Photonic crystal surface-emitting laser element and method for manufacturing the same

Abstract

A first cladding layer (2) of a first conductivity type, an active layer (4), Al-containing layers (5,6), a photonic crystal layer (7), and a second cladding layer (8) of a second conductivity type are formed in order on a semiconductor substrate (1). The photonic crystal layer (7) has a base material layer (7a) and a region of different refractive indices (7b) that is periodically formed in the base material layer (7a) in a plan view and is made of a material with a different refractive index than the base material layer (7a). The Al-containing layers (5,6) contain Al as a constituent element. The bottom of the region of different refractive indices (7b) is in contact with the Al-containing layers (5,6). At the interface between the region of different refractive indices (7b) and the Al-containing layers (5,6), the Al-containing layers (5,6) have Al-O bonds or Al-N bonds.

Description

[Technical Field]

[0001] This disclosure relates to a photonic crystal surface-emitting laser element and a method for manufacturing the same. [Background technology]

[0002] In recent years, photonic crystal surface-emitting laser elements have been developed. For example, a photonic crystal surface-emitting laser element has been disclosed that has a photonic crystal in which regions with different refractive indices from the surrounding area are periodically arranged in the plane, an active layer formed near the crystal, and a pair of electrodes flanking them (see, for example, Patent Document 1). In such a photonic crystal surface-emitting laser element, light is generated by carrier recombination in the gain region of the active layer, and this light is diffracted in the in-plane direction by the photonic crystal, producing a feedback effect that forms a standing wave of light of a specific wavelength. This amplifies the light of the specific wavelength, causing laser oscillation. A portion of the laser light thus generated is radiated perpendicular to the plane due to the effect of the photonic crystal, and this can be used as output light.

[0003] In photonic crystal surface-emitting laser devices, it is crucial that the light generated in the active layer is efficiently diffracted by the photonic crystal. Therefore, it is necessary to increase the light distribution ratio (optical confinement in the photonic crystal layer) within the layer where the photonic crystal is formed. If optical confinement in the photonic crystal layer is small, most of the light generated in the active layer is scattered without being diffracted by the photonic crystal, resulting in loss. On the other hand, if optical confinement in the photonic crystal layer is large, the light generated in the active layer is efficiently diffracted, allowing it to be confined within the gain region. This enables a reduction in the laser oscillation threshold and miniaturization of the laser device. Therefore, increasing optical confinement in the photonic crystal layer is important for improving laser characteristics.

[0004] To enhance photoconfinement in a photonic crystal layer, it is effective to reduce the distance between the active layer and the photonic crystal. However, when attempting to form a photonic crystal near the active layer after its formation, the following problems arise. Photonic crystals are formed by creating a periodic structure consisting of fine irregularities on a semiconductor substrate, and dry etching is used for this process. As a result, damage from dry etching may be introduced into the underlying active layer, potentially degrading the crystal quality of the active layer.

[0005] Therefore, a method has been proposed to induce laser oscillation in a higher-order waveguide mode (i.e., a mode having two peaks in the optical intensity distribution perpendicular to the plane) by providing a low-refractive-index layer between the active layer and the photonic crystal layer (see, for example, Patent Document 2). This allows for increased optical confinement of the photonic crystal while ensuring sufficient distance between the active layer and the photonic crystal layer to avoid damage due to dry etching. However, in order to obtain stable laser oscillation in a higher-order waveguide mode, it is necessary to control the relative distances between the low-refractive-index layer and the active layer and the photonic crystal layer, as well as the film thickness and refractive index of the low-refractive-index layer, making it difficult to achieve manufacturing stability.

[0006] Furthermore, a method has been disclosed in which periodic holes are formed by dry etching, the holes are then closed by epitaxial growth, and an active layer is formed on top of them (see, for example, Non-Patent Document 1). As a result, the active layer is not exposed to damage caused by dry etching. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent No. 5072402 [Patent Document 2] Japanese Patent No. 5836609 [Non-patent literature]

[0008] [Non-Patent Document 1] Optics Express, Vol.30, No.16, p29539

Summary of the Invention

Problems to be Solved by the Invention

[0009] In the method described in Non-Patent Document 1, after the holes formed by dry etching are completely closed by epitaxial growth, an active layer is formed. However, at the time when the holes are closed, irregularities occur on the surface. In order not to deteriorate the crystal quality of the active layer, it is necessary to grow a certain film thickness to flatten the irregularities. The active layer and the photonic crystal layer are separated by this film thickness. Therefore, there is a problem that the light confinement in the photonic crystal layer is reduced.

[0010] The present disclosure has been made to solve the above-described problems, and an object thereof is to obtain a photonic crystal surface-emitting laser device and a method for manufacturing the same that can prevent etching damage from reaching the active layer and enhance light confinement in the photonic crystal layer.

Means for Solving the Problems

[0011] The photonic crystal surface-emitting laser device according to the present disclosure includes a semiconductor substrate, a first cladding layer of a first conductivity type, an active layer, an Al-containing layer, a photonic crystal layer, and a second cladding layer of a second conductivity type, which are sequentially formed on the semiconductor substrate. The photonic crystal layer has a base material layer and a refractive index different region formed periodically in a plan view on the base material layer and made of a material having a different refractive index from the base material layer. The Al-containing layer contains Al as a constituent element, the bottom of the refractive index different region is in contact with the Al-containing layer, and the Al-containing layer has an Al-O bond or an Al-N bond at the interface between the refractive index different region and the Al-containing layer.

