The present invention relates to an apparatus having a light emitting diode comprising a structure having a Group III-nitride, and a method of making the apparatus.
It is desirable to improve the efficiency of light-emitting crystal structures, such as light-emitting diodes (LEDs), because this would increase their scope of use in commercial applications. Efficiency can be improved in two ways: increase the external efficiency or increase the internal efficiency.
An improvement in external efficiency is achieved by extracting more light out of the structure. As well known by those skilled in the art light-emitting crystal structures have a critical angle where light reflected beyond that angle gets reflected internally and does not exit the structure. E.g., only about 5 percent of the light generated in conventional planar LED passes out of the LED, the rest being internally reflected. Efforts to extract more light include texturing planar LEDs to reduce the amount of internally reflected light.
One embodiment is an apparatus comprising a structure comprising a group III-nitride and a junction between n-type and p-type group III-nitride therein, the structure having a pyramidal shape or a wedge shape.
Another embodiment is an apparatus comprising a light-emitting crystalline structure on a substrate. The structure has n-type and p-type barrier regions and a junction there between. The junction is located at one or more surfaces of the n-type and the p-type barrier regions that are inclined relative to a planar surface of the substrate.
Another embodiment is a method manufacturing an apparatus. The method comprises forming a light-emitting crystalline structure that includes forming a first barrier region on a substrate, the first barrier region having one or more inclined surfaces relative to a planar surface of the substrate. Forming the structure also includes forming a second barrier region over the first barrier region, to form a junction at the inclined surfaces. The first barrier region comprises one of an n-type or p-type semiconductor crystal, and the second barrier region comprises the other of the n-type or p-type semiconductor crystal.
The invention is best understood from the following detailed description, when read with the accompanying FIGUREs. Various features may not be drawn to scale and may be arbitrarily increased or reduced in size for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
The present invention benefits from the recognition that both the intrinsic and extrinsic efficiency can be improved by forming light-emitting crystalline structures having an inclined surface. An improvement in internal efficiency can be achieved by altering the properties of the crystal material itself. Certain crystals, however, have no inversion symmetry along certain crystal axes, which causes the crystal to have an internal electric field. The internal electrical field detrimentally reduces the internal efficiency and can shift the wavelength of light emitted from such structures. Forming light emitting components of the structure on an inclined surface can render the structure a semi-polar or non-polar crystal, thereby decreasing or eliminating the internal electric field of the structure. Additionally, forming the structure on an inclined surface reduces the amount of light that gets internally reflected, thereby improving external efficiency.
One embodiment of the invention is an apparatus.
The inclined surface 125 refers to a grown or wet etch-revealed surface of one of the n-type and p-type barrier regions 110, 115 that deviates from the horizontal planar surface 127 of the substrate 130 that the structure 105 is located on. E.g., for n-type and p-type barrier regions 110, 115 that comprise a group III-nitride, the horizontal plane 127 can correspond to a (0001) or a (000
The term group III-nitride as used herein refers to a metal nitride or metal alloy nitride, where the metal comprises one or more atoms from Group III of the Periodic Table of Elements. Examples include aluminum nitride, gallium nitride, indium nitride, or combinations thereof. In some cases, the n-type and p-type barrier regions 110, 115 include dopants to form an n-type and p-type material. Examples of suitable n-type and p-type dopants include silicon and magnesium, respectively.
The examples to follow feature Wurtzite crystal structures comprising group III-nitrides. However, one skilled in the art would appreciate that the invention could be applied to any light emitting crystal structure having non-inversion symmetry. Examples of other light-emitting crystal structures having non-inversion symmetry include Group II-VI compounds, such as Zinc Oxide (ZnO), Magnesium Zinc Oxide (MgZnO), Cadmium Zinc Oxide (CdZnO) and combinations thereof.
As illustrated in
It is advantageous to form structures 105 that provide one or more inclined surface 125 having the desired angle 135. In some preferred embodiments, the apparatus 100 can comprise a structure 105 comprising group III-nitrides and a junction 120 between n-type and p-type group III-nitrides therein 110, 115, the structure 105 having a pyramidal shape or a wedge shape.
