Atomic Layer Deposition of Titanium Oxide-Based Films for Semiconductor Applications-Effects of Precursor and Operating Conditions.

Two widely used atomic layer deposition precursors, Tetrakis (dimethylamido) titanium (TDMA-Ti) and titanium tetrachloride (TiCl4), were investigated for use in the deposition of TiOx-based thin films as a passivating contact material for solar cells. This study revealed that both precursors are suited to similar deposition temperatures (150 °C). Post-deposition annealing plays a major role in optimising the titanium oxide (TiOx) film passivation properties, improving minority carrier lifetime (τeff) by more than 200 µs. Aluminium oxide deposited together with titanium oxide (AlOy/TiOx) reduced the sheet resistance by 40% compared with pure TiOx. It was also revealed that the passivation quality of the (AlOy/TiOx) stack depends on the precursor and ratio of AlOy to TiOx deposition cycles.

CMOS Image Sensor for Broad Spectral Range with >90% Quantum Efficiency.

Even though the recent progress made in complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) has enabled numerous applications affecting our daily lives, the technology still relies on conventional methods such as antireflective coatings and ion-implanted back-surface field to reduce optical and electrical losses resulting in limited device performance. In this work, these methods are replaced with nanostructured surfaces and atomic layer deposited surface passivation. The results show that such surface nanoengineering applied to a commercial backside illuminated CIS significantly extends its spectral range and enhances its photosensitivity as demonstrated by >90% quantum efficiency in the 300-700 nm wavelength range. The surface nanoengineering also reduces the dark current by a factor of three. While the photoresponse uniformity of the sensor is seen to be slightly better, possible scattering from the nanostructures can lead to increased optical crosstalk between the pixels. The results demonstrate the vast potential of surface nanoengineering in improving the performance of CIS for a wide range of applications.

Boron-Implanted Black Silicon Photodiode with Close-to-Ideal Responsivity from 200 to 1000 nm.

Detection of UV light has traditionally been a major challenge for Si photodiodes due to reflectance losses and junction recombination. Here we overcome these problems by combining a nanostructured surface with an optimized implanted junction and compare the obtained performance to state-of-the-art commercial counterparts. We achieve a significant improvement in responsivity, reaching near ideal values at wavelengths all the way from 200 to 1000 nm. Dark current, detectivity, and rise time are in turn shown to be on a similar level. The presented detector design allows a highly sensitive operation over a wide wavelength range without making major compromises regarding the simplicity of the fabrication or other figures of merit relevant to photodiodes.

Efficient surface passivation of germanium nanostructures with 1% reflectance.

Germanium (Ge) is a vital element for applications that operate in near-infrared wavelengths. Recent progress in developing nanostructured Ge surfaces has resulted in >99% absorption in a wide wavelength range (300-1700 nm), promising unprecedented performance for optoelectronic devices. However, excellent optics alone is not enough for most of the devices (e.g. PIN photodiodes and solar cells) but efficient surface passivation is also essential. In this work, we tackle this challenge by applying extensive surface and interface characterization including transmission electron microscopy and x-ray photoelectron spectroscopy, which reveals the limiting factors for surface recombination velocity (SRV) of the nanostructures. With the help of the obtained results, we develop a surface passivation scheme consisting of atomic-layer-deposited aluminum oxide and sequential chemical treatment. We achieve SRV as low as 30 cm s-1combined with ∼1% reflectance all the way from ultraviolet to NIR. Finally, we discuss the impact of the achieved results on the performance of Ge-based optoelectronic applications, such as photodetectors and thermophotovoltaic cells.

Optoelectronic properties of black silicon fabricated by femtosecond laser in ambient air: exploring a large parameter space.

We study the surface morphology, optical absorption (400-1100 nm), and carrier lifetime of black silicon fabricated by femtosecond (fs) laser in air. We explore a large laser parameter space, for which we adopt a single parameter ξ to describe the cumulative fluence delivered to the sample. We also study the laser-oxidized surface layer by measuring its photoluminescence spectra and comparing its effect on the aforementioned properties. Our study in a broad range of ξ is instructive in choosing laser parameters when targeting different applications.

Electron Injection in Metal Assisted Chemical Etching as a Fundamental Mechanism for Electroless Electricity Generation.

Metal-assisted chemical etching (MACE) is a widely applied process for fabricating Si nanostructures. As an electroless process, it does not require a counter electrode, and it is usually considered that only holes in the Si valence band contribute to the process. In this work, a charge carrier collecting p-n junction structure coated with silver nanoparticles is used to demonstrate that also electrons in the conduction band play a fundamental role in MACE, and enable an electroless chemical energy conversion process that was not previously reported. The studied structures generate electricity at a power density of 0.43 mW/cm2 during MACE. This necessitates reformulating the microscopic electrochemical description of the Si-metal-oxidant nanosystems to separately account for electron and hole injections into the conduction and valence band of Si. Our work provides new insight into the fundamentals of MACE and demonstrates a radically new route to chemical energy conversion by solar cell-inspired devices.

Grass-like alumina coated window harnesses the full omnidirectional potential of black silicon photodiodes.

