WO2021260529A1

WO2021260529A1 - Ultraviolet light-emitting diode based on n-polar alingan alloys and method

Info

Publication number: WO2021260529A1
Application number: PCT/IB2021/055456
Authority: WO
Inventors: Zhe ZHUANG; Daisuke Iida; Kazuhiro Ohkawa
Original assignee: King Abdullah University Of Science And Technology
Priority date: 2020-06-25
Filing date: 2021-06-21
Publication date: 2021-12-30

Abstract

An ultraviolet (UV) light emitting diode (LED) (400) includes a substrate (202); a semiconductor buffer layer (204) that is N-polar and is formed on the substrate, wherein the N-polar is achieved by having (1) a nitrogen atom layer in the semiconductor buffer layer at a distal surface from the substrate (202) and (2) a metal atom layer in the semiconductor buffer layer at a proximal surface from the substrate (202); an n-type layer (210) formed over the semiconductor buffer layer (204); an active layer (212) formed over the n-type layer (210) and configured to generate UV light; a p-type layer (214) formed over the active layer (212), which in tandem with the n-type layer (210) form a pn junction; a first electrode (610) formed over the p-type layer (214); and a second electrode (620) formed directly over the n-type layer (210).

Description

ULTRAVIOLET LIGHT-EMITTING DIODE BASED ON N-POLAR

ALINGAN ALLOYS AND METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63/044,046, filed on June 25, 2020, entitled “ULTRAVIOLET LIGHT-EMITTING DIODES BASED ON N-POLAR ALUMINUM INDIUM GALLIUM NITRIDE ALLOYS,” the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

TECHNICAL FIELD

[0002] Embodiments of the subject matter disclosed herein generally relate to a deep-ultraviolet light-emitting diode, and more particularly, to an N-polar nitride- based ultraviolet light-emitting diode with enhanced internal quantum efficiency and suppressed efficiency droop.

DISCUSSION OF THE BACKGROUND

[0003] The Ill-nitride semiconductors have been widely used in optoelectronic devices. In the past two decades, InGaN alloy based visible light-emitting diodes (LEDs) have achieved great success in many applications such as solid-state lighting, display backlights and interior/exterior lighting. Meanwhile, another alloy based on AllnGaN could be used as the active material for ultraviolet (UV) LEDs and has attracted rather considerable attention in application of water purification, food/air sterilization, skin curing and so on. However, the traditional AllnGaN alloy grown on a c-plane (0001) sapphire substrate is usually metal-polar (i.e. , Ga- polar/AI-polar/ln-polar), which generates an opposite electric field, which blocks the electron/hole injection into the quantum wells. At large injection currents, the metal- polar UV LEDs suffer from a serious efficiency droop due to severe electron overflowing.

[0004] The AIGaN-based UV LEDs have attracted considerable attention to replace toxic mercury-based UV lamps. There are a wide range of potential applications in this regard, such as water purification, UV curing, environmental sensing, disinfection, and plant-growth lighting. Many techniques were proposed to develop high-power AIGaN-based UV LEDs, for example, utilizing vicinal substrates with large misfit angles, a nanometer-scale thin p-AIN as a p-ohmic contact layer, and thin-film LEDs by removing SiC substrates. However, the external quantum efficiency (EQE) of state-of-art AIGaN-based deep-UV LEDs is 20.3%, so there is a need for further improvements compared to commercial InGaN-based visible LEDs. [0005] One troublesome problem is that the AIGaN-based UV LEDs suffer from poor carrier injection, resulting in relatively low injection efficiency. The low injection efficiency is commonly attributed to weak carrier confinement in the active regions, severe electron leakage with a reduced potential barrier, and low hole concentrations due to the high activation energy of Mg acceptors. [0006] Many efforts have been devoted to promoting the carrier injection with various approaches, such as Mg delta-doping or modulation-doping, polarization- induced p-doping techniques, multiple quantum barriers with different Al composition, and a composite p-AIGaN/AIN electron-blocking heterostructure. However, the existing devices still suffer from poor carrier injection.

[0007] Thus, there is a need for an LED device that overcomes the poor carrier injection and have a high IQE value.

BRIEF SUMMARY OF THE INVENTION

[0008] According to an embodiment, there is an ultraviolet (UV) light emitting diode (LED) that includes a substrate, a semiconductor buffer layer that is N-polar and is formed on the substrate, wherein the N-polar is achieved by having (1) a nitrogen atom layer in the semiconductor buffer layer at a distal surface from the substrate and (2) a metal atom layer in the semiconductor buffer layer at a proximal surface from the substrate, an n-type layer formed over the semiconductor buffer layer, an active layer formed over the n-type layer and configured to generate UV light, a p-type layer formed over the active layer, which in tandem with the n-type layer form a pn junction, a first electrode formed over the p-type layer, and a second electrode formed directly over the n-type layer.

[0009] According to another embodiment, there is a UV LED that includes a substrate, a first electrode located on the substrate, a p-type layer located on the first electrode, an active layer located on the p-type layer and configured to generate UV light within one or more quantum well layers, an n-type layer located on the active layer and which in tandem with the p-type layer form a pn junction, and a second electrode located on the n-type layer. The active layer is N-polar, and the N-polar is achieved by having (1) a nitrogen atom layer, in the one or more quantum well layers, at a proximal surface from the substrate, and (2) a metal atom layer, in the one or more quantum well layers, at a distal surface from the substrate.

