Group III nitride based quantum well light emitting device structures

<u>Cross-Reference to Related Applications</u>

This application is a continuation-in-part of United States Patent Application Serial No. <patcit id="pcit0001" dnum="US10140796B"><text>10/140,796 filed May 7, 2002</text></patcit> entitled <i>GROUP III NITRIDE BASED LIGHT EMITTING DIODE STRUCTURES WITH A QUANTUM WELL AND SUPERLATTICE, GROUP III NITRIDE BASED QUANTUM WELL STRUCTURES AND GROUP III NITRIDE BASED SUPERLATTICE STRUCTURES</i> which claims the benefit of, and priority from, Provisional Application Serial No. <patcit id="pcit0002" dnum="US60294445B"><text>60/294,445, filed May 30, 2001</text></patcit> entitled <i>MULTI-QUANTUM WELL LIGHT EMITTING DIODE STRUCTURE</i>, Provisional Application Serial No. <patcit id="pcit0003" dnum="US60294308B"><text>60/294,308, filed May 30, 2001</text></patcit> entitled <i>LIGHT EMITTING DIODE STRUCTURE WITH SUPERLATTICE STRUCTURE</i> and Provisional Application Serial No. <patcit id="pcit0004" dnum="US60294378B"><text>60/294,378, filed.May 30, 2001</text></patcit> entitled <i>LIGHT EMITTING DIODE STRUCTURE WITH MULTI-QUANTUM WELL AND SUPERLATTICE STRUCTURE,</i> the disclosures of which are hereby incorporated herein by reference in their entirety as if set forth fully herein.

<u>Field of the Invention</u>

This invention relates to microelectronic devices and fabrication methods therefor, and more particularly to structures which may be utilized in Group III nitride semiconductor devices, such as light emitting diodes (LEDs).

<u>Background of the Invention</u>

Light emitting diodes are widely used in consumer and commercial applications. As is well known to those having skill in the art, a light emitting diode generally includes a diode region on a microelectronic substrate, The microelectronic substrate may comprise, for example, gallium arsenide, gallium phosphide, alloys thereof, silicon carbide and/or sapphire. Continued developments in LEDs have resulted in highly efficient and mechanically robust light sources that can cover the visible spectrum and beyond. These attributes, coupled with the potentially long service life of solid state devices, may enable a variety of new display applications, and may place LEDs in a position to compete with the well entrenched incandescent lamp.

One difficulty in fabricating Group III nitride based LEDs, such as gallium nitride based LEDs, has been with the fabrication of high quality gallium nitride. Typically, gallium nitride LEDs have been fabricated on sapphire or silicon carbide substrates. Such substrates may result in mismatches between the crystal lattice of the substrate and the gallium nitride. Various techniques have been employed to overcome potential problems with the growth of gallium nitride on sapphire and/or silicon carbide. For example, aluminum nitride (AlN) may be utilized as a buffer between a silicon carbide substrate and a Group III active layer, particularly a gallium nitride active layer. Typically, however, aluminum nitride is insulating rather than conductive. Thus, structures with aluminum nitride buffer layers typically require shorting contacts that bypass the aluminum nitride buffer to electrically link the conductive silicon carbide substrate to the Group III nitride active layer.

Alternatively, conductive buffer layer materials such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), or combinations of gallium nitride and aluminum gallium nitride may allow for elimination of the shorting contacts typically utilized with AlN buffer layers. Typically, eliminating the shorting contact reduces the epitaxial layer thickness, decreases the number of fabrication steps required to produce devices, reduces the overall chip size, and/or increases the device efficiency. Accordingly, Group III nitride devices may be produced at lower cost with a higher performance. Nevertheless, although these conductive buffer materials offer these advantages, their crystal lattice match with silicon carbide is less satisfactory than is that of aluminum nitride.

The above described difficulties in providing high quality gallium nitride may result in reduced efficiency the device. Attempts to improve the output of Group III nitride based devices have included differing configurations of the active regions of the devices. Such attempts have, for example, included the use of single and/or double heterostructure active regions. Similarly, quantum well devices with one or more Group III nitride quantum wells have also been described. While such attempts have improved the efficiency of Group III based devices, further improvements may still be achieved.

<u>Summary of the Invention</u>

The present invention provides a light emitting diode in accordance with claim 1 and a method of fabricating a light emitting diode in accordance with claim 8.

Some embodiments of the present invention provide Group III nitride based light emitting devices and methods of fabricating Group III nitride based light emitting devices that include an n-type Group III nitride layer, a Group III nitride based active region on the n-type Group III nitride layer and including at least one quantum well structure, a Group III nitride layer including indium on the active region, a p-type Group III nitride layer including aluminium on the Group III nitride layer including indium, a first contact on the n-type Group III nitride layer and a second contact on the p-type Group III nitride layer.

