Vertical resonator-type light-emitting element

Abstract

The invention provides a vertical resonator type light emitting element. The light emitting diode has low threshold current density and high luminous efficiency, and the service life is improved. A vertical resonator surface emitting laser (10) is provided with: a semiconductor DBR (12); an n-type semiconductor layer (13) formed on the semiconductor DBR (12); an active layer (15) provided on the n-type semiconductor layer (13); an intermediate layer (16) provided on the active layer (15); an electron blocking layer (17) that is provided on the intermediate layer (16) and contains Al in the composition; a p-type semiconductor layer (18) which is provided on the electron blocking layer (17), contains Al in a component, and is doped with an impurity; and a dielectric DBR (25) provided on the p-type semiconductor layer (18). The electron blocking layer (17) is a component inclined layer in which the Al component decreases in the direction away from the active layer (15), and the impurity concentration of the impurities has a peak value within the electron blocking layer (17) and decreases from the peak value in the direction toward the active layer (15).

Description

Technical Field

[0001] The present invention relates to a vertical resonator type light emitting element. Background Art

[0002] Conventionally, a vertical resonator type light emitting element such as a vertical cavity surface emitting laser (VCSEL) having a structure in which light resonates perpendicular to the substrate surface and is emitted in a direction perpendicular to the substrate surface has been known.

[0003] In comparison with a light emitting diode (LED) or the like, the active layer of the vertical resonator type light emitting element is required to have a high carrier density. Various developments have been made to improve the injection efficiency of the vertical resonator type light emitting element and to increase the light output.

[0004] Patent Document 1 discloses a nitride semiconductor laser diode having a compositionally graded region including a region formed of AlGaN in which the Al composition of the AlGaN continuously increases in a direction away from the active layer in order to improve the light emission efficiency of the nitride semiconductor laser diode.

[0005] In addition, Patent Document 2 discloses an ultraviolet light emitting element having a nitride semiconductor utilizing a polarization doping effect, wherein the polarization doping effect is achieved by a compositionally graded layer in which the Al composition y decreases in a direction away from the substrate.

[0006] Patent Document 3 discloses a semiconductor wafer having an electron blocking layer containing Mg as a p-type dopant.

[0007] Patent Document 4 discloses a nitride semiconductor light emitting element having a compositionally graded layer, wherein, in order to improve the hole injection efficiency into the active layer, the Al composition value in the compositionally graded layer is compositionally graded in a manner of decreasing toward a side where the sum of the spontaneous polarization and the piezoelectric polarization is negative.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2021-184456

[0011] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2022-041738

[0012] Patent Document 3: Japanese Patent No. 6830098

[0013] Patent Document 4: Japanese Patent No. 6192378 Summary of the Invention

[0014] Problems to be Solved by the Invention

[0015] In a vertical resonator light-emitting element in the prior art, there are problems that the threshold current needs to be further reduced, and the light-emitting efficiency and lifespan need to be improved.

[0016] An object of the present invention is to provide a vertical resonator type light-emitting element having a low threshold current density, a high light-emitting efficiency, and an improved lifespan.

[0017] Means for Solving the Problems

[0018] A vertical resonator type light-emitting element in one aspect of the present invention includes:

[0019] A first mirror;

[0020] An n-type semiconductor layer formed on the first mirror;

[0021] An active layer provided on the n-type semiconductor layer;

[0022] An intermediate layer provided on the active layer;

[0023] An electron blocking layer provided on the intermediate layer and containing Al in its composition;

[0024] A p-type semiconductor layer provided on the electron blocking layer and doped with impurities; and

[0025] A second mirror provided on the p-type semiconductor layer,

[0026] The electron blocking layer is a composition gradient layer in which the Al composition decreases in a direction away from the active layer,

[0027] The impurity concentration of the impurities has a peak within the electron blocking layer and decreases in a direction approaching the active layer from the peak. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a cross-sectional view schematically showing the structure of a vertical cavity surface emitting laser according to a first embodiment of the present invention.

[0029] Figure 2 is a diagram schematically showing the band structure of the conduction band of a vertical cavity surface emitting laser.

[0030] Figure 3 is a diagram magnifying and schematically showing the band structure of the conduction band of an electron blocking layer (EBL) and its vicinity.

[0031] Figure 4It is a diagram showing the SIMS analysis curve of the semiconductor layer of a vertical cavity surface emitting laser.

[0032] Figure 5 It is a band diagram schematically showing a pn structure composed of an n-type semiconductor, an active layer (light-emitting layer), and a p-type semiconductor.

[0033] Figure 6A It is a diagram for conceptually explaining holes generated by piezoelectric polarization.

[0034] Figure 6B It is a graph showing the band diagram of an electron blocking layer (EBL).

[0035] Figure 6C It is a graph showing the simulation result of the generated hole concentration with respect to the distance z from the interface of the active layer side of the electron blocking layer.

[0036] Figure 7 It is the hole concentration generated in the electron blocking layer (cm -3 ) plotted against the average composition gradient CS (% / nm) of Al in the electron blocking layer.

[0037] Figure 8A It is a diagram showing an example of the Al composition distribution of the electron blocking layer.

[0038] Figure 8B It is a diagram showing another example of the Al composition distribution of the electron blocking layer.

[0039] Figure 8C It is a diagram showing another example of the Al composition distribution of the electron blocking layer.

[0040] Figure 9 Schematically shows the standing wave of the electric field intensity in the semiconductor layer from the active layer to the dielectric DBR in the vertical cavity surface emitting laser of Example 1 (Ex.1).

[0041] Figure 10 It is a graph showing the device characteristics (light output - current density characteristics) of the vertical cavity surface emitting laser of Example 2, the vertical cavity surface emitting lasers of Comparative Example 1 and Comparative Example 2.

[0042] Figure 11 It is a cross-sectional view schematically showing the structure of the vertical cavity surface emitting laser of the second embodiment. Detailed Embodiment

[0043] Hereinafter, preferred embodiments of the present invention will be described, but the above embodiments can be appropriately changed and combined. In addition, in the following description and drawings, substantially the same or equivalent parts are denoted by the same reference numerals for explanation.

