ULTRAVIOLET SEMICONDUCTOR LIGHT EMISSION ELEMENT

Abstract

A high-efficiency ultraviolet semiconductor light-emitting element with high output characteristics and high reliability is provided. The ultraviolet semiconductor light-emitting element comprises an n-type coating layer consisting of an Al X Ga 1-X N-type layer formed on a single-crystal AlN substrate; an active quantum well layer formed on the n-type coating layer and composed of an AlGaN layer; an electron barrier layer as an Al Y1 Ga 1-Y1 N-layer formed on the active quantum well layer; and a p-type coating layer as an Al Y2 Ga 1-Y2A p-type N-layer formed on the electron barrier layer. The active quantum well layer comprises barrier layers of the same crystal composition, at least one sub-quantum well layer, and a main quantum well layer, the latter being the quantum well layer closest to the electron barrier layer. The at least one sub-quantum well layer and the main quantum well layer are separated by the barrier layer. The at least one sub-quantum well layer has the same crystal composition and thickness. The main quantum well layer has a thickness of at least 1.2 times that of each of the sub-quantum well layers.

Description

TECHNICAL AREA

[0001] The present invention relates to an ultraviolet semiconductor light emission element or semiconductor light emission element for ultraviolet light, and in particular to a nitrite semiconductor light emission element which emits deep ultraviolet light. TECHNICAL BACKGROUND

[0002] Recently, semiconductor light-emitting elements with a light emission wavelength band in the deep ultraviolet region have received attention as light sources that have effects of inactivating and sterilizing bacteria and viruses.

[0003] For example, patent literature 1 discloses that by controlling the thickness of a quantum well layer in an active layer, the compositions of the quantum well layer and barrier or blocking layers, etc., it is possible to realize a deep ultraviolet light emitting element with high emission efficiency, which can also achieve stable operation in a region with strong current input.

[0004] Patent literature 2 discloses a semiconductor light emission element for deep ultraviolet light in which a p-type semiconductor layer is co-doped with Si. Citation list of patent literature Patent literature 1: Japanese patent no. 6466653 Patent Literature 2: Japanese Patent Publication No. 2022-167231 Summary of the invention: Technical problem

[0005] For conventional semiconductor light-emitting elements for ultraviolet light, studies of semiconductor layers have been carried out on the stacked structure, compositions, impurity concentrations, layer thicknesses, etc. of the layers, but it has been difficult to realize elements with sufficiently high efficiency and high or strong output.

[0006] Furthermore, degradation or deterioration of the element progresses early when it is driven with high current to obtain high optical output, and it has been difficult to achieve characteristics with both high or strong output and high reliability (long lifetime).

[0007] The present application has been drawn up with regard to such a problem and is intended to provide a semiconductor light emission element for ultraviolet light with high efficiency, strong or high output characteristics and high reliability. Solution to the problem

[0008] A semiconductor light-emitting element for ultraviolet light according to an embodiment of the present invention has the following features: a substrate made from a single crystal AIN; an n-type cladding or coating layer, which is an Al X Ga 1-X The N-layer is of the n-type, which is formed on the substrate; an active quantum well layer formed on the n-type coating layer and featuring an AlGaN layer; an electron blocking or electron barrier layer, which is an Al Y1 Ga 1-Y1 The N-layer is formed on the active quantum well layer; and a p-type cladding or coating layer, which is an Al Y2 Ga 1-Y2 The N-layer is of the p-type, which is formed on the electron barrier layer, wherein the active quantum well layer comprises barrier layers with the same crystal composition, at least one sub-quantum well layer and a main quantum well layer, which is a quantum well layer that is closest to the electron barrier layer, wherein the at least one sub-quantum well layer and the main quantum well layer are separated from each other by the barrier layers, where at least one sub-quantum well layer has the same crystal composition and layer thickness, and wherein the at least one sub-quantum well layer and the main quantum well layer have the same crystal composition, and wherein the main quantum well layer has a layer thickness of at least 1.2 times the layer thickness of each sub-quantum well layer. Brief description of the drawings Fig. Figure 1 is a cross-sectional view that schematically illustrates the structure of a semiconductor light-emitting element for ultraviolet light according to an embodiment of the present invention. Fig. Figure 2 is a diagram that schematically illustrates a band diagram of an ultraviolet LED. Fig. Figure 3 is a table showing the configurations of layers of an example ultraviolet LED (EMB). Fig. Figure 4 is a table showing the layer thicknesses of quantum well layers and barrier layers of an active layer in ultraviolet LEDs of the example (EMB) and a comparison example (CMP). Fig. Figure 5 illustrates STEM or scanning transmission electron microscope images of a cross-section of semiconductor layers in the ultraviolet LEDs of the example and the comparison example. Fig. Figure 6 is a figure illustrating histograms of optical outputs for samples of the ultraviolet LEDs of the example and the comparison example. Fig. Figure 7 is a figure illustrating the forward voltage Vf and the emission wavelength λ of the ultraviolet LEDs of the example and the comparison example. Fig. Figure 8 is a figure illustrating an optical output hold ratio relative to an initial optical output for the ultraviolet LEDs of the example and the comparison example. Fig. Figure 9 is a diagram illustrating the relationship between a quantum well layer thickness Lz and the ground levels of electrons and holes. Fig. Figure 10 is a diagram that schematically illustrates a band diagram of the active layer. Fig. Figure 11 is a figure illustrating EL spectra of the ultraviolet LEDs of the example and the comparison example. Fig. Figure 12 is a diagram that schematically illustrates the relationship between the layer thickness of a quantum well layer and electron and hole wave functions. Fig. Figure 13A is a figure illustrating the relationship between the layer thickness of a final barrier layer and the initial optical output or optics output. Fig. 13B is a SIMS profile showing a Mg concentration in a depth direction from a p-type coating layer to a main quantum well layer. Description of exemplary implementations

[0009] Advantageous examples of the present invention, described below, can be suitably modified and combined. In the following description and in the accompanying drawings, parts that are essentially identical or equivalent to each other are designated by the same reference numeral. [Structure of an ultraviolet semiconductor light-emitting element]

[0010] Fig. Figure 1 is a cross-sectional view that schematically illustrates the structure of a semiconductor light-emitting element 10 for ultraviolet light, or ultraviolet semiconductor light-emitting element, according to an embodiment of the present invention. The ultraviolet semiconductor light-emitting element 10 is an ultraviolet light-emitting diode (hereinafter also referred to as ultraviolet LED 10) and can be manufactured, for example, by metal-organic chemical vapor deposition (MOCVD), but is not limited to this method.

