Indium gallium nitride-based ohmic contact layers for gallium nitride-based devices

Light emitting devices include a gallium nitride-based epitaxial structure that includes an active light emitting region and a gallium nitride-based outer layer, for example gallium nitride. A indium nitride-based layer, such as indium gallium nitride, is provided directly on the outer layer. A reflective metal layer or a transparent conductive oxide layer is provided directly on the indium gallium nitride layer opposite the outer layer. The indium gallium nitride layer forms a direct ohmic contact with the outer layer. An ohmic metal layer need not be used. Related fabrication methods are also disclosed.

FIELD OF THE INVENTION

This invention relates to semiconductor devices, such as light emitting devices, and methods of fabricating same, and more particularly to ohmic contacts for gallium nitride-based devices and methods of fabricating same.

BACKGROUND OF THE INVENTION

Light emitting diodes and laser diodes are well known solid state electronic devices capable of generating light upon application of a sufficient voltage. Light emitting diodes and laser diodes may be generally referred to as light emitting devices (“LEDs”). Light emitting devices generally include a p-n junction formed in an epitaxial layer grown on a substrate such as sapphire, silicon, silicon carbide, gallium arsenide and the like. The wavelength distribution of the light generated by the LED generally depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that make up the active light emitting region of the device.

Typically, an LED chip includes a substrate, an N-type epitaxial region formed on the substrate and a P-type epitaxial region formed on the N-type epitaxial region (or vice-versa). In order to facilitate the application of a voltage to the device, an anode ohmic contact is formed on a P-type region of the device (typically, an exposed P-type epitaxial layer) and a cathode ohmic contact is formed on an N-type region of the device (such as the substrate or an exposed N-type epitaxial layer). The ohmic contact may typically include an ohmic metal layer, such as platinum, palladium, nickel, titanium, gold, tin or combinations thereof. The ohmic metal layer is generally provided to reduce the forward or turn-on voltage of the device.

SUMMARY OF THE INVENTION

Light emitting devices according to some embodiments of the present invention include a gallium nitride-based epitaxial structure that includes an active light emitting region and a gallium nitride-based outer layer, for example binary gallium nitride. The active light emitting region may include multiple quantum well structures with indium gallium nitride-based wells. An indium gallium nitride-based layer is provided directly on the gallium nitride-based outer layer. In some embodiments, the indium gallium nitride-based layer has a higher percentage indium than the wells. A reflective metal layer is provided directly on the indium gallium nitride-based layer opposite the gallium nitride-based outer layer. However, in other embodiments, an ohmic metal layer is also provided between the indium gallium nitride-based layer and the reflective metal layer.

In some embodiments, the indium gallium nitride-based layer is undoped (i.e., not intentionally doped) and the gallium nitride-based outer layer is doped P-type. In other embodiments, the indium gallium nitride-based layer may be between about 5 Å and about 100 Å thick. In still other embodiments, the gallium nitride-based outer layer may include a nonplanar outer surface, and the indium gallium nitride-based layer is also nonplanar. In some embodiments, the reflective metal layer comprises silver, nickel-silver alloy and/or aluminum. Other embodiments also include a barrier layer on the reflective metal layer opposite the indium gallium nitride-based layer, and a bonding layer on the barrier layer opposite the reflective metal layer. Yet other embodiments may include a silicon carbide or other substrate on the gallium nitride-based epitaxial structure opposite the outer layer.

Light emitting devices according to other embodiments of the invention include a gallium nitride-based epitaxial structure that includes an active light emitting region and a gallium nitride-based outer layer, for example binary gallium nitride. The active light emitting region may include multiple quantum well structures with indium gallium nitride-based wells. An indium gallium nitride-based layer is provided directly on the gallium nitride-based outer layer. In some embodiments, the indium gallium nitride-based layer has a higher percentage indium than the wells. A transparent conductive spacer layer is provided directly on the indium gallium nitride-based layer opposite the gallium nitride-based outer layer. A reflective metal layer is provided directly on the transparent conductive spacer layer opposite the indium gallium nitride-based layer. Dopings, thicknesses, barrier layers, bonding layers, nonplanar layers and/or substrates may be provided as was described above. In some embodiments, the transparent conductive spacer layer may be sufficiently thick to space the active light emitting region apart from the reflective metal layer, so as to increase, and in some embodiments maximize, reflection of light from the reflective layer.