[0044] [First Embodiment]

[0045] Figure 1 FIG. 6 is a cross-sectional view schematically showing the structure of a vertical cavity surface emitting laser 10 according to a first embodiment of the present invention. In the present embodiment, the vertical cavity surface emitting laser 10 is a nitride surface emitting laser composed of III-V nitride semiconductor layers.

[0046] The vertical cavity surface emitting laser 10 is formed by epitaxially growing a semiconductor DBR (Distributed Bragg Reflector) 12, an n-type semiconductor layer 13, an active layer 15 composed of multiple quantum wells, an intermediate layer 16, an electron blocking layer (EBL: Electron Blocking Layer) 17, and a p-type semiconductor layer 18 in this order on a substrate 11.

[0047] The substrate 11 is a GaN substrate. The substrate 11 is a +C-plane GaN substrate inclined by 0.5° in the direction from the C-plane to the M-plane and inclined by 0 ± 0.1° in the direction to the A-plane.

[0048] Crystal growth of the semiconductor layers is performed by metalorganic vapor phase epitaxy (MOVPE). A base GaN layer 11B with a thickness of about 1 μm is grown on the substrate 11, and a semiconductor DBR 12 serving as a distributed Bragg reflector is formed on the base GaN layer 11B.

[0049] The semiconductor DBR 12 (first mirror) is formed by laminating 42 pairs of n-type GaN films and AlInN films. It should be noted that each semiconductor film of the semiconductor DBR 12 has a film thickness of λ / 4n (n is the refractive index of each semiconductor film) of the emission wavelength λ of the active layer 15.

[0050] An n-type semiconductor layer 13 (layer thickness: 350 nm) doped with Si (silicon) is grown on the semiconductor DBR 12.

[0051] A barrier layer (barrier layer) 15B and a quantum well layer (well layer) 15W are alternately formed on the n-type semiconductor layer 13 to form an active layer 15 having 4 quantum well layers 15W. The barrier layer 15B is composed of GaInN (layer thickness: 3 nm), and the well layer 15W is composed of GaN (layer thickness: 4 nm). It should be noted that the composition and layer thickness of the barrier layer 15B and the well layer 15W can be appropriately selected according to the desired emission wavelength, emission characteristics, etc.

[0052] On the final well layer 15WL, which is the final layer of the active layer 15, an undoped GaN with a layer thickness of 120 nm is grown as the intermediate layer (LB) 16 of the final barrier layer. That is, the intermediate layer 16 is the layer between the active layer 15 and the electron blocking layer (EBL) 17.

[0053] Next, an electron blocking layer (EBL) 17 composed of Al x Ga 1-x N (Al composition: x) doped with Mg (magnesium) as an impurity is grown.

[0054] Next, an 83-nm Mg-doped p-GaN layer is grown as the p-type semiconductor layer 18 on the electron blocking layer 17 (p-AlGaN).

[0055] The peripheral part is etched to form a cylindrical mesa structure in such a way that the wafer grown in the above manner reaches the inside of the n-type semiconductor layer 13.

[0056] The peripheral part of the p-type semiconductor layer 18, which is the uppermost semiconductor layer of this mesa structure, is etched by dry etching to a depth of about 20 nm to form a concave part, and a p-type semiconductor layer 18 with a cylindrical mesa convex part is formed.

[0057] In the concave part of the p-type semiconductor layer 18 formed by etching, an insulating film (SiO 2 )21 with a thickness of 20 nm for lateral current and light confinement is deposited. Thus, the concave part of the p-type semiconductor layer 18 is flattened, and a current narrowing structure is formed to form a cylindrical (central axis: CX) current injection region.

[0058] Next, an ITO (indium tin oxide) film with a thickness of 20 nm is formed as the transparent conductive film 22 on the p-type semiconductor layer 18 and the insulating film 21.

[0059] Next, a dielectric (Nb 2 O 5 ) film with a thickness of 38 nm is formed as the spacer layer 24. The spacer layer 24 functions as a phase adjustment layer.

[0060] And a film of the dielectric DBR25 (second mirror) is formed on the spacer layer 24. The dielectric DBR25 is composed of 10.5 pairs of SiO 2 (11 layers) and Nb 2 O 5 (10 layers). It should be noted that the dielectric DBR25 is preferably formed coaxially with the cylindrical mesa of the p-type semiconductor layer 18.

[0061] Next, an n - electrode 27 is formed on the concave portion of the outer peripheral part of the n - type semiconductor layer 13, and a p - electrode 28 is formed on the transparent conductive film 22. Further, the back surface of the substrate 11 is polished to form an AR (anti - reflection) coating 29 (anti - reflection film) composed of two layers of Nb 2 O 5 / SiO 2 . Thus, the formation of the vertical cavity surface emitting laser 10 is completed.

[0062] It should be noted that the composition and layer thickness of the above - mentioned intermediate layer (LB) 16 are only examples. That is, the intermediate layer 16 is described as a GaN layer, but a nitride semiconductor layer of other compositions such as InGaN, AlGaN, InAlGaN, etc. can also be used. In addition, although the intermediate layer 16 is an undoped layer, dopants diffused from the electron blocking layer 17 or the p - type semiconductor layer 18 can also be mixed in.

[0063] In addition, the layer thickness of the electron blocking layer 17 is only an example. The electron blocking layer 17 can have a layer thickness of, for example, 3 - 30 nm. In addition, the electron blocking layer 17 is composed of a nitride semiconductor containing Al in its composition. Moreover, the electron blocking layer 17 is formed as a composition gradient layer in which the Al composition varies in the layer thickness direction. Details will be described later.

[0064] The electron blocking layer 17 is described as a p - type semiconductor layer (p - AlGaN), but the electron blocking layer 17 can also be a p - type semiconductor layer containing Al in its composition, which is grown as an i - layer and doped with impurities (Mg) diffused from the p - type semiconductor layer 18.

[0065] In addition, the case where the active layer 15 is a quantum well active layer and has four quantum well layers 15W is exemplified, but as long as it has at least one quantum well layer. In addition, the active layer 15 is not limited to a quantum well active layer. A so - called bulk - type active layer can also be used.

