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WO2015123566A1 - Monolithically integrated white light-emitting devices - Google Patents

Monolithically integrated white light-emitting devices

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Publication number
WO2015123566A1
WO2015123566A1 PCT/US2015/015894 US2015015894W WO2015123566A1 WO 2015123566 A1 WO2015123566 A1 WO 2015123566A1 US 2015015894 W US2015015894 W US 2015015894W WO 2015123566 A1 WO2015123566 A1 WO 2015123566A1
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WO
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Prior art keywords
nitride
light
device
ill
emitting
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PCT/US2015/015894
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French (fr)
Inventor
Shuji Nakamura
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The Regents Of The University Of California
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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L33/00Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/08Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a plurality of light emitting regions, e.g. laterally discontinuous light emitting layer or photoluminescent region integrated within the semiconductor body
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S5/00Semiconductor lasers
    • H01S5/40Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
    • H01S5/4025Array arrangements, e.g. constituted by discrete laser diodes or laser bar
    • H01S5/4031Edge-emitting structures
    • H01S5/4043Edge-emitting structures with vertically stacked active layers
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L33/00Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/44Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the coatings, e.g. passivation layer or anti-reflective coating
    • H01L33/46Reflective coating, e.g. dielectric Bragg reflector
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S5/00Semiconductor lasers
    • H01S5/02Structural details or components not essential to laser action
    • H01S5/026Monolithically integrated components, e.g. waveguides, monitoring photo-detectors, drivers
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S5/00Semiconductor lasers
    • H01S5/04Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
    • H01S5/041Optical pumping
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S5/00Semiconductor lasers
    • H01S5/10Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/1028Coupling to elements in the cavity, e.g. coupling to waveguides adjacent the active region, e.g. forward coupled [DFC] structures
    • H01S5/1032Coupling to elements comprising an optical axis that is not aligned with the optical axis of the active region
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S5/00Semiconductor lasers
    • H01S5/10Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/12Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feed-back lasers (DFB-lasers)
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S5/00Semiconductor lasers
    • H01S5/20Structure or shape of the semiconductor body to guide the optical wave; Confining structures perpendicular to the optical axis, e.g. index- or gain-guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • H01S5/2004Confining in the direction perpendicular to the layer structure
    • H01S5/2018Optical confinement, e.g. absorbing-, reflecting- or waveguide-layers
    • H01S5/2027Reflecting region or layer, parallel to the active layer, e.g. to modify propagation of the mode in the laser or to influence transverse modes
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S5/00Semiconductor lasers
    • H01S5/30Structure or shape of the active region; Materials used for the active region
    • H01S5/32Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures
    • H01S5/3202Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures grown on specifically orientated substrates, or using orientation dependent growth
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S5/00Semiconductor lasers
    • H01S5/30Structure or shape of the active region; Materials used for the active region
    • H01S5/32Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures
    • H01S5/3211Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures characterised by special cladding layers, e.g. details on band-discontinuities
    • H01S5/3214Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures characterised by special cladding layers, e.g. details on band-discontinuities comprising materials from other groups of the periodic system than the materials of the active layer, e.g. ZnSe claddings and GaAs active layer
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S5/00Semiconductor lasers
    • H01S5/30Structure or shape of the active region; Materials used for the active region
    • H01S5/32Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures
    • H01S5/323Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
    • H01S5/32308Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser emitting light at a wavelength less than 900 nm
    • H01S5/32341Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser emitting light at a wavelength less than 900 nm blue laser based on GaN or GaP
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01SDEVICES USING STIMULATED EMISSION
    • H01S5/00Semiconductor lasers
    • H01S5/40Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
    • H01S5/4025Array arrangements, e.g. constituted by discrete laser diodes or laser bar
    • H01S5/4087Array arrangements, e.g. constituted by discrete laser diodes or laser bar emitting more than one wavelength

Abstract

A monolithically integrated white light-emitting device, including light- emitting diodes (LEDs) and laser diodes (LDs), comprised of a Ill-nitride substrate and semipolar, nonpolar or polar Ill-nitride layers. A first device structure has a smaller band gap than a second device structure; the first device structure is optically- pumped by the second device structure; the second device structure is electrically injected; and emissions from the first and second device structures are mixed to generate white light. These emissions may be yellow light mixed with blue light, or green and red light mixed with blue light.

Description

MONOLITHICALLY INTEGRATED WHITE LIGHT-EMITTING DEVICES

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C Section 119(e) of the following co-pending and commonly-assigned application:

U.S. Provisional Application Serial No. 61/940,187, filed on February 14, 2014, by Shuji Nakamura, entitled "MONOLITHICALLY INTEGRATED WHITE LIGHT- EMITTING DEVICES," attorneys' docket number 30794.547-US-P1 (2014-546-1); which application is incorporated by reference herein.

This application is related to the following co-pending and commonly- assigned U.S. patent applications:

P.C.T. International Patent Application Serial No. PCT/US 14/72752, filed on December 30, 2014, by Robert M. Farrell, Shuji Nakamura, and Claude C. A.

Weisbuch, entitled "MONOLITHIC INTEGRATION OF OPTICALLY PUMPED AND ELECTRICALLY INJECTED III-NITRIDE LEDS," attorneys' docket number 30794.542-WO-U1 (2014-416-2), application claims the benefit under 35 U.S.C Section 119(e) of the following co-pending and commonly-assigned U.S. Provisional Patent Application Serial No. 61/921,829, filed on December 30, 2013, by Robert M. Farrell, Shuji Nakamura, and Claude C. A. Weisbuch, entitled "MONOLITHIC INTEGRATION OF OPTICALLY PUMPED AND ELECTRICALLY INJECTED III-NITRIDE LEDS," attorneys' docket number 30794.542-US-P1 (2014-416-1); both of which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

This invention is related to the monolithically integrated white light-emitting 2. Description of the Related Art.

(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets [Ref. x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled "References." Each of these publications is incorporated by reference herein.)

This invention describes a structure for improving the performance of III- nitride light-emitting devices. The term "Ill-nitrides" refers to any alloy composition of the (Ga,Al,In,B)N semiconductors having the formula GawAlxInyBzN where 0 < w < 1, 0 < x < l, 0 < y < 1, and 0 < z < 1.

The usefulness of Ill-nitrides has been well established for the fabrication of visible and ultraviolet optoelectronic devices and high power electronic devices. Current state-of-the-art Ill-nitride thin films, heterostructures, and devices are grown along the polar [0001] c-axis. The total polarization of such films consists of spontaneous and piezoelectric polarization contributions, both of which originate from the single polar [0001] c-axis of the wurtzite Ill-nitride crystal structure. When Ill-nitride heterostructures are grown pseudomorphically, polarization discontinuities are formed at surfaces and interfaces within the crystal. These discontinuities lead to the accumulation or depletion of carriers at surfaces and interfaces, which in turn produce electric fields. Since the alignment of these polarization-induced electric fields coincides with the typical polar [0001] c-plane growth direction of III -nitride thin films and heterostructures, these fields have the effect of "tilting" the energy bands of Ill-nitride devices.

