Conference Paper

Outstanding as-deposited surface passivation by Industrial PECVD aluminum oxide

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Aluminum oxide has been highlighted as a promising surface passivation layer for p-type silicon surface. To-date, most of the studies have focused on aluminum oxide layers deposited with atomic layer deposition systems which have lower throughput than industrial plasma-based systems. In this study, the effects of deposition conditions on the electrical and optical properties of aluminum oxide deposited by an industrial plasma enhanced chemical vapor deposition system are presented. Low saturation current density of 1.9 fA/cm 2 was achieved by as deposited layer on p-type Czochralski wafer. The most significant deposition process factor for high quality surface passivation was found to be the gas flow rate ratio between nitrous oxide and tri-methyl-aluminum.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... Assuming the used wafers have a similar initial bulk quality, it can be expected that τ eff would follow the changes in the surface passivation. However, the difference between J 0s and τ eff in the resulting model indicates a possible modification of the bulk quality which occurs during the low temperature deposition process, as also reported in (Kim et al., 2016). ...
Article
Plasma-enhanced chemical-vapor deposition systems are commonly used to deposit aluminum oxide (AlOx) on silicon wafers in the production of silicon solar cells. This study investigates the impact of the deposition conditions on the obtained surface passivation quality of p-type wafers. It is the first to investigate the impact of all the five main process parameters. The ratio between the microwave power and the total gas flow rate is identified as a critical condition to form thermally stable layers. We find that the most significant parameter for high quality surface passivation is the flow rate ratio of nitrous oxide (N2O) to tri-methyl-aluminum (TMA). Higher flow rate ratio ([N2O]/[TMA]) is required to achieve better passivation for fired wafers, whereas lower ratio is preferred when the firing process is not permissible. Elastic-recoil detection analysis reveals that the gas flow rate ratio has a significant impact on the likely direction of hydrogen released from the layer during firing (either to the interface with the silicon wafer or to the environment). Surprisingly, the atomic concentration of aluminum and oxygen is found to be almost stoichiometric regardless of the wide range of the gas flow rate ratios studied in this paper.
Article
We present a subnanometer scale study of the interface between crystalline silicon (c-Si) and aluminum oxide (AlO $_{x}$ ) deposited by an industrial type, remotely plasma-enhanced chemical vapor deposition (PECVD) system. We investigate the origin of the negative charge density within the AlO $_{x}$ layer and the interface defect density. In order to elucidate the source of the negative charge in subnanoscale, we study various AlO $_{x}$ layers with different properties, such as total charge (Q $_{tot}$ ) and interface defect density (D $_{it}$ ) in both as-deposited and thermally treated conditions using scanning transmission electron microscopy and electron energy loss spectroscopy (EELS). The spatially resolved EELS analysis of aluminum (Al) at the c-Si/AlO $_{x}$ interface reveals changes in its chemical configuration through the thermal process. It is found that the ratio of tetrahedrally- to octahedrally-coordinated Al at the c-Si/AlO $_{x}$ interface is a key factor that linearly correlates with the resulting Q $_{tot}$ . However, the ratio in the AlO $_{x}$ layer (far from the interface) stays constant and is the same for both as-deposited and thermally-treated samples, which seems to show a different aspect of PECVD AlO $_{x}$ layers to those presented in previous reports regarding plasma-assisted atomic layer deposited Al $_{2}$ O $_{3}$ . We also highlight the possibility of the combined impact on D $_{it}$ from the interfacial layer's thickness and the ratio of silicon dioxide to the tetrahedral Al peaks.
