JP6037449B2 - Nitride semiconductor optical device and manufacturing method thereof - Google Patents

Description

  The present invention relates to a nitride semiconductor optical device and a method for manufacturing the same, and more particularly to a nitride semiconductor optical device in which the operating voltage of the device is reduced.

  Conventionally, in a nitride semiconductor optical device, gallium nitride or sapphire is used as a substrate material. An example of a nitride semiconductor optical device is disclosed in Patent Document 1, and the nitride semiconductor optical device disclosed in Patent Document 1 employs a ridge structure as an element structure. In such a nitride semiconductor optical device, a semiconductor multilayer including an active layer and a p-type cladding layer is grown on a substrate using metal organic chemical vapor deposition (MOCVD). Using a combination of lithography and dry etching, a mask is formed on the upper surface of the semiconductor multilayer. A ridge stripe (hereinafter referred to as a ridge) is formed by etching from the upper surface of the semiconductor multilayer to a part of the p-type cladding layer using the mask.

JP 2009-021424 A

  In nitride semiconductor optical devices, further reduction in operating voltage is required from the viewpoint of improving device performance and reliability.

  One of the causes of the deterioration of the characteristics of the nitride semiconductor optical device is etching damage that occurs in the p-type cladding layer in the step of forming a mask and forming a ridge using the mask. In this process, dry etching is generally used, but it is known that the crystal layer is damaged by charged particles and products generated during the etching.

  The etching used in this process includes a first etching and a second etching. The first etching is to form an insulating film such as a silicon oxide film on the upper side of the semiconductor multilayer and to etch this in a stripe shape. The second etching is to etch the semiconductor multilayer (p-type cladding layer) using the striped insulating film as a mask. The p-type nitride semiconductor has a remarkably low carrier activation rate of about several percent, so that it is considered that carriers are trapped by damage due to dry etching, the operating voltage rises, and the characteristics deteriorate.

  In order to solve such a problem, it can be considered that the second etching for etching the semiconductor multilayer is changed from dry etching to wet etching. However, when wet etching is used for the second etching, the selectivity in the vertical and horizontal directions of the etching is lowered, so that the dimensional controllability of the ridge width and depth is deteriorated. Since the width and depth of the ridge are important parameters in terms of device characteristics, the device yield decreases. In addition, nitride semiconductor crystals are difficult to wet etch due to their chemical stability.

  In order to clarify the problem in dry etching, the inventors performed first etching, which forms a mask by etching a silicon oxide film in a stripe shape, by wet etching instead of dry etching. Etching was performed by dry etching to produce a nitride semiconductor optical device. When the operating voltage of such a nitride semiconductor optical device was measured, the operating voltage of the device was reduced. As a result, when the first etching is performed by dry etching, the region where the semiconductor multilayer is damaged by dry etching extends to the upper portion of the ridge, that is, the upper portion of the p-type cladding layer in contact with the upper electrode (ohmic electrode). Obtained knowledge. This is presumed to be the same in the second etching, and even in the second etching, dry etching, the semiconductor multilayer is damaged. If the second etching can be changed to wet etching, the damage is more damaged. Can be said to be less.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a nitride semiconductor device in which damage due to etching is reduced and a method for manufacturing the same.

  (1) In order to solve the above-described problem, a method for manufacturing a nitride semiconductor optical device according to the present invention includes a semiconductor multilayer formation step in which a semiconductor multilayer including an active layer and an upper cladding layer in order is stacked on a substrate. An amorphous layer is formed on the upper surface of the semiconductor multilayer, an amorphous layer forming step, an insulating film is formed on the upper surface of the amorphous layer, and an ridge is formed on the upper surface of the insulating film. A resist film mask forming step of forming a resist film mask having a corresponding shape, and an insulating film mask formation in which a first etching is performed on the insulating film using the resist film mask to form an insulating film mask And using the insulating film mask, a second etching, which is a dry etching, is performed on the amorphous layer and the semiconductor multilayer to form a ridge. And forming step, wherein removing the amorphous layer remaining on the surface of the semiconductor multilayer comprises an amorphous layer removing step.

  (2) In the method for manufacturing a nitride semiconductor optical device according to (1), in the amorphous layer forming step, the amorphous layer may be formed of GaN.

  (3) In the method for manufacturing a nitride semiconductor optical device according to (1), the amorphous layer may be formed of AlN in the amorphous layer forming step.

