Patent Application
20110057220
This application claims priority to Japanese Patent Application No. 2009-207764 filed on Sep. 9, 2009, the disclosure of which including the specification, the drawings, and the claims is hereby incorporated by reference in its entirety.
The present disclosure relates to nitride semiconductor light-emitting devices, and more particularly to nitride semiconductor light-emitting devices having facet protection films.
Nowadays, various semiconductor laser diodes are broadly used as light sources for optical disk devices. Among others, a blue-violet semiconductor laser diode using a III-V nitride semiconductor, such as gallium nitride (GaN), emits light in a short-wavelength region (400 nm band), which enables a smaller size of light collection spot on an optical disk as compared with light in a red region and in an infra-red region, as a light source for the next-generation high-density optical disk (Blu-ray Disc (registered trademark)). This is advantageous for improvement in playback operation and record density of optical disks, and thus blue-violet semiconductor laser diodes are becoming commonplace as well as indispensable.
In order to achieve high density and high-speed write operation, a novel disk utilizing a blue-violet semiconductor laser diode requires a high-power blue-violet semiconductor laser diode with high reliability. In an aluminum gallium arsenide (AlGaAs)-based semiconductor laser diode or an aluminum gallium indium phosphide (AlGaInP)-based semiconductor laser diode used in a conventional CD (compact disc) and DVD (digital versatile disc), dielectric films made of oxide are formed over cavity facets as protection films in order to prevent degradation and optical damage of the cavity facets.
However, in case of a GaN-based blue-violet laser diode, if a facet protection film made of oxide is formed over a cavity facet, then the cavity facet is oxidized by oxygen in the facet protection film as well as oxygen in the air. This causes facet degradation of a semiconductor laser diode.
Thus, an attempt to shield the cavity facets from oxygen to reduce degradation thereof caused by oxidation of the facets by providing a layer made of aluminum nitride (AlN) over a facet protection film is described, for example, in Japanese Patent Publication No. 2007-103814 etc.
In addition, for example, Japanese Patent Publication No. 2008-186837 etc. describe an attempt to prevent increase of drive current (Top) of a laser diode by exploiting a fact that if a facet protection film is formed of a first film made of aluminum nitride (AlN) and a second film made of aluminum oxide (Al2O3), and if the film thickness “d” of the second film satisfies R(d, n)>R(d, n+1) and d>λ/n (where R is the reflectance, n is the refractive index, and λ is the light wavelength), then the refractive index of the second film increases and the reflectance thereof decreases due to change in properties of the second film during laser operation.
However, high-power operation with an optical output of 160 mW or higher, which is applied to, for example, Blu-ray Disc on which dual layer recording can be performed at 4× or faster, cannot be stably achieved only with the configuration described above.
Generally, facet degradation causes an increase of operating current and a decrease of COD (catastrophic optical damage) optical output which results in COD, thereby hinders high-power operation. For this reason, it is known that continued high-power operation will cause operating current to be gradually increased, or to be rapidly increased to stop the light emission. Thus, the present inventors have conducted various studies, focusing on oxidation of a boundary between a cavity facet and an aluminum nitride (AlN) film and light absorption by the aluminum nitride film. As a result, it has been found that forming a facet protection film by a thin aluminum nitride film having an orientation where the crystal axis is different by 90° from the cavity facet is effective in reducing facet degradation because light absorption by the facet protection film can be reduced and oxidation toward the cavity facet can be reduced.
In addition, in case of a configuration in which an aluminum oxide (Al2O3) film is provided over a cavity facet interposing an aluminum nitride film therebetween, it has been found that it is also required to take into consideration an effect of oxygen permeability of an aluminum oxide film and stress thereof on a boundary between a cavity facet and an aluminum nitride film. That is, from a viewpoint of stress, aluminum oxide, which has relatively small stress, is suitable for a facet-coating film. However, since aluminum oxide has a low crystallization temperature to crystallize from an amorphous state, a change in properties readily occurs on boundaries with, and on surfaces of, semiconductor crystals (a region with changed properties is referred to hereinafter as “crystallized region”) if high-energy blue-violet laser light is emitted for a long period of time. As such, in a semiconductor laser diode, continued high-power operation causes the properties of an aluminum oxide film, which serves as a facet-coating film, to change from the surface, and thus causes the stress and the refractive index thereof to change gradually.