Article
The surface passivation quality and the bulk lifetime of boron-doped p-type Czochralski silicon wafers were studied in response to dark annealing at 175 °C, using in situ effective lifetime measurements. We investigated non-fired and fired silicon nitride (SiNx), aluminum oxide (AlOx) capped with SiNx, and thermally-grown silicon oxide (SiO2). Modulation in surface passivation quality and bulk lifetime was detected only in cases where hydrogen is assumed to be released into the silicon wafer from the dielectric (AlOx/SiNx stack and fired SiNx layer). Interestingly, the degradation of both the surface and the bulk were followed by a recovery. It is also interesting to note that the changes in the surface and the bulk seem to be related, as the surface degradation starts when the initial bulk degradation ends. This study indicates a possible involvement of hydrogen in both the degradation and the recovery processes. The evolution of the effective lifetime as a function of time is similar to that reported for carrier-induced degradation in multicrystalline wafers; however, occurring on a different time scale. Hence, these findings may also be valuable for investigation of other degradation mechanisms in different silicon materials.
Conference Paper
Full-text available
The efficiency potential of mass-produced PERC cells is investigated by considering incremental improvements of existing fabrication technologies and of p-type wafer materials. Using numerical device simulations, we predict that the continuous developments of industrial-type PERC cells may lead to cell efficiencies beyond 24 %. The main point here is to recognize the dominating losses in each improvement step and to improve existing fabrication technology solely tailored to these losses. For this purpose, we present a method to identify the key technology for the next efficiency level. Our study shows that the key points of near-future for the improvements of PERC cells are: advanced emitter structures such as selective emitters, boron-added Al paste for the rear, wafers with 1 ms lifetime, multi wires instead of busbars and 10 µm narrow fingers with high aspect ratio. The predicted efficiencies lead us to the conclusion that PERC cells will dominate the market and define the moving target for some time to come.
Article
Full-text available
Currently, aluminum oxide stacked with silicon nitride (Al2O3/SiNx:H) is a promising rear passivation material for high-efficiency P-type passivated emitter and rear cell (PERC). It has been indicated that atomic layer deposition system (ALD) is much more suitable to prepare high-quality Al2O3 films than plasma-enhanced chemical vapor deposition system and other process techniques. In this study, an ultrafast, non-vacuum spatial ALD with the deposition rate of around 10 nm/min, developed by our group, is hired to deposit Al2O3 films. Upon post-annealing for the Al2O3 films, the unwanted delamination, regarded as blisters, was found by an optical microscope. This may lead to a worse contact within the Si/Al2O3 interface, deteriorating the passivation quality. Thin stoichiometric silicon dioxide films prepared on the Si surface prior to Al2O3 fabrication effectively reduce a considerable amount of blisters. The residual blisters can be further out-gassed when the Al2O3 films are thinned to 8 nm and annealed above 650°C. Eventually, the entire PERC with the improved triple-layer SiO2/Al2O3/SiNx:H stacked passivation film has an obvious gain in open-circuit voltage (V oc) and short-circuit current (J sc) because of the increased minority carrier lifetime and internal rear-side reflectance, respectively. The electrical performance of the optimized PERC with the V oc of 0.647 V, J sc of 38.2 mA/cm2, fill factor of 0.776, and the efficiency of 19.18% can be achieved.
Article
Full-text available
Abstract The charge dynamics and the interface defect state density of AlOx/SiNx passivation stacks deposited by plasma-enhanced chemical vapor deposition (PECVD) on crystalline silicon (c-Si) wafers are investigated. High frequency (1 MHz) capacitance voltage (C-V) measurements were performed on stacks in the as deposited state and after an annealing step. C-V sweeps reveal an initially high negative charge density for the as deposited sample, activated by the thermal budget during SiNx deposition. However, this charge state is unstable and reduced owning to electron detrapping and emission into the c-Si upon applying moderate voltages. In the annealed sample, the AlOx/SiNx stack has a stable negative fixed charge. Both for as deposited and for annealed samples, applying a positive or negative constant gate voltage stress (Vstress) enhances or reduces the negative effective charge density (Qox,eff), respectively. Injection of charges from the c-Si into traps in the AlOx/SiNx stack is identified as the mechanism responsible for this behavior. We conclude that in addition to fixed negative charges trapping of negative charges near the interface is a crucial mechanism contributing to the total effective negative charge of the stack. Their contribution depends on the temperature and duration of the thermal treatment. Additionally, a large Vstress leads to generation of additional Si dangling bond defects over the entire c-Si bang gap at the c-Si/AlOx interface.
