Patent Application
20100230714
The present invention relates to a method for producing a gallium nitride based compound semiconductor light emitting device, in particular, a method for producing a gallium nitride based compound semiconductor light emitting device having excellent light emitting properties, and also relates to a gallium nitride based compound semiconductor light emitting device and a lamp using the same.
Priority is claimed on Japanese Patent Application No. 2006-080883, filed Mar. 23, 2006, the content of which is incorporated herein by reference.
In recent years, a GaN-based compound semiconductor material that is a nitride based semiconductor has been attracting attention as a semiconductor material for a light emitting device emitting short wavelength light. Using various oxides or group III-V compounds such as a sapphire single crystal as a substrate, a GaN-based compound semiconductor is formed on this substrate using a metalorganic chemical vapor deposition (MOCVD) method, a molecular beam epitaxy (MBE) method, or the like.
Characteristics of GaN-based compound semiconductor materials include small current diffusion in the lateral direction. Due to this, an electric current is applied only to the semiconductor located immediately below an electrode and the light emitted in a light emitting layer is screened by the electrode, which makes it difficult to emit light from the light emitting device to the outside. Accordingly, with such a light emitting device, a translucent positive electrode is usually used and light is emitted through the translucent positive electrode.
Conventional translucent positive electrodes have been configured as a layered structure in which an oxide such as Ni or Co is combined with a contact metal such as Au. In addition, in recent years, a layered structure, in which translucency is enhanced by reducing the film thickness of the contact metal as much as possible due to the use of oxides having higher electrical conductivity such as indium tin oxide (ITO), is adopted as a positive electrode, and it is configured so that the light from the light emitting layer can be efficiently emitted to the outside.
The external quantum efficiency is used as an indicator for the improvements in the output of such light emitting devices. When this external quantum efficiency is high, it is possible to say that the light emitting device has a high output.
The external quantum efficiency is represented by the product of internal quantum efficiency and light emission efficiency.
The internal quantum efficiency refers to the ratio at which the energy of an electric current applied to the device is converted to light. On the other hand, the light emission efficiency refers to the ratio of light emitted to the outside to light generated from the inside of a semiconductor crystal.
The internal quantum efficiency of the light emitting devices described above is said to have increased up to about 70 to 80% due to the improvements in the state or structure of the crystal, and thus satisfactory effects with respect to the amount of applied electric current have been attained.
However, in light emitting diodes in general and not only in the GaN-based compound semiconductor, the light emission efficiency with respect to the amount of electric current applied is low in most cases and it cannot be said that light emitted inside the devices is sufficiently emitted to the outside considering the amount of electric current applied.
The cause for this low light emission efficiency is that since the light emitting layer of a GaN-based compound semiconductor has a refractive index of about 2.5 which is considerably higher than that of air, which is 1, and has a small critical angle of about 25°, light is repeatedly reflected/absorbed inside the crystal and cannot be emitted to the outside.
In order to improve the light emission efficiency of light emitting devices, a gallium nitride based compound semiconductor light emitting device has been proposed in which light emission efficiency is improved by roughening a light emitting surface to provide various angles within the light emitting surface (for example, refer to Patent Document 1).
However, in the light emitting device described in Patent Document 1, a large resistance is generated between a translucent positive electrode and a p-type semiconductor layer due to the damage caused by the dry etching process which results in a problem of an extremely large driving voltage.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. Hei 6-291368
The present invention is made in view of the above problems and its object is to provide a method for producing a gallium nitride based compound semiconductor light emitting device that is excellent in terms of light emission efficiency and is also capable of operating at a low driving voltage, a gallium nitride based compound semiconductor light emitting device, and a lamp using the device.
The present inventors completed the present invention as a result of intensive studies in order to solve the above problems.
That is, the present invention relates to the following.
A first aspect of the present invention is a method for producing a gallium nitride based compound semiconductor light emitting device, the method characterized by including a first crystal growth step in which an n-type semiconductor layer, a light emitting layer, and a first p-type semiconductor layer which are formed of a gallium nitride based compound semiconductor are laminated in this order on a substrate; a second crystal growth step in which a second p-type semiconductor layer formed of a gallium nitride based compound semiconductor is further laminated thereon; an uneven pattern forming step in which an uneven pattern is formed on a surface of the first p-type semiconductor layer after the first crystal growth step and before the second crystal growth step; and a heat treatment step in which a heat treatment is carried out after the uneven pattern forming step.
A second aspect of the present invention is the method for producing a gallium nitride based compound semiconductor light emitting device according to the above first aspect characterized in that the heat treatment step is carried out in an oxygen atmosphere.
A third aspect of the present invention is the method for producing a gallium nitride based compound semiconductor light emitting device according to the above first aspect or second aspect characterized in that a heat treatment temperature in the heat treatment step is greater than or equal to 250° C.
A fourth aspect of the present invention is the method for producing a gallium nitride based compound semiconductor light emitting device according to the above first aspect or second aspect characterized in that a heat treatment temperature in the heat treatment step is greater than or equal to 500° C.
A fifth aspect of the present invention is the method for producing a gallium nitride based compound semiconductor light emitting device according to any one of the above first aspect to fourth aspect characterized in that the uneven pattern forming step includes the following intermediate steps (a) and (b):
(a) a step for forming a mask on the first p-type semiconductor layer; and
(b) a step for dry etching a surface of the first p-type semiconductor layer.
