Vertical cavity light-emitting element

Abstract

To provide a vertical cavity light-emitting element having a low threshold current density, high luminous efficiency, and improved lifespan.SOLUTION: A vertical cavity light-emitting element includes: a first reflection mirror; an n-type semiconductor layer formed on the first reflection mirror; an active layer provided on the n-type semiconductor layer; an intermediate layer provided on the active layer; an electron barrier layer provided on the intermediate layer and having a composition containing Al; a p-type semiconductor layer which is provided on the electron barrier layer, has a composition containing Al, and is doped with an impurity; and a second reflection mirror provided on the p-type semiconductor layer. The electron barrier 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 impurity has a peak within the electron barrier layer, and decreases in the direction approaching the active layer from the peak.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a vertical cavity light emitting device. [Background technology]

[0002] BACKGROUND ART Conventionally, vertical cavity light emitting devices such as vertical cavity surface emitting lasers (VCSELs) have been known which have a structure in which light is resonated perpendicular to a substrate surface and emitted in a direction perpendicular to the substrate surface.

[0003] Vertical cavity light emitting devices require a higher carrier density in the active layer than light emitting diodes (LEDs) etc. Various developments have been made to increase the injection efficiency of vertical cavity light emitting devices and improve their 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 ratio continuously increases in the direction away from the active layer, in order to increase the light-emitting efficiency of the laser diode.

[0005] Furthermore, Patent Document 2 discloses an ultraviolet light emitting device made of a nitride semiconductor that utilizes the polarization doping effect of a compositionally graded layer in which the Al composition y decreases in the 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 device having a compositionally graded layer in which the Al composition decreases toward the side where the sum of spontaneous polarization and piezoelectric polarization becomes negative, in order to improve the efficiency of hole injection into the active layer. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent Publication No. 2021-184456 [Patent Document 2] Japanese Patent Publication No. 2022-041738 [Patent Document 3] Patent No. 6830098 [Patent Document 4] Patent No. 6192378 Summary of the Invention [Problem to be solved by the invention]

[0009] In conventional vertical cavity light emitting devices, further reduction of the threshold current and improvement of the light emitting efficiency and lifespan have been issues to be addressed.

[0010] An object of the present invention is to provide a vertical cavity light emitting device having a low threshold current density, high luminous efficiency and improved lifetime. [Means for solving the problem]

[0011] A vertical cavity light emitting device according to one embodiment of the present invention comprises: a first reflecting mirror; an n-type semiconductor layer formed on the first reflecting mirror; an active layer provided on the n-type semiconductor layer; an intermediate layer provided on the active layer; an electron barrier layer provided on the intermediate layer and containing Al; a p-type semiconductor layer doped with impurities and provided on the electron barrier layer; a second reflecting mirror provided on the p-type semiconductor layer, the electron barrier layer is a compositionally graded layer in which the Al composition decreases in a direction away from the active layer, The impurity concentration of the impurity has a peak in the electron barrier layer and decreases from the peak in the direction approaching the active layer. [Brief explanation of the drawings]

[0012] [Figure 1] 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. [Figure 2] FIG. 2 is a diagram schematically illustrating the band structure of the conduction band of a vertical-cavity surface-emitting laser. [Figure 3] 1 is an enlarged schematic diagram showing the band structure of an electron blocking layer (EBL) and the conduction band in its vicinity. [Figure 4] FIG. 1 shows SIMS analysis profiles of semiconductor layers of a vertical cavity surface emitting laser. [Figure 5] 1 is a band diagram showing a schematic band structure of a pn junction structure consisting of an n-type semiconductor, an active layer (light-emitting layer), and a p-type semiconductor. [Figure 6A] FIG. 1 is a diagram for conceptually explaining holes generated by piezoelectric polarization. [Figure 6B] FIG. 1 shows a band diagram of an electron blocking layer (EBL). [Figure 6C] 6C is a graph showing the simulation results of the generated hole concentration versus the distance z from the interface on the active layer side of the electron barrier layer shown in FIG. 6B. [Figure 7] 1 is a graph plotting the concentration (cm −3 ) of holes generated in the electron barrier layer against the average Al composition gradient CS (% / nm) of the electron barrier layer. [Figure 8A] FIG. 10 is a diagram showing another example of the Al composition profile of the electron barrier layer. [Figure 8B] FIG. 10 is a diagram showing another example of the Al composition profile of the electron barrier layer. [Figure 8C] FIG. 10 is a diagram showing another example of the Al composition profile of the electron barrier layer. [Figure 9] 1 is a diagram schematically showing standing waves of electric field intensity in semiconductor layers from an active layer to a dielectric DBR in a vertical-cavity surface-emitting laser of Example 1 (Ex. 1). FIG. [Figure 10] 10 is a graph showing device characteristics (optical output-current density characteristics) of the vertical cavity surface emitting laser of Example 2, and the vertical cavity surface emitting lasers of Comparative Examples 1 and 2. [Figure 11]FIG. 4 is a cross-sectional view schematically showing the structure of a vertical cavity surface emitting laser according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] In the following, preferred embodiments of the present invention will be described, but these may be modified and combined as appropriate. In the following description and accompanying drawings, substantially the same or equivalent parts are designated by the same reference numerals.

[0014] [First embodiment] 1 is a cross-sectional view schematically illustrating the structure of a vertical-cavity surface-emitting laser 10 according to a first embodiment of the present invention. In this embodiment, the vertical-cavity surface-emitting laser 10 is a nitride surface-emitting laser made of III-V nitride-based semiconductor layers.