Article
Full-text available
In this publication, the activation and degradation of the passivation quality of plasma-enhanced chemical vapor deposited aluminum oxide (Al2O3) layers with different thicknesses (10 nm, 20 nm, and 110 nm) on crystalline silicon (c-Si) during long and high temperature treatments are investigated. As indicated by Fourier Transform Infrared Spectroscopy, the concentration of tetrahedral and octahedral sites within the Al2O3 layer changes during temperature treatments and correlates with the amount of negative fixed charges at the Si/Al2O3 interface, which was detected by Corona Oxide Characterization of Semiconductors. Furthermore, during a temperature treatment at 820 °C for 30 min, the initial amorphous Al2O3 layer crystallize into the γ-Al2O3 structure and was enhanced by additional oxygen as was proven by x-ray diffraction measurements and underlined by Density Functional Theory simulations. The crystallization correlates with the increase of the optical density up to 20% while the final Al2O3 layer thickness decreases at the same time up to 26%. All observations described above were detected to be Al2O3 layer thickness dependent. These observations reveal novel aspects to explain the temperature induced passivation and degradation mechanisms of Al2O3 layers at a molecular level like the origin of the negative fixe charges at the Si/SiOx/Al2O3 interface or the phenomena of blistering. Moreover, the crystal phase of Al2O3 does not deliver good surface passivation due to a high concentration of octahedral sites leading to a lower concentration of negative fixed charges at the interface.
Article
Full-text available
In this research, Al2O3 films were grown by remote plasma-enhanced atomic layer deposition using a nonpyrophoric precursor, dimethylaluminum isopropoxide (DMAI), and oxygen plasma. After optimization, the growth rate was determined to be ∼1.5 Å/cycle within a growth window of 25–220 °C; the higher growth rate than reported for thermal atomic layer deposition was ascribed to the higher reactivity of the plasma species compared with H2O and the adsorption of active oxygen at the surface, which was residual from the oxygen plasma exposure. Both effects enhance DMAI chemisorption and increase the saturation density. In addition, a longer oxygen plasma time was required at room temperature to complete the reaction and decrease the carbon contamination below the detection limit of x-ray photoemission spectroscopy. The properties of the subsequent Al2O3 films were measured for different temperatures. When deposited at 25 °C and 200 °C, the Al2O3 films demonstrated a single Al-O bonding state as measured by x-ray photoemission spectroscopy, a similar band gap of 6.8±0.2 eV as determined by energy loss spectroscopy, a similar index of refraction of 1.62±0.02 as determined by spectroscopic ellipsometry, and uniform growth with a similar surface roughness before and after growth as confirmed by atomic force microscopy. However, the room temperature deposited Al2O3 films had a lower mass density (2.7 g/cm3 compared with 3.0 g/cm3) and a higher atomic ratio of O to Al (2.1 compared with 1.6) as indicated by x-ray reflectivity and Rutherford backscattering spectroscopy, respectively.
Article
Full-text available
We investigated hydrogenated aluminum oxide (a-Al1-xOx:H) as a high quality rear surface passivation layer of crystalline silicon solar cells. The a-Al1-xOx:H films were deposited by plasma-enhanced chemical vapor deposition (PECVD) using a mixture of trimethylaluminum (TMA), carbon dioxide (CO2), and hydrogen (H2) at a low substrate temperature of about 200 °C. The ratio of CO2 to TMA during deposition and thermal annealing after the film deposition are the key factors in achieving high quality passivation. A 28-nm-thick a-Al1-xOx:H film deposited by PECVD showed a low surface recombination velocity of about 10 cm/s.
