Thick Germanium-on-Nothing Structures by Annealing Microscale Hole Arrays With Straight Sidewall Profiles

Jeong, Mun Goung; Kim, Taeyeong; Lee, Bong Jae; Lee, Jungchul

doi:10.1109/jmems.2021.3139094

Cited by 7 publications

(5 citation statements)

References 13 publications

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“…As explained in the previous section, pillars tend to form at random locations, with a density determined by the porous structure (porosity, wall profile, and surface chemistry) and thermal profile and ambient 15 . The interplay between these many parameters and the difficulty in controlling germanium surface ageing make it very difficult to achieve a reproducible and uniform random pillar density.…”

Section: From Ge Wafer To Wafer‐scale Detachable Ge Foilmentioning

confidence: 99%

“…The third function that Ge wafers should fulfill for high‐efficiency multijunction solar cells is that of bottom cell. Since the thickness of the GeON seed layer is limited to a few micrometers, 15 it requires thickening up if no advanced light trapping schemes are implemented 19 . Thickening can be done in situ by homoepitaxy right after reorganization.…”

Section: From 1‐μm Up To 30‐μm‐thick Geon Foil With High Growth‐rate ...mentioning

confidence: 99%

“…The suspended foil thickness is directly determined by the pore pitch and diameter 14 . A practical limit is set by the anneal time and temperature that are necessary to ensure the Ge surface diffusion for complete pore closure and merging and can be estimated around 5 μm, 15 which is on the low side for a Ge bottom cell in a typical multijunction solar cell to absorb enough infrared light 16 . This thickness can thus further be increased by epitaxial growth.…”

Section: Introductionmentioning

confidence: 99%

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Wafer‐scale Ge epitaxial foils grown at high growth rates and released from porous substrates for triple‐junction solar cells

Depauw

Porret

Moelants

et al. 2022

Progress in Photovoltaics

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Germanium is listed as a critical raw material, and for environmental and economic sustainability reasons, strategies for lower consumption must be implemented. A promising approach is Ge lift‐off concepts, which enable to re‐use the substrate multiple times. Our concept is based on the Ge‐on‐Nothing approach that is the controlled restructuring at high temperature of a macroporous Ge surface, forming a Ge foil weakly attached to its parent wafer. Its suitability as III–V epitaxy seed and support substrate has previously been demonstrated with proof‐of‐concept solar cells. This work focuses on bringing this concept to the next level, by upscaling the detachable area to a full 200‐mm wafer scale, increasing foil thickness for sufficient light absorption in the Ge bottom cell, and improving the control on the strength that is bonding the suspended foil to its parent. By introducing a new high growth‐rate epitaxy process from GeCl4, and by engineering the GeON structure to introduce pillars with ad hoc density and shape, we fabricated P‐type foils with tunable boron doping up to 15 μm in thickness and 11 cm × 11 cm in area, for which the detachment strength could be adapted to the stresses induced by the solar cell process steps. The surface roughness and the electrical and crystal qualities of these foils were inspected by AFM, SIMS, SRP, ECCI, and TEM to check the GeCl4‐based epitaxy conditions and to check that the ad hoc pillars were not introducing any damage. Small‐area triple‐junction lattice‐matched GaInP/GaInAs/Ge solar cells were fabricated on 7‐μm‐thick Ge foils with various pillar densities and on a standard reference Ge wafer. The III–V layer nucleation was virtually the same on both substrates and the solar cells on the GeON foils performed in the same way as the cells on the Ge wafer, albeit a small loss in short‐circuit current and open‐circuit voltage that can be attributed to the thickness reduction and absence of rear‐side passivation. We conclude that it is possible to gain control on the GeON detachability and upscale the concept to areas relevant for the space PV industry, proving that porous germanium is a serious candidate for replacement of bulk Ge wafers in view of a more sustainable multijunction solar cell process.

