We report on electroluminescence spectroscopy experiments demonstrating room-temperature light emission from heavily alloyed SiGe quantum dots, for which the light emission properties are enhanced by incorporated split-[110] self-interstitials. The quantum dots are formed during molecular beam epitaxy deposition of Si0.6Ge0.4 alloys on n-doped silicon-on-insulator substrates. To create the split-[110] self-interstitials the quantum dots were co-implantated in-situ using Si and Ge ions. The hybrid emitters were further embedded into the intrinsic region of a p-i-n diode structure to enable electrical pumping. Similar to previous theoretical results on unstrained Ge-based quantum dots containing these defects, radiative direct transitions are possible for these SiGe light emitters. However, in SiGe dots these transitions are not at the Brillouin zone center. Instead, first-principles calculations indicate that the presence of the split-[110] self-interstitial defect in strained and unstrained SiGe can lead to optically direct transitions in momentum space in the X-direction of the Brillouin zone.

The realization of scalable telecom light emitters that can be monolithically implemented into large-scale Si integration technology has been a driving factor in the field of Si photonics for the last decades. Applications are envisioned in the short and medium-range data transfer and for realizing scalable, Si-based quantum information technology. For the latter, CMOS-compatible sources that can be controlled on the single photon level are needed. In solid-state materials, group-III-V quantum dots (QDs) are one of the leading platforms for realizing single photon emitters of excellent optical quality [1,2]. However, their emission outside the telecom band and the difficulty of combining them with the mature Si photonics components such as waveguides remain an ongoing problem. In the group-IV (SiGe) system, the bandgap of the bulk materials is indirect, and electron and hole states in QDs are spatially separated due to the type-II band alignment. These material properties lead to relatively long radiative lifetimes and small emission efficiencies. However, for Si/SiGe QDs grown on silicon on insulator substrates (SOI), these efficiencies can be strongly and deterministically Purcell-enhanced by aligning photonic crystal cavities with single QDs [3]. One necessary prerequisite is, therefore, to achieve perfect nucleation site control of the QDs on the substrates [4,5]. This nucleation control can be achieved by deterministic substrate patterning combined with Ge growth in the supersaturation regime of the two-dimensional wetting layer [6], utilizing Ge's extensive surface diffusion lengths at growth temperatures above 600°C [7]. In such a way, it is possible to grow isolated QDs with virtually arbitrary inter-QD distance [4,5]. Here, one isolated QD was grown on a substrate area of the size of 100´100 µm2 [8]. We demonstrate the accurate QD positioning in PhC cavities [3,8], particularly in bichromatic cavities. Bichromatic cavities, a relatively novel design of photonic crystal cavity, offer ultra-high quality factor, and therefore Purcell enhancement, that is achievable with simple design rules [9]. Unlike in standard line defect cavities, in the present bichromatic layout, the line defect consists of a periodic sequence of holes, leaving only narrow Si regions. For this reason, an extremely high accuracy for QD positioning is required, which can be provided by interlacing site control of QD growth and PhC cavity formation. In micro photoluminescence spectra, the modes of single cavities are clearly observed. From the dependence of the mode intensities on the temperature and the excitation intensity, modes fed by the single QD in the cavity can be identified. We observe ultra-high quality factors of up to 100,000 for the modes coupled to QDs [8]. For resonators loaded with a QD, such high Q-factor values are record-high for the silicon-on-insulator integrated optics platform. Results on the excited state lifetime will be discussed. [1] D. Huber et al., Phys. Rev. Lett. 121, 33902 (2018) [2] N. Somaschi et al., Nature Photonics 10, 340 (2016) [3] M. Schatzl et al., ACS Photonics 4, 665 (2017) [4] M. Grydlik et al., Nanotechnology 24, 105601 (2013) [5] M. Brehm, et al., Nanotechnology 28, 392001 (2017) [6] M. Brehm, et al., Physical Review B 80 (20), 205321 (2009) [7] M. Grydlik et al., Physical Review B 88 (11), 115311 (2013) [8] T. Poempool et al., Optics Express 31 (10), 15564-15578 (2023) [9] A. Simbula et al., APL Photonics 2, 056102 (2017).

