WO2015089379A1

WO2015089379A1 - High power semipolar {30-3-1} light-emitting diodes with low current droop and low thermal droop

Info

Publication number: WO2015089379A1
Application number: PCT/US2014/069975
Authority: WO
Inventors: Yuji Zhao; Sang Ho Oh; Daniel L. BECERRA; Steven P. Denbaars; Shuji Nakamura
Original assignee: The Regents Of The University Of California
Priority date: 2013-12-13
Filing date: 2014-12-12
Publication date: 2015-06-18

Abstract

A high power semipolar {30-3-1 } light-emitting diode (LED) with low current droop and low thermal droop. Specifically, a thick active layer is used in order to reduce efficiency droop. For example, the LED's efficiency is higher than 40% with an active layer thickness larger than 10 nm. Experimental data shows the LED's efficiency droop is less than 1% at 35 A/cm², 5% at 50 A/cm², 10% at 100 A/cm², and/or 20% at 1000 A/cm². Consequently, the LED's efficiency droop is lower than that of a polar (c-plane) LED operating at a similar current density with a similar indium composition. Moreover, the LED has a small device area less than about 0.1 mm² in which the LED's light output power is over about 1 W.

Description

HIGH POWER SEMIPOLAR {30-3-1 } LIGHT-EMITTING DIODES

WITH LOW CURRENT DROOP AND LOW THERMAL DROOP

CROSS REFERENCE TO RELATED APPLICATIONS

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. 61/915,658, filed on December 13, 2013, by Yuji Zhao, Sang Ho Oh, Daniel L. Becerra, Steven P. DenBaars, and Shuji Nakamura, entitled "HIGH POWER SEMIPOLAR {30-3-1 } LIGHT-EMITTING DIODES WITH LOW CURRENT DROOP AND THERMAL DROOP," attorneys ' docket number 30794.540-US-P1 (2014-415-1);

which application is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

This invention is related to high power semipolar {30-3-1 } light-emitting diodes (LEDs) with low current droop and low thermal droop.

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, e.g., [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.)

The efficiency of conventional (Al, Ga, In)N-based LEDs grown on the polar c- plane drops dramatically at high current densities. This problem, known as efficiency droop, presents significant challenges to the fabrication of high power and high efficiency devices. [Refs. 1-3] The droop effect is further exacerbated for the thin (2-3 nm) quantum wells (QWs) used in conventional c-plane devices - a device structure used to compensate the low electron-hole wavefunction overlap and quantum confined Stark effect (QCSE). [Refs. 4-5] This leads to an increased current density in the active region and a further increased droop effect on c-plane devices. More complex structures, such as Indium Gallium Nitride (InGaN) / Gallium Nitride (GaN) multiple-quantum-wells (MQWs), were utilized to enhance the efficiency and reduce the droop on c-plane devices. [Refs. 4-5] However, the results are less than satisfactory due to the issues such as carrier transport, non-uniform carrier distributions in the wells, and still present polarization-related effects. Additionally, the phenomenon of thermal droop, where the external quantum efficiency (EQE) of devices decreases with increasing temperature, while less studied than current droop, is also an issue for Ill-nitride LEDs. [Refs. 6-7]

Alternatively, growth on nonpolar or semipolar planes of GaN holds great promise for high performance devices, due to the reduction of the polarization fields and the QCSE. Certain semipolar devices have already shown improved performance with reduced efficiency droop. For example, LEDs grown on the semipolar (20-2-1) plane have been demonstrated with low efficiency droop up to 400 A/cm². [Refs. 8-9] It was argued that the low polarization-related electric fields and high-quality InGaN layer allows for LED structures with a relatively thick active layer which are favorable for reducing efficiency droop. [Ref. 9] The carrier density dependent Auger recombination effect, recently shown to be a primary contributor to droop [Ref. 10] should be reduced for these thicker active layers. Additional advantages including high optical polarization ratio, high indium incorporation, small wavelength shift, and narrow spectral linewidth were also reported for semipolar (20-2-1) LEDs and lasers. [Refs. 11-13]