[0011] In the ultraviolet LED 10, a cladding or coating layer 12 of the n-type (AlGaN layer of the n-type), an active layer 13, an electron barrier layer 14 (AlGaN layer of the p-type), a cladding or coating layer 15 of the p-type (AlGaN layer of the p-type) and a contact layer 16 of the p-type (GaN layer of the p-type) are stacked on a substrate 11 in the order mentioned above by epitaxial growth or epitaxial cultivation.

[0012] Fig. Figure 2 schematically illustrates a band diagram of the ultraviolet LED 10. The ultraviolet LED 10 is described in more detail below with reference to the Fig. 1 and Fig. 2 described.

[0013] Substrate 11 is a single-crystal AIN substrate with a dislocation density of 10 8 cm -2or less and is hereinafter referred to as the AIN substrate. In an AlGaN-based semiconductor material, which forms the ultraviolet light-emitting element or ultraviolet light-emitting element of the present invention, as described in OPTICS EXPRESS, Vol. 25, No. 16, A639 (2017), it is known, for example, that the light emission efficiency drops sharply when the dislocation density is 10 7 up to 10 8 cm -2 exceeds. Thus, the dislocation density of the single-crystal AIN substrate is preferably as low as possible, and in particular 10 6 cm -2 or less, preferably 10 4 cm -2 or less. By using such an AIN substrate 11 with low dislocation density, it is possible to reduce the dislocation density in the active layer 13, which will be described later, 10 7 cm -2 or to reduce it, without decreasing the light emission efficiency.

[0014] The growth surface (surface) of the AIN substrate 11 of the present invention is not specifically limited and can be a growth surface, such as a C-plane or an M-plane, but is preferably a C-plane, which is typically used as the growth surface of an AlGaN-based material. In a case where a C-plane is used as the crystal growth plane, the substrate is also preferably an off-substrate which is slightly inclined relative to the C-plane, for purposes such as improving the smoothness of an AlGaN layer grown on the AIN substrate 11. The angle of inclination from the C-plane is not specifically limited and can be suitably determined to obtain a smooth AlGaN layer, but is typically selected in the range of 0.1 to 1.0°.Furthermore, the inclination direction from the C-plane is not specifically restricted and can be selected appropriately, for example in the A-axis direction or the M-axis direction; however, it is preferred that the M-axis direction be selected, which results in high linearity of steps.

[0015] If the surface roughness of the AIN substrate 11 is high, it can, for example, cause abnormal growth of the AlGaN layer on the substrate, and thus the surface roughness (RSM) is preferably 1.0 nm or less and particularly preferably 0.5 nm or less. To obtain such a smooth surface or to remove a damaged layer that forms on the substrate surface during a manufacturing process of the substrate, the substrate surface is preferably subjected to a CMP treatment (CMP = Chemical Machine Polishing).

[0016] If the absorption coefficient of the substrate for ultraviolet light emitted by the active layer is high, the total amount of ultraviolet light that can be extracted to the outside decreases, resulting in a reduction in light emission efficiency. Therefore, the absorption coefficient of the AIN substrate and an AIN layer of an AIN sample or AIN template is preferably 20 cm⁻¹. -1 or less and especially 10 cm -1 or less. By setting the absorption coefficient to 10 cm. -1 or less it is possible to ensure a linear permeability (of 90% or more) even if the plate thickness of the AIN substrate 11 is, for example, 100 µm.

[0017] The coating layer 12 of the n-type (Al X Ga 1-XThe n-type AlGaN layer is a conductive layer of the n type doped with silicon (Si). In the ultraviolet semiconductor light-emitting element, ultraviolet light emitted by a light-emitting layer typically passes through the n-type AlGaN layer 12 and the substrate 11 and is emitted outwards. Increasing the aluminum composition or the aluminum content of the n-type AlGaN layer widens the band gap, allowing shorter wavelengths of ultraviolet light to pass through. Therefore, the aluminum composition or the aluminum content of the n-type AlGaN layer can be appropriately controlled to achieve sufficient transparency for the emission wavelength of the desired ultraviolet light.

[0018] The coating layer 12 of the n-type can be formed from a multitude of layers having different Al compositions or Al content (X), and can furthermore be a composition- or concentration-graded layer in which the Al content is gradually graded in the stacking direction. For example, a stacked structure is possible which includes a first coating layer 12A of the n-type (Al X1 Ga 1-X1 N-layer of n-type) and a second coating layer 12B of n-type (Al X2 Ga 1-X2 N-layer).

[0019] The first coating layer 12A of the n-type is, for example, a graded layer with respect to composition, in which the Al composition or Al content X1 decreases from 1.0 to 0.75 in the stacking direction (growth direction), and the second coating layer 12B of the n-type is, for example, a graded layer with respect to composition, in which the Al composition or Al content X2 decreases from 0.75 to 0.70. It should be noted that the Al content at the interface between the first coating layer 12A of the n-type and the second coating layer 12B of the n-type is preferably the same.

[0020] The thickness of the n-type coating layer 12 is not specifically restricted and can be determined appropriately; however, if the thickness of the n-type coating layer 12 is too large, a lattice strain may occur between the AIN substrate 11 and the n-type coating layer 12, which makes dislocation likely, and thus the total thickness of the n-type coating layer 12 is preferably set in a range of 0.5 to 2.0 µm.

[0021] In a case where the coating layer 12 of the n-type has the stacked structure described above, which is formed by the first coating layer 12A of the n-type (Al X1 Ga 1-X1 N-layer of the n-type) and the second coating layer 12B of the n-type (Al X2 Ga 1-X2For example, a stacked structure can be used in which the first n-type coating layer 12A has a thickness of 200 nm, and the second n-type coating layer 12B has a thickness of 1000 nm. The thicknesses of the first and second n-type coating layers 12A and 12B are not limited to the values ​​described as examples and can be appropriately determined so that the total thickness is 2.0 µm or less.

