Buried Voids Research Articles

Enhancing lead-free photovoltaic performance: Minimizing buried surface voids in tin perovskite films through weakly polar solvent pre-treatment strategy

Buried interfacial voids have always been a notorious phenomenon observed in the fabrication of lead perovskite films. The existence of interfacial voids at the buried interface will capture the carrier, suppress carrier transport efficiencies, and affect the stability of photovoltaic devices. However, the impact of these buried interfacial voids on tin perovskites, a promising avenue for advancing lead-free photovoltaics, has been largely overlooked. Here, we utilize an innovative weakly polar solvent pre-treatment strategy (WPSPS) to mitigate buried interfacial voids of tin perovskites. Our investigation reveals the presence of numerous voids in tin perovskites during annealing, attributed to trapped dimethyl sulfoxide (DMSO) used in film formation. The WPSPS method facilitates accelerated DMSO evaporation, effectively reducing residual DMSO. Interestingly, the WPSPS shifts the energy level of PEDOT:PSS downward, making it more aligned with the perovskite. This alignment enhances the efficiency of charge carrier transport. As the result, tin perovskite film quality is significantly improved, achieving a maximum power conversion efficiency approaching 12% with only an 8.3% efficiency loss after 1700 h of stability tests, which compares well with the state-of-the-art stability of tin-based perovskite solar cells.

Ge-on-insulator fabrication based on Ge-on-nothing technology

Ge-on-Insulator (GOI) is considered to be a necessary structure for novel Ge-based devices. This paper proposes an alternative approach for fabricating GOI based on the Ge-on-Nothing (GeON) template. In this approach, a regular macropore array is formed by lithography and dry etching. These pores close and merge upon annealing, forming a suspended monocrystalline Ge membrane on one buried void. GOI is fabricated by direct bonding of GeON on Si carrier substrates, using an oxide bonding interface, and subsequent detachment. The fabricated GOI shows uniform physical properties as demonstrated using micro-photoluminescence measurements. Its electrical characteristics and cross-sectional structure are superior to those of Smart-CutTM GOI. To demonstrate its application potential, back-gate GOI capacitors and MOSFETs are fabricated. Their characteristics nicely agree with the theoretically calculated one and show typical MOSFET operations, respectively, which indicates promising Ge crystallinity. This method, therefore, shows the potential to provide high-quality GOI for advanced Ge application devices.

Elimination of buried interfacial voids for efficient perovskite solar cells

Establishing optimal interfacial contact is crucial for enhancing the efficiency of perovskite solar cells (PSCs). However, formation of voids at buried interface of perovskite films often hinders this critical objective. Our investigation reveals that these buried interfacial voids predominantly manifest at sites characterized by large-angle pits on rough substrate, due to their elevated nucleation barriers compared to their small-angle counterparts. To address this challenge and promote uniform nucleation and growth across all sites, regardless of their angles, we develop an innovative precursor regulation strategy to reduce the nucleation barrier discrepancy between different sites by introducing nonstoichiometric homologous ions or by facilitating the formation of intermediate phases. Notably, the incorporation of FACl additive into the FAPbI3 precursor combines the dual benefits of this approach, yielding high-quality perovskite film with intimate contact and reduced defect density. Consequently, this leads to a significant enhancement in the photovoltaic performance of PSCs.

Application of ambient noise tomography for deep void detection

This study aims to take the advantage of ambient noise recordings rich in low-frequency components for deep site characterization. We investigate the capabilities of a recently developed ambient noise tomography (ANT) method for imaging deep buried voids via both synthetic and field experiments. A challenging synthetic model with two deep voids was used to demonstrate the practicality of this ANT approach. To further test the method's capability, we conducted a field experiment at a bridge construction site in Miami, Florida, which contained a large and deep void (28 m to 44 m depth). The cross-correlation functions (CCFs) of the traffic noise recordings were directly inverted to provide subsurface S-wave velocity profiles. The results demonstrate that the method is capable of imaging deep voids. The in-situ standard penetration test (SPT) data was then compared to the inverted S-wave velocity obtained by the ANT approach. It shows that the trend of Vs and SPT values are generally in agreement, including the identification of the void and its depth. The field results suggest that the ANT is a useful geophysical tool for roadway imaging, particularly for detection of deep voids that are difficult to be imaged by active-seismic methods.

