Metal-assisted chemically etched silicon nanopillars hosting telecom photon emitters

Around the world, specialists are working on implementing quantum information technologies. One important path involves light: Looking ahead, single light packages, also known as light quanta or photons, could transmit data that is both coded and effectively tap proof. To this end, new photon sources are required that emit single light quanta in a controlled fashion—and on demand. Only recently has it been discovered that silicon can host sources of single-photons with properties suitable for quantum communication. So far, however, no one has known how to integrate the sources into modern photonic circuits.

Silicon has recently been proven to be instrumental for hosting sources of single photons emitting in the strategic optical telecommunication O-band (1260–1360 nm). The origin of the telecom single-photon emitters is a carbon-irradiation induced damage center in silicon, the so-called G-center. These single-photon emitters are created at sufficiently low carbon concentrations, whereby two or more of the G-center defects do not interact with each other. The scalability, brightness, purity, and collection efficiency of these single-photon emitters are key issues in bringing them closer to practical applications. A scalable photonic platform that houses single-photon emitters integrated into photonic structures, such as nanopillars, solid immersion lenses, or high-quality optical microcavities, is thus beneficial. Among solid immersion lenses and optical microcavities, nanopillars are the simplest photonic structures from a fabrication standpoint to efficiently scale and collect single photons with relatively low numerical aperture objectives. Solid immersion lenses, for example, require the fabrication of hemispheres with a large curvature and sufficient accuracy to collect photons through a large numerical aperture objective near the hemisphere.

For the first time, a team led by Dr. Yonder Berencén at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has now presented an appropriate production technology using silicon nanopillars: A chemical etching method followed by ion bombardment. Their research is published in the Journal of Applied Physics.

Silicon and single-photon sources in the telecommunication field have long been the missing link in speeding up the development of quantum communication by optical fibers. Now they have created the necessary preconditions for it. Although single-photon sources have been fabricated in materials like diamonds, only silicon-based sources generate light particles at the right wavelength to proliferate in optical fibers—a considerable advantage for practical purposes.

The researchers achieved this technical breakthrough by choosing a wet etching technique what is known as MacEtch (metal-assisted chemical etching) rather than the conventional dry etching techniques for processing the silicon on a chip. These standard methods, which allow the creation of silicon photonic structures, use highly reactive ions. These ions induce light-emitting defects caused by the radiation damage in the silicon. However, they are randomly distributed and overlay the desired optical signal with noise. Metal-assisted chemical etching, on the other hand does not generate these defects instead, the material is etched away chemically under a kind of metallic mask. Using the MacEtch method, researchers initially fabricated the simplest form of a potential light wave-guiding structure: silicon nanopillars on a chip. They then bombarded the finished nanopillars with carbon ions, just as they would with a massive silicon block, and thus generated photon sources embedded in the pillars. Employing the new etching technique means the size, spacing, and surface density of the nanopillars can be precisely controlled and adjusted to be compatible with modern photonic circuits. Per square millimeter chip, thousands of silicon nanopillars conduct and bundle the light from the sources by directing it vertically through the pillars.

The researchers varied the diameter of the pillars because they hoped this would mean they could perform single defect creation on thin pillars and actually generate a single photon source per pillar, It didn’t work perfectly the first time. By comparison, even for the thinnest pillars, the dose of our carbon bombardment was too high. But now it’s just a short step to single photon sources. This is a step on which the team is already working intensively because the new technique has also unleashed something of a race for future applications.

The monolithic integration of a single-photon source with reconfigurable photonic elements and single-photon detectors in a single silicon chip would result in quantum photonic integrated circuits that would harness quantum phenomena for secure communication and computation. These quantum photonic integrated circuits could be fabricated using the same equipment and methods as for computer circuits, avoiding the high cost of developing new technology.

