JP2006024703A - Nitride semiconductor laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a high-quality nitride semiconductor laser device capable of preventing a p-electrode from peeling while satisfying the refractive index as an embedded film and at the same time obtaining a film with uniform and good composition in the inside of the surface as well as in a thickness direction. <P>SOLUTION: The nitride semiconductor laser device comprises an active layer sandwiched between a first and a second nitride semiconductor layer, and an electrode formed via the embedded film on the second nitride semiconductor layer. The nitride semiconductor laser device is constituted such that the embedded film may vary from the side of the second nitride semiconductor layer to the surface side with respect to (1) refractive index, (2) oxygenation ratio, (3) density, and/or (4) surface roughness. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

Today, semiconductor lasers using nitride semiconductors are considered capable of oscillating in a wide wavelength range from the ultraviolet to the red, and their application range is large-capacity and high-density information recording / reproduction such as DVD. It is expected not only for use in an optical disc system capable of recording, but also as a light source for laser printers, optical networks, and the like.
In a nitride semiconductor laser element, for example, an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer are stacked in this order on a substrate, and electrodes are formed on the n-type and p-type nitride semiconductor layers, respectively. Has been configured.

Further, in the laser element having such a laminated structure, a striped ridge is formed on the surface of the p-type nitride semiconductor layer, and a buried film is formed on the exposed surface of the p-type nitride semiconductor layer on both sides of the ridge. Is formed. Thus, an effective refractive index type waveguide structure can be obtained.
In this way, by forming a ridge and further forming a buried film on both sides of the ridge, an effective refractive index distribution is ensured in the waveguide region directly below the ridge and the region immediately below both sides of the ridge. The confinement is optimized, the transverse mode is controlled, and the threshold is also controlled.
In addition, the one having a striped waveguide structure is provided with a striped electrode on the flat p-type nitride semiconductor layer surface or having a striped opening without providing the ridge as described above. By forming an insulating film (embedded film) and providing an electrode on the insulating film (embedded film), the current injection region is controlled in a stripe shape, and further, the current confining layer is provided in the semiconductor layer. These have no ridges on the surface, so that the yield at the time of chip formation is excellent, and there is no ridge formation step, and there are advantages such as a small number of steps.

As the buried film, a dielectric film such as TiO ₂ , ZrO ₂ , HfO ₂ , CeO ₂ , In ₂ O ₃ , Nd ₂ O ₃ , Sb ₂ O ₃ , SnO ₂ , Ta ₂ O ₅ and ZnO is used. (For example, Patent Document 1).
Japanese Patent Laid-Open No. 10-270792

  A dielectric film as shown in Patent Document 1 usually has a refractive index specific to each substance. For this reason, in order to obtain a desired refractive index as a buried film, an appropriate material is used in combination. However, even if a material that satisfies the refractive index is selected, the material may not function sufficiently as a buried film of the laser element due to its film formation method and physical properties.

  That is, the above-described dielectric film is usually formed by sputtering or the like, and is formed as an oxide while reacting with oxygen on the target surface. However, there are portions in the target plane that are likely to react with oxygen and portions that are difficult to react, so in the formed buried film, a uniform oxide may not be formed in the in-plane direction or in the film thickness direction. . As a result, an appropriate refractive index cannot be obtained as a buried film, and light confinement or the like cannot be controlled.

  Further, not only when the ridge is formed in the laser element but also when the ridge is not formed, that is, by providing a stripe-shaped opening in the insulating film (embedded film) from the semiconductor layer to the buried film. Even in the case of forming electrodes across the substrate, depending on the material, there is a problem in that the adhesion with the p-electrode formed thereon is poor, the electrodes are peeled off, and the product yield is lowered.

  The present invention has been made in view of the above problems, and while satisfying the refractive index as a buried film, it is possible to obtain a film having a desired function in the plane and in the film thickness direction, and to remove the p electrode. An object is to obtain a high-quality nitride semiconductor laser device.

  In the nitride semiconductor laser device of the present invention, a first nitride semiconductor layer, an active layer, and a second nitride semiconductor layer are stacked in this order, and an electrode is formed on the second nitride semiconductor layer via a buried film. In the nitride semiconductor laser device configured as described above, the embedded film has (1) a refractive index, (2) an oxygen ratio and / or (3) density from the second nitride semiconductor layer side to the surface side. Is characterized by changing. Alternatively, the buried film is formed of at least two layers, and the surface roughness is different between the layer on the second nitride semiconductor layer side and the layer on the surface side.

  In the nitride semiconductor laser device, whether the refractive index of the buried film is larger on the surface side than the second nitride semiconductor layer side, or is the oxygen ratio of the buried film smaller on the surface side than the second nitride semiconductor layer side? The density of the buried film is larger on the surface side than the second nitride semiconductor layer side, and / or the surface of the outermost layer side of the buried film (when the buried film is a laminated film of two or more layers) It is preferable that the buried layer is the flattest, in other words, the surface roughness is small.

In particular, the refractive index of the buried film on the second nitride semiconductor layer side is more preferably 2.20 or less, and the refractive index of the buried film on the surface side is more preferably 2.20.
The embedded film may be composed of a single layer or a film having a laminated structure of two or more layers.
Further, the buried film contains oxygen or is composed of an oxide at least on the second nitride semiconductor layer side, and is preferably composed of oxygen and zirconium.
In the nitride semiconductor laser element, it is preferable that a ridge is formed on the surface of the second nitride semiconductor layer.

According to the nitride semiconductor laser device of the present invention, the refractive index, oxygen ratio, density, or surface roughness changes in the film thickness direction, so that the surface side of the buried film, that is, the side in contact with the electrode is very Since a film having a good quality can be disposed, the adhesion with the electrode can be further strengthened. As a result, peeling of the electrodes can be prevented, and a product with a high yield can be provided with high quality.
Further, when the buried film is formed in a laminated structure of two or more layers, it becomes possible to control the film thickness of the buried film itself while ensuring a desired film quality, such as obtaining a desired refractive index. It becomes easy to design a semiconductor laser device.
In particular, when the buried film is formed of the same constituent elements such as oxygen and zirconium, there is no interface between different materials, so the adhesion within the laminated film is good, and the refractive index and adhesion to the electrode are good. Both sexes can be satisfied.

In the nitride semiconductor laser device of the present invention, typically, a first nitride semiconductor layer, an active layer, and a second nitride semiconductor layer are stacked in this order, and a part of the embedded film is formed on the second nitride semiconductor layer. The nitride semiconductor laser device is configured by forming an electrode through the electrode.
Here, the material for the buried film can be appropriately selected according to the characteristics to be obtained. For example, it may be a film containing at least oxygen, or a film containing a metal such as Zr, Hf, Ce, In, Sb, Sn, Ta, Zn, Si, Al, and Hf. Further, it may be a film containing oxygen and these metals. Specifically, oxide films of these metals are preferably used. Especially, it is preferable that the constituent elements include the same or the same constituent elements in the film thickness direction, and the composition ratios thereof are different. For example, a film containing oxygen and zirconium (hereinafter referred to as “zirconium oxide”) is preferably included in a part thereof, and the entire embedded film is more preferably composed of oxygen and zirconium.

  The buried film of the present invention has (1) the refractive index, (2) the oxygen ratio, and (3) the density from the second nitride semiconductor layer side to the surface side (that is, the electrode side), in other words, in the film thickness direction. And / or (4) a film with varying surface roughness. Only one of these may be changed, two or more may be changed, or all may be changed. Furthermore, a film in which the particle size of the material constituting the embedded film is changed may be used.

  In the present invention, “changes from the second nitride semiconductor layer side to the surface side” may change gradually, may change stepwise, or may change so as to increase. , It may change so as to become smaller, or may change randomly. Further, in any case, whether it is a gradual change or a gradual change, the embedded film itself may be formed of a single layer, or two or more layers are laminated. May be formed. Especially, it is preferable to form with the laminated film from which two or more layers differ in a structural element.

By this change, it is intended to secure and improve the adhesion with the electrode while fully exerting the function as the buried film without impairing various functions.
For example, the outermost film (including both the outermost surface region of the single-layer film and the outermost film side of the laminated film, the same shall apply hereinafter) is provided with a film having a higher refractive index than the film located immediately below it. It becomes. In other words, it is preferable that a film having a refractive index smaller than that of the uppermost layer is disposed immediately below the uppermost layer. For example, the refractive index of the uppermost film is suitably a film of 2.21 or more, 2.22 or more, or 2.23 or more, which is larger than 2.20. Further, it is appropriate that the film on the second nitride semiconductor side has a refractive index of 2.20 or less.

