This application is based upon and claims the benefit of priority of the Japanese Patent Application No. 2008-180656, filed on Jul. 10, 2008, the entire contents of which are incorporated herein by reference.
1. Field
The present invention relates to a semiconductor light emitting device and its manufacture method, and more particularly to a nitride semiconductor light emitting device and its manufacture method.
2. Related Art
External emission efficiency of a nitride semiconductor light emitting device is desired to be further improved. Description will be made by using a typical nitride semiconductor light emitting device as an example. On a transparent substrate such as sapphire, a GaN buffer layer grown at a low temperature, an n-type GaN layer, an emission layer and a p-type GaN layer are grown and laminated in this order by metal organic chemical vapor deposition (MOCVD) or the like. Band gap of GaInN formed by replacing a portion of Ga of GaN with In becomes narrower than that of GaN, whereas band gap of GaAlN formed by replacing a portion of Ga of GaN with Al becomes broader than that of GaN. AlN may be epitaxially grown on GaN. AlxInyGaN (0≦x≦1, 0≦y≦1, 0<z≦1, x+y+z=1) is called nitride semiconductor. It is possible to adjust the characteristics of a semiconductor light emitting device, by selecting compositions of AlGaInN.
Emission in an emission layer is omnidirectional. Light propagating toward a flat surface and reaching at an incidence angle of a critical angle or larger is totally reflected at the surface. It is not easy to externally emit light once totally reflected at the surface. It is known that if texture (irregularity) is formed on the surface, it becomes possible to improve an external emission efficiency.
Japanese Patent Publication No. 3469484 proposes a method of processing the surface of a p-type nitride semiconductor layer by dry etching or ion milling to form texture (irregularity).
There is a possibility that dry etching or ion milling may damage a processed layer and increase contact resistance. Development is being made to further improve external emission efficiency.
It is an object of the present invention to provide a nitride semiconductor light emitting device with an improved external emission efficiency and a method for manufacturing a nitride semiconductor light emitting device.
According to one aspect of the present invention, there is provided a nitride semiconductor light emitting device comprising:
a substrate for growing nitride semiconductor of a hexagonal crystal structure;
a first nitride semiconductor layer of a first conductivity type formed above the substrate;
an active layer formed on the first nitride semiconductor layer for emitting light when current flows;
a second nitride semiconductor layer of a second conductivity type opposite to the first conductivity type, formed on the active layer;
texture formed above at least a partial area of the second nitride semiconductor layer and having a plurality of protrusions of a pyramid shape, each of the protrusions including a lower layer made of nitride semiconductor doped with impurities of the second conductivity type and an upper layer made of nitride semiconductor not intentionally doped with impurities; and
a transparent electrode covering surfaces of the second nitride semiconductor layer and the texture.
The present inventors have studied improvement of external emission efficiency of a semiconductor light emitting device by forming texture on the device surface. It is expected that in order not to influence an emission mechanism, it is preferable to improve external emission efficiency by forming an additional structure without changing the structure of a light emitting device. An additional structure capable of reducing total reflection is desired. It is desired that emission efficiency itself will not be lowered even if an additional structure is provided. The present inventors propose that by following these fundamental policies, surface texture capable of improving external emission efficiency is grown on the emission structure.
Nitride semiconductor is made of AlxInyGa2N (0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1) having a crystal structure of hexagonal symmetry. The buffer layer 11 is, for example, a GaN layer grown at a low temperature and crystallized through high temperature anneal. The buffer layer 11 and underlying layer 12 are auxiliary constituent elements to be used for crystal growth of the semiconductor light emitting device, are not positive constituent elements, and non-doped. The n-type layer 21 is, for example, an Si-doped n-type GaN layer. The active layer 22 is an emission region of a multiple quantum well constituted of a GaN/GaN repetitive multilayer. The emission layer may also be formed by a double hetero structure such as GaN/InGaN/GaN or a non-doped InGaN layer. The clad layer 13 is a wide gap layer providing the active layer with a carrier/light confinement effect, and, for example, a p-type AlGaN layer. The p-type layer 23 has preferably low resistance, and is, for example, an Mg-doped p-type GaN layer.
Each protrusion of the texture 30 has a six-sided pyramid shape originating from the crystal structure of AlxInyGazN, and is constituted of, e.g., an Mg-doped p-type GaN lower layer 31 and a non-doped GaN upper layer 32. The texture 30 has a function of improving external emission efficiency. The transparent electrode 40 is, for example, an ITO layer. The p-side pad-electrode 50 is, for example, a TiAu layer. The n-side electrode 60 is, for example, a TiAl layer.