Light emitting devices according to yet other embodiments of the present invention include a gallium nitride-based epitaxial structure that includes an active light emitting region and a gallium nitride-based outer layer, for example binary gallium nitride. A binary indium nitride layer is provided directly on the gallium nitride-based outer layer. A reflective metal layer or a transparent conductive oxide layer is provided directly on the binary indium nitride layer opposite the gallium nitride-based outer layer. Accordingly, a binary indium nitride layer forms a direct ohmic contact with a gallium nitride-based outer layer. An ohmic metal layer need not be used, although it may be provided between the binary indium nitride layer and the gallium nitride-based outer layer in other embodiments.

In still other embodiments of the present invention, the reflective metal layer may comprise silver, nickel and/or aluminum, and the transparent conductive oxide layer may comprise indium tin oxide. Moreover, some embodiments may provide a barrier layer on the reflective metal layer opposite the binary indium nitride layer and a bonding layer on the barrier layer opposite the reflective metal layer. Other embodiments may provide a bond pad on the transparent conductive oxide layer opposite the binary indium nitride layer. Moreover, a silicon carbide or other substrate may be provided on the gallium nitride-based epitaxial structure opposite the outer layer.

Light emitting devices according to still other embodiments of the invention include a gallium nitride-based epitaxial structure that includes an active light emitting region. An indium gallium nitride-based layer including therein clusters of elemental indium and/or binary indium nitride is provided on the gallium nitride-based epitaxial structure. A reflective metal layer or a transparent conductive oxide layer is provided on the indium gallium nitride-based layer opposite the gallium nitride-based epitaxial structure. In some embodiments, the indium gallium nitride-based layer including therein clusters of elemental indium and/or binary indium nitride is an indium gallium nitride layer including therein clusters of elemental indium and/or binary indium nitride. Remaining layers in the device including the reflective metal layer, the transparent conductive oxide layer, the barrier layer, the bond pad and/or the substrate may be provided as was described above.

Embodiments of the present invention have been described above in connection with light emitting devices. However, analogous methods of forming light emitting devices may also be provided according to other embodiments of the present invention. In some embodiments, a gallium nitride-based structure that includes an active light emitting region and a gallium nitride-based outer layer, for example binary gallium nitride, may be epitaxially formed. An indium gallium nitride layer may be epitaxially formed directly on the outer layer. A reflective metal layer or a transparent conductive layer may be formed directly on the indium gallium nitride layer. The reflective layer may be formed by electron beam and/or sputter deposition without annealing, or a subsequent anneal may take place. The gallium nitride-based outer layer may be fabricated by exposing the gallium nitride-based structure to sources of gallium, nitrogen and a P-type dopant, and the indium gallium nitride layer may be formed by further exposing the gallium nitride-based structure to the sources of gallium and nitrogen, and to a source of indium, while terminating exposure to the P-type dopant. Other structures may be formed as described herein.

DETAILED DESCRIPTION

It will be understood that when an element such as a layer, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. It will be understood that if part of an element is referred to as “outer,” it is closer to the outside of the device than other parts of the element. Furthermore, relative terms such as “beneath” or “overlies” may be used herein to describe a relationship of one layer or region to another layer or region relative to a substrate or base layer as illustrated in the figures. It will be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. Finally, the term “directly” means that there are no intervening elements. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items and may be abbreviated as “/”.

Embodiments of the invention are described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized embodiments of the 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 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, a region illustrated or described as a rectangle will, typically, have rounded or curved features due to normal manufacturing tolerances. 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 invention.