A sixth aspect of the present invention is the method for producing a gallium nitride based compound semiconductor light emitting device according to the above fifth aspect characterized in that the mask is formed on the first p-type semiconductor layer in the step (a) by carrying out a patterning process using a resist material.
A seventh aspect of the present invention is the method for producing a gallium nitride based compound semiconductor light emitting device according to the above fifth aspect characterized in that the step (a) includes a sub-step for forming a metal thin film on the first p-type semiconductor layer and a sub-step for carrying out a heat treatment after forming the metal thin film.
An eighth aspect of the present invention is the method for producing a gallium nitride based compound semiconductor light emitting device according to the above seventh aspect characterized in that the metal thin film formed in the step (a) remains in the first p-type semiconductor layer or on a surface thereof after a dry etching process in the step (b).
A ninth aspect of the present invention is the method for producing a gallium nitride based compound semiconductor light emitting device according to the above seventh aspect or eighth aspect characterized in that the metal thin film is made of a low melting point metal or a low melting point alloy which has a melting point of 100° C. to 450° C.
A tenth aspect of the present invention is the method for producing a gallium nitride based compound semiconductor light emitting device according to the above seventh aspect or eighth aspect characterized in that the metal thin film is made of Ni or a Ni alloy.
An eleventh aspect of the present invention is the method for producing a gallium nitride based compound semiconductor light emitting device according to any one of the above seventh aspect to tenth aspect characterized in that the metal thin film is made of a low melting point metal selected from the group consisting of Ni, Au, Sn, Ge, Pb, Sb, Bi, Cd, and In or a low melting point alloy containing at least one of these metals.
A twelfth aspect of the present invention is a gallium nitride based compound semiconductor light emitting device characterized by including an n-type semiconductor layer, a light emitting layer, and a first p-type semiconductor layer which are formed of a gallium nitride-based compound semiconductor and are laminated in this order on a substrate; a second p-type semiconductor layer formed of a gallium nitride based compound semiconductor further laminated thereon; and an uneven pattern formed on a surface of the first p-type semiconductor layer.
A thirteenth aspect of the present invention is the gallium nitride based compound semiconductor light emitting device according to the above twelfth aspect characterized in that a translucent positive electrode is laminated on the second p-type semiconductor layer, and the second p-type semiconductor layer and the translucent positive electrode are foamed unevenly so as to follow the uneven pattern formed on the surface of the first p-type semiconductor layer.
A fourteenth aspect of the present invention is a gallium nitride based compound semiconductor light emitting device obtained by any one of the above methods of first aspect to eleventh aspect.
A fifteenth aspect of the present invention is a lamp characterized in that any one of the above gallium nitride based compound semiconductor light emitting devices of twelfth aspect to fourteenth aspect is used.
According to the method for producing a gallium nitride based compound semiconductor light emitting device of the present invention, a gallium nitride based compound semiconductor light emitting device that is excellent in terms of light emission efficiency and is capable of operating at a low driving voltage is obtained.
In addition, the production method of the present invention has effects, especially the following effects (1) and (2).
(1) an uneven pattern with no damage caused by the dry etching process can be formed on the surface of the first p-type semiconductor layer.
(2) A crystal plane inclined with respect to the substrate normal direction is exposed in the surface since the second p-type semiconductor layer is laminated and regrown. Due to this, the step coverage characteristics of the translucent positive electrode improve.
Due to these effects, a gallium nitride based compound semiconductor light emitting device that is excellent in terms of light emission efficiency and is capable of operating at a low driving voltage is obtained.
In addition, since the lamp of the present invention is one that uses gallium nitride based compound semiconductor light emitting device of the present invention, a lamp that is highly reliable and has excellent light emitting properties is obtained.
1: Light emitting device; 2: Lamp; 11: Substrate; 12: Buffer layer; 13: n-type semiconductor layer; 14: Light emitting layer; 15: First p-type semiconductor layer; 16: p-contact layer (second p-type semiconductor layer); 17: Translucent positive electrode; 18: Positive electrode bonding pad; 19: Negative electrode bonding pad
One embodiment of the method for producing a gallium nitride based compound semiconductor light emitting device (hereinafter also referred to as a light emitting device), the gallium nitride based compound semiconductor light emitting device, and the lamp using the device which are the present invention will be described below while referring to
Note that the present invention is not limited to each element of the following embodiment and, for example, constituting elements in this embodiment may be combined where appropriate.
The method of the present invention for producing a gallium nitride based compound semiconductor light emitting device includes a first crystal growth step in which an n-type semiconductor layer 13, a light emitting layer 14, and a first p-type semiconductor layer 15 which are formed of a gallium nitride-based compound semiconductor are laminated in this order on a substrate 11 as shown in
In the method of the present invention for producing a light emitting device, driving voltage of the light emitting device is lowered and its light emission efficiency is improved by forming an uneven pattern having no damage caused by the etching process on the surface of the first p-type semiconductor layer 15 of the light emitting surface.
Examples of the major phenomena caused by the etching damages include an extreme reduction of impurity concentration in the surface of the p-type semiconductor layer. Various proposals have been made including the method described in Japanese Unexamined Patent Application, First Publication No. 2000-196152 in order to restore this damage.
