JP4639571B2 - Nitride semiconductor laser device and manufacturing method thereof - Google Patents

Description

[0001]
[Industrial application fields]
  The present invention relates to a nitride semiconductor (In_aAl_bGa_1-abN, 0 ≦ a, 0 ≦ b, a + b ≦ 1).
[0002]
[Prior art]
  We have produced a nitride semiconductor laser device including an active layer on a nitride semiconductor substrate, and have achieved the world's first continuous oscillation at room temperature of 10,000 hours or more (ICNS'97 Proceedings, October 27-31, 1997, P444-446, and Jpn. J. Appl. Phys. Vol. 36 (1997) pp. L1568-1571, Part 2, No. 12A, 1 December 1997). As a basic structure, partially formed SiO on the sapphire substrate₂A plurality of nitride semiconductor layers serving as laser element structures are stacked on a nitride semiconductor substrate made of n-GaN selectively grown through a film. (For details, refer to Jpn.J.Appl.Phys.Vol.36)
[0003]
[Problems to be solved by the invention]
  FIG. 6 is a schematic cross-sectional view showing one structure of a conventional laser element. This figure is almost the same as the figure shown in the above-mentioned J.J.A.P. As shown in this figure, in the conventional laser element, p-Al_0.14Ga_0.86A ridge is provided above a p-side cladding layer made of an N / GaN superlattice structure and a p-side contact layer made of p-GaN. The ridge is formed across the side surface of the ridge and the plane of the p-side cladding layer.₂An insulating film is formed, and a p-electrode electrically connected to the p-GaN layer through the insulating film is formed. The stripe width of the ridge is adjusted to be very narrow, for example, 10 μm or less. It is difficult to form an ohmic p-electrode on the outermost surface of the ridge having a narrow stripe width, and to bond directly on the p-electrode. It is. Therefore, as shown in this figure, a p-pad electrode that is electrically connected to the p-electrode and has a larger area than the p-electrode is formed via an insulating film formed on the surface of the p-side cladding layer. .
[0004]
  However, SiO₂Since the insulating film made of this material is usually formed using PVD technology such as sputtering or vapor deposition, it is highly insulating SiO₂In many cases, the insulating property of the Si oxide tends to be insufficient. If the insulation is insufficient, a current flows through the p-side cladding layer other than the ridge, which causes the threshold value to rise. In addition, the p-side cladding layer has a thickness of 1.0 μm or less, which is very thin compared to other semiconductor materials. Further, since the p-side cladding layer is grown from a nitride semiconductor containing Al, fine holes (pits) are formed in the cladding layer. Likely to happen. If the insulating film formed on the p-side cladding layer has insufficient insulation, a current may flow from the pits, causing a short circuit.
[0005]
  On the other hand, regarding the electrode forming method, a very fine operation is required to provide an ohmic p-electrode on the surface of the p-side contact layer having a narrow stripe width. If an insulating film with a uniform film thickness is not provided on the surface of the p-side cladding layer when forming the insulating film, current concentrates on the thin film, causing a short circuit. Furthermore, when forming ridge stripes, SiO₂Is used as a mask, the protective film on the top of the cladding layer is made of the same SiO₂It is difficult in terms of the process because both are dissolved in the same hydrofluoric acid.
[0006]
  Accordingly, an object of the present invention is to provide a highly reliable laser element in which a highly insulating insulating film is provided on the surface of the p-side cladding layer in the laser element in which the ridge stripe is provided on the p-layer side. It is another object of the present invention to provide an electrode forming method that can easily form an electrode through an insulating film formed by a simple method. Furthermore, the threshold value of the laser element is lowered by providing a novel electrode structure.
[0007]
[Means for Solving the Problems]
  The nitride semiconductor laser device of the present invention includes a p-side cladding layer containing a first p-type nitride semiconductor on an active layer, and a second p-type nitride semiconductor on the p-side cladding layer. In a nitride semiconductor laser device in which a p-side contact layer is stacked, etched from the p-side contact layer side, and provided with a stripe-shaped waveguide region in a layer below the p-side contact layer, the stripe An insulating film other than Si oxide and having a refractive index smaller than that of the nitride semiconductor is formed on both sides of the stripe of the waveguide and on the plane of the nitride semiconductor layer continuous with the side surface. An electrode is provided on the surface of the contact layer in the uppermost layer of the stripe, and the insulating film is an oxide containing at least one element selected from Zr and Hf, BN, Si Characterized in that it consists of at least one of the.
  Furthermore, a plane of the nitride semiconductor continuous with both side surfaces of the stripe is on the substrate side with respect to the lower end surface of the p-side cladding layer.
  Furthermore, a p-side light guide layer is provided between the active layer and the p-side cladding layer.
  Furthermore, the p-side clad layer has a superlattice structure including at least one nitride semiconductor layer containing Al and having nitride semiconductor layers having different band gap energies.
  A p-side contact layer including a second p-type nitride semiconductor is stacked on the p-side cladding layer including the first p-type nitride semiconductor, and etched from the p-side contact layer side. In a nitride semiconductor laser device in which a stripe-shaped waveguide region is provided in a layer below the p-side contact layer, both sides of the stripe of the stripe waveguide and a plane of the nitride semiconductor layer continuous with the side surface An insulating film other than Si oxide and having a refractive index smaller than that of a nitride semiconductor is formed, and an electrode is provided on the surface of the contact layer at the uppermost layer of the stripe via the insulating film.Be.
