JP3804494B2 - Nitride semiconductor laser device - Google Patents

Description

[0001]
[Field of the Invention]
The present invention is a nitride semiconductor _{(In X Al Y Ga 1-} X-Y N, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1) relates to a laser device made of.
[0002]
[Prior art]
Nitride semiconductors have been studied as semiconductor materials that oscillate from green to ultraviolet. A nitride semiconductor laser device is disclosed in, for example, Japanese Patent Laid-Open No. 6-152072. This publication shows a double heterostructure laser in which an active layer is sandwiched between lattice-matched clad layers. The device structure is a gain-guided laser such as an electrode stripe type, a mesa stripe type, or a heteroisolation stripe type. A buried hetero stripe type refractive index guided laser or the like is shown.
[0003]
In general, in gain-guided laser elements, the current spreads in the cladding layer, so the transverse mode laser light is controlled to obtain a transverse mode light that is stable in a single mode and the astigmatic difference is reduced. Thus, a refractive index guided laser element is used in which a portion corresponding to a lateral direction of the active layer, that is, a direction parallel to the laser resonance direction is sandwiched between materials having a refractive index lower than that of the active layer. The refractive index guided laser element disclosed in the above publication also has an active layer in the lateral direction sandwiched between i-type InAlGaN, but a quaternary mixed crystal nitride semiconductor is very difficult to grow, for example, several μm. There is a drawback that it is difficult to grow with a thick film.
[0004]
[Problems to be solved by the invention]
In order to perform optical confinement in the lateral direction of the laser light of the nitride semiconductor laser element, it is desirable to select a material that is more realistic and effectively perform optical confinement. Accordingly, the present invention has been made in view of such circumstances, and an object of the present invention is to obtain a stable laser element by performing optical confinement in the transverse mode of the active layer of the nitride semiconductor laser element. is there.
[0005]
[Means for Solving the Problems]
The laser device of the present invention has an active layer that oscillates between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer, and is parallel to the resonance direction of the laser beam. A dielectric multilayer film reflecting the emission wavelength of the multilayered active layer is formed on the surfaces of the active layer and the p-type nitride semiconductor layer, and a metal thin film is formed on the surface of the dielectric multilayer film. It is characterized by being.
[0006]
Further, the dielectric multilayer film is made of SiO ₂ and TiO ₂ . Further, the metal thin film is a metal thin film that reflects light of the active layer. The metal thin film is one selected from Al, Ag, Ni, Cr, and Pt. The active layer is a nitride semiconductor containing In.
[0007]
[Action]
In the laser device of the present invention, a dielectric thin film and a metal thin film are formed on the surfaces of the n-type nitride semiconductor, the active layer, and the p-type nitride semiconductor in a direction parallel to the laser resonance direction. In other words, the lateral mode of the laser beam can be confined by covering the side surface of the active layer in the direction parallel to the resonance direction of the laser beam with the dielectric thin film and the metal thin film. Easy to obtain laser light.
[0008]
In addition, if the metal thin film is exposed on the surface, the positive electrode and the negative electrode may be short-circuited via the conductive material. Therefore, by forming a second dielectric thin film on the surface of the metal thin film, Can be prevented.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a schematic sectional view showing the structure of the laser element of the present invention, and FIG. 2 is a perspective view showing the shape of the laser element of FIG. 1 is a cross-sectional view of the laser element of FIG. 2 taken along a direction parallel to the resonance surface. The basic structure of this laser device is that an n-type contact layer 2, an n-type optical confinement layer 3, an n-type light guide layer 4, an active layer 5 and a p-type light guide made of a nitride semiconductor are formed on an insulating substrate 1. It is a laminated structure of the layer 6, the p-type optical confinement layer 7, and the p-type contact layer 8. Further, striped etching is performed from the uppermost p-type contact layer 8 side to the n-type contact layer 2 to narrow the width of the active layer so that the current is concentrated in the active layer. A positive electrode 30 is provided on the p-type contact layer 8 of the resonator that has been striped by etching, and a negative electrode 20 is provided on the n-type contact layer 2 in a direction parallel to the positive electrode 30.
[0010]
The substrate 1 is often an insulating substrate such as sapphire (Al ₂ O ₃ , A plane, C plane, R plane), spinel (MgAl ₂ O ₄ , 111 plane), but in addition, SiC, MgO, Si, ZnO A conventionally known substrate made of a single crystal such as is used. In this figure, an insulating substrate is used.