Conference Paper
Full-text available
We present independently confirmed efficiencies of 21.4% for PERC cells with plasma-assisted atom-ic-layer-deposited (plasma ALD) Al2O3 rear passivation and 20.7% for cells with thermal ALD-Al2O3 . Additionally, we evaluate three different industrially relevant techniques for the deposition of surface-passivating Al2O3 layers on 1-Ohmcm p-type silicon wafers, namely high-rate spatial ALD (spatial ALD), plasma-enhanced chemical vapour deposi-tion (PECVD) and reactive sputtering. Using spatial ALD and PECVD, surface recombination velocities (SRVs) below 10 cm/s are obtained. Sputtered Al2O3 layers still provide an SRV of 35 – 70 cm/s. Despite their lower passiva-tion quality compared to the Al2O3 films deposited by spatial ALD and by PECVD, we demonstrate that the sputtered Al2O3 layers are still suitable for the fabrication of 20.1% efficient PERC cells. After firing at ~800°C in a conveyor-belt furnace the SRV provided by the Al 2 O 3 films deposited by spatial ALD is still below 20 cm/s, indicating an ex-cellent firing stability. Both PECVD and sputtered Al 2 O 3 passivation layers degrade to SRVs larger than 100 cm/s af-ter firing. Hence, the firing stability of PECVD and – in particular – sputtered Al 2 O 3 needs further optimisation.
Article
Full-text available
A preceding companion paper provides a general introduction to Variable Angle Spectroscopic Ellipsometry (VASE), and also describes many typical applications of the technique. In this paper, more advanced VASE applications are discussed. These applications rely on recent advances in ellipsometric hardware, which allow extremely accurate ellipsometric data to be acquired over a broad spectral range, from the IR to VUV. This instrumentation can also quantitatively measure the optical response of non-isotropic samples. Advanced data analysis techniques are also presented.
Article
Full-text available
Variable angle spectroscopic ellipsometry (VASE) is important for metrology in several industries, and is a powerful technique for research on new materials and processes. Sophisticated instrumentation and software for VASE data acquisition and analysis is available for the most demanding research applications, while simple to use software enables the use of VASE for routine measurements as well. This article gives a basic introduction to the theory of ellipsometry, references `classic' papers, and shows typical VASE applications. In the following companion paper, more advanced applications are discussed.
Article
Full-text available
An accurate quantitative description of the Auger recombination rate in silicon as a function of the dopant density and the carrier injection level is important to understand the physics of this fundamental mechanism and to predict the physical limits to the performance of silicon based devices. Technological progress has permitted a near suppression of competing recombination mechanisms, both in the bulk of the silicon crystal and at the surfaces. This, coupled with advanced characterization techniques, has led to an improved determination of the Auger recombination rate, which is lower than previously thought. In this contribution we present a systematic study of the injection-dependent carrier recombination for a broad range of dopant concentrations of high-purity n-type and p-type silicon wafers passivated with state-of-the-art dielectric layers of aluminum oxide or silicon nitride. Based on these measurements, we develop a general parametrization for intrinsic recombination in crystalline silicon at 300 K consistent with the theory of Coulomb-enhanced Auger and radiative recombination. Based on this improved description we are able to analyze physical aspects of the Auger recombination mechanism such as the Coulomb enhancement.
Article
Full-text available
In order to utilize the full potential of solar cells fabricated on n -type silicon, it is necessary to achieve an excellent passivation on B-doped emitters. Experimental studies on test structures and theoretical considerations have shown that a negatively charged dielectric layer would be ideally suited for this purpose. Thus, in this work the negative-charge dielectric Al <sub>2</sub> O <sub>3</sub> was applied as surface passivation layer on high-efficiency n -type silicon solar cells. With this front surface passivation layer, a confirmed conversion efficiency of 23.2% was achieved. For the open-circuit voltage V<sub> oc </sub> of 703.6 mV , the upper limit for the emitter saturation current density J<sub>0e</sub> , including the metalized area, has been evaluated to be 29 fA / cm <sup>2</sup> . This clearly shows that an excellent passivation of highly doped p -type c- Si can be obtained at the device level by applying Al <sub>2</sub> O <sub>3</sub> .