However, more radical device structures and material advances are required to further improve the device performance. 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 high power semipolar {30-3-1 } Ill-nitride LED with low current droop and low thermal droop. Specifically, a thick active layer is used in order to reduce efficiency droop. For example, the LED's efficiency is higher than 40% with an active layer thickness larger than 10 nm. Experimental data shows the LED's efficiency droop is less than 1% at 35 A/cm², 5% at 50 A/cm², 10% at 100 A/cm², and/or 20% at 1000 A/cm². Consequently, the LED's efficiency droop is lower than that of a polar (c-plane) LED operating at a similar current density with a similar indium composition. Moreover, the LED has a small device area less than about 0.1 mm² in which the LED's light output power is over about 1 W at a current density of about 1 kA/cm².

In one embodiment, the present invention includes both an opto-electronic device and a method of fabricating the opto-electronic device, wherein the device comprises a III -nitride light emitting device structure grown on a semipolar {30-3-1 } plane of a GaN substrate. The Ill-nitride light emitting device structure includes an active region comprised of a single quantum well (SQW) or a double heterostructure (DH), e.g., the active region is comprised of an InGaN layer sandwiched between two GaN layers, wherein the active region has a thickness of about 10-100 nanometers (nm).

Specifically, the Ill-nitride light emitting device structure comprises:

a GaN substrate with a growth surface that comprises the semipolar {30-3-1 } plane, wherein the GaN substrate has a roughened backside for extracting light from the Ill-nitride light emitting device structure;

an n-type GaN layer, formed on or above the GaN substrate;

the active region comprised of an InGaN layer sandwiched between two GaN layers, formed on or above the n-type GaN layer; a p-type Aluminum Gallium Nitride (AlGaN) electron blocking layer (EBL), formed on or above the active region;

a p-type GaN layer, formed on or above the p-type AlGaN electron blocking layer;

a p-type transparent conductive layer, formed on or above the p-type GaN layer; a p-type pad, formed on or above the p-type transparent conductive layer; and an n-type pad, formed on or above the n-type GaN layer;

wherein an active area of the Ill-nitride light emitting device structure is 0.1 mm² or less.

The Ill-nitride light emitting device structure has an external quantum efficiency droop that is reduced as compared to the external quantum efficiency droop of a similar Ill-nitride light emitting device structure with a thinner active region operating at a similar current density.

The Ill-nitride light emitting device structure also generates a light output power (LOP) that is greater than the light output power of a Ill-nitride light emitting device structure grown on a semipolar (20-2-1) plane with a similar Indium composition and active region thickness and operating at a similar current density, when the active region has a thickness greater than about 30 nm.

In addition, the Ill-nitride light emitting device structure has an external quantum efficiency droop that is lower than the external quantum efficiency droop of a Ill-nitride light emitting device structure grown on a polar {0001 } c-plane with a similar Indium composition and operating at a similar current density.

Finally, the III -nitride light emitting device structure has a thermal droop that is lower than a III -nitride light emitting device structure grown on a polar {0001 } plane with a similar Indium composition and operating at a similar current density. BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 1 is a schematic of an LED device structure according to one embodiment of the present invention.

FIG. 2(a) is a simulated band diagram for 10 nm thick violet-emitting single- quantum-well at 100 A/cm² on c-plane GaN and FIG. 2(b) is a simulated band diagram for 10 nm thick violet-emitting single-quantum-well at 100 A/cm² on (30-3-1) GaN.

FIG. 3 is a graph of calculated Matthews-Blakeslee critical thickness for various semipolar planes of GaN.

FIG. 4 is a graph of active layer thickness (nm) vs. quick test (QT) power for {30- 3-1 } and {20-2-1 } LEDs with different active layer thicknesses.

FIG. 5(a) is a graph of electroluminescence light output power (LOP) vs.

thickness of the active region comparing (20-2-1) and (30-3-1) semipolar planes.

FIGS. 5(b)-5(d) are panchromatic cathodo luminescence images of (20-2-1) LED structures for 20, 40, and 60 nm thick active regions, respectively.

FIGS. 5(e)-5(g) are panchromatic cathodo luminescence images of (30-3-1) LED structures for 20, 40, and 60 nm thick active regions, respectively.

