Light emitting devices (LEDs) are an important class of solid-state devices that convert electric energy to light Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
The cost and conversion efficiency of LEDs are important factors in determining the rate at which this new technology will replace conventional light sources and be utilized in high power applications. Many high power applications require multiple LEDs to achieve the needed power levels. Individual LEDs are limited to a few watts. In addition, LEDs generate light in relatively narrow spectral bands. Hence, in applications requiring a light source of a particular color, the light from a number of LEDs with spectral emission in different optical bands is combined. Hence, the cost of many light sources based on LEDs is many times the cost of the individual LEDs.
The conversion efficiency of individual LEDs is an important factor in addressing the cost of high power LED light sources. Electrical power that is not converted to light in the LED is converted to heat that raises the temperature of the LED. Heat dissipation places a limit on the power level at which an LED operates. In addition, the LEDs must be mounted on structures that provide heat dissipation, which, in turn, further increases the cost of the light sources. Hence, if the conversion efficiency of an LED can be increased, the maximum amount of light that can be provided by a single LED can also be increased, and hence, reduce the number of LEDs needed for a given light source. In addition, the cost of operation of the LED is also inversely proportional to the conversion efficiency. Hence, there has been a great deal of work directed to improving the conversion efficiency of LEDs.
The spectral band generated by an LED, in general, depends on the materials from which the LED is made. LEDs commonly include an active layer of semiconductor material sandwiched between additional layers. For the purposes of this discussion, an LED can be viewed as having three layers, the active layer sandwiched between two other layers. These layers are typically deposited on a substrate such as sapphire. It should be noted that each of these layers typically includes a number of sub-layers. The final LED chip is often encapsulated in a clear medium such as epoxy. To simplify the following discussion, it will be assumed that the light that leaves the LED exits through the outer layer that is furthest from the substrate. This layer will be referred to as the top layer in the following discussion.
Improvements in materials have led to improvements in the efficiency of light generated in the active layer. However, a significant fraction of the light generated in the active layer is lost. Light that is generated in the active layer must pass through the top layer before exiting the LED. Since the active layer emits light in all directions, the light from the active region strikes the boundary between the top layer and the encapsulating material at essentially all angles from 0 to 90 degrees relative to the normal direction at the boundary. Light that strikes the boundary at angles that are greater than the critical angle is totally reflected at the boundary. This light is redirected toward the substrate and is likewise reflected back into the LED. As a result, the light is trapped within the LED until it strikes the end of the LED or is absorbed by the material in the LED. In the case of conventional GaN-based LEDs on sapphire substrates approximately 70% of the light emitted by the active layer is trapped between the sapphire substrate and the outer surface of the GaN. Much of this light is lost, and hence, the light conversion efficiency of these devices is poor.
Several techniques have been described to improve light extraction from LEDs, and hence, improve the light conversion efficiency of these devices. In one class of techniques, the top surface of the LED is converted from a smooth planar surface to a rough surface. Some of the light that is reflected at the top surface will return to the top surface at a location in which that light is now within the critical angle, and hence, escape rather than being again reflected. This technique depends on introducing scattering features into the top surface of the LED either by etching the surface, by depositing the features, or by growing the top layer under conditions that result in non-planar growth. Depositing the scattering features typically requires some form of lithography, and hence, significantly increases the cost of the resultant LEDs. Growing the surface so that it is non-planar is difficult to control and reproduce. Hence, etching the surface has significant advantages if the etching operation can be carried out without the use of lithographic masking operations or other costly fabrication steps.
To provide a suitable rough surface, the etching technique needs to create features that have a size that is a significant fraction of the wavelength of light generated by the LED. If the features are small compared to the wavelength of the generated light, the light will not be scattered by the features. Since the thickness of the top layer is typically only a few wavelengths of light at most, the etching technique must remove a significant amount of the top layer. In the case of LEDs constructed from GaN, this presents a serious problem, since the outer surface of the top layer is very difficult to etch. Hence, techniques based on exposing the surface to an etchant have failed to provide the desired surface features at an acceptable cost.