Article
Full-text available
We measure surface recombination velocities (SRVs) below 10 cm/s on p-type crystalline silicon wafers passivated by atomic–layer–deposited (ALD) aluminium oxide (Al2O3) films of thickness ≥10 nm. For films thinner than 10 nm the SRV increases with decreasing Al2O3 thickness. For ultrathin Al2O3 layers of 3.6 nm we still attain a SRV < 22 cm/s on 1.5 Ω cm p-Si and an exceptionally low SRV of 1.8 cm/s on high-resistivity (200 Ω cm) p-Si. Ultrathin Al2O3 films are particularly relevant for the implementation into solar cells, as the deposition rate of the ALD process is extremely low compared to the frequently used plasma-enhanced chemical vapour deposition of silicon nitride (SiNx). Our experiments on silicon wafers passivated with stacks composed of ultrathin Al2O3 and SiNx show that a substantially improved thermal stability during high-temperature firing at 830 °C is obtained for the Al2O3/SiNx stacks compared to the single-layer Al2O3 passivation. Al2O3/SiNx stacks are hence ideally suited for the implementation into industrial-type silicon solar cells where the metal contacts are made by screen-printing and high-temperature firing of metal pastes. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Article
A novel method for the measurement of the saturation current of emitters is described. The transient decay of photogenerated carriers is measured on a lightly doped sample containing the emitter profile of interest. In high-level injection, the recombination rate from the emitter is proportional to the square of the carrier concentration, so it can be separated from the linear bulk and surface recombination. Since no ohmic connections are required, this technique is uniquely suited to the study of passivated emitters. Furthermore, the emitter saturation current of a given profile can be measured both with and without a passivation layer, so that the relative sources of emitter recombination can be determined. The results of measurement on a variety of phosphorous and boron emitters are presented.
Article
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. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)
Article
We report an outstanding level of surface passivation for both n+ and p+ silicon by AlOx/SiNx dielectric stacks deposited in an inline plasma-enhanced chemical vapor deposition (PECVD) reactor for a wide range of sheet resistances. Extremely low emitter saturation current densities (J0e) of 12 and 200 fA/cm2 are obtained on 165 and 25 Ω/sq n+ emitters, respectively, and 8 and 45 fA/cm2 on 170 and 30 Ω/sq p+ emitters, respectively. Using contactless corona-voltage measurements and device simulations, we demonstrate that the surface passivation mechanism on both n+ and p + silicon is primarily due to a relatively low interface defect density of <;1011 eV-1cm-2 in combination with a moderate fixed negative charge density of (1-2) × 1012 cm-2. From advanced modeling, the fundamental surface recombination velocity parameter is shown to be in the order of 104 cm/s for PECVD AlOx/SiNx passivated heavily doped n+ and p+ silicon surfaces.
Article
Excellent passivation of boron emitters is realised using AlOx/SiNx dielectric stacks deposited in an industrial inline plasma-enhanced chemical vapour deposition reactor. Very low emitter saturation current density (J0e) values of 10 and 45 fA/cm2 are obtained for 180 and 30 Ω/sq planar p+ emitters, respectively. For textured p+ emitters, the J0e was found to be 1.5–2 times higher compared with planar emitters. The required thermal activation of the AlOx films is performed in a standard industrial fast-firing furnace, making the developed passivation stack industrially viable. We also show that an AlOx thickness of 5 nm in the AlOx/SiNx stack is sufficient for obtaining a J0e of 18 fA/cm2 for planar 80 Ω/sq p+ emitters, which corresponds to a 1-sun open-circuit voltage limit of the solar cell of 736 mV at 25 °C. Copyright © 2012 John Wiley & Sons, Ltd.
