JP4768452B2 - Optical semiconductor device and method for manufacturing the same, optical disc device, and optical transmission system - Google Patents

Description

  The present invention relates to an optical semiconductor device, in particular, an optical semiconductor device such as a semiconductor laser device in which light propagates in a crystal, and a method for manufacturing the same, and more particularly to formation of a p-side ohmic electrode of such an optical semiconductor device. . Furthermore, the present invention relates to an optical disk device and an optical transmission system provided with such a semiconductor laser device.

  A large number of optical semiconductor devices typified by semiconductor laser devices are used in optical disk devices, optical transmission systems, and the like, and their development is underway energetically.

  Conventionally, as an ohmic electrode of an optical semiconductor device, AuGe / Ni is generally used for an n-type GaAs-based semiconductor material, and AuZn is generally used for a p-type GaAs-based semiconductor material. However, in order to ensure the long-term reliability of the optical semiconductor device, it is important to prevent the diffusion of Au into the semiconductor layer. Therefore, as the p-side electrode, instead of alloy-type AuZn, Ti is used. A method using a non-alloy type electrode such as / Pt / Au has become the mainstream in recent years (see, for example, Patent Document 1: JP-A-7-211982, Patent Document 2: JP-A 2000-114660).

  Ti-based ohmic electrodes are superior in terms of long-term reliability because Ti, which is an electrode material, does not diffuse deeply into the semiconductor layer due to the influence of heat or electricity, and is a ridge waveguide semiconductor laser element In particular, it is frequently used when an electrode is formed near a light waveguide (see Japanese Patent Application Laid-Open No. 2000-114660).

  In addition to the Ti / Pt / Au described above, there are Mo / Au, W, WSi, WN, WAl, and the like as the non-alloy type metal (Patent Document 3: Japanese Patent Publication No. 2906407).

  However, it has been found that an optical semiconductor device having such a non-alloy type p-side ohmic electrode has a problem of a decrease in light propagation efficiency, although long-term reliability is improved. This is due to the fact that non-alloyed metals that have been used so far generally have a refractive index comparable to that of the semiconductor layer. For example, in the non-alloy type metal described above, the refractive index of Ti is 3.03 to 3.6 at a wavelength of 650 nm to 1.5 μm, and the refractive index of Mo is 3.56 at a wavelength of 622 nm to 1.6 μm. The refractive index of W is 2.83 to 3.34 at wavelengths of 800 nm to 1.31 μm. In this way, the metal material formed so as to be in contact with the semiconductor layer has a refractive index of around 3, while the refractive index of the semiconductor material forming the waveguide structure is about 3, so that the refractive index of the both is almost the same. Absent.

Such a non-alloy type metal material having a refractive index relatively similar to that of the semiconductor layer is a distribution of light propagating in the crystal, particularly when it is disposed in the vicinity of a waveguide through which light propagates. In some cases, light leaks in the electrode direction. In general, the light absorption coefficient of a metal material is 10 ⁴ to 10 ⁵ times larger than that of a semiconductor material. Therefore, if light leaks to the metal material forming the electrode, very large light absorption occurs, and the propagation efficiency is remarkably lowered.
JP-A-7-211982 (2nd page) JP 2000-114660 A (page 3, FIG. 1) Patent Publication No. 2906407 (2nd page)

  Therefore, the main problem of the present invention is that a light that can realize a p-side ohmic electrode that has the same contact resistance and long-term reliability as the above-described conventional non-alloyed electrode and that does not deteriorate the propagation efficiency due to light absorption. An object of the present invention is to provide a method for manufacturing a semiconductor device.

  A further object of the present invention is to provide an optical semiconductor device having long-term reliability and good propagation efficiency by using such a manufacturing method, as well as an optical disk device equipped with such an optical semiconductor device, and The object is to provide an optical transmission system.

  As a means for solving the problem of light absorption by the metal material used as the ohmic electrode described above, for light in a wavelength range of 450 nm to more than 1 μm (about 1.55 μm) generally used in an optical semiconductor device. It is conceivable to use Al (aluminum) having a refractive index of about 0.5 to 2 and sufficiently smaller than that of the semiconductor layer. However, details of a method for forming a p-side ohmic electrode made of Al that has the same contact resistance and long-term reliability as a conventional non-alloy ohmic electrode have not been clarified.

  The present invention provides an optical semiconductor device manufacturing method capable of realizing a p-type Al ohmic electrode having good contact resistance and reliability by taking the following measures.

That is, in the method of manufacturing an optical semiconductor device of the present invention, a p-type In _x Ga _1-x As layer (where 0 ≦ x ≦) having a p-type doping concentration of 5 × 10 ¹⁹ cm ⁻³ or more is formed on a substrate. 0.5) as a top layer, forming a semiconductor layer stack, cleaning the surface of the p-type In _x Ga _1-x As layer with at least an acid, and cleaning the p-type After depositing an Al layer , a Pt layer, and an Au layer in this order on the surface of the In _x Ga _1-x As layer, heat treatment is performed at a temperature exceeding 450 ° C. and not more than 550 ° C., and the p-type In _x Ga It is characterized by comprising a step of forming an ohmic electrodes made of a multilayer film Al / Pt / Au on _{1-x as} layer.

  The acid is preferably hydrofluoric acid, particularly buffered hydrofluoric acid, but sulfuric acid, phosphoric acid, and the like can also be used.

In one embodiment, the step of cleaning the surface of the p-type In _x Ga _1-x As layer includes a first cleaning step of cleaning using hydrofluoric acid as the acid for 30 seconds to 5 minutes.

The reason why the p-type doping concentration of the p _- type In _x Ga _1-x As layer is set to 5 × 10 ¹⁹ cm ⁻³ or more is to reduce the contact resistance with the Al ohmic electrode layer in contact with this layer. .

