Surface Passivation Schemes Research Articles

Tunnel oxide passivating contact enabled by polysilicon on ultra-thin SiO2 for advanced silicon radiation detectors

Conventional silicon junction detectors encounter significant carrier recombination within the heavily doped p+ and n+ layers, as well as beneath the metal contact regions, creating the so-called “dead layers”, especially on the detector side. In this study, we present the tunnel oxide passivating contact with doped polysilicon on oxide, which demonstrates exceptional surface passivation and carrier selectivity. The key innovation lies in an ultra-thin (~ 1.5 nm) interfacial oxide layer that facilitates efficient majority carrier transportation via tunneling while effectively block minority carriers. Remarkably low saturation current densities, ranging from 5 to 10 fA/cm2 even with the metal contact, underscore the superiority of both n-type and p-type tunnel oxide passivating contacts. In contrast, conventional p–n junction or high-low junction exhibit saturation current densities ranging from 10 to 90 fA/cm2 in the studied p+ and n+ layers with surface passivation schemes due to Auger recombination and surface recombination, and 1000–6000 fA/cm2 with metal contacts due to intense metal-induced recombination at the interface. These findings indicate the potential and superiority of implementing n-type tunnel oxide passivating contact on the detector side and p-type contact on the back side for advanced silicon radiation detectors. This approach would enable thorough collection of generated charge carriers along the track of incident ionizing radiation particles, leading to improved energy resolution and reduced noise levels.

Low Surface Recombination in Hexagonal SiGe Alloy Nanowires: Implications for SiGe-Based Nanolasers.

Monolithic integration of silicon-based electronics and photonics could open the door toward many opportunities including on-chip optical data communication and large-scale application of light-based sensing devices in healthcare and automotive; by some, it is considered the Holy Grail of silicon photonics. The monolithic integration is, however, severely hampered by the inability of Si to efficiently emit light. Recently, important progress has been made by the demonstration of efficient light emission from direct-bandgap hexagonal SiGe (hex-SiGe) alloy nanowires. For this promising material, realized by employing a nanowire structure, many challenges and open questions remain before a large-scale application can be realized. Considering that for other direct-bandgap materials like GaAs, surface recombination can be a true bottleneck, one of the open questions is the importance of surface recombination for the photoluminescence efficiency of this new material. In this work, temperature-dependent photoluminescence measurements were performed on both hex-Ge and hex-SiGe nanowires with and without surface passivation schemes that have been well documented and proven effective on cubic silicon and germanium to elucidate whether and to what extent the internal quantum efficiency (IQE) of the wires can be improved. Additionally, time-resolved photoluminescence (TRPL) measurements were performed on unpassivated hex-SiGe nanowires as a function of their diameter. The dependence of the surface recombination on the SiGe composition could, however, not be yet addressed given the sample-to-sample variations of the state-of-the-art hex-SiGe nanowires. With the aforementioned experiments, we demonstrate that at room temperature, under high excitation conditions (a few kW cm-2), the hex-(Si)Ge surface is most likely not a bottleneck for efficient radiative emission under relatively high excitation conditions. This is an important asset for future hex(Si)Ge optoelectronic devices, specifically for nanolasers.

Precisely nanostructured HfO2 rear passivation layers for ultra‐thin Cu(In,Ga)Se2