Next-generation Si-based applications ranging from short-range (quantum) data transfer to sensing could highly benefit from potential progress in group-IV-based telecom light emitters that can be readily implemented with Si Photonics components [1]. However, since Si is known to be an indirect band gap semiconductor, light emission is limited and inhibits the integration of efficient emitters, such as, e.g. light-emitting diodes and lasers on Si Photonic platforms. In this presentation, we stress that a particular type of epitaxial Ge on Si quantum dots allows for overcoming the indirect bandgap limitations of Si and Ge and, thus, enables pronounced light emission even at room temperature and above [2]. These nanostructures consist of ~2 nm high Ge-rich quantum dots on Si(001) substrate epitaxially created via strain-driven formation. However, in contrast to commonly used nanostructures, we intentionally incorporate point defects into these nanostructures during their growth through in-situ low-energy Ge ion implantation [3-4]. Therefore, the point defects can be confined within the 2 nm thick Ge layer while the surrounding Si matrix remains perfectly crystalline. We will discuss the superior light-emission properties of these so-called defect-enhanced quantum dots (DEQDs) and present the current theoretical understanding of the defect’s effects on the band structure. The latter leads to the enhanced light emission from DEQDs and the opening of direct recombination channels in k-space [5,6]. In addition to signs for optically pumped lasing from DEQDs embedded in microdisk resonators, implementing these emitters into the i-region of p-i-n diodes is remarkably straightforward since the matrix material is crystalline Si [7,8]. To showcase the superior properties of these zero-dimensional light emitters, we highlight light emission from DEQD light-emitting diodes with exceptional temperature stability up to 100°C [9]. This behavior starkly contrasts conventional Si/Ge light emitters, for which the light emission is typically confined to cryogenic temperatures. For optimizing the light emission from this interlaced defect/nanostructure complex [10], we elaborate on the testing of crucial parameters related to the temperature stability of the DEQDs [8,11], the scalability of light emission with increasing the density of the DEQDs [12] and passivation of detrimental defects in the Si matrix [13]. [1] I. A. Fischer, et al., APL Photonics 7 , 050901 (2022). [2 ] M. Brehm, Silicon Photonics IV, 67-103, Silicon Photonics IV: Innovative Frontiers, edited by David J. Lockwood and Lorenzo Pavesi, Springer series Topics in Applied Physics (2021). [3] M. Grydlik, et al., Nano Lett. 16 , 6802–6807 (2016). [4] M. Grydlik, et al., ACS Photonics 3 , 298–303 (2016). [5] F. Murphy-Armando, et al., Physical Review B 103 (8), 085310 (2021). [6] M. Brehm, et al., to be published [7] H. Groiss et al., Semicond. Sci. Technol. 32 , 02LT01 (2017). [8] L. Spindlberger et al., Crystals 10 , 351 (2020). [9] P. Rauter et al., ACS Photonics 5 , 431-438 (2018). [10] M. Brehm and M. Grydlik, Nanotechnology 28 , 392001 (2017). [11] L. Spindlberger et al., Physica Status Solidi (a) 216 , 1900307 (2019). [12] H. Groiss et al., Semicond. Sci. Technol. 32 , 02LT01 (2017). [13] L. Spindlberger et al., Appl. Phys. Lett. 118 , 083104 (2021).

Si Photonics is poised to bring vast benefits to data transfer and sensing applications. Many necessary components for Si photonics have already reached the level of commercialization. Most troublesome, however, is the development of a suitable light source that can be readily integrated with Si technology and that fulfills mandatory emission characteristics [1-3]. Silicon’s indirect band gap makes the implementation of useful light-emitting diodes and lasers on Si Photonic platforms challenging, as light emission is confined to cryogenic temperatures. In this paper, we emphasize that combining group-IV-based nanostructures and point defects can lead to direct bandgap recombination paths in Ge/Si and SiGe/Si systems, leading to pronounced luminescence emission at room temperature and above [4]. The Ge/Si and SiGe/Si nanostructures (quantum dots) can be formed via strain-driven self-assembly, while the defects are intentionally created using low-energy ion implantation [4-7]. The latter allows defects to be confined within the quantum dots while the surrounding Si matrix remains crystalline. Due to the band structure differences of Si and Ge, the effect of resulting defects that are of the split-interstitial type are vastly different for Ge and SiGe alloys. We performed first-principles calculations to show that in the case of Ge on Si, direct bandgap recombination is enabled at the Brillouin zone center, while for SiGe alloys, direct recombination is only possible in X-direction of the Brillouin zone [8-9]. In both cases, however, pronounced photoluminescence light emission from these defect-enhanced quantum dots at room temperature is observed [6,7] This is also true for electrically-driving these light emitters in p-i-n diodes [9,10] for which the luminescence emission is barely decaying up to a sample temperature of 100°C [10]. We note that the sample structure can be modified and improved post-growth using thermal annealing and passivation of parasitic defects using hydrogen [11,12]. References: [1] D. Thomason et al., Journal of Optics 18, 073003 (2016). [2] D. Liang, et al., Light, Advanced Manufacturing 2, 59 (2021). [3] S. Wirths et al., Nature Photonics 9, 88 (2015). [4] I. A. Fischer, et al., APL Photonics 7, 050901 (2022). [5] M. Brehm, Silicon Photonics IV, 67-103, Silicon Photonics IV: Innovative Frontiers, edited by David J. Lockwood and Lorenzo Pavesi, Springer series Topics in Applied Physics (2021). [6] M. Grydlik, et al., Nano Lett. 16, 6802–6807 (2016). [7] M. Grydlik, et al., ACS Photonics 3, 298–303 (2016). [8] F. Murphy-Armando, et al., Physical Review B 103 (8), 085310 (2021). [9] M. Brehm, et al., to be published [10] P. Rauter et al., ACS Photonics 5, 431-438 (2018). [11] L. Spindlberger et al., Crystals 10, 351 (2020). [12] L. Spindlberger et al., Appl. Phys. Lett. 118, 083104 (2021).

