1. Field of the Invention
The present invention relates to a light-emitting element using a nitride semiconductor, and more particularly, the present invention relates to a nitride semiconductor light-emitting element which prevents threading dislocations from extending into an active layer and increasing leakage currents by reliably generating pits underneath the active layer, thus improving brightness.
2. Description of the Related Art
Conventional light-emitting elements using a nitride semiconductor are formed by growing a nitride semiconductor lamination portion that contains a buffer layer and a light-emitting layer formation portion on, for example, a sapphire substrate, etching a portion of this semiconductor lamination portion to expose a conductor formation layer on the lower side of the semiconductor lamination portion, and respectively providing a lower electrode on the exposed surface of the lower-side conductor formation layer and an upper electrode on the upper surface of the semiconductor lamination portion. Meanwhile, in order to avoid complications involving the use of such an insulating substrate, adhesion of contamination caused by etching, or the like, a method is also considered in which a semiconductor substrate formed of SiC is used as the substrate, and a light-emitting portion is formed thereon by laminating a nitride semiconductor.
When a sapphire substrate is used as the substrate, lattice mismatching with the nitride semiconductor layer laminated thereon reaches approximately 14%, so that complete lattice matching cannot be accomplished. Furthermore, even if an SiC substrate is used, there is a high degree of lattice mismatching, so that complete lattice matching cannot be accomplished. Accordingly, an extremely large number of crystal defects are produced in the nitride semiconductor layer that is caused to grow on the substrate, and the crystal defects also extend in the vertical direction through the nitride semiconductor layer that is laminated thereon, so that numerous crystal defects referred to as threading dislocations are present. The density of the threading dislocations becomes 1×108/cm2 or more, and when the threading dislocations extend through the active layer, leakage occurs via the threading dislocations, with the threading dislocations acting as non-radiative recombination centers. Therefore, there is a problem in that the emission efficiency (internal quantum efficiency) drops. In order to solve such a problem, a method is considered in which recessed portions called pits are formed in the tip end portions of the threading dislocations in the layer underneath the active layer such that the threading dislocations do not extend into the active layer, thus stopping the threading dislocations, and during the growth of the active layer, these threading dislocations are prevented from extending into the active layer by forming portions of the active layer that extend to the threading dislocations as recessed portions, and subsequently embedding these recessed portions. For example, see Japanese Patent Application Kokai No. 2000-232238.
As described above, in order to form pits, it is necessary to perform etching following the growth of the active layer or to grow a pit generation layer at a low temperature of 800° C. or less. Inserting an etching step during epitaxial growth not only complicates the manufacturing process, but also creates the following problem. Because the element is taken out of a growth furnace and placed into an air atmosphere and then is chemically treated, contamination on the growth surface is generated, which lowers the crystal characteristics of the regrowing gallium nitride-type compound. Furthermore, even in cases where pits are generated as a result of the growth at a low temperature, as is also described in Japanese Patent Application Kokai No. 2000-232238, the following problems are encountered. Specifically, if the growth temperature is too low, the fundamental film quality deteriorates, and if the growth temperature is too high, pits cannot be generated reliably.
In order to overcome the problems described above, preferred embodiments of the present invention provide a nitride semiconductor light-emitting element which suppresses and minimizes reactive currents and non-radiative recombination centers by providing, as an underlying layer of the active layer, a pit formation layer that reliably generates pits, while maintaining a good film quality, so that the internal quantum efficiency is improved, and the light-emitting characteristics are also improved.
In addition, preferred embodiments of the present invention provide a nitride semiconductor light-emitting element having a construction that makes it possible to improve the internal quantum efficiency by further preventing leakage currents.
A nitride semiconductor light-emitting element according to a preferred embodiment of the present invention includes a substrate and a nitride semiconductor lamination portion provided on the substrate and at least an active layer for forming a light-emitting portion, wherein a pit formation layer is disposed on the substrate side of the active layer in a superlattice structure of a nitride semiconductor, and the pit formation layer is arranged to generate pits in end portions of threading dislocations that are generated in the nitride semiconductor layer on the side of the substrate.
In this description, nitride semiconductor refers to a compound of Ga, which is a group III element, and N, which is a group V element, or a compound in which a portion or all of Ga, which is a group III element, is substituted by another group III element such as Al or In, and/or a nitride in which a portion of Na, which is a group V element, is substituted by another group V element such as P or As. Furthermore, the term pits refers to recessed portions formed in the end portions of threading dislocations in the shape of a cone or in the shape of a truncated cone, for example.
