Ultraviolet semiconductor light emitting element

Abstract

To provide an ultraviolet semiconductor light emitting element which has high efficiency, high output characteristics and high reliability.SOLUTION: An ultraviolet semiconductor light emitting element includes: an n-type cladding layer which is an n-type AlXGa1-XN layer formed on a single crystal AlN substrate; a quantum well active layer which is formed on the n-type cladding layer and is composed of an AlGaN layer; an electron blocking layer which is an AlY1Ga1-Y1N layer formed on the quantum well active layer; and a p-type cladding layer which is a p-type AlY2Ga1-Y2N layer formed on the electron blocking layer. The quantum well active layer includes: a barrier layer having the same crystal composition; and at least one sub quantum well layer and a main quantum well layer which is a quantum well layer closest to the electron blocking layer, the sub quantum well layer and the main quantum well layer being separated from each other by the barrier layer. The at least one sub quantum well layer has the same crystal composition and the same layer thickness, the sub quantum well layer and the main quantum well layer have the same crystal composition, and the main quantum well layer has a layer thickness that is not less than 1.2 times the layer thickness of the sub quantum well layer.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an ultraviolet semiconductor light-emitting element, and more particularly to a nitride semiconductor light-emitting element that emits deep ultraviolet light. [Background technology]

[0002] In recent years, semiconductor light-emitting elements that emit light in the deep ultraviolet region have been attracting attention as light sources that have the effect of inactivating and sterilizing bacteria and viruses.

[0003] For example, Patent Document 1 discloses that by controlling the thickness of the quantum well layer of the active layer, the compositions of the quantum well layer and the barrier layer, etc., a deep ultraviolet light-emitting device with high luminous efficiency and capable of stable operation even in a high injection current region is realized.

[0004] Moreover, Patent Document 2 discloses a deep ultraviolet semiconductor light emitting device in which a p-type semiconductor layer is co-doped with Si. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6466653 [Patent Document 2] JP 2022-167231 A Summary of the Invention [Problem to be solved by the invention]

[0006] In conventional ultraviolet semiconductor light-emitting devices, various studies have been conducted on the stacked structure, composition, impurity concentration, layer thickness, etc. of the semiconductor layers, but it has been difficult to realize a device with sufficiently high efficiency and high output.

[0007] Furthermore, when driven with a large current in order to obtain high optical output, the element deteriorates quickly, making it difficult to achieve both high output characteristics and high reliability (long life).

[0008] The present application has been made in view of the above problems, and has an object to provide an ultraviolet semiconductor light-emitting element that has high efficiency, high output characteristics, and high reliability. [Means for solving the problem]

[0009] An ultraviolet semiconductor light emitting device according to one embodiment of the present invention comprises: A substrate made of single crystal AlN; The n-type Al formed on the substrate X Ga 1-X an n-type cladding layer, which is an N layer; a quantum well active layer formed on the n-type cladding layer and made of an AlGaN layer; The Al formed on the quantum well active layer Y1 Ga 1-Y1 an electron blocking layer, which is an N layer; The p-type Al formed on the electron blocking layer Y2 GaN layer 1-Y2 and a p-type cladding layer, The quantum well active layer has a barrier layer having the same crystal composition and a at least one sub-quantum well layer and a main quantum well layer, the main quantum well layer being the quantum well layer closest to the electron blocking layer, the main quantum well layer being spaced apart from one another; The at least one sub-quantum well layer has the same crystal composition and layer thickness; The sub-quantum well layer and the main quantum well layer have the same crystal composition, and the main quantum well layer has a thickness 1.2 times or more that of the sub-quantum well layer. [Brief description of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view illustrating a schematic structure of an ultraviolet semiconductor light-emitting element according to an embodiment of the present invention. [Diagram 2] FIG. 1 is a schematic diagram showing a band diagram of an ultraviolet LED. [Diagram 3] 1 is a table showing the configuration of each layer of an ultraviolet LED according to an embodiment (EMB). [Figure 4]1 is a table showing thicknesses of quantum well layers and barrier layers in the active layer in ultraviolet LEDs of an embodiment (EMB) and a comparative example (CMP). [Diagram 5] 4 shows cross-sectional STEM images of semiconductor layers of ultraviolet LEDs according to an example and a comparative example. [Figure 6] FIG. 13 is a diagram showing histograms of light output of ultraviolet LED samples of an example and a comparative example. [Figure 7] FIG. 2 is a diagram showing the forward voltage Vf and the emission wavelength λ of ultraviolet LEDs of an example and a comparative example. [Figure 8] FIG. 1 is a diagram showing the light output maintenance rate versus the initial light output of ultraviolet LEDs in an example and a comparative example. [Figure 9] FIG. 13 is a diagram showing the relationship between the quantum well layer thickness Lz and the ground levels of electrons and holes. [Figure 10] FIG. 2 is a diagram illustrating a band diagram of an active layer. [Figure 11] FIG. 2 is a diagram showing EL spectra of ultraviolet LEDs of an example and a comparative example. [Figure 12] FIG. 2 is a diagram illustrating a relationship between the layer thickness of a quantum well layer and the wave functions of electrons and holes. [Figure 13A] FIG. 13 is a diagram showing the relationship between the layer thickness of the final barrier layer and the initial light output. [Figure 13B] 1 is a SIMS profile showing the Mg concentration in the depth direction from the p-type cladding layer to the main quantum well layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] In the following, preferred embodiments of the present invention will be described, which may be modified and combined as appropriate. In the following description and accompanying drawings, substantially the same or equivalent parts are designated by the same reference numerals. [Structure of ultraviolet semiconductor light-emitting element] 1 is a cross-sectional view showing a schematic structure of an ultraviolet semiconductor light-emitting element 10 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 an ultraviolet LED 10), and can be manufactured by, for example, but not limited to, a metal-organic chemical vapor deposition (MOCVD) method.