[0012] A method for manufacturing a photonic crystal surface-emitting laser element according to this disclosure comprises the steps of: forming a first cladding layer of a first conductivity type, an active layer, an Al-containing layer containing Al as a constituent element, and a base material layer consisting of a photonic crystal base material in order on a semiconductor substrate; forming an etching mask on the base material layer in which a plurality of openings are periodically formed, and performing dry etching using the etching mask until the Al-containing layer is exposed by penetrating the base material layer to form a plurality of through holes in the base material layer in a plan view; and forming a second cladding layer of a second conductivity type on the base material layer in which the plurality of through holes are formed, characterized in that the dry etching is performed in an atmosphere containing at least one of oxygen atoms and nitrogen atoms. [Effects of the Invention]

[0013] In this disclosure, the Al-containing layer has Al-O bonds or Al-N bonds at the interface between the different refractive index regions of the photonic crystal and the Al-containing layer. These act as barriers against dry etching, preventing etching damage from reaching the active layer. Since it is not necessary to increase the thickness of the Al-containing layer to prevent etching damage, and the photonic crystal layer and the active layer can be brought closer together, photoconfinement in the photonic crystal layer can be enhanced. [Brief explanation of the drawing]

[0014] [Figure 1] This is a cross-sectional view showing a photonic crystal surface-emitting laser element according to Embodiment 1. [Figure 2] This is a top view showing a photonic crystal surface-emitting laser element according to Embodiment 1. [Figure 3] This is a bottom view showing a photonic crystal surface-emitting laser element according to Embodiment 1. [Figure 4] This is a cross-sectional view showing a method for manufacturing a photonic crystal surface-emitting laser element according to Embodiment 1. [Figure 5] This is a cross-sectional view showing a method for manufacturing a photonic crystal surface-emitting laser element according to Embodiment 1. [Figure 6]This is a plan view showing the etching mask. [Figure 7] This is a plan view that is an enlarged portion of Figure 6. [Figure 8] This is a cross-sectional view showing a method for manufacturing a photonic crystal surface-emitting laser element according to Embodiment 1. [Figure 9] This is a plan view showing the base material layer in which pairs of through holes are formed. [Figure 10] This is a plan view that is an enlarged portion of Figure 9. [Figure 11] This is a cross-sectional view showing a method for manufacturing a photonic crystal surface-emitting laser element according to Embodiment 1. [Figure 12] This is a magnified cross-sectional view of the vicinity of a pair of through-holes immediately after dry etching. [Figure 13] This is a magnified cross-sectional view of the vicinity of a pair of through-holes immediately after dry etching. [Figure 14] This graph shows the current-peak light output characteristics when a pulsed current is applied to a photonic crystal surface-emitting laser element. [Figure 15] This is an electron microscope image of a vacancy cross-section of a photonic crystal surface-emitting laser element corresponding to the embodiment. [Figure 16] This is an electron microscope image of a vacancy cross-section of a photonic crystal surface-emitting laser element corresponding to the comparative example. [Figure 17] This figure shows the process of dry etching the base material layer in Embodiment 5. [Modes for carrying out the invention]

[0015] A photonic crystal surface-emitting laser element and its manufacturing method according to an embodiment will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals, and repetition of the description may be omitted.

[0016] Embodiment 1 FIG. 1 is a cross-sectional view showing a photonic crystal surface emitting laser device according to Embodiment 1. FIG. 2 is a top view showing the photonic crystal surface emitting laser device according to Embodiment 1. FIG. 3 is a bottom view showing the photonic crystal surface emitting laser device according to Embodiment 1.

[0017] A first cladding layer 2, a first guide layer 3, an active layer 4, a second guide layer 5, a carrier blocking layer 6, and a photonic crystal layer 7 are sequentially formed on a semiconductor substrate 1. The semiconductor substrate 1 is made of n-type GaAs. The first cladding layer 2 is an n-type Al x Ga 1―x As (x = 0.37) with a thickness of 800 nm. The first guide layer 3 is an i-type Al x Ga 1―x As (x = 0.15) with a thickness of 80 nm. The second guide layer 5 is an i-type Al x Ga 1―x As (x = 0.15) with a thickness of 20 nm. Here, the i-type means an intrinsic semiconductor. The carrier blocking layer 6 is a p-type Al x Ga 1―x As (x = 0.45) with a thickness of 25 nm.

[0018] The active layer 4 has a multiple quantum well structure in which well layers 4a and barrier layers 4b are alternately stacked. The number of layers is 3 for the well layers 4a and 2 for the barrier layers 4b. The well layer 4a is an i-type InGaAs with a thickness of 10 nm. The barrier layer 4b is Al x Ga 1―x As (x = 0.15) with a thickness of 20 nm. When current is injected into the active layer 4, light in a wavelength band corresponding to the energy bandgap of the well layer 4a is generated. The center wavelength of the PL (Photoluminescence) emission of the active layer is 936 nm.

[0019] Since the Al composition of the carrier blocking layer 6 is larger than the Al compositions of the layers constituting the active layer 4 and the second guide layer 5, the energy bandgap of the carrier blocking layer 6 is larger than the maximum energy bandgap of the layers constituting the active layer 4 and the second guide layer 5. Therefore, the carrier blocking layer 6 has a function of suppressing electron overflow. This point will be described later.

[0020] The photonic crystal layer 7 comprises a base layer 7a and a region of different refractive indices 7b, which is periodically formed in a plan view on the base layer 7a and consists of a material with a different refractive index than the base layer 7a. The base layer 7a is made of p-type GaAs with a thickness of 235 nm. The bottom of the region of different refractive indices 7b is in contact with the carrier block layer 6. At the interface between the region of different refractive indices 7b and the carrier block layer 6, the carrier block layer 6 has Al-O bonds or Al-N bonds. This point will also be described later.

