Vertical resonator type light-emitting element

Abstract

To provide a vertical resonator type light-emitting element of a low threshold current and high emission efficiency.SOLUTION: A vertical resonator type light-emitting element comprises: an n-type semiconductor layer formed on a first reflection mirror; an active layer consisting of multiple quantum wells formed on the n-type semiconductor layer; a final barrier layer formed on a final quantum well of the active layer; an electron barrier layer formed on the final barrier layer; a p-type semiconductor layer formed on the electron barrier layer; a dielectric spacer layer formed on the p-type semiconductor layer; and a second reflection mirror formed on the spacer layer. The number of bellies of standing wave, included in the electron barrier layer and the p-type semiconductor layer, by light emission from the active layer is 1, the number of nodes is 0 or 1, and the active layer and the final barrier layer satisfy an expression (3).SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a vertical cavity surface emitting laser (VCSEL), particularly a vertical cavity surface emitting laser having a multiple quantum well active layer.

Background Art

[0002] Conventionally, vertical cavity surface emitting lasers (VCSELs) and other vertical cavity surface emitting devices having a structure that resonates light perpendicular to the substrate surface and emits light in a direction perpendicular to the substrate surface are known.

[0003] In vertical cavity surface emitting devices, a multiple quantum well (MQW) structure is generally adopted in the active layer in order to obtain low threshold current and high-efficiency light emission characteristics.

[0004] For example, Patent Document 1 describes an edge-emitting nitride semiconductor laser device having a structure aimed at reducing the electron and hole concentrations in the p-side optical guide layer between the final quantum well layer and the electron barrier layer to improve the internal quantum efficiency.

[0005] Patent Document 2 describes a surface-emitting semiconductor laser provided with first to fourth semiconductor multilayer mirrors and having an adjusted Al composition or impurity concentration in the semiconductor multilayer mirrors in order to promote lateral carrier diffusion at low temperatures. <00>

[0006] Patent Document 3 describes a vertical cavity surface emitting laser aimed at promoting the supply of current to the active region and reducing the threshold current. The vertical cavity surface emitting laser has an insulating layer having an opening, a translucent electrode covering the opening, and a mirror made of a dielectric material provided on the opening through the translucent electrode, and a conductive material is provided between the insulating layer and the mirror.

Prior Art Documents

Patent Documents

[0007] [Patent Document 1] Japanese Patent Publication No. 2014-131019 [Patent Document 2] Japanese Patent Publication No. 2009-194102 [Patent Document 3] Japanese Patent Publication No. 2011-29607 [Overview of the project] [Problems that the invention aims to solve]

[0008] In conventional vertical resonator light-emitting devices, further reduction of the threshold current and improvement of luminous efficiency were challenges.

[0009] The inventors of this application have found that improving the non-uniformity of holes and electrons within the multiple quantum well active layer leads to a significant improvement in device characteristics. The present invention is based on this finding and aims to provide a vertical resonator type light-emitting element with low threshold current and high luminous efficiency. [Means for solving the problem]

[0010] A vertical resonator type light-emitting element according to one embodiment of the present invention is The first reflective mirror, An n-type semiconductor layer formed on the first reflective mirror, An active layer consisting of multiple quantum wells formed on the n-type semiconductor layer, A final barrier layer formed on the final quantum well of the active layer, An electron barrier layer formed on the aforementioned final barrier layer, A p-type semiconductor layer formed on the electron barrier layer, A dielectric spacer layer formed on the p-type semiconductor layer, The spacer layer has a second reflective mirror formed on it, The number of antinodes and nodes of the standing waves generated by the light emitted from the active layer, contained in the electron barrier layer and the p-type semiconductor layer, is 0 or 1, respectively. The thickness of the final barrier layer and the active layer are each set to H fb ,H qw , the refractive index of the final barrier layer is n fb , the equivalent refractive index of the active layer is n qw In this case, the active layer and the final barrier layer are given by the following equation

[0011]

number

[0012] It satisfies the requirements.

[0013] A vertical resonator type light-emitting element according to another embodiment of the present invention is: The first reflective mirror, A first n-type semiconductor layer formed on the first reflective mirror, An active layer consisting of multiple quantum wells formed on the n-type semiconductor layer, A final barrier layer formed on the final quantum well of the active layer, An electron barrier layer formed on the aforementioned final barrier layer, A p-type semiconductor layer formed on the electron barrier layer, A tunnel junction layer is formed on the p-type semiconductor layer as a current-constricting layer, A second n-type semiconductor layer formed by embedding the tunnel junction layer, The present invention comprises a second reflective mirror formed on the second n-type semiconductor layer, The number of antinodes and nodes of the standing waves generated by the light emitted from the active layer, contained in the electron barrier layer and the p-type semiconductor layer, is 0 or 1, respectively. The thickness of the final barrier layer and the active layer are each set to H fb ,H qw , the refractive index of the final barrier layer is n fb , the equivalent refractive index of the active layer is n qw In this case, the active layer and the final barrier layer are given by the following equation