In c-plane wurtzite Ill-nitride quantum wells, the "tilted" energy bands spatially separate the electron and hole wavefunctions. This spatial charge separation reduces the oscillator strength of radiative transitions and red-shifts the emission wavelength. These effects are manifestations of the quantum confined Stark effect (QCSE) and have been thoroughly analyzed for Ill-nitride quantum wells (QWs). [Refs. 4-7] Additionally, the large polarization-induced electric fields can be partially screened by injected carriers, [Ref. 8] making the emission characteristics difficult to engineer accurately.

One approach to decreasing polarization effects in Ill-nitride devices is to grow the devices on nonpolar planes of the crystal. These include the {11-20} planes, known collectively as a-planes, and the { 10-10} planes, known collectively as m- planes. Such planes contain equal numbers of Gallium (Ga) and Nitrogen (N) atoms per plane and are charge-neutral. Subsequent nonpolar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction.

Another approach to reducing polarization effects in Ill-nitride devices is to grow the devices on semipolar planes of the crystal. The term "semipolar plane" can be used to refer to any plane that cannot be classified as c-plane, a-plane, or m-plane. In crystallographic terms, a semipolar plane would be any plane that has at least two nonzero h, i, or k Miller indices and a nonzero 1 Miller index. Subsequent semipolar layers are equivalent to one another, so the bulk crystal will have reduced polarization along the growth direction.

Despite these inherent advantages, challenges still remain for nonpolar and semipolar device growth. For example, when green, yellow or red Ill-nitride light emitting diodes (LEDs) and laser diodes (LDs) are grown with active regions with high Indium (In) contents, the active region can form extended defects and can easily be degraded by subsequent high temperature growth steps. [Refs. 1,2] In particular, the growth of subsequent p-type layers can be especially damaging, as these layers usually need to be grown at elevated growth temperatures to ensure adequate p-type conduction. [Ref. 3] In order to minimize the damage to green, yellow or red InGaN QWs, the growth temperature of the subsequent p-type layers were decreased, which caused the high operation voltage of the devices, caused by the poor crystal quality and poor activation of Magnesium (Mg) acceptors of p-type layers.

Thus, there is a need in the art for improved methods of fabricating Ill-nitride LEDs and LDs. The present invention satisfies this need. SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a monolithically integrated white light-emitting device, including LEDs and LDs, comprised of one more Ill-nitride layers grown on or above one or more surfaces of a substrate, wherein the substrate is a Ill-nitride substrate and the Ill-nitride layers are semipolar, nonpolar or polar III- nitride layers.

In this opto-electronic device, a first Ill-nitride light-emitting device structure has a smaller band gap than a second Ill-nitride light-emitting device structure; the first Ill-nitride light-emitting device structure is optically pumped by the second III- nitride light-emitting device structure; the second Ill-nitride light-emitting device structure is electrically injected; and emissions from the first Ill-nitride light-emitting device structure are mixed with emissions from the second Ill-nitride light-emitting device structure to generate white light.

In this opto-electronic device, an active region of the first Ill-nitride light- emitting device structure is clad on both sides only by undoped or n-type III -nitride layers and is not clad by p-type Ill-nitride layers, and an active region of the second III -nitride light-emitting device is clad on opposite sides by n-type and p-type III- nitride layers.

In one or more embodiments, the emissions from the first Ill-nitride light- emitting device structure are yellow light, and the emissions from the second III- nitride light-emitting device structure are blue light. In other embodiments, the emissions from the first III -nitride light-emitting device structure are green and red light, and the emissions from the second III -nitride light-emitting device structure are blue light. In all embodiments, the mixing of emissions from the first Ill-nitride light- emitting device structure with emissions from the second Ill-nitride light-emitting device structure results in white light. BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers represent corresponding parts throughout:

FIG. 1 is a cross-sectional schematic of a monolithically integrated two-laser design where the LDs are grown on opposite sides of the substrate.

FIG. 2 is a cross-sectional schematic of a monolithically integrated LD and LED where the LD and LED are grown on opposite sides of the substrate and white emission appears from the LED's side.

FIG. 3 is a cross-sectional schematic of a monolithically integrated LD and LEDs where the LD and LEDs are grown on opposite sides of the substrate and white emission appears from the LEDs' side.

FIG. 4 is a cross-sectional schematic of a monolithically integrated LEDs where the blue LED and the green and red LEDs are grown on opposite sides of the substrate and white emission appears from the green and red LEDs' side.

FIG. 5 is a cross-sectional schematic of a monolithically integrated two-laser design where both LDs are grown on the same side of the substrate.

FIG. 6 is a cross-sectional schematic of a monolithically integrated two-laser design where both LDs are grown on the same side of the substrate, and a grating and an indium-tin-oxide (ITO) cladding are used to improve the overlap between the blue and yellow LDs' active regions and waveguides.

FIG. 7 is a cross-sectional schematic of a monolithically integrated LD and LED design where both the LD and LED are grown on the same side of the substrate, and a grating and ITO cladding are used to scatter the blue LD's emission to the yellow LED's active region for the excitement of yellow LED's active region to generate a white emission.

FIG. 8 is a cross-sectional schematic of a monolithically integrated LD and LEDs design where both the LD and LEDs are grown on the same side of the substrate, and a grating and ITO cladding are used to scatter the blue LD's emission to the green and red LEDs' active regions for the excitement of the green and red LEDs' active regions to generate a white emission.

FIG. 9 is a flowchart illustrating the process steps for a method of fabricating an opto-electronic device according to the embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference is made to a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

Overview

The motivation underlying the present invention is that nonpolar and semipolar (In)GaN LEDs and LDs have been theorized to have higher gain and higher radiative efficiency than polar c-plane (In)GaN devices, due to valence band splitting and lower polarization fields. Nonetheless, nonpolar and semipolar Ill-nitride LEDs and LDs are limited by strain and hole mobility/injection efficiency.

Optical pumping of nonpolar and semipolar Ill-nitride LEDs and LDs can address these problems. Monolithically integrating a blue light emitting Ill-nitride LEDs and LDs to optically pump yellow or green and red light emitting Ill-nitride LEDs and LDs on the same chip can thus lead to a white light emitting device resulting from the mixture of the blue light with either the yellow light or the green and red light.

