CN115377267A

CN115377267A - Ultraviolet semiconductor light emitting element

Info

Publication number: CN115377267A
Application number: CN202210534737.6A
Authority: CN
Inventors: 木下亨; 茂木正彦
Original assignee: Stanley Electric Co Ltd
Current assignee: Stanley Electric Co Ltd
Priority date: 2021-05-21
Filing date: 2022-05-17
Publication date: 2022-11-22
Also published as: JP2022179037A

Abstract

The invention provides an ultraviolet semiconductor light emitting element capable of improving external quantum efficiency, high efficiency and high output. An ultraviolet semiconductor light emitting element comprising: a substrate composed of single-crystal AlN; and a semiconductor structure layer formed by sequentially epitaxially growing an n-type AlGaN layer, an active layer, and Mg-doped p-type Al on a substrate _Y1 Ga _1‑ _Y1 N layer and Mg-doped p-type Al _Y2 Ga _1‑Y2 An N layer in which 0.5. Ltoreq. Y1. Ltoreq.1.0, 0.5. Ltoreq. Y2. Ltoreq.1.0, Y2. Ltoreq. Y1, the active layer having a light emission peak wavelength in the range of 210nm to 300nm, exhibiting a main peak and a secondary peak derived from Mg in a light emission spectrum by applying a current to the active layer, and having a driving current density of 20mA/mm ² Time of dayThe peak intensity of the peak relative to the intensity of the main peak is 3% to 15%.

Description

Ultraviolet semiconductor light emitting element

Technical Field

The present invention relates to an ultraviolet semiconductor light emitting element, and more particularly to a nitride semiconductor light emitting element that emits ultraviolet light.

Background

In recent years, semiconductor light emitting elements that emit light in the deep ultraviolet region have been attracting attention as light sources having an inactivating effect on bacteria and viruses and a sterilizing effect.

For example, patent document 1 discloses a nitride semiconductor light-emitting element in which the external quantum efficiency is improved by controlling the dopant concentration at the interface between the light-emitting layer and another semiconductor layer.

[ Prior art documents ]

[ patent document ]

[ patent document 1] Japanese patent application laid-open No. 2016-098632

Disclosure of Invention

[ problems to be solved by the invention ]

In conventional ultraviolet semiconductor light-emitting devices, the layer structure, composition, impurity concentration, layer thickness, and the like of the semiconductor layer have been studied for each layer, but it has been difficult to realize a device having sufficiently high efficiency and high output.

In particular, as described in patent document 1, it is suggested that when the main peak emission intensity decreases with an increase in the secondary peak emission intensity in the emission wavelength spectrum, the light emission efficiency decreases due to an increase in the non-radiative transition.

The inventors of the present application have obtained the following findings: it is difficult to sufficiently improve the light emission efficiency by simply reducing the emission intensity of the secondary peak in the emission wavelength spectrum. The present invention has been made based on the above-described findings, and an object thereof is to provide an ultraviolet semiconductor light-emitting element which can improve external quantum efficiency, has high luminous efficiency, and has high output.

[ means for solving problems ]

An ultraviolet semiconductor light-emitting element according to an embodiment of the present invention includes:

a substrate composed of single-crystal AlN; and

a semiconductor structure layer formed by sequentially epitaxially growing an n-type AlGaN layer, an active layer and Mg-doped p-type Al on the substrate _Y1 Ga _1-Y1 N layer and Mg-doped p-type Al _Y2 Ga _1-Y2 N layer, wherein Y1 is more than or equal to 0.5 and less than or equal to 1.0, Y2 is more than or equal to 0.5 and less than or equal to 1.0, Y1 is more than or equal to Y2,

the light emission peak wavelength of the active layer is in the range of 210nm to 300nm,

a main peak and a secondary peak derived from Mg in a luminescence spectrum by applying a current to the active layer, and a driving current density of 20mA/mm ² The intensity (S) of the secondary peak at that time is 3% to 15% of the peak intensity (S/M) of the intensity (M) of the primary peak.

In addition, an ultraviolet semiconductor light emitting element according to another embodiment of the present invention includes:

a substrate composed of single-crystal AlN; and

a semiconductor structure layer epitaxially grown on the substrate sequentially with an n-type AlGaN layer, an active layer, and a Mg-doped p-type Al _Y1 Ga _1-Y1 N layer and Mg-doped p-type Al _Y2 Ga _1-Y2 N layer, wherein Y1 is more than or equal to 0.5 and less than or equal to 1.0, Y2 is more than or equal to Y1,

the p-type Al _Y1 Ga _1-Y1 Mg concentration of the N layer is 3.0X 10 ¹⁹ cm ^-3 ～5.0×10 ¹⁹ cm ^-3 In the range of (1), the p-type Al _Y2 Ga _1-Y2 Mg concentration of the N layer is 2.0X 10 ¹⁹ cm ^-3 ～1.0×10 ²⁰ cm ^-3 In the range of (a) to (b),

the p-type Al _Y1 Ga _1-Y1 The thickness of the N layer is in the range of 4 nm-10 nm.

Drawings

Fig. 1 is a cross-sectional view schematically showing the structure of an ultraviolet semiconductor light-emitting element 10 according to an embodiment of the present invention.

Fig. 2 is a diagram schematically showing a Band diagram (Band diagram) of the ultraviolet LED 10.

Fig. 3A is a table collectively showing characteristics of ultraviolet LEDs obtained from wafers having different Mg concentrations in the spacer layer 13S and the p-type AlGaN layer 15.

Fig. 3B is a graph plotting the light emission efficiency (EQE: external quantum efficiency) of the ultraviolet LED obtained from each wafer and the concentration of the spacer layer 13S.

FIG. 4 is a diagram showing an example of emission spectra of the ultraviolet LED element 10 (example: EMB) according to the embodiment and the ultraviolet LED of the comparative example (CMP).

