Patent Application
20130308667
This is a continuation of International Application No. PCT/JP2011/003840 filed on Jul. 5, 2011, which claims priority to Japanese Patent Application No. 2011-014032 filed on Jan. 26, 2011. The entire disclosures of these applications are incorporated by reference herein.
The present disclosure relates to semiconductor light emitting devices and the light emitting apparatuses, and more particularly to semiconductor light emitting devices using nitride semiconductors and light emitting apparatuses.
Semiconductor light emitting devices emit light with high light output and with high directivity, and therefore, such devices have been actively developed as light sources of image display apparatuses such as laser displays and projectors, or light sources of industry processing equipment such as laser welding devices. In the field of image display apparatuses, laser devices and superluminescent diode (SLD) devices have been particularly developed, and in the field of industry processing equipment, laser devices and laser array devices have been particularly developed.
An indium aluminium gallium phosphide (InAlGaP) based material is used for semiconductor light emitting devices in which the emission wavelength ranges from red region (about 650 nm) to infrared region (about 1000 nm), and an indium aluminium gallium nitride (InAlGaN) based material is used for semiconductor light emitting devices in which the emission wavelength ranges from ultraviolet region (about 350 nm) to green region (about 530 nm).
In particular, it has been considered that the market for the semiconductor light emitting devices using the InAlGaP-based material would grow in the future as light sources for displays, and such devices have been actively developed. The semiconductor light emitting devices for use in such applications have been required to output light exceeding 1 W. In order to achieve semiconductor light emitting devices outputting light exceeding 1 W, it is necessary to improve the operating efficiency and the heat dissipation of the semiconductor light emitting devices. In order to reduce an increase in fabrication cost, the semiconductor light emitting devices have been also required to have a structure that can be easily manufactured.
A ridge structure in which stripe-shaped raised portions (ridges) are formed by removing a part of a cladding layer has been known as a structure that allows a semiconductor light emitting device such as a laser device to operate with high efficiency. In the ridge structure, in order to confine current and light, an insulating film whose refractive index is smaller than that of the cladding layer is formed so as to cover the cladding layer except the upper surface of the ridge. The insulating film covers the cladding layer, thereby making it possible not only to efficiently confine light to an optical waveguide, but also to confine current injected into an active layer. As a result, the luminous efficiency of the semiconductor light emitting device is improved to achieve the high-efficient operation of the semiconductor light emitting device.
In order to improve the heat dissipation of a semiconductor light emitting device, the semiconductor light emitting device has been generally connected to a heat sink. It has become the mainstream to form, on a substrate made of gallium nitride (GaN), the semiconductor light emitting device in which an InAlGaN-based material is used and an emission wavelength ranges from ultraviolet region to blue region. Since the thermal conductivity of the GaN substrate is 130 W/(m·K) which is higher than that of a GaAs substrate, heat dissipation via the substrate can be expected. Therefore, a junction-up mounting in which the GaN substrate and the heat sink are connected together has been generally used. In the junction-up mounting, heat generated in the active layer is mainly dissipated via the substrate located directly under the active layer. However, part of the heat generated in the active layer is conducted to a ridge side. The heat having been conducted to the ridge is conducted to an electrode formed on the ridge. Part of the heat having been conducted to the electrode is dissipated to the air, and part of the heat is conducted to a cladding layer. However, the efficiency of heat dissipation to the air is low. An insulating film is generally formed around the ridge, as described above. Silicon dioxide (SiO2) that is a representative example of the insulating film has a low thermal conductivity of approximately 1.3 W/(m·K). Therefore, the heat conduction efficiency from the electrode to the cladding layer is low, and the heat is stored in the electrode. In a high-power semiconductor light emitting device outputting light of more than 1 Watt, electric power of several watts are supplied to a region of several hundred micrometers square, and therefore, improvement of the heat dissipation from a side closer to the ridge is an important problem to be solved.
In order to heat dissipation from the ridge, it has been considered to provide an uneven portion in a cladding layer except the ridge (for example, see Japanese Patent Publication No. 2006-173265). The uneven portion is provided to increase a surface area of the cladding layer, and therefore, improvement of the heat dissipation is expected.