The refractive index can be measured by a spectroscopic ellipsometer using ellipsometry. Specifically, J. et al. A. For example, HS-190 manufactured by WOOLLAM can be used.
For example, in the case where the laminated film is composed of at least two layers, it is appropriate that the refractive indexes of the at least two layers are different. The lower layer film is preferably set to have a refractive index smaller than that of the upper layer film. Further, the lower layer film has a refractive index of 2.20 or less, 2.15 or less, 2.10 or less, 2.05 or less, 2.00. Hereinafter, it is more preferable that the refractive index of the upper layer film is greater than 2.20, 2.21 or more, 2.22 or more, or 2.23 or more. The refractive index of the upper and lower layer films is a value obtained by separately measuring the refractive index of each film in a state where the upper and lower films are laminated.

Alternatively, the outermost film is a film having an oxygen ratio different from that of the film located immediately below the outermost film.
For example, when the buried film is formed of at least two layers, it is appropriate that the oxygen ratio of the at least two layers is different, and the lower layer film has a larger oxygen ratio than the upper layer film. It is preferable to be set. Here, the ratio of oxygen is different means that the ratio of the number of oxygen elements to the total number of elements in each film is different even if it is slight. The number of each element can be measured using Auger electron spectroscopy or the like.

From another point of view, in particular, when the buried film has a laminated structure of two or more layers, the film on the outermost surface is arranged with a film having a surface roughness different from that of the film located immediately below.
For example, the surface roughness of the lower layer film is set to be rougher than that of the upper layer film (the outermost layer), in other words, the surface of the outermost layer is preferably the flattest among the buried layers. . The surface roughness can be measured as roughness using a spectroscopic ellipsometer, or using an atomic force microscope (AFM) or the like.

Furthermore, from another point of view, the outermost film is a film having a density different from that of the film located immediately below the outermost film.
For example, when the buried film is formed of at least two layers, it is appropriate that the densities are different, and it is preferable that the lower layer film is formed to have a lower density than the upper layer film.
In addition, the outermost film is a film having a crystal structure or crystal grain size different from that of the film located immediately below.

For example, when the buried film is formed of at least two layers, it is appropriate that the crystal structure or the crystal grain size is different, and the lower layer film is formed with smaller crystal grains than the upper layer film. Alternatively, it is preferably formed of a crystal structure that is not dense (rough).
The density, crystal structure, or crystal grain size is, for example, a method of observing a cross section by transmission electron microscopy (TEM), a method of observing by scanning electron microscopy (SEM), an electron diffraction pattern, an X-ray diffraction pattern, or the like It can measure by the method of measuring, the method of observing with an ultra-thin film evaluation apparatus, etc.

  The buried film can be formed by a method known in the art. For example, vapor deposition method, sputtering method, reactive sputtering method, ion beam assisted vapor deposition method, ion plating method, laser ablation method, CVD method, spray method, spin coating method, dipping method or a combination of two or more of these methods Various methods such as a method or a combination of these methods and an oxidation treatment (heat treatment) can be used. In the case where the embedded film is a single layer, the refractive index and the like can be changed by changing the film forming conditions by one film forming method.

  Specifically, when forming a buried film (for example, zirconium oxide), a method of adopting different film forming methods for the upper and lower films, such as forming the lower film by sputtering and the upper film by vapor deposition, can be mentioned. It is done. At this time, first, a film may be formed by sputtering using a target made of zirconium oxide, and then a zirconium film may be formed by vapor deposition, and heat treatment may be performed during or after the film formation. After the formation, heat treatment may be performed to form a zirconium oxide film. Further, the degree of vacuum may be changed gradually or suddenly during film formation by each method (for example, within a range of about 0.01 to 2.0 pa), or the substrate temperature may be changed gradually or suddenly. Good (for example, within a range of about 400 to 1200 ° C.).

When forming a buried film by sputtering, zirconium oxide or zirconium is used as a target, and from a gas having a high oxygen partial pressure (about 1 × 10 ⁻³ to 1 × 10 ⁻² Pa) as a sputtering gas, gradually or rapidly. A method of switching to a gas having a small oxygen partial pressure (about 1 × 10 ⁻⁵ to 1 × 10 ⁻³ Pa) or zero, using zirconium oxide or zirconium as a target, and gradually or rapidly lowering the film formation rate, The RF power is gradually or suddenly reduced (the range to be reduced is about 50 to 500 W), or the distance between the target and the substrate is gradually or suddenly changed (the range to be changed is 0.2 to the original distance). The pressure is gradually or rapidly decreased when forming a film using zirconium oxide or zirconium as a method and target. (The pressure range to be reduced is about 0.1 to 2.0 pa) and the like. Furthermore, when forming the buried film by the sputtering method, there is a method in which the temperature of the substrate is gradually or rapidly increased or decreased (the temperature range to be changed is about 50 to 500 ° C.). Thereafter, heat treatment may be optionally performed.

In addition, when zirconium oxide is formed by ion plating, oxygen gas is turned into plasma from the middle of the film formation, and this oxygen plasma is taken into the zirconium film, and film formation is performed by CVD. In this case, a method of controlling the flow rate of the oxygen gas or the raw material oxygen-containing gas can be mentioned.
In addition, heat treatment in multiple stages, such as forming a zirconium film halfway, then heat-treating, and subsequently forming and heat-treating, may be used.

  Examples of the heat treatment method include lamp annealing, annealing with a heating furnace, and steam oxidation. Further, as the oxidation treatment after the zirconium film is formed, electron beam irradiation, a method of irradiating oxygen ions using an ion gun, or laser ablation may be used. The degree of heat treatment and oxidation treatment can be appropriately adjusted depending on the material to be obtained, the film thickness, and the like.

Furthermore, these methods may be arbitrarily combined.
Among them, a method (specifically, thermal oxidation) in which oxygen is contained in the formed zirconium film after forming the zirconium film is preferable. Thereby, variation in oxygen (oxidation) in the in-plane direction of the obtained zirconium oxide can be prevented, and a film having a uniform composition in the in-plane direction can be formed.

  In order to minimize damage to the surface of the second nitride semiconductor layer at the initial stage of the buried film formation, it is preferable to form the film at a mild condition at the initial stage of film formation regardless of the type of the buried film. For example, when the sputtering method is used, the film forming rate is set low or the RF power is set low. It is appropriate to perform the film formation under mild conditions at the initial stage of the film formation, for example, with a film thickness of about 100 mm or less.

  The film thickness of the buried film can be adjusted as appropriate depending on the material used and the film forming method. For example, when the film is formed of only a zirconium oxide film, the film thickness is about 0.01 to 1 μm. Is about 0.01 to 1 μm (including the case of the uppermost layer formed of at least two layers) and the total film thickness of about 0.01 to 1 μm when formed of at least two layers. From another viewpoint, when a ridge is formed, the film thickness can be lower than the height of the ridge. When the ridge is formed, the buried film is also formed on the side surface of the ridge, and the film thickness is different from the buried film formed on the exposed surface of the p-type semiconductor layer on both sides of the ridge. The above film thickness refers to the film thickness of the buried film formed on the exposed surface of the p-type semiconductor layer, and the ridge side surface is slightly thinner than that.

  The buried film is formed on a second nitride semiconductor layer, which will be described later, in a region other than a region where the second nitride semiconductor layer and an electrode, which will be described later, are in direct contact and are electrically connected. The position, size, shape, and the like of the connection region between the second nitride semiconductor layer and the electrode are not particularly limited, and a part of the surface of the second nitride semiconductor layer, for example, the second nitride semiconductor layer Illustrated is almost the entire upper surface of the striped ridge formed on the surface.

Examples of the electrode that is in contact with the embedded film to obtain an electrical connection can be exemplified by all materials, film thickness, and shape that can be used as an electrode. For example, zinc (Zn), nickel (Ni), platinum (Pt) palladium (Pd), rhodium (Rh), ruthenium (Ru), osmium (Os), iridium (Ir), titanium (Ti), zirconium (Zr) , Hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), cobalt (Co), iron (Fe), manganese (Mn), molybdenum (Mo), chromium (Cr), tungsten (W) Metals such as lanthanum (La), copper (Cu), silver (Ag), and yttrium (Y), alloys; single layer films or laminated films such as conductive oxides such as ITO, ZnO, and SnO ₂ . Among them, those having low resistance are preferable. Specifically, Ni—Au, Ni—Au—Pt, Pd—Pt, and Ni—Pt based electrode materials, and electrodes different from the above-mentioned electrodes are preferable. Ti-Al-based, V-Pt, V-Pt-Au-based, Ti-Al-Ti-Pt-based, W-Al-W-based, Ti-Mo-Ti-Pt-based electrode materials, and the like. These electrodes can be formed with a film thickness of about 100 nm to 10 μm, for example.