In addition, the heat treatment of the Al layer is performed at a temperature exceeding 450 ° C. and not more than 550 ° C., but at 450 ° C. or less, regardless of the high p-type doping concentration of the p _- type In _x Ga _1-x As layer, This is because the contact resistance between the Al ohmic electrode layer and the p-type In _x Ga _1-x As layer is sufficiently low, specifically, it cannot be reduced to 10 ⁻⁶ Ωcm ² units. This is because the semiconductor layer (that is, the active layer) related to the generation of light is likely to have an adverse effect because it approaches the crystal growth temperature.

As is clear from the above, the manufacturing method of the present invention uses In _x Ga _1-x As (0 ≦ x ≦ 0) doped with p-type doping at 5 × 10 ¹⁹ cm ⁻³ or more as the semiconductor layer forming the ohmic electrode. .5), the surface of the semiconductor layer is cleaned with an acid to clean the surface, and an Al layer , a Pt layer, and an Au layer are deposited in this order, and the temperature exceeds 450 ° C., preferably Is formed by heat treatment at a high temperature in the range of 500 ° C. or higher and 550 ° C. or lower to form an Al ohmic electrode layer, so that a sufficiently good contact resistance (10 ⁻⁶ Ωcm ² units) is obtained. be able to.

In the present specification, the p-type In _x Ga _1-x As layer is also referred to as a “p-type GaAs layer” when the In composition ratio x is 0, and when the In composition ratio x is other than 0, It will also be referred to as a “p-type InGaAs layer”.

  An Al ohmic electrode layer of an optical semiconductor device (for example, a semiconductor laser device) manufactured by this manufacturing method is used for light in a wavelength range of 450 nm to more than 1 μm (about 1.55 μm) generally used in an optical semiconductor device. Therefore, it was confirmed that the contact resistance and long-term reliability equivalent to those of conventional non-alloyed electrodes can be secured. Therefore, the optical semiconductor device manufactured by this method is suppressed in reduction in propagation efficiency due to light absorption, and is excellent in terms of performance and long-term reliability.

  By the way, it is considered that the Al ohmic electrode layer of the optical semiconductor device manufactured by this manufacturing method has a contact resistance equivalent to that of the conventional non-alloy type electrode for the following reason.

In the manufacturing method of the present invention, before the step of depositing Al on the p-type In _x Ga _1-x As layer, the step of cleaning the surface of this layer with at least an acid such as hydrofluoric acid is performed. However, this cleaning process removes an oxide film such as a natural oxide film present on the surface of the p-type In _x Ga _1-x As layer. Then, with the surface oxide film removed, an Al layer is deposited and heat treatment is performed at a temperature within the above range, thereby at least partially at the interface between the In _x Ga _1-x As layer and the Al layer. In addition, a thin alloy layer of In _x Ga _1-x As and Al not containing oxygen atoms can be formed. Even if the alloy layer is not formed, there is no oxide film that increases the resistance value between the In _x Ga _1-x As layer and the Al layer. As a result, it is considered that a low contact resistance is realized.

  In addition, the Al ohmic electrode layer has long-term reliability because the Al layer is heat-treated at a temperature sufficiently higher than the operating temperature of the optical semiconductor device, so that the resulting Al ohmic electrode layer This is because the resistance value does not change over a long period of time and is stable. In addition, Al in the Al ohmic electrode layer does not diffuse deeply into the semiconductor layer due to heat or electric current, and this also provides long-term reliability.

In consideration of the purpose and significance of the cleaning process, the cleaning process is performed immediately before the Al deposition process as much as possible, and a process that promotes surface oxidation of the semiconductor layer such as O ₂ plasma processing is not performed before the Al deposition. This is very important.

  In one embodiment, the hydrofluoric acid used in the first cleaning step is buffered hydrofluoric acid diluted 10 to 100 times with ammonium fluoride or pure water.

  As the hydrofluoric acid used in the first cleaning step, such buffered hydrofluoric acid is used because a semiconductor material containing Al (AlGaAs, AlGaInP, etc.) often used as an optical confinement (cladding) layer in an optical semiconductor device. This is to obtain a sufficient surface cleaning effect while preventing erosion by the etching action during the cleaning step. When the dilution factor is less than 10, the optical confinement (cladding) layer is easily etched. On the other hand, when the dilution ratio exceeds 100 times, the surface cleaning effect becomes insufficient.

The step of cleaning the surface of the p-type In _x Ga _1-x As layer may further include a second cleaning step of cleaning with pure water for at least 5 minutes after cleaning with the hydrofluoric acid.

The second cleaning step is performed in order to sufficiently remove hydrofluoric acid on the surface of the p-type In _x Ga _1-x As layer. After washing with pure water, heat drying processes such as baking should be avoided.

  The combination of the first cleaning step and the second cleaning step realizes an optimal pretreatment for forming an Al ohmic electrode layer without affecting the light confinement layer.

In one embodiment, the optical semiconductor device has a ridge having the p-type In _x Ga _1-x As layer as an uppermost layer, and the step of forming the semiconductor layer stack includes at least the substrate, an n-type cladding layer, an active layer, a p-type cladding layer, and a p-type In _x Ga _1-x As layer having a p-type doping concentration of 5 × 10 ¹⁹ cm ⁻³ or more (where 0 ≦ x ≦ 0 .5) sequentially crystal growth, a region outside the ridge of the p-type In _x Ga _1-x As layer, and a region outside the ridge at least on the In _x Ga _1-x As layer side of the p-type cladding layer And removing the region to form the ridge.

  Here, the “outside ridge region” is a region on the side of the region where the ridge is to be formed in the semiconductor layer stack before the ridge formation, and after the ridge formation, the semiconductor layer It is a region on the side of the ridge in the laminate.

