ULTRAVIOLET LIGHT-EMITTING DEVICES INCORPORATING GRADED LAYERS AND COMPOSITIONAL OFFSETS

Abstract

In various embodiments, light-emitting devices incorporate graded layers with compositional offsets at one or both end points of the graded layer to promote formation of two-dimensional carrier gases and polarization doping, thereby enhancing device performance.

Claims

1-25. (canceled)

26. An ultraviolet (UV) light-emitting device comprising: a substrate having an Al.sub.uGa.sub.1-uN top surface, wherein 0u1.0; an active, light-emitting device structure disposed over the substrate, the light-emitting device structure comprising a multiple-quantum well layer comprising a plurality of periods each comprising a strained Al.sub.xGa.sub.1-xN barrier and a strained Al.sub.yGa.sub.1-yN quantum well, x and y being different by an amount facilitating confinement of charge carriers in the multiple-quantum well layer; an electron blocking layer disposed over the multiple-quantum well layer, the electron blocking layer comprising Al.sub.vGa.sub.1-vN, wherein v>y and v>x; a graded Al.sub.zGa.sub.1-zN layer disposed over the electron blocking layer, a composition of the graded Al.sub.zGa.sub.1-zN layer being graded in Al concentration z such that the Al concentration z decreases in a direction away from the light-emitting device structure; a p-doped Al.sub.wGa.sub.1-wN cap layer disposed over the graded Al.sub.zGa.sub.1-zN layer, wherein 0w0.4; and a metallic contact disposed over the p-doped Al.sub.wGa.sub.1-wN cap layer and comprising at least one metal, wherein at an interface between the graded Al.sub.zGa.sub.1-zN layer and the electron blocking layer, the Al concentration z of the graded Al.sub.zGa.sub.1-zN layer is less than the Al concentration v of the electron blocking layer.

27. The device of claim 26, wherein, at the interface between the graded Al.sub.zGa.sub.1-zN layer and the electron blocking layer, the Al concentration z of the graded Al.sub.zGa.sub.1-zN layer is less than the Al concentration v of the electron blocking layer by an amount no less than 0.03.

28. The device of claim 26, wherein, at the interface between the graded Al.sub.zGa.sub.1-zN layer and the electron blocking layer, the Al concentration z of the graded Al.sub.zGa.sub.1-zN layer is less than the Al concentration v of the electron blocking layer by an amount no more than 0.85.

29. The device of claim 26, wherein, at the interface between the graded Al.sub.zGa.sub.1-zN layer and the electron blocking layer, the Al concentration z of the graded Al.sub.zGa.sub.1-zN layer is less than the Al concentration v of the electron blocking layer by an amount no less than 0.03 and by an amount no more than 0.85.

30. The device of claim 26, wherein 0.4u1.0.

31. The device of claim 26, wherein 0w0.2.

32. The device of claim 26, wherein the graded Al.sub.zGa.sub.1-zN layer is undoped.

33. The device of claim 26, wherein a p-type dopant concentration within the graded Al.sub.zGa.sub.1-zN layer is less than 10.sup.13 cm.sup.3.

34. The device of claim 26, wherein the Al concentration z of the graded Al.sub.zGa.sub.1-zN layer at an interface between the graded Al.sub.zGa.sub.1-zN layer and the p-doped Al.sub.wGa.sub.1-wN cap layer is approximately equal to the Al concentration w of the p-doped Al.sub.wGa.sub.1-wN cap layer.

35. The device of claim 26, wherein the Al concentration w of the p-doped Al.sub.wGa.sub.1-wN cap layer is approximately 0.

36. The device of claim 26, wherein the substrate comprises doped or undoped AlN.

37. The device of claim 26, wherein the UV light-emitting device comprises a light-emitting diode.

38. The device of claim 26, wherein the UV light-emitting device comprises a laser.

39. The device of claim 26, wherein a thickness of the p-doped Al.sub.wGa.sub.1-wN cap layer is no less than 1 nm and no greater than 50 nm.

40. The device of claim 26, further comprising an n-doped Al.sub.nGa.sub.1-nN bottom contact layer disposed between the substrate and the multiple-quantum well layer, wherein y<n<x.

41. The device of claim 26, further comprising a reflection layer disposed over at least a portion of the p-doped Al.sub.wGa.sub.1-wN cap layer, the reflection layer having a reflectivity to light emitted by the light-emitting device structure larger than a reflectivity to light emitted by the light-emitting device structure of the metallic contact.

42. The device of claim 41, wherein the reflection layer comprises Al.

43. The device of claim 41, further comprising a transmissive layer disposed between at least a portion of the reflection layer and the p-doped Al.sub.wGa.sub.1-wN cap layer, the transmissive layer having a transmissivity to light emitted by the light-emitting structure larger than a transmissivity to light emitted by the light-emitting structure of the metallic contact.

44. The device of claim 43, wherein the transmissive layer comprises at least one of silicon oxide, silicon nitride, aluminum oxide, or gallium oxide.