However, the method proposed in Japanese Unexamined Patent Application, First Publication No. 2000-196152 does not fully restore the damage which results in a subsequent increase in the driving voltage. The present inventors discovered that the etching damage are sufficiently restored and the driving voltage reduces due to the method described below.
In
In the light emitting device of the present invention, an uneven pattern is formed on the surface of the first p-type semiconductor layer 15 and a p-contact layer 16 is further laminated thereon as in the example shown in
The light emitting device of the present invention will be described in detail below.
For the substrate 11, known substrate materials including oxide single crystals such as a sapphire single crystal (Al2O3; A-plane, C-plane, M-plane, and R-plane), a spinel single crystal (MgAl2O4), a ZnO single crystal, a LiAlO2 single crystal, a LiGaO2 single crystal and a MgO single crystal, a Si single crystal, a SiC single crystal, a GaAs single crystal, an AlN single crystal, a GaN single crystal, and boride single crystals such as a ZrB2 single crystal are used. Also in the present invention, any substrate materials including these known substrate materials can be used without any limitations. Among these, a sapphire single crystal and a SiC single crystal are particularly preferable.
Note that the plane direction of a substrate is not particularly limited. In addition, the substrate may be a just substrate or a substrate having an off angle.
The n-type semiconductor layer 13, the light emitting layer 14, and the first p-type semiconductor layer 15 which are formed of a gallium nitride-based compound semiconductor are usually laminated on the substrate 11 with the buffer layer 12 interposed therebetween. Note that there are cases where the buffer layer 12 is not required depending on the type of substrate used or the growth conditions of an epitaxial layer.
In addition, in the light emitting device of the present invention, an uneven pattern is formed on the surface of the first p-type semiconductor layer 15 and the p-contact layer (second p-type semiconductor layer) 16 is further laminated on the first p-type semiconductor layer 15 as in the example shown in
Various kinds of gallium nitride based compound semiconductors have been known which are represented by general formula AlXGaYInZN1-AMA (0≦X<1, 0≦Y≦1, 0≦Z≦1, as well as X+Y+Z=1, and the symbol M represents a group V element different from nitrogen (N) and 0≦A<1). The present invention can also use any kind of gallium nitride based compound semiconductor represented by the general formula AlXGaYInZN1-AMA (0≦X<1, 0≦Y≦1, 0≦Z≦1, as well as X+Y+Z=1, and the symbol M represents a group V element different from nitrogen (N) and 0≦A<1) including those already known gallium nitride based compound semiconductors.
The gallium nitride based compound semiconductor can contain other group III elements in addition to Al, Ga, and In and elements such as Ge, Si, Mg, Ca, Zn, Be, P, As, and B can also be contained if necessary. Moreover, the elements contained are not limited to those added intentionally and there are also cases where the semiconductor contains impurities that are inevitably contained depending on the film forming conditions and the like as well as the trace impurities contained in the source materials and the reaction tube materials.
The method for growing gallium nitride based compound semiconductor is not particularly limited and all the methods that are known to grow a nitride semiconductor such as the metalorganic chemical vapor deposition (MOCVD) method, the hydride vapor phase epitaxy (HVPE) method, and the molecular beam epitaxy (MBE) method can be applied. The preferable growing method is the MOCVD method in view of its film thickness controllability and mass productivity.
In the MOCVD method, hydrogen (H2) or nitrogen (N2), trimethylgallium (TMG) or triethylgallium (TEG), trimethylaluminum (TMA) or triethylaluminum (TEA), trimethylindium (TMI) or triethylindium (TEI), and ammonia (NH3), hydrazine (N2H4), or the like are used as a carrier gas; a Ga source, an Al source, and an In source which are group III materials; and an N source which is a group V material, respectively. In addition, as a dopant, monosilane (SiH4) or disilane (Si2H6), and germane gas (GeH4) or organic germanium compounds such as tetramethylgermanium ((CH3)4Ge) and tetraethylgermanium ((C2H5)4Ge) can be used in the n-type semiconductor as a Si source material and a Ge source material, respectively.
In the MBE method, elemental germanium can also be used as a doping source. For example, biscyclopentadienyl magnesium (Cp2Mg) or bisethylcyclopentadienyl magnesium (EtCp2Mg) is used as an Mg source material for the p-type semiconductor.
The n-type semiconductor layer 13 is usually configured from a underlying layer, an n-contact layer, and an n-cladding layer. The n-contact layer can also serve as an underlying layer and/or an n-cladding layer.
The underlying layer is preferably constituted from an AlXGa1-XN layer (0≦X≦1, preferably 0≦X≦0.5 and more preferably 0≦X≦0.1). Film thickness of the underlying layer is preferably greater than or equal to 0.1 μm, more preferably greater than or equal to 0.5 μm, and most preferably greater than or equal to 1 μm. The AlXGa1-XN layer with satisfactory crystallinity is likely to be obtained by making the film thickness greater than or equal to 1 μm.
Although n-type impurities may be doped in the underlying layer as long as the concentration thereof is within the range of 1×1017 to 1×1019/cm3, it is preferable that the underlying layer is undoped (that is, the concentration is <1×1017/cm3) in view of the maintenance of satisfactory crystallinity. Although n-type impurities are not particularly limited, examples thereof include Si, Ge, and Sn and Si and Ge are preferable.