[0066] Moreover, the p - type semiconductor layer 18 can also be composed of multiple semiconductor layers including layers with different compositions and / or doping concentrations and undoped layers. Similarly, the n - type semiconductor layer 13 can also be composed of multiple semiconductor layers.

[0067] The case where the dielectric DBR 25 is composed of SiO 2 films and Nb 2 O 5 films is exemplified, but it can also be composed of dielectric films with different refractive indices based on other combinations. The dielectric DBR 25 can also be composed of a semiconductor DBR composed of semiconductor films with different refractive indices.

[0068] Figure 2It is a diagram schematically showing the band structure of the conduction band of the vertical-cavity surface-emitting laser 10. It should be noted that the band structure from the active layer 15 to the p-type semiconductor layer 18 is shown in the figure.

[0069] The active layer 15 is composed of four well layers (Well layer) 15W, namely QW1 to QW4, and barrier layers 15B provided between these well layers. The quantum well layer QW4 adjacent to the intermediate layer (LB) 16 is the final well layer 15WL.

[0070] The intermediate layer 16 has a layer thickness t1, the electron blocking layer 17 has a layer thickness t2, and the p-type semiconductor layer 18 has a layer thickness t3.

[0071] [Al composition of the electron blocking layer]

[0072] Figure 3 It is a diagram that magnifies and schematically shows the band structure of the electron blocking layer 17 (EBL) and its vicinity in the conduction band.

[0073] In addition, similar to Figure 2 , the left side of the figure is the active layer 15 (ACT) side.

[0074] As Figure 3 shown, the electron blocking layer 17 is formed as an Al composition gradient layer in which the Al composition decreases in the direction away from the active layer 15 (ACT). Specifically, the Al composition (x) of the electron blocking layer 17 linearly decreases from x1 (%) to x2 (%).

[0075] It should be noted that in the specification of this application, the "composition gradient" of the Al composition (x) is not limited to the case where the Al composition is linear or monotonically decreasing. It is only necessary that the Al composition (x1) at the interface on the active layer 15 side (the interface between the electron blocking layer 17 and the intermediate layer 16) is the highest, and the Al composition (x2) at the interface between the electron blocking layer 17 and the p-type semiconductor layer 18 is the lowest. In addition, the composition (x) includes a constant part within the range from the interface on the active layer 15 side to the interface between the electron blocking layer 17 and the p-type semiconductor layer 18.

[0076] In addition, Figure 3 schematically shows the concentration distribution (dashed line) of Mg as a p-dopant. As shown by the concentration distribution of Mg, the Mg concentration decreases in the direction approaching the active layer 15 from the interface between the intermediate layer 16 and the electron blocking layer 17, and has a peak near the end of the electron blocking layer 17 (the interface with the p-type semiconductor layer 18). It should be noted that the vicinity of the interface between the electron blocking layer 17 and the p-type semiconductor layer 18 refers to a position at least closer to the p-type semiconductor layer 18 than the center of the electron blocking layer 17.

[0077] In addition, the distribution of the Mg concentration shows a peak and valley near the interface between the electron blocking layer 17 and the p-type semiconductor layer 18, that is, at a position corresponding to the interface and on the p-type semiconductor layer 18 side relative to the peak of the above-mentioned Mg concentration. That is, the distribution of the Mg concentration has a depression at this interface position, and a peak and valley of this depression is formed near this interface.

[0078] This Mg concentration distribution is caused by providing an electron blocking layer 17 (AlGaN layer) between the intermediate layer 16 (GaN layer) and the p-type semiconductor layer 18 (p-GaN layer).

[0079] Figure 4 The figure shows the distribution of the semiconductor layers of the vertical cavity surface emitting laser 10 analyzed by SIMS (Secondary Ion Mass Spectrometry). The analysis is performed from the surface of the p-type semiconductor layer 18. In the figure, the right direction is the depth direction, which is the direction from the p-type semiconductor layer 18 toward the active layer 15. That is, it should be noted that in Figure 2 and Figure 3 , the positional relationship between the active layer 15 and the electron blocking layer 17 on the horizontal axis is opposite. That is, in Figure 4 , the direction away from the active layer 15 (ACT) is the left direction in the figure.

[0080] As Figure 4 shown, within the electron blocking layer 17, the Al component of the electron blocking layer 17 has a peak near the intermediate layer 16 and decreases in the direction away from the active layer 15.

[0081] On the other hand, the Mg concentration decreases in the electron blocking layer 17 in the direction approaching the active layer 15. That is, the Mg concentration distribution has a concentration gradient opposite to the compositional gradient of Al.

[0082] That is, the electron blocking layer 17 has a compositional gradient with a high Al component on the active layer 15 side and a low Al component on the p-type semiconductor layer 18 side. At the same time, the Mg concentration in the electron blocking layer 17 decreases in the direction approaching the active layer 15.

[0083] It should be noted that even when the impurity (Mg) diffuses from the p-type semiconductor layer 18 into the electron blocking layer 17, the Mg concentration in the electron blocking layer 17 shows the same distribution as described above. That is, the Mg concentration shows this peak and valley and decreases in the direction approaching the active layer 15.

[0084] [Effective Barrier of Electron Blocking Layer, Mg Concentration, Hole Generation Based on Piezoelectric Polarization]

[0085] Hereinafter, the relationship between the effective barrier of the electron blocking layer and the concentration of Mg as a p-dopant, and hole generation based on piezoelectric polarization will be described.

[0086] In the past, by increasing the Mg concentration in the electron blocking layer to improve the injection efficiency of carriers, the high efficiency of laser diodes (LDs) and vertical cavity surface emitting lasers (VCSELs) has been achieved.

[0087] However, in order to achieve the long life of vertical cavity surface emitting lasers, it is necessary to reduce the sharpness of the Mg concentration and reduce the Mg concentration on the active layer side. On the other hand, if the Mg concentration on the active layer side is reduced, there is a problem of reduced external quantum efficiency.

[0088] In particular, compared with LEDs, semiconductor lasers such as VCSELs have a driving current density and an electron / hole concentration in the light emitting layer that are about two orders of magnitude higher. Therefore, if the Mg concentration in the electron blocking layer is reduced, there is a problem that the effective barrier energy is reduced and high-energy electrons leak to the p-layer side, resulting in a reduction in injection efficiency.

