Single SiGe Quantum Dot Emission Deterministically Enhanced in a High-Q Photonic Crystal Resonator

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 $Q\sim 10^5$ 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

Single SiGe Quantum Dots in Photonic Crystal Cavities and Defect-Enhanced Ge Quantum Dots for Optoelectronics

As microelectronic components get smaller and smaller, the influence of the metallic interconnects on the processing speed and on the energy consumption increases dramatically. To circumvent the interconnect bottleneck, the metallic wirings on a Si-chip could be replaced by optical interconnects. The desired compatibility to already established CMOS-technology led to the rise of Silicon Photonics. The biggest challenge and \enquote However, their optical properties never matched the expectations due to the spatial separation of electrons and holes and the indirect nature of the bandgap of both constituents. One way to enhance the spontaneous emission rate is to embed SiGe QDs in photonic crystal cavities, taking advantage of the Purcell effect. In the first part of this work, micro-photoluminescence (-PL) measurements on single SiGe QDs embedded in L3 photonic crystal cavities are discussed. The relative position of the QD to the cavity is systematically changed. As predicted by the Purcell effect, the emission rate depends heavily on the spatial overlap of the emitter and the cavity mode. With this approach, the local density of states was mapped for individual L3 cavity modes. As suggested by M. Schatzl in her PhD Thesis, the single exponential decay at low excitation powers of single SiGe QDs embedded in an H1 cavity could be a sign of single photon emission. Hanbury-Brown and Twiss experiments were conducted to validate this statement. However, no anti bunching was observed. The second part of this work deals with defect engineering of Ge QDs; a novel approach for the realization of Si-based light emitters. Single defects are introduced into the QDs by Ge ion implantation during their growth and subsequent annealing. These defects lead to deep, localized electron states below the conduction band edge of Ge. Optically direct transitions are observed up to room temperature (RT) by recombination of electrons bound to those states with holes confined inside the QDs. These defect-enhanced Ge quantum dots (DEQDs) can be used as active gain material for CMOS-compatible laser sources. Here, various approaches to improve the RT PL yield of DEQDs are presented. Both in-situ and post-growth sample treatments are discussed. In-situ treatment includes the influence of doping and thus, the increase of charge carriers in the layer system. For an ideal Sb doping concentration, we observe a PL enhancement of more than a factor of two, due to the supply of additional electrons. The influence of the ion implantation species into the QDs (e.g. using Si instead of Ge as implanted species) is discussed. An increase of PL yield is observed if the implantation conditions for Si ions are chosen properly. Furthermore, we will discuss the influence of post-growth treatment on the PL yield of DEQDs, i.e. annealing, hydrogen implantation, and passivation. Combining the aforementioned approaches leads to an optimization of DEQDs as gain material for electrically pumped CMOS-compatible lasers operating at RT.Die stetige Miniaturisierung der in der Mikroelektronik eingesetzten Bauelemente führt zu einem steigenden negativen Einfluss der metallischen Leitungen auf die Rechenleistung und den Energieverbrauch. Dieses Problem könnte durch den Einsatz optischer Leitungen umgangen werden. Die Entwicklung optischer Systeme, welche mit der CMOS-Technologie kompatibel sind, ist die Aufgabe der sogenannten Silizium Photonik. Die größte Herausforderung und der „heilige Gral“ dieser Entwicklung bleiben bis heute CMOS-kompatible Laserquellen, welche im Telekombereich 1300nm - 1550nm Licht emittieren. Auf Silizium gewachsene Silizium-Germanium Quantenpunkte (SiGe QDs) galten ab deren Entdeckung zu Beginn der 1990er Jahre als vielversprechende Kandidaten. Allerdings konnten sie aufgrund der örtlichen Trennung der Elektronen von den Löchern, und der indirekten Natur der Bandlücke von Si und Ge nie die an sie gestellten Erwartungen im Bezug auf deren optischen Eigenschaften erfüllen. Eine Möglichkeit die Emission von SiGe QDs zu erhöhen ist es den Purcell Effekt zu nutzen, indem QDs in Resonatoren in der Form von photonischen Kristallen platziert werden. Im ersten Teil dieser Arbeit werden Mikro-Photolumineszenz ( -PL) Messungen an einzelnen SiGe QDs diskutiert, welche sich innerhalb von sogenannten L3 Resonatoren befinden. Die Position des Quantenpunktes innerhalb des Resonators wird systematisch verändert wodurch sich der örtliche Überlapp zwischen Quantenpunkt und Resonator-Mode ändert. Da der Purcell Effekt unter anderem stark von diesem Überlapp abhängt, ist es möglich anhand der gemessenen Emissionsintensitäten die lokalen Zustandsdichten einzelner Resonator-Moden auszumessen. Im Rahmen ihrer Doktorarbeit hatte Magdalena Schatzl bei zeitaufgelösten -PL Messungen an einzelnen SiGe Quantenpunkten in H1 Resonatoren einfach exponentielle Zerfälle bei niedrigen Anregungsleistungen gemessen. Dies wurde als mögliches Zeichen für Einzelphotonenemission interpretiert. Um diese Vermutung zu überprüfen, wurden Hanbury-Brown and Twiss Experimente durchgeführt. Allerdings wurde kein Antibunching -ein Zeichen für Einzelphotonenemission- beobachtet. Im zweiten Teil dieser Arbeit wird ein gänzlich neuer Ansatz zur Steigerung der Lichtemission im SiGe System diskutiert. Die optischen Eigenschaften werden durch das gezielte Einbringen von Störstellen in den Quantenpunkt erheblich verbessert (defect-enhanced Quantum Dots, DEQDs). Während des Wachstums werden die Ge QDs mit Ge-Ionen implantiert und anschließend thermisch behandelt. Diese Störstellen führen zu örtlich stark lokalisierten Zuständen unter der Leitungsbandkante von Ge. Elektronen tunneln in diese Zustände und rekombinieren mit den Löchern, welche auch innerhalb des Quantenpunktes gebunden sind. DEQDs können als aktives Lasermaterial in CMOS-kompatiblen Laserquellen genutzt werden. Verschiedene in-situ und ex-situ Möglichkeiten zur Steigerung der PL-Ausbeute bei Raumtemperatur werden diskutiert. Werden DEQDs mit Sb dotiert, wird auf Grund der höheren Zahl an freien Ladungsträgern eine deutliche Steigerung der PL beobachtet. Statt Ge-Ionen zu implantieren, ist es auch möglich andere Elemente zur Bildung von Störstellen einzubringen. Wird Si implantiert, führt dies zu einer deutlichen Steigerung der PL. Ex-situ Behandlungen umfassen nachträgliche thermische Behandlungen und die Implantation von Wasserstoffionen zur Defektpassivierung. All diese erwähnten Möglichkeiten werden zur Optimierung von DEQDs genutzt, um diese in Zukunft als aktives Material in elektrisch betriebenen CMOS-kompatiblen Lasern einzusetzen.submitted by Lukas Spindlberger, BSc.Universität Linz, Masterarbeit, 2017(VLID)232485