Fig. 5 is a graph plotting the light emission efficiency (EQE: external quantum efficiency) and the S/M ratio (%) of the ultraviolet LED10 obtained from the wafer (EX 1) of example 1 and the wafer (EX 2) of example 2.

Fig. 6 is a diagram showing an example of SIMS imaging (profile) of the LED elements of the Example (EMB) and the comparative example (CMP).

Fig. 7 is a conceptual diagram for explaining the relationship between the S/M ratio and the light emission efficiency or the carrier injection amount in examples (EMB 1, EMB 2) and comparative example (CMP).

Description of the reference symbols

10: ultraviolet semiconductor light-emitting element, 11: substrate, 12: n-type AlGaN layer, 12A: first n type Al _X1 Ga _1-X1 N layer, 12B: second n type Al _X2 Ga _1-X2 N layer, 13: active layer, 13Q: quantum well structure layer, 13S: spacer layer, 14: p type Al _Y1 Ga _1-Y1 N layer, 15: p type Al _Y2 Ga _1-Y2 N layer, 16: and a p-type GaN layer.

Detailed Description

Preferred embodiments of the present invention will be described below, but they may be appropriately changed or combined. In the following description and the drawings, the same or equivalent parts are denoted by the same reference numerals.

[ Structure of ultraviolet semiconductor light-emitting element ]

Fig. 1 is a sectional view schematically showing the 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 LED element 10), and is manufactured by, for example, a Metal Organic Chemical Vapor Deposition (MOCVD) method.

The LED element 10 is formed by sequentially stacking an n-type AlGaN layer 12, an active layer 13, a p-type AlGaN layer 14, a p-type AlGaN layer 15, and a p-type GaN layer 16 on a substrate 11 by epitaxial growth.

Fig. 2 schematically shows an energy band diagram of the LED element 10. This is explained in more detail with reference to fig. 1 and 2.

The substrate 11 has a dislocation density of 10 ⁸ cm ^-2 The following single crystal AlNThe substrate is hereinafter referred to as an AlN substrate 11. It is known that, in the AlGaN semiconductor material constituting the ultraviolet light-emitting element of the present invention, the dislocation density exceeds 10, for example, as described in OPTICS EXPRESS Vol.25No.16A639 (2017) ⁷ cm ^-2 ～10 ⁸ cm ^-2 The light emission efficiency is drastically reduced. Therefore, the lower the dislocation density of the single-crystal AlN substrate is, the more preferable is 10 ⁶ cm ^-2 Hereinafter, it is more preferably 10 ⁴ cm ^-2 The following. By using such an AlN substrate 11 having a low dislocation density, the dislocation density in an active layer 13 described later can be set to 10 without lowering the light emission efficiency ⁷ cm ^-2 The following.

The growth surface (surface) of the AlN substrate 11 of the present invention is not particularly limited, and may be a C-plane, M-plane, or other growth surface, but the growth surface of an AlGaN-based material is preferably a commonly used C-plane. Further, when the C-plane is used as the crystal growth plane, an OFF substrate slightly inclined from the C-plane is preferable for the purpose of improving the smoothness of the AlGaN layer grown on the AlN substrate 11. The angle of inclination from the C-plane is not particularly limited, and may be appropriately determined so as to obtain a smooth AlGaN layer, and is usually selected within a range of 0.1 ° to 1.0 °. The direction inclined from the C-plane is not particularly limited, and the a-axis direction, the M-axis direction, and the like may be appropriately selected.

Further, when the surface roughness of the AlN substrate 11 is large, the large roughness causes abnormal growth of the AlGaN layer grown on the substrate, and therefore the surface Roughness (RMS) is preferably 1.0nm or less, and more preferably 0.5nm or less. In order to obtain such a smooth surface or to remove a damaged layer formed on the substrate surface in the process of manufacturing the substrate, it is preferable to perform Chemical Mechanical Polishing (CMP) on the substrate surface.

Further, if the substrate has a large absorption coefficient for ultraviolet light emitted from the active layer, the total amount of ultraviolet light extracted to the outside is reduced, which may result in a decrease in light emission efficiency. Therefore, the absorption coefficients of the AlN substrate and the AlN layer of the AlN template are preferably 20cm ^-1 It is more preferably 10cm or less ^-1 The following. By setting to 10 ^-1 cm or less, for example, even if the AlN substrate 11 has a thickness of 100 μm, a linear transmittance of 90% or more can be secured.

The n-type AlGaN layer 12 is an n-type conductive layer doped with Si (silicon). In the ultraviolet semiconductor light emitting element, ultraviolet light emitted from the light emitting layer is generally transmitted through the n-type AlGaN layer 12 and the substrate 11 and emitted to the outside. Since the band gap of the n-type AlGaN layer becomes larger as the Al composition of the n-type AlGaN layer becomes larger, and ultraviolet light having a shorter wavelength can be transmitted in accordance with this, the Al composition of the n-type AlGaN layer may be appropriately determined so that sufficient transmittance can be obtained for the emission wavelength of desired ultraviolet light. The n-type AlGaN layer may be formed of a plurality of layers having different Al compositions, or may be a composition gradient layer having an Al composition that is inclined in the stacking direction. For example, the first n-type Al _X1 Ga _1-X1 N layer 12A and second N-type Al _X2 Ga _1-X2 A laminated structure of N layers 12B. First n type Al _X1 Ga _1-X1 The N layer 12A is, for example, a compositionally-graded layer in which the Al composition X1 decreases from 1.0 to 0.75 in the stacking direction (growth direction), a second N-type Al _X2 Ga _1-X2 The N layer 12B is, for example, a composition gradient layer in which the Al composition X2 is reduced from 0.75 to 0.70. Further, the first n-type Al is preferable _X1 Ga _1-X1 N layer 12A and second N-type Al _X2 Ga _1-X2 The Al composition at the interface of the N layer 12B is equal.

