WO2008144337A1

WO2008144337A1 - Light emitting diode based on multiple double-heterostructures (quantum wells) with rare earth doped active regions

Info

Publication number: WO2008144337A1
Application number: PCT/US2008/063597
Authority: WO
Inventors: Madis Raukas; Kailash C. Mishra; Joseph Laski
Original assignee: Osram Sylvania Inc
Current assignee: Osram Sylvania Inc
Priority date: 2007-05-16
Filing date: 2008-05-14
Publication date: 2008-11-27
Anticipated expiration: 2009-11-16

Abstract

A light-emitting diode having a light-emitting surface is described. The light-emitting diode comprises a series of quantum well layers wherein one or more of the quantum well layers is doped with one or more activator ions that emit light and the quantum well layers are arranged in a sequence whereby the light emission from a lower quantum well layer is not significantly absorbed by quantum well layers between the lower quantum well layer and the light-emitting surface.

Description

LIGHT EMITTING DIODE BASED ON MULTIPLE DOUBLE-HETEROSTRUCTURES (QUANTUM WELLS) WITH RARE EARTH DOPED ACTIVE REGIONS

Cross Reference to Related Appiication

[0001] This appiication claims the benefit of U.S. Provisional Appiication No.

60/938,209, filed 5/16/2007.

Technical Field

[0002] This invention pertains to solid state light sources based on GaN-AIGaN- InGaN alloys with rare earth doping. Gallium, aluminum and indium nitride alloys with terbium, europium and/or other rare earth ions have been previously proposed as active layers for phosphoriess white light emitting double heterostructure (DHS) LEDs (See, US Patent Publication No. 2005/0253162).

Background of the Invention

[0003] Luminescence from various activator ions doped into a nitride semiconductor alloy is based on energy transfer from host material (the alloy) to the activator ion (the rare earth). The efficiency of this energy transfer depends on several factors including the band gap (BG) energies of the host or band gap emission (BGE). It is possible to optimize the electronic structure and BGE energy of the host for one particular type of dopant ion by changing the alloy composition so that the luminescence efficiency of this host-activator combination is at its maximum. However, for generating better quality white light, more than one activator type for multiple color components is needed. It may be difficult to achieve optimum conditions for the energy transfer to multiple activators, it may prove equally hard to simultaneously incorporate notable concentrations of various activator ions in the same lattice for efficacious white light emission.

Summary of the invention

[0004] It is an object of the invention to obviate the disadvantages of the prior art. [0005] it is another object of the invention to provide a light-emitting diode having a novel multiple quantum well structure.

[0006] it is a further object of the invention to provide a white-light-emitting diode.

[0007] In accordance with these and other objects and advantages, there is provided a light-emitting diode having a light-emitting surface, the light-emitting diode comprising a plurality of quantum well layers wherein one or more of the quantum well layers is doped with one or more activator ions that emit light and the quantum well layers are arranged in a sequence whereby the light emission from a lower quantum wel! layer is not significantly absorbed by quantum well layers between the lower quantum well layer and the light emitting surface. Preferably, the quantum well layers are arranged in a sequence of increasing band gap energy and comprise an AIGaInN alloy.

[0008] In another aspect of the invention, the light-emitting diode includes a stack of quantum well layers comprising a first quantum well layer that emits a red light and is doped with Eu³⁺, a second quantum well layer that emits a yellow light and is doped with Dy³⁺, a third quantum well layer that emits a green light and is doped with Tb³⁺, and a fourth quantum well layer that emits a blue light, the quantum well layers being arranged in order according to their emission wavelength wherein the first quantum well layer is furthest from the light-emitting surface.

Brief Description of the Drawing

[0009] The Figure is illustrates a AIGaln-based multiple quantum well (MQW) structure with a combination of active regions emitting in red (Eu³⁺), yellow (Dy³⁺), green (Tb³⁺), and blue (undoped) spectral regions.

Detailed Description of the Invention

[0010] Generation of white light with a desired correlated color temperature (CCT) and color rendering index (CRl) is achievable in devices having multiple active layers (or thin quantum wells, QWs). Each layer is based on an alloy composition optimized for a particular host-activator combination which produces the desired color with maximum quantum efficiency. Such layers are stacked according to their emission wavelengths and the number and thickness of layers from each type is determined by the overall optimum conditions for desired optica! and electrical performance.

[0011] Active layers of DHSs (or quantum wells) in nitride-based LEDs doped with the rare earth ions of more than one type (for example, trivalent Tm+Tb+Eu or Dy+Eu, to name only two combinations) are expected to emit light that is perceived as white due to simultaneous luminescence from the host and dopants in various parts of the visible spectrum. Instead of relaxing across the BG as in pure semiconductors, the recombination of injected electrons and holes may involve the dopant ion with a probability determined by a number of factors. White light is formed with or without a significant contribution due to the host alloy itself (BGE, typically in the blue spectral range), depending on the choice and concentration of these activator ions. Each rare earth ion (in this case Pr, Eu, Tb, Dy, Er or Tm in a +3 state) exhibits multiple emission lines across the visible spectrum arising from 4f-4f electronic transitions. These transitions change very little from host to host and due to one or two dominant transitions generate a light of a particular color characteristic to that ion (e.g. red for Eu³⁺, green for Tb³⁺ or Er³⁺). At the same time, the transitions involving excited states of the outer orbitals of the rare earth (such as 5d) or host ions (as in charge transfer) may shift significantly in energy with various cation-ligand combinations of the lattice. This affects strongly the luminescence efficiency of each ion in that lattice.