[0021] A second cladding layer 8, a reflective layer 9, and a contact layer 10 are formed sequentially on the photonic crystal layer 7. The reflective layer 9 is made of p-type Al x Ga 1―x A 67nm thick first layer made of As (x=0.1) and p-type Al x Ga 1―x The reflective layer 9 consists of 18 alternating layers of a 78 nm thick second layer made of As (x=0.9). The combined optical thickness of the first and second layers corresponds to half the oscillation wavelength. Such a reflective layer 9 functions as a distributed Bragg reflector (DBR). That is, due to the effect of the photonic crystal, a portion of the laser light is diffracted away from the semiconductor substrate 1 (in the +z direction), but the reflective layer 9 reflects this light in the -z direction with high reflectivity, contributing to the output light. With the above configuration of the reflective layer 9, the energy reflectivity for light in the wavelength range of 936 ± 6 nm is 98% or higher.

[0022] The second cladding layer 8 is p-type Al x Ga 1―x The material is As (x=0.4). The thickness of the second cladding layer 8 is adjusted to approximately 940-1080 nm so that the interference between the light diffracted in the -z direction by the photonic crystal effect (hereinafter referred to as substrate-side diffracted light) and the light diffracted in the +z direction and further reflected in the -z direction by the reflective layer 9 (hereinafter referred to as DBR reflected light) is constructive. The contact layer 10 is p-type GaAs with a thickness of 290 nm.

[0023] A protective insulating film 11 is formed on the contact layer 10. The protective insulating film 11 has a circular opening 11a with a diameter L1. L1 = 400 μm. A p electrode 12 having a square planar shape is formed on the contact layer 10 and the protective insulating film 11, which are exposed at the opening 11a. The p electrode 12 is made of Ti / Au, Cr / Au, or Ti / Pt / Au.

[0024] An anti-reflective film 13 and an n-electrode 14 are formed on the back side of the semiconductor substrate 1. The anti-reflective film 13 is circular with a diameter L3. The n-electrode 14 has a square planar shape and has a circular opening 14a inside that encloses the anti-reflective film 13. The n-electrode 14 is a metal multilayer film having, for example, AuGe in contact with the semiconductor substrate 1 and an outermost layer of Au.

[0025] The anti-reflective film 13 transmits and outputs the combined light of the substrate-side diffracted light and DBR reflected light emitted from the back side of the substrate (-z direction). Therefore, the value of L3 is set to be approximately the same as or larger than L2 (the diameter of the region in which the laser light spreads in the in-plane direction), which will be described later. A dielectric monolayer or dielectric multilayer film is used as the anti-reflective film 13. For example, SiO2 or SiN can be used as the dielectric monolayer film. For example, SiO2 can be used as the dielectric multilayer film. x The formula / SiO2 (0≦x<2) is used. The thickness of the anti-reflective coating 13 is adjusted to obtain high transmittance. In particular, when the anti-reflective coating 13 is a dielectric single layer film, the transmittance is maximized by setting the optical thickness of the anti-reflective coating 13 to λ / 4 (where λ is the oscillation wavelength).

[0026] The following describes the operating principle of the photonic crystal surface-emitting laser element. When current is injected into the photonic crystal surface-emitting laser element with the p electrode 12 as the anode and the n electrode 14 as the cathode, light emission occurs due to carrier recombination in the active layer 4. The emission region at this time is determined by the current injection region into the active layer. Since the p electrode 12 is closer to the active layer 4 in the substrate-perpendicular direction (z direction) than the n electrode 14, light emission occurs in the region corresponding to the contact region (circular region with diameter L1) of the p electrode 12. The spectrum of this emission has a broadened range around the wavelength corresponding to the energy band gap of the well layer 4a (around 936 nm in this embodiment).

[0027] The resulting light emission is selectively amplified at specific wavelengths due to the feedback effect of the photonic crystal, causing laser oscillation. The wavelength at this time (oscillation wavelength) is approximately equal to the period a of the photonic crystal within the medium. In the direction perpendicular to the semiconductor substrate 1, the average refractive index of the active layer 4 is higher than that of the first cladding layer 2 and the second cladding layer 8, so a waveguide mode with a peak in light distribution intensity is formed near the active layer 4. The closer the distance between the region of different refractive indices 7b of the photonic crystal layer 7 and the active layer 4 in the direction perpendicular to the substrate (z direction), the stronger the feedback effect of the photonic crystal becomes. Therefore, forming the region of different refractive indices 7b close to the active layer 4 is necessary for efficient light amplification.

[0028] Next, a method for manufacturing a photonic crystal surface-emitting laser element according to this embodiment will be described. Figures 4, 5, 8, and 11 are cross-sectional views showing a method for manufacturing a photonic crystal surface-emitting laser element according to Embodiment 1.

[0029] First, as shown in Figure 4, the first cladding layer 2, the first guide layer 3, the active layer 4, the second guide layer 5, the carrier block layer 6, and the base material layer 7a are epitaxially grown on the semiconductor substrate 1 in that order.

[0030] Next, as shown in Figure 5, a silicon oxide film 15 is formed on the base material layer 7a by plasma CVD, creating multiple pairs of apertures 15a consisting of two types of apertures. This silicon oxide film 15, with its periodically formed pairs of apertures 15a, serves as an etching mask for transferring the photonic crystal pattern. EB exposure (electron beam exposure) and dry etching are used for patterning the pairs of apertures 15a. Here, Figure 6 is a plan view showing the etching mask. Figure 7 is an enlarged plan view of a part of Figure 6. Figure 5 corresponds to a cross-section along AA' in Figure 6.