Various embodiments of semiconductor light emitting devices will be described herein. As used herein, the term “semiconductor light emitting device” may include a light emitting diode, laser diode and/or other semiconductor device which includes one or more semiconductor light emitting layers, which may include silicon, silicon carbide, gallium nitride and/or other semiconductor materials. A light emitting device may or may not include a substrate such as a sapphire, silicon, silicon carbide, aluminum nitride and/or other microelectronic substrate(s). A light emitting device may include one or more contact layers which may include metal and/or other conductive layers. In some embodiments, ultraviolet, blue and/or green light emitting diodes may be provided. Red and/or amber LEDs may also be provided. The design and fabrication of semiconductor light emitting devices are understood by those having skill in the art and need not be described in detail herein. For example, the semiconductor light emitting devices may be gallium nitride-based light emitting diodes or lasers fabricated on a silicon carbide substrate such as those devices manufactured and sold by Cree, Inc. of Durham, N.C.

Finally, when referring to a Group III nitride compound, the word “-based” means that additional Group III elements may be included in the compound. Thus, for example, a gallium nitride-based layer includes binary, ternary, quaternary, etc. compounds that include gallium nitride (GaN), and an indium gallium nitride-based layer includes quaternary and higher compounds that include indium gallium nitride (InGaN). In contrast, the terms “elemental”, “binary”, “ternary”, “quaternary”, etc. mean that additional Group III materials are not included in the compound. Thus, for example, binary gallium nitride excludes ternary, quaternary, etc. compounds, and ternary indium gallium nitride excludes quaternary, etc. compounds. However, all of the above defined terms do not preclude the Group III nitride compound from being doped P-type and/or N-type using, for example, P-type dopants such as magnesium and/or N-type dopants such as silicon.

Conventional LEDs based on Group III nitrides typically use an ohmic metal layer between the device layer and (1) a reflective metal layer in the case of flip-chip LEDs, or (2) a metal bond pad in the case of conventional (i.e., non-flip-chip) LEDs. The ohmic metal layer may include platinum, palladium, titanium, gold, tin or combinations thereof. Without the ohmic metal layer, the device may suffer from high forward or turn-on voltage (Vf).

Some embodiments of the present invention may arise from a recognition that, even though ohmic metal layers are generally used in light emitting devices, the ohmic metal layer still may produce an undesirably high forward voltage. Moreover, an ohmic metal layer may undesirably absorb at least some emission from the active layer, even if the ohmic metal layer is made very thin. Some embodiments of the present invention can eliminate the need for a separate ohmic metal layer, or can expand the range of metals that can form ohmic contacts to nitride-based devices. Moreover, embodiments of the invention may also be used with an ohmic metal layer to further reduce the forward voltage.

Some embodiments of the present invention can provide an indium nitride-based layer, such as an indium gallium nitride-based layer, directly on a gallium nitride-based outer layer, for example binary gallium nitride, and a reflective metal layer or a transparent conductive oxide layer directly on the indium nitride-based layer. The indium nitride-based layer, such as an indium gallium nitride-based layer, can provide a low bandgap contact layer that is undoped (i.e., not intentionally doped) or is intentionally doped N-type or P-type. For flip-chip mounting, a reflective metal layer may be provided directly on the indium nitride-based ohmic contact layer. For standard devices (i.e., non-flip-chip), there is no need to cover the surface of the indium nitride-based contact layer with a metal ohmic contact. In either case, forward voltage of the resulting device may be reduced, and light emission may be increased.

Stated differently, a reflective metal layer, such as silver, does not provide a good ohmic contact to conventional gallium nitride-based LEDs. However, some embodiments of the present invention provide an intermediate layer of indium nitride-based material, such as indium gallium nitride-based material, that can allow a good ohmic contact to be formed, so that the need for a separate ohmic metal layer may be eliminated. Moreover, when the indium nitride-based layer, such as the indium gallium nitride-based layer, is directly on a gallium nitride-based outer layer, for example binary gallium nitride, an even better ohmic contact may be provided.