In some embodiments, such as shown in
In other embodiments, such as shown in
It is desirable for the structures 105 to have a plurality of inclined surfaces 125 because this increases the external efficiency of the structure 105. For instance, having a plurality of inclined surfaces 125 corresponding to the six facets 140 of the hexagonal pyramid structure 105 (
As further illustrated in
All three of the n-type region 110, the p-type region 115, and the quantum well 305 are pyramid-shaped when the structure 105 is pyramid-shaped. Alternately all three of the n-type region 110, the p-type region 115, and the quantum well 305 are wedge-shaped when the structure 105 is wedge-shaped. Having such configurations beneficially improves the internal light emission efficiency because it provides the inclined surface 125 needed to reduce the structure's 105 internal electric field. These configurations also advantageously improve the external light emission efficiency of the structure 105. That is, there will be less internally reflected light from a quantum well 305 located on the inclined surface 125 compared to light from a quantum well of similar composition, but located on a planar surface.
The composition of the group III-nitride of the quantum well 305 is different than the compositions of the group III-nitrides of n-type and p-type barrier regions 110, 115. It is important to select the compositions of the quantum well 305 and the n-type and p-type barrier regions 110, 115 so as to configure the n-type and p-type barrier regions 110, 115 to have a larger band gap than the quantum well 305. Group III-nitrides having aluminum (e.g. AlxGa1-xN) will cause these materials to have a wider band gap, while a group III-nitride having indium (e.g. InyGa1-yN) causes these materials to have a narrower band gap.
For example, in some preferred embodiments, the n-type and p-type barrier regions 110, 115 comprises gallium nitride, while the quantum well 305 comprises an alloy of indium and gallium (e.g., indium gallium nitride). In other embodiments, the n-type and p-type barrier regions 110, 115 comprise an aluminum gallium alloy (e.g., aluminum gallium nitride) and the quantum well 305 comprises gallium nitride. In still other embodiments, n-type and p-type barrier regions 110, 115 comprise an aluminum-rich aluminum gallium alloy (e.g., AlGaN having a ratio of Al:Ga:N of about 80:20:100) and the quantum well 305 comprises an aluminum-poor aluminum gallium alloy (e.g., AlGaN having a ratio of Al:Ga:N of about 60:40:100 AlGaN). In still other cases, the n-type and p-type barrier regions 110, 115 comprise indium aluminum nitride or indium gallium aluminum nitride, and the quantum well 305 comprises indium gallium nitride. One skilled in the art would appreciate that other combinations of group III-nitrides alloys could be used.
As further illustrated in
Each quantum well layer 310 is preferably interposed between barrier layers 315. In cases where there is more than one quantum well layer 310, the quantum well layers 310 are separated from one another by barrier layers with a larger bandgap than the quantum well layers. In some preferred embodiments, each barrier layer 315 has a thickness 320 of about 10 to 50 Angstroms, and each quantum well layer 310 has a thickness 325 of about 5 to 50 Angstroms. In some preferred embodiments the quantum well 305 comprises 1 to 8 quantum well layers 310. Having multiple quantum well layers 310 beneficially increases the probability of carrier capture into the quantum well 305. However, if there is too large a number of quantum well layers 310, then carriers may not be distributed evenly through the different layers 310.
The quantum well region 310 and barrier layers 315 can comprise any of the combinations of the material described above for the quantum well 305 and the n-type and p-type regions 110, 115, respectively. It is preferable for the compositions of the quantum well region 310 and barrier layers 315 to be configured so that the quantum well region 310 has a narrow band gap and the barrier layers 315 has a wider band gap.