AbstractThe quest for material‐efficient Cu(In,Ga)Se2 (CIGS) solar cells encourages the development of ultra‐thin absorbers. Their use reduces material consumption and energy usage during production by increasing the throughput. Thereby, both the bill of materials as well as the energy and capital costs are reduced. However, because thin absorbers are prone to increase back contact recombination, back surface passivation schemes are necessary to reach a similar or higher conversion efficiency than for absorbers with conventional thickness. Here, we investigate nanostructured hafnium oxide (HfO2) rear passivation layers for ultra‐thin CIGS solar cells. We fabricate regular arrays of point contacts with 200 nm diameter through HfO2 layers with thicknesses between 7 and 40 nm using electron beam lithography and reactive ion etching. The current–voltage curves of solar cells with a 500 nm thick CIGS absorber layer and the nanostructured passivation layer show improved performance concerning Voc and Jsc compared to non‐passivated reference devices. Furthermore, external quantum efficiency and optical reflection confirm an effective passivation behavior, with an average efficiency increase of up to 1.2% for the cells with the 40 nm thick HfO2 layer. In addition, simulation work shows that even 40 nm thick HfO2 passivation layers have only a minimal effect on the optical properties of ultra‐thin CIGS solar cells, and hence, the photocurrent increase verified experimentally stems from electrical improvements caused by the HfO2 layer passivation effect. We also investigate the impact of ultra‐thin (0.3, 0.6, 1.3, and 2.5 nm) non‐patterned HfO2 passivation layers on the same type of solar cells. However, these results showed no improvement in solar cell performance, despite an increase in the current density with layer thickness.

Status of Al2O3/TiO2‐Based Antireflection and Surface Passivation for Silicon Solar Cells

Inhibiting charge recombination effectively by surface passivation is very important for high‐efficiency silicon solar cells. With the emergence of passivated emitter and rear cells (PERCs), which surpass conventional solar cells, the need for a novel passivation scheme is increasing. Passivation layer stacks using negatively charged Al2O3 have been applied successfully to realize high‐efficiency PERC solar cells. Herein, the developments of surface passivation by Al2O3/TiO2‐based layers are reviewed. The topics range from synthesis methods and optical properties as antireflection coatings to the electrical properties related to the material, interface, and passivation quality. Ultrathin Al2O3/TiO2‐based layers generally feature a combination of field‐effect passivation by negative fixed charges, a low interface defect density, and adequate humidity stability. Among the various deposition methods, atomic layer deposition is focused on, which can form dual layers and nanolaminates. How the features of atomic‐layer‐deposited Al2O3/TiO2‐based layers can be utilized for interface engineering and tailoring the properties of surface passivation schemes is discussed.

Interplay of Various Charge Sources in AlGaN/GaN Epi-Stack Governing HEMT Breakdown

In this work, we have revealed the interplay of various charge sources (surface, polarization, and buffer) and their relative concentrations across the AlGaN/GaN epi-stack governing the electric field distribution and the breakdown mechanism in high electron mobility transistors (HEMTs). The investigations are carried out for Schottky, metal-insulator-semiconductor (MIS), and p-GaN gate stacks while accounting for possible GaN buffer types (Fe-doped and C-doped). A strong correlation between 3-terminal (3T) breakdown voltage, gate design, and relative concentration of various charges was found. % increase in 3T breakdown voltage when physical dimensions were doubled was also found to be strongly correlated with the relative concentration of various charges for both Fe- and C-doped buffer HEMTs. On the other hand, the 2T (source–drain, without gate, and GaN channel below gate) breakdown voltage was independent of all other parameters except buffer properties and physical dimensions. Physical insights are developed to explain the dependence of electric field distribution, carrier injection, and HEMT breakdown on the surface states, polarization charge, and buffer traps, as well as their relative concentrations for both the buffer types and all three gate types. These insights will help to design efficient surface passivation schemes and resolve ambiguities, often observed in experiments, in terms of location of peak electric field (drain side, or gate side or both) as well as OFF-state conduction and breakdown mechanism (gate injection, or punchthrough, or parasitic conduction through buffer or avalanche generation).