45. An ultraviolet (UV) light-emitting device comprising: a substrate having an Al.sub.uGa.sub.1-uN top surface, wherein 0u1.0; an active, light-emitting device structure disposed over the substrate, the light-emitting device structure comprising a multiple-quantum well layer comprising a plurality of periods each comprising a strained Al.sub.xGa.sub.1-xN barrier and a strained Al.sub.yGa.sub.1-yN quantum well, x and y being different by an amount facilitating confinement of charge carriers in the multiple-quantum well layer; a graded Al.sub.zGa.sub.1-zN layer disposed over the multiple-quantum well layer, a composition of the graded Al.sub.zGa.sub.1-zN layer being graded in Al concentration z such that the Al concentration z decreases in a direction away from the light-emitting device structure; a p-doped Al.sub.wGa.sub.1-wN cap layer disposed over the graded Al.sub.zGa.sub.1-zN layer, wherein 0w0.4; and a metallic contact disposed over the p-doped Al.sub.wGa.sub.1-wN cap layer and comprising at least one metal, wherein at an interface between the graded Al.sub.zGa.sub.1-zN layer and the p-doped Al.sub.wGa.sub.1-wN cap layer, the Al concentration w of the p-doped Al.sub.wGa.sub.1-wN cap layer is less than the Al concentration z of the graded Al.sub.zGa.sub.1-zN layer.

46. The device of claim 45, wherein, at the interface between the graded Al.sub.zGa.sub.1-zN layer and the p-doped Al.sub.wGa.sub.1-wN cap layer, the Al concentration w of the p-doped Al.sub.wGa.sub.1-wN cap layer is less than the Al concentration z of the graded Al.sub.zGa.sub.1-zN layer by an amount no less than 0.03.

47. The device of claim 45, wherein, at the interface between the graded Al.sub.zGa.sub.1-zN layer and the p-doped Al.sub.wGa.sub.1-wN cap layer, the Al concentration w of the p-doped Al.sub.wGa.sub.1-wN cap layer is less than the Al concentration z of the graded Al.sub.zGa.sub.1-zN layer by an amount no more than 0.85.

48. The device of claim 45, wherein, at the interface between the graded Al.sub.zGa.sub.1-zN layer and the p-doped Al.sub.wGa.sub.1-wN cap layer, the Al concentration w of the p-doped Al.sub.wGa.sub.1-wN cap layer is less than the Al concentration z of the graded Al.sub.zGa.sub.1-zN layer by an amount no less than 0.03 and by an amount no more than 0.85.

49. The device of claim 45, wherein 0.4u1.0.

50. The device of claim 45, wherein 0w0.2.

51. The device of claim 45, wherein the graded Al.sub.zGa.sub.1-zN layer is undoped.

52. The device of claim 45, wherein a p-type dopant concentration within the graded Al.sub.zGa.sub.1-zN layer is less than 10.sup.13 cm.sup.3.

53. The device of claim 45, further comprising, disposed between the multiple-quantum well layer and the graded Al.sub.zGa.sub.1-zN layer, an electron blocking layer comprising Al.sub.vGa.sub.1-vN, wherein vy and vx.

54. The device of claim 53, wherein the Al concentration z of the graded Al.sub.zGa.sub.1-zN layer at an interface between the graded Al.sub.zGa.sub.1-zN layer and the electron blocking layer is approximately equal to the Al concentration v of the electron blocking layer.

55. The device of claim 45, wherein the Al concentration w of the p-doped Al.sub.wGa.sub.1-wN cap layer is approximately 0.

56. The device of claim 45, wherein the substrate comprises doped or undoped

57. The device of claim 45, wherein the UV light-emitting device comprises a light-emitting diode.

58. The device of claim 45, wherein the UV light-emitting device comprises a laser.

59. The device of claim 45, wherein a thickness of the p-doped Al.sub.wGa.sub.1-wN cap layer is no less than 1 nm and no greater than 50 nm.

60. The device of claim 45, further comprising an n-doped Al.sub.nGa.sub.1-nN bottom contact layer disposed between the substrate and the multiple-quantum well layer, wherein y<n<x.

61. The device of claim 45, further comprising a reflection layer disposed over at least a portion of the p-doped Al.sub.wGa.sub.1-wN cap layer, the reflection layer having a reflectivity to light emitted by the light-emitting device structure larger than a reflectivity to light emitted by the light-emitting device structure of the metallic contact.

62. The device of claim 61, wherein the reflection layer comprises Al.

63. The device of claim 61, further comprising a transmissive layer disposed between at least a portion of the reflection layer and the p-doped Al.sub.wGa.sub.1-wN cap layer, the transmissive layer having a transmissivity to light emitted by the light-emitting structure larger than a transmissivity to light emitted by the light-emitting structure of the metallic contact.

64. The device of claim 63, wherein the transmissive layer comprises at least one of silicon oxide, silicon nitride, aluminum oxide, or gallium oxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:

[0024] FIG. 1 is a schematic cross-section of a light-emitting device structure in accordance with various embodiments of the invention;

[0025] FIG. 2 is a graph of aluminum concentration as a function of depth from the surface for a nitride-based light-emitting device structure in accordance with various embodiments of the invention;

[0026] FIG. 3A is a schematic plan view of a light-emitting device structure incorporating an electrode layer and a reflection layer in accordance with various embodiments of the invention;

[0027] FIG. 3B is a schematic cross-section of the light-emitting device structure of FIG. 3A;

[0028] FIGS. 4 and 5 are schematic cross-sections of light-emitting device structures incorporating electrode layers and reflection layers in accordance with various embodiments of the invention;

[0029] FIGS. 6 and 7 are schematic cross-sections of light-emitting device structures incorporating electrode layers, reflection layers, and transmissive layers in accordance with various embodiments of the invention; and

[0030] FIG. 8 is a schematic cross-section of a light-emitting device structure having two top contacts in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