For example, aluminum oxide has a refractive index of 1.66 in an amorphous state, and even though a specific refractive index of a crystallized region is unknown, sapphire, which is a crystallized material, for example, has a refractive index of 1.76. This provides an estimation of a change within a range of approximately 0.1 or less.
In view of the foregoing, it is an object of the present disclosure to provide a facet protection film which reduces an increase of current associated with a decrease of COD optical output over time due to facet degradation during operation and can withstand high-power laser operation over a long period of time.
In order to achieve the above object, a nitride semiconductor light-emitting device includes a laminate structure formed of a plurality of nitride semiconductor layers including a light-emitting layer, and having cavity facets facing each other, a first protection film made of aluminum nitride, formed over a light-emitting facet of the cavity facets, and a second protection film made of aluminum oxide having a refractive index of n1, formed over the first protection film, where the second protection film has a crystallized surface at least in a region facing a light-emitting region on the cavity facets; the thickness (t) of the second protection film satisfies λ/(2·n1)<t<3λ/(4·n1) (where λ is a wavelength of the output light); and a second reflectance R(n2) (where n2 is a refractive index of the aluminum oxide which has been crystallized) of the light-emitting region in the cavity facets is lower than a first reflectance R(n1) of a region surrounding the light-emitting region in the cavity facets.
According to a nitride semiconductor light-emitting device of the present invention, since the first protection film is made of aluminum nitride, the cavity facet is shielded from oxygen. This hinders oxygen permeation during laser operation, and allows reduction of facet degradation. In addition, the second protection film is made of aluminum oxide, the film thickness satisfies λ(2·n1)<t<3λ(4·n1), and the facet reflectances satisfy R(n2)<R(n1). Thus, it is possible to prevent an increase of operating current due to crystallization of the second protection film. This can reduce facet degradation due to increase of operating current during laser operation, and this can also reduce facet degradation associated with increase in stress.
In the nitride semiconductor light-emitting device according to the present invention, the aluminum nitride forming the first protection film may have an orientation where the crystal axis is different by 90° from the optical cavity facets.
With this configuration, the first protection film has a dense film characteristic, thereby further hinders oxygen permeation during laser operation, and thus allows facet degradation in the cavity facet to be further reduced. Accordingly, high-power operation of 160 mW or higher can be stably achieved.
In the nitride semiconductor light-emitting device according to the present invention, the first protection film may have a film thickness of 4 nm or more and 20 nm or less.
This configuration can prevent variation in oxygen permeability of the cavity facet due to variation in the film thickness of the aluminum nitride film. In addition, since the film thickness of the aluminum nitride film of 20 nm or less can reduce the laser light absorption according to the extinction coefficient of the aluminum nitride film, facet degradation during laser operation can be reduced. Moreover, the film thickness of the aluminum nitride film of 4 nm or more can reduce crystallization on the side of the first protection film in the second protection film.
In the nitride semiconductor light-emitting device according to the present invention, the extinction coefficient of the first protection film may be 0.005 or less over a range of oscillation wavelength of the light emitted from the light-emitting layer.
This configuration can reduce light absorption by the first protection film facing the light-emitting region, and thus can further reduce facet degradation in the cavity facet.
In the nitride semiconductor light-emitting device according to the present invention, a reflectance of a facet-coating film formed of the first protection film and the second protection film may be 8% or more and 13% or less.
This configuration can reduce degradation of noise characteristics. In addition, the reflectance of the facet-coating film of 13% or less allows the optical density of an end portion of the light-emitting region to be set to a small value.
The nitride semiconductor light-emitting device according to the present invention may further include a third protection film formed over a light-reflecting facet opposite the light-emitting facet of the cavity facets, and having the same configuration as that of the first protection film, and a fourth protection film formed over the third protection film, and having the same configuration as that of the second protection film.
This configuration can reduce facet degradation due to oxidation of the light-reflecting facet.