In further embodiments of the present invention, the Group III nitride layer including indium also includes aluminium. For example, the Group III nitride layer including indium may include InAlGaN. The Group III nitride layer including indium may also include InGaN. The Group III nitride layer including indium may be from about 20 to about 320 Ǻ thick.

In particular embodiments of the present invention, the Group III nitride layer including indium includes a layer of InAlGaN having a higher Al composition in a region distal from the active region than is present in a region proximate the active region. In some embodiments, the InAlGaN layer may be continuously graded. In other embodiments, the InAlGaN layer may include a plurality of the InAlGaN layers having different Al and/or In compositions.

In further embodiments of the present invention, the Group III nitride layer including indium includes a first layer of In_xAl_yGa_1-x-yN, where 0&lt;x≤0.2 and 0≤y&lt;0.4 and a second layer of In_wAl_zGa_1-w-zN, where 0&lt;w≤0.2 and y≤z&lt;1. The first layer may have a thickness of from about 10 to about 200 Ǻ and the second layer may have a thickness of from about 10 to about 120 Ǻ. In particular embodiments, the first layer has a thickness of about 80 Ǻ, x=0.1 and y=0.25 and the second layer has a thickness of about 30 Ǻ, w=0.05 and z=0.55.

In additional embodiments of the present invention, the light emitting devices further include a p-type Group III nitride layer disposed between the second contact and the p-type Group III nitride layer including aluminium. The p-type Group III nitride layer disposed between the second contact and the p-type Group III nitride layer including aluminium may also include indium. The p-type Group III nitride layer including aluminium may also include indium.

Document <patcit id="pcit0005" dnum="US20030006418A"><text>US2003/0006418</text></patcit> describes a light emitting diode having a Group Ill nitride based active region. An undoped AIGaN capping region is positioned on the active region. In certain embodiments of the present invention, the light emitting devices include a silicon carbide substrate disposed between the first contact and the n-type Group III nitride layer.

Some embodiments of the present invention provide light emitting devices and methods of fabricating light emitting devices that include an n-type gallium nitride based layer on a substrate, a gallium nitride based active region on the n-type gallium nitride based layer and include at least one quantum well structure, a gallium nitride based layer including indium on the active region, a p-type gallium nitride based layer including aluminum on the gallium nitride based layer including indium, a first contact on the n-type gallium nitride based layer and a second contact on the p-type gallium nitride based layer.

In particular embodiments of the present invention, the n-type gallium nitride layer includes an n-type AlGaN layer on the substrate and an n-type GaN layer on the n-type AlGaN layer. The gallium nitride based active region may include a plurality of InGaN/GaN quantum wells.

In further embodiments of the present invention, the p-type gallium nitride based layer includes a p-type AlGaN layer on the gallium nitride based layer including indium and a p-type GaN layer on the p-type AlGaN layer. The second contact is on the p-type GaN layer. The gallium nitride based layer including indium may include a first layer of In_xAl_yGa_1-x-yN, where 0&lt;x≤0.2 and 0≤y≤0.4 and a second layer of In_wAl_zGa_1-w-zN, where 0&lt;w≤0.2 and y≤z&lt;1. The first layer may have a thickness of from about 10 to about 200 Å and the second layer may have a thickness of from about 10 to about 120 Å. In particular embodiments of the present invention, the first layer has a thickness of about 80 Å, x=0.1 and y=0.25 and the second layer has a thickness of about 30 Å, w=0.05 and z=0.55.

In still further embodiments of the present invention, the substrate is a silicon carbide substrate and the first contact is on the silicon carbide substrate opposite the n-type AlGaN layer.

<u>Brief Description of the Drawings</u>

Other features of the present invention will be more readily understood from the following detailed description of specific embodiments thereof when read in conjunction with the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001"><b>Figure 1</b></figref> is a schematic illustration of a Group III nitride light emitting diode incorporating embodiments of the present invention;</li><li><figref idref="f0002"><b>Figure 2</b></figref> is a schematic illustration of a Group III nitride light emitting diode incorporating further embodiments of the present invention;</li><li><figref idref="f0003"><b>Figure 3</b></figref> is a schematic illustration of a quantum well structure and a multi-quantum well structure according to additional embodiments of the present invention; and</li><li><figref idref="f0004"><b>Figure 4</b></figref> is a schematic illustration of a Group III nitride light emitting diode incorporating further embodiments of the present invention.</li></ul>

<u>Detailed Description of Preferred Embodiments</u>

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. As used herein the term "and/or" includes any and all combinations of one or more of the associated listed items.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

It will be understood that when an element such as a layer, region or substrate is referred to as being "on" or extending "onto" another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" or extending "directly onto" another element, there are no intervening elements present. It will also be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present. Like numbers refer to like elements throughout the specification.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.