  By using this manufacturing method, the contact resistance and long-term reliability equivalent to those of conventional non-alloy electrodes can be realized, while the influence on the light distribution in the waveguide can be eliminated, and the light propagation efficiency is not lowered. A ridge waveguide type optical semiconductor device having an ohmic electrode on the ridge can be obtained.

  The optical disk device of the present invention is characterized by including a semiconductor laser device manufactured by using any one of the above methods.

  Since this optical disk device has higher light propagation efficiency of the semiconductor laser device serving as the light source than the optical disk device using the above-described conventional semiconductor laser device having a non-alloy type p-side electrode, it has long-term reliability. High-speed writing with lower power consumption is possible while maintaining

  The optical transmission system according to the present invention includes a semiconductor laser device manufactured using any one of the above methods.

  According to the present invention, it is possible to provide an optical transmission module that is more efficient than before, and to improve the performance of the optical transmission system.

Furthermore, an optical semiconductor device according to another aspect of the present invention is an optical semiconductor device having a wavelength of emitted light of 0.6 to 1.55 μm, and is formed on a substrate made of GaAs or InP and the substrate. A semiconductor layer stack including a p-type In _x Ga _1-x As layer (here, 0 ≦ x ≦ 0.5) having a p-type doping concentration of 5 × 10 ¹⁹ cm ⁻³ or more as a top layer, p-type an in _x Ga is formed in _1-x as layer, characterized in that a p-side ohmic electrode made of a multilayer film of Al / Pt / Au to the bottom layer of the a l layer.

As is apparent from the above description related to the manufacturing method of the present invention, this optical semiconductor device has almost no decrease in propagation efficiency due to light absorption by the electrode material, and the p-side ohmic electrode and the p-type In _x. The contact resistance between Ga _1-x As layers is low, and it is excellent in terms of performance and long-term reliability.

When the p-side ohmic electrode is formed of a multilayer film having an Al layer as the lowermost layer, the uppermost layer is preferably an Au layer. However, when a two-layer film of Al and Au is used, a high-resistance layer is often formed by an alloying reaction between Al and Au. Therefore, it is desirable to insert a metal layer having a barrier effect between them. Such metals, other Ti, Pt group element typified by Pt, W and, WN, alloy containing tungsten such as WSi, Mo, by reasons Cr or the like, which will be described later in the present invention Pt Is used .

  On the other hand, when the p-side ohmic electrode is composed of an Al layer alone, the thickness of the Al layer may be at least 50 nm, and preferably 100 nm or more. If the influence of stress is removed, the thickness of the Al layer is preferably as thick as possible, but considering the manufacturing cost and time, it is practical to set the thickness to 500 nm or less.

The optical semiconductor device may be a ridge waveguide semiconductor device. In this case, the p-type In _x Ga _1-x As layer is the uppermost layer of the ridge.

As apparent from the above description, according to the present invention, a conventional non-alloy electrode equivalent contact resistance and long-term reliability can be ensured, further, there is no reduction in the propagation efficiency due to light absorption Al / Pt / Au It is possible to provide a method for manufacturing an optical semiconductor device having an ohmic electrode, and the manufactured optical semiconductor device has almost no decrease in propagation efficiency due to light absorption, and is excellent in terms of performance and long-term reliability. It becomes.

  Further, according to the present invention, it is possible to provide an optical disc apparatus capable of writing at high speed with lower power consumption than a conventional optical disc apparatus.

  Further, according to the present invention, it is possible to provide an optical transmission module that is more efficient than before, and to improve the performance of the optical transmission system.

  Hereinafter, an optical semiconductor device, a manufacturing method thereof, an optical disk device, and an optical transmission system according to the present invention will be described in detail with reference to embodiments shown in the drawings.

  In the following description, “n−” and “p−” represent n-type and p-type, respectively. Further, throughout this specification, “upper” means a direction away from the substrate, and “lower” means a direction closer to the substrate. Crystal growth proceeds from “down” to “up”. In the following description, an example in which the present invention is applied to a ridge waveguide type semiconductor laser device is taken up, but the present invention is also applicable to other optical semiconductor devices.

[First embodiment]
FIG. 1 shows the structure of the semiconductor laser device according to the first embodiment of the present invention.

  This semiconductor laser device is a red or infrared semiconductor laser device having a wavelength of emitted light of 0.6 to 1.55 μm. An n-GaAs buffer layer 102 and an n-AlGaAs lower cladding layer are formed on an n-GaAs substrate 101. 103, an AlGaAs lower guide layer 104, a multi-strain quantum well active layer 105, an AlGaAs upper guide layer 106, a p-AlGaAs first upper cladding layer 107, and a p-InGaAsP semiconductor layer 108 are sequentially stacked.

As an example of the p-AlGaAs second upper cladding layer 109 and the p ⁺ -GaAs contact layer 110 (p-In _x Ga _1-x As layer) so as to form a forward mesa stripe-shaped ridge 130 on the semiconductor layer 108. In this case, x = 0) is provided.

  The Al layer (layer thickness: 50 nm) 111a is the lowest layer on the top and side surfaces of the ridge 130 and the upper portion of the semiconductor layer 108, followed by the Ti layer (layer thickness: 10 nm) 111b and the Au layer (layer thickness: 300 nm). ) 111c, and a p-side electrode 111 made of a multilayer metal thin film formed in this order.

  An n-side electrode 112 made of a multilayer metal thin film of AuGe / Ni / Au is formed on the back surface of the substrate 101 as another electrode layer.

  Next, a method for manufacturing the semiconductor laser device of FIG. 1 will be described with reference to FIGS. The semiconductor laser device is actually manufactured in units of wafers. However, in order to simplify the drawings, FIGS. 2 to 4 are not for the entire wafer but for one chip (chip size: 500 μm × 250 μm). Only shows.