Road sinkhole detection with 2D ambient noise tomography

Seismic methods are often used for detection of precollapsed sinkholes (voids) under roadways for remediation to minimize risks to the safety of the traveling public. Although active-source seismic methods can provide accurate subsurface profiles, they require closing the traffic flow for hours during testing and may potentially cause sinkhole collapse due to ground perturbation by source excitation. To address these issues, we have developed a new 2D ambient noise tomography (2D ANT) method for imaging voids under roadways. Instead of using the approximated Green’s function, whose required assumption of energy balance at both sides of each receiver pair is rarely satisfied, the crosscorrelation function of traffic noise recordings is inverted directly to obtain velocity structures. To adopt the concepts of seismic interferometry and derive the model structural kernel, passing-by vehicles are assumed to be moving sources along the receiver array. The source power-spectrum density is determined via the reverse time imaging approach to approximate the source distribution. The 2D ANT method is first demonstrated on a realistic synthetic model with the accurate recovery of the model variable layers and a buried void. To demonstrate its effectiveness on real-world problems, we successfully apply it to field data for assessment of a repaired sinkhole under U.S. Route 441, Florida, USA. The field experimental result finds that the method is capable of resolving the subsurface S-wave velocity ([Formula: see text]) structure and detecting a low-velocity anomaly. The inverted [Formula: see text] profile from 2D ANT generally agrees with that of 2D active-source full-waveform inversion, including the [Formula: see text] value and depth of the anomaly. To our best knowledge, this is the first study to directly invert the waveform crosscorrelation of traffic noise recordings to extract material property at the engineering meter scale (<30 m depth).

Sinkhole detection with 3D full seismic waveform tomography

Sinkhole collapse may result in significant property damage and even loss of life. Early detection of sinkhole attributes (buried voids, raveling zones) is critical to limit the cost of remediation. One of the most promising ways to obtain subsurface imaging is 3D seismic full-waveform inversion. For demonstration, a recently developed 3D Gauss-Newton full-waveform inversion (3D GN-FWI) method is used to detect buried voids, raveling soils, and characterize variable subsurface soil/rock layering. It is based on a finite-difference solution of 3D elastic wave equations and Gauss-Newton optimization. The method is tested first on a data set constructed from the numerical simulation of a challenging synthetic model and subsequently on field data collected from two separate test sites in Florida. For the field tests, receivers and sources are placed in uniform 2D surface grids to acquire the seismic wavefields, which then are inverted to extract the 3D subsurface velocity structures. The inverted synthetic results suggest that the approach is viable for detecting voids and characterizing layering. The field seismic results reveal that the 3D waveform analysis identified a known manmade void (plastic culvert), unknown natural voids, raveling, as well as laterally variable soil/rock layering including rock pinnacles. The results are confirmed later by standard penetration tests, including depth to bedrock, two buried voids, and a raveling soil zone. Our study provides insight into the application of the 3D seismic FWI technique as a powerful tool in detecting shallow voids and other localized subsurface features.

