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. 2026 Jan 23;16:6061. doi: 10.1038/s41598-026-36743-4

Mie-mediated edge-enhanced Raman scattering of vertically-stacking ge quantum-dots/Si-SiN array for enhancing photoluminescence and photodetection

Shih-Hsiang Yang 1, Maria Isabel Alonso 2, Horng-Chih Lin 1, Pei-Wen Li 1,✉
PMCID: PMC12902106  PMID: 41578014

Abstract

We reported vertically-stacking architecture of Si-Si3N4 embedded Ge quantum dots (QDs) array for quantum nanophotonics. Salient features of ordered, stacking Ge QDs lie in good control over the QD size, spherical-shape, and spatial location without relying on lithographic definition. Mie-mediated interaction of Ge QDs with Si nanolayers is characterized by edge enhanced Raman scattering (EERS) of Si and photoluminescence of Ge QDs at the visible−near infrared bands. Ge QDs PIN diodes with 40 nm-wide intrinsic region showcase high gain× bandwidth > 107 GHz A/A at zero bias for self-powered, energy-efficient photodetection.

Keywords: germanium, quantum dots, EERS

Subject terms: Materials science, Nanoscience and technology, Optics and photonics, Physics

Introduction

Si photonics not only fuels energy-efficient, high-performance computation1−4 via optical interconnections but also unleashes innovative revolution of quantum nanophotonics for photodetection and light emission at the nanoscale. Integrating quantum optics with advanced Si nanophotonic architectures paves new ways towards on-chip quantum information, computing, and sensing5,6. However, the fundamental classical limit of diffraction restricts the minimum achievable spatial resolution of Si nanophotonics that remains to be addressed for practical applications. Recently, high refractive index Ge/Si nanostructures7–9 or quantum dots (QDs)10–12 leveraging Mie resonances is an active and compelling research subject for their promise in highly efficient control over light propagation and localization at the spatial scales far below the optical wavelength, surpassing the classical limits imposed by diffraction. Mie resonance of high refractive index Ge/Si nanostructures or QDs also induces large enhancement of local electromagnetic (EM) fields, boosting Raman signals by orders of magnitude and enabling surface enhanced Raman scattering (SERS)13, edge enhanced Raman scattering (EERS)14, or tip-enhanced Raman scattering (TERS)15,16. Therefore, Mie resonance of Ge/Si nanophotonics offers compact and robust solutions for low-loss metasurfaces with rich optical functionalities5,6 and for advancing the implementation of high efficiency Si-based nanoemitters17 and photodetectors10 without the concern of Joule damping associated with the free electron response of metallic nanoparticles in traditional plasmonics. In this context, Raman scattering serves as a powerful characterization tool, acting as a means to examine structural aspects like strain18 and composition, 19 as well as the amplification of optical processes resulting from strong near fields20.

Advanced manufacturable CMOS technology is highly envisaged to implement Ge/Si resonant nanophotonics from fabrication perspectives, however, off-the-shelf nanometer-scale transistor processes cannot be readily transferred. Instead, holistic design, process compatibility, high-temperature thermal stability, and architecture flexibility from 2D to 3D arrays of embedded QDs structures are essential. One of the technical challenges is the growth of embedded Ge QDs within designed Si-based dielectric resonators with high degree control of QD size, shape, and spatial location for nanophotonic active devices.

Encouragingly, several attempts have been made onto the growth of embedded Ge QDs array on top of SiO2/Si layers using combined processes of epitaxy, condensation, annealing, and/or lithography of Ge or SiGe films21–23. However, self-assembled Ge QDs usually grow in irregular shapes and need suitable seeding layers for strategically placing Ge QDs within Si-based dielectric resonators. On the other hand, enhanced absorption and emission efficiency per unit volume could be achieved in single QDs layer compared to their counterparts of quantum-wells or films,24 so it is essential to increase the QDs spatial density laterally and vertically for improving the total absorption/emission volume from practical device perspectives.