[0012] The ultraviolet LED 10 has an n-type cladding layer (n-type AlGaN layer) 12, an active layer 13, an electron blocking layer (p-type AlGaN layer) 14, a p-type cladding layer (p-type AlGaN layer) 15, and a p-type contact layer (p-type GaN layer) 16 stacked in this order on a substrate 11 by epitaxial growth.

[0013] 2 is a schematic diagram showing a band diagram of the ultraviolet LED 10. The ultraviolet LED 10 will be described in more detail below with reference to FIGS.

[0014] The substrate 11 has a dislocation density of 10 8 cm -2 The AlN substrate is a single crystal AlN substrate, which is hereinafter referred to as an AlN substrate 11. In the AlGaN-based semiconductor material constituting the ultraviolet light-emitting device of the present invention, for example, as described in OPTICS EXPRESS Vol. 25 No. 16 A639 (2017), the dislocation density is 10 7 ~10 8 cm -2 It is known that the light emission efficiency drops sharply when the dislocation density exceeds 10 6 cm -2 More preferably, 10 4 cm -2 By using the AlN substrate 11 having such a low dislocation density, the dislocation density in the active layer 13 described later can be reduced to 10% or less without decreasing the light emission efficiency. 7 cm -2 It can be the following:

[0015] The growth surface (surface) of the AlN substrate 11 of the present invention is not particularly limited, and may be a growth surface such as a C-plane or an M-plane, but is preferably a C-plane, which is generally used as a growth surface for AlGaN-based materials. Furthermore, when the C-plane is used as the crystal growth surface, it is preferable that the substrate is an OFF substrate slightly inclined from the C-plane for the purpose of improving the smoothness of the AlGaN layer grown on the AlN substrate 11. The inclination angle from the C-plane is not particularly limited, and may be appropriately determined so as to obtain a smooth AlGaN layer, but is usually selected in the range of 0.1 to 1.0°. The inclination direction from the C-plane is also not particularly limited, and may be appropriately selected such as an A-axis direction or an M-axis direction, but it is preferable to select the M-axis direction, which increases the linearity of the step edge.

[0016] Furthermore, since a large surface roughness of the AlN substrate 11 can cause abnormal growth of the AlGaN layer on the substrate, the surface roughness (RSM) is preferably 1.0 nm or less, and more preferably 0.5 nm or less. In order to obtain such a smooth surface or to remove a damaged layer formed on the substrate surface during the substrate manufacturing process, the substrate surface is preferably subjected to chemical mechanical polishing (CMP) processing.

[0017] In addition, if the absorption coefficient of the substrate for the ultraviolet light emitted from the active layer is large, the total amount of ultraviolet light that can be extracted to the outside is reduced, which may lead to a decrease in the light emission efficiency. Therefore, the absorption coefficient of the AlN substrate and the AlN layer of the AlN template is preferably 20 cm -1 Less than 10 cm, more preferably -1 Less than 10cm -1 By setting it to the following value, even if the thickness of the AlN substrate 11 is 100 μm, for example, it is possible to ensure an in-line transmittance of 90% or more.

[0018] n-type cladding layer (n-type Al X Ga 1-XThe n-type AlGaN layer 12 is an n-type conductive layer doped with Si (silicon). In an ultraviolet semiconductor light-emitting element, ultraviolet light emitted from the light-emitting layer is usually emitted to the outside after passing through the n-type AlGaN layer 12 and the substrate 11. As the Al composition of the n-type AlGaN layer increases, the band gap of the n-type AlGaN layer increases, and accordingly, ultraviolet light of a shorter wavelength can be transmitted through the n-type AlGaN layer. Therefore, the Al composition of the n-type AlGaN layer may be appropriately determined so as to obtain sufficient transmittance for the desired emission wavelength of ultraviolet light.

[0019] The n-type cladding layer 12 may be formed of a plurality of layers having different Al compositions (X), and may be a compositionally graded layer in which the Al composition is graded in the stacking direction. For example, the first n-type cladding layer (n-type Al X1 Ga 1-X1 N layer) 12A and the second n-type cladding layer (n-type Al X2 Ga 1-X2 N layer) 12B.

[0020] The first n-type cladding layer 12A is, for example, a composition gradient layer in which the Al composition X1 decreases from 1.0 to 0.75 in the stacking direction (growth direction), and the second n-type cladding layer 12B is, for example, a composition gradient layer in which the Al composition X2 decreases from 0.75 to 0.70. Note that it is preferable that the Al compositions at the interfaces between the first n-type cladding layer 12A and the second n-type cladding layer 12B are equal.

[0021] Furthermore, the thickness of the n-type cladding layer 12 is not particularly limited and may be determined as appropriate. However, if the thickness of the n-type cladding layer 12 is too thick, lattice relaxation occurs between the AlN substrate 11 and the n-type cladding layer 12, making dislocations more likely to occur. Therefore, it is preferable to set the total thickness of the n-type cladding layer 12 in the range of 0.5 to 2.0 μm.

[0022] For example, the n-type cladding layer 12 may be the first n-type cladding layer (n-type Al X1 Ga 1-X1 N layer) 12A and the second n-type cladding layer (n-type Al X2 Ga 1-X2In the case of a laminated structure consisting of a first n-type cladding layer 12A and a second n-type cladding layer 12B, a laminated structure in which the first n-type cladding layer 12A has a thickness of 200 nm and the second n-type cladding layer 12B has a thickness of 1000 nm may be used. Naturally, the thicknesses of the first and second n-type cladding layers 12A and 12B are not limited to the exemplified values, and may be appropriately determined so that the total thickness is 2.0 μm or less.