[0015] The vertical cavity surface emitting laser 10 is formed by crystal growth on a substrate 11, in the order given, of a semiconductor DBR (Distributed Bragg Reflector) 12, an n-type semiconductor layer 13, an active layer 15 consisting of a multiple quantum well, an intermediate layer 16, an electron blocking layer (EBL) 17, and a p-type semiconductor layer 18.

[0016] The substrate 11 is a GaN substrate, and is a +C-plane GaN substrate tilted from the C-plane toward the M-plane by 0.5° and toward the A-plane by 0±0.1°.

[0017] Crystal growth of the semiconductor layer was performed by metalorganic vapor phase epitaxy (MOVPE). An underlying GaN layer 11B with a thickness of approximately 1 μm was grown on substrate 11, and a semiconductor DBR 12, which is a distributed Bragg reflector, was formed on underlying GaN layer 11B.

[0018] The semiconductor DBR 12 (first reflecting mirror) was formed by stacking 42 pairs of n-type GaN films and AlInN films. Each semiconductor film of the semiconductor DBR 12 had a thickness that was λ / 4n (n is the refractive index of each semiconductor film) of the emission wavelength λ of the active layer 15.

[0019] On the semiconductor DBR 12, an n-type semiconductor layer 13 (layer thickness: 350 nm) which was an n-type GaN layer doped with Si (silicon) was grown.

[0020] Barrier layers 15B and quantum well layers 15W were alternately formed on the n-type semiconductor layer 13 to form an active layer 15 having four quantum well layers 15W. The barrier layers 15B were made of GaInN (thickness: 3 nm), and the well layers 15W were made of GaN (thickness: 4 nm). The compositions and thicknesses of the barrier layers 15B and well layers 15W can be selected appropriately depending on the desired emission wavelength, emission characteristics, etc.

[0021] An intermediate layer (LB) 16, which is the final barrier layer, is grown to a thickness of 120 nm by undoped GaN on the final well layer 15WL, which is the final layer of the active layer 15. That is, the intermediate layer 16 is a layer between the active layer 15 and the electron barrier layer (EBL) 17.

[0022] Next, we investigated Al doped with Mg (magnesium) as an impurity. x Ga 1-x An electron barrier layer (EBL) 17 made of N (Al composition: x) was grown.

[0023] Subsequently, an Mg-doped p-GaN layer was grown to a thickness of 83 nm as a p-type semiconductor layer 18 on the electron barrier layer 17 (p-AlGaN).

[0024] The wafer grown as described above was etched at its outer periphery so as to reach the inside of the n-type semiconductor layer 13, thereby forming a cylindrical mesa structure.

[0025] The p-type semiconductor layer 18, which is the uppermost semiconductor layer of the mesa structure, was dry-etched to a depth of approximately 20 nm at its outer periphery to form a recess, thereby forming the p-type semiconductor layer 18 having a cylindrical mesa protrusion.

[0026] An insulating film (SiO2) 21 for lateral current and light confinement was deposited to a thickness of 20 nm in the recess of the p-type semiconductor layer 18 formed by etching. This flattened the recess of the p-type semiconductor layer 18, formed a current confinement structure, and formed a cylindrical (center axis: CX) current injection region.

[0027] Next, an ITO (indium tin oxide) film was formed as a transparent conductive film 22 on the p-type semiconductor layer 18 and the insulating film 21 to a thickness of 20 nm.

[0028] Subsequently, a dielectric (Nb2O5) film was formed to a thickness of 38 nm as the spacer layer 24. The spacer layer 24 functions as a phase adjustment layer.

[0029] Furthermore, a dielectric DBR 25 (second reflecting mirror) was deposited on the spacer layer 24. The dielectric DBR 25 consisted of 10.5 pairs of SiO2 (11 layers) and Nb2O5 (10 layers). Note that the dielectric DBR 25 is preferably formed so as to be coaxial with the cylindrical mesa of the p-type semiconductor layer 18.

[0030] Next, an n-electrode 27 was formed on the recess in the outer periphery of the n-type semiconductor layer 13, and a p-electrode 28 was formed on the transparent conductive film 22. The back surface of the substrate 11 was polished, and an AR (anti-reflection) coating 29 (anti-reflection film) consisting of two layers of Nb2O5 / SiO2 was formed. This completed the formation of the vertical cavity surface emitting laser 10.

[0031] The composition and thickness of the intermediate layer (LB) 16 described above are merely examples. Although the intermediate layer 16 has been described as a GaN layer, nitride semiconductor layers of other compositions, such as InGaN, AlGaN, or InAlGaN, may also be used. Furthermore, although the intermediate layer 16 is described as an undoped layer, it may contain dopants diffused from the electron barrier layer 17 or the p-type semiconductor layer 18.

[0032] The thickness of the electron barrier layer 17 is merely an example. The electron barrier layer 17 may have a thickness of, for example, 3 to 30 nm. The electron barrier layer 17 is made of a nitride semiconductor containing Al in its composition. The electron barrier layer 17 is formed as a compositionally graded layer in which the Al composition changes in the thickness direction. Details will be described later.

[0033] Although the electron barrier layer 17 has been described as a p-type semiconductor layer (p-AlGaN), it may be a p-type semiconductor layer containing Al in its composition, grown as an i-layer and mixed with impurities (Mg) diffused from the p-type semiconductor layer 18.

[0034] Although the active layer 15 is a quantum well active layer having four quantum well layers 15W, it is sufficient that the active layer 15 has at least one quantum well layer. The active layer 15 is not limited to a quantum well active layer. An active layer having a so-called bulk structure may also be used.