[0014]

number

[0015] It satisfies the requirements. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic cross-sectional view showing the structure of a vertical-cavity surface-emitting laser 10 according to a first embodiment of the present invention. [Figure 2] This diagram schematically shows the band structure of the conduction band of a vertical-cavity surface-emitting laser 10. [Figure 3] This figure schematically shows the standing wave SW 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). [Figure 4] This figure schematically shows the standing wave switch (SW) in the semiconductor layer from the active layer to the dielectric DBR in the vertical-cavity surface-emitting laser of Comparative Example 1 (Cmp.1). [Figure 5] This table shows the layer thickness of the semiconductor layer, the number of nodes (ND) (NND), and the number of antinodes (AN) (NAN) of the vertical-cavity surface-emitting lasers 10 in Examples 1-3 (Ex.1-Ex.3) and the vertical-cavity surface-emitting lasers in Comparative Examples 1 and 2 (Cmp.1 and Cmp.2). [Figure 6A] This graph shows the optical output of the vertical-cavity surface-emitting laser 10 in Example 1 (Ex.1) as a function of the injection current. [Figure 6B] This graph shows the voltage as a function of the injection current of the vertical cavity surface-emitting laser 10 in Example 1 (Ex.1). [Figure 6C] This graph shows the differential resistance of the vertical cavity surface-emitting laser 10 in Example 1 (Ex.1) with respect to the injection current. [Figure 7A] This graph shows the optical output as a function of the injection current for the vertical-cavity surface-emitting laser in Comparative Example 1 (Cmp.1). [Figure 7B] This graph shows the voltage as a function of the injection current of the vertical-cavity surface-emitting laser in Comparative Example 1 (Cmp.1). [Figure 7C] This graph shows the differential resistance of the vertical-cavity surface-emitting laser of Comparative Example 1 (Cmp.1) with respect to the injection current. [Figure 8A] This figure shows the calculated electron concentration distribution of each well layer during laser oscillation of the surface-emitting laser 10 in Example 1. [Figure 8B] This figure shows the calculated electron concentration distribution of each well layer during laser oscillation of the surface-emitting laser of Comparative Example 1. [Figure 9] This figure schematically shows the standing wave SW in the semiconductor layers from the active layer 15 to the dielectric DBR 25 in the vertical cavity surface-emitting laser 10 of Example 2. [Figure 10A] This figure shows the calculation results of the electron concentration distribution in each well layer during laser oscillation of the surface-emitting laser 10 in Example 2. [Figure 10B] This figure shows the calculated electron concentration distribution of each well layer during laser oscillation of the surface-emitting laser of Comparative Example 2. [Figure 11] This figure shows the calculated electron concentration distribution of each well layer during laser oscillation of the surface-emitting laser 10 in Example 3. [Figure 12] This is a schematic cross-sectional view showing the structure of a vertical-cavity surface-emitting laser 50 according to a second embodiment of the present invention. [Modes for carrying out the invention]

[0017] Preferred embodiments of the present invention will be described below, but these may be modified and combined as appropriate. In the following description and accompanying drawings, substantially identical or equivalent parts will be denoted by the same reference numerals. [First Embodiment] Figure 1 is a schematic cross-sectional view showing 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 a GaN (gallium nitride) semiconductor layer.

[0018] The vertical-cavity surface-emitting laser 10 is formed on a substrate 11 by sequentially growing a semiconductor DBR (Distributed Bragg Reflector) 12, an n-type semiconductor layer 13, an active layer 15 consisting of multiple quantum wells, a final barrier layer 16, an electron barrier layer (EBL) 17, and a p-type semiconductor layer 18 in that order.

[0019] The substrate 11 is a GaN substrate, and is a C-plane GaN substrate tilted 0.5° from the C-plane in the M-plane direction and 0±0.1° in the A-plane direction.

[0020] Crystal growth of the semiconductor layer was performed using metal-organic vapor deposition (MOVPE). A base GaN layer 11B with a thickness of approximately 1 μm was grown on the substrate 11, and a distributed Bragg reflector semiconductor DBR 12 was formed on the base GaN layer 11B.

[0021] The semiconductor DBR12 (first reflective mirror) was formed by stacking 42 pairs of n-type GaN and AlInN films. Each semiconductor film in the semiconductor DBR12 has a thickness of λ / 4n (where n is the refractive index of each semiconductor film) of the emission wavelength λ of the active layer 15.

[0022] An n-type semiconductor layer 13 (thickness: 350 nm), which is an n-type GaN layer doped with Si (silicon), was grown on the semiconductor DBR12.

[0023] An active layer 15 having four quantum well layers 15W was formed by alternately forming barrier layers 15B and quantum well layers 15W on an n-type semiconductor layer 13. The barrier layer 15B is made of GaInN (thickness: 3 nm), and the well layer 15W is made of GaN (thickness: 4 nm). The composition and thickness of the barrier layer 15B and the well layer 15W can be appropriately selected according to the desired emission wavelength, emission characteristics, etc.

[0024] An undoped GaN layer with a thickness of 120 nm was grown as the final barrier layer (LB) 16 on the final well layer 15WL, which is the final layer of the active layer 15.

[0025] Next, an electron barrier layer (EBL) 17 made of Mg (magnesium-doped AlGaN (Al composition: 0.30)) with a thickness of 10 nm was grown. Subsequently, a p-GaN layer of 83 nm was grown on the electron barrier layer 17 (p-AlGaN) as a p-type semiconductor layer 18.

[0026] The wafer grown in the manner described above was etched at its outer periphery to form a cylindrical mesa structure, reaching the interior of the n-type semiconductor layer 13.

[0027] The p-type semiconductor layer 18, which is the uppermost semiconductor layer of the mesa structure, was etched at its outer periphery to a depth of approximately 20 nm by dry etching to form a recess, thereby creating a p-type semiconductor layer 18 having a cylindrical mesa protrusion.

