We present an on-chip LED based on native Si, fabricated in an open foundry CMOS node. This LED has remarkable characteristics such as its sub-wavelength emission area, broad spectrum, high spatial intensity, high bandwidth, and high reproducibility, which make it an ideal light source for various imaging and sensing systems. Two prototypes, a holographic microscope and a LIDAR, are built employing this LED. Our work demonstrates the possibility of integrating monolithic light sources with other photonic and electronic components on a single photonic chip.
Silicon technologies have been developed for both electronics and photonics. Future demands call for further innovation in each field separately, but also depend on our ability to bring the best of both worlds together through integrated solutions. For decades, the pursuit of all-silicon electronic-photonic integration has been hindered by the lack of a native light source due to silicon’s indirect bandgap. Here, we discuss the potential for micro- and nano-scale light sources realized in microelectronic CMOS technology without any modification or postprocessing. High brightness is realized by exploiting the well-passivated silicon surfaces available in CMOS to realize efficient light emission despite the indirect bandgap. NIR emission at the silicon bandgap is demonstrated and exploited to demonstrate chip-to-chip optical links and sensors utilizing only silicon light sources.
As the best performing light emitting diodes (LEDs) are approaching the conventional limit of unity efficiency, a unique heat-pump operating mode of the devices has been proposed to address this problem, in which case lattice heat is pumped from the phonon field of the device into the incoherent photon field of emission at the expense of consuming zero-entropy electrical power. To better understand the potential of visible LEDs for further efficiency improvement in this mode, we present a thermodynamic framework that allows us to estimate the Carnot limit for their wall-plug efficiency (WPE) at different operating conditions. We find that the theoretical efficiency limit drops at higher light intensities but can still be well above 100% even at 10 W/cm^2. Ideally, realizing such high efficiency at useful output powers requires the device to possess an external quantum efficiency (EQE) close to unity. Here we are able to introduce dissipation into the thermodynamic model and thus determine a minimum EQE required for an LED to achieve unity WPE. In addition, the thermodynamic study for visible LEDs yields one surprising result. The first observation of above-unity WPE was on a heated mid-infrared (2.2 um) LED, and the subsequent demonstration at room temperature necessarily required a longer-wavelength 3.4 um device in order to realize sufficient carrier injection for measurable optical output. On the contrary, this thermodynamic analysis indicates that at useful optical powers – and hence useful cooling powers – visible LEDs of shorter wavelength are expected to show higher cooling at a lower current density.
It is known that the wall-plug efficiency (WPE) of a light-emitting diode (LED) can exceed unity and that electroluminescence cooling (ELC) happens in this scenario. However, it is difficult to observe the associated temperature drop due to the relatively small cooling power and the overwhelming heat flux from the ambient. In this work, we design a photonic crystal (PhC) enhanced LED which has smaller surface area as well as thermal mass compared with an encapsulated LED. We also present thermal models to evaluate the temperature drop of the LED in air and vacuum.
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