As the electrode material, when it is formed over almost the entire surface of the buried film, an electrode material that functions as a light absorbing film may be selected. As a result, so-called oozing light that cannot be completely confined by the buried film alone can be absorbed by the absorbing film. Thereby, a laser beam having a good FFP can be obtained.
The nitride semiconductor laser device of the present invention is usually formed by laminating a first nitride semiconductor layer, an active layer, and a second nitride semiconductor layer in this order on a substrate.

As the substrate that can be used here, a different kind of substrate from the nitride semiconductor may be used, or a nitride semiconductor substrate may be used. Examples of the heterogeneous substrate include, for example, an insulating substrate such as sapphire, spinel (MgA1 ₂ O ₄ ) having any one of the C-plane, R-plane, and A-plane, SiC (including 6H, 4H, and 3C), A nitride semiconductor such as an oxide substrate lattice-matched with ZnS, ZnO, GaAs, Si, and a nitride semiconductor can be grown, and a conventionally known substrate material can be used. Among them, sapphire and spinel are mentioned.

  In the case of using a heterogeneous substrate, (1) a buffer layer (low temperature growth layer) and a base layer made of a nitride semiconductor (for example, AlN, GaN, AlGaN, InGaN, etc., preferably GaN), and (2) ELOG (Epitaxially Laterally). Overgrowth) Nitride semiconductor layer grown, (3) Nitride semiconductor layer grown laterally from the side of the opening of the nitride semiconductor layer in which the opening is formed, (4) Selection using light irradiation, etc. It is preferable to form a laminated structure formed by combining one or more of the grown nitride semiconductor layers and the like. In other words, it is preferable that a nitride semiconductor layer using lateral growth or selective growth of a nitride semiconductor is formed. Thereby, the crystallinity of the nitride semiconductor layer formed thereon is improved.

As the nitride semiconductor layer grown by ELOG, for example, a nitride semiconductor layer is grown on a heterogeneous substrate, and a mask region and a non-mask region made of a protective film on which the growth of the nitride semiconductor is difficult are striped. In addition to growth in the film thickness direction, the nitride semiconductor grows in the lateral direction by growing the nitride semiconductor from the non-mask region, and the nitride semiconductor is formed on the mask region. Examples include grown layers. Examples of the protective film used in this case include oxides such as silicon oxide (SiO _x ), titanium oxide (TiO _x ), and zirconium oxide (ZrO _x ), and nitrides such as silicon nitride (Si _x N _y ) and titanium nitride. In addition to materials, silicon nitride oxide, and multilayer films thereof, there are refractory metals having a melting point of 1200 ° C. or higher, such as tungsten, titanium, and tantalum.

In the case where a heterogeneous substrate is used, a nitride semiconductor layer as a base before forming an element structure is grown on the heterogeneous substrate, and then the heterogeneous substrate is removed by a method such as polishing to obtain a single substrate of nitride semiconductor Alternatively, the heterogeneous substrate may be removed during or after the element structure is formed.
An example of the nitride semiconductor substrate is a substrate made of the nitride semiconductor described above.
In addition, the GaN substrate has a region where the crystal defect is small or partially, for example, about 1 × 10 ⁷ cm ⁻² or less, preferably about 1 × 10 ⁶ cm ⁻² at least on the surface portion. What you have is appropriate. Furthermore, it may have an off-angle angle of about 0.01 to 0.3 °, and further a step-like off-angle angle. Thereby, the generation of fine cracks can be prevented inside the n-type and p-type nitride semiconductor layers and the active layer constituting the device. Furthermore, the nitride semiconductor layer such as a buffer layer formed on the substrate or the substrate is an n-type impurity (for example, Si, Sn, Ge, etc.) in a range of about 1 × 10 ^{16 to} 5 × 10 ²¹ cm ^−3, for example. Se, C, Ti, etc.) may be contained.

Of the first nitride semiconductor layer, the active layer, and the second nitride semiconductor layer formed on the substrate, the first and second nitride semiconductor layers include, for example, AlN, GaN, AlGaN, AlInGaN, InN, and the like III -V group nitride semiconductor layer is mentioned. Among these, a nitride semiconductor layer containing Al is suitable. Specifically _{In y Al z Ga 1-y} -z N (0 ≦ y, 0 ≦ z, y + z ≦ 1), in _{particular, Al x Ga 1-x N} (0 <x <1) and the like are preferable. These semiconductor layers may be any of a single layer, a stacked layer, or a superlattice structure. For example, a superlattice structure of a nitride semiconductor layer containing Al and a nitride semiconductor layer having a composition different from that of the nitride semiconductor layer containing Al, specifically, Al _a Ga _1-a N (0 <a ≦ 1) and a superlattice structure of Al _b Ga _1-b N (0 ≦ b <1). When forming a superlattice structure, for example, a structure in which two layers having different compositions are alternately stacked may be used, but one layer or both layers are alternately changed while changing the composition and / or film thickness. A laminated structure may also be used.

  The first nitride semiconductor layer is usually a clad layer, a light guide layer, a crack prevention layer, a contact layer, a cap layer, etc. between the clad layer and an active layer described later or on the opposite side of the clad layer from the active layer. However, the second nitride semiconductor layer is a clad layer, an electron confinement layer, a light guide layer, a cap layer, a contact layer, etc. between the active layer and the clad layer described later or on the opposite side of the clad layer from the active layer. However, you may be comprised by combining 1 type (s) or 2 or more types.

The first and second nitride semiconductor layers may be formed by methods known in the art such as MOVPE, MOCVD (metal organic chemical vapor deposition), HVPE (halide vapor deposition), MBE (molecular beam vapor deposition). It can form by either. The nitride semiconductor layer has n-type or p-type conductivity by being doped with p-type impurities (for example, Mg, Zn, Cd, Be, Ca, Ba, etc.) or the above-described n-type impurities. It may be undoped. An example of the doping concentration is about 1 × 10 ^{16 to} 5 × 10 ²⁰ cm ⁻³ .

The active layer is suitably those formed by a nitride semiconductor layer containing In, in _{particular, In s Al t Ga 1-} s-t N (0 <s ≦ 1,0 ≦ t <1,0 < It is preferable to use a nitride semiconductor represented by s + t ≦ 1). By containing Al, light can be emitted in the ultraviolet region, and light can be emitted on the long wavelength side, and light emission in a wide range up to about 360 to 580 nm becomes possible. The nitride semiconductor may be non-doped, n-type impurity doped, or p-type impurity doped, but is preferably non-doped or n-type impurity doped. Thereby, high output can be achieved in the nitride semiconductor device.

  The active layer may be formed of any of a single layer, a multilayer, or a quantum well (multi-quantum, single quantum well) structure. In the case of the quantum well structure, a nitride semiconductor containing In is used for at least the well layer. By using a multiple quantum well structure, it is possible to improve the output and lower the oscillation threshold. As the quantum well structure of the active layer, a structure in which well layers and barrier layers are alternately stacked can be used. Further, the barrier layer sandwiched between the well layers is not limited to one layer (well layer / barrier layer / well layer), and two or more barrier layers may be referred to as “well layer / barrier layer”. A plurality of layers may be provided as (1) / barrier layer (2) / barrier layer (3) /. In addition, as for the active layer, either the well layer or the barrier layer may be disposed in the outermost layer.

An appropriate thickness of the active layer is, for example, about 100 to 3000 mm. In particular, in the case of a quantum well structure, the film thickness of the well layer and the number of well layers are not particularly limited. For example, the film thickness is in the range of about 10 to 300 mm, so that V _f , threshold current Density can be reduced. It is appropriate that the number of well layers is 1 or more. By setting the number of well layers to 2, a decrease in threshold current density and an improvement in life characteristics are recognized. The thickness and composition of the barrier layer are not particularly limited, but the In mixed crystal ratio is lower than the well layer so that a band gap energy difference is provided between the well layer and the band gap energy is larger than that of the well layer. It is preferable to use a nitride semiconductor containing In or a nitride semiconductor containing GaN or Al. The thickness of the barrier layer is, for example, 500 mm or less, preferably in the range of about 10 to 300 mm.

