arXiv subjects

Mahmoud R. M. Atalla

Publications and source records attributed to Mahmoud R. M. Atalla.

14 recordsLinked to original sources

arXiv2026

Reduced Dark Current in Cd0.9Zn0.1Te Detector Arrays by Aluminium Oxide Passivation

Contrary to the prevailing view that processing cadmium zinc telluride (CZT) semiconductors above 150 °C leads to irreversible device degradation, here we show that Al2O3 passivation layer deposited by atomic layer deposition at 250 °C not only preserves CZT detector performance but also substantially improves it. Pixelated metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) CZT detectors were passivated with a thin atomic-layer-deposited Al2O3 film and characterized electrically. The passivated devices exhibited no measurable degradation, even under a high-bias operation of 1000 V. Instead, the dark current decreased by approximately a factor of five, while the interpixel leakage current was reduced by nearly one order of magnitude, from ~41 nA to ~2-3 nA at -200 V. Passivation also produced highly uniform dark-current characteristics across adjacent pixels and completely eliminated current-voltage hysteresis, indicating suppression of defect-assisted charge transport. Furthermore, no significant difference in dark current was observed between the MS and MIS detectors after passivation, suggesting that the Al2O3 layer dominates the surface electrical behavior. These results demonstrate that optimized Al2O3 passivation at 250 °C is fully compatible with high-performance CZT detector processing and provides a practical route toward lower-noise, higher-spectral-resolution X-ray imaging systems.

physics.app-ph↗

arXiv2026

Mid-Infrared Thermal Radiation Harvesting using Uncooled Narrow Bandgap GeSn Thermophotovoltaic cell

Thermophotovoltaic (TPV) cells are increasingly attractive for applications in industrial waste heat harvesting, aerospace energy management, and compact power generation. Deploying midwave-infrared (MWIR) TPV in practical applications requires narrow-bandgap semiconductors that not only absorb low-energy photons but also integrate with scalable, low-cost platforms. Although high-performance TPV devices have been demonstrated using III-V materials such as InAs, GaSb, and InGaAs(P), their use remains limited by cost and substrate size. With this perspective, narrow bandgap GeSn alloys are a promising alternative that extend group-IV absorption into the MWIR while being silicon-compatible. Although the potential of GeSn TPV cells has been predicted, no experimental demonstration has been reported. Here, proof-of-concept Ge$_{0.91}$Sn$_{0.09}$ p-i-n TPV diodes (1 mm diameter) grown on silicon were fabricated and their performance was benchmarked against commercial InAs and extended-InGaAs devices. Measurements at 300 K under 2.33 $μ$m laser and $\sim$1500 K SiC Globar illumination revealed peak responsivity of $\sim$ 0.2 A/W at $\sim$ 1.7 $μ$m, and an output power of $\sim$ 0.41 mW/cm$^2$. These devices show trends comparable to those of the InAs diode under identical conditions, although at reduced absolute levels. To assess the intrinsic performance potential, Poisson-drift-diffusion modeling incorporating experimentally calibrated emitter emissivity predicts power densities exceeding 1 W/cm$^2$ under moderate MWIR thermal illumination, indicating that the present devices operate far below their fundamental limits and are primarily constrained by defect-assisted recombination and transport losses. These results establish GeSn as a scalable, silicon-compatible MWIR TPV platform and highlight a larger performance potential achievable through material and device optimization.

physics.app-ph↗

arXiv2025

Polarization-sensitive GeSn Mid-Infrared Membrane Photodetectors with Integrated Plasmonic Metasurface

Germanium-Tin (GeSn) semiconductors are promising for mid-infrared optoelectronics owing to their silicon compatibility, tunable bandgap, and potential for room-temperature operation. Released GeSn membranes provide an additional degree of freedom to extend the operation wavelength through epitaxial strain relaxation, while their transferability expands design flexibility. On the other hand, metasurfaces have become an effective strategy to engineer light--matter interaction, and their integration with photodetectors can enhance performance and introduce new functionalities. Here, we demonstrate a mid-infrared photodetector consisting of a transfer-printed Ge$_{0.89}$Sn$_{0.11}$ membrane integrated with an Au plasmonic metasurface. The photodetector exhibits a wavelength cutoff exceeding 3.0~$μ$m with nearly fourfold increase in responsivity at 2.5~$μ$m as compared to unreleased films, attributed to Fabry--Pérot resonance. Furthermore, the integration with an anisotropic metasurface yields detectors with strong polarization sensitivity, achieving a measured contrast ratio of $\sim$4:1 between orthogonal polarizations. Moreover, the operation wavelength of the photodetector can be selectively tuned by varying the geometric scale of the metasurface. The experimental results show excellent agreement with simulations, confirming the effectiveness and versatility of this integrated metasurface--membrane design.