Article
The reduction in electronic recombination losses by the passivation of silicon surfaces is a critical enabler for high-efficiency solar cells. In 2006, aluminum oxide (Al2O3) nanolayers synthesized by atomic layer deposition (ALD) emerged as a novel solution for the passivation of p- and n-type crystalline Si (c-Si) surfaces. Today, high efficiencies have been realized by the implementation of ultrathin Al2O3 films in laboratory-type and industrial solar cells. This article reviews and summarizes recent work concerning Al2O3 thin films in the context of Si photovoltaics. Topics range from fundamental aspects related to material, interface, and passivation properties to synthesis methods and the implementation of the films in solar cells. Al2O3 uniquely features a combination of field-effect passivation by negative fixed charges, a low interface defect density, an adequate stability during processing, and the ability to use ultrathin films down to a few nanometers in thickness. Although various methods can be used to synthesize Al2O3, this review focuses on ALD—a new technology in the field of c-Si photovoltaics. The authors discuss how the unique features of ALD can be exploited for interface engineering and tailoring the properties of nanolayer surface passivation schemes while also addressing its compatibility with high-throughput manufacturing. The recent progress achieved in the field of surface passivation allows for higher efficiencies of industrial solar cells, which is critical for realizing lower-cost solar electricity in the near future.
Article
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 5 nm/s as well as excellent surface recombination velocities below 10 cm/s after firing. A fixed negative charge of −4×1012 cm−2 is measured for ICP AlOx single layers with an interface state density of 11.0×1011 eV−1 cm−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 (15.6×15.6 cm2) 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.
Article
Spectroscopic ellipsometry has become an essential metrology tool for the semiconductor industry. It is widely used where precise film thicknesses and optical constants are required. Faster measurement speeds are opening new doors for ellipsometry in thin-film processing environments, primarily due to its ability to maintain high precision when measuring very thin or multilayered films (such as gate oxides). The technique works well for all varieties of films, including semiconductors, dielectrics, metals, and polymer coatings.
Article
The surface passivation properties of aluminium oxide (Al2O3) on crystalline Si are compared with the traditional passivation system of silicon nitride (SiNx). It is shown that Al2O3 has fundamental advantages over SiNx when applied to the rear of p-type silicon solar cells as well as to the p+ emitter of n-type silicon solar cells. Special emphasis is paid to the transfer of Al2O3 into industrial solar cell production. We compare different Al2O3 deposition techniques suitable for mass production such as ultrafast spatial atomic layer deposition, inline plasma-enhanced chemical vapour deposition and reactive sputtering. Finally, we review the most recent cell results with Al2O3 passivation and give a brief outlook on the future prospects of Al2O3 in silicon solar cell production.
Article
Recently, a simple yet powerful carrier lifetime technique for semiconductor wafers has been introduced that is based on the simultaneous measurement of the light-induced photoconductance of the sample and the corresponding light intensity [Appl. Phys. Lett. 69, 2510 (1996)]. In combination with a light pulse from a flash lamp, this method allows the injection level dependent determination of the effective carrier lifetime in the quasi-steady-state mode as well as the quasi-transient mode. For both cases, approximate solutions (those for steady-state and transient conditions) of the underlying semiconductor equations have been used. However, depending on the actual lifetime value and the time dependence of the flash lamp, specific systematic errors in the effective carrier lifetime arise from the involved approximations. In this work, we present a generalized analysis that avoids these approximations and hence substantially extends the applicability of the quasi-steady-state and quasi-transient methods beyond their previous limits. © 1999 American Institute of Physics.
Article
Excellent surface passivation of c-Si has been achieved by Al2O3 films prepared by plasma-assisted atomic layer deposition, yielding effective surface recombination velocities of 2 and 13 cm/s on low resistivity n- and p-type c-Si, respectively. These results obtained for ∼ 30 nm thick Al2O3 films are comparable to state-of-the-art results when employing thermal oxide as used in record-efficiency c-Si solar cells. A 7 nm thin Al2O3 film still yields an effective surface recombination velocity of 5 cm/s on n-type silicon.
