The present invention relates to a nitride-based semiconductor light-emitting device and a method for fabricating such a device. More particularly, the present invention relates to a GaN-based semiconductor light-emitting device such as a light-emitting diode or a laser diode that operates at wavelengths over the entire visible radiation range, which covers the ultraviolet, blue, green, orange and white parts of the spectrum, and that is expected to be applied to various fields of technologies including display, illumination and optical information processing in the near future.
A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting device because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors (which will be referred to herein as “GaN-based semiconductors” and which are represented by the formula AlxGayInzN (where 0≦x, y, z≦1 and x+y+z=1)) have been researched and developed particularly extensively. As a result, blue-ray-emitting light-emitting diodes (LEDs), green-ray-emitting LEDs and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products (see Patent Documents Nos. 1 and 2, for example).
When a semiconductor device is fabricated using GaN-based semiconductors, a sapphire wafer, an SiC wafer, an Si wafer or any other appropriate wafer is used as a wafer on which a crystal of GaN-based semiconductors needs to be grown. No matter which of these wafers is used, however, it is always difficult to achieve a sufficient degree of lattice matching between the wafer and the GaN-based semiconductor crystal (i.e., to realize a coherent growth). As a result, a lot of dislocations (including edge dislocations, spiral dislocations and mixed dislocations) will usually be produced inside the GaN-based semiconductor crystal and will have as high a density as approximately 1×109 cm−2 when a sapphire wafer or an SiC wafer is used, for example. Consequently, an increase in threshold current and a decrease in reliability will be unavoidable as for a semiconductor laser diode, and an increase in power dissipation and a decrease in efficiency or reliability will be inevitable as for an LED. Also, some existent GaN wafers may certainly have a lower density of dislocations but its crystal would have a lot of residual strain. That is why even if a GaN-based semiconductor crystal is grown on such a wafer, it should be difficult to go without experiencing a similar problem.
Thus, a so-called “epitaxial lateral overgrowth (ELO)” technique has been proposed as a method for reducing the density of dislocations in a GaN-based semiconductor crystal. Such a method will effectively reduce the number of threading dislocations in a system with a high degree of lattice misfit. If a GaN-based semiconductor crystal is grown by ELO on each of the wafers described above, the upper part of the seed crystal will have a region in which there are a lot of dislocations at a density of approximately 1×109 cm−2, but the density of locations can be reduced to around 1×107 cm−2 in a laterally growing region. And if an active region, which is an electron injected region, is defined over such a region with fewer dislocations, the reliability can be increased.
Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2001-308462
Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2003-332697
The present inventors, however, discovered that even such a GaN-based semiconductor light-emitting device, of which the crystal was grown by ELO, was not without a different kind of problem. Specifically, when a GaN-based semiconductor crystal, which had been grown by ELO, was analyzed with an X-ray micro beam, there was a non-uniform distribution of strain within a plane of the GaN-based semiconductor crystal. Such a non-uniform distribution of strain is not beneficial because it would make the emission intensity non-uniform within that plane.
It is therefore an object of the present invention to suppress the non-uniform strain in a nitride-based semiconductor light-emitting device, of which the crystal has been grown by ELO process.
A nitride-based semiconductor light-emitting device according to the present invention has a nitride-based semiconductor multilayer structure. The nitride-based semiconductor multilayer structure includes: an active layer including an AlaInbGacN crystal layer (where a+b+c=1, a≧0, b≧0 and c≧0); an AldGaeN overflow suppressing layer (where d+e=1, d>0, and e≧0); and an AlfGagN layer (where f+g=1, f≧0, and f<d). The AldGaeN overflow suppressing layer is arranged between the active layer and the AlfGagN layer. The AldGaeN overflow suppressing layer includes an In-doped layer that is doped with In at a concentration of 1×1016 atms/cm3 to 8×1018 atms/cm3. A normal to the principal surface of the nitride-based semiconductor multilayer structure defines an angle of 1 to 5 degrees with respect to a normal to an m plane.
In one preferred embodiment, the nitride-based semiconductor multilayer structure is made up of semiconductor layers that are tilted in either a c-axis direction or an a-axis direction.
In another preferred embodiment, the nitride-based semiconductor multilayer structure is arranged on a GaN substrate, and a normal to the principal surface of the GaN substrate defines an angle of 1 to 5 degrees with respect to a normal to an m plane.
In still another preferred embodiment, the In-doped layer forms a part of the AldGaeN overflow suppressing layer and is arranged closest to the active layer.
In yet another preferred embodiment, the In-doped layer is a half or less as thick as the AldGaeN overflow suppressing layer.
In yet another preferred embodiment, the more distant from the active layer, the lower the In concentration of the In-doped layer.
In yet another preferred embodiment, an undoped GaN layer is arranged between the active layer and the AldGaeN overflow suppressing layer.