The quantum well layer 310 can comprise one type of group III-nitride, while the barrier layers 315 can comprise another type of group III-nitride. E.g., the quantum well layer 310 can comprise InGaN, while the barrier layers 315 comprises GaN. In some preferred embodiments, the quantum well layer 310 comprises InGaN having a ratio of In:Ga:N ranging from about 15:85:100 to 20:80:100, and even more preferably, about 17:83:100. This composition is advantageous because the internal electric fields of the quantum well layers 310 are substantially reduced when located on an inclined surface 120 having an angle 135 of e.g., about 55 to 65° with respect to the substrate 130. Such angles 135 are attained for the facets 140, 210 of pyramidal (
In some embodiments, the quantum well 305 has a thickness 330 ranging from about 2.5 to 5 nanometers. In cases where the quantum well comprises a plurality of quantum well layers 310 and barrier layers 315, the thickness 330 refers to the sum of the thicknesses of these layers 310, 315. In some cases, it is desirable for the variation in the thickness 330 of the quantum well 305 (or its component layers 310, 315) throughout the entire structure 105 to be about ±5% or less. This low thickness variation is desirable because it minimizes the range of wavelengths of light emitted from the structure 105. Thin-film deposition techniques such as molecular beam epitaxy (MBE) can be used, e.g., to fabricate such a low thickness variation layers of quantum well 305. In other cases, however, where a broader range of emitted wavelengths is desired, other techniques, such CVD can be used to produce quantum wells 305 whose thickness 330 varies by more than ±5%. E.g., the thickness 330 within any one structure 105 can range from about 2.5 to 5 nanometers.
In the above discussion of the examples shown in
In some embodiments, the outer layer 340, comprising either one of an n-type or p-type group III-nitride that has a thickness 355 ranging from about 50 to 500 nanometers.
As further illustrated in
As shown in
The pyramid 360 formed by a partial wet etch of the base 365. Wet etch processes can be designed to remove material from a specific surface of crystal structures to reveal the pyramid 360. Examples of such wet-etch processes are presented in U.S. Pat. No. 6,986,693 to Chowdhury et al., which is incorporated by reference herein in its totality. For example, the nitrogen-polar (N-polar) surface of a Group III-nitrides crystal is more susceptible to a base wet etch than a metal-polar (M-polar) surface. One skilled in the art would understand that N-polar surface refers to a face of a Group III-nitride Wurtzite structure having a straight bond (in a tetragonal bonding configuration) from a Nitrogen atom to a Group III metal atom. An M-polar surface refers to a face having the straight bond from a Group III metal atom to the Nitrogen atom. The base wet etch etches the {1
As further illustrated in
Another aspect of the invention is a method of manufacturing an apparatus.
Preferred embodiments of the layer of first barrier region 605 comprise a grown M-polar surface 610 and a grown N-polar surface 615. That is, the layer of first barrier region 605 grown on the barrier region seed layer 407 has the M-polar surface 610, while the layer of first barrier region 605 grown on the substrate 410 that is exposed in the opening 430 has the N-polar surface 615. It is advantageous for the first barrier region 605 to have both the M-polar and N-polar surfaces 610, 615 because this allows one to predefine the location on the substrate 410 where the inclined surface of the first barrier region 605 will be formed.
For instance, as shown in
As further illustrated in
Similarly, the second contact 1205 does not need to cover the entire second barrier region 805 formed on each inclined surface 705 of the structure. When the second barrier region 805 forms a uniform coating over interconnected pyramids 710 and the base 730 of the first barrier region 605, then a second contact 1205 touching any portion of the second barrier region 805 is also in electrical contact with the inclined surfaces 705 and the junction 810 of the structure 405. Having the structure 405 comprise a uniform coating of second barrier region 805 over a plurality interconnected pyramids 710 advantageously allows one more flexibility as to the placement of the second contact 1205. This avoids the need to align the second contact 1205 with a specific location on each pyramid 710, which can problematic because the exact location of where a pyramid 710 will form by the wet etch process can be unpredictable.
Although the embodiments have been described in detail, those of ordinary skill in the art should understand that they could make various changes, substitutions and alterations herein without departing from the scope of the invention.
This Application is a Divisional of U.S. application Ser. No. 11/456,428 filed on Jul. 10, 2006, to Hock Min Ng, entitled “LIGHT-EMITTING CRYSTAL STRUCTURES,” currently allowed; commonly assigned with the present invention and incorporated herein by reference in it entirety.
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of DAAE 30-03-D-1013 awarded by the U.S. Army ARD (Picatinny Arsenal).
Number | Date | Country | |
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Parent | 11456428 | Jul 2006 | US |
Child | 12852877 | US |