[0028] An insulating film (SiO2) 21 for lateral current and photoconfinement was deposited to a thickness of 20 nm in the recesses of the p-type semiconductor layer 18 formed by etching. As a result, the recesses of the p-type semiconductor layer 18 were planarized, a current-constricting structure was formed, and a cylindrical (central axis: CX) current injection region was created.

[0029] Next, an ITO (indium tin oxide) film with a thickness of 20 nm was deposited on the p-type semiconductor layer 18 and the insulating film 21 as a transparent conductive film 22.

[0030] Next, a dielectric (Nb2O5) was deposited as a spacer layer 24 to a thickness of 38 nm. The spacer layer 24 functions as a phase adjustment layer.

[0031] Furthermore, a dielectric DBR25 (second reflective mirror) was deposited on the spacer layer 24. The dielectric DBR25 consists of 10.5 pairs of SiO2 (11 layers) and Nb2O5 (10 layers). Preferably, the dielectric DBR25 is formed coaxially with the cylindrical mesa of the p-type semiconductor layer 18.

[0032] 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. Furthermore, the back surface of the substrate 11 was polished to form an AR (anti-reflective) coating 29 consisting of two layers of Nb2O5 / SiO2. With these steps completed, the formation of the vertical-cavity surface-emitting laser 10 was finished.

[0033] The composition and thickness of the final barrier layer (LB) 16 described above are merely examples. In other words, although the final barrier layer 16 was described as a GaN layer, nitride semiconductor layers of other compositions, such as InGaN, AlGaN, InAlGaN, etc., may also be used. Furthermore, although the final barrier layer 16 was described as an undoped layer, it may contain dopants diffused from the electron barrier layer 17 or the p-type semiconductor layer 18.

[0034] Furthermore, the composition and thickness of the electron barrier layer 17 are merely illustrative. The electron barrier layer 17 may have a thickness of, for example, 3 to 30 nm, and the Al composition may be adjusted within the range of 10 to 70%.

[0035] Although the electron barrier layer 17 was described as a p-type semiconductor layer (p-AlGaN), it may also be a p-type semiconductor layer grown as an i-layer and containing dopants diffused from the p-type semiconductor layer 18.

[0036] Furthermore, although the example given shows that the active layer 15 has four quantum well layers 15W, it is sufficient to have multiple quantum well layers.

[0037] Furthermore, the p-type semiconductor layer 18 may be composed of multiple semiconductor layers, including layers with different compositions and / or doping concentrations, and an undoped layer. Similarly, the n-semiconductor layer 13 may also be composed of multiple semiconductor layers.

[0038] Although the example given shows the dielectric DBR25 consisting of an SiO2 film and an Nb2O5 film, it may also be composed of dielectric films with different refractive indices in other combinations. Alternatively, it may be composed of a semiconductor DBR made up of semiconductor films with different refractive indices.

[0039] FIG. 2 schematically shows the band structure of the conduction band of the vertical cavity surface emitting laser 10. The band structure from the active layer 15 to the p-type semiconductor layer 18 is shown.

[0040] The active layer 15 consists of four well layers 15W of QW1 to QW4 and barrier layers 15B provided therebetween. The quantum well layer QW4 adjacent to the final barrier layer (LB) 16 is the final well layer 15WL.

[0041] The final barrier layer 16 has a layer thickness t1, the electron barrier layer 17 has a layer thickness t2, and the p-type semiconductor layer 18 has a layer thickness t3. [Conditions for a highly efficient vertical cavity light emitting device] The inventor of the present application has obtained knowledge about the conditions that a vertical cavity light emitting device with low internal loss and high efficiency should satisfy.

[0042] That is, first, the first interface inside the resonator with a high reflection mirror such as a DBR (Distributed Bragg Reflector) or a diffraction grating is defined as the phase 0” (reference).

[0043] The refractive index n of each layer in the resonator i and the layer thickness t i , and using the emission wavelength λ, the total phase information of the standing wave in the resonator is expressed by the following formula (1).

[0044]

Equation

[0045] Note that the combined information of the standing wave at a predetermined position inside the layer is considered by replacing the thickness from the interface with other layers on the resonator side to the predetermined position with t i and considering.

[0046] And the first condition is that 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 barrier layer 17, which are p regions, are each 0 or 1 (N AN= 0 or N AN =1, N ND = 0 or N ND =1)

[0047] Furthermore, the thickness of each layer of the transparent conductive film 22 (ITO), the p-type semiconductor layer 18, and the electron barrier layer 17 is set to H ITO ,H GaN ,H EB , each refractive index n ITO ,n GaN ,n EB When the wavelength is λ, it is preferable that the following equation (2) is satisfied.

[0048]

number

[0049] Furthermore, the second condition is that the number of nodes ND and antinodes AN of the standing wave SW contained within the final barrier layer 16 is 1 or more (N ND ≥1 and N AN ≥1).

[0050] Note that the number of nodes ND and antinodes AN of the standing wave SW is N. ND , N AN This can be determined by using the above equation (1) to calculate the total phase in the p region and the final barrier layer 16, respectively, and then obtaining the number of elements in the p region and the final barrier layer 16 whose phase is kπ (k=1,2,...) and the number of elements whose phase is (2l-1)π / 2 (l=1,2,...).

[0051] Equation (1) = k / 2 (k=1,2,...) indicates the position where the standing wave becomes an antinode AN, and equation (1) = (2l-1) / 4 (l=1,2,...) indicates the position where the standing wave becomes a node ND. That is, N within the p-type semiconductor layer 18, the electron barrier layer 17, and the final barrier layer 16. ND , N AN This can be determined by counting how many of the above-mentioned antinode AN and node ND locations are included within the stacking range of each layer.