We have demonstrated planar arrays of Si-Si3N4 embedded Ge spherical QDs with controlled size tunability at the designated spatial locations, using thermal oxidation of poly-SiGe lithographically-patterned structures.25−27 Here, we advanced the fabrication of ordered, vertically-stacking Si-Si3N4 embedded Ge QDs array using wavy layout design in combination with corrugated sidewall engineering. Engineering advantages of our stacking Si-Si3N4 embedded Ge QDs approach result from precise control over the Ge QD size and spherical-shape at the designated spatial locations with high degree of thermal stability. Diameter and stacking density of Ge QDs are essentially determined by the specially-designed, corrugated sidewall profile of Si-nanoantenna ridges without relying on lithographic definition. In this work, Raman spectral, line-scan, and mapping analyses in our vertically-stacking Si-Si3N4 embedded Ge spherical QDs array reveal EERS at the sidewalls of corrugated Si nanostrips in combination with Mie-induced SERS of Si longitudinal optical (LO) phonon mode between Ge QDs and the Si nanoridges, facilitating the implementation of PIN photodiodes with high bandwidth and zero-bias responsivity.

Results and discussion

Vertically-stacking Si/Si3N4 embedded Ge QDs array

Our Si/Si3N4 embedded Ge QDs were created in a self-assembled and self-organized manner via Ostwald ripening and migration of Ge nanocrystals within growing SiO2, Si3N4 and Si during the thermal oxidation of poly-SiGe spacer layers. Details of Ge QD growth and migration mechanism have been elaborated elsewhere28,29. The heart of fabricating vertically-stacking Ge QDs in this work is the controlled tunability of wavy layout design and corrugated sidewall profile of Si-Si3N4 ridges, allowing the strategic placement of Ge QDs at the designated concave notches due to the built-up of large compressive stress during the thermal oxidation of poly-SiGe spacer layer (Fig. 1). TCAD simulation reveals that thermal oxidation continuously accumulates positive pressure at the sidewalls of Si3N4/Si ridge. In particular, the compressive stress at the concave notches is considerably higher than that at the convex regions (Fig. 1(a)−(c)). Highly-stressed concave notches not only are subject to a reduced oxidation rate due to impeded diffusion of oxidants, but also serve as nucleation sites for Ostwald ripening of Ge QDs. Ultimately, Ge QDs tend to be autonomously positioned within each designated sidewall notch of Si/Si3N4 ridges once the poly-SiGe spacer layer is completely oxidized (Fig. 1(c)).

Fig. 1.

Fig. 1

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SEM micrographs and corresponding simulated pressure of oxidized Si corrugated ridge/Si3N4-poly-SiGe spacer layers for oxidation time of (a) 0 min, (b) 17 min, and (c) 22 min at 900 °C in an H2O ambient.

3D transmission electron microscopy (TEM) (Fig. 2(a)) and plan-view/cross-sectional electron dispersive x-ray spectroscopy (EDS) (Fig. 2(b)/2(c)) examinations on arrays of 4 μm-long and 220 nm-thick Si nanoridges with wavy patterns (in a pitch = 100 nm) on the x-y plane and corrugated sidewall profile (55 nm per notch) along the z-direction show that, Ge QDs with averaged diameter of 40±3.8 nm deterministically reside at the designated concave notches of Si3N4/Si ridges laterally (the y-direction as seen in Fig. 2(b)) and vertically (the z-direction as seen in Fig. 2(c)) thanks to the wavy-pattern layout design and Bosch etching, respectively. The lateral and vertical position offsets of Ge QDs within the designated notches are 10 nm and < 5 nm extracted from 120 Ge QDs. Notably, Ge QDs penetrate the overlayer of Si3N4 and even get in touch with the Si ridge, as indicated by the appearance of a 1–2 nm-thick interfacial layer of SiO2 surrounding the Ge QD from the EDS mapping micrograph of elemental oxygen (Fig. 2(d)). The stacking QD number directly relies on the sidewall notch number that is tailored by the cycles of Bosch etching. The diameter of Ge QDs is essentially determined by the total Ge content per sidewall stack that is controllably tunable by designing the wavy layout and adjusting parameters of Bosch etching and poly-Si1 − xGex deposition.