[0008]
  In the laser element of the present invention, the plane of the nitride semiconductor continuous with the side surface of the stripe is closer to the substrate side from the lower end surface to the p-side contact layer direction of 0.2 μm in the film thickness direction of the p-side cladding layer.There may be.
[0009]
  Further, the plane of the nitride semiconductor continuous with the side surface of the stripe is below the lower end surface of the p-side cladding layer.There may be.This greatly reduces the threshold value of the laser element to 2/3 or less, which is very preferable. The lower end surface of the p-side cladding layer refers to the interface between the base layer on which the p-side cladding layer is formed and the p-side cladding layer. Moreover, 0.2 μm in the p-side contact layer direction from the lower end surface indicates a state in which 0.2 μm of the p-side cladding layer remains from the interface.
[0010]
  Furthermore, in the laser element of the present invention, the stripe width has a range of 4 μm to 0.5 μm.May be.More preferably, it adjusts to 3 micrometers-1 micrometer. If the width is larger than 4 μm, the transverse mode tends to be multimode, and if it is smaller than 0.5 μm, it is difficult to form a stripe, and the contact area with the electrode is small, so the threshold value is likely to rise.
[0011]
  As the insulating film, it is desirable to select at least one of oxides containing at least one element selected from the group consisting of Ti, V, Zr, Nb, Hf, and Ta, BN, SiC, and AlN. Preferably, oxides of Zr and Hf, BN, and SiC are used. Note that SiC is an insulator because it becomes amorphous in film formation by PVD such as sputtering and vapor deposition, and SiC that does not contain n-type and p-type impurities is also an insulator.
[0012]
  The laser element of the present inventionAs a manufacturing method ofAfter a p-side contact layer including a second p-type nitride semiconductor is stacked on the p-side cladding layer including the first p-type nitride semiconductor, a stripe-shaped first layer is formed on the surface of the p-side contact layer. A first step of forming one protective film, and a portion of the nitride semiconductor where the first protective film is not formed is etched through the first protective film, and a stripe shape is formed directly under the protective film. The second step of forming the waveguide region, and after the second step, the second protective film, which is made of a material different from the first protective film and has an insulating property, is etched on the side surface of the stripe waveguide and A third step of forming the nitride semiconductor layer exposed on the plane, and after the third step, the first protective film is removed, and the second protective film and the uppermost p-type nitride semiconductor layer A fourth process for forming an electrode electrically connected to the p-side contact layer on the surface of Characterized by including and.
[0013]
  As a method for manufacturing the laser element of the present invention,In the second step, the etching stop is a planar surface of the nitride semiconductor located on the substrate side from the lower end surface in the p-side contact layer direction of 0.2 μm in the film thickness direction of the p-side cladding layer. Needless to say, etching stop is a layer that stops etching. After this etching stop, a stripe-shaped waveguide region is formed in the nitride semiconductor, and the plane of the nitride semiconductor continuous with the side surface of the stripe is exposed.
[0014]
  In the second step, the etching stop is a plane of the nitride semiconductor located on the substrate side with respect to the lower end surface of the p-side cladding layer. By setting the etching stop to the nitride semiconductor plane located on the substrate side with respect to the lower end surface of the p-side cladding layer, the threshold value of the laser element is significantly lowered.
[0015]
  The laser element of the present inventionAs a manufacturing method ofIn the first step, after forming a first protective film on substantially the entire upper surface of the p-type nitride semiconductor layer and forming a stripe-shaped third protective film on the first protective film, The first protective film is preferably formed by a step of etching the first protective film in a stripe shape through the third protective film. That is, this shows a method for forming the first protective film.
[0016]
  The laser element of the present inventionAs a manufacturing method ofIn the first step, the first protective film is preferably formed by a lift-off method.
[0017]
  FirstThe protective film 1 is made of an oxide of Si, and the second protective film is an oxide containing at least one element selected from the group consisting of Ti, V, Zr, Nb, Hf, and Ta, or BN At least one of SiC, SiC, and AlN is selected, and an oxide of Zr and Hf, BN, and SiC are more preferably used as the second protective film. When the first protective film and the second protective film are made of these materials, only the first protective film can be removed by lift-off due to the solubility difference and the etching rate difference of the protective film.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
  FIG. 1 is a schematic cross-sectional view showing a partial structure of a nitride semiconductor wafer for explaining the steps of the electrode forming method of the present invention, and is perpendicular to the stripe waveguide formed by etching, that is, The figure at the time of cut | disconnecting in a parallel direction with respect to the resonance surface is shown. In the first step of the present invention, as shown in FIG. 1C, a stripe-shaped first protective film 61 is formed on the p-side contact layer 13 as the uppermost layer.
[0019]
  The first protective film 61 is not particularly limited in insulation, and any material may be used as long as it has a difference from the etching rate of the nitride semiconductor. For example, Si oxide (SiO₂In order to provide a difference in solubility from the second protective film to be formed later, it is more easily dissolved in acid than the second protective film. Select the material. As the acid, hydrofluoric acid is preferably used, and therefore Si oxide is preferably used as a material that is easily dissolved in hydrofluoric acid. The stripe width (W) of the first protective film is adjusted to 4 μm to 0.5 μm, preferably 3 μm to 1 μm. The stripe width of the first protective film 61 roughly corresponds to the stripe width of the waveguide region.