Furthermore, relative terms, such as "lower" or "bottom" and "upper" or "top," may be used herein to describe one element's relationship to another elements as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in the Figures is turned over, elements described as being on the "lower" side of other elements would then be oriented on "upper" sides of the other elements. The exemplary term "lower", can therefore, encompasses both an orientation of "lower" and "upper," depending of the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as "below" or "beneath" other elements would then be oriented "above" the other elements. The exemplary terms "below" or "beneath" can, therefore, encompass both an orientation of above and below.

Embodiments of the present invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments of the present invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the present invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an etched region illustrated or described as a rectangle will, typically, have rounded or curved features. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region of a device and are not intended to limit the scope of the present invention.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed "adjacent" another feature may have portions that overlap or underlie the adjacent feature.

Although various embodiments of LEDs disclosed herein include a substrate, it will be understood by those skilled in the art that the crystalline epitaxial growth substrate on which the epitaxial layers comprising an LED are grown may be removed, and the freestanding epitaxial layers may be mounted on a substitute carrier substrate or submount which may have better thermal, electrical, structural and/or optical characteristics than the original substrate. The invention described herein is not limited to structures having crystalline epitaxial growth substrates and may be utilized in connection with structures in which the epitaxial layers have been removed from their original growth substrates and bonded to substitute carrier substrates.

Embodiments of the present invention will be described with reference to <figref idref="f0001"><b>Figure 1</b></figref> that illustrates a light emitting diode (LED) structure <b>40.</b> The LED structure <b>40</b> of <figref idref="f0001"><b>Figure 1</b></figref> includes a substrate <b>10,</b> which is preferably 4H or 6H n-type silicon carbide. Substrate <b>10</b> may also comprise sapphire, bulk gallium nitride or another suitable substrate. Also included in the LED structure <b>40</b> of <figref idref="f0001"><b>Figure 1</b></figref> is a layered semiconductor structure comprising gallium nitride-based semiconductor layers on substrate <b>10.</b> Namely, the LED structure <b>40</b> illustrated includes the following layers: a conductive buffer layer <b>11,</b> a first silicon-doped GaN layer <b>12,</b> a second silicon doped GaN layer <b>14,</b> a superlattice structure <b>16</b> comprising alternating layers of silicon-doped GaN and/or InGaN, an active region <b>18,</b> which may be provided by a multi-quantum well structure, an undoped GaN and/or AlGaN layer <b>22,</b> an AlGaN layer <b>30</b> doped with a p-type impurity, and a GaN contact layer <b>32,</b> also doped with a p-type impurity. The structure further includes an n-type ohmic contact <b>23</b> on the substrate <b>10</b> and a p-type ohmic contact <b>24</b> on the contact layer <b>32.</b>

Buffer layer <b>11</b> is preferably n-type AlGaN. Examples of buffer layers between silicon carbide and group III-nitride materials are provided in <patcit id="pcit0006" dnum="US5393993A"><text>U.S. Patents 5,393,993</text></patcit> and <patcit id="pcit0007" dnum="US5523589A"><text>5,523,589</text></patcit>, and <patcit id="pcit0008" dnum="US154363A" dnum-type="L"><text>U.S. Application Serial No. 09/154,363</text></patcit> entitled "Vertical Geometry InGaN Light Emitting Diode" assigned to the assignee of the present invention, the disclosures of which are incorporated by reference as if fully set forth herein. Similarly, embodiments of the present invention may also include structures such as those described in United States Patent No. <patcit id="pcit0009" dnum="US6201262B"><text>6,201,262</text></patcit> entitled "Group III Nitride Photonic Devices on Silicon Carbide Substrates With Conductive Buffer Interlay Structure," the disclosure of which is incorporated herein by reference as if set forth fully herein.

GaN layer <b>12</b> is preferably between about 500 and 4000 nm thick inclusive and is most preferably about 1500 nm thick. GaN layer <b>12</b> may be doped with silicon at a level of about 5x10¹⁷ to 5x10¹⁸ cm^-3. GaN layer <b>14</b> is preferably between about 10 and 500 Å thick inclusive, and is most preferably about 80 Å thick. GaN layer <b>14</b> maybe doped with silicon at a level of less than about 5x10¹⁹ cm^-3.