The benefits of optical pumping include the following:

· No use of Mg in the green, yellow and red light emitting III -nitride

LEDs and LDs, wherein Mg is used for p-type doping of GaN, which is not present in the green, yellow and red light emitting III -nitride LEDs and LDs, and is a dominant source of loss in III -nitride LEDs and LDs [Ref. 14], No higher temperature growth of p-type GaN layers immediately after the growth of the active region of the green, yellow and red light emitting Ill-nitride LEDs and LDs, which eliminates thermal damage [Ref. 15],

Wider design space, because carrier transport not an issue,

Wide barriers to control strain [Ref. 16], and

Many periods for increased confinement [Ref. 17].

The structures and functions that provide these benefits are described in more detail below.

Technical Disclosure

As noted above, when green, yellow or red Ill-nitride LEDs and LDs are grown with active regions with high In contents, the active region can form extended defects and can easily be degraded by subsequent high temperature growth steps. [Refs. 1 ,2] In particular, the growth of subsequent p-type layers can be especially damaging, as these layers usually need to be grown at elevated growth temperatures to ensure adequate p-type conduction. [Ref. 3]

One way to avoid high temperature growth steps after the active region for green, yellow or red Ill-nitride LEDs and LDs is to design a structure that does not involve the growth of p-type layers after the active region, which is typically comprised of InGaN QWs. Since electrical injection in such a structure cannot be possible, a second Ill-nitride LED or LD is needed to optically pump the green, yellow and/or red Ill-nitride LED or LD. This second Ill-nitride LED or LD needs to emit light at a shorter wavelength region of the optical spectrum (e.g., in the blue or violet region), so that it can be absorbed by the active region of the green, yellow and/or red III -nitride LED or LD.

For the sake of simplicity, the longer wavelength (smaller band gap)

"pumped" LEDs and LDs are referred to herein as a first Ill-nitride device structure and may comprise one or more "green, yellow or red" LEDs and LDs, although they may emit at a wavelength other than green, yellow or red. Note that the emitting regions (active regions) of the optically-pumped smaller band gap Ill-nitride light- emitting device structure may include at least nanopoles, nanopillars, quantum wires, quantum dots and/or quantum wells.

Likewise, the shorter wavelength (larger band gap) "pumping" LEDs and LDs are referred herein to as a second Ill-nitride device structure and may comprise one or more "blue" LEDs and LDs, although they may emit at a wavelength other than blue, such as ultraviolet (UV) or violet. Note that the emitting region (active region) of the electrically-injected larger band gap III -nitride light-emitting device structure may comprise LEDs, LDs, and/or super luminescent LEDs.

Moreover, the use of first and second do not imply an order in fabrication, and the first or green, yellow or red LEDs and LDs may be fabricated before or after the second or blue LEDs and LDs.

The following discussion presents a number of examples for illustrating the concept of monolithic integration of optically-pumped and electrically-injected white Ill-nitride light-emitting devices. These examples involve blue light emitting LDs or LEDs, and green, yellow or red light emitting LDs or LEDs, but the concepts can easily be extended to LEDs and LDs emitting other colors, as well as light-emitting devices of any other emission wavelength, to make white emitting devices. Thus, these examples are not meant to limit the scope of this invention to LEDs and LDs or particular emission wavelengths, but rather to illustrate the essential ideas of the invention to make a white emission.

A critical component of this scheme is that the light emitted by the blue pumping LD or LED needs to be properly coupled to the green, yellow or red pumped LD or LED to ensure efficient power transfer to make a white emission. This involves limiting unwanted light scattering, limiting unwanted absorption, and maximizing light absorption by the active region of the green, yellow or red LD or LED. FIG. 1 shows a cross-sectional schematic of one of the preferred

embodiments, which involves a monolithically integrated two-laser device 100 design, where the LDs are grown on opposite sides of a substrate 102. Specifically, the device 100 structure is based on a {20-21 } free-standing GaN substrate 102. The layers for the electrically-injected blue LD are grown on a (20-2-1) top surface 104 of the substrate 102 and comprise n-type III -nitride layers 106, a blue LD active region and waveguide 108, p-type Ill-nitride layers 110, an ITO layer 112, a grating structure 114, and a mirror 116. The layers for the optically-pumped yellow LD are grown on a (20-21) bottom surface 118 of the substrate 100 and comprise n-type Ill-nitride layers 120, a green LD active region and waveguide 122, n-type III -nitride layers 124, a grating structure 126, and a mirror 128. An n-type contact by metal or oxide is formed on an n-type layer and a p-type contact by metal or oxide is formed on a p- type layer, but these contacts are not shown in this view. When a forward bias is applied, the device 100 emits white light 132 from the side.

In fabricating the device 100, the electrically-injected blue LD is first grown on the (20-2-1) side of the double-side-polished {20-21 } substrate 102. This orientation has already been used to demonstrate blue LDs with an output power of 2.15 W, a differential efficiency of 49%, and a peak external quantum efficiency (EQE) of 39% [Ref. 10], indicating that it is an appropriate orientation for producing high efficiency pump sources.

Next, the optically-pumped yellow LD is grown on the (20-21) side of the double-side -polished {20-21 } substrate 102. This orientation has already been used to demonstrate green LDs with an output power greater than 100 mW, wall plug efficiencies of 7-9%, and lifetimes greater than 5000 hours [Refs. 11,12], indicating that it is an appropriate orientation for producing high performance yellow LDs.

The yellow LD 122 is clad by n-type layers 120, 124 on both sides of the LD 122, lowering the temperature of any growth steps that follow the growth of the active region 122 compared to a typical electrically-injected LD. This should help prevent the formation of extended defects in the active region 122 during subsequent growth steps and should significantly improve the performance of the yellow LD 122. [Refs. 1,2]

By mixing the blue laser emission and the yellow laser emission, white laser emission 132 is obtained from the side wall (or the facets). In order to mix both emissions effectively to make a white emission, additional mixing tools are needed. However, no phosphor is used for this white LD and therefore reliability is determined by the Ill-nitride layers, not by phosphors, resulting in much better expected reliability.

Several schemes can be used to increase the coupling of the blue LD light into the yellow LD.

One such scheme is shown in FIG. 1, which, as noted above, involves a monolithically integrated two LDs design, where each LD is grown on opposite sides of the substrate 102. First, the blue LD 108 is grown and then the yellow LD 122 is grown on the opposite side of the substrate 102 to prevent damage to the yellow LD 122. After growing the LDs, a ridge waveguide is etched into the top surface of the blue LD, ITO is deposited on the p-type cladding layer as a part of the cladding layer and Ohmic contact layers, a grating 114 is etched into ITO 112, and a mirror 116 is deposited on top of the ITO 112.

On the other side of the side of the substrate 102, a ridge waveguide is etched into the top surface of the yellow LD 122, ITO 126 is deposited on the n-type cladding layer 124 as a part of the cladding layer, a grating 128 is etched into the ITO 126, and a mirror 130 is deposited on top of the ITO 126. The grating 114 etched into ITO 112 of the blue LD 108 is intended to diffract light that is guided by the waveguide of the blue LD 108 into light that is directed towards the yellow LD 122. Likewise, the mirror 116 that is on top of the blue LD 108 is intended to reflect light that is propagating away from the blue LD 108 toward the yellow LD 122.