[0020]
  FIGS. 1A and 1B show specific steps for forming the first protective film 61. That is, as shown in FIG. 1A, a first protective film 61 is formed on almost the entire surface of the p-side contact layer 13, and then a striped third film is formed on the first protective film 61. The protective film 63 is formed. Thereafter, as shown in FIG. 1B, if the third protective film 63 is removed after the first protective film 61 is etched with the third protective film 63 attached, the third protective film 63 is removed. A stripe-shaped first protective film 61 as shown in FIG. It is also possible to perform etching from the p-side contact layer 13 side by changing the etching gas or the etching means with the third protective film 63 attached.
[0021]
  A lift-off method can also be used to form the stripe-shaped first protective film 61 as shown in FIG. That is, a photoresist having a shape in which stripe-shaped holes are formed is formed, a first protective film is formed on the entire surface of the photoresist, and then the photoresist is dissolved and removed to contact the p-side contact layer. This is a means for leaving only the first protective film. It is to be noted that, when the first protective film having a stripe shape is formed by the lift-off method, a stripe having a substantially vertical end face and a uniform shape can be obtained by etching as shown in FIGS. 1 (a) and 1 (b). It tends to be easy.
[0022]
  Next, in the second step of the present invention, as shown in FIG. 1 (d), from the p-side contact layer 13 where the first protective film 61 is not formed via the first protective film 61. Etching is performed to form a striped waveguide region corresponding to the shape of the protective film immediately below the first protective film 61. When etching is performed, the structure and characteristics of the laser element differ depending on the position of the etch stop. The etch stop may be stopped at any nitride semiconductor layer as long as it is a layer below the p-side contact layer. In the example shown in FIG. 1, the middle of the p-side cladding layer 12 under the p-side contact layer 13 is used as an etch stop. When the substrate side from the lower end face of the p-side cladding layer is 0.2 μm away from the p-side contact layer direction 0.2 μm, the stripe becomes a ridge and a refractive index waveguide type laser element can be obtained. The lower end surface refers to the surface of the lowermost clad layer with respect to the thickness direction. When the optical guide layer is located under the clad layer as described above, the interface between the guide layer and the clad layer is lowered. It corresponds to the end face. When the etch stop is set above the lower end surface, the etching time is shortened and the etching rate is easily controlled, which is convenient in terms of production technology. Further, since the p-side cladding layer exists under the stripe, the threshold value tends to increase, but an element with few shorts between electrodes can be easily achieved.
[0023]
  On the other hand, although not shown in FIG. 1, the etch stop may be a nitride semiconductor below the lower end surface of the p-side cladding layer. When the layer on the substrate side with respect to the lower end surface is used as an etch stop, although the exposed area of the side surface is large and the electrode tends to be short-circuited, the threshold tends to be remarkably lowered, which is preferable.
[0024]
  As the etching means, for example, when dry etching such as RIE (reactive ion etching) is used, in order to etch the first protective film made of Si oxide frequently used in the first step, CF is used.₄It is desirable to use a fluorine compound-based gas such as Cl, which is often used in other III-V compound semiconductors to etch nitride semiconductors in the second step.₂, CCl₄, SiCl₄It is desirable to use a chlorine-based gas such as that because the selectivity with Si oxide can be increased.
[0025]
  Next, in the third step, as shown in FIG. 1E, a second protective film 62 made of a material different from that of the first protective film 61 and having insulating properties is formed on the side surface of the striped waveguide. And the nitride semiconductor layer exposed by etching (in FIG. 1e, the p-side cladding layer 12). Since the first protective film 61 is made of a material different from that of the second protective film 62, the first protective film 61 has selectivity with the second protective film with respect to the etching means. Therefore, if only the first protective film 61 is removed later with, for example, hydrofluoric acid, the second continuous with both the surface of the p-type cladding layer 12 and the side surface of the stripe as shown in FIG. The protective film 62 can be formed. By continuously forming the second protective film, high insulation can be maintained. In addition, when the second protective film 62 is formed continuously from the first protective film 61, it can be formed with a uniform film thickness on the p-side cladding layer 12, so that the film thickness is less likely to be uneven. Concentration of current due to non-uniform thickness does not occur. Since the etch stop is in the middle of the p-side cladding layer 12 in the second step, the second protective film 62 is formed on the plane of the p-side cladding layer in FIG. It goes without saying that the second protective film is naturally formed on the plane of the nitride semiconductor layer that has been etched-stopped below the side cladding layer 12.
[0026]
  The material of the second protective film is SiO₂It is desirable to form with a material other than the above, preferably an oxide containing at least one element selected from the group consisting of Ti, V, Zr, Nb, Hf, Ta, BN, SiC, AlN, Among these, it is particularly preferable to use an oxide of Zr or Hf, BN, or SiC. Some of these materials have a property of being slightly dissolved in hydrofluoric acid. However, if the insulating layer of the laser element is used, SiO2 can be used as a buried layer.₂Tend to be much more reliable. Also, oxide-based thin films formed in the gas phase such as PVD and CVD are less likely to be oxides in which the elements and oxygen are equivalently reacted, and thus the reliability of the oxide-based thin film tends to be insufficient. However, PVD, CVD oxides, BN, SiC, and AlN of the elements selected in the present invention tend to be more reliable in terms of insulation than Si oxides. In addition, it is very convenient as a buried layer of a laser element when an oxide whose refractive index is smaller than that of a nitride semiconductor (for example, other than SiC) is selected. Furthermore, when the first protective film 61 is made of Si oxide, since it has selectivity for hydrofluoric acid with respect to the Si oxide, the side surface of the stripe waveguide as shown in FIG. When the stripe is formed continuously on the plane (etch stop layer) and the surface of the first protective film 61, when only the first protective film 61 is removed by a lift-off method, FIG. The second protective film 62 having a uniform film thickness with respect to the plane can be formed as shown in FIG.