A manufacturing method according to the present invention is a method for fabricating a nitride-based semiconductor light-emitting device having a nitride-based semiconductor multilayer structure. The method includes the steps of: (a) forming an active layer including an AlaInbGacN crystal layer (where a+b+c=1, a≧0, b≧0 and c≧0) as a portion of the nitride-based semiconductor multilayer structure; (b) forming an AldGaeN overflow suppressing layer (where d+e=1, d>0, and e≧0) as another portion of the nitride-based semiconductor multilayer structure; and forming an AlfGagN layer (where f+g=1, f≧0, g≧0 and f<d) as still another portion of the nitride-based semiconductor multilayer structure. The step (b) includes forming an In-doped layer that is doped with In at a concentration of 1×1016 atms/cm3 to 8×1018 atms/cm3 in the AldGaeN overflow suppressing layer. A normal to the principal surface of the nitride-based semiconductor multilayer structure defines an angle of 1 to 5 degrees with respect to a normal to an m plane.
In one preferred embodiment, the nitride-based semiconductor multilayer structure is made up of semiconductor layers that are tilted in either a c-axis direction or an a-axis direction.
In another preferred embodiment, the In-doped layer forms a part of the AldGaeN overflow suppressing layer and is arranged closest to the active layer.
In still another preferred embodiment, the In-doped layer is a half or less as thick as the AldGaeN overflow suppressing layer.
Another nitride-based semiconductor light-emitting device according to the present invention has a nitride-based semiconductor multilayer structure, which includes: an active layer including an AlaInbGacN crystal layer (where a+b+c=1, a≧0, b≧0 and c≧0); an AldGaeN overflow suppressing layer (where d+e=1, d>0, and e≧0); and an AlfGagN layer (where f+g=1, f≧0, g≧0 and f<d). The AldGaeN overflow suppressing layer is arranged between the active layer and the AlfGagN layer. The AldGaeN overflow suppressing layer includes an In-doped layer that is doped with In at a concentration of 1×1016 atms/cm3 to 1×1019 atms/cm3. A normal to the principal surface of the nitride-based semiconductor multilayer structure defines an angle of 1 to 5 degrees with respect to a normal to an m plane. And the In-doped layer is a half or less as thick as the AldGaeN overflow suppressing layer.
Another manufacturing method according to the present invention is a method for fabricating a nitride-based semiconductor light-emitting device having a nitride-based semiconductor multilayer structure. The method includes the steps of: (a) forming an active layer including an AlaInbGacN crystal layer (where a+b+c=1, a≧0, b≧0 and c≧0) as a portion of the nitride-based semiconductor multilayer structure; (b) forming an AldGaeN overflow suppressing layer (where d+e=1, d>0, and e≧0) as another portion of the nitride-based semiconductor multilayer structure; and forming an AlfGagN layer (where f+g=1, f≧0, g≧0 and f<d) as still another portion of the nitride-based semiconductor multilayer structure. The step (b) includes forming an In-doped layer that is doped with In at a concentration of 1×1016 atms/cm3 to 1×1019 atms/cm3 in the AldGaeN overflow suppressing layer. A normal to the principal surface of the nitride-based semiconductor multilayer structure defines an angle of 1 to 5 degrees with respect to a normal to an m plane. And the In-doped layer is a half or less as thick as the AldGaeN overflow suppressing layer.
According to the present invention, an In-doped layer including In at a concentration of 1×1016 atms/cm3 to 1×1019 atms/cm3 is formed in an AldGaeN layer, thereby minimizing the occurrence of non-uniform strain in a nitride-based semiconductor light-emitting device. As a result, it is possible to prevent the nitride-based semiconductor light-emitting device from having a non-uniform in-plane distribution of emission. In addition, according to the present invention, even when a GaN substrate, of which the principal surface defines a tilt angle of 1 to 5 degrees with respect to an m plane, is used, the same effect will also be achieved as in a situation where an m-plane GaN substrate (of which the principal surface defines a tilt angle of less than 1 degree with respect to the m plane) is used.
a) is a cross-sectional view schematically illustrating a second specific preferred embodiment of the present invention, and
a) is a graph showing how the degrees of internal quantum efficiency and internal loss change according to the thickness of an overflow suppressing layer (i.e., AldGaeN layer) 36, and
Portions (a) and (b) of
a) and 5(b) show the results of a rocking curve measurement that was carried out on a nitride-based semiconductor light-emitting device, of which the AldGaeN layer 36 had no In-doped layer 35.
a) and 6(b) are photographs showing results of evaluation that was carried out, using cathode luminescence, on a structure including a GaN layer instead of the In-doped layer 35 and on a structure including an InGaN layer instead of the In-doped layer 35, respectively.