On the other side of the substrate 102, the grating 128 etched into the ITO 126 of the yellow LD 122 is intended to diffract blue light that is directed away from the optical cavity of the blue LD 108 into light that is guided by the waveguide of the yellow LD 122. Similarly, a mirror 130 is deposited on top of the yellow LD 122 to maximize the absorption of light that is directed perpendicular to the cavity of the yellow LD 122. These efforts are all intended to maximize absorption of blue LD 108 emission by the yellow LD 122.

FIG. 2 shows a cross-sectional schematic of another preferred embodiment, which involves a monolithically integrated LD and LED device 200 design, where the LD and LED are grown on opposite sides of a substrate 202. Again, the device 100 structure is based on a {20-21 } free-standing GaN substrate 202. The layers for the electrically-injected blue LD are grown on a (20-2-1) top surface 204 of the substrate 202 and comprise n-type Ill-nitride layers 206, a blue LD active region and waveguide 208, p-type Ill-nitride layers 210, an ITO layer 212, a grating structure 214, and a mirror 216. The layers for the optically-pumped yellow LED are grown on a (20-21) bottom surface 218 of the substrate 200 and comprise n-type Ill-nitride layers 220, a yellow LED active region and waveguide 222, n-type Ill-nitride layers 224, and an n-contact 226. A p-type contact by metal or oxide is formed on a p-type layer, but this contact is not shown in this view. When a forward bias is applied, the device 200 emits white light 228 from the bottom.

In fabricating the device 200, the blue LD 208 is grown first and then the yellow LED 222 is grown on the opposite side of the substrate 202 to prevent damage to the yellow LED 222. After growing the blue LD 208 and yellow LED 222, a ridge waveguide is etched into the top surface of the blue LD 208, ITO 212 is deposited on the p-type cladding layer 210 as a part of the cladding layer and Ohmic contact layers, a grating 214 is etched into ITO 212, and a mirror 216 is deposited on top of the ITO 212. These efforts are all intended to maximize the blue LD 208 emission intensity towards the yellow LED 222.

On the other side of the substrate 202, one or more yellow emitting InGaN QWs are formed in the yellow LED active region 222, sandwiched by n-type GaN barriers. The number of QWs and barriers are determined by the intensity of yellow emission necessary in order to achieve a good white emission 228 color balance. In this structure, n-type GaN can be grown at a relatively lower temperatures in comparison with that of p-type GaN. Thus, the heat damage of the yellow InGaN QWs can be minimized during the subsequent growth of n-type GaN instead of p-type GaN.

The blue LD 208 emission effectively excites the yellow QWs and a strong white light emission 228 is observed from the yellow LED 222 side.

FIG. 3 shows a cross-sectional schematic of another preferred embodiment, which involves a monolithically integrated LD and LEDs device 300 design, where the LD and LEDs are grown on opposite sides of a substrate 302. Again, the device 300 structure is based on a {20-21 } free-standing GaN substrate 302. The layers for the electrically-injected blue LD are grown on a (20-2-1) top surface 304 of the substrate 302 and comprise n-type Ill-nitride layers 306, a blue LD active region and waveguide 308, p-type Ill-nitride layers 310, an ITO layer 312, a grating structure 314, and a mirror 316. The layers for the optically-pumped green and red LEDs are grown on a (20-21) bottom surface 318 of the substrate 302 and comprise n-type III- nitride layers 320, a green LED active region 322, n-type Ill-nitride layers 324, a red LED active region 326, n-type Ill-nitride layers 328, and an n-type contact 330. A p- type contact by metal or oxide is formed on a p-type layer, but this contact is not shown in this view. When a forward bias is applied, the device 300 emits white light 332 from the bottom.

In fabricating the device 300, the blue LD 308 is grown first and then the green and red LEDs 322, 326 are grown on the opposite side of the substrate 302 to prevent damage to the green and red LEDs 322, 326. After growing the blue LD 308 and the green and red LEDs 322, 326, a ridge waveguide is etched into the top surface of the blue LD 308, ITO 312 is deposited on the p-type cladding layer 310 as a part of the cladding layer and Ohmic contact layers, a grating 314 is etched into the ITO 312, and a mirror 316 is deposited on top of the ITO 312. These efforts are all intended to maximize the blue LD 308 emission intensity towards the green and red LEDs 322, 326. On the other side of the substrate 302, green and red emitting InGaN QWs are formed in the green and red LEDs 322, 326, sandwiched by n-type GaN barriers. The number of green and red QWs and the barriers for each are determined by the intensity of green and red emissions necessary in order to achieve a good white emission 332 color balance. In this structure, n-type GaN can be grown at a relatively lower temperature in comparison with that of p-type GaN. Thus, the heat damage of the green and red InGaN QWs can be minimized during the subsequent growth of n- type GaN without p-type GaN.

The blue LD 308 emission effectively excites the green and red LEDs 322, 326 and a strong white emission 332 is observed from the green and red LED 322, 326 side. The Color Rendering Index (CRI) should be higher because the white emission 332 is composed of three primary colors of blue, green and red emissions, wherein the blue emission directly comes from the blue LD 308.

FIG. 4 shows a cross-sectional schematic of another preferred embodiment, which involves a monolithically integrated LEDs device 400 design, where the blue LED and green/red LEDs are grown on opposite sides of a substrate 402. Again, the device 400 structure is based on a {20-21 } free-standing GaN substrate 402. The layers for the electrically-injected blue LD are grown on a (20-2-1) top surface 404 of the substrate 402 and comprise n-type Ill-nitride layers 406, a blue LD active region 408, p-type Ill-nitride layers 410, and a mirror 412. The layers for the optically- pumped green and red LEDs are grown on a (20-21) bottom surface 414 of the substrate 402 and comprise n-type Ill-nitride layers 416, a green LED active region 418, n-type Ill-nitride layers 420, a red LED active region 422, n-type Ill-nitride layers 424, and an n-type contact 426. A p-type contact by metal or oxide is formed on a p-type layer, but this contact is not shown in this view. When a forward bias is applied, the device 400 emits white light 428 from the bottom.

In fabricating the device 400, the blue LED 408 is grown first, and then the green and red LEDs 418, 422 are grown on the opposite side of the substrate 402 to prevent damage to the green and red LEDs 418, 422. After growing the green and red LEDs 418, 422, a mirror 412 which has a high reflectivity, such as silver (Ag) or other metals, is deposited on top of the p-type layer 410. These efforts are all intended to maximize the blue LD 408 emission intensity towards the green and red LEDs 418, 422.

