Color-Tunable and Phosphor-Free White-Light Multilayered Light-Emitting Diodes

A tightly integrated 3-D RGB light-emitting diode (LED) stack is demonstrated. Chips of identical dimensions are stacked on top of each other, with wire bonds embedded within. This is achieved by integrating laser-micromachined channels onto the sapphire face of InGaN LEDs, serving to accommodate wire bonds from the chip beneath. The resultant structure eliminates leakage of monochromatic light from individual chips, producing optimally mixed emission through the top aperture. The device can emit a wide range of colors and is an efficient phosphor-free white-light LED as well. When emitting at correlated color temperatures (CCTs) of 2362 K, 5999 K, and 7332 K, the device generates ~ 20 lm/W, exhibiting performance invariant of CCT. Thermal characteristics of this multilayered device are investigated via infrared thermometry.

I. Introduction

An incandescent lamp's color temperature changes with the temperature of the tungsten element, although the emission remains broadband throughout. Fluorescent lighting emits with fixed spectral characteristics. To generate different colors from such light sources, filters are used to remove the unwanted spectral components, incurring energy losses. Light-emitting diodes (LEDs), on the other hand, produce monochromatic radiation by nature; by mixing the emissions from multiple LEDs, a wide range of colors across the visible spectrum can be obtained. Solutions based on this concept, in the form of RGB LEDs whereby chips emitting the primary colors are bonded onto the same package adjacent to each other, are now available and have been adopted on LED panel displays [1]. The technological progresses of blue-light-emitting InGaN quantum well (QW) and red-light-emitting AlInGaP QW LEDs have resulted in promising device characteristics [2]. However, the strong charge separation in InGaN QWs results in low internal quantum efficiencies at longer wavelengths (high In concentration); fortunately, several methods have been pursued to suppress this effect [3]–[6]. Apart from relying on AlInGaP, several recent approaches have been proposed to achieve red-light-emitting LEDs based on III-nitride technology [7]–[9]. Such developments make RGB emitters more promising than ever. Nevertheless, a major drawback of this approach is the spatial color variations giving rise to nonideal color mixing as emission cones from the discrete devices do not overlap with each other completely [10]. Consequently, the dimensions of chips in RGB LEDs are typically kept small , which also set limitations on the overall output power that can be delivered. Additionally, diffusers are often used to overcome this problem, although optical losses of ∼20% are inevitable [11], together with a loss of color sharpness and richness. In view of such limitations, the stacked LED architecture has been proposed, whereby RGB LED chips are physically stacked on top of each other. The light paths of the three devices become aligned to each other, producing broad-band emission that is naturally mixed without additional optics. The rationale for adopting this design has been explained in [12] and [13].