[0089] Figure 5 It is a band diagram schematically showing the band structure of a pn structure composed of an n-type semiconductor, an active layer (light emitting layer), and a p-type semiconductor.

[0090] When the voltage applied to the pn junction is set to V A , the Fermi level is set to E F , and the band gap is set to E g , the effective barrier height Beff that determines the injection efficiency of carriers injected into the active layer is represented by B eff = E g - V A - E F .

[0091] The Fermi level EF changes with the Mg concentration. Specifically, when the Mg concentration (acceptor concentration) is reduced, the Fermi level EF rises and the effective barrier height Beff decreases. When the effective barrier height Beff decreases, the blocking effect of the electron blocking layer decreases and the injection efficiency of electrons decreases.

[0092] Figure 6A It is a diagram for conceptually explaining holes generated by piezoelectric polarization. In addition, Figure 6B represents the band diagram of the electron blocking layer 17; Figure 6C is a graph showing the simulation results of the hole concentration with respect to Figure 6B the distance z (distance in the stacking direction) from the interface of the electron blocking layer 17 shown in

[0093] Note that piezoelectric polarization occurs in crystal systems that are asymmetric crystal structures (for example, hexagonal crystals). The following simulation is performed for the case where an electron blocking layer made of AlGaN is formed on the +C plane of a GaN substrate.

[0094] As Figure 6A shown, polarization charges are generated by piezoelectric polarization in the electron blocking layer 17 (AlGaN) with an Al inclined component (left figure). The larger the Al component, the greater the polarization. Holes are generated in the electron blocking layer 17 by this polarization charge (right figure).

[0095] As Figure 6B shown, in the electron blocking layer 17, the Al content on the active layer 15 side is 50%, and the Al content on the p-GaN layer 18 side is 15%. In addition, the electron blocking layer 17 has a layer thickness of 10 nm.

[0096] As Figure 6C shown, it can be seen that holes with a concentration of 1.9×10 19 cm -3 ~2.0×10 19 cm -3 are generated in the electron blocking layer 17.

[0097] That is to say, it can be known that even if the doping amount of Mg is reduced, a high hole concentration can be obtained. Therefore, the Fermi level will not rise, and the effective barrier height can be ensured. That is, high injection efficiency can be maintained while reducing the doping concentration of Mg.

[0098] Referring again to Figure 4 the SIMS data, the Mg concentration has peaks and valleys near the interface between the electron blocking layer 17 and the p-type semiconductor layer 18 (p-GaN layer) (Mg concentration: about 3×10 18 cm -3 ).

[0099] By forming peaks and valleys in the Mg concentration between the electron blocking layer 17 and the p-type semiconductor layer 18 (p-GaN layer), the total amount of Mg concentration can be reduced, and the device lifetime can be extended.

[0100] It should be noted that from the viewpoint of device lifetime, the Mg concentration at the interface between the electron blocking layer 17 and the intermediate layer 16 is preferably less than 1.0×10 19 cm -3 , and more preferably less than 5.0×10 18 cm -3 .

[0101] In addition, as Figure 4 shown, a peak of hydrogen (H) is formed at the position corresponding to the peak and valley of the Mg concentration. That is, hydrogen is captured by the part with the Mg concentration peak and valley to form a peak of hydrogen.

[0102] Therefore, the entry of hydrogen into the active layer 15 can be suppressed, and the light output characteristics such as the voltage-current characteristics of the vertical cavity surface emitting laser 10 can be stabilized.

[0103] [Al composition distribution of the electron blocking layer]

[0104] Figure 7 is a graph plotted with the hole concentration (cm -3 ) generated in the electron blocking layer versus the average composition of Al in the electron blocking layer with an inclination CS (% / nm).

[0105] In order to maintain the injection efficiency of the vertical cavity surface emitting laser 10, it is necessary to generate a hole concentration of 5.0×10 18 cm -3 or more. Therefore, according to the line graph of Figure 7 , the average composition slope CS is preferably 0.9 (% / nm) or more.

[0106] It should be noted that except for the case where the Al composition shows a linear inclination, the average composition inclination CS (i.e., CS = (x1 - x2) / t2) is preferably 0.9 (% / nm) or more.

[0107] It should be noted that the Al composition distribution of the electron blocking layer 17 is not limited to the above-mentioned linear composition inclination. As long as the Al composition of the electron blocking layer 17 is the highest on the active layer 15 side of the electron blocking layer 17 and the lowest on the p-type semiconductor layer 18 side.

[0108] On the other hand, if the product of the average Al composition (%) of the electron blocking layer 17 and the layer thickness (nm) is a certain value or more, damage due to stress occurs in the electron blocking layer 17 and cracks are generated. Specifically, if the product of the average Al composition (%) and the layer thickness t2 (nm) exceeds 700 (%·nm), cracks may be generated. For example, when the average Al composition of the electron blocking layer 17 is 70% and the layer thickness t2 is 10 nm, cracks are generated. Therefore, the product of the average Al composition (%) and the layer thickness t2 (nm) in the electron blocking layer 17 only needs to be less than 700 (%·nm).

[0109] For example, as Figure 8A shown, the electron blocking layer 17 may have the following Al composition distribution: the Al composition decreases stepwise from the active layer 15 side to the p-type semiconductor layer 18 side.

[0110] Or, as Figure 8B shown, the electron blocking layer 17 may also have the following Al composition distribution: it is composed of an inclined composition part where the Al composition decreases from the active layer 15 side and a constant composition part where the Al composition is constant from this inclined composition part to the p-type semiconductor layer 18.

[0111] Or, as Figure 8CAs shown, the electron blocking layer 17 may also have the following Al composition distribution: It is composed of a constant composition part where the Al composition is constant from the active layer 15 side and a sloped composition part where the Al composition decreases from this constant composition part to the p-type semiconductor layer 18.

[0112] Alternatively, the electron blocking layer 17 may also have an Al composition distribution formed by combining the constituent parts of the above Al composition distribution.