Impact of AlGaN/GaN Interface and Passivation on the Robustness of Low-Noise Amplifiers

Poststress dc characteristics of AlGaN/GaN HEMTs can be used to study the effect of high-power stress on the noise figure (NF) and gain of low-noise amplifiers (LNAs) subjected to large input overdrives. This enables a shift from circuit- to transistor-level measurements to investigate the impact of variations in HEMT design parameters on the robustness (including both recovery time and survivability) by mimicking LNA operation. Using this method, a tradeoff between survivability and recovery time is demonstrated for different AlGaN/GaN interface profiles (sharp interface, standard interface, and AlN interlayer). Furthermore, the impact of different surface passivation schemes (Si-rich, Si-poor, and bilayer SiN x ) on robustness is investigated. The bilayer passivation, which features low leakage current and small gain compression under overdrive stress, exhibits relatively weak survivability. The mechanisms influencing the robustness are analyzed based on transistor physics. The short recovery time is mainly due to impeding the injection of hot electrons into surface traps and high reverse current, whereas the survivability is dependent on the local or global peak electrical fields around the gate under high power stress.

Photoluminescence of carbon dots prepared by ball milling and their application in Hela cell imaging

Carbon dots are small carbon nanoparticles with various surface passivation schemes. The deliberate functionalization of carbon nanoparticles and carbonization of organic or carbon-rich species are two main synthesis routes for carbon dots. However, these two synthesis routes suffer from drawbacks such as critical reaction conditions and uncontrollable structure and performance. In our present work, a hybrid approach combining the advantageous characteristics of these two synthesis routes was applied to prepare carbon dots. The spherical and hydrophilic carbon dots were prepared by high-energy ball milling of active carbon and potassium carbonate. The carbon dots prepared are disperse and fine with an average size of 3.5 nm and a size distribution of 1.6–6.0 nm. They exhibit a nearly excitation-independent and high photostable blue photoluminescence behavior with the maximal emission wavelength at 430 nm. They also show an excellent photostability under high NaCl concentrations or long UV exposure time and a stable photoluminescence behavior in a wide pH range from 3 to 11. They are nearly not cytotoxic to Hela cells, can be taken up by Hela cells, and applied to Hela cell imaging. Our hybrid approach did not use any strong basic and can be scaled up for large-scale preparation of carbon dots.

Carbon dots for energy conversion applications

Quantum dots (QDs), generally referring to semiconductor nanocrystals that display the quantum confinement effect, have been widely pursued for many energy conversion applications. More recently, carbon dots (CDots), which are small carbon nanoparticles with various surface passivation schemes, have been found to possess optical properties and photoinduced redox characteristics resembling those of conventional semiconductor QDs and thus are amenable to some of the same uses in energy conversions. Among the various carbon nanomaterials, fullerenes have been extensively investigated for their use as critical components in optoelectronic devices and systems. Carbon nanoparticles, representing a largely ignored nanoscale carbon allotrope, are in fact more effective in some of the same functions, which are materialized and much enhanced upon the surface passivation of the nanoparticles in CDots. In this perspective article on CDots for energy conversion applications, the optical properties and redox characteristics of CDots, including the related mechanistic framework and its relationship to the use of CDots as potent photocatalysts for the conversion of CO2 into small organic molecules, are highlighted. Also highlighted are results from representative studies using CDots in light-emitting diodes and various solar cells to demonstrate their excellent potential for a wide range of roles in optoelectronic devices and systems. Issues and opportunities in the further development of the CDots platform and related technologies are discussed.

Systematic Toxicity Evaluations of High-Performance Carbon "Quantum" Dots.

Carbon dots (CDots) in a general structure of small carbon nanoparticles with various surface passivation schemes have emerged to represent a new class of carbon nanomaterials in now a rapidly advancing and expanding research field. Among various synthesis methods, the use of pre-processed and selected small carbon nanoparticles for deliberate chemical functionalization of the particle surface with organic molecules have produced high-performance and structurally better defined CDots. Specifically, small organic molecules 2,2'-(ethylenedioxy)bis(ethylamine) and 3-ethoxypropylamine were used for the effective surface passivation of the carbon nanoparticles via chemical functionalization to yield CDots that are brightly fluorescent and also structurally ultra-compact, amenable to various desired cell imaging applications. Thus, a systematic evaluation of these CDots on their cytotoxicity profiles is necessary, and performed in this study by using a diverse selection of cell lines. Also for fluorescence imaging, CDots were modified with their encapsulating selected organic dyes for much enhanced red/near-IR fluorescence emissions. These modified CDots with the dyes as guest were also evaluated for their cytotoxicity profiles. The results suggest that the CDots without and with the guest dyes are generally nontoxic to the selected cell lines, further supporting the notion that CDots can be used as high-performance yet nontoxic bioimaging agents.