  (4) The method for manufacturing a nitride semiconductor optical device according to any one of (1) to (3), wherein in the amorphous layer forming step, the amorphous layer is stacked with a layer thickness of 50 nm or more and 100 nm or less. May be.

  (5) In the method for manufacturing a nitride semiconductor optical device according to any one of (1) to (4), the first etching may be dry etching.

  (6) A nitride semiconductor optical device according to the present invention includes a semiconductor multilayer forming step of laminating a semiconductor multilayer including an active layer and an upper cladding layer in this order on a substrate, and an amorphous layer on the upper surface of the semiconductor multilayer. Laminating an amorphous layer forming step; laminating an insulating film on the upper surface of the amorphous layer; forming an insulating film on the upper surface of the amorphous layer; and forming a resist film mask having a shape corresponding to the ridge on the upper surface of the insulating film; Using the resist film mask forming step, and using the resist film mask, the insulating film is first etched to form an insulating film mask, and using the insulating film mask, A ridge forming step of performing a second etching which is a dry etching on the amorphous layer and the semiconductor multilayer to form a ridge, and an upper surface of the semiconductor multilayer Removing the amorphous layer remaining, and the amorphous layer removing step, may be fabricated include.

  According to the present invention, a nitride semiconductor device in which damage caused by etching is reduced and a method for manufacturing the nitride semiconductor device are provided.

1 is a cross-sectional view of a nitride semiconductor optical device according to an embodiment of the present invention. It is sectional drawing which shows the manufacturing process of the nitride semiconductor optical element concerning the 1st Embodiment of this invention. It is sectional drawing which shows the manufacturing process of the nitride semiconductor optical element concerning the 1st Embodiment of this invention. It is sectional drawing which shows the manufacturing process of the nitride semiconductor optical element concerning the 1st Embodiment of this invention. It is sectional drawing which shows the manufacturing process of the nitride semiconductor optical element concerning the 1st Embodiment of this invention. It is sectional drawing which shows the manufacturing process of the nitride semiconductor optical element concerning the 1st Embodiment of this invention. It is sectional drawing which shows the manufacturing process of the nitride semiconductor optical element concerning the 1st Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described specifically and in detail based on the drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments, and the repetitive description thereof will be omitted. In addition, the drawings shown below are merely examples of the embodiment, and the size of the drawings and the scales described in this example do not necessarily match.

  FIG. 1 is a cross-sectional view of a nitride semiconductor optical device 1 according to an embodiment of the present invention. The cross section shown in FIG. 1 is a cross section perpendicular to the light emission direction. The nitride semiconductor optical device 1 according to this embodiment is a nitride semiconductor optical device having a ridge structure, and is, for example, a light emitting device such as a laser device or an LED. As shown in FIG. 1, a semiconductor multilayer is formed on an n-type GaN substrate 10 whose plane orientation is (0001). The semiconductor multilayer consists of an n-type cladding layer 11 (lower cladding layer), an n-type guide layer 12 (lower guide layer), a multiple quantum well active layer 13, and a p-type guide layer in order from the upper surface of the n-type GaN substrate 10. 14 (upper guide layer) and p-type cladding layer 15 (upper cladding layer). A ridge is formed in the p-type cladding layer 15 in a region above the optical waveguide formed in the multiple quantum well active layer 13. Of the semiconductor multilayer, the portion where the ridge is formed is referred to as a ridge portion 15A. On the upper surface of the p-type cladding layer 15, silicon oxide films 19 are formed on both side surfaces of the ridge portion 15A and a region where the ridge portion 15A is not formed. An upper electrode 20 (p-type electrode) is formed so as to cover the p-type cladding layer 15 and the silicon oxide film 19. Here, the silicon oxide film 19 is not formed on the upper surface of the ridge, and the upper surface of the ridge is electrically connected to the upper electrode 20. Although not shown in FIG. 1 for simplicity, a p-type contact layer is actually formed on the upper surface of the ridge, and the upper electrode 20 is connected to the p-type via the p-type contact layer. It is electrically connected to the cladding layer. A lower electrode 21 (n-type electrode) is formed on the back surface (lower surface) of the n-type GaN substrate 10.