On the other side of the substrate 402, green and red InGaN QWs are formed in the green and red LEDs 418, 422, which are sandwiched by n-type GaN barriers. The number of green and red QWs and barriers for each are determined by the intensity of green and red emissions necessary in order to achieve a good white color balance. In this structure, n-type GaN can be grown at a relatively lower temperature in comparison with that of p-type GaN. Thus, the heat damage of the green and red InGaN QWs can be minimized during the subsequent growth of n-type GaN without p-type GaN.

The blue LD 408 emission effectively excites the green and red LEDs 418, 422 and a strong white emission 428 is observed from the green and red LEDs 418, 422 side. The Color Rendering Index (CRI) should be higher because the white emission 428 is composed of three primary colors of blue, green and red emissions, wherein the blue emission directly comes from the blue LED 408.

FIG. 5 shows a cross-sectional schematic of another preferred embodiment, which involves a monolithically integrated two-laser device 500 design, where both LDs are grown on the same side of the substrate 502. [Ref. 9] Again, the device 500 structure is based on a {20-21 } free-standing GaN substrate 502. The layers for both the optically-pumped yellow LD and the electrically-injected blue LD are grown on one side of the substrate 502 and comprise n-type Ill-nitride layers 504, a yellow LD active region and waveguide 506, n-type Ill-nitride layers 508, a blue LD active region and waveguide 510, p-type Ill-nitride layers 512, and a p-type contact 514. An n-type contact 516 is formed on an opposite side of the substrate 502. When a forward bias is applied, the device 500 emits white light 518 from the side.

In fabricating the device 500, the optically-pumped yellow LD 506 is first grown on the {20-21 } substrate 502. The yellow LD 506 is clad by n-type layers 504, 508 on both sides of the yellow LD active region 506, which can stabilize the active region of the yellow LD 506 during the higher temperature growth of the blue LD 510. [Ref. 9] Next, the electrically-injected blue LD 510 is grown directly on top of the yellow LD 506. After growing the yellow and blue LDs 506, 510, a ridge waveguide is etched into the surface of the blue LD 510 and electrical contacts 514, 516 are made to the blue LD 510. Next, high-reflectivity (HR) coatings (not shown) are applied to one facet of the blue LD 510, HR coatings (not shown) are applied to one facet of the yellow LD 506, and anti-reflective (AR) coatings (not shown) are applied to the other facet of the blue and yellow LDs 510, 506.

As indicated by the transverse lateral mode profile 520, the device 500 is designed so that there is overlap between the transverse mode of the blue LD 510 and the QWs of the yellow LD 506, as represented by the shaded area of the transverse mode profile 520 over the yellow LD 506. If the overlap is too low, there will not be adequate power transfer between the blue and yellow LDs 510, 506, whereas if the overlap is too high the loss in the blue LD 510 may be too high and it may not lase. Thus, the device needs to be carefully designed to ensure the proper amount of power transfer between the blue and yellow LDs 510, 506 in order to obtain lasing from both blue and yellow LDs 510, 506 for the white emission 518 lasing by mixing of both emissions.

As a means to improve the overlap between the blue and yellow guided waves, ITO partial cladding, gratings and mirror can be used, as shown in FIG. 1. The ITO and grating should be optimized to increase the overlapping among two LDs, which is different from FIG. 1. In such a case, the optical modes of the blue laser waveguide are pushed away from the surface and can interact strongly with the yellow LD active region. This modification is shown in FIG. 6.

FIG. 6 shows a cross-sectional schematic of another embodiment, which involves a monolithically integrated two-laser device 600 design, where both LDs are grown on the same side of the substrate 602. [Ref. 9] Again, the device 600 structure is based on a {20-21 } free-standing GaN substrate 602. The layers for both the optically-pumped yellow LD and the electrically-injected blue LD are grown on one side of the substrate 602 and comprise n-type Ill-nitride layers 604, a yellow LD active region and waveguide 606, n-type Ill-nitride layers 608, a blue LD active region and waveguide 610, p-type Ill-nitride layers 612, an ITO layer 614, a grating structure 616, and a mirror 618. An n-type contact 620 is formed on an opposite side of the substrate 602. A p-type contact by metal or oxide is formed on a p-type layer, but this contact is not shown in this view. When a forward bias is applied, the device 600 emits white light 622 from the side.

As indicated by the transverse lateral mode profile 624, the device 600 is designed so that there is overlap between the transverse mode of the blue LD 610 and the QWs of the yellow LD 606, as represented by the shaded area of the transverse mode profile 624 over the yellow LD 606. The overlap is designed for adequate power transfer between the blue and yellow LDs 610, 606, but not an overlap that is too high and results in to great a loss in the blue LD 610.

As a means to improve the overlap between the blue and yellow guided modes, a surface photonic crystal with a high index contrast can also be used, such as one that contains air holes. In such a case, the optical modes of the blue LD waveguide 610 are pushed away from the surface and can interact strongly with the yellow LD active region 606.

FIG. 7 shows a cross-sectional schematic of another embodiment, which involves a monolithically integrated LD and LED device 700 design, where both the LD and LED are grown on the same side of the substrate 702. [Ref. 9] Again, the device 700 structure is based on a {20-21 } free-standing GaN substrate 702. The layers for both the optically-pumped yellow LED and the electrically -injected blue LD are grown on one side of the substrate 702 and comprise n-type Ill-nitride layers 704, a yellow LED active region 706, n-type Ill-nitride layers 708, a blue LD active region and waveguide 710, p-type Ill-nitride layers 712, an ITO layer 714, a grating structure 716, and a mirror 718. An n-type contact 720 is formed on an opposite side of the substrate 702. A p-type contact by metal or oxide is formed on a p-type layer, but this contact is not shown in this view. When a forward bias is applied, the device 700 emits white light 722 from the bottom.

In fabricating the device 700, the optically-pumped yellow LED 706 is first grown on the {20-21 } GaN substrate 702. An InGaN QW of the yellow LED 706 is clad by n-type layers 704, 708 on both sides of the yellow LED active region 706, which can stabilize the active region 706 comprised of the InGaN QWs during the higher temperature growth of the blue LD 710. [Ref. 9] Next, the electrically-injected blue LD 710 is grown directly on top of the yellow LED 706. After growing the blue LD 710 and yellow LED 706, a ridge waveguide is etched into the top surface of the blue LD 710, ITO 714 is deposited on the p-type cladding layer 712 as a part of the cladding layer and Ohmic contact layers, a grating 716 is etched into the ITO 714, a mirror 718 is deposited on top of the ITO 714, and electrical contacts 720 are made to the blue LD 710. Next, high-reflectivity (HR) coatings (not shown) are applied to both facets of the blue LD 710. These efforts are all intended to maximize the blue emission towards the yellow LED 706.