As illustrated in
Although a thickness of the semiconductor laminate 20 is drawn exaggerated in the drawings, a height of a protrusion of the texture 30 is considerably thicker than that of the p-type layer 23. Contact area between the transparent electrode 40 and p-type layer 23 is reduced corresponding in amount to the area covered with the texture 30. If the texture 30 is all made of non-doped GaN, sufficient current injection into the active layer 22 becomes difficult. If the texture 30 is all made of Mg-doped p-type GaN, light absorption therein by the dopant becomes large and external emission efficiency lowers. With usual vapor deposition, a dopant concentration becomes higher at the position near the distal end (apex) portion, and light absorption increases. A non-doped distal end portion of each protrusion of the texture is better from this reason. It is therefore preferable that each protrusion of the texture 30 has a structure having a doped lower layer 31 and a non-doped upper layer 32.
With reference to
A non-doped GaN underlying layer 12 is grown by MOCVD, by supplying TMG: 45 μmol/min and NH3: 4.4 LM for 60 minutes at a substrate temperature of 1000° C. A film thickness is about 3 μm. In this manner, an underlying surface is obtained having good crystallinity suitable for epitaxial growth of the nitride semiconductor light emission structure. An Si-doped n-type GaN layer 21 is grown by MOCVD, by supplying TMG: 45 μmol/min, SiH4: 2.7 μmol/min, and NH3: 4.4 LM for 60 minutes at a substrate temperature of 1000° C. A film thickness is about 3 μm, and a carrier concentration is about 5×1018 cm−3. An active layer 22 is grown on the n-type layer 21.
As illustrated in
For example, an InGaN well layer 22w having a thickness of about 2.2 nm is grown by supplying trimethylindium TMI: 10 μmol/min and NH3: 4.4 LM for 33 seconds at a substrate temperature of, e.g., 700° C. Next, by maintaining the substrate temperature at 700° C., a GaN barrier layer 22b having a film thickness of about 15 nm is grown by supplying TMG: 3.6 μmol/min and NH3: 4.4 LM for 320 seconds. The same crystal growth is repeated five cycles to complete the MQW active layer 22.
Reverting to
An Mg-doped p-type GaN layer 23 is grown on the clad layer 13 by MOCVD by supplying TMG: 18 μmol/min, CP2Mg: 2.7×10−7 μmol/min, and NH3: 4.4 LM for 7 minutes at a substrate temperature of 870° C. A film thickness is about 150 nm, and a carrier concentration is about 1×1018 cm−3.
As illustrated in
Instead of a circular opening, an opening such as a rectangular shape and a hexagonal shape aligned with crystalline axes may also be used. If a circular opening is used, it is not necessary to align the circular opening with axis directions of underlying crystal. The bottom surface of crystal grown in the circular opening is a regular hexagonal shape, and a length of each diagonal line of the regular hexagonal shape is approximately equal to the diameter L of the opening. For example, circular openings having a diameter L=3 μm are disposed in a close packed structure at a pitch p=2 μm. L is preferably 1 μm or longer. If the texture 30 is too small, a sufficient size for an emission wavelength cannot be realized.
As illustrated in
As illustrated in
As illustrated in
As illustrated in
Sc≧(3·31/2)/8·L2(=0.65 L2) (1)
At L=3 μm, Sc≧5.85 μm2.
Six side walls of the six-sided pyramid of the texture 30 correspond to (1-101), (10-11), (−1011), (01-11), (0-111) and (−1101) planes of a wurtzite crystal structure. An angle α between a ridge and a bottom surface of the six-sided pyramid, a bottom angle β of a side wall constituted of an isosceles triangle, and an angle γ between a side wall and a bottom surface can be estimated from a crystal structure and lattice constants. For example, if the main constituent material of the texture 30 is GaN, α≈58°, β≈75° and γ≈62°. A height H of the six-sided pyramid of the texture 30 is represented by:
H=(L/2)·tan α(=0.8 L) (2)
At L=3 μm, H=2.4 μm.
A preferable height h of the lower layer 31 obtained by rearranging the equation (1) is given by:
h≧31/2/4·tan γ(1−(1−cos γ)1/2)·L (3)
If the main constituent material of the texture 30 is GaN, the angles are substituted into the equations (2) and (3) (as in parentheses). A lattice constant and growth mode may change depending upon a composition, a dope amount, growth temperature and the like, and α, β, and γ may increase or decrease slightly.