First, as shown in FIG. 2, an n-GaAs buffer layer 102, an n-AlGaAs lower cladding layer 103, an AlGaAs lower guide layer 104, a multi-strain quantum well activity are formed on the (100) plane of an n-GaAs substrate (wafer) 101. Layer 105, AlGaAs upper guide layer 106, p-AlGaAs first upper cladding layer 107, InGaAsP semiconductor layer 108 (layer thickness: 15 nm, Zn doping concentration: 1 × 10 ¹⁷ m ⁻³ ), p-AlGaAs semiconductor layer 109 ′ ( Layer thickness: 1.28 μm, Zn doping concentration: 2.4 × 10 ¹⁸ cm ⁻³ ), p ⁺ -GaAs semiconductor layer 110 ′ (layer thickness: 0.5 μm, Zn doping concentration: 5 × 10 ¹⁹ cm ⁻³ ) Are sequentially grown by MOCVD (metal organic chemical vapor deposition).

  The multiple strain quantum well active layer 105 includes an InGaAs compression strain quantum well layer (strain 0.7%, layer thickness: 4.6 nm, two layers) and an InGaAsP tensile strain barrier layer (strain 0.1%, band gap Eg≈). 1.60 eV, layer thickness from substrate side: 3 layers of 21.5 nm, 7.9 nm and 21.5 nm, the closest to the substrate 101 is the n-side barrier layer, and the farthest is the p-side barrier layer ) Are alternately arranged. Note that the structure of the multi-strain quantum well active layer 105 is not directly related to the essence of the present invention, so the detailed cross-sectional structure of the multi-strain quantum well active layer 105 is not shown.

  Next, as shown in FIG. 2, a resist mask 113 (mask width 3.5 μm) is formed on a region (referred to as “ridge region”) 113a (see FIG. 1) in which a ridge 130 is to be formed in the semiconductor layer 110 ′. It is produced by a photolithography process. The resist mask 113 is formed so as to extend in a stripe shape in the <0-11> direction corresponding to the direction in which the ridge 130 to be formed extends (direction perpendicular to the paper surface).

Next, as shown in FIG. 3, by using this resist mask 113 as a mask, portions of the semiconductor layers 110 ′ and 109 ′ that are outside the ridge 113b (see FIG. 1) located on both sides of the resist mask 113 are etched. By removing the ridge 130, a forward mesa stripe-shaped ridge 130 composed of the p-AlGaAs second upper cladding layer 109 and the p ⁺ -GaAs contact layer 109 is formed. In this way, the semiconductor layer stack 102-110 having the ridge 130 is obtained.

  At the time of this etching, the p-InGaAsP semiconductor layer 108 different from the material of these semiconductor layers is provided below the semiconductor layers 110 ′ and 109 ′, so that a highly accurate ridge can be formed using selective etching. is there. In addition, the p-InGaAsP semiconductor layer 108 does not contain Al and is not easily oxidized, and has a great effect on maintaining long-term current confinement (reliability) of the completed semiconductor laser device, and has a low barrier to holes. There is also an effect that the hole injection efficiency can be improved.

  After the etching is completed, the resist mask 113 is removed. The semiconductor layer 108 in the outer ridge region 113b is exposed by etching.

  Next, the surface of the manufactured ridge 130 and the semiconductor layer 108 is cleaned for 1 minute using a buffered hydrofluoric acid solution diluted 10 times with ammonium fluoride (first cleaning step). Thereafter, rinsing with pure water is performed for 10 minutes (second cleaning step). The first cleaning step removes the natural oxide film present on the surfaces of the ridge 130 and the semiconductor layer 108, and the second cleaning step provides sufficient buffered hydrofluoric acid solution present on the surfaces of the ridge 130 and the semiconductor layer 108. Removed. Since the buffered hydrofluoric acid solution diluted 10 times with ammonium fluoride is used in the first cleaning step, it is possible to prevent the semiconductor material containing Al from being subjected to unnecessary erosion. Although the dilution ratio may be further increased, it should be suppressed to 100 times or less in order to ensure sufficient detergency.

Next, as shown in FIG. 4, using an electron beam vapor deposition method, metal is formed in the order of Al layer 111a (layer thickness: 50 nm) / Ti layer 111b (layer thickness: 10 nm) / Au layer 111c (layer thickness: 300 nm). A thin film is laminated and subsequently heated using an rapid thermal annealing (RTA) apparatus at 500 ° C. for 3 minutes in an N ₂ atmosphere to perform an ohmic treatment of the Al material. Thus, the p-side ohmic electrode 111 is formed. Since the surface oxide film is removed from the surface of the p ⁺ -GaAs contact layer 109 by the cleaning process, the interface 120 between the Al layer 111a and the p ⁺ -GaAs contact layer 110 is not shown in the heat treatment process. An alloy thin film of Al and GaAs is formed. Even if an alloy thin film is not formed, oxygen atoms do not exist at the interface between the Al layer 111a and the p ⁺ -GaAs contact layer 110. Therefore, combined with the high p-type doping concentration of the contact layer 110, the contact resistance is lowered.

  Since the RTA apparatus can precisely control the temperature, the heating time is as short as 1 to 5 minutes, and this apparatus is suitable for mass production. However, instead of the RTA apparatus, a simple heating apparatus in which heater wires are locally arranged around the glass tube may be used. However, in this case, it is necessary to set the heating time to about 10 to 15 minutes, which is longer than that using the RTA apparatus.

  In addition, in the electrode layer deposition process of the p-side ohmic electrode 111, a sputtering method or the like can be applied in addition to the vacuum vapor deposition method such as the electron beam vapor deposition method used in this embodiment. Regardless of which method is used, the process from the deposition of the lowermost Al layer 111a to the deposition of the uppermost Au layer 111c is carried out consistently without being exposed to the air in the middle, whereby the lowermost layer of the electrode is obtained. As a result, the Al layer formed as is not oxidized during the electrode forming step, and the electrode resistance can be reduced.