[0052] Furthermore, the thickness of the final barrier layer 16 and the active layer 15 is set to Hfb ,H qw In this case, it is preferable that the following equation (3) is satisfied. Here, n fb n is the refractive index of the final barrier layer 16. qw This is the equivalent refractive index of the active layer 15.

[0053]

number

[0054] Furthermore, (i) it is preferable that the final barrier layer 16 has a thickness of λ / 4 or more. That is, it is preferable that the following equation (4) is satisfied.

[0055]

number

[0056] Also, (ii) the number of nodes ND included in the final barrier layer 16 is two or more (N ND ≥2), the number of abdominal ANs is 1 or more (N AN It is preferable that (≥1). If (i) and (ii) are satisfied, it is also preferable that the following formula (5) is satisfied.

[0057]

number

[0058] Furthermore, it is preferable that the thickness of the active layer 15 is λ / 8 or less, that is, that it satisfies the following equation (6). In this case, the optical confinement loss of the active layer 15 can be reduced.

[0059]

number

[0060] With the above configuration, the optical electric field strength from the p-type semiconductor layer 18 to the electron barrier layer 17 is increased. Furthermore, the presence of an antinode AN with a high optical electric field strength within the final barrier layer 16, along with at least one node ND, excites the active layer 15 to a degree that yields a large optical gain.

[0061] Furthermore, when the internal light intensity is increased by the highly reflective semiconductor DBR12 and dielectric DBR25, electrons and holes in the final barrier layer 16 are excited by the internal light, and holes accumulated at the interface between the electron barrier layer 17 and the p-type semiconductor layer 18 are drawn into the active layer 15, resulting in switching-like hole injection into the active layer 15.

[0062] 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, enabling the realization of an efficient surface-emitting laser. [Consideration of the mechanism of characteristic improvement] By configuring the laser as described above, the uniformity of each carrier (electrons and holes) in the well layer 15W of the active layer 15 is improved, enabling the realization of an efficient surface-emitting laser. We will now consider the mechanism by which this improvement in efficiency and other characteristics can be achieved.

[0063] The mechanism for improving performance is thought to be related to the switching-like injection of holes to eliminate carrier inhomogeneity in the multiple quantum wells. In this surface-emitting laser, the active layer (multiple quantum wells) is designed to align with the antinodes of standing waves in the center. 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.

[0064] Therefore, the final barrier layer 16 adjacent to the active layer 15 will experience a decrease in the electric field intensity of light. However, by positioning the final barrier layer 16 so that it is located where, or near, an antinode of a standing wave different from that of the active layer is located, as described above, the light intensity of this layer increases significantly near the threshold, generating carriers (electrons and holes), which is presumed to have the effect of rapidly pulling in holes on the p-type semiconductor layer 18 side of the electron barrier layer 17. This pull-in is more likely to occur when the electrical electric field gradient in the p-region is high.

[0065] This effect is thought to be due to an increase in the hole concentration accumulating on the p-type semiconductor layer 18 side and an increase in the electrical electric field gradient of the electron barrier layer 17. When holes are pulled in and switched, the uniformity of the carrier distribution in the multiple quantum wells is improved and internal losses are reduced. Therefore, in some cases, due to laser oscillation, the differential resistance and drive current have a minimum value (dR / dI=0,d) near the threshold. 2 V / dI 2 In some cases, properties not seen in typical surface-emitting lasers (VCSELs), such as (=0), can be obtained. Therefore, the position of the antinode of the final barrier layer 16, the thinning of the p-layer (electron barrier layer 17 and p-type semiconductor layer 18), and the light intensity such that there is strong vertical light feedback and optical gain due to the high-reflectivity mirror are important factors. [Example 1] Figure 3 schematically shows the 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.

[0066] In Example 1 (Ex.1), there is one node ND and one antinode AN of the standing wave SW within the final barrier layer 16. That is, the number of nodes ND is N ND The number of abdominal ANs is N AN Therefore, N ND =1,N AN = 1. Furthermore, at the high-reflection mirror interface A using dielectric DBR25, the position of the antinode AN of the standing wave SW is as shown in Figure 3.

[0067] Furthermore, in the p-region electron barrier layer (EBL) 17 and p-type semiconductor layer (p-GaN) 18, there is one standing wave SW node ND and one antinode AN, respectively (N ND =1,N AN (=1). Furthermore, there is one standing wave SW node ND within the transparent conductive film (ITO) 22 (N ND =1) is preferable.

[0068] This configuration strengthens the electrical electric field from the p-type semiconductor layer 18 to the electron barrier layer 17. Furthermore, the presence of an antinode with a high optical electric field intensity within the final barrier layer 16, along with at least one node, excites the active layer 15 to a degree that yields a large optical gain.

[0069] Furthermore, when the internal light intensity is increased by the highly reflective semiconductor DBR12 and dielectric DBR25, electrons and holes in the final barrier layer 16 are excited by the internal light, and holes accumulated at the interface between the electron barrier layer 17 and the p-type semiconductor layer 18 are drawn into the active layer 15, resulting in switching-like hole injection into the active layer 15.