As described above, according to a nitride semiconductor light-emitting device of the present invention, since increase of operating current due to crystallization of the protection films made of aluminum oxide can be reduced, and oxidation and stress at the boundary between the cavity facet and the first protection film can be reduced, a facet protection film which can withstand high-power operation for a long period of time can be achieved. As a result, high-power operation of 160 mW or higher, applicable to, for example, Blu-ray Disc on which dual layer recording can be performed at 4× or faster, can be performed.
An example embodiment will be described below with reference to the drawings.
As shown in
In the laminate structure 40, a part of the p-type cladding layer 16 and the p-type contact layer 17 are processed into a ridge-stripe geometry extending along the longitudinal direction of a cavity. The width of the ridge-stripe portion is, for example, about 1.4 μm. The cavity length is, for example, 800 μm. The chip width is, for example, 200 μm.
A p-side contact electrode 19 made of palladium (Pd)/platinum (Pt) is formed over the top surface of the ridge-stripe portion so as to contact the p-type contact layer 17. A dielectric film 18 is formed over the exposed portion of the p-type cladding layer 16 other than the top surface of the ridge-stripe portion. A p-side interconnect electrode 20 made of titanium (Ti)/platinum (Pt)/gold (Au) is formed over the p-side contact electrode 19 in the ridge-stripe portion and the dielectric film 18. In addition, an n-side contact electrode 21 made of Ti/Pt/Au is formed over a surface (rear surface) on the opposite side of the substrate 10 from the laminate structure 40.
A low-reflection front-facet-coating film (not shown) formed of aluminum nitride (AlN) and aluminum oxide (Al2O3) films is formed over an emitting facet, which is the front facet of the cavity. In addition, a high-reflection rear-facet-coating film (not shown) formed of an aluminum oxide (Al2O3) film and six cycles of silicon dioxide (SiO2) and zirconium dioxide (ZrO2) films stacked thereover is formed over a reflecting facet, which is the rear facet of the cavity.
A method for fabricating the nitride semiconductor light-emitting device configured as described above will be described below.
First, the laminate structure 40 constituting the nitride semiconductor light-emitting device is formed on the principal surface of the n-type substrate 10 by crystal growth using a metal-organic chemical vapor deposition (MOCVD) technique.
Next, for example, a mask film made of SiO2 etc. is formed over the top surface of the laminate structure 40 for use as a mask for forming a ridge-stripe structure using a plasma-enhanced chemical vapor deposition (CVD) technique etc., and the portion of the mask film other than the ridge-stripe portion is etched by lithography and an etching technique using hydrofluoric acid (HF) etc. The ridge-stripe portion is formed by performing etching down to the inside of the p-type cladding layer 16 by, for example, a dry etching apparatus having an ISM (inductively super magnetron) etc. using the mask film remaining on the ridge-stripe portion. Thereafter, the mask film is removed, and then the dielectric film 18 made of, for example, SiO2 etc. is formed over the entire surface of the laminate structure 40. After this, only the ridge-stripe portion is opened by lithography, and the dielectric film 18 is etched using hydrofluoric acid (HF) etc. This forms the dielectric film 18 as a current-blocking layer. Following this, a metal film made of Pd/Pt, which is to be the p-side contact electrode 19, is formed by a vacuum evaporation technique, and then the p-side contact electrode 19 is formed by removing the metal film other than the portion over the ridge-stripe portion using a lift-off technique. Then, the p-side interconnect electrode 20 is formed over the dielectric film 18 including the p-side contact electrode 19 by a vacuum evaporation technique.