Growth temperature for growing the underlying layer is preferably 800 to 1,200° C. and it is more preferable to adjust it within the range of 1,000 to 1,200° C. The underlying layer with satisfactory crystallinity is obtained when it is grown within this temperature range. In addition, it is preferable to adjust the pressure inside the MOCVD growth furnace within the range of 15 to 40 kPa.
Similar to the underlying layer, the n-contact layer is preferably constituted from an AlXGa1-XN layer (0≦X≦1, preferably 0≦X≦0.5 and more preferably 0≦X≦0.1). In addition, it is preferable that n-type impurities are doped and it is preferable to contain n-type impurities within a concentration range of 1×1017 to 1×1019/cm3, more preferably 1×1018 to 1×1019/cm3 for maintaining a satisfactory ohmic contact with the negative electrode, suppressing the generation of cracks, and maintaining satisfactory crystallinity. Although n-type impurities are not particularly limited, examples thereof include Si, Ge, and Sn and Si and Ge are preferable. Preferable range of the growth temperature for the n-contact layer is the same as that of the underlying layer.
It is preferable that the gallium nitride based compound semiconductor constituting the n-contact layer has the same composition as that of the underlying layer and it is preferable to set the total thickness of these film layers within the range of 1 to 20 μm, more preferably 2 to 15 μm, and even more preferably 3 to 12 μm. The crystallinity of semiconductor is satisfactorily maintained when the sum of the film thickness of the n-contact layer and the underlying layer is within this range.
It is preferable to provide an n-cladding layer between the n-contact layer and the light emitting layer 14. By providing an n-cladding layer, the areas with impaired smoothness generated on the outermost surface of the n-contact layer can be filled. The n-cladding layer can be formed with AlGaN, GaN, GaInN, or the like. In addition, it is also possible to configure a heterojunction of these structures or a superlattice structure where these structures are laminated several times. Needless to say, when GaInN is used, it is desirable that its bandgap is larger than that of GaInN in the light emitting layer.
Although the film thickness of the n-cladding layer is not particularly limited, it is preferably within the range of 0.005 to 0.5 μm and more preferably within the range of 0.005 to 0.1 μm.
In addition, the n-type dopant concentration in the n-cladding layer is preferably within the range of 1×1017 to 1×1020/cm3 and more preferably within the range of 1×1018 to 1×1019/cm3. It is preferable that the dopant concentration be within this range for maintaining satisfactory crystallinity and for reducing the operating voltage of the light emitting device.
A light emitting layer constituted of a gallium nitride based compound semiconductor, preferably a gallium nitride based compound semiconductor made of Ga1-SInSN (0<S<0.4) is usually used as the light emitting layer 14 that is laminated on the n-type semiconductor layer 13.
Although the film thickness of the light emitting layer 14 is not particularly limited, the film thickness where the quantum effect is achieved, that is, the critical film thickness is preferable and it is within the range of 1 to 10 nm, for example, and more preferably within the range of 2 to 6 nm. It is preferable that the film thickness be within the above range from a viewpoint of emission power.
In addition, apart from the abovementioned single quantum well (SQW) structure, the light emitting layer may also be configured as a multiple quantum well (MQW) structure constituted from the abovementioned Ga1-SInSN layer as a well layer and an AlcGa1-cN (0≦c<0.3 as well as b>c) barrier layer having a higher bandgap energy than that of the well layer. Moreover, the well layer and the barrier layer may be doped with impurities.
Growth temperature of the AlcGa1-cN barrier layer is preferably greater than or equal to 700° C., and it is more preferable to grow the layer at a temperature of 800° C. to 1,100° C. since satisfactory crystallinity is achieved. In addition, the GaInN well layer is grown at a temperature of 600 to 900° C., more preferably 700 to 900° C. In other words, the growth temperature is preferably changed for different layers in order to achieve satisfactory crystallinity of the MQW structure.
The first p-type semiconductor layer 15 is usually constituted from a p-cladding layer and a p-contact layer. However, the p-contact layer may also serve as the p-cladding layer.
The p-cladding layer is not particularly limited as long as it has a composition that has a higher bandgap energy than that of the light emitting layer and it can confine carries in the light emitting layer 14, and the preferable examples thereof include an AldGa1-dN (0≦d≦0.4, preferably 0.1≦d≦0.3) layer. It is preferable that the p-cladding layer be formed from such an AlGaN layer from the viewpoint of the confinement of carriers in the light emitting layer 14.
Although the film thickness of the p-cladding layer is not particularly limited, it is preferably within the range of 1 to 400 nm and more preferably within the range of 5 to 100 nm.
The p-type dopant concentration in the p-cladding layer is preferably within the range of 1×1018 to 1×1021/cm3 and more preferably within the range of 1×1019 to 1×102°/cm3. Satisfactory p-type crystals are obtained without reducing the crystallinity thereof when the p-type dopant concentration is within the abovementioned range.
The p-contact layer is a gallium nitride based compound semiconductor layer containing at least AleGa1-eN (0≦e<0.5, preferably, 0≦e≦0.2, more preferably, 0≦e≦0.1). It is preferable that the Al composition be within the above range for maintaining satisfactory crystallinity and for satisfactory ohmic contact with a p-ohmic electrode.