A nitride semiconductor having a higher band gap energy level than the active layer is embedded inside the recessed portions formed in this active layer continuously with the pits formed in the pit formation layer, so that the injection of electrons and positive holes can be suppressed and minimized without any void portion remaining, which is preferable.
In specific terms, the above-mentioned active layer has a multiquantum well structure including InxGa1-xN (0<x≦1) and AlyInzGa1-y-zN (0≦y<1, 0≦z<1, 0≦y+z<1, and z<x), and the pit formation layer has a superlattice structure including of 10 to 50 pairs of InaGa1-aN (0<a≦1) and AlbIncGa1-b-cN (0≦b<1, 0≦c<1, 0≦b+c<1, c<a<x)
An embedded layer formed from undoped AlrGa1-rN (0≦r<1) is provided on the active layer on the side opposite from the substrate, and portions of the embedded layer are embedded inside the recessed portions in the active layer, thus making it possible to lower the carrier concentration and to suppress the injection of positive holes. Therefore, this is preferable for suppressing and minimizing leakage currents.
By providing n-type or p-type barrier layers formed from AlsGa1-sN (0≦s<1) on the substrate side of the pit formation layer and on the embedded layer on the side opposite from the active layer, it is possible to effectively close in the carrier in the active layer.
With preferred embodiments of the present invention, because the pit formation layer has a superlattice structure, pits can be reliably generated without taking into consideration the reduction of the growth temperature in order to generate pits as a result of the threading dislocations striking the interfaces in the semiconductor layer for forming superlattices. Accordingly, it is not necessary to lower the growth temperature to an extreme extent, and the film quality of the nitride semiconductor layer can be maintained at a favorable level. Furthermore, because a superlattice structure is used, it is possible to maintain the film quality in an even more favorable manner, to reduce the series resistance, and to improve the light-emitting characteristics.
Moreover, because pits are formed in the tip end portions of the threading dislocations before the threading dislocations reach the active layer, the threading dislocations stop and are contained in the pit formation layer without extending into the active layer. Meanwhile, although the recessed portions extend into the active layer after the threading dislocations stop, the recessed portions are filled with the material of the embedded layer or barrier layers, so that the recessed portions do not remain “as is,” and therefore, do not create any problem in terms of reliability. Because such an embedded layer or barrier layers have a higher band gap energy level than the active layer, the injection of electrons and positive holes is much less likely to occur than in the original active layer, so that the current (non-radiative recombination centers) flowing through this region is reduced to an extreme degree and effectively minimized. As a result, it is possible to eliminate the problems of increasing the leakage current and lowering the internal quantum efficiency caused by the threading dislocations directly reaching the active layer, to achieve a reduction in the leakage current and a reduction in non-radiative recombination at the threading dislocations, and to improve the internal quantum efficiency. Accordingly, a nitride semiconductor light-emitting element having a large output can be obtained.
As a result of portions of the undoped embedded layer being embedded inside the recessed portions that are continuous with the pits, it is possible to reduce the carrier concentration, to further reduce reactive currents, and to improve the internal quantum efficiency.
Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.
FIGS. 2(a) and 2(b) show enlarged explanatory diagrams of a pit portion of the construction shown in
Nitride semiconductor light-emitting elements according to various preferred embodiments of the present invention will be described with reference to the figures. A sectional explanatory diagram of one preferred embodiment of the present invention is shown in
In order to suppress a leakage current which occurs as a result of threading dislocations that tend to be generated in the nitride semiconductor layer extending into the active layer for forming the light-emitting portion, the present preferred embodiment of the present invention preferably has a construction that makes it possible to reliably generate pits and to increase the internal quantum efficiency without lowering the film quality even if a layer for generating the pits is inserted, while also adopting a construction in which the pits are formed in the tip end portions of the threading dislocations in the layer underneath the active layer, thus preventing the threading dislocations from extending into the active layer.