[0012] The energy transfer from the host lattice to the dopant ion may proceed via more than one mechanism with the effectiveness of each channel depending on several factors. Some of them, like the position of host energy levels, the size of the BG and hence also the BGE energy relative to the positioning of ground and excited states of the dopant can be modified by varying the host alloy composition. This allows for the optimization of dopant luminescence efficiency in nitride alloys via changes in cation composition, for example by tuning the Ga/ln ratio in In_xGa₁^_xN. However, it is unlikely that rare earth ions emitting through very different excitation mechanisms (cf. Eu³⁺ and Tb³⁴) will achieve optimum efficiencies for the same alloy composition.

[0013] A DHS consists of an active layer sandwiched between the two cladding layers that confine the charge carriers to the recombination region due to a larger band gap (an alloy with different cation composition and no rare earth doping) than that of the active region. Band discontinuities much larger than kT help in concentrating the electrons and holes for effective recombination (quantum efficiency). The thickness of the active layer in a typical DHS is application optimized and ranges from ~100nm to ~1Q.00.nm while in a QW it is of several nanometers. QWs are typically used in multiples (MQWs). For the latter, the confinement layers control the flow of carriers between the adjacent wells. Their height and thickness, in turn, are dictated by achieving efficient, homogeneous charge carrier distribution. Higher barriers may be used for overall confinement of electrons and holes in the MQW region. Coupling or stacking the DHSs follows the same general rationale but since providing homogeneous current distribution and extracting of the light becomes increasingly difficult for thick layers, we describe the structure from here on as consisting of QWs.

[0014] In present case, each single QW would constitute a combination of the dopant rare earth ion and Al_xln_yGai_-x-_yN alloy with an optimized composition (x,y) to produce maximum desired luminescence efficiency of the desired color (or color combination with alloy emission).

[0015] As shown in the Figure, the QWs 5 in the LED 7 are stacked in the sequence of increasing band gap and emission energy so that the red emission (Eu³⁺) from the lowest QW in the stack is furthest from the topmost, light-emitting surface 2. (See, energy diagram to left of LED 7). This way, the re-absorption of light by the host alloys is largely avoided. The choice of activators and their concentrations, number of QWs of each type as well as all other parameters will be determined corresponding to the application and considering the maximum possible efficacy. In a preferred embodiment, the AIGaln-based MQW structure is constructed with active regions emitting in red (Eu³⁺), yellow (Dy³⁺), green (Tb³⁺), and blue (undoped) spectral regions and are stacked in order according to their emission wavelength with the red-emitting layer being furthest from the LED's light-emitting surface.

[0016] While there have been shown and described what are at present considered to be preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.

Claims

Claims:We claim:

1. A light-emitting diode having a ϋght-emitting surface, the Sight-emitting diode comprising a plurality of quantum well layers wherein one or more of the quantum well layers is doped with one or more activator ions that emit light and the quantum well layers are arranged in a sequence whereby the light emission from a lower quantum well layer is not significantly absorbed by quantum well layers between the lower quantum well layer and the light emitting surface.

2. The light-emitting diode of claim 1 wherein the quantum well layers are arranged in a sequence of increasing band gap energy.

3. The light-emitting diode of claim 1 wherein the quantum well layers are comprised of an AIGaInN alloy.

4. The light-emitting diode of claim 1 wherein each quantum well layer that is doped with an activator ion contains a different type of activator ion than the other quantum well layer(s) that are doped with activator ion(s).

5. The light-emitting diode of claim 1 wherein the quantum well layers comprise at least one red-emitting, one yellow-emitting, one green-emitting and one blue-emitting quantum layer.

6. The light-emitting diode of claim 5 wherein the red-, yellow-, green- and blue- emitting quantum wells are stacked in order according to their emission wavelength with the red-emitting quantum layer being furthest from the light-emitting surface.

7. A light-emitting diode having a light-emitting surface, the light-emitting diode including a stack of quantum well layers comprising a first quantum well layer that emits a red light and is doped with Eu³⁺, a second quantum well layer that emits a yellow light and is doped with Dy³⁺, a third quantum well layer that emits a green light and is doped with Tb³⁺, and a fourth quantum well layer that emits a blue light, the quantum well layers being arranged in order according to their emission wavelength wherein the first quantum well layer is furthest from the light-emitting surface.

8. The light-emitting diode of claim 7 wherein the quantum well layers are comprised of an AIGaInN alloy.