[0022] The Si concentration doped in the n-type coating layer 12 can be suitably determined to achieve a desired n-type conductivity; however, from the perspective of reducing the resistance of the n-type coating layer 12, the Si concentration is preferably 1 × 10 18 up to 1 × 10 20 cm -3 and in particular preferably 5 × 10 18up to 5 × 10 19 cm -3 .

[0023] The Si doping concentration can be constant in the thickness direction of the n-type coating layer 12, or a modulation or variable doping regime, in which the Si concentration varies in the thickness direction, can be employed. It should be noted that the Si and Mg concentrations, which will be described later, can be measured by a well-established secondary ion mass spectrometry analysis (SIMS analysis; SIMS = Secondary ion mass spectrometry). In the present application, the Si and Mg concentrations are quantitative values, respectively, determined using standard samples of AIN and Al. 0,65 Ga 0,35 N and GaN are determined for an AIN layer or an AlGaN layer and a GaN layer.

[0024] The active layer 13 (ACT) has a multi-quantum well structure (MQW = Multi quantum well) consisting of a multitude of barrier layers 13B made of Al A1 Ga 1-A1 are made of N layers, and consists of three quantum well layers QS1, QS2 and QM, which are made of Al A2 Ga 1-A2 N-layers are made.

[0025] In particular, the active layer 13 (ACT) has a main quantum well layer QM and two sub-quantum well layers QS1 and QS2. Specifically, the active layer 13 (ACT) has the sub-quantum well layers (QS1 and QS2) and the main quantum well layer (QM) that are sequentially provided from the side of the substrate 11.

[0026] In particular, the active layer 13 has a first barrier layer 13B1 (with a layer thickness TB1) provided on the n-type coating layer 12, a second barrier layer 13B2 (with a layer thickness TB2) provided between the first sub-quantum well layer QS1 and the second sub-quantum well layer QS2, a third barrier layer 13B3 (with a layer thickness TB3) provided between the second sub-quantum well layer QS2 and the main quantum well layer QM, and a final barrier layer 13L (with a layer thickness TL) provided between the main quantum well layer QM and the electron barrier layer 14 (Al Y1 Ga 1-Y1 N-layer of the p-type) is provided.

[0027] The first sub-quantum well layer QS1 and the second sub-quantum well layer QS2 have the same crystal composition and layer thickness TS. The first sub-quantum well layer QS1, the second sub-quantum well layer QS2, and the main quantum well layer QM have the same crystal composition (A2).

[0028] In the present description, “equal” with respect to crystal composition and layer thickness essentially encompasses identity and refers to an identity to a degree that is achieved in the crystal growth of semiconductor layers.

[0029] The barrier layers 13B, in other words, the first barrier layer 13B1, the second barrier layer 13B2, the third barrier layer 13B3, and the last barrier layer 13L, have the same crystal composition (A1). It should be noted that, unless they differ specifically from one another, these barrier layers are collectively referred to as the barrier layers 13B.

[0030] It should be noted that the layer thickness (TB2 in the case described above) of each barrier layer of each sub-quantum well layer is preferably the same as the layer thickness (TB3 in the case described above) of the barrier layer between the main quantum well layer QM and the sub-quantum well layer that is closest to the main quantum well layer QM.

[0031] The main quantum well layer QM is the quantum well layer closest to the electron barrier layer 14. The two sub-quantum well layers QS1 and QS2 have the same layer thickness TS, and the main quantum well layer QM has a layer thickness TM that is greater than that of the sub-quantum well layers QS1 and QS2 (TS < TM).

[0032] It should be noted that, although the case where two sub-quantum well layers are provided is described as an example, at least one sub-quantum well layer QS1, QS2 ... QSn (where n is an integer of one or more) may be provided. In a case where a plurality of sub-quantum well layers are provided, the sub-quantum well layers have the same crystal composition and layer thickness. It should be noted that, unless they are specifically distinguished from one another, the sub-quantum well layers are collectively referred to as sub-quantum well layers QS.

[0033] The active layer 13 has a peak light emission wavelength in the range of 200 to 360 nm. The wavelength of the light emitted by the active layer 13 is determined by the Al composition or Al content and the layer thickness of the main quantum well layer QM, and thus the Al composition and the layer thickness can be determined in a suitable manner so that a desired emission wavelength in the wavelength range described above is obtained.

[0034] It should be noted that the peak light emission wavelength of the active layer 13 is preferably a wavelength in a UVC region, which is a short wavelength region in the deep ultraviolet range, and in particular in the range of 200 to 280 nm, which has an excellent sterilization effect.

[0035] The main quantum well layer QM, the sub-quantum well layers QS1 and QS2, and the barrier layers 13B can be n-type layers doped with Si. The main quantum well layer QM, the sub-quantum well layers QS1 and QS2, and the barrier layers 13B can be Si-doped layers, wherein each of the main quantum well layer QM and the sub-quantum well layers QS1 and QS2 can be doped with Si, or wherein only the barrier layers 13B can be doped with Si. The Si concentration to be doped is not specifically restricted, but is preferably in the range of 1 × 10⁻⁶. 17 up to 5 × 10 18 cm -3 .

[0036] It should be noted that a p-type dopant, such as Mg, may be present in the last barrier layer 13L. The Mg present in the last barrier layer 13L may result from diffusion doping due to diffusion from the electron barrier layer 14 (AlY1 Ga 1-Y1 N-layer of the p-type) which will be described later, or may result from an intentional Mg doping.

[0037] The electron barrier layer 14 (EBL) of the active layer 13 has a function of suppressing an overflow of electrons introduced into the active layer 13 into the p-type coating layer 15 (Al Y2 Ga 1-Y2 N-layer of the p-type), which will be described later. Thus, the electron barrier layer 14 (Al Y1 Ga 1-Y1 N-layer) preferably has a larger bandgap than that of the active layer 13 and the p-type coating layer 15, and the Al composition Y1 of the electron barrier layer 14 is preferably set in the range of 0.8 < Y1 < 1.0.

[0038] As the emission wavelength shortens, the Al composition or Al fraction of the AlGaN layer grown epitaxially on substrate 11 increases, and in a case where the emission wavelength is shorter than 270 nm, the Al composition or Al fraction Y1 is preferably 0.9 ≤ Y1 ≤ 1.0 to adequately demonstrate the function of the electron barrier layer. It should be noted that in the present example, AIN (Y1 = 1) is the (Al Y1 Ga 1-Y1 N-layer) 14 is used.

