What do transistors, solar cells, microLEDs, and thin-film transistors have in common? At first glance, the answer might seem straightforward: they all rely on semiconductor materials such as silicon (Si), germanium (Ge), gallium arsenide (GaAs), indium phosphide (InP), and gallium nitride (GaN), among others. However, there’s much more to their story. These devices are also part of a broader technological trend—one that increasingly demands form factors with higher surface-to-bulk ratios to maximize performance while minimizing costs. Take silicon transistors, for example. They’ve transformed from planar devices into 3D architectures like finFETs and gate-all-around nanosheets to keep pace with Moore’s Law. Similarly, silicon solar cells have grown thinner over time, reducing material usage while improving efficiency.For microLEDs, smaller devices enable high resolution and compact displays, which are essential for immersive experiences and applications such as augmented reality. Across these advances, another common thread emerges: the critical role of atomic-scale processing techniques, particularly atomic layer deposition (ALD). ALD is becoming increasingly indispensable in the industrial fabrication of these devices, enabling the deposition of precise and conformal films on semiconductor surfaces. One key application is surface passivation, where thin films are engineered to minimize charge carrier recombination at surface sites, boosting device performance and efficiency.
Surface passivation is the topic of this blog post, which highlights a review paper that we have recently published in the Journal of Vacuum Science and Technology A: Surface passivation approaches for silicon, germanium, and III–V semiconductors. Please note that this review paper is open access and therefore free to read and download for everyone. Also most figures in the review paper are licensed a Creative Commons Attribution (CC BY) license. Several are included in this post and can be downloaded from the ImageBase.
Surface passivation by thin films has been an important research topic within our group in the past 20 years. We have published many papers about it and this includes a number of review articles which will be listed below. But let’s start with answering a few questions to delve a bit deeper into the topic.
Question 1: What is surface passivation of semiconductors and why is it important?
Semiconductors are used in so-called semiconductor devices due to the unique ability to control the charge carriers in semiconductors, enabling functionalities such as current regulation, charge carrier generation and separation, or recombination control of the charge carriers. However, achieving precise and optimal control requires suppressing adverse effects, including the trapping and recombination of charge carriers at defects. These defects, particularly abundant at semiconductor surfaces where the lattice is disrupted, can hamper device performance and significantly reduce device efficiency. Surface passivation refers to the effect that minimizes the influence of electrically active defects at the semiconductor surface, reducing undesired recombination of free electrons (present in the conduction band) and holes (missing electrons in the valence band). This is especially critical as devices increasingly adopt designs with higher surface-to-volume ratios. Passivation can be achieved through chemical treatments or by applying thin films. Note that the semiconductor surface often becomes an interface with another material, making surface passivation synonymous with interface passivation. The figure below illustrates key elementary processes related to charge carrier generation, recombination, and passivation mechanisms, showcasing the critical role of surface passivation in improving device performance.
Question 2: How can thin films passivate surfaces and which aspects need to be taken into account?
Surface recombination requires three “ingredients”: electronic defect sites where recombination can occur as well as electrons and holes which can actually recombine with each other. Effective passivation can be achieved by removing or reducing any of these. Reducing the density of defect sites, for example saturating “dangling bonds” with chemical bonds, lowers the interface defect density (𝐷𝑖𝑡) and this approach is labelled as chemical passivation. Reducing the density of one type of charge carrier (electrons or holes) near the surface using an electric field is another approach. This is labelled field-effect passivation and can be induced by band engineering or by applying a thin film with fixed charges (𝑄𝑓) or a different work function near the interface with the semiconductor.
The figure below outlines critical factors influencing surface passivation. These include the characteristics inherent to the semiconductor and its surface, specifications of the passivation layer applied, and properties the interlayer that often forms between the two. Additionally, pre- and post-treatments are essential considerations for optimizing surface passivation.
Question 3: Why are atomic scale processing methods becoming more important for surface passivation?
Historically, either surface passivation was not really critical for device performance or techniques like physical vapor deposition (PVD), chemical vapor deposition (CVD), and plasma-enhanced CVD (PECVD) were sufficient for achieving surface passivation by applying thin films. These methods offered fairly high deposition rates and allowed for high-temperature processing to achieve adequate film quality. While still widely used, emerging trends in device technology have increased the demand for atomic-scale control. Devices now often require higher levels of surface passivation and also often extremely thin films, high uniformity across large areas, and compatibility with advanced surface topologies, including advanced 3D-structured surfaces. In addition, some devices impose constraints on processing temperatures or demand exceptionally high-quality layers. These challenges make methods like atomic layer deposition (ALD) and plasma-enhanced ALD highly desirable. ALD provides precise control over film thickness and composition, ensuring uniformity over large areas and conformality over 3D-structured surface at the atomic scale. Beyond film deposition, surface preparation steps like the removal of a damage layer or simply surface cleaning have grown in importance. Techniques such as atomic layer etching (ALE) or atomic layer cleaning are emerging as critical processes to ensure pristine surfaces for subsequent passivation. The figure below illustrates how atomic-scale processing methods can address these challenges, emphasizing the growing role of advanced techniques like ALD, ALE, and atomic layer cleaning in the fabrication of modern semiconductor devices.