FIG. 1is a cross-sectional view of a conventional flip-chip LED. As shown inFIG. 1, a conventional flip-chip LED10includes a gallium nitride-based epitaxial structure, also referred to as “LED epi”28that includes an active light emitting region11, a gallium nitride-based outer layer12of, for example, binary gallium nitride that may be doped P-type, also referred to as a P—GaN-based layer and a gallium nitride-based layer18of, for example, binary gallium nitride that may be doped N-type, also referred to as an N—GaN-based layer. An ohmic metal layer13is provided on the P—GaN-based layer12and an ohmic metal layer23is provided on the N—GaN-based layer18. The ohmic metal layers13/23may include platinum, palladium, nickel, titanium, gold, tin and/or combinations thereof. A reflective metal layer14, for example comprising silver, nickel silver alloy and/or aluminum is provided on the ohmic metal layer13. A barrier layer15, comprising, for example, platinum, is provided on the reflective metal layer14. A bonding layer16, for example comprising gold, is provided on the barrier layer15and is used to bond the device to a carrier17, such as a submount. A substrate may be provided on the active region11in some embodiments.

FIG. 2is a cross-sectional view of a conventional non-flip-chip LED20that includes a substrate21, an LED epi region28, including an active region11, a P—GaN-based outer layer12and an N—GaN-based layer18, as was described above, and an ohmic metal layer13as was described above. The ohmic layer13provides current spreading over the entire top surface of the device. A bond pad22, such as a nickel/gold bond pad, is provided on the ohmic metal layer13.

FIG. 3is a cross-sectional view of a flip-chip light emitting device according to various embodiments of the present invention. As shown inFIG. 3, these devices30may include an active region11, a P—GaN-based layer12, a reflective metal layer14, a barrier layer15, a bonding layer16and a carrier17, as was described above in connection withFIG. 1. The active region11also may generally include a multi-quantum well (MQW) structure that includes indium gallium nitride-based wells and gallium nitride-based barriers. A binary indium nitride layer33is provided directly on the gallium nitride-based outer layer12. Moreover, the reflective metal layer14is provided directly on the binary indium nitride layer33opposite the gallium nitride-based outer layer12. A separate ohmic metal layer13is not provided. However, in other embodiments, an ohmic metal layer may be provided to further decrease the forward voltage. The active region11, the outer layer12and the binary indium nitride layer33may be fabricated as an LED epitaxial structure38.

Still referring toFIG. 3, an N-type gallium nitride-based layer18may be provided on the active region11opposite the P—GaN-based layer12, and may also be included as part of the LED epi structure38. The N—GaN-based layer18may be provided on a substrate19, which may comprise silicon carbide, sapphire and/or other transparent substrates. The substrate19is shown in dashed line because it can be removed after forming the LED epi structure38on the substrate19, so that it need not be present in the final structure.

Moreover, when the N—GaN-based layer18and/or the substrate19are included, one or more surfaces thereof may be roughened to allow improved light extraction. Thus, the interface between the N—GaN-based layer18and the substrate19may be nonplanar (e.g., textured and/or roughened) and/or the outer surface of the substrate19remote from the N—GaN-based layer18may be nonplanar. Nonplanar surfaces may be provided by adjusting growth parameters and/or by texturing/roughening a planar surface after it is grown.

FIG. 4illustrates a non-flip-chip LED according to various embodiments of the present invention. As shown inFIG. 4, these non-flip-chip LEDs40may include a substrate21, an active region11, an N—GaN-based layer18, an outer layer12and a bond pad22, as was described above in connection withFIG. 2. However, a binary indium nitride layer43is provided directly on the P—GaN-based outer layer12. Moreover, a transparent conductive oxide layer44, such as indium tin oxide (ITO), is provided directly on the binary indium nitride layer43opposite the outer layer12, to provide the requisite current spreading. A separate ohmic metal layer13is not provided. However, it may be provided in other embodiments. The active region11, the GaN-based layers12and13, and the binary indium nitride layer43may be fabricated as an LED epitaxial structure48.

The binary indium nitride layer33and/or43ofFIGS. 3and/or4, respectively, can provide a lower bandgap than the outer P—GaN-based layer12of these figures, and can also provide relatively low resistivity, to thereby provide a good ohmic contact.