[0010] According to still another embodiment, there is a method for making an UV LED, and the method includes providing a substrate, forming a semiconductor buffer layer on the substrate by nitridation, wherein the semiconductor buffer layer is N-polar, and the N-polar is achieved by depositing (1) a nitrogen atom layer in the semiconductor buffer layer at a distal surface from the substrate, and (2) a metal atom layer in the semiconductor buffer layer at a proximal surface from the substrate, forming an n-type layer over the semiconductor buffer layer, forming an active layer over the n-type layer, wherein the active layer is configured to generate UV light, forming a p-type layer over the active layer, which in tandem with the n-type layer form a pn junction, depositing a first electrode over the p-type layer, and depositing a second electrode directly over the n-type layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0012] Figures 1A to 1C schematically illustrate N-polar and metal-polar semiconductor devices;

[0013] Figure 2 illustrates how an N-polar buffer layer is formed on a substrate;

[0014] Figure 3 illustrates a UV LED having an N-polar active layer;

[0015] Figures 4A and 4B illustrate possible structures of a transition layer that is part of the UV LED;

[0016] Figures 5A to 5E illustrate possible structures of the N-polar active layer;

[0017] Figure 6 is a schematic diagram of a UV LED device having N-polar layers;

[0018] Figure 7 is a schematic diagram of another UV LED device having N- polar layers that emit UV light from an N-polar side;

[0019] Figure 8 is a schematic diagram of yet another UV LED device having N-polar layers that emit UV light from a metal-polar side;

[0020] Figure 9 is a schematic diagram of a UV LED device that has its original substrate and other layers removed; [0021] Figure 10 is a schematic diagram of the UV LED device from Figure 9 being attached to another substrate and being configured to emit UV light from a metal-polar side;

[0022] Figure 11 is a schematic diagram of another UV LED device that is configured to emit UV light from the metal-polar side;

[0023] Figure 12 is a schematic diagram of a UV LED device that is used for calculating various parameters related to the generated UV light;

[0024] Figure 13 illustrates the current density dependency of the UV LED device of Figure 12;

[0025] Figures 14A and 14B illustrate the energy band diagrams of a metal- polar and N-polar UV LED device, respectively; and

[0026] Figure 15 is a flow chart of a method for manufacturing a UV LED device with N-polar structure.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to an UV LED diode that uses an N-polar configuration to improve the electron/hole injection and to suppress the electron overflow. However, the embodiments to be discussed next are not limited to UV LEDs, but may be applied to other semiconductor devices.

[0028] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0029] According to an embodiment, an N-polar AIGaN-based UV LED with various Al contents in the quantum wells (QWs) and barriers is investigated and shown to retain higher IQE values even when the acceptor concentrations in the p- layers were one order of magnitude lower. The enhanced performance originates from the higher injection efficiencies of the N-polar structures in terms of efficient carrier injection into QWs and suppressed electron overflow at high current densities. [0030] In contrast to the traditional metal-polar LEDs, the N-polar LEDs present opposite electric fields in the whole epitaxial structures [1] As a result, the N-polar devices have demonstrated enhanced carrier injection in both simulations and experiments [2, 3] This advantage of the N-polar structures was demonstrated to be possible to suppress the efficiency droop in visible InGaN-based LEDs [4] and achieved a flat EQE curve up to 400 A/cm² even without electron blocking layers [5] [0031] Generally, high-quality N-polar nitride materials can be grown on foreign substrates such as c-plane sapphire, SiC and Si (111), with misoriented angles. Researchers have obtained a smooth surface of N-polar GaN films by introducing indium as a surfactant during growth. N-polar InGaN-based visible LEDs have been fabricated [6, 7], but they could not exhibit a better luminescence compared to the metal-polar counterparts. This weak performance of the existing N- polar LEDs originates from the higher impurity concentrations of the N-polar GaN barriers in the active region compared to the metal-polar counterparts when grown by metalorganic vapor phase epitaxy. The impurity concentrations could be reduced by increasing the growth temperature [8] Thus, unlike N-polar InGaN visible LEDs, N-polar AIGaN UV LEDs grown at high temperatures are promising to reduce the impurity concentrations and obtain a better luminescence compared to the metal- polar devices. Besides, N-polar AIN templates were fabricated on 4H-SiC substrates with a misfit angle. The X-ray rocking curve full width half maximums (FWHMs) were 203 arcsec and 389 arcsec for (002) and (102) reflections, respectively. These FWHMs demonstrated that high-quality N-polar AIN templates were possible to be achieved by optimal growth conditions. Very recently, researchers have started to investigate the polarization effect in AIGaN-based UV LEDs and proposed compositional grading electron blocking layers [9] and a lateral-polarity structure with metal-polar and N-polar domains on the same wafer [10] However, the polarization effect is strongly dependent on the Al contents in the LED structures.

[0032] AllnGaN alloy-based UV LEDs are usually grown on a sapphire substrate. Before growing the UV LED structures, an AIN buffer layer is first deposited on the sapphire substrate. The AIN buffer layer is usually Al-polar (i.e., metal-polar) under the traditional surface treatment of sapphire substrate. As a result, all the epitaxial AllnGaN alloy layers on the Al-polar AIN buffer layer are also metal-polar (AI-polar/Ga-polar/l n-polar). The electric fields generated in these metal- polar AllnGaN alloy layers are misoriented for the injection efficiency of the UV LEDs. Therefore, to improve the efficiency, these UV LEDs require an opposite electric field, which assists the electron/hole injection and suppresses the electron overflow.