[0035] Furthermore, p-type semiconductor layer 18 may be composed of a plurality of semiconductor layers including layers with different compositions and / or doping concentrations and an undoped layer. Similarly, n-semiconductor layer 13 may be composed of a plurality of semiconductor layers.

[0036] Although the dielectric DBR 25 has been illustrated as being made of an SiO2 film and an Nb2O5 film, it may be made of other combinations of dielectric films with different refractive indices, or may be made of a semiconductor DBR made of semiconductor films with different refractive indices.

[0037] 2 is a diagram showing a schematic diagram of the band structure of the conduction band of vertical cavity surface emitting laser 10. The band structure from active layer 15 to p-type semiconductor layer 18 is shown.

[0038] The active layer 15 is made up of four well layers 15W, QW1 to QW4, and barrier layers 15B provided between them. The quantum well layer QW4 adjacent to the intermediate layer (LB) 16 is the final well layer 15WL.

[0039] The intermediate layer 16 has a thickness t1, the electron barrier layer 17 has a thickness t2, and the p-type semiconductor layer 18 has a thickness t3.

[0040] [Al composition of electron barrier layer] 3 is an enlarged schematic diagram showing the band structure of the electron barrier layer 17 (EBL) and the conduction band in the vicinity thereof. As in FIG. 2, the left side of the diagram is the active layer 15 (ACT) side.

[0041] 3, the electron barrier 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 barrier layer 17 decreases linearly from x1 (%) to x2 (%).

[0042] In this specification, the "composition gradient" of the Al composition (x) is not limited to a linear or monotonically decreasing Al composition. It is sufficient that the Al composition (x1) is highest at the interface on the active layer 15 side (the interface with the intermediate layer 16) and the Al composition (x2) is lowest at the interface with the p-type semiconductor layer 18. Furthermore, a portion where the composition (x) is constant from the interface on the active layer 15 side toward the interface with the p-type semiconductor layer 18 may be included.

[0043] 3 also shows a schematic concentration profile (broken line) of Mg, a p-dopant. As shown in the Mg concentration profile, the Mg concentration decreases from the interface between the intermediate layer 16 and the electron barrier layer 17 toward the active layer 15, and reaches a peak near the end of the electron barrier layer 17 (the interface with the p-type semiconductor layer 18). Note that the vicinity of the interface with the p-type semiconductor layer 18 here refers to a position at least closer to the p-type semiconductor layer 18 than the center of the electron barrier layer 17.

[0044] The Mg concentration profile also exhibits a bottom near the interface between the electron barrier layer 17 and the p-type semiconductor layer 18, i.e., at a position corresponding to the interface and closer to the p-type semiconductor layer 18 than the above-mentioned Mg concentration peak. That is, the Mg concentration profile has a dip at the interface, and the bottom of the dip is formed near the interface.

[0045] Such a Mg concentration profile is due to the presence of the electron barrier layer 17 (AlGaN layer) between the intermediate layer 16 (GaN layer) and the p-type semiconductor layer 18 (p-GaN layer).

[0046] 4 shows a profile analyzed by SIMS (Secondary Ion Mass Spectrometry) of the semiconductor layer of the vertical-cavity surface-emitting laser 10. The analysis was performed starting from the surface of the p-type semiconductor layer 18, and the right direction in the figure is the depth direction, which is the direction from the p-type semiconductor layer 18 to the active layer 15. That is, it should be noted that the positional relationship between the active layer 15 and the electron barrier layer 17 on the horizontal axis is reversed from that in FIGS. 2 and 3. That is, in FIG. 4, the direction away from the active layer 15 (ACT) is the left direction in the figure.

[0047] As shown in FIG. 4, in the electron barrier layer 17, the Al composition of the electron barrier layer 17 has a peak near the electron barrier layer 17 and decreases in the direction away from the active layer 15.

[0048] On the other hand, the Mg concentration decreases in the electron barrier layer 17 in the direction approaching the active layer 15. That is, the Mg concentration profile has a concentration gradient in the opposite direction to the Al composition gradient.

[0049] That is, the electron barrier layer 17 has a composition gradient in which the Al composition is high on the active layer 15 side and low on the p-type semiconductor layer 18 side, and the Mg concentration in the electron barrier layer 17 decreases in the direction toward the active layer 15.

[0050] Even if the impurity (Mg) diffuses from the p-type semiconductor layer 18 to the electron barrier layer 17, the Mg concentration in the electron barrier layer 17 exhibits a profile similar to that described above. That is, the Mg concentration exhibits the bottom and decreases in the direction approaching the active layer 15.

[0051] [Effective barrier of electron barrier layer, Mg concentration, hole generation due to piezoelectric polarization] The relationship between the effective barrier of the electron barrier layer and the concentration of the p-dopant Mg, and hole generation due to piezoelectric polarization will be explained below.

[0052] Conventionally, increasing the Mg concentration in the electron barrier layer has been used to increase the carrier injection efficiency and improve the efficiency of laser diodes (LDs) and vertical-cavity surface-emitting lasers (VCSELs).

[0053] However, to extend the lifetime of vertical-cavity surface-emitting lasers, it was necessary to reduce the steepness of the Mg concentration gradient and to lower the Mg concentration on the active layer side.However, reducing the Mg concentration on the active layer side posed the problem of lowering the external quantum efficiency.

[0054] In particular, in semiconductor lasers such as VCSELs, the driving current density and electron / hole concentration in the light-emitting layer are about two orders of magnitude higher than in LEDs, so reducing the Mg concentration in the electron barrier layer reduces the effective barrier energy, causing high-energy electrons to leak to the p-layer side and reducing injection efficiency.

