Monolithic integration of light-emitting diode and photodetector for monitoring and regulation of light output

Abstract

Light-emitting diode (LED) is adopted in increasingly wide applications like general lighting, display back lighting and automotive lighting while they suffer from inevitable degradation and fluctuation which will lead to light output variations and non-uniformities which may be ignorable in normal applications but would be a serious concern when it comes to some specific conditions like medical, biological and scientific applications where stable and constant light intensities must be obtained. Traditional method to generate constant light output over time is to install an external photodetector (PD) for monitoring the light output so that a feedback signal, which is the photocurrent generated inside the PD, could be obtained for controlling the light output. However high noise levels will be introduced from ambient environment thus the accuracy of PD will be affected. Furthermore, the whole detection system would be bulky since it needs extra components like a beam splitter to couple the light to PD which will diminish the inherent advantages of the LED including efficiency, robustness and compactness. In view of the existing problems for external detection mentioned above, a novel structure of monolithic integration of LED and PD is proposed. The LED and PD have the same structure so that they could be fabricated together, although they function in different modes. Standard photolithography including positive and negative photolithography is involved followed by inductively coupled plasma etching. The key step is the deep etch which is used to electrically separately the LED and PD. Complete characterization is carried out to verify the functionality of the on-chip detection system. Light coupling path is studied through ray-trace simulation which shows light channeling along the underlying sapphire substrate is the main coupling path. Only 0.3% of the light is coupled to PD which means majority of the light from the LED can emit as normal. The photocurrent generated is small but sufficient to perform the functionality. The responsivity spectra is measured together with the electroluminescence (EL) spectrum whose overlap is around half width of EL spectrum. Further measurement for verifying the electrical and optical characteristics of LED and PD are also presented. Finally the natural degradation test and light output regulation are performed. As for natural degradation, after being operated nearly 500 hours, the LED shows a decrease of light output by ~9% and ~0.7% fluctuation on average over 1 hour. As for the device equipped with PD feedback regulation circuits, the light output can be maintained at a constant level and the fluctuation can be decreased to ~0.2% over 600 hours. This design opens a new horizon on monitoring and regulating the light output. Through monolithic integration, the whole system would be highly efficient, robust and compact. A new application is proposed and preliminarily studied. By mixing three LEDs of orange, green and blue colors, white light emission can be achieved. By integrating each LED with an on-chip PD, it is possible to maintain not only the light output but also the color chromaticity.