As illustrated in <figref idref="f0001"><b>Figure 1</b></figref><b>,</b> a superlattice structure <b>16</b> according to embodiments of the present invention includes alternating layers of In_XGa_1-XN and In_YGa_1-YN, wherein X is between 0 and 1 inclusive and X is not equal to Y. Preferably, X=0 and the thickness of each of the alternating layers of InGaN is about 5-40 Å thick inclusive, and the thickness of each of the alternating layers of GaN is about 5-100 Å thick inclusive. In certain embodiments, the GaN layers are about 30 Å thick and the InGaN layers are about 15 Å thick. The superlattice structure <b>16</b> may include from about 5 to about 50 periods (where one period equals one repetition each of the In_XGa_1-XN and In_YGa_1-YN layers that comprise the superlattice). In one embodiment, the superlattice structure <b>16</b> comprises 25 periods. In another embodiment, the superlattice structure <b>16</b> comprises 10 periods. The number of periods, however, may be decreased by, for example, increasing the thickness of the respective layers. Thus, for example, doubling the thickness of the layers may be utilized with half the number of periods. Alternatively, the number and thickness of the periods may be independent of one another.

Preferably, the superlattice <b>16</b> is doped with an n-type impurity such as silicon at a level of from about 1x10¹⁷ cm^-3 to about 5x10¹⁹ cm^-3. Such a doping level may be actual doping or average doping of the layers of the superlattice <b>16.</b> If such doping level is an average doping level, then it may be beneficial to provide doped layers adjacent the superlattice structure <b>16</b> that provide the desired average doping which the doping of the adjacent layers is averaged over the adjacent layers and the superlattice structure <b>16.</b> By providing the superlattice <b>16</b> between substrate <b>10</b> and active region <b>18,</b> a better surface may be provided on which to grow InGaN-based active region <b>18.</b> While not wishing to be bound by any theory of operation, the inventors believe that strain effects in the superlattice structure <b>16</b> provide a growth surface that is conducive to the growth of a high-quality InGaN-containing active region. Further, the superlattice is known to influence the operating voltage of the device. Appropriate choice of superlattice thickness and composition parameters can reduce operating voltage and increase optical efficiency.

The superlattice structure <b>16</b> may be grown in an atmosphere of nitrogen or other gas, which enables growth of higher-quality InGaN layers in the structure. By growing a silicon-doped InGaN/GaN superlattice on a silicon-doped GaN layer in a nitrogen atmosphere, a structure having improved crystallinity and conductivity with optimized strain may be realized.

In certain embodiments of the present invention, the active region <b>18</b> may comprise a single or multi-quantum well structure as well as single or double heterojunction active regions. In particular embodiments of the present invention, the active region <b>18</b> comprises a multi-quantum well structure that includes multiple InGaN quantum well layers separated by barrier layers (not shown in <figref idref="f0001"><b>Figure 1</b></figref>).

Layer <b>22</b> is provided on active region <b>18</b> and is preferably undoped GaN or AlGaN between about 0 and 120 Å thick inclusive. As used herein, undoped refers to a not intentionally doped. Layer <b>22</b> is preferably about 35 Å thick. If layer <b>22</b> comprises AlGaN, the aluminum percentage in such layer is preferably about 10-30% and most preferably about 24%. The level of aluminum in layer <b>22</b> may also be graded in a stepwise or continuously decreasing fashion. Layer <b>22</b> may be grown at a higher temperature than the growth temperatures in quantum well region <b>25</b> in order to improve the crystal quality of layer <b>22.</b> Additional layers of undoped GaN or AlGaN may be included in the vicinity of layer <b>22.</b> For example, LED <b>1</b> may include an additional layer of undoped AlGaN about 6-9Å thick between the active region <b>18</b> and the layer <b>22.</b>

An AlGaN layer <b>30</b> doped with a p-type impurity such as magnesium is provided on layer <b>22.</b> The AlGaN layer <b>30</b> may be between about 0 and 300 Å thick inclusive and is preferably about 130 Å thick. A contact layer <b>32</b> of p-type GaN is provided on the layer <b>30</b> and is preferably about 1800 Å thick. Ohmic contacts <b>24</b> and <b>25</b> are provided on the p-GaN contact layer <b>32</b> and the substrate <b>10,</b> respectively.

<figref idref="f0002"><b>Figure 2</b></figref> illustrates further embodiments of the present invention incorporating a multi-quantum well active region. The embodiments of the present invention illustrated in <figref idref="f0002"><b>Figure 2</b></figref> include a layered semiconductor structure <b>100</b> comprising gallium nitride-based semiconductor layers grown on a substrate <b>10.</b> As described above, the substrate <b>10</b> may be SiC, sapphire or bulk gallium nitride. As is illustrated in <figref idref="f0002"><b>Figure 2</b></figref><b>,</b> LEDs according to particular embodiments of the present invention may include a conductive buffer layer <b>11,</b> a first silicon-doped GaN layer <b>12,</b> a second silicon doped GaN layer <b>14,</b> a superlattice structure <b>16</b> comprising alternating layers of silicon-doped GaN and/or InGaN, an active region <b>125</b> comprising a multi-quantum well structure, an undoped GaN or AlGaN layer <b>22,</b> an AlGaN layer <b>30</b> doped with a p-type impurity, and a GaN contact layer <b>32,</b> also doped with a p-type impurity. The LEDs may further include an n-type ohmic contact <b>23</b> on the substrate <b>10</b> and a p-type ohmic contact <b>24</b> on the contact layer <b>32.</b> In embodiments of the present invention where the substrate <b>10</b> is sapphire, the n-type ohmic contact <b>23</b> would be provided on n-type GaN layer <b>12</b> and/or n-type GaN layer <b>14.</b>