Additional discussion of the binary indium nitride layers33,43will now be provided. In some embodiments, these layers are undoped, i.e., not intentionally doped. These layers may have a thickness of between about 5 Å and about 100 Å, and, in some embodiments, about 20 Å. In other embodiments, the binary indium nitride layer33,43may be intentionally doped N-type or P-type. These layers may typically be N-type as grown. However, N-type dopants, such as silicon, may also be used. For P-type layers, magnesium (Mg) may be used as a dopant. However, magnesium doped layers may also be N-type if the amount of magnesium is not sufficient for the layer to be P-type. The binary indium nitride layer33,43may be grown at the same conditions as wells in the active region11. The binary indium nitride-based layer may be pit-free or may manifest pits as measured by an atomic force microscope or scanning electron microscope.

Moreover, inFIG. 3, the reflective metal layer14may be deposited so that it is ohmic in its as-deposited state. For example, electron beam deposition and/or sputter deposition may be used. Alternatively, the reflective metal14may be non-ohmic as deposited and then may be annealed, for example between about 200° C. and about 700° and, in some embodiments, at about 300° C., for between about 1 minute and 30 minutes, and, in some embodiments, for about 15 minutes, to make the reflective metal14ohmically contact the binary indium nitride layer33.

FIGS. 5 and 6illustrate other flip-chip and non-flip-chip embodiments of the present invention, respectively. InFIGS. 5 and 6, these LEDs50and60include an indium gallium nitride-based layer53that includes therein clusters54of elemental indium and/or binary indium nitride. Growth conditions may be controlled, for example by decreasing the growth temperature to less than about 750° C., to provide an indium gallium nitride-based layer53with clusters54of elemental indium and/or binary indium nitride therein. These clusters54may be of average size of less than about 100 Å, and, in some embodiments, less than about 50 Å in diameter. These clusters54may have even lower bandgaps, such as a bandgap of less than about 1 eV, and in some embodiments, of about 0.7 eV, and may provide conductive paths that can further reduce overall forward voltage. These layers may also have an indium composition of between about 5 atomic percent and about 30 atomic percent. Other parameters of the indium gallium nitride-based layer53may be as was described above for layers33and/or43.

It will also be understood that other embodiments ofFIGS. 5 and 6need not provide the indium gallium nitride-based layer53including therein clusters54of elemental indium and/or binary indium nitride directly on a binary gallium nitride layer12or directly on an active region11. Moreover, still other embodiments ofFIGS. 5 and 6need not provide the reflective metal layer14or the ITO layer44directly on the indium gallium nitride-based layer53including therein clusters54of elemental indium and/or binary indium nitride.

FIG. 7is a cross-sectional view of non-flip-chip LEDs according to other embodiments of the present invention. These non-flip-chip LEDs70include a silicon carbide substrate21that may be 4 H or 6 H N-type silicon carbide. In other embodiments, the substrate21can include sapphire, bulk gallium nitride, bulk aluminum nitride or other suitable substrates. In some embodiments, the substrate21can be a growth substrate on which the epitaxial layers78forming the LED structure70are formed. In other embodiments, the substrate21can be a carrier substrate to which the epitaxial layers78are transferred. In other embodiments, the substrate21can be removed altogether, as is understood in the art.

Still referring toFIG. 7, the LED epi structure78includes a silicon doped binary GaN layer75on the substrate21. One or more buffer layers76, such as a silicon doped ternary aluminum gallium nitride buffer layer, may be provided between the substrate21and the binary GaN layer75. An N-type superlattice (SLS) structure74can be formed on the binary GaN layer75. For example, 25 repetitions of ternary indium gallium nitride/binary gallium nitride may be provided. A multi-quantum well (MQW)73may be formed on the superlattice74. In some embodiments, the MQW may comprise 6 to 8 repetitions of ternary indium gallium nitride/binary gallium nitride. An undoped quaternary aluminum indium gallium nitride layer72may be formed on the MQW73, and a ternary aluminum gallium nitride layer71doped with a P-type dopant, such as magnesium, may be formed on the quaternary aluminum indium gallium nitride layer72. A gallium nitride-based layer, such as a binary gallium nitride layer12also doped with a P-type dopant, such as magnesium, may be formed on the ternary aluminum gallium nitride layer71. The active region of the device70may include the GaN:Si layer75, the superlattice74and the multi-quantum well73. An InN-based layer33,43,53is provided on the GaN-based layer12as was described above. An ITO layer44and a bond pad22are provided as was described above.