  The second nitride semiconductor layer may not have a ridge formed on the surface thereof, but preferably has a ridge formed thereon. The ridge is provided to define a region for confining current inside the active layer, a so-called waveguide. The size of the ridge is not particularly limited, and the width on the bottom side is wide and the forward mesa shape that the stripe width decreases as it approaches the top surface, conversely, the reverse mesa shape that the width of the stripe decreases as it approaches the bottom surface of the ridge, The shape may have a side surface perpendicular to the laminated surface, or may be a shape in which these are combined. The height of the ridge can be appropriately adjusted depending on the film thickness of the second nitride semiconductor layer, and examples thereof include about 0.1 to 2 μm, and further about 0.2 to 1 μm. The width of the ridge is, for example, about 0.1 to 10 μm, more preferably about 1 to 7 μm, but may not be all the same in the longitudinal direction (resonator direction).

In the nitride semiconductor laser element of the present invention, an electrode that is also electrically connected to the first nitride semiconductor layer is formed. This electrode can be formed of the same material as the electrode material described above. The formation position may be on the same side (upper side) as the electrode formed on the second nitride semiconductor layer with respect to the substrate, or may be on a different side (lower side).
Furthermore, the laser element is used by being divided into unit chips from the wafer, but a bar-shaped laser is formed by forming a scribe groove on the back side of the substrate, breaking from the nitride semiconductor layer side, and cleaving. Can do. Alternatively, a bar-shaped laser may be formed by etching such as RIE. The cleavage plane of the nitride semiconductor layer is suitably the M-plane, A-plane, R-plane, etc. of the nitride semiconductor, and particularly preferably the M-plane, (1-100 plane), etc. It becomes the resonator surface. Further, the etching surface of the nitride semiconductor layer is not limited to the M surface or the like, and may be any surface. Note that a film that functions as a protective film and / or functions as a reflectance control film is preferably formed on the resonator surface.

For example, one or more of TiO ₂ , ZrO ₂ , HfO ₂ , Sc ₂ O ₃ , Y ₂ O ₃ , MgO, Al ₂ O ₃ , Si ₃ N ₄ or ThO ₂ as a high refractive index material, low Examples of the refractive index material include one or more of SiO ₂ , ThF ₄ , LaF ₃ , MgF ₂ , LiF, NaF or Na ₃ AlF _6. These high refractive index materials, low refractive index materials, Are preferably used in appropriate combination. This film can be formed by laminating one to several tens of layers with a thickness of several tens to several μm depending on the oscillation wavelength.

Hereinafter, a laser element structure as shown in FIG. 1 will be described as an example.
Example 1
First, as a dissimilar substrate, an off-angle angle θ = 0.02 °, a step difference of about 20 angstroms, a terrace width W of about 800 angstroms, an orientation flat surface as an A surface, and a step C perpendicular to the A surface are mainly used. A sapphire substrate 101 having a surface is prepared.

The sapphire substrate 101 is set in a reaction vessel, the temperature is set to 510 ° C., hydrogen is used as a carrier gas, ammonia and TMG (trimethyl gallium) are used as a source gas, and a low-temperature growth buffer made of GaN on the sapphire substrate 101. The layer is grown to a thickness of 200 Angstroms.
Thereafter, only TMG is stopped, the temperature is raised to 1050 ° C., and when it reaches 1050 ° C., TMG and ammonia are used as source gases, and a GaN layer made of undoped GaN is grown to a thickness of 2.5 μm. The obtained GaN layer is processed to have a stripe width of 4 μm and a window width of 16 μm. Thereafter, on the GaN layer processed into a stripe shape, undoped GaN is grown to a thickness of about 20 μm and planarized to form the base layer 102.

Next, a buffer layer (not shown) made of Al _0.02 Ga _0.98 N is formed on the base layer 102 at a temperature of 1100 ° C. using TMG, TMA (trimethylaluminum), and ammonia. The film is grown with a thickness of 5 μm.
On the obtained buffer layer, an n-type contact layer 103 made of Al _0.02 Ga _0.98 N doped with Si at 1100 ° C. is grown to a thickness of 4 μm.

Next, the crack prevention layer 104 made of In _0.10 Ga _0.90 N doped with Si at 5 × 10 ¹⁸ / cm ³ at a temperature of 920 ° C. using TMG, TMI (trimethylindium), and ammonia is set to 0.0. Growing with a film thickness of 15 μm.
At a temperature of 1000 ° C., an A layer made of undoped Al _0.07 Ga _0.93 N is grown to a thickness of 25 mm, and then Si is used at 5 × 10 ¹⁸ / cm ³ using silane gas as an impurity gas. A B layer made of doped GaN is grown to a thickness of 25 mm. Then, the operation of alternately laminating the A layer and the B layer is repeated 400 times to laminate the A layer and the B layer, and the n-type clad layer 105 made of a multilayer film (superlattice structure) having a total film thickness of 2 μm is grown. .

The n-type cladding layer may not have a superlattice structure, and may be formed of, for example, Si-doped Al _0.035 Ga _0.965 N (film thickness: 2 μm).
On the clad layer 105, an n-type light guide layer 106 made of GaN and having a thickness of 0.15 μm is grown at the same temperature.
Next, at a temperature of 970 ° C., a barrier layer (140Å) made of Si-doped In _0.02 Ga _0.98 N, a well layer (70Å) made of undoped In _0.1 Ga _0.9 N, and undoped Of In _0.02 Ga _0.98 N are stacked in this order, and a well layer and a barrier layer are stacked on top of each other to form an active layer 107 having a quantum well structure. To do.

The active layer 107 is made of Al _0.25 Ga _0.75 N doped with 1 × 10 ¹⁹ / cm ^{3 of} Mg using Cp ₂ Mg (cyclopentadienyl magnesium) as an impurity gas at the same temperature. A p-type electron confinement layer 108 is grown to a thickness of 10 nm.
The p-type light guide layer 109 made of GaN and having a thickness of 0.15 μm is grown at a temperature of 1000 ° C. The p-type light guide layer 109 is doped with Mg by diffusion of Mg from adjacent layers such as the p-type electron confinement layer 108 and the p-type cladding layer 110 described later. It can be a doped layer.

Subsequently, an A layer made of undoped Al _0.1 Ga _0.9 N is grown to a thickness of 25 mm at 1000 ° C., and a B layer made of Mg-doped GaN is grown to a thickness of 25 mm to form an A layer and a B layer. The p-type cladding layer 110 made of a superlattice layer having a total thickness of 0.45 μm is grown by repeating the operation of alternately stacking 90 times.
Note that the p-type cladding layer may not have a superlattice structure, and may be formed of Mg-doped Al _0.05 Ga _0.95 N (film thickness 0.45 μm), for example.

Finally, a p-type contact layer 111 made of p-type GaN doped with 1 × 10 ²⁰ / cm ^{3 of} Mg is grown on the p-type cladding layer 110 at a temperature of 1000 ° C. to a thickness of 150 mm. The p-type contact layer 111 preferably has a high carrier concentration of 1 × 10 ¹⁷ / cm ³ or more.
In the reaction vessel, the wafer is annealed at 700 ° C. in a nitrogen atmosphere to further reduce the resistance of the p-type layer.

  After the nitride semiconductor is grown in this way to form a laminated structure, the wafer is taken out of the reaction vessel, and one of the p-type semiconductor layer, the active layer, and the n-type semiconductor layer is removed from the uppermost p-type contact layer 111. The portions are sequentially etched to expose the surface of the n-type contact layer that forms the n-electrode. At this time, not only the region where the n-electrode is formed, but also etching is performed so that the end surface perpendicular to the stripe-shaped waveguide region is exposed, so that this surface can be used as the resonator surface. This resonator surface forming step may be performed simultaneously with the n-type contact layer exposure or may be performed in a separate step.

Next, a first protective film made of Si oxide (mainly SiO ₂ ) is formed to a thickness of 0.5 μm on almost the entire surface of the uppermost p-type contact layer (upper contact layer) 111. 1 protective film is processed to a stripe width of 1.6 μm. Subsequently, using the first protective film as a mask, the p-type contact layer 111, the p-type cladding layer 110, and a part of the p-type light guide layer 109 are etched, so that the film thickness of the p-type light guide layer 109 is increased. A ridge stripe is formed by etching to a depth of 0.1 μm.

On the first protective film, using a sputtering apparatus, a target made of zirconium and argon as a sputtering gas, an RF power of 170 W, an underlayer film of 700 ＲＦ, and an RF power of 100 W, 300 Å The upper and lower films obtained were thermally oxidized at 600 ° C. for 20 minutes in an oxygen atmosphere.
Next, the wafer is immersed in hydrofluoric acid, and the first protective film is removed by a lift-off method. As a result, the first protective film provided on the p-type contact layer 111 is removed, and the p-type contact layer is exposed. Zirconium oxide 2 is formed on both sides of the ridge and on the upper surface of the p-type light guide layer 109. A buried film 162 made of a laminated film of layers was formed.