[0011]
n-type contact layer 2 is _{In X Al Y Ga 1-X} -Y N (0 ≦ X, 0 ≦ Y, X + Y ≦ 1) can be composed of, particularly GaN, InGaN, in GaN doped with Si among them By configuring, an n-type layer having a high carrier concentration can be obtained, and a preferable ohmic contact with the negative electrode 20 can be obtained, so that the threshold current of the laser element can be lowered. As the material of the negative electrode 20, a metal or an alloy such as Al, Ti, W, Cu, Zn, Sn, or In is preferable. Nitride semiconductors, not limited to GaN, have n-type properties due to nitrogen vacancies that can be formed inside the crystal even in non-doped (undoped state), but donor impurities such as Si, Ge, and Sn are grown during crystal growth. By doping, a nitride semiconductor having a high carrier concentration and a preferable n-type characteristic can be obtained.
[0012]
The n-type optical confinement layer 3 is made of an n-type nitride semiconductor containing Al, and preferably is made of binary mixed crystal or ternary mixed crystal Al _Y Ga _1-Y N (0 <Y ≦ 1). Thus, a material having good crystallinity can be obtained, and the refractive index difference with the active layer is increased to be effective for confining the longitudinal mode of the laser beam. This layer is preferably grown with a thickness of usually 0.1 μm to 1 μm. If it is thinner than 0.1 μm, it will be difficult to act as a light confinement layer, and if it is thicker than 1 μm, cracks are likely to occur in the crystal, and device fabrication tends to be difficult.
[0013]
The n-type light guide layer 4 is composed of an n-type nitride semiconductor containing In or n-type GaN, and is preferably a ternary mixed crystal or a binary mixed crystal In _X Ga _1-X N (0 ≦ X <1). ). This layer is usually preferably grown to a thickness of 100 Å to 1 μm. In particular, by using InGaN or GaN, the next active layer 5 can easily have a quantum well structure.
[0014]
As described above, the active layer 5 is made of a nitride semiconductor containing In, and preferably has a ternary mixed crystal In _X Ga _1-X N (0 <X <1). Since the ternary mixed crystal InGaN has better crystallinity than the quaternary mixed crystal, the light emission output is improved. Among them, a multi-quantum well structure (MQW: Multi-quantum structure) in which an active layer is preferably formed by stacking a well layer made of In _X Ga _1-X N and a barrier layer made of a nitride semiconductor having a larger band gap than the well layer. -well). Similarly, the barrier layer is preferably a ternary mixed crystal In _{X ′} Ga _{1−X ′} N (0 ≦ X ′ <1, X ′ <X), and well + barrier + well +. The multiple quantum well structure is configured by stacking as described above. When the active layer is MQW in which InGaN is stacked as described above, a high-power LD between about 365 nm and 660 nm can be realized by light emission between quantum levels. Furthermore, when a barrier layer made of InGaN is stacked on the well layer, the barrier layer made of InGaN is softer than GaN and AlGaN crystals. Therefore, since the thickness of the AlGaN cladding layer can be increased, laser oscillation can be realized. Further, InGaN and GaN have different crystal growth temperatures. For example, in the MOVPE method, InGaN is grown at 600 to 800 ° C., whereas GaN is grown at a temperature higher than 800 ° C. Therefore, if a well layer made of InGaN is grown and then a barrier layer made of GaN is grown, it is necessary to raise the growth temperature. When the growth temperature is raised, the previously grown InGaN well layer is decomposed, so it is difficult to obtain a well layer with good crystallinity. Furthermore, the thickness of the well layer is only a few tens of angstroms, and it becomes difficult to produce the MQW when the thin well layer is decomposed. On the other hand, in the present invention, since the barrier layer is also InGaN, the well layer and the barrier layer can be grown at the same temperature. Therefore, since the well layer formed previously is not decomposed, MQW having good crystallinity can be formed. This shows the most preferable mode of MQW, but any composition can be used as long as the band gap energy of the barrier layer is made larger than that of the well layer, such as InGaN for the well layer, GaN for the barrier layer, and AlGaN. good.
[0015]
The total film thickness of the active layer 5 having a multiple quantum well structure is preferably adjusted to 100 angstroms or more. If the thickness is less than 100 angstroms, the output is not sufficiently increased and laser oscillation tends to be difficult. If the thickness of the active layer is too thick, the output tends to decrease, and it is desirable to adjust it to 1 μm or less, more preferably 0.5 μm or less. If it is thicker than 1 μm, the crystallinity of the active layer is deteriorated, or the laser beam spreads in the active layer and the threshold current tends to increase.