FIG. 6 is a graph of current vs. normalized external quantum efficiency of semipolar (30-3-1) LEDs with different active layer thicknesses at different current densities.

FIG. 7 is a graph of current density vs. light output power and external quantum efficiency of semipolar (30-3-1) LEDs.

FIG. 8(a) is a graph of absolute external quantum efficiency vs. current density and FIG. 8(b) is a graph of normalized external quantum efficiency vs. current density. FIG. 9 is a graph of current density vs. external quantum efficiency and temperature for semipolar (30-3-1) LEDs showing thermal droop data at different temperatures.

FIGS. 10(a) and 10(b) are graphs of external quantum efficiency vs. current density measured at different temperatures for devices with a 20 nm thick active region, as shown in FIG. 10(a), and a 100 nm thick active region, as shown in FIG. 10(b).

FIG. 11(a) is a graph of output power and external quantum efficiency vs. current density for a packaged LED device with a 15 nm thick single-quantum- well active region and FIG. 11(b) is a graph of peak wavelength and full-width-half-maximum for the device as a function of current density.

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 present invention is directed to a Group-Ill nitride based LED grown on a semipolar {30-3-1 } plane, wherein the LED has low efficiency droop and low thermal droop. In addition, a Group-Ill nitride LED grown on a semipolar {30-3-1 } plane will have a lower QCSE and reduced polarization fields, as compared to a Group-Ill nitride based LED grown on a polar {00001 } c-plane, which leads to improved device performance, such as increased efficiency and reduced wavelength blue shift. These properties of semipolar {30-3-1 } Group-Ill nitride LEDs allows for a wider design space to address the problem of efficiency droop at high current densities. In this regard, the present invention discloses high power and low droop violet- emitting InGaN/GaN LEDs fabricated on semipolar {30-3-1 } free-standing GaN substrates using conventional metal organic chemical vapor deposition (MOCVD) and processed and packaged using conventional techniques, wherein the semipolar {30-3-1 } plane has a high inclination angle (-80°) with respect to the polar {0001 } c-plane. The low polarization-related effects and large critical thickness on the semipolar {30-3-1 } plane enables radical LED structures with very thick InGaN/GaN active regions, including single-quantum-wells (SQWs) and double heterostructures (DHs), with thickness ranging from 10 to 100 nm.

Simulated band diagrams showed reduced polarization fields on the {30-3-1 } plane. In addition, the calculated critical thickness for misfit dislocation formation is higher on the {30-3-1 } plane than on other semipolar planes, such as {20-2-1 }, allowing for thicker active regions than prior devices to further reduce droop. The higher critical thickness was confirmed with defect characterization via cathodo luminescence.

Moreover, the {30-3-1 } plane for Group-Ill nitride has higher critical thickness as compared to other planes, which facilitate LED structures with a thicker active region.

For the exploration of efficiency droop in InGaN/GaN LEDs, the nonradiative Auger recombination has been implicated as the cause of efficiency droop. By using thicker active regions on {30-3-1 } planes, Auger recombination can be reduced, and the carrier concentration in the active region can be reduced, which will reduce the efficiency droop for LEDs.

Experimental results for both current droop and thermal droop are presented for LEDs with different active region thicknesses. A trend is demonstrated in lower efficiency droop for devices with thicker active regions.

These observed results were utilized to demonstrate a high-performance low- droop small-area (0.1 mm²) semipolar (30-3-1) Ill-nitride LED that has a peak emission at a violet emission wavelength with a light output power of about 1 W at a current density of about 1 kA/cm² that was fabricated using this thick active region design.

Technical Disclosure

FIG. 1 is a schematic of an exemplary Ill-nitride light emitting device structure

100 epitaxially grown by conventional MOCVD on a semipolar (30-3-1) plane of a freestanding GaN substrate 102 supplied by Mitsubishi Chemical Corporation. However, in alternative embodiments, the device structure may be grown on a nonpolar GaN substrate, a c-plane GaN substrate, or a c-plane sapphire substrate.