Antimicrobial Carbon-Based Nanomaterials

Infectious diseases caused by bacterial pathogens have been a serious threat to public health for decades and remain one of the major concerns of our society. Control and prevention of pathogen contamination are effective ways to reduce the risk of infectious disease. The advancement of nanotechnology has brought various nanomaterials with remarkable properties for a wide range of applications, including the antimicrobial fields. Carbon-based nanomaterials represent a class of relatively safe and cost-effective materials yet with desirable antimicrobial properties which are applicable to the antimicrobial fields. This research explored the antimicrobial activity of two types of carbon-based nanomaterials and their composites: carbon nanotubes (CNTs) and carbon “quantum” dots. We investigated the applications of single-walled CNTs (SWNTs), multi-walled CNTs (MWNTs), CNTs-Ag, CNTs-chemicals/natural peptides, CNTs-NIR, CNTs-coated surfaces, for inactivation of bacterial pathogens, bacterial spores, and viruses, inhibition of biofilms, as well as for capturing and concentrating bacterial pathogens (Fig. 1). The results indicated that SWNTs effectively inactivated Salmonella, E. coli cells, Bacillus anthracis cells in suspensions, but did not effective in inactivating bacterial spores. The microbial activity of SWNTs was concentration-, treatment time-, buffer-, CNTs’ length- and surface group- dependent. SWNTs coupled with H2O2 or NaOCl had the synergistic effect contributed by the two individual antimicrobial mechanisms of SWNTs and the oxidizing antimicrobial chemicals, and resulted in much stronger sporicidal effect compared to treatment with H2O2 or NaOCl alone at the same concentrations, doubling the log reduction of viable spore number (~3.3 log vs. ~1.6 log). MWNTs were not effective in inactivating bacterial cells in suspension, but MWNTs modified surfaces, including MWNT forest on silicon wafer and MWNT sheet on poly(methyl methacrylate) (PMMA) film, significantly increased surface hydrophobicity and enhanced the attachment of spores on their surfaces compared to the uncoated substrates, respectively, showing their potential as adsorbents for removal of Bacillus spores from fluids. Carbon dots (CDots), defined as small carbon nanoparticles with various surface passivation schemes, with their optical properties and photocatalytic functions, were evaluated for their photoinduced bactericidal functions, with the results suggesting that the dots were highly effective in bacteria-killing with visible light illumination (Fig.1). Correlations between CDots property, including CDots quantum yield and surface functionalization, and antimicrobial activity were studied. Concentration- and time- dependency of CDots’ antimicrobial activity was examined. Mechanistic implications of the results will be discussed. Challenges and opportunities in the development of carbon dots into a new class of effective visible/natural light-responsible bactericidal agents for a variety of bacteria control applications will be discussed Acknowledgement: The research was supported by NSF grants: DMR# 1701399 and NIH grant R15GM114752. Figure 1

Taking monocrystalline silicon to the ultimate lifetime limit

A central quantity to assess the high quality of monocrystalline silicon (on scales beyond mere purity) is the minority charge carrier lifetime. We demonstrate that the lifetime in high purity float zone material can be improved beyond existing observations, thanks to a deeper understanding of grown-in defects and how they can be permanently annihilated. In a first step we investigate the influence of several process sequences on the lifetime by applying a low temperature superacid passivation treatment. We find that a pre-treatment consisting of an oxidation at 1050 °C followed by a POCl3 diffusion at 900 °C can improve the lifetime by deactivating or eliminating grown-in defects. Then, pre-treated wafers of different float zone materials are passivated with three state-of-the-art layer stacks. Very high effective lifetime values are measured, thereby demonstrating the high quality of the surface passivation schemes and the pre-treated silicon wafers. The measured effective lifetimes exceed previous records, and we report an effective lifetime of 225 ms measured on a 200 µm thick 100 Ω cm n-type silicon wafer symmetrically passivated with a layer stack of a thin thermally grown oxide and a polycrystalline layer (the TOPCon layer stack).