Portion (a) of
Portions (a) and (b) of
Portions (a) and (b) of
Portions (a) and (b) of
a) and 13(b) are graphs showing how the emission intensity (in arbitrary unit) changed with the time delay (ns) in a -comparative example and in the second preferred embodiment of the present invention, respectively.
a) to 15(c) are cross-sectional views illustrating respective manufacturing process steps according to the second preferred embodiment of the present invention.
a) and 16(b) are cross-sectional views illustrating respective manufacturing process steps according to the second preferred embodiment of the present invention.
a) and 17(b) are cross-sectional views illustrating respective manufacturing process steps according to the second preferred embodiment of the present invention.
a) and 18(b) are cross-sectional views illustrating respective manufacturing process steps according to the second preferred embodiment of the present invention.
a) and 20(b) are cross-sectional views illustrating respective manufacturing process steps according to the second preferred embodiment of the present invention.
a) and 23(b) schematically illustrate the crystal structures of nitride-based semiconductors, of which the principal surfaces are a c plane and an m plane, respectively, as viewed on a cross section thereof that intersects with the principal surface of the substrate at right angles.
a) schematically illustrates the crystal structure (i.e., the wurtzite crystal structure) of a GaN substrate and
a) and 28(b) are cross-sectional views showing the relation between the principal surface of a GaN substrate and an m plane.
a) and 29(b) are cross-sectional views illustrating the crystal structure of the principal surface of a GaN substrate 10a and its surrounding region.
Hereinafter, a first specific preferred embodiment of a nitride-based semiconductor light-emitting device according to the present invention will be described with reference to
As shown in
The nitride-based semiconductor multilayer structure 12 includes an active layer 13 including an AlaInbGacN crystal layer (where a+b+c=1, a≧0, b≧0 and c≧0) and an AldGaeN layer 14 (where d+e=1, d>0, and e≧0), which is located on the other side of the active layer 13 opposite to the selectively grown layer 11.
The AldGaeN layer 14 includes a layer 15 that includes In at a concentration of 1×1016 atms/cm3 to 1×1019 atms/cm3 (which will be simply referred to herein as an “In-doped layer”).
A GaN-based semiconductor crystal including In as one of its constituent elements (or parent elements) is already known. In general, if In is supposed to be one of the constituent elements of a crystal, then In should be included in a concentration that is high enough to have some impact on the physical properties of a GaN-based semiconductor crystal.
If the In concentration was 1×1019 cm−3, for example, In would account for just 1% of the entire composition. In that case, the physical property would be almost no different from a situation where no In is included. That is why if In is supposed to be one of such essential constituent elements of a crystal, then the In concentration should be higher than 1×1019 cm−3 (e.g., 1×1020 cm−3 or more). That is to say, the In concentration of the In-doped layer 15 of this preferred embodiment is lower than the one in a situation where In is one of the essential elements of a crystal.
The present inventors discovered that there was a non-uniform distribution of strain within a plane of a GaN-based semiconductor crystal that had been grown by ELO process. According to this preferred embodiment, such a non-uniform distribution of strain can be smoothed out by forming the In-doped layer 15 as a part of the AldGaeN layer 14. It is not quite clear at this time exactly how and why it works. But we find it proper to reason that if In atoms, which are bigger in size than Al or Ga atoms, are included in a concentration that is merely as high as that of a normal dopant, a moderate degree of strain should be caused and would probably contribute to smoothing out that non-uniform distribution of strain. As for a GaN-based semiconductor crystal including In as one of its essential constituent elements, on the other hand, In should account for such a high percentage of the entire composition that the lattice constant would increase so much as to produce excessive strain inside the crystal.
In
The selectively grown layer 11 may be either an AlxGayInzN crystal layer (where x+y+z=1) that has been formed on a substrate or a part of a GaN substrate. Those structures and their manufacturing methods will be described as second and third specific preferred embodiments of the present invention.
Hereinafter, a second specific preferred embodiment of a nitride-based semiconductor light-emitting device according to the present invention will be described with reference to
As shown in
A recess 22 has been cut through the AluGavInwN layer 20 and a selective growth mask 23 is arranged at the bottom of the recess 22. The selective growth mask 23 is made of a dielectric film, an amorphous insulating film or a metal film.
As the thickness of the selective growth mask 23 is less than the depth of the recess 22, there is an air gap 25, which is surrounded with the side surfaces of the recess 22 and the lower surface of the AlxGayInzN crystal layer 30 and which is located over the mask 23.
The AlxGayInzN crystal layer 30 includes a dopant of a first conductivity type (which may be n-type, for example) and has grown using, as its seed crystal, at least a part of a surface region of the AluGavInwN layer 20 that is not covered with the selective growth mask 23 (such a surface region will be referred to herein as a “seed crystal region 24”). And a portion of the AlxGayInzN crystal layer 30 that is located over the air gap 25 has grown laterally.
If the air gap 25 is left, then the selective growth mask 23 does not contact with the laterally grown portion of the AlxGayInzN crystal layer 30. As a result, the interfacial stress can be minimized and that laterally grown portion of the AlxGayInzN crystal layer 30 comes to have a crystal axis with a decreased tilt angle. Consequently, an AlxGayInzN crystal layer 30 with a sufficiently low density of dislocations can be obtained in a broad area (i.e., the laterally grown region) of the AluGavInwN layer 20 except for its seed crystal region that is in contact with the AlxGayInzN crystal layer 30.