[0027]
  Next, in the fourth step of the present invention, as shown in FIG. 1 (f), after removing the first protective film 61, next, as shown in FIG. 1 (g), the second protective film 62. A p-electrode electrically connected to the p-side contact layer is formed on the p-side contact layer 13. In the present invention, since the second protective film is formed first, it is not necessary to form the p electrode only in the contact layer with a narrow stripe width, and it can be formed in a large area. In addition, an electrode material that also serves as an ohmic contact can be selected to form an ohmic and an electrode that serves as a bonding electrode together.
[0028]
  In the nitride semiconductor laser device, when forming a striped waveguide region, dry etching is used because etching is difficult by wet etching. In dry etching, since the selectivity between the first protective film and the nitride semiconductor is regarded as important, SiO as the first protective film is used.₂Is used. However, SiO₂Is used for the second protective film formed on the plane of the etch-stopped layer, because the insulation is insufficient and is the same material as the first protective film, so only the first protective film is used. It becomes difficult to remove. Therefore, in the present invention, the second protective film is made of SiO.₂If a material other than the above is used, selectivity with the first protective film can be obtained, and the nitride semiconductor is not etched after the formation of the second protective film. Therefore, the second protective film has an etching rate with the nitride semiconductor. There is no problem with this.
[0029]
【Example】
[Example 1]
  FIG. 2 is a schematic cross-sectional view showing the structure of a laser device according to an embodiment of the present invention, and shows a view when cut in a direction perpendicular to the stripe waveguide. Hereinafter, Example 1 is demonstrated based on this figure.
[0030]
(Underlayer 2)
  A heterogeneous substrate 1 made of sapphire having a 1-inch φ and C-plane as the main surface is set in a MOVPE reaction vessel, and the temperature is set to 500 ° C.₃), A buffer layer made of GaN is grown to a thickness of 200 angstroms. After growing the buffer layer, the temperature is set to 1050 ° C., and the underlayer 2 made of GaN is grown to a thickness of 4 μm. This underlayer functions as an underlayer for forming a protective film partially on the surface and then performing selective growth of the nitride semiconductor substrate.
[0031]
(Protective film 3)
  After the underlayer growth, the wafer is taken out from the reaction vessel, a striped photomask is formed on the surface of the underlayer, and a PVD apparatus is used to reduce the stripe width to 10 μm and the stripe interval (window) to 2 μm.₂A protective film 3 is formed. As for the shape of the protective film, a nitride semiconductor substrate with fewer crystal defects to be grown is obtained when the area of the protective film is made larger than that of the stripe window. As a material of the protective film, for example, silicon oxide (SiO_X), Silicon nitride (Si_XN_Y), Titanium oxide (TiO_X), Zirconium oxide (ZrO)_XIn addition to oxides and nitrides such as), and multilayer films thereof, metals having a melting point of 1200 ° C. or higher can be used. These protective film materials withstand the nitride semiconductor growth temperature of 600 ° C. to 1100 ° C., and have a property that the nitride semiconductor does not grow or hardly grow on the surface thereof. The protective film 3 is a protective film for growing the nitride semiconductor substrate 4 and is different from the protective film of the method of the present invention.
[0032]
(Nitride semiconductor substrate 4)
  After forming the protective film, the wafer is set again in the MOVPE reaction vessel, the temperature is set to 1050 ° C., and the nitride semiconductor substrate 4 made of undoped GaN is grown to a thickness of 20 μm using TMG and ammonia. Since this nitride semiconductor substrate is grown laterally on the protective film 3, the crystal defect is 10%.⁵Pieces / cm²Compared to the following and the underlayer 2, the number is reduced by two orders of magnitude or more.
[0033]
(N-side contact layer 5)
  Next, ammonia and TMG are used, and silane gas is used as an impurity gas. On the nitride semiconductor substrate 1, 3 × 10 Si is added at 1050 ° C.¹⁸/cm³An n-side contact layer 5 made of doped GaN is grown to a thickness of 4 μm.
[0034]
(Crack prevention layer 6)
  Next, using TMG, TMI (trimethylindium), and ammonia, the temperature is set to 800 ° C. and In_0.06Ga_0.94A crack prevention layer 6 made of N is grown to a thickness of 0.15 μm. This crack prevention layer can be omitted.
[0035]
(N-side cladding layer 7)
  Subsequently, undoped Al was used at 1050 ° C. using TMA (trimethylaluminum), TMG, and ammonia._0.16Ga_0.84A layer made of N is grown to a film thickness of 25 Å, then TMA is stopped, silane gas is flowed, and Si is 1 × 10 × 10.¹⁹/cm³A layer made of doped n-type GaN is grown to a thickness of 25 Å. These layers are alternately laminated to form a superlattice layer, and an n-side cladding layer 7 made of a superlattice having a total thickness of 1.2 μm is grown.