As was described above, a method for performing etching after growing an active layer and a method for growing a nitride semiconductor layer at a low temperature have been proposed in order to generate pits. In this method for growing a nitride semiconductor layer at a low temperature, it has been commonly known that if the growth temperature is high, pit generation cannot be reliably accomplished, and that if the growth temperature is low, the film quality of the nitride semiconductor layer drops. When the film quality of the nitride semiconductor layer drops, the carrier cannot be moved smoothly, and series resistance increases, so that the internal quantum efficiency ends up being lowered. Therefore, as a result of repeated diligent studies by the present inventors, the following discovery has been made. If a pit formation layer for generating the pits is formed to have a superlattice structure, the pits can be generated reliably because of increased interfaces, even without lowering the growth temperature of the nitride semiconductor layer very much. Furthermore, it is possible to improve the film quality and to increase the carrier concentration when a superlattice structure is used, compared to a case in which a bulk layer is grown even in the case of growth at the same temperature. Therefore, a pit formation layer 4 having an extremely high film quality and low resistance can be provided.
In specific terms, for example, as an enlarged sectional explanatory diagram and a perspective explanatory diagram of a pit portion are shown in
The pit formation layer 4 is a layer for stopping the threading dislocations and forming pits. The formation of the pits is accomplished preferably by growing the nitride semiconductor layer at approximately 600° C. to 850° C. However, the growth of an InGaN-type compound cannot be accomplished unless the temperature is originally set at a low temperature because the decomposition temperature of In is low. Therefore, pits tend to be generated when growing an InGaN-type compound. However, pits can still be generated as a result of the growth at a low temperature even with the use of a GaN or AlGaN-type compound. Because a superlattice structure is formed in preferred embodiments of the present invention, pits can be reliably generated even without lowering the temperature to an extreme extent.
For example, as exaggerated explanatory diagrams are shown in
In preferred embodiments of the present invention, because the layer for generating the recessed portions 14 is formed to have a superlattice structure, pits tend to be generated in the interfaces of the superlattice structure, so that pits can be reliably generated without significantly lowering the growth temperature. Moreover, because the lamination is accomplished using a superlattice structure, the film quality is improved, and in the case of the formation using an n-type layer, the carrier concentration can be increased. Therefore, as will be described later, by making the nitride that is embedded inside the recessed portions 14 using a material with a high band gap energy level and not doping this material, electrons completely avoid the recessed portions, pass through the portions of the pit formation layer 4 where no pits are formed, and are recombined with positive holes in the active layer 5 without any recessed portions 14, so that the light is emitted. Specifically, the nitride semiconductor that is embedded inside the recessed portions 14 has a high band gap, and therefore does not contribute to emission of light, so that it is desirable that the recombination of electrons and positive holes do not occur inside these recessed portions 14. Furthermore, even if a current flows, this current is a reactive current, so that it is better if no current flows. However, in preferred embodiments of the present invention, because a construction is used in which no current tends to flow to the pits (recessed portions 14) even if the carrier concentration is increased with the pit formation layer 4 having a superlattice structure, the current can be utilized extremely effectively.
Substantially the same construction as in the prior art is preferably used except for this pit formation layer 4 and embedded layer 6 (described later). In the example shown in
The semiconductor lamination portion 9 has a light-emitting layer formation portion having a double heterojunction structure in which the active layer 5 is formed from a material having a band gap energy level corresponding to the emission wavelength, and barrier layers (n-type layer 3 and p-type layer 7) having a higher band gap energy level than the active layer are provided above and below this active layer 5. In preferred embodiments of the present invention, the pit formation layer 4 is provided on the substrate side of the active layer 5. Furthermore, in the example shown in
In the example shown in
The formation of an n-type layer can be accomplished by mixing Se, Si, Ge, or Te as an impurity raw material gas of H2Se, SiH4, GeH4TeH4, or the like into a reactive gas, and the formation of a p-type layer can be accomplished by mixing Mg or Zn as an organic metal gas of cyclopentadienyl magnesium (Cp2Mg) or dimethyl zinc (DMZn) into a raw material gas. In the case of an n-type layer, however, N tends to evaporate during the film formation even without mixing any impurity, so that an n-type layer can naturally be formed. Therefore, this property can also be utilized.
In the example shown in
The embedded layer 6 can be formed to have a thickness of about 0.005 μm to about 0.1 μm from, for example, undoped AlrGa1-rN (0≦r<1, e.g., r=0). The embedded layer 6 is used so as to be embedded inside the recessed portions 14 that are formed so as to extend from the pit formation layer 4 over to the active layer 5, and as a result of the growth at a high temperature of about 900° C. to about 1200° C., the embedding inside the recessed portions 14 can be accomplished. The embedded layer 6 may have the same composition as the p-type layer 7, or a different composition. However, as the mixed crystal ratio r of Al becomes higher, the embedding effect is greater, and the band gap energy level is higher. Therefore, a higher mixed crystal ratio of Al is preferable from the standpoint of suppressing and minimizing the electron injection into the recessed portions 14. It is preferable that the embedded layer 6 be an undoped layer because the carrier movement can easily be suppressed. However, the electron movement can be suppressed by using a material having a high band gap energy level, so that portions of the p-type layer 7 are embedded inside the recessed portions 14 during the growth of this p-type layer even without providing any embedded layer 6.