[0031] FIG. 1 schematically depicts a light-emitting device structure 100 in accordance with the present invention. Light-emitting device structures 100 in accordance with embodiments of the invention may include, consist essentially of, or consist of, for example, light-emitting diodes or lasers. As shown, the device structure 100 includes a substrate 105, which in various embodiments includes, consists essentially of, or consists of a semiconductor substrate. In various embodiments, the top surface of substrate 105 (i.e., the surface on which other layers in device structure 100 are disposed) is Al.sub.uGa.sub.1-uN, where u0.4 (and u1.0). All or a portion of the substrate 105 may be undoped (i.e., not intentionally doped) or intentionally doped with one or more species of dopants. The substrate 105 may be substantially entirely composed of the Al.sub.uGa.sub.1-uN material (e.g., AlN), or the substrate 105 may include, consist essentially of, or consist of a different material (e.g., silicon carbide, silicon, MgO, Ga.sub.2O.sub.3, alumina, ZnO, GaN, InN, and/or sapphire) with the Al.sub.uGa.sub.1-uN material formed thereover by e.g., epitaxial growth; such material may be substantially fully lattice relaxed and may have a thickness of, e.g., at least 1 m.

[0032] In various embodiments, the substrate 105 need not be transparent to radiation emitted by the device structure 100 (e.g., UV radiation), since it may be partially or substantially removed during device fabrication. Semiconductor substrate 105 may be miscut such that the angle between its c-axis and its surface normal is between approximately 0 and approximately 4. In various embodiments, the misorientation of the surface of substrate 105 is less than approximately 0.3, e.g., for substrates 105 that are not deliberately or controllably miscut. In other embodiments, the misorientation of the surface of substrate 105 is greater than approximately 0.3, e.g., for substrates 105 that are deliberately and controllably miscut. In various embodiments, the direction of the miscut is towards the a-axis. The surface of substrate 105 may have a group-III (e.g., Al) polarity, and may be planarized, e.g., by chemical-mechanical polishing. The RMS surface roughness of substrate 105 is preferably less than approximately 0.5 nm for a 10 m10 m area. In some embodiments, atomic-level steps are detectable on the surface when probed with an atomic-force microscope. The threading dislocation density of substrate 105 may be measured using, e.g., etch pit density measurements after a 5 minute KOHNaOH eutectic etch at 450 C. In various embodiments, the threading dislocation density is less than approximately 210.sup.3 cm.sup.2. In some embodiments substrate 105 has an even lower threading dislocation density. Substrate 105 may be topped with a homoepitaxial layer (not shown) that includes, consists essentially of, or consists of the same material present in or on substrate 105, e.g., AlN.

[0033] The various layers of device structure 100 disposed over substrate 105 may be formed by any of a variety of different techniques, e.g., epitaxial growth techniques such as chemical vapor deposition (CVD) methods such as metallorganic CVD (MOCVD).

[0034] Device structure 100 also includes a bottom contact layer 110 disposed over the substrate 105. In various embodiments, the bottom contact layer 110 is n-type doped, e.g., doped with an impurity such as P, As, Sb, C, H, F, O, Mg, and/or Si. The bottom contact layer 110 may include, consist essentially of, or consist of, for example, Al.sub.xGa.sub.1-xN, and the aluminum concentration x may be approximately the same as, or different then, the corresponding aluminum content in substrate 105. In an embodiment, an optional graded buffer layer (not shown) is disposed above substrate 105 and below bottom contact layer 110. The graded buffer layer may include, consist essentially of, or consist of one or more semiconductor materials, e.g., Al.sub.xGa.sub.1-xN. In various embodiments, the graded buffer layer has a composition approximately equal to that of substrate 105 at an interface therewith in order to promote two-dimensional growth and avoid deleterious islanding (such islanding may result in undesired elastic strain relief and/or surface roughening in the graded buffer layer and subsequently grown layers). The composition of the graded buffer layer at an interface with bottom contact layer 110 may be chosen to be close to (e.g., approximately equal to) that of the desired active region of the device (e.g., the Al.sub.xGa.sub.1-xN concentration that will result in the desired wavelength emission from the light-emitting device). In an embodiment, the graded buffer layer includes, consists essentially of, or consists of doped or undoped Al.sub.xGa.sub.1-xN graded from an Al concentration x of approximately 100% to an Al concentration x of approximately 60%.

[0035] The bottom contact layer 110 may have a thickness sufficient to prevent current crowding after device fabrication and/or to stop on during etching to fabricate contacts. For example, the thickness of bottom contact layer 320 may range from approximately 100 nm to approximately 500 nm, or from approximately 100 nm to approximately 2 m. When utilizing a bottom contact layer 110, the final light-emitting device may be fabricated with back-side contacts. In various embodiments, bottom contact layer 110 will have high electrical conductivity even with a small thickness due to the low defect density maintained when the layer is pseudomorphic. As utilized herein, a pseudomorphic film is one where the strain parallel to the interface between the film and an underlying layer or substrate is approximately that needed to distort the lattice in the film to match that of the substrate (or a relaxed, i.e., substantially unstrained, layer over the substrate and below the pseudomorphic film). Thus, the parallel strain in a pseudomorphic film will be nearly or approximately equal to the difference in lattice parameters between an unstrained substrate parallel to the interface and an unstrained epitaxial layer parallel to the interface.