One or more yellow InGaN QWs are formed in the yellow LED 706, sandwiched by n-type GaN barriers. The number of InGaN QWs and barriers are determined by the intensity of yellow emission necessary in order to achieve a good white color balance. In this structure, n-type GaN can be grown at a relatively lower temperature in comparison with that of p-type GaN. Thus, the heat damage of the yellow InGaN QWs can be minimized during the subsequent growth of n-type GaN instead of p-type GaN.

The blue LD 710 emission effectively excites the yellow LED 706 and a strong white emission 722 is observed from the LED 706 side, wherein the blue emission directly comes from the blue LD 710.

FIG. 8 shows a cross-sectional schematic of an another embodiment, which involves a device 800 design comprised of monolithically integrated LD and LEDs, where both LD and LEDs are grown on the same side of the substrate 802. [Ref. 9] Again, the device 800 structure is based on a {20-21 } free-standing GaN substrate 802. The layers for both the optically-pumped red and green LEDs and the electrically-injected blue LD are grown on one side of the substrate 802 and comprise n-type Ill-nitride layers 804, a red LED active region 806, n-type Ill-nitride layers 808, a green LED active region 810, n-type Ill-nitride layers 812, a blue LD active region and waveguide 814, p-type Ill-nitride layers 816, an ITO layer 818, a grating structure 820, and a mirror 822. An n-type contact 824 is formed on an opposite side of the substrate 802. A p-type contact by metal or oxide is formed on a p-type layer, but this contact is not shown in this view. When a forward bias is applied, the device 800 emits white light 826 from the bottom.

In fabricating the device 800, the optically-pumped red and green LEDs 806,

810 are first grown on the {20-21 } GaN substrate 802. The InGaN QWs of the red and green LEDs 806, 810 are clad by n-type layers 804, 808, 812 on both sides of the red and green LED active regions 806, 810, which can stabilize the active regions of the red and green LEDs 806, 810 during the higher temperature growth of the blue LD 814. [Ref. 9] Next, the electrically-injected blue LD 814 is grown directly on top of the red and green LEDs 806, 810. After growing the blue LD 814 and the red and green LEDs 806, 810, a ridge waveguide is etched into the top surface of the blue LD 814, ITO 818 is deposited on the p-type cladding layer 816 as a part of the cladding and Ohmic contact layers, a grating 820 is etched into the ITO 818, and a mirror 822 is deposited on top of the ITO 818. Next, high-reflectivity (HR) coatings (not shown) are applied to both facets of the blue LD 814. These efforts are all intended to maximize the blue LD 814 emission towards the red and green LEDs 806, 810.

The QWs of the red and green LEDs 806, 810 are sandwiched by n-type GaN barriers 804, 808, 812. The number of QWs and barriers are determined by the intensity of red and green emissions necessary in order to achieve a good white emission 826 color balance. In this structure, n-type GaN can be grown at a relatively lower temperature in comparison with that of p-type GaN. Thus, the heat damage of the red and green InGaN QWs can be minimized during the subsequent growth of n- type GaN instead of p-type GaN. The blue LD 814 emission effectively excites the red and green LEDs 806, 810, and a strong white emission 826 is observed from the red and green LEDs 806, 810 side. The Color Rendering Index (CRI) should be higher because the white emission 826 is composed of three primary colors of blue, green and red emissions.

Another way to increase absorption by the yellow LD and the green, yellow or red LED active region is to use a separate confinement heterostructure. In that case, the absorption of the blue LD light can take place in the waveguide region with a low concentration in indium and can enable the growth of thick absorbing layers. The photoexcited carriers can then relax down in energy into the green, yellow or red light-emitting region, which can be made very thin, thus alleviating the need to grow thick green, yellow or red light-emitting layers. In effect, the structure separates the functions of absorption and emission of the green, yellow or red LEDs or LDs. For example, the blue LD could emit at 450 nm, and the waveguide region of the yellow LD could absorb at 490 nm, while the yellow light-emitting layers would emit around 550 nm. Typical thicknesses for the yellow light-emitting layers would be ~3 nm per QW and for the waveguide region would be -50 nm on each side of the active region.

Process Steps for the Invention

FIG. 9 is a flowchart illustrating the process steps for a method of fabricating an opto-electronic device according to one embodiment of the present invention.

Block 900 represents the step of fabricating at least first and second Ill-nitride light-emitting device structures comprised of one more Ill-nitride layers grown on or above one or more surfaces of a substrate, wherein the substrate is a Ill-nitride substrate and the Ill-nitride layers are semipolar, nonpolar or polar Ill-nitride layers.

In this opto-electronic device, the first Ill-nitride light-emitting device structure has a smaller band gap than the second Ill-nitride light-emitting device structure; the first III -nitride light-emitting device structure is optically-pumped by the second Ill-nitride light-emitting device structure; the second Ill-nitride light- emitting device structure is electrically injected; and emissions from the first III- nitride light-emitting device structure are mixed with emissions from the second III- nitride light-emitting device structure to generate white light.

In this opto-electronic device, an active region of the first Ill-nitride light- emitting device structure is clad on both sides only by undoped or n-type III -nitride layers and is not clad by p-type Ill-nitride layers, and an active region of the second III -nitride light-emitting device structure is clad on opposite sides by n-type and p- type III -nitride layers.

In one or more embodiments, the emissions from the first Ill-nitride light- emitting device structure are yellow light, and the emissions from the second III- nitride light-emitting device structure are blue light. In other embodiments, the emissions from the first III -nitride light-emitting device structure are green and red light, and the emissions from the second III -nitride light-emitting device structure are blue light. In all embodiments, the mixing of emissions from the first Ill-nitride light- emitting device structure with emissions from the second Ill-nitride light-emitting device structure results in white light.

In one or more embodiments, the first and second III -nitride light-emitting device structures are grown on opposite sides of the substrate. In this embodiment, the second Ill-nitride light-emitting device structure is grown first, and the first III- nitride light-emitting device structure is grown after the second III -nitride light- emitting device structure, wherein the substrate is a {20-21} Ill-nitride substrate, the first Ill-nitride light-emitting device structure is grown on or above a (20-21) or (20- 2-1) surface of the {20-21} Ill-nitride substrate, and the second III -nitride light- emitting device structure is grown on or above a (20-2-1) or (20-21) surface of the {20-21} Ill-nitride substrate, respectively.