physics.app-ph↗

arXiv2025

Waveguide-Coupled Mid-Infrared GeSn Membrane Photodetectors on Silicon-on-Insulator

Silicon photonics has thrived in telecommunications over recent decades, and its extension to the mid-infrared range has the potential to unlock valuable opportunities for sensing, imaging, and free-space communications. With this perspective, germanium-tin (GeSn) alloy has been extensively investigated as a silicon-compatible semiconductor with bandgap tunability that covers this entire spectral range. Indeed, a variety of GeSn-based high-performance optoelectronic devices have been demonstrated, confirming the potential of this system for mid-infrared applications. However, the integration of these devices onto silicon photonic platforms remains underexplored. Herein, we demonstrate the fabrication and integration, through transfer-printing, of strain-relaxed GeSn membranes onto silicon-on-insulator waveguides to create integrated detectors operating up to 3.1 $μ$m at room temperature. Two different designs of waveguide structures are evaluated to study the coupling efficiency between the passive structures and the active membrane detector. A responsivity reaching 0.36 A/W at an operation wavelength of 2.33 $μ$m is measured under a bias of 1 V. Moreover, the fabrication resulted in multiple working devices exhibiting similar performance using a single transfer printing step, demonstrating the scalability of the proposed approach.

physics.app-ph↗

arXiv2024

Transfer-printed multiple Ge$_{0.89}$Sn$_{0.11}$ membrane mid-infrared photodetectors

Due to their narrow band gap and compatibility with silicon processing, germanium-tin (Ge$_{1-x}$Sn$_x$) alloys are a versatile platform for scalable integrated mid-infrared photonics. These semiconductors are typically grown on silicon wafers using Ge as an interlayer. However, the large lattice mismatch in this heteroepitaxy protocol leads to the build-up of compressive strain in the grown layers. This compressive strain limits the material quality and its thermal stability besides expanding the band gap, thereby increasing the Sn content needed to cover a broader range in the mid-infrared. Released Ge$_{1-x}$Sn$_x$ membranes provide an effective way to mitigate these harmful effects of the epitaxial strain and control the band gap energy while enabling the hybrid integration onto different substrates. With this perspective, herein strain-relaxed Ge$_{0.89}$Sn$_{0.11}$ membranes are fabricated and subsequently transfer-printed with metal contacts to create multiple photodetectors in a single transfer step. The resulting photodetectors exhibit an extended photodetection cutoff reaching a wavelength of $3.1 \,μ$m, coupled with a significant reduction in the dark current of two orders of magnitude as compared to as-grown photoconductive devices. The latter yields a reduced cutoff of $2.8 \,μ$m due to the inherent compressive strain. Furthermore, the impact of chemical treatment and annealing on the device performance was also investigated showing a further reduction in the dark current. The demonstrated transfer printing, along with the use of an adhesive layer, would allow the transfer of multiple GeSn membranes onto virtually any substrate. This approach paves the way for scalable fabrication of hybrid optoelectronic devices leveraging the tunable band gap of Ge$_{1-x}$Sn$_x$ in the mid-wave infrared range.