[0036]
(N-side light guide layer 8)
  Subsequently, the silane gas is stopped, and an n-side light guide layer 8 made of undoped GaN is grown at 1050 ° C. to a thickness of 0.1 μm. The n-side light guide layer 8 may be doped with n-type impurities.
[0037]
(Active layer 9)
  Next, the temperature is set to 800 ° C. and Si-doped In_0.05Ga_0.95A barrier layer of N is grown to a thickness of 100 Å, and then at the same temperature, undoped In_0.2Ga_0.8A well layer made of N is grown to a thickness of 40 Å. A barrier layer and a well layer are alternately stacked twice, and finally an active layer having a multi-quantum well structure (MQW) having a total film thickness of 380 Å is grown by ending with the barrier layer. The active layer may be undoped as in this embodiment, or may be doped with n-type impurities and / or p-type impurities. Impurities may be doped into both the well layer and the barrier layer, or one of them may be doped. Note that if the n-type impurity is doped only in the barrier layer, the threshold value tends to decrease.
[0038]
(P-side cap layer 10)
  Next, the temperature is increased to 1050 ° C., and TMG, TMA, ammonia, Cp₂Mg (cyclopentadienylmagnesium) is used and has a band gap energy larger than that of the p-side light guide layer 11.²⁰/cm³Doped p-type Al_0.3Ga_0.7A p-side cap layer 7 made of N is grown to a thickness of 300 angstroms.
[0039]
(P-side light guide layer 11)
  Next, Cp₂The Mg and TMA are stopped, and the p-side light guide layer 11 made of undoped GaN having a band gap energy smaller than that of the p-side cap layer 10 is grown at a thickness of 0.1 μm at 1050 ° C.
[0040]
(P-side cladding layer 12)
  Subsequently, undoped Al at 1050 ° C._0.16Ga_0.84A layer of N is grown to a thickness of 25 Angstroms, followed by Cp₂After stopping Mg and TMA, a layer made of undoped GaN is grown to a thickness of 25 Å, and a p-side cladding layer 12 made of a superlattice layer having a total thickness of 0.6 μm is grown. When the p-side cladding layer is made of a superlattice in which at least one nitride semiconductor layer containing Al is included and nitride semiconductor layers having different bandgap energies are stacked, impurities are heavily doped into one of the layers. Although so-called modulation doping tends to improve the crystallinity, both may be doped in the same manner. The cladding layer 12 is a nitride semiconductor layer containing Al, preferably Al_XGa_1-XA superlattice structure including N (0 <X <1) is desirable, and a superlattice structure in which GaN and AlGaN are stacked is more preferable. By making the p-side cladding layer 12 a superlattice structure, the Al mixed crystal ratio of the entire cladding layer can be increased, so that the refractive index of the cladding layer itself is reduced and the band gap energy is increased. It is very effective in lowering. Furthermore, since the superlattice is used, the number of pits generated in the cladding layer itself is less than that of the superlattice, so that the probability of short-circuiting is reduced.
[0041]
(P-side contact layer 13)
  Finally, at 1050 ° C., 1 × 10 5 Mg was deposited on the p-side cladding layer 9.²⁰/cm³A p-side contact layer 13 made of doped p-type GaN is grown to a thickness of 150 Å. The p-side contact layer is p-type In_XAl_YGa_1-XYN (0.ltoreq.X, 0.ltoreq.Y, X + Y.ltoreq.1), preferably Mg-doped GaN, provides the most preferable ohmic contact with the p-electrode 20. Since the contact layer 13 is a layer for forming an electrode, 1 × 10¹⁷/cm³It is desirable to have the above high carrier concentration. 1 × 10¹⁷/cm³If it is lower than that, it tends to be difficult to obtain a preferable ohmic with the electrode. Furthermore, when the composition of the contact layer is GaN, a preferable ohmic with the electrode material is easily obtained.
[0042]
  The wafer on which the nitride semiconductor has been grown as described above is taken out of the reaction vessel, and SiO 2 is deposited on the surface of the uppermost p-side contact layer.₂A protective film is formed, and SiCl is formed using RIE (reactive ion etching).₄Etching with gas exposes the surface of the n-side contact layer 5 where the n-electrode is to be formed, as shown in FIG. In order to etch a nitride semiconductor deeply in this way, a protective film is SiO.₂Is the best.
[0043]
  Next, the electrode forming method of the present invention will be described in detail. First, as shown in FIG. 1A, an Si oxide (mainly SiO 2) is formed on almost the entire surface of the uppermost p-side contact layer 13 by a PVD apparatus.₂) Is formed to a thickness of 0.5 μm, a mask having a predetermined shape is put on the first protective film 61, and a third protective film 63 made of photoresist is formed. The stripe width is 2 μm and the thickness is 1 μm.
[0044]
  Next, as shown in FIG. 1B, after the third protective film 63 is formed, CF is performed by an RIE (reactive ion etching) apparatus.₄Using gas, the first protective film is etched using the third protective film 63 as a mask to form stripes. Thereafter, the first protective film 61 having a stripe width of 2 μm can be formed on the p-side contact layer 13 as shown in FIG.
[0045]
  Further, as shown in FIG. 1 (d), after forming the first protective film 61 in a stripe shape, SiCl is again performed by RIE.₄The p-side contact layer 13 and the p-side cladding layer 12 are etched using gas to form a striped waveguide region (in this case, a ridge stripe). When forming the stripe, it is very preferable that the cross-sectional shape of the stripe is a forward mesa shape as shown in FIG. This stripe shape is applicable to all shapes of the stripe waveguide of the present invention.