[0033] Thus, in this embodiment, the N-polar AllnGaN alloy is used for an UV LED. Because the polarization for the N-polar AllnGaN alloys is opposite to the polarization of the metal-polar AllnGaN alloys, the electric field can be generated in an opposite direction. This opposite electric field can contribute to the enhancement of the injection efficiency, leading to efficient UV LEDs. The term N-polar means in this application that for a given layer 100 made of an alloy, for example, including metal atoms and nitrogen atoms, the top side of the alloy layer 100 is the nitrogen atom layer 104 (i.e. , the distal surface of the layer 100, from the substrate 110, is made of the nitrogen atom layer 104 that includes nitrogen atoms), and the bottom surface of the alloy layer 100 is made of the metal atom layer 102 (i.e., the proximal surface to the substrate 110 is the metal atom layer), as shown in Figure 1A. The bottom surface of the alloy layer 100 is considered to be proximal to the substrate 110 on which the alloy is grew and the top surface of the alloy layer is considered to be distal from the substrate 110. There is only one proximal and one distal surface for a given alloy layer. For a metal-polar alloy layer 120, which is shown in Figure 1 B, the metal atom layer 102 forms the top surface of the alloy layer and the nitrogen atom layer 104 forms the bottom surface of the alloy layer, i.e., it is located next to the substrate 110. The metal-polar material shown in Figures 1A and 1B may include any metal, e.g., Al, In, Ga, etc. Figure 1C illustrates the metal-polar and N-polar configurations at the atom level for GaN.

[0034] To obtain N-polar AllnGaN alloys, an N-polar buffer layer is first grown on a substrate. Figure 2 shows a N-polar buffer layer 204 formed on a substrate 202. Instead of the traditional surface treatment in hydrogen gas, which is conducive to a metal-polar layer, a nitridation process [11, 12] in ammonia or a mixture of hydrogen, ammonia and nitrogen gases was carried out for obtaining the N-polar buffer layer 204. As a result, the buffer layer 204 will transform the subsequent layers of the device to N-polar layers. The N-polar buffer layer 204 can be N-polar AIN or N-polar AlxGai-xN, where x can take plural values. Based on this process, an N-polar UV LED 300 can be grown as illustrated in Figure 3. Based on the N-polar buffer layer 204, all the epitaxial layers grown in top of the buffer layer 204 will maintain the N- polar orientation during the growth procedures. In this embodiment, the N-polar UV LED 300 further includes, in this order, grown in top of each other, an undopped layer 206, a transition layer 208, an n-type layer 210, an active layer 212, a first p- type layer 214, a second p-type layer 216, and a third p-type layer 218. The top p- type layer 218 can be p++GaN or p++AIGaN. While Figure 3 show all these layers, it is also possible to remove one or more of the layers, except for the active layer, and one n-type and one p-type layers.

[0035] The material of the substrate 202 is not limited only to Si as long as a semiconductor layer can be grown on its upper surface. This technique is not only for grown on c-plane sapphire substrate but also it can allow to use other materials such as SiC, AIN, GaN, and Si. The SiC and sapphire substrates can be with misorientation angles from 0.1° to 10° towards different planes such as a-plane or m- plane. The Si substrate can be with a miscut of 0.1° to 10° towards <11-2> and <1- 10>. The GaN and AIN substrates are N-polar.

[0036] The buffer layer 204 can be formed by N-polar GaN, AIN, and/or AIGaN. On the other hand, for example, when using an N-polar AIN as the substrate instead of the heterogeneous substrate, the buffer layer 204 may be omitted as the substrate itself would provide the N-polar original seed. In this regard, note that the N-polar UV LED 300 needs one initial layer to be N-polar for being able to make the subsequent layers to also be N-polar.

[0037] The undoped layer 206 may include AIN and/or AIGaN. In general, AIN is often used. The thickness of this layer is preferable to be from 10 to 10,000 nm, and more preferably between 100 and 4,000 nm. The transition layer 208 may include AIN and/or AIGaN. One possible structure of the transition layer 208 includes plural AlxGai xN (410)/AIN (420) superlattices (SLs), as shown in Figure 4A. Another possible structure for the transition layer 208 is shown in Figure 4B and includes only multiple Al_xGai-_xN layers 430-I. The Al content in these layers may be x1, x2, and xn, with x1 > x2 > ... > xn. The total thickness of the transition layer 208 is preferable to be from 20 to 1,000 nm, and more preferably between 100 and 300 nm.

[0038] The n-type layer 210 may include AIGaN and/or AllnGaN. The material may not have a uniform composition. The n-type layer 210 may be an n-side contacting layer when an n-electrode is provided as a last layer of the device 300. This layer can be n-type doping with a concentration from 1 ^c 10¹⁶ to 1 ^c 10²¹ cm^-3. The n-type layer 210 does not have to have a uniform doping. The thickness is preferable to be from 10 to 10,000 nm, and more preferably between 1,000 and 3,000 nm.