[0113] [Improving the injection efficiency of carriers into the multi-quantum well active layer]

[0114] As described in detail above, by the vertical cavity surface emitting laser 10 formed in such a manner as to satisfy the above Al composition distribution and impurity (Mg) concentration, it is possible to achieve the improvement of carrier injection efficiency and reliability (element lifetime), which is the subject of this application.

[0115] Moreover, in the case where a multi-quantum well (MQW) structure composed of multiple quantum well layers is applied to the active layer, by further satisfying the following conditions, it is possible to achieve higher carrier injection efficiency and longer lifetime.

[0116] First, define the interface between a high reflector such as a DBR (Distributed Bragg Reflector) or a diffraction grating and the innermost part of the resonator as the phase "0" (reference).

[0117] Using the refractive index n i and layer thickness t i of each layer in the resonator and the emission wavelength λ, the total phase information of the standing wave in the resonator can be expressed by the following formula (1).

[0118] [Equation 1]

[0119]

[0120] Note that regarding the combined information of the standing wave at a specified position inside the layer, consider it after replacing the thickness from the interface with other layers on the resonator side to the specified position with t i and then.

[0121] And the content of the first condition is as follows: The number N AN of antinodes AN and the number N ND of nodes ND of the standing wave SW included in the p-type semiconductor layer 18 and the electron blocking layer 17 as the p region AN are each 0 or 1 (N AN = 0 or N ND = 1, N ND = 0 or N

[0122] Note that when the thicknesses of the transparent conductive film 22 (ITO), p-type semiconductor layer 18, and electron blocking layer 17 are set to H ITO , H GaN , H EB , and the refractive indices are set to n ITO , n GaN , n EB and the wavelength is set to λ, it is preferably to satisfy the following formula (2).

[0123] [Formula 2]

[0124]

[0125] Moreover, the content of the second condition is as follows: The number of nodes ND and antinodes AN of the standing wave SW included in the intermediate layer 16 is 1 or more (N ND ≥1 and N AN ≥1).

[0126] Note that the number of nodes ND and antinodes AN of the standing wave SW, N ND , N AN can be obtained by the following method: Calculate the total phase in the p region and the intermediate layer 16 using the above formula (1), and based on the number of phases of kπ (k = 1, 2,...) and the number of phases of (2l - 1)π / 2 (l = 1, 2,...) in the p region and the intermediate layer 16.

[0127] Formula (1) = k / 2 (k = 1, 2,...) represents the position where the standing wave becomes an antinode AN, and formula (1) = (2l - 1) / 4 (l = 1, 2,...) represents the position where the standing wave becomes a node ND. That is, N ND and N AN in the p-type semiconductor layer 18, electron blocking layer 17, and intermediate layer 16 are obtained by counting the number of positions of the above antinodes AN and nodes ND included in the stacking range of each layer.

[0128] In addition, when the layer thicknesses of the intermediate layer 16 and the active layer 15 are set to H fb , H qw respectively, it is preferably to satisfy the following formula (3). Here, n fb is the refractive index of the intermediate layer 16, and n qw is the equivalent refractive index of the active layer 15.

[0129] [Formula 3]

[0130]

[0131] Among them,

[0132] Moreover, (i) the intermediate layer 16 preferably has a layer thickness of λ / 4 or more. That is, it preferably satisfies the following formula (4).

[0133] [Formula 4]

[0134]

[0135] In addition, (ii) preferably, the number of nodes ND included in the intermediate layer 16 is two or more (N ND ≥2), and the number of antinodes AN is 1 or more (N AN ≥1). When (i) and (ii) are satisfied, it is further preferred that the following formula (5) is satisfied.

[0136] [Formula 5]

[0137]

[0138] In addition, preferably, the layer thickness of the active layer 15 is λ / 8 or less, that is, it satisfies the following formula (6). In this case, the optical confinement loss of the active layer 15 can be reduced.

[0139] [Formula 6]

[0140]

[0141] Constructed in the above-described manner, the electric field strength of light from the p-type semiconductor layer 18 to the electron blocking layer 17 is enhanced. In addition, in the intermediate layer 16, there is an antinode AN where the electric field strength of light is high, and there is at least one node ND. As a result, the active layer 15 is excited to a degree where a large optical gain can be obtained.

[0142] Moreover, when the internal light intensity increases due to the action of the semiconductor DBR12 and the dielectric DBR25 as high reflectors, electrons and holes in the intermediate layer 16 are excited by the internal light, and holes accumulated at the interface between the electron blocking layer 17 and the p-type semiconductor layer 18 are extracted into the active layer 15, and hole injection for the switching of the active layer 15 occurs.

[0143] As a result, it is possible to realize a surface-emitting laser with good efficiency in which the uniformity of carriers (electrons and holes) in each of the multiple well layers 15W of the active layer 15 is improved.

[0144] [Examination of the mechanism for improving characteristics]

[0145] Constructed in the above-described manner, it is possible to realize a surface-emitting laser with good efficiency in which the uniformity of carriers (electrons and holes) in each of the well layers 15W of the active layer 15 is improved. Examine the mechanism for improving characteristics such as efficiency like this.

[0146] It is considered that the mechanism of characteristic improvement is related to the case of switchingly injecting holes in order to eliminate the carrier inhomogeneity of the multiple quantum wells. In this surface-emitting laser, the antinode of the standing wave is designed to be aligned with the center of the active layer (multiple quantum wells). That is, by arranging the active layer at a position where the electric field of light is large, the interaction between light and electron-hole recombination is increased.

[0147] Therefore, the intermediate layer 16 adjacent to the active layer 15 becomes the direction in which the electric field intensity of light decreases. However, as described above, by arranging the intermediate layer 16 that is different from the active layer and includes the position where the antinode of the standing wave is located or the vicinity of the antinode, it can be speculated that the light intensity of this layer near the threshold of the laser oscillation current rises significantly, generating carriers (electrons and holes), and having the effect of suddenly introducing holes to the p-type semiconductor layer 18 side of the electron blocking layer 17. This introduction is likely to occur when the electric field gradient in the p-region is high.