[0023] The concentration of Si doped in the n-type cladding layer 12 may be appropriately determined so as to obtain a desired n-type conductivity. From the viewpoint of reducing the resistance value of the n-type cladding layer 12, however, it is preferable to dope the n-type cladding layer 12 with a Si concentration of 1×10 18 ~1×10 20 cm -3 It is preferable that the number of the particles is 5×10 18 ~5×10 19 cm -3 It is preferable that:

[0024] The Si doping concentration may be constant in the layer thickness direction in the n-type cladding layer 12, or it may be modulated doping in which the Si concentration varies in the layer thickness direction. The Si concentration and the Mg concentration described below can be measured by known Secondary Ion Mass Spectrometry (SIMS) analysis. The Si concentration and Mg concentration in this application are measured using AlN, AlGaN, and GaN layers, respectively. 0.65 Ga 0.35 Quantitative values ​​using standard samples of N and GaN are used.

[0025] The active layer (ACT) 13 is Al A1 Ga 1-A1 A plurality of barrier layers 13B each made of an AlN layer A2 Ga 1-A2 It has a multi-quantum well (MQW) structure consisting of three quantum well layers QS1, QS2, and QM, each consisting of N layers.

[0026] More specifically, the active layer (ACT) 13 has one main quantum well layer QM and two sub-quantum well layers QS1 and QS2. More specifically, the active layer (ACT) 13 has the sub-quantum well layers QS1 and QS2 and the main quantum well layer QM, which are provided in this order from the substrate 11 side.

[0027] More specifically, the active layer 13 includes a first barrier layer 13B1 (thickness: TB1) provided on the n-type cladding layer 12, a second barrier layer 13B2 (thickness: TB2) provided between the first sub-quantum well layer QS1 and the second sub-quantum well layer QS2, a third barrier layer 13B3 (thickness: TB3) provided between the second sub-quantum well layer QS2 and the main quantum well layer QM, and a third barrier layer 13B4 (thickness: TB5) provided between the main quantum well layer QM and an electron block layer (p-type Al Y1 Ga 1-Y1 A final barrier layer 13L (layer thickness: TL) is provided between the first layer 13A and the second layer 14.

[0028] The first sub-quantum well layer QS1 and the second sub-quantum well layer QS2 have the same crystal composition and layer thickness TS. In addition, 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). In this specification, the term "same" with respect to the crystal composition and layer thickness includes "substantially same" and refers to the degree of sameness obtained in the crystal growth of the semiconductor layer.

[0029] The barrier layers 13B, i.e., the first barrier layer 13B1, the second barrier layer 13B2, the third barrier layer 13B3 and the final barrier layer 13L, have the same crystal composition (A1). When there is no need to distinguish between each of these multiple barrier layers, they will be collectively referred to as the barrier layer 13B.

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

[0031] The main quantum well layer QM is the quantum well layer closest to the electron blocking layer 14. Also, 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 (TS < TM) larger than that of the sub-quantum well layers QS1 and QS2.

[0032] Although the case where two sub-quantum well layers are provided is shown as an example, at least one sub-quantum well layer QS1, QS2, ···, QSn (n is an integer of 1 or more) may be provided. When a plurality of sub-quantum well layers are provided, the plurality of sub-quantum well layers have the same crystal composition and layer thickness. When not particularly distinguishing each of the plurality of sub-quantum well layers, they will be collectively described as the sub-quantum well layer QS.

[0033] The emission peak wavelength of the active layer 13 is in the range of 200 to 360 nm. Since the wavelength of the light emitted from the active layer 13 is determined by the Al composition and layer thickness of the main quantum well layer QM, the Al composition and layer thickness can be appropriately determined so as to obtain a desired emission wavelength in the above wavelength range.

[0034] Note that it is preferable that the emission peak wavelength of the active layer 13 is in the wavelength range of the UVC region, which is the deep ultraviolet short wavelength region, particularly in the range of 200 - 280 nm, which is excellent in bactericidal action.

[0035] Also, the main quantum well layer QM, the sub-quantum well layers QS1 and QS2, and the barrier layer 13B can also be n-type layers doped with Si. The main quantum well layer QM, the sub-quantum well layers QS1 and QS2, and the barrier layer 13B may be Si-doped layers, or either the main quantum well layer QM or the sub-quantum well layers QS1 and QS2 may be doped with Si, or only the barrier layer 13B may be doped with Si. The Si concentration to be doped is not particularly limited, but is preferably in the range of 1×10 17 ~5×10 18 cm -3 range.

[0036] Note that the final barrier layer 13L may contain a p-type dopant such as Mg. Here, the Mg contained in the final barrier layer 13L may be due to diffusion doping caused by diffusion from the electron blocking layer (p-type Al Y1 Ga 1-Y1 N layer) 14 described later, or it may be intentionally doped with Mg. The electron blocking layer (EBL: Electron Blocking Layer) 14 on the active layer 13 has a function of suppressing the overflow of electrons injected into the active layer 13 into the p-type cladding layer (p-type Al Y2 Ga 1-Y2 N layer) 15 described later. Therefore, the electron blocking layer (Al Y1 Ga 1-Y1 N layer) 14 has a larger bandgap than the active layer 13 and the p-type cladding layer 15, and it is preferable that the Al composition Y1 of the electron blocking layer 14 is determined in the range of 0.8 < Y1 ≦ 1.0.

[0037] As the emission wavelength becomes shorter, the Al composition of the AlGaN layer epitaxially grown on the substrate 11 increases. When the emission wavelength is shorter than 270 nm, in order to sufficiently exhibit the function as an electron blocking layer, it is preferable that 0.9 ≦ Y1 ≦ 1.0 for the Al composition Y1. In this embodiment, Al Y1 Ga 1-Y1 N layer 14 uses AlN (Y1 = 1).

[0038] Also, the electron blocking layer 14 may be an undoped layer or may be doped with a p-type dopant as long as it can exhibit the function as an electron blocking layer.

[0039] As the p-type dopant material in the electron blocking layer 14, Mg (magnesium), Zn (zinc), Be (beryllium), C (carbon), etc. can be used. In particular, it is preferable to use Mg which is generally used as the p-type dopant material for the AlGaN layer, and Mg is also used in the embodiments of the present invention described later.