Fig. 2.

Fig. 2

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(a) 3D TEM as well as (b) plan-view and (c)/(d) cross-sectional EDS elemental maps for nitrogen, oxygen, germanium, and silicon of Si/Si3N4 embedded Ge QDs.

Edge enhanced Raman scattering in stacking Ge QDs/Si

Analysis of Raman spectra from Ge/Si structures provides quantitative information about composition and strain. Raman spectral characteristics (Fig. 3) in our samples show distinct phonon lines centered at 312 cm− 1 and 515.9 cm−1−520.7 cm− 1, corresponding to Ge optical and LO-Si modes, respectively, measured from Si/Si3N4 ridges with and without Ge QDs at their sidewalls. The Ge mode is blue-shifted by + 10.5 cm− 1 relative to the bulk Ge mode at 301.5 cm− 1. The possible contributions to this blue-shift are Si intermixing and/or strain. No phonon line at 400–410 cm− 1 is observed, indicating inappreciable Si-Ge mixing in Si/Si3N4-embedded Ge QDs, in contrast to other recently studied nanostructures30. This observation agrees with the EDS structural analyses indicating pure Ge composition of the Ge QDs. A possible red-shift of the Raman phonon mode due to quantum phonon confinement is insignificant and not considered for our 40±3.8 nm sized Ge QDs since the exciton Bohr radius of Ge is approximately 24.9 nm. So, our experimentally observed blue shift is assigned solely to strain, indicating the Ge QDs at the sidewalls of Si/Si3N4-ridges being in compressive strain of -2.5% in average. Another interesting finding of notes from the Raman spectra is that the Si modes from the corrugated Si/Si3N4 ridges with and without Ge QDs are red shifted with respect to the LO-Si mode at 520.7 cm– 1 of bulk Si. Therefore, the Si in the ridges with and without Ge QDs are in tensile strain of + 0.57% and + 0.27%, respectively.

Fig. 3.

Fig. 3

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Raman spectra of Si ridges with and without QDs taken at the ridges and at the field.

In addition, Raman scattering is an effective probe for mapping local EM fields because sharp nanostructures (such as tips or QD surfaces) confine the light into extremely localized, intense EM fields near their surfaces, boosting the weak Raman signal by orders of magnitude. These enhancements are demonstrated from Raman linescan analysis shown in Fig. 4(a). In the reference sample of Si ridges with no Ge QDs, the intensity of LO-Si mode at the smooth edge (produced by the straight layout design as seen in Fig. 4(b)) is stronger than that of the Si film (away from the edge) by 3-fold with either vertical or horizontal incident laser polarization. In contrast, the LO-Si signal displays distinct polarization dependence in the corrugated Si ridges that are produced by the wavy layout design as seen in Fig. 4(b). Relative to the LO-Si intensity in the Si film, the enhancement factor of LO-Si signal in corrugated Si ridges increases from 3 to 11 when changing the incident laser polarization from horizontal (across the ridges) to vertical (along the ridges). This is the so-called EERS effect, which was first experimentally discovered from 20 nm-wide single Si nanowires14. An enhancement in the LO-Si signal at the edge of Si nanowires is attributed to a high concentration of optical electric field (E-field) when both incident and scattered lights are polarized parallel to the nanowire. Moreover, Raman linescan (Fig. 4(c)) and mapping micrographs (Fig. 4(d), (e)) clearly show that ordered, size-uniform Ge QDs at the sidewalls of corrugated Si ridges (Fig. 4(f, g)) improve the enhancement factor of LO-Si mode to 4.5 and 15 with incident horizontal and vertical laser polarization, respectively. In contrast, at the lateral edge of Si film, randomly distributed, size-fluctuated Ge QDs (as seen in Fig. 4(h)) contribute no additional improvement in the enhancement factor of LO-Si signal.