As mentioned earlier, our group has been investigating surface passivation of semiconductor surfaces for over 20 years. Initially, our focus was on silicon surfaces, particularly for applications in silicon solar cells. This work has resulted in numerous publications, including several key review papers:
- Status and prospects of Al₂O₃-based surface passivation schemes for silicon solar cells, G. Dingemans and W.M.M. Kessels, J. Vac. Sci. Technol. A 30, 040802 (2012).[1]
- Passivating contacts for crystalline silicon solar cells: from concepts and materials to prospects, J. Melskens, B.W.H. van de Loo, B. Macco, L.E. Black, S. Smit, and W.M.M. Kessels, IEEE J. Photovoltaic 8, 373 (2018).[2]
- Explorative studies of novel silicon surface passivation materials: considerations and lessons learned, L.E. Black, B.W.H. van de Loo, B. Macco, J. Melskens, W.J.H. Berghuis, and W.M.M. Kessels, Sol. Energy Mater. Sol. Cells 188, 182 (2018).[3]
- Atomic layer deposition of conductive and semiconductive oxides, B. Macco, and W.M.M. Kessels, Appl. Phys. Rev. 9, 041313 (2022).[4]
More recently, we have expanded our research to other semiconductors, such as germanium (Ge) and indium phosphide (InP)[5–8]. For these materials, as well as related semiconductors like gallium arsenide (GaAs) and gallium nitride (GaN), considerably less research has been conducted, despite their growing importance. The figure below clearly highlights this gap.
Our latest review paper addresses the state-of-the-art in surface passivation for silicon and these emerging semiconductor materials. It also illustrates some typical applications of these semiconductors, see the figure below.
We encourage readers to explore our review paper for more details and a better understanding. However, we would like to conclude this blog with a few intriguing observations and insights into future perspectives:
High-volume manufacturing of ALD Al2O3 passivation layers for silicon solar cells
As also outlined in an earlier blog on ALD for silicon solar cells, ALD has become a cornerstone in the production of high-performance Al₂O₃ surface passivation layers for silicon solar cells, playing a critical role in enhancing the efficiency of PERC and, more recently, TOPCon solar cells. Initially, both batch ALD and spatial ALD techniques were employed to meet the stringent throughput demands for high-volume manufacturing, but batch ALD has since become the dominant technology due to its scalability and cost-effectiveness. Today, billions of solar cells are coated with ALD Al₂O₃ annually, highlighting the pivotal role of ALD in advancing solar cell efficiency and sustainability. This transformative impact of ALD on solar cell manufacturing will soon be explored further in an upcoming blog post.
Exploring new passivation layers and concepts, enabled by ALD
Over the past decade, atomic layer deposition (ALD) has significantly expanded the range of passivation layers available, moving beyond conventional materials like Al₂O₃ and SiO₂ to explore novel options offering enhanced or specialized functionality. A notable breakthrough from our group is the development of POₓ/Al₂O₃ stacks, which feature an exceptionally high positive fixed charge, making them ideal for passivating n-type silicon. ALD has also been central to the revolution in silicon solar cell technology through passivating contacts layers that not only provide excellent surface passivation but also selectively allow electrons or holes to pass (see also earlier blogs one, two, three). A variety of ALD materials, including TiO₂, Nb₂O₅, Ta₂O₅, and MgO, have been explored for these advanced contact layers. More recently, we introduced highly efficient passivating contacts based on Al-doped ZnO, which will be the focus of an upcoming blog.[9] Furthermore, the lessons learned and materials developed for silicon can now be leveraged to explore novel applications in other semiconductor devices, including Ge, InP, and GaN.
Tailored Passivation Stacks for Different Semiconductors
Each semiconductor presents unique challenges and considerations for achieving effective passivation, often requiring the careful engineering of nanolayered stacks. For example, passivating germanium is significantly more difficult than silicon due to the highly defective and unstable nature of germanium’s native oxide. This challenge partly explains why silicon, despite its lower electron mobility, became the preferred material for transistors. To address this, we developed a passivation strategy combining amorphous silicon (deposited via PECVD) for excellent chemical passivation without germanium oxide formation, followed by PEALD Al₂O₃ to introduce high fixed charge at the interface for field-effect passivation.[7] Another breakthrough in stack engineering was achieved with InP, where surface phosphor vacancies readily form, creating deep-level defects. An ALD POₓ layer provided a phosphorus reservoir to mitigate these defects, but its hygroscopic nature posed stability issues. By adding an Al₂O₃ capping layer to form a POₓ/Al₂O₃ stack, we achieved exceptional passivation for InP, with similar strategies later extending to silicon and germanium.[10] These examples highlight how tailored passivation stacks can unlock high-performance surfaces for diverse semiconductors.