[0040] The p-type dopant material may be uniformly doped in the stacking direction of the electron blocking layer 14, or the concentration of the dopant material may be varied in the stacking direction. For example, a stacked structure consisting of an undoped AlN layer 14A (Y1=1) and a Mg (magnesium)-doped p-type AlN layer 14B from the side in contact with the active layer may be used. The p-type dopant concentration in the electron blocking layer 14 is set to 1.0×10 19 ~8.0×10 19 cm -3 is preferably 3.0×10 19 ~5.0×10 19 cm -3、 Particularly preferably, 3.0×10 19 ~4.0×10 19 cm -3 It is.

[0041] Moreover, the electron blocking layer 14 preferably has a layer thickness in the range of 4 to 10 nm. If the layer thickness is less than 4 nm, the effect as an electron blocking layer is small due to the tunneling effect, and if the layer thickness is 10 nm or more, the efficiency of hole injection decreases.

[0042] p-type cladding layer (p-type Al Y2 Ga 1-Y2 N layer) 15 is p-type Al Y1 Ga 1-Y1 It is formed on the N layer 14 and functions as a cladding layer doped with Mg. The above-mentioned materials can be used as the p-type dopant material without any restrictions. Y1 Ga 1-Y1 As with the N layer 14, it is preferable to use Mg.

[0043] In the ultraviolet light emitting device of the present invention, the Mg concentration in the p-type cladding layer 15 is 2.0×10 19 ~1.0×10 20 cm -3 is preferably 2.0×10 19 ~5.0×1019 cm -3 By setting the Mg concentration in the p-type cladding layer 15 within the above range, high luminous efficiency can be obtained.

[0044] p-type cladding layer (p-type Al Y2 Ga 1-Y2 In the case of a structure in which Y2 is a constant value in the stacking direction, the Al composition Y2 of the p-type cladding layer 15 preferably exceeds the Al composition of the barrier layer of the active layer and is equal to or less than the Al composition Y1 of the electron blocking layer 14. By setting the Al composition Y2 of the p-type cladding layer 15 in the above range, a high carrier overflow suppression effect can be obtained even when the injection current amount of the ultraviolet light emitting device is large. To obtain a higher effect, the difference between the Al composition of the barrier layer of the active layer and the Al composition Y2 of the p-type cladding layer 15 is preferably 0.5 to 1.0.

[0045] Furthermore, the Al composition Y2 of the p-type cladding layer 15 is preferably larger than the Al composition of the n-type cladding layer 12, which enhances the effect of suppressing carrier overflow to the p-type layer and increases the luminous efficiency of the ultraviolet light-emitting device.

[0046] The p-type cladding layer 15 may be a compositionally graded layer in which the Al composition Y2 changes in the stacking direction. In particular, the structure is preferably such that the Al composition Y2 decreases in the stacking direction from the side in contact with the electron blocking layer 14. This provides a polarization doping effect in the p-type cladding layer 15, making it easier to obtain a higher hole concentration, and as a result, the efficiency of hole injection into the active layer is increased. For example, when the emission wavelength is 270 nm or less, the Al composition on the side in contact with the electron blocking layer 14 is preferably 0.95 to 1.0, and the Al composition in the surface layer of the p-type cladding layer 15 on the opposite side is preferably 0.60 to 0.85. By adopting such a structure, the above-mentioned polarization doping effect can be enhanced and transparency to the emission wavelength can be maintained, making it easier to obtain high emission efficiency.

[0047] The thickness of the p-type cladding layer 15 is not particularly limited, but may be appropriately determined within the range of 10 to 150 nm. If the thickness of the p-type cladding layer 15 is less than 10 nm, it becomes difficult to obtain the above-mentioned effect of suppressing carrier overflow. On the other hand, if the thickness is thick and exceeds 150 nm, the resistance value of the p-type cladding layer 15 increases, resulting in an increase in the operating voltage of the ultraviolet light-emitting device. From this viewpoint, the thickness of the p-type cladding layer 15 is preferably 40 to 120 nm, and more preferably 50 to 100 nm. In the embodiment of the present invention, a compositionally graded layer in which the Al composition Y2 decreases in the growth direction from the Al composition of the electron blocking layer 14 (the Al composition Y2 decreases from 1.0 to 0.8) is adopted. The thickness of the p-type cladding layer 15 is set to 60 nm.

[0048] The p-type cladding layer (p-type Al Y2 Ga 1-Y2 The p-type cladding layer 15 is co-doped with p-type impurities acting as acceptors and n-type impurities acting as donors. It is preferable that the p-type cladding layer 15 is co-doped, but this is not limitative. Co-doping will be described below.

[0049] Examples of p-type impurities that can be used to dope the p-type cladding layer 15 include magnesium (Mg), zinc (Zn), beryllium (Be), and carbon (C). Of these, it is preferable to use Mg, which is generally used as a p-type dopant material for AlGaN semiconductors. Examples of n-type impurities that can be used include germanium (Ge), selenium (Se), sulfur (S), and oxygen (O). Of these, it is preferable to use Si, which is generally used as an n-type dopant material.

[0050] In the p-type cladding layer 15 of this embodiment, the ratio (Nd / Na) of the n-type impurity concentration (Nd) to the p-type impurity concentration in the p-type cladding layer 15 satisfies formula (1).

[0051] 0.009≦(Nd / Na)<0.185...Equation (1) Furthermore, it is more preferable that Nd / Na satisfies any one of the following formulas.

[0052] 0.009≦(Nd / Na)<0.135...Equation (2) 0.038≦(Nd / Na)<0.185...Equation (3) By co-doping the p-type cladding layer 15 so that the p-type impurity concentration ratio (Nd / Na) satisfies any one of the above formulas (1) to (3), an ultraviolet LED with high luminous efficiency and good output maintenance rate (element life) is realized.