The Ill-nitride light emitting device structure 100 may comprise an LED that includes an n-type GaN (n-GaN) layer 104, an InGaN/GaN active region 106, a p-type AlGaN (p-AlGaN) electron blocking layer (EBL) 108, a p-type GaN (p-GaN) layer 110, an indium-tin-oxide (ITO) current spreading layer 112, and two Ti/Au pads (a p-pad 114 on the ITO layer 112 and an n-pad 116 on a Ti/Al/Ni/Au contact 118 on the n-GaN layer 104).

The active region 106 (also referred to herein as the active layer) may be comprised of one or more relatively thick light emitting InGaN/GaN layers. These layers may comprise a single-quantum-well or a double heterostructure, both of which include a single InGaN layer sandwiched between two GaN layers, with the active region 106 thickness ranging from 10 nm to 100 nm.

For the LED fabrication, a rectangular mesa pattern (active area of 0.1 mm²) was formed by conventional lithography and chlorine-based inductively coupled plasma (ICP) etching after the ITO current spreading layer 112 was deposited by electron beam evaporation.

The Ti/Au p-pad 112 and n-pad 114 were deposited by electron beam evaporation and a conventional lift-off process. The backside (bottom) of the substrate 102 was roughened to have conical features, which improves the light extraction efficiency, using a procedure described elsewhere. [Ref. 19]

The devices 100 were then diced and mounted on a silver header and

encapsulated in silicone (not shown).

Encapsulated devices were tested in both DC and pulsed mode with 1 KHz frequency and 1% duty cycle to prevent self- heating effects. The tests were done at room temperature with forward currents up to 1000 A/cm². The device structure of the present invention had an external quantum efficiency droop less than 1% at 35 A/cm², 5% at 50 A/cm², 10% at 100 A/cm², and/or 20% at 1000 A/cm².

Experimental Data

FIGS. 2(a) and 2(b) demonstrate the calculated band diagram profiles for polar (1000) c-plane III -nitride LEDs and semipolar (30-3-1) plane Ill-nitride LEDs, each with a 10 nm thick Ino.12Gao.9N/GaN active region, powered at a current density of 100 A/cm², using the commercial SiLENSe package developed by the STR Group. The potential distributions were calculated by solving the Schrodinger-Poisson equations self- consistently and include strain and polarization effects. The details for the methods used to calculate the band diagrams can be found in [Ref. 12].

In FIG. 2(a), for c-plane (0001) InGaN/GaN devices, it is well understood that, due to the large polarization-related electric fields, the electrons and holes in the active region will be pushed apart in opposite directions, which results in a distorted energy band diagram profile, a low electron-hole wavefunction overlap, and subsequently a low radiative recombination efficiency.

In FIG. 2(b), for semipolar (30-3-1) InGaN/GaN devices, on the other hand, the polarization-related electric fields are much smaller than the c-plane case and a flat band diagram profile with high electron-hole wavefunction overlap can be obtained. It is noteworthy that this low wavefunction overlap problem will become more prominent for c-plane structures with increased active layer thickness, while the semipolar (30-3-1) structures can maintain a high wavefunction overlap (>0.9) with very thick active regions.

FIG. 3 presents the Matthews-Blakeslee equilibrium critical thickness values

(calculated under the assumption of isotropic elasticity) vs. Indium compositions for InGaN/GaN structures on various semipolar planes, e.g., (30-3-1), (20-2-1), and (10-1-1). [Ref. 14] Growth of thick strained heterostructures on nonpolar and semipolar planes often leads to the formation of misfit dislocations (MDs) at heterointerfaces via dislocation glide on the available slip planes. [Ref. 15] The driving force for such dislocation glide originates from the resolved shear stress on the slip plane. Since the preferred slip plane in wurtzite GaN is the basal c-plane, this is referred to as basal plane (BP) slip. The resolved stress first increases with inclination angle from the c-plane to a maximum at about 45°, then decreases with increasing inclination angle from the c-plane. Therefore, semipolar planes with high inclination angles (with respect to the c-plane), such as the (30-3-1) plane, will have an increased critical thickness for strained InGaN layers compared to other planes, due to the reduction in the resolved shear stress on the basal plane. Relaxation has been observed both above and below this calculated thickness. [Refs. 16-17]