  When the obtained embedded film 162 was evaluated with a spectroscopic ellipsometer (HS-190 manufactured by JA WOOLLAM), the refractive index of the lower layer film was about 1.98 with respect to 400 nm light, and the refractive index of the upper layer film was It was about 2.23. Further, the roughness (surface roughness) of the spectroscopic ellipsometer was measured as a positive value for the lower layer film, measured as zero for the upper layer film, and it was confirmed that the surface roughness of the lower layer film was rougher than that of the upper layer film. Further, it was confirmed that the distribution of oxygen in the wafer surface was uniform and suppressed as compared with the buried film formed using a zirconium oxide target.

In addition, when the cross section of the buried film 162 was observed with an SEM and a TEM, the interface between the lower layer film and the upper layer film was clearly observed, and the lower layer film had fine grains and the upper layer film had coarse grains and corners. It was.
Subsequently, a p-electrode 120 made of Ni / Au (100 Å / 1500 Å) is formed on the exposed surface of the p-type contact layer 111 on the upper surface of the ridge and the buried film 162. The p-electrode 120 is formed over the second protective film 162 with a stripe width of 30 μm as shown in FIG.

Before or after the formation of the p-electrode 120, a striped n-electrode 121 made of Ti / Al (100Å / 5000Å) is formed in a direction parallel to the stripe on the surface of the n-type contact layer 103 that has already been exposed.
Next, a mask having a desired shape is formed on the p-electrode 120 and the n-electrode 121, and a film 164 made of a SiO ₂ single layer is provided.

Thereafter, a pad electrode 122 made of Ni—Ti—Au (1000 Å−1000 Å−8000 Å) is provided on the p electrode 120, and a pad electrode 123 made of Ni—Ti—Au (1000 Å−1000 Å−8000 Å) is provided on the n electrode 121. Provide.
After forming the n electrode 121 and the p electrode 120 as described above, a part of the exposed surface of the n-type contact layer is further etched to expose sapphire. Next, the film 164 made of SiO ₂ is removed, and the sapphire substrate 101 of the wafer is polished to 200 μm.

Subsequently, the wafer is divided into bars, and at least the light emitted from the active layer is efficiently resonated at least on the light emitting surface side of the main laser light. A protective film made of Al ₂ O ₃ is formed using a sputtering apparatus so as to have a refractive index difference from the resonator surface. Next, a dielectric multilayer film (not shown) composed of a total of seven layers in which (SiO ₂ / ZrO ₂ ) was repeatedly layered three times on Al ₂ O ₃ was formed on the opposite resonator surface.

Finally, the wafer was divided into chips in the direction parallel to the p-electrode to obtain a laser element (resonator length 650 μm).
About the obtained laser element, the adhesiveness of a buried film and a p electrode was evaluated. First, the obtained semiconductor device was subjected to ultrasonic treatment for 30 minutes twice, then treated with ultrasonic waves for 30 minutes, heat-treated at 180 ° C. for 30 minutes, and further immersed in cold water. I went twice. Then, it processed for 60 minutes with ultrasonic waves, heat-processed for 30 minutes at 220-300 degreeC, and also immersed in cold water, this process was performed 8 times, and finally, it processed with ultrasonic waves for 60 minutes.
Even by such a series of ultrasonic treatment and thermal shock test, it was confirmed that the p-type electrode was not peeled off and could sufficiently withstand the subsequent manufacturing process.

On the other hand, for comparison, except that the lower layer film is set to 1000 mm and a single-layer buried film not forming the upper layer film is formed, a laser element is formed in the same manner as in the above-described embodiment, and ultrasonic treatment and thermal shock test are similarly performed. went.
As a result, it was confirmed that the p-type electrode was peeled off in the first 30 minutes of ultrasonic treatment and thermal shock test.

Further, in the obtained laser element, the buried film is configured to have two different refractive indexes, that is, a buried film having a low refractive index and a high refractive index is formed from the side close to the waveguide. Therefore, only the fundamental mode of emitted light can be confined, and higher-order modes can escape from the waveguide and be absorbed by the electrode. As a result, a desired FFP can be obtained.
Furthermore, since the buried film in the obtained laser element is configured to have a low density on the p-type nitride semiconductor layer side, the dielectric constant can be lowered only on the p-type nitride semiconductor layer side. it can. Thereby, it becomes possible to contribute to the response at higher speed.

Example 2
First, a sapphire substrate 101 similar to that in Example 1 is prepared.

The sapphire substrate 101 is set in a reaction vessel, the temperature is set to 510 ° C., hydrogen is used as a carrier gas, ammonia and TMG (trimethyl gallium) are used as a source gas, and a low-temperature growth buffer made of GaN on the sapphire substrate 101. The layer is grown to a thickness of 200 Angstroms.
Thereafter, only TMG is stopped, the temperature is raised to 1050 ° C., and when it reaches 1050 ° C., TMG and ammonia are used as source gases, and a GaN layer made of undoped GaN is grown to a thickness of 2.5 μm. The obtained GaN layer is processed to have a stripe width of 4 μm and a window width of 16 μm. Thereafter, on the GaN layer processed into a stripe shape, undoped GaN is grown to a thickness of about 20 μm and planarized to form the base layer 102.

Next, a buffer layer (not shown) made of Al _0.02 Ga _0.98 N is formed on the base layer 102 at a temperature of 1100 ° C. using TMG, TMA (trimethylaluminum), and ammonia. The film is grown with a thickness of 5 μm.
On the obtained buffer layer, an n-type contact layer 103 made of Al _0.02 Ga _0.98 N doped with Si at 1100 ° C. is grown to a thickness of 4 μm.

Next, the crack prevention layer 104 made of In _0.10 Ga _0.90 N doped with Si at 5 × 10 ¹⁸ / cm ³ at a temperature of 920 ° C. using TMG, TMI (trimethylindium), and ammonia is set to 0.0. Growing with a film thickness of 15 μm.
At a temperature of 1000 ° C., an A layer made of undoped Al _0.13 Ga _0.87 N is grown to a thickness of 25 mm, and subsequently, Si is used as an impurity gas and Si is 5 × 10 ¹⁸ / cm ^3. A B layer made of doped Al _0.09 Ga _0.91 N is grown to a thickness of 25 mm. Then, the operation of alternately laminating the A layer and the B layer is repeated 120 times to laminate the A layer and the B layer, and the n-type cladding layer 105 made of a multilayer film (superlattice structure) having a total film thickness of 0.6 μm is formed. Grow.

On the clad layer 105, an n-type light guide layer 106 made of Al _0.065 Ga _0.935 N and having a thickness of 0.15 μm is grown at the same temperature.
Next, at a temperature of 970 ° C., a barrier layer (70 cm) made of Si-doped Al _0.15 Ga _0.85 N and a well layer made of undoped In _0.02 Al _0.03 Ga _0.95 N ( 100Å) and a barrier layer (50Å) made of undoped Al _0.15 Ga _0.85 N are stacked in this order to form an active layer 107 having a quantum well structure.

The active layer 107 is made of Al _0.3 Ga _0.7 N doped with 1 × 10 ¹⁹ / cm ^{3 of} Mg using Cp ₂ Mg (cyclopentadienyl magnesium) as an impurity gas at the same temperature. A p-type electron confinement layer 108 is grown to a thickness of 13 nm.
The p-type light guide layer 109 having a thickness of 0.15 μm made of Al _0.065 Ga _0.935 N is grown at a temperature of 1000 ° C. The p-type light guide layer 109 is doped with Mg by diffusion of Mg from adjacent layers such as the p-type electron confinement layer 108 and the p-type cladding layer 110 described later. It can be a doped layer.

Subsequently, an A layer made of undoped Al _0.13 Ga _0.87 N is grown to a thickness of 25 mm at 1000 ° C., and a B layer made of Mg doped Al _0.09 Ga _0.91 N is grown to a thickness of 25 mm. The p-type cladding layer 110 made of a superlattice layer having a total thickness of 0.45 μm is grown by repeating the operation of alternately growing the A layer and the B layer 90 times.
Finally, a p-type contact layer 111 made of p-type GaN doped with 1 × 10 ²⁰ / cm ^{3 of} Mg is grown on the p-type cladding layer 110 at a temperature of 1000 ° C. to a thickness of 150 mm. The p-type contact layer 111 preferably has a high carrier concentration of 1 × 10 ¹⁷ / cm ³ or more.