[0039] As long as the function of the electron barrier layer can be demonstrated or realized, the electron barrier layer 14 can be an undoped layer or can be doped with a p-type dopant.

[0040] Magnesium (Mg), zinc (Zn), beryllium (Be), carbon (C), etc., can be used as p-type doping materials in the electron barrier layer 14. In particular, it is preferable to use Mg, which is typically used as a p-type doping material for AlGaN layers, and Mg is also used in the example of the present invention, which will be described later.

[0041] The p-type doping material can be uniformly doped in the stacking direction of the electron barrier layer 14, and the concentration of the doping material can be varied in the stacking direction. For example, a stacked structure can be used in which an undominated AIN layer 14A (Y1 = 1) and a Mg-doped (magnesium-doped) p-type AIN layer 14B are stacked sideways in contact with the active layer. The concentration of the p-type dopant in the electron barrier layer 14 is preferably 1.0 × 10⁻⁶ 19 up to 8.0 × 10 19 cm -3 , in particular preferably 3.0 × 10 19 up to 5.0 × 10 19 cm -3 , and in particular preferably 3.0 × 10 19 up to 4.0 × 10 19 cm -3, namely from the perspective that the function of the electron barrier layer is achieved and the efficiency of carrier introduction or carrier injection into the light-emitting layer is improved.

[0042] The electron barrier layer 14 preferably has a thickness in the range of 4 to 10 nm. If the thickness is less than 4 nm, the effect of the electron barrier layer is small due to the tunneling effect, and if the thickness is 10 nm or more, the hole injection efficiency decreases.

[0043] The coating layer 15 of the p-type (Al Y2 Ga 1-Y2 N-layer of the p-type) is on the Al Y1 Ga 1-Y1 N-layer 14 is formed and acts as a cladding or coating layer doped with Mg. The p-type doping material can be any of the materials described above without restriction; however, it is preferable to use Mg in the electron barrier layer 14 (Al Y1 Ga1-Y1 to use the N-layer of the p-type).

[0044] In the ultraviolet light emission element of the present invention, the Mg concentration or the Mg content in the coating layer 15 of the p-type is preferably 2.0 × 10 19 up to 1.0 × 10 20 cm -3 and in particular preferably 2.0 × 10 19 up to 5.0 × 10 19 cm -3 By setting the Mg concentration in the coating layer 15 of the p-type to the areas described above, it is possible to obtain a high light emission efficiency.

[0045] In the case of a structure in which Y2 is constant in the stacking direction, it is preferable that the Al composition or the Al content Y2 of the coating layer 15 of the p-type (Al Y2 Ga 1-Y2The Al composition of each barrier layer of the active layer (N-layer of the p-type) exceeds the Al composition of Y1 of the electron barrier layer 14 and is equal to or less than the Al composition Y1 of the electron barrier layer 14. By setting the Al composition or Al content Y2 of the coating layer 15 of the p-type to the range described above, it is possible to obtain a strong suppression effect for carrier overflow, even when the amount of introduced or injected current of the ultraviolet light element is large. To achieve a greater effect, the difference between the Al composition of each barrier layer of the active layer and the Al composition Y2 of the coating layer 15 of the p-type is preferably 0.5 to 1.0.

[0046] The Al composition Y2 of the coating layer 15 of the p-type is preferably greater than the Al composition of the coating layer 12 of the n-type, and accordingly the effect of suppressing a carrier overflow is improved compared to the layer of the p-type, and the light emission efficiency of the ultraviolet light emission element can be increased.

[0047] The p-type coating layer 15 can be a graded layer with respect to composition, in which the Al composition or Al content Y2 varies in the stacking direction. In particular, a structure is provided in which the Al composition Y2 decreases in the stacking direction from the side in contact with the electron barrier layer 14. This results in a polarization doping effect in the p-type coating layer 15, which makes it easier to achieve a higher hole concentration and consequently increases the efficiency of hole injection into the active layer. For example, in a case where the emission wavelength is 240 nm or less, the Al composition on the side in contact with the electron barrier layer 14 is preferably 0.95 to 1.0, and the Al composition of the surface layer of the p-type coating layer 15 on the opposite side is preferably 0.60 to 0.85.By using such a structure, it is possible to enhance the polarization doping effect described above while maintaining transparency for the emission wavelength, making it easier to achieve high light emission efficiency.

[0048] The thickness of the p-type coating layer 15 is not specifically limited and can be suitably determined in the range of 10 to 150 nm. If the thickness of the p-type coating layer 15 is less than 10 nm, it is difficult to achieve the carrier overflow suppression effect described above, and if the thickness is large and exceeds 150 nm, the resistance of the p-type coating layer 15 increases, resulting in an increase in the operating voltage of the ultraviolet light-emitting element. From this perspective, the thickness of the p-type coating layer 15 is preferably 40 to 120 nm and particularly preferably 50 to 100 nm. In the embodiment of the present invention, a graded layer with respect to composition is used, in which the Al composition is...The Al content Y2 decreases in one growth direction from the Al composition of the electron barrier layer 14 (the Al composition Y2 decreases from 1.0 to 0.8). The layer thickness of the p-type coating layer 15 is 60 nm.

[0049] The coating layer 15 of the p-type (Al Y2 Ga 1-Y2 The N-layer of the p-type coating in the present embodiment is co-doped with a p-type impurity, which serves as an acceptor, and with an n-type impurity, which serves as a donor. It should be noted that the p-type coating layer 15 is preferably co-doped, but this is not limited to co-doping. The following describes the co-doping process.

[0050] The p-type impurity doped into the p-type coating layer 15 can be magnesium (Mg), zinc (Zn), beryllium (Be), carbon (C), or similar. Of these, Mg is preferably used, as it is typically used as a p-type doping material for AlGaN semiconductors. The n-type impurity can be silicon (Si), germanium (Ge), selenium (Se), sulfur (S), oxygen (O), or similar. Of these, silicon (Si) is preferably used, as it is typically used as an n-type doping material.