[0148] It is considered that this effect is due to the increase in the hole concentration accumulated on the p-type semiconductor layer 18 side and the increase in the electric field gradient of the electron blocking layer 17. It is considered that through this switchable hole introduction, the uniformity of the carrier distribution in the multiple quantum wells is improved and the internal loss is reduced. Therefore, depending on the situation, characteristics that are not seen in the characteristics of ordinary surface-emitting lasers (VCSELs) can be obtained due to laser oscillation, such as the differential resistance near the threshold and the minimum value of the drive current (dR / dI = 0, d 2 V / dI 2 = 0). Therefore, the position of the antinode of the intermediate layer 16 and the thinning of the p-layer (electron blocking layer 17 and p-type semiconductor layer 18) and the feedback of strong light in the vertical direction caused by the high reflector and the light intensity of the optical gain become important factors.

[0149] [Example 1]

[0150] Figure 9 FIG. is a diagram schematically showing the standing wave SW of the electric field intensity of light emitted from the active layer 15 in the semiconductor layer from the active layer 15 to the dielectric DBR 25 in the vertical-cavity surface-emitting laser 10 of Example 1 (Ex.1) of the present embodiment.

[0151] As described above, in the vertical-cavity surface-emitting laser 10 of Example 1 (Ex.1), an undoped GaN layer with a layer thickness of 120 nm is provided as the intermediate layer (LB) 16.

[0152] In addition, the layer thickness of the electron blocking layer 17 is 10 nm, and it has a composition gradient in which the Al component decreases from 50% to 15% in the direction away from the active layer 15.

[0153] In addition, a p-GaN layer with a layer thickness of 83 nm is provided on the electron blocking layer 17 as the p-type semiconductor layer 18.

[0154] In Example 1, the electron blocking layer 17 is doped with Mg at a doping concentration of 1.0×10 18 ~1.0×10 19 cm 3 , and the p-type semiconductor layer 18 is doped with Mg at a doping concentration of 1.0×10 18 cm 3 or more. However, it is not limited to the above doping concentration, and can be appropriately set according to the composition, layer thickness, etc. of the p-side semiconductor layers such as the intermediate layer 16, the electron blocking layer 17, and the p-type semiconductor layer 18.

[0155] In Example 1 (Ex.1), there is a node (ND) of a standing wave SW and an anti-node (AN) of a standing wave SW in the intermediate layer 16 respectively. That is, if the number of nodes ND is set to N ND and the number of anti-nodes AN is set to N AN , then N ND = 1, and N AN = 1. It should be noted that, as Figure 9 shown, at the mirror interface A with high reflection based on the dielectric DBR25, it is the position of the anti-node AN of the standing wave SW.

[0156] In addition, in the electron blocking layer (EBL) 17 and the p-type semiconductor layer (p-GaN) 18 as the p region, there are a node ND and an anti-node AN of a standing wave SW respectively (N ND = 1, N AN = 1). Moreover, preferably, there is a node ND of a standing wave SW in the transparent conductive film (ITO) 22 (N ND = 1).

[0157] By configuring in this way, the electric field strength increases in the direction from the p-type semiconductor layer 18 to the electron blocking layer 17. In addition, there is an anti-node with a high electric field strength of light in the intermediate layer 16, and there is at least one node. Thus, the active layer 15 is excited to an extent that a large optical gain can be obtained.

[0158] Moreover, when the internal light intensity increases due to the action of the semiconductor DBR12 and the dielectric DBR25 as high reflection mirrors, the electrons and holes in the intermediate layer 16 are excited by the internal light, and the holes accumulated at the interface between the electron blocking layer 17 and the p-type semiconductor layer 18 are extracted into the active layer 15, and a switching hole injection into the active layer 15 is generated.

[0159] This situation promotes the uniformity of the carrier concentration among multiple quantum wells, and improves the internal loss, external differential quantum efficiency, and slope efficiency.

[0160] That is, it is possible to improve the uniformity of carriers (electrons and holes) in multiple well layers 15W, and achieve a high-efficiency surface-emitting laser.

[0161] In addition, it is possible to provide a vertical resonator type light-emitting element that can reduce the operating current and improve the reliability (lifetime) of the element.

[0162] [Element characteristics: Effect of reducing Mg concentration]

[0163] The vertical cavity surface emitting lasers 10 of Example 2 of the present embodiment, and the vertical cavity surface emitting lasers of Comparative Example 1 and Comparative Example 2 were fabricated, and the element characteristics were evaluated. The evaluation results will be described below.

[0164] [Example 2]

[0165] Figure 10 It is a graph showing the element characteristics (light output - current density characteristics) of the vertical cavity surface emitting laser 10 (Ex.2) of Example 2, and the vertical cavity surface emitting lasers of Comparative Example 1 (Cmp.1) and Comparative Example 2 (Cmp.2).

[0166] Example 2 (Ex.2) is a vertical cavity surface emitting laser 10 having an electron blocking layer 17. The layer thickness of the electron blocking layer 17 is 10 nm, and it has a composition gradient in which the Al composition decreases from 50% to 15% in the direction away from the active layer 15 (refer to Figure 6B ).

[0167] In addition, the doping concentration of Mg in the electron blocking layer 17 is 0.25×10 19 cm -3 , which is 1 / 4 of the concentration of Comparative Example 1 below.

[0168] The vertical cavity surface emitting laser of Comparative Example 1 (Cmp.1) has an electron blocking layer with the same layer thickness (10 nm) as that of Example 2. Comparative Example 1 is different from Example 2 in that the electron blocking layer has a constant Al composition (Al composition: 30%). In addition, the doping concentration of Mg is 1.0×10 19 cm -3 .

[0169] The vertical cavity surface emitting laser of Comparative Example 2 (Cmp.2) is different from the vertical cavity surface emitting laser of Comparative Example 1 in that the doping concentration of Mg is 1 / 2 of that of Comparative Example 1 (Cmp.1) (0.5×10 19 cm -3)。The other aspects are the same as those of the vertical cavity surface emitting laser of Comparative Example 1.

[0170] The light output characteristics of the vertical cavity surface emitting laser of Comparative Example 1 (Cmp.1) having an electron blocking layer in the prior art are shown by a dashed line. In Comparative Example 2, the doping concentration of Mg in the electron blocking layer was reduced to 1 / 2 of that in Comparative Example 1, but it was confirmed that the light output was significantly reduced compared to Comparative Example 1.