[0039] The active layer 212 may include a single quantum well structure (SQW) having a single well layer or a multiple quantum well structure (MQW) having a plurality of well layers and barrier layers. The stacking order may vary. The active layers may be stacked from the well layer or the barrier layer, and it may be similarly terminated with the well layer or the barrier layer. For example, the active layers may include barrier layers AIGaN or AllnGaN 510, and a well layer 520 made of AllnGaN, AllnN, or AIGaN, as shown in Figures 5A to 5E. That is, the barrier layer 510 and the well layer 520 form a pair, and a plurality of such pair layers are sequentially stacked on top of each other. The barrier layers can consist of a multiple stacking structure that includes AIN, AIGaN, and/or AllnGaN materials. The total thickness of the barrier layers 510 is preferably larger than that of the well layers 520. The well layer thickness is preferable to be from 0.3 to 5 nm, and more preferably between 1 and 3.5 nm. The total barrier thickness is preferable to be from 0.3 to 100 nm, and more preferably between 1 and 30 nm. It is preferable that the barrier layers and the well layers are not intentionally doped (i.e. , undoped) with n-type impurities. In the present application, the term “undoped” means less than 1 ^c 10¹⁷ cm^-3.

[0040] The first p-type layer 214 may include AIGaN and/or AllnGaN. In general, p-AIGaN has been often used as an electron blocking layer. In this embodiment, the Al composition is higher than the Al composition in the active layer 212 (both barrier layer or well layer). The Al composition in the first p-type layer 214 is not required to be constant. It can be graded to 1. The doping composition of the layer 214 can be from 1 ^c 10¹⁶ to 1 ^c 10²¹ cm^-3. In this embodiment, the first p-type layer 214 is not required to have a uniform doping. The thickness of the layer is preferable to be from 1 to 1000 nm, and more preferably between 5 and 100 nm. [0041] The second p-type layer 216 may include AIGaN and/or AllnGaN. In general, p-AIGaN has been often used as a p-type layer. The doping concentration of this layer can be from 1 ^c 10¹⁶ to 1 ^c 10²¹ cm^-3. In this embodiment, the second p- type layer 216 does not have to have a uniform doping. The thickness of the layer is preferable to be from 1 to 1,000 nm, and more preferably between 5 and 200 nm. [0042] The third p-type layer 218 may include an Mg-doped GaN, which is often used as the p-side contacting layer. The p-type Mg-doped AIGaN may be used The doping concentration of this layer may be from 1 ^c 10¹⁷ to 1 ^c 10²¹ cm^-3. In this embodiment, the third p-type layer 218 does not have to have a uniform doping. The thickness is preferable to be from 1 to 1,000 nm, and more preferably between 5 and 50 nm. However, this third p-layer 218 may be omitted in some embodiments.

[0043] It is noted that the UV LED 300 shown in Figure 3 illustrates the epitaxial structures of the N-polar UV LED. Because of the N-polar buffer layer 204, all the epitaxial layers of the UV LED will maintain the N-polarity during the subsequent growth procedures. Figure 6 shows a UV LED 400 having two electrodes so that an electrical current can be applied to generate the UV light. A p- electrode 610 is formed on the third p-type layer 218 and is highly reflected or transparent in the UV region, so that the UV light emitted from the active layer 212 can escape from the LED device. The p-electrode 610 may be made as a thin film of indium tin oxide (ITO) and ZnO. The thickness of the thin film is preferable to be from 10 to 500 nm, and more preferably between 50 and 200 nm. Also, the p-electrode 610 may be made of multiple metal layers for high reflection in the emission wavelength region corresponding to the active layer 212. These multiple metal layers can be Rh, Ti, Al, Ag, Cr, Ni, and/or Au. Figure 6 further shows an optional second p- electrode 612 formed over the p-electrode 610, and an n-electrode 620 formed over the n-type layer 210. The p-electrode 612 may include multiple metal layers, such as Ti, Cr, Ni, or Au. The n-type electrode 620 may include one or more of Ti/AI/Ti/Au. [0044] The UV LED 400 may be implemented with various layers and the composition of these layers may vary, as illustrated in Figures 7 and 8. The UV LED 700 is configured to have a light emission in the N-polar direction (or from the N- polar side) while the UV LED 800 is configured to have a light emission from the metal-polar direction (or from the metal-polar side). While these polarization directions are indicated in Figures 7 and 8, the figures also schematically indicate the metal 102 and the nitrogen 104 in the active layer 212, which in conjunction with the applied electrical field in the QW layers determine these polarizations.

[0045] The substrate 202 in the embodiments illustrated in Figures 7 and 8 is a (0001) sapphire, the buffer layer 204 is N-polar AIN, the undoped layer 206 is AIN, the transition layer 208 includes Alo.ssGaN/AIN superlattices, the n-type layer 210 is n-Alo.55Gao.45N, the active layer 212 is AI0.45Ga0.55N/AI0.55Ga0.45N SQWor MQWs, the first p-type layer 214 is p-Alo.7Gao.3N electron blocking layer (EBL), and the second p-type layer 216 is p-Alo.5Gao.5N. The LED 700 has no third p-type layer and the p- electrode 610 is made of randomly-organized Cu nanowires and the p-electrode 612 made of Ti/AI/Ti/Au while the LED 710 has the third p-type layer 218 made of p+- GaN, the p-electrode 610 is made of Ni/AI and the p-electrode 612 is made of Ti/Au. Both configurations have the n-electrode 620 made of Ti/AI/Ti/Au.