A side length of a regular triangle which is ⅙ of a regular hexagon of the bottom surface of the six-sided pyramid is represented by U (=L/2). An area of the regular triangle is S0=(31/2/4)U2. An apex of the six-sided pyramid is represented by P, and a triangle of one side wall is represented by PMN. The triangle PMN has a lower side length U at the lower side MN, a height (U/2) tan β=1.866 U and an area S1=(1/2)U·(1.866 U)=0.933 U2. Representing a triangle of each side wall formed by the upper layer 32 by PLK and a length V of a lower side LK by V=cU, an area S2 of the triangle PLK is S2=0.933 V2=0.933 c2U2. An area S of a trapezoid KLMN is S=S1−S2=(1−c2)·0.933 U2. A difference between the areas of the trapezoid and regular triangle is Sc−S0={(1−c2)·0.933−(31/2/4)}U2. An area difference becomes 0 if 0.933 (1−c2)=(31/2/4)=0.433, i.e., (1−c2)=0.464, i.e., c2=0.536, and i.e., c=0.732. Namely, the contact areas are almost equal if an upper layer height is 0.732×(height of the six-sided pyramid) or lower and if a lower layer height is 0.268×(height of the six-sided pyramid) or higher.
Taking some margin, it is preferable that the lower layer height is 0.3×the pyramid height or higher, and more preferably the lower layer height is 0.4×the pyramid height or higher. The manufactured nitride semiconductor light emitting device has a total height of 2.4 μm and a lower layer height of 1 μm. The lower layer height is about 0.42×the total height. A total sum of a contact area between the transparent electrode 40 and the p-type layer 23 and six-sided pyramid lower layer 31 is larger than the contact area between the transparent electrode 40 and the p-type layer 23 without forming the six-sided pyramid.
p1>L/2·((tan γ)2−1) (4)
Since light output from a six-sided pyramid can be substantially suppressed from entering an adjacent six-sided pyramid, the height h of the lower layer 31 can be increased to a limit height. Increasing the height of the lower layer 31 to a limit height means that a contact area between the transparent electrode 40 and p-type layers 23 and 31 can be increased to a maximum value. A current supply is therefore enhanced.
If light vertically or upwardly output from the surface of a six-sided pyramid is made not at all to enter again an adjacent six-sided pyramid, the pitch p becomes large so that there is a fear that an external emission loss becomes large because of total reflection at the surface of the p-type layer 23 not covered with a six-sided pyramid of the texture 30. In order to reduce the total reflection at the surface of the p-type layer 23, six-sided pyramids of the texture 30 are preferably made dense. If six-sided pyramids of the texture 30 are too dense, lights output once from six-sided pyramids of the texture 30 enter again the lower layer 31 of another six-sided pyramid of the texture 30 so that there is a fear that the external emission efficiency is lowered by optical absorption by dopant in the lower layer.
As illustrated in
p2≦L/2−((tan γ)2−1)
In this case, six-sided pyramids of the texture 30 are disposed in such a manner that light will not enter again the lower layer 31 of an adjacent six-sided pyramid of the texture 30. If light enters again the lower layer 31 of another six-sided pyramid of the texture 30, external emission efficiency is lowered by optical absorption by the impurities doped therein. Since the upper layer 32 is non-doped, optical absorption by impurities does not exist in principle.
In order to ensure a contact area between the transparent electrode and p-type layer, a height h of the lower layer 31 represented by the formula (3) is retained. This condition is represented by:
p2>31/2/4·tan γ(1−(1−cos γ)1/2)((tan γ)2−1)·L
The pitch p2 between six-sided pyramids is represented by:
31/2/4·tan γ(1−(1−cos γ)1/2)((tan γ)2−1)·L≦p2≦L/2·((tan γ)2−1) (5)
In this case, if a height h of the lower layer is in a range of:
31/2/4·tan γ(1−(1−cos γ)1/2)·L≦h≦p2(cos γ/sin γ)/(1−(cos γ/sin γ)2 (6)
light will not enter again the lower layer so that six-sided pyramids can be disposed densely. A condition satisfying both the formulas (5) and (6) is preferable.
Although the present invention has been described in connection with the embodiments, the present invention is not limited thereto. For example, compositions of nitride semiconductor layers may be changed in accordance with a desired emission wavelength etc.. The positions of the n-type layer 21 and p-type layer 23 may be exchanged. In this case, dopant to be doped in the lower layer 31 of the texture 30 becomes n-type, for example, S1. Further, it is obvious for those skilled in the art that various modifications, improvements, combinations and the like are possible.
Number | Date | Country | Kind |
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2008-180656 | Jul 2008 | JP | national |
Number | Name | Date | Kind |
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6495862 | Okazaki et al. | Dec 2002 | B1 |
7244957 | Nakajo et al. | Jul 2007 | B2 |
Number | Date | Country |
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3469484 | Sep 2003 | JP |
Number | Date | Country | |
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20100006876 A1 | Jan 2010 | US |