It has also been considered to directly connect an electrode to a side surface of a ridge with no insulating film interposed therebetween (for example, see Japanese Patent Publication No. H03-156988). A contact electrode forming an ohmic contact with the ridge is formed on the upper surface of the ridge, and an interconnect electrode made of a metal forming a Schottky junction with the cladding layer is formed on the entire surface of the cladding layer including the upper surface of the contact electrode and the side surface of the ridge. The interconnect electrode has a thermal conductivity far larger than that of the insulating film, and therefore, the improvement of the heat dissipation from a side closer to the ridge can be expected.
However, the above conventional structures have the following problems. First, if an uneven portion is provided in the cladding layer, the heat dissipation cannot be significantly improved since the insulating film is formed between the electrode and the cladding layer. The fabrication process is complicated, causing an increase in fabrication cost.
When an interconnect electrode is directly formed on the cladding layer, the heat dissipation from the ridge can be improved. However, the interconnect electrode contacting the side surface of the ridge allows light absorption to occur, resulting in the reduction of the luminous efficiency. Furthermore, the ridge generally has a side surface that is almost perpendicular to the wafer surface, and a rising edge of the ridge is a large step. If the interconnect electrode is formed in the step, disconnection of the interconnect electrode is likely to occur. If the disconnection of interconnect electrode occurs, electric power is nonuniformly injected, and heat generation and light absorption increase, leading to a cause of deterioration of the device characteristics.
It is a problem of the present disclosure to solve the above problems, and to provide a semiconductor light emitting device which significantly improves the heat dissipation without reducing the luminous efficiency.
A semiconductor light emitting device of the present disclosure has a structure having: an insulating film covering a portion of a second cladding layer that includes a side surface of a ridge and exposing a portion of the second cladding layer; and a second electrode contacting the upper surface of a contact electrode, the upper surface of the insulating film, and the exposed portion of the second cladding layer from the insulating film.
Specifically, a first example semiconductor light emitting device includes: a nitride semiconductor layer including a first cladding layer, a light emitting layer, and a second cladding layer which are sequentially provided on a substrate, the second cladding layer including at least one ridge; an insulating film provided on a portion of the second cladding layer including the at least one ridge; a first electrode provided on the at least one ridge; and a second electrode provided to contact the first electrode, the insulating film, and a portion of the second cladding layer exposed from the insulating film, wherein the insulating film is provided on a side surface of the at least one ridge and a region adjoining the at least one ridge.
The first example semiconductor light emitting device includes the insulating film provided on a portion of the second cladding layer including the at least one ridge, and the second electrode formed to contact the first electrode, the insulating film, and a portion of the second cladding layer exposed from the insulating film. Therefore, in a portion in which the second electrode and the second cladding layer are directly connected together, heat can be efficiently conducted between the second electrode and the second cladding layer. Accordingly, when a junction-down mounting is performed, of the heat generated in the light emitting layer, the heat having been conducted to the side of the ridge can be efficiently conducted to a heat sink connected to a portion closer to the substrate, and the efficiency of heat dissipation can be improved. When a junction-up mounting is performed, the heat can be efficiently conducted to a heat sink connected to a portion closer to the electrode, and the efficiency of heat dissipation can be improved. The insulating film covers a region including a side surface of the ridge, and therefore, the light absorption around the ridge by the second electrode can be reduced, and the luminous efficiency can be improved.
In the first example semiconductor light emitting device, a width of a portion of the insulating film provided on the region adjoining the at least one ridge may be 1 μm or more and 10 μm or less, and may be preferably 1 μm or more and 2 μm or less.
In the first example semiconductor light emitting device, a nitrogen density of a portion of the second cladding layer around an interface with the second electrode may be smaller than a nitrogen density of an inside portion of the second cladding layer.
In the first example semiconductor light emitting device, the second electrode may be made of a material having a work function smaller than that of the first electrode.
In the first example semiconductor light emitting device, the second cladding layer may have a recessed/raised structure provided in a portion which is not covered with the insulating film.
In this case, the recessed/raised structure may be provided in a stripe pattern extending in parallel to the at least one ridge, and may be provided in a grid pattern.
In the first example semiconductor light emitting device, the at least one ridge includes a plurality of ridges, and the second cladding layer may have the plurality of ridges.