The thickness of the n-type AlGaN layer is not particularly limited and may be determined as appropriate, but if the thickness of the n-type AlGaN layer becomes too thick, the AlN substrate 11 and the n-type AlGaN layer 12 are lattice-relaxed and dislocations are likely to occur, and therefore the total thickness of the n-type AlGaN layer 12 is preferably set to a range of 0.5 μm to 2.0 μm. For example, the n-type AlGaN layer 12 is made of the first n-type Al described above _X1 Ga _1-X1 N layer 12A and second N-type Al _X2 Ga _1-X2 In the case of the laminated structure of the N layer 12B, the first N-type Al is used _X1 Ga _1-X1 The N layer 12A has a layer thickness of 200nm and is a second N-type Al _X2 Ga _1-X2 The N layer 12B may have a laminated structure having a layer thickness of 1000 nm. Of course, the film thicknesses of the first and second n-type AlGaN layers are not limited to the illustrated numbers, and are appropriately determinedThe total film thickness may be set to 2.0 μm or less.

Further, the doped Si concentration may be appropriately determined so as to obtain desired n-type conductivity, but is preferably 1 × 10 from the viewpoint of reducing the resistance value of the n-type AlGaN layer ¹⁸ cm ^-3 ～1×10 ²⁰ cm ^-3 More preferably 5X 10 ¹⁸ cm ^-3 ～5×10 ¹⁹ cm ^-3 . The Si doping concentration may be constant in the film thickness direction in the n-type AlGaN layer, or may be modulated doping having a different Si concentration in the film thickness direction. The Si concentration and the Mg concentration described later can be measured by a known Secondary Ion Mass Spectrometry (SIMS) analysis. In the present application, the use of AlN and Al as the Si concentration and the Mg concentration for the AlN layer, the AlGaN layer and the GaN layer, respectively _0.65 Ga _0.35 N and quantitative values of GaN standard samples.

The active layer (ACT) 13 has a quantum well structure composed of a barrier layer 13B and a well layer 13W, wherein the barrier layer 13B is made of Al _A1 Ga _1-A1 N layer, the well layer 13W is made of Al _A2 Ga _1-A2 And N layers. The active layer 13 has a spacer layer 13S described later. The light emission peak wavelength of the active layer 13 is in the range of 210 to 300 nm. Since the wavelength of light emitted from the active layer 13 is determined by the Al composition and the film thickness of the well layer, the Al composition and the film thickness can be appropriately determined so as to obtain a desired emission wavelength within the above wavelength range.

For example, the thickness of the well layer is set in the range of 2nm to 10nm, and the Al composition can be determined so as to obtain a desired light emission wavelength. The Al composition and the film thickness of the barrier layer are not particularly limited, and for example, the Al composition may be set in the range of A2 < A1. Ltoreq.1.0, and the film thickness may be set in the range of 2nm to 15 nm. The well layer and the barrier layer may be n-type layers doped with Si. Both the well layer and the barrier layer may be Si doped layers, or may be a structure in which only the well layer or only the barrier layer is doped with Si. The concentration of doped Si is not particularly limited, but is preferably 1X 10 ¹⁷ cm ^-3 ～5×10 ¹⁸ cm ^-3 The range of (1). In addition, the number of layers of quantum well is not peculiarThe Quantum Well structure may be a Multi Quantum Well (MQW) structure in which a plurality of Well layers are formed, or may be a Single Quantum Well (SQW). The number of well layers is preferably appropriately determined within a range of 1 to 5.

As shown in fig. 2, in the active layer 13, a quantum well layer 13W (i.e., p-type Al closest to the p-type Al described later) from the end of the n-type AlGaN layer 12 to the uppermost quantum well layer 13W is formed _Y1 Ga _1-Y1 Quantum well layer 13W of N layer 14) as quantum well structure layer 13Q, spacer layer 13S is provided on quantum well layer 13W of the uppermost layer and p-type Al _Y1 Ga _1-Y1 Between the N layers 14.

The Al composition of the spacer layer 13S may be optimally selected according to the light emission wavelength or in the process of improving the light emission efficiency as follows. The barrier layer may have the same composition as the barrier layer 13B of the quantum well structure, or may have the barrier layer 13B and p-type Al _Y1 Ga _1-Y1 Intermediate composition of the N layer 14. Alternatively, the spacer layer 13S may be composed of a layer having the same composition as the barrier layer 13B and a layer having the intermediate composition formed as an upper layer thereof. Further, the spacer layer 13S may be a layer of a gradient composition or a layer of a step composition whose composition changes in a stepwise manner.

Specifically, the embodiment of the present invention is exemplified by the case where Al doped with Si is used _0.6 Ga _0.4 N layer as barrier layer (barrier layer) 13B, undoped Al _0.5 Ga _0.5 In a 3-layer Multiple Quantum Well (MQW) structure in which the N layer is a Quantum Well layer (Well layer) 13W, the spacer layer 13S has the same composition as the barrier layer 13B, but in order to improve the injection efficiency of carriers into the active layer 13W, the Al composition (Y) of the spacer layer 13S is preferably Y =0.55 to 0.65, and more preferably Y =0.57 to 0.62.

Although the thickness of the spacer layer 13S is not particularly limited, when the thickness is increased, the physical distance between the p-type layer and the well layer 13W increases, and thus the injection efficiency of carriers into the active layer 13W tends to decrease. Therefore, the thickness of the spacer layer 13S is preferably in the range of 2nm to 15nm, and more preferably in the range of 4nm to 10 nm.

The spacer layer 13S is the same as the barrier layer described aboveAlso, a structure doped with Si is possible. The doped Si concentration is not particularly limited, but is preferably 1X 10 ¹⁷ cm ^-3 ～5×10 ¹⁸ cm ^-3 The range of (1).