Here, the n-type cladding layer 11 is an n-type AlGaN layer having a thickness of 5 μm and doped with Si at a carrier concentration of 5 × 10 ¹⁸ cm ⁻³ . The n-type guide layer 12 is an n-type GaN layer having a thickness of 2 μm and doped with Si at a carrier concentration of 1 × 10 ¹⁹ cm ⁻³ . The multiple quantum well active layer 13 is a multiple quantum well layer in which an undoped In _0.1 Ga _0.9 N well layer having a thickness of 10 nm and an undoped GaN barrier layer having a thickness of 10 nm are alternately stacked. The number of well layers in the multiple quantum well active layer 13 is ten. An undoped layer (i layer) refers to a semiconductor layer that is not intentionally doped with impurities. The p-type guide layer 14 is a p-type GaN layer having a thickness of 2 μm and doped with Mg at a carrier concentration of 1 × 10 ¹⁸ cm ⁻³ . The p-type cladding layer 15 is a p-type AlGaN layer having a thickness of 2 μm and doped with Mg at a carrier concentration of 5 × 10 ¹⁷ cm ⁻³ .

  Hereinafter, a method for manufacturing a nitride semiconductor optical device according to the embodiment will be described with reference to the drawings. 2 to 7 are cross-sectional views showing the manufacturing steps of the nitride semiconductor optical device according to this embodiment.

  First, an n-type GaN substrate 10 is prepared, and a semiconductor multilayer is laminated on its main surface (upper surface). That is, the n-type cladding layer 11, the n-type guide layer 12, the multiple quantum well active layer 13, the p-type guide layer 14, and the p-type cladding layer 15 are grown in order (semiconductor multilayer formation step). A GaN amorphous layer 16 (amorphous layer) having a thickness of 60 nm is continuously grown on the upper surface of the semiconductor multilayer (p-type cladding layer 15) (amorphous layer forming step). FIG. 2 shows a cross section after the process. The semiconductor multilayer and the GaN amorphous layer 16 can be grown by metal organic chemical vapor deposition (MOCVD), and can be easily changed from a crystalline layer to an amorphous layer by controlling the growth temperature. For example, an AlGaN or GaN crystal layer is usually grown at about 1100 ° C., and an InGaN crystal layer is grown at about 800 ° C. because In easily evaporates. On the other hand, the GaN amorphous layer 16 is usually laminated by low-temperature growth at about 300 ° C.

  In addition, although the amorphous layer laminated | stacked on the upper side of a semiconductor multilayer in the said embodiment is formed with GaN, it is not limited to this. AlN has a growth temperature for amorphization higher than that of GaN, and can be laminated by low temperature growth, usually about 500 ° C. From the viewpoint of controlling the growth temperature, it is desirable to form it with AlN. The amorphous layer may be formed of AlGaN that is an intermediate composition of GaN and AlN, or may be another nitride amorphous layer. Furthermore, if it says, as long as it is the layer made amorphous (amorphous), you may form with the other material.

  Next, a silicon oxide film 17 is deposited (deposited) on the upper surface of the GaN amorphous layer 16 by using, for example, chemical vapor deposition (CVD) (insulating film forming step). Here, the silicon oxide film 17 is shown as an example of an insulating film, but is not limited to this, and may be another insulating film. A photoresist film 18 is applied on the upper surface of the silicon oxide film 17, and the photoresist film 18 is formed into a predetermined shape using a known photolithography technique (resist film mask forming step). FIG. 3 shows a cross section after the process. Note that the photoresist film 18 having a predetermined shape is a resist film mask. Here, the predetermined shape corresponds to the shape of the ridge portion 15A formed in the semiconductor multilayer, and the predetermined shape has a predetermined width (first width) on the opposite side from the end face on the emission side. The stripe shape extends to the end face (in a direction perpendicular to the paper surface of FIG. 3). The predetermined width (first width) is the lateral length of the photoresist film 18 shown in FIG. 3 and is substantially equal to the lateral width of the ridge portion 15A shown in FIG. The width of the resist film mask generated by the etching is different from the width of the ridge portion 15A. Therefore, in designing the manufacturing method, the width of the resist film mask may be determined in consideration of the variation in the width in a later process.