The translucent conductive layer 10 including, for example, ZnO, is preferably formed to have a thickness of about 0.1 μm to about 10 μm on the semiconductor lamination portion 9, and the p-side electrode 11 is formed on a portion of this translucent conductive layer 10 with a laminated structure of Ti and Au. The material of this translucent conductive layer 10 is not limited to ZnO; a thin alloy layer of about 2 nm to about 100 nm including ITO or Ni and Au may also be used, as long as the material can cause a current to be diffused over the entire chip while allowing light to pass through. In the case of an Ni—Au layer, because this is a metal layer, if the layer is thick, translucency is lost, so that this layer is thinly formed. In the case of ZnO or ITO, however, light is allowed to pass through, so that a thick layer may be used. In the example shown in
The upper electrode 11 is preferably formed as a p-side electrode because the upper surface of the semiconductor lamination portion is a p-type layer in the example shown in
Next, a method for manufacturing a nitride semiconductor light-emitting element according to a preferred embodiment of the present invention will be described briefly using a specific example. First, an SiC substrate 1 is set inside an MOCVD (metalorganic chemical vapor deposition) apparatus, for example, and a component gas for a semiconductor layer that grows, i.e., a required gas selected from among, for example, trimethylgallium, trimethylaluminum, (in the case of forming an AlGaN-type layer), trimethylindium, ammonia gas, any of H2Se, SiH4, GeH4, and TeH4as an n-type dopant gas, and DMZn or Cp2Mg as a p-type dopant gas, is introduced together with an H2 gas or N2 gas used as the carrier gas. An n-type Al0.2Ga0.8N buffer layer 2 and an n-type layer 3 preferably formed of GaN are respectively laminated, for example, at a temperature of about 700° C. to about 1200° C. Then, the substrate temperature is reduced to approximately 760° C., for example, and first layers 41 preferably formed of, for example, In0.05Ga0.95N with a thickness of about 1 nm and second layers 42 preferably formed of, for example, GaN with a thickness of about 2 nm are laminated in approximately 20 pairs, thus forming a pit formation layer 4 having a superlattice structure. In this case, recessed portions 14 are formed in the end portions of threading dislocations 13.
Next, well layers preferably formed of, for example, In0.12Ga0.88N with a thickness of approximately 3 nm and barrier layers preferably formed of GaN with a thickness of approximately 18 nm are laminated in 5 pairs to form an active layer 5 having a multiquantum well (MQW) structure so as to have a thickness of approximately 0.1 μm as a whole. In this case, the recessed portions 14 formed in the pit formation layer 4 are also formed in the active layer 5 as recessed portions that are continuously widened. Afterwards, the substrate temperature is increased to approximately 1065° C., for example, and an embedded layer 6 preferably formed of, for example, GaN is formed into a film having a thickness of approximately 0.02 μm as an undoped layer. Subsequently, a p-type layer 7 preferably formed of, for example, GaN with a thickness of about 0.5 μm to about 2 μm is continuously formed, thus forming a light-emitting layer formation portion by the successive epitaxial growth of the respective layers described above.
Then, an SiO2 protective film is provided over the entire surface of the semiconductor lamination portion, and annealing is performed at about 400° C. to about 800° C. for approximately 20 to 60 minutes to activate the p-type layer 7. When the annealing is completed, a translucent conductive layer 10 preferably formed of ZnO is formed on the surface of the p-type layer 7 to a thickness of about 0.3 μm by placing a wafer inside a sputtering apparatus or vacuum evaporation apparatus, and a p-side electrode 11 is formed by forming a film of Ti, Al, or the like. Afterwards, the thickness of the SiC substrate 1 is reduced by performing lapping on the back surface side of the SiC substrate 1, and a metal film of Ti, Au, or the like is similarly formed on the back surface of the substrate 1, thus forming a lower electrode 12. Finally, a nitride semiconductor light-emitting element chip is obtained by forming a chip by scribing.