In addition, it is preferable that the p-type dopant concentration be within the range of 1×1018 to 1×1021/cm3 for maintaining a satisfactory ohmic contact, preventing the occurrence of cracks, and maintaining satisfactory crystallinity, and more preferably, the concentration is within the range of 5×1019 to 5×1020/cm3.
Although p-type impurities are not particularly limited, preferable examples thereof include Mg.
The thickness of the p-contact layer is not particularly limited, but is preferably within the range of 0.2 to 4 μm and more preferably 0.4 to 2 μm. It is preferable that the film thickness be within this range from a viewpoint of emission power.
An uneven pattern is formed on the surface of the first p-type semiconductor layer in the light emitting device of the present invention as in the example shown in
In the present invention, by adopting a configuration where an uneven pattern is formed on the surface of the first p-type semiconductor layer 15, which is the light emitting surface, a light emitting device with excellent light emitting properties can be achieved due to the reduction in the driving voltage of the light emitting device and the improvements in the light emission efficiency.
As described earlier, in the light emitting device of the present invention, the p-contact layer (second p-type semiconductor layer) 16 is laminated on the first p-type semiconductor layer 15, in which an uneven pattern is formed on the surface thereof. In addition, in the example shown in
Preferable film thickness of the p-contact layer 16 differs depending on the method used in the step for forming an uneven pattern on the surface of the first p-type semiconductor layer, on which the p-contact layer 16 is laminated. For example, when the formation of an uneven pattern is conducted by preparing a common resist mask with any given pattern due to a photolithography method, the film thickness is preferably within the range of 50 nm to 500 nm.
When the film thickness of the p-contact layer 16 is too large, the uneven pattern will be planarized. Accordingly, it is not preferable from the viewpoint of light emission. On the other hand, when the film thickness of the p-contact layer 16 is too small, an inclined crystal plane does not develop, and thus it is not preferable from the viewpoint of the step coverage characteristics of a translucent positive electrode.
As shown in the example of
For example, metals such as Au, Ni, Co, Cu, Pd, Pt, Rh, Os, Ir, and Ru can be used for the translucent positive electrode 17. In addition, the translucent positive electrode may contain a transparent oxide such as ITO, NiO, and CoO. The transparent oxide may be contained in the abovementioned metal film in the form of a mass or may be contained by being superposed on the abovementioned metal film in the form of a layer. These known materials and structures can be adopted without any limitations.
In addition, the translucent positive electrode 17 can be formed, for example, by using materials that include at least one of the transparent oxides such as ITO(In2O3—SnO2), AZnO(ZnO—Al2O3), IZnO(In2O3—ZnO), and GZO(ZnO—GeO2) by a means that has been well known in this technical field. Moreover, the translucent positive electrode can adopt any structures without any limitations including the structures already known.
The translucent positive electrode 17 may be formed so as to cover substantially the entire surface of the p-contact layer 16, or it may be formed in a lattice shape or a tree shape with gaps present. After the translucent positive electrode 17 is formed, a thermal annealing process may be conducted for alloying or increasing transparency, or it may not be conducted.
In addition, as shown in
A positive electrode bonding pad 18 is provided on the translucent positive electrode 17 as shown in the example of
As the materials for the positive electrode bonding pad 18, various structures using, for example, Au, Al, Ni, Cu, or the like are known and these known materials and structures can be used without any limitations.
It is preferable that the thickness of the positive electrode bonding pad 18 be within the range of 100 to 1000 nm. In addition, the bonding pad has characteristics that, as the thickness thereof increases, bondability is improved. Therefore, it is more preferable that that thickness of the positive electrode bonding pad 18 be greater than or equal to 300 nm. In addition, it is preferable that the thickness of the positive electrode bonding pad be less than or equal to 500 nm from the viewpoint of manufacturing cost.
A negative electrode bonding pad 19 is formed so as to come into contact with the n-type semiconductor layer 13 as shown in the example of
For this reason, when the negative electrode bonding pad 19 is formed, the light emitting layer 14 and the p-type semiconductor layer 15 are partially removed to expose the n-contact layer of the n-type semiconductor layer 13, and the negative electrode bonding pad 19 is formed on the exposed portion.
As the materials for the negative electrode bonding pad 19, negative electrodes having various compositions and structures are known and these known negative electrodes can be used without any limitations. They can be formed by a means that has been well known in this technical field.
As described earlier, the method of the present invention for producing a gallium nitride based compound semiconductor light emitting device includes a first crystal growth step in which an n-type semiconductor layer 13, a light emitting layer 14, and a first p-type semiconductor layer 15 which are formed of a gallium nitride based compound semiconductor are laminated in this order on a substrate 11 as shown in
In the method for producing a light emitting device of the present invention, an uneven pattern is formed on a wafer (uneven pattern forming step), in which the aforementioned first crystal growth step is completed. Then a heat treatment step is conducted, and thereafter, a second crystal growth step, in which a p-type semiconductor layer alone is laminated again, is conducted. In the production method of the present invention, the heat treatment step and the second crystal growth step play a role in rediffusing impurities to the portions where the concentration of impurities is reduced due to the dry etching process.