Packaged photodiodes suffer from Fresnel reflection from the package window glass, especially at high angles of incidence. This has a notable impact particularly on black silicon (b-Si) photodiodes, which have extreme sensitivity. In this work, we show that by adding a simple grass-like alumina antireflection (AR) coating on the window glass, excellent omnidirectional sensitivity and high external quantum efficiency (EQE) of b-Si photodiodes can be retained. We demonstrate that EQE increases at all angles, and up to 15% absolute increases in EQE at a 70° angle of incidence compared to conventional uncoated glass. Furthermore, even at the incidence angle of 50°, the double-sided coating provides higher EQE than bare glass at normal incidence. Our results demonstrate that grass-like alumina coatings are efficient and omnidirectional AR coatings for photodiode package windows in a wide wavelength range across the visible spectrum to near-infrared radiation.

Efficient photon capture on germanium surfaces using industrially feasible nanostructure formation.

Nanostructured surfaces are known to provide excellent optical properties for various photonics devices. Fabrication of such nanoscale structures to germanium (Ge) surfaces by metal assisted chemical etching (MACE) is, however, challenging as Ge surface is highly reactive resulting often in micron-level rather than nanoscale structures. Here we show that by properly controlling the process, it is possible to confine the chemical reaction only to the vicinity of the metal nanoparticles and obtain nanostructures also in Ge. Furthermore, it is shown that controlling the density of the nanoparticles, concentration of oxidizing and dissolving agents as well as the etching time plays a crucial role in successful nanostructure formation. We also discuss the impact of high mobility of charge carriers on the chemical reactions taking place on Ge surfaces. As a result we propose a simple one-step MACE process that results in nanoscale structures with less than 10% surface reflectance in the wavelength region between 400 and 1600 nm. The method consumes only a small amount of Ge and is thus industrially viable and also applicable to thin Ge layers.

Decreasing Interface Defect Densities via Silicon Oxide Passivation at Temperatures Below 450 °C.

Low-temperature (LT) passivation methods (<450 °C) for decreasing defect densities in the material combination of silica (SiOx) and silicon (Si) are relevant to develop diverse technologies (e.g., electronics, photonics, medicine), where defects of SiOx/Si cause losses and malfunctions. Many device structures contain the SiOx/Si interface(s), of which defect densities cannot be decreased by the traditional, beneficial high temperature treatment (>700 °C). Therefore, the LT passivation of SiOx/Si has long been a research topic to improve application performance. Here, we demonstrate that an LT (<450 °C) ultrahigh-vacuum (UHV) treatment is a potential method that can be combined with current state-of-the-art processes in a scalable way, to decrease the defect densities at the SiOx/Si interfaces. The studied LT-UHV approach includes a combination of wet chemistry followed by UHV-based heating and preoxidation of silicon surfaces. The controlled oxidation during the LT-UHV treatment is found to provide an until now unreported crystalline Si oxide phase. This crystalline SiOx phase can explain the observed decrease in the defect density by half. Furthermore, the LT-UHV treatment can be applied in a complementary, post-treatment way to ready components to decrease electrical losses. The LT-UHV treatment has been found to decrease the detector leakage current by a factor of 2.

Low-Temperature Plasma-Enhanced Atomic Layer Deposition of SiO2 Using Carbon Dioxide.

In this work, we report the successful growth of high-quality SiO2 films by low-temperature plasma-enhanced atomic layer deposition using an oxidant which is compatible with moisture/oxygen sensitive materials. The SiO2 films were grown at 90 °C using CO2 and Bis(tertiary-butylamino)silane as process precursors. Growth, chemical composition, density, optical properties, and residual stress of SiO2 films were investigated. SiO2 films having a saturated growth-per-cycle of ~ 1.15 Å/cycle showed a density of ~ 2.1 g/cm3, a refractive index of ~ 1.46 at a wavelength of 632 nm, and a low tensile residual stress of ~ 30 MPa. Furthermore, the films showed low impurity levels with bulk concentrations of ~ 2.4 and ~ 0.17 at. % for hydrogen and nitrogen, respectively, whereas the carbon content was found to be below the measurement limit of time-of-flight elastic recoil detection analysis. These results demonstrate that CO2 is a promising oxidizing precursor for moisture/oxygen sensitive materials related plasma-enhanced atomic layer deposition processes.

Black silicon significantly enhances phosphorus diffusion gettering.

Black silicon (b-Si) is currently being adopted by several fields of technology, and its potential has already been demonstrated in various applications. We show here that the increased surface area of b-Si, which has generally been considered as a drawback e.g. in applications that require efficient surface passivation, can be used as an advantage: it enhances gettering of deleterious metal impurities. We demonstrate experimentally that interstitial iron concentration in intentionally contaminated silicon wafers reduces from 1.7 × 1013 cm-3 to less than 1010 cm-3 via b-Si gettering coupled with phosphorus diffusion from a POCl3 source. Simultaneously, the minority carrier lifetime increases from less than 2 µs of a contaminated wafer to more than 1.5 ms. A series of different low temperature anneals suggests segregation into the phosphorus-doped layer to be the main gettering mechanism, a notion which paves the way of adopting these results into predictive process simulators. This conclusion is supported by simulations which show that the b-Si needles are entirely heavily-doped with phosphorus after a typical POCl3 diffusion process, promoting iron segregation. Potential benefits of enhanced gettering by b-Si include the possibility to use lower quality silicon in high-efficiency photovoltaic devices.