In the reaction vessel, the wafer is annealed at 700 ° C. in a nitrogen atmosphere to further reduce the resistance of the p-type layer.
After the nitride semiconductor is grown in this way to form a laminated structure, the wafer is taken out of the reaction vessel, and one of the p-type semiconductor layer, the active layer, and the n-type semiconductor layer is removed from the uppermost p-type contact layer 111. The portions are sequentially etched to expose the surface of the n-type contact layer that forms the n-electrode. At this time, not only the region for forming the n-electrode but also the resonance surface is formed at the same time. As described above, even in a region including a position where the nitride semiconductor element is cleaved or divided in a later step and / or a region including the periphery thereof, an effect such as stress relaxation can be obtained by exposing the surface of the n-type contact layer. There is a case.

Next, a first protective film made of Si oxide (mainly SiO ₂ ) is formed to a thickness of 0.5 μm on almost the entire surface of the uppermost p-type contact layer (upper contact layer) 111. 1 protective film is processed to a stripe width of 1.6 μm. Subsequently, using the first protective film as a mask, the p-type contact layer 111, the p-type cladding layer 110, and a part of the p-type light guide layer 109 are etched, so that the film thickness of the p-type light guide layer 109 is increased. A ridge stripe is formed by etching to a depth of 0.1 μm.
On the first protective film, using a sputtering apparatus, a target made of zirconium and argon as a sputtering gas, an RF power of 170 W, an underlayer film of 1700 mm, and an RF power of 100 W, 300 mm The upper and lower films obtained were thermally oxidized at 600 ° C. for 20 minutes in an oxygen atmosphere.

Next, the wafer is immersed in hydrofluoric acid, and the first protective film is removed by a lift-off method. As a result, the first protective film provided on the p-type contact layer 111 is removed, and the p-type contact layer is exposed. Zirconium oxide 2 is formed on both sides of the ridge and on the upper surface of the p-type light guide layer 109. A buried film 162 made of a laminated film of layers was formed.
When the obtained embedded film 162 was evaluated with a spectroscopic ellipsometer (HS-190 manufactured by JA WOOLLAM), the refractive index of the lower layer film was about 1.98 with respect to 400 nm light, and the refractive index of the upper layer film was It was about 2.23. Further, the roughness (surface roughness) of the spectroscopic ellipsometer was measured as a positive value for the lower layer film, measured as zero for the upper layer film, and it was confirmed that the surface roughness of the lower layer film was rougher than that of the upper layer film. Further, it was confirmed that the distribution of oxygen in the wafer surface was uniform and suppressed as compared with the buried film formed using a zirconium oxide target.

In addition, when the cross section of the buried film 162 was observed with an SEM and a TEM, the interface between the lower layer film and the upper layer film was clearly observed, and the lower layer film had fine grains and the upper layer film had coarse grains and corners. It was.
Subsequently, a p-electrode 120 made of Ni / Au / Pt (100 Å / 1500 Å / 1500 形成) is formed on the exposed surface of the p-type contact layer 111 on the ridge upper surface and the buried film 162. The p-electrode 120 is formed over the second protective film 162 with a stripe width of 30 μm as shown in FIG.

Before or after the formation of the p-electrode 120, a striped n-electrode 121 made of Ti / Al (100Å / 5000Å) is formed in a direction parallel to the stripe on the surface of the n-type contact layer 103 that has already been exposed.
Next, a mask having a desired shape is formed on the p-electrode 120 and the n-electrode 121, and a single layer film 164 made of SiO ₂ is provided.
Subsequently, an adhesion layer made of Ti / Pt (100/500) is formed.

Thereafter, a pad electrode 122 made of Pt / Au (3000/6000 Å) is provided on the p-electrode 120, and a pad electrode 123 made of Ni—Ti—Au (1000Å-1000Å-8000Å) is provided on the n-electrode 121.
After forming the n-electrode 121 and the p-electrode 120 as described above, the sapphire substrate 101 of the wafer is polished to 200 μm, and then a part of the n-type contact layer described above is further etched to expose the substrate. Let Next, the film made of SiO ₂ is removed, and the sapphire substrate of the wafer is polished to a thickness of 200 μm.
Subsequently, the light emitted from the active layer is efficiently resonated at least on the emission surface side of the main laser beam, and in particular, the resonator surface on the opposite surface side has a refractive index difference from the resonator surface on the light emission surface side. Thus, using a sputtering apparatus, a protective film made of Nb ₂ O ₅ or Al ₂ O ₃ is formed, and then on the resonator surface on the light reflection side, on ZrO ₂ or Al ₂ O ₃ , ( A dielectric multilayer film (not shown) composed of a total of nine layers in which SiO ₂ / ZrO ₂ ) was repeatedly laminated four times was formed. Finally, the wafer was cut into chips in a direction parallel to the p-electrode to obtain a laser element (resonator length: 650 μm).

For the obtained laser element, the adhesion between the buried film and the p-electrode was evaluated in the same manner as in Example 1. As a result, it was confirmed that the p-type electrode was not peeled off and could sufficiently withstand the subsequent manufacturing process.
On the other hand, for comparison, except that the lower layer film is set to 1000 mm and a single-layer buried film not forming the upper layer film is formed, a laser element is formed in the same manner as in the above-described embodiment, and ultrasonic treatment and thermal shock test are similarly performed. went.

As a result, it was confirmed that the p-type electrode was peeled off in the first 30 minutes of ultrasonic treatment and thermal shock test.
Further, in the obtained laser element, the buried film is configured to have two different refractive indexes, that is, a buried film having a low refractive index and a high refractive index is formed from the side close to the waveguide. Therefore, only the fundamental mode of emitted light can be confined, and higher-order modes can escape from the waveguide and be absorbed by the electrode. As a result, a desired FFP can be obtained.
Furthermore, since the buried film in the obtained laser element is configured to have a low density on the p-type nitride semiconductor layer side, the dielectric constant can be lowered only on the p-type nitride semiconductor layer side. it can. Thereby, it becomes possible to contribute to the response at higher speed.

Example 3
In the laser element of this embodiment, the buried film 162 is first formed as a lower layer film using a target made of zirconium and a sputtering gas of argon gas, and as an upper layer film, a mixture of argon gas and oxygen gas is mixed. A laser having the same configuration is substantially the same as in Example 1, except that a two-layered film of zirconium oxide is formed using a gas as a sputtering gas, and then heat treatment is performed in the same manner. An element was obtained.
Note that the pressure of the sputtering gas when forming the lower layer film is about 0.1 to 2.0 Pa, and the partial pressure of the oxygen gas when forming the upper layer film is 1 × 10 ⁻³ to 1 × 10 ^−1. About Pa.
As in Example 1, the obtained laser element was confirmed to have a uniform in-plane oxygen distribution and good adhesion to the electrode.

Example 4
In the laser element of this embodiment, the buried film 162 is first formed by sputtering, using a target made of zirconium and a sputtering gas of argon gas as a lower layer film, and a film thickness of 610 mm at a pressure of about 1.5 pa. Except that a two-layered film of zirconium oxide was formed using the same target and sputtering gas as the upper layer film and a film thickness of 320 mm at a pressure of about 0.1 pa. Specifically, a laser device having the same configuration was obtained by the same method as in Example 1.

When the density of the obtained upper layer film and lower layer film was measured by X-ray diffraction, it was about 5.7 g / cm for the upper layer film and 4.19 g / cm for the lower layer film, and the upper layer film was denser than the lower layer film. It was confirmed that it has a simple structure.
In addition, it was confirmed that the obtained laser element had a uniform oxygen in-plane distribution and good adhesion to the electrode, as in Example 1.

Example 5
In the laser element of this embodiment, the buried film 162 is first formed by sputtering using a target made of zirconium and a sputtering gas of argon gas at a pressure of about 1.5 pa, and the pressure is gradually increased. Is reduced to about 0.1 pa, the total film thickness is 1000 mm, and a two-layered film of zirconium oxide is formed in substantially the same manner as in Example 1, except that A laser device having the structure was obtained.

When the density of the buried film thus obtained was measured by X-ray diffraction, it was about 5.7 g / cm at the extremely thin part on the surface side and 4.19 g at the extremely thin part on the p-type nitride semiconductor layer side. / Cm, and it was confirmed to have a structure in which the density increases from the p-type nitride semiconductor layer side to the surface side.
In addition, it was confirmed that the obtained laser element had a uniform oxygen in-plane distribution and good adhesion to the electrode, as in Example 1.

Example 6
The laser device of this example has the same configuration as that of Example 1 except that the buried film 162 is first used as a lower layer film, and a vacuum deposition method is used instead of the sputtering method. A laser element was obtained.
As in Example 1, the obtained laser element was confirmed to have a uniform in-plane oxygen distribution and good adhesion to the electrode.