Fig. 4.

Fig. 4

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(a) Raman linescan profile of LO-Si signals in the Si film/edge/ ridges with no Ge QDs. (b) plan-view SEM micrograph showing corrugated Si ridges and patterned straight-edge of Si films. Raman (c) linescan profile and mapping micrographs of (d) Ge optical mode and (e) LO-Si mode of corrugated Si ridges and the Si film with Ge QDs at the sidewalls and lateral edge. Cross-sectional TEM micrographs of (f) across the Si ridges, (g) along the Si ridges, and (h) along the edge of Si film.

The Ge optical mode at the nanoridges is also strongly enhanced with respect to the too faint intensity observed at the lateral edge, see Fig. 4(d). This Ge optical mode appears to have stronger polarization dependence than LO-Si mode, as evidenced by a large increase in the peak intensity by a factor of 7 when changing the incident laser polarization from horizontal to vertical (Fig. 4(c)), consistent with the additional EERS effect caused by the nanoridges.

In summary, enhanced Raman effects are a probe for near EM fields that can also enhance other optical signals such as light absorption and emission, indicating the suitability of the Si/Si3N4-embedded Ge QDs stacking structure for these functionalities. Here, Raman linescan and mapping analysis suggests enhancement of optical signals in this structure both due to the nanoridge edges (EERS) as well as Mie effect mediated Si Raman scattering enhancement caused by Ge QDs.

Interactions of excitation light with Si ridges, which have corrugated sidewalls in combination with ordered, size-uniform Ge QDs or scattered, size fluctuated Ge QDs, were analyzed using commercially available software, COMSOL Multiphysics. Simulation shows that the E-field, H-field, and Poynting vector or surface power density (E×H) appear to be highly dependent on the light polarization relative to the ridge direction. It is clearly seen in Fig. 5(a) that when the light polarization is parallel to the corrugated Si ridges, E-field, H-field, and power density are localized within the ridge. Instead, highly concentrated E-field and power density appear at the tips of sidewall notches in combination with the H-field being localized in the ridge core when the light polarization is across the ridges (Fig. 5(b)). Based on these simulations, the larger enhancement observed in y-polarization could be due to high near fields affecting a bigger volume of Si resulting in a larger contribution to the total enhanced Raman signal. As an additional possibility, the narrow ridge geometry has been suggested to provide a photonic waveguide effect31 in which the y-polarization could be more effectively coupled from/to the far field than the x-polarization.

Fig. 5.

Fig. 5

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Schematic diagram of simulated structure and calculated mapping of electrical field, magnetic field, and power density when the light is along and across the ridge direction. Corrugated Si ridge when the light polarization is (a) parallel and (b) perpendicular to the ridge direction. Corrugated Si ridge with (c) ordered, uniform size Ge QDs and (d) randomly-distributed, size-fluctuated Ge QDs at the sidewall notches. Light polarized in x-direction.

Strikingly, the inclusion of ordered, size-uniform Ge QDs at the sidewall notches makes the highly concentrated E-field occur at the notches instead of the tips (Fig. 5(c)). Ge QDs appear to couple their intense EM field with the intimate Si ridge, not only concentrating large enhancement in the E-field locally at the interface of Ge QDs/Si ridge but also enhancing H-field at the core. The strong near fields across the ridges in presence of Ge QDs can be related to the antenna effect,32 ultimately originated in Mie resonant behavior of the Ge nanospheres. However, in this case, the relatively low refractive index contrast between the Si ridge and the Ge QDs results in a low antenna quality factor. Conversely, its advantage in this case is that the resulting resonance becomes broad, that is, both spectrally and spatially quite extended33. Hence, the observed enhancements result from the combined EERS and Mie effects. Still, the larger observed signals in y-polarization again suggest the important contribution from the ridge geometry as a photonic element. The enhancement in |E|, |H|, and |E×H| is suppressed or dispersed when the Ge QDs randomly distribute among the sidewalls with size fluctuation (Fig. 5(d)). In summary, calculated distributions of E-field, H-field, and power density support our experimental observation of enhanced Raman signals of LO-Si mode at the corrugated Si ridges and at the edge of Si film (Fig. 4).