In the first example semiconductor light emitting device, the second cladding layer may include a plurality of layers having different compositions from each other or different materials from each other.
A second example semiconductor light emitting device includes: a nitride semiconductor layer including a first cladding layer, a light emitting layer, and a second cladding layer which are sequentially provided on a substrate; at least one ridge provided on the second cladding layer and including a first electrode made of a material which is transparent for an emission wavelength of light; an insulating film provided on a portion of the second cladding layer; and a second electrode provided to contact the at least one ridge, the insulating film, and a portion of the second cladding layer exposed from the insulating film, wherein the insulating film is provided on a side surface of the at least one ridge and a region adjoining the at least one ridge.
An example semiconductor light emitting apparatus includes: a heat sink; and the semiconductor light emitting device of the present disclosure mounted on the heat sink. The substrate of the semiconductor light emitting device may face the heat sink or the second cladding layer of the semiconductor light emitting device may face the heat sink.
The semiconductor light emitting device of the present disclosure can significantly improve heat dissipation without reducing luminous efficiency.
As illustrated in
The substrate 101 may be, e.g., n-type hexagonal GaN substrate having a (0001) principal surface. The nitride semiconductor layer 103 includes an n-type cladding layer 131, an n-type optical guide layer 132, a barrier layer (not shown), the active layer 133, a p-type optical guide layer 134, a carrier overflow suppression layer (OFS layer: not shown), a p-type cladding layer 135, and a p-type contact layer 136 which are sequentially formed from the substrate 101. The n-type cladding layer 131 may be, e.g., an n-type aluminum gallium nitride (AlGaN) layer having a thickness of 2 μm, and the n-type optical guide layer 132 may be, e.g., an n-type gallium nitride (GaN) layer having a thickness of 0.1 μm. The barrier layer may be, e.g., an indium gallium nitride (InGaN) layer. The active layer 133 may be, e.g., a quantum well active layer made of InGaN. The quantum well active layer may be formed of, e.g., a triple quantum well structure. The p-type optical guide layer 134 may be, e.g., a p-type GaN layer having a thickness of 0.1 μm. The OFS layer may be, e.g., an AlGaN layer having a thickness of 10 nm. The p-type cladding layer 135 may be, e.g., a strained superlattice layer having a thickness of 0.48 μm, and formed of a lamination of 160 cycles of p-type AlGaN layers each having a thickness of 1.5 nm and GaN layers each having a thickness of 1.5 nm. The p-type contact layer 136 may be, e.g., a p-type GaN layer having a thickness of 0.05 μm.
The nitride semiconductor layer 103 may be formed by, for example, a metal-organic chemical vapor deposition (MOCVD) technique. The ridge 103A may be formed by selectively removing the p-type contact layer 136 and the p-type cladding layer 135 after forming the nitride semiconductor layer 103, as illustrated in
A stripe width W0 of the ridge 103A will be now described. In general, in a ridge-type semiconductor laser device, light in a stripe width direction (transverse direction) is confined by a difference in effective refractive index between the ridge and the vicinity thereof. If the stripe width W0 is smaller than about 2 μm, a semiconductor laser device generally operates in a single-mode in which a horizontal transverse mode that is a transverse electric field distribution with respect to an active layer has a single-peak. If the stripe width W0 is about 2 μm or more, the semiconductor laser device operates in a multi-mode in which plural modes coexist. The stripe width W0 is extended to allow the semiconductor laser device to operate in the multi-mode, whereby the light density, the threshold carrier density, and the heat generation density in a light-emitting end facet can be reduced, and the output power can be increased, compared with the device in the single-mode. However, if the stripe width W0 is made too wide, the area of the ridge is increased, and as a result, an injected current is increased, heat is generated in the semiconductor laser device, and the semiconductor laser device cannot provide laser oscillation. The present inventors have studied the stripe width W0, and found that, if the stripe width W0 is 20 μm or less, the semiconductor laser device can provide laser oscillation. They also found that, if the stripe width W0 is 6-8 μm, the light output is maximum. Therefore, the stripe width W0 of the ridge 103A is preferably about 2 μm or more and about 20 μm or less, and more preferably about 6-8 μm. The condition in which the semiconductor device has the stripe width W0 of less than about 2 μm, and operates in the single-mode may be also utilized.