Data of ALD Al2O3 rear surface passivation, Al2O3 PERC cell performance, and cell efficiency loss mechanisms of Al2O3 PERC cell.

This data article is related to the recently published article '20.8% industrial PERC solar cell: ALD Al2O3 rear surface passivation, efficiency loss mechanisms analysis and roadmap to 24%' (Huang et al., 2017) [1]. This paper is about passivated emitter and rear cell (PERC) structures and it describes the quality of the Al2O3 rear-surface passivation layer deposited by atomic layer deposition (ALD), in relation to the processing parameters (e.g. pre-clean treatment, deposition temperature, growth per cycle, and film thickness) and to the cell efficiency loss mechanisms. This dataset is made public in order to contribute to the limited available public data on industrial PERC cells, to be used by other researchers.

Data of the recombination loss mechanisms analysis on Al2O3 PERC cell using PC1D and PC2D simulations.

This data article is related to our recently published article ('20.8% industrial PERC solar cell: ALD Al2O3 rear surface passivation, efficiency loss mechanisms analysis and roadmap to 24%', Huang et al., 2017 [1]) where we have presented a systematic evaluation of the overall cell processing and a cost-efficient industrial roadmap for PERC cells. Aside from the information already presented in Huang et al., 2017 [1], here we provide data related to Sectin 3 in Huang et al., 2017 [1] concerning the analysis of the recombination losses׳ mechanisms by PC1D V5.9 and PC2D simulations (Clugston and Basore, 1997, Basore and Cabanas-Holmen, 2011, Cabanas-Holmen and Basore, 2012 and Cabanas-Holmen and Basore, 2012.) [2], [3], [4], [5] on our current industrial Al2O3 PERC cell. The data include: i) PC2D simulations on J02, ii) the calculation of series resistance and back surface recombination velocity (BSRV) on the rear side metallization of PERC cell for the case of a point contact, and iii) the PC1D simulation on the cumulative photo-generation and recombination along the distance from the front surface. Finally, the roadmap of the solar cell efficiency for an industrial PERC technology up to 24% is presented, with the aim of providing a potential guideline for industrial researchers.

Silicon dioxide mask by plasma enhanced atomic layer deposition in focused ion beam lithography.

In this work, focused ion beam (FIB) lithography was developed for plasma enhanced atomic layer deposited (PEALD) silicon dioxide SiO2 hard mask. The PEALD process greatly decreases the deposition temperature of the SiO2 hard mask. FIB Ga+ ion implantation on the deposited SiO2 layer increases the wet etch resistivity of the irradiated region. A programmed exposure in FIB followed by development in a wet etchant enables the precisely defined nanoscale patterning. The combination of FIB exposure parameters and the development time provides greater freedom for optimization. The developed process provides high pattern dimension accuracy over the tested range of 90-210 nm. Utilizing the SiO2 mask developed in this work, silicon nanopillars with 40 nm diameter were successfully fabricated with cryogenic deep reactive ion etching and the aspect ratio reached 16:1. The fabricated mask is suitable for sub-100 nm high aspect ratio silicon structure fabrication.

Wave optical simulation of the light trapping properties of black silicon surface textures.

Due to their low reflectivity and effective light trapping properties black silicon nanostructured surfaces are promising front side structures for thin crystalline silicon solar cells. For further optimization of the light trapping effect, particularly in combination with rear side structures, it is necessary to simulate the optical properties of black silicon. Especially, the angular distribution of light in the silicon bulk after passage through the front side structure is relevant. In this paper, a rigorous coupled wave analysis of black silicon is presented, where the black silicon needle shaped structure is approximated by a randomized cone structure. The simulated absorptance agrees well with measurement data. Furthermore, the simulated angular light distribution within the silicon bulk shows that about 70% of the light can be subjected to internal reflection, highlighting the good light trapping properties.

Black silicon solar cells with interdigitated back-contacts achieve 22.1% efficiency.

The nanostructuring of silicon surfaces--known as black silicon--is a promising approach to eliminate front-surface reflection in photovoltaic devices without the need for a conventional antireflection coating. This might lead to both an increase in efficiency and a reduction in the manufacturing costs of solar cells. However, all previous attempts to integrate black silicon into solar cells have resulted in cell efficiencies well below 20% due to the increased charge carrier recombination at the nanostructured surface. Here, we show that a conformal alumina film can solve the issue of surface recombination in black silicon solar cells by providing excellent chemical and electrical passivation. We demonstrate that efficiencies above 22% can be reached, even in thick interdigitated back-contacted cells, where carrier transport is very sensitive to front surface passivation. This means that the surface recombination issue has truly been solved and black silicon solar cells have real potential for industrial production. Furthermore, we show that the use of black silicon can result in a 3% increase in daily energy production when compared with a reference cell with the same efficiency, due to its better angular acceptance.