Next, after the rear surface of the n-type substrate 10 is ground to a thickness of about 80 μm, the n-side contact electrode 21 is formed over the ground rear surface by a vacuum evaporation technique. The laminate structure 40 as shown in
After this, a primary cleavage is performed along the (101-0) plane, which is the plane orientation of the n-type substrate 10, by scribing equipment and breaking equipment, etc. Note that, for purposes herein, a negative sign “−” added to a Miller index for a plane orientation expediently represents an inversion of the Miller index integer immediately before the negative sign. In this embodiment, as described previously, a cavity having a length of 800 μm is formed. In this process, the cleavage is performed while a plurality of light-emitting devices in the form of a wafer are protected by placing the plurality of light-emitting devices in the form of a wafer before the cleavage on an adhesive sheet and a protection sheet is also applied by adhesion on the top surfaces of the plurality of light-emitting devices. Accordingly, when the primary cleavage is performed, the cavity facets of the light-emitting devices are exposed to an atmosphere interposed between the adhesive sheet and the protection sheet, and after the primary cleavage is performed, the cavity facets of the light-emitting devices may contact the adhesive sheet etc. Thus, component materials included in the adhesive sheet etc. may adhere to the cavity facets. Among the component materials included in the adhesive sheet etc. is a siloxane-based component, and siloxane contains silicon (Si). Therefore, adhesion of silicon to the cavity facets may have a significant effect on long-term reliability of a GaN-based nitride light-emitting device. Accordingly, in this embodiment, the primary cleavage is performed using an adhesive sheet and a protection sheet which do not contain silicon.
Next, a facet-coating process in which a protection film made of a dielectric film is formed over each cavity facet of a laser bar including the plurality of light-emitting devices obtained by the primary cleavage using an electron cyclotron resonance (ECR) system will be described.
An inlet window 106 is provided on the opposite side of the plasma chamber 104 from the film deposition chamber 105, and microwave introduced through a microwave inlet 103 is introduced into the plasma chamber 104 through the inlet window 106. ECR plasma is generated in the plasma chamber 104 by the introduced microwave and the magnetic field generated by the magnetic coils 112.
The film deposition chamber 105 is vacuum evacuated through an evacuation outlet 102, while argon (Ar) gas, oxygen (O2) gas, and nitrogen (N2) gas are introduced into the film deposition chamber 105 through a gas inlet 101. A stage 111 is disposed at a location facing the plasma chamber 104 in the film deposition chamber 105. A laser bar cleaved by the primary cleavage is introduced into the film deposition chamber 105 and held on the stage 111 so that the cavity facets are exposed to ECR plasma. The inner surface of the plasma chamber 104 is covered by members made of quartz in order to protect the plasma chamber 104 from ECR plasma. That is, an end plate 107, inner tubes 108, and window plates 109 on the top and at the bottom of the respective inner tubes 108 are respectively provided as the quartz members covering the inner surface of the plasma chamber 104.
It is preferable that a cleaning process of the cavity facets be performed before forming facet-coating films, by performing plasma cleaning using Ar gas. In the cleaning process, if only plasma exposure is performed without applying a bias voltage to the target materials 110 in the ECR system, the target materials 110 are not sputtered. That is, the cleaning process can be performed by generating plasma with no bias applied. Note that the cleaning process may be performed using a mixture gas of Ar gas and N2 gas instead of using only Ar gas.
First, the protection film (front-facet-coating film) on the emitting-facet side will be described using
Next, after the first protection film 31 has been formed, Ar gas and O2 gas are introduced to the film deposition chamber 105 where the first protection film 31 has been formed, and an aluminum oxide (Al2O3) film is deposited as the second protection film 32 in a similar manner. Note that in this embodiment, the material of the target materials 110 for forming the AlN and the Al2O3 films respectively as the first and the second protection films 31 and 32 is aluminum (Al), and thus the films are deposited in an Al target chamber.
When the AlN film, which is the first protection film 31, is formed, the deposition rate can be reduced by deposition at high partial pressure of nitrogen (N2) gas as shown in
Furthermore, since the deposition condition is such that the N2 partial pressure is high, that is, the Ar partial pressure is low, plasma damage by the Ar gas in the plasma chamber 104 is reduced, thereby reducing wear caused by etching the quartz members (the end plate 107, the inner tubes 108, and the window plates 109). If the Ar partial pressure is high, this wear occurs when each quartz member is exposed to plasma and the etching process progresses because the mass of Ar is relatively large. As such, since the Ar partial pressure can be set low in this embodiment, the attachment of silicon (Si) and oxygen (O), which are main components of each quartz member, to the boundary between the cavity facet 30 and the first protection film 31 can be reduced. This allows the light absorption of laser light due to a formation of silicon oxides (SiOx) at the boundary etc. to be reduced, thereby allows induction of heat generation and degradation of the cavity facet 30 to be reduced.