[0036] As shown in FIG. 1, device structure also includes an active light-emitting device structure (i.e., one or more layers (e.g., quantum wells) configured for the emission of light in response to an applied voltage) disposed over the bottom contact layer 110. For example, the active light-emitting device structure may include, consist essentially of, or consist of a multiple-quantum well (MQW) layer 115 disposed above bottom contact layer 110. In various embodiments, MQW layer 115 is disposed directly on the bottom contact layer 110. In other embodiments, an optional layer (e.g., an undoped layer including, consisting essentially of, or consisting of an undoped semiconductor material such as AlGaN) may be disposed between the bottom contact layer 110 and the MQW layer 115. The MQW layer 115 may be doped with the same doping polarity as the bottom contact layer 110, e.g., n-type doped. The MQW layer 115 may include, consist essentially of, or consist of one or more quantum wells separated by (or surrounded on both sides by) barriers. For example, each period of MQW layer 115 may feature an Al.sub.xGa.sub.1-xN quantum well and an Al.sub.yGa.sub.1-yN barrier, where x is different from y. Typically, y is greater than 0.4 for light-emitting devices designed to emit light having a wavelength less than 300 nm and may be greater than 0.7 for shorter-wavelength emitters. It may even be greater than 0.9 for devices designed to emit at wavelengths shorter than 250 nm. The value of x will, at least in part, determine the emission wavelength of the device. For emission wavelengths longer than 280 nm, x may be as low as 0.2. For wavelengths between 250 nm and 280 nm, x may vary between 0.2 and 0.7. For wavelengths shorter than 250 nm, x may be greater than 0.6. In various embodiments, the difference between x and y is large enough to obtain good confinement of the electrons and holes in the active region, thus enabling high ratio of radiative recombination to non-radiative recombination. In an embodiment, the difference between x and y is approximately 0.25, e.g., x is approximately 0.5 and y is approximately 0.75. MQW layer 115 may include a plurality of such periods, and may have a total thickness ranging from 20 nm to 100 nm, or less than approximately 50 nm.

[0037] In various embodiments of the invention, an electron-blocking layer 120 may be disposed over the active light-emitting device structure of device structure 100 (e.g., disposed above MQW layer 115). The electron-blocking layer 120 typically has a wider band gap than that of a band gap within the MQW layer 115 (e.g., a band gap of the barrier layers therewithin). In various embodiments, the electron-blocking layer 120 may include, consist essentially of, or consist of e.g., Al.sub.xGa.sub.1-xN, and electron-blocking layer 120 may be doped. For example, the electron-blocking layer 120 may be doped with the same doping polarity as that of bottom contact layer 110 and/or MQW layer 115 (e.g., n-type doped). In various embodiments, the value of x in the electron-blocking layer 120 is greater than the value of the Al mole fraction in the barrier layers used in the MQW layer 115. For longer wavelength devices with emission wavelengths greater than 300 nm, x may be as low as 0.4 and may be greater than 0.7 for shorter wavelength devices. It may even be greater than 0.9 for devices designed to emit at wavelengths shorter than 250 nm. Electron-blocking layer 120 may have a thickness that may range, for example, between approximately 10 nm and approximately 50 nm, or even between approximately 10 nm and approximately 30 nm. In various embodiments of the invention, the electron-blocking layer 120 is sufficiently thin (e.g., thickness less than about 30 nm, or less than about 20 nm) so as to facilitate carrier (e.g., hole) tunneling through the electron-blocking layer 120. In various embodiments of the invention, the electron-blocking layer 120 is omitted from device structure 100.

[0038] As shown in FIG. 1, device structure 100 may also include a graded layer 125 disposed above the electron-blocking layer 120 (or above the active light-emitting device structure in embodiments in which electron-blocking layer 120 is omitted), and a cap layer 130 may be disposed over the graded layer 125. The cap layer 130 may be doped with a doping polarity opposite of that of the bottom contact layer 110, e.g., p-type doped with one or more dopants such as Mg, Be, and/or Zn. In other embodiments, the cap layer 130 may be undoped, as carriers (e.g., holes) may be injected from an electrode into a two-dimensional carrier gas disposed at the interface between the cap layer 130 and the graded layer 125. (While in exemplary embodiments described herein the cap layer 130 is doped p-type and the bottom contact layer 110 is doped n-type, embodiments in which the doping polarities of these layers are switched are within the scope of the present invention; in such embodiments, the electron-blocking layer 120, if present, may be considered to be a hole-blocking layer, as understood by those of skill in the art.) The cap layer 130 may have a thickness ranging from, e.g., approximately 1 nm to approximately 100 nm, or approximately 1 nm to approximately 50 nm, or approximately 1 nm to approximately 20 nm. In various embodiments, the cap layer 130 includes, consists essentially of, or consists of Al.sub.xGa.sub.1-xN, and in various embodiments the aluminum concentration x may range from 0 (i.e., pure GaN) to approximately 0.2.

[0039] Analysis of graded layer 125 may be performed by using various analysis methods such as X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDX), X-ray fluorescence analysis (XRF), auger electron spectroscopy (AES), secondary ion mass spectrometry (SIMS), electron energy loss spectrum (EELS), and/or an atom probe (AP). For example, graded layer 125 may exhibit a gradient in composition exceeding a measurement accuracy of one or more such analytical techniques.

[0040] The device structure 100 may also incorporate a metallic contact to facilitate electrical contact to the device. For example, the metallic contact may include or consist essentially of an electrode layer 135 disposed above or on the cap layer 130. The composition and/or shape of the electrode layer 135 are not particularly limited as long as it enables the injection of carriers (e.g., holes) into the cap layer 130. In embodiments in which holes are injected into a p-type doped nitride-based semiconductor cap layer 130, the electrode layer 135 may include, consist essentially of, or consist of one or more metals having large work functions, e.g., Ni, Au, Pt, Ag, Rh, and/or Pd, alloys or mixtures of two or more of these metals, or oxide-based and/or transparent electrode materials such as indium tin oxide (ITO). Electrode layers 135 in accordance with embodiments of the invention are not limited to these materials. The thickness of the electrode layer 135 may be, for example, between approximately 10 nm and approximately 100 nm, or between approximately 10 nm and approximately 50 nm, or between approximately 10 nm and approximately 30 nm, or between approximately 25 nm and approximately 40 nm.