3D full-waveform inversion in time-frequency domain: Field data application

We present a 3D Gauss-Newton full-waveform inversion (3D GN-FWI) method in the time-frequency domain for detection of subsurface anomalies. The use of Gauss-Newton approach is particularly important for near-surface imaging, in which acquired seismic wavefields are dominated by Rayleigh waves. The inverse Hessian matrix utilized in this approach acts as a weighting function to reduce the dominancy of Rayleigh waves, increase the contribution of body waves, and thus help resolve deeper structures. However, the Gauss-Newton method requires a huge amount of computer memory to store derivative wavefields (Jacobian matrix). To address this issue, the presented 3D GN-FWI method exploits advantages of the combined time-frequency domain. Specifically, the forward wave simulation is done in the time-domain to generate wavefields at multiple frequencies simultaneously without the requirement of an inverse matrix solver, while inversion is conducted in the frequency-domain to significantly reduce the required computer memory. Synthetic and field experimental datasets are used to assess the capability of the presented waveform method. The synthetic result shows that a variable profile with a buried void is well recovered. For the field experiment, a large mobile shaker was used to induce wavefields at individual frequencies with consistent magnitudes required for the presented frequency-domain inversion. The wavefields were recorded with uniform 2D grids of sources and receivers on the ground surface, and analyzed to obtain 3D subsurface wave velocity profiles. The seismic result is consistent with an invasive standard penetration test (SPT), including the identified bedrock depth and buried void.

Experimental Study on GPR Detection of Voids inside and behind Tunnel Linings

Ground penetrating radar (GPR) is considered an effective tool to detect tunnel lining voids. In this paper, an experimental study was carried out using a physical tunnel lining model to evaluate the performances of different antenna frequencies. We built a 4.2 m–long, 4.2 m–wide, and 2.0 m–high experimental model to simulate the secondary lining, initial lining, and surrounding rock of a tunnel structure. In the model, we created four categories of voids, which are voids in secondary and initial linings, a delamination between the secondary and initial linings, a delamination between the initial lining and sand, and a void buried in the sand, to simulate real cases in tunnel engineering. The GPR wave velocities in the sand and concrete of the model were measured using the reflection method for the calibration of void depth. We employed a commercial GPR system equipped with antennae of different centre frequencies to detect the voids. GPR data were processed using a conventional data processing flow, and the performances of different frequencies were examined. The results show that the 1000 MHz centre frequency GPR is capable of characterizing shallow buried voids in the secondary lining but is not able to penetrate into the initial lining. The 250 MHz centre frequency GPR system is not advised to detect voids in or behind tunnel linings due to its low resolving power for voids of centimetre sizes. The 500 MHz centre frequency GPR system is optimal for void detection because it demonstrated a balanced performance of resolving ability and investigation depth. The findings of this work could be useful references for antenna selection and data processing in real GPR applications.

X-ray Imaging of Functional Three-Dimensional Nanostructures on Massive Substrates.

To investigate the performance of three-dimensional (3D) nanostructures, it is vital to study their internal structure with a methodology that keeps the device fully functional and ready for further integration. To this aim, we introduce here traceless X-ray tomography (TXT) that combines synchrotron X-ray holographic tomography with high X-ray photon energies (17 keV) in order to study nanostructures “as is” on massive silicon substrates. The combined strengths of TXT are a large total sample size to field-of-view ratio and a large penetration depth. We study exemplary 3D photonic band gap crystals made by CMOS-compatible means and obtain real space 3D density distributions with 55 nm spatial resolution. TXT identifies why nanostructures that look similar in electron microscopy have vastly different nanophotonic functionality: one “good” crystal with a broad photonic gap reveals 3D periodicity as designed; a second “bad” structure without a gap reveals a buried void, and a third “ugly” one without gap is shallow due to fabrication errors. Thus, TXT serves to nondestructively differentiate between the possible reasons of not finding the designed and expected performance and is therefore a powerful tool to critically assess 3D functional nanostructures.

Application of 2D full-waveform tomography on land-streamer data for assessment of roadway subsidence