Optical emission from vertically-stacking Ge QDs/Si system

Our previous works[26, 34, 35 have already reported a continuously tunable range of process-controlled Ge QD diameter (dQD) from 5 to 90 nm and quantum-confinement induced size-dependent PL peak energy (EPL) that could be described by EPL (eV) = 0.79 + 310/(dQD)2. In this work, we fabricated vertically-stacking Ge QDs with tunable size of 35–50 nm by tailoring sidewall-notch size via Bosch etching, as seen in the inset TEM micrograph of Fig. 6(a). Cathodoluminescence (CL) spectra taken from the stacking Ge QDs locating at the sidewalls of Si/Si3N4 ridges reveal sharp peaks at λ = 660 nm, 1120 nm, and 1280 nm (Fig. 6(a)). The observed CL peak at λ = 660 nm might be contributed by 17–20 nm sized Ge QDs due to quantum-size effects and/or optically active centers located at the interface of SiO2/Ge QDs/Si and Si3N4/Ge QDs/Si. Extensive TEM examinations from 120 Ge QDs show that studied Ge QDs have an averaged diameter of 40 nm±3.8 nm, suggesting inappreciable small-sized (17–20 nm) Ge QDs. Compared to weak or nearly invisible emission observed from the reference samples of solely SiO2 or Si3N4 films on top of Si substrates (containing no QDs), the distinct CL peak at λ = 660 nm appears to originate from Ge QDs mediated luminescence enhancement of optically active centers located at the interface of SiO2/Si36 and Si3N4/SiO237 by the Mie scattering of Si/Si3N4 embedded Ge QDs. The power (P)-dependence of CL spectra reveal that the intensity (ICL) of CL peak at λ = 660 nm follows a power law of ICL ∝ P0.67, suggesting radiative recombination of free electrons/holes and defects between Ge QDs and Si3N4/SiO2 matrices. The CL peak at λ = 660 nm possesses thermal stability over the temperature range of 10–300 K in terms of nearly intact peak wavelength (λ = 664 nm at T = 10 K to λ = 658 nm at T = 300 K) and a slight increase in the full-width-at-half-maximum (FWHM) from 47 nm at T = 170 K to 62 nm at T = 300 K (Fig. 6(b)). For comparison, the Si peak at 1120 nm has a red shift of 100 nm with increasing temperature from 10 K to 300 K (not shown here). Extracted activation energy of these optically active centers is 18 meV from the Arrhenius plot of integrated CL peak intensity (Fig. 6(c)).

Fig. 6.

Fig. 6

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Temperature-dependent (a) CL spectra of stacking Si/Si3N4 embedded Ge QDs. (b) CL peak wavelength and FWHM and (c) Arrhenius plot of integrated CL peak intensity at λ = 660 nm. (d) QD size dependency of CL intensity per unit volume. (e) PL spectrum of stacking Si/Si3N4 embedded Ge QDs measured at T = 15 K.

On the other hand, strong CL peaking at λ = 1150–1350 nm presumably comes from exciton recombination in Ge QDs with diameters ranging from 33 to 48 nm due to quantum confinement effect26,34,35. The CL-estimated QD sizes agree well with the values of 35–50 nm observed from TEM examinations (Fig. 6(a)). Normalized CL intensity per unit volume appears to be inversely proportional to QD diameter in a form of CLIntensity ∝ 1/d2QD (Fig. 6(d)). Emission efficiency per unit volume gets higher with reducing QD diameter, which is attributed to an enhanced oscillator strength as a result of quantum confinement effects38,39. Photoluminescence (PL) spectra measured at T = 15 K (Fig. 6(e)) agrees well with CL spectra in terms of peak wavelength and linewidth.

