Photonic crystal light emitting device
A photonic crystal structure is formed in an n-type region of a III-nitride semiconductor structure including an active region sandwiched between an n-type region and a p-type region. A reflector is formed
Photonic crystal light emitting device
A photonic crystal structure is formed in an n-type region of a III-nitride semiconductor structure including an active region sandwiched between an n-type region and a p-type region. A reflector is formed on a surface of the p-type region opposite the active region. In some embodiments, the growth substrate on which the n-type region, active region, and p-type region are grown is removed, in order to facilitate forming the photonic crystal in an an-type region of the device, and to facilitate forming the reflector on a surface of the p-type region underlying the photonic crystal. The photonic crystal and reflector form a resonant cavity, which may allow control of light emitted by the active region.
1. Field of Invention
The present invention relates to semiconductor light emitting devices including photonic crystal structures.
2. Description of Related Art
Light emitting diodes (“LEDs”) are technologically and economically advantageous solid state light sources. LEDs are capable of reliably providing light with high brightness, hence in the past decades they have come to play a critical role in numerous applications, including flat-panel displays, traffic lights, and optical communications. An LED includes a forward biased p-n junction. When driven by a current, electrons and holes are injected into the junction region, where they recombine and release their energy by emitting photons. The quality of an LED can be characterized, for example, by its extraction efficiency, which measures the intensity of the emitted light for a given number of photons generated within the LED chip. The extraction efficiency is limited, for example, by the emitted photons suffering multiple total internal reflections at the walls of the high refractive index semiconductor medium. As a result, the emitted photons do not escape into free space, leading to poor extraction efficiencies, typically less than 30%.
In the past thirty years, various approaches have been proposed to enhance the extraction efficiency of LEDs. The extraction efficiency can be increased, for example, by enlarging the spatial angle in which the emitted photons can escape by developing suitable geometries, including cubic, cylindrical, pyramidal, and dome like shapes. However, none of these geometries can entirely eliminate losses from total internal reflection.
A further source of loss is the reflection caused by the refractive index mismatch between the LED and the surrounding media. While such losses could be reduced with an anti-reflection coating, complete cancellation of reflection can be achieved only at a specific photon energy and one angle of incidence.
U.S. Pat. No. 5,955,749, entitled “Light Emitting Device Utilizing a Periodic Dielectric Structure,” granted to J. Joannopoulos et al., describes an approach to the problem of enhancing the extraction efficiency. According to U.S. Pat. No. 5,955,749, a photonic crystal is created by forming a lattice of holes completely through the semiconductor layers of the light emitting diode. The lattice of holes creates a medium with a periodically modulated dielectric constant, affecting the way light propagates through the medium. The photons of the light emitting diode can be characterized by their spectrum or dispersion relation, describing the relation between the energy and the wavelength of the photons. The relationship may be plotted, yielding a photonic band diagram consisting of energy bands, or photonic bands, separated by band gaps. Though the photonic band diagram is analogous to the spectrum of electrons in crystalline lattices as expressed in an electronic band diagram, the photonic band diagram is unrelated to the electronic band diagram. When a photonic crystal is formed in an LED it affects how light propagates in the structure. Therefore if the proper lattice spacing is chosen, light that would otherwise have been trapped in the structure by total internal reflection can now escape, increasing the extraction of the LED. Also, alternative lattices can reduce the photon mode volume in the LED structure increasing the radiative rate or internal efficiency of the LED active layer.
In an effort to explore the usefulness of photonic crystals for light generation, U.S. Pat. No. 5,955,749 gives a partial description of a theoretical structure of a photonic crystal device.
U.S. Pat. No. 5,955,749 describes an n-doped layer, an active layer, a p-doped layer, and a lattice of holes formed in these layers. However, the device of U.S. Pat. No. 5,955,749 is not operational and therefore is not a LED. First, electrodes are not described, even though electrodes are needed for the successful operation of a photonic crystal LED (“PXLED”). Though the fabrication of electrodes in regular LEDs is known in the art, for PXLEDs neither the fabrication of electrodes, nor their influence on the operation of the PXLED is obvious. For example, suitably aligning the mask of the electrode layer with the lattice of holes may require new fabrication techniques. Also, electrodes are typically thought to reduce the extraction efficiency as they reflect a portion of the emitted photons back into the LED, and absorb another portion of the emitted light.
Second, U.S. Pat. No. 5,955,749 proposes fabricating photonic crystal light emitting devices from GaAs. GaAs is indeed a convenient and hence popular material for fabricating regular LEDs. However, it has a high surface recombination velocity of about 106 cm/sec as described, for example, by S. Tiwari in “Compound Semiconductor Devices Physics,” Academic Press (1992). The surfac