In the semiconductor multilayer structure 50, there is an active layer 32, including an AlaInbGacN crystal layer (where a+b+c=1, a≧0, b≧0 and c≧0), over the AlxGayInzN crystal layer 30. In this case, the active layer 32 is an electron injected region of the nitride-based semiconductor light-emitting device 100. On the active layer 32, arranged is an AldGaeN layer 36 (where d+e=1, d>0, and e≧0) of a second conductivity type (e.g., p-type). In this preferred embodiment, the AldGaeN layer 36 is doped with Mg. In a GaN-based material, electrons have a small effective mass. That is why if the amount of drive current is increased in an LED or a laser structure that uses GaN-based materials, a greater number of electrons will overflow and the efficiently will decrease. Thus, to minimize such an overflow, the AldGaeN layer 36 is provided as an overflow suppressing layer on one side of the active layer 32 closer to p-type layers. The AldGaeN layer 36 preferably has a thickness of 10 nm to 200 nm.
An undoped GaN layer 34 is sandwiched between the active layer 32 and the AldGaeN layer 36.
An In-doped layer 35 forms a lower part of the AldGaeN layer 36. Considering that the closer to the active layer 32 the In-doped layer 35 is, the more perfectly the non-uniform distribution of strain in the active layer 32 can be smoothed out, the In-doped layer 35 preferably forms a lower part of the AldGaeN layer 36 (i.e., closest to the active layer 32). The In-doped layer 35 preferably has a thickness of 10 nm to 100 nm.
On the AldGaeN layer 36 including the In-doped layer 35, arranged is a GaN layer 38 of the second conductivity type (e.g., p-type). The GaN layer 38 functions as an electricity guide layer for guiding holes from a p-electrode to the active layer. The GaN layer 38 does not have to be made of GaN only but could be made of an AlfGagN layer (where f+g=1, f≧0, and g≧0). Nevertheless, the Al mole fraction of the AlfGagN layer should be lower than that of the AldGaeN layer (the overflow suppressing layer) 36 (i.e., f<d should be satisfied). The AldGaeN layer (overflow suppressing layer) 36 is arranged between the active layer 32 and the GaN layer 38.
And on the GaN layer 38, arranged is a contact layer 40 of p+-GaN.
b) shows how the atomic concentration varies in a region of the light-emitting device 100 of this preferred embodiment that covers the range from the active layer 32 through the AldGaeN layer 36. In
And a portion of the AldGaeN layer 36 including In at a concentration of 1×1016 atms/cm3 to 1×1019 atms/cm3 corresponds to the In-doped layer 35. In the AldGaeN layer 36 of this preferred embodiment, the In concentration in the In-doped layer 35 decreases toward the upper surface of the AldGaeN layer 36 (i.e., in the positive x-axis direction).
The AldGaeN layer (overflow suppressing layer) 36 has so high an Al mole fraction of nearly 20% as to have a greater bandgap than any other semiconductor layer that is in contact with the AldGaeN layer 36. As a result, strain is produced between the AldGaeN layer 36 and other semiconductor layers that are in contact with the AldGaeN layer 36 and affects the active layer 32.
The distance between the AldGaeN layer 36 and the active layer 32 can be changed by adjusting the thickness of the undoped GaN layer 34.
These results reveal that to minimize the overflow, the AldGaeN layer 36 preferably has an increased thickness and that to reduce the influence of the strain on the active layer 32, the undoped GaN layer 34 is preferably provided.
Next, the non-uniform distribution of strain that was newly discovered by the present inventors (i.e., the non-uniform distribution of strain within a plane of a GaN based semiconductor crystal) will be described with reference to
The present inventors evaluated a nitride-based semiconductor light-emitting device with no In-doped layer 35 by carrying out a rocking curve measurement using X-ray micro beams in SPring8 (Super Photon ring-8 GeV).
SPring8 is a huge synchrotron radiation facility, which is located in Harima Science Park City, Hyogo, JAPAN. In that facility, a group of accelerators for accelerating and storing electrons and radiation beams generated can be used. On the other hand, the “rocking curve measurement” (which is also called “θ scan method”) is a method for estimating a variation in X-ray intensity at a peak of diffraction by scanning a sample for a very small angular ω range with the Bragg diffraction angle 2θ fixed at a (0002) diffraction peak position.