Further, mg is contained in the spacer layer 13S. The Mg contained in the spacer layer 13S may be p-type Al from the description below _Y1 Ga _1-Y1 Diffusion doping by diffusion of the N layer 14 layer, or may be intentionally doped with Mg. The present inventors found that the light emission efficiency tends to increase as the concentration of Mg contained in the spacer layer 13 increases. This effect is considered to be because the spacer layer 13S contains Mg as a p-type dopant, and the distance between the p-type layer and the active layer is substantially close, and as a result, the carrier injection efficiency is improved. Specifically, the concentration of Mg contained in the spacer layer 13S is preferably 5.0X 10 ¹⁷ cm ^-3 ～5.0×10 ¹⁸ cm ^-3 More preferably 7.0X 10 ¹⁷ cm ^-3 ～3.0×10 ¹⁸ cm ^-3 Most preferably 1.0X 10 ¹⁸ cm ^-3 ～2.5×10 ¹⁸ cm ^-3 。

In SIMS analysis performed to determine the Mg concentration, a sample is irradiated with primary ions (oxygen) to determine the amount of secondary ions (Mg) ejected from the material, and quantitative evaluation is performed. Generally, in the SIMS analysis, since impurities are pushed into a sample by primary ions and uniform primary ion irradiation is difficult due to the surface roughness of the sample, the impurity concentration tends to be smeared in the depth direction, and since the etching rate differs depending on the material of the sample (in the example of the present application, the Al composition of the AlGaN layer), it is difficult to obtain accuracy in the depth direction of several nanometers. Therefore, the Mg concentration of the spacer layer is controlled by p-type Al _Y1 Ga _1-Y1 The peak position of the Mg concentration of the N layer 14 is defined as the Mg concentration at a position shifted by 20nm toward the active layer 13 side (evaluation position).

P-type Al on the active layer 13 _Y1 Ga _1-Y1 The N layer 14 serves to suppress electrons injected into the active layer 13 from being transferred to p-type Al described later _Y2 Ga _1-Y2 Electron blocking layer (EBL:electron Blocking Layer) functions. Thus, al _Y1 Ga _1-Y1 The N layer 14 has a lower conductivity than the active layer 13 and p-type Al _Y2 Ga _1-Y2 The large band gap of the N layer 15 determines Al within the range of Y1 more than 0.8 and less than or equal to 1.0 _Y1 Ga _1-Y1 Al of the N layer 14 constitutes Y1. The Al composition of the AlGaN layer epitaxially grown on the substrate 11 increases as the emission wavelength becomes shorter, and when the emission wavelength is shorter than 270nm, the Al composition Y1 is preferably 0.9. Ltoreq. Y1. Ltoreq.1.0 in order to sufficiently exhibit the function as an electron blocking layer. In addition, in the present example, alN (Y1 = 1) was used as Al _Y1 Ga _1-Y1 N layers 14.

In addition, al _Y1 Ga _1-Y1 The N layer 14 may be an undoped layer or may be doped with a p-type dopant so long as it can function as an electron blocking layer. As Al _Y1 Ga _1-Y1 As the p-type dopant material in the N layer 14, mg (magnesium), zn (zinc), be (beryllium), C (carbon), or the like can Be used. In particular, mg that is commonly used is preferably used as the p-type dopant material of the AlGaN layer, and Mg is also used in the embodiment of the present invention described later. May be in Al _Y1 Ga _1-Y1 The N layer 14 is uniformly doped with the p-type dopant material in the stacking direction, and the concentration of the dopant material may be changed in the stacking direction. For example, a stacked structure may be formed from the undoped AlN layer 14A (Y1 = 1) and the p-type AlN layer 14B doped with Mg (magnesium) from the side in contact with the active layer. From the viewpoint of obtaining a function as an electron blocking layer and improving the injection efficiency of carriers into a light-emitting layer, al _Y1 Ga _1-Y1 The p-type dopant concentration in the N layer 14 is preferably 1.0 × 10 ¹⁹ cm ^-3 ～8.0×10 ¹⁹ cm ^-3 More preferably 3.0X 10 ¹⁹ cm ^-3 ～5.0×10 ¹⁹ cm ^-3 Particularly preferably 3.0X 10 ¹⁹ cm ^-3 ～4.0×10 ¹⁹ cm ^-3 。

In addition, p-type Al _Y1 Ga _1-Y1 The N layer 14 preferably has a layer thickness in the range of 4nm to 10 nm. This is because if the wavelength is less than 4nm, the effect as an electron blocking layer due to the tunneling effect is small, and if the wavelength is 10nm or more, hole (hole) injection occursA decrease in entrance efficiency.

p type Al _Y2 Ga _1-Y2 The N layer 15 is formed on the p-type Al _Y1 Ga _1-Y1 The N layer 14 functions as a p-type clad layer doped with Mg. The above-mentioned materials can be used without limitation for the p-type dopant material, but are preferably used together with Al _Y1 Ga _1-Y1 Mg is similarly used for the N layer 14. In the ultraviolet light emitting element of the present invention, p-type Al _Y2 Ga _1-Y2 The Mg concentration in the N layer 15 is preferably 2.0X 10 ¹⁹ cm ^-3 ～1.0×10 ²⁰ cm ^-3 More preferably 2.0X 10 ¹⁹ cm ^-3 ～5.0×10 ¹⁹ cm ^-3 . By adding p-type Al _Y2 Ga _1-Y2 The Mg concentration in the N layer 15 is in the above range, and S/M is 3% to 15%, whereby high light emission efficiency can be obtained.