We report the resonantly enhanced radiative emission from a single SiGe quantum dot (QD), which is deterministically embedded into a bichromatic photonic crystal resonator (PhCR) at the position of its largest modal electric field by a scalable method. By optimizing our molecular beam epitaxy (MBE) growth technique, we were able to reduce the amount of Ge within the whole resonator to obtain an absolute minimum of exactly one QD, accurately positioned by lithographic methods relative to the PhCR, and an otherwise flat, a few monolayer thin, Ge wetting layer (WL). With this method, record quality (Q) factors for QD-loaded PhCRs up to Q~1e5 are achieved. A comparison with control PhCRs on samples containing a WL but no QDs is presented, as well as a detailed analysis of the dependence of the resonator-coupled emission on temperature, excitation intensity, and emission decay after pulsed excitation. Our findings undoubtedly confirm a single QD in the center of the resonator as a potentially novel photon source in the telecom spectral range.

For the development of photonic integrated circuits, it is mandatory to implement light sources on a Si-on-insulator (SOI) platform. However, point defects in the Si matrix and, e.g., at the Si/SiO2 interface act as nonradiative recombination channels, drastically limiting the performance of Si-based light emitters. In this Letter, we study how these defects can be healed by applying an advanced hydrogenation process, recently developed in photovoltaic research for the passivation of performance-limiting defects in Si solar cells. Upon hydrogenation, we observe an increase in the room temperature photoluminescence (PL) yield by a factor of more than three for defect-enhanced quantum dots (DEQDs) grown on float-zone Si substrates, revealing the potential of this technique to passivate detrimental defects. For DEQDs grown using SOI substrates, the PL yield enhancement even exceeds a factor of four, which we attribute to the additional passivation of defects originating from the substrate. The results for SOI substrates are of particular interest due to their relevance for future photonic integrated circuits.

The Si/SiGe heterosystem would be ideally suited for the realization of complementary metal-oxide-semiconductor (CMOS)-compatible integrated light sources, but the indirect band gap, exacerbated by a type-II band offset, makes it challenging to achieve efficient light emission. We address this problem by strain engineering in ordered arrays of vertically close-stacked SiGe quantum dot (QD) pairs. The strain induced by the respective lower QD creates a preferential nucleation site for the upper one and strains the upper QD as well as the Si cap above it. Electrons are confined in the strain pockets in the Si cap, which leads to an enhanced wave function overlap with the heavy holes near the upper QD’s apex. With a thickness of the Si spacer between the stacked QDs below 5 nm, we separated the functions of the two QDs: The role of the lower one is that of a pure stressor, whereas only the upper QD facilitates radiative recombination of QD-bound excitons. We report on the design and strain engineering of the QD pairs via strain-dependent Schrödinger-Poisson simulations, their implementation by molecular beam epitaxy, and a comprehensive study of their structural and optical properties in comparison with those of single-layer SiGe QD arrays. We find that the double QD arrangement shifts the thermal quenching of the photoluminescence signal at higher temperatures. Moreover, detrimental light emission from the QD-related wetting layers is suppressed in the double-QD configuration.

While light-emitting nanostructures composed of group-IV materials fulfil the mandatory compatibility with CMOS-fabrication methods, factors such as the structural stability of the nanostructures upon thermal annealing, and the ensuing photoluminescence (PL) emission properties, are of key relevance. In addition, the possibility of improving the PL efficiency by suitable post-growth treatments, such as hydrogen irradiation, is important too. We address these issues for self-assembled Ge quantum dots (QDs) that are co-implanted with Ge ions during their epitaxial growth. The presence of defects introduced by the impinging Ge ions results in pronounced PL-emission at telecom wavelengths up to room temperature (RT) and above. This approach allows us to overcome the severe limitations of light generation in the indirect-band-gap group-IV materials. By performing in-situ annealing, we demonstrate a high PL-stability of the defect-enhanced QD (DEQD) system against thermal treatment up to 600 °C for at least 2 h, even though the Ge QDs are structurally affected by Si/Ge intermixing via bulk diffusion. The latter, in turn, allows for emission tuning of the DEQDs over the entire telecom wavelength range from 1.3 µm to 1.55 µm. Two quenching mechanisms for light-emission are discussed; first, luminescence quenching at high PL recording temperatures, associated with the thermal escape of holes to the surrounding wetting layer; and second, annealing-induced PL-quenching at annealing temperatures >650 °C, which is associated with a migration of the defect complex out of the QD. We show that low-energy ex-situ proton irradiation into the Si matrix further improves the light emission properties of the DEQDs, whereas proton irradiation-related optically active G-centers do not affect the room temperature luminescence properties of DEQDs.