[0051] As regards the p-type coating layer 15 of the present embodiment, the ratio (Nd / Na) of the concentration Nd of the n-type impurity in the p-type coating layer 15 to the concentration Na of the n-type impurity satisfies the condition of expression (1). 0.009≤(Nd / Na)<0.185

[0052] The ratio Nd / Na satisfies in particular the condition of each of the expressions (2) and (3) below. 0.009≤(Nd / Na)<0.135 0.038≤(Nd / Na)<0.185

[0053] By co-doping the p-type coating layer 15 in such a way that the Nd / Na ratio of the p-type impurity concentration satisfies the condition of each of expressions (1) to (3) above, it is possible to realize an ultraviolet LED with high luminous efficacy and to maintain a favorable output ratio (element lifetime).

[0054] The amount of p-type impurity doped into the p-type coating layer 15 is preferably 1 × 10 17 up to 1.2 × 10 20 cm -3As theoretically shown in J. Appl. Phys., Vol. 95, No. 8, April 15 (2004), the amount of nitrogen defects, which is considered a deterioration factor, is assumed to increase along with the amount of p-type impurities in the p-type coating layer 15. In a case where the amount of p-type impurities is 1.2 × 10 20 cm -3 If the output exceeds the limit, the amount of nitrogen defects initially formed is correspondingly too large, making it difficult to maintain a strong output sustainment ratio or a consistent output.

[0055] As the concentration of p-type impurities decreases, maintaining high light emission efficiency becomes difficult. This is due to a reduction in output resulting from a decrease in hole concentration and an increase in minority carrier (electron) mobility, either when the Al composition Y2 is constant or when the Al composition Y2 is graded. Therefore, the concentration of p-type impurities can be appropriately set within the range described above, taking this trade-off into account. However, a concentration of 1 × 10⁻⁶ is preferable. 19 up to 5 × 10 19 cm -3 , and in particular preferably 1 × 10 19 up to 4 × 10 19 cm -3, in order to obtain a higher retention ratio of the expenditure or a more consistent expenditure and a high expenditure.

[0056] The amount of n-type impurities doped into the p-type coating layer 15 is preferably 1.1 × 10 18 or more and 9.0 × 10 18 cm -3 or less and in particular 1.8 × 10 18 or more and 8.0 × 10 18 cm -3 or less. With this amount of n-type impurities, it is possible to obtain the light emission element 10 with high light emission efficiency.

[0057] The concentrations of the p-type and n-type impurities doped into the p-type coating layer 15 can be constant within the layer or can vary in concentration along the stacking direction. For example, the side in contact with the electron barrier layer 14 can be a co-doping layer, and the remaining part of the p-type coating layer 15 can be a layer doped without n-type impurities.

[0058] The p-type contact layer 16 (p-type GaN layer), doped with a p-type dopant, can be formed on the p-type coating layer 15 to reduce the contact resistance with an electrode. The p-type dopant material can be any of the known p-type dopant materials described above, but for the same reason, Mg is preferably used. The Mg doping concentration in the p-type GaN layer 16 is not specifically restricted, but is preferably 1 × 10⁻⁶ 18 up to 2 × 10 20 cm -3 to reduce the resistance or specific resistance of the p-type GaN layer and the contact resistance. The thickness of the p-type GaN layer 16 is not specifically limited and can be suitably determined in the range of 5 to 500 nm.

[0059] It should be noted that all layers of the AlGaN layers 12, 13, 14 and 15, except for the p-type GaN layer 16, are epitaxially grown in a lattice-matched state with the AIN substrate 11 and thus have low dislocation densities equivalent to that of the AIN substrate 11. In particular, the layers have dislocation densities of 10 8 cm -2 or less.

[0060] It should be noted that the ultraviolet semiconductor light emission element of the present invention is the ultraviolet LED 10 (light-emitting diode) in the above description, but can be configured as a semiconductor laser element (LD).

[0061] The following describes a method for producing the ultraviolet LED 10 with the structure described above. The ultraviolet LED 10 of the present invention can be produced by a well-known crystal growth process, such as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Of these, MOCVD is preferred due to its high productivity and widespread industrial use. The Group III source gas (Al-Ga) and the Group V source gas (N) used in the present invention are not specifically limited and can be well-known source gases.

[0062] For example, gases such as trimethylaluminium, triethylaluminium, trimethylgallium, or triethylgallium can be used as the group III source gas. Ammonia is still typically used as the group V source gas.

[0063] Well-known materials can be used as dopant source gases for Mg and Si without restriction, and for example bis(cyclopentadienyl)magnesium, monosilane and tetraethysilane can be used.

[0064] The element layers of the ultraviolet LED 10 are grown by supplying the source gases described above to the substrate 11 together with carrier gas, such as hydrogen and / or nitrogen.

[0065] The delivery ratio (V / III ratio) of the Group III source gas and the Group V source gas can be determined in a suitable manner to obtain desired characteristics, and it is preferably set in the range of 500 to 10000.

[0066] The growth temperature of the element layers forming the ultraviolet LED 10 is not specifically limited and can be determined in a suitable manner to obtain desired characteristics of the layers and the ultraviolet LED; however, the element layers are preferably grown at 1000 to 1200°C and preferably at 1000 to 1150°C. Example

[0067] The following describes in particular the present invention using an example in which the ultraviolet LED 10 is generated with a light emission wavelength of 256 nm, but the present invention is not limited thereto. (a) Structure of the element

[0068] Fig. Figure 3 is a table showing the configurations of layers of the manufactured ultraviolet LED 10 of an example (EMB). Fig. Figure 4 is a table showing the layer thicknesses of the quantum well layers and barrier layers of the active layer in the ultraviolet LED 10 of the example (EMB) and an ultraviolet LED of a comparison example (CMP). It should be noted that the ultraviolet LED of the comparison example (CMP) has the same structure as the ultraviolet LED 10 of the example (EMB), except that the layer thicknesses of the quantum well layers and the barrier layers are different.

[0069] As in Fig. As illustrated in Figure 4, in the ultraviolet LED 10 of the example and in the ultraviolet LED of the comparison example, the quantum well layers and the last barrier layer 13L are undoped layers, and the first to third barrier layers 13B1 to 13B3 are doped with Si in a concentration of 1.0 × 10 18 cm -3 doped. It should be noted that the last barrier layer 13L can be a doped layer. The Si doping concentration of the first to third barrier layers 13B1 to 3B3 or of the last barrier layer 13L is preferably within the range of 1.0 × 10 17 cm -3 up to 5.0 × 10 18 cm -3 The first to third barrier layers 13B1 to 13B3 can be undoped layers.