[0016]
Next, the p-type light guide layer 6 is made of a nitride semiconductor containing In or GaN, and preferably grows binary mixed crystal or ternary mixed crystal In _Y Ga _1-Y N (0 <Y ≦ 1). Let me. The light guide layer 6 is preferably grown to a thickness of usually 100 angstroms to 1 μm. In particular, by using InGaN or GaN, the next p-type light confinement layer 7 can be grown with good crystallinity. A p-type nitride semiconductor can be obtained by doping an acceptor impurity such as Zn, Mg, Be, Cd, and Ca during crystal growth. Among these, Mg exhibits the most preferable p-type characteristics. Further, after crystal growth, annealing at 400 ° C. or higher in an inert gas atmosphere can obtain a p-type with even lower resistance.
[0017]
The p-type confinement layer 7 is made of a p-type nitride semiconductor containing Al, and preferably is made of binary mixed crystal or ternary mixed crystal Al _Y Ga _1-Y N (0 <Y ≦ 1). Good crystallinity is obtained. Like the n-type optical confinement layer, this p-type optical confinement layer is desirably grown to a thickness of 0.1 μm to 1 μm. By using a p-type nitride semiconductor containing Al, such as AlGaN, The difference in refractive index is effectively increased and acts as an optical confinement layer for the longitudinal mode laser light.
[0018]
The p-type contact layer 8 can be composed of p-type In _X Al _Y Ga _1- XYN (0 ≦ X, 0 ≦ Y, X + Y ≦ 1), especially InGaN, GaN, among which Mg is doped When p-type GaN is used, a p-type layer having the highest carrier concentration can be obtained, good ohmic contact with the positive electrode 30 can be obtained, and the threshold current can be lowered. As a material of the positive electrode 30, a metal or alloy having a relatively high work function such as Ni, Pd, Ir, Rh, Pt, Ag, Au, or the like is likely to obtain an ohmic property.
[0019]
Next, in the laser device of the present invention, as shown in FIG. 1, a p-type contact layer 8, a p-type light confinement layer 7, a p-type light guide layer 6, an active layer 5, an n-type light guide layer 4, and an n-type light. The confinement layer 3 and the n-type contact layer are etched in stripes. The light emitted from the active layer 5 resonates in the length direction of the stripe and oscillates. Longitudinal light is controlled by n-type and p-type light confinement layers. The light in the lateral direction is confined by the dielectric thin film 40 and the metal thin film 50 formed on the etching end face in a direction parallel to the resonance direction of the laser light.
[0020]
The stripe width of the p-type contact layer 8 is not particularly limited, but when adjusted to 10 μm or less, more preferably 5 μm or less, and most preferably 3 μm or less, the astigmatic difference of the laser decreases and the threshold current also decreases. . As the etching means, dry etching is preferably used. For example, reactive ion etching, ion milling, ECR etching, focused ion beam etching, ion beam assist etching, or the like can be used.
[0021]
Next, in order to form the dielectric thin film 40, conventional vapor deposition means such as plasma CVD, sputtering, molecular beam evaporation, etc. can be used. Examples of the material include SiOx, SiN, AlN, Al High dielectric materials such as ₂ O ₃ can be used. Alternatively, a thin film of these dielectrics may be stacked in multiple layers to form a dielectric multilayer film that reflects light in the lateral direction of the active layer 5. The dielectric thin film can be formed with a film thickness of about 0.01 μm to 50 μm, for example.
[0022]
Further, in the laser element of the present invention, the metal thin film 50 that reflects the light of the active layer is formed on the dielectric thin film 40. By forming the metal thin film 50, the lateral light of the active layer 5 can be completely confined. As a material for the metal thin film, it is desirable to select a material having a high reflectance with respect to nitride semiconductor laser light (eg, 380 nm to 550 nm) such as Al, Ag, Ni, Cr, Pt.
[0023]
FIG. 3 is a schematic cross-sectional view showing the structure of a laser device according to another embodiment of the present invention, and shows a state where the laser device is connected to a heat sink via a conductive material 90 such as solder. 3 differs from FIGS. 1 and 2 in that a second dielectric thin film 41 is further formed on the metal thin film 50.
[0024]
The second dielectric thin film 41 is formed on the surface of the metal thin film 50 and acts to prevent a short circuit between the electrodes. In other words, when the laser chip is connected face-down to a base such as a heat sink or submount, the positive and negative electrodes are connected while the conductive material such as solder that connects the electrode and base touches the metal thin film. Doing so may cause a short circuit between the electrodes. Therefore, by covering the surface of the metal thin film 50 with the second dielectric thin film 41, a short circuit between the electrodes can be prevented. In particular, as shown in this figure, it is more effective if the second dielectric thin film 41 is continuously formed so as to cover the interface between the dielectric thin film 40 and the metal thin film 50 previously formed.
