Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
2000-01-14
2001-07-10
Pham, Long (Department: 2823)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
C438S022000, C438S047000
Reexamination Certificate
active
06258618
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed towards improving light extraction from light emitting devices.
BACKGROUND
Light extraction from a semiconductor light-emitting device (LED) is limited due to the large optical refractive index, (n~2.2-3.8) of the semiconductor material relative to the surrounding ambient, typically air (n~1) or transparent epoxy (n~1.5). The amount of extraction depends heavily on the macroscopic geometry of the LED and the three-dimensional emission profile of light generated within the active region or light-emitting layer. Before it can escape, most of the light generated within the device is attenuated by the absorbance in the surrounding materials, e.g. epitaxial layers, confining regions, substrate, die attach materials, and electrical contacts.
Typical devices generate photons at the p-n junction that are emitted into a wide range of directions (nearly isotropic emission). As a result, a large percentage of emitted light rays are incident at the device/ambient interface at angles greater than the critical angle for exiting the semiconductor. These rays are internally reflected and are susceptible to absorption within the device. For a typical GaN-based LED, only~11% of photons are incident on the top surface within the critical angle (for transmission into epoxy). The remaining light undergoes at least one internal reflection before escaping the chip.
The internally-reflected light in AlInGaN LEDs is particularly susceptible to absorption by the p-layer contact. These contacts must cover essentially the entire p-n junction emitting area because current cannot spread laterally in the semiconductor layers. Since the conductivity of the p-type epitaxial layers is extremely low, typically >20000 &OHgr;/square, current is confined to directly under the contact metal or to within ~1 &mgr;m of the contact edge.
To allow light to escape, AlInGaN LEDs use p- contacts made with extremely thin metal layers. They are typically between 50 and 500 Å thick and made from AuNi or similar alloys. While these thin, ‘semi-transparent’ layers pass most of the light that strikes them near normal incidence, typically greater than 20% of this light is absorbed.
There are several problems with semi-transparent contacts. First, the contacts absorb a large fraction of the LED light. Although they can pass as much as~80% of light at near-normal incidence, they are relatively absorptive at angles greater than the critical angle (for light to escape from the LED). Since most LED light is internally reflected, it encounters the partially absorbing contact many times. For LEDs made on sapphire substrates,~70% of the emitted light becomes trapped between the absorptive metal surface and the substrate. Since the contact metal rapidly attenuates the intensity of this light, these semi-transparent metal films can absorb the majority of the emitted light.
A second problem is that because the semi-transparent metal films are extremely thin, on the order of a few hundred angstroms, the films do not completely cover rough semiconductor surfaces. This is particularly disadvantageous since rough surfaces can improve light extraction. Semi-transparent films do not conduct current evenly over rough or uneven surfaces and can become discontinuous. This causes the LED to emit light unevenly, or not at all in portions of the device.
A third problem with thin metals is that they are easily scratched, hence creating a discontinuous conducting surface. This makes them difficult to handle, complicating the LED fabrication process.
SUMMARY OF THE INVENTION
The present invention is a AlInGaN light-emitting device that includes a finely-patterned, reflective p-contact that increases light extraction efficiency. These contacts allow light to pass through them because they contain an array of closely spaced openings. Thus, light impinging on the contacts either passes through the openings or is reflected back into the LED with very little loss. This significantly reduces the absorption of light that occurs in conventional semi-transparent contacts. The contact provides low-resistance current spreading (even over rough semiconductor surfaces) since it can be made of thick, highly conducting metal. Openings in the contact metal occupy 20-80% of the overall contact area, and are finely space to achieve transparency and uniform light emission. The contact is made from highly reflective material such as Ag, Al, or Rh. This insures that light that does not pass through the contact is reflected with very little optical absorption. An optional dielectric encapsulant, e.g. silicon dioxide, silicon nitride, aluminum nitride, or aluminum oxide, may be deposited over the p-type contact. The surface of the active region may be optionally roughened with etched features that are self-aligned with the openings in the metal. These features scatter internally-reflected light, increasing the opportunities for light extraction. A reflective layer may be applied to the bottom surface of the LED to further improve light-extraction efficiency.
REFERENCES:
patent: 5909459 (1999-06-01), Ishibashi et al.
patent: 5917202 (1999-06-01), Haitz et al.
patent: 5994723 (1999-11-01), Sekii et al.
patent: 6107644 (2000-08-01), Shakuda et al.
Lumileds Lighting, US, LLC
Ogonowsky Brian D.
Pham Long
Skjerven Morrill & MacPherson LLP
Sung Jenny K.
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