After the formation of the p-side ohmic electrode 111, as shown in FIG. 1, the substrate 101 is ground from the back surface side to a desired thickness (here, about 100 μm) by a lapping method. Then, by using resistance heating vapor deposition from the back side, AuGe alloy (Au 88% and Ge 12% alloy, layer thickness: 100 nm), Ni (layer thickness: 15 nm), Au (layer thickness: 300 nm) are used as the n-side electrode material. ), And is heated in an N ₂ atmosphere at 390 ° C. for 1 minute using an RTA apparatus to perform an alloy process on the n-side electrode material. Thus, the n-side electrode 112 is formed.

  After the wafer obtained through the above steps is divided into a plurality of bars having a desired resonator length (here, 500 μm), end coating is performed on the bars, and the bars are further formed into chips (500 μm × 250 μm). Divide into The chip after the division is fixed on a stem (not shown) using In glue. Then, an Au wire (not shown) for electrical connection with an external circuit is bonded on the p-side electrode 111. This completes the semiconductor laser device.

  The semiconductor laser device described above is a ridge waveguide semiconductor laser having an Al ohmic electrode layer 111a (as the lowermost layer in this example) immediately above the ridge waveguide. By forming an Al ohmic electrode layer in the vicinity of the optical waveguide instead of the conventional non-alloyed ohmic electrode such as Ti, the following effects can be obtained.

  That is, unlike the conventional p-type ohmic electrode made of the non-alloy type metal material described above, Al which is an ohmic electrode material formed in the vicinity of the waveguide of this embodiment is, for example, the first of AMERICAN INSTITUTE OF PHYSICS HANDBOOK THIRD EDITION. As shown in Chapter 6, pages 124-125 (extracted in the table below), the refractive index of light in the wavelength range of 450 nm to more than 1 μm (about 1.55 μm) generally used in optical semiconductor devices. Is as small as about 0.5 to 2. That is, the Al material has a sufficiently large refractive index difference (Δn ≧ 1) with respect to the semiconductor material. Therefore, by using Al as an electrode material for forming in the vicinity of the optical waveguide, the effect of confining the oscillated laser light in the semiconductor layer can be increased. For this reason, it is possible to prevent light from leaking to the electrode. As a result, the semiconductor laser device manufactured in this embodiment has almost no decrease in propagation efficiency due to light absorption.

table

  In the table above, the refractive index of Al for wavelengths between 0.950 μm and 2.0 μm is not listed, but according to calculations performed by the inventors, 1.31 μm is 1.37, It was 1.48 at .55 μm.

Thus, although it is an Al material effective for maintaining high light propagation efficiency of the optical semiconductor device, this is made into a semiconductor layer made of p-type In _x Ga _1-x As (0 ≦ x ≦ 0.5). On the other hand, in order to use as a good p-side ohmic electrode, there are two major points in the manufacturing process.

First the first point, before the p-type _In x _{Ga 1-x} As depositing the Al on the semiconductor layer, acid the p-type _In x _{Ga 1-x} As semiconductor layer surface, preferably hydrofluoric acid in, It is to carry out a cleaning process. It is important that this step is performed as much as possible immediately before the Al deposition step, and that a step that promotes surface oxidation of the semiconductor layer, such as O ₂ plasma treatment, is performed after the hydrofluoric acid cleaning and before the Al deposition. In an optical semiconductor device, a semiconductor material containing Al (such as AlGaAs or AlGaInP) is often used as an optical confinement (cladding) layer. However, the semiconductor material containing Al is useless by the hydrofluoric acid cleaning step. In order to prevent erosion, the hydrofluoric acid used for cleaning is preferably diluted 10 times or more with ammonium fluoride or pure water. When the dilution factor is less than 10, the optical confinement (cladding) layer is easily etched. Moreover, the dilution rate should not exceed 100 times. If it exceeds 100 times, the surface cleaning effect becomes insufficient. Further, following the hydrofluoric acid cleaning step, it is preferable to perform rinsing with pure water for 5 minutes or more to sufficiently remove hydrofluoric acid from the semiconductor surface. However, after washing with pure water, a drying process such as baking should be avoided.

  In the washing step, hydrofluoric acid, particularly buffered hydrofluoric acid is preferably used as described above, but acids such as sulfuric acid and phosphoric acid can also be used.

  If the semiconductor surface after crystal growth is kept extremely clean and Al can be deposited promptly, it is theoretically possible to omit the cleaning step. However, in order to obtain stable device characteristics with high yield, it is simpler and more reliable to perform the cleaning step.

The second important point is that after the Al deposition process, annealing is performed at a temperature exceeding 450 ° C., preferably 500 ° C. or more and 550 ° C. or less. as a semiconductor layer for forming a p-side ohmic electrode layer, 5 × ¹⁰ 19 ^{cm -3} or more (in this embodiment, 5 × ¹⁰ 19 ^{cm -3)} to the p-type doped _{In x Ga 1-x As (} 0 ≦ x ≦ 0.5), and in addition to the first acid cleaning step, when the RTA apparatus is used, a sufficiently good contact resistance (with a heating time of about 1 to 5 minutes) 10 ⁻⁶ Ωcm ² units) can be obtained. According to the inventor's experimental results, the contact resistance between the Al ohmic electrode layer and the p-type In _x Ga _1-x As layer is 2 × 10 ^{−6 to} 5 × 10 ⁻ if a temperature in the range of 500 to 550 ° C. is used. ⁶ Ωcm ² was achieved.

  The p-type ohmic electrode formed in this way does not change its contact resistance even when a heat treatment at around 400 ° C. is applied as an n-side electrode alloy treatment thereafter, and Ti, which is a typical non-alloy electrode material, is used. And its contact resistance and long-term reliability are equivalent. In addition, as described above, since it has a sufficiently low refractive index, it is possible to avoid reducing the light propagation efficiency of the semiconductor laser device that is the manufactured optical semiconductor device.