[0041] As mentioned above, embodiments of the present invention feature a graded layer 125 disposed between the cap layer 130 and the electron-blocking layer 120 (or the MQW layer 115 in embodiments in which the electron-blocking layer 120 is omitted). The graded layer 125 typically includes, consists essentially of, or consists of a nitride semiconductor, e.g., a mixture or alloy of Ga, In, and/or Al with N. The compositional gradient within graded layer 125 may be substantially continuous or stepped, and the grading rate within the graded layer 125 may be substantially constant or may change one or more times within the thickness of graded layer 125. The graded layer 125 may be undoped. In other embodiments, the graded layer 125 is doped n-type or p-type with one or more dopants, e.g., C, H, F, O, Mg, Be, Zn, and/or Si. The thickness of the graded layer 125 may be, for example, between approximately 5 nm and approximately 100 nm, between approximately 10 nm and approximately 50 nm, or between approximately 20 nm and approximately 40 nm. In various embodiments, the epitaxial growth process utilized to form the various layers of the device structure 100 may be temporarily halted between growth of the graded layer 125 and the underlying layer and/or the overlying layer. In various embodiments, the graded layer 125 is pseudomorphically strained to one or more of the underlying layers.

[0042] In general, the composition as a function of thickness of the graded layer 125 trends from that of the underlying layer (e.g., electron-blocking layer 120 or MQW layer 115) toward that of the overlying layer (e.g., cap layer 130). However, in various embodiments, the composition of the graded layer 125 at its interface with electron-blocking layer 120 does not match that of electron blocking layer 120, and/or the composition of the graded layer 125 at its interface with cap layer 130 does not match that of cap layer 130. FIG. 2 shows a (not-to-scale) representation of the aluminum concentration within various layers of an exemplary device structure 100 as a function of depth, where the top surface of the device is toward the right side of the figure. As shown in FIG. 2, the device structure 100 may incorporate a compositional jump, i.e., an offset or discontinuity in composition, at one or both of the interfaces to graded layer 125 to facilitate the formation of a two-dimensional hole gas at one or both of the interfaces during device operation. The compositional difference(s) between the graded layer 125 and the electron blocking layer 120 and/or the cap layer 130 may be analyzed via one or more of a variety of analysis methods such as X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDX), X-ray fluorescence analysis (XRF), auger electron spectroscopy (AES), secondary ion mass spectrometry (SIMS), and/or electron energy loss spectrum (EELS).

[0043] As shown in FIG. 2, for exemplary device structures 100 in which various layers contain different amounts of aluminum, the device structure 100 may incorporate a compositional offset 200 in aluminum concentration; namely, the aluminum concentration at the initial, bottom portion of the graded layer 125 (e.g., at the interface between the graded layer 125 and the layer underlying it) may be lower than that of the electron-blocking layer 120 and/or the MQW layer 115. This compositional difference 200 may be at least 0.05, or at least 0.1, or at least 0.2, and it may be as large as 0.55, or as large as 0.75 or 0.85, in various embodiments of the invention. Exemplary device structures 100 may also, or instead, incorporate a compositional offset 205 in aluminum concentration; namely, the aluminum concentration at the final, top portion of the graded layer (e.g., at the interface between graded layer 125 and the layer overlying it) may be higher than that of the cap layer 130. This compositional difference 205 may be at least 0.05, or at least 0.1, or at least 0.2, and it may be as large as 0.55, or as large as 0.75 or 0.85, in various embodiments of the invention. Hole injection into the active region (e.g., MQW layer 115) may be enhanced via the compositional offset 200, and hole injection between the active region and the cap layer 130 and/or electrode thereover (and/or carrier and/or current spreading within cap layer 130) may be enhanced via the compositional offset 205. Two-dimensional carrier gases (e.g., two-dimensional hole gases) may thus advantageously form during operation of the light-emitting device proximate compositional offsets 200, 205. In various embodiments of the invention incorporating a compositional offset 205 between the graded layer 125 and the cap layer 130, the cap layer may have a thickness no greater than approximately 20 nm (e.g., between approximately 1 nm and approximately 20 nm, between approximately 1 nm and approximately 10 nm, between approximately 5 nm and approximately 15 nm, or between approximately 5 nm and approximately 10 nm) for greater responsivity of the carriers within the two-dimensional hole gas proximate the compositional offset 205;

[0044] thicker cap layers 130 may result in loss of at least a portion of the benefits afforded by the two-dimensional hole gas.

[0045] Hole concentrations within the graded layer 125 of approximately 210.sup.19 cm.sup.3 or more, or even 310.sup.19 cm.sup.3 or more may be achieved through polarization doping without impurity doping (e.g., even being substantially free of doping impurities), as modeled using SiLENSe software available from the STR Group, Inc. of Richmond, Va. In general, polarization doping is enabled by the polarization in nitride materials that is due to the difference in electronegativity between the metal atoms and the nitrogen atoms. This results in a polarization field along asymmetric directions in the wurtzite crystal structure. In addition, strain in the layers may result in additional piezoelectric polarization fields and thus additional polarization doping. These fields create fixed charges at abrupt interfaces (e.g., two-dimensional sheets) or graded composition layers (e.g., three-dimensional volumes), which results in mobile carriers of the opposite sign. The magnitude of the total charge may be defined by, for example, the difference in Al compositions within the graded layer, i.e., the difference between the starting composition and the final composition. The concentration of carriers is defined by the total charge divided by the graded layer thickness. A very high carrier concentration may be achieved by a high composition change over a small thickness, while a lower composition change or larger grading thickness typically results in a smaller carrier concentration; however, for a given composition change the total number of carriers may be approximately constant.