[0070] Fig. Figure 5 are cross-sectional STEM images or scanning transmission electron microscope images of a cross-section, obtained by viewing semiconductor layers in the example and in the comparison example using a scanning transmission electron microscope (STEM).

[0071] As in Fig. As illustrated in Figure 5, the quantum well layers and barrier layers in the example and in the comparison example were evaluated based on the scanning transmission electron microscope images of the cross-section to check the layer thicknesses ( Fig. 4) The figure shows the STEM images of the comparison example (CMP) and the example (EMB) on the left and right sides for comparison.

[0072] It should be noted that in the ultraviolet LED 10 of the example, the layer thicknesses of the main quantum well layer QM and the last barrier layer 13L are approximately twice as large as those of the ultraviolet LED of the comparison example (see Fig. 4) It should be noted that the layer thicknesses of the sub-quantum well layers QS1 and QS2 in the ultraviolet LED of the example are essentially the same as the layer thicknesses of the first to third quantum well layers in the ultraviolet LED of the comparison example ( Fig. 4). (b) Characteristics of the elements and discussion (b-1) Optical output characteristics

[0073] Fig. Figure 6 illustrates histograms of optical outputs (mW) for samples of the example ultraviolet LED (EMB) and the comparison example ultraviolet LED (CMP), and Fig. Figure 7 illustrates a forward voltage Vf and an emission wavelength λ of the samples of the ultraviolet LED of the example and the ultraviolet LED of the comparison example.

[0074] As in Fig. As illustrated in Figure 6, the average optical output of the ultraviolet LED 10 in the example was 75 mW, which was a considerable improvement compared to the average optical output of 50 mW for the ultraviolet LED in the comparison example. Furthermore, the average external differential quantum efficiency (EQE) of the ultraviolet LED 10 in the example was 3.1%, which was an improvement compared to the average external differential quantum efficiency of 2.1% for the ultraviolet LED in the comparison example.

[0075] As in Fig. As illustrated in Figure 7, the average Vf (avg. = average) of the forward voltage Vf for the ultraviolet LED 10 of the example was 6.9 V, and no significant difference was observed compared to the 7.1 V of the ultraviolet LED of the comparison example.

[0076] The average emission wavelength λ (avg. = average) was 256.0 nm for the example ultraviolet LED 10, which was shifted 4 nm towards the red end compared to 261.0 nm for the comparison ultraviolet LED. The emission wavelength of the example ultraviolet LED corresponds to an emission wavelength resulting from recombination between quantum levels in the main quantum well layer QM, which has a thickness twice that of the quantum well layer of the comparison ultraviolet LED (in other words, the thickness of the sub-quantum well layer of the example ultraviolet LED 10). Accordingly, it was found that light emission from the example ultraviolet LED 10 is dominated by light emission from the main quantum well layer QM. (b-2) Reliability (lifespan of the element)

[0077] Fig. Figure 8 shows a retention ratio (%) of the optical output, or uniformity of the optical output, which shows the rate of change of the optical output relative to an initial optical output (arbitrary unit or average unit) for the ultraviolet LED 10 of the example (EMB) and the ultraviolet LED of the comparison example (CMP). Specifically, the retention ratio or uniformity (%) of the optical output was defined as the rate of change of the optical output at room temperature (RT) after 100 hours from the start of the power supply, while maintaining a constant driving current of the element and using the initial optical output (average or arbitrary unit) as a reference. It should be noted that the retention ratio of the optical output was measured by selecting samples with different optical outputs from those in Figure 8. Fig. 6 illustrated samples. In addition, a comparison example (CMP0) is also shown (outlined by a dashed line), in which the coating layer 15 of the p-type (Al Y2 Ga 1-Y2 N-layer of the p-type) is not co-doped.

[0078] The optical output retention ratio of the ultraviolet LED 10 of the example is improved for all initial optical outputs, and additionally it is significantly improved for high or strong initial outputs, especially because of the low dependence of the optical output retention ratio on the initial optical output. (c) Discussion of the structure of the elements (c-1) Improvement of the external differential quantum efficiency (EQE) and the optical output

[0079] Firstly, in ultraviolet semiconductor light-emitting elements, the hole concentration in a p-type semiconductor layer is typically low, and the hole mobility is lower than that of the electrons, resulting in low external quantum efficiency and optical output.

[0080] It is assumed that the reason for the increase in optical output of the ultraviolet LED 10 of the present example is an increase in the efficiency of hole injection into the active layer 13 from the p-type coating layer 15 and the electron barrier layer 14.

[0081] In other words, it is assumed that an increase in the thickness of the main quantum well layer QM, which is closest to the electron barrier layer 14, leads to an increase in the efficiency of the uptake of hole carriers from the electron barrier layer 14 by the main quantum well layer QM, which results in an increase in the amount of carriers that contribute to light emission, thereby increasing the probability of recombination, and accordingly increasing the external differential quantum efficiency (EQE) and the optical output.

[0082] Furthermore, it is assumed that, as schematically shown in Fig. Figure 9 illustrates that when the thickness Lz of the quantum well layer is increased, the grounding levels (quantum levels En1 and Eh1) of electrons and holes approach the band ends of the quantum well, in other words, the conduction band Ec and the valence band Ev, and accordingly, radiative recombination or radiation recombination becomes more likely to occur, which increases the optical output. (c-2) Dominance of light emission from the main quantum well layer QM

[0083] Fig. Figure 10 schematically illustrates a band diagram of the active layer 13. Fig. Figure 11 illustrates an electroluminescence spectrum (EL spectrum) of the ultraviolet LED 10 of the example (EMB) and that of the ultraviolet LED 10 of the comparison example (CMP). It should be noted that the spectra are EL spectra for injection and introduction at 450 mA, respectively. The dominance of light emission from the main quantum well layer QM in the ultraviolet LED 10 of the example is described below. (i) First, it is assumed that, as in Fig. Figure 10 schematically illustrates the quantum well layer QM closest to a p-type semiconductor layer (in the example the electron barrier layer 14), which traps and confines most of the holes, since the hole concentration of the p-type semiconductor layer is low and the mobility of the holes is lower than that of the electrons. (ii) As described above, the thickness TM of the main quantum well layer QM in the example ultraviolet LED 10 was set to 7.9 nm, which was approximately twice the thickness TS (=3.9 nm) of the sub-quantum well layers QS1 and QS2. The emission wavelength of the example ultraviolet LED 10 was shifted 4 nm towards the red compared to the ultraviolet LED 10 of the comparison example, and this wavelength shift corresponds to a stimulation result for the increase (4 nm) in the quantum well layer thickness. (iii) As in Fig. Figure 1 illustrates that the full width at half maximum (FWHM) of the EL spectrum of the ultraviolet LED 10 of the example is 8.5 nm, which is not wider than the full width at half maximum (FWHM) of 9.4 nm of the ultraviolet LED of the comparison example.

