AlGaInN-based LED having thick epitaxial layer for improved light extraction

#The invention relates generally to light-emitting diodes and more particularly to AlGaInN-based light-emitting diodes.

#Light emitting diodes (LEDs) are well known solid state devices that can generate light having a peak wavelength in a specific region of the light spectrum. LEDs are typically used as illuminators, indicators and displays. Aluminum-gallium-indium-nitride (AlGaInN)-based LEDs can emit light having a peak wavelength in the blue and green region of the visible light spectrum with greater luminous intensity than other conventional LEDs. Due to their superior luminance, the AlGaInN-based LEDs have attracted much attention in recent years.

#An exemplary prior art AlGaInN-based LED is schematically illustrated in FIG. 1. The AlGaInN-based LED 10 includes a sapphire (Al.sub.2 O.sub.3) substrate 12 and a multi-layered epitaxial structure 14. The multi-layered epitaxial structure includes an upper AlGaInN region 16, an active region 18, and a lower AlGaInN region 20. The term "AlGaInN" is defined herein as a material composed of one or more elements from a set that includes aluminum, gallium, indium, and nitrogen. The upper AlGaInN region and the lower AlGaInN region are made of multiple epitaxial layers of AlGaInN. The upper AlGaInN region is made of p-type AlGaInN epitaxial layers, while the lower AlGaInN region is made of n-type and/or undoped AlGaInN epitaxial layers. The active region includes one or more layers of indium-gallium-nitride (InGaN) that form(s) what is/are commonly referred to as quantum well(s). The typical thickness of the upper AlGaInN region is less than 0.5 micrometers, while the typical thickness of the lower AlGaInN region is approximately 2 to 3 micrometers. The active region thickness is typically less than 0.2 micrometers. Therefore, the typical maximum thickness of the multi-layered epitaxial structure is approximately 3.7 micrometers.

#FIG. 1 is a schematic diagram of a prior art AlGaInN-based LED.

#FIG. 2 is a schematic diagram of an AlGaInN-based LED having a thick multi-layered epitaxial structure in accordance with the present invention.

#FIG. 3 shows a plot of extraction efficiency vs. epitaxial thickness from numerical ray-trace modeling calculations.

#With reference to FIG. 2, an aluminum-gallium-indium-nitride (AlGaInN)-based light-emitting diode (LED) 32 in accordance with the present invention is shown. The AlGaInN-based LED includes a substrate 34 and a multi-layered epitaxial structure 36. As compared to multi-layered epitaxial structures of conventional AlGaInN-based LEDs, such as the multi-layered epitaxial structure 14 of LED 10 of FIG. 1, the multi-layered epitaxial structure 36 of the AlGaInN-based LED 32 is substantially thicker. The increased thickness of the multi-layered epitaxial structure increases the portion of the light generated by an active region 38 that escapes from the LED 32 through the sides of the multi-layered epitaxial structure. The manner in which the thick multi-layered epitaxial structure facilitates the increased emission of light from the LED will be described in detail below.

#The multi-layered epitaxial structure 36 includes an upper AlGaInN region 40, the active region 38, and a lower AlGaInN region 42. The upper and lower AlGaInN regions include multiple epitaxial layers of AlGaInN. The upper AlGaInN region is made of p-type AlGaInN epitaxial layers, while the lower AlGaInN region is made of n-type AlGaInN epitaxial layers and undoped epitaxial layers. The portion of the lower AlGaInN region having the n-type AlGaInN epitaxial layers is identified by the reference numeral 44. The portion of the lower AlGaInN region having the undoped epitaxial layers is identified by the reference numeral 46. The undoped epitaxial layers may be layers of AlGaInN or other AlGaInN-based material. The active region includes at least one AlGaInN light-emitting layer that forms the quantum well(s). For visible light emission, the light-emitting layer may be comprised of indium-gallium-nitride (InGaN). The multi-layered epitaxial structure has an approximate thickness of 4 micrometers or greater. In one embodiment, the thickness of the multi-layered epitaxial structure is approximately 7 micrometers. In another embodiment, the thickness of the multi-layered epitaxial structure is approximately 15 micrometers. However, the thickness of the multi-layered epitaxial structure may be greater than 15 micrometers.

#A plot of extraction efficiency vs. epitaxial thickness from numerical ray-trace modeling calculations is shown in FIG. 3. Because of issues such as cracking due to strain, it is not obvious that high-quality active regions can be grown atop thick AlGaInN structures. For certain choices of buffer layer and lower AlGaInN layer composition and doping (heavy or light, n-type or p-type, or undoped), high quality active regions are achieved and in fact increased quantum efficiency is observed. Strain can also be controlled by the use of AlGaN layers or AlGaInN-based superlattice structures to balance tensile and compressive forces within the epitaxial structure and minimize defect formation which can reduce the internal quantum efficiency of the active region.