a PECVD SiNy layer. We achieve independently confirmed conversion efficiencies of up to 20.1% for large-area (15.6×15.6cm2) PERC solar cells with screen-printed metal contacts and ICP AlOx/SiNy rear side passivation on standard boron-doped Czochralski-grown silicon wafers. The internal quantum efficiency reveals an effective rear surface recombination velocity Srear of (90±30)cm/s and an internal rear reflectance Rb of (91±1)% which demonstrates the excellent rear surface passivation of the ICP AlOx/SiNy layer stack.

Surface Passivation and Interface Properties of Bulk GaAs and Epitaxial-GaAs/Ge Using Atomic Layer Deposited TiAlO Alloy Dielectric

High quality surface passivation on bulk-GaAs substrates and epitaxial-GaAs/Ge (epi-GaAs) layers were achieved by using atomic layer deposited (ALD) titanium aluminum oxide (TiAlO) alloy dielectric. The TiAlO alloy dielectric suppresses the formation of defective native oxide on GaAs layers. X-ray photoelectron spectroscopy (XPS) analysis shows interfacial arsenic oxide (As(x)O(y)) and elemental arsenic (As) were completely removed from the GaAs surface. Energy dispersive X-ray diffraction (EDX) analysis and secondary ion mass spectroscopy (SIMS) analysis showed that TiAlO dielectric is an effective barrier layer for reducing the out-diffusion of elemental atoms, enhancing the electrical properties of bulk-GaAs based metal-oxide-semiconductor (MOS) devices. Moreover, ALD TiAlO alloy dielectric on epi-GaAs with AlGaAs buffer layer realized smooth interface between epi-GaAs layers and TiAlO dielectric, yielding a high quality surface passivation on epi-GaAs layers, much sought-after for high-speed transistor applications on a silicon platform. Presence of a thin AlGaAs buffer layer between epi-GaAs and Ge substrates improved interface quality and gate dielectric quality through the reduction of interfacial layer formation (Ga(x)O(y)) and suppression of elemental out-diffusion (Ga and As). The AlGaAs buffer layer and TiAlO dielectric play a key role to suppress the roughening, interfacial layer formation, and impurity diffusion into the dielectric, which in turn largely enhances the electrical property of the epi-GaAs MOS devices.

High-quality surface passivation of silicon using native oxide and silicon nitride layers

We report on the attainment of high quality surface passivation of crystalline silicon using facile native oxide and plasma enhanced chemical vapour deposition SiNx. Using systematic measurements of excess carrier density dependent minority carrier lifetime, it is observed that the inferred interface defect density decreases with increasing native oxide thickness while the interface charge density remains unchanged with thickness, which ranges from 0.2 Å to 10 Å. A surface recombination velocity of 8 cm/s is attained corresponding to a native oxide layer thickness of ∼10 Å. Similar chemically grown oxide layer followed by SiNx deposition is shown to yield comparable passivation, indicating practical viability of the passivation scheme.

Passivation characteristics of SiNx/a-Si and SiNx/Si-rich-SiNx stacked layers on crystalline silicon

High-quality surface passivation is essential for obtaining crystalline silicon (c-Si) solar cells with high energy-conversion efficiency. Silicon-nitride (SiNx)/amorphous-silicon (a-Si) stacked layers prepared by catalytic chemical vapor deposition (Cat-CVD), also referred to as hot-wire CVD, demonstrate excellent performance as surface passivation layers for c-Si and realize a dramatically low surface recombination velocity (SRV). However, the use of a more transparent material than a-Si is required because the a-Si layer absorbs sunlight. Here, Si-rich SiNx films, as a more transparent material, with various atomic ratios of silicon/nitrogen (Si/N) are investigated as a replacement for a-Si films. The use of SiNx/Si-rich-SiNx stacked layers as passivation films on c-Si wafers results in a SRV as low as 5cm/s and 30% improvement of the transparency at the wavelength of 400nm compared to that of the SiNx/a-Si stacked layers. Moreover, after the annealing process, the passivation characteristics of the stacked layer were significantly improved, with a SRV as low as 3cm/s.