[0084] If the sub-quantum well layers QS1 and QS2 contribute to the light emission to a certain extent, a light emission peak or peak would appear in the FWHM due to light emission at 261.0 nm from the sub-quantum well layers QS1 and QS2 and light emission at 265.0 nm from the main quantum well layer QM, or an increase would be observed.

[0085] However, no increase in the full width at half maximum (FWHM) is observed in the EL spectrum of the example ultraviolet LED 10; instead, a decrease is observed. Thus, it is assumed that the light emission from the sub-quantum well layers QS1 and QS2 is extremely small.

[0086] It should be noted that the reduction in the half-width (8.5 nm) of the EL spectrum of the ultraviolet LED 10 of the example compared to the half-width (9.4 nm) of the ultraviolet LED 10 of the comparison example (CMP) is probably due to a reduction in the rate of change of the ground levels with respect to the layer thickness fluctuation, since the layer thickness TM of the main quantum well layer QM is large. (c-3) Functions of the sub-quantum well layers

[0087] The functions of the sub-quantum well layers QS, which are provided in active layer 13, are described below. Again with reference to Fig. In a case where the layer thickness TB3 of the third barrier layer 13B3 between the main quantum well layer QM and the sub-quantum well layer QS2 adjacent to the main quantum well layer QM is small (for example, several nm), electrons that are confined or trapped in the sub-quantum well layer QS exhibit tunneling motion and are injected into the main quantum well layer QM with a lower ground level due to the tunneling effect.

[0088] It is assumed that electron tunneling enables confinement and injection of carriers into the main quantum well layer QM with a thick well layer and a reduction in electron concentration excess due to electron tunneling, thereby increasing the probability of radiative recombination and increasing the output.

[0089] Since the hole concentration and hole mobility are lower in the valence band, most holes are trapped by the third quantum well layer. Accordingly, the sub-quantum well layer QS functions by improving the carrier confinement in the main quantum well layer QM.

[0090] From the perspective of electron tunneling, the layer thickness TB3 of the third barrier layer 13B3 is preferably 10 nm or less, and particularly preferably 7 nm or less. The layer thickness TB3 is further preferably 6 nm or less. (c-4) Layer thickness of the main quantum well layer

[0091] Fig. Figure 12 is a diagram that schematically represents electron and hole wave forms in cases where the layer thickness Lz of a quantum well layer is small (left side of the diagram) and large (right side of the diagram).

[0092] Due to a difference in crystal composition between the quantum well layer and each barrier layer, a lattice strain arises between the quantum well layer and the barrier layer, generating a piezoelectric field. Consequently, the electron and hole wave functions in the quantum well layer are misaligned, and their overlap decreases, reducing the probability of a radiative transition and thus the light emission efficiency. Since the thickness Lz of the quantum well layer is increased, the wave function overlap decreases, and the light emission efficiency decreases accordingly.

[0093] In a case where an Al-GaN layer is stacked on the AIN substrate, compressive strain is applied due to the pulling of the underlying layer, specifically because of the difference in the lattice constant. As the AlGaN layer thickens, lattice relaxation occurs when the critical film thickness is reached, and threading or displacement is generated in the AlGaN layer, reducing the internal quantum efficiency and, consequently, the light emission efficiency.

[0094] In the example described above, the layer thickness TM of the quantum well layer QM is twice as large as the layer thickness TS of the sub-quantum top layers QS1 and QS2, however, the invention is not limited thereto.

[0095] A simulation was performed using band gap modeling software for a semiconductor device (“SILENSe”), varying the layer thickness of the sub-quantum well layers QS1 and QS2 and the main quantum well layer QM based on the composition and layer thickness of a stacked AlGaN body.

[0096] According to the simulation, the internal quantum efficiency decreases as the quantum well layer thickness increases, and it is effective when the quantum well layer thickness is at least 1.2 times the layer thickness of the sub-quantum well layer QS2. Furthermore, the layer thickness TM of the main quantum well layer QM is preferably at least 1.5 times the layer thickness TS of the sub-quantum top layer QS (of the sub-quantum well layer QS2) adjacent to the main quantum well layer QM, so that the electron tunneling effect from the sub-quantum well layer QS2 to the main quantum well layer QM and the electron confinement effect described above can be adequately demonstrated.

[0097] The layer thickness TM of the main quantum well layer QM is preferably 16 nm or less, which is the critical film thickness in a case where an AlGaN layer is stacked on an AIN substrate. Furthermore, based on a simulation of the overlap integration between electrons and holes, the layer thickness TM is further preferably 10 nm or less, as this is the range in which the integral value becomes significant. (c-5) Thickness of the last barrier layer

[0098] Fig. Figure 13A illustrates the relationship between the thickness TL of the last barrier layer 13L and the initial optical output. The optical output increases as the thickness TL of the last barrier layer 13L increases. This is because the hole injection efficiency decreases as the thickness of the last barrier layer 13L increases.

[0099] Thus, the layer thickness TL of the last barrier layer 13L is preferably 30 nm or less, more preferably 27 nm or less and even more preferably 21 nm or less.

[0100] Fig. 13B is a SIMS profile showing a Mg concentration (concentration of a p-type dopant) in a depth direction from the p-type coating layer 15 to the main quantum well layer QM.