Surface passivation of silicon photonic devices with high surface-to-volume-ratio nanostructures

Recently, a high-speed and high-efficiency silicon photodiode (PD) was demonstrated by enabling light trapping micro/nanostructured holes. While attractive for light manipulation, these high surface-to-volume-ratio nanostructures, which are created by top-down dry etching processes, can also bring other challenges, such as creating silicon surface damage and crystalline defects. To reduce the dark current level and minimize surface recombination, successful passivation is a vital step to achieving the ultimate performance of the silicon PD based on photon-trapping structures. In this paper, we present and compare several effective silicon surface passivation schemes including plasma-enhanced chemical vapor deposited SiO2 and Si3N4, hydrofluoric acid (HF) treatment, thermal oxidation, and low ion energy dry etch. These passivation techniques can reduce the dark current by more than 4 orders of magnitude, bringing it down to the nanoampere level. Among these passivation schemes, HF treatment is relatively simple and cost effective, while other techniques may alter the light trapping characteristics of silicon PDs to some extent, thus affecting their external quantum efficiency. The passivation techniques discussed in this paper are CMOS compatible and can also be applied to other silicon photonic devices, such as photovoltaics.

Dimension- and Surface-Tailored ZnO Nanowires Enhance Charge Collection in Quantum Dot Photovoltaic Devices

The use of zinc oxide (ZnO) nanowires improves charge collection, and consequently power conversion efficiency, in quantum dot (QD) based photovoltaic devices. However, the role of the nanowire geometry (e.g., density, length, and morphology, etc.) relative to the QD properties remains unexplored, in part due to challenges with controlled nanowire synthesis. Here, we independently tailor nanowire length and the active device layer thickness to study charge collection in lead sulfide (PbS) QD photovoltaic devices. We then demonstrate consistently high internal quantum efficiency in these devices by applying quantum efficiency and total reflectance measurements. Our results show that significant losses originate from ZnO nanowire–QD interfacial recombination, which we then successfully overcome by using nanowire surface passivation. This geometry-tailored approach is generally applicable to other nanowire–QD systems, and the surface passivation schemes will play a significant role in future development of n...

Characteristic Excitation Wavelength Dependence of Fluorescence Emissions in Carbon “Quantum” Dots

Carbon “quantum” dots (CDots), generally defined as small carbon nanoparticles with various surface passivation schemes, have emerged to represent a rapidly advancing and expanding research field. ...

Long-Term Stability of Aluminum Oxide Based Surface Passivation Schemes Under Illumination at Elevated Temperatures

The stability of passivation layers under the conditions of field application of solar modules is a crucial parameter. We have performed an experiment to test the stability of aluminum oxide based passivation schemes under halogen lamp illumination of 1 sun equivalent intensity at 75 °C. We compare aluminum oxide layers with atomic layer deposition and plasma-enhanced chemical vapor deposition with and without an amorphous silicon nitride capping layer. Different thermal treatments are performed to activate the passivation or simulate solar cell contact firing. All passivation schemes are tested on full four inch 1 Ω·cm p- and n-type float-zone silicon wafers. The quality and stability of the surface passivation are tested by means of repeated charge carrier lifetime measurements. No significant surface passivation degradation due to light-soaking is evident on capped samples for several hundred hours. N-type samples are demonstrated to maintain excellent passivation quality for at least 3700 h and only slight degradation of p-type samples is found.