physics.app-ph↗

arXiv2024

Extended-SWIR High-Speed All-GeSn PIN Photodetectors on Silicon

There is an increasing need for silicon-compatible high bandwidth extended-short wave infrared (e-SWIR) photodetectors (PDs) to implement cost-effective and scalable optoelectronic devices. These systems are quintessential to address several technological bottlenecks in detection and ranging, surveillance, ultrafast spectroscopy, and imaging. In fact, current e-SWIR high bandwidth PDs are predominantly made of III-V compound semiconductors and thus are costly and suffer a limited integration on silicon besides a low responsivity at wavelengths exceeding $2.3 \,μ$m. To circumvent these challenges, Ge$_{1-x}$Sn$_{x}$ semiconductors have been proposed as building blocks for silicon-integrated high-speed e-SWIR devices. Herein, this study demonstrates a vertical all-GeSn PIN PDs consisting of p-Ge$_{0.92}$Sn$_{0.08}$/i-Ge$_{0.91}$Sn$_{0.09}$/n-Ge$_{0.89}$Sn$_{0.11}$ and p-Ge$_{0.91}$Sn$_{0.09}$/i-Ge$_{0.88}$Sn$_{0.12}$/n-Ge$_{0.87}$Sn$_{0.13}$ heterostructures grown on silicon following a step-graded temperature-controlled epitaxy protocol. The performance of these PDs was investigated as a function of the device diameter in the $10-30 \,μ$m range. The developed PD devices yield a high bandwidth of 12.4 GHz at a bias of 5V for a device diameter of $10 \,μ$m. Moreover, these devices show a high responsivity of 0.24 A/W, a low noise, and a $2.8 \,μ$m cutoff wavelength thus covering the whole e-SWIR range.

physics.app-ph↗

arXiv2023

Continuous-wave GeSn light emitting diodes on silicon with $2.5 \, μ$m room-temperature emission

Silicon-compatible short- and mid-wave infrared emitters are highly sought-after for on-chip monolithic integration of electronic and photonic circuits to serve a myriad of applications in sensing and communication. To address this longstanding challenge, GeSn semiconductors have been proposed as versatile building blocks for silicon-integrated optoelectronic devices. In this regard, this work demonstrates light-emitting diodes (LEDs) consisting of a vertical PIN double heterostructure p-Ge$_{0.94}$Sn$_{0.06}$/i-Ge$_{0.91}$Sn$_{0.09}$/n-Ge$_{0.95}$Sn$_{0.05}$ grown epitaxially on a silicon wafer using germanium interlayer and multiple GeSn buffer layers. The emission from these GeSn LEDs at variable diameters in the 40-120 $μ$m range is investigated under both DC and AC operation modes. The fabricated LEDs exhibit a room temperature emission in the extended short-wave range centered around 2.5 $μ$m under an injected current density as low as 45 A/cm$^2$. By comparing the photoluminescence and electroluminescence signals, it is demonstrated that the LED emission wavelength is not affected by the device fabrication process or heating during the LED operation. Moreover, the measured optical power was found to increase monotonically as the duty cycle increases indicating that the DC operation yields the highest achievable optical power. The LED emission profile and bandwidth are also presented and discussed.

physics.optics↗

arXiv2023

Group IV Mid-Infrared Thermophotovoltaic Cells on Silicon

Compound semiconductors have been the predominant building blocks for the current mid-infrared thermophotovoltaic devices relevant to sub-2000 K heat conversion and power beaming. However, the prohibitively high cost associated with these technologies limits their broad adoption. Herein, to alleviate this challenge we introduce an all-group IV mid-infrared cell consisting of GeSn alloy directly on a silicon wafer. This emerging class of semiconductors provides strain and composition as degrees of freedom to control the bandgap energy thus covering the entire mid-infrared range. The proposed thermophotovoltaic device is composed of a fully relaxed Ge$_{0.83}$Sn$_{0.17}$ double heterostructure corresponding to a bandgap energy of 0.29 eV. A theoretical framework is derived to evaluate cell performance under high injection. The black-body radiation absorption is investigated using the generalized transfer matrix method thereby considering the mixed coherent/incoherent layer stacking. Moreover, the intrinsic recombination mechanisms and their importance in a narrow bandgap semiconductor were also taken into account. In this regard, the parabolic band approximation and Fermi's golden rule were combined for an accurate estimation of the radiative recombination rate. Based on these analyses, power conversion efficiencies of up to 9% are predicted for Ge$_{0.83}$Sn$_{0.17}$ thermophotovoltaic cells under black-body radiation at temperatures in the 500-1500 K range. A slight improvement in the efficiency is observed under the frontside illumination but vanishes below 800 K, while the use of a backside reflector improves the efficiency across the investigated black-body temperature range. The effects of the heterostructure thickness, surface recombination velocity, and carrier lifetime are also elucidated and discussed.