It is possible to configure the uneven pattern forming step of the present invention to include the following intermediate steps (a) and (b).
(a) a step for forming a mask on the first p-type semiconductor layer; and
(b) a step for dry etching the surface of the first p-type semiconductor layer.
In addition, it is possible to configure the uneven pattern forming step of the present invention so that the abovementioned step (a) includes a sub-step for forming a metal thin film on the first p-type semiconductor layer and a sub-step for carrying out a heat treatment after forming the metal thin film.
The method for forming an uneven pattern on the surface of the first p-type semiconductor layer will be described in detail below using the example of a light emitting device 1 shown in
First, a wafer is prepared, in which the n-type semiconductor layer 13, the light emitting layer 14, and the first p-type semiconductor layer 15 which are formed of a gallium nitride based compound semiconductor are laminated in this order on the substrate 11 with the buffer layer 12 interposed therebetween, and a common resist mask is formed on the wafer with any given pattern due to the photolithography method. Shape of the pattern at this stage may be circular, elliptic, polygonal, stripe, or the like and is selected depending on the size and shape of the light emitting device, the emission wavelength, or the like. Thereafter, the first p-type semiconductor layer 15 is etched using a common dry etching apparatus. Etching depth is preferably equal to or less than the film thickness of the first p-type semiconductor layer 15. The resist mask is removed after the dry etching process due to the organic cleaning process or the like.
Note that a metal mask (metal thin film) may be used in addition to the resist mask. When a metal mask is used, there are two ways for using it; i.e., the mask may be used in a similar manner to that of the resist mask described earlier or the mask may be formed with granules thereon by exploiting the agglomeration effects of metals. When a mask is formed by taking advantage of the agglomeration effects, the metal material is preferably a metal with a low melting point selected from the group consisting of Ni, Au, Sn, Ge, Pb, Sb, Bi, Cd, and In or an alloy with a low melting point containing at least one of these metals. In addition, the aforementioned metal with a low melting point and the alloy with a low melting point preferably have a melting point within the range of 100° C. and 450° C.
The mask is formed on the wafer with any given pattern using these metals and employing a common film forming method such as a deposition method, a sputtering method, or the like, and thereafter, the metal is agglomerated by applying a heat treatment suited for the material. For example, when Ni or an Ni alloy is used, it is preferable to conduct a heat treatment within a temperature range of 300° C. and 600° C. in the presence of oxygen. A preferable shape in terms of light emission can be obtained by conducting a heat treatment in the above condition. An uneven pattern which reflects that of the granules of the mask is formed on the surface of the first p-type semiconductor layer by conducting a dry etching process.
Note that in the production method of the light emitting device of the present invention, it may be configured so that the metal thin film formed in the aforementioned step (a) remains in the first p-type semiconductor layer 15 or on the surface thereof after the dry etching process in the aforementioned step (b). The contact resistance between the first p-type semiconductor layer 15 and the p-contact layer (second p-type semiconductor layer) 16 can be reduced even further by the remaining metal thin film.
The heat treatment step will be described below.
In the heat treatment step of the present invention, the treatment temperature is preferably greater than or equal to 250° C. and more preferably greater than or equal to 500° C. In addition, the heat treatment is preferably conducted in the presence of oxygen. However, it should be noted that when the heat treatment temperature is greater than or equal to 1,100° C., it will be likely that the MQW structure is damaged and the reverse voltage characteristics thereof may be adversely affected.
A light emitting device with a low driving voltage is obtained by conducting a heat treatment in the above conditions.
In addition, although the role of oxygen in this heat treatment step is not clear yet, satisfactory experimental results are obtained when it is present (refer to the Examples described later).
Since it is not preferable to conduct a heat treatment step in the presence of oxygen in an MOCVD furnace where hydrogen is used for safety reasons, it is preferable to carry out the heat treatment step in a heating apparatus other than the MOCVD furnace.
The second crystal growth step will be described in detail below.
In the present invention, an uneven pattern is formed by the aforementioned uneven pattern forming step and the heat treatment step on the surface of the first p-type semiconductor layer 15 that is laminated in the first crystal growth step, and thereafter, the p-contact layer (second p-type semiconductor layer) is further laminated in the second crystal growth step.
The p-contact layer laminated in the second crystal growth step of the present invention is a gallium nitride based compound semiconductor layer containing at least AleGa1-eN (0≦e<0.5, preferably, 0≦e≦0.2, more preferably, 0≦e≦0.1) as described earlier. It is preferable that the Al composition be within the above range for maintaining satisfactory crystallinity and for satisfactory ohmic contact with a p-ohmic electrode. In addition, it is preferable that the p-type dopant concentration be within the range of 1×1018 to 1×1021/cm3 for maintaining a satisfactory ohmic contact, preventing the occurrence of cracks, and maintaining satisfactory crystallinity, and more preferably, the concentration is within the range of 5×1019 to 5×1020/cm3. Although p-type impurities are not particularly limited, preferable examples thereof include Mg.