The insulating film 105 is formed on the surface of the ridge 103A and a region (of p-type cladding layer 135) adjoining the ridge 103A. The insulating film 105 serves a current confining layer confining current injected into the active layer 133. It also confines light to the optical waveguide. The insulating film 105 may be, e.g., a SiO2 film having a thickness of about 300 nm. The insulating film 105 may be formed as follows, for example. As illustrated in
The p-side electrode 107 includes a first electrode 171 serving as a contact electrode contacting the p-type contact layer 136, a second electrode 173 serving as an interconnect electrode, and a third electrode 175 serving as a pad electrode. The first electrode 171 forms an ohmic contact with the p-type contact layer 136, and may be a laminated film comprising palladium having a thickness of 50 nm and platinum having a thickness of 50 nm. The second electrode 173 contacts the first electrode 171, the insulating film 105, and a portion of the p-type cladding layer 135 that is not covered with the insulating film 105, and a width of the second electrode 173 in a direction intersecting the ridge 103A may be 150 μm, and a length of the second electrode 173 in a direction parallel to the ridge 103A may be 500 μm. The second electrode 173 may be a laminated film comprising, e.g., titanium having a thickness of 50 nm, platinum having a thickness of 200 nm, and gold having a thickness of 100 nm. The third electrode 175 may be gold having a thickness of 20 μm. The first electrode 171 and the second electrode 173 may be formed by, e.g., electron beam deposition and lift-off, and the third electrode 175 may be formed by, e.g., electroplating. In order to allow the first electrode 171 to form an ohmic contact, the first electrode 171 may be sintered at the temperature of, e.g., 400° C.
The n-side electrode 109 may be a laminated film comprising, e.g., titanium having a thickness of 5 nm, platinum having a thickness of 10 nm, gold having a thickness of 100 nm. The back surface of the substrate 101 may be polished with, e.g., diamond slurry, to reduce the thickness of the substrate 101 to about 80 μm, and then, the n-side electrode 109 may be formed by, e.g., electron beam deposition.
After such a structure is formed on the wafer, the wafer is cleaved into chips such that a width of each of the chips in a direction intersecting the ridge 103A is, e.g., 200 μm and a length of each of the chips in a direction parallel to the ridge 103A is, e.g., 800 μm to form the semiconductor light emitting device 100.
Next, a semiconductor light emitting apparatus on which the semiconductor light emitting device 100 of the embodiment is mounted will be described.
The efficiency of heat dissipation of the semiconductor light emitting device of the embodiment can be improved for the following reasons. As illustrated in
In contrast, as illustrated in
The p-side electrode 107 may have a large thickness to sufficiently obtain the advantage by the heat dissipation paths N2 and N3. In the embodiment, since the third electrode 175 has a thickness of about 20 μm, the efficiency of the heat dissipation path N2 and N3 can be improved.
The luminous efficiency in the semiconductor light emitting device 100 of the embodiment can be sufficiently secured for the following reasons.
In view of the above, in order that light absorption can be ignored while the heat dissipation is secured, the width W1 of the region covered with insulating film 105 may be in the range of about 1-10 μm, and more preferably in the range of about 1-2 μm. In the embodiment, if the width W1 is 1 μm, the light absorption can be almost ignored, and the heat dissipation can be maximized.
Next, the semiconductor light emitting device 100 of the embodiment can improve the disconnection of the electrode for the following reasons.
In contrast, in the semiconductor light emitting device 100 of the embodiment, the insulating film 105 is provided on a portion of the upper surface of the p-type cladding layer 135 which includes the side surface of the ridge 103A and a region adjoining the side surface of the ridge 103A, as illustrated in
The semiconductor light emitting device 100 of the embodiment includes a portion in which the second electrode 173 and the p-type cladding layer 135 are directly connected together. Therefore, leakage current may flow from the second electrode 173 into the p-type cladding layer 135. However, the work function of p-type GaN is higher than that of a metal, generally used as an electrode material, such as nickel, palladium, titanium, gold, platinum, copper, aluminum, tantalum, tungsten, and chromium, and therefore, it is difficult to provide an ohmic contact between the p-side electrode and the p-type cladding layer. Therefore, a heavily doped contact layer made of p-type GaN is formed to have a thin thickness, and is sintered to alloy the p-side electrode. Such a structure can allow the carrier to tunnel a potential barrier to reduce the resistive connection of the p-side electrode. In the semiconductor light emitting device 100 of the embodiment, no heavily doped layer exists at a connection interface between the second electrode 173 and the p-type cladding layer 135. Therefore, the second electrode 173 and the p-type cladding layer 135 forms a Schottky junction, and current hardly flows between the second electrode 173 and the p-type cladding layer 135. If only the first electrode 171 is alloyed, it may be sintered after the formation of the first electrode 171 and before the formation of the second electrode 173.