[0046]
  After forming the ridge stripe, the wafer is transferred to a PVD apparatus, and as shown in FIG. 1 (e), Zr oxide (mainly ZrO₂The second protective film 62 is formed continuously on the first protective film 61 and on the p-side cladding layer 12 exposed by etching to a thickness of 0.5 μm.
[0047]
  After forming the second protective film 62, the wafer is heat-treated at 600 ° C. In this way SiO₂When a material other than the above is formed as the second protective film, after the second protective film is formed, by performing heat treatment at 300 ° C. or higher, preferably 400 ° C. or higher, and below the decomposition temperature of the nitride semiconductor (1200 ° C.), The second protective film is difficult to dissolve in the dissolving material (hydrofluoric acid) of the first protective film, and it is more desirable to add this step.
[0048]
  Next, the wafer is immersed in hydrofluoric acid, and as shown in FIG. 1F, the first protective film 61 is removed by a lift-off method.
[0049]
  Next, as shown in FIG. 1G, the p-electrode 20 made of Ni / Au is formed on the surface of the p-side contact layer exposed by removing the first protective film 61 on the p-side contact layer 13. To do. However, the p electrode 20 has a stripe width of 100 μm and is formed over the second protective film 62 as shown in FIG.
[0050]
  After the formation of the second protective film, the n-electrode 21 made of Ti / Al is formed in the direction parallel to the stripes on the surface of the n-side contact layer 5 exposed first.
[0051]
  As described above, the sapphire substrate of the wafer on which the n-electrode and the p-electrode are formed is polished to 70 μm, and then cleaved in a bar shape from the substrate side in the direction perpendicular to the stripe-shaped electrode. A resonator is formed on the 11-00 plane, a plane corresponding to the side surface of the hexagonal columnar crystal = M plane). SiO on the resonator surface₂And TiO₂A dielectric multilayer film is formed, and finally the bar is cut in a direction parallel to the p-electrode to obtain a laser element as shown in FIG. The resonator length is preferably 300 to 500 μm.
[0052]
  When this laser element was placed on a heat sink and each electrode was wire-bonded and laser oscillation was attempted at room temperature, the oscillation wavelength was 400 to 420 nm, the threshold current density was 2.9 kA / cm.²Showed continuous oscillation at room temperature. Furthermore, even when the current value was increased to increase the output to 40 mW, no short circuit occurred in the element itself, and continuous oscillation continued for 50 hours or more.
[0053]
  On the other hand, the second protective film is made of SiO.₂However, when the output was 40 mW, there was a case where a short circuit occurred immediately between the electrodes.
[0054]
[Example 2]
  In Example 1, after forming the stripe waveguide, when forming the second protective film 62, a laser is formed in the same manner as in the example except that BN is continuously formed with a film thickness of 0.5 μm by a PVD apparatus. When an element was obtained, a laser element having substantially the same characteristics as in Example 1 was obtained.
[0055]
[Example 3]
  In Example 1, when the second protective film 62 is formed after the stripe waveguide is formed, the PVD apparatus uses an Hf oxide (mainly HfO₂) Was continuously formed with a film thickness of 0.5 μm, and a laser element was obtained in the same manner as in the example. As a result, a laser element having substantially the same characteristics as in Example 1 was obtained.
[0056]
[Example 4]
  In Example 1, when the second protective film 62 was formed, a laser element was obtained in the same manner as in Example except that SiC was continuously formed with a film thickness of 0.5 μm with a PVD apparatus. A laser element having substantially the same characteristics as in Example 1 was obtained.
[0057]
[Example 5]
  In Example 1, when the second protective film 62 is formed, the PVD apparatus uses a Ti oxide (mainly TiO 2).₂) Was continuously formed with a film thickness of 0.5 μm, and the laser element was obtained in the same manner as in the example. As a result, the threshold value was slightly high, and the life was slightly shortened to 40 hours at an output of 40 mW.
[0058]
[Example 6]
  In Example 1, when the second protective film 62 is formed, a VD oxide (mainly V₂O₃) Was continuously formed with a film thickness of 0.5 μm, and a laser device was obtained in the same manner as in the example. As a result, the characteristic was almost the same as that of the example 5.
[0059]
[Example 7]
  In Example 1, when the second protective film 62 is formed, the PVD apparatus uses an Nb oxide (mainly Nb₂O₅) Was continuously formed with a film thickness of 0.5 μm, and a laser device was obtained in the same manner as in the example. As a result, the characteristic was almost the same as that of the example 5.
[0060]
[Example 8]
  In Example 1, when the second protective film 62 is formed, a Ta oxide (mainly Ta oxide) is used in the PVD apparatus.₂O₅) Was continuously formed with a film thickness of 0.5 μm, and a laser device was obtained in the same manner as in the example. As a result, almost the same characteristics as in Example 5 were obtained.
[0061]
[Example 9]
  In Example 1, when the second protective film 62 was formed, a laser element was obtained in the same manner as in Example except that AlN was continuously formed with a film thickness of 0.5 μm with a PVD apparatus. The characteristic almost the same as that of Example 5 was exhibited.