[0070] As a result, the uniformity of carriers (electrons and holes) in the four well layers (QW1 to QW4) is improved, enabling the realization of an efficient surface-emitting laser. [Comparative Example 1] Figure 4 schematically shows the standing wave SW of the optical electric field intensity in the semiconductor layers from the active layer 15 to the dielectric DBR 25 in the vertical-cavity surface-emitting laser of Comparative Example 1 (Cmp.1). Figure 5 shows the layer thickness of the semiconductor layers and the number of nodes ND N of the vertical-cavity surface-emitting lasers 10 of Examples 1 to 3 (Ex.1 to Ex.3) of this embodiment and the vertical-cavity surface-emitting lasers of Comparative Examples 1 and 2 (Cmp.1, Cmp.2). ND , Number of abdominal ANs N AN This is a table showing the results.

[0071] The vertical-cavity surface-emitting laser of Comparative Example 1 (Cmp.1) differs significantly from the vertical-cavity surface-emitting laser 10 of Example 1 (Ex.1) in that the thickness of the final barrier layer (LB) 16 is 10 nm. That is, in the vertical-cavity surface-emitting laser of Comparative Example 1, there are no standing wave nodes ND and antinodes AN within the final barrier layer 16 (N ND =0,N AN (=0). Except for the difference in film thickness shown in Figure 5, the configuration is the same as in Example 1.

[0072] Figures 6A, 6B, and 6C show the measurement results for the vertical-cavity surface-emitting laser 10 of Example 1 (Ex.1), and are graphs showing the characteristics of optical output, voltage, and differential resistance with respect to injection current, respectively. Figures 7A, 7B, and 7C show the measurement results for the vertical-cavity surface-emitting laser of Comparative Example 1 (Cmp.1), and are graphs showing the characteristics of optical output, voltage, and differential resistance with respect to injection current, respectively.

[0073] As shown in Figure 6A, the vertical-cavity surface-emitting laser 10 of Example 1 (Ex.1) demonstrated laser oscillation characteristics at low threshold currents. Furthermore, as shown in Table 1 below, the slope efficiency and external differential quantum efficiency were improved compared to the vertical-cavity surface-emitting laser of Comparative Example 1 (Cmp.1).

[0074] [Table 1]

[0075] As shown in Table 1, Comparative Example 1 has lower slope efficiency and external differential quantum efficiency compared to Example 1. Analysis revealed that the internal loss of the laser in Comparative Example 1 is higher than that of Example 1. The internal loss of the laser in Comparative Example 1 was comparable to that of conventionally reported internal losses. Furthermore, the IV characteristics of Comparative Example 1 exhibited similar behavior to those described in prior art literature, and no minimum value or negative resistance of the differential resistance near the threshold current was observed in the prior art.

[0076] Specifically, as shown in Figures 6B and 6C, in the vertical-cavity surface-emitting laser 10 of this embodiment, some devices exhibited voltage fluctuations near the threshold current. It was found that the differential resistance decreased sharply near the threshold current, and that the differential resistance and drive current sometimes reached minimum values ​​(dR / dI=0,d 2 V / dI 2 (=0, R: resistance, I: current). In more severe cases, the differential resistance may enter the negative region.

[0077] Such characteristics are not seen in conventional surface-emitting lasers. In the vertical-cavity surface-emitting laser 10 of this embodiment, the thickness of the p-region (electron barrier layer 17 and p-type semiconductor layer 18) is thin, and a large electrical electric field is applied to the p-region layer. It is thought that holes accumulated at the interface between the electron barrier layer 17 and the p-type semiconductor layer 18 (p-GaN) are induced by electrons and holes generated in the final barrier layer 16 and flow rapidly into the active layer 15. This promotes uniformity of carrier concentration between multiple quantum wells, improving internal loss, external differential quantum efficiency, and slope efficiency, and reducing or minimizing the differential resistance near the threshold current.

[0078] More specifically, the reduction in internal loss is thought to be due to the uniform distribution of carrier density in each 15W well layer. Figures 8A and 8B show the calculated results of the carrier density (electron concentration) distribution in each 15W well layer during laser oscillation of the surface-emitting lasers of Example 1 and Comparative Example 1, respectively.

[0079] Figures 8A and 8B show the electron concentration normalized with the well layer with the lowest carrier density (QW2) set to 1. In Comparative Example 1, the non-uniformity is more than 3 times, but in Example 1, it is suppressed to about 2 times. This non-uniformity is thought to cause optical loss in the well layer with the lowest carrier density (QW2), resulting in a large internal loss.

[0080] Furthermore, in conventional surface-emitting lasers, the low external differential quantum efficiency and high differential resistance before laser oscillation are thought to be due to the presence of holes with large effective masses and piezoelectric fields, which makes it difficult for holes to move, resulting in hole and electron inhomogeneity within the multiple quantum well layer.

[0081] In Comparative Example 1, there is one node ND and one antinode AN in the p region (electron barrier layer 17 and p-type semiconductor layer 18) (N ND =1,N AN =1), the first condition is met, but there are no nodes ND and antinodes AN within the final barrier layer 16 (N ND =0,N AN =0), the second condition is not met. It is important that both the first and second conditions are met.

[0082] As described above, the vertical-cavity surface-emitting laser 10 of this embodiment achieves a reduction in internal loss, and a reduction in differential resistance occurs near the threshold current. Furthermore, near the threshold current, the differential resistance and drive current reach minimum values ​​(dR / dI=0,d 2 V / dI 2 It may also have a value of =0.