physics.app-ph↗

arXiv2023

Extended-SWIR GeSn LEDs with reduced footprint and power consumption

CMOS-compatible short- and mid-wave infrared emitters are highly coveted for the monolithic integration of silicon-based photonic and electronic integrated circuits to serve a myriad of applications in sensing and communications. In this regard, a group IV germanium-tin (GeSn) material epitaxially grown on silicon (Si) emerges as a promising platform to implement tunable infrared light emitters. Indeed, upon increasing the Sn content, the bandgap of GeSn narrows and becomes direct, making this material system suitable for developing an efficient silicon-compatible emitter. With this perspective, microbridge PIN GeSn LEDs with a small footprint of $1,520$ $μ$m$^2$ are demonstrated and their operation performance is investigated. The spectral analysis of the electroluminescence emission exhibits a peak at $2.31$ $μ$m and it red-shifts slightly as the driving current increases. It is found that the microbridge LED operates at a dissipated power as low as $10.8$ W at room temperature and just $3$ W at $80$ K. This demonstrated low operation power is comparable to that reported for LEDs having a significantly larger footprint reaching $10^6$ $μ$m$^2$. The efficient thermal dissipation of the current design helped to reduce the heat-induced optical losses, thus enhancing light emission. Further performance improvements are envisioned through thermal and optical simulations of the microbridge design. The use of GeSnOI substrate for developing a similar device is expected to improve optical confinement for the realization of electrically driven GeSn lasers.

physics.optics↗

arXiv2022

Micrometer-thick, atomically random Si0.06Ge0.90Sn0.04 for silicon-integrated infrared optoelectronics

A true monolithic infrared photonics platform is within reach if strain and bandgap energy can be independently engineered in SiGeSn semiconductors. Herein, we investigate the structural and optoelectronic properties of a 1.5 μm-thick Si0.06Ge0.90Sn0.04 layer that is nearly lattice-matched to a Ge on Si substrate. Atomic-level studies demonstrate high crystalline quality and uniform composition and show no sign of short-range ordering and clusters. Room temperature spectroscopic ellipsometry and transmission measurements show direct bandgap absorption at 0.83 eV and a reduced indirect bandgap absorption at lower energies. Si0.06Ge0.90Sn0.04 photoconductive devices operating at room temperature exhibit dark current and spectral responsivity (1 A/W below 1.5 μm wavelengths) similar to Ge on Si devices, with the advantage of a near-infrared band gap tunable by alloy composition. These results underline the relevance of SiGeSn semiconductors in implementing a group IV material platform for silicon-integrated infrared optoelectronics.

cond-mat.mtrl-sci↗

arXiv2022

Extended-SWIR Photodetection in All-Group IV Core/Shell Nanowires

Group IV Ge1-xSnx semiconductors hold the premise of enabling broadband silicon-integrated infrared optoelectronics due to their tunable bandgap energy and directness. Herein, we exploit these attributes along with the enhanced lattice strain relaxation in Ge/Ge0.92Sn0.08 core-shell nanowire heterostructures to implement highly responsive, room-temperature short-wave infrared nanoscale photodetectors. Atomic-level studies confirm the uniform shell composition and its higher crystallinity with respect to thin films counterparts. The demonstrated Ge/Ge0.92Sn0.08 p-type field-effect nanowire transistors exhibit superior optoelectronic properties achieving simultaneously a relatively high mobility, a high ON/OFF ratio, and a high responsivity, in addition to a broadband absorption in the short-wave infrared range. Indeed, the reduced bandgap of the Ge0.92Sn0.08 shell yields an extended cutoff wavelength of 2.1 um, with a room-temperature responsivity reaching 2.7 A/W at 1550 nm. These results highlight the potential of Ge/Ge1-xSnx core/shell nanowires as silicon-compatible building blocks for nanoscale integrated infrared photonics.