[0031] Next, as shown in Figure 8, a silicon oxide film 15 in which multiple pairs of openings 15a are periodically formed is used as an etching mask to perform dry etching until the carrier block layer 6 is exposed at least partially through the base layer 7a, thereby forming multiple pairs of through-holes 7c in the base layer 7a in a plan view. These multiple pairs of through-holes 7c become the photonic crystal pattern of the photonic crystal layer 7. SiCl4 / Ar is used as the etching gas. Instead of SiCl4, BCl3, Cl2, HI may be used, or a mixture of several of these may be used. Also, Xe may be used instead of Ar.

[0032] Furthermore, Al-O bonds are formed on the exposed surface of the carrier block layer 6. This mechanism will be described later. Subsequently, the silicon oxide film 15 is removed by wet etching. This transfers the photonic crystal pattern to the base material layer 7a.

[0033] Figure 9 is a plan view showing the base material layer in which the through-hole pair is formed. Figure 10 is an enlarged plan view of a part of Figure 9. Regarding the planar shapes of the two types of apertures constituting the through-hole pair 7c, one is rectangular and the other is square. The centroids of the two types of apertures are separated from each other by a relative distance of dx in the x direction and dy in the y direction. The two types of apertures are arranged in a square lattice with a lattice constant a and the directions of the fundamental translation vectors are in the x and y directions. The lattice constant a is set to be equal to the oscillation wavelength in the medium. In this embodiment, a = 276 nm. Photonic crystals (double-lattice photonic crystals) using such an aperture pair consisting of two types of apertures are disclosed in Patent Document 1, Non-Patent Document 1, etc., and by adjusting the shape, area balance, and relative distance (dx, dy) of each aperture, the diffraction effect can be controlled with a higher degree of freedom compared to a single-lattice photonic crystal in which one type of aperture is arranged in a lattice, making it easier to obtain desired laser characteristics. In this embodiment, both dx and dy are 0.25a.

[0034] Regarding the formation range of the photonic crystal pattern, it is desirable to make it sufficiently large to extend beyond the current injection region, considering that light generated in the current injection region (circular aperture 11a with diameter L1) propagates in the xy-plane direction while being diffracted by the photonic crystal and spreads outwards. That is, if the light spreads out to a circular region of about diameter L2 while being diffracted by the photonic crystal, the photonic crystal formation range should include the circular region of diameter L2 or beyond.

[0035] Next, as shown in Figure 11, the through-hole pairs 7c are closed by epitaxial growth of GaAs, the same material as the base layer 7a, on top of the base layer 7a. The vacancy pairs with closed through-hole pairs 7c become the different refractive index regions 7b. The base layer 7a incorporating the different refractive index regions 7b is the photonic crystal layer 7. Each vacancy pair consists of two vacancies. In the xy plane, the centroids of the two vacancies are separated by a relative distance of dx in the x direction and dy in the y direction. They are arranged in a square lattice with lattice constant a and the directions of the fundamental translation vectors being in the x and y directions. Here, a = 276 nm and dx = dy = 0.25 a. Next, the second cladding layer 8, the reflective layer 9, and the contact layer 10 are epitaxially grown on top of the photonic crystal layer 7 in that order.

[0036] Next, as shown in Figures 1 to 3, a protective insulating film 11 having a circular opening 11a with a diameter L1 is formed on the contact layer 10. A p-electrode 12 having a square planar shape is formed on the opening 11a and the protective insulating film 11. A silicon nitride film with a thickness of 100 nm formed by plasma CVD is used as the protective insulating film 11, and patterning is performed using photolithography and dry etching. EB deposition is used to form the p-electrode 12, and photolithography and lift-off are used for patterning.

[0037] Next, the back surface of the semiconductor substrate 1 is ground to reduce its thickness to 100 μm. Subsequently, a circular anti-reflective film 13 with a diameter of L3 is formed on the back surface of the semiconductor substrate 1. Sputtering is used to form the anti-reflective film 13, and photolithography and dry etching are used for patterning.

[0038] Next, an n-electrode 14 is formed, having a square planar shape and a circular opening 14a inside that encloses the anti-reflective coating 13. EB deposition is used to form the n-electrode 14, and photolithography and lift-off are used for patterning. Finally, the photonic crystal surface-emitting laser element according to this embodiment is manufactured by separating the elements by cleaving them apart.

[0039] The formation of the through-hole pair 7c by dry etching described above will now be explained in detail. Figures 12 and 13 are enlarged cross-sectional views of the vicinity of the through-hole pair immediately after dry etching. These cross-sections correspond to the cross-section along AA' in Figure 9 and show the cross-section of the rectangular hole (hereinafter referred to as rectangular hole 7d) within the through-hole pair 7c. Furthermore, Figures 12 and 13 correspond to cases where dry etching is stopped at different stopping positions.

[0040] Figure 12 shows the case where dry etching is stopped when the lower carrier block layer 6 is exposed at a point in the bottom of the rectangular hole 7d. Generally, when forming holes with a high aspect ratio (hole depth ÷ hole diameter) by dry etching, a phenomenon called shadowing is known where reactive particles irradiated from the plasma are blocked near the side walls of the hole, causing a decrease in the etching rate. Due to this effect, the etching rate decreases relatively near the side walls of the hole, and the carrier block layer 6 is first exposed in the center of the rectangular hole 7d, forming an exposed region 6a. Dry etching is performed using a silicon oxide film 15 as an etching mask, but the silicon oxide film is also physically etched by irradiation with reactive particles from the plasma. As a result, oxygen atoms detached from the silicon oxide film flow into the plasma and reattach inside the hole. When oxygen atoms reattach to the exposed region 6a of the carrier block layer 6, the oxygen atoms bond with Al, a constituent element of the carrier block layer 6, and an Al-O bond is formed on the surface of the exposed region 6a (hereinafter referred to as the exposed surface). Generally, with respect to bonds between different atoms, the greater the difference in electronegativity between the atoms, the greater the bond energy tends to be. Al and O have a large difference in electronegativity, and are known to have a larger bond energy compared to semiconductor materials. For this reason, exposed surfaces with Al-O bonds are less susceptible to dry etching and function as a barrier protecting the layer below the exposed surface from the intrusion of reactive particles. This effect prevents a decrease in the crystal quality of the layer below the base material layer 7a.