As described above with reference to <figref idref="f0001"><b>Figure 1</b></figref><b>,</b> buffer layer <b>11</b> is preferably n-type AlGaN. Similarly, GaN layer <b>12</b> is preferably between about 500 and 4000 nm thick inclusive and is most preferably about 1500 nm thick. GaN layer <b>12</b> may be doped with silicon at a level of about 5x10¹⁷ to 5x10¹⁸ cm^-3. GaN layer <b>14</b> is preferably between about 10 and 500 Å thick inclusive, and is most preferably about 80 Å thick. GaN layer <b>14</b> may be doped with silicon at a level of less than about 5x10¹⁹ cm^-3. The superlattice structure <b>16</b> may also be provided as described above with reference to <figref idref="f0001"><b>Figure 1</b></figref><b>.</b>

The active region <b>125</b> comprises a multi-quantum well structure that includes multiple InGaN quantum well layers <b>120</b> separated by barrier layers <b>118.</b> The barrier layers <b>118</b> comprise In_XGa_1-XN where 0≤X&lt;1. Preferably the indium composition of the barrier layers <b>118</b> is less than that of the quantum well layers <b>120,</b> so that the barrier layers <b>118</b> have a higher bandgap than quantum well layers <b>120.</b> The barrier layers <b>118</b> and quantum well layers <b>120</b> may be undoped (<i>i.e.</i> not intentionally doped with an impurity atom such as silicon or magnesium). However, it may be desirable to dope the barrier layers <b>118</b> with Si at a level of less than 5x10¹⁹ cm^-3, particularly if ultraviolet emission is desired.

In further embodiments of the present invention, the barrier layers <b>118</b> comprise Al_XIn_YGa_1-X-YN where 0&lt;X&lt;1, 0≤Y&lt;1 and X+Y ≤1. By including aluminum in the crystal of the barrier layers <b>118,</b> the barrier layers <b>118</b> may be lattice-matched to the quantum well layers <b>120,</b> thereby providing improved crystalline quality in the quantum well layers <b>120,</b> which increases the luminescent efficiency of the device.

Referring to <figref idref="f0003"><b>Figure 3</b></figref><b>,</b> embodiments of the present invention that provide a multi-quantum well structure of a gallium nitride based device are illustrated. The multi-quantum well structure illustrated in <figref idref="f0003"><b>Figure 3</b></figref> may provide the active region of the LEDs illustrated in <figref idref="f0001"><b>Figure 1</b></figref> and/or <figref idref="f0002"><b>Figure 2</b></figref><b>.</b> As seen in <figref idref="f0003"><b>Figure 3</b></figref><b>,</b> an active region <b>225</b> comprises a periodically repeating structure <b>221</b> comprising a well support layer <b>218a</b> having high crystal quality, a quantum well layer <b>220</b> and a cap layer <b>218b</b> that serves as a protective cap layer for the quantum well layer <b>220.</b> When the structure <b>221</b> is grown, the cap layer <b>218b</b> and the well support layer <b>218a</b> together form the barrier layer between adjacent quantum wells <b>220.</b> Preferably, the high quality well support layer <b>218a</b> is grown at a higher temperature than that used to grow the InGaN quantum well layer <b>220.</b> In some embodiments of the present invention, the well support layer <b>218a</b> is grown at a slower growth rate than the cap layer <b>218b.</b> In other embodiments, lower growth rates may be used during the lower temperature growth process and higher growth rates utilized during the higher temperature growth process. For example, in order to achieve a high quality surface for growing the InGaN quantum well layer <b>220,</b> the well support layer <b>218a</b> may be grown at a growth temperature of between about 700 and 900°C. Then, the temperature of the growth chamber is lowered by from about 0 to about 200 °C to permit growth of the high-quality InGaN quantum well layer <b>220.</b> Then, while the temperature is kept at the lower InGaN growth temperature, the cap layer <b>218b</b> is grown. In that manner, a multi-quantum well region comprising high quality InGaN layers may be fabricated.