[0046] In another embodiment, as illustrated in Figure 9, the substrate 202 and the first layers 204-208 can be removed so that the LED 900 is obtained, which starts with the n-type layer 210. All the other layers shown in Figure 3 are maintained for this LED. Further, it is possible to make the n-electrode 620 on top of the n-type layer 210 (after flipping the LED 900 to arrive at the configuration shown in Figure 10), and to add the p-type electrodes 610 and 612 on the third p-type layer 218. The second p-type electrode 612 may be attached to a substrate 1002, as shown in Figure 10. The substrate 1002 may be made of the same materials as the substrate 202. Thus, the UV LED 1000 in Figure 10 is still N-polar but is configured to emit the UV light from the metal-polar side 102. A specific implementation of a vertical UV LED 1100 with emission from the metal-polar direction is illustrated in Figure 11. The specific metal concentrations for the various layers are illustrated in the figure. For example, the n-type layer 210, active layer 212, first p-type layer 214, second p-type layer 216 and third p-type layer 218 can be the same in this embodiment. The n-type layer 210 is n-Alo.55Gao.45N ((1 pm, Si=3 ^c 10¹⁸ cm^-3), the active layer 212 includes 5 pairs of undoped Alo.45Gao.55N (2.5 nm) MQWs with Alo.55Gao.45N (12 nm) barriers, the EBL layer 214 includes p-Alo.7Gao.3N (10 nm, Mg=3 ^c 10¹⁹ cm^-3), the layer 216 includes p-Alo.55Gao.45N (100 nm, Mg=5 ^c 10¹⁹ cm^-3), and the 218 layer includes p+- GaN (20 nm, Mg=1 ^c 10²⁰ cm^-3). The n-electrode and p-electrode 620 and 610 are Ti/AI/Ti/Au and Ni/AI/Ti/Au, respectively. The supporting substrate 202 is silicon. [0047] The performance of the N-polar AIGaN-based UV LEDs discussed above are now numerically examined for various Al contents in QWs and barriers. The maximum IQEs were calculated for N-polar UV LEDs with electroluminescence (EL) peaks at 258 to 327 nm and compared to those of metal-polar LEDs. The carrier concentrations, radiative recombination rates and band-diagrams were simulated to explain the enhanced performance of the N-polar LED structures. Furthermore, the IQE performance obtained with different levels of acceptor doping was investigated for both types of LEDs.

[0048] The selected structure 1200 of an AIGaN-based UV LED shown in Figure 12 has been used for these simulations. The UV LED 1200 has a 500-nm- thick n-type contact layer 210 with Si doping concentration of 2^c10¹⁸ cm^-3, an active region 212 that includes 5 pairs of 3-nm-thick QWs layers 520 and 10-nm-thick barrier layers 510, followed by a 10-nm-thick p-type EBL layer 214, and finally, a 50- nm-thick p-type contact layer 216. The Mg doping concentration of the p-type EBL 214 and contact layer 216 is 5^c10¹⁹ cm^-3. The ionization energy of Si is set to be 13 meV, and that of Mg scales linearly from 170 (GaN) to 470 meV (AIN) with Al composition increasing. All layers are AIGaN alloys with an assumed dislocation density of 2^c10⁸ cm^-2. The conduction and valence band offset ratio for AIGaN alloy is set to be 0.7/0.3.

[0049] The parameters Xw and XB indicate the Al contents in the QW layers 520 and barrier layers 510. N-type and p-type contact layers have the same Al contents with barriers in the active region, while the p-type EBL has an Al content of XB+0.15. Mg acceptors diffuse to underlayers at high growth temperatures above 1000°C, so it is assumed that the Mg concentration is linearly graded from 0 to 5x10¹⁹ cm^-3 for the last barrier layer. The device area was 0.001 cm² (316 pm ^c 316 pm), and the operation temperature of the LEDs was set as 300 K. A device structure with the same parameters for a metal-polar UV LED is considered as a reference device for comparison.

[0050] The peak wavelengths of the metal-polar and N-polar UV LEDs were investigated using different Al contents in both the QW and barrier layers. It was found that the EL peak wavelengths of both the metal-polar and N-polar LEDs varied from 327 to 258 nm as the Al contents in QWs (Xw) varied from 0.2 to 0.6. However, when the difference in Al content between the barrier and QW layers (XB-XW) was varied from 0.1 to 0.2, the EL peak shifts were small for both types of LEDs.

[0051] To compare the differences between the metal-polar and N-polar devices, the peak shifts were calculated and mapped. Positive values mean that the EL peaks from N-polar LEDs exhibit redshifts compared to those of the metal-polar LEDs. In most cases, the values of the peak shifts were found to be within ±1 nm, meaning that the metal-polar and N-polar LEDs had similar EL peaks as long as the Al contents in QW and barrier layers were the same. When the difference XB-XW was less than 0.13, all N-polar LEDs exhibited redshifts compared to the metal-polar counterparts. However, when the difference XB-XW was greater than 0.13, blue shifts were obtained from N-polar devices with high-AI AIGaN QW layers.

[0052] To explain these peak shift behaviors, the electric fields in the QW layers were calculated in two cases for Xw=0.2 and Xw=0.6. The electric fields of the metal-polar and N-polar structures have opposite directions, so the absolute values of the electric fields were compared. In the case of Xw=0.2, the metal-polar and N- polar QW layers exhibited almost the same absolute electric fields except that the first and last N-polar QW layers from the p-side were a little higher value than those of the metal-polar QW layers. As a result, the N-polar QW layers suffered a bit more from the quantum-confined Stark effect (QCSE) than the metal-polar ones did.