[0055] FIG. 5 is a band diagram that schematically shows the band structure of a pn junction structure consisting of an n-type semiconductor, an active layer (light-emitting layer), and a p-type semiconductor.

[0056] The effective barrier height B determines the efficiency of carrier injection into the active layer. eff is the voltage applied to the pn junction, V A , Fermi level E F , the band gap is E g Then, B eff =E g -V A -E F It is expressed as:

[0057] Fermi level E F changes depending on the Mg concentration. Specifically, when the Mg concentration (acceptor concentration) is decreased, the Fermi level E F rises, and the effective barrier height B eff The effective barrier height B effIf the barrier effect of the electron barrier layer decreases, the electron injection efficiency decreases.

[0058] Fig. 6A is a diagram for conceptually explaining holes generated by piezoelectric polarization. Fig. 6B shows a band diagram of the electron barrier layer 17. Fig. 6C is a graph showing the simulation results of the hole concentration versus the distance z (distance in the stacking direction) from the interface on the active layer 15 side of the electron barrier layer 17 shown in Fig. 6B.

[0059] Piezoelectric polarization occurs in a crystal system with an asymmetric crystal structure (for example, a hexagonal crystal). The following simulation was performed for a case where an electron barrier layer made of AlGaN was formed on the +C plane of a GaN substrate.

[0060] As shown in Figure 6A, polarization charges are generated in the Al-gradient electron barrier layer 17 (AlGaN) due to piezoelectric polarization (left diagram). The larger the Al composition, the larger the polarization. These polarization charges generate holes in the electron barrier layer 17 (right diagram).

[0061] 6B, electron barrier layer 17 has an Al composition of 50% on the active layer 15 side and 15% on the p-GaN layer 18 side. Electron barrier layer 17 has a thickness of 10 nm.

[0062] As shown in FIG. 6C, the electron barrier layer 17 has a concentration of 1.9×10 19 cm -3 ~2.0×10 19 cm -3 It can be seen that holes with a concentration of

[0063] In other words, a high hole concentration can be obtained even with a reduced Mg doping amount, and the effective barrier height can be secured without increasing the Fermi level. In other words, high injection efficiency can be maintained while reducing the Mg doping concentration.

[0064] Referring again to the SIMS data in FIG. 4, the Mg concentration is at the bottom (Mg concentration: about 3×10 18 cm -3 )

[0065] By forming a bottom of the Mg concentration between the electron barrier layer 17 and the p-type semiconductor layer 18 (p-GaN layer), the total amount of Mg concentration can be reduced, and the device life can be extended.

[0066] From the viewpoint of device life, the Mg concentration at the interface between the electron barrier layer 17 and the intermediate layer 16 is 1.0×10 19 cm -3 Preferably, it is less than 5.0 × 10 18 cm -3 It is preferable that it is less than 10 ...

[0067] Furthermore, a hydrogen (H) peak is formed at a position corresponding to the bottom of the Mg concentration, as shown in Figure 4. In other words, hydrogen is trapped by the portion having the bottom of the Mg concentration, forming the hydrogen peak.

[0068] Therefore, hydrogen is prevented from entering the active layer 15, and the optical output characteristics such as the voltage-current characteristics of the vertical cavity surface emitting laser 10 are stabilized.

[0069] [Al composition profile of electron barrier layer] FIG. 7 shows the hole concentration (cm -3 ) is plotted against the average Al composition gradient CS (% / nm) of the electron barrier layer 17.

[0070] The generated hole concentration is 5.0×10 18 cm -3 Since this or more is necessary to maintain the injection efficiency of the vertical cavity surface emitting laser 10, it is clear from the graph of FIG. 7 that the average composition gradient CS is preferably 0.9 (% / nm) or more.

[0071] In addition, except when the Al composition is linearly graded, the average composition gradient CS (ie, CS=(x1-x2) / t2) is preferably 0.9 (% / nm) or more.

[0072] The Al composition profile of the electron barrier layer 17 is not limited to the linear composition gradient described above. The Al composition of the electron barrier layer 17 may be highest on the active layer 15 side of the electron barrier layer 17 and lowest on the p-type semiconductor layer 18 side.

[0073] On the other hand, if the product (nm) of the average Al composition (%) and the layer thickness of the electron barrier layer 17 exceeds a certain level, stress-induced breakdown occurs in the electron barrier layer 17, resulting in cracks. Specifically, cracks may occur if the product of the average Al composition (%) and the layer thickness t2 (nm) exceeds 700 (% nm). For example, cracks may occur if the average Al composition of the electron barrier layer 17 is 70% and the layer thickness t2 is 10 nm. Therefore, it is sufficient that the product of the average Al composition (%) and the layer thickness t2 (nm) of the electron barrier layer 17 is less than 700 (% nm).

[0074] For example, as shown in FIG. 8A, the electron barrier layer 17 may have an Al composition profile in which the Al composition decreases stepwise from the active layer 15 side to the p-type semiconductor layer 18 side.

[0075] Alternatively, as shown in FIG. 8B , the electron barrier layer 17 may have an Al composition profile consisting of a gradient composition portion in which the Al composition decreases from the active layer 15 side, and a constant composition portion in which the Al composition is constant from the gradient composition portion to the p-type semiconductor layer 18.

[0076] Alternatively, as shown in FIG. 8C , the electron barrier layer 17 may have an Al composition profile consisting of a constant composition portion where the Al composition is constant from the active layer 15 side and a gradient composition portion where the Al composition decreases from the constant composition portion to the p-type semiconductor layer 18.