[0053] The amount of p-type impurities doped into the p-type cladding layer 15 is 1×10 17 ~1.2×10 20 cm -3 As theoretically shown in J. Appl. Phys., Vol. 95, No. 8, 15 April (2004), the amount of nitrogen defects, which are considered to be a cause of degradation, is thought to increase with the amount of p-type impurities in the p-type cladding layer 15. Therefore, when the amount of p-type impurities is 1.2×10 20 cm -3 If it exceeds this value, the amount of nitrogen defects formed in the initial stage becomes too large, making it difficult to obtain a high output maintenance rate.

[0054] Furthermore, when the p-type impurity concentration is low, particularly when the Al composition Y2 is constant, the hole concentration decreases and the minority carrier (electron) mobility increases, and when the Al composition Y2 is inclined, the minority carrier (electron) mobility increases, causing a decrease in output and making it difficult to obtain high luminous efficiency. Therefore, the p-type impurity concentration can be appropriately determined within the above range, taking such a trade-off into consideration, but in order to obtain a higher output maintenance rate and high output, it is recommended to set the p-type impurity concentration to 1×10 19 ~5×10 19 cm -3 It is preferable that the concentration is 1×10 19 ~4×10 19 cm -3 It is.

[0055] The amount of n-type impurities doped into the p-type cladding layer 15 is 1.1×10 18 Above 9.0×10 18 cm -3 It is preferable that the number of the particles is 1.8×10 or less, and more preferably, 1.8×10 18 Above 8.0×10 18 cm -3 With these amounts of n-type impurities, a light emitting device 10 with high luminous efficiency can be obtained.

[0056] The concentrations of the p-type impurities and n-type impurities doped into the p-type cladding layer 15 may be constant within the layer, or may vary in the stacking direction. For example, the side in contact with the electron blocking layer 14 may be a co-doped layer, and the remaining p-type cladding layer 15 may be a layer that is not doped with n-type impurities.

[0057] On the p-type cladding layer 15, a p-type contact layer (p-type GaN layer) 16 doped with a p-type dopant may be formed in order to reduce the contact resistance with the electrode. The above-mentioned known p-type dopant materials can be used as the p-type dopant material, but for the same reason, it is preferable to use Mg. The Mg doping concentration in the p-type GaN layer 16 is not particularly limited, but in order to reduce the resistance value in the p-type GaN layer and the contact resistance, it is preferable to use a doping concentration of 1×10 18 ~2×10 20 cm -3 In addition, the thickness of the p-type GaN layer 16 is not particularly limited, and may be appropriately determined within the range of 5 to 500 nm.

[0058] All of the AlGaN layers 12, 13, 14, and 15, except for the p-type GaN layer 16, are grown in a state of lattice matching with the AlN substrate 11, and therefore have a low dislocation density equivalent to that of the AlN substrate 11. 5 cm -2 The dislocation density is as follows:

[0059] In the above description, the ultraviolet semiconductor light emitting element of the present invention is described as an ultraviolet LED 10 (light emitting diode), but it may be configured as a semiconductor laser element (LD: Laser Diode).

[0060] Next, a method for manufacturing the ultraviolet LED 10 having the structure described above will be described. The ultraviolet LED 10 of the present invention can be manufactured by known crystal growth methods such as metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Among these, the MOCVD method is preferred because it has high productivity and is widely used industrially. The group III (Al, Ga) source gas and group V (N) source gas used in the present invention can be any known source gas without any particular restrictions.

[0061] For example, the Group III source gas may be trimethylaluminum, triethylaluminum, trimethylgallium, triethylgallium, etc. The Group V source gas is usually ammonia.

[0062] Furthermore, as the dopant source gas for Mg and Si, known materials can be used without any restrictions, such as biscyclopentadienyl magnesium, monosilane, and tetraethylsilane.

[0063] The above-mentioned source gases are supplied onto the substrate 11 together with a carrier gas such as hydrogen and / or nitrogen to grow the element layers of the ultraviolet LED 10.

[0064] The supply ratio of the Group III source gas to the Group V source gas (V / III ratio) may be appropriately determined so as to obtain desired characteristics, but is preferably set within the range of 500 to 10,000.

[0065] The growth temperature of the element layers constituting the ultraviolet LED 10 is not particularly limited and may be appropriately determined so as to obtain the desired characteristics of each layer and the characteristics of the ultraviolet LED 10. However, growth at a temperature of 1000 to 1200°C is preferable, and 1000 to 1150°C is more preferable. EXAMPLES

[0066] In the following, the present invention will be specifically described using an example in which an ultraviolet LED 10 with an emission wavelength of 265 nm was fabricated, but the present invention is not limited to this example. (a) Device structure Fig. 3 is a table showing the configuration of each layer of the fabricated ultraviolet LED 10 of the example (EMB). Also, Fig. 4 is a table showing the layer thicknesses of the quantum well layer and barrier layer of the active layer in the ultraviolet LED 10 of the example (EMB) and the ultraviolet LED of the comparative example (CMP). The ultraviolet LED of the comparative example (CMP) has the same structure as the ultraviolet LED 10 of the example (EMB) except for the layer thicknesses of the quantum well layer and barrier layer.

[0067] As shown in FIG. 4, in the ultraviolet LED 10 of the embodiment and the ultraviolet LED of the comparative example, each quantum well layer and the final barrier layer 13L are undoped layers, and the first to third barrier layers 13B1 to 13B3 have a 1.0×10 18 cm -3 The final barrier layer 13L may be a doped layer. The first to third barrier layers 13B1 to 13B3 or the final barrier layer 13L are doped with Si at a concentration of 1.0×10 17 cm -3 ~5.0×10 18 cm -3 It is preferable that the thickness is within the range of 13B1 to 13B3. The first to third barrier layers 13B1 to 13B3 may be undoped layers.