The intentional merging of epitaxial Ge on Si(001) quantum dots with optically active defect sites promises low‐cost applications such as room temperature (RT) light emitters in Si photonics. Despite recent progress in this field, important benchmarks, for example, the thermal stability of such a combination of low‐dimensional nanosystems, as well as the curing of parasitic charge‐carrier recombination channels, have been barely investigated thus far. Herein, the structural robustness of defect‐enhanced quantum dots (DEQDs) is examined under millisecond flash lamp annealing (FLA), carried out at sample temperatures up to 800 °C. Changes in the optical DEQD properties are investigated using photoluminescence spectroscopy performed in a sample temperature range from 10 to 300 K. It is demonstrated that FLA—in contrast to in situ thermal annealing—leads to only negligible modifications of the electronic band alignment. Moreover, upon proper conditions of FLA, the RT emission intensity of DEQDs is improved by almost 50% with respect to untreated reference samples.

We study the optical properties of tetravalent vanadium impurities in 4H silicon carbide (4H SiC). Emission from two crystalline sites is observed at wavelengths of 1.28 \mum and 1.33 \mum, with optical lifetimes of 163 ns and 43 ns. Group theory and ab initio density functional supercell calculations enable unequivocal site assignment and shed light on the spectral features of the defects. We conclude with a brief outlook on applications in quantum photonics.

Optical and optoelectronic properties of all-group-IV quantum dot light-emitters for which the optical properties are enhanced by in-situ implantation of heavy ions (DEQDs) are investigated. Research directions towards more efficient light-emission from DEQDs as well as considerations towards the microstructural investigation of DEQD-defects are elaborated.

As recently demonstrated, defect-enhanced Ge quantum dots (Ge-DEQDs) in a crystalline Si matrix can be employed as CMOS-compatible gain material in optically pumped lasers. Due to the stability of their optical properties up to temperatures beyond 300 K, the Ge-DEQD system is a highly promising candidate for the realization of an electrically pumped group-IV laser source for integration in a monolithic optoelectronic platform fit for room-temperature operation. We report on the realization of light-emitting diodes based on Ge-DEQDs operating at telecom wavelengths and above room temperature. The DEQD electroluminescence characteristics were studied spectrally resolved as a function of driving current and device temperature. The experimental results show that the excellent optical properties of Ge-DEQDs are maintained under electrical pumping at high current densities and at device temperatures of at least 100 degrees C. Furthermore, the emission intensity scales with the number of quantum dot layers embedded into the p-i-n diode structures, thus, indicating the scalability of the approach for large gain material volumes. The presented results form an essential step toward the future demonstration of a CMOS-compatible, electrically pumped room-temperature laser based on Ge-DEQDs.

Efficient coupling to integrated high-quality-factor cavities is crucial for the employment of germanium quantum dot (QD) emitters in future monolithic silicon-based optoelectronic platforms. We report on strongly enhanced emission from single Ge QDs into L3 photonic crystal resonator (PCR) modes based on precise positioning of these dots at the maximum of the respective mode field energy density. Perfect site control of Ge QDs grown on prepatterned silicon-on-insulator substrates was exploited to fabricate in one processing run almost 300 PCRs containing single QDs in systematically varying positions within the cavities. Extensive photoluminescence studies on this cavity chip enable a direct evaluation of the position-dependent coupling efficiency between single dots and selected cavity modes. The experimental results demonstrate the great potential of the approach allowing CMOS-compatible parallel fabrication of arrays of spatially matched dot/cavity systems for group-IV-based data transfer or quantum optical systems in the telecom regime.

In this work, we show that the room-temperature photoluminescence intensity from Ge ion-bombarded (GIB) epitaxial Ge on Si quantum dots (QD) can be improved by their vertical stacking. We stress that the growth of GIB-QD multilayers is more demanding compared to all-crystalline epitaxial QDs, as a consequence of local amorphous regions within the GIB-QDs required during their genesis. We show that in spite of those amorphous regions, for accurately chosen growth temperatures of the Si spacer layers separating the GIB-QD layers, multiple GIB-QD layers can be stacked without detrimental break-down of epitaxial growth. Compared to a single GIB-QD layer, we observe a 650% increase in PL intensity for an eleven-layer GIB-QD stack, indicating that such multilayers are promising candidates as gain material for all-group-IV nano-photonic lasers.