In this regard, it is preferable that the film thickness of the first protection film 31 made of AlN be set to 4 nm or more and 20 nm or less. The film thickness of the AlN film of 4 nm or more can reduce variation in oxygen permeability of the cavity facet 30 due to variation in the film thickness of the AlN film, and more particularly, can reduce variation in the COD level. In addition, the film thickness of the AlN film of 20 nm or less can reduce the laser light absorption according to the extinction coefficient of the AlN film, and thus can reduce the decrease of the COD level during laser operation as compared to when the film thickness is large. In this embodiment, the film thickness of the first protection film 31 is 10 nm.
It is preferable that the film thickness of the second protection film 32 made of aluminum oxide (Al2O3) be set so that the reflectance at the emitting facet (front facet) is 8% or more and 13% or less. A lower reflectance causes optical feedback noise to occur, which will raise a major issue when this light-emitting device is used in an optical pickup. Although it may not apply depending on the design of a specific optical pickup, it is preferable that the reflectance of the front facet be 8% or more because, from a viewpoint of optical feedback noise, the relative intensity noise (RIN) needs to be −125 dB/Hz or less as shown in
Here, since blue-violet laser light has high optical energy and the Al2O3 forming the second protection film 32 has a low crystallization temperature, a problem exists in that a change in properties occurs due to crystallization of Al2O3 particularly at the boundary with the air on or near the outermost surface of the second protection film 32, which is considered to have a high energy level, by prolonged laser-light irradiation. Even though the change in properties due to crystallization of the second protection film 32 is notable especially within a range of oscillation wavelength of blue-violet laser light, there is no limitation on the oscillation wavelength of the light-emitting device because the change in properties occurs independently of the oscillation wavelength. Note that in this embodiment, the oscillation wavelength of the light-emitting device is 405 nm.
Furthermore,
As shown in
In this embodiment, under a condition of high light-output in which the temperature is 70° C. and the light output is 160 mW of a continuously energized light-emission, change in the differential efficiency (Se) has more effect on the drive current (lop) than change in the threshold current value (Ith). This causes an operating-current change rate (ΔIop) before and after the continuously energized light-emission to increase in a region where the reflectance increases, and to decrease in a region where the reflectance decreases.
In the regions 1-5 of respective film thickness ranges, since crystallization of Al2O3 progresses gradually and the facet reflectance changes monotonously, the film thickness of the Al2O3 film needs to be set to a value within a region where the reflectance decreases in order to reduce the change in the operating-current change rate (ΔIop) during a continuously energized light-emission. Accordingly, in this embodiment, the film thickness is 145 nm which belongs to the region 3 as shown in
In
As shown in
The cause of the rapid degradations occurred in the regions 1, 2, 4, and 5 shown in
First, the issue on oxygen diffusion will be described.
Oxygen diffusion is significantly reduced by using a c-axis oriented AlN film as the first protection film 31. However, since oxygen diffusion proceeds from the air through the second and the first protection films 32 and 31 to the cavity facet 30, each film thickness of the first and the second protection films 31 and 32 also has significant effect on oxygen diffusion. In the regions 1 and 2, the film thickness of the second protection film 32 is insufficient to prevent oxygen diffusion, and this is considered to cause the rapid degradation to occur on and near the cavity facet 30.
Next, the issue on stress will be described.
Since the AlN forming the first protection film 31 is a nitride, the AlN film has very high stress (tensile stress). It is known that only with the AlN film, this stress causes rapid degradation to occur on the cavity facet 30 in a short period of time. Therefore, it is indispensable to provide the second protection film 32. Here, in discussing the stress, if, for example, SiO2 etc. having similar tensile stress to AlN is used, then rapid degradation still occurs on the cavity facet 30 because stress on the cavity facet 30 further increases. Meanwhile, as in the regions 4 and 5, if the second protection film 32 made of Al2O3 having compressive stress is deposited to a relatively large thickness, compressive stress becomes high. It is thought that this produces stress on the cavity facet 30, which causes the rapid degradation to occur as shown in
For these reasons, such a phenomenon is presumably caused by crystallization of the second protection film 32 made of Al2O3 during operation, during which oxygen diffusion is facilitated and stress is locally applied to the light-emitting region of the laminate structure 40. Thus, from a viewpoint of oxygen diffusion and stress, it can be understood that the film thickness of the second protection film 32 made of Al2O3 needs a restriction to ensure long-term reliability. Meanwhile, in the region 3 shown in
Note that as can be seen from
Furthermore, as shown in
As can be seen from
As can be seen from
It is preferable that the extinction coefficient of the first protection film 31 made of AlN be 0.005 or less. Setting the extinction coefficient of the first protection film 31 to 0.005 or less allows light absorption in the light-permeated portion in the first protection film 31 to be reduced. As a result, as shown in
Next, the rear-facet-coating film (reflective film) on the reflecting-facet side will be described.