[0068] FIG. 5 shows cross-sectional STEM images of the semiconductor layers of the example and comparative example observed with a scanning transmission electron microscope (STEM). As shown in Figure 5, the quantum well layer and barrier layer of the example and comparative example were evaluated using cross-sectional STEM images to confirm the layer thickness (Figure 4). The STEM image of the comparative example (CMP) is shown on the left side of the figure, while the STEM image of the example (EMB) is shown on the right side for comparison.

[0069] It can be seen that in the ultraviolet LED 10 of the embodiment, the thickness of the main quantum well layer QM and the final barrier layer 13L are each about twice as thick as those in the ultraviolet LED of the comparative example (see FIG. 4). The thicknesses of the sub-quantum well layers QS1 and QS2 of the ultraviolet LED 10 of the embodiment are approximately the same as the thicknesses of the first to third quantum well layers of the ultraviolet LED of the comparative example (FIG. 4).

[0070] (b) Device characteristics and considerations (b-1) Optical output characteristics FIG. 6 shows a histogram of the light output (mW) of the ultraviolet LED 10 of the embodiment (EMB) and the ultraviolet LED sample of the comparative example (CMP), and FIG. 7 shows the forward voltage Vf and emission wavelength λ of the ultraviolet LED 10 of the embodiment and the ultraviolet LED sample of the comparative example.

[0071] 6, the ultraviolet LED 10 of the embodiment had an average light output of 75 mW, which was a significant improvement over the average light output of the ultraviolet LED of the comparative example, which was 50 mW. In addition, the ultraviolet LED 10 of the embodiment had an average external differential efficiency (EQE) of 3.1%, which was an improvement over the average external differential efficiency of the ultraviolet LED of the comparative example, which was 2.1%.

[0072] As shown in FIG. 7, the average value Vf(avg.) of the forward voltage Vf was 6.9 V for the ultraviolet LED 10 of the embodiment, which was not significantly different from 7.1 V for the ultraviolet LED of the comparative example.

[0073] The average value λ (avg.) of the emission wavelength λ of the ultraviolet LED 10 of the embodiment is 265.0 nm, which is 4 nm longer than the 261.0 nm of the ultraviolet LED of the comparative example. The emission wavelength of the ultraviolet LED 10 of the embodiment matches the emission wavelength due to recombination of energy between quantum levels of the main quantum well layer QM, which has a layer thickness twice that of the quantum well layer of the ultraviolet LED of the comparative example (i.e., the sub-quantum well layer thickness of the ultraviolet LED 10 of the embodiment). In other words, it was found that the emission of the ultraviolet LED 10 of the embodiment is dominated by the emission of light by the main quantum well layer QM.

[0074] (b-2) Reliability (element life) FIG. 8 shows the light output maintenance rate (%), which indicates the rate of change in light output relative to the initial light output (arbitrary unit) of the ultraviolet LED 10 of the embodiment (EMB) and the ultraviolet LED of the comparative example (CMP). More specifically, at room temperature (RT), the driving current of the element was kept constant, and the initial light output (arbitrary unit) was used as the reference, and the rate of change in light output 100 hours after the start of current application was taken as the light output maintenance rate (%). Samples with different light outputs were selected from the samples shown in FIG. 6 to measure the light output maintenance rate. In addition, the p-type cladding layer (p-type Al Y2 Ga 1-Y2 A comparative example (CMP0) in which the N layer 15 is not co-doped is also plotted (shown surrounded by a dashed line).

[0075] The light output maintenance rate of the ultraviolet LED 10 of the embodiment is improved at all initial light outputs. In addition, since the light output maintenance rate has less dependency on the initial light output, the improvement is particularly remarkable at high initial light outputs.

[0076] (c) Consideration of element structure (c-1) Improvement of external differential efficiency (EQE) and light output First, in ultraviolet semiconductor light-emitting elements, the hole concentration in the p-type semiconductor layer is generally low, and the mobility of holes is lower than that of electrons, resulting in low external quantum efficiency and low light output.

[0077] The reason for the increase in light output in the ultraviolet LED 10 of this embodiment is believed to be that the efficiency of hole injection from the p-type cladding layer 15 and the electron blocking layer 14 to the active layer 13 is increased.

[0078] That is, by increasing the thickness of the main quantum well layer QM closest to the electron blocking layer 14, the efficiency of capturing hole carriers from the electron blocking layer 14 by the main quantum well layer QM is increased, and the amount of carriers contributing to light emission is increased, which in turn increases the recombination probability and results in increased external differential efficiency (EQE) and light output.

[0079] Furthermore, as shown diagrammatically in Figure 9, increasing the quantum well layer thickness Lz brings the ground levels of electrons and holes (quantum levels En1, Eh1) closer to the band edges of the quantum well, i.e., the conduction band Ec and the valence band Ev, making radiative recombination easier and increasing the light output.

[0080] (c-2) Light emission from the primary quantum well layer QM is dominant Fig. 10 is a schematic diagram of a band diagram of the active layer 13. Fig. 11 shows the EL (electroluminescence) spectra of the ultraviolet LED 10 of the embodiment (EMB) and the ultraviolet LED of the comparative example (CMP). The EL spectra are obtained when 450 mA is injected. The fact that the emission from the main quantum well layer QM is dominant in the ultraviolet LED 10 of the embodiment will be described below.

[0081] (i) First, as shown in FIG. 10, the hole concentration in the p-type semiconductor layer is low and the mobility of holes is lower than that of electrons. Therefore, it is considered that the quantum well layer QM closest to the p-type semiconductor layer (electron blocking layer 14 in the embodiment) captures and confines most of the holes.