[0041] As shown in Figure 12, at the point when the carrier block layer 6 is exposed, the side walls of the holes have a tapered shape due to the shadowing effect. After at least a portion of the through-hole penetrates the base material layer 7a, dry etching is continued until the tapered shape of the side walls of the through-hole disappears. As a result, as shown in Figure 13, the through-hole becomes vertical, and the width of the through-hole does not narrow from the top to the bottom of the photonic crystal layer 7. Also, corresponding to the disappearance of the tapered shape, the exposed area 6a expands to the position of the side wall of the rectangular hole 7d. At this time, the exposed surface is also slightly etched by irradiation with reactive particles, but as mentioned above, the Al-O bond has a high bond energy, and the etching rate is smaller compared to the base material layer 7a which is composed only of Ga-As bonds. For this reason, while the side walls of the through-hole become vertical as dry etching progresses, the amount of downward recession of the exposed surface with Al-O bonds is small, and as a result the bottom of the rectangular hole 7d becomes generally flat. Furthermore, despite the recession of the exposed surface, the formation of new Al-O bonds occurs simultaneously directly beneath it, so the exposed surface recedes downward while remaining protected by Al-O bonds. As long as the carrier block layer 6 has residual thickness in the exposed region 6a, the Al-O bonds on the exposed surface protect the layer below the base material layer 7a from irradiation by reactive particles, preventing a decrease in crystal quality.

[0042] Figures 12 and 13 both show a rectangular hole 7d penetrating the base material layer 7a, but they differ in whether or not the bottom side wall is tapered. During manufacturing, variations in the etching rate occur, so even if the etching time is set with the case of Figure 12 in mind, etching may unintentionally stop before the carrier block layer is exposed, or the situation may unintentionally approach that of Figure 13, within the range of manufacturing variations. Variations in the etching stop position cause variations in the shape of the vacancy pair and its positional relationship with the active layer, resulting in variations in laser characteristics. Therefore, it is desirable to set the dry etching time longer with the case of Figure 13 in mind. Specifically, the etching time should be set so that etching progresses to the extent that the tapered side wall disappears even when the etching rate decreases, and so that the carrier block layer 6 retains some thickness in the exposed region 6a even when the etching rate increases. By doing so, it is possible to suppress variations in hole shape while avoiding etching damage extending to the layers below the base material layer 7a.

[0043] In the above, we described one of the through-hole pairs in a double-lattice photonic crystal (a rectangular hole in this embodiment). However, if the area of ​​the planar shape differs between the two holes (a square hole in this embodiment), the timing at which the bottom of the hole penetrates the base material layer will differ. That is, due to the microloading effect, where the etching rate decreases as the opening area decreases, the hole with a larger area in planar shape penetrates the base material layer 7a first, exposing the carrier block layer 6 below. Therefore, depending on the timing of stopping the dry etching, three situations 1 to 3 shown in Table 1 can be expected. In this case as well, following the idea of ​​suppressing the variation in hole shape described above, it is desirable to set the etching time longer with the intention of achieving situation 3 (i.e., so that the hole with the smaller area in planar shape and lower etching rate approaches the state shown in Figure 13). [Table 1] TIFF0007910699000001.tif53161

[0044] The through-hole pair 7c penetrates the base material layer 7a and is in close proximity to the active layer 4 only through the carrier block layer 6 and the second guide layer 5. Therefore, the feedback effect by the photonic crystal can be maximized. In addition, an Al-O bond is formed at the interface between the bottom of the through-hole pair 7c and the carrier block layer 6. This prevents etching damage from reaching the active layer.

[0045] Furthermore, the carrier block layer 6 has an Al composition ratio (Al x Ga 1―x As(x) is greater than the Al composition ratio of the barrier layer 4b, the second guide layer 5, and the second cladding layer 8, and therefore the energy band gap is larger than that of the barrier layer 4b, the second guide layer 5, and the second cladding layer 8. Thus, the carrier block layer 6 functions as a barrier when electrons supplied from the semiconductor substrate 1 flow beyond the active layer 4 to the second cladding layer. In other words, it can confine electrons within the active layer 4, promote carrier recombination, and increase luminescence efficiency. Due to the above effect, electron overflow can be suppressed even during high-temperature operation or high-current operation, and good laser characteristics can be obtained. On the other hand, since the carrier block layer 6 has a lower refractive index than other layers, if the film thickness is thick, it may repel light and reduce optical confinement in the active layer 4 and the photonic crystal layer 7. In this embodiment, the film thickness of the carrier block layer 6 is thinned to 25 nm, so the above reduction in optical confinement can be avoided while the effect of suppressing electron overflow can be obtained.

[0046] In the laser oscillation state, the laser light is diffracted by the photonic crystal in the xy plane and spreads into a circular region of approximately diameter L2, while a portion of it is also diffracted in the ±z direction within the same region, becoming substrate-side diffracted light and DBR-reflected light. The substrate-side diffracted light and DBR-reflected light interfere with each other to form a composite light that passes through the anti-reflective film 13. This interference depends on the optical path difference between the substrate-side diffracted light and the DBR-reflected light, and the optical path of the DBR-reflected light changes depending on the thickness of the second cladding layer 8 (therefore, constructive interference occurs when the thickness of the second cladding layer is within a certain range). In this way, the composite light is radiated towards the substrate in a circular region of diameter L2, but since the anti-reflective film 13 exists over a circular region of diameter L3 (≧L2), the composite light passes through the anti-reflective film 13 without being blocked.