In-Situ Cleaning: A Key to Effective Passivation
In-situ cleaning has emerged as a critical step in the passivation of semiconductors, particularly those with unstable native oxides like germanium and gallium arsenide. By integrating surface cleaning either as a pretreatment or during the deposition process itself (self-cleaning), oxidation can be effectively prevented, preserving the surface integrity. For germanium, we discovered that the POₓ/Al₂O₃ passivation stack exhibits a surprising self-cleaning effect, removing the native GeOₓ and contributing to its exceptional passivation performance. We hypothesize that this self-cleaning stems from oxygen migration from the GeOₓ layer to the POₓ layer and/or from the formation of phosphoric acid during POₓ deposition, which is known to etch GeOₓ.[6] These findings highlight the potential of self-cleaning processes to enhance passivation, and exploring similar effects for other semiconductors could unlock new pathways for achieving high-performance surfaces.
Passivating GaN for Next-Generation Power Electronics and Displays
Passivation of gallium nitride (GaN) surfaces is crucial for its performance in high-electron-mobility transistors (HEMTs) used in power semiconductors and in microLED displays. In HEMTs, effective passivation reduces surface states that can trap charges, improving device reliability and efficiency. Similarly, for microLEDs, surface passivation minimizes non-radiative recombination at the GaN interface, boosting brightness and performance. Atomic layer deposition (ALD) has proven valuable in these applications. In-situ cleaning processes also play a vital role here, as removing contaminants or native oxides prior to passivation is essential for stable and effective surface passivation. For example, research has shown that ALD in conjunction with in-situ cleaning by pulsing of trimethyl aluminum as a reducing agent can significantly enhance GaN device performance by preventing reoxidation and ensuring clean interfaces. These advancements underscore the importance of combining ALD with innovative cleaning strategies to fully leverage GaN’s potential in next-generation electronics and displays.
References
[1] G. Dingemans, W.M.M. Kessels, Status and prospects of Al2O3-based surface passivation schemes for silicon solar cells, J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 30 (2012) 040802. https://doi.org/10.1116/1.4728205
[2] J. Melskens, B.W.H. van de Loo, B. Macco, L.E. Black, S. Smit, W.M.M. Kessels, Passivating Contacts for Crystalline Silicon Solar Cells: From Concepts and Materials to Prospects, IEEE J. Photovoltaics. 8 (2018) 373–388. https://doi.org/10.1109/JPHOTOV.2018.2797106.
[3] L.E. Black, B.W.H. Van De Loo, B. Macco, J. Melskens, W.M.M. Kessels, Explorative studies of novel silicon surface passivation materials : Considerations and lessons learned, Sol. Energy Mater. Sol. Cells. 188 (2018) 182–189. https://doi.org/10.1016/j.solmat.2018.07.003.
[4] B. Macco, W.M.M. Kessels, Atomic layer deposition of conductive and semiconductive oxides, Appl. Phys. Rev. 9 (2022) 041313. https://doi.org/10.1063/5.0116732.
[5] R.J. Theeuwes, J. Melskens, W. Beyer, U. Breuer, L.E. Black, W.J.H. Berghuis, B. Macco, W.M.M. Kessels, POx/Al2O3 stacks for surface passivation of Si and InP, Sol. Energy Mater. Sol. Cells. 246 (2022) 111911. https://doi.org/10.1016/j.solmat.2022.111911.
[6] R.J. Theeuwes, W.J.H. Berghuis, B. Macco, W.M.M. Kessels, Excellent passivation of germanium surfaces by POx/Al2O3 stacks, Appl. Phys. Lett. 123 (2023). https://doi.org/10.1063/5.0164028.
[7] W.J.H. Berghuis, J. Melskens, B. Macco, R.J. Theeuwes, L.E. Black, M.A. Verheijen, W.M.M. Kessels, Excellent surface passivation of germanium by a-Si:H/Al2O3 stacks, J. Appl. Phys. 130 (2021) 135303. https://doi.org/10.1063/5.0064808.
[8] W.J.H. Berghuis, M. Helmes, J. Melskens, R.J. Theeuwes, W.M.M. (Erwin) Kessels, B. Macco, Extracting surface recombination parameters of germanium–dielectric interfaces by corona-lifetime experiments, J. Appl. Phys. 131 (2022) 195301. https://doi.org/10.1063/5.0091759.
[9] B. Macco, B.W.H. van de Loo, M. Dielen, D.G.J.A. Loeffen, B.B. van Pelt, N. Phung, J. Melskens, M.A. Verheijen, W.M.M. Kessels, Atomic-layer-deposited Al-doped zinc oxide as a passivating conductive contacting layer for n+-doped surfaces in silicon solar cells, Sol. Energy Mater. Sol. Cells. 233 (2021) 111386. https://doi.org/10.1016/j.solmat.2021.111386.
[10] L.E. Black, A. Cavalli, M.A. Verheijen, J.E.M. Haverkort, E.P.A.M. Bakkers, W.M.M.. Kessels, Effective Surface Passivation of InP Nanowires by Atomic-Layer-Deposited Al2O3 with POx Interlayer, Nano Lett. 17 (2017) 6287–6294. https://doi.org/10.1021/acs.nanolett.7b02972.
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