[0077] Alternatively, the electron barrier layer 17 may have an Al composition profile that is a combination of these.

[0078] [Improving the efficiency of carrier injection into multiple quantum well active layers] As described above in detail, the vertical cavity surface emitting laser 10 formed to satisfy the above-mentioned Al composition profile and impurity (Mg) concentration achieves the improvement of carrier injection efficiency and reliability (device life), which are the objectives of the present application.

[0079] Furthermore, when a multiple quantum well (MQW) structure consisting of multiple quantum well layers is used for the active layer, higher carrier injection efficiency and longer life can be achieved by further satisfying the following conditions.

[0080] First, the first interface between a highly reflective mirror such as a DBR (Distributed Bragg Reflector) or a diffraction grating and the inside of the resonator is defined as phase 0″ (reference).

[0081] Refractive index n of each layer in the resonator i and layer thickness t i , and the emission wavelength λ, the total phase information of the standing waves in the resonator is expressed by the following equation (1):

[0082]

number

[0083] The synthesized information of the standing wave at a predetermined position inside the layer is calculated by dividing the thickness from the interface with other layers on the resonator side to the predetermined position by t i Let's think about it in this way.

[0084] The first condition is the number N of antinodes AN of the standing wave SW included in the p-type semiconductor layer 18 and the electron barrier layer 17, which are p-regions. AN and the number N of clauses ND ND are 0 or 1 (N AN =0 or N AN =1, N ND =0 or N ND =1).

[0085] The thickness of each of the transparent conductive film 22 (ITO), the p-type semiconductor layer 18, and the electron barrier layer 17 is HITO ,H GaN ,H EB , each refractive index is n ITO ,n GaN ,n EB and the wavelength is λ, it is preferable that the following formula (2) is satisfied.

[0086]

number

[0087] Furthermore, the second condition is that the number of nodes ND and antinodes AN of the standing wave SW contained in the intermediate layer 16 is 1 or more (N ND ≧1 and N AN ≧1).

[0088] The number of nodes ND and antinodes AN of the standing wave SW is N ND , N AN can be obtained by calculating the total phase in the p region and the intermediate layer 16 using the above formula (1), and then finding the number of times the phase is kπ (k=1, 2,...) in the p region and the intermediate layer 16 and the number of times the phase is (2l-1)π / 2 (l=1, 2,...).

[0089] The equation (1) = k / 2 (k = 1, 2, ) indicates the position where the standing wave has an antinode AN, and the equation (1) = (2l-1) / 4 (l = 1, 2, ) indicates the position where the standing wave has a node ND. ND , N AN is obtained by counting the number of positions that become the above antinodes AN and nodes ND included within the stacking range of each layer.

[0090] The thicknesses of the intermediate layer 16 and the active layer 15 are H fb ,H qw When n is set as the number of saturations, it is preferable to satisfy the following formula (3): fb is the refractive index of the intermediate layer 16, n qw is the equivalent refractive index of the active layer 15.

[0091]

number

[0092] Furthermore, (i) it is preferable that the intermediate layer 16 has a thickness of λ / 4 or more, that is, it is preferable that the following formula (4) is satisfied.

[0093]

number

[0094] Also, (ii) the number of nodes ND included in the intermediate layer 16 is two or more (N ND ≥ 2), the number of abdominal ANs is 1 or more (N AN ≧1). When (i) and (ii) are satisfied, it is preferable that the following formula (5) be satisfied.

[0095]

number

[0096] Furthermore, it is preferable that the thickness of the active layer 15 is λ / 8 or less, that is, that the following formula (6) is satisfied: In this case, the optical confinement loss of the active layer 15 can be reduced.

[0097]

number

[0098] The above-described configuration strengthens the electric field strength of light from the p-type semiconductor layer 18 to the electron barrier layer 17. Furthermore, the presence of an antinode AN where the electric field strength of light is high and at least one node ND in the intermediate layer 16 excites the active layer 15 to an extent that a large optical gain is obtained.

[0099] Furthermore, when the internal light intensity is increased by the semiconductor DBR 12 and the dielectric DBR 25, which are 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 barrier layer 17 and the p-type semiconductor layer 18 are extracted by the active layer 15, resulting in switching-like hole injection into the active layer 15.

[0100] As a result, the uniformity of carriers (electrons and holes) in each of the multiple well layers 15W of the active layer 15 is improved, and an efficient surface-emitting laser can be realized.

[0101] [Consideration of the mechanism of characteristic improvement] The above-described configuration improves the uniformity of carriers (electrons and holes) in each well layer 15W of the active layer 15, thereby realizing an efficient surface-emitting laser. The mechanism by which such improvements in efficiency and other characteristics can be achieved will now be considered.

[0102] The mechanism for improving the characteristics is thought to be related to the switching injection of holes to eliminate carrier non-uniformity in the multiple quantum wells. This surface-emitting laser is designed so that the antinode of the standing wave is aligned with the center of the active layer (multiple quantum wells). In other words, by placing the active layer where the electric field of light is large, the interaction between light and electron-hole recombination is increased.

[0103] Therefore, the electric field intensity of light tends to decrease in the intermediate layer 16 adjacent to the active layer 15. However, by arranging the intermediate layer 16 so that an antinode of a standing wave different from that of the active layer is located or so that it extends to the vicinity of the antinode, as described above, it is presumed that the optical intensity of this layer increases significantly near the threshold of the laser oscillation current, generating carriers (electrons and holes), which has the effect of instantly drawing in holes on the p-type semiconductor layer 18 side of the electron barrier layer 17. This drawing is likely to occur when the electric field gradient in the p region is high.