As p-type Al _Y2 Ga _1-Y2 When the Al composition Y2 of the N layer 15 has a structure in which Y2 has a constant value in the stacking direction, it is preferable that the Al composition of the barrier layer of the active layer be higher than the Al composition of the barrier layer of the active layer and be Al _Y1 Ga _1-Y1 The Al composition of the N layer 14 is not more than Y1. By adding p-type Al _Y2 Ga _1-Y2 The Al composition Y2 of the N layer 15 is set to the above range, and even when the amount of injection current of the ultraviolet light emitting element is large, a preferable effect of suppressing carrier overflow can be obtained. To obtain better effect, al composition of barrier layer of active layer and p-type Al _Y2 Ga _1-Y2 The difference in Al composition Y2 of the N layer 15 is preferably 0.5 to 1.0. In addition, p-type Al is preferable _Y2 Ga _1-Y2 The Al composition Y2 of the N layer 15 is larger than that of the N-type AlGaN layer, and therefore, the effect of suppressing the overflow of carriers to the p-type layer is improved, and the light emission efficiency of the ultraviolet light emitting element can be improved.

In addition, p-type Al _Y2 Ga _1-Y2 The N layer 15 may be a composition gradient layer in which the Al composition Y2 changes in the stacking direction. The following structure is particularly preferred: from Al to _Y1 Ga _1-Y1 From the side where the N layers 14 meet, the Al composition Y2 becomes smaller in the stacking direction. Thereby, in p-type Al _Y2 Ga _1-Y2 Since the N layer 15 has a polarization doping effect, a higher hole concentration is easily obtained, and as a result, holes are injected into the active layerThe entrance efficiency becomes high. For example, when the emission wavelength is 270nm or less, it is preferable to use Al _Y1 Ga _1-Y1 The Al composition of the side where the N layers 14 are in contact is 0.95 to 1.0, and p-type Al of the opposite side is preferable _Y2 Ga _1-Y2 The Al composition in the surface layer of the N layer 15 is 0.60 to 0.85. By adopting such a structure, the polarization doping effect described above can be improved, and since transparency can be maintained for the emission wavelength, high emission efficiency can be easily obtained.

In addition, p-type Al _Y2 Ga _1-Y2 The thickness of the N layer 15 is not particularly limited, and may be appropriately determined within a range of 10nm to 150 nm. If p-type Al _Y2 Ga _1-Y2 When the thickness of the N layer 15 is less than 10nm, the above-described effect of suppressing carrier overflow is hardly obtained, while when the thickness is increased to more than 150nm, p-type Al is present _Y2 Ga _1-Y2 The resistance value of the N layer 15 becomes large, resulting in an increase in the operating voltage of the ultraviolet light emitting element. From such a viewpoint, p-type Al _Y2 Ga _1-Y2 The thickness of the N layer 15 is preferably 40nm to 120nm, particularly preferably 50nm to 100nm. In an embodiment of the present invention, Y2 is composed of p-type Al with Al _Y1 Ga _1-Y1 The Al composition of the N layer 14 starts as a composition-inclined layer that decreases in the growth direction (Al composition Y2 decreases from 1.0 to 0.8). In addition, p-type Al _Y2 Ga _1-Y2 The thickness of the N layer 15 was 60nm.

In p-type Al _Y2 Ga _1-Y2 On the N layer 15, a p-type GaN layer 16 doped with a p-type dopant may be formed in order to reduce contact resistance with the electrode. As the p-type dopant material, the known p-type dopant material described above can be used, but Mg is preferably used for the same reason. The Mg doping concentration in the p-type GaN layer 16 is not particularly limited, but is preferably 1 × 10 in order to reduce the resistance value and the contact resistance in the p-type GaN layer ¹⁸ cm ^-3 ～2×10 ²⁰ cm ^-3 . The thickness of the p-type GaN layer 16 is not particularly limited, and may be appropriately determined within the range of 5nm to 500 nm.

In addition, all layers of AlGaN layers 12, 13, 14, and 15 except for p-type GaN layer 16 are crystal-grown in a state of lattice-matching with AlN substrate 11, and thus have a lattice matching with alN substrate 11 has an equivalent low dislocation density. Specifically, has a value of 10 ⁵ cm ^-2 The following dislocation density.

The LED element 10 is described as a Light Emitting Diode (LED), but may be configured as a semiconductor Laser element (LD). Next, a method for manufacturing the ultraviolet LED having the above-described structure will be described. The ultraviolet LED10 of the present invention can be manufactured by a known crystal growth method such as a Metal Organic Chemical Vapor Deposition (MOCVD) method or a Molecular Beam Epitaxy (MBE) method. Among them, the MOCVD method which is industrially widely used with high productivity is preferable. The group III (Al, ga) source gas and the group V (N) source gas used in the present invention may be any known source gases without any particular limitation.

For example, as the group III source gas, a gas such as trimethylaluminum, triethylaluminum, trimethylgallium, or triethylgallium may be used. Further, ammonia is generally used as the group V source gas.

In addition, as the dopant source gases of Mg and Si, known materials may be used without limitation, and for example, biscyclopentadienylmagnesium, monosilane, tetraethylsilane, and the like may be used.

The element layer of the ultraviolet LED10 is grown by supplying the above-described source gas onto the substrate 11 together with a carrier gas such as hydrogen and/or nitrogen.

The ratio of the amounts of the group III source gas and the group V source gas to be supplied (V/III ratio) may be appropriately determined so that desired characteristics can be obtained, and is preferably set within the range of 500 to 10000.

The growth temperature of the element layers constituting the ultraviolet LED10 is not particularly limited, and may be appropriately determined so as to obtain desired characteristics of each layer and characteristics of the ultraviolet LED10, and the element layers are preferably grown at 1000 to 1200 ℃.

[ examples ]

Hereinafter, the present invention will be specifically described with reference to examples in which ultraviolet LEDs having an emission wavelength of 265nm are produced, but the present invention is not limited to the examples.

[ production of LED element ]

Growing ultraviolet LED elementsAs a substrate of the layer, an AlN single crystal substrate manufactured by the method described in Applied Physics Express 5 (2012) 122101 was used. Specifically, the present invention relates to a laminated substrate including a C-plane AlN seed substrate produced by a Physical Vapor Transport (PVT) method and an AlN thick film grown by a hydride vapor phase growth (HVPE) method. The AlN substrate had a dislocation density of 10 ⁵ cm ^-2 Below, 5X 5 μm ² Surface Roughness (RMS) in the range of 0.1nm.