[0047] Figure 14 is a graph showing the current-peak optical output characteristics when a pulsed current is applied to a photonic crystal surface-emitting laser element. The pulse width of the pulsed current is 50 nsec, and the repetition frequency is 2 kHz. The comparative example and this embodiment differ in the stopping position of the dry etching for the formation of the through-hole pair 7c. The film thickness of the second cladding layer 8 was adjusted in both the comparative example and this embodiment so that the interference between the substrate-side diffracted light and the DBR reflected light is constructive interference. The film thickness of the second cladding layer 8 in the comparative example is 975 nm. The film thickness of the second cladding layer 8 in this embodiment is 1010 nm. The other configurations are the same in the comparative example and this embodiment. From the graph in Figure 14, it can be seen that this embodiment has a lower threshold current and higher optical output than the comparative example. The oscillation wavelength was 940 nm for the comparative example and 930 nm for this embodiment. Although both wavelengths deviate from the central wavelength of PL emission (936 nm), the PL emission intensity at both wavelengths (940 nm and 930 nm) was equivalent. Therefore, the difference in optical output between the comparative example and this embodiment in Figure 14 is not due to a difference in oscillation wavelength. This difference in oscillation wavelength is due to the difference in the depth of the through-hole pair (or void pair). In this embodiment, where the holes are deeper, the average refractive index of the photonic crystal layer is lower, resulting in a shorter oscillation wavelength.

[0048] Figure 15 is an electron microscope image of a vacancy cross-section of a photonic crystal surface-emitting laser element corresponding to the embodiment. Figure 16 is an electron microscope image of a vacancy cross-section of a photonic crystal surface-emitting laser element corresponding to the comparative example. Figures 15 and 16 correspond to cross-sections along BB' in Figure 9, and vacancy pairs are observed simultaneously in one cross-section. In this embodiment, the etching time was set so that both vacancy pairs reached the etching stop position corresponding to Figure 13. As a result, as shown in Figure 15, the vacancy pairs are in contact with the carrier block layer 6, and the carrier block layer 6 has residual thickness. On the other hand, in the comparative example, as shown in Figure 16, the vacancy pairs and the carrier block layer 6 are separated. The high optical output obtained in this embodiment is because the close proximity of the vacancy pairs and the active layer 4 strengthened the feedback effect by the photonic crystal, resulting in efficient amplification of light. Furthermore, the current-peak optical output characteristics of this embodiment show that laser oscillation occurs at a low threshold (approximately 0.7 A) and linearity is maintained up to 50 A. This indicates that, in addition to the fact that the crystal quality of the layer below the photonic crystal layer 7 is good and free of defects, the remaining thickness of the carrier block layer 6 is secured, resulting in an effect of suppressing electron overflow even when a large current is injected.

[0049] Alternatively, the reflective layer 9 may be omitted, and instead, a material that reflects light with high reflectivity may be used for the p electrode 12. Furthermore, for example, a material that absorbs light may be used for the p electrode 12, eliminating the need to contribute diffracted light in the +z direction to the output light. In this case, adjustment of the thickness of the second cladding layer 8 to generate constructive interference becomes unnecessary.

[0050] In this embodiment, a silicon oxide film 15 was used as a mask to supply oxygen atoms during dry etching for the formation of through-hole pairs 7c, but a silicon nitride film may also be used as a mask. The silicon nitride film is physically etched in a dry etching atmosphere, nitrogen atoms flow into the plasma, and Al-N bonds are formed on the exposed surface of the carrier block layer 6. There is a large difference in electronegativity between Al atoms and nitrogen atoms, and the bond energy of the Al-N bond is greater than that of the Ga-As bond. Therefore, the layer having Al-N bonds functions as an etching barrier layer. However, since Al-N has a lower bond energy than Al-O, it is desirable to supply oxygen atoms and use Al-O bonds as the etching barrier to obtain barrier properties.

[0051] In this embodiment, the different refractive index region 7b is formed by closing the through-hole pair 7c formed by dry etching to create a vacancy pair, but the configuration of the different refractive index region 7b is not limited to this. For example, the different refractive index region 7b may be formed by embedding an insulating film such as a silicon oxide film or a semiconductor layer with a different composition from the base material layer 7a in the through-hole pair 7c. However, if the different refractive index region 7b is made up of vacancies, the refractive index of the vacancies is 1, so the refractive index difference with the base material layer 7a becomes larger compared to when other materials (insulating films, semiconductor materials, etc.) are used. The larger the difference in refractive index between the base material layer 7a and the different refractive index region 7b, the stronger the feedback effect of the photonic crystal can be, so it is desirable that the different refractive index region 7b be made up of vacancies.

[0052] As described above, in this embodiment, a silicon oxide film or a silicon nitride film is used as an etching mask in the dry etching of the base material layer 7a. Therefore, dry etching is performed in an atmosphere containing at least one of oxygen atoms and nitrogen atoms until the base material layer 7a is penetrated and the carrier block layer 6, which is an Al-containing layer, is exposed at the bottom of the through-hole pair 7c. As a result, at the interface between the different refractive index region 7b of the photonic crystal layer 7 and the carrier block layer 6, the carrier block layer 6 has Al-O bonds or Al-N bonds. Since these function as a barrier against dry etching, etching damage can be prevented from reaching the active layer 4, and excellent laser properties can be obtained. Furthermore, since it is not necessary to thicken the carrier block layer 6 to prevent etching damage, the photonic crystal layer 7 and the active layer 4 can be brought closer together, so that light confinement in the photonic crystal layer 7 can be enhanced.