Roadways are key components of the modern transportation system. Therefore, assessment of roadway subsidence is critical to the health and safety of the traveling public. Existing seismic refraction and waveform tomography methods can be used for subsidence evaluation; however, the data acquisition time is significant because they require multiple source impacts (shots) along a test line. To mitigate the negative impact caused by closing the traffic flow under seismic testing, a land-streamer seismic testing system and waveform analysis are developed. An existing 2D Gauss-Newton full-waveform inversion (FWI) method is extended for analysis of the land-streamer waveform data. The main advantage of using land-streamer waveform data is that geophones are not coupled to test materials and source-receiver offsets are fixed; thus, the whole test system can be moved along the roadway quickly for data acquisition. To demonstrate the effectiveness of land-streamer waveform data, the FWI method was tested on synthetic and field data sets. The synthetic result reveals that buried voids can be well-characterized by the land-streamer waveform analysis. Field data were collected on asphalt pavement using a 24 channel land streamer and a propelled energy generator to induce seismic wave energy. The test system was towed by a pickup truck along a roadway with an on-going subsidence (repaired sinkhole). The data were collected over 277.5 m distance at a 3 m interval, and the total data acquisition time was approximately 1 h. The field data result indicates that the waveform analysis was able to delineate low-velocity soil zones and laterally variable bedrock. The FWI results are also compared with multichannel analysis of surface wave (MASW) results. The 2D [Formula: see text] profiles from the FWI and MASW methods are consistent; however, the FWI method provides more detailed information ([Formula: see text] of [Formula: see text] cells) of low-velocity anomalies for assessment of roadway subsidence.

Porous structure formation in ion irradiated germanium

The ion beam induced modification of amorphous germanium is characterised by the formation of voids close to the sample surface and the transformation into a sponge-like porous surface layer at high ion fluences. This extreme structural modification of the sample surface is independent of the (heavy) ion species used and accompanied by a strong volume expansion. Nevertheless, recently it was demonstrated that buried voids (and buried sponge-like layers) can be formed in the depth of the projected ion range, however, only for the irradiation with I-ions at high ion fluences. Thus, the ion species and their chemical properties seem to play an important role in the structural modification around the projected ion range. In this paper we investigate the influence of the ion species on the ion beam induced void formation in Ge for room temperature irradiation with 380keV I- and Au-ions as a function of the ion fluence. Independent of the ion species, a strong volume expansion is observed caused by void formation and the transformation into a sponge-like porous surface layer. For both ion species used, the final porous layers are structurally identical as established by cross section and plan view electron microscopy investigations. Further ion irradiation of the sponge-like porous structure, however, leads to significant differences in the ion beam induced structural evolution. For the Au-ion irradiation the porous layer remains nearly unchanged, whereas for the irradiation with I-ions a transformation from sponge-like to netlike porous layers occurs which is accompanied again by an extreme volume expansion. The underlying mechanism will be discussed based on chemical properties of the implanted ions.

Detection of a buried void in an elastic layer of constant thickness: The anti-plane problem

This work is concerned with an anti-plane inverse problem about identification of a buried void in an elastic layer of constant thickness. It is assumed that a certain load is applied to the upper boundary surface, to generate wave fields inside the layer. Then, a numerical algorithm is proposed to restore the position and size of the buried void. The input data is given by the amplitude of oscillations known over a certain finite-length interval on the upper face of the layer. Some numerical examples demonstrate the stability of the proposed algorithm.

Structural modifications of low-energy heavy-ion irradiated germanium

Heavy-ion irradiation of crystalline germanium (c-Ge) results in the formation of a homogeneous amorphous germanium (a-Ge) layer at the surface. This a-Ge layer undergoes structural modification such as a strong volume expansion accompanied by drastic surface blackening with further ion irradiation. In the present paper we investigate the mechanism of this ion-induced structural modification in a-Ge basically for the irradiation with I ions (3 and 9 MeV) at room and low temperature as a function of ion fluence for the ion incidence angles of $\ensuremath{\Theta}={7}^{\ensuremath{\circ}}$ and $\ensuremath{\Theta}={45}^{\ensuremath{\circ}}$. For comparison, Ag- and Au-ion irradiations were performed at room temperature as a function of the ion fluence. At fluences two orders of magnitude above the amorphization threshold, morphological changes were observed for all irradiation conditions used. Over a wide range of ion fluences we demonstrate that the volume expansion is caused by the formation of voids at the surface and in the depth of the projected ion range. At high ion fluences the amorphous layer transforms into a porous structure as established by cross section and plan view electron microscopy investigations. However, the formation depth of the surface and buried voids as well as the shape and the dimension of the final porous structure depend on the ion fluence, ion species, and irradiation temperature and will be discussed in detail. The rate of the volume expansion (i.e., porous layer formation) depends linearly on the value of ${\ensuremath{\epsilon}}_{n}$. This clearly demonstrates that the structural changes are determined solely by the nuclear energy deposited within the amorphous phase. In addition, at high ion fluences all perpendicular ion irradiations lead to a formation of a microstructure at the surface, whereas for nonperpendicular ion irradiations a nonsaturating irreversible plastic deformation (ion hammering) without a microstructure formation is observed. For the irradiation with ion energies of several MeV, the effect of plastic deformation shows a linear dependence on the ion fluence. Based on these results, we provide an explanation for the differences in surface morphology observed for different angles of incidence of the ion beam will be discussed in detail.