In these embodiments, the device may further be comprised of: a grating formed on or above the second Ill-nitride light-emitting device structure that diffracts light that is guided by a waveguide of the second Ill-nitride light-emitting device structure into light that is directed toward the first Ill-nitride light-emitting device structure; a mirror formed on or above the second Ill-nitride light-emitting device structure that reflects light that is propagating away from the first Ill-nitride light- emitting device structure towards the first Ill-nitride light-emitting device structure; a grating formed on or above the first Ill-nitride light-emitting device structure that diffracts light that is directed away from the first Ill-nitride light-emitting device structure into light that is guided by a waveguide of the first III -nitride light-emitting device structure; and/or a mirror formed on or above the first Ill-nitride light-emitting device structure that reflects light that is directed perpendicular to an optical cavity of the first Ill-nitride light-emitting device structure.

In other embodiments, the first and second Ill-nitride light-emitting device structures are grown on the same side of the substrate, wherein the substrate is a {20- 21} Ill-nitride substrate, and both the first and second III -nitride light-emitting device structures are grown on or above a (20-21) surface of the {20-21} substrate or a (20- 2-1) surface of the {20-21} substrate. In this embodiment, the first Ill-nitride light- emitting device structure is grown first, and the second Ill-nitride light-emitting device structure is grown after the first III -nitride light-emitting device structure, and the first and second Ill-nitride light-emitting device structures share n-type Ill-nitride layers. The undoped or n-type Ill-nitride layers on both sides of the active region of the first Ill-nitride light-emitting device structure stabilize the active region of the first Ill-nitride light-emitting device structure during growth of the second Ill-nitride light-emitting device structure.

In these embodiments, the device may further be comprised of: a photonic crystal to improve overlap of the second Ill-nitride light-emitting device structure's optical modes with the active region of the first Ill-nitride light-emitting device structure; and/or a separate confinement heterostructure to increase absorption of the second Ill-nitride light-emitting device structure's optical modes by the active region of the first Ill-nitride light-emitting device structure.

In all embodiments, a transparent conductive oxide material may be used as a cladding layer in the first Ill-nitride light-emitting device structure, in the second III- nitride light-emitting device structure, or in both the first and second Ill-nitride light- emitting device structure structures.

Finally, Block 902 represents the resulting opto-electronic device.

Modifications and Alternatives for the Invention

The scope of this invention covers more than just the semipolar orientations ({20-21 }, (20-21) and (20-2-1)) cited in the technical description. This idea is also pertinent to all semipolar planes that can be used for growing high performance III- nitride LEDs and LDs. The term "semipolar plane" can be used to refer to any plane that cannot be classified as c-plane, a-plane, or m-plane. In crystallographic terms, a semipolar plane would be any plane that has at least two nonzero h, i, or k Miller indices and a nonzero 1 Miller index. Subsequent semipolar layers are equivalent to one another so the bulk crystal will have reduced polarization along the growth direction.

Likewise, the scope of this patent is also pertinent to all nonpolar planes that can be used for growing high performance Ill-nitride LEDs and LDs. These include the {11-20} planes, known collectively as a-planes, and the {10-10} planes, known collectively as m-planes. Such planes contain equal numbers of gallium and nitrogen atoms per plane and are charge-neutral. Subsequent nonpolar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction.

Finally, the scope of this patent is also pertinent to all polar planes that can be used for growing high performance Ill-nitride LEDs and LDs. These include the (0001) plane, known as Ga-polar c-plane, and the (000-1) plane, known as N-polar c- plane. Such planes are terminated by a single type of atom and are polarized along the growth direction.

The scope of this invention also covers Ill-nitride LEDs and LDs with various active region designs. This idea is also pertinent to III -nitride LEDs and LDs with active regions with only one QW, active regions with more than one QW, active regions with QWs of any thickness, active regions with QWs of any alloy composition, active regions with barriers of any thickness, and active regions with barriers of any alloy composition.

Additional impurities or dopants can also be incorporated into the Ill-nitride layers or thin films described in this invention. For example, Fe, Mg, Si, and Zn are frequently added to various layers in Ill-nitride heterostructures to alter the conduction properties of those and adjacent layers. The use of such dopants and others not listed here are within the scope of the invention.

The Ill-nitride LEDs and LDs described above were comprised of multiple homogenous layers. However, the scope of this invention also includes III -nitride LEDs and LDs comprised of multiple layers having varying or graded compositions such as separate confinement heterostructures.

Moreover, substrates other than free-standing semipolar GaN could be used for Ill-nitride thin film growth. The scope of this invention includes the growth of Ill-nitride thin films on all possible crystallographic orientations of all possible foreign substrates. These foreign substrates include, but are not limited to, sapphire, silicon carbide, silicon, zinc oxide, boron nitride, lithium aluminate, lithium niobate, germanium, aluminum nitride, lithium gallate, partially substituted spinels, and quaternary tetragonal oxides sharing the y-LiA102 structure.

Furthermore, variations in Ill-nitride nucleation (or buffer) layers and nucleation layer growth methods are acceptable for the practice of this invention. The growth temperature, growth pressure, orientation, and composition of the nucleation layers need not match the growth temperature, growth pressure, orientation, and composition of the subsequent thin films and heterostructures. The scope of this invention includes the growth of Ill-nitride thin films on all possible substrates using all possible nucleation layers and nucleation layer growth methods.

The scope of this invention also covers Ill-nitride layers or thin films grown on epitaxial laterally overgrown (ELO) III -nitride templates. The ELO technique is a method of reducing the density of threading dislocations (TD) in subsequent epitaxial layers. Reducing the TD density leads to improvements in device performance. For c-plane Ill-nitride LEDs and LDs, these improvements include increased output powers, increased internal quantum efficiencies, longer device lifetimes, and reduced threshold current densities. [Ref. 13] These advantages will be pertinent to all III- nitride LEDs and LDs grown on ELO templates.

Free-standing III -nitride substrates may also be created by removing a foreign substrate from a thick Ill-nitride layer, by sawing a bulk Ill-nitride ingot or boule into individual Ill-nitride wafers, or by any other possible crystal growth or wafer manufacturing technique. The scope of this invention includes the growth of III- nitride layers or thin films on all possible free-standing Ill-nitride wafers created by all possible crystal growth methods and wafer manufacturing techniques.

Advantages and Benefits of the Invention

The realization of high efficient white semipolar or nonpolar Ill-nitride LEDs and LDs would potentially allow for multiple advances in the manufacturability of white Ill-nitride LEDs and LDs. Growth of III -nitride LEDs and LDs on semipolar or nonpolar planes could significantly improve device performance by decreasing polarization-induced electric fields and reducing the valence band density states through unbalanced biaxial strain induced splitting of the heavy hole and light hole bands. Decreasing polarization-induced electric fields should increase the radiative efficiency in III -nitride LEDs. Likewise, decreasing polarization-induced electric fields and reducing the valence band density states should decrease the current densities necessary to generate optical gain in Ill-nitride LDs. This should lead to significantly less heating in nitride LEDs and LDs, which should result in higher efficiency, longer device lifetimes and higher production yields for device

manufacturers.