  Using the photoresist film 18 having a predetermined shape formed in the resist film mask forming step as a mask, the silicon oxide film 17 is subjected to dry etching (first dry etching) using, for example, a fluorine-based gas, The silicon oxide film 17 is patterned (insulating film mask forming step). That is, the patterned silicon oxide film 17 is an insulating film mask. FIG. 4 shows a cross section after the process. When the silicon oxide film 17 in the region outside the resist film mask is removed by dry etching (first dry etching), the layer disposed below the silicon oxide film 17 is damaged by the dry etching. Of the layer, not only the region where the upper silicon oxide film 17 is removed by dry etching but also the region where the silicon oxide film 17 remains on the upper side (the region below the resist film mask) is damaged. it is conceivable that. In this embodiment, the GaN amorphous layer 16 is disposed below the silicon oxide film 17, and it is considered that the GaN amorphous layer 16 is damaged by dry etching. Note that the shape of the mucosal mask corresponds to the shape of the resist film mask. That is, like the shape of the resist film mask, it has a stripe shape with a predetermined width (second width) extending from the end face on the emission side to the end face on the opposite side. The predetermined width (second width) of the insulating film mask is substantially equal to the predetermined width (first width) of the resist film mask, but the width varies due to dry etching. Note that here, the first etching is dry etching, but the first etching is etching with respect to the insulating film, and the first etching may be wet etching. Although dry etching is desirable from the viewpoint of the accuracy of patterning the insulating film into a predetermined shape in the insulating film mask formation step, damage due to dry etching can be avoided by using wet etching as the first etching.

  In order to use the patterned silicon oxide film 17 as a mask, the photoresist film 18 is removed (resist film removing step). Next, using the silicon oxide film 17 patterned in the insulating film mask forming step as a mask, the GaN amorphous layer 16 and the p-type cladding layer 15 are dry-etched using, for example, a chlorine-based gas (second etching). A ridge portion 15A is formed by performing dry etching (ridge forming step). FIG. 5 shows a cross section after the process. The GaN amorphous layer 16 and the p-type cladding layer 15 outside the insulating film mask are removed by dry etching (second dry etching). As shown in FIG. 5, the dry etching is performed on the upper surface of the semiconductor multilayer. By starting from the middle and stopping in the middle of the p-type cladding layer 15, a ridge portion 15A having a convex cross-sectional shape is formed in the p-type cladding layer 15. The ridge (ridge portion 15A) is also called a low mesa. In the low mesa, the mesa structure is formed in a layer located above the active layer. The active layer is formed so that both sides of the region to be the optical waveguide further spread outward. On the other hand, in the high mesa, the active layer on both sides of the region to be the optical waveguide is removed, and the mesa structure extends to both sides of the active layer.

  The width and depth of the ridge portion 15A are important parameters that affect the phenomenon that current injected into the element spreads on both sides of the optical waveguide when the element is driven, and it is very important to perform dimensional control with high accuracy. . Although wet etching has an advantage of being hardly damaged by etching compared to dry etching, it is difficult to control the dimensions, and thus it is difficult to form the ridge portion 15A by wet etching. Therefore, the second etching is dry etching. Even when the second etching is performed, the layer disposed immediately below the silicon oxide film 17 is more damaged by dry etching, but here the GaN amorphous layer 16 to be removed later is disposed. The damage to the p-type cladding layer 15 can be reduced.

  Next, wet etching is performed using, for example, a hydrofluoric acid chemical solution to remove the silicon oxide film 17 (insulating film mask) remaining above the ridge portion 15A (insulating film mask removing step). Furthermore, wet etching is performed using, for example, a phosphoric acid-containing liquid (other acids may be mixed as appropriate) to remove the GaN amorphous layer 16 remaining on the upper surface of the semiconductor multilayer (p-type cladding layer 15). (Amorphous layer removal step). The wet etching is preferably performed by heating to about 150 ° C. FIG. 6 shows a cross section after the process. In wet etching using a phosphoric acid-containing solution, the GaN amorphous layer 16 and the p-type cladding layer 15 which is a crystal layer have high etching selectivity and hardly etch the p-type cladding layer 15, so that the dimensional controllability of the ridge portion 15A is improved. Has been improved.

  Further, a silicon oxide film 19 is stacked (deposited) on the upper side of the p-type cladding layer 15 by using, for example, a CVD method. Subsequently, a photoresist film (not shown) is formed in a predetermined shape, wet etching is performed using the photoresist film as a mask, and the silicon oxide film 19 formed on the uppermost surface of the ridge portion 15A is removed (semiconductor). Multilayer upper surface insulating film forming step). FIG. 7 shows a cross section after the process. For the etching in this step, for example, a hydrofluoric acid chemical solution is used. By etching, the uppermost surface of the semiconductor multilayer (ridge) is exposed.