[0062]
[Example 10]
  In Example 1, after growing the p-side contact layer 13, a photoresist having a 2 μm stripe-shaped opening is formed on the p-side contact layer 13 with a thickness of 0.5 μm, and then the photoresist From above, Si oxide (mainly SiO₂The first protective film 61 is formed to a thickness of 0.5 μm. Thereafter, the photoresist is dissolved and removed by a lift-off method to form a first protective film 61 having a stripe width of 2 μm as shown in FIG. Thereafter, a laser device was fabricated in the same manner as in Example 1. As a result, a laser device having substantially the same characteristics as in Example 1 was obtained.
[0063]
[Example 11]
  FIG. 3 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention. Embodiment 11 will be described below with reference to this drawing.
[0064]
(Nitride semiconductor substrate 4 ')
  In Example 1, after forming the stripe-shaped protective film 3 on the surface of the underlayer 2, the wafer is set again in the MOVPE reaction vessel, the temperature is set to 1050 ° C., TMG and ammonia are used, and undoped GaN is 5 μm. Grow with film thickness. Thereafter, the wafer is transferred to an HVPE (hydride vapor phase epitaxy) apparatus, and a nitride semiconductor substrate 4 ′ made of undoped GaN is grown to a thickness of 200 μm using Ga metal, HCl gas, and ammonia as raw materials. As described above, when a nitride semiconductor is grown on the protective film 3 by the MOVPE method and then a GaN thick film having a thickness of 100 μm or more is grown by the HVPE method, the crystal defects are reduced by an order of magnitude or more compared to the first embodiment. . After the growth of the nitride semiconductor substrate 4 ′, the wafer is taken out of the reaction vessel, and the sapphire substrate 1, the buffer layer 2, the protective film 3, and the undoped GaN layer are removed by polishing to form the nitride semiconductor substrate 4 ′ alone.
[0065]
  Thereafter, in the same manner as in Example 1, the layers from the n-side contact layer 5 to the p-side contact layer 13 are laminated on the nitride semiconductor substrate 4 ′ opposite to the polishing side.
[0066]
  After the growth of the p-side contact layer 13, the stripe-shaped first protective film 61 is formed in the same manner as in Example 1, and then the etching stop is used as the surface of the n-side contact layer 5 in the second step. After that, ZrO was performed in the same manner as in the examples.₂3 is formed on the side surface of the stripe waveguide and the surface of the n-side contact layer 5, and then electrodes are formed on the respective contact layers to obtain a structure as shown in FIG. A laser element is used. When forming the resonance surface, the cleavage surface of the nitride semiconductor substrate is the same M surface as in the first embodiment. This laser device has a threshold current density of 1.8 kA / cm as compared with Example 1.²The life has been improved by more than 3 times.
[0067]
[Example 12]
  FIG. 4 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention. Embodiment 11 will be described below with reference to this drawing.
[0068]
  In Example 11, when producing the nitride semiconductor substrate 4 ′, silane gas was added to the raw material in the HVPE apparatus, and 1 × 10 Si was added.¹⁸/cm³A nitride semiconductor substrate 4 ″ made of doped GaN is grown to a thickness of 200 μm. Si concentration is 1 × 10¹⁷/cm³~ 5x10¹⁹/cm³It is desirable to be in the range. After the growth of the nitride semiconductor substrate 4 ″, the sapphire substrate 1, the buffer layer 2, the protective film 3, and the undoped GaN layer are polished and removed in the same manner as in Example 1 to obtain a single nitride semiconductor substrate 4 ″.
[0069]
  Next, on the nitride semiconductor substrate 4 ″, the crack prevention layer 6 to the p-side contact layer 13 are laminated and grown in the same manner as in the first embodiment.
  After the growth of the p-side contact layer 13, the stripe-shaped first protective film 61 is formed in the same manner as in Example 1. Then, in the second step, the etching stop is the surface of the n-side cladding layer 7 shown in FIG. 4. And After that, ZrO was performed in the same manner as in the examples.₂After forming the second protective film 62 containing as a main component on the side surface of the stripe waveguide and the surface of the n-side cladding layer 7, the p-electrode 20 is formed through the second protective film. On the other hand, n electrode 21 is formed on almost the entire back surface of the nitride semiconductor substrate. After the electrodes are formed, the resonance surface is produced by cleaving at the M-plane of the nitride semiconductor substrate to obtain a laser element having a structure as shown in FIG. It was.
[0070]
  In the step of manufacturing the laser device of Example 12, when the etching stop is various semiconductor layers stacked on the nitride semiconductor substrate in the second step, the threshold current density and the etching depth of the laser device are obtained. Shows the relationship. A is 0.1 μm from the upper end surface of the p-side cladding layer 12, B is the p-side cladding layer remaining at 0.2 μm, C is the center of the p-side light guide layer 11, and D is the n-side light guide. The center of the layer 8, E is the center of the n-side cladding layer 7, and F is the position 0.1 μm from the top surface of the substrate. As shown in FIG. 5, it can be seen that the threshold is remarkably lowered when the etching stop is a layer closer to the substrate than the lower end surface of the p-side cladding layer. Also, when etched deeper than point B, 2.0 kA / cm²The following threshold current density is obtained: 2.0 kA / cm²When the threshold value is further increased, the laser element tends to be easily cut off when continuously transmitting at a high output for 500 hours or more.