  In addition, when other ohmic electrodes (for example, n-side electrodes made of AuGe, AuZn, or the like) that require alloying at a lower temperature than the Al ohmic electrode are used at the same time as in this embodiment, heat treatment is included. After performing the Al ohmic electrode forming step, it is preferable to perform another ohmic electrode forming step. By performing electrode formation in this order, each of the Al ohmic electrode and the ohmic electrode such as AuGe or AuZn can have a good contact resistance.

  The present invention is particularly effective for an optical semiconductor device having a relatively small ohmic junction area in which an ohmic junction is formed only in a narrow region immediately above the ridge structure. For example, in this embodiment, the chip size is 500 μm × 250 μm, but in the case of a conventional buried regrowth type optical semiconductor element, the entire surface of the above 500 μm × 250 μm can be an ohmic junction region. In such a ridge waveguide type optical semiconductor element, the ohmic junction region is reduced to 500 μm × several μm.

  That is, even if the contact resistance between the metal and semiconductor layers for the same area is the same, if the ohmic junction area is reduced, the electrode resistance is deteriorated by the reduction. For example, in the case described above, the degree of deterioration is proportional to the decrease in the chip width and is several tens of times or more. In other words, the contact resistance improvement effect becomes more prominent as the ohmic junction area decreases. Therefore, the manufacturing method of the present invention capable of forming an electrode with improved contact resistance is particularly effective for such an optical semiconductor element having a relatively small ohmic junction area.

In addition, when a current is passed between the p-side electrode 111 and the n-side electrode 112 of the semiconductor laser device described above, a Schottky junction is formed between the semiconductor layer 108 on the side of the ridge 130 and the p-side electrode 111. Is interrupted, and a current flows only through an ohmic junction between the p ⁺ -GaAs contact layer 110 and the p-side electrode 111 provided on the uppermost portion of the ridge 130. Thereby, current confinement is realized. As described above, since the crystal growth process in the manufacturing stage can be completed only once, it can be formed at low cost.

In this embodiment, the SCH (having a guide layer between the active layer and the clad layer)
Although the semiconductor laser device having the structure (Separate Confinement Heterostructure) has been described as an example, of course, the present invention is not limited to the semiconductor laser device described above, and any optical semiconductor device in which an ohmic electrode is formed in the vicinity of the optical waveguide. Of course it can be applied. Needless to say, each layer thickness, material change, and the like can be added without departing from the spirit of the present invention, such as adding an intermediate layer for smooth crystal growth.

For example, in this embodiment, the contact layer 110 is made of p ⁺ -GaAs containing no In component, but may be p-In _x Ga _1-x As (0 <x ≦ 0.5). When p-In _x Ga _1-x As (0 <x ≦ 0.5) is used as the contact layer 110, the problem of crystallinity degradation due to lattice mismatch with GaAs serving as a substrate is suppressed. Therefore, a graded layer p-In _{0 → x} Ga _{1 → (1-x)} As (0 ≦ x ≦ 0.5) between In and Ga between the p-AlGaAs semiconductor layer 109 and the contact layer 110 is used. It is preferable to provide a composition transition layer whose composition is gradually changed.

  In this embodiment, the p-side electrode 111 is composed of the Al layer 111a (layer thickness: 50 nm) / Ti layer 111b (layer thickness: 10 nm) / Au layer 111c (layer thickness: 300 nm), but is an intermediate layer. Instead of the Ti layer, a Pt group element typified by Pt, an alloy containing tungsten such as W, WN, WSi, Mo, Cr, or the like may be used. In any case, it is possible to prevent an increase in resistance due to the formation of the AlAu alloy layer that can occur when the intermediate layer is not formed. In particular, when Pt is used as the intermediate layer, the diffusion of Au can be satisfactorily prevented. Moreover, since the Pt layer and the Au layer are very difficult to oxidize, the oxidation of the Al layer as the lowermost layer of the p-side electrode can be prevented well.

  Further, the p-side electrode 111 may be composed of an Al layer having a layer thickness of 50 nm or more, more preferably 100 nm to 500 nm.

  In this embodiment, the p-type upper cladding layer is formed of the p-type first cladding layer 107 and the p-type second cladding layer 109, and the p-type second cladding layer 109 is used as the lower layer of the ridge 130. The upper mold cladding layer may be composed of one layer, and the lower layer of the ridge 130 may be formed on the contact layer 110 side portion of the p-type upper cladding layer.

The substrate 101 may use InP instead of GaAs. In this case, as the contact layer 110, it is preferable to use p-InGaAs, particularly In _0.53 Ga _0.47 As, which lattice matches with InP serving as a substrate.

[Second Embodiment]
FIG. 5 shows an example of the structure of the optical disc apparatus 200 according to the present invention. This is for writing data on the optical disc 201 or reproducing the written data, and the wavelength manufactured using the configuration of the first embodiment described above as the light emitting element used at that time. A semiconductor laser device 202 in which the composition and layer thickness of the active layer are adjusted so as to oscillate in the 780 nm band is provided.

  This optical disk device will be described in more detail. At the time of writing, the signal light L emitted from the semiconductor laser device 202 is converted into parallel light by the collimator lens 203, passes through the beam splitter 204, and the polarization state is adjusted by the λ / 4 polarizing plate 205, and then the objective lens 206. Is condensed and irradiated onto the optical disc 201. At the time of reading, the optical disc 201 is irradiated with a laser beam not carrying a data signal along the same path as at the time of writing. This laser beam is reflected on the surface of the optical disc 201 on which data is recorded, passes through the laser beam irradiation objective lens 206, the λ / 4 polarizing plate 205, and then is reflected by the beam splitter 204 to change the angle by 90 °. The light is condensed by the light receiving element objective lens 207 and is incident on the signal detecting light receiving element 208. The recorded data signal is converted into an electric signal by the intensity of the laser beam incident in the signal detection light-receiving element 208, and is reproduced by the signal light reproducing circuit 209 to the original signal.