Thermal stability of defectenhanced Ge on Si quantum dot luminescence upon millisecond flash lamp annealing

The intentional merging of epitaxial Ge on Si(001) quantum dots with optically active defect sites promises lowcost 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 lowdimensional nanosystems, as well as the curing of parasitic chargecarrier recombination channels, have been barely investigated thus far. Herein, the structural robustness of defectenhanced quantum dots (DEQDs) is examined under millisecond flash lamp annealing (FLA), carried out at sample temperatures up to 800C. Changes in the optical DEQD properties are investigated using photoluminescence spectroscopy performed in a sample temperature range from 10 to 300K. It is demonstrated that FLAin contrast to in situ thermal annealingleads 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.FWF P29137-N36LIT-2016-1-YOU-002Horizon 2020 731473(VLID)441801

Room-Temperature Group-IV LED Based on Defect-Enhanced Ge Quantum Dots

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 °C. Furthermore, the emission intensity scales with the number of quantum dot layers embedded into the <i>p</i>-<i>i</i>-<i>n</i> 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

Supplementary document for Single SiGe Quantum Dot Emission Deterministically Enhanced in a High-Q Photonic Crystal Resonator - 6176570.pdf

Additional Information to the main paper

Enhanced Telecom Emission from Single Group-IV Quantum Dots by Precise CMOS-Compatible Positioning in Photonic Crystal Cavities

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