Kinetic factors, such as Peierls barriers and/or existing threading dislocation geometries, are suggested to be possible causes of this. [Ref. 17] Additionally, relaxation along other planes besides the basal c-plane has been observed. [Ref.18] The activation of these other slip systems, which is referred to as two-dimensional (2D) relaxation or non-basal plane (NBP) slip, can depend on those same kinetic factors that cause the deviation from the calculated basal plane relaxation critical thickness, and are also plane dependent. [Ref. 18] The misfit dislocations caused by these non-basal plane slip systems can have a more dramatic effect on device performance, as shown below. FIG. 4 is a graph of active layer thickness (nm) vs. Quick Test (QT) power (a.u.) that shows that III -nitride LEDs grown on the (30-3-1) plane can maintain high efficiency up to an active layer thickness of about 60 nm, while the performance of other LEDs, such as those grown on a semipolar (20-2-1) plane, show reduced performance with increased active layer thickness. This graph shows similar performance for active layer thicknesses up to about 20 nm, but significantly better performance for (30-3-1) III- nitride LEDs as compared to (20-2-1) Ill-nitride LEDs for active layer thickness greater than about 30 nm.

FIG. 5(a) shows the electroluminescence (EL) light-output-power (LOP) as a function of active layer thickness for III -nitride LED structures grown on both the (20-2- 1) and (30-3-1) planes. At low active layer thickness (< 20 nm), the (20-2-1) LED showed comparable performance with the (30-3-1) LED. When the active layer of the (20-2-1) LED exceeds its critical thickness (~ 20 nm), however, the (20-2-1) device performance drops dramatically.

In contrast, the (30-3-1) devices showed higher LOP performance with thicker active layer structures than the (20-2-1) devices due to the higher critical thickness of the (30-3-1) devices, i.e., the active region of a (30-3-1) device has a critical thickness of about 30 nm. Consequently, what this shows is that the Ill-nitride light emitting device structure of the present invention generates an LOP that is greater than the LOP of a III- nitride light emitting device structure grown on a semipolar (20-2-1) plane with a similar Indium composition and active region thickness and operating at a similar current density, when the active region has a thickness greater than about 30 nm.

The optical performance of the devices was further illuminated by material characterization.

FIGS. 5(b)-5(g) are representative panchromatic cathodo luminescence (CL) images demonstrating the progression of defect generation and stress relaxation in (20-2- 1) and (30-3-1) Ill-nitride LEDs. Specifically, FIGS. 5(b)-5(d) are panchromatic CL images of (20-2-1) LED structures for 20, 40, and 60 nm thick active regions, respectively, and FIGS. 5(e)-5(g) are panchromatic CL images of (30-3-1) LED structures for 20, 40, and 60 nm thick active regions, respectively.

In FIG. 5(b), at 20 nm, a few dark lines parallel to the a-direction (BP MDs) are visible for the (20-2-1) devices. The presence of very small NBP MDs can also be seen, indicating the onset of performance degrading 2D relaxation. Starting from 40 nm, in FIG. 5(c), a significant amount of non-basal plane dark defects are seen on (20-2-1) devices, indicating that the InGaN layers were experiencing stronger 2D relaxation as the thickness increased, corresponding to the low output power. This increases for 60 nm with (20-2-1) devices, in FIG. 5(d).

The (30-3-1) structures showed similar trends, in FIGS. 5(e), 5(f) and 5(g).

However, due to the increased critical thickness, no NBP MDs are seen on the 20 nm images for (30-3-1) devices in FIG. 5(e), and significantly fewer NBP MDs for the 40 nm and 60 nm images, in FIGS. 5(f) and 5(g), respectively. This indicates that the (30-3-1) samples were less relaxed compared to (20-2-1) devices, and it also indicates that BP MDs do not have a large impact on device performance, especially for thick InGaN layers. This is consistent with the device results in FIG. 5(a).