[0082] (ii) As described above, in the ultraviolet LED 10 of the embodiment, the thickness TM of the main quantum well layer QM is set to 7.9 nm, which is approximately twice the thickness TS (=3.9 nm) of the sub-quantum well layers QS1, QS2. The ultraviolet LED 10 of the embodiment emits light with a wavelength 4 nm longer than that of the ultraviolet LED of the comparative example, and this wavelength shift amount matches the wavelength shift amount in the simulation results for an increase in the quantum well layer thickness (4 nm).

[0083] (iii) As shown in FIG. 11, the full width at half maximum (FWHM) of the EL spectrum of the ultraviolet LED 10 of the embodiment is 8.5 nm, which is not as wide as the full width at half maximum (FWHM) of the ultraviolet LED of the comparative example, which is 9.4 nm.

[0084] In other words, if the sub-quantum well layers QS1 and QS2 contribute to the light emission to a certain extent, an emission peak should be observed due to emission at 261.0 nm from the sub-quantum well layers QS1 and QS2 and emission at 265.0 nm from the main quantum well layer QM, or an increase in the half-width of the EL spectrum should be observed.

[0085] However, the full width at half maximum FWHM does not increase but rather decreases in the EL spectrum of the UV LED 10 of the embodiment, and it is therefore presumed that the light emission from the sub-quantum well layers QS1 and QS2 is very small.

[0086] In addition, the half-width (8.5 nm) of the EL spectrum of the ultraviolet LED 10 of the embodiment is The reason why the half-width is smaller than that of the example (CMP) ultraviolet LED (9.4 nm) is thought to be because the rate of change of the ground state with respect to the fluctuation of the layer thickness is lower due to the thicker layer thickness TM of the main quantum well layer QM.

[0087] (c-3) Function of the sub-quantum well layer The following describes the function of the sub-quantum well layer QS provided in the active layer 13. Referring again to Fig. 10, when 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, a few nm), electrons confined in the sub-quantum well layer QS2 tunnel due to the tunnel effect and are injected into the main quantum well layer QM, which has a low ground level.

[0088] It is believed that electron tunneling has the effect of confining and injecting carriers into the thick main quantum well layer QM, and that electron tunneling can reduce electron overflow, thereby further increasing the probability of radiative recombination and increasing output power.

[0089] On the other hand, in the valence band, the hole concentration is low and the hole mobility is small, so most of the holes are captured in the third quantum well layer. Therefore, the sub-quantum well layer QS has the function of enhancing the confinement of carriers in the main quantum well layer QM.

[0090] The thickness TB3 of the third barrier layer 13B3 is preferably 10 nm or less in terms of electron tunneling, more preferably 7 nm or less, and even more preferably 6 nm or less.

[0091] (c-4) Thickness of the main quantum well layer FIG. 12 is a diagram showing a schematic diagram of wave functions of electrons and holes when the quantum well layer thickness Lz is small (left side of the diagram) and large (right side of the diagram).

[0092] Due to the difference in crystal composition between the quantum well layer and the barrier layer, lattice distortion occurs between the quantum well layer and the barrier layer, generating a piezoelectric field. As a result, the electrons and holes in the quantum well layer are misaligned with each other, and the overlap decreases, which reduces the probability of luminescence transition and the luminescence efficiency. Increasing the thickness Lz of the quantum well layer reduces the overlap of the wave functions, resulting in a decrease in the luminescence efficiency.

[0093] Furthermore, when an AlGaN layer is stacked on an AlN substrate, the difference in lattice constant causes it to be pulled by the underlayer, resulting in compressive strain. When the AlGaN layer is made thicker and reaches its critical thickness, lattice relaxation occurs, causing threading dislocations in the AlGaN layer, reducing the internal quantum efficiency and light emission efficiency.

[0094] In the above embodiment, the thickness TM of the main quantum well layer QM is twice the thickness TS of the sub-quantum well layers QS1 and QS2, but the present invention is not limited to this. Using band gap modeling software for semiconductor devices ("SiLENSe"), simulations were performed by varying the layer thicknesses of the sub-quantum well layers QS1, QS2 and the main quantum well layer QM based on the composition and layer thickness of the AlGaN stack. According to this simulation, it was found that the internal quantum efficiency increases with increasing quantum well layer thickness, and is effective when the thickness is 1.2 times or more the thickness of the sub-quantum well layer QS2. In order to fully effectively exert the above-mentioned electron tunneling effect from the sub-quantum well layer QS2 to the main quantum well layer QM and the electron confinement effect, it is even more preferable that the thickness TM of the main quantum well layer QM is 1.5 times or more the thickness TS of the sub-quantum well layer QS (sub-quantum well layer QS2) adjacent to the main quantum well layer QM.

[0095] On the other hand, the thickness TM of the main quantum well layer QM is preferably 16 nm or less, which is the critical thickness when an AlGaN layer is laminated on an AlN substrate, and more preferably 10 nm or less, which is the range in which the integral value is significant based on a simulation of the overlap integral of electrons and holes.

[0096] (c-5) Thickness of the final barrier layer 13A shows the relationship between the thickness TL of the final barrier layer 13L and the initial light output. Increasing the thickness TL of the final barrier layer 13L increases the light output. This is because the efficiency of hole injection decreases as the thickness of the final barrier layer 13L increases.

[0097] Therefore, the layer thickness TL of the final barrier layer 13L is preferably 30 nm or less, more preferably 27 nm or less, and even more preferably 21 nm or less.

[0098] FIG. 13B is a SIMS profile showing the concentration of Mg (p-type dopant) in the depth direction from the p-type cladding layer 15 to the primary quantum well layer QM.

[0099] According to the Mg concentration profile, the Mg concentration is 1×10 18 cm -3 That is all. Mg is 1×10 18 cm -3 When the primary quantum well layer QM contains a large number of nitrogen defects at a concentration equal to or greater than this, the device life is degraded. Therefore, it is preferable that the thickness TL of the final barrier layer 13L is 9 nm or more.