  In the optical disk device of the second embodiment, since the semiconductor laser device 202 that has high light propagation efficiency and can be manufactured at low cost is used, high-speed writing can be performed and power consumption can be greatly reduced. It was. Therefore, an optical disc apparatus with less environmental load can be provided at low cost.

  Here, an example in which the semiconductor laser element 202 oscillating at a wavelength of 780 nm is applied to a recording / reproducing optical disc apparatus has been described. However, other wavelength bands (for example, 650 nm band) manufactured by applying the manufacturing method of the first embodiment are described. Needless to say, the present invention can also be applied to an optical disk device provided with the semiconductor laser device (1).

[Third embodiment]
FIG. 6 is a cross-sectional view showing an optical transmission module 300 used in the optical transmission system according to the third embodiment of the present invention. FIG. 7 is a perspective view showing a light source portion, and FIG. 8 is a schematic view of an optical transmission system. In the third embodiment, an InGaAs-based semiconductor laser element (laser chip) 301 having an oscillation wavelength of 890 nm manufactured using the method for manufacturing an optical semiconductor device described in the first embodiment as a light source, and silicon ( Si) pin photodiodes are used. In the present embodiment, by providing the same optical transmission module 300 on both sides (for example, a terminal and a server) that perform communication, an optical transmission system that transmits and receives an optical signal between both optical transmission modules 300 is configured. The

  In FIG. 6, a pattern of both positive and negative electrodes for driving a semiconductor laser is formed on a circuit board 306. As shown in the drawing, a recess 306a having a depth of 300 μm is provided in a portion on which a laser chip is mounted. A laser mount (mounting material) 310 on which the laser chip 301 is mounted is fixed to the recess 306a with solder. The flat portion 313 (see FIG. 7) of the positive electrode 312 of the laser mount 310 is electrically connected to a laser driving positive electrode portion (not shown) on the circuit board 306 by a wire 307a. The recess 306a has a depth that does not hinder the emission of laser light, and the roughness of the surface does not affect the emission angle.

  The light receiving element 302 is also mounted on the circuit board 306, and an electric signal is taken out by the wire 307b. In addition, an IC circuit 308 for laser driving / reception signal processing is mounted on the circuit board 306.

  Next, an appropriate amount of a liquid silicon resin 309 is dropped on a portion where the laser mount 310 is mounted. In the silicon resin 309, a filler that diffuses light is mixed. The silicon resin 309 stays in the recess 306a due to surface tension, covers the laser mount 310, and is fixed to the recess 306a. In the third embodiment, the recess 306a is provided on the circuit board 306 and the laser mount 310 is mounted. However, as described above, the silicon resin 309 remains on the laser chip surface and its vicinity due to surface tension. 306a is not necessarily provided.

  Thereafter, it is heated at 80 ° C. for about 5 minutes to be cured until it forms a jelly. Next, it is covered with a transparent epoxy resin mold 303. Above the laser chip 301, a lens part 304 for controlling the radiation angle is formed, and above the light receiving element 302, a lens part 305 for condensing the signal light is integrally formed as a molded lens. .

  Next, the laser mount 310 will be described with reference to FIG. As shown in FIG. 7, a laser element 301 is die-bonded to an L-shaped heat sink 311 using In glue. The laser chip 301 is the InGaAs-based semiconductor laser element described in the first embodiment, and the chip lower surface 301b is coated with a high reflection film, while the laser chip upper surface 301a is coated with a low reflection film. ing. These reflective films also serve as protection for the end face of the laser chip.

  A positive electrode 312 is fixed to the base 311 b of the heat sink 311 with an insulator so as not to be electrically connected to the heat sink 311. The positive electrode 312 and the electrode region 301c provided on the Schottky junction on the surface of the laser chip 301 are connected by a gold wire 307c. As described above, the laser mount 310 is fixed to the negative electrode (not shown) of the circuit board 306 in FIG. 6 by soldering, and the flat part 313 on the upper side of the positive electrode 312 and the positive electrode part (see FIG. (Not shown) with a wire 307a. By forming such wiring, the optical transmission module 300 that can obtain the laser beam 314 by oscillation is completed.

  As described above, the optical transmission module 300 of the third embodiment uses a semiconductor laser device that is highly efficient, has low element resistance, and can be manufactured at low cost. Compared to the conventional case, it can be significantly reduced, and the module unit price can be reduced. Since the optical transmission system using the optical transmission module 300 operates with low power consumption, the load on the environment can be reduced, and the optical transmission system can be configured at a low price. Further, when this optical transmission system is mounted on a portable device, the battery driving time can be made longer than before, and the portable device can be used more comfortably.

  As described above, by providing the same optical transmission module 300 on both sides that perform communication, an optical transmission system that transmits and receives optical signals between both optical transmission modules 300 is configured. FIG. 8 shows a configuration example of an optical transmission system using the optical transmission module 300. This optical transmission system includes the optical transmission module 300 in a base station 315 installed on the ceiling of a room, and the same optical transmission module 300 ′ as above (indicated with “for distinction”) in a personal computer 316. I have. The optical signal emitted from the light source of the optical transmission module 300 ′ on the personal computer 316 side is received by the light receiving element of the optical transmission module 300 on the base station 315 side. The optical signal emitted from the light source of the optical transmission module 300 on the base station 315 side is received by the light receiving element of the optical transmission module 300 on the personal computer 315 side. In this way, data communication using light (infrared rays) can be realized.

  Note that the optical semiconductor device manufacturing method, optical disk device, and optical transmission system of the present invention are not limited to the illustrated examples described above. For example, the detailed configuration of the optical semiconductor device (such as the layer thickness and the number of layers of the well layer / barrier layer) can of course be variously changed without departing from the scope of the present invention.