Film thickness of the p-contact layer differs depending on the method used in the step for forming an uneven pattern on the surface of the first p-type semiconductor layer, on which the p-contact layer is laminated. For example, when the formation of an uneven pattern is conducted by preparing a common resist mask with any given pattern due to a photolithography method, the film thickness is preferably within the range of 50 to 500 nm. When the film thickness of the p-contact layer is too large, the uneven pattern will be planarized. Accordingly, it is not preferable from the viewpoint of light emission. On the other hand, when the film thickness of the p-contact layer is too small, an inclined crystal plane does not develop, and thus it is not preferable from the viewpoint of the step coverage characteristics of a translucent positive electrode.
According to the method for producing a gallium nitride based compound semiconductor light emitting device of the present invention, a gallium nitride based compound semiconductor light emitting device that is excellent in terms of light emission efficiency and is capable of operating at a low driving voltage is obtained by adopting the abovementioned configuration.
In addition, the production method of the present invention has effects, especially the following effects (1) and (2).
(1) an uneven pattern with no damage caused by the dry etching process can be formed on the surface of the first p-type semiconductor layer.
(2) A crystal plane inclined with respect to the substrate normal direction is exposed in the surface since the second p-type semiconductor layer is laminated and regrown. Due to this, the step coverage characteristics of the translucent positive electrode improve.
Due to these effects, a gallium nitride based compound semiconductor light emitting device that is excellent in terms of light emission efficiency and is capable of operating at a low driving voltage is obtained.
A lamp can be formed by providing a transparent cover to the gallium nitride based compound semiconductor light emitting device of the present invention due to, for example, a known means for those skilled in the art. In addition, it is possible to form a white lamp by combining the gallium nitride based compound semiconductor light emitting device of the present invention with a cover including a phosphor.
The lamp 2 can be produced by a conventionally known method using the light emitting device 1 of the present invention shown in
Note that the lamp of the present invention can be used for various types of lamps, such as a general-purpose cannonball shaped lamp, a side view type lamp as a backlight of a mobile phone, or a top view type lamp used for a display device.
Since the light emitting device of the present invention has excellent light emission efficiency, it is possible to achieve a lamp having excellent light emitting properties.
Next, the light emitting device of the present invention and the lamp using the device will be described in more detail with reference to Examples. However, the present invention is not limited to these Examples.
A gallium nitride based compound semiconductor layer was laminated on a sapphire substrate 1 with a buffer layer 12 made of AlN interposed therebetween. The gallium nitride based compound semiconductor layer was formed by laminating the n-type semiconductor layer 13, the light emitting layer 14, and the first p-type semiconductor layer 15 in this order. The n-type semiconductor layer 13 was formed by laminating an undoped GaN underlying layer with a thickness of 8 μm, a Ge-doped n-type GaN contact layer with a thickness of 2 μm, and an n-type In0.1 Ga0.9N cladding layer with a thickness of 0.02 μm in this order. The light emitting layer 14 having a multiple quantum well structure was formed by providing a barrier layer on five laminates of a Si-doped GaN barrier layer with a thickness of 16 nm and an In0.06Ga0.94N well layer with a thickness of 2.5 nm. The p-type semiconductor layer 15 was formed by laminating a Mg-doped p-type Al0.07Ga0.93N cladding layer with a thickness of 0.01 μm and a Mg-doped p-type Al0.02Ga0.98N contact layer with a thickness of 0.5 μm. The semiconductor was configured such that light was emitted from the surface of the semiconductor layer.
Next, a resist mask was formed in a dot pattern on the aforementioned p-contact layer using a common photolithography method. After the dry etching process at a depth of 0.4 μm, the resist mask was removed by the organic cleaning process. Then an annealing process was conducted at 600° C. for 1 minute in an atmosphere where the oxygen concentration was 1%.
Next, a Mg-doped p-type Al0.02Ga0.98N contact layer 16 with a thickness of 0.1 μm was further laminated on the p-type semiconductor layer 15, in which the uneven pattern was formed on the surface thereof.
In this structure, the carrier concentration of the n-type GaN contact layer was 1×1019 cm3, and the doping amount of Si in the GaN barrier layer was 1×1017 cm−3. The carrier concentration of the p-type AlGaN contact layer was 5×1018 cm−3, and the doping amount of Mg in the p-type AlGaN cladding layer was 5×1019 cm−3.
In addition, the laminated structure of the gallium nitride based compound semiconductor layer (reference numerals 12, 13, 14, 15, and 16 in
Then, the n-type GaN contact layer in a region for forming the negative electrode was exposed from the gallium nitride based compound semiconductor layer by a reactive ion etching method.
First, a resist was uniformly applied on the entire surface of the p-type semiconductor layer to faun an etching mask, and the resist was then removed from the region for forming the negative electrode by a known lithography technique. Then, the laminated structure was put into a vacuum deposition apparatus, and Ni and Ti were deposited at a pressure of 4×10−4 Pa or less by an electron beam method to form films with thicknesses of 50 nm and 300 nm, respectively. Thereafter, the resist and a metal film other than the region for forming the negative electrode were removed by a lift-off technique.
Then, a semiconductor laminated substrate was mounted on the electrode in an etching chamber of the reactive ion etching apparatus and the pressure of the etching chamber was reduced to 10−4 Pa. Thereafter, the semiconductor laminate was etched until the n-type GaN contact layer was exposed by supplying Cl2, serving as an etching gas. After the etching process, the semiconductor laminate was taken out from the reactive ion etching apparatus, and the etching mask was removed by nitric acid and fluoric acid.