[0100] (c-6) Codoping of p-type cladding layer The p-type cladding layer (p-type Al Y2 Ga 1-Y2 The N layer 15 is co-doped with p-type impurities that act as acceptors and n-type impurities that act as donors.

[0101] Referring again to FIG. 8, it can be seen that the ultraviolet LED of the comparative example (CMP0, shown surrounded by a dashed line in the figure) in which the p-type cladding layer is not co-doped has a higher light output than the ultraviolet LED of the comparative example (CMP) in which the p-type cladding layer is co-doped, but the light output maintenance rate (%) is lower and reliability (element life) is deteriorated.

[0102] In this embodiment, Si co-doping and a thick final barrier layer 13L are used. By co-doping the p-type cladding layer 15 with Si, the amount of nitrogen defects generated in the p-type cladding layer 15 can be reduced. In addition, by using the thick final barrier layer 13L, the diffusion of nitrogen defects due to heat during current flow is suppressed, resulting in improved optical output and reliability (element life).

[0103] Although the case where the p-type cladding layer 15 is co-doped (FIG. 4) has been described, the electron blocking layer 14 may also be co-doped. (c-7) Emission wavelength and composition of AlGaN layer The AlGaN layer formed on the AlN substrate, i.e., the n-type cladding layer (n-type Al X Ga 1-X N layer) 12, active layer 13, electron blocking layer (Al Y1 Ga 1-Y1 N layer) 14 and p-type cladding layer (p-type Al Y1 Ga 1-Y1 The N layer 15 is made of AlGaN with an Al composition (X, Y1, Y2, etc.), which is the ratio of Al to group III elements, of 100% to 50%, and the ultraviolet LED 10 is preferably a light emitting element that emits light in the UVC region with a wavelength of 200 to 280 nm.

[0104] The Al composition is set to 50% or more in consideration of the occurrence of dislocations due to an increase in the amount of lattice strain in the AlN substrate. According to theoretical calculations using the energy balance model (People and Bean's formula), when the Al composition is 40% or less, the critical film thickness is 45 nm. However, since this is close to the total thickness of the active layer 15 of this embodiment (44.3 nm in the above example), it is preferable that the Al composition is 50% or more so that lattice relaxation does not occur inside the active layer 15 and a decrease in optical output does not occur. In addition, the absorption loss when deep ultraviolet light (for example, with a wavelength of 265 nm), which has the highest sterilization effect, is extracted from the AlN substrate side is taken into consideration.

[0105] As described above in detail, according to the present invention, it is possible to provide an ultraviolet semiconductor light-emitting element having high efficiency, high output characteristics, and high reliability. [Explanation of symbols]

[0106] 10: Ultraviolet semiconductor light emitting element 11: Substrate 12: n-type cladding layer (n-type AlGaN layer) 12A: First n-type cladding layer (n-type Al X1 Ga 1-X1 N layer) 12B: Second n-type cladding layer (Al X2 Ga 1-X2 N layer) 13: Quantum well active layer 13B1-13B3: Barrier layer 13L: Final barrier layer 14: Electron blocking layer (p-type Al Y1 Ga 1-Y1 N layer) 15: p-type cladding layer (p-type Al Y2 Ga 1-Y2 N layer) 16: p-type contact layer (p-type GaN layer) QM: Primary quantum well layer QS1, QS2: Sub-quantum well layers

Claims

1. a substrate made of single crystal AlN; n-type Al formed on the substrate Ｘ Ga １－Ｘ an n-type cladding layer, which is an N layer; a quantum well active layer formed on the n-type cladding layer and made of an AlGaN layer; Al formed on the quantum well active layer Ｙ１ Ga １－Ｙ１ an electron blocking layer, which is an N layer; p-type Al formed on the electron blocking layer Ｙ２ a p-type cladding layer that is a Ga1-Y2N layer; the quantum well active layer includes barrier layers having the same crystal composition, at least one sub-quantum well layer separated from the barrier layers by the barrier layers, and a main quantum well layer which is the quantum well layer closest to the electron blocking layer; the at least one sub-quantum well layer has the same crystal composition and layer thickness; the sub-quantum well layer and the main quantum well layer have the same crystal composition, and the main quantum well layer has a thickness that is 1.2 times or more the thickness of the sub-quantum well layer; The final barrier layer between the primary quantum well layer and the electron blocking layer has a thickness in the range of 9 to 27 nm. Ultraviolet semiconductor light-emitting element.

2. A substrate made of single crystal AlN; an n-type cladding layer formed on the substrate, the n-type cladding layer being an n-type Al x Ga 1-x N layer; a quantum well active layer formed on the n-type cladding layer and made of an AlGaN layer; an electron blocking layer formed on the quantum well active layer, the electron blocking layer being an Al Y1 Ga 1-Y1 N layer; a p-type cladding layer which is a p-type Al Y 2 Ga 1-Y 2 N layer formed on the electron blocking layer; the quantum well active layer includes barrier layers having the same crystal composition, at least one sub-quantum well layer separated from the barrier layers by the barrier layers, and a main quantum well layer which is the quantum well layer closest to the electron blocking layer; the at least one sub-quantum well layer has the same crystal composition and layer thickness; the sub-quantum well layer and the main quantum well layer have the same crystal composition, and the main quantum well layer has a thickness that is 1.2 times or more the thickness of the sub-quantum well layer; the p-type cladding layer is co-doped with p-type impurities that act as acceptors and n-type impurities that act as donors; The electron blocking layer is co-doped with p-type impurities that act as acceptors and n-type impurities that act as donors. Ultraviolet semiconductor light-emitting element.

3. 3. The ultraviolet semiconductor light emitting device according to claim 1, wherein the thickness of the main quantum well layer is 16 nm or less.

4. 3. The ultraviolet semiconductor light emitting device according to claim 1, 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.

5. 3. The ultraviolet semiconductor light emitting device according to claim 1, wherein the emission wavelength of said quantum well active layer is in the range of 200 to 280 nm.

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