[0046] In an exemplary device structure 100 having a compositional offset 200 between electron-blocking layer 120 and graded layer 125, an electron-blocking layer 120 including, consisting essentially of, or consisting of Al.sub.0.8Ga.sub.0.2N or Al.sub.0.85Ga.sub.0.15N is formed over MQW layer 115. Prior to formation of a cap layer 130 including, consisting essentially of, or consisting of GaN, a graded layer 125 is formed over electron-blocking layer 120. The graded layer 125 may be graded in composition from, for example, Al.sub.0.75Ga.sub.0.25N to GaN over a thickness of approximately 30 nm. The graded layer 125 may be formed by, e.g., MOCVD, and in this embodiment is formed by ramping the flow of TMA and TMG (by ramping the flow of hydrogen through their respective bubblers) from the conditions utilized to form a layer of lower Al mole fraction than the electron-blocking layer 120 to 0 standard cubic centimeters per minute (sccm) and 6.4 sccm, respectively, over a period of approximately 24 minutes, thus resulting in a monotonic grade from Al.sub.0.75Ga.sub.0.25N to GaN (all of the other growth conditions are substantially fixed). The thickness of the graded layer 125 in this exemplary embodiment is approximately 30 nm. It should be emphasized that this particular embodiment is exemplary, and embodiments of the invention feature graded layers 125 having various compositional end points (i.e., various compositions at interfaces with underlying and/or overlying layers) and compositional offsets 200 and/or 205.

[0047] In various embodiments of the invention, one or more (or even all) of the layers of device structure 100 may be pseudomorphically strained, similar to device layers described in U.S. patent application Ser. No. 12/020,006, filed on Jan. 25, 2008, U.S. patent application Ser. No. 12/764,584, filed on Apr. 21, 2010, and U.S. patent application Ser. No. 14/208,379, filed on Mar. 13, 2014, the entire disclosure of each of which is incorporated by reference herein. Thus, as detailed therein, in various embodiments, one or more of the layers of device structure 100 may be pseudomorphic and may have a thickness greater than its predicted (e.g., via the Maxwell-Blakeslee theory) critical thickness. Moreover, the collective layer structure of device structure 100 may have a total thickness greater than the predicted critical thickness for the layers considered collectively (i.e., for a multiple-layer structure, the entire structure has a predicted critical thickness even when each individual layer would be less than a predicted critical thickness thereof considered in isolation). In other embodiments, one or more layers of device structure 100 are pseudomorphically strained and cap layer 130 is partially or substantially fully relaxed. For example, the lattice mismatch between cap layer 130 and substrate 105 and/or MQW layer 115 may be greater than approximately 1%, greater than approximately 2%, or even greater than approximately 3%. In an exemplary embodiment, cap layer 130 includes, consists essentially of, or consists of undoped or doped GaN, substrate 105 includes, consists essentially of, or consists of doped or undoped AlN (e.g., single-crystal AlN), and MQW layer 115 includes, consists essentially of, or consists of multiple Al.sub.0.55Ga.sub.0.45N quantum wells interleaved with Al.sub.0.75Ga.sub.0.25N barrier layers, and cap layer 130 is lattice mismatched by approximately 2.4%. Cap layer 130 may be substantially relaxed, i.e., may have a lattice parameter approximately equal to its theoretical unstrained lattice constant. A partially or substantially relaxed cap layer 130 may contain strain-relieving dislocations having segments threading to the surface of cap layer 130 (such dislocations may be termed threading dislocations). The threading dislocation density of a relaxed cap layer 130 may be larger than that of substrate 105 and/or layers underlying cap layer 130 by, e.g., one, two, or three orders of magnitude, or even larger.

[0048] Embodiments of the present invention may incorporate a reflection layer for reflecting at least a portion of the light generated by device structure 100 back away from the reflection layer (e.g., back toward substrate 105 or back contact layer 110). In various embodiments, the electrode layer 135 may form a good ohmic contact to the cap layer 130 but may not be reflective to the light (e.g., UV light) generated within the active device structure of device structure 100 (e.g., MQW layer 115). For example, the electrode layer 135 may include, consist essentially of, or consist of Ni/Au that is not reflective to UV light. In an exemplary embodiment, the contact resistivity of the electrode layer 135 to the cap layer 130 is less than approximately 1.0 m-cm.sup.2, or even less than approximately 0.5 m-cm.sup.2. Contact resistance between various metal-based layers (e.g., electrode layer 135 and/or the reflection layer) and the semiconductor-based layers they contact (e.g., cap layer 130) may be determined via, e.g., a transmission line measurement (TLM) method such as the one described in Solid State Electronics, Vol. 25, No. 2, pp. 91-94.