On the AlN substrate, an AlN layer (100 nm), a first n-type AlGaN layer (200 nm), and a second n-type AlGaN layer (1000 nm) were grown by an MOCVD apparatus. The first n-type AlGaN layer and the second n-type AlGaN layer are compositionally-tilted layers, and the first n-type AlGaN layer has an Al composition decreasing from 1.0 to 0.75 from the side in contact with the AlN layer, and the second n-type AlGaN layer has an Al composition decreasing from 0.75 to 0.70 from the side in contact with the first n-type AlGaN layer. In addition, the Si concentration in the n-type AlGaN layer was controlled to 1X 10 ¹⁹ cm ^-3 。

Next, a 3-weight quantum well layer composed of a barrier layer and a well layer was grown, wherein the barrier layer was made of n-type Al _0.6 Ga _0.4 N (7 nm), the well layer is made of Al _0.5 Ga _0.5 N (4 nm). Si concentration of the barrier layer is controlled to 1 x 10 ¹⁸ cm ^-3 。

Next, the electron blocking layer 14 made of AlN (10 nm) was grown. The electron blocking layer on the side contacting the barrier layer was an undoped layer 14A (2 nm), and the remaining electron blocking layer 14B was doped with 4. + -. 1X 10 ¹⁹ cm ^-3 And (3) Mg of (1).

Then, a p-type AlGaN layer 15 (60 nm) is grown. The p-type AlGaN layer 15 is a composition gradient layer in which the Al composition decreases from 1.0 to 0.8 from the side in contact with the electron blocking layer. The Mg concentration of the p-type AlGaN layer 15 was 2X 1.0X 10 ¹⁹ cm ^-3 ～1.0×10 ²⁰ cm ^-3 A plurality of wafers varied within a range of (1).

Next, the p-type GaN layer 16 (270 nm) was grown to complete the growth of the deep ultraviolet LED element layer. Mg concentration in the p-type GaN layer was 5X 10 ¹⁹ cm ^-3 。

Next, after etching was performed by ICP dry etching until the second n-type AlGaN layer was exposed, an n-type electrode made of Ti/Au was formed, and heat treatment was performed at 900 ℃. Subsequently, a p-electrode made of Ni/Au was formed on the p-type GaN layer, and heat treatment was performed at 500 ℃ in an oxygen atmosphere.

Subsequently, the back surface (the side opposite to the ultraviolet LED element layer) of the AlN substrate was mechanically polished until the HVPE AlN thick resist was exposed, thereby completing the ultraviolet LED. Subsequently, the substrate was cut into a chip shape of 0.75mm × 0.95mm by dicing, and flip-chip bonded to a ceramic submount to complete an ultraviolet LED element, and spectrum and characteristic evaluation was performed.

Fig. 3A is a table showing the Mg concentrations of the spacer layer 13S and the p-type AlGaN layer 15 of the fabricated wafer, and the element characteristics of the LED obtained from these wafers. In addition, in the following, exponentiation may be, for example, 1.0 × 10 ¹⁹ Expressed as 1.0e19.

In the upper and lower stages of each column in the figure, element characteristics according to the difference in the growth temperature of the wafer are shown separately. Specifically, the upper stage shows the element characteristics when the growth temperatures of the p-type AlN layer 14 and the p-type AlGaN layer 15 are 1115 ℃ and 1090 ℃, respectively (growth temperature conditions: GT 1), and the lower stage shows the element characteristics when the temperatures are slightly lower (20 ℃ lower) than the growth temperature conditions GT1 and are 1095 ℃ and 1070 ℃, respectively (growth temperature conditions: GT 2).

In each column, the maximum EQE (external quantum efficiency) among the elements obtained from each wafer and the S/M ratio of the element in which the maximum EQE is obtained are shown in parentheses.

Fig. 3B is a graph in which the maximum EQE (external quantum efficiency) of the element obtained from each wafer shown in the upper stage of fig. 3A is plotted with the concentration of the spacer layer 13S as the horizontal axis.

[ element evaluation (luminescence spectrum) ]

Fig. 4 is a diagram showing an example of emission spectra of the ultraviolet LED element 10 (example: EMB) according to the embodiment of the present invention and the ultraviolet LED of the comparative example (CMP). Further, the drive current density J was 20mA/mm ² The luminescence spectrum of (c).

In the emission spectrum of Example (EMB), emission peaks from the active layer 13, i.e., a MAIN peak MAIN having an intensity M and a SUB peak SUB having an intensity S, were observed. Hereinafter, for convenience of explanation and understanding, the intensity and the peak value are also expressed as a main peak value M or a sub-peak value S using the same reference numerals.

The secondary peak S is observed at a long wavelength side of about 30nm from the primary peak M. On the other hand, no clear secondary peak S was observed in the emission spectrum of comparative example (CMP).

The secondary peak S observed in the emission spectrum of Example (EMB) can be interpreted as light emission through the impurity level of p-type, that is, light emission derived from Mg, depending on the wavelength thereof. Further, as the current density increases, light emission in the active layer becomes dominant, and light emission from Mg decreases, so that the ratio of the light emission intensity S of the sub-peak to the light emission intensity M of the main peak (hereinafter referred to as the S/M ratio) decreases.

Therefore, from the viewpoint of detection sensitivity, the S/M ratio at a low current density was used as a criterion for evaluation. Specifically, the drive current density J was set to 20mA/mm ² The S/M ratio (%) at that time was used as a reference for evaluation.

[ element evaluation (light-emitting characteristics and S/M ratio) ]

FIG. 5 is a graph plotting Mg concentration of 2.0E18cm from the spacer layer 13S ^-3 And the Mg concentration of the p-type AlGaN layer 15 is 4.0E19cm ^-3 The Mg concentration of the wafer (example 1 ^-3 And the Mg concentration of the p-type AlGaN layer 15 is 1.0E20cm ^-3 The wafer (example 2.