[0053] In nitride LEDs, QCSE shifts the emission peaks to a longer wavelength, so that the N-polar LEDs with more QCSE showed small red peak shifts compared to the corresponding metal-polar devices. In contrast, the opposite result was obtained for Xw=0.6. Most of the N-polar QW layers exhibited distinctly lower electric fields, which resulted in less QCSE in the QW layers and small blue peak shifts. Therefore, the different peak shift behavior is attributed to the different features of the electric field in the metal-polar and N-polar QW layers. [0054] Although IQE values depend on the current density, the maximum IQE values (IQEmax) were extracted for comparison. When Xw is less than 0.35 (corresponding to peak wavelengths greater than 300 nm), both metal-polar and N- polar LEDs exhibit similar IQEmax mapping. This means that the N-polar UV LEDs are not better at achieving higher IQE values at emission wavelengths above 300 nm. Both IQEmax values depend on the XB-XW rather than XB, and they saturate at around 45% in the range of XB-XW ³ 0.16. This behavior indicates that the value of the XB-XW=0.16 is enough to design QWs structures for both metal-polar and N-polar UV LEDs with peak wavelengths above 300 nm. It is not necessary to introduce AIGaN barriers with higher Al content (XB > Cu\Aq.16) because such AIGaN barriers might have a risk to cause more lattice mismatches with QW layers and introduce defects in them during experimental epitaxy.

[0055] However, when the Xw is greater than 0.35 (corresponding to peak wavelengths less than 300 nm), there are different behaviors in the IQEmax mappings of the metal-polar and N-polar UV LEDs. The N-polar LEDs retained high IQEmax values, while those of the metal-polar LEDs degraded significantly by increasing the Xw. Especially for the deep-UV region (peak wavelength £ 280 nm) with Xw ³ 0.45, the IQEmax values of the N-polar LEDs were enhanced by 1.10-1.50 times in comparison to those of the metal-polar LEDs. The enhancements demonstrate that the N-polar structures are superior to the metal-polar ones in the deep-UV region. For efficient N-polar deep-UV LEDs, the difference XB-XW can be as low as 0.13. [0056] Typical examples of metal-polar and N-polar UV LEDs were investigated and the values of XB and Xwwere 0.52 and 0.65 for both samples, respectively. Figure 13 shows the current-density dependencies of the IQEs for these two UV LEDs. The IQE of the metal-polar deep-UV LED reached a maximum value of 35.6% at 30 A/cm² and then decreased with the current density. Its efficiency droop was prominent. The droop value is defined herein as

where p_max and p_j are the maximum efficiency and the efficiency at the current density of J, respectively.

[0057] From equation (1), the IQE droop value was obtained to be 48.1% at 300 A/cm². In contrast, the IQE values of the N-polar UV LEDs increased with the current density, reached a maximum value of 44.4% at 138 A/cm², and exhibited no significant droop even at 300 A/cm², as shown in Figure 13. The IQE droop value for the N-polar LED was as low as 3.4% at 300 A/cm², demonstrating that the N-polar LED structures discussed above could effectively suppress the IQE droop.

[0058] The injection efficiency influences the IQE so that the current-density dependence of the injection efficiency is also plotted in Figure 13 for comparison. The injection efficiency curves for the metal-polar and N-polar LEDs exhibited droop values of 58.5% and 3.9% at 300 A/cm², respectively. These droop values in the injection efficiency are consistent with those in the IQE for metal-polar and N-polar LEDs. This demonstrates that the suppressed IQE droop in the N-polar structures was mainly due to the improvement of the injection efficiency. [0059] P-type doping in high-AI AIGaN layers is an obstacle in achieving highly efficient deep-UV LEDs. For Al contents of devices having Xw=0.52 in QWs and XB=0.65 in barriers, the maximum IQE values of metal-polar deep-UV LEDs were from 35% to less than 10% when the acceptor concentration of the p-type region varied from 5*10¹⁹ cm^-3 to 5^c10¹⁸ cm^-3. Therefore, a higher doping level of p- AIGaN layers was required to improve the efficiency of these LEDs.

[0060] In the case of the N-polar devices, the lower p-doping levels also resulted in the lower maximum IQE values, but the higher efficiency levels were maintained. The IQE curve for the N-polar LED with [Mg] = 5^c10¹⁸ cm^-3 was better than that of the metal-polar one with [Mg] = 5*10¹⁹ cm^-3. This result indicates that the N-polar structures do not require very high acceptor doping in p-layers like metal- polar ones do, which is greatly beneficial for the epitaxial growth of the N-polar deep- UV LEDs. Since the Mg activation would become more difficult in high-AI p-layers, this result also explains the reason why the N-polar structures were superior to the metal-polar ones in the deep-UV region.

[0061] The carrier transport was also studied to clarify the improved IQE performance of the N-polar LEDs. The electron and hole currents in the case of 198 mA (198 A/cm²) were calculated, and the electron and hole currents are normalized by their counterparts in the n-type and p-type regions, respectively. Each current can indicate an overflow current of each carrier. Electron and hole currents in the N-polar devices become zero at the opposite cladding layer, indicating that both overflow currents were well suppressed. However, the electron current for the metal-polar LED does not reach zero but remains 0.5 at the p-layer. The value is almost the same as 1 - h i_njMetal-poiar· As a result, the N-polar devices have improved injection efficiency.