The active regions <b>125</b> and <b>225</b> of <figref idref="f0002"><b>Figures 2</b></figref> and <figref idref="f0003"><b>3</b></figref> are preferably grown in a nitrogen atmosphere, which may provide increased InGaN crystal quality. The barrier layers <b>118,</b> the well support layers <b>218a</b> and/or the cap layers <b>218b</b> may be between about 50 - 400 Å thick inclusive. The combined thickness of corresponding ones of the well support layers <b>218a</b> and the cap layers <b>218b</b> may be from about 50-400 Å thick inclusive. Preferably, the barrier layers <b>118</b> the well support layers <b>218a</b> and/or the cap layers <b>218b</b> are greater than about 90 Å thick and most preferably are about 225 Å thick. Also, it is preferred that the well support layers <b>218a</b> be thicker than the cap layers <b>218b.</b> Thus, the cap layers <b>218b</b> are preferably as thin as possible while still reducing the desorption of Indium from or the degradation of the quantum well layers <b>220.</b> The quantum well layers <b>120</b> and <b>220</b> may be between about 10 - 50 Å thick inclusive. Preferably, the quantum well layers <b>120</b> and <b>220</b> are greater than 20 Å thick and most preferably are about 25 Å thick. The thickness and percentage of indium in the quantum well layers <b>120</b> and <b>220</b> may be varied to produce light having a desired wavelength. Typically, the percentage of indium in quantum well layers <b>120</b> and <b>220</b> is about 25-30%, however, depending on the desired wavelength, the percentage of indium has been varied from about 5% to about 50%.

In preferred embodiments of the present invention, the bandgap of the superlattice structure <b>16</b> exceeds the bandgap of the quantum well layers <b>120.</b> This may be achieved by by adjusting the average percentage of indium in the superlattice <b>16.</b> The thickness (or period) of the superlattice layers and the average Indium percentage of the layers should be chosen such that the bandgap of the superlattice structure <b>16</b> is greater than the bandgap of the quantum wells <b>120.</b> By keeping the bandgap of the superlattice <b>16</b> higher than the bandgap of the quantum wells <b>120,</b> unwanted absorption in the device may be minimized and luminescent emission may be maximized. The bandgap of the superlattice structure <b>16</b> may be from about 2.95 eV to about 3.35 eV. In a preferred embodiment, the bandgap of the superlattice structure <b>16</b> is about 3.15 eV.

In additional embodiments of the present invention, the LED structure illustrated in <figref idref="f0002"><b>Figure 2</b></figref> includes a spacer layer <b>17</b> disposed between the superlattice <b>16</b> and the active region <b>125.</b> The spacer layer <b>17</b> preferably comprises undoped GaN. The presence of the optional spacer layer <b>17</b> between the doped superlattice <b>16</b> and active region <b>125</b> may deter silicon impurities from becoming incorporated into the active region <b>125.</b> This, in turn, may improve the material quality of the active region <b>125</b> that provides more consistent device performance and better uniformity. Similarly, a spacer layer may also be provided in the LED structure illustrated in <figref idref="f0001"><b>Figure 1</b></figref> between the superlattice <b>16</b> and the active region <b>18.</b>

Returning to <figref idref="f0002"><b>Figure 2</b></figref><b>,</b> the layer <b>22</b> may be provided on the active region <b>125</b> and is preferably undoped GaN or AlGaN between about 0 and 120 Å thick inclusive. The layer <b>22</b> is preferably about 35 Å thick. If the layer <b>22</b> comprises AlGaN, the aluminum percentage in such layer is preferably about 10-30% and most preferably about 24%. The level of aluminum in the layer <b>22</b> may also be graded in a stepwise or continuously decreasing fashion. The layer <b>22</b> may be grown at a higher temperature than the growth temperatures in the active region <b>125</b> in order to improve the crystal quality of the layer <b>22.</b> Additional layers of undoped GaN or AlGaN may be included in the vicinity of layer <b>22.</b> For example, the LED illustrated in <figref idref="f0002"><b>Figure 2</b></figref> may include an additional layer of undoped AlGaN about 6-9Å thick between the active regions <b>125</b> and the layer <b>22.</b>

An AlGaN layer <b>30</b> doped with a p-type impurity such as magnesium is provided on layer <b>22.</b> The AlGaN layer <b>30</b> may be between about 0 and 300 Å thick inclusive and is preferably about 130 Å thick. A contact layer <b>32</b> of p-type GaN is provided on the layer <b>30</b> and is preferably about 1800 Å thick. Ohmic contacts <b>24</b> and <b>25</b> are provided on the p-GaN contact layer <b>32</b> and the substrate <b>10,</b> respectively. Ohmic contacts <b>24</b> and <b>25</b> are provided on the p-GaN contact layer <b>32</b> and the substrate <b>10,</b> respectively.