The structure of the sample (nitride-based semiconductor light-emitting device) that was used in that measurement is shown in portions (a) and (b) of
First of all, X-ray micro beams, which had been condensed to the order of sub-micrometers, were generated by using a zone plate and slits in combination and then made to be incident on the sample 100a. Then, the sample was scanned for a very small angular W range with the Bragg diffraction angle 2θ fixed at the angle a at which a (0002) diffraction peak appeared, thereby measuring the X-ray diffraction intensities. Specifically, with the Bragg diffraction angle 2θ fixed at around 28.8 degrees, the angle ω of the sample was changed from 28.5 degrees into 29.15 degrees, thereby measuring X-ray diffraction intensities at respective angles ω. Thereafter, similar measurements were repeatedly carried out a number of times with points of measurement on the sample 100a changed in a step of 0.5 μm in <11-20> directions.
a) and 5(b) show results of the measurements. In
In
Among other things, close attention should be paid to the fact that a region d with low lightness appeared at a ω of approximately 28.65 degrees at an ordinate of 60 μm but at a ω of approximately 28.9 degrees at an ordinate of 80 μ m as shown in
a) and 6(b) are photographs showing results of evaluation that was carried out on semiconductor multilayer structures using cathode luminescence. Specifically,
As can be seen from
If the arrangement of multiple layers in the GaN-based light-emitting device 100 were significantly changed from a typical one, then some conditions for avoiding such non-uniform strain distribution even when the ELO process is adopted could be found. In that case, however, the arrangement of layers should be changed so significantly that it might be difficult for the GaN-based semiconductor light-emitting device to exhibit desired properties or its life or reliability might have to be sacrificed.
With these considerations in mind, the present inventors discovered that the occurrence of such a non-uniform strain distribution could be avoided by providing the In-doped layer 35 intentionally for the AldGaeN layer 36 as shown in
In the photograph shown in
In
According to the preferred embodiment described above, since the In-doped layer 35 forms part of the AldGaeN layer 36, the non-uniform strain distribution that would otherwise be seen often in a conventional device can be reduced in this nitride-based semiconductor light-emitting device 100. In addition, a number of crystal defects to be caused by such a non-uniform strain distribution can also be reduced. As a result, non-uniform in-plane emission can be eliminated.
Next, the effects to be achieved by this preferred embodiment will be described. According to this preferred embodiment, since the AlxGayInzN crystal layer 30 is grown laterally over the air gap 25 as shown in
As shown in portion (a) of
Hereinafter, it will be described with reference to the accompanying drawings how different conventional and our (this invention's) approaches to the strain problem are.
Portions (a) and (b) of
On the other hand, portions (a) and (b) of
As described above, the conventional approach deals with unidirectional strain, while the approach of this preferred embodiment deals with the non-uniform strain distribution. On top of that, the conventional approach is adopted to reduce the strain but our approach is adopted to smooth out the non-uniform strain distribution, not to reduce the strain. In these respects, our approach is different from theirs.
Portions (a) and (b) of
Portion (a) of
Specifically, in a region at a depth of more than 0.5 μm under the surface, the dopant In had a concentration of 1.0×1017 atms/cm3, while a region at a depth of around 0.5 μm under the surface had an In concentration, which decreased gradually toward the surface and eventually went approximately zero. In the range that was doped with In, a region including In at a concentration of 1×1016 atms/cm3 to 1×1019 atms/cm3 is the In-doped layer 35. On the other hand, portion (b) of
Hereinafter, the life of emission produced by GaN based semiconductor light-emitting devices will be described with reference to
Next, the luminous efficacy of GaN based semiconductor light-emitting devices will be described with reference to
As can be seen from
On top of that, it was also confirmed that the nitride-based semiconductor light-emitting device of this preferred embodiment could reduce the operating voltage Vop by approximately 1 V compared to the conventional one and could cut down the power dissipation.
Hereinafter, it will be described with reference to
First of all, a substrate 10 is provided as shown in
Next, as shown in
Subsequently, as shown in
Thereafter, as shown in
Next, as shown in
It should be noted that with the air gap 25 left, the AlxGayInzN crystal layer 30 and the selective growth mask can avoid contact with each other, thus minimizing the interfacial stress between them and decreasing the tilt angle of the crystallographic axis of the AlxGayInzN crystal layer 30. Consequently, the AlxGayInzN crystal layer 30 can have a lower density of dislocations. However, the air gap does not always have to be left and the AlxGayInzN crystal layer 30 may contact with the selective growth mask 23.
Subsequently, as shown in
Next, as shown in
Thereafter, as shown in
This effect of smoothing out the non-uniform strain distribution would be achieved by striking an adequate balance between the lattice strains inside the active layer 32 and inside the In-doped layer 35. That is why to smooth out the non-uniform strain distribution, the In-doped layer 35 should have strain energy that is high enough to affect the strain of the active layer 32. Generally speaking, the greater the thickness, the higher the strain energy. Specifically, if the In-doped layer 35 has a thickness of 10 nm or more, the strain energy of the In-doped layer 35 will have some influence on the active layer 32. And if the In-doped layer 35 has a thickness of 30 nm or more, the strain energy given by the In-doped layer 35 to the active layer 32 will have rather great influence, which is proved by results of elasticity calculations. However, if the In-doped layer 35 had a thickness of more than 100 nm, then excessive strain energy, which is almost as high as that of a layer including In as an essential constituent element, would be produced to lessen the expected effect. For that reason, the In-doped layer 35 preferably has a thickness of 100 nm or less.