Example 7
In the laser element of this embodiment, when the buried film 162 is first formed, a lower layer film is formed using an ion gun, and about 10 ¹² oxygen ions / cm ² on the wafer surface (p-type semiconductor layer). A laser device having the same configuration was obtained by a method substantially similar to that of Example 5 except that the irradiation was performed at the same time.
As in Example 1, the obtained laser element was confirmed to have a uniform in-plane oxygen distribution and good adhesion to the electrode.

Example 8
In the laser element of this embodiment, the buried film 162 is first formed by a vacuum deposition method when the lower layer film is formed, and the deposition rate is set to 50 秒 / sec. When the upper layer film is formed, the vacuum film method is used. A laser device having the same configuration was obtained in substantially the same manner as in Example 1 except that the film formation rate was reduced to 5 liters / second to form a two-layered zirconium oxide film.
As in Example 1, the obtained laser element was confirmed to have a uniform in-plane oxygen distribution and good adhesion to the electrode.

Example 9
As shown in FIG. 2, in the laser element of this embodiment, when the buried film 62 is first formed as the lower layer film a, the film formation rate is 5 liters / second by a vacuum evaporation method, and the zirconium oxide film is 100 liters thick. The thickness of the middle layer film b is 700 nm with a film thickness of Al ₂ O ₃ , HfO ₂ or SiO ₂ by sputtering, and the upper film c is 200 mm with a film formation rate of 5 mm / second by vacuum deposition. A laser device having the same configuration was obtained by a method substantially similar to that in Example 1 except that zirconium oxide was formed.
In addition to the effects of the first embodiment, the obtained laser element can further ensure the insulation resistance of the buried film 62, and a more reliable laser element can be obtained.

Example 10
First, the same sapphire substrate as in Example 1 is prepared.
This sapphire substrate is set in a reaction vessel, the temperature is set to 510 ° C., hydrogen is used as a carrier gas, ammonia and TMG (trimethyl gallium) are used as a source gas, and a low-temperature growth buffer layer made of GaN is formed on the sapphire substrate. Grow with a film thickness of 200 Å. Thereafter, only TMG is stopped, the temperature is raised to 1050 ° C., and when it reaches 1050 ° C., TMG and ammonia are used as source gases, and a GaN layer made of undoped GaN is grown to a thickness of 2.5 μm.
The obtained GaN layer is processed to have a stripe width of 4 μm and a window width of 16 μm. Thereafter, undoped GaN is grown on the GaN layer processed into a stripe shape by about 20 μm, and is flattened to form an underlayer. After the underlayer is grown, the wafer is taken out of the reaction vessel, a striped photomask is formed on the surface of the underlayer, and a SiO 2 having a stripe width of 10 to 300 μm and a stripe interval (window) of 5 to 300 μm is formed by a CVD apparatus. ₂ is formed.
After forming the protective film, the wafer is transferred to an HVPE (hydride vapor phase epitaxy) apparatus, and a nitride semiconductor made of GaN is grown to a thickness of 100 μm using Ga metal, HCl gas and ammonia as raw materials.
At this time, when an n-electrode is formed on the back surface, GaN is grown by oxygen or Si doping. Further, when an n-electrode is formed on the surface, it may be non-doped.

Next, a buffer layer (not shown) made of Al _0.02 Ga _0.98 N is used on the grown GaN layer at a temperature of 1100 ° C. using TMG, TMA (trimethylaluminum), and ammonia. Growing with a film thickness of 0.5 μm.
When forming an n-electrode on the surface, an n-type contact layer 103 made of Al _0.02 Ga _0.98 N doped with Si at 1100 ° C. is grown on the obtained buffer layer to a thickness of 4 μm.

At a temperature of 1000 ° C., an A layer made of undoped Al _0.07 Ga _0.93 N is grown to a thickness of 25 mm, and then Si is used at 5 × 10 ¹⁸ / cm ³ using silane gas as an impurity gas. A B layer made of doped GaN is grown to a thickness of 25 mm. Then, the operation of alternately laminating the A layer and the B layer is repeated 400 times to laminate the A layer and the B layer, and an n-type cladding layer made of a multilayer film (superlattice structure) having a total film thickness of 2 μm is grown.

The n-type cladding layer may not have a superlattice structure, and may be formed of, for example, Si-doped Al _0.035 Ga _0.965 N (film thickness: 2 μm).
On the clad layer, an n-type light guide layer made of GaN having a thickness of 0.15 μm is grown at the same temperature.
Next, at a temperature of 970 ° C., a barrier layer (140Å) made of Si-doped In _0.02 Ga _0.98 N, a well layer (70Å) made of undoped In _0.1 Ga _0.9 N, and undoped Of In _0.02 Ga _0.98 N are stacked in this order, and a well layer and a barrier layer are stacked one by one to form an active layer having a quantum well structure. .

On the active layer, p is made of Al _0.25 Ga _0.75 N doped with 1 × 10 ¹⁹ / cm ^{3 of} Mg using Cp ₂ Mg (cyclopentadienyl magnesium) as an impurity gas at the same temperature. A type electron confinement layer is grown to a thickness of 10 nm.
The p-type light guide layer 109 made of GaN and having a thickness of 0.15 μm is grown at a temperature of 1000 ° C. This p-type light guide layer is doped with Mg by diffusion of Mg from adjacent layers such as a p-type electron confinement layer and a p-type cladding layer described later. it can.

Subsequently, an A layer made of undoped Al _0.1 Ga _0.9 N is grown to a thickness of 25 mm at 1000 ° C., and a B layer made of Mg-doped GaN is grown to a thickness of 25 mm to form an A layer and a B layer. The p-type cladding layer made of a superlattice layer having a total film thickness of 0.45 μm is grown by repeating the operation of alternately stacking 90 times.
Note that the p-type cladding layer may not have a superlattice structure, and may be formed of Mg-doped Al _0.05 Ga _0.95 N (film thickness 0.45 μm), for example.

Finally, a p-type contact layer made of p-type GaN doped with Mg at 1 × 10 ²⁰ / cm ³ is grown on the p-type cladding layer at a temperature of 1000 ° C. to a thickness of 150 mm. The p-type contact layer preferably has a high carrier concentration of 1 × 10 ¹⁷ / cm ³ or more.
In the reaction vessel, the wafer is annealed at 700 ° C. in a nitrogen atmosphere to further reduce the resistance of the p-type layer.
After the nitride semiconductor is grown in this way to form a laminated structure, the wafer is taken out of the reaction vessel, and one of the p-type semiconductor layer, the active layer, and the n-type semiconductor layer is removed from the uppermost p-type contact layer 111. The portions are sequentially etched to expose the surface of the n-type contact layer that forms the n-electrode. Note that this step can be omitted when an n-electrode is provided on the back side.

Next, a first protective film made of Si oxide (mainly SiO ₂ ) is formed to a thickness of 0.5 μm on almost the entire surface of the uppermost p-type contact layer (upper contact layer). This protective film is processed to a stripe width of 1.6 μm. Subsequently, using the first protective film as a mask, the p-type contact layer, the p-type cladding layer, and a part of the p-type light guide layer are etched, and the film thickness of the p-type light guide layer becomes 0.1 μm. A ridge stripe is formed by etching to a certain depth.

On the first protective film, using a sputtering apparatus, a target made of zirconium and argon as a sputtering gas, an RF power of 170 W, an underlayer film of 700 ＲＦ, and an RF power of 100 W, 300 Å The upper and lower films obtained were thermally oxidized at 600 ° C. for 20 minutes in an oxygen atmosphere.
Next, the wafer is immersed in hydrofluoric acid, and the first protective film is removed by a lift-off method. As a result, the first protective film provided on the p-type contact layer is removed, and the p-type contact layer is exposed, and two zirconium oxide layers are formed on both sides of the ridge and on the upper surface of the p-type light guide layer. A buried film made of a laminated film was formed.

  When the obtained embedded film was evaluated with a spectroscopic ellipsometer (HS-190 manufactured by JA WOOLLAM), the refractive index of the lower layer film was about 1.98 with respect to 400 nm light, and the refractive index of the upper layer film was about It was 2.23. Further, the roughness (surface roughness) of the spectroscopic ellipsometer was measured as a positive value for the lower layer film, measured as zero for the upper layer film, and it was confirmed that the surface roughness of the lower layer film was rougher than that of the upper layer film. Further, it was confirmed that the distribution of oxygen in the wafer surface was uniform and suppressed as compared with the buried film formed using a zirconium oxide target.