[0049] Thus, in various embodiments the electrode layer 135 may be utilized in conjunction with a reflection layer that reflects light generated within the active device structure of device structure 100. For example, as shown in FIGS. 3A and 3B, a reflection layer that is at least partially reflective to the light emitted by device structure 100 may at least partially surround (or even, in some embodiments, at least partially overlap) the electrode layer 135. Depending upon the wavelength of light emitted by the device structure 100, the reflection layer 300 may include, consist essentially of, or consist of, for example, Ag, Rh, and/or Al, a multi-layer dielectric film, or one or more fluoroplastics (e.g., polytetrafluoroethylene (PTFE)), although embodiments of the invention are not limited to such materials. For example, Ag is substantially reflective to various wavelengths of visible light while Al is substantially reflective to various wavelengths of UV light (e.g., >90% reflectivity to light having a wavelength of approximately 265 nm). As shown, electrode layer 135 is still in contact with at least portions of cap layer 130, enabling good ohmic contact to the device structure 100 even if the reflection layer 300 does not form a good ohmic contact to cap layer 130.

[0050] Various different configurations of the electrode layer 135 and/or the reflection layer 300 are possible within embodiments of the present invention. For example, as shown in FIG. 4, the reflection layer 300 may cover the entirety of the electrode layer 135. In such embodiments, the reflection layer 300 may, but does not necessarily, cover substantially the entire top surface of device structure 100. As shown in FIG. 5, in embodiments of the present invention portions (or discrete areas) of the reflection layer 300 may be disposed directly on the cap layer 130, and portions of the electrode layer 135 may overlap such portions of the reflection layer 300 while a portion of the electrode layer 135 makes direct contact with the cap layer 130.

[0051] In embodiments of the invention, the device structure may also incorporate a transmissive layer 600 in conjunction with electrode layer 135 and/or reflection layer 300, as shown in FIG. 6. The transmissive layer 600 may include, consist essentially of, or consist of a material having a greater transmissivity to the light emitted by device structure 100 and may even, in some embodiments, be substantially transparent to such light. For example, the transmissive layer 600 may include, consist essentially of, or consist of an insulating material and/or an oxide and/or a nitride such as silicon oxide, silicon nitride, aluminum oxide, and/or gallium oxide. As shown in FIG. 6, the transmissive layer 600 may be disposed between the reflection layer 300 and the cap layer 130, and no portion of the reflection layer 300 directly contacts the cap layer. Alternatively, as shown in FIG. 7, at least portions of the reflection layer 300 may contact the cap layer 130, and the transmissive layer 600 is disposed between other portions of the reflection layer 300 and the cap layer 130.

[0052] In various embodiments of the invention, electrical contact to the device structure 100 may be made via electrode layer 135 disposed on the top of the device structure and via a contact disposed on the back side of the device structure 100. For example, a back contact may be disposed on the back side of the substrate 105, or, in embodiments in which the substrate 105 is removed after fabrication of the other layers of device structure 105, a back contact may be disposed on the back side of the back contact layer 110. Alternatively, as shown in FIG. 8, in one or more regions various upper layers of the device structure 100 may be removed (e.g., via chemical or dry/plasma etching), and a back contact 800 may be disposed on the top of back contact layer 110 or on the top of an unremoved portion of the back contact layer 110. The back contact 800 may include, consist essentially of, or consist of, for example, one or more of the materials described herein for the electrode layer 135.

[0053] Embodiments of the invention may utilize photon-extraction techniques described in the '093 application. Such techniques include surface treatment (e.g., roughening, texturing, and/or patterning), substrate thinning, substrate removal, and/or the use of rigid lenses with thin intermediate encapsulant layers. Exemplary substrate-removal techniques include laser lift-off, as described in High brightness LEDs for general lighting applications using the new Thin GaNTechnology, V. Haerle, et al., Phys. Stat. Sol. (a) 201, 2736 (2004), the entire disclosure of which is incorporated by reference herein.

[0054] In embodiments in which the device substrate is thinned or removed, the back surface of the substrate may be ground, for example, with a 600 to 1800 grit wheel. The removal rate of this step may be purposefully maintained at a low level (approximately 0.3-0.4 m/s) in order to avoid damaging the substrate or the device layers thereover. After the optional grinding step, the back surface may be polished with a polishing slurry, e.g., a solution of equal parts of distilled water and a commercial colloidal suspension of silica in a buffered solution of KOH and water. The removal rate of this step may vary between approximately 10 m/min and approximately 15 m/min. The substrate may be thinned down to a thickness of approximately 200 m to approximately 250 m, or even to a thickness of approximately 20 m to approximately 50 m, although the scope of the invention is not limited by this range. In other embodiments, the substrate is thinned to approximately 20 m or less, or even substantially completely removed. The thinning step is preferably followed by wafer cleaning in, e.g., one or more organic solvents. In one embodiment of the invention, the cleaning step includes immersion of the substrate in boiling acetone for approximately 10 minutes, followed by immersion in boiling methanol for approximately 10 minutes.

EXAMPLES

Example 1

[0055] Three different device structures similar to that depicted in FIG. 1 were fabricated to investigate the effect of compositional offsets in accordance with embodiments of the present invention on internal device efficiency. Device structure A lacked compositional offsets 200, 205 and thus lacked two-dimensional hole gases. Specifically, device structure A incorporated an Al.sub.xGa.sub.1-xN electron-blocking layer having an Al content of 85%, and the overlying graded layer had an Al content of 85% at its interface with the electron-blocking layer. Device structure B incorporated a compositional offset 200 in accordance with embodiments of the present invention. Specifically, device structure B incorporated an Al.sub.xGa.sub.1-xN electron-blocking layer having an Al content of 85%, and the overlying graded layer had an Al content of 75% at its interface with the electron-blocking layer. Device structure C incorporated a compositional offset between the electron-blocking layer and the overlying graded layer, but in the direction opposite that depicted in FIG. 2, i.e., an Al mole fraction at the initiation of the graded layer greater than that of the underlying layer. Specifically, device structure C incorporated an Al.sub.xGa.sub.1-xN electron-blocking layer having an Al content of 85%, and the overlying graded layer had an Al content of 95% at its interface with the electron-blocking layer.