[0171] On the other hand, in Example 2 (Ex.2) having an electron blocking layer with an Al composition gradient, it was confirmed that even when the doping concentration of Mg was reduced to 1 / 4, light output characteristics exceeding those of Comparative Example 1 could be obtained, achieving high luminous efficiency.

[0172] That is, it was confirmed that the vertical cavity surface emitting laser 10 according to Example 2 could significantly reduce the doping concentration of Mg and achieve high luminous efficiency.

[0173] In addition, by having the electron blocking layer 17 with an Al composition gradient, the doping concentration of Mg can be reduced and high injection efficiency can be maintained.

[0174] Therefore, the operating current can be reduced, and a vertical cavity type light emitting element with improved element reliability (lifetime) can be provided.

[0175] [Example 3]

[0176] In the vertical cavity surface emitting laser 10 of Example 3 (Ex.3), the layer thickness of the intermediate layer 16 is 220 nm, which is thicker than 120 nm in Example 1.

[0177] In addition, the layer thickness of the p-region (the total layer thickness of the electron blocking layer 17 and the p-type semiconductor layer 18) is 101 nm (20 + 81 nm), which is thicker than 93 nm (10 + 83 nm) in Example 1.

[0178] In addition, the layer thickness of the electron blocking layer 17 is 20 nm, and it has a composition gradient in which the Al composition decreases from 50% to 15% in the direction away from the active layer 15.

[0179] The configuration other than the above is the same as that of Example 2.

[0180] In Example 3, due to the thickness of the intermediate layer 16, there are two nodes ND and two antinodes AN in the intermediate layer 16 (N ND = 2, N AN = 2). In addition, there is one node ND and one antinode AN in the p-region (the electron blocking layer 17 and the p-type semiconductor layer 18) respectively (N ND = 1, N AN = 1).

[0181] Therefore, since the above first condition and second condition are satisfied, it is possible to solve the reduction in the efficiency of the surface-emitting laser caused by the carrier non-uniformity between the well layers of the quantum well active layer, and thus a surface-emitting laser with high efficiency can be provided.

[0182] As in this embodiment, it is possible to thicken the intermediate layer 16 to satisfy the first condition and the second condition, and thus it is possible to suppress the deterioration of the device characteristics and the lifetime caused by the migration of dopants (such as Mg).

[0183] In addition, by having an electron blocking layer 17 with an inclined Al composition, it is possible to reduce the doping concentration of Mg and maintain a high injection efficiency.

[0184] Therefore, it is possible to provide a vertical cavity surface-emitting device that can reduce the operating current and improve the reliability (lifetime) of the device.

[0185] [Second Embodiment]

[0186] Figure 11 FIG. is a cross-sectional view schematically showing the structure of a vertical cavity surface-emitting laser 50 according to a second embodiment of the present invention. The vertical cavity surface-emitting laser 50 of this embodiment has a tunnel junction layer as a current narrowing structure.

[0187] The vertical cavity surface-emitting laser 50 is a nitride surface-emitting laser having the same structure as the vertical cavity surface-emitting laser 10 of the above first embodiment except that it has a tunnel junction layer 31.

[0188] That is, the electron blocking layer 17 has a compositional gradient in which the Al composition on the active layer 15 side is higher than the Al composition on the p-type semiconductor layer 18 side, and has a concentration distribution in which the Mg concentration in the electron blocking layer 17 decreases in the direction approaching the active layer 15.

[0189] More specifically, the vertical cavity surface-emitting laser 50 is formed by sequentially growing semiconductor DBR 12, n-type semiconductor layer 13, active layer 15 composed of multiple quantum wells, final barrier layer 16, electron blocking layer (EBL: Electron Blocking Layer) 17, and p-type semiconductor layer 18 on the substrate 11 by crystal growth. In addition, the composition, layer thickness, impurity concentration, etc. of each semiconductor layer are also the same as those of the vertical cavity surface-emitting laser 10 of the first embodiment.

[0190] In the vertical cavity surface-emitting laser 50, a tunnel junction layer 31 is provided on the p-type semiconductor layer 18, and the tunnel junction layer 31 is composed of a p + -type semiconductor layer 31A with a high impurity concentration of -GaN and an n + -type semiconductor layer 31B with a high impurity concentration of -GaN.

[0191] The tunnel junction layer 31 has a cylindrical mesa shape. For example, the tunnel junction layer 31 has a layer thickness of 20 nm and a diameter (central axis CX) of 4 μm. It should be noted that the tunnel junction layer 31 is formed by etching the p-type semiconductor layer 31A, the n-type semiconductor layer 31B, and the p-type semiconductor layer 18 to a depth of, for example, 25 nm.

[0192] In addition, the tunnel junction layer 31 is buried in the n-type semiconductor layer 32 (second n-type semiconductor layer) which is n-GaN.

[0193] On the n-type semiconductor layer 32, a semiconductor DBR 35 (second DBR) composed of n-AlInN and n-GaN is provided. The semiconductor DBR 35 is composed of, for example, 46 pairs of n-AlInN / GaN.

[0194] The outer peripheries of the respective semiconductor layers and the semiconductor DBR 35 are etched so as to reach inside the n-type semiconductor layer 13, and a surface-emitting laser having a cylindrical mesa structure coaxial with the central axis CX of the tunnel junction layer 31 is formed.

[0195] An n electrode 27 is formed on the surface of the outer periphery of the n-type semiconductor layer 13. In addition, when viewed from above (in the case of observing from a direction perpendicular to the semiconductor DBR 35), a p electrode 36 having a diameter larger than that of the tunnel junction layer 31 and having a circular shape with an opening coaxial with the central axis CX is located on the semiconductor DBR 35.

[0196] On the back surface of the vertical-cavity surface-emitting laser 50, an AR (antireflection) coating 29 composed of two layers of Nb 2 O 5 / SiO 2 is formed.