With preferred embodiments of the present invention, because a pit formation layer having a superlattice structure is provided before threading dislocations reach the active layer, pits can be reliably generated in some of the interfaces in the superlattice structure of the pit formation layer without significantly lowering the growth temperature. Accordingly, the threading dislocations are reliably stopped underneath the active layer without extending up to the active layer, and nitride semiconductor having a higher band gap energy level is embedded inside the recessed portions in the active layer, so that a leakage current can be reduced to a great extent. Furthermore, because the pit formation layer for generating pits is formed to have a superlattice structure, the film quality of the semiconductor layer is good, the carrier concentration can be increased, the series resistance can be reduced, and a current can be utilized even more effectively. As a result, the recombination of positive holes and electrons can occur in portions of the active layer where no recessed portions are formed, so that the internal quantum efficiency can be improved considerably with very little wasted current.
In the above-mentioned example, a conductive SiC substrate is preferably used as the substrate. However, even when a sapphire substrate is used, if a pit formation layer having a superlattice structure is similarly provided on the lower side of the active layer, pits are reliably formed, and at the same time, a pit formation layer preferably formed of a nitride semiconductor layer and having a high film quality can be provided on the lower side of the active layer. The construction of the semiconductor lamination portion may be the same as the above-mentioned laminated structure. In cases where the substrate is formed of sapphire, however, the electrode cannot be taken out from the back surface of the substrate. Therefore, by forming a buffer layer or substrate-side nitride semiconductor layer as an undoped layer, the crystal characteristics can also be improved. Such an example is shown in
In
Furthermore, the undoped high-temperature buffer layer 3a is used to improve the crystal characteristics of the laminated semiconductor layer of a gallium nitride-type compound, and for this reason, the first layer growing at a high temperature is undoped. Moreover, with regard to the p-type layer 7 and contact layer 7a, the formation of layers containing Al on the side of the active layer 5 is indicated as a preferred example from the standpoint of the effect of closing in the carrier as described above. In addition, because the substrate is an insulator, there is no need to form the buffer layer 2 as a conductive layer, and AlN may also be used.
A translucent conductive layer 10 and a p-side electrode 11 are formed on this semiconductor lamination portion just as in the example described above, and a portion of the laminated semiconductor layer is removed by etching, so that an n-side electrode 12 is formed on the exposed n-type layer 3 by means of a laminated structure preferably formed of, for example, Al, Mo, and Au. The Al layer is laminated with a thickness of about 5 nm to about 20 nm (e.g., about 10 nm), the Mo layer is laminated with a thickness of about 30 nm to about 100 nm (e.g., about 50 nm), and the Au layer is laminated with a thickness of about 0.2 μm to about 1 μm (e.g., about 0.25 μm). A thermal treatment of rapid heating (RTA) is performed for approximately 5 seconds at about 600° C. Although a portion of the Al layer diffuses to the gallium nitride-type compound, the respective metal layers do not form an alloy with each other as a result of the Mo layer acting as a barrier layer, and the Au layer that is not formed into an alloy is secured on the surface of the n-type electrode 12, so that the bonding characteristics of wire bonding can be improved. Furthermore, a passivation film including SiO2 or the like (not shown) is provided on the entire surface, excluding the surfaces of the p-side electrode 11 and n-side electrode 12.
In the example previously described, the SiC substrate is preferably formed as an n-type layer, and p-type layers are preferably formed toward the surface. This is because the use of this construction is convenient for performing annealing for the purpose of activating the p-type layers. However, it would also be possible to form the substrate and layers that are present on the substrate side of the active layer as p-type layers. Furthermore, the nitride semiconductor is not limited to the above-mentioned examples, and may also be constructed from a nitride material expressed as a general formula of AlpGaqIn1-p-qN (0≦p≦1, 0≦q≦1, and 0≦p+q<1). Moreover, a compound may also be used in which a portion of this N is substituted by another group V element.
Furthermore, the light-emitting layer formation portion is preferably formed to have a double heterojunction structure using a sandwich construction in which the active layer is held between the n-type layer and p-type layer. However, this light-emitting layer may also be constructed similarly by further inserting another semiconductor layer such as a guide layer between any of the layers or using a single heterojunction structure or homo p-n junction structure. In this case, the active layer defines the light-emitting portion.
In addition, the above-mentioned example is an example of LED. In the case of a semiconductor laser, however, the internal quantum efficiency can also be improved by similarly providing a pit formation layer having a superlattice structure on the lower side of the active layer.
While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.
Number | Date | Country | Kind |
---|---|---|---|
2005-344170 | Nov 2005 | JP | national |