FIG. 6 shows the normalized external quantum efficiency (EQE) of the semipolar {30-3-1 } III -nitride LEDs with different active layer thickness at different current densities J (A/cm²). The LEDs show reduced EQE droop with increasing active layer thickness, i.e., the device structure has an EQE droop that is reduced as compared to the EQE droop of a similar Ill-nitride light emitting device structure with a thinner active region operating at a similar current density. For example, the LED's efficiency is higher than 40% at a current density of about 900 A/cm² with an active layer thickness larger than 10 nm, more than 60% at a current density of about 900 A/cm² with an active layer thickness larger than 20 nm, about 80% at a current density of about 900 A/cm² with an active layer thickness larger than 40 nm, and more than 80% at a current density of about 900 A/cm² with an active layer thickness larger than 100 nm. This is consistent with the theory and physical expectation.

FIG. 7 shows both the LOP and EQE of the semipolar (30-3-1) Ill-nitride LEDs with current densities up to about 900 A/cm². These LEDs show outstanding LOP and EQE at extremely high current densities. For example, the LED's LOP is more than 200 mW at a current density of about 200 A/cm², is more than 400 mW at a current density of about 400 A/cm², is more than 600 mW at a current density of about 600 A/cm², is less than about 800 mW at a current density of about 800 A/cm², and peaks at more than 800 mW at a current density of less than about 900 A/cm². In another example, the LED's EQE peaks at about 40% at a current density of less than about 100 A/cm², is more than 35% at a current density of about 200 A/cm², is about 35% at a current density of about 400 A/cm², is more than 30% at a current density of about 600 A/cm , is more than 30% at a current density of about 800 A/cm², and is more than 30% at a current density of about 900 A/cm².

FIG. 8(a) presents the EQE as a function of current density for (30-3-1) Ill-nitride

LEDs with various active layer thicknesses under pulsed operation (1% duty cycle). The LEDs with the thinnest active regions (10 nm) had a higher peak efficiency. Specifically, the Ill-nitride light emitting device structure of the present invention has an EQE that is greater than about 40% when the active region has a thickness of about 10 nm. It can be seen that the efficiency decreases with increasing active region thickness, and is also shifted to higher current densities.

FIG. 8(b) shows the same data as FIG. 8(a) plotted with normalized EQE vs. current density on a logarithmic scale. The data in this figure show a trend with lower EQE droop with increasing active region thickness.

A decrease can also be seen in EQE as the temperature increases, as has been identified elsewhere. [Ref. 7] This is known as thermal droop. FIG. 9 shows thermal droop results for (30-3-1) Ill-nitride LEDs in a graph of current density vs. EQE. The EQE decreases with increasing current density, with lower temperatures exhibiting less droop in EQE. Nonetheless, (30-3-1) Ill-nitride LEDs show improved thermal droop as compared to conventional c-plane LEDs. For example, at 100 °C, the EQE of (30-3-1) LEDs drops around 10%, while the EQE of the conventional c- plane LEDs drops around 20%>. Thus, the Ill-nitride light emitting device structure of the present invention has a thermal EQE droop that is lower than a Ill-nitride light emitting device structure grown on a polar {0001 } plane with a similar Indium composition and operating at a similar current density.

FIGS. 10(a) and 10(b) also show EQE vs. current density at different device temperatures for different active region thicknesses in semipolar (30-3-1) III -nitride LEDs.

In order to quantify the thermal droop, a factor known as the hot/cold (H/C) factor is used. This is defined as: hot/cold factor = EQEi₀o°c / EQE₂₀°c- FIG. 10(a) shows a representative curve for the 20 nm thick active region device.

As the active regions became thicker, the thermal performance suffers. The devices with relatively thin active regions (thicknesses of about 10-40 nm had similar performance) showed improved thermal characteristics with only about 10% EQE droop at 120°C, corresponding to a H/C factor of 0.9, similar to that of devices with thick active regions on the (20-2-1) plane. [Ref. 7]

FIG. 10(b) shows the performance of the III -nitride light emitting device structure of the present invention, which has a thermal EQE droop that is about 20% at a temperature of about 120°C, corresponding to a H/C factor of 0.8, when the active region has a thickness of about 100 nm.

The above studies were used to optimize a device structure to obtain the highest performance. An active region thickness of 15 nm was selected and a device was fabricated as discussed above, but packaged using a vertical transparent packaging method. [Ref. 20]

FIG. 11(a) shows the EQE and LOP of the small-area 0.1 mm² semipolar (30-3- III -nitride LED with the 15 nm thick active region under pulsed operation (1% duty cycle). This figure includes the following data:

This data shows a peak EQE above about 50% and drooping to only about 33% at a current density of 1 kA/cm² to generate the light output power of about 1 W.