Similarly to the emitting-facet side, a cleaning process is performed before forming the reflective film provided on the reflecting facet of the cavity. Then, Ar gas and O2 gas are introduced to the film deposition chamber 105 of the ECR system shown in
In addition, as shown in
Thus, in the reflective film provided on the reflecting facet also, use of an AlN film as the third protection film 31A allows crystallization on the cavity-facet side of the Al2O3 film, which is the fourth protection film 32A, to be reduced. Furthermore, since oxygen permeation to the cavity facet can be reduced, degradation of the cavity facet occurring on the reflecting-facet side during laser operation can be prevented. In addition, providing the terminating film 34 made of SiO2 on the outermost surface of the reflective film prevents crystallization at or near the atmospheric boundary which would be caused by the terminating film made of ZrO2 having a low crystallization temperature as shown in Table 1 below, thereby allowing the light-emitting device to achieve a stable laser operation.
After this, a laser bar including a plurality of light-emitting devices receives a secondary cleavage into dice, and thus laser chips are obtained.
Next, a mounting process will be described. Each of the laser chips described below is firmly fixed on a submount made of, for example, aluminum nitride (AlN) or silicon carbide (SiC), on which solder material is arranged, and then is mounted on a stem. Then, wires made of gold (Au) for current supply are respectively connected to the p-side interconnect electrode 20 and to an interconnect electrode of the submount connected to the n-side contact electrode 21. Following this, a cap having a laser-light exit window is attached by fusion in order to isolate the laser chip from the ambient air, thus a nitride semiconductor light-emitting device is obtained.
An operation at room temperature of a nitride semiconductor light-emitting device achieved by this embodiment has shown that the threshold current value is 30 mA, the slope efficiency is 1.5 W/A, the oscillation wavelength is 405 nm, and a continuous oscillation occurs (or continuous wave (CW) is generated). In addition, a reliability test is performed with CW driving under a condition of high temperature and high power (70° C., 160 mW), and it has been shown that stable operation is possible over 1000 hours or longer.
An oscillation wavelength within a range from 395 nm to 420 nm is more preferable for a nitride semiconductor light-emitting device according to this embodiment.
In this way, in a semiconductor light-emitting device according to this embodiment, the protection film provided on each cavity facet has a stacked configuration of the first protection film 31 made of AlN and the second protection film 32 made of Al2O3; the film thickness “t” of the Al2O3 film, which is the second protection film 32, is set to satisfy λ/(2·n1)<t<3λ/(4·n1); and the facet reflectances on the emitting facet is set to satisfy Rf(n2)<Rf(n1) (where n1 is the initial refractive index, and n2 is the refractive index after crystallization). This significantly reduces oxidation and stress occurring at the boundary between the corresponding cavity facet and the first protection film 31 made of AlN, thereby can reduce facet degradation during laser operation. Thus, since reliability and durability of a light-emitting device can be greatly improved, high-power operation of 160 mW or higher, applicable to, for example, Blu-ray Disc on which dual layer recording can be performed at 4× or faster, can be performed.
As has been described, since the nitride semiconductor light-emitting device according to this embodiment can reduce oxidation and stress at the boundary between the cavity facet and the first protection film, and can reduce facet degradation, a facet protection film which can withstand a prolonged high-power laser operation can be achieved, and thus the present invention is useful for nitride semiconductor light-emitting devices such as light sources for optical pickups etc.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-207764 | Sep 2009 | JP | national |