Deep confined karst detection, analysis and paleo-hydrology reconstruction at a basin-wide scale using new geophysical interpretation of borehole logs

summary Deep karst voids can be identified by a new method of geophysical interpretation of commonly used borehole logs in deeply confined carbonate aquifers. We show that deep, buried karst voids can be characterized by combining this geophysical interpretation together with geological and hydrological data, and with known speleological constraints. We demonstrate how this characterization can reveal past hydrological regimes and allow mapping of karst distribution on a basin-wide scale. A combined analysis of geophysical, geological, hydrological, and speleological data in the confined Yarkon–Taninim aquifer, Israel, led us to reconstruct past groundwater levels at different relief and sea levels, with the karst voids as a marker for long-term flow close to the water table. Paleo-canyons along the Mediterranean Sea shoreline strongly affected the region’s paleo-hydrology, by serving as major outlets of the aquifer during most of the Cenozoic. We conclude that intensive karstification was promoted by flow periods of longer duration and/or higher flux and flow velocities close to the aquifer’s past and present outlets. In addition, we suggest that karst voids found under shallow confinement were developed by renewed aggressivity due to hypogene water rising in cross-formational flow becoming mixed with fresh lateral water flow from the east.

Detection of buried reference structures by use of atomic force acoustic microscopy

The miniaturization of micro- and nanoelectronic components requires new methods for the inspection of buried inner structures at the nanoscale. We used the atomic force acoustic microscopy technique (AFAM) to image subsurface defects. This technique combines high lateral resolution with the capability to determine local elastic properties of materials near the surface. As the structures buried near the surface change the effective tip-sample contact stiffness it is possible to detect them. For the verification of the detection capabilities of AFAM we fabricated well-defined buried void structures with different geometries and dimensions. Large, thin, plate like structures of silicon nitride with a local filling were our first test samples. Then, sets of nine small, square, thin plates with thicknesses increasing stepwise from 30 to 270 nm were etched in a thinned silicon wafer. The last two samples contained wedge structures of widths varying between 1.6 and 10 μm. Our results showed that it was possible to detect buried void structures at depths between 180 and 900 nm. We also observed that the depths at which the buried defects can be detected by the use of the AFAM method depend on the defect dimensions and geometry, and on the mismatch in the elastic properties of the sample and the defects. The experimental results obtained for the groups of small, thin plates were verified by quantitative analysis via finite element method (FEM) simulations.

Nondestructive 3-D imaging of femtosecond laser written volumetric structures using optical coherence microscopy

Nondestructive three-dimensional imaging of femtosecond laser-written buried structures is demonstrated using optical coherence microscopy providing lateral and depth resolution on a micron scale. This high speed technique, which requires no sample preparation, enables the visualization of volumetric structural modification created deep in transparent dielectric medium with high signal/noise contrast. Images of buried void structures with dimensions as large as 190 μm in length were obtained without shadowing effects impugning the image fidelity.