[0025]
[Example]
Embodiments of the present invention will be described below with reference to FIGS. After the substrate 1 made of spinel (MgAl ₂ O ₄ , 111 plane) is placed in the reaction vessel of the MOVPE apparatus, TMG (trimethylgallium) and ammonia are used as source gases and GaN is formed on the surface of the sapphire substrate at a temperature of 500 ° C. A buffer layer consisting of 200 angstroms was grown. This buffer layer has the effect of relaxing the lattice mismatch between the substrate and the nitride semiconductor, and it is also possible to grow AlN, AlGaN or the like. It is known that the growth of this buffer layer improves the crystallinity of the n-type nitride semiconductor grown on the substrate. However, the buffer layer may not be grown depending on the growth method, the type of the substrate, and the like. .
[0026]
Subsequently, the temperature was raised to 1050 ° C., and an n-type contact layer 2 made of Si-doped GaN was grown to a thickness of 4 μm using TMG, ammonia as source gas, and SiH ₄ (silane) gas as a donor impurity.
[0027]
Next, the temperature is lowered to 750 ° C., a crack prevention layer made of Si-doped In0.1Ga0.9N is grown to a thickness of 500 angstroms using TMG, TMI (trimethylindium), ammonia as the source gas, and silane gas as the impurity gas. It was. Although this crack prevention layer is not specifically shown, an n-type optical confinement layer 3 made of an n-type nitride semiconductor containing In, preferably InGaN, which is grown next by InGaN, is formed. A thick film can be grown. In the case of an LD, it is necessary to grow layers that become a light confinement layer and a light guide layer with a film thickness of, for example, 0.1 μm or more. Conventionally, when a thick AlGaN film is grown directly on the GaN or AlGaN layer, it was difficult to fabricate the device because cracks were formed in the AlGaN grown later. It is possible to prevent the layer 3 from cracking. Moreover, even if the light confinement layer 3 to be grown next is grown as a thick film, it can be grown with good film quality. The crack prevention layer is preferably grown with a film thickness of 100 Å or more and 0.5 μm or less. If it is thinner than 100 angstroms, it is difficult to act as a crack prevention as described above, and if it is thicker than 0.5 μm, the crystal itself tends to turn black. The crack prevention layer can be omitted depending on the growth method and the growth apparatus.
[0028]
Next, the temperature is set to 1050 ° C., and the n-type optical confinement layer 3 made of Si-doped n-type Al 0.3 Ga 0.7 N is set to 0 using TEG, TMA (trimethylaluminum) as the source gas, and silane gas as the impurity gas. The film was grown with a thickness of 5 μm.
[0029]
Subsequently, TMG and ammonia were used as the source gas and silane gas was used as the impurity gas, and an n-type light guide layer 4 made of Si-doped n-type GaN was grown to a thickness of 500 angstroms.
[0030]
Next, the active layer 5 was grown using TMG, TMI, and ammonia as source gases. The active layer is maintained at a temperature of 750 ° C., and a well layer made of non-doped In 0.2 Ga 0.8 N is first grown to a thickness of 25 Å. Next, a barrier layer made of non-doped In0.01Ga0.95N is grown to a thickness of 50 angstroms at the same temperature only by changing the molar ratio of TMI. This operation was repeated 13 times, and finally a well layer was grown to grow an active layer having a multiple quantum well structure with a total film thickness of 0.1 μm.
[0031]
After growing the active layer 5, the temperature is set to 1050 ° C., TMG, TMA, ammonia, Cp2Mg (cyclopentadienylmagnesium) is used as an acceptor impurity source, and a p-type cap layer made of Mg-doped p-type Al0.2Ga0.8N is made 100 Grown with angstrom thickness. Although this p-type cap layer is not particularly illustrated, the cap layer prevents the active layer made of InGaN from decomposing by growing it to a thickness of 1 μm or less, more preferably 10 Å or more and 0.1 μm or less. Further, by growing the p-type cap layer 48 made of a p-type nitride semiconductor containing Al on the active layer, the light emission output is remarkably improved. On the other hand, if the p layer in contact with the active layer is GaN, the output of the device is reduced to about 1/3. This is presumed to be because AlGaN tends to be p-type compared to GaN and has the effect of suppressing decomposition of InGaN during the growth of the p-type cap layer, but the details are unknown. If the thickness of the p-type cap layer is greater than 1 μm, the layer itself tends to crack, and device fabrication tends to be difficult. This p-type cap layer can also be omitted.
[0032]
Next, while maintaining the temperature at 1050 ° C., a p-type light guide layer 6 made of Mg-doped p-type GaN using TMG, ammonia, and Cp 2 Mg was grown to a thickness of 500 Å. As described above, when the second p-type light guide layer is made of InGaN or GaN, the following optical confinement layer containing Al can be grown with good crystallinity.