FIG. 8is a cross-sectional view of a flip-chip light emitting device according to other embodiments of the present invention. As shown inFIG. 8, these devices80may include an active region11, a P—GaN-based outer layer12, a reflective metal layer14, a barrier layer15, a bonding layer16and a carrier17, as was described above, for example, in connection withFIG. 3. An N—GaN-based layer18and/or a substrate19also may be provided. An undoped indium gallium nitride-based layer83is provided directly on the gallium nitride-based outer layer12. Moreover, the reflective metal layer14is provided directly on the undoped indium gallium nitride-based layer83. A separate ohmic metal layer13is not provided, although it may be provided in other embodiments. The active region11, the outer layer12, and the undoped indium gallium nitride-based layer83may be fabricated as an LED epitaxial structure88.

More specifically, the indium gallium nitride-based layer83may have a thickness of between about 5 Å and about 100 Å in some embodiments, and, in some embodiments, may be about 20 Å thick. An indium composition of about 5 atomic percent to about 30 atomic percent may be provided, and, in some embodiments, about 10 atomic percent indium may be provided. In other embodiments, the indium composition in the indium gallium nitride-based layer83may be equal to or greater than the indium composition of the wells of the active region11. Moreover, the indium gallium nitride-based layer83may be undoped, i.e., not intentionally doped. More specifically, N- and P-doping sources may not be used during the epitaxial growth of the indium gallium nitride-based layer83. It will be understood that some dopants may still be found in the indium gallium nitride-based layer83due to, for example, residual dopants in the epitaxial system, so that an actual doping of magnesium from about 1×1015cm−3to about 1×1022cm−3may be present in some embodiments, and, in some embodiments, a magnesium doping level of about 1×1017cm−3may be found. In other embodiments, the layer83may be intentionally doped N- or P-type.

The undoped indium gallium nitride-based layer83may be fabricated directly on the P—GaN-based outer layer12by continuing to expose the LED epi structure88to a source of gallium and a source of nitrogen while turning on the source of indium and also turning off the source of magnesium or other P-type dopants.

The indium gallium nitride-based layer83may further act to decrease the bandgap, without being unduly absorptive. Accordingly, the thickness and/or indium content of the indium gallium nitride-based layer83may be selected to provide acceptable electrical conduction without excessively absorbing emitted radiation from the active region11. Accordingly, in some embodiments, an indium gallium nitride layer that is about 20 Å in thickness, having about 10 atomic percent weight indium and not being intentionally doped may satisfy these criteria. In other embodiments, the indium gallium nitride-based layer83may be made even thinner, so as to not be unduly absorptive, even though higher indium content than the indium gallium nitride quantum wells of the active region is provided. Thus, if the indium gallium nitride wells of the active region11have an indium content of between about 20% and 40%, an even higher indium content may be provided in the undoped indium gallium nitride-based layer83, provided that this layer is sufficiently thin so as to not excessively absorb emitted radiation from the active region.