[0056] Light-emitting devices were fabricated from the three different device structures, and the compositional offset 200 in accordance with embodiments of the invention provided significant gains in device efficiency (e.g., average internal efficiency) when compared to device structure Athe average internal efficiency of device structure B was approximately 40%, a factor of two greater than the average internal efficiency of device structure A (which was approximately 20%). In contrast, the compositional offset in the opposite direction in device structure C resulted in a significant reduction in device performance, even compared to the device lacking any compositional offset, as the average internal efficiency was less than 5%. Internal efficiency as used in this example is a calculated estimation of the product of the internal quantum efficiency and injection efficiency using a predetermined and constant extraction efficiency and measured output powers before and after thinning the substrate. Specifically, the internal efficiency IE was calculated as:

[00001]$\mathrm{IE}=\frac{L}{e^{-.Math..Math.t}0.047E_{}I}$

where L is the output power, is the absorption coefficient of the substrate, t is the thickness of the substrate, E.sub. is the bandgap of the peak light emission, and l is the input current. The constant 0.047 is the calculated fraction of light escaping through a smooth planar substrate using an index of refraction appropriate for AlN at the wavelength of the peak light emission (the value would increase for a substrate with a smaller index of refraction).

Example 2

[0057] An n-type AlGaN back-contact layer, an AlGaN multiple quantum well, an AlGaN graded layer in which the Al composition was continuously graded from 85% to 20% from the multiple quantum well toward a p-type GaN cap layer, and the p-type GaN cap layer (having a thickness of 10 nm) were deposited in series on an AlN substrate.

[0058] Thus, at the interface between the graded layer and the cap layer there was a compositional offset in Al content of 20%. A portion of the layer structure was dry etched using a chlorine-based gas so that a portion of the n-type AlGaN back-contact layer was exposed. Then, an n-type electrode composed of an alloy of Ti, Al, Ni, and Au was disposed on the exposed n-type AlGaN back-contact layer. A contact electrode composed of an alloy of Ni and Au was disposed on a portion of the p-type GaN cap layer, and a reflection layer composed of Al was disposed on an exposed portion of the p-type GaN cap layer and on the contact electrode, thereby obtaining a light-emitting device in accordance with embodiments of the present invention. When a current was caused to flow between the n-type electrode and the contact electrode, light emission having a wavelength of 275 nm and a power of 3.2 mW was obtained with a current of 100 mA.

Example 3

[0059] An n-type AlGaN back-contact layer, an AlGaN multiple quantum well, an AlGaN graded layer in which the Al composition was continuously graded from 85% to 20% from the multiple quantum well toward a p-type GaN cap layer, and the p-type GaN cap layer (having a thickness of 10 nm) were deposited in series on an AlN substrate. Thus, at the interface between the graded layer and the cap layer there was a compositional offset in Al content of 20%. A portion of the layer structure was dry etched using a chlorine-based gas so that a portion of the n-type AlGaN back-contact layer was exposed. Then, an n-type electrode composed of an alloy of Ti, Al, Ni, and Au was disposed on the exposed n-type AlGaN back-contact layer. A contact electrode composed of an alloy of Ni and Au was disposed on a portion of the p-type GaN cap layer, thereby obtaining a light-emitting device in accordance with embodiments of the present invention. When a current was caused to flow between the n-type electrode and the contact electrode, light emission having a wavelength of 275 nm and a power of 2.7 mW was obtained with a current of 100 mA.

Comparative Example

[0060] An n-type AlGaN back-contact layer, an AlGaN multiple quantum well, an AlGaN graded layer in which the Al composition was continuously graded from 85% to 0% from the multiple quantum well toward a p-type GaN cap layer, and the p-type GaN cap layer (having a thickness of 10 nm) were deposited in series on an AlN substrate.

[0061] Thus, at the interface between the graded layer and the cap layer there was no compositional offset in Al content. A portion of the layer structure was dry etched using a chlorine-based gas so that a portion of the n-type AlGaN back-contact layer was exposed. Then, an n-type electrode composed of an alloy of Ti, Al, Ni, and Au was disposed on the exposed n-type AlGaN back-contact layer. A contact electrode composed of an alloy of Ni and Au was disposed on a portion of the p-type GaN cap layer, thereby obtaining a light-emitting device. When a current was caused to flow between the n-type electrode and the contact electrode, light emission having a wavelength of 275 nm and a power of 2.0 mW was obtained with a current of 100 mA. The lower emission power exhibited by this device compared with that of Examples 2 and 3 demonstrates one benefit of devices in accordance with embodiments of the present invention.

[0062] It is noted that the Examples provided herein were designed merely to compare device structures in order to demonstrate the efficacy of various embodiments of the present invention, and do not represent devices further optimized to produce state-of-the-art output power. For example, the structure described in Example 3 has been utilized in devices in which the AlN substrate was thinned to 200 microns after fabrication to reduce substrate absorption and which were packaged in a standard surface mounted design (SMD) package with good thermal conductivity (10 K/W). Device output powers greater than 38 mW were obtained when operated at an input current of 400 mA with a peak wavelength of approximately 270 nm. Even greater powers would be anticipated with the addition of the reflection layer described in Example 2.

[0063] The terms and expressions employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.