<figref idref="f0004"><b>Figure 4</b></figref> illustrates further embodiments of the present invention incorporating a Group III-nitride layer incorporating Indium on the active region of the device. For example, an InAlGaN cap structure may be provided. The embodiments of the present invention illustrated in <figref idref="f0004"><b>Figure 4</b></figref> include a layered semiconductor structure <b>400</b> comprising gallium nitride-based semiconductor layers grown on a substrate <b>10.</b> As described above, the substrate <b>10</b> may be SiC, sapphire or bulk gallium nitride., . In particular embodiments of the present invention, the substrate <b>10</b> is a SiC substrate having a thickness of from about 50 to about 800 µm and in some embodiments, about 100 µm.

As is illustrated in <figref idref="f0004"><b>Figure 4</b></figref><b>,</b> LEDs according to particular embodiments of the present invention may include a conductive buffer layer <b>11,</b> a first silicon-doped GaN layer <b>12,</b> a second silicon doped GaN layer <b>14,</b> a superlattice structure <b>16</b> comprising alternating layers of silicon-doped GaN and/or InGaN, an active region <b>125</b> comprising a multi-quantum well structure, an undoped AlInGaN layer <b>40,</b> an AlGaN layer <b>30</b> doped with a p-type impurity, and a GaN contact layer <b>32,</b> also doped with a p-type impurity. The LEDs may further include an n-type ohmic contact <b>23</b> on the substrate <b>10</b> and a p-type ohmic contact <b>24</b> on the contact layer <b>32.</b> In embodiments of the present invention where the substrate <b>10</b> is sapphire, the n-type ohmic contact <b>23</b> would be provided on n-type GaN layer <b>12</b> and/or n-type GaN layer <b>14.</b>

As described above with reference to <figref idref="f0001"><b>Figures 1</b></figref> and <figref idref="f0002"><b>2</b></figref><b>,</b> the buffer layer <b>11</b> may be n-type AlGaN. For example, the buffer layer <b>11</b> may be AlGaN doped with Si and having a thickness of from about 100 to about 10.000 Å. In certain embodiments the thickness is about 1500 Å. The GaN layer <b>12</b> may be doped with Si and may have a thickness of from about 5000 to 50,000 Å thick inclusive and, in particular embodiments, is about 18,000 Å thick. The GaN layer <b>12</b> may be doped with silicon at a level of about 5x10¹⁷ to 5x10¹⁸ cm^-3. The superlattice structure <b>16</b> may also be provided as described above with reference to <figref idref="f0001"><b>Figure 1</b></figref><b>.</b> For example, the superlattice structure <b>16</b> may have from 3 to 35 periods of InGaN/GaN. The thickness of the periods may be from about 30 to about 100 Å. In particular embodiments of the present invention, twenty five (25) periods of InGaN/GaN are provided with the thickness of a period of layers being about 70 Å and the thickness of the GaN or InGaN layer being about 15 Å with the other layer making up the remainder.

The active region <b>325</b> may include a multi-quantum well structure that includes multiple InGaN quantum well layers <b>320</b> separated by barrier layers <b>318.</b> The barrier layers <b>318</b> comprise In_XGa_1-XN where 0≤X&lt;1. Preferably the indium composition of the barrier layers <b>318</b> is less than that of the quantum well layers <b>320,</b> so that the barrier layers <b>318</b> have a higher bandgap than quantum well layers <b>320.</b> The barrier layers <b>318</b> and quantum well layers <b>320</b> may be undoped (<i>i.e.</i> not intentionally doped with an impurity atom such as silicon or magnesium). However, it may be desirable to dope the barrier layers <b>318</b> with Si at a level of less than 5x10¹⁹ cm^-3, particularly if ultraviolet emission is desired.

In further embodiments of the present invention, the barrier layers <b>318</b> comprise Al_XIn_YGa_1-X-YN where 0&lt;X&lt;1, 0≤Y&lt;1 and X+Y ≤1. By including aluminum in the crystal of the barrier layers <b>318,</b> the barrier layers <b>318</b> may be lattice-matched to the quantum well layers <b>320,</b> thereby allowing improved crystalline quality in the quantum well layers <b>320,</b> which can increase the luminescent efficiency of the device.

The active region <b>325</b> may also be provided as illustrated in <figref idref="f0003"><b>Figure 3</b></figref> and described above with reference to <figref idref="f0001 f0002 f0003"><b>Figures 1</b> through <b>3</b></figref><b>.</b> In particular embodiments of the present invention, the active region <b>325</b> includes 3 or more quantum wells and in certain embodiments, eight (8) quantum wells are provided. The thickness of the quantum well structures may be from about 30 to about 250 Å. In particular embodiments of the present invention, the thickness of a quantum well structure may be about 120 Å with the thickness of the well layer being about 25 Å.