The In-doped p-Al0.14Ga0.86N layer to be the In-doped layer 35 is preferably grown at a low temperature (of 805° C. to 910° C., for example). By growing the layer at such a low temperature, the molar amount of In supplied can be kept rather small. The present inventors also discovered and confirmed via experiments that if the AIGaN layer started to be grown with In supplied and if the growth temperature was gradually raised from a relatively low temperature to a relatively high one with time, the crystallinity improved. In that case, the AlGaN layer may start to be grown at 910° C. and then the temperature may be raised to 940° C., for example.
According to this preferred embodiment, In may be introduced so as to have either varying concentrations or a constant concentration as shown in portion (a) or (b) of FIG. 12. If In is introduced to have varying concentrations, then
In may be introduced to have a concentration of 1.0×1017 atms/cm3 (which is indicated as 1E+17 in the graph shown in portion (b) of
The In-doped layer 35 of this preferred embodiment has an In concentration, which is approximately as high as a normal dopant concentration and which is lower than an In concentration (which is 1×1020 cm−3 or more and which may be 1×1022 atms/cm3, for example) in a situation where In is one of the essential constituent elements of a crystal.
According to this preferred embodiment, an MOCVD process is adopted as a method for growing the respective layers epitaxially. In this process, TMG is used as a source material for Ga and an organic metal such as TMA is used as source material for Al. The temperatures of those organic metals are controlled in a thermostat. By introducing a hydrogen gas into the thermostat, an organic metal in a number of moles to be determined by the temperature and vapor pressure at that point in time is decomposed and mixed with the hydrogen gas. In this case, by adjusting the flow rate of the hydrogen gas using a mass flow controller, the number of moles of the organic metal that reaches the substrate per unit time (which will be referred to herein as “molar amount of the source material supplied”) is controlled. For example, when an InGaN layer is grown as the active layer 32 (i.e., if In is supplied as one of the essential constituent elements of a crystal), In needs to be supplied at a relatively high rate (e.g., to have a concentration of 1×1022 atms/cm3), and therefore, a 1,000 cc/min mass flow controller is used. On the other hand, when the In-doped layer 35 of this preferred embodiment is grown, the required concentration of In (of 1.0×1017 atms/cm3, for example) is smaller than when the active layer 32 is grown. For that reason, if a 1,000 cc/min mass flow controller were used in that case, then it would be difficult to control the rate of In supplied. Thus, in order to control the rate of supplying In appropriately enough to form the In-doped layer 35, it is preferred that a 10 cc/min mass flow controller be used and that the temperature of the thermostat be lowered than when the active layer 32 is formed. Thus, to grow the In-doped layer 35, one line and a mass flow controller are preferably provided for the manufacturing facility where the respective layers are grown.
Next, as shown in
Thereafter, as shown in
Subsequently, as shown in
After that, as shown in
In the nitride-based semiconductor light-emitting device 100 of this preferred embodiment, when a voltage is applied between the n- and p-electrodes 42 and 41, holes will be injected from the p-electrode 41 into the active layer 32 and electrons will be injected from the n-electrode 42 into the active layer 32, thereby producing an emission at a wavelength of 450 nm, for example.
In the preferred embodiment described above, the AldGaeN layer 36 is supposed to have an Al mole fraction of 14 at % and the InGaN layer is supposed to have an In mole fraction of 10 at %. However, any other composition could be adopted as well. Also, the In-doped layer 35 and the p-AldGaeN layer 36 could be deposited directly on the active layer 32 with the undoped GaN layer 34 omitted.
Hereinafter, a third specific preferred embodiment of a nitride-based semiconductor light-emitting device according to the present invention will be described with reference to
As shown in
The GaN substrate 60 of this preferred embodiment has been formed by carrying out an ELO process. Specifically, to obtain the GaN substrate 60, a thick GaN layer to be the GaN substrate may be deposited on a sapphire wafer (not shown) with its surface partially masked with silicon dioxide, for example, but with the rest of the surface exposed. In that case, once the GaN layer has been formed, the sapphire wafer may be removed. Alternatively, titanium may be deposited in a net pattern on a GaN layer that has been formed on the sapphire wafer and then a GaN layer to be the GaN substrate may be deposited thereon. According to this method, once the upper GaN layer is obtained, the lower one is separated using the titanium net as a boundary.
In this preferred embodiment, the GaN substrate 60 is formed by ELO process, and therefore, has a non-uniform strain distribution.
The semiconductor multilayer structure 70 has the same structure as the counterpart 50 of the second preferred embodiment described above. Specifically, there is an active layer 32, including an AlaInbGacN crystal layer (where a+b+c=1, a≧0, b≧0 and c≧0), over the AlxGayInzN crystal layer 30. In this case, the active layer 32 is an electron injected region of the nitride-based semiconductor light-emitting device 200. On the active layer 32, arranged is an AldGaeN layer 36 (d+e=1, d>0, and e≧0) of a second conductivity type (e.g., p-type). In this preferred embodiment, the AldGaeN layer 36 is doped with Mg. Also, in this preferred embodiment, an undoped GaN layer 34 is sandwiched between the active layer 32 and the AldGaeN layer 36.