[0083] The emission spectrum and gain spectrum before laser oscillation can be broadly distributed to high energies according to the Fermi-Dirac distribution function, and it is preferable that the gain peak and emission peak are on the higher energy (shorter wavelength) side than the oscillation wavelength. Furthermore, it is preferable to have a thick well layer, for example, 4 nm or more, so that a second energy level (quantum energy level in the conduction band) is formed in the quantum well.

[0084] Furthermore, whether the emission peak is on the high-energy side can be confirmed by examining the optical spectrum before oscillation and observing that the light intensity or energy is greater at wavelengths shorter than the oscillation wavelength. Additionally, by utilizing this hole injection effect, it is possible to easily induce an optical switching phenomenon by optical excitation while keeping the current fixed at the pre-oscillation current, or to induce self-pulsation operation in a surface-emitting laser.

[0085] When utilizing the optical switching phenomenon, a power supply (not shown) connected to the vertical-cavity surface-emitting laser 10 supplies a current slightly below the threshold current, causing light to be shone onto the vertical-cavity surface-emitting laser 10 from an external source. The external light can be shone using a light source separate from the vertical-cavity surface-emitting laser 10. [Example 2] In Example 1 (Ex.1), there is one node ND and one antinode AN of the standing wave SW within the final barrier layer 16 (N ND =1,N AN =1), there is one node ND and one antinode AN in the p region (electron barrier layer 17 and p-type semiconductor layer 18) (N ND =1,N AN I explained the case where =1).

[0086] The structure of the vertical-cavity surface-emitting laser 10 of Example 2 (Ex.2) will be described with reference to Figure 9. Figure 9 is a schematic diagram showing the standing wave SW of the optical electric field intensity in the semiconductor layers from the active layer 15 to the dielectric DBR 25 in the vertical-cavity surface-emitting laser 10 of Example 2.

[0087] As shown in Figure 5, in Example 2, the thickness of the final barrier layer 16 is 70 nm, which is thinner than the 120 nm in Example 1. Also, the thickness of the p-region (total thickness of the electron barrier layer 17 and the p-type semiconductor layer 18) is 65 nm (20 + 45 nm), which is thinner than the 93 nm (10 + 83 nm) in Example 1.

[0088] Furthermore, the Al composition of the electron barrier layer 17 was set to 0.15. Other than the points mentioned above, the procedure is the same as in Example 1.

[0089] In the vertical-cavity surface-emitting laser 10 of Example 2, there is one node ND and one antinode AN within the final barrier layer 16 (N ND =1,N AN =1), there is one antinode AN in the p region (electron barrier layer 17 and p-type semiconductor layer 18), and no node ND exists (N AN =1,N ND =0). [Comparative Example 2] The vertical-cavity surface-emitting laser of Comparative Example 2 (Cmp.2) differs significantly from the vertical-cavity surface-emitting lasers 10 of Example 1 (p-region layer thickness: 93nm) and Example 2 (p-region layer thickness: 65nm) in that the p-region layer thickness (total layer thickness of electron barrier layer 17 and p-type semiconductor layer 18) is 397nm (10+387nm). Aside from the above, it is the same as Example 1.

[0090] As shown in Figure 5, in the vertical-cavity surface-emitting laser of Comparative Example 2, there is one node ND and two antinodes AN of the standing wave SW within the final barrier layer 16 (N ND =1,N AN =2). Also, in the p region (electron barrier layer 17 and p-type semiconductor layer 18), there are four nodes ND and four antinodes AN (N ND =4,N AN =4).

[0091] In other words, the second condition described above (the number of nodes ND and antinodes AN contained within the final barrier layer 16) is satisfied, but the first condition (the number of nodes ND and antinodes AN contained within the electron barrier layer 17 and the p-type semiconductor layer 18) is not satisfied.

[0092] Figures 10A and 10B show the calculated carrier density (electron concentration) distribution of each well layer at 15W during laser oscillation of the surface-emitting lasers of Example 2 and Comparative Example 2, respectively.

[0093] Figures 10A and 10B show the data normalized with the well layer with the lowest carrier density (QW2) set to 1. In Comparative Example 2, the non-uniformity is 3.5 times or more, but in Example 2, it is suppressed to about 2 times.

[0094] This non-uniformity is thought to cause optical loss in the well layer with the lowest carrier density (QW2), leading to increased internal loss.

[0095] From the above, it can be seen that simply satisfying the conditions for the final barrier layer (the second condition) is not enough to obtain an efficient surface-emitting laser.

[0096] Furthermore, although slightly, the carrier homogeneity was higher in Example 2 than in Example 1, and in the electron barrier layer 17 and the p-type semiconductor layer 18, N ND =1,N AN =1 is greater than N ND =0,N AN =1 is preferable. [Example 3] As shown in Figure 5, in Example 3 (Ex.3), the thickness of the final barrier layer 16 is 220 nm, which is thicker than the 120 nm in Example 1. Also, 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 Example 1. Furthermore, the Al composition of the electron barrier layer 17 was set to 0.15. Except for the points mentioned above, it is the same as Example 1.

[0097] In Example 3, reflecting the thickness of the final barrier layer 16, there are two nodes ND and two antinodes AN within the final barrier layer 16 (N ND =2,N AN (=2). Also, in the p region (electron barrier layer 17 and p-type semiconductor layer 18), there is one node ND and one antinode AN (N ND =1,N AN =1).

[0098] Figure 11 shows the calculated carrier density (electron concentration) distribution of each well layer 15W during laser oscillation of the surface-emitting laser 10 of Example 3.