[0104] This effect is believed to be due to the increase in the concentration of holes accumulated on the p-type semiconductor layer 18 side and the increase in the electric field gradient of the electron barrier layer 17. This switching-like hole attraction is believed to improve the uniformity of the carrier distribution in the multiple quantum well and reduce internal loss. Therefore, in some cases, the differential resistance and drive current reach their minimum values ​​(dR / dI=0, d 2 V / dI 2 In some cases, characteristics not seen in ordinary vertical cavity surface emitting lasers (VCSELs), such as (=0), can be obtained. Therefore, the important factors are the position of the antinode of the intermediate layer 16, thinning of the p-layer (electron barrier layer 17 and p-type semiconductor layer 18), and optical intensity that provides strong optical feedback and optical gain in the vertical direction due to the high-reflection mirror.

[0105] [Example 1] FIG. 9 is a diagram showing a standing wave SW of the electric field intensity of light emitted from the active layer 15 in the semiconductor layers from the active layer 15 to the dielectric DBR 25 in the vertical cavity surface emitting laser 10 of Example 1 (Ex. 1) of this embodiment.

[0106] As described above, the vertical cavity surface emitting laser 10 of Example 1 (Ex. 1) is provided with an undoped GaN layer having a thickness of 120 nm as the intermediate layer (LB) 16.

[0107] The electron barrier layer 17 has a thickness of 10 nm and has a composition gradient in which the Al composition decreases from 50% to 15% in the direction away from the active layer 15.

[0108] Furthermore, on the electron barrier layer 17, a p-GaN layer having a thickness of 83 nm is provided as a p-type semiconductor layer 18.

[0109] In Example 1, the electron barrier layer 17 contains 1.0×10 18 ~1.0×10 19 cm 3 The p-type semiconductor layer 18 is doped with Mg at a doping concentration of 1.0×10 18 cm 3Mg is doped at the above doping concentrations. However, the doping concentrations are not limited to these and can be set appropriately depending on the compositions, thicknesses, etc. of the p-side semiconductor layers such as the intermediate layer 16, the electron barrier layer 17, and the p-type semiconductor layer 18.

[0110] In Example 1 (Ex. 1), there is one node ND and one antinode AN of the standing wave SW in the intermediate layer 16. That is, the number of nodes ND is N ND , the number of anteroposterior regions is N AN Then, N ND =1,N AN = 1. At the high-reflection mirror interface A of the dielectric DBR 25, the position is at the antinode AN of the standing wave SW, as shown in FIG.

[0111] In addition, the electron barrier layer (EBL) 17 and the p-type semiconductor layer (p-GaN) 18, which are p-regions, each have one node ND and one antinode AN of the standing wave SW (N ND =1,N AN Furthermore, one node ND of the standing wave SW exists in the transparent conductive film (ITO) 22 (N ND =1) is preferred.

[0112] This configuration strengthens the electric field strength from the p-type semiconductor layer 18 to the electron barrier layer 17. In addition, the presence of an antinode with high optical field strength and at least one node in the intermediate layer 16 excites the active layer 15 to an extent that a large optical gain is obtained.

[0113] Furthermore, when the internal light intensity is increased by the semiconductor DBR 12 and the dielectric DBR 25, which are 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 barrier layer 17 and the p-type semiconductor layer 18 are extracted by the active layer 15, resulting in switching-like hole injection into the active layer 15.

[0114] This promotes uniformity of carrier concentration among the multiple quantum wells, resulting in improvements in internal loss, external differential quantum efficiency, and slope efficiency.

[0115] That is, the uniformity of carriers (electrons and holes) in the multiple well layers 15W is improved, and an efficient surface-emitting laser can be realized.

[0116] It is also possible to provide a vertical cavity light emitting device that can reduce the operating current and has improved reliability (lifespan) of the device.

[0117] [Element characteristics: Effect of reducing Mg concentration] The vertical cavity surface emitting laser 10 of Example 2 of this embodiment and the vertical cavity surface emitting lasers of Comparative Examples 1 and 2 were fabricated and the device characteristics were evaluated. The evaluation results will be described below.

[0118] [Example 2] FIG. 10 is a graph showing device characteristics (light output vs. current density characteristics) of the vertical cavity surface emitting laser 10 of Example 2 (Ex.2), the vertical cavity surface emitting lasers of Comparative Example 1 (Cmp.1) and Comparative Example 2 (Cmp.2).

[0119] Example 2 (Ex.2) is a vertical-cavity surface-emitting laser 10 having an electron barrier layer 17 with a layer thickness of 10 nm and a composition gradient in which the Al composition decreases from 50% to 15% in the direction away from the active layer 15 (see FIG. 6B).

[0120] The doping concentration of Mg in the electron barrier layer 17 is 0.25×10 19 cm -3 This is one-fourth the concentration of Comparative Example 1 below.

[0121] The vertical cavity surface emitting laser of Comparative Example 1 (Cmp. 1) has an electron barrier layer with the same layer thickness (10 nm) as that of Example 2. Comparative Example 1 differs from Example 2 in that it has an electron barrier layer with a constant Al composition (Al composition: 30%). In addition, the Mg doping concentration is 1.0 × 10 19 cm -3 is.

[0122] The vertical cavity surface emitting laser of Comparative Example 2 (Cmp. 2) has a Mg doping concentration that is half that of Comparative Example 1 (Cmp. 1) (0.5 × 10 19 cm -3 ) which is different from Comparative Example 1. In other respects, it is the same as the vertical-cavity surface-emitting laser of Comparative Example 1.