In both the wafers (EX 1, EX 2) of example 1 and example 2, the luminous efficiency increased with the increase in the S/M ratio.

In the case of example 1 (EX 1), the S/M ratio was 4.0%, and the emission efficiency (EQE) showed a maximum of 3.4%, after which the emission efficiency slowly decreased as the S/M ratio increased.

In the case of example 2 (EX 2), the S/M ratio was 10.9%, and the emission efficiency (EQE) exhibited a maximum value of 2.8%, after which the emission efficiency decreased as the S/M ratio increased.

Referring to the results of examples 1 and 2 (EX 1 and EX 2), it is understood that when the S/M ratio is 3% to 15%, higher luminous efficiency is obtained than when the sub-peak is not found. Further, referring to the results of example 1 (EX 1), it is understood that when the S/M ratio is 3% to 6%, extremely high light emission efficiency is obtained.

Referring again to fig. 3A and 3B, the maximum value EQE of the emission efficiency (EQE) is shown for the LED element 10 of each wafer in which the Mg concentration of the above-described spacer layer 13S and p-type AlGaN layer 15 is different _max (%) and S/M ratio (%).

From these results, it is understood that if the Mg concentration of the spacer layer 13S is 5.0E17 cm ^-3 ～5.0E18cm ^-3 In the range of (2), the Mg concentration of the p-type AlGaN layer 15 is 2.0E19cm ^-3 ～1.0E20cm ^-3 In this range, high emission efficiency (EQE) can be obtained.

It is particularly preferable that the Mg concentration of the spacer layer 13S is 7.0E17cm ^-3 ～3.0E18cm ^-3 In the range of (2), the Mg concentration of the p-type AlGaN layer 15 is 2.0E19cm ^-3 ～5.0E19cm ^-3 Within the range of (1).

[ relationship between S/M ratio and light-emitting characteristics ]

Regarding the emission intensity, for example, patent document 1 describes that an increase in non-radiative transition in the active layer is caused by an increase in the secondary peak intensity, and that the main peak emission intensity is reduced with an increase in the secondary peak intensity, as a result of which the external quantum efficiency is reduced.

In addition, it is shown that the sub-peak intensity decreases as the Mg concentration of the spacer layer, which is a layer between the quantum well layer and the EBL layer (AlN layer), decreases, and it is described that the preferable range of the Mg concentration in the spacer layer is 1E17cm ^-3 The following.

The light emission efficiency (internal quantum efficiency) of the LED semiconductor layer is determined by the following formula. The EQE (external quantum efficiency) described above is represented by the product of the internal quantum efficiency and the efficiency of extracting light from the LED semiconductor layers.

Internal quantum efficiency = recombination probability in the active layer x injection efficiency of carriers into the active layer

As described in the above equation, it is known that in the conventional structure, when the Mg concentration of the p layer in the vicinity of the active layer is high, the light emission efficiency is lowered. This is explained because the Mg impurity level is physically close to the active layer, and therefore carriers are consumed in the transition process via the Mg impurity level, so that the carriers injected into the active layer are reduced.

The inventors of the present application examined the structures and growth conditions (Mg supply flow rate, growth temperature) of the p-AlN layer 14 and the p-AlGaN layer 15, and the Mg concentration of the spacer layer 13S determined in response to the results of these growth conditions, in order to improve the light emission efficiency of the ultraviolet LED.

Specifically, as a result of examining the Mg concentration of the p-AlN layer 14 and the p-AlGaN layer 15, it was found that it was difficult to sufficiently improve the light emission efficiency if the sub-peak intensity was simply decreased, and the light emission efficiency tended to decrease as the sub-peak intensity decreased.

Further, the inventors have focused on the structures of the p-AlN layer 14 and the p-AlGaN layer 15, particularly the relationship between the thickness of the p-AlN layer 14 and the Mg concentration of the p-AlGaN layer 15 and the diffusion doping concentration of the spacer layer 13S, and have obtained findings about improvement in the light emission efficiency of the ultraviolet LED.

(Mg concentration of spacer layer)

Fig. 6 shows an example of the results of SIMS (Secondary Ion Mass Spectrometry) measurement of the LED element 10 of the Example (EMB) of the present invention and the LED element of the comparative example (CMP). Specifically, the Mg concentration in the depth direction (horizontal axis) of the LED is shown. In addition, the positions of the respective layers of the semiconductor layer are shown in the upper column of SIMS imaging. It should be noted that, as described above, the depth of the horizontal axis does not accurately reflect the film thickness of each layer, and therefore, al is shown _0.65 Ga _0.35 Etch depth of N standard samples.

Referring to an image (solid line) of the LED element 10 of Example (EMB), mg diffuses in the spacer layer 13S. On the other hand, referring to the image (broken line) of the LED element of comparative example (CMP), it is understood that the Mg concentration in the spacer layer 13S is drastically decreased and the diffusion of Mg is small.

More specifically, an LED element was produced in which the p-AlN layer 14 and the p-AlGaN layer 15 had the same layer thickness and the growth temperature and the Mg concentration were varied. Then, for these LED elements, the Mg concentration at a position (evaluation position) shifted from the peak position of the Mg concentration of the p-AlN layer 14 to the active layer 13 side by 20nm was measured. Further, the light emission efficiency of these LED elements was measured, and the correlation with the Mg concentration at the evaluation position was examined.

As a result, it was found that the Mg concentration of the spacer layer 13S has a correlation with the light emission efficiency. Specifically, the Mg concentration at this evaluation position of the spacer layer 13S was 5.0E17 cm ^-3 ～5.0E18cm ^-3 High luminous efficiency is obtained, and further, at 7.0E17cm ^-3 ～3.0E18cm ^-3 Higher luminous efficiency is obtained. On the other hand, the Mg concentration was 8.0E16cm ^-3 When the ratio S/M is 1% or less, high luminous efficiency cannot be obtained. Further, the Mg concentration was 7.0E18cm ^-3 When S/M is 80% or more, high luminous efficiency cannot be obtained.