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Section: From Ge Wafer To Wafer‐scale Detachable Ge Foilmentioning

confidence: 99%

Section: From 1‐μm Up To 30‐μm‐thick Geon Foil With High Growth‐rate ...mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Wafer‐scale Ge epitaxial foils grown at high growth rates and released from porous substrates for triple‐junction solar cells

Depauw

Porret

Moelants

et al. 2022

Progress in Photovoltaics

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show abstract

“…The Ge hole patterns were annealed in a high vacuum furnace (2

\times

10 −6 Torr) at 890 °C to accelerate the surface diffusion. [ 21 ] Then, four structures were sampled after different annealing durations: 5, 15, 60, and 150 min. Surface image and topography of each structure were respectively acquired using OM and AFM, as shown in the correlations in Figure 2B.…”

Section: Methodsmentioning

confidence: 99%

Predicting AFM Topography from Optical Microscope Images Using Deep‐Learning

Jeong

Kim

Lee

et al. 2022

Advanced Intelligent Systems

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Atomic force microscopy (AFM) is routinely used as a metrological tool among diverse scientific and engineering disciplines. A typical AFM, however, is intrinsically limited by low throughput and is inoperable under extreme conditions. Thus, this work attempts to provide an alternative to a conventional optical microscope (OM) by training a deep learning model to predict surface topography from surface OM images. The feasibility of this novel methodology is shown with germanium‐on‐nothing (GON) samples, which are self‐assembled structures that undergo surface and sub‐surface morphological transformations upon high‐temperature annealing. Their transformed surface topographies are predicted based on the OM‐AFM correlation of three different surfaces, bearing an error of about 15% with a 1.72× resolution upscale from OM to AFM. The OM‐based approach brings about significant improvement in topography measurement throughput (equivalent to OM acquisition rate, up to 200 frames per second) and area (≈1 mm2). Furthermore, this method is operable even under extreme environments when an in situ measurement is impossible. Based on such competence, the model's simultaneous application in further specimen analysis, namely surface morphological classification and simulation of dynamic surfaces’ transformation is also demonstrated.

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“…Silicon-on-nothing (SON) 1 – 3 and germanium-on-nothing (GON) 4 , 5 are established as a promising fabrication methodology, with their main advantage of unmatched process simplicity for fabricating micro and nanoscale cavities 6 , 7 . Depending on the initial DRIE hole patterns, the annealed cavities will form a shape of either a sphere, a circular pipe, or a plate 8 .…”

Section: Introductionmentioning

confidence: 99%

PCA-based sub-surface structure and defect analysis for germanium-on-nothing using nanoscale surface topography

Jeong

Kim

Lee

et al. 2022

Sci Rep

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Empty space in germanium (ESG) or germanium-on-nothing (GON) are unique self-assembled germanium structures with multiscale cavities of various morphologies. Due to their simple fabrication process and high-quality crystallinity after self-assembly, they can be applied in various fields including micro-/nanoelectronics, optoelectronics, and precision sensors, to name a few. In contrast to their simple fabrication, inspection is intrinsically difficult due to buried structures. Today, ultrasonic atomic force microscopy and interferometry are some prevalent non-destructive 3-D imaging methods that are used to inspect the underlying ESG structures. However, these non-destructive characterization methods suffer from low throughput due to slow measurement speed and limited measurable thickness. To overcome these limitations, this work proposes a new methodology to construct a principal-component-analysis based database that correlates surface images with empirically determined sub-surface structures. Then, from this database, the morphology of buried sub-surface structure is determined only using surface topography. Since the acquisition rate of a single nanoscale surface micrograph is up to a few orders faster than a thorough 3-D sub-surface analysis, the proposed methodology benefits from improved throughput compared to current inspection methods. Also, an empirical destructive test essentially resolves the measurable thickness limitation. We also demonstrate the practicality of the proposed methodology by applying it to GON devices to selectively detect and quantitatively analyze surface defects. Compared to state-of-the-art deep learning-based defect detection schemes, our method is much effortlessly finetunable for specific applications. In terms of sub-surface analysis, this work proposes a fast, robust, and high-resolution methodology which could potentially replace the conventional exhaustive sub-surface inspection schemes.

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Thick Germanium-on-Nothing Structures by Annealing Microscale Hole Arrays With Straight Sidewall Profiles

Cited by 7 publications

References 13 publications

Wafer‐scale Ge epitaxial foils grown at high growth rates and released from porous substrates for triple‐junction solar cells

Wafer‐scale Ge epitaxial foils grown at high growth rates and released from porous substrates for triple‐junction solar cells

Predicting AFM Topography from Optical Microscope Images Using Deep‐Learning

PCA-based sub-surface structure and defect analysis for germanium-on-nothing using nanoscale surface topography

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