Advanced metallization of rear surface passivated metal wrap through silicon solar cells

Abstract We present metal wrap through passivated emitter and rear cells (MWT-PERC) on p-type Czochralski silicon (Cz-Si) using advanced metallization approaches. Front surface metallization is performed either with a one-step approach (conventional thick film screen printing) or with a more advanced two-step approach, consisting of aerosol printing of a silver seed layer and subsequent silver electroplating. High short circuit current densities of up to 39.9 mA/cm 2 indicate excellent light trapping and decreased front side shading due to the absence of busbars as well as high quality surface passivation. Both metallization technologies allow for conversion efficiencies of 18.7% for 125 x 125 mm 2 sized Cz-Si MWT-PERC solar cells. Furthermore, we show the possibility of via metallization by the use of two-step seed and plate technology. Via resistances similar to those for screen printed via metallization are achieved. The seed and plate technology therefore forms a promising approach for via metallization of next generation MWT-PERC solar cells.

Measuring the role of surface chemistry in silicon microphotonics

Utilizing a high quality factor (Q∼1.5×106) optical microresonator to provide sensitivity down to a fractional surface optical loss of αs′∼10−7, we show that the optical loss within Si microphotonic components can be dramatically altered by Si surface preparation, with αs′∼1×10−5 measured for chemical oxide surfaces as compared to αs′⩽1×10−6 for hydrogen-terminated Si surfaces. These results indicate that the optical properties of Si surfaces can be significantly and reversibly altered by standard microelectronic treatments, and that stable, high optical quality surface passivation layers will be critical in future Si micro- and nanophotonic systems.

Industrial high-rate (∼5 nm/s) deposited silicon nitride yielding high-quality bulk and surface passivation under optimum anti-reflection coating conditions

High-quality surface and bulk passivation of crystalline silicon solar cells has been obtained under optimum anti-reflection coating properties by silicon nitride (a-SiNx:H) deposited at very high deposition rates of ∼5 nm/s. These a-SiNx:H films were deposited using the expanding thermal plasma (ETP) technology under regular processing conditions in an inline industrial-type reactor with a nominal throughput of 960 solar cells/hour. The low surface recombination velocities (50–70 cm/s) were obtained on p-type silicon substrates (8·4 Ω cm resistivity) for as-deposited and annealed films within the broad refractive index range of 1·9–2·4, which covers the optimum bulk passivation and anti-reflection coating performance reached at a refractive index of ∼2·1. Copyright © 2005 John Wiley & Sons, Ltd.

High‐quality surface passivation of silicon solar cells in an industrial‐type inline plasma silicon nitride deposition system

AbstractWe have studied the surface passivation of silicon by deposition of silicon nitride (SiN) in an industrial‐type inline plasma‐enhanced chemical vapor deposition (PECVD) reactor designed for the continuous coating of silicon solar cells with high throughput. An optimization study for the passivation of low‐resistivity p‐type silicon has been performed exploring the dependence of the film quality on key deposition parameters of the system. With the optimized films, excellent passivation properties have been obtained, both on undiffused p‐type silicon and on phosphorus‐diffused n+ emitters. Using a simple design, solar cells with conversion efficiencies above 20% have been fabricated to prove the efficacy of the inline PECVD SiN. The passivation properties of the films are on a par with those of high‐quality films prepared in small‐area laboratory PECVD reactors. Copyright © 2004 John Wiley & Sons, Ltd.