[0053] Embodiment 2 In this embodiment, the carrier block layer 6 is omitted from the configuration of Embodiment 1. In this case, the base material layer 7a is directly above the second guide layer 5. Therefore, the second guide layer 5 is exposed during the formation of through-hole pairs 7c by dry etching. The second guide layer 5 is an Al-containing layer that includes Al as a constituent element. Therefore, by using a silicon oxide film or silicon nitride film as an etching mask during the dry etching of the base material layer 7a, Al and oxygen or nitrogen atoms flowing in from the plasma form Al-O bonds or Al-N bonds on the exposed surface of the second guide layer 5. Therefore, as long as the second guide layer 5 has residual thickness below the exposed surface, the Al-O bonds or Al-N bonds on the exposed surface can protect the layer below the photonic crystal layer 7 from irradiation by reactive particles and prevent a decrease in crystal quality. Furthermore, since the through-hole pairs 7c approach the active layer 4 only through the second guide layer 5, light confinement in the photonic crystal layer 7 can be further enhanced, and the feedback effect by the photonic crystal can be further strengthened. Such a configuration is suitable for operation at low temperatures where electron carrier overflow is not a problem, or for operation at low currents (low light output).

[0054] Embodiment 3 Embodiment 1 is a photonic crystal surface-emitting laser element with an oscillation wavelength in the 940 nm band using a GaAs substrate, but this embodiment is a photonic crystal surface-emitting laser element with an oscillation wavelength in the communication wavelength band (oscillation wavelengths of 1.3 μm and 1.55 μm) using an InP substrate. In this case, the first cladding layer 2 is n-type InP. The active layer 4 is i-type Al x Ga y In 1―x-y It has a quantum well structure made of As. The first guide layer 3 is n-type Al x Ga y In 1―x-y It consists of As. The second guide layer 5 is type i Al x Ga y In 1―x-y It consists of As. The base material layer 7a is InP or Ga x In 1―x As y P 1―y The carrier block layer 6 is made of p-type AlInAs. The first guide layer 3 and the second guide layer 5 may consist of multiple layers with different compositions x and y. The second guide layer 5 may also be doped with p-type. The second cladding layer 8 is p-type InP. The minimum energy band gap of the first guide layer 3 and the second guide layer 5 is the same as or greater than the maximum energy band gap of the active layer 4. The energy band gap of the carrier block layer 6 is greater than the maximum energy band gap of the active layer 4 and the second guide layer 5.

[0055] In this embodiment as well, by using a silicon oxide film or silicon nitride film as an etching mask during dry etching of the base material layer 7a, Al-O or Al-N bonds are formed in the carrier block layer 6. Due to the difference in electronegativity, the bond energies of Al-O and Al-N are greater than the bond energies of Ga or In and As or P. Therefore, the Al-O or Al-N bonds protect the layer below the photonic crystal layer 7 from irradiation by reactive particles and prevent a decrease in crystal quality. Furthermore, since the energy band gap of the carrier block layer 6 is greater than the maximum value of the energy band gap of the active layer 4 and the second guide layer 5, it functions as a barrier when electrons supplied from the substrate side jump over the active layer 4 and flow into the second cladding layer 8 side. Therefore, electron overflow is suppressed even during high-temperature operation or high-current operation, and good laser characteristics can be obtained. In this embodiment, the active layer 4 is made of i-type Ga x In 1―x As y P 1―y A single or multiple quantum well structure consisting of a first guide layer 3 made of n-type Ga x In 1―x As y P 1―y A single-layer or multi-layer structure consisting of, a second guide layer 5 of i-type or p-type Ga x In 1―x As y P 1―y It may also be a single-layer or laminated structure consisting of these.

[0056] Note that the carrier block layer 6 in Embodiment 3 can be omitted. In this case, the material directly beneath the base material layer 7a is i-type Al. x Ga y In 1―x-yThe second guide layer 5 is made of As. Therefore, the second guide layer 5 is exposed during the formation of through-hole pairs 7c by dry etching. The second guide layer 5 is an Al-containing layer that includes Al as a constituent element. Therefore, by using a silicon oxide film or silicon nitride film as an etching mask during the dry etching of the base material layer 7a, Al and oxygen or nitrogen atoms flowing in from the plasma form Al-O bonds or Al-N bonds on the exposed surface of the second guide layer 5. Therefore, as long as the second guide layer 5 has residual thickness below the exposed surface, the Al-O bonds or Al-N bonds on the exposed surface can protect the layer below the photonic crystal layer 7 from irradiation by reactive particles and prevent a decrease in crystal quality. In addition, since the through-hole pairs 7c approach the active layer 4 only through the second guide layer 5, light confinement in the photonic crystal layer 7 can be further enhanced, and the feedback effect by the photonic crystal can be further strengthened. Such a configuration is suitable for operation at low temperatures where electron carrier overflow is not a problem, or for operation at low currents (low light output).

[0057] Embodiment 4 In this embodiment, an etching gas containing at least one of oxygen atoms and nitrogen atoms is used in the dry etching of the base material layer 7a. For example, O2, N2, and N2O are used as etching gases. This allows oxygen atoms or nitrogen atoms to be supplied to the exposed surface of the carrier block layer 6.

[0058] When a gas containing oxygen or nitrogen atoms is supplied into the plasma, the oxygen or nitrogen atoms dissociate. When the dissociated oxygen or nitrogen atoms reach the exposed region 6a, they bond with Al, a constituent element of the carrier block layer 6, at the exposed surface, forming Al-O bonds or Al-N bonds. Therefore, the Al-O bonds or Al-N bonds at the exposed surface protect the layer below the photonic crystal layer 7 from irradiation by reactive particles and prevent a decrease in crystal quality.