[0123] The optical output characteristics of the vertical cavity surface emitting laser of Comparative Example 1 (Cmp. 1) having a conventional electron barrier layer are shown by the broken line. In Comparative Example 2, the Mg doping concentration of the electron barrier layer was reduced to half that of Comparative Example 1, but it was confirmed that the optical output was significantly reduced compared to Comparative Example 1.

[0124] On the other hand, in Example 2 (Ex. 2) having an Al composition gradient electron barrier layer, it was confirmed that light output characteristics exceeding those of Comparative Example 1 were obtained even when the Mg doping concentration was reduced to 1 / 4, and high luminous efficiency was obtained.

[0125] That is, it was confirmed that the vertical cavity surface emitting laser 10 of Example 2 can significantly reduce the Mg doping concentration and obtain high luminous efficiency.

[0126] Furthermore, the electron barrier layer 17 having an Al composition gradient can reduce the Mg doping concentration while maintaining high injection efficiency.

[0127] Therefore, it is possible to provide a vertical cavity light emitting device that can reduce the operating current and has improved reliability (lifespan) of the device.

[0128] [Example 3] In the vertical cavity surface emitting laser 10 of Example 3 (Ex. 3), the thickness of the intermediate layer 16 is 220 nm, which is thicker than the 120 nm of Example 1.

[0129] The thickness of the p-region (total thickness of the electron barrier layer 17 and the p-type semiconductor layer 18) is 101 nm (20+81 nm), which is thicker than the 93 nm (10+83 nm) in the first embodiment.

[0130] The electron barrier layer 17 has a thickness of 20 nm and has a composition gradient in which the Al composition decreases from 50% to 15% in the direction away from the active layer 15.

[0131] The configuration other than the above is the same as in the second embodiment.

[0132] In Example 3, reflecting the fact that the intermediate layer 16 is thick, there are two nodes ND and two antinodes AN in the intermediate layer 16 (N ND =2,N AN = 2). In addition, the p-region (electron barrier layer 17 and p-type semiconductor layer 18) has one node ND and one antinode AN (N ND =1,N AN =1).

[0133] Therefore, since the first and second conditions are satisfied, it is possible to eliminate the decrease in efficiency of the surface-emitting laser caused by carrier non-uniformity between the well layers of the quantum well active layer, and to provide a highly efficient surface-emitting laser.

[0134] As in this embodiment, the intermediate layer 16 can be thickened so as to satisfy the first and second conditions, and therefore deterioration of the device characteristics and life span due to migration of dopants (Mg, etc.) can be suppressed.

[0135] Furthermore, the electron barrier layer 17 having an Al composition gradient can reduce the Mg doping concentration while maintaining high injection efficiency.

[0136] Therefore, it is possible to provide a vertical cavity light emitting device that can reduce the operating current and has improved reliability (lifetime) of the device.

[0137] [Second embodiment] 11 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 as a current confinement structure.

[0138] The vertical cavity surface emitting laser 50 is a nitride surface emitting laser having a configuration similar to that of the vertical cavity surface emitting laser 10 of the first embodiment described above, except that it has a tunnel junction layer 31 .

[0139] That is, the electron barrier layer 17 has a composition gradient in which the Al composition on the active layer 15 side is higher than that on the p-type semiconductor layer 18 side, and the Mg concentration in the electron barrier layer 17 has a concentration profile in which it decreases in the direction toward the active layer 15.

[0140] More specifically, vertical cavity surface emitting laser 50 is formed by successively growing crystals of semiconductor DBR 12, n-type semiconductor layer 13, active layer 15 consisting of a multiple quantum well, final barrier layer 16, electron blocking layer (EBL) 17, and p-type semiconductor layer 18 in this order on substrate 11. The composition, thickness, impurity concentration, etc. of each semiconductor layer are also the same as those of vertical cavity surface emitting laser 10 of the first embodiment.

[0141] In the vertical cavity surface emitting laser 50, a p-type semiconductor layer 18 is formed on the p-type semiconductor layer 18. + The p-type semiconductor layer 31A is made of GaN with a high impurity concentration, and the n-type semiconductor layer 31B is made of GaN with a high impurity concentration. + A tunnel junction layer 31 is provided, which is made of an n-type semiconductor layer 31B made of -GaN and having a high impurity concentration.

[0142] 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 of 4 μm (center axis CX). 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 25 nm, for example.

[0143] Furthermore, the tunnel junction layer 31 is buried with an n-type semiconductor layer 32 (second n-type semiconductor layer) made of n-GaN.

[0144] A semiconductor DBR 35 (second DBR) made of n-AlInN and n-GaN is provided on the n-type semiconductor layer 32. The semiconductor DBR 35 is made of, for example, 46 pairs of n-AlInN / GaN.

[0145] The outer periphery of each semiconductor layer and the semiconductor DBR 35 is etched so as to reach the inside of 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.

[0146] An n-electrode 27 is formed on the surface of the outer periphery of the n-type semiconductor layer 13. In addition, a p-electrode 36 having a diameter larger than that of the tunnel junction layer 31 and a circular opening coaxial with the central axis CX in top view (when viewed from a direction perpendicular to the semiconductor DBR 35) is formed on the semiconductor DBR 35.

[0147] On the back surface of the vertical cavity surface emitting laser 50, an AR (anti-reflection) coating 29 consisting of two layers of Nb2O5 / SiO2 is formed.