Stacking Ge-QD PIN photodiodes of 40 nm-wide intrinsic region at zero-bias self-power mode

Our stacking Si/Si3N4-embedded Ge QDs array facilitates the implementation of PIN photodiodes with an ultra-thin intrinsic region (Win ~ 40 nm) of Ge QDs, which are placed between inter-digitated N+-c-Si nanoridge/P+- poly-Si in a self-aligned manner (Fig. 7). Notably, the Win is essentially determined by the QD diameter rather than the resolution of lithography and overlay alignment.

Fig. 7.

Fig. 7

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Stacking Ge-QD PIN photodiodes with an ultrathin (40 nm-wide Ge QDs) intrinsic region and inter-digitated P+/N+ electrodes on SOI. (a) false-color cross-sectional scanning electron microscopy (SEM) micrographs showing the formation of corrugated Si nanoridges, Si3N4 and poly-SiGe overlayers, and stacking Ge QDs using the processes described in the Methods section and Fig. 9, (b) plan-view optical microscopy and SEM, (c) cross-sectional and (d) STEM/EDS micrographs of Ge-QD PIN photodiodes.

Figure 8(a) displays current-voltage (I-V) characteristics of Ge-QD PIN photodiodes under various power illumination at λ = 850 nm. Low dark current of 10 pA and < 0.1 pA is achieved when the Ge-QD PIN photodiodes are biased at VP = -1 V and 0 V, respectively (Fig. 8(a)). At least ten Ge-QD photodiodes were measured, showing good uniformity of I-V characteristics under illumination at λ = 850–1550 nm. High efficiency photocarrier generation within the stacking Ge QDs at λ = 850 nm illumination is evidenced by open-circuit voltage (VOC), photoresponsivity (ℜ), and 3dB frequency (f3dB) as high as + 0.8 V, 5 mA/W, and 22 GHz, respectively, at 0 V in Fig. 8(b)−(d). High gain and high bandwidth of our Ge-QD photodiodes at zero bias indicates the advantage in energy-efficient photodetection, since our self-powered Ge-QD photodiode is driven by a large built-in E-field with no need for an external power supply. Compared to our previously-reported planar Ge QD PIN photodiodes40 and phototransistors41, the dark current, ℜ, and f3dB are significantly improved thanks to an ingenious combination of an ultra-thin intrinsic region with stacking Ge QDs and Si nanoridges for efficient light absorption. Similar to our previous reports,34, 42 the reference photodiodes without Ge QDs show invisible difference between photocurrent at λ = 850 nm and dark current (not shown here).

Fig. 8.

Fig. 8

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Power-dependent (a) I-V characteristics, (b) VOC, and (c) ℜ. (d) bias-dependent f3dB of vertically-stacking Ge-QD PIN photodiodes at λ = 850 nm illumination.

Conclusion

A combination of layout design, Bosch etching, and self-assembled growth enables precise control over the stacking density, position, and diameter of Ge QDs, paving the way for the fabrication of highly ordered and tunable Ge QDs/Si-based nanostructures. Raman spectral and mapping analyses reveal EERS at the sidewalls of corrugated Si ridges in combination with Mie-induced SERS of LO-Si between Ge QDs and the Si ridges. Ultimately, our vertically-stacking Ge QDs/Si–Si3N4 array offers a versatile and scalable platform for designing quantum light emitters and photodetectors with high thermal stability and 3D integration into SiN waveguided nanophotonic systems, promising advancements in energy-efficient ultrafine sensing and optical interconnect technologies.

