The integration of various new materials with and beyond silicon may lead to improvements in data centers, photovoltaic cells, x-ray machines, and displays.

By Hank Hogan

Alone, nothing is perfect. So when various materials in photonics applications work in tandem, a better performing hybrid approach arises, and teamwork prevails. Examples of such hybrid photonics can be found in existing and emerging commercial products. However, to successfully compete with incumbent technologies, hybrids must integrate different materials and simultaneously achieve required performance and reliability.

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Hybrid organic-inorganic metal-halide perovskites can be used in displays. The material produces various colors depending on chemical composition. Courtesy of Helio Display Materials.

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One place where a hybrid approach is both successful and commercially important is in data centers, said Eric Mounier, an analyst at the Lyon, France-based research firm Yole Développement. Increasingly, the centers use optical transceivers based on hybrid photonics for data communication. Silicon and related oxides or nitrides provide the optical waveguide function, and other semiconductors handle different tasks.

“In the receiver, for example, InP [indium phosphide] or Ge [germanium] is used because of the good performances in photodetection,” Mounier said. “For the transmitter, InP is used for the uncooled laser source because of the lasing capability of this material.”

A silicon base offers the best platform for the integration of electronic and optical components. But because light cannot be generated from silicon, a hybrid material approach is the only possibility for achieving successful and fully functional integration.

Integration with silicon

InP, a III-V semiconductor, can be incorporated into a silicon photonic chip in various ways. Santa Clara, Calif.-based Intel grows InP on relatively small 2- or 3-in. wafers. The wafer is diced into little pieces and the pieces are integrated at specific spots in silicon photonic chips, which are fabricated on a 300-mm (12-in.) wafer.

When the two materials are brought together, there are benefits, according to Robert Blum, general manager for new business at Intel’s Silicon Photonics Product Division. “In addition to the full suite of passive silicon photonics components, we can integrate indium phosphide photodiodes and active components such as lasers with very different wavelengths,” he said.

This ability to integrate makes it possible to multiplex various lasers and data channels on the same chip. Multiple lasers enable higher data rates because, for example, four 25-Gb/s channels can be combined to create a 100-Gb/s connection. As the speed capability of individual channels increases, the total bandwidth realized by multiplexing can, too.

Blum said electronic and photonic chips may both be built largely out of silicon but with various fabrication technologies. At the same time, there is, of course, an increasing demand for faster data communication rates. One solution is to combine everything — hybrid photonics and electronics — into a single package that puts the photonics as close as possible to the electronics, an approach Intel followed when it created a 12.8-Tb/s (12,800-Gb/s) network switch. He said such approaches to packaging for data communications, as well as for sensing, are a big focus for Intel.

The company is keeping an eye on emerging photonic materials. Integration with silicon could open up a range of new applications in sensing, biotech, and elsewhere, Blum said.

Perovskite advantages

Potential new photonic materials are metal-halide perovskites, a class of compounds that is itself a hybrid. Perovskites that detect or emit light may contain, for instance, lead (a heavy metal) or metal-like iodine in a hybrid configuration of organic-inorganic components.

Metal-halide perovskites are relative newcomers to photonics applications, but they offer significant advantages, according to Matthew Beard, a senior research fellow at the National Renewable Energy Laboratory of Golden, Colo. Beard is also director of the Center for Hybrid Organic-Inorganic Semiconductors for Energy, which studies these unusual materials.

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Receiver: • Photodiode (InP or Ge) • TIA (Si) • Couplers, Isolator, and Lenses (Glass, Polymer, Other Materials) Also: Basic Light Routing Such as WG, Mux/Demux, Couplers (InP, Si, Silica)

Transmitter: • Uncooled Laser Source (InP) • MZ Driver (Si) • Modulator (LiNbO3, Si, Polymer Possible) • Couplers, Isolator, and Lenses (Glass, Polymer, Other Materials)

The inside of a transceiver. Transceivers are made of photonic integrated circuits composed of various materials, including hybrid silicon photonics with InP or Ge integrated into a silicon platform. Courtesy of Yole Développement.

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A hybrid silicon photonic wafer. Chips incorporate nonsilicon semiconductors to emit and detect light. Courtesy of Intel.

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Their hybrid nature allows tuning of optical, electronic, and other properties, Beard said. Also, metal-halide perovskites are solution processable, which means they can be deposited via ink jet or by spin coating onto an optionally flexible substrate. These perovskites are defect tolerant as well, so the extreme cleanliness required for processing silicon or other semiconductors is unnecessary.

The new material has already appeared in a form of hybrid photonics: the tandem photovoltaic (PV) cell. In this approach, which is nearing large-scale commercial production, a perovskite layer sits atop a silicon solar cell. The top layer absorbs short-wavelength light while the silicon bottom layer absorbs longer wavelengths. The result is a higher light-to-electricity conversion efficiency than would otherwise be possible. This may make a hybrid solution worth a premium. (For more on this topic, see “Photonic Technologies Energize Sustainability” in the July issue of Photonics Spectra.)

Yarnton, England-based Oxford PV is bringing the tandem PV cell technique to market. The fact that the technology has reportedly met requirements for commercial PV installations is important, Beard said.