[0059] Embodiment 5 Figure 17 shows the process of dry etching the base material layer in Embodiment 5. A stage 17 is provided inside the chamber 16 of the dry etching apparatus. A semiconductor substrate 1 with the etching mask shown in Figure 5 is placed on the stage 17, and a stage protection member 18 is arranged around it. The stage protection member 18 has at least an oxide or nitride on its surface. For example, quartz can be used as the stage protection member 18. Alternatively, a member coated with a silicon oxide film or a silicon nitride film of SiC may be used as the stage protection member 18. That is, in dry etching, at least one of silicon oxide and silicon nitride is used for the member inside the chamber 16 of the dry etching apparatus.

[0060] The reactive particles irradiated from the plasma physically etch (sputter) not only the wafer to be processed but also the stage protection member 18. As a result, desorbed atoms 19 flow into the plasma. The desorbed atoms 19 are oxygen atoms if the surface of the stage protection member 18 is an oxide, and nitrogen atoms if the stage protection member 18 is a nitride. When these oxygen or nitrogen atoms reach the exposed region 6a, they bond with Al, a constituent element of the carrier block layer 6, at the exposed surface, forming Al-O bonds or Al-N bonds. The Al-O bonds or Al-N bonds on the exposed surface protect the layer below the photonic crystal layer 7 from irradiation by reactive particles, preventing a decrease in crystal quality.

[0061] Furthermore, Embodiment 4 or 5 may be combined with Embodiments 1 to 3. That is, oxygen atoms or nitrogen atoms may be supplied by multiple means among etching masks, additive gases, and chamber members. By doing so, the amount of oxygen atoms or nitrogen atoms supplied can be increased, ensuring that the exposed surface is covered with Al-O bonds or Al-N bonds, and enhancing the etching barrier effect on the layer below the photonic crystal layer 7.

[0062] In the above embodiment, a double-lattice photonic crystal was used, but the invention is not limited to this. For example, a single-lattice photonic crystal in which a single pattern such as a circle or a right triangle is arranged in a square lattice may be used. Alternatively, a photonic crystal in which one or more patterns are arranged in a triangular lattice may be used. [Explanation of Symbols]

[0063] 1 Semiconductor substrate, 2 First cladding layer, 4 Active layer, 5 Second guide layer (Al-containing layer), 6 Carrier block layer (Al-containing layer), 7 Photonic crystal layer, 7a Base material layer, 7b Different refractive index region, 7c Through-hole pair (Through-hole), 8 Second cladding layer, 15 Silicon oxide film (Etching mask), 15a Aperture pair (Aperture), 18 Stage protection member (Chamber member)

Claims

1. Semiconductor substrate and The semiconductor substrate comprises, in order, a first cladding layer of a first conductivity type, an active layer, an Al-containing layer, a photonic crystal layer, and a second cladding layer of a second conductivity type, The photonic crystal layer comprises a base material layer and regions with different refractive indices that are periodically formed in the base material layer in a plan view and are made of a material with a different refractive index than the base material layer. The aforementioned Al-containing layer contains Al as a constituent element. The bottom of the region with different refractive indices is in contact with the Al-containing layer. A photonic crystal surface-emitting laser element characterized in that the Al-containing layer has Al-O bonds or Al-N bonds at the interface between the different refractive index region and the Al-containing layer.

2. The photonic crystal surface emitting laser element according to claim 1, characterized in that the energy band gap of the Al-containing layer is larger than the maximum energy band gap of the layer constituting the active layer.

3. The photonic crystal surface emitting laser element according to claim 1 or 2, characterized in that the width of the different refractive index region does not narrow from the top to the bottom of the photonic crystal layer.

4. The photonic crystal surface-emitting laser element according to claim 1 or 2, characterized in that the region with different refractive indices consists of vacancies.

5. A process of sequentially forming a first cladding layer of a first conductivity type, an active layer, an Al-containing layer containing Al as a constituent element, and a base material layer consisting of a photonic crystal base material on a semiconductor substrate, A step of forming an etching mask on the base material layer in which a plurality of openings are periodically formed, and performing dry etching using the etching mask until the Al-containing layer is exposed by penetrating the base material layer, thereby forming a plurality of periodically formed through holes in the base material layer in a plan view, The process includes forming a second cladding layer of a second conductivity type on the base material layer having the plurality of through holes formed thereon, A method for manufacturing a photonic crystal surface-emitting laser element, characterized by performing the dry etching in an atmosphere containing at least one of oxygen atoms and nitrogen atoms.

6. The method for manufacturing a photonic crystal surface-emitting laser element according to claim 5, characterized in that the energy band gap of the Al-containing layer is larger than the maximum energy band gap of the layer constituting the active layer.

7. A method for manufacturing a photonic crystal surface-emitting laser element according to claim 5 or 6, characterized in that the dry etching is continued after at least a portion of the through-hole penetrates the base material layer until the hem of the side wall of the through-hole disappears.

8. The method for manufacturing a photonic crystal surface-emitting laser element according to claim 5 or 6, further comprising the step of epitaxially growing a semiconductor on the base material layer to close the through-holes.

9. The method for manufacturing a photonic crystal surface-emitting laser element according to claim 5 or 6, characterized in that a silicon oxide film or a silicon nitride film is used as the etching mask in the dry etching process.

10. The method for manufacturing a photonic crystal surface emitting laser element according to claim 5 or 6, characterized in that an etching gas containing at least one of oxygen atoms and nitrogen atoms is used in the dry etching.

11. The method for manufacturing a photonic crystal surface emitting laser element according to claim 5 or 6, characterized in that at least one of silicon oxide and silicon nitride is used as an internal component of the chamber of the dry etching apparatus in the dry etching process.

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