Methods

Vertically-stacking Si3N4/Si-embedded Ge spherical QDs array and PIN photodiodes

The fabrication of vertically-stacking Ge spherical QDs embedded within Si3N4/Si nanostrips and PIN photodiodes started with SOI wafers, comprising a 220 nm-thick, p-type (~ 2 × 1015 cm− 3), single-crystalline Si (100) layer on top of 3 μm-thick buried oxide (BOX). Firstly, Si active areas were patterned using optical lithography and plasma etching (C4F8/SF6 = 43 sccm/37.5 sccm), followed by Arsenic ion implantation (5 × 1015 cm− 2 and 100 keV) generating the N+-regions.

Next, 220 nm-thick Si nanoridges with corrugated sidewalls along the z-direction and wavy patterns on the x-y plane were produced using combined processes of electron-beam lithography (EBL) and Bosch etching (Fig. 9(a)). To do this, PMMA A2 was coated onto the SOI wafer via a two-step spinning process of 1000 rpm/10 s and then 5800 rpm/60 s, followed by a soft bake at 180 °C for 5 min, resulting in the PMMA A2 thickness of approximately 60–80 nm. Then, wavy layout-patterns with a pitch of 100 nm on the x-y plane were lithographically delineated using EBL with a dose of 420 µC/cm2 and MIBK: IPA (1:3) development for 50 s. The corrugated sidewall profile of Si nanoridges was created by the Bosch etching process with a chamber pressure of 20 mTorr and a chuck temperature of 25 °C. The Bosch etching process comprises repeated cycles of a three-step process: (1) polymer deposition on the sidewalls and bottom surface of etched Si trenches by CFx ions (SF6/C4F8/ICP power/ RF bias power/step time = 10 sccm/ 250 sccm/ 600 W/ 0 W/ 1.4 s), (2) bottom polymer etching by SFx ions (SF6/C4F8 /ICP power/ RF bias power/step time = 100 sccm/10 sccm/ 500 W/ 30 W/ 1 s), and (3) Si isotropic etching by SFx ions (SF6/C4F8 /ICP power/ RF bias power/step time = 200 sccm/10 sccm/ 625 W/ 0 W/ 0.3 s).

Fig. 9.

Fig. 9

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Process flow and schematic diagrams for the fabrication of vertically-stacking Si/Si3N4 embedded Ge QDs PIN photodiodes.

Subsequently, 20 nm-thick Si3N4 and 25 nm-thick poly-Si0.85Ge0.15 layers were deposited for conformally encapsulating Si corrugated nanoridges using low-pressure chemical vapor deposition (LPCVD) (Fig. 9(b)). Following the thermal oxidation at 900 °C for 20 min (with the flow rates of O2/H2 = 3.0 slm/4.2 slm and a two-step furnace ramping process of 30 °C/min from 400 °C to 850 °C and then 10 °C/min from 850 °C to 900 °C), vertically-stacking, 40 nm ± 3.8 nm-sized Ge QDs were produced within each designated sidewall notch of N+-Si/Si3N4 nanoridges in a self-organized manner (Fig. 9(c)).

The growing SiO2 layer encapsulating Ge QDs by virtue of the selective oxidation of the Si content of the original poly-SiGe layer was then removed using diluted hydrofluoric acid (HF : H2O = 1 : 50), followed by a bi-layer deposition of 2 nm-thick SiO2 and 150 nm-thick poly-Si using LPCVD. The poly-Si layer was doped by Boron ion implantation (5 × 1015 cm− 2, 20 keV) in combination with thermal activation at 900 °C for 1 min and 875 °C for 15 min. Following direct etch-back of the P+-poly-Si layer using C4F8/SF6 plasma, inter-digitated N+-single-crystalline Si stripe/Ge QDs/P+-poly-Si structure was formed in a self-aligned manner.