[0197] In addition, the tunnel junction layer 31 is composed of a p + -GaN layer and an n + -GaN layer, but semiconductor layers of other compositions, such as GaInN, etc., can also be used. For the p + -GaN layer, for example, Mg can be used as an impurity (dopant). In addition, the doping concentration of Mg is preferably 4×10 19 cm 3 or more. For the n + -GaN layer or the n + -GaInN layer, high doping of preferably 1×10 cm or more is preferred.

[0198] (Correspondence with Examples 1 to 3)

[0199] The first condition and the second condition for the vertical cavity surface emitting laser 50 can be considered the same as those for the vertical cavity surface emitting laser 10 in the first embodiment.

[0200] Specifically, in the vertical cavity surface emitting laser 50 of Example 4, the interface between the semiconductor DBR 35 and the n-type semiconductor layer 32 corresponds to the phase reference (phase = 0). The antinode of the standing wave is located at this reference position.

[0201] In addition, the n-type semiconductor layer 32 (second n-type semiconductor layer) corresponds to the spacer layer 24 in the above embodiments. More specifically, the portion of the n-type semiconductor layer 32 between the tunnel junction layer 31 and the semiconductor DBR 35 corresponds to the spacer layer 24 in the first embodiment.

[0202] Moreover, preferably, the tunnel junction layer 31 corresponds to the transparent conductive film (ITO) 22 in the first embodiment, and there is a node ND (N ND = 1) of the standing wave SW in the tunnel junction layer 31.

[0203] In addition, the conditions (first condition) for the number of nodes ND and antinodes AN of the standing wave SW included in the electron barrier layer 17 and the p-type semiconductor layer 18 as the p region and the conditions (second condition) for the number of nodes ND and antinodes AN included in the final barrier layer 16 are the same as those in the first embodiment.

[0204] According to the vertical cavity surface emitting laser 50 of the second embodiment, even if the doping concentration of impurities (dopants) is reduced, high injection efficiency can be maintained. Therefore, the operating current can be reduced, and a vertical cavity type light emitting element with improved element reliability (lifetime) can be provided.

[0205] As described in detail above, according to the present invention, a vertical cavity type light emitting element having a low threshold current density, high luminous efficiency, and improved lifetime can be provided.

[0206] Symbol Explanation

[0207] 10, 50: Vertical cavity surface emitting laser

[0208] 11: Substrate

[0209] 12: Semiconductor DBR (first mirror)

[0210] 13: n-type semiconductor layer

[0211] 15: Active layer

[0212] 15B: Barrier layer (barrier layer)

[0213] 15W: Quantum well layer (Well layer)

[0214] 16: Intermediate layer (LB)

[0215] 17: Electron blocking layer

[0216] 18: p-type semiconductor layer

[0217] 21: Insulating film

[0218] 22: Transparent conductive film

[0219] 24: Spacer layer

[0220] 25: Dielectric DBR (Second mirror)

[0221] 31: Tunnel junction layer

[0222] 31A: p-type semiconductor layer with high impurity concentration

[0223] 31B: n-type semiconductor layer with high impurity concentration

[0224] 32: n-type semiconductor layer (Second n-type semiconductor layer)

[0225] 35: Semiconductor DBR.

Claims

1. A vertical resonator type light emitting element composed of III-V nitride semiconductors, characterized in that it comprises: A first mirror; An n-type semiconductor layer formed on the first mirror; An active layer provided on the n-type semiconductor layer; An intermediate layer provided on the active layer; An electron blocking layer provided on the intermediate layer and containing Al in its composition; A p-type semiconductor layer provided on the electron blocking layer and doped with impurities; and A second mirror provided on the p-type semiconductor layer, The electron blocking layer is a composition gradient layer in which the Al composition decreases in the direction away from the active layer, The impurity concentration of the impurities has a peak in the electron blocking layer and decreases in the direction approaching the active layer from the peak.

2. The vertical resonator type light emitting element according to claim 1, Characterized in that Holes are generated by piezoelectric polarization in the electron blocking layer, and the concentration of holes generated by the piezoelectric polarization is 5.0×10 18 atoms / cm 3 or more.

3. The vertical resonator type light emitting element according to claim 1, Characterized in that The average composition gradient of the Al composition in the electron blocking layer is 0.9% / nm or more.

4. The vertical resonator type light emitting element according to claim 1, Characterized in that The impurity concentration has a peak and valley at a position corresponding to the interface between the p-type semiconductor layer and the electron blocking layer.

5. The vertical resonator type light emitting element according to claim 1, Characterized in that The impurity concentration of the impurities at the interface between the electron blocking layer and the intermediate layer is less than 1.0×10 19 cm -3 .

6. The vertical resonator type light emitting element according to claim 1, Characterized in that The impurity concentration at the interface between the electron blocking layer and the intermediate layer is less than 5.0×10 18 cm -3 .

7. The vertical resonator type light emitting element according to claim 1, Characterized in that The intermediate layer is an undoped layer.

8. A vertical resonator type light emitting element, Characterized in that It comprises: A first mirror; An n-type semiconductor layer provided on the first mirror; An active layer composed of multiple quantum wells and provided on the n-type semiconductor layer; An intermediate layer provided on the final quantum well of the active layer; An electron blocking layer provided on the intermediate layer and containing Al in its composition; A p-type semiconductor layer provided on the electron blocking layer and doped with impurities; A spacer layer provided on the p-type semiconductor layer; and A second mirror provided on the spacer layer, The electron blocking layer is a composition gradient layer in which the Al composition decreases in the direction away from the active layer, The impurity concentration of the impurities has a peak in the electron blocking layer and decreases in the direction approaching the active layer from the peak, The number of antinodes and the number of nodes of the standing wave generated by the light emission from the active layer contained in the electron blocking layer and the p-type semiconductor layer are each 0 or 1, When the layer thicknesses of the intermediate layer and the active layer are set to H fb , H qw , when the refractive index of the intermediate layer is set to n fb , and the equivalent refractive index of the active layer is set to n qw , the active layer and the intermediate layer satisfy the following formula (1), (Equation 1) 9. The vertical resonator type light emitting element according to claim 8, Characterized in that The number of nodes contained in the intermediate layer is two or more, and the number of antinodes contained in the intermediate layer is one or more.

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