FIG. 11(b) shows the wavelength and full width at half maximum (FWHM) of the emission spectrum. This figure shows that the Ill-nitride light emitting device structure of the present invention has a very small wavelength shift (e.g., less than about 5 nm) and exhibits an emission spectrum that has a low FWHM (below about 18 nm) up to a current density of about 1 kA/cm².

In summary, LED devices with very thick active regions of 10-100 nm were grown on the semipolar (30-3-1) plane of a GaN substrate. Utilizing the low polarization fields and high critical thickness of this plane, high-power low-droop LEDs were fabricated. Electroluminescence and cathodoluminescence confirmed fewer 2D dislocations on (30-3-1) devices led to higher performance at very thick active regions. A trend in lower droop with thicker active region thickness was demonstrated. The results of this study led to the fabrication of a small-area 0.1 mm² LED that has an LOP of about 1 W or greater for a current density of about 1000 A/cm² or more. References

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Nomenclature

The terms "Group-Ill nitride" or "Ill-nitride" or "nitride" as used interchangeably herein refer to any composition or material related to (Al,Ga,In)N semiconductors having the formula Al_xGa_yIn_zN where 0 < x < l, 0 < y < l, 0 < z < l, and x + y + z = 1. These terms as used herein are intended to be broadly construed to include respective nitrides of the single species, Al, Ga, and In, as well as binary, ternary and quaternary compositions of such Group-Ill metal species. Accordingly, these terms include, but are not limited to, the compounds of A1N, GaN, InN, AlGaN, AlInN, InGaN, and AlGalnN. When two or more of the (Al,Ga,In)N component species are present, all possible compositions, including stoichiometric proportions as well as off-stoichiometric proportions (with respect to the relative mole fractions present of each of the (Al, Ga, In)N component species that are present in the composition), can be employed within the broad scope of this invention. Further, compositions and materials within the scope of the invention may further include quantities of dopants and/or other impurity materials and/or other inclusional materials.

This invention also covers the selection of particular crystal orientations, directions, terminations and polarities of Group-Ill nitrides. When identifying crystal orientations, directions, terminations and polarities using Miller indices, the use of braces, { }, denotes a set of symmetry-equivalent planes, which are represented by the use of parentheses, ( ). The use of brackets, [ ], denotes a direction, while the use of brackets, < >, denotes a set of symmetry-equivalent directions.

Many Group-Ill nitride devices are grown along a "polar" orientation, namely a c-plane {0001 } of the crystal, although this results in an undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. One approach to decreasing polarization effects in Group-Ill nitride devices is to grow the devices along nonpolar or semipolar orientations of the crystal.

The term "nonpolar" includes the {1 1-20} planes, known collectively as a-planes, and the {10-10} planes, known collectively as m-planes. Such planes contain equal numbers of Group III 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.

The term "semipolar" 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 crystal will have reduced polarization along the growth direction. Conclusion

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:

a Ill-nitride light emitting device structure grown on a semipolar {30-3-1 } plane; the Ill-nitride light emitting device structure including an active region comprised of a single quantum well or a double heterostructure;

the active region having a thickness of about 10-100 nanometers (nm); and the Ill-nitride light emitting device structure having an external quantum efficiency (EQE) droop that is reduced as compared to the EQE droop of a similar III- nitride light emitting device structure with a thinner active region operating at a similar current density.

2. The device of claim 1, wherein the III -nitride light emitting device structure is grown on the semipolar {30-3-1 } plane of a Gallium Nitride (GaN) substrate.

3. The device of claim 1, wherein the III -nitride light emitting device structure has a light output power (LOP) of about 1 watt (W) or greater for a current density of about 1000 amperes per centimeter square (A/cm²) or more.

4. The device of claim 3, wherein the active region has a thickness of about

15 nm for a peak external quantum efficiency (EQE) above about 50% and drooping to only about 33% at the current density of about 1000 A/cm² to generate the light output power of about 1 W.