[0101] According to the Mg concentration profile, the Mg concentration is 1 × 10 18 cm -3 or more in the range of 9 nm from the electron barrier layer 14 (EBL). When Mg enters the main quantum well layer QM at a concentration of 1 × 10 18 cm -3or more occurs, in other words, if a large amount of nitrogen defects enters the main quantum well layer QM, the lifetime of the element is degraded, and thus the layer thickness TL of the last barrier layer 13L is preferably 9 nm or more. (c-6) Co-doping of the p-type coating layer

[0102] The coating layer 15 of the p-type (Al Y2 Ga 1-Y2 The N-layer of the p-type of the present embodiment is co-doped with a p-type impurity serving as an acceptor and with an n-type impurity serving as a donor.

[0103] Again with reference to Fig. 8. It is understandable that in the ultraviolet LED of the comparison example (CMP0; surrounded by a dashed line in the drawing), where the p-type coating layer is not co-doped, the optical output increases compared to that of the co-doped ultraviolet LED of the comparison example (CMP), but the retention ratio of the optical output (%) decreases, resulting in a deterioration of the reliability (lifespan of the element).

[0104] In the present embodiment, Si-Co doping and the thickness of the final barrier layer 13L are used. Co-doping the p-type coating layer 15 with Si reduces the number of nitrogen defects generated in the p-type coating layer 15. Furthermore, the use of the thick final barrier layer 13L improves optical output and reliability (element lifetime) by synergistically suppressing the diffusion of nitrogen defects caused by heat during power supply or energy input.

[0105] It should be noted that the case where the coating layer 15 of the p-type is co-doped ( Fig. 4) as described above, however the electron barrier layer 14 can also be co-doped. (c-7) Emission wavelength and composition of the AlGaN layer

[0106] AlGaN layers formed on an AlGaN substrate, in other words, the coating layer 12 of the n-type (Al X1 Ga 1-X N-layer of the n-type), the active layer 13, the electron barrier layer 14 (Al Y1 Ga 1-Y1 N-layer) and the coating layer 15 of the p-type (Al Y1 Ga 1-Y1 N-layer of the p-type) are preferably made of AlGaN with an Al composition (X, Y1, Y2 etc.) of 100% to 50%, which is the Al ratio among the group III elements, and the ultraviolet LED 10 is preferably a light-emitting element or light-emitting element which emits light in the UVC region with a wavelength of 200 to 280 nm.

[0107] The reason for the Al composition of 50% or more is to prevent dislocation generation due to an increase in the extent of lattice strain with respect to the AIN substrate. According to the theoretical calculation based on the energy balance model (People and Bean formula), the critical thickness is 45 nm when the Al composition or Al content is 40% or less, which is close to the total thickness (44.3 nm in the example described above) of the active layer 13 of the preferred embodiment. Therefore, the Al composition is preferably 50% or more so that lattice strain does not occur in the active layer 13 and there is no reduction in optical output. Furthermore, absorption loss is taken into account when deep ultraviolet light (for example, with a wavelength of 265 nm) is extracted from the side of the AIN substrate, which has the highest sterilization effect.

[0108] As described in more detail above, according to the present invention it is possible to provide an ultraviolet semiconductor light emission element with high efficiency, high or strong output characteristics and high reliability. Reference symbol list 10 Ultraviolet semiconductor light emission element 11 Substrat 12. Coating layer of the n-type (AINGaN layer) 12A first coating layer of the n-type (Al X1 Ga 1-X1 N-layer of the n-type) 12B second coating layer of the n-type (Al X2 Ga 1-X2 N-layer of the n-type) 13 active quantum well layers 13B1 to 13B3 barrier layer 13L last barrier layer 14 Electron barrier layer (Al Y1 Ga 1-Y1 N-layer of the p-type) 15 p-type coating layer (Al Y2 Ga 1-Y2 N-layer of the p-type 16 p-type contact layer (p-type GaN layer) QM Main Quantum Well Layer QS1, QS2 sub-quantum well layer QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP 2022-167231

[0004] Cited non-patent literature

[0000] OPTICS EXPRESS, Vol. 25, No. 16, A639 (2017

[0013] J. Appl. Phys., Vol. 95, No. 8, April 15 (2004

[0054]

Claims

[1] Ultraviolet semiconductor light emission element comprising: a substrate made of single-crystal AIN; an n-type cladding or coating layer, which is an Al X Ga 1-X The N-layer is of the n-type, which is formed on the substrate; an active quantum well layer formed on the n-type coating layer and featuring an AlGaN layer; an electron barrier layer that is an Al Y1 Ga 1-Y1 The N-layer is formed on the active quantum well layer; and a p-type cladding or coating layer, which is an Al Y2 Ga 1-Y2 The N-layer is of the p-type, which is formed on the electron barrier layer, wherein the active quantum well layer comprises barrier layers with the same crystal composition, at least one sub-quantum well layer and one main quantum well layer which is the quantum well layer closest to the electron barrier layer, wherein the at least one sub-quantum well layer and the main quantum well layer are separated from each other by the barrier layers, wherein at least one sub-quantum well layer has / have the same composition and layer thickness, and wherein the at least one sub-quantum well layer and the main quantum well layer have the same crystal composition, and wherein the main quantum well layer has a layer thickness of at least 1.2 times the layer thickness of each sub-quantum well layer. [2] Ultraviolet semiconductor light emission element according to claim 1, wherein the layer thickness of the main quantum well layer is 16 nm or less. [3] Ultraviolet semiconductor light emission element according to claim 1 or 2, wherein the thickness of the barrier layer between the main quantum well layer and the sub-quantum well layer adjacent to the main quantum well layer is 7 nm or less. [4] Ultraviolet semiconductor light emission element according to claim 1 or 2, wherein a final barrier layer, which is the barrier layer between the main quantum well layer and the electron barrier layer, has a layer thickness in the range of 9 to 27 nm. [5] Ultraviolet semiconductor light emission element according to claim 1 or 2, wherein the p-type coating layer is co-doped with a p-type impurity serving as an acceptor and is co-doped with an n-type impurity serving as a donor. [6] Ultraviolet semiconductor light emission element according to claim 5, wherein the electron barrier layer is co-doped with a p-type impurity serving as an acceptor and an n-type impurity serving as a donor. [7] Ultraviolet semiconductor light emission element according to claim 1, wherein the active quantum well layer has an emission wavelength in a range of 200 to 280 nm.

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