In general, the bonding of nitrogen to other atoms is weaker than that of gallium, and therefore, nitrogen is likely to be removed from a semiconductor crystal. If the ridge of the p-type cladding layer 135 is formed by dry etching, the surface of the p-type cladding layer 135 is modified due to the dry etching, and a N hole from which nitrogen is removed. As a result, the etched surface is a low-nitrogen layer from which many nitrogen atoms are removed and which has a nitrogen density lower than that of the non-etched portion. The nitrogen density of a portion of the p-type cladding layer 135 around a connection interface with the second electrode 173 is smaller than that of the inside portion of the p-type cladding layer 135. If the N hole serves a donor, and the concentration of the N hole is substantially the same as the acceptor concentration of the p-type cladding layer 135, an inactive layer is formed. The inactive layer serves as a high-resistance layer, and current leakage can be reduced. If the concentration of the N hole is higher than the acceptor concentration of the p-type cladding layer 135, an n-type conversion layer is formed. If the n-type conversion layer is formed between the second electrode 173 and the p-type cladding layer 135, it serves as an npn junction and can prevent the occurrence of current leakage. In the embodiment, the p-type cladding layer 135 is dry etched, thereby simultaneously forming the ridge and the N hole, and heat dissipation can be improved without an increase in fabrication cost.
The modification of the surface increases with increasing the etching power. However, too large power causes a problem where controllability of the ridge shape and the etching depth is deteriorated. The present inventors found that the power that can both cause modification of the surface and maintain the controllability is about 100-200 W. If the p-type cladding layer 135 is heavily etched and the etched surface reaches the n-type cladding layer, current leakage occurs. As a result of the study of the present inventors, if the thickness of the p-type cladding layer 135 is 10 nm or more after etching, leakage current can be sufficiently controlled.
In order to further cause modification of the surface, for example, ion irradiation, plasma treatment, electron beam irradiation, and wet etching by phosphoric acid solution and alkaline solution may be performed after dry etching. A metal electrode that reacts to N may be in contact with the surface. For example, a metal electrode such as titanium (Ti) or vanadium (V) is in contact with the p-type cladding layer 135, whereby such a metal material is bonded to N on the surface to form, e.g., TiN or VN, and therefore, the N hole can be formed on the surface of the p-type cladding layer 135.
In order to form a high-resistance layer, for example, ion implantation or annealing may be performed. An ion such as iron, zinc, or boron is implanted into the p-type cladding layer 135, whereby an inactive region can be formed to increase the resistance of the surface of the p-type cladding layer 135. The modified surface of the p-type cladding layer 135 is likely to be oxidized, and therefore, the surface is annealed in an oxygen atmosphere, whereby the surface can be oxidized. The present inventors found that the annealing at a temperature in the range of 400-1000° C. can form a high-resistance layer that reduces current leakage. The oxidation reaction proceeds according to the crystal defect density. Therefore, after the process of damaging the surface is performed as described above, the surface is oxidized, thereby facilitating the oxidation reaction. In the embodiment, after the first electrode 171 is formed, sintering is performed in an oxygen atmosphere, thereby making it possible to form a contact junction in the first electrode 17 while oxidizing the etched surface. Such a process can reduce leakage current without increasing a fabrication cost.