The LED structure illustrated in <figref idref="f0004"><b>Figure 4</b></figref> may also include a spacer layer disposed between the superlattice <b>16</b> and the active region <b>325</b> as described above.

Returning to <figref idref="f0004"><b>Figure 4</b></figref><b>,</b> a Group III-nitride capping layer <b>40</b> that includes Indium may be provided on the active region <b>325</b> and, more specifically, on the quantum well <b>320</b> of the active region <b>325.</b> The Group III-nitride capping layer <b>40</b> may include InAlGaN between about 10 and 320 Å thick inclusive. The capping layer <b>40</b> may be of uniform composition, multiple layers of different compositions and/or graded composition. In particular embodiments of the present invention, the capping layer <b>40</b> includes a first capping layer having a composition of In_xAl_yGa_1-x-yN, where 0&lt;x≤0.2 and 0≤y≤0.4 and has a thickness of from about 10 to about 200 Å and a second capping layer having a composition of In_wAl_zGa_1-w-zN, where 0&lt;w≤0.2 and y≤z&lt;1 and has a thickness of from about 10 to about 120 Å. In certain embodiments of the present invention, the first capping layer has a thickness of about 80 Å, x=0.1 and y=0.25 and the second capping layer has a thickness of about 30 Å, w=0.05 and z=0.55. The capping layer <b>40</b> may be grown at a higher temperature than the growth temperatures in the active region <b>325</b> in order to improve the crystal quality of the layer <b>40.</b> Additional layers of undoped GaN or AlGaN may be included in the vicinity of layer <b>40.</b> For example, a thin layer of GaN may be provided between a last quantum well layer and the capping layer <b>40.</b> The capping layer <b>40</b> that includes indium may be more closely lattice matched to the quantum wells of the active region <b>325</b> and may provide a transition from the lattice structure of the active region <b>325</b> to the lattice structure of the p-type layers. Such a structure may result in increased brightness of the device.

An AlGaN hole injection layer <b>42</b> doped with a p-type impurity such as magnesium is provided on the capping layer <b>40.</b> The AlGaN layer <b>42</b> may be between about 50 and 2500 Å thick inclusive and, in particular embodiments, is about 150 Å thick. The AlGaN layer <b>42</b> may be of the composition of AlGa_1-xN, where 0≤x≤0.4. In particular embodiments of the present invention, x=0.23 for.the AlGaN layer <b>42.</b> The AlGaN layer <b>42</b> may be doped with Mg. In some embodiments of the present invention, the layer <b>42</b> may also include Indium.

A contact layer <b>32</b> of p-type GaN is provided on the layer <b>42</b> and may be from about 250 to abut 10,000 Å thick and in some embodiments, about 1500 Å thick. In some embodiments, the contact layer <b>32</b> may also include Indium. Ohmic contacts <b>24</b> and <b>25</b> are provided on the p-GaN contact layer <b>32</b> and the substrate <b>10,</b> respectively. Ohmic contacts <b>24</b> and <b>25</b> are provided on the p-GaN contact layer <b>32</b> and the substrate <b>10,</b> respectively.

Electroluminescence (EL) testing was performed on LED wafers having devices with and without the indium containing capping layer, in particular, an InAlGaN capping layer, as illustrated in <figref idref="f0004"><b>Figure 4</b></figref><b>.</b> The EL test is an on-wafer test that measures the brightness of LED epitaxial structures. This test is not influenced by the LED fabrication method, chip shaping, or packaging method. Approximately 176 wafers with the structure including the indium containing layer and 615 wafers without the indium containing layer were tested. Both structures were grown continuously on a number of reactors. The reactors were all essentially the same (i.e. none have any special modification for increased brightness, all have been and continue to be suitable for production use). The data from the wafers was binned and shows that the structure with the indium containing layer was approximately 1.15 to 1.25 times brighter than the structure without the indium containing layer.

While embodiments of the present invention have been described with multiple quantum wells, the benefits from the teachings of the present invention may also be achieved in single quantum well structures. Thus, for example, a light emitting diode may be provided with a single occurrence of the structure <b>221</b> of <figref idref="f0003"><b>Figure 3</b></figref> as the active region of the device. Thus, while different numbers of quantum wells may be utilized according to embodiments of the present invention, the number of quantum wells will typically range from 1 to 10 quantum wells.

While embodiments of the present invention have been described with reference to gallium nitride based devices, the teachings and benefits of the present invention may also be provided in other Group III nitrides. Thus, embodiments of the present invention provide Group III nitride based superlattice structures, quantum well structures and/or Group III nitride based light emitting diodes having superlattices and/or quantum wells.

In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.