Integrated study of the sinkhole development site on the Western shores of the Dead Sea using geophysical methods

This paper presents a combination of high‐resolution seismic diffraction, reflection, refraction, continuous vertical electric sounding (CVES), ground‐penetrating radar (GPR) and microgravity methods to study the structure and properties of the shallow subsurface at the sinkhole development site in the Nahal Hever southern area (on the shore of the Dead Sea) in Israel. These methods have different resolutions and penetration depths. The methods are complementary to each other and reveal useful information about the subsurface. Each one investigates certain characteristics, but when combined they provide us with new information about the subsurface. The seismic refraction method is utilized to map salt layers and to detect dissolved zones. The analysis of all of the seismic refraction data obtained at the Dead Sea area, verified by the information gained from a borehole, has shown that the P‐wave velocity of the salt layer varies from 2900 to 4250 m/s. The velocity of 2900 m/s was used as the minimum velocity criterion for salt‐layer identification at this site.A low‐velocity zone (VP < 2800 m/s) revealed within a high‐velocity area ( m/s) is interpreted as a dissolved salt or non‐saline unit. The site is characterized by two significant geological features. The first feature is a water‐filled cave, detected recently by a borehole in the depth interval 23–28.5 m, and the second feature is a depression at the surface. Geophysical results show that the depression site is presently characterized as a heavily perturbed zone in both lateral and vertical directions. Soil decompaction in the subsurface is clearly identified by the CVES method as a high‐resistivity anomaly. This decompaction is assumed to be associated with the voids and fractures occurring down to a depth of 100–120 m (by diffraction data) and in the uppermost subsurface (by GPR data). The zone containing the cave is characterized by a lateral (funnel) and vertical (faults) distortion of the subsurface structure (by seismic reflection data) and has a faint diffraction anomaly in the shallow subsurface. A slightly increased resistivity anomaly appears in the depth interval 6–12 m (from CVES data). Drilling data reveal this interval as a decompaction zone. No GPR anomalies were detected at the buried cave site. The microgravity survey at the buried void site in 1999 detected an anomaly that is very similar to the funnel‐shaped one.

Strong coupling of light to flat metals via a buried nanovoid lattice: the interplay of localized and free plasmons

We study the optical plasmonic properties of metal surfaces which have a periodic lattice of voids buried immediately beneath their flat upper surface. Light reflection spectra calculated in the framework of a self-consistent electromagnetic multiple-scattering layer-KKR approach exhibit two types of plasmon resonances originating from the excitation of different plasmon modes: surface plasmon-polaritons propagating on the planar surface of metal and Mie plasmons localized in the buried voids. Coupling between these two types of plasma oscillation leads to an enhancement of the surface plasmon-polariton resonances even for close-packed void lattices. Our theoretical model quantitatively agrees with experimental results, demonstrating that planar surfaces can exhibit strong plasmonic field enhancements.

Characterization of Fiber/Matrix Interfaces Using X-Ray Microtopography

The structure and strength of fiber/matrix interfaces in fiber-reinforced composites are crucial in determining the overall mechanical behavior of these materials. To investigate the integrity and structure of such an interface on the microstructure scale, a model composite consisting of a single crystal Al 2 O 3 fiber in an Al matrix was investigated with X-ray microdiffraction. The intensity of the (30.0) and (22.0) reflections of Al 2 O 3 was monitored in topography mode using an X-ray beam focused by a tapered capillary to spot sizes ranging from 5 to 25 μm. Significant changes in this intensity revealed buried voids along a nominally intact interface. These changes also provided information about the distribution of residual stresses along the interface. The discussion includes advantages and some complications with this technique for measuring residual stresses in composites.

Quantitative metrology study of Cu/SiO2 interconnect structures using fluorescence x-ray microscopy

We demonstrate the capability of fluorescence x-ray microscopy with a 0.25 μm beam for in situ measurements of Cu-wiring interconnects of submicron dimensions. We are able to measure submicron line widths, lengths, and thicknesses of both Cu and W structures, and a Ta liner in the test vehicle, to the absolute accuracy of 0.03 μm, and a relative accuracy of ∼4% in lateral dimensions, and ∼10% in heights. The shape of a buried electromigration void was also determined. This nanoscale nondestructive characterization technique promises to be powerful for a variety of materials systems.