It is a cross-sectional schematic diagram of the semiconductor laser apparatus produced using the manufacturing method of the optical semiconductor device of 1st embodiment of this invention. It is a schematic diagram for demonstrating the manufacturing process of the optical semiconductor device of this invention, and represents the state which provided the photomask for ridge formation after a crystal growth process. It is a schematic diagram for demonstrating the manufacturing process of the optical semiconductor device of this invention, and represents the state after the etching process for ridge formation. It is a schematic diagram for demonstrating the manufacturing process of the optical semiconductor device of this invention, and represents the state after the vapor deposition process of a p side electrode. It is the schematic of the optical disk apparatus of 2nd embodiment of this invention. It is the schematic of the optical transmission module used for the optical transmission system of 3rd embodiment of this invention. It is a perspective view of the light source concerning the optical transmission system of 3rd embodiment of this invention. It is a perspective view which shows the structural example of the optical transmission system of 3rd embodiment of this invention.

101 n-GaAs substrate 102 n-GaAs buffer layer 103 n-AlGaAs lower cladding layer 104 AlGaAs lower guide layer 105 multiple strain quantum well active layer 106 AlGaAs upper guide layer 107 p-AlGaAs first upper cladding layer 108 p-InGaAsP semiconductor layer 109 p-AlGaAs second upper cladding layer 110 p ⁺ -GaAs contact layer 111 p-side electrode 111a Al layer 111b Ti layer 111c Au layer 112 n-side electrode 113 resist mask 113a ridge formation region 113b ridge formation outside region 120 Al layer and contact Interface 130 with layer Ridge 200 Optical disc device 201 Optical disc 202 Semiconductor laser device 203 Collimator lens 204 Beam splitter 205 λ / 4 polarizing plates 206 and 207 Objective lens 208 Light receiving element 09 Signal light reproduction circuit 300 Optical transmission module 301 Laser chip 301a Low reflection film 301b High reflection film 301c Schottky bonded electrode region 302 Light receiving element 303 Epoxy resin mold 304, 305 Lens part 306 Circuit board 306a Recessed parts 307a, 307b, 307c Wire 308 IC circuit 309 Silicon resin 310 Laser mount 311 Heat sink 311b Base 312 Positive electrode 313 Flat part 314 Laser beam 315 Base station 316 Personal computer

Claims (10)

A semiconductor layer stack including a p-type In _x Ga _1-x As layer (here, 0 ≦ x ≦ 0.5) having a p-type doping concentration of 5 × 10 ¹⁹ cm ⁻³ or more as a top layer on a substrate. Forming a step;
Cleaning the surface of the p-type In _x Ga _1-x As layer with at least an acid;
After depositing an Al layer , a Pt layer, and an Au layer in this order on the surface of the cleaned p-type In _x Ga _1-x As layer, heat treatment is performed at a temperature exceeding 450 ° C. and not more than 550 ° C., method of manufacturing an optical semiconductor device characterized by comprising the step of forming the ohmic electrodes made of a multilayer film of the p-type in _x Ga _1-x as layer in Al / Pt / Au. In the manufacturing method of the optical semiconductor device according to claim 1,
The step of cleaning the surface of the p-type In _x Ga _1-x As layer includes a _first cleaning step of cleaning with hydrofluoric acid as the acid for 30 seconds to 5 minutes. Production method. In the manufacturing method of the optical semiconductor device according to claim 2,
The step of cleaning the surface of the p-type In _x Ga _1-x As layer further includes a second cleaning step of cleaning with pure water for at least 5 minutes after cleaning with the hydrofluoric acid. Manufacturing method of optical semiconductor device. In the manufacturing method of the optical semiconductor device according to claim 2,
The method of manufacturing an optical semiconductor device, wherein the hydrofluoric acid is buffered hydrofluoric acid diluted 10 to 100 times with ammonium fluoride or pure water. In the manufacturing method of the optical semiconductor device according to any one of claims 1 to 4,
The optical semiconductor device has a ridge having the p-type In _x Ga _1-x As layer as an uppermost layer,
The step of forming the semiconductor layer stack includes
On the substrate, at least an n-type cladding layer, an active layer, a p-type cladding layer, and a p-type In _x Ga _1-x As layer having a p-type doping concentration of 5 × 10 ¹⁹ cm ⁻³ or more (here And 0 ≦ x ≦ 0.5) sequentially crystal growth,
The p-type In _x Ga _1-x As layer outer ridge region and the p-type cladding layer at least the In _x Ga _1-x As layer side ridge outer region are removed to form the ridge. A method of manufacturing an optical semiconductor device.   6. An optical disk device comprising a semiconductor laser device manufactured by using the manufacturing method according to claim 1.   An optical transmission system comprising an optical transmission module having a semiconductor laser device manufactured by using the manufacturing method according to claim 1. An optical semiconductor device having a wavelength of emitted light of 0.6 to 1.55 μm,
A substrate made of GaAs or InP;
A semiconductor formed on the substrate and including a p-type In _x Ga _1-x As layer (where 0 ≦ x ≦ 0.5) having a p-type doping concentration of 5 × 10 ¹⁹ cm ⁻³ or more as a top layer. A layer stack;
The p-type an In _x Ga is formed on the _1-x As layer, the optical semiconductor device being characterized in that a p-side ohmic electrode made of a multilayer film of Al / Pt / Au to the bottom layer of the A l layer . The optical semiconductor device according to claim 8,
An alloy layer of In _x Ga _1-x As (0 ≦ x ≦ 0.5) and Al is formed between the p-type In _x Ga _1-x As layer and the Al layer. An optical semiconductor device. The optical semiconductor device according to claim 8 or 9,
The optical semiconductor device is a ridge waveguide semiconductor device, wherein the p-type In _x Ga _1-x As layer is the uppermost layer of the ridge.

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