Detailed study of the composition of hydrogenated SiNx layers for high-quality silicon surface passivation

We have analyzed the role of the bond densities of a-SiNx:H films on the passivation properties at the SiNx:H/Si interface. The films are deposited onto silicon wafers by plasma enhanced chemical vapor deposition using a 13.56 MHz direct plasma system and a SiH4/N2/H2 gas mixture. Fourier transform infrared spectroscopy measurements are performed in order to obtain the bonding concentration of Si–Si, Si–H, Si–N and N–H. The passivation properties are deduced by lifetime measurements using a microwave-detected photoconductance decay technique. Carrier lifetimes of the SiNx:H-passivated silicon wafers of up to 1200 μs correlate to surface recombination velocities, Seff, as low as 4–6 cm/s. This means that the films provide excellent passivation of silicon surfaces, which is necessary for high-efficiency solar cells. The Si–H bond density and the total bond density are considered as measures of the passivation quality. Models for the formation of K+ centers and for the passivation pathways during the plasma deposition are proposed. The addition of a further hydrogen source to the plasma gas (H2) leads to a better defect passivation of Si dangling bonds during the deposition.

Al-enhanced PECVD SiNx induced hydrogen passivation in string ribbon silicon

The effectiveness of manufacturable gettering and passivation technologies is investigated for their ability to improve the quality of a promising Si photovoltaic material. The results of this study indicate that a lifetime enhancement of 30 μs is attained when a backside screen-printed aluminum layer and a thin film of SiNx, applied by plasma-enhanced chemical vapor deposition (PECVD), are simultaneously annealed at 850°C in a lamp-heated belt furnace. Based on the results of this study, a model is proposed to describe the Al-enhanced SiNx induced hydrogen defect passivation in String Ribbon silicon due to the simultaneous anneal. According to this model, three factors play an important role: i) the release of hydrogen from the SiNx film into the substrate; ii) the retention of hydrogen at defect sites in silicon; and iii) the generation of vacancies at the Al−Si interface due to the alloying process which increases the incorporation of hydrogen and creates a chemical potential gradient which enhances the migration of hydrogen in the substrate. A PC1D device simulation indicates that screen-printed cell efficiencies approaching 16% can be achieved if the gettering and passivation treatments examined in this study are employed, the substrate thickness is reduced, and a high-quality surface passivation scheme is applied.

Comprehensive study of rapid, low-cost silicon surface passivation technologies

A comprehensive and systematic investigation of low-cost surface passivation technologies is presented for achieving high-performance silicon devices such as solar cells. Most commercial solar cells today lack adequate surface passivation, while laboratory cells use conventional furnace oxides (CFO) for high-quality surface passivation involving an expensive and lengthy high-temperature step. This investigation tries to bridge the gap between commercial and laboratory cells by providing fast, low-cost methods for effective surface passivation. This paper demonstrates for the first time, the efficacy of TiO/sub 2/, thin (<10 nm) rapid thermal oxide (RTO), and PECVD SiN individually and in combination for (phosphorus diffused) emitter and (undiffused) back surface passivation. The effects of emitter sheet resistance, surface texture, and three different SiN depositions (two direct PECVD systems and one remote plasma system) were investigated. The effects of post-growth/deposition treatments such as forming gas anneal (FGA) and firing of screen printed contacts were also examined. This study reveals that the optimum passivation scheme consisting of a thin RTO with a SiN cap followed by a very short 730/spl deg/C anneal can 1) reduce the emitter saturation current density, J/sub 0e/, by a factor of >15 for a 90 /spl Omega//sq. emitter, 2) reduce J/sub 0e/ by a factor of >3 for a 40 /spl Omega//sq, emitter, and 3) reduce S/sub back/ below 20 cm/s on 1.3 /spl Omega/cm p-Si. Furthermore, this double-layer RTO+SiN passivation is relatively independent of the deposition conditions (direct or remote) of the SiN film and is more stable under heat treatment than SiN or RTO alone. Model calculations are also performed to show that the RTO+SiN surface passivation scheme may lead to 17%-efficient thin screen-printed cells even with a low bulk lifetime of 20 /spl mu/s.