Stability of effective lifetime of float-zone silicon wafers with AlOx surface passivation schemes under illumination at elevated temperature

For solar cell application, the stability of interface passivation quality to in-field conditions is crucial. We have performed an experiment to test the resilience of different aluminium oxide based passivation schemes to illumination at 75 °C. Different thermal treatments to activate the passivation and/or simulate contact firing were performed before light soaking. The experiment was performed on 1 Ωcm float-zone silicon of both p- and n-type doping. The study demonstrates that good passivation quality can be achieved both by atomic layer deposition and by PECVD and that addition of silicon nitride capping layers greatly enhances thermal stability. On p-type wafers a severe but temporary degradation of the electrical quality of the wafer bulk was observed during the first hours upon application of such capping layers. Besides this effect, reasonable temporal stability of the effective lifetime was observed for p-type samples while n-type samples featured excellent long-term stability.

Steady-state and time-resolved fluorescence studies on interactions of carbon “quantum” dots with nitrotoluenes

Carbon “quantum” dots or carbon dots, generally defined as small carbon nanoparticles with various surface passivation schemes, have emerged to represent a rapidly advancing and expanding research field. A wide variety of technological applications of carbon dots have been pursued, including especially their uses as fluorescence probes or sensors. In the work reported here, carbon dots obtained from the surface chemical functionalization of small carbon nanoparticles with 2,2′-(ethylenedioxy)bis(ethylamine) (EDA) were selected as structurally compact and defined fluorescence probes, and their interactions with nitrotoluene compounds were studied on the basis of fluorescence quenching in steady-state and time-resolved measurements. The fluorescence quenching of the carbon dots by the nitrotoluenes, which are commonly used as signature compounds for nitroaromatic explosives, was found to be extremely efficient in ambient solution, though still at the very upper-limit of diffusion-control. There were some static quenching contributions at high nitrotoluene quencher concentrations, especially for the compound that is more electron deficient, which could be rationalized in terms of some near-neighbor charge transfer interactions. Mechanistic implications of the results and the potential for carbon dots to serve as highly sensitive fluorescence probes in the detection of nitroaromatic explosives are discussed.

Remote Molecular Doping of Colloidal Quantum Dot Photovoltaics

In recent years colloidal quantum dot (CQD) photovoltaics have developed rapidly because of novel device architectures and robust surface passivation schemes. Achieving controlled net doping remains an important unsolved challenge for this field. Herein we present a general molecular doping platform for CQD solids employing a library of metal–organic complexes. Low effective ionization energy and high electron affinity complexes are shown to produce n- and p-doped CQD solids. We demonstrate the obvious advantage in solar cells by p-doping the CQD absorber layer. Employing photoemission spectroscopy, we identify two doping concentration regimes: lower concentrations lead to efficient doping, while higher concentrations also cause large surface dipoles creating energy barriers to carrier flow. Utilizing the lower concentration regime, we remove midgap electrons leading to 25% enhancement in the power conversion efficiency relative to undoped cells. Given the vast number of available metal–organic complexes, this approach opens new and facile routes to tuning the properties of CQDs for various applications without necessarily resorting to new ligand chemistries.

Advances in Surface Passivation Schemes for High Efficiency c-Silicon Solar Cells

Advances in Surface Passivation Schemes for High Efficiency c-Silicon Solar Cells

Visible-Light-Activated Bactericidal Functions of Carbon "Quantum" Dots.

Carbon dots, generally defined as small carbon nanoparticles with various surface passivation schemes, have emerged as a new class of quantum-dot-like nanomaterials, with their optical properties and photocatalytic functions resembling those typically found in conventional nanoscale semiconductors. In this work, carbon dots were evaluated for their photoinduced bactericidal functions, with the results suggesting that the dots were highly effective in bacteria-killing with visible-light illumination. In fact, the inhibition effect could be observed even simply under ambient room lighting conditions. Mechanistic implications of the results are discussed and so are opportunities in the further development of carbon dots into a new class of effective visible/natural light-responsible bactericidal agents for a variety of bacteria control applications.