Reactive ion etching (RIE) is the method of choice in mainstream silicon integrated circuit manufacturing to transfer lithographically defined photoresist patterns into electronic and photonic materials with great fidelity. Yet, crystal damage along with contamination defects can be introduced during the RIE process of silicon since its surface is exposed to bombardment by energetic particles. Consequently, RIE of silicon also produces radiation damage defects with sharp luminescence lines, such as the X- (1192 nm), W- (1218 nm), G- (1278 nm), I- (1284 nm), T- (1326 nm), C- (1570 nm), and P-line (1616 nm).7,13,14 The formation of these optically active radiation damage defects by RIE has been demonstrated to depend on the conditions of the plasma treatment, type of plasma gases, and the substrate characteristics, such as the residual amount of impurities in the pristine sample. Subsequent annealing can be performed to mitigate the RIE-related crystal damage, although damage mitigation by annealing might not be effective for destroying all the radiation damage centers or can simply lead to the formation of the T- and I-centers. These centers are typically formed upon silicon bombardment with energetic particles followed by annealing at around 450 C. Investigation and understanding of radiation damage mitigation of Si processed by RIE combined with annealing for practical quantum applications would result in a laborious and time-consuming process, regardless of the feasibility of RIE for producing Si nanopillars.16

Alternatively, metal-assisted chemical etching (MACEtch) is a low-cost and promising etching technique for defect-free pattern transfer in silicon, which enables the fabrication of high-aspect-ratio structures for applications in photovoltaics, x-ray optics, energy storage, and sensors. MACEtch is a wet-chemical process in which vertical etch fronts are obtained in silicon using only a thin-film noble metal mask (e.g., Au, Ag, Pt, Pd) and an etching solution consisting of a mixture of a strong oxidizing agent (e.g., H2O2) and hydrofluoric acid (HF). Si is oxidized only in the region where it is in contact with the metal mask, which catalyzes the oxidation process by the injection of holes into the Si valence band. The HF subsequently dissolves the formed SiO2.

In particular, MACEtch enables precise positioning of aligned Si nanopillars as well as control of diameter, length, spacing, and density. Wafer-scale vertically aligned arrays of Si nanopillars are demonstrated by metal-assisted chemical etching of silicon. This involves the use of an anodic aluminum oxide as a pattering mask of a thin metallic film on a Si substrate. A high density of nanopillars in the order of 1010pillars/cm2 is accomplished with average pillar diameters below 20 nm. Typically, the diameter is readily controlled by varying the pore diameter of the porous alumina film along with the thickness of the deposited metal film of choice.

They research team demonstrated the integration of 1278 nm emitting G-centers in a two-dimensional platform of arrays of silicon nanopillars fabricated by MACEtch followed by carbon implantation. Our top-down nanofabrication approach enables the production of thousands of nanopillars per square millimeter, each hosting an ensemble of telecom photon emitters. Moreover, the structures are free of fabrication-related G-center defects. Compared to that of bulk Si, they found an improved photoluminescence (PL) of the G center-related emission along with a waveguiding effect of the 1278 nm-photon emission along individual pillars. This effect is confirmed by the reduction by a factor of 3.5 of the full width at half maximum (FWHM) of the emitted G-center light along the pillar when compared to that of bulk Si.

In summary, the authors demonstrated the integration of 1278 nm emitting G-centers in two-dimensional arrays of silicon nanopillars. They successfully proposed a low-cost, top-down nanofabrication process which allows for scalability and enables the production of thousands of nanopillars per square millimeter, each hosting telecom photon emitters. They  demonstrated that metal-assisted chemical etching is an effective pathway for optically active defect-free pattern transfer in silicon, resulting in high-aspect ratio nanopillars with a 5 𝜇m-pitch that is comparable to that of integrated photonic circuits. They proved a vertically directed waveguiding effect of the G-center-related emission along individual pillars, which is accompanied by improved PL of the G-centers compared to that of bulk Si. According to the authors that these results hold great promise for hosting and scaling up single-spin color centers and single-photon emitters in silicon coupled to silicon pillars. In turn, focused ion beam, with its sub-micrometer writing resolution, could become the technique of choice for quasi-deterministically creating single color centers integrated into silicon photonic structures.