In addition, when the cross-section of the buried film was observed by SEM and TEM, the interface between the lower layer film and the upper layer film was clearly observed, and the lower layer film was finer and the upper layer film was coarser and the corner was observed. .
Subsequently, a p-electrode made of Ni / Au (100Å / 1500Å) is formed on the exposed surface of the p-type contact layer on the upper surface of the ridge and on the buried film. The p-electrode is formed over the second protective film with a stripe width of 15 μm. Further, an n-electrode made of Ti / Pt / Au (100 Å / 1500 Å / 3000 Å) is formed on the n-type contact layer. Note that this step is omitted when the n-electrode is formed on the back surface.

Next, a mask having a desired shape is formed on the p-electrode, and a film composed of a single layer of SiO ₂ is provided.
Thereafter, a pad electrode made of Ni—Ti—Au (1000 Å−1000 Å−8000 Å) is provided on the p electrode.
After forming the p-electrode as described above, the sapphire substrate of the wafer is polished to a total film thickness of 100 μm. After that, the entire surface of the wafer is irradiated with excimer laser light to separate the sapphire substrate and the nitride semiconductor layer.

  The back side of the obtained wafer becomes a GaN layer, and Au / Sn is formed as a eutectic material on the back side. When forming the n electrode on the back surface, the n electrode made of Ti / Pt / Au (100 Å / 1500 Å / 3000 Å) is formed.

Subsequently, by dividing the wafer in a direction perpendicular to the ridge by cleavage to form a resonator surface, the wafer is formed into a bar shape, and further, the active layer emits light at least on the main laser beam emission surface side. A protective film made of Al ₂ O ₃ is formed using a sputtering apparatus so that light is efficiently resonated, and in particular, the resonator surface on the opposite surface side has a refractive index difference from the resonator surface on the light output surface side. To do. Next, a dielectric multilayer film (not shown) composed of a total of seven layers in which (SiO ₂ / ZrO ₂ ) was repeatedly laminated three times on Al ₂ O ₃ was formed on the light reflecting side resonator surface.

Finally, the wafer was divided into chips in a direction parallel to the p-electrode to obtain a laser element (resonator length is 600 μm).
When the adhesion between the buried film and the p-electrode was evaluated for the obtained laser element, the same result as in Example 1 was obtained.
Further, when the average surface roughness of the obtained upper layer film and lower layer film was measured by using AFM, the upper layer film was about 4.5 × 10 ⁻¹ nm, and the lower layer film was about 1 It was confirmed that the upper layer film had a smaller surface roughness, that is, a flatter surface.

Further, it was confirmed that the laser element can have a longer life in addition to the effects of the first embodiment.
In Example 10, the heterogeneous substrate (sapphire) is finally removed to form the structure of only the nitride semiconductor layer. However, the nitride semiconductor layer is formed on the heterogeneous substrate of sapphire, GaAs, SiC, or the like. The heterogeneous substrate may be removed by growing the film, and then a functional layer including an active layer or the like may be stacked. Further, a commercially available nitride semiconductor substrate may be used. In any case, it is possible to provide an electrode on the back surface when the nitride semiconductor is exposed on the back surface side and when the conductivity is good and the ohmic property and adhesion with the electrode material are excellent. Therefore, it is preferable that the laser element itself can be downsized.

Example 11
At least up to the active layer is formed in the same manner as in Example 1 (preferably up to the p-type guide layer, more preferably up to half the thickness of the p-type guide layer), and an electric current made of aluminum nitride is formed thereon A constriction layer (film thickness of 300 mm) is formed. The current confinement layer is formed so as to be disposed on both sides above the waveguide region. Since the current confinement layer only needs to be above the active layer, it may be between the p-type guide layer and the p-type cladding layer, or may be formed in the p-type cladding layer. Moreover, not only a single layer but a multilayer structure in which a plurality of layers having different compositions is laminated may be used. The film thickness is preferably such that at least no current flows or has a high resistance, and the current path needs to be controllable in a stripe shape. Furthermore, it can be adjusted to a value that can realize a desired light confinement efficiency, depending on the material, the position to be formed (distance from the active layer), and the like.

Subsequently, a p-type nitride semiconductor layer such as a p-type guide layer is stacked on the current confinement layer in the same manner as in the first embodiment.
Thereafter, a buried film similar to that of the first embodiment is formed on the p-type contact layer and in a region corresponding to the current confinement layer, and the p-type contact layer is further covered with a p-type layer so as to cover a part of the buried film. An electrode is formed.

Note that the n electrode and the like are formed in the same manner as in Example 10.
Since the obtained laser element is provided with a current confinement layer instead of the formation of the ridge, the conduction path can be controlled in a stripe shape and a material having a low refractive index such as AlN can be used. As in the case of the laser element in which is formed, it is possible to efficiently confine light.

  In addition, as in Example 1, it was confirmed that the adhesion between the buried film and the p-electrode was good.

  The present invention provides not only a laser diode element (LD) but also a light emitting diode element (LED), a light emitting element such as a super photoluminescence diode, a light receiving element such as a solar cell or a photosensor, or an electronic device such as a transistor or a power device. The present invention can be widely applied to nitride semiconductor elements that need to ensure adhesion between a film made of the same material as the buried film and the electrode material, as used in the above.

It is a schematic cross section explaining the laser element structure of one embodiment of the present invention. It is a schematic cross section explaining the structure of the embedded film in the laser device of another embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Substrate 102 Buffer layer 103 n-type contact layer 104 Crack prevention layer 105 n-type cladding layer 106 n-type light guide layer 107 active layer 108 p-type electron confinement layer 109 p-type light guide layer 110 p-type cladding layer 111 p-type contact Layer 120 p electrode 121 n electrode 122 p pad electrode 123 n pad electrodes 62 and 162 buried film 164 film a lower layer film b middle layer film c upper layer film

Claims (15)

A nitride semiconductor in which a first nitride semiconductor layer, an active layer, and a second nitride semiconductor layer are stacked in this order, and an electrode is formed on the second nitride semiconductor layer through a part of a buried film. A laser element,
The nitride semiconductor laser device, wherein the buried film has a refractive index varying from the second nitride semiconductor layer side to the surface side. The nitride semiconductor laser element according to claim 1, wherein the refractive index of the buried film is larger on the surface side than on the second nitride semiconductor layer side. 3. The nitride semiconductor laser device according to claim 1, wherein the refractive index of the buried film on the second nitride semiconductor layer side is 2.20 or less and the refractive index of the buried film on the surface side is larger than 2.20. A nitride semiconductor in which a first nitride semiconductor layer, an active layer, and a second nitride semiconductor layer are stacked in this order, and an electrode is formed on the second nitride semiconductor layer through a part of a buried film. A laser element,
The nitride semiconductor laser device, wherein the buried film has an oxygen ratio that changes from the second nitride semiconductor layer side to the surface side. The nitride semiconductor laser device according to claim 4, wherein the oxygen ratio of the buried film is smaller on the surface side than on the second nitride semiconductor layer side. A nitride semiconductor in which a first nitride semiconductor layer, an active layer, and a second nitride semiconductor layer are stacked in this order, and an electrode is formed on the second nitride semiconductor layer through a part of a buried film. A laser element,
The nitride semiconductor laser element, wherein the buried film has a density that varies from the second nitride semiconductor layer side to the surface side. The nitride semiconductor laser element according to claim 6, wherein the density of the buried film is higher on the surface side than on the second nitride semiconductor layer side. A nitride semiconductor in which a first nitride semiconductor layer, an active layer, and a second nitride semiconductor layer are stacked in this order, and an electrode is formed on the second nitride semiconductor layer through a part of a buried film. A laser element,
The nitride semiconductor laser device, wherein the buried film is formed of at least two layers, and the surface roughness of the layer on the second nitride semiconductor layer side and the layer on the surface side are different. 9. The nitride semiconductor laser device according to claim 8, wherein the surface of the layer on the outermost surface side of the buried film is flattest among the buried layers. The nitride semiconductor laser element according to claim 1, wherein the buried film is formed of a single layer. The nitride semiconductor laser element according to claim 1, wherein the buried film is formed of a film having a laminated structure of two or more layers. The nitride semiconductor laser element according to claim 1, wherein the buried film contains oxygen at least on the second nitride semiconductor layer side. The nitride semiconductor laser element according to claim 1, wherein the buried film is made of an oxide at least on the second nitride semiconductor layer side. The nitride semiconductor laser device according to claim 1, wherein the buried film is made of oxygen and zirconium. The nitride semiconductor laser element according to claim 1, wherein a ridge is formed on a surface of the second nitride semiconductor layer.

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