Next, a Ni contact metal layer with a thickness of 1 nm and a current spreading layer made of ITO were formed only in the region for forming the positive electrode on the p-type AlGaN contact layer 16 employing a known photolithography technique and a known lift-off technique. In the formation of the current spreading layer, the substrate having the gallium nitride based compound semiconductor layer laminated thereon was first put into a vacuum sputtering apparatus and ITO with a thickness of 1 μm was deposited on the Ni contact metal layer.
Then, the substrate was taken out from the vacuum chamber, and by performing a heat treatment in order to improve transparency, the translucent positive electrode 17 was formed.
Next, a first layer made of Au, a second layer made of Ti, a third layer made of Al, a fourth layer made of Ti, and a fifth layer made of Au were sequentially laminated on a portion of the translucent positive electrode by a known procedure usually called lift-off and the same deposition method as described above, thereby forming the positive electrode bonding pad 18.
Then, the negative electrode bonding pad 19 was formed on the exposed n-type GaN contact layer by the following procedures. A resist was uniformly applied on the entire surface of the n-type GaN contact layer, and the resist was removed from a portion for forming the negative electrode on the exposed n-type GaN contact layer by a known lithography technique. Then, Ti and Au were respectively deposited with a thickness of 100 nm and 200 nm on the semiconductor by a general vacuum deposition method in this order, thereby forming the negative electrode bonding pad 19. Then, the resist was removed by a known method.
Then, the back surface of the substrate 11 in the wafer having the positive electrode and the negative electrode formed thereon as described above was ground/polished such that the thickness of the substrate 11 was reduced to 80 μm, and a laser scriber was used to mark the semiconductor. Then, the semiconductor was cut into chips each having a 350 μm square.
The chip obtained as described above was mounted on a stem and an electric current of 20 mA was applied thereto to measure a forward voltage, which was 3.3 V. The emission power was 13 mW.
In addition, the chip was mounted to a TO-18 package, and the emission power thereof was measured by a tester. When an electric current of 20 mA was applied, the emission power was 20 mW. The emission power was about 1.3-fold that of a chip, in which no uneven pattern was formed in the p-type semiconductor layer. In addition, in terms of light emission distribution in the light emitting surface, it was verified that light was emitted from the entire surface of the positive electrode.
The light emitting devices of Examples 2 to 6 and Comparative Example 1 were produced in a similar manner to that of Example 1 except that the heat treatment conditions were changed to those shown in the following Table 1. In addition, the same evaluations as those made in Example 1 were made and the results thereof are shown in the following Table 1.
As shown in Table 1, the emission powers were greater than or equal to 12 mW and the driving voltages were within the range of 3.3 and 3.8 V in all the light emitting devices of Examples 2 to 6, in which the formation of an uneven pattern on the surface of the first p-type semiconductor layer 15 and the heat treatment thereto were conducted and the p-contact layer 16 is further laminated thereon in the conditions specified by the production method of the present invention.
On the other hand, in the light emitting device of Comparative Example 1 where a heat treatment was not conducted after the formation of an uneven pattern on the surface of the first p-type semiconductor layer, the emission power was low (10 mW) and the driving voltage was high (4.0 V) compared to those exhibited by the light emitting devices of the respective Examples.
An AlN buffer layer 12 and gallium nitride based compound semiconductor layers 13 to 15 were laminated on a sapphire substrate in a similar manner to that in Example 1. Then, the resulting laminated body was put in a deposition apparatus and Ni was deposited with a thickness of 150 nm on the entire surface of the p-contact layer.
Next, a heat treatment was performed at a temperature of 600° C. in an atmosphere where the oxygen concentration was 0.2% to agglomerate the thin film of the deposited Ni, thereby forming a mask made of metal particles. The mask was subjected to a dry etching process at a depth of 0.4 μm. It was verified by microscopic observation that the trace amount of Ni remained on the surface after the dry etching process.
Then, an annealing process was conducted at 600° C. for 1 minute in an atmosphere where the oxygen concentration was 1%. Thereafter, a Mg-doped p-type Al0.02Ga0.98N contact layer with a thickness of 0.1 μm was further laminated thereon.
The method for forming the positive electrode and the negative electrode, the process for grinding/polishing the back surface of the substrate, and the method for dividing semiconductor laminates into chips were the same as those employed in Example 1. The obtained chip was mounted on a stem and an electric current of 20 mA was applied thereto to measure a forward voltage, which was 3.2 V. The integrating sphere output was 14 mW.
In addition, the chip was mounted to a TO-18 package, and the emission power thereof was measured by a tester. When an electric current of 20 mA was applied, the emission power was 22 mW. In addition, in terms of light emission distribution in the light emitting surface, it was verified that light was emitted from the entire surface of the positive electrode.
It is apparent from the abovementioned results that the light emitting device obtained by the production method of the present invention has excellent light emission efficiency and high device characteristics.
The present invention can be applied to a method for producing a gallium nitride based compound semiconductor light emitting device, in particular, a method for producing a gallium nitride based compound semiconductor light emitting device having excellent light emitting properties, and can also be applied to a gallium nitride based compound semiconductor light emitting device and a lamp using the same.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-080883 | Mar 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/056007 | 3/23/2007 | WO | 00 | 9/19/2008 |