FIG. 9is a cross-sectional view of a flip-chip light emitting device according to still other embodiments of the present invention. These devices90may be similar to devices80ofFIG. 8, except that the LED epi structure98includes a P—GaN-based layer92that has a nonplanar (e.g., roughened or textured) surface, illustrated by a wavy line inFIG. 9, and the undoped indium gallium nitride-based layer93is also nonplanar. Thus, a conformal nonplanar layer of undoped indium gallium nitride-based material93may be provided on the nonplanar surface of the P—GaN-based layer92. It will be understood that inFIG. 9, the thicknesses of the layers are not drawn to scale, and that layer93is typically much thinner than layer92, so that layer93may be regarded as a thin conformal coating on P—GaN-based layer92. The nonplanar surface of P—GaN-based layer92may enhance extraction of light, and the provision of the nonplanar undoped InGaN layer93may compensate for the voltage drop that may otherwise be presented by the P—GaN-based outer layer92. Accordingly, improved light extraction may be obtained without the need to pay a voltage penalty. It will be understood that the nonplanar layers12and/or93may be grown in a nonplanar form by adjusting the growth parameters and/or planar layers may be nonplanarized or further nonplanarized after growth.

FIG. 10is a cross-sectional view of a flip-chip light emitting device according to still other embodiments of the present invention. As shown inFIG. 10, these devices100may include an LED epi region88/98, as was described above in connection withFIGS. 8and/or9. However, a transparent conductive spacer104is added directly on the undoped InGaN-based layer83/93, and the reflective metal layer14is provided directly on the transparent conductive spacer104. The transparent conductive spacer104, which may comprise a transparent conductive oxide layer, such as indium tin oxide (ITO), zinc oxide (ZnO), or nickel oxide (NiO), having a thickness of between about 5 Å and about 5,000 Å may be provided.

The transparent conductive spacer layer104may perform a unique function in embodiments ofFIG. 10. Typically, transparent conductive oxides are used to provide current spreading, while allowing light to pass therethrough. However, in embodiments ofFIG. 10, current spreading may not be needed, because the undoped InGaN-based layer83/93provides a good ohmic contact. Nonetheless, by providing a transparent conductive layer104, the efficiency of the reflective metal layer14may be increased and, in some embodiments, optimized, by providing a desired, and in some embodiments optimum, spacing between the active region11and the reflective metal layer14.

Stated differently, by moving the reflective metal14further away from the active region11, an increased, and in some embodiments maximized, percentage of reflection may be provided. The thickness of the transparent conductive layer may selected based on the distance between the active region11and the reflective metal layer14, the composition of the reflective metal layer14, the frequency of light that is being emitted by the active region11and/or other parameters, to increase or maximize the reflection from the reflective metal layer14. In some embodiments, the spacing may be selected as a function of the wavelength of the light, to reduce destructive interference between the incoming and reflected light, and thereby provide enhanced or maximum reflected light. In some embodiments, a spacing of one quarter the wavelength of light, or multiples of one quarter the wavelength of the emitted light, may be used. Thus, a transparent conductive oxide, such as ITO, may be used as a transparent conductive spacer in a flip-chip LED, to enhance the efficiency of the reflective metal layer14. It will be understood that embodiments ofFIG. 10may be used in combination with the nonplanar P—GaN layer92and nonplanar undoped InGaN layer93ofFIG. 9.

FIGS. 11 and 12are cross-sectional views of flip-chip light emitting devices according to yet other embodiments of the invention. The devices110ofFIG. 11may be similar to devices80ofFIG. 8, and the devices120ofFIG. 12may be similar to the devices90ofFIG. 9, except that an ohmic metal layer113,123, respectively, is added between the undoped InGaN layer83/93, and the reflective metal layer14. The ohmic metal layer113/123may comprise nickel. As shown in embodiments ofFIGS. 11 and 12, the addition of an ohmic metal layer may further decrease the forward voltage of the light emitting device when used in combination with the undoped InGaN layer83/93.

Embodiments of the invention have been described above in connection with light emitting devices. However, ohmic contact structures as described herein may also be used for gallium nitride-based devices that are not light emitting. For example, contact structures as described herein may be used in connection with gallium nitride-based High Electron Mobility Transistor (HEMT) devices. In HEMT devices, layers12and33ofFIG. 3may be used on an active HEMT region and layers12and53ofFIG. 5may be used on an active HEMT region. Accordingly, gallium nitride-based devices that are not light emitting may also be provided according to other embodiments of the present invention.