  Finally, the upper electrode 20 electrically connected to the p-type cladding layer 15 is formed by depositing a metal film on the upper surface of the n-type GaN substrate 10 and patterning using, for example, a vacuum deposition method. . Further, after polishing the back surface (lower surface) of the n-type GaN substrate 10 and reducing the substrate thickness to about 100 μm, a metal film is deposited on the entire back surface of the n-type GaN substrate 10 to form a lower electrode. (Electrode forming step). Furthermore, by cleaving the substrate, the nitride semiconductor optical device 1 according to this embodiment is manufactured as shown in FIG.

  The nitride semiconductor and the manufacturing method thereof according to the embodiment of the present invention have been described above. The main feature of the present invention is that the manufacturing method includes an amorphous layer forming step of laminating an amorphous layer on the upper surface of the semiconductor multilayer. In the insulating film mask forming step of forming an insulating film mask by performing the first etching on the insulating film formed on the upper surface of the amorphous layer, the amorphous layer is damaged by the first etching. The semiconductor multilayer formed under the amorphous layer is hardly damaged. Then, an amorphous layer is removed by an amorphous layer removing step that removes the amorphous layer remaining on the upper surface of the semiconductor multilayer (ridge), thereby manufacturing a nitride semiconductor optical device in which damage due to etching is reduced. I can do it. Thereby, the operating voltage of the nitride semiconductor optical device can be reduced. In the amorphous layer removing step, for example, by adopting wet etching having high etching selectivity between the amorphous layer and the semiconductor multilayer, such as a phosphoric acid-containing liquid, it is possible to ensure the dimensional controllability of the ridge formed in the semiconductor multilayer. And improvement of the characteristics of the element is realized. As described above, a nitride semiconductor optical device with improved performance and reliability can be manufactured.

  In this embodiment, the film thickness of the GaN amorphous layer 16 is 60 nm, but the present invention is not limited to this. A thicker amorphous layer 16 is desirable from the viewpoint of absorbing damage due to the first etching. However, it is desirable to make the GaN amorphous layer 16 thinner from the viewpoint of removing the GaN amorphous layer 16 in the second etching (mesa forming step) and nitride amorphous removing step. Considering both, it is desirable that the GaN amorphous layer 16 and the other nitride amorphous layers are laminated with a film thickness of 50 nm or more and 100 nm or less.

  Further, the present invention is not limited to the embodiment, and it is needless to say that various changes can be made without departing from the scope of the invention. For example, in the present embodiment, the n-type GaN substrate 10 whose plane orientation is (0001) is used as the substrate, but the present invention is not limited to this, and a nitride semiconductor such as sapphire or SiC (silicon carbide) is used. Other materials may be used as long as they can grow. Also, the plane orientation of the substrate is not limited to (0001), and other plane orientations may be used.

  DESCRIPTION OF SYMBOLS 1 Nitride semiconductor optical device, 10 n-type GaN substrate, 11 n-type clad layer, 12 n-type guide layer, 13 Multiple quantum well active layer, 14 p-type guide layer, 15 p-type clad layer, 15A ridge part, 16 GaN Amorphous layer, 18 photoresist film, 19 silicon oxide film, 20 upper electrode, 21 lower electrode.

Claims (5)

A semiconductor multilayer forming step of laminating a semiconductor multilayer including an active layer and an upper cladding layer in order on the substrate;
An amorphous layer forming step of laminating an amorphous layer on the upper surface of the semiconductor multilayer; and
An insulating film forming step of laminating an insulating film on the upper surface of the amorphous layer;
Forming a resist film mask having a shape corresponding to the ridge on the upper surface of the insulating film;
Using the resist film mask, the insulating film is first etched to form an insulating film mask; and
Using the insulating film mask to form a ridge by performing a second etching that is a dry etching on the amorphous layer and the semiconductor multilayer;
Removing the amorphous layer remaining on the upper surface of the semiconductor multilayer; and an amorphous layer removing step;
A method for producing a nitride semiconductor optical device, comprising: A method for producing a nitride semiconductor optical device according to claim 1,
The method for manufacturing a nitride semiconductor optical device, wherein in the amorphous layer forming step, the amorphous layer is formed of GaN. A method for producing a nitride semiconductor optical device according to claim 1,
The method for manufacturing a nitride semiconductor optical device, wherein in the amorphous layer forming step, the amorphous layer is formed of AlN. A method for manufacturing a nitride semiconductor optical device according to any one of claims 1 to 3,
In the amorphous layer forming step, the amorphous layer is laminated with a layer thickness of not less than 50 nm and not more than 100 nm. A method for manufacturing a nitride semiconductor optical device according to claim 1,
The method of manufacturing a nitride semiconductor optical device, wherein the first etching is dry etching.

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