In order to reduce leakage current between the second electrode 173 and the p-type cladding layer 135, it is preferable to use, as the second electrode, an electrode material having a work function that is as small as possible. In order to supply current to only the ridge 103A, it is preferable to use, as the first electrode 171, a material having a work function larger than that of the second electrode 173. For example, the combination of palladium and titanium, the combination of nickel and titanium, the combination of nickel and chromium, and the combination of nickel and aluminum are preferably used as the combination of the first electrode 171 and the second electrode 173. Each of the first electrode and the second electrode may be a laminated film formed by a plural materials or may be an alloy. The combination of the plural materials can improve adherence, and reduce deterioration of the electrode characteristics due to, e.g., oxidation.
The recessed/raised portions 103B may has any structure as long as a large contact area between the nitride semiconductor layer 103 and the second electrode 173 is secured. For example, stripe-shaped raised portions and stripe-shaped recessed portions extending in parallel to the ridge 103A may be alternately formed, as illustrated in
Although the recessed/raised portions 103B may be formed by any methods, the formation process can be simplified if the recessed/raised portions 103B and the ridge 103A are formed by the same process. For example, as illustrated in
The recessed/raised portions 103B may be arranged in a grid pattern, as illustrated in
If the light output does not have to be adjusted in each of the ridges 103A, each of the second electrode 173 and the third electrode 175 may be continuously formed in all the ridges 103A. In such a structure, the heat generated in a portion of the active layer 133 located directly under each of the ridges 103A can be efficiently dispersed into the semiconductor light emitting device 100B, and the semiconductor light emitting device 100B is allowed to have a uniform temperature. Therefore, a uniform emission strength of light emitted from each of the ridge 103A can be also obtained.
In the first and second modifications, the second electrode 173 and the p-type contact layer 136 are in contact with each other on the raised portions. However, if the second electrode 173 is formed after the first electrode 171 and the p-type contact layer 136 are sintered, an ohmic contact is not formed between the second electrode 173 and the p-type contact layer 136, and current hardly flows from the second electrode 173 to the p-type contact layer 136.
The embodiment and the previous modifications illustrate the example in which one p-type cladding layer is formed. The p-type cladding layer may have a laminated structure made of a plurality of layers having different compositions from each other and different materials from each other.
When the second layer 135B is formed to include p-type impurities at a low concentration, current leakage from the interconnect electrode 173 can be further reduced.
In the present modification, the recessed/raised portions may be provided as well as the first modification. Also, a plurality of ridges may be provided as well as the second modification.
The embodiment and the previous modifications illustrate the example in which the ridge is integrally formed with the p-type cladding layer. The ridge may be formed by a layer having a different composition from the p-type cladding layer or a different material from the p-type cladding layer.
As illustrated in
Such a structure can reduce the thickness of the p-type cladding layer 135. The thickness of the p-type cladding layer 135 can be, e.g., 0.1-0.2 μm. It has been generally known that an InAlGaN-based nitride semiconductor of p-type has a high electrical resistance, and the thickness of the p-type cladding layer 135 is reduced, thereby making it possible to reduce a series resistance and an operating voltage. Since the power supplied to the device can be reduced by the simple process, it is useful to reduce the amount of heat generation. The p-type contact layer 136 and the second electrode 173 are in contact with each other, whereby the heat can be efficiently conducted from the side of the second electrode 173 to the side of the nitride semiconductor layer 103, and from the side of the nitride semiconductor layer 103 to the side of the second electrode 173. Therefore, even when a junction-down mounting or a junction-up mounting is performed, the heat can be efficiently dissipated.
As illustrated in
The p-type contact layer 136 expect a portion thereof located directly under the ridge 103E is removed by etching, whereby a low-nitrogen layer can be formed. The formation of the low-nitrogen layer further reduce current leakage from the second electrode 173 serving as an interconnect electrode. The etching for forming the ridge 103E may be ICP etching using, e.g., Cl2 gas.
In the present modification, the recessed/raised portions may be provided as well as the first modification. Also, a plurality of ridges may be provided as well as the second modification.
The semiconductor light emitting device of the present disclosure can significantly improve heat dissipation without reducing luminous efficiency. In particular, the semiconductor light emitting device and the light emitting apparatus of the present disclosure is useful as, e.g., a semiconductor light emitting device using a nitride semiconductor and a light emitting apparatus.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-014032 | Jan 2011 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2011/003840 | Jul 2011 | US |
| Child | 13951455 | US |