That is, the concentration of Mg diffused into the spacer layer 13 is at least 7.0E17cm ^-3 With the above constitution, a high luminous efficiency can be obtained, and the luminous efficiency is at least 1.0E18cm ^-3 In this case, higher luminous efficiency can be obtained.

It is also found that when the layer thickness of the p-AlN layer 14 is 10nm or less, the Mg flow rate in the p-AlN layer 14 is increased to raise the growth temperature, thereby improving the light emission efficiency.

(relationship between luminous efficiency and S/M ratio)

As shown by the evaluation results of the Mg concentration and the light emission efficiency in the spacer layer 13 of the LED element and the S/M ratio and the light emission efficiency evaluation results of fig. 5 described above, it is considered that the diffusion of Mg into the spacer layer 13 is equivalent to the observation of the secondary peak in the light emission spectrum.

In other words, it is understood that the diffusion of Mg in the spacer layer 13 results in a secondary peak in the emission spectrum, and that high emission efficiency can be obtained when there is a certain range of diffusion of Mg, that is, a secondary peak having a certain intensity. This is considered to be because the Mg concentration of the spacer layer 13 in the vicinity of the active layer 13 becomes high, and thus the injection efficiency of carriers into the active layer 13 becomes high.

Fig. 7 is a conceptual diagram for explaining the relationship between the S/M ratio and the light emission efficiency or the carrier injection amount in the examples (EMB 1, EMB 2) of the present invention and the comparative example (CMP) in which the Mg concentration of the spacer layer is low.

As described above, in the study of the Mg concentrations of the p-AlN layer 14 and the p-AlGaN layer 15, it is found from the relationship between the S/M ratio and the luminous efficiency that there is an optimum range of the S/M ratio in terms of improving the luminous efficiency.

Therefore, it is presumed that there is a trade-off relationship between the light emission transition amount and the carrier injection amount. First, an increase in the intensity of the secondary peak means consumption of carriers (reduction of light emission transition of the main peak in the active layer), and thus the light emission transition amount decreases as the S/M ratio increases (solid line). On the other hand, due to the increase in the S/M ratio (diffusion of Mg into the spacer layer 13), the carrier injection amount increases (EMB 1, EMB2: one-dot chain line, and CMP: broken line).

Since the light emission efficiency is the product of the light emission transition probability and the carrier injection efficiency, the S/M ratio has an optimum value (intersection of the light emission transition probability and the carrier injection efficiency). In addition, in the case of example EMB2 in which the Mg flux and the growth temperature were increased as compared to example EMB1, the S/M ratio had a lower optimum value. On the other hand, in the case of comparative example (CMP), the Mg concentration of the spacer layer was low and the carrier injection amount was small, and therefore, the light emission efficiency was improved as the sub-peak intensity was reduced, but the light emission efficiency was smaller than that in the cases of examples (EMB 1, EMB 2).

As described above, by examining the Mg concentrations of the spacer layer 13S, the p-AlN layer 14 and the p-AlGaN layer 15, it is known from the relationship between the S/M ratio and the luminous efficiency that the luminous efficiency is the maximum when the S/M ratio is 3% to 15%. Further, it is found that when the S/M ratio is 3% to 6%, higher luminous efficiency can be obtained.

As described above in detail, according to the present invention, it is possible to provide an ultraviolet semiconductor light emitting element having high injection efficiency of carriers into an active layer, high efficiency, and high output.

Claims

1. An ultraviolet semiconductor light emitting element comprising:

a substrate composed of single-crystal AlN; and

a main peak and a secondary peak derived from Mg in a light emission spectrum by applying a current to the active layer, and a driving current density of 20mA/mm ² The peak intensity ratio (S/M) of the intensity (S) of the secondary peak to the intensity (M) of the primary peak is 3 to 15%.

2. The ultraviolet semiconductor light-emitting element according to claim 1,

the p-type Al _Y1 Ga _1-Y1 Mg concentration of N layer is 3.0 × 10 ¹⁹ cm ^-3 ～5.0×10 ¹⁹ cm ^-3 In the range of (1), the p-type Al _Y2 Ga _1-Y2 Mg concentration of the N layer is 2.0X 10 ¹⁹ cm ^-3 ～1.0×10 ²⁰ cm ^-3 Within the range of (1).

3. The ultraviolet semiconductor light-emitting element according to claim 1,

the p-type Al _Y2 Ga _1-Y2 Mg concentration of the N layer is 2.0X 10 ¹⁹ cm ^-3 ～5.0×10 ¹⁹ cm ^-3 In the presence of a surfactant.

4. The ultraviolet semiconductor light-emitting element according to any one of claims 1 to 3,

the p-type Al _Y1 Ga _1-Y1 The thickness of the N layer is in the range of 4nm to 10 nm.

5. The ultraviolet semiconductor light-emitting element according to any one of claims 1 to 4,

the p-type Al _Y1 Ga _1-Y1 The N layer is a p-type AlN layer.

6. The ultraviolet semiconductor light-emitting element according to any one of claims 1 to 5,

the ratio (S/M) of the intensity (S) of the secondary peak to the intensity (M) of the primary peak is 3% to 6%.

7. An ultraviolet semiconductor light emitting element comprising:

a substrate composed of single-crystal AlN; and

the p-type Al _Y1 Ga _1-Y1 Mg concentration of N layer is 3.0 × 10 ¹⁹ cm ^-3 ～5.0×10 ¹⁹ cm ^-3 In the range of (1), the p-type Al _Y2 Ga _1-Y2 Mg concentration of N layer is 2.0 × 10 ¹⁹ cm ^-3 ～1.0×10 ²⁰ cm ^-3 In the range of (a) to (b),