[0033]
Subsequently, a p-type optical confinement layer 7 made of Mg-doped Al0.3Ga0.7N was grown to a thickness of 0.5 μm using TMG, TMA, ammonia, and Cp2Mg.
[0034]
Subsequently, a p-type contact layer 8 made of Mg-doped p-type GaN was grown to a thickness of 0.5 μm using TMG, ammonia, and Cp 2 Mg.
[0035]
The nitride semiconductor laminated wafer as described above is taken out of the reaction vessel, and selectively etched from the uppermost p-type contact layer 8 side with a reactive ion etching (RIE) apparatus to form a negative electrode. The surface of the n-type contact layer was exposed, and the p-type contact layer 8 to the n-type contact layer 2 were etched into a stripe shape. The stripe width was 10 μm.
[0036]
A portion of the nitride semiconductor wafer on which the positive and negative electrodes are to be formed is masked, and a dielectric multilayer film 40 made of SiO ₂ and TiO ₂ is applied to the etching end face by a plasma CVD apparatus. Formed at 10 μm. Needless to say, the dielectric multilayer film is designed to reflect the emission wavelength of the active layer. The dielectric thin film is a multilayer film, but may be formed of a single film such as Al ₂ O ₃ or SiO ₂ .
[0037]
Next, a metal thin film 50 made of Al was formed on the dielectric thin film 40 to a thickness of 0.1 μm by vapor deposition.
[0038]
Further, a second dielectric thin film 41 made of SiO ₂ was formed on the metal thin film 50 to a thickness of 1 μm by the same plasma CVD method.
[0039]
A striped positive electrode 30 made of Ni and Au is formed on the p-type contact layer 8 via an insulating film 11, and a striped negative electrode made of Ti and Al is formed on the n-type contact layer 2 previously exposed. 20 was formed.
[0040]
The wafer as described above was first divided at a position parallel to the striped electrode, and then divided in a direction perpendicular to the electrode, and the divided surface divided in the perpendicular direction was polished into a mirror surface. A reflecting mirror was formed on the resonance surface according to a conventional method to obtain a laser chip. As shown in FIG. 3, when this laser chip was placed on a heat sink in which the electrode pattern 100 was previously formed and pulsed at room temperature, a laser oscillation of 410 nm was exhibited at a threshold current density of ² kA / cm ² .
[0041]
【The invention's effect】
As described above, in the laser element of the present invention, the active layer in the direction parallel to the resonance direction of the laser beam and the cladding layer are covered with the dielectric thin film and the metal, so that the optical confinement in the complete transverse mode is achieved. And stable laser light can be obtained. Furthermore, a dielectric thin film, a metal thin film, etc. can form a thin film easily compared with a nitride semiconductor. Therefore, if a nitride semiconductor is to be formed, after etching the cladding layer and the active layer in a stripe shape, a mask must be formed and selective growth must be performed at a high temperature. Even if it is not a nitride semiconductor growth apparatus such as MOCVD or MBE, it can be formed by another simple CVD apparatus, which is very useful in terms of production technology.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing the structure of a laser device according to an embodiment of the present invention.
2 is a perspective view showing the shape of the laser element of FIG. 1. FIG.
FIG. 3 is a schematic cross-sectional view showing the structure of a laser device according to another embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... substrate 2 ... n-type contact layer 3 ... n-type light confinement layer 4 ... n-type light guide layer 5 ... active layer 6 ... p-type light Guide layer 5 ... p-type optical confinement layer 6 ... p-type contact layer 40 ... dielectric thin film 50 ... metal thin film 41 ... second dielectric thin film

Claims (5)

  An active layer that oscillates between the n-type nitride semiconductor layer and the p-type nitride semiconductor layer, and is parallel to the resonance direction of the laser beam, the active layer, and the p A nitride multilayer film is formed on the surface of the type nitride semiconductor layer to reflect the emission wavelength of the multilayered active layer, and a metal thin film is formed on the surface of the dielectric multilayer film. Semiconductor laser device. The nitride semiconductor laser element according to claim 1, wherein the dielectric multilayer film is made of SiO ₂ and TiO ₂ .   3. The nitride semiconductor laser device according to claim 1, wherein the metal thin film is a metal thin film that reflects light of an active layer.   6. The nitride semiconductor laser device according to claim 1, wherein the metal thin film is one selected from Al, Ag, Ni, Cr, and Pt.   5. The nitride semiconductor laser element according to claim 1, wherein the active layer is a nitride semiconductor containing In.

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