Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With reflector – opaque mask – or optical element integral...
Reexamination Certificate
2000-07-20
2002-08-06
Wilson, Allan R. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With reflector, opaque mask, or optical element integral...
C257S432000, C257S436000, C372S043010
Reexamination Certificate
active
06429463
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of art for producing semiconductor electronic and optoelectronic devices, especially light-emitting diodes (LEDs), laser diodes, solar cells, photodiodes, photodetectors, and integrated circuits.
BACKGROUND OF THE INVENTION
The utility of many semiconductor optoelectronic devices is based on their efficiency in converting input electrical power into output optical power, as for example, in light-emitting diodes (LEDs) or laser diodes; or conversely, on their efficiency in converting input optical power into output electrical power, as for example, in photovoltaic solar cells or photodiodes. This invention relates both to 1. semiconductor device designs in which these conversions can be effected more efficiently, and 2. processes for fabricating such devices.
Light-Emitting Diodes
Light-emitting diodes (LEDs) are semiconductor devices that convert electrical power into optical power. They are used for, among other things, displays, indicator lights, and fiber optic light sources. The operation of a light emitting diode is based on physical phenomena that are well understood and quantitatively characterized.
In LEDs, light of a specified wavelength range is generated by radiative processes in which an excess (non-equilibrium) number of minority charge carriers (i.e., electrons in material of p-type conductivity; or holes in material of n-type conductivity) combine with majority charge carriers (i.e., holes in material of p-type conductivity; or electrons in material of n-type conductivity), emitting photons with a characteristic energy close to that of the bandgap of the material. This radiative phenomena is especially efficient in certain semiconductors, and in particular in III-V compound semiconductors such as gallium arsenide (GaAs), indium phosphide (InP), gallium nitride (GaN), their related alloys such as aluminum gallium arsenide (AlGaAs), aluminum gallium indium phosphide (AlGaInP) and indium gallium nitride (InGaN), in II-VI compound semiconductors such as zinc selenide (ZnSe), zinc sulfide (ZnS) and related materials, and less efficiently in other semiconductors such as silicon carbide (SiC). The emission spectrum or color of the generated light is determined to a large degree by the bandgap of the semiconductors and to a lesser degree by impurity doping.
One of the most efficient means to produce an excess concentration of minority carriers in a semiconductor utilizes the mechanism of minority carrier injection. By applying a voltage to a junction formed between two judiciously chosen materials, at least one of which is a semiconductor, an excess concentration of minority carriers in an excited energy state can be created in the semiconductor. These injected excess minority carriers combine with majority carriers (which are present due to impurity doping of the semiconductor). A fraction of the recombination phenomena is radiative in that a photon of light is generated for some fraction of recombination events.
The most common device for the purpose of injecting minority carriers is the p-n junction diode which designates the structure formed between a material which exhibits p-type conductivity (conduction primarily by positive charge carriers, i.e., holes) and a material which exhibits n-type conductivity (conduction primarily by negative charge carriers, i.e., electrons). Other structures for injecting minority carriers are metal-semiconductor junctions (Schottky barriers), and metal-insulator-semiconductor (MIS) junctions. Although Schottky or MIS junctions are not as efficient for minority carrier injection as p-n junctions, they are sometimes used for LEDs in cases where the choice of semiconductor material makes it difficult or impossible to form a p-n junction
Some of the minority carriers so injected into a luminescent semiconductor material such as GaAs combine with majority carriers (which are present at a relatively high concentration due to impurity doping) through various radiative (i.e., luminescent or light-emitting) processes. Other minority carriers combine with majority carriers by non-radiative processes and such processes represent a loss in LED efficiency.
The light generated by the radiative processes is isotropic in the sense that there is no preferred direction of the luminescence. Because of the high refractive index of most semiconductors, only a relatively small fraction (5 to 10%) of the generated light can escape through the front surface. Most of the light which is not at near-normal incidence to the front surface is internally reflected from the front surface. It is only the fraction of generated light that is transmitted through the front surface that constitutes the useful optical output of the LED. The remaining light is confined and undergoes multiple internal reflections and is mostly re-absorbed in the epitaxial layers, in the substrate, or at metal contacts. This re-absorption of luminescence is a major loss factor in LEDs.
To assess the importance of various loss mechanisms, it is instructive to view the total or overall efficiency
TOT
of an LED as the product of three efficiency components:
TOT
=
INJ RAD C
The injection efficiency is the fraction of electrical current which constitutes the injected minority carriers. In hetero-junction LEDs, the injection efficiency can be close to 1. The radiative efficiency is the fraction of injected minority carriers which undergo radiative combination with emission of a photon. In well-developed LED materials, such as GaAs, the radiative efficiency can also be close to 1. In conventional LEDs, the coupling efficiency is typically on the order of 0.01 to 0.1. As described previously, this is a consequence of the isotropic nature of the luminescence and the high refractive index of semiconductors, such that only a small fraction of the luminescence escapes through the front surface of the LED. It is obvious that any further significant improvements in LED efficiency must come by way of improvements in the coupling efficiency.
Approaches to reduce the coupling losses include structuring, “lensing” or texturing the front emitting surface of the LED. In practice, these techniques are too complicated for low-cost LED production or do not sufficiently improve over-all LED efficiency to justify their use in commercial LEDs.
In its application to LEDs, the invention relates primarily to improvements in coupling efficiency. It is an object of the invention to incorporate features in the LED structure, which result in dramatic enhancement of the coupling efficiency. In particular, a reflecting layer sandwiched between the luminescent semiconductor layers and the supporting semiconductor substrate and which we term a “buried mirror” is used to improve the LED coupling efficiency. The buried mirror reduces absorption losses and enhances so-called photon recycling effects. Photon recycling refers to phenomena whereby luminescent photons are absorbed in the semiconductor material, thus generating new minority carriers which in turn recombine radiatively to generate photons. These additional photons produced by photon recycling effects can make a significant contribution to the optical power output of the LED. The buried mirror design effectively enhances and exploits recycling phenomena in LEDs as described by SCHNITZER et al. in
Applied Physics Letters
62, 2 1993.
In the buried mirror LED design, the thickness of the epitaxial device layers can be adjusted to form a resonant optical cavity. Such a resonant cavity can be used to provide narrower emission angles, narrower emission spectral widths, increase quantum efficiencies, and increase modulation speeds.
Solar Cells
A solar cell is a device for converting light into electrical power. The invention can be applied to solar cells which are comprised of epitaxial layers as is common with GaAs-based solar cells, or less commonly, with silicon solar cells. The use of a buried mirror can improve the performance of a solar cell in several ways. The absorption of light in the solar cells
Astropower, Inc.
Connolly Bove & Lodge & Hutz LLP
Wilson Allan R.
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