[0062] The electron and hole concentrations in the metal-polar and N-polar QW layers at 198 mA were also calculated. The N-polar QWs have higher electron and hole concentrations than the metal-polar QWs. This result supports the better carrier confinement in QWs for N-polar LEDs. Using these carrier profiles in the QW region, it is possible to calculate the radiative recombination rate (krad). The radiative efficiency is another important factor of IQE. The spatial profiles of krad were also investigated. The values of krad for N-polar LEDs are almost twice the values of the metal-polar ones. The high radiative recombination originates from the higher electron and hole concentrations in N-polar QW layers, which provides two advantages. First, more electrons and holes in N-polar QW layers enhance the radiative recombination process. Second, the increased electrons and holes screened the electric fields in N-polar QW layers, which reduces the QCSE and enhances the overlap of the electron and hole wave functions.

[0063] To examine the underlying physics in the carrier transport, the energy band diagrams of the metal-polar and N-polar deep-UV LEDs are shown in Figures 14A and 14B. The effective barrier height for electrons and holes was estimated from the difference between their quasi-Fermi levels and the conduction and valence bands, respectively. The effective barriers for carrier injection and overflow blocking are referred herein as the injection barrier and the blocking barrier, respectively. N- polar structures have higher blocking barriers for both electrons and holes, indicating strong suppression of the overflow current, which explains the normalized electron and hole currents discussed above. Figure 14A shows that the electrons for the metal-polarized LED device are facing an increased blocking energy 1410, while Figure 14B shows the opposite for the N-polar LED device, i.e., the energy 1420 promotes electron flow through the layers. Note that Figures 14A and 14B illustrate the barrier layers 510 and the QW layers 520. The same is true for the holes, which are plotted at the bottom in Figures 14A and 14B.

[0064] In terms of the injection barrier, the N-polar LEDs have a lower height for both electrons and holes, as shown in Figure 14B. This lower height illustrates that both the electrons and holes could be injected into the N-polar QWs more easily, resulting in efficient carrier injection. Besides, the electric fields in the N-polar QWs and barriers were able to oppose the escape of electrons and holes in QWs, leading to better carrier confinement. This improved carrier injection and confinement in N- polar structures support the higher carrier concentrations.

[0065] Based on these calculations it was found that the N-polar UV LEDs exhibited higher maximum IQE values and less efficiency droop than metal-polar ones, especially in the deep-UV region. Even when the acceptor concentration of the N-polar p-layers was one order of magnitude lower, the N-polar deep-UV LEDs still had higher IQE values than the metal-polar devices. This better performance occurred because the N-polar structures helped to enhance the carrier injection into the QW layers and provided better carrier confinement in the QW layers. Also, the N- polar structures avoid carrier leakages at high current densities. The simulation data illustrate the great potential of N-polar structures for achieving high-efficiency deep-

UV LEDs. [0066] A method for making a UV LED is now discussed with regard to Figure

15. The method includes a step 1500 of providing a substrate, a step 1502 of forming a semiconductor buffer layer on the substrate by nitridation, wherein the semiconductor buffer layer is N-polar, and the N-polar is achieved by having (1) a nitrogen atom layer in the semiconductor buffer layer predominantly at a distal surface from the substrate and (2) a metal atom layer in the semiconductor buffer layer predominantly at a proximal surface from the substrate, a step 1504 of forming an n-type layer over the semiconductor buffer layer, a step 1506 of forming an active layer over the n-type layer, wherein the active layer is configured to generate UV light, a step 1508 of forming a p-type layer over the active layer, which in tandem with the n-type layer form a pn junction, a step 1510 of depositing a first electrode over the p-type layer, and a step 1512 of depositing a second electrode directly over the n-type layer.

[0067] The disclosed embodiments provide a UV LED that uses N-polar Al alloy to suppress the electron flow and to improve electron/hole injection. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. [0068] Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. [0069] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

References

The entire content of all the publications listed herein is incorporated by reference in this patent application.

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[2] J. Verma, J. Simon, V. Protasenko, T. Kosel, H. G. Xing, and D. Jena, “N-polar Ill-nitride quantum well light-emitting diodes with polarization-induced doping,” Appl. Phys. Lett. 99(17), 171104 (2011).

[3] H. Tao, S. Xu, J. Zhang, P. Li, Z. Lin, and Y. Hao, “Numerical Investigation on the Enhanced Performance of N-Polar AIGaN-Based Ultraviolet Light-Emitting Diodes With Superlattice p-Type Doping,” IEEE Trans. Electron Devices 66(1), 478-484 (2019).

[4] G. Deng, Y. Zhang, Y. Yu, L. Yan, P. Li, X. Han, L. Chen, D. Zhao, and G. Du, “Simulation and fabrication of N-polar GaN-based blue-green light-emitting diodes with p-type AIGaN electron blocking layer,” J. Mater. Sci. : Mater. Electron. 29(11), 9321-9325 (2018).

[5] F. Akyol, D. N. Nath, S. Krishnamoorthy, P. S. Park, and S. Rajan, “Suppression of electron overflow and efficiency droop in N-polar GaN green light emitting diodes,” Appl. Phys. Lett. 100(11), 111118 (2012).

[6] K. Shojiki, T. Tanikawa, J. H. Choi, S. Kuboya, T. Hanada, R. Katayama, and T. Matsuoka, “Red to blue wavelength emission of N-polar (000(1 )over-bar) InGaN light-emitting diodes grown by metalorganic vapor phase epitaxy,” Appl. Phys. Express 8(6), 061005 (2015).