Also, an In-doped layer 35 forms at least a part of the AldGaeN layer 36. In the example illustrated in
On the AldGaeN layer 36 including the In-doped layer 35, arranged is a GaN layer 38 of the second conductivity type (e.g., p-type). And on the GaN layer 38, arranged is a contact layer 40, which is made of p+-GaN in this preferred embodiment.
According to this preferred embodiment, by providing the In-doped layer 35, it is possible to prevent the semiconductor multilayer structure 70 from producing a non-uniform in-plane distribution of strain and to reduce the number of crystal defects to be caused by such a non-uniform strain distribution. As a result, non-uniform in-plane emission can be eliminated.
Furthermore, as such a non-uniform strain distribution is produced more easily in a non-polar GaN as described above, the significance of this preferred embodiment would further increase in such an application.
Hereinafter, a fourth specific preferred embodiment of a nitride-based semiconductor light-emitting device according to the present invention will be described.
Currently, it is proposed that a substrate, of which the principal surface is a non-polar plane (e.g., a so-called m plane” that is a (10-10) plane that intersects with the [10-10] directions at right angles), be used to make an LED or a laser diode. Such a substrate is called an “m-plane GaN substrate”.
As shown in
a) schematically illustrates the crystal structure of a nitride-based semiconductor, of which the principal surface is a c plane, as viewed on a cross section thereof that intersects with the principal surface of the substrate at right angles. On the other hand,
Just like the nitride-based semiconductor light-emitting device 100 shown in
Such a GaN substrate, of which the principal surface is an m plane, may be obtained by growing a thick GaN crystal on a c-plane sapphire substrate, and then dicing the GaN crystal perpendicularly to the c plane of the sapphire substrate. Alternatively, a nitride-based semiconductor layer may be grown epitaxially on an m-plane GaN substrate. Even so, the nitride-based semiconductor layer also has an m plane as its principal surface.
The configuration and manufacturing process of this preferred embodiment are quite the same as what has already been described for the first preferred embodiment except that the principal surfaces of the GaN substrate 10 and the semiconductor multilayer structure 50 are m planes and that the selective growth is not carried out in this preferred embodiment. Thus, the description thereof will be omitted herein.
In an m-plane GaN substrate currently available, there is a distribution of dislocation densities within a plane. For example, the densities of dislocations in an en-plane GaN-based substrate may vary within the range of 105 cm−2 to 106 cm−2 within its plane, for example. And due to such a variation in the density of dislocations, there is a non-uniform strain distribution within a plane of the m-plane GaN substrate. That is why if a semiconductor layer is formed on such an m-plane GaN substrate, a non-uniform strain distribution will be produced within a plane of the semiconductor layer even if no selective growth is carried out. As a result, a semiconductor device that uses an m-plane GaN substrate will have a decreased quantum efficiency. When the present inventors actually irradiated such an m-plane GaN substrate with an X-ray, split peaks of X-ray diffraction were observed and it was confirmed that the degree of their split was greater than in a GaN substrate, of which the principal surface was a c plane (which will be referred to herein as a “c-plane GaN substrate”). These results revealed that a more complicated non-uniform strain distribution was produced in an m-plane GaN substrate than in a c-plane GaN substrate. Consequently, as far as the m-plane GaN substrate is concerned, it is particularly important to smooth out such a non-uniform strain distribution.
In was added had an In concentration of 7×1017 cm−3. It can be seen that the sample with the additive In clearly had a higher emission intensity and a better quantum efficiency than the sample with no additive In.
It should be noted that the surface (more particularly, the principal surface) of an actual m-plane semiconductor layer does not always have to be perfectly parallel to an m plane but may define a very small tilt angle (which is greater than 0 degrees but less than ±1 degree) with respect to an m plane. According to the manufacturing process technologies currently available, it is difficult to make a substrate or a semiconductor layer so that their surface is 100% parallel to an m plane. That is to say, if an m-plane substrate or an m-plane semiconductor layer is made by current manufacturing process technologies, the actual surface will slightly tilt with respect to the ideal m plane. However, as the tilt angle and tilt direction will vary from one manufacturing process to another, it is difficult to accurately control the tilt angle and tilt direction of the surface.
In some cases, the surface (or the principal surface) of a substrate or a semiconductor layer is tilted intentionally by one degree or more with respect to an m plane. In the preferred embodiment of the present invention to be described below, both a GaN substrate and nitride semiconductor layers grown thereon have their surface (principal surface) tilted intentionally with respect to an m plane.
Such a substrate, of which the principal surface defines a tilt angle of at least one degree with respect to an m plane just like the GaN substrate 10a of this preferred embodiment does, is generally called an “off-axis substrate”. The off-axis substrate is obtained by polishing the surface of a substrate that has been sliced off from a single crystal ingot so that its principal surface will be tilted intentionally in a particular direction with respect to an m plane.