[0099] Compared to Comparative Examples 1 and 2, a significant improvement in uniformity was confirmed. Furthermore, Example 3 showed a slight improvement compared to Example 1, with a configuration that increased the number of antagonisms AN and nodes ND in the final barrier layer, i.e., N ND and / or N AN It was found that it is possible to achieve the desired high-efficiency surface-emitting laser even with a configuration of two or more elements.

[0100] Therefore, while there is no effect when the number of antinodes AN and node ND in the final barrier layer is insufficient (Comparative Example 1) or when the number of antinodes AN and node ND in the p region is too large (Comparative Example 2), when the first and second conditions described above are met, hole injection switching occurs, eliminating the decrease in efficiency of the surface-emitting laser due to carrier non-uniformity between each well layer of the quantum well active layer, and providing a highly efficient surface-emitting laser. [Second Embodiment] Figure 12 is a schematic cross-sectional view 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-constricting structure.

[0101] The vertical-cavity surface-emitting laser 50 is a nitride surface-emitting laser having the same configuration as Example 1 of the vertical-cavity surface-emitting laser 10 according to the first embodiment.

[0102] In other words, the vertical cavity surface-emitting laser 50 is formed on a substrate 11 by sequentially growing a semiconductor DBR 12, an n-type semiconductor layer 13, an active layer 15 consisting of multiple quantum wells, a final barrier layer 16, an electron barrier layer (EBL) 17, and a p-type semiconductor layer 18 in that order. Furthermore, the composition, thickness, and impurity concentration of each semiconductor layer are the same as those of the vertical cavity surface-emitting laser 10.

[0103] In the vertical cavity surface-emitting laser 50 according to the second embodiment, after growing the p-type semiconductor layer 18, + -GaN is a high-impurity p-type semiconductor layer 31A and n + A tunnel junction layer 31 is grown from an n-type semiconductor layer 31B which is GaN and has a high impurity concentration.

[0104] Next, the tunnel junction layer 31 and the p-type semiconductor layer 18 are etched in a cylindrical shape from the upper surface of the tunnel junction layer 31 to the interior of the p-type semiconductor layer 18 to form a cylindrical mesa structure.

[0105] For example, the thickness of the tunnel junction layer 31 is 20 nm, and the diameter of the cylindrical mesa structure is 4 μm (central axis CX). Furthermore, the mesa structure is formed using dry etching, in which the tunnel junction layer 31 and the p-type semiconductor layer 18 are etched to a depth of, for example, 25 nm.

[0106] Subsequently, an n-GaN n-type semiconductor layer 32 (second n-type semiconductor layer) is grown again using the MOVPE apparatus, and the tunnel junction layer 31 is embedded.

[0107] Next, a semiconductor DBR35 (second DBR) consisting of n-AlInN and n-GaN is formed. The semiconductor DBR35 consists of, for example, 46 pairs of n-AlInN / GaN.

[0108] Next, the outer periphery of the wafer was etched to reach the interior of the n-type semiconductor layer 13, forming a surface-emitting laser with a cylindrical mesa structure coaxial with the central axis CX of the tunnel junction layer 31.

[0109] Next, an n-electrode 27 was formed on the outer surface of the n-type semiconductor layer 13. In addition, a p-electrode 36 was formed on the semiconductor DBR 35, having a larger diameter than the tunnel junction layer 31 and a circular opening coaxial with the central axis CX, when viewed from above (from a direction perpendicular to the semiconductor DBR 35).

[0110] Furthermore, the back surface of the substrate 11 was polished to form an AR (anti-reflective) coating 29 consisting of two layers of Nb2O5 / SiO2. With this, the formation of the vertical cavity surface-emitting laser 10 was completed.

[0111] Furthermore, the tunnel junction layer 31 is p + -GaN layer and n + -Although it is constructed with a GaN layer, semiconductor layers of other compositions, such as GaInN, may also be used. + -The GaN layer can use, for example, Mg as an impurity (dopant). The Mg doping concentration is 4 × 10⁻⁶. 19 cm 3 The above is preferable. + -GaN layer or n +For the -GaInN layer, a high doping of 1×10 18 cm 3 or more is preferable. [Correspondence with the First Embodiment] In the vertical cavity surface emitting laser 50 according to the second embodiment, the first and second conditions can be considered in the same manner as in the case of the first embodiment.

[0112] Specifically, in the vertical cavity surface emitting laser 50 according to the second embodiment, the interface between the semiconductor DBR 35 and the n-type semiconductor layer 32 corresponds to the phase reference (0). In the reference, the antinode of the standing wave is located.

[0113] Also, the n-type semiconductor layer 32 (the second n-type semiconductor layer) corresponds to the spacer layer 24 in the first embodiment. 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.

[0114] Furthermore, the tunnel junction layer 31 corresponds to the transparent conductive film (ITO) 22 in the first embodiment, and it is preferable that there is one node ND of the standing wave SW in the tunnel junction layer 31 (N ND =1).

[0115] Also, the conditions (the first condition) of 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 which are p regions, and the conditions (the second condition) of the number of nodes ND and antinodes AN included in the final barrier layer 16 are the same as the conditions in the first embodiment.

[0116] As described in detail above, according to the present invention, a vertical cavity light emitting device with a low threshold current and high luminous efficiency can be provided.

[0117] In the above-described embodiments, a vertical cavity light emitting device using a nitride-based semiconductor has been described, but it is also possible to apply it to a vertical cavity light emitting device using a semiconductor of another crystal system.