Article
In this work, we present a systematic study of the surface recombination velocity of boron-diffused Si wafer passivated with plasma-enhanced chemical-vapor-deposited (PECVD) AlOx layers. Saturation current densities in the range of 5.2–38 fA cm− 2 (at 300 K) were achieved on planar surfaces. In particular, we present an industrially relevant boron emitter, allowing for about 700 mV open circuit voltage on textured surfaces, after a firing process. This high passivation quality could be achieved using AlOx passivation layers deposited by PECVD. The passivation quality is found to be equivalent to AlOx layers deposited by plasma-assisted atomic-layer-deposition. A wide range of surface doping concentration (2.5–70 × 1018 cm− 3) was investigated. The emitters used here allow a high resolution in the determination of the surface recombination velocity upper limit. Our simulations, based on Fermi-Dirac statistics, indicate that only very shallow emitters can be used to further increase the resolution on the determination of the surface recombination velocity.
Article
Thin layers of Al2O3 always require a thermal post-deposition treatment to activate the passivation on crystalline silicon surfaces. In this work, we studied the influence of different thermal post-deposition treatments for the activation of passivating ALD Al2O3 single layers and Al2O3/SiNx stacks. For the stacks, especially with less than 5 nm Al2O3, a short high temperature process at ∼800 °C results in a remarkably lower surface recombination compared to a commonly applied annealing at 425 °C. We observed that four ALD cycles of Al2O3 are sufficient to reach the full potential of surface passivation, and even with one atomic layer of Al2O3 (one ALD cycle) emitter saturation current densities as low as 45 fA/cm2 can be reached on boron-diffused emitters.
Article
We measure surface recombination velocities (SRVs) below 10 cm/s on low-resistivity (1.4 cm) p-type crystalline silicon wafers passivated with plasma-assisted and thermal atomic layer deposited (ALD) aluminium oxide (Al2O3) films. Ultrathin Al2O3 films (< 5 nm) are particularly relevant for the implementation into solar cells, as the deposition rate of the ALD process is very low compared to e.g. plasma-enhanced chemical vapor deposition (PECVD). Hence, we examine the passivation quality of a stack consisting of an ultrathin Al2O3 passivation layer deposited by ALD and a SiNx capping layer deposited by PECVD. Our experiments show a substantial improvement of the thermal stability during firing at 810 °C for the Al2O3/SiNx stacks compared to a single Al2O3 layer. We report on a „regeneration effect‟ observed for Al2O3 single layers after firing, where the degraded passivation is significantly improved after annealing at 400 °C and also by illumination at room temperature using a halogen lamp. Nevertheless, for Al2O3/SiNx stacks we measure SRVs < 15 cm/s after firing, whereas for Al2O3 single layers the regenerated SRVs are in the range of 10-30 cm/s. Al2O3/SiNx stacks are hence ideally suited for the implementation into industrial-type silicon solar cells, although „regenerated‟ Al2O3 single layers should result in a comparable cell performance.
Article
From lifetime measurements, including a direct experimental comparison with thermal Si O <sub>2</sub> , a- Si : H , and as-deposited a- Si N <sub>x</sub>: H , it is demonstrated that Al <sub>2</sub> O <sub>3</sub> provides an excellent level of surface passivation on highly B-doped c- Si with doping concentrations around 10<sup>19</sup> cm <sup>-3</sup> . The Al <sub>2</sub> O <sub>3</sub> films, synthesized by plasma-assisted atomic layer deposition and with a high fixed negative charge density, limit the emitter saturation current density of B-diffused p<sup>+</sup> -emitters to ∼10 and ∼30 fA / cm <sup>2</sup> on ≫100 and 54 Ω/ sq sheet resistance p<sup>+</sup> -emitters, respectively. These results demonstrate that highly doped p -type Si surfaces can be passivated as effectively as highly doped n -type surfaces.