However, when green, yellow or red III -nitride LEDs and LDs are grown with active regions with high indium contents, the active region can form extended defects and can easily be degraded by subsequent high temperature growth steps. [Refs. 1,2] In particular, the growth of subsequent p-type layers can be especially damaging, as these layers usually need to be grown at elevated growth temperatures to ensure adequate p-type conduction. [Ref. 3] In order to minimize the damage to green, yellow or red InGaN QWs, the growth temperature of subsequent p-type layers may be decreased, which results in high operation voltage of the devices, caused by the poor crystal quality and poor activation of the Mg acceptors of the p-type layers.

One way to avoid high temperature growth steps after the active region for the Ill-nitride LDs is to design a structure that does not involve the growth of p-type layers after the active region. Such a configuration has the potential for creating green, yellow or red Ill-nitride LEDs and LDs with improved performance and higher wall plug efficiency, which should have applications in high efficient white emitting devices, portable projectors, heads-up displays, lighting and laser TVs.

By mixing of the shorter wavelength exciting emission, such as blue or violet, and the optically-pumped green, yellow or red emission, white emission would be obtained. Conventional white LEDs use blue LEDs with yellow and red phosphors, but the phosphors have a degradation problem under high excitation irradiation and high temperature operations. On the other hand, phosphor-free white emitting devices, such as this invention, would have no degradation problems.

References

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This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:
1. An opto-electronic device, comprising:
at least first and second Ill-nitride light-emitting device structures comprised of one more Ill-nitride layers grown on or above one or more surfaces of a substrate; wherein:
the first III -nitride light-emitting device structure has a smaller band gap than the second Ill-nitride light-emitting device structure;
the first III -nitride light-emitting device structure is optically-pumped by the second Ill-nitride light-emitting device structure;
the second Ill-nitride light-emitting device structure is electrically injected; and
emissions from the first III -nitride light-emitting device structure are mixed with emissions from the second Ill-nitride light-emitting device structure to generate white light.
2. The device of claim 1, wherein the emissions from the first III -nitride light-emitting device structure are yellow light, and the emissions from the second III- nitride light-emitting device structure are blue light.
3. The device of claim 1, wherein the emissions from the first III -nitride light-emitting device structure are green and red light, and the emissions from the second Ill-nitride light-emitting device structure are blue light.
4. The device of claim 1, wherein the first and second Ill-nitride light- emitting device structures are grown on opposite sides of the substrate.
5. The device of claim 4, wherein the second Ill-nitride light-emitting device structure is grown first, and the first Ill-nitride light-emitting device structure is grown after the second Ill-nitride light-emitting device structure.
6. The device of claim 4, wherein the substrate is a {20-21 } Ill-nitride substrate, the first Ill-nitride light-emitting device structure is grown on or above a (20-21) or (20-2-1) surface of the {20-21} Ill-nitride substrate, and the second III- nitride light-emitting device structure is grown on or above a (20-2-1) or (20-21) surface of the {20-21} Ill-nitride substrate.
7. The device of claim 1, wherein the first and second Ill-nitride light- emitting device structures are grown on the same side of the substrate.
8. The device of claim 7, wherein the substrate is a {20-21} Ill-nitride substrate, and both the first and second Ill-nitride light-emitting device structures are grown on or above a (20-21) surface of the {20-21} substrate or a (20-2-1) surface of the {20-21} substrate.
9. The device of claim 7, wherein the first Ill-nitride light-emitting device structure is grown first, and the second Ill-nitride light-emitting device structure is grown after the first Ill-nitride light-emitting device structure, so that undoped or n- type doped cladding or barrier layers can stabilize an active region of the first III- nitride light-emitting device structure during a higher temperature growth of the second Ill-nitride light-emitting device structure.
10. The device of claim 1, wherein an active region of the first Ill-nitride light-emitting device structure is clad on both sides only by undoped or n-type III- nitride layers and is not clad by p-type III -nitride layers.
11. The device of claim 1 , wherein an active region of the second III- nitride light-emitting device structure is clad on opposite sides by n-type and p-type Ill-nitride layers.
12. The device of claim 1, wherein a transparent conductive oxide material is used as a cladding layer in the first Ill-nitride light-emitting device structure, in the second Ill-nitride light-emitting device structure, or in both the first and second III- nitride light-emitting device structures.
13. The device of claim 1, further comprising a grating on or above the second Ill-nitride light-emitting device structure that diffracts light that is guided by a waveguide of the second Ill-nitride light-emitting device structure into light that is directed toward the first Ill-nitride light-emitting device structure.
14. The device of claim 1, further comprising a mirror on or above the second Ill-nitride light-emitting device structure that refiects light that is propagating away from the first Ill-nitride light-emitting device structure towards the first III- nitride light-emitting device structure.
15. The device of claim 1, further comprising a grating on or above the first Ill-nitride light-emitting device structure that diffracts light that is directed away from the first Ill-nitride light-emitting device structure into light that is guided by a waveguide of the first Ill-nitride light-emitting device structure.
16. The device of claim 1 , further comprising a mirror on or above the first
III -nitride light-emitting device structure that reflects light that is directed
perpendicular to an optical cavity of the first Ill-nitride light-emitting device structure.
17. The device of claim 1, further comprising a photonic crystal to improve overlap of the second Ill-nitride light-emitting device structure's optical modes with an active region of the first Ill-nitride light-emitting device structure.
18. The device of claim 1 , further comprising a separate confinement heterostructure to increase absorption of the second Ill-nitride light-emitting device structure's optical modes by an active region of the first Ill-nitride light-emitting device structure.
19. The device of claim 1, wherein the substrate is a Ill-nitride substrate.
20. The device of claim 1, wherein the III -nitride layers are semipolar, nonpolar or polar Ill-nitride layers.
21. The device of claim 1, the second Ill-nitride light-emitting device structure is a laser diode (LD), a light emitting diode (LED) or a super luminescent LED.
22. The device of claim 1, wherein the first III -nitride light-emitting device structure comprises an emitting region which include at least nanopoles, nanopillars, quantum wires, quantum dots or quantum wells.
23. A method of fabricating an opto-electronic device, comprising:
growing at least first and second Ill-nitride light-emitting device structures comprised of one more Ill-nitride layers grown on or above one or more surfaces of a substrate;
wherein:
the first III -nitride light-emitting device structure has a smaller band gap than the second Ill-nitride light-emitting device structure; the first III -nitride light-emitting device structure is optically-pumped by the second Ill-nitride light-emitting device structure;
the second Ill-nitride light-emitting device structure is electrically injected; and
emissions from the first III -nitride light-emitting device structure are mixed with emissions from the second Ill-nitride light-emitting device structure to generate white light.
PCT/US2015/015894 2014-02-14 2015-02-13 Monolithically integrated white light-emitting devices WO2015123566A1 (en)

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