In such an off-axis substrate currently available, of which the principal surface defines a tilt angle of 1 to 5 degrees with respect to an m plane, there is a distribution of dislocation densities (which may fall within the range of 105 cm−2 to 106 cm−2) within its plane. And due to such a variation in the density of dislocations, there is a non-uniform strain distribution within a plane of that GaN substrate 10a. That is why if a semiconductor layer is formed on such a GaN substrate 10a, a non-uniform strain distribution will be produced within a plane of the semiconductor layer even if no selective growth is carried out. Also, if a semiconductor multilayer structure 50a is stacked on the principal surface of such a GaN substrate 10a, then the surface (i.e., the principal surface) of those semiconductor layers will also tilt with respect to an m plane.
The configuration and manufacturing process of this preferred embodiment are the same as those of the fourth preferred embodiment of the present invention described above except that the respective principal surfaces of the GaN substrate 10a and the semiconductor multilayer structure 50a have been defined by such an off-axis process with respect to an m plane. And they are also the same as those of the first through third preferred embodiments of the present invention described above except that the respective principal surfaces of the GaN substrate 10a and the semiconductor multilayer structure 50a have been defined by such an off-axis process with respect to an m plane and that no selective growth is carried out. Thus, the same configuration and manufacturing process as those of the first through fourth preferred embodiments of the present invention described above will not be described in detail again. According to this preferred embodiment, after the semiconductor multilayer structure 50a has been formed on the GaN substrate 10a, part or all of the GaN substrate 10a could be removed.
Next, it will be described with reference to FIG. exactly how the GaN substrate 10a and semiconductor multilayer structure 50a of this preferred embodiment are tilted.
a) schematically illustrates the crystal structure (i.e., the wurtzite crystal structure) of the GaN substrate. The crystal structure will look as in
The c planes of a GaN crystal include +c planes and −c planes. Specifically, a +c plane is a (0001) plane on which Ga atoms are exposed, and is sometimes called a “Ga plane”. On the other hand, a −c plane is a (000-1) plane on which N (nitrogen) atoms are exposed, and is sometimes called an “N plane”. +c and −c planes are parallel to each other and both intersect with an m plane at right angles. Although c planes have polarities and can be classified into +c and −c planes in this manner, it is meaningless to classify non-polar a planes into +a and −a planes.
The +c-axis direction shown in
If the principal surface of a GaN substrate defines a tilt angle of at least one degree with respect to an m plane, it means that a normal to the principal surface of that GaN substrate defines a tilt angle of at least one degree with respect to a normal to the m plane.
Next, turn to
In each of the examples illustrated in
According to this preferred embodiment, when the tilt angle falls within the range of 1 degree to 5 degrees or the range of −5 degrees to −1 degree, the effect of the present invention can be achieved as significantly as in a situation where the tilt angle is greater than 0 degrees but less than ±1 degree. The reason will be described with reference to
If GaN based compound semiconductor layers are formed on such a GaN substrate 10a, the principal surface of those GaN based compound semiconductor layers will also have the same shape as that of the GaN substrate 10a. That is to say, the principal surface of those GaN based compound semiconductor layers will also have a number of steps and will also tilt overall with respect to the m plane.
The same phenomenon would also be observed even if the principal surface normal vector tilts toward neither the +c plane nor the −c plane but faces any other direction. But the same can be said even if the principal surface normal vector tilts in the a-axis direction, for example, and if the tilt angle falls within the range of 1 to 5 degrees.
Therefore, even if a nitride-based compound semiconductor light-emitting device has a GaN substrate 10a, of which the principal surface defines a tilt angle of 1 to 5 degrees in any arbitrary direction with respect to an m plane and if a layer including In at a concentration of 1×1016 atms/cm3 to 1×1019 aims/cm3 forms part of its AldGaeN layer, then the nitride-based semiconductor light-emitting device will not have a non-uniform strain distribution. Consequently, it is possible to prevent the nitride-based semiconductor light-emitting device from producing non-uniform emission within its plane.
It should be noted that if the absolute value of the tilt angle θ were greater than 5 degrees, then a piezoelectric field will be generated to decrease the internal quantum efficiency. If such a piezoelectric field were generated excessively, it would be much less meaningful to make a semiconductor light-emitting device by m-plane growth. For that reason, according to the present invention, the absolute value of the tilt angle θ is defined to at most 5 degrees. Nevertheless, even if the tilt angle θ is defined to be exactly 5 degrees, the actual tilt angle θ could be 5±1 degrees due to some variation involved with the real-world manufacturing process. It is difficult to totally eliminate such a variation involved with the manufacturing process and such a slight angular variation would never ruin the effect of the present invention and is negligible.
While the present invention has been described with respect to preferred embodiments thereof, the disclosed invention may be modified in numerous ways and may assume many embodiments other than those specifically described above.
The present invention provides a GaN-based semiconductor light-emitting device that hardly has a non-uniform strain distribution.
Number | Date | Country | Kind |
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2009-278619 | Dec 2009 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2010/007112 | 12/7/2010 | WO | 00 | 8/29/2011 |