[0118] Furthermore, while semiconductor DBRs or dielectric DBRs were given as examples of reflective mirrors constituting the resonator, the system is not limited to these. For example, single-layer reflective mirrors or diffraction gratings can also be used.

[0119] Furthermore, although the vertical resonator type light-emitting element of the present invention has been described using the MOVPE method as an example, it can also be manufactured by other known crystal growth methods such as molecular beam epitaxy (MBE). [Explanation of Symbols]

[0120] 10: Vertical Cavity Surface Emitting Laser 11: Circuit board 12: DBR (Reflective Mirror) 13: n-type semiconductor layer 15:Active layer 15B: Barrier layer 15W: Quantum well layer (well layer) 16: Final Barrier Layer (LB) 17: Electron barrier layer 18: p-type semiconductor layer 21: Insulating Film 22: Transparent conductive film 24: Spacer layer 25: DBR (Reflective Mirror) 31: Tunnel junction layer 31A: p-type semiconductor layer with high impurity concentration 31B: n-type semiconductor layer with high impurity concentration 32: n-type semiconductor layer (second n-type semiconductor layer) 35: Semiconductor DBR

Claims

1. The first reflective mirror, An n-type semiconductor layer formed on the first reflective mirror, An active layer consisting of multiple quantum wells formed on the n-type semiconductor layer, A final barrier layer formed on the final quantum well of the active layer, An electron barrier layer formed on the aforementioned final barrier layer, A p-type semiconductor layer formed on the electron barrier layer, A dielectric spacer layer formed on the p-type semiconductor layer, The dielectric spacer layer comprises a second reflective mirror formed on the dielectric spacer layer, The number of antinodes and nodes of the standing waves generated by the light emitted from the active layer, contained in the electron barrier layer and the p-type semiconductor layer, is 0 or 1, respectively. The thickness of the final barrier layer and the active layer are each set to H ｆｂ , H ｑｗ , the refractive index of the final barrier layer is n ｆｂ , the equivalent refractive index of the active layer is n ｑｗ When the emission wavelength from the active layer is denoted as λ, the active layer and the final barrier layer are given by the following equation [Mathematics 1] Satisfying the conditions, A vertical resonator type light-emitting element, wherein the number of nodes and antinodes of the standing wave contained within the final barrier layer is one or more.

2. The p-type semiconductor layer and the dielectric spacer layer are provided with a transparent conductive film, The vertical resonator type light-emitting element according to claim 1, wherein the transparent conductive film is provided such that the nodes of the standing wave exist within the transparent conductive film.

3. The aforementioned final barrier layer is λ / 4n ｆｂ A vertical resonator type light-emitting element according to claim 1 or 2, having the above layer thickness.

4. The vertical resonator type light-emitting element according to claim 1 or 2, wherein the number of nodes and the number of antinodes included in the final barrier layer are two or more.

5. The thickness of the active layer is λ / 8n ｑｗ The vertical resonator type light-emitting element according to any one of claims 1 to 4, which is as follows:

6. The vertical resonator type light-emitting element has a minimum value of differential resistance due to laser oscillation near a threshold current, as described in any one of claims 1 to 5.

7. The vertical resonator type light-emitting element according to any one of claims 1 to 6, wherein the first reflective mirror is a semiconductor DBR (Distributed Bragg Reflector) and the second reflective mirror is a dielectric DBR.

8. The first reflective mirror, A first n-type semiconductor layer formed on the first reflective mirror, An active layer consisting of multiple quantum wells formed on the n-type semiconductor layer, A final barrier layer formed on the final quantum well of the active layer, An electron barrier layer formed on the aforementioned final barrier layer, A p-type semiconductor layer formed on the electron barrier layer, A tunnel junction layer is formed on the p-type semiconductor layer as a current-constricting layer, A second n-type semiconductor layer formed by embedding the tunnel junction layer, The present invention comprises a second reflective mirror formed on the second n-type semiconductor layer, The number of antinodes and nodes of the standing waves generated by the light emitted from the active layer, contained in the electron barrier layer and the p-type semiconductor layer, is 0 or 1, respectively. Let the layer thicknesses of the final barrier layer and the active layer be H ｆｂ , H ｑｗ , let the refractive index of the final barrier layer be n ｆｂ , let the equivalent refractive index of the active layer be n ｑｗ , when the light emission wavelength from the active layer is λ, the active layer and the final barrier layer satisfy the following formula [Math 2] Satisfying the conditions, A vertical resonator type light-emitting element, wherein the number of nodes and antinodes of the standing wave contained within the final barrier layer is one or more.

9. The vertical resonator type light-emitting element according to claim 8, wherein the tunnel junction layer is provided such that the nodes of the standing wave exist within the tunnel junction layer.

10. The aforementioned final barrier layer is λ / 4n ｆｂ A vertical resonator type light-emitting element according to claim 8 or 9, having the above layer thickness.

11. The vertical resonator type light-emitting element according to claim 8 or 9, wherein the number of nodes and the number of antinodes included in the final barrier layer are two or more.

12. The thickness of the active layer is λ / 8n ｑｗ The vertical resonator type light-emitting element according to any one of claims 8 to 11, which is as follows:

13. The vertical resonator type light-emitting element according to any one of claims 8 to 12, wherein the vertical resonator type light-emitting element has a minimum value of differential resistance due to laser oscillation near the threshold current.

14. The vertical resonator type light-emitting element according to any one of claims 8 to 13, wherein the first reflective mirror and the second reflective mirror are semiconductor DBRs.

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