[0148] The tunnel junction layer 31 is p + -GaN layer and n + Although the semiconductor layer is made of GaN, other semiconductor layers such as GaInN may also be used. + The GaN layer can be doped with Mg, for example. The doping concentration of Mg is 4×10 19 cm 3 More than this is preferable. + -GaN layer or n + - 1 × 10 for GaInN layer 18 cm 3 Higher doping levels than this are preferred.

[0149] (Correspondence with Examples 1 to 3) The first and second conditions in the vertical cavity surface emitting laser 50 can be considered to be similar to those in the vertical cavity surface emitting laser 10 of the first embodiment.

[0150] 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). An antinode of the standing wave is located at this reference position.

[0151] Moreover, the n-type semiconductor layer 32 (second n-type semiconductor layer) corresponds to the spacer layer 24 in the above example. 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.

[0152] Furthermore, the tunnel junction layer 31 corresponds to the transparent conductive film (ITO) 22 in the first embodiment, and one node ND of the standing wave SW exists in the tunnel junction layer 31 (N ND =1) is preferred.

[0153] Furthermore, the condition (first condition) for the number of nodes ND and antinodes AN of the standing wave SW contained in the electron barrier layer 17 and the p-type semiconductor layer 18, which are p regions, and the condition (second condition) for the number of nodes ND and antinodes AN contained in the final barrier layer 16 are the same as those in the first embodiment.

[0154] According to the vertical cavity surface emitting laser 50 of the second embodiment, high injection efficiency can be maintained even when the doping concentration of impurities (dopants) is reduced, and therefore, it is possible to provide a vertical cavity light emitting device that can reduce the operating current and has improved reliability (lifetime) of the device.

[0155] As described above in detail, according to the present invention, it is possible to provide a vertical cavity light emitting device having a low threshold current density, high luminous efficiency, and an improved lifespan. [Explanation of symbols]

[0156] 10,50: Vertical cavity surface emitting laser 11: Circuit board 12: DBR (reflecting mirror) 13: n-type semiconductor layer 15:Active layer 15B: Barrier layer 15W: Quantum well layer (well layer) 16: Middle layer (LB) 17: Electron barrier layer 18: p-type semiconductor layer 21: insulating film 22: Transparent conductive film 24: Spacer layer 25: DBR (reflecting mirror) 31: Tunnel junction layer 31A: Highly impurity-concentrated p-type semiconductor layer 31B: Highly impurity-concentrated n-type semiconductor layer 32: n-type semiconductor layer (second n-type semiconductor layer) 35: Semiconductor DBR

Claims

1. a first reflecting mirror; an n-type semiconductor layer formed on the first reflecting mirror; an active layer provided on the n-type semiconductor layer; an intermediate layer provided on the active layer; an electron barrier layer provided on the intermediate layer and containing Al; a p-type semiconductor layer doped with impurities and provided on the electron barrier layer; a second reflecting mirror provided on the p-type semiconductor layer, the electron barrier layer is a compositionally graded layer having a composition gradient in which the Al composition decreases in a direction away from the active layer, a vertical cavity light emitting device made of a III-V nitride semiconductor, wherein the impurity concentration has a peak near the interface between the electron barrier layer and the p-type semiconductor layer and decreases in a direction approaching the active layer.

2. Holes are generated in the electron barrier layer by piezoelectric polarization, and the concentration of holes generated by the piezoelectric polarization is 5.0×10 １８ atoms / cm ３ 2. The vertical cavity light emitting device according to claim 1, wherein:

3. 2. The vertical cavity light emitting device according to claim 1, wherein the average composition gradient of the Al composition of the electron barrier layer is 0.9% / nm or more.

4. 2. The vertical cavity light emitting device according to claim 1, wherein the impurity concentration has a bottom at a position corresponding to the interface between the p-type semiconductor layer and the electron barrier layer.

5. The impurity concentration of the impurity at the interface between the electron barrier layer and the intermediate layer is 1.0×10 １９ cm －３ 10. The vertical cavity light emitting device of claim 1, wherein the .lambda.

6. The impurity concentration at the interface between the electron barrier layer and the intermediate layer is 5.0×10 １８ cm －３ 10. The vertical cavity light emitting device of claim 1, wherein the .lambda.

7. 2. The vertical cavity light emitting device according to claim 1, wherein the intermediate layer is an undoped layer.

8. a first reflecting mirror; an n-type semiconductor layer provided on the first reflecting mirror; an active layer formed of a multiple quantum well on the n-type semiconductor layer; an intermediate layer disposed on the final quantum well of the active layer; an electron barrier layer provided on the intermediate layer and containing Al; a p-type semiconductor layer doped with impurities and provided on the electron barrier layer; a spacer layer provided on the p-type semiconductor layer; a second reflecting mirror provided on the spacer layer, the electron barrier layer is a compositionally graded layer having a composition gradient in which the Al composition decreases in a direction away from the active layer, the impurity concentration of the impurity has a peak near the interface between the electron barrier layer and the p-type semiconductor layer and decreases in a direction approaching the active layer, the number of antinodes and the number of nodes of a standing wave caused by light emission from the active layer, which are contained in the electron barrier layer and the p-type semiconductor layer, are 0 or 1, respectively; The thickness of the intermediate layer and the active layer is H ｆｂ , H ｑｗ , the refractive index of the intermediate layer is n ｆｂ , the equivalent refractive index of the active layer is n ｑｗ When the above formula is satisfied, the active layer and the intermediate layer are [Equation 1] Vertical cavity light emitting device that satisfies the above requirements.

9. 9. The vertical cavity light emitting device according to claim 8, wherein the number of nodes and the number of antinodes included in the intermediate layer are two or more and one or more, respectively.

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