Finally, a 500-nm-thick passivation oxide layer of SiO2 was deposited by plasma-enhanced CVD (PECVD) at 350 °C using SiH4/N2O = 40 sccm/460 sccm, followed by contact-hole patterning using optical lithography and plasma (CHF3/CH4/Ar = 20 sccm/ 1 sccm/ 100 sccm) etching. A 500-nm-thick Al layer was deposited by sputtering and subsequently, the ground-signal-ground (GSG) Al pads were lithographically patterned. A forming gas (95% N2/5% H2) annealing at 400 °C for 10 min completed the fabrication of Ge-QD PIN photodiodes.

Raman, cathodoluminescence and photoluminescence characterization

To probe local fields within vertically-stacking Ge QDs/Si-Si3N4 nanostructures, Raman images were obtained with the WITec alpha300 R confocal setup, using 488 nm laser excitation at room temperature and a Nikon Fluor (100×/0.9 NA) objective, giving an estimated spot resolution of 0.33 μm. The pixel size for the images was 0.1 × 0.1 µm2, as given by the scanning movement of a piezo-motor-driven sample stage.

Temperature-dependent (10–300 K) CL and PL measurements were conducted to investigate optical emission from Ge QDs/Si-Si3N4 system. Steady-state PL was excited from Si3N4/Si-embedded Ge QDs system using the 532 nm line of an Nd: YAG laser and was detected using a monochromator with a slit width of 2 mm in combination with an InGaAs photodetector. CL characteristics were measured using the Attolight Allalin CL SEM EM026100, equipped with field emission of electrons with beam energy: 2–10 KeV/probe current: 30 pA-100 nA, giving the spatial resolution < 10 nm. The collection optics features a NA of 0.71, providing field of view > 300 μm in diameter. Spectral analysis was conducted using an Andor Kymera328i spectrometer with a configurable slit ranging from 10 μm to 2.5 mm. UV–VIS detection was performed with an ANDOR DU920P-BEX2 detector, covering a wavelength range of 200–1060 nm. NIR detection was conducted using IRMPT17, external cooled photomultiplier (PMT, 300–1700 nm). The sample was mounted on a cryogenic nanopositioning stage, operating in a temperature range from 10 K to 300 K by using liquid He.

Photocurrent characterization

Current-voltage (I-V) characteristics of Ge QDs photodiodes were conducted in a Lakeshore 4RX-4 K probe station equipped with five DC/RF probe arms and one fiber optic probe arm. DC/RF (ZN50R) probes are used for DC biasing, low noise shielded, low-leakage guarded, and low-/high-frequency measurements. The fiber optic probe arm features a continuous, vacuum-sealed length of IRVIS (400 nm to 2100 nm) fiber with core/cladding diameter of 200/240 µm from SMA connector to probe end for maximal optical throughput. I-V characteristics are measured using Keysight B1500A semiconductor analyzer equipped with B1517A high-resolution source monitor unit, E5288A atto sense and switch unit. The null current measured at room temperature is < 10 fA. Frequency response was measured using vector network analyzer (Anritsu MS4640B), E-O modulator (iXBlue ModBox-850 nm-VNA-40 GHz), and calibrated with O-E converter (Anritsu MN4765B).

Author contributions

Y.S.H. conducted the fabrication and COMSOL simulation of stacking Ge-QDs array and Ge QDs P-I-N photodetector. M.I.A conducted µ-Raman measurement, data analysis and manuscript preparation. L.H.C. contributed to data analysis. L.P.W. conceived the study, supervised the work, and contributed to data analysis and manuscript preparation. All authors read and approved the final manuscript.

Funding

This work was financially supported by Honghai Research Institute, MA-tek, and National Science and Technology Council, Taiwan 114UA90042, 114-2119-M-A49-005, 113-2927-I-A49-503 and 112-2221-E-A49-162-MY3. MIA acknowledges projects CEX2023-001263-S and PID2022-141956NB-I00 of MICIU (Spain), as well as the CSIC (Spain)-NSTC (Taiwan) collaboration BILTW22002.

Data availability

The datasets used and analyzed during the current study available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

References

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The datasets used and analyzed during the current study available from the corresponding author on reasonable request.

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