[0071]
【The invention's effect】
  As described above, in the laser device of the present invention, the protective film having excellent insulation and high reliability is formed on the side surface of the stripe waveguide region and the plane of the nitride semiconductor continuous with the side surface. Even when an electrode is formed on the protective film and a high current flows, the life of the laser element can be extended without causing a short circuit between the electrodes. Furthermore, by setting the position to the substrate side from the lower end surface of the p-side cladding layer, it is possible to manufacture a laser element having a significantly reduced threshold. Further, according to the method of the present invention, since a protective film can be formed on the cladding layer with a substantially uniform film thickness, current concentration hardly occurs. In addition, since the material of the first protective film is different from the material of the second protective film, the electrode can be formed with high reproducibility using the selectivity of the protective film by the etching means. As described above, the present invention is very useful in order to increase the life of the laser element from now on.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing a partial structure of a wafer obtained in each step for explaining each step of a method of the present invention.
FIG. 2 is a schematic cross-sectional view showing the structure of a laser device according to an embodiment of the present invention.
FIG. 3 is a schematic cross-sectional view showing the structure of a laser device according to another embodiment of the present invention.
FIG. 4 is a schematic cross-sectional view showing the structure of a laser device according to another embodiment of the present invention.
FIG. 5 is a diagram showing a relationship between an etching stop layer and a threshold current density of a laser element.
FIG. 6 is a schematic cross-sectional view showing the structure of a conventional laser element.
[Explanation of symbols]
  1 ... Different substrates
  2 ... Underlayer
  3 ... Protective film for growth of nitride semiconductor substrate
  4, 4'4 "... nitride semiconductor substrate
  5 ... n-side contact layer
  6 ... Crack prevention layer
  7 ... n-side cladding layer
  8 ... n-side light guide layer
  9 ... Active layer
10 ... p-side cap layer
11 ... p-side light guide layer
12 ... p-side cladding layer
13 ... p-side contact layer
61... First protective film
62 ... Second protective film
63: Third protective film
20 ... p electrode
21 ... n electrode

Claims (10)

A p-side cladding layer including a first p-type nitride semiconductor is stacked on the active layer, and a p-side contact layer including a second p-type nitride semiconductor is stacked on the p-side cladding layer. In the nitride semiconductor laser element etched from the p-side contact layer side and provided with a striped waveguide region in a layer below the p-side contact layer,
An insulating film other than Si oxide and having a refractive index smaller than that of the nitride semiconductor is formed on both sides of the stripe of the stripe waveguide and on the plane of the nitride semiconductor layer continuous with the side surface. An electrode is provided on the surface of the contact layer in the uppermost layer of the stripe via an insulating film,
The nitride semiconductor laser device, wherein the insulating film is made of at least one of an oxide containing at least one element selected from Zr and Hf, BN, and SiC.   2. The nitride semiconductor laser device according to claim 1, wherein a plane of the nitride semiconductor continuous with both side surfaces of the stripe is on the substrate side with respect to a lower end surface of the p-side cladding layer.   The nitride semiconductor laser element according to claim 1, further comprising a p-side light guide layer between the active layer and the p-side cladding layer.   4. The superlattice structure according to claim 1, wherein at least one of the p-side cladding layers includes a nitride semiconductor layer containing Al and has a band gap energy different from each other. 5. 2. The nitride semiconductor laser device according to item 1. After a p-side contact layer including a second p-type nitride semiconductor is stacked on the p-side cladding layer including the first p-type nitride semiconductor, a stripe-shaped first layer is formed on the surface of the p-side contact layer. A first step of forming one protective film, and a portion of the nitride semiconductor where the first protective film is not formed is etched through the first protective film, and a stripe shape is formed directly under the protective film. The second step of forming the waveguide region, and after the second step, the second protective film, which is made of a material different from the first protective film and has an insulating property, is etched on the side surface of the stripe waveguide and A third step of forming the nitride semiconductor layer exposed on the plane, and after the third step, the first protective film is removed, and the second protective film and the uppermost p-type nitride semiconductor layer A fourth process for forming an electrode electrically connected to the p-side contact layer on the surface of Provided with a door,
The method for manufacturing a nitride semiconductor laser element, wherein the second protective film is made of an insulating material other than Si oxide. In the second step, the etching stop is a plane of the nitride semiconductor in the film thickness direction of the p-side cladding layer and on the substrate side from the lower end surface of the p-side cladding layer to the p-side contact layer direction of 0.2 μm. A method for manufacturing a nitride semiconductor laser device according to claim 5 . 6. The method of manufacturing a nitride semiconductor laser device according to claim 5, wherein, in the second step, the etching stop is a plane of the nitride semiconductor located on the substrate side with respect to the lower end surface of the p-side cladding layer . In the first step, a first protective film is formed on substantially the entire surface of the p-side contact layer, and after forming a striped third protective film on the first protective film, the third protective film is formed. The nitride semiconductor according to any one of claims 5 to 7 , wherein the first protective film is formed by a step of etching the first protective film in a stripe shape through the protective film. A method for manufacturing a laser element. 9. The method for manufacturing a nitride semiconductor laser element according to claim 5, wherein in the first step, the first protective film is formed by a lift-off method. The first protective film is made of an oxide of Si, and the second protective film is an oxide containing at least one element selected from the group consisting of Ti, V, Zr, Nb, Hf, and Ta, or BN , SiC, manufacturing method of the nitride semiconductor laser device according to any one of claims 5-9, characterized in that comprises at least one of AlN.

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