Article
Surface recombination velocities as low as 10 cm/s have been obtained by treated atomic layer deposition (ALD) of Al2O3 layers on p-type CZ silicon wafers. Low surface recombination is achieved by means of field induced surface passivation due to a high density of negative charges stored at the interface. In comparison to a diffused back surface field, an external field source allows for higher band bending, that is, a better performance. While this process yields state of the art results, it is not suited for large-scale production. Preliminary results on an industrially viable, alternative process based on a pseudo-binary system containing Al2O3 are presented, too. With this process, surface recombination velocities of 500–1000 cm/s have been attained on mc-Si wafers.
Article
Aluminum oxide layers can provide excellent passivation for lowly and highly doped p-type silicon surfaces. Fixed negative charges induce an accumulation layer at the p-type silicon interface, resulting in very effective field-effect passivation. This paper presents highly negatively charged (Q(ox)=-2.1 X 10(12) cm(-2)) aluminum oxide layers produced using an inline plasma-enhanced chemical vapor deposition system, leading to very low effective recombination velocities (similar to 10 cm s(-1)) on low-resistivity p-type substrates. A minimum static deposition rate (100 nm min(-1)) at least one order of magnitude higher than atomic layer deposition was achieved on a large carrier surfaces (similar to 1 m(2)) without significantly reducing the resultant passivation quality.
Comparison of the thermal stability of single Al 2 O 3 layers and Al 2 O 3 /SiN x stacks for the surface passiviation of siliconGeneralized analysis of quasi-steady-state and quasi-transient measurements of carrier lifetimes in semiconductors
  • B Veith
  • F Werner
  • D Zielke
  • R Brendel
  • J Schmidt
  • H Nagel
  • C Berge
  • A G Aberle
  • B D Johs
  • J A Woollam
  • C M Herzinger
  • J N Hilfiker
  • R A Synowicki
  • C L Bungay
B. Veith, F. Werner, D. Zielke, R. Brendel, and J. Schmidt, "Comparison of the thermal stability of single Al 2 O 3 layers and Al 2 O 3 /SiN x stacks for the surface passiviation of silicon," Energy Procedia, vol. 8, pp. 307-312, 2011. [30] H. Nagel, C. Berge, and A. G. Aberle, "Generalized analysis of quasi-steady-state and quasi-transient measurements of carrier lifetimes in semiconductors," Journal of Applied Physics, vol. 86, pp. 6218-6221, 1999. [31] B. D. Johs, J. A. Woollam, C. M. Herzinger, J. N. Hilfiker, R. A. Synowicki, and C. L. Bungay, "Overview of variable-angle spectroscopic ellipsometry (VASE): II. Advanced applications," in Optical Metrology, 1999, pp. 29-58. [32]
Investigation on the passivated Si/Al
  • S.-Y Lien
  • C.-H Yang
  • K.-C Wu
  • C.-Y Kung
Comparison of the thermal stability of single Al
  • B Veith
  • F Werner
  • D Zielke
  • R Brendel
  • J Schmidt
Excellent boron emitter passivation for high-efficiency Si wafer solar cells using AlO/SiN
  • S Duttagupta
  • F Lin
  • K D Shetty
  • A G Aberle
  • B Hoex
Industrially relevant Al
  • J Schmidt
  • F Werner
  • B Veith
  • D Zielke
  • R Bock
  • V Tiba
Progress in surface passivation of heavily hoped n-type and p-type silicon by plasma-deposited AlO/SiN dielcetric stacks
  • S F M Ma
  • T Lin
  • A G Aberle
  • B Hoex
  • S Fa-Jun
  • J M Hilbe
J. M. Hilbe, "STATISTICA 7," The American Statistician, 2012.
Measurement of the emitter saturation current by a contactless photoconductivity decay method
  • D E Kane
  • R R M Swanson
D. E. Kane and R. R.M. Swanson, "Measurement of the emitter saturation current by a contactless photoconductivity decay method," in the 18th IEEE photovoltaic specialists conference, 1985, pp. 578-583.