physics.optics↗

arXiv2022

Dark current in monolithic extended-SWIR GeSn PIN photodetectors

The monolithic integration of extended short-wave infrared (e-SWIR) photodetectors (PDs) on silicon is highly sought-after to implement manufacturable, cost-effective sensing and imaging technologies. With this perspective, GeSn PIN PDs have been the subject of extensive investigations because of their bandgap tunability and silicon compatibility. However, due to growth defects, these PDs suffer a relatively high dark current density as compared to commercial III-V PDs. Herein, we elucidate the mechanisms governing the dark current in $2.6 \, μ$m GeSn PDs at a Sn content of $10$ at.%. It was found that in the temperature range of $293 \, $K -- $363 \,$K and at low bias, the diffusion and Shockley-Read-Hall (SRH) leakage mechanisms dominate the dark current in small diameter ($20 \, μ$m) devices, while combined SRH and trap assisted tunneling (TAT) leakage mechanisms are prominent in larger diameter ($160 \, μ$m) devices. However, at high reverse bias, TAT leakage mechanism becomes dominant regardless of the operating temperature and device size. The effective non-radiative carrier lifetime in these devices was found to reach $\sim 300$ -- $400$ ps at low bias. Owing to TAT leakage current, however, this lifetime reduces progressively as the bias increases.

physics.app-ph↗

arXiv2021

Recrystallization and Interdiffusion Processes in Laser-Annealed Strain-Relaxed Metastable Ge$_{0.89}$Sn0$_{.11}$

The prospect of GeSn semiconductors for silicon-integrated infrared optoelectronics brings new challenges related to the metastability of this class of materials. As a matter of fact, maintaining a reduced thermal budget throughout all processing steps of GeSn devices is essential to avoid possible material degradation. This constraint is exacerbated by the need for higher Sn contents along with an enhanced strain relaxation to achieve efficient mid-infrared devices. Herein, as a low thermal budget solution for post-epitaxy processing, we elucidate the effects of laser thermal annealing (LTA) on strain-relaxed Ge$_{0.89}$Sn0$_{.11}$ layers and Ni-Ge$_{0.89}$Sn0$_{.11}$ contacts. Key diffusion and recrystallization processes are proposed and discussed in the light of systematic microstructural studies. LTA treatment at a fluence of 0.40 J/cm2 results in a 200-300 nm-thick layer where Sn atoms segregate toward the surface and in the formation of Sn-rich columnar structures in the LTA-affected region. These structures are reminiscent to those observed in the dislocation-assisted pipe-diffusion mechanism, while the buried GeSn layers remain intact. Moreover, by tailoring the LTA fluence, the contact resistance can be reduced without triggering phase separation across the whole GeSn multi-layer stacking. Indeed, a one order of magnitude decrease in the Ni-based specific contact resistance was obtained at the highest LTA fluence, thus confirming the potential of this method for the functionalization of direct bandgap GeSn materials.

physics.app-ph↗

arXiv2020

All-Group IV membrane room-temperature mid-infrared photodetector

Strain engineering has been a ubiquitous paradigm to tailor the electronic band structure and harness the associated new or enhanced fundamental properties in semiconductors. In this regard, semiconductor membranes emerged as a versatile class of nanoscale materials to control lattice strain and engineer complex heterostructures leading to the development of a variety of innovative applications. Herein we exploit this quasi-two-dimensional platform to tune simultaneously the lattice parameter and bandgap energy in group IV GeSn semiconductor alloys. As Sn content is increased to reach a direct band gap, these semiconductors become metastable and typically compressively strained. We show that the release and transfer of GeSn membranes lead to a significant relaxation thus extending the absorption wavelength range deeper in the mid-infrared. Fully released Ge$_{0.83}$Sn$_{0.17}$ membranes were integrated on silicon and used in the fabrication of broadband photodetectors operating at room temperature with a record wavelength cutoff of 4.6 $μ$m, without compromising the performance at shorter wavelengths down to 2.3 $μ$m. These membrane devices are characterized by two orders of magnitude reduction in dark current as compared to devices processed from as-grown strained epitaxial layers. The latter exhibit a content-dependent, shorter wavelength cutoff in the 2.6-3.5 $μ$m range, thus highlighting the role of lattice strain relaxation in shaping the spectral response of membrane photodetectors. This ability to engineer all-group IV transferable mid-infrared photodetectors lays the groundwork to implement scalable and flexible sensing and imaging technologies exploiting these integrative, silicon-compatible strained-relaxed GeSn membranes.

physics.app-ph↗