[7] Y. Wang, R. Shimma, T. Yamamoto, H. Hayashi, K. Shiohama, K. Kurihara, R. Hasegawa, and K. Ohkawa, “The effect of plane orientation on indium incorporation into InGaN/GaN quantum wells fabricated by MOVPE,” J. Cryst. Growth 416, 164- 168 (2015).

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Claims

WHAT IS CLAIMED IS:

1. An ultraviolet (UV) light emitting diode (LED) (400) comprising: a substrate (202); a semiconductor buffer layer (204) that is N-polar and is formed on the substrate, wherein the N-polar is achieved by having (1) a nitrogen atom layer in the semiconductor buffer layer at a distal surface from the substrate (202) and (2) a metal atom layer in the semiconductor buffer layer at a proximal surface from the substrate (202); an n-type layer (210) formed over the semiconductor buffer layer (204); an active layer (212) formed over the n-type layer (210) and configured to generate UV light; a p-type layer (214) formed over the active layer (212), which in tandem with the n-type layer (210) form a pn junction; a first electrode (610) formed over the p-type layer (214); and a second electrode (620) formed directly over the n-type layer (210).

2. The UV LED of Claim 1, wherein each of the n-type layer (210), the active layer (212), and the p-type layer (214) are N-polar.

3. The UV LED of Claim 2, wherein the semiconductor buffer layer is AIN, the n-type layer is AIGaN, the active layer includes quantum well layers including AllnN, and barrier layers including AllnGaN, and the p-type layer is AIGaN.

4. The UV LED of Claim 3, wherein a concentration of Al in each of the n-type layer (210), the active layer (212), and the p-type layer (214) is different from the other layers.

5. The UV LED of Claim 1 , further comprising: an undoped layer made of a same material as the semiconductor buffer layer; and a transition layer made on the undoped layer, wherein the undoped layer and the transition layer are located between the semiconductor buffer layer and the n-type layer.

6. The UV LED of Claim 5, wherein the transition layer includes plural pairs of layers, each pair including an AIN layer and an AIGaN layer.

7. The UV LED of Claim 1, wherein the p-type layer includes first and second p-type layers, each of the first and second p-type layers including different concentrations of Al and Ga.

8. The UV LED of Claim 1, wherein the first electrode includes randomly- organized Cu nanowires and the second electrode includes Ti, Al, and Au.

9. The UV LED of Claim 1, wherein the UV light is generated at a N-polar distal side of the active layer.

10. The UV LED of Claim 1 , further comprising: a p-type doped GaN layer formed on the p-type layer, wherein the UV light is generated at a metal-polar proximal side of the active layer.

11. An ultraviolet (UV) light emitting diode (LED) (1000) comprising: a substrate (1002); a first electrode (610) located on the substrate (1002); a p-type layer (214) located on the first electrode (610); an active layer (212) located on the p-type layer (214) and configured to generate UV light within one or more quantum well layers (520); an n-type layer (210) located on the active layer (212) and which in tandem with the p-type layer (214) form a pn junction; and a second electrode (620) located on the n-type layer (210), wherein the active layer (212) is N-polar, and the N-polar is achieved by having (1) a nitrogen atom layer, in the one or more quantum well layers (520), at a proximal surface from the substrate (1002), and (2) a metal atom layer, in the one or more quantum well layers, at a distal surface from the substrate (1002).

12. The UV LED of Claim 11 , wherein each of the n-type layer (210), the active layer (212), and the p-type layer (214) are N-polar.

13. The UV LED of Claim 12, wherein the p-type layer is AIGaN, the n-type layer is AIGaN, the one or more quantum well layers includes AllnN, and barrier layers of the active layer include AllnGaN.

14. The UV LED of Claim 13, wherein a concentration of Al in each of the n- type layer (210), the active layer (212), and the p-type layer (214) is different from the other layers.

15. The UV LED of Claim 11 , wherein there are no undoped layer made of a same material as the semiconductor buffer layer and no transition layer made on the undoped layer.

16. The UV LED of Claim 11 , wherein the p-type layer includes first and second p-type layers, each of the first and second p-type layers including different concentrations of Al and Ga.

17. The UV LED of Claim 11, wherein the first electrode includes randomly- organized Cu nanowires and the second electrode includes Ti, Al, and Au.

18. The U V LED of Claim 11 , wherein the U V light is generated at a metal- polar distal side of the active layer.

19. The UV LED of Claim 11, further comprising: a p-type doped GaN layer formed on the p-type layer, wherein the first electrode is sandwiched between the p-type dope GaN layer and the substrate.

20. A method for making an ultraviolet (UV) light emitting diode (LED) (400), the method comprising: providing (1500) a substrate (202); forming (1502) a semiconductor buffer layer (204) on the substrate (202) by nitridation, wherein the semiconductor buffer layer (204) is N-polar, and the N-polar is achieved by depositing (1) a nitrogen atom layer in the semiconductor buffer layer at a distal surface from the substrate (202), and (2) a metal atom layer in the semiconductor buffer layer at a proximal surface from the substrate (202); forming (1504) an n-type layer (210) over the semiconductor buffer layer

(204); forming (1506) an active layer (212) over the n-type layer (210), wherein the active layer (212) is configured to generate UV light; forming (1508) a p-type layer (214) over the active layer (212), which in tandem with the n-type layer (210) form a pn junction; depositing (1510) a first electrode (610) over the p-type layer (214); and depositing (1512) a second electrode (620) directly over the n-type layer

(210).