Commercial photovoltaics must meet stringent guidelines for longevity and reliability, including decades-long functionality standards. This requirement — combined with the need to outperform an incumbent, largely silicon-only technology — makes PVs a challenge to manufacture.

“That’s one of the most demanding applications,” Beard said. Thus, the fact that the tandem cell seems poised for commercial deployment could be viewed as an endorsement of the hybrid.

Sensing and emitting

Metal-halide perovskites work well in PV settings because the material absorbs light readily over a broad spectrum, Beard said. These same attributes, though, mean perovskites may shine in another area: sensing. While such detection can be done in the visible, the presence of lead may mean the hybrid material will excel at sensing x-rays. Lead stops these photons better than silicon or other semiconductors, making it possible to build potentially superior and more compact x-ray detectors based on perovskites.

“These materials absorb x-rays very well and can either emit photons with high efficiency, or transport x-ray generated charges with high efficiency,” said Jinsong Huang, University of North Carolina professor of applied physical sciences. “They are easy to make — from nanomaterials to thin films and bulk single crystals — using a scalable solution process.”

Huang formed a company, Perotech Inc., to commercialize the material. One of the goals with the company’s products is to reduce the x-ray dose given to patients through the development and use of a more efficient detector. Because flexible surfaces can be coated with perovskites, the use of this hybrid could lead to an x-ray detector that could be wrapped around patients.

According to Huang, one aspect that needs to be better understood is the radiation dose stability of the perovskite detectors. Ideally, after repeated exposure to x-rays, there would be no difference in detector response, but x-rays are energetic. So they may cause physical changes in the detector and potentially a shift in how it reacts to x-rays. However, Huang said perovskites have a self-recovery capability, which may mitigate any damage. He added that more research in this area is needed.

In addition to sensing light, perovskites can also emit it, either by downconverting incoming illumination or in response to an electrical current. Oxford, England-based startup Helio Display Materials is commercializing the technology.

Converting wavelengths

According to Helio CEO Simon Jones, initial applications involve converting blue light into other colors, such as the red and green needed for a full-color display. LEDs efficiently generate blue light, but generating red or green is indirect because it involves filtering, Jones said. As a result, the process is inefficient and drains battery charge.

So the industry wants a material for direct conversion. Jones said perovskites, as a result of their material characteristics, provide significant advantages for this direct route.

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Perovskites, a hybrid material, can be deposited onto flexible surfaces and used to generate or detect light. Courtesy of Helio Display Materials.

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“Perovskites offer very high absorption and conversion efficiency so that a thin layer is all that is needed to fully convert the blue light. Also, perovskites have a very narrow emission spectrum, which is required to maximize color gamut,” he said.

Compared with the existing approach, perovskites can increase the efficiency of the light conversion process by as much as 70% while improving color rendering, Jones said. One result could be portable devices with longer-lasting run times that deliver higher-quality images.

Perovskites are also electroluminescent, which means they could potentially compete with OLEDs, quantum dots, and other technologies that create their own light. The organic-inorganic hybrid material may have an advantage because only a thin layer is needed, and the layer can be processed as a solution, or printed via ink jet. Jones said the output spectrum of perovskites is defined by its chemical composition, which makes them simpler to manufacture at scale than some alternatives.

However, no material is perfect. For perovskites, lead’s presence in the highest-performing configuration is an issue. Because the perovskite films are thin, the amount of lead is small, and Jones said his company’s products will meet European RoHS (Restriction of Hazardous Substances) requirements that restrict lead’s use in electrical and electronic equipment. For most other applications, the perovskite may be so well encapsulated as to render nearly impossible the escape of lead into the environment. Still, research is underway to further reduce any potential leakage for PV applications.

Additionally, perovskites can have longevity problems, particularly if current flows through the material. Strides are being made in this area, Jones said. Helio has demonstrated good reliability and color output stability results, but more work must be — and is being — done to meet product requirements. The company expects to have a qualified manufacturing process ready to ramp up for volume production early in 2024.

If these efforts succeed, hybrid photonics will have new materials along with old. Applications will benefit because combinations will move closer to the ideal of perfection than any one single material.

“The objective is really to take benefits from all the different advantages of the different performance of the materials,” Yole’s Mounier said.

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How Heliomatrix Delivers 70% More Power Efficiency for LCDs

In a conventional LCD, light originates from a blue LED and is converted to white light with phosphors. It is then filtered to make the red, green, and blue light needed for the RGB subpixels that make up the image. The filtering works by blocking all unwanted colors; therefore, considerable power is lost.

With in-pixel color conversion, there is no white-light step and blue light is directly available for the blue subpixels. The blue light is converted to red and green light as appropriate in the red and green subpixels, leading to a 70% increase in efficiency.

Heliomatrix is a perovskite-based color conversion material under development for this application. Perovskites have uniquely high light absorption, which means that a thinner layer is needed to fully convert the blue light to red or green. Thinner layers mean faster and lower-cost production processes, less material usage, and easier optical design.

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A schematic illustration of the increased efficiency of in-pixel color conversion with Heliomatrix materials. Other optical losses are similar between the two architectures. Courtesy of Helio Display Materials.

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There are 68 suppliers of Displays in the Photonics Marketplace.

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