5. The device of claim 4, wherein the III -nitride light emitting device structure has a wavelength shift of less than about 5 nm up to the current density of about 1000 A/cm².

6. The device of claim 4, wherein the Ill-nitride light emitting device structure exhibits an emission spectrum that has a full width at half maximum (FWHM) below about 18 nm up to the current density of about 1000 A/cm².

7. The device of claim 1, wherein the active region has a critical thickness of about 30 nm.

8. The device of claim 1, wherein the active region is comprised of an Indium Gallium Nitride (InGaN) layer sandwiched between two Gallium Nitride (GaN) layers.

9. The device of claim 1, wherein the III -nitride light emitting device structure has an external quantum efficiency (EQE) that is greater than 40% when the active region has a thickness larger than 10 nm.

10. The device of claim 1, wherein the III -nitride light emitting device structure has an external quantum efficiency (EQE) that:

peaks at about 40% at a current density of less than about 100 A/cm²,

is more than 35% at a current density of about 200 A/cm²,

is more than 30% at a current density of about 400 A/cm²,

is more than 30% at a current density of about 600 A/cm²,

is more than 30% at a current density of about 800 A/cm², and/or

is more than 30% at a current density of about 900 A/cm².

11. The device of claim 1 , wherein the III -nitride light emitting device structure has an external quantum efficiency (EQE) droop less than 1% at 35 A/cm², 5% at 50 A/cm², 10% at 100 A/cm², and/or 20% at 1000 A/cm².

12. The device of claim 1, wherein the III -nitride light emitting device structure has an external quantum efficiency (EQE) droop that is lower than the EQE droop of a Ill-nitride light emitting device structure grown on a polar {0001 } c-plane with a similar Indium composition and operating at a similar current density.

13. The device of claim 1, wherein the III -nitride light emitting device structure has a thermal droop that is about 10% when the active region has a thickness of about 10-40 nm.

14. The device of claim 1, wherein the III -nitride light emitting device structure has a thermal droop that is about 20% when the active region has a thickness of about 100 nm.

15. The device of claim 1, wherein the III -nitride light emitting device structure has a thermal droop that is lower than a Ill-nitride light emitting device structure grown on a polar {0001 } plane with a similar Indium composition and operating at a similar current density.

16. The device of claim 1, wherein the Ill-nitride light emitting device structure generates a light output power (LOP) that is greater than the LOP of a Ill-nitride light emitting device structure grown on a semipolar (20-2-1) plane with a similar Indium composition and active region thickness and operating at a similar current density, when the active region has a thickness greater than about 30 nm.

17. The device of claim 1, wherein the III -nitride light emitting device structure has a light output power (LOP) that is:

more than 200 mW at a current density of about 200 A/cm²,

more than 400 mW at a current density of about 400 A/cm²,

more than 600 mW at a current density of about 600 A/cm²,

less than about 800 mW at a current density of about 800 A/cm², and/or more than 800 mW at a current density of less than about 900 A/cm².

18. The device of claim 1, wherein the III -nitride light emitting device structure has a peak emission at a violet emission wavelength.

19. The device of claim 1, further comprising:

a Gallium Nitride (GaN) substrate with a growth surface that comprises the semipolar {30-3-1 } plane, wherein the GaN substrate has a roughened backside for extracting light from the Ill-nitride light emitting device structure;

an n-type GaN layer, formed on or above the GaN substrate;

the active region comprised of an Indium Gallium Nitride (InGaN) layer sandwiched between two GaN layers, formed on or above the n-type GaN layer;

a p-type Aluminum Gallium Nitride (AlGaN) electron blocking layer (EBL), formed on or above the active region;

a p-type transparent conductive layer, formed on or above the p-type GaN layer; a p-type pad, formed on or above the p-type transparent conductive layer; and an n-type pad, formed on or above the n-type GaN layer; wherein an active area of the Ill-nitride light emitting device structure is 0.1 mm² or less.

20. A method of fabricating an opto-electronic device, comprising:

fabricating a Ill-nitride light emitting device structure grown on a semipolar {30- 3-1 } plane;

the Ill-nitride light emitting device structure including an active region comprised of a single quantum well or a double heterostructure;